28.10: Integrins- Bidirectional Cell Adhesion Receptors

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Learning Goals 

(Learning goals written by Claude, Sonnet 4.6, Anthropic)

Integrin Structure and Ligand Recognition

• Describe the domain architecture of integrin αβ heterodimers—including the α subunit's seven-bladed β-propeller head, I-domain (where present), thigh, calf-1, and calf-2 domains, and the β subunit's PSI, hybrid, βI, and four EGF-repeat domains—and explain how the short cytoplasmic tails (20–70 residues in α; up to 1000 residues in β4) connect extracellular ligand binding to intracellular cytoskeletal and signaling proteins.
• Classify the four major integrin heterodimer families (leukocyte/β2, collagen-binding/β1, RGD-recognizing, and laminin-binding) by their shared subunit, ECM ligand specificity, and representative tissue functions, and explain how divalent cations (Ca²⁺, Mg²⁺, Mn²⁺) bound at MIDAS and ADMIDAS motifs facilitate ligand coordination—including the binding of aspartate in RGD motifs to Mg²⁺.

Conformational States and Bidirectional Activation

• Describe the three conformational states of integrin heterodimers—bent-closed (inactive, low affinity), extended-closed (intermediate affinity), and extended-open (high affinity)—and explain how the transition between states is driven mechanochemically by lateral cytoskeletal forces on the β subunit rather than tensile forces along the receptor length.
• Explain inside-out integrin activation: describe how intracellular signals (elevated Ca²⁺, DAG, and Rap1-GEF activity) recruit talin and kindlin to the NPxY motifs on the β cytoplasmic tail, disrupt the α-β salt bridge through talin's F3/PTB domain interaction, and drive the conformational extension that increases ligand-binding affinity.
• Explain outside-in integrin signaling: describe how multivalent ECM ligand binding triggers integrin clustering, autophosphorylation of focal adhesion kinase (FAK) at Tyr397, recruitment and activation of Src family kinases, and downstream activation of Rho-family GTPases (Rac, Cdc42, Rho) that remodel the actin cytoskeleton and regulate cell adhesion, migration, and gene expression.

Integrins in Inflammation, Infection, and Disease

• Explain how β2 integrins on leukocytes mediate the sequential steps of inflammation—rolling, tethering, firm adhesion to ICAM-expressing endothelium, and transendothelial migration—and describe how inflammatory cytokines rapidly upregulate β2 integrin affinity through inside-out signaling to promote leukocyte recruitment to sites of tissue injury.
• Describe how integrins cooperate with growth factor receptors (EGFR, PDGFR, VEGFR, insulin receptor) and Toll-like receptors to amplify inflammatory and angiogenic signaling, and explain how αVβ6 and αVβ8 integrins uniquely activate latent TGF-β1 and TGF-β3 by binding their RGDLXXL/I motifs, linking integrin biology to immune regulation and tissue remodeling.

This section was derived almost completely, with some modifications and additions, from the following source: 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. https://doi.org/10.1038/s41390-020-01177-9. Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/. Consult the original article for specific references.

Introduction

Integrins link the extracellular matrix to the intracellular cytoskeleton through a single transmembrane alpha-helical segment. They work with growth factor receptors to regulate cell survival, migration, and division. They contain a very large extracellular domain, which comprises most of the protein, and a very small intracellular domain.

Integrins are heterodimeric transmembrane cell adhesion molecules 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, playing essential roles in apoptosis, tissue repair, and all processes critical to inflammation, infection, and angiogenesis.

Integrins are a family of ubiquitous αβ heterodimeric receptors that exist in multiple conformations and interact with diverse ligands. These molecules mediate interactions between cells and the extracellular matrix (ECM), thereby playing a critical role in signaling and homeostasis. By facilitating dynamic linkages between the intracellular actin cytoskeleton and the ECM, integrins also transduce external and internal mechanochemical cues and bidirectional signaling across the plasma membrane. Integrins are involved in diverse 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 of integrin activation and signaling; their roles in inflammation, infection, and angiogenesis; and current advances in integrin-targeted therapies. Understanding the factors that regulate integrin structure, function, and signaling would enable the identification of new therapeutic targets.

