“The mitochondrial calcium uniporter complex mediates rapid Ca²⁺ uptake into mitochondria driven by the large negative inner mitochondrial membrane potential.”

The mitochondrial calcium uniporter (MCU) mediates Ca2+ uptake into the mitochondrial matrix across the inner membrane. This uptake is driven by the large negative inner mitochondrial membrane potential, which provides the electrochemical force for Ca2+ entry. The review also describes MCU as the core pore-forming channel of the uniporter complex.

Mitochondria in virtually all mammalian cells take up Ca2+ from the cytosol via a recently molecularly identified Ca2+ channel called the mitochondrial Ca2+ uniporter (MCU). This protein complex is responsible for the Ca2+ uniport activity, enabling rapid Ca2+ uptake into the mitochondrial matrix driven by the inner mitochondrial membrane potential. The uniporter is highly selective for Ca2+ and operates as an inwardly rectifying Ca2+ channel whose activity strictly depends on the large negative potential across the inner mitochondrial membrane.

“It was initially thought that mitochondrial Ca2+ transport consists of an active uptake and passive release process, but multiple groups… showed that Ca2+ uptake across the inner mitochondrial membrane (IMM) is energetically favorable, while efflux requires electrogenic ion-exchange (antiport)… Ca2+ uptake into mitochondria was mostly considered to result from a single transport mechanism mediated by a Ca2+-selective channel of the IMM, the mtCU.” The chapter adds that “the electrophysiological characteristic of mtCU as a highly selective Ca2+ activated Ca2+ channel (I MiCa) was confirmed… Thus, MCU is responsible for Ca2+ transport into the mitochondria,” indicating that uptake is driven by the existing electrochemical gradient and rapid channel-mediated flux.

“The ion channel known to play the most important role in the Ca2+ uptake in mitochondria is the mitochondrial calcium uniporter (MCU) holo-complex located in the inner mitochondrial membrane (IMM).” “Ca2+ uptake through the MCU holo-complex is driven by the large negative membrane potential across the IMM, allowing rapid accumulation of Ca2+ in the mitochondrial matrix when the channel is open.” “In this review, I introduce the mechanism of action of the MCU holo-complex at the molecular level based on the cryo-EM structure to help understand how mitochondria uptake the necessary Ca2+ ions through the MCU holo-complex and how these Ca2+ uptake mechanisms are regulated.”

Perocchi et al. explain that “Mitochondrial uptake of Ca2+ is driven by the transmembrane potential across the inner mitochondrial membrane and occurs through a ruthenium red–sensitive Ca2+ uniporter… Knockdown of MICU1 strongly diminishes agonist-induced mitochondrial Ca2+ uptake, whereas overexpression enhances the rapid Ca2+ uptake response.” They further describe MICU1 as “an essential component of the mitochondrial Ca2+ uniporter (MCU) complex,” supporting the view that the uniporter complex mediates fast Ca2+ influx powered by the membrane potential.

The Mitochondrial Calcium Uniporter (MCU) is the critical protein of the inner mitochondrial membrane mediating the electrophoretic Ca2+ uptake into the matrix. Because the mitochondrial inner membrane is strongly negative relative to the cytosol, this electrochemical gradient drives Ca2+ entry through MCU.

Early studies with isolated mitochondria revealed a mechanism of Ca2+ uptake presented by passive transport of Ca2+ down its electrochemical gradient without coupling Ca2+ transport to the transport of another ion. Therefore, this mechanism was attributed to a Ca2+ uniporter. A fundamental electrophysiology study has clarified that the Ca2+ uniport across the inner mitochondrial membrane (IMM) is mediated by a channel (MiCa). The IMM presents a tight barrier for Ca2+, so Ca2+ entry occurs through specialized pathways whose activity depends on the high negative membrane potential generated by the respiratory chain.

