Mitochondria-associated membranes (MAMs): An emerging platform connecting energy and immune sensing to metabolic flexibility

Exploring the therapeutic effects of sulforaphane: an in-depth review on endoplasmic reticulum stress modulation across different disease contexts

The endoplasmic reticulum (ER) is an intracellular organelle that contributes to the folding of proteins and calcium homeostasis. Numerous elements can disrupt its function, leading to the accumulation of proteins that are unfolded or misfolded in the lumen of the ER, a condition that is known as ER stress. This phenomenon can trigger cell death through the activation of apoptosis and inflammation. Glucoraphanin (GRA) is the predominant glucosinolate found in cruciferous vegetables. Various mechanical and biochemical processes activate the enzyme myrosinase, leading to the hydrolysis of glucoraphanin into the bioactive compound sulforaphane. Sulforaphane is an organosulfur compound that belongs to the isothiocyanate group. It possesses a wide range of activities and has shown remarkable potential as an anti-inflammatory, antioxidant, antitumor, and anti-angiogenic substance. Additionally, sulforaphane is resistant to oxidation, has been demonstrated to have low toxicity, and is considered well-tolerable in individuals. These properties make it a valuable natural dietary supplement for research purposes. Sulforaphane has been demonstrated as a potential candidate drug molecule for managing a range of diseases, primarily because of its potent antioxidant, anti-inflammatory, and anti-apoptotic properties, which can be mediated by modulation of ER stress pathways. This review seeks to cover a wealth of data supporting the broad range of protective functions of sulforaphane, improving various diseases, such as cardiovascular, central nervous system, liver, eye, and reproductive diseases, as well as diabetes, cancer, gastroenteritis, and osteoarthritis, through the amelioration of ER stress in both in vivo and in vitro studies.

Mitochondria-associated endoplasmic reticulum membrane as a mediator of vanadium-induced endoplasmic reticulum quality control in duck brains

Vanadium (V) plays a crucial role in normal cells, but excess V causes multi-organ toxicity, including neurotoxicity. Mitochondria-associated endoplasmic reticulum membrane (MAM) is a dynamic structure between endoplasmic reticulum (ER) and mitochondria that mediates ER quality control (ERQC). To explore the effects of excess V on MAM and ERQC in the brain, 72 ducks were randomly divided into two groups: the control group (basal diet) and the V group (30 mg V/kg basal diet). On days 22 and 44, brain tissues were collected for histomorphological observation and determination of trace element contents. In addition, the mRNA and protein levels of MAM and ERQC-related factors in the brain were analyzed. Results show that excessive V causes the imbalance of trace elements, the integrity disruption of MAM, rupture of ER and autophagosomes formation. Moreover, it inhibits IP3R and VDAC1 co-localization, down-regulates the expression levels of MAM-related factors, but up-regulates the expression levels of ERQC and autophagy related factors. Together, results indicate that V exposure causes disruption of MAM and activates ERQC, which is further causing autophagy.

When Our Best Friend Becomes Our Worst Enemy: The Mitochondrion in Trauma, Surgery, and Critical Illness

Common for major surgery, multitrauma, sepsis, and critical illness, is a whole-body inflammation. Tissue injury is able to trigger a generalized inflammatory reaction. Cell death causes release of endogenous structures termed damage associated molecular patterns (DAMPs) that initiate a sterile inflammation. Mitochondria are evolutionary endosymbionts originating from bacteria, containing molecular patterns similar to bacteria. These molecular patterns are termed mitochondrial DAMPs (mDAMPs). Mitochondrial debris released into the extracellular space or into the circulation is immunogenic and damaging secondary to activation of the innate immune system. In the circulation, released mDAMPS are either free or exist in extracellular vesicles, being able to act on every organ and cell in the body. However, the role of mDAMPs in trauma and critical care is not fully clarified. There is a complete lack of knowledge how they may be counteracted in patients. Among mDAMPs are mitochondrial DNA, cardiolipin, N-formyl peptides, cytochrome C, adenosine triphosphate, reactive oxygen species, succinate, and mitochondrial transcription factor A. In this overview, we present the different mDAMPs, their function, release, targets, and inflammatory potential. In light of present knowledge, the role of mDAMPs in the pathophysiology of major surgery and trauma as well as sepsis, and critical care is discussed.

