(l to r) David Pincus, Eric Solís, Vlad Denic
No matter where you fall on the spectrum of libertarianism this election year, you can hopefully appreciate that the quality of your everyday life depends on various government bodies. Just take away the Cambridge fire department and watch the fumbled crepe flambé in the Engert lab kitchen turn into MCB website news (“Chair Alex Schier Feels the Bern”). Much like our local science community, the cell is filled with protein inhabitants that are on the verge of setting their neighbors on fire. This undesirable behavior comes from the cellular process that takes newly born polypeptides lacking shape and function and shapes them into hard-working members of their community. However, until they are folded properly, unfolded polypeptides have a tendency to glom on to one another and lead to the formation of extremely stable protein aggregates. If you have ever fried an egg, you have already witnessed the awesome power of protein aggregation. Yet cells can resist becoming fried at hot temperatures by using proteins called chaperones that block illicit interactions between unfolded polypeptides. A key insight from previous work was that a transcription factor called Heat shock factor 1 (Hsf1) is required for increased chaperone production at elevated temperatures. This is a sensible adaptation for coping with stress because heat makes proteins unfold and this in turn necessitates more chaperones to counter the rise in protein aggregation. However, many other transcription factors besides Hsf1 are activated by heat and this made it historically challenging to tease apart the role of Hsf1 in conferring thermotolerance.
In our paper, we began by asking why are cells unable to live without Hsf1 even at physiological temperature. Our approach was to engineer cells so that by adding a specific drug we were able to force Hsf1 rapidly out of its nuclear residence and into the cytoplasm. Cells didn’t seem to be initially affected by this drug treatment but after a couple of hours they started to accumulate protein aggregates in their cytoplasm and soon after that, they stopped dividing and eventually lysed. To go back to the original analogy, we took away Hsf1’s function, and the egg fried itself at room temperature. Using a variety of genomic tools, we measured the immediate gene expression changes caused by Hsf1 nuclear export and defined a decrease in the expression of just eighteen genes, which with one exception encoded chaperones, including Hsp70 and Hsp90. Remarkably, when we engineered cells to transgenically co-express just Hsp70 and Hps90 under the control of a heterologous promoter, we could genetically ablate Hsf1 without compromising cell growth and viability. In follow up work, we found that heat stress alters the expression of hundreds of genes via transcription factors distinct from Hsf1. Nonetheless, elevated Hsf1 activity protects cells from heat stress by up-regulating expression of a small number of chaperone genes. Lastly, we carried out a comparative study of non-transformed mammalian cells that express Hsf1 as a non-essential gene. We found that high basal chaperone expression was unaffected by genetic ablation of Hsf1 arguing that a distinct transcription factor is responsible. By contrast, heat-induced up-regulation of a handful of chaperones, including mammalian Hsp70 and Hsp90, was still Hsf1-dependent. This difference in the logic of chaperone gene control (basal and heat-induced in yeast vs. heat-induced only in mammals) warrants further investigation but could be an evolutionary adaptation to cell non-autonomous thermoregulation in metazoans.
Protein aggregation is something that is rarely seen in young human individuals but is prevalent in the elderly. In particular, many devastating neurodegenerative disorders, such as Alzheimer’s disease, are associated with the presence of protein aggregates in the affected neurons. Our future goal is to study the effects of aging on Hsf1 function and use these insights to develop new tools for reversing protein aggregation in old cells. To end by going back to the beginning, we hope that this work will help restore the fried egg back to its original state.
Read more in Molecular Cell, PDF