Hyperactive Vasopressin Receptors and Disturbed Water Homeostasis
http://www.100md.com
《新英格兰医药杂志》
Regulation of water homeostasis is of vital importance for all terrestrial organisms. In most mammals, including humans, the maintenance of water balance is critically dependent on water intake, the sensation of thirst, and the regulation of water excretion in the kidney, which is under the control of the antidiuretic hormone arginine vasopressin (AVP).1 In the past decade, our insight into the AVP-mediated renal concentration mechanism has substantially improved with the elucidation of the roles of crucial molecular players in this process. The identification of disease-causing genes in hereditary disorders of water balance has been extremely helpful in identifying these pivotal molecules.
In normal physiology, AVP is secreted into the circulation by the posterior pituitary gland, in response to an increase in serum osmolality or a decrease in effective circulating volume (see diagram). In the kidney, AVP binds to the V2 vasopressin reeptor in the basolateral membranes of collecting-duct cells in the last portion of the nephron (see diagram). Occupancy of this receptor results, by means of signaling through a guanine nucleotide–binding regulatory protein (G protein), in mediated activation of adenylate cylase and the formation of cyclic adenosine monophosphate (cAMP). cAMP, in turn, activates protein kinase A, which promotes the fusion of cytoplasmic vesicles containing aquaporin-2 water-channel proteins with the apical membrane. As a result, this normally water-tight membrane becomes water-permeable. Driven by the osmotic gradient of sodium, water is then transcellularly reabsorbed, entering the cells through aquaporin-2 in the apical membrane and leaving the cells for the interstitium through aquaporin-3 and aquaporin-4, which reside in the basolateral membrane. The withdrawal of AVP results in endocytosis of the water channels, restoring the water-impermeable state of the apical membrane.
Physiology of Water Homeostasis in Humans (Panel A) and Pathway of AVP Signaling in Renal Collecting-Duct Cells Involved in Regulating Water Excretion (Panel B).
The cells of the collecting duct are polarized into an apical surface that faces the lumen of the collecting duct, through which the tubular fluid flows, and a basolateral surface that faces the circulating blood. The relevant disorders of water balance include the syndrome of inappropriate antidiuretic hormone secretion (SIADH), congenital nephrogenic diabetes insipidus (CNDI), and nephrogenic syndrome of inappropriate antidiuresis (NSIAD). The aquaporin-2 water channel is regulated by vasopressin, and the aquaporin-3 and aquaporin-4 water channels are constitutively expressed. AVP denotes arginine vasopressin, and cAMP cyclic adenosine monophosphate.
Proof of principle for the critical roles of both the V2 receptor and aquaporin-2 in orchestrating the AVP-mediated renal concentration mechanism came from the identification of mutations in the genes encoding these proteins in patients with congenital nephrogenic diabetes insipidus.2 This rare genetic disorder is characterized by a failure to concentrate urine despite normal or elevated levels of AVP. Polyuria and polydipsia, usually associated with hypernatremia, are typical features of this disorder. The disease is particularly serious in infants, in whom recurrent episodes of dehydration may result in severe neurologic deficits, growth retardation, or even death. Mutations in the gene encoding the V2 receptor (AVPR2) cause the X-linked form of congenital nephrogenic diabetes insipidus, and mutations in the gene encoding aquaporin-2 are responsible for both the autosomal recessive and autosomal dominant forms.
The V2 receptor is a typical member of the large superfamily of G protein–coupled receptors, which includes receptors for hormones, neurotransmitters, light, odorants, lipids, and ions. Once an agonist binds to such a receptor, an intracellular signal cascade is induced through the coupling and activation of a G protein. G protein–coupled receptors play key roles in almost all physiological functions, and mutations in the genes encoding these receptors have been identified in patients with a variety of inherited and acquired disorders, such as retinitis pigmentosa, hypothyroidism and hyperthyroidism, dwarfism, fertility disorders, obesity, and even cancer.3 Most such mutations that are associated with disease are inactivating mutations, resulting in reduced or total lack of response to the agonist. These loss-of-function mutations are almost exclusively germ-line mutations.
Activating (gain-of-function) mutations in G protein–coupled receptors, which induce constitutive receptor activation in the absence of the agonist, occur less frequently and consist primarily of somatic mutations that have been identified in certain adenomas and malignant tumors. Such activating mutations with germ-line transmission have been identified in only a few inherited disorders (see table).
Diseases Caused by Activating (Gain-of-Function) Mutations in G Protein–Coupled Receptors.
At present, more than 180 different mutations in AVPR2 have been described. These mutations all cause congenital nephrogenic diabetes insipidus and, consistently, are inactivating mutations, resulting in receptor malfunction at different levels, such as reduced receptor expression at the cell surface or disturbances in hormone binding and G protein coupling.
