Cell adhesion is the interaction of a cell with a neighboring cell or with the underlying extracellular matrix, via specialized multi-protein adhesive structures:
Cells interact with each other, and their substrate, throughout their lifetime. These interactions can be transient, such as at the immunological synapse, or they can be long-lived, such as at a neuromuscular junction. These complex cellular structures involve many proteins; from receptor molecules to structural scaffolding proteins. Significant differences in composition exist between an adhesion complex that interacts with the cellular substrate, or extracellular matrix, and one that interacts with another cell. Despite the differences however their fundamental function remains the same; to enable cellular communication through the generation and transduction of mechanical signals. While cell-cell adhesions serve as cellular ‘handshakes’, cell-matrix adhesions allow a cell to pull against its substrate to either measure the substrate rigidity, or to pull the cell forward.
Cell adhesions can be described as a functional extension of the actin cytoskeleton. Indeed, all adhesion types are linked physically to the actin filament network, and the dynamic processes of actin filament polymerization and disassembly are intertwined with the turnover and function of the adhesions complexes. Cell adhesions are mediated by either transmembrane cell-adhesion molecules (CAMs), which binding similar partner proteins on opposing cells, or adhesion receptors, which bind various ligands. These proteins are integral to the formation of adhesions and essentially link the intracellular space to the extracellular space to help relay information to the cell interior about the surroundings.
Cells adhere to the ECM, or to other cells, via complexes that can collectively be called anchoring junctions (reviewed in ). These multiprotein complexes are found in all cell types where they they stabilize the cells position, provide stability and rigidity, and support tissue integrity by holding cell sheets together. Anchoring junctions also form a tight seal between neighboring cells to restrict the flow of molecules between cells and from one side of the tissue to the other. Lastly, anchoring junctions regulate the motility of both single cells and cellular masses through their substrates. These anchor points are highly dynamic, primarily associated with actin filaments, and come in many different forms.
Features of anchoring junctions
There are three main features of anchoring junctions:
There are four main types of anchoring junctions:
Several types of anchoring junctions have been identified with each involved in distinct types of adhesion.
Ligand-receptor binding is followed by the rapid association of other proteins to the intracellular portion of the receptor; this reinforcement of the adhesion domain is controlled by adhesion receptor mobility in the membrane . Such change in forces can affect mechanosensory molecules to activate intracellular signal transduction cascades (e.g. the Rho family of GTPases) and mechanotransduction events that mediate a number of diverse processes such as cell proliferation, fate, migration, shape and polarization  (reviewed in ).
Various types of cell-matrix receptors exist. These include:
Anchoring junctions are multiprotein complexes. Crucial to the formation of these junctions are cell adhesion molecules (CAMs).
CAMs have many distinct domains that allow them to mediate cell-cell contacts by binding to specific partner proteins; when these interactions occur between apposed cells they are described as either homophilic (i.e. binding to the same kind of CAM molecule) or heterophilic (binding to a different kind of CAM molecule). Furthermore, CAMs can mediate interactions between cells of the same type (aka homotypic adhesion) or between different cell types (aka heterotypic adhesion).
CAMs are grouped into four main families:
This family of glycoproteins includes over 100 members divided into 6 subfamilies; type I classical cadherins , type II atypical cadherins, desmosomal cadherins, flamingo cadherins, proto-cadherins and several ungrouped members. Cadherins can be identified through common motifs in their extracellular domains termed cadherin repeats. Not all cadherins are involved in cell-cell adhesion, though type I and type II cadherins have well established roles in this process . Both of these subfamilies contain cadherin repeats within their extracellular domains, with the outermost cadherin repeats facilitating extracellular interactions with cadherins on apposing cells (transinteractions). Type I cadherins can in addition engage in lateral interactions on the same cell (cis interactions). Intercellular interactions between cadherins can occur between those of the same type (homophilic binding) or a different type (heterophilic binding). Intracellular interactions involve the cytoplasmic domains of the cadherins. In the case of type I cadherins these interactions can be used to identify this subfamily, namely through their ability to bind catenins via their cytoplasmic tails. Catenins form part of the bridge connecting adherens junctions to the actin cytoskeleton. It should be noted that individual cadherin interactions are weak. The strength of cadherin-based adhesive junctions comes from the clustering of multiple, weak cadherin-cadherin interactions .
