CYCLODEXTRINS COUNTERACT PROTEIN AGGREGATION IN A SPINOBULBAR MUSCULAR ATROPHY

Spinobulbar Muscular Atrophy (SBMA) is a inherited X-linked neurodegenerative disease caused by an abnormal expansion of a CAG repeat, encoding a polyglutamine tract (polyQ), in the amino-terminal region of the androgen receptor (AR). The AR is a nuclear hormone receptor and it functions as a transcription factor after binding its specific ligand testosterone (T) in the C-terminal ligand-binding domain (LBD). The elongated polyQ causes the AR to form T-dependent intracellular aggregates in the spinal cord and brainstem and causes the death of those motoneurons. Cyclodextrins (Cdx) are water soluble, non-toxic, cyclic oligosaccharides usually used as vehicle to deliver poorly soluble drugs. Cdx have been validated as artificial chaperones for the refolding of enzymes, but their mechanisms remain unclear. In stress conditions, it has been demonstrated that Cdx prevent incorrect interactions between aberrantly exposed amino acids thus avoiding the formation of a stable beta-sheet structure. Preliminary studies obtained using circular dichroism analyses on an in vitro model, have already suggested that mutant AR presented a decrease in alpha-helix content in favor of an increase of beta-sheet amount when compared to the physiological AR. Thus, protein misfolding may be one of the main causes of aggregation. We also demonstrated that transient transfection of mutant AR in immortalized motoneurons (NSC34) caused proteasome impairment and intracellular accumulation of insoluble AR. Apparently these effects are due to an ineffective protein degradation of the mutant AR. We therefore wanted to investigate whether Cdx could enhance AR solubility and turnover, restoring proteasome activity or stimulating other cellular degradation pathways. Treatment with Cdx decreased aggregates and AR insolubile species. and affected in general AR levels. The reduced aggregation rate had also a positive impact on proteasome activity, measured through a YFPu reporter system. It is conceivable that these beneficial effects of Cdx are mediated by an enhancement of AR clearance mediated by a more effective proteasome activity or stimulating alternative cellular degradation pathways. Moreover, altogether these effects led to a reduced motoneuronal toxicity. These preliminary results, therefore, suggests that Cdx could be tested in further experiments as a potential treatment for neurodegenerative diseases caused by aggregation-prone proteins