An inductive interaction in 4-cell stage C. elegans embryos involves APX-1 expression in the signalling cell

Notch signaling without the APH-2/nicastrin subunit of gamma secretase in Caenorhabditis elegans germline stem cells

The final step in Notch signaling activation is the transmembrane cleavage of Notch receptor by γ secretase. Thus far, genetic and biochemical evidence indicates that four subunits are essential for γ secretase activity in vivo: presenilin (the catalytic core), APH-1, PEN-2, and APH-2/nicastrin. Although some γ secretase activity has been detected in APH-2/nicastrin-deficient mammalian cell lines, the lack of biological relevance for this activity has left the quaternary γ secretase model unchallenged. Here, we provide the first example of in vivo Notch signal transduction without APH-2/nicastrin. The surprising dispensability of APH-2/nicastrin is observed in Caenorhabditis elegans germline stem cells (GSCs) and contrasts with its essential role in previously described C. elegans Notch signaling events. Depletion of GLP-1/Notch, presenilin, APH-1, or PEN-2 causes a striking loss of GSCs. In contrast, aph-2/nicastrin mutants maintain GSCs and exhibit robust and localized expression of the downstream Notch target sygl-1. Interestingly, APH-2/nicastrin is normally expressed in GSCs and becomes essential under conditions of compromised Notch function. Further insight is provided by reconstituting the C. elegans γ secretase complex in yeast, where we find that APH-2/nicastrin increases but is not essential for γ secretase activity. Together, our results are most consistent with a revised model of γ secretase in which the APH-2/nicastrin subunit has a modulatory, rather than obligatory role. We propose that a trimeric presenilin-APH-1-PEN-2 γ secretase complex can provide a low level of γ secretase activity, and that cellular context determines whether or not APH-2/nicastrin is essential for effective Notch signal transduction.

Mechanisms of lineage specification in Caenorhabditis elegans

The studies of cell fate and lineage specification are fundamental to our understanding of the development of multicellular organisms. Caenorhabditis elegans has been one of the premiere systems for studying cell fate specification mechanisms at single cell resolution, due to its transparent nature, the invariant cell lineage, and fixed number of somatic cells. We discuss the general themes and regulatory mechanisms that have emerged from these studies, with a focus on somatic lineages and cell fates. We next review the key factors and pathways that regulate the specification of discrete cells and lineages during embryogenesis and postembryonic development; we focus on transcription factors and include numerous lineage diagrams that depict the expression of key factors that specify embryonic founder cells and postembryonic blast cells, and the diverse somatic cell fates they generate. We end by discussing some future perspectives in cell and lineage specification.

Defect-buffering cellular plasticity increases robustness of metazoan embryogenesis

Developmental processes are intrinsically robust so as to preserve a normal-like state in response to genetic and environmental fluctuations. However, the robustness and potential phenotypic plasticity of individual developing cells under genetic perturbations remain to be systematically evaluated. Using large-scale gene perturbation, live imaging, lineage tracing, and single-cell phenomics, we quantified the phenotypic landscape of C. elegans embryogenesis in >2,000 embryos following individual knockdown of over 750 conserved genes. We observed that cellular genetic systems are not sufficiently robust to single-gene perturbations across all cells; rather, gene knockdowns frequently induced cellular defects. Dynamic phenotypic analyses revealed many cellular defects to be transient, with cells exhibiting phenotypic plasticity that serves to alleviate, correct, and accommodate the defects. Moreover, potential developmentally related cell modules may buffer the phenotypic effects of individual cell position changes. Our findings reveal non-negligible contributions of cellular plasticity and multicellularity as compensatory strategies to increase developmental robustness.

The extra-embryonic space and the local contour are crucial geometric constraints regulating cell arrangement

In multicellular systems, cells communicate with adjacent cells to determine their positions and fates, an arrangement important for cellular development. Orientation of cell division, cell-cell interactions (i.e. attraction and repulsion) and geometric constraints are three major factors that define cell arrangement. In particular, geometric constraints are difficult to reveal in experiments, and the contribution of the local contour of the boundary has remained elusive. In this study, we developed a multicellular morphology model based on the phase-field method so that precise geometric constraints can be incorporated. Our application of the model to nematode embryos predicted that the amount of extra-embryonic space, the empty space within the eggshell that is not occupied by embryonic cells, affects cell arrangement in a manner dependent on the local contour and other factors. The prediction was validated experimentally by increasing the extra-embryonic space in the Caenorhabditis elegans embryo. Overall, our analyses characterized the roles of geometrical contributors, specifically the amount of extra-embryonic space and the local contour, on cell arrangements. These factors should be considered for multicellular systems.

