PLGA-based control release of Noggin blocks the premature fusion of cranial sutures caused by retinoic acid

Direct BMP signaling to chordoblasts is required for the initiation of segmented notochord sheath mineralization in zebrafish vertebral column development

The vertebral column, with the centra as its iteratively arranged building blocks, represents the anatomical key feature of the vertebrate phylum. In contrast to amniotes, where vertebrae are formed from chondrocytes and osteoblasts deriving from the segmentally organized neural crest or paraxial sclerotome, teleost vertebral column development is initiated by chordoblasts of the primarily unsegmented axial notochord, while sclerotomal cells only contribute to later steps of vertebrae formation. Yet, for both mammalian and teleostean model systems, unrestricted signaling by Bone Morphogenetic Proteins (BMPs) or retinoic acid (RA) has been reported to cause fusions of vertebral elements, while the interplay of the two signaling processes and their exact cellular targets remain largely unknown. Here, we address this interplay in zebrafish, identifying BMPs as potent and indispensable factors that, as formerly shown for RA, directly signal to notochord epithelial cells/chordoblasts to promote entpd5a expression and thereby metameric notochord sheath mineralization. In contrast to RA, however, which promotes sheath mineralization at the expense of further collagen secretion and sheath formation, BMP defines an earlier transitory stage of chordoblasts, characterized by sustained matrix production/col2a1 expression and concomitant matrix mineralization/entpd5a expression. BMP-RA epistasis analyses further indicate that RA can only affect chordoblasts and their further progression to merely mineralizing cells after they have received BMP signals to enter the transitory col2a1/entpd5a double-positive stage. This way, both signals ensure consecutively for proper mineralization of the notochord sheath within segmented sections along its anteroposterior axis. Our work sheds further light onto the molecular mechanisms that orchestrate early steps of vertebral column segmentation in teleosts. Similarities and differences to BMP's working mechanisms during mammalian vertebral column formation and the pathomechanisms underlying human bone diseases such as Fibrodysplasia Ossificans Progressiva (FOP) caused by constitutively active BMP signaling are discussed.

Altered BMP-Smad4 signaling causes complete cleft palate by disturbing osteogenesis in palatal mesenchyme

As the major receptor mediated BMP signaling in craniofacial development, Bmpr1a expression was detected in the anterior palatal shelves from E13.5 and the posterior palatal shelves from E14.5. However, inactivating BMP receptor in the mesenchyme only leads to anterior cleft palate or submucous cleft palate. The role of BMP signaling in posterior palatal mesenchyme and palatal osteogenesis is still unknown. In this study, a secreted BMP antagonist, Noggin was over-expressed by Osr2-cre^KI to suppress BMP signaling intensively in mouse palatal mesenchyme, which made the newborn mouse displaying complete cleft palate, a phenotype much severer than the anterior or submucous cleft palate. Immunohistochemical analysis indicated that in the anterior and posterior palatal mesenchyme, the canonical BMP-Smad4 signaling was dramatically down-regulated, while the non-canonical BMP signaling pathways were altered little. Although cell proliferation was reduced only in the anterior palatal mesenchyme, the osteogenic condensation and Osterix distribution were remarkably repressed in the posterior palatal mesenchyme by Noggin over-expression. These findings suggested that BMP-Smad4 signaling was essential for the cell proliferation in the anterior palatal mesenchyme, and for the osteogenesis in the posterior palatal mesenchyme. Interestingly, the constitutive activation of Bmpr1a in palatal mesenchyme also caused the complete cleft palate, in which the enhanced BMP-Smad4 signaling resulted in the premature osteogenic differentiation in palatal mesenchyme. Moreover, neither the Noggin over-expression nor Bmpr1a activation disrupted the elevation of palatal shelves. Our study not only suggested that BMP signaling played the differential roles in the anterior and posterior palatal mesenchyme, but also indicated that BMP-Smad4 signaling was required to be finely tuned for the osteogenesis of palatal mesenchyme.