BDNF/TrkB Signaling in the Brain–Kidney Axis Under Functional Stress

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review report

Central and Peripheral Actions of Brain-Derived Neurotrophic 2 Factor during Functional Stress

Introduction

Clear explanation of the following points may be beneficial

The HPA axis and CRH regulation by BDNF

Cardiovascular parallels (cardiomyocyte protection)

Renal fibrosis as a pathological context

The bidirectional nature of brain–kidney communication

 

Section 2: "When Functional Stress Becomes a Renal Threat"

1. The section introduces vascular stiffness and pulse pressure as modifiers of podocyte vulnerability, then immediately jumps to BDNF/TrkB as a candidate mechanism. The mechanistic bridge between hemodynamic load à podocyte cytoskeletal stress à BDNF involvement is assumed rather than explained. A transitional sentence is needed.

2."recurrence, because AKI is associated with increased long-term risk of CKD and worse outcomes with repeated events"

This phrasing conflates two distinct claims: a) AKI → CKD risk (a chronic progression argument) b) Repeated AKI → worse outcomes (a cumulative injury argument)

These are related but mechanistically different and deserve separation for clarity.

3.The section ends with a strong hedging statement — "remains hypothetical and requires direct experimental validation" — which is scientifically honest. However, this somewhat undermines the motivation for the entire review if the central mechanism is immediately flagged as purely hypothetical without acknowledging any existing supportive evidence.

Section 3: "BDNF Biosynthesis, Cellular Distribution, and Signaling"

The text alternates between "TrkB-FL" and "TrkB.FL" (the figure uses dot notation, the text uses hyphen notation). A consistent format should be used throughout.

Excellent! Let me do a thorough critical analysis of Sections 4, 5, and 6.

Section 4: "Central Effects of BDNF"

1. Repetition from Section 3 Several sentences in Section 4 are near-identical to content already presented in Section 3
2. a) TrkB expression in hippocampus, cerebral cortex, astrocytes, and microglia (lines 202–207) essentially repeats lines 159–164 from Section 3
3. b) The microglial BDNF modulation point is repeated almost word by word
4. Scientific Ambiguity — Contradictory Phenotypes "BDNF deficiency [is linked] to altered HPA axis reactivity, with both hyperreactive and hyporeactive phenotypes reported"

 

The authors acknowledge contradictory findings but offer no explanation or resolution — no discussion of why opposite phenotypes occur. Simply listing contradictory results without interpretation is scientifically unsatisfying in a review article.

3. Speculative BBB Claim: "BBB permeability may increase during severe or prolonged stress, which could theoretically facilitate access of peripheral factors to the CNS". The word "theoretically" signals speculation, which is fine, but the very next sentence introduces a neuroinflammatory risk without clarifying whether this is also theoretical or evidence-based. The level of certainty shifts without signposting, which could confuse readers.
4. Relevance to Review Focus Section 4 focuses heavily on central/CNS effects of BDNF — synaptic plasticity, memory, dendritic spines, PSD-95, synaptophysin. While contextually useful, this content is tangentially related to the review's core focus on the brain–kidney axis and podocyte protection. The section could be significantly trimmed without loss of the main argument.

 Section 5: "Peripheral Effects of BDNF"

1. Unresolved Contradiction on Platelet BDNF Origin: "The origin of platelet-derived BDNF remains debated". The authors mention this debate but do not present the opposing view — what is the alternative to megakaryocyte synthesis? Is it uptake from plasma? Endothelial transfer? A review should at minimum outline both sides of a stated debate.
2. Vague Generalization : "BDNF is most often described as a protective factor that enhances resistance to hypoxia and oxidative stress and stabilizes cytoskeletal and junctional architecture". The phrase "most often described" is imprecise for a scientific review. It should specify in which tissues, under which conditions, and cite the primary evidence directly rather than generalizing across all peripheral tissues.

 

Section 6: "Renal Effects of BDNF"

1. Internal Repetition Lines 256–262 and lines 262–267 essentially repeat the same information about BDNF localization: "BDNF has been detected in podocytes and tubular epithelial cells" (line 259–260). "BDNF has been described in proximal tubular cells, collecting duct epithelium, and renal vascular endothelium" (lines 262–263). Please rewrite appropriately.
2. Isoform Inconsistency (Recurring)

    "Podocytes predominantly express the truncated TrkB isoform TrkB-T1, whereas full-length TrkB has been reported mainly in selected experimental models" . This is an important claim, but it contradicts earlier content in Section 3, which states that analyses of isolated glomeruli detected both TrkB-FL and a truncated TrkB isoform without specifying that TrkB-FL is limited to experimental models. The two statements need to be reconciled explicitly.

3. microRNA Pathway — Directionality Needs Clarification "microRNA-134 is downregulated and microRNA-132 is upregulated, which increases LIMK1 translation and phosphorylation"The mechanism described is biologically plausible but the causal chain is compressed. It is not clearly explained: a) How does microRNA-134 downregulation specifically increase LIMK1? (miR-134 suppresses LIMK1, so its downregulation releases LIMK1 — this should be stated explicitly) b) How does microRNA-132 upregulation fit into this same pathway — is it additive, parallel, or synergistic?

A reader unfamiliar with miRNA biology would struggle to follow this without more explicit mechanistic explanation.

4. BDNF Molecular Weight — Minor Inaccuracy Risk

    "Monomeric BDNF is approximately 14 kDa and about 27 kDa as a dimer"

The mature monomeric BDNF is approximately 13–14 kDa, which is correct. However, the authors use this as an argument for potential filtration barrier traversal without acknowledging charge. BDNF is a basic protein (pI ~10) and the glomerular basement membrane carries a negative charge, which would actually impede passage of cationic proteins. This electrochemical factor is not discussed, which is a notable scientific omission given the context. 

Section 7: "BDNF as a Biomarker of Kidney Damage"

1. Circular Referencing

References [22,23] are cited repeatedly throughout this short section — appearing in nearly every sentence. This suggests the entire section rests on a very narrow evidence base (essentially 2 studies), which should be explicitly acknowledged as a major limitation rather than obscured by repeated citation.

2. Interpretive Ambiguity — Correlation Direction- "Urinary BDNF mRNA levels also correlate negatively with the urinary albumin-to-creatinine ratio (uACR), suggesting an association with better preservation of the glomerular filtration barrier" This interpretation assumes higher urinary BDNF mRNA = better barrier preservation. However, an alternative interpretation — that higher urinary BDNF mRNA reflects increased local synthesis as a stress response to barrier damage — is not addressed. Both interpretations are biologically plausible and should be discussed.
3. Unresolved Chicken-and-Egg Problem-"whether BDNF primarily reflects systemic stress and platelet activation or acts as an active participant in pathogenic cascades" -This is a fundamental interpretive question that the section raises but does not attempt to resolve or even discuss further. For a review article, simply flagging this without any analytical framework is insufficient.

 Section 8: "Neuron and Podocyte: A Shared Actin-Dependent Organizational Principle"

1. Repetition of miRNA Mechanism: The miR-132/miR-134 → LIMK1 → cofilin pathway is described for the third time across the manuscript (previously in Sections 3 and 6). While brief here, this repetition is unnecessary and should be replaced with a cross-reference to the earlier section.
2. Subheading Formatting Inconsistency: The subsections "Small Rho GTPases", "The LIMK1–cofilin axis", and "Cytoskeletal protein synthesis" are formatted as bold subheadings without numbering, while all other sections in the manuscript use numbered headings. This is a formatting inconsistency that should be standardized.
3. Scientific Oversimplification — Rho GTPases- "Cdc42 promotes the initiation of new protrusions (filopodia) and de novo actin assembly, whereas Rac1 drives peripheral actin polymerization" -While broadly accurate, the text presents Rac1 and Cdc42 as operating independently. In reality, crosstalk between Rac1, Cdc42, and RhoA is essential to cytoskeletal outcomes — RhoA in particular counterbalances Rac1/Cdc42 activity and is relevant to podocyte stress responses. Its complete omission here is a notable scientific gap.

