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

Endothelial MICU1 protects against vascular inflammation and atherosclerosis by inhibiting mitochondrial calcium uptake

Mitochondrial dysfunction fuels vascular inflammation and atherosclerosis. Mitochondrial calcium uptake 1 (MICU1) maintains mitochondrial Ca2+ homeostasis. However, the role of MICU1 in vascular inflammation and atherosclerosis remains unknown. Here, we report that endothelial MICU1 prevents vascular inflammation and atherosclerosis by maintaining mitochondrial homeostasis. We observed that vascular inflammation was aggravated in endothelial cell-specific Micu1 knockout mice (Micu1ECKO) and attenuated in endothelial cell-specific Micu1 transgenic mice (Micu1ECTg). Furthermore, hypercholesterolemic Micu1ECKO mice also showed accelerated development of atherosclerosis, while Micu1ECTg mice were protected against atherosclerosis. Mechanistically, MICU1 depletion increased mitochondrial Ca2+ influx, thereby decreasing the expression of the mitochondrial deacetylase sirtuin 3 (SIRT3) and the ensuing deacetylation of superoxide dismutase 2 (SOD2), leading to the burst of mitochondrial reactive oxygen species (mROS). Of clinical relevance, we observed decreased MICU1 expression in the endothelial layer covering human atherosclerotic plaques and in human aortic endothelial cells exposed to serum from patients with coronary artery diseases (CAD). Two-sample Wald ratio Mendelian randomization further revealed that increased expression of MICU1 was associated with decreased risk of CAD and coronary artery bypass grafting (CABG). Our findings support MICU1 as an endogenous endothelial resilience factor that protects against vascular inflammation and atherosclerosis by maintaining mitochondrial Ca2+ homeostasis.

Dependence of mitochondrial calcium signalling and dynamics on the disaggregase, CLPB

Cells utilize protein disaggregases to avoid abnormal protein aggregation that causes many diseases. Among these, caseinolytic peptidase B protein homolog (CLPB) is localized in the mitochondrial intermembrane space and linked to human disease. Upon CLPB loss, MICU1 and MICU2, regulators of the mitochondrial calcium uniporter complex (mtCU), and OPA1, a main mediator of mitochondrial fusion, become insoluble but the functional outcome remains unclear. In this work we demonstrate that CLPB is required to maintain mitochondrial calcium signalling and fusion dynamics. CLPB loss results in altered mtCU composition, interfering with mitochondrial calcium uptake independently of cytosolic calcium and mitochondrial membrane potential. Additionally, OPA1 decreases, and aggregation occurs, accompanied by mitochondrial fragmentation. Disease-associated mutations in the CLPB gene present in skin fibroblasts from patients also display mitochondrial calcium and structural changes. Thus, mtCU and fusion activity are dependent on CLPB, and their impairments might contribute to the disease caused by CLPB variants.

Mechanisms of dual modulatory effects of spermine on the mitochondrial calcium uniporter complex

The mitochondrial Ca2+ uniporter is the Ca2+ channel responsible for mitochondrial Ca2+ uptake. It plays crucial physiological roles in regulating oxidative phosphorylation, intracellular Ca2+ signaling, and cell death. The uniporter contains the pore-forming MCU subunit, the auxiliary EMRE protein, and the regulatory MICU1 subunit, which blocks the MCU pore under resting cellular Ca2+ concentrations. It has been known for decades that spermine, a biological polyamine ubiquitously present in animal cells, can enhance mitochondrial Ca2+ uptake, but the underlying mechanisms remain incompletely understood. In this study, we demonstrate that spermine exerts both potentiation and inhibitory effects on the uniporter. At physiological concentrations, spermine binds to membranes and disrupts MCU-MICU1 interactions, thereby opening the uniporter to import more Ca2+. However, at millimolar concentrations, spermine also inhibits the uniporter by targeting the pore-forming region in a MICU1-independent manner. These findings provide molecular insights into how cells can use spermine to control the critical processes of mitochondrial Ca2+ signaling and homeostasis.