Proximity Proteome Analysis Reveals Novel TREM2 Interactors in the ER-Mitochondria Interface of Human Microglia

Triggering receptor expressed on myeloid cells 2 (TREM2) plays a central role in microglial biology and the pathogenesis of Alzheimer's disease (AD). Besides DNAX-activating protein 12 (DAP12), a communal adaptor for TREM2 and many other receptors, other cellular interactors of TREM2 remain largely elusive. We employed a 'proximity labeling' approach using a biotin ligase, TurboID, for mapping protein-protein interactions in live mammalian cells. We discovered novel TREM2-proximal proteins with diverse functions, including those localized to the Mitochondria-ER contact sites (MERCs), a dynamic subcellular 'hub' implicated in a number of crucial cell physiology such as lipid metabolism. TREM2 deficiency alters the thickness (inter-organelle distance) of MERCs, a structural parameter of metabolic state, in microglia derived from human induced pluripotent stem cells. Our TurboID-based TREM2 interactome study suggest novel roles for TREM2 in the structural plasticity of the MERCs, raising the possibility that dysregulation of MERC-related TREM2 functions contribute to AD pathobiology.

TRPV1 Channels Are New Players in the Reticulum-Mitochondria Ca2+ Coupling in a Rat Cardiomyoblast Cell Line

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.

Hepatitis C virus replication requires integrity of mitochondria-associated ER membranes

Background & Aims

Chronic HCV infection causes cellular stress, fibrosis and predisposes to hepatocarcinogenesis. Mitochondria play key roles in orchestrating stress responses by regulating bioenergetics, inflammation and apoptosis. To better understand the role of mitochondria in the viral life cycle and disease progression of chronic hepatitis C, we studied morphological and functional mitochondrial alterations induced by HCV using productively infected hepatoma cells and patient livers.

Methods

Biochemical and imaging assays were used to assess localization of cellular and viral proteins and mitochondrial functions in cell cultures and liver biopsies. Cyclophilin D (CypD) knockout was performed using CRISPR/Cas9 technology. Viral replication was quantified by quantitative reverse-transcription PCR and western blotting.

Results

Several HCV proteins were found to associate with mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs), the points of contact between the ER and mitochondria. Downregulation of CypD, which is known to disrupt MAM integrity, reduced viral replication, suggesting that MAMs play an important role in the viral life cycle. This process was rescued by ectopic CypD expression. Furthermore, HCV proteins were found to associate with voltage dependent anion channel 1 (VDAC1) at MAMs and to reduce VDAC1 protein levels at MAMs in vitro and in patient biopsies. This association did not affect MAM-associated functions in glucose homeostasis and Ca²⁺ signaling.

Conclusions

HCV proteins associate specifically with MAMs and MAMs play an important role in viral replication. The association between viral proteins and MAMs did not impact Ca²⁺ signaling between the ER and mitochondria or glucose homeostasis. Whether additional functions of MAMs and/or VDAC are impacted by HCV and contribute to the associated pathology remains to be assessed.

Impact and implications

Hepatitis C virus infects the liver, where it causes inflammation, cell damage and increases the long-term risk of liver cancer. We show that several HCV proteins interact with mitochondria in liver cells and alter the composition of mitochondrial subdomains. Importantly, HCV requires the architecture of these mitochondrial subdomains to remain intact for efficient viral replication.

Balancing life and death: BCL-2 family members at diverse ER-mitochondrial contact sites

The outer mitochondrial membrane is a busy place. One essential activity for cellular survival is the regulation of membrane integrity by the BCL-2 family of proteins. Another critical facet of the outer mitochondrial membrane is its close approximation with the endoplasmic reticulum. These mitochondrial-associated membranes (MAMs) occupy a significant fraction of the mitochondrial surface and serve as key signaling hubs for multiple cellular processes. Each of these pathways may be considered as forming their own specialized MAM subtype. Interestingly, like membrane permeabilization, most of these pathways play critical roles in regulating cellular survival and death. Recently, the pro-apoptotic BCL-2 family member BOK has been found within MAMs where it plays important roles in their structure and function. This has led to a greater appreciation that multiple BCL-2 family proteins, which are known to participate in numerous functions throughout the cell, also have roles within MAMs. In this review, we evaluate several MAM subsets, their role in cellular homeostasis, and the contribution of BCL-2 family members to their functions.

