Extracellular Matrix (With Diagram) | Cell Biology

In this article we will discuss about Extracellular Matrix:- 1. Subject-Matter of Extracellular Matrix 2. Types of Extracellular Matrix 3. Extracellular Matrix on Cell Surface Receptors. 

Subject-Matter of Extracellular Matrix:

Animal tissue is not only composed of cells but also contains many types of extracellular space or intercellular space. These spaces are again filled up by many types of macro­molecules constituting the extracellular matrix.

The extracellular matrix has some specialised functions such as, strength, filtration, adhesion etc. The macromolecules that constitute the extracellular matrix are mainly secreted locally by the cells. In most of the connective tissues the macromolecules are secreted by fibroblast (Fig. 4.9). In some specialised connective tissues, such as cartilage and bone, they are secreted by chondroblasts and osteoblasts, re­spectively.

The extracellular space is more or less syn­onymous of intercellular space which is a space between the membranes of two cells and have a width between 200-300 A for closely applied cells. It is also true that in some tissues, the extracellular space and its matrix is a part of an organised structure such as basement membrane or connective tissue stroma.

Types of Extracellular Matrix:

The extracellular matrix is made of three main types of extracellular macromolecules:

(i) Polysaccharide glycosaminoglycan’s (com­monly known as mucopolysaccharides) or GAGs which are usually linked covalently to proteins in the form of proteoglycans;

(ii) Fibrous proteins of two functional types:

(a) Mainly structural (e.g., collagen and elastin) and

(b) Mainly adhesive (e.g., fibronectin and laminin);

(iii) Specialised extracellular matrix or basal lamina.

(i) Glycosaminoglycan:

It is a long, un-branched linear polysaccharide chains and consists of repeating disaccharide units in which one of two sugars is always either N-acetyl glucosamine or N-acetylgalactosamine. Hence it is named glycosaminoglycan.

The second sugar of glycosaminoglycan is a uronic acid. In most of the cases, the amino sugar is sulfated. Due to presence of large numbers of carboxyl and sulfate group on most of their sugar residues, glycosaminoglycan’s are highly acidic and negatively charged.

There are four main classes of glycosaminoglycan’s:

(i) Hyaluronic acid,

(ii) Chondroitin sulfate and dermatan sulfate,

(iii) Heparan sulfate and heparin, and

(iv) Keratan sulfate.

These can be distinguished on the basis of sugar residue, the type of linkage and number and location of sulfate groups (Table 4.1). These are distributed in the extracellular ma­trix of different tissues. The amount of gly­cosaminoglycan is usually less than 10% by the weight of the amount of the fibrous proteins.

Hyaluronic acid consisting of several thou­sand simple sugar residues, is made for regular repeating sequence of non-sulphated disaccha­ride units. Each unit contains glucuronic acid and N-acetyl glucosamine. Hyaluronic acid is thought to facilitate cell migration during tissue morphogenesis and wound repair.

It is also an important constituent of joint fluid where it serves as a lubricant. It is also evident that excess hyaluronic acid is degraded by the enzyme hyaluronidase.

In most cases, glycosaminoglycan’s exist in combination with proteins, the complex be­ing termed a proteoglycans. It is made of core protein linked with numerous un-branched glycosaminoglycan’s. A serine residue of the polypeptide chain of core protein is first linked with three sugar residues such as xylose, galac­tose, galactose (known as link trisaccharide) which, in turn, are attached with glycosamino-glycan.

A proteoglycan aggregate from fetal bovine cartilage is made of 100 proteoglycan monomers which are non-covalently bound to a single hyaluronic acid chain through two link proteins that bind to both the core protein of the proteoglycan and to the hyaluronic acid chain.

Individual proteoglycan monomers con­sist of a central core protein to which large num­ber of the sulfated glycosaminoglycan’s chon­droitin sulfate and keratan sulphate are at­tached.

(ii) Fibrous Protein:

A. Structural Fibrous Protein:

(a) Collagen:

The major fibre-forming structural proteins of the extracellular matrix are collagens. The fibrillar collagens are generally rope-like, triple- stranded helical molecules that aggregate into long cable-like fibrils in the extracellular space.

