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

Elucidating Solution Structures of Cyclic Peptides Using Molecular Dynamics Simulations

Protein-protein interactions are vital to biological processes, but the shape and size of their interfaces make them hard to target using small molecules. Cyclic peptides have shown promise as protein-protein interaction modulators, as they can bind protein surfaces with high affinity and specificity. Dozens of cyclic peptides are already FDA approved, and many more are in various stages of development as immunosuppressants, antibiotics, antivirals, or anticancer drugs. However, most cyclic peptide drugs so far have been natural products or derivatives thereof, with de novo design having proven challenging. A key obstacle is structural characterization: cyclic peptides frequently adopt multiple conformations in solution, which are difficult to resolve using techniques like NMR spectroscopy. The lack of solution structural information prevents a thorough understanding of cyclic peptides' sequence-structure-function relationship. Here we review recent development and application of molecular dynamics simulations with enhanced sampling to studying the solution structures of cyclic peptides. We describe novel computational methods capable of sampling cyclic peptides' conformational space and provide examples of computational studies that relate peptides' sequence and structure to biological activity. We demonstrate that molecular dynamics simulations have grown from an explanatory technique to a full-fledged tool for systematic studies at the forefront of cyclic peptide therapeutic design.

Central diabetes insipidus in children: Diagnosis and management

Central diabetes insipidus (CDI) is a complex disorder in which large volumes of dilute urine are excreted due to arginine-vasopressin deficiency, and it is caused by a variety of conditions (genetic, congenital, inflammatory, neoplastic, traumatic) that arise mainly from the hypothalamus. The differential diagnosis between diseases presenting with polyuria and polydipsia is challenging and requires a detailed medical history, physical examination, biochemical approach, imaging studies and, in some cases, histological confirmation. Magnetic resonance imaging is the gold standard method for evaluating the sellar-suprasellar region in CDI. Pituitary stalk size at presentation is variable and can change over time, depending on the underlying condition, and other brain areas or other organs - in specific diseases - may become involved during follow up. An early diagnosis and treatment are preferable in order to avoid central nervous system damage and the risk of dissemination of germ cell tumor, or progression of Langerhans Cell Histiocytosis, and in order to start treatment of additional pituitary defects without further delay. This review focuses on current diagnostic work-up and on the role of neuroimaging in the differential diagnosis of CDI in children and adolescents. It provides an update on the best approach for diagnosis - including novel biochemical markers such as copeptin - treatment and follow up of children and adolescents with CDI; it also describes the best approach to challenging situations such as post-surgical patients, adipsic patients, patients undergoing chemotherapy and/or in critical care.

Protein Misfolding Diseases and Therapeutic Approaches

Protein folding is the process by which a polypeptide chain acquires its functional, native 3D structure. Protein misfolding, on the other hand, is a process in which protein fails to fold into its native functional conformation. This misfolding of proteins may lead to precipitation of a number of serious diseases such as Cystic Fibrosis (CF), Alzheimer's Disease (AD), Parkinson's Disease (PD), and Amyotrophic Lateral Sclerosis (ALS) etc. Protein Quality-control (PQC) systems, consisting of molecular chaperones, proteases and regulatory factors, help in protein folding and prevent its aggregation. At the same time, PQC systems also do sorting and removal of improperly folded polypeptides. Among the major types of PQC systems involved in protein homeostasis are cytosolic, Endoplasmic Reticulum (ER) and mitochondrial ones. The cytosol PQC system includes a large number of component chaperones, such as Nascent-polypeptide-associated Complex (NAC), Hsp40, Hsp70, prefoldin and T Complex Protein-1 (TCP-1) Ring Complex (TRiC). Protein misfolding diseases caused due to defective cytosolic PQC system include diseases involving keratin/collagen proteins, cardiomyopathies, phenylketonuria, PD and ALS. The components of PQC system of Endoplasmic Reticulum (ER) include Binding immunoglobulin Protein (BiP), Calnexin (CNX), Calreticulin (CRT), Glucose-regulated Protein GRP94, the thiol-disulphide oxidoreductases, Protein Disulphide Isomerase (PDI) and ERp57. ER-linked misfolding diseases include CF and Familial Neurohypophyseal Diabetes Insipidus (FNDI). The components of mitochondrial PQC system include mitochondrial chaperones such as the Hsp70, the Hsp60/Hsp10 and a set of proteases having AAA+ domains similar to the proteasome that are situated in the matrix or the inner membrane. Protein misfolding diseases caused due to defective mitochondrial PQC system include medium-chain acyl-CoA dehydrogenase (MCAD)/Short-chain Acyl-CoA Dehydrogenase (SCAD) deficiency diseases, hereditary spastic paraplegia. Among therapeutic approaches towards the treatment of various protein misfolding diseases, chaperones have been suggested as potential therapeutic molecules for target based treatment. Chaperones have been advantageous because of their efficient entry and distribution inside the cells, including specific cellular compartments, in therapeutic concentrations. Based on the chemical nature of the chaperones used for therapeutic purposes, molecular, chemical and pharmacological classes of chaperones have been discussed.

