Structural and mechanistic insights into MICU1 regulation of mitochondrial calcium uptake

Mitochondrial calcium uniporter complex: Unveiling the interplay between its regulators and calcium homeostasis

The mitochondrial calcium uniporter complex (MCUc), serving as the specific channel for calcium influx into the mitochondrial matrix, is integral to calcium homeostasis and cellular integrity. Given its importance, ongoing research spans various disease models to understand the properties of the MCUc in pathophysiological contexts, but reported a different conclusion. Therefore, this review delves into the profound connection between MCUc-mediated calcium transients and cellular signaling pathways, mitochondrial dynamics, metabolism, and cell death. Additionally, we shed light on the recent advancements concerning the structural intricacies and auxiliary components of the MCUc in both resting and activated states. Furthermore, emphasis is placed on novel extrinsic and intrinsic regulators of the MCUc and their therapeutic implications across a spectrum of diseases. Meanwhile, we employed molecular docking simulations and identified candidate traditional Chinese medicine components with potential binding sites to the MCUc, potentially offering insights for further research on MCUc modulation.

Supralinear Dependence of the IP3 Receptor-to-Mitochondria Local Ca2+ Transfer on the Endoplasmic Reticulum Ca2+ Loading

Calcium signal propagation from endoplasmic reticulum (ER) to mitochondria regulates a multitude of mitochondrial and cell functions, including oxidative ATP production and cell fate decisions. Ca2+ transfer is optimal at the ER-mitochondrial contacts, where inositol 1,4,5-trisphosphate (IP3) receptors (IP3R) can locally expose the mitochondrial Ca2+ uniporter (mtCU) to high [Ca2+] nanodomains. The Ca2+ loading state of the ER (Ca2 + ER) can vary broadly in physiological and pathological scenarios, however, the correlation between Ca2 + ER and the local Ca2+ transfer is unclear. Here, we studied IP3-induced Ca2+ transfer to mitochondria at different Ca2 + ER in intact and permeabilized RBL-2H3 cells via fluorescence measurements of cytoplasmic [Ca2+] ([Ca2+]c) and mitochondrial matrix [Ca2+] ([Ca2+]m). Preincubation of intact cells in high versus low extracellular [Ca2+] caused disproportionally greater increase in [Ca2+]m than [Ca2+]c responses to IP3-mobilizing agonist. Increasing Ca2 + ER by small Ca2+ boluses in suspensions of permeabilized cells supralinearly enhanced the mitochondrial Ca2+ uptake from IP3-induced Ca2+ release. The IP3-induced local [Ca2+] spikes exposing the mitochondrial surface measured using a genetically targeted sensor appeared to linearly correlate with Ca2 + ER, indicating that amplification happened in the mitochondria. Indeed, overexpression of an EF-hand deficient mutant of the mtCU gatekeeper MICU1 reduced the cooperativity of mitochondrial Ca2+ uptake. Interestingly, the IP3-induced [Ca2+]m signal plateaued at high Ca2 + ER, indicating activation of a matrix Ca2+ binding/chelating species. Mitochondria thus seem to maintain a "working [Ca2+]m range" via a low-affinity and high-capacity buffer species, and the ER loading steeply enhances the IP3R-linked [Ca2+]m signals in this working range.

Preserving and enhancing mitochondrial function after stroke to protect and repair the neurovascular unit: novel opportunities for nanoparticle-based drug delivery

The neurovascular unit (NVU) is composed of vascular cells, glia, and neurons that form the basic component of the blood brain barrier. This intricate structure rapidly adjusts cerebral blood flow to match the metabolic needs of brain activity. However, the NVU is exquisitely sensitive to damage and displays limited repair after a stroke. To effectively treat stroke, it is therefore considered crucial to both protect and repair the NVU. Mitochondrial calcium (Ca²⁺) uptake supports NVU function by buffering Ca²⁺ and stimulating energy production. However, excessive mitochondrial Ca²⁺ uptake causes toxic mitochondrial Ca²⁺ overloading that triggers numerous cell death pathways which destroy the NVU. Mitochondrial damage is one of the earliest pathological events in stroke. Drugs that preserve mitochondrial integrity and function should therefore confer profound NVU protection by blocking the initiation of numerous injury events. We have shown that mitochondrial Ca²⁺ uptake and efflux in the brain are mediated by the mitochondrial Ca²⁺ uniporter complex (MCU_cx) and sodium/Ca²⁺/lithium exchanger (NCLX), respectively. Moreover, our recent pharmacological studies have demonstrated that MCU_cx inhibition and NCLX activation suppress ischemic and excitotoxic neuronal cell death by blocking mitochondrial Ca²⁺ overloading. These findings suggest that combining MCU_cx inhibition with NCLX activation should markedly protect the NVU. In terms of promoting NVU repair, nuclear hormone receptor activation is a promising approach. Retinoid X receptor (RXR) and thyroid hormone receptor (TR) agonists activate complementary transcriptional programs that stimulate mitochondrial biogenesis, suppress inflammation, and enhance the production of new vascular cells, glia, and neurons. RXR and TR agonism should thus further improve the clinical benefits of MCU_cx inhibition and NCLX activation by increasing NVU repair. However, drugs that either inhibit the MCU_cx, or stimulate the NCLX, or activate the RXR or TR, suffer from adverse effects caused by undesired actions on healthy tissues. To overcome this problem, we describe the use of nanoparticle drug formulations that preferentially target metabolically compromised and damaged NVUs after an ischemic or hemorrhagic stroke. These nanoparticle-based approaches have the potential to improve clinical safety and efficacy by maximizing drug delivery to diseased NVUs and minimizing drug exposure in healthy brain and peripheral tissues.

MICU1 deficiency alters mitochondrial morphology and cytochrome C release

The mitochondrial inner boundary membrane harbors a protein called MICU1, which is sensitive to Ca²⁺ and binds to the MICOS components Mic60 and CHCHD2. Changes in the mitochondrial cristae junction structure and organization in MICU1^-/- cells lead to increased cytochrome c release, membrane potential rearrangement, and changes in mitochondrial Ca²⁺ uptake dynamics. These findings shed new light on the multifaceted role of MICU1, highlighting its involvement not only as an interaction partner and regulator of the MCU complex but also as a crucial determinant of mitochondrial ultrastructure and, thus, an essential player in processes initiating apoptosis.

The mitochondrial calcium uniporter complex–A play in five acts

Mitochondrial Ca²⁺ (mitCa²⁺) uptake controls both intraorganellar and cytosolic functions. Within the organelle, [Ca²⁺] increases regulate the activity of tricarboxylic acid (TCA) cycle enzymes, thus sustaining oxidative metabolism and ATP production. Reactive oxygen species (ROS) are also generated as side products of oxygen consumption. At the same time, mitochondria act as buffers of cytosolic Ca²⁺ (cytCa²⁺) increases, thus regulating Ca²⁺-dependent cellular processes. In pathological conditions, mitCa²⁺ overload triggers the opening of the mitochondrial permeability transition pore (mPTP) and the release of apoptotic cofactors. MitCa²⁺ uptake occurs in response of local [Ca²⁺] increases in sites of proximity between the endoplasmic reticulum (ER) and the mitochondria and is mediated by the mitochondrial Ca²⁺ uniporter (MCU), a highly selective channel of the inner mitochondrial membrane (IMM). Both channel and regulatory subunits form the MCU complex (MCUC). Cryogenic electron microscopy (Cryo-EM) and crystal structures revealed the correct assembly of MCUC and the function of critical residues for the regulation of Ca²⁺ conductance.

