The Extracellular Matrix - Biochemistry Chapter

50 C H A P T E R The Extracellular Matrix Kathleen M. Botham, PhD, DSc & Robert K. Murray, MD, PhD O B J EC T IVES...

50 C H A P T E R The Extracellular Matrix Kathleen M. Botham, PhD, DSc & Robert K. Murray, MD, PhD O B J EC T IVES Indicate the importance of the extracellular matrix (ECM) and its components in health and disease. After studying this chapter, Describe the structural and functional properties of collagen and elastin, the you should be able to: major proteins of the ECM. Indicate the major features of fibrillin, fibronectin, and laminin, other important proteins of the ECM. Describe the properties and general features of the synthesis and degradation of glycosaminoglycans and proteoglycans, and their contributions to the ECM. Give a brief account of the major biochemical features of bone and cartilage. BIOMEDICAL IMPORTANCE COLLAGEN IS THE MOST Most mammalian cells are located in tissues where they are ABUNDANT PROTEIN IN THE ANIMAL WORLD ECM surrounded by a complex extracellular matrix (ECM) often ① Structur ral referred to as “connective - tissue,” which protects the organs + 25 % Collagen, the major component of most connective tissues, - > ② specializand also provides elasticity where required (eg, in blood ves- zed sels, lungs, and skin). The ECM contains three major classes constitutes approximately &* 25% of the protein of mammals. It ③ Proteog of biomolecules:- structural proteins, for example, collagen, provides an extracellular framework for all metazoan animals and exists in virtually every animal tissue. At least 28 types of - lycans elastin, and fibrillin; certain - specialized proteins such as fibronectin and laminin, which form a mesh of fibers that collagen made up of over 30 distinct polypeptide chains (each are embedded in the third class,- proteoglycans. The ECM encoded by a separate gene) have been identified in human tis- is involved in many processes, both normal and pathologic, sues (Table 50–1). Although several of these are present only for example, it plays important roles in development, in in small proportions, they may play important roles in deter- inflammatory states, and in the spread of cancer cells. Cer- mining the physical properties of specific tissues. In addition, tain components of the ECM play a part in both rheumatoid a number of proteins (eg, the C1q component of the comple- arthritis and osteoarthritis. Several diseases (eg, osteogen- ment system, pulmonary surfactant proteins SPA and SPD) esis imperfecta and a number of types of the Ehlers-Danlos that are not classified as collagens have collagen-like domains syndrome) are due to genetic disturbances of the synthesis of in their structures; these proteins are sometimes referred to as collagen, a major ECM component. Specific components of “noncollagen collagens.” proteoglycans (the glycosaminoglycans; GAGs) are affected in the group of genetic disorders known as the mucopoly- COLLAGENS HAVE A TRIPLE saccharidoses. Changes occur in the ECM during the aging process. This chapter describes the basic biochemistry of the HELIX STRUCTURE three major classes of biomolecules found in the ECM and All collagen types have a=triple helical structure made up of illustrates their biomedical significance. Major biochemi- three polypeptide chain subunits (α chains). In some colla- cal features of two specialized forms of ECM—bone and gens, the entire molecule is a triple helix, whereas in others cartilage—and of a number of diseases involving them are only a fraction of the structure may be in this form. Mature also briefly considered. ecollagen type I belongs to the former type; each polypeptide 592  CHAPTER 50 The Extracellular Matrix 593 TABLE 50–1 Types of Collagen and Their –Gly – X – Y – Gly – X – Y – Gly – X – Y – Amino acid sequence Tissue Distribution G G G G Type Distribution X Y Y Y Y G Alpha-chain X X X X I Noncartilaginous connective tissues, including bone, tendon, skin 1.4 nm Triple helix II Cartilage, vitreous humor III Extensible connective tissues, including skin, lung, vascular system IV Basement membranes Triple helix molecule V Minor component in tissues containing collagen I N C (300 nm) VI Muscle and most connective tissues VII Dermal-epidermal junction 67 nm Fibril VIII Endothelium and other tissues Over- Gap lap zone IX Tissues containing collagen II zone X Hypertrophic cartilage FIGURE 50–1 Molecular features of collagen structure XI Tissues containing collagen II from the primary sequence to the fibril. Each individual polypeptide chain is twisted into a left-handed helix of three XII Tissues containing collagen I residues (Gly-X-Y) per turn, and three of these chains are then wound into a right-handed superhelix. The triple helices are then XIII Many tissues, including neuromuscular junctions and skin assembled into a quarter-staggered alignment to form fibrils. XIV Tissues containing collagen I This arrangement leads to areas where there is complete overlap of the molecules alternating with areas where there is a gap, XV Associated with collagens close to basement membranes giving the fibrils a regular banded appearance. (Modified and in many tissues including in eye, muscle, microvessels redrawn from Eyre DR: Collagen: molecular diversity in the body’s XVI Many tissues protein scaffold. Science 1980;207:1315. Reprinted with permis- sion from AAAS.) XVII Epithelia, skin hemidesmosomes XVIII Associated with collagens close to basement membranes, close structural homologue of XV accommodated in the limited space available in the central XIX Rare, basement membranes, rhabdomyosarcoma cells core of the triple helix. This - repeating structure, represented XX Many tissues, particularly corneal epithelium as (Gly-X-Y)n, is an absolute requirement for the formation of XXI Many tissues the triple helix. While X and Y can be any other amino acids, the X positions are often proline and the Y positions are often XXII Tissue junctions, including cartilage-synovial fluid, hair hydroxyproline. Proline and hydroxyproline confer E rigidity Hydroxyproline follicle–dermis * ↳ formed by on the collagen molecule. - Hydroxyproline is formed by the by droxylation of proline cat , - XXIII Limited in tissues, mainly transmembrane and shed forms posttranslational hydroxylation of peptide-bound proline a lyzed by prolyl hydroxylase ↳ cofactors : XXIV Developing cornea and bone residues catalyzed by the enzyme prolyl hydroxylase, whose Ferrousirond cofactors are- ascorbic acid (vitamin C) and - >α-ketoglutarate. (Vit. C) XXV Brain ③ X-Ketogluta- Lysines in the Y position may also be posttranslationally mod- rate XXVI Testis, ovary ified to hydroxylysine through the action of- lysyl hydroxylase, XXVII Embryonic cartilage and other developing tissues, an enzyme with similar cofactors. Some of these hydroxyly- cartilage in adults sines may be further modified by the addition of galactose XXVIII Basement membrane around Schwann cells or galactosyl-glucose through an- O-glycosidic linkage (see Chapter 46), a glycosylation site that is unique to collagen. Some collagen types form long rod-like fibers in tissues. These are assembled by lateral association of these triple heli- subunit is twisted into a left-handed polyproline helix of three cal units into& fibrils (10-300 nm in diameter) in a “quarter- residues per turn forming an α chain. Three of these are then staggered” alignment such that each is displaced longitudinally wound into a right-handed triple- or superhelix, forming a from its neighbor by slightly less than one-quarter of its length rod-like molecule 1.4 nm in diameter and about 300-nm long (Figure 50–1). Fibrils, in turn, associate into thicker fibers (Figure 50–1).