Characterization of ubiquilin proteins and assessment of ubiquilin-1 overexpression as a therapy for Huntington's disease

Abstract

Huntington's disease (HD) is a hereditary neurodegenerative disorder characterized by motor dysfunction, cognitive decline, and premature death. Currently there is no cure for HD, and most therapies focus on alleviating the symptoms of the disease. One approach in the effort to develop meaningful therapies treating the underlying pathology of HD is to enhance cellular proteostasis pathways. Ubiquilins represent a conserved family of proteins that share a common structure consisting of an N-terminal UBL domain and a C-terminal UBA domain. This allows these proteins to act as shuttle factors in the ubiquitin proteasome system (UPS), endoplasmic reticulum associated degradation (ERAD), and autophagy pathways. Mutations in ubiquilin proteins cause neurodegeneration, presumably due to loss of protein function. Overexpression of ubiquilin-1 suppresses polyglutamine toxicity in both cell culture and Caenorhabditis elegans models of HD. Moreover, knockdown of ubiquilins exacerbates toxicity, shortens the lifespan of worms, impairs autophagosome formation, and reduces clearance of ERAD substrates. Here, we characterized the expression of ubiquilins 1-4 in the brains of wild-type and R6/2 HD model mice. We found that ubiquilins 1, 2, and 4, but not 3, are expressed in the brain, and that they all colocalized with huntingtin (HTT) inclusion bodies. In addition, a progressive loss of ubiquilin-1 was observed that correlated with disease progression. This finding coupled with previous overexpression data suggested the possibility that ubiquilin-1 overexpression would be protective in R6/2 mice. To test this, transgenic mice were generated that express human ubiquilin-1 under the control of the THY1.2 promoter, which drives transgene expression in neurons. These mice were then crossed with R6/2 mice in order to produce R6/2-UBQLN1 mice. These mice lived 20% longer than R6/2 mice and displayed delayed accumulation of inclusions in the hippocampus and cortex, and decreased ER stress in the hippocampus. Experiments in NB2a neuroblastoma cells indicate that the observed reduction in inclusions is dependent on proteasomal degradation. Despite these findings, ubiquilin-1 overexpression did not reduce aggregate load in the striatum or improve motor function in R6/2-UBQLN1 mice. I discuss the possible reasons why this could have occurred and outline a strategy for use of ubiquilin-1 for HD therapy.