The unc-86 gene product couples cell lineage and cell identity in C. elegans

Conditions for dye-filling of sensory neurons in Caenorhabditis elegans

Dye-filling is a common method used to stain Caenorhabditis elegans sensory neurons in vivo. While the amphids and phasmids are easy to stain, a subset of sensory neurons, the IL2 neurons, are difficult to stain reproducibly. Here we examined the conditions under which the IL2 neurons take up the lipophilic fluorescent dye DiI. We find that IL2 dye-filling depends on salt concentration, but not osmolarity. Low salt prior and during incubation is important for efficient dye uptake. Additional parameters that affect dye-filling are the speed of shaking during incubation and the addition of detergents. Our modified dye-filling procedure provides a reliable method to distinguish mutant alleles that stain amphids and phasmids, IL2 neurons, or both. An additional benefit is that it can also stain the excretory duct. The method allows genetic screens to be performed to identify mutants that selectively affect only one of the sensory structures or the excretory duct.

RSBP-1 is a membrane-targeting subunit required by the Galpha(q)-specific but not the Galpha(o)-specific R7 regulator of G protein signaling in Caenorhabditis elegans

Regulator of G protein signaling (RGS) proteins inhibit G protein signaling by activating Galpha GTPase activity, but the mechanisms that regulate RGS activity are not well understood. The mammalian R7 binding protein (R7BP) can interact with all members of the R7 family of RGS proteins, and palmitoylation of R7BP can target R7 RGS proteins to the plasma membrane in cultured cells. However, whether endogenous R7 RGS proteins in neurons require R7BP or membrane localization for function remains unclear. We have identified and knocked out the only apparent R7BP homolog in Caenorhabditis elegans, RSBP-1. Genetic studies show that loss of RSBP-1 phenocopies loss of the R7 RGS protein EAT-16, but does not disrupt function of the related R7 RGS protein EGL-10. Biochemical analyses find that EAT-16 coimmunoprecipitates with RSBP-1 and is predominantly plasma membrane-associated, whereas EGL-10 does not coimmunoprecipitate with RSBP-1 and is not predominantly membrane-associated. Mutating the conserved membrane-targeting sequence in RSBP-1 disrupts both the membrane association and function of EAT-16, demonstrating that membrane targeting by RSBP-1 is essential for EAT-16 activity. Our analysis of endogenous R7 RGS proteins in C. elegans neurons reveals key differences in the functional requirements for membrane targeting between members of this protein family.

SUMO regulates the assembly and function of a cytoplasmic intermediate filament protein in C. elegans

Sumoylation is a reversible posttranslational modification that plays roles in many processes, including transcriptional regulation, cell division, chromosome integrity, and DNA damage response. Using a proteomics approach, we identified approximately 250 candidate targets of sumoylation in C. elegans. One such target is the cytoplasmic intermediate filament (cIF) protein named IFB-1, which is expressed in hemidesmosome-like structures in the worm epidermis and is essential for embryonic elongation and maintenance of muscle attachment to the cuticle. In the absence of SUMO, IFB-1 formed ectopic filaments and protein aggregates in the lateral epidermis. Moreover, depletion of SUMO or mutation of the SUMO acceptor site on IFB-1 resulted in a reduction of its cytoplasmic soluble pool, leading to a decrease in its exchange rate within epidermal attachment structures. These observations indicate that SUMO regulates cIF assembly by maintaining a cytoplasmic pool of nonpolymerized IFB-1, and that this is necessary for normal IFB-1 function.

Single-cell transcriptional analysis of taste sensory neuron pair in Caenorhabditis elegans

The nervous system is composed of a wide variety of neurons. A description of the transcriptional profiles of each neuron would yield enormous information about the molecular mechanisms that define morphological or functional characteristics. Here we show that RNA isolation from single neurons is feasible by using an optimized mRNA tagging method. This method extracts transcripts in the target cells by co-immunoprecipitation of the complexes of RNA and epitope-tagged poly(A) binding protein expressed specifically in the cells. With this method and genome-wide microarray, we compared the transcriptional profiles of two functionally different neurons in the main C. elegans gustatory neuron class ASE. Eight of the 13 known subtype-specific genes were successfully detected. Additionally, we identified nine novel genes including a receptor guanylyl cyclase, secreted proteins, a TRPC channel and uncharacterized genes conserved among nematodes, suggesting the two neurons are substantially different than previously thought. The expression of these novel genes was controlled by the previously known regulatory network for subtype differentiation. We also describe unique motif organization within individual gene groups classified by the expression patterns in ASE. Our study paves the way to the complete catalog of the expression profiles of individual C. elegans neurons.

N-Glycosylation regulates fibroblast growth factor receptor/EGL-15 activity in Caenorhabditis elegans in vivo

The regulation of cell function by fibroblast growth factors (FGFs) classically occurs through a dual receptor system of a tyrosine kinase receptor (FGFR) and a heparan sulfate proteoglycan co-receptor. Mutations in some consensus N-glycosylation sites in human FGFR result in skeletal disorders and craniosynostosis syndromes, and biophysical studies in vitro suggest that N-glycosylation of FGFR alters ligand and heparan sulfate binding properties. The evolutionarily conserved FGFR signaling system of Caenorhabditis elegans has been used to assess the role of N-glycosylation in the regulation of FGFR signaling in vivo. The C. elegans FGF receptor, EGL-15, is N-glycosylated in vivo, and genetic substitution of specific consensus N-glycosylation sites leads to defects in the maintenance of fluid homeostasis and differentiation of sex muscles, both of which are phenotypes previously associated with hyperactive EGL-15 signaling. These phenotypes are suppressed by hypoactive mutations in EGL-15 downstream signaling components or activating mutations in the phosphatidylinositol 3-kinase pathway, respectively. The results show that N-glycans negatively regulate FGFR activity in vivo supporting the notion that mutation of N-glycosylation sites in human FGFR may lead to inappropriate activation of the receptor.

Somatic cAMP signaling regulates MSP-dependent oocyte growth and meiotic maturation in C. elegans

Soma-germline interactions control fertility at many levels, including stem cell proliferation, meiosis and gametogenesis, yet the nature of these fundamental signaling mechanisms and their potential evolutionary conservation are incompletely understood. In C. elegans, a sperm-sensing mechanism regulates oocyte meiotic maturation and ovulation, tightly coordinating sperm availability and fertilization. Sperm release the major sperm protein (MSP) signal to trigger meiotic resumption (meiotic maturation) and to promote contraction of the follicle-like gonadal sheath cells that surround oocytes. Using genetic mosaic analysis, we show that all known MSP-dependent meiotic maturation events in the germline require Galpha(s)-adenylate cyclase signaling in the gonadal sheath cells. We show that the MSP hormone promotes the sustained actomyosin-dependent cytoplasmic streaming that drives oocyte growth. Furthermore, we demonstrate that efficient oocyte production and cytoplasmic streaming require Galpha(s)-adenylate cyclase signaling in the gonadal sheath cells, thereby providing a somatic mechanism that coordinates oocyte growth and meiotic maturation with sperm availability. We present genetic evidence that MSP and Galpha(s)-adenylate cyclase signaling regulate oocyte growth and meiotic maturation in part by antagonizing gap-junctional communication between sheath cells and oocytes. In the absence of MSP or Galpha(s)-adenylate cyclase signaling, MSP binding sites are enriched and appear clustered on sheath cells. We discuss these results in the context of a model in which the sheath cells function as the major initial sensor of MSP, potentially via multiple classes of G-protein-coupled receptors. Our findings highlight a remarkable similarity between the regulation of meiotic resumption by soma-germline interactions in C. elegans and mammals.