It is a hydrophobic protein. This protein is found in all multicellular animals and is secreted mainly by connective tissue cells. The basic molecular unit of collagen is tropocollagen or pro-collagen which is 300 nm in length and 1.5 nm wide. It is made of three polypeptide chains that are coiled together to form a triple helical structure.

The major portion of three polypeptide chains of tropocollagen called a-chain (about 1.000 amino acid long) has an a-helix organisation with short non-helical segments of 16-25 residues at both ends that are called tclopeptides.

The amino acid composition of the polypeptide chain of collagen is very simple; they have a large amount of proline and many of the proline and lysine residues are hydroxylated. So far, about 20 distinct a-chains of collagen have been identified. These are encoded by separate genes.

Different combination, and permutations of these genes are expressed in different tissues. So, various combinations of the 20 types of a-chain will theoretically constitute more than thousand types of collagen molecules.

So far, about five isotypes of collagen based on slight differences (Table 4.2) in the organisation of the polypeptide and association with other molecules—such as polysaccharide and glycoprotein—have been found.

These are types I, II, III, IV, and V. Types I, II, III, and V are fibrillar collagens, while type IV is non-fibrillar and assemble into a sheet-like meshwork that constitutes a major part of all basal laminae along with fibronectin and laminin.

Collagen synthesis and fibrillogenesis is a complex multistage process that involves sev­eral intra- and extracellular steps (Fig. 4.10):

The individual collagen polypeptide chains are first synthesised on membrane-bound ribosomes and then exported into the lumen of en­doplasmic reticulum as larger precursor called the pro a-chain. Proline is a ring structure which stabilizes a left-handed helical configu­ration in each a-chain with three amino acid residues per turn.

Glycine is the smallest amino acid regularly spaced at every third residue throughout the central region of the a-chain. In the lumen of the endoplasmic reticulum, proline and lysine are hydroxylated to form hydroxyproline and hydroxyserine, respectively. Each pro a-chain has an extra non-helical segment at their amino—and carboxyl terminal ends.

The extra segment is called telopeptides. Each pro α-chains then combines with other two pro a-chain and forms a triple stranded helical molecule called the pro-collagen or tropocollagen.

The pro-collagen molecules are then secreted to the extracellular space and are converted into collagen molecules in the extracellular space by the removal of the telopeptide. Several collagen monomer molecules combine with each other to form much larger collagen fibrils (10-300 nm in diameter). Further, several collagen fibrils aggregate to form a collagen fibre.

When isolated collagen fibrils are fixed, stained and viewed in an electron microscope, they show a cross-striation appearance. This pattern indicates the packing arrangement of the individual collagen monomer in the fibril where they are staggered, so that the adjacent molecules are displaced to a distance of 67 nm. This arrangement gives rise to the striations (Fig. 4.11 and 4.12).

 

Type IV collagen is a non-fibrillar collagen and is thought to assemble into a multilayered network which forms the core of all basal lam­ina. Each monomer or a-chain consists of three parts head or C-terminal globular domain, mid­dle pieces or triple helical domains and N- terminal tail.

During assemblage, every fibril to monomers undergoes rapid head to head as­sociation via C-terminal globular domains and forms dimers. After then, lateral association of dimers take place via triple helical domains to form a sheet-like network.

In sheet-like polygonal meshwork, N-terminal tails projects above and below the plane of meshwork. Lastly, slow covalent associations via N-terminal tails take place to form a multilayered network of sheets (Fig. 4.13).

(b) Elastin:

Elastin is a fibrillar cross-linked, random-coil, hydrophobic, non-glycosylated protein that gives the elasticity of the tissues—such as skin, blood vessels and lungs—in order to function. This protein is rich in proline and glycine and contains little amount of hydroxyproline and hydroxyserine.

It is secreted into the extracellular space and forms an extensive cross-linked network of fibres and sheets that can stretch and recoil like a rubber band and imparts the elasticity to the extracellular matrix. Elastin fibre also contains a glycoprotein which is distributed as micro-fibrils on the elastin fibre surface.

