Search Results (57)

Mitochondria-ER contact sites (MERCS) are vital for mitochondrial dynamics, lipid exchange, Ca²⁺ homeostasis, and energy metabolism. We examined whether mitochondrial metabolism changes during the cell cycle depend on MERCS dynamics and are regulated by the outer mitochondrial protein mitochondrial rho GTPase 1 (MIRO1). Wound healing was assessed in mice with fibroblast-specific deletion of MIRO1. Wild-type and MIRO1^-/- fibroblasts and vascular smooth muscle cells were evaluated for proliferation, cell cycle progression, number of MERCS, distance, and protein composition throughout the cell cycle. Restoration of MIRO1 mutants was used to test the role of MIRO1 domains; Ca²⁺ transients and mitochondrial metabolism were evaluated using biochemical, immunodetection, and fluorescence techniques. MERCS increased in number during G1/S compared with during G0, which was accompanied by a notable rise in protein–protein interactions involving VDAC1 and IP3R as well as GRP75 and MIRO1 by proximity-ligation assays. Split-GFP ER/mitochondrial contacts of 40 nm also increased. Mitochondrial Ca²⁺ concentration ([Ca²⁺]), membrane potential, and ATP levels correlated with the formation of MERCS during the cell cycle. MIRO1 deficiency blocked G1/S progression and the cell-cycle-dependent formation of MERCS and altered ER Ca²⁺ release and mitochondrial Ca²⁺ uptake. MIRO1 mutants lacking the Ca²⁺-sensitive EF hands or the transmembrane domain did not rescue cell proliferation or the formation of MERCS. MIRO1 controls an increase in the number of MERCS during cell cycle progression and increases mitochondrial [Ca²⁺], driving metabolic activity and proliferation through its EF hands. Full article

Focal swellings of dendrites (“dendritic blebbing”) together with structural damage of mitochondria and the endoplasmic reticulum (ER) are morphological hallmarks of glutamate neurotoxicity, also known as excitotoxicity. These pathological alterations are generally thought to be caused by the so-called “overactivation” of N-methyl-D-aspartate receptors (NMDARs). Here, we demonstrate that the activation of extrasynaptic NMDARs, specifically when forming a protein–protein complex with TRPM4, drives these pathological traits. In contrast, strong activation of synaptic NMDARs fails to induce cell damage despite evoking plateau-type calcium signals that are comparable to those generated by activation of the NMDAR/TRPM4 complex, indicating that high intracellular calcium levels per se are not toxic to neurons. Using confocal laser scanning microscopy and transmission electron microscopy, we show that disrupting the NMDAR/TRPM4 complex using the recently discovered small-molecule TwinF interface inhibitor FP802 inhibits the NMDA-induced neurotoxicity-associated dendritic blebbing and structural damage to mitochondria and the ER. It also prevents, at least in part, the disruption of ER–mitochondria contact sites. These findings establish the NMDAR/TRPM4 complex as the trigger for the structural damage of dendrites and intracellular organelles associated with excitotoxicity. They also suggest that activation of the NMDAR/TRPM4 complex, in addition to inducing high-amplitude, plateau-type calcium signals, generates a second signal required for glutamate neurotoxicity (“two-hit hypothesis”). As structural damage to organelles, particularly mitochondria, is a common feature of many human neurodegenerative diseases, including Alzheimer’s disease and amyotrophic lateral sclerosis (ALS), TwinF interface inhibitors have the potential to provide neuroprotection across a broad spectrum of these diseases. Full article

Neural progenitor cells (NPCs) are often used to study the subcellular mechanisms underlying differentiation into neurons in vitro. Works published to date have focused on the pathways that distinguish undifferentiated NPCs from mature neurons, neglecting the earlier and intermediate stages of this process. Current evidence suggests that mitochondria interaction with the ER is fundamental to a wide range of intracellular processes. However, it is not clear whether and how the mitochondria–ER interactions differ between NPCs and their differentiated counterparts. Here we take advantage of the widely used NPC line LUHMES to provide hints on the mitochondrial dynamic trait changes that occur during the first stage of their maturation into dopaminergic-like neurons. We observed that the morphology of mitochondria, their interaction with the ER, and the expression of several mitochondria–ER contact site resident proteins change, which suggests the potential contribution of mitochondria dynamics to NPC differentiation. Further studies will be needed to explore in depth these changes, and their functional outcomes, which may be relevant to the scientific community focusing on embryonic neurogenesis and developmental neurotoxicity. Full article

