Huntington's disease-associated ankyrin repeat palmitoyl transferases are rate-limiting factors in lysosome formation and fusion

Abstract

Protein palmitoylation in the Golgi apparatus is critical for the appropriate sorting of various proteins belonging to secretory and lysosomal systems, and defective palmitoylation can lead to the onset of severe pathologies. HIP14 and HIP14L ankyrin repeat-containing palmitoyl transferases were linked to the pathogenesis of Huntingtons disease, however, how perturbation of these Golgi resident enzymes contributes to neurological disorders is yet to be understood. In this study, we investigated the function of Hip14 and Patsas - the Drosophila orthologs of HIP14 and HIP14L respectively - to uncover their role in secretory and lysosomal membrane trafficking. Using larval salivary gland, a well-established model of the regulated secretory pathway, we found that these PAT enzymes equally contribute to the proper maturation and crinophagic degradation of glue secretory granules by mediating their fusion with the endo-lysosomal compartment. We also revealed that Patsas and Hip14 are both required for lysosomal acidification and biosynthetic transport of various lysosomal hydrolases, and we demonstrated that the rate of secretory granule-lysosome fusion and subsequent acidification positively correlates with the level of Hip14. Furthermore, Hip14 is also essential for proper lysosome formation and neuronal function in adult brains. Finally, we found that the over-activation of lysosomal biosynthetic transport and lysosomal fusions by the expression of the constitutively active form of Rab2 could compensate for the lysosomal dysfunction caused by the loss of Patsas or Hip14 both in larval salivary glands and neurons. Therefore, we demonstrated that ankyrin repeat palmitoyl transferases may act as rate-limiting factors in lysosomal fusions and provide genetic evidence that defective protein palmitoylation and the subsequent lysosomal dysfunction can contribute to the onset of Huntingtons disease-like symptoms. Author SummaryGrowing body of evidence suggests that decreased activity of HIP14 and HIP14L palmitoyl transferases caused by accumulation of mutant Huntingtin and the subsequent alterations in protein palmitoylation play a critical role in the onset of Huntingtons disease (HD). However, which cellular processes are perturbed and eventually lead to the emergence of HD due to impaired palmitoylation is still poorly understood. In our study, we used a Drosophila model to uncover the role of Hip14 and Patsas (the fly ortholog of HIP14L) in secretory and lysosomal membrane trafficking. We found that silencing of these transferases in larval salivary glands equally disrupts secretory granule lysosome-fusion, their subsequent acidification and also the trafficking of lysosomal hydrolases. While overexpression of Hip14 resulted in the acceleration of these processes. We also observed that neuron-specific loss of Hip14 not only perturbs lysosome formation but also results in a progressive decline in neuromuscular functions. Importantly, both lysosomal and neuronal defects emerging in Hip14 and Patsas deficient backgrounds could be restored by hyperactivation of Rab2 GTPase mediated lysosome formation and fusion. These findings suggest that Hip14 and Patsas protect from the onset of HD like symptoms by acting as rate-limiting factors of lysosome formation and fusion.