Most cadherins adhere by homophilic interactions (i.e. they bind to the same type of cadherin) but certain types (e.g. E-cadherin) also adhere by heterophilic interactions (i.e. they bind other types of cadherin). Cadherin association is sensitive to extracellular calcium (hence their name, calcium adhering). The interactions can take place laterally on the same cell, called a cis interaction, or between two cells, called a trans interaction (see Figure below).
The classical cadherins (e.g. E-, N-, and P-cadherins) are the most common family members. Classical cadherins interact directly with p120ctn at their transmembrane region and through their cytoplasmic tails to beta (β)-catenin or plakoglobin(i.e. gamma [γ]-catenin). The correct function and stability of the cadherins requires these associations (reviewed in ). β-catenin binds tightly to classical cadherins before they are transported to the cell surface ). Cadherins further interact indirectly with other adaptor proteins (e.g. alpha [α]-catenin, vinculin, EPLIN,α-actinin, zyxin) to form linkages between the cell membrane and the actin cytoskeleton (reviewed in ). Desmosomes in contrast, have two specialized cadherins that interact with specific adaptor proteins (e.g. plakoglobin, plakophilin, desmoplakin) to form links with the intermediate filaments.
Members of this family include vascular and neural cell adhesions molecules (VCAM and NCAM), intercellular adhesion molecules (ICAM) and the Nectins and nectin-like (Necl) proteins. Nectins in particular are involved in the formation of cadherin-based cell-cell junctions , mediating initial cell-cell contacts via nectin-nectin or nectin-Necl binding and establishing links to the actin cytoskeleton via nectin-afadin binding . Of the four major groups of CAMs, IgCAMs are the only group that function independently of calcium.
The Ig superfamily is a large group of cell surface molecules that includes members such as:
Most ICAMs are expressed mainly by immune cells and endothelial cells, however brain-specific forms also exist (e.g. ICAM-5 aka TLCN) . All ICAMs appear to share lymphocyte function-associated antigen-1 (LFA-1, CD11a/CD18, αLβ2 integrin) as their counter receptor . LFA-1 integrin is found on the surface of leukocytes where it modulates adhesion-dependent events that are essential for immune system activities. In the brain, LFA-1 expression appears to be restricted to resident macrophages (microglia) and its expression is tied to microglia activation . ICAM-1 and LFA-1 binding is magnesium-dependent  and the sites for LFA-1 binding lie in the first two amino-terminal Ig domains of ICAM-1; the residues involved in binding to LFA-1 are conserved in other ICAMs .
Nectins and nectin-like molecules (Necls) are expressed in a number of cell types where they have been shown to be important for cell-cell adhesion and the formation of stable junctions (e.g adherens junctions). Nectins and Necls also play a role in various cellular activities including cell polarization, migration, growth and cell fate (reviewed in ). Nectin and Necls interact with and share a number of binding partners through their cytoplasmic domain, however, only nectins bind to afadin, an F-actin binding protein.
α-catenin has broad activity that contributes to processes such as differentiation (i.e. commitment to a particular cell type), embryonic and tissue development, and cell migration (reviewed in ). α-catenin is concentrated at cell-cell adhesion sites, e.g., tight junctions , and adherens junctions (reviewed in ), through its association with a related family member, beta (β)-catenin; this binding interaction is controlled by phosphorylation of either α- or β-catenin  (reviewed in ) and phosphorylated β-catenin is expected to compete with homodimerziation of α-catenin . Dimerization of α-catenin creates a complex with functional domains at both ends that preferentially binds actin filaments, in contrast to the monomer which prefers E-cadherin-β-catenin complexes .
Current models suggest that α-catenin promotes stronger adhesions in a few ways:
1) α-catenin may foster lateral clustering and activation of cadherins 
These CAMs form heterodimers comprising an alpha and beta subunit and are commonly known to facilitate cell-matrix interactions (e.g. at focal adhesions) via their interactions with extracellular matrix proteins. However they are also capable of mediating cell-cell interactions through their interactions with IgCAMs – a process vital in mounting immune responses via leukocytes .
Three members constitute this family, E-selectin (endothelial), L-selectin (leukocyte) and P-selectin (platelet), all of which bind to fucosylated carbohydrates . For example P-selectin on leukocytes binds PSGL-1 (P-selectin glycoprotein ligand-1) on endothelial cells.