p66Shc is an adaptor protein and one of the cellular fate regulators since it modulates mitogenic signaling pathways, mitochondrial function, and reactive oxygen species (ROS) production. p66Shc is localized mostly in the cytosol and endoplasmic reticulum (ER); however, under oxidative stress, p66Shc is post-translationally modified and relocates to mitochondria. p66Shc was found in the intermembrane space, where it interacts with cytochrome c, contributing to the hydrogen peroxide generation by the mitochondrial respiratory chain. Our previous studies suggested that p66Shc is localized also in mitochondria-associated membranes (MAM). MAM fraction consists of mitochondria and mostly ER membranes. Contact sites between ER and mitochondria host proteins involved in multiple processes including calcium homeostasis, apoptosis, and autophagy regulation. Thus, p66Shc in MAM could participate in processes related to cell fate determination. Due to reports on various and conditional p66Shc intracellular localization, in the present paper, we describe the allocation of p66Shc pools in different subcellular compartments in mouse liver tissue and HepG2 cell culture. We provide additional evidence for p66Shc localization in MAM. In the present study, we use precisely purified subcellular fraction isolated by differential centrifugation-based protocol from control mouse liver tissue and HepG2 cells and from cells treated with hydrogen peroxide to promote mitochondrial p66Shc translocation. We performed controlled digestion of crude mitochondrial fraction, in which the degradation patterns of p66Shc and MAM fraction marker proteins were comparable. Moreover, we assessed the distribution of the individual ShcA isoforms (p46Shc, p52Shc, and p66Shc) in the subcellular fractions and their contribution to the total ShcA in control mice livers and HepG2 cells. In conclusion, we showed that a substantial pool of p66Shc protein resides in MAM in control conditions and after oxidative stress induction. Full article

The cytoskeleton mediates fundamental cellular processes by organizing inter-organelle interactions. Pathogenic variants of inverted formin 2 (INF2) CAAX isoform, an actin assembly factor that is predominantly expressed in the endoplasmic reticulum (ER), are linked to focal segmental glomerulosclerosis (FSGS) and Charcot–Marie–Tooth (CMT) neuropathy. To investigate how pathogenic INF2 variants alter ER integrity, we used high-resolution live imaging of HeLa cells. Cells expressing wild-type (WT) INF2 showed a predominant tubular ER with perinuclear clustering. Cells expressing INF2 FSGS variants that cause mild and intermediate disease induced more sheet-like ER, a pattern similar to that seen for cells expressing WT-INF2 that were treated with actin and microtubule (MT) inhibitors. Dual CMT-FSGS INF2 variants led to more severe ER dysmorphism, with a diffuse, fragmented ER and coarse INF2 aggregates. Proper organization of both F-actin and MT was needed to modulate the tubule vs. sheet conformation balance, while MT arrays regulated spatial expansion of tubular ER in the cell periphery. Pathogenic INF2 variants also induced mitochondria fragmentation and dysregulated mitochondria distribution. Such mitochondrial abnormalities were more prominent for cells expressing CMT-FSGS compared to those with FSGS variants, indicating that the severity of the dysfunction is linked to the degree of cytoskeletal disorganization. Our observations suggest that pathogenic INF2 variants disrupt ER continuity by altering interactions between the ER and the cytoskeleton that in turn impairs inter-organelle communication, especially at ER–mitochondria contact sites. ER continuity defects may be a common disease mechanism involved in both peripheral neuropathy and glomerulopathy. Full article

