Systematic Identification of MCU Modulators by Orthogonal Interspecies Chemical Screening

Myocardial transcriptomic analysis of diabetic patients with aortic stenosis: key role for mitochondrial calcium signaling

BACKGROUND

Type 2 diabetes (T2D) is a frequent comorbidity encountered in patients with severe aortic stenosis (AS), leading to an adverse left ventricular (LV) remodeling and dysfunction. Metabolic alterations have been suggested as contributors of the deleterious effect of T2D on LV remodeling and function in patients with severe AS, but so far, the underlying mechanisms remain unclear. Mitochondria play a central role in the regulation of cardiac energy metabolism.

OBJECTIVES

We aimed to explore the mitochondrial alterations associated with the deleterious effect of T2D on LV remodeling and function in patients with AS, preserved ejection fraction, and no additional heart disease.

METHODS

We combined an in-depth clinical, biological and echocardiography phenotype of patients with severe AS, with (n = 34) or without (n = 50) T2D, referred for a valve replacement, with transcriptomic and histological analyses of an intra-operative myocardial LV biopsy.

RESULTS

T2D patients had similar AS severity but displayed worse cardiac remodeling, systolic and diastolic function than non-diabetics. RNAseq analysis identified 1029 significantly differentially expressed genes. Functional enrichment analysis revealed several T2D-specific upregulated pathways despite comorbidity adjustment, gathering regulation of inflammation, extracellular matrix organization, endothelial function/angiogenesis, and adaptation to cardiac hypertrophy. Downregulated gene sets independently associated with T2D were related to mitochondrial respiratory chain organization/function and mitochondrial organization. Generation of causal networks suggested a reduced Ca2+ signaling up to the mitochondria, with the measured gene remodeling of the mitochondrial Ca2+ uniporter in favor of enhanced uptake. Histological analyses supported a greater cardiomyocyte hypertrophy and a decreased proximity between the mitochondrial VDAC porin and the reticular IP3-receptor in T2D.

CONCLUSIONS

Our data support a crucial role for mitochondrial Ca2+ signaling in T2D-induced cardiac dysfunction in severe AS patients, from a structural reticulum-mitochondria Ca2+ uncoupling to a mitochondrial gene remodeling. Thus, our findings open a new therapeutic avenue to be tested in animal models and further human cardiac biopsies in order to propose new treatments for T2D patients suffering from AS.

TRIAL REGISTRATION

URL: https://www.

CLINICALTRIALS

gov ; Unique Identifier: NCT01862237.

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.

Unravelling the complexity of the mitochondrial Ca2+ uniporter: regulation, tissue specificity, and physiological implications

Calcium (Ca2+) signalling acts a pleiotropic message within the cell that is decoded by the mitochondria through a sophisticated ion channel known as the Mitochondrial Ca2+ Uniporter (MCU) complex. Under physiological conditions, mitochondrial Ca2+ signalling is crucial for coordinating cell activation with energy production. Conversely, in pathological scenarios, it can determine the fine balance between cell survival and death. Over the last decade, significant progress has been made in understanding the molecular bases of mitochondrial Ca2+ signalling. This began with the elucidation of the MCU channel components and extended to the elucidation of the mechanisms that regulate its activity. Additionally, increasing evidence suggests molecular mechanisms allowing tissue-specific modulation of the MCU complex, tailoring channel activity to the specific needs of different tissues or cell types. This review aims to explore the latest evidence elucidating the regulation of the MCU complex, the molecular factors controlling the tissue-specific properties of the channel, and the physiological and pathological implications of mitochondrial Ca2+ signalling in different tissues.

The mitochondrial calcium uniporter: Balancing tumourigenic and anti-tumourigenic responses

Increased malignancy and poor treatability associated with solid tumour cancers have commonly been attributed to mitochondrial calcium (Ca2+) dysregulation. The mitochondrial Ca2+ uniporter complex (mtCU) is the predominant mode of Ca2+ uptake into the mitochondrial matrix. The main components of mtCU are the pore-forming mitochondrial Ca2+ uniporter (MCU) subunit, MCU dominant-negative beta (MCUb) subunit, essential MCU regulator (EMRE) and the gatekeeping mitochondrial Ca2+ uptake 1 and 2 (MICU1 and MICU2) proteins. In this review, we describe mtCU-mediated mitochondrial Ca2+ dysregulation in solid tumour cancer types, finding enhanced mtCU activity observed in colorectal cancer, breast cancer, oral squamous cell carcinoma, pancreatic cancer, hepatocellular carcinoma and embryonal rhabdomyosarcoma. By contrast, decreased mtCU activity is associated with melanoma, whereas the nature of mtCU dysregulation remains unclear in glioblastoma. Furthermore, we show that numerous polymorphisms associated with cancer may alter phosphorylation sites on the pore forming MCU and MCUb subunits, which cluster at interfaces with EMRE. We highlight downstream/upstream biomolecular modulators of MCU and MCUb that alter mtCU-mediated mitochondrial Ca2+ uptake and may be used as biomarkers or to aid in the development of novel cancer therapeutics. Additionally, we provide an overview of the current small molecule inhibitors of mtCU that interact with the Asp residue of the critical Asp-Ile-Met-Glu motif or through other allosteric regulatory mechanisms to block Ca2+ permeation. Finally, we describe the relationship between MCU- and MCUb-mediating microRNAs and mitochondrial Ca2+ uptake that should be considered in the discovery of new treatment approaches for cancer.

The mitochondrial calcium uniporter inhibitor Ru265 increases neuronal excitability and reduces neurotransmission via off-target effects

BACKGROUND AND PURPOSE

Excitotoxicity due to mitochondrial calcium (Ca2+) overloading can trigger neuronal cell death in a variety of pathologies. Inhibiting the mitochondrial calcium uniporter (MCU) has been proposed as a therapeutic avenue to prevent calcium overloading. Ru265 (ClRu(NH3)4(μ-N)Ru(NH3)4Cl]Cl3) is a cell-permeable inhibitor of the mitochondrial calcium uniporter (MCU) with nanomolar affinity. Ru265 reduces sensorimotor deficits and neuronal death in models of ischemic stroke. However, the therapeutic use of Ru265 is limited by the induction of seizure-like behaviours.

EXPERIMENTAL APPROACH

We examined the effect of Ru265 on synaptic and neuronal function in acute brain slices and hippocampal neuron cultures derived from mice, in control and where MCU expression was genetically abrogated.

KEY RESULTS

Ru265 decreased evoked responses from calyx terminals and induced spontaneous action potential firing of both the terminal and postsynaptic principal cell. Recordings of presynaptic Ca2+ currents suggested that Ru265 blocks the P/Q type channel, confirmed by the inhibition of currents in cells exogenously expressing the P/Q type channel. Measurements of presynaptic K+ currents further revealed that Ru265 blocked a KCNQ current, leading to increased membrane excitability, underlying spontaneous spiking. Ca2+ imaging of hippocampal neurons showed that Ru265 increased synchronized, high-amplitude events, recapitulating seizure-like activity seen in vivo. Importantly, MCU ablation did not suppress Ru265-induced increases in neuronal activity and seizures.

CONCLUSIONS AND IMPLICATIONS

Our findings provide a mechanistic explanation for the pro-convulsant effects of Ru265 and suggest counter screening assays based on the measurement of P/Q and KCNQ channel currents to identify safe MCU inhibitors.

