The type I serine/threonine kinase receptor Alk8/Lost-a-fin is required for Bmp2b/7 signal transduction during dorsoventral patterning of the zebrafish embryo

Genetic analysis of the mammalian transforming growth factor-beta superfamily

Members of the TGF-beta superfamily, which includes TGF-betas, growth differentiation factors, bone morphogenetic proteins, activins, inhibins, and glial cell line-derived neurotrophic factor, are synthesized as prepropeptide precursors and then processed and secreted as homodimers or heterodimers. Most ligands of the family signal through transmembrane serine/threonine kinase receptors and SMAD proteins to regulate cellular functions. Many studies have reported the characterization of knockout and knock-in transgenic mice as well as humans or other mammals with naturally occurring genetic mutations in superfamily members or their regulatory proteins. These investigations have revealed that TGF-beta superfamily ligands, receptors, SMADs, and upstream and downstream regulators function in diverse developmental and physiological pathways. This review attempts to collate and integrate the extensive body of in vivo mammalian studies produced over the last decade.

BMP signalling regulates anteroposterior endoderm patterning in zebrafish

In vertebrates, the embryonic dorsoventral asymmetry is regulated by the bone morphogenetic proteins (Bmp) activity gradient. In the present study, we have used dorsalized swirl (bmp2b) and ventralized chordino (chordin) zebrafish mutants to investigate the effects of dorsoventral signalling on endoderm patterning and on the differentiation and positioning of its derivatives. Alterations of dorsoventral Bmp signalling do not perturb the induction of endodermal precursors, as shown by normal amounts of cells expressing cas and sox17 in swirl and chordino gastrulae, but affect dramatically the expression pattern of her5, a regulator of endoderm anteroposterior patterning in zebrafish. In particular, increased levels of Bmp signalling in chordino gastrulae are associated with a markedly reduced her5 expression domain, that may be abolished by injecting bmp2b mRNA. Conversely, in swirl mutants, lacking Bmp2b, the her5 expression domain is expanded. Thus, a gradient of Bmp2b signalling defines the extension of the her5 expression domain at gastrulation and the allocation of anterior endodermal precursors. A balanced Bmp2b signalling is also required for the normal development of the pancreas, as shown by the sharp reduction of the pancreatic primordium in swirl embryos and its expansion in chordino mutants. In the latter, at 3 days post-fertilization, the increased Bmp signalling does not compromise the endocrine/exocrine pancreas compartmentalization, but the right/left positioning of the pancreas and liver is randomized. Our results suggest that by regulating the expression of her5, the Bmp2b/Chordin gradient directs the anteroposterior patterning of endoderm in zebrafish embryos.

The novel type I serine-threonine kinase receptor Alk8 binds TGF-beta in the presence of TGF-betaRII

The TGF-beta superfamily consists of an array of ligands including BMP, TGF-beta, activin, and nodal subfamilies. The extensive range of biological effects elicited by TGF-beta family signaling is due in part to the large numbers and promiscuity of types I and II TGF-beta family member receptors. Alk8 is a novel type I TGF-beta family member receptor first identified in zebrafish [Dev. Dyn. 211 (4) (1998) 352], which participates in BMP signaling pathways [Development 128 (6) (2001) 849; Development 128 (6) (2001) 859; Mech. Dev. 100 (2) (2001) 275; J. Dent. Res. 80 (11) (2001) 1968]. Here we report that Alk8 also forms active signaling complexes with TGF-beta in the presence of TGF-betaRII. These results expand the signaling repertoire of zAlk8 by demonstrating an ability to participate in two distinct TGF-beta subfamily signaling pathways.

Zebrafish DeltaNp63 is a direct target of Bmp signaling and encodes a transcriptional repressor blocking neural specification in the ventral ectoderm

Bone morphogenetic proteins (Bmps) promote ventral specification in both the mesoderm and the ectoderm of vertebrate embryos. Here we identify zebrafish DeltaNp63, encoding an isoform of the p53-related protein p63, as an ectoderm-specific direct transcriptional target of Bmp signaling. DeltaNp63 itself acts as a transcriptional repressor required for ventral specification in the ectoderm of gastrulating embryos. Loss of DeltaNp63 function leads to reduced nonneural ectoderm followed by defects in epidermal development during skin and fin bud formation. In contrast, forced DeltaNp63 expression blocks neural development and promotes nonneural development, even in the absence of Bmp signaling. Together, DeltaNp63 fulfills the criteria to be the neural repressor postulated by the "neural default model."

