The lin-12 locus specifies cell fates in Caenorhabditis elegans

Proteasomal regulation of the proliferation vs. meiotic entry decision in the Caenorhabditis elegans germ line

Reproductive fitness in many animals relies upon a tight balance between the number of cells that proliferate in the germ line and the number of cells that enter meiosis and differentiate as gametes. In the Caenorhabditis elegans germ line, the GLP-1/Notch signaling pathway controls this balance between proliferation and meiotic entry. Here we describe the identification of the proteasome as an additional regulator of this balance. We show that a decrease in proteasome activity, through either genetic mutation or RNAi to core components of the proteasome, shifts this balance toward excess germ-line proliferation. We further demonstrate that there are likely two or more proteasome targets that contribute to excess germ-line proliferation when proteasome activity is reduced. One of these targets is likely a component or regulator of the Notch-signaling pathway, while the other functions on one of the two major redundant genetic pathways downstream of GLP-1/Notch signaling. We propose a model in which the proteasome degrades proteins that are necessary for proliferation as cells switch from proliferation to meiotic entry.

Transmission dynamics of heritable silencing induced by double-stranded RNA in Caenorhabditis elegans

Heritable silencing effects are gene suppression phenomena that can persist for generations after induction. In the majority of RNAi experiments conducted in Caenorhabditis elegans, the silencing response results in a hypomorphic phenotype where the effects recede after the F1 generation. F2 and subsequent generations revert to the original phenotype. Specific examples of transgenerational RNAi in which effects persist to the F2 generation and beyond have been described. In this study, we describe a systematic pedigree-based analysis of heritable silencing processes resulting from initiation of interference targeted at the C. elegans oocyte maturation factor oma-1. Heritable silencing of oma-1 is a dose-dependent process where the inheritance of the silencing factor is unequally distributed among the population. Heritability is not constant over generational time, with silenced populations appearing to undergo a bottleneck three to four generations following microinjection of RNA. Transmission of silencing through these generations can be through either maternal or paternal gamete lines and is surprisingly more effective through the male gametic line. Genetic linkage tests reveal that silencing in the early generations is transmitted independently of the original targeted locus, in a manner indicative of a diffusible epigenetic element.

SKR-1, a homolog of Skp1 and a member of the SCF(SEL-10) complex, regulates sex-determination and LIN-12/Notch signaling in C. elegans

Sex-determination in Caenorhabditis elegans requires regulation of gene transcription and protein activity and stability. sel-10 encodes a WD40-repeat-containing F-box protein that likely mediates the ubiquitin-mediated degradation of important sex-determination factors. Loss of sel-10 results in a mild masculinization of hermaphrodites, whereas dominant alleles of sel-10, such as sel-10(n1074), cause a more severe masculinization, including a reversal of the life versus death decision in sex-specific neurons. To investigate about how sel-10 regulates sex-determination, we conducted a sel-10(n1074) suppressor screen and isolated a weak loss-of-function allele of skr-1, one of 21 Skp1-related genes in C. elegans. Skp1, Cullin, and F-box proteins, such as SEL-10, are components of the SCF E3 ubiquitin-ligase complex. We present genetic evidence that the sel-10(n1074) masculinization phenotype is dependent upon skr-1 and cul-1 activity. Furthermore, we show that the SKR-1(M140I) weak loss-of-function mutation interferes with SKR-1/SEL-10 binding. Unexpectedly, we found that the G567E substitution in SEL-10 caused by the n1074 allele impairs the binding of SEL-10 to SKR-1 and the dimerization of SEL-10, which may be important for SEL-10 function. Our results suggest that SKR-1, CUL-1 and SEL-10 constitute an SCF E3 ligase complex that plays an important role in modulating sex-determination and LIN-12/Notch signaling in C. elegans.

The CSL transcription factor LAG-1 directly represses hlh-6 expression in C. elegans

The Caenorhabditis elegans gene hlh-6 is expressed specifically in pharyngeal glands, one of five distinct pharyngeal cell types. Expression of hlh-6 is controlled by a discrete set of cis-regulatory elements, including a negative element called HRL1. Here we demonstrate that HRL1 is a functional binding site for LAG-1, the CSL transcriptional effector of Notch in C. elegans, and that regulation of hlh-6 by LAG-1 is direct. Regulation of hlh-6 by LAG-1 is strictly negative: removal of HRL1 or LAG-1 regulation results in ectopic expression of hlh-6, but does not affect expression in pharyngeal glands. Furthermore, direct regulation of hlh-6 expression does not appear to involve Notch signaling, contrary to the canonical mechanism by which CSL factors regulate target genes. We also identify an additional cis-regulatory element in the hlh-6 promoter that, together with previously identified elements, is sufficient to overcome repression by LAG-1 and activate hlh-6 expression in pharyngeal glands.

Notch signalling is required for both dauer maintenance and recovery in C. elegans

The Notch signalling pathway is conserved among higher metazoans and is used repeatedly throughout development to specify distinct cell fates among populations of equipotent cells. Mounting evidence suggests that Notch signalling may also be crucial in neuronal function in postmitotic, differentiated neurons. Here, we demonstrate a novel role for the canonical Notch signalling pathway in postmitotic neurons during a specialised ;diapause-like' post-embryonic developmental stage in C. elegans called dauer. Our data suggest that cell signalling downstream of the developmental decision to enter dauer leads to the activation of Notch-responding genes in postmitotic neurons. Consistent with this, we demonstrate that glp-1, one of the two C. elegans Notch receptors, and its ligand lag-2 are expressed in neurons during the dauer stage, and both genes are required to maintain this stage in a daf-7/TGFbeta dauer constitutive background. Our genetic data also suggest that a second Notch receptor, lin-12, functions upstream of, or in parallel with, insulin-like signalling components in response to replete growth conditions to promote dauer recovery. Based on our findings, cues associated with the onset of dauer ultimately trigger a glp-1-dependent Notch signalling cascade in neurons to maintain this developmental state. Then, as growth conditions improve, activation of the LIN-12 Notch receptor cooperates with the insulin-like signalling pathway to signal recovery from the dauer stage.

Toward verified biological models

The last several decades have witnessed a vast accumulation of biological data and data analysis. Many of these data sets represent only a small fraction of the system's behavior, making the visualization of full system behavior difficult. A more complete understanding of a biological system is gained when different types of data (and/or conclusions drawn from the data) are integrated into a larger-scale representation or model of the system. Ideally, this type of model is consistent with all available data about the system, and it is then used to generate additional hypotheses to be tested. Computer-based methods intended to formulate models that integrate various events and to test the consistency of these models with respect to the laboratory-based observations on which they are based are potentially very useful. In addition, in contrast to informal models, the consistency of such formal computer-based models with laboratory data can be tested rigorously by methods of formal verification. We combined two formal modeling approaches in computer science that were originally developed for non-biological system design. One is the inter-object approach using the language of live sequence charts (LSCs) with the Play-Engine tool, and the other is the intra-object approach using the language of statecharts and Rhapsody as the tool. Integration is carried out using InterPlay, a simulation engine coordinator. Using these tools, we constructed a combined model comprising three modules. One module represents the early lineage of the somatic gonad of C. elegans in LSCs, while a second more detailed module in statecharts represents an interaction between two cells within this lineage that determine their developmental outcome. Using the advantages of the tools, we created a third module representing a set of key experimental data using LSCs. We tested the combined statechart-LSC model by showing that the simulations were consistent with the set of experimental LSCs. This small-scale modular example demonstrates the potential for using similar approaches for verification by exhaustive testing of models by LSCs. It also shows the advantages of these approaches for modeling biology.

