EEL-1, a Hect E3 ubiquitin ligase, controls asymmetry and persistence of the SKN-1 transcription factor in the early C. elegans embryo

MBL-1 and EEL-1 affect the splicing and protein levels of MEC-3 to control dendrite complexity

Transcription factors (TFs) play critical roles in specifying many aspects of neuronal cell fate including dendritic morphology. How TFs are accurately regulated during neuronal morphogenesis is not fully understood. Here, we show that LIM homeodomain protein MEC-3, the key TF for C. elegans PVD dendrite morphogenesis, is regulated by both alternative splicing and an E3 ubiquitin ligase. The mec-3 gene generates several transcripts by alternative splicing. We find that mbl-1, the orthologue of the muscular dystrophy disease gene muscleblind-like (MBNL), is required for PVD dendrite arbor formation. Our data suggest mbl-1 regulates the alternative splicing of mec-3 to produce its long isoform. Deleting the long isoform of mec-3(deExon2) causes reduction of dendrite complexity. Through a genetic modifier screen, we find that mutation in the E3 ubiquitin ligase EEL-1 suppresses mbl-1 phenotype. eel-1 mutants also suppress mec-3(deExon2) mutant but not the mec-3 null phenotype. Loss of EEL-1 alone leads to excessive dendrite branches. Together, these results indicate that MEC-3 is fine-tuned by alternative splicing and the ubiquitin system to produce the optimal level of dendrite branches.

The giant E3 ligase HUWE1 is linked to tumorigenesis, spermatogenesis, intellectual disability, and inflammatory diseases

E3 ubiquitin ligases determine the substrate specificity and catalyze the ubiquitination of lysine residues. HUWE1 is a catalytic HECT domain-containing giant E3 ligase that contains a substrate-binding ring structure, and mediates the ubiquitination of more than 40 diverse substrates. HUWE1 serves as a central node in cellular stress responses, cell growth and death, signal transduction, etc. The expanding atlas of HUWE1 substrates presents a major challenge for the potential therapeutic application of HUWE1 in a particular disease. In addition, HUWE1 has been demonstrated to play contradictory roles in certain aspects of tumor progression in either an oncogenic or a tumor-suppressive manner. We recently defined novel roles of HUWE1 in promoting the activation of multiple inflammasomes. Inflammasome activation-mediated immune responses might lead to multifunctional effects on tumor therapy, inflammation, and autoimmune diseases. In this review, we summarize the known substrates and pleiotropic functions of HUWE1 in different types of cells and models, including its involvement in development, cancer, neuronal disorder and infectious disease. We also discuss the advances in cryo-EM-structural analysis for a functional-mechanistic understanding of HUWE1 in modulating the multitudinous diverse substrates, and introduce the possibility of revisiting the comprehensive roles of HUWE1 in multiple aspects within one microenvironment, which will shed light on the potential therapeutic application of targeting giant E3 ligases like HUWE1.

Involvement of HECT-type E3 ubiquitin ligase genes in salt chemotaxis learning in Caenorhabditis elegans

The ubiquitin-proteasome system is associated with various phenomena including learning and memory. In this study, we report that E3 ubiquitin ligase homologs and proteasome function are involved in taste avoidance learning, a type of associative learning between starvation and salt concentrations, in Caenorhabditis elegans. Pharmacological inhibition of proteasome function using bortezomib causes severe defects in taste avoidance learning. Among 9 HECT-type ubiquitin ligase genes, loss-of-function mutations of 6 ubiquitin ligase genes cause significant abnormalities in taste avoidance learning. Double mutations of those genes cause lethality or enhanced defects in taste avoidance learning, suggesting that the HECT-type ubiquitin ligases act in multiple pathways in the processes of learning. Furthermore, mutations of the ubiquitin ligase genes cause additive effects on taste avoidance learning defects of the insulin-like signaling mutants. Our findings unveil the consequences of aberrant functions of the proteasome and ubiquitin systems in learning behavior of Caenorhabditis elegans.

