Dominant unc-37 mutations suppress the movement defect of a homeodomain mutation in unc-4, a neural specificity gene in Caenorhabditis elegans

Function of the C. elegans T-box factor TBX-2 depends on interaction with the UNC-37/Groucho corepressor

T-box transcription factors are important regulators of development in all animals, and altered expression of T-box factors has been identified in an increasing number of diseases and cancers. Despite these important roles, the mechanism of T-box factor activity is not well understood. We have previously shown that the Caenorhabditis elegans Tbx2 subfamily member TBX-2 functions as a transcriptional repressor to specify ABa-derived pharyngeal muscle, and that this function depends on SUMOylation. Here we show that TBX-2 function also depends on interaction with the Groucho-family corepressor UNC-37. TBX-2 interacts with UNC-37 in yeast two-hybrid assays via a highly conserved engrailed homology 1 (eh1) motif located near the TBX-2 C-terminus. Reducing unc-37 phenocopies tbx-2 mutants, resulting in a specific loss of anterior ABa-derived pharyngeal muscles and derepression of the tbx-2 promoter. Moreover, double mutants containing hypomorphic alleles of unc-37 and tbx-2 exhibit enhanced phenotypes, providing strong genetic evidence that unc-37 and tbx-2 share common functions in vivo. To test whether interaction with UNC-37 is necessary for TBX-2 activity, we developed a transgene rescue assay using a tbx-2 containing fosmid and found that mutating the tbx-2 eh1 motif reduced rescue of a tbx-2 null mutant. These results indicate that TBX-2 function in vivo depends on interaction with UNC-37. As many T-box factors contain eh1 motifs, we suggest that interaction with Groucho-family corepressors is a common mechanism contributing to their activity.

A Caenorhabditis elegans locomotion phenotype caused by transgenic repeats of the hlh-17 promoter sequence

Transgene technology is one of the most heavily relied upon tools in modern biological research. Expression of an exogenous gene within cells, for research and therapeutic applications, nearly always includes promoters and other regulatory sequences. We found that repeats of a non-protein coding transgenic sequence produced profound changes to the behavior of the nematode Caenorhabditis elegans. These changes were produced by a glial promoter sequence but, unexpectedly, major deficits were observed specifically in backward locomotion, a neuron-driven behavior. We also present evidence that this behavioral phenotype is transpromoter copy number-dependent and manifests early in development and is maintained into adulthood of the worm.

On-chip analysis of C. elegans muscular forces and locomotion patterns in microstructured environments

The understanding of force interplays between an organism and its environment is imperative in biological processes. Noticeably scarce from the study of C. elegans locomotion is the measurement of the nematode locomotion forces together with other important locomotive metrics. To bridge the current gap, we present the investigation of C. elegans muscular forces and locomotion metrics (speed, amplitude and wavelength) in one single assay. This assay uses polydimethylsiloxane (PDMS) micropillars as force sensing elements and, by variation of the pillar arrangement, introduces microstructure. To show the usefulness of the assay, twelve wild-type C. elegans sample worms were tested to obtain a total of 4665 data points. The experimental results lead to several key findings. These include: (1) maximum force is exerted when the pillar is in contact with the middle part of the worm body, (2) C. elegans locomotion forces are highly dependent on the structure of the surrounding environment, (3) the worms' undulation frequency and locomotion speed increases steadily from the narrow spacing of 'honeycomb' design to the wider spacing of 'lattice' pillar arrangement, and (4) C. elegans maintained their natural sinusoidal movement in the microstructured device, despite the existence of PDMS micropillars. The assay presented here focuses on wild type C. elegans, but the method can be easily applied to its mutants and other organisms. In addition, we also show that, by inverting the measurement device, worm locomotion behaviour can be studied in various substrate environments normally unconducive to flexible pillar fabrication. The quantitative measurements demonstrated in this work further improve the understanding of C. elegans mechanosensation and locomotion.

