Tespa1 is a novel component of mitochondria-associated endoplasmic reticulum membranes and affects mitochondrial calcium flux

Tetrameric, active PKM2 inhibits IP3 receptors, potentially requiring GRP75 as an additional interaction partner

Pyruvate kinase M2 (PKM2) is a key glycolytic enzyme interacting with the inositol 1,4,5-trisphosphate receptor (IP3R). This interaction suppresses IP3R-mediated cytosolic [Ca2+] rises. As PKM2 exists in monomeric, dimeric and tetrameric forms displaying different properties including catalytic activity, we investigated the molecular determinants of PKM2 enabling its interaction with IP3Rs. Treatment of HeLa cells with TEPP-46, a compound stabilizing the tetrameric form of PKM2, increased both its catalytic activity and the suppression of IP3R-mediated Ca2+ signals. Consistently, in PKM2 knock-out HeLa cells, PKM2C424L, a tetrameric, highly active PKM2 mutant, but not inactive PKM2K270M or the less active PKM2K305Q, suppressed IP3R-mediated Ca2+ release. Surprisingly, however, in vitro assays did not reveal a direct interaction between purified PKM2 and either the purified Fragment 5 of IP3R1 (a.a. 1932-2216) or the therein located D5SD peptide (a.a. 2078-2098 of IP3R1), the presumed interaction sites of PKM2 on the IP3R. Moreover, on-nucleus patch clamp of heterologously expressed IP3R1 in DT40 cells devoid of endogenous IP3Rs did not reveal any functional effect of purified wild-type PKM2, mutant PKM2 or PKM1 proteins. These results indicate that an additional factor mediates the regulation of the IP3R by PKM2 in cellulo. Immunoprecipitation of GRP75 using HeLa cell lysates co-precipitated IP3R1, IP3R3 and PKM2. Moreover, the D5SD peptide not only disrupted PKM2:IP3R, but also PKM2:GRP75 and GRP75:IP3R interactions. Our data therefore support a model in which catalytically active, tetrameric PKM2 suppresses Ca2+ signaling via the IP3R through a multiprotein complex involving GRP75.

KRAP regulates mitochondrial Ca2+ uptake by licensing IP3 receptor activity and stabilizing ER-mitochondrial junctions

Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) are high-conductance channels that allow the regulated redistribution of Ca2+ from the endoplasmic reticulum (ER) to the cytosol and, at specialized membrane contact sites (MCSs), to other organelles. Only a subset of IP3Rs release Ca2+ to the cytosol in response to IP3. These 'licensed' IP3Rs are associated with Kras-induced actin-interacting protein (KRAP, also known as ITPRID2) beneath the plasma membrane. It is unclear whether KRAP regulates IP3Rs at MCSs. We show, using simultaneous measurements of Ca2+ concentration in the cytosol and mitochondrial matrix, that KRAP also licenses IP3Rs to release Ca2+ to mitochondria. Loss of KRAP abolishes cytosolic and mitochondrial Ca2+ signals evoked by stimulation of IP3Rs via endogenous receptors. KRAP is located at ER-mitochondrial membrane contact sites (ERMCSs) populated by IP3R clusters. Using a proximity ligation assay between IP3R and voltage-dependent anion channel 1 (VDAC1), we show that loss of KRAP reduces the number of ERMCSs. We conclude that KRAP regulates Ca2+ transfer from IP3Rs to mitochondria by both licensing IP3R activity and stabilizing ERMCSs.

Genome-wide epistasis analysis reveals gene-gene interaction network on an intermediate endophenotype P-tau/Aβ42 ratio in ADNI cohort

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the most common cause of dementia in the elderly worldwide. The exact etiology of AD, particularly its genetic mechanisms, remains incompletely understood. Traditional genome-wide association studies (GWAS), which primarily focus on single-nucleotide polymorphisms (SNPs) with main effects, provide limited explanations for the "missing heritability" of AD, while there is growing evidence supporting the important role of epistasis. In this study, we performed a genome-wide SNP-SNP interaction detection using a linear regression model and employed multiple GPUs for parallel computing, significantly enhancing the speed of whole-genome analysis. The cerebrospinal fluid (CSF) phosphorylated tau (P-tau)/amyloid-[Formula: see text] (A[Formula: see text]) ratio was used as a quantitative trait (QT) to enhance statistical power. Age, gender, and clinical diagnosis were included as covariates to control for potential non-genetic factors influencing AD. We identified 961 pairs of statistically significant SNP-SNP interactions, explaining a high-level variance of P-tau/A[Formula: see text] level, all of which exhibited marginal main effects. Additionally, we replicated 432 previously reported AD-related genes and found 11 gene-gene interaction pairs overlapping with the protein-protein interaction (PPI) network. Our findings may contribute to partially explain the "missing heritability" of AD. The identified subnetwork may be associated with synaptic dysfunction, Wnt signaling pathway, oligodendrocytes, inflammation, hippocampus, and neuronal cells.

Tespa1 deficiency reduces the antitumour immune response by decreasing CD8+T cell activity in a mouse Lewis lung cancer model

Thymocyte-expressed, positive selection-associated 1 (Tespa1) is a key molecule in T-cell development and has been linked to immune diseases. However, its role in antitumour CD8+T cell immunity remains unclear. Here, we demonstrated that Tespa1 plays an important role in antitumour CD8+T cell immunity. First, compared with wild-type (WT) mice, Lewis lung cancer cells grew faster in Tespa1 knockout (Tespa1-/-) mice, with reduced apoptosis, and decreased CD8+T cells in peripheral blood and tumor tissues. Second, the proportion of CD8+T and Th1 cells in the splenocytes of Tespa1-/- mice was lower than that in WT mice. Third, Tespa1-/- CD8+ tumor-infiltrating lymphocytes (TILs) showed weakened proliferation, invasion, cytotoxicity, and protein expression of IL-2 signalling pathway components compared to WT CD8+TILs. Furthermore, PD-1 expression in CD8+TILs was higher in Tespa1-/- than in WT mice. Lastly, CD8+TILs in WT mice improved the antitumour ability of Tespa1-/- mice. In conclusion, these findings suggest that Tespa1 plays a critical role in the tumor immune system by regulating CD8+T cells.

IP3 receptors: An "elementary" journey from structure to signals

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are large tetrameric channels which sit mostly in the membrane of the endoplasmic reticulum (ER) and mediate Ca2+ release from intracellular stores in response to extracellular stimuli in almost all cells. Dual regulation of IP3Rs by IP3 and Ca2+ itself, upstream "licensing", and the arrangement of IP3Rs into small clusters in the ER membrane, allow IP3Rs to generate spatially and temporally diverse Ca2+ signals. The characteristic biphasic regulation of IP3Rs by cytosolic Ca2+ concentration underpins regenerative Ca2+ signals by Ca2+-induced Ca2+-release, while also preventing uncontrolled explosive Ca2+ release. In this way, cells can harness a simple ion such as Ca2+ as a near-universal intracellular messenger to regulate diverse cellular functions, including those with conflicting outcomes such as cell survival and cell death. High-resolution structures of the IP3R bound to IP3 and Ca2+ in different combinations have together started to unravel the workings of this giant channel. Here we discuss, in the context of recently published structures, how the tight regulation of IP3Rs and their cellular geography lead to generation of "elementary" local Ca2+ signals known as Ca2+ "puffs", which form the fundamental bottleneck through which all IP3-mediated cytosolic Ca2+ signals must first pass.

