What's your position? the Xenopus cement gland as a paradigm of regional specification

Pitx1 regulates cement gland development in Xenopus laevis through activation of transcriptional targets and inhibition of BMP signaling

The cement gland in Xenopus laevis has long been used as a model to study the interplay of cell signaling and transcription factors during embryogenesis. It has been shown that an intermediate level of Bone Morphogenetic Protein (BMP) signaling is essential for cement gland formation. In addition, several transcription factors have been linked to cement gland development. One of these, the homeodomain-containing protein Pitx1, can generate ectopic cement gland formation; however, the mechanisms underlying this process remain obscure. We report here, for the first time, a requirement for Pitx proteins in cement gland formation, in vivo: knockdown of both pitx1 and the closely related pitx2c inhibit endogenous cement gland formation. Pitx1 transcriptionally activates cement gland differentiation genes through both direct and indirect mechanisms, and functions as a transcriptional activator to inhibit BMP signaling. This inhibition, required for the expression of pitx genes, is partially mediated by Pitx1-dependent follistatin expression. Complete suppression of BMP signaling inhibits induction of cement gland markers by Pitx1; furthermore, we find that Pitx1 physically interacts with Smad1, an intracellular transducer of BMP signaling. We propose a model of cement gland formation in which Pitx1 limits local BMP signaling within the cement gland primordium, and recruits Smad1 to activate direct downstream targets.

Tbx3 represses bmp4 expression and, with Pax6, is required and sufficient for retina formation

Vertebrate eye formation begins in the anterior neural plate in the eye field. Seven eye field transcription factors (EFTFs) are expressed in eye field cells and when expressed together are sufficient to generate retina from pluripotent cells. The EFTF Tbx3 can regulate the expression of some EFTFs; however, its role in retina formation is unknown. Here, we show that Tbx3 represses bmp4 transcription and is required in the eye field for both neural induction and normal eye formation in Xenopus laevis Although sufficient for neural induction, Tbx3-expressing pluripotent cells only form retina in the context of the eye field. Unlike Tbx3, the neural inducer Noggin can generate retina both within and outside the eye field. We found that the neural and retina-inducing activity of Noggin requires Tbx3. Noggin, but not Tbx3, induces Pax6 and coexpression of Tbx3 and Pax6 is sufficient to determine pluripotent cells to a retinal lineage. Our results suggest that Tbx3 represses bmp4 expression and maintains eye field neural progenitors in a multipotent state; then, in combination with Pax6, Tbx3 causes eye field cells to form retina.

The Nodal signaling pathway controls left-right asymmetric development in amphioxus

Background

Nodal is an important determinant of the left-right (LR) body axis in bilaterians, specifying the right side in protostomes and non-chordate deuterostomes as opposed to the left side in chordates. Amphioxus represents an early-branching chordate group, rendering it especially useful for studying the character states that predate the origin of vertebrates. However, its anatomy, involving offset arrangement of axial structures, marked asymmetry of the oropharyngeal region, and, most notably, a mouth positioned on the left side, contrasts with the symmetric arrangement of the corresponding regions in other chordates.

Results

We show that the Nodal signaling pathway acts to specify the LR axis in the cephalochordate amphioxus in a similar way as in vertebrates. At early neurula stages, Nodal switches from initial bilateral to the left-sided expression and subsequently specifies the left embryonic side. Perturbation of Nodal signaling with small chemical inhibitors (SB505124 and SB431542) alters expression of other members of the pathway and of left/right-sided, organ-specific genes. Upon inhibition, larvae display loss of the innate alternation of both somites and axons of peripheral nerves and loss of left-sided pharyngeal structures, such as the mouth, the preoral pit, and the duct of the club-shaped gland. Concomitantly, the left side displays ectopic expression of otherwise right-sided genes, and the larvae exhibit bilaterally symmetrical morphology, with duplicated endostyle and club-shaped gland structures.

