Integrins are heterodimeric transmembrane cell adhesion molecules made up of alpha (α) and beta (β) subunits arranged in numerous dimeric pairings. These complexes have varying affinities to extracellular ligands. Integrins regulate cellular growth, proliferation, migration, signaling, and cytokine activation and release and thereby play important roles in cell proliferation and migration, apoptosis, tissue repair, as well as in all processes critical to inflammation, infection, and angiogenesis. This review presents current evidence from human and animal studies on integrin structure and molecular signaling, with particular emphasis on signal transduction in infants. We have included evidence from our own laboratory studies and from an extensive literature search in databases PubMed, EMBASE, Scopus, and the electronic archives of abstracts presented at the annual meetings of the Pediatric Academic Societies. To avoid bias in identification of existing studies, key words were short-listed prior to the actual search both from anecdotal experience and from PubMed’s Medical Subject Heading (MeSH) thesaurus.
Integrins are a family of ubiquitous αβ heterodimeric receptors that interact with numerous ligands in physiology and disease. Integrins play a key role in cell proliferation, tissue repair, inflammation, infection, and angiogenesis.
This review summarizes current evidence from human and animal studies on integrin structure and molecular signaling and promising role in diseases of inflammation, infection, and angiogenesis in infants.
This review shows that integrin receptors and ligands are novel therapeutic targets of clinical interest and hold promise as novel therapeutic targets in the management of several neonatal diseases.
Integrins are a family of ubiquitous αβ heterodimeric receptors that exist in multiple conformations and interact with a diverse group of ligands. These molecules mediate interactions between cells and of these cells with the extracellular matrix (ECM) and thereby serve a critical role in signaling and homeostasis. By facilitating dynamic linkages between the intracellular actin cytoskeleton and the ECM, integrins also transduce both external and internal mechanochemical cues and bi-directional signaling across the plasma membrane.1,2 Integrins are involved in a diverse range of body processes, including cellular survival, inflammation, immunity, infection, thrombosis, angiogenesis, and malignancy. In this review, we highlight the structure and function of integrins; the mechanisms involved in integrin activation and signaling; their role in inflammation, infection, and angiogenesis; and discuss current advances in integrin-targeted therapies. Understanding the factors that regulate integrin structure, function, and signaling would enable us to identify new therapeutic targets.
Structure of integrins
In mammals, the family of integrins is comprised of 24 αβ pairs of heterodimeric transmembrane adhesion receptors and cell-surface proteins. These pairings are known to involve 18 α and 8 β subunits (Fig. 1),3 and their non-covalent associations involve an α and another β subunit (Fig. 2).4 The αβ pairings of integrin subunits dictate the specificity of the integrin to a particular ligand, modulate formation of intracellular adhesion complexes, and regulate downstream signaling.1 Six α (α1–6) and seven β (β1–7) subunits are known to form several unique αβ subunit associations (Fig. 1). Interestingly, the earliest discovered integrins, lymphocyte function-associated antigen 1 (integrin αLβ2) and macrophage antigen 1 (integrin αMβ2), derive their specificity from specific α subunits, but these share the same β subunit.5
Integrin α subunit family
The integrin α subunits carry a 200 amino acid “inserted” domain, the I-domain (αI). When present on an integrin, the αI domain is an exclusive ligand-binding site. αI integrins have 13 extracellular domains in 2 subunits, which interact with a variety of ligands. The I-domains are seen in 6 out of the 15 integrin α subunits.
