Natural genetic variation and an alternative physiological state modify polyglutamine aggregation and toxicity in C. elegans

Many human diseases are caused by mutations that induce misfolding and aggregation of the affected proteins, and are thought to result from failures in proteostasis. Pathways involved in maintaining proteostasis, when activated in model organisms, can suppress protein aggregation, but their activation can also have detrimental effects on organism's physiology. Therefore, it is imperative to identify physiologically compatible targets of these pathways. Prominent characteristics of protein aggregation diseases include cell-specific susceptibility to the toxic effects of a ubiquitously expressed mutant protein and the variation in disease onset among individuals with the same mutant aggregation-prone protein. These characteristics suggest there are genetic or physiological modifiers which modulate both the susceptibility of cells and individuals to protein aggregation and the age of disease onset. Using a model expressing muscle-specific polyglutamine-containing transgene (Q40), we asked how natural genetic variation among wild isolates of C. elegans modulates the behavior of aggregation-prone proteins, and whether the adaptive physiological state, dauer, can protect against protein aggregation. Identifying the genetic modifiers of protein aggregation and their mechanism may point to new potential therapeutic targets. A large modifier locus from one wild isolate, DR1350, was identified that caused two genetically separable phenotypes: an overall increase in Q40 aggregation and a switch in susceptibility (referred to as Swis) of muscle cells to aggregation. I found that increased Q40 aggregation was caused by variants in a regulatory region of atg-5 that caused increased activation of autophagy. Although autophagy is thought to clear aggregation, I found that direct activation of autophagy increased aggregation in muscles but decreased it in the intestine. These findings show that interaction between autophagy and protein aggregation is dependent on the cellular environment. In the second part of this dissertation, I investigated the Swis phenotype, where the head muscles that are the most resistant to aggregation in the laboratory strain became the most susceptible in the wild DR1350 strain. A suppressor screen combined with genome sequencing revealed a nonsense mutation in UNC-54 (myosin), which suppressed the Swis phenotype. These findings provide the first evidence for myosin controlling susceptibility to polyglutamine aggregation, and provide insight into possible mechanisms for cell vulnerability. To identify physiological modifiers of protein aggregation that can activate proteostasis pathways without detrimental consequences, I employed the C. elegans alternative physiological program dauer diapause. Because dauer animals are protected against multiple proteotoxic stresses, but are able to return to normal development, I tested whether activation of dauer diapause can suppress Q40 aggregation. Surprisingly, activation of dauer did not suppress Q40 aggregation, but instead uncoupled aggregation from its associated muscle dysfunction. I found that [alpha]-crystallin chaperone HSP-12.6 can delay the muscle dysfunction when expressed in aging animals, without causing detrimental effects. Thus, physiological activation of an organisms' proteostasis pathways can identify intrinsic modulators of protein aggregation toxicity. Unexpectedly, HSP-12.6 does not coaggregate with Q40, but may maintain myofilament structure during cellular stress, suggesting a novel mechanism for a small heat-shock protein. Collectively, these findings advance our understanding of modifier pathways of protein aggregation and provide insight into potential targets that can be modulated without negatively affecting the organism.