Reabsorption of Sodium Chloride — Lessons from the Chloride Channels
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《新英格兰医药杂志》
Under normal conditions, most of the sodium chloride filtered by the kidney (often more than 99 percent) is reabsorbed, and one can think of the tubular functions that permit this reabsorption as involving salt (solute) and water (solvent). When salt is lost, water goes along with it, and the various salt-losing tubulopathies are often very difficult to control. These conditions can be distinguished by their symptoms and biochemical characteristics, which has been helpful in their management.
Hypokalemic salt-losing tubulopathies (called Bartter's syndromes) are a group of clinically and genetically distinct inherited renal disorders.1 In 1962, Bartter and colleagues detailed the features of two patients (5 and 25 years of age) with persistent hypokalemic alkalosis, hyposthenuria, dehydration, and failure to thrive.2 Both had hyperaldosteronism but normal blood pressure, and histologic analysis of their kidneys revealed hyperplasia and hypertrophy of the juxtaglomerular apparatus. Bartter's syndrome is now recognized as a genetically heterogeneous disorder that affects the thick ascending limb of the loop of Henle, typically presents during the neonatal period, and is associated with hypercalciuria and nephrocalcinosis. Gitelman's variant of Bartter's syndrome, although similar to the classic syndrome in many ways, affects the distal tubule of the nephron, is usually diagnosed at a later age, and is associated with hypocalciuria and hypomagnesemia accompanied by predominantly muscular signs and symptoms. In fact, given their relatively mild phenotype, one might speculate that Bartter's first patients may, in fact, have had this distal tubular disorder.
Salt is reabsorbed at several sites along the renal tubule. Sixty percent of the sodium chloride filtered by the glomerulus is reabsorbed in the proximal tubule of the nephron, largely by means of sodium–hydrogen exchange. The thick ascending limb of the loop of Henle is the site affected in various forms of Bartter's syndrome. Thirty percent of the filtered sodium chloride is reabsorbed in the thick ascending limb (see Figure) through the apically expressed sodium–potassium–chloride cotransporter NKCC2, which uses the sodium gradient across the membrane to transport chloride and potassium into the cell. The potassium ions must be recycled through the apical membrane by the potassium channel, ROMK. Sodium leaves the cell actively through the basolateral sodium–potassium ATPase. In contrast, chloride diffuses passively through two basolateral chloride channels, ClC-Ka and ClC-Kb. Both of these channels must bind to the beta subunit barttin in order to be transported to the cell surface. Normally, sodium chloride enters through the NKCC2 cotransporter and exits through the chloride channels; the tubule recycles necessary potassium ions in the tubular lumen through the ROMK channel. Abnormalities in any of these proteins of the thick ascending limb can lead to a salt-losing tubulopathy.
Figure. Schematic Representation of Transepithelial Salt Resorption in a Cell of the Thick Ascending Limb of the Loop of Henle.
Four previously defined types of Bartter's syndrome (types I, II, III, and IV) are attributable to recessive mutations in the genes that encode the NKCC2 cotransporter, the potassium channel (ROMK), one of the chloride channels (ClC-Kb), and barttin, respectively. A fifth type of Bartter's syndrome has now been shown to be a digenic disorder that is attributable to loss-of-function mutations in the genes that encode the chloride channels ClC-Ka and ClC-Kb. As a result, calcium (Ca2+), magnesium (Mg2+), potassium (K+), and ammonium (NH4+) cannot be reabsorbed in the paracellular space. The recycling of potassium maintains a lumen-positive gradient (+8 mV). Paracellin-1 is necessary for the paracellular transport of calcium and magnesium.
