Thorax: first published as 10.1136/thx.41.6.418 on 1 June 1986. Downloaded from Thorax 1986;41:418-428 Review article Lung collagen: more than scaffolding Collagens of various types are the major group of be given to its role in two disorders: pulmonary proteins in the lung. They are present in all major fibrosis and emphysema. structures, including airways, blood vessels, the inter- stitium of the lung parenchyma, and the basement Collagen structure and metabolism membranes of epithelial and endothelial cells. Any alterations in quantity, structure, or the geometry of This section offers a brief summary of what is cur- their distribution would be likely to alter lung func- rently known about the structure and metabolism of tion. Changes would have particularly dramatic collagen. Readers are referred elsewhere for more de- effects if they occurred in the interstitium, where the tailed reviews ofcollagen biochemistry.2 An appre- distance between air and blood may be as little as ciation of the pathways described here may be useful 50 nm.1 for an understanding of what follows, but some read- A collagen molecule typically has a rod like ers may wish to pass directly to the next section on structure, 300 nm in length and 3 nm in diameter, lung collagen. forming tightly packed fibrils with diameters up to Our perception of collagen has changed dra- about 100 nm. Any large deposits of collagen or col- matically over the last 10 years. One of the major lagen deposition with incorrect orientation within the breakthroughs has been the discovery of collagen interstitium may severely impede gas exchange; while isotypes and the recognition that there is a family of loss of collagen from the interstitium would lead to collagens-each having distinctive structural and disruption of the normal alveolar network, and loss metabolic characteristics, and in some cases different from basement membranes would compromise their functions. There are thought to be at least 11 different http://thorax.bmj.com/ role controlling entry ofmaterials to and from cells in collagens, coded by a group of about 20 genes6 (ta- the lung. Not surprisingly perhaps against this back- ble). Each of these so called collagen types comprises ground, lung diseases occur in which imbalances in three polypeptide chains (a chains), which intertwine collagen structure or metabolism appear to play a in a right handed triple helix. The individual chains part. This editorial review describes some of the de- typically contain about 1000 amino acids, and a char- velopments in our understanding oflung collagen and acteristic of all of them is a high proportion of gly- its role in pulmonary disease. Particular attention will cine, proline, and hydroxyproline residues. A large of the on September 24, 2021 by guest. Protected copyright. Address for reprint requests: Dr Geoffrey J Laurent, Lung Biochem- proportion polypeptide chain contains repeat istry Unit, Department of Thoracic Medicine, Cardiothoracic Insti- units of the structure Gly-X-Y, where X and Y are tute, London SW3 6HP. either proline, alanine, or hydroxyproline. It is likely Collagen types and their likely distribution in the lung Type Likely distribution in the lung Molecular structure Likely cells responsiblefor bulk ofreproduction Large bronchi, blood vessels, interstitium Fibroblasts I (trimer) Presence in lung uncertain [ai(I)] Fibroblasts II Bronchial and tracheal cartilage [ax(II)1 Chondrocytes III Large bronchi, blood vessels, interstitium [a;(III)b3 Fibroblasts IV Basement membranes Endothelial and epithelial cells V Basement membranes and interstitium [O(IV)12a (IV) Fibroblasts Structure uncertain, may be comprised of 3 distinct molecules VI Blood vessels and interstitium codistributing aO(VI)a2(VI)a3(VI) Fibroblasts with types I and III collagen VII Uncertain [al(VII)]3 Uncertain VIII Uncertain Uncertain Endothelial cells Ix Cartilage a1(IX)a2(IX)a3(IX) Chondrocytes x Cartilage [a21(X)13 Chondrocytes xI Cartilage Uncertain Chondrocytes 418 Thorax: first published as 10.1136/thx.41.6.418 on 1 June 1986. Downloaded from Lung collagen: more than scaffolding 419 Chromatin RNA polymerase NUCLEUS Transcription (1) (2) CYTOPLASM I. .. ................. ... polypeptide with "signal peptide' Translation ,1% RER lumen g_ _ribosomes (movingakx-igmRNA) Rough endoplasmic reticulum (RER)L mRNA pro collagen NH2 COOH a chain (3) 4 OH OH I hydroxylation (4) glycosylation (5) Golgi apparatus disulphide bond formation (6) H ;~O I OH OH procollagen molecule OH 4 OH http://thorax.bmj.com/ ,F ' OH StZ removal of N and Fibril £S> C terminal propeptides assembly site OH 3w ~~~~~~~~~(3) EXTRACELLAR SPACE )lecular nk on September 24, 2021 by guest. Protected copyright. molecular slink (7) Collagen fibril extracellular