Extracellular matrix proteins and their cellular receptors, integrins, play a fundamental role in keratinocyte adhesion and migration. During wound healing, keratinocytes detach, migrate until the two epithelial sheets confront, and then regenerate the basement membrane. We examined the expression of different integrins and their putative ligands in keratinocytes during human mucosal wound healing. Migrating keratinocytes continuously expressed kalinin but not the other typical components of the basement membrane zone: type IV collagen, laminin, and type VII collagen. When the epithelial sheets confronted each other, these missing basement membrane components started to appear gradually through the entire wound area. The expression of integrin 81 subunit was increased in keratinocytes during migration. The #,B-associated a2 and a3 subunits were expressed constantly by wound keratinocytes whereas the a5 subunit was present only in keratinocytes during reepithelialization. Furthermore, migrating cells started to express a,-integrins which were not present in the nonaffected epithelium. All keratinocytes also expressed the a684 integrin during migration. In the migrating cells, the distribution of integrins was altered. In normal mucosa, ,61-integrins were located mainly on the lateral plasma membrane and aQ84 at the basal surface of basal keratinocytes in the nonaffected tissue. In wounds, integrins were found in filopodia of migrating keratinocytes, and also surrounding cells in several cell layers of the migrating sheet. The results indicate that migrating keratinocytes in deep human wounds enlarge their integrin repertoire. The changes in integrin expression take place concomitantly with changes in the basement membrane composition, suggesting a close interplay of these two groups of molecules during wound healing. (J. Clin. Invest. 1993. 92:1425-1435

In the integrin family, the collagen receptors form a structurally and functionally distinct subgroup. Two members of this subgroup, ␣ 1 ␤ 1 and ␣ 2 ␤ 1 integrins, are known to bind to monomeric form of type I collagen. However, in tissues type I collagen monomers are organized into large fibrils immediately after they are released from cells. Here, we studied collagen fibril recognition by integrins. By an immunoelectron microscopy method we showed that integrin ␣ 2 I domain is able to bind to classical D-banded type I collagen fibrils. However, according to the solid phase binding assay, the collagen fibril formation appeared to reduce integrin ␣ 1 I and ␣ 2 I domain avidity to collagen and to lower the number of putative ␣I domain binding sites on it. Respectively, cellular ␣ 1 ␤ 1 integrin was able to mediate cell spreading significantly better on monomeric than on fibrillar type I collagen matrix, whereas ␣ 2 ␤ 1 integrin appeared still to facilitate both cell spreading on fibrillar type I collagen matrix and also the contraction of fibrillar type I collagen gel. Additionally, ␣ 2 ␤ 1 integrin promoted the integrin-mediated formation of long cellular projections typically induced by fibrillar collagen. Thus, these findings suggest that ␣ 2 ␤ 1 integrin is a functional cellular receptor for type I collagen fibrils, whereas ␣ 1 ␤ 1 integrin may only effectively bind type I collagen monomers. Furthermore, when the effect of soluble ␣I domains on type I collagen fibril formation was tested in vitro, the observations suggest that integrin type collagen receptors might guide or even promote pericellular collagen fibrillogenesis.A fibril-forming type I collagen, a ubiquitous protein in all vertebrates, is known to provide mechanical stability for tissues and serve as a functional environment for cells (1). Depending on the physical properties of the tissue, type I collagen fibrils are arranged with different suprafibrillar architectures and diameters. Thus, narrow fibrils (ϳ20 nm) in highly ordered arrangement occur in the cornea, where optical transparency is important, whereas large diameter fibrils (ϳ500 nm) provide high tensile strength in mature tendon (2).The mechanism of type I collagen fibril formation has been under extensive research for decades. In tissues, type I collagen is synthesized as a monomeric precursor, which is secreted by exocytosis into the extracellular space. In addition to the triple helical collagenous domain, the precursor contains noncollagenous C-and N-propeptides, which are linked to the triple helical domain by short sequences called telopeptides (3). After the enzymatic removal of propeptides, the solubility of collagen monomers decreases, and they spontaneously form fibrils, assisted by remaining nonhelical telopeptides (1, 4). Evidently, collagen molecules themselves contain all the information needed for fibril assembly. Therefore, in physiological conditions, acid-solubilized collagen monomers form tissue-type long fibrils with characteristic axial periodic structure also in v...

