Groff Jeffrey R, Smith Gregory D
Department of Applied Science, College of William and Mary, Williamsburg, Virginia, USA.
Biophys J. 2008 Jul;95(1):135-54. doi: 10.1529/biophysj.107.119982. Epub 2008 Mar 21.
Puffs and sparks are localized intracellular Ca(2+) elevations that arise from the cooperative activity of Ca(2+)-regulated inositol 1,4,5-trisphosphate receptors and ryanodine receptors clustered at Ca(2+) release sites on the surface of the endoplasmic reticulum or the sarcoplasmic reticulum. While the synchronous gating of Ca(2+)-regulated Ca(2+) channels can be mediated entirely though the buffered diffusion of intracellular Ca(2+), interprotein allosteric interactions also contribute to the dynamics of ryanodine receptor (RyR) gating and Ca(2+) sparks. In this article, Markov chain models of Ca(2+) release sites are used to investigate how the statistics of Ca(2+) spark generation and termination are related to the coupling of RyRs via local [Ca(2+)] changes and allosteric interactions. Allosteric interactions are included in a manner that promotes the synchronous gating of channels by stabilizing neighboring closed-closed and/or open-open channel pairs. When the strength of Ca(2+)-mediated channel coupling is systematically varied (e.g., by changing the Ca(2+) buffer concentration), simulations that include synchronizing allosteric interactions often exhibit more robust Ca(2+) sparks; however, for some Ca(2+) coupling strengths the sparks are less robust. We find no evidence that the distribution of spark durations can be used to distinguish between allosteric interactions that stabilize closed channel pairs, open channel pairs, or both in a balanced fashion. On the other hand, the changes in spark duration, interspark interval, and frequency observed when allosteric interactions that stabilize closed channel pairs are gradually removed from simulations are qualitatively different than the changes observed when open or both closed and open channel pairs are stabilized. Thus, our simulations clarify how changes in spark statistics due to pharmacological washout of the accessory proteins mediating allosteric coupling may indicate the type of synchronizing allosteric interactions exhibited by physically coupled RyRs. We also investigate the validity of a mean-field reduction applicable to the dynamics of a ryanodine receptor cluster coupled via local [Ca(2+)] and allosteric interactions. In addition to facilitating parameter studies of the effect of allosteric coupling on spark statistics, the derivation of the mean-field model establishes the correct functional form for cooperativity factors representing the coupled gating of RyRs. This mean-field formulation is well suited for use in computationally efficient whole cell simulations of excitation-contraction coupling.
钙瞬变和钙火花是局部性的细胞内钙离子浓度升高，它们源于内质网或肌浆网表面钙离子释放位点处聚集的钙离子调节型肌醇1,4,5 -三磷酸受体和兰尼碱受体的协同活动。虽然钙离子调节型钙离子通道的同步门控可以完全通过细胞内钙离子的缓冲扩散来介导，但蛋白质间的变构相互作用也有助于兰尼碱受体（RyR）门控和钙火花的动力学过程。在本文中，钙离子释放位点的马尔可夫链模型被用于研究钙火花产生和终止的统计学特征如何与通过局部钙离子浓度变化和变构相互作用实现的兰尼碱受体耦合相关。变构相互作用以一种通过稳定相邻的关闭 - 关闭和/或开放 - 开放通道对来促进通道同步门控的方式被纳入模型。当钙离子介导的通道耦合强度系统地变化时（例如，通过改变钙离子缓冲液浓度），包含同步变构相互作用的模拟通常会展现出更稳定的钙火花；然而，对于某些钙离子耦合强度，钙火花的稳定性会降低。我们没有发现证据表明火花持续时间的分布可用于区分以平衡方式稳定关闭通道对、开放通道对或两者的变构相互作用。另一方面，当从模拟中逐渐去除稳定关闭通道对的变构相互作用时观察到的火花持续时间、火花间隔和频率的变化，与稳定开放通道对或同时稳定关闭和开放通道对时观察到的变化在性质上有所不同。因此，我们的模拟阐明了由于介导变构耦合的辅助蛋白的药理学洗脱导致的火花统计学变化如何可能指示物理耦合的兰尼碱受体所表现出的同步变构相互作用的类型。我们还研究了适用于通过局部钙离子浓度和变构相互作用耦合的兰尼碱受体簇动力学的平均场简化的有效性。除了便于对变构耦合对火花统计学的影响进行参数研究外，平均场模型的推导还为代表兰尼碱受体耦合门控的协同因子确定了正确的函数形式。这种平均场公式非常适合用于兴奋 - 收缩耦合的高效全细胞模拟计算。
