Like other peptide hormones, PTH interacts through a receptor on the plasma membrane of target cells. This same receptor binds PTHrP . The type 1 PTH/ PTHrP receptor (PTHR1) is a seven- transmembrane G- protein- linked receptor that has the ‘signature’ GPCR topology, a seven- membrane- spanning, ‘serpentine’ domain, as well as an extracellular ligand- binding do main and an intracellular COOH- terminal domain. It is a member of group B of the GPCR superfamily. The receptor can couple to the stimulatory G protein, Gs , leading to increased adenylate cyclase activity, the generation of cAMP, and activation of the protein kinase A (PKA) pathway, and can couple to Gq , leading to an increase in the protein kinase C (PKC) pathway and to an increase in IP3, diacylglycerol, and intracellular Ca2+. As with other GPCRs, PTHR1 undergoes cyclical receptor activation, desensitization, and internalization. After ligand binding and endocytosis, the PTHR1 is either recycled to the cell membrane or targeted for degradation. High circulating levels of PTH in hyper parathyroid states have been associated with hormonal desensitization in target tissues. Arrestins contribute to the desensitization of both Gs and Gq mediated PTHR1 signalling. PTHR1 activation and internalization can be selectively dissociated. PTHR1 signal ling can be modified by scaffolding proteins such as the Na+/ H+ exchanger regulatory factor (NHERF) 1 and 2 through PDZ1 and PDZ2 domains. PTHR1 signalling via the cAMP pathway, leading to PKA activation, results in phosphorylation of the cyclic AMP response element binding protein (CREB). CREB binds to the CRE in the promoter region of many genes and transcription ally modulates their expression. The difference in the extent of anabolic versus catabolic activity of PTH and PTHrP analogues may be explained, at least in part, by the way in which these two pep tides interact at two PTHR1 conformations, R0 and RG. Higher affinity for R0 is associated with increased internalization, more cyclic AMP generation and longer-lived catabolic effects, whereas greater binding to RG is associated with shorter‐duration anabolic responses.
The PTHR1 is highly expressed in kidney and bone, the primary target tissues of PTH, but is also expressed in a wide variety of embryonic and adult tissues, including cartilage, liver, brain, smooth muscle, spleen, testis, and skin. In most of these tissues, the receptor appears to mediate the autocrine/ paracrine actions of locally produced and secreted PTHrP. Nevertheless, PTHrP may also exert some of its bioactivity through domains of the molecule that do not interact with PTHR1.
The human PTH/ PTHrP receptor gene (PTHR1) localizes to chromosome 3p21.1- 22. A second related receptor which is the product of a distinct gene (PTHR2 on chromosome 2q33), and which binds PTH, TIP39, but not PTHrP, has been identified. It is expressed in brain, pancreas, testis, and placenta, and its endogenous ligand is TIP39.
Direct evidence that the PTHR1 mediates the calcium homeostatic actions of PTH and the skeletal growth plate actions of PTHrP in humans has come from the study of rare genetic disorders. Jansen’s metaphyseal chondrodysplasia (JMC) is inherited in an autosomal dominant fashion although most reported cases are sporadic. The disorder comprises short- limbed dwarfism secondary to severe growth plate abnormalities, asymptomatic hypercalcaemia, and hypophosphatemia. There is increased bone resorption similar to that in primary hyperparathyroidism and urinary cAMP levels are elevated, but circulating PTH and PTHrP levels are low or un detectable. Although PTHR1 is found widely in fetal and adult tis sues, it is most abundant in three major organs, the kidney, bone, and metaphyseal growth plate. The changes in mineral ion homeostasis and the growth plate in JMC are caused by heterozygous gain- of- function mutations (Figure 1) in the PTHR1 giving rise to constitutively active receptors.
Fig1. Schematic representation of the type 1 human PTH/ PTHrP receptor. The locations of the H223R, T410P, and I458R activating mutations identified in patients with Jansen’s metaphyseal chondrodysplasia, the R104X, P132L, V365del- 1fsX505, and D373- 383 inactivating mutations found in patients with Blomstrand chondrodysplasia, the R485X Eiken syndrome mutation, the G121E, A122T, R150C and R255H endochondromatosis mutations, and the E155X primary failure of tooth eruption (PFE) mutation, are indicated. Splice- site mutations that would result in predicted mutant C351fsX485 and E182fsX203 proteins have been identified in additional PFE cases.
Inactivating or loss- of- function mutations in the PTHR1 have been implicated in the molecular pathogenesis of Blomstrand lethal chondrodysplasia (BLC). This rare disease is characterized by advanced endochondral bone maturation, short- limbed dwarfism, abnormal breast and tooth morphogenesis, and fetal death, thus mimicking the phenotype of Pthr1- less mice. The majority of BLC cases were born to phenotypic ally normal, consanguineous parents, suggesting an autosomal recessive mode of inheritance. Mutant PTHR1s (Figure 1) identified in BLC fetuses fail to bind ligand or stimulate cAMP or inositol phosphate production. A milder form of recessively inherited skeletal dysplasia, known as Eiken syndrome, has been linked to mutations of PTHR1, suggesting a wider range of skeletal phenotypes to this gene. Dominantly acting heterozygous PTHR1 mutations have been identified in familial, non- syndromic primary failure of tooth eruption. Heterozygous PTHR1 mutations have been identified in endochondromas of patients with endochondromatosis (Ollier’s disease), a familial disorder with evidence of autosomal dominance characterized by multiple benign cartilage tumours, and a predisposition to malignant chondrosarcomas. As many patients with Ollier’s dis ease do not apparently have PTHR1 mutations the condition may be genetically heterogeneous.
Heterozygous inactivating mutations in the GNAS1 gene encoding Gαs cause an ~50% reduction in amount/ activity of the protein leading to resistance to PTH and other hormones in the dis order, pseudohypoparathyroidism (PHP) type 1a. In contrast, patients with PHP type 1b have end- organ resistance to PTH without the typical physical stigmata— termed Albright’s hereditary osteodystrophy— of PHP type 1a. Linkage to chromosome 20q13.3, which includes the GNAS1 locus was established in kindreds with PHP type 1b. In addition, the genetic defect is imprinted paternally and is inherited in the same fashion as the PTH resistance in kindreds with PHP type 1a, and in a mouse model heterozygous for ablation of the GNAS gene. In PHP type 1b patients, mutations some distance upstream of the GNAS1 coding regions affect the normal differential methylation of maternal and paternal alleles leading to silencing of the GNAS gene specifically in the renal proximal tubules.
PTH controls renal phosphate reabsorption. Mutations in the genes encoding the two renal sodium phosphate cotransporters, NPT2a and NPT2c, have been identified in a few patients with hyperphosphaturia. The sodium- hydrogen exchanger regulatory factor 1 (NHERF1) interacts with the PTHR1 and NPT2a. Study of hyperphosphaturic patients referred initially for nephrolithiasis or osteopaenia identified a few cases having NHERF1 mutations that could contribute to the renal phosphate loss.
