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Mitochondria take up Ca 2+ through the mitochondrial calcium uniporter complex to regulate energy production, cytosolic Ca 2+ signalling and cell death 1 , 2 . In mammals, the uniporter complex (uniplex) contains four core components: the pore-forming MCU protein, the gatekeepers MICU1 and MICU2, and an auxiliary subunit, EMRE, essential for Ca 2+ transport 3 – 8 . To prevent detrimental Ca 2+ overload, the activity of MCU must be tightly regulated by MICUs, which sense changes in cytosolic Ca 2+ concentrations to switch MCU on and off 9 , 10 . Here we report cryo-electron microscopic structures of the human mitochondrial calcium uniporter holocomplex in inhibited and Ca 2+ -activated states. These structures define the architecture of this multicomponent Ca 2+ -uptake machinery and reveal the gating mechanism by which MICUs control uniporter activity. Our work provides a framework for understanding regulated Ca 2+ uptake in mitochondria, and could suggest ways of modulating uniporter activity to treat diseases related to mitochondrial Ca 2+ overload. Cryo-electron microscopy reveals the structures of the mitochondrial calcium uniporter holocomplex in low- and high-calcium conditions, showing the gating mechanism that underlies uniporter activation in response to intracellular calcium signals.

Calcium ions (Ca2+) are some of the most versatile signalling molecules, and they have many physiological functions, prominently including muscle contraction, neuronal excitability, cell migration and cell growth. By sequestering and releasing Ca2+, mitochondria serve as important regulators of cellular Ca2+. Mitochondrial Ca2+ also has other important functions, such as regulation of mitochondrial metabolism, ATP production and cell death. In recent years, identification of the molecular machinery regulating mitochondrial Ca2+ accumulation and efflux has expanded the number of (patho)physiological conditions that rely on mitochondrial Ca2+ homeostasis. Thus, expanding the understanding of the mechanisms of mitochondrial Ca2+ regulation and function in different cell types is an important task in biomedical research, which offers the possibility of targeting mitochondrial Ca2+ machinery for the treatment of several disorders.

Calcium (Ca2+) signalling is of paramount importance to immunity. Regulated increases in cytosolic and organellar Ca2+ concentrations in lymphocytes control complex and crucial effector functions such as metabolism, proliferation, differentiation, antibody and cytokine secretion and cytotoxicity. Altered Ca2+ regulation in lymphocytes leads to various autoimmune, inflammatory and immunodeficiency syndromes. Several types of plasma membrane and organellar Ca2+-permeable channels are functional in T cells. They contribute highly localized spatial and temporal Ca2+ microdomains that are required for achieving functional specificity. While the mechanistic details of these Ca2+ microdomains are only beginning to emerge, it is evident that through crosstalk, synergy and feedback mechanisms, they fine-tune T cell signalling to match complex immune responses. In this article, we review the expression and function of various Ca2+-permeable channels in the plasma membrane, endoplasmic reticulum, mitochondria and endolysosomes of T cells and their role in shaping immunity and the pathogenesis of immune-mediated diseases.Regulated calcium signalling, in particular downstream of the T cell receptor, is crucial for many T cell effector functions. This Review provides an overview of the numerous membrane and organellar calcium-permeable channels that are coordinated to fine-tune T cell immunity.

Mitochondrial calcium ( m Ca 2+ ) has a central role in both metabolic regulation and cell death signalling, however its role in homeostatic function and disease is controversial 1 . Slc8b1 encodes the mitochondrial Na + /Ca 2+ exchanger (NCLX), which is proposed to be the primary mechanism for m Ca 2+ extrusion in excitable cells 2 , 3 . Here we show that tamoxifen-induced deletion of Slc8b1 in adult mouse hearts causes sudden death, with less than 13% of affected mice surviving after 14 days. Lethality correlated with severe myocardial dysfunction and fulminant heart failure. Mechanistically, cardiac pathology was attributed to m Ca 2+ overload driving increased generation of superoxide and necrotic cell death, which was rescued by genetic inhibition of mitochondrial permeability transition pore activation. Corroborating these findings, overexpression of NCLX in the mouse heart by conditional transgenesis had the beneficial effect of augmenting m Ca 2+ clearance, preventing permeability transition and protecting against ischaemia-induced cardiomyocyte necrosis and heart failure. These results demonstrate the essential nature of m Ca 2+ efflux in cellular function and suggest that augmenting m Ca 2+ efflux may be a viable therapeutic strategy in disease. Conditional deletion of the mitochondrial Na + /Ca 2+ exchanger NCLX in adult mouse hearts causes sudden death due to mitochondrial calcium overload, whereas its overexpression limits cell death elicited by ischaemia reperfusion injury and heart failure. Homeostasis 'chalked up' to mitochondrial calcium exchange The relevance of mitochondrial calcium exchange in vivo has been controversial. Here, John Elrod and colleagues explore the physiological significance of this process in the mouse heart. Conditional deletion of the mitochondrial sodium–calcium exchanger NCLX in adult mouse hearts caused sudden death owing to mitochondrial calcium overload and necrotic cell death. Conversely, overexpression of NCLX in mouse hearts limited cell death caused by ischaemia reperfusion injury, the tissue damage caused when blood rushes back to a site that has suffered a lack of oxygen. The authors conclude that cardiomyocyte mitochondrial calcium exchange is a prominent mitochondrial regulatory mechanism in cardiac disease.