A Caenorhabditis elegans model for epithelial-neuronal transdifferentiation

Understanding transdifferentiation-the conversion of one differentiated cell type into another-is important from both basic science and clinical perspectives. In Caenorhabditis elegans, an epithelial cell named Y is initially part of the rectum but later appears to withdraw, migrate, and then become a motor neuron named PDA. Here, we show that this represents a bona fide transdifferentiation event: Y has epithelial hallmarks without detectable neural characteristics, and PDA has no residual epithelial characteristics. Using available mutants and laser microsurgery, we found that transdifferentiation does not depend on fusion with a neighboring cell or require migration of Y away from the rectum, that other rectal epithelial cells are not competent to transdifferentiate, and that transdifferentiation requires the EGL-5 and SEM-4 transcription factors and LIN-12/Notch signaling. Our results establish Y-to-PDA transdifferentiation as a genetically tractable model for deciphering the mechanisms underlying cellular plasticity in vivo.

Transcriptional control of cell-cycle quiescence during C. elegans development

During the development of the C. elegans reproductive system, cells that give rise to the vulva, the vulval precursor cells (VPCs), remain quiescent for two larval stages before resuming cell division in the third larval stage. We have identified several transcriptional regulators that contribute to this temporary cell-cycle arrest. Mutation of lin-1 or lin-31, two downstream targets of the Receptor Tyrosine kinase (RTK)/Ras/MAP kinase cascade that controls VPC cell fate, disrupts the temporary VPC quiescence. We found that the LIN-1/Ets and LIN-31/FoxB transcription factors promote expression of CKI-1, a member of the p27 family of cyclin-dependent kinase inhibitors (CKIs). LIN-1 and LIN-31 promote cki-1/Kip-1 transcription prior to their inhibition through RTK/Ras/MAPK activation. Another mutation identified in the screen defined the mdt-13 TRAP240 Mediator subunit. Further analysis of the multi-subunit Mediator complex revealed that a specific subset of its components act in VPC quiescence. These components substantially overlap with the CDK-8 module implicated in transcriptional repression. Taken together, strict control of cell-cycle quiescence during VPC development involves transcriptional induction of CKI-1 and transcriptional regulation through the Mediator complex. These transcriptional regulators represent potential molecular connections between development and the basic cell-cycle machinery.

Dorsoventral patterning of the C. elegans postembryonic mesoderm requires both LIN-12/Notch and TGFbeta signaling

The C. elegans postembryonic mesodermal lineage arises from a single cell M, which generates distinct dorsal and ventral cell types. We have previously shown that mutations in the Schnurri homolog sma-9 cause ventralization of the M lineage and that wild-type SMA-9 antagonizes the Sma/Mab TGFbeta pathway to promote dorsal M lineage fates [Foehr, M.L., Lindy, A.S., Fairbank, R.C., Amin, N.M., Xu, M., Yanowitz, J., Fire, A.Z., Liu, J., 2006. An antagonistic role for the C. elegans Schnurri homolog SMA-9 in modulating TGFbeta signaling during mesodermal patterning. Development 133, 2887-2896]. Interestingly, loss-of-function mutations in the Notch receptor lin-12 cause dorsalization of the M lineage [Greenwald, I.S., Sternberg, P.W., Horvitz, H.R., 1983. The lin-12 locus specifies cell fates in Caenorhabditis elegans. Cell 34, 435-444]. We have found that although LIN-12 protein is present in both the dorsal and ventral M lineage cells, its ligands LAG-2 and APX-1 are asymmetrically localized in cells adjacent to ventral M-derived cells, and may function redundantly in promoting ventral M lineage fates. To investigate how LIN-12/Notch signaling interacts with SMA-9 and Sma/Mab TGFbeta signaling in regulating M lineage patterning, we generated double and triple mutant combinations among lin-12, sma-9 and dbl-1 (the ligand for the Sma/Mab TGFbeta pathway) and examined their M lineage phenotypes. Our results suggest that the LIN-12/Notch pathway and the Sma/Mab TGFbeta pathway function independently in regulating dorsoventral patterning of the M lineage, with LIN-12/Notch required for ventral M lineage fates, and SMA-9 antagonism of TGFbeta signaling required for dorsal M lineage fates. Our work provides a model for how combined Notch and TGFbeta signaling regulates the developmental potential of two equipotent cells along the dorsoventral axis.

The C. elegans ROR receptor tyrosine kinase, CAM-1, non-autonomously inhibits the Wnt pathway

Inhibitors of Wnt signaling promote normal development and prevent cancer by restraining when and where the Wnt pathway is activated. ROR proteins, a class of Wnt-binding receptor tyrosine kinases, inhibit Wnt signaling by an unknown mechanism. To clarify how RORs inhibit the Wnt pathway, we examined the relationship between Wnts and the sole C. elegans ROR homolog, cam-1, during C. elegans vulval development, a Wnt-regulated process. We found that loss and overexpression of cam-1 causes reciprocal defects in Wnt-mediated cell-fate specification. Our molecular and genetic analyses revealed that the CAM-1 extracellular domain (ECD) is sufficient to non-autonomously antagonize multiple Wnts, suggesting that the CAM-1/ROR ECD sequesters Wnts. A sequestration model is supported by our findings that the CAM-1 ECD binds to several Wnts in vitro. These results demonstrate how ROR proteins help to refine the spatial pattern of Wnt activity in a complex multicellular environment.

Co-regulation by Notch and Fos is required for cell fate specification of intermediate precursors during C. elegans uterine development

The Notch pathway is the key signal for many cell fate decisions in the nematode Caenorhabditis elegans including the uterine pi cell fate, crucial for a proper uterine-vulval connection and egg laying. Expression of the egl-13 SOX domain transcription factor is specifically upregulated upon induction of the pi lineage and not in response to other LIN-12/Notch-mediated decisions. We determined that dual regulation by LIN-12 and FOS-1 is required for egl-13 expression at specification and for complete rescue of egl-13 mutants. We found that fos-1 mutants exhibit uterine defects and fail to express pi markers. We show that FOS-1 is expressed at pi cell specification and can bind in vitro to egl-13 upstream regulatory sequence (URS) as a heterodimer with C. elegans Jun.

Genetic control of cellular interactions in Caenorhabditis elegans development

During development of the nematode, Caenorhabditis elegans, cell interactions play a significant role in controlling cell fate. Regulatory cells in the somatic gonad control proliferation in the germline and induce formation of the vulva in the hypodermis. In the early embryo, mesodermal cells are induced in a process similar to embryonic induction. In addition, interactions between precursor cells of equivalent developmental potential direct one cell to adopt one fate and the other to adopt a second fate. Two genes have been identified in C. elegans that appear to mediate cell interactions. The glp-1 gene is required for embryonic induction and continued germline proliferation; lin-12 is required for cells of equal developmental potential to adopt different fates. Genetic mosaics indicate that glp-1 may be part of the receiving mechanism in controlling germline proliferation.

