Integrins

What are integrins?

The transmembrane receptors known as integrins facilitate cell adhesion to both the extracellular matrix (ECM) and the surface receptors of nearby cells. Mediating cell adhesion and migration and preserving tissue integrity are the roles of integrins. They exist as noncovalent heterodimers of α- and β-subunits on the surface of almost all animal cells with more than one cell. Integrins are given names according to the composition of their α- and β-subunits, for instance, α1β1 and αIIbβ3. Each of the 18 α- and 8 β-subunits that make up the integrin family in humans is encoded by a different set of genes. There are a total of twenty-four functional integrin heterodimers, with each α-subunit pairing with a limited number of β-subunits. In turn, only a small subset of cell types express each pair. A unique set of integrins is thus present in the repertoire of every cell type. Since various integrins have diverse preferences when it comes to binding ligands, these repertoires reveal the sticky preferences of various cell types. There are situations where different integrins can identify the same ligand. The extracellular matrix protein fibronectin is bound to by a large number of integrins. Integrins can differ in their recognition selectivity even within the same ECM protein. So, according to Humphries et al. (2006), integrin αVβ3 attaches to an Arg-Gly-Asp (RGD) motif in fibronectin, whereas integrin α4β1 attaches to a Leu-Asp-Val (LDV) motif located in a separate region of fibronectin. Even proteins that are completely unrelated to one another in terms of evolution can include these short peptide motifs, which can act as binding sites for a wide variety of integrins. A large number of integrins, such as αIIbβ3, αvŲ3, αvŲ5, and α5β1, can bind to proteins that contain the RGD motif. For instance, the RGD sequence in the matrix protein tiggrin is recognized by the Drosophila PS2 integrin, demonstrating that this integrin-binding motif interaction is retained in phylogeny. The presence of integrin-binding motifs like RGD exemplifies how the specificity of adhesion receptor recognition has driven convergent development.

A single transmembrane domain (TMD) is present in every integrin subunit, dividing the big ligand-binding head domain at the N-terminal from the usually short cytoplasmic tail region at the C-terminal (less than 60 amino acids). Integrins govern embryogenesis, cell growth, proliferation, apoptosis, and the immunological response in addition to serving as cellular glue to assist adhesion and as feet to facilitate movement. Integrins also originate transmembrane signals.

Integrin structure

Integrins are created when the α- and β subunits of two type I transmembrane glycoproteins bind noncovalently. The section outside of cells, which makes of about 700 amino acids for α- and 1000 amino acids for β subunits, forms long stalks and a region that binds ligands globularly. A portion of the subunit that helps with ligand binding and produces the globular head region is made up of seven sets of approximately 60 amino acids that fold into a structure resembling the trimeric G protein β subunit, which is a seven-bladed β propeller. A little over half of the integrin a subunits contain an insert called the I-domain inside the β-propeller. This domain plays a direct role in ligand binding and seems to be closely related to the small G proteins and the trimeric G protein α subunit. Part of the globular head region of all integrin β subunits is a "I-like domain" that plays a role in ligand binding, especially in integrins that do not include α I-domains. Both the I domain and the I-like domain have metal-binding motifs that are structurally similar; the former has a "metal ion-dependent adhesion site" (MIDAS). Like tiny G proteins, integrin ligand binding affects the coordination of the metal ion and opens up the I-domains from a closed to an open conformation, just like GTP hydrolysis changes the coordination of a Mg²⁺ ion, which causes conformational changes and the loss of effector protein connection. An increase in ligand affinity and subsequent strengthening of adhesions are two potential outcomes of this process.

With the exception of the β4 tail, which contains four fibronectin type III repeats and is around a thousand amino acids long, the length of the integrin cytoplasmic tails is less than 75 amino acids. A notable similarity exists between the β cytoplasmic tails and the α cytoplasmic tails, but the latter are significantly different, with the exception of a conserved GFFKR motif adjacent to the transmembrane region, which plays a crucial role in the interaction with the former.9 It has been reported that several cytoskeletal and signaling proteins bind to β cytoplasmic tails, and some of these proteins have been observed to interact with certain α tails. The majority of β tails have an NPxY motif that has the ability to interact to numerous cytoskeletal and signaling proteins that have a PTB-domain. There may be a way to regulate integrin connections at the cytoplasmic face by tyrosine phosphorylation in the NPxY motif. Most, if not all, integrin-mediated functions rely on the cytoplasmic tails to mediate the integrin-cytoskeletal interaction by recruiting several actin filament-binding proteins.

Various α and β subunit variants are produced by alternative mRNA splicing in both the extracellular and intracellular areas. There is some evidence that these variations can vary in how they affect signal transduction pathways or which ligands they bind to. Important functional distinctions are suggested by the selection for particular splice variants in specific cell types at specific differentiation phases. In order to promote muscular contraction, it is likely that the transition to α7A and β1D variations that occurs with final differentiation of skeletal muscle cells reflects the necessity for a greater interaction with laminin and a stronger link to the cytoskeleton.

Integrins function

(1) Matrix Assembly

Many different kinds of ECM components rely on integratins for their assembly. In basement membranes, β1 integrins function together with the dystroglycan receptor to encourage the production and assembly of laminin chains into a multivalent network. This network is then used to include collagen and other components. Embryogenesis and wound healing rely on matrices rich in fibronectin for cell movement. Fibronectin matrix construction relies on the functional interaction of integrins with syndecans. The lack of α5β1 can be somewhat compensated for by other integrins, but its involvement is crucial. The reason why α5β1 promotes fibronectin matrix construction so efficiently might be because it effectively boosts the activity of RhoA, a Rho GTPase that is necessary for fibronectin fibrillogenesis. RhoA is responsible for actomyosin-driven contractility.

