The role of the calcium-sensing receptor in the pathophysiology of secondary hyperparathyroidism

Calcium-sensing receptor-mediated macrophage polarization improves myocardial remodeling in spontaneously hypertensive rats

Chronic inflammation is a key element in the progression of essential hypertension (EH). Calcium plays a key role in inflammation, so its receptor, the calcium-sensing receptor (CaSR), is an essential mediator of the inflammatory process. Compelling evidence suggests that CaSR mediates inflammation in tissues and immune cells, where it mediates their activity and chemotaxis. Macrophages (Mφs) play a major role in the inflammatory response process. This study provided convincing evidence that R568, a positive regulator of CaSR, was effective in lowering blood pressure in spontaneously hypertensive rats (SHRs), improving cardiac function by alleviating cardiac hypertrophy and fibrosis. R568 can increase the content of CaSR and M2 macrophages (M2Mφs, exert an anti-inflammatory effect) in myocardial tissue, reduce M1 macrophages (M1Mφs), which have a pro-inflammatory effect in this process. In contrast, NPS2143, a negative state regulator of CaSR, exerted the opposite effect in all of the above experiments. Following this study, R568 increased CaSR content in SHR myocardial tissue, lowered blood pressure, promoted macrophages to M2Mφs and improved myocardial fibrosis, but interestingly, both M1Mφs and M2Mφs were increased in the peritoneal cavity of SHRs, the number of M2Mφs remained lower than M1Mφs. In vitro, R568 increased CaSR content in RAW264.7 cells (a macrophage cell line), regulating intracellular Ca2+ ([Ca2+]i) inhibited NOD-like receptor family protein 3 (NLRP3) inflammasome activation and ultimately prevented its conversion to M1Mφs. The results showed that a decrease in CaSR in hypertensive rats causes further development of hypertension and cardiac damage. EH myocardial remodeling can be improved by CaSR overexpression by suppressing NLRP3 inflammasome activation and macrophage polarization toward M1Mφs and increasing M2Mφs.

Mechanism of calcitriol regulating parathyroid cells in secondary hyperparathyroidism

A common consequence of chronic renal disease is secondary hyperparathyroidism (SHPT) and is closely related to the mortality and morbidity of uremia patients. Secondary hyperparathyroidism (SHPT) is caused by excessive PTH production and release, as well as parathyroid enlargement. At present, the mechanism of cell proliferation in secondary hyperparathyroidism (SHPT) is not completely clear. Decreased expression of the vitamin D receptor (VDR) and calcium-sensing receptor (CaSR), and 1,25(OH)2D3 insufficiency all lead to a decrease in cell proliferation suppression, and activation of multiple pathways is also involved in cell proliferation in renal hyperparathyroidism. The interaction between the parathormone (PTH) and parathyroid hyperplasia and 1,25(OH)2D3 has received considerable attention. 1,25(OH)2D3 is commonly applied in the therapy of renal hyperparathyroidism. It regulates the production of parathormone (PTH) and parathyroid cell proliferation through transcription and post-transcription mechanisms. This article reviews the role of 1,25(OH)2D3 in parathyroid cells in secondary hyperparathyroidism and its current understanding and potential molecular mechanism.

Asymmetric activation of the calcium-sensing receptor homodimer

The calcium-sensing receptor (CaSR), a cell-surface sensor for Ca²⁺, is the master regulator of calcium homeostasis in humans and is the target of calcimimetic drugs for the treatment of parathyroid disorders¹. CaSR is a family C G-protein-coupled receptor² that functions as an obligate homodimer, with each protomer composed of a Ca²⁺-binding extracellular domain and a seven-transmembrane-helix domain (7TM) that activates heterotrimeric G proteins. Here we present cryo-electron microscopy structures of near-full-length human CaSR in inactive or active states bound to Ca²⁺ and various calcilytic or calcimimetic drug molecules. We show that, upon activation, the CaSR homodimer adopts an asymmetric 7TM configuration that primes one protomer for G-protein coupling. This asymmetry is stabilized by 7TM-targeting calcimimetic drugs adopting distinctly different poses in the two protomers, whereas the binding of a calcilytic drug locks CaSR 7TMs in an inactive symmetric configuration. These results provide a detailed structural framework for CaSR activation and the rational design of therapeutics targeting this receptor.

Intraoperative verification of parathyroid glands in primary and secondary hyperparathyroidism using near-infrared autofluorescence (IOPA)

Intraoperative verification of parathyroid glands relies on visual identification by the surgeon and, with some time delay, on serum parathormon measurements and frozen section. Fluorescence imaging, however, is an instant on-table method for direct visualization of parathyroid tissue which is known to exhibit increased autofluorescence intensity when exposed to near-infrared light. In this retrospective observational study, we evaluate the clinical use of this method in a series of patients with primary and secondary hyperparathyroidism. A total of 66 adenomatous and hyperplastic parathyroid glands were examined with intraoperative autofluorescence in 39 patients with primary and secondary hyperparathyroidism using a near-infrared system (KARL STORZ GmbH & Co. KG). The specimens were verified by conventional histology. Fifty-seven of 66 histologically proven adenomatous/hyperplastic glands exhibited autofluorescence. The sensitivity of near-infrared autofluorescence was 0.9 in pHPT and 0.83 in sHPT, respectively. The positive predictive value was 0.93 in pHPT and 1.0 in sHPT, respectively. Near-infrared autofluorescence guidance presents an innovative instant surgical imaging tool with sensitivity in detecting adenomatous and hyperplastic parathyroid glands comparable to current intraoperative methods. Due to its elegant and tracer-free design combined with low follow-up costs, this method can be useful for routine use.

