Heat shock proteins and Drosophila aging
Research Highlights
►Review of the expression and function of hsps in response to stress and aging. ►The role of the HSF pathway and the JNK/Foxo pathway. ►Antagonistic pleiotropy, sexual antagonistic pleiotropy, p53 and foxo. ►Protein turnover, proteotoxicity, and the role of hsps in aging. ►Hsps as predictive biomarkers of aging and life span.
Introduction
The laboratory fruit-fly, Drosophila melanogaster, has been a leading model for the study of aging for almost a century because of its ease of culture, short life cycle, and highly developed genetics and molecular biology (Partridge and Tower, 2008). Moreover, the heat shock response and the Hsp genes were first discovered in Drosophila, as revealed by the “puffing” pattern of polytene chromosomes in response to heat and oxidative stress (Ritossa, 1962, Ritossa, 1996). Since then the Drosophila model has been key to understanding the regulation of the heat shock response, the function of the Hsps, and the intimate connection between the Hsps and the aging process.
At a genetic level, aging in Drosophila and other organisms is thought to result from antagonistic pleiotropy of gene function between developmental stages and the sexes (Hughes and Reynolds, 2005, Magwire et al., 2010, Tower, 2006, Tower and Arbeitman, 2009). The force of natural selection declines with age, allowing for the accumulation of inherited alleles with late-acting deleterious effects (“mutation accumulation”). These same aging-promoting alleles may be maintained by positive selection if they provide a benefit earlier in life (“antagonistic plieotropy”). Because the sexes are subject to different and sometimes opposing selective pressures, inherited alleles may accumulate that are relatively beneficial to one sex but relatively deleterious to the other sex, or detrimental to both sexes (“sexual antagonistic plieotropy”). Antagonistic pleiotropy is one possible explanation for why pathways that promote growth and differentiation, such as the insulin/IGF1-like signaling (IIS) pathway, the target-of-rapamycin (TOR) pathway, and the sex-determination pathway (X/Y) are such important modulators of aging, as differentiation should be epistatic to the antagonist–pleiotropic activities of many genes (Fig. 1). For example, these growth and differentiation-promoting pathways may act by up-regulating expression of life-span shortening genes, and down-regulating expression of longevity-promoting genes (such as Hsps), as was observed for IIS in C. elegans (Ayyadevara et al., 2008, Curran et al., 2009, Kenyon, 2010, Murphy et al., 2003). One of the first genes suspected of exhibiting antagonistic pleiotropy between developmental stages was mammalian p53, based upon its beneficial function in young animals to prevent cancer, but a possible detrimental effect in old animals by promoting counter-productive cell senescence or apoptosis (Campisi, 2003). Recently Drosophila p53 has been shown to exhibit sexual antagonistic pleiotropy, as indicated by opposing effects of p53 mutations and transgenes on life span in males versus females (Shen and Tower, 2010, Waskar et al., 2009). Interestingly the sexual-dimorphism of p53 transgene effects on life span were found to be regulated by the IIS target transcription factor Foxo, consistent with a role for IIS and Foxo in promoting sexual antagonistic pleiotropy (Shen and Tower, 2010). The activity of p53 is regulated by Hsps, making p53 one likely component of the mechanism(s) by which Hsps can affect aging and life span.
Mechanistically, aging appears to involve an imbalance between damage and repair of macromolecules, including DNA damage and mutations, loss of epigenetic regulation, and telomere erosion (Vijg, 2008), as well as the response of the animal and its cells to such damage (Campisi and Vijg, 2009). Proteins are particularly subject to aging-related damages, including cleavage, covalent modifications, oxidative lesions, glycation, crosslinking, and denaturation (Semba et al., 2010, Stadtman, 2006). The correct synthesis, folding, and turnover of proteins is one of the most energetically costly functions of the cell, and proteotoxicity is a key component of aging and aging-related diseases (Morimoto and Cuervo, 2009). Mitochondrial malfunction is increasingly implicated in the aging process, and abnormal mitochondria have been found to accumulate with age in several Drosophila tissues (Calleja et al., 1993, Fleming et al., 1985, Schwarze et al., 1998, Sohal, 1975, Walker and Benzer, 2004). The resultant oxidative stress and reduction in ATP synthesis may play a central role in the creation and accumulation of abnormal proteins during aging (Bueler, 2009, Linford and Pletcher, 2009, Marzetti et al., 2009, Morrow and Tanguay, 2008).
