Beyond the Matrix: The Many Non-ECM Ligands for Integrins

Abstract

The traditional view of integrins portrays these highly conserved cell surface receptors as mediators of cellular attachment to the extracellular matrix (ECM), and to a lesser degree, as coordinators of leukocyte adhesion to the endothelium. These canonical activities are indispensable; however, there is also a wide variety of integrin functions mediated by non-ECM ligands that transcend the traditional roles of integrins. Some of these unorthodox roles involve cell-cell interactions and are engaged to support immune functions such as leukocyte transmigration, recognition of opsonization factors, and stimulation of neutrophil extracellular traps. Other cell-cell interactions mediated by integrins include hematopoietic stem cell and tumor cell homing to target tissues. Integrins also serve as cell-surface receptors for various growth factors, hormones, and small molecules. Interestingly, integrins have also been exploited by a wide variety of organisms including viruses and bacteria to support infectious activities such as cellular adhesion and/or cellular internalization. Additionally, the disruption of integrin function through the use of soluble integrin ligands is a common strategy adopted by several parasites in order to inhibit blood clotting during hematophagy, or by venomous snakes to kill prey. In this review, we strive to go beyond the matrix and summarize non-ECM ligands that interact with integrins in order to highlight these non-traditional functions of integrins.

Keywords: bacteria; counterreceptor; disintegrin; extracellular matrix; growth factor; hormone; immune system; integrin; pathogen; stem cell; venom; virus.

Figure 1

Integrin heterodimers and their ligands. Integrins are heterodimeric cell surface receptors that bind extracellular matrix (ECM) molecules. In addition to this role, integrins also bind many non-ECM ligands. Integrin subunits connected by a ray represent heterodimeric α/β binding partners. The inner ring depicts integrin heterodimers grouped into families based upon their classical binding profile. These families include RGD receptors, collagen (GFOGER) receptors, laminin receptors, or leukocyte-specific receptors. Within the outer ring, the non-ECM ligands of these families are listed. Non-ECM ligands include growth factors, hormones, venomous compounds, disintegrins, bacterial proteins, fungal polysaccharides, viruses, polyphenols, and counterreceptors.

Figure 2

Integrins act as “double agents” during Helicobacter pylori infection in the stomach, serving to potentiate bacterial pathogenicity while also aiding in the immune response. H. pylori bacteria in the gastric lumen bind integrins on gastric epithelial cells in order to inject the virulence factor CagA. As shown in the magnified view of this process, docking of α5β1 integrin is achieved through integrin affinity for the RGD motif of the CagL protein component of the type IV secretion system (T4SS). Integrin α5β1-mediated stabilization of the T4SS facilitates the translocation of CagA while activating intracellular kinases. Once in the cytosol, CagA is phosphorylated by Src family kinases (SFKs) and Abelson (ABL) kinases, which potentiates its virulence. Phospho-CagA activates Src homology 2 domain-containing phosphatase-2 (SHP-2) and mitogen-activated protein kinase (MAPK) signaling, triggering cytoskeletal remodeling. CagA disrupts cell-cell junctions, activates the nuclear factor-κB (NF-κB) pathway, and stimulates cytokine production. Alternatively, CagL docking with αVβ5 integrin on gastric G cells activates integrin-linked kinase (ILK), which stimulates epidermal growth factor receptor (EGFR) and MAPK activation, inducing gastrin production. These mechanisms increase the permeability of the gastric epithelium, which aids H. pylori dissemination into the underlying lamina propria. This stimulates an inflammatory response causing the release of the antimicrobial peptide LL-37 from gastric epithelial cells and recruitment of immune cells from the blood stream. As shown in the magnified view of the recruitment process, leukocytes first stick to inflamed endothelium through selectin binding, which facilitates integrin-mediated tight adhesion. This leads to leukocyte extravasation into the lamina propria, where neutrophils and macrophages phagocytize bacteria. Phagocytosis is mediated through integrin recognition of the opsonization factors LL-37 and complement. Neutrophil extracellular traps (NETs) are stimulated through integrin interaction with pathogens.

Figure 3

Viruses hijack integrins for adhesion and infectivity. Virus families use specific integrins in order to adhere to target cells for the purposes of internalization and infectivity. Members of the family Adenoviridae are non-enveloped viruses with icosahedral capsids that have penton base structures which facilitate RGD-dependent docking with αVβ1, αVβ3, αVβ5, and α5β1 integrins as well as the RGD-independent engagement of α3β1. Adenoviruses also target αMβ2 integrin through an undetermined mechanism. Birnaviridae contains members who employ a fibronectin-mimicking IDA peptide to bind α4β1 integrin. Members of the Flaviviridae family have an RGD-containing E-protein which binds αVβ3 integrin. Viruses in the family Hantaviridae target the plexin-semaphorin-integrin (PSI) domain of αVβ3 and αIIbβ3 integrins. Herpesviridae has members that employ a few different mechanisms of integrin engagement for the purposes of viral entry. The envelope protein BMRF-2 contains an RGD sequence that docks α5β1 integrin. The envelope proteins gH and gL dock with αVβ5, αVβ6, and αVβ8. Another envelope protein, known as gB, contains both an RGD motif and disintegrin-like domain, which affords viral targeting of αVβ3, αVβ5, α2β1, α3β1, α6β1, and α9β1 integrins. Members of the Picornaviridae family use capsid proteins to target integrins. The targeting of α2β1 integrin proceeds in an RGD-independent manner, while αVβ1, αVβ3, αVβ6, and α5β1 integrins are bound in an RGD-dependent fashion. Reoviridae contains members which employ a DGE sequence within a VP4 capsid protein to engage α2β1. Additionally, the reovirus VP7 capsid protein has a GPR tripepetide which recognizes αXβ2, an LDV tripeptide that ligates α4β1, and a novel NEWLCNPDM amino acid sequence that targets αVβ3. Togaviridae has members which have a collagen-mimicking spike protein that docks α1β1 integrin.

Figure 4

Integrins serve as cell surface receptors for growth factors, hormones, and small molecules. Various growth factors use integrins as cell surface receptors. Angiopoietin-like proteins (ANGPTLs) bind α5β1 and αVβ3 integrins to facilitate a host of cellular effects. Pro-TGFβ is activated by αVβ3, αVβ5, αVβ6, and αVβ8 through the integrin-dependent dissociation of an RGD-containing latency-associated peptide (LAP), thus converting it to its active form. Activated TGFβ acts as a master regulator of fibrosis, among other roles. Vascular endothelial growth factor (VEGF) ligates α3β1, α9β1, αVβ3, and other αV-containing integrins, resulting in cellular effects that promote angiogenesis and lymphangiogenesis. The polyphenol trans-resveratrol, which is derived from grapevines, binds the β3 subunit of αVβ3 integrin near the RGD recognition pocket. This binding event induces extracellular signal-regulated kinase (ERK) activation and p53-dependent apoptosis, while promoting angiostasis. Like trans-resveratrol, the active form of testosterone (DHT) also binds the β3 subunit of αVβ3 integrin near the RGD pocket. DHT-αVβ3 interaction inhibits trans-resveratrol-induced effects and stimulates cellular proliferation. The thyroid hormones, T3 and T4, utilize αVβ3 integrin as a cell surface receptor to activate a range of signaling molecules which induce angiogenesis. When binding to αVβ3 integrin, the thyroid hormone analog tetrac blocks T3/T4 integrin-induced effects.