The Emerging Role of Nutritional Vitamin D in Secondary Hyperparathyroidism in CKD

In chronic kidney disease (CKD), hyperphosphatemia induces fibroblast growth factor-23 (FGF-23) expression that disturbs renal 1,25-dihydroxy vitamin D (1,25D) synthesis; thereby increasing parathyroid hormone (PTH) production. FGF-23 acts on the parathyroid gland (PTG) to increase 1α-hydroxylase activity and results in increase intra-gland 1,25D production that attenuates PTH secretion efficiently if sufficient 25D are available. Interesting, calcimimetics can further increase PTG 1α-hydroxylase activity that emphasizes the demand for nutritional vitamin D (NVD) under high PTH status. In addition, the changes in hydroxylase enzyme activity highlight the greater parathyroid 25-hydroxyvitmain D (25D) requirement in secondary hyperparathyroidism (SHPT); the higher proportion of oxyphil cells as hyperplastic parathyroid progression; lower cytosolic vitamin D binding protein (DBP) content in the oxyphil cell; and calcitriol promote vitamin D degradation are all possible reasons supports nutritional vitamin D (NVD; e.g., Cholecalciferol) supplement is crucial in SHPT. Clinically, NVD can effectively restore serum 25D concentration and prevent the further increase in PTH level. Therefore, NVD might have the benefit of alleviating the development of SHPT in early CKD and further lowering PTH in moderate to severe SHPT in dialysis patients.

Histology and immunohistochemistry of the parathyroid glands in renal secondary hyperparathyroidism refractory to vitamin D or cinacalcet therapy

BACKGROUND

Cinacalcet is a new effective treatment of secondary hyperparathyroidism (SHPT) in hemodialysis patients (HP), but the alterations of parathyroid gland (PTG) hyperplasia determined by cinacalcet and vitamin D have not been extensively investigated in humans.

METHODS

We performed histological analyses of 94 PTGs removed from 25 HP who underwent parathyroidectomy (PTx) because of SHPT refractory to therapy with vitamin D alone (group A=13 HP and 46 PTGs) or associated with cinacalcet (group B=12 HP and 48 PTGs). The number, weight, the macroscopic cystic/hemorrhagic changes, and type of hyperplasia of PTG (nodular=NH, diffuse=DH) were assessed. In randomly selected HP of group A (4 HP and 14 PTGs) and group B (4 HP and 15 PTGs), the labeling index of cells positive to Ki-67 and TUNEL and the semiquantitative score of immunohistochemistry staining of vitamin D receptor, calcium-sensing receptor, and vascular endothelial growth factor-α (VEGF-α) were measured in the entire PTGs and in the areas with DH or NH.

RESULTS

The number and weight of single and total PTG of each HP were similar in the two groups as well as the number of PTG with macroscopic cystic/hemorrhagic areas. TUNEL, Ki-67, and VEGF-α scores were higher in NH than in DH areas.

CONCLUSION

This observational study of a highly selected population of HP, submitted to PTx because SHPT refractory to therapy, shows that the macroscopic, microscopic, and immunochemistry characteristics of PTG in HP who received or did not receive cinacalcet before PTx did not differ significantly.

The extracellular calcium-sensing receptor, CaSR, in fetal development

In fetal mammals, serum levels of both total and ionized calcium significantly exceed those in the adult. This relative fetal hypercalcemia is crucial for skeletal development and is maintained irrespectively of maternal serum calcium levels. Elegant studies by Kovacs and Kronenberg have previously addressed the role of the CaSR in creating and maintaining this relative fetal hypercalcemia, through the regulation of parathyroid hormone-related peptide secretion. More recently we have shown that the CaSR is widely distributed throughout the developing fetus, where the receptor plays major, unexpected roles in ensuring growth and maturation of several organs. In this article, we present evidence for a role of the CaSR in the control of skeletal development, and how fetal hypercalcemia, acting through the CaSR, regulates lung development.

Regulation of mouse lung development by the extracellular calcium-sensing receptor, CaR

Postnatal lung function is critically dependent upon optimal embryonic lung development. As the free ionized plasma calcium concentration ([Ca(2+)](o)) of the fetus is higher than that of the adult, the process of lung development occurs in a hypercalcaemic environment. In the adult, [Ca(2+)](o) is monitored by the G-protein coupled, extracellular calcium-sensing receptor (CaR), but neither its ontogeny nor its potential role in lung development are known. Here, we demonstrate that CaR is expressed in the mouse lung epithelium, and that its expression is developmentally regulated, with a peak of expression at embryonic day 12.5 (E12.5) and a subsequent decrease by E18, after which the receptor is absent. Experiments carried out using the lung explant culture model in vitro show that lung branching morphogenesis is sensitive to [Ca(2+)](o), being maximal at physiological adult [Ca(2+)](o) (i.e. 1.0-1.3 mM) and lowest at the higher, fetal (i.e. 1.7 mM) [Ca(2+)](o). Administration of the specific CaR positive allosteric modulator, the calcimimetic R-568, mimics the suppressive effects of high [Ca(2+)](o) on branching morphogenesis while both phospholipase C and PI3 kinase inhibition reverse these effects. CaR activation suppresses cell proliferation while it enhances intracellular calcium signalling, lung distension and fluid secretion. Conditions which are restrictive either to branching or to secretion can be rescued by manipulating [Ca(2+)](o) in the culture medium. In conclusion, fetal Ca(2+)(o), acting through a developmentally regulated CaR, is an important extrinsic factor that modulates the intrinsic lung developmental programme. Our observations support a novel role for the CaR in preventing hyperplastic lung disease in utero.