ECM Remodeling Events during EC‐Pericyte Tube Co‐Assembly in 3D Matrices

Abstract

We have been investigating the molecular events controlling capillary tube formation in 3D matrices, whereby human endothelial cells (ECs) and pericytes co‐assemble to form tubes with associated pericytes that are polarized on the EC abluminal surface. Dynamic EC‐pericyte interactions lead to vascular basement membrane matrix assembly in either 3D collagen or fibrin matrices, a process that requires the presence of both cell types and which occurs between the cells. Normally, pericyte recruitment leads to more narrow EC tube diameters that occur as a result of basement membrane formation. A major emphasis of our laboratory over the past several years has been to define the growth factor requirements for EC tubulogenesis and EC‐pericyte tube co‐assembly. We have developed models of these processes under serum‐free defined conditions, which require the addition of SCF, IL‐3, SDF‐1alpha, FGF‐2 and insulin (Factors). Addition of these Factors allows for both EC tubulogenesis and EC‐pericyte tube co‐assembly. In contrast, the addition of VEGF or the combination of VEGF and FGF‐2 fails to support these events. Of great interest is that we have identified four additional EC‐derived growth factors that are necessary for EC‐pericyte tube co‐assembly and they are: PDGF‐BB, HB‐EGF, TGFbeta1, and ActivinB. Blockade of these growth factors (individually or combinations) or their receptors leads to marked decreases in pericyte recruitment, pericyte proliferation and vascular basement membrane formation in vitro and in vivo during quail development. In addition, EC tube widths increased when these growth factors are disrupted. Overall, these data indicate that 9 or more growth factors are necessary for human capillaries to assemble in vitro; a novel concept regarding vascular morphogenesis.