The Caenorhabditis elegans lin-12 gene encodes a transmembrane protein with overall similarity to Drosophila Notch

Intracellular cell-autonomous association of Notch and its ligands: a novel mechanism of Notch signal modification

Notch (N) and its ligands, Delta (Dl) and Serrate (Ser), are membrane-spanning proteins with EGF repeats. They play an essential role in mediating proliferation and segregated differentiation of stem cells. One of the prominent features of N signal system is that its ligands are anchored to the plasma membrane, which allows the ligand/receptor association only between the neighboring cells. Various lines of evidences have verified this intercellular signal transmission, but there also have been implications that expression of Dl or Ser interferes cell-autonomously with the ability of the cell to receive N signal, implying that N and its ligands may interact in the same cell. Here, we demonstrate that N, Dl, and Ser cell-autonomously form homomeric or heteromeric complexes. The cell-autonomous heteromeric complexes are not present on the cell surface, implying that the association occurs in the endoreticulum or Golgi apparatus. Expression of Dl or Ser cell-autonomously reduces the N-mediated HES-5 promoter activity, indicating that the cell-autonomous association alters the N signal receptivity. Intracellular deletion of Dl shows elevated activity of this dominant-negative effect. In vivo overexpression study suggests that the cell-autonomous function of Dl and Ser is independent of the ligand specificity and may be modulated by Fringe (Fg), which inhibits the formation of the cell-autonomous Dl/N or Ser/N complex.

Pattern formation during C. elegans vulval induction

Studies of C. elegans vulval development provide insights into the process of pattern formation during animal development. The invariant pattern of vulval precursor cell fates is specified by the integration of at least two signaling systems. Recent findings suggest that multiple, partially redundant mechanisms are involved in patterning the vulval precursor cells. The inductive signal activates the LET-60/RAS signaling pathway and induces the 1 degree fate, whereas the lateral signal mediated by LIN-12/Notch is required for specification of the 2 degrees fate. Several regulatory pathways antagonize the RAS signaling pathway and specify the non-vulval 3 degrees fate in the absence of induction. The temporal and spatial regulation of VPC competence and production of the inductive and the lateral signal are precisely coordinated to ensure the wild-type vulval pattern.

REF-1, a protein with two bHLH domains, alters the pattern of cell fusion in C. elegans by regulating Hox protein activity

Hox genes control the choice of cell fates along the anteroposterior (AP) body axis of many organisms. In C. elegans, two Hox genes, lin-39 and mab-5, control the cell fusion decision of the 12 ventrally located Pn.p cells. Specific Pn.p cells fuse with an epidermal syncytium, hyp7, in a sexually dimorphic pattern. In hermaphrodites, Pn.p cells in the mid-body region remain unfused whereas in males, Pn.p cells adopt an alternating pattern of syncytial and unfused fates. The complexity of these fusion patterns arises because the activities of these two Hox proteins are regulated in a sex-specific manner. MAB-5 activity is inhibited in hermaphrodite Pn.p cells and thus MAB-5 normally only affects the male Pn.p fusion pattern. Here we identify a gene, ref-1, that regulates the hermaphrodite Pn.p cell fusion pattern largely by regulating MAB-5 activity in these cells. Mutation of ref-1 also affects the fate of other epidermal cells in distinct AP body regions. ref-1 encodes a protein with two basic helix-loop-helix domains distantly related to those of the hairy/Enhancer of split family. ref-1, and another hairy homolog, lin-22, regulate similar cell fate decisions in different body regions along the C. elegans AP body axis.

The germline in C. elegans: origins, proliferation, and silencing

Germ cells are essential for reproduction, yet the molecular mechanisms that underlie their unique development are only beginning to be understood. Here we review important events that lead to the establishment of the germline and the initiation of meiotic development in C. elegans. Formation of the germline begins in the pregastrulation embryo, where it depends on polarization along the anterior/posterior axis and on the asymmetric segregation of P granules and associated factors. During postembryonic development, the germline expands using the GLP-1/Notch signaling pathway to promote proliferation and regulate entry into meiosis. Throughout their development, germ cells also employ unique "silencing" mechanisms to regulate their genome and protect themselves against unwanted expression from repetitive sequences including transposable elements. Together these mechanisms preserve the health and reproductive potential of the germline.

Cloning and characterization of a potential transcriptional activator of human gamma-globin genes

Hybrids produced by fusing human fetal erythroblasts (HFE) with mouse erythroleukemia (MEL) cells initially produce predominantly or exclusively human gamma-globin and switch to human beta globin expression as time in culture advances. One explanation for the initially predominant expression of gamma-globin gene in these hybrids is the presence of trans-acting factors that activate gamma-globin gene transcription. We used differential display of hybrids before and after the gamma to beta switch as well as fetal liver and adult erythroblasts to identify cDNAs that could be candidates for potential gamma gene activators. Identically sized amplicons which were present in fetal liver erythroblasts and in the hybrids expressing only gamma-globin but were absent in the adult erythroblasts and in the same hybrids after they had switched to beta globin expression were cloned and sequenced. Fifty pairs of cDNAs fitting these criteria were chosen for further analysis. The sequences of the two members of 48 pairs differed from each other, revealing the low efficiency of this experimental approach. One clone pair coded for human proteosome subunit X. The second pair coded for a protein containing an acidic domain in the N-terminus and three consecutive CDC10/SW16/ankyrin repeats in the C-terminus. Transactivation assays in the yeast hybrid system and transient transfection assays in COS cells showed that a potent trans-activating domain resides in the N-terminus of this protein. Northern blot and RT-PCR assays showed that this gene is expressed in several fetal tissues but not in adult tissues. Stable transfection assays provided evidence that the product of this gene may increase the level of gamma mRNA in HFE x MEL cell hybrids that undergo the gamma to beta switch, suggesting that this new gene encodes a protein that may function as gamma gene activator.