Handed asymmetry, handedness reversal and mechanisms of cell fate determination in nematode embryos

Embryos of the nematode Caenorhabditis elegans exhibit left-right asymmetry with an invariant handedness. The embryonic cell lineage is asymmetrical: although the animal is generally bilaterally symmetrical with only a few left-right asymmetries, many of its contralaterally analogous cells arise via different lineages on the two sides of the embryo. Larvae and adults also exhibit left-right asymmetries with a handedness that is normally invariant. The frequency of animals with opposite handedness was increased among the progeny of adults exposed to the mutagen ethyl methanesulphonate and among animals that developed from embryos treated in early cleavage with chitinase to destroy the egg shell. Reversal of embryonic handedness was accomplished directly by micromanipulation at the 6-cell stage, resulting in mirror-image but otherwise normal development into healthy, fertile animals with all the usual left-right asymmetries reversed. This demonstrates that (1) the handedness of cell positions in the 6-cell embryo dictates handedness throughout development; (2) at this stage the pair of anterior blastomeres on the right is equivalent to the pair on the left; and (3) the extensive differences in fates of lineally homologous cells on the two sides of the animal must be dictated by cellular interactions, most of which are likely to occur early in embryogenesis and appear to have been conserved in widely diverged nematode species.

The Bro1-domain protein, EGO-2, promotes Notch signaling in Caenorhabditis elegans

In Caenorhabditis elegans, as in other animals, Notch-type signaling mediates numerous inductive events during development. The mechanism of Notch-type signaling involves proteolytic cleavage of the receptor and subsequent transport of the receptor intracellular domain to the nucleus, where it acts as a transcriptional regulator. Notch-type signaling activity is modulated by post-translational modifications and endocytosis of ligand and receptor. We previously identified the ego-2 (enhancer of glp-1) gene as a positive regulator of germline proliferation that interacts genetically with the GLP-1/Notch signaling pathway in the germline. Here, we show that ego-2 positively regulates signaling in various tissues via both GLP-1 and the second C. elegans Notch-type receptor, LIN-12. ego-2 activity also promotes aspects of development not known to require GLP-1 or LIN-12. The EGO-2 protein contains a Bro1 domain, which is known in other systems to localize to certain endosomal compartments. EGO-2 activity in the soma promotes GLP-1 signaling in the germline, consistent with a role for EGO-2 in production of active ligand. Another C. elegans Bro1-domain protein, ALX-1, is known to interact physically with LIN-12/Notch. We document a complex phenotypic interaction between ego-2 and alx-1, consistent with their relationship being antagonistic with respect to some developmental processes and agonistic with respect to others.

Predictive modeling of signaling crosstalk during C. elegans vulval development

Caenorhabditis elegans vulval development provides an important paradigm for studying the process of cell fate determination and pattern formation during animal development. Although many genes controlling vulval cell fate specification have been identified, how they orchestrate themselves to generate a robust and invariant pattern of cell fates is not yet completely understood. Here, we have developed a dynamic computational model incorporating the current mechanistic understanding of gene interactions during this patterning process. A key feature of our model is the inclusion of multiple modes of crosstalk between the epidermal growth factor receptor (EGFR) and LIN-12/Notch signaling pathways, which together determine the fates of the six vulval precursor cells (VPCs). Computational analysis, using the model-checking technique, provides new biological insights into the regulatory network governing VPC fate specification and predicts novel negative feedback loops. In addition, our analysis shows that most mutations affecting vulval development lead to stable fate patterns in spite of variations in synchronicity between VPCs. Computational searches for the basis of this robustness show that a sequential activation of the EGFR-mediated inductive signaling and LIN-12 / Notch-mediated lateral signaling pathways is key to achieve a stable cell fate pattern. We demonstrate experimentally a time-delay between the activation of the inductive and lateral signaling pathways in wild-type animals and the loss of sequential signaling in mutants showing unstable fate patterns; thus, validating two key predictions provided by our modeling work. The insights gained by our modeling study further substantiate the usefulness of executing and analyzing mechanistic models to investigate complex biological behaviors.

Systems biology aims to gain a system-level understanding of living systems. To achieve such an understanding, we need to establish the methodologies and techniques to understand biological systems in their full complexity. One such attempt is to use methods designed for the construction and analysis of complex computerized systems to model biological systems. Describing mechanistic models in biology in a dynamic and executable language offers great advantages for representing time and parallelism, which are important features of biological behavior. In addition, automatic analysis methods can be used to ensure the consistency of computational models with biological data on which they are based. We have developed a dynamic computational model describing the current mechanistic understanding of cell fate determination during C. elegans vulval development, which provides an important paradigm for studying animal development. Our model is realistic, reproduces up-to-date experimental observations, allows in silico experimentation, and is analyzable by automatic tools. Analysis of our model provides new insights into the temporal aspects of the cell fate patterning process and predicts new modes of interaction between the signaling pathways involved. These biological insights, which were also validated experimentally, further substantiate the usefulness of dynamic computational models to investigate complex biological behaviors.

LIN-61, one of two Caenorhabditis elegans malignant-brain-tumor-repeat-containing proteins, acts with the DRM and NuRD-like protein complexes in vulval development but not in certain other biological processes

Vulval development in Caenorhabiditis elegans is inhibited by the redundant functions of the synthetic multivulva (synMuv) genes. At least 26 synMuv genes have been identified, many of which appear to act via transcriptional repression. Here we report the molecular identification of the class B synMuv gene lin-61, which encodes a protein composed of four malignant brain tumor (MBT) repeats. MBT repeats, domains of approximately 100 amino acids, have been found in multiple copies in a number of transcriptional repressors, including Polycomb-group proteins. MBT repeats are important for the transcriptional repression mediated by these proteins and in some cases have been shown to bind modified histones. C. elegans contains one other MBT-repeat-containing protein, MBTR-1. We demonstrate that a deletion allele of mbtr-1 does not cause a synMuv phenotype nor does mbtr-1 appear to act redundantly with or in opposition to lin-61. We further show that lin-61 is phenotypically and biochemically distinct from other class B synMuv genes. Our data indicate that while the class B synMuv genes act together to regulate vulval development, lin-61 functions separately from some class B synMuv proteins in other biological processes.

Similar requirements for CDC-42 and the PAR-3/PAR-6/PKC-3 complex in diverse cell types

During animal development, a complex of Par3, Par6 and atypical protein kinase C (aPKC) plays a central role in cell polarisation. The small G protein Cdc42 also functions in cell polarity and has been shown in some cases to act by regulating the Par3 complex. However, it is not yet known whether Cdc42 and the Par3 complex widely function together in development or whether they have independent functions. For example, many studies have implicated Cdc42 in cell migrations, but the Par3 complex has only been little studied, with conflicting results. Here we examine the requirements for CDC-42 and the PAR-3/PAR-6/PKC-3 complex in a range of different developmental events. We found similar requirements in all tissues examined, including polarised growth of vulval precursors and seam cells, migrations of neuroblasts and axons, and the development of the somatic gonad. We also propose a novel role for primordial germ cells in mediating coalescence of the Caenorhabditis elegans gonad. These results indicate that CDC-42 and the PAR-3/PAR-6/aPKC complex function together in diverse cell types.

