The Role of Astrocytes and Alpha-Synuclein in Parkinson’s Disease: A Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Parkinson’s disease (PD) is one of the most common neurodegenerative diseases that tortured a significant population all over the world. Patients with PD exhibits progressive motor deficits, postural instability and bradykinesia which significantly compromise their life quality. However, the etiology of PD still remains unclear and development of PD therapy has always been a challenge for scientists and health professionals. Here, Brash-Arias et al. summarized published literature and knowledge on how major factors including astrocytes and alpha-synuclein contribute to PD progression and pathogenesis. The authors developed this question and potential solution by including three major sections: 1. How astrocytes contribute to PD progression and pathogenesis; 2. How a-synuclein contributes to PD progression and pathogenesis; 3. What therapeutic strategy may help cure PD. This review was written in fluent and professional English, and also referenced proper literature.

 However, there are several major issues that need to be changed:

1.    In line 79-83, the authors defined two groups of reactive astrocytes in PD including A1 astrocytes that promote neuronal death and PD and A2 astrocytes that are protective to neuronal cells and slow down PD progression. In the following paragraphs, the authors primarily focused on the A1 astrocytes and made a good summary on how this group of reactive astrocytes causes PD pathogenesis. Although A1 astrocytes are much more relevant to the key topic: PD progression and pathogenesis, it is also critical to understand how A2 astrocytes provide protective roles for neuronal cell types. Without that, it will be very difficult to have a complete understanding of the roles of astrocytic processes in PD pathogenesis. In this sense, it is equally important to include paragraphs in the relevant section that explicit functional roles of A2 astrocytes and summarize their functions in neuroprotection with relevant references.

2.    In line 84-85, a number of pathological processes that reactive astrocytes (mainly A1 astrocytes) contribute to were listed here including: neuroinflammation, neuroprotection deficits, erroneous neurotransmitter uptake as well as oxidative stress/ mitochondrial damage. And then, the following paragraph provided detailed information on the aspects of neuroinflammation, neuroprotection deficits and neurotransmitter issues. However, it lacks a summary on how reactive astrocytes may contribute to oxidative stress and eventually cause mitochondria damage. It would be very interesting to understand this since mitochondria damage may be one of the reasons for neuronal death.

3.    The section of “3. Alpha-synuclein” may need a systemic reorganization. During this section, the authors talked about potential mechanisms on how alpha-synuclein pathology causes its aggregation in neurons and astrocytes, including: 1. Mutations of alpha-synuclein sequence; 2. Formation of a-synuclein oligomers; 3. Down-regulation of ubiquitin-proteosome system and consequent inhibition of protein elimination. After that, the gear got completely switched and the author started to talk about the spreading of alpha-synuclein between neurons and astrocytes. Indeed, both topics are essential to be included here in the section for alpha-synuclein, which helps understand multiple dimensions of protein biology and pathology. However, it broke the logic and disrupted the fluency of the paper by adding spreading mechanisms (tagged as “3.1”) right after the introduction part that summarized the three pathology triggers. It will be necessary to insert an paragraph as a transition in between the parts and briefly mention how alpha-synuclein pathology that induced by three factors mentioned above causes its prion-like spreading pattern. More specifically, what differences does each factor contribute to the spreading of alpha-synuclein?

Moreover, since the section of “Alpha-syn: its prion-like spreading and presence in the peripheral and enteric nervous system” was tagged as “3.1”, there is supposedly a following section with tag 3.2, 3.3, etc. Are those sections missing here? If not, the “section 3.1” can be organized with the rest of the section without a specific tag or be organized and tagged as an individual section rather than part of the section 3.  

4.    In the line 168-169, it was claimed that “It was found that α-syn can propagate from the gastrointestinal tract, through the vagus nerve, to the brain, supporting the Braak staging hypothesis.” However, there is a lack of molecular and cellular mechanisms that were summarized in this section. An addition of relevant mechanisms will help the understanding of how alpha-synuclein proteins commute between neuronal cells and astrocytes.

