Absence of Ataxin-3 Leads to Enhanced Stress Response in C. elegans

Abstract

Ataxin-3, the protein involved in Machado-Joseph disease, is able to bind ubiquitylated substrates and act as a deubiquitylating enzyme in vitro, and it has been involved in the modulation of protein degradation by the ubiquitin-proteasome pathway. C. elegans and mouse ataxin-3 knockout models are viable and without any obvious phenotype in a basal condition however their phenotype in stress situations has never been described.Considering the role of ataxin-3 in the protein degradation pathway, we analyzed the effects of heat shock, a known protein homeostasis stressor, in C. elegans ataxin-3 (ATX-3) knockout animals. We found that ATX-3 mutants have an exacerbated stress response and survive significantly better than wild type animals when subjected to a noxious heat shock stimulus. This increased thermotolerance of mutants was further enhanced by pre-exposure to a mild heat shock. At a molecular level, ATX-3 mutants have a distinct transcriptomic and proteomic profile with several molecular chaperones abnormally up-regulated during heat shock and recovery, consistent with the observed resistance phenotype.The improved thermotolerancein ATX-3 mutants is independent of heat shock factor 1, the maestro of the heat shock response, but fully dependent on DAF-16, a critical stress responsive transcription factor involved in longevity and stress resistance. We also show that the increased thermotolerance of ATX-3 mutants is mainly due to HSP-16.2, C12C8.1 and F44E5.5 given that the knockdown of these heat shock proteins using RNA interference causes the phenotype to revert.This report suggests that the absence of ATX-3 activates the DAF-16 pathway leading to an overexpression of molecular chaperones, which yields knockout animals with an improved capacity for dealing with deleterious stimuli.

Toward therapeutic targets for SCA3: Insight into the role of Machado–Joseph disease protein ataxin-3 in misfolded proteins clearance

“…By the time the mice are 80 weeks old, no differences are observed between WT and KO. Similarly, the absence of ataxin-3 in C. elegans did not downregulate or otherwise affect basal mRNA levels of chaperones, and surprisingly, the ataxin-3 mutants exhibited an exacerbated stress response and survived significantly better compared to wild-type animals when both were placed under a noxious heat shock stress (Rodrigues et al, 2011). When worms were heatshocked from the baseline temperature (20 8C) to the lethal temperature (35 8C), the survival curves showed no differences between wild-type (N2) and ataxin-3 mutants (gk193, tm1689) (Kuhlbrodt et al, 2011;Rodrigues et al, 2011).…”

“…Similarly, the absence of ataxin-3 in C. elegans did not downregulate or otherwise affect basal mRNA levels of chaperones, and surprisingly, the ataxin-3 mutants exhibited an exacerbated stress response and survived significantly better compared to wild-type animals when both were placed under a noxious heat shock stress (Rodrigues et al, 2011). When worms were heatshocked from the baseline temperature (20 8C) to the lethal temperature (35 8C), the survival curves showed no differences between wild-type (N2) and ataxin-3 mutants (gk193, tm1689) (Kuhlbrodt et al, 2011;Rodrigues et al, 2011). It should be noted that when worms were heatshocked from a stress-threshold temperature (25 8C) to the lethal temperature (35 8C), gk193 showed a significantly increased resistance to stress as compared to WT, and this phenomenon was exacerbated by an additional pre-exposure to a non-lethal heat shock (30 8C, 2 h).…”

“…First the association of ataxin-3 and VCP supports rather than antagonizes protein turnover. Worms lacking ataxin-3 or mice containing an ataxin-3 mutant allele exhibit an increased stress response, suggesting that they bear defects in protein homeostasis regulation (Hubener et al, 2011;Rodrigues et al, 2011). Further, the degradation of p97 substrates is inhibited in ataxin-3 knockout worms (Kuhlbrodt et al, 2011).…”

“…There may be a stress threshold for a cell to judge which pathway it should use. The accumulation of increased endogenous stress induced by ataxin-3 deficiency may have made it easier to reach this threshold, thus potentiating the exogenous stress response in the worms (Rodrigues et al, 2011). However, more evidence is needed to verify this hypothesis, as the existing data do not prove it.…”

Role of the ubiquitin–proteasome system in nervous system function and disease: using C. elegans as a dissecting tool

“…Intriguingly, and perhaps counter-intuitively, ATX-3 knock-out C. elegans strains show an improved response to heat stress, with higher expression of chaperones [222], suggesting a negative regulatory role for this protein in the stress response or perhaps an adaptation of the knockout strains to a permanent proteotoxic stress. Another insight to the ATX-3-mediated mechanisms of lifespan regulation was given by the finding that CDC48 and ATX-3 synergistically cooperate in ubiquitin-mediated proteolysis and in ageing regulation.…”