A novel super-resolution STED microscopy analysis approach to observe spatial MCU and MICU1 distribution dynamics in cells

The uptake of Ca2+ by mitochondria is an important and tightly controlled process in various tissues. Even small changes in the key proteins involved in this process can lead to significant cellular dysfunction and, ultimately, cell death. In this study, we used stimulated emission depletion (STED) microscopy and developed an unbiased approach to monitor the sub-mitochondrial distribution and dynamics of the mitochondrial calcium uniporter (MCU) and mitochondrial calcium uptake 1 (MICU1) under resting and stimulated conditions. To visualize the inner mitochondrial membrane, the STED-optimized dye called pkMitoRed was used. The study presented herein builds on the previously verified exclusive localization of MICU1 in the intermembrane space, and that MCU moves exclusively laterally along the inner mitochondrial membrane (IMM). We applied a multi-angled arrow histogram to analyze the distribution of proteins within mitochondria, providing a one-dimensional view of protein localization along a defined distance. Combining this with optimal transport colocalization enabled us to further predict submitochondrial protein distribution. Results indicate that in HeLa cells Ca2+ elevation yielded MCU translocation from the cristae membrane (CM) to the inner boundary membrane (IBM). In AC16 cardiomyocyte cell line, MCU is mainly located at the IBM under resting conditions, and it translocates to the CM upon rising Ca2+. Our data describe a novel unbiased super-resolution image analysis approach. Our showcase sheds light on differences in spatial distribution dynamics of MCU in cell lines with different MICU1:MCU abundance.

The roles of mitochondria in global and local intracellular calcium signalling

Activation of Ca2+ channels in Ca2+ stores in organelles and the plasma membrane generates cytoplasmic calcium ([Ca2+]c) signals that control almost every aspect of cell function, including metabolism, vesicle fusion and contraction. Mitochondria have a high capacity for Ca2+ uptake and chelation, alongside efficient Ca2+ release mechanisms. Still, mitochondria do not store Ca2+ in a prolonged manner under physiological conditions and lack the capacity to generate global [Ca2+]c signals. However, mitochondria take up Ca2+ at high local [Ca2+]c signals that originate from neighbouring organelles, and also during sustained global elevations of [Ca2+]c. Accumulated Ca2+ in the mitochondria stimulates oxidative metabolism and upon return to the cytoplasm, can produce spatially confined rises in [Ca2+]c to exert control over processes that are sensitive to Ca2+. Thus, the mitochondrial handling of [Ca2+]c is of physiological relevance. Furthermore, dysregulation of mitochondrial Ca2+ handling can contribute to debilitating diseases. We discuss the mechanisms and relevance of mitochondria in local and global calcium signals.

Sirtuin-1 Regulates Mitochondrial Calcium Uptake Through Mitochondrial Calcium Uptake 1 (MICU1)

Mitochondria play a central role in cell biological processes, functioning not only as producers of ATP but also as regulators of Ca2+ signaling. Mitochondrial calcium uptake occurs primarily through the mitochondrial calcium uniporter channel (mtCU), with the mitochondrial calcium uptake subunits 1, 2, and 3 (MICU1, MICU2, and MICU3) serving as the main regulatory components. Dysregulated mitochondrial calcium uptake is a hallmark of cellular degeneration. Sirtuin 1 (SIRT1), a key regulator of cellular metabolism, plays a critical role in aging and various neurodegenerative conditions. By blocking SIRT1 using EX527 or shSIRT1, we observed mitochondrial structural fragmentation as well as intensified and prolonged mitochondrial calcium overload. Our study revealed a direct interaction between SIRT1 and MICU1. Notably, SIRT1 inhibition resulted in reduced MICU1 expression, hence led to mitochondrial calcium overload, illustrating the unconventional role of SIRT1 in governing mitochondrial function.