Section 9: "Strenuous Physical Exercise and the Blood-Brain Barrier"

1. Section Title Mismatch: The section is titled "Strenuous Physical Exercise and the Blood-Brain Barrier" but covers much more — including renal biomarkers of AKI, circulating BDNF dynamics, and systemic inflammation. The BBB content, while interesting, occupies only the second half. A more accurate title would be something like "Systemic Effects of Extreme Exercise on the Brain–Kidney Axis".
2. S100B Specificity Caveat — The authors correctly note that S100B is not brain-specific. However, they do not suggest what alternative or complementary biomarkers of BBB disruption would be more reliable in this context (e.g., neurofilament light chain, GFAP). For a review, this omission leaves the reader without a practical alternative.
3. "transient increases in BBB permeability may facilitate humoral signaling to the CNS and thereby affect behavior and cognitive performance". This is a multi-step speculative chain — BBB disruption → humoral access → behavioral change — presented without clearly flagging each inferential step. The authors should use more explicit hedging language at each step.
4. The proposed mechanisms of BBB permeability increase (inflammation, hyperthermia, dehydration, serotonergic effects, reduced cerebral perfusion) are presented as a flat list without any indication of their relative importance, evidence quality, or interaction. A brief ranking or acknowledgment of which mechanisms have the strongest evidence would strengthen this section.

 Section 10: "BDNF in Kidney Disease"

1. Section 10 substantially overlaps with Section 7 on BDNF as a biomarker. Specifically: The Endlich et al. findings (KIM-1, NPHS1 associations) are discussed in both sections. The reduced serum / increased urinary BDNF pattern in proteinuric conditions is mentioned in both. The CKD cohort associations are covered in both. These two sections should either be merged or clearly differentiated — Section 7 focusing purely on biomarker utility and Section 10 on mechanistic disease-context implications.
2. Urinary Bladder Analogy — Weak Extrapolation- "In the urinary bladder, BDNF has been implicated in regenerative smooth muscle responses...this example cannot be directly extrapolated to the kidney". The authors themselves acknowledge this analogy cannot be directly extrapolated, which raises the question of why it is included at all. If it cannot be extrapolated, its scientific utility in this context is questionable and it risks confusing rather than clarifying the argument.
3. Weak Causal Language Throughout

The section relies heavily on phrases like: "may reflect", "remains ambiguous", "remains limited", "supports an association" etc. While epistemically honest, the density of hedging language without any synthesis of what IS known with reasonable confidence makes the section feel inconclusive. A brief paragraph summarizing what the evidence does support with reasonable certainty would improve the section's contribution.

 Section 11: "Translational Perspectives"

1. Given that translational perspectives are central to the review's stated purpose — improving risk assessment and developing renoprotective strategies — this section is very short. It identifies knowledge gaps but offers minimal concrete translational roadmap.
2. "prolonged pathway activation could also drive maladaptive cytoskeletal remodeling". This is an important safety concern that is introduced for the first time here, near the end of the manuscript. Given its significance for any therapeutic application, it should have been flagged earlier — ideally in Section 6 or 8 — and then revisited here with more depth.
3. The abstract mentions the discussion of "strategies to protect kidney function in physically stressed individuals" but Section 11 does not deliver concrete strategies. There is no discussion of exercise prescription modifications, BDNF-enhancing pharmacological approaches, monitoring protocols for at-risk athletes or dietary or hydration interventions. This is a significant gap between the abstract and the manuscript.
4. Repetition of Knowledge Gaps : The three knowledge gaps listed in Section 11 (causal isolation of BDNF, TrkB isoform roles, long-term safety) largely repeat what was already flagged in earlier sections, particularly the abstract and introduction.

For the minor and typographic errors, refere the attached document

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Comments for author File: Comments.pdf

Author Response

Reviewer 1 Report:

Central and Peripheral Actions of Brain-Derived Neurotrophic 2 Factor during Functional Stress

Introduction

Clear explanation of the following points may be beneficial

The HPA axis and CRH regulation by BDNF

Cardiovascular parallels (cardiomyocyte protection)

Renal fibrosis as a pathological context

The bidirectional nature of brain–kidney communication

Summary Response: We thank the reviewer for this constructive suggestion. In the revised Introduction, we expanded the conceptual framing of the brain–kidney axis by (a) explicitly describing the role of BDNF in HPA-axis regulation, including CRH-linked stress reactivity, as a mechanistic entry point for neuroendocrine modulation of systemic hemodynamics during functional stress [19]; (b) adding cardiovascular parallels by summarizing evidence that BDNF/TrkB signaling supports cardiomyocyte viability and can confer protection under ischemic stress conditions [23,24]; (c) incorporating renal fibrosis as a relevant pathological context in which BDNF/TrkB has been implicated in remodeling processes [18]; and (d) clarifying the bidirectional nature of brain–kidney communication, emphasizing that central autonomic/neuroendocrine outputs shape renal hemodynamics while renal perturbations can engage afferent and humoral signaling that feeds back to central stress circuits. These additions were introduced to improve clarity and to better motivate the review’s mechanistic focus on BDNF/TrkB signaling under functional stress.

Section 2: "When Functional Stress Becomes a Renal Threat"

The section introduces vascular stiffness and pulse pressure as modifiers of podocyte vulnerability, then immediately jumps to BDNF/TrkB as a candidate mechanism. The mechanistic bridge between hemodynamic load à podocyte cytoskeletal stress à BDNF involvement is assumed rather than explained. A transitional sentence is needed.

Response : We thank the reviewer for this helpful suggestion. In the revised manuscript, we added an explicit transitional bridge linking vascular stiffness and elevated pulse pressure to increased pulsatile load and mechanical strain at the glomerular filtration barrier, which challenges podocyte actin cytoskeletal stability and predisposes to foot process effacement. We then use this hemodynamic-to-cytoskeletal coupling to motivate consideration of candidate resilience pathways converging on actin regulation, including BDNF/TrkB signaling. This addition clarifies the mechanistic rationale for introducing BDNF/TrkB in this section and improves conceptual continuity.

Changes in the manuscript:

Section 2 now includes a mechanistic transition from hemodynamic load – podocyte actin remodeling/effacement risk – rationale for actin-convergent resilience pathways (BDNF/TrkB).

2."recurrence, because AKI is associated with increased long-term risk of CKD and worse outcomes with repeated events"

This phrasing conflates two distinct claims: a) AKI → CKD risk (a chronic progression argument) b) Repeated AKI → worse outcomes (a cumulative injury argument)

These are related but mechanistically different and deserve separation for clarity.

Response: We thank the reviewer for this important clarification. We revised the text to separate these two related but distinct concepts. The revised wording first notes that even a single episode of AKI is associated with an increased long-term risk of CKD and adverse long-term outcomes in longitudinal cohorts, and then separately states that repeated AKI episodes are linked to additional risk consistent with a cumulative-injury framework. This restructuring improves conceptual precision and avoids conflating chronic progression with cumulative injury effects.

Changes in the manuscript:

Section 2 now presents the recurrence rationale as two distinct statements (“First… Second…”) with corresponding citations [29–31].

3.The section ends with a strong hedging statement — "remains hypothetical and requires direct experimental validation" — which is scientifically honest. However, this somewhat undermines the motivation for the entire review if the central mechanism is immediately flagged as purely hypothetical without acknowledging any existing supportive evidence.

Response: We thank the reviewer for this valuable point. In the revised manuscript, we retained appropriate epistemic caution but adjusted the framing to avoid presenting the central mechanism as purely hypothetical. Specifically, we now acknowledge that the BDNF/TrkB hypothesis is supported by mechanistic evidence from experimental podocyte/glomerular injury models linking BDNF/TrkB signaling to actin-regulatory programs and proteinuria-related outcomes, while clearly stating that the dominant in vivo route and relevance under human exercise-related renal stress remain to be established. This revision preserves scientific rigor while maintaining the rationale for the review.