MCU complex: Exploring emerging targets and mechanisms of mitochondrial physiology and pathology

BACKGROUND

Globally, the onset and progression of multiple human diseases are associated with mitochondrial dysfunction and dysregulation of Ca2+ uptake dynamics mediated by the mitochondrial calcium uniporter (MCU) complex, which plays a key role in mitochondrial dysfunction. Despite relevant studies, the underlying pathophysiological mechanisms have not yet been fully elucidated.

AIM OF REVIEW

This article provides an in-depth analysis of the current research status of the MCU complex, focusing on its molecular composition, regulatory mechanisms, and association with diseases. In addition, we conducted an in-depth analysis of the regulatory effects of agonists, inhibitors, and traditional Chinese medicine (TCM) monomers on the MCU complex and their application prospects in disease treatment. From the perspective of medicinal chemistry, we conducted an in-depth analysis of the structure-activity relationship between these small molecules and MCU and deduced potential pharmacophores and binding pockets. Simultaneously, key structural domains of the MCU complex in Homo sapiens were identified. We also studied the functional expression of the MCU complex in Drosophila, Zebrafish, and Caenorhabditis elegans. These analyses provide a basis for exploring potential treatment strategies targeting the MCU complex and provide strong support for the development of future precision medicine and treatments.

KEY SCIENTIFIC CONCEPTS OF REVIEW

The MCU complex exhibits varying behavior across different tissues and plays various roles in metabolic functions. It consists of six MCU subunits, an essential MCU regulator (EMRE), and solute carrier 25A23 (SLC25A23). They regulate processes, such as mitochondrial Ca2+ (mCa2+) uptake, mitochondrial adenosine triphosphate (ATP) production, calcium dynamics, oxidative stress (OS), and cell death. Regulation makes it a potential target for treating diseases, especially cardiovascular diseases, neurodegenerative diseases, inflammatory diseases, metabolic diseases, and tumors.

Inhibition of mitochondrial calcium transporters alters adp-induced platelet responses

INTRODUCTION

ADP-stimulated elevation of cytosolic Ca2+ is an important effector mechanism for platelet activation. The rapidly elevating cytosolic Ca2+ is also transported to mitochondrial matrix via Mitochondrial Ca2+ Uniporter (MCU) and extruded via Na+/Ca2+/Li+ Exchanger (NCLX). However, the exact contribution of MCU and NCLX in ADP-mediated platelet responses remains incompletely understood.

METHODS AND RESULTS

The present study aimed to elucidate the role of mitochondrial Ca2+ transport in ADP-stimulated platelet responses by inhibition of MCU and NCLX with mitoxantrone (MTX) and CGP37157 (CGP), respectively. As these inhibitory strategies are reported to cause distinct effects on matrix Ca2+ concentration, we hypothesized to observe opposite impact of MTX and CGP on ADP-induced platelet responses. Platelet aggregation profiling was performed by microplate-based spectrophotometery while p-selectin externalization and integrin αIIbβ3 activation were analyzed by fluorescent immunolabeling using flow cytometery. Our results confirmed the expression of both MCU and NCLX mRNAs with relatively low abundance of NCLX in human platelets. In line with our hypothesis, MTX caused a dose-dependent inhibition of ADP-induced platelet aggregation without displaying any cytotoxicity. Likewise, ADP-induced p-selectin externalization and integrin αIIbβ3 activation was also significantly attenuated in MTX-treated platelets. Concordantly, inhibition of NCLX with CGP yielded an accelerated ADP-stimulated platelet aggregation which was associated with an elevation of p-selectin surface expression and αIIbβ3 activation.

CONCLUSION

Together, these findings uncover a vital and hitherto poorly characterized role of mitochondrial Ca2+ transporters in ADP-induced platelet activation.

Accumulation of endogenous free radicals is required to induce titan-like cell formation in Cryptococcus neoformans

IMPORTANCE

Cryptococcus neoformans is an excellent model to investigate fungal pathogenesis. This yeast can produce "titan cells," which are cells of an abnormally larger size that contribute to the persistence of the yeast in the host. In this work, we have used a new approach to characterize them by identifying drugs that inhibit this process. We have used a repurposing off-patent drug library, combined with an automatic method to image and analyze fungal cell size. In this way, we have identified many compounds that inhibit this transition. Interestingly, several compounds were antioxidants, allowing us to confirm that endogenous ROS and mitochondrial changes are important for titan cell formation. This work provides new evidence of the mechanisms required for titanization. Furthermore, the future characterization of the inhibitory mechanisms of the identified compounds by the scientific community will contribute to better understand the role of titan cells in virulence.

Negative modulation of mitochondrial calcium uniporter complex protects neurons against ferroptosis

Ferroptosis is an iron- and reactive oxygen species (ROS)-dependent form of regulated cell death, that has been implicated in Alzheimer's disease and Parkinson's disease. Inhibition of cystine/glutamate antiporter could lead to mitochondrial fragmentation, mitochondrial calcium ([Ca2+]m) overload, increased mitochondrial ROS production, disruption of the mitochondrial membrane potential (ΔΨm), and ferroptotic cell death. The observation that mitochondrial dysfunction is a characteristic of ferroptosis makes preservation of mitochondrial function a potential therapeutic option for diseases associated with ferroptotic cell death. Mitochondrial calcium levels are controlled via the mitochondrial calcium uniporter (MCU), the main entry point of Ca2+ into the mitochondrial matrix. Therefore, we have hypothesized that negative modulation of MCU complex may confer protection against ferroptosis. Here we evaluated whether the known negative modulators of MCU complex, ruthenium red (RR), its derivative Ru265, mitoxantrone (MX), and MCU-i4 can prevent mitochondrial dysfunction and ferroptotic cell death. These compounds mediated protection in HT22 cells, in human dopaminergic neurons and mouse primary cortical neurons against ferroptotic cell death. Depletion of MICU1, a [Ca2+]m gatekeeper, demonstrated that MICU is protective against ferroptosis. Taken together, our results reveal that negative modulation of MCU complex represents a therapeutic option to prevent degenerative conditions, in which ferroptosis is central to the progression of these pathologies.

Electro-metabolic coupling in multi-chambered vascularized human cardiac organoids

The study of cardiac physiology is hindered by physiological differences between humans and small-animal models. Here we report the generation of multi-chambered self-paced vascularized human cardiac organoids formed under anisotropic stress and their applicability to the study of cardiac arrhythmia. Sensors embedded in the cardiac organoids enabled the simultaneous measurement of oxygen uptake, extracellular field potentials and cardiac contraction at resolutions higher than 10 Hz. This microphysiological system revealed 1 Hz cardiac respiratory cycles that are coupled to the electrical rather than the mechanical activity of cardiomyocytes. This electro-mitochondrial coupling was driven by mitochondrial calcium oscillations driving respiration cycles. Pharmaceutical or genetic inhibition of this coupling results in arrhythmogenic behaviour. We show that the chemotherapeutic mitoxantrone induces arrhythmia through disruption of this pathway, a process that can be partially reversed by the co-administration of metformin. Our microphysiological cardiac systems may further facilitate the study of the mitochondrial dynamics of cardiac rhythms and advance our understanding of human cardiac physiology.

