No abstract

G protein-coupled receptor (GPCR) internalization is crucial for the termination of GPCR activity, and in some cases is associated with G protein-independent signaling and endosomal receptor signaling. To date, internalization has been studied in great detail for class A GPCRs; whereas it is not well established to what extent the observations can be generalized to class C GPCRs, including the extracellular calcium-sensing receptor (CaSR). The CaSR is a prototypical class C GPCR that maintains stable blood calcium (Ca 21) levels by sensing minute changes in extracellular free Ca 21. It is thus necessary that the activity of the CaSR is tightly regulated, even while continuously being exposed to its endogenous agonist. Previous studies have used overexpression of intracellular proteins involved in GPCR trafficking, pathway inhibitors, and cell-surface expression or functional desensitization as indirect measures to investigate CaSR internalization. However, there is no general consensus on the processes involved, and the mechanism of CaSR internalization remains poorly understood. The current study provides new insights into the internalization mechanism of the CaSR. We have used a state-of-the-art time-resolved fluorescence resonance energy transfer-based internalization assay to directly measure CaSR internalization in real-time. We demonstrate that the CaSR displays both constitutive and concentration-dependent Ca 21-mediated internalization. For the first time, we conclusively show that CaSR internalization is sensitive to immediate positive and negative modulation by the CaSR-specific allosteric modulators N-(3-[2-chlorophenyl]propyl)-(R)-a-methyl-3-methoxybenzylamine (NPS R-568) and 2-chloro-6-[(2R)-2hydroxy-3-[(2-methyl-1-naphthalen-2-ylpropan-2-yl)amino] propoxy]benzonitrile (NPS 2143), respectively. In addition, we provide compelling evidence that CaSR internalization is b-arrestin-dependent while interestingly being largely independent of G q/11 and G i/o protein signaling. SIGNIFICANCE STATEMENT A novel highly efficient cell-based real-time internalization assay to show that calcium-sensing receptor (CaSR) internalization is b-arrestin-dependent and sensitive to modulation by allosteric ligands.

The adhesion G protein‐coupled receptors [ADGRs/class B2 G protein‐coupled receptors (GPCRs)] constitute an ancient family of GPCRs that have recently been demonstrated to play important roles in cellular and developmental processes. Here, we describe a first insight into the structure‐function relationship of ADGRs using the family member ADGR subfamily G member 4 (ADGRG4)/GPR112 as a model receptor. In a bio‐informatics approach, we compared conserved, functional elements of the well‐characterized class A and class B1 secretin‐like GPCRs with the ADGRs. We identified several potential equivalent motifs and subjected those to mutational analysis. The importance of the mutated residues was evaluated by examining their effect on the high constitutive activity of the N‐terminally truncated ADGRG4/GPR112 in a 1‐receptor‐1‐G protein Saccharomyces cerevisiae screening system and was further confirmed in a transfected mammalian human embryonic kidney 293 cell line. We evaluated the results in light of the crystal structures of the class A adenosine A2A receptor and the class B1 corticotropin‐releasing factor receptor 1. ADGRG4 proved to have functionally important motifs resembling class A, class B, and combined elements, but also a unique highly conserved ADGR motif (H3.33). Given the high conservation of these motifs and residues across the adhesion GPCR family, it can be assumed that these are general elements of ADGR function.—Peeters, M. C., Mos, I., Lenselink, E. B., Lucchesi, M., IJzerman, A. P., Schwartz, T. W. Getting from A to B—exploring the activation motifs of the class B adhesion G protein‐coupled receptor subfamily G member 4/GPR112. FASEB J. 30, 1836–1848 (2016). http://www.fasebj.org

Context The calcium-sensing receptor (CaSR) is essential to maintain a stable calcium concentration in serum. Spermatozoa are exposed to immense changes in concentrations of CaSR ligands such as calcium, magnesium, and spermine during epididymal maturation, in the ejaculate, and in the female reproductive environment. However, the role of CaSR in human spermatozoa is unknown. Objective This work aimed to investigate the role of CaSR in human spermatozoa. Methods We identified CaSR in human spermatozoa and characterized the response to CaSR agonists on intracellular calcium, acrosome reaction, and 3′,5′-cyclic adenosine 5′-monophosphate (cAMP) in spermatozoa from men with either loss-of-function or gain-of-function mutations in CASR and healthy donors. Results CaSR is expressed in human spermatozoa and is essential for sensing extracellular free ionized calcium (Ca2+) and Mg2+. Activators of CaSR augmented the effect of sperm-activating signals such as the response to HCO3– and the acrosome reaction, whereas spermatozoa from men with a loss-of-function mutation in CASR had a diminished response to HCO3–, lower progesterone-mediated calcium influx, and were less likely to undergo the acrosome reaction in response to progesterone or Ca2+. CaSR activation increased cAMP through soluble adenylyl cyclase (sAC) activity and increased calcium influx through CatSper. Moreover, external Ca2+ or Mg2+ was indispensable for HCO3– activation of sAC. Two male patients with a CASR loss-of-function mutation in exon 3 presented with normal sperm counts and motility, whereas a patient with a loss-of-function mutation in exon 7 had low sperm count, motility, and morphology. Conclusion CaSR is important for the sensing of Ca2+, Mg2+, and HCO3– in spermatozoa, and loss-of-function may impair male sperm function.