Date of Award

2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Chemical and Biomolecular Engineering

First Advisor

Scott L. Diamond

Abstract

Platelet aggregation is one of the body's first responses to vascular damage to prevent blood loss; upon injury to the endothelium platelets react to the exposed extracellular matrix and undergo a host of intracellular biochemical changes enabling them to activate and form a "plug" at the site of injury. Internally, platelets respond to their environment by exhibiting a sharp rise in cytosolic calcium that triggers a series of chemical and morphological changes which are critical to platelet activation and subsequent clot propagation. This thesis develops a mechanistic, computational model of platelet calcium regulation using coupled sets of ordinary differential equations. This thesis extends previous work modeling calcium release mediated by inositol 1,4,5-trisphosphate (IP3) to engineer what is the first, to date, complete model of store-operated calcium entry (SOCE) integrated into a systems model for calcium signaling. SOCE is a ubiquitous extracellular calcium entry pathway which is activated by calcium store depletion, is seen in many cells types and is yet to be fully understood. Our model for SOCE regulation consists of diffusion-limited dimerization of the calcium sensor STIM1, followed by fast, cytosolic calcium-dependent association of STIM1 dimers with Orai1 channels in the plasma membrane resulting in graded store-operated channel activation. Appropriate model resting states were characterized using a dense Monte Carlo technique on an initial condition sampling space constrained by available data on species concentrations and protein copy numbers. From this set of resting configurations, following application of physiologic IP3 stimuli, we selected for resting states exhibiting calcium dynamics that are in agreement with experimental data. We also selected for states presenting significant SOCE current based on differences in cytosolic calcium between simulations run with and without extracellular calcium. Low resting levels of IP3 are required for system robustness and for simultaneous appropriate dynamic response to physiologic agonists. Platelets require a resting electrical potential across the membrane surrounding the calcium stores of greater than -70 mV in order to exhibit significant agonist-induced calcium release.

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