The E3 ligase parkin maintains mitochondrial integrity by increasing linear ubiquitination of NEMO.

Parkin, a RING-between-RING-type E3 ubiquitin ligase associated with Parkinson's disease, has a wide neuroprotective activity, preventing cell death in various stress paradigms. We identified a stress-protective pathway regulated by parkin that links NF-κB signaling and mitochondrial integrity via linear ubiquitination. Under cellular stress, parkin is recruited to the linear ubiquitin assembly complex and increases linear ubiquitination of NF-κB essential modulator (NEMO), which is essential for canonical NF-κB signaling. As a result, the mitochondrial guanosine triphosphatase OPA1 is transcriptionally upregulated via NF-κB-responsive promoter elements for maintenance of mitochondrial integrity and protection from stress-induced cell death. Parkin-induced stress protection is lost in the absence of either NEMO or OPA1, but not in cells defective for the mitophagy pathway. Notably, in parkin-deficient cells linear ubiquitination of NEMO, activation of NF-κB, and upregulation of OPA1 are significantly reduced in response to TNF-α stimulation, supporting the physiological relevance of parkin in regulating this antiapoptotic pathway.

Ret rescues mitochondrial morphology and muscle degeneration of Drosophila Pink1 mutants.

Parkinson's disease (PD)-associated Pink1 and Parkin proteins are believed to function in a common pathway controlling mitochondrial clearance and trafficking. Glial cell line-derived neurotrophic factor (GDNF) and its signaling receptor Ret are neuroprotective in toxin-based animal models of PD. However, the mechanism by which GDNF/Ret protects cells from degenerating remains unclear. We investigated whether the Drosophila homolog of Ret can rescue Pink1 and park mutant phenotypes. We report that a signaling active version of Ret (Ret(MEN2B) rescues muscle degeneration, disintegration of mitochondria and ATP content of Pink1 mutants. Interestingly, corresponding phenotypes of park mutants were not rescued, suggesting that the phenotypes of Pink1 and park mutants have partially different origins. In human neuroblastoma cells, GDNF treatment rescues morphological defects of PINK1 knockdown, without inducing mitophagy or Parkin recruitment. GDNF also rescues bioenergetic deficits of PINK knockdown cells. Furthermore, overexpression of Ret(MEN2B) significantly improves electron transport chain complex I function in Pink1 mutant Drosophila. These results provide a novel mechanism underlying Ret-mediated cell protection in a situation relevant for human PD.

The loss of LRPPRC function induces the mitochondrial unfolded protein response.

The inactivation of the LRPPRC gene, which has previously been associated with the neurodegenerative French Canadian Leigh Syndrome, results in a decrease in the production of mitochondria-encoded subunits of complex IV, thereby causing a reduction in complex IV activity. Previously we have shown that reducing complex IV activity triggers a compensatory and conserved mitochondrial hyperfusion response. We now demonstrate that LRPPRC knock-down in mammalian cells leads to an imbalance between mitochondria-encoded and nuclear-encoded subunits of complex IV and that this imbalance triggers the mitochondrial unfolded protein response (UPR(mt)). The inactivation of the LRPPRC-like gene mma-1 in C. elegans also induces UPR(mt), which demonstrates that this response is conserved. Furthermore, we provide evidence that mitochondrial hyperfusion and UPR(mt) are coordinated but mediated by genetically distinct pathways. We propose that in the context of LRPPRC mma-1 knock-down, mitochondrial hyperfusion helps to transiently maintain mitochondrial ATP production while UPR(mt) participates in the restoration of mitochondrial proteostasis. Mitochondrial proteostasis is not only critical in pathophysiology but also during aging, as proteotoxic stress has been shown to increase with age. Therefore, we speculate that the coordination of these two mitochondrial stress responses plays a more global role in mitochondrial proteostasis.

Parkin cooperates with GDNF/RET signaling to prevent dopaminergic neuron degeneration.

Parkin and the glial cell line-derived neurotrophic factor (GDNF) receptor RET have both been independently linked to the dopaminergic neuron degeneration that underlies Parkinson's disease (PD). In the present study, we demonstrate that there is genetic crosstalk between parkin and the receptor tyrosine kinase RET in two different mouse models of PD. Mice lacking both parkin and RET exhibited accelerated dopaminergic cell and axonal loss compared with parkin-deficient animals, which showed none, and RET-deficient mice, in which we found moderate degeneration. Transgenic expression of parkin protected the dopaminergic systems of aged RET-deficient mice. Downregulation of either parkin or RET in neuronal cells impaired mitochondrial function and morphology. Parkin expression restored mitochondrial function in GDNF/RET-deficient cells, while GDNF stimulation rescued mitochondrial defects in parkin-deficient cells. In both cases, improved mitochondrial function was the result of activation of the prosurvival NF-κB pathway, which was mediated by RET through the phosphoinositide-3-kinase (PI3K) pathway. Taken together, these observations indicate that parkin and the RET signaling cascade converge to control mitochondrial integrity and thereby properly maintain substantia nigra pars compacta dopaminergic neurons and their innervation in the striatum. The demonstration of crosstalk between parkin and RET highlights the interplay in the protein network that is altered in PD and suggests potential therapeutic targets and strategies to treat PD.