Date Published: October , 2011
Publisher: Blackwell Publishing Ltd
Author(s): Cathy Slack, Maria E Giannakou, Andrea Foley, Martin Goss, Linda Partridge.
The insulin/insulin-like growth factor-like signaling (IIS) pathway in metazoans has evolutionarily conserved roles in growth control, metabolic homeostasis, stress responses, reproduction, and lifespan. Genetic manipulations that reduce IIS in the nematode worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse have been shown not only to produce substantial increases in lifespan but also to ameliorate several age-related diseases. In C. elegans, the multitude of phenotypes produced by the reduction in IIS are all suppressed in the absence of the worm FOXO transcription factor, DAF-16, suggesting that they are all under common regulation. It is not yet clear in other animal models whether the activity of FOXOs mediate all of the physiological effects of reduced IIS, especially increased lifespan. We have addressed this issue by examining the effects of reduced IIS in the absence of dFOXO in Drosophila, using a newly generated null allele of dfoxo. We found that the removal of dFOXO almost completely blocks IIS-dependent lifespan extension. However, unlike in C. elegans, removal of dFOXO does not suppress the body size, fecundity, or oxidative stress resistance phenotypes of IIS-compromised flies. In contrast, IIS-dependent xenobiotic resistance is fully dependent on dFOXO activity. Our results therefore suggest that there is evolutionary divergence in the downstream mechanisms that mediate the effects of IIS. They also imply that in Drosophila, additional factors act alongside dFOXO to produce IIS-dependent responses in body size, fecundity, and oxidative stress resistance and that these phenotypes are not causal in IIS-mediated extension of lifespan.
The insulin/insulin-like growth factor (IGF)-like signaling (IIS) pathway of metazoans regulates such diverse processes as growth, developmental timing, body size, metabolism, stress responses, reproduction, and lifespan (Kenyon, 2005; Giannakou & Partridge, 2007). Genetic manipulations that inhibit IIS in the nematode worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse not only increase lifespan but also delay the onset of age-related pathology and disease (Tatar et al., 2003; Kenyon, 2005; Bonkowski et al., 2006; Cohen et al., 2006; Wessells & Bodmer, 2007; Selman et al., 2008; Wessells et al., 2009). Direct downstream targets of IIS in worms, flies, and mammals are the FOXO (Forkhead bOX-containing protein, subfamily O) proteins, a highly conserved family of transcription factors. Phosphorylation of FOXOs by the insulin-activated protein kinases PKB/AKT and SGK leads to their sequestration within the cytoplasm and, as a result, transcriptional inactivation of target gene expression (van der Horst & Burgering, 2007; Partridge & Bruning, 2008). Several direct FOXO target genes have been identified that function during cell cycle control, metabolism, apoptosis, and the regulation of cellular stress responses (Greer & Brunet, 2005, 2008; Partridge & Bruning, 2008; Salih & Brunet, 2008). Hence, the activation of FOXOs and their target genes has been under intense study to identify the transcriptional changes associated with IIS-dependent lifespan extension.
As a result of the pleiotropic effects of IIS on animal physiology, extension of lifespan by reduced IIS is often accompanied by other phenotypic responses, including reduced or delayed reproduction, growth inhibition, increased stress resistance, and metabolic dysregulation. In C. elegans, all of the phenotypic outcomes of reduced IIS are under a common regulatory mechanism, because they are all dependent on the transcriptional activity of the FOXO transcription factor, DAF-16 (Kenyon et al., 1993; Dillin et al., 2002). In Drosophila and mammals, many of the same physiological traits are affected by reduced IIS, but a requirement for FOXO transcriptional activity in mediating all of the phenotypic responses to reduced IIS, especially lifespan extension, in these other animal models is less well understood. In this study, we have combined a novel deletion mutant of dfoxo that is devoid of dfoxo mRNA expression with several models of reduced IIS in Drosophila to investigate the consequences of dfoxo removal on lifespan, fecundity, development, growth, and stress resistance.