TOR controls translation initiation and early G1 progression in yeast.

Saccharomyces cerevisiae cells treated with the immunosuppressant rapamycin or depleted for the targets of rapamycin TOR1 and TOR2 arrest growth in the early G1 phase of the cell cycle. Loss of TOR function also causes an early inhibition of translation initiation and induces several other physiological changes characteristic of starved cells entering stationary phase (G0). A G1 cyclin mRNA whose translational control is altered by substitution of the UBI4 5' leader region (UBI4 is normally translated under starvation conditions) suppresses the rapamycin-induced G1 arrest and confers starvation sensitivity. These results suggest that the block in translation initiation is a direct consequence of loss of TOR function and the cause of the G1 arrest. We propose that the TORs, two related phosphatidylinositol kinase homologues, are part of a novel signaling pathway that activates eIF-4E-dependent protein synthesis and, thereby, G1 progression in response to nutrient availability. Such a pathway may constitute a checkpoint that prevents early G1 progression and growth in the absence of nutrients.     

Deletion of the intestinal peptide transporter affects insulin and TOR signaling in Caenorhabditis elegans

The mammalian intestinal peptide transporter PEPT1 mediates the uptake of di- and tripeptides from the gut lumen into intestinal epithelial cells and acts in parallel with amino acid transporters. Here we address the importance of the PEPT1 orthologue PEP-2 for the assimilation of dietary protein and for overall protein nutrition in Caenorhabditis elegans. pep-2 is expressed specifically along the apical membrane of the intestinal cells, and in pep-2 deletion mutant animals, uptake of intact peptides from the gut lumen is abolished. The consequences are a severely retarded development, reduced progeny and body size, and increased stress tolerance. We show here that pep-2 cross-talks with both the C. elegans target of rapamycin (TOR) and the DAF-2/insulin-signaling pathways. The pep-2 mutant enhances the developmental and longevity phenotypes of daf-2, resulting, among other effects, in a pronounced increase in adult life span. Moreover, all aspects of a weak let-363/TOR RNA interference phenotype are intensified by pep-2 deletion, indicating that pep-2 function upstream of TOR-mediated nutrient sensing. Our findings provide evidence for a predominant role of the intestinal peptide transporter for the delivery of bulk quantities of amino acids for growth and development, which consequently affects signaling pathways that regulate metabolism and aging.     

TOR and PKA signaling pathways converge on the protein kinase Rim15 to control entry into G0

The highly conserved Tor kinases (TOR) and the protein kinase A (PKA) pathway regulate cell proliferation in response to growth factors and/or nutrients. In Saccharomyces cerevisiae, loss of either TOR or PKA causes cells to arrest growth early in G(1) and to enter G(0) by mechanisms that are poorly understood. Here we demonstrate that the protein kinase Rim15 is required for entry into G(0) following inactivation of TOR and/or PKA. Induction of Rim15-dependent G(0) traits requires two discrete processes, i.e., nuclear accumulation of Rim15, which is negatively regulated both by a Sit4-independent TOR effector branch and the protein kinase B (PKB/Akt) homolog Sch9, and release from PKA-mediated inhibition of its protein kinase activity. Thus, Rim15 integrates signals from at least three nutrient-sensory kinases (TOR, PKA, and Sch9) to properly control entry into G(0), a key developmental process in eukaryotic cells.     

PYP-1, inorganic pyrophosphatase, is required for larval development and intestinal function in C. elegans

Inorganic pyrophosphatase (PPase) catalyzes the hydrolysis of inorganic pyrophosphate (PPi) into phosphate (Pi), which provides a thermodynamic driving force for important biosynthetic reactions. The nematode Caenorhabditis elegans gene C47E12.4 encodes a PPase (PYP-1) which shows 54% amino acid identity with human PPase. PYP-1 exhibits specific enzyme activity and is mainly expressed in the intestinal and nervous system. A null mutant of pyp-1 reveals a developmental arrest at early larval stages and exhibits gross defects in intestinal morphology and function. The larval arrest phenotype was successfully rescued by reintroduction of the pyp-1 gene, suggesting that PYP-1 is required for larval development and intestinal function in C. elegans.     