Summary: The local contour and the extra-embryonic space, the empty space within the eggshell not occupied by embryonic cells, are important geometric constraints in cell arrangement of nematode embryos.

The Notch signaling network in muscle stem cells during development, homeostasis, and disease

Skeletal muscle stem cells have a central role in muscle growth and regeneration. They reside as quiescent cells in resting muscle and in response to damage they transiently amplify and fuse to produce new myofibers or self-renew to replenish the stem cell pool. A signaling pathway that is critical in the regulation of all these processes is Notch. Despite the major differences in the anatomical and cellular niches between the embryonic myotome, the adult sarcolemma/basement-membrane interphase, and the regenerating muscle, Notch signaling has evolved to support the context-specific requirements of the muscle cells. In this review, we discuss the diverse ways by which Notch signaling factors and other modifying partners are operating during the lifetime of muscle stem cells to establish an adaptive dynamic network.

FLIRT: fast local infrared thermogenetics for subcellular control of protein function

FLIRT (fast local infrared thermogenetics) is a microscopy-based technology to locally and reversibly manipulate protein function while simultaneously monitoring the effects in vivo. FLIRT locally inactivates fast-acting temperature-sensitive mutant proteins. We demonstrate that FLIRT can control temperature-sensitive proteins required for cell division, Delta-Notch cell fate signaling, and germline structure in Caenorhabditis elegans with cell-specific and even subcellular precision.

From "the Worm" to "the Worms" and Back Again: The Evolutionary Developmental Biology of Nematodes

Since the earliest days of research on nematodes, scientists have noted the developmental and morphological variation that exists within and between species. As various cellular and developmental processes were revealed through intense focus on Caenorhabditis elegans, these comparative studies have expanded. Within the genus Caenorhabditis, they include characterization of intraspecific polymorphisms and comparisons of distinct species, all generally amenable to the same laboratory culture methods and supported by robust genomic and experimental tools. The C. elegans paradigm has also motivated studies with more distantly related nematodes and animals. Combined with improved phylogenies, this work has led to important insights about the evolution of nematode development. First, while many aspects of C. elegans development are representative of Caenorhabditis, and of terrestrial nematodes more generally, others vary in ways both obvious and cryptic. Second, the system has revealed several clear examples of developmental flexibility in achieving a particular trait. This includes developmental system drift, in which the developmental control of homologous traits has diverged in different lineages, and cases of convergent evolution. Overall, the wealth of information and experimental techniques developed in C. elegans is being leveraged to make nematodes a powerful system for evolutionary cellular and developmental biology.

Cell-intrinsic and -extrinsic mechanisms promote cell-type-specific cytokinetic diversity

Cytokinesis, the physical division of one cell into two, is powered by constriction of an actomyosin contractile ring. It has long been assumed that all animal cells divide by a similar molecular mechanism, but growing evidence suggests that cytokinetic regulation in individual cell types has more variation than previously realized. In the four-cell Caenorhabditis elegans embryo, each blastomere has a distinct cell fate, specified by conserved pathways. Using fast-acting temperature-sensitive mutants and acute drug treatment, we identified cell-type-specific variation in the cytokinetic requirement for a robust formin^CYK-1-dependent filamentous-actin (F-actin) cytoskeleton. In one cell (P2), this cytokinetic variation is cell-intrinsically regulated, whereas in another cell (EMS) this variation is cell-extrinsically regulated, dependent on both Src^SRC-1 signaling and direct contact with its neighbor cell, P2. Thus, both cell-intrinsic and -extrinsic mechanisms control cytokinetic variation in individual cell types and can protect against division failure when the contractile ring is weakened.

The successful division of one cell into two is essential for all organisms to live, grow and reproduce. For an animal cell, the nucleus – the compartment containing the genetic material – must divide before the surrounding material. The rest of the cell, called the cytoplasm, physically separates later in a process known as cytokinesis.

Cytokinesis in animal cells is driven by the formation of a ring in the middle of the dividing cell. The ring is composed of myosin motor proteins and filaments made of a protein called actin. The movements of the motor proteins along the filaments cause the ring to contract and tighten. This pulls the cell membrane inward and physically pinches the cell into two. For a long time, the mechanism of cytokinesis was assumed to be same across different types of animal cell, but later evidence suggested otherwise. For example, in liver, heat and bone cells, cytokinesis naturally fails during development to create cells with two or more nuclei. If a similar ‘failure’ happened in other cell types, it could lead to diseases such as cancers or blood disorders. This raised the question: what are the molecular mechanisms that allow cytokinesis to happen differently in different cell types?