Parkinson’s disease (PD) is a disease of an unknown origin. Despite that, decades of research have provided considerable evidence that alpha-synuclein (αSyn) is central to the pathogenesis of disease. Mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) are functional domains formed at contact sites between the ER and mitochondria, with a well-established function of MAMs being the control of lipid homeostasis within the cell. Additionally, there are numerous proteins localized or enriched at MAMs that have regulatory roles in several different molecular signaling pathways required for cellular homeostasis, such as autophagy and neuroinflammation. Alterations in several of these signaling pathways that are functionally associated with MAMs are found in PD. Taken together with studies that find αSyn localized at MAMs, this has implicated MAM (dys)function as a converging domain relevant to PD. This review will highlight the many functions of MAMs and provide an overview of the literature that finds αSyn, in addition to several other PD-related proteins, localized there. This review will also detail the direct interaction of αSyn and αSyn-interacting partners with specific MAM-resident proteins. In addition, recent studies exploring new methods to investigate MAMs will be discussed, along with some of the controversies regarding αSyn, including its several conformations and subcellular localizations. The goal of this review is to highlight and provide insight on a domain that is incompletely understood and, from a PD perspective, highlight those complex interactions that may hold the key to understanding the pathomechanisms underlying PD, which may lead to the targeted development of new therapeutic strategies. Full article

Ongoing technical and bioinformatics improvements in mass spectrometry (MS) allow for the identifying and quantifying of the enrichment of increasingly less-abundant proteins in individual fractions. Accordingly, this study reassessed the proteome of mouse liver peroxisomes by the parallel isolation of peroxisomes from a mitochondria- and a microsome-enriched prefraction, combining density-gradient centrifugation with a semi-quantitative SWATH-MS proteomics approach to unveil novel peroxisomal or peroxisome-associated proteins. In total, 1071 proteins were identified using MS and assessed in terms of their distribution in either high-density peroxisomal or low-density gradient fractions, containing the bulk of organelle material. Combining the data from both fractionation approaches allowed for the identification of specific protein profiles characteristic of mitochondria, the ER and peroxisomes. Among the proteins significantly enriched in the peroxisomal cluster were several novel peroxisomal candidates. Five of those were validated by colocalization in peroxisomes, using confocal microscopy. The peroxisomal import of HTATIP2 and PAFAH2, which contain a peroxisome-targeting sequence 1 (PTS1), could be confirmed by overexpression in HepG2 cells. The candidates SAR1B and PDCD6, which are known ER-exit-site proteins, did not directly colocalize with peroxisomes, but resided at ER sites, which frequently surrounded peroxisomes. Hence, both proteins might concentrate at presumably co-purified peroxisome-ER membrane contacts. Intriguingly, the fifth candidate, OCIA domain-containing protein 1, was previously described as decreasing mitochondrial network formation. In this work, we confirmed its peroxisomal localization and further observed a reduction in peroxisome numbers in response to OCIAD1 overexpression. Hence, OCIAD1 appears to be a novel protein, which has an impact on both mitochondrial and peroxisomal maintenance. Full article

Pyruvate kinase M (PKM) 2 was described to interact with the inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) and suppress its activity. To further investigate the physiological importance of the PKM2:IP₃R interaction, we developed and characterized HeLa PKM2 knockout (KO) cells. In the HeLa PKM2 KO cells, the release of Ca²⁺ to the cytosol appears to be more sensitive to low agonist concentrations than in HeLa wild-type (WT) cells. However, upon an identical IP₃-induced Ca²⁺ release, Ca²⁺ uptake in the mitochondria is decreased in HeLa PKM2 KO cells, which may be explained by the smaller number of contact sites between the ER and the mitochondria. Furthermore, in HeLa PKM2 KO cells, mitochondria are more numerous, though they are smaller and less branched and have a hyperpolarized membrane potential. TAT-D5SD, a cell-permeable peptide representing a sequence derived from IP₃R1 that can disrupt the PKM2:IP₃R interaction, induces Ca²⁺ release into the cytosol and Ca²⁺ uptake into mitochondria in both HeLa WT and PKM2 KO cells. Moreover, TAT-D5SD induced apoptosis in HeLa WT and PKM2 KO cells but not in HeLa cells completely devoid of IP₃Rs. These results indicate that PKM2 separately regulates cytosolic and mitochondrial Ca²⁺ handling and that the cytotoxic effect of TAT-D5SD depends on IP₃R activity but not on PKM2. However, the tyrosine kinase Lck, which also interacts with the D5SD sequence, is expressed neither in HeLa WT nor PKM2 KO cells, and we can also exclude a role for PKM1, which is upregulated in HeLa PKM2 KO cells, indicating that the TAT-D5SD peptide has a more complex mode of action than anticipated. Full article

