Characterization of hypertonic stress-induced protein damage and the cellular mechanisms for defense and repair in C. elegans

Abstract

Proteostasis is maintained by a complex network of genes and processes which includes core synthesis and degradation machineries as well as chemical and protein chaperones. Much of what is known about the function and organization of the proteostasis network stems from analyzing how cells respond to genetic or environmental perturbation of proteomic integrity. Recent evidence points to a critical role for the proteostasis network in survival of hypertonic environments, but the proteotoxic effects of hypertonic stress remain largely undescribed. Employing the many experimental advantages of the nematode C. elegans, we provide the first detailed description of the nature and extent of protein damage caused by hypertonic stress. Misfolding and aggregation of diverse reporters and endogenous proteins are rapid and widespread in vivo. Additionally, we demonstrate that acclimation of C. elegans to a mild hypertonic environment activates unknown proteostasis activities capable of preventing aggregation during extreme hypertonic stress.

To define novel aspects of the hypertonic stress response and extend our understanding of cellular proteostasis strategies, we employ genetic and pharmacological approaches in determining the mechanism by which hypertonic acclimation enhances proteostasis. We hypothesize that chemical chaperones, protein chaperones, proteolysis machineries, and/or protein synthesis must be involved. Surprisingly, hypertonicity- or mutation-induced accumulation of glycerol, an organic osmolyte widely believed to act as a chemical chaperone in vivo, does not directly ameliorate protein damage during stress or aging. Protein chaperone expression is not transcriptionally upregulated. Further, hypertonic stress actually reduces protein degradation, an effect not reversed by acclimation. We demonstrate for the first time that suppression of protein translation during an environmental stress directly enhances proteostasis by preventing aggregation of extant proteins. Combined with recent observations that inhibition of translation extends lifespan and occurs naturally in response to other proteotoxic stressors, this finding suggests that translational reprogramming represents a conserved mechanism by which cells reduce the population of nascent, damage-prone proteins to enhance the availability and effectiveness of pre-existing chaperones.