An interphase contractile ring reshapes primordial germ cells to allow bulk cytoplasmic remodeling

Some cells discard undesired inherited components in bulk by forming large compartments that are subsequently eliminated. Caenorhabditis elegans primordial germ cells (PGCs) jettison mitochondria and cytoplasm by forming a large lobe that is cannibalized by intestinal cells. Although PGCs are nonmitotic, we find that lobe formation is driven by constriction of a contractile ring and requires the RhoGEF ECT-2, a RhoA activator also essential for cytokinesis. Whereas centralspindlin activates ECT-2 to promote cytokinetic contractile ring formation, we show that the ECT-2 regulator NOP-1, but not centralspindlin, is essential for PGC lobe formation. We propose that lobe contractile ring formation is locally inhibited by the PGC nucleus, which migrates to one side of the cell before the cytokinetic ring assembles on the opposite cortex. Our findings reveal how components of the cytokinetic contractile ring are reemployed during interphase to create compartments used for cellular remodeling, and they reveal differences in the spatial cues that dictate where the contractile ring will form.

Roles of the HUWE1 ubiquitin ligase in nervous system development, function and disease

Huwe1 is a highly conserved member of the HECT E3 ubiquitin ligase family. Here, we explore the growing importance of Huwe1 in nervous system development, function and disease. We discuss extensive progress made in deciphering how Huwe1 regulates neural progenitor proliferation and differentiation, cell migration, and axon development. We highlight recent evidence indicating that Huwe1 regulates inhibitory neurotransmission. In covering these topics, we focus on findings made using both vertebrate and invertebrate in vivo model systems. Finally, we discuss extensive human genetic studies that strongly implicate HUWE1 in intellectual disability, and heighten the importance of continuing to unravel how Huwe1 affects the nervous system.

Genome-wide identification of the CLAVATA3/EMBRYO SURROUNDING REGION (CLE) family in grape (Vitis vinifera L.)

Background

CLE genes play various biological roles in plant growth and development, as well as in responses to environmental stimuli.

Results

In the present study, we identified nine CLE genes in the grape genome using an effective identification method. We analyzed the expression profiles of grape CLE genes in different tissues and under environmental different stimuli. VvCLE3 was expressed in shoot apical meristem (SAM) enriched regions, and VvCLE6 was expressed in shoot tissue without SAM. When grapes were infected with bois noir, VvCLE2 was up-regulated. Under ABA treatment, VvCLE3 was down-regulated. VvCLE6 was up-regulated under high temperature stress. We found that VvCLE6 and VvCLE1 were highly expressed in root tissue. In addition, we compared the characteristics of CLEs from grape and other plant species. The CLE family in Sphagnum fallax underwent positive selection, while the CLE family in grape underwent purifying selection. The frequency of optimal codons and codon adaptation index of rice and grape CLE family members were positively correlated with GC content at the third site of synonymous codons, indicating that the dominant evolutionary pressure acting on rice and grape CLE genes was mutation pressure. We also found that closely related species had higher levels of similarity in relative synonymous codon usage in CLE genes. The rice CLE family was biased toward C and G nucleotides at third codon positions. Gene duplication and loss events were also found in grape CLE genes.

Conclusion

These results demonstrate an effective identification method for CLE motifs and increase the understanding of grape CLEs. Future research on CLE genes may have applications for grape breeding and cultivation to better understand root and nodulation development.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5944-2) contains supplementary material, which is available to authorized users.

