Patterning the spine

Vertebral pattern and morphology is determined during embryonic segmentation

BACKGROUND

The segmented nature of the adult vertebral column is based on segmentation of the paraxial mesoderm during early embryogenesis. Disruptions to embryonic segmentation, whether caused by genetic lesions or environmental stress, result in adult vertebral pathologies. However, the mechanisms linking embryonic segmentation and the details of adult vertebral morphology are poorly understood.

RESULTS

We induced border defects using two approaches in zebrafish: heat stress and misregulation of embryonic segmentation genes tbx6, mesp-ba, and ripply1. We assayed vertebral length, regularity, and polarity using microscopic and radiological imaging. In population studies, we find a correlation between specific embryonic border defects and specific vertebral defects, and within individual fish, we trace specific adult vertebral defects to specific embryonic border defects.

CONCLUSIONS

Our data reveal that transient disruptions of embryonic segment border formation led to significant vertebral anomalies that persist through adulthood. The spacing of embryonic borders controls the length of the vertebra. The positions of embryonic borders control the positions of ribs and arches. Embryonic borders underlie fusions and divisions between adjacent spines and ribs. These data suggest that segment borders have a dominant role in vertebral development.

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.

Chordacentrum mineralization is delayed in zebrafish betaglycan-null mutants

BACKGROUND

The Transforming Growth Factor β (TGFβ) family is a group of related proteins that signal through a type I and type II receptors. Betaglycan, also known as the type III receptor (Tgfbr3), is a coreceptor for various ligands of the TGFβ family that participates in heart, liver and kidney development as revealed by the tgfbr3-null mouse, as well as in angiogenesis as revealed by Tgfbr3 downregulation in morphant zebrafish.

RESULTS

Here, we present CRISPR/Cas9-derived zebrafish Tgfbr3-null mutants, which exhibited unaltered embryonic angiogenesis and developed into fertile adults. One reproducible phenotype displayed by these Tgfbr3-null mutants is delayed chordacentra mineralization, which nonetheless does not result in vertebral abnormalities in the adult fishes. We also report that the canonical TGFβ signaling pathway is needed for proper chordacentra mineralization and that Tgfbr3 absence decreases this signal in the notochordal cells responsible for this process.

CONCLUSION

Betaglycan's "ligand presentation" function contributes to the optimal TGFβ signaling required for zebrafish chordacentra mineralization.

Heads and tails: The notochord develops differently in the cranium and caudal fin of Atlantic Salmon (Salmo salar, L.)

While it is well known that the notochord of bony fishes changes over developmental time, less is known about how it varies across different body regions. In the development of the Atlantic salmon, Salmo salar L., cranial and caudal ends of the notochord are overlaid by the formation of the bony elements of the neurocranium and caudal fin, respectively. To investigate, we describe how the notochord of the cranium and caudal fin changes from embryo to spawning adult, using light microscopy, SEM, TEM, dissection, and CT scanning. The differences are dramatic. In contrast to the abdominal and caudal regions, at the ends of the notochord vertebrae never develop. While the cranial notochord builds a tapering, unsegmented cone of chordal bone, the urostylic notochordal sheath never ossifies: adjacent, irregular bony elements form from the endoskeleton of the caudal fin. As development progresses, two previously undescribed processes occur. First, the bony cone of the cranial notochord, and its internal chordocytes, are degraded by chordoclasts, an undescribed function of the clastic cell type. Second, the sheath of the urostylic notochord creates transverse septae that partly traverse the lumen in an irregular pattern. By the adult stage, the cranial notochord is gone. In contrast, the urostylic notochord in adults is robust, reinforced with septae, covered by irregularly shaped pieces of cellular bone, and capped with an opistural cartilage that develops from the sheath of the urostylic notochord. A previously undescribed muscle, with its origin on the opistural cartilage, inserts on the lepidotrich ventral to it.

Notochordal Signals Establish Phylogenetic Identity of the Teleost Spine

The spine is a defining feature of the vertebrate body plan. However, broad differences in vertebral structures and morphogenetic strategies occur across vertebrate groups, clouding the homology between their developmental programs. Analysis of a zebrafish mutant, spondo, whose spine is dysmorphic, prompted us to reconstruct paleontological evidence, highlighting specific transitions during teleost spine evolution. Interestingly, the spondo mutant recapitulates characteristics present in basal fishes, not found in extant teleosts. Further analysis of the mutation implicated the teleost-specific notochord protein, Calymmin, as a key regulator of spine patterning in zebrafish. The mutation in cmn results in loss of notochord sheath segmentation, altering osteoblast migration to the developing spine, and increasing sensitivity to somitogenesis defects associated with congenital scoliosis in amniotes. These data suggest that signals from the notochord define the evolutionary identity of the spine and demonstrate how simple shifts in development can revert traits canalized for about 250 million years.

Notochord vacuoles absorb compressive bone growth during zebrafish spine formation

The vertebral column or spine assembles around the notochord rod which contains a core made of large vacuolated cells. Each vacuolated cell possesses a single fluid-filled vacuole, and loss or fragmentation of these vacuoles in zebrafish leads to spine kinking. Here, we identified a mutation in the kinase gene dstyk that causes fragmentation of notochord vacuoles and a severe congenital scoliosis-like phenotype in zebrafish. Live imaging revealed that Dstyk regulates fusion of membranes with the vacuole. We find that localized disruption of notochord vacuoles causes vertebral malformation and curving of the spine axis at those sites. Accordingly, in dstyk mutants the spine curves increasingly over time as vertebral bone formation compresses the notochord asymmetrically, causing vertebral malformations and kinking of the axis. Together, our data show that notochord vacuoles function as a hydrostatic scaffold that guides symmetrical growth of vertebrae and spine formation.

New Insights into the Control of Cell Fate Choices and Differentiation by Retinoic Acid in Cranial, Axial and Caudal Structures

Retinoic acid (RA) signaling is an important regulator of chordate development. RA binds to nuclear RA receptors that control the transcriptional activity of target genes. Controlled local degradation of RA by enzymes of the Cyp26a gene family contributes to the establishment of transient RA signaling gradients that control patterning, cell fate decisions and differentiation. Several steps in the lineage leading to the induction and differentiation of neuromesodermal progenitors and bone-producing osteogenic cells are controlled by RA. Changes to RA signaling activity have effects on the formation of the bones of the skull, the vertebrae and the development of teeth and regeneration of fin rays in fish. This review focuses on recent advances in these areas, with predominant emphasis on zebrafish, and highlights previously unknown roles for RA signaling in developmental processes.