NAFLD: Mechanisms, Treatments, and Biomarkers

Nonalcoholic fatty liver disease (NAFLD), recently renamed metabolic-associated fatty liver disease (MAFLD), is one of the most common causes of liver diseases worldwide. NAFLD is growing in parallel with the obesity epidemic. No pharmacological treatment is available to treat NAFLD, specifically. The reason might be that NAFLD is a multi-factorial disease with an incomplete understanding of the mechanisms involved, an absence of accurate and inexpensive imaging tools, and lack of adequate non-invasive biomarkers. NAFLD consists of the accumulation of excess lipids in the liver, causing lipotoxicity that might progress to metabolic-associated steatohepatitis (NASH), liver fibrosis, and hepatocellular carcinoma. The mechanisms for the pathogenesis of NAFLD, current interventions in the management of the disease, and the role of sirtuins as potential targets for treatment are discussed here. In addition, the current diagnostic tools, and the role of non-coding RNAs as emerging diagnostic biomarkers are summarized. The availability of non-invasive biomarkers, and accurate and inexpensive non-invasive diagnosis tools are crucial in the detection of the early signs in the progression of NAFLD. This will expedite clinical trials and the validation of the emerging therapeutic treatments.

In Vivo Pravastatin Treatment Reverses Hypercholesterolemia Induced Mitochondria-Associated Membranes Contact Sites, Foam Cell Formation, and Phagocytosis in Macrophages

Statins are successful drugs used to treat hypercholesterolemia, a primary cause of atherosclerosis. In this work, we investigated how hypercholesterolemia and pravastatin treatment impact macrophage and mitochondria functions, the key cell involved in atherogenesis. By comparing bone marrow-derived macrophages (BMDM) of wild-type (WT) and LDL receptor knockout (LDLr^−/−) mice, we observed hypercholesterolemia increased the number of contact sites at mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs), enhanced mitochondrial hydrogen peroxide release, altered the gene expression of inflammatory markers, and increased oxidized LDL (ox-LDL) uptake and phagocytic activity. Three months of in vivo pravastatin treatment of LDLr^−/− mice reversed the number of contact sites at the MAM, ox-LDL uptake, and phagocytosis in LDLr^−/− BMDM. Additionally, pravastatin increased BMDM mitochondrial network branching. In peritoneal macrophages (PMs), hypercholesterolemia did not change MAM stability, but stimulated hydrogen peroxide production and modulated gene expression of pro- and anti-inflammatory markers. It also increased mitochondrial branching degree and had no effects on ox-LDL uptake and phagocytosis in PM. Pravastatin treatment increased superoxide anion production and changed inflammation-related gene expression in LDLr^−/− PM. In addition, pravastatin increased markedly the expression of the mitochondrial dynamics-related genes Mfn2 and Fis1 in both macrophages. In summary, our results show that hypercholesterolemia and pravastatin treatment affect macrophage mitochondria network structure as well as their interaction with the endoplasmic reticulum (ER). These effects impact on macrophage conversion rates to foam cell and macrophage phagocytic capacity. These findings associate MAM stability changes with known mechanisms involved in atherosclerosis progression and resolution.

Targeting ER-Mitochondria Signaling as a Therapeutic Target for Frontotemporal Dementia and Related Amyotrophic Lateral Sclerosis

Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) are two major neurodegenerative diseases. FTD is the second most common cause of dementia and ALS is the most common form of motor neuron disease. These diseases are now known to be linked. There are no cures or effective treatments for FTD or ALS and so new targets for therapeutic intervention are required but this is hampered by the large number of physiological processes that are damaged in FTD/ALS. Many of these damaged functions are now known to be regulated by signaling between the endoplasmic reticulum (ER) and mitochondria. This signaling is mediated by "tethering" proteins that serve to recruit ER to mitochondria. One tether strongly associated with FTD/ALS involves an interaction between the ER protein VAPB and the mitochondrial protein PTPIP51. Recent studies have shown that ER-mitochondria signaling is damaged in FTD/ALS and that this involves breaking of the VAPB-PTPIP51 tethers. Correcting disrupted tethering may therefore correct many other downstream damaged features of FTD/ALS. Here, we review progress on this topic with particular emphasis on targeting of the VAPB-PTPIP51 tethers as a new drug target.