Modulation of BMP activity in dorsal-ventral pattern formation by the chordin and ogon antagonists

We analyzed the interactions between mutations in antagonistic BMP pathway signaling components to examine the roles that the antagonists play in regulating BMP signaling activity. The dorsalized mutants swirl/bmp2b, snailhouse/bmp7, lost-a-fin/alk8, and mini fin/tolloid were each analyzed in double mutant combinations with the ventralized mutants chordino/chordin and ogon, whose molecular nature is not known. Similar to the BMP antagonist chordino, we found that the BMP ligand mutants swirl/bmp2b and snailhouse/bmp7 are also epistatic to the putative BMP pathway antagonist, ogon, excluding a class of intracellular antagonists as candidates for ogon. In ogon;mini fin double mutants, we observed a mutual suppression of the ogon and mini fin mutant phenotypes, frequently to a wild type phenotype. Thus, the Tolloid/Mini fin metalloprotease that normally cleaves and inhibits Chordin activity is dispensable, when Ogon antagonism is reduced. These results suggest that Ogon encodes a Tolloid and Chordin-independent antagonistic function. By analyzing genes whose expression is very sensitive to BMP signaling levels, we found that the absence of Ogon or Chordin antagonism did not increase the BMP activity remaining in swirl/bmp2b or hypomorphic snailhouse/bmp7 mutants. These results, together with other studies, suggest that additional molecules or mechanisms are essential in generating the presumptive gastrula BMP activity gradient that patterns the dorsal-ventral axis. Lastly we observed a striking increased penetrance of the swirl/bmp2b dominant dorsalized phenotype, when Chordin function is also absent. Loss of the BMP antagonist Chordin is expected to increase BMP signaling levels in a swirl heterozygote, but instead we observed an apparent decrease in BMP signaling levels and a loss of ventral tail tissue. As has been proposed for the fly orthologue of chordin, short gastrulation, our paradoxical results can be explained by a model whereby Chordin both antagonizes and promotes BMP activity.

Morpholino oligos: making sense of antisense?

Since morpholino oligos were first introduced as a means to inhibit gene function in embryos, in the Spring of 2000, they have been tested in a range of model organisms, including sea urchin, ascidian, zebrafish, frog, chick, and mouse. This review surveys the results of these studies and examines the successes and limitations of the approach for targeting maternal and zygotic gene function. The evidence so far suggests that, with careful controls, morpholinos provide a relatively simple and rapid method to study gene function.

Eye defects in receptor protein-tyrosine phosphatase alpha knock-down zebrafish

Receptor protein-tyrosine phosphatase alpha (RPTP alpha) is highly expressed in the developing retina of different species, but little is known about its function there. Here, we report that injection of antisense morpholinos in zebrafish embryos reduced RPTP alpha expression to almost nondetectable levels up to 3 days postfertilization (dpf). RPTP alpha was detectable again from 4 dpf onward. RPTP alpha knock-down resulted in smaller eyes. Examination of sections of the retina at different developmental stages demonstrated that already at 28 hours postfertilization (hpf) fewer cells were present in the retina of RPTP alpha-morpholino-injected embryos. At 3 dpf, the layered organization of the retina was absent. In addition, the morphology and labeling with an axon specific antibody, acetylated tubulin, demonstrated that most cells appeared to be undifferentiated. Strikingly, at 5 dpf the lamination of the retina was partially restored, concomitant with re-expression of RPTP alpha protein. Although cells in the retina were now differentiated, the layering of the retina remained disrupted and significant gaps were observed in the amacrine cell layer. Therefore, knock-down of RPTP alpha protein provides evidence that RPTP alpha is essential for normal retinal development.

Conservation and divergence in molecular mechanisms of axis formation

Genetic screens in Drosophila melanogaster have helped elucidate the process of axis formation during early embryogenesis. Axis formation in the D. melanogaster embryo involves the use of two fundamentally different mechanisms for generating morphogenetic activity: patterning the anteroposterior axis by diffusion of a transcription factor within the syncytial embryo and specification of the dorsoventral axis through a signal transduction cascade. Identification of Drosophila genes involved in axis formation provides a launch-pad for comparative studies that examine the evolution of axis specification in different insects. Additionally, there is similarity between axial patterning mechanisms elucidated genetically in Drosophila and those demonstrated for chordates such as Xenopus. In this review we examine the postfertilization mechanisms underlying axis specification in Drosophila. Comparative data are then used to ask whether aspects of axis formation might be derived or ancestral.

Regulation of tooth development by the novel type I TGFbeta family member receptor Alk8

We have recently identified, in zebrafish, a novel type I receptor of the TGFbeta family, alk8, that participates in Bmp signaling pathways to mediate early dorsoventral patterning of neurectodermal and mesendodermal tissues. Since Bmps play significant roles in tooth specification, initiation, and differentiation, we hypothesized that alk8 may play a role in directing the Bmp-mediated epithelial mesenchymal cell interactions regulating tooth development. Immunohistochemical analysis demonstrates that Alk8 is expressed in developing zebrafish and mouse teeth. Examination of tooth development in zebrafish with disrupted alk8 signaling revealed specific defects in tooth development. Ectopic expression of constitutively active Alk8 results in the formation of elongated tooth structures, while expression of dominant-negative Alk8 results in arrested tooth development at the bud stage. These results are consistent with the established requirements for Bmp signaling in tooth development and demonstrate that Alk8 is a key regulator of tooth development.