Protruding vulva mutants identify novel loci and Wnt signaling factors that function during Caenorhabditis elegans vulva development

The Caenorhabditis elegans vulva develops from the progeny of three vulval precursor cells (VPCs) induced to divide and differentiate by a signal from the somatic gonad. Evolutionarily conserved Ras and Notch extracellular signaling pathways are known to function during this process. To identify novel loci acting in vulval development, we carried out a genetic screen for mutants having a protruding-vulva (Pvl) mutant phenotype. Here we report the initial genetic characterization of several novel loci: bar-1, pvl-4, pvl-5, and pvl-6. In addition, on the basis of their Pvl phenotypes, we show that the previously identified genes lin-26, mom-3/mig-14, egl-18, and sem-4 also function during vulval development. Our characterization indicates that (1) pvl-4 and pvl-5 are required for generation/survival of the VPCs; (2) bar-1, mom-3/mig-14, egl-18, and sem-4 play a role in VPC fate specification; (3) lin-26 is required for proper VPC fate execution; and (4) pvl-6 acts during vulval morphogenesis. In addition, two of these genes, bar-1 and mom-3/mig-14, are known to function in processes regulated by Wnt signaling, suggesting that a Wnt signaling pathway is acting during vulval development.

Mutations in cye-1, a Caenorhabditis elegans cyclin E homolog, reveal coordination between cell-cycle control and vulval development

We have identified strong loss-of-function mutations in the C. elegans cyclin E gene, cye-1. Mutations in cye-1 lead to the underproliferation of many postembryonic blast lineages as well as defects in fertility and gut-cell endoreduplication. In addition, cye-1 is required maternally, but not zygotically for embryonic development. Our analysis of vulval development in cye-1 mutants suggests that a timing mechanism may control the onset of vulval cell terminal differentiation: once induced, these cells appear to differentiate after a set amount of time, rather than a specific number of division cycles. cye-1 mutants also show an increase in the percentage of vulval precursor cells (VPCs) that adopt vulval cell fates, indicating that cell-cycle length can play a role in the proper patterning of vulval cells. By analyzing cul-1 mutants, we further demonstrate that vulval cell terminal differentiation can be uncoupled from associated changes in vulval cell division planes.

Nematode development: an evolutionary fugue

Recent studies of vulva development in the nematode Pristionchus pacificus have identified cell interactions that do not appear to occur in Caenorhabditis elegans. The new results underscore the diversity of patterning mechanisms that can produce structures with similar cellular morphology.

Myotonic dystrophy in Ancient Egypt

Amenhotep IV, better known as Akhenaton, the heretical pharaoh, was a king of the New Kingdom of Ancient Egypt. Statues and reliefs of him show an unhealthy man whose body has abnormal features. By studying the pictures of Akhenaton (the mummy has not yet been found), we conclude that he may have been affected by myotonic dystrophy (MD). Moreover, the available data on his family suggest that MD may have caused the end of the royal bloodline of the Eighteenth Dynasty.

Caenorhabditis elegans twist plays an essential role in non-striated muscle development

The basic helix-loop-helix (bHLH) transcription factor Twist plays a role in mesodermal development in both invertebrates and vertebrates. In an effort to understand the role of the unique Caenorhabditis elegans Twist homolog, hlh-8, we analyzed mesodermal development in animals with a deletion in the hlh-8 locus. This deletion was predicted to represent a null allele because the HLH domain is missing and the reading frame for the protein is disrupted. Animals lacking CeTwist function were constipated and egg-laying defective. Both of these defects were rescued in transgenic mutant animals expressing wild-type hlh-8. Observing a series of mesoderm-specific markers allowed us to characterize the loss of hlh-8 function more thoroughly. Our results demonstrate that CeTwist performs an essential role in the proper development of a subset of mesodermal tissues in C. elegans. We found that CeTwist was required for the formation of three out of the four non-striated enteric muscles born in the embryo. In contrast, CeTwist was not required for the formation of the embryonically derived striated muscles. Most of the post-embryonic mesoderm develops from a single lineage. CeTwist was necessary for appropriate patterning in this lineage and was required for expression of two downstream target genes, but was not required for the expression of myosin, a marker of differentiation. Our results suggest that mesodermal patterning by Twist is an evolutionarily conserved function.

EAT-20, a novel transmembrane protein with EGF motifs, is required for efficient feeding in Caenorhabditis elegans

The pharynx of Caenorhabditis elegans is a neuromuscular organ responsible for feeding, concentrating food by its pumping movement. A class of mutants, the eat mutants, are defective in this behavior. We have identified a novel eat gene, eat-20, encoding a unique transmembrane protein with three EGF motifs. Staining with a specific polyclonal antibody reveals that EAT-20 is expressed predominantly in the pharyngeal muscles and a subset of neurons. Some hypodermal cells also express EAT-20. eat-20 mutant animals are starved, have smaller brood sizes, and have prolonged egg-laying periods. The starvation apparently results from pharyngeal pumping defects, including a reduced pumping rate and "slippery pumping," in which the contents of the pharynx sometimes move rostrally. However, electrical activity of eat-20 mutants appears normal by electropharyngeogram.

Genetic regulation of somite formation

Segmentation of the paraxial mesoderm into somites requires a strategy distinct from the division of a preexisting field of cells, as seen in the segmentation of the vertebrate hindbrain into rhombomeres and the formation of the body plan of invertebrates. Each new somite forms from the anterior end of the segmental plate; therefore, the conditions for establishing the anterior-posterior boundary must be re-created prior to the formation of the next somite. It has been established that regulation of this process is native to the anterior end of the segmental plate, however, the components of a genetic pathway are poorly understood. A growing library of candidate genes has been generated from hybridization screens and sequence homology searches, which include cell adhesion molecules, cell surface receptors, growth factors, and transcription factors. With the increasing accessibility of gene knockout technology, many of these genes have been tested for their role in regulating somitogenesis. In this chapter, we will review the significant advances in our understanding of segmentation based on these experiments.

