In addition to studying how cells eliminate protein aggregates en bloc by selective autophagy, we are also interested in how they prevent protein aggregation in the first place even in the absence of external folding stress. Our focus is on regulatory mechanisms that balance protein synthesis in the cell with the cell’s capacity for folding newly-synthesized proteins and destroying proteins that are terminally misfolded. A critical component of these protein homeostasis (proteostasis) mechanisms are transcription factors capable of responding to unfolded proteins by driving expression of genes encoding molecular chaperones and other proteostasis effectors.
Defining a conserved transcriptional network for staving off proteostasis collapse
Heat shock factor 1 (Hsf1) is a conserved eukaryotic transcription factor involved in proteostasis. Despite its eponymous association with heat stress, Hsf1 in the budding yeast S. cerevisiae is essential even at physiological temperature. We have recently used a chemical genetics approach to acutely inactivate the essential function of yeast Hsf1, which allowed us to define a transcriptional program that maintains cell proliferation and viability by preventing cytosolic protein aggregation. Specifically, rapid drug-dependent nuclear export of yeast Hsf1 (in minutes) resulted in cell growth arrest after several hours of drug treatment, which was associated with massive protein aggregation and eventual cell death (see top half of cartoon below). A combination of genome-wide sequencing approaches allowed us to define 18 genes, all but one encoding chaperones, whose basal expression is directly controlled by Hsf1. Turning to the core function of mammalian Hsf1 in two different cell lines for comparison, we found that a distinct transcription factor drives basal chaperone gene expression but that heat stress enables Hsf1 to induce expression of nine genes in both cell types–eight out of which are chaperone genes homologous to those controlled by Hsf1 in yeast–above their basal levels. Lastly, we were able to mammalianize yeast basal chaperone gene expression by placing Hsf1 gene targets under the control of Hsf1-independent promoters–most critically, the genes encoding the cytosolic chaperones Hsp70 and Hsp90–to stave off the lethal proteostasis collapse in the absence of Hsf1 (see bottom half of cartoon below).
Age-induced proteostasis collapse
Young cells are free of protein aggregates because they are proficient at maintaining proteostasis. By contrast, accumulation of protein aggregates is a hallmark of many aging models and age-induced inhibition of Hsf1 homologs is believed to be one of the mechanisms responsible. We are using proteomics and cell microscopy approaches to systematically compare protein aggregation induced by replicative aging in yeast cells versus chemical genetic inhibition of Hsf1. These studies will provide a better mechanistic understanding of the causes and consequences of proteostasis collapse during replicative lifespan.