Schwann cells use a novel collagen-dependent mechanism for fibronectin fibril assembly

Cultured rat Schwann cells were stimulated to deposit fibrillar extracellular matrix by treatment with ascorbic acid in the absence of nerve cells. Immunofluoresence staining of the matrix showed that it contains collagens types I and IV, fibronectin and perlecan but not laminin. Collagen type IV, fibronectin and perlecan co-distributed completely in the matrix fibrils, whereas collagen type I was present in only a subset of these fibrils. Time course studies indicated that collagen type I fibrils appear at late stages of matrix formation. Digestion of Schwann cell extracellular matrix with collagenase effectively disrupted most of the matrix including fibronectin fibrils. This was in contrast with fibroblasts, where collagenase treatment removed collagen with no visible effect on fibronectin fibrils. alpha5 integrin was expressed on the cell surface of Schwann cells and partially codistributed with fibronectin-containing fibrils. This suggests that the inability of Schwann cells to deposit fibronectin-containing matrix through a conventional, collagen-independent mechanism was not due to the lack of fibronectin-binding integrins on their cell surface. Polyclonal anti-fibronectin antibodies inhibited the deposition of fibronectin into the matrix fibrils, whereas collagen type IV fibrils were generally unaffected. Growth of Schwann cells on collagen type IV-coated substrate in the absence of ascorbate induced deposition of fine fibronectin fibrils. These results suggest that Schwann cells use an apparently novel, collagen type IV-dependent mechanism for the deposition of fibronectin into their extracellular matrix.