Bioenergetics of neurons inhibit the translocation response of Parkin following rapid mitochondrial depolarization

Abstract

Recent studies delineate a pathway involving familial Parkinson's disease (PD)-related proteins PINK1 and Parkin, in which PINK1-dependent mitochondrial accumulation of Parkin targets depolarized mitochondria towards degradation through mitophagy. The pathway has been primarily characterized in cells less dependent on mitochondria for energy production than neurons. Here we report that in neurons, unlike other cells, mitochondrial depolarization by carbonyl cyanide m-chlorophenyl hydrazone did not induce Parkin translocation to mitochondria or mitophagy. PINK1 overexpression increased basal Parkin accumulation on neuronal mitochondria, but did not sensitize them to depolarization-induced Parkin translocation. Our data suggest that bioenergetic differences between neurons and cultured cell lines contribute to these different responses. In HeLa cells utilizing usual glycolytic metabolism, mitochondrial depolarization robustly triggered Parkin-mitochondrial translocation, but this did not occur in HeLa cells forced into dependence on mitochondrial respiration. Declining ATP levels after mitochondrial depolarization correlated with the absence of induced Parkin-mitochondrial translocation in both HeLa cells and neurons. However, intervention allowing neurons to maintain ATP levels after mitochondrial depolarization only modestly increased Parkin recruitment to mitochondria, without evidence of increased mitophagy. These data suggest that changes in ATP levels are not the sole determinant of the different responses between neurons and other cell types, and imply that additional mechanisms regulate mitophagy in neurons. Since the Parkin-mitophagy pathway is heavily dependent on bioenergetic status, the unique metabolic properties of neurons likely influence the function of this pathway in the pathogenesis of PD.

The pathways of mitophagy for quality control and clearance of mitochondria

“…In fact, it has been reported that unlike other cell types, depolarization of neuronal mitochondria does not lead to Parkin recruitment or their autophagic clearance. 102 On the other hand, recruitment of Parkin to depolarized neuronal mitochondria and their Parkin-dependent mitophagy have been observed in other studies. 69,86 Clearly, further work is required to address the relevance of the PINK1/Parkin-mediated pathway of mitophagy in neurons.…”

Quality Control of Mitochondrial Proteostasis

“…Thus, stabilization of Pink1 at the OM and recruitment of Parkin to the mitochondrial OM is crucial to induce mitophagy Chan et al 2011). It will be interesting to define the contribution of blocked OM protein degradation and mitophagy to the progression of Parkinson's disease, especially with the recent observation that Parkin does not appear to be recruited to mitochondria in neuronal cells (Van Laar et al 2011).…”

Role of Glucose Metabolism and ATP in Maintaining PINK1 Levels during Parkin-mediated Mitochondrial Damage Responses

“…Mitochondrial Depolarization-induced Parkin Mitochondrial Recruitment and Elevated PINK1 Levels Correlate with Intracellular ATP Levels-Previous studies have shown that the rapid loss of ATP after mitochondrial depolarization could be one of the reasons behind poor Parkin-mitochondrial translocation in neurons or HeLa cells forced into dependence on mitochondrial respiration (24). Because HeLa cells and immortalized cell lines generally utilize glycolytic metabolism for energy production, it is our expectation that glucose withdrawal coupled with mitochondrial depolarization would severely suppress intracellular ATP levels.…”

“…Although several studies have verified this observation in neuronally derived (4,23) and non-neuronal immortalized cell lines (2,4,5), the PINK1/Parkin mitophagy pathway has been proven to be less robust in neurons (24). To date, research in this field predominantly relied on ionophores and inhibitor drugs to compromise the mitochondrial integrity (3,(25)(26)(27).…”

“…Consequently, the differing bioenergetics resulting from this large scale mitochondrial damage may contribute to this observed variance, in which immortalized cell lines exhibit the Warberg effect to rely on glycolysis for a significant portion of their ATP production (28 -31), whereas neurons rely primarily on oxidative phosphorylation for ATP production (32). Although the involvement of ATP was suggested to influence the mitophagy pathway in previous studies (24,33), definitive evidence demonstrating the necessity for ATP in the PINK1/Parkin pathway, as well as the mechanism that explains this phenomenon, has yet to be explored.…”