C. elegans CARMIL negatively regulates UNC-73/Trio function during neuronal development

Whereas many molecules that promote cell and axonal growth cone migrations have been identified, few are known to inhibit these processes. In genetic screens designed to identify molecules that negatively regulate such migrations, we identified CRML-1, the C. elegans homolog of CARMIL. Although mammalian CARMIL acts to promote the migration of glioblastoma cells, we found that CRML-1 acts as a negative regulator of neuronal cell and axon growth cone migrations. Genetic evidence indicates that CRML-1 regulates these migrations by inhibiting the Rac GEF activity of UNC-73, a homolog of the Rac and Rho GEF Trio. The antagonistic effects of CRML-1 and UNC-73 can control the direction of growth cone migration by regulating the levels of the SAX-3 (a Robo homolog) guidance receptor. Consistent with the hypothesis that CRML-1 negatively regulates UNC-73 activity, these two proteins form a complex in vivo. Based on these observations, we propose a role for CRML-1 as a novel regulator of cell and axon migrations that acts through inhibition of Rac signaling.

The ALP-Enigma protein ALP-1 functions in actin filament organization to promote muscle structural integrity in Caenorhabditis elegans

Mutations that affect the Z-disk-associated ALP-Enigma proteins have been linked to human muscular and cardiac diseases. Despite their clear physiological significance for human health, the mechanism of action of ALP-Enigma proteins is largely unknown. In Caenorhabditis elegans, the ALP-Enigma protein family is encoded by a single gene, alp-1; thus C. elegans provides an excellent model to study ALP-Enigma function. Here we present a molecular and genetic analysis of ALP-Enigma function in C. elegans. We show that ALP-1 and alpha-actinin colocalize at dense bodies where actin filaments are anchored and that the proper localization of ALP-1 at dense bodies is dependent on alpha-actinin. Our analysis of alp-1 mutants demonstrates that ALP-1 functions to maintain actin filament organization and participates in muscle stabilization during contraction. Reducing alpha-actinin activity enhances the actin filament phenotype of the alp-1 mutants, suggesting that ALP-1 and alpha-actinin function in the same cellular process. Like alpha-actinin, alp-1 also interacts genetically with a connectin/titin family member, ketn-1, to provide mechanical stability for supporting body wall muscle contraction. Taken together, our data demonstrate that ALP-1 and alpha-actinin function together to stabilize actin filaments and promote muscle structural integrity.

The N- or C-terminal domains of DSH-2 can activate the C. elegans Wnt/beta-catenin asymmetry pathway

Dishevelleds are modular proteins that lie at the crossroads of divergent Wnt signaling pathways. The DIX domain of dishevelleds modulates a beta-catenin destruction complex, and thereby mediates cell fate decisions through differential activation of Tcf transcription factors. The DEP domain of dishevelleds mediates planar polarity of cells within a sheet through regulation of actin modulators. In Caenorhabditis elegans asymmetric cell fate decisions are regulated by asymmetric localization of signaling components in a pathway termed the Wnt/beta-catenin asymmetry pathway. Which domain(s) of Disheveled regulate this pathway is unknown. We show that C. elegans embryos from dsh-2(or302) mutant mothers fail to successfully undergo morphogenesis, but transgenes containing either the DIX or the DEP domain of DSH-2 are sufficient to rescue the mutant phenotype. Embryos lacking zygotic function of SYS-1/beta-catenin, WRM-1/beta-catenin, or POP-1/Tcf show defects similar to dsh-2 mutants, including a loss of asymmetry in some cell fate decisions. Removal of two dishevelleds (dsh-2 and mig-5) leads to a global loss of POP-1 asymmetry, which can be rescued by addition of transgenes containing either the DIX or DEP domain of DSH-2. These results indicate that either the DIX or DEP domain of DSH-2 is capable of activating the Wnt/beta-catenin asymmetry pathway and regulating anterior-posterior fate decisions required for proper morphogenesis.

Intracellular amyloid formation in muscle cells of Abeta-transgenic Caenorhabditis elegans: determinants and physiological role in copper detoxification

BACKGROUND

The amyloid beta-peptide is a ubiquitous peptide, which is prone to aggregate forming soluble toxic oligomers and insoluble less-toxic aggregates. The intrinsic and external/environmental factors that determine Abeta aggregation in vivo are poorly understood, as well as the cellular meaning of this process itself. Genetic data as well as cell biological and biochemical evidence strongly support the hypothesis that Abeta is a major player in the onset and development of Alzheimer's disease. In addition, it is also known that Abeta is involved in Inclusion Body Myositis, a common myopathy of the elderly in which the peptide accumulates intracellularly.

RESULTS

In the present work, we found that intracellular Abeta aggregation in muscle cells of Caenorhabditis elegans overexpressing Abeta peptide is affected by two single amino acid substitutions, E22G (Arctic) and V18A (NIC). Both variations show decrease intracellular amyloidogenesis compared to wild type Abeta. We show that intracellular amyloid aggregation of wild type Abeta is accelerated by Cu2+ and diminished by copper chelators. Moreover, we demonstrate through toxicity and behavioral assays that Abeta-transgenic worms display a higher tolerance to Cu2+ toxic effects and that this resistance may be linked to the formation of amyloid aggregates.

CONCLUSION

Our data show that intracellular Abeta amyloid aggregates may trap excess of free Cu2+ buffering its cytotoxic effects and that accelerated intracellular Abeta aggregation may be part of a cell protective mechanism.

Regulatory logic of neuronal diversity: terminal selector genes and selector motifs

Individual neuronal cell types are defined by the expression of unique batteries of terminal differentiation genes. The elucidation of the cis-regulatory architecture of several distinct, single neuron type-specific gene batteries in Caenorhabditis elegans has revealed a strikingly simple cis-regulatory logic, in which small cis-regulatory motifs are activated in postmitotic neurons by autoregulating transcription factors (TFs). Loss of the TFs results in the loss of the identity of the individual neuron type. I propose to term these TFs "terminal selector genes" and their cognate cis-regulatory target sites "terminal selector motifs." Terminal selector genes assign individual neuronal identities by directly controlling the expression of downstream, terminal differentiation genes and act in specific regulatory network configurations. The simplicity of the cis-regulatory logic on which the terminal selector gene concept is based may contribute to the evolvability of neuronal diversity.

Sarcomeric actin organization is synergistically promoted by tropomodulin, ADF/cofilin, AIP1 and profilin in C. elegans

Sarcomeric organization of thin and thick filaments in striated muscle is important for the efficient generation of contractile forces. Sarcomeric actin filaments are uniform in their lengths and regularly arranged in a striated pattern. Tropomodulin caps the pointed end of actin filaments and is a crucial regulator of sarcomere assembly. Here, we report unexpected synergistic functions of tropomodulin with enhancers of actin filament dynamics in Caenorhabditis elegans striated muscle. Pointed-end capping by tropomodulin inhibited actin filament depolymerization by ADF/cofilin in vitro. However, in vivo, the depletion of tropomodulin strongly enhanced the disorganization of sarcomeric actin filaments in ADF/cofilin mutants, rather than antagonistically suppressing the phenotype. Similar phenotypic enhancements by tropomodulin depletion were also observed in mutant backgrounds for AIP1 and profilin. These in vivo effects cannot be simply explained by antagonistic effects of tropomodulin and ADF/cofilin in vitro. Thus, we propose a model in which tropomodulin and enhancers of actin dynamics synergistically regulate elongation and shortening of actin filaments at the pointed end.