Until now, the term Bartter's syndrome (Online Mendelian Inheritance in Man number 241200) has referred to a group of autosomal recessive disorders caused by inactivating mutations in one of four genes that encode the membrane proteins of the thick ascending limb of the loop of Henle (see Figure). Studies involving these encoded proteins have elucidated the way in which sodium chloride enters and exits the renal tubular cell, is reabsorbed, and contributes to the hypertonicity of the renal medulla. The types of Bartter's syndrome are numbered according to the order in which their molecular defects were delineated: loss-of-function mutations in NKCC2 (the gene encoding the NKCC2 cotransporter) were discovered first (type I), and those in BSND (the gene encoding barttin, an essential subunit for ClC-Ka and ClC-Kb) were discovered last (type IV). Barttin is special in that it is expressed in the inner ear as well as in the kidney and serves, together with ClC-Ka and ClC-Kb, to maintain a high potassium concentration and a low sodium concentration in the endolymph. Thus, loss-of-function mutations in BSND are associated with deafness.3
In this issue of the Journal, Schlingmann and coauthors (pages 1314 –1319) describe a patient with severe renal salt wasting and deafness, a clinical picture that clearly suggests the presence of loss-of-function mutations in the BSND gene. However, during the evaluation of the affected child, it was found that the complete coding region of the BSND gene was normal. Instead, a large deletion in the gene encoding ClC-Kb (CLCNKB) gene and a point mutation in the gene encoding ClC-Ka (CLCNKA) were identified, and the latter was expressed in heterologous cells, confirming the effect of the missense mutation. The consequence of homozygous mutations in both CLCNKA and CLCNKB is that chloride cannot escape from the interior of the cell and cannot be reabsorbed in the inner medulla. Consequently, sodium chloride is lost into the urine, positive lumenal voltage is abolished, and calcium, magnesium, potassium, and ammonium cannot be reabsorbed in the paracellular space as they normally would be. The result is hypercalciuria. Finally, the lack of accumulation of sodium chloride in the inner medulla results in polyuria and polydipsia — though not a "pure" polyuria–polydipsia phenotype, since there is a loss not only of water, but also of sodium, potassium, chloride, and calcium.
This picture is quite different from that of the pure polyuria–polydipsia phenotype caused by mutations in the genes expressed in the collecting duct that encode the vasopressin V2 receptor (AVPR2) or the aquaporin water channel (AQP2).4 The diabetes insipidus caused by either of these mutations is an illustration of an isolated abnormality in water selectivity. Normally, the principal cells of the collecting ducts permit water without ions to permeate the cell; mutations render the principal cells impermeable. Indeed, such pure polyuria–polydipsia syndromes resulting from mutations in AVPR2 or AQP2 are somewhat easier to diagnose and treat than are tubular disorders such as Bartter's syndrome, in which both salt and water are lost.
However, both simple and complex polyuria–polydipsia syndromes that are manifested during the perinatal period present major medical and molecular challenges. Complex polyuria–polydipsia syndromes constitute an enormous perinatal conundrum, since polyhydramnios often leads to prematurity and, in the perinatal period, requirements for large amounts of water and sodium chloride. Furthermore, increased prostaglandin production, with its attendant signs, must be controlled with prostaglandin inhibitors, and the molecular challenge is substantial as well. The molecular delineation of the genetic defects that result in tubulopathies can lead to a better understanding of their physiology. However, the DNA sequencing of the genes that encode transporters and channels (as well as their subunits) is not a trivial matter and must be complemented by experiments determining expression patterns. The Xenopus oocytes that have been used for such studies are transfected cells rather than "real" polarized cells of the thick ascending limb of the loop of Henle surrounded by the sophisticated hypertonic environment of the renal medulla.
The complex polyuria–polydipsia syndrome described by Schlingmann et al. is attributable to the concomitant loss-of-function mutations in both CLCNKA and CLCNKB; the syndrome results in ion selectivity, demonstrating the means whereby a renal tubular cell lets one type of ion (chloride) through the lipid membrane to the exclusion of others. It thus provides yet another example of the molecular basis of Bartter's syndrome (see Figure).
The contributions of Roderick McKinnon and Peter Agre to solving these two complementary problems of the resorption of renal solute and renal solvent earned them the 2003 Nobel Prize in chemistry.5 We live in a fascinating time in which clinical syndromes can be deciphered at the molecular and even the atomic level.
Source Information
From the Department of Medicine and the Membrane Protein Study Group, University of Montreal (D.G.B.); and the Department of Human Genetics and Medicine, McGill University (T.M.F.) — both in Montreal.
References
Peters M, Jeck N, Reinalter S, et al. Clinical presentation of genetically defined patients with hypokalemic salt-losing tubulopathies. Am J Med 2002;112:183-190.
Bartter FC, Pronove P, Gill JR Jr, MacCardle RC. Hyperplasia of the juxtaglomerular complex with hyperaldosteronism and hypokalemic alkalosis: a new syndrome. Am J Med 1962;33:811-828.
Hubner CA, Jentsch TJ. Ion channel diseases. Hum Mol Genet 2002;11:2435-2445.
Bichet DG, Fujiwara TM. Nephrogenic diabetes insipidus. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The metabolic and molecular bases of inherited disease. 8th ed. Vol. 3. New York: McGraw-Hill, 2001:4181-204.