SEL-2, theC. elegansneurobeachin/LRBA homolog, is a negative regulator oflin-12/Notchactivity and affects endosomal traffic in polarized epithelial cells

The vulval precursor cells (VPCs) of Caenorhabditis elegans are polarized epithelial cells that adopt a precise pattern of fates through regulated activity of basolateral LET-23/EGF receptor and apical LIN-12/Notch. During VPC patterning, there is reciprocal modulation of endocytosis and trafficking of both LET-23 and LIN-12. We identified sel-2 as a negative regulator of lin-12/Notch activity in the VPCs, and found that SEL-2 is the homolog of two closely related human proteins, neurobeachin(also known as BCL8B) and LPS-responsive, beige-like anchor protein (LRBA). SEL-2, neurobeachin and LRBA belong to a distinct subfamily of BEACH-WD40 domain-containing proteins. Loss of sel-2 activity leads to basolateral mislocalization and increased accumulation of LIN-12 in VPCs in which LET-23 is not active, and to impaired downregulation of basolateral LET-23 in VPCs in which LIN-12 is active. Downregulation of apical LIN-12 in the VPC in which LET-23 is active is not affected. In addition, in sel-2 mutants, the polarized cells of the intestinal epithelium display an aberrant accumulation of the lipophilic dye FM4-64 when the dye is presented to the basolateral surface. Our observations indicate that SEL-2/neurobeachin/LRBA is involved in endosomal traffic and may be involved in efficient delivery of cell surface proteins to the lysosome. Our results also suggest that sel-2 activity may contribute to the appropriate steady-state level of LIN-12 or to trafficking events that affect receptor activation.

Distinct roles of the Pumilio and FBF translational repressors during C. elegans vulval development

The C. elegans PUF and FBF proteins regulate various aspects of germline development by selectively binding to the 3' untranslated region of their target mRNAs and repressing translation. Here, we show that puf-8, fbf-1 and fbf-2 also act in the soma where they negatively regulate vulvaI development. Loss-of-function mutations in puf-8 cause ectopic vulval differentiation when combined with mutations in negative regulators of the EGFR/RAS/MAPK pathway and suppress the vulvaless phenotype caused by mutations that reduce EGFR/RAS/MAPK signalling. PUF-8 acts cell-autonomously in the vulval cells to limit their temporal competence to respond to the extrinsic patterning signals. fbf-1 and fbf-2, however, redundantly inhibit primary vulval cell fate specification in two distinct pathways acting in the soma and in the germline. The FBFs thereby ensure that the inductive signal selects only one vulval precursor cell for the primary cell fate. Thus, translational repressors regulate various aspects of vulval cell fate specification, and they may play a conserved role in modulating signal transduction during animal development.

sli-3 negatively regulates the LET-23/epidermal growth factor receptor-mediated vulval induction pathway in Caenorhabditis elegans

The LIN-3-LET-23-mediated inductive signaling pathway plays a major role during vulval development in C. elegans. Studies on the components of this pathway have revealed positive as well as negative regulators that function to modulate the strength and specificity of the signal transduction cascade. We have carried out genetic screens to identify new regulators of this pathway by screening for suppressors of lin-3 vulvaless phenotype. The screens recovered three loci including alleles of gap-1 and a new gene represented by sli-3. Our genetic epistasis experiments suggest that sli-3 functions either downstream or in parallel to nuclear factors lin-1 and sur-2. sli-3 synergistically interacts with the previously identified negative regulators of the let-23 signaling pathway and causes excessive cell proliferation. However, in the absence of any other mutation sli-3 mutant animals display wild-type vulval induction and morphology. We propose that sli-3 functions as a negative regulator of vulval induction and defines a branch of the inductive signaling pathway. We provide evidence that sli-3 interacts with the EGF signaling pathway components during vulval induction but not during viability and ovulation processes. Thus, sli-3 helps define specificity of the EGF signaling to induce the vulva.

N-ethylmaleimide sensitive factor is required for fusion of the C. elegans uterine anchor cell

The fusion of the Caenorhabditis elegans uterine anchor cell (AC) with the uterine-seam cell (utse) is an excellent model system for studying cell-cell fusion, which is essential to animal development. We obtained an egg-laying defective (Egl) mutant in which the AC fails to fuse with the utse. This defect is highly specific: other aspects of utse development and other cell fusions appear to occur normally. We find that defect is due to a missense mutation in the nsf-1 gene, which encodes N-ethylmaleimide-sensitive factor (NSF), an intracellular membrane fusion factor. There are two NSF-1 isoforms, which are expressed in distinct tissues through two separate promoters. NSF-1L is expressed in the uterus, including the AC. We find that nsf-1 is required cell-autonomously in the AC for its fusion with the utse. Our results establish AC fusion as a paradigm for studying cell fusion at single cell resolution and demonstrate that the NSF ATPase is a key player in this process.

Presenilin Function in Caenorhabditis elegans

The genome of the nematode Caenorhabditis elegans contains homologs of several genes associated with familial Alzheimer's disease in humans. apl-1 encodes a transmembrane protein belonging to the amyloid precursor protein family, sel-12 and hop-1 are the two somatically expressed presenilin genes that resemble PS1 and PS2 on both a structural and a functional level. Mutations in the sel-12-encoded presenilin gene cause defective Notch/lin-12 signaling and result in reduced egg-laying, caused by cell specification and cell attachment defects. spr-1, spr-3, spr-4 and spr-5 were identified as the suppressors of the egg-laying defect of presenilin/sel-12 loss of function mutants in genetic suppressor screens. The corresponding proteins are C. elegans homologs of human REST, CoREST and LSD1, respectively. REST/NSRF (Re1 silencing transcription factor/neural-restrictive silencing factor) is a transcriptional repressor that blocks the expression of neuronal genes in non-neuronal tissues in vertebrates. CoREST is a conserved histone deacetylase and demethylase-containing co-repressor complex possessing a potential chromatin-modifying activity. It is recruited to the promoter via REST-mediated DNA binding. LSD1 is a flavin-dependent demethylase of histone H3. Mutations in spr-1, spr-3, spr-4 and spr-5 genes suppress the egg-laying phenotype of sel-12 loss of function mutants by derepressing the expression of the second C. elegans presenilin gene, hop-1.

EMB-4: a predicted ATPase that facilitates lin-12 activity in Caenorhabditis elegans

The sel-6 gene was previously identified in a screen for suppressors of the egg-laying defect associated with hypermorphic alleles of lin-12 (Tax et al. 1997). Here we show that sel-6 and two other previously defined genes, mal-2 and emb-4, are the same gene, now called "emb-4." We perform a genetic and molecular characterization of emb-4 and show that it functions cell autonomously as a positive regulator of lin-12 activity. Viable alleles identified as suppressors of lin-12 are partial loss-of-function mutations, whereas the null phenotype encompasses a range of lethal terminal phenotypes that apparently are not related to loss of lin-12/Notch signaling. emb-4 encodes a large nuclearly localized protein containing a predicted ATPase domain and has apparent orthologs in fission yeast, plants, and animals.