Integrin β subunit family
In humans, integrin β subunits have a cytoplasmic tail that have <75 amino acids in length, except the β4 tail that is about 1000 amino acids long (includes four fibronectin type III repeats).3 The integrin β tails have one or two NPxY/F motifs (x refers to any amino acid) that recognize protein modules, phosphotyrosine-binding domains, that are involved in several signaling and cytoskeletal proteins at the cytoplasmic face of the plasma membrane through phosphorylation of the tyrosine (Y) in the NPxY/F motif.3 The integrin β subunit family includes β1–7, which bind the α subunits in different combinations. The most frequently seen β subunit integrin heterodimers is β1. Although β2 integrins show functional overlap, the corresponding α subunit defines its individual functional properties.6 The β2/CD18 chain has also received attention because of its involvement in several inflammatory receptors such as CD11a/CD18, αLβ2, lymphocyte function-associated antigen-1 (LFA-1); CD11b/CD18, αMβ2, Mac-1, complement receptor 3 (CR3); CD11c/CD18 (αXβ2, p150.95, CR4); and CD11d/CD18, αDβ2; Fig. 1). In these β2 integrins, the α subunits bind specific ligands such as the intercellular adhesion molecules (ICAMs). The non-I-domain α subunits in other integrins, such as the laminin-binding α3, α6, and α7, and others that recognize the arginine (R), glycine (G), aspartic acid (D) (RGD) motif (αV, α8, α5, and αIIb), are also closely related to each other.7 The α subunit of each integrin is the primary determinant of its extracellular ligand specificity. The β chain binds acidic residues in ICAMs and in cytoplasmic adapters such as paxillin, talins, and kindlins to facilitate cellular adhesions with the ECM. Integrins interact with the actin cytoskeleton through the talin- and kindlin-binding motifs present in the cytoplasmic domains of their β subunits.8
Characteristics of specific integrin heterodimers
Integrin αβ heterodimers are divided into four classes (leukocyte, collagen-binding, Arg-Gly-Asp (RGD)-binding, and laminin-binding integrins; Fig. 1) based on evolutionary associations, ligand specificity, and restricted expression on white blood cells (β2 and β7 integrins). Leucocyte integrins have a common β2 chain that is linked to CD-18 and bind receptors such as ICAM and plasma proteins such as complement components C3b and C4b.9 Collagen-binding integrins have a common β1 chain that binds various α chains in integrins α1β1, α2β1, α10β1, and α11β1. The α2β1 integrin binds its primary ligand, collagen,10 and chondroadherin, a matrix protein.11 The RGD-binding integrins have a common αV chain or β1 chain. The RGD peptide motif was first discovered in fibronectin12 but was later found in several other ECM proteins, such as fibronectin,9 osteopontin,13 vitronectin,14 von Willebrand factor (VWF),15 and laminin.16 Among the 24 human integrin subtypes known to date, eight integrin dimers recognize the tripeptide RGD motif within ECM proteins, namely: αVβ1, αVβ3, αVβ5, αVβ6, αVβ8, α5β1, α8β1, and αIIbβ3. Laminin-binding integrins (α3β1, α6β1, α7β1, and α6β4) mediate the adhesion of cells to basement membranes in various tissues.9 The α4β1, α9β1, and α4β7 integrin family binds fibronectin in a RGD-independent manner.9
Integrin–ligand binding and consequent activation
The structure and function of integrins are complex. Integrins bind numerous extracellular ligands, intracellular signaling molecules, and the cytoskeleton in a bivalent-cation-dependent manner with varying specificities. Integrins also have many states with multiple conformations and affinities.
Mechanism of integrin ligand binding and conformational states
Integrins bind cell-surface ligands to promote cellular interactions with the ECM and with other cells in the transduction of complex signals that modulate many cellular processes, such as adhesion, migration, and differentiation. These soluble, ECM, or cell surface-bound ligands may include growth factors, structural constituents of the ECM, proteases, cytokines, plasma proteins, microbial pathogens, or receptors specific to immune cells. The affinity and avidity of a ligand may change actively by inside-out signaling in specific pathways. Ligand affinity may vary with the strength of interaction and dissociation of a monovalent protein and its ligand, where ligand avidity refers to its ability to form multiple combinations of bonds.17
Integrins exist primarily in three conformational states: bent–closed (inactive; the predominant state), extended–closed (active; low affinity or intermediate state), and the extended–open (active; high affinity).18 The affinity of integrins to various inhibitory and stimulatory ligands is modulated by bivalent cations, which induce a range of conformational changes in integrins ranging from a folded, inactive, and low-affinity state to a high-affinity conformation (Fig. 2).19 These conformational changes in the extracellular domains of integrins modulate both ligand binding and downstream cellular signaling.
