Basement membranes: molecular organization and function in development and disease

L

Basement membranes: molecular or&h&tion function in development and disease J.C. Schittny Department

of Pathology,

Robert

Wood

Current

and P.D. Yurchenco Johnson

Opinion

Medical

in Cell Biology

Introduction Basement membranes are deposited by endothelial, epithelial and some mesenchymal cells and are produced as the first extracellular matrix during early embryonic development. Typically these matrices are sheet-like structures which can be resolved into electron-dense (lamina densa) and electron-lucent (lamina lucida) layers after heavy metal staining. They provide mechanical support for resident cells, serve as semipermeable barriers between tissue compartments, and act as regulators of cellular attachment, migration and differentiation. We can distinguish two levels of structure which affect function. First, there are interacting sites located in various basement membrane components: these determinants bind neighboring matrix macromolecules to produce a supramolecular architecture and transmit signals to cells by binding to their surfaces. Second, the architectures generated by these inter-actions provide an additional order of structure and biological information. Sieving and mechanical support are clearly dependent on architecture. While many of the observed cell-matrix interactions are dependent on primary basement membrane determinants, these interactions can also be mod&d by architecture. For example, in comeal epithelial basement membrane, the interior cell binding region (Pl) of laminin is located at the interface of the lamina lucida and densa and is probably not accessible to this epithelial cell surface [ 11. The long arm globule (which contains a second cell-binding site in its terminal globule), on the other hand, favors three major orientations and can interact with both matrix and cells [l].

Structure of basement membrane and their molecular organization

ah

components

School,

New Jersey, USA

1989, 1:983988

Collagen IV and laminin are capable of self-assembly and may form a double polymer in many basement membranes. Components such as fibronectin and SPARC(‘secreted protein, acidic, rich in cysteine’) are also found in basement membranes, but are not unique to these matrices, and may not be required for underlying architecture. Type VII is an example of a component which serves to bridge basement membranes to adjacent cells and stroma (Keene et al, J Cell Bioll987, 104:611621). Type IV collagen

Collagen IV molecules (540 kD) are -400 nm long [al(N)]z[a2(lV)] heterotrimers [2,3] organized into an N-terminal triple-helical-rich 30~1 long rod (7S), a major triple-helical rod which contains over 20 irregularly spaced ‘non-collagenous’ interruptions, and a Cterminal globular domain (NCl, Fig. 1) [2,3] (Wood, FEBS Lett 1988, 2275-g). A number of the interruptions contribute to the flexibility of the collagen IV molecule (Hofmann, J Mol Bioll984, 172:325-343) and may play a role in self-assembly and provide binding sites for other components. Recently the structure of insect collagen IV has also been elucidated (Lunstrum, J Biol Ckm 1988, 263:183X&18327; Blumberg, J Biol C7xm-z1988, 26318328-18337). Insect and mammalian collagen lV share similar N- and C-terminal regions and 11 of the interruptions, suggesting their structural importance in many species. Mammalian collagen IV can self-assemble into a network consisting of Nterminal tetramers (7S), C-terminal dimers (NCl), and lateral associations between the collagen triple helices [3]. N-terminal tetmmer (75) formation, while initially formed through non-covalent interactions (Yurchenco, Biochemistry 1984, 23:1839-18501, becomes stabilized by disulfide and Iysyl ox&se-derived crosslinks in a specitic geometry (Siebold et al, Eur J Bicxzbem 1987, 168:56!&575).

Several classes of structural and cell-interactive components can be identified in basement membranes. Collagen IV, laminin, entactin (nidogen) and heparan sulfate proteoglycans are unique to basement membranes and appear to be essential in forming underlying architecture.

