The lin-12 locus specifies cell fates in Caenorhabditis elegans

Histone methylation restrains the expression of subtype-specific genes during terminal neuronal differentiation in Caenorhabditis elegans

Although epigenetic control of stem cell fate choice is well established, little is known about epigenetic regulation of terminal neuronal differentiation. We found that some differences among the subtypes of Caenorhabditis elegans VC neurons, particularly the expression of the transcription factor gene unc-4, require histone modification, most likely H3K9 methylation. An EGF signal from the vulva alleviated the epigenetic repression of unc-4 in vulval VC neurons but not the more distant nonvulval VC cells, which kept unc-4 silenced. Loss of the H3K9 methyltransferase MET-2 or H3K9me2/3 binding proteins HPL-2 and LIN-61 or a novel chromodomain protein CEC-3 caused ectopic unc-4 expression in all VC neurons. Downstream of the EGF signaling in vulval VC neurons, the transcription factor LIN-11 and histone demethylases removed the suppressive histone marks and derepressed unc-4. Behaviorally, expression of UNC-4 in all the VC neurons caused an imbalance in the egg-laying circuit. Thus, epigenetic mechanisms help establish subtype-specific gene expression, which are needed for optimal activity of a neural circuit.

Canonical RTK-Ras-ERK signaling and related alternative pathways

Receptor Tyrosine Kinase (RTK)-Ras-Extracellular signal-regulated kinase (ERK) signaling pathways control many aspects of C. elegans development and behavior. Studies in C. elegans helped elucidate the basic framework of the RTK-Ras-ERK pathway and continue to provide insights into its complex regulation, its biological roles, how it elicits cell-type appropriate responses, and how it interacts with other signaling pathways to do so. C. elegans studies have also revealed biological contexts in which alternative RTK- or Ras-dependent pathways are used instead of the canonical pathway.

A network model for the specification of vulval precursor cells and cell fusion control in Caenorhabditis elegans

The vulva of Caenorhabditis elegans has been long used as an experimental model of cell differentiation and organogenesis. While it is known that the signaling cascades of Wnt, Ras/MAPK, and NOTCH interact to form a molecular network, there is no consensus regarding its precise topology and dynamical properties. We inferred the molecular network, and developed a multivalued synchronous discrete dynamic model to study its behavior. The model reproduces the patterns of activation reported for the following types of cell: vulval precursor, first fate, second fate, second fate with reversed polarity, third fate, and fusion fate. We simulated the fusion of cells, the determination of the first, second, and third fates, as well as the transition from the second to the first fate. We also used the model to simulate all possible single loss- and gain-of-function mutants, as well as some relevant double and triple mutants. Importantly, we associated most of these simulated mutants to multivulva, vulvaless, egg-laying defective, or defective polarity phenotypes. The model shows that it is necessary for RAL-1 to activate NOTCH signaling, since the repression of LIN-45 by RAL-1 would not suffice for a proper second fate determination in an environment lacking DSL ligands. We also found that the model requires the complex formed by LAG-1, LIN-12, and SEL-8 to inhibit the transcription of eff-1 in second fate cells. Our model is the largest reconstruction to date of the molecular network controlling the specification of vulval precursor cells and cell fusion control in C. elegans. According to our model, the process of fate determination in the vulval precursor cells is reversible, at least until either the cells fuse with the ventral hypoderm or divide, and therefore the cell fates must be maintained by the presence of extracellular signals.

LIN-12/Notch signaling instructs postsynaptic muscle arm development by regulating UNC-40/DCC and MADD-2 in Caenorhabditis elegans

The diverse cell types and the precise synaptic connectivity between them are the cardinal features of the nervous system. Little is known about how cell fate diversification is linked to synaptic target choices. Here we investigate how presynaptic neurons select one type of muscles, vm2, as a synaptic target and form synapses on its dendritic spine-like muscle arms. We found that the Notch-Delta pathway was required to distinguish target from non-target muscles. APX-1/Delta acts in surrounding cells including the non-target vm1 to activate LIN-12/Notch in the target vm2. LIN-12 functions cell-autonomously to up-regulate the expression of UNC-40/DCC and MADD-2 in vm2, which in turn function together to promote muscle arm formation and guidance. Ectopic expression of UNC-40/DCC in non-target vm1 muscle is sufficient to induce muscle arm extension from these cells. Therefore, the LIN-12/Notch signaling specifies target selection by selectively up-regulating guidance molecules and forming muscle arms in target cells.

DOI:http://dx.doi.org/10.7554/eLife.00378.001

The development of the nervous system involves the formation of complex networks of connections between diverse cell types, such as motor neurons, interneurons and pyramidal cells. However, the mechanisms by which individual cells are programmed to acquire particular identities, and how they are instructed to form connections with other specific cells, remain unclear.

In many species, the Notch signaling pathway has a role in setting up these networks. Notch is a transmembrane protein, which means that it has one component inside the cell and another outside. When a ligand binds to the extracellular part of Notch, this causes the receptor to break in two. The intracellular domain then travels to the nucleus where it can influence gene expression.

The nematode worm (C. elegans), which has two Notch receptors, is often used to study the formation of neuronal networks because each worm has only around 300 neurons, and they are connected in roughly the same way in each worm. C. elegans relies on two types of cell that are very similar to each other—type-1 and type-2 vulval muscle cells—to lay eggs, and the neurons that trigger egg-laying form synaptic connections on specialized structures called muscle arms. However, these structures are found only in type-2 vulval muscle.

To investigate the mechanisms underlying the formation of the egg-laying circuit, Li et al. screened large numbers of mutant worms to find animals that lacked muscle arms. They identified a number of such mutants, which laid fewer eggs compared to wild-type worms, and found that they all had mutations in genes that encode for proteins or ligands that are involved in the LIN-12/Notch pathway. This pathway mediates cell–cell interactions that help to specify cell fates.

Li et al. showed that type-2 vulval muscle cells develop muscle arms when their neighbors—type-1 vulval muscle cells and vulval epithelial cells—produce enough ligand to activate the LIN-12 Notch receptor on the type-2 vulval muscle cells. They also identified two of the downstream targets of LIN-12, and found that artificially expressing one of these in type-1 vulval muscle cells is sufficient to trigger the formation of muscle arms.

The work of Li et al. provides further evidence that the Notch signalling pathway, which is well known for its role in early development, also acts at later developmental stages to determine cell fate and patterns of connectivity.

