The CaSR is a class C GPCR that is critical for metabolism. In cells dedicated to the control of mineral metabolism, the CaSR is most sensitive to Ca2+ ions but also binds and responds to amino acids (AAs) and AA analogs, and this latter function is emphasised in hormone-secreting cells of the gut that detect protein breakdown products. Recent studies of CaSR structures have clarified the locations of AA and Ca2+o binding sites (1, 2, 3): the canonical class C orthosteric binding site is occupied not by Ca2+o but by AAs or AA analogs.
The Ca2+o-stimulated CaSR activates Gq/11-mediated PI-PLC and ERK phosphorylation. Surprisingly, however, AAs do not stimulate Gq/11-dependent PLC activity leading to the now discredited notion that AAs are allosteric modulators with biased signalling properties.
An alternative explanation for the lack of effect of AAs on PLC is suggested by the conclusion from structural studies that AAs act as co-agonists with Ca2+o. According to this idea, certain cell-types release endogenous AA analogs to permit Ca2+o-dependent activation and AA-dependent activation of PLC can only be observed when endogenous activators are removed.
In this study we tested whether AAs stimulate CaSR-mediated PLC and pERK in CaSR-expressing HEK-293 cells after removal of conditioned media. After washing and exposure to CaSR activators, the cells were lysed and samples were quantified for IP1 and pERK. Concentration-response analyses revealed increases in EC50 values for Ca2+o and the CaSR activator, L-Phe, restored maximum Ca2+o-sensitivity.
We conclude that some cell-types are insensitive to changes in AA concentrations because they release endogenous AA analogs and are thus primed to respond specifically to Ca2+o. In addition, we predict that CaSR-expressing cells that are primarily sensitive to AAs are not equipped to release CaSR-activating AA analogs.