- Email: [email protected]
The Roles of the Skeleton and Phosphorus in the CKD Mineral Bone Disorder
The Roles of the Skeleton and Phosphorus in the CKD Mineral Bone Disorder Keith A. Hruska and Suresh Mathew The CKD mineral bone disorder is a new term coined to describe the multiorgan system failure that is a major component of the excess cardiovascular mortality and morbidity complicating decreased kidney function. This syndrome embodies new discoveries of organ-to-organ communication including the skeletal hormone fibroblast growth factor-23 (FGF-23), which signals the status of skeletal mineral deposition to the kidney. The CKD mineral bone disorder begins with mild decreases in kidney function (stage 2 CKD) affecting the skeleton, as marked by increased FGF-23 secretion. At this stage, the stimulation of cardiovascular risk has begun and the increases in FGF-23 levels are strongly predictive of cardiovascular events. Later in CKD, hyperphosphatemia ensues when FGF-23 and hyperparathyroidism are no longer sufficient to maintain phosphate excretion. Hyperphosphatemia has been shown to be a direct stimulus to several cell types including vascular smooth muscle cells migrating to the neointima of atherosclerotic plaques. Phosphorus stimulates FGF-23 secretion by osteocytes and expression of the osteoblastic transcriptome, thereby increasing extracellular matrix mineralization in atherosclerotic plaques, hypertrophic cartilage, and skeletal osteoblast surfaces. In CKD, the skeleton positively contributes to hyperphosphatemia through excess bone resorption and inhibition of matrix mineralization. Thus, through the action of phosphorus, FGF-23, and other newly discovered skeletal hormones, such as osteocalcin, the skeleton plays an important role in the occurrence of cardiovascular morbidity in CKD. Q 2011 by the National Kidney Foundation, Inc. All rights reserved. Key Words: Mineral bone disorder, CKD
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he CKD mineral bone disorder (CKD-MBD) is a term coined by the Kidney Disease Improving Global Outcomes Foundation1 to replace the term renal osteodystrophy (ROD), used for recognition of several pathophysiological discoveries of the 21st century. The first of these pathophysiological discoveries is that the skeletal remodeling disorders caused by CKD contribute directly to the heterotopic mineralization, especially vascular calcification, and the disordered mineral metabolism that accompany CKD.2,3 Second, the disorders in mineral metabolism associated with CKD are key factors contributing to the excess mortality observed in CKD.4,5 Third, CKD or renal injury impairs skeletal anabolism, thereby decreasing osteoblast function and bone formation rates.3,6 In short, a multiorgan system that fails in CKD has been defined involving the kidney, skeleton, parathyroid glands, and the cardiovasculature (Fig 1).
pathology has been detected in early CKD, and the earliest histologic bone abnormalities in CKD-MBD are seen after a relatively mild reduction in the glomerular filtration rate (creatinine clearance rates of between 40 and 70 mL/min, stage 2 CKD).7 In addition, elevated parathyroid hormone (PTH) levels may be observed before any detectable changes in the serum phosphorus, calcitriol, or calcium.8 If hyperparathyroidism is prevented or treated, a low turnover osteodystrophy, the adynamic bone disorder, is observed, uncovering the effects of kidney injury on the skeleton. By stage 5 CKD (creatinine clearance, ,15 mL/min), skeletal histologic pathology is found in virtually all patients.9 The increasing incidence of CKD in the United States and across the world and the role of CKD-MBD in its high mortality make ROD a major health issue for Americans and all developed societies.10-12
Pathobiology The initial skeletal abnormalities of CKD-MBD are marked by the stimulation of fibroblast growth factor23 (FGF-23) secretion by osteocytes, which signal that the skeleton has been affected by kidney disease. Skeletal
From Division of Pediatric Nephrology, Department of Pediatrics, Washington University, St. Louis, MO. Address correspondence to Keith A. Hruska, MD, Division of Pediatric Nephrology, Department of Pediatrics, Washington University, Campus Box 8208, 5th Floor MPRB, 660 South Euclid Avenue, St. Louis, MO 63110. E-mail: [email protected] Ó 2011 by the National Kidney Foundation, Inc. All rights reserved. 1548-5595/$36.00 doi:10.1053/j.ackd.2011.01.001
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Pathogenesis Renal injuries give rise to a loss of skeletal anabolism manifested as a decrease in bone formation rates that derive from osteoblast activity.6 The increase in FGF-23 levels is the first indication that the skeleton has been affected by CKD. The loss of anabolism owing to kidney injury occurs in the presence of normal PTH, vitamin D, calcium, and phosphorus levels, but it is not usually observed because abnormalities in these factors stimulate an adaptation, that is, PTH secretion and secondary hyperparathyroidism. The sustained increase in PTH levels produced through adaptation to CKD results in an unwanted disorder of skeletal remodeling, a high turnover ROD or osteitis fibrosa.
Advances in Chronic Kidney Disease, Vol 18, No 2 (March), 2011: pp 98-104
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Figure 1. Failure of a multiorgan system in CKD. The discoveries that disorders of mineral metabolism are causally linked to mortality, that hyperphosphatemia causes vascular calcification, and that kidney failure directly impairs skeletal anabolism establishes a multiorgan system which fails in CKD. The system consists of the kidney, the skeleton, the intestine, the vasculature, and the heart. The system is established by direct connections between each organ, which have not been completely defined in the case of kidney–bone and kidney–heart. However, the contribution of the skeleton and the kidney to hyperphosphatemia, the role of hyperphosphatemia in causing vascular calcification, and the effects of vascular calcification on the heart have been established. The kidney–intestine connection is represented by calcitriol and the intestine–bone connection by serotonin and an intestinal phosphatonin under study.
Pathogenetic Factors in CKD-MBD Fibroblast Growth Factor-23 FGF-23 is the original phosphatonin (phosphate excretion regulating hormone) discovered in studies of autosomal dominant hypophosphatemic rickets and oncogenic osteomalacia.13,14 FGF-23 levels progressively rise during the course of CKD15,16 in conjunction with PTH, serving to maintain phosphate homeostasis in the early stages of renal disease. FGF-23 is produced mainly by osteocytes and to some extent by osteoblasts, and it is a mineralization inhibitor. It is unclear whether FGF-23 contributes to the mineralization defects observed in skeletal remodeling in CKD.17 FGF-23 represents a direct bone–kidney connection in the multiorgan system involved in CKDMBD, but it is unclear whether it has direct actions to stimulate cardiovascular events of which its high levels are very predictive.18 FGF-23 functions through signal transduction mediated by the FGF receptors (FGFR), mainly FGFR1c, and FGF-23 stimulation of cell signals through the FGFR requires the co-receptor Klotho.19 This is demonstrated by the identical phenotype displayed by mice deficient in FGF-23 or in Klotho. The phenotype that includes hyperphosphatemia and vascular calcification can be rescued by a low phosphate diet or by breeding in the conditions involving double deficiency of the vitamin D receptor (VDR) and FGF-23/Klotho. Thus, the distribution of Klotho is thought to delineate the target tissues of FGF-23, which are the kidney distal tubule and the parathyroid gland. What remains unclear is how FGF-23 sends signals to the proximal tubule, which has a low expression rate of Klotho, and the cardiovasculature and skeleton, which do not express Klotho.
