Non-fibrillar collagenous proteins synthesized by rat mesangial cells

Recent studies have shown that nonrenal extracellular matrices are composed of collagenous proteins with properties that are different from those of fibrillar collagens and type IV collagen. The structures of these newly described collagens suggest that they may provide connections between specific matrix molecules and, in so doing, partially determine the three-dimensional structure of the matrix. The molecular composition and organization of normal and diseased mesangial matrix are incompletely understood. As a means to further understand the structure of the mesangial matrix, we have further defined the collagenous proteins synthesized by rat mesangial cells. By using monospecific antibodies, we have shown that these cells secrete types III and IV collagen. Metabolic labeling and electrophoretic analysis of proteins isolated from the culture medium revealed that mesangial cells also synthesize and secrete short-chain collagenous proteins of 80, 75, and 69 kDa which differ in their binding to concanavalin A. Cell-free translation of mesangial mRNA demonstrated the presence of two additional collagenase sensitive proteins, 40 and 35 kDa in size, which were not detected in the culture medium. These results demonstrate that mesangial cells synthesize several short-chain collagenous proteins which may be similar to short-chain collagens isolated from nonrenal tissues. Further definition of these proteins will lead to a greater understanding of the extracellular matrix elaborated by mesangial cells.