* Glycine residues occur at every third position (1-20 μm in diameter). Because the quarter staggered align- of the triple helical portion of the α chain. This is necessary ment results in regularly spaced gaps between the triple heli- because glycine is the only amino acid small enough to be cal molecules in the array, fibers have a banded appearance in  594 SECTION X Special Topics (B) connective tissues. In some tissues, for example tendons, fibers Collagen Undergoes Extensive associate into even larger bundles, which may have a diameter of up to 500 μm. Collagen fibers are further stabilized by the Posttranslational Modifications formation of- covalent cross-links, both within and between theNewly synthesized collagen undergoes extensive posttrans- ① Preprocolla- covalent triple helical units. These cross-links form through the action lational modification before becoming part of a mature cross- linkages gen extracellular collagen fiber (Table 50–2). Like most secreted ↳ formed by lysyl Oxidase lysyl oxidase, a copper-dependent enzyme that oxidatively of - signal peptide ↓ leas preprocollar deaminates the ε-amino groups of certain lysine and hydroxy- proteins, collagen is synthesized on ribosomes in a precur- gen to ER lysine residues, yielding reactive aldehydes. Such aldehydes sor form,- preprocollagen, which contains a leader or signal can form aldol condensation products with other lysine- or sequence that directs the polypeptide chain into the lumen ② Procollagen hydroxylysine-derived aldehydes or form Schiff bases with - of the endoplasmic reticulum (ER) (see Chapter 49). As it Hydroxylation Glycosylation the ε-amino groups of unoxidized lysines or hydroxylysines. enters the ER, this leader sequence is removed enzymatically. Disulfidelin- ① Hydroxylation of proline and lysine residues and glycosyl- Kage These reactions, after further chemical rearrangements, result Triple helix ation of hydroxylysines in the procollagen molecule also take formation in the stable covalent cross-links that are important for the W tensile strength of the fibers. Histidine may also be involved place at this site. The procollagen molecule contains polypep- ③ Tropocolla- in certain cross-links. tide extensions (extension peptides) of 20 to 35 kDa at both gen The main fibril-forming collagens in skin and bone and in its amino- and carboxyl-terminal ends, which are not present ↓ Propeptidase TypeIcollag is cartilage, respectively, are types I and II, although other col-in mature collagen. Both extension peptides contain cysteine cross-link Type ↳ It collagen cartilage lagens also adopt this structure. In addition, however, there residues. While the amino terminal propeptide forms only S are many nonfibril-forming collagens and their structures and intrachain disulfide bonds, the carboxyl-terminal propeptides ④ collagen functions are described briefly in the section below. form both intrachain and interchain disulfide bonds. Forma- tion of these disulfide bonds assists in the- registration of the three collagen molecules to form the triple helix, winding from Some Collagen Types Do Not Form Fibrils the carboxyl-terminal end. After formation of the triple helix, Several collagen types do not form fibrils in tissues no further hydroxylation of proline or lysine or glycosylation (Figure 50–2). They are characterized by interruptions of of hydroxylysines can take place. - Self-assembly is a cardinal the triple helix with stretches of protein lacking Gly-X-Y principle in the biosynthesis of collagen. repeat sequences. Thus, areas of globular structure are Following secretion from the cell by way of the Golgi appara- interspersed in the triple helical structure. - Network-like tus, extracellular enzymes called procollagen aminoproteinase collagens such as - - type IV form networks in basement and procollagen carboxyproteinase remove the extension pep- membranes; fibril-associated collagens with interrupted tides at the amino- and carboxyl-terminal ends, respectively, triple helices (FACITs), as their name indicates, have inter- forming the monomeric units of collagen, termed- tropocolla- ruptions in the triple helical domains; - e beaded filaments gen. Cleavage of the propeptides may occur within crypts or consist of long chains of collagen molecules which have a folds in the cell membrane. Once the propeptides are removed, regular beaded appearance; - collagen VII forms the main the tropocollagen molecules, containing approximately 1000 part of - anchoring fibrils in epithelial tissues; - trans- amino acids per α chain, spontaneously assemble into collagen - membrane collagens have short intracellular N-terminal fibers. These are further stabilized by the formation of inter- domains and extracellular domains with long interrupted and intrachain cross-links through the action of lysyl oxidase, triple helices; -multiplexins are collagens with multiple as described previously. triple helix domains and interruptions. * TABLE 50–2 Order and Location of Processing of the most common Fibrillar Collagen Precursor * I Fibril-forming Network-like IV, VIII, X Intracellular OI, II, III, V, XI, XXIV, XXVII FACITs 1. Cleavage of signal peptide IX, XII, XIV, XVI, XIX, XX, XXI, XXII 2. Hydroxylation of prolyl residues and some lysyl residues; glycosylation of some hydroxylysyl residues Multiplexins COLLAGEN 3. Formation of intrachain and interchainOS–S bonds in extension XV, XVIII peptides Beaded filaments ↳ disulfide VI, XXVI, XXVIII 4. Formation of triple helix Transmembrane Extracellular XIII, XVII, XXIII, XXV Anchoring fibrils 1. Cleavage of amino- and carboxyl-terminal propeptides VII 2. Assembly of collagen fibers in quarter-staggered alignment 3. Oxidative deamination of h -amino groups of lysyl and FIGURE 50–2 Classification of collagens according to the hydroxylysyl residues to aldehydes structures they form. FACIT, fibril-associated collagen with inter- 4. Formation of intra- and interchain cross-links via Schiff bases and rupted triple helices; multiplexin, multiple triple helix domains and aldol condensation products interruptions.  