Structure of integrins

In mammals, the family of integrins comprises 24 αβ pairs of heterodimeric transmembrane adhesion receptors and cell-surface proteins. These pairings involve 18 α and 8 β subunits as shown in Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Integrin heterodimers consist of numerous combinations of α and β subunits.

With respect to ligand specificity, integrins are generally classified as collagen-binding integrins (α₁β₁, α₂β₁, α₁₀β₁, and α₁₁β₁), Arg-Gly-Asp (RGD)-recognizing integrins (α₅β₁, α_Vβ₁, α_Vβ₃, α_Vβ₅, α_Vβ₆, α_Vβ₈, and α_IIbβ₃), laminin-binding integrins (α₃β₁, α₆β₁, α₇β₁, and α₆β₄), and leukocyte integrins (α_Lβ₂, α_Mβ₂, α_Xβ₂, and α_Dβ₂).

The right-hand side of Figure \(\PageIndex{1}\) shows that the β₂ integrin subunit (CD18) can pair with one of the four α subunits (α_L-CD11a, α_M-CD11b, α_X-CD11c, and α_D-CD11d), forming leukocyte function-associated antigen-1, Mac1/CR3 (macrophage-1 antigen, complement receptor 3), 150.95/CR4 (complement receptor 4), and CD18/CD11d, respectively.

The structure of the heterodimers and their non-covalent associations are shown in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Structurally, the αβ integrin subunits are type 1 transmembrane proteins.

Each subunit comprises one large multi-domain extracellular segment, one transmembrane helix, and a short cytoplasmic tail. The extracellular region interacts with extracellular matrix (ECM) ligands and contains about 1104 residues in the α subunit and 778 in the β subunit, with shorter cytoplasmic domains of 30–50 residues. The short cytoplasmic tails comprise 20–70 amino acids and mediate interactions with intracellular cytoskeletal and signaling proteins.

In response to intracellular or extracellular stimuli, integrin activation occurs through ligand binding or changes in the cytoplasmic domains, resulting in the elongation and separation of the legs. Integrins appear in a closed or “bent” conformation on resting cells and display a low binding affinity for ligands, rendering them inactive to ligand binding or signal transduction. In contrast, once activated, the integrin adopts an open conformation, leading to high affinity. In a closed conformation, integrins show low ligand-binding affinity, partly due to the bend in the center of the α and β subunits, which brings the ligand-binding site within 5 nm of the cell surface. However, when the conformation is open, the two subunits straighten, increasing integrin affinity for the ligand. The initial binding of extracellular ligands causes separation of the cytoplasmic domains, allowing interaction with signal transduction and cytoskeletal molecules during outside-in signaling. In contrast, the separation of the cytoplasmic domains by talin and other activators activates the head to enable ligand binding during inside-out signaling.

The αβ pairings of integrin subunits dictate the specificity of the integrin to a particular ligand, modulate the formation of intracellular adhesion complexes, and regulate downstream signaling. Six α (α_1–6) and seven β (β_1–7) subunits are known to form several unique αβ subunit associations, as shown in Figure \(\PageIndex{1}\). Interestingly, the earliest-discovered integrins, lymphocyte function-associated antigen 1 (αLβ2) and macrophage antigen 1 (α_Mβ₂), derive their specificity from distinct α subunits but share the same β subunit.

Integrin α subunit family

The integrin α subunits carry a 200-amino acid “inserted” domain, the I-domain (αI). The αI domain is an exclusive ligand-binding site when present on an integrin. αI integrins have 13 extracellular domains across 2 subunits and interact with various ligands. The I-domains are seen in 6 out of the 15 integrin α subunits.

Integrin alpha-1/beta-1 is a receptor for laminin and collagen. It recognizes the proline-hydroxylated sequence G-F-P-G-E-R in collagen. The human α₁ subunit (1179 amino acids) has the domain structure shown in Figure \(\PageIndex{3}\):

Figure \(\PageIndex{3}\): Domain structure of human integrin α₁

The green represents the Van Willibrand Factor Type A domain. The middle pinkish domains are FG-GAP extracellular domains, which are repeated up to 7 times in alpha-integrins. The orangish domain at the very C-terminus is the transmembrane helix domain (1142-1164). This membrane protein is very different from those we have seen before, as it has just a 15-amino-acid C-terminal tail exposed in the cytoplasm.

