Browsing by Subject "Siamesische Zwillinge"

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Funktionelle Analyse der Histondeacetylase 6 sowie experimentelle Modellierung von Lateralitätsdefekten während der Links-Rechts-Achsenentwicklung von Xenopus laevis und Paracentrotus lividus
(2017) Tisler, Matthias; Blum, Martin
Vertebrates display an asymmetric positioning of the visceral organs, which is also denominated as left-right body axis. During embryogenesis, an asymmetric gene expression is detectable that is initiated by an evolutionary conserved mechanism of symmetry breakage, which is conserved among deuterostomes. During neurula stages, rotating motile mono-cilia at the so called left-right organizer (LRO) generate an asymmetric stimulus known as extracellular leftward fluid flow that is essential for the unilateral left asymmetric gene expression of the Nodal cascade. Spontaneous mutations or the experimentally induced loss of function of genes influencing ciliogenesis at the LRO, the induction of the Nodal cascade or its propagation lead to left-right defects. Left-right defects are frequently observed in human conjoined twins. Thoracopagous, dicephalic conjoined twins display defects in the arrangement of the inner organs, that are solely reported from the twin located to the right side. While left twins orient the inner organs wildtypically, right twins show a randomization of the left-right axis. The functional cause of the inverted arrangement regarding the right twin has remained enigmatic. It has been hypothesized that the observed laterality determination in conjoined twins, like in wildtype embryos, was dependent on leftward flow. In the course of this thesis, the known unilaterlal left-sided induction of the Nodal cascade in the left conjoined twin, as in singelton embryos, can be linked to leftward flow. The artificial induction of a second body axis leads to a subsequent duplication of the LRO during development. During flow stages endogenous and induced LROs locate in close proximity and display a partial fusion of cell populations. Anti-sense Morpholino Oligomeres or methylcelluose mediated loss of cilia motility lead to a loss of markergene expression in the left-lateral plate mesoderm of the left twin. By combining differential gain- and loss-of-function strategies, it was possible to link the establishment of laterality in conjoined twins to the leftward flow and, moreover, to manipulate it an a predictable manner. The cause of this hitherto enigmatic laterality defects in conjoined twins can therefore be explained by the evolutionary conserved mechanism of left-right establishment. Although the general mechanism of symmetry breakage has been characterized, novel candidate genes are continously beeing identified that act at a specific sequence of this process. The candidate gene histonedeacetylase 6 (hdac6) was shown to impact on left-right development. Anti-sense Morpholino Oligomere induced loss-of-function experiments led to left-right defects in a dose dependent manner regarding, the induction of the genes of the Nodal cascade, indicating a function of hdac6 before fluid flow induced regulation of dand5 mRNA. Taken together: histonedeacetylase 6 acts as modulator of canonical Wnt-signaling in the transcriptional induction of the Wnt-dependent transcription of foxj1, a master control gene of the biogenesis of motile cilia. Loss of Hdac6 leads to defects regarding the ciliogenesis of motile cilia at the LRO as well as the multiciliated epidermis of the embryo. The here presented results represent the first developmental hdac6 loss-of-function phenotype, which was so far not know from Hdac6-/- mice. These experiments shed a new light on the differential in vivo function of this unique histondeacetylase during development. Even though the asymmetric positioning of the inner organs is restricted to vertebrates, the asymmetric expression of the Nodal cascade turns out to be evolutionary conserved among deuterostomes. Comparable to vertebrate species, larvae of the sea urchin (Paracentrotus lividus, Echinodermata) display an asymmetric expression of the Nodal cascade in the ectoderm an during gastrula stages. Experiments from this work could demonstrate that also in sea urchin embryos the asymmetric gene expression depends on motile cilia. The archenteron of gastrula stage embryos was identified and described as homologous structure to vertebrate LROs. Deciliation experiments at different time points of development induce laterality defects and point towards a symmetry breakage during early gastrulation. By this experiments, the cilia dependent establishment of left-right asymmetry is described as a common synapomorphy of the deuterostomes beeing conserved from sea urchin to vertebrates, shedding a new light on the establishment of asymmetric gene expression.
Towards a unifying model of symmetry breakage in Xenopus laevis : serotonin signaling and the cilia-driven leftward flow
(2011) Thumberger, Thomas; Blum, Martin
Orientation of the three vertebrate body axes anterior-posterior (AP), dorso-ventral (DV) and left-right (LR) is specified during early embryogenesis. Whereas the formation of the AP and DV axes is well understood, it is not finally resolved how and when the left and right sides get molecularly distinct. All deuterostomes analyzed so far, however, display an asymmetric left-sided expression of the TGF-β factor Nodal during embryonic development which precedes asymmetric organogenesis. In zebrafish, medaka, mouse and rabbit embryos a cilia-driven extracellular leftward fluid flow was shown to be causal for the left asymmetric induction of the Nodal gene cascade during early neurulation. In X. laevis, leftward flow was also shown to be driven by a mono-ciliated epithelium in the posterior part of the archenteron roof (gastrocoel roof plate, GRP). Mechanical blockage of this current resulted in laterality defects. Despite the apparent evolutionary conservation of flow, an earlier mechanism to specify the LR axis during early cleavage stages has been reported in X. laevis. Based on mostly inhibitor experiments, the so-called 'ion-flux' hypothesis was put forward which proposes an electrogenic transport and asymmetric accumulation of determinants as early as at the 32-64 cell stage. The monoamine serotonin is the core-effector of this hypothesis and was reported to asymmetrically accumulate at the ventral right blastomeres of early cleavage stage embryos. The aim of this study was to investigate putative interactions of the two apparently opposing mechanisms for breaking the initial LR symmetry of the Xenopus zygote. Reinvestigation of serotonin localization could not confirm the initial report. Further, serotonin signaling was shown to be necessary for LR axis formation on the dorsal but not ventral side, more specifically as a competence factor for the canonical Wnt-pathway. Detailed analyses of specimens impaired for serotonin signaling revealed requirement of serotonin signaling for specification of the superficial mesoderm (SM) which gives rise to the GRP and, consequently, to leftward flow. Leftward flow thus indirectly depends on dorsal serotonin signaling. In a further part of the present thesis, a re-examination of laterality in Siamese twins was performed. It has been known since the earliest experimental investigations of laterality that in induced and naturally occurring Siamese twins the left twin consistently displays wildtype orientation of the visceral organs whereas the orientation in the right twin is randomized. In experimentally induced conjoined twins, this observation holds true regardless of which twin is the induced. A model of symmetry breakage, in order to be plausible, thus should also be able to account for this phenomenon. When experimentally induced twins were analyzed for leftward flow, in the majority of cases a continuous leftward flow was observed, i.e. both twins shared one GRP. Thus, laterality cue(s) get translocated towards the far left side, i.e. only the left embryo receives the wildtype asymmetric information, regardless if it is the induced or endogenous twin. In rare case X. laevis conjoined axes developed far apart from one another such that two separate GRPs and individual leftward flows were observed, a condition that enabled both axes to exhibit a left-sided Nodal cascade. These experiments strongly suggest that Spemann's organizer itself is necessary and sufficient to establish all three body axes. In conclusion, the present analysis of laterality determination in the frog Xenopus supports evolutionary conservation of leftward flow as symmetry breaking event, as previously reported for mouse, rabbit and bony fish.