Effects of Laminar Fluid Shear Stress on the Function of Adult Stem Cells

The objective of this doctoral thesis was to investigate the effects of laminar fluid shear stress on the function three types of adult stem cells: mesenchymal stem cells, neural crest stem cells and multipotent vascular stem cells. These three types of stem cells represent potential cell sources for vascular tissue engineering. Before these stem cells can be used to create tissue engineered vascular grafts, a thorough understanding of the effects of hemodynamic forces on their function is necessary. Much like cyclical stretch and normal pressure, fluid shear stress plays a major role in the microenvironment of the blood vessel wall but the effects of this mechanical force on the function of these adult stem cells is not well understood./nFirst, we began by investigating the effects of laminar fluid shear stress on TGF-beta1/SMAD2 signaling in human mesenchymal stem cells. Human mesenchymal cells (hMSCs) are multipotent fibroblast-like cells, which are found primarily in the bone marrow. MSCs are a potential cell source for tissue engineering because of their ease of isolation and expansion, their multipotency and their low immunogenicity. We found that exposing hMSCs to fluid flow promotes transforming growth factor 1 (TGF-beta1) signaling in a receptor-dependent manner. The mechanism explaining this phenomenon, however, was unclear. Based on our results, we rejected several hypotheses to explain the observed phenomenon: shear force transmission through the glycocalyx; changes in membrane fluidity; and changes in the internalization of TGF-beta; receptors. We were able to show that the increase in TGF-beta1/Smad2 signaling when hMSC are exposed to fluid flow was not caused by shear stress but instead, by an increase in the flow rate. We were also able to show that shear stress inhibits TGF-beta1/Smad2 signaling./nSecond, we examined the effects of laminar fluid shear stress on the function of human neural crest stem cells. Neural crest stem cells (NCSCs) are multipotent cells that give rise to various tissues during the embryonic development of vertebrates. NCSCs were derived from induced pluripotent stem cells and, then, exposed to a laminar fluid shear stress of 10 dynes/cm2 for various time periods. We found that laminar fluid shear stress increased NCSC proliferation. We also showed that fluid shear stress increased the activation of ERK1/2 in a time dependent manner. In addition, we observed that exposure to laminar fluid shear stress did not affect myogenic, neurogenic or osteogenic differentiation in NCSCs. We found, however, that exposure to fluid shear stress prevented adipogenic differentiation./nThird, we explored the effects of laminar fluid shear stress on the function of multipotent vascular stem cells. Multipotent vascular stem cells (MVSCs) are adult stem cells that have recently been discovered in the medial layer of blood vessels. MVSCs have been shown to give rise to a variety of cell types including: schwann cells, peripheral neurons, smooth muscle cells, adipocytes, chondrocytes and osteocytes. MVSCs were isolated from rat carotid arteries. These cells were then expanded without differentiation and exposed to a laminar fluid shear stress of 6 dynes/cm2. We found that laminar fluid shear stress increased rat MVSC proliferation. We also showed that fluid shear stress increased the activation of ERK1/2 in a time dependent manner in MVSCs. Laminar fluid shear stress also caused a decrease in the gene expression of smooth muscle cell markers and an increase in the expression of osteoblastic differentiation genes. In addition, we observed that exposure to fluid shear stress did not affect myogenic, osteogenic and neurogenic cell differentiation in MVSCs./nThe results of the aforementioned studies provide new clues in efforts to elucidate the mechanobiology of adult stem cells but further investigation into the effects of mechanical stimulation on the function of human MSCs, NCSCs and MVSCs will be necessary to provide a rational basis for the use of these cells in tissue engineering applications.

Advisor: Li, Song