The Klotho gene encodes a type-1 membrane protein that is related to b-glucuronidases.20 Genetic variants in KLOTHO are associated with human aging,21 and Klotho is a circulating protein that declines with age and CKD. Reduced production of Klotho in CKD may be one of the factors underlying the degenerative processes of arteriosclerosis, osteoporosis, and skin atrophy seen in CKD. In addition, the parathyroid hyperplasia that accompanies CKD results in decreased Klotho expression in the parathyroid glands. This produces FGF-23 resistance and impaired parathyroid suppression by FGF-23. Klotho deficiency as discussed in the preceding section produces premature aging, hyperphosphatemia, and vascular calcification, which are relieved by lowering calcitriol activity.22-24 Phosphorus Because renal injury decreases nephron number, phosphate retention results from a reduction in filtered phosphate and is reversed through FGF-23- and PTH-mediated reductions in tubular epithelial phosphate transport. Phosphorus is a direct stimulant to the osteocytes for regulation of FGF-23 secretion. The kinetics of the osteocyte phosphate sensor are unknown, and thus, it is unclear whether subdetectable changes in the serum phosphorus are controlling the early increases in FGF-23 in kidney diseases or whether changes in bone formation are the initial stimulus for increased FGF-23 secretion. The increase in phosphate excretion per remaining nephron restores phosphate homeostasis at the cost of higher FGF-23 and PTH levels, and maintains normal phosphate excretion. In stage 4 and 5 CKD, when renal injury is severe enough that the glomerular filtration rate reaches levels of ,30% of normal, hyperphosphatemia becomes fixed owing to insufficient renal excretion despite high PTH and FGF-23 levels.25 Studies demonstrate that failure of calcium and phosphorous deposition into the skeleton or excess resorption of the skeleton also contribute to abnormal calcium and phosphorus levels in CKD and end-stage kidney disease (ESKD).3,26 Hyperphosphatemia decreases serum calcium through physicochemical binding and suppresses 1a-hydroxylase activity, which results in further lowering of circulating calcitriol levels. Moreover, a direct stimulatory effect of phosphorus on parathyroid gland cells, independent of calcium and calcitriol, results in increased secretion and nodular hyperplasia of parathyroid gland cells.27,28 Finally, hyperphosphatemia is a signaling mechanism for induction of heterotopic mineralization of the vasculature in CKD and ESRD.29,30 We have shown that phosphorus directly stimulates matrix mineralization by vascular smooth muscle cells isolated from atherosclerotic human blood vessels through stimulation of osteoblastic gene expression. In the presence of high levels of phosphorus, mineralization is blocked by suppression of an osteoblastic transcription factor, osterix.31
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Hyperphosphatemia Serum inorganic phosphorus (Pi) concentrations are generally maintained at 2.5 to 4.5 mg/dL or 0.75 to 1.45 mM in adults, whereas the normal range in children is between 3.5 and 7 mg/dL. Hyperphosphatemia may be the consequence of an increased intake of Pi, a decreased excretion of Pi, or translocation of Pi from tissue breakdown into the extracellular fluid.32 Because the kidneys are able to excrete phosphate very efficiently over a wide range of dietary intake, hyperphosphatemia most frequently results from renal insufficiency and the attendant inability to excrete Pi. However, in metabolic bone disorders, such as osteoporosis and ROD, the skeleton is a poorly recognized contributor to the serum phosphorus. The bulk of total body phosphate (85%) is present in the bone as part of the mineralized extracellular matrix. This phosphate pool is accessible, but in a limited manner, through bone resorption. Other than bone storage, phosphate is a predominantly intracellular anion with an estimated concentration of approximately 100 mmol/L, most of which is either complexed or bound to proteins or lipids. Serum phosphorus concentration varies with age, time of day, fasting state, and season. It is higher in children than in adults. Phosphorus levels exhibit a diurnal variation, with the lowest phosphate level occurring near noon. Serum phosphorus concentration is regulated by diet, hormones, and physical factors such as pH. Importantly, because phosphate moves in and out of cells under several influences, the serum concentration of phosphorus poorly reflects phosphate stores. Clinical and translational studies demonstrate that excess bone resorption contributes to the level of serum phosphorus that is poorly appreciated. Even in clinical situations where bone formation is decreased, such as adynamic ROD and osteoporosis, a variability in the serum phosphorus is produced when bone formation is stimulated (Fig 2).3,6 For instance, in low turnover osteodystrophy treated with a skeletal anabolic agent, when phosphorus intake is constant, the serum phosphorus falls despite no change or decrease in phosphate excretion (Fig 2B). The most important short-term consequences of hyperphosphatemia are hypocalcemia and tetany, which occur most commonly in patients with an increased Pi load from any source, exogenous or endogenous. By contrast, soft-tissue calcification and secondary hyperparathyroidism are long-term consequences of hyperphosphatemia that occur mainly in patients with renal insufficiency and decreased renal Pi excretion. Phosphorus directly affects the activity of both boneforming (osteoblasts)33 and bone-resorbing (osteoclasts)34 cells. The actions of phosphorus on osteoblasts are similar to those in vascular smooth muscle cells undergoing Pi-stimulated osteoblastic transition and have been dis-
cussed later in the text. The actions of Pi to stimulate Pi uptake in osteoclasts are a necessary component of adenosine triphosphate production and the high rates of energy utilization by these cells.34 Thus, Pi is supportive of bone resorption and the uncoupling of formation and resorption that occurs in CKD.
Heterotopic Mineralization Extraskeletal calcification associated with hyperphosphatemia is usually seen in patients with chronic kidney failure, diabetes, severe atherosclerosis, or aging. Recent basic, translational, and clinical research studies have discovered the pathogenesis and the consequences of this phenomenon.31,35-37 Several inhibitors of vascular calcification have been discovered including osteoprotegerin,38 osteopontin,39,40 matrix Gla protein,41 the Klotho gene product,42 and Smad 643 through phenotyping transgenic knockout mice. These substances constitute an inherent defense against heterotopic mineralization, which is breached in the disease environment. In the setting of CKD, hyperphosphatemia has been identified as a major factor contributing to the forces favoring mineralization.44 In contrast to the breach of defense theory of vascular calcification, there is significant evidence that vascular cells undergo osteoblastic transition, including expression of the osteoblast transcription factors RUNX2/Cbfal, osterix, MSX2, and DlX5. As a result, the osteoblastic transcriptosome, including the marker protein osteocalcin, and vascular mineralization is observed.45-48 Experimental models have demonstrated that elevated phosphate is a direct stimulus of this transformation.29,49-51 The finding of vascular calcification and the role of hyperphosphatemia contributes much more than academic significance. Calcification of the neointima or the tunica media, including the large blood vessels, coronary arteries, and heart valves, in patients with renal failure and those with diabetes is associated with a high morbidity and mortality from systolic hypertension, congestive heart failure, coronary artery disease, and myocardial infarction.52-59 The mechanisms of calcification of the atherosclerotic neointima and the tunica media differ, and osteoblastic transition of neointimal mesenchymal lineage cells has been established as the mechanism of atherosclerotic calcification.31,60 Another manifestation of vascular calcification in more peripheral arteries, calciphylaxis, is also associated with hyperphosphatemia and carries a poor prognosis.61-63 As a result, both vascular calcification and hyperphosphatemia are independent risk factors for mortality. Hyperphosphatemia as a result of renal failure plays a critical role in development of secondary hyperparathyroidism, ROD, and mortality.3,44,55,64 Several actions of Pi contribute to these complications. These include hyperphosphatemia-induced hypocalcemia through physicochemical interactions and expression of trans-
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Figure 2. Skeletal remodeling contributes to phosphate balance and serum phosphorus levels. (A) The phosphate balance diagram is amplified to show that the serum phosphorus is a small component of a rapidly exchangeable phosphorus pool comprising cellular phosphorus and the bone mineralization front. (B) When bone formation is decreased (adynamic bone disorders), the exchangeable pool size is diminished and intestinal absorption from food intake will produce larger fluctuations in the serum phosphorus. These fluctuations are sufficient to activate the signaling actions of the serum inorganic phosphorus, even though the fasting serum inorganic phosphorus is normal. Stimulation of bone anabolism increases the exchangeable phosphorus pool size and decreases serum phosphorus fluctuations. In end-stage kidney disease, treatment of secondary hyperparathyroidism with a calcimimetic, which does not affect phosphate absorption, decreases the serum phosphate level, demonstrating the role of the skeleton in hyperphosphatemia.
forming growth factor alpha (TGF-a) and the epidermal growth factor receptor in parathyroid chief cells leading to hyperplasia and increased PTH secretion.65,66 Additionally, hyperphosphatemia directly inhibits vitamin D synthesis. Finally, we and others have shown direct action of hyperphosphatemia in stimulating vascular calcification.3 In patients with advanced renal failure, the enhanced phosphate load from PTHmediated osteolysis may ultimately become the dominant influence on serum phosphorus levels (Fig 2B). This phenomenon may account for the correlation between serum phosphorus levels and the severity of osteitis fibrosa cystica in patients maintained on chronic hemodialysis. Hyperphosphatemia also plays a critical role in the development of vascular calcification, as discussed pre-
viosuly.3,29 There is a direct relationship between defective orthotopic mineralization (bone formation) in CKD and increased heterotopic mineralization.3,49,67-69 Our data3,70 and that of Price and colleagues71 and Morshita and colleagues49 demonstrate that increasing bone formation will lower phosphate levels and diminish vascular calcification in CKD. Calcitriol Deficiency When FGF-23 secretion is increased in early CKD, stimulation of 24-hydroxylase and inhibition of the 1a-hydroxylase enzymes lead to decrease in calcitriol levels. The result is highly prevalent vitamin D deficiency and decreasing calcitriol levels as CKD advances. The
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decrease in functioning nephron mass further contributes to decreased calcitriol production by proximal tubular 25-hydroxy cholecalciferol 1a-hydroxylase,72 which in conjunction with the increased phosphate load in remaining nephrons leads to overt calcitriol deficiency. The latter in turn decreases intestinal calcium absorption and leads to hypocalcemia. Calcitriol deficiency in cases of advanced kidney failure in turn diminishes tissue levels of VDRs, in particular, the VDR of parathyroid gland cells.73 Because the chief cell VDR suppresses the expression of pre-pro-PTH messenger ribonucleic acid, lower circulating calcitriol levels together with a low number of VDRs in patients with ESKD result in stimulation of both synthesis and secretion of PTH.74
hyperplastic glands, promoting parathyroid gland resistance to calcitriol and calcium. Sustained elevation in PTH levels, although adaptive to maintain osteoblast surfaces, produces an abnormal phenotype of osteoblast function with relatively less type 1 collagen and more receptor activator of nuclear factor kappa B ligand (RANKL) ligand production than anabolic osteoblasts. This leads to a high turnover osteodystrophy, PTH receptor desensitization, and excess bone resorption.
Hypocalcemia As CKD progresses, hypocalcemia develops owing to decreased intestinal calcium absorption. Low blood levels of ionized calcium stimulate PTH secretion, whereas high calcium concentrations suppress it. The action of calcium on parathyroid gland chief cells is mediated through a calcium sensor, a G-protein coupled plasma membrane receptor (CASR) expressed in chief cells, kidney tubular epithelia, and widely throughout the body at lower levels.75,76 The short-term stimulation of PTH secretion induced by low calcium is because of exocytosis of PTH packaged in granules, whereas longer-term stimulation results from an increase in the number of cells that secrete PTH. More prolonged hypocalcemia induces changes in intracellular PTH degradation and mobilization of a secondary storage pool. Within days or weeks of the onset of hypocalcemia, pre-pro-PTH messenger ribonucleic acid expression is stimulated. This effect is exerted through a negative calcium response element located in the upstream flanking region of the gene for PTH. Expression of the calcium receptor has been shown to be suppressed by calcitriol deficiency and stimulated by calcitriol administration, suggesting an additional regulatory mechanism of the active vitamin D metabolite on PTH production. The decreased number of calcium-sensing receptors with low circulating calcitriol may, at least in part, explain the relative insensitivity of parathyroid gland cells to calcium in patients undergoing dialysis.