Davies et al. investigated this question using embryos of the worm Caenorhabditis elegans at a stage in their development when they consist of just four cells. The proteins forming the contractile ring in this worm are the same as those in humans. However, in the worm, the contractile ring can easily be damaged using chemical inhibitors or by mutating the genes that encode its proteins.

Davies et al. show that when the contractile ring was damaged, two of the four cells in the worm embryo still divided successfully. This result indicates the existence of new mechanisms to divide the cytoplasm that allow division even with a weak contractile ring. In a further experiment, the embryos were dissected to isolate each of the four cells. Davies et al. saw that one of the two dividing cells could still divide on its own, while the other cell could not. This shows that this new method of cytokinesis is regulated both by factors inherent to the dividing cell and by external signals from other cells. Moreover, one of these extrinsic signals was found to be a signaling protein that had previously been implicated in human cancers.

Future work will determine if these variations in cytokinesis between the different cell types found in the worm apply to humans too; and, more importantly from a therapeutic standpoint, if these new mechanisms exist in human cancers.

Establishment of Signaling Interactions with Cellular Resolution for Every Cell Cycle of Embryogenesis

Intercellular signaling interactions play a key role in breaking fate symmetry during animal development. Identification of signaling interactions at cellular resolution is technically challenging, especially in a developing embryo. Here, we develop a platform that allows automated inference and validation of signaling interactions for every cell cycle of Caenorhabditis elegans embryogenesis. This is achieved by the generation of a systems-level cell contact map, which consists of 1114 highly confident intercellular contacts, by modeling analysis and is validated through cell membrane labeling coupled with cell lineage analysis. We apply the map to identify cell pairs between which a Notch signaling interaction takes place. By generating expression patterns for two ligands and two receptors of the Notch signaling pathway with cellular resolution using the automated expression profiling technique, we are able to refine existing and identify novel Notch interactions during C. elegans embryogenesis. Targeted cell ablation followed by cell lineage analysis demonstrates the roles of signaling interactions during cell division in breaking fate symmetry. Finally, we describe the development of a website that allows online access to the cell-cell contact map for mapping of other signaling interactions by the community. The platform can be adapted to establish cellular interactions from any other signaling pathway.

Reciprocal signaling by Wnt and Notch specifies a muscle precursor in the C. elegans embryo

The MS blastomere produces one-third of the body wall muscles (BWMs) in the C. elegans embryo. MS-derived BWMs require two distinct cell-cell interactions, the first inhibitory and the second, two cell cycles later, required to overcome this inhibition. The inductive interaction is not required if the inhibitory signal is absent. Although the Notch receptor GLP-1 was implicated in both interactions, the molecular nature of the two signals was unknown. We now show that zygotically expressed MOM-2 (Wnt) is responsible for both interactions. Both the inhibitory and the activating interactions require precise spatiotemporal expression of zygotic MOM-2, which is dependent upon two distinct Notch signals. In a Notch mutant defective only in the inductive interaction, MS-derived BWMs can be restored by preventing zygotic MOM-2 expression, which removes the inhibitory signal. Our results suggest that the inhibitory interaction ensures the differential lineage specification of MS and its sister blastomere, whereas the inductive interaction promotes the expression of muscle-specifying genes by modulating TCF and β-catenin levels. These results highlight the complexity of cell fate specification by cell-cell interactions in a rapidly dividing embryo.

Combinatorial decoding of the invariant C. elegans embryonic lineage in space and time

Understanding how a single cell, the zygote, can divide and differentiate to produce the diverse animal cell types is a central goal of developmental biology research. The model organism Caenorhabditis elegans provides a system that enables a truly comprehensive understanding of this process across all cells. Its invariant cell lineage makes it possible to identify all of the cells in each individual and compare them across organisms. Recently developed methods automate the process of cell identification, allowing high-throughput gene expression characterization and phenotyping at single cell resolution. In this Review, we summarize the sequences of events that pattern the lineage including establishment of founder cell identity, the signaling pathways that diversify embryonic fate, and the regulators involved in patterning within these founder lineages before cells adopt their terminal fates. We focus on insights that have emerged from automated approaches to lineage tracking, including insights into mechanisms of robustness, context-specific regulation of gene expression, and temporal coordination of differentiation. We suggest a model by which lineage history produces a combinatorial code of transcription factors that act, often redundantly, to ensure terminal fate.