The ER-mitochondria interface, where Ca2+ and cell death meet

Endoplasmic reticulum (ER)-mitochondria contact sites are crucial to allow Ca²⁺ flux between them and a plethora of proteins participate in tethering both organelles together. Inositol 1,4,5-trisphosphate receptors (IP₃Rs) play a pivotal role at such contact sites, participating in both ER-mitochondria tethering and as Ca²⁺-transport system that delivers Ca²⁺ from the ER towards mitochondria. At the ER-mitochondria contact sites, the IP₃Rs function as a multi-protein complex linked to the voltage-dependent anion channel 1 (VDAC1) in the outer mitochondrial membrane, via the chaperone glucose-regulated protein 75 (GRP75). This IP₃R-GRP75-VDAC1 complex supports the efficient transfer of Ca²⁺ from the ER into the mitochondrial intermembrane space, from which the Ca²⁺ ions can reach the mitochondrial matrix through the mitochondrial calcium uniporter. Under physiological conditions, basal Ca²⁺ oscillations deliver Ca²⁺ to the mitochondrial matrix, thereby stimulating mitochondrial oxidative metabolism. However, when mitochondrial Ca²⁺ overload occurs, the increase in [Ca²⁺] will induce the opening of the mitochondrial permeability transition pore, thereby provoking cell death. The IP₃R-GRP75-VDAC1 complex forms a hub for several other proteins that stabilize the complex and/or regulate the complex's ability to channel Ca²⁺ into the mitochondria. These proteins and their mechanisms of action are discussed in the present review with special attention for their role in pathological conditions and potential implication for therapeutic strategies.

Mitochondrial Dysfunction in Cardiac Arrhythmias

Electrophysiological and structural disruptions in cardiac arrhythmias are closely related to mitochondrial dysfunction. Mitochondria are an organelle generating ATP, thereby satisfying the energy demand of the incessant electrical activity in the heart. In arrhythmias, the homeostatic supply-demand relationship is impaired, which is often accompanied by progressive mitochondrial dysfunction leading to reduced ATP production and elevated reactive oxidative species generation. Furthermore, ion homeostasis, membrane excitability, and cardiac structure can be disrupted through pathological changes in gap junctions and inflammatory signaling, which results in impaired cardiac electrical homeostasis. Herein, we review the electrical and molecular mechanisms of cardiac arrhythmias, with a particular focus on mitochondrial dysfunction in ionic regulation and gap junction action. We provide an update on inherited and acquired mitochondrial dysfunction to explore the pathophysiology of different types of arrhythmias. In addition, we highlight the role of mitochondria in bradyarrhythmia, including sinus node dysfunction and atrioventricular node dysfunction. Finally, we discuss how confounding factors, such as aging, gut microbiome, cardiac reperfusion injury, and electrical stimulation, modulate mitochondrial function and cause tachyarrhythmia.

Mitochondria-associated endoplasmic reticulum membranes: A promising toxicity regulation target

Mitochondria-associated endoplasmic reticulum membranes (MAMs) are dynamic suborganelle membranes that physically couple endoplasmic reticulum (ER) and mitochondria to provide a platform for exchange of intracellular molecules and crosstalk between the two organelles. Dysfunctions of mitochondria and ER and imbalance of intracellular homeostasis have been discovered in the research of toxics. Cellular activities such as oxidative stress, ER stress, Ca²⁺ transport, autophagy, mitochondrial fusion and fission, and apoptosis mediated by MAMs are closely related to the toxicological effects of various toxicants. These cellular activities mediated by MAMs crosstalk with each other. Regulating the structure and function of MAMs can alleviate the damage caused by toxicants to some extent. In this review, we discuss the relationships between MAMs and the mechanisms of toxicological effects, and highlight MAMs as a potential target for protection against toxicants.

IP3R at ER-Mitochondrial Contact Sites: Beyond the IP3R-GRP75-VDAC1 Ca2+ Funnel

Membrane contact sites (MCS) circumvent the topological constraints of functional coupling between different membrane-bound organelles by providing a means of communication and exchange of materials. One of the most characterised contact sites in the cell is that between the endoplasmic reticulum and the mitochondrial (ERMCS) whose function is to couple cellular Ca2+ homeostasis and mitochondrial function. Inositol 1,4,5-trisphosphate receptors (IP3Rs) on the ER, glucose-regulated protein 75 (GRP 75) and voltage-dependent anion channel 1 (VDAC1) on the outer mitochondrial membrane are the canonical component of the Ca2+ transfer unit at ERMCS. These are often reported to form a Ca2+ funnel that fuels the mitochondrial low-affinity Ca2+ uptake system. We assess the available evidence on the IP3R subtype selectivity at the ERMCS and consider if IP3Rs have other roles at the ERMCS beyond providing Ca2+. Growing evidence suggests that all three IP3R subtypes can localise and regulate Ca2+ signalling at ERMCS. Furthermore, IP3Rs may be structurally important for assembly of the ERMCS in addition to their role in providing Ca2+ at these sites. Evidence that various binding partners regulate the assembly and Ca2+ transfer at ERMCS populated by IP3R-GRP75-VDAC1, suggesting that cells have evolved mechanisms that stabilise these junctions forming a Ca2+ microdomain that is required to fuel mitochondrial Ca2+ uptake.

Mitochondria-associated endoplasmic reticulum membranes and cardiac hypertrophy: Molecular mechanisms and therapeutic targets

Cardiac hypertrophy has been shown to compensate for cardiac performance and improve ventricular wall tension as well as oxygen consumption. This compensatory response results in several heart diseases, which include ischemia disease, hypertension, heart failure, and valvular disease. Although the pathogenesis of cardiac hypertrophy remains complicated, previous data show that dysfunction of the mitochondria and endoplasmic reticulum (ER) mediates the progression of cardiac hypertrophy. The interaction between the mitochondria and ER is mediated by mitochondria-associated ER membranes (MAMs), which play an important role in the pathology of cardiac hypertrophy. The function of MAMs has mainly been associated with calcium transfer, lipid synthesis, autophagy, and reactive oxygen species (ROS). In this review, we discuss key MAMs-associated proteins and their functions in cardiovascular system and define their roles in the progression of cardiac hypertrophy. In addition, we demonstrate that MAMs is a potential therapeutic target in the treatment of cardiac hypertrophy.