The cell signaling adaptor protein EPS-8 is essential for C. elegans epidermal elongation and interacts with the ankyrin repeat protein VAB-19

Background

The epidermal cells of the C. elegans embryo undergo coordinated cell shape changes that result in the morphogenetic process of elongation. The cytoskeletal ankyrin repeat protein VAB-19 is required for cell shape changes and localizes to cell-matrix attachment structures. The molecular functions of VAB-19 in this process are obscure, as no previous interactors for VAB-19 have been described.

Methodology/Principal Findings

In screens for VAB-19 binding proteins we identified the signaling adaptor EPS-8. Within C. elegans epidermal cells, EPS-8 and VAB-19 colocalize at cell-matrix attachment structures. The central domain of EPS-8 is necessary and sufficient for its interaction with VAB-19. eps-8 null mutants, like vab-19 mutants, are defective in epidermal elongation and in epidermal-muscle attachment. The eps-8 locus encodes two isoforms, EPS-8A and EPS-8B, that appear to act redundantly in epidermal elongation. The function of EPS-8 in epidermal development involves its N-terminal PTB and central domains, and is independent of its C-terminal SH3 and actin-binding domains. VAB-19 appears to act earlier in the biogenesis of attachment structures and may recruit EPS-8 to these structures.

Conclusions/Significance

EPS-8 and VAB-19 define a novel pathway acting at cell-matrix attachments to regulate epithelial cell shape. This is the first report of a role for EPS-8 proteins in cell-matrix attachments. The existence of EPS-8B-like isoforms in Drosophila suggests this function of EPS-8 proteins could be conserved among other organisms.

FMRFamide neuropeptides and acetylcholine synergistically inhibit egg-laying by C. elegans

Egg-laying behavior of the Caenorhabditis elegans hermaphrodite is regulated by G protein signaling pathways. Here we show that the egg laying-defective mutant egl-6(n592) carries an activating mutation in a G protein-coupled receptor that inhibits C. elegans egg-laying motor neurons in a G(o)-dependent manner. Ligands for EGL-6 are Phe-Met-Arg-Phe-NH(2) (FMRFamide)-related peptides encoded by the genes flp-10 and flp-17. flp-10 is expressed in both neurons and non-neuronal cells. The major source of flp-17 peptides is a pair of presumptive sensory neurons, the BAG neurons. Genetic analysis of the egl-6 pathway revealed that the EGL-6 neuropeptide signaling pathway functions redundantly with acetylcholine to inhibit egg-laying. The retention of embryos in the uterus of the C. elegans hermaphrodite is therefore under the control of a presumptive sensory system and is inhibited by the convergence of signals from neuropeptides and the small-molecule neurotransmitter acetylcholine.

unc-3-dependent repression of specific motor neuron fates in Caenorhabditis elegans

unc-3 encodes the Caenorhabditis elegans ortholog of the Olf-1/Early B cell factor family of transcription factors, which in vertebrates regulate development and differentiation of B lymphocytes, adipocytes, and cells of the nervous system. unc-3 mutants are uncoordinated in locomotion. Here we show that unc-3 represses a VC-like motor neuron program in the VA and VB motor neurons, which in wild-type animals control backwards and forwards locomotion, respectively. We identify a physical interaction between UNC-3 and the C2H2 zinc finger transcription factor PAG-3, the mammalian homologs of which are coexpressed in olfactory epithelium and hematopoietic cells. Our data explain the locomotory defects of unc-3 mutants and suggest that interactions between unc-3 and pag-3 orthologs in other species may be functionally important.

Caenorhabditis elegans teneurin, ten-1, is required for gonadal and pharyngeal basement membrane integrity and acts redundantly with integrin ina-1 and dystroglycan dgn-1

The Caenorhabditis elegans teneurin ortholog, ten-1, plays an important role in gonad and pharynx development. We found that lack of TEN-1 does not affect germline proliferation but leads to local basement membrane deficiency and early gonad disruption. Teneurin is expressed in the somatic precursor cells of the gonad that appear to be crucial for gonad epithelialization and basement membrane integrity. Ten-1 null mutants also arrest as L1 larvae with malformed pharynges and disorganized pharyngeal basement membranes. The pleiotropic phenotype of ten-1 mutant worms is similar to defects found in basement membrane receptor mutants ina-1 and dgn-1 as well as in the mutants of the extracellular matrix component laminin, epi-1. We show that the ten-1 mutation is synthetic lethal with mutations of genes encoding basement membrane components and receptors due to pharyngeal or hypodermal defects. This indicates that TEN-1 could act redundantly with integrin INA-1, dystroglycan DGN-1, and laminin EPI-1 in C. elegans development. Moreover, ten-1 deletion sensitizes worms to loss of nidogen nid-1 causing a pharynx unattached phenotype in ten-1;nid-1 double mutants. We conclude that TEN-1 is important for basement membrane maintenance and/or adhesion in particular organs and affects the function of somatic gonad precursor cells.

The C. elegans F-spondin family protein SPON-1 maintains cell adhesion in neural and non-neural tissues

The F-spondin family of extracellular matrix proteins has been implicated in axon outgrowth, fasciculation and neuronal cell migration, as well as in the differentiation and proliferation of non-neuronal cells. In screens for mutants defective in C. elegans embryonic morphogenesis, we identified SPON-1, the only C. elegans member of the spondin family. SPON-1 is synthesized in body muscles and localizes to integrin-containing structures on body muscles and to other basement membranes. SPON-1 maintains strong attachments of muscles to epidermis; in the absence of SPON-1, muscles progressively detach from the epidermis, causing defective epidermal elongation. In animals with reduced integrin function, SPON-1 becomes dose dependent, suggesting that SPON-1 and integrins function in concert to promote the attachment of muscles to the basement membrane. Although spon-1 mutants display largely normal neurite outgrowth, spon-1 synergizes with outgrowth defective mutants, revealing a cryptic role for SPON-1 in axon extension. In motoneurons, SPON-1 acts in axon guidance and fasciculation, whereas in interneurons SPON-1 maintains process position. Our results show that a spondin maintains cell-matrix adhesion in multiple tissues.

Natural variation in Pristionchus pacificus insect pheromone attraction involves the protein kinase EGL-4

The geographical mosaic theory of coevolution predicts that different local species interactions will shape population traits, but little is known about the molecular factors involved in mediating the specificity of these interactions. Pristionchus nematodes associate with different scarab beetles around the world, with Pristionchus pacificus isolated primarily from the oriental beetle in Japan. In particular, the constituent populations of P. pacificus represent a rare opportunity to study multiple specialized interactions and the mechanisms that influence population traits at the genetic level. We identified a component of the cGMP signaling pathway to be involved in the natural variation for sensing the insect pheromone ETDA, using targeted introgression lines, exogenous cGMP treatment, and a null egl-4 allele. Our data strongly implicate egl-4 as one of several loci involved in behavioral variation in P. pacificus populations. That EGL-4 homologs have been independently implicated for behavioral variations in other invertebrate models suggests that EGL-4 may act as a modulator for interspecies behavioral repertoires across large phylogenetic distances.