Dissection of genetic pathways in C. elegans

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

The UNC-4 homeobox protein represses mab-9 expression in DA motor neurons in Caenorhabditis elegans

The T-box transcription factor mab-9 has been shown to be required for the correct fate of the male-specific blast cells B and F, normal posterior hypodermal morphogenesis, and for the correct axon migration of motor neurons that project circumferential commissures to dorsal muscles. In this study, an RNAi screen designed to identify upstream transcriptional regulators of mab-9 showed that silencing of unc-4 (encoding a paired-class homeodomain protein) increases mab-9::gfp expression in the nervous system, specifically in posterior DA motor neurons. Over-expression of unc-4 from a heat-shock promoter has the opposite effect, causing repression of mab-9 in various cells. We find that mab-9 expression in unc-37 mutants is also elevated in DA motor neurons, consistent with known roles for UNC-37 as a co-repressor with UNC-4. These results identify mab-9 as a novel target of the UNC-4/UNC-37 repressor complex in motor neurons, and suggest that mis-expression of mab-9 may contribute to the neuronal wiring defects in unc-4 and unc-37 mutants.

The Groucho ortholog UNC-37 interacts with the short Groucho-like protein LSY-22 to control developmental decisions in C. elegans

Transcriptional co-repressors of the Groucho/TLE family are important regulators of development in many species. A subset of Groucho/TLE family members that lack the C-terminal WD40 domains have been proposed to act as dominant-negative regulators of Groucho/TLE proteins, yet such a role has not been conclusively proven. Through a mutant screen for genes controlling a left/right asymmetric cell fate decision in the nervous system of the nematode C. elegans, we have retrieved loss-of-function alleles in two distinct loci that display identical phenotypes in neuronal fate specification and in other developmental contexts. Using the novel technology of whole-genome sequencing, we find that these loci encode the C. elegans ortholog of Groucho, UNC-37, and, surprisingly, a short Groucho-like protein, LSY-22, that is similar to truncated Groucho proteins in other species. Besides their phenotypic similarities, unc-37 and lsy-22 show genetic interactions and UNC-37 and LSY-22 proteins also physically bind to each other in vivo. Our findings suggest that rather than acting as negative regulators of Groucho, small Groucho-like proteins may promote Groucho function. We propose that Groucho-mediated gene regulatory events involve heteromeric complexes of distinct Groucho-like proteins.

Inhibition of cortical neuron differentiation by Groucho/TLE1 requires interaction with WRPW, but not Eh1, repressor peptides

In both invertebrates and vertebrates, transcriptional co-repressors of the Groucho/transducin-like Enhancer of split (Gro/TLE) family regulate a number of developmental mechanisms, including neuronal differentiation. The pleiotropic activity of Gro/TLE depends on context-specific interactions with a variety of DNA-binding proteins. Most of those factors engage Gro/TLE through two different types of short peptide motifs, the WRP(W/Y) tetrapeptide and the Engrailed homology 1 (Eh1) sequence (FXIXXIL). The aim of this study was to elucidate the contribution of WRP(W/Y) and Eh1 motifs to mammalian Gro/TLE anti-neurogenic activity. Here we describe point mutations within the C-terminal WD40 repeat domain of Gro/TLE1 that do not perturb protein folding but disrupt the ability of Gro/TLE1 to inhibit the differentiation of cerebral cortex neural progenitor cells into neurons. One of those mutations, L743F, selectively blocks binding to Hes1, an anti-neurogenic basic helix-loop-helix protein that harbors a WRPW motif. In contrast, the L743F mutation does not disrupt binding to Engrailed1 and FoxG1, which both contain Eh1 motifs, nor to Tcf3, which binds to the Gro/TLE N terminus. These results demonstrate that the recruitment of transcription factors harboring WRP(W/Y) tetrapeptides is essential to the anti-neurogenic function of Gro/TLE1.

