Hyperparathyroidism is an endocrine disorder linked to vitamin D deficiency. Reduced vitamin D receptor (VDR) activity promotes parathyroid hormone (PTH) hypersecretion by increasing heterodimerization of the type B γ-aminobutyric acid receptor 1 (GABAB1R) with the extracellular Ca2+-sensing receptor (CaSR) in parathyroid cells; however, endogenous activators of the heterodimers are unknown. We uncovered increased expression of amyloid-β peptide cleaved from the amyloid-β precursor protein (APP) in parathyroid cells of patients with hyperparathyroidism and aging mice manifesting vitamin D deficiency and the ability of exogenous amyloid-β to promote tonic PTH secretion from cultured murine or human parathyroid glands. Conversely, parathyroid cell-specific App gene deletion reduced tonic PTH secretion and lowered serum PTH concentrations in mice. The absence of the amyloid-β effect on PTH secretion in parathyroid cells lacking CaSR or GABAB1R supported direct interactions of amyloid-β with the receptor heterodimers. In situ proteomic profiling of parathyroid cells of patients with hyperparathyroidism correlated lower serum 25-hydroxyvitamin D concentrations with increased GABAB1R/CaSR heterodimer expression, β-amyloidogenesis, and phosphorylation of Tau, a downstream effector of amyloid-β. Concurrent ablation of App or the Tau-encoding Mapt gene prevented tonic PTH hypersecretion in parathyroid cell-specific Vdr knockout mice. Likewise, weekly administration of an amyloid-β-neutralizing antibody suppressed tonic PTH hypersecretion and synergized with daily administration of cinacalcet, which activates CaSR homodimers, to reduce serum PTH concentrations in aged mice. These data demonstrated amyloid-β actions in driving tonic PTH secretion by activating GABAB1R/CaSR heterodimers and the potential of targeting amyloid-β to treat hyperparathyroidism due to vitamin D deficiency.

BACKGROUND:Impaired mineral ion metabolism is a hallmark of CKD-metabolic bone disorder. It can lead to pathologic vascular calcification and is associated with an increased risk of cardiovascular mortality. Loss of calcium-sensing receptor (CaSR) expression in vascular smooth muscle cells exacerbates vascular calcification in vitro. Conversely, vascular calcification can be reduced by calcimimetics, which function as allosteric activators of CaSR. METHODS:To determine the role of the CaSR in vascular calcification, we characterized mice with targeted Casr gene knockout in vascular smooth muscle cells ( SM22α CaSR Δflox/Δflox ). RESULTS:Vascular smooth muscle cells cultured from the knockout (KO) mice calcified more readily than those from control (wild-type) mice in vitro. However, mice did not show ectopic calcifications in vivo but they did display a profound mineral ion imbalance. Specifically, KO mice exhibited hypercalcemia, hypercalciuria, hyperphosphaturia, and osteopenia, with elevated circulating fibroblast growth factor 23 (FGF23), calcitriol (1,25-D3), and parathyroid hormone levels. Renal tubular α-Klotho protein expression was increased in KO mice but vascular α-Klotho protein expression was not. Altered CaSR expression in the kidney or the parathyroid glands could not account for the observed phenotype of the KO mice. CONCLUSIONS:These results suggest that, in addition to CaSR's established role in the parathyroid-kidney-bone axis, expression of CaSR in vascular smooth muscle cells directly contributes to total body mineral ion homeostasis.

In brief: A specific anion binding site senses moderate changes of inorganic phosphate levels leading to rapid inhibition of CaSR responsiveness and PTH secretion in a murine and human in vitro model system.

