Parkin-dependent regulation of the MCU complex component MICU1

Abstract

The mitochondrial Ca2+ uniporter machinery is a multiprotein complex composed by the Ca2+ selective pore-forming subunit, the mitochondrial uniporter (MCU), and accessory proteins, including MICU1, MICU2 and EMRE. Their concerted action is required to fine-tune the uptake of Ca2+ into the mitochondrial matrix which both sustains cell bioenergetics and regulates the apoptotic response. To adequately fulfil such requirements and avoid impairment in mitochondrial Ca2+ handling, the intracellular turnover of all the MCU components must be tightly regulated. Here we show that the MCU complex regulator MICU1, but not MCU and MICU2, is rapidly and selectively degraded by the Ubiquitin Proteasome System (UPS). Moreover, we show that the multifunctional E3 ubiquitin ligase Parkin (PARK2), whose mutations cause autosomal recessive early-onset Parkinson’s disease (PD), is a potential candidate involved in this process since its upregulation strongly decreases the basal level of MICU1. Parkin was found to interact with MICU1 and, interestingly, Parkin Ubl-domain, but not its E3-ubquitin ligase activity, is required for the degradation of MICU1, suggesting that in addition to the well documented role in the control of Parkin basal auto-inhibition, the Ubl-domain might exert important regulatory functions by acting as scaffold for the proteasome-mediated degradation of selected substrates under basal conditions, i.e. to guarantee their turnover. We have found that also MICU2 stability was affected upon Parkin overexpression, probably as a consequence of increased MICU1 degradation. Our findings support a model in which the PD-related E3 ubiquitin ligase Parkin directly participates in the selective regulation of the MCU complex regulator MICU1 and, indirectly, also of the MICU2 gatekeeper, thus indicating that Parkin loss of function could contribute to the impairment of the ability of mitochondria to handle Ca2+ and consequently to the pathogenesis of PD.

Mitochondrial calcium uniporter complex modulation in cancerogenesis

“…The AKT-mediated MICU1 phosphorylation abolishes MICU1's gatekeeping function on MCU, leading to higher mitochondrial Ca 2+ content at resting conditions and ROS production [103] (Figure 2). We observed that phosphor-MICU1 accumulates in a non-mature form that is rapidly degraded (potentially by the ubiquitinproteasome system, as recently proposed [104]), leading to concomitant loss of the binding partner MICU2 and a disordered MCU complex composition [103]. The expression of a nonphosphorylatable MICU1 mutant restores normal mitochondrial Ca 2+ and ROS levels and inhibits AKT-mediated tumor growth in vivo, suggesting that the MICU1dependent regulation of mitochondrial Ca 2+ and ROS homeostasis is a crucial element in AKTdriven tumorigenesis.…”

Increased Levels of cAMP by the Calcium-Dependent Activation of Soluble Adenylyl Cyclase in Parkin-Mutant Fibroblasts

“…Furthermore, our study confirms, as previously described, the involvement of parkin in the control of cellular Ca 2 + levels, suggesting altered interactions between the endoplasmic reticulum and the mitochondria, and a potential different efficiency for the main Ca 2+ transport mechanisms. An additional element to the link between parkin and mitochondria-related calcium homeostasis is provided by a recent study, showing that parkin selectively regulates the turnover of the MCU accessory protein MICU1 [77]. MICU1 is a regulatory subunit of the MCU complex that is found to exert a stimulatory effect on mitochondrial Ca 2+ uptake [78].…”

PINK1/Parkin Mediated Mitophagy, Ca2+ Signalling, and ER–Mitochondria Contacts in Parkinson’s Disease

“…In basal conditions, Parkin was found to ubiquitylate MICU1, which is rapidly degraded via the proteasome system. In this way, Parkin indirectly also controls MICU2 levels, as MICU2 stability depends on MICU1 [102]. Even in this case, the Parkin Ubl domain is important for this action because the expression of a ∆Ubl Parkin mutant has no effect on MICU1 turnover.…”