Hansen M et al. (2016) Trends Cell Biol "Does Longer Lifespan Mean Longer Healthspan?"

Hansen M, Kennedy BK

[Trends Cell Biol, 2016]

Once thought to be impossible, it is now clear that changing the activity of several conserved genetic pathways can lead to lifespan extension in experimental organisms. In humans, however, the goal is to extend healthspan, the functional and disease-free period of life. Are the current pathways to lifespan extension also improving healthspan?

Hansen M et al. (2013) Cell Metab "Reproduction, fat metabolism, and life span: what is the connection?"

Flatt T, Hansen M, Aguilaniu H

[Cell Metab, 2013]

Reduced reproduction is associated with increased fat storage and prolonged life span in multiple organisms, but the underlying regulatory mechanisms remain poorly understood. Recent studies in several species provide evidence that reproduction, fat metabolism, and longevity are directly coupled. For instance, germline removal in the nematode Caenorhabditis elegans promotes longevity in part by modulating lipid metabolism through effects on fatty acid desaturation, lipolysis, and autophagy. Here, we review these recent studies and discuss the mechanisms by which reproduction modulates fat metabolism and life span. Elucidating the relationship between these processes could contribute to our understanding of age-related diseases including metabolic disorders.

Hansen M et al. (2007) Aging Cell "Lifespan extension by conditions that inhibit translation in Caenorhabditis elegans."

Crawford D, Hansen M, Kenyon C, Libina N, Lee SJ, Taubert S

[Aging Cell, 2007]

Many conditions that shift cells from states of nutrient utilization and growth to states of cell maintenance extend lifespan. We have carried out a systematic lifespan analysis of conditions that inhibit protein synthesis. We find that reducing the levels of ribosomal proteins, ribosomal-protein S6 kinase or translation-initiation factors increases the lifespan of Caenorhabditis elegans. These perturbations, as well as inhibition of the nutrient sensor target of rapamycin (TOR), which is known to increase lifespan, all increase thermal-stress resistance. Thus inhibiting translation may extend lifespan by shifting cells to physiological states that favor maintenance and repair. Interestingly, different types of translation inhibition lead to one of two mutually exclusive outputs, one that increases lifespan and stress resistance through the transcription factor DAF-16/FOXO, and one that increases lifespan and stress resistance independently of DAF-16. Our findings link TOR, but not sir-2.1, to the longevity response induced by dietary restriction (DR) in C. elegans, and they suggest that neither TOR inhibition nor DR extends lifespan simply by reducing protein synthesis.

Hansen M et al. (2008) PLoS Genet "A role for autophagy in the extension of lifespan by dietary restriction in C. elegans."

Kenyon C, Hansen M, Chandra A, Onken B, Mitic LL, Driscoll M

[PLoS Genet, 2008]

In many organisms, dietary restriction appears to extend lifespan, at least in part, by down-regulating the nutrient-sensor TOR (Target Of Rapamycin). TOR inhibition elicits autophagy, the large-scale recycling of cytoplasmic macromolecules and organelles. In this study, we asked whether autophagy might contribute to the lifespan extension induced by dietary restriction in C. elegans. We find that dietary restriction and TOR inhibition produce an autophagic phenotype and that inhibiting genes required for autophagy prevents dietary restriction and TOR inhibition from extending lifespan. The longevity response to dietary restriction in C. elegans requires the PHA-4 transcription factor. We find that the autophagic response to dietary restriction also requires PHA-4 activity, indicating that autophagy is a transcriptionally regulated response to food limitation. In spite of the rejuvenating effect that autophagy is predicted to have on cells, our findings suggest that autophagy is not sufficient to extend lifespan. Long-lived daf-2 insulin/IGF-1 receptor mutants require both autophagy and the transcription factor DAF-16/FOXO for their longevity, but we find that autophagy takes place in the absence of DAF-16. Perhaps autophagy is not sufficient for lifespan extension because although it provides raw material for new macromolecular synthesis, DAF-16/FOXO must program the cells to recycle this raw material into cell-protective longevity proteins.