In vivo intracellular Ca2+ profiles after eccentric rat muscle contractions: addressing the mechanistic bases for repeated bout protection

Eccentric contractions (ECC) are accompanied by the accumulation of intracellular calcium ions ([Ca2+]i) and induce skeletal muscle damage. Suppressed muscle damage in repeated bouts of ECC is well characterized; however, whether it is mediated by altered Ca2+ profiles remains unknown. We tested the hypothesis that repeated ECC suppresses Ca2+ accumulation via adaptations in Ca2+ regulation. Male Wistar rats were divided into two groups: ECC single bout (ECC-SB) and repeated bout (ECC-RB). Tibialis anterior (TA) muscles were subjected to ECC (40 times, 5 sets) once (ECC-SB) or twice 14 days apart (ECC-RB). Under anesthesia, the TA muscle was loaded with Ca2+ indicator Fura 2-AM, and the 340/380 nm ratio was evaluated as [Ca2+]i. Ca2+ handling proteins were measured by Western blots. ECC induced [Ca2+]i increase in both groups, but ECC-RB evinced a markedly suppressed [Ca2+]i (Time: P < 0.01, Group: P = 0.0357). Five hours post-ECC, in contrast to the localized [Ca2+]i accumulation in ECC-SB, ECC-RB exhibited lower and more uniform [Ca2+]i (P < 0.01). In ECC-RB, mitochondria Ca2+ uniporter complex (MCU) components MCU and MICU2 were significantly increased pre-second ECC bout (P < 0.01), and both SERCA1 and MICU1 were better preserved after contractions (P < 0.01). Fourteen days after novel ECC, skeletal muscle mitochondrial Ca2+ regulating proteins were elevated. Following subsequent ECC, [Ca2+]i accumulation and muscle damage were suppressed and SERCA1 and MICU1 preserved. These findings suggest that tolerance to a subsequent ECC bout is driven, at least in part, by enhanced mitochondrial and sarcoplasmic reticulum Ca2+ regulation.NEW & NOTEWORTHY We demonstrated a reduced [Ca2+]i profile with suppressed muscle damage after a repeated bout of ECC in vivo: the ECC-induced immediate [Ca2+]i increase was suppressed and the persistence of increased [Ca2+]i with localized accumulation was diminished after repeated ECC. This effect occurred consonant with the upregulation of the mitochondrial Ca2+ uniporter complex and better preservation of SERCA1 and MICU1. These findings suggest that the mechanistic bases for repeated bout protection involve adaptation of Ca2+ regulation.

MICU1 and MICU2, two peas in a pod or entirely different fruits?

Fluctuations in mitochondrial matrix Ca2+ plays a critical role in matching energy production to cellular demand through direct effects on oxidative phosphorylation and ATP production. Disruption in mitochondrial Ca2+ homeostasis, particularly under pathological conditions such as ischemia or heart failure, can lead to mitochondrial dysfunction, energy deficit, and eventually death of cardiomyocytes. The primary channel regulating acute mitochondrial Ca2+ influx is the mitochondrial Ca2+ uniporter (mtCU), which is regulated by the mitochondrial Ca2+ uptake (MICU) proteins that were examined here.

MICUs protect the heart by regulating mitochondrial calcium

Regulation of mitochondrial calcium uptake by the mitochondrial calcium uniporter (mtCU) complex is crucial for heart function. In a recent study, Hasan et al. demonstrated that mitochondrial calcium uptake (MICU)1 and MICU2, regulatory subunits of the complex, help maintain calcium homeostasis in cardiac mitochondria, providing potential targets for therapies aimed at improving mitochondrial function in heart disease.

