Chemical chaperone 4-phenylbutylate reduces mutant protein accumulation in the endoplasmic reticulum of arginine vasopressin neurons in a mouse model for familial neurohypophysial diabetes insipidus

Reticulon 4 reflects endoplasmic reticulum stress in arginine vasopressin neurons

Arginine vasopressin (AVP) is synthesized in the magnocellular supraoptic nucleus and paraventricular nuclei of the hypothalamus, where AVP neurons function under a consistently high demand for AVP production. AVP neurons are subject to endoplasmic reticulum (ER) stress even under basal conditions, and this ER stress is further exacerbated when AVP production increases due to dehydration. Reticulon (RTN) is essential for ER formation and stabilization and plays a critical role in membrane morphogenesis within the ER. This study aimed to investigate the expression of RTN family members in hypothalamic AVP neurons. Fluorescence immunohistochemistry and in-situ hybridization were employed to examine the expression of RTN family members in hypothalamic AVP neurons of adult male mice. Water deprivation and treatment with a chemical chaperone 4-phenylbutyric acid were used to increase and decrease the ER stress of AVP neurons, respectively. Among the RTN family members, only RTN4 was found to be expressed in hypothalamic AVP neurons. RTN4 was colocalized with ER organelle markers, including immunoglobulin heavy chain binding protein and calnexin. Furthermore, RTN4 expression increased during ER stress induced by water deprivation. On the other hand, increased RTN4 expression by water deprivation was attenuated by 4-phenylbutyric acid treatment. Our results suggest that RTN4 expression in AVP neurons is closely associated with ER stress caused by increased protein production in neuroendocrine cells.

Simplified drug efficacy evaluation system for vasopressin neurodegenerative disease using mouse disease-specific induced pluripotent stem cells

Familial neurohypophyseal diabetes insipidus (FNDI) is a degenerative disorder in which vasopressin-secreting neurons degenerate over time due to the production of mutant proteins. We have demonstrated therapeutic effects of chemical chaperones in an FNDI mouse model, but the complexity and length of this evaluation were problematic. In this study, we established disease-specific mouse induced pluripotent stem cells (iPSCs) from FNDI-model mice and differentiated vasopressin neurons that produced mutant proteins. Fluorescence immunostaining showed that chemical chaperones appeared to protect vasopressin neurons generated from iPSCs derived from FNDI-model mice. Although KCL stimulation released vasopressin hormone from vasopressin neurons generated from FNDI-derived iPSCs, vasopressin hormone levels did not differ significantly between baseline and chaperone-added culture. Semi-quantification of vasopressin carrier protein and mutant protein volumes in vasopressin neurons confirmed that chaperones exerted a therapeutic effect. This research provides fundamental technology for creating in vitro disease models using human iPSCs and can be applied to therapeutic evaluation of various degenerative diseases that produce abnormal proteins.

Differentiation of human induced pluripotent stem cells into hypothalamic vasopressin neurons with minimal exogenous signals and partial conversion to the naive state

Familial neurohypophyseal diabetes insipidus (FNDI) is a degenerative disease of vasopressin (AVP) neurons. Studies in mouse in vivo models indicate that accumulation of mutant AVP prehormone is associated with FNDI pathology. However, studying human FNDI pathology in vivo is technically challenging. Therefore, an in vitro human model needs to be developed. When exogenous signals are minimized in the early phase of differentiation in vitro, mouse embryonic stem cells (ESCs)/induced pluripotent stem cells (iPSCs) differentiate into AVP neurons, whereas human ESCs/iPSCs die. Human ESCs/iPSCs are generally more similar to mouse epiblast stem cells (mEpiSCs) compared to mouse ESCs. In this study, we converted human FNDI-specific iPSCs by the naive conversion kit. Although the conversion was partial, we found improved cell survival under minimal exogenous signals and differentiation into rostral hypothalamic organoids. Overall, this method provides a simple and straightforward differentiation direction, which may improve the efficiency of hypothalamic differentiation.

