Functional genomic approaches to understanding molecular chaperones and stress responses

Heat shock transcription factor 1 is activated as a consequence of lymphocyte activation and regulates a major proteostasis network in T cells critical for cell division during stress

Heat shock transcription factor 1 (HSF1) is a major transcriptional regulator of the heat shock response in eukaryotic cells. HSF1 is evoked in response to a variety of cellular stressors, including elevated temperatures, oxidative stress, and other proteotoxic stressors. Previously, we demonstrated that HSF1 is activated in naive T cells at fever range temperatures (39.5°C) and is critical for in vitro T cell proliferation at fever temperatures. In this study, we demonstrated that murine HSF1 became activated to the DNA-binding form and transactivated a large number of genes in lymphoid cells strictly as a consequence of receptor activation in the absence of apparent cellular stress. Microarray analysis comparing HSF1(+/+) and HSF1(-/-) gene expression in T cells activated at 37°C revealed a diverse set of 323 genes significantly regulated by HSF1 in nonstressed T cells. In vivo proliferation studies revealed a significant impairment of HSF1(-/-) T cell expansion under conditions mimicking a robust immune response (staphylococcal enterotoxin B-induced T cell activation). This proliferation defect due to loss of HSF1 is observed even under nonfebrile temperatures. HSF1(-/-) T cells activated at fever temperatures show a dramatic reduction in cyclin E and cyclin A proteins during the cell cycle, although the transcription of these genes was modestly affected. Finally, B cell and hematopoietic stem cell proliferation from HSF1(-/-) mice, but not HSF1(+/+) mice, were also attenuated under stressful conditions, indicating that HSF1 is critical for the cell cycle progression of lymphoid cells activated under stressful conditions.

Modulation of chaperone gene expression in mutagenized Saccharomyces cerevisiae strains developed for recombinant human albumin production results in increased production of multiple heterologous proteins

The yeast Saccharomyces cerevisiae has been successfully established as a commercially viable system for the production of recombinant proteins. Manipulation of chaperone gene expression has been utilized extensively to increase recombinant protein production from S. cerevisiae, focusing predominantly on the products of the protein disulfide isomerase gene PDI1 and the hsp70 gene KAR2. Here we show that the expression of the genes SIL1, LHS1, JEM1, and SCJ1, all of which are involved in regulating the ATPase cycle of Kar2p, is increased in a proprietary yeast strain, developed by several rounds of random mutagenesis and screening for increased production of recombinant human albumin (rHA). To establish whether this expression contributes to the enhanced-production phenotype, these genes were overexpressed both individually and in combination. The resultant strains showed significantly increased shake-flask production levels of rHA, granulocyte-macrophage colony-stimulating factor, and recombinant human transferrin.

Human endoplasmic reticulum mannosidase I is subject to regulated proteolysis

In the early secretory pathway, opportunistic cleavage of asparagine-linked oligosaccharides by endoplasmic reticulum (ER) mannosidase I targets misfolded glycoproteins for dislocation into the cytosol and destruction by 26 S proteasomes. The low basal concentration of the glycosidase is believed to coordinate the glycan cleavage with prolonged conformation-based ER retention, ensuring that terminally misfolded glycoproteins are preferentially targeted for destruction. Herein the intracellular fate of human ER mannosidase I was monitored to determine whether a post-translational process might contribute to the regulation of its intracellular concentration. The transiently expressed recombinant human glycosidase was subject to rapid intracellular turnover in mouse hepatoma cells, as was the endogenous mouse ortholog. Incubation with either chloroquine or leupeptin, but not lactacystin, led to intracellular stabilization, implicating the involvement of lysosomal acid hydrolases. Inhibition of protein synthesis with cycloheximide led to intracellular depletion of the glycosidase and concomitant ablation of asparagine-linked glycoprotein degradation, confirming the physiologic relevance of the destabilization process. Metabolic incorporation of radiolabeled phosphate, detection by anti-phosphoserine antiserum, and the stabilizing effect of general serine kinase inhibition implied that ER mannosidase I is subjected to regulated proteolysis. Stabilization in response to genetically engineered removal of the amino-terminal cytoplasmic tail, a postulated regulatory domain, and colocalization of green fluorescent protein fusion proteins with Lamp1 provided two additional lines of evidence to support the hypothesis. A model is proposed in which proteolytically driven checkpoint control of ER mannosidase I contributes to the establishment of an equitable glycoprotein quality control standard by which the efficiency of asparagine-linked glycoprotein conformational maturation is measured.

Genome-scale approaches for discovering novel nonconventional splicing substrates of the Ire1 nuclease

Three different genome-scale screens indicate that the HAC1 mRNA is the only substrate for the Ire1 nuclease in yeast.

Background

The unfolded protein response (UPR) allows intracellular feedback regulation that adjusts the protein-folding capacity of the endoplasmic reticulum (ER) according to need. The signal from the ER lumen is transmitted by the ER-transmembrane kinase Ire1, which upon activation displays a site-specific endoribonuclease activity. Endonucleolytic cleavage of the intron from the HAC1 mRNA (encoding a UPR-specific transcription factor) is the first step in a nonconventional mRNA splicing pathway; the released exons are then joined by tRNA ligase. Because only the spliced mRNA is translated, splicing is the key regulatory step of the UPR.

Results

We developed methods to search for additional mRNA substrates of Ire1p in three independent lines of genome-wide analysis. These methods exploited the well characterized enzymology and genetics of the UPR and the yeast genome sequence in conjunction with microarray-based detection. Each method successfully identified HAC1 mRNA as a substrate according to three criteria: HAC1 mRNA is selectively cleaved in vitro by Ire1; the HAC1 mRNA sequence contains two predicted Ire1 cleavage sites; and HAC1 mRNA is selectively degraded in tRNA ligase mutant cells.

Conclusion

Within the limits of detection, no other mRNA satisfies any of these criteria, suggesting that a unique nonconventional mRNA-processing mechanism has evolved solely for carrying out signal transduction between the ER and the nucleus. The approach described here, which combines biochemical and genetic 'fractionation' of mRNA with a novel application of cDNA microarrays, is generally applicable to the study of pathways in which RNA metabolism and alternative splicing have a regulatory role.

Lowered temperature set point for activation of the cellular stress response in T-lymphocytes

The induction of heat shock protein gene expression in response to stress is critical for the ability of organisms to cope with and survive exposure to these stresses. However, most studies on HSF1-mediated induction of hsp70 gene expression have utilized immortalized cell lines and temperatures above the physiologically relevant range. For these reasons much less is known about the heat shock response as it occurs in mammalian cells within tissues in the intact organism. To gain insight into this area we determined the temperature thresholds for activation of HSF1 DNA binding in different mouse tissues. We have found that HSF1 DNA binding activity and hsp70 synthesis are induced in spleen cells at significantly lower temperatures relative to cells of other tissues, with a temperature threshold for activation (39 degrees C) that is within the physiological range for fever. Furthermore, we found that the lowered temperature set point for induction of the stress response in spleen is specific to T-lymphocytes residing within this tissue and is not exhibited by B-lymphocytes. This lowered threshold is also observed in T-lymphocytes isolated from lymph nodes, suggesting that it is a general property of T-lymphocytes, and is seen in different mouse strains. Fever is an early event in the immune response to infection, and thus activation of the cellular stress response in T-lymphocytes by fever temperatures could serve as a way to give these cells enough time to express hsps in anticipation of their function in the coming immune response. The induced hsps likely protect these cells from the stressful conditions that can exist during the immune response, for example increasing their protection against stress-induced apoptosis.