Figure \(\PageIndex{4}\) shows an interactive iCn3D model of the predicted structure of human Integrin alpha-1 (AlphaFold, P56199).

Figure \(\PageIndex{4}\): Predicted structure of human integrin alpha-1 (AlphaFold, P56199) (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...9r15PAYYbNbit8

The color coding is gray spacefill for the C-terminal transmembrane helix, yellow spacefill for the inhibitor binding pocket, and magenta for the collagen-binding site.

Integrin β subunit family

In humans, integrin β subunits have a cytoplasmic tail of <75 amino acids, except the β₄ tail, which is about 1000 amino acids long (includes four fibronectin type III repeats). The integrin β tails contain one or two NPxY/F motifs (where x represents any amino acid) that recognize protein modules, specifically phosphotyrosine-binding domains, which are involved in various signaling and cytoskeletal proteins at the cytoplasmic face of the plasma membrane. This recognition occurs through the phosphorylation of the tyrosine (Y) residue in the NPxY/F motif. The integrin β subunit family includes β_1–7, which bind the α subunits in different combinations. The most frequently seen β subunit integrin heterodimers are β₁.

Although β₂ integrins show functional overlap, the corresponding α subunit defines its individual functional properties. The β2/CD18 chain has also received attention for its involvement in several inflammatory receptors, including αLβ2 (LFA-1), αMβ2 (Mac-1), and complement receptor 3 (CR3). 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 (α_V, α₈, α₅, and α_IIb), are also closely related to each other.

• The α subunit of each integrin is the primary determinant of its extracellular ligand specificity.
• The β chain binds acidic residues on ICAMs and to cytoplasmic adapters such as paxillin, talins, and kindlins, facilitating cellular adhesion to the ECM. Integrins interact with the actin cytoskeleton through the talin- and kindlin-binding motifs in the cytoplasmic domains of their β subunits.

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 (as shown in Figure \(\PageIndex{1}\), based on evolutionary associations, ligand specificity, and restricted expression on white blood cells (β₂ and β₇ integrins).

• Leucocyte integrins have a common β₂ chain linked to CD-18 and bind receptors such as ICAM and plasma proteins such as complement components C3b and C4b.
• Collagen-binding integrins share a common β1 chain that binds various α chains, including α₁β₁, α₂β₁, α₁₀β₁, and α₁₁β₁. The α₂β₁ integrin binds its primary ligand, collagen, and chondroadherin, a matrix protein.
• The RGD-binding integrins share an αV or β₁ chain. The RGD peptide motif was first discovered in fibronectin and was later found in several other ECM proteins, including osteopontin, vitronectin, von Willebrand factor (VWF), and laminin.

Among the 24 human integrin subtypes known to date, eight integrin dimers recognize the RGD tripeptide motif in ECM proteins, namely α_Vβ₁, α_Vβ₃, α_Vβ₅, α_Vβ₆, α_Vβ₈, α₅β₁, α₈β₁, and α_IIbβ₃. Laminin-binding integrins (α₃β₁, α₆β₁, α₇β₁, and α₆β₄) mediate the adhesion of cells to basement membranes in various tissues. The α₄β₁, α₉β₁, and α₄β₇ integrin family binds fibronectin in an RGD-independent manner.

Figure \(\PageIndex{5}\) shows an interactive iCn3D model of the structure of α₆β₁ integrin in complex with laminin-511 (7CEC)

Figure \(\PageIndex{5}\): Structure of α₆β₁ integrin in complex with laminin-511 (7CEC) (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...CxCTt6caaWrPd6

The cyan chain is integrin α₆ and the magenta chain is integrin β₁. The laminin chains are α (brown), β (orange), and γ (red). Note that each of the laminin subunits interacts with the α₆β₁ integrin. Two carboxylates on the C-terminal region of the laminin γ chain interact with the metal ion-dependent binding sites on the integrin beta subunit and an Asp 189 in the alpha subunit.

Integrin–ligand binding and consequent activation

Integrins have complex structures and functions. They bind numerous extracellular ligands, intracellular signaling molecules, and cytoskeletal components 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 mediate cellular interactions with the ECM and other cells, transducing 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.

