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Tight regulation of calcium levels is required for many critical biological functions. The Ca2+-sensing receptor (CaSR) expressed by parathyroid cells controls blood calcium concentration by regulating parathyroid hormone (PTH) secretion. However, CaSR is also expressed in other organs, such as the kidney, but the importance of extraparathyroid CaSR in calcium metabolism remains unknown. Here, we investigated the role of extraparathyroid CaSR using thyroparathyroidectomized, PTH-supplemented rats. Chronic inhibition of CaSR selectively increased renal tubular calcium absorption and blood calcium concentration independent of PTH secretion change and without altering intestinal calcium absorption. CaSR inhibition increased blood calcium concentration in animals pretreated with a bisphosphonate, indicating that the increase did not result from release of bone calcium. Kidney CaSR was expressed primarily in the thick ascending limb of the loop of Henle (TAL). As measured by in vitro microperfusion of cortical TAL, CaSR inhibitors increased calcium reabsorption and paracellular pathway permeability but did not change NaCl reabsorption. We conclude that CaSR is a direct determinant of blood calcium concentration, independent of PTH, and modulates renal tubular calcium transport in the TAL via the permeability of the paracellular pathway. These findings suggest that CaSR inhibitors may provide a new specific treatment for disorders related to impaired PTH secretion, such as primary hypoparathyroidism.

α-klotho was identified as a gene associated with premature aging-like phenotypes characterized by short lifespan. In mice, we found the molecular association of α-Klotho (α-Kl) and Na⁺,K⁺-adenosine triphosphatase (Na⁺,K⁺-ATPase) and provide evidence for an increase of abundance of Na⁺,K⁺-ATPase at the plasma membrane. Low concentrations of extracellular free calcium ([Ca²⁺]e) rapidly induce regulated parathyroid hormone (PTH) secretion in an α-Kl- and Na⁺,K⁺-ATPase-dependent manner. The increased Na⁺ gradient created by Na⁺,K⁺-ATPase activity might drive the transepithelial transport of Ca²⁺ in cooperation with ion channels and transporters in the choroid plexus and the kidney. Our findings reveal fundamental roles of α-Kl in the regulation of calcium metabolism.

The identification of the calcium-sensing receptor (CaSR) and the clarification of its role as the major regulator of parathyroid gland function have important implications for understanding the pathogenesis and evolution of secondary hyperthyroidism in chronic kidney disease (CKD). Signaling through the CaSR has direct effects on three discrete components of parathyroid gland function, which include parathyroid hormone (PTH) secretion, PTH synthesis, and parathyroid gland hyperplasia. Disturbances in calcium and vitamin D metabolism that arise owing to CKD diminish the level of activation of the CaSR, leading to increases in PTH secretion, PTH synthesis, and parathyroid gland hyperplasia. Each represents a physiological adaptive response by the parathyroid glands to maintain plasma calcium homeostasis. Studies of genetically modified mice indicate that signal transduction via the CaSR is a key determinant of parathyroid cell proliferation and parathyroid gland hyperplasia. Because enlargement of the parathyroid glands has important implications for disease progression and disease severity, it is possible that clinical management strategies that maintain adequate calcium-dependent signaling through the CaSR will ultimately prove useful in diminishing parathyroid gland hyperplasia and in modifying disease progression.

Autosomal dominant familial isolated hypoparathyroidism (AD-FIH) is caused by a Cys [rightward arrow] Arg mutation (C18R) in the hydrophobic core of the signal peptide of human preproparathyroid hormone (PPTH). Although this mutation impairs secretion of the hormone, the mechanism by which one mutant allele produces the autosomal-dominant disease is unexplained. Using transfected HEK293 cells, we demonstrate that the expressed mutant hormone is trapped intracellularly, predominantly in the endoplasmic reticulum (ER). This ER retention was found to be toxic for the cells, which underwent apoptosis, as evident from the marked increase in the number of cells staining positive for Annexin V binding and for the TUNEL reaction. The cells producing mutant hormone also had marked up-regulation of the ER stress-responsive proteins, BiP and PERK, as well as the proapoptotic transcription factor, CHOP. Up-regulation of these markers of the unfolded protein response supported a causal link between the ER stress and the cell death cascade. When the C18R PPTH was expressed in the presence of 4-phenylbutyric acid, which is a pharmacological chaperone, intracellular accumulation was reduced and normal secretion was restored. This treatment also produced remarkable reduction of ER stress signals and protection against cell death. These data implicate ER stress-induced cell death as the underlying mechanism for AD-FIH and suggest that the pharmacological manipulation of this pathway by using chemical chaperones offers a therapeutic option for treating this disease.