Mechanisms and significance of tissue-specific MICU regulation of the mitochondrial calcium uniporter complex

Mitochondrial Ca2+ uptake, mediated by the mitochondrial Ca2+ uniporter, regulates oxidative phosphorylation, apoptosis, and intracellular Ca2+ signaling. Previous studies suggest that non-neuronal uniporters are exclusively regulated by a MICU1-MICU2 heterodimer. Here, we show that skeletal-muscle and kidney uniporters also complex with a MICU1-MICU1 homodimer and that human/mouse cardiac uniporters are largely devoid of MICUs. Cells employ protein-importation machineries to fine-tune the relative abundance of MICU1 homo- and heterodimers and utilize a conserved MICU intersubunit disulfide to protect properly assembled dimers from proteolysis by YME1L1. Using the MICU1 homodimer or removing MICU1 allows mitochondria to more readily take up Ca2+ so that cells can produce more ATP in response to intracellular Ca2+ transients. However, the trade-off is elevated ROS, impaired basal metabolism, and higher susceptibility to death. These results provide mechanistic insights into how tissues can manipulate mitochondrial Ca2+ uptake properties to support their unique physiological functions.

An unexpected effect of risperidone reveals a nonlinear relationship between cytosolic Ca2+ and mitochondrial Ca2+ uptake

Mitochondria actively contribute to cellular Ca²⁺ homeostasis. The molecular mechanisms of mitochondrial Ca²⁺ uptake and release are well characterized and are attributed to the multi-protein assembly of the mitochondrial Ca²⁺ uniporter complex (MCUC) and the mitochondrial sodium-calcium exchanger (NCLX), respectively. Hence, Ca²⁺ transfer from the endoplasmic reticulum (ER) and store-operated Ca²⁺ entry (SOCE) into the mitochondrial matrix has been quantitatively visualized on the subcellular level using targeted fluorescent biosensors. However, a correlation between the amplitude of cytosolic Ca²⁺ elevation with that in the mitochondrial matrix has not been investigated in detail so far. In the present study, we combined the Ca²⁺-mobilizing agonist histamine with the H₁-receptor antagonist risperidone to establish a well-tunable experimental approach allowing the correlation between low, slow, high, and fast cytosolic and mitochondrial Ca²⁺ signals in response to inositol 1,4,5-trisphosphate (IP₃)-triggered ER Ca²⁺ release. Our present data confirm a defined threshold in cytosolic Ca²⁺, which is necessary for the activation of mitochondrial Ca²⁺ uptake. Moreover, our data support the hypothesis of different modes of mitochondrial Ca²⁺ uptake depending on the source of the ion (i.e., ER vs SOCE).

A controversial issue: Can mitochondria modulate cytosolic calcium and contraction of skeletal muscle fibers?

Mitochondria are characterized by a high capacity to accumulate calcium thanks to the electrochemical gradient created by the extrusion of protons in the respiratory chain. Thereby calcium can enter crossing the inner mitochondrial membrane via MCU complex, a high-capacity, low-affinity transport mechanism. Calcium uptake serves numerous purposes, among them the regulation of three dehydrogenases of the citric cycle, apoptosis via permeability transition, and, in some cell types, modulation of cytosolic calcium transients. This Review is focused on mitochondrial calcium uptake in skeletal muscle fibers and aims to reanalyze its functional impact. In particular, we ask whether mitochondrial calcium uptake is relevant for the control of cytosolic calcium transients and therefore of contractile performance. Recent data suggest that this may be the case, at least in particular conditions, as modified expression of MCU complex subunits or of proteins involved in mitochondrial dynamics and ablation of the main cytosolic calcium buffer, parvalbumin.

The Splicing of the Mitochondrial Calcium Uniporter Genuine Activator MICU1 Is Driven by RBFOX2 Splicing Factor during Myogenic Differentiation

Alternative splicing, the process by which exons within a pre-mRNA transcript are differentially joined or skipped, is crucial in skeletal muscle since it is required both during myogenesis and in post-natal life to reprogram the transcripts of contractile proteins, metabolic enzymes, and transcription factors in functionally distinct muscle fiber types. The importance of such events is underlined by the numerosity of pathological conditions caused by alternative splicing aberrations. Importantly, many skeletal muscle Ca²⁺ homeostasis genes are also regulated by alternative splicing mechanisms, among which is the Mitochondrial Ca²⁺ Uniporter (MCU) genuine activator MICU1 which regulates MCU opening upon cell stimulation. We have previously shown that murine skeletal muscle MICU1 is subjected to alternative splicing, thereby generating a splice variant-which was named MICU1.1-that confers unique properties to the mitochondrial Ca²⁺ uptake and ensuring sufficient ATP production for muscle contraction. Here we extended the analysis of MICU1 alternative splicing to human tissues, finding two additional splicing variants that were characterized by their ability to regulate mitochondrial Ca²⁺ uptake. Furthermore, we found that MICU1 alternative splicing is induced during myogenesis by the splicing factor RBFOX2. These results highlight the complexity of the alternative splicing mechanisms in skeletal muscle and the regulation of mitochondrial Ca²⁺ among tissues.

mtROS Induced via TLR-2-SOCE Signaling Plays Proapoptotic and Bactericidal Role in Mycobacterium fortuitum-Infected Head Kidney Macrophages of Clarias gariepinus

The mechanisms underlying Mycobacterium fortuitum-induced mycobacteriosis remain unexplored. Using head kidney macrophages (HKM) from catfish (Clarias gariepinus), we report that Ca²⁺ surge across mitochondrial-Ca²⁺ uniporter (MICU), and consequent mitochondrial ROS (mtROS) production, is imperative for mycobactericidal activity. Inhibition of mtROS alleviated HKM apoptosis and enhanced bacterial survival. Based on RNA interference (RNAi) and inhibitor studies, we demonstrate that the Toll-like receptor (TLR)-2–endoplasmic reticulum (ER) stress–store-operated calcium entry (SOCE) axis is instrumental for activating the mt-Ca²⁺/mtROS cascade in M. fortuitum-infected HKM. Additionally, pharmacological inhibition of mtROS attenuated the expression of CHOP, STIM1, and Orai1, which suggests a positive feedback loop between ER-stress-induced SOCE and mtROS production. Elevated tumor necrosis factor alpha (TNF-α) levels and caspase-8 activity were observed in HKM consequent to M. fortuitum infection, and our results implicate that mtROS is crucial in activating the TNF-mediated caspase-8 activation. Our results for the first time demonstrate mitochondria as an innate immune signaling center regulating mycobacteriosis in fish. We conclude that M. fortuitum-induced persistent SOCE signaling leads to mtROS production, which in turn activates the TNF-α/caspase-8 axis culminating in HKM apoptosis and bacterial clearance.

MICU1-dependent mitochondrial calcium uptake regulates lung alveolar type 2 cell plasticity and lung regeneration

Lung alveolar type 2 (AT2) cells are progenitors for alveolar type 1 (AT1) cells. Although many factors regulate AT2 cell plasticity, the role of mitochondrial calcium (_mCa²⁺) uptake in controlling AT2 cells remains unclear. We previously identified that the miR-302 family supports lung epithelial progenitor cell proliferation and less differentiated phenotypes during development. Here, we report that a sustained elevation of miR-302 in adult AT2 cells decreases AT2-to-AT1 cell differentiation during the Streptococcus pneumoniae–induced lung injury repair. We identified that miR-302 targets and represses the expression of mitochondrial Ca²⁺ uptake 1 (MICU1), which regulates _mCa²⁺ uptake through the _mCa²⁺ uniporter channel by acting as a gatekeeper at low cytosolic Ca²⁺ levels. Our results reveal a marked increase in MICU1 protein expression and decreased _mCa²⁺ uptake during AT2-to-AT1 cell differentiation in the adult lung. Deletion of Micu1 in AT2 cells reduces AT2-to-AT1 cell differentiation during steady-state tissue maintenance and alveolar epithelial regeneration after bacterial pneumonia. These studies indicate that _mCa²⁺ uptake is extensively modulated during AT2-to-AT1 cell differentiation and that MICU1-dependent _mCa²⁺ uniporter channel gating is a prominent mechanism modulating AT2-to-AT1 cell differentiation.