Piscataway,

C-terminal

globular

dimerization

is accom-

plished through a reshuflling of disulfide bonds [4,5]. Lateral associations are formed reversibly with one chain staggered with respect to the other (Yurchenco, 1984). The completed network (Fig. 2), with its tie branching iilaments and globular domains, has been visualized

Abbreviations bFCF-basic

fibroblast

growth

factor; EHS-Engelbreth-Holm-Swarm; NCAM-neural cell adhesion

@ Current

Science

CBM-glomerular

basic

membrane;

molecule.

Ltd ISSN 0955+674

983

984

Extracellular

matrix

in situ in the basement membranes of the amnion and Engelbreth-Holm-Swarm (EHS) tumor [6] (Yurchenco, J cell Bioll987, 105:255!%2568). . laminin

1988,21:286-297). Using a combination of specific crosslinking and gel-filtration, Pa&son et al 193 have shown that laminin fragment El’ forms small oligomers in the presence of calcium whereas smaller fragments such as El (which lacks globular domains) have lost this ability. Recent studies indicate that domains E4 and El’ (see Fig. 3), are critical for laminin self assembly (Schittny and Yurchenco, unpublished data). These data also indicate that the self-assembly domains of the short arms are most likely the terminal globules. Entactin (nidogen) is a 150kD dumbbell-shaped tyrosine-sulfated glycoprotein about 2Onm in length. It has recently been sequenced by two groups [ 10,111. The barlike domain between the globules possesses five cysteinerich repeats similar to those found in laminin. An RGD sequence is located in one of the repeats and may promote cell attachment. The C-terminal globular domain of entactin (nidogen) strongly binds laminin in the vicinity of the cross (Paulsson et al, Eur J Biocbem 1987, 166: I 1-16) and also interacts with collagen lV [ 11 I. The spatial relationships between laminin and collagen assemblies may be maintained through an entactin (nidogen) bridge [2] (Mann et al, EM730 J 1989, 8:65) although direct but low-affinity laminin-collagen IV interactions (Charonis et al, J Cell Biof 1985,100~18481853) may also contribute. Iaminin also binds heparin, and to a lesser extent, heparan sulfates. The major site of heparin attachment in laminin is located in the distal moiety of the long arm globule (fragment E3), probably in a basic amino-acid rich region of subdomain G5 [7]. Recently it has been proposed that other heparin sites are present in proximal regions of a short arm of laminin (Skubitz et al, J Biol m 1988, 263:4861-4868) and a site in the internal globule of the Bl chain short arm (Charonis et al, J Cell BioIl988, 107:12531260). Our data (Yurchenco et al, unpublished data) indicate that sites other than the G domain site are likely to be of low affinity. Furthermore, it is unclear whether the internal short arm globule site is of physiological relevance since the purified elastase fragment El0 containing the putative heparin-binding sequence is found not to interact with heparin [2] (Mann et al, Eur J Biocbem 1988, 178:71+X)). Because

and enactinlnidogen

Iaminin ( 8OOkD) is composed of three polypeptide chains (Bl, B2, A) in which three short arms and one long arm extend from a common vertex [7] (Engel, JMol BioIl981, 150:97T120; Fig. 3). with the recent elucidation of the mouse A chain sequence 171, the entire sequence of all three chains is now known [7] (Sasaki et al, Proc Nat1 Acud Sci USA 1987,84:93%939; Sasaki and Yamada, J Biol Gem 1987,262:17111-17117). The structure predicted from the sequence is consistent with the observed morphology; however, the data indicate that there should be a fourth A chain globular domain adjacent to the vertex and the long arm globular region is subdivided into five disulfide-stabilized subdomains, GlG5 (see Kleinman and Weeks, pp 964-967, this issue). It is becoming increasingly evident that the different structural domains of laminin are involved in different interactions, with both cells and matrix components. Recently it has been shown that there is a variant form of larninin with three arms which lacks a recognizable A chain [8]. Other forms of laminin exist as well (see Kleinman and Weeks, this issue) but until now the functional differences of these variants have been largely unknown. Four-armed laminin shows several domain-specific binding interactions in z&-o which are probably important for basement membrane architecture. One of these interactions is that of self-assembly which involves several laminin domains, requires divalent cation, and has the characteristics of a cooperative nucleation-propagation reaction. The first step is temperature-dependent, with the formation of small oligomers, while the second step is calcium-dependent, with the conversion of oligomers to larger aggregates (Yurchenco, J Biol Gem 1985, 260:76367644). It has been found that each laminin can bind 16 calcium atoms 191, that one to two of these interactions are of sufficient affinity ( N 10 pmol/litre) to account for aggregation, and that this calcium binding influences protein conformation (Ietoumeau, J Neurosci Res