The activation of integrins increases the affinity of these molecules to extracellular ligands. Integrin tail domains play a critical role in these steps, and any genetic mutations in these parts of integrins can disrupt downstream intracellular signaling.20 Integrin-mediated signaling across cell membranes is typically bi-directional and termed “outside-in” and “inside-out” signaling.20,21 When integrins interact with ECM ligands, a conformational change allows adherence to downstream adaptor molecules in the cell-membrane plane.22 Once clustered, integrins are able to recruit and activate kinases such as Src family kinases, focal adhesion and scaffold molecules such as the adaptor protein p130CRK-associated substrate/breast cancer anti-estrogen resistance 1 (p130CAS/BCAR1).22 These integrin-associated complexes include discrete active and inactive integrin organizations, which can activate unique signaling pathways.23,24
The extracellular domains of integrins are known to undergo a diverse range of conformational changes that alter the ligand-binding domains. In the cytoplasmic tails of integrins, α-helices are seen as heterodimers,25 and the β-strands often bind intracellular proteins, such as talin or filamin.26,27 The cytoplasmic tail may undergo several specific conformational changes to bind a range of other signal transducers.28,29
Integrin bi-directional inside-out and outside-in signaling
Mechanical stress30 and extracellular chemicals31 can induce rapid conformational changes to cause inside-out activation of integrins.32 Integrins display bi-directional signaling across the plasma membrane. Ligand binding induces extracellular-to-cytoplasm signal transduction, and inside-out signaling or priming regulates integrin-ligand binding conformations (Fig. 2). During integrin activation and signaling, the cytoplasmic tail acts as both a receptor and transmitter of signals. Specifically, during inside-out signaling, the activating signals make an impression on the cytoplasmic tail to induce large conformational changes to the extracellular domain, thereby transforming the integrin from a resting to an active state.33 During outside-in signaling, the binding of a ligand to the extracellular domain of active integrin transmits a conformational change to the cytoplasmic tail, which leads to the activation of kinases and adaptor molecules in the cytosol.1 In contrast, talin and kindlin interaction with the β-cytoplasmic tail can trigger inside-out signaling, leading to integrin activation, clustering, and recruitment of intercellular adaptor proteins to strengthen cellular adhesion. Talin is a large dimeric actin-binding protein and a major regulator of integrin activation, and the regulation of talin–integrin interactions is important in the control of integrin activation and signaling pathways.33 Direct interactions between the talin head and the short cytoplasmic tails of β integrin subunits disrupt inhibitory interactions between α and β integrin subunits.33 This leads to conformational changes in the integrin extracellular domains and consequent increase in their ligand affinity. The role of kindlins are not clearly defined, but they are structurally related to the talin head. The synergistic binding of talin and kindlin to β integrin cytoplasmic tails induces integrin activation by disrupting the α–β interactions at the transmembrane and the cytoplasmic domains.33,34
Integrins in inflammation and infection
In the resting state, β2 integrins are expressed specifically on leucocyte receptors. During inflammation, the inflammatory cytokines activate these integrins and promote cellular adherence to the counter-receptors such as ICAMs and promote phagocytosis and cytotoxic killing. Integrin receptors on leukocytes, such as the macrophage-1 antigen (Mac-1, also known as CR3, αMβ2, CD11b/CD18) interact with platelet antigens such as the glycoprotein Ibα (GPIbα) during inflammation. Integrins bind to the pro-domain of transforming growth factor (TGF)-β1 to activate it and promote its secretion. The pro-TGF-βs are biosynthesized and stored in tissues in latent forms, and integrins αVβ6 and αVβ8 can uniquely bind and activate pro-TGF-β1 and pro-TGF-β3. The αVβ6 integrin is known to specifically bind the RGDLXXL/I motif in TGF-β1 and TGF-β3.35
