Xenopus blastulae show regional differences in competence for mesoderm induction: correlation with endogenous basic fibroblast growth factor levels

Freeze-substitution: origins and applications

Freeze-substitution is a physicochemical process in which biological specimens are immobilized and stabilized for microscopy. Water frozen within cells is replaced by organic solvents at subzero temperatures. Freeze-substitution is widely used for ultrastructural and immunocytochemical analyses of cells by transmission and scanning electron microscopy. Less well recognized is its superiority over conventional chemical fixation in preserving labile and rare tissue antigens for immunocytochemistry by light microscopy. In the postgenome era, the focus of molecular genetics will shift from analyzing DNA sequence structure to elucidating the function of gene networks, the intercellular effects of polygenetic diseases, and the conformational rearrangements of proteins in situ. Novel strategies will be needed to integrate knowledge of chemical structures of normal and abnormal macromolecules with the physiology and developmental biology of cells and tissues from whole organisms. This review summarizes the progress and future prospects of freeze-substitution for such explorations.

Time course of ion channel development in Xenopus muscle induced in vitro by activin

During the process of mesoderm specification in Xenopus embryos, cells of the equatorial region are induced to form mesoderm in response to signals from the underlying endodermal cells. One mesodermal cell type resulting from this in vivo induction is skeletal muscle, which has a very specific and tightly regulated course of electrical and morphological development. Previously, electrical development could be analyzed only after neurulation, once myocytes could be morphologically identified. In vitro, activin triggers a cascade of events leading to the development of specific mesodermal tissues, including skeletal muscle; however, the precise role of activin in vivo is less clear. Much is now known about the mechanism and control of activin action, but very little is known about the subsequent time course of differentiation of activin-induced muscle. Such muscle is routinely identified by the presence of a small number of specific markers which, although they accurately confirm the presence of muscle, give little indication of the time course or quantitative aspects of muscle development. One of the most important functional aspects of muscle development is the acquisition of the complex electrical properties which allow it to function normally. Here we assess the ability of activin to drive in vitro the normal highly regulated sequence of electrical development in skeletal muscle. We find that in most, but not all, respects the normal time course of development of voltage-gated ion currents is well reproduced in activin-induced muscle. This characterization strengthens the case for activin as an agent capable of inducing the detailed developmental program of muscle and now allows for analysis of the regulation of electrical development prior to neurulation.

Expression of alternatively spliced and canonical basic fibroblast growth factor mRNAs in the early embryo and developing heart

Previous studies in this laboratory have revealed the presence of substantial deposits of basic fibroblast growth factor (bFGF; FGF-2) in the myocardium from the earliest stages of heart development (Parlow et al. [1991] Dev. Biol. 146:139-147) and that an autocrine supply of bFGF is required for myocardial cell proliferation (Sugi et al. [1993] Dev, Biol, 157:28-37). Recently, an alternatively spliced isoform of bFGF, termed alt-bFGF, was described during later stages of embryogenesis, after heart morphogenesis is complete (Borja et al. [1993] Dev. Biol. 157:110-118). Because the antibody and nucleic acid probes used in our previous studies would have recognized canonical as well as alt-bFGF proteins and mRNAs, we have examined the expression of alt-and canonical bFGF mRNAs at early stages of embryogenesis, during which the initial differentiative and morphogenetic phases of heart development occur (Hamburger-Hamilton stages 3-24). Reverse transcription/polymerase chain reaction (RT/PCR) analysis detected the presence of both alt-bFGF and bFGF mRNAs in whole embryos as early as stage 3 and in the developing heart from the time of its initial appearance at stage 9. The presence of alt-bFGF mRNA was corroborated by RNase protection analysis which, in assessing RNA from whole embryos, revealed increasing levels of alt-bFGF mRNA between stages 5-18, suggesting that expression of alt-bFGF is developmentally regulated. Utilization of a probe that simultaneously protects segments of both alt- and canonical bFGF mRNAs indicated that alt-bFGF was the more abundant FGF isoform in the developing embryo until stage 24, when equivalent expression of each isoform was detected. Similar analysis revealed that alt-bFGF was the more abundant isoform in the embryonic heart, but that its relative expression was not decreased at stage 24.

bFGF as a possible morphogen for the anteroposterior axis of the central nervous system in Xenopus