Systematic quantification of developmental phenotypes at single-cell resolution during embryogenesis

Current imaging technology provides an experimental platform in which complex developmental processes can be observed at cellular resolution over an extended time frame. New computational tools are essential to achieve a systems-level understanding of this high-content information. We have devised a structured approach to systematically analyze complex in vivo phenotypes at cellular resolution, which divides the task into a panel of statistical measurements of each cell in terms of cell differentiation, proliferation and morphogenesis, followed by their spatial and temporal organization in groups and the cohesion within the whole specimen. We demonstrate the approach to C. elegans embryogenesis with in toto imaging and automated cell lineage tracing. We define statistical distributions of the wild-type developmental behaviors at single-cell resolution based on over 50 embryos, cumulating in over 4000 distinct, developmentally based measurements per embryo. These methods enable statistical quantification of abnormalities in mutant or RNAi-treated embryos and a rigorous comparison of embryos by testing each measurement for the probability that it would occur in a wild-type embryo. We demonstrate the power of this structured approach by uncovering quantitative properties including subtle phenotypes in both wild-type and perturbed embryos, transient behaviors that lead to new insights into gene function and a previously undetected source of developmental noise and its subsequent correction.

POS-1 and GLD-1 repress glp-1 translation through a conserved binding-site cluster

RNA-binding proteins (RBPs) coordinate cell fate specification and differentiation in a variety of systems. RNA regulation is critical during oocyte development and early embryogenesis, in which RBPs control expression from maternal mRNAs encoding key cell fate determinants. The Caenorhabditis elegans Notch homologue glp-1 coordinates germline progenitor cell proliferation and anterior fate specification in embryos. A network of sequence-specific RBPs is required to pattern GLP-1 translation. Here, we map the cis-regulatory elements that guide glp-1 regulation by the CCCH-type tandem zinc finger protein POS-1 and the STAR-domain protein GLD-1. Our results demonstrate that both proteins recognize the glp-1 3' untranslated region (UTR) through adjacent, overlapping binding sites and that POS-1 binding excludes GLD-1 binding. Both factors are required to repress glp-1 translation in the embryo, suggesting that they function in parallel regulatory pathways. It is intriguing that two equivalent POS-1-binding sites are present in the glp-1 3' UTR, but only one, which overlaps with a translational derepression element, is functional in vivo. We propose that POS-1 regulates glp-1 mRNA translation by blocking access of other RBPs to a key regulatory sequence.

Functional roles of Notch signaling in the cnidarian Nematostella vectensis

Notch signaling is among the oldest of known Metazoan signaling pathways and is used in a multitude of developmental contexts to effect cellular differentiation, specification and the maintenance of stem cell state. Here we report the isolation and expression of the canonical Notch signaling pathway in the early branching metazoan Nematostella vectensis (Anthozoa, Cnidaria) during embryonic and larval development. We have used pharmacological treatment, morpholino knockdown, and dominant negative misexpression experiments to demonstrate that Notch signaling acts to mediate cnidogenesis, the development of cnidarian-specific neural effecter cells. Notch signaling often results in the transcriptional activation of NvHes genes, a conserved family of bHLH transcription factors. A loss of Notch signaling through use of pharmacological inhibition or knock-down of the Notch effecter gene Suppressor of Hairless Su(H) similarly results in a loss of cnidocyte cell fate. We also provide evidence that Notch signaling is responsible for certain aspects of neurogenesis in developing N. vectensis planula in which disruption of Notch cleavage via the pharmacological agent DAPT results in increased expression of neural marker genes in vivo. This data suggests that Notch signaling acting on components of the developing nervous system is an ancient role of this pathway. The shared requirement of Notch signaling for the development of both cnidocytes and neurons further supports the hypothesis that cnidocytes and neurons share common origins as multifunctional sensory-effecter cells.

Cell fate specification in the C. elegans embryo

Cell specification requires that particular subsets of cells adopt unique expression patterns that ultimately define the fates of their descendants. In C. elegans, cell fate specification involves the combinatorial action of multiple signals that produce activation of a small number of "blastomere specification" factors. These initiate expression of gene regulatory networks that drive development forward, leading to activation of "tissue specification" factors. In this review, the C. elegans embryo is considered as a model system for studies of cell specification. The techniques used to study cell fate in this species, and the themes that have emerged, are described.

