Uncontrolled mitochondrial calcium uptake underlies the pathogenesis of neurodegeneration in MICU1-deficient mice and patients

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.

Calcium (Ca2+) hemostasis, mitochondria, autophagy, and mitophagy contribute to Alzheimer's disease as early moderators

This review rigorously investigates the early cerebral changes associated with Alzheimer's disease, which manifest long before clinical symptoms arise. It presents evidence that the dysregulation of calcium (Ca2+) homeostasis, along with mitochondrial dysfunction and aberrant autophagic processes, may drive the disease's progression during its asymptomatic, preclinical stage. Understanding the intricate molecular interplay that unfolds during this critical period offers a window into identifying novel therapeutic targets, thereby advancing the treatment of neurodegenerative disorders. The review delves into both established and emerging insights into the molecular alterations precipitated by the disruption of Ca2+ balance, setting the stage for cognitive decline and neurodegeneration.

MICU1's calcium sensing beyond mitochondrial calcium uptake

The discovery of MICU1 as gatekeeper of mitochondrial calcium (mCa2+) entry has transformed our understanding of mCa2+ flux. Recent studies revealed an additional role of MICU1 as a Ca2+ sensor at MICOS (mitochondrial contact site and cristae organizing system). MICU1's presence at MICOS suggests its involvement in coordinating Ca2+ signaling and mitochondrial ultrastructure. Besides its role in Ca2+ regulation, MICU1 influences cellular signaling pathways including transcription, epigenetic regulation, metabolism, and cell death, thereby affecting human health. Here, we summarize recent findings on MICU1's canonical and noncanonical functions, and its relevance to human health and diseases.

Mitochondrial calcium uniporter deficiency in dentate granule cells remodels neuronal metabolism and impairs reversal learning

The mitochondrial calcium uniporter (MCU) is the main route of calcium (Ca2+) entry into neuronal mitochondria. This channel has been linked to mitochondrial Ca2+ overload and cell death under neurotoxic conditions, but its physiologic roles for normal brain function remain poorly understood. Despite high expression of MCU in excitatory hippocampal neurons, it is unknown whether this channel is required for learning and memory. Here, we genetically down-regulated the Mcu gene in dentate granule cells (DGCs) of the hippocampus and found that this manipulation increases the overall respiratory activity of mitochondrial complexes I and II, augmenting the generation of reactive oxygen species in the context of impaired electron transport chain. The metabolic remodeling of MCU-deficient neurons also involved changes in the expression of enzymes that participate in glycolysis and the regulation of the tricarboxylic acid cycle, as well as the cellular antioxidant defenses. We found that MCU deficiency in DGCs does not change circadian rhythms, spontaneous exploratory behavior, or cognitive function in middle-aged mice (11-13 months old), when assessed with a food-motivated working memory test with three choices. DGC-targeted down-regulation of MCU significantly impairs reversal learning assessed with an 8-arm radial arm water maze but does not affect their ability to learn the task for the first time. Our results indicate that neuronal MCU plays an important physiologic role in memory formation and may be a potential therapeutic target to develop interventions aimed at improving cognitive function in aging, neurodegenerative diseases, and brain injury.

Oxidative stress-mediated proapoptosis signaling: A novel theory on the mechanism underlying the pathogenesis of burning mouth syndrome

BACKGROUND

Burning mouth syndrome (BMS) is a chronic oral pain disorder characterized by a generalized burning sensation in the oral mucosa without apparent medical or dental causes. Despite various hypotheses proposed to explain BMS pathogenesis, a clear understanding of the cellular-level events and associated histologic and molecular findings is lacking. Advancing our understanding of BMS pathogenesis could facilitate the development of more targeted therapeutic interventions.

TYPES OF STUDIES REVIEWED

The authors conducted an extensive literature search and review of cellular mechanisms, focusing on evidence-based data that support a comprehensive hypothesis for BMS pathogenesis. The authors explored novel and detailed mechanisms that may account for the characteristic features of BMS.

RESULTS

The authors proposed that BMS symptoms arise from the uncontrolled activation of proapoptotic transmembrane calcium permeable channels expressed in intraoral mucosal nerve fibers. Elevated levels of reactive oxygen species or dysfunctional antiapoptosis pathways may lead to uncontrolled oxidative stress-mediated apoptosis signaling, resulting in upregulation of transmembrane transient receptor potential vanilloid type 1 and P2X 3 calcium channels in nociceptive fibers. Activation of these channels can cause nerve terminal depolarization, leading to generation of action potentials that are centrally interpreted as pain.