B. Adhesive Fibrous Protein:

The extracellular matrix contains several ad­hesive fibrous glycoproteins that bind to both cells and other matrix macromolecules and, ultimately, help cells stick to the extracellu­lar matrix. Fibronectin and laminin are the examples of best characterised large adhesive glycoproteins in the extracellular matrix.

(a) Fibronectin:

Fibronectin is a glycoprotein. It is made of two polypeptide chains which are similar but not identical. The two polypeptides are joined by two disulfide bonds near the carboxyl terminus. Each chain is folded into a series of globu­lar domains joined by a flexible polypeptide segments (Fig. 4.14).

Individual domains are specialised for binding to a particular molecule or to a cell. For example, one domain binds to collagen, another to heparin, another to specific receptors on the surface of various types of cells, and so on. In this way fibronectin builds up the close organisation of the matrix and help cells attach to it.

Fibronectin occurs in three forms:

1. A Soluble Dineric Form:

Called plasma fi­bronectin—which circulates in the blood and other body fluid. The main function of this fibronectin is to enhance blood clotting, wound healing and phagocytosis.

2. Oligomers of Fibronectin:

Called cell-sur­face fibronectin—which are occasionally found to attach on the cell surface and helps cell to cell attachment.

3. Highly Insoluble Fibrillar Fibronectin:

Called matrix fibronectin—which help cell adhere to the matrix.

(b) Lamina:

Laminin is an adhesive glycoprotein. It is secreted specially by epithelial cells. This protein is a major part of all basal laminae. It binds the epithelial cells to type IV collagen of basal Lamina. Laminin is composed of three multi-domain polypeptide chains, such as A chain, B₁ chain and B₂ chain (Fig. 4.15).

It has a rather peculiar asymmetric cross-shaped structure with an extended long arm ending with a large domain at one pole and three short arms having two globular domains in each arm at the opposite end.

In the middle portion both B₁ and B₂ chains make a double helical configuration around the straight central A chain. Three chains are held together by disulfide bond. Each chain is made of more than 1,500 amino acid residues. Laminin has high-affinity binding sites for other components of the basal lamina.

Laminin is the first extracellular matrix pro­tein to appear in the embryo. In the kidney it acts a major barrier to filtration. When this protein deposits in the glomerular basement membrane, antibodies are produced against laminin and severely affect the kidney func­tions. Laminin is increased in basement mem­branes of diabetic patients. Antibodies are also found in Chagas disease.

(iii) Specialised Extracellular Matrix Basal Laminae:

Basal lamina is a continuous thin mat or sheet like specialised extracellular structure that un­derlies all epithelial cells. Individual muscle cells, fat cells, Schwann cells are wrapped by basal lamina. It is actually linked to the plasma membranes of different types of cell by specific receptors.

The basal lamina separate these cells from the connective tissue. In the glomerulus of the kidney, the basal lamina lie between two cell sheets and forms a porous filter that allows water, ions and small molecules in blood to cross into the urinary space while retaining protein and cells in the blood.

Basal lamina is also able to determine cell polarity, influence cell metabolism, organise the proteins in neighbouring plasma membrane, induces cell differentiation and also facilitate cell migration.

The macromolecules that comprise the basal lamina are synthesised by the cells that sit on it. The precise composition of basal lamina varies from tissue to tissue but, in general, it is made of huge quantity of type IV collagen, together with proteoglycan —primarily heparan sulfate and some glycoproteins like laminin and enlactin.

In cross-sectional view, most of the basal lamina consists of two distinct layers—an electron-lucent layer, i.e., lamina lucida or rara, which remains in close contact with plasma membrane of the epithelial cells that sit on it; and an electron-dense layer, or lamina densa, that is present just below the lamina lucida.

In some cases, a third layer, i.e., lamina reticular is also found below the lamina densa and connects the underlying connective tissue (Fig. 4.16). It is made of collagen fibrils. Lamina lucida and lamina densa are unitedly called basal lamina. Lamina reticularis plus basal lamina constitute the basement membrane.

