Discovery of the genes in putative GnRH signaling pathway with focus on characterization of GnRH-like receptor transcripts in the brain and ovary of the oriental river prawn Macrobrachium nipponense

Vertebrate gonadotropin-releasing hormone (GnRH) plays a central role in the regulation of gonadial development by binding with its specific membrane receptor, GnRH receptor (GnRHR), which is an incipient key node in the GnRH signaling pathway. To explore the potential GnRH pathway in invertebrates, in this study, we reanalyze the transcriptomic library data of the oriental prawn Macrobrachium nipponense achieved previously in our laboratory. A total of 26 transcripts representing the genes assigned to the vertebrate GnRH signaling cascade were identified, including a transcript encoding a GnRH-like receptor (Mn-GnRHR). The Mn-GnRHR transcript contains an open reading frame (ORF) of 1083bp, encoding a protein of 360 amino acid residues. The putative protein is characterized by seven typical transmembrane (TM) domains of vertebrates GnRHR and is most closely related to the invertebrate Octopus vulgaris GnRHR as demonstrated by phylogenetic analysis. Reverse-transcription PCR revealed that the Mn-GnRHR mRNA was distributed predominantly in the brain, gonad, eyestalk, muscle and weakly or not in other tissues examined. In situ hybridization further demonstrated that the Mn-GnRHR mRNA was mainly distributed in the small- and medium-sized neurons located in the protocerebrum and deutocerebrum. In the ovary,the Mn-GnRHR mRNA was localized in the ooplasm of previtellogenic and vitellogenic oocytes and meiotically-mature oocytes as well as the follicular cells around oocytes. Quantitative real time PCR showed that the Mn-GnRHR transcript was expressed stably in ovaries at various stages. The above data provided the first evidence of the presence of GnRH-like receptors in the crustacean, which may function through GnRH signaling pathway to regulate ovarian development.