Transgenesis and neuronal ablation in parasitic nematodes: revolutionary new tools to dissect host-parasite interactions

Ease of experimental gene transfer into viral and prokaryotic pathogens has made transgenesis a powerful tool for investigating the interactions of these pathogens with the host immune system. Recent advances have made this approach feasible for more complex protozoan parasites. By contrast, the lack of a system for heritable transgenesis in parasitic nematodes has hampered progress toward understanding the development of nematode-specific cellular responses. Recently, however, significant strides towards such a system have been made in several parasitic nematodes, and the possible applications of these in immunological research should now be contemplated. In addition, methods for targeted cell ablation have been successfully adapted from Caenorhabditis elegans methodology and applied to studies of neurobiology and behaviour in Strongyloides stercoralis. Together, these new technical developments offer exciting new tools to interrogate multiple aspects of the host-parasite interaction following nematode infection.

Genetic screens for Caenorhabditis elegans mutants defective in left/right asymmetric neuronal fate specification

We describe here the results of genetic screens for Caenorhabditis elegans mutants in which a single neuronal fate decision is inappropriately executed. In wild-type animals, the two morphologically bilaterally symmetric gustatory neurons ASE left (ASEL) and ASE right (ASER) undergo a left/right asymmetric diversification in cell fate, manifested by the differential expression of a class of putative chemoreceptors and neuropeptides. Using single cell-specific gfp reporters and screening through a total of almost 120,000 haploid genomes, we isolated 161 mutants that define at least six different classes of mutant phenotypes in which ASEL/R fate is disrupted. Each mutant phenotypic class encompasses one to nine different complementation groups. Besides many alleles of 10 previously described genes, we have identified at least 16 novel "lsy" genes ("laterally symmetric"). Among mutations in known genes, we retrieved four alleles of the miRNA lsy-6 and a gain-of-function mutation in the 3'-UTR of a target of lsy-6, the cog-1 homeobox gene. Using newly found temperature-sensitive alleles of cog-1, we determined that a bistable feedback loop controlling ASEL vs. ASER fate, of which cog-1 is a component, is only transiently required to initiate but not to maintain ASEL and ASER fate. Taken together, our mutant screens identified a broad catalog of genes whose molecular characterization is expected to provide more insight into the complex genetic architecture of a left/right asymmetric neuronal cell fate decision.

Cell-specific microarray profiling experiments reveal a comprehensive picture of gene expression in the C. elegans nervous system

BACKGROUND

With its fully sequenced genome and simple, well-defined nervous system, the nematode Caenorhabditis elegans offers a unique opportunity to correlate gene expression with neuronal differentiation. The lineal origin, cellular morphology and synaptic connectivity of each of the 302 neurons are known. In many instances, specific behaviors can be attributed to particular neurons or circuits. Here we describe microarray-based methods that monitor gene expression in C. elegans neurons and, thereby, link comprehensive profiles of neuronal transcription to key developmental and functional properties of the nervous system.

RESULTS

We employed complementary microarray-based strategies to profile gene expression in the embryonic and larval nervous systems. In the MAPCeL (Microarray Profiling C. elegans cells) method, we used fluorescence activated cell sorting (FACS) to isolate GFP-tagged embryonic neurons for microarray analysis. To profile the larval nervous system, we used the mRNA-tagging technique in which an epitope-labeled mRNA binding protein (FLAG-PAB-1) was transgenically expressed in neurons for immunoprecipitation of cell-specific transcripts. These combined approaches identified approximately 2,500 mRNAs that are highly enriched in either the embryonic or larval C. elegans nervous system. These data are validated in part by the detection of gene classes (for example, transcription factors, ion channels, synaptic vesicle components) with established roles in neuronal development or function. Of particular interest are 19 conserved transcripts of unknown function that are also expressed in the mammalian brain. In addition to utilizing these profiling approaches to define stage-specific gene expression, we also applied the mRNA-tagging method to fingerprint a specific neuron type, the A-class group of cholinergic motor neurons, during early larval development. A comparison of these data to a MAPCeL profile of embryonic A-class motor neurons identified genes with common functions in both types of A-class motor neurons as well as transcripts with roles specific to each motor neuron type.

CONCLUSION

We describe microarray-based strategies for generating expression profiles of embryonic and larval C. elegans neurons. These methods can be applied to particular neurons at specific developmental stages and, therefore, provide an unprecedented opportunity to obtain spatially and temporally defined snapshots of gene expression in a simple model nervous system.

