Type XIII collagen: a novel cell adhesion component present in a range of cell–matrix adhesions and in the intercalated discs between cardiac muscle cells

Abstract

Recent analysis of type XIII collagen surprisingly showed that it is anchored to the plasma membranes of cultured cells via a transmembrane segment near its amino terminus. Here we demonstrate that type XIII collagen is concentrated in cultured skin fibroblasts and several other human mesenchymal cell lines in the focal adhesions at the ends of actin stress fibers, co-localizing with the known focal adhesion components talin and vinculin. This co-occurrence was also observed in rapidly forming adhesive structures of spreading and moving fibroblasts and in disrupting focal adhesions following microinjection of the Rho-inhibitor C3 transferase into the cells, suggesting that type XIII collagen is an integral focal adhesion component. Moreover, it appears to have an adhesion-related function since cell-surface expression of type XIII collagen in cells with weak basic adhesiveness resulted in improved cell adhesion on selected culture substrata. In tissues type XIII collagen was found in a range of integrin-mediated adherens junctions including the myotendinous junctions and costameres of skeletal muscle as well as many cell–basement membrane interfaces. Some cell–cell adhesions were found to contain type XIII collagen, most notably the intercalated discs in the heart. Taken together, the results strongly suggest that type XIII collagen has a cell adhesion-associated function in a wide array of cell–matrix junctions.

Introduction

The collagens are a family of 19 distinct types of extracellular matrix proteins, divided into a fibril-forming and a non-fibril-forming group on the basis of their supramolecular complexes (Kielty et al., 1993, Bateman et al., 1996). Type XIII is a non-fibril-forming collagen that has been well characterized at the primary structural level, but like many of the newer collagen types, its function has yet to be resolved (Pihlajaniemi and Rehn, 1995). We have shown recently that the amino terminal NC1 domains of the human and mouse chains contain a hydrophobic membrane-spanning segment that anchors the molecule to the plasma membrane of the cells that express this protein, in a configuration with a 40-residue amino terminal cytosolic domain and a larger extracellular, mostly collagenous carboxyterminal domain (Hägg et al., 1998). Accordingly, type XIII collagen and type XVII collagen, which is a hemidesmosomal transmembrane protein (Burgeson and Christiano, 1997), are considered to form a subgroup of their own within the family of collagens. These two non-fibrillar collagen types are not structurally homologous except that both are predicted to be type II transmembrane proteins with a single transmembrane domain near the amino terminus. With respect to tissue distribution, type XVII collagen is confined to hemidesmosomes while in situ hybridization data suggest that type XIII collagen has a wide tissue distribution (Sandberg et al., 1989, Juvonen et al., 1993, Burgeson and Christiano, 1997).

The human and mouse type XIII collagen chains are highly similar in structure, both consisting of three collagenous domains, COL1–3, interspersed and flanked by four non-collagenous domains, NC1–4. Analysis of recombinant type XIII collagen shows that it can form homotrimers with the triple-helical collagenous domains (Snellman et al., 2000). The precursor-mRNAs encoding type XIII collagen undergo complex alternative splicing, affecting both collagenous and non-collagenous sequences of the ectodomain (Pihlajaniemi and Tamminen, 1990, Juvonen and Pihlajaniemi, 1992, Juvonen et al., 1992, Juvonen et al., 1993, Peltonen et al., 1997).

This study was undertaken to investigate the possibility that type XIII collagen may be a component of cellular junctions, as suggested by its co-localization with the β3 integrin subunit in spreading human HT-1080 fibrosarcoma cells (Hägg et al. 1998). In addition to the previously described antibodies, a rabbit polyclonal antibody against the NC1-domain of mouse type XIII collagen was produced, and the location of type XIII collagen in cultured human mesenchymal cells and several mouse tissues was determined by immunofluorescence microscopy. The putative adhesive role of type XIII collagen was studied by expressing it in cultured insect cells. The present observations underline the transmembrane nature of type XIII collagen and suggest that its function is probably related to cell adhesion.