Modulation of Ca2+ signaling by antiapoptotic Bcl-2 versus Bcl-xL: From molecular mechanisms to relevance for cancer cell survival

Members of the Bcl-2-protein family are key controllers of apoptotic cell death. The family is divided into antiapoptotic (including Bcl-2 itself, Bcl-xL, Mcl-1, etc.) and proapoptotic members (Bax, Bak, Bim, Bim, Puma, Noxa, Bad, etc.). These proteins are well known for their canonical role in the mitochondria, where they control mitochondrial outer membrane permeabilization and subsequent apoptosis. However, several proteins are recognized as modulators of intracellular Ca²⁺ signals that originate from the endoplasmic reticulum (ER), the major intracellular Ca²⁺-storage organelle. More than 25 years ago, Bcl-2, the founding member of the family, was reported to control apoptosis through Ca²⁺ signaling. Further work elucidated that Bcl-2 directly targets and inhibits inositol 1,4,5-trisphosphate receptors (IP₃Rs), thereby suppressing proapoptotic Ca²⁺ signaling. In addition to Bcl-2, Bcl-xL was also shown to impact cell survival by sensitizing IP₃R function, thereby promoting prosurvival oscillatory Ca²⁺ release. However, new work challenges this model and demonstrates that Bcl-2 and Bcl-xL can both function as inhibitors of IP₃Rs. This suggests that, depending on the cell context, Bcl-xL could support very distinct Ca²⁺ patterns. This not only raises several questions but also opens new possibilities for the treatment of Bcl-xL-dependent cancers. In this review, we will discuss the similarities and divergences between Bcl-2 and Bcl-xL regarding Ca²⁺ homeostasis and IP₃R modulation from both a molecular and a functional point of view, with particular emphasis on cancer cell death resistance mechanisms.

Alcohol use disorder is associated with DNA methylation-based shortening of telomere length and regulated by TESPA1: implications for aging

Chronic heavy alcohol consumption is associated with increased mortality and morbidity and often leads to premature aging; however, the mechanisms of alcohol-associated cellular aging are not well understood. In this study, we used DNA methylation derived telomere length (DNAmTL) as a novel approach to investigate the role of alcohol use on the aging process. DNAmTL was estimated by 140 cytosine phosphate guanines (CpG) sites in 372 individuals with alcohol use disorder (AUD) and 243 healthy controls (HC) and assessed using various endophenotypes and clinical biomarkers. Validation in an independent sample of DNAmTL on alcohol consumption was performed (N = 4219). Exploratory genome-wide association studies (GWAS) on DNAmTL were also performed to identify genetic variants contributing to DNAmTL shortening. Top GWAS findings were analyzed using in-silico expression quantitative trait loci analyses and related to structural MRI hippocampus volumes of individuals with AUD. DNAmTL was 0.11-kilobases shorter per year in AUD compared to HC after adjustment for age, sex, race, and blood cell composition (p = 4.0 × 10-12). This association was partially attenuated but remained significant after additionally adjusting for BMI, and smoking status (0.06 kilobases shorter per year, p = 0.002). DNAmTL shortening was strongly associated with chronic heavy alcohol use (ps < 0.001), elevated gamma-glutamyl transferase (GGT), and aspartate aminotransferase (AST) (ps < 0.004). Comparison of DNAmTL with PCR-based methods of assessing TL revealed positive correlations (R = 0.3, p = 2.2 × 10-5), highlighting the accuracy of DNAmTL as a biomarker. The GWAS meta-analysis identified a single nucleotide polymorphism (SNP), rs4374022 and 18 imputed ones in Thymocyte Expressed, Positive Selection Associated 1(TESPA1), at the genome-wide level (p = 3.75 × 10-8). The allele C of rs4374022 was associated with DNAmTL shortening, lower hippocampus volume (p < 0.01), and decreased mRNA expression in hippocampus tissue (p = 0.04). Our study demonstrates DNAmTL-related aging acceleration in AUD and suggests a functional role for TESPA1 in regulating DNAmTL length, possibly via the immune system with subsequent biological effects on brain regions negatively affected by alcohol and implicated in aging.

The Potential of Small Molecules to Modulate the Mitochondria-Endoplasmic Reticulum Interplay in Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disease affecting a growing number of elderly individuals. No disease-modifying drugs have yet been identified despite over 30 years of research on the topic, showing the need for further research on this multifactorial disease. In addition to the accumulation of amyloid β-peptide (Aβ) and hyperphosphorylated tau (p-tau), several other alterations have been associated with AD such as calcium (Ca2+) signaling, glucose-, fatty acid-, cholesterol-, and phospholipid metabolism, inflammation, and mitochondrial dysfunction. Interestingly, all these processes have been associated with the mitochondria-endoplasmic reticulum (ER) contact site (MERCS) signaling hub. We and others have hypothesized that the dysregulated MERCS function may be one of the main pathogenic pathways driving AD pathology. Due to the variety of biological processes overseen at the MERCS, we believe that they constitute unique therapeutic targets to boost the neuronal function and recover neuronal homeostasis. Thus, developing molecules with the capacity to correct and/or modulate the MERCS interplay can unleash unique therapeutic opportunities for AD. The potential pharmacological intervention using MERCS modulators in different models of AD is currently under investigation. Here, we survey small molecules with the potential to modulate MERCS structures and functions and restore neuronal homeostasis in AD. We will focus on recently reported examples and provide an overview of the current challenges and future perspectives to develop MERCS modulators in the context of translational research.

Mitochondrial Ca2+ Homeostasis: Emerging Roles and Clinical Significance in Cardiac Remodeling

Mitochondria are the sites of oxidative metabolism in eukaryotes where the metabolites of sugars, fats, and amino acids are oxidized to harvest energy. Notably, mitochondria store Ca²⁺ and work in synergy with organelles such as the endoplasmic reticulum and extracellular matrix to control the dynamic balance of Ca²⁺ concentration in cells. Mitochondria are the vital organelles in heart tissue. Mitochondrial Ca²⁺ homeostasis is particularly important for maintaining the physiological and pathological mechanisms of the heart. Mitochondrial Ca²⁺ homeostasis plays a key role in the regulation of cardiac energy metabolism, mechanisms of death, oxygen free radical production, and autophagy. The imbalance of mitochondrial Ca²⁺ balance is closely associated with cardiac remodeling. The mitochondrial Ca²⁺ uniporter (mtCU) protein complex is responsible for the uptake and release of mitochondrial Ca²⁺ and regulation of Ca²⁺ homeostasis in mitochondria and consequently, in cells. This review summarizes the mechanisms of mitochondrial Ca²⁺ homeostasis in physiological and pathological cardiac remodeling and the regulatory effects of the mitochondrial calcium regulatory complex on cardiac energy metabolism, cell death, and autophagy, and also provides the theoretical basis for mitochondrial Ca²⁺ as a novel target for the treatment of cardiovascular diseases.

