Cloning and characterisation of gonadotropin-releasing hormone (GnRH) receptors in the cichlid (Haplochromis burtoni) and the zebrafish (Danio rerio)
Master Thesis
2003
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University of Cape Town
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Abstract
The identification of multiple forms of gonadotropin-releasing hormone (GnRH) in a single species is becoming a common occurrence. The highly conserved chicken GnRH II is present along with one or two other GnRHs, composing a combination unique to particular species. This multifunctional peptide is widely distributed through the central nervous system and peripheral tissues. Also, endogenous GnRHs demonstrate distinct patterns of spatial expression within the brain, suggesting they may have separate functions. In addition to being the primary regulator of gonadotropin secretion in vertebrates, GnRH is also involved in the release of GH and prolactin and may fulfil a possible neuromodulatory role. GnRHs exert their actions through the stimulation of distinct GnRH receptors on pituitary gonadotrophs. The presence of multiple GnRH receptor subtypes has been demonstrated in several species and is likely to be a common characteristic of most vertebrates. This thesis describes the cloning and characterisation of GnRH receptors in two species of teleost fish, Haplochromis burtoni (cichlid) and Dania rerio (zebrafish). A type I GnRH receptor has previously been shown to exist in the cichlid. In the present study degenerate primers designed to extracellular loop three of the mammalian GnRH receptors were used to identify a second putative receptor subtype from cichlid (Haplochramis burtoni) genomic DNA. Furthermore, a near full-length cDNA, encompassing transmembrane domain 1 through to transmembrane domain 7 of the GnRH receptor, was cloned from cichlid RNA by reverse transcriptase PCR. This region of the receptor shares approximately 80% amino acid homology with corresponding regions of type III GnRH receptors previously identified in species of perciform fish. Partial sequences of a type IA and a type lB GnRH receptor have previously been identified in the zebrafish. Two sets of degenerate primers were used to elucidate the possible existence of a third receptor in the zebrafish using both genomic DNA and RNA. However, this strategy failed to result in the amplification of novel receptor subtypes in the zebrafish. Controversy surrounds the developmental origins of GnRH neurons and their temporal expression in relation to GnRH receptors. The zebrafish is a model organism, widely used for the study of reporter gene expression during development. Hence an attempt was made to isolate the zebrafish GnRH receptor genes using a genomic DNA library and identify the promoter regions for use as reporter genes in the study of GnRH and GnRH receptor expression during development. Southern blot analysis revealed six genomic clones with sequences homologous to zebrafish GnRH receptor cDNA. Comparison with genomic and cDNA sequences of other GnRH receptors revealed that those regions of the genomic clones that were sequenced only encoded exons 2 and 3. The presence of large introns in the GnRH receptor gene made it difficult to identify genomic clones containing the entire gene and the promoter region. The cloning of part of the zebrafish GnRH receptor genes will make their complete characterisation somewhat less problematic since an idea of their basic intron/exon structure has been obtained. Exons 2 and 3 of the zebrafish type IA and type IB GnRH receptor genes show a high degree of conservation when compared to the same regions of the goldfish type IA and type IB GnRH receptor cDNAs, demonstrating approximately 90% homology in both cases. In this study sequence information was obtained for the regions between transmembrane domains 4 and 7, and 3 and 7 of the zebrafish type IA and type IB GnRH receptor genes, respectively, and was subsequently used clone zebrafish GnRH receptor full-length cDNAs. This study describes the discovery of a type III GnRH receptor in the cichlid but suggests its presence may be restricted to only certain orders of teleost since a type III receptor was not identified in the zebrafish on this occasion. The information acquired from this study may help to reveal patterns, which relate the presence of particular GnRHs and GnRH receptors in single species to specific reproductive requirements.
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Bibliography: leaves 75-89.
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Reference:
Morley, M. 2003. Cloning and characterisation of gonadotropin-releasing hormone (GnRH) receptors in the cichlid (Haplochromis burtoni) and the zebrafish (Danio rerio). University of Cape Town.