Positive and negative signals modulate formation of the Xenopus cement gland

Activation of goblet-cell stress sensor IRE1β is controlled by the mucin chaperone AGR2

Intestinal goblet cells are secretory cells specialized in the production of mucins, and as such are challenged by the need for efficient protein folding. Goblet cells express Inositol-Requiring Enzyme-1β (IRE1β), a unique sensor in the unfolded protein response (UPR), which is part of an adaptive mechanism that regulates the demands of mucin production and secretion. However, how IRE1β activity is tuned to mucus folding load remains unknown. We identified the disulfide isomerase and mucin chaperone AGR2 as a goblet cell-specific protein that crucially regulates IRE1β-, but not IRE1α-mediated signaling. AGR2 binding to IRE1β disrupts IRE1β oligomerization, thereby blocking its downstream endonuclease activity. Depletion of endogenous AGR2 from goblet cells induces spontaneous IRE1β activation, suggesting that alterations in AGR2 availability in the endoplasmic reticulum set the threshold for IRE1β activation. We found that AGR2 mutants lacking their catalytic cysteine, or displaying the disease-associated mutation H117Y, were no longer able to dampen IRE1β activity. Collectively, these results demonstrate that AGR2 is a central chaperone regulating the goblet cell UPR by acting as a rheostat of IRE1β endonuclease activity.

Molecular biology of microRNA-342 during tumor progression and invasion

Cancer is considered as one of the main causes of human deaths and health challenges in the world. Various factors are involved in the high death rate of cancer patients, including late diagnosis and drug resistance that result in treatment failure and tumor recurrence. Invasive diagnostic methods are one of the main reasons of late tumor detection in cancer patients. Therefore, it is necessary to investigate the molecular tumor biology to introduce efficient non-invasive markers. MicroRNAs (miRNAs) are involved in regulation of the cellular mechanisms such as cell proliferation, apoptosis, and migration. MiRNAs deregulations have been also frequently shown in different tumor types. Here, we discussed the molecular mechanisms of miR-342 during tumor growth. MiR-342 mainly functions as a tumor suppressor by the regulation of transcription factors and signaling pathways such as WNT, PI3K/AKT, NF-kB, and MAPK. Therefore, miR-342 mimics can be used as a reliable therapeutic strategy to inhibit the tumor cells growth. The present review can also pave the way to introduce the miR-342 as a non-invasive diagnostic/prognostic marker in cancer patients.

NEIL1 and NEIL2 DNA glycosylases protect neural crest development against mitochondrial oxidative stress

Base excision repair (BER) functions not only in the maintenance of genomic integrity but also in active DNA demethylation and epigenetic gene regulation. This dual role raises the question if phenotypic abnormalities resulting from deficiency of BER factors are due to DNA damage or impaired DNA demethylation. Here we investigate the bifunctional DNA glycosylases/lyases NEIL1 and NEIL2, which act in repair of oxidative lesions and in epigenetic demethylation. Neil-deficiency in Xenopus embryos and differentiating mouse embryonic stem cells (mESCs) leads to a surprisingly restricted defect in cranial neural crest cell (cNCC) development. Neil-deficiency elicits an oxidative stress-induced TP53-dependent DNA damage response, which impairs early cNCC specification. Epistasis experiments with Tdg-deficient mESCs show no involvement of epigenetic DNA demethylation. Instead, Neil-deficiency results in oxidative damage specific to mitochondrial DNA, which triggers a TP53-mediated intrinsic apoptosis. Thus, NEIL1 and NEIL2 DNA glycosylases protect mitochondrial DNA against oxidative damage during neural crest differentiation.

The face of animals with a backbone is formed in great part by a group of cells called cranial neural crest cells. When too few of these cells are made, the skull and the face can become deformed. For example, the jaw- or cheekbones can be underdeveloped or there may be defects in the eyes or ears. These types of abnormalities are among the most common birth defects known in humans.

NEIL1 and NEIL2 are mouse proteins with two roles. On the one hand, they help protect DNA from damage by acting as so-called ‘base excision repair enzymes’, meaning they remove damaged building blocks of DNA. On the other hand, they help remove a chemical group known as a methyl from DNA building blocks in a process called demethylation, which is involved both in development and disease. Previous research by Schomacher et al. in 2016 showed that, in frogs, the absence of a similar protein called Neil2, leads to deformities of the face and skull.

Han et al. – who include some of the researchers involved in the 2016 study – have now used frog embryos and mouse embryonic stem cells to examine the role of the NEIL proteins in cranial neural crest cells. Stem cells can become any type of cell in the body, but when NEIL1 and NEIL2 are missing, these cells lose the ability to become cranial neural crest cells.

To determine whether the effects of removing NEIL1 and NEIL2 were due to their role in DNA damage repair or demethylation, Han et al. removed two proteins, each involved in one of the two processes. Removing APEX1, which is involved in DNA damage repair, had similar effects to the removal of NEIL1 and NEIL2, while removing TDG, which only works in demethylation, did not. This indicates that NEIL1 and NEIL2’s role in DNA damage repair is likely necessary for stem cells to become cranial neural crest cells.

Although NEIL1 and NEIL2 are part of the DNA repair machinery, Han et al. showed that when stem cells turn into cranial neural crest cells, these proteins are not protecting the cell’s genomic DNA. Instead, they are active in the mitochondria, the compartments of the cell responsible for producing energy, which have their own DNA. Mitochondria use oxygen to produce energy, but by-products of these reactions damage mitochondrial DNA, explaining why mitochondria need NEIL1 and NEIL2. These results suggest that antioxidants, which are molecules that protect the cells from the damaging oxygen derivatives, may help prevent deformities in the face and skull. This theory could be tested using mice that do not produce proteins involved in base excision repair, which could be derived from the cells lacking NEIL1 and NEIL2.

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.

AGR2 diagnostic value in nasopharyngeal carcinoma prognosis

BACKGROUND

Anterior Gradient (AGR) 2 concentration increases in the serum of tumor patients, and their diagnostic and prognostic significances were evaluated in some tumors. The previous works showed that AGR2 high express in nasopharyngeal carcinoma (NPC) biopsy tissues. However, whether AGR2 serves as a diagnostic and prognostic marker for NPC remains unclear.

