Calcium influx through the mitochondrial calcium uniporter holocomplex, MCU_cx

Ca²⁺ flux into the mitochondrial matrix through the MCU holocomplex (MCU_cx) has recently been measured quantitatively and with milliseconds resolution for the first time under physiological conditions in both heart and skeletal muscle. Additionally, the dynamic levels of Ca²⁺ in the mitochondrial matrix ([Ca²⁺]_m) of cardiomyocytes were measured as it was controlled by the balance between influx of Ca²⁺ into the mitochondrial matrix through MCU_cx and efflux through the mitochondrial Na⁺ / Ca²⁺ exchanger (NCLX). Under these conditions [Ca²⁺]_m was shown to regulate ATP production by the mitochondria at only a few critical sites. Additional functions attributed to [Ca²⁺]_m continue to be reported in the literature. Here we review the new findings attributed to MCU_cx function and provide a framework for understanding and investigating mitochondrial Ca²⁺ influx features, many of which remain controversial. The properties and functions of the MCU_cx subunits that constitute the holocomplex are challenging to tease apart. Such distinct subunits include EMRE, MCUR1, MICUx (i.e. MICU1, MICU2, MICU3), and the pore-forming subunits (MCU_pore). Currently, the specific set of functions of each subunit remains non-quantitative and controversial. The more contentious issues are discussed in the context of the newly measured native MCU_cx Ca²⁺ flux from heart and skeletal muscle. These MCU_cx Ca²⁺ flux measurements have been shown to be a highly-regulated, tissue-specific with femto-Siemens Ca²⁺ conductances and with distinct extramitochondrial Ca²⁺ ([Ca²⁺]_i) dependencies. These data from cardiac and skeletal muscle mitochondria have been examined quantitatively for their threshold [Ca²⁺]_i levels and for hypothesized gatekeeping function and are discussed in the context of model cell (e.g. HeLa, MEF, HEK293, COS7 cells) measurements. Our new findings on MCU_cx dependent matrix [Ca²⁺]_m signaling provide a quantitative basis for on-going and new investigations of the roles of MCU_cx in cardiac function ranging from metabolic fuel selection, capillary blood-flow control and the pathological activation of the mitochondrial permeability transition pore (mPTP). Additionally, this review presents the use of advanced new methods that can be readily adapted by any investigator to enable them to carry out quantitative Ca²⁺ measurements in mitochondria while controlling the inner mitochondrial membrane potential, ΔΨ_m.