ADrosophilamodel of neuronal ceroid lipofuscinosisCLN4reveals a hypermorphic gain of function mechanism

AbstractThe autosomal dominant neuronal ceroid lipofuscinoses (NCL)CLN4is caused by mutations in the synaptic vesicle (SV) protein CSPα, which is a critical co-chaperone of Hsc70 protecting synapses from activity-dependent degeneration. We developed the first animal models ofCLN4by expressing eitherCLN4mutant human CSPα (hCSPα) orDrosophilaCSP (dCSP) in fly neurons. Similar to patients,CLN4mutations induced excessive oligomerization of mutant hCSPα and premature lethality in a dose-dependent manner. Instead of being localized to SVs, mostCLN4mutant hCSPα abnormally accumulated in axons and somata, and co-localized with ubiquitinated proteins and the prelysosomal markers HRS and LAMP1. Ultrastructurally, abnormal multi-laminar membrane structures were frequently observed in axons and somata next to degenerative abnormalities. The lethality, oligomerization and prelysosomal accumulation induced byCLN4mutations was attenuated by reducing wild type (WT) dCSP levels and enhanced by increasing WT dCSP or hCSPα levels, which indicates that bothCLN4alleles resemble dominant hypermorphic gain of function mutations. Furthermore, reducing the gene dosage of Hsc70 also attenuatedCLN4phenotypes. Taken together, we suggest thatCLN4alleles resemble dominant hypermorphic gain of function mutations that drive excessive oligomerization and impair membrane trafficking.