H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 93
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In effect, the integrins through adapter proteins transmit the intracellular forces generated by the actin cytoskeleton toextracellular fibronectin. Gradually, the initially formedfibronectin fibrils mature into highly stable matrix components by covalent cross-linking. In some electron micrographic images, exterior fibronectin fibrils appear to bealigned in a seemingly continuous line with bundles of actinfibers within the cell (Figure 6-26b). These observations andthe results from other studies provided the first example ofa molecularly well defined adhesion receptor (i.e., an integrin) forming a bridge between the intracellular cytoskeletonand the extracellular matrix components—a phenomenonnow known to be widespread.223Hyaluronan, a highly hydrated GAG, is a major component of the ECM of migrating and proliferating cells.Certain cell-surface adhesion receptors bind hyaluronan tocells.■Large proteoglycan aggregates containing a centralhyaluronan molecule noncovalently bound to the core protein of multiple proteoglycan molecules (e.g., aggrecan)contribute to the distinctive mechanical properties of thematrix (see Figure 6-22).■Fibronectins are abundant multiadhesive matrix proteinsthat play a key role in migration and cellular differentiation.
They contain binding sites for integrins and ECMcomponents (collagens, proteoglycans) and can thus attachcells to the matrix (see Figure 6-23).■The tripeptide RGD sequence (Arg-Gly-Asp), found infibronectins and some other matrix proteins, is recognizedby several integrins.■6.5 Adhesive Interactionsand Nonepithelial CellsAfter adhesive interactions in epithelia form during differentiation, they often are very stable and can last throughout thelife span of epithelial cells or until the cells undergo differentiation into loosely associated nonpolarized mesenchymalcells, the epithelial–mesenchymal transition. Although suchlong-lasting (nonmotile) adhesion also exists in nonepithelialtissues, some nonepithelial cells must be able to crawl acrossor through a layer of extracellular matrix or other cells.
Inthis section, we describe various cell-surface structures innonepithelial cells that mediate long-lasting adhesion andtransient adhesive interactions that are especially adapted forthe movement of cells. The detailed intracellular mechanismsused to generate the mechanical forces that propel cells andmodify their shapes are covered in Chapter 19.KEY CONCEPTS OF SECTION 6.4The Extracellular Matrix of Nonepithelial TissuesConnective tissue, such as tendon and cartilage, differsfrom other solid tissues in that most of its volume is madeup of extracellular matrix (ECM) rather than cells.■The synthesis of fibrillar collagen (e.g., types I, II, andIII) begins inside the cell with the chemical modificationof newly made chains and their assembly into triplehelical procollagen within the endoplasmic reticulum.
After secretion, procollagen molecules are cleaved, associatelaterally, and are covalently cross-linked into bundles calledfibrils, which can form larger assemblies called fibers (seeFigure 6-20).■The various collagens are distinguished by the ability oftheir helical and nonhelical regions to associate into fibrils, to form sheets, or to cross-link other collagen types(see Table 6-1).■Integrin-Containing Adhesive StructuresPhysically and Functionally Connect the ECMand Cytoskeleton in Nonepithelial CellsAs already discussed in regard to epithelia, integrin-containinghemidesmosomes connect epithelial cells to the basal laminaand, through adapter proteins, to intermediate filaments of thecytoskeleton (see Figure 6-1).
In nonepithelial cells, integrins inthe plasma membrane also are clustered with other moleculesin various adhesive structures called focal adhesions, focalcontacts, focal complexes, 3D adhesions, and fibrillar adhesions and in circular adhesions called podosomes (Chapter14). These structures are readily observed by fluorescence microscopy with the use of antibodies that recognize integrinsor other coclustered molecules (Figure 6-27). Like cell–matrixanchoring junctions in epithelial cells, the various adhesivestructures attach nonepithelial cells to the extracellular matrix;224CHAPTER 6 • Integrating Cells into Tissues EXPERIMENTAL FIGURE 6-27(a) Focal adhesion(b) 3D adhesionIntegrins cluster into adhesive structureswith various morphologies innonepithelial cells. Immunofluorescencemethods were used to detect adhesivestructures (green) on cultured cells.Shown here are focal adhesions (a) and 3Dadhesions (b) on the surfaces of humanfibroblasts.
Cells were grown directly onthe flat surface of a culture dish (a) oron a three-dimensional matrix of ECMcomponents (b). The shape, distribution, andcomposition of the integrin-based adhesionsformed by cells vary, depending on cultureconditions. [Part (a) from B. Geiger et al., 2001,Nature Rev. Mol. Cell Biol. 2:793. Part (b) courtesyof K. Yamada and E. Cukierman; see E. Cukiermanet al., 2001, Science 294:1708–12.]they also contain dozens of intracellular adapter and associated proteins that mediate attachment to cytoskeletal actin filaments and activate adhesion-dependent signals for cellgrowth and cell motility.Although found in many nonepithelial cells, integrincontaining adhesive structures have been studied most frequently in fibroblasts grown in cell culture on flat glass orplastic surfaces (substrata).