Clapham DE. Symmetry, selectivity, and the 2003 Nobel Prize. Cell 2003;115:641-646.(Daniel G. Bichet, M.D., a)
Hypokalemic salt-losing tubulopathies (called Bartter's syndromes) are a group of clinically and genetically distinct inherited renal disorders.1 In 1962, Bartter and colleagues detailed the features of two patients (5 and 25 years of age) with persistent hypokalemic alkalosis, hyposthenuria, dehydration, and failure to thrive.2 Both had hyperaldosteronism but normal blood pressure, and histologic analysis of their kidneys revealed hyperplasia and hypertrophy of the juxtaglomerular apparatus. Bartter's syndrome is now recognized as a genetically heterogeneous disorder that affects the thick ascending limb of the loop of Henle, typically presents during the neonatal period, and is associated with hypercalciuria and nephrocalcinosis. Gitelman's variant of Bartter's syndrome, although similar to the classic syndrome in many ways, affects the distal tubule of the nephron, is usually diagnosed at a later age, and is associated with hypocalciuria and hypomagnesemia accompanied by predominantly muscular signs and symptoms. In fact, given their relatively mild phenotype, one might speculate that Bartter's first patients may, in fact, have had this distal tubular disorder.
Salt is reabsorbed at several sites along the renal tubule. Sixty percent of the sodium chloride filtered by the glomerulus is reabsorbed in the proximal tubule of the nephron, largely by means of sodium–hydrogen exchange. The thick ascending limb of the loop of Henle is the site affected in various forms of Bartter's syndrome. Thirty percent of the filtered sodium chloride is reabsorbed in the thick ascending limb (see Figure) through the apically expressed sodium–potassium–chloride cotransporter NKCC2, which uses the sodium gradient across the membrane to transport chloride and potassium into the cell. The potassium ions must be recycled through the apical membrane by the potassium channel, ROMK. Sodium leaves the cell actively through the basolateral sodium–potassium ATPase. In contrast, chloride diffuses passively through two basolateral chloride channels, ClC-Ka and ClC-Kb. Both of these channels must bind to the beta subunit barttin in order to be transported to the cell surface. Normally, sodium chloride enters through the NKCC2 cotransporter and exits through the chloride channels; the tubule recycles necessary potassium ions in the tubular lumen through the ROMK channel. Abnormalities in any of these proteins of the thick ascending limb can lead to a salt-losing tubulopathy.
Figure. Schematic Representation of Transepithelial Salt Resorption in a Cell of the Thick Ascending Limb of the Loop of Henle.
Four previously defined types of Bartter's syndrome (types I, II, III, and IV) are attributable to recessive mutations in the genes that encode the NKCC2 cotransporter, the potassium channel (ROMK), one of the chloride channels (ClC-Kb), and barttin, respectively. A fifth type of Bartter's syndrome has now been shown to be a digenic disorder that is attributable to loss-of-function mutations in the genes that encode the chloride channels ClC-Ka and ClC-Kb. As a result, calcium (Ca2+), magnesium (Mg2+), potassium (K+), and ammonium (NH4+) cannot be reabsorbed in the paracellular space. The recycling of potassium maintains a lumen-positive gradient (+8 mV). Paracellin-1 is necessary for the paracellular transport of calcium and magnesium.