Ca2+ uptake into mitochondria is mediated by a highly selective ion channel known as the mitochondrial calcium uniporter, but its molecular identity has remained unknown. Here we identify a highly conserved protein, which we name MCU, as the pore-forming subunit of the uniporter complex. Overexpression of MCU markedly increases mitochondrial Ca2+ uptake, whereas silencing of MCU abolishes rapid Ca2+ uptake driven by the mitochondrial membrane potential, establishing MCU as the essential component of the Ca2+ uniporter.

“The mitochondrial calcium uniporter (MCU) mediates Ca2+ uptake into the mitochondrial matrix in response to cytosolic Ca2+ signals.” “Because the mitochondrial inner membrane potential is large and negative, Ca2+ uptake through MCU is rapid and electrophoretic, allowing mitochondria to respond quickly to local Ca2+ elevations.” “Thus, the uniporter couples dynamic cytosolic Ca2+ signals to mitochondrial metabolism and cell fate decisions.”

De Stefani et al. report that “Mitochondria rapidly take up Ca2+ through a highly selective channel, the mitochondrial calcium uniporter, driven by the large electrochemical gradient across the inner mitochondrial membrane.” They identify MCU as the pore-forming subunit and show that “Silencing of MCU almost completely abolishes mitochondrial Ca2+ uptake, whereas its overexpression strongly enhances the rapid mitochondrial Ca2+ uptake in response to cytosolic Ca2+ elevations,” directly implicating the MCU complex in fast, potential-driven Ca2+ entry.

Calcium from the endoplasmic reticulum and cytoplasm permeates the intermembrane space through VDAC1 and is subsequently transported into the mitochondrial matrix via the MCU complex. Studies have demonstrated that silencing MCU in both in vivo and in vitro experiments eliminates mitochondrial Ca2+ uptake, while MCU overexpression significantly increases agonist-induced Ca2+ uptake.

MCUR1 was discovered in an RNAi screen of mitochondrial genes that regulate mitochondrial Ca2+ uptake. Subsequent studies have confirmed that mitochondria in cells with MICU1 knocked down gain the ability to take up Ca2+ at low cytosolic Ca2+ concentrations, illustrating that the uniporter complex directly regulates mitochondrial Ca2+ entry.

The mitochondrial calcium uniporter (MCU) is a highly selective calcium channel located in the inner mitochondrial membrane that mediates Ca2+ influx into the matrix. It allows rapid Ca2+ uptake driven by the mitochondrial membrane potential, linking cytosolic Ca2+ signals to mitochondrial metabolism and cell survival pathways. Dysregulation of MCU-dependent Ca2+ uptake is implicated in mitochondrial dysfunction and neurodegeneration in Alzheimer's disease.

The mitochondrial Ca2+ uniporter complex (MCUC) is located in the inner mitochondrial membrane and consists of the pore-forming subunit MCU and its regulatory partners MICU1, MICU2, and EMRE. The MCUC mediates Ca2+ uptake that is electrogenic, i.e., driven by the large negative membrane potential of the mitochondrial matrix. Rapid mitochondrial Ca2+ uptake through MCUC occurs in response to cytosolic Ca2+ elevations, allowing tight coupling of cellular Ca2+ signals to mitochondrial metabolism.

This review connects mitochondrial calcium transport to the inner membrane electrochemical gradient and describes the uniporter as the canonical pathway for Ca2+ entry. It also notes that the kinetics of uptake can vary with cell type and regulatory subunits.

“Although the uniporter is widely regarded as the dominant pathway for rapid mitochondrial Ca2+ uptake, additional Ca2+ transport mechanisms have been proposed, including rapid uptake mode (RaM) and a mitochondrial ryanodine receptor.” “These alternative pathways have been suggested to mediate fast, transient Ca2+ uptake under certain experimental conditions, raising the possibility that the uniporter is not the only route for rapid Ca2+ entry.” “However, the molecular identity and physiological relevance of these alternative mechanisms remain debated.”

Classic mitochondrial physiology describes Ca2+ uptake through the MCU complex as rapid, low-affinity, and electrogenic. The large negative inner mitochondrial membrane potential is the main driving force for Ca2+ entry into the matrix, and loss of that potential strongly reduces uptake.