The effect of scaffold physical properties on endothelial cell function

Abstract

Endothelial cells (EC) are ubiquitous - as vascular epithelial cells they line the inner surface of all vessels and are the contact surface with flowing blood. Macrovascular EC are the first line barrier between flowing blood and mural structures. The microvasculature includes EC-lined vessels that contact virtually every cell in the body. These EC are potent bioregulatory cells, modulating thrombosis, inflammation and control over mural smooth muscle cells and vascular health. The biochemical roles of EC can be retained when cells are embedded within three-dimensional matrices without recapitulation of the full vessel architecture. Within these matrices, surface and structural properties impose a set of forces on the embedded EC. Indeed, substrata pore size and modulus have profound effects on phenotype and function of a range of cell types. In the first part of this work, we examined the effect of pore size, matrix relative density and modulus on matrix-embedded EC growth and secretion and found a greater biological dependence on modulus than pore size or density. In the second part of this work, we examined the effect of isolated changes in modulus on BC growth, secretion of growth regulators, and modulation of smooth muscle cell growth. EC growth is maximal at intermediate moduli over a range from 50 Pa- 1500 Pa. Secretion of heparan sulfate proteoglycans (HSPGs), which inhibit smooth muscle cell growth, is maximal at low moduli and flat at high moduli. Secretion of growth factors such as FGF2 and PDGF-BB were also modulus responsive. Inhibition of smooth muscle cell growth rose as modulus decreased from 510 Pa to 50 Pa and was the result of a balance between increased HSPG secretion and reduced secretion of vasoactive growth factors. Changes in endothelial function correlated with extracellular matrix gene and integrin aP 3 and c41 expression. Changes in the forces experienced by the cell - a change in substrate modulus - cause the cell to alter its ECM and integrin expression in an effort to return the force balance to normal, leading to downstream effects on cell function. While growth stimulatory function largely conserved, growth inhibitory function was altered to a much larger degree. In the final part of this work, we examined the effect of scaffold modulus on EC response to inflammatory stimuli, and attempted to correlate it to changes in smooth muscle cell regulation and integrin expression. While cytokine secretion was independent of modulus, surface expression of ICAM- 1 and VCAM-1, and induction of CD4' T cell proliferation followed a similar pattern to smooth muscle cell inhibition, suggesting that similar mechanisms may be involved in their regulation by substrate modulus. Alteration of scaffold modulus has a profound impact on EC function including growth regulation and inflammatory response. The model offered in this thesis - wherein EC attempt to neutralize changes in environmental force balance by altering ECM and integrin expression, leading to changes in downstream function - offers insight into how environmental changes effect functional changes in ECs.