Inhibition of germline proliferation during C. elegans dauer development requires PTEN, LKB1 and AMPK signalling

In C. elegans, reduced insulin-like signalling induces developmental quiescence, reproductive delay and lifespan extension. We show here that the C. elegans orthologues of LKB1 and AMPK cooperate during conditions of reduced insulin-like signalling to establish cell cycle quiescence in the germline stem cell population, in addition to prolonging lifespan. The inactivation of either protein causes aberrant germline proliferation during diapause-like ;dauer' development, whereas the loss of AMPK uncouples developmental arrest from lifespan extension. Reduced TGF-beta activity also triggers developmental quiescence independent of the insulin-like pathway. Our data suggest that these two signalling pathways converge on the C. elegans PTEN orthologue to coordinate germline proliferation with somatic development during dauer formation, via the regulation of AMPK and its upstream activator LKB1, rather than through the canonical insulin-like signalling cascade. In humans, germline mutations in TGF-beta family members, PTEN or LKB1 result in related tumour-predisposing syndromes. Our findings establish a developmental relationship that may underscore their shared, characteristic aetiology.     

A Werner syndrome protein homolog affects C. elegans development, growth rate, life span and sensitivity to DNA damage by acting at a DNA damage checkpoint

A Werner syndrome protein homolog in C. elegans (WRN-1) was immunolocalized to the nuclei of germ cells, embryonic cells, and many other cells of larval and adult worms. When wrn-1 expression was inhibited by RNA interference (RNAi), a slight reduction in C. elegans life span was observed, with accompanying signs of premature aging, such as earlier accumulation of lipofuscin and tissue deterioration in the head. In addition, various developmental defects, including small, dumpy, ruptured, transparent body, growth arrest and bag of worms, were induced by RNAi. The frequency of these defects was accentuated by gamma-irradiation, implying that they were derived from spontaneous or induced DNA damage. wrn-1(RNAi) worms showed accelerated larval growth irrespective of gamma-irradiation, and pre-meiotic germ cells had an abnormal checkpoint response to DNA replication blockage. These observations suggest that WRN-1 acts as a checkpoint protein for DNA damage and replication blockage. This idea is also supported by an accelerated S phase in wrn-1(RNAi) embryonic cells. wrn-1(RNAi) phenotypes similar to those of Werner syndrome, such as premature aging and short stature, suggest wrn-1-deficient C. elegans as a useful model organism for Werner syndrome.     

Structural requirements for the tissue-specific and tissue-general functions of the Caenorhabditis elegans epidermal growth factor LIN-3.

Caenorhabditis elegans lin-3 encodes a homolog of the epidermal growth factor (EGF) family of growth factors. LIN-3 is the inductive signal for hermaphrodite vulval differentiation, and it is required for animal viability, hermaphrodite fertility, and the specification of anterior cell fates in the male B cell lineage. We describe the cloning of a lin-3 homolog from C. briggsae, sequence comparison of C. elegans lin-3 with C. briggsae lin-3, and the determination of molecular lesions in alleles of C. elegans lin-3, including three new alleles. We also analyzed the severity of phenotypes caused by the new and existing alleles of lin-3. Correlation of mutant phenotypes and their molecular lesions, as well as sequence comparison between two species, reveal that the EGF motif and the N-terminal portion of the cytoplasmic domain are important for the functions of LIN-3 in all tissues, while the C-terminal portion of the cytoplasmic domain is involved in the tissue-specific functions of lin-3. We discuss how the structure of lin-3 contributes to its functions in multiple developmental processes.     

Developmental stage- and DNA damage-specific functions of C. elegans FANCD2

In this study, we set out to investigate the role of Fanconi anemia complementation group D2 protein (FANCD2) in developmental stage-specific DNA damage responses in Caenorhabditis elegans. A mutant C. elegans strain containing a deletion in the gene encoding the FANCD2 homolog, FCD-2, exhibited egg-laying defects, precocious oogenesis, and partial defects in fertilization. The mutant strain also had a lower hatching rate than the wild-type after gamma-irradiation of embryos, but not after the irradiation of pachytene stage germ cells. This mutation sensitized pachytene stage germ cells to the genotoxic effects of photoactivated psoralen, as seen by a greatly reduced hatching rate and increased chromosomal aberrations. This mutation also enhanced physiological M-phase arrest and apoptosis. Taken together, our data reveal that the C. elegans FANCD2 homolog participates in the repair of spontaneous DNA damage and DNA crosslinks, not only in proliferating cells but also in pachytene stage cells, and it may have an additional role in double-stranded DNA break repair during embryogenesis.     