Inflammatory Mediators, Acidosis, and Aluminum
Hyperparathyroidism All of the mechanisms discussed previously result in increased production of PTH and nodular hyperplasia of the parathyroid glands in CKD. The size of the parathyroid glands progressively increases during CKD and in patients undergoing dialysis, paralleling serum PTH levels. This increase in gland size is mainly because of diffuse cellular hyperplasia. Monoclonal chief cell growth also develops, resulting in the formation of nodules. Nodular hyperplastic glands express fewer VDRs and calcium-sensing receptors, as compared with diffuse
Hypogonadism Patients with ESKD display various states of gonadal dysfunction. Estrogen and testosterone deficiency significantly contribute to CKD-MBD pathogenesis.
Inflammatory mediators, acidosis, and aluminum are all potentially critical factors in CKD-MBD that have either not been well studied or have been clinically eliminated, and lack of space prevents further description of their role. Other Factors Some patients with CKD are treated with glucocorticoids, which have an effect on bone metabolism. Patients maintained on chronic dialysis retain b2-microglobulin. Alterations in growth factors and other hormones including leptin, serotonin, and others involved in the regulation of bone remodeling, are disordered in CKD/ESRD. Their dysregulation affects bone remodeling and contributes to the development of CKD-MBD.
Conclusions Recent advances have led to the naming of CKD-MBD as a syndrome contributing to cardiovascular mortality associated with diseases of the kidney. Even more recent discoveries demonstrate that CKD-MBD begins soon after the onset of kidney disease and is marked by increased secretion of the skeletal hormone FGF-23. The CKD-MBD in stage 2 CKD involves the following: the effects of kidney injury on the skeleton; the stimulation of FGF-23 secretion by osteocytes; the actions of the skeleton and of the kidney on the vasculature to cause calcification; and decreased CYP27B1 activity and increased CYP24A1 activity causing vitamin D and calcitriol levels to trend downward. During this stage (stage 2 CKD), Pi, calcium, PTH, and calcitriol levels are normal. Progression of kidney disease leads to increasing defects in Pi and calcium homeostasis and the development of hyperparathyroidism. Therapy for CKD-MBD should begin with agents that stimulate the skeleton to decrease FGF-23 production and vitamin D supplementation, if not calcitriol replacement. Clinical trials are needed to demonstrate that successful intervention in CKD-MBD improves clinical outcomes.
Renal Osteodystrophy
Key Points CKD-MBD is a new term for a syndrome complicating CKD that includes the skeletal and mineral disorders of CKD and their contribution to cardiovascular mortality and events. CKD-MBD begins early in stage 2 CKD as abnormalities in skeletal remodeling develop. These abnormalities in remodeling are marked by increased secretion in FGF-23. Finally, the stimulation of vascular calcification, a component of the increased cardiovascular risk of CKD-MBD, is already detectable in stage 2 CKD. In the early stages of CKD-MBD, the serum Pi, calcium, PTH, and calcitriol levels are normal. Phosphorus is a signaling molecule, which directly affects osteocyte signaling to increase FGF-23 secretion, directly stimulates vascular smooth muscle cells and osteoblasts to increase matrix mineralization and osteoblastic transcription factor gene expression, directly regulates osteoclastic bone resorption, and directly regulates proximal tubular phosphate transport and 1a-hydroxylase activity (calcitriol synthesis). The disorders of skeletal remodeling directly contribute to the level of the serum phosphorus through excess bone resorption and abnormal bone formation (decreased or increased). The skeleton further contributes to the syndrome of CKD-MBD through production of skeletal hormones, FGF-23 and osteocalcin, and the skeletal factor, sclerostin.
Acknowledgments The writing of this chapter was supported by NIH grants DK070790 and AR41677.
References 1. Olgaard K. KDIGO: Clinical Guide to Bone and Mineral Metabolism in CKD New York, NY: National Kidney Foundation; 2006. 2. Mathew S, Lund R, Strebeck F, et al. Reversal of the adynamic bone disorder and decreased vascular calcification in chronic kidney disease by sevelamer carbonate therapy. J Am Soc Nephrol. 2007;18:122-130. 3. Davies MR, Lund RJ, Mathew S, et al. Low turnover osteodystrophy and vascular calcification are amenable to skeletal anabolism in an animal model of chronic kidney disease and the metabolic syndrome. J Am Soc Nephrol. 2005;16:917-928. 4. Stevens LA, Djurdjev O, Cardew S, et al. Calcium, phosphate, and parathyroid hormone levels in combination and as a function of dialysis duration predict mortality: evidence for the complexity of the association between mineral metabolism and outcomes. J Am Soc Nephrol. 2004;15:770-779. 5. Block GA, Hulbert-Shearon TE, Levin NW, et al. Association of serum phosphorus and calcium X phosphate product with mortality risk in chronic hemodialysis patients: a national study. Am J Kidney Dis. 1998;31:607-617. 6. Lund RJ, Davies MR, Brown AJ, et al. Successful treatment of an adynamic bone disorder with bone morphogenetic protein-7 in a renal ablation model. J Am Soc Nephrol. 2004;15:359-369. 7. Malluche HH, Ritz E, Lange HP. Bone histology in incipient and advanced renal failure. Kidney Int. 1976;9:355-362.
103
8. Craver L, Marco MP, Martinez I, et al. Mineral metabolism parameters throughout chronic kidney disease stages 1-5–achievement of K/DOQI target ranges. Nephrol Dial Transplant. 2007;22:1171-1176. 9. Malluche HH, Faugere MC. Renal bone disease 1990: challenge for nephrologists. Kidney Int. 1990;38:193-211. 10. Foley RN, Parfrey PS, Sarnak MJ. Clinical epidemiology of cardiovascular disease in chronic renal disease. Am J Kidney Dis. 1998;32:S112-S119. 11. Slinin Y, Foley RN, Collins AJ. Calcium, phosphorus, parathyroid hormone, and cardiovascular disease in hemodialysis patients: the USRDS waves 1, 3, and 4 study. J Am Soc Nephrol. 2005;16:1788-1793. 12. Shlipak MG, Sarnak MJ, Katz R, et al. Cystatin C and the risk of death and cardiovascular events among elderly persons. N Engl J Med. 2005;352:2049-2060. 13. White KE, Carn G, Lorenz-Depiereux B, et al. Autosomal dominant hypophosphatemic rickets is associated with mutations in FGF23. Nat Genet. 2000;26:345-348. 14. White KE, Jonsson KB, Carn G, et al. The autosomal dominant hypophosphatemic rickets (ADHR) gene is a secreted polypeptide overexpressed by tumors that cause phosphate wasting. J Clin Endocrinol Metab. 2001;86:497-500. 15. Larsson T, Nisbeth U, Ljunggren O, et al. Circulating concentration of FGF-23 increases as renal function declines in patients with chronic kidney disease, but does not change in response to variation in phosphate intake in healthy volunteers. Kidney Int. 2003;64:2272-2279. 16. Shimada T, Muto T, Urakawa I, et al. Mutant FGF-23 responsible for autosomal dominant hypophosphatemic rickets is resistant to proteolytic cleavage and causes hypophosphatemia in vivo. Endocrinology. 2002;143:3179-3182. 17. Juppner H, Wolf M, Salusky IB. FGF-23: more than a regulator of renal phosphate handling? J Bone Miner Res. 2010;25:2091-2097. 18. Gutierrez OM, Mannstadt M, Isakova T, et al. Fibroblast growth factor 23 and mortality among patients undergoing hemodialysis. N Engl J Med. 2008;359:584-592. 19. Kurosu H, Ogawa Y, Miyoshi M, et al. Regulation of fibroblast growth factor-23 signaling by Klotho. J Biol Chem. 2006;281:6120-6123. 20. Matsumura Y, Aizawa H, Shiraki-Iida T, et al. Identification of the human klotho gene and its two transcripts encoding membrane and secreted klotho protein. Biochem Biophys Res Commun. 1998;242:626-630. 21. Kuro-o M, Matsumura Y, Aizawa H, et al. Mutation of the mouse klotho gene leads to a syndrome resembling ageing. Nature. 1997;390:45-51. 22. Tsujikawa H, Kurotaki Y, Fujimori T, et al. Klotho, a gene related to a syndrome resembling human premature aging, functions in a negative regulatory circuit of vitamin D endocrine system. Mol Endocrinol. 2003;17:2393-2403. 23. Imura A, Tsuji Y, Murata M, et al. 6Klotho as a regulator of calcium homeostasis. Science. 2007;316:1615-1618. 24. Medici D, Razzaque MS, DeLuca S, et al. FGF-23-Klotho signaling stimulates proliferation and prevents vitamin D-induced apoptosis. J Cell Biol. 2008;182:459-465. 25. Slatopolsky E, Robson AM, Elkan I, et al. Control of phosphate excretion in uremic man. J Clin Invest. 1968;47:1865-1874. 26. Kurz P, Monier-Faugere M-C, Bognar B, et al. Evidence for abnormal calcium homeostasis in patients with adynamic bone disease. Kidney Int. 1994;46:855-861. 27. Moallem E, Kilav R, Silver J, et al. RNA-protein binding and posttranscriptional regulation of parathyroid hormone gene expression by calcium and phosphate. J Biol Chem. 1998;273:5253-5259. 28. Naveh-Many T, Rahamimov R, Livni N, et al. Parathyroid cell proliferation in normal and chronic renal failure rats. J Clin Invest. 1995;96:1786-1793. 29. Jono S, McKee MD, Murry CE, et al. Phosphate regulation of vascular smooth muscle cell calcification. Circ Res. 2000;87:e10-e17. 30. Li X, Yang HY, Giachelli CM. Role of the sodium-dependent phosphate cotransporter, pit-1, in vascular smooth muscle cell calcification. Circ Res. 2006;98:905-912. 31. Mathew S, Tustison KS, Sugatani T, et al. The mechanism of phosphorus as a cardiovascular risk factor in chronic kidney disease. J Am Soc Nephrol. 2008;19:1092-1105.
