Chapter 13
Signal Transduction to and from Adhesion Molecules
Here we consider the adherence of cells to surfaces or to other cells and ask how this affects their responses to soluble agonists such as growth
factors; also, how soluble agonists affect cellular adherence, itself a signalling event important in the maintenance of stem cell compartments and the epithelial mesenchymal transition (see page 432). The molecules that effect adhesion serve both as targets for signals that are generated within cells (inside-out signalling) and as receptors for extracellular signals (outside-
in signalling). These two aspects are well exemplified in the regulation of survival, proliferation, and differentiation, and in leukocyte trafficking (also discussed in the following chapter). This list is far from complete. Adhesion molecules are of prime importance in the functioning of synapses, nerve cell differentiation, differentiation of keratinocytes, gene expression in epithelial cells, thymic selection, and T lymphocyte activation.
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Adhesion molecules
Adhesion molecules were originally thought of merely as a sort of glue. We now recognize that they also act as signalling molecules and are properly described as receptors (Figure 13.1). However, the ligands that interact with adhesion molecules are generally insoluble, frequently adhesion molecules themselves. They are presented on adjacent cells or by the extracellular matrix on the surfaces of epithelia or endothelia, or by the interstitial matrix of connective tissue.
Adhesion molecules first came to light around 1970, as a result of investigations of brain development. It was realized that the very precise organization of neuronal cells in the central nervous system must require a dynamic process of cell guidance and cell adhesion. This would drive the
direction-seeking processes of neurite outgrowth and synapse formation. Two main ideas were considered.1 The first suggested that during development, in order to establish precise cell–cell contacts, the interacting cells must
each present unique adhesion molecules that fit each other like a lock and key (chemoaffinity hypothesis). The second idea was that the set of adhesion molecules is limited, but their binding capacity could be modulated over time. For instance, developing neuronal cells would all offer the same molecule and during outgrowth, this would be in a low-affinity state. The cell might then convert the adhesion molecule into its high-affinity state, to promote binding to its counterpart on a nearby cell.
It now appears that there is truth in both these propositions. The number of adhesion molecules is certainly limited and their capacity to interact with counter receptors is regulated by their levels of expression (presence or absence) and also by their binding state (affinity). In the realm of immunology,
FIG 13.1 Three levels of communication. Cells are not only in contact with soluble ligands (e.g. hormones and cytokines), but also with each other and with the extracellular matrix. These contacts
are mediated by adhesion molecules, which in many instances should be regarded as receptors that convey signals into the cells.
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the set of adhesion molecules expressed on a cell surface and their state of activation has been called the ‘area code’.2
Clear evidence for specific adhesion interactions came from studies of the re-aggregation of disaggregated tissues. Tissue cells dispersed by treatment with trypsin, which strips proteins from the surface, only recover their adherent properties after a period in culture. The need for a recovery period suggested that new adhesion molecules must be expressed on the cell surface. The re-aggregation can be prevented by monovalent Fab1 fragments prepared from antibodies that bind to cell surface epitopes (Figure 13.2). The possibility of weak non-specific interactions was precluded. It followed that there is a limited number of specialized molecules that determine cell–cell interactions. These are the adhesion molecules and the first to be discovered was the neuronal cell adhesion molecule (NCAM).3
Early attempts to identify individual adhesion molecules were hampered by the lack of specific antibodies. The key was provided by the advent of monoclonal antibodies and their application to questions regarding cell adhesion. This led to the identification of Mac-1, which is expressed on the surface of macrophages and plays a pivotal role in the binding of leukocytes to the vascular endothelium.4 It also determines the binding of the serum complement factor iC3b, which, together with the Fc receptor, mediates activation of the respiratory burst. Later, a monoclonal antibody that recognizes LFA-1 (lymphocyte function-associated antigen-1) was used to show that the binding of cytotoxic T lymphocytes to their target cells is also mediated by specific adhesion molecules.5
The many abbreviations used in this chapter are collected together at the end of the chapter.
Complement is the term originally used to refer to the heat-labile factor in serum that causes immune cytolysis, the lysis of antibody-coated cells. It now refers to the
entire functionally related system comprising at least 20 distinct serum proteins. As well as being the effector of cytolysis, it has other functions.
FIG 13.2 Reassembly of retinal dispersed cells: discovery of NCAM.
(a) The re-assembly of dispersed cells is prevented by antibodies that recognize cell membrane proteins. By
this approach, the role of the adhesion molecule NCAM in the maintenance of tissue integrity was discovered. Since intact bivalent antibodies would have forced the cells to aggregate, it was essential to use monovalent Fab fragments that are unable to form cross-links.(drawing of image from Brackenbury et al.3). (b) Generation of the fragments Fc, Fab1 (monovalent), and Fab2 (divalent) by enzyme digestion of immunoglobulin G.