Therapeutic Strategies Targeting Mitochondrial Calcium Signaling: A New Hope for Neurological Diseases?

Calcium (Ca²⁺) is a versatile secondary messenger involved in the regulation of a plethora of different signaling pathways for cell maintenance. Specifically, intracellular Ca²⁺ homeostasis is mainly regulated by the endoplasmic reticulum and the mitochondria, whose Ca²⁺ exchange is mediated by appositions, termed endoplasmic reticulum-mitochondria-associated membranes (MAMs), formed by proteins resident in both compartments. These tethers are essential to manage the mitochondrial Ca²⁺ influx that regulates the mitochondrial function of bioenergetics, mitochondrial dynamics, cell death, and oxidative stress. However, alterations of these pathways lead to the development of multiple human diseases, including neurological disorders, such as amyotrophic lateral sclerosis, Friedreich's ataxia, and Charcot-Marie-Tooth. A common hallmark in these disorders is mitochondrial dysfunction, associated with abnormal mitochondrial Ca²⁺ handling that contributes to neurodegeneration. In this work, we highlight the importance of Ca²⁺ signaling in mitochondria and how the mechanism of communication in MAMs is pivotal for mitochondrial maintenance and cell homeostasis. Lately, we outstand potential targets located in MAMs by addressing different therapeutic strategies focused on restoring mitochondrial Ca²⁺ uptake as an emergent approach for neurological diseases.

Cristae junction as a fundamental switchboard for mitochondrial ion signaling and bioenergetics

OPA1 and MICU1 are both involved in the regulation of mitochondrial Ca2+ uptake and the stabilization of the cristae junction, which separates the inner mitochondrial membrane into the interboundary membrane and the cristae membrane. In this mini-review, we focus on the synergetic control of OPA1 and MICU1 on the cristae junction that serves as a fundamental regulator of multiple mitochondrial functions. In particular, we point to the critical role of an adaptive cristae junction permeability in mitochondrial Ca2+ signaling, spatial H+ gradients and mitochondrial membrane potential, metabolic activity, and apoptosis. These characteristics bear on a distinct localization of the oxidative phosphorylation machinery, the FoF1-ATPase, and mitochondrial Ca2+uniporter (MCU) within sections of the inner mitochondrial membrane isolated by the cristae junction and regulated by proteins like OPA1 and MICU1. We specifically focus on the impact of MICU1-regulated cristae junction on the activity and distribution of MCU within the complex ultrastructure of mitochondria.

Mitochondrial calcium uniporter affects neutrophil bactericidal activity during Staphylococcus aureus infection

Neutrophils simultaneously restrict Staphylococcus aureus dissemination and facilitate bactericidal activity during infection through the formation of neutrophil extracellular traps (NETs). Neutrophils that produce higher levels of mitochondrial superoxide undergo enhanced terminal NET formation (suicidal NETosis) in response to S. aureus; however, mechanisms regulating mitochondrial homeostasis upstream of neutrophil antibacterial processes are not fully resolved. Here, we demonstrate that mitochondrial calcium uptake 1 (MICU1)-deficient (MICU1^-/-) neutrophils accumulate higher levels of calcium and iron within the mitochondria in a mitochondrial calcium uniporter (MCU)-dependent manner. Corresponding with increased ion flux through the MCU, mitochondrial superoxide production is elevated, thereby increasing the propensity for MICU1^-/- neutrophils to undergo suicidal NETosis rather than primary degranulation in response to S. aureus. Increased NET formation augments macrophage killing of bacterial pathogens. Similarly, MICU1^-/- neutrophils alone are not more antibacterial towards S. aureus, but rather enhanced suicidal NETosis by MICU1^-/- neutrophils facilitates increased bactericidal activity in the presence of macrophages. Similarly, mice with a deficiency in MICU1 restricted to cells expressing LysM exhibit lower bacterial burdens in the heart with increased survival during systemic S. aureus infection. Coinciding with the decrease in S. aureus burdens, MICU1^-/- neutrophils in the heart produced higher levels of mitochondrial superoxide and undergo enhanced suicidal NETosis. These results demonstrate that ion flux by the MCU affects the antibacterial function of neutrophils during S. aureus infection.

Ru360 and Mitoxantrone inhibit MCU channel to relieve liver steatosis induced by high-fat diet

BACKGROUND AND PURPOSE

Nonalcoholic fatty liver disease (NAFLD) affects over 25% of the general population and lacks an effective treatment. Recent evidence implicates disrupted mitochondrial calcium homeostasis in the pathogenesis of hepatic steatosis.

EXPERIMENTAL APPROACH

In this study, mitochondrial calcium uniporter (MCU) was inhibited through classical genetic approaches, viral vectors or small molecule inhibitors in vivo to study its role in hepatic steatosis induced by HFD. In vitro, MCU was overexpressed or inhibited to change mitochondrial calcium homeostasis; endoplasmic reticulum-mitochondrial linker was adopted to increase mitochondria-associated membranes (MAM); and MICU1-EF hand mutant was used to decrease the sensitivity of mitochondrial calcium uptake 1 (MICU1) to calcium and block MCU channel.

KEY RESULTS

Here we found that inhibition of liver MCU by AAV virus and classical genetic approaches can alleviate HFD-induced liver steatosis. MCU regulates mitochondrial calcium homeostasis and affects lipid accumulation in liver cells. In addition, a HFD in mice enlarged the MAM. The high calcium environment produced by MAM invalidated the function of MICU1 and led to persistent open of MCU channels. Therefore, it caused mitochondrial calcium overload and liver fat deposition. Inhibition of MAM and MCU alleviated HFD-induced hepatic steatosis. MCU inhibitors (Ru360 and mitoxantrone) can block MCU channels and reduce mitochondrial calcium levels. Intraperitoneal injection of MCU inhibitors (0.01 μM/kg bodyweight) can alleviate HFD-induced hepatic steatosis.

CONCLUSION AND IMPLICATIONS

These findings provide molecular insights into the way HFD disrupts mitochondrial calcium homeostasis and identified MCU as a promising drug target for the treatment of hepatic steatosis.

Mitochondrial calcium exchange in physiology and disease

The uptake of calcium into and extrusion of calcium from the mitochondrial matrix is a fundamental biological process that has critical effects on cellular metabolism, signaling, and survival. Disruption of mitochondrial calcium (_mCa²⁺) cycling is implicated in numerous acquired diseases such as heart failure, stroke, neurodegeneration, diabetes, and cancer, and is genetically linked to several inherited neuromuscular disorders. Understanding the mechanisms responsible for _mCa²⁺ exchange therefore holds great promise for the treatment of these diseases. The past decade has seen the genetic identification of many of the key proteins that mediate mitochondrial calcium uptake and efflux. Here, we present an overview of the phenomenon of _mCa²⁺ transport, and a comprehensive examination of the molecular machinery that mediates calcium flux across the inner mitochondrial membrane: the mitochondrial uniporter complex (consisting of MCU, EMRE, MICU1, MICU2, MICU3, MCUB, and MCUR1), NCLX, LETM1, the mitochondrial ryanodine receptor, and the mitochondrial permeability transition pore. We then consider the physiological implications of _mCa²⁺ flux and evaluate how alterations in _mCa²⁺ homeostasis contribute to human disease. This review concludes by highlighting opportunities and challenges for therapeutic intervention in pathologies characterized by aberrant _mCa²⁺ handling and by summarizing critical unanswered questions regarding the biology of _mCa²⁺ flux.