DOI:http://dx.doi.org/10.7554/eLife.00378.002

SEL-10/Fbw7-dependent negative feedback regulation of LIN-45/Braf signaling in C. elegans via a conserved phosphodegron

The conserved E3 ubiquitin ligase component named SEL-10 in Caenorhabditis elegans and Fbw7 in mammals targets substrates for ubiquitin-mediated degradation through a high-affinity binding site called a Cdc4 phosphodegron (CPD). As many known substrates of Fbw7 are oncoproteins, the identification of new substrates may offer insight into cancer biology as well as aspects of proteome regulation. Here, we evaluated whether the presence of an evolutionarily conserved CPD would be a feasible complement to proteomics-based approaches for identifying new potential substrates. For functional assessments, we focused on LIN-45, a component of the signal transduction pathway underlying vulval induction and the ortholog of human Braf, an effector of Ras in numerous cancers. Our analysis demonstrates that LIN-45 behaves as a bona fide substrate of SEL-10, with mutation of the CPD or loss of sel-10 resulting in increased activity and protein stability in vivo. Furthermore, during vulval induction, the downstream kinase MPK-1/ERK is also required for LIN-45 protein degradation in a negative feedback loop, resulting in degradation of LIN-45 where ERK is highly active. As the CPD consensus sequence is conserved in human Braf, we propose that Fbw7 may also regulate Braf stability in some cell contexts. We discuss the implications of our findings for vulval development in C. elegans, the potential applicability to human Braf, and the value of a CPD-based predictive approach for human Fbw7 substrates.

Deconvolving the roles of Wnt ligands and receptors in sensing and amplification

Establishment of cell polarity is crucial for many biological processes including cell migration and asymmetric cell division. The establishment of cell polarity consists of two sequential processes: an external gradient is first sensed and then the resulting signal is amplified and maintained by intracellular signaling networks usually using positive feedback regulation. Generally, these two processes are intertwined and it is challenging to determine which proteins contribute to the sensing or amplification process, particularly in multicellular organisms. Here, we integrated phenomenological modeling with quantitative single-cell measurements to separate the sensing and amplification components of Wnt ligands and receptors during establishment of polarity of the Caenorhabditis elegans P cells. By systematically exploring how P-cell polarity is altered in Wnt ligand and receptor mutants, we inferred that ligands predominantly affect the sensing process, whereas receptors are needed for both sensing and amplification. This integrated approach is generally applicable to other systems and will facilitate decoupling of the different layers of signal sensing and amplification.

Notch and the awesome power of genetics

Notch is a receptor that mediates cell-cell interactions in animal development, and aberrations in Notch signal transduction can cause cancer and other human diseases. Here, I describe the major advances in the Notch field from the identification of the first mutant in Drosophila almost a century ago through the elucidation of the unusual mechanism of signal transduction a little over a decade ago. As an essay for the GENETICS Perspectives series, it is my personal and critical commentary as well as an historical account of discovery.

Cellular reprogramming processes in Drosophila and C. elegans

The identity of individual cell types in a multicellular organism appears to be continuously maintained through active processes but is not irreversible. Changes in the identity of individual cell types can be brought about through ectopic mis-expression of regulatory factors, but in a number of cases also occurs in normal development. I will review here these natural cellular reprogramming processes occurring in the invertebrate model organisms Caenorhabditis elegans and Drosophila melanogaster. Furthermore, I will discuss the issue of why only certain cell types can be converted during induced reprogramming processes evoked by ectopic expression of regulatory factors and how recent work in model systems have shown that this cellular context-dependency can be manipulated.

Notch signaling inhibits axon regeneration

Many neurons have limited capacity to regenerate their axons after injury. Neurons in the mammalian central nervous system do not regenerate, and even neurons in the peripheral nervous system often fail to regenerate to their former targets. This failure is likely due in part to pathways that actively restrict regeneration; however, only a few factors that limit regeneration are known. Here, using single-neuron analysis of regeneration in vivo, we show that Notch/lin-12 signaling inhibits the regeneration of mature C. elegans neurons. Notch signaling suppresses regeneration by acting autonomously in the injured cell to prevent growth cone formation. The metalloprotease and gamma-secretase cleavage events that lead to Notch activation during development are also required for its activity in regeneration. Furthermore, blocking Notch activation immediately after injury improves regeneration. Our results define a postdevelopmental role for the Notch pathway as a repressor of axon regeneration in vivo.

Geometry, epistasis, and developmental patterning

Developmental signaling networks are composed of dozens of components whose interactions are very difficult to quantify in an embryo. Geometric reasoning enumerates a discrete hierarchy of phenotypic models with a few composite variables whose parameters may be defined by in vivo data. Vulval development in the nematode Caenorhabditis elegans is a classic model for the integration of two signaling pathways; induction by EGF and lateral signaling through Notch. Existing data for the relative probabilities of the three possible terminal cell types in diverse genetic backgrounds as well as timed ablation of the inductive signal favor one geometric model and suffice to fit most of its parameters. The model is fully dynamic and encompasses both signaling and commitment. It then predicts the correlated cell fate probabilities for a cross between any two backgrounds/conditions. The two signaling pathways are combined additively, without interactions, and epistasis only arises from the nonlinear dynamical flow in the landscape defined by the geometric model. In this way, the model quantitatively fits genetic experiments purporting to show mutual pathway repression. The model quantifies the contributions of extrinsic vs. intrinsic sources of noise in the penetrance of mutant phenotypes in signaling hypomorphs and explains available experiments with no additional parameters. Data for anchor cell ablation fix the parameters needed to define Notch autocrine signaling.

Somatic muscle specification during embryonic and post-embryonic development in the nematode C. elegans

Myogenesis has proved to be a powerful paradigm for understanding cell fate specification and differentiation in many model organisms. Studies of somatic bodywall muscle (BWM) development in Caenorhabditis elegans allow us to define, with single cell resolution, the distinct hierarchies of transcriptional regulators needed for myogenesis throughout development. Although all 95 BWM cells appear uniform after differentiation, there are several different regulatory cascades employed embryonically and post-embryonically. These, in turn, are integrated into multiple extrinsic cell signaling events. The convergence of these different pathways on the key nodal point, that is the activation of the core muscle module, commits individual cells to myogenesis. Comparisons of myogenesis between C. elegans and other model systems provide insights into the evolution of contractile cell types, demonstrating the conservation of regulatory schemes for muscles throughout the animal kingdom.