CHAPTER 50 The Extracellular Matrix 595 The same cells that secrete collagen also secrete& fibronec- TABLE 50–3 Diseases Caused by Mutations in Collagen tin, a large glycoprotein present on cell surfaces, in the extra- & Genes or by Deficiencies in the Activities of Enzymes cellular matrix, and in blood (see below). Fibronectin binds Involved in the Posttranslational Biosynthesis collagen fibers during aggregation and alters the kinetics of of Collagen fiber formation in the pericellular matrix. Associated with Gene or Enzyme Affected Diseasea proteogly- fibronectin and procollagen in this matrix are the- COL1A1, COL1A2 Osteogenesis imperfecta type 1b cans heparan sulfate and chondroitin sulfate (see below). In & Osteoporosis type IX collagen, a minor collagen type from cartilage, fact, - Ehlers-Danlos syndrome, subtype contains an attached glycosaminoglycan chain. Such interac- arthrochalasia tions may serve to regulate the formation of collagen fibers COL2A1 Severe chondrodysplasia and to determine their orientation in tissues. Osteoarthritis Once formed, collagen is relatively metabolically stable. COL3A1 Ehlers-Danlos syndrome, subtype However, its breakdown is increased during starvation and vascular various inflammatory states. Excessive production of colla- COL4A3-COL4A6 Alport syndrome (autosomal and gen occurs in a number of conditions, for example, hepatic X-linked) cirrhosis. COL7A1 Epidermolysis bullosa, dystrophic A Number of Genetic & Deficiency COL10A1 Schmid metaphyseal chondrodysplasia Diseases Result From Abnormalities in COL5A1, COL5A2, COL1A1 Ehlers-Danlos syndrome, subtype the Synthesis of Collagen classical More than 30 genes encode the collagens, and they are desig- COL3A1, tenascin XB (TNXB) Ehlers-Danlos syndrome, subtype nated according to the procollagen type and their constituent hypermobility α chains, called proα chains. Collagens may be homotrimeric, & containing three identical proα chains, or heterotrimeric, Lysyl hydroxylase Ehlers-Danlos syndrome, subtype kyphoscoliosis where the proα chains are different. For example, type I col- ADAM metallopeptidase Ehlers-Danlos syndrome, subtype lagen is heterotrimeric, containing two proα1(I) and one with thrombospondin type 1 dermatosparaxis proα2(I) chains (the arabic number refers to the proα chain, motif (ADAMTS2) (also called and the roman numeral in parentheses indicates the collagen procollagen N-proteinase) type), while type II collagen is homotrimeric, having three Lysyl oxidase Menkes diseasec proα1(II) chains. Collagen genes have the prefix COL followed by the type in arabic numerals, then an A and the number of a Genetic linkage to collagen genes has been shown to a few other conditions not listed here. the proα chain they encode. Thus, COL1A1 and COL1A2 b Eight different types of osteogenesis imperfecta are recognized, but most cases are are the genes for the proα1 and 2 chains of type I collagen, caused by mutations in the COL1A1 and COL1A2 genes. c Secondary to a deficiency of copper (see Chapter 52). COL2A1 is the gene for the proα1 chain of type II collagen, and so on. The pathway of collagen biosynthesis is complex, involving subtypes are more common, while the other three,- kyphosco- at least eight enzyme-catalyzed posttranslational steps. Thus, liosis, arthrochalasis, and - e dermatosparaxis are extremely it is not surprising that a number of diseases (Table 50–3) rare. The vascular subtype is the most serious because of its are due to mutations in collagen genes or in genes encod- tendency for spontaneous rupture of arteries or the bowel, ing some of the enzymes involved in these posttranslational reflecting abnormalities in - type III collagen. Patients with modifications. Diseases affecting bone (eg, osteogenesis kyphoscoliosis exhibit progressive curvature of the spine (sco- imperfecta) and cartilage (eg, the chondrodysplasias) will be liosis) and a tendency to ocular rupture due to a deficiency discussed later in this chapter. - collagen 1 I11 v , , of lysyl hydroxylase. A deficiency of procollagen N-proteinase * Ehlers-Danlos syndrome (formerly called Cutis hyper- (ADAM metallopeptidase with thrombospondin type 1 motif elastica), comprises a group of inherited disorders whose [ADAMTS2]), causing formation of abnormal thin, irregu- principal clinical features are -hyperextensibility of the skin, lar collagen fibrils, results in dermatosparaxis, manifested by abnormal tissue fragility, and - - increased joint mobility. The marked fragile and sagging skin. clinical picture is variable, reflecting underlying extensive The Alport - syndrome (hereditary nephritis) is the name genetic heterogeneity. A number of forms of the disease given to a number of genetic disorders (both X-linked and caused by genetic defects in proteins involved in the synthe- autosomal) affecting - type IV collagen, a network-like col- sis and assembly of- collagens type I, III, and V are known, lagen which forms part of the structure of the basement and since 1997 the Villefranche classification of six subtypes membranes of the renal- glomeruli, inner ear, and -eye (see based on their phenotype and molecular defects has been discussion of laminin, below). Mutations in several genes used (Table 50–4). The- hypermobility, vascular and classical encoding type IV collagen fibers have been demonstrated.  596 SECTION X Special Topics (B) TABLE 50–4 The Villefranche Classificationa of Ehlers-Danlos Syndrome Subtypes Subtype Name Defect in Incidence Clinical Signs - Hypermobility Type III collagen, tenascin Xb 1:10,000-15,000 Joint hypermobility, skin abnormalities, osteoarthritis, severe pain - Classical Types I and V collagen 1:20,000-30,000 Similar to the hypermobility subtype, but with more severe skin abnormalities and less severe joint changes - Vascular - Type III collagen 1:100,000 Fragile blood vessels and organs, small stature, thin and translucent skin, easy bruising > Kyphoscoliosis - Lysyl hydroxylase Arthrochalasis - Type I collagen (synovial fluid) and in↳ vitreous humor in the eye, as well as in embryonic - tissues. It is thought to play an important role in the matrix relative to proteins. permitting- cell migration during morphogenesis and wound repair. Its ability to attract water into the ECM triggers loosen- Various Glycosaminoglycans Exhibit ing of the matrix, aiding this process. The high concentrations of hyaluronic acid together with chondroitin sulfates present Differences in Structure & Have cartilage contribute to its- in - compressibility (see below). Characteristic Distributions and Diverse Functions Chondroitin Sulfates (Chondroitin 4-Sulfate & The seven GAGs named above differ from each other in a Chondroitin 6-Sulfate) number of the following properties: amino sugar composi- - Proteoglycans linked to chondroitin sulfate by the Xyl-Ser tion, uronic acid composition, linkages between these com- O-glycosidic bond are prominent components of - cartilage ponents, chain length of the disaccharides, the presence or (see below). The repeating disaccharide is similar to that absence of sulfate groups and their positions of attachment found in hyaluronic acid, containing - GlcUA but with &GalNAc to the constituent sugars, the nature of the core proteins to replacing GlcNAc. The GalNAc is substituted with sulfate at which they are attached, the nature of the linkage to core either its 4′ or its 6′ position, with approximately one sulfate β1,4 β1,3 β1,4 β1,3 β1,4 Hyaluronic acid: GlcUA GlcNAc GlcUA GlcNAc β1,4 β1,3 β1,4 β1,3 β1,3 β1,4 β Chondroitin sulfates: GlcUA GalNAc GlcUA Gal Gal Xyl Ser 4- or 6-Sulfate β an) GlcNAc Asn (keratan sulfate I) ,M Ac Keratan sulfates (GlcN β1,4 β1,3 β1,4 β1,3 I and II: GlcNAc Gal GlcNAc