The Motor Circuit

Caenorhabditis elegans motor neurons control a range of activities including locomotion, foraging, defecation, and gender-specific functions. In this chapter,we focus primarily on motor neurons that regulate body movement, with particular emphasis on those in the ventral nerve cord (VNC). We describe the basic architecture and development of the motor circuit, genes that specify motor neuron fates, and models of how the motor circuit controls locomotion. We identify surprising similarities between the structure and development of the nematode and vertebrate axial nerve cords and speculate about the potential roles of conserved families of transcription factors in the evolution of these motor circuits.

The Schizosaccharomyces pombe corepressor Tup11 interacts with the iron-responsive transcription factor Fep1

The Schizosaccharomyces pombe fep1(+) gene encodes a GATA transcription factor that represses the expression of iron transport genes in response to elevated iron concentrations. This transcriptional response is altered only in strains harboring a combined deletion of both tup11(+) and tup12(+) genes. This suggests that Tup11 is capable of negatively regulating iron transport gene expression in the absence of Tup12 and vice versa. The tup11(+)- and tup12(+)-encoded proteins resemble the Saccharomyces cerevisiae Tup1 corepressor. Using yeast two-hybrid analysis we show that Tup11 and Fep1 physically interact with each other. The C-terminal region from amino acids 242 to 564 of Fep1 is required for interaction with Tup11. Within this region, a minimal domain encompassing amino acids 405-541 was sufficient for Tup11-Fep1 association. Deletion mapping analysis revealed that the WD40-repeat sequence motifs of Tup11 are necessary for its interaction with Fep1. Analysis of Tup11 mutants with single amino acid substitutions in the WD40 repeats suggested that the Fep1 transcription factor interacts with a putative flat upper surface on the predicted beta-propeller structure of this motif. Further analysis by in vivo coimmunoprecipitation showed that Tup11 and Fep1 are physically associated. In vitro pull-down experiments further verified a direct interaction between the Fep1 C terminus and the Tup11 C-terminal WD40 repeat domain. Taken together, these results describe the first example of a physical interaction between a corepressor and an iron-sensing factor controlling the expression of iron uptake genes.

Crystal structure of the C-terminal WD40 repeat domain of the human Groucho/TLE1 transcriptional corepressor

Groucho (Gro)/TLE proteins are transcriptional corepressors that lack inherent DNA binding but interact with DNA-bound transcription factors and histones, and recruit histone deacetylases. Groucho-mediated repression is essential in embryonic development and involved in regulation of Wnt signaling in adult tissue. We have determined the 1.6 A crystal structure of a C-terminal fragment of human Groucho/TLE1, comprising part of the Ser/Pro-rich region and a seven-bladed beta propeller WD40 repeat domain, implicated in protein-protein interactions. The structure confirms the relationship to the yeast Tup1 corepressor, but reveals important structural differences specific to the metazoan system. Analysis of missense mutations in the C. elegans Groucho homolog UNC-37 identifies sites of interaction with repression effectors, and suggests an induced fit binding site for eh1 domains of Engrailed-type transcription factors.

The art and design of genetic screens: caenorhabditis elegans

The nematode Caenorhabditis elegans was chosen as a model genetic organism because its attributes, chiefly its hermaphroditic lifestyle and rapid generation time, make it suitable for the isolation and characterization of genetic mutants. The most important challenge for the geneticist is to design a genetic screen that will identify mutations that specifically disrupt the biological process of interest. Since 1974, when Sydney Brenner published his pioneering genetic screen, researchers have developed increasingly powerful methods for identifying genes and genetic pathways in C. elegans.