CONCLUSIONS AND PRACTICAL IMPLICATIONS

The authors present a novel hypothesis for BMS pathogenesis, highlighting the role of proapoptotic transmembrane calcium permeable channels and oxidative stress-mediated apoptosis signaling in the development of BMS symptoms. Understanding these underlying mechanisms could provide new insights into the development of targeted therapeutic interventions for BMS. Additional research is warranted to validate this hypothesis and explore potential avenues for effective management of BMS.

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.

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.

Mitochondrial Proteomes in Neural Cells: A Systematic Review

Mitochondria are ancient endosymbiotic double membrane organelles that support a wide range of eukaryotic cell functions through energy, metabolism, and cellular control. There are over 1000 known proteins that either reside within the mitochondria or are transiently associated with it. These mitochondrial proteins represent a functional subcellular protein network (mtProteome) that is encoded by mitochondrial and nuclear genomes and significantly varies between cell types and conditions. In neurons, the high metabolic demand and differential energy requirements at the synapses are met by specific modifications to the mtProteome, resulting in alterations in the expression and functional properties of the proteins involved in energy production and quality control, including fission and fusion. The composition of mtProteomes also impacts the localization of mitochondria in axons and dendrites with a growing number of neurodegenerative diseases associated with changes in mitochondrial proteins. This review summarizes the findings on the composition and properties of mtProteomes important for mitochondrial energy production, calcium and lipid signaling, and quality control in neural cells. We highlight strategies in mass spectrometry (MS) proteomic analysis of mtProteomes from cultured cells and tissue. The research into mtProteome composition and function provides opportunities in biomarker discovery and drug development for the treatment of metabolic and neurodegenerative disease.

The calcium connection: exploring the intricacies of calcium signaling in plant-microbe interactions

The process of plant immune response is orchestrated by intracellular signaling molecules. Since plants are devoid of a humoral system, they develop extensive mechanism of pathogen recognition, signal perception, and intricate cell signaling for their protection from biotic and abiotic stresses. The pathogenic attack induces calcium ion accumulation in the plant cells, resulting in calcium signatures that regulate the synthesis of proteins of defense system. These calcium signatures induct different calcium dependent proteins such as calmodulins (CaMs), calcineurin B-like proteins (CBLs), calcium-dependent protein kinases (CDPKs) and other signaling molecules to orchestrate the complex defense signaling. Using advanced biotechnological tools, the role of Ca2+ signaling during plant-microbe interactions and the role of CaM/CMLs and CDPKs in plant defense mechanism has been revealed to some extent. The Emerging perspectives on calcium signaling in plant-microbe interactions suggest that this complex interplay could be harnessed to improve plant resistance against pathogenic microbes. We present here an overview of current understanding in calcium signatures during plant-microbe interaction so as to imbibe a future direction of research.

The Mitochondrial Calcium Uniporter (MCU): Molecular Identity and Role in Human Diseases

Calcium (Ca2+) ions act as a second messenger, regulating several cell functions. Mitochondria are critical organelles for the regulation of intracellular Ca2+. Mitochondrial calcium (mtCa2+) uptake is ensured by the presence in the inner mitochondrial membrane (IMM) of the mitochondrial calcium uniporter (MCU) complex, a macromolecular structure composed of pore-forming and regulatory subunits. MtCa2+ uptake plays a crucial role in the regulation of oxidative metabolism and cell death. A lot of evidence demonstrates that the dysregulation of mtCa2+ homeostasis can have serious pathological outcomes. In this review, we briefly discuss the molecular structure and the function of the MCU complex and then we focus our attention on human diseases in which a dysfunction in mtCa2+ has been shown.

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.

Mitochondria as intracellular signalling organelles. An update

Traditionally, mitochondria are known as "the powerhouse of the cell," responsible for energy (ATP) generation (by the electron transport chain, oxidative phosphorylation, the tricarboxylic acid cycle, and fatty acid ß-oxidation), and for the regulation of several metabolic processes, including redox homeostasis, calcium signalling, and cellular apoptosis. The extensive studies conducted in the last decades portray mitochondria as multifaceted signalling organelles that ultimately command cells' survival or death. Based on current knowledge, we'll outline the mitochondrial signalling to other intracellular compartments in homeostasis and pathology-related mitochondrial stress conditions here. The following topics are discussed: (i) oxidative stress and mtROS signalling in mitohormesis, (ii) mitochondrial Ca²⁺ signalling; (iii) the anterograde (nucleus-to-mitochondria) and retrograde (mitochondria-to-nucleus) signal transduction, (iv) the mtDNA role in immunity and inflammation, (v) the induction of mitophagy- and apoptosis - signalling cascades, (vi) the mitochondrial dysfunctions (mitochondriopathies) in cardiovascular, neurodegenerative, and malignant diseases. The novel insights into molecular mechanisms of mitochondria-mediated signalling can explain mitochondria adaptation to metabolic and environmental stresses to achieve cell survival.

