Caenorhabditis elegans SDF-9 enhances insulin/insulin-like signaling through interaction with DAF-2

The neurodegenerative effects of selenium are inhibited by FOXO and PINK1/PTEN regulation of insulin/insulin-like growth factor signaling in Caenorhabditis elegans

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Insulin/insulin-like signaling reduction alters selenium-induced neurodegeneration.

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Selenium induces nuclear translocation of DAF-16/FOXO3a.

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DAF-16 overexpression decreases GABAergic and cholinergic motor neuron degeneration.

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Loss of DAF-18/PTEN increases sensitivity to selenium-induced movement deficits.

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Glutathione requires DAF-18/PINK-1 to improve selenium-induced movement deficits.

Exposures to high levels of environmental selenium have been associated with motor neuron disease in both animals and humans and high levels of selenite have been identified in the cerebrospinal fluid of patients with amyotrophic lateral sclerosis (ALS). We have shown previously that exposures to high levels of sodium selenite in the environment of Caenorhabditis elegans adult animals can induce neurodegeneration and cell loss resulting in motor deficits and death and that this is at least partially caused by a reduction in cholinergic signaling across the neuromuscular junction. Here we provide evidence that reduction in insulin/insulin-like (IIS) signaling alters response to high dose levels of environmental selenium which in turn can regulate the IIS pathway. Most specifically we show that nuclear localization and thus activation of the DAF-16/forkhead box transcription factor occurs in response to selenium exposure although this was not observed in motor neurons of the ventral cord. Yet, tissue specific expression and generalized overexpression of DAF-16 can partially rescue the neurodegenerative and behavioral deficits observed with high dose selenium exposures in not only the cholinergic, but also the GABAergic motor neurons. In addition, two modifiers of IIS signaling, PTEN (phosphatase and tensin homolog, deleted on chromosome 10) and PINK1 (PTEN-induced putative kinase 1) are required for the cellular antioxidant reduced glutathione to mitigate the selenium-induced movement deficits. Studies have suggested that environmental exposures can lead to ALS or other neurological diseases and this model of selenium-induced neurodegeneration developed in a genetically tractable organism provides a tool for examining the combined roles of genetics and environment in the neuro-pathologic disease process.

TATN-1 mutations reveal a novel role for tyrosine as a metabolic signal that influences developmental decisions and longevity in Caenorhabditis elegans

Recent work has identified changes in the metabolism of the aromatic amino acid tyrosine as a risk factor for diabetes and a contributor to the development of liver cancer. While these findings could suggest a role for tyrosine as a direct regulator of the behavior of cells and tissues, evidence for this model is currently lacking. Through the use of RNAi and genetic mutants, we identify tatn-1, which is the worm ortholog of tyrosine aminotransferase and catalyzes the first step of the conserved tyrosine degradation pathway, as a novel regulator of the dauer decision and modulator of the daf-2 insulin/IGF-1-like (IGFR) signaling pathway in Caenorhabditis elegans. Mutations affecting tatn-1 elevate tyrosine levels in the animal, and enhance the effects of mutations in genes that lie within the daf-2/insulin signaling pathway or are otherwise upstream of daf-16/FOXO on both dauer formation and worm longevity. These effects are mediated by elevated tyrosine levels as supplemental dietary tyrosine mimics the phenotypes produced by a tatn-1 mutation, and the effects still occur when the enzymes needed to convert tyrosine into catecholamine neurotransmitters are missing. The effects on dauer formation and lifespan require the aak-2/AMPK gene, and tatn-1 mutations increase phospho-AAK-2 levels. In contrast, the daf-16/FOXO transcription factor is only partially required for the effects on dauer formation and not required for increased longevity. We also find that the controlled metabolism of tyrosine by tatn-1 may function normally in dauer formation because the expression of the TATN-1 protein is regulated both by daf-2/IGFR signaling and also by the same dietary and environmental cues which influence dauer formation. Our findings point to a novel role for tyrosine as a developmental regulator and modulator of longevity, and support a model where elevated tyrosine levels play a causal role in the development of diabetes and cancer in people.

