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Identification of novel palmitoyl acyl transferases and characterization of the role of Huntingtin palmitoylation in Huntington Disease Huang, Kun

Abstract

In neurons, modification by the lipid palmitate regulates trafficking and function of signaling molecules, neurotransmitter receptors and associated synaptic scaffolding proteins. HIP14 (huntingtin interacting protein 14) is the first identified and characterized mammalian palmitoyl transferase that regulates this process. I have shown that HIP14 has striking effects on modulating trafficking and function of many proteins important for synapse formation and plasticity such as PSD-95, a postsynaptic scaffolding molecule. The importance of the finding that HIP14 is a neuronal palmitoyl transferase is further emphasized by our recent discovery that huntingtin protein folding, trafficking and function are regulated by the enzyme HIP14. Expansion of the polyglutamine tract in huntingtin as seen in Huntington Disease (HD) results in reduced association with HIP14 and decreased palmitoylation of huntingtin, which contributes to the formation of inclusion bodies and enhanced neuronal toxicity. By manipulating HIP14 levels through expression or knockdown, we can manipulate the number of huntingtin inclusion bodies and neuronal cell viability. Overall, these discoveries offer novel mechanism for HD pathogenesis and provide new approaches to therapy for HD. The tight association of HIP14 with wild-type huntingtin, which differs from other known enzyme-substrate interactions, indicates that huntingtin serves other functions beyond being a substrate of HIP14. I have discovered that, in vitro, wild-type huntingtin may facilitate activity of HIP14 to palmitoylate other neuronal substrates such as SNAP25, PSD95 and GAD65. By contrast, mutant htt does not act this way, probably due to lack of interaction with HIP14. Furthermore, immunoprecipitated HIP14 from huntingtin+/- mice also exhibits less enzyme activity in palmitoylating GST-SNAP25 in vitro, suggesting that decreased huntingtin expression compromises HIP14 activity. In vivo, using Acyl Biotin Exchange assay, I have also found that palmitoylation of a number of presynaptic and postsynaptic proteins that are involved in neurotransmission are reduced in huntingtin+/- mice. This study not only ascribes an important biochemical function to wild-type huntingtin, but also suggests that defects in protein palmitoylation in general due to mutant huntingtin lack of ability to facilitate HIP14 activity may contribute to the pathogenesis of HD.

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