Systematic mapping of mitochondrial calcium uniporter channel (MCUC)-mediated calcium signaling networks

The mitochondrial calcium uniporter channel (MCUC) mediates mitochondrial calcium entry, regulating energy metabolism and cell death. Although several MCUC components have been identified, the molecular basis of mitochondrial calcium signaling networks and their remodeling upon changes in uniporter activity have not been assessed. Here, we map the MCUC interactome under resting conditions and upon chronic loss or gain of mitochondrial calcium uptake. We identify 89 high-confidence interactors that link MCUC to several mitochondrial complexes and pathways, half of which are associated with human disease. As a proof-of-concept, we validate the mitochondrial intermembrane space protein EFHD1 as a binding partner of the MCUC subunits MCU, EMRE, and MCUB. We further show a MICU1-dependent inhibitory effect of EFHD1 on calcium uptake. Next, we systematically survey compensatory mechanisms and functional consequences of mitochondrial calcium dyshomeostasis by analyzing the MCU interactome upon EMRE, MCUB, MICU1, or MICU2 knockdown. While silencing EMRE reduces MCU interconnectivity, MCUB loss-of-function leads to a wider interaction network. Our study provides a comprehensive and high-confidence resource to gain insights into players and mechanisms regulating mitochondrial calcium signaling and their relevance in human diseases.

Calcium signaling in mitochondrial intermembrane space

The mitochondrial intermembrane space (IMS) is a highly protected compartment, second only to the matrix. It is a crucial bridge, coordinating mitochondrial activities with cellular processes such as metabolites, protein, lipid, and ion exchange. This regulation influences signaling pathways for metabolic activities and cellular homeostasis. The IMS harbors various proteins critical for initiating apoptotic cascades and regulating reactive oxygen species production by controlling the respiratory chain. Calcium (Ca2+), a key intracellular secondary messenger, enter the mitochondrial matrix via the IMS, regulating mitochondrial bioenergetics, ATP production, modulating cell death pathways. IMS acts as a regulatory site for Ca2+ entry due to the presence of different Ca2+ sensors such as MICUs, solute carriers (SLCs); ion exchangers (LETM1/SCaMCs); S100A1, mitochondrial glycerol-3-phosphate dehydrogenase, and EFHD1, each with unique Ca2+ binding motifs and spatial localizations. This review primarily emphasizes the role of these IMS-localized Ca2+ sensors concerning their spatial localization, mechanism, and molecular functions. Additionally, we discuss how these sensors contribute to the progression and pathogenesis of various human health conditions and diseases.

MICU1 and MICU2 control mitochondrial calcium signaling in the mammalian heart

Activating Ca2+-sensitive enzymes of oxidative metabolism while preventing calcium overload that leads to mitochondrial and cellular injury requires dynamic control of mitochondrial Ca2+ uptake. This is ensured by the mitochondrial calcium uptake (MICU)1/2 proteins that gate the pore of the mitochondrial calcium uniporter (mtCU). MICU1 is relatively sparse in the heart, and recent studies claimed the mammalian heart lacks MICU1 gating of mtCU. However, genetic models have not been tested. We find that MICU1 is present in a complex with MCU in nonfailing human hearts. Furthermore, using murine genetic models and pharmacology, we show that MICU1 and MICU2 control cardiac mitochondrial Ca2+ influx, and that MICU1 deletion alters cardiomyocyte mitochondrial calcium signaling and energy metabolism. MICU1 loss causes substantial compensatory changes in the mtCU composition and abundance, increased turnover of essential MCU regulator (EMRE) early on and, later, of MCU, that limit mitochondrial Ca2+ uptake and allow cell survival. Thus, both the primary consequences of MICU1 loss and the ensuing robust compensation highlight MICU1's relevance in the beating heart.

Self-Sulfhydrated, Nitro-Fixed Albumin Nanoparticles as a Potent Therapeutic Agent for the Treatment of Acute Liver Injury