Arbiter of differentiation and death: Notch signaling meets apoptosis

Notch-ligand interactions are a highly conserved mechanism that regulates cell fate decisions. Over the past few years, numerous observations have shown that this mechanism operates to regulate cell differentiation in an enormous variety of developmental and cell maturation processes. Recent studies indicate that in addition to cell differentiation, Notch signaling has direct effects on proliferation and programmed cell death. The picture emerging from these findings suggests that, depending on cellular and developmental context, Notch signaling may function as a general "arbiter" of cell fate, regulating differentiation potential, rate of proliferation, and apoptotic cell death. In this review, we briefly summarize the current knowledge of the structure and function of Notch receptors and discuss the recent evidence that Notch signaling regulates apoptotic cell death. The possible mechanisms of this effect and its potential implications for developmental biology, immunobiology, neuropathology, and tumor biology are discussed.

The lin-11 LIM domain transcription factor is necessary for morphogenesis of C. elegans uterine cells

The Caenorhabditis elegans hermaphrodite egg-laying system comprises several tissues, including the uterus and vulva. lin-11 encodes a LIM domain transcription factor needed for certain vulval precursor cells to divide asymmetrically. Based on lin-11 expression studies and the lin-11 mutant phenotype, we find that lin-11 is also required for C. elegans uterine morphogenesis. Specifically, lin-11 is expressed in the ventral uterine intermediate precursor (pi) cells and their progeny (the utse and uv1 cells), which connect the uterus to the vulva. Like (pi) cell induction, the uterine lin-11 expression responds to the uterine anchor cell and the lin-12-encoded receptor. In wild type animals, the utse, which forms the planar process at the uterine-vulval interface, fuses with the anchor cell. We found that, in lin-11 mutants, utse differentiation was abnormal, the utse failed to fuse with the anchor cell and a functional uterine-vulval connection was not made. These findings indicate that lin-11 is essential for uterine-vulval morphogenesis.

Lateral signaling mediated by axon contact and calcium entry regulates asymmetric odorant receptor expression in C. elegans

C. elegans detects several odorants with the bilaterally symmetric pair of AWC olfactory neurons. A stochastic, coordinated decision ensures that the candidate odorant receptor gene str-2 is expressed in only one AWC neuron in each animal--either the left or the right neuron, but never both. An interaction between the two AWC neurons generates asymmetric str-2 expression in a process that requires normal axon guidance and probably AWC axon contact. This interaction induces str-2 expression by reducing calcium signaling through a voltage-dependent Ca2+ channel and the CaM kinase II UNC-43. CaMKII activity acts as a switch in the initial decision to express str-2; thus, calcium signals can define distinct cell types during neuronal development. A cGMP signaling pathway that is used in olfaction maintains str-2 expression after the initial decision has been made.

Competence and commitment of Caenorhabditis elegans vulval precursor cells

Multipotent Caenorhabditis elegans vulval precursor cells (VPCs) choose among three fates (1 degrees, 2 degrees, and 3 degrees ) in response to two intercellular signals: the EGF family growth factor LIN-3 induces 1 degrees fates at high levels and 2 degrees fates at low levels; and a signal via the receptor LIN-12 induces 2 degrees fates. If the level of LIN-3 signal is reduced by a lin-3 hypomorphic mutation, the daughters of the VPC closest to the anchor cell (AC), P6.p, are induced by the AC. By expressing LIN-3 as a function of time in LIN-3-deficient animals, we find that both VPCs and the daughters of VPCs are competent to respond to LIN-3, and VPC daughters lose competence after fusing with the hypodermis. We also demonstrate that the daughters of VPCs specified to be 2 degrees can respond to LIN-3, indicating that 2 degrees VPCs are not irreversibly committed. We propose that maintenance of VPC competence after the first cell cycle and the prioritization of the 1 degrees fate help ensure that P6.p will become 1 degrees. This mechanism of competence regulation might have been maintained from ancestral nematode species that used induction both before and after VPC division and serves to maximize the probability that a functional vulva is formed.

p24 proteins and quality control of LIN-12 and GLP-1 trafficking in Caenorhabditis elegans

Mutations in the Caenorhabditis elegans sel-9 gene elevate the activity of lin-12 and glp-1, which encode members of the LIN-12/NOTCH family of receptors. Sequence analysis indicates SEL-9 is one of several C. elegans p24 proteins. Allele-specific genetic interactions suggest that reducing sel-9 activity increases the activity of mutations altering the extracellular domains of LIN-12 or GLP-1. Reducing sel-9 activity restores the trafficking to the plasma membrane of a mutant GLP-1 protein that would otherwise accumulate within the cell. Our results suggest a role for SEL-9 and other p24 proteins in the negative regulation of transport of LIN-12 and GLP-1 to the cell surface, and favor a role for p24 proteins in a quality control mechanism for endoplasmic reticulum-Golgi transport.

Characterization of Chlamydomonas reinhardtii zygote-specific cDNAs that encode novel proteins containing ankyrin repeats and WW domains

Genes that are expressed only in the young zygote are considered to be of great importance in the development of an isogamous green alga, Chlamydomonas reinhardtii. Clones representing the Zys3 gene were isolated from a cDNA library prepared using zygotes at 10 min after fertilization. Sequencing of Zys3 cDNA clones resulted in the isolation of two related molecular species. One of them encoded a protein that contained two kinds of protein-to-protein interaction motifs known as ankyrin repeats and WW domains. The other clone lacked the ankyrin repeats but was otherwise identical. These mRNA species began to accumulate simultaneously in cells beginning 10 min after fertilization, and reached maximum levels at about 4 h, after which time levels decreased markedly. Genomic DNA gel-blot analysis indicated that Zys3 was a single-copy gene. The Zys3 proteins exhibited parallel expression to the Zys3 mRNAs at first, appearing 2 h after mating, and reached maximum levels at more than 6 h, but persisted to at least 1 d. Immunocytochemical analysis revealed their localization in the endoplasmic reticulum, which suggests a role in the morphological changes of the endoplasmic reticulum or in the synthesis and transport of proteins to the Golgi apparatus or related vesicles.

T-cell fate

Recent studies suggest that lineage commitment steps, which occur during T-cell differentiation, follow principles in common with fate specification in simple invertebrates. Here we review T-cell development from the perspective of developmental biology. We present models for alpha beta vs gamma delta and CD4 vs CD8 lineage commitment that are consistent with previously published and newly presented experiments.

