The type 2 inositol 1,4,5-trisphosphate receptor, emerging functions for an intriguing Ca²⁺-release channel

Abstract

The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) type 2 (IP3R2) is an intracellular Ca²⁺-release channel located on the endoplasmic reticulum (ER). IP3R2 is characterized by a high sensitivity to both IP3 and ATP and is biphasically regulated by Ca²⁺. Furthermore, IP3R2 is modulated by various protein kinases. In addition to its regulation by protein kinase A, IP3R2 forms a complex with adenylate cyclase 6 and is directly regulated by cAMP. Finally, in the ER, IP3R2 is less mobile than the other IP3R isoforms, while its functional properties appear dominant in heterotetramers. These properties make the IP3R2 a Ca²⁺ channel with exquisite properties for setting up intracellular Ca²⁺ signals with unique characteristics. IP3R2 plays a crucial role in the function of secretory cell types (e.g. pancreatic acinar cells, hepatocytes, salivary gland, eccrine sweat gland). In cardiac myocytes, the role of IP3R2 appears more complex, because, together with IP3R1, it is needed for normal cardiogenesis, while its aberrant activity is implicated in cardiac hypertrophy and arrhythmias. Most importantly, its high sensitivity to IP3 makes IP3R2 a target for anti-apoptotic proteins (e.g. Bcl-2) in B-cell cancers. Disrupting IP3R/Bcl-2 interaction therefore leads in those cells to increased Ca²⁺ release and apoptosis. Intriguingly, IP3R2 is not only implicated in apoptosis but also in the induction of senescence, another tumour-suppressive mechanism. These results were the first to unravel the physiological and pathophysiological role of IP3R2 and we anticipate that further progress will soon be made in understanding the function of IP3R2 in various tissues and organs.

Keywords: Apoptosis; Cancer; Heart; IP(3); Secretion; Senescence.

Figure 1. Linear representation of the IP₃R2 based on the human sequence demonstrating the interaction sites for its major regulators

The IP₃R2 is represented in blue; the 5 functional domains are indicated. In the channel domain, the 6 transmembrane helices as well as the connecting loops are depicted in green. The specific regulatory mechanisms discussed in the text are shown: identified phosphorylation sites are represented in dark blue, interaction sites for ATP and for Bcl-2 are in orange and the recently described Gly²⁴⁹⁸Ser mutation in the pore domain affecting IP₃R2 function [126] is depicted in red.

Figure 2. Modulation of IP₃R2 single-channel activity by IP₃, Ca²⁺ and ATP

Representative single channel recordings of IP₃R2 expressed in DT40 TKO cells using the “on-nucleus” configuration of the patch-clamp technique. In A, channel activity was stimulated with a maximal [IP₃] (10 μM) at the indicated [Ca²⁺] and [ATP] (10 μM, in blue; 5 mM, in black). The pooled data in B reveal that channel activity stimulated by maximal [IP₃] is modulated by [Ca²⁺] in a biphasic manner and that this relationship is unaffected by increasing the [ATP]. In C, channel activity was stimulated with a sub-maximal [IP₃] (1 μM) at the indicated [Ca²⁺] and [ATP] (10 μM, in blue; 5 mM, in black). The pooled data in D demonstrate that while channel activity is also biphasically regulated by [Ca²⁺] at sub-maximal [IP₃], the maximally achievable open probability, at each [Ca²⁺], is, in contrast to what happens at a maximal [IP₃], markedly potentiated in the presence of a high [ATP]. Modified from [79], with permission.

Figure 3. “Park and Drive” model for IP₃R1 and IP₃R2 gating

An increase in IP₃R1 (A) and IP₃R2 (B) channel activity in the presence of activating ligands is characterized by an increase in channel “bursting” without altering the intraburst kinetics. The bursts have subtype specific characteristics. A gating scheme for both channels can minimally be described by three states; one open state (in green) and two closed states (in red). Bursting activity is represented by rapid transitions between the open state (O) and a short-lived closed state (C₁) representing the “Drive Mode” of the channel. In the interburst intervals, the channel is effectively “Parked” in a long-lived closed state (C₂). For both IP₃R1 (A) and IP₃R2 (B) increasing the concentrations of activating ligands solely alters the transition from C₂ to C₁. However, ligands both increase the likelihood that IP₃R1 will leave the parked state to drive mode, as well as reciprocally decreasing the chances it will return to this state, thus extending the period of bursting (A). In the case of IP₃R2, ligands only destabilize the parked state resulting in an increase in bursting episodes of relatively constant duration (B).