Exogenous polysulfhydryls (R-SH) supplementation and nitric oxide (NO) gas molecules delivery provide essential antioxidant buffering pool components and anti-inflammatory species in cellular defense against injury, respectively. Herein, the intermolecular disulfide bonds in bovine serum albumin (BSA) molecules were reductively cleaved under native and mild conditions to expose multiple sulfhydryl groups (BSA-SH), then sulfhydryl-nitrosylated (R-SNO), and nanoprecipitated to form injectable self-sulfhydrated, nitro-fixed albumin nanoparticles (BSA-SNO NPs), allowing albumin to act as a NO donor reservoir and multiple sulfhydryl group transporter while also preventing unfavorable oxidation and self-cross-linking of polysulfhydryl groups. In two mouse models of ischemia/reperfusion-induced and endotoxin-induced acute liver injury (ALI), a single low dosage of BSA-SNO NPs (S-nitrosothiols: 4 μmol·kg-1) effectively attenuated oxidative stress and systemic inflammation cascades in the upstream pathophysiology of disease progression, thus rescuing dying hepatocytes, regulating host defense, repairing microcirculation, and restoring liver function. By mechanistically upregulating the antioxidative signaling pathway (Nrf-2/HO-1/NOQ1) and inhibiting the inflammatory cytokine storm (NF-κB/p-IκBα/TNF-α/IL-β), BSA-SNO NPs blocked the initiation of the mitochondrial apoptotic signaling pathway (Cyto C/Bcl-2 family/caspase-3) and downregulated the cell pyroptosis pathway (NLRP3/ASC/IL-1β), resulting in an increased survival rate from 26.7 to 73.3%. This self-sulfhydrated, nitro-fixed functionalized BSA nanoformulation proposes a potential drug-free treatment strategy for ALI.

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.

MICU3 Regulates Mitochondrial Calcium and Cardiac Hypertrophy

BACKGROUND

Calcium (Ca2+) uptake by mitochondria occurs via the mitochondrial Ca2+ uniporter. Mitochondrial Ca2+ uniporter exists as a complex, regulated by 3 MICU (mitochondrial Ca2+ uptake) proteins localized in the intermembrane space: MICU1, MICU2, and MICU3. Although MICU3 is present in the heart, its role is largely unknown.

METHODS

We used CRISPR-Cas9 to generate a mouse with global deletion of MICU3 and an adeno-associated virus (AAV9) to overexpress MICU3 in wild-type mice. We examined the role of MICU3 in regulating mitochondrial calcium ([Ca2+]m) in ex vivo hearts using an optical method following adrenergic stimulation in perfused hearts loaded with a Ca2+-sensitive fluorophore. Additionally, we studied how deletion and overexpression of MICU3, respectively, impact cardiac function in vivo by echocardiography and the molecular composition of the mitochondrial Ca2+ uniporter complex via Western blot, immunoprecipitation, and Blue native-PAGE analysis. Finally, we measured MICU3 expression in failing human hearts.

RESULTS

MICU3 knock out hearts and cardiomyocytes exhibited a significantly smaller increase in [Ca2+]m than wild-type hearts following acute isoproterenol infusion. In contrast, heart with overexpression of MICU3 exhibited an enhanced increase in [Ca2+]m compared with control hearts. Echocardiography analysis showed no significant difference in cardiac function in knock out MICU3 mice relative to wild-type mice at baseline. However, mice with overexpression of MICU3 exhibited significantly reduced ejection fraction and fractional shortening compared with control mice. We observed a significant increase in the ratio of heart weight to tibia length in hearts with overexpression of MICU3 compared with controls, consistent with hypertrophy. We also found a significant decrease in MICU3 protein and expression in failing human hearts.

CONCLUSIONS

Our results indicate that increased and decreased expression of MICU3 enhances and reduces, respectively, the uptake of [Ca2+]m in the heart. We conclude that MICU3 plays an important role in regulating [Ca2+]m physiologically, and overexpression of MICU3 is sufficient to induce cardiac hypertrophy, making MICU3 a possible therapeutic target.