104
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32. Hruska KA, Levi M, Slatopolsky E. Disorders of phosphorus, calcium, and magnesium metabolism. In: Schrier RW, ed. Diseases of the Kidney and Urinary Tract. Chap 88. Philadelphia, PA: Lippincott Williams & Wilkins; 2005. 33. Beck GR Jr. Inorganic phosphate as a signaling molecule in osteoblast differentiation. J Cell Biochem. 2003;90:234-243. 34. Gupta A, Guo X-L, Alvarez UM, et al. Regulation of sodium-dependent phosphate transport in osteoclasts. J Clin Invest. 1997;100:538-549. 35. Hruska KA, Mathew S, Saab G. Bone morphogenetic proteins in vascular calcification. Circ Res. 2005;97:105-114. 36. Doherty TM, Asotra K, Fitzpatrick LA, et al. Calcification in atherosclerosis: bone biology and chronic inflammation at the arterial crossroads. Proc Natl Acad Sci U S A. 2004;100:11201-11206. 37. Vattikuti R, Towler DA. Osteogenic regulation of vascular calcification: an early perspective. Am J Physiol Endocrinol Metab. 2004; 286:E686-E696. 38. Bucay N, Sarosi I, Dunstan CR, et al. Osteoprotegerin-deficient mice develop early onset osteoporosis and arterial calcification. Genes Dev. 1998;12:1260-1268. 39. Jono S, Peinado C, Giachelli CM. Phosphorylation of osteopontin is required for inhibition of vascular smooth muscle cell calcification. J Biol Chem. 2000;275:20197-20203. 40. Demer LL, Tintut Y. Osteopontin: between a rock and a hard plaque. Circ Res. 1999;84:250-252. 41. Luo G, Ducy P, McKee MD, et al. Spontaneous calcification of arteries and cartilage in mice lacking matrix GLA protein. Nature. 1997;386:78-81. 42. Kawaguchi H, Manabe N, Miyaura C, Chikuda H, Nakamura K, Kuro-o M. Independent impairment of osteoblast and osteoclast differentiation in klotho mouse exhibiting low-turnover osteopenia. J Clin Invest. 1999;104:229-237. 43. Galvin KM, Donovan MJ, Lynch CA, et al. A role for Smad6 in development and homeostasis of the cardiovascular system. Nat Genet. 2000;24:171-174. 44. Block GA, Port FK. Re-evaluation of risks associated with hyperphosphatemia and hyperparathyroidism in dialysis patients: recommendations for a change in management. Am J Kidney Dis. 2000;35:1226-1237. 45. Demer L. Lipid hypothesis of cardiovascular calcification. Circulation. 1997;95:297-298. 46. Tintut Y, Parhami F, Bostr€ om K, et al. cAMP stimulates osteoblastlike differentiation of calcifying vascular cells. J Biol Chem. 1998;273:7547-7553. 47. Tintut Y, Parhami F, Le V, et al. Inhibition of osteoblast-specific transcription factor Cbfal by the cAMP pathway in osteoblastic cells. Ubiquitin/proteasome-dependent regulation. J Biol Chem. 1999; 274:28875-28879. 48. Bostr€ om K, Watson KE, Stanford WP, et al. Atherosclerotic calcification: relation to developmental osteogenesis. Am J Cardiol. 1995; 75:88B-91B. 49. Morishita K, Shirai A, Kubota M, et al. The progression of aging in klotho mutant mice can be modified by dietary phosphorus and zinc. J Nutr. 2001;131:3182-3188. 50. Steitz SA, Speer MY, Curinga G, et al. Smooth muscle cell phenotypic transition associated with calcification. Circ Res. 2001; 89:1147-1154. 51. Chen NX, O’Neill KD, Duan D, et al. Phosphorus and uremic serum up-regulate osteopontin expression in vascular smooth muscle cells. Kidney Int. 2002;62:1724-1731. 52. Blacher J, Guerin AP, Pannier B, et al. Arterial calcifications, arterial stiffness, and cardiovascular risk in end-stage renal disease. Hypertension. 2001;38:938-942. 53. Chertow GM, Burke SK, Raggi P. Sevelamer attenuates the progression of coronary and aortic calcification in hemodialysis patients. Kidney Int. 2002;62:245-252. 54. Cozzolino M, Dusso AS, Slatopolsky E. Role of calcium-phosphate product and bone-associated proteins on vascular calcification in renal failure. J Am Soc Nephrol. 2001;12:2511-2516.
55. Fatica RA, Dennis VW. Cardiovascular mortality in chronic renal failure: hyperphosphatemia, coronary calcification, and the role of phosphate binders. Cleve Clin J Med. 2002;69:S21-S27. 56. Iribarren C, Sidney S, Sternfeld B, et al. Calcification of the aortic arch: risk factors and association with coronary heart disease, stroke, and peripheral vascular disease. JAMA. 2000;283:2810-2815. 57. Newman AB, Naydeck BL, Sutton-Tyrrell K, et al. Relationship between coronary artery calcification and other measures of subclinical cardiovascular disease in older adults. Arterioscler Thromb Vasc Biol. 2002;22:1674-1679. 58. Raggi P, Boulay A, Chasan-Taber S, et al. Cardiac calcification in adult hemodialysis patients. A link between end-stage renal disease and cardiovascular disease? J Am Coll Cardiol. 2002;39:695-701. 59. Reslerova M, Moe SM. Vascular calcification in dialysis patients: pathogenesis and consequences. Am J Kidney Dis. 2003;41:S96-S99. 60. Shao JS, Cai J, Towler DA. Molecular mechanisms of vascular calcification: lessons learned from the aorta. Arterioscler Thromb Vasc Biol. 2006;26:1423-1430. 61. Ahmed S, O’Neill KD, Hood AF, et al. Calciphylaxis is associated with hyperphosphatemia and increased osteopontin expression by vascular smooth muscle cells. Am J Kidney Dis. 2001;37:1267-1276. 62. Block GA. Control of serum phosphorus: implications for coronary artery calcification and calcific uremic arteriolopathy (calciphylaxis). Curr Opin Nephrol Hypertens. 2001;10:741-747. 63. Wilmer WA, Magro CM. Calciphylaxis: emerging concepts in prevention, diagnosis, and treatment. Semin Dial. 2002;15:172-186. 64. Slatopolsky E, Finch J, Denda M, et al. Phosphorus restriction prevents parathyroid gland growth. High phosphorus directly stimulates PTH secretion in vitro. J Clin Invest. 1996;97:2534-2540. 65. Cozzolino M, Lu Y, Sato T, et al. A critical role for enhanced-TGFalpha and EGFR expression in the initiation of parathyroid hyperplasia in experimental kidney disease. Am J Physiol Renal Physiol. 2005;289:F1096-F1102. 66. Martin DR, Ritter CS, Slatopolsky E, et al. Acute regulation of parathyroid hormone by dietary phosphate. Endocrinol Metab. 2005; 289:E729-E734. 67. Lund RJ, Huq N, Davies MR, et al. Efficacy of BMP-7 treatment of adynamic renal osteodystrophy. J Am Soc Nephrol. 2002;13:578A. 68. Bucay N, Sarosi I, Dunstan CR, et al. Osteoprotegerin-deficient mice develop early onset osteoporosis and arterial calcification. Genes Dev. 1998;12:1260-1268. 69. Hak AE, Pols HAP, van Hemert AM, et al. Progression of aortic calcification is associated with metacarpal bone loss during menopause a population-based longitudinal study. Arterioscler Thromb Vasc Biol. 2000;20:1926-1931. 70. Danen EHJ, Sonneveld P, Brakebusch C, et al. The fibronectinbinding integrins a5b1 and avb3 differentially modulate RhoAGTP loading, organization of cell matrix adhesions, and fibronectin fibrillogenesis. J Cell Biol. 2002;159:1071-1086. 71. Price PA, June HH, Buckley JR, et al. Osteoprotegerin inhibits artery calcification induced by Warfarin and by vitamin D. Arterioscler Thromb Vasc Biol. 2001;21:1610-1616. 72. Goodman WG, Quarles LD. Development and progression of secondary hyperparathyroidism in chronic kidney disease: lessons from molecular genetics. Kidney Int. 2007;74:276-288. 73. Naveh-Many T, Marx R, Keshet E, et al. Regulation of 1,25-dihydroxyvitamin D3 receptor gene expression by 1,25-dihydroxyvitamin D3 in the parathyroid in vivo. J Clin Invest. 1990;86:1968-1975. 74. Silver J, Russell J, Sherwood LM. Regulation by vitamin D metabolites of messenger ribonucleic acid for preproparathyroid hormone in isolated bovine parathyroid cells. Proc Natl Acad Sci U S A. 1985;82:4270-4273. 75. Brown EM, Gamba G, Riccardi D, et al. Cloning and characterization of an extracellular Ca21-sensing receptor from bovine parathyroid. Nature. 1993;366:575-580. 76. Brown EM, Hebert SC. A cloned Ca21 sensing receptor: a mediator of direct effects of extracellular Ca21 on renal function? J Am Soc Nephrol. 1995;6:1530-1540.
