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 was challenged the observation that impairment of the UPS induces autophagy, and that conditional knockout of autophagy in the mouse brain leads to neurodegeneration with ubiquitin-positive pathology. Recently, we discovered that impairment of the UPS leads to induction of compensatory autophagy in vivo. Furthermore, we used molecular genetic approaches to identify HDAC6, a microtubule-associated deacetylase, as an essential link between the UPS and autophagy. Most importantly, we determined that augmentation of HDAC6 function rescues neurodegeneration in an autophagy-dependent manner. In ongoing studies, we are employing biochemical, cell biological and molecular genetic approaches to elucidate the mechanism whereby HDAC6 influences autophagic degradation of ubiquitinated substrates. These projects use cell culture, Drosophila and transgenic mouse models. Our long term goal is to exploit protein degradation pathways to accelerate clearance of misfolded proteins to provide effective treatment for proteopathies such as Parkinson’s disease, polyglutamine diseases, and motor neuron diseases.
HDAC6 rescues degeneration in flies with proteasome impairment and in a fly model of SBMA that exhibits impaired UPS function. (a-e) Scanning electron microscopy (SEM) images of fly eyes expressing DTS7 with or without the indicated HDAC6 transgenes. (a) Normal eyes in DTS7 flies reared at 22oC. (b) Rough eyes in DTS7 flies reared at 28oC. Degeneration was suppressed by expression of dHDAC6 (c) or hHDAC6 (d), but not a catalytically dead mutant of hHDAC6 (e). (f-j) SEM images of fly eyes expressing AR52 with or without the indicated HDAC6 transgenes. (f) Normal eyes in AR52 flies reared without DHT. (g) Rough eyes in AR52 flies reared with DHT. Degeneration was suppressed by expression of dHDAC6 (h) or hHDAC6 (i), but not a catalytically dead mutant of hHDAC6 (j). (k-p) Detection of UPS reporter in imaginal eye discs from third instar larvae by confocal microscopy. High level fluorescence was found in flies expressing GFP (k, positive control), but fluorescence was barely detectable in control flies expressing CL1-GFP (l, negative control). CL1-GFP accumulates in DTS7 flies with temperature-dependent proteasome impairment (compare m to n) and in AR52 flies with ligand-dependent degeneration (compare o to p). The retinal phenotypes of 200 to >1000 flies of each genotype were examined. (DHT, dihydrotestosterone).