Hybrid Modeling of Cell Signaling and Transcriptional Reprogramming and Its Application in C. elegans Development

Modeling of signal driven transcriptional reprogramming is critical for understanding of organism development, human disease, and cell biology. Many current modeling techniques discount key features of the biological sub-systems when modeling multiscale, organism-level processes. We present a mechanistic hybrid model, GESSA, which integrates a novel pooled probabilistic Boolean network model of cell signaling and a stochastic simulation of transcription and translation responding to a diffusion model of extracellular signals. We apply the model to simulate the well studied cell fate decision process of the vulval precursor cells (VPCs) in C. elegans, using experimentally derived rate constants wherever possible and shared parameters to avoid overfitting. We demonstrate that GESSA recovers (1) the effects of varying scaffold protein concentration on signal strength, (2) amplification of signals in expression, (3) the relative external ligand concentration in a known geometry, and (4) feedback in biochemical networks. We demonstrate that setting model parameters based on wild-type and LIN-12 loss-of-function mutants in C. elegans leads to correct prediction of a wide variety of mutants including partial penetrance of phenotypes. Moreover, the model is relatively insensitive to parameters, retaining the wild-type phenotype for a wide range of cell signaling rate parameters.

Mindbomb 1, an E3 ubiquitin ligase, forms a complex with RYK to activate Wnt/β-catenin signaling

MIB1 ubiquitin ligase–mediated regulation of internalization of the Wnt receptor RYK is necessary for response to Wnt3a ligand in cell culture and C. elegans.

Receptor-like tyrosine kinase (RYK) functions as a transmembrane receptor for the Wnt family of secreted protein ligands. Although RYK undergoes endocytosis in response to Wnt, the mechanisms that regulate its internalization and concomitant activation of Wnt signaling are unknown. We discovered that RYK both physically and functionally interacts with the E3 ubiquitin ligase Mindbomb 1 (MIB1). Overexpression of MIB1 promotes the ubiquitination of RYK and reduces its steady-state levels at the plasma membrane. Moreover, we show that MIB1 is sufficient to activate Wnt/β-catenin (CTNNB1) signaling and that this activity depends on endogenous RYK. Conversely, in loss-of-function studies, both RYK and MIB1 are required for Wnt-3A–mediated activation of CTNNB1. Finally, we identify the Caenorhabditis elegans orthologue of MIB1 and demonstrate a genetic interaction between ceMIB and lin-18/RYK in vulva development. These findings provide insights into the mechanisms of Wnt/RYK signaling and point to novel targets for the modulation of Wnt signaling.

Notch and Ras promote sequential steps of excretory tube development in C. elegans

Receptor tyrosine kinases and Notch are crucial for tube formation and branching morphogenesis in many systems, but the specific cellular processes that require signaling are poorly understood. Here we describe sequential roles for Notch and Epidermal growth factor (EGF)-Ras-ERK signaling in the development of epithelial tube cells in the C. elegans excretory (renal-like) organ. This simple organ consists of three tandemly connected unicellular tubes: the excretory canal cell, duct and G1 pore. lin-12 and glp-1/Notch are required to generate the canal cell, which is a source of LIN-3/EGF ligand and physically attaches to the duct during de novo epithelialization and tubulogenesis. Canal cell asymmetry and let-60/Ras signaling influence which of two equivalent precursors will attach to the canal cell. Ras then specifies duct identity, inducing auto-fusion and a permanent epithelial character; the remaining precursor becomes the G1 pore, which eventually loses epithelial character and withdraws from the organ to become a neuroblast. Ras continues to promote subsequent aspects of duct morphogenesis and differentiation, and acts primarily through Raf-ERK and the transcriptional effectors LIN-1/Ets and EOR-1. These results reveal multiple genetically separable roles for Ras signaling in tube development, as well as similarities to Ras-mediated control of branching morphogenesis in more complex organs, including the mammalian kidney. The relative simplicity of the excretory system makes it an attractive model for addressing basic questions about how cells gain or lose epithelial character and organize into tubular networks.

Quantitative variation in autocrine signaling and pathway crosstalk in the Caenorhabditis vulval network

BACKGROUND

Biological networks experience quantitative change in response to environmental and evolutionary variation. Computational modeling allows exploration of network parameter space corresponding to such variations. The intercellular signaling network underlying Caenorhabditis vulval development specifies three fates in a row of six precursor cells, yielding a quasi-invariant 3°3°2°1°2°3° cell fate pattern. Two seemingly conflicting verbal models of vulval precursor cell fate specification have been proposed: sequential induction by the EGF-MAP kinase and Notch pathways, or morphogen-based induction by the former.

RESULTS

To study the mechanistic and evolutionary system properties of this network, we combine experimental studies with computational modeling, using a model that keeps the network architecture constant but varies parameters. We first show that the Delta autocrine loop can play an essential role in 2° fate specification. With this autocrine loop, the same network topology can be quantitatively tuned to use in the six-cell-row morphogen-based or sequential patterning mechanisms, which may act singly, cooperatively, or redundantly. Moreover, different quantitative tunings of this same network can explain vulval patterning observed experimentally in C. elegans, C. briggsae, C. remanei, and C. brenneri. We experimentally validate model predictions, such as interspecific differences in isolated vulval precursor cell behavior and in spatial regulation of Notch activity.

CONCLUSIONS

Our study illustrates how quantitative variation in the same network comprises developmental patterning modes that were previously considered qualitatively distinct and also accounts for evolution among closely related species.

The C. elegans SoxC protein SEM-2 opposes differentiation factors to promote a proliferative blast cell fate in the postembryonic mesoderm

The proper development of multicellular organisms requires precise regulation and coordination of cell fate specification, cell proliferation and differentiation. Abnormal regulation and coordination of these processes could lead to disease, including cancer. We have examined the function of the sole C. elegans SoxC protein, SEM-2, in the M lineage, which produces the postembryonic mesoderm. We found that SEM-2/SoxC is both necessary and sufficient to promote a proliferating blast cell fate, the sex myoblast fate, over a differentiated striated bodywall muscle fate. A number of factors control the specific expression of sem-2 in the sex myoblast precursors and their descendants. This includes direct control of sem-2 expression by a Hox-PBC complex. The crucial nature of the HOX/PBC factors in directly enhancing expression of this proliferative factor in the C. elegans M lineage suggests a possible more general link between Hox-PBC factors and SoxC proteins in regulating cell proliferation.

Ras effector switching promotes divergent cell fates in C. elegans vulval patterning

The C. elegans vulva is patterned by epidermal growth factor (EGF) activation of Ras to control 1° fate, and 1° fate induces antagonistic Notch-dependent 2° fate. Furthermore, a spatial EGF gradient, in addition to inducing 1° fate, directly contributes to 2° fate via an unknown pathway. We find that in addition to its canonical effector, Raf, vulval Ras utilizes an exchange factor for the Ral small GTPase (RalGEF), such that Ras-RalGEF-Ral antagonizes Ras-Raf pro-1° fate activity. Consistent with its restricted expression pattern, Ral participates in EGF pro-2° activity. Thus, we have delineated a Ras effector-switching mechanism whereby position within the morphogen gradient dictates that Ras effector usage is switched to RalGEF from Raf to promote 2° instead of 1° fate. Our observations define the utility of Ras effector switching during normal development and may provide a possible mechanistic basis for cell and cancer-type differences in effector dependency and activation.

