Mutations in the vasopressin prohormone involved in diabetes insipidus impair endoplasmic reticulum export but not sorting

Familial neurohypophysial diabetes insipidus is characterized by vasopressi n deficiency caused by heterozygous expression of a mutated vasopressin pro hormone gene. To elucidate the mechanism of this disease, we stably express ed five vasopressin prohormones with a mutation in the neurophysin moiety ( NP14G-->R, NP47E-->G, NP47 Delta E, NP57G-->S, and NP65G-->V) in the neuroe ndocrine cell lines Neuro-2A and PC12/PC2, Metabolic labeling demonstrated that processing and secretion of all five mutants was impaired, albeit to d ifferent extents (NP65G-->V greater than or equal to NP14G-->R > NP47 Delta E greater than or equal to NP47E-->G > NP57G-->S), Persisting endoglycosid ase H sensitivity revealed these defects to be due to retention of mutant p rohormone in the endoplasmic reticulum. Mutant prohormones that partially p assed the endoplasmic reticulum were normally targeted to the regulated sec retory pathway. Surprisingly, this also included mutants with mutations in residues involved in binding of vasopressin to neurophysin, a process impli cated in targeting of the prohormone. To mimick the high expression in vaso pressin-producing neurons, mutant vasopressin prohormones were transiently expressed in Neuro-2A cells. Immunofluorescence displayed formation of larg e accumulations of mutant prohormone in the endoplasmic reticulum, accompan ied by redistribution of an endoplasmic reticulum marker. Our data suggest that prolonged perturbation of the endoplasmic reticulum eventually leads t o degeneration of neurons expressing mutant vasopressin prohormones, explai ning the dominant nature of the disease.