Changes in the manuscript:

Section 2 conclusion was revised to balance supportive mechanistic model evidence with the need for human causal validation (including explicit reference to podocyte injury model evidence [23]).

Section 3: "BDNF Biosynthesis, Cellular Distribution, and Signaling"

The text alternates between "TrkB-FL" and "TrkB.FL" (the figure uses dot notation, the text uses hyphen notation). A consistent format should be used throughout.

Excellent! Let me do a thorough critical analysis of Sections 4, 5, and 6.

Response: We thank the reviewer for noting this inconsistency. We standardized TrkB isoform notation across the entire manuscript and figure legends, using a single consistent format (TrkB-FL, TrkB-T1, TrkB-T2) throughout the text and figures. This ensures uniform terminology and prevents confusion between isoforms.

Changes in the manuscript:

Global edit: unified TrkB isoform notation in Sections 3–11, Figure 1 legend, and all cross-references.

Section 4: "Central Effects of BDNF"

Repetition from Section 3 Several sentences in Section 4 are near-identical to content already presented in Section 3

1. a) TrkB expression in hippocampus, cerebral cortex, astrocytes, and microglia (lines 202–207) essentially repeats lines 159–164 from Section 3
2. b) The microglial BDNF modulation point is repeated almost word by word

Scientific Ambiguity — Contradictory Phenotypes "BDNF deficiency [is linked] to altered HPA axis reactivity, with both hyperreactive and hyporeactive phenotypes reported"

The authors acknowledge contradictory findings but offer no explanation or resolution — no discussion of why opposite phenotypes occur. Simply listing contradictory results without interpretation is scientifically unsatisfying in a review article.

Speculative BBB Claim: "BBB permeability may increase during severe or prolonged stress, which could theoretically facilitate access of peripheral factors to the CNS". The word "theoretically" signals speculation, which is fine, but the very next sentence introduces a neuroinflammatory risk without clarifying whether this is also theoretical or evidence-based. The level of certainty shifts without signposting, which could confuse readers.

Relevance to Review Focus Section 4 focuses heavily on central/CNS effects of BDNF — synaptic plasticity, memory, dendritic spines, PSD-95, synaptophysin. While contextually useful, this content is tangentially related to the review's core focus on the brain–kidney axis and podocyte protection. The section could be significantly trimmed without loss of the main argument.

Response: We thank the reviewer for this detailed and helpful critique. We substantially revised Section 4 to improve focus and conceptual clarity. First, we removed repetitive descriptions of TrkB distribution across CNS regions and glial compartments and replaced them with a brief cross-reference to Section 3. Second, we addressed the apparent contradiction in HPA-axis phenotypes by adding a short interpretive framework explaining how opposite outcomes can arise across models due to differences in experimental context (developmental versus adult manipulations, region/circuit specificity, stressor characteristics, and compensatory feedback adaptations), and we emphasize that BDNF acts as a modulator of stress-circuit set points rather than enforcing a unidirectional change in HPA output. Third, we revised the BBB paragraph to maintain consistent hedging, explicitly framing BBB permeability changes and downstream neuroinflammatory implications as context-dependent and uncertain in humans. Finally, we trimmed “neuroplasticity textbook” details (e.g., synaptic protein examples) and retained only CNS elements directly relevant to the review’s brain–kidney axis logic (stress control, autonomic regulation, and barrier considerations).

Changes in the manuscript:

Section 4 rewritten: removed redundancy with Section 3 and added cross-reference.

Added interpretive explanation for divergent HPA phenotypes (context dependence).

Revised BBB wording to align certainty/hedging and clarify human relevance.

Trimmed synaptic/memory-specific details to preserve focus on brain–kidney axis relevance.

Section 5: "Peripheral Effects of BDNF"

Unresolved Contradiction on Platelet BDNF Origin: "The origin of platelet-derived BDNF remains debated". The authors mention this debate but do not present the opposing view — what is the alternative to megakaryocyte synthesis? Is it uptake from plasma? Endothelial transfer? A review should at minimum outline both sides of a stated debate.

Vague Generalization : "BDNF is most often described as a protective factor that enhances resistance to hypoxia and oxidative stress and stabilizes cytoskeletal and junctional architecture". The phrase "most often described" is imprecise for a scientific review. It should specify in which tissues, under which conditions, and cite the primary evidence directly rather than generalizing across all peripheral tissues. 

Comment 5.1 (Platelet BDNF origin): The manuscript states that the origin of platelet-derived BDNF is debated but does not outline alternative explanations

Response: We agree and revised Section 5 to briefly outline non-mutually exclusive explanations for platelet-associated BDNF. Specifically, we now distinguish between (a) endogenous loading during megakaryocyte/platelet biogenesis and (b) acquisition from the extracellular milieu, including uptake/binding of soluble BDNF and potential transfer within platelet–endothelium interfaces during vascular activation. We also emphasize that circulating measurements are strongly shaped by platelet activation and sampling matrix, making these mechanisms difficult to disentangle in human studies.

Changes in the manuscript:

Section 5 now includes a short, explicit clarification of two competing/non-exclusive hypotheses for platelet-associated BDNF and reiterates serum vs plasma/platelet-poor plasma interpretive constraints.

Comment 5.2 (Vague generalization): “Most often described” is imprecise, as the statement should be tissue- and context-specific.

Response: We agree and revised the wording to avoid broad generalization. In the revised Section 5, we replaced “most often described” with tissue- and context-specific statements and cited representative peripheral evidence, including cardiomyocyte viability under ischemic stress and endothelial barrier/junctional stability in inflammatory or hypoxic contexts, as examples of reported BDNF/TrkB–linked stress-resilience programs.

Changes in the manuscript:

Section 5 now uses context- and cell type–dependent phrasing and anchors peripheral examples to cardiovascular and endothelial evidence with primary citations.

Section 6: "Renal Effects of BDNF"

Internal Repetition Lines 256–262 and lines 262–267 essentially repeat the same information about BDNF localization: "BDNF has been detected in podocytes and tubular epithelial cells" (line 259–260). "BDNF has been described in proximal tubular cells, collecting duct epithelium, and renal vascular endothelium" (lines 262–263). Please rewrite appropriately.

Isoform Inconsistency (Recurring)

    "Podocytes predominantly express the truncated TrkB isoform TrkB-T1, whereas full-length TrkB has been reported mainly in selected experimental models" . This is an important claim, but it contradicts earlier content in Section 3, which states that analyses of isolated glomeruli detected both TrkB-FL and a truncated TrkB isoform without specifying that TrkB-FL is limited to experimental models. The two statements need to be reconciled explicitly.

microRNA Pathway — Directionality Needs Clarification "microRNA-134 is downregulated and microRNA-132 is upregulated, which increases LIMK1 translation and phosphorylation"The mechanism described is biologically plausible but the causal chain is compressed. It is not clearly explained: a) How does microRNA-134 downregulation specifically increase LIMK1? (miR-134 suppresses LIMK1, so its downregulation releases LIMK1 — this should be stated explicitly) b) How does microRNA-132 upregulation fit into this same pathway — is it additive, parallel, or synergistic?

A reader unfamiliar with miRNA biology would struggle to follow this without more explicit mechanistic explanation.

BDNF Molecular Weight — Minor Inaccuracy Risk

    "Monomeric BDNF is approximately 14 kDa and about 27 kDa as a dimer"

The mature monomeric BDNF is approximately 13–14 kDa, which is correct. However, the authors use this as an argument for potential filtration barrier traversal without acknowledging charge. BDNF is a basic protein (pI ~10) and the glomerular basement membrane carries a negative charge, which would actually impede passage of cationic proteins. This electrochemical factor is not discussed, which is a notable scientific omission given the context. 