-. 100 I < _’

I

200 I

300 I

I

400 nm I,+l

Fig. 1. Schematic structure of collagen IV monomer. Two ul(lV) and one a2fIV) chains form collagen IV. This m 400 nm long molecule possesses an N-terminal (75) domain, major helical region, and a C-terminal globular (NC11 domain. Irregularly spaced and sized interruptions of the gly-xaa-yaa sequence are indicated with lines and boxes. The disultide-linked, loop (residues 6fS-685) is unique for the &2(W) chain. Drawing based on work of Kuhn and colleagues 131.

Basement

Single Lateral

molecular

associations

chair

Branch

.

ii

C-tyir$

heparin can both displace heparan sulfate chains from laminin and drive the equilibrium of the laminin polymerization further towards aggregation, we have postulated that heparin can act as a specific regulator of the structure and function of laminin in basement membranes in vivo (Yurchenco. In Structure, Interaction and Assembly of Cytoskeletal and Extracellular Proteins edited by Aebi and Engel. Springer Series in Biophysics No. 3. Springer, 1989, pp 357-366). The recent finding that heparin also binds type IV collagen and inhibits collagen polymerization (Tsilibary et al, J Biol cbem 1988, 263:19112-191181, and that a heparan sulfate proteoglycan binds various fibrillar collagens (Koda et al, J Biol cbem 1985, 260:8157+X82) suggests an additional role of these glycosaminoglycan chains in matrix self-assemW. Heparan

sulfate

proteoglycan

Heparan sulfate chains are important for the ionic control of liltration through basement membranes (Farquhar. In Cell Biology of Extracellular Matrix edited by Hey. Plenum Press, 1981, pp 335-378). Wo different kinds of proteoglycan, distinguished by size and buoyant density, have been described (Fig. 4). The high-density class contains a number of smaller forms: while some of the high-

point

gl;bules

membranes

Schittny

and Yurchenco

Fig. 2. Model of the collagen IV network. The network appears as a complex, three-dimensional meshwork of branching filaments. Four N-terminal ends are crosslinked to form the 75 domain and two C-terminal globules are dimerized. Lateral associated collagen IV molecules produce an irregular network and appear, in various regions, as a supertwisted helix 161.

density forms represent proteolytic degradation products of the low-density form, others (Fujiwara et d, Eur J Biodem 1984,143:145-157; Kato etal, JBiol C&m 1987, 262:718&7188) appear to be unique. The low-density form is a single species with an 8Onm long multidomain protein core (- 5OOkD), subdivided into an array of about six globular domains, with three heparan sulfate chains (each lOO-170~1 long) attached at one end (Paulsson et al, J Mol Bioll987,197:297-313). The multiple domain structure suggests multiple interactions. Sequence analysis of core regions has revealed homology to the cysteine-rich repeats found in laminin and to the cell-adhesion protein neural cell adhesion molecule (NCAM), which belongs to the immunoglobulin-gene superfamify [ 121. The proteoglycan is tightly bound to EHS tumor matrix and chaotropic agents (4-6mol/litre guanidine) are required for its extraction. The core protein can bind to itself to form linear dimers and small stellate oligomers (Yurchenco et al, J Biol C&em 1987, 262:17668-17676) and the heparan sulfate chains can weakly bind to laminin and to the NC1 domain of col-

lagen IV (Fujiwara et al, 1984). The low-density heparan sulfate proteoglycan core antigen is present in the basement membranes of many vascularized tissues (Kate et al, J Cell Biol1988, 106:2203-2210) including the renal glomerulus (Klein et al, J Ceil Biol1988, 106:96%970).