MITOL-mediated DRP1 ubiquitylation and degradation promotes mitochondrial hyperfusion in CMT2A-linked MFN2 mutant

Mutations in Mitofusin2 (MFN2), associated with the pathology of the debilitating neuropathy, Charcot-Marie-Tooth type 2A (CMT2A) are known to alter mitochondrial morphology. One such abundant MFN2 mutant, R364W results in the generation of elongated, interconnected mitochondria. However, the mechanism leading to this mitochondrial aberration remains poorly understood. Here we show that mitochondrial hyperfusion in the presence of R364W-MFN2 is due to increased degradation of DRP1. The Ubiquitin E3 ligase MITOL is known to ubiquitylate both MFN2 and DRP1. Interaction with and its subsequent ubiquitylation by MITOL is stronger in presence of WT-MFN2 than R364W-MFN2. This differential interaction of MITOL with MFN2 in the presence of R364W-MFN2 renders the ligase more available for DRP1 ubiquitylation. Multimonoubiquitylation and proteasomal degradation of DRP1 in R364W-MFN2 cells in the presence of MITOL eventually leads to mitochondrial hyperfusion. Here we provide a mechanistic insight into mitochondrial hyperfusion, while also reporting that MFN2 can indirectly modulate DRP1 - an effect not shown before.

A Review of the Role of Endo/Sarcoplasmic Reticulum-Mitochondria Ca2+ Transport in Diseases and Skeletal Muscle Function

The physical contact site between a mitochondrion and endoplasmic reticulum (ER), named the mitochondria-associated membrane (MAM), has emerged as a fundamental platform for regulating the functions of the two organelles and several cellular processes. This includes Ca²⁺ transport from the ER to mitochondria, mitochondrial dynamics, autophagy, apoptosis signalling, ER stress signalling, redox reaction, and membrane structure maintenance. Consequently, the MAM is suggested to be involved in, and as a possible therapeutic target for, some common diseases and impairment in skeletal muscle function, such as insulin resistance and diabetes, obesity, neurodegenerative diseases, Duchenne muscular dystrophy, age-related muscle atrophy, and exercise-induced muscle damage. In the past decade, evidence suggests that alterations in Ca²⁺ transport from the ER to mitochondria, mediated by the macromolecular complex formed by IP₃R, Grp75, and VDAC1, may be a universal mechanism for how ER-mitochondria cross-talk is involved in different physiological/pathological conditions mentioned above. A better understanding of the ER (or sarcoplasmic reticulum in muscle)-mitochondria Ca²⁺ transport system may provide a new perspective for exploring the mechanism of how the MAM is involved in the pathology of diseases and skeletal muscle dysfunction. This review provides a summary of recent research findings in this area.

Beclin-1 improves mitochondria-associated membranes in the heart during endotoxemia

Mitochondria‐associated membranes (MAMs) are essential to mitochondria. This study was to determine whether endotoxemia rearranges MAMs in the heart, and whether Beclin‐1 regulates this process. Wild‐type mice and mice with a cardiac‐specific overexpression of Beclin‐1 (Becn1‐Tg), or a heterozygous knockout of Beclin‐1 (Becn1^+/−) were given lipopolysaccharide (LPS) challenge. In the heart, the ultrastructure of MAMs was examined by electron microscopy and the histology evaluated by immunostaining. Additionally, MAMs were isolated by ultracentrifugation, and their content and function were quantified. The effects of Beclin‐1‐activating peptide (TB‐peptide) on MAMs were also examined. Data showed that endotoxemia decreased both the total mass and the function of MAMs, and these deficiencies became worse in Becn1^+/− mice but were alleviated in Becn1‐Tg and TB‐peptide‐treated mice. Responses of myocardial MAMs to LPS and to TB‐peptide were additionally examined in AC16 human cardiomyocytes. In vitro findings recaptured the effects of LPS and TB‐peptide in cardiomyocytes; the challenge of LPS reduced the level and activity of MAMs, and TB‐peptide attenuated this defect. Together, the results suggest a new function of Beclin‐1 in improving cardiac MAMs during endotoxemia, providing a mechanism for the previously identified role of Beclin‐1 in protection of mitochondria and cardiac function.