Cloning of inv, a gene that controls left/right asymmetry and kidney development

Most vertebrate internal organs show a distinctive left/right asymmetry. The inv (inversion of embryonic turning) mutation in mice was created previously by random insertional mutagenesis; it produces both a constant reversal of left/right polarity (situs inversus) and cyst formation in the kidneys. Asymmetric expression patterns of the genes nodal and lefty are reversed in the inv mutant, indicating that inv may act early in left/right determination. Here we identify a new gene located at the inv locus. The encoded protein contains 15 consecutive repeats of an Ank/Swi6 motif at its amino terminus. Expression of the gene is the highest in the kidneys and liver among adult tissues, and is seen in presomite-stage embryos. Analysis of the transgenic genome and the structure of the candidate gene indicate that the candidate gene is the only gene that is disrupted in inv mutants. Transgenic introduction of a minigene encoding the candidate protein restores normal left/right asymmetry and kidney development in the inv mutant, confirming the identity of the candidate gene.

The Hox gene lin-39 is required during C. elegans vulval induction to select the outcome of Ras signaling

The Ras signaling pathway specifies a variety of cell fates in many organisms. However, little is known about the genes that function downstream of the conserved signaling cassette, or what imparts the specificity necessary to cause Ras activation to trigger different responses in different tissues. In C. elegans, activation of the Ras pathway induces cells in the central body region to generate the vulva. Vulval induction takes place in the domain of the Hox gene lin-39. We have found that lin-39 is absolutely required for Ras signaling to induce vulval development. During vulval induction, the Ras pathway, together with basal lin-39 activity, up-regulates lin-39 expression in vulval precursor cells. We find that if lin-39 function is absent at this time, no vulval cell divisions occur. Furthermore, if lin-39 is replaced with the posterior Hox gene mab-5, then posterior structures are induced instead of a vulva. Our findings suggest that in addition to permitting vulval cell divisions to occur, lin-39 is also required to specify the outcome of Ras signaling by selectively activating vulva-specific genes.

Characterization of the role of cadherin in regulating cell adhesion during sea urchin development

During development, the modulation of cadherin adhesive function is proposed to control various morphogenetic events including epithelial-mesenchymal conversions and tubulogenesis, although the mechanisms responsible for regulating cadherin activity during these events remain unclear. In order to gain insights into the regulation of cadherin function during morphogenesis, we utilized the sea urchin embryo as a model system to study the regulation of cadherin localization during epithelial-mesenchymal conversion and convergent-extension movements. Polyclonal antibodies raised against the cytoplasmic domain of a cloned sea urchin cadherin recognize three major polypeptides of M(r) 320, 140, and 125 kDa and specifically stain adherens junctions, and to a lesser extent, lateral membrane domains in all epithelial tissues of the embryo. Analysis of embryos during gastrulation demonstrates that changes in cadherin localization are observed in cells undergoing an epithelial-mesenchymal conversion. Ingression of primary mesenchyme cells is accompanied by the rapid loss of junctional cadherin staining and the coincident accumulation of cadherin in intracellular organelles. These data are consistent with the idea that the deadhesion of mesenchymal cells from neighboring epithelial cells involves the regulated endocytosis of cell surface cadherin molecules. Conversely, neither cadherin abundance nor localization is altered in cells of the gut which undergo convergent-extension movements during the formation of the archenteron. This observation indicates that these movements do not require the loss of junctional cadherin molecules. Instead, the necessary balance between adhesion and motility may be achieved by regulating the expression of different subtypes of cadherin molecules or modifying interactions between cadherins and catenins, proteins that bind the cytoplasmic domain of cadherin and are necessary for cadherin adhesive function. To address cadherin function at the molecular level, we used a partial cDNA representing the conserved cytoplasmic domain to identify a novel cadherin molecule in the sea urchin Lytechinus variegatus. The deduced amino acid sequence of LvG-cadherin (for Goliath-cadherin) predicts that it is a transmembrane protein with an apparent relative molecular mass of 303 kDa. The cytoplasmic domain shows significant sequence identity to that of vertebrate classic cadherins. However, the extracellular domain is distinguished from its vertebrate counterparts by both an increased number of cadherin-specific repeats and the presence of four EGF-like repeats proximal to the transmembrane domain. Taken together, these data are consistent with the hypothesis that the sea urchin possesses several cadherins, including a novel member of the cadherin family, and that the dynamic regulation of cadherin localization plays a role in epithelial to mesenchymal conversions during gastrulation.

Immunolocalization of Drosophila eye-specific diacylgylcerol kinase, rdgA, which is essential for the maintenance of the photoreceptor

The Drosophila retinal degeneration A (rdgA) mutant has photoreceptor cells that degenerate within a week after eclosion. The degeneration starts with the disruption of the subrhabdomeric cisternae (SRC), which are the organelles essential for the transport of phospholipids to the photoreceptive membranes. Our previous biochemical and molecular studies suggested that the rdgA gene encodes an eye-specific diacylglycerol kinase (DGK). In this study, we show that retinal degeneration is prevented by the introduction of the eye-DGK gene in the rdgA mutant genome, suggesting that the DGK activity is crucial for the maintenance of the photoreceptor. Furthermore, by immunohistochemical analysis, we have demonstrated that the rdgA protein is predominantly associated with the SRC, suggesting that the conversion from diacylglycerol (DG) to phosphatidic acid (PA) most actively occurs in SRC. The analysis of the eyes of mutants homozygous for rdgA and eye-protein kinase C mutations indicates that retinal degeneration is caused by the deficiency of PA rather than excessive accumulation of DG. From these data, we conclude that the production of PA in the SRC membranes is essential for the maintenance of the photoreceptor.