METHODS

42 healthy volunteers, 34 breast cancer patients and 124 NPC patients were enrolled into this study, and the serum samples were collected from these healthy volunteers, breast cancer patients and NPC patients. Concomitantly, 79 frozen nasopharyngeal specimens consisted of 65 NPC tissues and 14 normal nasopharyngeal tissues were enrolled in the observation. The enzyme linked immunosorbent assay (ELISA) was used to estimate AGR2 concentration in the serum samples, and AGR2 mRNA expressions in the frozen tissue samples were detected by real time RT-PCR.

RESULTS

The real time RT-PCR results showed that AGR2 mRNA level was increased in NPC tissues compared with the normal nasopharyngeal tissues (p<0.05). The ELISA data showed that AGR2 concentration in NPC serum was significantly higher in NPC patient serums than that in the health population (p<0.05). And, AGR2 expression showed a correlation with tumor node metastasis (TNM) grade (p<0.05) and Recurrence (p<0.05). Moreover, the cumulative survival rate of patients with high concentration of AGR2 was significantly lower than that of patients with low concentration of AGR2 (p<0.05), and the cumulative hazard rate of patients with high concentration of AGR2 was significantly higher than that with low concentration of AGR2 (p<0.05).

CONCLUSION

Serum AGR2 can be used as a serum marker for clinical prognosis of nasopharyngeal carcinoma. However, serum AGR2 levels could not provide advantages in clinical practice for the differential diagnosis of cancer.

Neil DNA glycosylases promote substrate turnover by Tdg during DNA demethylation

DNA 5-methylcytosine is a dynamic epigenetic mark which plays important roles in development and disease. In the Tet-Tdg demethylation pathway, methylated cytosine is iteratively oxidized by Tet dioxygenases and unmodified cytosine is restored via thymine DNA glycosylase (Tdg). Here we show that human NEIL1 and NEIL2 DNA glycosylases coordinate abasic site processing during TET–TDG DNA demethylation. NEIL1 and NEIL2 cooperate with TDG during base excision: TDG occupies the abasic site and is displaced by NEILs, which further process the baseless sugar, thereby stimulating TDG substrate turnover. In early Xenopus embryos Neil2 cooperates with Tdg to remove oxidized methylcytosines and to specify neural crest development together with Tet3. Thus, Neils function as AP lyases in the coordinated AP site hand-over during oxidative DNA demethylation.

Coco is a dual activity modulator of TGFβ signaling

The TGFβ signaling pathway is a crucial regulator of developmental processes and disease. The activity of TGFβ ligands is modulated by various families of soluble inhibitors that interfere with the interactions between ligands and receptors. In an unbiased, genome-wide RNAi screen to identify genes involved in ligand-dependent signaling, we unexpectedly identified the BMP/Activin/Nodal inhibitor Coco as an enhancer of TGFβ1 signaling. Coco synergizes with TGFβ1 in both cell culture and Xenopus explants. Molecularly, Coco binds to TGFβ1 and enhances TGFβ1 binding to its receptor Alk5. Thus, Coco acts as both an inhibitor and an enhancer of signaling depending on the ligand it binds. This finding raises the need for a global reconsideration of the molecular mechanisms regulating TGFβ signaling.

AGR3 in breast cancer: prognostic impact and suitable serum-based biomarker for early cancer detection

Blood-based early detection of breast cancer has recently gained novel momentum, as liquid biopsy diagnostics is a fast emerging field. In this study, we aimed to identify secreted proteins which are up-regulated both in tumour tissue and serum samples of breast cancer patients compared to normal tissue and sera. Based on two independent tissue cohorts (n = 75 and n = 229) and one serum cohort (n = 80) of human breast cancer and healthy serum samples, we characterised AGR3 as a novel potential biomarker both for breast cancer prognosis and early breast cancer detection from blood. AGR3 expression in breast tumours is significantly associated with oestrogen receptor α (P<0.001) and lower tumour grade (P<0.01). Interestingly, AGR3 protein expression correlates with unfavourable outcome in low (G1) and intermediate (G2) grade breast tumours (multivariate hazard ratio: 2.186, 95% CI: 1.008-4.740, P<0.05) indicating an independent prognostic impact. In sera analysed by ELISA technique, AGR3 protein concentration was significantly (P<0.001) elevated in samples from breast cancer patients (n = 40, mainly low stage tumours) compared to healthy controls (n = 40). To develop a suitable biomarker panel for early breast cancer detection, we measured AGR2 protein in human serum samples in parallel. The combined AGR3/AGR2 biomarker panel achieved a sensitivity of 64.5% and a specificity of 89.5% as shown by receiver operating characteristic (ROC) curve statistics. Thus our data clearly show the potential usability of AGR3 and AGR2 as biomarkers for blood-based early detection of human breast cancer.

AGR2, an endoplasmic reticulum protein, is secreted into the gastrointestinal mucus

The MUC2 mucin is the major constituent of the two mucus layers in colon. Mice lacking the disulfide isomerase-like protein Agr2 have been shown to be more susceptible to colon inflammation. The Agr2^−/− mice have less filled goblet cells and were now shown to have a poorly developed inner colon mucus layer. We could not show AGR2 covalently bound to recombinant MUC2 N- and C-termini as have previously been suggested. We found relatively high concentrations of Agr2 in secreted mucus throughout the murine gastrointestinal tract, suggesting that Agr2 may play extracellular roles. In tissue culture (CHO-K1) cells, AGR2 is normally not secreted. Replacement of the single Cys in AGR2 with Ser (C81S) allowed secretion, suggesting that modification of this Cys might provide a mechanism for circumventing the KTEL endoplasmic reticulum retention signal. In conclusion, these results suggest that AGR2 has both intracellular and extracellular effects in the intestine.