Targeting Acute Myeloid Leukemia Stem Cells Through Perturbation of Mitochondrial Calcium

We previously reported that acute myeloid leukemia stem cells (LSCs) are uniquely reliant on oxidative phosphorylation (OXPHOS) for survival. Moreover, maintenance of OXPHOS is dependent on BCL2, creating a therapeutic opportunity to target LSCs using the BCL2 inhibitor drug venetoclax. While venetoclax-based regimens have indeed shown promising clinical activity, the emergence of drug resistance is prevalent. Thus, in the present study, we investigated how mitochondrial properties may influence mechanisms that dictate venetoclax responsiveness. Our data show that utilization of mitochondrial calcium is fundamentally different between drug responsive and non-responsive LSCs. By comparison, venetoclax-resistant LSCs demonstrate a more active metabolic (i.e., OXPHOS) status with relatively high steady-state levels of calcium. Consequently, we tested genetic and pharmacological approaches to target the mitochondrial calcium uniporter, MCU. We demonstrate that inhibition of calcium uptake sharply reduces OXPHOS and leads to eradication of venetoclax-resistant LSCs. These findings demonstrate a central role for calcium signaling in the biology of LSCs and provide a therapeutic avenue for clinical management of venetoclax resistance.

Mitochondrial Calcium Overload Plays a Causal Role in Oxidative Stress in the Failing Heart

Heart failure is a serious global health challenge, affecting more than 6.2 million people in the United States and is projected to reach over 8 million by 2030. Independent of etiology, failing hearts share common features, including defective calcium (Ca2+) handling, mitochondrial Ca2+ overload, and oxidative stress. In cardiomyocytes, Ca2+ not only regulates excitation-contraction coupling, but also mitochondrial metabolism and oxidative stress signaling, thereby controlling the function and actual destiny of the cell. Understanding the mechanisms of mitochondrial Ca2+ uptake and the molecular pathways involved in the regulation of increased mitochondrial Ca2+ influx is an ongoing challenge in order to identify novel therapeutic targets to alleviate the burden of heart failure. In this review, we discuss the mechanisms underlying altered mitochondrial Ca2+ handling in heart failure and the potential therapeutic strategies.

Beyond the TCA cycle: new insights into mitochondrial calcium regulation of oxidative phosphorylation

While mitochondria oxidative phosphorylation is broadly regulated, the impact of mitochondrial Ca2+ on substrate flux under both physiological and pathological conditions is increasingly being recognized. Under physiologic conditions, mitochondrial Ca2+ enters through the mitochondrial Ca2+ uniporter and boosts ATP production. However, maintaining Ca2+ homeostasis is crucial as too little Ca2+ inhibits adaptation to stress and Ca2+ overload can trigger cell death. In this review, we discuss new insights obtained over the past several years expanding the relationship between mitochondrial Ca2+ and oxidative phosphorylation, with most data obtained from heart, liver, or skeletal muscle. Two new themes are emerging. First, beyond boosting ATP synthesis, Ca2+ appears to be a critical determinant of fuel substrate choice between glucose and fatty acids. Second, Ca2+ exerts local effects on the electron transport chain indirectly, not via traditional allosteric mechanisms. These depend critically on the transporters involved, such as the uniporter or the Na+-Ca2+ exchanger. Alteration of these new relationships during disease can be either compensatory or harmful and suggest that targeting mitochondrial Ca2+ may be of therapeutic benefit during diseases featuring impairments in oxidative phosphorylation.

Increased mitochondrial free Ca2+ during ischemia is suppressed, but not eliminated by, germline deletion of the mitochondrial Ca2+ uniporter

Mitochondrial Ca2+ overload is proposed to regulate cell death via opening of the mitochondrial permeability transition pore. It is hypothesized that inhibition of the mitochondrial Ca2+ uniporter (MCU) will prevent Ca2+ accumulation during ischemia/reperfusion and thereby reduce cell death. To address this, we evaluate mitochondrial Ca2+ in ex-vivo-perfused hearts from germline MCU-knockout (KO) and wild-type (WT) mice using transmural spectroscopy. Matrix Ca2+ levels are measured with a genetically encoded, red fluorescent Ca2+ indicator (R-GECO1) using an adeno-associated viral vector (AAV9) for delivery. Due to the pH sensitivity of R-GECO1 and the known fall in pH during ischemia, hearts are glycogen depleted to decrease the ischemic fall in pH. At 20 min of ischemia, there is significantly less mitochondrial Ca2+ in MCU-KO hearts compared with MCU-WT controls. However, an increase in mitochondrial Ca2+ is present in MCU-KO hearts, suggesting that mitochondrial Ca2+ overload during ischemia is not solely dependent on MCU.

Structure-Activity Relationships of Metal-Based Inhibitors of the Mitochondrial Calcium Uniporter

The mitochondrial calcium uniporter (MCU) is a transmembrane protein that is responsible for mediating mitochondrial calcium (mCa2+ ) uptake. Given this critical function, the MCU has been implicated as an important target for addressing various human diseases. As such, there has a been growing interest in developing small molecules that can inhibit this protein. To date, metal coordination complexes, particularly multinuclear ruthenium complexes, are the most widely investigated MCU inhibitors due to both their potent inhibitory activities as well as their longstanding use for this application. Recent efforts have expanded the metal-based toolkit for MCU inhibition. This concept paper summarizes the development of new metal-based inhibitors of the MCU and their structure-activity relationships in the context of improving their potential for therapeutic use in managing human diseases related to mCa2+ dysregulation.

MICU1 controls the sensitivity of the mitochondrial Ca2+ uniporter to activators and inhibitors

Mitochondrial Ca2+ homeostasis loses its control in many diseases and might provide therapeutic targets. Mitochondrial Ca2+ uptake is mediated by the uniporter channel (mtCU), formed by MCU and is regulated by the Ca2+-sensing gatekeeper, MICU1, which shows tissue-specific stoichiometry. An important gap in knowledge is the molecular mechanism of the mtCU activators and inhibitors. We report that all pharmacological activators of the mtCU (spermine, kaempferol, SB202190) act in a MICU1-dependent manner, likely by binding to MICU1 and preventing MICU1's gatekeeping activity. These agents also sensitized the mtCU to inhibition by Ru265 and enhanced the Mn2+-induced cytotoxicity as previously seen with MICU1 deletion. Thus, MCU gating by MICU1 is the target of mtCU agonists and is a barrier for inhibitors like RuRed/Ru360/Ru265. The varying MICU1:MCU ratios result in different outcomes for both mtCU agonists and antagonists in different tissues, which is relevant for both pre-clinical research and therapeutic efforts.

The mitochondrial calcium uniporter (MCU) activates mitochondrial respiration and enhances mobility by regulating mitochondrial redox state

Regulation of mitochondrial redox balance is emerging as a key event for cell signaling in both physiological and pathological conditions. However, the link between the mitochondrial redox state and the modulation of these conditions remains poorly defined. Here, we discovered that activation of the evolutionary conserved mitochondrial calcium uniporter (MCU) modulates mitochondrial redox state. By using mitochondria-targeted redox and calcium sensors and genetic MCU-ablated models, we provide evidence of the causality between MCU activation and net reduction of mitochondrial (but not cytosolic) redox state. Redox modulation of redox-sensitive groups via MCU stimulation is required for maintaining respiratory capacity in primary human myotubes and C. elegans, and boosts mobility in worms. The same benefits are obtained bypassing MCU via direct pharmacological reduction of mitochondrial proteins. Collectively, our results demonstrate that MCU regulates mitochondria redox balance and that this process is required to promote the MCU-dependent effects on mitochondrial respiration and mobility.

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.