Comment 6.1 (Internal repetition on BDNF localization): The section repeats BDNF localization statements across renal compartments.

Response: We agree and revised Section 6 to remove redundant phrasing and present renal BDNF localization in a single compact description. The revised text now summarizes BDNF presence across nephron segments and renal vascular structures once, and then transitions to compartment-relevant implications (local paracrine signaling and stress-response programs) without repeating the same localization information.

Changes in the manuscript:

Section 6 now contains a consolidated localization paragraph (nephron segments, renal endothelium, and glomerular-related contexts) without duplicate “detected/described” statements.

Comment 6.2 (Isoform inconsistency: glomeruli vs podocytes): Statements about TrkB-FL in podocytes conflict with Section 3 reporting both TrkB-FL and truncated isoforms in isolated glomeruli.

Response: We agree and reconciled these statements explicitly. In the revised manuscript, we clarify that isoform detection depends on the biological level examined: isolated glomeruli represent a mixed-cell preparation (podocytes, endothelial and mesangial cells) in which both TrkB-FL and truncated variants can be detected, whereas podocyte-focused datasets more consistently report predominant expression of truncated TrkB isoforms (often TrkB-T1) with more variable TrkB-FL detection depending on species, injury context, and experimental system. We also distinguish expression evidence from isoform-specific functional validation.

Changes in the manuscript:

Section 6 now includes an explicit reconciliation sentence linking glomerular mixed-cell detection to podocyte-focused isoform reporting and adds cautious language regarding isoform-specific function.

Comment 6.3 (miRNA pathway directionality): The causal chain miR-134/miR-132 → LIMK1 → cofilin is compressed and unclear.

Response: We agree and expanded the mechanistic explanation to make directionality explicit. The revised text now states that miR-134 acts as a negative regulator of LIMK1 translation and that its downregulation can relieve repression of LIMK1, increasing LIMK1 availability. We also clarify that miR-132 upregulation has been reported alongside these changes and may act in a coordinated manner to support cytoskeletal remodeling programs, while the LIMK1–cofilin consequence (cofilin phosphorylation and reduced actin depolymerization) is described stepwise.

Changes in the manuscript:

Section 6 now provides a clear, stepwise causal explanation of the miRNA–LIMK1–cofilin axis rather than a compressed statement.

Comment 6.4 (BDNF molecular weight and filtration argument): The text uses molecular size to imply filtration without addressing charge selectivity.

Response: We agree and revised this discussion to incorporate charge-dependent constraints of the filtration barrier. The revised text retains the mature BDNF size range (~13–14 kDa monomer; ~27 kDa dimer) but explicitly states that glomerular handling depends on size and electrostatic interactions with negatively charged barrier components. We therefore frame access of circulating BDNF to glomerular cells as unresolved and emphasize that dominant delivery routes (local renal production vs vascular/endothelial transport mechanisms and other pathways) remain a major limitation requiring targeted validation.

Changes in the manuscript:

Section 6 now includes an explicit charge-selectivity caveat and reframes “size” as insufficient alone for inferring glomerular access, highlighting source/delivery as an unresolved limitation.

Section 7: "BDNF as a Biomarker of Kidney Damage"

Circular Referencing

References [22,23] are cited repeatedly throughout this short section — appearing in nearly every sentence. This suggests the entire section rests on a very narrow evidence base (essentially 2 studies), which should be explicitly acknowledged as a major limitation rather than obscured by repeated citation.

Interpretive Ambiguity — Correlation Direction- "Urinary BDNF mRNA levels also correlate negatively with the urinary albumin-to-creatinine ratio (uACR), suggesting an association with better preservation of the glomerular filtration barrier" This interpretation assumes higher urinary BDNF mRNA = better barrier preservation. However, an alternative interpretation — that higher urinary BDNF mRNA reflects increased local synthesis as a stress response to barrier damage — is not addressed. Both interpretations are biologically plausible and should be discussed.

Unresolved Chicken-and-Egg Problem-"whether BDNF primarily reflects systemic stress and platelet activation or acts as an active participant in pathogenic cascades" -This is a fundamental interpretive question that the section raises but does not attempt to resolve or even discuss further. For a review article, simply flagging this without any analytical framework is insufficient.

Comment 7.1 (Circular referencing / narrow evidence base): References [22,23] appear in nearly every sentence, suggesting the section rests on a very narrow evidence base that should be acknowledged explicitly.

Response: We agree and revised Section 7 to be more explicit about the limited evidence base rather than obscuring it through repeated citation. The revised section now states upfront that current urinary-sediment transcript evidence is derived from a small number of studies, with [22,23] representing a substantial fraction of the direct evidence. We also streamlined the narrative to reduce repetitive citation while maintaining accurate attribution.

Changes in the manuscript:

Section 7 now explicitly acknowledges the narrow evidence base and reduces repetitive citation density while preserving key references.

Comment 7.2 (Interpretive ambiguity: uACR correlation direction): The negative correlation between urinary BDNF mRNA and uACR is interpreted as better barrier preservation, but an alternative “stress-response/local synthesis” interpretation is not discussed.

Response: We agree and revised the interpretation to present both biologically plausible explanations. The revised text now states that the negative association can be interpreted either as reflecting more effective barrier preservation/local protective programs, or alternatively as reflecting a compensatory stress response and/or shifts in urinary sediment composition during injury that do not straightforwardly indicate barrier preservation. This revision avoids one-directional inference from correlation.

Changes in the manuscript:

Section 7 now includes a dual-interpretation paragraph for the uACR association rather than a single directional conclusion.

Comment 7.3 (Chicken-and-egg: marker vs mediator): The section raises “marker vs mediator” but provides no analytical framework for distinguishing these possibilities.

Response: We agree and strengthened the analytical framing. In the revised Section 7, we explicitly position BDNF-related readouts as context-dependent candidate biomarkers at present and outline the type of study designs needed to distinguish systemic/platelet-driven signals from local renal responses (paired blood matrices including platelet-poor plasma, urinary sediment transcripts, podocyte-centered endpoints, and longitudinal sampling across stress exposures and recovery trajectories). This provides a concrete framework for resolving directionality.

Changes in the manuscript:

Section 7 now includes an explicit “marker vs mediator” framework and specifies endpoint-driven longitudinal designs needed to establish directionality.

 Section 8: "Neuron and Podocyte: A Shared Actin-Dependent Organizational Principle"

Repetition of miRNA Mechanism: The miR-132/miR-134 → LIMK1 → cofilin pathway is described for the third time across the manuscript (previously in Sections 3 and 6). While brief here, this repetition is unnecessary and should be replaced with a cross-reference to the earlier section.

Subheading Formatting Inconsistency: The subsections "Small Rho GTPases", "The LIMK1–cofilin axis", and "Cytoskeletal protein synthesis" are formatted as bold subheadings without numbering, while all other sections in the manuscript use numbered headings. This is a formatting inconsistency that should be standardized.

Scientific Oversimplification — Rho GTPases- "Cdc42 promotes the initiation of new protrusions (filopodia) and de novo actin assembly, whereas Rac1 drives peripheral actin polymerization" -While broadly accurate, the text presents Rac1 and Cdc42 as operating independently. In reality, crosstalk between Rac1, Cdc42, and RhoA is essential to cytoskeletal outcomes — RhoA in particular counterbalances Rac1/Cdc42 activity and is relevant to podocyte stress responses. Its complete omission here is a notable scientific gap.

Comment 8.1 (Repetition of miRNA mechanism): The miR-132/miR-134 → LIMK1 → cofilin pathway is repeated and should be replaced with a cross-reference.

Response: We agree and removed the repeated miRNA description from Section 8. In the revised manuscript, we now provide a brief cross-reference to the detailed miRNA–LIMK1–cofilin discussion presented earlier, rather than repeating the mechanism.

Changes in the manuscript:

Section 8 now replaces the repeated miRNA mechanism with a cross-reference to the earlier detailed description (Section 6).