Transcriptional control of Notch signaling by a HOX and a PBX/EXD protein during vulval development in C. elegans

The Notch signaling pathway controls growth, differentiation and patterning in divergent animal phyla; in humans, defective Notch signaling has been implicated in cancer, stroke and neurodegenerative disorders. Despite its developmental and medical significance, little is known about the factors that render cells to become competent for Notch signaling. Here we show that during vulval development in the nematode Caenorhabditis elegans the HOX protein LIN-39 and its EXD/PBX-like cofactor CEH-20 are required for LIN-12/Notch-mediated lateral signaling that specifies the 2 degrees vulval cell fate. Inactivation of either lin-39 or ceh-20 resulted in the misspecification of 2 degrees vulval cells and suppressed the multivulva phenotype of lin-12(n137) gain-of-function mutant animals. Furthermore, both LIN-39 and CEH-20 are required for the expression of basal levels of the genes encoding the LIN-12/Notch receptor and one of its ligands in the vulval precursor cells, LAG-2/Delta/Serrate, rendering them competent for the subsequent lin-12/Notch induction events. Our results suggest that the transcription factors LIN-39 and CEH-20, which function at the bottom of the RTK/Ras and Wnt pathways in vulval induction, serve as major integration sites in coordinating and transmitting signals to the LIN-12/Notch cascade to regulate vulval cell fates.

The claudin superfamily protein nsy-4 biases lateral signaling to generate left-right asymmetry in C. elegans olfactory neurons

Early in C. elegans development, signaling between bilaterally symmetric AWC olfactory neurons causes them to express different odorant receptor genes. AWC left-right asymmetry is stochastic: in each animal, either the left or the right neuron randomly becomes AWC(ON), and the other neuron becomes AWC(OFF). Here we show that the nsy-4 gene coordinates the lateral signaling that diversifies AWC(ON) and AWC(OFF) neurons. nsy-4 mutants generate 2 AWC(OFF) neurons, as expected if communication between the AWC neurons is lost, whereas overexpression of nsy-4 results in 2 AWC(ON) neurons. nsy-4 encodes a transmembrane protein related to the gamma subunits of voltage-activated calcium channels and the claudin superfamily; it interacts genetically with calcium channels and antagonizes a calcium-to-MAP kinase cascade in the neuron that becomes AWC(ON). Genetic mosaic analysis indicates that nsy-4 functions both cell-autonomously and nonautonomously in signaling between AWC neurons, providing evidence for lateral signaling and feedback that coordinate asymmetric receptor choice.

An antagonistic role for theC. elegansSchnurri homolog SMA-9 in modulating TGFβ signaling during mesodermal patterning

In C. elegans, the Sma/Mab TGFβ signaling pathway regulates body size and male tail patterning. SMA-9, the C. elegans homolog of Schnurri, has been shown to function as a downstream component to mediate the Sma/Mab TGFβ signaling pathway in these processes. We have discovered a new role for SMA-9 in dorsoventral patterning of the C. eleganspost-embryonic mesoderm, the M lineage. In addition to a small body size, sma-9 mutant animals exhibit a dorsal-to-ventral fate transformation within the M lineage. This M lineage defect of sma-9 mutants is unique in that animals carrying mutations in all other known components of the TGFβ pathway exhibit no M lineage defects. Surprisingly, mutations in the core components of the Sma/Mab TGFβ signaling pathway suppressed the M lineage defects of sma-9 mutants without suppressing their body size defects. We show that this suppression specifically happens within the M lineage. Our studies have uncovered an unexpected role of SMA-9 in antagonizing the TGFβ signaling pathway during mesodermal patterning,suggesting a novel mode of function for the SMA-9/Schnurri family of proteins.

The C. elegans HP1 homologue HPL-2 and the LIN-13 zinc finger protein form a complex implicated in vulval development

HP1 proteins are essential components of heterochromatin and contribute to the transcriptional repression of euchromatic genes via the recruitment to specific promoters by corepressor proteins including TIF1 and Rb. The Caenorhabditis elegans HP1 homologue HPL-2 acts in the "synMuv" (synthetic multivulval) pathway, which defines redundant negative regulators of a Ras signaling cascade required for vulval induction. Several synMuv genes encode for chromatin-associated proteins involved in transcriptional regulation, including Rb and components of the Mi-2/NuRD and TIP60/NuA4 chromatin remodeling complexes. Here, we show that HPL-2 physically interacts in vitro and in vivo with the multiple zinc finger protein LIN-13, another member of the synMuv pathway. A variant of the conserved PXVXL motif found in many HP1-interacting proteins mediates LIN-13 binding to the CSD of HPL-2. We further show by in vivo localization studies that LIN-13 is required for HPL-2 recruitment in nuclear foci. Our data suggest that the LIN-13/HPL-2 complex may physically link a subset of the Rb related synMuv proteins to chromatin.

Cell fate-specific regulation of EGF receptor trafficking during Caenorhabditis elegans vulval development

By controlling the subcellular localization of growth factor receptors, cells can modulate the activity of intracellular signal transduction pathways. During Caenorhabditis elegans vulval development, a ternary complex consisting of the LIN-7, LIN-2 and LIN-10 PDZ domain proteins localizes the epidermal growth factor receptor (EGFR) to the basolateral compartment of the vulval precursor cells (VPCs) to allow efficient receptor activation by the inductive EGF signal from the anchor cell. We have identified EGFR substrate protein-8 (EPS-8) as a novel component of the EGFR localization complex that links receptor trafficking to cell fate specification. EPS-8 expression is upregulated in the primary VPCs, where it creates a positive feedback loop in the EGFR/RAS/MAPK pathway. The membrane-associated guanylate kinase LIN-2 recruits EPS-8 into the receptor localization complex to retain the EGFR on the basolateral plasma membrane, and thus allow maximal receptor activation in the primary cell lineage. Low levels of EPS-8 in the neighboring secondary VPCs result in the rapid degradation of the EGFR, allowing these cells to adopt the secondary cell fate. Extracellular signals thus regulate EGFR trafficking in a cell type-specific manner to control pattern formation during organogenesis.

Intercellular coupling amplifies fate segregation during Caenorhabditis elegans vulval development

During vulval development in Caenorhabditis elegans, six precursor cells acquire a spatial pattern of distinct cell fates. This process is guided by a gradient in the soluble factor, LIN-3, and by direct interactions between neighboring cells mediated by the Notch-like receptor, LIN-12. Genetic evidence has revealed that these two extracellular signals are coupled: lateral cell-cell interactions inhibit LIN-3-mediated signaling, whereas LIN-3 regulates the extent of lateral signaling. To elucidate the quantitative implications of this coupled network topology for cell patterning during vulval development, we developed a mathematical model of LIN-3/LIN-12-mediated signaling in the vulval precursor cell array. Our analysis reveals that coupling LIN-3 and LIN-12 amplifies cellular perception of the LIN-3 gradient and polarizes lateral signaling, both of which enhance fate segregation beyond that achievable by an uncoupled system.

