XB-ART-8601Development 2001 Jun 01;12812:2301-8. doi: 10.1242/dev.128.12.2301.
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Primitive erythropoiesis in the Xenopus embryo: the synergistic role of LMO-2, SCL and GATA-binding proteins.
Hematopoietic stem cells are derived from ventral mesoderm during vertebrate development. Gene targeting experiments in the mouse have demonstrated key roles for the basic helix-loop-helix transcription factor SCL and the GATA-binding protein GATA-1 in hematopoiesis. When overexpressed in Xenopus animal cap explants, SCL and GATA-1 are each capable of specifying mesoderm to become blood. Forced expression of either factor in whole embryos, however, does not lead to ectopic blood formation. This apparent paradox between animal cap assays and whole embryo phenotype has led to the hypothesis that additional factors are involved in specifying hematopoietic mesoderm. SCL and GATA-1 interact in a transcriptional complex with the LIM domain protein LMO-2. We have cloned the Xenopus homolog of LMO-2 and show that it is expressed in a similar pattern to SCL during development. LMO-2 can specify hematopoietic mesoderm in animal cap assays. SCL and LMO-2 act synergistically to expand the blood island when overexpressed in whole embryos. Furthermore, co-expression of GATA-1 with SCL and LMO-2 leads to embryos that are ventralized and have blood throughout the dorsal-ventral axis. The synergistic effect of SCL, LMO-2 and GATA-1, taken together with the findings that these factors can form a complex in vitro, suggests that this complex specifies mesoderm to become blood during embryogenesis.
PubMed ID: 11493549
Article link: Development
Species referenced: Xenopus laevis
Genes referenced: gata1 lmo2 tal1
Article Images: [+] show captions
|Fig. 3. XLMO-2 expression pattern in embryos. RNA in situ hybridization analysis was performed with full-length antisense digoxigenin-labeled probes on albino Xenopus laevis embryos. (A) Stage 21; lateral, dorsal and ventral views of the same embryo showing extensive staining in the nascent ventral blood island (VBI), the tailbud region and the dorsal lateral plate (DLP) mesoderm (black arrowheads). (B) Stage 26; lateral view. Staining is evident in the VBI, DLP (black arrowhead) and tailbud region. Staining is also seen in the brain and in the eye. (C) Stage 33; lateral view. Expression of XLMO-2 is maintained at a high level in the VBI but has diminished in the DLP region (black arrowhead). Staining in the tailbud has condensed to a small region at the tip of the outgrowing tail. (D) Stage 37; lateral view. Staining is evident throughout the circulatory system as XLMO-2 expression is maintained in circulating primitive erythrocytes (asterisked arrowhead). Expression in the tailbud region has condensed to the very tip of the tail. (E) Transverse section of a stage 26 embryo (ventral half only). XLMO-2 stains the ventral blood island (hematopoietic) mesoderm. (F) Transverse section of a stage 21 embryo stained with XLMO-2. Staining is diffuse throughout the mesoderm of the nascent tail bud.|
|Fig. 4. Comparison of XLMO-2, GATA-1 and SCL expression patterns. RNA in situ hybridization analysis reveals that these genes have very similar expression patterns. Each gene is expressed at a high level in the VBI. GATA-1 expression is restricted to erythrocytes whereas SCL and XLMO-2 are also expressed in other embryonic tissues such as the brain and tailbud.|
|Fig. 7. Synergistic activity of LMO-2, SCL and GATA-1 expression in whole embryos. (Top) Embryos were injected at the one cell stage with synthetic mRNAs encoding SCL and LMO-2 (1 ng). Embryos were cultured to stage 33 and stained for tadpole α-globin protein by immunohistochemistry. (Bottom) Tadpoles injected with SCL, LMO-2 or GATA-1 alone develop normally and globin expression is limited to the ventral blood island (VBI, black arrows). Combination of SCL and LMO-2 mRNA results in developmental abnormalities. Head structures are diminished or completely missing and globin expression extends laterally around the embryo, reaching at least half way up the body. Co-injection of LMO-2, SCL and GATA-1 results in severe developmental abnormalities: dorsal/anterior structures are absent and globin staining is evident throughout the body axis.|
|gata1 ( GATA binding protein 1 (globin transcription factor 1)) gene expression in Xenopus laevis embryos, NF stage 41, as assayed by in situ hybridization. Lateral view: anterior right, dorsal up. Image copyrighted by Company of Biologists Limited|