The C. elegans even-skipped homologue, vab-7, specifies DB motoneurone identity and axon trajectory

Locomotory activity is defined by the specification of motoneurone subtypes. In the nematode, C. elegans, DA and DB motoneurones innervate dorsal muscles and function to induce movement in the backwards or forwards direction, respectively. These two neurone classes express separate sets of genes and extend axons with oppositely directed trajectories; anterior (DA) versus posterior (DB). The DA-specific homeoprotein UNC-4 interacts with UNC-37/Groucho to repress the DB gene, acr-5 (nicotinic acetylcholine receptor subunit). We show that the C. elegans even-skipped-like homoedomain protein, VAB-7, coordinately regulates different aspects of the DB motoneurone fate, in part by repressing unc-4. Wild-type DB motoneurones express VAB-7, have posteriorly directed axons, express ACR-5 and lack expression of the homeodomain protein UNC-4. In a vab-7 mutant, ectopic UNC-4 represses acr-5 and induces an anteriorly directed DB axon trajectory. Thus, vab-7 indirectly promotes DB-specific gene expression and posteriorly directed axon outgrowth by preventing UNC-4 repression of DB differentiation. Ectopic expression of VAB-7 also induces DB traits in an unc-4-independent manner, suggesting that VAB-7 can act through a parallel pathway. This work supports a model in which a complementary pair of homeodomain transcription factors (VAB-7 and UNC-4) specifies differences between DA and DB neurones through inhibition of the alternative fates. The recent findings that Even-skipped transcriptional repressor activity specifies neurone identity and axon guidance in the mouse and Drosophila motoneurone circuit points to an ancient origin for homeoprotein-dependent mechanisms of neuronal differentiation in the metazoan nerve cord.

Regulation of neurotransmitter vesicles by the homeodomain protein UNC-4 and its transcriptional corepressor UNC-37/groucho in Caenorhabditis elegans cholinergic motor neurons

Motor neuron function depends on neurotransmitter release from synaptic vesicles (SVs). Here we show that the UNC-4 homeoprotein and its transcriptional corepressor protein UNC-37 regulate SV protein levels in specific Caenorhabditis elegans motor neurons. UNC-4 is expressed in four classes (DA, VA, VC, and SAB) of cholinergic motor neurons. Antibody staining reveals that five different vesicular proteins (UNC-17, choline acetyltransferase, Synaptotagmin, Synaptobrevin, and RAB-3) are substantially reduced in unc-4 and unc-37 mutants in these cells; nonvesicular neuronal proteins (Syntaxin, UNC-18, and UNC-11) are not affected, however. Ultrastructural analysis of VA motor neurons in the mutant unc-4(e120) confirms that SV number in the presynaptic zone is reduced ( approximately 40%) whereas axonal diameter and synaptic morphology are not visibly altered. Because the UNC-4-UNC-37 complex has been shown to mediate transcriptional repression, we propose that these effects are performed via an intermediate gene. Our results are consistent with a model in which this unc-4 target gene ("gene-x") functions at a post-transcriptional level as a negative regulator of SV biogenesis or stability. Experiments with a temperature-sensitive unc-4 mutant show that the adult level of SV proteins strictly depends on unc-4 function during a critical period of motor neuron differentiation. unc-4 activity during this sensitive larval stage is also required for the creation of proper synaptic inputs to VA motor neurons. The temporal correlation of these events may mean that a common unc-4-dependent mechanism controls both the specificity of synaptic inputs as well as the strength of synaptic outputs for these motor neurons.

Regulation of neurotransmitter vesicles by the homeodomain protein UNC-4 and its transcriptional corepressor UNC-37/groucho in Caenorhabditis elegans cholinergic motor neurons

Motor neuron function depends on neurotransmitter release from synaptic vesicles (SVs). Here we show that the UNC-4 homeoprotein and its transcriptional corepressor protein UNC-37 regulate SV protein levels in specific Caenorhabditis elegans motor neurons. UNC-4 is expressed in four classes (DA, VA, VC, and SAB) of cholinergic motor neurons. Antibody staining reveals that five different vesicular proteins (UNC-17, choline acetyltransferase, Synaptotagmin, Synaptobrevin, and RAB-3) are substantially reduced in unc-4 and unc-37 mutants in these cells; nonvesicular neuronal proteins (Syntaxin, UNC-18, and UNC-11) are not affected, however. Ultrastructural analysis of VA motor neurons in the mutant unc-4(e120) confirms that SV number in the presynaptic zone is reduced ( approximately 40%) whereas axonal diameter and synaptic morphology are not visibly altered. Because the UNC-4-UNC-37 complex has been shown to mediate transcriptional repression, we propose that these effects are performed via an intermediate gene. Our results are consistent with a model in which this unc-4 target gene ("gene-x") functions at a post-transcriptional level as a negative regulator of SV biogenesis or stability. Experiments with a temperature-sensitive unc-4 mutant show that the adult level of SV proteins strictly depends on unc-4 function during a critical period of motor neuron differentiation. unc-4 activity during this sensitive larval stage is also required for the creation of proper synaptic inputs to VA motor neurons. The temporal correlation of these events may mean that a common unc-4-dependent mechanism controls both the specificity of synaptic inputs as well as the strength of synaptic outputs for these motor neurons.