Role of protein aggregation and degradation in autosomal dominant neurohypophyseal diabetes insipidus

This review focuses on the cellular and molecular aspects underlying familial neurohypophyseal diabetes insipidus (DI), a rare disorder that is usually transmitted in an autosomal-dominant fashion. The disease, manifesting in infancy or early childhood and gradually progressing in severity, is caused by fully penetrant heterozygous mutations in the gene encoding prepro-vasopressin-neurophysin II, the precursor of the antidiuretic hormone arginine vasopressin (AVP). Post mortem studies in affected adults have shown cell degeneration in vasopressinergic hypothalamic nuclei. Studies in cells expressing pathogenic mutants and knock-in rodent models have shown that the mutant precursors are folding incompetent and fail to exit the endoplasmic reticulum (ER), as occurs normally with proteins that have entered the regulated secretory pathway. A portion of these mutants is eliminated via ER-associated degradation (ERAD) by proteasomes after retrotranslocation to the cytosol. Another portion forms large disulfide-linked fibrillar aggregates within the ER, in which wild-type precursor is trapped. Aggregation capacity is independently conferred by two domains of the prohormone, namely the AVP moiety and the C-terminal glycopeptide (copeptin). The same domains are also required for packaging into dense-core secretory granules and regulated secretion, suggesting a disturbed balance between the physiological self-aggregation at the trans-Golgi network and avoiding premature aggregate formation at the ER in the disease. The critical role of ERAD in maintaining physiological water balance has been underscored by experiments in mice expressing wild-type AVP but lacking critical components of the ERAD machinery. These animals also develop DI and show amyloid-like aggregates in the ER lumen. Thus, the capacity of the ERAD is exceeded in autosomal dominant DI, which can be viewed as a neurodegenerative disorder associated with the formation of amyloid ER aggregates. While DI symptoms develop prior to detectable cell death in transgenic DI mice, the eventual loss of vasopressinergic neurons is accompanied by autophagy, but the mechanism leading to cell degeneration in autosomal dominant neurohypophyseal DI still remains unknown.

Lessons from animal models of endocrine disorders caused by defects of protein folding in the secretory pathway

Most peptide hormones originate from secretory protein precursors synthesized within the endoplasmic reticulum (ER). In this specialized organelle, the newly-made prohormones must fold to their native state. Completion of prohormone folding usually occurs prior to migration through the secretory pathway, as unfolded/misfolded prohormones are retained by mechanisms collectively known as ER quality control. Not only do most monomeric prohormones need to fold properly, but many also dimerize or oligomerize within the ER. If oligomerization occurs before completion of monomer folding then when a poorly folded peptide prohormone is retained by quality control mechanisms, it may confer ER retention upon its oligomerization partners. Conversely, oligomerization between well-folded and improperly folded partners might help to override ER quality control, resulting in rescue of misfolded forms. Both scenarios appear to be possible in different animal models of endocrine disorders caused by genetic defects of protein folding in the secretory pathway. In this paper, we briefly review three such conditions, including familial neurohypophyseal diabetes insipidus, insulin-deficient diabetes mellitus, and hypothyroidism with defective thyroglobulin.

A novel AVP gene mutation in a Turkish family with neurohypophyseal diabetes insipidus

PURPOSE

Familial neurohypophyseal diabetes insipidus (FNDI) is a rare, autosomal dominant, inherited disorder which is characterized by severe polydipsia and polyuria generally presenting in early childhood. In the present study, we aimed to analyze the AVP gene in a Turkish family with FNDI.

METHODS

Four patients with neurohypophyseal diabetes insipidus and ten healthy members of the family were studied. Diabetes insipidus was diagnosed by the water deprivation test in affected family members. Mutation analysis was performed by sequencing the whole coding region of AVP-NPII gene using DNA isolated from peripheral blood samples.

