Mtx2 directs zebrafish morphogenetic movements during epiboly by regulating microfilament formation

Abstract

The homeobox transcription factor Mtx2 is essential for epiboly, the first morphogenetic movement of gastrulation in zebrafish. Morpholino knockdown of Mtx2 results in stalling of epiboly and lysis due to yolk rupture. However, the mechanism of Mtx2 action is unknown. The role of mtx2 is surprising as most mix/bix family genes are thought to have roles in mesendoderm specification. Using a transgenic sox17-promoter driven EGFP line, we show that Mtx2 is not required for endoderm specification but is required for correct morphogenetic movements of endoderm and axial mesoderm. During normal zebrafish development, mtx2 is expressed at both the blastoderm margin and in the zebrafish equivalent of visceral endoderm, the extra-embryonic yolk syncytial layer (YSL). We show that formation of the YSL is not Mtx2 dependent, but that Mtx2 directs spatial arrangement of YSL nuclei. Furthermore, we demonstrate that Mtx2 knockdown results in loss of the YSL F-actin ring, a microfilament structure previously shown to be necessary for epiboly progression. In summary, we propose that Mtx2 acts within the YSL to regulate morphogenetic movements of both embryonic and extra-embryonic tissues, independently of cell fate specification.

ApoA-II Directs Morphogenetic Movements of Zebrafish Embryo by Preventing Chromosome Fusion during Nuclear Division in Yolk Syncytial Layer

“…YSNs from wild-type embryos were significantly dispersed at greater than about 10 m length scales, with a maximum incidence at around 22 m as previously mentioned (21). The spatial organization of YSNs was seen in apoa2 MO embryos on length-scales smaller than 6 m and showed aggregation in some regions within eYSL and iYSL (Fig.…”

“…During blastulation, gastrulation, and early segmentation, zebrafish YSN display several highly patterned movements to organize YSL (18). We injected the vital dye SYTOX Green into the YSL at the 512-cell and 1k-cell stages to inspect the spatial organization of YSN as described previously (21). The YSN within the ring at the blastoderm margin appeared abnormal in distribution and morphology in embryos injected with apoa2 MO (Fig.…”

“…Capped apoa2 mRNA was synthesized from its cDNA subcloned in the pcDNA3.1(ϩ) vector using mMACHINE T7 Ultra Kit (Ambion). SYTOX Green, which acts to specifically label nuclei, was injected into yolk at 1k-cell stage, at 50 M, as described (21).…”

“…Convergence and extension are key steps for dorsoventral (DV) patterning during embryonic gastrulation (21). Therefore, to determine whether apoa2 also functioned in DV patterning, we examined the expression of a variety of DV marker genes in embryos, which were injected with apoa2 MO1 at the 1-cell stage and exhibited the "survival" phenotype (Fig.…”

“…Mtx2 Expression and the F-actin Ring in YSL Are Altered in apoa2 Morphants-The proper organization of YSL has been shown to be essential for initiation and progression of epiboly in teleosts (21,31). Apoa2 is dominantly expressed in the YSL and performs a function in zebrafish embryonic gastrulation.…”