Cellular Model for the Analysis of IRBIT-Dependent Regulation of the Type 1 IP<sub>3</sub> Receptor

Abstract

In vertebrate genomes, three genes encode subunits of IP3 receptors, including IP3R1, IP3R2, and IP3R3. Despite high homology between different subunits, homotetrameric IP3 receptors formed by IP3R1, IP3R2, and IP₃R3 in the endoplasmic reticulum membrane are markedly distinct by their functional features and regulatory mechanisms. It was particularly reported that IP3R1 is specifically regulated by the IP3R binding protein released with IP₃ (IRBIT), which competes with IP3 for binding to IP3R1. In turn, affinity of IRBIT/IP₃R1 binding is regulated by phosphorylation of IRBIT. By using the CRISPR/Cas9 approach to edit the genome of HEK-293 cells, two monoclonal cell lines were generated as a platform for uncovering a role of IRBIT and associated regulatory circuits in control of the IP₃R1 activity. In one line, HEK-IP3R1, IP₃R2, and IP3R3 genes were disrupted, while IP₃R1 was remained functional. Based on this line, the HEK-IP3R1/DIRBIT line was generated, wherein IRBIT (AHCYL1) gene was inactivated. The comparative analysis of ACh-induced Ca2+ signaling in cells of both lines was performed by employing the Ca2+ dye Fluo-4 and Ca2+ imaging. It was particularly shown that ACh mobilized Ca2+ in cells of both lines, which responded to the agonist at widely varied doses in an “all-or-nothing” manner. Yet, HEK-IP₃R1/DIRBIT cells turned out to be less sensitive to ACh compared to HEK-IP₃R1 cells.