Characterization of MCH-gene-overprinted-polypeptide-immunoreactive material in hypothalamus reveals an inhibitory role of pro-somatostatin1-64 on somatostatin secretion

The melanin-concentrating hormone (MCH) gene encodes two proteins, pro-MCH and MCH-gene-overprinted polypeptide (MGOP), produced through alternative splicing of the primary transcript. Our initial purpose was to characterize the MGOP-immunoreactive material. First, MGOP mRNA was clearly found in rat and mouse hypothalami but Western blot analysis failed to unambiguously identify MGOP in protein extracts. Immunohistochemical experiments with wild-type and MCH gene-null mice demonstrated genuine expression of MGOP confined to the MCH-containing neurons in the lateral hypothalamus area and the presence of an 'MGOP-like' antigen in periventricular nucleus and arcuate nucleus neurons and their area of projection. This suggested a colocalization in somatostatin (SRIF) hypophysiotropic neurons. Further characterization, using SRIF gene-null mice and Western blot analysis with recombinant proteins, revealed that the MGOP-like product was pro-SRIF1-64. The role of pro-SRIF1-64 on fetal hypothalamic neurons was evaluated and a strong tonic inhibitory effect on SRIF secretion was found. These results (i) indicate that MGOP expression is restricted to the MCH neurons in the lateral hypothalamus and that MGOP-like immunoreactivity outside this system corresponds to pro-SRIF1-64, and (ii) provide the first evidence for a negative feedback regulation by pro-SRIF1-64 on SRIF secretion, suggesting new mechanisms by which the pro-region of a neuropeptide precursor may control the regulated secretion of a neuropeptide derived from the same precursor.