Integrins exist primarily in three conformational states: bent–closed (inactive; the predominant state), extended–closed (active; low affinity or intermediate state), and extended–open (active; high affinity). 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, as shown in Figure \(\PageIndex{2}\). These conformational changes in the extracellular domains of integrins modulate both ligand binding and downstream cellular signaling.

Integrin activation

The activation of integrins increases their affinity for extracellular ligands. Integrin tail domains play a critical role in these steps, and genetic mutations in these regions can disrupt downstream intracellular signaling. Integrin-mediated signaling across cell membranes is typically bidirectional and termed “outside-in” and “inside-out” signaling. When integrins interact with ECM ligands, a conformational change enables their association with downstream adaptor molecules in the cell membrane plane. Once clustered, integrins can recruit and activate kinases such as Src family kinases and focal adhesion kinase, and scaffold molecules such as the adaptor protein p130CRK-associated substrate/breast cancer anti-estrogen resistance 1 (p130CAS/BCAR1). These integrin-associated complexes include discrete active and inactive integrin organizations that can activate distinct signaling pathways.

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 and β-turns are connected with random coils.  The β-strands often bind intracellular proteins, such as talin or filamin. The cytoplasmic tail may undergo several specific conformational changes to bind a range of other signal transducers.

This section is derived from Mechanobiology. https://www.mbi.nus.edu.sg/mbinfo/ho...rin-activated/. Creative Commons Attribution-NonCommercial 4.0 International License.

Integrins can be activated from two directions: from the inside by regulated protein binding to their cytoplasmic tails, and from the outside by multivalent ligand binding. In either case, talin binding to the integrin β tails is an essential and final common step. Though the two processes are conceptually separate, they are mutually cooperative, i.e, one can lead to the other. Some structural studies using force application to mimic ligand/intracellular protein interactions suggested that the combined action of these two events favors the transition from a closed, low-affinity to an open, high-affinity conformation of integrin. Activation triggers bidirectional signaling, which is crucial for various anchorage-dependent events such as adhesion, cell spreading, migration, polarity, and ECM organization, thereby driving physiological changes. Figure \(\PageIndex{6}\) shows the different states of the integrin dimer in inside-out and outside-in signaling.

Figure \(\PageIndex{6}\) Integrin dimer in inside-out and outside-in signaling.

(A) shows integrin in a low-affinity, inactive, bent conformation. (B1) and (B2) show inside-out integrin activation by cytoplasmic proteins or outside-in integrin activation via extracellular matrix (ECM) ligands, both of which lead to the complete extension of the extracellular domains. (C) shows a high-affinity and active integrin characterized by separating the cytoplasmic leg domains.

Inside-out signaling

Signals received by other receptors foster the binding of talin and kindlin to the cytoplasmic end of the integrin β subunit at sites of actin polymerization. Substantial information on the signaling pathway leading to activation is available for integrin αIIbβ3.

Talin binds to the integrin β-tail via the F3 phosphotyrosine-binding (PTB) domain, forming a unique interaction with the membrane-proximal (MP) region of the integrin (NPxY motif). This permits competition between a conserved lysine on talin and an aspartic acid on integrin α, which is essential for α/β salt-bridge disruption and sufficient for integrin activation. Additional interactions via the basic patches in the FERM subdomain F2 help orient the β-subunit, promoting spatial separation of the cytoplasmic domains.

Kindlin is also an essential co-activator of integrin and binds to a membrane distal NxxY motif on β-integrin via its FERM F3 subdomain. A preceding threonine patch on integrins β1 and β3 that gets phosphorylated and a tryptophan on kindlin F3 are also required for binding. However, kindlins are not known to activate integrins independently but may render integrin-specific effects.

Outside-in signaling

Ligand binding to the external domain causes conformational changes that increase ligand affinity, modify protein-interaction sites in the cytoplasmic domains, and, hence, the resulting signals.

In addition to conformational changes that extend integrin dimers, multivalent ligand binding leads to integrin clustering, activating the Src family of kinases (SFKs) through autophosphorylation. SFKs phosphorylate tyrosines of the integrin cytoplasmic domain (NPxY motifs) and other proteins, leading to:

1. control of ligand binding strength
2. alteration of binding with signaling molecules (kinases, GTPases, and adaptors), which constitute dynamic adhesion structures such as focal adhesions and podosomes

Nevertheless, whether clustering triggers outside-in signaling to facilitate integrin activation or occurs after integrin activation is uncertain.