Gal 1,6 α 6-Sulfate 6-Sulfate GalNAc Thr (Ser) (keratan sulfate II) Gal-NeuAc 6-Sulfate Heparin and α1,4 α1,4 α1,4 β1,4 α1,4 β1,3 β1,3 β1,4 β heparan sulfate: IdUA GlcN GlcUA GlcNAc GlcUA Gal Gal Xyl Ser 2-Sulfate SO3– or Ac β1,4 α1,3 β1,4 β1,3 β1,4 β1,3 β1,3 β1,4 β Dermatan sulfate: IdUA GalNAc GlcUA GalNAc GlcUA Gal Gal Xyl Ser 2-Sulfate 4-Sulfate FIGURE 50–10 Structures of glycosaminoglycans and their attachments to core proteins. (Ac, acetyl; Asn, L-asparagine; Gal, D-galac- tose; GalN, D-galactosamine; GlcN, D-glucosamine; GlcUA, D-glucuronic acid; IdUA, L-iduronic acid; Man, D-mannose; NeuAc, N-acetylneuraminic acid; Ser, L-serine; Thr, L-threonine; Xyl, L-xylose.) The summary structures are qualitative representations only and do not reflect, for example, the uronic acid composition of hybrid glycosaminoglycans such as heparin and dermatan sulfate, which contain both L-iduronic and D-glucuronic acid. Hyaluronic acid has no covalent attachemnt to protein. Chondroitin sulfates, heparin, heparan sulfate, and dermatan sulfate attach to a Ser on the core protein via the Gal-Gal-Xyl link trisaccharide. Keratan sulfate I links to a core protein Asn via GlcNAc and Keratan sulfate II to a Ser (or Thr) via GalNAc. 602 SECTION X Special Topics (B) TABLE 50–6 Properties of Glycosaminoglycans GAG Sugars Sulfatea Protein Linkage Location Hyaluronic acid - GlcNAc, GLcUA – None Skin, synovial fluid, bone, cartilage, vitreous humor, embryonic tissues Chondroitin sulfate = GalNAc, GlcUA GalNAc Xyl-Ser; associated with Cartilage, bone, CNS HA via link proteins Keratan sulfate I and II - GlcNAc, Gal GlcNAc GlcNAc-Asn (KS I) Cornea, cartilage, loose connective tissue GalNAc-Thr (KS II) Heparin - Gln, IdUA GlcN Ser Mast cells, liver, lung, skin GlcN IdUA T Heparan sulfate GlcN, GlcUA GlcN Xyl-Ser Skin, kidney basement membrane Dermatan sulfate GalNAc, IdUA, GalNAc Xyl-Ser Skin, wide distribution (GlcUA) IdUA a The sulfate is attached to various positions of the sugars indicated (see Figure 50–10). Note that all GAGs except the keratan sulfates contain a uronic acid which may be glucuronic or iduronic acid. being present per disaccharide unit. Chondroitin sulfates have (Figure 50–11). Most of the amino groups of the GlcN resi- an important role in maintaining the structure of the ECM. dues are N-sulfated, but a few are acetylated (GlcNAc). The They are located at sites of calcification in endochondral6bone GlcN also carries a sulfate attached to carbon 6. and are a major component of & cartilage. They are found in The vast majority of the uronic acid residues are IdUA. high amounts in the ECM of the central nervous system and, Initially, all of the uronic acids are GlcUA, but a 5′-epimerase in addition to their structural function, are thought to act as converts approximately 90% of the GlcUA residues to IdUA signaling molecules in the prevention of the repair of nerve after the polysaccharide chain is formed. The protein mol- endings after injury. ecule of the heparin proteoglycan is unique, consisting exclusively of serine and glycine residues. Approximately, Keratan Sulfates I & II two-thirds of the serine residues contain GAG chains, usu- As shown in Figure 50–10, the- keratan sulfates consist of ally of 5 to 15 kDa but occasionally much larger. Heparin is repeating - Gal-GlcNAc disaccharide units containing - sulfate found in the granules of mast - cells and also in &liver, lung, attached to the 6′ position of GlcNAc or occasionally of Gal. and Oskin. It is an important- anticoagulant. It is released into - Keratan sulfate I was originally isolated from the - cornea, the blood from capillary walls by the action of lipoprotein while-> keratan sulfate II came from - cartilage. The two GAGs lipase and it binds with factors IX and XI, but its most impor- differ in the structural links to the core proteins, and as I or II, tant interaction is with plasma antithrombin (discussed in the classification is based on the different linkage to the core Chapter 55). protein (Figure 50–10). In the eye, they lie between collagen fibrils and play a critical role in corneal transparency. Changes Heparan Sulfate in proteoglycan composition found in corneal scars disappear This molecule is present in a proteoglycan found on many when the cornea heals. extracellular cell surfaces. It contains & GlcN with O fewer - N-sulfates than heparin, and, unlike heparin, its predominant Heparin GlcUA. Heparan sulfates are associated with uronic acid is - The repeating disaccharide & heparin contains -glucosamine the=plasma membrane of cells, with their core proteins actually - and either of the - (GlcN) two uronic acids (GlcUA or IdUA) spanning that membrane. In this, they may act as - receptors CH2OSO3– CH2OSO3– CH2OSO3– CO2– CH2OSO3– O O O O O O O CO2– CO2– O OH OH O OH OH O OH OH O OH O O O O HNSO3– OSO3– HNSO3– OH HNSO3– OH HNAc GlcN IdUA GlcN IdUA GlcN GlcUA GlcNAc FIGURE 50–11 Structure of heparin. Structural features typical of heparin are shown. Each repeating disaccharide contains glucosamine (GlcN) and either D-glucuronic (GlcUA) or L-iduronic acid (IdUA). A few GlcN residues are acetylated (GlcNAc). The sequence of variously substituted repeating disaccharide units has been arbitrarily selected. Non-O-sulfated or 3-O-sulfated glucosamine residues may also occur. (Modified and redrawn from Lindahl U, et al: Structure and biosynthesis of heparin-like polysaccharides. Fed Proc 1977;36:19.)  CHAPTER 50 The Extracellular Matrix 603 and may also participate in the mediation of the C cell growth exhibit - relatively slow turnover, their half-lives being days to and -cell–cell communication. The attachment of cells to their weeks. substratum in culture is mediated at least in part by heparan Understanding of the degradative pathways for GAGs, as sulfate. This proteoglycan is also found in the- basement mem- in the case of glycoproteins (see Chapter 46) and glycosphin- brane of the kidney along with type IV collagen and laminin - golipids (see Chapter 24), has been greatly aided by elucida- (see above), where it plays a major role in determining the tion of the specific enzyme deficiencies that occur in certain charge selectiveness of glomerular filtration. inborn errors of metabolism. When GAGs are involved, these inborn errors are called - mucopolysaccharidoses (MPSs) Dermatan Sulfate (Table 50–8). This substance is widely distributed in animal tissues. Its Degradation of GAGs is carried out by a battery of structure is similar to that of chondroitin sulfate, except that lysosomal hydrolases. These include endoglycosidases, exo- in place of a GlcUA in β-1,3 linkage to GalNAc it contains an glycosidases, and sulfatases, generally acting in sequence. IdUA in an α-1,3 linkage to GalNAc. Formation of the IdUA The MPSs (Table 50–8) share a common mechanism of cau- occurs, as in heparin and heparan sulfate, by 5′-epimerization sation involving a mutation in a gene encoding a lysosomal of GlcUA. Because this is regulated by the degree of sulfation hydroxylase responsible for the degradation of one or more and because sulfation is incomplete,- dermatan sulfate con- GAGs. This leads to a defect in the enzyme and the accu- tains