Role Of BMPRIA In Mesenchymal Stromal Cells In Bone Remodeling

The bone undergoes life-long remodeling,which is carried out by bone forming osteoblasts and bone resorbing osteoclasts.Bone mass and quality can be regulated at the level of osteoblastogenesis/bone formation,osteoclastogenesis/bone resorption,and coupling between bone formation and resorption.Some osteogenic regulators,e.g.,transforming growth factor-?1(TGF-?1),bone morphogenetic proteins(BMPs),and insulin growth factor-1(IGF-1),released from bone matrix after resorption or synthesized by osteoclasts are important coupling factors.Besides,osteoblasts produce some of the master regulators of osteoclastogenesis,e.g.,macrophage colony stimulating factor(M-CSF),receptor activator of nuclear factor-kappaB ligand(RANKL),and osteoprotegerin(OPG).Disruption of the coupling between bone formation and resorption can cause bone-related diseases.BMPs activate the canonical Smadl/5/8 and non-canonical Takl-MAPK pathways via BMP receptors ? and ? to regulate skeletal development and bone remodeling.Specific ablation of Bmpr 1a in immature osteoblasts,osteoblasts,or osteocytes results in an increase in trabecular bone mass,yet opposite results have been reported regarding the underlying mechanisms Moreover,the role for BMPRIA-mediated signaling in mesenchymal stromal cells(MSCs)has not been explored.Here,we specifically ablated Bmprla in MSCs in adult mice to study the function of BMPR1A in bone remodeling and found that the mutant mice showed an increase in trabecular and cortical bone mass,which was accompanied by a decrease in bone formation rate and a greater decrease in bone resorption rate.Decreased bone formation was associated with a defect in MSC osteogenic differentiation whereas decreased bone resorption was associated with a decrease in RANKL production and osteoclastogenesis However,ablation of Takl,a critical non-canonical signaling molecule downstream of BMP receptors,in MSCs at adult stage did not affect bone remodeling.These results suggest that BMP signaling through BMPRIA controls MSC osteogenic differentiation/bone formation and RANKL expression/osteoclastogenesis in adult mice independent of Takl signalingThe Notch signaling pathway plays vital roles in the regulation of cell proliferation,differentiation and cell fate determination in many tissues and cell types.This study aimed to examine the effects of Notch1 ablation on corneal homeostasis and wound healing by utilizing the Twist2-Cre;Notch1f/f mice model in which Notch1 gene was deleted in corneal stroma.Limbal stem cells(LSCs),located in the basal epithelial layer of the limbus,are in charge of corneal homeostasis and repair.Yet,how LSC self-renewal and differentiation are controlled remains largely elusive.Here,we showed that the corneal stromal layer can be labelled by MSC marker Twist2 using Twist2-Cre;Rosa-td Tomato reporter mice.These cells are derived from the neural crest during embryo development,and can be renewed by stromal stem cells.Depletion of these cells can lead to a limbal stem cell deficiency,a severe condition in which the cornea losses its ability to regenerate by itself.While previous studies mainly focused on functions of Notch signaling in corneal epithelial and retinal cells,the present study checked the roles of this signaling in the stromal layer.Ablation of Notch1 in corneal stroma led to an abnormal corneal morphogenesis including overgrowth of the stromal and epithelial layer at 7 weeks of age and eventual formation of plaque onto central cornea and loss of vision.The corneal epithelium layer of mutant mice showed a shift from corneal epithelium to skin-like epidermis,and vascularization which did not involve inflammation.Moreover,the Notch1-deficient mice showed slow regeneration following alkali-induced injury when compared with age-and sex-matched control littermates.To test the molecular mechanism that might play role in developing fibrosis of stromal layer and overgrowth of epithelial layer in absence of Notch1,we checked Wnt/?-catenin-mediated signaling pathway.To do so,we performed immunofluorescence staining for ?-Catenin and quantitative q RT-PCR for several Wnt genes.We found in mutant mice that corneal epithelial and limbal stem cells showed an increase in the activation of ?-Catenin and corneal stroma showed a significant upregulation of expression of the Wnt genes.Therefore,the results of this study suggest that stromal cells could regulate corneal homeostasis via secreting Wnt molecules.Thus,Notch1 might inhibit the expression of Wnt/?-Catenin signaling pathway which could be involved in phenotype development of Notch1-deficient mice.