Metallothionein-vasopressin fusion gene expression in transgenic mice. Nephrogenic diabetes insipidus and brain transcripts localized to magnocellular neurons

Arginine vasopressin (AVP) is a potent neuroactive and vasoactive nonapeptide encoded in and processed from a precursor, preproarganine vasopressin-neurophysin II (preproAVP-NPII). To study the physiologic consequences of a genetic model of chronic hypervasopressinemia transgenic mice were produced by introduction of a mouse metallothionein-rat-ppAVP-NPII fusion gene into the germ line of mice. Three stable transgenic pedigrees were analyzed through several generations. Levels of immunoreactive AVP and neurophysin (NP) in sera, livers, kidneys, intestines, pancreas, and brains were markedly elevated. Chromatographic analyses showed sera levels of approximately 400 pg/ml (normal 0-20 pg/ml) of authentic AVP nonapeptide and serum osmolalities were elevated, 315.4 ± 1.4 mosm/liter (control, 307.3 ± 1.1), consistent with a state of mild nephrogenic diabetes insipidus. Brain levels of immunoreactive AVP in transgenic mice were 3-4-fold elevated 145 ± 15 ng/g versus 31 ± 7 (controls). Although immunoreactive AVP in livers and intestines, and to some extent kidneys, consisted predominantly of unprocessed precursors, in brain and pancreas >90% of AVP consisted of processed bioactive nonapeptide, as determined by chromatography and measurements of cAMP-generation in LLC-PK₁ cells. Immunocytochemistry localized immunoreactive AVP to the exocrine pancreas and to the magnacellular neurons (SON and PVN) of the hypothalamus. Expression of the fusion gene in the hypothalamus was further demonstrated by Northern analyses of fusion gene specific transcripts and in situ histohybridization. Although the fusion gene contained only 35 base pais of 5'-flanking DNA and the ppAVP-NPII gene, a tentative neuronal cell-specific expression element, ^-17GCCCAGCC^-10 resides in the sequence and may confer neuron-specific expression to the fusion gene.