Expression of the Ghrelin Axis in the Mouse: An Exon 4-Deleted Mouse Proghrelin Variant Encodes a Novel C Terminal Peptide

Ghrelin, an n-octanoylated 28-amino-acid peptide capable of inducing GH secretion and food intake in humans and rats, is the endogenous ligand for the GH secretagogue receptor (GHS-R). Here we describe the expression and tissue distribution of the ghrelin/GHS-R axis in the mouse. We also report for the first time the identification of a novel mouse ghrelin mRNA variant in which there is a complete deletion of exon 4. Translation of this variant mRNA yields a protein containing ghrelin and an alternative C-terminal domain with a unique C-terminal peptide sequence. RT-PCR with primers specific for mouse ghrelin was used to demonstrate the mRNA expression of the full preproghrelin transcript and the exon 4-deleted variant in multiple mouse tissues. Real-time PCR was also employed to quantitate mRNA expression of ghrelin, the novel isoform and a previously reported ghrelin gene variant, ghrelin gene-derived transcript. We also demonstrated the tissue expression of the functional GHS-R in the mouse. Immunohistochemistry, employing antibodies raised against the mature human n-octanoylated ghrelin peptide and the putative C-terminal peptide encoded by the exon 4-deleted proghrelin variant, was used to demonstrate protein expression of ghrelin and the variant in multiple mouse tissues including stomach, kidney, and reproductive tissues. The coexpression of ghrelin and its receptor in a wide range of murine tissues suggests varied autocrine/paracrine roles for these peptides. Exon 4-deleted proghrelin, a novel mouse proghrelin isoform with a unique C-terminal peptide sequence, is also widely expressed in the mouse and thus may possess biological activity in these tissues.