These conditions only poorly approximate the three-dimensional ECM environment that normally surrounds such cells in vivo. When fibroblasts arecultured in three-dimensional ECM matrices derived fromcells or tissues, they form adhesions to the three-dimensionalECM substratum, called 3D adhesions.
These structures dif-fer somewhat in composition, shape, distribution, and activity from the focal or fibrillar adhesions seen in cells growingon the flat substratum typically used in cell-culture experiments (see Figure 6-27). Cultured fibroblasts with these“more natural” anchoring junctions display greater adhesionand mobility, increased rates of cell proliferation, and spindleshaped morphologies more like those of fibroblasts in tissuesthan do cells cultured on flat surfaces.
These observations indicate that the topological, compositional, and mechanical(e.g., flexibility) properties of the extracellular matrix all playa role in controlling the shape and activity of a cell. Tissuespecific differences in these matrix characteristics probablycontribute to the tissue-specific properties of cells.TABLE 6-2 Selected Vertebrate Integrins*SubunitCompositionPrimary CellularDistributionLigands11Many typesMainly collagens; also laminins21Many typesMainly collagens; also laminins41Hematopoietic cellsFibronectin; VCAM-151FibroblastsFibronectinL2T lymphocytesICAM-1, ICAM-2M2MonocytesSerum proteins (e.g., C3b, fibrinogen,factor X); ICAM-1IIb3PlateletsSerum proteins (e.g., fibrinogen, vonWillebrand factor, vitronectin); fibronectin64Epithelial cellsLaminin*The integrins are grouped into subfamilies having a common subunit.
Ligands shown in red are CAMs; all others areECM or serum proteins. Some subunits can have multiply spliced isoforms with different cytosolic domains.SOURCE:R. O. Hynes, 1992, Cell 69:11.6.5 • Adhesive Interactions and Nonepithelial Cells225blood vessel formation, leukocyte function, the response toinfection (inflammation), bone remodeling, and hemostasis.Diversity of Ligand–Integrin InteractionsContributes to Numerous Biological ProcessesAlthough most cells express several distinct integrins thatbind the same ligand or different ligands, many integrins areexpressed predominantly in certain types of cells.
Table 6-2lists a few of the numerous integrin-mediated interactionswith ECM components or CAMs or both. Not only do manyintegrins bind more than one ligand, but several of their ligands bind to multiple integrins.All integrins appear to have evolved from two ancientgeneral subgroups: those that bind RGD-containing molecules (e.g., fibronectin) and those that bind laminin. For example, 51 integrin binds fibronectin, whereas the widelyexpressed 11 and 21 integrins, as well as the 64 integrin expressed by epithelial cells, bind laminin. The 1, 2,and several other integrin subunits contain a distinctive inserted domain, the I-domain. The I-domain in some integrins(e.g., 11 and 21) mediates binding to various collagens.Other integrins containing subunits with I-domains are expressed exclusively on leukocytes and hematopoietic cells;these integrins recognize cell-adhesion molecules on othercells, including members of the Ig superfamily (e.g., ICAMs,VCAMs), and thus participate in cell–cell adhesion.The diversity of integrins and their ECM ligands enablesintegrins to participate in a wide array of key biologicalprocesses, including the migration of cells to their correctlocations in the formation the body plan of an embryo(morphogenesis) and in the inflammatory response.
Theimportance of integrins in diverse processes is highlightedby the defects exhibited by knockout mice engineered to havemutations in each of almost all of the integrin subunit genes.These defects include major abnormalities in development,Cell–Matrix Adhesion Is Modulated by Changesin the Binding Activity and Numbers of IntegrinsCells can exquisitely control the strength of integrinmediated cell–matrix interactions by regulating the ligandbinding activity of integrins or their expression or both. Suchregulation is critical to the role of these interactions in cellmigration and other functions.Many, if not all, integrins can exist in two conformations:a low-affinity (inactive) form and a high-affinity (active)form (Figure 6-28).
The results of structural studies and experiments investigating the binding of ligands by integrinshave provided a model of the changes that take place whenintegrins are activated. In the inactive state, the heterodimer is bent, the conformation of the ligand-binding siteat the tip of the molecule allows only low-affinity ligandbinding, and the cytoplasmic C-terminal tails of the two subunits are closely bound together. In the “straight,” activestate, alterations in the conformation of the domains thatform the binding site permit tighter (high-affinity) ligandbinding, and the cytoplasmic tails separate.These structural models also provide an attractive explanation for the ability of integrins to mediate outside-in andinside-out signaling.
The binding of certain ECM moleculesor CAMs on other cells to the bent, low-affinity structurewould force the molecule to straighten and consequentlyseparate the cytoplasmic tails. Intracellular adapters could“sense” the separation of the tails and, as a result, either bindor dissociate from the tails. The changes in these adapters FIGURE 6-28 Model for integrinβ propellerβ propeller βA domainβA domainActivationLigandαβactivation.
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