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

Abstract

Endoplasmic reticulum (ER)-mitochondria contact sites are critical structures for cellular function. They are implicated in a plethora of cellular processes, including Ca²⁺ signalling and mitophagy, the selective degradation of damaged mitochondria. Phosphatase and tensin homolog (PTEN)-induced kinase (PINK) and Parkin proteins, whose mutations are associated with familial forms of Parkinson's disease, are two of the best characterized mitophagy players. They accumulate at ER-mitochondria contact sites and modulate organelles crosstalk. Alterations in ER-mitochondria tethering are a common hallmark of many neurodegenerative diseases including Parkinson's disease. Here, we summarize the current knowledge on the involvement of PINK1 and Parkin at the ER-mitochondria contact sites and their role in the modulation of Ca²⁺ signalling and mitophagy.

Keywords: Ca2+; ER–mitochondria tethering; PINK1; Parkin; mitophagy.

Figure 1

The canonical phosphatase and tensin homolog (PTEN)-induced kinase 1 (PINK1)/Parkin pathway. (A) In healthy mitochondria, PINK1 is constitutively imported via translocase of the outer membrane (TOM)/translocase of the inner membrane (TIM)23 complexes to the inner membrane (IMM), cleaved by two proteases (mitochondrial processing peptidase (MPP) and presenilin-associated rhomboid-like (PARL)) and retro-translocated to the cytosol, where it is degraded. (B) When ΔΨ_M is dissipated, Adenine nucleotide translocator ANT inhibits TIM23-mediated import of PINK1, which is not processed and accumulates on the outer membrane (OMM). Here, a supercomplex is formed, composed by TOM complex subunits and PINK1 homodimers, facilitating PINK1 autophosphorylation and activation. Once activated, PINK1 phosphorylates ubiquitinated substrates on the OMM and thus recruits and phosphorylates Parkin. Phosphorylated Parkin starts to ubiquitylate several proteins on the OMM, which are new substrates of PINK1 phosphorylation—a positive feedback loop is initiated, leading to the coating of damaged mitochondria with phospho-ubiquitin chain, red arrows. The MITOL mitochondrial E3 ubiquitin ligase, seems to be fundamental for the introduction of the initial ubiquitination. Phospho-ubiquitin chains are bound by two mitophagy adaptors, Nuclear domain 10 protein 52 (NDP52) and Optineurin, blue arrows. The two adaptors recruit autophagosomes via Microtubule-associated protein 1A/1B-light chain 3 (LC3) binding, allowing the engulfment of dysfunctional mitochondria. In parallel, PINK1 interacts with Beclin-1 and Parkin with Ambra1, further stimulating autophagosome formation, blue arrows.

Figure 2

PINK1, Parkin, and Ca²⁺. (A) PINK1 is involved in mitochondrial Ca²⁺ handling. In fact, its depletion leads to mitochondrial Ca²⁺ overload. The mechanisms underlying this PINK1 function are not clear, but it has been suggested as being a possible, controversial PINK1-dependent regulation of mitochondrial Ca²⁺ influx via the modulation of Ca²⁺ uptake or efflux via the modulation of the mitochondrial Na⁺/Ca²⁺ exchanger (NCLX). PINK1 also phosphorylates the leucine zipper-EF-hand-containing transmembrane protein 1 (LETM1) by increasing its activity. (B) Parkin involvement in Ca²⁺ homeostasis is more characterized. It regulates mitochondrial Ca²⁺ uptake protein 1 MICU1 turnover, and indirectly also that of mitochondrial Ca²⁺ uptake protein 2 (MICU2), whose stability depends on MICU1. MICU1 and 2 are the positive and negative regulators of the mitochondrial uniporter pore subunit (MCU), respectively. By decreasing MICU levels, Parkin is able to regulate mitochondrial Ca²⁺ handling. Parkin also interacts with and ubiquitylate phospholipase Cγ1 (PLCγ1), responsible for the generation of the second messenger inositol 1,4,5 trisphosphate (IP₃) and diacylglycerol (DAG). Parkin deficiency leads to a PLC-dependent increase of intracellular Ca²⁺ levels, possibly due to excessive activation of PLCγ1. Two additional Parkin substrates have been recently identified: the ATPase Na⁺/K⁺ transporting subunit alpha 2 (ATP1A2), possibly involved in modulating Ca²⁺ dynamics, and Hippocalcin, a Ca²⁺ sensor that can also regulate voltage-dependent Ca²⁺ channels.

Figure 3

PINK1, Parkin, and endoplasmic reticulum (ER)–mitochondria tethering. PINK1 and Parkin are found enriched at ER–mitochondria contact sites. Upon treatment with CCCP, PINK1 localization is markedly increased in the ER–mitochondria interface, where it recruits Beclin-1, a pro-autophagic protein. ER–mitochondria juxtaposition and omegasome formation (a process that occurs at ER–mitochondrial contact sites) are enhanced upon PINK1-mediated Beclin-1 recruitment, red arrows. Miro1 protein, an OMM Rho GTPase 1, which tethers mitochondria to microtubules and regulates their movement, is a PINK1 substrate, red arrow.