The Role And Mechanism Of Hedgehog Pathway In Mesenchymal Stem Cells On Bone Development And Bone Tumor Formation

Bone tumors are common primary tumors,of which chondromas and osteosarcoma are the two most common tumors in the skeletal system.These two kinds of tumors usually occur at the long bones,which not only affects the daily life of the patients,but also is prone to metastasis.Because it is not sensitive to traditional radiotherapy and chemotherapy,surgical resection is currently the only effective treatment for this type of tumor.Therefore,research about the molecular and cellular mechanisms of its occurrence and development will help to develop safer and more effective personalized treatments.Previous studies have found that mutations in a variety of genes can directly lead to the development of chondroma or osteosarcoma.However,the exact cell origin and genetic mutations involved in different tumors remain to be elucidated.The Hedgehog signaling pathway is a class of evolutionarily conserved signaling pathways that play an important role in embryonic pattern development and organogenesis,as well as in the regulation of adult stem cell maintenance and tissue homeostasis after birth.Many studies have shown that Abnormal activation of hedgehog pathway can also be involved in a variety of neoplastic diseases.Over activation of the hedgehog signaling pathway was observed during the development and deterioration of bone tumors caused by mutations in different genes.Although a large number of studies have confirmed that the hedgehog signaling pathway itself plays an important role in embryonic bone and cartilage development,its direct role in bone tumors is still not fully understood.Here we use Prrx1-CreERT;Ptch1^f/f mice to study the functions of hedgehog signaling in mesenchymal stem cells?MSCs?and bone tumor development during adolescence.The results showed that the specific activation of hedgehog signaling pathway in mesenchymal stem cells induced the occurrence of chondroma and osteosarcoma in the long bone of mutant mice,and the occurrence of tumor was accompanied by morphological abnormalities of articular joints and osteoarthritic-like lesions.By lineage tracing experiments,we confirmed that chondromas and osteosarcoma in mutant mice were all derived from Prrx1⁺cells.Further molecular mechanism experiments showed that Ptch1 knockout mutant mice activated Wnt signaling by enhancing the binding of hedgehog downstream transcription factor Gli1 to Wnt5a and Wnt6 promoters,thereby promoting the proliferation and abnormal differentiation of mesenchymal stem cells.Blocking the secretion of Wnt signaling by the small molecule compound IWP2 can effectively rescue the bone tumor and joint deformity phenotype in Prrx1-CreERT;Ptch1^f/f mice.These results demonstrate that abnormally activated hedgehog signaling induces changes in the Wnt paracrine signaling pathway,causing changes in cell fate,resulting in abnormal differentiation and growth disorders of chondrocytes and osteoblasts,ultimately leading to tumorigenesis.Taken together,our study demonstrates that the abnormally activated hedgehog signaling pathway in mesenchymal cells directly leads to the simultaneous occurrence of different types of bone tumors by regulating the Wnt signaling pathway.It is suggested that Ptch1 knockout mice can be used as a good animal model for studying bone tumors.The hedgehog signaling pathway can also be used as an effective target for the treatment of bone tumors.At the same time,our research also extends the definition of Prrx1⁺cells,which are not only bone marrow internal cartilage and osteogenic progenitor cells,but also define a class of osteoblast precursors on the periosteum to understand bone tumor cells.