Autophagy and insulin/TOR signaling in Caenorhabditis elegans pcm-1 protein repair mutants

Biological responses due to nutrient deprivation in the nematode Caenorhabditis elegans, including L1 diapause and autophagy during dauer formation, can be mediated through the linked DAF-2/insulin/IGF receptor and target-of-rapamycin (TOR) kinase pathways. Here we discuss how altered insulin/TOR signaling may underlie the previously reported phenotypes of worms with a null mutation in the pcm-1 gene that results in reduced autophagy during dauer formation and decreased L1 arrest survival. PCM-1 encodes a protein repair methyltransferase and mutants of the encoding pcm-1 gene are incapable of converting spontaneously damaged l-isoaspartyl residues in cellular proteins to normal forms by this pathway. We speculate that PCM-1 may function either directly or indirectly as an inhibitor of insulin/TOR signaling, perhaps in a role to balance autophagy with alternative protein degradation pathways that may be more specific for recognizing age-damaged proteins.     

Phosphoinositide-3-OH-kinase inhibitor LY294002 prevents activation of Ancylostoma caninum and Ancylostoma ceylanicum third-stage infective larvae

The developmentally arrested hookworm infective larva resumes development only after encountering specific host-mediated cues during invasion. These cues activate a signaling pathway that culminates in the resumption of development. In Ancylostoma caninum, activation is characterised by the resumption of feeding and the release of excretory/secretory products required for infection. The dauer stage of the free-living nematode Caenorhabditis elegans is a developmentally arrested stage analogous to the hookworm infective larva. Dauer larvae exit developmental arrest in response to environmental cues that indicate favorable conditions for reproduction and growth. Because of the similarity between dauer recovery and activation, exit from dauer provides a model for hookworm larval activation. An insulin-signaling pathway has been implicated in controlling exit from developmental arrest in both C. elegans dauers and A. caninum larvae. To further investigate the role of insulin signaling in hookworm larval activation, the phosphatidylinositol-3-OH kinase inhibitor LY294002 was tested for its effect on in vitro activation using the resumption of feeding as a marker for activation. LY294002 prevented feeding in A. caninum infective larvae stimulated with host serum filtrate and a glutathione-analogue, the muscarinic agonist arecoline, or the cell permeable cGMP-analogue 8-bromo-cGMP. Similar results were seen with the congeneric hookworm Ancylostoma ceylanicum. These data suggest that an insulin-signaling pathway mediates activation in hookworm larvae, as in C. elegans, and that the phosphatidylinositol-3-OH kinase inhibitor acts downstream of the cGMP and muscarinic signaling steps in the pathway. In A. caninum, LY294002 had no effect on the release of excretory/secretory products associated with activation, suggesting that the secretory pathway diverges from the activation pathway upstream of the phosphatidylinositol-3-OH kinase step. These results provide additional support for the insulin-signaling pathway as the primarily pathway for activation to parasitism in hookworm larvae.     

jkk-1 and mek-1 regulate body movement coordination and response to heavy metals through jnk-1 in Caenorhabditis elegans

