Chapter 15

1. Chapter 15 The extracellular matrix and cell adhesion By George Plopper

2. 15.1 Introduction • Cell-cell junctions are specialized protein complexes that allow neighboring cells to: • adhere to one another • communicate with one another • The extracellular matrix is a dense network of proteins that: • lies between cells • is made by the cells within the network

3. 15.1 Introduction • Cells express receptors for extracellular matrix proteins. • The proteins in the extracellular matrix and cell junctions control: • the three-dimensional organization of cells in tissues • the growth, movement, shape, and differentiation of these cells

4. 15.2 A brief history of research on the extracellular matrix • The study of the extracellular matrix and cell junctions has occurred in four historical stages. • Each is defined by the technological advances that allowed increasingly detailed examination of these structures. • Current research in this field is focused on determining how the proteins in the extracellular matrix and cell junctions control cell behavior.

5. 15.3 Collagen provides structural support to tissues • The principal function of collagens is to provide structural support to tissues. • Collagens are a family of over 20 different extracellular matrix proteins. • Together they are the most abundant proteins in the animal kingdom.

6. 15.3 Collagen provides structural support to tissues • All collagens are organized into triple helical, coiled-coil “collagen subunits.” • They are composed of three separate collagen polypeptides. • Collagen subunits are: • secreted from cells • then assembled into larger fibrils and fibers in the extracellular space

7. 15.3 Collagen provides structural support to tissues • Mutations of collagen genes can lead to a wide range of diseases, from mild wrinkling to brittle bones to fatal blistering of the skin.

8. 15.4 Fibronectins connect cells to collagenous matrices • The principal function of the extracellular matrix protein fibronectin is to connect cells to matrices that contain fibrillar collagen. • At least 20 different forms of fibronectin have been identified. • All of them arise from alternative splicing of a single fibronectin gene.

9. 15.4 Fibronectins connect cells to collagenous matrices • The soluble forms of fibronectin are found in tissue fluids. • The insoluble forms are organized into fibers in the extracellular matrix.

10. 15.4 Fibronectins connect cells to collagenous matrices • Fibronectin fibers consist of crosslinked polymers of fibronectin homodimers. • Fibronectin proteins contain six structural regions. • Each has a series of repeating units.

11. 15.4 Fibronectins connect cells to collagenous matrices • Fibrin, heparan sulfate proteoglycan, and collagen: • bind to distinct regions in fibronectin • integrate fibronectin fibers into the extracellular matrix network • Some cells express integrin receptors that bind to the Arg-Gly-Asp (RGD) sequence of fibronectin.

12. 15.5 Elastic fibers impart flexibility to tissues • The principal function of elastin is to impart elasticity to tissues. • Elastin monomers (known as tropoelastin subunits) are organized into fibers. • The fibers are so strong and stable they can last a lifetime.

13. 15.5 Elastic fibers impart flexibility to tissues • The strength of elastic fibers arises from covalent crosslinks formed between lysine side chains in adjacent elastin monomers. • The elasticity of elastic fibers arises from the hydrophobic regions, which: • are stretched out by tensile forces • spontaneously reaggregate when the force is released

14. 15.5 Elastic fibers impart flexibility to tissues • Assembly of tropoelastin into fibers: • occurs in the extracellular space • is controlled by a threestep process • Mutations in elastin give rise to a variety of disorders, ranging from mild skin wrinkling to death in early childhood.

15. 15.6 Laminins provide an adhesive substrate for cells • Laminins are a family of extracellular matrix proteins. • They are found in virtually all tissues of vertebrate and invertebrate animals. • The principal functions of laminins are: • to provide an adhesive substrate for cells • to resist tensile forces in tissues

16. 15.6 Laminins provide an adhesive substrate for cells • Laminins are heterotrimers comprising three different subunits wrapped together in a coiled-coil configuration. • Laminin heterotrimers do not form fibers. • They bind to linker proteins that enable them to form complex webs in the extracellular matrix.

17. 15.6 Laminins provide an adhesive substrate for cells • A large number of proteins bind to laminins, including more than 20 different cell surface receptors.

18. 15.7 Vitronectin facilitates targeted cell adhesion during blood clotting • Vitronectin is an extracellular matrix protein. • It circulates in blood plasma in its soluble form. • Vitronectin can bind to many different types of proteins, such as: • collagens • integrins • clotting factors • cell lysis factors • extracellular proteases

19. 15.7 Vitronectin facilitates targeted cell adhesion during blood clotting • Vitronectin facilitates blood clot formation in damaged tissues. • In order to target deposition of clotting factors in tissues, vitronectin must convert from the soluble form to the insoluble form, which binds clotting factors.

