FGF4 regulates blood and muscle specification in Xenopus laevis

Xbra modulates the activity of linker region phosphorylated Smad1 during Xenopus development

The Bmp/Smad1 pathway plays a crucial role in developmental processes and tissue homeostasis. Mitogen-activated protein kinase (Mapk)/Erk mediated phosphorylation of Smad1 in the linker region leads to Smad1 degradation, cytoplasmic retention and inhibition of Bmp/Smad1 signaling. While Fgf/Erk pathway has been documented to inhibit Bmp/Smad1 signaling, several studies also suggests the cooperative interaction between these two pathways in different context. However, the precise role and molecular pathway of this collaborative interaction remain obscure. Here, we identified Xbra induced by Fgf/Erk signaling as a factor in a protective mechanism for Smad1. Xbra physically interacted with the linker region phosphorylated Smad1 to make Xbra/Smad1/Smad4 trimeric complex, leading to Smad1 nuclear localization and protecting it from ubiquitin-mediated proteasomal degradation. This interaction of Xbra/Smad1/Smad4 led to sustained nuclear localization of Smad1 and the upregulation of lateral mesoderm genes, while concurrently suppression of neural and blood forming genes. Taken together, the results suggests Xbra-dependent cooperative interplays between Fgf/Erk and Bmp/Smad1 signaling during lateral mesoderm specification in Xenopus embryos.

Rapalog-induced cell adhesion molecule inhibits mesoderm migration in Xenopus embryos by increasing frequency of adhesion to the ectoderm

During the gastrula stage of Xenopus laevis, mesodermal cells migrate on the blastocoel roof (BCR) toward the animal pole. In this process, mesodermal cells directly adhere to the BCR via adhesion molecules, such as cadherins, which in turn trigger a repulsive reaction through factors such as Eph/ephrin. Therefore, the mesoderm and BCR repeatedly adhere to and detach from each other, and the frequency of this adhesion is thought to control mesoderm migration. Although knockdown of cadherin or Eph/ephrin causes severe gastrulation defects, these molecules have been reported to contribute not only to boundary formation but also to the internal function of each tissue. Therefore, it is possible that the defect caused by knockdown occurs due to tissue function abnormalities. To address this problem, we developed a method to specifically induce adhesion between different tissues using rapalog (an analog of rapamycin). When adhesion between the BCR and mesoderm was specifically enhanced by rapalog, mesoderm migration was strongly suppressed. Furthermore, we confirmed that rapalog significantly increased the frequency of adhesion between the two tissues. These results support the idea that the adhesion frequency controls mesoderm migration, and demonstrate that our method effectively enhances adhesion between specific tissues in vivo.

Fgf-driven Tbx protein activities directly induce myf5 and myod to initiate zebrafish myogenesis

Skeletal muscle derives from dorsal mesoderm formed during vertebrate gastrulation. Fibroblast growth factor (Fgf) signalling cooperates with Tbx transcription factors to promote dorsal mesoderm formation, but their role in myogenesis has been unclear. Using zebrafish, we show that dorsally derived Fgf signals act through Tbx16 and Tbxta to induce slow and fast trunk muscle precursors at distinct dorsoventral positions. Tbx16 binds to and directly activates the myf5 and myod genes, which are required for commitment to myogenesis. Tbx16 activity depends on Fgf signalling from the organiser. In contrast, Tbxta is not required for myf5 expression, but binds a specific site upstream of myod that is not bound by Tbx16 and drives (dependent on Fgf signals) myod expression in adaxial slow precursors, thereby initiating trunk myogenesis. After gastrulation, when similar muscle cell populations in the post-anal tail are generated from tailbud, declining Fgf signalling is less effective at initiating adaxial myogenesis, which is instead initiated by Hedgehog signalling from the notochord. Our findings suggest a hypothesis for ancestral vertebrate trunk myogenic patterning and how it was co-opted during tail evolution to generate similar muscle by new mechanisms.

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Highlighted Article: Tbx16 and Tbxta activate myf5 and myod directly during the earliest myogenesis in zebrafish, and Fgf signalling acts through Tbx16 to drive myogenesis in trunk but not tail.

Expression of apoptosis and myogenesis related genes during prenatal life in two divergent breeds of pigs