T
he CKD mineral bone disorder (CKD-MBD) is a term coined by the Kidney Disease Improving Global Outcomes Foundation1 to replace the term renal osteodystrophy (ROD), used for recognition of several pathophysiological discoveries of the 21st century. The first of these pathophysiological discoveries is that the skeletal remodeling disorders caused by CKD contribute directly to the heterotopic mineralization, especially vascular calcification, and the disordered mineral metabolism that accompany CKD.2,3 Second, the disorders in mineral metabolism associated with CKD are key factors contributing to the excess mortality observed in CKD.4,5 Third, CKD or renal injury impairs skeletal anabolism, thereby decreasing osteoblast function and bone formation rates.3,6 In short, a multiorgan system that fails in CKD has been defined involving the kidney, skeleton, parathyroid glands, and the cardiovasculature (Fig 1).
pathology has been detected in early CKD, and the earliest histologic bone abnormalities in CKD-MBD are seen after a relatively mild reduction in the glomerular filtration rate (creatinine clearance rates of between 40 and 70 mL/min, stage 2 CKD).7 In addition, elevated parathyroid hormone (PTH) levels may be observed before any detectable changes in the serum phosphorus, calcitriol, or calcium.8 If hyperparathyroidism is prevented or treated, a low turnover osteodystrophy, the adynamic bone disorder, is observed, uncovering the effects of kidney injury on the skeleton. By stage 5 CKD (creatinine clearance, ,15 mL/min), skeletal histologic pathology is found in virtually all patients.9 The increasing incidence of CKD in the United States and across the world and the role of CKD-MBD in its high mortality make ROD a major health issue for Americans and all developed societies.10-12
Pathobiology The initial skeletal abnormalities of CKD-MBD are marked by the stimulation of fibroblast growth factor23 (FGF-23) secretion by osteocytes, which signal that the skeleton has been affected by kidney disease. Skeletal
From Division of Pediatric Nephrology, Department of Pediatrics, Washington University, St. Louis, MO. Address correspondence to Keith A. Hruska, MD, Division of Pediatric Nephrology, Department of Pediatrics, Washington University, Campus Box 8208, 5th Floor MPRB, 660 South Euclid Avenue, St. Louis, MO 63110. E-mail: [email protected] Ó 2011 by the National Kidney Foundation, Inc. All rights reserved. 1548-5595/$36.00 doi:10.1053/j.ackd.2011.01.001
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Pathogenesis Renal injuries give rise to a loss of skeletal anabolism manifested as a decrease in bone formation rates that derive from osteoblast activity.6 The increase in FGF-23 levels is the first indication that the skeleton has been affected by CKD. The loss of anabolism owing to kidney injury occurs in the presence of normal PTH, vitamin D, calcium, and phosphorus levels, but it is not usually observed because abnormalities in these factors stimulate an adaptation, that is, PTH secretion and secondary hyperparathyroidism. The sustained increase in PTH levels produced through adaptation to CKD results in an unwanted disorder of skeletal remodeling, a high turnover ROD or osteitis fibrosa.
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Figure 1. Failure of a multiorgan system in CKD. The discoveries that disorders of mineral metabolism are causally linked to mortality, that hyperphosphatemia causes vascular calcification, and that kidney failure directly impairs skeletal anabolism establishes a multiorgan system which fails in CKD. The system consists of the kidney, the skeleton, the intestine, the vasculature, and the heart. The system is established by direct connections between each organ, which have not been completely defined in the case of kidney–bone and kidney–heart. However, the contribution of the skeleton and the kidney to hyperphosphatemia, the role of hyperphosphatemia in causing vascular calcification, and the effects of vascular calcification on the heart have been established. The kidney–intestine connection is represented by calcitriol and the intestine–bone connection by serotonin and an intestinal phosphatonin under study.
Pathogenetic Factors in CKD-MBD Fibroblast Growth Factor-23 FGF-23 is the original phosphatonin (phosphate excretion regulating hormone) discovered in studies of autosomal dominant hypophosphatemic rickets and oncogenic osteomalacia.13,14 FGF-23 levels progressively rise during the course of CKD15,16 in conjunction with PTH, serving to maintain phosphate homeostasis in the early stages of renal disease. FGF-23 is produced mainly by osteocytes and to some extent by osteoblasts, and it is a mineralization inhibitor. It is unclear whether FGF-23 contributes to the mineralization defects observed in skeletal remodeling in CKD.17 FGF-23 represents a direct bone–kidney connection in the multiorgan system involved in CKDMBD, but it is unclear whether it has direct actions to stimulate cardiovascular events of which its high levels are very predictive.18 FGF-23 functions through signal transduction mediated by the FGF receptors (FGFR), mainly FGFR1c, and FGF-23 stimulation of cell signals through the FGFR requires the co-receptor Klotho.19 This is demonstrated by the identical phenotype displayed by mice deficient in FGF-23 or in Klotho. The phenotype that includes hyperphosphatemia and vascular calcification can be rescued by a low phosphate diet or by breeding in the conditions involving double deficiency of the vitamin D receptor (VDR) and FGF-23/Klotho. Thus, the distribution of Klotho is thought to delineate the target tissues of FGF-23, which are the kidney distal tubule and the parathyroid gland. What remains unclear is how FGF-23 sends signals to the proximal tubule, which has a low expression rate of Klotho, and the cardiovasculature and skeleton, which do not express Klotho.
The Klotho gene encodes a type-1 membrane protein that is related to b-glucuronidases.20 Genetic variants in KLOTHO are associated with human aging,21 and Klotho is a circulating protein that declines with age and CKD. Reduced production of Klotho in CKD may be one of the factors underlying the degenerative processes of arteriosclerosis, osteoporosis, and skin atrophy seen in CKD. In addition, the parathyroid hyperplasia that accompanies CKD results in decreased Klotho expression in the parathyroid glands. This produces FGF-23 resistance and impaired parathyroid suppression by FGF-23. Klotho deficiency as discussed in the preceding section produces premature aging, hyperphosphatemia, and vascular calcification, which are relieved by lowering calcitriol activity.22-24 Phosphorus Because renal injury decreases nephron number, phosphate retention results from a reduction in filtered phosphate and is reversed through FGF-23- and PTH-mediated reductions in tubular epithelial phosphate transport. Phosphorus is a direct stimulant to the osteocytes for regulation of FGF-23 secretion. The kinetics of the osteocyte phosphate sensor are unknown, and thus, it is unclear whether subdetectable changes in the serum phosphorus are controlling the early increases in FGF-23 in kidney diseases or whether changes in bone formation are the initial stimulus for increased FGF-23 secretion. The increase in phosphate excretion per remaining nephron restores phosphate homeostasis at the cost of higher FGF-23 and PTH levels, and maintains normal phosphate excretion. In stage 4 and 5 CKD, when renal injury is severe enough that the glomerular filtration rate reaches levels of ,30% of normal, hyperphosphatemia becomes fixed owing to insufficient renal excretion despite high PTH and FGF-23 levels.25 Studies demonstrate that failure of calcium and phosphorous deposition into the skeleton or excess resorption of the skeleton also contribute to abnormal calcium and phosphorus levels in CKD and end-stage kidney disease (ESKD).3,26 Hyperphosphatemia decreases serum calcium through physicochemical binding and suppresses 1a-hydroxylase activity, which results in further lowering of circulating calcitriol levels. Moreover, a direct stimulatory effect of phosphorus on parathyroid gland cells, independent of calcium and calcitriol, results in increased secretion and nodular hyperplasia of parathyroid gland cells.27,28 Finally, hyperphosphatemia is a signaling mechanism for induction of heterotopic mineralization of the vasculature in CKD and ESRD.29,30 We have shown that phosphorus directly stimulates matrix mineralization by vascular smooth muscle cells isolated from atherosclerotic human blood vessels through stimulation of osteoblastic gene expression. In the presence of high levels of phosphorus, mineralization is blocked by suppression of an osteoblastic transcription factor, osterix.31
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Hyperphosphatemia Serum inorganic phosphorus (Pi) concentrations are generally maintained at 2.5 to 4.5 mg/dL or 0.75 to 1.45 mM in adults, whereas the normal range in children is between 3.5 and 7 mg/dL. Hyperphosphatemia may be the consequence of an increased intake of Pi, a decreased excretion of Pi, or translocation of Pi from tissue breakdown into the extracellular fluid.32 Because the kidneys are able to excrete phosphate very efficiently over a wide range of dietary intake, hyperphosphatemia most frequently results from renal insufficiency and the attendant inability to excrete Pi. However, in metabolic bone disorders, such as osteoporosis and ROD, the skeleton is a poorly recognized contributor to the serum phosphorus. The bulk of total body phosphate (85%) is present in the bone as part of the mineralized extracellular matrix. This phosphate pool is accessible, but in a limited manner, through bone resorption. Other than bone storage, phosphate is a predominantly intracellular anion with an estimated concentration of approximately 100 mmol/L, most of which is either complexed or bound to proteins or lipids. Serum phosphorus concentration varies with age, time of day, fasting state, and season. It is higher in children than in adults. Phosphorus levels exhibit a diurnal variation, with the lowest phosphate level occurring near noon. Serum phosphorus concentration is regulated by diet, hormones, and physical factors such as pH. Importantly, because phosphate moves in and out of cells under several influences, the serum concentration of phosphorus poorly reflects phosphate stores. Clinical and translational studies demonstrate that excess bone resorption contributes to the level of serum phosphorus that is poorly appreciated. Even in clinical situations where bone formation is decreased, such as adynamic ROD and osteoporosis, a variability in the serum phosphorus is produced when bone formation is stimulated (Fig 2).3,6 For instance, in low turnover osteodystrophy treated with a skeletal anabolic agent, when phosphorus intake is constant, the serum phosphorus falls despite no change or decrease in phosphate excretion (Fig 2B). The most important short-term consequences of hyperphosphatemia are hypocalcemia and tetany, which occur most commonly in patients with an increased Pi load from any source, exogenous or endogenous. By contrast, soft-tissue calcification and secondary hyperparathyroidism are long-term consequences of hyperphosphatemia that occur mainly in patients with renal insufficiency and decreased renal Pi excretion. Phosphorus directly affects the activity of both boneforming (osteoblasts)33 and bone-resorbing (osteoclasts)34 cells. The actions of phosphorus on osteoblasts are similar to those in vascular smooth muscle cells undergoing Pi-stimulated osteoblastic transition and have been dis-
cussed later in the text. The actions of Pi to stimulate Pi uptake in osteoclasts are a necessary component of adenosine triphosphate production and the high rates of energy utilization by these cells.34 Thus, Pi is supportive of bone resorption and the uncoupling of formation and resorption that occurs in CKD.