Expression of BMP signalling pathway members in the developing zebrafish inner ear and lateral line

In this paper we describe the mRNA expression patterns of members of the bone morphogenetic protein (BMP) signalling pathway in the developing zebrafish ear. bmp2b, 4, and 7 are expressed in discrete areas of otic epithelium, some of which correspond to sensory patches. bmp2b and 4 mark the developing cristae before and during the appearance of differentiated hair cells. bmp4 is also expressed in a dorsal, non-sensory region of the ear. Expression of bmps in cristae is conserved between zebrafish, chick, and mouse, but there are also notable differences in ear expression patterns between these species. Of five zebrafish BMP antagonists, only one (follistatin) shows significant expression in the otic epithelium. The type I receptor bmpr-IB shows localised expression in the ear epithelium. Mediators of BMP signalling, smad1 and smad5, are expressed in statoacoustic and lateral line ganglia; smad5 is also expressed at low levels throughout the ear epithelium. An inhibitory smad, smad6, is expressed laterally in the ear epithelium. Lateral line primordia and neuromasts also express bmp2b, 4, follistatin, smad1, and smad5. The conservation of bmp expression in cristae among different species adds weight to the growing evidence that BMPs are required for the development of the vertebrate ear.

New developments for TGFbeta

A recent FASEB meeting was held in Tucson, Arizona that encompassed TGFbeta superfamily signaling pathways and their roles in development. This review focuses on the developmental biology presented at the meeting.

A novel sox gene, 226D7, acts downstream of Nodal signaling to specify endoderm precursors in zebrafish

Vertebrate endoderm development has recently become the focus of intense investigation. We have identified a novel sox gene, 226D7, which is important in zebrafish endoderm development. 226D7 was isolated by an in situ hybridization screening for genes expressed in the yolk syncytial layer (YSL) at the blastula stage. 226D7 is expressed mainly in the YSL at this stage and, during gastrulation, its expression is also detected in the forerunner cells and endodermal precursor cells. The expression of 226D7 is positively regulated by Nodal signaling. The knockdown of 226D7 using morpholino antisense oligonucleotides results in a lack of sox17-expressing endodermal precursor cells during gastrulation, and, consequently, lacks endodermal derivatives such as gut tissue. The effect is strictly restricted to the endodermal lineage, while the mesoderm is normally formed, a phenotype that is nearly identical to that of the casanova mutant (Dev. Biol. 215 (1999) 343). We further demonstrate that overexpression of 226D7 increases the number of sox17-expressing endodermal progenitor cells without upregulating the expression of the Nodal genes, cyclops and squint. Region-specific knockdown and overexpression of 226D7 by injection into the YSL suggest that 226D7 in the YSL is not involved in endoderm formation and 226D7 in the endoderm progenitor cells is important for endoderm development. Taken together, our data demonstrate that 226D7 is a downstream target of Nodal signal and a critical transcriptional regulator of early endoderm formation.

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.

Axis formation and patterning in zebrafish

A large collection of mutations affecting zebrafish embryogenesis was described in 1996. The cloning of the affected genes has now provided novel insights into the role and regulation of signaling by BMP, Nodal, Wnt, FGF, Hedgehog, Delta, Slit, retinoic acid and lipids. Detailed analyses have revealed a complex genetic network that patterns the early embryo.

Bmp2b and Oep promote early myocardial differentiation through their regulation of gata5

Members of both the bone morphogenetic protein (Bmp) and EGF-CFC families have been implicated in vertebrate myocardial development. Zebrafish swirl (swr) encodes Bmp2b, a member of the Bmp family required for patterning the dorsoventral axis. Zebrafish one-eyed pinhead (oep) encodes a maternally and zygotically expressed member of the EGF-CFC family essential for Nodal signaling. Both swr/bmp2b and oep mutants exhibit severe defects in myocardial development. swr/bmp2b mutants exhibit reduced or absent expression of nkx2.5, an early marker of the myocardial precursors. Embryos lacking zygotic oep (Zoep mutants) display cardia bifida and, as we show here, also display reduced or absent nkx2.5 expression. Recently, we have demonstrated that the zinc finger transcription factor Gata5 is an essential regulator of nkx2.5 expression. In this paper, we investigate the relationships between bmp2b, oep, gata5, and nkx2.5. We show that both swr/bmp2b and Zoep mutants exhibit defects in gata5 expression in the myocardial precursors. Forced expression of gata5 in swr/bmp2b and Zoep mutants restores robust nkx2.5 expression. Moreover, overexpression of gata5 in Zoep mutants restores expression of cmlc1, a myocardial sarcomeric gene. These results indicate that both Bmp2b and Oep regulate gata5 expression in the myocardial precursors, and that Gata5 does not require Bmp2b or Oep to promote early myocardial differentiation. We conclude that Bmp2b and Oep function at least partly through Gata5 to regulate nkx2.5 expression and promote myocardial differentiation. We integrate these and other data to propose a pathway of the molecular events regulating early myocardial differentiation in zebrafish.