Emerging roles for the pro-oncogenic anterior gradient-2 in cancer development

Clinical studies have defined the core 'genetic blueprint' of a cancer cell, but this information does not necessarily predict the cancer phenotype. Signalling hubs that mediate such phenotype have been identified largely using OMICS platforms that measure dynamic molecular changes within the cancer cell landscape. The pro-oncogenic protein anterior gradient 2 (AGR2) is a case in point; AGR2 has been shown using a range of expression platforms to be involved in asthma, inflammatory bowel disease, cell transformation, cancer drug resistance and metastatic growth. AGR2 protein is also highly overexpressed in a diverse range of human cancers and can be secreted and detected in extracellular fluids, thus representing a compelling pro-oncogenic signalling intermediate in human cancer. AGR2 belongs to the protein disulphide isomerase family with all the key features of an endoplasmic reticulum-resident protein-this gives clues into how it might function as an oncoprotein through the regulation of protein folding, maturation and secretion that can drive metastatic cell growth. In this review, we will describe the known aspects of AGR2 molecular biology, including gene structure and regulation, emerging protein interaction networks and how its subcellular localization mediates its biological functions. We will finally review the cases of AGR2 expression in human cancers, the pathophysiological consequences of AGR2 overexpression, its potential role as a tumour biomarker that predicts the response to therapy and how the AGR2 pathway might form the basis for drug discovery programmes aimed at targeting protein folding/maturation pathways that mediate secretion and metastasis.

Gadd45a and Gadd45g regulate neural development and exit from pluripotency in Xenopus

Gadd45 genes encode a small family of multifunctional stress response proteins, mediating cell proliferation, apoptosis, DNA repair and DNA demethylation. Their role during embryonic development is incompletely understood. Here we identified Xenopus Gadd45b, compared Gadd45a, Gadd45b and Gadd45g expression during Xenopus embryogenesis, and characterized their gain and loss of function phenotypes. Gadd45a and Gadd45g act redundantly and double Morpholino knock down leads to pleiotropic phenotypes, including shortened axes, head defects and misgastrulation. In contrast, Gadd45b, which is expressed at very low levels, shows little effect upon knock down or overexpression. Gadd45ag double Morphants show reduced neural cell proliferation and downregulation of pan-neural and neural crest markers. In contrast, Gadd45ag Morphants display increased expression of multipotency marker genes including Xenopus oct4 homologs as well as gastrula markers, while mesodermal markers are downregulated. The results indicate that Gadd45ag are required for early embryonic cells to exit pluripotency and enter differentiation.

Rspo3 binds syndecan 4 and induces Wnt/PCP signaling via clathrin-mediated endocytosis to promote morphogenesis

The R-Spondin (Rspo) family of secreted Wnt modulators is involved in development and disease and holds therapeutic promise as stem cell growth factors. Despite growing biological importance, their mechanism of action is poorly understood. Here, we show that Rspo3 binds syndecan 4 (Sdc4) and that together they activate Wnt/PCP signaling. In Xenopus embryos, Sdc4 and Rspo3 are essential for two Wnt/PCP-driven processes-gastrulation movements and head cartilage morphogenesis. Rspo3/PCP signaling during gastrulation requires Wnt5a and is transduced via Fz7, Dvl, and JNK. Rspo3 functions by inducing Sdc4-dependent, clathrin-mediated endocytosis. We show that this internalization is essential for PCP signal transduction, suggesting that endocytosis of Wnt-receptor complexes is a key mechanism by which R-spondins promote Wnt signaling.

Ventralization of an indirect developing hemichordate by NiCl₂ suggests a conserved mechanism of dorso-ventral (D/V) patterning in Ambulacraria (hemichordates and echinoderms)

One of the earliest steps in embryonic development is the establishment of the future body axes. Morphological and molecular data place the Ambulacraria (echinoderms and hemichordates) within the Deuterostomia and as the sister taxon to chordates. Extensive work over the last decades in echinoid (sea urchins) echinoderms has led to the characterization of gene regulatory networks underlying germ layer specification and axis formation during embryogenesis. However, with the exception of recent studies from a direct developing hemichordate (Saccoglossus kowalevskii), very little is known about the molecular mechanism underlying early hemichordate development. Unlike echinoids, indirect developing hemichordates retain the larval body axes and major larval tissues after metamorphosis into the adult worm. In order to gain insight into dorso-ventral (D/V) patterning, we used nickel chloride (NiCl₂), a potent ventralizing agent on echinoderm embryos, on the indirect developing enteropneust hemichordate, Ptychodera flava. Our present study shows that NiCl₂ disrupts the D/V axis and induces formation of a circumferential mouth when treated before the onset of gastrulation. Molecular analysis, using newly isolated tissue-specific markers, shows that the ventral ectoderm is expanded at expense of dorsal ectoderm in treated embryos, but has little effect on germ layer or anterior-posterior markers. The resulting ventralized phenotype, the effective dose, and the NiCl₂ sensitive response period of Ptychodera flava, is very similar to the effects of nickel on embryonic development described in larval echinoderms. These strong similarities allow one to speculate that a NiCl₂ sensitive pathway involved in dorso-ventral patterning may be shared between echinoderms, hemichordates and a putative ambulacrarian ancestor. Furthermore, nickel treatments ventralize the direct developing hemichordate, S. kowalevskii indicating that a common pathway patterns both larval and adult body plans of the ambulacrarian ancestor and provides insight in to the origin of the chordate body plan.

A divergent substrate-binding loop within the pro-oncogenic protein anterior gradient-2 forms a docking site for Reptin

Anterior gradient-2 (AGR2) functions in a range of biological systems, including goblet cell formation, limb regeneration, inhibition of p53, and metastasis. There are no well-validated binding proteins for AGR2 protein despite the wealth of data implicating an important cellular function in vertebrates. The yeast two-hybrid system was used to isolate the ATP binding protein Reptin as an AGR2-interacting protein. AGR2 formed a stable complex in human cell lysates with Reptin, thus validating Reptin as an AGR2 binding protein in cells. Reptin was also shown to be overproduced in a panel of primary breast cancer biopsy specimens, relative to normal adjacent tissue from the same patient, suggesting a role in cancer growth in vivo. Mutations were made at the two ATP binding motifs in Reptin to evaluate the effects of ATP on Reptin-AGR2 complex stability. Loss-of-ATP binding mutations at the Walker A motif (K83A) or gain-of-ATP binding mutations at the Walker B motif (D299N) resulted in Reptin mutants with altered oligomerization, thermostability, and AGR2 binding properties. These data indicate that the two ATP binding motifs of Reptin play a role in regulating the stability of the AGR2-Reptin complex. The minimal region of AGR2 interacting with Reptin was localized using overlapping peptide libraries derived from the AGR2 protein sequence. The Reptin docking site was mapped to a divergent octapeptide loop in the AGR2 superfamily between amino acids 104 and 111. Mutations at codon Y104 or F111 in full-length AGR2 destabilized the binding of Reptin. These data highlight the existence of a protein docking motif on AGR2 and an ATP-regulated peptide-binding activity for Reptin. This knowledge has implications for isolating other AGR2-interacting proteins, for developing assays to isolate small molecules that target the Reptin ATP binding site, and for measuring the effects of the Reptin-AGR2 complex in cancer cell growth.

