REVIEW ARTICLE COLLAGEN METABOLISM COLLAGEN METABOLISM Types of Collagen 228 Structure of Collagen Molecules 230 Synthesis and Processing of Procollagen Polypeptides 232 Transcription and Translation 233 Posttranslational Modifications 233 Extracellular Processing of Procollagen and Collagen Fibrillogenesis 240 Functions of Collagen in Connective rissue 243 Collagen Degradation 245 Regulation of the Metabolism of Collagen 246 Heritable Diseases of Collagen 247 Recessive Dermatosparaxis 248 Recessive Forms of EDS 251 EDS VI 251 EDS VII 252 EDS V 252 Lysyl Oxidase Deficiency in the Mouse 253 X-Linked Cutis Laxa 253 Menke's Kinky Hair Syndrome 253 Homocystinuria 254 EDS IV 254 Dominant Forms of EDS 254 Dominant Collagen Packing Defect I 255 Dominant and Recessive Forms of Osteogenesis Imperfecta 258 Dominant and Recessive Forms of Cutis Laxa 258 The Marfan Syndrome 259 Acquired Diseases and Repair Processes Affecting Collagen 259 Acquired Changes in the Types of Collagen Synthesis 260 Acquired Changes in Amounts of Collagen Synthesized 263 Acquired Changes in Hydroxylation of Proline and Lysine 264 Acquired Changes in Collagen Cross-Links 265 Acquired Defects in Collagen Degradation 267 Conclusion 267 Bibliography 267 Collagen Metabolism A Comparison of Diseases of Collagen and Diseases Affecting Collagen Ronald R. Minor, VMD, PhD COLLAGEN CONSTITUTES approximately one third of the body's total protein, and changes in synthesis and/or degradation of colla- gen occur in nearly every disease process. There are also a number of newly described specific diseases of collagen in both man and domestic animals. Thus, an understanding of the synthesis, deposition, and turnover of collagen is important for the pathologist, the clinician, and the basic scientist alike. Fortunately, during the last 10 years much progress has oc- curred in our understanding of the metabolism of collagen. Much of this progress is discussed in a number of recent books and reviews.3,5-27 A sig- nificant part of this progress has been due to studies of the specific defects in the metabolism of collagen in disease of both man and domestic ani- mals.18"19'22-24'26'27 In fact, it appears likely that it was the discovery of the disease dermatosparaxis in cattle in Texas"3 and BeigiUM12L11263L that quickly led to the discovery of procollagen, the precursor of collagen. Vet- erinarians, therefore, played a key role in the initial steps leading to the recent major advances in our knowledge of the metabolism of collagen. By virtue of our constant exposure to the full range of diseases affecting a wide range of animals, veterinary pathologists are in an excellent position to identify and characterize the primary metabolic diseases of collagen, as well as the many diseases in which secondary changes in the metabolism of collagen are important. Consequently, in this review we will briefly dis- cuss the current state of knowledge of the structure, synthesis, secretion, deposition, and turnover of collagen. This discussion will serve as a basis for a review of both the primary diseases of collagen discovered to date and the more prevalent disease processes in which changes in the metabo- lism of collagen are a prominent feature. From the Department of Pathology, New York State College of Veterinary Medicine, Cornell University, Ithaca, New York. Supported in part by USPHS Research Grants HL-20635 and AM-20793. Prepared for the 29th Annual Meeting of the American College of Veterinary Pathologists, San Antonio, Texas. Accepted for publication August 1, 1979. Address reprint requests to Dr. Ronald R. Minor, New York State College of Veterinary Medicine, Ithaca, NY 14852. 