Conclusions

We demonstrate that Nodal signaling is necessary for establishing the LR embryonic axis and for developing profound asymmetry in amphioxus. Our data suggest that initial symmetry breaking in amphioxus and propagation of the pathway on the left side correspond with the situation in vertebrates. However, the organs that become targets of the pathway differ between amphioxus and vertebrates, which may explain the pronounced asymmetry of its oropharyngeal and axial structures and the left-sided position of the mouth.

Electronic supplementary material

The online version of this article (doi:10.1186/2041-9139-6-5) contains supplementary material, which is available to authorized users.

Development and evolution of the vertebrate primary mouth

The vertebrate oral region represents a key interface between outer and inner environments, and its structural and functional design is among the limiting factors for survival of its owners. Both formation of the respective oral opening (primary mouth) and establishment of the food-processing apparatus (secondary mouth) require interplay between several embryonic tissues and complex embryonic rearrangements. Although many aspects of the secondary mouth formation, including development of the jaws, teeth or taste buds, are known in considerable detail, general knowledge about primary mouth formation is regrettably low. In this paper, primary mouth formation is reviewed from a comparative point of view in order to reveal its underestimated morphogenetic diversity among, and also within, particular vertebrate clades. In general, three main developmental modes were identified. The most common is characterized by primary mouth formation via a deeply invaginated ectodermal stomodeum and subsequent rupture of the bilaminar oral membrane. However, in salamander, lungfish and also in some frog species, the mouth develops alternatively via stomodeal collar formation contributed both by the ecto- and endoderm. In ray-finned fishes, on the other hand, the mouth forms via an ectoderm wedge and later horizontal detachment of the initially compressed oral epithelia with probably a mixed germ-layer derivation. A very intriguing situation can be seen in agnathan fishes: whereas lampreys develop their primary mouth in a manner similar to the most common gnathostome pattern, hagfishes seem to undergo a unique oropharyngeal morphogenesis when compared with other vertebrates. In discussing the early formative embryonic correlates of primary mouth formation likely to be responsible for evolutionary-developmental modifications of this area, we stress an essential role of four factors: first, positioning and amount of yolk tissue; closely related to, second, endoderm formation during gastrulation, which initiates the process and constrains possible evolutionary changes within this area; third, incipient structure of the stomodeal primordium at the anterior neural plate border, where the ectoderm component of the prospective primary mouth is formed; and fourth, the prime role of Pitx genes for establishment and later morphogenesis of oral region both in vertebrates and non-vertebrate chordates.

Keeping two animal systems in one lab - a frog plus fish case study

For two decades, my lab has been studying development using two vertebrate animals, the frog Xenopus and the zebrafish, Danio. This has been both productive and challenging. The initial rationale for the choice was to compare the same process in two species, as a means to find commonalities that may carry through all vertebrates. As time progressed, however, each species has become exploited for its specific attributes, more than for comparative studies. Maintaining two species simultaneously has been challenging, as has the division of research between the two and making sure that lab members know both systems well enough to communicate productively. Other significant issues concern funding for disparate research, figuring out how to make contributions to both fish and frog communities, and being accepted as a member of two communities. I discuss whether this dual allegiance has been a good idea.

Conservation, development, and function of a cement gland-like structure in the fish Astyanax mexicanus

The larvae of the fish Astyanax mexicanus transiently develop a flat and adhesive structure on the top of their heads that we have called "the casquette" (cas, meaning "hat"). We hypothesized that the cas may be a teleostean homolog of the well-studied Xenopus cement gland, despite their different positions and structures. Here we show that the cas has an ectodermal origin, secretes mucus, expresses bone morphogenic protein 4 (Bmp4) and pituitary homeobox 1/2 (Pitx1/2), is innervated by the trigeminal ganglion and serotonergic raphe neurons, and has a role in the control and the development of the larval swimming behavior. These developmental, connectivity, and behavioral functional data support a level of deep homology between the frog cement gland and the Astyanax cas and suggest that attachment organs can develop in varied positions on the head ectoderm by recruitment of a Bmp4-dependent developmental module. We also show that the attachment organs of the cichlid Tilapia mariae larvae display some of these features. We discuss the possibility that these highly diversified attachment glands may be ancestral to chordates and have been lost repetitively in many vertebrate classes.