Ectodermal (Animal Cap) Layer Separations in Xenopus laevis

INTRODUCTIONIn Xenopus laevis, the blastula animal cap comprises two morphologically distinct cell layers, an outer monolayer termed the epithelial layer, which consists of tightly adherent pigmented cells, and an inner layer several cells thick termed the sensorial layer, which consists of loosely adherent cells. It is possible to isolate cell layers and to test their developmental potential and response to induction. This protocol describes how to separate the cell layers most easily, that is, by dissecting an intact Xenopus embryo.

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.

Xenopus Xotx2 and Drosophila otd share similar activities in anterior patterning of the frog embryo

Despite the obvious anatomical differences between the fly and the vertebrate body plans, several genes involved in their development are largely conserved. In this work we provide evidence that overexpression of the Drosophila orthodenticle (otd) gene in Xenopus laevis has a similar effect to that of its homolog Xotx2. Injections of otd mRNA in whole embryos lead to posterior truncations and to induction of ectopic cement glands, similar to Xotx2 injections. In animal cap assays, otd, like Xotx2, is able to activate the cement gland marker XAG and to suppress the expression of the epidermal marker XK81. Finally, as assayed by Einsteck transplantation assays, otd, like Xotx2, is able to respecify a tail/trunk organizer to a head organizer. In this work we also show that Xotx2 and otd share molecular functions that regulate early regional specification of the Xenopus anterior neural plate. Gain-of-function experiment targeting low doses of either otd or Xotx2 mRNAs in the neural plate promote reduction of Xrx1 and Xbf1 expression domain; no changes are observed for the anterior mesodermal marker Xgsc, the dorsal diencephalic marker Xbh1, and the midbrain/hindbrain marker Xen2. otd/Xotx2 inhibition activity of Xrx1 and Xbf1 expression is consistent with the strong inhibition of Xfgf8 expression in the anterior neural ridge observed upon otd/Xotx2 mRNA injection.

R-Spondin2 is a secreted activator of Wnt/beta-catenin signaling and is required for Xenopus myogenesis

We have carried out a small pool expression screen for modulators of the Wnt/beta-catenin pathway and identified Xenopus R-spondin2 (Rspo2) as a secreted activator of this cascade. Rspo2 is coexpressed with and positively regulated by Wnt signals and synergizes with Wnts to activate beta-catenin. Analyses of functional interaction with components of the Wnt/beta-catenin pathway suggest that Rspo2 functions extracellularly at the level of receptor ligand interaction. In addition to activating the Wnt/beta-catenin pathway, Rspo2 overexpression blocks Activin, Nodal, and BMP4 signaling in Xenopus, raising the possibility that it may negatively regulate the TGF-beta pathway. Antisense Morpholino experiments in Xenopus embryos and RNAi experiments in HeLa cells reveal that Rspo2 is required for Wnt/beta-catenin signaling. In Xenopus embryos depleted of Rspo2, the muscle markers myoD and myf5 fail to be activated and later muscle development is impaired. Thus, Rspo2 functions in a positive feedback loop to stimulate the Wnt/beta-catenin cascade.

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

The correct positioning of organs during embryonic development requires multiple cues. The Xenopus cement gland is a mucus-secreting epithelium that is a simple model for organogenesis, allowing detailed analysis of this complex process. The cement gland forms at a conserved anterior position, where embryonic ectoderm and endoderm touch. In all deuterostomes, this region will form the stomodeum (primitive mouth) and, in some aquatic larva, will also form a cement gland. In recent years, a model has been put forward suggesting that an intermediate level of BMP signaling in the ectoderm leads to cement gland formation. We propose an alternative model whereby, during gastrulation, the cement gland (CG) is positioned by the overlap of three domains, corresponding to anterodorsal identity (AD), ventrolateral identity (VL), and ectodermal outer layer identity (EO), defining the equation (AD + VL + EO = CG). Anterodorsal identity requires a contribution by the transcription factor Otx2 while ventrolateral identity requires the BMP4 signaling pathway. These postional cues are integrated to activate cement gland differentiation. This integration appears to require intermediate steps, including expression of pitx genes, and members of the ATF/CREB and Ets transcription factor families.

Expression zones of three novel genes abut the developing anterior neural plate of Xenopus embryo

We identified three novel genes that were expressed within the anterior non-neural ectoderm of Xenopus early neurula embryos. The expression of these genes was observed in the different areas complementary to the expression zone of a homeodomain gene Xanf-1 in the anterior neural plate. One of these genes, a Ras-like GTP-ase Ras-dva, marked the anterior placodal ectoderm area; a second, an Agr family homologous gene, XAgr2, was expressed in the anterior-most ectoderm in the cement gland primordium, and a third, novel gene Nlo was expressed in the lateral neural folds. The genes were transiently expressed in the developing cement and hatching gland primordia, and repressed in the mature cement and hatching glands. XAgr2 and Nlo were also expressed in the otic vesicles, and Ras-dva was expressed in the dorso-lateral column of the neural tube.

Intrinsic differences between the superficial and deep layers of the Xenopus ectoderm control primary neuronal differentiation

In Xenopus, primary neurons differentiate early, in the deep layer of the neuroectoderm. In contrast, the neural precursors of the superficial layer continue to proliferate. We report that superficial layer precursors differ from deep layer precursors in that they are refractory to the neuronal-promoting activity of bHLH genes, dominant-negative X-Delta-1, FGF-8, or signals from the organizer. In this system, neuronal differentiation is guided by an early established, intrinsic, cell-autonomous difference in the competence of the precursor cells to differentiate. This difference may be controlled in part by ESR6e, a bHLH gene of the Enhancer-of-split family, which is expressed in the superficial layer of the late blastula and when expressed ectopically suppresses primary neurogenesis in the deep layer.

otx2 expression in the ectoderm activates anterior neural determination and is required for Xenopus cement gland formation

We previously showed that otx2 regulates Xenopus cement gland formation in the ectoderm. Here, we show that otx2 is sufficient to direct anterior neural gene expression, and that its activity is required for cement gland and anterior neural determination. otx2 activity at midgastrula activates anterior and prevents expression of posterior and ventral gene expression in whole embryos and ectodermal explants. These data suggest that part of the mechanism by which otx2 promotes anterior determination involves repression of posterior and ventral fates. A dominant negative otx2-engrailed repressor fusion protein (otx2-En) ablates endogenous cement gland formation, and inhibits expression of the mid/hindbrain boundary marker engrailed-2. Ectoderm expressing otx2-En is not able to respond to signals from the mesoderm to form cement gland, and is impaired in its ability to form anterior neural tissue. These results compliment analyses in otx2 mutant mice, indicating a role for otx2 in the ectoderm during anterior neural patterning.