Two Microplitis demolitor polydnavirus mRNAs expressed in hemocytes of Pseudoplusia includens contain a common cysteine-rich domain

Microplitis demolitor is a polydnavirus-carrying wasp that parasitizes the larval stage of Pseudoplusia includens. A previous study indicated that M. demolitor polydnavirus (MdPDV) infects primarily hemocytes in parasitized hosts. Thereafter, several alterations that compromise the immune response of P. includens toward the developing parasitoid occur in hemocytes. In this study, we identified two MdPDV mRNAs (1.0 and 1.5 kb) expressed in P. includens hemocytes that have homology to the viral genomic clone pMd-2. Corresponding 1.0- and 1.5-kb cDNA clones (MdPi455 and MdPi59) were isolated from an MdPDV-infected hemocyte cDNA library. Nucleotide sequence analysis of the cDNA clones confirmed that the 1.5- and 1.0-kb mRNAs have significant regions of homology. Sequence alignment revealed that the gene, OMd1.0, encoding the 1.0-kb mRNA is present in pMd-2. This gene contains two introns and three exons that agree with the sequence for MdPi455. In contrast, the 1.5-kb mRNA is likely encoded by a related gene located on the same MdPDV genomic DNA as is OMd1.0. The predicted peptide sequences for the 1.0- and 1.5-kb transcripts contain a cysteine-rich region at their 5' ends that have some similarity with epidermal growth factor-like motifs. Hybridization studies revealed that both mRNAs are expressed in granular cells and plasmatocytes, the primary classes of hemocytes involved in defense against M. demolitor and other parasites.

Nematode development: evolutionary detours of a pivotal cell

The anchor cell plays a central role in organizing the reproductive structures of the nematode Caenorhabditis elegans. Recent studies show that significant alterations in the origin, function and fate of this key regulatory cell have occurred during the course of nematode evolution.

Calcium stabilizes fibrillin-1 against proteolytic degradation

The calcium-binding epidermal growth factor (cbEGF)-like domain is a structural motif that is present in many matrix proteins throughout the animal kingdom from invertebrates to mammals. This module has been demonstrated to bind calcium in the micromolar range. However, little is known about the functional consequences of calcium binding to proteins that contain this structural element. We used fibrillin-1, an extracellular matrix protein consisting of approximately 60% cbEGF-like motifs, as a model system to study stabilizing effects of calcium in protease degradation assays. Authentic human fibrillin-1 and recombinant human fibrillin-1 subdomains, spanning the whole molecule, showed significantly slower proteolytic degradation in the presence of CaCl2 than in the presence of EDTA, demonstrating that calcium stabilizes the structure of fibrillin-1 and protects the molecule against proteolytic degradation. Information about cleavage sites protected by calcium was obtained with a new recombinant subdomain, rF17 (Asp 952-Val 1527), comprising the longest stretch of cbEGF-like motifs in the center of the fibrillin-1 molecule. The most sensitive sites for trypsin and endoproteinase Glu-C were observed in cbEGF-like motifs 11 (Met 1034 and Asn 1046), 12 (Ser 1103), and 17 (Thr 1318). Since most of the currently known mutations in fibrillin-1 are found within cbEGF-like motifs and are predicted to disrupt calcium binding, we suggest that these mutations render fibrillin-1 more susceptible to proteolytic cleavage, and this might be one of the reasons why these mutations result in Marfan's syndrome.

lin-12 and glp-1 are required zygotically for early embryonic cellular interactions and are regulated by maternal GLP-1 signaling in Caenorhabditis elegans

Cell-cell interactions mediated by LIN-12 and GLP-1, members of the LNG (LIN-12, Notch, GLP-1) family of receptors, are required to specify numerous cell fates during development of the nematode Caenorhabditis elegans. Maternally expressed GLP-1 participates in two of at least four sequential inductive interactions that specify the fates of early embryonic descendants of the AB founder cell. We report that GLP-1 and LIN-12, and apparently their ligand, LAG-2, as well as a downstream component, LAG-1, are required in the latter two inductions. We find that LAG-2 is expressed in the signaling cells and LIN-12 is expressed in cells receiving the inductions, consistent with their proposed roles as ligand and receptor, respectively. Furthermore, we report that maternal GLP-1 activity is required (1) to repress early zygotic lag-2 expression and (2) to activate zygotic lin-12 expression in the early embryo. The patterning of both receptor and ligand expression by maternal GLP-1 signaling establishes competence for the zygotic LNG-mediated cellular interactions and localizes these interactions to the appropriate cells. We propose that activation of maternal GLP-1 regulates zygotic lin-12 and lag-2 expression by a regulatory mechanism analogous to that described for the post-embryonic gonad.

Morphogenesis of the C. elegans hermaphrodite uterus

We have undertaken electron micrographic reconstruction of the Caenorhabditis elegans hermaphrodite uterus and determined the correspondence between cells defined by their lineage history and differentiated cell types. In this organ, many cells do not move during morphogenesis and the cell lineage may function to put cells where they are needed. Differentiated uterine cell types include the toroidal ut cells that make structural epithelium, and specialized utse and uv cells that make the connection between the uterus and the vulva. A cell fate decision in which the anchor cell (AC) induces adjacent ventral uterine intermediate precursor cells to adopt the pi fate, rather than the ground state rho, has profound consequences for terminal differentiation: all pi progeny are directly involved in making the uterine-vulval connection whereas all rho progeny contribute to ut toroids or the uterine-spermathecal valve. In addition to specifying certain uterine cell fates, the AC also induces the vulva. Its multiple inductions thereby function to coordinate the connection of an internal to an external epithelium. The AC induces the pi cells and ultimately fuses with a subset of their progeny. This is an example of reciprocal cell-cell interaction that can be studied at single cell resolution. The AC is thus a transitory cell type that plays a pivotal role in organizing the morphogenesis of the uterine-vulval connection.

Asymmetric distribution of the C. elegans HAM-1 protein in neuroblasts enables daughter cells to adopt distinct fates

One mechanism of generating cellular diversity is to distribute developmental potential asymmetrically to daughter cells at mitosis. Two observations described in this report suggest that the C. elegans HAM-1 protein functions in dividing neuroblasts to produce daughter cells that adopt distinct fates. First, HAM-1 is asymmetrically distributed to the periphery of certain mitotic cells, ensuring that it will be inherited by only one daughter cell. Second, ham-1 mutations disrupt the asymmetric divisions of five neuroblasts. In one of these divisions, loss of ham-1 function causes the daughter cell that does not inherit HAM-1 to adopt the fate of the daughter cell that normally inherits HAM-1. We propose that asymmetric distribution of HAM-1 enables daughter cells to adopt distinct fates.

