Adult onset neuronal ceroid lipofuscinosis (ANCL) is a fatal lysosomal storage disease caused by two known dominant mutations in the gene encoding cysteine string protein-? (CSP?): CSP?L115R and CSP?L116?. CSP? forms a chaperone complex with SGT (small glutamine-rich tetratricopeptide repeat-containing protein) and Hsp70/Hsc70 (heat shock protein/cognate 70 kDa) to chaperone the synaptic SNARE protein SNAP-25. It is surprising that mutations in CSP? lead to lysosomal pathology because its role has only been clarified in the context of synaptic function. I have recently found that SNAP-23, a homolog of SNAP-25, is also a client of the CSP?/SGT/Hsc70 chaperone complex. This interaction was found via (i) immunoprecipitation of CSP? from wild type mouse brain followed by tandem mass spectrometry identification of SNAP-23, (ii) reduced protein levels of SNAP-23 in CSP? knockout (CSP?-/-) mouse brains, and (iii) co-immunoprecipitation of SNAP-23 with each member of the CSP?/SGT/Hsc70 chaperone complex. Importantly, SNAP-23 mediates Ca2+-dependent fusion of lysosomes with the plasma membrane by forming a SNARE-complex with VAMP-7 and syntaxin-4. In support of this function, I have identified diminished Ca2+-dependent lysosomal exocytosis in CSP?-/- primary neurons by measuring cell surface exposure of the LAMP-1 luminal domain following intracellular Ca2+ induction with ionomycin. Altogether, these preliminary data draw a new and direct connection between CSP? dysfunction and lysosomal pathology in ANCL by means of impaired SNAP-23 function. Key gaps remain in our understanding of how mutations in CSP? cause the pathological cascade of ANCL: a) how ANCL mutations in CSP? affect chaperoning of SNAP-23, and b) how SNAP-23 dysfunction leads to lysosomal pathology with lipofuscin accumulation. My hypothesis is that ANCL mutations in CSP? prevent the CSP?/SGT/Hsc70 complex from chaperoning the lysosomal SNARE protein SNAP-23, disrupting lysosomal exocytosis and leading to lipofuscin accumulation. This hypothesis will be addressed using a multi-disciplinary approach including primary cortical neurons from CSP?-/- mice, biochemical assays and lentiviral rescue experiments. Experiments will be carried out by means of two proposed specific aims:
Aim 1 will clarify how ANCL mutations affect CSP??s chaperoning of SNAP-23:
Aim 2 will elucidate how ANCL mutations in CSP? affect lysosomal exocytosis, leading to lipofuscinosis. Completion of these aims will lead to a detailed understanding of the pathological cascade of ANCL, opening future avenues for testing therapeutics strategies.
Despite a known genetic cause, no effective treatments exist for the fatal lysosomal storage disorder adult onset neuronal ceroid lipofuscinosis (ANCL). The aim of this proposal is to delineate the pathological cascade of ANCL that is caused by these genetic mutations. This understanding will be essential to testing future corrective strategies for the benefit of society.