The murine homolog of the fibroblast growth factor-5 (FGF-5) gene has been cloned, and the sequence of the gene's three exons has been determined. The murine gene and the previously isolated human FGF-5 gene are substantially homologous within the coding sequences and in upstream sequences, which contain an additional open reading frame. We have used a portion of the murine gene as probe to detect FGF-5 RNA in adult mouse tissues by both Northern blot and in situ hybridization methods. FGF-5 RNA is present at low levels in widely distributed areas of the central nervous system. Several loci of FGF-5 expression could be localized by in situ hybridization and include portions of the cerebral cortex, hippocampus, and thalamus. Neuronal expression accounts for at least some of the FGF-5 RNA synthesized in the central nervous system. Fibroblast growth factors (FGFs) constitute a family of mitogenic proteins with related primary structures. Each mammalian factor, of which seven are now known, is encoded by a distinct gene (1)(2)(3)(4)(5)(6)(7)(8). FGFs are mitogenic towards a broad spectrum of mesodermal and ectodermal cells (9) and can act also as inducers (10-13) and inhibitors (14) of developmental pathways. Fibroblasts and endothelial cells can respond to several different FGFs (9). By contrast, keratinocyte growth factor, the most recently characterized FGF, does not stimulate fibroblast growth (15). Hence, FGFs may have overlapping but distinct spectra of activities.FGF-5 is a growth factor discovered in our laboratory as the product of a human oncogene detected by DNA transfection assays (6, 16). The protein is mitogenic towards fibroblasts and endothelial cells in vitro (6), but the natural targets for FGF-5 action in vivo have yet to be ascertained. As a first step towards an understanding of Genomic Library Screening. Murine NIH 3T3 DNA partially digested with Sau3aI was cloned into EMBL4 A phage DNA to generate one library, while another consisting of mouse spleen DNA cloned into A phage EMBL3 was obtained from F. Costantini. Libraries were screened by a standard procedure (17) using as probe human FGF-5 cDNA (6) labeled with 32P by random hexamer priming (18 RNA Filter Blot Hybridization. Tissues were dissected from 8-week-old C57BL/6 mice, and the RNA was isolated after solubilization in guanidinium isothiocyanate (21). RNA was also isolated from seven components of brain dissected by standard procedure (22). RNA filter blot hybridization after agarose gel electrophoresis followed a standard protocol (23) using a 32P-labeled Pst I/Sac I 450-bp DNA fragment containing exon 3 of the murine FGF-5 gene (see Fig. 1 *The sequences reported in this paper have been deposited in the GenBank data base (accession nos. M37821-4 for the murine FGF-5 gene and M37825 for the corrected human FGF-5 cDNA). 8022The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this f...

No abstract

Fate-mapping experiments in the mouse have revealed that the primitive streak can be divided into three functional regions: the proximal region gives rise to germ cells and the extra-embryonic mesoderm of the yolk sac; the distal region generates cardiac mesoderm and node-derived axial mesendoderm; and the middle streak region produces the paraxial, intermediate and lateral plate mesoderm of the trunk. To gain insight into the mechanisms that mediate the assembly of the primitive streak into these functional regions, we have cloned and functionally identified the gene disrupted in the amnionless (amn) mouse, which has a recessive, embryonic lethal mutation that interferes specifically with the formation and/or specification of the middle primitive streak region during gastrulation. Here we report that the gene Amn encodes a novel type I transmembrane protein that is expressed exclusively in the extra-embryonic visceral endoderm layer during gastrulation. The extracellular region of the Amn protein contains a cysteine-rich domain with similarity to bone morphogenetic protein (BMP)-binding cysteine-rich domains in chordin, its Drosophila melanogaster homolog (Short gastrulation) and procollagen IIA (ref. 3). Our findings indicate that Amn may direct the production of trunk mesoderm derived from the middle streak by acting in the underlying visceral endoderm to modulate a BMP signaling pathway.

The primitive streak is the defining feature of the gastrulating mouse embryo. Currently, little is known in the mouse about the mechanisms that mediate the assembly of the primitive streak or about the signaling pathways that specify the different types of mesoderm and endoderm generated from the streak. To gain insight into primitive streak assembly and function, we have conducted a detailed phenotypic characterization of amnionless, a transgene-induced insertional mouse mutation that arrests embryonic development during gastrulation. Our histological and molecular analyses, when examined in the context of the mouse gastrula fate map, lead to the model that middle streak formation is specifically impaired in the amnionless mutant. Significantly, these observations argue that the formation of the middle streak is mediated by a pathway that is genetically separable from those that direct the specification of the distal and proximal streak regions. Intriguingly, our findings from wt ES cell left and right arrow amnionless-/- blastocyst chimeras indicate that this proposed separate pathway for middle streak formation is dependent on amnionless gene functions in the visceral endoderm.