MICU1 Serves as a Molecular Gatekeeper to Prevent In Vivo Mitochondrial Calcium Overload

Abstract

MICU1 is a component of the mitochondrial calcium uniporter, a multiprotein complex that also includes MICU2, MCU, and EMRE. Here, we describe a mouse model of MICU1 deficiency. MICU1−/− mitochondria demonstrate altered calcium uptake and deletion of MICU1 results in significant, but not complete, perinatal mortality. Similar to afflicted patients, viable MICU1−/− mice manifest marked ataxia and muscle weakness. Early in life, these animals display a range of biochemical abnormalities including increased resting mitochondrial calcium levels, altered mitochondrial morphology, and reduced ATP. Older MICU1−/− mice show marked, spontaneous improvement, coincident with improved mitochondrial calcium handling and an age-dependent reduction in EMRE expression. Remarkably, deleting one allele of EMRE helps normalize calcium uptake while simultaneously rescuing the high perinatal mortality observed in young MICU1−/− mice. Together, these results demonstrate that MICU1 serves as a molecular gatekeeper preventing calcium overload and suggests that modulating the calcium uniporter could have widespread therapeutic benefits.

MICU2 Restricts Spatial Crosstalk between InsP 3 R and MCU Channels by Regulating Threshold and Gain of MICU1-Mediated Inhibition and Activation of MCU

“…Clearance of bath Ca 2+ in MICU1_KO cells was blocked by Ru360 (1 μM – not shown), indicating that it was mediated by MCU. These results recapitulate previous observations that MCU-mediated Ca 2+ uptake in low [Ca 2+ ] c is strongly enhanced in the absence of MICU1 (Mallilankaraman et al, 2012; Csordás et al, 2013; Patron et al, 2014; Liu et al, 2016). Using a genetically-encoded Ca 2+ indicator targeted to the mitochondrial matrix (mito-CAR-GECO1; K D = 490 nM; Figures 1C) (Wu et al, 2013), [Ca 2+ ] m in WT cells was ~100 nM whereas it was increased over 3-fold to ~300–400 nM in MICU1_KO cells (Figure 1D).…”

“…In the absence of MICU1, MCU becomes constitutively active, in agreement with previous reports (Mallilankaraman et al, 2012; Csordás et al, 2013; Patron et al, 2014; Liu et al, 2016). Whereas some previous studies were confounded by reduced MICU2 expression in cells with MICU1 knocked-down or deleted, here we observed similar constitutive activity even when MICU2 was re-expressed to normal levels in MICU1_KO cells.…”

“…Here, we have for the first time quantified the new steady-state [Ca 2+ ] m in cells lacking MICU1. Perhaps surprisingly, resting [Ca 2+ ] m was ~300–400 nM, which while elevated compared with ~100 nM in WT cells, is not excessively high, suggesting that mitochondria in MICU1_KO cells may not be as Ca 2+ over-loaded as has been inferred (Mallilankaraman et al, 2012; Logan et al, 2013; Antony et al, 2016; Liu et al, 2016; Bhosale et al, 2017). Several mechanisms could account for why [Ca 2+ ] m is not elevated to much higher levels.…”

“…The importance of understanding regulation of MCU-channel activity by MICU proteins is underscored by human loss-of-function mutations in MICU1 that result in apparent mitochondrial-Ca 2+ overload and cause early-onset muscle weakness and CNS defects (Logan et al, 2013; Lewis-Smith et al, 2016). Similar pathology and effects on mitochondrial [Ca 2+ ] observed in surviving MICU1_KO mice suggested that constitutive MCU-activity in vivo caused disease pathogenesis (Liu et al, 2016). …”

Julia Liu

“…We found that when MICU1, one component of the uniporter, is deleted, the uniporter loses its gatekeeper ability and lets calcium in even under basal, unstimulated conditions, leading to calcium overload in the matrix. 2 There are a number of different diseases that are characterized by having mitochondrial calcium overload, such as Parkinson's, Alzheimer's, and muscular dystrophy, so understanding how to prevent or mitigate that therapeutically might help us treat these disorders.…”