Although in vitro evidence suggests two c-Jun N-terminal kinase (JNK) kinases, MKK4 and MKK7, transactivate JNK, in vivo confirmation is incomplete. In fact, JNK deficiency may differ from the composite deficiency of MKK4 and MKK7 in Drosophila and mice. Recently, the Caenorhabditis elegans homolog of human JNK, jnk-1, and two MKK-7s, mek-1 and jkk-1, were cloned. Here we characterize jnk-1, which encodes two isoforms JNK-1 alpha and JNK-1 beta. A null allele, jnk-1(gk7), yielded worms with defective body movement coordination and modest mechanosensory deficits. Similarly to jkk-1 mutants, elimination of GABAergic signals suppressed the jnk-1(gk7) locomotion defect. Like mek-1 nulls, jnk-1(gk7) showed copper and cadmium hypersensitivity. Conditional expression of JNK-1 isoforms rescued these defects, suggesting that they are not due to developmental errors. While jkk-1 or mek-1 inactivation mimicked jnk-1(gk7) locomotion and heavy metal stress defects, respectively, mkk-4 inactivation did not, but rather yielded defective egg laying. Our results delineate at least two different JNK pathways through jkk-1 and mek-1 in C.elegans, and define interaction between MKK7, but not MKK4, and JNK.     

Cdx2 determines the fate of postnatal intestinal endoderm

Knock out of intestinal Cdx2 produces different effects depending upon the developmental stage at which this occurs. Early in development it produces histologically ordered stomach mucosa in the midgut. Conditional inactivation of Cdx2 in adult intestinal epithelium, as well as specifically in the Lgr5-positive stem cells, of adult mice allows long-term survival of the animals but fails to produce this phenotype. Instead, the endodermal cells exhibit cell-autonomous expression of gastric genes in an intestinal setting that is not accompanied by mesodermal expression of Barx1, which is necessary for gastric morphogenesis. Cdx2-negative endodermal cells also fail to express Sox2, a marker of gastric morphogenesis. Maturation of the stem cell niche thus appears to be associated with loss of ability to express positional information cues that are required for normal stomach development. Cdx2-negative intestinal crypts produce subsurface cystic vesicles, whereas untargeted crypts hypertrophy to later replace the surface epithelium. These observations are supported by studies involving inactivation of Cdx2 in intestinal crypts cultured in vitro. This abolishes their ability to form long-term growing intestinal organoids that differentiate into intestinal phenotypes. We conclude that expression of Cdx2 is essential for differentiation of gut stem cells into any of the intestinal cell types, but they maintain a degree of cell-autonomous plasticity that allows them to switch on a variety of gastric genes.     

Deficiency of Caenorhabditis elegans RecQ5 homologue reduces life span and increases sensitivity to ionizing radiation

Gene expression and RNA interference phenotypes were investigated for a Caenorhabditis elegans homologue (Ce-RCQ-5) of human RecQ5 protein. Expression of the mRNA was observed by in situ hybridization from earliest embryogenesis and gradually decreased during late embryogenesis. Ce-RCQ-5 was immuno-localized in the nuclei of embryos, germ cells, and oocytes and also in the nuclei of various somatic cells of larvae and adults. Despite ubiquitous expression in postembryonic cells, RCQ-5 protein expression was highest in intestinal cells, which was confirmed by tagging the gene expression with green fluorescence protein. When endogenous Ce-rcq-5 gene expression was inhibited by RNA interference, no clear phenotypes were observed during development. However, C. elegans life span was reduced by 37% due to RNA interference of rcq-5 gene, suggesting its possible role in maintenance of genomic stability, as has been ascribed to other RecQ family DNA helicases. In addition, C. elegans became significantly more sensitive to ionizing radiation after inhibition of rcq-5 gene expression, indicating an involvement of C. elegans RCQ-5 in a cellular response to DNA damage, possibly in DNA repair.     

PCR amplification of putative gpa-2 and gpa-3 orthologs from the (A+T)-rich genome of Strongyloides stercoralis

Two G protein alpha subunit genes orthologous to gpa-2 and gpa-3 in Caenorhabditis elegans have been identified in the parasitic nematode, Strongyloides stercoralis. These genes mediate chemosensory signal transduction regulating dauer arrest in C. elegans. In the parasite, they represent candidate mediators for regulation of the choice between free-living and parasitic life cycles, the obligatory developmental arrest of infective larvae, and reactivation of development after infection. The (A+T) content of these genes is 72.2% for coding sequences, 90% for introns, and 84.1% for 5' and 3' flanking regions, requiring the use of low extension temperatures for long distance PCR. The possible significance of conserved structural motifs of these proteins is discussed.