Exon shuffling and other ways of module exchange

Thanks to recent improvements in techniques used for the detection of homologies, it is now clear that module exchange played a major role in protein evolution. Analysis of the genes of various modular proteins has identified a large number of cases where gene assembly was facilitated by intronic recombination--i.e., the proteins were formed by exon shuffling. Studies of the principles and mechanistic details of exon shuffling, however, revealed that this powerful evolutionary mechanism could become significant only after the appearance of spliceosomal introns typical of higher eukaryotes. Although exon shuffling is the most efficient way of constructing modular proteins, recent studies on the evolution of multidomain proteins of prokaryotes emphasize that intronic recombination is not an absolute prerequisite of module exchange.

Coordinated morphogenesis of epithelia during development of the Caenorhabditis elegans uterine-vulval connection

Development of the nematode egg-laying system requires the formation of a connection between the uterine lumen and the developing vulval lumen, thus allowing a passage for eggs and sperm. This relatively simple process serves as a model for certain aspects of organogenesis. Such a connection demands that cells in both tissues become specialized to participate in the connection, and that the specialized cells are brought in register. A single cell, the anchor cell, acts to induce and to organize specialization of the epidermal and uterine epithelia, and registrates these tissues. The inductions act via evolutionarily conserved intercellular signaling pathways. The anchor cell induces the vulva from ventral epithelial cells via the LIN-3 growth factor and LET-23 transmembrane tyrosine kinase. It then induces surrounding uterine intermediate precursors via the receptor LIN-12, a founding member of the Notch family of receptors. Both signaling pathways are used multiple times during development of Caenorhabditis elegans. The outcome of the signaling is context-dependent. Both inductions are reciprocated. After the anchor cell has induced the vulva, it stretches toward the induced vulval cells. After the anchor cell has induced specialized uterine intermediate precursor cells, it fuses with a subset of their progeny.

Murine central neurons express a novel member of the cdc10/SWI6 motif-containing protein superfamily

V-1 protein is a novel member of the cdc10/SWI6 motif-containing protein superfamily several members of which have been demonstrated to play crucial roles in the regulation of intracellular signaling. In the present study we examined the distribution of V-1 mRNA in the murine central nervous system (CNS). Northern analysis revealed the expression of V-1 mRNA in various regions of the brain with the following rank order: hippocampus, cerebellum > cerebral cortex, olfactory bulb, medulla oblongata, pons > thalamus. In situ hybridization also showed that V-1 mRNA is widely distributed in various regions of the brain, with parallel expression levels to those revealed by Northern analysis. Immunohistochemical analysis revealed that the V-1 protein exists in various types of neurons, mainly in cell bodies but also in dendrites, axons and possibly in synaptic areas. These expression patterns of the V-1 gene in the murine CNS suggest that the V-1 protein performs some common function in different classes of neurons. We found no significant difference in the expression level of V-1 mRNA in cerebellar granule cells between the control and mutant mice of Purkinje cell degeneration (pcd). In comparison with our previous data obtained in another mutant, staggerer, we discussed the effects of target deprivation on the expression of V-1 mRNA in cerebellar granule cells.

The germ line regulates somatic cyst cell proliferation and fate during Drosophila spermatogenesis

Spermatogenesis relies on the function of germ-line stem cells, as a continuous supply of differentiated spermatids is produced throughout life. In Drosophila, there must also be somatic stem cells that produce the cyst cells that accompany germ cells throughout spermatogenesis. By lineage tracing, we demonstrate the existence of such somatic stem cells and confirm that of germ-line stem cells. The somatic stem cells likely correspond to the ultrastructurally described cyst progenitor cells. The stem cells for both the germ-line and cyst lineage are anchored around the hub of non-dividing somatic cells located at the testis tip. We then address whether germ cells regulate the behavior of somatic hub cells, cyst progenitors and their daughter cyst cells by analyzing cell proliferation and fate in testes in which the germ line has been genetically ablated. Daughter cyst cells, which normally withdraw from the cell cycle, continue to proliferate in the absence of germ cells. In addition, cells from the cyst lineage switch to the hub cell fate. Male-sterile alleles of chickadee and diaphanous, which are deficient in germ cells, exhibit similar cyst cell phenotypes. We conclude that signaling from germ cells regulates the proliferation and fate of cells in the somatic cyst lineage.

Symmetry breakage in the development of one-armed gonads in nematodes

Whereas the hermaphrodite gonad of Caenorhabditis elegans has two symmetric arms (didelphy), the female/hermaphrodite gonad of many nematode species features a single anterior arm (monodelphy). We examined how gonadal cell lineages and intercellular signalling evolve to generate these diverse structures. In C. elegans, the two arms develop symmetrically from two somatic precursor cells, Z1 (anterior) and Z4 (posterior). Each first gives rise to one distal tip cell (which promotes arm growth and germ line proliferation), two ovary precursors and three uterine precursors in the center of the developing gonad. In monodelphic species, Z1 and Z4 have different fates. The first visible asymmetry between them is in the relative timing of their divisions, followed by asymmetric cell movements. The putative posterior distal tip cell is then eliminated in all but one species by programmed cell death. In some species the posterior ovary precursors form a small vestigial posterior arm, the post-vulval sac; in other species, they stay undivided, or die. In Cephalobus sp. PS1197, the specific fate of Z4 progeny is induced by Z1 (or its daughters). In the uterus in C. elegans, symmetric lateral signalling between Z1.ppp and Z4.aaa renders them equally likely to become the anchor cell, which links the uterus to the vulva. In the different monodelphic species, anchor cell specification is biased, or fully fixed, to a descendant of either Z1 or Z4. Replacement regulation upon anchor cell ablation is conserved in some species, but lost in others, leading to a mosaic-type development. Differentiation between Z1 and Z4 is thus manifested at this later stage in the breakage of symmetry of cell interactions in the ventral uterus.