A "latent niche" mechanism for tumor initiation

Stem cells, their niches, and their relationship to cancer are under intense investigation. Because tumors and metastases acquire self-renewing capacity, mechanisms for their establishment may involve cell-cell interactions similar to those between stem cells and stem cell niches. On the basis of our studies in Caenorhabditis elegans, we introduce the concept of a "latent niche" as a differentiated cell type that does not normally contact stem cells nor act as a niche but that can, under certain conditions, promote the ectopic self-renewal, proliferation, or survival of competent cells that it inappropriately contacts. Here, we show that ectopic germ-line stem cell proliferation in C. elegans is driven by a latent niche mechanism and that the molecular basis for this mechanism is inappropriate Notch activation. Furthermore, we show that continuous Notch signaling is required to maintain ectopic germ-line proliferation. We highlight the latent niche concept by distinguishing it from a normal stem cell niche, a premetastatic niche and an ectopic niche. One of the important distinguishing features of this mechanism for tumor initiation is that it could operate in the absence of genetic changes to the tumor cell or the tumor-promoting cell. We propose that a latent niche mechanism may underlie tumorigenesis and metastasis in humans.

Early embryonic programming of neuronal left/right asymmetry in C. elegans

BACKGROUND

Nervous systems are largely bilaterally symmetric on a morphological level but often display striking degrees of functional left/right (L/R) asymmetry. How L/R asymmetric functional features are superimposed onto an essentially bilaterally symmetric structure and how nervous-system laterality relates to the L/R asymmetry of internal organs are poorly understood. We address these questions here by using the establishment of L/R asymmetry in the ASE chemosensory neurons of C. elegans as a paradigm. This bilaterally symmetric neuron pair is functionally lateralized in that it senses a distinct class of chemosensory cues and expresses a putative chemoreceptor family in a L/R asymmetric manner.

RESULTS

We show that the directionality of the asymmetry of the two postmitotic ASE neurons ASE left (ASEL) and ASE right (ASER) in adults is dependent on a L-/R-symmetry-breaking event at a very early embryonic stage, the six-cell stage, which also establishes the L/R asymmetric placement of internal organs. However, the L/R asymmetry of the ASE neurons per se is dependent on an even earlier anterior-posterior (A/P) Notch signal that specifies embryonic ABa/ABp blastomere identities at the four-cell stage. This Notch signal, which functions through two T box genes, acts genetically upstream of a miRNA-controlled bistable feedback loop that regulates the L/R asymmetric gene-expression program in the postmitotic ASE cells.

CONCLUSIONS

Our results link adult neuronal laterality to the generation of the A/P axis at the two-cell stage and raise the possibility that neural asymmetries observed across the animal kingdom are similarly established by very early embryonic interactions.

Genetic Networks in the Early Development of Caenorhabditis elegans

One of the best-studied model organisms in biology is Caenorhabditis elegans. Because of its simple architecture and other biological advantages, considerable data have been collected about the regulation of its development. In this review, currently available data concerning the early phase of embryonic development are presented in the form of genetic networks. We performed computer simulations of regulatory mechanisms in embryonic development, and the results are described and compared with experimental observations.

Chromatin regulation during C. elegans germline development

Recent studies in Caenorhabditis elegans implicate PcG- and NuRD-like chromatin regulators in the establishment and maintenance of germline-soma distinctions. Somatic cells appear to utilize NuRD-related nucleosome-remodeling factors to overwrite germline-specific chromatin states that are specified through PcG-like activities. The germline, in turn, may rely on an asymmetrically inherited inhibitor to prevent chromatin reorganization that would otherwise erase pluripotency.

HcSTK, a Caenorhabditis elegans PAR-1 homologue from the parasitic nematode, Haemonchus contortus

A putative serine/threonine protein kinase (HcSTK) from the parasitic nematode Haemonchus contortus was characterised at the mRNA and amino acid levels. HcSTK displays a high level of identity (85-93% in the catalytic domain) with proteins of the PAR-1/MARK serine/threonine protein kinase (STK) subfamily, which represent signal transduction molecules involved in establishing and maintaining polarity in proliferating and differentiating cells. The transcript of hcstk is expressed in different developmental stages (second-, third-, fourth-stage larvae and adults) and various organs (muscle, intestine and reproductive) of H. contortus. In addition, there are several isoforms which appear to relate to a single gene. The expression profile of hcstk is similar to that of Caenorhabditis elegans PAR-1, and the level of sequence identity among members of the PAR-1/MARK STK subfamily, representing a range of species of vertebrates (e.g. humans and rodents), invertebrates (e.g. insects and C. elegans) and yeast, suggests that HcSTK may be involved in a conserved signal transduction pathway.

