Functional GHRH receptor carboxyl terminal isoforms in normal and dwarf (dw) rats

GHRH plays a critical role in pituitary somatotroph development and function, actions which are mediated by a G-protein coupled receptor (GHRHr) that has been recently cloned. PCR amplification of rat pituitary mRNA using primers that span the GHRHr coding region resulted in two distinct products. When sequenced, the two isoforms were identical through bp 1278 of the GHRH coding region. However, the novel variant, which we have termed GHRHrbeta, contains a 131 bp deletion (1279-1408) and resumes at bp 1409 in the 3'UTR of the previously identified transcript (GHRHralpha). The identical isoforms were present in pituitaries from dwarf (dw) rats. The predicted amino acid sequence for the alternate receptor isoform differs from the published amino acid sequence at the extreme carboxyl terminus, with the last 5 amino acids of the published sequence replaced and an additional 17 amino acids added to the sequence. When translated in vitro or expressed as an epitope-tagged construct in non-GHRHr containing cell lines, the GHRHrbeta mRNA produces a 42 kDa protein product, appropriately larger than the 40 kDa product of GHRHralpha mRNA. Furthermore, GHRHrbeta retains the ability to promote cAMP generation in response to GHRH when expressed in non-GHRHr containing cell lines. These results indicate the presence of a splice variant of rat GHRHr mRNA present in normal and dw rat pituitary that codes for a functional receptor protein with an alternate carboxyl terminal domain. These findings raise the possibilities of target cell regulation of GHRH response, modulation of response through receptor isoform interactions and the involvement of multiple intracellular signaling pathways.