β2 integrins promote recruitment of leukocytes to the sites of inflammation by promoting the adhesion of circulating leukocytes to vascular endothelium, transendothelial migration,36,37 the formation of immunological synapses in leucocytes,38 and inflammatory signaling in involved cells.39 Activated β2 integrins on dendritic cells (DCs) may act as negative regulators of DC migration in certain conditions and may also regulate T cell activation.40,41 β integrins on the leukocyte surface are also involved in the tethering, rolling, and adhesion of leukocytes to activated endothelial cells.42 β2 integrins can also initiate intracellular signaling pathways in macrophages and neutrophils and stimulate cytokine secretion from these cells either directly or in synergy with Toll-like receptors (TLRs).43 Integrins may also integrate the impact of the epidermal growth factor receptor, platelet-derived growth factor receptor, insulin receptor, met receptor superfamily (hepatocyte growth factor receptor), and the vascular endothelial growth factor receptor (VEGFR) in inflammatory cells.44
β2 integrins are important regulators of adhesion, leukocyte recruitment, and immunological signaling. These integrins mediate adhesive interactions between myeloid cells, endothelial cells, antigen-presenting cells, T cells and the ECM.45 L-selectin, the CCR7 chemokine receptor, interacts with specific carbohydrate epitopes on the endothelium and promotes leukocyte rolling and transmigration through the vascular endothelium.46 Leukocyte rolling induces a rapid, although transient, increase in the affinity of the β1 and β2 integrins to the endothelial ligands.47,48 Conformational changes in the structure of the inserted (I) domain of the αL subunit of LFA-149 enhance firm leukocyte adhesion under shear flow.31,49
Role of integrins in neonatal organs during normal development and inflammation
Integrins in the lung during normal development and in inflammation
Integrins and receptor tyrosine kinases act with cytokine and growth factors to modulate the extracellular signal-regulated kinase and phosphatidylinositol 3-kinase (PI3K)-AKT signaling pathways during regeneration, inflammation, developmental, and pathological processes in the developing lung.2,44,50,51 The ECM in the lung contains collagen, fibronectin, laminin, and entactin,52 and alterations in the formation and structure of the ECM during normal development, healing from injury, or in chronic lung disease could lead to profound alterations in the lung structure.53,54 For instance, fibronectin in the ECM promotes integrin-mediated cellular migration and differentiation of cells during lung development.55 β1 integrin activates several signaling pathways, particularly the PI3K/AKT pathway activated during wound healing in the presence of collage VI in the lung ECM.56 β1 integrins play a critical role in alveolar homeostasis, as seen in chronic lung disease depicted in β1 integrin-deficient mice.57 In addition, β1 integrin-deficient alveolar epithelial cells produce excessive monocyte chemoattractant protein 1 and reactive oxygen species, suggesting that β1 integrins may be involved in alveolar homeostasis.58 In murine models of bronchopulmonary dysplasia, perinatal exposure to lipopolysaccharide and increased expression of interleukin-33 may activate neutrophils and promote fibronectin degradation in alveolar epithelial cells.59 Other studies have noted increased expression of integrin α2β1 on mast cells and activation/release of inflammatory cytokines.60,61 Similar findings have been noted in murine models with Listeria monocytogenes infections.62 Mice deficient in integrin α263 and integrin αIIb64 show defective platelet interaction with collagen. α2β1 integrin-null mice have normal angiogenesis but may have altered angiogenic responses during injury repair.65 In contrast, integrin β1 knockout mice may have altered development and are not viable, indicating an essential role of β1 during development. Table 1 outlines murine models of integrins, their target tissues, and signaling.