Comment 8.2 (Subheading formatting inconsistency): Subsections are bolded but not numbered, unlike the rest of the manuscript.

Response: We agree and standardized formatting across the manuscript. In the revised version, the Section 8 subsections are now presented as numbered subheadings consistent with the overall structure.

Changes in the manuscript:

Section 8 subsections are now formatted as numbered headings (e.g., 8.1, 8.2, 8.3).

Comment 8.3 (Rho GTPases oversimplification; omission of RhoA): Rac1/Cdc42 are presented without acknowledging essential crosstalk with RhoA.

Response: We agree and revised this subsection to acknowledge crosstalk among small Rho GTPases. Specifically, we added RhoA-driven actomyosin/contractility as a counterbalancing pathway that modulates Rac1/Cdc42 outcomes and is relevant to podocyte stress responses. This improves biological accuracy while keeping the discussion concise.

Changes in the manuscript:

Section 8 now explicitly includes RhoA and notes functional crosstalk among Rac1, Cdc42, and RhoA in shaping actin-dependent outcomes.

Section 9: "Strenuous Physical Exercise and the Blood-Brain Barrier"

Section Title Mismatch: The section is titled "Strenuous Physical Exercise and the Blood-Brain Barrier" but covers much more — including renal biomarkers of AKI, circulating BDNF dynamics, and systemic inflammation. The BBB content, while interesting, occupies only the second half. A more accurate title would be something like "Systemic Effects of Extreme Exercise on the Brain–Kidney Axis".

S100B Specificity Caveat — The authors correctly note that S100B is not brain-specific. However, they do not suggest what alternative or complementary biomarkers of BBB disruption would be more reliable in this context (e.g., neurofilament light chain, GFAP). For a review, this omission leaves the reader without a practical alternative.

"transient increases in BBB permeability may facilitate humoral signaling to the CNS and thereby affect behavior and cognitive performance". This is a multi-step speculative chain — BBB disruption → humoral access → behavioral change — presented without clearly flagging each inferential step. The authors should use more explicit hedging language at each step.

The proposed mechanisms of BBB permeability increase (inflammation, hyperthermia, dehydration, serotonergic effects, reduced cerebral perfusion) are presented as a flat list without any indication of their relative importance, evidence quality, or interaction. A brief ranking or acknowledgment of which mechanisms have the strongest evidence would strengthen this section.

Comment 9.1 (Section title mismatch): The title focuses on BBB but the section covers broader systemic and renal aspects.

Response: We agree and revised the section title to reflect its broader scope, including systemic stress physiology, renal stress/AKI biomarkers, and BDNF dynamics in addition to BBB-related considerations.

Changes in the manuscript:

Section 9 title updated to better match content (systemic effects of extreme exercise on the brain–kidney axis).

Comment 9.2 (S100B specificity; suggest complementary biomarkers): The section notes S100B is not brain-specific but does not propose alternatives (e.g., GFAP, NfL).

Response: We agree and expanded the discussion to suggest complementary blood biomarkers with higher CNS relevance that may help contextualize CNS stress in extreme-exercise settings. We added GFAP (astroglial injury/stress marker) and neurofilament light chain (NfL; neuroaxonal injury marker), while explicitly noting that these markers are not direct BBB-permeability measures and require careful interpretation under prolonged exertion and systemic inflammation.

Changes in the manuscript:

Section 9 now includes GFAP and NfL as complementary CNS-relevant biomarkers and clarifies their interpretive limitations.

Comment 9.3 (Multi-step speculative chain BBB - humoral access - behavior): The causal chain is presented too directly because stronger hedging is needed at each step.

Response: We agree and rewrote this statement to explicitly present it as a multi-step hypothesis. The revised wording separates (a) reported BBB permeability shifts in some contexts, (b) the conditional possibility of increased access of circulating mediators to CNS-facing interfaces, and (c) the further conditional and context-dependent nature of any behavioral/cognitive effects, which are framed as hypothetical rather than established causality.

Changes in the manuscript:

Section 9 now uses stepwise hedging and explicitly labels the BBB–behavior link as a hypothesis.

Comment 9.4 (Flat list of BBB mechanisms; no ranking): Mechanisms are listed without indicating relative evidence strength.

Response: We agree and added a brief evidence-weighting statement. The revised text notes that the most consistently supported contributors in exercise literature relate to systemic inflammation/oxidative stress and thermal–hydration strain (hyperthermia and hypohydration), while altered perfusion/oxygenation is plausible but more context-dependent, whereas receptor-specific neuromodulatory mechanisms (e.g., serotonergic effects) are presented as lower-certainty contributors in endurance exercise settings.

Changes in the manuscript:

Section 9 now includes a short qualitative ranking of proposed BBB contributors by consistency of evidence.

 Section 10: "BDNF in Kidney Disease"

Section 10 substantially overlaps with Section 7 on BDNF as a biomarker. Specifically: The Endlich et al. findings (KIM-1, NPHS1 associations) are discussed in both sections. The reduced serum / increased urinary BDNF pattern in proteinuric conditions is mentioned in both. The CKD cohort associations are covered in both. These two sections should either be merged or clearly differentiated — Section 7 focusing purely on biomarker utility and Section 10 on mechanistic disease-context implications.

Urinary Bladder Analogy — Weak Extrapolation- "In the urinary bladder, BDNF has been implicated in regenerative smooth muscle responses...this example cannot be directly extrapolated to the kidney". The authors themselves acknowledge this analogy cannot be directly extrapolated, which raises the question of why it is included at all. If it cannot be extrapolated, its scientific utility in this context is questionable and it risks confusing rather than clarifying the argument.

Weak Causal Language Throughout

The section relies heavily on phrases like: "may reflect", "remains ambiguous", "remains limited", "supports an association" etc. While epistemically honest, the density of hedging language without any synthesis of what IS known with reasonable confidence makes the section feel inconclusive. A brief paragraph summarizing what the evidence does support with reasonable certainty would improve the section's contribution.

Comment 10.1 (Overlap with Section 7 on biomarker content): Section 10 substantially overlaps with Section 7; the two sections should be merged or clearly differentiated.

Response: We agree and revised the structure to clearly differentiate these sections. In the revised manuscript, Section 7 focuses strictly on biomarker interpretation and methodological constraints (including urinary sediment transcripts, uACR associations, and blood matrix/platelet confounding), whereas Section 10 has been reframed to emphasize disease-context biology and mechanistic implications in CKD/proteinuric states without repeating biomarker associations. This restructuring reduces redundancy and improves conceptual coherence.

Changes in the manuscript:

Section 7 refocused on biomarker utility/interpretation; Section 10 refocused on CKD disease-context mechanisms, with removal of overlapping biomarker-specific content from Section 10.

Comment 10.2 (Urinary bladder analogy): The bladder analogy is acknowledged as non-extrapolatable and risks confusing the argument.

Response: We agree and removed the urinary bladder analogy from Section 10 to avoid weak extrapolation and to maintain focus on kidney-relevant evidence and mechanisms.

Changes in the manuscript:

Urinary bladder analogy deleted from Section 10.

Comment 10.3 (Weak causal language; inconclusive tone): The section contains dense hedging without synthesis of what is known with reasonable confidence.

Response: We agree and strengthened the synthesis. In the revised Section 10, we retained appropriate caution but added a short “reasonable confidence” paragraph summarizing the main points supported by current evidence (renal detectability/dynamic regulation of BDNF/TrkB axis components in chronic injury contexts; mechanistic support from glomerular injury models for actin-regulatory programs relevant to podocyte integrity; and major confounding of circulating BDNF interpretation by platelet biology and systemic comorbidities). This improves readability and provides a clearer take-home message.

Changes in the manuscript:

Section 10 now includes a concise synthesis paragraph (“reasonable confidence”) to balance cautious language with clear conclusions.