Dickkopf1 is required for embryonic head induction and limb morphogenesis in the mouse

Dickkopf1 (Dkk1) is a secreted protein that acts as a Wnt inhibitor and, together with BMP inhibitors, is able to induce the formation of ectopic heads in Xenopus. Here, we show that Dkk1 null mutant embryos lack head structures anterior of the midbrain. Analysis of chimeric embryos implicates the requirement of Dkk1 in anterior axial mesendoderm but not in anterior visceral endoderm for head induction. In addition, mutant embryos show duplications and fusions of limb digits. Characterization of the limb phenotype strongly suggests a role for Dkk1 both in cell proliferation and in programmed cell death. Our data provide direct genetic evidence for the requirement of secreted Wnt antagonists during embryonic patterning and implicate Dkk1 as an essential inducer during anterior specification as well as a regulator during distal limb patterning.

Pitx1 and Pitx2c are required for ectopic cement gland formation in Xenopus laevis

The mucus secreting cement gland is the anterior-most ectodermal organ of the Xenopus embryo. The homeobox genes Pltx1 and Pitx2c are expressed in the cement gland primordium. Misexpression of both genes induced ectopic cement gland tissue in whole embryos and transcription of the marker genes Xag1 and Xag2 in animal cap explant cultures. Antisense morpholino oligonucleotides against Pitx1 and Pitx2c inhibited ectopic cement gland formation induced by otx2. Gene knock downs generated by morpholino oligonucleotides were specific and could be rescued by coinjection of Pitx mRNAs. These data demonstrate for the first time the requirement of specific genes for cement gland formation by loss-of-function experiments. genesis 30:144--148, 2001.

Xenopus ADAM 13 is a metalloprotease required for cranial neural crest-cell migration

BACKGROUND

Cranial neural-crest (CNC) cells originate from the lateral edge of the anterior neuroepithelium and migrate to form parts of the peripheral nervous system, muscles, cartilage, and bones of the face. Neural crest-cell migration involves the loss of adhesion from the surrounding neuroepithelium and a corresponding increase in cell adhesion to the extracellular matrix (ECM) present in migratory pathways. While proteolytic activity is likely to contribute to the regulation of neural crest-cell adhesion and migration, the role of a neural crest-specific protease in these processes has yet to be demonstrated. We previously showed that CNC cells express ADAM 13, a cell surface metalloprotease/disintegrin. Proteins of this family are known to act in cell-cell adhesion and as sheddases. ADAMs have also been proposed to degrade the ECM, but this has not yet been shown in a physiological context.

RESULTS

Using a tissue transplantation technique, we show that Xenopus CNC cells overexpressing wild-type ADAM 13 migrate along the same hyoid, branchial, and mandibular pathways used by normal CNC cells. In contrast, CNC cell grafts that express protease-defective ADAM 13 fail to migrate along the hyoid and branchial pathways. In addition, ectopic expression of wild-type ADAM 13 results in a gain-of-function phenotype in embryos, namely the abnormal positioning of trunk neural-crest cells. We further show that explanted embryonic tissues expressing wild-type, but not protease-defective, ADAM 13 display decreased cell-matrix adhesion. Purified ADAM 13 can cleave fibronectin, and tissue culture cells that express wild-type, but not protease-defective, ADAM 13 can remodel a fibronectin substrate.

CONCLUSIONS

Our findings support the hypothesis that the protease activity of ADAM 13 plays a critical role in neural crest-cell migration along defined pathways. We propose that the ADAM 13-dependent modification of ECM and/or other guidance molecules is a key step in the directed migration of the CNC.

Early anteroposterior division of the presumptive neurectoderm in Xenopus

We analyze the timing of neural patterning in Xenopus and the mechanism by which the early pattern is generated. With regard to timing, we show that by early gastrula, two domains of the anteroposterior (A/P) pattern exist in the presumptive neurectoderm, since the opl gene is expressed throughout the future neural plate, while the fkh5 gene is expressed only in more posterior ectoderm. By mid-gastrula, this pattern has become more elaborate, with an anterior domain defined by expression of opl and otx2, a middle domain defined by expression of opl and fkh5, and a posterior domain defined by expression of opl, fkh5 and HoxD1. Explant assays indicate that the late blastula dorsal ectoderm is specified as the anterior domain, but is not yet specified as middle or posterior domains. With regard to the mechanism by which the A/P pattern is generated, gain and loss of function assays indicate that quantitatively and qualitatively different factors may be involved in inducing the early A/P neural pattern. These data show that neural patterning occurs early in Xenopus and suggest a molecular basis for initiating this pattern.

A zebrafish forebrain-specific zinc finger gene can induce ectopic dlx2 and dlx6 expression

Identification of the earliest forebrain-specific markers should facilitate the elucidation of molecular events underlying vertebrate forebrain determination and specification. Here we report the sequence and characterization of fez (forebrain embryonic zinc finger), a gene that is specifically expressed in the embryonic forebrain of zebrafish. Fez encodes a putative nuclear zinc finger protein that is highly conserved in Drosophila, zebrafish, Xenopus, mouse, and human. In zebrafish, the expression of fez becomes detectable at the anterior edge of the presumptive neuroectoderm by 70% epiboly. During the segmentation period, its expression is completely restricted to the rostral region of the prospective forebrain. At approximately 24 h postfertilization, fez expression is mostly confined to the telencephalon and the anterior-ventral region of the diencephalon. Although fez expression is present in one-eyed pinhead (oep) and cyclops (cyc) zebrafish mutants, the pattern is altered. Forced expression of fez induces ectopic expression of dlx2 and dlx6, two genes involved in brain development. Knockdown of fez function using a morpholino-based antisense oligo inhibited dlx2 expression in the ventral forebrain. Our studies indicate that fez is one of the earliest markers specific for the anterior neuroectoderm and it may play a role in forebrain development by regulating Dlx gene expression.