Integrins in intestinal inflammation and in necrotizing enterocolitis (NEC)
The regulation of intestinal leukocyte responses is vital to maintaining immune homeostasis and prevention of intestinal inflammatory conditions. Integrin αvβ5 is expressed on neonatal intestinal macrophages; the expression is developmentally regulated and is not dependent on microbial colonization. These integrins bind different ECM components, such as laminins, collagens, and fibronectin, and are known to coordinate epithelial cell adhesion and movement.4,66 These integrins recognize the RGD tri-peptide sequence present in ECM proteins, such as fibronectin and vitronectin.67,68 The integrin αVβ5 can be found in both focal adhesions and in clathrin-coated membrane domains.69,70
Integrin αvβ8 plays an important role in epithelial homeostasis and is a major activator of TGF-β expression.71 α3 and β1 integrins, which are known to increase epithelial migration, are upregulated by bacterial products during NEC.72,73 TLR4 signaling on enterocytes promotes the efflux of β1 integrins from the cytoplasm toward the cell membrane and enhances cell–matrix contacts that limit cellular movement.74 In other studies, Besner and colleagues have examined the role of E-cadherin and integrins in NEC and showed that the growth factor, heparin-bound epidermal growth factor, can promote intestinal restitution in NEC through its effects on integrin–ECM interaction and intercellular adhesions.75 Intestinal epithelial cells also express α3β1, another set of integrins of translational importance. In NEC, increased epithelial expression of α3β1 may impair the migration of epithelial cells needed for mucosal wound healing.74 However, the same α3 integrins are also required for morphologic differentiation of the intestinal epithelium in the developing intestine.76 Despite the physiological needs of the β1 integrins, therapeutic targeting of these molecules may still be possible with information on the best timing and the possibility of regionally focused intervention.
Integrins in the developing eye and in retinopathy of prematurity
In the developing eye, disruption of the oxygen supply to the retina can disrupt neuronal dysfunction needed for transduction and transmission of photosensitive visual signals to the occipital lobe and other cognitive centers. Integrin α2β1 and VEGF interact closely in several intracellular angiogenic signaling (Fig. 3).77,78 Cyclic peptides selectively inhibit αVβ3 and αVβ5, and are potent inhibitors of endothelial cell invasion and differentiation induced by VEGF-A or fibroblast growth factor-2.78 Integrin αVβ3 works synergistically with VEGF to activate angiogenesis in endothelial cells via VEGFR‐2 phosphorylation.79 Endothelial cells are the primary cells expressing both VEGFR-2 and α2β1 integrin.80 Proteoglycans such as decorin and perlecan in the ECM of the eye can modulate α2β1 and play a vital role in angiogenesis.80 The C-terminal fragment of perlecan, known as endorepellin, has an opposite effect and blocks angiogenesis through antagonism of VEGFR-2 and α2β1 integrin on endothelial cells.80 Retinal pigment epithelial cells express beta‐8 integrin at the surface, and the knockdown of beta-8 integrin significantly decreased retinal pigment epithelial cell migration in wound-healing assays.81
The retinal tissue has one of the body’s highest metabolic demands, placing it at risk of injury from oxidative stress, metabolic derangements, and consequent pathologic neovascularization seen in retinopathy of prematurity (ROP) and other proliferative retinal vitreoretinopathies. ROP is a bi-phasic disease of retinal vascular development due to dysregulation of VEGF.82,83 In phase 1, VEGF is downregulated during exposure to hyperoxia, while in phase 2, VEGF is upregulated in relative/true hypoxia. VEGF is known to have several isoforms; VEGFA165 is the predominant isoform in the eye with multiple pro- and anti-angiogenic splice variants.84 In a newborn mouse model of oxygen-induced retinopathy (OIR), oxidative stress from fluctuating hyperoxia and hypoxia leads to altered vascular development with tortuous arteries, dilated veins, and capillary attrition, akin to human ROP.82,85 These changes persist in adult mice with long-term abnormalities in vascularization, structure, and function both in vivo and histologically.86,87
Integrin-targeted therapy holds promise in ROP. Targeting α2β1 integrin expression on endothelial cells mitigates OIR,88 and the administration of 3-[3-(6-guanidino-1-oxoisoindolin-2-yl) propanamido]-3-(pyridin-3-yl) propanoic acid dihydrochloride, a novel non-peptide αvβ3 antagonist, can inhibit retinal neovascularization.89 There are exciting possibilities that endothelial α2β1 may be therapeutic target in pathological angiogenesis.