Role of mitochondria in the myopathy of juvenile dermatomyositis and implications for skeletal muscle calcinosis

OBJECTIVES

To elucidate mechanisms contributing to skeletal muscle calcinosis in patients with juvenile dermatomyositis.

METHODS

A well-characterized cohorts of JDM (n = 68), disease controls (polymyositis, n = 7; juvenile SLE, n = 10, and RNP + overlap syndrome, n = 12), and age-matched health controls (n = 17) were analyzed for circulating levels of mitochondrial (mt) markers including mtDNA, mt-nd6, and anti-mitochondrial antibodies (AMAs) using standard qPCR, ELISA, and novel-in-house assays, respectively. Mitochondrial calcification of affected tissue biopsies was confirmed using electron microscopy and energy dispersive X-ray analysis. A human skeletal muscle cell line, RH30, was used to generate an in vitro calcification model. Intracellular calcification is measured by flow cytometry and microscopy. Mitochondria were assessed for mtROS production and membrane potential by flow cytometry and real-time oxygen consumption rate by Seahorse bioanalyzer. Inflammation (interferon-stimulated genes) was measured by qPCR.

RESULTS

In the current study, patients with JDM exhibited elevated levels of mitochondrial markers associated with muscle damage and calcinosis. Of particular interest are AMAs predictive of calcinosis. Human skeletal muscle cells undergo time- and dose-dependent accumulation of calcium phosphate salts with preferential localization to mitochondria. Calcification renders skeletal muscle cells mitochondria stressed, dysfunctional, destabilized, and interferogenic. Further, we report that inflammation induced by interferon-alpha amplifies mitochondrial calcification of human skeletal muscle cells via the generation of mitochondrial reactive oxygen species (mtROS).

CONCLUSIONS

Overall, our study demonstrates the mitochondrial involvement in the skeletal muscle pathology and calcinosis of JDM and mtROS as a central player in the calcification of human skeletal muscle cells. Therapeutic targeting of mtROS and/or upstream inducers, such as inflammation, may alleviate mitochondrial dysfunction, leading to calcinosis. AMAs can potentially identify patients with JDM at risk for developing calcinosis.

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.

MICU1 occludes the mitochondrial calcium uniporter in divalent-free conditions

Mitochondrial Ca2+ uptake is mediated by the mitochondrial uniporter complex (mtCU) that includes a tetramer of the pore-forming subunit, MCU, a scaffold protein, EMRE, and the EF-hand regulatory subunit, MICU1 either homodimerized or heterodimerized with MICU2/3. MICU1 has been proposed to regulate Ca2+ uptake via the mtCU by physically occluding the pore and preventing Ca2+ flux at resting cytoplasmic [Ca2+] (free calcium concentration) and to increase Ca2+ flux at high [Ca2+] due to cooperative activation of MICUs EF-hands. However, mtCU and MICU1 functioning when its EF-hands are unoccupied by Ca2+ is poorly studied due to technical limitations. To overcome this barrier, we have studied the mtCU in divalent-free conditions by assessing the Ru265-sensitive Na+ influx using fluorescence-based measurement of mitochondrial matrix [Na+] (free sodium concentration) rise and the ensuing depolarization and swelling. We show an increase in all these measures of Na+ uptake in MICU1KO cells as compared to wild-type (WT) and rescued MICU1KO HEK cells. However, mitochondria in WT cells and MICU1 stable-rescued cells still allowed some Ru265-sensitive Na+ influx that was prevented by MICU1 in excess upon acute overexpression. Thus, MICU1 restricts the cation flux across the mtCU in the absence of Ca2+, but even in cells with high endogenous MICU1 expression such as HEK, some mtCU seem to lack MICU1-dependent gating. We also show rearrangement of the mtCU and altered number of functional channels in MICU1KO and different rescues, and loss of MICU1 during mitoplast preparation, that together might have obscured the pore-blocking function of MICU1 in divalent-free conditions in previous studies.

[Decreased Expression of Mitochondrial Calcium Uptake Protein 1 Leads to Skeletal Muscle Dysfunction in Septic Mice]

Objective

To observe the effect of sepsis on skeletal muscle function and to explore the role of skeletal muscle mitochondrial calcium uptake protein 1 (MICU1).