BRAP-2 promotes DNA damage induced germline apoptosis in C. elegans through the regulation of SKN-1 and AKT-1

As part of the DNA damage response (DDR) network, the tumour suppressor Breast cancer susceptibility gene 1 (BRCA1) is activated to facilitate DNA repair, transcription and cell cycle control. BRC-1, the Caenorhabditis elegans ortholog of BRCA1, has conserved function in DNA double strand break repair, wherein a loss of brc-1 results in high levels of germline apoptosis. BRAP2/IMP was initially identified as a BRCA1 associated binding protein and previously we have shown that the C. elegans brap-2 deletion mutant experiences BRC-1 dependent larval arrest when exposed to low concentrations of paraquat. Since BRC-1 function in the germline is conserved, we wanted to determine the role of BRAP-2 in DNA damage induced germline apoptosis in C. elegans. We examined levels of germ cell death following DNA damage and found that brap-2(ok1492) mutants display reduced levels of germline apoptosis when compared to the wild type, and the loss of brap-2 significantly reduced germ cell death in brc-1 mutant animals. We also found increased mRNA levels of skn-1 following DNA damage in brap-2 mutants and that skn-1 RNAi knockdown in brap-2;brc-1 double mutants and a loss of pmk-1 mutation in brap-2 mutants increased apoptosis to wild type levels, indicating that brap-2 promotion of cell survival requires PMK-1 and SKN-1. Since mammalian BRAP2 has been shown to bind the AKT phosphatase PHLPP1/2, it suggests that BRAP2 could be involved in the Insulin/Insulin-like growth factor Signaling (IIS) pathway. We found that this interaction is conserved between the C. elegans homologs and that a loss of akt-1 in brap-2 mutants increased germline apoptosis. Thus in response to DNA damage, our findings suggest that BRAP-2 is required to attenuate the pro-cell survival signals of AKT-1 and PMK-1/SKN-1 to promote DNA damage induced germline apoptosis.

The HECT Family Ubiquitin Ligase EEL-1 Regulates Neuronal Function and Development

Genetic changes in the HECT ubiquitin ligase HUWE1 are associated with intellectual disability, but it remains unknown whether HUWE1 functions in post-mitotic neurons to affect circuit function. Using genetics, pharmacology, and electrophysiology, we show that EEL-1, the HUWE1 ortholog in C. elegans, preferentially regulates GABAergic presynaptic transmission. Decreasing or increasing EEL-1 function alters GABAergic transmission and the excitatory/inhibitory (E/I) balance in the worm motor circuit, which leads to impaired locomotion and increased sensitivity to electroshock. Furthermore, multiple mutations associated with intellectual disability impair EEL-1 function. Although synaptic transmission defects did not result from abnormal synapse formation, sensitizing genetic backgrounds revealed that EEL-1 functions in the same pathway as the RING family ubiquitin ligase RPM-1 to regulate synapse formation and axon termination. These findings from a simple model circuit provide insight into the molecular mechanisms required to obtain E/I balance and could have implications for the link between HUWE1 and intellectual disability.

E3 Ubiquitin Ligases Neurobiological Mechanisms: Development to Degeneration

Cells regularly synthesize new proteins to replace old or damaged proteins. Deposition of various aberrant proteins in specific brain regions leads to neurodegeneration and aging. The cellular protein quality control system develop various defense mechanisms against the accumulation of misfolded and aggregated proteins. The mechanisms underlying the selective recognition of specific crucial protein or misfolded proteins are majorly governed by quality control E3 ubiquitin ligases mediated through ubiquitin-proteasome system. Few known E3 ubiquitin ligases have shown prominent neurodevelopmental functions, but their interactions with different developmental proteins play critical roles in neurodevelopmental disorders. Several questions are yet to be understood properly. How E3 ubiquitin ligases determine the specificity and regulate degradation of a particular substrate involved in neuronal proliferation and differentiation is certainly the one, which needs detailed investigations. Another important question is how neurodevelopmental E3 ubiquitin ligases specifically differentiate between their versatile range of substrates and timing of their functional modulations during different phases of development. The premise of this article is to understand how few E3 ubiquitin ligases sense major molecular events, which are crucial for human brain development from its early embryonic stages to throughout adolescence period. A better understanding of these few E3 ubiquitin ligases and their interactions with other potential proteins will provide invaluable insight into disease mechanisms to approach toward therapeutic interventions.