Efficient production of monoclonal antibodies recognizing specific structures in Caenorhabditis elegans embryos using an antigen subtraction method

Monoclonal antibodies (mAbs) have been widely used to probe molecular components of specific cell types or cellular structures. We have developed a method to enrich antigens of low abundance in heterogeneous molecule mixtures by subtracting abundant antigens. The subtracted immunogen mixture is then used for immunization, which significantly increases the production of mAbs that exhibit specific staining patterns. By applying this "antigen subtraction" method to the embryonic extract of Caenorhabditis elegans, we have successfully isolated 35 mAbs that recognize specific structures, including P granules, muscles, the pharynx, and subsets of hypodermal cells; some of the mAbs revealed previously unreported cellular structures. This antigen subtraction approach can be used in various applications to produce mAbs against relatively scarce antigens in complex molecular mixtures. The mAbs will be useful tools for developmental and cell biological studies.

Genome integrity is regulated by the Caenorhabditis elegans Rad51D homolog rfs-1

Multiple mechanisms ensure genome maintenance through DNA damage repair, suppression of transposition, and telomere length regulation. The mortal germline (Mrt) mutants in Caenorhabditis elegans are defective in maintaining genome integrity, resulting in a progressive loss of fertility over many generations. Here I show that the high incidence of males (him)-15 locus, defined by the deficiency eDf25, is allelic to rfs-1, the sole rad-51 paralog group member in C. elegans. The rfs-1/eDf25 mutant displays a Mrt phenotype and mutant animals manifest features of chromosome fusions prior to the onset of sterility. Unlike other Mrt genes, rfs-1 manifests fluctuations in telomere lengths and functions independently of telomerase. These data suggest that rfs-1 is a novel regulator of genome stability.

Different isoforms of the C. elegans FGF receptor are required for attraction and repulsion of the migrating sex myoblasts

The Caenorhabditis elegans FGF receptor, EGL-15, is alternatively-spliced to yield two major isoforms that differ in their extracellular domains. The EGL-15(5A) isoform is necessary for the gonadal chemoattraction of the migrating sex myoblasts (SMs), while the EGL-15(5B) isoform is required for viability. Here we show that 5A is predominantly expressed in the M lineage, which gives rise to the migrating SMs and their sex muscle descendants, while 5B is predominantly expressed in the hypodermis. Tissue-specific expression, however, explains only part of the functional differences between these two receptor isoforms. 5A can carry out the reciprocal essential function of 5B when expressed in the hypodermis, but 5B is incapable of carrying out SM chemoattraction. Our data, therefore, indicate that the structural differences in these two isoforms contribute to their functional differences. Two lines of evidence indicate that the 5B isoform also plays a role in SM migration, implicating it in the repulsion that is observed when the chemoattraction is compromised. Thus, structural differences in the extracellular domains of these two isoforms can specify either attraction to or repulsion from the gonad.

Cellular and molecular determinants targeting the Caenorhabditis elegans PHR protein RPM-1 to perisynaptic regions

Caenorhabditis elegans RPM-1 is a member of a conserved protein family, the PHR proteins, that includes human Pam, mouse Phr1, zebrafish Esrom, and Drosophila Highwire. PHR proteins play important roles in the development of the nervous system. In particular, mutations in rpm-1 cause a disruption of synaptic architecture, affecting the distribution of synaptic vesicles and the number of presynaptic densities. Using antibodies against RPM-1, we determined the localization of the endogenous RPM-1 protein in wild-type and in several mutants that affect synaptic development. Our analyses show that, in mature neurons, RPM-1 resides in a distinct region that is close to, but does not overlap with, the synaptic exo- and endocytosis domains. The localization of RPM-1 occurs independently of several proteins that function in the transport or assembly of synapse components, and its abundance is partially dependent on its binding partner the F-box protein FSN-1. RPM-1 has been shown to target the MAPKKK DLK-1 for degradation. We show that activated DLK-1 may be preferentially targeted for degradation. Furthermore, using transgene analysis, we identified a critical role of the conserved PHR domain of RPM-1 in its subcellular localization.

Identification of major classes of cholinergic neurons in the nematode Caenorhabditis elegans

The neurotransmitter acetylcholine (ACh) is specifically synthesized by the enzyme choline acetyltransferase (ChAT). Subsequently, it is loaded into synaptic vesicles by a specific vesicular acetylcholine transporter (VAChT). We have generated antibodies that recognize ChAT or VAChT in a model organism, the nematode Caenorhabditis elegans, in order to examine the subcellular and cellular distributions of these cholinergic proteins. ChAT and VAChT are found in the same neurons, including more than one-third of the 302 total neurons present in the adult hermaphrodite. VAChT is found in synaptic regions, whereas ChAT appears to exist in two forms in neurons, a synapse-enriched form and a more evenly distributed possibly cytosolic form. We have used antibodies to identify the cholinergic neurons in the body of larval and adult hermaphrodites. All of the classes of putative excitatory motor neurons in the ventral nerve cord appear to be cholinergic: the DA and DB neurons in the first larval stage and the AS, DA, DB, VA, VB, and VC neurons in the adult. In addition, several interneurons with somas in the tail and processes in the tail or body are cholinergic; sensory neurons are generally not cholinergic. Description of the normal pattern of cholinergic proteins and neurons will improve our understanding of the role of cholinergic neurons in the behavior and development of this model organism.

The C. elegans L1CAM homologue LAD-2 functions as a coreceptor in MAB-20/Sema2 mediated axon guidance

The L1 cell adhesion molecule (L1CAM) participates in neuronal development. Mutations in the human L1 gene can cause the neurological disorder CRASH (corpus callosum hypoplasia, retardation, adducted thumbs, spastic paraplegia, and hydrocephalus). This study presents genetic data that shows that L1-like adhesion gene 2 (LAD-2), a Caenorhabditis elegans L1CAM, functions in axon pathfinding. In the SDQL neuron, LAD-2 mediates dorsal axon guidance via the secreted MAB-20/Sema2 and PLX-2 plexin receptor, the functions of which have largely been characterized in epidermal morphogenesis. We use targeted misexpression experiments to provide in vivo evidence that MAB-20/Sema2 acts as a repellent to SDQL. Coimmunoprecipitation assays reveal that MAB-20 weakly interacts with PLX-2; this interaction is increased in the presence of LAD-2, which can interact independently with MAB-20 and PLX-2. These results suggest that LAD-2 functions as a MAB-20 coreceptor to secure MAB-20 coupling to PLX-2. In vertebrates, L1 binds neuropilin1, the obligate receptor to the secreted Sema3A. However, invertebrates lack neuropilins. LAD-2 may thus function in the semaphorin complex by combining the roles of neuropilins and L1CAMs.

