ACTA OPHTHALMOLOGICA

Vol. 70 (1992) Suppl. 202

INTEGRINS AS RECEPTORS FOR EXTRACELLULAR MATRIX PROTEINS IN HUMAN CORNEA lsmo Virtanenl, Kaarina Tewol, Matti Korhonenl, Taru PBBllysaho’ and Tim0 Tervo2 Department of Anatomy’, University of Helsinki, Helsinki and Department of Ophthalmology2, Helsinki University Central Hospital, Helsinki, Finland

Abstract. Extracellular matrix (ECM) proteins form distinct protein families that play a role during tissue maturation, wound healing and maintenance of tissue architecture. Recent studies show that there are tissue type -specific variations in their expression. ECM proteins function by complexing with each other and also by interacting with their cellular receptors, called integrins. Integrins are heterodimeric membrane glycoproteinsthat are partly cell type -specifically expressed in human tissues. Like other stratified epithelia, corneal epithelium expresses a,SI, a&, aJ3,.4, and avPl integrins that mediate attachment to the basement membrane and cell-cell interactions.

may change during embryonal development (Kolega et al., 1989). We have recently shown that in the kidney, for instance, both cellular FNs (Laitinen et al., 1991) and tenascin can be found in developing BMs but no longer in adult BMs. Similarly, during wound healing in the cornea, some of the ECM proteins, i.e. cellular FNs and tenascin (Tervo et al., 1989, in press), may reemerge, apparently providing a suitable surface for the migrating epithelium.

Key words: Cornea - extracellular matrix proteins - integrins - fibronectin.

I ntegri ns

The extracellular matrix (ECM) proteins include components of basement membranes (BM) such as laminins, type IV collagen and nidogen (Martin et al., 1988). Collagens form a group of ECM proteins consisting of at least 13 distinct proteins, some of which are cell type-specifically expressed in tissues. Fibronectins (FN), on the other hand, are found both soluble in plasma and insoluble in tissues (Ruoslahti, 1988). Tenascins are a newly characterized group of glycoproteins found particularly in developing and malignant tissues, and which share sequence homology with fibronectins (Chiquet-Ehrismann, 1990; Koukoulis et al.,

The functions of ECM proteins appear to be mediated by heterodimeric transmembrane glycoproteins called integrins. The integrin receptor for FN was the first t o be characterized (Pytela et al., 1985); since then numerous new proteins have been found. All the known integrins are heterodimeric proteins having one a subunit and one B subunit. Thus, there are now at least 7 known p subunits and even more a subunits. The S, integrin subunit can complex with at least 8 different a subunits, and accordingly the affinities of these complexes may be for collagens, laminins and/or fibronectins (Albelda & Buck, 1990; Ruoslahti, 1991). Bz integrins mediate leucocyte adhesion. p, integrins comprise receptors for vitronectin, FN, fibrinogen and thrombospondin. Furthermore, a single a subunit may complex with several p subunits: the a, subunit forms the laminin receptor when complexed with

1991).

Many recent studies show that the expression of ECM proteins and the composition of BMs 18

lntegrins as receptors for ECM proteins

the PI subunit, but a component of hemidesmosomes when complexed with the P4 subunit. Similarly, the a, subunit may be complexed with either the PI, p3 or the P5 subunit; the resulting complexes then have different ligand specificities (for a review, see Albelda 8z Buck, 1990; Ruoslahti, 1991). The known integrin complexes and their specificities are indicated in Fig. 1. The functional integrin receptor is a heterodimer composed of one a and one p subunit (Buck et al., 1986). Uncomplexed integrin subunits do not reach the cell surface, but are degraded in the cytoplasm (Cheresh 8z Spiro, 1987; Heino et al., 1989). The lingand binding site is formed by sequences from both subunits (Buck et al., 1986).

a

LM, COII,

LM, coil FN, EP

FN (alt)

a

FN (ROD)

a a

LM

FN (ROD)