Deficiency of WFS1 leads to the impairment of AVP secretion under dehydration in male mice

Wolfram syndrome (WS) is mainly caused by mutations in the WFS1 gene and characterized by diabetes mellitus, optic atrophy, hearing loss, and central diabetes insipidus (CDI). WFS1 is an endoplasmic reticulum (ER)-resident transmembrane protein, and Wfs1 knockout (Wfs1-/-) mice, which have been used as a mouse model for WS, reportedly manifested impairment of glucose tolerance due to pancreatic β-cell loss. In the present study, we examined water balance, arginine vasopressin (AVP) secretion, and ER stress in AVP neurons of the hypothalamus in Wfs1-/- mice. There were no differences in urine volumes between Wfs1-/- and wild-type mice with free access to water. Conversely, when mice were subjected to intermittent water deprivation (WD) for 20 weeks, during which water was unavailable for 2 days a week, urine volumes were larger in Wfs1-/- mice, accompanied by lower urine AVP concentrations and urine osmolality, compared to wild-type mice. The mRNA expression of immunoglobulin heavy chain binding protein, a marker of ER stress, was significantly increased in the supraoptic nucleus and paraventricular nuclei in Wfs1-/- mice compared to wild-type mice after WD. Our results thus showed that Wfs1 knockout leads to a decrease in AVP secretion during dehydration, which could explain in part the mechanisms by which Wfs1 mutations cause CDI in humans.

Familial neurohypophyseal diabetes insipidus: clinical, genetic and functional studies of novel mutations in the arginine vasopressin gene

PURPOSE

Familial neurohypophyseal diabetes insipidus (FNDI) is a rare disorder characterized by childhood-onset progressive polyuria and polydipsia due to mutations in the arginine vasopressin (AVP) gene. The aim of the study was to describe the clinical and molecular characteristics of families with neurohypophyseal diabetes insipidus.

METHODS

Five Portuguese families with autosomal dominant FNDI underwent sequencing of the AVP gene and the identified mutations were functionally characterized by in vitro studies.

RESULTS

Three novel and two recurrent heterozygous mutations were identified in the AVP gene. These consisted of one initiation codon mutation in the signal peptide coding region (c.2T > C, p.Met1?), three missense mutations in the neurophysin II (NPII) coding region (c.154T > C, p.Cys52Arg; c.289C > G, p.Arg97Gly; and c.293G > C, p.Cys98Ser), and one nonsense mutation in the NPII coding region (c.343G > T, p.Glu115Ter). In vitro transfection of neuronal cells with expression vectors containing each mutation showed that the mutations resulted in intracellular retention of the vasopressin prohormone. Patients showed progressive symptoms of polyuria and polydipsia, but with wide variability in severity and age at onset. No clear genotype-phenotype correlation was observed.

CONCLUSION

The intracellular accumulation of mutant vasopressin precursors supports the role of cellular toxicity of the mutant proteins in the etiology of the disorder and explains the progressive onset of the disorder. These findings further expand the AVP mutational spectrum in FNDI and contribute to the understanding of the molecular pathogenic mechanisms involved in FNDI.

Arginine vasopressin-Venus reporter mice as a tool for studying magnocellular arginine vasopressin neurons

Arginine vasopressin (AVP) synthesized in the magnocellular neurons of the hypothalamus is transported through their axons and released from the posterior pituitary into the systemic circulation to act as an antidiuretic hormone. AVP synthesis and release are precisely regulated by changes in plasma osmolality. Magnocellular AVP neurons receive innervation from osmosensory and sodium-sensing neurons, but previous studies showed that AVP neurons per se are osmosensitive as well. In the current study, we made AVP-Venus reporter mice and showed that Venus was expressed exclusively in AVP neurons and was upregulated under water deprivation. In hypothalamic organotypic cultures from the AVP-Venus mice, Venus-labeled AVP neurons in the supraoptic and paraventricular nuclei survived for 1 month, and Venus expression was upregulated by forskolin. Furthermore, in dissociated Venus-labeled magnocellular neurons, treatment with NaCl, but not with mannitol, decreased Venus fluorescence in the soma of the AVP neurons. Thus, Venus expression in AVP-Venus transgenic mice, as well as in primary cultures, faithfully showed the properties of intrinsic AVP expression. These findings indicate that AVP-Venus mice as well as the primary hypothalamic cultures could be useful for studying magnocellular AVP neurons.