RESULTS

Urine osmolality was low (<300 mOsm/kg) during water deprivation test, and an increase more than 50 % in urine osmolality and recovery of the symptoms were observed by the administration of desmopressin in all patients. Plasma copeptin levels were lower than expected according to plasma osmolality. Pituitary MRI revealed partial empty sella with a bright spot in index patient and a normal neurohypophysis in the other affected subjects. Genetic screening revealed a novel, heterozygous mutation designated as c.-3A>C in all patients.

CONCLUSION

c.-3A>C mutation in 5'UTR of AVP gene in this family might lead to the truncation of signal peptide, aggregation of AVP in the cytoplasm instead of targeting in the endoplasmic reticulum, thereby could disrupt AVP secretion without causing neuronal cytotoxicity, which might explain the presence of bright spot. The predicted effect of this mutation should be investigated by further in vitro molecular studies.

Disordered structural ensembles of vasopressin and oxytocin and their mutants

Vasopressin and oxytocin are intrinsically disordered cyclic nonapeptides belonging to a family of neurohypophysial hormones. Although unique in their functions, these peptides differ only by two residues and both feature a tocin ring formed by the disulfide bridge between first and sixth cysteine residues. This sequence and structural similarity are experimentally linked to oxytocin agonism at vasopressin receptors and vasopressin antagonism at oxytocin receptors. Yet single- or double-residue mutations in both peptides have been shown to have drastic impacts on their activities at either receptor, and possibly the ability to bind to their neurophysin carrier protein. In this study we perform molecular dynamics simulations of the unbound native and mutant sequences of the oxytocin and vasopressin hormones to characterize their structural ensembles. We classify the subpopulations of these structural ensembles on the basis of the distributions of radius of gyration and secondary structure and hydrogen-bonding features of the canonical tocin ring and disordered tail region. We then relate the structural changes observed in the unbound form of the different hormone sequences to experimental information about peptide receptor binding, and more indirectly, carrier protein binding affinity, receptor activity, and protease degradation. This study supports the hypothesis that the structural characteristics of the unbound form of an IDP can be used to predict structural or functional preferences of its functional bound form.

A novel variation in the AVP gene resulting in familial neurohypophyseal diabetes insipidus in a large Italian kindred

Familial neurohypophyseal diabetes insipidus (FNDI) is mostly an autosomal dominant inherited disorder presenting with severe polydipsia and polyuria typically in early childhood. To date, 69 different variations in the AVP gene encoding the AVP prohormone have been identified in autosomal dominant FNDI (adFNDI). In this study we present a family of seven generations, in which a novel variation in the AVP gene seems to cause adFNDI. Clinical assessment by 24 h urine collection, water deprivation test, desmopressin (dDAVP) challenge, and magnetic resonance imaging (MRI) of the posterior pituitary are presented. The diagnosis of adFNDI was confirmed by direct DNA sequence analysis of the AVP gene. Inheritance pattern and clinical history clearly pointed towards adFNDI. Inability of concentrating urine upon dehydration was demonstrated by a water deprivation test, and neurohypophyseal diabetes insipidus was strongly suspected after dDAVP administration, during which renal concentration ability quadrupled. MRI revealed a very weak pituitary "bright spot" in each of six subjects and a further reduction in the size of the neurohypophysis in a 7-year follow-up MRI scan in one subject. DNA sequence analysis revealed heterozygousity for a novel g.1785T > C gene variation predicting a p.Leu63Pro substitution in four affected subjects. Genetic testing in the diagnostic evaluation of families in which diabetes insipidus segregates is highly recommended in that interpretation of clinical assessments can be difficult. Furthermore, presymptomatic diagnosis can ease the parental concern of the carrier status of their offspring, and also avoid unnecessary surveillance of those being unaffected.

A missense mutation in the arginine-vasopressin neurophysin-II gene causes autosomal dominant neurohypophyseal diabetes insipidus in a Chinese family

BACKGROUND

Familial neurohypophyseal diabetes insipidus, an autosomal dominant disorder, is mostly caused by mutations in the genes that encode AVP or its intracellular binding protein, neurophysin-II. The mutations lead to aberrant preprohormone processing and progressive destruction of AVP-secreting cells, which gradually manifests a progressive polyuria and polydipsia during early childhood, and a disorder of water homeostasis.