;

a x

02+r p o 4

0 5

P P

The functions and interactions of integrins have been studied in detail using cell cultures. Many of the integrins (a,P,, aaPl, avP3and a,PJ become concentrated into specialized cellular structures called focal adhesions when they contact their ligand (Burridge et al., 1988; Ylanne et al., 1989). Such an association, which is detectable as a polarized distribution, can also be revealed in vivo for p1integrins (Korhonen et al., 1990). Although only a few studies have so far been completed on the expression of integrins in developing, normal, and malignant tissues, it is already clear that integrins show cell type-specific variations in their occurrence in tissues (Korhonen et al., 1990a, b; Virtanen et al., 1990). Even within a single organ, different cells may utilize distinct integrins to recognize BM proteins. We have studied n detail the expression of integrins in the developing and adult human kidney (Korhonen et al., 1990a, b). In the adult kidney, for instance, all tubular epithelial cells appear to utilize asplintegrin as a BM receptor; and distal tubules also utilize a3P1 and a2p1integrins; whereas glomerular podocytes and capillaries distinctly prefer the a,PIand a2PIintegrin complexes, respectively. In glomeruli the a6Plintegrin complex is only transiently expressed during early development. The a6PI integrin that recognizes the E8 fragment of laminin (Aumailley et al., 1990), is laclc-

LM, COII

4

6 6

Cell type-specific expression of integrins in human tissues

a a

,x; - ; ;

1

M ;Z 1;-

Ilh

FB, VWF. VN, FN (ROD)

v

FB. vWF, VN, OP, ESP I (ROD)

a a

6,

a a

\’

BM VN (ROD)

4

Peywa Patch Addressin

Fig. 1. The integrin family. The diagram shows the currently known subunits, the subunit combinations, and ligands. The integrins binding the RGD sequence are

also indicated. FN = fibronectin; VN = vitronectin; FB = fibrinogen; LM = laminin; EP = epiligrin; vWF = von Willebrand factor; Coll = collagen; OP = osteopontin; BSP 1 = bone sialoprotein 1; ICAM-I = intercellular adhesion molecules; FX = factor X; BM = basement membrane; C3bi = complement component C3bi; FN alt = fibronectin alternatively spliced domain.

ing in some other epithelial cells, such as those in the thyroid gland; and the a$, integrin shows there a similar, polarized distribution. The thyroid gland also expresses the a, subunit in abundance, as do many carcinoma cells in culture (Virtanen et al., 1990). The a5PIintegrin complex, specific for FN, is seldom expressed in human tissues, being found almost exclusively in some smooth muscle cells (Virtanen et al., 1990). Thus, its abundant presence in cultured cells may reflect a kind of “in vitro activation.” The a$, integrin appears, however, to also emerge in malignant cells upon increasing malignancy (Koukoulis et al., in press) and in invading cells, such as the intermediate trophoblast (Korhonen et al., 1991).

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lsmo Virtanen et al.

Fig. 2. ab integrin immunoreactivity in the cornea is seen in the lateral membranes of basal cells, and is especially bright basally along the BM (a). integrin immunoreactivity (b) as well as that of a2 and a3

integrins, is seen in the lateral membranes of the basal cells, as well as in the suprabasal cells; in contrast, the fi4 integrin immunoreactivity is solely confined to the basal aspect of the basal cells (c) (x 250).