Dissection of genetic pathways in C. elegans

With unique genetic and cell biological strengths, C. elegans has emerged as a powerful model system for studying many biological processes. These processes are typically regulated by complex genetic networks consisting of genes. Identifying those genes and organizing them into genetic pathways are two major steps toward understanding the mechanisms that regulate biological events. Forward genetic screens with various designs are a traditional approach for identifying candidate genes. The completion of the genome sequencing in C. elegans and the advent of high-throughput experimental techniques have led to the development of two additional powerful approaches: functional genomics and systems biology. Genes that are discovered by these approaches can be ordered into interacting pathways through a variety of strategies, involving genetics, cell biology, biochemistry, and functional genomics, to gain a more complete understanding of how gene regulatory networks control a particular biological process. The aim of this review is to provide an overview of the approaches available to identify and construct the genetic pathways using C. elegans.

Stochastic mechanisms of cell fate specification that yield random or robust outcomes

Although cell fate specification is tightly controlled to yield highly reproducible results and avoid extreme variation, developmental programs often incorporate stochastic mechanisms to diversify cell types. Stochastic specification phenomena are observed in a wide range of species and an assorted set of developmental contexts. In bacteria, stochastic mechanisms are utilized to generate transient subpopulations capable of surviving adverse environmental conditions. In vertebrate, insect, and worm nervous systems, stochastic fate choices are used to increase the repertoire of sensory and motor neuron subtypes. Random fate choices are also integrated into developmental programs controlling organogenesis. Although stochastic decisions can be maintained to produce a mosaic of fates within a population of cells, they can also be compensated for or directed to yield robust and reproducible outcomes.

LIN-14 inhibition of LIN-12 contributes to precision and timing of C. elegans vulval fate patterning

Studies of C. elegans vulval development have illuminated mechanisms underlying cell fate specification and elucidated intercellular signaling pathways [1]. The vulval precursor cells (VPCs) are spatially patterned during the L3 stage by the EGFR-Ras-MAPK-mediated inductive signal and the LIN-12/Notch-mediated lateral signal. The pattern is both precise and robust [2] because of crosstalk between these pathways [3]. Signaling is also regulated temporally, because constitutive activation of the spatial patterning pathways does not alter the timing of VPC fate specification [4, 5]. The heterochronic genes, including the microRNA lin-4 and its target lin-14, constitute a temporal control mechanism used in different contexts [6-8]. We find that lin-4 specifically controls the activity of LIN-12/Notch through lin-14, but not other known targets, and that persistent lin-14 blocks LIN-12 activity without interfering with the key events of LIN-12/Notch signal transduction. In the L2 stage, there is sufficient lin-14 activity to inhibit constitutive lin-12. Our results suggest that lin-4 and lin-14 contribute to spatial patterning through temporal gating of LIN-12. We propose that in the L2 stage, lin-14 sets a high threshold for LIN-12 activation to help prevent premature activation of LIN-12 by ligands expressed in other cells in the vicinity, thereby contributing to the precision and robustness of VPC fate patterning.

Re-programming of C. elegans male epidermal precursor fates by Wnt, Hox, and LIN-12/Notch activities

In Caenorhabditiselegans males, different subsets of ventral epidermal precursor (Pn.p) cells adopt distinct fates in a position-specific manner: three posterior cells, P(9-11).p, comprise the hook sensillum competence group (HCG) with three potential fates (1 degrees , 2 degrees , or 3 degrees ), while eight anterior cells, P(1-8).p, fuse with the hyp7 epidermal syncytium. Here we show that activation of the canonical BAR-1 beta-catenin pathway of Wnt signaling alters the competence of P(3-8).p and specifies ectopic HCG-like fates. This fate transformation requires the Hox gene mab-5. In addition, misexpression of mab-5 in P(1-8).p is sufficient to establish HCG competence among these cells, as well as to generate ectopic HCG fates in combination with LIN-12 or EGF signaling. While increased Wnt signaling induces predominantly 1 degrees HCG fates, increased LIN-12 or EGF signaling in combination with MAB-5 overexpression promotes 2 degrees HCG fates in anterior Pn.p cells, suggesting distinctive functions of Wnt, LIN-12, and EGF signaling in specification of HCG fates. Lastly, wild-type mab-5 function is necessary for normal P(9-11).p fate specification, indicating that regulation of ectopic HCG fate formation revealed in anterior Pn.p cells reflect mechanisms of pattern formation during normal hook development.

Caenorhabditis elegans as a model for stem cell biology

We review the application of Caenorhabditis elegans as a model system to understand key aspects of stem cell biology. The only bona fide stem cells in C. elegans are those of the germline, which serves as a valuable paradigm for understanding how stem-cell niches influence maintenance and differentiation of stem cells and how somatic differentiation is repressed during germline development. Somatic cells that share stem cell-like characteristics also provide insights into principles in stem-cell biology. The epidermal seam cell lineages lend clues to conserved mechanisms of self-renewal and expansion divisions. Principles of developmental plasticity and reprogramming relevant to stem-cell biology arise from studies of natural transdifferentiation and from analysis of early embryonic progenitors, which undergo a dramatic transition from a pluripotent, reprogrammable condition to a state of committed differentiation. The relevance of these developmental processes to our understanding of stem-cell biology in other organisms is discussed.

sel-11 and cdc-42, two negative modulators of LIN-12/Notch activity in C. elegans

Background

LIN-12/Notch signaling is important for cell-cell interactions during development, and mutations resulting in constitutive LIN-12/Notch signaling can cause cancer. Loss of negative regulators of lin-12/Notch activity has the potential for influencing cell fate decisions during development and the genesis or aggressiveness of cancer.

Methodology/Principal Findings

We describe two negative modulators of lin-12 activity in C. elegans. One gene, sel-11, was initially defined as a suppressor of a lin-12 hypomorphic allele; the other gene, cdc-42, is a well-studied Rho GTPase. Here, we show that SEL-11 corresponds to yeast Hrd1p and mammalian Synoviolin. We also show that cdc-42 has the genetic properties consistent with negative regulation of lin-12 activity during vulval precursor cell fate specification.

Conclusions/Significance

Our results underscore the multiplicity of negative regulatory mechanisms that impact on lin-12/Notch activity and suggest novel mechanisms by which constitutive lin-12/Notch activity might be exacerbated in cancer.

