Genetic Modifiers of Adult-Onset Neuronal Ceroid Lipofuscinosis (ANCL)

Abstract

Cysteine-string protein α (CSPα) is a molecular co-chaperone located at the synapse that functions to promote vesicle fusion. Two mutations in the cysteine-string domain (CSD) of human CSPα, L115R and L116Δ, are known to cause adult-onset neuronal ceroid lipofuscinosis (ANCL). ANCL is a neurodegenerative disease characterized by lysosomal accumulations in the brain and leads to early death. Currently the mechanisms of disease progression are unknown, making potential therapeutics difficult to discover. Previously, our lab created a Drosophila model of ANCL that can recapitulate most of the disease pathology. Using this model, our lab performed a F1 candidate screen for genetic modifiers that alter the rough surface and depigmentation seen in the eye of L116Δ mutant flies. Several modifiers of L116Δ toxicity were uncovered, including Hsp40, Hsp110, and Hip14. Hip14 is the enzyme responsible for the palmitoylation of CSP at the synapse, while Hsp110 and Hsp40 are molecular chaperones that participate in the refolding of misfolded client proteins. Using a Drosophila model expressing a human L116Δ transgene, I examined the effects of Hsp40, Hsp110 and Hip14 expression on the number of CSP positive endosomes in the larval VNC. Overexpression of Hsp110 and Hsp40 had no significant effect on the number of endosomes, although unexpectedly, both Hsp40 RNAi and Hsp110 RNAi animals exhibited few endosomal accumulations. Western blot analysis indicates that only Hsp40 overexpression increases the formation of HMW accumulations, although levels of the lipidated monomer do not show significant changes in any of the examined genotypes. This suggests that the toxicity of mutant hCSP is not related to the number of hCSP positive accumulations. Loss of one functional copy of Hip14 does not alter the number of observed endosomes, while reducing Hip14 expression through a RNAi transgene causes CSP positive punctae to localize only to a few cell bodies of the neuropil. Taken together these results suggest that Hip14 availability at the synapse is necessary for inducing the formation of hCSP positive endosomal accumulations.