The collagen family of extracellular matrix proteins has played a fundamental role in the evolution of multicellular animals. At the present, 28 triple helical proteins have been named as collagens and they can be divided into several subgroups based on their structural and functional properties. In tissues, the cells are anchored to collagenous structures. Often the interaction is indirect and mediated by matrix glycoproteins, but cells also express receptors, which have the ability to directly bind to the triple helical domains in collagens. Some receptors bind to sites that are abundant in all collagens. However, increasing evidence indicates that the coevolution of collagens and cell adhesion mechanisms has given rise to receptors that bind to specific motifs in collagens. These receptors may also recognize the different members of the large collagen family in a selective manner. This review summarizes the present knowledge about the properties of collagen subtypes as cell adhesion proteins.

Collagenase-3 (matrix metalloproteinase-13, MMP-13) is a recently identified human MMP with an exceptionally wide substrate specificity and restricted tissue-specific expression. Here we show that MMP-13 expression is induced in normal human skin fibroblasts cultured within three-dimensional collagen gel resulting in production and proteolytic activation of MMP-13. Induction of MMP-13 mRNAs by collagen gel was potently inhibited by blocking antibodies against ␣ 1 and ␣ 2 integrin subunits and augmented by activating antibody against ␤ 1 integrin subunit, indicating that both ␣ 1 ␤ 1 and ␣ 2 ␤ 1 integrins mediate the MMP-13-inducing cellular signal generated by three-dimensional collagen. Collagen-related induction of MMP-13 expression was dependent on tyrosine kinase activity, as it was abolished by treatment of fibroblasts with tyrosine kinase inhibitors genistein and herbimycin A. Contact of fibroblasts to three-dimensional collagen resulted in simultaneous activation of mitogen-activated protein kinases (MAPKs) in three distinct subgroups: extracellular signal-regulated kinase (ERK)1 and ERK2, Jun N-terminal kinase/ stress-activated protein kinase, and p38. Induction of MMP-13 expression was inhibited by treatment of fibroblasts with a specific p38 inhibitor, SB 203580, whereas blocking the ERK1,2 pathway (Raf/MEK1,2/ERK1,2) by PD 98059, a selective inhibitor of MEK1,2 activation potently augmented MMP-13 expression. Furthermore, specific activation of ERK1,2 pathway by 12-O-tetradecanoylphorbol-13-acetate markedly suppressed MMP-13 expression in dermal fibroblasts in collagen gel. These results show that collagen-dependent induction of MMP-13 in dermal fibroblasts requires p38 activity, and is inhibited by activation of ERK1,2. Therefore, the balance between the activity of ERK1,2 and p38 MAPK pathways appears to be crucial in regulation of MMP-13 expression in dermal fibroblasts, suggesting that p38 MAPK may serve as a target for selective inhibition of collagen degradation, e.g. in chronic dermal ulcers. Controlled degradation of extracellular matrix (ECM)1 is essential in physiologic situations involving connective tissue remodeling, such as tissue morphogenesis, angiogenesis, and tissue repair. On the other hand, excessive breakdown of connective tissue plays an important role in the pathogenesis of, e.g., rheumatoid arthritis, osteoarthritis, atherosclerosis, periodontitis, autoimmune blistering disorders of skin, and dermal photoaging, as well as in invasion and metastasis of tumor cells (see Refs. 1 and 2). Matrix metalloproteinases (MMPs) are a family of structurally related zinc-dependent endopeptidases collectively capable of degrading essentially all ECM components, and they apparently play an important role in ECM remodeling in the physiologic and pathologic situations mentioned above. At present, human MMP gene family contains 16 members, which can be divided into subgroups of collagenases, gelatinases, stromelysins, membrane-type MMPs, and novel MMPs based on their structure and substrate speci...