Browsing by Person "Bitzer, Eva"

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Establishment of a new in vitro culture system and functional analysis of sonic hedgehog and FGF8 in the determination of laterality in the rabbit embryo
(2008) Bitzer, Eva; Blum, Martin
Cilia-driven leftward flow plays a pivotal role in the determination of left-right (LR) asymmetry. In mammals, this extracellular fluid flow is produced by motile monocilia situated on the posterior notochordal plate (PNC). The PNC is homologous to superficial mesoderm derived structures of other species like the gastrocoel roof plate (GRP) in frog and Kupffer's vesicle (KV) in fish. Directional fluid flow created at these structures subsequently leads to the initiation of the left-specifying Nodal signalling cascade in the left lateral plate (LPM). The rabbit develops via a flat blastodisc phase representing the archetypical mode of mammalian embryogenesis. These specific advantages of the rabbit were employed in this study to further examine the role of two central determinants of laterality, namely Sonic hedgehog (Shh) and FGF8, and also extended by the design of a new in vitro culture technique. In this new method, the so-called ring culture, a medium-filled plastic ring was placed upon the extraembryonic tissue of the explanted embryo. In contrast to the semi-dry standard culture method, this setting corresponded more to the in vivo conditions of gastrulating/neurulating rabbit embryos in the uterus. It therefore facilitated stable development of laterality in most cases also when presomite stages were taken into culture. Subsequent analysis showed that this was due to improved development of the PNC. Conversely, in standard-cultured embryos showing altered LR marker gene expression, maturation of the PNC was impaired leading to a disturbance of leftward flow. This study also provided evidence that cilia-driven leftward flow is indispensable for the determination of laterality in rabbit embryos. When the flow was blocked during culture by methylcellulose-containing medium embryos displayed altered LR marker gene expression in a very high proportion. The unilateral gain-of-function of Shh revealed important differences between rabbit and chick embryos. In rabbit, Shh induced right-sided marker gene expression only in the 2 somite stage, whereas in chick this inductive effect lasted from stage 4 until up to the 1-2 somite stage. This indicated that in rabbit Shh works in conjunction with the flow, which has not been described up to now in chick. The systemic inhibition of Shh signalling by cyclopamine led to bilateral expression of LR marker genes in rabbit. This was due to disruption of the floor plate and therefore the loss of the restrictive midline barrier function of Lefty expression as described in Shh mutant mice. FGF8 has a right-sided repressive function in the rabbit implicated in the transfer of laterality cues. In the present study it could be shown that this repressive effect is epistatic to cilia-driven leftward flow, because it also functioned when the flow was blocked. The systemic inhibition of FGF8 signalling with SU5402 caused loss of LR marker gene expression prior to the 2 somite stage but did not influence ciliogenesis or the setup of cilia-driven leftward flow. Taken together, this suggested a dual function for FGF8 signalling: First, it is needed to confer competence to the lateral plate and second, during the 2 somite stage, it is needed for the transfer of LR cues.