The role of Xenopus dickkopf1 in prechordal plate specification and neural patterning

Dickkopf1 (dkk1) encodes a secreted WNT inhibitor expressed in Spemann's organizer, which has been implicated in head induction in Xenopus. Here we have analyzed the role of dkk1 in endomesoderm specification and neural patterning by gain- and loss-of-function approaches. We find that dkk1, unlike other WNT inhibitors, is able to induce functional prechordal plate, which explains its ability to induce secondary heads with bilateral eyes. This may be due to differential WNT inhibition since dkk1, unlike frzb, inhibits Wnt3a signalling. Injection of inhibitory antiDkk1 antibodies reveals that dkk1 is not only sufficient but also required for prechordal plate formation but not for notochord formation. In the neural plate dkk1 is required for anteroposterior and dorsoventral patterning between mes- and telencephalon, where dkk1 promotes anterior and ventral fates. Both the requirement of anterior explants for dkk1 function and their ability to respond to dkk1 terminate at late gastrula stage. Xenopus embryos posteriorized with bFGF, BMP4 and Smads are rescued by dkk1. dkk1 does not interfere with the ability of bFGF to induce its immediate early target gene Xbra, indicating that its effect is indirect. In contrast, there is cross-talk between BMP and WNT signalling, since induction of BMP target genes is sensitive to WNT inhibitors until the early gastrula stage. Embryos treated with retinoic acid (RA) are not rescued by dkk1 and RA affects the central nervous system (CNS) more posterior than dkk1, suggesting that WNTs and retinoids may act to pattern anterior and posterior CNS, respectively, during gastrulation.

Different activities of the frizzled-related proteins frzb2 and sizzled2 during Xenopus anteroposterior patterning

In a search for factors that regulate patterning of the Xenopus anteroposterior (A/P) axis, particularly the anterior ectoderm, we isolated two members of the Frizzled-related protein (FRP) gene family that are thought to encode antagonists of Wnt signaling. frzb2 is expressed in head mesoderm while sizzled2 is expressed in ventral ectoderm and mesoderm, tissues that modulate anterior fates. Consistent with a role for these genes in A/P patterning, ectopically expressed frzb2 inhibited head formation, while sizzled2 dorsalized embryos, causing expansion of the head. The different activities of frzb2 and sizzled2 may be explained by their interaction with distinct proteins since frzb2 is an inhibitor of Xwnt8 activity, while sizzled2 is unable to inhibit the activity of Xwnt8 or any other Xwnt tested. The data suggest that anteroposterior patterning is modulated by multiple components of the Wnt signaling pathway.

Vertebrate anteroposterior patterning: the Xenopus neurectoderm as a paradigm

This review discusses formation of the vertebrate anteroposterior (AP) axis, focusing on the dorsal ectoderm, which gives rise to the nervous system, using the frog Xenopus as a model. After summarizing classical models of AP neural patterning, we describe recent molecular studies that are encouraging re-examination of these models. Such studies have shown that AP ectodermal patterning occurs by the onset of gastrulation, much earlier than previously thought. The identity of tissues that determine AP pattern is discussed, and the definition of the Organizer is reconsidered. The activity of factors secreted by inducing tissues in early patterning decisions is assessed and formulated into a revised model for Xenopus AP neural patterning. Finally, AP ectodermal patterning in Xenopus dorsal ectoderm is compared to that of other germ layers, and to other vertebrates.

Xotx5b, a new member of the Otx gene family, may be involved in anterior and eye development in Xenopus laevis

We describe the cloning, expression pattern and functional overexpression analysis of Xotx5b, a new member of the Otx gene family in Xenopus laevis. Early expression of Xotx5b resembles that of Xotx2, being detected in the organizer region at early gastrula stage, and, shortly after, also in anterior neuroectoderm. During neurula stages Xotx5b exhibits a changing and dynamic pattern of expression. After neural tube closure, Xotx5b is expressed in the eye and pineal gland, both involved in photoreception. Overexpression of Xotx5b has a similar effect to that of Xotx2, producing posterior truncations and inducing ectopic cement gland and neural tissue in whole embryos. In animal cap assays, Xotx5b and Xotx2 are both able to activate XAG, to strongly suppress the expression of the epidermal marker XK81, and to reciprocally activate each other. Finally, in einsteck transplantation assays, Xotx5b is able to respecify a tail/trunk organizer to a head organizer.

Coincidence of otx2 and BMP4 signaling correlates with Xenopus cement gland formation

We previously showed that otx2 activates ectopic formation of the Xenopus cement gland only in ventrolateral ectoderm, defining a region of the embryo permissive for cement gland formation. In this paper, we explore the molecular identity of this permissive area. One candidate permissive factor is BMP4, whose putative graded inhibition by factors such as noggin has been proposed to activate both cement gland and neural fates. Several lines of evidence are presented to suggest that BMP signaling and otx2 work together to activate cement gland formation. First, BMP4 is highly expressed in the cement gland primordium together with otx2. Second, cement gland formation in isolated ectoderm is always accompanied by coexpression of otx2 and BMP4 RNA, whether cement gland is induced by otx2 or by the BMP protein inhibitor noggin. Third, BMP signaling can modulate otx2 activity, such that increasing BMP signaling preferentially inhibits neural induction by otx2, while decreasing BMP signaling prevents cement gland formation. In addition, we show that a hormone-inducible otx2 activates both ectopic neural and cement gland formation within the cement gland permissive region, in a pattern reminiscent of that found in the embryo. We discuss this observation in view of a model that BMP4 and otx2 work together to promote cement gland formation.

An anterior signalling centre in Xenopus revealed by the homeobox gene XHex

BACKGROUND

Signals from anterior endodermal cells that express the homeobox gene Hex initiate development of the most rostral tissues of the mouse embryo. The dorsal/anterior endoderm of the Xenopus gastrula, which expresses Hex and the putative head-inducing gene cerberus, is proposed to be equivalent to the mouse anterior endoderm. Here, we report the origin and signalling properties of this population of cells in the early Xenopus embryo.

