Caenorhabditis elegans lin-25: cellular focus, protein expression and requirement for sur-2 during induction of vulval fates

The Signaling Network Controlling C. elegans Vulval Cell Fate Patterning

EGF, emitted by the Anchor Cell, patterns six equipotent C. elegans vulval precursor cells to assume a precise array of three cell fates with high fidelity. A group of core and modulatory signaling cascades forms a signaling network that demonstrates plasticity during the transition from naïve to terminally differentiated cells. In this review, we summarize the history of classical developmental manipulations and molecular genetics experiments that led to our understanding of the signals governing this process, and discuss principles of signal transduction and developmental biology that have emerged from these studies.

Integration of EGFR and LIN-12/Notch Signaling by LIN-1/Elk1, the Cdk8 Kinase Module, and SUR-2/Med23 in Vulval Precursor Cell Fate Patterning in Caenorhabditis elegans

Six initially equivalent, multipotential Vulval Precursor Cells (VPCs) in Caenorhabditis elegans adopt distinct cell fates in a precise spatial pattern, with each fate associated with transcription of different target genes. The pattern is centered on a cell that adopts the "1°" fate through Epidermal Growth Factor Receptor (EGFR) activity, and produces a lateral signal composed of ligands that activate LIN-12/Notch in the two flanking VPCs to cause them to adopt "2°" fate. Here, we investigate orthologs of a transcription complex that acts in mammalian EGFR signaling-lin-1/Elk1, sur-2/Med23, and the Cdk8 Kinase module (CKM)-previously implicated in aspects of 1° fate in C. elegans and show they act in different combinations for different processes for 2° fate. When EGFR is inactive, the CKM, but not SUR-2, helps to set a threshold for LIN-12/Notch activity in all VPCs. When EGFR is active, all three factors act to resist LIN-12/Notch, as revealed by the reduced ability of ectopically-activated LIN-12/Notch to activate target gene reporters. We show that overcoming this resistance in the 1° VPC leads to repression of lateral signal gene reporters, suggesting that resistance to LIN-12/Notch helps ensure that P6.p becomes a robust source of the lateral signal. In addition, we show that sur-2/Med23 and lin-1/Elk1, and not the CKM, are required to promote endocytic downregulation of LIN-12-GFP in the 1° VPC. Finally, our analysis using cell fate reporters reveals that both EGFR and LIN-12/Notch signal transduction pathways are active in all VPCs in lin-1/Elk1 mutants, and that lin-1/Elk1 is important for integrating EGFR and lin-12/Notch signaling inputs in the VPCs so that the proper gene complement is transcribed.

Comparative genomics supports a deep evolutionary origin for the large, four-module transcriptional mediator complex

The multisubunit Mediator (MED) complex bridges DNA-bound transcriptional regulators to the RNA polymerase II (PolII) initiation machinery. In yeast, the 25 MED subunits are distributed within three core subcomplexes and a separable kinase module composed of Med12, Med13 and the Cdk8-CycC pair thought to control the reversible interaction between MED and PolII by phosphorylating repeated heptapeptides within the Rpb1 carboxyl-terminal domain (CTD). Here, MED conservation has been investigated across the eukaryotic kingdom. Saccharomyces cerevisiae Med2, Med3/Pgd1 and Med5/Nut1 subunits are apparent homologs of metazoan Med29/Intersex, Med27/Crsp34 and Med24/Trap100, respectively, and these and other 30 identified human MED subunits have detectable counterparts in the amoeba Dictyostelium discoideum, indicating that none is specific to metazoans. Indeed, animal/fungal subunits are also conserved in plants, green and red algae, entamoebids, oomycetes, diatoms, apicomplexans, ciliates and the 'deep-branching' protists Trichomonas vaginalis and Giardia lamblia. Surprisingly, although lacking CTD heptads, T. vaginalis displays 44 MED subunit homologs, including several CycC, Med12 and Med13 paralogs. Such observations have allowed the identification of a conserved 17-subunit framework around which peripheral subunits may be assembled, and support a very ancient eukaryotic origin for a large, four-module MED. The implications of this comprehensive work for MED structure-function relationships are discussed.

