Structural basis of neurophysin hormone specificity: Geometry, polarity, and polarizability in aromatic ring interactions

Familial neurohypophyseal diabetes insipidus--an update

Although molecular research has contributed significantly to our knowledge of familial neurohypophyseal diabetes insipidus (FNDI) for more than a decade, the genetic background and the pathogenesis still is not understood fully. Here we provide a review of the genetic basis of FNDI, present recent progress in the understanding of the molecular mechanisms underlying its development, and survey diagnostic and treatment aspects. FNDI is, in 87 of 89 kindreds known, caused by mutations in the arginine vasopressin (AVP) gene, the pattern of which seems to be largely revealed as only few novel mutations have been identified in recent years. The mutation pattern, together with evidence from clinical, cellular, and animal studies, points toward a pathogenic cascade of events, initiated by protein misfolding, involving intracellular protein accumulation, and ending with degeneration of the AVP producing magnocellular neurons. Molecular research has also provided an important tool in the occasionally difficult differential diagnosis of DI and the opportunity to perform presymptomatic diagnosis. Although FNDI is treated readily with exogenous administration of deamino-D-arginine vasopressin (dDAVP), other treatment options such as gene therapy and enhancement of the endoplasmic reticulum protein quality control could become future treatment modalities.

A novel heterozygous missense mutation in the vasopressin moiety is identified in a Japanese person with neurohypophyseal diabetes insipidus

The autosomal dominant familial neurohypophyseal diabetes insipidus (adFNDI) is caused by diverse mutations in one allele of the gene that encodes the arginine vasopressin (AVP) precursor protein, AVP-neurophysin II (AVP-NP II). Most of the mutations identified so far are located in either the signal peptide or NP II moiety. Two recently published mutations in the AVP gene identified in kindreds with adFNDI predict a substitution of histidine for tyrosine at position 2 and a deletion of phenylalanine at position 3 in AVP. They are unique among adFNDI mutations in that they are the only adFNDI mutations that affect amino acid residues in the AVP moiety of the pro-hormone. Here, we report a novel heterozygous missense mutation in the AVP moiety of the AVP-NP II gene in a Japanese person with neurohypophyseal diabetes insipidus (DI). This mutation occurs at position 2 in AVP and predicts a substitution of serine for tyrosine (Y21S). It is expected to interfere with normal binding of AVP with NP II, and thus result in misfolding of the precursor proteins. The data of this study support the notion that mutations affecting the AVP moiety can result in the initiation of the pathological processes.

Differential cellular handling of defective arginine vasopressin (AVP) prohormones in cells expressing mutations of the AVP gene associated with autosomal dominant and recessive familial neurohypophyseal diabetes insipidus

An unusual mutation in the arginine vasopressin (AVP) gene, predicting a P26L amino acid substitution of the AVP prohormone, is associated with autosomal recessive familial neurohypophyseal diabetes insipidus (FNDI). To investigate whether the cellular handling of the P26L prohormone differed from that of the Y21H prohormone associated with autosomal dominant inheritance of FNDI, the mutations were examined by heterologous expression in cell lines. Immunoprecipitation demonstrated retarded processing and secretion of the Y21H prohormone, whereas the secretion of the P26L prohormone seemed to be unaffected. Confocal laser scanning microscopy showed accumulation of the Y21H prohormone in the endoplasmic reticulum, whereas the P26L prohormone and/or processed products were localized in secretory granules in the cellular processes. RIA analysis showed reduced amounts of immunoreactive Y21H-AVP and P26L-AVP in the cell culture medium. Thus, the recessive mutation does not seem to affect the intracellular trafficking but rather the final processing of the prohormone. Our results provide an important negative control in support of the hypothesis that autosomal dominant inheritance of FNDI is caused by mutations in the AVP gene that alter amino acid residues important for folding and/or dimerization of the neurophysin II moiety of the AVP prohormone and subsequent transport from the endoplasmic reticulum.