Nemo-like kinase-myocyte enhancer factor 2A signaling regulates anterior formation in Xenopus development

The development of anterior neural structure in Xenopus laevis requires the inhibition of bone morphogenic protein 4 and Wnt signaling. We previously reported that Nemo-like kinase (NLK) negatively regulates Wnt signaling via the phosphorylation of T-cell factor/lymphoid enhancer factor. However, the molecular events occurring downstream of NLK pathways in early neural development remain unclear. In the present study, we identified the transcription factor myocyte enhancer factor 2A (MEF2A) as a novel substrate for NLK. NLK regulates the function of Xenopus MEF2A (xMEF2A) via phosphorylation, and this modification can be inhibited by the depletion of endogenous NLK. In Xenopus embryos, the depletion of either NLK or MEF2A results in a severe defect in anterior development. The endogenous expression of anterior markers was blocked by the depletion of endogenous Xenopus NLK (xNLK) or xMEF2A but, notably, not by the depletion of other xMEF2 family proteins, xMEF2C and xMEF2D. Defects in head formation or the expression of the anterior marker genes caused by the depletion of endogenous xMEF2A could be eliminated by the expression of wild-type xMEF2A, but not xMEF2A containing mutated xNLK phosphorylation sites. Furthermore, the expression of xNLK-induced anterior markers was efficiently blocked by the depletion of endogenous xMEF2A in animal pole explants. These results show that NLK specifically regulates the MEF2A activity required for anterior formation in Xenopus development.

Evolutionary modification of mouth position in deuterostomes

In chordates, the oral ectoderm is positioned at the anterior neural boundary and is characterized by pituitary homeobox (Pitx) and overlapping Dlx and Six3 expressions. Recent studies have shown that the ectoderm molecular map is also conserved in hemichordates and echinoderms. However, the mouth develops in a more posterior position in these animals, in a domain characterized by Nkx2.1 and Goosecoid expression, in a manner similar to that observed in protostomes. Furthermore, BMP signaling antagonizes mouth development in echinoderms and hemichordates, but seems to promote oral ectoderm specification in chordates. Conversely, Nodal signaling appears to be required for oral ectoderm specification in sea urchins but not in chordates. The Nodal/BMP antagonism at work during ectoderm patterning thus seems to constitute a conserved feature in deuterostomes, and mouth relocation may have been accompanied by a change in the influence of BMP/Nodal signals on oral ectoderm specification. We suggest that the mouth primordium was located at the anterior neural boundary, in early chordate evolution. In extant chordate embryos, subsequent mouth positioning differ between urochordates and vertebrates, presumably as a consequence of surrounding tissues remodelling. We illustrate these morphogenetic movements by means of morphological data obtained by the confocal imaging of ascidian tailbud embryos, and provide a table for determining the tailbud stages of this model organism.

Positioning the extreme anterior in Xenopus: cement gland, primary mouth and anterior pituitary

The extreme anterior of the deuterostome embryo is unusual in that ectoderm and endoderm are directly juxtaposed, without intervening mesoderm. In all vertebrates, this region gives rise to the anterior pituitary, the primary mouth and, in most frogs, to the mucus-secreting cement gland. Using the frog Xenopus laevis as a paradigm, we suggest that, initially, the extreme anterior forms a homogenous domain characterized by expression of pitx genes. Subsequently, this domain becomes subdivided to form these three different structures under the influence of different inductive signals from surrounding tissues.