Whether an adhesion is formed between two cells, or between a cell and its substrate, alterations to the actin cytoskeleton occur. This is because the adhesion complex must connect to the cytoskeleton in order to facilitate its function. Several proteins help facilitate this process including Ena/VASP which associates with components of the Arp2/3 complex-mediated actin assembly module. These are required for actin dynamics at sites of cadherin-cadherin binding . The association of Mena and VASP may be modulated by signal-mediated phosphorylation (reviewed in ); VASP phosphorylation prevents it from interacting with other cadherin-complex proteins (e.g. zyxin) .
Other proteins located at sites of cell adhesion include the adaptor proteins which connect the adhesion molecules to the cytoskeleton and signaling molecules. Examples include parvin, paxillin, talin, tensin, vinculin and zyxin.
Parvin is found in several cell types and at many locations in the cell such as: the leading edge of migrating cells and at sites of growing adhesions; it extends from mature FAs; and it partially localizes with stress fibers . Parvin co-localizes completely with talin in FAs and with fibers along the cell body . As parvin is not usually found along the entire length of stress fibers , these central fibers more likely resemble tensin-rich fibrillar adhesions (FBs) . Parvin is a member of a triad known as IPP (ILK-PINCH-parvin) which controls the maturation of cell-matrix adhesions by forming a permissive platform for tensin recruitment . Parvin contains numerous potential phosphorylation consensus sequences for kinases such as protein kinase C  and extracellular signal-regulated protein kinase ; phosphoryation of parvin increases during cell adhesion/spreading .
Paxillin is a multidomain scaffolding protein that is a key platform for bringing together signaling molecules, structural components, and regulatory proteins that control the adhesion and organization of the internal cytoskeleton for processes such as cell migration (reviewed in ). Paxillin contains five amino-terminal leucine-aspartic acid (LD1-5) motifs and four carboxy-terminal LIM (Lin11, Isl-1, Mec-3) domains; the LD and LIM domains mediate protein-protein interactions with a number of structural and regulatory proteins (see figure at right).
Paxillin contains a number of likely phosphorylation sites for serine/threonine kinases (e.g. protein kinase C) and tyrosine kinases  and is phosphorylated in response to various growth factors and adhesion stimuli both in vitro  and in vivo (reviewed in ). Phosphorylation of the LIM domains has been suggested to influence cellular adhesion to fibronectin as well as paxillin localization to focal adhesions .
In migrating cells, paxillin appears to remodel from older to newer adhesions at the leading edge to become one of the first proteins found at cell-matrix adhesion sites . Paxillin largely contributes to cytoskeleton dynamics by regulating the activity of the Rho family of GTPases and by coordinating their association with specific ligands and downstream effector systems ; for example, paxillin-integrin binding is sufficient for regulating signal transduction through Rac1 GTPase . It has also been recently shown to coordinate membrane trafficking and hence directional migration based on physical cues .
Integrin tail binding occurs via the F3 phosphotyrosine binding (PTB) domain via a unique interaction with the integrin membrane proximal region, which is sufficient for integrin activation . The basic patches on all subdomains can dock onto the plasma membrane and further enhance integrin activation. Specific interactions through basic residues on F3 are also essential for integrin clustering .
Both F2 and F3 contribute to actin binding, with the F3 binding pocket being the same that binds integrin and PIPKIγ90 as well thus linking these adhesion components . F3 also binds to layilin (a hyaluronan receptor) and signaling molecules FAK (reviewed in ). The neck region is susceptible to cleavage by calpain 2 .
The rod contains an additional integrin-binding site (IBS2), two actin-binding sites (ABD) and several vinculin-binding sites that are shown to be exposed by stretch in response to force, both in vitro  and in vivo . Vinculin binding reinforces and increases the stability of adhesion sites . Talin also contains numerous potential phosphorylation sites  which are suggested to directly or indirectly regulate the association of talin with other factors (reviewed in ).
Agonist stimulation has been shown to trigger a signaling pathway for membrane targeting of talin/ activation of integrin αIIbβ3 , involving small GTPase Rap1, Rap-GEF or protein kinase C and adaptor protein RIAM .
