A Drosophila model of neuronal ceroid lipofuscinosis CLN4 reveals a hypermorphic gain of function mechanism

Abstract

The autosomal dominant neuronal ceroid lipofuscinoses (NCL) CLN4 is caused by mutations in the synaptic vesicle (SV) protein CSP, which is a critical co-chaperone of Hsc70 protecting synapses from activity-dependent degeneration. We developed the first animal models of CLN4 by expressing either CLN4 mutant human CSP (hCSP) or Drosophila CSP (dCSP) in fly neurons. Similar to patients, CLN4 mutations induced excessive oligomerization of mutant hCSP and premature lethality in a dose-dependent manner. Instead of being localized to SVs, most CLN4 mutant hCSP 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 by CLN4 mutations was attenuated by reducing wild type (WT) dCSP levels and enhanced by increasing WT dCSP or hCSP levels, which indicates that both CLN4 alleles resemble dominant hypermorphic gain of function mutations. Furthermore, reducing the gene dosage of Hsc70 also attenuated CLN4 phenotypes. Taken together, we suggest that CLN4 alleles resemble dominant hypermorphic gain of function mutations that drive excessive oligomerization and impair membrane trafficking.