21st century neontology and the comparative development of the vertebrate skull

Classic neontology (comparative embryology and anatomy), through the application of the concept of homology, has demonstrated that the development of the gnathostome (jawed vertebrate) skull is characterized both by a fidelity to the gnathostome bauplan and the exquisite elaboration of final structural design. Just as homology is an old concept amended for modern purposes, so are many of the questions regarding the development of the skull. With due deference to Geoffroy-St. Hilaire, Cuvier, Owen, Lankester et al., we are still asking: How are bauplan fidelity and elaboration of design maintained, coordinated, and modified to generate the amazing diversity seen in cranial morphologies? What establishes and maintains pattern in the skull? Are there universal developmental mechanisms underlying gnathostome autapomorphic structural traits? Can we detect and identify the etiologies of heterotopic (change in the topology of a developmental event), heterochronic (change in the timing of a developmental event), and heterofacient (change in the active capacetence, or the elaboration of capacity, of a developmental event) changes in craniofacial development within and between taxa? To address whether jaws are all made in a like manner (and if not, then how not), one needs a starting point for the sake of comparison. To this end, we present here a "hinge and caps" model that places the articulation, and subsequently the polarity and modularity, of the upper and lower jaws in the context of cranial neural crest competence to respond to positionally located epithelial signals. This model expands on an evolving model of polarity within the mandibular arch and seeks to explain a developmental patterning system that apparently keeps gnathostome jaws in functional registration yet tractable to potential changes in functional demands over time. It relies upon a system for the establishment of positional information where pattern and placement of the "hinge" is driven by factors common to the junction of the maxillary and mandibular branches of the first arch and of the "caps" by the signals emanating from the distal-most first arch midline and the lamboidal junction (where the maxillary branch meets the frontonasal processes). In this particular model, the functional registration of jaws is achieved by the integration of "hinge" and "caps" signaling, with the "caps" sharing at some critical level a developmental history that potentiates their own coordination. We examine the evidential foundation for this model in mice, examine the robustness with which it can be applied to other taxa, and examine potential proximate sources of the signaling centers. Lastly, as developmental biologists have long held that the anterior-most mesendoderm (anterior archenteron roof or prechordal plate) is in some way integral to the normal formation of the head, including the cranial skeletal midlines, we review evidence that the seminal patterning influences on the early anterior ectoderm extend well beyond the neural plate and are just as important to establishing pattern within the cephalic ectoderm, in particular for the "caps" that will yield medial signaling centers known to coordinate jaw development.

Grainyhead-like 3, a transcription factor identified in a microarray screen, promotes the specification of the superficial layer of the embryonic epidermis

The Xenopus ectoderm consists of two populations of cells, superficial polarised epithelial cells and deep, non-epithelial cells. These two cell types differ in their developmental fate. In the neural ectoderm, primary neurons are derived only from the deep cells. In the epidermal ectoderm, superficial cells express high levels of differentiation markers, while most of the deep cells do not differentiate until later when they produce the stratified adult epidermis. However, few molecular differences are known between the deep and superficial cells. Here, we have undertaken a systematic approach to identify genes that show layer-restricted expression by microarray analysis of deep and superficial cells at the gastrula stage, followed by wholemount in situ hybridisation. We have identified 32 differentially expressed genes, of which 26 show higher expression in the superficial layer and 6 in the deep layer and describe their expression at the gastrula and neurula stage. One of the identified genes is the transcription factor Grhl3, which we found to be expressed in the superficial layer of the gastrula ectoderm and the neurula epidermis. By using markers identified in this work, we show that Grlh3 promotes superficial gene expression in the deep layer of the epidermis. Concomitantly, deep layer specific genes are switched off, showing that Grlh3 can promote deep cells to take on a superficial cell identity in the embryonic epidermis.

BMP-3 is a novel inhibitor of both activin and BMP-4 signaling in Xenopus embryos

In Xenopus, the biological effects of BMP-3 oppose those of ventralizing BMPs, but the mechanism for this antagonism remains unclear. Here, we demonstrate that BMP-3 is a dorso-anteriorizing factor in Xenopus embryos that interferes with both activin and BMP signaling. BMP-3 acts by binding to ActRIIB, the common type II receptor for these proteins. Once BMP-3 binds to ActRIIB, it cannot be competed off by excess ligand making a receptor complex that is unable to activate R-Smads and transduce signal. Consistent with a model where BMP-3 interferes with activin and BMPs through a shared receptor, we show that overexpression of BMP-3 can only be rescued by co-injection of xActRIIB. Our results identify BMP-3 as a novel antagonist of both activin and BMPs and uncover how some of the diverse developmental processes that are regulated by both activin and BMP signaling can be modulated during embryogenesis.