Vertebrate neural development is initiated during gastrulation by the inductive action of the dorsal mesoderm (Spemann's organizer in amphibians) on neighbouring ectoderm, which eventually gives rise to the central nervous system from forebrain to spinal cord. Here we present evidence that bFGF can mimic the organizer action by inducing Xenopus ectoderm cells in culture to express four position-specific neural markers (XeNK-2, En-2, XIHbox1 and XIHbox6) along the anteroposterior axis. bFGF also induced the expression of a general neural marker NCAM but not the expression of immediate-early mesoderm markers (goosecoid, noggin, Xbra and Xwnt-8), suggesting that bFGF directly neuralized ectoderm cells without forming mesodermal cells. The bFGF dose required to induce the position-specific markers was correlated with the anteroposterior location of their expression in vivo, with lower doses eliciting more anterior markers and higher doses more posterior markers. These data indicate that bFGF or its homologue is a promising candidate for a neural morphogen for anteroposterior patterning in Xenopus. Further, we showed that the ability of ectoderm cells to express the anterior markers in response to bFGF was lost by mid-gastrula, before the organizer mesoderm completely underlies the anterior dorsal ectoderm. Thus, an endogenous FGF-like molecule released from the involuting organizer may initiate the formation of the anteroposterior axis of the central nervous system during the early stages of gastrulation by forming a concentration gradient within the plane of dorsal ectoderm.

Quantitative export of FGF-2 occurs through an alternative, energy-dependent, non-ER/Golgi pathway

Although basic fibroblast growth factor (bFGF/FGF-2) is found outside cells, it lacks a conventional signal peptide sequence; the mechanism underlying its export from cells is therefore unknown. Using a transient COS-1 cell expression system, we have identified a novel membrane-associated transport pathway that mediates export of FGF-2. This export pathway is specific for the 18-kD isoform of FGF-2, is resistant to the anti-Golgi effects of Brefeldin A, and is energy-dependent. In FGF-2-transfected COS-1 cells, this ER/Golgi-independent pathway appears to be constitutively active and functions to quantitatively export metabolically-labeled 18-kD FGF-2. Co-transfection and co-immunoprecipitation experiments, using a vector encoding the cytoplasmic protein neomycin phosphotransferase, further demonstrated the selectivity of this export pathway for FGF-2. When neomycin phosphotransferase was appended to the COOH-terminus of 18-kD FGF-2, the chimera was exported. However, the transmembrane anchor sequence of the integral membrane glycoprotein (G protein) of vesicular stomatitis virus (VSV) blocked export. The chimeric protein localized to the plasma membrane with its FGF-2 domain extracellular and remained cell-associated following alkaline carbonate extraction. Taken together, the data suggest that FGF-2 is "exported" from cells via a unique cellular pathway, which is clearly distinct from classical signal peptide-mediated secretion. This model system provides a basis for the development and testing of therapeutic agents which may block FGF-2 export. Such an intervention may be of considerable use for the treatment of angiogenesis-dependent diseases involving FGF-2.

Nucleo-cytoplasmic translocation and secretion of fibroblast growth factor-2 during avian gastrulation

The expression and distribution of the fibroblast growth factor-2 (FGF-2 or bFGF) proteins during early avian embryogenesis has been analysed in detail. Three FGF-2 protein isoforms of 18.5, 20.0 and 21.5 kDa are expressed during gastrulation of chicken embryos. Using whole mount immunohistochemistry, these proteins were found to be predominantly nuclear in prestreak blastodiscs during mesoderm induction. Distribution of positive cells in the epiblast was mosaic, whereas all cells of the forming hypoblast expressed the FGF-2 proteins. During primitive streak formation, the proteins started to translocate to the cytoplasm in epiblast cells but remained nuclear in the hypoblast. The FGF-2 proteins became predominantly cytoplasmic in all cells during the subsequent developmental stages. Their highest levels were detected in endodermal cells underlying Hensen's node and the newly formed notochord, the dorsal apex of all epiblast cells and, most interestingly, in the extra-cellular basal lamina separating the epiblast from newly formed mesoderm. Heparin and suramin treatment of these advanced embryos (stage 4) revealed a dose-dependent inhibition on the regression of Hensen's node and formation of mesodermal derivatives such as somites. The results are discussed with respect to current models on FGF-mediated functions during vertebrate mesoderm induction and regionalization.

