The laminins

ht. J. Biochem. Cell Bid. Vol. 28, No. 9,pp. 957-959, 1996

Pergamon PII: S1357-2725(96)00042-S

C o p y r i g h t 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1357-2725/96 $15.00 + 0.00

MOLECULES IN FOCUS

The Laminins KATHERINE M. MALINDA,

HYNDA K. KLEINMAN”

Laboratory of Developmental Biology, Building 30, Room 433, National Institute of Dental Research, Bethseda, MD 20892, U.S.A. The laminlns are a family of extracellular matrix glycoproteins localized in the basement membrane that separates epithellal cells from the underlying stroma. They are also found in basement membrane surrounding fat, muscle and peripheral nerve cells. The lamininsare large trimeric glycoproteins comprising three disulphide-bonded chains. They are the first extracellular matrix molecules to be observed in the developing embryo and have potent biological activities. In addition, there have been new developments in the numberand localization of the homologues of the laminin chains and the role of laminin in neuromuscular disease. Their primary role is in cell-matrix attachment, but many additional biological activities, including promoting cell growth and migration,tmuourgrowth and metastases, neurite outgrowth, nerve regeneration, woundrepair and graft survival, have been demonstrated. Many of these biological activities are duplicated by proteolytic fragments of lamlnin and by small laminin-derivedsynthetic peptides. These lamininderlved peptides may be useful clinical reagents for accelerating wound healing with minimal scarring or for blocking tumour metastases. Copyright 0 1996 Elsevler Science Ltd Keywords: Basement membrane Biological activity

Extracellular matrix

Lamimn Cell attachment protein

Int. J. Biochem. Cell Biol. (1996) 28, 957-959

INTRODUCTION

Laminin was first purified and named in 1979 (Timpl et al., 1979) and had originally been thought to be a third chain of type IV collagen, but resistance to collagenase indicated its non-collagenous nature. Laminin was first isolated from a tumour, but antibody localization studies and chemical analysis indicated that it was also present in authentic basement membranes in various tissues. Although laminin is present in most tissues, it represents only a small portion of the total protein, thus making the tumour the primary source of large quantities of material, It is now recognized that there is a family of laminins, since many homologues of the various chains have been isolated and others probably *To whom all correspondence should be addressed. Tel.: 301-496-4069; fax: 301-402-0897; e-mail: kleinman @yoda.nidr.nih.gov. Received 10 October 1995; accepted 26 March 1996.

exist (Kleinman et al., 1993; Timpl and Brown, 1994). For convenience, the homologues of the chains are now numbered based on their relationship to the specific chains, i.e. ~1, ~2, 013, etc. (Burgeson et al., 1994). The various homologues (chains CL, B, r) form heterotrimeric molecules that, in part, have tissue-specific locations (see Table 1). Many of these variant molecules and others yet to be determined may also have tissue-specific functions. STRUCTURE

Laminin exists as a cruciform-like structure formed by three chains: c(, B, y. These chains are structurally homologous, but have little similarity in the level of their amino acid sequence. Various isoforms of the chains (i.e. ~1, ~12, ot3, etc.) also share varying degrees of homology. The c1 chains (400 kDa) have three globular domains at their amino terminus which are separated by EGF-like repeats. They also have a coiled-coil domain and a large globular

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Molecules in Focus

New name Laminin- 1 Laminin-2 Laminin-3 Laminin-4 Laminin-5 Laminin-6 Laminin-7

Table 1. Nomenclature for laminins Chain composition

alPly (many tissues) a2plyl (muscle, nerve) al p2rl (neuromuscular junction, renal glomeruli)

a2p2yl a3P3y2 a3plyl a3p2yl

(dermoepidermal junction) (dermoepidermal junction)

domain at the carboxy terminus containing five globules. The p and y chains are shorter than the 01 chains, containing only two globular domains and two EGF repeats at the amino terminus, and lacking the large globular domain at the carboxy terminus. Assembly of the intact molecule occurs by the interaction of the three chains at the carboxy end, and is mediated through distinct sites in the a-helical region of the coiled-coil domain critical for chain specific assembly of the laminin molecule (Utani et al., 1994). Disulfide bonds link the chains near the carboxy terminus, while the amino termini remain free, giving laminin its classic cruciform structure. FUNCTION

Laminins have structural and biological roles in the basement membrane (Nurcombe, 1992; Kleinman et al., 1993). Since laminin can bind many of the other components of the basement membrane, including collagen IV, perlecan and entactin, as well as binding to itself, it is likely to play a role in organizing and possibly initiating the formation of the basement membrane matrix. This hypothesis is supported by the fact that, in the developing embryo, laminin is the first extracellular matrix molecule to be synthesized. Laminin is also very biologically active as demonstrated by in vitro and in vivo studies. Many activities have been defined for laminin and several cellular receptors for laminin molecules have been identified. The adhesion of both normal and malignant epithelial cells to laminin was the first activity shown for laminin, and, subsequently, specific adhesion-promoting peptide sequences have been identified (some are indicated in Fig. 1). Laminin also promotes the malignant phenotype. For example, cells cultured on laminin in vitro form more tumours in vivo than cells not cultured on laminin. In addition, cells selected by adhesion to laminin in vitro also form more tumours in vivo than the parental cells, non-adherent cells or fibronectin-adherent cells.

