Functional roles of MICU1 and MICU2 in mitochondrial Ca2+ uptake (original) (raw)
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MICU3 is a tissue-specific enhancer of mitochondrial calcium uptake
Cell Death & Differentiation
The versatility and universality of Ca 2+ as intracellular messenger is guaranteed by the compartmentalization of changes in [Ca 2+ ]. In this context, mitochondrial Ca 2+ plays a central role, by regulating both specific organelle functions and global cellular events. This versatility is also guaranteed by a cell type-specific Ca 2+ signaling toolkit controlling specific cellular functions. Accordingly, mitochondrial Ca 2+ uptake is mediated by a multimolecular structure, the MCU complex, which differs among various tissues. Its activity is indeed controlled by different components that cooperate to modulate specific channeling properties. We here investigate the role of MICU3, an EF-hand containing protein expressed at high levels, especially in brain. We show that MICU3 forms a disulfide bond-mediated dimer with MICU1, but not with MICU2, and it acts as enhancer of MCU-dependent mitochondrial Ca 2+ uptake. Silencing of MICU3 in primary cortical neurons impairs Ca 2+ signals elicited by synaptic activity, thus suggesting a specific role in regulating neuronal function.
Biophysical Journal, 2013
Mitochondrial Ca 2+ (Ca 2+ m ) uptake is mediated by an inner membrane Ca 2+ channel called the uniporter. Ca 2+ uptake is driven by the considerable voltage present across the inner membrane (DJ m ) generated by proton pumping by the respiratory chain. Mitochondrial matrix Ca 2+ concentration is maintained five to six orders of magnitude lower than its equilibrium level, but the molecular mechanisms for how this is achieved are not clear. Here, we demonstrate that the mitochondrial protein MICU1 is required to preserve normal [Ca 2+ ] m under basal conditions. In its absence, mitochondria become constitutively loaded with Ca 2+ , triggering excessive reactive oxygen species generation and sensitivity to apoptotic stress. MICU1 interacts with the uniporter pore-forming subunit MCU and sets a Ca 2+ threshold for Ca 2+ m uptake without affecting the kinetic properties of MCU-mediated Ca 2+ uptake. Thus, MICU1 is a gatekeeper of MCU-mediated Ca 2+ m uptake that is essential to prevent [Ca 2+ ] m overload and associated stress.
MICU1 encodes a mitochondrial EF hand protein required for Ca2+ uptake
Nature, 2010
Mitochondrial calcium uptake plays a central role in cell physiology by stimulating ATP production, shaping cytosolic calcium transients, and regulating cell death. The biophysical properties of mitochondrial calcium uptake have been studied in detail, but the underlying proteins remain elusive. Here, we utilize an integrative strategy to predict human genes involved in mitochondrial calcium entry based on clues from comparative physiology, evolutionary genomics, and organelle proteomics. RNA interference against 13 top candidates highlighted one gene that we now call mitochondrial calcium uptake 1 (MICU1). Silencing MICU1 does not disrupt mitochondrial respiration or membrane potential but abolishes mitochondrial calcium entry in intact and permeabilized cells, and attenuates the metabolic coupling between cytosolic calcium transients and activation of matrix dehydrogenases. MICU1 is associated with the organelle's inner membrane and has two canonical EF hands that are essential for its activity, suggesting a role in calcium sensing. MICU1 represents the founding member of a set of proteins required for high capacity mitochondrial calcium entry. Its discovery may lead to the complete molecular characterization of mitochondrial calcium uptake pathways, and offers genetic strategies for understanding their contribution to normal physiology and disease. The uptake of calcium (Ca 2+) by vertebrate mitochondria was first documented nearly 50 years ago1,2. These early studies revealed that suspensions of isolated mitochondria can transport and buffer massive amounts of Ca 2+ across the inner membrane. This high capacity "uniporter" mechanism is classically defined by its dependence on membrane potential, sensitivity to ruthenium red, and activity when extramitochondrial calcium concentrations are in the micromolar range. Subsequent studies, using genetically encoded Users may view, print, copy, download and text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:
Molecular cell, 2014
Mitochondrial calcium accumulation was recently shown to depend on a complex composed of an inner-membrane channel (MCU and MCUb) and regulatory subunits (MICU1, MCUR1, and EMRE). A fundamental property of MCU is low activity at resting cytosolic Ca 2+ concentrations, preventing deleterious Ca 2+ cycling and organelle overload. Here we demonstrate that these properties are ensured by a regulatory heterodimer composed of two proteins with opposite effects, MICU1 and MICU2, which, both in purified lipid bilayers and in intact cells, stimulate and inhibit MCU activity, respectively. Both MICU1 and MICU2 are regulated by calcium through their EF-hand domains, thus accounting for the sigmoidal response of MCU to [Ca 2+ ] in situ and allowing tight physiological control. At low [Ca 2+ ], the dominant effect of MICU2 largely shuts down MCU activity; at higher [Ca 2+ ], the stimulatory effect of MICU1 allows the prompt response of mitochondria to Ca 2+ signals generated in the cytoplasm. Molecular Cell 53, 1-12, March 6, 2014 ยช2014 Elsevier Inc. 1 Please cite this article in press as: Patron et al., MICU1 and MICU2 Finely Tune the Mitochondrial Ca 2+ Uniporter by Exerting Opposite Effects on MCU Activity, Molecular Cell (2014), http://dx.