Role of Nrf2/HO-1 system in development, oxidative stress response and diseases: an evolutionarily conserved mechanism

The multifunctional regulator nuclear factor erythroid 2-related factor (Nrf2) is considered not only as a cytoprotective factor regulating the expression of genes coding for anti-oxidant, anti-inflammatory and detoxifying proteins, but it is also a powerful modulator of species longevity. The vertebrate Nrf2 belongs to Cap 'n' Collar (Cnc) bZIP family of transcription factors and shares a high homology with SKN-1 from Caenorhabditis elegans or CncC found in Drosophila melanogaster. The major characteristics of Nrf2 are to some extent mimicked by Nrf2-dependent genes and their proteins including heme oxygenase-1 (HO-1), which besides removing toxic heme, produces biliverdin, iron ions and carbon monoxide. HO-1 and their products exert beneficial effects through the protection against oxidative injury, regulation of apoptosis, modulation of inflammation as well as contribution to angiogenesis. On the other hand, the disturbances in the proper HO-1 level are associated with the pathogenesis of some age-dependent disorders, including neurodegeneration, cancer or macular degeneration. This review summarizes our knowledge about Nrf2 and HO-1 across different phyla suggesting their conservative role as stress-protective and anti-aging factors.

Natural Genetic Variation Influences Protein Abundances in C. elegans Developmental Signalling Pathways

Complex traits, including common disease-related traits, are affected by many different genes that function in multiple pathways and networks. The apoptosis, MAPK, Notch, and Wnt signalling pathways play important roles in development and disease progression. At the moment we have a poor understanding of how allelic variation affects gene expression in these pathways at the level of translation. Here we report the effect of natural genetic variation on transcript and protein abundance involved in developmental signalling pathways in Caenorhabditis elegans. We used selected reaction monitoring to analyse proteins from the abovementioned four pathways in a set of recombinant inbred lines (RILs) generated from the wild-type strains N2 (Bristol) and CB4856 (Hawaii) to enable quantitative trait locus (QTL) mapping. About half of the cases from the 44 genes tested showed a statistically significant change in protein abundance between various strains, most of these were however very weak (below 1.3-fold change). We detected a distant QTL on the left arm of chromosome II that affected protein abundance of the phosphatidylserine receptor protein PSR-1, and two separate QTLs that influenced embryonic and ionizing radiation-induced apoptosis on chromosome IV. Our results demonstrate that natural variation in C. elegans is sufficient to cause significant changes in signalling pathways both at the gene expression (transcript and protein abundance) and phenotypic levels.

E3 ubiquitin ligases promote progression of differentiation during C. elegans embryogenesis

Regulated choice between cell fate maintenance and differentiation provides decision points in development to progress toward more restricted cell fates or to maintain the current one. Caenorhabditis elegans embryogenesis follows an invariant cell lineage where cell fate is generally more restricted upon each cell division. EMS is a progenitor cell in the four-cell embryo that gives rise to the endomesoderm. We recently found that when ubiquitin-mediated protein degradation is compromised, the anterior daughter of EMS, namely MS, reiterates the EMS fate. This observation demonstrates an essential function of ubiquitin-mediated protein degradation in driving the progression of EMS-to-MS differentiation. Here we report a genome-wide screen of the ubiquitin pathway and extensive lineage analyses. The results suggest a broad role of E3 ligases in driving differentiation progression. First, we identified three substrate-binding proteins for two Cullin-RING ubiquitin ligase (CRL) E3 complexes that promote the progression from the EMS fate to MS, namely LIN-23/β-TrCP and FBXB-3 for the CRL1/SCF complex and ZYG-11/ZYG-11B for the CRL2 complex. Genetic analyses suggest these E3 ligases function through a multifunctional protein OMA-1 and the endomesoderm lineage specifier SKN-1 to drive differentiation. Second, we found that depletion of components of the CRL1/SCF complex induces fate reiteration in all major founder cell lineages. These data suggest that regulated choice between self-renewal and differentiation is widespread during C. elegans embryogenesis as in organisms with regulative development, and ubiquitin-mediated protein degradation drives the choice towards differentiation. Finally, bioinformatic analysis of time series gene expression data showed that expression of E3 genes is transiently enriched during time windows of developmental stage transitions. Transcription factors show similar enrichment, but not other classes of regulatory genes. Based on these findings we propose that ubiquitin-mediated protein degradation, like many transcription factors, function broadly as regulators driving developmental progression during embryogenesis in C. elegans.