Mitochondrial calcium signaling mediated transcriptional regulation of keratin filaments is a critical determinant of melanogenesis

Mitochondria are versatile organelles that regulate several physiological functions. Many mitochondria-controlled processes are driven by mitochondrial Ca2+ signaling. However, role of mitochondrial Ca2+ signaling in melanosome biology remains unknown. Here, we show that pigmentation requires mitochondrial Ca2+ uptake. In vitro gain and loss of function studies demonstrated that Mitochondrial Ca2+ Uniporter (MCU) is crucial for melanogenesis while the MCU rheostats, MCUb and MICU1 negatively control melanogenesis. Zebrafish and mouse models showed that MCU plays a vital role in pigmentation in vivo. Mechanistically, MCU controls activation of transcription factor NFAT2 to induce expression of three keratins (keratin 5, 7 and 8), which we report as positive regulators of melanogenesis. Interestingly, keratin 5 in turn modulates mitochondrial Ca2+ uptake thereby this signaling module acts as a negative feedback loop that fine-tunes both mitochondrial Ca2+ signaling and melanogenesis. Mitoxantrone, an FDA approved drug that inhibits MCU, decreases physiological melanogenesis. Collectively, our data demonstrates a critical role for mitochondrial Ca2+ signaling in vertebrate pigmentation and reveal the therapeutic potential of targeting MCU for clinical management of pigmentary disorders. Given the centrality of mitochondrial Ca2+ signaling and keratin filaments in cellular physiology, this feedback loop may be functional in a variety of other pathophysiological conditions.

Mitochondrial Ion Channels

Mitochondria are involved in multiple cellular tasks, such as ATP synthesis, metabolism, metabolite and ion transport, regulation of apoptosis, inflammation, signaling, and inheritance of mitochondrial DNA. The majority of the correct functioning of mitochondria is based on the large electrochemical proton gradient, whose component, the inner mitochondrial membrane potential, is strictly controlled by ion transport through mitochondrial membranes. Consequently, mitochondrial function is critically dependent on ion homeostasis, the disturbance of which leads to abnormal cell functions. Therefore, the discovery of mitochondrial ion channels influencing ion permeability through the membrane has defined a new dimension of the function of ion channels in different cell types, mainly linked to the important tasks that mitochondrial ion channels perform in cell life and death. This review summarizes studies on animal mitochondrial ion channels with special focus on their biophysical properties, molecular identity, and regulation. Additionally, the potential of mitochondrial ion channels as therapeutic targets for several diseases is briefly discussed.

Targeting the mitochondrial Ca2+ uniporter complex in cardiovascular disease

Cardiovascular diseases (CVDs), the leading cause of death worldwide, share in common mitochondrial dysfunction, in specific a dysregulation of Ca²⁺ uptake dynamics through the mitochondrial Ca²⁺ uniporter (MCU) complex. In particular, Ca²⁺ uptake regulates the mitochondrial ATP production, mitochondrial dynamics, oxidative stress, and cell death. Therefore, modulating the activity of the MCU complex to regulate Ca²⁺ uptake, has been suggested as a potential therapeutic approach for the treatment of CVDs. Here, the role and implications of the MCU complex in CVDs are presented, followed by a review of the evidence for MCU complex modulation, genetically and pharmacologically. While most approaches have aimed within the MCU complex for the modulation of the Ca²⁺ pore channel, the MCU subunit, its intra- and extra- mitochondrial implications, including Ca²⁺ dynamics, oxidative stress, post-translational modifications, and its repercussions in the cardiac function, highlight that targeting the MCU complex has the translational potential for novel CVDs therapeutics.

Investigation of Cobalt(III) Cage Complexes as Inhibitors of the Mitochondrial Calcium Uniporter

The mitochondrial calcium uniporter (MCU) mediates uptake of calcium ions (Ca2+) into the mitochondria, a process that is vital for maintaining normal cellular function. Inhibitors of the MCU, the most promising of which are dinuclear ruthenium coordination compounds, have found use as both therapeutic agents and tools for studying the importance of this ion channel. In this study, six Co3+ cage compounds with sarcophagine-like ligands were assessed for their abilities to inhibit MCU-mediated mitochondrial Ca2+ uptake. These complexes were synthesized and characterized according to literature procedures and then investigated in cellular systems for their MCU-inhibitory activities. Among these six compounds, [Co(sen)]3+ (3, sen = 5-(4-amino-2-azabutyl)-5-methyl-3,7-diaza-1,9-nonanediamine) was identified to be a potent MCU inhibitor, with IC50 values of inhibition of 160 and 180 nM in permeabilized HeLa and HEK293T cells, respectively. Furthermore, the cellular uptake of compound 3 was determined, revealing moderate accumulation in cells. Most notably, 3 was demonstrated to operate in intact cells as an MCU inhibitor. Collectively, this work presents the viability of using cobalt coordination complexes as MCU inhibitors, providing a new direction for researchers to investigate in future studies.

Mitochondrial calcium cycling in neuronal function and neurodegeneration

Mitochondria are essential for proper cellular function through their critical roles in ATP synthesis, reactive oxygen species production, calcium (Ca2+) buffering, and apoptotic signaling. In neurons, Ca2+ buffering is particularly important as it helps to shape Ca2+ signals and to regulate numerous Ca2+-dependent functions including neuronal excitability, synaptic transmission, gene expression, and neuronal toxicity. Over the past decade, identification of the mitochondrial Ca2+ uniporter (MCU) and other molecular components of mitochondrial Ca2+ transport has provided insight into the roles that mitochondrial Ca2+ regulation plays in neuronal function in health and disease. In this review, we discuss the many roles of mitochondrial Ca2+ uptake and release mechanisms in normal neuronal function and highlight new insights into the Ca2+-dependent mechanisms that drive mitochondrial dysfunction in neurologic diseases including epilepsy, Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. We also consider how targeting Ca2+ uptake and release mechanisms could facilitate the development of novel therapeutic strategies for neurological diseases.

Dinuclear nitrido-bridged osmium complexes inhibit the mitochondrial calcium uniporter and protect cortical neurons against lethal oxygen-glucose deprivation

Dysregulation of mitochondrial calcium uptake mediated by the mitochondrial calcium uniporter (MCU) is implicated in several pathophysiological conditions. Dinuclear ruthenium complexes are effective inhibitors of the MCU and have been leveraged as both tools to study mitochondrial calcium dynamics and potential therapeutic agents. In this study, we report the synthesis and characterization of Os245 ([Os₂(μ-N)(NH₃)₈Cl₂]³⁺) which is the osmium-containing analogue of our previously reported ruthenium-based inhibitor Ru265. This complex and its aqua-capped analogue Os245' ([Os₂(μ-N)(NH₃)₈(OH₂)₂]⁵⁺) are both effective inhibitors of the MCU in permeabilized and intact cells. In comparison to the ruthenium-based inhibitor Ru265 (k _obs = 4.92 × 10^-3 s^-1), the axial ligand exchange kinetics of Os245 are two orders of magnitude slower (k _obs = 1.63 × 10^-5 s^-1) at 37 °C. The MCU-inhibitory properties of Os245 and Os245' are different (Os245 IC₅₀ for MCU inhibition = 103 nM; Os245' IC₅₀ for MCU inhibition = 2.3 nM), indicating that the axial ligands play an important role in their interactions with this channel. We further show that inhibition of the MCU by these complexes protects primary cortical neurons against lethal oxygen-glucose deprivation. When administered in vivo to mice (10 mg kg^-1), Os245 and Os245' induce seizure-like behaviors in a manner similar to the ruthenium-based inhibitors. However, the onset of these seizures is delayed, a possible consequence of the slower ligand substitution kinetics for these osmium complexes. These findings support previous studies that demonstrate inhibition of the MCU is a promising therapeutic strategy for the treatment of ischemic stroke, but also highlight the need for improved drug delivery strategies to mitigate the pro-convulsant effects of this class of complexes before they can be implemented as therapeutic agents. Furthermore, the slower ligand substitution kinetics of the osmium analogues may afford new strategies for the development and modification of this class of MCU inhibitors.