Structure of non-(1-84) PTH fragments secreted by parathyroid glands in primary and secondary hyperparathyroidism. Non-(1-84) parathyroid hormone (PTH) fragments are large circulating carboxyl-terminal (C) fragments with a partially preserved amino-terminal (N) structure. hPTH (7-84), a synthetic surrogate, has been demonstrated to exert biologic effects in vivo and in vitro which are opposite to those of hPTH (1-34) on the PTH/PTHrP type I receptor through a C-PTH receptor. We wanted to determine the N structure of non-(1-84) PTH fragments. Parathyroid cells isolated from glands obtained at surgery from three patients with primary hyperparathyroidism and three patients with secondary hyperparathyroidism were incubated with 35S-methionine to internally label their secretion products. Incubations were performed for 8 hours at the patient-ionized calcium concentration and in the presence of various protease inhibitors. The supernatant was fractionated by high-performance liquid chromatography (HPLC) and fractions were analyzed with PTH assays having (1 to 4) and (12 to 23) epitopes, respectively. The serum of each patient was similarly analyzed. Peaks of immunoreactivity identified were submitted to sequence analysis to recover the 35S-methionine residues in positions 8 and 18. Three regions of interest were identified with PTH assays. They corresponded to non-(1-84) PTH fragments (further divided in regions 3 and 4), a peak of N-PTH migrating in front of hPTH (1-84) (region 2) and a peak of immunoreactivity corresponding to the elution position of hPTH (1-84) (region 1). The last corresponded to a single sequence starting at position 1. Region 2 gave similar results in all cases (a major signal starting at position 1) but also sometimes minor sequences starting at position 4 or 7. Regions 3 and 4 always identified a major sequence starting at positions 7 and minor sequences starting at positions 8, 10, and 15. Surprisingly, a major signal starting at position 1 was also present in region 3. The HPLC profile obtained from a given patient's parathyroid cells was qualitatively similar to the one obtained with his/her serum in each case. These results indicate that non-(1-84) PTH fragments are composed of a family of fragments which may be generated by specific or progressive cleavage at the N region. The longest fragment starts at position 4 and the shortest at position 15. A peptide starting at position 7 appears as the major component of non-(1-84) PTH fragments. The generation process is similar to the one described for smaller C-PTH fragments a number of years ago, suggesting a similar production mechanism and source for all C-PTH fragments.

Apart from its function as co-receptor for fibroblast growth factor-23 (FGF23), Klotho is thought to regulate insulin signaling, intracellular oxidative stress, and parathyroid hormone (PTH) secretion in an FGF23 independent fashion. Here, we crossed Klotho deficient (Kl⁻/⁻) mice with vitamin D receptor (VDR) mutant mice to examine further vitamin D independent functions of Klotho. All mice were fed a rescue diet enriched with calcium, phosphorus, and lactose to prevent hyperparathyroidism in VDR mutants, and were killed at 4 weeks of age after double fluorochrome labeling. Kl⁻/⁻ mice displayed hypercalcemia, hyperphosphatemia, dwarfism, organ atrophy, azotemia, pulmonary emphysema, and osteomalacia. In addition, glucose and insulin tolerance tests revealed hypoglycemia and profoundly increased peripheral insulin sensitivity in Kl⁻/⁻ mice. Compound mutants were normocalcemic and normophosphatemic, did not show premature aging or organ atrophy, and were phenocopies of VDR mutant mice in terms of body weight, bone mineral density, bone metabolism, serum calcium, serum phosphate, serum PTH, gene expression in parathyroid glands, as well as urinary calcium and phosphate excretion. Furthermore, ablation of vitamin D signaling in double mutants completely normalized glucose and insulin tolerance, indicating that Klotho has no vitamin D independent effects on insulin signaling. Histomorphometry of pancreas islets showed similar beta cell volume per body weight in all groups of animals. In conclusion, our findings cast doubt on a physiologically relevant vitamin D and Fgf23 independent function of Klotho in the regulation of glucose metabolism, bone turnover, and steady-state PTH secretion in vivo.

Summary In this study, a strategy of the construction of leaky strains for the extracellular production of target proteins was exploited, in which the genes mrcA, mrcB, pal and lpp (as a control) from Escherichia coli were knocked out by using single‐ and/or double‐gene deletion methods. Then the recombinant strains for the expression of exogenous target proteins including Trx‐hPTH (human parathyroid hormone 1–84 coupled with thioredoxin as a fusion partner) and reteplase were reconstructed to test the secretory efficiency of the leaky strains. Finally, the fermentation experiments of the target proteins from these recombinant leaky strains were carried out in basic media (Modified R media) and complex media (Terrific Broth media) in flasks or fermenters. The results demonstrated that the resultant leaky strains were genetically stable and had a similar growth profile in the complex media as compared with the original strain, and the secretory levels of target proteins into Modified R media from the strains with double‐gene deletion (up to 88.9%/mrcA lpp‐pth) are higher than the excretory levels from the strains with single‐gene deletion (up to 71.1%/lpp‐pth) and the host E. coli JM109 (DE3) (near zero). The highest level of extracellular production of Trx‐hPTH in fermenters is up to 680 mg l−1. Nine genetically stable leaky mutants were obtained. The leaky levels of Trx‐hPTH in TB medium are higher than those in MR medium.The highest level of extracellular production of Trx‐hPTH in fermenters is up to 680 mg/l