Comments on the Quality of English Language

The language quality of this manuscript is good except for certain words and sentences that needs to be changed. For example, in line 54-57, a sentence stated as "Astrocytes are non-neuronal cells... maintaining ionic, metabolic, neurotransmission... among other functions" apparently contains grammatical errors by using "ionic" and "metabolic" as nouns. Changes may be done by adding words "homeostasis" after "metabolic" and making the phrase as "maintaining ionic and metabolic homeostasis". 

Author Response

1. In line 79-83, the authors defined two groups of reactive astrocytes in PD including A1 astrocytes that promote neuronal death and PD and A2 astrocytes that are protective to neuronal cells and slow down PD progression. In the following paragraphs, the authors primarily focused on the A1 astrocytes and made a good summary on how this group of reactive astrocytes causes PD pathogenesis. Although A1 astrocytes are much more relevant to the key topic: PD progression and pathogenesis, it is also critical to understand how A2 astrocytes provide protective roles for neuronal cell types. Without that, it will be very difficult to have a complete understanding of the roles of astrocytic processes in PD pathogenesis. In this sense, it is equally important to include paragraphs in the relevant section that explicit functional roles of A2 astrocytes and summarize their functions in neuroprotection with relevant references.

We appreciate the reviewer's suggestion and agree with it. Additional information has been integrated to better understand this topic.

“In 2017, it was discovered that neuroinflammation and ischemia induced two types of reactive astrocytes, which they named A1 (harmful) and A2 (protective) due to their nomenclature (analogous to the M1 and M2 macrophage and microglia paradigm) [24]. In this report, the authors demonstrated in an in vivo mouse model with acute CNS injury that A1 astrocytes are induced by activated microglia and lose their normal functions as astrocytes although they acquire a new neurotoxic function, generating neuronal death and decreased phagocytic capacity. Likewise, through a co-culture of control astrocytes and A1 astrocytes from retinal ganglion cells, it was found that astrocytes generate a soluble toxin that rapidly kills subsets of CNS neurons and mature oligodendrocytes, as well as the ability to promote neuronal survival and growth [24].

While evidence supports that A1 astrocytes lose their neuroprotective functions and contribute to neuronal death; conversely, A2 astrocytes produce antioxidant molecules in response to oxidative stress and protect dopaminergic neurons in rat PD models, both in vitro and in vivo [25].

Several studies on the detrimental effects of A1 astrocytes on neurons have been reported in recent years; however, the protective effects of A2 astrocytes remain to be fully explored. So far, genetic profiling has shown that A2 astrocytes induced by mouse middle cerebral artery occlusion, upregulate beneficial neurotrophic factors and inflammatory factors such as cardiotrophin-like cytokine factor 1 (CLCF1), hypoxia-inducing factor (HIF), IL-6, IL-10 and thrombospondins, which promote neuronal survival and remodeling [16, 26).

Additionally, it has been demonstrated in primary mouse astrocytes that overexpression of the chemokine-like signaling protein Prokineticin-2 (PK2) induces the A2 astrocytic phenotype as well as upregulation of key protective genes and A2 reactivity markers (PTX3, SPHK1, TM4SF1 and Nrf2). This response was accompanied by an increase in Glutamate/Aspartate transporter (GLAST) expression and glutamate uptake, indicating that A2 astrocytes can reduce extracellular glutamate through the positive regulation of GLAST, preventing neuronal excitotoxicity [27].”

2. In line 84-85, a number of pathological processes that reactive astrocytes (mainly A1 astrocytes) contribute to were listed here including: neuroinflammation, neuroprotection deficits, erroneous neurotransmitter uptake as well as oxidative stress/ mitochondrial damage. And then, the following paragraph provided detailed information on the aspects of neuroinflammation, neuroprotection deficits and neurotransmitter issues. However, it lacks a summary on how reactive astrocytes may contribute to oxidative stress and eventually cause mitochondria damage. It would be very interesting to understand this since mitochondria damage may be one of the reasons for neuronal death.