Molecular mechanisms mediating tonic secretion of parathyroid hormone (PTH) in response to hypocalcaemia and hyperparathyroidism (HPT) are unclear. Here we demonstrate increased heterocomplex formation between the calcium-sensing receptor (CaSR) and metabotropic γ-aminobutyric acid (GABA) B1 receptor (GABAB1R) in hyperplastic parathyroid glands (PTGs) of patients with primary and secondary HPT. Targeted ablation of GABAB1R or glutamic acid decarboxylase 1 and 2 in PTGs produces hypocalcaemia and hypoparathyroidism, and prevents PTH hypersecretion in PTGs cultured from mouse models of hereditary HPT and dietary calcium-deficiency. Cobinding of the CaSR/GABAB1R complex by baclofen and high extracellular calcium blocks the coupling of heterotrimeric G-proteins to homomeric CaSRs in cultured cells and promotes PTH secretion in cultured mouse PTGs. These results combined with the ability of PTG to synthesize GABA support a critical autocrine action of GABA/GABAB1R in mediating tonic PTH secretion of PTGs and ascribe aberrant activities of CaSR/GABAB1R heteromer to HPT.

Extracellular phosphate regulates its own renal excretion by eliciting concentration-dependent secretion of parathyroid hormone (PTH). However, the phosphate-sensing mechanism remains unknown and requires elucidation for understanding the aetiology of secondary hyperparathyroidism in chronic kidney disease (CKD). The calcium-sensing receptor (CaSR) is the main controller of PTH secretion and here we show that raising phosphate concentration within the pathophysiologic range for CKD significantly inhibits CaSR activity via non-competitive antagonism. Mutation of residue R62 in anion binding site-1 abolishes phosphate-induced inhibition of CaSR. Further, pathophysiologic phosphate concentrations elicit rapid and reversible increases in PTH secretion from freshly-isolated human parathyroid cells consistent with a receptor-mediated action. The same effect is seen in wild-type murine parathyroid glands, but not in CaSR knockout glands. By sensing moderate changes in extracellular phosphate concentration, the CaSR represents a phosphate sensor in the parathyroid gland, explaining the stimulatory effect of phosphate on PTH secretion.

The complex vertebrate skeleton depends on regulated cell activities to lay down protein matrix and mineral components of bone. As a distinctive vertebrate characteristic, bone is a storage site for physiologically-important calcium ion. The extracellular calcium-sensing receptor (CaSR) is linked to homeostatic regulation of calcium through its expression in endocrine glands that secrete calcium homeostatic hormones, in Ca2+- and ion-transporting epithelia, and in skeleton. Since CaSR is restricted in its presence to the chordate–vertebrate evolutionary lineage, we propose there to be important functional ties between CaSRs and vertebrate skeleton in the context of that group's characteristic form of calcium-mineralized skeleton. Since little is known about CaSR in the skeletal biology of non-mammalian vertebrates, reverse transcription-polymerase chain reaction (RT-PCR), in situ hybridization and immunohistochemistry were applied to adult and embryonic zebrafish to reveal CaSR transcript and protein expression in several tissues, including, among these, chondrocytes and developing bone and notochord as components in skeletal development. Morpholino oligonucleotide (MO) knockdown technique was used to probe CaSR role(s) in the zebrafish model system. By RT-PCR assessment, injection of a splice-inhibiting CaSR MO reduced normally-spliced Casr gene transcript expression measured at 2days postfertilization (dpf). Corresponding to the knockdown of normally-spliced mRNA by the CaSR MO, we observed a morphant phenotype characterized by stunted growth and disorganization of the notochord and axial skeleton by 1dpf. We conclude that, like its critically important role in normal bone development in mammals, CaSR is essential in skeletogenesis in fishes.

