159 - In vitro Characterization of Potential Pig Calcium-sensing Receptor (CaSR) Ligands and Cell Signaling Pathways Related to Casr Activation by a Dual-luciferase Reporter Assay

The calcium-sensing receptor (CaSR) is a pivotal regulator of calcium homeostasis. Our previous study has found that pig CaSR (pCaSR) is widely expressed in intestinal segments in weaned piglets. To characterize the activation of pCaSR by potential ligands and related cell signaling pathways, a dual-luciferase reporter assay was employed for the ligands screening and molecular docking was utilized to predict the binding mode of identified ligands. Our results showed that the dual-luciferase reporter assay system was well suited for pCaSR research and its ligand screening. The extracellular calcium activated pCaSR in a concentration-dependent manner with a half-maximal effective concentration (EC₅₀) = 4.74 mM through the Gq/11 signaling pathway, EC₅₀ = 2.85 mM through extracellular signal-regulated kinases 1 and 2 (ERK1/2) activation signaling pathway, and EC₅₀ = 2.26 mM through the Ras homolog family member A (RhoA) activation signaling pathway. Moreover, the activation of pCaSR stimulated by extracellular calcium showed biased agonism through three main signaling pathways: ERK1/2 phosphorylation signaling, Gq/11 signaling, and G12/13 signaling. Both L-Tryptophan and α-casein (90-95) could activate the pCaSR in the presence of extracellular calcium. Furthermore, we characterized the L-tryptophan binding pocket formed by pCaSR residues TRP 70, SER 147, ALA168, SER 169, SER 170, ASP 190, GLU 297, ALA 298, and ILE 416, as well as the α-casein (90-95) binding pocket formed by pCaSR residues PRO188, ASN189, GLU191, HIS192, LYS225, LEU242, ASP480, VAL486, GLY487, VAL513, and TYR514. In conclusion, similar to the human CaSR, the pCaSR also shows biased agonism through three main signaling pathways and both α-casein (90-95) and L-tryptophan are agonists for pCaSR. Furthermore, the binding sites of α-casein (90-95) and L-tryptophan are mainly located within the extracellular domain of pCaSR.