We appreciate the reviewer's observation; it undoubtedly enriches our work. The following information has been added.

"In most pathological conditions of the brain astrocytes often undergo changes in both morphology and function, a process referred to as reactive astrogliosis. This process has as key characteristics hypertrophy of astrocytic processes and increased expression of intermediate filaments, primarily glial fibrillary acidic protein (GFAP) [20].

Particularly, reactive astrocytes respond to acute cellular stress and work to limit damage to the CNS, but chronic reactive astrogliosis can result in the sustained production of reactive oxygen and nitrogen species (ROS/RNS), as well as the release of proinflammatory molecules, which promotes neuronal injury and neurotoxicity [13].

Although, the exact signaling pathways by which reactive astrogliosis promotes dopaminergic neuronal death are not fully understood, it is believed that astrogliosis can have both beneficial and detrimental effects on dopaminergic neurons [23]. Increasing evidence reveals that reactive astrocytes produce proinflammatory cytokines (TNF-α, IL-1β, IL-6 and interferon-γ), which marks the initiation of neuronal apoptosis through the activation of caspases 3 and 8, as well as cytochrome c [21, 22, 47,48]. Furthermore, reactive astrocytes release nitric oxide into the extracellular space, causing increased lipid peroxidation, mitochondrial damage, and DNA strand breaks, eventually leading to neuronal injury and death [49]."

3. The section of “3. Alpha-synuclein” may need a systemic reorganization. During this section, the authors talked about potential mechanisms on how alpha-synuclein pathology causes its aggregation in neurons and astrocytes, including: 1. Mutations of alpha-synuclein sequence; 2. Formation of a-synuclein oligomers; 3. Down-regulation of ubiquitin-proteosome system and consequent inhibition of protein elimination. After that, the gear got completely switched and the author started to talk about the spreading of alpha-synuclein between neurons and astrocytes. Indeed, both topics are essential to be included here in the section for alpha-synuclein, which helps understand multiple dimensions of protein biology and pathology. However, it broke the logic and disrupted the fluency of the paper by adding spreading mechanisms (tagged as “3.1”) right after the introduction part that summarized the three pathology triggers. It will be necessary to insert an paragraph as a transition in between the parts and briefly mention how alpha-synuclein pathology that induced by three factors mentioned above causes its prion-like spreading pattern. More specifically, what differences does each factor contribute to the spreading of alpha-synuclein?

Agree with the reviewer. His suggestion has been considered.

“The triplication of the SNCA gene locus leads to increased α-syn levels, has been associated with a higher risk of developing PD [11]. The mutations in α-syn (A53T, A30P and E46K) also increase the probability of developing PD since they alter the secondary structure of this protein, promoting its neuronal and astrocytic aggregation [45]. Furthermore, alleles within a Rep1 polymorphic region, 10 kB upstream of the α-synuclein gene promoter [46], have been associated with increased α-syn mRNA expression in human and mouse neurons [47, 48] as well as in the substantia nigra in humans [49]. These findings suggest that increased α-syn mRNA concentration and α-syn protein are triggering factors for the development and progression of PD [50].

Under physiological conditions, α-syn can bind to the lipid membrane to promote the assembly of the SNARE complex and the formation of stable tetramers resistant to physiological aggregation processes [51]. When the balance between α-syn production and clearance is disrupted, this protein aggregates and unfolds into oligomers, then amyloid fibrils, and finally into LB [50]. The α-syn aggregates exhibit diverse structures, ranging from soluble oligomeric ring-shaped, rope-shaped, or spherical forms (protofibrils) to insoluble fibrils [52-54]. These fibrils are thought to form the basis of Lewy bodies, although it is controversial whether the smaller protofibrils or the larger amyloid fibrils are the toxic α-syn species that cause neuronal cell death [55, 56]. Among the multiple ways by which α-syn oligomers can induce cytotoxicity are mitochondrial damage, endoplasmic reticulum stress, synaptic impairment, excitotoxicity, neuroinflammation, proteostasis loss, and cell apoptosis [57].