Vigorous motor activity in Caenorhabditis elegans requires efficient clearance of dopamine mediated by synaptic localization of the dopamine transporter DAT-1

The catecholamine dopamine (DA) functions as a powerful modulatory neurotransmitter in both invertebrates and vertebrates. As in man, DA neurons in the nematode Caenorhabditis elegans express a cocaine-sensitive transporter (DAT-1), presumably to regulate synaptic DA signaling and limit DA spillover to extrasynaptic sites, although evidence supporting this is currently lacking. In this report, we describe and validate a novel and readily quantifiable phenotype, swimming-induced paralysis (SWIP) that emerges in DAT-1-deficient nematodes when animals exert maximal physical activity in water. We verify the dependence of SWIP on DA biosynthesis, vesicular packaging, synaptic release, and on the DA receptor DOP-3. Using DAT-1 specific antibodies and GFP::DAT-1 fusions, we demonstrate a synaptic enrichment of DAT-1 that is achieved independently of synaptic targeting of the vesicular monoamine transporter (VMAT). Importantly, dat-1 deletions and point mutations that disrupt DA uptake in cultured C. elegans neurons and/or impact DAT-1 synaptic localization in vivo generate SWIP. SWIP assays, along with in vivo imaging of wild-type and mutant GFP::DAT-1 fusions identify a distal COOH terminal segment of the transporter as essential for efficient somatic export, synaptic localization and in vivo DA clearance. Our studies provide the first description of behavioral perturbations arising from altered trafficking of DATs in vivo in any organism and support a model whereby endogenous DA actions in C. elegans are tightly regulated by synaptic DAT-1.

A microcosm of the biomedical research experience for upper-level undergraduates

The skill set required of biomedical researchers continues to grow and evolve as biology matures as a natural science. Science necessitates creative yet critical thinking, persuasive communication skills, purposeful use of time, and adeptness at the laboratory bench. Teaching these skills can be effectively accomplished in an inquiry-based, active-learning environment at a primarily undergraduate institution. Cell Biology Techniques, an upper-level cell biology laboratory course at St. John Fisher College, features two independent projects that take advantage of the biology of the nematode Caenorhabditis elegans, a premier yet simple model organism. First, students perform a miniature epigenetic screen for novel phenotypes using RNA interference. The results of this screen combined with literature research direct students toward a singe gene that they attempt to subclone in the second project. The biology of the chosen gene/protein also becomes an individualized focal point with respect to the content of the laboratory. Progress toward course goals is evaluated using written, oral, and group-produced assignments, including a concept map. Pre- and postassessment indicates a significant increase in the understanding of broad concepts in cell biological research.

Remarkable longevity and stress resistance of nematode PI3K-null mutants

The great majority of lifespan-augmenting mutations were discovered in the nematode Caenorhabditis elegans. In particular, genetic disruption of insulin-like signaling extends longevity 1.5- to 3-fold in the nematode, and to lesser degrees in other taxa, including fruit flies and mice. C. elegans strains bearing homozygous nonsense mutations in the age-1 gene, which encodes the class-I phosphatidylinositol 3-kinase catalytic subunit (PI3K(CS)), produce progeny that were thought to undergo obligatory developmental arrest. We now find that, after prolonged developmental times at 15-20 degrees C, they mature into extremely long-lived adults with near-normal feeding rates and motility. They survive to a median of 145-190 days at 20 degrees C, with nearly 10-fold extension of both median and maximum adult lifespan relative to N2DRM, a long-lived wild-type stock into which the null mutant was outcrossed. PI3K-null adults, although a little less thermotolerant, are considerably more resistant to oxidative and electrophilic stresses than worms bearing normal or less long-lived alleles. Their unprecedented factorial gains in survival, under both normal and toxic environments, are attributed to elimination of residual and maternally contributed PI3K(CS) or its products, and consequent modification of kinase signaling cascades.

Optical imaging of cell fusion and fusion proteins in Caenorhabditis elegans

Cell fusion is a very dynamic process in which the entire membrane and cellular contents of two or more cells merge into one. Strategies developed to understand the component processes that make up a full fusion event require imaging to be performed over a range of space and time scales. These strategies must cover detection of nanometer-sized pores, monitoring cytoplasmic diffusion and the dynamic localization of proteins that induce fusion competence, and three-dimensional reconstruction of multinucleated cells. Caenorhabditis elegans' small size, predictable development, and transparent body make this organism optimal for microscopic investigations. In this chapter, focus is placed on light microscopy techniques that have been used thus far to study developmental fusion events in C. elegans and the insights that have been gained from them. There is also a general overview of the developmental timing of the cell fusion events. Additionally, several protocols are described for preparing both fixed and live specimens at various developmental stages of C. elegans for examination via optical microscopy.

Neuronal signaling modulates protein homeostasis in Caenorhabditis elegans post-synaptic muscle cells

Protein homeostasis maintains proper intracellular balance by promoting protein folding and clearance mechanisms while minimizing the stress caused by the accumulation of misfolded and damaged proteins. Chronic expression of aggregation-prone proteins is deleterious to the cell and has been linked to a wide range of conformational disorders. The molecular response to misfolded proteins is highly conserved and generally studied as a cell-autonomous process. Here, we provide evidence that neuronal signaling is an important modulator of protein homeostasis in post-synaptic muscle cells. In a forward genetic screen in Caenorhabditis elegans for enhancers of polyglutamine aggregation in muscle cells, we identified unc-30, a neuron-specific transcription factor that regulates the synthesis of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). We used additional sensors of protein conformational states to show that defective GABA signaling or increased acetylcholine (ACh) signaling causes a general imbalance in protein homeostasis in post-synaptic muscle cells. Moreover, exposure to GABA antagonists or ACh agonists has a similar effect, which reveals that toxins that act at the neuromuscular junction are potent modifiers of protein conformational disorders. These results demonstrate the importance of intercellular communication in intracellular homeostasis.

Control of sex-specific apoptosis in C. elegans by the BarH homeodomain protein CEH-30 and the transcriptional repressor UNC-37/Groucho

Apoptosis is essential for proper development and tissue homeostasis in metazoans. It plays a critical role in generating sexual dimorphism by eliminating structures that are not needed in a specific sex. The molecular mechanisms that regulate sexually dimorphic apoptosis are poorly understood. Here we report the identification of the ceh-30 gene as a key regulator of sex-specific apoptosis in Caenorhabditis elegans. Loss-of-function mutations in ceh-30 cause the ectopic death of male-specific CEM neurons. ceh-30 encodes a BarH homeodomain protein that acts downstream from the terminal sex determination gene tra-1, but upstream of, or in parallel to, the cell-death-initiating gene egl-1 to protect CEM neurons from undergoing apoptosis in males. The second intron of the ceh-30 gene contains two adjacent cis-elements that are binding sites for TRA-1A and a POU-type homeodomain protein UNC-86 and acts as a sensor to regulate proper specification of the CEM cell fate. Surprisingly, the N terminus of CEH-30 but not its homeodomain is critical for CEH-30's cell death inhibitory activity in CEMs and contains a conserved eh1/FIL domain that is important for the recruitment of the general transcriptional repressor UNC-37/Groucho. Our study suggests that ceh-30 defines a critical checkpoint that integrates the sex determination signal TRA-1 and the cell fate determination and survival signal UNC-86 to control the sex-specific activation of the cell death program in CEMs through the general transcription repressor UNC-37.

