Expression and preliminary characterization of human MICU2

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.

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.

Mitochondrial calcium exchange in physiology and disease

The uptake of calcium into and extrusion of calcium from the mitochondrial matrix is a fundamental biological process that has critical effects on cellular metabolism, signaling, and survival. Disruption of mitochondrial calcium (mCa2+) cycling is implicated in numerous acquired diseases such as heart failure, stroke, neurodegeneration, diabetes, and cancer and is genetically linked to several inherited neuromuscular disorders. Understanding the mechanisms responsible for mCa2+ exchange therefore holds great promise for the treatment of these diseases. The past decade has seen the genetic identification of many of the key proteins that mediate mitochondrial calcium uptake and efflux. Here, we present an overview of the phenomenon of mCa2+ transport and a comprehensive examination of the molecular machinery that mediates calcium flux across the inner mitochondrial membrane: the mitochondrial uniporter complex (consisting of MCU, EMRE, MICU1, MICU2, MICU3, MCUB, and MCUR1), NCLX, LETM1, the mitochondrial ryanodine receptor, and the mitochondrial permeability transition pore. We then consider the physiological implications of mCa2+ flux and evaluate how alterations in mCa2+ homeostasis contribute to human disease. This review concludes by highlighting opportunities and challenges for therapeutic intervention in pathologies characterized by aberrant mCa2+ handling and by summarizing critical unanswered questions regarding the biology of mCa2+ flux.

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

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

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The uniporter contains multiple subunits regulating various cellular processes

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

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

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

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

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

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

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

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

Expression and Characterization of MICU2, a Ca2+ Sensor Protein

MICU2 is a Ca²⁺ sensor protein of mitochondrial uniporter which is a highly selective Ca²⁺ channel mediating mitochondrial Ca²⁺ uptake to regulate cell death, metabolism, and cytoplasmic Ca²⁺ signaling. Here we describe the procedures for protein preparation of various MICU2 constructs, which have enabled successful in vitro characterizations of MICU2 including interaction with MICU1 using pull-down assays and oligomerization using multi-angle laser light scattering.

MICU1 and MICU2 Play an Essential Role in Mitochondrial Ca2+ Uptake, Growth, and Infectivity of the Human Pathogen Trypanosoma cruzi

The mitochondrial Ca²⁺ uptake in trypanosomatids, which belong to the eukaryotic supergroup Excavata, shares biochemical characteristics with that of animals, which, together with fungi, belong to the supergroup Opisthokonta. However, the composition of the mitochondrial calcium uniporter (MCU) complex in trypanosomatids is quite peculiar, suggesting lineage-specific adaptations. In this work, we used Trypanosoma cruzi to study the role of orthologs for mitochondrial calcium uptake 1 (MICU1) and MICU2 in mitochondrial Ca²⁺ uptake. T. cruzi MICU1 (TcMICU1) and TcMICU2 have mitochondrial targeting signals, two canonical EF-hand calcium-binding domains, and localize to the mitochondria. Using the CRISPR/Cas9 system (i.e., clustered regularly interspaced short palindromic repeats with Cas9), we generated TcMICU1 and TcMICU2 knockout (-KO) cell lines. Ablation of either TcMICU1 or TcMICU2 showed a significantly reduced mitochondrial Ca²⁺ uptake in permeabilized epimastigotes without dissipation of the mitochondrial membrane potential or effects on the AMP/ATP ratio or citrate synthase activity. However, none of these proteins had a gatekeeper function at low cytosolic Ca²⁺ concentrations ([Ca²⁺]_cyt), as occurs with their mammalian orthologs. TcMICU1-KO and TcMICU2-KO epimastigotes had a lower growth rate and impaired oxidative metabolism, while infective trypomastigotes have a reduced capacity to invade host cells and to replicate within them as amastigotes. The findings of this work, which is the first to study the role of MICU1 and MICU2 in organisms evolutionarily distant from animals, suggest that, although these components were probably present in the last eukaryotic common ancestor (LECA), they developed different roles during evolution of different eukaryotic supergroups. The work also provides new insights into the adaptations of trypanosomatids to their particular life styles.IMPORTANCE Trypanosoma cruzi is the etiologic agent of Chagas disease and belongs to the early-branching eukaryotic supergroup Excavata. Its mitochondrial calcium uniporter (MCU) subunit shares similarity with the animal ortholog that was important to discover its encoding gene. In animal cells, the MICU1 and MICU2 proteins act as Ca²⁺ sensors and gatekeepers of the MCU, preventing Ca²⁺ uptake under resting conditions and favoring it at high cytosolic Ca²⁺ concentrations ([Ca²⁺]_cyt). Using the CRISPR/Cas9 technique, we generated TcMICU1 and TcMICU2 knockout cell lines and showed that MICU1 and -2 do not act as gatekeepers at low [Ca²⁺]_cyt but are essential for normal growth, host cell invasion, and intracellular replication, revealing lineage-specific adaptations.

MICU2 Restricts Spatial Crosstalk between InsP3R and MCU Channels by Regulating Threshold and Gain of MICU1-Mediated Inhibition and Activation of MCU

Ca²⁺ entry into mitochondria is mediated by the Ca²⁺ uniporter-channel complex containing MCU, the Ca²⁺-selective pore, and associated regulatory proteins. The roles of MICU proteins are controversial. MICU1 was proposed to be necessary for MCU activity, whereas subsequent studies suggested it inhibits the channel in the low-cytoplasmic Ca²⁺ ([Ca²⁺]_c) regime, a mechanism referred to as "gatekeeping," that imposes a [Ca²⁺]_c threshold for channel activation at ∼1-3 μM. Here, we measured MCU activity over a wide range of quantitatively controlled and recorded [Ca²⁺]_c. MICU1 alone can mediate gatekeeping as well as highly cooperative activation of MCU activity, whereas the fundamental role of MICU2 is to regulate the threshold and gain of MICU1-mediated inhibition and activation of MCU. Our results provide a unifying model for the roles of the MICU1/2 heterodimer in MCU-channel regulation and suggest an evolutionary role for MICU2 in spatially restricting Ca²⁺ crosstalk between single inositol 1,4,5-trisphosphate receptor (InsP₃R) and MCU channels.

