A prominent feature of late-onset neurodegenerative diseases is accumulation of misfolded protein in vulnerable neurons. When levels of misfolded protein overwhelm degradative pathways, the result is cellular toxicity and neurodegeneration. Cellular mechanisms for degrading misfolded protein include the ubiquitin-proteasome system (UPS), the main non-lysosomal degradative pathway for ubiquitinated proteins, and autophagy, a lysosome-mediated degradative pathway. The UPS and autophagy have long been viewed as complementary degradation systems with no point of intersection. This view has been challenged by two observations suggesting an apparent interaction: impairment of the UPS induces autophagy in vitro, and conditional knockout of autophagy in the mouse brain leads to neurodegeneration with ubiquitin-positive pathology. It is not known whether autophagy is strictly a parallel degradation system, or whether it is a compensatory degradation system when the UPS is impaired; furthermore, if there is a compensatory interaction between these systems, the molecular link is not known. Here we show that autophagy acts as a compensatory degradation system when the UPS is impaired in Drosophila melanogaster, and that histone deacetylase 6 (HDAC6), a microtubule-associated deacetylase that interacts with polyubiquitinated proteins, is an essential mechanistic link in this compensatory interaction. We found that compensatory autophagy was induced in response to mutations affecting the proteasome and in response to UPS impairment in a fly model of the neurodegenerative disease spinobulbar muscular atrophy. Autophagy compensated for impaired UPS function in an HDAC6-dependent manner. Furthermore, expression of HDAC6 was sufficient to rescue degeneration associated with UPS dysfunction in vivo in an autophagy-dependent manner. This study suggests that impairment of autophagy (for example, associated with ageing or genetic variation) might predispose to neurodegeneration. Morover, these findings suggest that it may be possible to intervene in neurodegeneration by augmenting HDAC6 to enhance autophagy.

Mitochondrial dysfunction is associated with many human diseases. There has not been a systematic genetic approach for identifying regulators of basal mitochondrial biogenesis and function in higher eukaryotes. We performed a genome-wide RNA interference (RNAi) screen in Drosophila cells using mitochondrial Citrate synthase (CS) activity as the primary readout. We screened 13,071 dsRNAs and identified 152 genes that modulate CS activity. These modulators are involved in a wide range of biological processes and pathways including mitochondrial-related functions, transcriptional and translational regulation, and signaling pathways. Selected hits among the 152 genes were further analyzed for their effect on mitochondrial CS activity in transgenic flies or fly mutants. We confirmed a number of gene hits including HDAC6, Rpd3(HDAC1), CG3249, vimar, Src42A, klumpfuss, barren, and smt3 which exert effects on mitochondrial CS activities in vivo, demonstrating the value of Drosophila genome-wide RNAi screens for identifying genes and pathways that modulate mitochondrial function.