Conserved mechanism of Wnt signaling function in the specification of vulval precursor fates in C. elegans and C. briggsae

The C. elegans hermaphrodite vulva serves as a paradigm for understanding how signaling pathways control organ formation. Previous studies have shown that Wnt signaling plays important roles in vulval development. To understand the function and evolution of Wnt signaling in Caenorhabditis nematodes we focused on C. briggsae, a species that is substantially divergent from C. elegans in terms of the evolutionary time scale yet shares almost identical morphology. We isolated mutants in C. briggsae that display multiple pseudo-vulvae resulting from ectopic VPC induction. We cloned one of these loci and found that it encodes an Axin homolog, Cbr-PRY-1. Our genetic studies revealed that Cbr-pry-1 functions upstream of the canonical Wnt pathway components Cbr-bar-1 (beta-catenin) and Cbr-pop-1(tcf/lef) as well as the Hox target Cbr-lin-39 (Dfd/Scr). We further characterized the pry-1 vulval phenotype in C. briggsae and C. elegans using 8 cell fate markers, cell ablation, and genetic interaction approaches. Our results show that ectopically induced VPCs in pry-1 mutants adopt 2° fates independently of the gonad-derived inductive and LIN-12/Notch-mediated lateral signaling pathways. We also found that Cbr-pry-1 mutants frequently show a failure of P7.p induction. A similar, albeit low penetrant, defect is also observed in C. elegans pry-1 mutants. The genetic analysis of the P7.p induction defect revealed that it was caused by altered regulation of lin-12 and its transcriptional target lip-1 (MAP kinase phosphatase). Thus, our results provide evidence for LIN-12/Notch-dependent and independent roles of Wnt signaling in promoting 2 degrees VPC fates in both nematode species.

Role of glycans and glycosyltransferases in the regulation of Notch signaling

The evolutionarily conserved Notch signaling pathway plays broad and important roles during embryonic development and in adult tissue homeostasis. Unlike most other pathways used during animal development, Notch signaling does not rely on second messengers and intracellular signaling cascades. Instead, pathway activation results in the cleavage of the Notch intracellular domain and its translocation into the nucleus, where it functions as a transcriptional co-activator of the Notch target genes. To ensure tight spatial and temporal regulation of a pathway with such an unusually direct signaling transduction, animal cells have devised a variety of specialized modulatory mechanisms. One such mechanism takes advantage of decorating the Notch extracellular domain with rare types of O-linked glycans. In this review, we will discuss the genetic and biochemical data supporting the notion that carbohydrate modification is essential for Notch signaling and attempt to provide a brief historical overview of how we have learned what we know about the glycobiology of Notch. We will also summarize what is known about the contribution of specific nucleotide-sugar transporters to Notch biology and the roles-enzymatic and non-enzymatic-played by specific glycosyltransferases in the regulation of this pathway. Mutations in the Notch pathway components cause a variety of human diseases, and manipulation of Notch signaling is emerging as a powerful tool in regenerative medicine. Therefore, studying how sugar modification modulates Notch signaling provides a framework for better understanding the role of glycosylation in animal development and might offer new tools to manipulate Notch signaling for therapeutic purposes.

The FoxF/FoxC factor LET-381 directly regulates both cell fate specification and cell differentiation in C. elegans mesoderm development

Forkhead transcription factors play crucial and diverse roles in mesoderm development. In particular, FoxF and FoxC genes are, respectively, involved in the development of visceral/splanchnic mesoderm and non-visceral mesoderm in coelomate animals. Here, we show at single-cell resolution that, in the pseudocoelomate nematode C. elegans, the single FoxF/FoxC transcription factor LET-381 functions in a feed-forward mechanism in the specification and differentiation of the non-muscle mesodermal cells, the coelomocytes (CCs). LET-381/FoxF directly activates the CC specification factor, the Six2 homeodomain protein CEH-34, and functions cooperatively with CEH-34/Six2 to directly activate genes required for CC differentiation. Our results unify a diverse set of studies on the functions of FoxF/FoxC factors and provide a model for how FoxF/FoxC factors function during mesoderm development.

Bias and evolution of the mutationally accessible phenotypic space in a developmental system

Genetic and developmental architecture may bias the mutationally available phenotypic spectrum. Although such asymmetries in the introduction of variation may influence possible evolutionary trajectories, we lack quantitative characterization of biases in mutationally inducible phenotypic variation, their genotype-dependence, and their underlying molecular and developmental causes. Here we quantify the mutationally accessible phenotypic spectrum of the vulval developmental system using mutation accumulation (MA) lines derived from four wild isolates of the nematodes Caenorhabditis elegans and C. briggsae. The results confirm that on average, spontaneous mutations degrade developmental precision, with MA lines showing a low, yet consistently increased, proportion of developmental defects and variants. This result indicates strong purifying selection acting to maintain an invariant vulval phenotype. Both developmental system and genotype significantly bias the spectrum of mutationally inducible phenotypic variants. First, irrespective of genotype, there is a developmental bias, such that certain phenotypic variants are commonly induced by MA, while others are very rarely or never induced. Second, we found that both the degree and spectrum of mutationally accessible phenotypic variation are genotype-dependent. Overall, C. briggsae MA lines exhibited a two-fold higher decline in precision than the C. elegans MA lines. Moreover, the propensity to generate specific developmental variants depended on the genetic background. We show that such genotype-specific developmental biases are likely due to cryptic quantitative variation in activities of underlying molecular cascades. This analysis allowed us to identify the mutationally most sensitive elements of the vulval developmental system, which may indicate axes of potential evolutionary variation. Consistent with this scenario, we found that evolutionary trends in the vulval system concern the phenotypic characters that are most easily affected by mutation. This study provides an empirical assessment of developmental bias and the evolution of mutationally accessible phenotypes and supports the notion that such bias may influence the directions of evolutionary change.

A conserved tetraspanin subfamily promotes Notch signaling in Caenorhabditis elegans and in human cells

The cytosolic domain of Notch is a membrane-tethered transcription factor. Ligand binding ultimately leads to gamma-secretase cleavage within the transmembrane domain, allowing the intracellular domain to translocate to the nucleus and activate target gene transcription. Constitutive Notch signaling has been associated with human cancers such as T cell acute lymphoblastic leukemia (T-ALL). As tetraspanins have been implicated in many different signaling processes, we assessed their potential contribution to Notch signaling. We used a genetic assay in Caenorhabditis elegans to identify TSP-12 as a positive factor for Notch activity in several cellular contexts. Then, using a cell culture system, we showed that two human TSP-12 orthologs, TSPAN33 and TSPAN5, promote Notch activity and are likely to act at the gamma-secretase cleavage step. We also acquired evidence for functional redundancy among tetraspanins in both C. elegans and human cells. Selective inhibition of tetraspanins may constitute an anti-NOTCH therapeutic approach to reduce gamma-secretase activity.

