Mutant vasopressin precursors that cause autosomal dominant neurohypophyseal diabetes insipidus retain dimerization and impair the secretion of wild-type proteins.
Autosomal dominant familial neurohypophyseal diabetes insipidus is caused by mutations in the arginine vasopressin (AVP) gene. We demonstrated recently that mutant AVP precursors accumulate within the endoplasmic reticulum of neuronal cells, leading to cellular toxicity. In this study, the possibility that mutant AVP precursors interact with wild-type (WT) proteins to alter their processing and function was explored. WT and mutant precursors were epitope-tagged to allow them to be distinguished in transfected cells. An in vivo cross-linking reaction revealed homo- and heterodimer formation between WT and mutant precursors. Mutant precursors were also shown to impair intracellular trafficking of WT precursors from the endoplasmic reticulum to the Golgi apparatus. In addition to the cytotoxicity caused by mutant AVP precursors, the interaction between the WT and mutant precursors suggests that a dominant-negative mechanism may also contribute to the pathogenesis of familial neurohypophyseal diabetes insipidus. (+info)
Blood pressure reduction and diabetes insipidus in transgenic rats deficient in brain angiotensinogen.
Angiotensin produced systemically or locally in tissues such as the brain plays an important role in the regulation of blood pressure and in the development of hypertension. We have established transgenic rats [TGR(ASrAOGEN)] expressing an antisense RNA against angiotensinogen mRNA specifically in the brain. In these animals, the brain angiotensinogen level is reduced by more than 90% and the drinking response to intracerebroventricular renin infusions is decreased markedly compared with control rats. Blood pressure of transgenic rats is lowered by 8 mmHg (1 mmHg = 133 Pa) compared with control rats. Crossbreeding of TGR(ASrAOGEN) with a hypertensive transgenic rat strain exhibiting elevated angiotensin II levels in tissues results in a marked attenuation of the hypertensive phenotype. Moreover, TGR(ASrAOGEN) exhibit a diabetes insipidus-like syndrome producing an increased amount of urine with decreased osmolarity. The observed reduction in plasma vasopressin by 35% may mediate these phenotypes of TGR(ASrAOGEN). This new animal model presenting long-term and tissue-specific down-regulation of angiotensinogen corroborates the functional significance of local angiotensin production in the brain for the central regulation of blood pressure and for the pathogenesis of hypertension. (+info)
Hypertonic saline test for the investigation of posterior pituitary function.
The hypertonic saline test is a useful technique for distinguishing partial diabetes insipidus from psychogenic polydipsia, and for the diagnosis of complex disorders of osmoreceptor and posterior pituitary function. However, there is little information concerning its use in childhood. The experience of using this test in five children (11 months to 18 years) who presented diagnostic problems is reported. In two patients, in whom water deprivation tests were equivocal or impractical, an inappropriately low antidiuretic hormone (ADH) concentration (< 1 pmol/l) was demonstrated in the presence of an adequate osmotic stimulus (plasma osmolality > 295 mosmol/kg). In two children--one presenting with adipsic hypernatraemia and the other with hyponatraemia complicating desmopressin treatment of partial diabetes insipidus--defects of osmoreceptor function were identified. Confirming a diagnosis of idiopathic syndrome of inappropriate ADH secretion (SIADH) was possible in a patient with no other evidence of pituitary dysfunction. The hypertonic saline test was well tolerated, easy to perform, and diagnostic in all cases. (+info)
Pituitary involvement by Wegener's granulomatosis: a report of two cases.
We describe two cases of pituitary involvement by Wegener's granulomatosis. At initial presentation, or during subsequent disease "flares," a pattern of pituitary abnormality was suggested. During periods of remission, we found the pituitary returned to a nearly normal appearance. Loss of the normal posterior pituitary T1 hyper-intensity matched a clinical persistence of diabetes insipidus, suggesting there is permanent damage to this structure by the initial disease process. (+info)
Autosomal recessive familial neurohypophyseal diabetes insipidus with continued secretion of mutant weakly active vasopressin.
Familial neurohypophyseal diabetes insipidus is an autosomal dominant disorder characterized by post-natal development of arginine vasopressin (AVP) deficiency due to mutations in the AVP gene. All published mutations affect the signal peptide or the neurophysin-II carrier protein and are presumed to interfere with processing of the preprohormone, leading to neuronal damage. We studied an unusual Palestinian family consisting of asymptomatic first cousin parents and three children affected with neurohypophyseal diabetes insipidus, suggesting autosomal recessive inheritance. All three affected children were homozygous and the parents heterozygous for a single novel mutation (C301->T) in exon 1, replacing Pro7 of mature AVP with Leu (Leu-AVP). Leu-AVP was a weak agonist with approximately 30-fold reduced binding to the human V2 receptor. Measured by radioimmunoassay with a synthetic Leu-AVP standard, serum Leu-AVP levels were elevated in all three children and further increased during water deprivation to as high as 30 times normal. The youngest child (2 years old) was only mildly affected but had Leu-AVP levels similar to her severely affected 8-year-old brother, suggesting that unknown mechanisms may partially compensate for a deficiency of active AVP in very young children. (+info)
Mechanism of endoplasmic reticulum retention of mutant vasopressin precursor caused by a signal peptide truncation associated with diabetes insipidus.