PLoS ONE, 2013
Mitochondrial calcium uptake is present in nearly all vertebrate tissues and is believed to be critical in shaping calcium signaling, regulating ATP synthesis and controlling cell death. Calcium uptake occurs through a channel called the uniporter that resides in the inner mitochondrial membrane. Recently, we used comparative genomics to identify MICU1 and MCU as the key regulatory and putative pore-forming subunits of this channel, respectively. Using bioinformatics, we now report that the human genome encodes two additional paralogs of MICU1, which we call MICU2 and MICU3, each of which likely arose by gene duplication and exhibits distinct patterns of organ expression. We demonstrate that MICU1 and MICU2 are expressed in HeLa and HEK293T cells, and provide multiple lines of biochemical evidence that MCU, MICU1 and MICU2 reside within a complex and cross-stabilize each other's protein expression in a cell-type dependent manner. Using in vivo RNAi technology to silence MICU1, MICU2 or both proteins in mouse liver, we observe an additive impairment in calcium handling without adversely impacting mitochondrial respiration or membrane potential. The results identify MICU2 as a new component of the uniporter complex that may contribute to the tissue-specific regulation of this channel.
MICU1 Controls Both the Threshold and Cooperative Activation of the Mitochondrial Ca2+ Uniporter
Cell Metabolism, 2013
Mitochondrial Ca 2+ uptake via the uniporter is central to cell metabolism, signaling, and survival. Recent studies identified MCU as the uniporter's likely pore and MICU1, an EF-hand protein, as its critical regulator. How this complex decodes dynamic cytoplasmic [Ca 2+ ] ([Ca 2+ ] c ) signals, to tune out small [Ca 2+ ] c increases yet permit pulse transmission, remains unknown. We report that loss of MICU1 in mouse liver and cultured cells causes mitochondrial Ca 2+ accumulation during small [Ca 2+ ] c elevations but an attenuated response to agonist-induced [Ca 2+ ] c pulses. The latter reflects loss of positive cooperativity, likely via the EF-hands. MICU1 faces the intermembrane space and responds to [Ca 2+ ] c changes. Prolonged MICU1 loss leads to an adaptive increase in matrix Ca 2+ binding, yet cells show impaired oxidative metabolism and sensitization to Ca 2+ overload. Collectively, the data indicate that MICU1 senses the [Ca 2+ ] c to establish the uniporter's threshold and gain, thereby allowing mitochondria to properly decode different inputs.
Pathological consequences of MICU1 mutations on mitochondrial calcium signalling and bioenergetics
Biochimica et Biophysica Acta (BBA) - Molecular Cell Research, 2017
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 Ca 2+ 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 Ca 2+ uptake through loss of MICU1 raises resting [Ca 2+ ] m , initiating a futile Ca 2+ cycle, whereby continuous mitochondrial Ca 2+ influx is balanced by Ca 2+ efflux through the sodium calcium exchanger (NLCX m). Thus, inhibition of NCLX m by CGP-37157 caused rapid mitochondrial Ca 2+ 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 [Ca 2+ ] 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 Ca 2+ 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.