Methods

A total of 40 specific-pathogen-free (SPF) healthy male C57BL/6J mice were randomly assigned to 4 groups, a sham operation group (Sham group, n=8), a sepsis modeling 6 h group (cecal ligation and puncture [CLP]-6 h group, n=10), a sepsis modeling 12 h group (CLP-12 h group, n=10), and a sepsis modeling 24 h group (CLP-24 h, n=12). The sepsis model was established by CLP. Mice in the Sham group only underwent laparotomic exploration of the cecum. Another 20 SPF mice were selected. The tibialis anterior muscle on one side was empty-transfected with adeno-associated virus (AAV) as controls (AAV-C), and the tibialis anterior muscle on the other side was transfected with AAV to enhance MICU1 expression (AAV-M). The mice were randomly assigned to two groups, a sham operation group (AAV-C-Sham and AAV-M-Sham, n=8) and a sepsis model 24 h group (AAV-C-CLP and AAV-M-CLP, n=12). The grip strength and compound muscle action potential (CMAP) of the tibialis anterior muscle were measured in each group at the corresponding time points. The levels of inflammatory factors, including tumor necrosis factor α (TNF-α) and interleukin 6 (IL-6), in the skeletal muscle were measured by ELISA. The morphological changes of skeletal muscle cells were observed through H&E staining. The expression levels of MICU1 and muscle atrophy-related proteins, including muscle RING-finger containing protein 1 (MuRF1) and muscle atrophy Fbox protein (MAFbx), were determined by Western blot. The expression levels of MICU1 mRNA in skeletal muscle were determined by RT-qPCR.

Results

Compared with mice in the Sham group, mice in the CLP group showed decreased body weight ( P<0.05); their grip strength decreased with the prolongation of CLP modeling time ( P<0.05); the amplitude of CMAP decreased, showing prolonged duration and latency ( P<0.05); the expression levels of inflammatory factors, including TNF-α and IL-6, in skeletal muscle increased gradually ( P<0.05); the fiber diameter and cross-sectional area of skeletal muscle decreased gradually with the prolongation of modeling time ( P<0.05); the protein expression levels of MuRF1and MAFbx proteins increased gradually ( P<0.05); the expression levels of MICU1 protein and mRNA decreased gradually ( P<0.05). There was no significant difference in all indices between AAV-M-Sham and AAV-C-Sham groups ( P>0.05). Compared with mice in the AAV-C-CLP group, mice in the AAV-M-CLP group showed increased grip strength ( P<0.05); the amplitude of CMAP increased, showing shortened duration and latency ( P <0.05); the fiber diameter and cross-sectional area of skeletal muscle increased ( P<0.05); the expression levels of MuRF1and MAFbx decreased ( P<0.05).

Conclusion

Sepsis leads to skeletal muscle dysfunction, which is related to the decrease in mitochondrial MICU1 expression.

Beyond the matrix: structural and physiological advancements in mitochondrial calcium signaling

Mitochondrial calcium (Ca2+) signaling has long been known to regulate diverse cellular functions, ranging from ATP production via oxidative phosphorylation, to cytoplasmic Ca2+ signaling to apoptosis. Central to mitochondrial Ca2+ signaling is the mitochondrial Ca2+ uniporter complex (MCUC) which enables Ca2+ flux from the cytosol into the mitochondrial matrix. Several pivotal discoveries over the past 15 years have clarified the identity of the proteins comprising MCUC. Here, we provide an overview of the literature on mitochondrial Ca2+ biology and highlight recent findings on the high-resolution structure, dynamic regulation, and new functions of MCUC, with an emphasis on publications from the last five years. We discuss the importance of these findings for human health and the therapeutic potential of targeting mitochondrial Ca2+ signaling.

IGF-1 boosts mitochondrial function by a Ca2+ uptake-dependent mechanism in cultured human and rat cardiomyocytes