The ubiquitin-proteasome system (UPS) and the lysosomal autophagy pathway (ALP) are the main pathways for eliminating overexpressed or misfolded proteins in cells to maintain protein homeostasis. It has been reported that UPS is the primary pathway for degrading α-syn, although once saturated, ALP also participates in the degradation process [58].

The reciprocal interaction between α-syn and proteasome function suggests a self-perpetuating process in which permanently elevated levels of α-syn impair the UPS, which in turn may lead to increased accumulation of α-syn. The detailed molecular mechanism by which α-syn is degraded by the proteasome is still not known. There is evidence in cell cultures that degradation can occur through a proteasome-dependent, but ubiquitin-independent pathway [59-61].

In 2011, in a human α-syn WT transgenic mouse model, it was found that the UPS degrades a-syn under conditions with an increased endogenous α-syn load and that, in contrast, autophagy is recruited to degrade a-syn only when intracellular levels of α-syn are elevated, providing a link between the proteasome, autophagy and synucleopathies, being one of the first evidence in an in vivo model between protein loading and the pathways recruited to maintain homeostasis [58].

Recently, it has been reported that down-regulation of UPS and ALP leads to the accumulation of α-syn oligomers, which in turn inhibits the protein elimination process.  By promoting the removal of α-syn oligomers, several research groups agree that targeting the signaling pathways involved in both systems can be an efficient way to restore proteostasis, becoming a potential and promising therapeutic target for PD [57, 58, 62-64]

The required paragraph has been added to have a better thread between the topics.

Collectively, the evidence suggests that all misfolded proteins in neurodegenerative diseases show prion seed-like effects, including α-syn. Findings have shown the detection of α-syn-positive Lewy bodies (LB) are detected in grafts from PD patients who received a transplant of embryonic midbrain cells, indicating the existence of host-to-graft transfer of α-syn pathology and a prion-like behavior [57].”

Moreover, since the section of “Alpha-syn: its prion-like spreading and presence in the peripheral and enteric nervous system” was tagged as “3.1”, there is supposedly a following section with tag 3.2, 3.3, etc. Are those sections missing here? If not, the “section 3.1” can be organized with the rest of the section without a specific tag or be organized and tagged as an individual section rather than part of the section 3.  

The reviewer's suggestion has been addressed.

4. In the line 168-169, it was claimed that “It was found that α-syn can propagate from the gastrointestinal tract, through the vagus nerve, to the brain, supporting the Braak staging hypothesis.” However, there is a lack of molecular and cellular mechanisms that were summarized in this section. An addition of relevant mechanisms will help the understanding of how alpha-synuclein proteins commute between neuronal cells and astrocytes.

The molecular and cellular mechanisms of Braak staging hypothesis have been aggregated.

"The discovery that misfolded α-syn exhibits properties similar to prions has raised interest in understanding the progression of PD according to the Braak staging system [9], which classifies the progression of PD into 6 stages. The first stages (1-2) are pre-symptomatic, characterized by the loss of non-motor functions, such as the loss of the sense of smell. At these stages there may be more prevalent Lewy neurites than LB, and the brain stem is the most affected. In the intermediate stages (3-4), patients lose motor functions, develop bradykinesia and rigidity. At this point, the disease passes to the striatum and LB are formed. In the final stages (5-6), patients have all other symptoms and the disease progresses to other parts of the brain and there may be neuronal loss [65].