Structural and functional properties of meprin β metalloproteinase with regard to cell signaling

The metalloproteinase meprin β plays an important role during collagen I deposition in the skin, mucus detachment in the small intestine and also regulates the abundance of different cell surface proteins such as the interleukin-6 receptor (IL-6R), the triggering receptor expressed on myeloid cells 2 (TREM2), the cluster of differentiation 99 (CD99), the amyloid precursor protein (APP) and the cluster of differentiation 109 (CD109). With that, regulatory mechanisms that control meprin β activity and regulate its release from the cell surface to enable access to distant substrates are increasingly important. Here, we will summarize factors that alternate meprin β activity and thereby regulate its proteolytic activity on the cell surface or in the supernatant. We will also discuss cleavage of the IL-6R and TREM2 on the cell surface and compare it to CD109. CD109, as a substrate of meprin β, is cleaved within the protein core, thereby releasing defined fragments from the cell surface. At last, we will also summarize the role of proteases in general and meprin β in particular in substrate release on extracellular vesicles.

Structural characterization of the mitochondrial Ca2+ uniporter provides insights into Ca2+ uptake and regulation

The mitochondrial uniporter is a Ca²⁺-selective ion-conducting channel in the inner mitochondrial membrane that is involved in various cellular processes. The components of this uniporter, including the pore-forming membrane subunit MCU and the modulatory subunits MCUb, EMRE, MICU1, and MICU2, have been identified in recent years. Previously, extensive studies revealed various aspects of uniporter activities and proposed multiple regulatory models of mitochondrial Ca²⁺ uptake. Recently, the individual auxiliary components of the uniporter and its holocomplex have been structurally characterized, providing the first insight into the component structures and their spatial relationship within the context of the uniporter. Here, we review recent uniporter structural studies in an attempt to establish an architectural framework, elucidating the mechanism that governs mitochondrial Ca²⁺ uptake and regulation, and to address some apparent controversies. This information could facilitate further characterization of mitochondrial Ca²⁺ permeation and a better understanding of uniporter-related disease conditions.

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The uniporter contains multiple subunits regulating various cellular processes

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Significant structural progresses have been made for the holo-complex of uniporter

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The holo-complex structures have inspired to propose several regulatory models

Sleep/wake calcium dynamics, respiratory function, and ROS production in cardiac mitochondria

Introduction

Incidents of myocardial infarction and sudden cardiac arrest vary with time of the day, but the mechanism for this effect is not clear. We hypothesized that diurnal changes in the ability of cardiac mitochondria to control calcium homeostasis dictate vulnerability to cardiovascular events.

Objectives

Here we investigate mitochondrial calcium dynamics, respiratory function, and reactive oxygen species (ROS) production in mouse heart during different phases of wake versus sleep periods.

Methods

We assessed time-of-the-day dependence of calcium retention capacity of isolated heart mitochondria from young male C57BL6 mice. Rhythmicity of mitochondrial-dependent oxygen consumption, ROS production and transmembrane potential in homogenates were explored using the Oroboros O2k Station equipped with a fluorescence detection module. Changes in expression of essential clock and calcium dynamics genes/proteins were also determined at sleep versus wake time points.

Results

Our results demonstrate that cardiac mitochondria exhibit higher calcium retention capacity and higher rates of calcium uptake during sleep period. This was associated with higher expression of clock gene Bmal1, lower expression of per2, greater expression of MICU1 gene (mitochondrial calcium uptake 1), and lower expression of the mitochondrial transition pore regulator gene cyclophilin D. Protein levels of mitochondrial calcium uniporter (MCU), MICU2, and sodium/calcium exchanger (NCLX) were also higher at sleep onset relative to wake period. While complex I and II-dependent oxygen utilization and transmembrane potential of cardiac mitochondria were lower during sleep, ROS production was increased presumably due to mitochondrial calcium sequestration.

Conclusions

Taken together, our results indicate that retaining mitochondrial calcium in the heart during sleep dissipates membrane potential, slows respiratory activities, and increases ROS levels, which may contribute to increased vulnerability to cardiac stress during sleep-wake transition. This pronounced daily oscillations in mitochondrial functions pertaining to stress vulnerability may at least in part explain diurnal prevalence of cardiac pathologies.

The mitochondrial permeability transition pore: an evolving concept critical for cell life and death

In this review, we summarize current knowledge of perhaps one of the most intriguing phenomena in cell biology: the mitochondrial permeability transition pore (mPTP). This phenomenon, which was initially observed as a sudden loss of inner mitochondrial membrane impermeability caused by excessive calcium, has been studied for almost 50 years, and still no definitive answer has been provided regarding its mechanisms. From its initial consideration as an in vitro artifact to the current notion that the mPTP is a phenomenon with physiological and pathological implications, a long road has been travelled. We here summarize the role of mitochondria in cytosolic calcium control and the evolving concepts regarding the mitochondrial permeability transition (mPT) and the mPTP. We show how the evolving mPTP models and mechanisms, which involve many proposed mitochondrial protein components, have arisen from methodological advances and more complex biological models. We describe how scientific progress and methodological advances have allowed milestone discoveries on mPTP regulation and composition and its recognition as a valid target for drug development and a critical component of mitochondrial biology.

Identification of a novel MICU1 nonsense variant causes myopathy with extrapyramidal signs in an Iranian consanguineous family

BACKGROUND

Ca²⁺ as a universal second messenger regulates basic biological functions including cell cycle, cell proliferation, cell differentiation, and cell death. Lack of the protein mitochondrial calcium uptake1 (MICU1), which has been regarded as a gatekeeper of Ca ions, leads to the abnormal mitochondrial Ca²⁺ handling, excessive production of reactive oxygen species (ROS), and increased cell death. Mutations in MICU1 gene causes a very rare neuromuscular disease, myopathy with extrapyramidal signs (MPXPS), due to primary alterations in mitochondrial calcium signaling which demonstrates the key role of mitochondrial Ca²⁺ uptake. To date, 13 variants have been reported in MICU1 gene in 44 patients presented with the vast spectrum of symptoms.

CASE PRESENTATION

Here, we report a 44-year-old Iranian patient presented with learning disability, muscle weakness, easy fatigability, reduced tendon reflexes, ataxia, gait disturbance, elevated hepatic transaminases, elevated serum creatine kinase (CK), and elevated lactate dehydrogenase (LDH). We identified a novel nonsense variant c.385C>T; p.(R129*) in MICU1 gene by whole exome sequencing (WES) and segregation analysis.

CONCLUSIONS

Our finding along with previous studies provides more evidence on the clinical presentation of the disease caused by pathogenic mutations in MICU1. Finding more variants and expanding the spectrum of the disease increases the diagnostic rate of molecular testing in screening of this kind of diseases and in turn improves the quality of counseling for at risk couples and helps them to minimize the risks of having affected children.

The mitochondrial Ca2+ uptake regulator, MICU1, is involved in cold stress-induced ferroptosis

Ferroptosis has recently attracted much interest because of its relevance to human diseases such as cancer and ischemia-reperfusion injury. We have reported that prolonged severe cold stress induces lipid peroxidation-dependent ferroptosis, but the upstream mechanism remains unknown. Here, using genome-wide CRISPR screening, we found that a mitochondrial Ca2+ uptake regulator, mitochondrial calcium uptake 1 (MICU1), is required for generating lipid peroxide and subsequent ferroptosis under cold stress. Furthermore, the gatekeeping activity of MICU1 through mitochondrial calcium uniporter (MCU) is suggested to be indispensable for cold stress-induced ferroptosis. MICU1 is required for mitochondrial Ca2+ increase, hyperpolarization of the mitochondrial membrane potential (MMP), and subsequent lipid peroxidation under cold stress. Collectively, these findings suggest that the MICU1-dependent mitochondrial Ca2+ homeostasis-MMP hyperpolarization axis is involved in cold stress-induced lipid peroxidation and ferroptosis.