UNC-4/UNC-37-dependent repression of motor neuron-specific genes controls synaptic choice in Caenorhabditis elegans

The UNC-4 homeoprotein and the Groucho-like corepressor UNC-37 specify synaptic choice in the Caenorhabditis elegans motor neuron circuit. In unc-4 mutants, VA motor neurons are miswired with inputs from interneurons normally reserved for their lineal sisters, the VB motor neurons. Here we show that UNC-4 and UNC-37 function together in VA motor neurons to repress VB-specific genes and that this activity depends on physical contact between UNC-37 and a conserved Engrailed-like repressor domain (eh1) in UNC-4. Missense mutations in the UNC-4 eh1 domain disrupt interactions between UNC-4 and UNC-37 and result in the loss of UNC-4-dependent repressor activity in vivo. A compensatory amino acid substitution in UNC-37 suppresses specific unc-4 alleles by restoring physical interactions with UNC-4 as well as UNC-4-dependent repression of VB-specific genes. We propose that repression of VB-specific genes by UNC-4 and UNC-37 is necessary for the creation of wild-type inputs to VA motor neurons. The existence of mammalian homologs of UNC-4 and UNC-37 indicates that a similar mechanism could regulate synaptic choice in the vertebrate spinal cord.

The Groucho-like transcription factor UNC-37 functions with the neural specificity gene unc-4 to govern motor neuron identity in C. elegans

Groucho and Tup1 are members of a conserved family of WD repeat proteins that interact with specific transcription factors to repress target genes. Here we show that mutations in WD domains of the Groucho-like protein, UNC-37, affect a motor neuron trait that also depends on UNC-4, a homeodomain protein that controls neuronal specificity in Caenorhabditis elegans. In unc-4 mutants, VA motor neurons assume the pattern of synaptic input normally reserved for their lineal sister cells, the VB motor neurons; the loss of normal input to the VAs produces a distinctive backward movement defect. Substitution of a conserved residue (H to Y) in the fifth WD repeat in unc-37(e262) phenocopies the Unc-4 movement defect. Conversely, an amino acid change (E to K) in the sixth WD repeat of UNC-37 is a strong suppressor of unc-37(e262) and of specific unc-4 missense mutations. We have previously shown that UNC-4 expression in the VA motor neurons specifies the wild-type pattern of presynaptic input. Here we demonstrate that UNC-37 is also expressed in the VAs and that unc-37 activity in these neurons is sufficient to restore normal movement to unc-37(e262) animals. We propose that UNC-37 and UNC-4 function together to prevent expression of genes that define the VB pattern of synaptic inputs and thereby generate connections specific to the VA motor neurons. In addition, we show that the WD repeat domains of UNC-37 and of the human homolog, TLE1, are functionally interchangeable in VA motor neurons which suggests that this highly conserved protein domain may also specify motor neuron identity and synaptic choice in more complex nervous systems.