Mitochondrial calcium uniporter channel gatekeeping in cardiovascular disease

The mitochondrial calcium (mCa2+) uniporter channel (mtCU) resides at the inner mitochondrial membrane and is required for Ca2+ to enter the mitochondrial matrix. The mtCU is essential for cellular function, as mCa2+ regulates metabolism, bioenergetics, signaling pathways and cell death. mCa2+ uptake is primarily regulated by the MICU family (MICU1, MICU2, MICU3), EF-hand-containing Ca2+-sensing proteins, which respond to cytosolic Ca2+ concentrations to modulate mtCU activity. Considering that mitochondrial function and Ca2+ signaling are ubiquitously disrupted in cardiovascular disease, mtCU function has been a hot area of investigation for the last decade. Here we provide an in-depth review of MICU-mediated regulation of mtCU structure and function, as well as potential mtCU-independent functions of these proteins. We detail their role in cardiac physiology and cardiovascular disease by highlighting the phenotypes of different mutant animal models, with an emphasis on therapeutic potential and targets of interest in this pathway.

High glucose impairs cognitive function through inducing mitochondrial calcium overload in Treg cells

High glucose has been proved to impair cognitive function in type 2 diabetes, but the underlying mechanisms remain elusive. Here, we found that high glucose increased transcription factors' SP1 O-GlcNAcylation in regulatory T (Treg) cells. Glycosylated SP1 further enhanced HDAC2 recruitment and histone deacetylation on Na+/Ca2+/Li+ exchanger (NCLX) promoter, which downregulated NCLX expression and led to mitochondrial calcium overload and oxidative damage, thereby promoting Treg cell dysfunction, M1 microglia polarization, and diabetes-associated cognitive impairment. Importantly, GLP-1 receptor agonist alleviated these deleterious effects via GLP-1-receptor-mediated upregulation of OGA and inhibition of SP1 O-GlcNAcylation in Treg cells. Our study highlighted a link between high-glucose-mediated SP1 O-GlcNAcylation and HDAC2/NCLX signaling in control of mitochondrial calcium concentrations in Treg cells. It also revealed a mechanism for linking Treg cell dysfunction and cognitive impairment in type 2 diabetes and provides an insight into the mechanism underlying the neuroprotective effects of GLP-1 receptor agonist.

Loss of mitochondrial Ca2+ uptake protein 3 impairs skeletal muscle calcium handling and exercise capacity

Mitochondrial calcium concentration ([Ca2+ ]m ) plays an essential role in bioenergetics, and loss of [Ca2+ ]m homeostasis can trigger diseases and cell death in numerous cell types. Ca2+ uptake into mitochondria occurs via the mitochondrial Ca2+ uniporter (MCU), which is regulated by three mitochondrial Ca2+ uptake (MICU) proteins localized in the intermembrane space, MICU1, 2, and 3. We generated a mouse model of systemic MICU3 ablation and examined its physiological role in skeletal muscle. We found that loss of MICU3 led to impaired exercise capacity. When the muscles were directly stimulated there was a decrease in time to fatigue. MICU3 ablation significantly increased the maximal force of the KO muscle and altered fibre type composition with an increase in the ratio of type IIb (low oxidative capacity) to type IIa (high oxidative capacity) fibres. Furthermore, MICU3-KO mitochondria have reduced uptake of Ca2+ and increased phosphorylation of pyruvate dehydrogenase, indicating that KO animals contain less Ca2+ in their mitochondria. Skeletal muscle from MICU3-KO mice exhibited lower net oxidation of NADH during electrically stimulated muscle contraction compared with wild-type. These data demonstrate that MICU3 plays a role in skeletal muscle physiology by setting the proper threshold for mitochondrial Ca2+ uptake, which is important for matching energy demand and supply in muscle. KEY POINTS: Mitochondrial calcium uptake is an important regulator of bioenergetics and cell death and is regulated by the mitochondrial calcium uniporter (MCU) and three calcium sensitive regulatory proteins (MICU1, 2 and 3). Loss of MICU3 leads to impaired exercise capacity and decreased time to skeletal muscle fatigue. Skeletal muscle from MICU3-KO mice exhibits a net oxidation of NADH during electrically stimulated muscle contractions, suggesting that MICU3 plays a role in skeletal muscle physiology by matching energy demand and supply.

