Phenotypic covariance of longevity, immunity and stress resistance in the Caenorhabditis nematodes

Background \ud Ageing, immunity and stresstolerance are inherent characteristics of all organisms. In animals, these traits are regulated, at least in part, by forkhead transcription factors in response to upstream signals from the Insulin/Insulin– like growth factor signalling (IIS) pathway. In the nematode Caenorhabditis elegans, these phenotypes are molecularly linked such that activation of the forkhead transcription factor DAF-16 both extends lifespan and simultaneously increases immunity and stress resistance. It is known that lifespan varies significantly among the Caenorhabditis species but, although DAF-16 signalling is highly conserved, it is unclear whether this phenotypic linkage occurs in other species. Here we investigate this phenotypic covariance by comparing longevity, stress resistance and immunity in four \ud Caenorhabditis species. \ud \ud Methodology/Principal Findings \ud We show using phenotypic analysis of DAF-16 influenced phenotypes that among four closely related Caenorhabditis nematodes, the gonochoristic species (Caenorhabditis remanei and Caenorhabditis brenneri) have diverged \ud significantly with a longer lifespan, improved stress resistance and higher immunity than the hermaphroditic species (C. elegans and Caenorhabditis briggsae). Interestingly, we also observe significant differences in expression levels between the daf-16 homologues in these species using Real-Time PCR, which positively correlate with the observed phenotypes. Finally, we provide additional evidence in support of a role for DAF-16 in regulating phenotypic coupling by using a combination of wildtype isolates, constitutively active daf-16 mutants and bioinformatic analysis. \ud \ud Conclusions \ud The gonochoristic species display a significantly longer lifespan (p < 0.0001)and more robust immune and stress response (p<0.0001, thermal stress; p<0.01, heavy metal stress; p<0.0001, pathogenic stress) than the hermaphroditic species. Our data suggests that divergence in DAF-16 mediated phenotypes may underlie many of the differences observed between these four species of Caenorhabditis nematodes. These findings are further supported by the correlative higher daf-16 expression levels among the gonochoristic species and significantly higher lifespan, immunity and stress tolerance in the constitutively active daf-16 hermaphroditic mutants

Phenotypic covariance of Longevity, Immunity and Stress Resistance in the Caenorhabditis Nematodes

Background: Ageing, immunity and stresstolerance are inherent characteristics of all organisms. In animals, these traits are regulated, at least in part, by forkhead transcription factors in response to upstream signals from the Insulin/Insulin–like growth factor signalling (IIS) pathway. In the nematode Caenorhabditis elegans, these phenotypes are molecularly linked such that activation of the forkhead transcription factor DAF-16 both extends lifespan and simultaneously increases immunity and stress resistance. It is known that lifespan varies significantly among the Caenorhabditis species but, although DAF-16 signalling is highly conserved, it is unclear whether this phenotypic linkage occurs in other species. Here we investigate this phenotypic covariance by comparing longevity, stress resistance and immunity in four Caenorhabditis species. \ud \ud Methodology/Principal Findings: We show using phenotypic analysis of DAF-16 influenced phenotypes that among four closely related Caenorhabditis nematodes, the gonochoristic species (Caenorhabditis remanei and Caenorhabditis brenneri) have diverged significantly with a longer lifespan, improved stress resistance and higher immunity than the hermaphroditic species (C. elegans and Caenorhabditis briggsae). Interestingly, we also observe significant differences in expression levels between the daf-16 homologues in these species using Real-Time PCR, which positively correlate with the observed phenotypes. Finally, we provide additional evidence in support of a role for DAF-16 in regulating phenotypic coupling by using a combination of wildtype isolates, constitutively active daf-16 mutants and bioinformatic analysis. \ud \ud Conclusions: The gonochoristic species display a significantly longer lifespan (p<0.0001) and more robust immune and stress response (p<0.0001, thermal stress; p<0.01, heavy metal stress; p<0.0001, pathogenic stress) than the hermaphroditic species. Our data suggests that divergence in DAF-16 mediated phenotypes may underlie many of the differences observed between these four species of Caenorhabditis nematodes. These findings are further supported by the correlative higher daf-16 expression levels among the gonochoristic species and significantly higher lifespan, immunity and stress tolerance in the constitutively active daf-16 hermaphroditic mutants

Co-Regulation of the DAF-16 Target Gene, cyp-35B1/dod-13, by HSF-1 in C. elegans Dauer Larvae and daf-2 Insulin Pathway Mutants

