Homologous and unique G protein alpha subunits in the nematode Caenorhabditis elegans.

A cDNA corresponding to a known G protein alpha subunit, the alpha subunit of Go (Go alpha), was isolated and sequenced. The predicted amino acid sequence of C. elegans Go alpha is 80-87% identical to other Go alpha sequences. An mRNA that hybridizes to the C. elegans Go alpha cDNA can be detected on Northern blots. A C. elegans protein that crossreacts with antibovine Go alpha antibody can be detected on immunoblots. A cosmid clone containing the C. elegans Go alpha gene (goa-1) was isolated and mapped to chromosome I. The genomic fragments of three other C. elegans G protein alpha subunit genes (gpa-1, gpa-2, and gpa-3) have been isolated using the polymerase chain reaction. The corresponding cosmid clones were isolated and mapped to disperse locations on chromosome V. The sequences of two of the genes, gpa-1 and gpa-3, were determined. The predicted amino acid sequences of gpa-1 and gpa-3 are only 48% identical to each other. Therefore, they are likely to have distinct functions. In addition they are not homologous enough to G protein alpha subunits in other organisms to be classified. Thus C. elegans has G proteins that are identifiable homologues of mammalian G proteins as well as G proteins that appear to be unique to C. elegans. Study of identifiable G proteins in C. elegans may result in a further understanding of their function in other organisms, whereas study of the novel G proteins may provide an understanding of unique aspects of nematode physiology.     

Two Neuronal G Proteins Are Involved in Chemosensation of the Caenorhabditis Elegans Dauer-Inducing Pheromone

Caenorhabditis elegans uses chemosensation to determine its course of development. Young larvae can arrest as dauer larvae in response to increasing population density, which they measure by a nematode-excreted pheromone, and decreasing food supply. Dauer larvae can resume development in response to a decrease in pheromone and increase in food concentration. We show here that two novel G protein alpha subunits (GPA-2 and GPA-3) show promoter activity in subsets of chemosensory neurons and are involved in the decision to form dauer larvae primarily through the response to dauer pheromone. Dominant activating mutations in these G proteins result in constitutive, pheromone-independent dauer formation, whereas inactivation results in reduced sensitivity to pheromone, and, under certain conditions, an alteration in the response to food. Interactions between gpa-2, gpa-3 and other genes controlling dauer formation suggest that these G proteins may act in parallel to regulate the neuronal decision making that precedes dauer formation.     

Hyperactivation of the G12-mediated signaling pathway in Caenorhabditis elegans induces a developmental growth arrest via protein kinase C

The G(12) type of heterotrimeric G-proteins play an important role in development and behave as potent oncogenes in cultured cells. However, little is known about the molecular nature of the components that act in the G(12)-signaling pathway in an organism. We characterized a C. elegans Galpha subunit gene, gpa-12, which is a homolog of mammalian G(12)/G(13)alpha, and found that animals defective in gpa-12 are viable. Expression of activated GPA-12 (G(12)QL) results in a developmental growth arrest caused by a feeding behavior defect that is due to a dramatic reduction in pharyngeal pumping. To elucidate the molecular nature of the signaling pathways in which G(12) participates, we screened for suppressors of the G(12)QL phenotype. We isolated 50 suppressors that contain mutations in tpa-1, which encodes two protein kinase C isoforms, TPA-1A and TPA-1B, most similar to PKCtheta/delta. TPA-1 mediates the action of the tumor promoter PMA. Expression of G(12)QL and treatment of wild-type animals with PMA induce an identical growth arrest caused by inhibition of larval feeding, which is dependent on TPA-1A and TPA-1B function. These results suggest that TPA-1 is a downstream target of both G(12) signaling and PMA in modulating feeding and growth in C. elegans. Taken together, our findings provide a potential molecular mechanism for the transforming capability of G(12) proteins.     