Control and integration of cell signaling pathways during C. elegans vulval development

Vulval development in the Caenorhabditis elegans hermaphrodite represents a simple, genetically tractable system for studying how cell signaling events control cell fate decisions. Current models suggest that proper specification of vulval cell fates relies on the integration of multiple signaling systems, including one that involves a receptor tyrosine kinase (RTK)-->Ras-->mitogen activated protein kinase (MAPK) cascade and one that involves a LIN-12/Notch family receptor. In this review, we first discuss how genetic strategies are being used to identify and analyze components that control vulval cell fate decisions. We then describe the different signaling systems that have been elucidated and how they relate to one another. Finally, we highlight several recently characterized genes that encode positive regulators, negative regulators or potential targets of the RTK-->Ras-->MAPK cascade involved in vulval induction.

Molecular genetic approaches to nociceptor development and function

The activation of peripheral nociceptors is the subject of intense scrutiny, because of its significance in pain regulation. Genetic approaches, including homology cloning, difference cloning and transgenic manipulation of mice are providing useful insights into nociceptor function. Recent work suggests that transcriptional regulators (for example, islet-I), which are expressed relatively selectively in sensory neurones, play a crucial role in defining cellular phenotype. Difference cloning has identified genes which encode both ligand-gated and voltage-gated ion channels expressed by small-diameter sensory neurones. The role of inflammatory mediators such as NGF in regulating nociceptor function has been clarified in mis-expression and deletion studies. An understanding of the mechanisms that regulate gene expression in nociceptors should provide new ways to manipulate nociceptor sensitivity, with potential significance for pain therapy.

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

During the 4-cell stage of C. elegans embryogenesis, the P2 blastomere provides a signal that allows two initially equivalent sister blastomeres, called ABa and ABp, to adopt different fates. Preventing P2 signalling in wild-type embryos results in defects in ABp development that are similar to those caused by mutations in the glp-1 and apx-1 genes, which are homologs of the Drosophila genes Notch and Delta, respectively. Previous studies have shown that GLP-1 protein is expressed in 4-cell stage embryos in both ABa and ABp. In this report, we show that APX-1 protein is expressed in the P2 blastomere and that a temperature-sensitive apx-1 mutant has a temperature-sensitive period between the 4-cell and 8-cell stages. We propose that APX-1 is part or all of the P2 signal that induces ABp to adopt a fate different than ABa.

T cell leukemia-associated human Notch/translocation-associated Notch homologue has I kappa B-like activity and physically interacts with nuclear factor-kappa B proteins in T cells

Translocation-associated Notch homologue (TAN-1), a gene originally cloned from the translocation breakpoint of a human T cell leukemia carrying a 9:7(q34.3) translocation, encodes a protein belonging to the Notch/Lin-12/Glp-1 receptor family. These receptors mediate the specification of numerous cell fates during development in invertebrates and vertebrates. The intracellular portion of Notch/TAN-1 contains six ankyrin repeats that are similar to those found in cytoplasmic I kappa B proteins. I kappa B proteins are specific inhibitors of nuclear factor (NF)-kappa B/Rel transcription factors. Here we show that TAN-1 has functional properties of an I kappa B-like regulator with specificity for the NF-kappa B p50 subunit. A recombinant polypeptide corresponding to the cytoplasmic portion of TAN-1 (TAN-1C) specifically inhibited the DNA binding of p50-containing NF-kappa B complexes. When overexpressed in an appropriate cell line, TAN-1C prevented kappa B-dependent transactivation in transient reporter gene assays in a fashion similar to the structurally related protein, Bcl-3. TAN-1C could activate kappa B-dependent gene expression by attenuating the inhibitory effect of an excess of p50 homodimers. Immunoprecipitation experiments showed that the TAN-1 from a T cell line is associated with NF-kappa B containing p50 and p65 subunits. These observations indicate that TAN-1C may directly engage NF-kappa B transcription factors and modulate nuclear gene expression.

lag-1, a gene required for lin-12 and glp-1 signaling in Caenorhabditis elegans, is homologous to human CBF1 and Drosophila Su(H)

The homologous receptors LIN-12 and GLP-1 mediate diverse cell-signaling events during development of the nematode Caenorhabditis elegans. These two receptors appear to be functionally interchangeable and have sequence similarity to Drosophila Notch. Here we focus on a molecular analysis of the lag-1 gene (lin-12 -and glp-1), which plays a central role in LIN-12 and GLP-1-mediated signal transduction. We find that the predicted LAG-1 protein is homologous to two DNA-binding proteins: human C Promoter Binding Factor (CBF1) and Drosophila Suppressor of Hairless (Su(H)). Furthermore, we show that LAG-1 binds specifically to the DNA sequence RTGGGAA, previously identified as a CBF-1/Su(H)-binding site. Finally, we report that the 5′ flanking regions and first introns of the lin-12, glp-1 and lag-1 genes are enriched for potential LAG-1-binding sites. We propose that LAG-1 is a transcriptional regulator that serves as a primary link between the LIN-12 and GLP-1 receptors and downstream target genes in C. elegans. In addition, we propose that LAG-1 may be a key component of a positive feedback loop that amplifies activity of the LIN-12/GLP-1 pathway.

Recent advances in understanding signal transduction pathways in worms and flies

One major challenge in the fields of signal transduction and pattern formation is to understand how multiple signals are integrated to determine cell fates. Two developmental systems, vulval development in Caenorhabditis elegans and axis formation during Drosophila melanogaster oogenesis, require the epidermal growth factor receptor tyrosine kinase and the NOTCH signaling pathways to specify cell fates. Current work in both systems has provided new opportunities to investigate the potential for the cross-talk between these different signaling pathways.

Heterochronic genes control cell cycle progress and developmental competence of C. elegans vulva precursor cells

Heterochronic genes control the timing of vulval development in the C. elegans hermaphrodite. lin-14 or lin-28 loss-of-function mutations cause the vulval precursor cells (VPCs) to enter S phase and to divide one larval stage earlier than in the wild type. A precocious vulva is formed by essentially normal cell lineage patterns, governed by the same intercellular signals as in the wild type. Mutations that prevent the normal developmental down-regulation of lin-14, activity delay or block VPC division and prevent vulval differentiation. A genetic pathway that includes lin-4, lin-14, and lin-28 controls when VPCs complete G1 and also controls when VPCs acquire the competence to respond to the intercellular patterning signals and express vulval fates.