lntegrins in stratified epithelial cells including the cornea The expression of integrins in the epidermis (Peltonen et al., 1989; Klein et al., 1990), gingiva (Hormia et al., 1990) and cornea (Lauweryns et al., 1991; Tervo et al., 1991) has been studied in detail. In all of these epithelia, the p,, a, and a3 subunits are found in basal and suprabasal cells throughout the cell membrane, most likely serving to mediate cell-cell interactions (Fig. 2b; Larjava et al., 1990). The most prominent integrin subunit in all stratified epithelia and myoepithelial cells, &, shows a strong, basally-confined immunoreactivity in the basal cells (Fig. 2c), probably in complex with the a6 subunit. The immunoreactivity of a6 in the cornea, however, is also seen in the lateral membranes of basal cells (Fig. 2a), suggesting that it may also be complexed with the 0,integrin. The ligand specificity of the a6P4complex is still unknown, but it appears to be a component of the hemidesmosome complex (Sonnenberg et al., 1991; Stepp et al., 1990). In contrast t o Lauweryns et al. (1991) we found no immunoreactivity for a4or a5integrins in human corneal epithelium. The distribution of integrins in the cornea appears to differ from that of other stratified epithelia, at least in the presence of the a, integrin in the basal cells (Tervo et al., 1991). In this respect it is of interest, however, that the adult corneal BM also contains 20

cFN (Tervo et al., 1986). Further studies of corneal wounding are needed to reveal whether distinct changes in integrin expression take place in the healing epithelium, as was recently found to be the case for activated epidermal keratinocytes (Guo et al., 1991).

Acknowledgements The original studies were supported by the Sigrid Justlius Foundation, University of Helsinki, the Finnish Academy of Medical Sciences, and by the Finnish Eye Foundation.

References Albelda S M & Buck C A (1990): Integrins and other cell adhesion molecules. FASEB J 4: 2868-2880. Aumailley M, Rimpl R & Sonnenberg A (1990): Antibody to integrin a6subunit specifically inhibits cellbinding to laminin fragment 8. Exp Cell Res 188: 55-60. Buck C , Shea E, Duggan K & Horwitz A (1986): Integrin (the CSAT antigen) functionality requires oligomeric integrity. J Cell Biol 103: 2421-2428. Burridge K, Fath K, Kelly T, Nuckolls G & Turner C (1988): Focal adhesion: transmembrane junctions

between the extracellular matrix and the cytoskeleton. Annu Rev Cell Biol 4: 487-525.

lntegrins as receptors for ECM proteins Cheresh D & Spiro R (1987): Biosynthetic and functional properties of an Arg-Gly-Asp-directed receptor involved in human melanoma cell attachment to vitronectin, fibrinogen, and von Willebrand factor. J Biol Chem 262: 17703-17711. Chiquet-Ehrismann R (1990): What distinguishes tenascin from fibronectin. FASEB . I4 2598-2604. Guo M, Kim L T, Akiyama S K, Gralnick H R, Yamada K M & Grinnell F (1991): Altered processing of integrin receptors during keratinocyte activation. Exp Cell Res 195: 315-322. Heino J, Ignotz R, Hemler M, Crouse C & Massague J (1989): Regulation of cell adhesion receptors by transforming growth factor-p. Concomitant regulation of integrins that share a common p, subunit. J Biol Chem 264: 380-388. Hormia M, Yllnne J & Virtanen I (1490):Expression of integrins in human gingiva. J Dent Res 69: 1817-1 823. Klein C E, Steinmayer T, Mattes J M, Kaufmann R & Weber L (1990): Integrins of normal human epidermis: differential expression, synthesis and molecular structure. Br J Dermatol 123: 171-178. Kolega J , Manabe M & Sun T-T (1989): Basement membrane heterogeneity and variation in corneal epithelial differentiation. Differentiation 42: 54-63, Korhonen M, Yllnne J, Laitinen L & Virtanen I (1990a): The distribtuion of PI and p3 integrins in human fetal and adult kidney. Lab Invest 62: 6 16-625. Korhonen M, Yllnne J, Laitinen L & Virtanen I (1990b): The a1-a6 subunits of integrins are characteristically expressed in distinct segments of developing and adult human nephron. J Cell Biol 111: 1245-1254. Korhonen M, Yllnne J, Laitinen L, Cooper HM, Quaranta V & Virtanen I (1991): Distribution of the a,-a6 integrin subunits in human developing and term placenta. Lab Invest 65: 347-356. Koukoulis G K, Gould V E, Howeedy A A & Virtanen I (1991): Tenascin in normal, reactive, hyperplastic and neoplastic tissues: biological and pathological implications. Human Pathol22: 636-643. Koukoulis G K, Virtanen I , Korhonen M, Laitinen L, Quaranta V & Gould V E (1991): Immunohistochemical localization of integrins in the normal, hyperplastic and neoplastic breast. Am J Pathol 139: 787-799. Laitinen L, Vartio T & Virtanen I (1991): Cellular fibronectins are differentially expressed in human fetal and adult kidney. Lab Invest 64: 492-498. Larjava H, Peltonen J, Sakiyama S K, Yamada S S, Gralnick H R, Uitto J & Yamada K M (1990): Novel function for p, integrins in keratinocyte cell-cell interactions. J Cell Biol 1 10: 803-8 15. Lauweryns B, van den Oord J J, Volpes R, Foets B & Missotten L (1991): Distribution of very late activation integrins in the human cornea. Invest Ophthalmol Vis Sci 32: 2079-2085.