Nucleotide sequences of Caenorhabditis elegans core histone genes. Genes for different histone classes share common flanking sequence elements

We have determined the nucleotide sequence of core histone genes and flanking regions from two of approximately 11 different genomic histone clusters of the nematode Caenorhabditis elegans. Four histone genes from one cluster (H3, H4, H2B, H2A) and two histone genes from another (H4 and H2A) were analyzed. The predicted amino acid sequences of the two H4 and H2A proteins from the two clusters are identical, whereas the nucleotide sequences of the genes have diverged 9% (H2A) and 12% (H4). Flanking sequences, which are mostly not similar, were compared to identify putative regulatory elements. A conserved sequence of 34 base-pairs is present 19 to 42 nucleotides 3' of the termination codon of all the genes. Within the conserved sequence is a 16-base dyad sequence homologous to the one typically found at the 3' end of histone genes from higher eukaryotes. The C. elegans core histone genes are organized as divergently transcribed pairs of H3-H4 and H2A-H2B and contain 5' conserved sequence elements in the shared spacer regions. One of the sequence elements, 5' CTCCNCCTNCCCACCNCANA 3', is located immediately upstream from the canonical TATA homology of each gene. Another sequence element, 5' CTGCGGGGACACATNT 3', is present in the spacer of each heterotypic pair. These two 5' conserved sequences are not present in the promoter region of histone genes from other organisms, where 5' conserved sequences are usually different for each histone class. They are also not found in non-histone genes of C. elegans. These putative regulatory sequences of C. elegans core histone genes are similar to the regulatory elements of both higher and lower eukaryotes. The coding regions of the genes and the 3' regulatory sequences are similar to those of higher eukaryotes, whereas the presence of common 5' sequence elements upstream from genes of different histone classes is similar to histone promoter elements in yeast.     

Strongyloides stercoralis: cell- and tissue-specific transgene expression and co-transformation with vector constructs incorporating a common multifunctional 3' UTR

Transgenesis is a valuable methodology for studying gene expression patterns and gene function. It has recently become available for research on some parasitic nematodes, including Strongyloides stercoralis. Previously, we described a vector construct, comprising the promoter and 3' UTR of the S. stercoralis gene Ss era-1 that gives expression of GFP in intestinal cells of developing F1 progeny. In the present study, we identified three new S. stercoralis promoters, which, in combination with the Ss era-1 3' UTR, can drive expression of GFP or the red fluorescent protein, mRFPmars, in tissue-specific fashion. These include Ss act-2, which drives expression in body wall muscle cells, Ss gpa-3, which drives expression in amphidial and phasmidial neurons and Ss rps-21, which drives ubiquitous expression in F1 transformants and in the gonads of microinjected P0 female worms. Concomitant microinjection of vectors containing GFP and mRFPmars gave dually transformed F1 progeny, suggesting that these constructs could be used as co-injection markers for other transgenes of interest. We have developed a vector "toolkit" for S. stercoralis including constructs with the Ss era-1 3' UTR and each of the promoters described above.     

Transforming growth factor-beta and insulin-like signalling pathways in parasitic helminths

The signal transduction pathways involved in regulating developmental arrest in the free-living nematode, Caenorhabditis elegans, are fairly well characterised. However, much less is known about how these processes may influence the developmental timing and maturation in helminth parasites. Here, we provide an overview of two signalling pathways implicated in the regulation of dauer larva formation in C. elegans, the insulin-like signalling pathway and the transforming growth factor-beta pathway, and explore what is known about these signalling pathways in a variety of parasitic helminths. Understanding the differences about how these pathways are affected by environmental cues in free-living versus parasitic species of helminths may provide insights into novel mechanisms for the control or prevention of helminth-induced disease.     

The Caenorhabditis elegans D2-like dopamine receptor DOP-2 physically interacts with GPA-14, a Gαi subunit