David G. Menter, Raymond N. DuBois, "Prostaglandins in Cancer Cell Adhesion, Migration, and Invasion", International Journal of Cell Biology, vol. 2012, Article ID 723419, 21 pages, 2012. https://doi.org/10.1155/2012/723419. Creative Commons Attribution License,

Let's look at another, more detailed representation of integrin states. Each αβ dimer recognizes a different intercellular adhesion molecule (ICAM), ligand, or protein substrate in the basement membrane or extracellular matrix. The α subunit dictates the ligand specificity by a seven-bladed β-propeller head domain connected to a leg support structure made of a "thigh", a "calf"-1, a "calf"-2, a transmembrane, and a cytoplasmic domain. The β subunit interacts with the cell cytoskeleton. It contains an N-terminal plexin-semaphorin-integrin (PSI) domain, a hybrid domain, a βI domain, four cysteine-rich epidermal growth factor (EGF) repeats, a transmembrane, and a cytoplasmic domain.

In many cases, the N-terminal β-I domain of a β subunit inserts into the 7-bladed β-propeller domain of an α subunit (α1, α2, α10, α11, αL, αM, αX, and αD) to form a bulbous-binding headpiece complex. The formation of integrin receptor complexes depends on divalent cations (i.e., Ca²⁺, Mn²⁺, Mg²⁺) that bind to metal-ion-dependent adhesion site (MIDAS) motifs in the α subunits and adjacent to MIDAS (ADMIDAS) motifs in β subunits found in the N-terminus of these receptors. Together, they join the α and β subunit termini to form an N-terminal headpiece. Figure \(\PageIndex{7}\) shows these detailed features of the integrin dimer structure.

Figure \(\PageIndex{7}\): Detailed domain/motif structures in integrin dimers. Menter and DuBois. https://doi.org/10.1155/2012/723419. Creative Commons Attribution License,

Three conformational states exist for α- and β-subunit complexes. (1) The unliganded conformation has a closed headpiece and a bent receptor structure with the EGF domains of the β-subunit touching the calf-1-calf-2 domains of the α-subunit. (2) The headpiece remains closed, but structural changes in the β-subunit EGF domains cause a separation from the calf-1-calf-2 domains of the α-subunits, causing an extended structure. (3) Conformational changes in the β6-α7 loops expose the ligand-binding site along with a complete separation of the β-subunit from the calf-1-calf-2 domains in the α-subunit. These conformational changes engage the specific integrin headpiece with its ligand.

Figure \(\PageIndex{8}\) shows an interactive iCn3D model of the headpiece of integrin α_IIbβ₃ in the headpiece extended and open conformation (3FCU)

Figure \(\PageIndex{8}\): Headpiece of integrin α_IIbβ₃ in the headpiece extended and open conformation (3FCU) (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...sG7piAXGhipg4A

The αIIb part of the headpiece is colored based on the secondary structure, with the yellow sheets comprising the beta propeller secondary structure motif. The gray chain is the β₃ chain. The side chains of the β₃ chain forming the binding interface between the α_IIb and β₃ chains are displayed as colored sticks. The three close metal ions (2 Ca²⁺ and 1 Mg²⁺) are important for ligand binding, with Mg²⁺ coordinating to the acidic side chains of integrin ligands. These metal ions are present before ligand binding. The RGD binding motif of some integrin ligands binds to Mg²⁺ through their aspartate. Without bound Mg²⁺, acidic side chains around the site would interfere with binding.

It appears that lateral forces are most important in activating integrins. This contrasts with tensile forces, which act along the length of the receptor. Tensile forces appear to stabilize the closed, extended low-affinity form. In contrast, lateral forces at the beta subunit, a mimic for moving cytoskeletal filaments, stabilized the open, extended, high-affinity form. This links conformational allosteric changes to cytoskeletal changes. The activation mechanism is hence mechanochemical.

