Active Negative Control of Collagen Fibrillogenesis in Vivo. Intracellular Cleavage of the Type I Procollagen Propeptides in Tendon Fibroblasts Without Intracellular Fibrils

It is established fact that type I collagen spontaneously self-assembles in vitro in the absence of cells or other macromolecules. Whether or not this is the situation in vivo was unknown. Recent evidence shows that intracellular cleavage of procollagen (the soluble precursor of collagen) to collagen can occur in embryonic tendon cells in vivo, and when this occurs, intracellular collagen fibrils are observed. A cause-and-effect relationship between intracellular collagen and intracellular fibrils was not established. Here we show that intracellular cleavage of procollagen to collagen occurs in postnatal murine tendon cells in situ. Pulse-chase analyses showed cleavage of procollagen to collagen via its two propeptide-retained intermediates. Furthermore, immunoelectron microscopy, using an antibody that recognizes the triple helical domain of collagen, shows collagen molecules in large-diameter transport compartments close to the plasma membrane. However, neither intracellular fibrils nor fibripositors (collagen fibril-containing plasma membrane protrusions) were observed. The results show that intracellular collagen occurs in murine tendon in the absence of intracellular fibrillogenesis and fibripositor formation. Furthermore, the results show that murine postnatal tendon cells have a high capacity to prevent intracellular collagen fibrillogenesis.