A modular cis-regulatory system controls isoform-specific pitx expression in ascidian stomodaeum

Pituitary homeobox (pitx) genes have been identified in vertebrates as critical molecular determinants of various craniofacial ontogenetic processes including pituitary organogenesis. Accordingly, a prominent conserved feature of pitx genes in chordates is their early expression in the anterior neural boundary (ANB) and oral ectoderm, also known as the stomodaeum. Here we used the ascidian model species Ciona intestinalis to investigate pitx gene organization and cis-regulatory logic during early stages of oral development. Two distinct Ci-pitx mRNA variants were found to be expressed in mutually exclusive embryonic domains. Ci-pitx and vertebrate pitx2 genes display remarkably similar exon usage and organization, suggesting ancestry of the pitx transcriptional unit and regulation in chordates. We next combined phylogenetic footprinting, transient transgenesis, and confocal imaging methods to study the Ci-pitx cis-regulatory system, with special emphasis on the regulation of isoform-specific ANB/stomodaeal expression. Among 10 conserved noncoding sequences (CNSs) interspersed in C. intestinalis and Ciona savignyi pitx loci, we identified two separate cis-regulatory modules (CRMs) that drive ANB/stomodaeal expression in complementary spatiotemporal patterns. We discuss the developmental relevance of these data that provide an entry point to investigate the gene regulatory networks (GRNs) that position and shape oral structures in chordates.

XEpac, a guanine nucleotide-exchange factor for Rap GTPase, is a novel hatching gland specific marker during theXenopus embryogenesis

cAMP is a second messenger controlling various cellular processes through cAMP-dependent protein kinase (cAPK, PKA) and cyclic nucleotide-gated ion channels. Recently, the PKA-independent-cAMP-mediated signaling pathway by means of exchange protein directly activated by cAMP (Epac) has been demonstrated. Epac is a guanine nucleotide-exchange factor (GEF) for Rap, a Ras-like small GTPase. To investigate this new target for cAMP in development, we have isolated Xepac, the Xenopus laevis homologue of Epac by cDNA library screening. Xepac (Xepac1) encodes 890 amino acids, which have 57% identity with human Epac1 and 59% with that of rat Epac1 in amino acids. Whole-mount in situ hybridization and reverse transcriptase-polymerase chain reaction analysis show that XEpac is expressed both maternally and zygotically and is restricted within the developing hatching gland. Intriguingly, overexpression of XEpac induces the anterior markers XAG-1 and XOtx2 and can convert ectoderm into cement- and hatching gland-expressing cells. These results suggest that XEpac contains anterior positional information.

Larval cement gland of frogs: Comparative development and morphology

The cement gland (CG) is a transient mucus-secreting organ, found in most anuran embryos and early larvae and located normally on the anteroventral side of the head. Its sticky secretion allows newly hatched larvae to attach to the egg jelly or to another support and remain hidden and stationary until feeding starts. Analysis of CG morphology in 20 anuran species from six families using scanning electron microscopy revealed five distinct patterns of development, which partly related to families. The five patterns are described, as well as additional details such as CG surface ciliation and asymmetry. Three species lacked a CG. This was expected in two cases, a late-hatching phyllomedusine hylid and a direct-developing eleutherodactylid, but not in the foam-nesting Leptodactylus fuscus, which hatches at the same stage as many species that develop a CG. Lack of the CG in L. fuscus suggests that its posthatching period in the foam nest may be obligate. In both L. fuscus and the phyllomedusine hylid, there remain morphological traces of CG development.