APH-1 is a multipass membrane protein essential for the Notch signaling pathway in Caenorhabditis elegans embryos

Early embryonic cells in Caenorhabditis elegans embryos interact through a signaling pathway closely related to the Notch signaling pathway in Drosophila and vertebrates. Components of this pathway include a ligand, receptor, the presenilin proteins, and a novel protein, APH-2, that is related to the Nicastrin protein in humans. Here we identify the aph-1 gene as a new component of the Notch pathway in Caenorhabditis elegans. aph-1 is predicted to encode a novel, highly conserved multipass membrane protein. We show that aph-1 and the presenilin genes share a similar function in that they are both required for proper cell-surface localization of APH-2/Nicastrin.

RNA and sex determination in Caenorhabditis elegans. Post-transcriptional regulation of the sex-determining tra-2 and fem-3 mRNAs in the Caenorhabditis elegans hermaphrodite

The Caenorhabditis elegans hermaphrodite sequentially produces sperm and oocytes from a single pool of precursors. Therefore, the hermaphrodite's germ line is the site of two major cell fate decisions: a germ cell precursor first undergoes a mitosis/meiosis decision and then a sperm/oocyte decision. While the mitosis/meiosis decision is governed by Notch/GLP-1 signalling, the sperm/oocyte decision relies on post-transcriptional regulation of two key mRNAs, tra-2 and fem-3. This review focuses on factors that are required for the silencing of these mRNAs, which results in the sequential production of sperm and oocytes. Most factors that regulate the expression of tra-2 and fem-3 are homologous to proteins involved in RNA regulation in yeast, mammals or Drosophila, suggesting that at least some of the molecular mechanisms regulating the two worm mRNAs have been conserved throughout evolution.

Identification of in vivo mRNA targets of GLD-1, a maxi-KH motif containing protein required for C. elegans germ cell development

Caenorhabditis elegans GLD-1, a KH motif containing RNA-binding protein of the GSG/STAR subfamily, controls diverse aspects of germ line development, suggesting that it may have multiple mRNA targets. We used an immunoprecipitation/subtractive hybridization/cloning strategy to identify 15 mRNAs that are putative targets of GLD-1 binding and regulation. For one target, the rme-2 yolk receptor mRNA, GLD-1 acts as a translational repressor to spatially restrict RME-2 accumulation, and thus yolk uptake, to late-stage oocytes. We found that GLD-1 binds sequences in both 5' coding and the 3' untranslated region of rme-2 mRNA. Initial characterization of the other 14 targets shows that (1) they are coexpressed with GLD-1; (2) they can have mutant/RNA-mediated interference depletion phenotypes indicating functions in germ line development or as maternal products necessary for early embryogenesis; and (3) GLD-1 may coregulate mRNAs corresponding to functionally redundant subsets of genes within two gene families. Thus, a diverse set of genes have come under GLD-1-mediated regulation to achieve normal germ line development. Previous work identified tra-2 as a GLD-1 target for germ line sex determination. Comparisons of GLD-1-mediated translational control of rme-2 and tra-2 suggests that the mechanisms may differ for distinct target mRNA species.

PIE-1 is a bifunctional protein that regulates maternal and zygotic gene expression in the embryonic germ line of Caenorhabditis elegans

The CCCH zinc finger protein PIE-1 is an essential regulator of germ cell fate that segregates with the germ lineage during the first cleavages of the Caenorhabditis elegans embryo. We have shown previously that one function of PIE-1 is to inhibit mRNA transcription. Here we show that PIE-1 has a second function in germ cells; it is required for efficient expression of the maternally encoded Nanos homolog NOS-2. This second function is genetically separable from PIE-1's inhibitory effect on transcription. A mutation in PIE-1's second CCCH finger reduces NOS-2 expression without affecting transcriptional repression and causes primordial germ cells to stray away from the somatic gonad, occasionally exiting the embryo entirely. Our results indicate that PIE-1 promotes germ cell fate by two independent mechanisms as follows: (1) inhibition of transcription, which blocks zygotic programs that drive somatic development, and (2) activation of protein expression from nos-2 and possibly other maternal RNAs, which promotes primordial germ cell development.