985

986

Extracellular

matrix

.

E4 (self-assembly)

Fig. 3. Domain

structure of laminin. The 61, 82 and A chains form an asymmetrical four arm structure. The locations of proteolytitally generated fragments and their associated functions are indicated felastase fragment El’, E3, E4, E8, El0 and pepsin fragment Pl’). Entactin fnidogen) tightly associates with laminin in the vicinity of the intersection of the arms (Paulsson et al., Eur j Biochem 1987, 16691-16).

Cell-basement development

membrane

interactions

and

The major structural components of the basement membrane interact with the cells that they reside upon. Iaminin, the most intensively studied (see Kleinman and Weeks, this issue), possesses cell interaction sites both near the intersection of its arms (Pl fragment) and towards the end of the long arm (E8 fragment). Entactin possesses an RGD sequence and interacts with cells [lo] (Durkin et al, 1988). The main triple helical region of collagen IV also exhibits cell interactions which may be mediated by members of the integrin family (Aumailley and Timpl, J Cell Bioll986, lO3:1569-1575; Tomaselli et aL, /Cell Biol107:1241-1252). Recently it has been found that 26-38kD surface proteins of hepatocytes can bind the core protein of the low-density heparan sulfate pro-

teoglycan and that the core will support cell attachment (Clement et al, J Biol cbem 1989, 264:12467-12471). It is becoming Increasingly apparent that basement membranes exert important influences in both embryogensis and repair. During embryonic development (Wu et UC, Dev Biol1983, 100:496505; I.&o et al, Dev Bioll980, 76:100-114) laminin is laid down as the first abundant basement membrane protein and during angiogenesis it is the first to be observed at the growing tip of a new capillary (Form et al, Lub Invest 1986,55:521-530). Here laminin may provide the only polymer scaffolding and may be the precursor to a more stable lamimn-collagen IV network. In contrast to collagen IV, which becomes rapidly covalently crosslinked after assembly, the laminin polymer is, initially at least, held together by a number of specific low-affiity interactions (Schittny and Yurchenco, unpublished data). Such a laminin matrix has the potential to be remodelled through readily reversible interactions, a feature expected to be useful in a developing basement membrane. The identiiication of variant forms of laminin of uncertain function and developmental time course raise new issues about basement membrane in development. Klein et aA [13] have shown that laminin plays an important role in the development of epithelial cell polarity. The B chains are constitutively expressed, whereas the appearance of the A chain is dependent on embryonic induction and coincides with the onset of cell polarization. Furthermore, antisera against the C-terminal globule of the A chain inhibit polarization of kidney epithelium. Proteoglycans also influence tissue development; it has been shown that interference with the biosynthesis of proteoglycans leads to anomalous development of basement membrane structure and the surrounding tissue in fetal kidneys (Ielongt et al, Dev Biol1988,128:256276). There is now a body of evidence that one of the functions of basement membrane heparan sulfate is to bind, immobilize and stablize basic libroblast growth factor (bFGF; Rifkin and Moscatelli, J Cell Bioll989, 109:1-6). Recently it has been shown that neurite outgrowth-promoting activity for PC12 cells, which can be influenced by the laminin long arm (E8 fragment), requires the presence of bFGF [ 141. This study raises questions about the regulatory relationships between laminin, bFGF, and heparin/heparan sulfate in neurite development.