Miro1 R272Q disrupts mitochondrial calcium handling and neurotransmitter uptake in dopaminergic neurons

The Rho GTPase Miro1, located at the mitochondrial outer membrane is known to properly distribute mitochondria to synapses, aid calcium buffering and initiate PINK1-Parkin mediated mitophagy. Several heterozygous RHOT1/Miro1 variants were identified in sporadic Parkinson's disease patients. Miro1 R272Q is located within a calcium binding domain, but the functional outcome of this point mutation and its contribution to the development of disease are unclear. To address this, we introduced a heterozygous RHOT1/Miro1 R272Q point mutation in healthy induced pluripotent stem cells. In dopaminergic neurons, Miro1 R272Q does not affect Miro1 protein levels, CCCP-induced mitophagy, nor mitochondrial movement yet causes the fragmentation of mitochondria with reduction of cristae and ATP5A. Inhibition of the mitochondrial calcium uniporter phenocopied Miro1 R272Q cytosolic calcium response to Thapsigargin in active neurons, a similar effect was observed during the calcium buffering phase in Miro1 knockdown neuroblastoma cells. Altered mitochondrial calcium regulation is associated with reduced mitochondrial respiration and reduced catecholamine neurotransmitter uptake. Synaptic changes are not coupled to dopamine distribution or dopamine transporters but are linked to Miro1 R272Q-related calcium handling via the mitochondria concomitant with defective dopamine regulation at the mitochondrial surface by monoamine oxidase. We conclude that the Miro1 R272Q heterozygous point mutation dampens mitochondrial-calcium regulation and mitochondrial capacity via events at the outer membrane that are sufficient to disrupt dopaminergic function.

Mitochondrial Ca2+ Signaling and Bioenergetics in Alzheimer's Disease

Alzheimer's disease (AD) is a hereditary and sporadic neurodegenerative illness defined by the gradual and cumulative loss of neurons in specific brain areas. The processes that cause AD are still under investigation and there are no available therapies to halt it. Current progress puts at the forefront the "calcium (Ca2+) hypothesis" as a key AD pathogenic pathway, impacting neuronal, astrocyte and microglial function. In this review, we focused on mitochondrial Ca2+ alterations in AD, their causes and bioenergetic consequences in neuronal and glial cells, summarizing the possible mechanisms linking detrimental mitochondrial Ca2+ signals to neuronal death in different experimental AD models.

Carboxylate-Capped Analogues of Ru265 Are MCU Inhibitor Prodrugs

The mitochondrial calcium uniporter (MCU) is a transmembrane protein that resides on the inner membrane of the mitochondria and mediates calcium uptake into this organelle. Given the critical role of mitochondrial calcium trafficking in cellular function, inhibitors of this channel have arisen as tools for studying the biological relevance of this process and as potential therapeutic agents. In this study, four new analogues of the previously reported Ru-based MCU inhibitor [ClRu(NH3)4(μ-N)Ru(NH3)4Cl]Cl3 (Ru265) are reported. These compounds, which bear axial carboxylate ligands, are of the general formula [(RCO2)Ru(NH3)4(μ-N)Ru(NH3)4(O2CR)]X3, where X = NO3- or CF3SO3- and R = H (1), CH3 (2), CH2CH3 (3), and (CH2)2CH3 (4). These complexes were fully characterized by IR spectroscopy, NMR spectroscopy, and elemental analysis. X-ray crystal structures of 1 and 3 were obtained, revealing the expected presence of both the linear Ru(μ-N)Ru core and axial formate and propionate ligands. The axial carboxylate ligands of complexes 1-4 are displaced by water in buffered aqueous solution to give the aquated compound Ru265'. The kinetics of these processes were measured by 1H NMR spectroscopy, revealing half-lives that span 5.9-9.9 h at 37 °C. Complex 1 with axial formate ligands underwent aquation approximately twice as fast as the other compounds. In vitro cytotoxicity and mitochondrial membrane potential measurements carried out in HeLa and HEK293T cells demonstrated that none of these four complexes negatively affects cell viability or mitochondrial function. The abilities of 1-4 to inhibit mitochondrial calcium uptake in permeabilized HEK293T cells were assessed and compared to that of Ru265. Fresh solutions of 1-4 are approximately 2-fold less potent than Ru265 with IC50 values in the range of 14.7-19.1 nM. Preincubating 1-4 in aqueous buffers for longer time periods to allow for the aquation reactions to proceed increases their potency of mitochondrial uptake inhibition to match that of Ru265. This result indicates that 1-4 are aquation-activated prodrugs of Ru265'. Finally, 1-4 were shown to inhibit mitochondrial calcium uptake in intact, nonpermeabilized cells, revealing their value as tools and potential therapeutic agents for mitochondrial calcium-related disorders.

MCU proteins dominate in vivo mitochondrial Ca2+ uptake in Arabidopsis roots

Ca2+ signaling is central to plant development and acclimation. While Ca2+-responsive proteins have been investigated intensely in plants, only a few Ca2+-permeable channels have been identified, and our understanding of how intracellular Ca2+ fluxes is facilitated remains limited. Arabidopsis thaliana homologs of the mammalian channel-forming mitochondrial calcium uniporter (MCU) protein showed Ca2+ transport activity in vitro. Yet, the evolutionary complexity of MCU proteins, as well as reports about alternative systems and unperturbed mitochondrial Ca2+ uptake in knockout lines of MCU genes, leave critical questions about the in vivo functions of the MCU protein family in plants unanswered. Here, we demonstrate that MCU proteins mediate mitochondrial Ca2+ transport in planta and that this mechanism is the major route for fast Ca2+ uptake. Guided by the subcellular localization, expression, and conservation of MCU proteins, we generated an mcu triple knockout line. Using Ca2+ imaging in living root tips and the stimulation of Ca2+ transients of different amplitudes, we demonstrated that mitochondrial Ca2+ uptake became limiting in the triple mutant. The drastic cell physiological phenotype of impaired subcellular Ca2+ transport coincided with deregulated jasmonic acid-related signaling and thigmomorphogenesis. Our findings establish MCUs as a major mitochondrial Ca2+ entry route in planta and link mitochondrial Ca2+ transport with phytohormone signaling.

MCU controls melanoma progression through a redox‐controlled phenotype switch

Melanoma is the deadliest of skin cancers and has a high tendency to metastasize to distant organs. Calcium and metabolic signals contribute to melanoma invasiveness; however, the underlying molecular details are elusive. The MCU complex is a major route for calcium into the mitochondrial matrix but whether MCU affects melanoma pathobiology was not understood. Here, we show that MCU_A expression correlates with melanoma patient survival and is decreased in BRAF kinase inhibitor‐resistant melanomas. Knockdown (KD) of MCU_A suppresses melanoma cell growth and stimulates migration and invasion. In melanoma xenografts, MCU_{A_KD} reduces tumor volumes but promotes lung metastases. Proteomic analyses and protein microarrays identify pathways that link MCU_A and melanoma cell phenotype and suggest a major role for redox regulation. Antioxidants enhance melanoma cell migration, while prooxidants diminish the MCU_{A_KD}‐induced invasive phenotype. Furthermore, MCU_{A_KD} increases melanoma cell resistance to immunotherapies and ferroptosis. Collectively, we demonstrate that MCU_A controls melanoma aggressive behavior and therapeutic sensitivity. Manipulations of mitochondrial calcium and redox homeostasis, in combination with current therapies, should be considered in treating advanced melanoma.

