Gli1+ Mediated Bone Repair Mechanisms: The Role of Gli1+ Mesenchymal Stem Cells within Calvarial Sutures in Craniofacial Bone Repair
Introduction. Craniofacial bones, including the bones of the face and calvarium, develop via intramembranous ossification due to the presence of little bone marrow1,2. Undifferentiated mesenchyme intervenes between calvarial bones to form sutures that remain active as sites of bone formation1,2. A critical sized defect will not heal without intervention. Current repair mechanisms include autologous and alloplastic sources but have limitations involving tissue morbidity and limited integration ability2. New repair mechanisms are needed2. Gli1+ cells in the suture mesenchyme of adult craniofacial bones have been identified3. Gli1 is the primary transcriptional activator of the hedgehog signaling pathway and has been linked to bone regeneration4. Gli1+ cells found in the suture mesenchyme provide a unique niche that may be implicated in bone repair. Methods. Gli1- LacZ, Gli1-CreERT2; R26- ZsGreenflox or Gli1-CreERT2; R26-tdTomatoflox, and CD1 mice were used3,5. To identify Gli-expressing cells, LacZ staining was done at postnatal days 0, 7, 14 and months 1 and 33. For time-dependent identification of Gli1+ cells, fluorescence was induced with tamoxifen at 1 month3. Gli1+ cells in the sagittal suture were detected at 5 days and 1, 2 and 8 months3. 2mm wide injury sites were drilled on suture, 0.5, 1 and 2 mm from suture in 6-8 week old CD1 mice. Healing was assessed after 1 month of injury5. Gli1-CreERT2; R26-tdTomatoflox mice were induced with tamoxifen at 1 month5. An injury was made 1mm from suture and was assessed after 2 weeks. Results. On postnatal day 0, Gli1+ cells were found throughout the periosteum, dura, and suture3. By one month of age Gli1+ cells were only found within the suture mesenchyme3. 5 days after induction with tamoxifen Gli1+ cells were detectable only within suture mesenchyme3. The presence of fluorescently tagged cells increased until the entire calvaria was labeled at 8 months3. 1-month post 2mm injury, the suture site was 100% healed while sites 0.5, 1, and 2 mm away were 80%, 50% and 20% healed respectively5. Fluorescently tagged Gli1+ cells were found between the suture and injury site made 1mm away5. Conclusions. Gli1+ cells are found within the sutures of calvarial bone and act as mesenchymal stem cells in post-natal mice3. The healing capacity of an injury site is inversely proportional to the distance from the suture5. Gli1+ cells can migrate from the cranial suture to other injury sites during bone regeneration5. Suture Gli1+ mesenchymal stem cells or Gli-engineered cells could be used in the future repair of critical sized calvarial defects.
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