The Role Of Wnt Signaling In Bone Fibrosis

Organ fibrosis is characterized by the accumulation of an extracellular matrix,permanent scarring and organ dysfunction in a number of diseases in skin,intestine,lung,liver and heart.Bone fibrosis,which is featured by deregulated new bone formation,has been observed in a number of malignancies including fibrous dysplasia,myeloid malignancies and tumor induced osteoblastic bone metastasis.It has been reported that the activation of Wnt signaling in the osteoblast precursors can promot pathologic bone fibrosis.Ectopic activation of?-catenin in osteoprogenitors by Osterix-Cre resembled the features of pathologic bone fibrosis.However,cell lineage analyses revealed that Osterix-Cre could label multiple stromal cells,indicating the cellular basis of bone fibrosis need further study in mouse model of bone fibrosis.Recent studies found that osteoblasts or osteoblast precursors can induce secondary myeloid malignancies and tumor bone metastasis with the expansion of immunosuppressive cells.However,relative little is known about the bone fibrosis mediated immunoregulation.In this study,we detected that ectopic expression of?-catenin by Col3.6-Cre displayed bone marrow fibrosis featured by dysregulated new bone formation,similar to the outcomes of activating?-catenin by Osterix-Cre.Cell lineage tracing showed that Col3.6-Cre could label a few osteoblast precursors and CD11b^-CD45^-CD115⁺Col1⁺fibrocytes,which expressed myofibroblast cell markers and profoundly expanded at trabecular and/or endosteal regions.These results indicated that bone fibrosis in our mouse model is sustained by the expansion of osteoblast precursors and fibrocytes.The Col3.6-Cre-targeted fibrocytes could be traced in the myeloid progenitors but not in osteoclast progenitors,which indicated that fibrocytes may be of myeloid origin,but are not intermediate cells of macrophage and fibroblast differentiation.We further confirmed a conserved pro-fibrotic signal cascade consisting of?-catenin/TGF?1/VEGF underlies the development of fibrocyte-mediated bone fibrosis in our mouse model.Moreover,we detected an increased frequency of CD11b⁺Gr1⁺myeloid cells,coupled with a corresponding decrease of frequency of c-Kit⁺CD11b⁺Gr1⁺undifferentiated immature myeloid cells.We also confirmed elevated frequency of CD11b^midCD11c^midF4/80^low and CD11b^midCD11c⁺F4/80⁺monocyte-derived dendritic cells at early differentiating stage in bone marrow and peripheral blood.In addition,we detected an increase of bone marrow stroma derived GM-CSF,a cytokine for differentiation of monocyte-derived dendritic cells.Furthermore,we detected an increased frequency of CD11b^midGr1^lowCD11c^mid and CD11b^midGr1^lowCD11c⁺myeloid cells featured by MHC-I^low and MHC-II^-,coupled with a corresponding decreased frequency of effector memory T cells in bone marrow and T cells in peripheral blood.It indicated that myeloid derived suppressor cells?MDSCs?such as immature monocyte-derived dendritic cells may be induced the bone fibrosis.Meanwhile,we observed an increased frequency of na?ve T cells,sustaining the antigen presenting function of fibrocytes.Taken together,our study revealed that fibrocytes-mediated bone fibrosis was involved in myeloid cell differentiation and other immunoregulation.In summary,by using Col3.6-Cre mediated activation of Wnt/?-catenin signaling,we set up another mouse model of bone fibrosis,in which the cell lineage labeled osteoblast precursors and a novel subset of monocyte-derived fibrocytes can expand as fibrotic effector cells.In addition,we also confirmed the deregulation of monocytic immature dendritic cells and the immune responses induced by the the fibrotic bone microenvironment.These results not only demonstrate the alternative targets of bone fibrosis but also the potential roles of bone marrow niche in the myeloid immune regulation.