The Hsps are defined by their ability to bind to denatured proteins, their ability to alter the folded structure of other proteins, and by the induction of their expression in response to stresses that cause protein denaturation, such as heat and oxidative stress (Morimoto, 2008, Morrow et al., 2006). By mediating either protein refolding or degradation, the Hsps counteract proteotoxicity and favor stress resistance. These functions of the Hsps may underlie their ability to favor life span and counteract aging-related malfunction in several systems (Lithgow and Walker, 2002, Morrow and Tanguay, 2003). In addition to their role in response to global protein damage, the Hsps also have more specific targets, and are involved in regulating pathways that are central to aging phenotypes such as protein turnover, cellular senescence, cancer, and apoptosis/programmed cell death (Morimoto and Cuervo, 2009, Tower, 2009), and the specific modulation of such pathways may also underlie the effects of Hsps on aging and life span. Finally, the expression of Hsps correlates with, and sometimes predicts, life span, making Hsps among the best-known biomarkers of aging in Drosophila and other animals (Johnson, 2006, Tower, 2009).
Section snippets
Hsp classes and functions
The heat shock proteins are molecular chaperones — a class of proteins characterized by their ability to modulate the structure and folding of other proteins (client proteins)(Kim et al., 2007). The expression of the Hsps is induced through the HSF (heat shock transcription factor) pathway, by stresses that cause protein denaturation (unfolding), such as heat stress, oxidative stress and other stresses (Fig. 2). Activated HSF binds to heat shock response elements (HSEs) in the promoters of the
Hsps and protein turnover
The Hsps can facilitate the refolding of denatured client proteins, and/or facilitate their entry into several degradation pathways, including the lysosome, autophagic vesicles, and the ubiquitin/proteosome pathway. By facilitating the clearance of damaged proteins, the Hsps are key components of the cells response to proteotoxicity, and can confer upon cells increased resistance to heat and other stresses.
Chaperone-mediated autophagy is a process where specific cytosolic proteins are targeted
Hsps as modulators of stress resistance and proteotoxicity
Hsps have been demonstrated to be key players in conferring resistance to heat and other stresses in Drosophila (Morrow and Tanguay, 2003, Vermeulen and Loeschcke, 2007). For example, the mutation of hsp22 decreases survival upon heat stress (Morrow et al., 2004a), as does mutation of all six copies of the hsp70 gene (Gong and Golic, 2006). Mutation of hsp83 sensitizes flies to the toxic effects of sleep deprivation (Shaw et al., 2002). Over-expression of certain Hsps can increase resistance to
Regulation of Hsp expression in response to proteotoxic stress and aging
Denaturation of proteins and the resultant exposure of hydrophobic residues is the main mechanism for the induction of Hsp genes by the transcription factor HSF (Fig. 2a). HSF is consitutively expressed, and normally resides in the cytoplasm in a complex with Hsps that hold HSF in an inactive state (Morimoto, 2002). When heat, ROS or other stress causes protein denaturation, this exposes hydrophobic residues that bind Hsps and titrate them away from HSF. This allows HSF to assume an active
Regulation of Hsp expression in response to ROS and aging, a role for JNK and Foxo
A subset of Hsp genes are regulated by the transcription factor Foxo in both Drosophila and C. elegans. Foxo is particularly relevant to aging because in C. elegans the Foxo homolog Daf16, along with HSF, has been shown to be required for the life span extension caused by reductions in IIS (Kenyon, 2010). Notably, RNAi inhibition of the expression of sHsp genes has been shown to block part of this life span extension, suggesting that induction of sHsps by Foxo and HSF is a part of the mechanism
Hsps as modulators of life span
Laboratory selection of Drosophila strains for increased life span produced flies with increased expression of sHsp genes as young adults, suggesting that increased sHsp expression might favor longevity (Kurapati et al., 2000). Consistent with this idea, mutation of hsp70 or hsp22 can reduce adult fly survival, and chemicals that increased life span, such as HDAC inhibitors, produced correlated increases in hsp70 and sHsp gene expression (Kang et al., 2002, Zhao et al., 2005). Direct testing of
Hsps are predictive biomarkers of life span
Drosophila undergo a number of functional declines that correlate with increased age and increased mortality rate, including decreased spontaneous movement, decreased climbing speed, decreased memory, decreased heart function, and decreased reproductive capacity (Grotewiel et al., 2005, Iliadi and Boulianne, 2010, Piazza et al., 2009, Reenan and Rogina, 2008). Correlated molecular changes include decreases in protein turnover system activities (ubiquitin/proteosome and autophagy/lysosomal),
Summary
Altered expression of Hsps was among the first molecular changes characterized during aging in Drosophila. Since then the Hsps have been shown to directly regulate aging phenotypes, including life span, stress resistance and function. The tractability of the Drosophila system promises to allow continued discovery of the role of Hsps in aging, and facilitate our understanding of human aging and disease.
Acknowledgements
The author was supported by a grant from the Department of Health and Human Services (AG011833) and by Scientific Opportunity Funds from the Genetics of Longevity Consortium (U19 AG032122).
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