Ionic calcium (Ca(2+)) supports essential functions within physiological systems, and consequently its concentration is homeostatically regulated within narrow bounds in the body fluids of animals through endocrine effects at ion-transporting osmoregulatory tissues. In vertebrates, extracellular Ca(2+) is detected at the cell surface by the extracellular calcium-sensing receptor (CaSR), a member of the G protein-coupled receptor (GPCR) superfamily. Interestingly, the taxonomic distribution of CaSRs is restricted to vertebrates, with some CaSR-like receptors apparently present in non-vertebrate chordates. Since bone is a known Ca(2+) storage site and is characteristically restricted to the vertebrate lineage, we hypothesized a functional association of CaSR with vertebrate skeleton that may have an ancient origin. Protein sequence alignment and phylogenetic analysis of vertebrate CaSRs and related GPCRs of the glutamate receptor-like family expose similarities and indel differences among these receptors, and reveal the evolutionary history of CaSRs. Evolutionary selection was tested statistically by evaluating the relationship between non-synonymous (replacement, dN) versus synonymous (silent, dS) amino acid substitution rates (as dN/dS) of protein-coding DNA sequences among branches of the estimated protein phylogeny. On a background of strong purifying selection (dN/dS<1) in the CaSR phylogeny, statistical evidence for adaptive evolution (dN/dS>1) was detected on some branches to major clades in the CaSR phylogeny, especially to the tetrapod vertebrate CaSRs and chordate CaSR-like branches. Testing also revealed overall purifying selection at the codon level. At some sites relaxation from strong purifying selection was seen, but evidence for adaptive evolution was not detected for individual sites. The results suggest purifying selection of CaSRs, and of adaptive evolution among some major vertebrate clades, reflecting clade specific differences in natural history and organismal biology, including skeletal involvement in calcium homeostasis.

Molecular phylogenetic analysis suggests that the extracellular calcium-sensing receptor (CaSR) emerged evolutionarily in association with the chordate-vertebrate lineage. Our studies overall explore the evolution of CaSRs, and the possible historical linkage of CaSRs to vertebrate skeleton as functional components of calcium homeostasis through regulated storage and/or release. We applied both reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry (IHC) to evaluate Casr gene and CaSR protein expression, respectively, in skeletal tissues of a cichlid teleost, the Mozambique tilapia (Oreochromis mossambicus). By RT-PCR, CaSR gene (Casr) expression was observed in skull and vertebral column (including notochordal tissues). Relative to skeleton, IHC revealed CaSR protein expression in notochordal sheath cells within the vertebral canal, in scleroblasts associated with body surface scales, and in chondrocytes within hyaline cartilage. Although closely apposed cells border the acellular bone in tilapia, these cells were only weakly immunostained. We conclude, therefore, that CaSR is expressed in skeletal tissues of tilapia, an advanced teleost fish, and that Casr may be part of a genetic network associated with vertebrate skeletal system. Our immunohistochemical examination also newly revealed CaSR protein expression in epidermis and red muscle of fishes.

Molecular phylogenetic analysis suggests that the extracellular calcium-sensing receptor (CaSR) emerged evolutionarily from an ancestral Class C G protein-coupled receptor (GPCR) related to pheromone and vomeronasal receptors. CaSRs are unknown among non-chordate animals, suggesting an association of CaSRs with the chordate-vertebrate lineage. Our studies overall explore the evolution of CaSRs, and the possible historical linkage of CaSRs to vertebrate skeleton as functional components of calcium homeostasis through storage and/or release. We applied both RT-PCR and immunohistochemistry to evaluate Casr gene and CaSR protein expression, respectively, in skeletal tissues of a cichlid teleost, the Mozambique tilapia (Oreochromis mossambicus). By RT-PCR, Casr gene expression was observed in skull and vertebrae (including notochordal tissues). Immunochemistry revealed CaSR protein expression in chondrocytes within hyaline cartilage, and in notochordal sheath cells within the vertebral canal. Observation of bone-associated CaSR immunoreactivity was more difficult, as a consequence, perhaps, of the acellular nature of bone in this advanced bony fish. Casr may be part of a genetic network associated with vertebrate skeletal system. Supported by Japan Society for the Promotion of Science (YT and SH), University of Tokyo AORI Visiting Professorship (CAL), and National Science Foundation (ALH).