(2) Cell Migration

Cellular motility is one of integrins' key functions, alongside mediating stable attachment. Metastatic malignancies are characterized by dysregulated migration, whereas embryonic development, immunological responses, and tissue healing all rely on cell migration. At the front of the migrating cells' lamellipodium is an actin cytoskeletal meshwork connected to stationary focal complexes, while at the back, F-actin stress fibers link big, inward sliding focal contacts, creating a polarized morphology. The lamellipodium can't stay still without adhering to either the ECM or other cells through integrins. Integrins are essential for the production of propulsive forces because they attach the actin cytoskeleton to the ECM in cell-matrix adhesions. While the exact mechanism by which the signals that regulate a migrating cell's polarity are organized remains a mystery, one possible component is the localization of different protein complexes at the front and back of the cell at locations of integrin-mediated adhesion. The expression of various integrins allows cells to change their motile activity, and various integrins encourage different types of motility.

(3) Signaling pathway

The regulation of differentiation, proliferation, and survival can be achieved by controlling the activity of intracellular signal transduction cascades through the engagement of integrins. As an example, in the absence of integrin-mediated adhesion, the activation of the Raf—MEK—ERK signaling pathway is weak and transitory for the majority of cell types, but it is powerful and persistent when adhesion is present. Amplification of signaling cascades may occur at cell-matrix adhesions due to the clustering of activated versions of signaling proteins such FAK, Src, p130Cas, ILK, and ERK. The process of signal amplification seems to be uncoupled from cell-anchorage when oncogenes are present: In transformed cells, FAK activity is dysregulated, and in response to cell adhesion, the tyrosine kinase c-Abl phosphorylates paxillin. However, in its oncogenic variation, Bcr-Abl, paxillin is constitutively bound in a multi-phosphoprotein complex. Protein synthesis may be locally linked to signal transduction since ribosomes and messenger RNA are also discovered attached to cell-matrix adhesions. One further direct way integrins can influence RTK signaling is by clustering and transactivating several RTKs, including PDGFR, EGFR, VEGFR, and others. In addition, the ECM has the ability to concentrate and alter growth factors, and integrin-mediated cell adhesion enables those substances to be presented to receptors for growth factors. Lastly, the distribution and stability of cell-matrix adhesions may be physically related to nuclear shape, chromatin structure, and gene expression through cytoskeletal interactions with the nucleus.

The cell cycle G1 phase is regulated by integrin-mediated cell adhesion via its effects on various signal transduction cascades. When it comes to driving S-phase entry, cyclin E-CDK2 activity is stimulated by integrins and RTKs working together. The role of integrins in organizing the actin cytoskeleton is likely to be crucial, regardless of the specific method cells employ to link integrins to G1 cell cycle development. Adhesion mediated by integrins is essential for the survival of the majority of adherent cell types; "anoikis" describes cell death in reaction to adhesion loss. In two-dimensional culture systems, cell adhesion mediated by Integrin boosts PI3K-mediated PKB/AKT activity, which in turn mediates survival signals. In three-dimensional cultures of mammary epithelial cells, α6β4 ligation enhances NFκB-mediated survival signals. The fact that unbound integrins can attract and activate caspase-8 to cause cell death in fully adherent tissues further supports the idea that a cell's ability to survive depends on the specific ECM environment in which it is expressed. The expression of differentiation-related genes is also regulated by integrins. In vitro, integrin-mediated adhesion regulates milk protein synthesis by mammary epithelial cells, myoblast cell formation and meromyosin expression by embryonic myoblasts, monocyte inflammatory cytokine production, and keratinocyte involucrin expression.

Integrins vs cadherins

Important for tissue homeostasis, wound healing, and development, cadherins mediate most cell-cell adhesions inside tissues. In a way similar to that of a zipper, members of this superfamily of calcium-dependent transmembrane proteins form "adherens junctions" that connect the cytoskeleton of neighboring cells to the adhesions they have formed. The ability of cadherins to detect stresses and initiate mechanotransductive signaling cascades is achieved through this connection. Integrins are the primary transmembrane receptors that regulate the contact between cells and their ECM, in contrast to cadherins, which are in charge of interactions between cells. Integrins form focal adhesions, which bring together other intracellular proteins, to create these linkages. As with cadherins, integrins are mechanotransducers that allow cells to sense and transform biochemical signals from the ECM's characteristics. Many cellular functions, including migration, proliferation, and differentiation, are driven by the signaling pathways activated by both types of adhesions. These pathways regulate cytoskeleton organization, signal transduction, and transcriptional events.

References

1. Barczyk M., et al., Integrins, Cell and tissue research, 2010, 339: 269-280.
2. Cox D., et al., Integrins as therapeutic targets: lessons and opportunities, Nature reviews Drug discovery, 2010, 9(10): 804-820.
3. Goodman S L., et al., Integrins as therapeutic targets, Trends in pharmacological sciences, 2012, 33(7): 405-412.
4. Kechagia J Z., et al., Roca-Cusachs P. Integrins as biomechanical sensors of the microenvironment, Nature reviews Molecular cell biology, 2019, 20(8): 457-473.
5. Weber G F., et al., Integrins and cadherins join forces to form adhesive networks, Journal of cell science, 2011, 124(8): 1183-1193.
6. Barcelona‐Estaje E., et al., You talking to me? Cadherin and integrin crosstalk in biomaterial design, Advanced Healthcare Materials, 2021, 10(6): 2002048.