Section 11: "Translational Perspectives"

Given that translational perspectives are central to the review's stated purpose — improving risk assessment and developing renoprotective strategies — this section is very short. It identifies knowledge gaps but offers minimal concrete translational roadmap.

"prolonged pathway activation could also drive maladaptive cytoskeletal remodeling". This is an important safety concern that is introduced for the first time here, near the end of the manuscript. Given its significance for any therapeutic application, it should have been flagged earlier — ideally in Section 6 or 8 — and then revisited here with more depth.

The abstract mentions the discussion of "strategies to protect kidney function in physically stressed individuals" but Section 11 does not deliver concrete strategies. There is no discussion of exercise prescription modifications, BDNF-enhancing pharmacological approaches, monitoring protocols for at-risk athletes or dietary or hydration interventions. This is a significant gap between the abstract and the manuscript.

Repetition of Knowledge Gaps : The three knowledge gaps listed in Section 11 (causal isolation of BDNF, TrkB isoform roles, long-term safety) largely repeat what was already flagged in earlier sections, particularly the abstract and introduction.

For the minor and typographic errors, refere the attached document

Comment 11.1 (Section too short; lacks translational roadmap): The section is very short and offers minimal concrete translational roadmap.

Response: We agree and substantially expanded Section 11 to provide a practical translational roadmap. The revised section now explicitly organizes translational implications into (i) risk stratification (conditions that convert exercise into a renal threat), (ii) mechanism-informed monitoring and inference strategies, (iii) near-term renoprotective domains that can be evaluated within standard sports medicine and nephrology frameworks, and (iv) testable directions for BDNF/TrkB as a candidate intervention axis with safety considerations.

Changes in the manuscript:

Section 11 rewritten and expanded to include a structured translational roadmap (risk – monitoring – interventions – testable mechanistic directions).

Comment 11.2 (Safety concern introduced only here): The maladaptive remodeling safety concern is introduced for the first time near the end and should be flagged earlier.

Response: We agree and flagged this safety consideration earlier in the manuscript. We added an explicit statement in the renal mechanistic section noting that sustained or non-physiological enhancement of BDNF/TrkB signaling could, in principle, promote maladaptive actin remodeling or reduce adaptive capacity under fluctuating mechanical demands, and we then revisit this point in Section 11 with greater depth.

Changes in the manuscript:

Safety caveat added in Section 6 (podocyte cytoskeletal remodeling context) and revisited in Section 11.

Comment 11.3 (Abstract promises strategies; Section 11 did not deliver): The abstract mentions “strategies to protect kidney function…” but Section 11 lacked concrete strategies.

Response: We agree and revised Section 11 to include concrete, near-term renoprotective domains relevant to physically stressed individuals. These now include hydration and heat-mitigation strategies, avoidance of NSAIDs around high-risk events, exposure scheduling and recovery considerations for repeated high-load stress, and individualized counseling for athletes with vascular or renal vulnerability, alongside mechanism-informed monitoring approaches.

Changes in the manuscript:

Section 11 now includes explicit, practical strategy domains aligned with the Abstract.

Comment 11.4 (Repetition of knowledge gaps): The knowledge gaps in Section 11 largely repeat earlier sections.

Response: We agree and reframed Section 11 away from reiterating knowledge gaps. The revised section now emphasizes “what next” in terms of testable, endpoint-driven study designs (source attribution, bioavailable fraction, isoform-specific actions, and glomerular targets) rather than repeating prior gap statements.

Changes in the manuscript:

Section 11 refocused from repeated “knowledge gaps” to testable translational priorities and endpoint-driven study designs.

 

Please see the attachment for a version of the article with all the corrections (marked by red color).

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This review focuses on BDNF/TrkB signaling in the brain–kidney axis during functional stress and its potential role in podocyte protection, covering a scientifically meaningful topic. However, the presentation would benefit from improved balance between central and peripheral mechanisms, stronger integration of human causal data, clearer elucidation of BDNF sources and TrkB isoform functions, and a dedicated Limitations section. Further refinement of writing, structure, and figure annotations is also suggested to enhance clarity and academic rigor. 

Major comment
1.The authors hypothesize that BDNF/TrkB signaling protects podocytes against exercise-related kidney injury, but the manuscript fails to clarify which source of BDNF (central, circulating, or local) mediates this effect or the dominant mechanism within the brain–kidney axis. This incomplete mechanistic chain significantly weakens the causal validity of the core conclusion.
2.The manuscript states that podocytes mainly express TrkB-T1, yet direct functional evidence for its roles in calcium signaling and cytoskeletal regulation remains limited. The authors should better distinguish between receptor expression and validated function, and use more cautious language to avoid overinterpretation.
3.The review frequently shifts among central, peripheral, and renal-specific effects of BDNF without a unified integrated model of the brain–kidney axis. This fragmented structure reduces conceptual coherence and impairs readability.
4.The authors attempt to link BDNF changes in acute exercise stress with those in chronic kidney disease, but these two conditions differ sharply in pathophysiology, BDNF source, and signaling patterns. Unwarranted direct comparison risks overextrapolation and undermines mechanistic consistency.
5.Circulating BDNF faces glomerular filtration barrier constraints, yet the relative contributions of systemic versus locally produced BDNF to podocyte signaling remain unclear. The authors should propose a testable hypothesis for the dominant BDNF source or explicitly note this as a major limitation.
6.The manuscript discusses BDNF as a potential clinical biomarker for renal injury, but current evidence is preliminary. We encourage more cautious claims and explicit acknowledgment of the limitations and confounding factors.

Minor comment
1.The title emphasizes "central and peripheral actions," but the abstract and main text focus mostly on renal/peripheral effects; central mechanisms are underrepresented. Revise title or expand central content.
2.In the Figure 1 legend, while the use of “?” to denote unknown functions of TrkB.T2 is appropriate, the legend should explicitly state that the schematic diagram is hypothetical in the context of renal cells, to avoid misinterpretation as established fact.
3.Definition of "functional stress" is inconsistent, the term is used interchangeably with extreme exercise, heat stress, dehydration, etc. A clear, consistent definition at first use is needed.
4.The manuscript demonstrates inconsistent usage of several key technical terms throughout the text, which affects clarity and academic rigor. The authors are encouraged to standardize terminology across the entire manuscript.
5.Limitations are adequately discussed but scattered throughout the text without a dedicated, explicitly labeled Limitations section.

Author Response

Reviewer 2:

This review focuses on BDNF/TrkB signaling in the brain–kidney axis during functional stress and its potential role in podocyte protection, covering a scientifically meaningful topic. However, the presentation would benefit from improved balance between central and peripheral mechanisms, stronger integration of human causal data, clearer elucidation of BDNF sources and TrkB isoform functions, and a dedicated Limitations section. Further refinement of writing, structure, and figure annotations is also suggested to enhance clarity and academic rigor. 

Summary Reply: We thank the reviewer for the constructive overall assessment. In the revised manuscript, we implemented substantial structural and conceptual revisions to improve balance, rigor, and clarity. Specifically, we (a) rebalanced central and peripheral content by trimming CNS-only neuroplasticity details and refocusing the central discussion on stress physiology and autonomic/neuroendocrine control relevant to the brain–kidney axis; (b) strengthened integration of human evidence by explicitly distinguishing mechanistic support from experimental models versus largely associative observations in humans and reframing causal language accordingly; (c) added an explicit causal framework distinguishing candidate BDNF sources (local renal vs circulating vs indirect central regulation), delivery routes, and target-cell specificity, and clarified TrkB isoform context across sections; (d) added a dedicated Limitations section consolidating key constraints (source attribution, bioavailable fraction, isoform-specific functions, and platelet/matrix confounding); and (e) refined writing and figure legends, including clarifying which elements are evidence-based versus conceptual in renal cells. We address the reviewer’s specific points in detail below.

