Regulation and Dynamic Behavior of the Heat Shock Transcription Factor Hsf-1 in C. Elegans

Eukaryotic cells respond to heat stress by activating the transcription factor HSF1. In addition to its role in stress response, HSF1 also functions in protein homeostasis, aging, innate immunity, and cancer. Despite prominent HSF1 involvement in processes pertinent to human health and disease, there are still gaps in our understanding of HSF1. For example, controversy exists regarding the localization of HSF1, the identity of HSF1 regulators, and the function and conservation of heat-induced HSF1 stress granules. Many of the physiological roles for HSF1 have been defined using the model organism Caenorhabditis elegans, yet little is known about how the molecular and biological properties of HSF-1 in C. elegans compare to HSF1 in other organisms, including humans. To address these questions, we generated animals expressing physiological levels of a GFP-tagged C. elegans HSF-1 protein. We studied the localization of HSF-1::GFP in vivo and observed its behavior upon heat shock in C. elegans. Furthermore, we conducted a genome-wide, RNAi-based screen for regulators of an HSF-1-dependent, heat shock-inducible transcriptional reporter. We found that in live C. elegans, HSF-1 localizes predominantly to the nucleus before and after heat shock. Following heat shock, HSF-1 redistributes into subnuclear puncta that share many characteristics with human nuclear stress granules, including rapid formation, reversibility, and colocalization with markers of active transcription. Granule formation in worms was affected by growth temperature, implying physiological regulation of this process. From our RNAi screen, we identified 44 regulators of HSF-1 target gene expression, the majority of which were positive regulators. One RNAi clone, encoding the worm homolog of the post-translational modifier SUMO, resulted in hyper-induction of the HSF-1 target after heat shock. Our findings from the screen suggest that basal repression of HSF-1 under low-temperature conditions may be very strict, and that sumoylation may be involved in downregulation of the activated heat stress response pathway. Our data also support a model of constitutively nuclear C. elegans HSF-1 and present evidence that HSF-1 nuclear stress granule formation may be an evolutionarily conserved phenomenon.