OBJECTIVE

We characterized the clinical and biochemical features, and sequenced the AVP neurophysin-II(AVP-NPII) gene of the affected individuals with autosomal dominant neurohypophyseal diabetes insipidus(ADNDI)to determine whether this disease was genetically determined.

PATIENTS AND METHODS

We obtained the histories of eight affected and four unaffected family individuals. The diagnosis of ADNDI was established using a water deprivation test and exogenous AVP administration. For molecular analysis, genomic DNA was extracted and the AVP-NPII gene was amplified using polymerase chain reaction and sequenced.

RESULTS

The eight affected individuals showed different spectra of age of onsets (7-15 years) and urine volumes (132-253 ml/kg/24 h). All affected individuals responded to vasopressin administration, with a resolution of symptoms and an increase in urine osmolality by more than 50%. The characteristic hyperintense signal in the posterior pituitary on T1-weighted magnetic resonance imaging was absent in six family members and present in one. Sequencing analysis revealed a missense heterozygous mutation 1516G > T (Gly17Val) in exon 2 of the AVP-NPII gene among the ADNDI individuals.

CONCLUSIONS

We identified a missense mutation in the AVP-NPII gene and the same mutation showed different spectra of age of onsets and urine volumes in a new Chinese family with ADNDI. The mutation may provide a molecular basis for understanding the characteristics of NPII and add to our knowledge of the pathogenesis of ADNDI, which would allow the presymptomatic diagnosis of asymptomatic subjects.

Polyuria and polydipsia in a young child: diagnostic considerations and identification of novel mutation causing familial neurohypophyseal diabetes insipidus

A 3-year 5-month-old boy was seen for second opinion regarding polydipsia and polyuria. Previously, a diagnosis of primary polydipsia was made after normal urine concentration after overnight water deprivation testing. The boy's father, paternal grandfather, and paternal aunt had diabetes insipidus treated with desmopressin acetate. Based on this young boy's symptoms, ability to concentrate urine after informal overnight water deprivation, and family history of diabetes insipidus, we performed AVP gene mutation testing. Analysis of the AVP gene revealed a novel mutation G54E that changes a normal glycine to glutamic acid, caused by a guanine to adenine change at nucleotide g.1537 (exon 2) of the AVP gene. Commonly, patients with familial neurohypophyseal diabetes insipidus (FNHDI) present within the first 6 years of life with progressively worsening polyuria and compensatory polydipsia. Since these patients have progressive loss of arginine vasopressin (AVP), they may initially respond normally to water deprivation testing and have normal pituitary findings on brain MRI. Genetic testing may be helpful in these patients, as well as preemptively diagnosing those with a mutation, thereby avoiding unnecessary surveillance of those unaffected.

A novel deletion partly removing the AVP gene causes autosomal recessive inheritance of early-onset neurohypophyseal diabetes insipidus

Familial neurohypophyseal diabetes insipidus (FNDI) typically presents with age-dependent penetrance and autosomal dominant inheritance caused by missense variations in one allele of the AVP gene encoding the arginine vasopressin (AVP) prohormone. We present the molecular genetic characteristics underlying an unusual form of FNDI occurring with very early onset and seemingly autosomal recessive inheritance. By DNA amplification and sequencing, we identified a novel variant allele of the AVP gene carrying a 10,396 base pair deletion involving the majority of the AVP gene as well as its regulatory sequences in the intergenic region between the AVP and the OXT gene, encoding the oxytocin prohormone. We found two chromosomes carrying the deletion in affected family members and one in unaffected family members suspected to transmit the deleted allele. Whole-genome array analysis confirmed the results and excluded the presence of any additional major pathogenic abnormalities. The deletion is predicted to abolish the transcription of the AVP gene, thus the fact that family members heterozygous for the deletion remain healthy argues, in general, against haploinsufficiency as the pathogenic mechanism FNDI. Accordingly, our data is strong support to the prevailing idea that dominant inheritance of FNDI is due to a dominant-negative effect exerted by variant AVP prohormone.

