CORDIS - EU research results



Bone is a highly dynamic tissue that is constantly dismantled and rebuilt, a process called bone remodeling. Maintenance of bone mass depends on the balanced activities of bone-resorbing osteoclasts (OCs) and bone-forming osteoblasts (OBs). Any imbalance of this system may cause a gain- or loss of bone mass. Aging-related bone loss and the resulting osteoporosis with its debilitating fragility fractures affect millions of people worldwide, representing a prevailing and further increasing medical and socio-economic challenge. We identified the gene 5’-TG-3’ interacting factor (Tgif) as being induced during OB differentiation, suggesting an important physiological role in OB function and bone formation. Indeed, mice in which Tgif had been deleted in the germ-line or specifically within the OB lineage demonstrated a balanced low turnover phenotype with bone formation and bone resorption as well as the numbers of OBs and OCs being equally reduced.
Since OB activity is critical for bone regeneration, we determined a potential role of Tgif in fracture repair. Briefly, long bones of Tgif-KO mice were fractured and stabilized using an intramedullary pin. Compared to wild-type mice, Tgif-KO mice demonstrated a reduced callus formation, a less consolidated fracture gap with abundant connective tissue, and only very little woven bone. Furthermore, the newly formed trabeculae were sparsely occupied by very few matrix-producing OBs, which resulted probably from an impaired OB migration.
These results suggest a functional role of Tgif in OB adhesion and migration. In support of this hypothesis, absence of Tgif in primary calvarial OBs resulted in a decreased adhesion to fibronectin- and collagen-I- coated surfaces, while Tgif expression greatly increased in wild-type calvarial OBs during adhesion in a time-dependent manner. In addition, osteoblastic cells over-expressing Tgif showed an improved migratory activity in an in vitro wound-healing assay. In primary calvarial OBs the velocity and the track length were reduced while the meandering index was increased compared to control as determined by live cell video microscopy. Furthermore, in a transwell assay Tgif-deficient OBs showed a reduced ability to pass the membrane.
In summary, our preliminary results demonstrate that Tgif has an important role in regulating bone remodeling since it is regulated during OB differentiation and since its absence impairs OB differentiation. Furthermore, deletion of Tgif, both globally and OB-specifically, results in a balanced low turnover phenotype. Furthermore, our data strongly suggest that Tgif plays an important role in fracture repair since it is required for proper OB adhesion and migration. Thus, we aim to investigate the underlying molecular mechanisms using in vitro and in vivo approaches, including a mouse model for in vivo cell lineage tracing and in vitro confocal microscopy to analyze the organization of the actin cytoskeleton in Tgif-deficient OBs. The proposed work will likely reveal new avenues for the future clinical care of aging-related bone loss by being potentially beneficial for patients suffering from osteoporotic bone loss and fractures (