Poster M5 - Stochastic Modeling of IP3R-Mediated Calcium Signaling in Brain Capillary Endothelial Cells

Introduction: Blood flow regulation in the brain is essential for delivering oxygen and nutrients to active neurons and is often impaired in neurodegenerative disorders like Alzheimer and Dementia. Endothelial Calcium (Ca2+) signaling plays a key role in this regulation. Ca2+ mobilization initiates the release of vasoactive substances like nitric oxide (NO), thereby modulating vascular tone, and allowing cerebral blood flow to meet the local metabolic demands. A recent study (Longden et al, 2021) has identified a high prevalence of Ca2+ activity in brain capillary endothelial cells (cEC). Ca2+ dynamics can be regulated by transmembrane currents, such as those through non-selective cation channels (NSC), as well as by the release of Ca2+ from intracellular stores. The IP3R is a critical component in intracellular Ca2+ signaling, mediating Ca2+ release from the endoplasmic reticulum (ER) in response to IP3 binding. This process is amplified through Ca2+-induced Ca2+-release (CICR), where the binding of Ca2+ further activates the receptor, enhancing Ca2+ release. However, IP3Rs can also be inhibited by Ca2+, where high intracellular Ca2+ levels bind to specific sites on the receptor, reducing its activity and thus serving as a negative feedback mechanism. The goal of this study is to combine our previous cEC Ca2+ dynamics model with a stochastic description of IP3R dynamics to elucidate the mechanisms underlying Ca2+ activity in cEC.

Materials and

Methods: An established model of cEC electrophysiology and Ca2+ dynamics (Silva, Kapela et al. 2007; Moshkforoush, et al. 2020) has been adapted to account for the stochastic opening of IP3Rs (Fig. 1). cEC subdomains feature a section of the ER with a cluster of IP3R channels (N=20). Key membrane and store components like non-selective cation channels (NSC), inward rectifier potassium channels (KIR), ATP-sensitive potassium channels (KATP), plasma membrane Ca2+ ATPase (PMCA), and sarcoplasmic reticulum Ca2+- ATPase pumps (SERCA) are distributed according to the subdomain-to-cell volume ratio. Each IP3R channel is modeled using the DeYoung-Keizer formalism (De Young and Keizer 1992) with three independent subunits with m (IP3 activated), n (Ca2+ activated), and h (Ca2+ inhibited) for a total of 9N two-state (Open-Close) gates. IP3R dynamics are captured using a four-state Continuous Time Markov chain for each gate type: all three gates closed, one gate open, two gates open, and all gates open. Kinetic parameters were selected to match IP3R activity at different Ca2+ and IP3 levels (Foskett et al. 2007). To model IP3R gate dynamics and cellular interactions efficiently in large-scale simulations, a fast kinetics assumption for the m-gate and stochastic opening of the n and h-gates using the Langevin approximation were employed. Simulations were performed in a cerebral microvascular network of 104 ECs.

Results, Conclusions, and Discussions: Simulations show that stochastic opening of IP3R in a cluster can generate robust Ca2+ events (Fig. 2) like those observed experimentally (Longden et al, 2021). The proposed model effectively demonstrates the dynamics and mechanisms underlying observed Ca2+ activity. Simulation results indicate that the stochastic openings of m and h gates within IP3R clusters induce fluctuations in local Ca2+ concentration, that sporadically, upon surpassing a concentration threshold, lead to robust CICR and the initiation of Ca2+ transients. The events are terminated by the inhibition of the IP3Rs at high Ca2+ levels. Additionally, the model supports experimental findings linking electrical and Ca2+ signaling. An increase in Ca2+ events upon membrane hyperpolarization is attributed to an increased transmembrane Ca2+ influx through NSC channels. Elevated Ca2+ enhances IP3R open probability and promotes faster recharging of the Ca2+ stores. The combined effect is a higher prevalence of Ca2+ events in the capillary network. Experiments and simulations support a novel electro-calcium coupling mechanism in the cerebral capillaries.

Acknowledgements (Optional): This work is supported by the National Institute of Neurological Disorders and Stroke and the National Institute of Aging of the NIH under award number R01NS19971.