Fibrillar aggregations of pathogenic pro-vasopressin mutants

Diabetes insipidus is a disregulation of water homeostasis characterized by large fluid turnover in the kidney. Water homeostasis is regulated by the hormone vasopressin by increasing reapsorption of water in the renal collecting duct. Autosomal dominant diabetes insipidus is caused by mutations in the precursor protein of vasopressin. More than 50 different such mutations are known. Mutant prohormones are retained within the cell and cause degeneration of vasopressinergic neurons in the hypothalamus by an unknown mechanism.
In this work we identified three novel dominant mutations of the pro-vasopressin gene. We further show that different mutations in the precursor lead to the formation of disulfide-linked homo-oligomers and large aggregates with fibrillar morphology in the endoplasmic reticulum of transfected cultured cells, both fibroblasts and neuronal. In addition, bacterially expressed and purified pro-vasopressin spontaneously formed fibrils in vitro under oxidizing conditions. Further mutagenesis experiments showed that the presence of cysteines, but no specific single cysteine, is essential for disulfide oligomerization and aggregation in vivo. Our findings place autosomal dominant neurohypophyseal diabetes insipidus in the class of neurodegenerative diseases associated with amyloid-like protein aggregations.
To identify the sequence responsible for aggregation, segments comprising ten amino acids throughout the precursor were replaced by a sequence composed almost entirely of prolines and glycines. Prolines and glycines have been shown to disrupt the formation of amyloids. Formation of large aggregates required the intact N-terminal ten amino acids. Disulfide-linked oligomers, however, were not abolished. These results indicate that the vasopressin moiety is essential for precursor aggregation and that disulfide-linked oligomers not necessarily lead to the formation of large aggregates.