Browsing by Subject "Bicc1"

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Establishment of the body axes in Xenopus laevis through goosecoid, myosin 1d and bicaudal c
(2021) Maerker, Markus Ferdinand; Schweickert, Axel
The bilaterian body plan consists of three body axes: the anteroposterior (AP; head-trunk/tail), the dorsoventral (DV; back-belly) and the left-right (LR; placement of inner organs) axis. Axis formation occurs during early embryogenesis and is critical for further development and viability of the embryo. In this comprehensive study three highly conserved determinants were functionally analyzed in the context of axis development. The first chapter of this work covers the autoregulatory, homeodomain containing, repressor gene goosecoid (gsc), whose most prominent expression marks the Spemann-(Mangold) organizer (SO). The SO is the primary dorsal signaling center and is instructive for tissue patterning along the DV and AP axes. Transplanting the SO or misexpressing gsc on the opposite ventral side of an embryo is sufficient to establish a new/secondary AP axis. However, its function during normal development in the SO remained enigmatic as the gsc loss of function (LOF) lead to no severe early developmental defects. To elucidate the function of gsc, timed gain of function (GOF) experiments were performed. Gsc efficiently repressed the planar cell polarity (PCP)/Wnt signaling pathway leading to severe gastrulation and neurulation defects. This novel Gsc function was correlated with two vertebrate specific domains, suggesting an evolutionary new function of Gsc with the emergence of jaws/neural crests in vertebrates. The second chapter of this study addresses the functions of Myosin1d (Myo1d) and Bicaudal c1 (Bicc1) during the LR axis determination in vertebrates. In this group LR symmetry breakage takes place at a ciliated epithelium called LR organizer (LRO). The initial cue for the asymmetric LR axis development is a cilia-driven leftward fluid flow. These cilia have to be correctly polarized through PCP/Wnt signaling. Interestingly, the invertebrate Drosophila melanogaster also displays a distinct LR axis but uses a cilia independent, yet not fully understood, mechanism. It depends on a myo1d homologous gene, myo31DF, and PCP. To unravel a potential common evolutionary origin of the bilaterian LR axis myo1d was analyzed during Xenopus laevis lateralization. Myo1d LOF experiments disturbed LR axis formation by compromising PCP dependent outgrowth and polarization of LRO cilia. These experiments link the PCP/Myosin based mechanism of flies to the newly evolved cilia/flow dependent mode of vertebrate LR axis determination suggesting actomyosin as common ancestral LR determinant. Contrary to Myo1d, Bicc1 was already described for its function during polarization of flow producing LRO cilia. However bicc1s expression is most prominent in the sensory LRO cells (sLRO). These cells detect the fluid flow and translate it into left-sided signaling of the morphogen Nodal1 and consequently asymmetric LR axis formation. These cells downregulate the expression of the secreted Nodal1 antagonist DAN domain family member 5 (dand5) in response to flow. Bicc1s function was re-evaluated with respect to its function in sLRO cells. Ex vivo and in vivo experiments involving GOF as well as LOF experiments showed that Bicc1 regulates both dand5 and nodal1 via a direct and indirect post-transcriptional mechanism, respectively. In the process of dand5 regulation several other LR determinants and regulatory events were linked with the Bicc1 dependent mechanism: Dicer1 dependent microRNA repression of dand5 and a proposed cation channel Polycystin 2 mediated Bicc1 modification. These results highlight the importance of a tightly controlled Dand5 protein level as decisive for the overall outcome of the LR symmetry breakage in vertebrates.