The Caenorhabditis elegans gene unc-76 and its human homologs define a new gene family involved in axonal outgrowth and fasciculation

The gene unc-76 (unc, uncoordinated) is necessary for normal axonal bundling and elongation within axon bundles in the nematode Caenorhabditis elegans. The UNC-76 protein and two human homologs identified as expressed sequence tags are not similar to previously characterized proteins and thus represent a new protein family. At least one of these human homologs can function in C. elegans, suggesting that it, like UNC-76, acts in axonal outgrowth. We propose that the UNC-76 protein, which is found in cell bodies and processes of all neurons throughout development, either has a structural role in the formation and maintenance of axonal bundles or transduces signals to the intracellular machinery that regulates axonal extension and adhesion.

pag-3, a Caenorhabditis elegans gene involved in touch neuron gene expression and coordinated movement

Mutations in a newly identified gene, pag-3, cause ectopic expression of touch neuron genes meo-7, mec-7lacZ and mec-4lacZ in the lineal sisters of the ALM touch neurons, the BDU neurons. Pag-3 mutants also show a reverse kinker uncoordinated phenotype. The first pag-3 allele was isolated in a screen for mutants with altered immunofluorescence staining patterns. Two additional pag-3 alleles were identified in a noncomplementation screen of 38,000 haploid genomes. All of the pag-3 alleles were recessive to wild type and cause the same phenotypes. Two-factor crosses, deficiency mapping and three-factor crosses located pag-3 to the right arm of the X chromosome between unc-3 and unc-7. Because recessive mutations in pag-3 result in expression of several touch cell specific genes in the BDU neurons, pag-3(+) must directly or indirectly suppress expression of these genes in the BDU neurons. Although pag-3 mutants did not show mec-3lacZ expression in their BDU neurons, expression of mec-7lacZ and mec-4lacZ in the BDU neurons of pag-3 mutants required mec-3(+).

Identification and cloning of unc-119, a gene expressed in the Caenorhabditis elegans nervous system

A spontaneous mutation affecting locomotion of the nematode Caenorhabditis elegans has been mapped to a new gene, unc-119. Phenotypic characterization of the mutants suggests the defect does not lie in the musculature and that the animals also have defects in feeding behavior and chemosensation. unc-119 has been physically mapped relative to a previously identified chromosomal break in linkage group III, and DNA clones covering the region can rescue the mutant phenotype in transgenic animals. Three more alleles at the locus, with identical phenotypes, have been induced and characterized, all of which are putative null alleles. The predicted UNC-119 protein has no significant similarity to other known proteins. Expression of an unc-119/lacZ fusion in transgenic animals is seen in many neurons, suggesting that the unc-119 mutant phenotype is due to a defect in the nervous system.

Expression of the unc-4 homeoprotein in Caenorhabditis elegans motor neurons specifies presynaptic input

In the nematode, Caenorhabditis elegans, VA and VB motor neurons arise from a common precursor cell but adopt different morphologies and synapse with separate sets of interneurons in the ventral nerve cord. A mutation that inactivates the unc-4 homeodomain gene causes VA motor neurons to assume the VB pattern of synaptic input while retaining normal axonal polarity and output; the disconnection of VA motor neurons from their usual presynaptic partners blocks backward locomotion. We show that expression of a functional unc-4-beta-galactosidase chimeric protein in VA motor neurons restores wild-type movement to an unc-4 mutant. We propose that unc-4 controls a differentiated characteristic of the VA motor neurons that distinguishes them from their VB sisters, thus dictating recognition by the appropriate interneurons. Our results show that synaptic choice can be controlled at the level of transcription in the post-synaptic neuron and identify a homeoprotein that defines a subset of cell-specific traits required for this choice.

Sequential signalling during Caenorhabditis elegans vulval induction

During the induction of the Caenorhabditis elegans vulva, cell signalling causes initially equipotent cells to express a reproducible pattern of cell fates. The position of the anchor cell determines the pattern of vulval precursor cell fates, such that the closest precursor cell (P6.p) expresses the primary cell fate, the next closest cells (P5.p and P7.p) both express the secondary cell fate, and each of the precursor cells located at a distance (P3.p, P4.p and P8.p) express the tertiary cell fate (Fig. 1a). We present data indicating that this stereotypical pattern of cell fates can be generated by sequential signals. We identified genetic mosaic animals in which P5.p and P7.p were defective in the anchor-cell signal-transduction pathway and observed that these cells adopted the secondary cell fate, indicating that anchor-cell signal transduction is not required for the expression of the secondary cell fate. These results suggest that the anchor cell induces P6.p to express the primary cell fate, and that P6.p subsequently induces P5.p and P7.p to express the secondary cell fate.