The Ca²⁺ release in microdomains formed by intercompartmental contacts, such as mitochondria-associated endoplasmic reticulum membranes (MAMs), encodes a signal that contributes to Ca²⁺ homeostasis and cell fate control. However, the composition and function of MAMs remain to be fully defined. Here, we focused on the transient receptor potential vanilloid 1 (TRPV1), a Ca²⁺-permeable ion channel and a polymodal nociceptor. We found TRPV1 channels in the reticular membrane, including some at MAMs, in a rat cardiomyoblast cell line (SV40-transformed H9c2) by Western blotting, immunostaining, cell fractionation, and proximity ligation assay. We used chemical and genetic probes to perform Ca²⁺ imaging in four cellular compartments: the endoplasmic reticulum (ER), cytoplasm, mitochondrial matrix, and mitochondrial surface. Our results showed that the ER Ca²⁺ released through TRPV1 channels is detected at the mitochondrial outer membrane and transferred to the mitochondria. Finally, we observed that prolonged TRPV1 modulation for 30 min alters the intracellular Ca²⁺ equilibrium and influences the MAM structure or the hypoxia/reoxygenation-induced cell death. Thus, our study provides the first evidence that TRPV1 channels contribute to MAM Ca²⁺ exchanges. Full article

Mitochondria are often referred to as the “powerhouse” of the cell. However, this organelle has many more functions than simply satisfying the cells’ metabolic needs. Mitochondria are involved in calcium homeostasis and lipid metabolism, and they also regulate apoptotic processes. Many of these functions require contact with the ER, which is mediated by several tether proteins located on the respective organellar surfaces, enabling the formation of mitochondria–ER contact sites (MERCS). Upon damage, mitochondria produce reactive oxygen species (ROS) that can harm the surrounding cell. To circumvent toxicity and to maintain a functional pool of healthy organelles, damaged and excess mitochondria can be targeted for degradation via mitophagy, a form of selective autophagy. Defects in mitochondria–ER tethers and the accumulation of damaged mitochondria are found in several neurodegenerative diseases, including Parkinson’s disease and amyotrophic lateral sclerosis, which argues that the interplay between the two organelles is vital for neuronal health. This review provides an overview of the different mechanisms of mitochondrial quality control that are implicated with the different mitochondria–ER tether proteins, and also provides a novel perspective on how MERCS are involved in mediating mitophagy upon mitochondrial damage. Full article

Exposure of living cells to non-thermal plasma produced in various electrical discharges affects cell physiology and often results in cell death. Even though plasma-based techniques have started finding practical applications in biotechnology and medicine, the molecular mechanisms of interaction of cells with plasma remain poorly understood. In this study, the involvement of selected cellular components or pathways in plasma-induced cell killing was studied employing yeast deletion mutants. The changes in yeast sensitivity to plasma-activated water were observed in mutants with the defect in mitochondrial functions, including transport across the outer mitochondrial membrane (∆por1), cardiolipin biosynthesis (∆crd1, ∆pgs1), respiration (ρ⁰) and assumed signaling to the nucleus (∆mdl1, ∆yme1). Together these results indicate that mitochondria play an important role in plasma-activated water cell killing, both as the target of the damage and the participant in the damage signaling, which may lead to the induction of cell protection. On the other hand, our results show that neither mitochondria-ER contact sites, UPR, autophagy, nor proteasome play a major role in the protection of yeast cells from plasma-induced damage. Full article

Alzheimer’s disease (AD) has no cure. Earlier, we showed that partial inhibition of mitochondrial complex I (MCI) with the small molecule CP2 induces an adaptive stress response, activating multiple neuroprotective mechanisms. Chronic treatment reduced inflammation, Aβ and pTau accumulation, improved synaptic and mitochondrial functions, and blocked neurodegeneration in symptomatic APP/PS1 mice, a translational model of AD. Here, using serial block-face scanning electron microscopy (SBFSEM) and three-dimensional (3D) EM reconstructions combined with Western blot analysis and next-generation RNA sequencing, we demonstrate that CP2 treatment also restores mitochondrial morphology and mitochondria-endoplasmic reticulum (ER) communication, reducing ER and unfolded protein response (UPR) stress in the APP/PS1 mouse brain. Using 3D EM volume reconstructions, we show that in the hippocampus of APP/PS1 mice, dendritic mitochondria primarily exist as mitochondria-on-a-string (MOAS). Compared to other morphological phenotypes, MOAS have extensive interaction with the ER membranes, forming multiple mitochondria-ER contact sites (MERCS) known to facilitate abnormal lipid and calcium homeostasis, accumulation of Aβ and pTau, abnormal mitochondrial dynamics, and apoptosis. CP2 treatment reduced MOAS formation, consistent with improved energy homeostasis in the brain, with concomitant reductions in MERCS, ER/UPR stress, and improved lipid homeostasis. These data provide novel information on the MOAS-ER interaction in AD and additional support for the further development of partial MCI inhibitors as a disease-modifying strategy for AD. Full article

