Crucial Role for Endothelial Cell α2β1 Integrin Receptor Clustering in Collagen‐Induced Angiogenesis

Abstract

Angiogenesis is a crucial mechanism of vascular growth and regeneration that requires biosynthesis and cross-linking of collagens in vivo and is induced by collagen in vitro. Here, we use an in vitro model in which apical Type I collagen gels rapidly induce angiogenesis in endothelial monolayers. We extend previous studies demonstrating the importance of the endothelial α2β1 integrin, a key collagen receptor, in angiogenesis by investigating the roles of receptor clustering and conformational activation. Immunocytochemical localization of α2β1 integrins in endothelial monolayers showed a concentration of integrins along cell-cell borders. After inducing angiogenesis with collagen, the receptors redistributed to apical cell surfaces, aligning with collagen fibers, which were also redistributed during angiogenesis. Levels of conformationally activated α2β1 integrins were unchanged during angiogenesis and undetected on endothelial cells binding collagen in suspension. We mimicked the polyvalency of collagen fibrils using antibody-coated polystyrene beads to cluster endothelial cell surface α2β1 integrins, which induced rapid angiogenesis in the absence of collagen gels. Clustering of αvβ3 integrins and PECAM-1 but not of α1 integrins also induced angiogenesis. Soluble antibodies alone had no effect. Thus, the angiogenic property of collagen may reside in its ability to ligate and cluster cell surface receptors such as α2β1 integrins. Furthermore, synthetic substrates that promote the clustering of select endothelial cell surface receptors mimic the angiogenic properties of Type I collagen and may have applications in promoting vascularization of engineered tissues.

Above the Matrix: Functional Roles for Apically Localized Integrins

“…Application of a type I collagen hydrogel to the apical surface of human umbilical vein endothelial cells (HUVECs) induces the reorganization of α2β1 integrins to the apical surface of the cells (Turner et al, 2020). It was further demonstrated that the activation of α2β1 integrins was an important step in rapid tube formation and angiogenesis (Turner et al, 2020).…”

“…Clustering of apical integrins, particularly activated integrins, appears to be a key component in inducing cell responses ranging from cell migration to increases in permeability (Ojakian and Schwimmer, 1994;Garbi et al, 1996;Zuk and Matlin, 1996;Kam et al, 2013;Walsh et al, 2015;Grassme et al, 2017;Stewart et al, 2017;Badaoui et al, 2020;Mang et al, 2020;Turner et al, 2020). Apical integrin clustering can also drive endocytosis which can be physiological, in the case of integrin turnover during cell migration, but also has the pathological consequences of facilitating infection by bacteria using apical integrins as receptors.…”

“…How clustering of apical integrins is linked to integrin conformation and activation is not well defined. Several studies have demonstrated that integrin activation, usually through ligand binding (Ojakian and Schwimmer, 1994;Garbi et al, 1996;Zuk and Matlin, 1996;Grassme et al, 2017;Badaoui et al, 2020;Mang et al, 2020;Turner et al, 2020), but not necessarily clustering (Xu et al, 2016) were necessary to cause cells to respond. Recent evidence demonstrates that β1 integrins associated with focal adhesions contain nanoclusters with distinct populations of both active and inactive integrins suggesting two independent pools depending on activation state (Spiess et al, 2018).…”

Successful treatment of acquired reactive perforating collagenosis with itraconazole

“…The collagen eliminated in RPC may be derived from the basement membrane zone [ 6 ]. Angiogenesis is a crucial mechanism of vascular growth and regeneration that requires biosynthesis and cross-linking of collagen in vivo and is induced by collagen in vitro [ 7 ]. The anti-angiogenic, anti-Hedgehog pathway, and anti-inflammatory properties of itraconazole have been widely reported [ 6 ].…”

Extracellular matrix: The proteins that function throughout the body

“…The article focused on the fibrillin‐1 expression in aorta and found that it was located to the medial elastic lamellae and that vascular smooth muscle and endothelial cells contribute to the synthesis and deposition (cover, Charbonneau et al, 2020). Vascular endothelial cells also respond to Type I collagen in vitro by clustering alpha2 beta1 integrin receptors as a mechanism of vascular growth and regeneration, and the authors propose that Type I collagen may have applications for promoting vascularization of engineered tissues (Turner et al, 2020). Another unique protein found in endothelial basement membranes is Willebrand factor A‐domain‐related protein (Fitzgerald, 2020).…”