Transcriptional upregulation of the C. elegans Hox gene lin-39 during vulval cell fate specification

Extracellular signaling pathways and transcriptional regulatory networks function during development to specify metazoan cell fates. During Caenorhabditis elegans vulval development, the specification of three vulval precursor cells (VPCs) requires the activity of Wnt, Notch, and Ras signaling pathways, and function of the Hox gene lin-39. LIN-39 protein levels are regulated in the VPCs by both Wnt and Ras signaling. In particular, activation of Ras signaling leads to an increase in LIN-39 protein in P6.p at the time of VPC fate specification. We wish to understand the regulation of lin-39 by these pathways. We first show that LIN-39 is a target for MAP kinase in vitro, suggesting that the Ras-dependent LIN-39 upregulation could be mediated post-translationally. To test this idea, we created transcriptional and translational lin-39::GFP fusions that include the entire lin-39 genomic region, allowing observation of lin-39 expression in live animals. The reporters express GFP in most, if not all, sites of expression previously observed by LIN-39 antibody staining. We used these constructs to show that at the time of vulval induction both lin-39::GFP reporters are upregulated in P6.p, indicating that the accumulation of high levels of LIN-39 protein detected previously corresponds to transcriptional upregulation of lin-39 expression. This transcriptional upregulation of lin-39 is dependent on Ras signaling. We tested the requirement for several transcription factors acting downstream of Ras signaling in the VPCs, and found that P6.p upregulation requires the transcription factors LIN-1 and LIN-25, but appears to be independent of LIN-31, SEM-4, EOR-1 and EOR-2.Finally, we found that when the Wnt pathway is over activated, expression from the transcriptional lin-39::GFP increases, suggesting that the Wnt pathway also regulates lin-39 at the transcriptional level.

lin-1 has both positive and negative functions in specifying multiple cell fates induced by Ras/MAP kinase signaling in C. elegans

lin-1 encodes an ETS domain transcription factor that functions downstream of a Ras/MAP kinase pathway mediating induction of the 1 degrees cell fate during vulval development in the C. elegans hermaphrodite. Mutants lacking lin-1 activity display a phenotype similar to that caused by mutations that constitutively activate let-60 Ras consistent with a model in which lin-1 is a repressor of the 1 degree fate whose activity is inhibited by phosphorylation by MPK-1 MAP kinase. Here, we show that, contrary the current model, lin-1 is required positively for the proper expression of several genes regulated by the pathway in cells adopting the 1 degrees cell fate. We show that the positive requirement for lin-1 is downstream of let-60 Ras and mpk-1 MAP kinase, and that it has a focus in the vulval precursor cells themselves. lin-1 alleles encoding proteins lacking a docking site for MPK-1 MAP kinase are defective in the positive function. We also show that lin-1 apparently has both positive and negative functions during the specification of the fates of other cells in the worm requiring Ras/MAP kinase signaling.

LIN-12/Notch activation leads to microRNA-mediated down-regulation of Vav in C. elegans

Cell-cell interactions and cross-talk between signaling pathways specify Caenorhabditis elegans vulval precursor cells (VPCs) to adopt a spatial pattern: a central "1 degrees " VPC, in which epidermal growth factor receptor (EGFR)-mitogen-activated protein kinase (MAPK) activity is high and LIN-12/Notch activity is low, flanked by two "2 degrees " VPCs, in which LIN-12/Notch activity is high and EGFR-MAPK activity is low. Here, we identify a microRNA gene, mir-61, as a direct transcriptional target of LIN-12 and show that expression of mir-61 promotes the 2 degrees fate. We also identify vav-1, the ortholog of the Vav oncogene, as a target of mir-61, and show that down-regulation of VAV-1 promotes lin-12 activity in specifying the 2 degrees fate. Our results suggest that lin-12, mir-61, and vav-1 form a feedback loop that helps maximize lin-12 activity in the presumptive 2 degrees VPCs.

Uncover Genetic Interactions in Caenorhabditis elegans by RNA Interference

RNA-mediated interference (RNAi) has emerged recently as one of the most powerful functional genomics tools. RNAi has been particularly effective in the nematode worm C. elegans where RNAi has been used to analyse the loss-of-function phenotypes of almost all predicted genes. In this review, we illustrate how RNAi has been used to analyse gene function in C. elegans as well as pointing to some future directions for using RNAi to examine genetic interactions in a systematic manner.

Evidence for functional redundancy between C. elegans ADAM proteins SUP-17/Kuzbanian and ADM-4/TACE

The ectodomain of LIN-12/Notch proteins is cleaved and shed upon ligand binding. In Caenorhabditis elegans, genetic evidence has implicated SUP-17, the ortholog of Drosophila Kuzbanian and mammalian ADAM10, as the protease that mediates this event. In mammals, however, biochemical evidence has implicated TACE, a different ADAM protein. We have investigated potential functional redundancy of sup-17 and the C. elegans ortholog of TACE, adm-4, by exploring their roles in cell fate decisions mediated by lin-12/Notch genes. We found that reduced adm-4 activity, like reduced sup-17 activity, suppresses an allele of glp-1 that encodes a constitutively active receptor. Furthermore, concomitant reduction of adm-4 and sup-17 activity causes the production of two anchor cells in the hermaphrodite gonad, instead of one--a phenotype associated with loss of lin-12 activity. Concomitant reduction of both sup-17 and adm-4 activity in hermaphrodites results in highly penetrant synthetic sterility, which appears to reflect a defect in the spermatheca. Expression of a truncated form of LIN-12 that mimics the product of ectodomain shedding rescues this fertility defect, suggesting that sup-17 and adm-4 may mediate ectodomain shedding of LIN-12 and/or GLP-1. Our results are consistent with the possibility that sup-17 and adm-4 are functionally redundant for at least a subset of LIN-12/Notch-mediated decisions in C. elegans.

New positive regulators of lin-12 activity in Caenorhabditis elegans include the BRE-5/Brainiac glycosphingolipid biosynthesis enzyme

Screens for suppressors of lin-12 hypermorphic alleles in C. elegans have identified core components and modulators of the LIN-12/Notch signaling pathway. Here we describe the recovery of alleles of six new genes from a screen for suppressors of the egg-laying defect associated with elevated lin-12 activity. The molecular identification of one of the new suppressor genes revealed it as bre-5, which had previously been identified in screens for mutations that confer resistance to Bt toxin in C. elegans. bre-5 is the homolog of D. melanogaster brainiac. BRE-5/Brainiac catalyzes a step in the synthesis of glycosphingolipids, components of lipid rafts that are thought to act as platforms for association among certain kinds of membrane-bound proteins. Reducing the activity of several other genes involved in glycosphingolipid biosynthesis also suppresses the effects of constitutive lin-12 activity. Genetic analysis and cell ablation experiments suggest that bre-5 functions prior to ligand-induced ectodomain shedding that activates LIN-12 for signal transduction.