Despite the fact that Braak's staging hypothesis in PD has been clinically validated both in vivo and in vitro, there is still a lack of understanding regarding the underlying molecular mechanisms. Furthermore, it is necessary to consider some documented inconsistencies [75], for example: in the study published by Braak and colleagues in 2002, only 30 of 413 cases of Lewy neurites in the dorsal motor nucleus of the vagus nerve were used to provide the basis of the stratification scheme. Likewise, no Lewy bodies were found in the SNpc of 30 of the sporadic PD patients [75]. Another study suggests that the Braak system does not allow classification of nearly 50% of PD cases, such is the case of Zaccai and collaborators who reported that only 51% of the 208 cases of autopsies in patients with a diagnosis of PD followed the Braak pattern [76]. Other report provided evidence invalidating a smooth and predictable rostro-caudal progression of the synucleinopathy abnormality in brains of people with PD [77].

Although these are some studies that debates the Braak staging system, there are important case statistics that allows to elucidate patterns of disease development, although more clinical, molecular, and pathological studies are required to strengthen this hypothesis."

5. In line 54-57, a sentence stated as "Astrocytes are non-neuronal cells... maintaining ionic, metabolic,.. among other functions" apparently contains grammatical errors by using "ionic" and "metabolic" as nouns. Changes may be done by adding words "homeostasis" after "metabolic" and making the phrase as "maintaining ionic and metabolic homeostasis”.

The grammatical errors were corrected.

Reviewer 2 Report

Comments and Suggestions for Authors

Brash-Arias et collegues submitted a review to collect all the available information about the role of astrocytes and alpha-synuclein in Parkinson's disease. The topic has a wide appeal and the review is well written and useful, but the references used to build it up ended in 2021 (with one more of 2022). I suggest to improve the text including other more recent papers too, remembering that a review, especially concerning a pathology like PD, should describe the state of the art of the field.  

1) the paper is strongly descriptive, without entering on the molecular details, like other review already published, but this could be a point of strenght if the purpose is to provide a tool that gives an overview to the reader.

2) the references used to write the review go up to 2021 (with one published in 2022). I suggest to improve the text including other more recent papers too. 

Author Response

1) the paper is strongly descriptive, without entering on the molecular details, like other review already published, but this could be a point of strenght if the purpose is to provide a tool that gives an overview to the reader.

We appreciate the reviewer's observation. This point has been addressed throughout the writing.

2) the references used to write the review go up to 2021 (with one published in 2022). I suggest to improve the text including other more recent papers too. 

Following the reviewer's suggestion, the following research works were added to our review:

• Fan, YY., Huo, J. A1/A2 astrocytes in central nervous system injuries and diseases: Angels or devils? Neurochem Int. 2021 148:105080. https://doi.org/1016/j.neuint.2021.105080.
• Bezard E, Dehay B. Maladie de Parkinson - Le rôle de la synucléine [Aggregation and spread of synuclein in Parkinson's disease]. Med Sci (Paris). 2022 38(1), pp. 45-51. https://doi.org/1051/medsci/2021241
• Zhong Y, Cai X, Ding L, Liao J, Liu X, Huang Y, Chen X, Long L. Nrf2 Inhibits the Progression of Parkinson's Disease by Upregulating AABR07032261.5 to Repress Pyroptosis. J Inflamm Res. 2022 Feb 2; 15:669-685. https://doi.org/2147/JIR.S345895
• Park, JS., Kam, TI., Lee, S., et al. Blocking microglial activation of reactive astrocytes is neuroprotective in models of Alzheimer's disease. Acta Neuropathol Commun. 2022 9(1):78. Published 2021 Apr 26. https://doi.org/1186/s40478-021-01180-z
• Wang T, Sun Y, Dettmer U. Astrocytes in Parkinson's Disease: From Role to Possible Intervention. Cells. 2023 12(19):2336. Sep 22. https://doi.org/3390/cells12192336
• Sheehan, P., Nadarajah, C., Kanan, M., Benitez, B., Davis, A., Musiek, E. An astrocyte BMAL1-BAG3 axis protects against alpha-synuclein and tau pathology. Neuron 2023 111, pp. 2383-2398 https://doi.org/10.1016/j.neuron.2023.05.006
• Ozoran H, Srinivasan R. Astrocytes and Alpha-Synuclein: Friend or Foe? J Parkinsons Dis. 2023 13(8), 1289-1301. https://doi.org/10.3233/JPD-230284