A physiological increase in cardiac workload results in adaptive cardiac remodeling, characterized by increased oxidative metabolism and improvements in cardiac performance. Insulin-like growth factor-1 (IGF-1) has been identified as a critical regulator of physiological cardiac growth, but its precise role in cardiometabolic adaptations to physiological stress remains unresolved. Mitochondrial calcium (Ca²⁺) handling has been proposed to be required for sustaining key mitochondrial dehydrogenase activity and energy production during increased workload conditions, thus ensuring the adaptive cardiac response. We hypothesized that IGF-1 enhances mitochondrial energy production through a Ca²⁺-dependent mechanism to ensure adaptive cardiomyocyte growth. We found that stimulation with IGF-1 resulted in increased mitochondrial Ca²⁺ uptake in neonatal rat ventricular myocytes and human embryonic stem cell-derived cardiomyocytes, estimated by fluorescence microscopy and indirectly by a reduction in the pyruvate dehydrogenase phosphorylation. We showed that IGF-1 modulated the expression of mitochondrial Ca²⁺ uniporter (MCU) complex subunits and increased the mitochondrial membrane potential; consistent with higher MCU-mediated Ca²⁺ transport. Finally, we showed that IGF-1 improved mitochondrial respiration through a mechanism dependent on MCU-mediated Ca²⁺ transport. In conclusion, IGF-1-induced mitochondrial Ca²⁺ uptake is required to boost oxidative metabolism during cardiomyocyte adaptive growth.

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.

From passage to inhibition: Uncovering the structural and physiological inhibitory mechanisms of MCUb in mitochondrial calcium regulation

Mitochondrial calcium (Ca²⁺ ) regulation is critically implicated in the regulation of bioenergetics and cell fate. Ca²⁺ , a universal signaling ion, passively diffuses into the mitochondrial intermembrane space (IMS) through voltage-dependent anion channels (VDAC), where uptake into the matrix is tightly regulated across the inner mitochondrial membrane (IMM) by the mitochondrial Ca²⁺ uniporter complex (mtCU). In recent years, immense progress has been made in identifying and characterizing distinct structural and physiological mechanisms of mtCU component function. One of the main regulatory components of the Ca²⁺ selective mtCU channel is the mitochondrial Ca²⁺ uniporter dominant-negative beta subunit (MCUb). The structural mechanisms underlying the inhibitory effect(s) exerted by MCUb are poorly understood, despite high homology to the main mitochondrial Ca²⁺ uniporter (MCU) channel-forming subunits. In this review, we provide an overview of the structural differences between MCUb and MCU, believed to contribute to the inhibition of mitochondrial Ca²⁺ uptake. We highlight the possible structural rationale for the absent interaction between MCUb and the mitochondrial Ca²⁺ uptake 1 (MICU1) gatekeeping subunit and a potential widening of the pore upon integration of MCUb into the channel. We discuss physiological and pathophysiological information known about MCUb, underscoring implications in cardiac function and arrhythmia as a basis for future therapeutic discovery. Finally, we discuss potential post-translational modifications on MCUb as another layer of important regulation.

PolyGA targets the ER stress-adaptive response by impairing GRP75 function at the MAM in C9ORF72-ALS/FTD

ER stress signaling is linked to the pathophysiological and clinical disease manifestations in amyotrophic lateral sclerosis (ALS). Here, we have investigated ER stress-induced adaptive mechanisms in C9ORF72-ALS/FTD, focusing on uncovering early endogenous neuroprotective mechanisms and the crosstalk between pathological and adaptive responses in disease onset and progression. We provide evidence for the early onset of ER stress-mediated adaptive response in C9ORF72 patient-derived motoneurons (MNs), reflected by the elevated increase in GRP75 expression. These transiently increased GRP75 levels enhance ER-mitochondrial association, boosting mitochondrial function and sustaining cellular bioenergetics during the initial stage of disease, thereby counteracting early mitochondrial deficits. In C9orf72 rodent neurons, an abrupt reduction in GRP75 expression coincided with the onset of UPR, mitochondrial dysfunction and the emergence of PolyGA aggregates, which co-localize with GRP75. Similarly, the overexpression of PolyGA in WT cortical neurons or C9ORF72 patient-derived MNs led to the sequestration of GRP75 within PolyGA inclusions, resulting in mitochondrial calcium (Ca²⁺) uptake impairments. Corroborating these findings, we found that PolyGA aggregate-bearing human post-mortem C9ORF72 hippocampal dentate gyrus neurons not only display reduced expression of GRP75 but also exhibit GRP75 sequestration within inclusions. Sustaining high GRP75 expression in spinal C9orf72 rodent MNs specifically prevented ER stress, normalized mitochondrial function, abrogated PolyGA accumulation in spinal MNs, and ameliorated ALS-associated behavioral phenotype. Taken together, our results are in line with the notion that neurons in C9ORF72-ALS/FTD are particularly susceptible to ER-mitochondrial dysfunction and that GRP75 serves as a critical endogenous neuroprotective factor. This neuroprotective pathway, is eventually targeted by PolyGA, leading to GRP75 sequestration, and its subsequent loss of function at the MAM, compromising mitochondrial function and promoting disease onset.