20. 15.8 Proteoglycans provide hydration to tissues • Proteoglycans consist of a central protein “core” to which long, linear chains of disaccharides, called glycosaminoglycans (GAGs), are attached. • GAG chains on proteoglycans are negatively charged. • This gives the proteoglycans a rodlike, bristly shape due to charge repulsion.

21. 15.8 Proteoglycans provide hydration to tissues • The GAG bristles act as filters to limit the diffusion of viruses and bacteria in tissues. • Proteoglycans attract water to form gels that: • keep cells hydrated • cushion tissues against hydrostatic pressure

22. 15.8 Proteoglycans provide hydration to tissues • Proteoglycans can bind to a variety of extracellular matrix components, including: • growth factors • structural proteins • cell surface receptors • Expression of proteoglycans is: • cell type specific • developmentally regulated

23. 15.9 Hyaluronan is a glycosaminoglycan enriched in connective tissues • Hyaluronan is a glycosaminoglycan. • It forms enormous complexes with proteoglycans in the extracellular matrix. • These complexes are especially abundant in cartilage. • There, hyaluronan is associated with the proteoglycan aggrecan, via a linker protein.

24. 15.9 Hyaluronan is a glycosaminoglycan enriched in connective tissues • Hyaluronan is highly negatively charged. • It binds to cations and water in the extracellular space. • This increases the stiffness of the extracellular matrix . • This provides a water cushion between cells that absorbs compressive forces. • Hyaluronan consists of repeating disaccharides linked into long chains.

25. 15.9 Hyaluronan is a glycosaminoglycan enriched in connective tissues • Unlike other glycosaminoglycans, hyaluronans chains are: • synthesized on the cytosolic surface of the plasma membrane • translocated out of the cell • Cells bind to hyaluronan via a family of receptors known as hyladherins. • Hyladherins initiate signaling pathways that control: • cell migration • assembly of the cytoskeleton

26. 15.10 Heparan sulfate proteoglycans are cell surface coreceptors • Heparan sulfate proteoglycans are a subset of proteoglycans. • They contain chains of the glycosaminoglycan heparan sulfate. • Most heparan sulfate is found on two families of membrane-bound proteoglycans: • the syndecans • the glypicans

27. 15.10 Heparan sulfate proteoglycans are cell surface coreceptors • Heparan sulfates are composed of distinct combinations of more than 30 different sugar subunits. • This allows for great variety in heparan sulfate proteoglycan structure and function. • Cell surface heparan sulfate proteoglycans: • are expressed on many types of cells • bind to over 70 different proteins

28. 15.10 Heparan sulfate proteoglycans are cell surface coreceptors • Cell surface heparan sulfate proteoglycans • assist in the internalization of some proteins • act as coreceptors for: • soluble proteins such as growth factors • insoluble proteins such as extracellular matrix proteins • Genetic studies in fruit flies show that heparan sulfate proteoglycans function in: • growth factor signaling • development

29. 15.11 The basal lamina is a specialized extracellular matrix • The basal lamina is a thin sheet of extracellular matrix • is composed of at least two distinct layers • is found at: • the basal surface of epithelial sheets • neuromuscular junctions

30. 15.11 The basal lamina is a specialized extracellular matrix • The basement membrane consists of the basal lamina connected to a network of collagen fibers. • The basal lamina functions as: • a supportive network to maintain epithelial tissues • a diffusion barrier • a collection site for soluble proteins such as growth factors • a guidance signal for migrating neurons

31. 15.11 The basal lamina is a specialized extracellular matrix • The components of the basal lamina vary in different tissue types. • But most share four principal extracellular matrix components: • sheets of collagen IV and laminin are held together by: • heparan sulfate proteoglycans • the linker protein nidogen

32. 15.12 Proteases degrade extracellular matrix components • Cells must routinely degrade and replace their extracellular matrix as a normal part of • development • wound healing

33. 15.12 Proteases degrade extracellular matrix components • Extracellular matrix proteins are degraded by specific proteases, which cells secrete in an inactive form. • These proteases are only activated in the tissues where they are needed. • Activation usually occurs by proteolytic cleavage of a propeptide on the protease.