Hansen, M. A. et al. (2017) International Worm Meeting "The role of ztf-16 in the maintenance of multipotency after dauer in C. elegans."

Bernstein, T. A., Karp, X., Daul, A. L., Lardie, B. K., Alessi, A. F., Montoye, E., Kim, J. K., Hansen, M. A., Matharoo, A., Rougvie, A. E.

[International Worm Meeting, 2017]

Most adult stem cells are multipotent and found in a state of reversible cell cycle arrest known as quiescence. However, the mechanisms that maintain multipotency during quiescence are relatively unknown. We use C. elegans as a model organism to study this process. C. elegans larvae develop through one of two different life histories: continuous or dauer. Dauer is a stress-resistant and developmentally arrested stage occurring after the second larval molt in response to adverse environmental conditions. If conditions improve, dauer larvae recover and complete development normally. C. elegans contains a set of stem cell-like seam cells that undergo specific division patterns at each larval stage, resulting in self-renewal and generation of hyp7 nuclei. During dauer, seam cells remain multipotent and quiescent, modeling mammalian stem cells. Following recovery from dauer, seam cells complete their development identically to larvae that did not experience dauer. Heterochronic genes ensure correct temporal control of seam cell division and differentiation. At adulthood, seam cells terminally differentiate, which can be visualized by the expression of the adult cell fate marker col-19::gfp. Intriguingly, many heterochronic genes that are required during continuous development are dispensable after dauer, suggesting that there is a separate genetic pathway operating during this time. To identify heterochronic genes that function after dauer, we performed a genetic screen for mutants with precocious post-dauer col-19::gfp expression. We screened 6000 genomes and isolated five independent alleles with high penetrance phenotypes. Using whole-genome sequencing, we found that two alleles contain mutations in ztf-16, which encodes a zinc finger transcription factor. We are currently confirming that ztf-16 is the causal mutation by rescue experiments and looking for phenocopy with RNAi and established ztf-16 mutations. Using established alleles plus our newly identified mutations, we find that ztf-16 is required to prevent precocious col-19::gfp expression during both continuous and dauer development. Furthermore, passage through dauer does not correct the ztf-16 precocious phenotype, making it unique among characterized precocious mutants. Ongoing genetic experiments are investigating the position of ztf-16 within the heterochronic pathway and determining its relevant expression pattern. Characterization of ztf-16 will shed light on mechanisms that maintain multipotency throughout quiescence.

Hansen, M. et al. (2019) International Worm Meeting "Determining the role of ztf-16 in regulating cell fate in stem cell-like cells after quiescence."

Kohtz, C., Brown, C., Kim, J., Vargus, M., Alessi, A., Hansen, M., Rougvie, A., Karp, X., Bernstein, T., Daul, A., Montoye, E.

[International Worm Meeting, 2019]

Multipotent adult stem cells typically exist in a state of reversible cell cycle arrest known as quiescence; however, the mechanisms maintaining multipotency during quiescence are relatively unknown. C. elegans contains stem cell-like seam cells that undergo stage-specific division patterns and eventual terminal differentiation. C. elegans larvae develop either continuously or through dauer-interrupted life history. Dauer occurs after the second larval molt in response to stress, and during dauer, seam cells remain multipotent and quiescent. If conditions improve, dauer larvae recover and their seam cells complete development normally. Heterochronic genes ensure correct temporal control of seam cell fate. Many heterochronic genes required during continuous development are dispensable after dauer, suggesting a separate developmental pathway. Through a genetic screen, we identified ztf-16, which encodes a transcription factor related to HBL-1. We find ztf-16 necessary to prevent precocious expression of the adult-specific marker col-19::gfp during both life histories. Based on the relationship between hbl-1 and ztf-16, we explored potential interactions between these genes in regulating L2 cell fate. During continuous development, but not post-dauer, the loss of ztf-16 enhances the cell fate defects of hbl-1 mutants. To further investigate L2 cell fates, we are now exploring the relationship between ztf-16 and the microRNAs that downregulate hbl-1 to allow progression to L3 cell fate. To determine the role of ztf-16 in adult cell fate, we examined genetic interactions between ztf-16 and lin-29. Unexpectedly, our data suggest that ztf-16 acts at least partially in parallel to lin-29 and uncouples the regulation of col-19::gfp from that of adult alae. We have also tagged the endogenous ztf-16 gene with both EGFP and mScarlet to characterize the expression of ZTF-16 throughout both life histories. By characterizing ztf-16, we hope to shed light on the mechanisms that control stem cell fate during and after quiescence.