Mitoxantrone alleviates hepatic steatosis induced by high-fat diet in broilers

Metabolic-dysfunction-associated fatty liver disease (MAFLD) is a common nutritional metabolic disease in poultry that seriously compromises the health of chickens and reduces the economic benefits of the industry. In this study, we investigated the therapeutic effect of mitoxantrone (MTX) on hepatic steatosis in broilers. We constructed a steatosis cell model in vitro by adding oleic acid and palmitic acid to chicken hepatocytes (LMH cells), to examine influence of MTX on fat deposition on LMH cells. To determine the effects of MTX on hepatic steatosis in broiler livers in vivo, broilers were fed a high-fat diet to establish a fatty liver model. Our data show that MTX reduced the triglyceride (TG) levels and total cholesterol levels in LMH cells. In the MAFLD chick model, MTX decreased mRNA abundance of hepatic-lipid-synthesis-related gene such as FASN and increased mRNA abundance of fatty-acid-β-oxidation-related genes such as CPT1, PPARα, and reduced hepatic TG levels. MTX also reduced serum lipid and the percentage of abdominal fat. These results suggest that MTX improves hepatic steatosis in broilers as well as reduces circulating lipid levels and fat accumulation in broilers. Our work provides a promising therapeutic strategy for MAFLD and excessive fat accumulation in broiler chickens.

Mitochondrial calcium uniporter deletion prevents painful diabetic neuropathy by restoring mitochondrial morphology and dynamics

ABSTRACT

Painful diabetic neuropathy (PDN) is an intractable complication affecting 25% of diabetic patients. Painful diabetic neuropathy is characterized by neuropathic pain accompanied by dorsal root ganglion (DRG) nociceptor hyperexcitability, resulting in calcium overload, axonal degeneration, and loss of cutaneous innervation. The molecular pathways underlying these effects are unknown. Using high-throughput and deep-proteome profiling, we found that mitochondrial fission proteins were elevated in DRG neurons from mice with PDN induced by a high-fat diet (HFD). In vivo calcium imaging revealed increased calcium signaling in DRG nociceptors from mice with PDN. Furthermore, using electron microscopy, we showed that mitochondria in DRG nociceptors had fragmented morphology as early as 2 weeks after starting HFD, preceding the onset of mechanical allodynia and small-fiber degeneration. Moreover, preventing calcium entry into the mitochondria, by selectively deleting the mitochondrial calcium uniporter from these neurons, restored normal mitochondrial morphology, prevented axonal degeneration, and reversed mechanical allodynia in the HFD mouse model of PDN. These studies suggest a molecular cascade linking neuropathic pain to axonal degeneration in PDN. In particular, nociceptor hyperexcitability and the associated increased intracellular calcium concentrations could lead to excessive calcium entry into mitochondria mediated by the mitochondrial calcium uniporter, resulting in increased calcium-dependent mitochondrial fission and ultimately contributing to small-fiber degeneration and neuropathic pain in PDN. Hence, we propose that targeting calcium entry into nociceptor mitochondria may represent a promising effective and disease-modifying therapeutic approach for this currently intractable and widespread affliction. Moreover, these results are likely to inform studies of other neurodegenerative disease involving similar underlying events.

Mitochondrial calcium homeostasis in hematopoietic stem cell: Molecular regulation of quiescence, function, and differentiation

Hematopoiesis is based on the existence of hematopoietic stem cells (HSC) with the capacity to self-proliferate and self-renew or to differentiate into specialized cells. The hematopoietic niche is the essential microenvironment where stem cells reside and integrate various stimuli to determine their fate. Recent studies have identified niche containing high level of calcium (Ca²⁺) suggesting that HSCs are sensitive to Ca²⁺. This is a highly versatile and ubiquitous second messenger that regulates a wide variety of cellular functions. Advanced methods for measuring its concentrations, genetic experiments, cell fate tracing data, single-cell imaging, and transcriptomics studies provide information into its specific roles to integrate signaling into an array of mechanisms that determine HSC identity, lineage potential, maintenance, and self-renewal. Accumulating and contrasting evidence, are revealing Ca²⁺ as a previously unacknowledged feature of HSC, involved in functional maintenance, by regulating multiple actors including transcription and epigenetic factors, Ca²⁺-dependent kinases and mitochondrial physiology. Mitochondria are significant participants in HSC functions and their responsiveness to cellular demands is controlled to a significant extent via Ca²⁺ signals. Recent reports indicate that mitochondrial Ca²⁺ uptake also controls HSC fate. These observations reveal a physiological feature of hematopoietic stem cells that can be harnessed to improve HSC-related disease. In this review, we discuss the current knowledge Ca²⁺ in hematopoietic stem cell focusing on its potential involvement in proliferation, self-renewal and maintenance of HSC and discuss future research directions.

Acute RyR1 Ca2+ leak enhances NADH-linked mitochondrial respiratory capacity

Sustained ryanodine receptor (RyR) Ca²⁺ leak is associated with pathological conditions such as heart failure or skeletal muscle weakness. We report that a single session of sprint interval training (SIT), but not of moderate intensity continuous training (MICT), triggers RyR1 protein oxidation and nitrosylation leading to calstabin1 dissociation in healthy human muscle and in in vitro SIT models (simulated SIT or S-SIT). This is accompanied by decreased sarcoplasmic reticulum Ca²⁺ content, increased levels of mitochondrial oxidative phosphorylation proteins, supercomplex formation and enhanced NADH-linked mitochondrial respiratory capacity. Mechanistically, (S-)SIT increases mitochondrial Ca²⁺ uptake in mouse myotubes and muscle fibres, and decreases pyruvate dehydrogenase phosphorylation in human muscle and mouse myotubes. Countering Ca²⁺ leak or preventing mitochondrial Ca²⁺ uptake blunts S-SIT-induced adaptations, a result supported by proteomic analyses. Here we show that triggering acute transient Ca²⁺ leak through RyR1 in healthy muscle may contribute to the multiple health promoting benefits of exercise.

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.

Metabolic adaptation to the chronic loss of Ca2+ signaling induced by KO of IP3 receptors or the mitochondrial Ca2+ uniporter

Calcium signaling is essential for regulating many biological processes. Endoplasmic reticulum inositol trisphosphate receptors (IP₃Rs) and the mitochondrial Ca²⁺ uniporter (MCU) are key proteins that regulate intracellular Ca²⁺ concentration. Mitochondrial Ca²⁺ accumulation activates Ca²⁺-sensitive dehydrogenases of the tricarboxylic acid (TCA) cycle that maintain the biosynthetic and bioenergetic needs of both normal and cancer cells. However, the interplay between calcium signaling and metabolism is not well understood. In this study, we used human cancer cell lines (HEK293 and HeLa) with stable KOs of all three IP₃R isoforms (triple KO [TKO]) or MCU to examine metabolic and bioenergetic responses to the chronic loss of cytosolic and/or mitochondrial Ca²⁺ signaling. Our results show that TKO cells (exhibiting total loss of Ca²⁺ signaling) are viable, displaying a lower proliferation and oxygen consumption rate, with no significant changes in ATP levels, even when made to rely solely on the TCA cycle for energy production. MCU KO cells also maintained normal ATP levels but showed increased proliferation, oxygen consumption, and metabolism of both glucose and glutamine. However, MCU KO cells were unable to maintain ATP levels and died when relying solely on the TCA cycle for energy. We conclude that constitutive Ca²⁺ signaling is dispensable for the bioenergetic needs of both IP₃R TKO and MCU KO human cancer cells, likely because of adequate basal glycolytic and TCA cycle flux. However, in MCU KO cells, the higher energy expenditure associated with increased proliferation and oxygen consumption makes these cells more prone to bioenergetic failure under conditions of metabolic stress.