A High-Throughput Screening Identifies MICU1 Targeting Compounds

Mitochondrial Ca²⁺ uptake depends on the mitochondrial calcium uniporter (MCU) complex, a highly selective channel of the inner mitochondrial membrane (IMM). Here, we screen a library of 44,000 non-proprietary compounds for their ability to modulate mitochondrial Ca²⁺ uptake. Two of them, named MCU-i4 and MCU-i11, are confirmed to reliably decrease mitochondrial Ca²⁺ influx. Docking simulations reveal that these molecules directly bind a specific cleft in MICU1, a key element of the MCU complex that controls channel gating. Accordingly, in MICU1-silenced or deleted cells, the inhibitory effect of the two compounds is lost. Moreover, MCU-i4 and MCU-i11 fail to inhibit mitochondrial Ca²⁺ uptake in cells expressing a MICU1 mutated in the critical amino acids that forge the predicted binding cleft. Finally, these compounds are tested ex vivo, revealing a primary role for mitochondrial Ca²⁺ uptake in muscle growth. Overall, MCU-i4 and MCU-i11 represent leading molecules for the development of MICU1-targeting drugs.

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An HTS identifies MCU-i4 and MCU-i11 as negative modulators of the MCU

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MCU-i4 and MCU-i11 bind MICU1

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MICU1 is required for the activity of MCU-i4 and MCU-i11

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MCU-i4 and MCU-i11 impair muscle cell growth

The molecular complexity of the Mitochondrial Calcium Uniporter

The role of mitochondria in regulating cellular Ca²⁺ homeostasis is crucial for the understanding of different cellular functions in physiological and pathological conditions. Nevertheless, the study of this aspect was severely limited by the lack of the molecular identity of the proteins responsible for mitochondrial Ca²⁺ uptake. In 2011, the discovery of the gene encoding for the Mitochondrial Calcium Uniporter (MCU), the selective channel responsible for mitochondrial Ca²⁺ uptake, gave rise to an explosion of studies aimed to characterize the composition, the regulation of the channel and its pathophysiological roles. Here, we summarize the recent discoveries on the molecular structure and composition of the MCU complex by providing new insights into the mechanisms that regulate MCU channel activity.

Physiological Ca2+ Transients Versus Pathological Steady-State Ca2+ Elevation, Who Flips the ROS Coin in Skeletal Muscle Mitochondria

Mitochondria are both the primary provider of ATP and the pivotal regulator of cell death, which are essential for physiological muscle activities. Ca²⁺ plays a multifaceted role in mitochondrial function. During muscle contraction, Ca²⁺ influx into mitochondria activates multiple enzymes related to tricarboxylic acid (TCA) cycle and oxidative phosphorylation, resulting in increased ATP synthesis to meet the energy demand. Pathophysiological conditions such as skeletal muscle denervation or unloading also lead to elevated Ca²⁺ levels inside mitochondria. However, the outcomes of this steady-state elevation of mitochondrial Ca²⁺ level include exacerbated reactive oxygen species (ROS) generation, sensitized opening of mitochondrial permeability transition pore (mPTP), induction of programmed cell death, and ultimately muscle atrophy. Previously, both acute and long-term endurance exercises have been reported to activate certain signaling pathways to counteract ROS production. Meanwhile, electrical stimulation is known to help prevent apoptosis and alleviate muscle atrophy in denervated animal models and patients with motor impairment. There are various mechanistic studies that focus on the excitation-transcription coupling framework to understand the beneficial role of exercise and electrical stimulation. Interestingly, a recent study has revealed an unexpected role of rapid mitochondrial Ca²⁺ transients in keeping mPTP at a closed state with reduced mitochondrial ROS production. This discovery motivated us to contribute this review article to inspire further discussion about the potential mechanisms underlying differential outcomes of physiological mitochondrial Ca²⁺ transients and pathological mitochondrial Ca²⁺ elevation in skeletal muscle ROS production.

Regulation of Cell Death by Mitochondrial Transport Systems of Calcium and Bcl-2 Proteins

Mitochondria represent the fundamental system for cellular energy metabolism, by not only supplying energy in the form of ATP, but also by affecting physiology and cell death via the regulation of calcium homeostasis and the activity of Bcl-2 proteins. A lot of research has recently been devoted to understanding the interplay between Bcl-2 proteins, the regulation of these interactions within the cell, and how these interactions lead to the changes in calcium homeostasis. However, the role of Bcl-2 proteins in the mediation of mitochondrial calcium homeostasis, and therefore the induction of cell death pathways, remain underestimated and are still not well understood. In this review, we first summarize our knowledge about calcium transport systems in mitochondria, which, when miss-regulated, can induce necrosis. We continue by reviewing and analyzing the functions of Bcl-2 proteins in apoptosis. Finally, we link these two regulatory mechanisms together, exploring the interactions between the mitochondrial Ca2+ transport systems and Bcl-2 proteins, both capable of inducing cell death, with the potential to determine the cell death pathway-either the apoptotic or the necrotic one.

The Mitochondrial Ca2+ Uptake and the Fine-Tuning of Aerobic Metabolism

Recently, the role of mitochondrial activity in high-energy demand organs and in the orchestration of whole-body metabolism has received renewed attention. In mitochondria, pyruvate oxidation, ensured by efficient mitochondrial pyruvate entry and matrix dehydrogenases activity, generates acetyl CoA that enters the TCA cycle. TCA cycle activity, in turn, provides reducing equivalents and electrons that feed the electron transport chain eventually producing ATP. Mitochondrial Ca²⁺ uptake plays an essential role in the control of aerobic metabolism. Mitochondrial Ca²⁺ accumulation stimulates aerobic metabolism by inducing the activity of three TCA cycle dehydrogenases. In detail, matrix Ca²⁺ indirectly modulates pyruvate dehydrogenase via pyruvate dehydrogenase phosphatase 1, and directly activates isocitrate and α-ketoglutarate dehydrogenases. Here, we will discuss the contribution of mitochondrial Ca²⁺ uptake to the metabolic homeostasis of organs involved in systemic metabolism, including liver, skeletal muscle, and adipose tissue. We will also tackle the role of mitochondrial Ca²⁺ uptake in the heart, a high-energy consuming organ whose function strictly depends on appropriate Ca²⁺ signaling.

Dysregulation of Mitochondrial Ca2+ Uptake and Sarcolemma Repair Underlie Muscle Weakness and Wasting in Patients and Mice Lacking MICU1

Muscle function is regulated by Ca²⁺, which mediates excitation-contraction coupling, energy metabolism, adaptation to exercise, and sarcolemmal repair. Several of these actions rely on Ca²⁺ delivery to the mitochondrial matrix via the mitochondrial Ca²⁺ uniporter, the pore of which is formed by mitochondrial calcium uniporter (MCU). MCU's gatekeeping and cooperative activation are controlled by MICU1. Loss-of-protein mutation in MICU1 causes a neuromuscular disease. To determine the mechanisms underlying the muscle impairments, we used MICU1 patient cells and skeletal muscle-specific MICU1 knockout mice. Both these models show a lower threshold for MCU-mediated Ca²⁺ uptake. Lack of MICU1 is associated with impaired mitochondrial Ca²⁺ uptake during excitation-contraction, aerobic metabolism impairment, muscle weakness, fatigue, and myofiber damage during physical activity. MICU1 deficit compromises mitochondrial Ca²⁺ uptake during sarcolemmal injury, which causes ineffective repair of the damaged myofibers. Thus, dysregulation of mitochondrial Ca²⁺ uptake hampers myofiber contractile function, likely through energy metabolism and membrane repair.

