There is a fundamental gap in understanding how small aggregates formed by mutant forms of the huntingtin (htt) protein with expanded polyQ tracts gain toxic biological properties causing Huntington's disease (HD) and, more specifically, how these proteins interact with cellular surfaces comprised of lipids. Continued existence of this gap represents an important problem because these interactions may represent a fundamental step in htt- induced cellular toxicity and understanding of this phenomenon can lead to new targets for therapeutic intervention. The long-term goal of our research is to understand the physicochemical aspects and molecular mechanisms of nanoscale, pathological self-assembly of biological macromolecules that lead to toxicity. The objective in this application is to elucidate the role of lipids on htt misfolding, aggregation, and related toxicity by determinng the impact htt aggregates have on the integrity of cellular and subcellular membranous surfaces. Our central hypothesis is that the binding of mutant htt and its aggregate forms to subcellular surfaces comprised predominately of lipids is a fundamental step associated with toxicity and is facilitated by lipid composition of the membrane and post-translational modifications (PTMs) of htt. This hypothesis is based on preliminary data demonstrating that mutant huntingtin (htt) fragments and synthetic polyQ peptides accumulate on and destabilize model lipid bilayers depending on the availability of specific flanking sequences, such as the firs 17 amino acids of htt or a polyproline domain. The rationale for the proposed work is that detailed knowledge of the aggregation process of mutant htt proteins at lipid membrane interfaces can be expected to ultimately lead to the development of novel therapeutic and neuroprotective targets for HD. Guided by our preliminary studies, this hypothesis will be tested by pursuing two specific aims: 1) Identify lipid components that play a role in the binding of htt o membranes;and 2) Determine how post-translational modification regulate the interaction between htt and lipid membranes. Under the first aim, scanning probe and a variety of vesicle-based assays will be employed to characterize and measure the endogenous interactions occurring between htt and membranes enriched with cholesterol, sphingomyelin, or GM1. Under the second aim, a combination of spectroscopic, mass spectrometry, and scanning probe techniques will be applied to the study of the role of biologically PTMs play in modulating htt/lipd interactions. The proposed research is innovative because it focuses on the impact of physical and mechanical consequences of htt (and its aggregate forms) binding and inserting into lipid membranes that may ultimately lead to cellular dysfunction and death. This contribution is significant because it is expected to provide mechanistic knowledge of htt interaction and aggregation at lipid surfaces, which potentially represents a fundamental step in HD pathology.
The proposed research is relevant to public health because understanding the role lipid interactions and post translational modifications play in the molecular mechanisms of pathological self-assembly huntingtin is ultimately expected to lead to improved therapies to prevent or treat Huntington's disease (and perhaps similar neurodegenerative diseases). Thus, the proposed research is relevant to the part of NIH's mission that pertains to acquiring fundamental knowledge aimed at alleviating the burden of human disease.
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