unc-94 encodes a tropomodulin in Caenorhabditis elegans

unc-94 is one of about 40 genes in Caenorhabditis elegans that, when mutant, displays an abnormal muscle phenotype. Two mutant alleles of unc-94, su177 and sf20, show reduced motility and brood size and disorganization of muscle structure. In unc-94 mutants, immunofluorescence microscopy shows that a number of known sarcomeric proteins are abnormal, but the most dramatic effect is in the localization of F-actin, with some abnormally accumulated near muscle cell-to-cell boundaries. Electron microscopy shows that unc-94(sf20) mutants have large accumulations of thin filaments near the boundaries of adjacent muscle cells. Multiple lines of evidence prove that unc-94 encodes a tropomodulin, a conserved protein known from other systems to bind to both actin and tropomyosin at the pointed ends of actin thin filaments. su177 is a splice site mutation in intron 1, which is specific to one of the two unc-94 isoforms, isoform a; sf20 has a stop codon in exon 5, which is shared by both isoform a and isoform b. The use of promoter-green fluorescent protein constructs in transgenic animals revealed that unc-94a is expressed in body wall, vulval and uterine muscles, whereas unc-94b is expressed in pharyngeal, anal depressor, vulval and uterine muscles and in spermatheca and intestinal epithelial cells. By Western blot, anti-UNC-94 antibodies detect polypeptides of expected size from wild type, wild-type-sized proteins of reduced abundance from unc-94(su177), and no detectable unc-94 products from unc-94(sf20). Using these same antibodies, UNC-94 localizes as two closely spaced parallel lines flanking the M-lines, consistent with localization to the pointed ends of thin filaments. In addition, UNC-94 is localized near muscle cell-to-cell boundaries.

C. elegans agrin is expressed in pharynx, IL1 neurons and distal tip cells and does not genetically interact with genes involved in synaptogenesis or muscle function

Agrin is a basement membrane protein crucial for development and maintenance of the neuromuscular junction in vertebrates. The C. elegans genome harbors a putative agrin gene agr-1. We have cloned the corresponding cDNA to determine the primary structure of the protein and expressed its recombinant fragments to raise specific antibodies. The domain organization of AGR-1 is very similar to the vertebrate orthologues. C. elegans agrin contains a signal sequence for secretion, seven follistatin domains, three EGF-like repeats and two laminin G domains. AGR-1 loss of function mutants did not exhibit any overt phenotypes and did not acquire resistance to the acetylcholine receptor agonist levamisole. Furthermore, crossing them with various mutants for components of the dystrophin-glycoprotein complex with impaired muscle function did not lead to an aggravation of the phenotypes. Promoter-GFP translational fusion as well as immunostaining of worms revealed expression of agrin in buccal epithelium and the protein deposition in the basal lamina of the pharynx. Furthermore, dorsal and ventral IL1 head neurons and distal tip cells of the gonad arms are sources of agrin production, but no expression was detectable in body muscles or in the motoneurons innervating them. Recombinant worm AGR-1 fragment is able to cluster vertebrate dystroglycan in cultured cells, implying a conservation of this interaction, but since neither of these proteins is expressed in muscle of C. elegans, this interaction may be required in different tissues. The connections between muscle cells and the basement membrane, as well as neuromuscular junctions, are structurally distinct between vertebrates and nematodes.

UNC-87, a calponin-related protein in C. elegans, antagonizes ADF/cofilin-mediated actin filament dynamics

Stabilization of actin filaments is critical for supporting actomyosin-based contractility and for maintaining stable cellular structures. Tropomyosin is a well-characterized ubiquitous actin stabilizer that inhibits ADF/cofilin-dependent actin depolymerization. Here, we show that UNC-87, a calponin-related Caenorhabditis elegans protein with seven calponin-like repeats, competes with ADF/cofilin for binding to actin filaments and inhibits ADF/cofilin-dependent filament severing and depolymerization in vitro. Mutations in the unc-87 gene suppress the disorganized actin phenotype in an ADF/cofilin mutant in the C. elegans body wall muscle, supporting their antagonistic roles in regulating actin stability in vivo. UNC-87 and tropomyosin exhibit synergistic effects in stabilizing actin filaments against ADF/cofilin, and direct comparison reveals that UNC-87 effectively stabilizes actin filaments at much lower concentrations than tropomyosin. However, the in vivo functions of UNC-87 and tropomyosin appear different, suggesting their distinct roles in the regulation of actomyosin assembly and cellular contractility. Our results demonstrate that actin binding via calponin-like repeats competes with ADF/cofilin-driven cytoskeletal turnover, and is critical for providing the spatiotemporal regulation of actin filament stability.

Transcriptional regulation of AQP-8, a Caenorhabditis elegans aquaporin exclusively expressed in the excretory system, by the POU homeobox transcription factor CEH-6

Due to the ever changing environmental conditions in soil, regulation of osmotic homeostasis in the soil-dwelling nematode Caenorhabditis elegans is critical. AQP-8 is a C. elegans aquaporin that is expressed in the excretory cell, a renal equivalent tissue, where the protein participates in maintaining water balance. To better understand the regulation of AQP-8, we undertook a promoter analysis to identify the aqp-8 cis-regulatory elements. Using progressive 5' deletions of upstream sequence, we have mapped an essential regulatory region to roughly 300 bp upstream of the translational start site of aqp-8. Analysis of this region revealed a sequence corresponding to a known DNA functional element (octamer motif), which interacts with POU homeobox transcription factors. Phylogenetic footprinting showed that this site is perfectly conserved in four nematode species. The octamer site's function was further confirmed by deletion analyses, mutagenesis, functional studies, and electrophoretic mobility shift assays. Of the three POU homeobox proteins encoded in the C. elegans genome, CEH-6 is the only member that is expressed in the excretory cell. We show that expression of AQP-8 is regulated by CEH-6 by performing RNA interference experiments. CEH-6's mammalian ortholog, Brn1, is expressed both in the kidney and the central nervous system and binds to the same octamer consensus binding site to drive gene expression. These parallels in transcriptional control between Brn1 and CEH-6 suggest that C. elegans may well be an appropriate model for determining gene-regulatory networks in the developing vertebrate kidney.

The UNC-45 chaperone mediates sarcomere assembly through myosin degradation in Caenorhabditis elegans

Myosin motors are central to diverse cellular processes in eukaryotes. Homologues of the myosin chaperone UNC-45 have been implicated in the assembly and function of myosin-containing structures in organisms from fungi to humans. In muscle, the assembly of sarcomeric myosin is regulated to produce stable, uniform thick filaments. Loss-of-function mutations in Caenorhabditis elegans UNC-45 lead to decreased muscle myosin accumulation and defective thick filament assembly, resulting in paralyzed animals. We report that transgenic worms overexpressing UNC-45 also display defects in myosin assembly, with decreased myosin content and a mild paralysis phenotype. We find that the reduced myosin accumulation is the result of degradation through the ubiquitin/proteasome system. Partial proteasome inhibition is able to restore myosin protein and worm motility to nearly wild-type levels. These findings suggest a mechanism in which UNC-45–related proteins may contribute to the degradation of myosin in conditions such as heart failure and muscle wasting.