Misfolding of Mutated Vasopressin Causes ER-Retention and Activation of ER-Stress Markers in Neuro-2a Cells

Arginine-vasopressin (AVP) is a peptide hormone normally secreted from neuroendocrine cells via the regulated secretory pathway. In Familial Neurohypophyseal Diabetes Insipidus (FNDI), an autosomal dominant form of central diabetes insipidus, mutations of pro-vasopressin appear to accumulate in the endoplasmic reticulum (ER) causing a lack of biologically active AVP in the blood. To investigate the effect of pro-vasopressin mutations regarding intracellular functions of protein targeting and secretion, we created two FNDI-associated amino acid substitution mutants, e.g., G14R, and G17V in frame with green fluorescent protein (GFP) and pro-vasopressin (VP) in frame with red fluorescent protein (VP-RFP). Fluorescence microscopy of Neuro-2a cells expressing these constructs revealed co-localization of VP-GFP and VP-RFP to punctate granules along the length and accumulating at the tips of neurites, characteristic of regulated secretory granules. In contrast, the two FNDI-associated amino acid substitution mutants, e.g., G14R-GFP, and G17VGFP, were localized to a perinuclear region of the Neuro-2a cells characteristic of the endoplasmic reticulum. Co-expression of these mutants with VP-RFP showed VP-RFP was retained in the ER, co-localized with the mutants suggesting the formation of heterodimers as found in FNDI. Stimulated secretion experiments indicated that VP-GFP was secreted in an inducible manner whereas, G14R-GFP and G17V-GFP were retained to nearly 100% within the cells. Analysis by western blotting and semi-quantitative RT-PCR indicated an increased protein and mRNA expression for an ER resident molecular chaperone, BiP. Further analysis of ER-storage disease-associated proteins such as caspase 12 and CHOP showed an increase in these as well. The results suggest that G14R-GFP and G17V-GFP are retained in the ER of Neuro-2a cells, resulting in up-regulation of the molecular chaperone BiP, and activation of the ER-storage disease-associated caspase cascade system.

Diverse vasopressin V2 receptor functionality underlying partial congenital nephrogenic diabetes insipidus

X-linked congenital nephrogenic diabetes insipidus (CNDI) is characterized by a defective renal response to the antidiuretic hormone (AVP) due to variations in the arginine vasopressin receptor 2 (AVPR2) gene. In a unique group of patients, the renal insensitivity to the effects of AVP is incomplete resulting in a partial phenotype. To investigate the molecular defects, two previously published variations in the AVPR2 gene, known to cause a partial CNDI phenotype, were expressed in transiently transfected human embryonic kidney cells. One variation (p.Arg104Cys) is located in the first extracellular loop and the other variation (p.Ser329Arg) is located in the intracellular COOH terminal of the receptor protein. Western blotting showed almost equal amounts of WT-V2R and Arg104Cys-V2R protein at steady state, whereas the level of Ser329Arg-V2R protein was lower. Confocal microscopy established that WT-V2R and Arg104Cys-V2R are localized on the cellular surface while the Ser329Arg-V2R primarily accumulates within the endoplasmic reticulum resulting in reduced surface expression. Ligand binding analysis demonstrated that the B(max) for cells expressing Arg104Cys-V2R and Ser329Arg-V2R were 14.8- and 2.5-fold lower than B(max) for WT-V2R, respectively. AVP affinity (1/K(d)) for WT-V2R and the Ser329Arg-V2R was similar while 1/K(d) for Arg104Cys-V2R was increased. cAMP assay revealed that cells expressing p.Arg104Cys-V2R or p.Ser329Arg-V2R produced 1.7- and 6.8-fold lower amounts of cAMP compared with WT-V2R, respectively. In conclusion, ligand binding and signal transduction capability are dependent on localization of the amino acid variation. Striking divergences at the level of receptor functionality may thus underlie similar clinical phenotypes in CNDI.

Progressive polyuria without vasopressin neuron loss in a mouse model for familial neurohypophysial diabetes insipidus