both IdUA-GalNAc and GlcUA-GalNAc disaccharides. mulation of the substrate GAGs in various tissues, including Dermatan sulfate has a widespread distribution in & tissues, the liver, spleen, bone, skin, and the central nervous system. and is the main GAG in & skin. Evidence suggests it may play The diseases are usually inherited in an autosomal recessive a part in blood coagulation, wound repair, and resistance to manner, with Hurler and Hunter syndromes being perhaps infection. the most widely studied. None is common. In general, these Proteoglycans are also found in intracellular loca- conditions are chronic and progressive and affect multiple tions such as the nucleus where they are thought to have organs. Many patients exhibit- organomegaly (eg, hepato- and a regulatory role in functions such as cell proliferation splenomegaly); severe abnormalities in the development of and transport of molecules between the nucleus and the cartilage and bone; abnormal facial appearance; and mental cytosol. The various functions of GAGs are summarized retardation. In addition, defects in hearing, vision, and the in Table 50–7. cardiovascular system may be present. Diagnostic tests include analysis of GAGs in urine or tissue biopsy samples; assay of suspected defective enzymes in white blood cells, fibroblasts Deficiencies of Enzymes That Degrade or serum; and test for specific genes. Prenatal diagnosis is Glycosaminoglycans Result in now sometimes possible using amniotic fluid cells or chori- Mucopolysaccharidoses onic villus biopsy samples. In some cases, a family history of a mucopolysaccharidosis is obtained. Both exo- and endoglycosidases degrade GAGs. Like most The term - “mucolipidosis” was introduced to denote other biomolecules, GAGs are subject to turnover, being both diseases that combined features common to both muco- synthesized and degraded. In adult tissues, GAGs generally polysaccharidoses and sphingolipidoses (see Chapter 24). In -sialidosis (mucolipidosis I, ML-I), various oligosaccha- TABLE 50–7 Some Functions of Glycosaminoglycans rides derived from glycoproteins and certain gangliosides and Proteoglycans accumulate in tissues. I-cell disease (ML-II) and Act as structural components of the ECM pseudo-Hurler polydystrophy (MLIII) are described in Have specific interactions with collagen, elastin, fibronectin, Chapter 46. The term “mucolipidosis” is retained because laminin, and other proteins such as growth factors it is still in relatively widespread clinical usage, but it is not As polyanions, bind polycations and cations appropriate for these two latter diseases since the mecha- Contribute to the characteristic turgor of various tissues Act as sieves in the ECM nism of their causation involves mislocation of certain Facilitate cell migration (HA) lysosomal enzymes. Genetic defects of the catabolism of Have role in compressibility of cartilage in weight bearing (HA, CS) the oligosaccharide chains of glycoproteins (eg, mannosi- Play role in corneal transparency (KS I and DS) dosis, fucosidosis) are also described in Chapter 46. Most Have structural role in sclera (DS) Act as anticoagulant (heparin) of these defects are characterized by increased excretion Are components of plasma membranes, where they may act as of various fragments of glycoproteins in the urine, which receptors and participate in cell adhesion and cell–cell interac- accumulate because of the metabolic block, as in the case of tions (eg, HS) Determine charge selectiveness of renal glomerulus (HS) the mucolipidoses. Are components of synaptic and other vesicles (eg, HS) Hyaluronidase is one important enzyme involved => Have a role in nuclear functions such as cell proliferation and in the catabolism of both hyaluronic acid and chondroi- transport of molecules between the nucleus and the cytosol tin sulfate. It is a widely distributed endoglycosidase that Abbreviations: CS, chondroitin sulfate; DS, dermatan sulfate; ECM, extracellular cleaves hexosaminidic linkages. From hyaluronic acid, the matrix; HA, hyaluronic acid; HS, heparan sulfate; KS I, keratan sulfate I. enzyme will generate a tetrasaccharide with the structure  604 SECTION X Special Topics (B) TABLE 50–8 The Mucopolysaccharidoses Disease Name Abbreviationa Enzyme Defective GAG(s) Affected Symptoms Hurler-, Scheie- Hurler- MPS I α-L-Iduronidase Dermatan sulfate, heparan Mental retardation, coarse facial Scheie syndrome sulfate features, hepatosplenomegaly, cloudy cornea Hunter syndrome MPS II Iduronate sulfatase Dermatan sulfate, heparan Mental retardation sulfate Sanfilippo syndrome A MPS IIIA Heparan sulfate-N-sulfataseb Heparan sulfate Delay in development, motor dysfunction Sanfilippo syndrome B MPS IIIB α-N-Acetylglucosaminidase Heparan sulfate As MPS IIIA Sanfilippo syndrome C MPS IIIC α-Glucosaminide Heparan sulfate As MPS IIIA N-acetyltransferase Sanfilippo syndrome D MPS IIID N-Acetylglucosamine Heparan sulfate As MPS IIIA 6-sulfatase Morquio syndrome A MPS IVA Galactosamine 6-sulfatase Keratan sulfate, chondroitin Skeletal dysplasia, short stature 6-sulfate Morquio syndrome B MPS IVB β-Galactosidase Keratan sulfate As MPS IVA Maroteaux-Lamy MPS VI N-Acetylgalactosamine Dermatan sulfate Curvature of the spine, short syndrome 4-sulfatasec stature, skeletal dysplasia, cardiac defects Sly syndrome MPS VII β-Glucuronidase Dermatan sulfate, heparan Skeletal dysplasia, short stature, sulfate, chondroitin 4-sulfate, hepatomegaly, cloudy cornea chondroitin 6-sulfate Natowicz syndrome MPS IX Hyaluronidase Hyaluronic acid Joint pain, short stature a The terms MPS V and MPS VIII are no longer used. b Also called sulfaminidase. c Also called arylsulfatase B. (GlcUAβ-1,3-GlcNAc-β-1,4)2, which can be degraded fur- these conditions. The amount of chondroitin sulfate in car- ↑ age : t cs ther by a β-glucuronidase and β-N-acetylhexosaminidase. A tilage diminishes with age, whereas the amounts of keratan genetic defect in hyaluronidase causes MPS IX, a lysosomal sulfate and hyaluronic acid increase. These changes may con- ↑ age : HA storage disorder in which hyaluronic acid accumulates in the tribute to the development of- osteoarthritis, as may increased & KS joints. activity of the enzyme aggrecanase, which acts to degrade aggrecan. Changes in the amounts of certain GAGs in the skin help to account for its characteristic alterations with aging. Proteoglycans Are Associated With In the past few years, it has become clear that in addition Major Diseases & With Aging to their structural role in the ECM, proteoglycans function as Hyaluronic acid may be important in permitting tumor cells signaling molecules which influence cell behavior, and they to migrate through the ECM. Tumor cells can induce fibro- are now believed to play a part in diverse diseases such as blasts to synthesize greatly increased amounts of this GAG, fibrosis, cardiovascular disease, and cancer. thereby facilitating their own spread. Some tumor cells have Major : - less heparan sulfate at their surfaces, and this may play a role BONE IS A MINERALIZED organic Type I : in the lack of adhesiveness that these cells display. The intima of the arterial wall contains- hyaluronic acid