Diseases

affecting

basement

membranes

A number of disorders of metabolic, inherited and immune origin involve basement membranes and affect functions of sieving and mechanical support. Diabetes

mellitus

The morphological hallmark of diabetic microvascular disease is generalized basement membrane thickening; in the kidney, a gradual increase in the mass of both glomerular basement membrane (GBM) and mesangial matrix occurs. These changes are associated with a pro-

Basement

membranes

Schittny and Yurchenco

be&s 1988, 37:532-539) that glycosylated laminin loses its ability in vitro to bind heparin suggests that non-enzymatic glycation may contribute to this decrease. T

Low density proteoglycans

\\ ‘a.,

Goadpasture’s \

\ -\ ‘-+e8ee@

t----4------, /

/’ ti

,“ :,

/

High density proteoglycans

,d’ / 4’

I

100 nm

I

Fig. 4. Basement membrane heparan sulfate proteoglycans. Protein cores are shown in black and heparan chains with dashed lines. A high-density form, which binds weakly to laminin and collagen (Fujiwara, Eur / Biochem.1984, ‘143145-1571, has a small core and four short heparan sulfate chains. The low-density form, which binds itself, possesses an elongated core subdivided into a tandem array of globular domains and two to three long heparan sulfate chains issuing from one pole. Adapted from Paulsson et a/. (/ MO/ Biol 1987, 197297).

gressive increase in GBM permeability followed by a decrease in glomerular filtration. One hypothesis is that accelerated non-enzymatic glycosylation (glycation) plays an important role in the development of diabetic microangiopathy (Brownlee et al, N Engl J Med 1988, 318:131+1321). Elevated blood glucose accelerates the binding of sugar to s-amino and N-terminal a-amino groups of proteins to form ketoamines, and ultimately inter- and intramolecular crosslinks. It is thought that there may be a substantial accumulation of such protein mocliiications in basement membrane, resulting in deleterious structural and functional changes. Type IV collagen, for example, has been found to be non-enzymatially glycosylated in viva above normal levels in diabetic GBMs (Uitto et al, Connect Tikme Res 1982,10:287-2%; Trueb et al, Co11Relat Res 1%4,4:23+251). Lens capsule basement membrane glycosylated in vitro develops covalent crosslinks which make the membrane brittle (Bailey and Kent. In The Maillard Reaction in Aging, Diabetes and Nutrition edited by Baynes and Monnier. Alan R Liss, 1989, p. 109). Nicholls and Mandel (L& Invest 1989, 60486-491) have reported that aminoguanidine, an inhibitor of non-enzymatic glycosylation, prevents the formation of advanced glycosylation end-products in the GBMs of diabetic mice. Decreased GBM heparan sulfate proteoglycan, reflecting a depletion of polyanionic changes distributed on the surface of the basement membrane, has also been considered as a cause of permeability changes (Mynderse et al, Lub Invest 1983,48:292-302; Rohrbach et al, Diabetes 1982,31:185-188; Parthasarathy and Spiro, Diubetes 1982, 31:738-741). While this decrease could be either due to a metabolic change or a result of glycation, the recent linding (Tarsio et al, Dia-

syndrome

This syndrome of acute glomerulonephritis and alveolar hemorrhages is caused by the damaging effects of anti-GBM antibodies which cross-react with alveolar basement membranes. Goodpasture antigen has been mapped to the C-terminal globular domain of type IV collagen (Kefalides, Lub Invest 1987, 56:1-3), in particular with a 28 kD monomer and 42-50 kD dimer (W&lander et al, Pm Nat1 Acud Sci (ISA 1984, 81:1544-1548) of a novel collagenous chain (designated u3[IV]) present in small amount in GBM [IS] (Butkowski eta& JBbZCbem 1987, 262~78747877). Hereditary

nephritis

Hematuria, progressive renal failure and hearing loss develop in Alport’s disease. An irmgular lamination and attenuation of the GBM is seen at the ultrastructural level. While the molecular basis of this disease is unclear, it has been reported that a 28kD monomer (Goodpasture antigen) from the globular domain of basement membrane collagen is missing (Kleppel et al, J Clin Invest 1987, 80:263266). The concept of a missing antigen is supported by kndings that Alport’s patients who receive renal transplants can develop antlglomerular antibodies (Noel et al, Adv Neplmll989,18:77-94). Blistering

