Spatial and temporal patterns of cell division during early Xenopus embryogenesis

The Effects of Photobiomodulation Therapy on Xenopus laevis Embryonic Epithelium

Background: Photobiomodulation therapy (PBMT) is widely used in clinical settings, particularly for treating oral mucositis. Although PBMT has shown promise in aiding tissue healing, its safety and potential effects on tumorigenesis remain controversial. Objective: Xenopus laevis (X. laevis) tadpoles have proven to be an effective in vivo model system to study how different therapies affect epithelial biology. Because of this, the goal of this study was to investigate how PBMT influences epithelial tissue in X. laevis tadpoles. Methods: X. laevis tadpoles were treated with PBMT three times, every 24 h, using a 660 nm low-level laser at a fluence of 2.08 J/cm2 and an irradiance of 0.208 W/cm2, resulting in a cumulative dose of 2.25 J. Tadpole tails were then fixed, and the abnormal tissue area was analyzed using fluorescent microscopy. Results: PBMT induced the formation of abnormal tissue structures along the epithelial edges. The altered tissue increased from 0.03% in control tadpoles to 0.37% in PBMT-treated tadpoles (p < 0.0001; Mann-Whitney nonparametric test), and the number of tadpoles displaying this abnormal phenotype increased from 3.8% of control tadpoles to 30.6% of PBMT-treated tadpoles (p < 0.001; chi-squared test). Conclusion: This study demonstrates that PBMT can have significant effects on the epithelial tissue of X. laevis. The PBMT-induced abnormal tissue structures represent a loss of tissue polarization and cellular organization along the tail edge, both of which are essential to maintaining tissue homeostasis. For the first time, we show that X. laevis may provide an in vivo model system for examining the effects and mechanisms of PBMT.

The Zebrafish Retina and the Evolution of the Onecut-Mediated Pathway in Cell Type Differentiation

Onecut/Hnf6 (Oc) genes play an important role in the proper formation of retinal cells in vertebrates, in particular horizontal, retinal ganglion and amacrine cells. However, it is not fully known how the unique and combined action of multiple Oc gene copies leads to the induction and differentiation of specific retinal cell types. To gain new insights on how Oc genes influence retina formation, we have examined the developmental role of oc1, oc2 and oc-like genes during eye formation in the non-mammalian vertebrate zebrafish Danio rerio. By using single and multiple morpholino knockdown of three zebrafish Oc genes we provide evidence for the independent and redundant role of each gene in the formation of photoreceptors and other retinal tissues. Through comparison of Oc genetic pathways in photoreceptor differentiation among chordates we demonstrate their mechanism of action through a series of conserved target genes involved in neural transmission.

Differential Sensitivity of Midline Patterning to Mitosis during and after Primitive Streak Extension

Background

Midline establishment is a fundamental process during early embryogenesis for Bilaterians . Midline patterning in nonamniotes can occur without mitosis, through Planar Cell Polarity (PCP) signaling. By contrast, amniotes utilize both cell proliferation and PCP signaling for patterning early midline landmark, the primitive streak (PS). This study examined their roles for midline patterning at post PS-extension.

Results

In contrast to PS extension stages, embryos under mitotic arrest during the post PS-extension preserved notochord (NC) extension and Hensen's node (HN)/PS regression judged by both morphology and marker genes, although they became shorter, and laterality was lost. Remarkably, no or background level of expression was detected for the majority of PCP core components in the NC-HN-PS area at post PS-extension stages, except for robustly detected prickle-1 . Morpholino knockdown of Prickle-1 showed little influence on midline patterning, except for suppressed embryonic growth. Lastly, associated with mitotic arrest-induced size reduction, midline tissue cells displayed hypertrophy.

Conclusion

Thus, the study has identified at least two distinct mitosis sensitivity phases during early midline pattering: One is PS extension that requires both mitosis and PCP, and the other is mitotic arrest-resistant midline patterning with little influence by PCP at post PS-extension stages.

Diphthamide deficiency promotes association of eEF2 with p53 to induce p21 expression and neural crest defects

Diphthamide is a modified histidine residue unique for eukaryotic translation elongation factor 2 (eEF2), a key ribosomal protein. Loss of this evolutionarily conserved modification causes developmental defects through unknown mechanisms. In a patient with compound heterozygous mutations in Diphthamide Biosynthesis 1 (DPH1) and impaired eEF2 diphthamide modification, we observe multiple defects in neural crest (NC)-derived tissues. Knockin mice harboring the patient's mutations and Xenopus embryos with Dph1 depleted also display NC defects, which can be attributed to reduced proliferation in the neuroepithelium. DPH1 depletion facilitates dissociation of eEF2 from ribosomes and association with p53 to promote transcription of the cell cycle inhibitor p21, resulting in inhibited proliferation. Knockout of one p21 allele rescues the NC phenotypes in the knockin mice carrying the patient's mutations. These findings uncover an unexpected role for eEF2 as a transcriptional coactivator for p53 to induce p21 expression and NC defects, which is regulated by diphthamide modification.