We aimed to evaluate apoptosis and myogenesis related genes expression in embryos and fetuses from two divergent genetic groups of pigs: Piau breed and a commercial line. Thirty females (15 Piau and 15 commercial line) were selected at 120 days of age. Estrous cycle was observed and on the third estrus females were considered sexually mature. Gilts were inseminated with semen from males of the respective breed. Three females from each breed were slaughtered at five different gestational ages: 15, 30, 45, 60 and 90 days. Whole embryos (15 and 30 d) and samples of longissimus dorsi muscle from fetuses (45, 60 and 90 d) were collected for RNA extraction. Expression of apoptosis and myogenesis related genes (BAX, BCL2, FGF4, IHH, HHIP, SHH, SOX2, WNT1 and WNT4) were evaluated by quantitative real time PCR. There was significant effect of interaction between breeds and gestational ages for all genes evaluated (P < 0.05). The BCL2 gene expression differed throughout pregnancy in Piau group with lower expression on day 15. The IHH gene expression throughout pregnancy was lower on days 15 and 60 in Piau group and lower on day 90 in commercial line group; and the SHH gene expression throughout pregnancy was higher on day 30 and lower on day 60 in Piau group and lower on day 45 in commercial group. The WNT1 gene expression along pregnancy was lower on day 15 and higher on days 30 and 45 in Piau group, and it was higher on day 30 than days 15 and 60 in commercial group. WNT4 gene expression throughout pregnancy was lower on day 15 in Piau group, and it was lower on day 30 in commercial pigs. Piau group presented higher expression of the FGF4 gene on days 45, 60 and 90, and commercial group showed higher expression on day 15 and 90. SOX2 gene expression was lower on day 15 in Piau pigs and it was constant throughout pregnancy in commercial group. Overall, the expression of IHH, SHH, WNT1, WNT4 and FGF4 genes were higher in commercial than Piau pigs on day 15, besides expression of BCL2 gene in Piau embryos was lower on day 15; these results might indicate that the muscle precursor cells are allowed to proliferate for a longer time in commercial than in Piau embryos by the balance of proliferative and apoptotic genes. Therefore, the expression differential between breeds can stimulate proliferation and differentiation of cells in different ways, explaining the postnatal differences in the muscularity between pigs from Piau breed and a commercial line.

The splicing factor PQBP1 regulates mesodermal and neural development through FGF signaling

Alternative splicing of pre-mRNAs is an important means of regulating developmental processes, yet the molecular mechanisms governing alternative splicing in embryonic contexts are just beginning to emerge. Polyglutamine-binding protein 1 (PQBP1) is an RNA-splicing factor that, when mutated, in humans causes Renpenning syndrome, an X-linked intellectual disability disease characterized by severe cognitive impairment, but also by physical defects that suggest PQBP1 has broader functions in embryonic development. Here, we reveal essential roles for PQBP1 and a binding partner, WBP11, in early development of Xenopus embryos. Both genes are expressed in the nascent mesoderm and neurectoderm, and morpholino knockdown of either causes defects in differentiation and morphogenesis of the mesoderm and neural plate. At the molecular level, knockdown of PQBP1 in Xenopus animal cap explants inhibits target gene induction by FGF but not by BMP, Nodal or Wnt ligands, and knockdown of either PQBP1 or WBP11 in embryos inhibits expression of fgf4 and FGF4-responsive cdx4 genes. Furthermore, PQBP1 knockdown changes the alternative splicing of FGF receptor-2 (FGFR2) transcripts, altering the incorporation of cassette exons that generate receptor variants (FGFR2 IIIb or IIIc) with different ligand specificities. Our findings may inform studies into the mechanisms underlying Renpenning syndrome.

Stochastic specification of primordial germ cells from mesoderm precursors in axolotl embryos

A common feature of development in most vertebrate models is the early segregation of the germ line from the soma. For example, in Xenopus and zebrafish embryos primordial germ cells (PGCs) are specified by germ plasm that is inherited from the egg; in mice, Blimp1 expression in the epiblast mediates the commitment of cells to the germ line. How these disparate mechanisms of PGC specification evolved is unknown. Here, in order to identify the ancestral mechanism of PGC specification in vertebrates, we studied PGC specification in embryos from the axolotl (Mexican salamander), a model for the tetrapod ancestor. In the axolotl, PGCs develop within mesoderm, and classic studies have reported their induction from primitive ectoderm (animal cap). We used an axolotl animal cap system to demonstrate that signalling through FGF and BMP4 induces PGCs. The role of FGF was then confirmed in vivo. We also showed PGC induction by Brachyury, in the presence of BMP4. These conditions induced pluripotent mesodermal precursors that give rise to a variety of somatic cell types, in addition to PGCs. Irreversible restriction of the germ line did not occur until the mid-tailbud stage, days after the somatic germ layers are established. Before this, germline potential was maintained by MAP kinase signalling. We propose that this stochastic mechanism of PGC specification, from mesodermal precursors, is conserved in vertebrates.

Inhibition of FGF signaling converts dorsal mesoderm to ventral mesoderm in early Xenopus embryos

In early vertebrate development, mesoderm induction is a crucial event regulated by several factors including the activin, BMP and FGF signaling pathways. While the requirement of FGF in Nodal/activin-induced mesoderm formation has been reported, the fate of the tissue modulated by these signals is not fully understood. Here, we examined the fate of tissues when exogenous activin was added and FGF signaling was inhibited in animal cap explants of Xenopus embryos. Activin-induced dorsal mesoderm was converted to ventral mesoderm by inhibition of FGF signaling. We also found that inhibiting FGF signaling in the dorsal marginal zone, in vegetal-animal cap conjugates or in the presence of the activin signaling component Smad2, converted dorsal mesoderm to ventral mesoderm. The expression and promoter activities of a BMP responsive molecule, PV.1 and a Spemann organizer, noggin, were investigated while FGF signaling was inhibited. PV.1 expression increased, while noggin decreased. In addition, inhibiting BMP-4 signaling abolished ventral mesoderm formation induced by exogenous activin and FGF inhibition. Taken together, these results suggest that the formation of dorso-ventral mesoderm in early Xenopus embryos is regulated by a combination of FGF, activin and BMP signaling.

