The Role Of ACVR1 In Regulating Odontoblast Polarization And Dentin Formation And Its Mechanism

Dentin,an important component of teeth,can bear chewing pressure,protect dental pulp and conduct sensations.The realization of these physiological and mechanical functions of dentin depends on the regularly arranged tubular dentin.In the bell stage of tooth germ,with the induction of enamel epithelium,the outer cells of dental papilla retreat from the cell cycle and become elongated.The nuclei move away from the basement membrane,and the odontoblast processes form near the basement membrane,with the organelles asymmetrically distributed.Then the odontoblasts establish cell polarity,differentiating into mature odontoblasts with the ability of secretion.Mature odontoblasts secrete matrix which further mineralizes to dentin.Cell polarity refers to the asymmetric distribution of cell morphology and contents along one or more axes.The process by which cells gradually become polar is called cell polarization.The polarization of odontoblasts is an important part of their differentiation process,which marks the change of cell morphology and function,and is the basis for the formation of regular dentin.Only when the tubular dentin is formed,the dentin fluid and nerve fibers can fill the regular dentin tubules,so that the teeth can have physiological functions such as sensory stimulation.In addition,the tubular structure of dentin provides greater mechanical support for the entire tooth.Therefore,the polarization of odontoblasts is of great significance to the formation of regular dentin and the subsequent normal functioning of the teeth.A variety of signaling pathways are involved in the differentiation of odontoblasts and the formation of dentin.Among them,bone morphogenetic protein(BMP)signaling pathway plays a crucial role.Conditional knockout of Smad4 gene(using Osr2-Cre)in BMP signaling pathway can lead to the suppression of the differentiation of odontoblasts and the disordered formation of dentin in mandibular molars in mice.In the BMP signaling pathways,type I BMP receptor plays a key role in signal transduction.Activin A receptor 1,ACVR1,is one of type I BMP receptors.Knockout of Acvr1 in the neural crest cells,results in a variety of phenotypes of craniofacial deformities in mice.Previous studies in our group showed that ACVR1 is expressed in dental mesenchyme.Deletion of Acvr1(Osterix-Cre)in the dental mesenchymal cells of mice results in the dentin formation defects of mandibular incisor at 21 days after birth.However,the specific role of ACVR1 in the polarization of odontoblast and the formation of dentin is still unclear.To study the effects of ACVR1 on odontoblast polarization and dentin formation,a mesenchymal specific Acvr1 knockout mouse model was established using the Cre-LoxP system.Mice genotyped Osterix-Cre;Acvr1 fx/-were designated as the experimental group,whereas Osterix-Cre;Acvr1 fx/+ were used as the control group.New born mice were harvested.HE staining was used to observe the formation of dentin,and the morphology of odontoblasts in the mandibular incisors.The arrangement of dentin collagenous fibers was observed by Sirius Red staining.Immunofluorescent staining was used to detect the localization of GM130 in the odontoblasts.The results of HE staining showed that the dentin of the mandibular incisors in the experimental group was disorganized,with osteodentin formation at the cusp area,compared with the control group.The odontoblasts in the incisor cervical loop were polygonal,and the shape of odontoblasts in the middle of the incisor(the initiation of dentin formation)were columnar and exhibited cell polarity,which were similar to the control group.However,at the incisor cusp,some of the odontoblasts were polygonal and lost cell polarity.The results of Sirius Red staining showed that the collagenous fibers in the dentin within the cusp area were disorganized in the experimental group,compared with the control group.The results of immunohistochemical staining showed that the relative position of Golgi apparatus to the nucleus in the odontoblasts of the experimental group was changed,compared with the control group.In conclusion,BMP type I receptor ACVR1 can maintain odontoblasts polarity and promote the formation of organized dentin.ACVR1 plays an important role in regulating dentin formation.