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Significance The endoplasmic reticulum (ER) deteriorates with age and fails to mount an effective stress response against misfolded proteins (UPRER), leading to protein folding disorders. However, preconditioning the ER using a mild ER stress (ER hormesis) can protect against future insults. We show that dietary restriction, an intervention that protects against protein misfolding disorders and increases life span across species, uses ER hormesis as a mechanism of action. Simply mimicking the ER hormesis in Caenorhabditis elegans by transient treatment with pharmacological reagents leads to delayed age-onset failure of UPRER, better capacity to process misfolded proteins, and increased life span. We also show that this process may be conserved in a mammalian cellular model of neurodegenerative disease., Unfolded protein response (UPR) of the endoplasmic reticulum (UPRER) helps maintain proteostasis in the cell. The ability to mount an effective UPRER to external stress (iUPRER) decreases with age and is linked to the pathophysiology of multiple age-related disorders. Here, we show that a transient pharmacological ER stress, imposed early in development on Caenorhabditis elegans, enhances proteostasis, prevents iUPRER decline with age, and increases adult life span. Importantly, dietary restriction (DR), that has a conserved positive effect on life span, employs this mechanism of ER hormesis for longevity assurance. We found that only the IRE-1–XBP-1 branch of UPRER is required for the longevity effects, resulting in increased ER-associated degradation (ERAD) gene expression and degradation of ER resident proteins during DR. Further, both ER hormesis and DR protect against polyglutamine aggregation in an IRE-1–dependent manner. We show that the DR-specific FOXA transcription factor PHA-4 transcriptionally regulates the genes required for ER homeostasis and is required for ER preconditioning-induced life span extension. Finally, we show that ER hormesis improves proteostasis and viability in a mammalian cellular model of neurodegenerative disease. Together, our study identifies a mechanism by which DR offers its benefits and opens the possibility of using ER-targeted pharmacological interventions to mimic the prolongevity effects of DR.

SummaryReductive stress leads to the loss of disulfide bond formation and induces the unfolded protein response of the endoplasmic reticulum (UPRER), necessary to regain proteostasis in the compartment. Here we show that peroxide accumulation during reductive stress attenuates UPRER amplitude by altering translation without any discernible effect on transcription. Through a comprehensive genetic screen in Saccharomyces cerevisiae, we identify modulators of reductive stress-induced UPRER and demonstrate that oxidative quality control (OQC) genes modulate this cellular response in the presence of chronic but not acute reductive stress. Using a combination of microarray and relative quantitative proteomics, we uncover a non-canonical translation attenuation mechanism that acts in a bipartite manner to selectively downregulate highly expressed proteins, decoupling the cell’s transcriptional and translational response during reductive ER stress. Finally, we demonstrate that PERK, a canonical translation attenuator in higher eukaryotes, helps in bypassing a ROS-dependent, non-canonical mode of translation attenuation.

Summary Insulin/IGF-1-like signalling (IIS) and dietary restriction (DR) are the two major modulatory pathways controlling longevity across species. Here, we show that both pathways license a common chromatin modifier, ZFP-1/AF10. The downstream transcription factors of the IIS and select DR pathways, DAF-16/FOXO or PHA-4/FOXA, respectively, both transcriptionally regulate the expression of zfp-1. ZFP-1, in turn, negatively regulates the expression of DAF-16/FOXO and PHA-4/FOXA target genes, apparently forming feed-forward loops that control the amplitude as well as the duration of gene expression. We show that ZFP-1 mediates this regulation by negatively influencing the recruitment of DAF-16/FOXO and PHA-4/FOXA to their target promoters. Consequently, zfp-1 is required for the enhanced longevity observed during DR and on knockdown of IIS. Our data reveal how two distinct sensor pathways control an overlapping set of genes, using different downstream transcription factors, integrating potentially diverse and temporally distinct nutritional situations.

The proteostasis network (PN) comprises a plethora of proteins that are dedicated to aid in protein folding and maintenance; some with overlapping functions. Despite this, there are multiple pathophysiological states associated with depletion of chaperones. This is counter-intuitive, assuming cells have the ability to re-program transcriptional outputs in accordance with its proteostasic limitations. Here, we have used S. cerevisiae to understand how cells respond to different types of proteostasis impairments. We monitored the proteostasis status and transcriptome of single deletions of fourteen different Protein Quality Control (PQC) genes. In most cases, cellular response did not activate proteostasis components or pathways that could either complement the function of the missing PQC gene or restore proteostasis. Over-expression of alternate machineries could restore part of the proteostasis defect in two representative PQC gene deletion strains. We posit that S. cerevisiae inherently lacks the ability to sense and respond optimally to defects in proteostasis caused due to deletion of specific PQC components.

Protein folding abnormalities are associated with the pathology of many diseases. This is surprising given the plethora of cellular machinery dedicated to aid protein folding. It is though that cellular response to proteotoxicity is generally sufficient, but may be compromised during pathological conditions. We asked if, in a physiological condition, cells have the ability to re-program transcriptional outputs in accordance with proteostasis demands. We have used S. cerevisiae to understand the response of cells when challenged with different proteostasis impairments, by removing one protein quality control (PQC) gene from the system at a time. Using 14 PQC deletions, we investigated the transcriptional response and find the mutants were unable to upregulate pathways that could complement the function of the missing PQC gene. To our surprise, cells have inherently a limited scope of response that is not optimally tuned; with transcriptomic responses being decorrelated with respect to the sign of their epistasis. We conclude that this non-optimality in proteotoxic response may limit the cellular ability to reroute proteins through alternate and productive machineries resulting in pathological states. We posit that epistasis guided synthetic biology approaches may be helpful in realizing the true potential of the cellular chaperone machinery.