Electrical recordings of the mitochondrial calcium uniporter in Xenopus oocytes

The mitochondrial calcium uniporter is a Ca²⁺ channel that has been hard to characterize electrophysiologically. Tsai and Tsai establish a method that permits efficient electrophysiological recordings of the human uniporter in Xenopus oocytes and demonstrate characteristic uniporter behaviour.

The mitochondrial calcium uniporter is a multisubunit Ca²⁺ channel that mediates mitochondrial Ca²⁺ uptake, a cellular process crucial for the regulation of oxidative phosphorylation, intracellular Ca²⁺ signaling, and apoptosis. In the last few years, genes encoding uniporter proteins have been identified, but a lack of efficient tools for electrophysiological recordings has hindered quantitative analysis required to determine functional mechanisms of this channel complex. Here, we redirected Ca²⁺-conducting subunits (MCU and EMRE) of the human uniporter to the plasma membrane of Xenopus oocytes. Two-electrode voltage clamp reveals inwardly rectifying Ca²⁺ currents blocked by a potent inhibitor, Ru360 (half maximal inhibitory concentration, ~4 nM), with a divalent cation conductivity of Ca²⁺ > Sr²⁺ > Ba²⁺, Mn²⁺, and Mg²⁺. Patch clamp recordings further reveal macroscopic and single-channel Ca²⁺ currents sensitive to Ru360. These electrical phenomena were abolished by mutations that perturb MCU-EMRE interactions or disrupt a Ca²⁺-binding site in the pore. Altogether, this work establishes a robust method that enables deep mechanistic scrutiny of the uniporter using classical strategies in ion channel electrophysiology.

High-affinity cooperative Ca2+ binding by MICU1-MICU2 serves as an on-off switch for the uniporter

The mitochondrial calcium uniporter is a Ca²⁺-activated Ca²⁺ channel that is essential for dynamic modulation of mitochondrial function in response to cellular Ca²⁺ signals. It is regulated by two paralogous EF-hand proteins-MICU1 and MICU2, but the mechanism is unknown. Here, we demonstrate that both MICU1 and MICU2 are stabilized by Ca²⁺ We reconstitute the MICU1-MICU2 heterodimer and demonstrate that it binds Ca²⁺ cooperatively with high affinity. We discover that both MICU1 and MICU2 exhibit affinity for the mitochondria-specific lipid cardiolipin. We determine the minimum Ca²⁺ concentration required for disinhibition of the uniporter in permeabilized cells and report a close match with the Ca²⁺-binding affinity of MICU1-MICU2. We conclude that cooperative, high-affinity interaction of the MICU1-MICU2 complex with Ca²⁺ serves as an on-off switch, leading to a tightly controlled channel, capable of responding directly to cytosolic Ca²⁺ signals.

Pathological consequences of MICU1 mutations on mitochondrial calcium signalling and bioenergetics

Loss of function mutations of the protein MICU1, a regulator of mitochondrial Ca^2 + uptake, cause a neuronal and muscular disorder characterised by impaired cognition, muscle weakness and an extrapyramidal motor disorder. We have shown previously that MICU1 mutations cause increased resting mitochondrial Ca²⁺ concentration ([Ca^2 +]_m). We now explore the functional consequences of MICU1 mutations in patient derived fibroblasts in order to clarify the underlying pathophysiology of this disorder. We propose that deregulation of mitochondrial Ca²⁺ uptake through loss of MICU1 raises resting [Ca²⁺]_m, initiating a futile Ca²⁺ cycle, whereby continuous mitochondrial Ca²⁺ influx is balanced by Ca²⁺ efflux through the sodium calcium exchanger (NLCX_m). Thus, inhibition of NCLX_m by CGP-37157 caused rapid mitochondrial Ca²⁺ accumulation in patient but not control cells. We suggest that increased NCLX activity will increase sodium/proton exchange, potentially undermining oxidative phosphorylation, although this is balanced by dephosphorylation and activation of pyruvate dehydrogenase (PDH) in response to the increased [Ca²⁺]_m. Consistent with this model, while ATP content in patient derived or control fibroblasts was not different, ATP increased significantly in response to CGP-37157 in the patient but not the control cells. In addition, EMRE expression levels were altered in MICU1 patient cells compared to the controls. The MICU1 mutations were associated with mitochondrial fragmentation which we show is related to altered DRP1 phosphorylation. Thus, MICU1 serves as a signal–noise discriminator in mitochondrial calcium signalling, limiting the energetic costs of mitochondrial Ca²⁺ signalling which may undermine oxidative phosphorylation, especially in tissues with highly dynamic energetic demands. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

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Loss of MICU1 protein expression in human fibroblasts increases resting mitochondrial calcium concentration ([Ca²⁺]_m).

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The increased mitochondrial Ca²⁺ uptake causes a futile Ca²⁺ cycle in MICU1 deficient cells.

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Increased [Ca²⁺]_mactivates pyruvate dehydrogenase (PDH) by activating PDH phosphatase, consequently dephosphorylating PDH.

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Loss of MICU1 leads to modifications of the MCU complex composition and mitochondrial fragmentation.