De novo inference of systems-level mechanistic models of development from live-imaging-based phenotype analysis

Elucidation of complex phenotypes for mechanistic insights presents a significant challenge in systems biology. We report a strategy to automatically infer mechanistic models of cell fate differentiation based on live-imaging data. We use cell lineage tracing and combinations of tissue-specific marker expression to assay progenitor cell fate and detect fate changes upon genetic perturbation. Based on the cellular phenotypes, we further construct a model for how fate differentiation progresses in progenitor cells and predict cell-specific gene modules and cell-to-cell signaling events that regulate the series of fate choices. We validate our approach in C. elegans embryogenesis by perturbing 20 genes in over 300 embryos. The result not only recapitulates current knowledge but also provides insights into gene function and regulated fate choice, including an unexpected self-renewal. Our study provides a powerful approach for automated and quantitative interpretation of complex in vivo information.

Depletion of a nucleolar protein activates xenobiotic detoxification genes in Caenorhabditis elegans via Nrf /SKN-1 and p53/CEP-1

The nucleolus has recently emerged as a major coordinator of cellular stress responses by regulating the tumor suppressor p53. However, it is not known if the nucleolus regulates the cap 'n' collar (CnC) transcription factors SKN-1 and Nrf2, which activate conserved antioxidant and detoxification responses in C. elegans and mammals, respectively. A screen for negative regulators of detoxification genes in C. elegans identified the conserved WD40 repeat containing protein WDR-46. This protein is highly conserved with yeast UTP7, which functions in 18S rRNA processing and assembly of the 40S small ribosomal subunit. WDR-46 is expressed in the nucleoli of multiple tissues in C. elegans and is required for rRNA processing. Mutation or silencing of WDR-46 activates the single C. elegans CnC homologue SKN-1 and increases expression of its target genes. Depletion of wdr-46 reduces lifespan and stress resistance and SKN-1 partially compensates. Lastly, the C. elegans p53 homologue CEP-1 is partially required for activation of gst-4 when wdr-46 or other ribosome processing genes are silenced but not when translation initiation genes are silenced suggesting that disruptions to nucleolar function can activate SKN-1 by a mechanism that involves p53/cep-1 and is independent of protein translation.

Dual regulation of the transcriptional activity of Nrf1 by β-TrCP- and Hrd1-dependent degradation mechanisms

A growing body of evidence suggests that Nrf1 is an inducible transcription factor that maintains cellular homeostasis. Under physiological conditions, Nrf1 is targeted to the endoplasmic reticulum (ER), implying that it translocates into the nucleus in response to an activating signal. However, the molecular mechanisms by which the function of Nrf1 is modulated remain poorly understood. Here, we report that two distinct degradation mechanisms regulate Nrf1 activity and the expression of its target genes. In the nucleus, β-TrCP, an adaptor for the SCF (Skp1-Cul1-F-box protein) ubiquitin ligase, promotes the degradation of Nrf1 by catalyzing its polyubiquitination. This activity requires a DSGLS motif on Nrf1, which is similar to the canonical β-TrCP recognition motif. The short interfering RNA (siRNA)-mediated silencing of β-TrCP markedly augments the expression of Nrf1 target genes, such as the proteasome subunit PSMC4, indicating that β-TrCP represses Nrf1 activation. Meanwhile, in the cytoplasm, Nrf1 is degraded and suppressed by the ER-associated degradation (ERAD) ubiquitin ligase Hrd1 and valosin-containing protein (VCP) under normal conditions. We identified a cytoplasmic degradation motif on Nrf1 between the NHB1 and NHB2 domains that exhibited species conservation. Thus, these results clearly suggest that both β-TrCP- and Hrd1-dependent degradation mechanisms regulate the transcriptional activity of Nrf1 to maintain cellular homeostasis.