The lamina densa is made primarily of type IV collagen with proteoglycan molecules lo­cated on both sides. Laminin is thought to be present mainly on the plasma membrane side of the lamina densa. It helps to bind epithelial cells to the lamina. On the other hand, fibronectin helps to bind the matrix macromolecules and connective tissues cells on the opposite side.

Functions of Basal Laminae:

The functions of basal laminae are varied. It acts as a molecular filter in the kidney glomeru­lus and regulates the passage of macromolecules from the blood into urine. It acts as a selective cellular barrier and pre­vents fibroblasts in the underlying connective tissue from making contact with the epithelial cells. It does not stop macrophages, lympho­cytes and nerve processes from passing through it.

The basal lamina helps to regenerate tissues after injury. When tissues are damaged, the basal lamina survives and makes a scaffolding along which regenerating cells can migrate. In this ways original tissue is recovered.

In the neuromuscular junction or the synapse (where a nerve cell transmits its stimulus to a skeletal muscle cell) the basal lamina helps to coordinate the organisation of the components on both sides of the synapse.

Extracellular Matrix on Cell Surface Receptors:

Cell surface contains many types of receptor proteins. Receptor is the site that communicates with the neighbouring cells as well as binds the matrix components and extracellular matrix components and often bind with their specific ligands either to trigger a metabolic reaction or to initiate the process of endocytosis.

Sometimes, due to presence of some specific receptors, cells have the capacity to recognize similar cells and permit them to adhere with one another or to associate themselves forming aggregates. Similarly, they may have property of dissociating with neighbouring cells by a process of contact inhibition.

On the basis of diverse functions, cell surface receptors can be classified into four major categories:

i. Matrix receptors

ii. Specific receptors

iii. Hormone receptors and

iv. Other macromolecule receptors.

i. Matrix Receptors:

In small group of cells, cell to cell contact or cell communication is often maintained by means of extracellular matrix. It is known that some proteoglycans are the integral components of plasma membrane. Their core protein may be either penetrated right through the lipid bilayer or covalently linked to it.

These proteoglycans bind in one hand to the plasma membrane and to extracellular matrix by other hand. Similarly, many cells bind to the extracellular matrix by the similar fashion.

As a result, cell to cell contact is established via extracellular matrix and cell surface bound proteoglycans. However, extracellular matrix component also bind to the cell surface via specific receptor glycoproteins. These receptors are known as matrix receptors.

They bind their ligand (a substance that binds to or fits or site) with relatively low affinity and are usually present at about 10-100 fold higher concentration than other receptors on the cell surface.

The best characterised or studied matrix receptor is the fibronectin receptors on mammalian fibroblasts. This receptor is a non-covalently associated complex of two distinct, high-molecular-weight polypeptide chains called α and β. It look.’ like a pin.

The projecting globular head is more than 20 nm in diameter. The head portion binds fibronectin outside the cell and the coiled tail portion is inserted through plasma membrane up to cytosol where it attaches to cytoskeleton viatalin protein.

The α and β chains are both glycosylated and are held together by non-covalent bonds. The α -chain is usually made at first as single 140,000 Dalton polypeptide chain which is then cleaved into one small trans membrane chain and one large extracellular chain that remain held together by a disulfide bond.

The β -chain is continuous and its extracellular part contains a repeating cysteine-rich region. Many other matrix receptors, such as collagen receptor, laminin receptors etc. have been characterised later on. But all these matrices are shown to be related to the fibronectin receptor and they are collectively called integrin’s.

In a variety of extracellular matrix protein contain a specific tripeptide sequence, i.e., Arg-Gly-Asp which is known as RGD sequence. This sequence is recognised by the matrix receptors that bind these protein.

There are at least three families within integrin’s. All these families have the same β – chain but differ in their α-chains. One family comprises of a fibroblast fibronectin receptor and at least five other members. The second family of receptor is found on blood platelets that binds with fibronectin and fibrogen during blood clotting.