Similar to conventional cell surface signal transducing receptors, integrins bind ligands and transmit information in an “outside-in signaling” as shown in the top panel of Figure \(\PageIndex{9}\). “Outside-in signaling” behavior typically involves the engagement of integrins with the extracellular matrix or ICAM surface receptors. When external factors bind to exposed ligand-binding sites on integrins, this results in conformational changes described in the previous section. Most ECM proteins exhibit multivalent or recurrent molecular patterns, which trigger integrin clustering. As cells engage the repetitive patterns in the ECM, these events coincide, activating intracellular signals. The myriad extracellular signals that cells encounter in their microenvironment mediate cell polarity, cytoskeletal structure, adhesion, migration, invasion, gene expression, cell survival, and proliferation.

Figure \(\PageIndex{9}\): Detailed view of "outside-in" and "inside-out" integrin signaling. Menter and DuBois, ibid

ECM ligands' “outside-in” binding to cell surface integrins stimulates conformational changes that activate focal adhesion kinase (FAK). FAK is then autophosphorylated on Tyrosine 397 near the catalytic domain, which binds Src. FAK contains a central kinase domain bordered by a FERM (protein 4.1, ezrin, radixin, and moesin homology) domain at the N-terminus and a focal adhesion targeting (FAT) sequence at the C-terminus. Activated Src interacts with human enhancer of filamentation1 (HEF1) and p130 CRK-associated substrate (p130CAS) scaffold proteins that help to regulate Src-FAK-Crk interactions with Rac positively. FAK also activates (PKL/Git2)-β-Pix complexes, and β-pix then serves as an exchange factor for Cdc42 or a scaffold protein to promote signaling via Rac and p21-activated protein kinases (PAK). FAK also interacts with actin-related proteins (ARP2 and ARP3), which are regulated by the Wiskott-Aldrich Syndrome Protein (WASP). ARP2/ARP3 initiates the polymerization of new actin filaments. FAK also influences actin contraction and polarization through another GTPase protein, Rho. The regulation of Rho GTPase hydrolysis of GTP (active) to GDP (inactive) form occurs through the opposing activities of guanine nucleotide exchange factor (GEFs). The GTPase regulator associated with FAK (GRAF) and p190RhoGAP blocks actin cytoskeleton changes.

In contrast, PDZRhoGEF and p190RhoGEF both serve to activate Rho. “Outside-in signaling” transfers integrin-mediated external signals to the inside of cells.“Inside-out signaling” depends on talin and kindlin. Both talin and kindlin contain FERM (4.1/ezrin/radixin/moesin) domains and a highly conserved C-terminal F3 domain. Talins bind β-integrin and actin at their C-termini, as well as vinculin. Kindlins bind integrins, the cell membrane, and various actin-adaptor proteins such as migfilin and integrin-linked kinase (ILK). Talin activation occurs through G protein-coupled receptors that increase cytoplasmic Ca²⁺ and diacylglycerol levels. This activates GEF activity with the Ras-proximate 1/Ras-related protein 1 (Rap1) GTPase. Rap1 then binds to the Rap1-GTP-interacting adaptor molecule (RIAM). RIAM recruits talin to the membrane and to the α- and β-integrin cytoplasmic domains. Kindlin interacts with the β-integrin cytoplasmic domain, stabilizing the activated state of the integrin complex. “Inside-out signaling” strengthens adhesive contacts and the appropriate force necessary for integrin-mediated cell migration, invasion, ECM remodeling, and matrix assembly.

In the case of “outside-in signaling” initiated by ECM proteins, a single ligand-binding event can trigger integrin activation. However, repetitive, regularly spaced molecular patterns provide a more effective stimulus. These adhesion-dependent sensory mechanisms initiate signal transduction within the cell by activating multiple pathways. Focal adhesions often form through cell interactions with the ECM substrate, initiating signal transduction via kinase cascades and other mechanisms.

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, promote cellular adherence to 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, α_Mβ₂, 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, thereby activating and promoting its secretion. The pro-TGF-βs are biosynthesized and stored in tissues in latent forms, and integrins α_Vβ₆ and α_Vβ₈ can uniquely bind and activate pro-TGF-β₁ and pro-TGF-β₃. The α_Vβ₆ integrin specifically binds the RGDLXXL/I motif in TGF-β₁ and TGF-β₃.