Familial neurohypophysial diabetes insipidus (FNDI), an autosomal dominant disorder, is mostly caused by mutations in the gene of neurophysin II (NPII), the carrier protein of arginine vasopressin (AVP). Previous studies suggest that loss of AVP neurons might be the cause of polyuria in FNDI. Here we analyzed knockin mice expressing mutant NPII that causes FNDI in humans. The heterozygous mice manifested progressive polyuria as do patients with FNDI. Immunohistochemical analyses revealed that inclusion bodies that were not immunostained with antibodies for mutant NPII, normal NPII, or AVP were present in the AVP cells in the supraoptic nucleus (SON), and that the size of inclusion bodies gradually increased in parallel with the increases in urine volume. Electron microscopic analyses showed that aggregates existed in the endoplasmic reticulum (ER) as well as in the nucleus of AVP neurons in 1-mo-old heterozygous mice. At 12 mo, dilated ER filled with aggregates occupied the cytoplasm of AVP cells, while few aggregates were found in the nucleus. Analyses with in situ hybridization revealed that expression of AVP mRNA was significantly decreased in the SON in the heterozygous mice compared with that in wild-type mice. Counting cells expressing AVP mRNA in the SON indicated that polyuria had progressed substantially in the absence of neuronal loss. These data suggest that cell death is not the primary cause of polyuria in FNDI, and that the aggregates accumulated in the ER might be involved in the dysfunction of AVP neurons that lead to the progressive polyuria.

Akt induces apoptosis in neuroblastoma cells expressing a C98X vasopressin mutant following autophagy suppression

Mutations in the arginine vasopressin (AVP)-neurophysin II (NP-II) gene that affect the folding and transport of the prohormone result in loss of secretion of the anti-diuretic hormone AVP from pituitary nerve terminals and cause autosomal dominant familial neurohypophyseal diabetes insipidus (adFNDI). One such mutation consists of the replacement of a Cys residue at position 98 with a stop codon (C98X) in the AVP precursor (corresponding to C67X in NP domain). In neuroblastoma cells over-expressing this truncated AVP precursor autophagy, a macromolecular degradation process, was shown to be essential for assuring cell survival. In the present study, we investigated the role of the Akt pro-survival signalling in the regulation of autophagy and of apoptosis linked with the handling of C98X AVP. Impairing autophagy-lysosomal sequestration or cathepsin D (CD)-mediated proteolysis triggered the activation of the intrinsic death pathway of apoptosis in C98X-expressing cells, but not in the wild-type -AVP-expressing cells. This was shown by the expression of a Vps34 dominant negative, which down-regulates the PI3k class III-dependent signalling needed for autophagosome (APH) formation, by genetic silencing as a result of RNA interference (RNAi) of Lamp2, a protein indispensable for the fusion of APHs with lysosomes, and by RNAi silencing of the lysosomal protease CD. Ectopic expression of either the wild-type or the mutated C98X AVP altered neither the expression nor the phosphorylation of the pro-survival signalling molecule Akt. Strikingly, the ectopic adenoviral-directed expression of a constitutively active Akt, instead of preserving cell survival, resulted in the suppression of autophagy, and precipitated Bax-mediated cell death. The present data demonstrate the need for autophagy-mediated degradation of mutated C98X peptides, which otherwise become toxic to the cell, and suggest that, in the presence of mis-folded proteins, the stimulation of the Akt signalling counteracts the beneficial effects of autophagy and precipitates cell death. It follows that growth factors impinging on the Akt pathway may have deleterious effect in neurones expressing mutant neuropeptides. This can provide an explanation for the late onset and progressive neuronal cell loss observed in hypothalamic magnocellular neurones of adFNDI patients.

Diabetes insipidus and, partially, low anxiety-related behaviour are linked to a SNP-associated vasopressin deficit in LAB mice

Following secretion from the posterior pituitary, the neuropeptide vasopressin (AVP) stimulates the kidney to retain water, and when released centrally it can contribute to anxiety- and depression-like behaviours. We hypothesized that CD1 mice bred for low trait anxiety (LAB) suffer from a deficit in AVP. Both osmotically stimulated peripheral secretion and intra-paraventricular nucleus (PVN) release of AVP were found decreased in LAB animals compared with normal anxiety (NAB) or high anxiety (HAB) controls. Consequently, in addition to their extreme non-anxiety, LAB mice showed signs of central diabetes insipidus (cDI), including increased fluid intake and reduced urine osmolality, as well as a pathological increase in plasma osmolality upon water deprivation. These cDI symptoms were attenuated by administration of a selective AVP V2 receptor agonist. A single nucleotide polymorphism (SNP) in exon 1 (C(+40)T) of the Avp gene of LAB animals causes an amino acid substitution in the signal peptide of the AVP precursor, and is likely to impair processing and trafficking of the precursor, as suggested by reduced axonal transport of AVP from the hypothalamic PVN, finally contributing to cDI symptoms and low trait anxiety. In an F2 panel, this SNP co-segregated with fluid intake and showed a partial contribution to low anxiety-related behaviour, indicated by its co-segregation with time spent on the open arms of the elevated plus-maze in a subset of F2 mice. Thus, the SNP-associated deficit in plasma and central AVP contributes to signs of cDI and, at least partially, to low trait anxiety, both features being typical of LAB animals.