A person may suffer from Glanzmann’s disease when the platelets con­tains less amount of receptors and it causes excessive bleeding.

The third family of integrin’s consists of receptors that are found on the surface of WBC. It is, again, two types of— LFA-1 (for lymphocyte function associated) and MAG-1 (found mainly on macrophages). These receptors are involved in both cell to cell and cell to matrix interaction and they are im­portant to fight against infection.

Sometimes, due to genetical cause, the cells are not able to synthesize β -chain. As a consequence, the WBC lacks the receptors and the person with such type of WBC may suffer repeated bacterial infection.”

ii. Specific Receptor:

Specific receptors actually mediate the response of cells to specific extracellular signals. The extracellular signalling substance is known as ligand and the cell contains the specific re­ceptors on its surface for receiving the ex­tracellular signal and is known as target cell. Specific receptors have a binding site with high affinity for a particular signalling substance.

When the signalling substance binds to the receptor the receptor-ligand complex initiates a sequence of reactions that changes the functions of the target cell. The specific signalling sub­stances may be neurotransmitter, pheromone, hormone, etc.

One cell may have two or more types of specific receptors, or various cell types may have different sets of specific receptors or the same specific receptor may occur on various cell types.

The ligand have no function except to bind at the specific site of the receptor. They are not always metabolised or internalised. The only function of the ligand is to change the properties of the receptors. At nerve-muscle junction or synapse, there is a specific receptor for acetylcholine, a neu­rotransmitter.

It has a specific binding site for acetylcholine and, normally, remains closed to inhibit mass inflow of ions from external environment. When acetylcholine binds with receptor at their specific binding site, it induces a conformational change [Fig. 4.17(a)] in the re­ceptor that opens as an ion channel.

The resul­tant flow of ions changes the electric potential across the cell membrane. In another example, ligand, e.g., leucocyte CD45 protein, when it binds its specific receptor causes activation of a phosphatase activity that removes a phosphate residue attached to a tyrosine on a substrate protein. In doing so, it alters the activity of that protein [Fig. 4.17(c)]. It is also known as catalytic receptor.

The binding of ligand to many cell surface receptors activates an enzyme that generates a short-lived increase in the concentration of an intracellular signalling compound termed a second messenger. For example, when a specific ligand binds receptor it activates the cytosolic synthesis of the second messenger 3′, 5′ cyclic GMP from GTP [Fig. 4.17(d)],

iii. Hormone Receptors:

Many hydrophilic hormones like Insulin, Glucagon, Gastrin, ACTH, FSH, LH, TSH, Epidermal growth factor (EGF), Epinephrine, Adrenaline, Serotonin etc. bind to cell-surface receptors of different target cells.

The receptors binding with hormones as ligands indirectly activate or inactivate a separate plasma- membrane-bound enzyme or ion-channel. The interaction between the receptor and the enzyme or ion channel is mediated by a third protein, called a GTP binding regulatory protein or G-protein.

The G-protein-linked receptors usually activate a chain of events that changes the concentration of one or more small intracellular mediators or second messenger. Cyclic AMP (cAMP), Ca²⁺ cyclic GMP(cGMP), inositol 1, 4, 5-triphosphate and 1, 2-diacylglycerol are the most important intracellular mediators or second messenger.

The elevated intracellular concentration of one or more such second messengers triggers a rapid change in the activity of one or more enzymes or non-enzymatic protein [Fig. 4.17(e)], Cyclic AMP is synthesised from ATP by the plasma membrane-bound enzyme adenylate cyclase and cAMP is hydrolysed by cyclic AMP phospho-diesterases to 5′-AMP. Similarly 3′, 5′ cyclic GTP is synthesised from GTP.

Again, when hormone binds with receptors, it activates G protein. Then concentration of cytosolic Ca²⁺ is elevated or Ca²⁺ is released from endoplasmic reticulum. Activation of G protein is preceded by the hydrolysis of an unusual plasma membrane, the phospholipid—phosphatidylinositol 4, 5 bi-phosphate. Hydrolysis of this phospholipid is by the plasma membrane-bound enzyme phospholipase C.