Endoplasmic reticulum-mitochondria signaling in neurons and neurodegenerative diseases

Recent advances have revealed common pathological changes in neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis with related frontotemporal dementia (ALS/FTD). Many of these changes can be linked to alterations in endoplasmic reticulum (ER)-mitochondria signaling, including dysregulation of Ca2+ signaling, autophagy, lipid metabolism, ATP production, axonal transport, ER stress responses and synaptic dysfunction. ER-mitochondria signaling involves specialized regions of ER, called mitochondria-associated membranes (MAMs). Owing to their role in neurodegenerative processes, MAMs have gained attention as they appear to be associated with all the major neurodegenerative diseases. Furthermore, their specific role within neuronal maintenance is being revealed as mutant genes linked to major neurodegenerative diseases have been associated with damage to these specialized contacts. Several studies have now demonstrated that these specialized contacts regulate neuronal health and synaptic transmission, and that MAMs are damaged in patients with neurodegenerative diseases. This Review will focus on the role of MAMs and ER-mitochondria signaling within neurons and how damage of the ER-mitochondria axis leads to a disruption of vital processes causing eventual neurodegeneration.

Endoplasmic Reticulum-Mitochondria Contacts: A Potential Therapy Target for Cardiovascular Remodeling-Associated Diseases

Cardiovascular remodeling occurs in cardiomyocytes, collagen meshes, and vascular beds in the progress of cardiac insufficiency caused by a variety of cardiac diseases such as chronic ischemic heart disease, chronic overload heart disease, myocarditis, and myocardial infarction. The morphological changes that occur as a result of remodeling are the critical pathological basis for the occurrence and development of serious diseases and also determine morbidity and mortality. Therefore, the inhibition of remodeling is an important approach to prevent and treat heart failure and other related diseases. The endoplasmic reticulum (ER) and mitochondria are tightly linked by ER-mitochondria contacts (ERMCs). ERMCs play a vital role in different signaling pathways and provide a satisfactory structural platform for the ER and mitochondria to interact and maintain the normal function of cells, mainly by involving various cellular life processes such as lipid metabolism, calcium homeostasis, mitochondrial function, ER stress, and autophagy. Studies have shown that abnormal ERMCs may promote the occurrence and development of remodeling and participate in the formation of a variety of cardiovascular remodeling-associated diseases. This review focuses on the structure and function of the ERMCs, and the potential mechanism of ERMCs involved in cardiovascular remodeling, indicating that ERMCs may be a potential target for new therapeutic strategies against cardiovascular remodeling-induced diseases.

Dynamic control of mitochondria-associated membranes by kinases and phosphatases in health and disease

Membrane-contact sites are getting more and more credit for their indispensable role in maintenance of cell function and homeostasis. In the last decades, the ER-mitochondrial contact sites in particular received a lot of attention. While our knowledge of ER-mitochondrial contact sites increases steadily, the focus often lies on a static exploration of their functions. However, it is increasingly clear that these contact sites are very dynamic. In this review, we highlight the dynamic nature of ER-mitochondrial contact sites and the role of kinases and phosphatases therein with a focus on recent findings. Phosphorylation events allow for rapid integration of information on the protein level, impacting protein function, localization and interaction at ER-mitochondrial contact sites. To illustrate the importance of these events and to put them in a broader perspective, we connect them to pathologies like diabetes type II, Parkinson's disease and cancer.

Advances in Intracellular Calcium Signaling Reveal Untapped Targets for Cancer Therapy

Intracellular Ca2+ distribution is a tightly regulated process. Numerous Ca2+ chelating, storage, and transport mechanisms are required to maintain normal cellular physiology. Ca2+-binding proteins, mainly calmodulin and calbindins, sequester free intracellular Ca2+ ions and apportion or transport them to signaling hubs needing the cations. Ca2+ channels, ATP-driven pumps, and exchangers assist the binding proteins in transferring the ions to and from appropriate cellular compartments. Some, such as the endoplasmic reticulum, mitochondria, and lysosomes, act as Ca2+ repositories. Cellular Ca2+ homeostasis is inefficient without the active contribution of these organelles. Moreover, certain key cellular processes also rely on inter-organellar Ca2+ signaling. This review attempts to encapsulate the structure, function, and regulation of major intracellular Ca2+ buffers, sensors, channels, and signaling molecules before highlighting how cancer cells manipulate them to survive and thrive. The spotlight is then shifted to the slow pace of translating such research findings into anticancer therapeutics. We use the PubMed database to highlight current clinical studies that target intracellular Ca2+ signaling. Drug repurposing and improving the delivery of small molecule therapeutics are further discussed as promising strategies for speeding therapeutic development in this area.

Adaptive optimization of the OXPHOS assembly line partially compensates lrpprc-dependent mitochondrial translation defects in mice

Mouse models of genetic mitochondrial disorders are generally used to understand specific molecular defects and their biochemical consequences, but rarely to map compensatory changes allowing survival. Here we took advantage of the extraordinary mitochondrial resilience of hepatic Lrpprc knockout mice to explore this question using native proteomics profiling and lipidomics. In these mice, low levels of the mtRNA binding protein LRPPRC induce a global mitochondrial translation defect and a severe reduction (>80%) in the assembly and activity of the electron transport chain (ETC) complex IV (CIV). Yet, animals show no signs of overt liver failure and capacity of the ETC is preserved. Beyond stimulation of mitochondrial biogenesis, results show that the abundance of mitoribosomes per unit of mitochondria is increased and proteostatic mechanisms are induced in presence of low LRPPRC levels to preserve a balance in the availability of mitochondrial- vs nuclear-encoded ETC subunits. At the level of individual organelles, a stabilization of residual CIV in supercomplexes (SCs) is observed, pointing to a role of these supramolecular arrangements in preserving ETC function. While the SC assembly factor COX7A2L could not contribute to the stabilization of CIV, important changes in membrane glycerophospholipid (GPL), most notably an increase in SC-stabilizing cardiolipins species (CLs), were observed along with an increased abundance of other supramolecular assemblies known to be stabilized by, and/or participate in CL metabolism. Together these data reveal a complex in vivo network of molecular adjustments involved in preserving mitochondrial integrity in energy consuming organs facing OXPHOS defects, which could be therapeutically exploited.

Cuillerier et al. investigate compensatory mechanisms underlying survival of mice with a liver-specific knockout of the mitochondrial mRNA-binding protein Lrpprc. They propose various mechanisms operating along the OXPHOS assembly line, including mitochondrial biogenesis, mitochondrial ribosome upregulation and preferential supercomplex assembly, that could compensate lack of LRPPRC and allow survival of these mice.

KRAP tethers IP3 receptors to actin and licenses them to evoke cytosolic Ca2+ signals

Regulation of IP₃ receptors (IP₃Rs) by IP₃ and Ca²⁺ allows regenerative Ca²⁺ signals, the smallest being Ca²⁺ puffs, which arise from coordinated openings of a few clustered IP₃Rs. Cells express thousands of mostly mobile IP₃Rs, yet Ca²⁺ puffs occur at a few immobile IP₃R clusters. By imaging cells with endogenous IP₃Rs tagged with EGFP, we show that KRas-induced actin-interacting protein (KRAP) tethers IP₃Rs to actin beneath the plasma membrane. Loss of KRAP abolishes Ca²⁺ puffs and the global increases in cytosolic Ca²⁺ concentration evoked by more intense stimulation. Over-expressing KRAP immobilizes additional IP₃R clusters and results in more Ca²⁺ puffs and larger global Ca²⁺ signals. Endogenous KRAP determines which IP₃Rs will respond: it tethers IP₃R clusters to actin alongside sites where store-operated Ca²⁺ entry occurs, licenses IP₃Rs to evoke Ca²⁺ puffs and global cytosolic Ca²⁺ signals, implicates the actin cytoskeleton in IP₃R regulation and may allow local activation of Ca²⁺ entry.