Coupled transmembrane mechanisms control MCU-mediated mitochondrial Ca2+ uptake

Ca2+ uptake by mitochondria regulates bioenergetics, apoptosis, and Ca2+ signaling. The primary pathway for mitochondrial Ca2+ uptake is the mitochondrial calcium uniporter (MCU), a Ca2+-selective ion channel in the inner mitochondrial membrane. MCU-mediated Ca2+ uptake is driven by the sizable inner-membrane potential generated by the electron-transport chain. Despite the large thermodynamic driving force, mitochondrial Ca2+ uptake is tightly regulated to maintain low matrix [Ca2+] and prevent opening of the permeability transition pore and cell death, while meeting dynamic cellular energy demands. How this is accomplished is controversial. Here we define a regulatory mechanism of MCU-channel activity in which cytoplasmic Ca2+ regulation of intermembrane space-localized MICU1/2 is controlled by Ca2+-regulatory mechanisms localized across the membrane in the mitochondrial matrix. Ca2+ that permeates through the channel pore regulates Ca2+ affinities of coupled inhibitory and activating sensors in the matrix. Ca2+ binding to the inhibitory sensor within the MCU amino terminus closes the channel despite Ca2+ binding to MICU1/2. Conversely, disruption of the interaction of MICU1/2 with the MCU complex disables matrix Ca2+ regulation of channel activity. Our results demonstrate how Ca2+ influx into mitochondria is tuned by coupled Ca2+-regulatory mechanisms on both sides of the inner mitochondrial membrane.

Regulation of Ca2+ exchanges and signaling in mitochondria

Mitochondrial calcium (_mCa²⁺) homeostasis also plays a key role in the buffering of cytosolic calcium (_cCa²⁺) and calcium transported into the mitochondrial matrix regulates cellular metabolism, migration and cell fate decisions. Recent work has highlighted the importance of mCa²⁺ homeostasis in regulating cellular function. The discovery of the mCa²⁺ uptake complex has shed new light on the role of _mCa²⁺ dynamics in cytoskeletal remodeling, mitochondrial shape and motility in cellular dynamics. Here we attempt to decipher the vast landscape of calcium regulatory effects of the mitochondria, the underlying mechanisms and the dynamics that control cellular function.

The structure of the MICU1-MICU2 complex unveils the regulation of the mitochondrial calcium uniporter

The MICU1-MICU2 heterodimer regulates the mitochondrial calcium uniporter (MCU) and mitochondrial calcium uptake. Herein, we present two crystal structures of the MICU1-MICU2 heterodimer, in which Ca²⁺ -free and Ca²⁺ -bound EF-hands are observed in both proteins, revealing both electrostatic and hydrophobic interfaces. Furthermore, we show that MICU1 interacts with EMRE, another regulator of MCU, through a Ca²⁺ -dependent alkaline groove. Ca²⁺ binding strengthens the MICU1-EMRE interaction, which in turn facilitates Ca²⁺ uptake. Conversely, the MICU1-MCU interaction is favored in the absence of Ca²⁺ , thus inhibiting the channel activity. This Ca²⁺ -dependent switch illuminates how calcium signals are transmitted from regulatory subunits to the calcium channel and the transition between gatekeeping and activation channel functions. Furthermore, competition with an EMRE peptide alters the uniporter threshold in resting conditions and elevates Ca²⁺ accumulation in stimulated mitochondria, confirming the gatekeeper role of the MICU1-MICU2 heterodimer. Taken together, these structural and functional data provide new insights into the regulation of mitochondrial calcium uptake.

Structural insights into the Ca2+-dependent gating of the human mitochondrial calcium uniporter

Mitochondrial Ca2+ uptake is mediated by an inner mitochondrial membrane protein called the mitochondrial calcium uniporter. In humans, the uniporter functions as a holocomplex consisting of MCU, EMRE, MICU1 and MICU2, among which MCU and EMRE form a subcomplex and function as the conductive channel while MICU1 and MICU2 are EF-hand proteins that regulate the channel activity in a Ca2+-dependent manner. Here, we present the EM structures of the human mitochondrial calcium uniporter holocomplex (uniplex) in the presence and absence of Ca2+, revealing distinct Ca2+ dependent assembly of the uniplex. Our structural observations suggest that Ca2+ changes the dimerization interaction between MICU1 and MICU2, which in turn determines how the MICU1-MICU2 subcomplex interacts with the MCU-EMRE channel and, consequently, changes the distribution of the uniplex assemblies between the blocked and unblocked states.

Structures reveal gatekeeping of the mitochondrial Ca2+ uniporter by MICU1-MICU2

The mitochondrial calcium uniporter is a Ca2+-gated ion channel complex that controls mitochondrial Ca2+ entry and regulates cell metabolism. MCU and EMRE form the channel while Ca2+-dependent regulation is conferred by MICU1 and MICU2 through an enigmatic process. We present a cryo-EM structure of an MCU-EMRE-MICU1-MICU2 holocomplex comprising MCU and EMRE subunits from the beetle Tribolium castaneum in complex with a human MICU1-MICU2 heterodimer at 3.3 Å resolution. With analogy to how neuronal channels are blocked by protein toxins, a uniporter interaction domain on MICU1 binds to a channel receptor site comprising MCU and EMRE subunits to inhibit ion flow under resting Ca2+ conditions. A Ca2+-bound structure of MICU1-MICU2 at 3.1 Å resolution indicates how Ca2+-dependent changes enable dynamic response to cytosolic Ca2+ signals.

Mitochondria, spermatogenesis, and male infertility - An update

The incorporation of mitochondria in the eukaryotic cell is one of the most enigmatic events in the course of evolution. This important organelle was thought to be only the powerhouse of the cell, but was later learnt to perform many other indispensable functions in the cell. Two major contributions of mitochondria in spermatogenesis concern energy production and apoptosis. Apart from this, mitochondria also participate in a number of other processes affecting spermatogenesis and fertility. Mitochondria in sperm are arranged in the periphery of the tail microtubules to serve to energy demand for motility. Apart from this, the role of mitochondria in germ cell proliferation, mitotic regulation, and the elimination of germ cells by apoptosis are now well recognized. Eventually, mutations in the mitochondrial genome have been reported in male infertility, particularly in sluggish sperm (asthenozoospermia); however, heteroplasmy in the mtDNA and a complex interplay between the nucleus and mitochondria affect their penetrance. In this article, we have provided an update on the role of mitochondria in various events of spermatogenesis and male fertility and on the correlation of mitochondrial DNA mutations with male infertility.

Structural Mechanisms of Store-Operated and Mitochondrial Calcium Regulation: Initiation Points for Drug Discovery

Calcium (Ca2+) is a universal signaling ion that is essential for the life and death processes of all eukaryotes. In humans, numerous cell stimulation pathways lead to the mobilization of sarco/endoplasmic reticulum (S/ER) stored Ca2+, resulting in the propagation of Ca2+ signals through the activation of processes, such as store-operated Ca2+ entry (SOCE). SOCE provides a sustained Ca2+ entry into the cytosol; moreover, the uptake of SOCE-mediated Ca2+ by mitochondria can shape cytosolic Ca2+ signals, function as a feedback signal for the SOCE molecular machinery, and drive numerous mitochondrial processes, including adenosine triphosphate (ATP) production and distinct cell death pathways. In recent years, tremendous progress has been made in identifying the proteins mediating these signaling pathways and elucidating molecular structures, invaluable for understanding the underlying mechanisms of function. Nevertheless, there remains a disconnect between using this accumulating protein structural knowledge and the design of new research tools and therapies. In this review, we provide an overview of the Ca2+ signaling pathways that are involved in mediating S/ER stored Ca2+ release, SOCE, and mitochondrial Ca2+ uptake, as well as pinpoint multiple levels of crosstalk between these pathways. Further, we highlight the significant protein structures elucidated in recent years controlling these Ca2+ signaling pathways. Finally, we describe a simple strategy that aimed at applying the protein structural data to initiating drug design.

