The Organization of Mitochondrial Quality Control and Life Cycle in the Nervous System In Vivo in the Absence of PINK1

Abstract

Maintenance of healthy mitochondria is crucial in cells, such as neurons, with high metabolic demands, and dysfunctional mitochondria are thought to be selectively degraded. Studies of chemically uncoupled cells have implicated PINK1 mitochondrial kinase, and Parkin E3 ubiquitin ligase in targeting depolarized mitochondria for degradation. However, the role of the PINK1/Parkin pathway in mitochondrial turnover is unclear in the nervous system under normal physiological conditions, and we understand little about the changes that occur in the mitochondrial life cycle when turnover is disrupted. Here, we evaluated the nature, location, and regulation of quality control in vivo using quantitative measurements of mitochondria in Drosophila nervous system, with deletion and overexpression of genes in the PINK1/Parkin pathway. We tested the hypotheses that impairment of mitochondrial quality control via suppression of PINK1 function should produce failures of turnover, accumulation of senescent mitochondria in the axon, defects in mitochondrial traffic, and a significant shift in the mitochondrial fission-fusion steady state. Although mitochondrial membrane potential was diminished by PINK1 deletion, we did not observe the predicted increases in mitochondrial density or length in axons. Loss of PINK1 also produced specific, directionally balanced defects in mitochondrial transport, without altering the balance between stationary and moving mitochondria. Somatic mitochondrial morphology was also compromised. These results strongly circumscribe the possible mechanisms of PINK1 action in the mitochondrial life cycle and also raise the possibility that mitochondrial turnover events that occur in cultured embryonic axons might be restricted to the cell body in vivo, in the intact nervous system.

Miro phosphorylation sites regulate Parkin recruitment and mitochondrial motility

“…We do not know whether the degradation of Miro in mammalian cells under normal physiological conditions is always an early "pro-clearance" step in a series that will ultimately produce mitophagy, or if there are levels of PINK1 activation that will stop the process with Miro degradation and a decrease in mitochondrial motility. In Drosophila, however, alterations in mitochondrial motility can be seen with manipulations of PINK1 and Parkin that do not appear to involve widespread mitophagy (15,46,58). If Miro degradation is a step toward clearance of either a segment of the OMM or mitophagy of the entire organelle (8,59), it is likely to be important, along with Mitofusin1/2 degradation, as a means to quarantine damaged mitochondria rapidly before the slower steps of autophagosome-dependent mitophagy or mitochondria-derived vesicle-based quality control.…”

Live imaging of mitochondrial dynamics in CNS dopaminergic neurons in vivo demonstrates early reversal of mitochondrial transport following MPP+ exposure

“…Retrograde transport of depolarized mitochondria may be particularly important in dopaminergic neurons, since mutations in proteins that link mitochondrial depolarization to their degradation (PINK1 and Parkin) cause recessive Parkinsonism (Narendra et al, 2008; Narendra et al, 2010). In addition, mitochondrial transport was recently found to be altered in larval segmental neurons from Drosophila lacking PINK1 (Devireddy et al, 2015), although dopaminergic neurons were not evaluated. Our working model is that MPP + uptake and concentration causes damage to mitochondria in dopaminergic terminals.…”

Mitostasis in Neurons: Maintaining Mitochondria in an Extended Cellular Architecture

“…Experimental conditions that poison all the mitochondria in a neuron will therefore also likely shut down the PINK1/Parkin pathway to mitophagy (Cai et al, 2012; Van Laar et al, 2011). There are additional routes to mitophagy that are not dependent on either PINK1 or Parkin or both (Roberts et al, 2016), and the relative contribution and extent of redundancy of these alternatives is not clear (Devireddy et al, 2015; Sung et al, 2016). Nor are the actions of PINK1 and Parkin necessarily restricted to triggering mitophagy (Johnson et al, 2012; Morais et al, 2014; Muller-Rischart et al, 2013); in particular, as mentioned above, they can also mediate formation of MDVs and it is not known what determines the choice between MDV formation and full-fledged mitophagy.…”

“…Less clear at present is the contribution of PINK1, Parkin, and autophagy to the overall maintenance of mitochondrial quality. In Drosophila, loss of PINK1 decreases the membrane potential of axonal mitochondria and causes structural abnormalities to the organelles (Devireddy et al, 2015), but knockout of PINK1 and Parkin in rodent models caused little or no neurodegeneration. Patients carrying these mutations generally do not have the onset of symptoms for three or four decades.…”