FGF signalling: diverse roles during early vertebrate embryogenesis

Fibroblast growth factor (FGF) signalling has been implicated during several phases of early embryogenesis, including the patterning of the embryonic axes, the induction and/or maintenance of several cell lineages and the coordination of morphogenetic movements. Here, we summarise our current understanding of the regulation and roles of FGF signalling during early vertebrate development.

The role of FGF signaling in the establishment and maintenance of mesodermal gene expression in Xenopus

FGF signaling is important for the formation of mesoderm in vertebrates, and when it is perturbed in Xenopus, most trunk and tail mesoderm fails to form. Here we have further dissected the activities of FGF in patterning the embryo by addressing its inductive and maintenance roles. We show that FGF signaling is necessary for the establishment of xbra expression in addition to its well-characterized role in maintaining xbra expression. The role of FGF signaling in organizer formation is not clear in Xenopus. We find that FGF signaling is essential for the initial specification of paraxial mesoderm but not for activation of several pan-mesodermal and most organizer genes; however, early FGF signaling is necessary for the maintenance of organizer gene expression into the neurula stage. Inhibition of FGF signaling prevents VegT activation of specific mesodermal transcripts. These findings illuminate how FGF signaling contributes to the establishment of distinct types of mesoderm.

Fibroblast growth factor controls the timing of Scl, Lmo2, and Runx1 expression during embryonic blood development

To program pluripotent cells into blood, a knowledge of the locations of precursors during their journey through the embryo and the signals they experience would be informative. The anterior (a) and posterior (p) ventral blood islands (VBIs) in Xenopus are derived from opposite sides of the pregastrula embryo. The aVBI goes through a “hemangioblast” state, characterized by coexpression of blood and endothelial genes at neurula stages, whereas the pVBI expresses these genes in a nonoverlapping fashion several hours later, after commitment to either a blood or an endothelial fate. We describe a novel role for fibroblast growth factor (FGF) in controlling the timing of Scl, Lmo2, and Runx1 expression in the 2 VBI compartments. Blocking FGF signaling during gastrulation expands expression at neurula stages into posterior regions. We show, by lineage labeling, explant analysis, and targeted blocking of FGF signaling, that this is due to the pVBI prematurely expressing these genes with the timing of the aVBI. In contrast, overexpression of FGF in aVBI precursors eliminates the anterior hemangioblast program. Using this information, we have recapitulated the anterior hemangioblast program in pluripotent cells in vitro by inhibiting FGF signaling in anterior mesoderm induced by activin and exposed to bone morphogenetic protein (BMP) signaling.

Hes6 is required for MyoD induction during gastrulation

The specification of mesoderm into distinct compartments sharing the same lineage restricted fates is a crucial step occurring during gastrulation, and is regulated by morphogenic signals such as the FGF/MAPK and activin pathways. One target of these pathways is the transcription factor XmyoD, which in early gastrulation is expressed in the lateral and ventral mesoderm. Expression of the hairy/enhancer of split transcription factor hes6, is also restricted to lateral and ventral mesoderm in gastrula stage Xenopus embryos, leading us to investigate whether it has a role in XmyoD regulation. In vivo, Xhes6 is required for FGF-mediated induction of XmyoD expression but not for induction of early mesoderm. The WRPW domain of Xhes6, which binds Groucho family transcriptional co-regulators, is essential for the XmyoD-inducing activity of Xhes6. Two Groucho proteins, Xgrg2 and Xgrg4, are expressed in lateral and ventral mesoderm, and inhibit expression of XmyoD. Xhes6 binds both Xgrg2 and Xgrg4 and relieves their inhibition of XmyoD expression. We also find that lowering Xhes6 expression levels blocks normal myogenic differentiation at tail bud stage. We conclude that Xhes6 is essential for XmyoD induction and acts by relieving Groucho-mediated repression of gene expression.

Transcriptional networks regulating hematopoietic cell fate decisions

PURPOSE OF REVIEW

We provide a summary of the temporal cascade of transcriptional networks giving rise to the hematopoietic stem cell (HSC) and controlling differentiation of the erythroid lineage from it. We focus on the mechanisms by which cell fate decisions are made and comment on recent developments and additions to the networks.

RECENT FINDINGS

A role for an SCL/LMO2 complex in HSC emergence, as well as in subsequent erythroid differentiation, has received support. Connections between the transcriptional networks and signaling molecules are being made but more work is needed in this area. Evidence that transcriptional cross-antagonistic switches underlie the choice between lineage pathways is increasing, and we highlight how the dynamics of earlier lineage decisions can influence later ones. Mathematical models are being built and reveal a surprising degree of power in these simple motifs to explain lineage choices.

SUMMARY

New links in the transcriptional networks underlying cell-fate decisions are constantly emerging, and their incorporation into the evolving networks will make mathematical modeling more precise in its predictions of cell behavior, which can be tested experimentally.