The branched actin nucleator Arp2/3 promotes nuclear migrations and cell polarity in the C. elegans zygote

Regulated movements of the nucleus are essential during zygote formation, cell migrations, and differentiation of neurons. The nucleus moves along microtubules (MTs) and is repositioned on F-actin at the cellular cortex. Two families of nuclear envelope proteins, SUN and KASH, link the nucleus to the actin and MT cytoskeletons during nuclear movements. However, the role of actin nucleators in nuclear migration and positioning is poorly understood. We show that the branched actin nucleator, Arp2/3, affects nuclear movements throughout embryonic and larval development in C. elegans, including nuclear migrations in epidermal cells and neuronal precursors. In one-cell embryos the migration of the male pronucleus to meet the female pronucleus after fertilization requires Arp2/3. Loss of Arp2/3 or its activators changes the dynamics of non-muscle myosin, NMY-2, and alters the cortical accumulation of posterior PAR proteins. Reduced establishment of the posterior microtubule cytoskeleton in Arp2/3 mutants correlates with reduced male pronuclear migration. The UNC-84/SUN nuclear envelope protein that links the nucleus to the MT and actin cytoskeleton is known to regulate later nuclear migrations. We show here it also positions the male pronucleus. These studies demonstrate a global role for Arp2/3 in nuclear migrations. In the C. elegans one-cell embryo Arp2/3 promotes the establishment of anterior/posterior polarity and promotes MT growth that propels the anterior migration of the male pronucleus. In contrast with previous studies emphasizing pulling forces on the male pronucleus, we propose that robust MT nucleation pushes the male pronucleus anteriorly to join the female pronucleus.

The EEL-1 ubiquitin ligase promotes DNA damage-induced germ cell apoptosis in C. elegans

E3 ubiquitin ligases target a growing number of pro- and anti-apoptotic proteins, including tumour suppressor p53, caspases, and the Bcl-2 family. The core apoptosis pathway is well conserved between mammals and Caenorhabditis elegans, but the extent to which ubiquitin ligases regulate apoptotic cell death is not known. To investigate the role of E3 ligases in apoptosis, we inhibited 108 of the 165 predicted E3 ubiquitin ligase genes by RNA interference and quantified apoptosis in the C. elegans germline after genotoxic stress. From this screen, we identified the homologous to E6-associated protein C terminus-domain E3 ligase EEL-1 as a positive regulator of apoptosis. Intriguingly, the human homologue of EEL-1, Huwe1/ARF-BP1/Mule/HectH9, has been reported to possess both pro- and anti-apoptotic functions through its ability to stimulate Mcl-1 and p53 degradation, respectively. Here, we demonstrate that eel-1 is required to promote DNA damage-induced germ cell apoptosis, but does not have a role in physiological germ cell apoptosis or developmental apoptosis in somatic tissue. Furthermore, eel-1 acts in parallel to the p53-like gene cep-1 and intersects the core apoptosis pathway upstream of the Bcl-2/Mcl-1 orthologue ced-9. Although ee1-1 mutants exhibit hypersensitivity to genotoxic stress they do not appear to be defective in DNA repair, suggesting a distinct role for EEL-1 in promoting damage-induced apoptosis in the germline.

Neuron-type specific regulation of a 3'UTR through redundant and combinatorially acting cis-regulatory elements

