Expression of transforming growth factor-β-responsive smads in cranial suture development and closure

The transforming growth factor-β (TGF-β) family of extracellular signaling molecules is heavily involved in developmental events, including patterning, formation, maintenance, and closure of the cranial suture. Several studies have demonstrated that TGF-βs are temporally and spatially localized to the suture and play a pivotal role in sutural state. These signals are translated into intracellular activity through a family of proteins known as smads. There are 8 known smads, with smads 1, 5, and 8 transducing BMP signals and smads 2 and 3 transducing TGF-β signals. Dimerization of any of these smads and smad 4 is necessary for phosphorylation and activation. Although many studies have delineated the presence of TGF-β during suture closure, no studies have determined smad activity. It was hypothesized that smad activity would change during sutural closure. Reverse transcription-polymerase chain reaction was used to determine whether TGF-β-responsive smads were present in the suture at which point they were immunohistochemically localized. A rat model was used in which the posterior intrafrontal suture fused during neonatal days 16 to 22. Time points before and after this event were analyzed for changes in smad expression and function. It was determined from these experiments that (1) the TGF-β-responsive smads 2, 3, and 4 are all present in the suture; (2) smads 2 and 4 are distributed in the region of the osteogenic front of the suture; and (3) smad 2/4 activity decreases in areas in which presumptive bone will form. These results add to the knowledge present about sutural development and may provide news targets to which therapeutics can be developed.