Reviewer 3 Report

Comments and Suggestions for Authors The review's topic, the interaction between astrocytes and alpha synuclein, points to a line of inquiry that would benefit from a minireview. Nevertheless, the material offered on this subject has already been briefly discussed in other reviews and does not offer any fresh information. The topic delves into the broad mechanisms of astrocytes and their role in Parkinson's disease (PD), although it falls short of other recent comprehensive reviews that are currently available in the field specifically on this topic (e.g., Wang et. al, Cells, 2023). Is it possible for the writers to discuss the more general subject of 'Interaction of non-neuronal cells with aSyn' in the review to include oligodendrocyte and glia? or just specifically expand on astrocyte-aSyn interactions and elaborate in that topic.   The section under "immunological biotherapies targeting aSyn" is unrelated to astrocytes and therefore unrelated to the rest of the topic. Additionally, the section lacks comprehensiveness and does not include all of the clinical directions that are being pursued at the moment (e.g. McFarthing et al, J. PD, 2023). Additional small molecule inhibitors (SynuClean-D, Pujols et al.; PcTs, Lamberto et al., etc.) have been found and tested against aSyn aggregation; however, they are not included in this list.  Expanding on the other areas and removing it from the review could be a better idea. Even if it is written and summarized effectively, section 3 on "Alpha Synuclein" might be condensed by referring to previous reviews in the field and concentrating just on the aspects of the interaction between astrocytes and synapses that are relevant to it (such as the part on prion propagation).    Although the paper is a useful resource, it could be beneficial to add some critical remarks on particular directions and cite any opposing papers. For instance, astrocytes have lower aSyn levels than neurons do (Booth et al., 2017), hence if the scenario of astrocyte overexpression of aSyn in [68] is comparable.   Broadly speaking, several of the parts might benefit from a deeper dive into more thorough explanation, including: (I) The final paragraph of Section 4: elaborates on the protective function beyond (ii) Section 5.1: Sertraline's relationship with astrocytes; The reason why NLY01 targets astrocytes specifically,   It could be beneficial to introduce paragraph 4 in Section 2 following paragraph 5.   Section 5: Combining therapy related to Nrf2 sections may be helpful.    Comments on the Quality of English Language

Well written, minor issues. 

Author Response

1. Is it possible for the writers to discuss the more general subject of 'Interaction of non-neuronal cells with aSyn' in the review to include oligodendrocyte and glia? or just specifically expand on astrocyte-aSyn interactions and elaborate in that topic.

 We thank the reviewer for his suggestion. We have expanded the topic on astrocyte/a-syn interactions.

“Astrocytes greatly contribute to neuronal survival through numerous mechanisms, such as the secretion of neurotrophins and antioxidants, clearance of α-synuclein, glutamate metabolism, fatty acid metabolism, and the transfer of healthy mitochondria to neurons [78]. However, the effects of astrocytic protection can be heterogeneous and depend on the brain region in which they are located and the loss of homeostatic balance of the CNS [79]. In contrast, reactive astrocytes are those that have undergone various cellular, molecular, and functional changes in response to injury or neurodegenerative diseases [78].

One of the hallmarks of PD is the formation of α-syn deposits. Several studies have demonstrated that astrocytes promote the formation and propagation of these protein deposits [13, 14, 80-82]. The first evidence of the close relationship between α-syn pathology and astrocytes was through the analysis of post mortem brain tissue from patients with PD. Particularly Wakabayashi et al., revealed that α-syn immunoreactive inclusions are frequently found in SNpc astrocytes of PD patients [14].

Toxic α-syn has been shown to induce mitochondrial damage, increased mitochondrial fragmentation, and affected mitophagy in neuronal cells [85-88]. Also, in human primary astrocytes α-syn was suggested to locate to mitochondria and cause reduced oxygen consumption [89]. In a study of primary astrocytes from mice infected with α-syn oligomers, mitochondrial damage with fragmentation patterns was confirmed, coupled with reduced mitochondrial functionality with abnormally high levels of reactive oxygen species (ROS), generating oxidative stress and neuronal death [85].