Left-right asymmetry in C. elegans intestine organogenesis involves a LIN-12/Notch signaling pathway

The C. elegans intestine is a simple tube consisting of a monolayer of epithelial cells. During embryogenesis, cells in the anterior of the intestinal primordium undergo reproducible movements that lead to an invariant, asymmetrical ‘twist’ in the intestine. We have analyzed the development of twist to determine how left-right and anterior-posterior asymmetries are generated within the intestinal primordium. The twist requires the LIN-12/Notch-like signaling pathway of C. elegans. All cells within the intestinal primordium initially express LIN-12, a receptor related to Notch; however, only cells in the left half of the primordium contact external, nonintestinal cells that express LAG-2, a ligand related to delta. LIN-12 and LAG-2 mediated interactions result in the left primordial cells expressing lower levels of LIN-12 than the right primordial cells. We propose that this asymmetrical pattern of LIN-12 expression is the basis for asymmetry in later cell-cell interactions within the primordium that lead directly to intestinal twist. Like the interactions that initially establish LIN-12 asymmetry, the later interactions are mediated by LIN-12. The later interactions, however, involve a different ligand related to delta, called APX-1. We show that the anterior-posterior asymmetry in intestinal twist involves the kinase LIT-1, which is part of a signaling pathway in early embryogenesis that generates anterior-posterior differences between sister cells.

aph-2 encodes a novel extracellular protein required for GLP-1-mediated signaling

In animal development, numerous cell-cell interactions are mediated by the GLP-1/LIN-12/NOTCH family of transmembrane receptors. These proteins function in a signaling pathway that appears to be conserved from nematodes to humans. We show here that the aph-2 gene is a new component of the GLP-1 signaling pathway in the early Caenorhabditis elegans embryo, and that proteins with sequence similarity to the APH-2 protein are found in Drosophila and vertebrates. During the GLP-1-mediated cell interactions in the C. elegans embryo, APH-2 is associated with the cell surfaces of both the signaling, and the responding, blastomeres. Analysis of chimeric embryos that are composed of aph-2(+) and aph-2(−) blastomeres suggests that aph-2(+) function may be provided by either the signaling or responding blastomere.

ASYMMETRIC CELL DIVISION IN PLANTS

Asymmetric cell divisions generate cells with different fates. In plants, where cells do not move relative to another cell, the specification and orientation of these divisions is an important mechanism to generate the overall cellular pattern during development. This review summarizes our knowledge of selected cases of asymmetric cell division in plants, in the context of recent insights into mechanisms underlying this process in bacteria, algae, yeast, and animals.

Early patterning of the C. elegans embryo

Studies of about 20 maternally expressed genes are providing an understanding of mechanisms of patterning and cell-fate determination in the early Caenorhabditis elegans embryo. The analyses have revealed that fates of the early blastomeres are specified by a combination of intrinsically asymmetric cell divisions and two types of cell-cell interactions: inductions and polarizing interactions. In this review we summarize the current level of understanding of the molecular mechanisms underlying these processes in the specification of cell fates in the pregastrulation embryo.

pos-1 encodes a cytoplasmic zinc-finger protein essential for germline specification in C. elegans

Germ cells arise during early C. elegans embryogenesis from an invariant sequence of asymmetric divisions that separate germ cell precursors from somatic precursors. We show that maternal-effect lethal mutations in the gene pos-1 cause germ cell precursors to inappropriately adopt somatic cell fates. During early embryogenesis, pos-1 mRNA and POS-1 protein are present predominantly in the germ precursors. POS-1 is a novel protein with two copies of a CCCH finger motif previously described in the germline proteins PIE-1 and MEX-1 in C. elegans, and in the mammalian TIS11/Nup475/TTP protein. However, mutations in pos-1 cause several defects in the development of the germline blastomeres that are distinct from those caused by mutations in pie-1 or mex-1. The earliest defect detected in pos-1 mutants is the failure to express APX-1 protein from maternally provided apx-1 mRNA, suggesting that POS-1 may have an important role in regulating the expression of maternal mRNAs in germline blastomeres.

Genetic requirements for PIE-1 localization and inhibition of gene expression in the embryonic germ lineage of Caenorhabditis elegans

In early Caenorhabditis elegans embryos, production of new mRNAs is inhibited in the germ lineage. This inhibition requires the germline factor PIE-1, and correlates with the absence in germline blastomeres of a phosphoepitope on RNA polymerase II (RNAPII-H5). We show that PIE-1 is uniformly distributed in oocytes and newly fertilized eggs, and becomes localized asymmetrically in the late one-cell stage. To begin to dissect the mechanisms required for PIE-1 localization and inhibition of RNAPII-H5 expression, we have examined the distribution of PIE-1 and RNAPII-H5 in maternal-effect mutants that disrupt embryonic development. We find that mutants that disrupt the asymmetric divisions of germline blastomeres mislocalize PIE-1, and activate RNAPII-H5 expression in the germ lineage. In contrast, mutants that alter somatic cell identities do not affect PIE-1 localization or RNAPII-H5 expression. Our observations suggest that PIE-1 represses mRNA transcription in each germline blastomere in a concentration-dependent manner. We also show that in wild-type, and in mutants where PIE-1 is mislocalized, the cellular and subcellular distribution of PIE-1 remarkably parallels that of the P granules, suggesting that the localizations of these two germline components are coordinately regulated.