CONNECTIVE TISSUE inorganic cast/ : and -chondroitin sulfate, dermatan sulfate, and- heparan sulfate Bone contains both organic and inorganic material. The Hydroxy a - proteoglycans. Of these proteoglycans, dermatan sulfate binds organic matter is mainly protein. The principal proteins of patite plasma low-density lipoproteins. In addition, dermatan sulfate bone are listed in Table 50–9; - type I collagen is the major pro- appears to be the major GAG synthesized by arterial smooth tein, comprising 90 to 95% of the organic material. Type V muscle cells. Because these cells proliferate in atherosclerotic collagen is also present in small amounts, as are a number of lesions in arteries, dermatan sulfate may play an important noncollagen proteins, some of which are relatively specific to role in development of the atherosclerotic plaque. bone. These are now believed to play an active part of the min- In various types of arthritis, proteoglycans may act as eralization process. The inorganic or mineral component is autoantigens, thus contributing to the pathologic features of mainly crystalline- hydroxyapatite—Ca10(PO4)6(OH)2—along  CHAPTER 50 The Extracellular Matrix 605 * TABLE 50–9 The Principal Proteins Found in Bonea Proteins Comments Collagens Collagen type I Approximately 90% of total bone protein. Composed of two α1(I) and one α2(I) chains. Collagen type V Minor component. Noncollagen proteins Plasma proteins Mixture of various plasma proteins. Proteoglycansb CS-PG I (biglycan) Contains two GAG chains; found in other tissues. CS-PG II (decorin) Contains one GAG chain; found in other tissues. CS-PG III Bone-specific. Bone SPARCc protein (osteonectin) Not bone-specific. Osteocalcin (bone Gla protein) Contains γ-carboxyglutamate (Gla) residues that bind to hydroxyapatite. Bone-specific. Osteopontin Not bone-specific. Glycosylated and phosphorylated. Bone sialoprotein Bone-specific. Heavily glycosylated, and sulfated on tyrosine. Bone morphogenetic proteins (BMPs) A family (at least 20) of secreted proteins with a variety of actions on bone; many induce ectopic bone growth. Osteoprotegerin Inhibits osteoclastogenesis a Noncollagen proteins are involved in the regulation of the mineralization process. A number of other proteins are also present in bone, including a tyrosine-rich acidic matrix protein (TRAMP), some growth factors (eg, TGF-β), and enzymes involved in collagen synthesis (eg, lysyl oxidase). b CS-PG, chondroitin sulfate–proteoglycan; these are similar to the dermatan sulfate PGs (DS-PGs) of cartilage. c SPARC, secreted protein acidic and rich in cysteine. with sodium, magnesium, carbonate, and fluoride; approxi- Osteoblasts- control mineralization by regulating the pas- mately 99% of the body’s E calcium is contained in bone (see sage of calcium and phosphate ions across their surface Chapter 44). Hydroxyapatite confers on bone the strength and membranes. Alkaline phosphatase, an enzyme in the cell resilience required for its physiologic roles. membrane, generates phosphate ions from organic phos- Bone is a dynamic structure that undergoes continuing phates. The mechanisms involved in mineralization are not cycles of remodeling, consisting of resorption (demineral- fully understood, but a number of factors, including tissue ization) followed by deposition of new bone tissue (miner- nonspecific alkaline phosphatase (TNAP), an isoenzyme of alization). This remodeling permits bone to adapt to both alkaline phosphatase, matrix vesicles, which contain calcium physical (eg, increases in weight bearing) and hormonal and phosphate and bud off from the osteoblast membrane signals. and type I collagen have been implicated. Mineralization The major cell types involved in bone resorption and first becomes evident in the gaps between successive col- deposition are - osteoclasts and osteoblasts, & respectively lagen molecules. Acidic phosphoproteins, such as- bone (Figure 50–12).- Osteocytes are found in mature bone and are sialoprotein and - & osteopontin, are thought to act as sites of also involved in the maintenance of the bone matrix. They are nucleation. These proteins contain RGD sequences for cell descended from osteoblasts and are very long-lived, with an attachment and motifs (eg, poly-Asp and poly-Glu stretches) average half-life of 25 years. that bind calcium and may provide an initial scaffold for & Osteoclasts are multinucleated cells derived from plu- mineralization. Some macromolecules, such as certain pro- ripotent hematopoietic stem cells. Osteoclasts possess an teoglycans and glycoproteins, can also act as inhibitors of apical membrane domain, exhibiting a ruffled border that nucleation. plays a key role in bone resorption (Figure 50–13). A special Bone consists of two types of tissue, = trabecular (also proton-translocating ATPase expels protons across the ruf- called cancellous or spongy) bone is found at the end of - = fled border into the resorption area, which is the microenvi- long bones close to the joints and is less dense than &com- ronment of low pH shown in the figure. This lowers the local pact (or cortical) bone, which, as well as being denser, - pH to 4.0 or less, thus increasing the solubility of hydroxy- is harder and stronger. It forms the cortex (or outer apatite and helping its breakdown into⑳ Ca2+, H3PO4 and shell) of most bones and accounts for about& 80% of the & H2CO3, and water, thus allowing demineralization to occur. weight of the human skeleton. It is estimated that approxi- Lysosomal acid proteases such as = cathepsins are also released mately &4% of compact bone andO 20% of trabecular bone to digest the now accessible matrix proteins.- Osteoblasts— is renewed annually in the typical healthy adult. Many mononuclear cells derived from pluripotent mesenchymal factors are involved in the regulation of bone metabo- precursors—synthesize most of the proteins found in bone lism. Some of which stimulate osteoblast activity (eg, (Table 50–9) as well as various growth factors and cyto- - parathyroid hormone and - 1,25-dihydroxycholecalciferol - Vit DE kines. They are responsible for the deposition of the new [see Chapter 44]) to promote mineralization, while others bone matrix O (osteoid) and its subsequent mineralization. inhibit it (eg, corticosteroids). - Parathyroid hormone and 606 SECTION X Special Topics (B) Osteoclast Mesenchyme Newly formed matrix (osteoid) Osteoblast Osteocyte Bone matrix FIGURE 50–12 Schematic illustration of the major cells present in the membranous bone. Osteoblasts (lighter color) are synthesizing type I collagen, which forms a matrix that traps cells. As this occurs, osteoblasts gradually differentiate to become osteocytes. (Reproduced, with permission, from Junqueira LC, Carneiro J: Basic Histology: Text & Atlas, 10th ed. McGraw-Hill, 2003.) Blood capillary Nucleus Osteoclast Golgi Nucleus Lysosomes CO2 + H2CO3 H2CO3 H+ + HCO3– Section of circumferential clear zone Ruffled border Microenvironment of low pH Bone matrix and lysosomal enzymes FIGURE 50–13 Schematic illustration of the role of the osteoclast in bone resorption. Lysosomal enzymes and hydrogen ions are released into the confined microenvironment created by the attachment between the bone matrix and the peripheral clear zone of the osteo- clast. The acidification of this confined space facilitates the dissolution of calcium phosphate from bone and is the optimal pH for the activity of lysosomal hydrolases. The bone matrix is thus removed, and the products of bone resorption are taken up into the cytoplasm of the osteoclast, probably digested further, and transferred into capillaries. The chemical equation shown refers to the action of carbonic anhydrase II, described in the text. (Reproduced, with permission, from Junqueira LC, Carneiro J: Basic Histology: Text & Atlas, 10th ed. McGraw-Hill, 2003.)  