Degradation of vasopressin precursor and pathogenic mutants in diabetes insipidus

The nonapeptide hormone, arginine vasopressin, plays a decisive role in the regulation of fluid balance by reducing free water clearance through reabsorption of water in the renal collecting ducts. Mutations in the gene encoding arginine vasopressin cause autosomal dominant neurohypophyseal diabetes insipidus, a disease characterized by excessive urine production and strong thirst. Post mortem examination of affected individuals suggests a selective degeneration of vasopressinergic neurons in the hypothalamus. On a molecular level, the disease is linked to a trafficking defect. Mutant vasopressin precursor is retained in the endoplasmic reticulum, while the wild-type is transported to mature secretory granules at synaptic processes. How this trafficking defect of the vasopressin precursor is interrelated with the degeneration of neurons is unknown. A plausible hypothesis is that mutant proteins, or degradation products thereof, are toxic to neurons. Accordingly, we analyzed the fate of mutant vasopressin precursor arrested in the endoplasmic reticulum of transfected cell lines. Proteasomal, but not lysosomal, inhibitors induced stabilization of mutant precursors and the accumulation of three distinct nonglycosylated cytosolic species: pre-pro-vasopressin, pro-vasopressin, and an N-terminally truncated form. These results provide evidence that mutant precursor, after translocation into the ER lumen, is retrotranslocated to the cytosol and degraded by the proteasome. Furthermore, a fraction of the newly synthesized precursor, even of wild-type, was found not to be translocated, but to be synthesized into the cytosol due to inefficiency of the vasopressin signal peptide. In autosomal dominant neurohypophyseal diabetes insipidus, neurotoxicity may thus result from degradation intermediates and/or by ER retention directly. Both mistargeted and retrotranslocated proteins add to the cytosolic pool of these degradation products. Neurodegeneration might occur in heterozygous individuals once a critical concentration of toxic material is exceeded.