Structure and Function of Mitochondria-Associated Endoplasmic Reticulum Membranes (MAMs) and Their Role in Cardiovascular Diseases

Abnormal function of suborganelles such as mitochondria and endoplasmic reticulum often leads to abnormal function of cardiomyocytes or vascular endothelial cells and cardiovascular disease (CVD). Mitochondria-associated membrane (MAM) is involved in several important cellular functions. Increasing evidence shows that MAM is involved in the pathogenesis of CVD. MAM mediates multiple cellular processes, including calcium homeostasis regulation, lipid metabolism, unfolded protein response, ROS, mitochondrial dynamics, autophagy, apoptosis, and inflammation, which are key risk factors for CVD. In this review, we discuss the structure of MAM and MAM-associated proteins, their role in CVD progression, and the potential use of MAM as the therapeutic targets for CVD treatment.

Phospholipid ebb and flow makes mitochondria go

Mitochondria, so much more than just being energy factories, also have the capacity to synthesize macromolecules including phospholipids, particularly cardiolipin (CL) and phosphatidylethanolamine (PE). Phospholipids are vital constituents of mitochondrial membranes, impacting the plethora of functions performed by this organelle. Hence, the orchestrated movement of phospholipids to and from the mitochondrion is essential for cellular integrity. In this review, we capture recent advances in the field of mitochondrial phospholipid biosynthesis and trafficking, highlighting the significance of interorganellar communication, intramitochondrial contact sites, and lipid transfer proteins in maintaining membrane homeostasis. We then discuss the physiological functions of CL and PE, specifically how they associate with protein complexes in mitochondrial membranes to support bioenergetics and maintain mitochondrial architecture.

The MAMs Structure and Its Role in Cell Death

The maintenance of cellular homeostasis involves the participation of multiple organelles. These organelles are associated in space and time, and either cooperate or antagonize each other with regards to cell function. Crosstalk between organelles has become a significant topic in research over recent decades. We believe that signal transduction between organelles, especially the endoplasmic reticulum (ER) and mitochondria, is a factor that can influence the cell fate. As the cellular center for protein folding and modification, the endoplasmic reticulum can influence a range of physiological processes by regulating the quantity and quality of proteins. Mitochondria, as the cellular "energy factory," are also involved in cell death processes. Some researchers regard the ER as the sensor of cellular stress and the mitochondria as an important actuator of the stress response. The scientific community now believe that bidirectional communication between the ER and the mitochondria can influence cell death. Recent studies revealed that the death signals can shuttle between the two organelles. Mitochondria-associated membranes (MAMs) play a vital role in the complex crosstalk between the ER and mitochondria. MAMs are known to play an important role in lipid synthesis, the regulation of Ca²⁺ homeostasis, the coordination of ER-mitochondrial function, and the transduction of death signals between the ER and the mitochondria. Clarifying the structure and function of MAMs will provide new concepts for studying the pathological mechanisms associated with neurodegenerative diseases, aging, and cancers. Here, we review the recent studies of the structure and function of MAMs and its roles involved in cell death, especially in apoptosis.

Integrative Genomic-Epigenomic Analysis of Clozapine-Treated Patients with Refractory Psychosis

Clozapine (CLZ) is the only antipsychotic drug that has been proven to be effective in patients with refractory psychosis, but it has also been proposed as an effective mood stabilizer; however, the complex mechanisms of action of CLZ are not yet fully known. To find predictors of CLZ-associated phenotypes (i.e., the metabolic ratio, dosage, and response), we explore the genomic and epigenomic characteristics of 44 patients with refractory psychosis who receive CLZ treatment based on the integration of polygenic risk score (PRS) analyses in simultaneous methylome profiles. Surprisingly, the PRS for bipolar disorder (BD-PRS) was associated with the CLZ metabolic ratio (pseudo-R2 = 0.2080, adjusted p-value = 0.0189). To better explain our findings in a biological context, we assess the protein-protein interactions between gene products with high impact variants in the top enriched pathways and those exhibiting differentially methylated sites. The GABAergic synapse pathway was found to be enriched in BD-PRS and was associated with the CLZ metabolic ratio. Such interplay supports the use of CLZ as a mood stabilizer and not just as an antipsychotic. Future studies with larger sample sizes should be pursued to confirm the findings of this study.

Molecular machineries and physiological relevance of ER-mediated membrane contacts

Membrane contact sites (MCSs) are defined as regions where two organelles are closely apposed, and most MCSs associated with each other via protein-protein or protein-lipid interactions. A number of key molecular machinery systems participate in mediating substance exchange and signal transduction, both of which are essential processes in terms of cellular physiology and pathophysiology. The endoplasmic reticulum (ER) is the largest reticulum network within the cell and has extensive communication with other cellular organelles, including the plasma membrane (PM), mitochondria, Golgi, endosomes and lipid droplets (LDs). The contacts and reactions between them are largely mediated by various protein tethers and lipids. Ions, lipids and even proteins can be transported between the ER and neighboring organelles or recruited to the contact site to exert their functions. This review focuses on the key molecules involved in the formation of different contact sites as well as their biological functions.

Mitochondria-Associated Endoplasmic Reticulum Membranes (MAMs) and Their Prospective Roles in Kidney Disease

Mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) serve as essential hubs for interorganelle communication in eukaryotic cells and play multifunctional roles in various biological pathways. A defect in ER-mitochondria signaling or MAMs dysfunction has pleiotropic effects on a variety of intracellular events, which results in disturbances of the mitochondrial quality control system, Ca2+ dyshomeostasis, apoptosis, ER stress, and inflammasome activation, which all contribute to the onset and progression of kidney disease. Here, we review the structure and molecular compositions of MAMs as well as the experimental methods used to study these interorganellar contact sites. We will specifically summarize the downstream signaling pathways regulated by MAMs, mainly focusing on mitochondrial quality control, oxidative stress, ER-mitochondria Ca2+ crosstalk, apoptosis, inflammasome activation, and ER stress. Finally, we will discuss how alterations in MAMs integrity contribute to the pathogenesis of kidney disease and offer directions for future research.

