Bmpr encodes a type I bone morphogenetic protein receptor that is essential for gastrulation during mouse embryogenesis

Evidence that mouse Bmp8a (Op2) and Bmp8b are duplicated genes that play a role in spermatogenesis and placental development

We have identified two highly conserved mouse genes encoding bone morphogenetic protein 8A (BMP8A/OP2) and 8B (BMP8B). The two loci are tightly linked on chromosome 4, suggesting that they arose through a recent gene duplication. Contrary to previous reports, neither gene is expressed in the early postimplantation mouse embryo (7.5-10.5 days post coitum) as judged by a variety of sensitive techniques. By contrast, high levels of Bmp8b RNA are found in the decidual cells of the uterus, and both genes are expressed in the trophoblast cells of the labyrinthine region of the placenta and in the inner root sheath of hair follicles of early postnatal skin. In addition, both Bmp8a and Bmp8b are expressed in the testis during specific stages of spermatogenesis, with the highest levels of RNA in stage 6-8 round spermatids after 3 weeks of age. Bmp8a and 8b are, therefore, the first members of the transforming growth factor beta (TGF beta)-related gene family to be found expressed in the germ cells of the testis, rather than in the somatic Sertoli cells. These results suggest that Bmp8a and 8b are not required for development of the embryo proper but regulate aspects of cell proliferation, survival and/or differentiation during spermatogenesis and placentation.

The gene encoding bone morphogenetic protein 8B is required for the initiation and maintenance of spermatogenesis in the mouse

Bone morphogenetic protein 8B (BMP8B) is a member of the TGFbeta superfamily of growth factors. In the mouse, Bmp8b is expressed in male germ cells of the testis and trophoblast cells of the placenta, suggesting that it has a role in spermatogenesis and reproduction. To investigate these possibilities, we have generated mice with a targeted mutation in Bmp8b. Here, we show that homozygous Bmp8b(tm1blh) mutant males exhibit variable degrees of germ-cell deficiency and infertility. Detailed analysis reveals two separable defects in the homozygous mutant testes. First, during early puberty (2 weeks old or younger) the germ cells of all homozygous mutants either fail to proliferate or show a marked reduction in proliferation and a delayed differentiation. Second, in adults, there is a significant increase in programmed cell death (apoptosis) of spermatocytes, leading to germ-cell depletion and sterility. Sertoli cells and Leydig cells appear relatively unaffected in mutants. This study therefore provides the first genetic evidence that a murine germ cell-produced factor, BMP8B, is required for the resumption of male germ-cell proliferation in early puberty, and for germ-cell survival and fertility in the adult.

The tumor suppressor gene Brca1 is required for embryonic cellular proliferation in the mouse

Mutations of the BRCA1 gone in humans are associated with predisposition to breast and ovarian cancers. We show here that Brca1+/- mice are normal and fertile and lack tumors by age eleven months. Homozygous Brca1(5-6) mutant mice die before day 7.5 of embryogenesis. Mutant embryos are poorly developed, with no evidence of mesoderm formation. The extraembryonic region is abnormal, but aggregation with wild-type tetraploid embryos does not rescue the lethality. In vivo, mutant embryos do not exhibit increased apoptosis but show reduced cell proliferation accompanied by decreased expression of cyclin E and mdm-2, a regulator of p53 activity. The expression of cyclin-dependent kinase inhibitor p21 is dramatically increased in the mutant embryos. Buttressing these in vivo observations is the fact that mutant blastocyst growth is grossly impaired in vitro. Thus, the death of Brca1(5-6) mutant embryos prior to gastrulation may be due to a failure of the proliferative burst required for the development of the different germ layers.

A role for BMP-4 in the development of subcutaneous cartilage

BMP-4 is a transforming growth factor beta (TGF- beta) superfamily member which plays important roles in various developmental processes in vertebrate embryogenesis. In this study the expression pattern of BMP-4 was examined during early chick and quail embryonic development. Transcripts were found in the lateral ridges of the neural plate during neurulation and in dorsal regions of the neural tube after its closure along the whole length of the neural axis. To investigate the role of BMP-4 expressed in dorsal regions of the neural tube, cells producing BMP4 were grafted ectopically in the paraxial mesoderm in E2 chick embryos at the level of somites 16-22. Ectopic BMP4 induced Msx-1 and Msx-2 gene expression in superficial ectoderm and superficial mesodermal cells which normally do not express these transcription factors. Moreover, it could be seen that in the E9 operated embryos, additional cartilage had been induced at the level of the graft. These results suggest that BMP4 produced by dorsal regions of the neural tube acts as an endogenous inducing signal in the activation of the genes of the Msx family which are likely to be part of the cascade of molecular events leading to subcutaneous cartilage formation. Therefore, BMP-4 expression in the tissues located dorsally in the neural tube and superficial ectoderm is proposed to be involved in the development of the spinous process of the vertebra.

MADR1, a MAD-related protein that functions in BMP2 signaling pathways

Components of the signaling pathways that lie downstream of Ser/Thr kinase receptors and are required for signaling by the TGF beta superfamily have been poorly defined. The Drosophila gene Mothers against dpp (MAD) and the C. elegans sma genes are implicated in these signaling pathways. We show that MAD functions downstream of DPP receptors and is required for receptor signaling. Phosphorylation of MADR1, a human homolog of MAD, is tightly regulated and rapidly induced by BMP2, but not TGF beta or activin. This phosphorylation is necessary for function, since a point mutant that yields a null phenotype in Drosophila is not phosphorylated. BMP2 treatment results in accumulation of MADR1 in the nucleus. MAD proteins may thus define a novel class of signaling molecules with nuclear function in Ser/Thr kinase receptor signaling pathways.

Hematopoiesis: how does it happen?

Hematopoiesis entails the generation of stem cells, the proliferation and maintenance of multipotential progenitors, and lineage commitment and maturation. During the past year, critical components of these steps have been defined. Notable are gene-targeting experiments in mice in which one or more hematopoietic lineages have been shown to be ablated upon inactivation of several nuclear regulatory proteins (GATA-2, Tal-1/SCL, Rbtn2/LMO2, PU.1, Ikaros, E2A, and Pax-5), and experiments that establish GATA-1 as a factor capable of programming at least three lineages (erythroid, thrombocytic, and eosinophilic) from a transformed avian progenitor cell.