Sex specific inflammatory profiles of cerebellar mitochondria are attenuated in Parkinson's disease

Response to inflammation is a key determinant in many diseases and their outcomes. Diseases that commonly affect older people are frequently associated with altered inflammatory processes. Neuroinflammation has been described in Parkinson's disease (PD) brain. PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and at the sub-cellular level, mitochondrial dysfunction is a key feature. However, there is evidence that a different region of the brain, the cerebellum, is involved in the pathophysiology of PD. We report relative levels of 40 pro- and anti-inflammatory cytokines measured in PD and control cerebellar mitochondria. These data were obtained by screening cytokine antibody arrays. In parallel, we present concentrations of 29 oxylipins and 4 endocannabinoids measured in mitochondrial fractions isolated from post-mortem PD cerebellum with age and sex matched controls. Our oxylipin and endocannabinoid data were acquired via quantitation by LC-ESI-MS/MS. The separate sample sets both show there are clearly different inflammatory profiles between the sexes in control samples. Sex specific profiles were not maintained in cerebellar mitochondria isolated from PD brains.

Disruption of endoplasmic reticulum-mitochondria tethering proteins in post-mortem Alzheimer's disease brain

Signaling between the endoplasmic reticulum (ER) and mitochondria regulates a number of key neuronal functions, many of which are perturbed in Alzheimer's disease. Moreover, damage to ER-mitochondria signaling is seen in cell and transgenic models of Alzheimer's disease. However, as yet there is little evidence that ER-mitochondria signaling is altered in human Alzheimer's disease brains. ER-mitochondria signaling is mediated by interactions between the integral ER protein VAPB and the outer mitochondrial membrane protein PTPIP51 which act to recruit and “tether” regions of ER to the mitochondrial surface. The VAPB-PTPIP51 tethers are now known to regulate a number of ER-mitochondria signaling functions including delivery of Ca²⁺from ER stores to mitochondria, mitochondrial ATP production, autophagy and synaptic activity. Here we investigate the VAPB-PTPIP51 tethers in post-mortem control and Alzheimer's disease brains. Quantification of ER-mitochondria signaling proteins by immunoblotting revealed loss of VAPB and PTPIP51 in cortex but not cerebellum at end-stage Alzheimer's disease. Proximity ligation assays were used to quantify the VAPB-PTPIP51 interaction in temporal cortex pyramidal neurons and cerebellar Purkinje cell neurons in control, Braak stage III-IV (early/mid-dementia) and Braak stage VI (severe dementia) cases. Pyramidal neurons degenerate in Alzheimer's disease whereas Purkinje cells are less affected. These studies revealed that the VAPB-PTPIP51 tethers are disrupted in Braak stage III-IV pyramidal but not Purkinje cell neurons. Thus, we identify a new pathogenic event in post-mortem Alzheimer's disease brains. The implications of our findings for Alzheimer's disease mechanisms are discussed.

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VAPB, PTPIP51 and IP3R1 levels are reduced in temporal cortex in late-stage Alzheimer's disease

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Proximity ligation assays reveal that the VAPB-PTPIP51 interaction is disrupted in temporal cortex pyramidal neurons in early (Braak stage III-IV) Alzheimer's disease

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We identify damage to the VAPB-PTPIP51 ER-mitochondria tethers as a new pathogenic event in Alzheimer’s disease.

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Disruption of the VAPB-PTPIP51 tethers may contribute to the neurodegenerative process in Alzheimer’s disease

Mitochondria, endoplasmic reticulum and innate immune dysfunction in mood disorders: Do Mitochondria-Associated Membranes (MAMs) play a role?

Mood disorders like major depression and bipolar disorder (BD) are among the most prevalent forms of mental illness. Current knowledge of the neurobiology and pathophysiology of these disorders is still modest and clear biological markers are still missing. Thus, a better understanding of the underlying pathophysiological mechanisms to identify potential therapeutic targets is a prerequisite for the design of new drugs as well as to develop biomarkers that help in a more accurate and earlier diagnosis. Multiple pieces of evidence including genetic and neuro-imaging studies suggest that mood disorders are associated with abnormalities in endoplasmic-reticulum (ER)-related stress responses, mitochondrial function and calcium signalling. Furthermore, deregulation of the innate immune response has been described in patients diagnosed with mood disorders, including depression and BD. These disease-related events are associated with functions localized to a subdomain of the ER, known as Mitochondria-Associated Membranes (MAMs), which are lipid rafts-like domains that connect mitochondria and ER, both physically and biochemically. This review will outline the current understanding of the role of mitochondria and ER dysfunction under pathological brain conditions, particularly in major depressive disorder (MDD) and BD, that support the hypothesis that MAMs can act in these mood disorders as the link connecting ER-related stress response and mitochondrial impairment, as well as a mechanisms behind sterile inflammation arising from deregulation of innate immune responses. The role of MAMs in the pathophysiology of these pathologies and its potential relevance as a potential therapeutic target will be discussed.