Hansen M et al. (2005) PLoS Genet "New genes tied to endocrine, metabolic, and dietary regulation of lifespan from a Caenorhabditis elegans genomic RNAi screen."

Hansen M, Kenyon C, Dillin A, Hsu AL

[PLoS Genet, 2005]

Most of our knowledge about the regulation of aging comes from mutants originally isolated for other phenotypes. To ask whether our current view of aging has been affected by selection bias, and to deepen our understanding of known longevity pathways, we screened a genomic Caenorhabditis elegans RNAi library for clones that extend lifespan. We identified 23 new longevity genes affecting signal transduction, the stress response, gene expression, and metabolism and assigned these genes to specific longevity pathways. Our most important findings are (i) that dietary restriction extends C. elegans' lifespan by down-regulating expression of key genes, including a gene required for methylation of many macromolecules, (ii) that integrin signaling is likely to play a general, evolutionarily conserved role in lifespan regulation, and (iii) that specific lipophilic hormones may influence lifespan in a DAF-16/FOXO-dependent fashion. Surprisingly, of the new genes that have conserved sequence domains, only one could not be associated with a known longevity pathway. Thus, our current view of the genetics of aging has probably not been distorted substantially by selection bias.

M Hansen et al. (2004) European Worm Meeting "GENOME-WIDE IDENTIFICATION OF LONGEVITY GENES IN C. ELEGANS USING SYSTEMATIC RNAI"

A Hsu, M Hansen, A Fraser, A Dillin, R Kamath, C Kenyon, J Ahringer

[European Worm Meeting, 2004]

Lifespan in C. elegans is determined by multiple genetic pathways and processes. The DAF-2 insulin/IGF-1 pathway, the reproductive system, food intake, and mitochondrial activity (1, 2, 3) have been shown to regulate aging (1, 4, 5). The forkhead transcriptionfactor DAF-16 is required for the lifespan extension observed in daf-2 mutants, as well as in animals defective in germline signaling (1). However, daf-16 is not required for the lifespan extension induced by pertubations of mitochondrial activities (2, 3) or in calorically restricted animals (5, 6). How are these pathways related and what genes work in these processes to regulate aging? To clarify how the known aging pathways and processes differ in regulating longevity and to identify new genes involved in lifespan regulation, we carried out a genome-wide RNAi screen. Specifically, we looked for genes that, when inhibited by RNAi, lead to a long-lived phenotype. In addition to known lifespan genes, we identified 32 new genes whose wild-type function is required to prevent lifespan extension. Reducing the function of these loci extended mean lifespan anywhere between 15-80%. The lifespan extension observed with 10 of these clones was dependent on daf-16, suggesting that they might encode new components in the DAF-2 insulin/IGF-1 signaling pathway, or, potentially, daf-2-independent, daf-16-dependent genes. We are currently characterizing these genes further. In contrast, the lifespan extension observed with the remaining 22 genes, including components of the mitochondrial respiratory chain (see 2), was not dependent on daf-16. We are continuing the epistasis analysis of these genes to determine their involvement, if any, in other known aging pathways. In summary, our genome-wide RNAi screen has identified over 30 new genes involved in lifespan regulation in C. elegans. We expect the study of these genes to provide valuable information about the mechanism of aging, and possibly to define new aging pathways. (1) Tatar et al., Science, 2003 (2) Dillin et al., Science, 2002 (3) Lee et al., Nature Genetics, 2002 (4) Arantes-Oliveira et al., Science, 2003 (5) Houthoofd et al., Exp Geron, 2003 (6) Lakowski and Hekimi, PNAS, 1998

Hipkiss AR (2007) Mech Ageing Dev "On why decreasing protein synthesis can increase lifespan."