An Arabidopsis mutant line lacking the mitochondrial calcium transport regulator MICU shows an altered metabolite profile

Plant metabolism is constantly changing and requires input signals for efficient regulation. The mitochondrial calcium uniporter (MCU) couples organellar and cytoplasmic calcium oscillations leading to oxidative metabolism regulation in a vast array of species. In Arabidopsis thaliana, genetic deletion of AtMICU leads to altered mitochondrial calcium handling and ultrastructure. Here we aimed to further assess the consequences upon genetic deletion of AtMICU. Our results confirm that AtMICU safeguards intracellular calcium transport associated with carbohydrate, amino acid, and phytol metabolism modifications. The implications of such alterations are discussed.

SEPSIS LEADS TO IMPAIRED MITOCHONDRIAL CALCIUM UPTAKE AND SKELETAL MUSCLE WEAKNESS BY REDUCING THE MICU1:MCU PROTEIN RATIO

Purpose: Intensive care unit-acquired weakness (ICUAW) is a severe neuromuscular complication that frequently occurs in patients with sepsis. The precise molecular pathophysiology of mitochondrial calcium uptake 1 (MICU1) and mitochondrial calcium uniporter (MCU) in ICUAW has not been fully elucidated. Here, we speculate that ICUAW is associated with MICU1:MCU protein ratio-mediated mitochondrial calcium ([Ca 2+ ] m ) uptake dysfunction. Methods: Cecal ligation and perforation (CLP) was performed on C57BL/6J mice to induce sepsis. Sham-operated animals were used as controls. Lipopolysaccharide (LPS) (5 μg/mL) was used to induce inflammation in differentiated C2C12 myoblasts. Compound muscle action potential (CMAP) was detected using a biological signal acquisition system. Grip strength was measured using a grip-strength meter. Skeletal muscle inflammatory factors were detected using ELISA kits. The cross-sectional area (CSA) of the tibialis anterior (TA) muscle was detected by hematoxylin and eosin staining. Cytosolic calcium ([Ca 2+ ] c ) levels were measured using Fluo-4 AM. Adeno-associated virus (AAV) was injected into TA muscles for 4 weeks to overexpress MICU1 prophylactically. A lentivirus was used to infect C2C12 cells to increase MICU1 expression prophylactically. Findings: The results suggest that sepsis induces [Ca 2+ ] m uptake disorder by reducing the MICU1:MCU protein ratio, resulting in skeletal muscle weakness and muscle fiber atrophy. However, MICU1 prophylactic overexpression reversed these effects by increasing the MICU1:MCU protein ratio. Conclusions: ICUAW is associated with impaired [Ca 2+ ] m uptake caused by a decreased MICU1:MCU protein ratio. MICU1 overexpression improves sepsis-induced skeletal muscle weakness and atrophy by ameliorating the [Ca 2+ ] m uptake disorder.

Mechanisms of dual modulatory effects of spermine on the mitochondrial calcium uniporter complex

The mitochondrial Ca 2 + uniporter mediates the crucial cellular process of mitochondrial Ca 2 + uptake, which regulates cell bioenergetics, intracellular Ca 2 + signaling, and cell death initiation. The uniporter contains the pore-forming MCU subunit, an EMRE protein that binds to MCU, and the regulatory MICU1 subunit, which can dimerize with MICU1 or MICU2 and under resting cellular [Ca 2 + ] occludes the MCU pore. It has been known for decades that spermine, which is ubiquitously present in animal cells, can enhance mitochondrial Ca 2 + uptake, but the underlying mechanisms remain unclear. Here, we show that spermine exerts dual modulatory effects on the uniporter. In physiological concentrations of spermine, it enhances uniporter activity by breaking the physical interactions between MCU and the MICU1-containing dimers to allow the uniporter to constitutively take up Ca 2 + even in low [Ca 2 + ] conditions. This potentiation effect does not require MICU2 or the EF-hand motifs in MICU1. When [spermine] rises to millimolar levels, it inhibits the uniporter by targeting the pore region in a MICU-independent manner. The MICU1-dependent spermine potentiation mechanism proposed here, along with our previous finding that cardiac mitochondria have very low MICU1, can explain the puzzling observation in the literature that mitochondria in the heart show no response to spermine.