Integrins in thrombosis and fibrosis
Platelet adhesion and signaling play key roles in hemostasis and thrombosis. Two platelet receptors, integrin αIIbβ3 and GPIbα, mediate the early and mid-stages of platelet adhesion in the vascular environment.90 GPIbα is a key part of the receptor for VWF, and its binding to VWF enables platelet rolling during the formation of thrombotic plugs at the sites of vascular injury.91,92,93 αIIbβ3 is expressed on both platelets and the endothelium, and upon activation, it promotes platelet adhesion and aggregation by cross-linking with soluble fibrinogen, fibronectin, and VWF.
In alloimmune thrombocytopenia, autoantibodies are frequently seen against integrin β3 and GPIbα.94,95 Intracranial hemorrhages may be seen more frequently in infants with anti-β3 integrin antibodies than in those with antibodies against GPIbα.96 Existing in vitro and in vivo data suggest that the β3 integrin may bind a wider range of ligands, including fibrinogen and VWF, and autoantibodies that block its function may induce a deeper functional deficit than the anti-GPIbα antibodies.97
Integrin dysregulation is implicated in the pathogenesis of numerous diseases with altered angiogenesis, inflammation, or in infectious diseases. In these conditions, therapeutic strategies may either directly target integrins or their ligands. Out of the 24 known human integrins, many have already been identified as therapeutic targets for monoclonal antibodies, peptides, and/or small molecules. In adult subjects, efforts are ongoing to target platelet integrin αIIbβ3 to prevent thrombotic complications after percutaneous vascular interventions, lymphocyte α4β1 and α4β7 integrins in the treatment of multiple sclerosis, and β7 integrins (α4β7 and αEβ7 integrins) in inflammatory bowel disease.98 Specifically, a humanized anti-α4 antibody (Natalizumab) works in reduction of inflammation in multiple sclerosis by blocking the α4β1–vascular cell adhesion molecule interaction or the α4β7–mucosal addressin cell adhesion molecule interaction on mucosal endothelium and blocking leukocyte trafficking across the blood–brain barrier.99 In another study, a micellar delivery vehicle decorated with an anti-angiogenic peptide has been shown to inhibit αVβ3-mediated neovascularization in endothelial cells.100 Several anticancer drugs have also been developed against integrin ligands or by using integrin-targeted encapsulated nanoparticles as vehicles to unload drugs into the vasculature of several tumors.101 In a mouse model of hepatic fibrosis, cyclic peptide-guided liposomes preferentially targeted the activated hepatic stellate cells (not quiescent ones) to treat the fibrotic phenotype.102 αVβ3 antagonists are being tried for the inhibition of retinal neovascularization and may have therapeutic value in ROP.89 In a mouse model of laser-induced choroidal neovascularization, intravenous injection of irradiated nanoparticles loaded with doxorubicin allowed nanoparticle accumulation in the neovascular lesions and reduced the size of neovascular lesions.103 Integrin antagonists may also be used in fibrotic diseases; IDL-2965 is being studied as a selective, highly potent, anti-fibrotic integrin antagonist in idiopathic pulmonary fibrosis.104 Small molecule pure antagonists, TDI-4161 and TDI-3761, have been designed to inhibit αVβ3-mediated cell adhesion to αVβ3 ligands.105 Further studies are needed to improve the specificity of anti-integrin drugs to improve both the safety profile and therapeutic success of these agents.
Enhanced understanding of integrin ligand interactions will enable development of therapies targeting specific receptors in order to modulate angiogenic, thrombotic, infections, and inflammatory disorders. Although numerous animal studies have shown promise in the clinical use of integrins as therapeutic targets, there is a need for clinical studies to confirm efficacy and safety in neonates and young infants. In this review, we have summarized and outlined the roles of integrins in inflammation, angiogenesis, and infectious conditions. Therapies could be targeted specifically to alpha subunits, but their overlapping roles are a critical factor to be considered. Further studies are needed both on molecular signaling and regulatory mechanisms of integrin function and the safety and efficacy in clinical settings.
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Mezu-Ndubuisi, O.J., Maheshwari, A. The role of integrins in inflammation and angiogenesis. Pediatr Res 89, 1619–1626 (2021). https://doi.org/10.1038/s41390-020-01177-9
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