Microglia monitor and protect neuronal function through specialized somatic purinergic junctions

Microglia are the main immune cells in the brain and have roles in brain homeostasis and neurological diseases. Mechanisms underlying microglia-neuron communication remain elusive. Here, we identified an interaction site between neuronal cell bodies and microglial processes in mouse and human brain. Somatic microglia-neuron junctions have a specialized nanoarchitecture optimized for purinergic signaling. Activity of neuronal mitochondria was linked with microglial junction formation, which was induced rapidly in response to neuronal activation and blocked by inhibition of P2Y12 receptors. Brain injury-induced changes at somatic junctions triggered P2Y12 receptor-dependent microglial neuroprotection, regulating neuronal calcium load and functional connectivity. Thus, microglial processes at these junctions could potentially monitor and protect neuronal functions.

Potential effect of germanium exposure on the risk of influenza-like illness in housewives in Shanxi Province, China

Few studies have examined the relationship between exposure to germanium (Ge) and the risk of influenza-like illness (ILI). Therefore, we investigated the association of Ge exposure and its interaction with single nucleotide polymorphisms (SNPs) related to Phase II metabolism on ILI risk among housewives in Shanxi Province, northern China. This cross-sectional study enrolled 373 housewives. Information on the housewives' characteristics and the frequency of ILI was collected by questionnaire. We analyzed the Ge concentrations in hair samples taken from near the scalp at the back of the head. Blood samples were used to identify SNPs related to Phase II metabolism. The results suggested that the hair Ge concentration was associated with ILI risk with an adjusted odds ratio and 95% confidence interval of 2.59 (1.61-4.19). A significant dose-response relationship was observed without or with adjusting for confounders. We did not observe any interaction effect between the hair Ge concentration and the SNPs on ILI risk. We found that high dietary consumption of meat and fried foods was positively correlated with the hair Ge concentration. Therefore, chronic Ge exposure may be a risk factor for an increased frequency of ILI in housewives.

A modified calcium retention capacity assay clarifies the roles of extra- and intracellular calcium pools in mitochondrial permeability transition pore opening

Calcium homeostasis is essential for cell survival and is precisely controlled by several cellular actors such as the sarco/endoplasmic reticulum and mitochondria. Upon stress induction, Ca²⁺ released from sarco/endoplasmic reticulum stores and from extracellular Ca²⁺ pools accumulates in the cytosol and in the mitochondria. This induces Ca²⁺ overload and ultimately the opening of the mitochondrial permeability transition pore (mPTP), promoting cell death. Currently, it is unclear whether intracellular Ca²⁺ stores are sufficient to promote the mPTP opening. Ca²⁺ retention capacity (CRC) corresponds to the maximal Ca²⁺ uptake by the mitochondria before mPTP opening. In this study, using permeabilized cardiomyocytes isolated from adult mice, we modified the standard CRC assay by specifically inducing reticular Ca²⁺ release to investigate the respective contributions of reticular Ca²⁺ and extracellular Ca²⁺ to mPTP opening in normoxic conditions or after anoxia-reoxygenation. Our experiments revealed that Ca²⁺ released from the sarco/endoplasmic reticulum is not sufficient to trigger mPTP opening and corresponds to ∼50% of the total Ca²⁺ levels required to open the mPTP. We also studied mPTP opening after anoxia-reoxygenation in the presence or absence of extracellular Ca²⁺ In both conditions, Ca²⁺ leakage from internal stores could not trigger mPTP opening by itself but significantly decreased the CRC. Our findings highlight how a modified CRC assay enables the investigation of the role of reticular and extracellular Ca²⁺ pools in the regulation of the mPTP. We propose that this method may be useful for screening molecules of interest implicated in mPTP regulation.

Lipid-dependent coupling of secretory cargo sorting and trafficking at the trans-Golgi network

In eukaryotic cells, the trans-Golgi network (TGN) serves as a platform for secretory cargo sorting and trafficking. In recent years, it has become evident that a complex network of lipid–lipid and lipid–protein interactions contributes to these key functions. This review addresses the role of lipids at the TGN with a particular emphasis on sphingolipids and diacylglycerol. We further highlight how these lipids couple secretory cargo sorting and trafficking for spatiotemporal coordination of protein transport to the plasma membrane.

