Polydom affects remodeling of lymphatic vessels in both mouse and zebrafish. Polydom deposited around lymphatic vessels seems to ensure Foxc2 upregulation in lymphatic endothelial cells, possibly via the Ang-2 and Tie1/Tie2 receptor system.

Integrin ␣8␤1 interacts with a variety of Arg-Gly-Asp (RGD)-containing ligands in the extracellular matrix. Here, we examined the binding activities of ␣8␤1 integrin toward a panel of RGD-containing ligands. Integrin ␣8␤1 bound specifically to nephronectin with an apparent dissociation constant of 0.28 ؎ 0.01 nM, but showed only marginal affinities for fibronectin and other RGD-containing ligands. The high-affinity binding to ␣8␤1 integrin was fully reproduced with a recombinant nephronectin fragment derived from the RGD-containing central "linker" segment. A series of deletion mutants of the recombinant fragment identified the LFEIFEIER sequence on the C-terminal side of the RGD motif as an auxiliary site required for high-affinity binding to ␣8␤1 integrin. Alanine scanning mutagenesis within the LFEIFEIER sequence defined the EIE sequence as a critical motif ensuring the high-affinity integrinligand interaction. Although a synthetic LFEIFEIER peptide failed to inhibit the binding of ␣8␤1 integrin to nephronectin, a longer peptide containing both the RGD motif and the LFEIF-EIER sequence was strongly inhibitory, and was ϳ2,000-fold more potent than a peptide containing only the RGD motif. Furthermore, trans-complementation assays using recombinant fragments containing either the RGD motif or LFEIFEIER sequence revealed a clear synergism in the binding to ␣8␤1 integrin. Taken together, these results indicate that the specific high-affinity binding of nephronectin to ␣8␤1 integrin is achieved by bipartite interaction of the integrin with the RGD motif and LFEIFEIER sequence, with the latter serving as a synergy site that greatly potentiates the RGD-driven integrin-ligand interaction but has only marginal activity to secure the interaction by itself.Integrins are a family of adhesion receptors that interact with a variety of extracellular ligands, typically cell-adhesive proteins in the extracellular matrix (ECM).2 They play mandatory roles in embryonic development and the maintenance of tissue architectures by providing essential links between cells and the ECM (1). Integrins are composed of two non-covalently associated subunits, termed ␣ and ␤. In mammals, 18 ␣ and 8 ␤ subunits have been identified, and combinations of these subunits give rise to at least 24 distinct integrin heterodimers.Based on their ligand-binding specificities, ECM-binding integrins are classified into three groups, namely laminin-, collagen-and RGD-binding integrins (2, 3), of which the RGD-binding integrins have been most extensively investigated. The RGD-binding integrins include ␣5␤1, ␣8␤1, ␣IIb␤3, and ␣V-containing integrins, and have been shown to interact with a variety of ECM ligands, such as fibronectin and vitronectin, with distinct binding specificities. The ␣8 integrin subunit was originally identified in chick nerves (4). Integrin ␣8␤1 is expressed in the metanephric mesenchyme and plays a crucial role in epithelial-mesenchymal interactions during the early stages of kidney morphogenesis.

Background: Polydom/SVEP1 is a putative extracellular matrix protein of unknown function. Results: Polydom/SVEP1 is a potent ligand for integrin ␣9␤1 and colocalizes with the integrin in vivo. Conclusion: Polydom/SVEP1 is a hitherto unknown high affinity ligand for integrin ␣9␤1. Significance: The identification of this high affinity ligand offers important clues toward better understanding of the consequences of integrin ␣9␤1-mediated cell-extracellular matrix interactions.