COS cells were transfected with polyglutamine-
expanded htt. The arrow indicates an inclusion body.
Grammatical modifiers like adjectives and adverbs describe other parts of speech, clarifying but not changing their meaning. In contrast, biological modifiers can dramatically alter the function of proteins. Palmitoylation attaches the fatty acid palmitate to cysteine residues. Because palmitoyl groups are highly hydrophobic, palmitoylated proteins tend to be membrane associated. Yanai et al. show that palmitoylation maintains the localization and function of huntingtin (htt) protein and suggest that palmitoylation is disrupted in Huntington disease in a recent article in Nature Neuroscience.
Huntington disease results from polyglutamine expansion of htt, a protein whose function in healthy cells is unknown. In most people, htt has 10-35 glutamine residues, but in people with Huntington disease, htt has 40 or more glutamine repeats. Trafficking of htt is often disrupted before the onset of disease symptoms.
The authors identified a highly conserved cysteine at amino acid 214 as the htt palmitoylation site. A cysteine-to-serine substitution (C214S) rendered htt resistant to palmitoylation. Polyglutamine-expanded (128 glutamine residues) htt showed a 50% reduction in palmitoylation relative to normal (15 residues) htt, suggesting that the disease-associated expansion of glutamine residues disrupts normal palmitoylation.
The authors found disease-associated protein inclusion bodies in neurons with disrupted palmitoylation. Neurons transfected with polyglutamine-expanded htt had more densely stained protein inclusion bodies than those transfected with normal htt, which rarely showed inclusions. The C214S mutation increased inclusion body formation regardless of htt glutamine status. Unlike cytoplasmic inclusion bodies, nuclear inclusion bodies indicate Huntington disease progression in people and in animal models. The authors observed nuclear inclusion bodies only in neurons with polyglutamine-expanded C214S htt. Regardless of glutamine expansion, the C214S mutation also increased vulnerability to excitotoxic injury.
HIP14 is a palmitoyl transferase that interacts with htt and localizes to the Golgi bodies. The authors overexpressed HIP14 in cortical neurons and found redistribution of htt to the Golgi bodies. Cells transfected with C214S htt showed no change in htt localization. In contrast, HIP14 short interfering RNA (siRNA), which knocks down expression of HIP14, induced cytoplasmic localization of htt protein. These data suggest that HIP14 regulates htt trafficking.
HIP14 overexpression in cortical neurons expressing polyglutamine-expanded htt decreased inclusion body formation, suggesting that palmitoylation might slow disease progression. YAC18 and YAC128 are mice transgenic for human htt with 18 and 128 glutamine repeats, respectively. The authors found reduced association with HIP14 and therefore reduced palmitoylation in YAC128 relative to YAC18 brains. HIP14 siRNA increased the number of inclusions in cultured cortical neurons and increased the vulnerability of neurons to NMDA excitoxicity.
Together, these data show the importance of palmitoylation to htt function and the dire consequences of its disruption. Because administration of exogenous HIP14 improved the condition of neurons with polyglutamine expansion of htt, HIP14 or inhibitors of enzymes that depalmitoylate htt may be important targets for drugs to slow the cellular progression of Huntington disease.