ABSTRACT: Dopaminergic inputs are sensed on the cell surface by the seven-transmembrane dopamine receptors that belong to a superfamily of G-protein-coupled receptors (GPCRs). Dopamine receptors are classified as D1-like or D2-like receptors based on their homology and pharmacological profiles. In addition to well established G-protein coupled mechanism of dopamine receptors in mammalian system they can also interact with other signaling pathways. In C. elegans four dopamine receptors (dop-1, dop-2, dop-3 and dop-4) have been reported and they have been implicated in a wide array of behavioral and physiological processes. We performed this study to assign the signaling pathway for DOP-2, a D2-like dopamine receptor using a split-ubiquitin based yeast two-hybrid screening of a C. elegans cDNA library with a novel dop-2 variant (DOP-2XL) as bait. Our yeast two-hybrid screening resulted in identification of gpa-14, as one of the positively interacting partners. gpa-14 is a Gα coding sequence and shows expression overlap with dop-2 in C. elegans ADE deirid neurons. In-vitro pull down assays demonstrated physical coupling between dopamine receptor DOP-2XL and GPA-14. Further, we sought to determine the DOP-2 region necessary for GPA-14 coupling. We generated truncated DOP-2XL constructs and performed pair-wise yeast two-hybrid assay with GPA-14 followed by in-vitro interaction studies and here we report that the third intracellular loop is the key domain responsible for DOP-2 and GPA-14 coupling. Our results show that the extra-long C. elegans D2-like receptor is coupled to gpa-14 that has no mammalian homolog but shows close similarity to inhibitory G-proteins. Supplementing earlier investigations, our results demonstrate the importance of an invertebrate D2-like receptor's third intracellular loop in its G-protein interaction.     

Two hypotheses to explain why RNA interference does not work in animal parasitic nematodes

RNA interference (RNAi) has been used extensively in model organisms such as Caenorhabditis elegans. Methods developed for RNAi in C. elegans have also been used in parasitic nematodes. However, RNAi in parasitic nematodes has been unsuccessful or has had limited success. Studies of genes essential for RNAi in C. elegans and of RNAi in Caenorhabditis spp. other than C. elegans suggest two complementary, and testable, hypotheses for the limited success of RNAi in animal parasitic nematodes. These are: (i) that the external supply of double stranded RNA (dsRNA) to parasitic nematodes is inappropriate to achieve RNAi and (ii) that parasitic nematodes are functionally defective in genes required to initiate RNAi from externally supplied dsRNA.     

Caenorhabditis elegans and the study of gene function in parasites.

The free-living nematode Caenorhabditis elegans is a tractable experimental model system for the study of both vertebrate and invertebrate biology. Its most significant advantages are its simplicity, both in anatomy and in genomic organization, and the elaborate methods that have been developed to attribute function to previously uncharacterized genes. Importantly, > 40% of parasitic nematode genes exhibit high levels of homology to genes within the C. elegans genome. Studying such genes using the C. elegans model should yield new insights into key molecules and their possible implications in parasite survival, leading to the discovery of new drug targets and vaccine candidates.     

Functional elements and domains inferred from sequence comparisons of a heat shock gene in two nematodes

Caenorhabditis elegans and Caenorhabditis briggsae are two closely related nematode species that are nearly identical morphologically. Interspecific cross-hybridizing DNA appears to be restricted primarily to coding regions. We compared portions of the hsp-3 homologs, two grp 78-like genes, from C. elegans and C. briggsae and detected regions of DNA identity in the coding region, the 5' flanking DNAs, and the introns. The hsp-3 homologs share approximately 98% and 93% identity at the amino acid and nucleotide levels, respectively. Using the nucleotide substitution rate at the silent third position of the codons, we have estimated a lower limit for the date of divergence between C. elegans and C. briggsae to be approximately 23-32 million years ago. The 5' flanking DNAs and one of the introns contain elements that are highly conserved between C. elegans and C. briggsae. Some of the regions of nucleotide identity in the 5' flanking DNAs correspond to previously detected identities including viral enhancer sequences, a heat shock element, and an element present in the regulatory regions of mammalian grp78 and grp94 genes. We propose that a comparison of C. elegans and C. briggsae sequences will be useful in the detection of potential regulatory and structural elements.     

RNAi in Haemonchus contortus: a potential method for target validation

RNA interference (RNAi) is a method for the functional analysis of specific genes, and is particularly well developed in the free-living nematode Caenorhabditis elegans. There have been several attempts to apply this method to parasitic nematodes. In a recent study undertaken in Haemonchus contortus, Geldhof and colleagues concluded that, although a mechanism for RNAi existed, the methods developed for RNAi in C. elegans had variable efficacy in this parasitic nematode. The potential benefits of RNAi are clear; however, further studies are required to characterize the mechanism present in parasitic nematodes, and to improve culture systems for these nematodes to monitor the long-term effects of RNAi. Only then could RNAi become a reliable assay of gene function.