Insulin/IGF-I-like signaling (IIS) has both cell autonomous and non-autonomous functions. In some cases, targets through which IIS regulates cell-autonomous functions, such as cell growth and metabolism, have been identified. In contrast, targets for many non-autonomous IIS functions, such as C. elegans dauer morphogenesis, remain elusive. Here, we report the use of genomic and genetic approaches to identify potential non-autonomous targets of C. elegans IIS. First, we used transcriptional microarrays to identify target genes regulated non-autonomously by IIS in the intestine or in neurons. C. elegans IIS controls expression of a number of stress response genes, which were differentially regulated by tissue-restricted IIS. In particular, expression of sod-3, a MnSOD enzyme, was not regulated by tissue-restricted IIS on the microarrays, while expression of hsp-16 genes was rescued back to wildtype by tissue restricted IIS. One IIS target regulated non-autonomously by age-1 was cyp-35B1/dod-13, encoding a cytochrome P450. Genetic analysis of the cyp-35B1 promoter showed both DAF-16 and HSF-1 are direct regulators. Based on these findings, we propose that hsf-1 may participate in the pathways mediating non-autonomous activities of age-1 in C. elegans

Molecular Time-Course and the Metabolic Basis of Entry into Dauer in Caenorhabditis elegans

When Caenorhabditis elegans senses dauer pheromone (daumone), signaling inadequate growth conditions, it enters the dauer state, which is capable of long-term survival. However, the molecular pathway of dauer entry in C. elegans has remained elusive. To systematically monitor changes in gene expression in dauer paths, we used a DNA microarray containing 22,625 gene probes corresponding to 22,150 unique genes from C. elegans. We employed two different paths: direct exposure to daumone (Path 1) and normal growth media plus liquid culture (Path 2). Our data reveal that entry into dauer is accomplished through the multi-step process, which appears to be compartmentalized in time and according to metabolic flux. That is, a time-course of dauer entry in Path 1 shows that dauer larvae formation begins at post-embryonic stage S4 (48 h) and is complete at S6 (72 h). Our results also suggest the presence of a unique adaptive metabolic control mechanism that requires both stage-specific expression of specific genes and tight regulation of different modes of fuel metabolite utilization to sustain the energy balance in the context of prolonged survival under adverse growth conditions. It is apparent that worms entering dauer stage may rely heavily on carbohydrate-based energy reserves, whereas dauer larvae utilize fat or glyoxylate cycle-based energy sources. We created a comprehensive web-based dauer metabolic database for C. elegans (www.DauerDB.org) that makes it possible to search any gene and compare its relative expression at a specific stage, or evaluate overall patterns of gene expression in both paths. This database can be accessed by the research community and could be widely applicable to other related nematodes as a molecular atlas

Does Reduced IGF-1R Signaling in Igf1r+/− Mice Alter Aging?

Mutations in insulin/IGF-1 signaling pathway have been shown to lead to increased longevity in various invertebrate models. Therefore, the effect of the haplo- insufficiency of the IGF-1 receptor (Igf1r+/−) on longevity/aging was evaluated in C57Bl/6 mice using rigorous criteria where lifespan and end-of-life pathology were measured under optimal husbandry conditions using large sample sizes. Igf1r+/− mice exhibited reductions in IGF-1 receptor levels and the activation of Akt by IGF-1, with no compensatory increases in serum IGF-1 or tissue IGF-1 mRNA levels, indicating that the Igf1r+/− mice show reduced IGF-1 signaling. Aged male, but not female Igf1r+/− mice were glucose intolerant, and both genders developed insulin resistance as they aged. Female, but not male Igf1r+/− mice survived longer than wild type mice after lethal paraquat and diquat exposure, and female Igf1r+/− mice also exhibited less diquat-induced liver damage. However, no significant difference between the lifespans of the male Igf1r+/− and wild type mice was observed; and the mean lifespan of the Igf1r+/− females was increased only slightly (less than 5%) compared to wild type mice. A comprehensive pathological analysis showed no significant difference in end-of-life pathological lesions between the Igf1r+/− and wild type mice. These data show that the Igf1r+/− mouse is not a model of increased longevity and delayed aging as predicted by invertebrate models with mutations in the insulin/IGF-1 signaling pathway