Hipkiss AR

[Mech Ageing Dev, 2007]

An explanation is offered for the increased lifespan of Caenorhabditis elegans when mRNA translation is inhibited due to loss of the initiation factor IFE-2 [Hansen, M., Taubert, T., Crawford, D., Libina, N., Lee, S.-J., Kenyon, C., 2007. Lifespan extension by conditions that inhibit translation in Caenorhabditis elegans. Ageing Cell 6, 95-110; Pan, K.Z., Palter, J.E., Rogers, A.N., Olsen, A., Chen, D., Lithgow, G.J., Kapahi, P., 2007. Inhibition of mRNA translation extends lifespan in Caenorhabditis elegans. Ageing Cell 6, 111-119; Syntichaki, P., Troulinaki, K., Tavernarakis, N., 2007. eIF4E function in somatic cells modulates ageing in Caenorhabditis elegans. Nature 445, 922-926]. It is suggested that the general reduction of protein synthesis, due to the decreased frequency of mRNA translation, also lowers the cellular load of erroneously synthesized polypeptides which the constitutive protein homeostatic apparatus (proteases and chaperones proteins) normally eliminates. This situation results in "spare" proteolytic and chaperone function which can then deal with those proteins modified post-synthetically, e.g. by oxidation and/or glycation, which are thought to contribute to the senescent phenotype. This increased availability of proteolytic and chaperone functions may thereby contribute to the observed increase in organism stress resistance and lifespan.

Gelino, S. et al. (2011) International Worm Meeting "Investigating the Dynamic Role of Autophagy in C. elegans Subjected to Dietary Restriction."

Gelino, S., Hansen, M.

[International Worm Meeting, 2011]

Multiple conserved pathways and processes can modulate lifespan, including dietary restriction (DR). The underlying mechanism for how DR promotes longevity is poorly understood, yet we and others have recently shown that autophagy is required for DR to extend lifespan in the nematode C. elegans. Autophagy is the major cellular pathway for degrading long-lived proteins and cytoplasmic organelles. To learn more about the role of autophagy in aging, we are investigating how autophagy may promote longevity at the cellular and molecular level during DR. Many long-lived mutants, including animals subjected to DR, share a common phenotype of increased stress resistance. Specifically, the heat stress resistance phenotype may contribute to extended longevity and be equally dependent on autophagy. To learn more about the molecular mechanism by which autophagy modulates lifespan, we asked whether autophagy is required for stress resistance in animals subjected to DR. We found that, as in longevity, autophagy is also required for the increase in heat stress resistance observed in animals subjected to DR. To further address the cellular basis of autophagy, we have utilized a tissue-specific RNA interference model to inactivate autophagy in specific tissues in animals subjected to DR. Specifically, we asked if autophagy-dependent lifespan modulation is specific to certain tissues in C. elegans. Knowledge of where in the animal that autophagy appears to have a rejuvenating effect will shed light on the underlying mechanism by which autophagy promotes longevity. In our preliminary studies, we have observed that the longevity function of autophagy is indeed restricted to specific tissues of the animal. Taken together, our studies suggest a broad physiological role for autophagy in C. elegans. Specifically, our findings indicate that certain tissues, in response to DR, specifically engage autophagy to increase stress resistance and promote lifespan in this multi-cellular organism. We propose that such tissues are more effective in removing damaged proteins and organelles that normally accumulate during the aging process, perhaps in response to a higher metabolic load in these specific tissues. The autophagic turn-over of such material, the nature of which is still to be identified, may prolong the youthfulness of a cell, tissue, and organism. This work was supported by the American Federation for Aging Research Foundation.