2.3. Collagens in the basement membrane of skeletal muscle
Basement membrane (BM) is a highly specialized sheet of connective tissue surrounding individual muscle fibers, blood vessels, Schwann’s cells and spindle capsule cells. Components of the BM are regulators of many biological activities such as cell growth, differentiation and migration which influence tissue development and repair (Erickson & Couchman 2000). Integrins attach muscle cells to ECM and serve as force-transmitters between ECM and contracting components inside the muscle cells. They connect laminin to the cell membrane to form the inner layer of basement membrane (Fig. 2) (Aumailley & Gayraud 1998). Type IV collagen molecules form a mesh-like structure outside the laminin layer and give stability to the BM (Kühn 1995). Laminin and type IV collagen are connected to each other by nidogen-1 in muscular basement membranes (Ries et al. 2001, Salmivirta et al. 2002). N-terminal 7-S domains of four type IV collagen molecules are covalently joined together (Risteli et al. 1980), while C-terminal noncollagenous (NC1) domains covalently connect two collagen IV molecules (Than et al. 2002).
Type VI collagen interacts with type IV (Kuo et al. 1997) and type I collagens (Bonaldo et al. 1990), providing a link between basement membranes and the surrounding matrix (Fig. 2). Collagens XV and XVIII comprise the multiplexin subfamily of nonfibrillar collagens (Erickson & Couchman 2000). They are located in the basement membrane zone and are known to bind nidogen-1, laminin-1-nidogen-1 complex, perlecan and fibulin (Myers et al. 1996, Sasaki et al. 2000, Tomono et al. 2002). Collagens XV and XVIII may have a role in stabilizing muscle cells (Eklund et al. 2001), although they also have an important physiological role as precursors of endostatins, which inhibit angiogenesis (O’Reilly et al. 1997, Sasaki et al. 2000).
Figure 1. Scanning electron micrographs of epimysium (EP, top right), perimysium (P, bottom) and endomysium (E, top left and bottom) of bovine semitendinosus muscle. Reprinted from Nishimura et al. (1994). Used with permission from S. Karger AG.
Type XIII collagen is a transmembrane protein (Hägg et al. 1998) capable of binding certain basement membrane proteins (Fig. 2) (Tu et al. 2002). Therefore it probably has a role as a link between muscle cell and its basement membrane (Kvist et al. 2001). Type XIII collagen is concentrated in the myotendinous junctions (Hägg et al. 2001), supporting the proposal of its important role in giving mechanical strength to cell-matrix junctions (Tu et al. 2002).