Repression of a potassium channel by nuclear hormone receptor and TGF-β signaling modulates insulin signaling in Caenorhabditis elegans

Transforming growth factor β (TGF-β) signaling acts through Smad proteins to play fundamental roles in cell proliferation, differentiation, apoptosis, and metabolism. The Receptor associated Smads (R-Smads) interact with DNA and other nuclear proteins to regulate target gene transcription. Here, we demonstrate that the Caenorhabditis elegans R-Smad DAF-8 partners with the nuclear hormone receptor NHR-69, a C. elegans ortholog of mammalian hepatocyte nuclear factor 4α HNF4α), to repress the exp-2 potassium channel gene and increase insulin secretion. We find that NHR-69 associates with DAF-8 both in vivo and in vitro. Functionally, daf-8 nhr-69 double mutants show defects in neuropeptide secretion and phenotypes consistent with reduced insulin signaling such as increased expression of the sod-3 and gst-10 genes and a longer life span. Expression of the exp-2 gene, encoding a voltage-gated potassium channel, is synergistically increased in daf-8 nhr-69 mutants compared to single mutants and wild-type worms. In turn, exp-2 acts selectively in the ASI neurons to repress the secretion of the insulin-like peptide DAF-28. Importantly, exp-2 mutation shortens the long life span of daf-8 nhr-69 double mutants, demonstrating that exp-2 is required downstream of DAF-8 and NHR-69. Finally, animals over-expressing NHR-69 specifically in DAF-28–secreting ASI neurons exhibit a lethargic, hypoglycemic phenotype that is rescued by exogenous glucose. We propose a model whereby DAF-8/R-Smad and NHR-69 negatively regulate the transcription of exp-2 to promote neuronal DAF-28 secretion, thus demonstrating a physiological crosstalk between TGF-β and HNF4α-like signaling in C. elegans. NHR-69 and DAF-8 dependent regulation of exp-2 and DAF-28 also provides a novel molecular mechanism that contributes to the previously recognized link between insulin and TGF-β signaling in C. elegans.

All animals must ensure metabolic homeostasis; if they fail to do so, diseases such as obesity and diabetes can develop. To maintain glucose balance, insulin is secreted upon glucose intake in a highly regulated and coordinated process. Previous studies suggested that the transforming growth factor beta (TGF-β) signaling pathway regulates insulin secretion in mammals. In the genetically tractable roundworm Caenorhabditis elegans, TGF-β and insulin signaling modulate larval development and aging, although the molecular link between insulin and TGF-β signaling remains poorly understood. In this study, we show that the TGF-β signaling component DAF-8 partners with NHR-69, a nuclear hormone receptor, to control the expression of the potassium channel exp-2, which in turn modulates the secretion of an insulin-like peptide. A loss-of-function exp-2 mutant exhibits increased insulin secretion and a shortened life span, whereas a gain-of-function mutant exhibits decreased insulin secretion. We also show that tissue-specific expression of nhr-69 in a pair of neurons that secrete neuropeptides causes reduced glucose content, increased insulin-like peptide levels and a lethargic phenotype. Because insulin and TGF-β signaling are linked to numerous diseases, our data may provide novel insights into the mechanisms contributing to pathophysiological changes.

EGG molecules couple the oocyte-to-embryo transition with cell cycle progression

The oocyte-to-embryo transition is a precisely coordinated process in which an oocyte becomes fertilized and transitions to an embryonic program of events. The molecules involved in this process have not been well studied. Recently, a group of interacting molecules in C. elegans have been described as coordinating the oocyte-to-embryo transition with the advancement of the cell cycle. Genes egg-3, egg-4, and egg-5 represent a small class of regulatory molecules known as protein-tyrosine phosphase-like proteins, which can bind phosphorylated substrates and act as scaffolding molecules or inhibitors. These genes are responsible for coupling the movements and activities of regulatory kinase mbk-2 with advancement of the cell cycle during the oocyte-to-embryo transition.