Basic fibroblast growth factor induces differentiation of neural tube and neural crest lineages of cultured ectoderm cells from Xenopus gastrula

The vertebrate nervous system is initially induced from a section of dorsal ectoderm by signal(s) from the underlying dorsal mesoderm during gastrulation. In an effort to identify the neural inducing factor(s) emanating from the dorsal mesoderm, we have examined the inductive action of various growth factors by applying them to ectoderm cells from Xenopus gastrulae (8- to 12.5-hour age; embryonic stage 9+ to 11 1/2) in a microculture system. Monoclonal antibodies that specifically recognize cellular differentiation antigens from three distinct ectoderm lineages (N1 for CNS neurons from neural tube, Me1 for melanophores from neural crest and E3 for skin epidermal cells from epidermal lineages, respectively) and a mesoderm lineage (Mu1 for muscle cells) were used as markers to monitor the differentiation of cultured ectoderm cells. We found that basic fibroblast growth factor (bFGF) was capable of specifically and reproducibly inducing gastrula ectoderm cells to produce CNS neurons and melanophores at concentrations as low as 5 pM, a value about 50-fold lower than that required to induce the formation of muscle cells from blastula animal cap cells (6-hour age; stage 8+). The induction of neural lineages by bFGF was correlated with a suppression of epidermal differentiation in a dose-dependent manner. bFGF never induced the formation of muscle cells from gastrula ectoderm cells even at concentrations as high as 5 nM. The response of ectoderm cells to bFGF changed dramatically during gastrulation. Ectoderm cells from early (8- to 9-hour age; stage 9+ to 10) gastrula gave rise to CNS neurons, but yielded few melanophores. As ectoderm cells were prepared from gastrulae of increasing age, their competence to form neurons was gradually lost, whereas the production of melanophores was enhanced and peaked in 11-hour gastrula (stage 10 1/2). The ability to form both neurons and melanophores was substantially reduced in 12.5-hour gastrula (stage 11 1/2). By examining ectoderm cells from the ventral and dorsal sides independently, it was also shown that during gastrulation the change in response to bFGF of the ventral ectoderm preceded that of the dorsal ectoderm. The state of competence of the ectoderm changed primarily due to intrinsic factors rather than by instruction from other parts of the gastrula embryo. This was shown by adding bFGF to cultures of ectoderm cells that were isolated at 9-hour (stage 10) and cultured for increasing periods to allow their autonomous development. The time course of both loss of neuronal competence and gain and loss of melanophore competence closely paralleled that observed in vivo during gastrulation.(ABSTRACT TRUNCATED AT 400 WORDS)

The community effect, dorsalization and mesoderm induction

The community effect is an interaction among muscle progenitor cells of amphibian gastrula, and is necessary for the initiation of muscle-specific gene expression. Dorsalization provides a signal that can convert ventral mesoderm cells to a muscle fate. Neither process involves mesoderm-inducing molecules. We suggest that the developmental significance of the community effect is to generate homogeneous but clearly demarcated groups of cells from progenitor cells arranged in a continuous gradient of developmental potential.

Genetic control of gastrulation in the mouse

In the past, understanding of the process of gastrulation in the mouse has primarily been based on morphological analyses. Recently, a number of molecules have been implicated in mesoderm induction and axis formation in Xenopus, and several of these exhibit unique patterns of expression during mouse gastrulation. These gene-expression data, together with fate mapping, ectopic expression experiments and mutational analysis, will now facilitate studies on the functional aspects of gastrulation in the mouse.

Fibroblast growth factor induces primitive streak formation in rabbit pre-implantation embryos in vitro

Culturing of rabbit pre-implantation embryos was performed in Ham's F10 medium supplemented with polyvinylpyrrolidone. Under these culture conditions, day 6 post coitum blastocysts increased their diameter within 24 h to 80% of that of day 7 blastocysts grown in vivo. Despite this substained growth, the embryonic disc remained undifferentiated with clear signs of degeneration after 24 h of culture. Basic fibroblast growth factor (bFGF) was able to overcome this developmental block. After 12 h of culture, day 6 blastocysts showed pear-shaped embryonic discs, and after 24 h, the primitive streak with Hensen's node was visible. The bFGF had no comparable effects on day 5 and day 7 blastocysts. The embryonic discs of day 5 blastocysts degenerated, even in the presence of bFGF, whereas day 7 blastocysts were able to form their primitive streak, also in the absence of bFGF. TGF beta 1 did not promote embryonic development in vitro. The data indicate that the onset of mesoderm formation in the rabbit is controlled by a growth factor of the FGF-family.