The E11 osteoblastic lineage marker is differentially expressed during fracture healing

Given the complexity of the fracture healing process and the involvement of a multitude of cells, we hypothesize that a very large number of genes would be transcriptionally regulated during the repair process. To identify genes that play a role during bone regeneration in cellular events, such as proliferation, migration, adhesion, and differentiation, we employed differential display and compared mRNA populations isolated from postfracture (PF) day 3 calluses to those of intact (contralateral) rat femurs. One such gene whose expression was upregulated at PF day 3 is identified as the osteoblastic lineage marker, E11 antigen. E11 is a cell membrane protein localized predominantly on osteoblasts and osteocytes. In this study we show that E11 mRNA expression is consistently upregulated during fracture repair, with elevated levels (tenfold) seen as early as PF day 3. These high levels of expression were maintained throughout all of the early stages of repair examined (PF day 3-21 calluses) and paralleled those of another osteoblastic marker, osteopontin. Similarly, high protein levels were detected throughout the reparative phase of the callus, particularly in osteoblasts, preosteocytes, and osteocytes, localized within the newly made osteoid. No labeling was detected in fibroblasts, proliferating chondrocytes, or hypertrophic chondrocytes, consistent with previous results. Taken together, these results suggest that the E11 antigen is indeed a suitable marker for both osteoblasts and osteocytes and that it plays a significant role in osteogenesis.