The molecules responsible for Ca 2+ -independent cell-cell adhesion belong mainly to the large

15 The molecules responsible for Ca 2+ -independent cell-cell adhesion belong mainly to the large

and ancient immunoglobulin (Ig) superfamily of proteins, so-called because they contain one or more Ig-like domains that are characteristic of anti-body molecules (discussed in Chapter 23). The best-studied example is the neural cell adhesion molecule (N-CAM), which is expressed by

a variety of cell types, including most nerve cells. It is the most prevalent of the Ca 2+ - independent cell-cell adhesion molecules in vertebrates, and, like cadherins, it is thought to bind cells together by a homophilic interaction (between N-CAM molecules on adjacent cells). Some Ig-like cell-cell adhesion proteins, however, use a heterophilic mechanism; some of these, called intercellular adhesion molecules (ICAMs), are expressed on activated endothelial cells, where they bind to integrins on the surface of white blood cells and thereby help to trap these blood cells at sites of inflammation.

There are at least 20 forms of N-CAM. Unlike the cadherins, each of which is encoded by a separate gene, the different N-CAM mRNAs are generated by alternative splicing of an RNA transcript produced from a single gene. The large extracellular part of the polypeptide chain in all forms of N-CAM is folded into five Ig-like domains. Most N-CAMs are single-pass transmembrane proteins with variable-sized intracellular domains, which are thought to be involved in cell signaling or binding to the cytoskeleton. One form does not cross the lipid bilayer and is attached to the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor, while another is secreted and may become incorporated into the extracellular matrix (Figure 19-27). Further variation arises from the glycosylation of N-CAM: some forms carry a large quantity of sialic acid (in the highly unusual form of several chains, each containing hundreds of repeating There are at least 20 forms of N-CAM. Unlike the cadherins, each of which is encoded by a separate gene, the different N-CAM mRNAs are generated by alternative splicing of an RNA transcript produced from a single gene. The large extracellular part of the polypeptide chain in all forms of N-CAM is folded into five Ig-like domains. Most N-CAMs are single-pass transmembrane proteins with variable-sized intracellular domains, which are thought to be involved in cell signaling or binding to the cytoskeleton. One form does not cross the lipid bilayer and is attached to the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor, while another is secreted and may become incorporated into the extracellular matrix (Figure 19-27). Further variation arises from the glycosylation of N-CAM: some forms carry a large quantity of sialic acid (in the highly unusual form of several chains, each containing hundreds of repeating

There is substantial evidence that N-CAM and its Ig-like relatives play an important part in vertebrate development. When antibodies against either N-CAM or another Ig-related neural cell- cell adhesion molecule called L1 are injected along the pathway of nerve processes growing from the retina to the brain, they disturb the normal growth pattern of the nerve processes. When used in culture, these antibodies inhibit the tendency of developing nerve cell processes to adhere to one another to form bundles (fascicles). Like N-cadherin, N-CAM is expressed in large amounts on cells of the developing neural tube, but when neural crest cells dissociate from the neural tube and migrate away, they lose N-CAM, only to reexpress it later when they reaggregate to form a neural ganglion (see Figure 19-22). As in the case of cadherins, N-CAM is also expressed transiently during critical stages in the development of many non-neural tissues.

Although cadherins and Ig family members are frequently expressed on the same cells, the adhesions mediated by the cadherins are much stronger, and they almost certainly play the major role in holding cells together, segregating cell collectives into discrete tissues, and maintaining tissue integrity. N-CAM and other members of the Ig family seem to contribute more to the regulation or fine-tuning of these adhesive interactions during development and regeneration. Thus an injection of N-cadherin mRNA into a fertilized frog egg results in the overexpression of N-cadherin in places where it is not normally expressed and leads to a gross disruption of normal tissue architecture. By contrast, the same experiment performed with N- CAM mRNA leads to relatively minor disturbances in development even though N-CAM is overexpressed in many abnormal locations.

The most critical test of the requirement for a protein in a particular biological process is not to overexpress it but instead to inhibit its production by disrupting the gene. While this can now be done in some vertebrates, it is most readily done in genetically tractable invertebrates such as Drosophila and the nematode C. elegans. A number of Ig-like proteins that mediate Ca 2+ - independent cell-cell adhesion have been defined in Drosophila. One of these, fasciclin II, is a close relative of N-CAM: like N-CAM, it has five Ig-like domains and operates by homophilic binding. It is expressed mainly on a subset of nerve cell processes and on some of the glial cells they contact during development. If both copies of the fasciclin II gene are inactivated by mutation, the gross structure of the nervous system is normal. However, at least two of the nerve cell processes that normally express fasciclin II and adhere together now fail to recognize each other and therefore do not form a bundle. This observation is consistent with the view that Ig-like cell-cell adhesion molecules play subtle but important roles in development.

Mu lt ip le Ty p e s o f Ce ll- S u rfa c e Mo le c u le s Ac t in P a ra lle l t o

Me d ia t e S e le c t iv e Ce ll- Ce ll a n d Ce ll- Ma t rix Ad h e s io n 16

Morphological, cell biological, and biochemical studies all indicate that even a single cell type Morphological, cell biological, and biochemical studies all indicate that even a single cell type

Unlike receptors for soluble chemical signals, which bind their specific ligand with high affinity, the receptors that bind to molecules on cell surfaces or in the extracellular matrix usually do so with relatively low affinity. The latter receptors therefore rely on the enormous increase in binding strength gained through simultaneous binding of multiple receptors to multiple ligands on an opposing cell or in the adjacent matrix. One could call this the "Velcro principle." We have seen, however, that the interaction of the extracellular binding domains of these cell-surface molecules is not enough to ensure cell adhesion: at least in the case of cadherins and, as we shall see, integrins, the adhesion molecules must also attach (via attachment proteins) to the cortical cytoskeleton inside the cell. The cytoskeleton is thought to assist and stabilize the lateral clustering of the adhesion molecules so as to facilitate multipoint binding, and it is also required to enable the adhering cell to exert traction on the adjacent cell or matrix (and vice versa) (Figure 19-29). Thus the mixture of specific types of cell-cell adhesion molecules and matrix receptors present on any two cells, as well as their concentration, cytoskeletal linkages, and distribution on the cell surface, will determine the total affinity with which the two cells bind to each other and to the matrix.

N o n ju n c t io n a l Co n t a c t s Ma y I n it ia t e Tis s u e - s p e c ific Ce ll- Ce ll Ad h e s io n s Th a t Ju n c t io n a l Co n t a c t s Th e n Orie n t a n d