Until now, the term Bartter's syndrome (Online Mendelian Inheritance in Man number 241200) has referred to a group of autosomal recessive disorders caused by inactivating mutations in one of four genes that encode the membrane proteins of the thick ascending limb of the loop of Henle (see Figure). Studies involving these encoded proteins have elucidated the way in which sodium chloride enters and exits the renal tubular cell, is reabsorbed, and contributes to the hypertonicity of the renal medulla. The types of Bartter's syndrome are numbered according to the order in which their molecular defects were delineated: loss-of-function mutations in NKCC2 (the gene encoding the NKCC2 cotransporter) were discovered first (type I), and those in BSND (the gene encoding barttin, an essential subunit for ClC-Ka and ClC-Kb) were discovered last (type IV). Barttin is special in that it is expressed in the inner ear as well as in the kidney and serves, together with ClC-Ka and ClC-Kb, to maintain a high potassium concentration and a low sodium concentration in the endolymph. Thus, loss-of-function mutations in BSND are associated with deafness.3
In this issue of the Journal, Schlingmann and coauthors (pages 1314 –1319) describe a patient with severe renal salt wasting and deafness, a clinical picture that clearly suggests the presence of loss-of-function mutations in the BSND gene. However, during the evaluation of the affected child, it was found that the complete coding region of the BSND gene was normal. Instead, a large deletion in the gene encoding ClC-Kb (CLCNKB) gene and a point mutation in the gene encoding ClC-Ka (CLCNKA) were identified, and the latter was expressed in heterologous cells, confirming the effect of the missense mutation. The consequence of homozygous mutations in both CLCNKA and CLCNKB is that chloride cannot escape from the interior of the cell and cannot be reabsorbed in the inner medulla. Consequently, sodium chloride is lost into the urine, positive lumenal voltage is abolished, and calcium, magnesium, potassium, and ammonium cannot be reabsorbed in the paracellular space as they normally would be. The result is hypercalciuria. Finally, the lack of accumulation of sodium chloride in the inner medulla results in polyuria and polydipsia — though not a "pure" polyuria–polydipsia phenotype, since there is a loss not only of water, but also of sodium, potassium, chloride, and calcium.
This picture is quite different from that of the pure polyuria–polydipsia phenotype caused by mutations in the genes expressed in the collecting duct that encode the vasopressin V2 receptor (AVPR2) or the aquaporin water channel (AQP2).4 The diabetes insipidus caused by either of these mutations is an illustration of an isolated abnormality in water selectivity. Normally, the principal cells of the collecting ducts permit water without ions to permeate the cell; mutations render the principal cells impermeable. Indeed, such pure polyuria–polydipsia syndromes resulting from mutations in AVPR2 or AQP2 are somewhat easier to diagnose and treat than are tubular disorders such as Bartter's syndrome, in which both salt and water are lost.
However, both simple and complex polyuria–polydipsia syndromes that are manifested during the perinatal period present major medical and molecular challenges. Complex polyuria–polydipsia syndromes constitute an enormous perinatal conundrum, since polyhydramnios often leads to prematurity and, in the perinatal period, requirements for large amounts of water and sodium chloride. Furthermore, increased prostaglandin production, with its attendant signs, must be controlled with prostaglandin inhibitors, and the molecular challenge is substantial as well. The molecular delineation of the genetic defects that result in tubulopathies can lead to a better understanding of their physiology. However, the DNA sequencing of the genes that encode transporters and channels (as well as their subunits) is not a trivial matter and must be complemented by experiments determining expression patterns. The Xenopus oocytes that have been used for such studies are transfected cells rather than "real" polarized cells of the thick ascending limb of the loop of Henle surrounded by the sophisticated hypertonic environment of the renal medulla.
The complex polyuria–polydipsia syndrome described by Schlingmann et al. is attributable to the concomitant loss-of-function mutations in both CLCNKA and CLCNKB; the syndrome results in ion selectivity, demonstrating the means whereby a renal tubular cell lets one type of ion (chloride) through the lipid membrane to the exclusion of others. It thus provides yet another example of the molecular basis of Bartter's syndrome (see Figure).
The contributions of Roderick McKinnon and Peter Agre to solving these two complementary problems of the resorption of renal solute and renal solvent earned them the 2003 Nobel Prize in chemistry.5 We live in a fascinating time in which clinical syndromes can be deciphered at the molecular and even the atomic level.
Source Information
From the Department of Medicine and the Membrane Protein Study Group, University of Montreal (D.G.B.); and the Department of Human Genetics and Medicine, McGill University (T.M.F.) — both in Montreal.
References
Peters M, Jeck N, Reinalter S, et al. Clinical presentation of genetically defined patients with hypokalemic salt-losing tubulopathies. Am J Med 2002;112:183-190.
Bartter FC, Pronove P, Gill JR Jr, MacCardle RC. Hyperplasia of the juxtaglomerular complex with hyperaldosteronism and hypokalemic alkalosis: a new syndrome. Am J Med 1962;33:811-828.
Hubner CA, Jentsch TJ. Ion channel diseases. Hum Mol Genet 2002;11:2435-2445.
Bichet DG, Fujiwara TM. Nephrogenic diabetes insipidus. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The metabolic and molecular bases of inherited disease. 8th ed. Vol. 3. New York: McGraw-Hill, 2001:4181-204.
Clapham DE. Symmetry, selectivity, and the 2003 Nobel Prize. Cell 2003;115:641-646.(Daniel G. Bichet, M.D., a)