The vasopressin system--from antidiuresis to psychopathology

Vasopressin is a neuropeptide with multiple functions. In addition to its predominantly antidiuretic action after peripheral secretion from the posterior pituitary, it seems to fulfill--together with its receptor subtype--all requirements for a neuropeptide system critically involved in higher brain functions, including cognitive abilities and emotionality. Following somatodendritic and axonal release in distinct brain areas, vasopressin acts as a neuromodulator and neurotransmitter in multiple and varying modes of interneuronal communication. Accordingly, changes in vasopressin expression and release patterns may have wide-spread consequences. As shown in mice, rats, voles, and humans, central vasopressin release along a continuum may be beneficial to the individual, serving to adjust physiology and behavior in stressful scenarios, possibly at the potential expense of increasing susceptibility to disease. Indeed, if over-expressed and over-released, it may contribute to hyper-anxiety and depression-like behaviors. A vasopressin deficit, in turn, may cause signs of both diabetes insipidus and total hypo-anxiety. The identification of genetic polymorphisms underlying these phenomena does not only explain individual variation in social memory and emotionality, but also help to characterize potential targets for therapeutic interventions. The capability of both responding to stressful stimuli and mediating genetic polymorphisms makes the vasopressin system a key mediator for converging (i.e., environmentally and genetically driven) behavioral regulation.

The diagnosis of children with central diabetes insipidus

Central diabetes insipidus is the end result of a number of different diseases that affect the hypothalamic-neurohypophyseal system. In many patients, especially children and young adults, it is caused by the destruction or degeneration of neurons that originate in the supraoptic and paraventricular nuclei of the hypothalamus. The known causes of these lesions include germinoma or craniopharyngioma; Langerhans cell histiocytosis; local inflammatory, autoimmune or vascular diseases; trauma resulting from surgery or an accident; sarcoidosis; metastases; and midline cerebral and cranial malformations. In rare cases, genetic defects in AVP synthesis that are inherited as autosomal dominant, autosomal recessive or X-linked recessive traits are the underlying cause. Accurate diagnostic differentiation is essential for both safe and effective disease management. Proper etiological diagnosis can be achieved via a series of steps that start with clinical observations and then progress, as needed, to more sophisticated methods. Indeed, magnetic resonance imaging (MRI) represents the examination method of choice for evaluating hypothalamic-pituitary-related endocrine diseases due to its ability to provide strongly-contrasted high-resolution multi-planar and spatial images. Specifically, MRI allows a detailed and precise anatomical study of the pituitary gland by differentiating between the anterior and posterior pituitary lobes. MRI identification of pituitary hyperintensity in the posterior part of the sella, now considered to be a clear marker of neurohypophyseal functional integrity, together with careful analysis of pituitary stalk shape and size, have provided the most striking recent findings contributing to the diagnosis and understanding of some forms of 'idiopathic' central diabetes insipidus.

Diabetes insipidus

Water balance is precisely regulated by vasopressin, thirst and the kidneys; plasma osmolality is maintained within a narrow range, despite large variations in normal water intake and loss. Disruption of these finely balanced mechanisms is common, however, and can be precipitated by various disease states. Diabetes insipidus is a disease process with several underlying pathogenic mechanisms--all ultimately resulting in excretion of large volumes of dilute urine. This review will discuss the pathophysiology, diagnosis and treatment of diabetes insipidus, with emphasis on disorders of osmoregulation following neurosurgery of the pituitary and hypothalamus.

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.

Protein Misfolding and Human Disease

Protein misfolding is a common event in living cells. In young and healthy cells, the misfolded protein load is disposed of by protein quality control (PQC) systems. In aging cells and in cells from certain individuals with genetic diseases, the load may overwhelm the PQC capacity, resulting in accumulation of misfolded proteins. Dependent on the properties of the protein and the efficiency of the PQC systems, the accumulated protein may be degraded or assembled into toxic oligomers and aggregates. To illustrate this concept, we discuss a number of very different protein misfolding diseases including phenylketonuria, Parkinson's disease, alpha-1-antitrypsin deficiency, familial neurohypophyseal diabetes insipidus, and short-chain acyl-CoA dehydrogenase deficiency. Despite the differences, an emerging paradigm suggests that the cellular effects of protein misfolding provide a common framework that may contribute to the elucidation of the cell pathology and guide intervention and treatment strategies of many genetic and age-dependent diseases.