Genetic control of fusion pore expansion in the epidermis of Caenorhabditis elegans

Developmental cell fusion is found in germlines, muscles, bones, placentae, and stem cells. In Caenorhabditis elegans 300 somatic cells fuse during development. Although there is extensive information on the early intermediates of viral-induced and intracellular membrane fusion, little is known about late stages in membrane fusion. To dissect the pathway of cell fusion in C. elegans embryos, we use genetic and kinetic analyses using live-confocal and electron microscopy. We simultaneously monitor the rates of multiple cell fusions in developing embryos and find kinetically distinct stages of initiation and completion of membrane fusion in the epidermis. The stages of cell fusion are differentially blocked or retarded in eff-1 and idf-1 mutants. We generate kinetic cell fusion maps for embryos grown at different temperatures. Different sides of the same cell differ in their fusogenicity: the left and right membrane domains are fusion-incompetent, whereas the anterior and posterior membrane domains fuse with autonomous kinetics in embryos. All but one cell pair can initiate the formation of the largest syncytium. The first cell fusion does not trigger a wave of orderly fusions in either direction. Ultrastructural studies show that epidermal syncytiogenesis require eff-1 activities to initiate and expand membrane merger.

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

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

The story of cell fusion: big lessons from little worms

The ability of two or more cells to unite to form a new syncytial cell has been utilized in metazoans throughout evolution to form many complex organs, such as muscles, bones and placentae. This requires migration, recognition and adhesion between cells together with fusion of their plasma membranes and rearrangement of their cytoplasmic contents. Until recently, understanding of the mechanisms of cell fusion was restricted to fusion between enveloped viruses and their target cells. The identification of new factors that take part in developmental cell fusion in C. elegans opens the way to understanding how cells fuse and what the functions of this process are. In this review, we describe current knowledge on the mechanisms and putative roles of developmental cell fusion in C. elegans and how cell fusion is regulated, together with other intercellular processes to promote organogenesis.

C. elegans EOR-1/PLZF and EOR-2 positively regulate Ras and Wnt signaling and function redundantly with LIN-25 and the SUR-2 Mediator component

In Caenorhabditis elegans, Ras/ERK and Wnt/beta-catenin signaling pathways cooperate to induce P12 and vulval cell fates in a Hox-dependent manner. Here we describe eor-1 and eor-2, two new positively acting nuclear components of the Ras and Wnt pathways. eor-1 and eor-2 act downstream or in parallel to ERK and function redundantly with the Mediator complex gene sur-2 and the functionally related gene lin-25, such that removal of both eor-1/eor-2 and sur-2/lin-25 mimics the removal of a main Ras pathway component. Furthermore, the eor-1 and eor-2 mutant backgrounds reveal an essential role for the Elk1-related gene lin-1. eor-1 and eor-2 also act downstream or in parallel to pry-1 Axin and therefore act at the convergence of the Ras and Wnt pathways. eor-1 encodes the ortholog of human PLZF, a BTB/zinc-finger transcription factor that is fused to RARalpha in acute promyelocytic leukemia. eor-2 encodes a novel protein. EOR-1/PLZF and EOR-2 appear to function closely together and cooperate with Hox genes to promote the expression of Ras- and Wnt-responsive genes. Further studies of eor-1 and eor-2 may provide insight into the roles of PLZF in normal development and leukemogenesis.