The maternal-to-zygotic transition in embryonic patterning of Caenorhabditis elegans

Maternal factors laid down in the oocyte regulate blastomere identities in the early Caenorhabditis elegans embryo by activating zygotic patterning genes and restricting their expression to the appropriate lineages. A number of early-acting zygotic genes that specify various cell fates have been identified recently and their temporal and spatial regulation by maternal factors has begun to be elucidated.

Maternal control of pattern formation in early Caenorhabditis elegans embryos

Genetic screens for recessive, maternal-effect, embryonic-lethal mutations have identified about 25 genes that control early steps of pattern formation in the nematode Caenorhabditis elegans. These maternal genes are discussed as belonging to one of three groups. The par group genes establish and maintain polarity in the one-cell zygote in response to sperm entry, defining an anterior/posterior body axis at least in part through interactions with the cyto-skeleton mediated by cortically localized proteins. Blastomere identity group genes act down-stream of the par group to specify the identities of individual embryonic cells, or blastomeres, using both cell autonomous and non-cell autonomous mechanisms. Requirements for the blastomere identity genes are consistent with previous studies suggesting that early asymmetric cleavages in the C. elegans embryo generate six "founder" cells that account for much of the C. elegans body plan. Intermediate group genes, most recently identified, may link the establishment of polarity in the zygote by par group genes to the localization of blastomere identity group gene functions. This review summarizes the known requirements for the members of each group, although it seems clear that additional regulatory genes controlling pattern formation in the early embryo have yet to be identified. An emerging challenge is to link the function of the genes in these three groups into interacting pathways that can account for the specification of the six founder cell identities in the early embryo, five of which produce somatic cell types and one of which produces the germline.

The LIN-12/Notch signaling pathway and its regulation

Notch, LIN-12, and GLP-1 are receptors that mediate a broad range of cell interactions during Drosophila and nematode development. Signaling by these receptors relies on a conserved pathway with three core components: DSL ligand, LNG receptor, and a CSL effector that links the receptor to its transcriptional response. Although key functional regions have been identified in each class of proteins, the mechanism for signal transduction is not yet understood. Diverse regulatory mechanisms influence signaling by the LIN-12/Notch pathway. Inductive signaling relies on the synthesis of ligand and receptor in distinct but neighboring cells. By contrast, lateral signaling leads to the transformation of equivalent cells that express both ligand and receptor into nonequivalent cells that express either ligand or receptor. This transformation appears to rely on regulatory feedback loops within the LIN-12/Notch pathway. In addition, the pathway can be regulated by intrinsic factors that are asymmetrically segregated during cell division or by extrinsic cues via other signaling pathways. Specificity in the pathway does not appear to reside in the particular ligand or receptor used for a given cell-cell interaction. The existence of multiple ligands and receptors may have evolved from the stringent demands placed upon the regulation of genes encoding them.

Cell fate determination in Drosophila

A major issue in development is to understand how local heterogeneities are interpreted to determine specific cell fates. The sense organs of Drosophila provide an accessible system for addressing this issue. Most sense organs comprise four types of cells, and their differentiation is the outcome of a complex developmental programme comprising several steps. Recent results illuminate, for several of these steps, the nature of the local heterogeneities and the mechanism used to interpret them in terms of cell fate decisions.

MEX-3 is a KH domain protein that regulates blastomere identity in early C. elegans embryos

After the first division of the C. elegans embryo, the posterior blastomere can produce numerous muscles while the anterior blastomere cannot. We show here that maternal-effect lethal mutations in the gene mex-3 cause descendants of the anterior blastomere to produce muscles by a pattern of development similar to that of a descendant of the wild-type posterior blastomere. mex-3 encodes a probable RNA-binding protein that is distributed unequally in early embryos and that is a component of germline-specific granules called P granules. We propose that MEX-3 contributes to anterior-posterior asymmetry by regulating one or more mRNAs involved in specifying the fate of the posterior blastomere.