Heterotopic Mineralization Extraskeletal calcification associated with hyperphosphatemia is usually seen in patients with chronic kidney failure, diabetes, severe atherosclerosis, or aging. Recent basic, translational, and clinical research studies have discovered the pathogenesis and the consequences of this phenomenon.31,35-37 Several inhibitors of vascular calcification have been discovered including osteoprotegerin,38 osteopontin,39,40 matrix Gla protein,41 the Klotho gene product,42 and Smad 643 through phenotyping transgenic knockout mice. These substances constitute an inherent defense against heterotopic mineralization, which is breached in the disease environment. In the setting of CKD, hyperphosphatemia has been identified as a major factor contributing to the forces favoring mineralization.44 In contrast to the breach of defense theory of vascular calcification, there is significant evidence that vascular cells undergo osteoblastic transition, including expression of the osteoblast transcription factors RUNX2/Cbfal, osterix, MSX2, and DlX5. As a result, the osteoblastic transcriptosome, including the marker protein osteocalcin, and vascular mineralization is observed.45-48 Experimental models have demonstrated that elevated phosphate is a direct stimulus of this transformation.29,49-51 The finding of vascular calcification and the role of hyperphosphatemia contributes much more than academic significance. Calcification of the neointima or the tunica media, including the large blood vessels, coronary arteries, and heart valves, in patients with renal failure and those with diabetes is associated with a high morbidity and mortality from systolic hypertension, congestive heart failure, coronary artery disease, and myocardial infarction.52-59 The mechanisms of calcification of the atherosclerotic neointima and the tunica media differ, and osteoblastic transition of neointimal mesenchymal lineage cells has been established as the mechanism of atherosclerotic calcification.31,60 Another manifestation of vascular calcification in more peripheral arteries, calciphylaxis, is also associated with hyperphosphatemia and carries a poor prognosis.61-63 As a result, both vascular calcification and hyperphosphatemia are independent risk factors for mortality. Hyperphosphatemia as a result of renal failure plays a critical role in development of secondary hyperparathyroidism, ROD, and mortality.3,44,55,64 Several actions of Pi contribute to these complications. These include hyperphosphatemia-induced hypocalcemia through physicochemical interactions and expression of trans-
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Figure 2. Skeletal remodeling contributes to phosphate balance and serum phosphorus levels. (A) The phosphate balance diagram is amplified to show that the serum phosphorus is a small component of a rapidly exchangeable phosphorus pool comprising cellular phosphorus and the bone mineralization front. (B) When bone formation is decreased (adynamic bone disorders), the exchangeable pool size is diminished and intestinal absorption from food intake will produce larger fluctuations in the serum phosphorus. These fluctuations are sufficient to activate the signaling actions of the serum inorganic phosphorus, even though the fasting serum inorganic phosphorus is normal. Stimulation of bone anabolism increases the exchangeable phosphorus pool size and decreases serum phosphorus fluctuations. In end-stage kidney disease, treatment of secondary hyperparathyroidism with a calcimimetic, which does not affect phosphate absorption, decreases the serum phosphate level, demonstrating the role of the skeleton in hyperphosphatemia.
forming growth factor alpha (TGF-a) and the epidermal growth factor receptor in parathyroid chief cells leading to hyperplasia and increased PTH secretion.65,66 Additionally, hyperphosphatemia directly inhibits vitamin D synthesis. Finally, we and others have shown direct action of hyperphosphatemia in stimulating vascular calcification.3 In patients with advanced renal failure, the enhanced phosphate load from PTHmediated osteolysis may ultimately become the dominant influence on serum phosphorus levels (Fig 2B). This phenomenon may account for the correlation between serum phosphorus levels and the severity of osteitis fibrosa cystica in patients maintained on chronic hemodialysis. Hyperphosphatemia also plays a critical role in the development of vascular calcification, as discussed pre-
viosuly.3,29 There is a direct relationship between defective orthotopic mineralization (bone formation) in CKD and increased heterotopic mineralization.3,49,67-69 Our data3,70 and that of Price and colleagues71 and Morshita and colleagues49 demonstrate that increasing bone formation will lower phosphate levels and diminish vascular calcification in CKD. Calcitriol Deficiency When FGF-23 secretion is increased in early CKD, stimulation of 24-hydroxylase and inhibition of the 1a-hydroxylase enzymes lead to decrease in calcitriol levels. The result is highly prevalent vitamin D deficiency and decreasing calcitriol levels as CKD advances. The
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decrease in functioning nephron mass further contributes to decreased calcitriol production by proximal tubular 25-hydroxy cholecalciferol 1a-hydroxylase,72 which in conjunction with the increased phosphate load in remaining nephrons leads to overt calcitriol deficiency. The latter in turn decreases intestinal calcium absorption and leads to hypocalcemia. Calcitriol deficiency in cases of advanced kidney failure in turn diminishes tissue levels of VDRs, in particular, the VDR of parathyroid gland cells.73 Because the chief cell VDR suppresses the expression of pre-pro-PTH messenger ribonucleic acid, lower circulating calcitriol levels together with a low number of VDRs in patients with ESKD result in stimulation of both synthesis and secretion of PTH.74
hyperplastic glands, promoting parathyroid gland resistance to calcitriol and calcium. Sustained elevation in PTH levels, although adaptive to maintain osteoblast surfaces, produces an abnormal phenotype of osteoblast function with relatively less type 1 collagen and more receptor activator of nuclear factor kappa B ligand (RANKL) ligand production than anabolic osteoblasts. This leads to a high turnover osteodystrophy, PTH receptor desensitization, and excess bone resorption.
Hypocalcemia As CKD progresses, hypocalcemia develops owing to decreased intestinal calcium absorption. Low blood levels of ionized calcium stimulate PTH secretion, whereas high calcium concentrations suppress it. The action of calcium on parathyroid gland chief cells is mediated through a calcium sensor, a G-protein coupled plasma membrane receptor (CASR) expressed in chief cells, kidney tubular epithelia, and widely throughout the body at lower levels.75,76 The short-term stimulation of PTH secretion induced by low calcium is because of exocytosis of PTH packaged in granules, whereas longer-term stimulation results from an increase in the number of cells that secrete PTH. More prolonged hypocalcemia induces changes in intracellular PTH degradation and mobilization of a secondary storage pool. Within days or weeks of the onset of hypocalcemia, pre-pro-PTH messenger ribonucleic acid expression is stimulated. This effect is exerted through a negative calcium response element located in the upstream flanking region of the gene for PTH. Expression of the calcium receptor has been shown to be suppressed by calcitriol deficiency and stimulated by calcitriol administration, suggesting an additional regulatory mechanism of the active vitamin D metabolite on PTH production. The decreased number of calcium-sensing receptors with low circulating calcitriol may, at least in part, explain the relative insensitivity of parathyroid gland cells to calcium in patients undergoing dialysis.