Previous name EHS laminin Merosin s-Laminin Merosin/+laminin Kalinimnicein k-Laminin k-Laminin

Furthermore, co-injection of laminin with tumour cells yields increased numbers of lung colonies. Several laminin-derived synthetic peptides have been found to influence the growth and metastasis of tumours (Yamada, 1991). A sequence on the /31 chain, YIGSR (tyr-ile-glyser-arg), reduces tumour growth and experimental lung colonization, in part, by decreasing the tumour blood supply. This peptide also has a direct effect on the tumour cells, and can induce apoptosis in vitro when presented to the cells in a multimeric form. The level of the 32/67-kDa laminin receptor expression, that also binds to YIGSR, has been found to correlate directly with malignancy in many murine tumour cells and in human colon cancer. Laminin also increases the secretion of collagenase IV that is involved in tumour spread. A sequence on the c( chain (ile-lys-val-ala-val) 01 chain

(400kD)

p chain

w3 w

LQVQLSIR Fig. 1. Schematic model of laminin-I. The location of several active sites defined by synthetic peptides is designated by the arrows. E8 and E3 define two proteolytic fragments. The G or globular domain is defined by brackets.

Molecules in Focus

has been found to promote protease activity, angiogenesis and the growth and metastases of tumours. The most potent activity of laminin seems to be with neurite outgrowth, where it is active in culture in the nanomolar range. Many types of primary and established central and peripheral neuronal cells respond to laminin (Nurcombe, 1992). Laminin can promote nerve regeneration in viuo over unusually long distances when incorporated into nerve guides, and has been found to increase the survival of nerve grafts in the brain. Two sites for neurite outgrowth have been identified on the a chain at the synthetic peptide level (IKVAV and LQVQLSIR), but these sites are not active with all lamininresponsive neuronal cells, and some cell-type specificity is observed in the neuronal cell response (Nomizu et al., 1995). This suggests that additional sites on laminin exist for neurite outgrowth and that more than one receptor for signaling neurite outgrowth may exist. Several receptors for laminin have been identified on neuronal cells, including integrins, non-integrins and amyloid precursor protein. These biological activities, as well as the ability of laminin to promote cell migration, growth and differentiation, suggest that laminin may have important clinical uses in wound repair and in the prevention of diseases. ROLE IN DISEASE PROCESSES

Laminin has been implicated in several acquired and inherited diseases. Since laminin is found in most tissues, is the first extracellular matrix protein in the developing embryo and is important in neural crest cell migration, defects in laminin may also be associated with human fetal loss, but this has not yet been firmly established. Laminin is highly antigenic and a number of diseases have been found to involve laminin autoantibodies, including Chagas’ disease, eclampsia, chronic mercury poisoning, and other autoimmune diseases. Interestingly, administration of laminin antibodies to pregnant animals can also cause spontaneous abortions. Antilaminin antibodies have been detected in monkeys with histories of reproductive failure, and sera from these monkeys cause neural tube defects in cultures of whole rat embryos. Furthermore, immunization of known breeder monkeys with either laminin or certain laminin peptides results in infertility.

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Mutations and deletions in the laminin a2 chain (formerly termed merosin) are associated with an autosomal recessive form of muscular dystrophy and with dysmyelination in dy mice and in congenital muscular dystrophy patients (Worton, 1995). The laminin a2 chain is a ligand for the a-dystroglycan which is a component of a complex that spans the sarcolemma, forming a link between the extracellular matrix and the subsarcolemmal cytoskeleton. It is hypothesized that the mutant laminin chain disrupts the basement membrane and leads to muscle degeneration and to some peripheral neuropathy. In summary, although discovered some 16 years ago, new developments on the structure of laminins and on its biologically active sites have defined some of the functions of laminin in organogenesis and in disease processes. Through the use of transgenic as well as knock-out mice, it is likely that additional functions of laminin will be defined. Potential applications for diagnostic and therapeutic use increase the excitement about the laminins. REFERENCES Burgeson R. E., Chiquet M., Deutzmann R., Ekblom P., Engel J., Kleinman H., Martin G. R., Menguzzi G., Paulsson M., Sanes J., Timpl R., Tryggvason K., Yamada Y. and Yurchenco P. D. (1994) A new nomenclature for the laminins. Matrix Bid. 14, 209-211. Kleinman H. K., Weeks B. S., Schnaper W. H., Kibbey M. C., Yamamura K. and Grant D. S. (1993) The laminins: a family of basement membrane glycoproteins important in cell differentiation and tumor metastases. Vit. Horm. 47, 161-186. Nomizu M., Kim W. H., Yamamura K., Utani A., Song S. Y., Otaka A., Roller P. P., Kleinman H. K. and Yamada Y. (1995) Identification of cell binding sites in the laminin ul chain carboxyl-terminal globular domain by systematic screening of synthetic peptides. J. Biol. Chem. 270, 20583-20590. Nurcombe V. (1992) Laminin in neural development. Pharmac.

Ther. 56, 241-264.

Timpl R. and Brown J. C. (1994) The laminins. Matrix Biol. 14, 275-28 1. Timpl R., Rohde H., Gehron Robey P., Rennard S. I., Foidart J. M. and Martin G. R. (1979) Laminin-a glycoprotein from basement membranes. J. Biol. Chem. 254, 9933-9937. Utani A., Nomizu M., Timpl R., Roller P. P. and Yamada Y. (1994) Laminin chain assembly. J. Biol. Chem. 269, 19167-19175. Worton R. (1995) Muscular dystrophies: diseases of the dystrophin-glycoprotein complex. Science 270, 755-756. Yamada K. M. (1991) Adhesive recognition sequences, J. Biol. Chem. 266, 1280912812.