Distinct Roles of Inositol 1,4,5-Trisphosphate Receptor Types 1 and 3 in Ca²⁺ Signaling

Three subtypes of inositol 1,4,5-trisphosphate receptor (IP₃R1, IP₃R2, and IP₃R3) Ca²⁺ release channel share basic properties but differ in terms of regulation. To what extent they contribute to complex Ca²⁺ signaling, such as Ca²⁺ oscillations, remains largely unknown. Here we show that HeLa cells express comparable amounts of IP₃R1 and IP₃R3, but knockdown by RNA interference of each subtype results in dramatically distinct Ca ²⁺ signaling patterns. Knockdown of IP₃R1 significantly decreases total Ca²⁺ signals and terminates Ca²⁺ oscillations. Conversely, knockdown of IP₃R3 leads to more robust and long lasting Ca²⁺ oscillations than in controls. Effects of IP₃R3 knockdown are surprisingly similar in COS-7 cells that predominantly (>90% of total IP₃R) express IP₃R3, suggesting that IP₃R3 functions as an anti-Ca ²⁺-oscillatory unit without contributing to peak amplitude of Ca ²⁺ signals, irrespective of its relative expression level. Therefore, differential expression of the IP₃R subtype is critical for various forms of Ca²⁺ signaling, and, particularly, IP₃R1 and IP₃R3 have opposite roles in generating Ca²⁺ oscillations.