The roles of PINK1, parkin, and mitochondrial fidelity in Parkinson's disease

Abstract

Understanding the function of genes mutated in hereditary forms of Parkinson's disease yields insight into disease etiology and reveals new pathways in cell biology. Although mutations or variants in many genes increase the susceptibility to Parkinson's disease, only a handful of monogenic causes of parkinsonism have been identified. Biochemical and genetic studies reveal that the products of two genes that are mutated in autosomal recessive parkinsonism, PINK1 and Parkin, normally work together in the same pathway to govern mitochondrial quality control, bolstering previous evidence that mitochondrial damage is involved in Parkinson's disease. PINK1 accumulates on the outer membrane of damaged mitochondria, activates Parkin's E3 ubiquitin ligase activity, and recruits Parkin to the dysfunctional mitochondrion. Then, Parkin ubiquitinates outer mitochondrial membrane proteins to trigger selective autophagy. This review covers the normal functions that PINK1 and Parkin play within cells, their molecular mechanisms of action, and the pathophysiological consequences of their loss.

Figure 1. Model of Parkin-induced Mitophagy

Dysfunctional mitochondria (Yellow Mitochondrion) fail to import and degrade PINK1 stabilizing it on the outer mitochondrial membrane. After PINK1 accumulation, PINK1 phosphorylates ubiquitin and Parkin to activate Parkin's E3 ligase activity. Parkin ubiquitinates substrates on the outer mitochondria for two divergent processes: autophagosome recruitment and ubiquitin proteasome degradation of ubiquitinated mitochondrial substrates. Fis1 is a receptor on the outer membrane that binding two proteins TBC1D15/TBC1D17 to govern the developing LC3 isolation membrane to generate the autophagosome around the damaged mitochondria. The autophagosome is then delivered to the lysosome for degradation.

Figure 2. Cartoon Depiction of the Location of Mutations in Monogenic PINK1 and Parkin PD Patients

* = heterozygous mutation; ins = insertion; / = compound heterozygous; fs = frame shift; dup = duplication; bp = base pairs; TM = transmembrane domain; MTS = mitochondrial targeting sequence; UBL = ubiquitin-like domain; RING = really interesting new gene domain; IBR = in between RING domain.

Figure 3. The Balance Between Mitochondrial Damage and its Removal Contributes to the Pathogenesis of PD

Healthy neurons efficiently remove damaged mitochondria by mitophagy as a quality control mechanism to ensure cell survival. Excess mitochondrial damage (from triggers such as MPTP exposure, paraquat, and aging-induced dysfunction) causes neuronal demise contributing to the pathogenesis of PD. Likewise, reducing the cells ability to remove damaged mitochondria (from the loss of Parkin or PINK1) may cause an accumulation of these dysfunctional organelles leading to early onset-PD.

Figure 4. PINK1 is Turned Over by Sequential Proteolysis

(Left) PINK1 is continuously imported into healthy mitochondria then degraded. Mitochondrial membrane potential drives mitochondrial import through the TIM complex of the mitochondria where proteases MPP and PARL cleave PINK1's mitochondrial targeting sequence and transmembrane domain. PARL's cleavage between Ala103 and Phe104 creates a free N-terminal Phenylalanine that is identified by N-degron type 2 E3 ubiquitin ligases targeting PINK1 to the ubiquitin proteasome. (Right) Depolarization of mitochondria or blocking mitochondrial import causes PINK1 to accumulate on the outer mitochondrial membrane. Tomm7 is an accessory subunit of the TOM complex that shunts and retains PINK1 on the outer mitochondrial membrane.

Figure 5. The Activation of Parkin Drives a Positive Feedback Loop Providing Ubiquitin as a Substrate for PINK1 to Complete Mitophagy

Mitochondrial dysfunction stabilizes PINK1 enabling it to phosphorylate ubiquitin and Parkin. This phosphorylation event activates Parkin, which ubiquitinates substrates on the outer membrane. These ubiquitin chains on the proteins of the outer mitochondrial membrane provide additional substrates for PINK1's kinase activity causing a strong positive feedback cycle assuring mitochondrial clearance. OMM = outer mitochondrial membrane.

Figure 6. Structural Representation of Parkin

(A) Ribbon diagram of the various domains in individual colors - cyan (Ubl), grey (RING0), green (RING1), violet (IBR), blue (Rep), and bronze (RING2). The residues Ser65 (red), Arg275 (blue), and Cys431 (yellow) are highlighted in stick representation. (B) Juxtaposition of the ribbon diagram over a surface presentation of Parkin (Trempe et al., 2013).

Figure 7. Electron Micrographs of Parkin-induced Mitophagy and Autophagosomal Engulfment of Mitochondria

Mouse embryonic fibroblasts overexpressing Parkin and treated with 20mM CCCP for 12 hours. White arrowhead points to autophagosome forming around mitochondria. Scale bar = 500 nm. Images from (Yoshii et al., 2011).