Neurons expressing palmitoylation-resistant mutant HTT form nuclear inclusions (top), increased expression of HIP14 influences normal distribution of HTT (bottom). Image courtesy of A. Yanai and colleagues, University of British Columbia, Canada.

Researchers from the University of British Columbia in Vancouver have gained new insight into the pathogenesis of Huntington's disease (HD). Their study shows that a particular modification to the huntingtin (HTT) protein is essential for its normal trafficking and function, and becomes disrupted in mutant HTT.

The progressive cognitive and motor decline that occurs in HD is associated, at the cellular level, with the formation of nuclear and cytoplasmic inclusions of aggregated HTT protein — caused by mutations that expand the polyglutamine (polyQ) tract of the protein. It was thought that these inclusions might contribute to the neuronal toxicity and eventual apoptosis that characterizes the pathogenesis of HD. However, mounting evidence suggests that they might instead be a marker rather than a cause of the disease. For example, in a mouse model of the disease (YAC128), the occurrence of inclusions is preceded by the occurrence of motor and cognitive decline. By contrast, in another mouse strain that expresses just a short portion of HTT protein, inclusions develop but the neuronal dysfunction and degeneration seen in YAC128 mice do not. So what does lead to toxicity and cell death in HD?

Palmitoylation is a recently identified post-translational protein modification that has been shown to regulate the trafficking and function of a number of membrane-associated proteins. Because normal HTT protein localizes to plasma and intracellular membranes, and has been shown to be palmitoylated (by huntingtin interacting protein, HIP14), the team considered that palmitoylation might also regulate HTT's normal function. And it does.

The team first identified which residue of HTT gets palmitoylated. Then, by preventing palmitoylation of both wild-type and mutant HTT, the team observed an increased formation of inclusions (not normally seen for the wild-type protein), indicating that the distribution of HTT was disrupted and suggesting a role for palmitoylation in normal trafficking and folding of HTT.

The authors identified that the amount of palmitoylation of HTT is significantly reduced in the presence of the expanded polyQ tract, suggesting that the expansion might interfere with palmitoylation of HTT by HIP14. To investigate whether reduced palmitoylation could contribute to neuronal toxicity in HD, the team measured the degree of apoptosis in cells expressing either wild-type or mutant HTT when exposed to NMDA (N-methyl-D-aspartate)-induced toxicity. Cells expressing mutant HTT showed increased apoptosis in response to NMDA compared with those expressing wild-type HTT, but importantly both cell types became significantly more susceptible when palmitoylation of the proteins was abolished.

The findings by Yanai and colleagues suggest that without correct palmitoylation the normal cellular distribution of HTT gets disturbed and, as a result, the protein becomes cytotoxic. Encouragingly, however, the team found that overexpression of HIP14 considerably reduced the number of inclusions in cells containing mutant HTT. Therefore, discovering how palmitoylation of mutant HTT might be increased (or depalmitoylation prevented) could lead to drugs with the potential to slow or even stop the progressive neurodegeneration in patients with HD.