New Insights in the IP3 Receptor and Its Regulation

The inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) is a Ca²⁺-release channel mainly located in the endoplasmic reticulum (ER). Three IP₃R isoforms are responsible for the generation of intracellular Ca²⁺ signals that may spread across the entire cell or occur locally in so-called microdomains. Because of their ubiquitous expression, these channels are involved in the regulation of a plethora of cellular processes, including cell survival and cell death. To exert their proper function a fine regulation of their activity is of paramount importance. In this review, we will highlight the recent advances in the structural analysis of the IP₃R and try to link these data with the newest information concerning IP₃R activation and regulation. A special focus of this review will be directed towards the regulation of the IP₃R by protein-protein interaction. Especially the protein family formed by calmodulin and related Ca²⁺-binding proteins and the pro- and anti-apoptotic/autophagic Bcl-2-family members will be highlighted. Finally, recently identified and novel IP₃R regulatory proteins will be discussed. A number of these interactions are involved in cancer development, illustrating the potential importance of modulating IP₃R-mediated Ca²⁺ signaling in cancer treatment.

The Interface Between ER and Mitochondria: Molecular Compositions and Functions

Mitochondria and endoplasmic reticulum (ER) are essential organelles in eukaryotic cells, which play key roles in various biological pathways. Mitochondria are responsible for ATP production, maintenance of Ca2+ homeostasis and regulation of apoptosis, while ER is involved in protein folding, lipid metabolism as well as Ca2+ homeostasis. These organelles have their own functions, but they also communicate via mitochondrial-associated ER membrane (MAM) to provide another level of regulations in energy production, lipid process, Ca2+ buffering, and apoptosis. Hence, defects in MAM alter cell survival and death. Here, we review components forming the molecular junctions of MAM and how MAM regulates cellular functions. Furthermore, we discuss the effects of impaired ER-mitochondrial communication in various neurodegenerative diseases.

IP3R-Grp75-VDAC1-MCU calcium regulation axis antagonists protect podocytes from apoptosis and decrease proteinuria in an Adriamycin nephropathy rat model

BACKGROUND

The mechanism of podocyte apoptosis is not fully understood. In addition, the role of the inositol 1,4,5-triphosphate receptor (IP₃R)/glucose-regulated protein 75 (Grp75)/voltage-dependent anion channel 1 (VDAC1)/mitochondrial calcium uniporter (MCU) calcium regulation axis, which is located at sites of endoplasmic reticulum (ER) mitochondria coupling, in the mechanism of podocyte apoptosis is unclear. This study aimed to understand the roles of this axis in podocyte apoptosis and explore potential targets for podocyte protection.

METHODS

The expression of IP₃R, Grp75, VDAC1, and MCU and mitochondrial Ca²⁺ were analyzed during Adriamycin- or angiotensin II-induced apoptosis in cultured mouse podocytes. The interaction between IP₃R, Grp75, and VDAC1 was investigated using co-immunoprecipitation experiments. The effects of IP₃R, Grp75, and MCU agonists and antagonists on mitochondrial Ca²⁺ and apoptosis were investigated in cultured podocytes. The podocyte-protective effects of an MCU inhibitor were further investigated in rats with Adriamycin-induced nephropathy.

RESULTS

Increased expression of IP₃R, Grp75, VDAC1 and MCU, enhanced interaction among the IP₃R-Grp75-VDAC1 complex, mitochondrial Ca²⁺ overload, and increased active caspase-3 levels were confirmed during Adriamycin- or angiotensin II-induced mouse podocyte apoptosis. Agonists of this axis facilitated mitochondrial Ca²⁺ overload and podocyte apoptosis, whereas specific antagonists against IP₃R, Grp75, or MCU prevented mitochondrial Ca²⁺ overload and podocyte apoptosis. A specific MCU inhibitor prevented Adriamycin-induced proteinuria and podocyte foot process effacement in rats.

CONCLUSIONS

This study identified a novel pathway in which the IP₃R-Grp75-VDAC1-MCU calcium regulation axis mediated podocyte apoptosis by facilitating mitochondrial Ca²⁺ overload. Antagonists that inhibit Ca²⁺ transfer from ER to mitochondria protected mouse podocytes from apoptosis. An MCU inhibitor protected podocytes and decreased proteinuria in rats with Adriamycin-induced nephropathy. Therefore, antagonists to this pathway have promise as novel podocyte-protective drugs.

Tespa1 Deficiency Dampens Thymus-Dependent B-Cell Activation and Attenuates Collagen-Induced Arthritis in Mice

Thymocyte-expressed, positive selection-associated 1 (Tespa1) plays an important role in both T cell receptor (TCR)-driven thymocyte development and in the FcεRI-mediated activation of mast cells. Herein, we show that lack of Tespa1 does not impair B cell development but dampens the in vitro activation and proliferation of B cells induced by T cell-dependent (TD) antigens, significantly reduces serum antibody concentrations in vivo, and impairs germinal center formation in both aged and TD antigen-immunized mice. We also provide evidence that dysregulated signaling in Tespa1-deficient B cells may be linked to CD40-induced TRAF6 degradation, and subsequent effects on 1-phosphatidylinositol-4,5-bisphosphate phosphodiesterase gamma-2 (PLCγ2) phosphorylation, MAPK activation, and calcium influx. Furthermore, we demonstrate that Tespa1 plays a critical role in pathogenic B cells, since Tespa1-deficient chimeric mice showed a lower incidence and clinical disease severity of collagen-induced arthritis. Overall, our study demonstrates that Tespa1 is essential for TD B cell responses, and suggests an important role for Tespa1 during the development of autoimmune arthritis.

HMGB1-induced asthmatic airway inflammation through GRP75-mediated enhancement of ER-mitochondrial Ca2+ transfer and ROS increased

Imbalanced T-helper (TH)1/Th2 response contributes significantly to asthma pathogenesis. Our study indicated that HMGB1 play an important role in the release of Th2-associated cytokines of asthma. However, the specific mechanism about HMGB1-induced imbalanced TH1/Th2 response is not known. In vivo, an OVA-induced asthma mouse model was set up and mice treated with anti-HMGB1 IgG. The mice treated with the anti-HMGB1 IgG ameliorated airway hyper-reactivity, disruption of Th1/Th2 balance and the upregulation of GRP75 induced by OVA. In vitro, the exposure of normal human bronchial epithelial cells to HMGB1 resulted in the upregulation of GRP75, proinflammatory cytokine production, enhanced ER-Mitochondrial Ca²⁺ transfer, and enhancement of reactive oxygen species (ROS). While HMGB1-induced these changes were attenuated by GRP75 siRNA treatment. Sequentially, pretreatment with 2-APB, SKF960365 (SKF) and Ru360 which inhibit ER-Mitochondrial Ca²⁺ transfer significantly lowered HMGB1-induced the generation of ROS and the release of Th2 cytokines in 16HBE cells. Meanwhile, N-acetylcysteine (NAC) significantly attenuated the HMGB1-mediated pro-inflammatory cytokines release. Therefore, these results indicate that GRP75-mediated ER-Mitochondrial Ca²⁺ transfer may be an important contributor in imbalanced of Th1/Th2 balance of asthma. Moreover, HMGB1 specifically induces the release of Th2 cytokines through GRP75-mediated enhancement of ER-Mitochondrial Ca²⁺ transfer and ROS increased.

