In vivo suppression of polyglutamine aggregation via co-condensation of the molecular chaperone DNAJB6

Abstract

Amyloidogenic protein aggregation is a hallmark of several human neurodegenerative conditions, including Alzheimers, Parkinsons, and Huntingtons disease (HD). Mutations and/or environmental stresses trigger conformational transition of specific proteins to amyloids, conferring them with gain of toxic function, which eventually leads to cell death in distinct brain areas. Cumulative data indicate that modulation of specific molecular chaperones can alleviate many of the pathological features of protein aggregation diseases. We previously showed that the Hsp70 co-chaperone DNAJB6 is among the strongest suppressors of amyloid aggregation, and that moderate DNAJB6 overexpression significantly extents lifespan of a mouse model of aggressive HD pathology. DNAJB6 alone delays amyloidogenic aggregation in vitro by several orders of magnitude at substoichiometric ratios, but its activity in cells is less efficient, albeit still markedly superior to most known anti-amyloidogenic agents. This suggests that downstream PQC factors are necessary for full DNAJB6-mediated suppression of aggregation in vivo, which might have to be co-stimulated in therapeutic strategies targeting DNAJB6 action. We explored here the PQC pathways required for optimal DNAJB6 inhibition of polyglutamine (polyQ) aggregation, focusing on the two main cellular proteolytic machineries: proteasomes and macroautophagy. Unexpectedly, DNAJB6 activity was largely insensitive to chemical blockage of either degradative pathway. Instead, live cell imaging unveiled a co-condensation mechanism of DNAJB6 with mobile polyQ assemblies. DNAJB6 was not required for polyQ condensation, but its expression increased the percentage of cells with mobile condensates by a factor of 3, suggesting that DNAJB6 prevents polyQ condensates to convert from the soluble to the solid state. This in turn, may keep the polyQ peptides competent for (regulated) degradation and accessible to factors allowing its extraction from the condensed state.