Clinical Features, Diagnosis and Molecular Studies of Familial Central Diabetes Insipidus

BACKGROUND

Familial central diabetes insipidus (DI) is rare and is characterised by polydipsia and polyuria with a variable age of onset. The evaluation of arginine vasopressin (AVP) secretion in these individuals has been reported infrequently and only in adulthood.

OBJECTIVE

To describe the clinical features, diagnosis and molecular investigation of children affected by familial central DI.

METHODS

Functional studies of AVP secretion were undertaken in children from two kindreds with familial central DI. The AVP-neurophysin II (AVP-NPII) gene was also sequenced in symptomatic individuals.

RESULTS

In affected individuals, the result of the water deprivation test may be inconclusive. However, the hypertonic saline test identified both the severe and partial forms of AVP deficiency. A novel mutation of the AVP-NPII gene was identified by direct gene sequencing in both families.

CONCLUSIONS

This report highlights the progressive decline in AVP secretion with increasing age in this disorder and the usefulness of mutational analysis in these families. In symptomatic individuals, the hypertonic saline test may be a useful second-line investigation for functional studies of AVP secretion where molecular diagnostics are unavailable.

Expression of three different mutations in the arginine vasopressin gene suggests genotype-phenotype correlation in familial neurohypophyseal diabetes insipidus kindreds

OBJECTIVE AND STUDY DESIGN

The autosomal dominant form of familial neurohypophyseal diabetes insipidus (adFNDI) is a rare disease characterized by a severe and progressive deficiency of AVP secondary to mutations in the gene encoding the AVP precursor. Whereas a number of studies have investigated the pathogenetic mechanisms behind the disease only few studies have included detailed clinical characterization of the affected patients, thereby making genotype-phenotype correlations difficult. The aims of the present study were to investigate the cellular effects of three different adFNDI mutations (A19T, L81P and C110X) by heterologous expression in a neurogenic cell line and to correlate these findings to the corresponding clinical phenotype as determined by extensive clinical tests.

RESULTS

The clinical studies showed a later age of onset in the family carrying the A19T mutation (3.4 years, range 2-9 years) compared with families with the L81P and C110X mutations [0.75 year, range 0.5-1 year and 1.0 year (n = 1), respectively]. No other differences could be demonstrated in the clinical phenotype between families. Expression studies showed that each of the three mutant genes caused significant reduction of the amount of immunoreactive AVP in the cell culture medium and severe impairment of the intracellular trafficking and processing of the AVP prohormone, supporting the disease causing nature of all three mutations. However, the A19T mutation was associated with some capacity for processing and trafficking consistent with the clinical observations. Immunoflourescence studies provided evidence of reticular accumulation of protein within the ER in the A19T and C110X mutants but a unique accumulation of much larger aggregates in the L81P, which were localized both within and immediately outside the ER.

CONCLUSION

The study suggests a genotype-phenotype correlation with regard to age of onset of diabetes insipidus symptoms and provides support by expression studies.

Current perspective on the pathogenesis of central diabetes insipidus

Diabetes insipidus is a heterogeneous condition characterised by polyuria and polydipsia caused by a lack of secretion of vasopressin, its physiological suppression following excessive water intake, or kidney resistance to its action. The clinical and laboratory diagnosis is confirmed by standard tests, but recent advances in molecular biology and imaging techniques have shed new light on the pathophysiology of this disease. In many patients, central diabetes insipidus is caused by a germinoma or craniopharyngioma; Langerhans' cell histiocytosis and sarcoidosis of the central nervous system; local inflammatory, autoimmune or vascular diseases; trauma from surgery or accident; and, rarely, genetic defects in vasopressin biosynthesis inherited as autosomal dominant or X-linked recessive traits. Thirty to fifty percent of cases are considered idiopathic. Tumour-associated central diabetes insipidus is uncommon in children younger than 5 years old. Biopsy of enlarged pituitary stalk should be reserved for patients with hypothalamic-pituitary mass and progressive thickening of the pituitary stalk since spontaneous recovery may occur. Molecular biology in selected patients may identify those with apparently idiopathic diabetes insipidus carrying the vasopressin-neurophysin II gene mutation.