Inflammatory Mediators, Acidosis, and Aluminum
Hyperparathyroidism All of the mechanisms discussed previously result in increased production of PTH and nodular hyperplasia of the parathyroid glands in CKD. The size of the parathyroid glands progressively increases during CKD and in patients undergoing dialysis, paralleling serum PTH levels. This increase in gland size is mainly because of diffuse cellular hyperplasia. Monoclonal chief cell growth also develops, resulting in the formation of nodules. Nodular hyperplastic glands express fewer VDRs and calcium-sensing receptors, as compared with diffuse
Hypogonadism Patients with ESKD display various states of gonadal dysfunction. Estrogen and testosterone deficiency significantly contribute to CKD-MBD pathogenesis.
Inflammatory mediators, acidosis, and aluminum are all potentially critical factors in CKD-MBD that have either not been well studied or have been clinically eliminated, and lack of space prevents further description of their role. Other Factors Some patients with CKD are treated with glucocorticoids, which have an effect on bone metabolism. Patients maintained on chronic dialysis retain b2-microglobulin. Alterations in growth factors and other hormones including leptin, serotonin, and others involved in the regulation of bone remodeling, are disordered in CKD/ESRD. Their dysregulation affects bone remodeling and contributes to the development of CKD-MBD.
Conclusions Recent advances have led to the naming of CKD-MBD as a syndrome contributing to cardiovascular mortality associated with diseases of the kidney. Even more recent discoveries demonstrate that CKD-MBD begins soon after the onset of kidney disease and is marked by increased secretion of the skeletal hormone FGF-23. The CKD-MBD in stage 2 CKD involves the following: the effects of kidney injury on the skeleton; the stimulation of FGF-23 secretion by osteocytes; the actions of the skeleton and of the kidney on the vasculature to cause calcification; and decreased CYP27B1 activity and increased CYP24A1 activity causing vitamin D and calcitriol levels to trend downward. During this stage (stage 2 CKD), Pi, calcium, PTH, and calcitriol levels are normal. Progression of kidney disease leads to increasing defects in Pi and calcium homeostasis and the development of hyperparathyroidism. Therapy for CKD-MBD should begin with agents that stimulate the skeleton to decrease FGF-23 production and vitamin D supplementation, if not calcitriol replacement. Clinical trials are needed to demonstrate that successful intervention in CKD-MBD improves clinical outcomes.
Renal Osteodystrophy
Key Points CKD-MBD is a new term for a syndrome complicating CKD that includes the skeletal and mineral disorders of CKD and their contribution to cardiovascular mortality and events. CKD-MBD begins early in stage 2 CKD as abnormalities in skeletal remodeling develop. These abnormalities in remodeling are marked by increased secretion in FGF-23. Finally, the stimulation of vascular calcification, a component of the increased cardiovascular risk of CKD-MBD, is already detectable in stage 2 CKD. In the early stages of CKD-MBD, the serum Pi, calcium, PTH, and calcitriol levels are normal. Phosphorus is a signaling molecule, which directly affects osteocyte signaling to increase FGF-23 secretion, directly stimulates vascular smooth muscle cells and osteoblasts to increase matrix mineralization and osteoblastic transcription factor gene expression, directly regulates osteoclastic bone resorption, and directly regulates proximal tubular phosphate transport and 1a-hydroxylase activity (calcitriol synthesis). The disorders of skeletal remodeling directly contribute to the level of the serum phosphorus through excess bone resorption and abnormal bone formation (decreased or increased). The skeleton further contributes to the syndrome of CKD-MBD through production of skeletal hormones, FGF-23 and osteocalcin, and the skeletal factor, sclerostin.
Acknowledgments The writing of this chapter was supported by NIH grants DK070790 and AR41677.
References 1. Olgaard K. KDIGO: Clinical Guide to Bone and Mineral Metabolism in CKD New York, NY: National Kidney Foundation; 2006. 2. Mathew S, Lund R, Strebeck F, et al. Reversal of the adynamic bone disorder and decreased vascular calcification in chronic kidney disease by sevelamer carbonate therapy. J Am Soc Nephrol. 2007;18:122-130. 3. Davies MR, Lund RJ, Mathew S, et al. Low turnover osteodystrophy and vascular calcification are amenable to skeletal anabolism in an animal model of chronic kidney disease and the metabolic syndrome. J Am Soc Nephrol. 2005;16:917-928. 4. Stevens LA, Djurdjev O, Cardew S, et al. Calcium, phosphate, and parathyroid hormone levels in combination and as a function of dialysis duration predict mortality: evidence for the complexity of the association between mineral metabolism and outcomes. J Am Soc Nephrol. 2004;15:770-779. 5. Block GA, Hulbert-Shearon TE, Levin NW, et al. Association of serum phosphorus and calcium X phosphate product with mortality risk in chronic hemodialysis patients: a national study. Am J Kidney Dis. 1998;31:607-617. 6. Lund RJ, Davies MR, Brown AJ, et al. Successful treatment of an adynamic bone disorder with bone morphogenetic protein-7 in a renal ablation model. J Am Soc Nephrol. 2004;15:359-369. 7. Malluche HH, Ritz E, Lange HP. Bone histology in incipient and advanced renal failure. Kidney Int. 1976;9:355-362.
103
8. Craver L, Marco MP, Martinez I, et al. Mineral metabolism parameters throughout chronic kidney disease stages 1-5–achievement of K/DOQI target ranges. Nephrol Dial Transplant. 2007;22:1171-1176. 9. Malluche HH, Faugere MC. Renal bone disease 1990: challenge for nephrologists. Kidney Int. 1990;38:193-211. 10. Foley RN, Parfrey PS, Sarnak MJ. Clinical epidemiology of cardiovascular disease in chronic renal disease. Am J Kidney Dis. 1998;32:S112-S119. 11. Slinin Y, Foley RN, Collins AJ. Calcium, phosphorus, parathyroid hormone, and cardiovascular disease in hemodialysis patients: the USRDS waves 1, 3, and 4 study. J Am Soc Nephrol. 2005;16:1788-1793. 12. Shlipak MG, Sarnak MJ, Katz R, et al. Cystatin C and the risk of death and cardiovascular events among elderly persons. N Engl J Med. 2005;352:2049-2060. 13. White KE, Carn G, Lorenz-Depiereux B, et al. Autosomal dominant hypophosphatemic rickets is associated with mutations in FGF23. Nat Genet. 2000;26:345-348. 14. White KE, Jonsson KB, Carn G, et al. The autosomal dominant hypophosphatemic rickets (ADHR) gene is a secreted polypeptide overexpressed by tumors that cause phosphate wasting. J Clin Endocrinol Metab. 2001;86:497-500. 15. Larsson T, Nisbeth U, Ljunggren O, et al. Circulating concentration of FGF-23 increases as renal function declines in patients with chronic kidney disease, but does not change in response to variation in phosphate intake in healthy volunteers. Kidney Int. 2003;64:2272-2279. 16. Shimada T, Muto T, Urakawa I, et al. Mutant FGF-23 responsible for autosomal dominant hypophosphatemic rickets is resistant to proteolytic cleavage and causes hypophosphatemia in vivo. Endocrinology. 2002;143:3179-3182. 17. Juppner H, Wolf M, Salusky IB. FGF-23: more than a regulator of renal phosphate handling? J Bone Miner Res. 2010;25:2091-2097. 18. Gutierrez OM, Mannstadt M, Isakova T, et al. Fibroblast growth factor 23 and mortality among patients undergoing hemodialysis. N Engl J Med. 2008;359:584-592. 19. Kurosu H, Ogawa Y, Miyoshi M, et al. Regulation of fibroblast growth factor-23 signaling by Klotho. J Biol Chem. 2006;281:6120-6123. 20. Matsumura Y, Aizawa H, Shiraki-Iida T, et al. Identification of the human klotho gene and its two transcripts encoding membrane and secreted klotho protein. Biochem Biophys Res Commun. 1998;242:626-630. 21. Kuro-o M, Matsumura Y, Aizawa H, et al. Mutation of the mouse klotho gene leads to a syndrome resembling ageing. Nature. 1997;390:45-51. 22. Tsujikawa H, Kurotaki Y, Fujimori T, et al. Klotho, a gene related to a syndrome resembling human premature aging, functions in a negative regulatory circuit of vitamin D endocrine system. Mol Endocrinol. 2003;17:2393-2403. 23. Imura A, Tsuji Y, Murata M, et al. 6Klotho as a regulator of calcium homeostasis. Science. 2007;316:1615-1618. 24. Medici D, Razzaque MS, DeLuca S, et al. FGF-23-Klotho signaling stimulates proliferation and prevents vitamin D-induced apoptosis. J Cell Biol. 2008;182:459-465. 25. Slatopolsky E, Robson AM, Elkan I, et al. Control of phosphate excretion in uremic man. J Clin Invest. 1968;47:1865-1874. 26. Kurz P, Monier-Faugere M-C, Bognar B, et al. Evidence for abnormal calcium homeostasis in patients with adynamic bone disease. Kidney Int. 1994;46:855-861. 27. Moallem E, Kilav R, Silver J, et al. RNA-protein binding and posttranscriptional regulation of parathyroid hormone gene expression by calcium and phosphate. J Biol Chem. 1998;273:5253-5259. 28. Naveh-Many T, Rahamimov R, Livni N, et al. Parathyroid cell proliferation in normal and chronic renal failure rats. J Clin Invest. 1995;96:1786-1793. 29. Jono S, McKee MD, Murry CE, et al. Phosphate regulation of vascular smooth muscle cell calcification. Circ Res. 2000;87:e10-e17. 30. Li X, Yang HY, Giachelli CM. Role of the sodium-dependent phosphate cotransporter, pit-1, in vascular smooth muscle cell calcification. Circ Res. 2006;98:905-912. 31. Mathew S, Tustison KS, Sugatani T, et al. The mechanism of phosphorus as a cardiovascular risk factor in chronic kidney disease. J Am Soc Nephrol. 2008;19:1092-1105.
