Smad1 and Smad8 Function Similarly in Mammalian Central Nervous System Development

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Abstract
Smads 1, 5, and 8 are the intracellular mediators for the bone morphogenetic proteins (BMPs), which play crucial roles during mammalian development. Previous research has shown that Smad1 is important in the formation of the allantois, while Smad5 has been shown to be critical in the process of angiogenesis. To further analyze the BMP-responsive Smads, we disrupted the murine Smad8 gene utilizing the Cre/loxP system. A Smad8 hypomorphic allele (Smad8Δexon3) was constructed that contains an in-frame deletion of exon 3, removing one-third of the MH2 domain and a small portion of the linker region. Xenopus injection assays indicated that this Smad8 deletion allele is still functional but has reduced ventralizing capability compared to the wild type. Although Smad8Δexon3/Δexon3 embryos are phenotypically normal, homozygotes of another hypomorphic allele of Smad8 (Smad83loxP) containing a neomycin cassette within intron 3, phenocopy an embryonic brain defect observed in roughly 22% of Smad1+/− embryos analyzed at embryonic day 11.5. These observations suggest that BMP-responsive Smads have critical functions in the development of the mammalian central nervous system.