Chapter 15

The extracellular matrix and cell adhesion

By George Plopper

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

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

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.

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.

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

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.

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.

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.

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.

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.

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.

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

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.

15.6 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

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.

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.

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

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.

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.

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

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

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.

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.

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

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

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

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

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

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

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

15.12 Proteases degrade extracellular matrix components

• Cells must routinely degrade and replace their extracellular matrix as a normal part of – development – wound healing

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.

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.

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.

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.

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.

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

15.13 Most integrins are receptors for extracellular matrix proteins • In vertebrates, there are many α and β integrin subunits.

– These combine to form at least 24 different αβ heterodimeric 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

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.

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.

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

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

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.

– F or example, upon ligand binding

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.

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.

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

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.

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.

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.

15.17 Septate junctions in invertebrates are similar to tight junctions • Septate junctions function principally as barriers to paracellular diffusion.

• Septate junctions perform two functions not associated with tight junctions: – they control cell growth and cell shape during development. • A special set of proteins unique to septate junctions performs these functions.

15.18 Adherens junctions link adjacent cells

• Adherens junctions are a family of related cell surface domains.

– They link neighboring cells together.

• Adherens junctions contain transmembrane cadherin receptors.

15.18 Adherens junctions link adjacent cells • The best-known adherens junction is the zonula adherens.

– It is located within the junctional complex that forms between neighboring epithelial cells in some tissues.

• Within the zonula adherens, adaptor proteins called catenins link cadherins to actin filaments.

15.19 Desmosomes are intermediate filamentbased cell adhesion complexes

• The principal function of desmosomes is to: – provide structural integrity to sheets of epithelial cells by linking the intermediate filament networks of cells.

15.19 Desmosomes are intermediate filament-based cell adhesion complexes • Desmosomes are components of the junctional complex.

• At least seven proteins have been identified in desmosomes. • The molecular composition of desmosomes varies in different cell and tissue types.

15.19 Desmosomes are intermediate filament-based cell adhesion complexes • Desmosomes function as both: – adhesive structures – signal transducing complexes • Mutations in desmosomal components result in fragile epithelial structures. – These mutations can be lethal, especially if they affect the organization of the skin.

15.20 Hemidesmosomes attach epithelial cells to the basal lamina

• Hemidesmosomes, like desmosomes, provide structural stability to epithelial sheets.

• Hemidesmosomes are found on the basal surface of epithelial cells.

– There, they link the extracellular matrix to the intermediate filament network via transmembrane receptors.

15.20 Hemidesmosomes attach epithelial cells to the basal lamina • Hemidesmosomes are structurally distinct from desmosomes.

• They contain at least six unique proteins.

15.20 Hemidesmosomes attach epithelial cells to the basal lamina • Mutations in hemidesmosome genes give rise to diseases similar to those associated with desmosomal gene mutations.

• The signaling pathways responsible for regulating hemidesmosome assembly are not well understood.

15.21 Gap junctions allow direct transfer of molecules between adjacent cells

• Gap junctions are protein structures that facilitate direct transfer of small molecules between adjacent cells. • They are found in most animal cells.

15.21 Gap junctions allow direct transfer of molecules between adjacent cells • Gap junctions consist of clusters of cylindrical gap junction channels, which: – project outward from the plasma membrane – span a 2-3 nm gap between adjacent cells • The gap junction channels consist of two halves, called connexons or hemichannels.

– Each consists of six protein subunits called connexins.

15.21 Gap junctions allow direct transfer of molecules between adjacent cells • Over 20 different connexin genes are found in humans.

– These combine to form a variety of connexon types.

• Gap junctions: – allow for free diffusion of molecules 1200 daltons in size – exclude passage of molecules 2000 daltons

15.21 Gap junctions allow direct transfer of molecules between adjacent cells • Gap junction permeability is regulated by opening and closing of the gap junction channels, a process called “

gating

.” • Gating is controlled by changes in – intracellular pH – calcium ion flux – direct phosphorylation of connexin subunits

15.21 Gap junctions allow direct transfer of molecules between adjacent cells • Two additional families of nonconnexin gap junction proteins have been discovered.

– This suggests that gap junctions evolved more than once in the animal kingdom.

15.22 Calcium-dependent cadherins mediate adhesion between cells

• Cadherins constitute a family of cell surface transmembrane receptor proteins that are organized into eight groups.

• The best-known group of cadherins is called the “classical cadherins.” – It plays a role in establishing and maintaining cell-cell adhesion complexes such as the adherens junctions.

15.22 Calcium-dependent cadherins mediate adhesion between cells • Classical cadherins function as clusters of dimers.

• The strength of adhesion is regulated by varying both: – the number of dimers expressed on the cell surface – the degree of clustering

15.22 Calcium-dependent cadherins mediate adhesion between cells • Classical cadherins bind to cytoplasmic adaptor proteins, called catenins.

– Catenins link cadherins to the actin cytoskeleton.

• Cadherin clusters regulate intracellular signaling by forming a cytoskeletal scaffold.

– This organizes signaling proteins and their substrates into a three-dimensional complex.

15.22 Calcium-dependent cadherins mediate adhesion between cells • Classical cadherins are essential for tissue morphogenesis, primarily by controlling: – specificity of cell-cell adhesion – changes in cell shape and movement

15.23 Calcium-independent NCAMs mediate adhesion between neural cells

• Neural cell adhesion molecules (NCAMs) are expressed only in neural cells.

• They function primarily as homotypic cell-cell adhesion and signaling receptors.

15.23 Calcium-independent NCAMs mediate adhesion between neural cells • Nerve cells express three different types of NCAM proteins.

– They arise from alternative splicing of a single NCAM gene.

15.23 Calcium-independent NCAMs mediate adhesion between neural cells • Some NCAMs are covalently modified with long chains of polysialic acid (PSA).

– This reduces the strength of homotypic binding. • This reduced adhesion may be important in developing neurons as they form and break contacts with other neurons.

15.24 Selectins control adhesion of circulating immune cells

• Selectins are cell-cell adhesion receptors expressed exclusively on cells in the vascular system. • Three forms of selectin have been identified: – L-selectin – P-selectin – E-selectin

15.24 Selectins control adhesion of circulating immune cells • Selectins function to arrest circulating leukocytes in blood vessels so that they can crawl out into the surrounding tissue.

• In a process called discontinuous cell cell adhesion, selectins on leukocytes bind weakly and transiently to glycoproteins on the endothelial cells.

– The leukocytes come to a “rolling stop” along the blood vessel wall.