34. 15.12 Proteases degrade extracellular matrix components • The matrix metalloproteinase (MMP) family is one of the most abundant classes of these proteases. • It can degrade all of the major classes of extracellular matrix proteins. • MMPs can activate one another by cleaving off their propeptides. • This results in a cascade-like effect of protease activation that can lead to rapid degradation of extracellular matrix proteins.

35. 15.12 Proteases degrade extracellular matrix components • ADAMs are a second class of proteases that degrade the extracellular matrix. • These proteases also bind to integrin extracellular matrix receptors. • Thus, they help regulate extracellular matrix assembly and degradation.

36. 15.12 Proteases degrade extracellular matrix components • Cells secrete inhibitors of these proteases to protect themselves from unnecessary degradation. • Mutations in the matrix metalloproteinase-2 gene give rise to numerous skeletal abnormalities in humans. • This reflects the importance of extracellular matrix remodeling during development.

37. 15.13 Most integrins are receptors for extracellular matrix proteins • Virtually all animal cells express integrins. • They are the most abundant and widely expressed class of extracellular matrix protein receptors. • Some integrins associate with other transmembrane proteins.

38. 15.13 Most integrins are receptors for extracellular matrix proteins • Integrins are composed of two distinct subunits, known as α and βchains. • The extracellular portions of both chains bind to extracellular matrix proteins • The cytoplasmic portions bind to cytoskeletal and signaling proteins.

39. 15.13 Most integrins are receptors for extracellular matrix proteins • In vertebrates, there are many αand βintegrin subunits. • These combine to form at least 24 different αβheterodimeric receptors. • Most cells express more than one type of integrin receptor. • The types of receptor expressed by a cell can change: • over time or • in response to different environmental conditions

40. 15.13 Most integrins are receptors for extracellular matrix proteins • Integrin receptors bind to specific amino acid sequences in a variety of extracellular matrix proteins. • All of the known sequences contain at least one acidic amino acid.

41. 15.14 Integrin receptors participate in cell signaling • Integrins are signaling receptors that control both: • cell binding to extracellular matrix proteins • intracellular responses following adhesion • Integrins have no enzymatic activity of their own. • Instead, they interact with adaptor proteins that link them to signaling proteins.

42. 15.14 Integrin receptors participate in cell signaling • Two processes regulate the strength of integrin binding to extracellular matrix proteins: • affinity modulation • varying the binding strength of individual receptors • avidity modulation • varying the clustering of receptors

43. 15.14 Integrin receptors participate in cell signaling • Changes in integrin receptor conformation are central to both types of modulation. • They can result from changes: • at the cytoplasmic tails of the receptor subunits or • in the concentration of extracellular cations

44. 15.14 Integrin receptors participate in cell signaling • In inside-out signaling, changes in receptor conformation result from intracellular signals that originate elsewhere in the cell. • For example, at another receptor • In outside-in signaling, signals initiated at a receptor are propagated to other parts of the cell. • For example, upon ligand binding

45. 15.14 Integrin receptors participate in cell signaling • The cytoplasmic proteins associated with integrin clusters vary greatly depending on: • the types of integrins and extracellular matrix proteins engaged. • The resulting cellular responses to integrin outside-in signaling vary accordingly. • Many of the integrin signaling pathways overlap with growth factor receptor pathways.

46. 15.15 Integrins and extracellular matrix molecules play key roles in development • Gene knockout by homologous recombination has been applied in mice to; • over 40 different extracellular matrix proteins • 21 integrin genes • Some genetic knockouts are lethal, while others have mild phenotypes.

47. 15.15 Integrins and extracellular matrix molecules play key roles in development • Targeted disruption of the β1 integrin gene has revealed that it plays a critical role in: • the organization of the skin • red blood cell development

48. 15.16 Tight junctions form selectively permeable barriers between cells • Tight junctions are part of the junctional complex that forms between adjacent epithelial cells or endothelial cells. • Tight junctions regulate transport of particles between epithelial cells.

49. 15.16 Tight junctions form selectively permeable barriers between cells • Tight junctions also preserve epithelial cell polarity by serving as a “fence.” • It prevents diffusion of plasma membrane proteins between the apical and basal regions.

50. 15.17 Septate junctions in invertebrates are similar to tight junctions • The septate junction: • is found only in invertebrates • is similar to the vertebrate tight junction • Septate junctions appear as a series of either straight or folded walls (septa) between the plasma membranes of adjacent epithelial cells.