Academic literature on the topic 'Spadetail'

Inhibition of fibroblast growth factor (FGF) signaling prevents trunk and tail formation in Xenopus and zebrafish embryos. While the T-box transcription factor Brachyury (called No Tail in zebrafish) is a key mediator of FGF signaling in the notochord and tail, the pathways activated by FGF in non-notochordal trunk mesoderm have been uncertain. Previous studies have shown that the spadetail gene is required for non-notochordal trunk mesoderm formation; spadetail mutant embryos have major trunk mesoderm deficiencies, but relatively normal tail and notochord development. We demonstrate here that spadetail encodes a T-box transcription factor with homologues in Xenopus and chick. Spadetail is likely to be a key mediator of FGF signaling in trunk non-notochordal mesoderm, since spadetail expression is regulated by FGF signaling. Trunk and tail development are therefore dependent upon the complementary actions of two T-box genes, spadetail and no tail. We show that the regulatory hierarchy among spadetail, no tail and a third T-box gene, tbx6, are substantially different during trunk and tail mesoderm formation, and propose a genetic model that accounts for the regional phenotypes of spadetail and no tail mutants.

Cell fate decisions in early embryonic cells are controlled by interactions among developmental regulatory genes. Zebrafish floating head mutants lack a notochord; instead, muscle forms under the neural tube. As shown previously, axial mesoderm in floating head mutant gastrulae fails to maintain expression of notochord genes and instead expresses muscle genes. Zebrafish spadetail mutant gastrulae have a nearly opposite phenotype; notochord markers are expressed in a wider domain than in wild-type embryos and muscle marker expression is absent. We examined whether these two phenotypes revealed an antagonistic genetic interaction by constructing the double mutant. Muscle does not form in the spadetail;floating head double mutant midline, indicating that spadetail function is required for floating head mutant axial mesoderm to transfate to muscle. Instead, the midline of spadetail;floating head double mutants is greatly restored compared to that of floating head mutants; the floor plate is almost complete and an anterior notochord develops. In addition, we find that floating head mutant cells can make both anterior and posterior notochord when transplanted into a wild-type host, showing that enviromental signals can override the predisposition of floating head mutant midline cells to make muscle. Taken together, these results suggest that repression of spadetail function by floating head is critical to promote notochord fate and prevent midline muscle development, and that cells can be recruited to the notochord by environmental signals.

Zebrafish paraxial protocadherin (papc) encodes a transmembrane cell adhesion molecule (PAPC) expressed in trunk mesoderm undergoing morphogenesis. Microinjection studies with a dominant-negative secreted construct suggest that papc is required for proper dorsal convergence movements during gastrulation. Genetic studies show that papc is a close downstream target of spadetail, gene encoding a transcription factor required for mesodermal morphogenetic movements. Further, we show that the floating head homeobox gene is required in axial mesoderm to repress the expression of both spadetail and papc, promoting notochord and blocking differentiation of paraxial mesoderm. The PAPC structural cell-surface protein may provide a link between regulatory transcription factors and the actual cell biological behaviors that execute morphogenesis during gastrulation.

We describe the isolation of the zebrafish MyoD gene and its expression in wild-type embryos and in two mutants with altered somite development, no tail (ntl) and spadetail (spt). In the wild-type embryo, MyoD expression first occurs in an early phase, extending from mid-gastrula to just prior to somite formation, in which cells directly adjacent to the axial mesoderm express the gene. In subsequent phases, during the anterior-to-posterior wave of somite formation and maturation, expression occurs within particular regions of each somite. In spt embryos, which lack normal paraxial mesoderm due to incorrect cell migration, early MyoD expression is not observed and transcripts are instead first detected in small groups of trunk cells that will develop into aberrant myotomal-like structures. In ntl embryos, which lack notochords and tails, the early phase of MyoD expression is also absent. However, the later phase of expression within the developing somites appears to occur at the normal time in the ntl mutants, indicating that the presomitogenesis and somitogenesis phases of MyoD expression can be uncoupled. In addition, we demonstrate that the entire paraxial mesoderm of wild-type embryos has the potential to express MyoD when Sonic hedgehog is expressed ubiquitously in the embryo, and that this potential is lost in some of the cells of the paraxial mesoderm lineage in no tail and spadetail embryos. We also show that MyoD expression precedes myogenin expression and follows or is coincident with expression of snaill in some regions that express this gene.

The dorsal longitudinal ascending (DoLA) interneurons are an uncommon, seemingly irregularly distributed interneuron type of the developing embryonic zebrafish spinal cord. For reasons not yet understood DoLA interneurons express tbx16, a T-box transcription factor originally recognised for its important role in mesodermal development. This is the only cell type expressing tbx16 in the developing spinal cord, making DoLA neurons one of the few neuronal types that can be identified by expression of a unique molecular marker. Throughout the natural world regularity in pattern formation is frequent; mechanisms that direct the production of regular patterns have been studied and many are well understood. The creation of irregular "patterns", especially in embryo development has been subjected to far less analysis. This is largely because studies in developmental biology frequently involve methods that disrupt regular patterning while the disruption of an irregular pattern is likely to result in similarly irregular pattern. The DoLA interneurons with their unique genetic marker offer a rare opportunity to investigate the mechanisms behind irregular patterns in development. This is of particular importance in the development of the spinal cord, as most of the known vertebrate spinal interneurons appear to have irregular distributions. The main focus of the research presented in this thesis has been to try to understand how the distribution pattern of DoLA interneurons is generated. This knowledge may then be extended to other spinal neurons and possibly to other irregular developmental patterns. In the work described in this thesis the distribution of DoLA interneurons has been extensively examined statistically. It was found that there is an underlying cryptic organisation to their peculiar distribution. This led to the surprising discovery that these neurons migrate rostrally a significant distance along the spinal cord. These neurons were also found in larval zebrafish at much older times than has previously been described, suggesting that they may play a role in post-embryonic stages. Notch signalling appears to have an influence on DoLA interneuron distribution since perturbing Presenilin (Psen) function affects the number of these cells. Interestingly, DoLA cell number is not affected when Psen1 function is inhibited but increases when Psen2 function is inhibited. Furthermore the wild type level of DoLA interneuron number can be partially rescued by inhibiting Psen1 function in combination with inhibition of Psen2 function. The creation of transgenic zebrafish lines where GFP is transcribed from tbx16 promoter sequence is described. These animals were produced to attempt to discover more about the patterning of DoLA interneurons and the function of tbx16 during development. Serendipitously, one of these transgenic lines expresses GFP in the commissural primary ascending (CoPA) interneurons. This led to the discovery that the CoPA interneurons are marked by mafba/valentino, revealing a new unique spinal neuron molecular marker.
Thesis (Ph.D.) -- University of Adelaide, School of Molecular and Biomedical Science, 2011