Molecular chaperones enhance the degradation of expanded polyglutamine repeat androgen receptor in a cellular model of spinal and bulbar muscular atrophy

Abstract

Spinal and bulbar muscular atrophy (SBMA) is one of a growing number of neurodegenerative diseases caused by a polyglutamine-encoding CAG trinucleotide repeat expansion, and is caused by an expansion within exon 1 of the androgen receptor (AR) gene. The family of polyglutamine diseases is characterized by the presence of ubiquitinated, intranuclear inclusions associated with molecular chaperones and 26S proteasome components, although the role of these inclusions in the pathogenesis of polyglutamine diseases remains unclear. The over-expression of molecular chaperones of the Hsp70 and Hsp40 families has been shown to modulate inclusion frequency and cellular toxicity. We developed a cell culture system which enables the quantitative analysis of the effects of molecular chaperones on the biochemical properties of an expanded repeat AR. Using this approach, we demonstrate that Hsp70 and its co-chaperone Hsp40 not only increase expanded repeat AR solubility, but function to enhance the degradation of expanded repeat AR through the proteasome. Furthermore, our studies indicate that these molecular chaperones significantly decrease the half-life of an expanded repeat AR. Molecular chaperone enhancement of protein degradation points to the modulation of molecular chaperones as a potential therapeutic target for polyglutamine diseases.

Firefly luciferase mutants as sensors of proteome stress

“…It has been shown in HeLa cells that DnaJB1 and Hsp70 translocate to nucleoli upon heat stress (Hattori et al, 1993;Terada and Mori, 2000). Moreover, it has been shown that DnaJB1 co-localizes with nuclear inclusions of polyQ proteins Seidel et al, 2011) and that it can facilitate the degradation of some substrates (Bailey et al, 2002). Therefore, we investigated the effect of Hsp40/DNAJB1 on the folding and degradation status of NES-FlucDM-EGFP.…”

“…It has been shown that DnaJB1 suppresses the aggregation of polyQ proteins (Rujano et al, 2007;Zijlstra et al, 2010) and also prevents luciferase aggregation during heat stress (Hageman et al, 2010). Moreover, it has been demonstrated that overexpression of Hsp70 and DnaJB1 facilitates the degradation of polyQ expanded forms of the androgen receptor (Bailey et al, 2002). Recently, it has also been shown that less characterized members of the DnaJB family, DnaJB6 and DnaJB8, bind non-foldable clients such as polyQ proteins and keep them in a state competent for (proteasomal) degradation (Hageman et al, 2010;Kampinga and Craig, 2010).…”

Proteasome-mediated Proteolysis of the Polyglutamine-expanded Androgen Receptor Is a Late Event in Spinal and Bulbar Muscular Atrophy (SBMA) Pathogenesis

“…Fast Migrating Species Contain Ubiquitin-Previous studies have shown the presence of ubiquitin and other proteasomal components associated with inclusions of polyglutamine-expanded AR in models of SBMA (7,13,18,34,35). If the fast migrating species represent inclusions or an inclusion-associated aggregation species, we would expect these species to be ubiquitinated.…”

Molecular chaperones as regulators of cell death

“…Overexpression of Hsj1A with Hsp70 reduced polyglutamine inclusion caused by mutant androgen receptor (containing polyglutamine repeat) of SBMA, probably by promoting ubiquitination, but not by protein folding of the substrate (Howarth et al, 2007). Hsp70 and Hsp40 were also reported as increasing the solubility of mutant androgen receptor as well as promoting the degradation of the protein through the proteasome (Bailey et al, 2002). Hsp90 inhibitor increases proteasome degradation of hyperphosphorylated tau protein, which is a primary component of neurofibrillary tangles in Alzheimer's disease (Dickey et al, 2006).…”