V-ATPase Regulates Retinal Progenitor Cell Proliferation During Eye Regrowth in Xenopus

Purpose: The induction of retinal progenitor cell (RPC) proliferation is a strategy that holds promise for alleviating retinal degeneration. However, the mechanisms that can stimulate RPC proliferation during repair remain unclear. Xenopus tailbud embryos successfully regrow functional eyes within 5 days after ablation, and this process requires increased RPC proliferation. This model facilitates identification of mechanisms that can drive in vivo reparative RPC proliferation. This study assesses the role of the essential H+ pump, V-ATPase, in promoting stem cell proliferation. Methods: Pharmacological and molecular loss of function studies were performed to determine the requirement for V-ATPase during embryonic eye regrowth. The resultant eye phenotypes were examined using histology and antibody markers. Misexpression of a yeast H+ pump was used to test whether the requirement for V-ATPase in regrowth is dependent on its H+ pump function. Results: V-ATPase inhibition blocked eye regrowth. Regrowth-incompetent eyes resulting from V-ATPase inhibition contained the normal complement of tissues but were much smaller. V-ATPase inhibition caused a significant reduction in reparative RPC proliferation but did not alter differentiation and patterning. Modulation of V-ATPase activity did not affect apoptosis, a process known to be required for eye regrowth. Finally, increasing H+ pump activity was sufficient to induce regrowth. Conclusions: V-ATPase is required for eye regrowth. These results reveal a key role for V-ATPase in activating regenerative RPC proliferation and expansion during successful eye regrowth.

Xenopus: An in vivo model for studying skin response to ultraviolet B irradiation

Ultraviolet B (UVB) in sunlight cause skin damage, ranging from wrinkles to photoaging and skin cancer. UVB can affect genomic DNA by creating cyclobutane pyrimidine dimers (CPDs) and pyrimidine-pyrimidine (6-4) photoproducts (6-4PPs). These lesions are mainly repaired by the nucleotide excision repair (NER) system and by photolyase enzymes that are activated by blue light. Our main goal was to validate the use of Xenopus laevis as an in vivo model system for investigating the impact of UVB on skin physiology. The mRNA expression levels of xpc and six other genes of the NER system and CPD/6-4PP photolyases were found at all stages of embryonic development and in all adult tissues tested. When examining Xenopus embryos at different time points after UVB irradiation, we observed a gradual decrease in CPD levels and an increased number of apoptotic cells, together with an epidermal thickening and an increased dendricity of melanocytes. We observed a quick removal of CPDs when embryos are exposed to blue light versus in the dark, confirming the efficient activation of photolyases. A decrease in the number of apoptotic cells and an accelerated return to normal proliferation rate was noted in blue light-exposed embryos compared with their control counterparts. Overall, a gradual decrease in CPD levels, detection of apoptotic cells, thickening of epidermis, and increased dendricity of melanocytes, emulate human skin responses to UVB and support Xenopus as an appropriate and alternative model for such studies.

Function of chromatin modifier Hmgn1 during neural crest and craniofacial development

The neural crest is a dynamic embryonic structure that plays a major role in the formation of the vertebrate craniofacial skeleton. Neural crest formation is regulated by a complex sequence of events directed by a network of transcription factors working in concert with chromatin modifiers. The high mobility group nucleosome binding protein 1 (Hmgn1) is a nonhistone chromatin architectural protein, associated with transcriptionally active chromatin. Here we report the expression and function of Hmgn1 during Xenopus neural crest and craniofacial development. Hmgn1 is broadly expressed at the gastrula and neurula stages, and is enriched in the head region at the tailbud stage, especially in the eyes and the pharyngeal arches. Hmgn1 knockdown affected the expression of several neural crest specifiers, including sox8, sox10, foxd3, and twist1, while other genes (sox9 and snai2) were only marginally affected. The specificity of this phenotype was confirmed by rescue, where injection of Hmgn1 mRNA was able to restore sox10 expression in morphant embryos. The reduction in neural crest gene expression at the neurula stage in Hmgn1 morphant embryos correlated with a decreased number of sox10- and twist1-positive cells in the pharyngeal arches at the tailbud stage, and hypoplastic craniofacial cartilages at the tadpole stage. These results point to a novel role for Hmgn1 in the control of gene expression essential for neural crest and craniofacial development. Future work will investigate the precise mode of action of Hmgn1 in this context.

