A Novel Nonlinear IP$_3$R State Transition Model and Calcium Oscillation

TL;DR

This paper introduces a new nonlinear model for IP3 receptor states that captures calcium oscillations and reveals a switch-like regulation mechanism by IP3 concentration, emphasizing the importance of a pre-activated state in calcium signaling.

Contribution

The model incorporates a pre-activated state based on electron microscopy data, providing a novel theoretical framework for IP3R dynamics and calcium oscillations.

Findings

Reproduces experimentally observed IP3R state distribution.

Demonstrates switch-like regulation of calcium oscillations by IP3.

Shows oscillations terminate abruptly beyond a secondary IP3 threshold.

Abstract

We present a novel nonlinear state transition model for inositol 1,4,5-trisphosphate receptors (IP$_3$Rs) that incorporates a pre-activated state, as suggested by electron microscopy observations. Our model provides a theoretical framework for the biphasic Ca$^{2+}$ dependence of IP$_3$Rs and accurately reproduces their experimentally observed state distribution under saturating IP$_3$ conditions. By integrating receptor dynamics with cytoplasmic and endoplasmic reticulum (ER) calcium exchange, we simulate IP$_3$R-mediated Ca$^{2+}$ oscillations governed by six key conformational states. A pivotal finding is that IP$_3$ regulates these oscillations in a switch-like manner: once a critical IP$_3$ concentration is reached, the system abruptly transitions to sustained, constant-amplitude oscillations that quickly terminate when the concentration exceeds a secondary threshold. These results underscore the crucial role of the pre-activated state in modulating calcium signaling.