disorders

of skin

In a number of blistering diseases, a splitting at the level of the epidennal basement membrane occurs, reflecting a loss of its support function. Some of these disorders involve type VII collagen anchoring hlaments, which connect basement membrane to stroma and are involved in the attachment of cells to basement membrane. Recently it has been found that the autoantibodies of epidemolysis bullosa acquisita bind the C-terminal globular domain of type VII collagen [ 161. In severe recessive dystrophic epidemrolysis bullosa there is an absence of detectable type VII collagen in affected skin [ 171.

Annotated reading

references

and recommended

a

Of interest

00

Of outstanding interest

1.

Scttn-r~~ JC, TIMPL R, ENGEL J:

0

electron

High

molutioa

immuno-

microscopic localization of fuoctiooal domains of

lamin@ oidogen, and heparao suEate prow&can in epithelial basement membraoe of mouse cornea reveals dilierem topological orientations. / CeU Biol 1!%8, 107:15!&1610. Nidogen (entactin), low-density hepatan sul.t%eproteoStyran and different domains of laminin were locaked using high-resolution immunoekctron microscopy. Different orientations of laminin wete identi6ed inside the epithelial basement membrane.

987

988 .-.

Extracellular

matrix

2. TIMPLR: $mcture and biolo@cal activity of basement meml . dlk&e proteins. Eur J Biofbem 1989, 183:487-502. Comprehensive review of the properties of the major basement membrane proteins. 3.

BRAZELD, POUNERR, OBEmUMER I, KUHN K: Human basemerit membrane col@en (tupe lV). The amino acid sequence of the a2(IV) chain and its comparison with the al(lV) chain reveals deletions in the al(lV) chain. Eur J Biocbem 1988, 172:35-42. This study adyzes the entire primary strucNre of CO!hgen Iv. 4. SIEBOUIB, DEUTZMANN R, KOHN K: The ivrangement of intral and intermokcular disullide bonds in the carboxyterminal, nontol@enous a88regation and cross-linking domain of basement-membrane type lV collagen. Eur J Bkxhem 1988, 176617-624. Evidence that the arrangement of intermolecular NC1 dimer disulphide bonds are formed by complete disullide exchange between cone spondin8 ‘knots’ of paired monomeric NC1 domains. l

5. 0

WEBER S, DOlz R, TLMPL R, Pmsm JH, ENGEL J: Reductive cleavage and reformation of the interchain and intrachain dhlEde bonds in the globular hexameric domain NC1 involved in network assembly of basement membrane collagen (type Iv). Eur J Biocbem 1988, 1752-236.

These SNdieS (451 ebkiate the mechanism lagen Iv via its C-terminal globular domain.

Of the ChetitiOn

Of COl-

6.

YURCHENCO PD, RUBEN GC: Type Iv collagen lateral associations in the MS tumor matrix: comparison with amniotic and in vlti networks. Am J P&d 1988, 132:278-291. Ah the visualization in situ of the collagen lV network in human am IIiOn, this Study now focuses on the structure of the coUa8en IV network in the EHS tumor and of an in vittu assembled network. l

7.

~M,KLEINMANHK,HIJEIERH,DEu~wANN R, YAMADAY: Lamin@ a multidomain protein. The A chain has a unique globular domain and homology with the basement membrane proteo@ycan and rhe laminin B chains. J Bid c&m 1988, 263:1653&16544. The sequence of mouse kminin A chain compkments earlier SNdieS and completes the entire primary structure of this protein. Based on homology between the 3 chains, a new domain model is proposed. l

a.