Recently, a report identified a link between the molecular circadian axis (BMAL1-BAG3 axis) in astrocytes and α-syn aggregation. It has been shown that silencing of the Bmal1^-/- clock gene in astrocytes was enough to prevent α-syn pathology in vivo and induce the activation of these cells. This response was associated with increased astrocytic phagocytosis of α-syn by BAG3 (macroautophagy chaperone) [94].

The mechanisms underlying α-syn internalization by astrocytes are thought to be different from neuronal mechanisms. In neurons, such internalization can occur in different ways, such as the interaction with heparin sulfates on the cell surface [95], Lag3 receptors [96] or the sodium-potassium transport subunit ATPase β3 [97]. However, in astrocytes it remains unclear, largely because they possess a unique interactome. Since astrocytic receptors that specifically bind to α-syn oligomers have not been identified so far, the development of therapeutic targets based on blocking astrocytic receptors continues to be studied [98].”

2. The section under "immunological biotherapies targeting aSyn" is unrelated to astrocytes and therefore unrelated to the rest of the topic.  Expanding on the other areas and removing it from the review could be a better idea. Even if it is written and summarized effectively.

We appreciate the reviewer's comment; However, we do not agree. Although that section of the article is not related to astrocytes, our work has 2 important bases related to Parkinson's disease: astrocytes and the protein alpha synuclein. That section in particular talks about current therapies focused on alpha synuclein.

3. The final paragraph of Section 4: elaborates on the protective function beyond (ii) Section 5.1: Sertraline's relationship with astrocytes; The reason why NLY01 targets astrocytes specifically, it could be beneficial to introduce paragraph 4 in Section 2 following paragraph 5.  Section 5: Combining therapy related to Nrf2 sections may be helpful.   

The reviewer's suggestion has been addressed.

“In complex with small Maf proteins, Nrf2 binds to antioxidant-responsive elements (AREs) that induce transcriptional activation of downstream genes encoding phase II antioxidant and detoxifying enzymes [103, 104]. It has been reported that in PD there is a dysfunction of this pathway, and its overexpression can increase the ability of astrocytes to reduce oxidative stress and inflammatory responses [105].

In addition to these findings, overexpression of Nrf2 in astrocytes delayed the onset of motor dysfunction and α-syn aggregation in mutant α-syn transgenic mice (A53T) [75]. Several studies have shown that curcumin, sulforaphane, and resveratrol can activate Nrf2 and exert neuroprotective effects against oxidative stress in animal models of neurodegenerative disorders [106-108].

Previous studies demonstrate that astrocyte-specific overexpression of Nrf2 was able to reduce chemically mediated neurotoxicity in models of PD and Huntington's disease [109, 110]. Another report showed that Nrf2 deficiency can exacerbate α-syn aggregation in mice using an adeno-associated viral vector expressing human α-syn [111]. In a Drosophila model with overexpression of a-syn and genetically increasing Nrf2, locomotor activity was restored [112]. The findings related to Nrf2 make it clear that this protein represents a key point for the development of new therapies targeting astrocytes in patients with PD.”

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

All the questions and suggestions in the first round have been addressed by the authors. No further changes will be needed. 

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have taken into account all the given suggestion and the paper has been correctly revised. 

Reviewer 3 Report

Comments and Suggestions for Authors

The revisions do not extensively address the concerns and still lack directionality. The material offered on this subject has already been briefly discussed in other reviews and does not offer any fresh information. The two topics (astrocytes and synuclein) make the review broad but lack depth. 

The alpha-synuclein portion of the review is not extensive and does not cover all the recent findings. e.g., many small-molecule inhibitors and antisense oligonucleotides are not covered in the inhibitors list. 

Comments on the Quality of English Language

No comments