In the fruit fly, Drosophila melanogaster, accumulation of osk mRNA at the posterior pole of the oocyte and local translation initiate assembly of the pole plasm, which is required for germ cell formation and posterior patterning of the embryo. I have used fluorescence in situ hybridization (FISH) in combination with immunofluorescence and laser scanning confocal microscopy to examine the spatial and temporal control of osk transcript localization and translation. Drosophila oocytes develop within cysts of 16 interconnected cells. One cell in each cyst differentiates to form the oocyte while the remaining cells form nurse cells that produce RNAs and proteins that are transported to the oocyte. osk mRNA is produced by the nurse cells and accumulates in the oocyte throughout oogenesis, but is only specifically localized to the posterior pole and translated during mid to late oogenesis. My studies help define distinct steps in the osk mRNA localization process. An early step in posterior localization is removal of osk mRNA from most of the cortex, leading to accumulation in the oocyte interior. This process requires microtubules, the microtubule motor protein Kinesin I, the actin binding protein Tropomyosin, and the RNA binding protein Staufen. Transcript then moves from the oocyte interior to the posterior pole through a microtubule independent process. The genes cappuccino, chickadee, spire, armitage, maelstrom, par-1 and gurken are all required for this next step in osk mRNA localization. The final capturing or tethering osk mRNA at the cortex requires an intact actin filament system, but additional components of this anchoring system remain to be identified. I also find that osk mRNA first begins to accumulate at the posterior pole during oogenesis stage 8, but protein is not detectable until stage 9. In addition, grk and par-1 mutations that block osk mRNA localization to the posterior pole and lead to transcript accumulation in the interior do not prevent translation; again, Osk protein production is only observed during stage 9 and later. These observations indicate that posterior localization is neither sufficient nor necessary to trigger osk mRNA translation, which appears to be under tight temporal control.

Ion channels and pumps play critical roles during sea urchin development including mediating the blocks to polyspermy, regulating left-right and dorsal-ventral axis specification, directing ventral PMC migration, and controlling biomineralization of the larval skeleton. We performed a screen of pharmacological ion channel inhibitors, and we chose two inhibitors to investigate further. First, we found that tricaine, a potent inhibitor of voltage-gated sodium channels (VGSCs), induces aberrant skeletal patterning in Lytechinus variegatus larvae. The larval skeleton is secreted by the primary mesenchyme cells (PMCs), which migrate within the blastocoel into a stereotypical pattern. We show that VGSC activity is required for normal PMC migration and skeletal patterning. Timed inhibitor studies identified VGSC activity as specifically required from early gastrula to the onset of late gastrula for normal skeletal patterning. Tricaine inhibits the voltage-gated sodium channel LvScn5a which is strongly expressed in the developing nervous system in pluteus larvae. We found that exogenous expression of an anesthetic-insensitive version of LvScn5a is sufficient to rescue hallmark tricaine-mediated skeletal patterning defects, demonstrating the specificity of the inhibitor. LvScn5a exhibits a ventrolateral ectodermal expression domain in gastrulating embryos that is spatiotemporally congruent with triradiate formation in the ventrolateral PMC clusters at the onset of skeletogenesis. This ectodermal territory normally expresses the patterning cue Wnt5, and we find that the expression of Wnt5 is dramatically spatially expanded by tricaine treatment. We also observe ectopic PMC clusters in tricaine-treated embryos. We found that knockdown of Wnt5 expression is sufficient to rescue tricaine-mediated skeletal patterning defects. These results are consistent with a model in which LvScn5a activity in the ventrolateral ectoderm functions to spatially restrict the expression of the ectodermal patterning cue Wnt5 that in turn induces PMC cluster formation. Together, these findings show that spatially restricted sodium channel activity regulates ectodermal cue expression that, in turn, regulates PMC differentiation and skeletal morphogenesis. Second, we show that V-type H⁺ ATPase (VHA) activity is required for specification of the dorsal-ventral (DV) axis. DV specification is controlled by the TGF-β signal Nodal that specifies the ventral territory and indirectly activates dorsal specification via induction of BMP 2/4 expression. Nodal expression occurs downstream of p38 MAPK, which is transiently, asymmetrically inactive on the presumptive dorsal side of the blastula embryo. VHA activity is required for that transient inactivation of p38 MAPK, and it is required for the subsequent spatial restriction of Nodal expression. We show that VHA inhibition is sufficient to induce global Nodal expression during the blastula stage, resulting in ventralization of the embryo. We show that this phenotype can be rescued by experimentally imposing asymmetric Nodal expression at the 4-cell stage. We discover a VHA-dependent voltage gradient across the DV axis and find that VHA activity is required for hypoxia inducible factor (HIF) activation. We show that neither hyperpolarization nor HIF activation is sufficient to perturb DV specification, which implicates a third unknown pathway connecting VHA activity and p38 MAPK symmetry breaking. These results are consistent with a model in which dorsal VHA activity is required to inhibit Nodal expression and signaling, potentially via dorsal p38 MAPK inhibition. Together, these studies demonstrate that ion channels are required for both DV specification and for normal skeletal patterning.