Differential diagnosis of familial diabetes insipidus

Diabetes insipidus (DI) is a syndrome characterized by the persistent excretion of abnormally large volumes of dilute urine. It can be caused by any of four fundamentally different abnormalities: deficient production of the antidiuretic hormone, arginine vasopressin (AVP) by magnocellular neurons that form the posterior pituitary (hypothalamic DI); impaired renal effects of AVP (nephrogenic DI); reduced AVP secretion due to excessive water intake (primary polydipsia); or degradation of AVP by placental vasopressinase (gestational DI). Each type of DI can be caused or potentiated by other disorders. Hypothalamic and nephrogenic DI can also be caused by mutation of the gene that encodes the AVP prohormone, the AVP-2 receptors in the kidney, or the aquaporin-2 water channels that mediate antidiuresis. Familial hypothalamic DI is usually transmitted in an autosomal dominant mode, but autosomal recessive or X-linked recessive forms also exist. Familial nephrogenic DI is usually transmitted in an X-linked recessive mode but can also be autosomal recessive or dominant. Hence the mode of inheritance does not always indicate the type of DI. Indirect methods of differential diagnosis are also unreliable and the pituitary MRI signal is diminished in both types of familial DI. Thus the determination of plasma AVP and/or the response to desmopressin therapy plus gene sequencing provides the best basis for effective management and family counseling.

Animal models for diabetes insipidus

The hormone arginine vasopressin (AVP) is a nonapeptide synthesized by hypothalamic magnocellular nuclei and secreted from the posterior pituitary into the bloodstream. It binds to AVP receptor 2 in the kidney to promote the insertion of aquaporin channels (AQP2) and antidiuretic responses. AVP secretion deficits produce central diabetes insipidus (CDI), while renal insensitivity to the antidiuretic effect of AVP causes nephrogenic diabetes insipidus (NDI). Hereditary and acquired forms of CDI and NDI generate hypotonic polyuria, polydipsia, hyperosmolality, and hypernatremia. The AVP mutant (Brattleboro) rat is the principal animal model of hereditary CDI, while neurohypophysectomy, pituitary stalk compression, hypophysectomy, and mediobasal hypothalamic lesions produce acquired CDI. In animals, hereditary NDI is mainly caused by mutations in AVP2R or AQP2 genes, while acquired NDI is most frequently induced by lithium. We report here on the determinants of the intake and excretion of water and mineral salts and on the different types of DI in humans. We then describe the hydromineral characteristics of these animal models and the responses observed after administration of hypertonic NaCl or when they are fed with low-sodium diets. Finally, we report on the effects of drugs such as AVP analogues and/or oxytocin, another neuropeptide that increases sodium excretion in animal models and humans with CDI, and sildenafil, a compound that increases the expression and function of AQP2 channels in animal models and humans with NDI.

Endoplasmic reticulum chaperone BiP/GRP78 knockdown leads to autophagy and cell death of arginine vasopressin neurons in mice

The immunoglobulin heavy chain binding protein (BiP), also referred to as 78-kDa glucose-regulated protein (GRP78), is a pivotal endoplasmic reticulum (ER) chaperone which modulates the unfolded protein response under ER stress. Our previous studies showed that BiP is expressed in arginine vasopressin (AVP) neurons under non-stress conditions and that BiP expression is upregulated in proportion to the increased AVP expression under dehydration. To clarify the role of BiP in AVP neurons, we used a viral approach in combination with shRNA interference for BiP knockdown in mouse AVP neurons. Injection of a recombinant adeno-associated virus equipped with a mouse AVP promoter and BiP shRNA cassette provided specific BiP knockdown in AVP neurons of the supraoptic (SON) and paraventricular nuclei (PVN) in mice. AVP neuron-specific BiP knockdown led to ER stress and AVP neuronal loss in the SON and PVN, resulting in increased urine volume due to lack of AVP secretion. Immunoelectron microscopy of AVP neurons revealed that autophagy was activated through the process of AVP neuronal loss, whereas no obvious features characteristic of apoptosis were observed. Pharmacological inhibition of autophagy by chloroquine exacerbated the AVP neuronal loss due to BiP knockdown, indicating a protective role of autophagy in AVP neurons under ER stress. In summary, our results demonstrate that BiP is essential for the AVP neuron system.

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.

Improved methods for the differentiation of hypothalamic vasopressin neurons using mouse induced pluripotent stem cells

High differentiation efficiency is one of the most important factors in developing an in vitro model from pluripotent stem cells. In this report, we improved the handling technique applied to mouse-induced pluripotent stem (iPS) cells, resulting in better differentiation into hypothalamic vasopressin (AVP) neurons. We modified the culture procedure to make the maintenance of iPS cells in an undifferentiated state much easier. Three-dimensional floating culture was demonstrated to be effective for mouse iPS cells. We also improved the differentiation method with regards to embryology, resulting in a greater number of bigger colonies of AVP neurons differentiating from mouse iPS cells. Fgf8, which was not used in the original differentiation method, increased iPS differentiation into AVP neurons. These refinements will be useful as a valuable tool for the modeling of degenerative disease in AVP neurons in vitro using disease-specific iPS cells in future studies.