The endoplasmic reticulum-mitochondria interface is perturbed in PARK2 knockout mice and patients with PARK2 mutations

Abstract

Mutations in PARK2, encoding the E3 ubiquitin protein ligase Parkin, are a common cause of autosomal recessive Parkinson's disease (PD). Loss of PARK2 function compromises mitochondrial quality by affecting mitochondrial biogenesis, bioenergetics, dynamics, transport and turnover. We investigated the impact of PARK2 dysfunction on the endoplasmic reticulum (ER)-mitochondria interface, which mediates calcium (Ca) exchange between the two compartments and is essential for Parkin-dependent mitophagy. Confocal and electron microscopy analyses showed the ER and mitochondria to be in closer proximity in primary fibroblasts from PARK2 knockout (KO) mice and PD patients with PARK2 mutations than in controls. Ca flux to the cytosol was also modified, due to enhanced ER-to-mitochondria Ca transfers, a change that was also observed in neurons derived from induced pluripotent stem cells of a patient with PARK2 mutations. Subcellular fractionation showed the abundance of the Parkin substrate mitofusin 2 (Mfn2), which is known to modulate the ER-mitochondria interface, to be specifically higher in the mitochondrion-associated ER membrane compartment in PARK2 KO tissue. Mfn2 downregulation or the exogenous expression of normal Parkin restored cytosolic Ca transients in fibroblasts from patients with PARK2 mutations. In contrast, a catalytically inactive PD-related Parkin variant had no effect. Overall, our data suggest that Parkin is directly involved in regulating ER-mitochondria contacts and provide new insight into the role of the loss of Parkin function in PD development.

Structure, function, and regulation of mitofusin‐2 in health and disease

“…The Parkinson's disease-associated proteins Parkin and PINK1 are involved in the ubiquitination of Mfn2 on damaged mitochondria Gegg & Schapira, 2011), which promotes local fission of the mitochondrial network thereby segregating damaged mitochondria for autophagic clearance (Ziviani, Tao & Whitworth, 2010). Higher levels of Mfn2 in Parkin knockout mice fibroblasts and Parkinson's disease patients were responsible for increased ER-mitochondria association (Gautier et al, 2016). Similarly, drosophila PINK1/Parkin mutants displayed defective mitochondria and activated PERK-mediated UPR signalling (Celardo et al, 2016).…”

From dysfunctional endoplasmic reticulum-mitochondria coupling to neurodegeneration

“…Parkin overexpression in nigral neurons also led to a slight increase in the percentage of mitochondria in contact with the ER (Zheng et al, 2017). In contrast, we and others recently associated Parkin and PINK1 dysfunction with an exacerbation of the ER-mitochondria interface (Celardo et al, 2016;Gautier et al, 2016). In particular, ER-mitochondria juxtaposition was found to be increased in fibroblasts from patients with PARK2 or PARK6 mutations compared to cells from healthy donors (Celardo et al, 2016;Gautier et al, 2016).…”

“…Controversial observations are not limited to the above mentioned studies, in which the role of Mfn2 in ER-mitochondria tethering was directly addressed by knockdown or knockout approaches in different model systems. They also apply to studies in which the degree of ER-mitochondria juxtaposition was correlated with endogenous Mfn2 levels: Mfn2 abundance was found to be either higher (Arruda et al, 2014;Gautier et al, 2016) or lower (Zheng et al, 2017) in models associated with an exacerbated ER-mitochondria interface.…”

“…Thus, Mfn2 silencing restored defects in models with either decreased or increased ER-mitochondria coupling (Area- Celardo et al, 2016;Gautier et al, 2016). We will discuss possible explanations for these recurrent controversies in section 5.…”

“…Approximately 10% of the cases are due to monogenic mutations . Physical and/or functional association with ER-mitochondria contacts was demonstrated in the case of the PD-related proteins α-synuclein (Cali et al, 2012;Guardia-Laguarta et al, 2014), DJ-1 , PINK1 (Celardo et al, 2016;Gelmetti et al, 2017) and Parkin Celardo et al, 2016;Gautier et al, 2016;Gelmetti et al, 2017;Van Laar et al, 2015;Zheng et al, 2017).…”