MCUB Regulates the Molecular Composition of the Mitochondrial Calcium Uniporter Channel to Limit Mitochondrial Calcium Overload During Stress

Abstract

Mitochondrial Calcium (mCa 2+) overload is a central event in myocardial-ischemia reperfusion (IR) injury that leads to metabolic derangement as well as activation of the mitochondrial permeability transition pore (mPTP). mPTP activation results in necrosis and loss of cardiomyocytes which results in acute death in some individuals while survivors are prone to developing heart failure and are predisposed to recurrent infarction events. mCa 2+ has also long been known to activate cellular bioenergetics implicating mCa 2+ in the highly metabolically demanding state of cardiac contractility. The mitochondrial calcium uniporter channel (mtCU) is a multi-subunit complex that resides in the inner mitochondrial membrane and is required for mitochondrial Ca 2+ (mCa 2+) uptake. Mitochondrial Calcium Uniporter B (MCUB, CCDC109B gene), a recently identified paralog of MCU, is reported to negatively regulate mCa 2+ uptake; however, its precise regulation of uniporter function and contribution to cardiac physiology remain unresolved. Size exclusion chromatography of mitochondria isolated from ventricular tissue revealed MCUB was undetectable in the high-molecular weight (MW) fraction of sham animals (~700kD, size of functional mtCU), but 24 hours following myocardial ischemia-reperfusion injury (IR) MCUB was clearly observed in the high-MW fraction. To investigate how MCUB contributes to mtCU regulation we created a stable MCUB-/-HeLa cell line using CRISPR-Cas9n. MCUB deletion increased histamine-mediated mCa 2+ transient amplitude by ~50% versus Wild-Type (WT) controls (mito-R-GECO1). Further, MCUB deletion increased mtCU capacitance (patch-clamp) and rate of [mCa 2+ ] uptake. Fast

Mitochondrial Calcification

“…In cardiac cells, with 37% of cell volume being mitochondria, such regulation through the higher expression of MCUb is crucial to prevent the massive accumulation of Ca 2+ by mitochondria and dysfunction and undesired cytosolic Ca 2+ buffering preventing heart contractile activity. Further, induction of MCUb expression was shown to be a stress-responsive mechanism to overcome calcium overload following cardiac injury [ 30 ]. Future studies in this area should explore how various physiological and pathological stimuli alter the ratios of MCU: MCUb and the consequences of such an altered expression on mitochondrial Ca 2+ uptake sensitivities/loading capacity of tissues and implications for tissue calcification.…”

Increased RyR2 activity is exacerbated by calcium leak-induced mitochondrial ROS

“…Indeed, as shown in Fig. 8e, f, CPVT is accompanied by a significant increase in expression levels of an inhibitory variant of MCU (MCUb; [41,58]), and consequently a ~ 200% increase in the MCUb/ MCU ratio. Taken together, our data suggest that hereditary enhancement in RyR2-complex activity affects mito-ROS production, exacerbating RyR2 dysfunction.…”

The Mitochondrial Ca2+ Uptake and the Fine-Tuning of Aerobic Metabolism

“…However, in the inducible cardiac-specific MCU knockout mouse, during the fight or flight response an increase in the mitochondrial ATP production is required to support the elevated energy demand of the augmented heart rate ( Kwong et al, 2015 ; Luongo et al, 2015 ). In addition, hearts expressing a dominant-negative MCU or overexpressing the MCUb treated with the β-adrenergic agonist isoproterenol show an impairment in the heart rate acceleration ( Wu et al, 2015 ; Lambert et al, 2019 ). In the conditional MCUb heart-specific transgenic mouse model mitochondrial bioenergetics was impaired because of increased PDH phosphorylation levels leading to a decrease in the maximal respiration and in the reserve capacity.…”

“…The impairment in the heart responsiveness was highlighted by the fact that these mice did not survive after IR injury. However, long-term induction of MCUb expression was accompanied by a rescue of the cardiac function and reduced infarct size, most likely because of compensatory mechanisms triggering PDH dephosphorylation and thus activation ( Lambert et al, 2019 ). In addition, a change in the stoichiometry of the MCU complex was observed in the hearts of these mice.…”

“…In addition, a change in the stoichiometry of the MCU complex was observed in the hearts of these mice. MCU, MICU1, and MICU2 levels were increased despite decreased association of MICU1 and MICU2 to the MCU complex ( Lambert et al, 2019 ).…”