Parkin-dependent mitophagy in the heart

Abstract

Mitochondria can undergo autophagic elimination for differing reasons, e.g. as part of a cell-wide macroautophagic response, as part of mitochondrial turnover during metabolic remodeling, or in the case of selective mitophagic destruction of dysfunctional mitochondria, during mitochondrial quality control. Multiple mechanistically distinct pathways converge upon, and activate, mitochondrial autophagy. Here, the evidence supporting a role for the prototypical mitochondrial quality control pathway, PINK1-Parkin mediated mitophagy, in cardiac homeostasis and heart disease is reviewed. Contrary to popular wisdom based on findings from non-cardiac systems, current data do not support a major role for Parkin-mediated mitophagy as a mechanism for constitutive mitochondrial housekeeping, and instead suggest that this pathway primarily functions in adult hearts as an inducible cardiac stress-response mechanism. Recent findings have also uncovered an unsuspected role for Parkin-mediated mitochondrial turnover in the normal perinatal transformation of myocardial metabolism.

Figure 1. Flow cytometry tracings demonstrating a range of mitochondrial membrane polarization in normal hearts

Membrane polarization was measured either by staining mitochondria isolated from normal mouse hearts with green 3,3’-Dihexyloxacarbocyanine Iodide (DiOC6; left) or red Tetramethylrhodamine ethyl ester (TMRE; right). Colored tracings are at baseline and black tracings are after mitochondrial depolarization with 50 nM FCCP (20 min @ room temperature). Overlap, indicating hypopolarized mitochondria, is shaded grey.

Figure 2. Parkin-dependent and – independent mechanisms of mitophagy

PINK1 stabilization initiates both pathways. Upper: phosphorylation of Mfn2 recruits cytosolic Parkin, and of ubiquitin activates Parkin, provoking ubiquitination of outer mitochondrial membrane proteins that attract atuophagosomal LC3 via docking protein p62. Lower: phosphorylation of ubiquitinated membrane proteins attracts autophagosomal LC3 via autophagy receptors optineurin (OPTN) or nuclear dot protein 52 (NDP52). The relative roles of each pathway in hearts and elsewhere are unknown.

Figure 3. Uncropped immunoblots of cytosolic Parkin in different types of mouse hearts

On the left are original data from reference demonstrating Parkin upregulation in hearts rendered acutely defective in mitochondrial fission by conditional ablation of Drp1 (30 μg protein/lane). On the right are original data from reference demonstrating Parkin upregulation in PINK1 knockout (KO) mouse hearts (21.5 μg protein/lane). In both sets of blots WT is wild-type (normal adult) heart. Parkin−/− is conditional cardiac-specific Parkin KO (left) and germ-line Parkin KO (right). Parkin transgenic (TG) mouse heart on left blot serves as positive control (1 μg protein). GAPDH is protein loading control. Note minimal expression of Parkin in WT compared to Parkin−/− hearts in both studies, and marked increase in Parkin protein with mitochondrial stress evoked by Drp1 or PINK1 ablation. Immunoblots on the right were provided by, and used with permission of, Dr. Asa Gustafsson (UCSD).