The TGF-β-related signalling system in mouse development

cDNA cloning and genomic organization of the mouse BMP type II receptor

The cDNA for the mouse bone morphogenetic protein type II receptor (BMPR-II) was isolated using the human counterpart as a probe and its genomic structure was determined. The cDNA encodes a protein of 1,038 amino acids with a single transmembrane domain, a serine/threonine kinase domain, and a long carboxy-terminal tail. The overall amino acid sequence identity between the mouse and the human BMPR-II is 96.6%. mRNA is widely distributed in various adult tissues. The gene is encoded by 13 exons spanning over 80 kb. Two large introns (intron 1 and 3) contribute to the majority of the gene size, as in the mouse activin type II receptor gene. The intron/exon boundaries were sequenced. The results suggest that alternative splicing can yield a shorter form of BMPR-II of 530 amino acids, as reported previously. Knowledge of the structure of the BMPR-II gene is essential for the understanding of the role of bone morphogenetic proteins in the developmental and physiological processes of animals.

Evidence that Mothers-against-dpp-related 1 (Madr1) plays a role in the initiation and maintenance of spermatogenesis in the mouse

We have cloned a mouse cDNA encoding a Mothers-against-dpp (MAD)-related protein, MADR1. Madr1 is ubiquitously expressed in the mouse embryo, indicating a broad function in a variety of tissue during embryogenesis, potentially relaying signals of numerous BMPs. However, its expression in the testis is strictly germ cell-specific and developmentally regulated. Testicular Madr1 expression starts in some seminiferous tubules at 2 weeks of age. After mid-puberty, a stage-specific Madr1 expression is established. During the cycling of the seminiferous epithelium, Madr1 expression initiates in the pachytene spermatocytes of stage V seminiferous tubules, peaks at stage X, then decreases as pachytene spermatocytes differentiate into secondary spermatocytes and then round spermatids. In the testis of adult Bmp8b homozygous mutant males, the Madr1- expressing pachytene spermatocytes are the first cell population to show increased apoptosis. These data suggest that MADR1 serves as a downstream component of the BMP8 signaling pathway during the differentiation of meiotic male germ cells.

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.

Bone morphogenetic protein-4 is required for mesoderm formation and patterning in the mouse

Bone morphogenetic protein-4 (BMP-4) is a member of the TGF-beta superfamily of polypeptide signaling molecules, closely related to BMP-2 and to Drosophila decapentaplegic (DPP). To elucidate the role of BMP-4 in mouse development the gene has been inactivated by homologous recombination in ES cells. Homozygous mutant Bmp-4tm1blh embryos die between 6.5 and 9.5 days p.c., with a variable phenotype. Most Bmp-4tm1blh embryos do not proceed beyond the egg cylinder stage, do not express the mesodermal marker T(Brachyury), and show little or no mesodermal differentiation. Some homozygous mutants develop to the head fold or beating heart/early somite stage or beyond. However, they are developmentally retarded and have truncated or disorganized posterior structures and a reduction in extraembryonic mesoderm, including blood islands. These results provide direct genetic evidence that BMP-4 is essential for several different processes in early mouse development, beginning with gastrulation and mesoderm formation. Moreover, in the presumed absence of zygotic ligand, it appears that homozygous mutants can be rescued partially by related proteins or by maternal BMP-4.