In this issue of the Journal (pages 1884-1890), Feldman and coauthors describe two unrelated male infants with euvolemic hyponatremia and serum hypo-osmolality, along with inappropriately elevated urine osmolality and urine sodium concentrations. At first glance, the disorder in these boys resembles the syndrome of inappropriate antidiuretic hormone secretion (SIADH), a relatively common disorder characterized by insufficient suppression of AVP secretion in relation to the degree of hypo-osmolality, which leads to inappropriate urine concentration. Since serum AVP levels were undetectably low in both boys, Feldman et al. ruled out SIADH and hypothesized that a hyperactive V2 receptor could be the underlying cause of the disorder. Sequencing of AVPR2 in the two patients revealed a hemizygous point mutation in each. In an in vitro functional assay, both mutations were shown to lead to the production of a constitutively active V2 receptor. The consequence was AVP-independent, and therefore inappropriate, activation of V2 receptor–mediated renal urine concentration. Remarkably, and for reasons that remain unexplained, neither patient showed any clinical or biochemical sign of constitutive activation of extrarenal V2 receptors, which are known to mediate increases in circulating coagulation and fibrinolytic factors and a decrease in diastolic blood pressure after AVP stimulation.
The two identified activating AVPR2 mutations result in substitutions of the same amino acid, located in a part of the receptor protein that is highly conserved among G protein–coupled receptors and that is involved in G protein coupling. Elucidation of the exact mechanism underlying the activation of these mutant V2 receptors will undoubtedly be of great help in clarifying the relationships between the structure and function of the V2 receptor and possibly other G protein–coupled receptors; it may even guide the design of rational drugs for V2 receptor–associated disease.
This report adds a new example to the currently short list of gain-of-function mutations as causes of genetic disorders of tubular transport. The identification and functional analysis of such disease-associated gain-of-function mutations, however, have proved important to our understanding of the physiology of renal tubular transport. For instance, functional analysis of activating mutations in the renal epithelial sodium channel identified in Liddle's syndrome, an autosomal dominant form of salt-sensitive hypertension, provided further insight into normal regulation of this channel. The activating mutations were shown to disrupt the interaction of the renal epithelial sodium channel with an important regulatory protein called Nedd4; failure of this interaction prevents the removal of the renal epithelial sodium channel from the cell surface, leading to constitutive expression at the cell membrane and increased sodium reabsorption. This mechanism accounts for the development of hypertension. An increased awareness of the possibility of gain-of-function mutations, triggered by the report of Feldman et al., may well lead, in the near future, to the elucidation of other unexplained genetic disorders of tubular transport.
Source Information
Dr. Knoers is a professor of clinical genetics at the Department of Human Genetics, Radboud University Nijmegen Medical Centre, Nijmegen, the Netherlands(Nine V.A.M. Knoers, M.D.,)
In normal physiology, AVP is secreted into the circulation by the posterior pituitary gland, in response to an increase in serum osmolality or a decrease in effective circulating volume (see diagram). In the kidney, AVP binds to the V2 vasopressin reeptor in the basolateral membranes of collecting-duct cells in the last portion of the nephron (see diagram). Occupancy of this receptor results, by means of signaling through a guanine nucleotide–binding regulatory protein (G protein), in mediated activation of adenylate cylase and the formation of cyclic adenosine monophosphate (cAMP). cAMP, in turn, activates protein kinase A, which promotes the fusion of cytoplasmic vesicles containing aquaporin-2 water-channel proteins with the apical membrane. As a result, this normally water-tight membrane becomes water-permeable. Driven by the osmotic gradient of sodium, water is then transcellularly reabsorbed, entering the cells through aquaporin-2 in the apical membrane and leaving the cells for the interstitium through aquaporin-3 and aquaporin-4, which reside in the basolateral membrane. The withdrawal of AVP results in endocytosis of the water channels, restoring the water-impermeable state of the apical membrane.
Physiology of Water Homeostasis in Humans (Panel A) and Pathway of AVP Signaling in Renal Collecting-Duct Cells Involved in Regulating Water Excretion (Panel B).
The cells of the collecting duct are polarized into an apical surface that faces the lumen of the collecting duct, through which the tubular fluid flows, and a basolateral surface that faces the circulating blood. The relevant disorders of water balance include the syndrome of inappropriate antidiuretic hormone secretion (SIADH), congenital nephrogenic diabetes insipidus (CNDI), and nephrogenic syndrome of inappropriate antidiuresis (NSIAD). The aquaporin-2 water channel is regulated by vasopressin, and the aquaporin-3 and aquaporin-4 water channels are constitutively expressed. AVP denotes arginine vasopressin, and cAMP cyclic adenosine monophosphate.