Understanding the molecular basis of Alzheimer's disease using a Caenorhabditis elegans model system

Alzheimer's disease (AD) is the major cause of dementia in the United States. At the cellular level, the brains of AD patients are characterized by extracellular dense plaques and intracellular neurofibrillary tangles whose major components are the beta-amyloid peptide and tau, respectively. The beta-amyloid peptide is a cleavage product of the amyloid precursor protein (APP); mutations in APP have been correlated with a small number of cases of familial Alzheimer's disease. APP is the canonical member of the APP family, whose functions remain unclear. The nematode Caenorhabditis elegans, one of the premier genetic workhorses, is being used in a variety of ways to address the functions of APP and determine how the beta-amyloid peptide and tau can induce toxicity. First, the function of the C. elegans APP-related gene, apl-1, is being examined. Although different organisms may use APP and related proteins, such as APL-1, in different functional contexts, the pathways in which they function and the molecules with which they interact are usually conserved. Second, components of the gamma-secretase complex and their respective functions are being revealed through genetic analyses in C. elegans. Third, to address questions of toxicity, onset of degeneration, and protective mechanisms, different human beta-amyloid peptide and tau variants are being introduced into C. elegans and the resultant transgenic lines examined. Here, we summarize how a simple system such as C. elegans can be used as a model to understand APP function and suppression of beta-amyloid peptide and tau toxicity in higher organisms.

Methylation and demethylation activities of a C. elegans MLL-like complex attenuate RAS signalling

The conserved Mixed Lineage Leukaemia (MLL) complex deposits activating methyl marks on histone tails through a methyltransferase (MT) activity. Here we provide in vivo evidence that in addition to methylation, the C. elegans MLL-like complex can remove specific methyl marks linked to repression of transcription. This supports the proposed model in which the MLL complex orchestrates both the deposition and the removal of methyl marks to activate transcription. We have uncovered the MLL-like complex in a large-scale RNAi screen designed to identify attenuators of RAS signalling during vulval development. We have also found that the histone acetyltransferase complex, NuA4/TIP60, cooperates with the C. elegans MLL-like complex in the attenuation of RAS signalling. Critically, we show that both complexes regulate a common novel target and attenuator of RAS signalling, AJM-1 (Apical Junction Molecule-1). Therefore, the C. elegans MLL-like complex cooperates with the NuA4/TIP60 complex to regulate the expression of a novel effector, AJM-1.

The roles of EGF and Wnt signaling during patterning of the C. elegans Bgamma/delta Equivalence Group

BACKGROUND

During development, different signaling pathways interact to specify cell fate by regulating transcription factors necessary for fate specification and morphogenesis. In Caenorhabditis elegans, the EGF-Ras and Wnt signaling pathways have been shown to interact to specify cell fate in three equivalence groups: the vulval precursor cells (VPCs), the hook competence group (HCG) and P11/12. In the VPCs, HCG and P11/12 pair, EGF and Wnt signaling positively regulate different Hox genes, each of which also functions during fate specification. In the male, EGF-Ras signaling is required to specify the Bgamma fate within the Bgamma/delta equivalence pair, while Notch signaling is required for Bdelta fate specification. In addition, TGF-beta signaling by dbl-1/dpp controls ceh-13/labial/Hox1 expression in Bgamma.

RESULTS

We show that EGF-Ras signaling is required for Bgamma expression of ceh-13/labial/Hox1. The transcription factors lin-1/ETS and lin-31/Forkhead, functioning downstream of the EGF pathway, as well as sur-2/MED23 (a component of the Mediator complex) also control ceh-13 expression in Bgamma. In addition, our results indicate that lin-44/Wnt, mom-2/Wnt and lin-17/Fz are necessary to maintain the division of Bgamma along a longitudinal axis. We also show that dbl-1/dpp acts either in parallel or downstream of EGF pathway to regulate ceh-13/Hox1 expression in Bgamma. Lastly, we find that a dbl-1/dpp null mutation did not cause any vulval or P12 defects and did not enhance vulval and P12 defects of reduction-of-function mutations of components of the EGF pathway.

CONCLUSIONS

ceh-13/labial/Hox1 expression in Bgamma is regulated by the EGF pathway and downstream factors lin-1/ETS lin-31/Forkhead and sur-2/MED23. Wnt signaling is required for proper Bgamma division, perhaps to orient the Bgamma mitotic spindle. Our results suggest that dbl-1/dpp is not required for VPC and P12 specification, highlighting another difference among these EGF-dependent equivalence groups.

hunchback and Ikaros-like zinc finger genes control reproductive system development in Caenorhabditis elegans

Here we provide evidence for a C2H2 zinc finger gene family with similarity to Ikaros and hunchback. The founding member of this family is Caenorhabditis elegans ehn-3, which has important and poorly understood functions in somatic gonad development. We examined the expression and function of four additional hunchback/Ikaros-like (HIL) genes in C. elegans reproductive system development. Two genes, ehn-3 and R08E3.4, are expressed in somatic gonadal precursors (SGPs) and have overlapping functions in their development. In ehn-3; R08E3.4 double mutants, we find defects in the generation of distal tip cells, anchor cells, and spermatheca; three of the five tissues derived from the SGPs. We provide in vivo evidence that C. elegans HIL proteins have functionally distinct zinc finger domains, with specificity residing in the N-terminal set of four zinc fingers and a likely protein-protein interaction domain provided by the C-terminal pair of zinc fingers. In addition, we find that a chimeric human Ikaros protein containing the N-terminal zinc fingers of EHN-3 functions in C. elegans. Together, these results lend support to the idea that the C. elegans HIL genes and Ikaros have similar functional domains. We propose that hunchback, Ikaros, and the HIL genes arose from a common ancestor that was present prior to the divergence of protostomes and deuterostomes.

Stochastic variation: from single cells to superorganisms

Observed phenotype often fails to correspond with genotype. Although it is well established that uncontrolled genetic modifier effects and environmental variability can affect phenotype, stochastic variation in gene expression can also contribute to phenotypic differences. Here we examine recent work that has provided insights into how fundamental physical properties of living cells, and the probabilistic nature of the chemical reactions that underlie gene expression, introduce noise. We focus on instances in which a stochastic decision initiates an event in the development of a multicellular organism and how that decision can be subsequently fixed. We present an example indicating that a similar interplay between an initial stochastic decision and subsequent fixation may underlie the regulation of reproduction in social insects. We argue, therefore, that stochasticity affects biological processes from the single-gene scale through to the complex organization of an ant colony, and represents a largely neglected component of phenotypic variation and evolution.