It yields two important products 1, 2 diacylglycerol which remain in the membrane and the water-soluble inositol 1, 4, 5, triphosphate.. 1, 2-diacylglycerol together with Ca²⁺ helps to activate a membrane-bound enzyme protein kinase C.

The activated protein kinase C, in turn, change the cellular inactive protein to active protein to bring about the cellular response. But inositol 1, 4, 5 triphosphate diffuses through cytosol to endoplasmic reticulum (Fig. 4.18) where it releases Ca²⁺ from the ER lumen into cytosol.

Other receptors, such as insulin receptor, epi­dermal growth factor receptor (EGF receptor), platelet-derived growth factor receptor (PDGE receptor) may not utilize a second message but act directly to modify the cytoplasmic protein by phosphorylating them [Fig. 4.17(b)].

These are catalytic receptor proteins and the best studied examples in animal cells are single- pass trans membrane tyrosine-specific protein kinase with their catalytic domain exposed on the cytoplasmic side of the plasma membrane.

When these receptors are activated by specific ligand (insulin, EGF, PDGF), they transfer the terminal phosphate group from ATP to the hydroxyl group on a tyrosine residue of the selected proteins in the target cells.

iv. Other Macromolecule Receptors:

In most animal cells, clathrinpits and vesicles provide an efficient pathway for taking up specific macromolecules from the extracellular fluid, a process called receptor mediated endocytosis. Clathrin is a fibrous protein.

The best studied receptor mediated endocytosis is the uptake of low density lipoproteins or LDL which help to transport cholesterol into the cell. LDL is a spherical particle of 22 nm diameter. It is made of a central core of cholesterol ester surrounded by a lipid monolayer and contains an apo-B-protein that organizes a site for bind­ing with receptors.

When the cell needs cholesterol for mem­brane synthesis, it make receptor protein for LDL. LDL particle binds with LDL receptors with a high degree of specificity. .The ligand- receptor complex makes a specialised depres­sion on the cell surface.

At the same time, clathrin molecules are internally deposited on the convex side of the depression. The mem­brane depression invaginates to form a pit which is coated by clathrin. Finally, the ligand receptor, complex in a coated pit pinches off to become a coated vesicle which is rapidly internised into the cytoplasm.

The clathrin coat then depolymerizes to clathrin triskelions, resulting in an uncoated vesicle or endosome. This endosome fuses with an uncoupling vesicle called the compartment of uncoupling of recep­tor and ligand (CURL) vesicle and its low pH (~ 5) causes the LDL particles to dissociate from the LDL receptors.

A receptor-rich re­gion buds off to form a separate vesicle that returns the LDL receptors back to the plasma membrane. A vesicle containing a LDL particle may fuse with another endosome but ultimately fuses with a primary lysosome to form a sec­ondary lysosome. There, the apo-B-protein of the LDL particle is degraded to amino acids and the cholesterol esters are finally hydrolysed into fatty acids and cholesterol.

If the LDL system is blocked or if it is defective, cholesterol may accumulate in the blood vessel and it leads to the formation atherosclerotic plaques in blood vessel walls.

Proteins, bacteria and viruses are inserted into animal cell by a special form of endocytosis which is known as phagocytosis. In order to be phagocytosed, the particle must bind to the surface of the cell which have a variety of spe­cialised surface receptors that are functionally linked to the phagocytic machinery of the cell.

Some receptors on the surface of polarised epithelial cells transfer specific macromolecules from one extracellular space to another by a process called transcytosis. For example, a new­born rat gets antibodies from its mother’s milk by transporting them across the epithelium of its gut.

The lumen of the gut is acidic and, at this low pH, the antibodies in the milk bind to specific receptors on the apical surface of the gut epithelial cells and are ingested via coated pits. The receptor-antibody complexes remain intact in endosome and fuse with basolateral domain of the plasma membrane.

Upon expo­sure to the neutral pH of the extracellular fluid, the antibodies dissociate from their receptors and then enter the newborn’s blood stream (Fig. 4.19).