Oxyphil cell parathyroid adenomas (OPA) are considered to be an uncommon cause of primary hyperparathyroidism (PHPT), and were historically thought to be clinically silent. It has been our clinical impression that these adenomas present more often than previously thought and may manifest a more severe form of primary hyperparathyroidism than classical adenoma. The aim of this study was to describe the incidence and clinical presentation of OPA. An observational case-control study was undertaken. The study group comprised patients undergoing parathyroidectomy for PHPT where the final pathology confirmed OPA. The controls were made up of an age- and sex-matched group of patients having parathyroidectomy in the same time period where the final pathology confirmed a classical or non-oxyphil adenoma. OPA were defined as parathyroid tumours containing >75 % oxyphilic cells. The OPA cases were obtained by reviewing all histopathology slides over an 11-year period (2002–12) where the reports contained the words ‘oxyphil’ or ‘oxyphilic’ parathyroid adenomas. These were then reviewed by two independent pathologists to confirm a diagnosis of OPA. The primary outcome measures were preoperative serum calcium and parathyroid hormone (PTH) levels. Secondary outcome measures were symptoms at presentation, accuracy of preoperative localization studies, parathyroid gland weight following surgery, and type of surgery undertaken. In the period 2002–2012, 2739 patients underwent surgery for PHPT. Following pathological review, 91 cases were confirmed as being OPA and formed the study group. A control group ( n  = 91) from the same period was selected following matching on the basis of age at presentation and sex. OPA were associated with higher preoperative serum calcium (10.84 versus 10.48 mg/dL, p  < 0.001) and parathyroid hormone (139 versus 64 ng/L, p  < 0.001). At presentation, a lower proportion of OPA cases had asymptomatic disease (15 versus 29 %, p  = 0.03). There was a trend toward a higher rate of renal calculi at presentation in the OPA group (9 versus 3 %, p  = 0.07). Preoperative ultrasound was less accurate in localization of OPA when compared with classical adenoma. The rate of minimally invasive surgery was 67 % for OPA and 78 % for the control group ( p  = 0.06). All patients were cured of hypercalcaemia at 6-month follow up. There was no significant difference in the weight of removed parathyroid tissue between the groups (868 mg for OPA versus 789 mg for the control group, p  = 0.6). OPA are frequently symptomatic and are associated with higher preoperative serum calcium and parathyroid hormone levels than classical types of parathyroid adenomas. OPA are less likely to be localised on preoperative ultrasound examination.

Fibroblast growth factor 23 (FGF23) is a phosphatonin that is secreted by osteocytes and osteoblasts in response to hyperphosphatemia and 1,25-dihydroxyvitamin D (1,25D). It acts on its receptor complex, Klotho–FGFR1c (fibroblast growth factor receptor 1 c-splicing form), in the distal convoluted tubule to repress renal phosphorus reabsorption in the proximal tubule and suppress the renal synthesis of 1,25D. Klotho–FGFR1c is also expressed in the parathyroid glands. FGF23 acts on the receptor complex in the parathyroid glands to decrease parathyroid hormone (PTH) gene expression and PTH secretion through activation of the MAPK pathway. In chronic kidney disease (CKD), both FGF23 and PTH are increased, implying resistance of the parathyroid glands to FGF23. There is a decrease in the Klotho–FGFR1c complex in the parathyroid glands in both experimental CKD and in patients with end-stage renal disease. In addition, in advanced experimental CKD, FGF23 has a decreased ability to inhibit PTH expression.

The present study was performed to investigate the association of calcium-sensing receptor (CaSR) genotypes with parathyroid hormone (PTH) secretion in hemodialysis patients. Subjects were 122 Japanese hemodialysis patients, including 39 patients with diabetes mellitus (DM). The CaSR polymorphisms tested were codon 990 in intracellular domain (A/A, A/G, and G/G groups) as well as T to C base change of intron 4 (T/T, T/C, and C/C groups). Statistical analysis of these polymorphisms revealed that the serum PTH level was significantly higher in the A/A group than in the G/G group in the former. In addition, the serum PTH level was also significantly higher in patients displaying C allele, as compared with the T/T group in the latter. This association of two polymorphisms with the serum PTH level was observed only in non-DM patients. Although two polymorphisms affected the PTH level independently, patients who possessed both genotypes (AAC+) had a markedly high level of PTH not only in the non-DM group but also in the DM group. The present findings indicate the possibility of the prediction for the extensive progression of secondary hyperparathyroidism through analyzing the CaSR polymorphisms in chronic hemodialysis patients.