TCR Signal Strength and T Cell Development

Thymocyte selection involves the positive and negative selection of the repertoire of T cell receptors (TCRs) such that the organism does not suffer autoimmunity, yet has the benefit of the ability to recognize any invading pathogen. The signal transduced through the TCR is translated into a number of different signaling cascades that result in transcription factor activity in the nucleus and changes to the cytoskeleton and motility. Negative selection involves inducing apoptosis in thymocytes that express strongly self-reactive TCRs, whereas positive selection must induce survival and differentiation programs in cells that are more weakly self-reactive. The TCR recognition event is analog by nature, but the outcome of signaling is not. A large number of molecules regulate the strength of the TCR-derived signal at various points in the cascades. This review discusses the various factors that can regulate the strength of the TCR signal during thymocyte development.

Computational properties of mitochondria in T cell activation and fate

In this article, we review how mitochondrial Ca²⁺ transport (mitochondrial Ca²⁺ uptake and Na⁺/Ca²⁺ exchange) is involved in T cell biology, including activation and differentiation through shaping cellular Ca²⁺ signals. Based on recent observations, we propose that the Ca²⁺ crosstalk between mitochondria, endoplasmic reticulum and cytoplasm may form a proportional–integral–derivative (PID) controller. This PID mechanism (which is well known in engineering) could be responsible for computing cellular decisions. In addition, we point out the importance of analogue and digital signal processing in T cell life and implication of mitochondrial Ca²⁺ transport in this process.

Tespa1 regulates T cell receptor-induced calcium signals by recruiting inositol 1,4,5-trisphosphate receptors

Thymocyte-expressed, positive selection-associated 1 (Tespa1) is important in T cell receptor (TCR)-driven thymocyte development. Downstream of the TCR, Tespa1 is a crucial component of the linker for activation of T cells (LAT) signalosome, facilitating calcium signalling and subsequent MAPK activation. However, it is unknown how Tespa1 elicits calcium signalling. Here, we show that inositol 1,4,5-trisphosphate receptor type 1 (IP3R1) is crucial for Tespa1-optimized, TCR-induced Ca²⁺ flux and thymocyte development. Upon TCR stimulation, Tespa1 directly interacts with IP3R1 and recruits it to the TCR complex, where IP3R1 is phosphorylated at Y353 by Fyn. This Tespa1-IP3R1 interaction is mediated by the F187 and F188 residues of Tespa1 and the amino-terminus of IP3R1. Tespa1-F187A/F188A mutant mice phenocopy Tespa1-deficient mice with impaired late thymocyte development due to reduced IP3R1 translocation to the TCR-proximal region. Our work elucidates the function of Tespa1 in T cell development and the regulation of TCR-induced Ca²⁺ signalling through IP3R1.

The thymocyte development protein Tespa1 is known to translate T cell receptor signals by affecting the calcium signalling cascade, but it is not clear how. Here the authors show that Tespa1 recruits IP3R1 to the TCR signalling complex.

Endoplasmic Reticulum-Mitochondria Communication Through Ca2+ Signaling: The Importance of Mitochondria-Associated Membranes (MAMs)

The execution of proper Ca²⁺ signaling requires close apposition between the endoplasmic reticulum (ER) and mitochondria. Hence, Ca²⁺ released from the ER is "quasi-synaptically" transferred to mitochondrial matrix, where Ca²⁺ stimulates mitochondrial ATP synthesis by activating the tricarboxylic acid (TCA) cycle. However, when the Ca²⁺ transfer is excessive and sustained, mitochondrial Ca²⁺ overload induces apoptosis by opening the mitochondrial permeability transition pore. A large number of regulatory proteins reside at mitochondria-associated ER membranes (MAMs) to maintain the optimal distance between the organelles and to coordinate the functionality of both ER and mitochondrial Ca²⁺ transporters or channels. In this chapter, we discuss the different pathways involved in the regulation of ER-mitochondria Ca²⁺ flux and describe the activities of the various Ca²⁺ players based on their primary intra-organelle localization.

Mitochondria-Associated Membranes and ER Stress

The endoplasmic reticulum (ER) is a crucial organelle for coordinating cellular Ca²⁺ signaling and protein synthesis and folding. Moreover, the dynamic and complex membranous structures constituting the ER allow the formation of contact sites with other organelles and structures, including among others the mitochondria and the plasma membrane (PM). The contact sites that the ER form with mitochondria is a hot topic in research, and the nature of the so-called mitochondria-associated membranes (MAMs) is continuously evolving. The MAMs consist of a proteinaceous tether that physically connects the ER with mitochondria. The MAMs harness the main functions of both organelles to form a specialized subcompartment at the interface of the ER and mitochondria. Under homeostatic conditions, MAMs are crucial for the efficient transfer of Ca²⁺ from the ER to mitochondria, and for proper mitochondria bioenergetics and lipid synthesis. MAMs are also believed to be the master regulators of mitochondrial shape and motility, and to form a crucial site for autophagosome assembly. Not surprisingly, MAMs have been shown to be a hot spot for the transfer of stress signals from the ER to mitochondria, most notably under the conditions of loss of ER proteostasis, by engaging the unfolded protein response (UPR). In this chapter after an introduction on ER biology and ER stress, we will review the emerging and key signaling roles of the MAMs, which have a root in cellular processes and signaling cascades coordinated by the ER.

Control of mitochondrial function and cell growth by the atypical cadherin Fat1

Mitochondrial products such as ATP, reactive oxygen species, and aspartate are key regulators of cellular metabolism and growth. Abnormal mitochondrial function compromises integrated growth-related processes such as development and tissue repair, as well as homeostatic mechanisms that counteract ageing and neurodegeneration, cardiovascular disease, and cancer. Physiologic mechanisms that control mitochondrial activity in such settings remain incompletely understood. Here we show that the atypical Fat1 cadherin acts as a molecular 'brake' on mitochondrial respiration that regulates vascular smooth muscle cell (SMC) proliferation after arterial injury. Fragments of Fat1 accumulate in SMC mitochondria, and the Fat1 intracellular domain interacts with multiple mitochondrial proteins, including critical factors associated with the inner mitochondrial membrane. SMCs lacking Fat1 (Fat1^KO) grow faster, consume more oxygen for ATP production, and contain more aspartate. Notably, expression in Fat1^KO cells of a modified Fat1 intracellular domain that localizes exclusively to mitochondria largely normalizes oxygen consumption, and the growth advantage of these cells can be suppressed by inhibition of mitochondrial respiration, which suggest that a Fat1-mediated growth control mechanism is intrinsic to mitochondria. Consistent with this idea, Fat1 species associate with multiple respiratory complexes, and Fat1 deletion both increases the activity of complexes I and II and promotes the formation of complex-I-containing supercomplexes. In vivo, Fat1 is expressed in injured human and mouse arteries, and inactivation of SMC Fat1 in mice potentiates the response to vascular damage, with markedly increased medial hyperplasia and neointimal growth, and evidence of higher SMC mitochondrial respiration. These studies suggest that Fat1 controls mitochondrial activity to restrain cell growth during the reparative, proliferative state induced by vascular injury. Given recent reports linking Fat1 to cancer, abnormal kidney and muscle development, and neuropsychiatric disease, this Fat1 function may have importance in other settings of altered cell growth and metabolism.