C. elegans PlexinA PLX-1 mediates a cell contact-dependent stop signal in vulval precursor cells

PLX-1 is a PlexinA transmembrane protein in Caenorhabditis elegans, and the transmembrane-type semaphorin, SMP-1, is a ligand for PLX-1. The SMP-1/PLX-1 system has been shown to be necessary for proper epidermal morphogenesis in the male tail and seam cells. Here, we show that the SMP-1/PLX-1 system also regulates vulval morphogenesis. In plx-1 and smp-1 mutants, hermaphrodites sometimes exhibit a protruding vulva or multiple vulva-like protrusions. Throughout the vulval development of plx-1 and smp-1 mutants, the arrangement of vulval cells is often disrupted. In the initial step of vulval morphogenesis, vulval precursor cells (VPCs) are generated normally but are subsequently arranged abnormally in mutants. Continuous observation revealed that plx-1 VPC fails to terminate longitudinal extension after making contact with neighbor VPCs. The arrangement defects of VPCs in plx-1 and smp-1 mutants are rescued by expressing the respective cDNA in VPCs. plx-1::egfp and smp-1::egfp transgenes are both expressed in all vulval cells, including VPCs, throughout vulval development. We propose that the SMP-1/PLX-1 system is responsible for a cell contact-mediated stop signal for VPC extension. Analyses using cell fate-specific markers showed that the arrangement defects of VPCs also affect cell fate specification and cell lineages, but in a relatively small fraction of plx-1 mutants.

The HMX homeodomain protein MLS-2 regulates cleavage orientation, cell proliferation and cell fate specification in the C. elegans postembryonic mesoderm

The proper formation of a complex multicellular organism requires the precise coordination of many cellular events, including cell proliferation, cell fate specification and differentiation. The C. elegans postembryonic mesodermal lineage, the M lineage, allows us to study mechanisms coordinating these events at single cell resolution. We have identified an HMX homeodomain protein MLS-2 in a screen for factors required for M lineage patterning. The MLS-2 protein is present in nuclei of undifferentiated cells in the early M lineage and in a subset of head neurons. In the M lineage, MLS-2 activity appears to be tightly regulated at the fourth round of cell division, coincident with the transition from proliferation to differentiation. A predicted null allele of mls-2, cc615, causes reduced cell proliferation in the M lineage, whereas a semi-dominant, gain-of-function allele, tm252, results in increased cell proliferation. Loss or overexpression of mls-2 also affects cleavage orientation and cell fate specification in the M lineage. We show that the increased cell proliferation in mls-2(tm252) mutants requires CYE-1, a G1 cell cycle regulator. Furthermore, the C. elegans Myod homolog HLH-1 acts downstream of mls-2 to specify M-derived coelomocyte cell fates. Thus MLS-2 functions in a cell type-specific manner to regulate both cell proliferation and cell fate specification.

The love-hate relationship between Ras and Notch

The Ras and Notch signaling pathways are used over and over again during development to control many different biological processes. Frequently, these two signaling pathways intersect to influence common processes, but sometimes they cooperate and sometimes they antagonize each other. The Caenorhabditis elegans vulva and the Drosophila eye are two classic paradigms for understanding how Ras and Notch affect cell fates, and how the two pathways work together to control biological pattern. Recent advances in these systems reveal some of the mechanisms by which Ras and Notch can interact. Similar types of interactions in mammals may be important for determining whether and how alterations in Ras or Notch lead to cancer.

lin-12 Notch functions in the adult nervous system of C. elegans

BACKGROUND

Notch signaling pathways are conserved across species and traditionally have been implicated in cell fate determination during embryonic development. Notch signaling components are also expressed postdevelopmentally in the brains of adult mice and Drosophila. Recent studies suggest that Notch signaling may play a role in the physiological, rather than developmental, regulation of neurons. Here, we investigate a new non-developmental role for Caenorhabditis elegans lin-12 Notch signaling in neurons regulating the spontaneous reversal rate during locomotion.

RESULTS

The spontaneous reversal rate of C. elegans during normal locomotion is constant. Both lin-12 gain and loss of function mutant animals had significantly increased reversal rates compared to wild type controls. These defects were caused by lin-12 activity, because the loss of function defect could be rescued by a wild type lin-12 transgene. Furthermore, overexpression of lin-12 recapitulated the gain-of-function defect. Increasing or decreasing lin-12 activity in the postdevelopmental adult animal was sufficient to rapidly and reversibly increase reversals, thereby excluding a developmental role for lin-12. Although lin-12 is expressed in the vulval and somatic gonad lineages, we find that these tissues play no role in regulating reversal rates. In contrast, altering lin-12 activity specifically in the nervous system was sufficient to increase reversals. These behavioral changes require components of the canonical lin-12 signaling cascade, including the ligand lag-2 and the transcriptional effector lag-1. Finally, the C. elegans AMPA/kainate glutamate receptor homolog glr-1 shows strong genetic interactions with lin-12, suggesting that glr-1 and/or other glutamate gated channels may be targets of lin-12 regulation.

CONCLUSION

Our results demonstrate a neuronal role for lin-12 Notch in C. elegans and suggest that lin-12 acutely regulates neuronal physiology to modulate animal behavior, without altering neuronal cell fate specification or neurite outgrowth. This is consistent with a role for Notch signaling in neurological disease with late onset symptoms.

Chromatin regulation and sumoylation in the inhibition of Ras-induced vulval development in Caenorhabditis elegans

In Caenorhabditis elegans, numerous 'synMuv' (synthetic multivulval) genes encode for chromatin-associated proteins involved in transcriptional repression, including an orthologue of Rb and components of the NuRD histone deacetylase complex. These genes antagonize Ras signalling to prevent erroneous adoption of vulval fate. To identify new components of this mechanism, we performed a genome-wide RNA interference (RNAi) screen. After RNAi of 16 757 genes, we found nine new synMuv genes. Based on predicted functions and genetic epistasis experiments, we propose that at least four post-translational modifications converge to inhibit Ras-stimulated vulval development: sumoylation, histone tail deacetylation, methylation, and acetylation. In addition, we demonstrate a novel role for sumoylation in inhibiting LIN-12/Notch signalling in the vulva. We further show that many of the synMuv genes are involved in gene regulation outside the vulva, negatively regulating the expression of the Delta homologue lag-2. As most of the genes identified in this screen are conserved in humans, we suggest that similar interactions may be relevant in mammals for control of Ras and Notch signalling, crosstalk between these pathways, and cell proliferation.