The cyclin-dependent kinase inhibitors, cki-1 and cki-2, act in overlapping but distinct pathways to control cell cycle quiescence during C. elegans development

Cyclin-dependent kinase inhibitors (CKIs) are major contributors to the decision to enter or exit the cell cycle. The Caenorhabditis elegans genome encodes two CKIs belonging to the Cip/Kip family, cki-1 and cki-2. cki-1 has been shown to act as a canonical negative regulator of cell cycle entry, while the role of cki-2 remains unclear. We identified cki-2 in a genome-wide RNAi screen to reveal genes essential for developmental cell cycle quiescence. Examination of cki-2 knockout animals revealed extra rounds of cell divisions, verifying a role in establishing or maintaining the temporary cell cycle arrest. Despite the overlapping defects, the pathways mediated by cki-1 and cki-2 are discrete since the extra cell phenotype conferred by a putative cki-2(null) mutation is enhanced upon additional loss of cki-1 activity. Moreover, the extra cell division defect of cki-2 is not increased with the additional loss of lin-35 Rb, as is seen with cki-1. Thus, both cki-1 and cki-2 mediate cell cycle quiescence, but our genetic and phenotypic analyses demonstrate that they act within distinct pathways to exert control over the cell cycle machinery.

Notch signaling: the core pathway and its posttranslational regulation

Notch signaling controls numerous cell-fate specification events in multicellular organisms, and dysregulated Notch signaling causes several diseases with underlying developmental defects. A key step in Notch receptor activation is its intramembrane proteolysis, which releases an intracellular fragment that participates directly in transcriptional regulation of nuclear target genes. Despite the apparent simplicity of this mechanism, a host of posttranslational processes regulate Notch activity during its synthesis and secretion, ligand-dependent activation at the surface, endocytic trafficking, and degradation. This review describes the core developmental logic of Notch signaling and how regulatory mechanisms tailor Notch pathway outputs to specific developmental scenarios.

A conserved Six-Eya cassette acts downstream of Wnt signaling to direct non-myogenic versus myogenic fates in the C. elegans postembryonic mesoderm

The subdivision of mesodermal cells into muscle and non-muscle cells is crucial to animal development. In the C. elegans postembryonic mesoderm, this subdivision is a result of an asymmetric cell division that leads to the formation of striated body wall muscles and non-muscle coelomocytes. Here we report that the Six homeodomain protein CEH-34 and its cofactor Eyes Absent, EYA-1, function synergistically to promote the non-muscle fate in cells also competent to form muscles. We further show that the asymmetric expression of ceh-34 and eya-1 is regulated by a combination of 1) mesodermal intrinsic factors MAB-5, HLH-1 and FOZI-1, 2) differential POP-1 (TCF/LEF) transcriptional activity along the anterior-posterior axis, and 3) coelomocyte competence factor(s). These factors are conserved in both vertebrates and invertebrates, suggesting a conserved paradigm for mesoderm development in metazoans.

C. elegans HLH-2/E/Daughterless controls key regulatory cells during gonadogenesis

The Caenorhabditis elegans distal tip cell (DTC) provides a niche for germline stem cells in both hermaphrodites and males. The hermaphrodite distal tip cell (hDTC) also provides "leader" function to control gonadal elongation and shape, while in males, leader function is allocated to the linker cell (LC). Therefore, the male distal tip cell (mDTC) serves as a niche but not as a leader. The C. elegans homolog of E/Daughterless, HLH-2, was previously implicated in hDTC specification. Here we report that HLH-2 is also critical for hDTC maintenance, hDTC niche function and hDTC expression of a lag-2/DSL ligand reporter. We also find that HLH-2 functions in males to direct linker cell specification and to promote both mDTC maintenance and the mDTC niche function. We conclude that HLH-2 functions in both sexes to promote leader cell specification and DTC niche function.

Predicting phenotypic diversity and the underlying quantitative molecular transitions

During development, signaling networks control the formation of multicellular patterns. To what extent quantitative fluctuations in these complex networks may affect multicellular phenotype remains unclear. Here, we describe a computational approach to predict and analyze the phenotypic diversity that is accessible to a developmental signaling network. Applying this framework to vulval development in C. elegans, we demonstrate that quantitative changes in the regulatory network can render ∼500 multicellular phenotypes. This phenotypic capacity is an order-of-magnitude below the theoretical upper limit for this system but yet is large enough to demonstrate that the system is not restricted to a select few outcomes. Using metrics to gauge the robustness of these phenotypes to parameter perturbations, we identify a select subset of novel phenotypes that are the most promising for experimental validation. In addition, our model calculations provide a layout of these phenotypes in network parameter space. Analyzing this landscape of multicellular phenotypes yielded two significant insights. First, we show that experimentally well-established mutant phenotypes may be rendered using non-canonical network perturbations. Second, we show that the predicted multicellular patterns include not only those observed in C. elegans, but also those occurring exclusively in other species of the Caenorhabditis genus. This result demonstrates that quantitative diversification of a common regulatory network is indeed demonstrably sufficient to generate the phenotypic differences observed across three major species within the Caenorhabditis genus. Using our computational framework, we systematically identify the quantitative changes that may have occurred in the regulatory network during the evolution of these species. Our model predictions show that significant phenotypic diversity may be sampled through quantitative variations in the regulatory network without overhauling the core network architecture. Furthermore, by comparing the predicted landscape of phenotypes to multicellular patterns that have been experimentally observed across multiple species, we systematically trace the quantitative regulatory changes that may have occurred during the evolution of the Caenorhabditis genus.

The diversity of metazoan life forms that we experience today arose as multicellular systems continually sampled new phenotypes that withstood ever changing selective pressures. This phenotypic diversification is driven by variations in the underlying regulatory network that instructs cells to form multicellular patterns and structures. Here, we computationally construct the phenotypic diversity that may be accessible through quantitative tuning of the regulatory network that drives multicellular patterning during C. elegans vulval development. We show that significant phenotypic diversity may be sampled through quantitative variations without overhauling the core regulatory network architecture. Furthermore, by comparing the predicted landscape of phenotypes to multicellular patterns that have been experimentally observed across multiple species, we systematically deduce the quantitative molecular changes that may have transpired during the evolution of the Caenorhabditis genus.