Mutation of the MAP kinase DYF-5 affects docking and undocking of kinesin-2 motors and reduces their speed in the cilia of Caenorhabditis elegans

In the cilia of the nematode Caenorhabditis elegans, anterograde intraflagellar transport (IFT) is mediated by two kinesin-2 complexes, kinesin II and OSM-3 kinesin. These complexes function together in the cilia middle segments, whereas OSM-3 alone mediates transport in the distal segments. Not much is known about the mechanisms that compartmentalize the kinesin-2 complexes or how transport by both kinesins is coordinated. Here, we identify DYF-5, a conserved MAP kinase that plays a role in these processes. Fluorescence microscopy and EM revealed that the cilia of dyf-5 loss-of-function (lf) animals are elongated and are not properly aligned into the amphid channel. Some cilia do enter the amphid channel, but the distal ends of these cilia show accumulation of proteins. Consistent with these observations, we found that six IFT proteins accumulate in the cilia of dyf-5(lf) mutants. In addition, using genetic analyses and live imaging to measure the motility of IFT proteins, we show that dyf-5 is required to restrict kinesin II to the cilia middle segments. Finally, we show that, in dyf-5(lf) mutants, OSM-3 moves at a reduced speed and is not attached to IFT particles. We propose that DYF-5 plays a role in the undocking of kinesin II from IFT particles and in the docking of OSM-3 onto IFT particles.

LIN-61, one of two Caenorhabditis elegans malignant-brain-tumor-repeat-containing proteins, acts with the DRM and NuRD-like protein complexes in vulval development but not in certain other biological processes

Vulval development in Caenorhabiditis elegans is inhibited by the redundant functions of the synthetic multivulva (synMuv) genes. At least 26 synMuv genes have been identified, many of which appear to act via transcriptional repression. Here we report the molecular identification of the class B synMuv gene lin-61, which encodes a protein composed of four malignant brain tumor (MBT) repeats. MBT repeats, domains of approximately 100 amino acids, have been found in multiple copies in a number of transcriptional repressors, including Polycomb-group proteins. MBT repeats are important for the transcriptional repression mediated by these proteins and in some cases have been shown to bind modified histones. C. elegans contains one other MBT-repeat-containing protein, MBTR-1. We demonstrate that a deletion allele of mbtr-1 does not cause a synMuv phenotype nor does mbtr-1 appear to act redundantly with or in opposition to lin-61. We further show that lin-61 is phenotypically and biochemically distinct from other class B synMuv genes. Our data indicate that while the class B synMuv genes act together to regulate vulval development, lin-61 functions separately from some class B synMuv proteins in other biological processes.

UNC-4 represses CEH-12/HB9 to specify synaptic inputs to VA motor neurons in C. elegans

In Caenorhabditis elegans, VA and VB motor neurons arise as lineal sisters but synapse with different interneurons to regulate locomotion. VA-specific inputs are defined by the UNC-4 homeoprotein and its transcriptional corepressor, UNC-37/Groucho, which function in the VAs to block the creation of chemical synapses and gap junctions with interneurons normally reserved for VBs. To reveal downstream genes that control this choice, we have employed a cell-specific microarray strategy that has now identified unc-4-regulated transcripts. One of these genes, ceh-12, a member of the HB9 family of homeoproteins, is normally restricted to VBs. We show that expression of CEH-12/HB9 in VA motor neurons in unc-4 mutants imposes VB-type inputs. Thus, this work reveals a developmental switch in which motor neuron input is defined by differential expression of transcription factors that select alternative presynaptic partners. The conservation of UNC-4, HB9, and Groucho expression in the vertebrate motor circuit argues that similar mechanisms may regulate synaptic specificity in the spinal cord.

Developmental expression of FOG-1/CPEB protein and its control in the Caenorhabditis elegans hermaphrodite germ line

The specification of a germ cell as sperm or oocyte and determination of cell number remain unsolved questions in developmental biology. This paper examines Caenorhabditis elegans FOG-1, a CPEB-related RNA-binding protein that controls the sperm fate. We find that abundant FOG-1 protein is observed transiently in germ cells just prior to their expression of an early sperm-differentiation marker. As the germline tissue elongates, abundant FOG-1 appears more and more distally as sperm become specified, but disappears when the germ line switches to oogenesis. This dynamic pattern is controlled by both globally acting and germline-specific sex-determining regulators. Importantly, the extent of FOG-1 expression corresponds roughly to sperm number in wild-type and mutants, altering sperm number. By contrast, three other key regulators of the sperm/oocyte decision do not similarly correspond to sperm number. We suggest that FOG-1 is precisely modulated in both time and space to specify sperm fate and control sperm number.

Touch sensitivity in Caenorhabditis elegans

The nematode Caenorhabditis elegans was the first organism for which touch insensitive mutants were obtained. The study of the genes defective in these mutants has led to the identification of components of a mechanosensory complex needed for specific cells to sense gentle touch to the body. Multiple approaches using genetics, cell biology, biochemistry, and electrophysiology have characterized a channel complex, containing two DEG/ENaC pore-forming subunits and several other proteins, that transduces the touch response. Other mechanical responses, sensed by other cells using a variety of other components, are less well understood in C. elegans. Many of these other senses may use TRP channels, although DEG/ENaC channels have also been implicated.

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.

Genetic control of sex differences in C. elegans neurobiology and behavior

As a well-characterized, genetically tractable animal, the nematode Caenorhabditis elegans is an ideal model to explore the connections between genes and the sexual regulation of the nervous system and behavior. The two sexes of C. elegans, males and hermaphrodites, have precisely defined differences in neuroanatomy: superimposed onto a "core" nervous system of exactly 294 neurons, hermaphrodites and males have 8 and 89 sex-specific neurons, respectively. These sex-specific neurons are essential for cognate sex-specific behaviors, including hermaphrodite egg-laying and male mating. In addition, regulated sex differences in the core nervous system itself may provide additional, though poorly understood, controls on behavior. These differences in the nervous system and behavior, like all known sex differences in the C. elegans soma, are controlled by the master regulator of C. elegans sex determination, tra-1. Downstream of tra-1 lie specific effectors of sex determination, including genes controlling sex-specific cell death and a family of regulators, the DM-domain genes, related to Drosophila doublesex and the vertebrate DMRT genes. There is no central (i.e., gonadal) regulator of sexual phenotype in the C. elegans nervous system; instead, tra-1 acts cell-autonomously in nearly all sexually dimorphic somatic cells. However, recent results suggest that the status of the gonad can be communicated to the nervous system to modulate sex-specific behaviors. Continuing research into the genetic control of neural sex differences in C. elegans is likely to yield insight into conserved mechanisms of cell-autonomous cross talk between cell fate patterning and sexual differentiation pathways.

Genetics of Egg-Laying in Worms

Genetic studies of behavior in the nematode Caenorhabditis elegans have provided an effective approach to investigate the molecular and cellular basis of nervous system function and development. Among the best studied behaviors is egg-laying, the process by which hermaphrodites deposit developing embryos into the environment. Egg-laying involves a simple motor program involving a small network of motorneurons and specialized smooth muscle cells, which is regulated by a variety of sensory stimuli. Analysis of egg-laying-defective mutants has provided insight into a number of conserved processes in nervous system development, including neurogenesis, cell migration, and synaptic patterning, as well as aspects of excitable cell signal transduction and neuromodulation.

