Fluid flow induced calcium response in osteoblasts: mathematical modeling

Mathematical modeling of intracellular calcium in presence of receptor: a homeostatic model for endothelial cell

Calcium is a ubiquitous molecule and second messenger that regulates many cellular functions ranging from exocytosis to cell proliferation at different time scales. In the vasculature, a constant adenosine triphosphate (ATP) concentration is maintained because of ATP released by red blood cells (RBCs). These ATP molecules continuously react with purinergic receptors on the surface of endothelial cells (ECs). Consequently, a cascade of chemical reactions are triggered that result in a transient cytoplasmic calcium (Ca[Formula: see text]), followed by return to its basal concentration. The mathematical models proposed in the literature are able to reproduce the transient peak. However, the trailing concentration is always higher than the basal cytoplasmic Ca[Formula: see text] concentrations, and the Ca[Formula: see text] concentration in endoplasmic reticulum (ER) remains lower than its initial concentration. This means that the intracellular homeostasis is not recovered. We propose, herein, a minimal model of calcium kinetics. We find that the desensitization of EC surface receptors due to phosphorylation and recycling plays a vital role in maintaining calcium homeostasis in the presence of a constant stimulus (ATP). The model is able to capture several experimental observations such as refilling of Ca[Formula: see text] in the ER, variation of cytoplasmic Ca[Formula: see text] transient peak in ECs, the resting cytoplasmic Ca[Formula: see text] concentration, the effect of removing ATP from the plasma on Ca[Formula: see text] homeostasis, and the saturation of cytoplasmic Ca[Formula: see text] transient peak with increase in ATP concentration. Direct confrontation with several experimental results is conducted. This work paves the way for systematic studies on coupling between blood flow and chemical signaling, and should contribute to a better understanding of the relation between (patho)physiological conditions and Ca[Formula: see text] kinetics.

Novel multi-functional fluid flow device for studying cellular mechanotransduction

Cells respond to their mechanical environment by initiating multiple mechanotransduction signaling pathways. Defects in mechanotransduction have been implicated in a number of pathologies; thus, there is need for convenient and efficient methods for studying the mechanisms underlying these processes. A widely used and accepted technique for mechanically stimulating cells in culture is the introduction of fluid flow on cell monolayers. Here, we describe a novel, multifunctional fluid flow device for exposing cells to fluid flow in culture. This device integrates with common lab equipment including routine cell culture plates and peristaltic pumps. Further, it allows the fluid flow treated cells to be examined with outcomes at the cell and molecular level. We validated the device using the biologic response of cultured UMR-106 osteoblast-like cells in comparison to a commercially available system of laminar sheer stress to track live cell calcium influx in response to fluid flow. In addition, we demonstrate the fluid flow-dependent activation of phospho-ERK in these cells, consistent with the findings in other fluid flow devices. This device provides a low cost, multi-functional alternative to currently available systems, while still providing the ability to generate physiologically relevant conditions for studying processes involved in mechanotransduction in vitro.

Surface chemistry regulates the sensitivity and tolerability of osteoblasts to various magnitudes of fluid shear stress

Scaffolds provide a physical support for osteoblasts and act as the medium to transfer mechanical stimuli to cells. To verify our hypothesis that the surface chemistry of scaffolds regulates the perception of cells to mechanical stimuli, the sensitivity and tolerability of osteoblasts to fluid shear stress (FSS) of various magnitudes (5, 12, 20 dynes/cm² ) were investigated on various surface chemistries (-OH, -CH₃ , -NH₂ ), and their follow-up effects on cell proliferation and differentiation were examined as well. The sensitivity was characterized by the release of adenosine triphosphate (ATP), nitric oxide (NO) and prostaglandin E₂ (PGE₂ ) while the tolerability was by cellular membrane integrity. The cell proliferation was characterized by S-phase cell fraction and the differentiation by ALP activity and ECM expression (fibronectin and type I collagen). As revealed, osteoblasts demonstrated higher sensitivity and lower tolerability on OH and CH₃ surfaces, yet lower sensitivity and higher tolerability on NH₂ surfaces. Observations on the focal adhesion formation, F-actin organization and cellular orientation before and after FSS exposure suggest that the potential mechanism lies in the differential control of F-actin organization and focal adhesion formation by surface chemistry, which further divergently mediates the sensitivity and tolerability of ROBs to FSS and the follow-up cell proliferation and differentiation. These findings are essentially valuable for design/selection of desirable surface chemistry to orchestrate with FSS stimuli, inducing appropriate cell responses and promoting bone formation. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 2978-2991, 2016.

A dynamic model for intracellular calcium response in fibroblasts induced by electrical stimulation

Regulation of intracellular calcium ion concentration ([Ca(2+)]in) in fibroblasts induced by exogenous electrical stimulation could be applied to control gene expressions selectively which in turn modulate the function of the fibroblasts. Regarding the mechanism for electric-field-induced Ca(2+) influx via voltage-gated Ca(2+) channels and/or stretch-activated cation channels in the fibroblasts, a dynamic mathematical model is proposed to quantify the [Ca(2+)]in dynamics in response to direct current or alternating current electric fields. Simulation results demonstrate that the changes in [Ca(2+)]in predicted by our dynamic model are consistent with the experimental data in the literature. The proposed dynamic model could provide not only more insights into the electric-field-induced intracellular Ca(2+) response but also a quantitative way to regulate the [Ca(2+)]in dynamics by controlling the external electrical stimulation.

A power-law rheology-based finite element model for single cell deformation

Physical forces can elicit complex time- and space-dependent deformations in living cells. These deformations at the subcellular level are difficult to measure but can be estimated using computational approaches such as finite element (FE) simulation. Existing FE models predominantly treat cells as spring-dashpot viscoelastic materials, while broad experimental data are now lending support to the power-law rheology (PLR) model. Here, we developed a large deformation FE model that incorporated PLR and experimentally verified this model by performing micropipette aspiration on fibroblasts under various mechanical loadings. With a single set of rheological properties, this model recapitulated the diverse micropipette aspiration data obtained using three protocols and with a range of micropipette sizes. More intriguingly, our analysis revealed that decreased pipette size leads to increased pressure gradient, potentially explaining our previous counterintuitive finding that decreased pipette size leads to increased incidence of cell blebbing and injury. Taken together, our work leads to more accurate rheological interpretation of micropipette aspiration experiments than previous models and suggests pressure gradient as a potential determinant of cell injury.