Figure 6 |. Model depicting the mechanism of CSPα oligomerization in ANCL and its alleviation by Fe-chelators.

CSPα^WT is heavily palmitoylated, allowing its localization to presynaptic membranes (panel 1), where it forms a functional chaperone complex with Hsc70 and SGT; which chaperones synaptic SNARE SNAP-25. The ANCL mutants CSPα^L115R and CSPα^L116Δ are inefficiently palmitoylated, and mislocalized away from synapses, where mutant CSPα’s J-domain (J Dom) recruits a CIA complex containing Hsc70, and ISCU – an Fe-S cluster transferring protein (panel 2). This molecular interaction leads to pathogenic oligomers of mutant CSPα: CSPα^WT expressed alone is not oligomerized beyond its native homodimerization (panel 3), while the loss of palmitoylation in the ANCL mutants CSPα^L115R and CSPα^L116Δ accompanies ectopic binding of their reactive Cys-strings to Fe-S clusters, cross-linking the mutant CSPα molecules into oligomers. These oligomers mainly reside away from the synapse, causing loss of CSPα’s chaperone function at the synapse (panel 4). In the CSPα^L115R or CSPα^L116Δ heterozygous cells, mutant CSPα is able to recruit CSPα^WT into oligomers, leading to less than hemizygous level of CSPα function at the synapse (panel 5). Iron-chelator treatment leads to reduction in the Fe-S cluster-bound oligomers, allowing improved CSPα function at the synapse (panel 6). This increase in functional CSPα leads to partial alleviation of the downstream SNAP-25 instability and lipofuscin accumulation phenotypes.