Heterogeneity of G protein activation by the calcium-sensing receptor

The calcium-sensing receptor (CaSR) is a G protein-coupled receptor that plays a fundamental role in extracellular calcium (Ca2+e) homeostasis by regulating parathyroid hormone release and urinary calcium excretion. The CaSR has been described to activate all four G protein subfamilies (Gαq/11, Gαi/o, Gα12/13, Gαs), and mutations in the receptor that cause hyper/hypocalcaemia, have been described to bias receptor signalling. However, many of these studies are based on measurements of second messengers or gene transcription that occurs many steps downstream of receptor activation and can represent convergence points of several signalling pathways. Therefore, to assess CaSR-mediated G protein activation directly, we took advantage of a recently described NanoBiT G protein dissociation assay system. Our studies, performed in HEK293 cells stably expressing CaSR, demonstrate that Ca2+e stimulation activates all Gαq/11 family and several Gαi/o family proteins, although Gαz was not activated. CaSR stimulated dissociation of Gα12/13 and Gαs from Gβ-subunits, but this occurred at a slower rate than that of other Gα-subunits. Investigation of cDNA expression of G proteins in three tissues abundantly expressing CaSR, the parathyroids, kidneys and pancreas, showed Gα11, Gαz, Gαi1 and Gα13 genes were highly expressed in parathyroid tissue, indicating CaSR most likely activates Gα11 and Gαi1 in parathyroids. In kidney and pancreas, the majority of G proteins were similarly expressed, suggesting CaSR may activate multiple G proteins in these cells. Thus, these studies validate a single assay system that can be used to robustly assess CaSR variants and biased signalling and could be utilised in the development of new pharmacological compounds targeting CaSR.

Molecular and clinical insights from studies of calcium-sensing receptor mutations

Twenty-five years have elapsed since the calcium-sensing receptor (CaSR) was first identified in bovine parathyroid and the receptor is now recognized as a fundamental contributor to extracellular Ca2+ (Ca2+ e) homeostasis, regulating parathyroid hormone release and urinary calcium excretion. The CaSR is a class C G-protein-coupled receptor (GPCR) that is functionally active as a homodimer and couples to multiple G-protein subtypes to activate intracellular signalling pathways. The importance of the CaSR in the regulation of Ca2+ e has been highlighted by the identification of >400 different germline loss- and gain-of-function CaSR mutations that give rise to disorders of Ca2+ e homeostasis. CaSR-inactivating mutations cause neonatal severe hyperparathyroidism, characterised by marked hypercalcaemia, skeletal demineralisation and failure to thrive in early infancy; and familial hypocalciuric hypercalcaemia, an often asymptomatic disorder associated with mild-moderately elevated serum calcium concentrations. Activating mutations are associated with autosomal dominant hypocalcaemia, which is occasionally associated with a Bartter's-like phenotype. Recent elucidation of the CaSR extracellular domain structure enabled the locations of CaSR mutations to be mapped and has revealed clustering in locations important for structural integrity, receptor dimerisation and ligand binding. Moreover, the study of disease-causing mutations has demonstrated that CaSR signals in a biased manner and have revealed specific residues important for receptor activation. This review presents the current understanding of the genetic landscape of CaSR mutations by summarising findings from clinical and functional studies of disease-associated mutations. It concludes with reflections on how recently uncovered signalling pathways may expand the understanding of calcium homeostasis disorders.

Disorders of the calcium-sensing receptor and partner proteins: insights into the molecular basis of calcium homeostasis

The extracellular calcium (Ca(2+) o)-sensing receptor (CaSR) is a family C G protein-coupled receptor, which detects alterations in Ca(2+) o concentrations and modulates parathyroid hormone secretion and urinary calcium excretion. The central role of the CaSR in Ca(2+) o homeostasis has been highlighted by the identification of mutations affecting the CASR gene on chromosome 3q21.1. Loss-of-function CASR mutations cause familial hypocalciuric hypercalcaemia (FHH), whereas gain-of-function mutations lead to autosomal dominant hypocalcaemia (ADH). However, CASR mutations are only detected in ≤70% of FHH and ADH cases, referred to as FHH type 1 and ADH type 1, respectively, and studies in other FHH and ADH kindreds have revealed these disorders to be genetically heterogeneous. Thus, loss- and gain-of-function mutations of the GNA11 gene on chromosome 19p13.3, which encodes the G-protein α-11 (Gα11) subunit, lead to FHH type 2 and ADH type 2, respectively; whilst loss-of-function mutations of AP2S1 on chromosome 19q13.3, which encodes the adaptor-related protein complex 2 sigma (AP2σ) subunit, cause FHH type 3. These studies have demonstrated Gα11 to be a key mediator of downstream CaSR signal transduction, and also revealed a role for AP2σ, which is involved in clathrin-mediated endocytosis, in CaSR signalling and trafficking. Moreover, FHH type 3 has been demonstrated to represent a more severe FHH variant that may lead to symptomatic hypercalcaemia, low bone mineral density and cognitive dysfunction. In addition, calcimimetic and calcilytic drugs, which are positive and negative CaSR allosteric modulators, respectively, have been shown to be of potential benefit for these FHH and ADH disorders.

Role of G-proteins in the differentiation of epiphyseal chondrocytes

Herein, we review the regulation of differentiation of the growth plate chondrocytes by G-proteins. In connection with this, we summarize the current knowledge regarding each family of G-protein α subunit, specifically, Gα(s), Gα(q/11), Gα(12/13), and Gα(i/o). We discuss different mechanisms involved in chondrocyte differentiation downstream of G-proteins and different G-protein-coupled receptors (GPCRs) activating G-proteins in the epiphyseal chondrocytes. We conclude that among all G-proteins and GPCRs expressed by chondrocytes, Gα(s) has the most important role and prevents premature chondrocyte differentiation. Receptor for parathyroid hormone (PTHR1) appears to be the major activator of Gα(s) in chondrocytes and ablation of either one leads to accelerated chondrocyte differentiation, premature fusion of the postnatal growth plate, and ultimately short stature.

Prolactin actions

Molecular genetics and other contemporary approaches have contributed to a better understanding of prolactin (PRL) actions at the cellular and organismal levels. In this review, several advances in knowledge of PRL actions are highlighted. Special emphasis is paid to areas of progress with consequences for understanding of human PRL actions. The impacts of these advances on future research priorities are analyzed.