RNAi screen of DAF-16/FOXO target genes in C. elegans links pathogenesis and dauer formation

The DAF-16/FOXO transcription factor is the major downstream output of the insulin/IGF1R signaling pathway controlling C. elegans dauer larva development and aging. To identify novel downstream genes affecting dauer formation, we used RNAi to screen candidate genes previously identified to be regulated by DAF-16. We used a sensitized genetic background [eri-1(mg366); sdf-9(m708)], which enhances both RNAi efficiency and constitutive dauer formation (Daf-c). Among 513 RNAi clones screened, 21 displayed a synthetic Daf-c (SynDaf) phenotype with sdf-9. One of these genes, srh-100, was previously identified to be SynDaf, but twenty have not previously been associated with dauer formation. Two of the latter genes, lys-1 and cpr-1, are known to participate in innate immunity and six more are predicted to do so, suggesting that the immune response may contribute to the dauer decision. Indeed, we show that two of these genes, lys-1 and clc-1, are required for normal resistance to Staphylococcus aureus. clc-1 is predicted to function in epithelial cohesion. Dauer formation exhibited by daf-8(m85), sdf-9(m708), and the wild-type N2 (at 27°C) were all enhanced by exposure to pathogenic bacteria, while not enhanced in a daf-22(m130) background. We conclude that knockdown of the genes required for proper pathogen resistance increases pathogenic infection, leading to increased dauer formation in our screen. We propose that dauer larva formation is a behavioral response to pathogens mediated by increased dauer pheromone production.

Polymorphic segmental duplication in the nematode Caenorhabditis elegans

BACKGROUND

The nematode Caenorhabditis elegans was the first multicellular organism to have its genome fully sequenced. Over the last 10 years since the original publication in 1998, the C. elegans genome has been scrutinized and the last gaps were filled in November 2002, which present a unique opportunity for examining genome-wide segmental duplications.

RESULTS

Here, we performed analysis of the C. elegans genome in search for segmental duplications using a new tool -- OrthoCluster -- we have recently developed. We detected 3,484 duplicated segments -- duplicons -- ranging in size from 234 bp to 108 Kb. The largest pair of duplicons, 108 kb in length located on the left arm of Chromosome V, was further characterized. They are nearly identical at the DNA level (99.7% identity) and each duplicon contains 26 putative protein coding genes. Genotyping of 76 wild-type strains obtained from different labs in the C. elegans community revealed that not all strains contain this duplication. In fact, only 29 strains carry this large segmental duplication, suggesting a very recent duplication event in the C. elegans genome.

CONCLUSION

This report represents the first demonstration that the C. elegans laboratory wild-type N2 strains has acquired large-scale differences.

A PP2A regulatory subunit regulates C. elegans insulin/IGF-1 signaling by modulating AKT-1 phosphorylation

The C. elegans insulin/IGF-1 signaling (IIS) cascade plays a central role in regulating life span, dauer, metabolism, and stress. The major regulatory control of IIS is through phosphorylation of its components by serine/threonine-specific protein kinases. An RNAi screen for serine/threonine protein phosphatases that counterbalance the effect of the kinases in the IIS pathway identified pptr-1, a B56 regulatory subunit of the PP2A holoenzyme. Modulation of pptr-1 affects IIS pathway-associated phenotypes including life span, dauer, stress resistance, and fat storage. We show that PPTR-1 functions by regulating worm AKT-1 phosphorylation at Thr 350. With striking conservation, mammalian B56beta regulates Akt phosphorylation at Thr 308 in 3T3-L1 adipocytes. In C. elegans, this ultimately leads to changes in subcellular localization and transcriptional activity of the forkhead transcription factor DAF-16. This study reveals a conserved role for the B56 regulatory subunit in regulating insulin signaling through AKT dephosphorylation, thereby having widespread implications in cancer and diabetes research.