Pathological consequences of MICU1 mutations on mitochondrial calcium signalling and bioenergetics

Abstract

Loss of function mutations of the protein MICU1, a regulator of mitochondrial Ca2 + uptake, cause a neuronal and muscular disorder characterised by impaired cognition, muscle weakness and an extrapyramidal motor disorder. We have shown previously that MICU1 mutations cause increased resting mitochondrial Ca2+ concentration ([Ca2 +]m). We now explore the functional consequences of MICU1 mutations in patient derived fibroblasts in order to clarify the underlying pathophysiology of this disorder. We propose that deregulation of mitochondrial Ca2+ uptake through loss of MICU1 raises resting [Ca2+]m, initiating a futile Ca2+ cycle, whereby continuous mitochondrial Ca2+ influx is balanced by Ca2+ efflux through the sodium calcium exchanger (NLCXm). Thus, inhibition of NCLXm by CGP-37157 caused rapid mitochondrial Ca2+ accumulation in patient but not control cells. We suggest that increased NCLX activity will increase sodium/proton exchange, potentially undermining oxidative phosphorylation, although this is balanced by dephosphorylation and activation of pyruvate dehydrogenase (PDH) in response to the increased [Ca2+]m. Consistent with this model, while ATP content in patient derived or control fibroblasts was not different, ATP increased significantly in response to CGP-37157 in the patient but not the control cells. In addition, EMRE expression levels were altered in MICU1 patient cells compared to the controls. The MICU1 mutations were associated with mitochondrial fragmentation which we show is related to altered DRP1 phosphorylation. Thus, MICU1 serves as a signal–noise discriminator in mitochondrial calcium signalling, limiting the energetic costs of mitochondrial Ca2+ signalling which may undermine oxidative phosphorylation, especially in tissues with highly dynamic energetic demands. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

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

“…Thus, in the absence of MICU1, constitutive influx and efflux constitutes an ongoing Ca 2+ cycling across the IMM that establishes a new, higher steady-state [Ca 2+ ] m . These observations corroborate those in fibroblasts from patients with genetic defects in MICU1 that result in absence of MICU1 expression (Bhosale et al, 2017). Here, we have for the first time quantified the new steady-state [Ca 2+ ] m in cells lacking MICU1.…”

“…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.…”

“…Nevertheless, pathogenesis in MICU1_KO animals is consistent with that expected from mitochondrial Ca 2+ overload (Liu et al, 2016). The relatively modest matrix free-Ca 2+ elevation under resting conditions may not be the (sole) cause of disease pathogenesis in animal models (Antony et al, 2016; Liu et al, 2016) and patients (Logan et al, 2013; Lewis-Smith et al, 2016; Bhosale et al, 2017). Rather, pathogenesis might be caused by sensitized gain-of-function responses to transient [Ca 2+ ] c signals up to the normal threshold of ~1.3 μM.…”

Structure of intact human MCU supercomplex with the auxiliary MICU subunits

“…The Glu 264 site is likely the same site in the MCU pore that was previously reported to bind cytosolic Ca 2+ with Kd ≤ 2 nM ( 6 ), and thus it should always be occupied by Ca 2+ under physiological conditions. The lowest cytosolic [Ca 2+ ] at which Ca 2+ permeation through the MCU pore can occur is ∼100 nM ( 22, 49, 50 ). At this concentration, Ca 2+ should start binding to the Asp 261 site, which reduces affinity for Ca 2+ binding at Glu 264 and makes permeation possible.…”

Mitochondrial junctions with cellular organelles: Ca2+ signalling perspective

“…This prevents futile Ca 2+ cycle and the associated bioenergetics costs required to maintain acceptably low mitochondrial Ca 2+ concentration. Such futile Ca 2+ cycle and ATP expenditure were recently demonstrated in cells harbouring MICU1 mutation by G. Bhosale and colleagues from M. Duchen’s laboratory [10]. Threshold created by MICU1 and MICU2 is an important mechanism for reducing the signal-to-noise ratio for the communication between Ca 2+ signalling and mitochondria.…”