Proteasomal ubiquitin receptor RPN-10 controls sex determination in Caenorhabditis elegans

The ubiquitin-binding RPN-10 protein serves as a ubiquitin receptor that delivers client proteins to the 26S proteasome. Although ubiquitin recognition is an essential step for proteasomal destruction, deletion of the rpn-10 gene in yeast does not influence viability, indicating redundancy of the substrate delivery pathway. However, their specificity and biological relevance in higher eukaryotes is still enigmatic. We report herein that knockdown of the rpn-10 gene, but not any other proteasome subunit genes, sexually transforms hermaphrodites to females by eliminating hermaphrodite spermatogenesis in Caenorhabditis elegans. The feminization phenotype induced by deletion of the rpn-10 gene was rescued by knockdown of tra-2, one of sexual fate decision genes promoting female development, and its downstream target tra-1, indicating that the TRA-2-mediated sex determination pathway is crucial for the Delta rpn-10-induced sterile phenotype. Intriguingly, we found that co-knockdown of rpn-10 and functionally related ubiquitin ligase ufd-2 overcomes the germline-musculinizing effect of fem-3(gf). Furthermore, TRA-2 proteins accumulated in rpn-10-defective worms. Our results show that the RPN-10-mediated ubiquitin pathway is indispensable for control of the TRA-2-mediated sex-determining pathway.

Serotonin targets the DAF-16/FOXO signaling pathway to modulate stress responses

Stress response is a fundamental form of behavioral and physiological plasticity. Here we describe how serotonin (5HT) governs stress behavior by regulating DAF-2 insulin/IGF-1 receptor signaling to the DAF-16/FOXO transcription factor at the nexus of development, metabolism, immunity, and stress responses in C. elegans. Serotonin-deficient tph-1 mutants, like daf-2 mutants, exhibit DAF-16 nuclear accumulation and constitutive physiological stress states. Exogenous 5HT and fluoxetine (Prozac) prevented DAF-16 nuclear accumulation in wild-type animals under stresses. Genetic analyses imply that DAF-2 is a downstream target of 5HT signaling and that distinct serotonergic neurons act through distinct 5HT receptors to influence distinct DAF-16-mediated stress responses. We suggest that modulation of FOXO by 5HT represents an ancient feature of stress physiology and that the C. elegans is a genetically tractable model that can be used to delineate the molecular mechanisms and drug actions linking 5HT, neuroendocrine signaling, immunity, and mitochondrial function.

Identification of a gonadotropin-releasing hormone receptor orthologue in Caenorhabditis elegans

BACKGROUND

The Caenorhabditis elegans genome is known to code for at least 1149 G protein-coupled receptors (GPCRs), but the GPCR(s) critical to the regulation of reproduction in this nematode are not yet known. This study examined whether GPCRs orthologous to human gonadotropin-releasing hormone receptor (GnRHR) exist in C. elegans.

RESULTS

Our sequence analyses indicated the presence of two proteins in C. elegans, one of 401 amino acids [GenBank: NP_491453; WormBase: F54D7.3] and another of 379 amino acids [GenBank: NP_506566; WormBase: C15H11.2] with 46.9% and 44.7% nucleotide similarity to human GnRHR1 and GnRHR2, respectively. Like human GnRHR1, structural analysis of the C. elegans GnRHR1 orthologue (Ce-GnRHR) predicted a rhodopsin family member with 7 transmembrane domains, G protein coupling sites and phosphorylation sites for protein kinase C. Of the functionally important amino acids in human GnRHR1, 56% were conserved in the C. elegans orthologue. Ce-GnRHR was actively transcribed in adult worms and immunoanalyses using antibodies generated against both human and C. elegans GnRHR indicated the presence of a 46-kDa protein, the calculated molecular mass of the immature Ce-GnRHR. Ce-GnRHR staining was specifically localized to the germline, intestine and pharynx. In the germline and intestine, Ce-GnRHR was localized specifically to nuclei as revealed by colocalization with a DNA nuclear stain. However in the pharynx, Ce-GnRHR was localized to the myofilament lattice of the pharyngeal musculature, suggesting a functional role for Ce-GnRHR signaling in the coupling of food intake with reproduction. Phylogenetic analyses support an early evolutionary origin of GnRH-like receptors, as evidenced by the hypothesized grouping of Ce-GnRHR, vertebrate GnRHRs, a molluscan GnRHR, and the adipokinetic hormone receptors (AKHRs) and corazonin receptors of arthropods.

CONCLUSION

This is the first report of a GnRHR orthologue in C. elegans, which shares significant similarity with insect AKHRs. In vertebrates, GnRHRs are central components of the reproductive endocrine system, and the identification of a GnRHR orthologue in C. elegans suggests the potential use of C. elegans as a model system to study reproductive endocrinology.

SYD-2 Liprin-α organizes presynaptic active zone formation through ELKS

A central event in synapse development is formation of the presynaptic active zone in response to positional cues. Three active zone proteins, RIM, ELKS (also known as ERC or CAST) and Liprin-alpha, bind each other and are implicated in linking active zone formation to synaptic vesicle release. Loss of function in Caenorhabditis elegans syd-2 Liprin-alpha alters the size of presynaptic specializations and disrupts synaptic vesicle accumulation. Here we report that a missense mutation in the coiled-coil domain of SYD-2 causes a gain of function. In HSN synapses, the syd-2(gf) mutation promotes synapse formation in the absence of syd-1, which is essential for HSN synapse formation. syd-2(gf) also partially suppresses the synaptogenesis defects in syg-1 and syg-2 mutants. The activity of syd-2(gf) requires elks-1, an ELKS homolog; but not unc-10, a RIM homolog. The mutant SYD-2 shows increased association with ELKS. These results establish a functional dependency for assembly of the presynaptic active zone in which SYD-2 plays a key role.

The C. elegans Sex-Determining GLI Protein TRA-1A Is Regulated by Sex-Specific Proteolysis

TRA-1A is the sole representative in Caenorhabditis elegans of the Gli transcription factor family. Its activity is required to specify all somatic female cell fates in XX hermaphrodites. We have found that TRA-1 protein levels are much higher in hermaphrodites than in males, and that the difference is attributable to the predominance in hermaphrodites of C-terminally truncated isoforms that are nearly undetectable in males. Our results support a model in which TRA-1A is negatively regulated by male-specific proteolysis that depends upon specific TRA-1A protein sequences and upon the activity of the fem genes. C-terminally truncated TRA-1 isoforms are stable and can inappropriately feminize XO males, suggesting that they escape this negative regulation. Thus, although C. elegans appears to lack a Hedgehog-signaling pathway, our results indicate that proteolytic processing and degradation of Gli family transcription factors, commonly seen during Hedgehog signaling in other organisms, also control C. elegans sex determination.

Some C. elegans class B synthetic multivulva proteins encode a conserved LIN-35 Rb-containing complex distinct from a NuRD-like complex

The Caenorhabditis elegans synthetic multivulva (synMuv) genes act redundantly to antagonize the specification of vulval cell fates, which are promoted by an RTK/Ras pathway. At least 26 synMuv genes have been genetically identified, several of which encode proteins with homologs that act in chromatin remodeling or transcriptional repression. Here we report the molecular characterization of two synMuv genes, lin-37 and lin-54. We show that lin-37 and lin-54 encode proteins in a complex with at least seven synMuv proteins, including LIN-35, the only C. elegans homolog of the mammalian tumor suppressor Rb. Biochemical analyses of mutants suggest that LIN-9, LIN-53, and LIN-54 are required for the stable formation of this complex. This complex is distinct from a second complex of synMuv proteins with a composition similar to that of the mammalian Nucleosome Remodeling and Deacetylase complex. The class B synMuv complex we identified is evolutionarily conserved and likely functions in transcriptional repression and developmental regulation.