Truncated mammalian Notch1 activates CBF1/RBPJk-repressed genes by a mechanism resembling that of Epstein-Barr virus EBNA2

The Notch/Lin-12/Glp-1 receptor family participates in cell-cell signaling events that influence cell fate decisions. Although several Notch homologs and receptor ligands have been identified, the nuclear events involved in this pathway remain incompletely understood. A truncated form of Notch, consisting only of the intracellular domain (NotchIC), localizes to the nucleus and functions as an activated receptor. Using both an in vitro binding assay and a cotransfection assay based on the two-hybrid principle, we show that mammalian NotchIC interacts with the transcriptional repressor CBF1, which is the human homolog of Drosophila Suppressor of Hairless. Cotransfection assays using segments of mouse NotchIC and CBF1 demonstrated that the N-terminal 114-amino-acid region of mouse NotchIC contains the CBF1 interactive domain and that the cdc10/ankyrin repeats are not essential for this interaction. This result was confirmed in immunoprecipation assays in which the N-terminal 114-amino-acid segment of NotchIC, but not the ankyrin repeat region, coprecipitated with CBF1. Mouse NotchIC itself is targeted to the transcriptional repression domain (aa179 to 361) of CBF1. Furthermore, transfection assays in which mouse NotchIC was targeted through Gal4-CBF1 or through endogenous cellular CBF1 indicated that NotchIC transactivates gene expression via CBF1 tethering to DNA. Transactivation by NotchIC occurs partially through abolition of CBF1-mediated repession. This same mechanism is used by Epstein-Barr virus EBNA2. Thus, mimicry of Notch signal transduction is involved in Epstein-Barr virus-driven immortalization.

The Caenorhabditis elegans gene lin-1 encodes an ETS-domain protein and defines a branch of the vulval induction pathway

The Caenorhabditis elegans gene lin-1 appears to act after the Ras-Raf-MEK-MAPK signaling cascade that mediates vulval induction. We show that lin-1 is a negative regulator of vulval cell fates and encodes an ETS-domain putative transcription factor containing potential MAPK phosphorylation sites. In lin-1 null mutants, the vulval precursor cells (VPCs) still respond to signaling from the gonadal anchor cell, indicating that lin-1 defines a branch of the inductive signaling pathway. We also provide evidence that the inductive and lateral signaling pathways are integrated to control the 1 degree and 2 degrees vulval cell fates after the point at which lin-1 acts in the inductive pathway and that VPCs can assess the relative rather than absolute levels of inductive and lateral signaling in determining whether to express the 1 degree or 2 degrees vulval cell fates.

APX-1 can substitute for its homolog LAG-2 to direct cell interactions throughout Caenorhabditis elegans development

The homologous LAG-2 and APX-1 membrane proteins are putative signaling ligands in the GLP-1/LIN-12 signal-transduction pathway in Caenorhabditis elegans. Normally, LAG-2 and APX-1 mediate distinct cell interactions. Here, we demonstrate that APX-1, which normally interacts with GLP-1 in the early embryo, can substitute for LAG-2 throughout development. When expressed under control of the lag-2 promoter, an apx-1 cDNA can completely rescue a lag-2 null mutant. To substitute for LAG-2, APX-1 must be able to interact with both GLP-1 and LIN-12 receptors and to mediate a variety of cell interactions during development. Therefore, APX-1 and LAG-2 are essentially equivalent in their ability to influence receptor activity. On the basis of this result, we suggest that the existence of multiple-signaling ligands in the LIN-12/GLP-1 signal transduction pathway does not reflect the evolution of functionally distinct proteins but rather the imposition of distinct controls of gene expression upon functionally similar proteins. Finally, we propose that the specification of distinct cell fates by the LIN-12/GLP-1 signal-transduction pathway relies on activities functioning downstream of the ligand and receptor, rather than on specific ligand-receptor interactions.

Establishment of left-right asymmetry in the Caenorhabditis elegans embryo: a multistep process involving a series of inductive events

Bilateral pairs of blastomeres derived from the founder cell AB, the anterior blastomere of the 2-cell stage, in the Caenorhabditis elegans embryo are initially equivalent in their developmental potential. Recently, we showed that an induction at the 12-cell stage by a blastomere called MS is necessary to establish the differences between left and right pairs of blastomeres in the anterior part of the embryo. Further analysis of the process of creating left-right asymmetry reveals that the induction at the 12-cell stage is only the first of a series of inductions establishing the left-right asymmetry of the embryo. We describe here two further inductions that create additional asymmetries in the posterior part of the embryo. One induction occurs at the 24-cell stage among AB descendants themselves. This induction is restricted to the left side of the embryo as a consequence of the fate changes induced by MS at the 12-cell stage. The second induction requires again blastomeres of the MS lineage and also occurs around the 24-cell stage. Together these inductions establish the fate differences observed in the development of left-right pairs of blastomeres in the embryo.

sur-2, a novel gene, functions late in the let-60 ras-mediated signaling pathway during Caenorhabditis elegans vulval induction

We describe here a new gene acting downstream of let-60 ras in the vulval signaling pathway of Caenorhabditis elegans. The sur-2 (suppressor of ras) gene is defined by eight mutations identified in a genetic screen for suppressors of the Multivulva phenotype of let-60(n1046), an activated let-60 ras mutation. sur-2 mutations result in pleiotropic, incompletely penetrant phenotypes that include a Vulvaless phenotype in hermaphrodites, defects in development of the male tail, gonadal abnormalities, and larval lethality, indicating a role for the sur-2 gene product in multiple developmental events. Genetic epistasis analyses suggest that sur-2 is required late in the vulval signaling pathway, downstream of let-60 Ras, and is likely to act downstream of the Raf/MAP Kinase cascade. We cloned the sur-2 gene by DNA-mediated transformation and have shown that it encodes a novel protein. We also show that a sur-2::lacZ transgene is expressed in the vulval precursor cells at the time of vulval determination.