Martin G R, Timpl R & Kiihn K (1988): Basement membrane proteins: structure and function. Adv Prot Chem 39: 1-50. Peltonen J, Larjava H, Jaakkola S, Gralnick H, Akiyama S K, Yamada S S, Yamada K M & Uitto J (1989): Localization of integrin receptors for fibronectin, collagen and laminin in human skin. J Clin Invest 84: 1916-1923. Pytela R, Pierschbacher M D & Ruoslahti E (1985): Identification and isolation of a 140 kd cell surface glycoprotein with properties expected of a fibronectin receptor. Cell 40: 191-198. Ruoslahti E (1988): Fibronectin and its receptors. Annu Rev Biochem 57: 375-413. Ruoslahti E (1991): Integrins. J Clin Invest 87: 1-5. Sonnenberg A, Calafat J , Janssen H, Daams H, van der Raaij-Helmer L M H, Falcioni R, Kennel S J, Aplin J D, Baker J, Loizidou M & Garrod D (1991): Integrin a,@, complex is located in hemidesmosomes, suggesting a major role in epidermal cellbasement membrane adhesion. J Cell Biol 113: 907-9 17. Stepp M A, Spurr-Michaud S, Tisdale A, Elwell J & Gipson I K (1990): asp4integrin heterodimer is a component of hemidesmosomes. Proc Natl Acad Sci USA 87: 8970-8974. Tervo T, Sulonen J , Valtonen S, Vannas A & Virtanen I(1986): Distribution of fibronectin in human and rabbit corneas. Exp Eye Res 42: 399-406. Tervo K, Tervo T, van Setten G-B, Tarkkanen A & Virtanen I (1989): Demonstration of tenascin-like immunoreactivity in rabbit corneal wounds. Acta Ophthalmol 67: 347-350. Tervo K, Tervo T, van Setten G-B & Virtanen I (1991): Integrins in human corneal epithelium. Cornea 10: 46 1 -465. Tervo K, van Setten G-B, Beuerman R W, Virtanen I, Tarkkanen A & Tervo T (1991): Expression of tenascin and cellular fibronectin in the rabbit cornea after anterior keratoectomy. Invest Ophthalmol Vis Sci 32: 2912-2918. Virtanen I, Korhonen M, Kariniemi A-L, Gould VE, Laitinen L & Ylanne J (1990): Integrins in human cells and tumors. Cell Differ Devel 32: 215-228. Ylanne J & Virtanen I (1989): The M, 140,000 fibronectin receptor complex in normal and virustransformed human fibroblasts and in fibrosarcoma cells: identical localization and function. Int J Cancer 43: 1126-1 136.

Author’s address: Ismo Virtanen, M.D. Department of Anatomy University of Helsinki Siltavuorenpenger 20 SF-00170 Helsinki Finland

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Integrins as receptors for extracellular matrix proteins in human cornea.

Extracellular matrix (ECM) proteins form distinct protein families that play a role during tissue maturation, wound healing and maintenance of tissue ...
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