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.

Mitochondrial Ca²⁺ is involved in heterogeneous functions, ranging from the control of metabolism and ATP production to the regulation of cell death. In addition, mitochondrial Ca²⁺ uptake contributes to cytosolic [Ca²⁺] shaping thus impinging on specific Ca²⁺-dependent events. Mitochondrial Ca²⁺ concentration is controlled by influx and efflux pathways: the former controlled by the activity of the mitochondrial Ca²⁺ uniporter (MCU), the latter by the Na⁺/Ca²⁺ exchanger (NCLX) and the H⁺/Ca²⁺ (mHCX) exchanger. The molecular identities of MCU and of NCLX have been recently unraveled, thus allowing genetic studies on their physiopathological relevance. After a general framework on the significance of mitochondrial Ca²⁺ uptake, this review discusses the structure of the MCU complex and the regulation of its activity, the importance of mitochondrial Ca²⁺ signaling in different physiological settings, and the consequences of MCU modulation on organ physiology.

Calcium ions are highly versatile intracellular signals that regulate many cellular processes. The key to achieving this pleiotropic role is the spatiotemporal control of calcium concentration evoked by an extensive molecular repertoire of signalling components. Among these, reactive oxygen species (ROS) signalling, together with calcium signalling, plays a crucial role in controlling several physiopathological events. Although initially considered detrimental by-products of aerobic metabolism, it is now widely accepted that ROS, in subtoxic levels, act as signalling molecules. However, dysfunctions in the mechanisms controlling the physiological ROS concentration affect cellular homeostasis, leading to the pathogenesis of various disorders.

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

Mitochondrial Ca(2+) uptake regulates a wide array of cell functions, from stimulation of aerobic metabolism and ATP production in physiological settings, to induction of cell death in pathological conditions. The molecular identity of the Mitochondrial Calcium Uniporter (MCU), the highly selective channel responsible for Ca(2+) entry through the IMM, has been described less than five years ago. Since then, research has been conducted to clarify the modulation of its activity, which relies on the dynamic interaction with regulatory proteins, and its contribution to the pathophysiology of organs and tissues. Particular attention has been placed on characterizing the role of MCU in cardiac and skeletal muscles. In this review we summarize the molecular structure and regulation of the MCU complex in addition to its pathophysiological role, with particular attention to striated muscle tissues. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.