ER-to-mitochondria miscommunication and metabolic diseases

Eukaryotic cells contain a variety of subcellular organelles, each of which performs unique tasks. Thus follows that in order to coordinate these different intracellular functions, a highly dynamic system of communication must exist between the various compartments. Direct endoplasmic reticulum (ER)-mitochondria communication is facilitated by the physical interaction of their membranes in dedicated structural domains known as mitochondria-associated membranes (MAMs), which facilitate calcium (Ca(2+)) and lipid transfer between organelles and also act as platforms for signaling. Numerous studies have demonstrated the importance of MAM in ensuring correct function of both organelles, and recently MAMs have been implicated in the genesis of various human diseases. Here, we review the salient structural features of interorganellar communication via MAM and discuss the most common experimental techniques employed to assess functionality of these domains. Finally, we will highlight the contribution of MAM to a variety of cellular functions and consider the potential role of MAM in the genesis of metabolic diseases. In doing so, the importance for cell functions of maintaining appropriate communication between ER and mitochondria will be emphasized.

Calcium signaling at ER membrane contact sites

Communication between organelles is a necessary consequence of intracellular compartmentalization. Membrane contact sites (MCSs) are regions where the membranes of two organelles come into close apposition allowing exchange of small molecules and ions including Ca²⁺. The ER, the cell's major Ca²⁺ store, forms an extensive and dynamic network of contacts with multiple organelles. Here we review established and emerging roles of ER contacts as platforms for Ca²⁺ exchange and further consider a potential role for Ca²⁺ in the regulation of MCS formation. We additionally discuss the challenges associated with the study of MCS biology and highlight advances in microscopy-based solutions. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

Disorders of phospholipid metabolism: an emerging class of mitochondrial disease due to defects in nuclear genes

The human nuclear and mitochondrial genomes co-exist within each cell. While the mitochondrial genome encodes for a limited number of proteins, transfer RNAs, and ribosomal RNAs, the vast majority of mitochondrial proteins are encoded in the nuclear genome. Of the multitude of mitochondrial disorders known to date, only a fifth are maternally inherited. The recent characterization of the mitochondrial proteome therefore serves as an important step toward delineating the nosology of a large spectrum of phenotypically heterogeneous diseases. Following the identification of the first nuclear gene defect to underlie a mitochondrial disorder, a plenitude of genetic variants that provoke mitochondrial pathophysiology have been molecularly elucidated and classified into six categories that impact: (1) oxidative phosphorylation (subunits and assembly factors); (2) mitochondrial DNA maintenance and expression; (3) mitochondrial protein import and assembly; (4) mitochondrial quality control (chaperones and proteases); (5) iron–sulfur cluster homeostasis; and (6) mitochondrial dynamics (fission and fusion). Here, we propose that an additional class of genetic variant be included in the classification schema to acknowledge the role of genetic defects in phospholipid biosynthesis, remodeling, and metabolism in mitochondrial pathophysiology. This seventh class includes a small but notable group of nuclear-encoded proteins whose dysfunction impacts normal mitochondrial phospholipid metabolism. The resulting human disorders present with a diverse array of pathologic consequences that reflect the variety of functions that phospholipids have in mitochondria and highlight the important role of proper membrane homeostasis in mitochondrial biology.

Mitochondrial fusion and fission proteins: novel therapeutic targets for combating cardiovascular disease

Mitochondria are no longer considered to be solely the static powerhouses of the cell. While they are undoubtedly essential to sustaining life and meeting the energy requirements of the cell through oxidative phosphorylation, they are now regarded as highly dynamic organelles with multiple functions, playing key roles in cell survival and death. In this review, we discuss the emerging role of mitochondrial fusion and fission proteins, as novel therapeutic targets for treating a wide range of cardiovascular diseases.

Tespa1 negatively regulates FcεRI-mediated signaling and the mast cell-mediated allergic response

Antigen-mediated cross-linking of IgE on mast cells triggers a signaling cascade that results in their degranulation and proinflammatory cytokine production, which are key effectors in allergic reactions. We show that the activation of mast cells is negatively regulated by the newly identified adaptor protein Tespa1. Loss of Tespa1 in mouse mast cells led to hyper-responsiveness to stimulation via FcεRI. Mice lacking Tespa1 also displayed increased sensitivity to IgE-mediated allergic responses. The dysregulated signaling in KO mast cells was associated with increased activation of Grb2-PLC-γ1-SLP-76 signaling within the LAT1 (linker for activation of T cells family, member 1) signalosome versus the LAT2 signalosome. Collectively, these findings show that Tespa1 orchestrates mast cell activation by tuning the balance of LAT1 and LAT2 signalosome assembly.

Acute high-altitude hypoxic brain injury: Identification of ten differential proteins

Hypobaric hypoxia can cause severe brain damage and mitochondrial dysfunction, and is involved in hypoxic brain injury. However, little is currently known about the mechanisms responsible for mitochondrial dysfunction in hypobaric hypoxic brain damage. In this study, a rat model of hypobaric hypoxic brain injury was established to investigate the molecular mechanisms associated with mitochondrial dysfunction. As revealed by two-dimensional electrophoresis analysis, 16, 21, and 36 differential protein spots in cerebral mitochondria were observed at 6, 12, and 24 hours post-hypobaric hypoxia, respectively. Furthermore, ten protein spots selected from each hypobaric hypoxia subgroup were similarly regulated and were identified by mass spectrometry. These detected proteins included dihydropyrimidinase-related protein 2, creatine kinase B-type, isovaleryl-CoA dehydrogenase, elongation factor Ts, ATP synthase beta-subunit, 3-mercaptopyruvate sulfurtransferase, electron transfer flavoprotein alpha-subunit, Chain A of 2-enoyl-CoA hydratase, NADH dehydrogenase iron-sulfur protein 8 and tropomyosin beta chain. These ten proteins are all involved in the electron transport chain and the function of ATP synthase. Our findings indicate that hypobaric hypoxia can induce the differential expression of several cerebral mitochondrial proteins, which are involved in the regulation of mitochondrial energy production.

Immune synapse: conductor of orchestrated organelle movement

To ensure proper cell function, intracellular organelles are not randomly distributed within the cell, but polarized and highly constrained by the cytoskeleton and associated adaptor proteins. This relationship between distribution and function was originally found in neurons and epithelial cells; however, recent evidence suggests that it is a general phenomenon occurring in many highly specialized cells including T lymphocytes. Recent studies reveal that the orchestrated redistribution of organelles is dependent on antigen-specific activation of and immune synapse (IS) formation by T cells. This review highlights the functional implications of organelle polarization in early T cell activation and examines recent findings on how the IS sets the rhythm of organelle motion and the spread of the activation signal to the nucleus.