Ventral mesodermal patterning in Xenopus embryos: Expression patterns and activities of BMP‐2 and BMP‐4

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Abstract
We provide a comparative analysis of the expression patterns and ventral mesoderm‐inducing properties of Xenopus BMP‐2 and BMP‐4. Transcripts for BMP‐2 and BMP‐4 are maternally stored in eggs, and zygotic expression of these genes is uniform in the ectoderm and mesoderm in late blastulae. During gastrulation, BMP‐2 is expressed at a low level throughout the ectoderm and marginal zone, but at early neurula stages a patch of dorso‐anterior cells displays enhanced expression. In contrast, BMP‐4 transcripts are restricted to the ventrolateral marginal zone during gastrulation, and in late gastrula and early neurula BMP‐4 is expressed in the epidermis but not the neural plate. At post‐neurula stages, BMP‐2 and BMP‐4 transcripts are associated with a variety of mesodermal structures, including the pharyngeal pouches, heart, blood island, and blastopore. At tailbud stages, BMP‐2 and BMP‐4 are expressed in neural tissues including the neural tube and brain. In mesoderm induction assays, BMP‐2 and BMP‐4 induce Xhox3, an early ventral‐posterior mesoderm marker, and larval βT1 globin, a marker for red blood cells. Induction of red blood cells in response to BMP‐4 was demonstrated by staining with a hemoglobin‐specific reagent. Little is known about factors that induce hematopoietic lineages in vertebrates, and these results provide evidence linking BMP activity and blood differentiation. Globin induction by BMP‐2 and BMP‐4 is blocked by co‐expression of a dominant‐negative activin receptor, suggesting that either endogenous activin signals are required for BMP‐mediated induction, or that the trancated activin receptor interferes with signaling by BMP receptors. In assays on marginal zone explants, we demonstrate that BMP‐4 respecifies dorsal mesoderm to form ventral mesoderm, consistent with its ability to induce blood and to ventralize embryos. BMP‐2, however, does not display such activity. The findings extend and support evidence that BMP‐2 and BMP‐4 function in ventral mesoderm induction and patterning in Xenopus. Our data furthermore high light the multiple functions these factors fulfill during early vertebrate embryogenesis.