CHAPTER 50 The Extracellular Matrix 607 ② 1,25-dihydroxycholecalciferol also stimulate bone resorb- * Osteopetrosis (marble bone disease), characterized by tion by increasing osteoclast activity, whereas - calcitonin increased bone density, is a rare condition characterized and -estrogens have the opposite effect. by inability to resorb bone. It is due to mutations in the gene (located on chromosome& 8q22) encoding carbonic anhydrase II BONE IS AFFECTED BY MANY (CA II), one of four isozymes of carbonic anhydrase present in human tissues. Deficiency of CA II in osteoclasts prevents nor- METABOLIC & GENETIC mal bone resorption, and osteopetrosis results. DISORDERS & Osteoporosis is a generalized progressive reduction A number of metabolic and genetic disorders affect bone, in bone tissue mass per unit volume, caused by an imbal- and some of the more important examples are listed in ance between bone resorption and deposition, and leads to Table 50–10. skeletal weakness. The primary type 1 condition commonly - Osteogenesis imperfecta (brittle bones) is character- occurs in women after the menopause. This is thought to ized by abnormal fragility of bones. The sclera of the eye be mainly due to lack of estrogen, which promotes bone is often abnormally thin and translucent and may appear resorption and decreases bone mineralization. Primary blue owing to a deficiency of connective tissue. Eight types & type 2 or senile osteoporosis occurs in both sexes post (I-VIII) of this condition have been recognized. Types I 75 years, although is more prevalent in women (ratio g 2:1 to IV are caused by mutations in the COL1A1 or COL1A2 female:male). The ratio of mineral to organic elements is genes or both. Type I is mild, but type II is severe and infants unchanged in the remaining normal bone. Fractures of var- born with the condition usually do not survive, and types III ious bones, such as the head of the femur, occur very easily and IV are progressive and/or deforming. Over 100 muta- and represent a huge burden to both the affected patients tions in these two genes have been documented and include and to society in general. partial gene deletions and duplications. Other mutations affect RNA splicing, and the most frequent type results in replacement of glycine by another bulkier amino acid, the - THE MAJOR COMPONENTS OF CARTILAGE ARE TYPE II Hyaline cartilage - affecting formation of the triple helix. In general, these muta- tions result in decreased expression of collagen or in struc- COLLAGEN & CERTAIN Type I turally abnormal pro chains that assemble into abnormal Elastic Cartilage PROTEOGLYCANS - fibrils, weakening the overall structure of bone. When one el a stin abnormal chain is present, it may interact with two normal There are three types of cartilage. The major type isc hyaline Fibroe- chains, but folding may be prevented, resulting in enzymatic (articular) cartilage and its principal proteins are listed in Id Stic - - Table 50–11. - degradation of all of the chains. This is called “procollagen Type II collagen is the major protein compo- cartila- ge suicide” and is an example of a dominant negative muta- & nent (Figure 50–14), and a number of other minor types of Type - tion, a result often seen when a protein consists of multiple collagen are also present. In addition to these components, the I different subunits. Types V to VIII are less common and are second type, - elastic cartilage, contains& elastin, and the third, caused by mutations in the genes for proteins involved in fibroelastic cartilage, containsS - type I collagen. Cartilage con- bone mineralization other than collagen. tains a number of proteoglycans, which play an important TABLE 50–10 Some Metabolic and Genetic Diseases Affecting Bone and Cartilage Condition Causes Condition Causes Dwarfism Often deficiency of growth Osteoporosis Age-related, estrogen deficiency following hormone, but many other menopause, mutations in genes affecting bone causes metabolism,a including the vitamin D receptor (VDR), estrogen receptor-α (ER-α), and COL1A1 Rickets Deficiency of vitamin D in Osteoarthritis Age-related cartilage degeneration, mutations in childhood various genesa including VDR, ER-α, and COL2A1 * Osteomalacia Deficiency of vitamin D in adults Chondrodysplasias Mutations in COL2A1 Hyperparathyroidism Excess parathyroid hormone Pfeiffer, Jackson-Weiss, and Mutations in the gene for fibroblast growth causing bone resorption Crouzon syndromesb factor receptor (FGFR) 1 and/or 2 Osteogenesis imperfecta Mutations in COL1A1 and Achondroplasia and Mutation in the gene for FGFR3 COL1A2 affecting the synthesis thanatophoric dysplasiac and structure of collagen Only a small number of cases. a In the Pfeiffer, Jackson-Weiss, and Crouzon syndromes, there is premature fusion of some bones in the skull (craniosynostosis). b Thanatophoric dysplasia in the most common neonatal lethal skeletal dysplasia. c  608 SECTION X Special Topics (B) TABLE 50–11 The Principal Proteins Found in Cartilage sulfate chains are located in domain C; both of these types of GAGs are bound covalently to the core protein. The core Proteins Comments protein also contains both O- and N-linked oligosaccharide Collagen proteins chains. Collagen type II 90–98% of total hyaline cartilage The other proteoglycans found in cartilage have simpler collagen. Composed of three α1(II) chains. structures than aggrecan. Collagens V, VI, IX, X, XI Type IX cross-links to type II collagen. - Chondronectin is involved in the attachment of type II Type XI may help control diameter of collagen to chondrocytes (the cells in cartilage). type II fibrils. Cartilage is an avascular tissue and obtains most of its Noncollagen proteins nutrients from synovial fluid. It exhibits slow but continuous Cartilage oligomeric An important structural component turnover. Various proteases (eg, collagenases and stromely- matrix protein (COMP) of cartilage. Regulates cell movement sin) synthesized by chondrocytes can degrade collagen and and attachment. the other proteins found in cartilage. Interleukin-1 (IL-1) and Aggrecan The major proteoglycan of cartilage. tumor necrosis factor α (TNF-α) appear to stimulate the pro- DS-PG I (biglycan)a Similar to CS-PG I of bone. DS-PG II (decorin) Similar to CS-PG II of bone. duction of such proteases, whereas TGF-β and insulin-like Chondronectin Promotes chondrocyte attachment to growth factor 1 (IGF-I) generally exert an anabolic influence type II collagen on the cartilage. a The core proteins of the proteoglycans DS-PG I and DS-PG II are homologous to those of CS-PG I and CS-PG II found in bone. A possible explanation is that osteo- CHONDRODYSPLASIAS ARE blasts lack the epimerase required to convert glucuronic acid to iduronic acid, the latter is found in dermatan sulfate. CAUSED BY MUTATIONS IN GENES ENCODING TYPE II COLLAGEN & role in its compressibility.