The
vasopressin gene encodes three
polypeptides besides the
signal peptide:
vasopressin,
neurophysin II (
neurophysin), and the carboxy-terminal
glycopeptide (
glycopeptide). Although the function of
vasopressin is well characterized, those of the latter two are not completely understood. In the present study, we investigated the effects of various mutations within the
neurophysin/
glycopeptide portion of the
vasopressin gene on
vasopressin secretion in vitro, to clarify the role of each
peptide in
vasopressin biosynthesis. Expression vectors containing the
vasopressin gene, either wild-type or various mutants, were transiently transfected into AtT20 cells, which are known to have the
enzymes necessary for the proper processing of the
vasopressin precursor
protein. The amount of
vasopressin secreted into the culture medium was estimated by specific radioimmunoassay. Variable degrees of decreased
vasopressin secretion were observed with mutant
vasopressin genes harboring deletions or amino acid substitutions in
neurophysin. The naturally-occurring frame-shift mutation in the hereditary
diabetes insipidus (Brattleboro) rat completely eliminated
vasopressin expression. In contrast, a missense mutation found in patients with familial
neurogenic diabetes insipidus only partially decreased
vasopressin secretion. Finally, the mutant
vasopressin gene lacking the N-linked glycosylation site in
glycopeptide had no effect on
vasopressin expression. Our data suggest that 1) intact
neurophysin is not indispensable for
vasopressin expression, although an altered structure of
neurophysin significantly affects the secretion of the
hormone; 2) the pathogenesis of
diabetes insipidus with the two naturally-occurring mutations found in the rat (Brattleboro rat) and human (familial
central diabetes insipidus) seem to be different; and 3) glycosylation of the carboxy-terminal
glycopeptide is not essential for the expression of
vasopressin.