3' Untranslated region (UTR)-dependent post-transcriptional regulation has emerged as a critical mechanism of controlling gene expression in various physiological contexts, including cellular differentiation events. Here, we examine the regulation of the 3'UTR of the die-1 transcription factor in a single neuron of the nematode C. elegans. This 3'UTR shows the intriguing feature of being differentially regulated across the animal's left/right axis. In the left gustatory neuron, ASEL, in which DIE-1 protein is normally expressed in adult animals, the 3'UTR confers no regulatory information, while in the right gustatory neuron, ASER, where DIE-1 is normally not expressed, this 3'UTR confers negative regulatory information. Here, we systematically analyze the cis-regulatory architecture of the die-1 3'UTR using a transgenic, in vivo assay system. Through extensive mutagenesis and sequence insertions into heterologous 3'UTR contexts, we describe three 25-base-pair (bp) sequence elements that are both required and sufficient to mediate the ASER-specific down-regulation of the die-1 3'UTR. These three 25-bp sequence elements operate in both a redundant and combinatorial manner. Moreover, there are not only redundant elements within the die-1 3'UTR regulating its left/right asymmetric activity but asymmetric 3'UTR regulation is itself redundant with other regulatory mechanisms to achieve asymmetric DIE-1 protein expression and function in ASEL versus ASER. The features of 3'UTR regulation we describe here may apply to some of the vast number of genes in animal genomes whose expression is predicted to be regulated through their 3'UTR.

Roles of the Wnt effector POP-1/TCF in the C. elegans endomesoderm specification gene network

In C. elegans the 4-cell stage blastomere EMS is an endomesodermal precursor. Its anterior daughter, MS, makes primarily mesodermal cells, while its posterior daughter E generates the entire intestine. The gene regulatory network underlying specification of MS and E has been the subject of study for more than 15 years. A key component of the specification of the two cells is the involvement of the Wnt/beta-catenin asymmetry pathway, which through its nuclear effector POP-1, specifies MS and E as different from each other. Loss of pop-1 function results in the mis-specification of MS as an E-like cell, because POP-1 directly represses the end-1 and end-3 genes in MS, which would otherwise promote an endoderm fate. A long-standing question has been whether POP-1 plays a role in specifying MS fate beyond repression of endoderm fate. This question has been difficult to ask because the only chromosomal lesions that remove both end-1 and end-3 are large deletions removing hundreds of genes. Here, we report the construction of bona fide end-1 end-3 double mutants. In embryos lacking activity of end-1, end-3 and pop-1 together, we find that MS fate is partially restored, while E expresses early markers of MS fate and adopts characteristics of both MS and C. Our results suggest that POP-1 is not critical for MS specification beyond repression of endoderm specification, and reveal that Wnt-modified POP-1 and END-1/3 further reinforce E specification by repressing MS fate in E. By comparison, a previous work suggested that in the related nematode C. briggsae, Cb-POP-1 is not required to repress endoderm specification in MS, in direct contrast with Ce-POP-1, but is critical for repression of MS fate in E. The findings reported here shed new light on the flexibility of combinatorial control mechanisms in endomesoderm specification in Caenorhabditis.

The NK-2 class homeodomain factor CEH-51 and the T-box factor TBX-35 have overlapping function in C. elegans mesoderm development

The C. elegans MS blastomere, born at the 7-cell stage of embryogenesis, generates primarily mesodermal cell types, including pharynx cells, body muscles and coelomocytes. A presumptive null mutation in the T-box factor gene tbx-35, a target of the MED-1 and MED-2 divergent GATA factors, was previously found to result in a profound decrease in the production of MS-derived tissues, although the tbx-35(-) embryonic arrest phenotype was variable. We report here that the NK-2 class homeobox gene ceh-51 is a direct target of TBX-35 and at least one other factor, and that CEH-51 and TBX-35 share functions. Embryos homozygous for a ceh-51 null mutation arrest as larvae with pharynx and muscle defects, although these tissues appear to be specified correctly. Loss of tbx-35 and ceh-51 together results in a synergistic phenotype resembling loss of med-1 and med-2. Overexpression of ceh-51 causes embryonic arrest and generation of ectopic body muscle and coelomocytes. Our data show that TBX-35 and CEH-51 have overlapping function in MS lineage development. As T-box regulators and NK-2 homeodomain factors are both important for heart development in Drosophila and vertebrates, our results suggest that these regulators function in a similar manner in C. elegans to specify a major precursor of mesoderm.