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Adhesion molecules not only link cells to surfaces but may also make the connection between the extracellular matrix and the cytoskeleton. Because it was perceived that this class integrates intracellular and extracellular events, they were called integrins.6 These proteins can bind to a wide range of extracellular matrix molecules, including fibronectin, fibrinogen, laminin, osteopontin, thrombospondin, vitronectin, and von Willebrand factor. A number of integrins bind these proteins by recognizing short sequences such
as RGD or EILDV.7 Mac-1 and LFA-1 share substantial sequence homology with the integrins and it was confirmed that they too are members of this family of adhesion molecules.
Naming names
Adhesion molecules have been given names that reflect their function (inter-cellular adhesion molecule-1, ICAM-1), their location and function (endothelium leukocyte adhesion molecule-1, ELAM-1), their need for Ca2 (Ca2 -dependent adherence, cadherins), their time of expression during T cell activation (very late antigen-4, VLA-4), or their integration of the extracellular matrix with the intracellular cytoskeleton (integrins). Of course, there are other names, often bestowed after cloning ( M integrin) or through recognition
by specific monoclonal antibodies (CD11b, cluster of differentiation 11b). If this is not all perfectly clear, then one should appreciate that CD18/CD11b is synonymous with Mac-1, which is synonymous with integrin M 2, and that CD62E is synonymous with ELAM-1 which is synonymous with E-selectin.L 2 is synonymous with LFA-1. Such are the problems of nomenclature when molecular cloning rubs shoulders with immunology, pharmacology, and all the rest.
Immunoglobulin superfamily
NCAM is a member of a large family of cell surface proteins that express repeated immunoglobulin-like domains at their extracellular N-termini. Proteins of the immunoglobulin superfamily that play a role in adhesion are called Ig-cellular adhesion molecules (Ig-CAMs). Their Ig-like, globular, loop structures are stabilized by sulfydryl bridges and are resistant to proteases (Figure 13.3a). The Ig-like domains are then subclassified as C1-, C2-, V-, and I-set (or -type), based on their similarity to the variable and constant regions of antibodies.
Examples of C2-set Ig-CAMs are VCAM-1 (vascular cell adhesion molecule-1),8 NCAM-1 and -2, PECAM (platelet endothelial cell adhesion molecule, CD31), and IGS4B (immunoglobulin superfamily member 4B) (Figure 13.3b). The latter contains two C2-set Ig-like domains and one V-set domain. There are two splice variants of VCAM-1 that express either five or seven Ig-like
C2-set domains. VCAM-1 is highly expressed on endothelial cells and certain
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fibroblasts when present in inflamed tissues. VCAM-1 binds integrins 4 1 (VLA-4) and D 2. Many of the Ig-CAM subfamilies have not yet been fully explored with respect to their function and to their signalling pathways.
ICAM
Intercellular adhesion molecules, ICAM-1–5, are a subfamily of Ig-CAMs with varying numbers of Ig-like C2-set domains. They bind the integrin L 2 (LFA- 1) and a diverse range of other ligands/counter-receptors. ICAM-1 and -2 are important for the recruitment of leukocytes into the tissues. Upon binding of LFA-1, they signal into the cell through members of the Rho family of GTPases. This mediates subsequent transendothelial migration through the loosening of the VE-cadherin cell–cell contact sites and by rearrangement of the actin cytoskeleton.10,11 This will be further elaborated in Chapter 14.
The interaction with the actin cytoskeleton is mediated through members of the ERM family (ezrin/moesin/radixin). ICAM-2 on vascular endothelial cells also supports homophilic interactions that may be involved in vascular tube formation during the process of angiogenesis (formation of new vasculature from a pre-existing vascular bed).12 ICAM-3 is restricted to leukocytes13 and ICAM-4 to red blood cells, constituting the Landsteiner–Wiener blood group.14 ICAM-5 is expressed in neurons15 (see Figure 13.3).
FIG 13.3 Adhesion molecules of the immunoglobulin superfamily. (a) Ig family adhesion molecules are characterized by repeated domain structures that are homologous to those in immunoglobulins (Ig domains). The disulfide bonds are indicated by red spheres. (b) There are several members of this family, all having a single membrane-spanning domain. They interact with different ligands (or counter-receptors). NCAM 120 is attached to the membrane by a glycosyl-phosphatidyl inositol anchor. (1epf9).
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