Structure and mechanism of the mitochondrial Ca2+ uniporter holocomplex

Mitochondria take up Ca²⁺ through the mitochondrial calcium uniporter complex to regulate energy production, cytosolic Ca²⁺ 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 Ca²⁺ transport^3-8. To prevent detrimental Ca²⁺ overload, the activity of MCU must be tightly regulated by MICUs, which sense changes in cytosolic Ca²⁺ 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 Ca²⁺-activated states. These structures define the architecture of this multicomponent Ca²⁺-uptake machinery and reveal the gating mechanism by which MICUs control uniporter activity. Our work provides a framework for understanding regulated Ca²⁺ uptake in mitochondria, and could suggest ways of modulating uniporter activity to treat diseases related to mitochondrial Ca²⁺ overload.

Variable Assembly of EMRE and MCU Creates Functional Channels with Distinct Gatekeeping Profiles

MCU is a Ca2+-selective channel that mediates mitochondrial Ca2+ influx. The human channel contains tetrameric pore-forming MCU, regulatory subunits MICU1/2, and EMRE that is required both for channel function and MICU1/2-mediated Ca2+ regulation. A structure of MCU with EMRE revealed a 4:4 stoichiometry, but the stoichiometry in vivo is unknown. Expression of tagged EMRE and MCU at a 1:10 ratio in cells lacking EMRE and MCU restored channel activity but not full channel gatekeeping. Increasing EMRE expression enhanced gatekeeping, raising the cytoplasmic Ca2+ concentration ([Ca2+]c) threshold for channel activation. MCU-EMRE concatemers creating channels with 1EMRE:4MCU restored Ca2+ uptake in cells, whereas cells expressing concatemers that enforced a 4EMRE:4MCU stoichiometry demonstrated enhanced channel gatekeeping. Concatemers enforcing 2EMRE/4MCU recapitulated the activity, gatekeeping, and size of endogenous channels. Thus, MCU does not require four EMRE, with most endogenous channels containing two, but complexes with 1-4 EMRE have activity with full or partial gatekeeping.

Reporting a rare form of myopathy, myopathy with extrapyramidal signs, in an Iranian family using next generation sequencing: a case report

Background

Myopathy with extrapyramidal signs (MPXPS) is an autosomal recessive mitochondrial disorder which is caused by mutation in mitochondrial calcium uptake 1 (MICU1) gene located on chromosome 10q22.1. Next Generation Sequencing (NGS) technology is the most effective method for identification of pathogenic variants with the ability to overcome some limitations which Sanger sequencing may encountered. There are few reports on this rare disease around the world and here in this study we first revealed genetic identification of two affected individuals in an Iranian family with a novel mutation.

Case presentation

The proband was a 5-year-old girl from consanguenous parents. She was first clinically suspicious of affected with limb-girdle muscular dystrophy (LGMD). Muscle biopsy studies and autozygosity mapping, using four short tandem repeat (STR) markers linked to 6 genes of the most prevalent forms of LGMD, ruled out calpainopathy, dysferlinopathy, and sarcoglycanopathis. DNA sample of the proband was sent for NGS. Whole exome sequencing (WES) revealed a novel mutation c.1295delA in exon 13 of MICU1 gene. This homozygous deletion creates a frameshift and a premature stop codon downstream of canonical EF4 calcium binding motif of MICU1. According to the American College of Medical Genetics and Genomics (ACMG) guidline for sequence interpretation, this variant was a pathogenic one. Sanger sequencing in all family members confirmed the results of the WES.

Conclusions

This study was the first report of MPXPS in Iranian population which also revealed a novel mutation in the MICU1 gene.

Insights Into the Role of Mitochondrial Ion Channels in Inflammatory Response

Mitochondria are the source of many pro-inflammatory signals that cause the activation of the immune system and generate inflammatory responses. They are also potential targets of pro-inflammatory mediators, thus triggering a severe inflammatory response cycle. As mitochondria are a central hub for immune system activation, their dysfunction leads to many inflammatory disorders. Thus, strategies aiming at regulating mitochondrial dysfunction can be utilized as a therapeutic tool to cure inflammatory disorders. Two key factors that determine the structural and functional integrity of mitochondria are mitochondrial ion channels and transporters. They are not only important for maintaining the ionic homeostasis of the cell, but also play a role in regulating reactive oxygen species generation, ATP production, calcium homeostasis and apoptosis, which are common pro-inflammatory signals. The significance of the mitochondrial ion channels in inflammatory response is still not clearly understood and will need further investigation. In this article, we review the different mechanisms by which mitochondria can generate the inflammatory response as well as highlight how mitochondrial ion channels modulate these mechanisms and impact the inflammatory processes.

Dimerization of MICU Proteins Controls Ca2+ Influx through the Mitochondrial Ca2+ Uniporter

The mitochondrial Ca²⁺ uniporter complex (MCUC) is responsible for Ca²⁺ influx into the mitochondrial matrix, playing critical roles in various mitochondrial functions. Eukaryotic MCUC consists of multiple subunits, and its Ca²⁺ influx activity is controlled by regulatory subunits, including mitochondrial Ca²⁺ uptake 1 (MICU1) and its paralogs (MICU2 and MICU3). However, the underlying mechanism remains unclear. Here, we determined multiple crystal structures of MICU2 and MICU3 from Homo sapiens. Our data demonstrate that distinct MICU protein N-domains determine the specific type of MICU dimers that perform the opposing roles in mitochondrial Ca²⁺ uptake at low cytosolic Ca²⁺ levels. In contrast, at high cytosolic Ca²⁺ levels, all MICU proteins undergo dimer rearrangement induced by Ca²⁺ binding, which releases the suppression of the MCUC pore-forming subunit and promotes the influx of large amounts of Ca²⁺. Altogether, our results elucidate the delicate mechanism of mitochondrial Ca²⁺ uptake regulation by MICU proteins.

Structure of the MICU1-MICU2 heterodimer provides insights into the gatekeeping threshold shift

Mitochondrial calcium uptake proteins 1 and 2 (MICU1 and MICU2) mediate mitochondrial Ca2+ influx via the mitochondrial calcium uniporter (MCU). Its molecular action for Ca2+ uptake is tightly controlled by the MICU1-MICU2 heterodimer, which comprises Ca2+ sensing proteins which act as gatekeepers at low [Ca2+] or facilitators at high [Ca2+]. However, the mechanism underlying the regulation of the Ca2+ gatekeeping threshold for mitochondrial Ca2+ uptake through the MCU by the MICU1-MICU2 heterodimer remains unclear. In this study, we determined the crystal structure of the apo form of the human MICU1-MICU2 heterodimer that functions as the MCU gatekeeper. MICU1 and MICU2 assemble in the face-to-face heterodimer with salt bridges and me-thio-nine knobs stabilizing the heterodimer in an apo state. Structural analysis suggests how the heterodimer sets a higher Ca2+ threshold than the MICU1 homodimer. The structure of the heterodimer in the apo state provides a framework for understanding the gatekeeping role of the MICU1-MICU2 heterodimer.