Cholesterol is a key component of all mammalian cell membranes. Disruptions in cholesterol metabolism have been observed in the context of various diseases, including neurodegenerative disorders such as Alzheimer’s disease (AD). The genetic and pharmacological blockade of acyl-CoA:cholesterol acyltransferase 1/sterol O-acyltransferase 1 (ACAT1/SOAT1), a cholesterol storage enzyme found on the endoplasmic reticulum (ER) and enriched at the mitochondria-associated ER membrane (MAM), has been shown to reduce amyloid pathology and rescue cognitive deficits in mouse models of AD. Additionally, blocking ACAT1/SOAT1 activity stimulates autophagy and lysosomal biogenesis; however, the exact molecular connection between the ACAT1/SOAT1 blockade and these observed benefits remain unknown. Here, using biochemical fractionation techniques, we observe cholesterol accumulation at the MAM which leads to ACAT1/SOAT1 enrichment in this domain. MAM proteomics data suggests that ACAT1/SOAT1 inhibition strengthens the ER-mitochondria connection. Confocal and electron microscopy confirms that ACAT1/SOAT1 inhibition increases the number of ER-mitochondria contact sites and strengthens this connection by shortening the distance between these two organelles. This work demonstrates how directly manipulating local cholesterol levels at the MAM can alter inter-organellar contact sites and suggests that cholesterol buildup at the MAM is the impetus behind the therapeutic benefits of ACAT1/SOAT1 inhibition. Full article

Neuroblastoma (NB) is the most common extracranial tumor of early childhood and accounts for 15% of all pediatric cancer mortalities. However, the precise pathways and genes underlying its progression are unknown. Therefore, we performed a differential gene expression analysis of neuroblastoma stage 1 and stage 4 + 4S to discover biological processes associated with NB progression. From this preliminary analysis, we found that NB samples (stage 4 + 4S) are characterized by altered expression of some proteins involved in mitochondria function and mitochondria–ER contact sites (MERCS). Although further analyses remain necessary, this review may provide new hints to better understand NB molecular etiopathogenesis, by suggesting that MERCS alterations could be involved in the progression of NB. Full article

The mitochondria-associated endoplasmic reticulum (ER) membrane (MAM) is the physical contact site between the ER and the mitochondria and plays a vital role in the regulation of calcium signaling, bioenergetics, and inflammation. Disturbances in these processes and dysregulation of the ER and mitochondrial homeostasis contribute to the pathogenesis of diabetic retinopathy (DR). However, few studies have examined the impact of diabetes on the retinal MAM and its implication in DR pathogenesis. In the present study, we investigated the proteomic changes in retinal MAM from Long Evans rats with streptozotocin-induced long-term Type 1 diabetes. Furthermore, we performed in-depth bioinformatic analysis to identify key MAM proteins and pathways that are potentially implicated in retinal inflammation, angiogenesis, and neurodegeneration. A total of 2664 unique proteins were quantified using IonStar proteomics-pipeline in rat retinal MAM, among which 179 proteins showed significant changes in diabetes. Functional annotation revealed that the 179 proteins are involved in important biological processes such as cell survival, inflammatory response, and cellular maintenance, as well as multiple disease-relevant signaling pathways, e.g., integrin signaling, leukocyte extravasation, PPAR, PTEN, and RhoGDI signaling. Our study provides comprehensive information on MAM protein changes in diabetic retinas, which is helpful for understanding the mechanisms of metabolic dysfunction and retinal cell injury in DR. Full article