A Yeast-Based Repurposing Approach for the Treatment of Mitochondrial DNA Depletion Syndromes Led to the Identification of Molecules Able to Modulate the dNTP Pool

Mitochondrial DNA depletion syndromes (MDS) are clinically heterogenous and often severe diseases, characterized by a reduction of the number of copies of mitochondrial DNA (mtDNA) in affected tissues. In the context of MDS, yeast has proved to be both an excellent model for the study of the mechanisms underlying mitochondrial pathologies and for the discovery of new therapies via high-throughput assays. Among the several genes involved in MDS, it has been shown that recessive mutations in MPV17 cause a hepatocerebral form of MDS and Navajo neurohepatopathy. MPV17 encodes a non selective channel in the inner mitochondrial membrane, but its physiological role and the nature of its cargo remains elusive. In this study we identify ten drugs active against MPV17 disorder, modelled in yeast using the homologous gene SYM1. All ten of the identified molecules cause a concomitant increase of both the mitochondrial deoxyribonucleoside triphosphate (mtdNTP) pool and mtDNA stability, which suggests that the reduced availability of DNA synthesis precursors is the cause for the mtDNA deletion and depletion associated with Sym1 deficiency. We finally evaluated the effect of these molecules on mtDNA stability in two other MDS yeast models, extending the potential use of these drugs to a wider range of MDS patients.

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.

Dysregulation of host cell calcium signaling during viral infections: Emerging paradigm with high clinical relevance

Viral infections are one of the leading causes of human illness. Viruses take over host cell signaling cascades for their replication and infection. Calcium (Ca2+) is a versatile and ubiquitous second messenger that modulates plethora of cellular functions. In last two decades, a critical role of host cell Ca2+ signaling in modulating viral infections has emerged. Furthermore, recent literature clearly implicates a vital role for the organellar Ca2+ dynamics (influx and efflux across organelles) in regulating virus entry, replication and severity of the infection. Therefore, it is not surprising that a number of viral infections including current SARS-CoV-2 driven COVID-19 pandemic are associated with dysregulated Ca2+ homeostasis. The focus of this review is to first discuss the role of host cell Ca2+ signaling in viral entry, replication and egress. We further deliberate on emerging literature demonstrating hijacking of the host cell Ca2+ dynamics by viruses. In particular, a variety of viruses including SARS-CoV-2 modulate lysosomal and cytosolic Ca2+ signaling for host cell entry and replication. Moreover, we delve into the recent studies, which have demonstrated the potential of several FDA-approved drugs targeting Ca2+ handling machinery in inhibiting viral infections. Importantly, we discuss the prospective of targeting intracellular Ca2+ signaling for better management and treatment of viral pathogenesis including COVID-19. Finally, we highlight the key outstanding questions in the field that demand critical and timely attention.

Mitochondrial cristae-remodeling protein OPA1 in POMC neurons couples Ca2+ homeostasis with adipose tissue lipolysis

Appropriate cristae remodeling is a determinant of mitochondrial function and bioenergetics and thus represents a crucial process for cellular metabolic adaptations. Here, we show that mitochondrial cristae architecture and expression of the master cristae-remodeling protein OPA1 in proopiomelanocortin (POMC) neurons, which are key metabolic sensors implicated in energy balance control, is affected by fluctuations in nutrient availability. Genetic inactivation of OPA1 in POMC neurons causes dramatic alterations in cristae topology, mitochondrial Ca2+ handling, reduction in alpha-melanocyte stimulating hormone (α-MSH) in target areas, hyperphagia, and attenuated white adipose tissue (WAT) lipolysis resulting in obesity. Pharmacological blockade of mitochondrial Ca2+ influx restores α-MSH and the lipolytic program, while improving the metabolic defects of mutant mice. Chemogenetic manipulation of POMC neurons confirms a role in lipolysis control. Our results unveil a novel axis that connects OPA1 in POMC neurons with mitochondrial cristae, Ca2+ homeostasis, and WAT lipolysis in the regulation of energy balance.

Mitochondrial Calcium Signaling as a Therapeutic Target for Alzheimer's Disease

Mitochondria absorb calcium (Ca2+) at the expense of the electrochemical gradient generated during respiration. The influx of Ca2+ into the mitochondrial matrix helps maintain metabolic function and results in increased cytosolic Ca2+ during intracellular Ca2+ signaling. Mitochondrial Ca2+ homeostasis is tightly regulated by proteins located in the inner and outer mitochondrial membranes and by the cross-talk with endoplasmic reticulum Ca2+ signals. Increasing evidence indicates that mitochondrial Ca2+ overload is a pathological phenotype associated with Alzheimer's Disease (AD). As intracellular Ca2+ dysregulation can be observed before the appearance of typical pathological hallmarks of AD, it is believed that mitochondrial Ca2+ overload may also play an important role in AD etiology. The high mitochondrial Ca2+ uptake can easily compromise neuronal functions and exacerbate AD progression by impairing mitochondrial respiration, increasing reactive oxygen species formation and inducing apoptosis. Additionally, mitochondrial Ca2+ overload can damage mitochondrial recycling via mitophagy. This review will discuss the molecular players involved in mitochondrial Ca2+ dysregulation and the pharmacotherapies that target this dysregulation. As most of the current AD therapeutics are based on amyloidopathy, tauopathy, and the cholinergic hypothesis, they achieve only symptomatic relief. Thus, determining how to reestablish mitochondrial Ca2+ homeostasis may aid in the development of novel AD therapeutic interventions.

Approved drugs ezetimibe and disulfiram enhance mitochondrial Ca2+ uptake and suppress cardiac arrhythmogenesis

BACKGROUND AND PURPOSE

Treatment of cardiac arrhythmia remains challenging due to severe side effects of common anti-arrhythmic drugs. We previously demonstrated that mitochondrial Ca²⁺ uptake in cardiomyocytes represents a promising new candidate structure for safer drug therapy. However, druggable agonists of mitochondrial Ca²⁺ uptake suitable for preclinical and clinical studies are still missing.

EXPERIMENTAL APPROACH

Herewe screened 727 compounds with a history of use in human clinical trials in a three-step screening approach. As a primary screening platform we used a permeabilized HeLa cell-based mitochondrial Ca²⁺ uptake assay. Hits were validated in cultured HL-1 cardiomyocytes and finally tested for anti-arrhythmic efficacy in three translational models: a Ca²⁺ overload zebrafish model and cardiomyocytes of both a mouse model for catecholaminergic polymorphic ventricular tachycardia (CPVT) and induced pluripotent stem cell derived cardiomyocytes from a CPVT patient.