The Use of Xenopus for Cell Biology Applications

Problems of cell biology and the molecular controls underpinning them have been studied in the remarkably versatile Xenopus systems for many years. This versatility is showcased in several accompanying protocols, which are introduced here. One protocol demonstrates how the Xenopus embryonic ectoderm can be used to study the effects of mechanical cell deformation; another illustrates how the developing eye can be used as a platform for determining cell-cycle length. Two protocols show how extracts from Xenopus embryos can be exploited to characterize the behavior of specific intracellular proteins-specifically, to determine protein phosphorylation status and the ability to bind to chromatin. Finally, because specific antibodies to Xenopus proteins are pivotal reagents for cell biology and biochemistry applications, four protocols describing how to generate, purify, and assay the specificity of antibodies raised against Xenopus proteins are included in hopes of stimulating the expansion of these critical resources across the Xenopus community.

Retinoic acid production, regulation and containment through Zic1, Pitx2c and Cyp26c1 control cranial placode specification

All paired sensory organs arise from a common precursor domain called the pre-placodal region (PPR). In Xenopus, Zic1 non-cell autonomously regulates PPR formation by activating retinoic acid (RA) production. Here, we have identified two Zic1 targets, the RA catabolizing enzyme Cyp26c1 and the transcription factor Pitx2c, expressed in the vicinity of the PPR as being crucially required for maintaining low RA levels in a spatially restricted, PPR-adjacent domain. Morpholino- or CRISPR/Cas9-mediated Cyp26c1 knockdown abrogated PPR gene expression, yielding defective cranial placodes. Direct measurement of RA levels revealed that this is mediated by a mechanism involving excess RA accumulation. Furthermore, we show that pitx2c is activated by RA and required for Cyp26c1 expression in a domain-specific manner through induction of FGF8. We propose that Zic1 anteriorly establishes a program of RA containment and regulation through activation of Cyp26c1 and Pitx2c that cooperates to promote PPR specification in a spatially restricted domain.

Surface contraction waves or cell proliferation waves in the presumptive neurectoderm during amphibian gastrulation: Mexican axolotl versus African clawed frog

This essay represents a critical analysis of the literary data on various types of waves occurring in the amphibian embryos during gastrulation. A surface contraction wave travels through the presumptive neurectoderm during Mexican axolotl gastrulation. This wave coincides temporally and spatially with involution of the inducing chordomesoderm and with the prospective neural plate. By contrast, there is no similar surface contraction wave during African clawed frog gastrulation. However, the clawed frog displays the waves of DNA synthesis and mitosis in the presumptive neurectoderm during gastrulation, whereas no such waves were discovered in axolotl gastrulae. These sets of experimental data are in accordance with the contemporary concept of considerable ontogenetic diversity of the class Amphibia.

Interplay of TRIM2 E3 Ubiquitin Ligase and ALIX/ESCRT Complex: Control of Developmental Plasticity During Early Neurogenesis

Tripartite motif 2 (TRIM2) drives neurite outgrowth and polarization, is involved in axon specification, and confers neuroprotective functions during rapid ischemia. The mechanisms controlling neuronal cell fate determination and differentiation are fundamental for neural development. Here, we show that in Xenopus, trim2 knockdown affects primary neurogenesis and neural progenitor cell survival. Embryos also suffer from severe craniofacial malformation, a reduction in brain volume, and the loss of motor sensory function. Using a high-throughput LC-MS/MS approach with GST-Trim2 as bait, we pulled down ALG-2 interacting protein X (Alix) from Xenopus embryonic lysates. We demonstrate that the expression of trim2/TRIM2 and alix/ALIX overlap during larval development and on a cellular level in cell culture. Interestingly, trim2 morphants showed a clustering and apoptosis of neural progenitors, which are phenotypic hallmarks that are also observed in Alix KO mice. Therefore, we propose that the interaction of Alix and Trim2 plays a key role in the determination and differentiation of neural progenitors via the modulation of cell proliferation/apoptosis during neurogenesis.

Morphogenetic mechanisms forming the notochord rod: The turgor pressure-sheath strength model

The notochord is a defining feature of chordates. During notochord formation in vertebrates and tunicates, notochord cells display dynamic morphogenetic movement, called convergent extension, in which cells intercalate and align at the dorsal midline. However, in cephalochordates, the most basal group of chordates, the notochord is formed without convergent extension. It is simply developed from mesodermal cells at the dorsal midline. This suggests that convergent extension movement of notochord cells is a secondarily acquired developmental attribute in the common ancestor of olfactores (vertebrates + tunicates), and that the chordate ancestor innovated the notochord upon a foundation of morphogenetic mechanisms independent of cell movement. Therefore, this review focuses on biological features specific to notochord cells, which have been well studied using clawed frogs, zebrafish, and tunicates. Attributes of notochord cells, such as vacuolation, membrane trafficking, extracellular matrix formation, and apoptosis, can be understood in terms of two properties: turgor pressure of vacuoles and strength of the notochord sheath. To maintain the straight rod-like structure of the notochord, these parameters must be counterbalanced. In the future, the turgor pressure-sheath strength model, proposed in this review, will be examined in light of quantitative molecular data and mathematical simulations, illuminating the evolutionary origin of the notochord.