The VAPB-PTPIP51 endoplasmic reticulum-mitochondria tethering proteins are present in neuronal synapses and regulate synaptic activity

Signaling between the endoplasmic reticulum (ER) and mitochondria regulates a number of key neuronal functions. This signaling involves close physical contacts between the two organelles that are mediated by "tethering proteins" that function to recruit regions of ER to the mitochondrial surface. The ER protein, vesicle-associated membrane protein-associated protein B (VAPB) and the mitochondrial membrane protein, protein tyrosine phosphatase interacting protein-51 (PTPIP51), interact to form one such tether. Recently, damage to ER-mitochondria signaling involving disruption of the VAPB-PTPIP51 tethers has been linked to the pathogenic process in Parkinson's disease, fronto-temporal dementia (FTD) and related amyotrophic lateral sclerosis (ALS). Loss of neuronal synaptic function is a key feature of Parkinson's disease and FTD/ALS but the roles that ER-mitochondria signaling and the VAPB-PTPIP51 tethers play in synaptic function are not known. Here, we demonstrate that the VAPB-PTPIP51 tethers regulate synaptic activity. VAPB and PTPIP51 localise and form contacts at synapses, and stimulating neuronal activity increases ER-mitochondria contacts and the VAPB-PTPIP51 interaction. Moreover, siRNA loss of VAPB or PTPIP51 perturbs synaptic function and dendritic spine morphology. Our results reveal a new role for the VAPB-PTPIP51 tethers in neurons and suggest that damage to ER-mitochondria signaling contributes to synaptic dysfunction in Parkinson's disease and FTD/ALS.

Disruption of Mitochondria-Associated Endoplasmic Reticulum Membrane (MAM) Integrity Contributes to Muscle Insulin Resistance in Mice and Humans

Modifications of the interactions between endoplasmic reticulum (ER) and mitochondria, defined as mitochondria-associated membranes (MAMs), were recently shown to be involved in the control of hepatic insulin action and glucose homeostasis, but with conflicting results. Whereas skeletal muscle is the primary site of insulin-mediated glucose uptake and the main target for alterations in insulin-resistant states, the relevance of MAM integrity in muscle insulin resistance is unknown. Deciphering the importance of MAMs on muscle insulin signaling could help to clarify this controversy. Here, we show in skeletal muscle of different mice models of obesity and type 2 diabetes (T2D) a marked disruption of ER-mitochondria interactions as an early event preceding mitochondrial dysfunction and insulin resistance. Furthermore, in human myotubes, palmitate-induced insulin resistance is associated with a reduction of structural and functional ER-mitochondria interactions. Importantly, experimental increase of ER-mitochondria contacts in human myotubes prevents palmitate-induced alterations of insulin signaling and action, whereas disruption of MAM integrity alters the action of the hormone. Lastly, we found an association between altered insulin signaling and ER-mitochondria interactions in human myotubes from obese subjects with or without T2D compared with healthy lean subjects. Collectively, our data reveal a new role of MAM integrity in insulin action of skeletal muscle and highlight MAM disruption as an essential subcellular alteration associated with muscle insulin resistance in mice and humans. Therefore, reduced ER-mitochondria coupling could be a common alteration of several insulin-sensitive tissues playing a key role in altered glucose homeostasis in the context of obesity and T2D.

The role of endoplasmic reticulum-mitochondria contact sites in the control of glucose homeostasis: an update

The contact sites that the endoplasmic reticulum (ER) forms with mitochondria, called mitochondria-associated membranes (MAMs), are a hot topic in biological research, and both their molecular determinants and their numerous roles in several signaling pathways are is continuously evolving. MAMs allow the exchange between both organelles of lipids, calcium (Ca²⁺), and likely reactive oxygen species, allowing adaptations of both cellular bioenergetics and cell fate depending of cellular needs or stresses. Therefore, it is not surprising that MAMs affect cellular metabolism. Nevertheless, recent arguments suggest that MAMs could also act as key hub of hormonal and/or nutrient signaling in several insulin-sensitive tissues, pointing a specific role of MAMs in the control of glucose homeostasis. Here, I provide a brief review and update on current key signaling roles of the MAMs in the control of glucose homeostasis in both health and metabolic diseases. Particularly, the relevance of ER-mitochondria miscommunication in the disruption of glucose homeostasis is analyzed in details in the liver, skeletal muscle, adipose tissue, and beta cells of the pancreas.