The structure of the core region of the mitochondrial calcium uniporter (MCU) is determined by NMR and electron microscopy, revealing that MCU is a homo-pentamer with a specific transmembrane helix forming a hydrophilic pore across the membrane, and representing one of the largest membrane protein structures characterized by NMR spectroscopy. Mitochondrial calcium uniporter structure Many mitochondria use an inner membrane transporter, called the uniporter, to uptake and buffer large amounts of Ca 2+ . The MCU (mitochondrial calcium uniporter) is the pore-forming and Ca 2+ -conducting subunit of the uniporter, but its primary sequence does not resemble any calcium channel known to date. These authors report the structure of the core region of the MCU using nuclear magnetic resonance spectroscopy and electron microscopy. They show that the MCU is a homo-oligomer with a specific transmembrane helix forming a hydrophilic pore across the membrane. In addition to having a previously unknown channel architecture, the MCU is one of the largest structures to have been characterized by NMR spectroscopy. Mitochondria from many eukaryotic clades take up large amounts of calcium (Ca 2+ ) via an inner membrane transporter called the uniporter. Transport by the uniporter is membrane potential dependent and sensitive to ruthenium red or its derivative Ru360 (ref. 1 ). Electrophysiological studies have shown that the uniporter is an ion channel with remarkably high conductance and selectivity 2 . Ca 2+ entry into mitochondria is also known to activate the tricarboxylic acid cycle and seems to be crucial for matching the production of ATP in mitochondria with its cytosolic demand 3 . Mitochondrial calcium uniporter (MCU) is the pore-forming and Ca 2+ -conducting subunit of the uniporter holocomplex, but its primary sequence does not resemble any calcium channel studied to date. Here we report the structure of the pore domain of MCU from Caenorhabditis elegans , determined using nuclear magnetic resonance (NMR) and electron microscopy (EM). MCU is a homo-oligomer in which the second transmembrane helix forms a hydrophilic pore across the membrane. The channel assembly represents a new solution of ion channel architecture, and is stabilized by a coiled-coil motif protruding into the mitochondrial matrix. The critical DXXE motif forms the pore entrance, which features two carboxylate rings; based on the ring dimensions and functional mutagenesis, these rings appear to form the selectivity filter. To our knowledge, this is one of the largest membrane protein structures characterized by NMR, and provides a structural blueprint for understanding the function of this channel.

Piezo1 is a bona fide mechanosensitive ion channel ubiquitously expressed in mammalian cells. The distribution of Piezo1 within a cell is essential for various biological processes including cytokinesis, cell migration, and wound healing. However, the underlying principles that guide the subcellular distribution of Piezo1 remain largely unexplored. Here, we demonstrate that membrane curvature serves as a key regulator of the spatial distribution of Piezo1 in the plasma membrane of living cells. Piezo1 depletes from highly curved membrane protrusions such as filopodia and enriches to nanoscale membrane invaginations. Quantification of the curvature-dependent sorting of Piezo1 directly reveals the in situ nano-geometry of the Piezo1-membrane complex. Piezo1 density on filopodia increases upon activation, independent of calcium, suggesting flattening of the channel upon opening. Consequently, the expression of Piezo1 inhibits filopodia formation, an effect that diminishes with channel activation. This study demonstrates that the curvature of the cell membrane directly regulates the spatial distribution of Piezo1, a widely expressed mechanosensitive ion channel. Piezo1 may flatten upon activation and can mechanically inhibit membrane dynamics