104
Hruska and Mathew
32. Hruska KA, Levi M, Slatopolsky E. Disorders of phosphorus, calcium, and magnesium metabolism. In: Schrier RW, ed. Diseases of the Kidney and Urinary Tract. Chap 88. Philadelphia, PA: Lippincott Williams & Wilkins; 2005. 33. Beck GR Jr. Inorganic phosphate as a signaling molecule in osteoblast differentiation. J Cell Biochem. 2003;90:234-243. 34. Gupta A, Guo X-L, Alvarez UM, et al. Regulation of sodium-dependent phosphate transport in osteoclasts. J Clin Invest. 1997;100:538-549. 35. Hruska KA, Mathew S, Saab G. Bone morphogenetic proteins in vascular calcification. Circ Res. 2005;97:105-114. 36. Doherty TM, Asotra K, Fitzpatrick LA, et al. Calcification in atherosclerosis: bone biology and chronic inflammation at the arterial crossroads. Proc Natl Acad Sci U S A. 2004;100:11201-11206. 37. Vattikuti R, Towler DA. Osteogenic regulation of vascular calcification: an early perspective. Am J Physiol Endocrinol Metab. 2004; 286:E686-E696. 38. Bucay N, Sarosi I, Dunstan CR, et al. Osteoprotegerin-deficient mice develop early onset osteoporosis and arterial calcification. Genes Dev. 1998;12:1260-1268. 39. Jono S, Peinado C, Giachelli CM. Phosphorylation of osteopontin is required for inhibition of vascular smooth muscle cell calcification. J Biol Chem. 2000;275:20197-20203. 40. Demer LL, Tintut Y. Osteopontin: between a rock and a hard plaque. Circ Res. 1999;84:250-252. 41. Luo G, Ducy P, McKee MD, et al. Spontaneous calcification of arteries and cartilage in mice lacking matrix GLA protein. Nature. 1997;386:78-81. 42. Kawaguchi H, Manabe N, Miyaura C, Chikuda H, Nakamura K, Kuro-o M. Independent impairment of osteoblast and osteoclast differentiation in klotho mouse exhibiting low-turnover osteopenia. J Clin Invest. 1999;104:229-237. 43. Galvin KM, Donovan MJ, Lynch CA, et al. A role for Smad6 in development and homeostasis of the cardiovascular system. Nat Genet. 2000;24:171-174. 44. Block GA, Port FK. Re-evaluation of risks associated with hyperphosphatemia and hyperparathyroidism in dialysis patients: recommendations for a change in management. Am J Kidney Dis. 2000;35:1226-1237. 45. Demer L. Lipid hypothesis of cardiovascular calcification. Circulation. 1997;95:297-298. 46. Tintut Y, Parhami F, Bostr€ om K, et al. cAMP stimulates osteoblastlike differentiation of calcifying vascular cells. J Biol Chem. 1998;273:7547-7553. 47. Tintut Y, Parhami F, Le V, et al. Inhibition of osteoblast-specific transcription factor Cbfal by the cAMP pathway in osteoblastic cells. Ubiquitin/proteasome-dependent regulation. J Biol Chem. 1999; 274:28875-28879. 48. Bostr€ om K, Watson KE, Stanford WP, et al. Atherosclerotic calcification: relation to developmental osteogenesis. Am J Cardiol. 1995; 75:88B-91B. 49. Morishita K, Shirai A, Kubota M, et al. The progression of aging in klotho mutant mice can be modified by dietary phosphorus and zinc. J Nutr. 2001;131:3182-3188. 50. Steitz SA, Speer MY, Curinga G, et al. Smooth muscle cell phenotypic transition associated with calcification. Circ Res. 2001; 89:1147-1154. 51. Chen NX, O’Neill KD, Duan D, et al. Phosphorus and uremic serum up-regulate osteopontin expression in vascular smooth muscle cells. Kidney Int. 2002;62:1724-1731. 52. Blacher J, Guerin AP, Pannier B, et al. Arterial calcifications, arterial stiffness, and cardiovascular risk in end-stage renal disease. Hypertension. 2001;38:938-942. 53. Chertow GM, Burke SK, Raggi P. Sevelamer attenuates the progression of coronary and aortic calcification in hemodialysis patients. Kidney Int. 2002;62:245-252. 54. Cozzolino M, Dusso AS, Slatopolsky E. Role of calcium-phosphate product and bone-associated proteins on vascular calcification in renal failure. J Am Soc Nephrol. 2001;12:2511-2516.
55. Fatica RA, Dennis VW. Cardiovascular mortality in chronic renal failure: hyperphosphatemia, coronary calcification, and the role of phosphate binders. Cleve Clin J Med. 2002;69:S21-S27. 56. Iribarren C, Sidney S, Sternfeld B, et al. Calcification of the aortic arch: risk factors and association with coronary heart disease, stroke, and peripheral vascular disease. JAMA. 2000;283:2810-2815. 57. Newman AB, Naydeck BL, Sutton-Tyrrell K, et al. Relationship between coronary artery calcification and other measures of subclinical cardiovascular disease in older adults. Arterioscler Thromb Vasc Biol. 2002;22:1674-1679. 58. Raggi P, Boulay A, Chasan-Taber S, et al. Cardiac calcification in adult hemodialysis patients. A link between end-stage renal disease and cardiovascular disease? J Am Coll Cardiol. 2002;39:695-701. 59. Reslerova M, Moe SM. Vascular calcification in dialysis patients: pathogenesis and consequences. Am J Kidney Dis. 2003;41:S96-S99. 60. Shao JS, Cai J, Towler DA. Molecular mechanisms of vascular calcification: lessons learned from the aorta. Arterioscler Thromb Vasc Biol. 2006;26:1423-1430. 61. Ahmed S, O’Neill KD, Hood AF, et al. Calciphylaxis is associated with hyperphosphatemia and increased osteopontin expression by vascular smooth muscle cells. Am J Kidney Dis. 2001;37:1267-1276. 62. Block GA. Control of serum phosphorus: implications for coronary artery calcification and calcific uremic arteriolopathy (calciphylaxis). Curr Opin Nephrol Hypertens. 2001;10:741-747. 63. Wilmer WA, Magro CM. Calciphylaxis: emerging concepts in prevention, diagnosis, and treatment. Semin Dial. 2002;15:172-186. 64. Slatopolsky E, Finch J, Denda M, et al. Phosphorus restriction prevents parathyroid gland growth. High phosphorus directly stimulates PTH secretion in vitro. J Clin Invest. 1996;97:2534-2540. 65. Cozzolino M, Lu Y, Sato T, et al. A critical role for enhanced-TGFalpha and EGFR expression in the initiation of parathyroid hyperplasia in experimental kidney disease. Am J Physiol Renal Physiol. 2005;289:F1096-F1102. 66. Martin DR, Ritter CS, Slatopolsky E, et al. Acute regulation of parathyroid hormone by dietary phosphate. Endocrinol Metab. 2005; 289:E729-E734. 67. Lund RJ, Huq N, Davies MR, et al. Efficacy of BMP-7 treatment of adynamic renal osteodystrophy. J Am Soc Nephrol. 2002;13:578A. 68. Bucay N, Sarosi I, Dunstan CR, et al. Osteoprotegerin-deficient mice develop early onset osteoporosis and arterial calcification. Genes Dev. 1998;12:1260-1268. 69. Hak AE, Pols HAP, van Hemert AM, et al. Progression of aortic calcification is associated with metacarpal bone loss during menopause a population-based longitudinal study. Arterioscler Thromb Vasc Biol. 2000;20:1926-1931. 70. Danen EHJ, Sonneveld P, Brakebusch C, et al. The fibronectinbinding integrins a5b1 and avb3 differentially modulate RhoAGTP loading, organization of cell matrix adhesions, and fibronectin fibrillogenesis. J Cell Biol. 2002;159:1071-1086. 71. Price PA, June HH, Buckley JR, et al. Osteoprotegerin inhibits artery calcification induced by Warfarin and by vitamin D. Arterioscler Thromb Vasc Biol. 2001;21:1610-1616. 72. Goodman WG, Quarles LD. Development and progression of secondary hyperparathyroidism in chronic kidney disease: lessons from molecular genetics. Kidney Int. 2007;74:276-288. 73. Naveh-Many T, Marx R, Keshet E, et al. Regulation of 1,25-dihydroxyvitamin D3 receptor gene expression by 1,25-dihydroxyvitamin D3 in the parathyroid in vivo. J Clin Invest. 1990;86:1968-1975. 74. Silver J, Russell J, Sherwood LM. Regulation by vitamin D metabolites of messenger ribonucleic acid for preproparathyroid hormone in isolated bovine parathyroid cells. Proc Natl Acad Sci U S A. 1985;82:4270-4273. 75. Brown EM, Gamba G, Riccardi D, et al. Cloning and characterization of an extracellular Ca21-sensing receptor from bovine parathyroid. Nature. 1993;366:575-580. 76. Brown EM, Hebert SC. A cloned Ca21 sensing receptor: a mediator of direct effects of extracellular Ca21 on renal function? J Am Soc Nephrol. 1995;6:1530-1540.