Autosomal dominant neurohypophyseal diabetes insipidus is caused by mutations in the gene encoding the vasopressin precursor protein, prepro-vasopressin-neurophysin II. We analyzed the molecular consequences of a mutation (DeltaG227) recently identified in a Swiss kindred that destroys the translation initiation codon. In COS-7 cells transfected with the mutant cDNA, translation was found to initiate at an alternative ATG, producing a truncated signal sequence that was functional for targeting and translocation but was not cleaved by signal peptidase. The mutant precursor was completely retained within the endoplasmic reticulum. The uncleaved signal did not affect folding of the neurophysin portion of the precursor, as determined by its protease resistance. However, formation of disulfide-linked aggregates indicated that it interfered with the formation of the disulfide bond in vasopressin, most likely by blocking its insertion into the hormone binding site of neurophysin. Preventing disulfide formation in the vasopressin nonapeptide by mutation of cysteine 6 to serine was shown to be sufficient to cause aggregation and retention. These results indicate that the DeltaG227 mutation induces translation of a truncated signal sequence that cannot be cleaved but prevents correct folding and oxidation of vasopressin, thereby causing precursor aggregation and retention in the endoplasmic reticulum. (+info)
Hypercalcemia in an euthyroid patient with secondary hypoadrenalism and diabetes insipidus due to hypothalamic tumor.
A 20-year-old Japanese man with a hypothalamic tumor (most likely germ-cell tumor) which caused secondary hypoadrenalism, hypogonadism and diabetes insipidus developed hypercalcemia and acute renal failure. The serum levels of intact PTH (iPTH), PTH-related protein (PTH-rP), 1,25-dihydroxy vitamin D (1,25- (OH)2 D), ACTH, cortisol, gonadotropins and testosterone were decreased, but his serum levels of triiodothyronine (T3) and thyroxine (T4) were within the normal range at admission, with depressed TSH and slightly increased thyroglobulin. The hypercalcemia was refractory to extensive hydration and calcitonin, but was ameliorated by pamidronate. After irradiation of the hypothalamic tumor, panhypopituitarism gradually developed. The patient has been normocalcemic for the last 2 years and is doing well under replacement therapy with glucocorticoid, L-thyroxine, methyltestosterone and 1-desamino D arginine vasopressin (dDAVP). As to the mechanism of euthyroidism at admission, transient destructive thyroiditis associated with hypopituitarism or delayed development of hypothyroidism following the hypoadrenalism was suggested. This is the first reported case of hypercalcemia in secondary hypoadrenalism due to hypothalamic tumor. Hypercalcemia was most likely induced by increased bone resorption, which was probably elicited by the combined effects of deficient glucocorticoid and sufficient thyroid hormones in addition to hypovolemia and reduced renal calcium excretion. Furthermore, severe dehydration due to diabetes insipidus and disturbance of thirst sensation caused by the hypothalamic tumor aggravated the hypercalcemia, leading to acute renal failure. (+info)
Mutations in the vasopressin prohormone involved in diabetes insipidus impair endoplasmic reticulum export but not sorting.
Familial neurohypophysial diabetes insipidus is characterized by vasopressin deficiency caused by heterozygous expression of a mutated vasopressin prohormone gene. To elucidate the mechanism of this disease, we stably expressed five vasopressin prohormones with a mutation in the neurophysin moiety (NP14G-->R, NP47E-->G, NP47DeltaE, NP57G-->S, and NP65G-->V) in the neuroendocrine cell lines Neuro-2A and PC12/PC2. Metabolic labeling demonstrated that processing and secretion of all five mutants was impaired, albeit to different extents (NP65G-->V >/= NP14G-->R > NP47DeltaE >/= NP47E-->G > NP57G-->S). Persisting endoglycosidase H sensitivity revealed these defects to be due to retention of mutant prohormone in the endoplasmic reticulum. Mutant prohormones that partially passed the endoplasmic reticulum were normally targeted to the regulated secretory pathway. Surprisingly, this also included mutants with mutations in residues involved in binding of vasopressin to neurophysin, a process implicated in targeting of the prohormone. To mimick the high expression in vasopressin-producing neurons, mutant vasopressin prohormones were transiently expressed in Neuro-2A cells. Immunofluorescence displayed formation of large accumulations of mutant prohormone in the endoplasmic reticulum, accompanied by redistribution of an endoplasmic reticulum marker. Our data suggest that prolonged perturbation of the endoplasmic reticulum eventually leads to degeneration of neurons expressing mutant vasopressin prohormones, explaining the dominant nature of the disease. (+info)