TRA-1/GLI controls the expression of the Hox gene lin-39 during C. elegans vulval development

The vulva of the Caenorhabditis elegans hermaphrodite develops from a subset of six vulval precursor cells (VPCs) by the combined effect of the Ras, Wingless and Notch signaling cascades, and of three redundant synMuv (synthetic Multivulva) pathways grouped into classes A, B and C. Here we show that signaling via the GLI- (Glioma-associated protein) like transcription factor TRA-1, which is the terminal regulator of the C. elegans sex determination cascade, is a newly discovered pathway specifying vulval cell fates. We found that TRA-1 accumulates in, and regulates the fusion process of, cells (including the VPCs and hypodermal cells) involved in vulval patterning. TRA-1 also influenced the expression of the Hox gene lin-39, a central regulator of vulval development. Furthermore, inactivation of tra-1, which transforms animals with hermaphrodite-specific karyotype into males, promoted vulval induction in synMuv A, but not in synMuv B, mutant background. This implies that TRA-1 interacts with the class B synMuv genes, many of which are involved in chromatin-mediated transcriptional repression of cell proliferation. These results may help to understand how compromised GLI activity in humans leads to cancer. Together, we suggest that the GLI protein family involved in several key developmental processes in both invertebrates and vertebrates regulates somatic cell fates through influencing, at least in part, the expression of specific Hox genes.

Plasticity and errors of a robust developmental system in different environments

Many developmental processes generate invariant phenotypes in a wide range of ecological conditions. Such robustness to environmental variation is a fundamental biological property, yet its extent, limits, and adaptive significance have rarely been assessed empirically. Here we tested how environmental variation affects vulval formation in Caenorhabditis nematodes. In different environments, a correct vulval pattern develops with high precision, but rare deviant patterns reveal the system's limits and how its mechanisms respond to environmental challenges. Key features of the apparent robustness are functional redundancy among vulval precursor cells and tolerance to quantitative variation in Ras, Notch, and Wnt pathway activities. The observed environmental responses and precision of vulval patterning vary within and between Caenorhabditis species. These results highlight the complex response of developmental systems to the environment and illustrate how a robust and invariant phenotype may result through cellular and molecular processes that are highly plastic--across environments and evolution.

A mutation in teg-4, which encodes a protein homologous to the SAP130 pre-mRNA splicing factor, disrupts the balance between proliferation and differentiation in the C. elegans germ line

Dividing stem cells can give rise to two types of daughter cells; self-renewing cells that have virtually the same properties as the parent cell, and differentiating cells that will eventually form part of a tissue. The Caenorhabditis elegans germ line serves as a model to study how the balance between these two types of daughter cells is maintained. A mutation in teg-4 causes over-proliferation of the stem cells, thereby disrupting the balance between proliferation and differentiation. We have cloned teg-4 and found it to encode a protein homologous to the highly conserved splicing factor subunit 3 of SF3b. Our allele of teg-4 partially reduces TEG-4 function. In an effort to determine how teg-4 functions in controlling stem cell proliferation, we have performed genetic epistasis analysis with known factors controlling stem cell proliferation. We found that teg-4 is synthetic tumorous with genes in both major redundant genetic pathways that function downstream of GLP-1/Notch signaling to control the balance between proliferation and differentiation. Therefore, teg-4 is unlikely to function specifically in either of these two genetic pathways. Further, the synthetic tumorous phenotype seen with one of the genes from these pathways is epistatic to glp-1, indicating that teg-4 functions downstream of glp-1, likely as a positive regulator of meiotic entry. We propose that a reduction in teg-4 activity reduces the splicing efficiency of targets involved in controlling the balance between proliferation and differentiation. This results in a shift in the balance towards proliferation, eventually forming a germline tumor.

Wnt and EGF pathways act together to induce C. elegans male hook development

Comparative studies of vulva development between Caenorhabditis elegans and other nematode species have provided some insight into the evolution of patterning networks. However, molecular genetic details are available only in C. elegans and Pristionchus pacificus. To extend our knowledge on the evolution of patterning networks, we studied the C. elegans male hook competence group (HCG), an equivalence group that has similar developmental origins to the vulval precursor cells (VPCs), which generate the vulva in the hermaphrodite. Similar to VPC fate specification, each HCG cell adopts one of three fates (1 degree, 2 degrees, 3 degrees), and 2 degrees HCG fate specification is mediated by LIN-12/Notch. We show that 2 degrees HCG specification depends on the presence of a cell with the 1 degree fate. We also provide evidence that Wnt signaling via the Frizzled-like Wnt receptor LIN-17 acts to specify the 1 degree and 2 degrees HCG fate. A requirement for EGF signaling during 1 degree fate specification is seen only when LIN-17 activity is compromised. In addition, activation of the EGF pathway decreases dependence on LIN-17 and causes ectopic hook development. Our results suggest that WNT plays a more significant role than EGF signaling in specifying HCG fates, whereas in VPC specification EGF signaling is the major inductive signal. Nonetheless, the overall logic is similar in the VPCs and the HCG: EGF and/or WNT induce a 1 degree lineage, and LIN-12/NOTCH induces a 2 degrees lineage. Wnt signaling is also required for execution of the 1 degree and 2 degrees HCG lineages. lin-17 and bar-1/beta-catenin are preferentially expressed in the presumptive 1 degree cell P11.p. The dynamic subcellular localization of BAR-1-GFP in P11.p is concordant with the timing of HCG fate determination.

OSM-11 facilitates LIN-12 Notch signaling during Caenorhabditis elegans vulval development

Notch signaling is critical for cell fate decisions during development. Caenorhabditis elegans and vertebrate Notch ligands are more diverse than classical Drosophila Notch ligands, suggesting possible functional complexities. Here, we describe a developmental role in Notch signaling for OSM-11, which has been previously implicated in defecation and osmotic resistance in C. elegans. We find that complete loss of OSM-11 causes defects in vulval precursor cell (VPC) fate specification during vulval development consistent with decreased Notch signaling. OSM-11 is a secreted, diffusible protein that, like previously described C. elegans Delta, Serrate, and LAG-2 (DSL) ligands, can interact with the lineage defective-12 (LIN-12) Notch receptor extracellular domain. Additionally, OSM-11 and similar C. elegans proteins share a common motif with Notch ligands from other species in a sequence defined here as the Delta and OSM-11 (DOS) motif. osm-11 loss-of-function defects in vulval development are exacerbated by loss of other DOS-motif genes or by loss of the Notch ligand DSL-1, suggesting that DOS-motif and DSL proteins act together to activate Notch signaling in vivo. The mammalian DOS-motif protein Deltalike1 (DLK1) can substitute for OSM-11 in C. elegans development, suggesting that DOS-motif function is conserved across species. We hypothesize that C. elegans OSM-11 and homologous proteins act as coactivators for Notch receptors, allowing precise regulation of Notch receptor signaling in developmental programs in both vertebrates and invertebrates.