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by Mingmei Wang, Chunlei Wan, Tao He, Chaojun Han, Kailian Zhu, John L.Waddington, Xuechu Zhen

Excerpt from the article published in Neuropharmacology, 15 September 2021  | https://doi.org/10.1016/j.neuropharm.2020.108360

Editor’s Highlights

• Mitophagy is impaired in the substantia nigra of MPTP-induced Parkinsonism mice.
• Sigma 1R dysfunction induces disturbance in depolarized mitochondria degradation under CCCP treatment in dopaminergic cells.
• Activation of Sig1R stimulates markers of mitophagy, likely through PINK1/Parkin pathway in dopaminergic cells and attenuates MPTP-induced motor disability.

Abstract

Mitochondria are essential for neuronal survival and function, and mitochondrial dysfunction plays a critical role in the pathological development of Parkinson’s disease (PD). Mitochondrial quality control is known to contribute to the survival of dopaminergic (DA) neurons, with mitophagy being a key regulator of the quality control system. In this study, we show that mitophagy is impaired in the substantia nigra pars compacta (SNc) of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mouse model of PD. Treatment with the sigma-1 receptor (Sig 1R) agonist 2-morpholin-4-ylethyl 1-phenylcyclohexane-1-carboxylate (PRE-084) reduced loss of DA neurons, restored motor ability and MPTP-induced damage to mitophagy activity in the SNc of PD-like mice. Additionally, knockdown of Sig 1R in SH-SY5Y DA cells inhibited mitophagy and enhanced 1-methyl-4-phenylpyridinium ion (MPP⁺) neurotoxicity, whereas application of the Sig 1R selective agonist SKF10047 promoted clearance of damaged mitochondria. Moreover, knockdown of Sig 1R in SH-SY5Y cells resulted in decreased levels of p-ULK1 (Unc-51 Like Autophagy Activating Kinase 1) (Ser⁵⁵⁵), p-TBK1 (TANK Binding Kinase 1) (Ser¹⁷²), p-ubiquitin (Ub) (Ser⁶⁵), Parkin recruitment, and stabilization of PTEN-induced putative kinase 1 (PINK1) in mitochondria. The present data provide the first evidence for potential roles of PINK1/Parkin in Sig 1R-modulated mitophagy in DA neurons.

Introduction

Parkinson’s disease (PD) is the second most common neurodegenerative disorder, affecting 2–3% of the population ≥65 years of age (Poewe et al., 2017). The pathology of PD is characterized by progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc). Although the majority of PD cases are sporadic, several genes such as PTEN-induced putative kinase 1 (PINK1) (Valente et al., 2004), parkin (Cookson, 2012), LRRK2 (Abou-Sleiman et al., 2006), DJ-1 (Bras et al., 2015) and α-synuclein (Mori et al., 2003) were identified as linked with familial forms of the disease. Notably, PINK1 is a 581-amino acid protein containing a C-terminal kinase domain and a mitochondrial targeting sequence at the N-terminus with serine/threonine kinase activity. Stabilization of PINK1 on depolarized mitochondria suggests it functions as a key factor in mitochondrial quality control systems (Pickrell and Youle, 2015). Mitochondrial dysfunction has been known to play a critical role in the pathogenesis of PD since the finding, in the late 1970s, that accidental exposure to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), an environmental toxin that disrupts mitochondrial respiratory chain function, led to an acute parkinsonian syndrome (Langston, 2017).

Mitophagy is generally accepted as one of the key modulators in regulation of mitochondrial homeostasis; it eliminates damaged mitochondria by autophagy pathway (Kim et al., 2007) and is known to be a critical process in mitochondrial function and cellular survival. More specifically, damaged or excessive mitochondria are engulfed by double-membrane structures known as autophagosomes, which then fuse with lysosomes to form autolysosomes, leading to degradation of their contents. PINK1 and Parkin are also involved in the regulation of mitochondrial functions: Parkin is recruited from the cytosol to depolarized mitochondria, and it promotes the selective clearance of damaged mitochondria via the autophagy pathway (Narendra et al., 2008); kinase activity of PINK1 is required for Parkin mitochondrial localization and activation (Matsuda et al., 2010; Narendra et al., 2010). As a Ub kinase, PINK1 phosphorylates both Ub and Parkin to activate Parkin E3 Ub ligase activity and recruits autophagy receptors to induce mitophagy (Chan et al., 2011; Kane et al., 2014; Lazarou et al., 2015; Rakovic et al., 2013). PD-associated Parkin mutations interfere with mitophagy flux in different stages (Lee et al., 2010); meanwhile, disease-associated PINK1 mutations also impair the degradation of depolarized mitochondria (Geisler et al., 2014). All of these findings indicate that the PINK1/Parkin pathway plays an important role in the regulation of mitophagy and may contribute to the pathological development of PD.

The endoplasmic reticulum (ER) protein sigma-1 receptor (Sig 1R) is a Ca²⁺-sensitive and ligand-operated receptor, chaperoning at the mitochondrion-associated ER membrane (MAM) (Hayashi and Su, 2007; Rousseaux and Greene, 2016). Sig 1R is widely expressed in the brain with complex functional roles that include neuroprotection, neuritogenesis, neuroinflammation, and neuroplasticity (Jia et al., 2018; Nguyen et al., 2017; Wu et al., 2015; Yu et al., 2008). Alteration of Sig 1R activity or expression has been associated with the pathological development of neuropsychiatric diseases such as PD, Huntington disease (HD), depression, and addiction (Francardo et al., 2014; Ryskamp et al., 2017; Sambo et al., 2018; Wang et al., 2016), and it has become an important target in drug development for these diseases. Recent studies have indicated that Sig 1R is involved in autophagy regulation. Alteration of Sig 1R expression or pharmacological activation has profound effects on autophagy, which is known to be associated with neurodegenerative diseases (Christ et al., 2019). Recently, Sig 1R is reported to regulate mitophagy (Yang et al., 2019), as Sig 1R deletion results in impaired autophagosome clearance in both retinal tissue and cultured cells, and this may be associated with impaired autophagosome and lysosome fusion. However, the mechanisms of and functional implications for Sig 1R-regulated mitophagy remain largely unknown.

The Sig 1R has been shown to be a promising drug target for PD (Francardo et al., 2014; Mancuso and Navarro, 2017; Su et al., 2016). Given the importance of mitochondria function in PD pathobiology, we investigated how Sig 1R regulates mitophagy in DA neurons and its functional roles in PD pathogenesis.