0002-9440/80/0109-225$01 .00 227 (© American Association of Pathologists 228 MINOR American Journal of Pathology Types of Collagen All collagen molecules are long, stiff rods consisting of a triple helix of three polypeptide chains called a-chains. There are, however, at least seven and possibly ten genetically different types of collagen molecules in the body (Table 1). These different molecules may contain either one type of a-chain or two genetically different a-chains (Table 1). The relative proportions of each of the different types of collagen are tis- sue-specific (Table 2).15,12,16,58,61-65,68,69,71-73,75,76,81-83,85,86,87 99 For example, in the dermis Type I collagen predominates, but small to moderate amounts of three other types of collagen are also present in the dermis. Type III collagen is present in reticulin fibers and in the walls of muscular arteries, while small amounts of Type V collagen are associated with smooth muscle cells in skin. In addition, Type IV procollagen is present in both vascular and epidermal basement membranes in the skin. The proportions of the different types of collagen change with time in the same tissue. For example, Type III collagen constitutes more than 60% of the collagen in fetal skin but makes up less than 20% of the colla- gen in adult skin.83 This has caused Type III collagen to be called "fetal collagen," but this term is misleading, for Type III collagen constitutes a significant proportion of the collagen in the wall of the gastrointes- tinal tract, muscular arteries, lung, and uterus of the adult ani- mal. 12,16,29,39,63,80,84,87 Even though each tissue may contain from two to four different types of collagen, there is no evidence that different types of collagen molecules polymerize in the same fibril. For example, Type I collagen forms the thick banded collagen fibrils that predominate in skin, fascia, ligaments, tendon, and bone. The fibrils of Type I collagen range from 50 to 300 nm in diameter, and they have the 68-nm banded staining pattern that is characteristic of collagen.'086'6" Each collagen fibril is surrounded by a thin layer of ruthenium-red-positive noncollagenous glycoprotein or pro- teoglycan.1'8 These fibrils are usually organized into fibers whose dimen- sions range from <1 ,A to >50 ,u in diameter, and each of these fibers is also surrounded by a thin layer of ruthenium-red-positive noncollagenous ma- terial.'"6 Therefore, the boundary of both fibrils and fibers of collagen is demarcated by this ruthenium-red-positive noncollagenous material. Type II collagen, in contrast, tends to form thin, unbanded fibrils that form a meshwork with aggregates of proteoglycans in cartilage,69 and Type III collagen is associated with argyrophilic glycoproteins in reticulin fibers.'0 To date, it is not known whether Type AB collagen makes up the thin fibers in the endomysium and perimysium or is a component of muscle basement membranes. It is generally assumed that Type AB colla- Vol. 98, No. 1 COLLAGEN METABOLISM 229 January 1980 cv) co co .04) c- V- r-- .0 -0) cls L)0 CY)co CD 0 4) .L0 -N 4) -0) (DU 1- (0) E co c 4) co C-D -'-0()v CC - v- .CQ - - -C,, 0 o N C6CD co ll- -0) I cNi c& 4) 0oCEo o0)0X 1o Z 0 .C )4 .5 a.2 Z 0 4). c 0~~~ 4) Cl) co cc 4)4)0 CE 4) ccE V2C> Co ° 0)4 4.cn co i co 0 c 0. C 04 4) . ~ ~ c00 00 > 0 04)4)o .0Co o4) .0 FLU E ~0 c E , >-co E Co ct$r < COvo-<0. ~E=ouF ~E o - xJ) cn - <1 Co 0C 4) C E 4) 4_ 4)a C0C ) m > > Q .0 oCo C > 7O 04) ) 4 4) 4) a 0)2'3: 4) 4) 0Co 0. 0. 0. 0. x 0- 0. >o C 4)4 .4) a- , o 7Q o N- 0 .ocl E O = .2 cor %00 0 0 O o 0 a00 l) - to *) =o* 4) .0 0). r -9 *E b N C,) - E 4) O 4)o o csN CO0 0'9 c b.0 CD* C m = 00 _ 0 E 'E °m CL ' iz 6- 6-7t > CO 4)0c Q.Co 0 a 6- 0 ~co E - . co E 0 c 4) C0) Co0) 0) 00 0 0 0 0 0 0 0 0000 0 ID C C E E a1.. L. 00 0 0 0 0 0 0 0 0000 0 0 _: 00 0 0 0 0 0 0 0000 0 <4) 't ._ 600 LU) i 6 U, 0 & U)L o U)L o 'a a 0)0) 0) 0) 0) 0) cD - 0 0) U) co 00 a 0 Co E E _- " E a C a c CO 0 co 0o 0) E 0C a) 0 4) .- C')>co_ 0O E m .- 0 .-L. EJ4 4) 0) F C o I- > i..0~~'- E 0~ 4 -, Cl = _. - CY cts o a.. 0 C b ts -* CD 0 m.0 o Zs ?s m x 230 MINOR American Journal of Pathology Table 2-Compositional Characteristics of Different Types of Collagen Partial amino acid composition of different a-chains (residues/1000 residues) a,(l)* a2(l)* a,(ll)t al(1II)4 aA§ aB§ a1(IV) || 80K¶ Amino acid Hyl 4 8 23 5 22 39 57 36 Lys 30 22 13 30 12 13 10 6 Arg 41 51 51 46 48 40 27 42 3-Hyp 1 0 1 0 7 10 12 1 4-Hyp 96 86 100 121 113 105 120 110 Pro 129 113 122 102 98 120 67 73 Gly 330 336 332 355 346 334 330 330 Ala 112 102 104 92 52 46 32 47 Glycosylated hydroxylysine Gal-Hyl ot 1 t 4t 1 t 3# 5# 3 2 GIc-Gal-Hyl 1 1 5 1 5 29 48 29 * Data from Piez et al.76 t Data from Miller.