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent Ca 2+ -mobilizing second messenger which uniquely mobilizes Ca 2+ from acidic endolysosomal organelles. However, the molecular identity of the NAADP receptor remains unknown. Given the necessity of the endolysosomal two-pore channel (TPC1 or TPC2) in NAADP signaling, we performed affinity purification and quantitative proteomic analysis of the interacting proteins of NAADP and TPCs. We identified a Sm-like protein Lsm12 complexed with NAADP, TPC1, and TPC2. Lsm12 directly binds to NAADP via its Lsm domain, colocalizes with TPC2, and mediates the apparent association of NAADP to isolated TPC2 or TPC2-containing membranes. Lsm12 is essential and immediately participates in NAADP-evoked TPC activation and Ca 2+ mobilization from acidic stores. These findings reveal a putative RNA-binding protein to function as an NAADP receptor and a TPC regulatory protein and provides a molecular basis for understanding the mechanisms of NAADP signaling. Nicotinic acid adenine dinucleotide phosphate (NAADP) potent Ca 2+ mobilizing second messenger which uniquely triggers Ca 2+ release from acidic endolysosomal organelles. Here the authors identify Lsm12 as an NAADP receptor essential for NAADP-evoked Ca 2+ release from lysosomes via NAADP binding on its Lsm domain.

The versatility and universality of Ca2+ as intracellular messenger is guaranteed by the compartmentalization of changes in [Ca2+]. In this context, mitochondrial Ca2+ plays a central role, by regulating both specific organelle functions and global cellular events. This versatility is also guaranteed by a cell type-specific Ca2+ signaling toolkit controlling specific cellular functions. Accordingly, mitochondrial Ca2+ uptake is mediated by a multimolecular structure, the MCU complex, which differs among various tissues. Its activity is indeed controlled by different components that cooperate to modulate specific channeling properties. We here investigate the role of MICU3, an EF-hand containing protein expressed at high levels, especially in brain. We show that MICU3 forms a disulfide bond-mediated dimer with MICU1, but not with MICU2, and it acts as enhancer of MCU-dependent mitochondrial Ca2+ uptake. Silencing of MICU3 in primary cortical neurons impairs Ca2+ signals elicited by synaptic activity, thus suggesting a specific role in regulating neuronal function.

Endoplasmic reticulum-mitochondria contacts (ERMCs) are restructured in response to changes in cell state. While this restructuring has been implicated as a cause or consequence of pathology in numerous systems, the underlying molecular dynamics are poorly understood. Here, we show means to visualize the capture of motile IP 3 receptors (IP3Rs) at ERMCs and document the immediate consequences for calcium signaling and metabolism. IP3Rs are of particular interest because their presence provides a scaffold for ERMCs that mediate local calcium signaling, and their function outside of ERMCs depends on their motility. Unexpectedly, in a cell model with little ERMC Ca 2+ coupling, IP3Rs captured at mitochondria promptly mediate Ca 2+ transfer, stimulating mitochondrial oxidative metabolism. The Ca 2+ transfer does not require linkage with a pore-forming protein in the outer mitochondrial membrane. Thus, motile IP3Rs can traffic in and out of ERMCs, and, when ‘parked’, mediate calcium signal propagation to the mitochondria, creating a dynamic arrangement that supports local communication. The formation and dissolution of ER-Mitochondria contacts is unclear. Here, authors show that the IP3 receptor traffics in and out of the contacts and, when trapped, improves calcium signaling to stimulate energy metabolism.

The essential role of ORAI1 channels in receptor-evoked Ca 2+ signaling is well understood, yet little is known about the physiological activation of the ORAI channel trio natively expressed in all cells. The roles of ORAI2 and ORAI3 have remained obscure. We show that ORAI2 and ORAI3 channels play a critical role in mediating the regenerative Ca 2+ oscillations induced by physiological receptor activation, yet ORAI1 is dispensable in generation of oscillations. We reveal that ORAI2 and ORAI3 channels multimerize with ORAI1 to expand the range of sensitivity of receptor-activated Ca 2+ signals, reflecting their enhanced basal STIM1-binding and heightened Ca 2+ -dependent inactivation. This broadened bandwidth of Ca 2+ influx is translated by cells into differential activation of NFAT1 and NFAT4 isoforms. Our results uncover a long-sought role for ORAI2 and ORAI3, revealing an intricate control mechanism whereby heteromerization of ORAI channels mediates graded Ca 2+ signals that extend the agonist-sensitivity to fine-tune transcriptional control. The essential role of ORAI1 channels in receptor-evoked Ca 2+ signaling is well understood, but the roles of ORAI2 and ORAI3 remained obscure. Here authors show that ORAI2 and ORAI3 channels multimerize with ORAI1 to expand the range of sensitivity of receptor-activated Ca 2+ signals, reflecting their enhanced basal STIM1-binding and heightened Ca 2+ -dependent inactivation.