EDGAR D, TIMPU R, THOENEN H: Structural requirements for the stimulation of neurite outgrowth by two variants of laminin and their inhibition by anribodies. J Ceff Bid 1988, 156:1299-1306. A 3-armed form of laminin has been characterized. A pting fezINI’C is that the C-terminal long-am globule, normally A-chain-derived, can be visualized in electron micrographs. It suggests the existence of a shorter akemative type of A chain. l

9. 0

PNJI.SSONM, SAIADIN K, IUVDU’EHRR Biding of Ca*+ in8uences susceptibiity of laminin to proteolytic digestion and interactions between domain-specific laminin frag merits. Eur J Biocbem 1988, 177:477-l. This Study focuses on the domain specilicity of the laminin self-assembly and the laminin binding affinity for calcium. 10. Du~lw ME, CHAKRAV~ S, BAKIXX BB, LIIJ S-H, FRIEDMANRL, l CHUNGAJZ: Amino acid sequence and domain structure of

entaccin. Homology with epidermal growth Hector precur. sor and low density lipoprotein receptor. J Cell Bid 1988, 107:274+2756. Sequence analyxis of enraccin reveals 2 globular domains, cysteine-rich repeats, potential calcium-biding sites, an RGD sequence, and identity to nidogen. 11. l

MANNYDEUIZMANN YAMADA Y, PAN T-c,

Ft, AUMAI~M M, TIMF-L R, RAIMoNDI L, CONWAY D, CHU M-L Amino acid se-

quence of mouse nidogen, a multidomain basement membrane protein with bind@ activity for laminin, collagen lV and cells. EMBO J 1989, 8:65-72. The sequences of nidogen and entactin [lo] have been published nearly simultaneously. !%quences are obtained from complementary DNA clones supported by Edman degradation of peptides. The dumbbell model of nidogen is supported. 12.

NODNANDM, HOIUGW E& IEDBEITER SR, VOGELI G, SAWI M, YAMADAY, HAsSElLJR Identification of cDNA clones encodin8 different domains of the basement membrane heparan sulfate proteoglycan. J Bid Cbem 1988, 263:1637+16387. About 40% of the sequence of the low-density heparan sulfate proteoglycan was am$zed. Homology to laminin and to NCAM was demonstrated. l

KLEIN G, IANGEGGERM, TM-L R, EKBLOMP: Role of laminin A chain in the development of epithelial ceil polarity. Cell 1988, 55:331-341. This study elucidates the role of the terminal globule of lamini& long arm for the development of renal epithelial cells. 13. l

14.

ROGELJS, KL~GSBIJRNM, A-IZMONR, KURROKAWAM, mov12 A, FUKS 2, VL~DA~KY I: Basic Ebroblast growth is an extracelh&ir matrix component required for supporting the proliferation of vascular endothelial cells and the differentiation of PC12 cells. J Cell Bid 1989, 109:823+331. This study provides evidence that bFGF is required for neurite oufgrowth. 2 questions arise. First, what is the relation between the contribution of the bFGF and that of the extracellular matrix components to cell proliferation and differentiation? Second, do ail cells require bFGF for this activity? l

SAUSJ, WIESL~NDERJ, L~NGEVEIDJPM, QUINONESS, HUDSON BG: Identification of the Goodpasture antigen as the a 3(lV) chain of col@en IV. J Bid &em 1988, 26331337413380. A new basement membrane collagen chain has been characterized. 15. l

16. a

WOODLEY DT, BURGESON RE, LUNSTRUM G, BRUCKNERTUDERMANI, REEsEMJ, BIUGGAMANRA Epidermolysii bullosa acquista antigen is the globular carboxyl terminus of type VII procollagen. J Clin Invest 1988, 81:683+%7. An autoimmune reaction against collagen W characterizes this disorder at a molecular level. 17.

BRUCKNER-TUDERMAN I RLJEGGER S, ODERMAT~ B, Mrtmas~l Y, WUWDER UW: Lack of type W collagen in una5zcted skin of patients with severe recessive dystrophic epidermolysis bullosa Dermatologica 1988, 176:57&L The molecular disorder in patients with severe rece&ve dystrophic epidermoiysii buUosa has been correlated to the absence of type W collagen l