Impaired trafficking of mutated AVP prohormone in cells expressing rare disease genes causing autosomal dominant familial neurohypophyseal diabetes insipidus

OBJECTIVE AND STUDY DESIGN

Two different mutations in the arginine vasopressin (AVP) gene associated with autosomal dominant familial neurohypophyseal diabetes insipidus (adFNDI) predict Y21H (AVP2) and V67A (NP36) amino acid substitutions of the AVP prohormone. They are unique in that they change, respectively, the AVP moiety and a region of the neurophysin II domain not so far affected by any mutations. To test whether they affect the cellular handling of the AVP prohormone in a similar manner to previously investigated mutations, they were examined by heterologous expression in cell lines.

RESULTS

Both mutations resulted in significantly reduced amounts of immunoreactive AVP in the cell culture medium as determined by radioimmunoassay analysis. Metabolic labelling combined with immunoprecipitation demonstrated that processing and secretion of the mutant prohormones was reduced but not prevented. Finally, confocal laser scanning microscopy showed that normal AVP prohormone and/or its processed products were localized in the tips of the cellular processes, whereas both mutant prohormones were accumulated in the endoplasmic reticulum (ER) and in the case of the V67A prohormone, also in perinuclear structures outside the ER.

CONCLUSION

Both mutations result in reduced AVP prohormone processing and secretion probably due to retention in the ER. This supports, at least partly, the hypothesis that the mutations lead to the production of a mutant hormone precursor that fails to fold and/or dimerize properly and, as a consequence, is retained by the ER protein quality control machinery. Perinuclear accumulation of the V67A prohormone outside the ER indicates that additional mechanisms could be involved.

Autosomal dominant neurohypophyseal diabetes insipidus due to substitution of histidine for tyrosine(2) in the vasopressin moiety of the hormone precursor

The autosomal dominant form of familial neurohypophyseal diabetes insipidus (adFNDI) has been linked to 40 different mutations of the gene encoding the vasopressin-neurophysin II (AVP-NPII) precursor. All of these mutations have been located in either the signal peptide or neurophysin II moiety. We now report a three-generation Turkish kindred in which severe adFNDI cosegregates with a novel missense mutation in the part of the AVP-NPII gene encoding the AVP moiety. This mutation (T-->C at position 285 in the genomic sequence) was found in only one allele and predicts a substitution of histidine for tyrosine at position 2 in AVP. Like other adFNDI mutations, this substitution is expected to impair folding and processing of the precursor, in this case by interfering with normal binding of the AVP and NPII moieties. It is associated clinically with inability to concentrate urine during fluid deprivation, a greater than 80% deficiency of AVP secretion, and absence of the posterior pituitary bright spot on magnetic resonance imaging. These findings are consistent with the hypothesis that mutations in the AVP-NPII gene cause adFNDI by directing the production of a folding incompetent precursor that prevents the expression of the normal allele via a cytotoxic effect on the magnocellular neurons.

Structures of an unliganded neurophysin and its vasopressin complex: implications for binding and allosteric mechanisms

The structures of des 1-6 bovine neurophysin-II in the unliganded state and as its complex with lysine vasopressin were determined crystallographically at resolutions of 2.4 A and 2.3 A, respectively. The structure of the protein component of the vasopressin complex was, with some local differences, similar to that determined earlier of the full-length protein complexed with oxytocin, but relatively large differences, probably intrinsic to the hormones, were observed between the structures of bound oxytocin and bound vasopressin at Gln 4. The structure of the unliganded protein is the first structure of an unliganded neurophysin. Comparison with the liganded state indicated significant binding-induced conformational changes that were the largest in the loop region comprising residues 50-58 and in the 7-10 region. A subtle binding-induced tightening of the subunit interface of the dimer also was shown, consistent with a role for interface changes in neurophysin allosteric mechanism, but one that is probably not predominant. Interface changes are suggested to be communicated from the binding site through the strands of beta-sheet that connect these two regions, in part with mediation by Gly 23. Comparison of unliganded and liganded states additionally reveals that the binding site for the hormone alpha-amino group is largely preformed and accessible in the unliganded state, suggesting that it represents the initial site of hormone protein recognition. The potential molecular basis for its thermodynamic contribution to binding is discussed.