The small heat shock proteins (sHsps) are evolutionarily conserved molecular chaperones which have a range of cytoprotective functions. The characteristic role of sHsps is the maintenance of protein homeostasis in cells; they do this primarily by preventing the aggregation of proteins in misfolded or unfolded states. Mutations in the sHsps are implicated in various neuropathies; specifically, sixteen mutations in Hsp27 have been identified that cause Charcot-Marie-Tooth disease or distal hereditary motor neuropathies. Interestingly, while Hsp27 is expressed in virtually every cell type, mutations in Hsp27 are exclusively associated with neuromuscular disorders.
The mechanism(s) by which pathogenic mutations of Hsp27 cause disease is unknown and is challenging to identify as Hsp27 plays a role in a wide range of cellular functions. It is well established that the tertiary and quaternary structure of Hsp27 is integral to its chaperone activity. As such, this work has focused on characterising the effect that seven pathogenic mutations have on the structure and chaperone function of Hsp27. These mutations span each of the key structural domains of the protein. Biophysical techniques, including intrinsic tryptophan fluorescence, circular dichroism and size-exclusion chromatography, were used to identify differences in the folding and oligomeric size and distribution of the mutant Hsp27 isoforms compared to the wild-type protein. Differences in the ability of these pathogenic Hsp27 mutants to prevent protein aggregation were assessed using in vitro assays employing various aggregation-prone target proteins. The overall results of these analyses show that each mutation has a variable effect on the structure and chaperone activity of Hsp27, highlighting the need to conduct cell-based assays to elucidate the effects of these mutations in the cellular environment. Thus, future work will be aimed at identifying intracellular binding partners of Hsp27, and investigating the role of Hsp27 in the formation and stabilisation of the cytoskeleton in neurons.