-Aggrecan (about 2 × 103 kDa) is FIBROBLAST GROWTH FACTOR the major proteoglycan. As shown in Figure 50–15, it has a RECEPTORS- Type I collagen very complex structure, containing several GAGs (hyaluronic > - Chondrodysplasias are a mixed group of hereditary disor- acid, chondroitin sulfate, and keratan sulfate) and both link ders affecting cartilage. They are manifested by short-limbed and core proteins. The core protein contains three domains: A, dwarfism and numerous skeletal deformities. A few are due to B, and C. Hyaluronic acid binds noncovalently to domain A of a variety of mutations in the COL2A1 gene, leading to abnor- the core protein as well as to the link protein, which stabilizes Stickler syn- mal forms of type II collagen. One example is the - the hyaluronate–core protein interactions. The keratan sul- drome, manifested by degeneration of the joint cartilage and & fate chains are located in domain B, whereas the chondroitin of the vitreous body of the eye. Domain : Hyaluronic acid A + HA B + KS C + CS Type II collagen fibril Hyaluronic acid Link protein Chondroitin sulfate Proteoglycan Core protein Collagen (type II) FIGURE 50–14 Schematic representation of the molecular organization in the cartilage matrix. Link proteins noncovalently bind the core protein (red) of proteoglycans to the linear hyaluronic acid molecules (gray). The chondroitin sulfate side chains of the proteoglycan electrostatically bind to the collagen fibrils, forming a cross-linked matrix. The oval outlines the area enlarged in the lower part of the figure. (Reproduced, with permission, from Junqueira LC, Carneiro J: Basic Histology: Text & Atlas, 10th ed. McGraw-Hill, 2003.)  CHAPTER 50 The Extracellular Matrix 609 Domain A Domain B Domain C Hyaluronate- binding region Core N-linked protein oligosaccharide Link protein Keratan Chondroitin O-linked Hyaluronic acid sulfate sulfate oligosaccharide FIGURE 50–15 Schematic diagram of aggrecan. A strand of hyaluronic acid is shown on the left. The core protein (about 210 kDa) has three major domains. Domain A, at its amino-terminal end, interacts with approximately five repeating disaccharides in hyaluronate. The link protein interacts with both hyaluronate and domain A, stabilizing their interactions. Approximately 30 keratan sulfate chains are attached, via GalNAc-Ser linkages, to domain B. Domain C contains about 100 chondroitin sulfate chains attached via Gal-Gal-Xyl-Ser linkages and about 40 O-linked oligosaccharide chains. One or more N-linked glycan chains are also found near the carboxyl terminal of the core protein. (Moran LA, et al: Biochemistry, 2nd ed., © 1994, pp. 9–43. Adapted by permission of Pearson Education, Inc., Upper Saddle River, NJ.) achondro- The best known of the chondrodysplasias is - plasia, the most common cause of short-limbed dwarfism. SUMMARY & The major components of the ECM are the structural Affected individuals have - short limbs, normal trunk size, proteins collagen, elastin, and fibrillin-1, a number of macrocephaly, and a variety of other skeletal abnormalities. - specialized proteins (eg, fibronectin and laminin), and various The condition is often inherited as an autosomal dominant proteoglycans. trait, but many cases are due to new mutations. Achondro- Collagen is the most abundant protein in the animal kingdom; plasia is not a collagen disorder but is due to mutations approximately 28 types have been isolated. All collagens in the gene encoding fibroblast growth factor receptor 3 contain greater or lesser stretches of triple helix and the (FGFR3). Fibroblast growth factors are a family of more - repeating structure (Gly-X-Y)n. than 20 proteins that affect the growth and differentiation The biosynthesis of collagen is complex, featuring many of cells of mesenchymal and neuroectodermal origin. Their posttranslational events, including hydroxylation of proline receptors are transmembrane proteins and form a subgroup and lysine. of four in the family of receptor tyrosine kinases. FGFR3 is Diseases associated with impaired synthesis of collagen include one member of this subgroup and mediates the actions of scurvy, osteogenesis imperfecta, Ehlers-Danlos syndrome A chondropl FGF3 on cartilage. In almost all cases of achondroplasia that (six subtypes), and Menkes disease. a sid have been investigated, the mutations were found to involve Elastin confers extensibility and elastic recoil on tissues. > - & substitutio nucleotide 1138 and resulted in substitution of arginine for Elastin lacks hydroxylysine, Gly-X-Y sequences, triple on of argi- glycine (residue number 380) in the transmembrane domain helical structure, and sugars, but contains desmosine and nine to gly- of the protein, rendering it inactive. No such mutation was isodesmosine cross-links not found in collagen. cine found in unaffected individuals. Fibrillin-1 is located in microfibrils. Mutations in the gene Other mutations in the same gene can result in hypo- encoding fibrillin-1 cause Marfan syndrome. The cytokine chondroplasia, thanatophoric dysplasia (types I and II) TGF-β appears to contribute to the cardiovascular pathology. (other forms of short-limbed dwarfism), and the SADDAN The GAGs are made up of repeating disaccharides containing a phenotype (severe achondroplasia with developmental delay uronic acid (glucuronic or iduronic) or hexose (galactose) and and acanthosis nigricans [the latter is a brown to black hyper- a hexosamine (galactosamine or glucosamine). Sulfate is also pigmentation of the skin]). frequently present. As indicated in Table 50–10, other skeletal dysplasias The major GAGs are hyaluronic acid, chondroitin 4- and (including certain craniosynostosis syndromes) are also due 6-sulfates, keratan sulfates I and II, heparin, heparan sulfate, to mutations in genes encoding FGF receptors. Another type and dermatan sulfate. of skeletal dysplasia, - > diastrophic dysplasia has been found to The GAGs are synthesized by the sequential actions of specific be due to mutation in a sulfate transporter. enzymes (glycosyltransferases, epimerases, sulfotransferases, 610 SECTION X Special Topics (B) etc) and are degraded by the sequential action of lysosomal hydrolases. Genetic deficiencies of the latter result in REFERENCES mucopolysaccharidoses (eg, the Hurler syndrome). Kadler KE, Baldock C, Bella J, Boot-Handford RP: Collagens at a glance. J Cell Sci 2007;120:1955. GAGs occur in tissues bound to various proteins (linker Kasper DL, Fauci AS, Hauser SL et al: Harrison’s Principles of proteins and core proteins), constituting proteoglycans. These Internal Medicine, 19th ed. McGrawHill Education, 2017. structures are often of very high molecular weight and serve Scriver CR, Beaudet AL, Valle D, et al (editors): The Metabolic and many functions in tissues. Molecular Bases of Inherited Disease, 8th e