Role of NO/cGMP pathway in endochondral bone formation

Previous work from our laboratory investigating the role of nitric oxide (NO) in growth plate chondrocytes demonstrated that NO enhanced alkaline phosphatase activity and collagen type X expression, which are two important markers of maturation. These effects were cGMP dependent. In addition, once chondrocytes became hypertrophic, NO induced cell death through a cGMP-independent pathway. Therefore, depending on the state of differentiation of cells, NO can act through distinct pathways and have different effects on chondrocytes. Objectives: The objective of this investigation is to expand these in vitro observations and determine the role of NO in endochondral bone formation using an organ culture approach. Methods: Tibia were removed from embryonic day 15.5 mice and cultured for one week in the absence or presence of different inhibitors/inducers of the NO/cGMP pathway. Whole limb measurements, histological studies, micro CT, and FTIR analyses were completed to determine the effects of these agents on endochondral bone development and growth. Results: Data shows that most of the inducers and inhibitors of the NO/cGMP pathway affected the growth rate of the mouse embryo tibia. We observed changes in the mineral content and the bone volume fraction in response to these pharmacological manipulations. Micro CT analysis reveals that the inhibitors/inducers of the pathway change the width, and shape of the limbs when compared to the control. Histological analyses show that these changes reflect alterations in the number of proliferating and hypertrophic chondrocytes in the treated limbs. Conclusion: The NO/cGMP pathway plays a major role during endochondral bone development and growth. Such pathway may offer novel therapeutic targets for growth anomalies. This work was supported by NIDCR (5K08DE017426).