The extracellular matrix and cell adhesion

Chapter 15

The extracellular matrix and cell adhesion

ByGeorge 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

• 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

15.1 Introduction

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.

• 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.

15.3 Collagen provides structural support to tissues

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.

• 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.

15.4 Fibronectins connect cells to collagenous matrices

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.

• 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.

15.5 Elastic fibers impart flexibility to tissues

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

• 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.

15.6 Laminins provide an adhesive substrate for cells

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

• 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.

15.7 Vitronectin facilitates targeted cell adhesion during blood clotting

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.

• 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

15.8 Proteoglycans provide hydration to tissues

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.

• 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

15.9 Hyaluronan is a glycosaminoglycan enriched in connective tissues

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

• 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

15.10 Heparan sulfate proteoglycans are cell surface coreceptors

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

• 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

15.11 The basal lamina is a specialized extracellular matrix

15.12 Proteases degrade extracellular matrix components• Cells must routinely degrade and

replace their extracellular matrix as a normal part of – development – wound healing

• 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.

15.12 Proteases degrade extracellular matrix components

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.

• 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.

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

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.

15.13 Most integrins are receptors for extracellular matrix proteins

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.

• 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.– For 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.

15.14 Integrin receptors participate in cell signaling

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.

• 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

15.15 Integrins and extracellular matrix molecules play key roles in 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.

• 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.

15.16 Tight junctions form selectively permeable barriers between cells

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.

• 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.

15.17 Septate junctions in invertebrates are similar to tight junctions

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.

• 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.

15.18 Adherens junctions link adjacent cells

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.

• 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.

15.19 Desmosomes are intermediate filament-based cell adhesion complexes

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.

• 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.

15.20 Hemidesmosomes attach epithelial cells to the basal lamina

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.

• 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.

15.21 Gap junctions allow direct transfer of molecules between adjacent cells

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.

• 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

15.22 Calcium-dependent cadherins mediate adhesion between cells

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.

• 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.

15.23 Calcium-independent NCAMs mediate adhesion between neural cells

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

• 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.

15.24 Selectins control adhesion of circulating immune cells