Major comment

1.The authors hypothesize that BDNF/TrkB signaling protects podocytes against exercise-related kidney injury, but the manuscript fails to clarify which source of BDNF (central, circulating, or local) mediates this effect or the dominant mechanism within the brain–kidney axis. This incomplete mechanistic chain significantly weakens the causal validity of the core conclusion.

Response: We thank the reviewer for this important comment. We agree that the initial version did not present a sufficiently explicit causal framework linking BDNF availability to podocyte signaling in the context of functional stress. In the revised manuscript, we added a dedicated “Causal framework for BDNF within the brain–kidney axis” paragraph at the end of the Introduction that explicitly distinguishes (a) major candidate sources of BDNF (local renal production vs circulating pools vs indirect central regulation), (b) circulating forms and measurement constraints (platelet-associated vs free BDNF; serum vs plasma/platelet-poor plasma), (c) plausible delivery routes to glomerular cells (paracrine renal signaling, endothelial transport mechanisms, and other potential routes for the free fraction discussed cautiously), and (d) target-cell specificity within the filtration apparatus (podocytes vs endothelial or tubular cells). We also revised the Abstract and Conclusions to avoid over-causal interpretation and to frame BDNF/TrkB as a candidate modulatory/resilience axis with supportive mechanistic evidence in podocyte injury models, while stating explicitly that the dominant in vivo route in humans under exercise-related renal stress remains unresolved and requires causal testing.

2.The manuscript states that podocytes mainly express TrkB-T1, yet direct functional evidence for its roles in calcium signaling and cytoskeletal regulation remains limited. The authors should better distinguish between receptor expression and validated function, and use more cautious language to avoid overinterpretation.

Response: We thank the reviewer for this important point. We agree that receptor expression does not necessarily imply validated receptor-isoform-specific function. In the revised manuscript, we now clearly distinguish between evidence that podocyte-focused datasets most consistently report predominant expression of truncated TrkB isoforms (often TrkB-T1) and direct functional evidence demonstrating isoform-specific roles in calcium signaling and cytoskeletal regulation. We revised wording in the renal sections to avoid overinterpretation by using cautious phrasing (e.g., “has been associated with”, “may contribute to”) and by explicitly stating that isoform-specific functional attribution in podocytes remains limited and should be considered provisional. We also consolidated this constraint in the dedicated Limitations section as a key unresolved issue and priority for future causal studies.

3.The review frequently shifts among central, peripheral, and renal-specific effects of BDNF without a unified integrated model of the brain–kidney axis. This fragmented structure reduces conceptual coherence and impairs readability.

Response: We thank the reviewer for this constructive comment. In the revised manuscript, we improved conceptual coherence by introducing an explicit unifying model of the brain–kidney axis and by strengthening transitions between central, peripheral, and renal sections. Specifically, we added an integrated causal framework at the end of the Introduction (“Causal framework for BDNF within the brain–kidney axis”) that organizes the discussion around BDNF sources (local renal vs circulating vs indirect central regulation), circulating forms and measurement constraints, plausible delivery routes to glomerular cells, and target-cell specificity. We also rebalanced the central content by trimming CNS-only neuroplasticity detail and refocusing Section 4 on stress physiology and autonomic/neuroendocrine control relevant to kidney hemodynamics, while the peripheral and renal sections were edited to reduce redundancy and to clarify how platelet-associated BDNF and local renal signaling fit within the same framework. In addition, we strengthened mechanistic bridges (e.g., hemodynamic load – podocyte actin stress – actin-convergent resilience pathways) to make the narrative flow explicit and improve readability.

4.The authors attempt to link BDNF changes in acute exercise stress with those in chronic kidney disease, but these two conditions differ sharply in pathophysiology, BDNF source, and signaling patterns. Unwarranted direct comparison risks overextrapolation and undermines mechanistic consistency.

Response: We thank the reviewer for this important concern. We agree that acute exercise-related renal stress and chronic kidney disease (CKD) represent distinct pathophysiological regimes with different injury milieus, time scales, and potential determinants of BDNF dynamics. In the revised manuscript, we reframed Section 10 to focus on CKD disease-context biology and mechanistic implications and relocated biomarker-centered associations (e.g., urinary sediment transcript correlations and serum/urinary patterns) to Section 7, thereby reducing overlap and avoiding direct extrapolation. We also explicitly stated that any parallels with acute exercise stress should be considered hypothesis-generating rather than directly translatable across regimes. This revision improves mechanistic consistency and clarifies that CKD is discussed primarily to illustrate context dependence of BDNF/TrkB signaling rather than to imply equivalence with exercise-related renal stress.

5.Circulating BDNF faces glomerular filtration barrier constraints, yet the relative contributions of systemic versus locally produced BDNF to podocyte signaling remain unclear. The authors should propose a testable hypothesis for the dominant BDNF source or explicitly note this as a major limitation.

Response: We thank the reviewer for this important point. In the revised manuscript, we explicitly treat dominant source attribution (systemic versus local renal BDNF) and delivery routes to glomerular cells as a major limitation and outline testable approaches to resolve it. First, the “Causal framework for BDNF within the brain–kidney axis” paragraph (end of the Introduction) distinguishes local renal production versus circulating pools and highlights source attribution and delivery routes to glomerular cells as key unresolved constraints. Second, in Section 6 we revised the discussion of glomerular access to incorporate filtration barrier selectivity beyond molecular size (including charge-dependent constraints) and state that the dominant route by which BDNF reaches glomerular cells (local renal production versus vascular/endothelial transport mechanisms and other pathways) remains insufficiently defined. Third, we consolidated these constraints in a dedicated Limitations section (source attribution and bioavailable fraction). Finally, we outline endpoint-driven, testable study designs - pairing platelet-controlled blood matrices (serum and platelet-poor plasma) with urinary sediment transcript profiling and podocyte-centered endpoints across defined stress exposures and recovery trajectories–to discriminate systemic/platelet-driven signals from local renal BDNF/TrkB activation and to test competing dominant-source hypotheses in exercise-related renal stress.

6.The manuscript discusses BDNF as a potential clinical biomarker for renal injury, but current evidence is preliminary. We encourage more cautious claims and explicit acknowledgment of the limitations and confounding factors.

Response: We thank the reviewer for this important point. We agree that the current evidence supporting BDNF as a clinical biomarker of renal injury is preliminary. In the revised manuscript, we adopted more cautious language and frame BDNF-related measures as candidate biomarkers rather than clinically validated indicators. We explicitly highlight key confounders that constrain interpretation, including platelet-associated BDNF pools and sampling methodology/matrix effects (serum vs plasma/platelet-poor plasma), and we emphasize the need for prospective validation with podocyte-centered endpoints and longitudinal sampling to establish directionality.

Changes in the manuscript:

Section 7 was rewritten to acknowledge the limited evidence base, discuss alternative interpretations of associations (including uACR relationships), highlight platelet/matrix confounding, and frame BDNF-related readouts as candidate biomarkers pending prospective validation.

Minor comment

1.The title emphasizes "central and peripheral actions," but the abstract and main text focus mostly on renal/peripheral effects; central mechanisms are underrepresented. Revise title or expand central content.

Response: We thank the reviewer for this helpful suggestion. We addressed this mismatch by revising the title to better reflect the manuscript’s primary focus on the brain–kidney axis under functional stress and podocyte resilience. Specifically, we changed the title from “Central and Peripheral Actions of Brain-Derived Neurotrophic Factor during Functional Stress” to “BDNF/TrkB Signaling in the Brain–Kidney Axis under Functional Stress.”

2.In the Figure 1 legend, while the use of “?” to denote unknown functions of TrkB.T2 is appropriate, the legend should explicitly state that the schematic diagram is hypothetical in the context of renal cells, to avoid misinterpretation as established fact.

Response: We thank the reviewer for this important suggestion. We revised the Figure 1 legend to explicitly state that, in the context of renal cells, the schematic should be interpreted as a conceptual/hypothetical framework rather than an established cell-type–specific pathway map. We also retained the “?” notation to indicate the limited evidence available for TrkB-T2–specific functions.