Role for calcium signaling in manganese neurotoxicity

BACKGROUND

Calcium is an essential macronutrient that is involved in many cellular processes. Homeostatic control of intracellular levels of calcium ions [Ca²⁺] is vital to maintaining cellular structure and function. Several signaling molecules are involved in regulating Ca²⁺ levels in cells and perturbation of calcium signaling processes is implicated in several neurodegenerative and neurologic conditions. Manganese [Mn] is a metal which is essential for basic physiological functions. However, overexposure to Mn from environmental contamination and workplace hazards is a global concern. Mn overexposure leads to its accumulation in several human organs particularly the brain. Mn accumulation in the brain results in a manganism, a Parkinsonian-like syndrome. Additionally, Mn is a risk factor for several neurodegenerative diseases including Parkinson's disease and Alzheimer's disease. Mn neurotoxicity also affects several neurotransmitter systems including dopaminergic, cholinergic and GABAergic. The mechanisms of Mn neurotoxicity are still being elucidated.

AIM

The review will highlight a potential role for calcium signaling molecules in the mechanisms of Mn neurotoxicity.

CONCLUSION

Ca²⁺ regulation influences the neurodegenerative process and there is possible role for perturbed calcium signaling in Mn neurotoxicity. Mechanisms implicated in Mn-induced neurodegeneration include oxidative stress, generation of free radicals, and apoptosis. These are influenced by mitochondrial integrity which can be dependent on intracellular Ca²⁺ homeostasis. Nevertheless, further elucidation of the direct effects of calcium signaling dysfunction and calcium-binding proteins activities in Mn neurotoxicity is required.

Development and Application of Sub-Mitochondrial Targeted Ca2 + Biosensors

Mitochondrial Ca2+ uptake into the mitochondrial matrix is a well-established mechanism. However, the sub-organellar Ca2+ kinetics remain elusive. In the present work we identified novel site-specific targeting sequences for the intermembrane space (IMS) and the cristae lumen (CL). We used these novel targeting peptides to develop green- and red- Ca2+ biosensors targeted to the IMS and to the CL. Based on their distinctive spectral properties, and comparable sensitivities these novel constructs were suitable to visualize Ca2+-levels in various (sub) compartments in a multi-chromatic manner. Functional studies that applied these new biosensors revealed that knockdown of MCU and EMRE yielded elevated Ca2+ levels inside the CL but not the IMS in response to IP3-generating agonists. Knockdown of VDAC1, however, strongly impeded the transfer of Ca2+ through the OMM while the cytosolic Ca2+ signal remained unchanged. The novel sub-mitochondrially targeted Ca2+ biosensors proved to be suitable for Ca2+ imaging with high spatial and temporal resolution in a multi-chromatic manner allowing simultaneous measurements. These informative biosensors will facilitate efforts to dissect the complex sub-mitochondrial Ca2+ signaling under (patho)physiological conditions.

MICU1 controls cristae junction and spatially anchors mitochondrial Ca2+ uniporter complex

Recently identified core proteins (MICU1, MCU, EMRE) forming the mitochondrial Ca²⁺ uniporter complex propelled investigations into its physiological workings. Here, we apply structured illumination microscopy to visualize and localize these proteins in living cells. Our data show that MICU1 localizes at the inner boundary membrane (IBM) due to electrostatic interaction of its polybasic domain. Moreover, this exclusive localization of MICU1 is important for the stability of cristae junctions (CJ), cytochrome c release and mitochondrial membrane potential. In contrast to MICU1, MCU and EMRE are homogeneously distributed at the inner mitochondrial membrane under resting conditions. However, upon Ca²⁺ elevation MCU and EMRE dynamically accumulate at the IBM in a MICU1-dependent manner. Eventually, our findings unveil an essential function of MICU1 in CJ stabilization and provide mechanistic insights of how sophistically MICU1 controls the MCU-Complex while maintaining the structural mitochondrial membrane framework.

The crystal structure of MICU2 provides insight into Ca2+ binding and MICU1-MICU2 heterodimer formation

The mitochondrial calcium uniporter (MCU) complex mediates the uptake of Ca²⁺ into mitochondria. Its activity is regulated by a heterodimer of MICU1 and MICU2, two EF-hand-containing proteins that act as the main gatekeeper of the uniporter. Herein we report the crystal structure of human MICU2 at 1.96 Å resolution. Our structure reveals a dimeric architecture of MICU2, in which each monomer adopts the canonical two-lobe structure with a pair of EF-hands in each lobe. Both Ca²⁺ -bound and Ca²⁺ -free EF-hands are observed in our structure. Moreover, we characterize the interaction sites within the MICU2 homodimer, as well as the MICU1-MICU2 heterodimer in both Ca²⁺ -free and Ca²⁺ -bound conditions. Glu242 in MICU1 and Arg352 in MICU2 are crucial for apo heterodimer formation, while Phe383 in MICU1 and Glu196 in MICU2 significantly contribute to the interaction in the Ca²⁺ -bound state. Based on our structural and biochemical analyses, we propose a model for MICU1-MICU2 heterodimer formation and its conformational transition from apo to a more compact Ca²⁺ -bound state, which expands our understanding of this co-regulatory mechanism critical for MCU's mitochondrial calcium uptake function.

Crosstalk between Calcium and ROS in Pathophysiological Conditions

Calcium ions are highly versatile intracellular signals that regulate many cellular processes. The key to achieving this pleiotropic role is the spatiotemporal control of calcium concentration evoked by an extensive molecular repertoire of signalling components. Among these, reactive oxygen species (ROS) signalling, together with calcium signalling, plays a crucial role in controlling several physiopathological events. Although initially considered detrimental by-products of aerobic metabolism, it is now widely accepted that ROS, in subtoxic levels, act as signalling molecules. However, dysfunctions in the mechanisms controlling the physiological ROS concentration affect cellular homeostasis, leading to the pathogenesis of various disorders.

AMPK-mediated activation of MCU stimulates mitochondrial Ca2+ entry to promote mitotic progression

The capacity of cells to alter bioenergetics in response to the demands of various biological processes is essential for normal physiology. The coordination of energy sensing and production with highly energy-demanding cellular processes, such as cell division, is poorly understood. Here, we show that a cell cycle-dependent mitochondrial Ca²⁺ transient connects energy sensing to mitochondrial activity for mitotic progression. The mitochondrial Ca²⁺ uniporter (MCU) mediates a rapid mitochondrial Ca²⁺ transient during mitosis. Inhibition of mitochondrial Ca²⁺ transients via MCU depletion causes spindle checkpoint-dependent mitotic delay. Cellular ATP levels drop during early mitosis, and the mitochondrial Ca²⁺ transients boost mitochondrial respiration to restore energy homeostasis. This is achieved through mitosis-specific MCU phosphorylation and activation by the mitochondrial translocation of energy sensor AMP-activated protein kinase (AMPK). Our results establish a critical role for AMPK- and MCU-dependent mitochondrial Ca²⁺ signalling in mitosis and reveal a mechanism of mitochondrial metabolic adaptation to acute cellular energy stress.

Redox regulation of ER and mitochondrial Ca2+ signaling in cell survival and death

Physiological signaling by reactive oxygen species (ROS) and their pathophysiological role in cell death are well recognized. This review focuses on two ROS targets that are key to local Ca²⁺ signaling at the ER/mitochondrial interface - notably, inositol trisphosphate receptors (IP₃Rs) and the mitochondrial calcium uniporter (MCU). Both transport systems are central to molecular mechanisms in cell survival and death. Methods for the measurement of the redox state of these proteins and for the detection of ROS nanodomains are described. Recent results on the redox regulation of these proteins are reviewed.