Early posterior/ventral fate specification in the vertebrate embryo.

One of the central questions in developmental biology is that of how one cell can give rise to all specialized cell types and organs in the organism. Within the embryo, all tissues are composed of cells derived from one or more of the three germ layers, the ectoderm, the mesoderm, and the endoderm. Understanding the molecular events that underlie both the specification and patterning of the germ layers has been a long-standing interest for developmental biologists. Recent years have seen a rapid advancement in the elucidation of the molecular players implicated in patterning the vertebrate embryo. In this review, we will focus solely on the ventral and posterior fate acquisition in the ventral-lateral domains of the pregastrula embryo. We will address the embryonic origins of various tissues and will present embryological and experimental evidence to illustrate how "classically defined" ventral and posterior structures develop in all three germ layers. We will discuss the status of our current knowledge by focusing on the African frog Xenopus laevis, although we will also gather evidence from other vertebrates, where available. In particular, genetic studies in the zebrafish and mouse have been very informative in addressing the requirement for individual genes in these processes. The amphibian system has enjoyed great interest since the early days of experimental embryology, and constitutes the best understood system in terms of early patterning signals and axis specification. We want to draw interest to the embryological origins of cells that will develop into what we have collectively termed "posterior" and "ventral" cells/tissues, and we will address the involvement of the major signaling pathways implicated in posterior/ventral fate specification. Particular emphasis is given as to how these signaling pathways are integrated during early development for the specification of posterior and ventral fates.

Expression and regulation of chicken fibroblast growth factor homologous factor (FHF)-4 during craniofacial morphogenesis.

Fibroblast growth factor homologous factors (FHFs) have been implicated in limb and nervous system development. In this paper we describe the expression of the cFHF-4 gene during chicken craniofacial development. cFHF-4 is expressed in the mesenchyme of the frontonasal process, and in the mesenchyme and ectoderm of the mandibular processes. The expression of cFHF-4 and other genes implicated in facial patterning have been analyzed in talpid(2) embryos or in the presence of exogenous retinoic acid. Talpid(2) mutants show abnormal patterns of gene expression, including up-regulation of cFHF-4 in the developing face, which correlate with defects in cartilage formation. By contrast, expression of cFHF-4 in the developing face is strongly downregulated by teratogenic doses of all-trans retinoic acid in a dose-dependent manner. Low levels of retinoic acid that produce distal upper beak truncations do not affect cShh, c-Patched-1, or c-Bmp-2 expression in the face, but downregulate cFHF-4 in the frontonasal process.

Expression and regulation of chicken fibroblast growth factor homologous factor (FHF)-4 at the base of the developing limbs.

Fibroblast growth factor homologous factors (FHFs) have been implicated in limb and nervous system development. In this paper we describe the expression of the cFHF-4 gene during early chicken development. cFHF-4 is expressed in the paraxial mesoderm, lateral ridge, and, most prominently, in the posterior-dorsal side of the base of each limb bud. The expression pattern of cFHF-4 at the base of the limbs is not altered by tissue grafts containing the zone of polarizing activity (ZPA), by implants of Shh-expressing cells, or by implants of beads containing retinoic acid, nor does it depend on the distal growth of the limb as it is not altered in limb buds that are surgically truncated. In three chicken mutants affecting limb patterning - talpid(2), limbless, and wingless - altered patterns of cFHF-4 expression are correlated with abnormal nerve plexus formation and altered patterns of limb bud innervation. Similarly, ectopic expression of cFHF-4 is correlated with a local induction of limb-like innervation patterns when beads containing FGF-2 are implanted in the flank. In these experiments, both ectopic innervation and ectopic expression of cFHF-4 in the flank were observed regardless of the size of the FGF-2-induced outgrowths. By contrast, ectopic expression of Shh and HoxD13 are seen only in the larger FGF-2-induced outgrowths. Taken together, these data suggest that cFHF-4 regulates or is coregulated with early events related to innervation at the base of the limbs.

Isoform diversity among fibroblast growth factor homologous factors is generated by alternative promoter usage and differential splicing.

Fibroblast growth factor (FGF) homologous factors-1, -2, -3, and -4 (FHFs 1-4; also referred to as FGFs 11-14) comprise a separate branch of the FGF family and have been implicated in the development of the nervous system and limbs. We report here the characterization of multiple isoforms of FHF-1, -2, -3, and -4 which are generated through the use of alternative start sites of transcription and splicing of one or more of a series of alternative 5'-exons. Several isoforms show different subcellular distributions when expressed in transfected tissue culture cells, and the corresponding differentially spliced transcripts show distinct expression patterns in developing and adult mouse tissues. Together with the evolutionary conservation of the FHF isoforms among human, mouse, and chicken, these data indicate that alternative promoter use and differential splicing are important regulatory processes in controlling the activities of this subfamily of FGFs.

Expression of chicken fibroblast growth factor homologous factor (FHF)-1 and of differentially spliced isoforms of FHF-2 during development and involvement of FHF-2 in chicken limb development.

Members of the fibroblast growth factor (FGF) family have been identified as signaling molecules in a variety of developmental processes, including important roles in limb bud initiation, growth and patterning. This paper reports the cloning and characterization of the chicken orthologues of fibroblast growth factor homologous factors-1 and -2 (cFHF-1/cFGF-12 and cFHF-2/cFGF-13, respectively). We also describe the identification of a novel, conserved isoform of FHF-2 in chickens and mammals. This isoform arises by alternative splicing of the first exon of the FHF-2 gene and is predicted to encode a polypeptide with a distinct amino-terminus. Whole-mount in situ hybridization reveals restricted domains of expression of cFHF-1 and cFHF-2 in the developing neural tube, peripheral sensory ganglia and limb buds, and shows that the two cFHF-2 transcript isoforms are present in non-overlapping spatial distributions in the neural tube and adjacent structures. In the developing limbs, cFHF-1 is confined to the posterior mesoderm in an area that encompasses the zone of polarizing activity and cFHF-2 is confined to the distal anterior mesoderm in a region that largely overlaps the progress zone. Ectopic cFHF-2 expression is induced adjacent to grafts of cells expressing Sonic Hedgehog and the zone of cFHF-2 expression is expanded in talpid2 embryos. In the absence of the apical ectodermal ridge or in wingless or limbless mutant embryos, expression of cFHF-1 and cFHF-2 is lost from the limb bud. A role for cFHF-2 in the patterning and growth of skeletal elements is implied by the observation that engraftment of developing limb buds with QT6 cells expressing a cFHF-2 isoform that is normally expressed in the limb leads to a variety of morphological defects. Finally, we show that a secreted version of cFHF-2 activates the expression of HoxD13, HoxD11, Fgf-4 and BMP-2 ectopically, consistent with cFHF-2 playing a role in anterior-posterior patterning of the limb.

Fibroblast growth factor (FGF) homologous factors: new members of the FGF family implicated in nervous system development.

Four new members of the fibroblast growth factor (FGF) family, referred to as fibroblast growth factor homologous factors (FHFs), have been identified by a combination of random cDNA sequencing, data base searches, and degenerate PCR. Pairwise comparisons between the four FHFs show between 58% and 71% amino acid sequence identity, but each FHF shows less than 30% identity when compared with other FGFs. Like FGF-1 (acidic FGF) and FGF-2 (basic FGF), the FHFs lack a classical signal sequence and contain clusters of basic residues that can act as nuclear localization signals. In transiently transfected 293 cells FHF-1 accumulates in the nucleus and is not secreted. Each FHF is expressed in the developing and adult nervous systems, suggesting a role for this branch of the FGF family in nervous system development and function.