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.

Reductive stress leads to the loss of disulfide bond formation and induces the unfolded protein response of the endoplasmic reticulum (UPR(ER)), necessary to regain proteostasis in the compartment. Here we show that peroxide accumulation during reductive stress attenuates UPR(ER) 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 UPR(ER) 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.

SummaryInsulin/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.

Several microRNAs have emerged as regulators of pathways that control aging. For example, miR‐228 is required for normal lifespan and dietary restriction (DR) mediated longevity through interaction with PHA‐4 and SKN‐1 transcription factors in Caenorhabditis elegans. miR‐229,64,65, and 66, a cluster of microRNAs located adjacent to each other on chromosome III, are in the same family as miR‐228, albeit with slight differences in the miR‐228 seed sequence. We demonstrate that, in contrast to the anti‐longevity role of miR‐228, the miR‐229‐66 cluster is required for normal C. elegans lifespan and for the longevity observed in mir‐228 mutants. miR‐229‐66 is also critical for lifespan extension observed under DR and reduced insulin signaling (IIS) and by constitutive nuclear SKN‐1. Both DR and low‐IIS upregulate the expression of the miRNA cluster, which is dependent on transcription factors PHA‐4, SKN‐1, and DAF‐16. In turn, the expression of SKN‐1 and DAF‐16 requires mir‐229,64,65,66. miR‐229‐66 targets the odd‐skipped‐related transcription factor, odd‐2 to regulate lifespan. Knockdown of odd‐2 increases lifespan, suppresses the short lifespan of mir‐229,64,65,66(nDf63) III mutants, and alters levels of SKN‐1 in the ASI neurons. Together with SKN‐1, the miRNA cluster also indirectly regulates several genes in the xenobiotic detoxification pathway which increases wild‐type lifespan and significantly rescues the short lifespan of mir‐229,64,65,66(nDf63) III mutants. Thus, by interacting with SKN‐1, miR‐229‐66 transduces the effects of DR and low‐IIS in lifespan extension in C. elegans. Given that this pathway is conserved, it is possible that a similar mechanism regulates aging in more complex organisms.

Aging is associated with the accumulation of damaged and misfolded proteins through a decline in the protein homeostasis (proteostasis) machinery, leading to various age-associated protein misfolding diseases such as Huntington’s or Parkinson’s. The efficiency of cellular stress response pathways also weakens with age, further contributing to the failure to maintain proteostasis. MicroRNAs (miRNAs or miRs) are a class of small, non-coding RNAs (ncRNAs) that bind target messenger RNAs at their 3′UTR, resulting in the post-transcriptional repression of gene expression. From the discovery of aging roles for lin-4 in C. elegans, the role of numerous miRNAs in controlling the aging process has been uncovered in different organisms. Recent studies have also shown that miRNAs regulate different components of proteostasis machinery as well as cellular response pathways to proteotoxic stress, some of which are very important during aging or in age-related pathologies. Here, we present a review of these findings, highlighting the role of individual miRNAs in age-associated protein folding and degradation across different organisms. We also broadly summarize the relationships between miRNAs and organelle-specific stress response pathways during aging and in various age-associated diseases.

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 thought 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.

The proteostasis network (PN) comprises a plethora of proteins that are dedicated to aid in protein folding; some with over-lapping 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. To this effect, we have used S. cerevisiae to understand the route a cell takes as a response when challenged with different proteostasis impairments. Using 14 single deletion strains of genes of Protein Quality Control (PQC) system, we quantify their proteostasis impairment and the transcriptional response. In most cases cellular response was incapable of restoring proteostasis. The response did not activate proteostasis components or pathways that could complement the function of the missing PQC gene. Over-expression of alternate machineries, could restore part of the proteostasis defect in deletion strains. We posit that epistasis guided synthetic biology approaches may be helpful in realizing the true potential of the cellular chaperone machinery.