Heat shock factor and molecular chaperones are well-known mediators of stress resistance. Thus, the finding that HSF-1 and molecular chaperones influence life span provides a molecular link between genes that regulate longevity and those that protect cells from stress. A large body of evidence accumulated in cellular and animal models of senescence has consistently indicated that heat shock gene expression is induced poorly in aged animals (Fargnoli et al., 1990
; Fawcett et al., 1994
). In instances where it has been tested, lack of heat shock gene induction corresponds with deficient oligomerization and DNA binding of HSF-1 without changes in the absolute levels of the protein (Fawcett et al., 1994
). However, it has been difficult to distinguish whether diminished activation of the heat shock response simply represents one of many cellular processes that decline during aging or whether HSF-1 itself participates in the regulation of cellular senescence and aging. Our present results support a direct role for HSF-1 as a regulator of longevity.
One potential concern is that the influence of HSF-1 inhibition on life span is a nonspecific effect resulting from loss of essential cellular functions. However, multiple lines of evidence support a central role for hsf-1
in the genetic regulation of aging. HSF-1 gain and loss of function have reciprocal effects on life span. Whereas loss of many essential genes might shorten life span, the set of factors that can extend life span is likely to be more specific. Suppression of dauer formation in age-1
mutants indicates that HSF-1 influences not only longevity but also a specific developmental checkpoint regulated by ILS. The failure of hsf-1(RNAi
) to further shorten life span of daf-16
null animals is also consistent with the suggestion that these factors function, at least in part, through a similar mechanism. In support of this, a recent report found similarly that HSF-1 participates in the regulation of life span through cooperation with ILS (Hsu et al., 2003
). Importantly, these workers found that hsf-1(RNAi
) suppressed longevity of age-1
animals but not isp-1
mutants that function through a different mechanism (Hsu et al., 2003
). Together, all of these observations strongly support the idea that HSF-1 is a bona fide regulator of aging through a mechanism similar to ILS.
The influence of ILS on life span is thought to be mediated through a neuroendocrine signaling pathway. This hypothesis is based on two major findings. First, genetic mosaic experiments have demonstrated daf-2
can function cell nonautonomously (Apfeld and Kenyon, 1998
). Second, the longevity of age-1
mutants could be reversed completely by expression of wild-type AGE-1 only in neurons, whereas expression of the rescuing constructs in muscle or intestinal cells had only minor effects on life span (Wolkow et al., 2000
). In contrast, expression of wild-type or HSF-1 DN had nearly identical effects whether they were expressed in neuronal or muscle cells (and to a lesser extent in intestinal cells). These observations suggest that HSF-1 exerts beneficial effects in multiple tissues throughout the organism to influence life span. Thus, these findings raise the intriguing possibility that regulatory signals from ILS in the nervous system could be conveyed to HSF-1 and molecular chaperones to mediate longevity in other tissues of the body, although this idea remains to be tested directly.
In contrast to the striking effects of hsf-1(RNAi
), suppression of age-1(hx546
) longevity by inactivation of individual chaperones was relatively small. Given the redundancy of chaperone function (Werner-Washburne et al., 1987
), it is somewhat surprising to observe any effects resulting from down-regulation of a single chaperone. However, the incremental effects of chaperones on age-1(hx546
) longevity and the essential role of HSF-1 in development may explain why these factors have not been identified in screens for genetic suppressors of daf-2/age-1
mutant phenotypes. Additionally, when RNAi for small heat shock proteins was initiated in embryos (this was possible because none of the genes they examined caused developmental phenotypes), suppression of age-1(hx546
) longevity was greater (~25%) than we observed by initiating RNAi in adults (Hsu et al., 2003
). However, in both of our studies, suppression resulting from down-regulation of any single chaperone was much less than that observed for hsf-1(RNAi
). An incremental role for many different molecular chaperones is also consistent with observations where overexpression of a single chaperone has only small effects on life span (Tower, 2000
; Yokoyama et al., 2002
; Walker and Lithgow, 2003
). Additionally, a recent study found that RNAi-mediated inhibition of individual DAF-16 targets identified by microarray analysis had much smaller effects in life span than inhibition of DAF-16 itself (Murphy et al., 2003
). Thus, the incremental role of each individual chaperone may reflect a general principle in genetic networks influencing longevity where upstream regulators, such as DAF-16 and HSF-1, have large effects on life span by orchestrating the activities of numerous effector molecules that each exert a small or specific form of protective influence.
In addition to its role in longevity, we provide evidence in this study that HSF-1 is required for dauer formation of age-1
mutants. This finding is intriguing because it implicates HSF-1 in a developmental switch that regulates the body plan of the organism. A remaining question of considerable interest is whether HSF-1 might be required for dauer formation mediated through pathways other than ILS, such as, transforming growth factor-β or cGMP signaling. At least part of the answer is provided by a recent report demonstrating that HSF-1 is required for both ILS and TGF-β-mediated dauer formation—although dauer formation of ILS mutants were considerably more sensitive to the inhibitory effects of hsf-1(RNAi
) (Walker et al., 2003
). Thus, it seems possible that HSF-1 may serve as a common modulator of dauer formation signaled by diverse regulatory pathways. Although the requirement for HSF-1 in dauer formation is unexpected, it is consistent with phenotypes of HSF-deficient Drosophila
suggesting HSF-1 regulates genes important for normal developmental progression and fertility (Jedlicka et al., 1997
). Additionally, many of the environmental conditions that induce dauer formation, including elevated temperature, high osmolarity, and starvation are themselves stressful. Whether HSF-1 acts by sensing these homeostatic challenges and activating genes necessary to facilitate and integrate stress induced dauer formation mediated by multiple pathways remains to be determined.
We have previously demonstrated that the reduced rate of aging in age-1
mutant animals is associated with delayed aggregation and toxicity of polyglutamine expansions that underlie Huntington's Disease and at least eight other mature-onset neurodegenerative disorders (Morley et al., 2002
). Here, we demonstrate that HSF-1 and molecular chaperones, factors essential for the maintenance of protein folding homeostasis, are themselves regulators of longevity. Additionally, inactivation of daf-
, or small heat shock proteins accelerated the aggregation of polyglutamine expansion proteins (Hsu et al., 2003
), supporting the idea that ILS could coordinately influence aging and protein aggregation through the action of DAF-16, HSF-1, and molecular chaperones. Together, these studies have revealed a common set of factors that link the genetic regulation of protein homeostasis, stress responsiveness, and longevity. This suggests that longevity and fitness may be, at least in part, a consequence of the efficient detection, capture, and resolution of misfolded and aggregation-prone proteins. Thus, these studies provide a new perspective on the way in which decreased ability to sense and respond to stress in old animals may contribute to declining cellular function and, in particular, increased susceptibility to diseases of protein misfolding observed during aging.