Talin is abundant specifically at sites of cell-ECM linkage  where it appears to be a key endpoint for multiple signaling pathways that lead to integrin activation (reviewed in ). Talin behaves as a prominent structural platform that is required for the initial linkage between the contractile cytoskeleton and sites of integrin/fibronectin adhesion .
During cell spreading, talin undergoes cycles of stretching and vinculin binding due to contractile forces on the rearward moving actin filaments . This phenomenon serves to convert matrix forces into biochemical signals at the adhesion site. Hence it not only organizes and stabilizes these initial linkages , but it also mediates signal transduction events through the integrins, vinculin and actin (reviewed in ).
The proteolytic cleavage of talin has been shown to be a critical event in the subsequent disassembly of other focal adhesion components  but not in integrin activation. Although talin is a key factor that translates mechanical forces into chemical responses primarily at sites of cell-matrix and cell-cell junctions, talin may also function in other cellular processes including membrane ruffling, cytokinesis, and phagocytosis (reviewed in ).
Tensin contains three actin-binding domains (ABDs) that allows it to form crosslinks along actin filaments; it also prevents actin assembly by capping actin filaments at the barbed end . Tensin has numerous phosphorylation sites and multiple protein interaction domains for both structural components (e.g. paxillin, β-integrin ) and signaling molecules (e.g. Src, phosphatidylinositol 3-kinase [PI3K], focal adhesion kinase [FAK])  (reviewed in ). Phosphorlyation of tensin corresponds with cell-ECM binding  and growth factor stimulation  (reviewed in ). Tensin forms a C-shaped structure  that binds focal adhesion components at both ends . Tensin is also proposed to form a dimer via its carboxy-terminus and this association may be dependent upon its phosphorylation state .
Tensin primarily localizes to sites of cell attachment such as focal adhesions  , elongated fibrillar structures (aka fibrillar adhesions) and possibly other adhesive junctions . Tensin serves as a link between signal transduction pathways and the actin cytoskeleton by forming a structural platform that regulates the assembly of focal adhesion components, phosphoproteins, and signaling molecules for processes such as cell migration  and tissue regeneration .
Vinculin frequently links adhesion receptors (e.g. integrins) to the contractile actin-myosin cytoskeleton by binding either talin through its amino-terminal globular head domain , or paxillin through its rod-like tail domain . Vinculin can also bind to lipids through the tail domain. The head and tail domains are linked by a flexible hinge that also contains binding sites for components of the actin polymerizing module (e.g. Arp2/3 complex , Ena/VASP proteins ).
Vinculin generally forms two structural states, an open (active) and closed (inactive) state, which are controlled by interaction(s) between the head and tail domains . Whether vinculin can bind to other factors depends both allosterically and sterically on the formation of the complete open state (reviewed in ). This in turn is favored by combinatorial binding of ligands namely talin, phosphatidylinositol 4,5-bis-phosphate [PIP2] and actin (reviewed in ). Phosphorylation at 4 residues has been proposed to prime vinculin for this complex formation and hence the activation process . Activation of vinculin influences its ability to form oligomers or other complxes in cells .
Vinculin recruitment to adhesion sites is mechanically regulated by ligand binding(e.g. other cells, extracellullar matrix) and activation of adhesion receptors. Physical restructuring of adhesion receptors such as integrins and their linked mechanosensors (e.g. talin) is transmitted to the cell interior to stimulate contraction of actin stress fibers; this promotes vinculin binding to these sites  and subsequent ordering of vinculin domains (reviewed in ). Interestingly, reduced cellular tension doesn’t lead to altered vinculin binding as has been observed for other structural components (e.g. zyxin ).
Vinculin N-terminal, when bound to talin, partially opens up and aids integrin clustering for FA growth, possibly by retaining the activated state of integrins . The C-terminal forms a mechanosensitive link between adhesion receptors and the actin cytoskeleton to help recruit other components of the actin linking module (e.g. α-actinin, paxillin) and influence the mechanical strength of the cell . Additionally, vinculin possesses actin filament capping activity . This needs an complete opening of vinculin structure allowing C-terminal of the tail to compete with formin mDia1 for actin barbed ends.
Vinculin also contributes to stability of focal adhesion under high forces by regulating contractile stress generation , thereby influencing the cell migration speed . Cells deficient in vinculin cannot form lamellipodia, assemble stress fibers, or spread efficiently over a substrate .