Trends in Pharmacological Sciences
ReviewNon-canonical signaling of the PTH receptor
Section snippets
PTHR: a paradoxical and medically crucial GPCR
Parathyroid hormone (PTH) and PTH-related protein (PTHrP) play crucial and distinct physiological roles by activating a common cell-surface receptor, the PTH type 1 receptor (hereafter termed PTHR), a family 2 GPCR. Circulating and homeostatic PTH regulates blood concentrations of calcium and phosphate ions, as well as vitamin D, by acting in bone (osteoblasts, osteocytes) and kidney (proximal and distal tubule cells). PTHrP, a paracrine hormone, controls cell differentiation and proliferation
Kinetics of PTHR activation
A combination of biochemical, pharmacological and optical techniques, including photoaffinity crosslinking using bisphenol A (BPA)-containing PTH analogs 3, 14, 15, 16, 17, 18, coupled with functional assays using mutant receptors and structurally modified ligands 19, 20, 21, 22, and Förster resonance energy transfer (FRET)-based approaches 23, 24, 25, 26, have revealed important insights about ligand–receptor (L–R) interaction mechanisms and rate-limiting reactions involved in the activation
Conformational selectivity of PTHR deactivation
As described above, the sequence of reactions involved in the activation of PTHR and GS proceed with similar kinetics and mechanisms in response to either PTH or PTHrP. By contrast, the mechanisms of signal termination are divergent (Box 1). Recent studies show that a brief pulse of PTH induces a long-lasting active state that is characterized by prolonged GS activation and sustained cAMP production even after PTH-bound PTHR internalizes to early endosomes. PTHrP dissociates rapidly from the
‘Non-canonical’ mode of PTHR signaling
Extensive studies of signaling by GPCRs, including but not limited to rhodopsin and the β2-adrenergic receptor (β2AR), have led to what is now considered a classical and general model of GPCR desensitization by arrestins 39, 40. In this ‘canonical’ model, arrestins engage active receptors after ligand binding has stimulated G-protein-coupled receptor kinases (GRKs) to phosphorylate residues on the C terminus of the receptor. Arrestin binding terminates GPCR signaling by preventing receptor–G
PTHR signaling stopped by retromer
Depletion of β-arrestins by siRNA reduces the level and duration of cAMP generation after PTH challenge whereas it increases cAMP induced by β2AR in response to isoproterenol, indicating again that β-arrestins do not desensitize cAMP generation by PTHR 26, 52. If arrestin does not prevent GS coupling from PTHR by steric inhibition, then it is necessary to ask what other protein could do this job. One possibility is that, like particular receptor tyrosine kinases [53], PTHR simply continues to
Concluding remarks
Studies discussed in this review suggest that PTHR can adopt multiple conformations that are stabilized by different ligands. This conformational selectivity in turn influences the downstream signaling responses in target cells. Understanding how these ligand-specific events occur is crucial to determine the molecular and cellular mechanisms underlying the anabolic and catabolic effects that PTHR ligands have on bone mass, depending on duration and timing of exposure. Based on available
Conflicts of interest
J-P.V. holds a patent on the technology of measuring GPCR activation/deactivation by FRET (EP 1581811B1; US8084575).
Acknowledgments
This work was supported by the National Institutes of Health (grant award R01 DK087688 to J-P.V.).
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