The role of small heat shock protein B8 (HspB8) in the removal of mutant androgen receptor in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a motorneuronal disease, caused by a polyglutamine (polyQ) expansion in the androgen receptor (AR) protein. SBMA is characterized by conformational changes in ARpolyQ that result in misfolding and protein aggregation. Using SBMA in vitro models expressing mutant AR (AR.Q46), we observed that the inactive and soluble AR.Q46 impairs the UPS activity, while Testosterone-activated AR.Q46 induced cytoplasmic aggregation leading to UPS desaturation and removing AR.Q46 from the soluble compartment into the aggregates waiting for an alternative clearance, as autophagy. It is known that chaperones facilitate the removal of misfolded proteins. We thus tested the potential of a small Heat Shock Protein (HSPB8) to facilitate mutant protein turnover and its effect on the degradative systems. HSPB8 overexpression decreased the AR.Q46 levels without affecting the proteasome and the inhibition of the proteasomal function did not block the HspB8. HspB8 might facilitate the AR.Q46 clearance acting through the macroautophagy. In fact, HspB8 stimulated the formation of LC3-II increasing the number of autophagosomes and the LC3 silencing correlated with the loss of HspB8 prodegradative effects on AR.Q46. Finally, we tested the effects of HspB8 overexpression on a longer polyQ tract (AR.Q112) because AR with Q46 and Q112 showed different biochemical properties and AR.Q112 has a higher tendency to aggregate than AR.Q46. Using transient and stable transfections with AR.Q112, which recapitulates the nuclear accumulation and aggregation observed in SBMA patients, we found that HspB8 was unable to induce AR.Q112 clearance and to counteract aggregation. All these data suggest that assisting protein folding with HspB8 we might reduce protein aggregation, stimulating the misfolded protein autophagic removal. Notably, HspB8 fail to suppress AR.Q112 aggregation suggesting that AR.Q46 and AR.Q112 might involve different mechanisms of neurotoxicity.