A lin-45 raf enhancer screen identifies eor-1, eor-2 and unusual alleles of Ras pathway genes in Caenorhabditis elegans

In Caenorhabditis elegans, the Ras/Raf/MEK/ERK signal transduction pathway controls multiple processes including excretory system development, P12 fate specification, and vulval cell fate specification. To identify positive regulators of Ras signaling, we conducted a genetic screen for mutations that enhance the excretory system and egg-laying defects of hypomorphic lin-45 raf mutants. This screen identified unusual alleles of several known Ras pathway genes, including a mutation removing the second SH3 domain of the sem-5/Grb2 adaptor, a temperature-sensitive mutation in the helical hairpin of let-341/Sos, a gain-of-function mutation affecting a potential phosphorylation site of the lin-1 Ets domain transcription factor, a dominant-negative allele of ksr-1, and hypomorphic alleles of sur-6/PP2A-B, sur-2/Mediator, and lin-25. In addition, this screen identified multiple alleles of two newly identified genes, eor-1 and eor-2, that play a relatively weak role in vulval fate specification but positively regulate Ras signaling during excretory system development and P12 fate specification. The spectrum of identified mutations argues strongly for the specificity of the enhancer screen and for a close involvement of eor-1 and eor-2 in Ras signaling.

Cell proliferation and growth in C. elegans

The cell division and differentiation events that occur during the development of the nematode Caenorhabditis elegans are nearly identical between different individuals, a feature that distinguishes this organism from larger and more complex metazoans, such as humans and Drosophila. In view of this discrepancy, it might be expected that the regulation of cell growth, division and differentiation in C. elegans would involve mechanisms separate from those utilized in larger animals. However, the results of recent genetic, molecular and cellular studies indicate that C. elegans employs an arsenal of developmental regulatory mechanisms quite similar to those wielded by its arthropod and vertebrate relatives. Thus, the nematode system is providing both novel and complementary insights into the general problem of how growth and patterning events are integrated in development. This review offers a general perspective on the regulation of cell division and growth in C. elegans, emphasizing recent studies of these crucial aspects of development.

Caenorhabditis elegans lin-25: a study of its role in multiple cell fate specification events involving Ras and the identification and characterization of evolutionarily conserved domains

Caenorhabditis elegans lin-25 functions downstream of let-60 ras in the genetic pathway for the induction of the 1 degrees cell fate during vulval development and encodes a novel 130-kD protein. The biochemical activity of LIN-25 is presently unknown, but the protein appears to function together with SUR-2, whose human homologue binds to Mediator, a protein complex required for transcriptional regulation. We describe here experiments that indicate that, besides its role in vulval development, lin-25 also participates in the fate specification of a number of other cells in the worm that are known to require Ras-mediated signaling. We also describe the cloning of a lin-25 orthologue from C. briggsae. Sequence comparisons suggest that the gene is evolving relatively rapidly. By characterizing the molecular lesions associated with 10 lin-25 mutant alleles and by assaying in vivo the activity of mutants lin-25 generated in vitro, we have identified three domains within LIN-25 that are required for activity or stability. We have also identified a sequence that is required for efficient nuclear translocation. We discuss how lin-25 might act in cell fate specification in C. elegans within the context of models for lin-25 function in cell identity and cell signaling.

Expression and function of members of a divergent nuclear receptor family in Caenorhabditis elegans

Nuclear receptors (NRs) are a large class of ligand-regulated transcriptional modulators that have been shown to play roles in many developmental processes. The Caenorhabditis elegans genome is predicted to encode a large and divergent family of NR proteins. The functions of most of these genes are unknown. As a first step toward defining their roles, we have initiated an expression and functional survey of a subset of these genes. In this study, we demonstrate expression of 21 of 28 NR genes examined, indicating that a large fraction of the predicted genes likely encode functional gene products. We show that five genes are expressed predominantly in neuronal cells, while others are expressed in multiple cell types. Interestingly, we find that eight genes are expressed exclusively in the lateral hypodermal (seam) cells. These eight genes share a high degree of overall homology and cluster in a neighbor-joining tree derived from sequence analysis of the NRs, suggesting that they arose by gene duplication from a common ancestor. We show that overexpression of each of three members of this subfamily results in similar developmental defects, consistent with a redundant role for these genes in the function of the lateral hypodermal cells.