| The integrity of bone is essential for normal function throughout life, because it provides a hard matrix on which the soft tissues can operate; facilitating protection of body organs, storage of micronutrients, and locomotion. Bone itself begins as a soft tissue secreted by the bone forming cells, the osteoblasts: these cells secrete the soft matrix and then regulate its mineralization to the hard skeleton. However, before mineralization can occur, the soft matrix, or "osteoid" secreted by the osteoblasts must be of proper microstructure such that the mature bone is formed on an underlying matrix that can withstand the tensile forces and storage requirements that will be placed upon it. Crosslinking of the extracellular matrix components by a class of enzymes, the transglutaminases, is thought to play an important role in assuring the quality of osteoid before mineralization occurs. The transglutaminases are a family of calcium-dependent enzymes that can stabilize protein networks by covalent crosslinks between glutamine side chains and lysines or other primary amines. Previous studies have indicated that the transglutaminases TG2 and FXIIIa are expressed in bone, and that they are capable of crosslinking components of the osteoid such as fibronectin and collagen in vitro. This study sought to identify elements regulating transglutaminase expression and cross-linking activity in maturing cultures of mouse calvarial preosteoblasts. The data confirm previous findings that FXIIIa, but not TG2, is externalized in differentiating osteoblasts. Newly realized is that this externalization is due to concurrent collagen type I secretion and deposition, and not due to direct affects of AA. Furthermore, externalization is required by the MAPK-pathway, but is apparently not mediated through collagen signaling via its receptor, the alpha 2beta1 integrin. In addition, collagen was found to not be a glutamine substrate donor for the transglutaminase-mediated crosslinking reaction, although fibronectin was. These data imply that although FXIIIa externalization is secondary to collagen secretion and that these two processes are linked in space and temporally, an alternative pathway to the primary collagen signaling pathway must feedback on the regulatory machinery of FXIIIa expression and localization. |
