During vasculogenesis and angiogenesis, endothelial cell responses to growth factors are modulated by the compositional and mechanical properties of a surrounding three-dimensional (3D) extracellular matrix (ECM) that is dominated by either cross-linked fibrin or type I collagen. While 3D-embedded endothelial cells establish adhesive interactions with surrounding ligands to optimally respond to soluble or matrix-bound agonists, the manner in which a randomly ordered ECM with diverse physico-mechanical properties is remodeled to support blood vessel formation has remained undefined. Herein, we demonstrate that endothelial cells initiate neovascularization by unfolding soluble fibronectin (Fn) and depositing a pericellular network of fibrils that serve to support cytoskeletal organization, actomyosin-dependent tension, and the viscoelastic properties of the embedded cells in a 3D-specific fashion. These results advance a new model wherein Fn polymerization serves as a structural scaffolding that displays adhesive ligands on a mechanically ideal substratum for promoting neovessel development.[Keywords: Actomyosin; angiogenesis; endothelial cells; extracellular matrix; fibronectin] Supplemental material is available at http://www.genesdev.org.

Laminins and collagens are extracellular matrix proteins that play essential roles in peripheral nervous system development. Laminin signals regulate Schwann cell proliferation and survival as well as actin cytoskeleton dynamics, which are essential steps for radial sorting and myelination of peripheral axons by Schwann cells. Collagen and their receptors promote Schwann cell adhesion, spreading, and myelination as well as neurite outgrowth. In this article, we will review the recent advances in the studies of laminin and collagen function in Schwann cell development. V V C

A heparin-binding glycoprotein was purified from conditioned medium of cultured rat Schwann cells. The protein, p200, which has an apparent molecular mass of approximately 200 kDa, was identified by its ability to bind the cell surface heparan sulfate proteoglycan Nsyndecan (syndecan-3) in a membrane overlay assay. Soluble heparin but not chondroitin sulfate inhibited the binding, suggesting the involvement of heparan sulfate chains of proteoglycan in the interaction. Purified p200 promoted the attachment and spreading of Schwann cells. Adhesion to p200 was blocked by heparin, suggesting that heparan sulfate proteoglycans are cell surface receptors for p200. The tissue distribution of p200 was determined by immunoblot analysis with antip200 antibodies. Among neonatal rat tissues examined p200 was detected only in sciatic nerve and, at lower levels, in skeletal muscle. p200 expression in sciatic nerve was detectable only during the first 2-3 weeks of postnatal development and was not detected in adult rats. Immunofluorescent staining of rat sciatic nerve showed that p200 was localized in the extracellular matrix surrounding individual Schwann cells-axon units. Two tryptic peptides from p200 were purified and sequenced. These contained multiple GXX collagen-like repeats. Bacterial collagenase digestion of p200 produced a product with an apparent molecular mass of approximately 90 kDa. These data suggest that Schwann cells secrete an apparently novel collagen-like adhesive protein that interacts with cells through cell surface heparan sulfate proteoglycans.During peripheral nerve development Schwann cells deposit basal lamina on the outer surface of the individual Schwann cell-axon units. Using primary co-cultures of Schwann cells and nerve cells, it was observed that under culture conditions in which Schwann cells do not assemble basal lamina (for instance in culture medium lacking ascorbate), the myelination and ensheathment of axons does not occur (1). The addition of exogenous basal lamina-like matrix such as Matrigel or certain basal lamina components restores the Schwann cell's ability to myelinate axons (2, 3). These findings demonstrate that Schwann cell contact with extracellular matrix (ECM) 1 is required for normal ensheathment and myelination of axons.In vivo and in vitro studies have identified a number of Schwann cell integrins, which are thought to mediate interaction of these cells with ECM (4 -6). The repertoire of Schwann cell integrins generally corresponds to the molecular composition of basal lamina. For example, Schwann cells express ␣ 6 ␤ 1 (4) and ␣ 2 ␤ 1 (6) integrin, which have been shown to function as receptors for laminin and/or collagen, both of which are secreted by Schwann cells and are present in the peripheral nerve ECM.In addition to integrins, Schwann cells possess cell surface proteoglycans that might also be involved in the interactions of the cells with the ECM, including basal lamina. It has been shown that glypican, a lipid-anchored cell surface proteoglycan of Schwann cells...

The results summarized above suggest that assembly of fibronectin is a fundamental biological process and that knowledge of the process of assembly may reveal new ways by which cells interact with extracellular molecules. Deposition of a fibronectin matrix seems to be regulated as tightly as synthesis of fibronectin or expression of adhesion receptors for fibronectin and is influenced profoundly by two products of blood coagulation--TGF-beta released from platelets and factor XIII activated by thrombin. Fibronectin assembly may be important in all sorts of physiological and pathophysiological processes. Cell A--for instance, a stromal cell--can influence the behavior of cell B--for instance, a lymphocyte--by assembling fibronectin made by cell C--for instance, a hepatocyte. We hope that the testable models of assembly presented in this paper will lead to new understanding of the process of assembly and suggest new modalities for treatment of diseases that result in fibrosis, damaged tissues, and neoplastic growth.