Activation profile of the Atlantic salmon (Salmo salar) calcium-sensing receptor (Casr) by selected L-amino acids

In mammals, the calcium-sensing receptor (CaSR) is involved in nutrient sensing and modulated by several amino acids. In teleosts, sequence homologues of the mammalian CaSR have been described but their function in sensing amino acids remains elusive, including in Atlantic salmon (Salmo salar), an important aquaculture species. This study investigated the activation of Atlantic salmon Casr (asCasr)-mediated signaling pathways-G_q, G_i, and ERK1/2-by six selected L-amino acids (histidine, tryptophan, phenylalanine, isoleucine, leucine and valine) and by Ca²⁺. Using a Flp-In-HEK293 cell line stably expressing asCasr, we confirmed activation of all three pathways. L-histidine, L-phenylalanine, and L-tryptophan triggered G_i signaling independent of Ca²⁺. Notably, no Ca²⁺ concentrations induced G_i activation, but IP₁ production increased in a concentration-dependent manner. L-histidine was the only amino acid to activate the G_q pathway without Ca²⁺, and this response was amplified by the presence of Ca²⁺. In the presence of 2.5 mM Ca²⁺, L-phenylalanine and L-tryptophan also activated G_q signaling in a concentration-dependent manner. Additionally, in the presence of 10 mM Ca²⁺, L-histidine, L-phenylalanine, and L-tryptophan triggered ERK phosphorylation. These findings establish asCasr as a functional homologue of mammalian CaSR, activated in a concentration-dependent manner by L-amino acids with an aromatic ring.