degradation (8) Fig I Likely steps in the pathway oflung collagen synthesis and degradation. that all the collagens that are currently well described major developments in our understanding of the' will be found in the lung, either in the parenchyma or pathways of collagen metabolism and the extensive in the higher airways. They are listed in the table and processing that occurs at intracellular and extra- discussed in more detail later. cellular sites. These include the following steps, many In addition to these advances in the appreciation of of which are illustrated in figure 1, with numbered the diversity of collagen structure, there have been cross references to this text. Thorax: first published as 10.1136/thx.41.6.418 on 1 June 1986. Downloaded from 420 Laurent 1 Extensive processing ofthe messenger RNAfor 5 Addition ofcarbohydrate moieties to the collagen polypeptide chains The gene for the a2(I) chain of type I collagen con- Carbohydrates, mostly glucose and galactose resi- tains many more bases than are required to code for dues, are covalantly bonded to hydroxylysine in reac- the polypeptide. The coding sequences (exons) are in- tions requiring the enzymes galactosyl transferase and terspersed throughout the gene, separated by about glucosyl transferase. The extent of glycosylation var- 50 intervening sequences (introns). During proces- ies greatly for the different collagens; it is particularly sing within the nucleus these sequences must high in type IV collagen, where these carbohydrate be specifically excised and the strands, subsequently moieties represent as much as 10% by weight of the ligated before transcription to the mRNA in the molecules. endoplasmic reticulum (fig 1). 6 Disulphide bonding between a chains For all collagens except type I disulphide bonds are 2 Synthesis with "signal" sequence formed between cysteine residues of adjacent chains, The initial transcript (pre-procollagen) has a short se- as well as interchain links between adjacent mole- quence of amino acids at one end of the molecule. cules. This sequence of predominantly hydrophobic amino acids functions to direct the molecule into the Golgi 7 Formation ofcovalant cross linked compounds apparatus, from which it is destined to be secreted between amino acids side chains from the cell. After the modifications described above, most of which take place in the Golgi apparatus, procollagen 3 Collagen synthesised in a precursorform molecules are secreted from cells via the secretory the poly- vacuoles (see fig 1). After removal of the propeptides, After removal of the "signal" sequence, collagens are assembled into fibrils in recesses of the peptide is still considerably longer than the chain that fibroblast plasma membrane. These fibrils are not sta- eventually forms fibrils in the extracellular matrix. chains There are peptide extensions at both ends of the mol- bilised until covalent cross links form between to as peptides), which do of adjacent molecules. The first step in this process is ecule (referred procollagen http://thorax.bmj.com/ not contain long repeat structures of the Gly-X-Y the modification of the lysine and hydroxylysine form and, apart from one short section, do not form residues in reactions requiring the copper dependent a triple helix. These extensions may have several func- enzyme lysyl oxidase. Cross linking then occurs tions: promoting formation of the triple helix, pre- via reactions between these products and unmodified venting premature fibril formation, and influencing lysine or hydroxylysine chains. have extracellular fibril formation. Possibly they also 8 Degradation ofprocollagen and collagen a role in the control of collagen production, with the Another recent development, discussed in detail later, N terminal propeptide exerting a negative feedback and is the finding that collagen turnover (synthesis on September 24, 2021 by guest. Protected copyright. on procollagen synthesis. The peptides are removed degradation) is more rapid in normal tissues than was by specific amino and carboxyproteases, which are originally believed. For this reason pathways of de- thought to act either at the membrane just before gradation, which are still relatively poorly under- secretion or some time after secretion from the cell, stood, are potentially important sites for regulation of depending on the collagen type. collagen homeostasis. The pathways of collagen de- gradation are complex and there are various sites, 4 Hydroxylation ofproline and lysine both intracellular and extracellular, where break- About one half of the proline residues in the pro- down may occur. Once it leaves the cell, the molecule collagen chain are hydroxylated after production of ages chemically with the formation of cross links that the molecule.