Huntington's disease (HD) is a neurodegenerative disorder arising from an inherited CAG expansion mutation within the HD gene, resulting in mutant huntingtin (Htt) protein containing an expanded polyglutamine (polyQ) tract. Degeneration of the striatum causes patients to suffer from progressive loss of voluntary motor control, psychiatric disturbances, and debilitating cognitive decline. A pathological hallmark of HD is the accumulation of mutant Htt and ubiquitin in intracellular deposits throughout the brain. The link between Htt aggregates and disease symptoms remains unclear, but improvements in both cellular and behavioral pathology have been observed in studies that silence the Htt transgene. In an inducible HD mouse model, transgene silencing leads to the elimination of aggregates in conjunction with amelioration of motor deficits. These findings imply that aggregate removal has the potential to alleviate the underlying cellular dysfunction associated with HD. Understanding the molecular mechanisms by which neurons dispose of aggregated proteins is a critical step towards the development of therapeutics for a variety of neurodegenerative disorders. Cellular model systems have suggested that Htt aggregates are broken down by a degradation process known as macroautophagy, in which cytosolic proteins are sequestered into a double-membrane structure that fuses with a lysosome to degrade its cargo. The macroautophagic machinery comprises a core group of autophagy-related proteins, such as Atg5, Atg7, and LC3. While this process was initially found to degrade proteins in bulk in response to starvation, it can also occur selectively for particular substrates. Our lab has identified a protein called Alfy (autophagy-linked FYVE protein) that mediates selective macroautophagy of aggregated proteins. In cells expressing mutant Htt, Alfy is essential for the clearance of aggregates. Additionally, overexpression of c-terminal Alfy decreases inclusions and protects against neurodegeneration in drosophila and primary neuronal models of polyQ expansion. This proposal seeks to address the role of Alfy-mediated selective macroautophagy in aging brain and in HD. Eliminating core macroautophagic proteins such as Atg7 from developing brain results in neurodegeneration and protein accumulation, but the role of selective and nonselective macroautophagy during aging remains largely unexplored in vivo. This project will examine this by eliminating Alfy or Atg7 in aging mice using a tamoxifen-inducible Cre to drive excision of a conditional allele (Aim 1). This will provide insight into the importance of macroautophagy in aging brain. Furthermore, this project will investigate Alfy- mediated macroautophagy of Htt aggregates in vivo, by eliminating Alfy in an inducible HD mouse model (Aim 2). This will address how removal of Alfy affects clearance of mutant Htt, and whether disease reversal is dependent upon clearance of aggregates.
The brains of Huntington's disease (HD) patients are burdened with deposits of mutant huntingtin protein, but the relationship of these aggregates to disease pathology is unclear. Using genetic manipulations in mice, this proposal will investigate a molecular mediator of aggregated protein breakdown, (1) in the aging brain and (2) in removal of mutant huntingtin aggregates. This project will provide an understanding of the importance of aggregated protein disposal during aging, and how it may be linked to neuropathological and behavioral recovery in HD, which are critical steps towards the identification of molecular targets for clinical intervention in HD and other neurological disorders.