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.

Evidence supporting the MICU1 occlusion mechanism and against the potentiation model in the mitochondrial calcium uniporter complex

The mitochondrial calcium uniporter is a Ca2+ channel that imports cytoplasmic Ca2+ into the mitochondrial matrix to regulate cell bioenergetics, intracellular Ca2+ signaling, and apoptosis. The uniporter contains the pore-forming MCU subunit, an auxiliary EMRE protein, and the regulatory MICU1/MICU2 subunits. Structural and biochemical studies have suggested that MICU1 gates MCU by blocking/unblocking the pore. However, mitoplast patch-clamp experiments argue that MICU1 does not block, but instead potentiates MCU via allosteric mechanisms. Here, we address this direct clash of the proposed MICU1 function. Supporting the MICU1-occlusion mechanism, patch-clamp demonstrates that purified MICU1 strongly suppresses MCU Ca2+ currents, and this inhibition is abolished by mutating the MCU-interacting K126 residue. Moreover, a membrane-depolarization assay shows that MICU1 prevents MCU-mediated Na+ flux into intact mitochondria under Ca2+-free conditions. Examining the observations underlying the potentiation model, we found that MICU1 occlusion was not detected in mitoplasts not because MICU1 cannot block, but because MICU1 dissociates from the uniporter complex. Furthermore, MICU1 depletion reduces uniporter transport not because MICU1 can potentiate MCU, but because EMRE is down-regulated. These results firmly establish the molecular mechanisms underlying the physiologically crucial process of uniporter regulation by MICU1.

Nicorandil protects podocytes via modulation of antioxidative capacity in acute puromycin aminonucleoside-induced nephrosis in rats

Nephrotic syndrome, characterized by proteinuria and hypoalbuminemia, results from the dysregulation of glomerular podocytes and is a significant cause of end-stage kidney disease. Patients with idiopathic nephrotic syndrome are generally treated with immunosuppressive agents; however, these agents produce various adverse effects. Previously, we reported the renoprotective effects of a stimulator of the mitochondrial ATP-dependent K⁺ channel (MitK_ATP), nicorandil, in a remnant kidney model. Nonetheless, the cellular targets of these effects remain unknown. Here, we examined the effect of nicorandil on puromycin aminonucleoside-induced nephrosis (PAN) rats, a well-established model of podocyte injury and human nephrotic syndrome. PAN was induced using a single intraperitoneal injection. Nicorandil was administered orally at 30 mg/kg/day. We found that proteinuria and hypoalbuminemia in PAN rats were significantly ameliorated following nicorandil treatment. Immunostaining and ultrastructural analysis under electron microscopy demonstrated that podocyte injury in PAN rats showed a significant partial attenuation following nicorandil treatment. Nicorandil ameliorated the increase in the oxidative stress markers nitrotyrosine and 8-hydroxy-2-deoxyguanosine in glomeruli. Conversely, nicorandil prevented the decrease in levels of the antioxidant enzyme manganese superoxide dismutase in PAN rats. We found that mitochondrial Ca²⁺ uniporter levels in glomeruli were higher in PAN rats than in control rats, and this increase was significantly attenuated by nicorandil. We conclude that stimulation of MitK_ATP by nicorandil reduces proteinuria by attenuating podocyte injury in PAN nephrosis, which restores mitochondrial antioxidative capacity, possibly through mitochondrial Ca²⁺ uniporter modulation. These data indicate that MitK_ATP may represent a novel target for podocyte injury and nephrotic syndrome.NEW & NOTEWORTHY Our findings suggest that the mitochondrial Ca²⁺ uniporter may be an upstream regulator of manganese superoxide dismutase and indicate a biochemical basis for the interaction between the ATP-sensitive K⁺ channel and Ca²⁺ signaling. We believe that our study makes a significant contribution to the literature because our results indicate that the ATP-sensitive K⁺ channel may be a potential therapeutic target for podocyte injury and nephrotic syndrome.