RESULTS

Xenopus anterior endoderm was found to derive in part from cells at the centre of the blastocoel floor that express XHex, the Xenopus cognate of Hex. Like their counterparts in the mouse embryo, these Hex-expressing blastomeres moved to the dorsal side of the Xenopus embryo as gastrulation commenced, and populated deep endodermal adjacent to Spemann's organiser. Experiments involving the induction of secondary axes confirmed that XHex expression was associated with anterior development. Ventral misexpression of XHex induced ectopic cerberus expression and conferred anterior signalling properties to the endoderm. Unlike the effect of misexpressing cerberus, these signals could not neuralise overlying ectoderm.

CONCLUSIONS

XHex expression reveals the unexpected origin of an anterior signalling centre in Xenopus, which arises in part from the centre of the blastula and localises to the deep endoderm adjacent to Spemann's organiser. Signals originating from these endodermal cells impart an anterior identity to the overlying ectoderm, but are insufficient for neural induction. The anterior movement of Hex-expressing cells in both Xenopus and mouse embryos suggests that this process is a conserved feature of vertebrate development.

Head in the WNT: the molecular nature of Spemann's head organizer

Formation of the head during vertebrate embryogenesis has been one of the most-studied topics in development, probably because we are such cephalized beings ourselves. Early experimenters found that the head is induced during gastrulation by Spemann's organizer. In 1999 we celebrate the 75th anniversary of the discovery of the organizer by Spemann and Mangold, a group of cells in amphibia that secretes powerful signalling molecules. Recently, advances have been made in identifying candidate head inducers. Not surprisingly, these inducers act in familiar molecular pathways, namely transforming growth factor beta (TGF-beta) and WNT signalling.

Ni2+ treatment causes cement gland formation in ectoderm explants of Xenopus laevis embryo

We found T-type calcium channel blocker Ni2+ can efficiently induce the formation of cement gland in Xenopus laevis animal cap explants. Another T-type specific calcium channel blocker Amiloride can also induce the formation of cement gland, while L-type specific calcium channel blocker Nifedipine has no inductive effect. These results may offer us an new approach to study the differentiation of cement gland through the change of intracelluar calcium concentration.

Neural induction. A bird's eye view

Since the discovery of the phenomenon of neural induction by Spemann and Mangold in 1924, considerable effort has been invested in identifying the signals produced by the organizer that are responsible for diverting the fate of cells from epidermal to neural. Substantial progress has been made only recently by the finding in amphibians that BMP4 is a neural inhibitor and epidermal inducer, and that endogenous antagonists of BMPs are secreted by the organizer. However, recent results in the chick point to the existence of other, upstream events required before BMP inhibition stabilizes neural fates. Here we take a critical view of the evidence for and against the view that BMP inhibition is a sufficient trigger for neural induction in different vertebrates.

Determination of the zebrafish forebrain: induction and patterning

We report an analysis of forebrain determination and patterning in the zebrafish Danio rerio. In order to study these events, we isolated zebrafish homologs of two neural markers, odd-paired-like (opl), which encodes a zinc finger protein, and fkh5, which encodes a forkhead domain protein. At mid-gastrula, expression of these genes defines a very early pattern in the presumptive neurectoderm, with opl later expressed in the telencephalon, and fkh5 in the diencephalon and more posterior neurectoderm. Using in vitro explant assays, we show that forebrain induction has occurred even earlier, by the onset of gastrulation (shield stage). Signaling from the early gastrula shield, previously shown to be an organizing center, is sufficient for activation of opl expression in vitro. In order to determine whether the organizer is required for opl regulation, we removed from late blastula stage embryos either the presumptive prechordal plate, marked by goosecoid (gsc) expression, or the entire organizer, marked by chordin (chd) expression. opl was correctly expressed after removal of the presumptive prechordal plate and consistently, opl was correctly expressed in one-eyed pinhead (oep) mutant embryos, where the prechordal plate fails to form. However, after removal of the entire organizer, no opl expression was observed, indicating that this region is crucial for forebrain induction. We further show that continued organizer function is required for forebrain induction, since beads of BMP4, which promotes ventral fates, also prevented opl expression when implanted during gastrulation. Our data show that forebrain specification begins early during gastrulation, and that a wide area of dorsal mesendoderm is required for its patterning.

Anterior specification of embryonic ectoderm: the role of the Xenopus cement gland-specific gene XAG-2

In a search for novel developmental genes expressed in a spatially restricted pattern in dorsal ectoderm of Xenopus we have identified XAG-2, a cement gland-specific gene with a putative role in ectodermal patterning. XAG-2 encodes a secreted protein, which is expressed in the anterior region of dorsal ectoderm from late gastrula stages onwards. Activation of XAG-2 transcription is observed in response to organizer-secreted molecules including the noggin, chordin, follistatin and cerberus gene products. Overexpression of XAG-2 but not of the related cement gland marker XAG-1 induces both cement gland differentiation and expression of anterior neural marker genes in the absence of mesoderm formation. Further, we show that XAG-2 signaling depends on an intact fibroblast growth factor (FGF) signal transduction pathway and that XAG-2-induced anterior neural fate of ectodermal cells can be transformed to a more posterior character by retinoic acid. Based on these findings we propose a role for XAG-2 in the specification of dorsoanterior ectodermal fate, i.e. in the formation of cement gland and induction of forebrain fate of Xenopus.

A small population of anterior cells patterns the forebrain during zebrafish gastrulation

During gastrulation in vertebrates, dorsal ectoderm is induced to form neural tissue that later gives rise to the brain and spinal cord. This induction depends on signals arising from a group of cells on the dorsal side of the gastrula. This group of cells constitutes the organizer. It is thought that the organizer initially induces neural tissue with anterior, or forebrain, character, and that other signals subsequently posteriorize neural tissue in the trunk. Here we show that development of the anterior central nervous system of the zebrafish embryo also depends on a small group of ectodermal cells located in the prospective head region. Removal of these ectodermal cells during gastrulation perturbs subsequent neural patterning and results in widespread cell death. Transplantation of these cells shows that they can induce forebrain-specific gene expression in more posterior regions of the neural plate. Our results indicate that an early step in neural patterning is the establishment of a small population of signalling cells within the most anterior region of the embryo. These cells are required for patterning and survival of the anterior brain.