Proof of principle for the critical roles of both the V2 receptor and aquaporin-2 in orchestrating the AVP-mediated renal concentration mechanism came from the identification of mutations in the genes encoding these proteins in patients with congenital nephrogenic diabetes insipidus.2 This rare genetic disorder is characterized by a failure to concentrate urine despite normal or elevated levels of AVP. Polyuria and polydipsia, usually associated with hypernatremia, are typical features of this disorder. The disease is particularly serious in infants, in whom recurrent episodes of dehydration may result in severe neurologic deficits, growth retardation, or even death. Mutations in the gene encoding the V2 receptor (AVPR2) cause the X-linked form of congenital nephrogenic diabetes insipidus, and mutations in the gene encoding aquaporin-2 are responsible for both the autosomal recessive and autosomal dominant forms.
The V2 receptor is a typical member of the large superfamily of G protein–coupled receptors, which includes receptors for hormones, neurotransmitters, light, odorants, lipids, and ions. Once an agonist binds to such a receptor, an intracellular signal cascade is induced through the coupling and activation of a G protein. G protein–coupled receptors play key roles in almost all physiological functions, and mutations in the genes encoding these receptors have been identified in patients with a variety of inherited and acquired disorders, such as retinitis pigmentosa, hypothyroidism and hyperthyroidism, dwarfism, fertility disorders, obesity, and even cancer.3 Most such mutations that are associated with disease are inactivating mutations, resulting in reduced or total lack of response to the agonist. These loss-of-function mutations are almost exclusively germ-line mutations.
Activating (gain-of-function) mutations in G protein–coupled receptors, which induce constitutive receptor activation in the absence of the agonist, occur less frequently and consist primarily of somatic mutations that have been identified in certain adenomas and malignant tumors. Such activating mutations with germ-line transmission have been identified in only a few inherited disorders (see table).
Diseases Caused by Activating (Gain-of-Function) Mutations in G Protein–Coupled Receptors.
At present, more than 180 different mutations in AVPR2 have been described. These mutations all cause congenital nephrogenic diabetes insipidus and, consistently, are inactivating mutations, resulting in receptor malfunction at different levels, such as reduced receptor expression at the cell surface or disturbances in hormone binding and G protein coupling.
In this issue of the Journal (pages 1884-1890), Feldman and coauthors describe two unrelated male infants with euvolemic hyponatremia and serum hypo-osmolality, along with inappropriately elevated urine osmolality and urine sodium concentrations. At first glance, the disorder in these boys resembles the syndrome of inappropriate antidiuretic hormone secretion (SIADH), a relatively common disorder characterized by insufficient suppression of AVP secretion in relation to the degree of hypo-osmolality, which leads to inappropriate urine concentration. Since serum AVP levels were undetectably low in both boys, Feldman et al. ruled out SIADH and hypothesized that a hyperactive V2 receptor could be the underlying cause of the disorder. Sequencing of AVPR2 in the two patients revealed a hemizygous point mutation in each. In an in vitro functional assay, both mutations were shown to lead to the production of a constitutively active V2 receptor. The consequence was AVP-independent, and therefore inappropriate, activation of V2 receptor–mediated renal urine concentration. Remarkably, and for reasons that remain unexplained, neither patient showed any clinical or biochemical sign of constitutive activation of extrarenal V2 receptors, which are known to mediate increases in circulating coagulation and fibrinolytic factors and a decrease in diastolic blood pressure after AVP stimulation.
The two identified activating AVPR2 mutations result in substitutions of the same amino acid, located in a part of the receptor protein that is highly conserved among G protein–coupled receptors and that is involved in G protein coupling. Elucidation of the exact mechanism underlying the activation of these mutant V2 receptors will undoubtedly be of great help in clarifying the relationships between the structure and function of the V2 receptor and possibly other G protein–coupled receptors; it may even guide the design of rational drugs for V2 receptor–associated disease.
This report adds a new example to the currently short list of gain-of-function mutations as causes of genetic disorders of tubular transport. The identification and functional analysis of such disease-associated gain-of-function mutations, however, have proved important to our understanding of the physiology of renal tubular transport. For instance, functional analysis of activating mutations in the renal epithelial sodium channel identified in Liddle's syndrome, an autosomal dominant form of salt-sensitive hypertension, provided further insight into normal regulation of this channel. The activating mutations were shown to disrupt the interaction of the renal epithelial sodium channel with an important regulatory protein called Nedd4; failure of this interaction prevents the removal of the renal epithelial sodium channel from the cell surface, leading to constitutive expression at the cell membrane and increased sodium reabsorption. This mechanism accounts for the development of hypertension. An increased awareness of the possibility of gain-of-function mutations, triggered by the report of Feldman et al., may well lead, in the near future, to the elucidation of other unexplained genetic disorders of tubular transport.
Source Information
Dr. Knoers is a professor of clinical genetics at the Department of Human Genetics, Radboud University Nijmegen Medical Centre, Nijmegen, the Netherlands(Nine V.A.M. Knoers, M.D.,)