β₂ integrins promote the recruitment of leukocytes to the sites of inflammation by promoting the adhesion of circulating leukocytes to vascular endothelium, transendothelial migration, the formation of immunological synapses in leukocytes, and inflammatory signaling in involved cells. β integrins on the leukocyte surface are also involved in the tethering, rolling, and adhesion of leukocytes to activated endothelial cells. β₂ integrins can also initiate intracellular signaling pathways in macrophages and neutrophils, thereby stimulating cytokine secretion from these cells, either directly or in synergy with Toll-like receptors (TLRs). Integrins may also mediate the effects of the epidermal growth factor receptor, platelet-derived growth factor receptor, insulin receptor, met receptor superfamily (hepatocyte growth factor receptor), and vascular endothelial growth factor receptor (VEGFR) in inflammatory cells.

β₂ integrins are essential 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. L-selectin, the CCR7 chemokine receptor, interacts with specific carbohydrate epitopes on the endothelium and promotes leukocyte rolling and transmigration through the vascular endothelium. Leukocyte rolling induces a rapid, although transient, increase in the affinity of the β1 and β2 integrins to the endothelial ligands. Conformational changes in the structure of the inserted (I) domain of the α_L subunit of LFA-1 enhance firm leukocyte adhesion under shear flow.     

Summary

(Summary written by Claude, Sonnet 4.6, Anthropic)

Integrins are heterodimeric transmembrane adhesion receptors that physically connect the extracellular matrix (ECM) to the intracellular actin cytoskeleton through a single transmembrane α-helix per subunit. The human integrin family comprises 24 distinct αβ combinations assembled from 18 α and 8 β subunits, each pairing conferring distinct ligand specificity and downstream signaling properties. The large extracellular domains—dominated by a seven-bladed β-propeller head in the α subunit and a plexin-semaphorin-integrin (PSI)/hybrid/βI organization in the β subunit—recognize ECM ligands including collagen (GFPGER motif), the tripeptide RGD sequence in fibronectin, vitronectin, and von Willebrand factor, as well as laminins and cell-surface ICAMs. Ligand coordination at the headpiece depends critically on divalent cations at MIDAS and ADMIDAS sites, with Mg²⁺ directly coordinating the aspartate of RGD-containing ligands.

Integrins are not constitutively active but exist in three interconvertible conformational states: bent-closed (predominant resting state, low affinity), extended-closed (intermediate affinity), and extended-open (high affinity). Transitions between these states are driven mechanochemically—lateral cytoskeletal forces on the β subunit stabilize the open, high-affinity conformation, while tensile forces along the receptor axis stabilize the closed, extended low-affinity form. Activation is bidirectional. In inside-out signaling, intracellular cascades—often initiated by GPCR-mediated Ca²⁺ and DAG elevation, Rap1-GEF activation, and RIAM-mediated membrane recruitment—converge on talin and kindlin binding to NPxY motifs in the β cytoplasmic tail, disrupting the α-β transmembrane salt bridge and extending the extracellular domains. In outside-in signaling, multivalent ECM ligand binding drives integrin clustering, autophosphorylation of focal adhesion kinase (FAK) at Tyr397, and sequential recruitment of Src family kinases, scaffold proteins (p130CAS, paxillin), and GEFs that activate Rac, Cdc42, and Rho GTPases to reorganize the actin cytoskeleton, establish cell polarity, and drive migration and ECM remodeling.

In inflammation, β2 integrins on leukocytes orchestrate the multistep adhesion cascade—rolling, firm adhesion, and transendothelial migration—by rapidly transitioning to high-affinity states in response to inflammatory cytokines, enabling binding to endothelial ICAMs. Specialized integrin pairs, particularly αVβ6 and αVβ8, uniquely activate latent TGF-β1 and TGF-β3 stored in the ECM, linking integrin engagement directly to anti-inflammatory and tissue-remodeling cytokine release. The extensive crosstalk between integrins and growth factor receptors (EGFR, VEGFR, insulin receptor) as well as Toll-like receptors positions integrins as central integrators of mechanical, adhesive, and biochemical signals that collectively regulate cell survival, proliferation, angiogenesis, and immunity—making them important targets for therapeutic intervention in inflammatory disease, cancer, and vascular disorders.