3.Definition of "functional stress" is inconsistent, the term is used interchangeably with extreme exercise, heat stress, dehydration, etc. A clear, consistent definition at first use is needed.

Response: We thank the reviewer for highlighting this terminology issue. In the revised manuscript, we added a clear definition of “functional stress” at first use and standardized its usage throughout the text. We now use “functional stress” as an umbrella term referring to acute, high-demand physiological states that impose combined hemodynamic and metabolic load on the kidney, and we treat extreme endurance exercise as the primary exemplar. Co-stressors such as heat stress and hypohydration/dehydration are described consistently as modifiers that amplify the stress exposure rather than being used interchangeably with the core term.

Changes in the manuscript:

Definition added at first use (Introduction/early Section 2) and terminology standardized across sections.

4.The manuscript demonstrates inconsistent usage of several key technical terms throughout the text, which affects clarity and academic rigor. The authors are encouraged to standardize terminology across the entire manuscript.

Response: We thank the reviewer for this helpful comment. In the revised manuscript, we performed a terminology audit and standardized key technical terms across the entire text and figure legends. Specifically, we unified TrkB isoform notation (TrkB-FL, TrkB-T1, TrkB-T2), standardized terminology for BDNF forms (mature BDNF vs proBDNF), harmonized blood-matrix terminology (serum, plasma, platelet-poor plasma) and platelet-related wording (platelet-associated BDNF). We also aligned figure legends with the main-text terminology to improve clarity and academic rigor.

Changes in the manuscript:

Terminology and notation standardized throughout the manuscript and figure legends.

Please see the attachment for a version of the article with all the corrections.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript (no. biology-4245853) presents a narrative review addressing the role of brain-derived neutrophic factor (BDNF) and TrkB signaling in the brain-kidney axis, with a particular emphasis on podocyte cytoskeletal integrity under conditions of functional stress such as extreme physical exercise.  The topic is timely and conceptually appealing, as it attempts to extend the well-established neurobiological functions of BDNF into renal physiology and pathophysiology.  However, in its current form, the manuscript remains largely descriptive and speculative, with insufficient mechanistic rigor and limited critical evaluation of existing evidence.  Several key conceptual and methodological issues need to be addressed before the manuscript can be considered for publication.

1. The authors should explicitly define a causal framework, including source of BDNF (circulating vs local renal production), mode of delivery (filtration, transcytosis, or paracrine signaling), and target cell specificity (podocytes vs endothelial or tubular cells).

2. The authors should clearly distinguish between platelet-bound and free BDNF, discuss methodological limitations in BDNF measurement, and reframe circulating BDNF as a context-dependent biomarker rather than a direct effector.

3. The authors should either provide stronger evidence linking BDNF to renal outcomes in exercise settings, or reframe BDNF as a modulatory factor rather than a primary driver.

 

<minor>

Figure 1: The signaling pathways are well illustrated but lack kidney-specific annotations.

Figure 2: The neuron–podocyte analogy is conceptually strong but would benefit from integration with experimental or clinical data.

Author Response

Reviewer 3 comment:
This manuscript (no. biology-4245853) presents a narrative review addressing the role of brain-derived neutrophic factor (BDNF) and TrkB signaling in the brain-kidney axis, with a particular emphasis on podocyte cytoskeletal integrity under conditions of functional stress such as extreme physical exercise.  The topic is timely and conceptually appealing, as it attempts to extend the well-established neurobiological functions of BDNF into renal physiology and pathophysiology.  However, in its current form, the manuscript remains largely descriptive and speculative, with insufficient mechanistic rigor and limited critical evaluation of existing evidence.  Several key conceptual and methodological issues need to be addressed before the manuscript can be considered for publication.

Summary Response: We thank the reviewer for the thoughtful overall assessment. In the revised manuscript, we strengthened mechanistic rigor and critical evaluation by introducing an explicit causal framework (sources/forms/routes/targets), clarifying isoform context and separating expression evidence from isoform-specific functional validation, and reframing the central narrative from causal claims to a candidate modulatory/resilience axis supported primarily by experimental podocyte injury models. We also expanded critical appraisal of human evidence, added a dedicated Limitations section, and refined figure legends to clearly distinguish conceptual elements from established renal cell–specific pathways. We address the reviewer’s specific points in detail below.

1. The authors should explicitly define a causal framework, including source of BDNF (circulating vs local renal production), mode of delivery (filtration, transcytosis, or paracrine signaling), and target cell specificity (podocytes vs endothelial or tubular cells).

Response 1: We thank the reviewer for this important suggestion. In the revised manuscript, we added an explicit causal framework to define (a) the major candidate sources of BDNF (local renal production or circulating pools, with an additional indirect central regulatory route), (b) the mode of delivery to glomerular cells (paracrine signaling within the kidney, endothelial transport mechanisms including receptor-mediated transcytosis, and the theoretical possibility of passive filtration of the free fraction constrained by size/charge selectivity), and (c) target-cell specificity within the filtration apparatus (podocytes versus endothelial or tubular cells). We also emphasize that dominant source attribution and delivery routes remain unresolved constraints and are highlighted as major limitations and priorities for causal testing.

2. The authors should clearly distinguish between platelet-bound and free BDNF, discuss methodological limitations in BDNF measurement, and reframe circulating BDNF as a context-dependent biomarker rather than a direct effector.

Response 2: We thank the reviewer for this important point. In the revised manuscript, we explicitly distinguish between platelet-associated (platelet-bound) BDNF and the free/bioavailable fraction, and we clarify that routine blood measurements do not directly report tissue-available BDNF. Specifically, we expanded the peripheral/biomarker sections to emphasize key methodological constraints,including the strong influence of platelet degranulation and sampling matrix (serum vs plasma/platelet-poor plasma), and we standardized terminology accordingly. We also reframed circulating BDNF throughout the manuscript as a context-dependent biomarker/readout of systemic stress and platelet activation, rather than a direct effector signal for renal cells, while noting that local renal BDNF/TrkB signaling may represent a distinct compartmental pathway. These measurement and interpretation constraints are additionally consolidated in the dedicated Limitations section.

We now explicitly distinguish platelet-associated versus free/bioavailable BDNF, highlight serum/plasma/platelet-poor plasma measurement constraints, and reframe circulating BDNF as a context-dependent biomarker rather than a direct renal effector (Sections 5 and 7; Causal framework in the Introduction; consolidated in Limitations; operationalized in Section 11)

3. The authors should either provide stronger evidence linking BDNF to renal outcomes in exercise settings, or reframe BDNF as a modulatory factor rather than a primary driver.

Response 3: We thank the reviewer for this important point. We agree that direct human causal evidence linking BDNF to renal outcomes in exercise settings remains limited. Accordingly, in the revised manuscript we reframed BDNF/TrkB signaling as a candidate modulatory/resilience axis rather than a primary causal driver of exercise-related renal injury. This reframing is reflected in the Abstract and Conclusions, where we explicitly note that current human evidence is insufficient to define dominant source attribution and delivery routes to glomerular cells, and we emphasize that causal testing will require endpoint-driven prospective designs with podocyte-centered outcomes.

<minor>

Figure 1: The signaling pathways are well illustrated but lack kidney-specific annotations.

Response: We thank the reviewer for this important comment. We have added details to Figure 1 specifically about the role of BDNF receptors in the context of kidney function.

Figure 2: The neuron–podocyte analogy is conceptually strong but would benefit from integration with experimental or clinical data.

Response: We thank the reviewer for this important comment. Figure 2, which illustrates the analogy between neurons and podocytes, as well as the text for this section of the review, are based on current scientific data (references 81-87). If necessary, we can supplement the text with specific experimental or clinical examples.

Please see the attachment for a version of the article with all the corrections.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Accept in present form

Reviewer 3 Report

Comments and Suggestions for Authors

I have no further concerns...
The authors have adequately addressed my previous concerns...