KEY RESULTS

We identifiedtwo candidate compounds, the clinically approved drugs ezetimibe and disulfiram, which stimulate SR-mitochondria Ca²⁺ transfer at nanomolar concentrations. This is significantly lower compared to the previously described mitochondrial Ca²⁺ uptake enhancers (MiCUps) efsevin, a gating modifier of the voltage-dependent anion channel 2, and kaempferol, an agonist of the mitochondrial Ca²⁺ uniporter. Both substances restored rhythmic cardiac contractions in a zebrafish cardiac arrhythmia model and significantly suppressed arrhythmogenesis in freshly isolated ventricular cardiomyocytes from a CPVT mouse model as well as induced pluripotent stem cell derived cardiomyocytes from a CPVT patient.

CONCLUSION AND IMPLICATIONS

Taken together we identified ezetimibe and disulfiram as novel MiCUps and efficient suppressors of arrhythmogenesis and as such as, promising candidates for future preclinical and clinical studies.

[The Molecular Mechanisms of Mitochondrial Calcium Uptake by Calcium Uniporter]

Mitochondria play a role as intracellular calcium stores as well as energy conversion functions. Excessive calcium accumulation in mitochondria induces cell death and induces diseases such as ischemia-reperfusion injury. Mitochondrial calcium uptake is considered to be mediated by calcium uniporters, which have attracted much attention as potential drug targets. Although calcium uniporter was shown to function as an ion channel, the molecular mechanisms have long been unclear. In this decade, the molecular composition of the calcium uniporter complex was discovered; the calcium uniporter consists of the 7 subunits. Each subunit has no structural similarity to other Ca ion channels; thus, the novel molecular mechanism of the Ca²⁺ uptake by calcium uniporter is of interest. Although calcium uniporter is conserved in human to warm, yeast lack mitochondrial calcium uptake activity. In the previous study, various subunits of mammalian calcium uniporter were expressed in the yeast mitochondria. As a result, although the expression of each subunit alone did not affect on the mitochondrial calcium uptake activity, the co-expression of mitochondrial calcium uniporter (MCU) and essential MCU regulator (EMRE) enabled to reconstitute calcium uptake activity in yeast mitochondria. This indicated that MCU and EMRE are key factors of the calcium uptake activity in mitochondria. This yeast reconstitution technique has also enabled us to perform detailed structure-function analysis of the MCU and EMRE. In this paper, we will discuss the molecular mechanism of Ca²⁺ uptake and the prospects for drug discovery.

Nanotherapeutic approaches to target mitochondria in cancer

Treatment of cancer cells exemplifies a difficult test in the light of challenges associated with the nature of cancer cells and the severe side effects too. After making a large number of trials using both traditional and advanced therapies (immunotherapy and hormone therapy), approaches to design new therapies have reached a saturation level. However, nanotechnology-based approaches exhibit higher efficacy and great potential to bypass many of such therapeutic limitations. Because of their higher target specificity, the use of nanoparticles offers incredible potential in cancer therapeutics. Mitochondria, acting as a factory of energy production in cells, reveal an important role in the death as well as the survival of cells. Because of its significant involvement in the proliferation of cancer cells, it is being regarded as an important target for cancer therapeutics. Numerous studies reveal that nanotechnology-based approaches to directly target the mitochondria may help in improving the survival rate of cancer patients. In the current study, we have detailed the significance of mitochondria in the development of cancer phenotype, as well as indicated it as the potential targets for cancer therapy. Our study further highlights the importance of different nanoparticle-based approaches to target mitochondria of cancer cells and the associated outcomes of different studies. Though, nanotechnology-based approaches to target mitochondria of cancer cells demonstrate a potential and efficient way in cancer therapeutics. Yet, further study is needed to overcome the linked limitations.

Cobalt amine complexes and Ru265 interact with the DIME region of the mitochondrial calcium uniporter

We report our investigation into the MCU-inhibitory activity of Co3+ complexes in comparison to Ru265. These compounds reversibly inhibit the MCU with nanomolar potency. Mutagenesis studies and molecular docking simulations suggest that the complexes operate through interactions with the DIME motif of the MCU pore.

Targeting mitochondrial ion channels for cancer therapy

Pharmacological targeting of mitochondrial ion channels is emerging as a promising approach to eliminate cancer cells; as most of these channels are differentially expressed and/or regulated in cancer cells in comparison to healthy ones, this strategy may selectively eliminate the former. Perturbation of ion fluxes across the outer and inner membranes is linked to alterations of redox state, membrane potential and bioenergetic efficiency. This leads to indirect modulation of oxidative phosphorylation, which is/may be fundamental for both cancer and cancer stem cell survival. Furthermore, given the crucial contribution of mitochondria to intrinsic apoptosis, modulation of their ion channels leading to cytochrome c release may be of great advantage in case of resistance to drugs triggering apoptotic events upstream of the mitochondrial phase. In the present review, we give an overview of the known mitochondrial ion channels and of their modulators capable of killing cancer cells. In addition, we discuss state-of-the-art strategies using mitochondriotropic drugs or peptide-based approaches allowing a more efficient and selective targeting of mitochondrial ion channel-linked events.

Identification and functional validation of FDA-approved positive and negative modulators of the mitochondrial calcium uniporter

The mitochondrial calcium uniporter (MCU), the highly selective channel responsible for mitochondrial Ca²⁺ entry, plays important roles in physiology and pathology. However, only few pharmacological compounds directly and selectively modulate its activity. Here, we perform high-throughput screening on a US Food and Drug Administration (FDA)-approved drug library comprising 1,600 compounds to identify molecules modulating mitochondrial Ca²⁺ uptake. We find amorolfine and benzethonium to be positive and negative MCU modulators, respectively. In agreement with the positive effect of MCU in muscle trophism, amorolfine increases muscle size, and MCU silencing is sufficient to blunt amorolfine-induced hypertrophy. Conversely, in the triple-negative breast cancer cell line MDA-MB-231, benzethonium delays cell growth and migration in an MCU-dependent manner and protects from ceramide-induced apoptosis, in line with the role of mitochondrial Ca²⁺ uptake in cancer progression. Overall, we identify amorolfine and benzethonium as effective MCU-targeting drugs applicable to a wide array of experimental and disease conditions.

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We screen an FDA-approved drug library for mitochondrial Ca²⁺ uptake modulators

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Amorolfine and benzethonium modulate MCU activity

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Amorolfine increases MCU-dependent mitochondrial metabolism and muscle size

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Benzethonium decreases MCU-dependent cancer cell growth and migration

Mitochondrial Ca2+ Signaling in Health, Disease and Therapy

The divalent cation calcium (Ca²⁺) is considered one of the main second messengers inside cells and acts as the most prominent signal in a plethora of biological processes. Its homeostasis is guaranteed by an intricate and complex system of channels, pumps, and exchangers. In this context, by regulating cellular Ca²⁺ levels, mitochondria control both the uptake and release of Ca²⁺. Therefore, at the mitochondrial level, Ca²⁺ plays a dual role, participating in both vital physiological processes (ATP production and regulation of mitochondrial metabolism) and pathophysiological processes (cell death, cancer progression and metastasis). Hence, it is not surprising that alterations in mitochondrial Ca²⁺ (mCa²⁺) pathways or mutations in Ca²⁺ transporters affect the activities and functions of the entire cell. Indeed, it is widely recognized that dysregulation of mCa²⁺ signaling leads to various pathological scenarios, including cancer, neurological defects and cardiovascular diseases (CVDs). This review summarizes the current knowledge on the regulation of mCa²⁺ homeostasis, the related mechanisms and the significance of this regulation in physiology and human diseases. We also highlight strategies aimed at remedying mCa²⁺ dysregulation as promising therapeutical approaches.