Palmitoylation, the addition of palmitate to proteins by palmitoyl acyltransferases (PATs), is an important regulator of synaptic protein localization and function. Many palmitoylated proteins and PATs have been implicated in neuropsychiatric diseases, including Huntington disease, schizophrenia, amyotrophic lateral sclerosis, Alzheimer disease, and X-linked intellectual disability. HIP14/DHHC17 is the most conserved PAT that palmitoylates many synaptic proteins. Hip14 hypomorphic mice have behavioral and synaptic deficits. However, the phenotype is developmental; thus, a model of post-developmental loss of Hip14 was generated to examine the role of HIP14 in synaptic function in the adult.

Huntington disease (HD) is an adult-onset neurodegenerative disease characterized by motor, cognitive, and psychiatric symptoms that is caused by a CAG expansion in the HTT gene. Palmitoylation is the addition of saturated fatty acids to proteins by DHHC palmitoylacyl transferases. HTT is palmitoylated by huntingtin interacting proteins 14 and 14-like (HIP14 and HIP14L or ZDHHC17 and 13 respectively). Mutant HTT is less palmitoylated and this reduction of palmitoylation accelerates its aggregation and increases cellular toxicity. Mouse models deficient in either Hip14 (Hip14(-/-)) or Hip14l (Hip14l(-/-)) develop HD-like phenotypes. The biological function of HTT palmitoylation and the role that loss of HTT palmitoylation plays in the pathogenesis of HD are unknown. To address these questions mice deficient for both genes were created. Loss of Hip14 and Hip14l leads to early embryonic lethality at day embryonic day 10-11 due to failed chorioallantoic fusion. The chorion is thickened and disorganized and the allantois does not fuse correctly with the chorion and forms a balloon-like shape compared to Hip14l(-/-); Hip14(+/+) littermate control embryos. Interestingly, the Hip14(-/-) ; Hip14(-/-) embryos share many features with the Htt(-/-) embryos, including folding of the yolk sac, a bulb shaped allantois, and a thickened and disorganized chorion. This may be due to a decrease in HTT palmitoylation. In Hip14(-/-); Hip14l(-/-) mouse embryonic fibroblasts show a 25% decrease in HTT palmitoylation compared to wild type cells. This is the first description of a double PAT deficient mouse model where loss of a PAT or multiple PATs results in embryonic lethality in mammals. These results reinforce the physiological importance of palmitoylation during embryogenesis.

Huntington disease is an adult onset neurodegenerative disease characterized by motor, cognitive, and psychiatric dysfunction, caused by a CAG expansion in the HTT gene. Huntingtin Interacting Protein 14 (HIP14) and Huntingtin Interacting Protein 14-like (HIP14L) are palmitoyl acyltransferases (PATs), enzymes that mediate the post-translational addition of long chain fatty acids to proteins in a process called palmitoylation. HIP14 and HIP14L interact with and palmitoylate HTT and are unique among PATs as they are the only two that have an ankyrin repeat domain, which mediates the interaction between HIP14 and HTT. These enzymes show reduced interaction with and palmitoylation of mutant HTT, leading to increased mutant HTT inclusion formation and toxicity. The interaction between HIP14 and HTT goes beyond that of only an enzyme-substrate interaction as HTT is essential for the full enzymatic activity of HIP14. It is important to further understand and characterize the interactions of HTT with HIP14 and HIP14L to guide future efforts to target and enhance this interaction and increase enzyme activity to remediate palmitoylation of HTT and their substrates, as well as to understand the relationship between the three proteins. HIP14 and HIP14L have been previously shown to interact with HTT amino acids 1-548. Here the interaction of HIP14 and HIP14L with N- and C-terminal HTT 1-548 deletion mutations was assessed. We show that HTT amino acids 1-548 were sufficient for full interaction of HTT with HIP14 and HIP14L, but partial interaction was also possible with HTT 1-427 and HTT 224-548. To further characterize the binding domain we assessed the interaction of HIP14-GFP and HIP14L-GFP with 15Q HTT 1-548Δ257-315. Both enzymes showed reduced but not abolished interaction with 15Q HTT 1-548Δ257-315. This suggests that two potential binding domains exist, one around residues 224 and the other around 427, for the PAT enzymes HIP14 and HIP14L.