The C. elegans homolog of the mammalian tumor suppressor Dep-1/Scc1 inhibits EGFR signaling to regulate binary cell fate decisions

Protein phosphorylation by kinases and the subsequent dephosphorylation by phosphatases are key mechanisms that regulate intracellular signal transduction during development. Here, we report the identification of the receptor protein tyrosine phosphatase DEP-1 as a negative regulator of the Caenorhabditis elegans EGF receptor. DEP-1 amplifies in the developing vulva and the excretory system the small differences in the amount of EGF signal received by equivalent precursor cells to achieve binary cell fate decisions. During vulval development, DEP-1 inhibits EGFR signaling in the secondary cell lineage in parallel with the NOTCH-mediated lateral inhibition, while EGFR signaling simultaneously down-regulates DEP-1 and NOTCH expression in the primary cell lineage. This regulatory network of inhibitors results in the full activation of the EGFR/RAS/MAPK pathway in the primary vulval cells and at the same time keeps the EGFR/RAS/MAPK pathway inactive in the adjacent secondary cells. Mammalian Dep-1/Scc1 functions as a tumor-suppressor gene in the intestinal epithelium. Thus, mutations in human Dep-1 may promote tumor formation through a hyperactivation of the EGF receptor.

Notch signaling, brain development, and human disease

The Notch signaling pathway is central to a wide array of developmental processes in a number of organ systems, including hematopoiesis, somitogenesis, vasculogenesis, and neurogenesis. These processes involve maintenance of stem cell self-renewal, proliferation, specification of cell fate or differentiation, and apoptosis. Recent studies have led to the recognition of the role of the Notch pathway in early neurodevelopment, learning, and memory, as well as late-life neurodegeneration. This review summarizes what is currently known about the role of the Notch pathway in neural stem cells, gliogenesis, learning and memory, and neurologic disease.

Computational insights into Caenorhabditis elegans vulval development

Studies of Caenorhabditis elegans vulval development provide a paradigm for pattern formation during animal development. The fates of the six vulval precursor cells are specified by the combined action of an inductive signal that activates the EGF receptor mitogen-activated PK signaling pathway (specifying a primary fate) and a lateral signal mediated by LIN-12/Notch (specifying a secondary fate). Here we use methods devised for the engineering of complex reactive systems to model a biological system. We have chosen the visual formalism of statecharts and use it to formalize Sternberg and Horvitz's 1989 model [Sternberg, P. W. & Horvitz, H. R. (1989) Cell 58, 679-693], which forms the basis for our current understanding of the interaction between these two signaling pathways. The construction and execution of our model suggest that different levels of the inductive signal induce a temporally graded response of the EGF receptor mitogen-activated PK pathway and make explicit the importance of this temporal response. Our model also suggests the existence of an additional mechanism operating during lateral specification that prohibits neighboring vulval precursor cells from assuming the primary fate.

Effect of experimental varicocele on the expressions of notch 1, 2, and 3 in rat testes: An immunohistochemical study

OBJECTIVE

To study expressions of Notch receptor isoforms (Notch 1, 2, and 3) in normal and varicocele-induced rat testes to examine their possible functions in cell fate.

DESIGN

Comparative and controlled study.

SETTING

Animal Care and Operation Unit, Akdeniz University.

ANIMAL(S)

Wistar male rats for experimental and control groups.

INTERVENTION(S)

The control group underwent a sham operation (n = 6). The experimental groups underwent partial ligation of the renal vein to induce an experimental varicocele and then were killed 9 (n = 6), 11 (n = 6), and 13 (n = 6) weeks after the induction of varicocele.

MAIN OUTCOME MEASURE(S)

All tissues were fixed and routinely processed for paraffin embedding. Subsequent immunohistochemical studies were performed.

RESULT(S)

In the sham-operation rat testes, Leydig cells and elongated spermatids were immunopositive for Notch 1. Notch-2 expression was present in Leydig cells, spermatogonia, and primary spermatocytes. Notch-3 expression was limited to Leydig cells. Varicocele formation diminished the expression of both Notch-1 and Notch-2 receptors as the varicocele formation progressed over time.

CONCLUSION(S)

The present study suggests that Notch 1 is related to the maturation of spermatids. Notch 2 is related to both proliferation and maturation of spermatogenic cells, whereas Notch 3 seems to be related to Leydig cell functions. The decrease of both Notch-1 and Notch-2 expression depended on the degree of varicocele development over time, indicating a potential role in varicocele-associated testicular dysfunction.

C. elegans LIN-18 is a Ryk ortholog and functions in parallel to LIN-17/Frizzled in Wnt signaling

Wnt proteins are intercellular signals that regulate various aspects of animal development. In Caenorhabditis elegans, mutations in lin-17, a Frizzled-class Wnt receptor, and in lin-18 affect cell fate patterning in the P7.p vulval lineage. We found that lin-18 encodes a member of the Ryk/Derailed family of tyrosine kinase-related receptors, recently found to function as Wnt receptors. Members of this family have nonactive kinase domains. The LIN-18 kinase domain is dispensable for LIN-18 function, while the Wnt binding WIF domain is required. We also found that Wnt proteins LIN-44, MOM-2, and CWN-2 redundantly regulate P7.p patterning. Genetic interactions indicate that LIN-17 and LIN-18 function independently of each other in parallel pathways, and different ligands display different receptor specificities. Thus, two independent Wnt signaling pathways, one employing a Ryk receptor and the other a Frizzled receptor, function in parallel to regulate cell fate patterning in the C. elegans vulva.

EGF signal propagation during C. elegans vulval development mediated by ROM-1 rhomboid

During Caenorhabditis elegans vulval development, the anchor cell (AC) in the somatic gonad secretes an epidermal growth factor (EGF) to activate the EGF receptor (EGFR) signaling pathway in the adjacent vulval precursor cells (VPCs). The inductive AC signal specifies the vulval fates of the three proximal VPCs P5.p, P6.p, and P7.p. The C. elegans Rhomboid homolog ROM-1 increases the range of EGF, allowing the inductive signal to reach the distal VPCs P3.p, P4.p and P8.p, which are further away from the AC. Surprisingly, ROM-1 functions in the signal-receiving VPCs rather than the signal-sending AC. This observation led to the discovery of an AC-independent activity of EGF in the VPCs that promotes vulval cell fate specification and depends on ROM-1. Of the two previously reported EGF splice variants, the longer one requires ROM-1 for its activity, while the shorter form acts independently of ROM-1. We present a model in which ROM-1 relays the inductive AC signal from the proximal to the distal VPCs by allowing the secretion of the LIN-3L splice variant. These results indicate that, in spite of their structural diversity, Rhomboid proteins play a conserved role in activating EGFR signaling in C. elegans, Drosophila, and possibly also in mammals.

sel-7, a positive regulator of lin-12 activity, encodes a novel nuclear protein in Caenorhabditis elegans

Suppressor genetics in C. elegans has identified key components of the LIN-12/Notch signaling pathway. Here, we describe a genetic and molecular characterization of the suppressor gene sel-7. We show that reducing or eliminating sel-7 activity suppresses the effects of constitutive lin-12 activity, enhances the effects of partially reduced lin-12 activity, and causes a synthetic Lin-12(0) phenotype when combined with a null mutation in the sel-12 presenilin gene. These observations suggest that sel-7 is a positive regulator of lin-12 activity. We also show that SEL-7 encodes a novel nuclear protein. Through yeast two-hybrid screening, we identified an apparent interaction partner, K08E3.8, that also interacts with SEL-8, a known component of the nuclear complex that forms upon LIN-12 activation. Our data suggest potential roles for SEL-7 in the assembly or function of this nuclear complex.