La formation des trois feuillets embryonnaires primordiaux et leur régionalisation selon les axes embryonnaires sont des étapes cruciales au cours du développement précoce. Dans ce contexte, nous avons montré que XDSCR6, un inducteur du mésoderme et des axes embryonnaires qui présente des propriétés dorsalisantes, interagit physiquement et fonctionnellement avec le facteur de transcription XSTAT3. Au cours des étapes précoces du développement, XSTAT3 est active pendant la gastrulation et l'activation anormale de cette protéine dans la région dorsale induit la ventralisation des tissus embryonnaires. Par ailleurs, nous avons montré que XDSCR6 et XSTAT3 présentent des rôles antagonistes in vivo au cours de la mise en place des axes embryonnaires. Cet antagonisme peut être expliqué par le fait que XDSCR6 régule négativement l'activité transcriptionnelle de XSTAT3 en modulant sa méthylation sur les résidus lysine. L'ensemble de nos résultats a permis de déterminer l'importance cruciale de cette modification post-traductionnelle dans les propriétés ventralisantes de XSTAT3. Par ailleurs, nous avons montré que la méthyltransférase XEZH2 méthyle et active XSTAT3 in vivo. Des travaux antérieurs de l'équipe avaient montré que les propriétés dorsalisantes de XDSCR6 reposent sur sa capacité à inhiber l'activité épigénétique répressive de XEZH2 sur les gènes du mésoderme dorsal. Ainsi, nos travaux suggèrent que XDSCR6 est un modulateur transcriptionnel situé à l'interface entre certains régulateurs chromatiniens et des facteurs de transcription pendant la mise en place des axes embryonnaires. / One of the most challenging questions in developmental biology is to understand how a totipotent zygote differentiates into an organism containing all cell lineages. The formation of the three germ layers and the establishment of embryonic axis are fundamental events during early development. In this context, we demonstrated that XDSCR6, a mesoderm and embryonic axis inducer that exhibits dorsalizing properties, physically and functionally interacts with the transcriptional factor XSTAT3. During early development, XSTAT3 is active throughout gastrulation step and its abnormal activation in dorsal region leads to embryonic tissues ventralization. Furthermore, we showed that XDSCR6 and XSTAT3 have antagonistic roles in vivo during axis formation. This antagonism can be explained by the fact that XDSCR6 negatively regulates the transcriptional activity of XSTAT3 by interfering with its methylation on lysine residues. Moreover, this post-translational modification plays a crucial role in the ventralization abilities of XSTAT3. In a previous study, it has been shown that XDSCR6 negatively regulates the XEZH2 repressive epigenetic activity on dorsal mesoderm genes. Thus, we propose that XDSCR6 is a transcriptional modulator acting between epigenetic regulators and transcriptional factors during embryonic axis formation.