Gα_s signaling controls intramembranous ossification during cranial bone development by regulating both Hedgehog and Wnt/β-catenin signaling

How osteoblast cells are induced is a central question for understanding skeletal formation. Abnormal osteoblast differentiation leads to a broad range of devastating craniofacial diseases. Here we have investigated intramembranous ossification during cranial bone development in mouse models of skeletal genetic diseases that exhibit craniofacial bone defects. The GNAS gene encodes Gαs that transduces GPCR signaling. GNAS activation or loss-of-function mutations in humans cause fibrous dysplasia(FD) or progressive osseous heteroplasia(POH) that shows craniofacial hyperostosis or craniosynostosis, respectively. We find here that, while Hh ligand-dependent Hh signaling is essential for endochondral ossification, it is dispensable for intramembranous ossification, where Gαsregulates Hh signaling in a ligand-independent manner. We further show that Gαscontrols intramembranous ossification by regulating both Hh and Wnt/β-catenin signaling. In addition, Gαsactivation in the developing cranial bone leads to reduced ossification but increased cartilage presence due to reduced cartilage dissolution, not cell fate switch. Small molecule inhibitors of Hh and Wnt signaling can effectively ameliorate cranial bone phenotypes in mice caused by loss or gain of Gnas function mutations, respectively. Our work shows that studies of genetic diseases provide invaluable insights in both pathological bone defects and normal bone development, understanding both leads to better diagnosis and therapeutic treatment of bone diseases.

A hierarchical vascularized engineered bone inspired by intramembranous ossification for mandibular regeneration

Mandibular defects caused by injuries,tumors,and infections are common and can severely affect mandibular function and the patient's appearance.However,mandible reconstruction with a mandibular bionic structure remains challenging.Inspired by the process of intramembranous ossification in mandibular development,a hierarchical vascularized engineered bone consisting of angiogenesis and osteogenesis modules has been produced.Moreover,the hierarchical vascular network and bone structure generated by these hierarchical vascularized engineered bone modules match the particular anatomical structure of the mandible.The ultra-tough polyion complex has been used as the basic scaffold for hierarchical vascularized engineered bone for ensuring better reconstruction of mandible function.According to the results of in vivo experiments,the bone regenerated using hierarchical vascularized engineered bone is similar to the natural mandibular bone in terms of morphology and genomics.The sonic hedgehog signaling pathway is specifically activated in hierarchical vascularized engineered bone,indicating that the new bone in hierarchical vascularized engineered bone underwent a process of intramembranous ossification identical to that of mandible development.Thus,hierarchical vascularized engineered bone has a high potential for clinical application in mandibular defect reconstruction.Moreover,the concept based on developmental processes and bionic structures provides an effective strategy for tissue regeneration.

The role of vascular endothelial growth factor in ossification

Osteogenesis and angiogenesis are two closely correlated processes during bone growth, development, remodelling and repair. Vascular endothelial growth factor （VEGF） is an essential mediator during the process of angiogenesis. Based on an extensive literature search, which was carried out using the PubMed database and the keywords of osteogenesis, VEGF, endochondral ossification and intramembranous ossification, this manuscript reviews the role of VEGF in ossification, with emphasis on its effect in endochondral and intramembranous ossification. Osteogenesis and angiogenesis are closely correlated processes. VEGF acts as an essential mediator durin~ these processes. It not only functions in bone an^io^enesis but also in various aspects of bone develooment.

Molecular mechanisms of intermuscular bone development in fish:a review

Intermuscular bones(IBs)are slender linear bones embedded in muscle,which ossify from tendons through a process of intramembranous ossification,and only exist in basal teleosts.IBs are essential for fish swimming,but they present a choking risk during human consumption,especially in children,which can lead to commercial risks that have a negative impact on the aquaculture of these fish.In this review,we discuss the morphogenesis and functions of IBs,including their underlying molecular mechanisms,as well as the advantages and disadvantages of different methods for IB studies and techniques for breeding and generating IB-free fish lines.This review reveals that the many key genes involved in tendon development,osteoblast differentiation,and bone formation,e.g.,scxa,msxC,sost,twist,bmps,and osterix,also play roles in IB development.Thus,this paper provides useful information for the breeding of new fish strains without IBs via genome editing and artificial selection.

Skeletal stem cells in bone development,homeostasis,and disease

Tissue-resident stem cells are essential for development and repair,and in the skeleton,this function is fulfilled by recently identified skeletal stem cells(SSCs).However,recent work has identified that SSCs are not monolithic,with long bones,craniofacial sites,and the spine being formed by distinct stem cells.Recent studies have utilized techniques such as fluorescence-activated cell sorting,lineage tracing,and single-cell sequencing to investigate the involvement of ssCs in bone development,homeostasis,and disease.These investigations have allowed researchers to map the lineage commitment trajectory of ssCs in different parts of the body and at different time points.Furthermore,recent studies have shed light on the characteristics of ssCs in both physiological and pathological conditions.This review focuses on discussing the spatiotemporal distribution of ssCs and enhancing our understanding of the diversity and plasticity of ssCs by summarizing recent discoveries.