Protective mechanisms of enhanced protein and organelle homeostasis by USP8 inhibition

Loss of protein and organelle homeostasis (proteostasis) is recognized as an important aggravating factor in the pathogenesis of neurodegenerative diseases. Post-mitotic cells like neurons are thought to be particularly sensitive to disruptions in proteostasis. Being high metabolic demanding, long-lived cells, with a complex architecture, neurons strongly depend on mechanisms of mitochondrial homeostasis and on the proteolytic activity of the proteasome and lysosome-autophagy systems. Both systems rely on ubiquitination of substrates as degradative signal, an event that is controlled by ubiquitin protein ligases, and can be counteracted by deubiquitinating enzymes (DUBs). We previously reported that genetic or pharmacological inhibition of Usp8, a DUB controlling endosomal trafficking, exerts beneficial effects in models of neurodegeneration. The molecular mechanism underlying the protective effect of Usp8 inhibition is unknown. In this work, we confirmed previous results on the protective effect of Usp8 downregulation in prolonging lifespan of flies (D. melanogaster) modeling PD. We extended these studies and found that Usp8 negatively regulates autophagy and mitophagy in neuronal cells of the flies’ brain. The mitophagic effect of Usp8 downregulation is independent of canonical Pink1/parkin mitophagy pathway, and possibly depends on the expression of mitochondrial pro-fusion protein Marf/Mfn, which we identified as potential Usp8 substrate.