MISC-1/OGC Links Mitochondrial Metabolism, Apoptosis and Insulin Secretion

We identified MISC-1 (Mitochondrial Solute Carrier) as the C. elegans orthologue of mammalian OGC (2-oxoglutarate carrier). OGC was originally identified for its ability to transfer α-ketoglutarate across the inner mitochondrial membrane. However, we found that MISC-1 and OGC are not solely involved in metabolic control. Our data show that these orthologous proteins participate in phylogenetically conserved cellular processes, like control of mitochondrial morphology and induction of apoptosis. We show that MISC-1/OGC is required for proper mitochondrial fusion and fission events in both C. elegans and human cells. Transmission electron microscopy reveals that loss of MISC-1 results in a decreased number of mitochondrial cristae, which have a blebbed appearance. Furthermore, our pull-down experiments show that MISC-1 and OGC interact with the anti-apoptotic proteins CED-9 and Bcl-xL, respectively, and with the pro-apoptotic protein ANT. Knock-down of misc-1 in C. elegans and OGC in mouse cells induces apoptosis through the caspase cascade. Genetic analysis suggests that MISC-1 controls apoptosis through the physiological pathway mediated by the LIN-35/Rb-like protein. We provide genetic and molecular evidence that absence of MISC-1 increases insulin secretion and enhances germline stem cell proliferation in C. elegans. Our study suggests that the mitochondrial metabolic protein MISC-1/OGC integrates metabolic, apoptotic and insulin secretion functions. We propose a novel mechanism by which mitochondria integrate metabolic and cell survival signals. Our data suggest that MISC-1/OGC functions by sensing the metabolic status of mitochondria and directly activate the apoptotic program when required. Our results suggest that controlling MISC-1/OGC function allows regulation of mitochondrial morphology and cell survival decisions by the metabolic needs of the cell

Sensory Perception of Food and Insulin-Like Signals Influence Seizure Susceptibility

Food deprivation is known to affect physiology and behavior. Changes that occur could be the result of the organism's monitoring of internal and external nutrient availability. In C. elegans, male mating is dependent on food availability; food-deprived males mate with lower efficiency compared to their well-fed counterparts, suggesting that the mating circuit is repressed in low-food environments. This behavioral response could be mediated by sensory neurons exposed to the environment or by internal metabolic cues. We demonstrated that food-deprivation negatively regulates sex-muscle excitability through the activity of chemosensory neurons and insulin-like signaling. Specifically, we found that the repressive effects of food deprivation on the mating circuit can be partially blocked by placing males on inedible food, E. coli that can be sensed but not eaten. We determined that the olfactory AWC neurons actively suppress sex-muscle excitability in response to food deprivation. In addition, we demonstrated that loss of insulin-like receptor (DAF-2) signaling in the sex muscles blocks the ability of food deprivation to suppress the mating circuit. During low-food conditions, we propose that increased activity by specific olfactory neurons (AWCs) leads to the release of neuroendocrine signals, including insulin-like ligands. Insulin-like receptor signaling in the sex muscles then reduces cell excitability via activation of downstream molecules, including PLC-γ and CaMKII

The significance of genome-wide transcriptional regulation in the evolution of stress tolerance.

It is widely recognized that stress plays an important role in directing the adaptive adjustment of an organism to changing environments. However, very little is known about the evolution of mechanisms that promote stress-induced variation. Adaptive transcriptional responses have been implicated in the evolution of tolerance to natural and anthropogenic stressors in the environment. Recent technological advances in transcriptomics provide a mechanistic understanding of biological pathways or processes involved in stress-induced phenotypic change. Furthermore, these studies are (semi) quantitative and provide insight into the reaction norms of identified target genes in response to specific stressors. We argue that plasticity in gene expression reaction norms may be important in the evolution of stress tolerance and adaptation to environmental stress. This review highlights the consequences of transcriptional plasticity of stress responses within a single generation and concludes that gene promoters containing a TATA box are more capable of rapid and variable responses than TATA-less genes. In addition, the consequences of plastic transcriptional responses to stress over multiple generations are discussed. Based on examples from the literature, we show that constitutive over expression of specific stress response genes results in stress adapted phenotypes. However, organisms with an innate capacity to buffer stress display plastic transcriptional responses. Finally, we call for an improved integration of the concept of phenotypic plasticity with studies that focus on the regulation of transcription. © Springer Science+Business Media B.V. 2010

Changes to the Fossil Record of Insects through Fifteen Years of Discovery

The first and last occurrences of hexapod families in the fossil record are compiled from publications up to end-2009. The major features of these data are compared with those of previous datasets (1993 and 1994). About a third of families (>400) are new to the fossil record since 1994, over half of the earlier, existing families have experienced changes in their known stratigraphic range and only about ten percent have unchanged ranges. Despite these significant additions to knowledge, the broad pattern of described richness through time remains similar, with described richness increasing steadily through geological history and a shift in dominant taxa, from Palaeoptera and Polyneoptera to Paraneoptera and Holometabola, after the Palaeozoic. However, after detrending, described richness is not well correlated with the earlier datasets, indicating significant changes in shorter-term patterns. There is reduced Palaeozoic richness, peaking at a different time, and a less pronounced Permian decline. A pronounced Triassic peak and decline is shown, and the plateau from the mid Early Cretaceous to the end of the period remains, albeit at substantially higher richness compared to earlier datasets. Origination and extinction rates are broadly similar to before, with a broad decline in both through time but episodic peaks, including end-Permian turnover. Origination more consistently exceeds extinction compared to previous datasets and exceptions are mainly in the Palaeozoic. These changes suggest that some inferences about causal mechanisms in insect macroevolution are likely to differ as well