Xenopus hindbrain patterning requires retinoid signaling

We have asked how posterior neural tissue is patterned in Xenopus by assaying the involvement of endogenous retinoic acid (RA) in this process and by using the labial Hox gene, HoxD1, as a posterior marker. Although RA is able to inhibit anterior gene expression and activate expression of more posterior genes, the normal role of retinoids in anteroposterior (A/P) patterning is unclear. HoxD1 is an early posterior neurectodermal marker, expressed from midgastrula with a later anterior expression limit in the future hindbrain. We previously showed that HoxD1 was induced as an immediate early response to retinoic acid in naive ectoderm (animal caps). Here, we use a truncated RARalpha2.2 receptor (RARDelta) to dominantly interfere with retinoid signaling. In embryos injected with RARDelta expression of HoxD1 is eliminated. Conjugates of ectoderm and dorsolateral mesoderm show that retinoid receptors are required in the ectoderm for HoxD1 induction. Further, expression of Krox-20 in r3 and r5 of the presumptive hindbrain is compressed into a single stripe that suggests elimination of r5. RARalpha2.2 expression almost precisely overlaps that of HoxD1, suggesting that this receptor may normally activate HoxD1. Expression of neither more anterior genes including cement gland, forebrain, and midbrain markers nor a more posterior spinal cord marker is affected by RARDelta. These data suggest that the posterior hindbrain is the region of the nervous system most sensitive to retinoid loss. Finally, we compare the ability of RA and fibroblast growth factor (FGF) to posteriorize isolated anterior neurectoderm and show that both factors can act directly on this substrate. RA acts in a more anterior domain than does FGF; however, neither factor is equivalent to the natural posteriorizing capacity of the posterior mesoderm. We propose that endogenous retinoid and FGF signals pattern largely nonoverlapping regions along the A/P axis and that posterior neural patterning requires multiple inducers.

Vertebrate head induction by anterior primitive endoderm

In vertebrates the antero-posterior organization of the embryonic body axis is thought to result from the activity of two separate centers, the head organizer and the trunk organizer, as operationally defined by Spemann in the 1920s. Current molecular studies have supported the existence of a trunk organizer activity while the presence of a distinct head inducing center has remained elusive. Mainly based on analyses of headless mutants in mice, it has been proposed that the anterior axial mesoderm plays a determining role in head induction. Recent gain- and loss-of-function studies in various organisms, however, provide compelling evidence that a largely ignored region, the anterior primitive endoderm, specifies rostral identity. In this review we discuss the emerging concept that the anterior primitive endoderm, rather than the prechordal plate mesoderm, induces head development in the vertebrate embryo.

Concentration-dependent patterning of the Xenopus ectoderm by BMP4 and its signal transducer Smad1

Morphogens are thought to establish pattern in early embryos by specifying several cell fates along a gradient of concentration; a well-studied example is the Drosophila protein decapentaplegic (DPP) acting in the wing disc. Recent work has established that bone morphogenetic protein 4 (BMP4), the vertebrate homologue of DPP, controls the fundamental choice between neural and epidermal fates in the vertebrate ectoderm, under the control of antagonists secreted by the organizer region of the mesoderm. We now show that BMP4 can act as a morphogen, evoking distinct responses in Xenopus ectodermal cells at high and low concentrations, in a pattern consistent with the positions of the corresponding cell types in the embryo. Moreover, this complex cellular response to extracellular BMP4 concentration does not require subsequent cell-cell communication and is thus direct, as required of a classical morphogen. We also show that the same series of cell types--epidermis, cement gland and neural tissue--can be produced by progressively inhibiting endogenous BMP signaling with specific antagonists, including the organizer factor noggin. Finally, expression of increasing doses of the signal transduction molecule Smad1 accurately reproduces the response to BMP4 protein. Since Smads have been shown to act in the nucleus, this finding implies a direct translation of extracellular morphogen concentration into transcription factor activity. We propose that a graded distribution of BMP activity controls the specification of several cell types in the gastrula ectoderm and that this extracellular gradient acts by establishing an intracellular and then nuclear gradient of Smad activity.

Identification of otx2 target genes and restrictions in ectodermal competence during Xenopus cement gland formation

The homeobox gene otx2 is a key regulator of positional identity in vertebrates, however its downstream target genes and mechanism of action are not known. We have analyzed otx2 function during formation of the Xenopus cement gland, an organ that expresses otx2. The cement gland forms at early neurula from extreme anterior ectoderm and corresponds to the chin primordium of mammals. Previous studies (Blitz, I. and Cho, K. (1995) Development 121, 993–1004; Pannese, M., Polo, C., Andreazzoli, M., Vignali, R., Kablar, B., Barsacchi, G. and Boncinelli, E. (1995) Development 121, 707–720) showed that misexpressed otx2 could activate cement gland formation. However, it was not clear whether this was a direct effect of otx2 or a secondary consequence of other tissues induced by otx2. In this study we ask whether otx2 activity is spatially and temporally restricted in the ectoderm and whether cement gland-specific genes are direct targets of otx2. In order to control the timing of otx2 activity, we constructed a dexamethasone-inducible otx2 protein (otx2-GR) by fusion with the ligand-binding domain of the glucocorticoid receptor. We conclude first, that regionally restricted factors regulate otx2 activity since otx2-GR is able to activate the cement gland markers XCG and XAG only in ventrolateral ectoderm, and never in the neural plate. Second, we show that temporal responsiveness of the ectoderm to otx2-GR is limited, beginning only at mid-gastrula but continuing as late as tailbud stages. Third, we show that otx2-GR activates expression of the cement gland differentiation marker XCG in the absence of protein synthesis, identifying a direct target of otx2. otx2-GR can also activate expression of the endogenous otx2 gene, defining an autoregulatory loop. Fourth, we show that otx2-GR is sufficient to overcome the inhibitory effects of retinoic acid on cement gland formation, indicating that this effect is caused by failure to express otx2. Corroboratively, we show that otx2 autoactivation is prevented by retinoic acid. Together, these findings suggest that otx2 directly controls cement gland differentiation, and that spatial and temporal modulation of otx2 activity limits cement gland formation to the front of the embryo.