Mcl1 protein levels and Caspase-7 executioner protease control axial organizer cells survival

BACKGROUND

Organizing centers are groups of specialized cells that secrete morphogens, thereby influencing development of their neighboring territories. Apoptosis is a form of programmed cell death reported to limit the size of organizers. Little is known about the identity of intracellular signals driving organizer cell death. Here we investigated in Xenopus the role of both the anti-apoptotic protein Myeloid-cell-leukemia 1 (Mcl1) and the cysteine proteases Caspase-3 and Caspase-7 in formation of the axial organizing center-the notochord-that derives from the Spemann organizer, and participates in the induction and patterning of the neuroepithelium.

RESULTS

We confirm a role for apoptosis in establishing the axial organizer in early neurula. We show that the expression pattern of mcl1 is coherent with a role for this gene in early notochord development. Using loss of function approaches, we demonstrate that Mcl1 depletion decreases neuroepithelium width and increases notochord cells apoptosis, a process that relies on Caspase-7, and not on Caspase-3, activity. Our data provide evidence that Mcl1 protein levels physiologically control notochord cells' survival and that Caspase-7 is the executioner protease in this developmental process.

CONCLUSIONS

Our study reveals new functions for Mcl1 and Caspase-7 in formation of the axial signalling center.

A dual function of FGF signaling in Xenopus left-right axis formation

Organ left-right (LR) asymmetry is a conserved vertebrate feature, which is regulated by left-sided activation of Nodal signaling. Nodal asymmetry is established by a leftward fluid-flow generated at the ciliated LR organizer (LRO). Although the role of fibroblast growth factor (FGF) signaling pathways during mesoderm development is conserved, diverging results from different model organisms suggest a non-conserved function in LR asymmetry. Here, we demonstrate that FGF is required during gastrulation in a dual function at consecutive stages of Xenopus embryonic development. In the early gastrula, FGF is necessary for LRO precursor induction, acting in parallel with FGF-mediated mesoderm induction. During late gastrulation, the FGF/Ca²⁺-branch is required for specification of the flow-sensing lateral LRO cells, a function related to FGF-mediated mesoderm morphogenesis. This second function in addition requires input from the calcium channel Polycystin-2. Thus, analogous to mesoderm development, FGF activity is required in a dual role for laterality specification; namely, for generating and sensing leftward flow. Moreover, our findings in Xenopus demonstrate that FGF functions in LR development share more conserved features across vertebrate species than previously anticipated.

Lignans from Bursera fagaroides Affect In Vivo Cell Behavior by Disturbing the Tubulin Cytoskeleton in Zebrafish Embryos

By using a zebrafish embryo model to guide the chromatographic fractionation of antimitotic secondary metabolites, seven podophyllotoxin-type lignans were isolated from a hydroalcoholic extract obtained from the steam bark of Bursera fagaroides. The compounds were identified as podophyllotoxin (1), β-peltatin-A-methylether (2), 5′-desmethoxy-β-peltatin-A-methylether (3), desmethoxy-yatein (4), desoxypodophyllotoxin (5), burseranin (6), and acetyl podophyllotoxin (7). The biological effects on mitosis, cell migration, and microtubule cytoskeleton remodeling of lignans 1–7 were further evaluated in zebrafish embryos by whole-mount immunolocalization of the mitotic marker phospho-histone H3 and by a tubulin antibody. We found that lignans 1, 2, 4, and 7 induced mitotic arrest, delayed cell migration, and disrupted the microtubule cytoskeleton in zebrafish embryos. Furthermore, microtubule cytoskeleton destabilization was observed also in PC3 cells, except for 7. Therefore, these results demonstrate that the cytotoxic activity of 1, 2, and 4 is mediated by their microtubule-destabilizing activity. In general, the in vivo and in vitro models here used displayed equivalent mitotic effects, which allows us to conclude that the zebrafish model can be a fast and cheap in vivo model that can be used to identify antimitotic natural products through bioassay-guided fractionation.

Multiscale analysis of architecture, cell size and the cell cortex reveals cortical F-actin density and composition are major contributors to mechanical properties during convergent extension

The large-scale movements that construct complex three-dimensional tissues during development are governed by universal physical principles. Fine-grained control of both mechanical properties and force production is crucial to the successful placement of tissues and shaping of organs. Embryos of the frog Xenopus laevis provide a dramatic example of these physical processes, as dorsal tissues increase in Young's modulus by six-fold to 80 Pascal over 8 h as germ layers and the central nervous system are formed. These physical changes coincide with emergence of complex anatomical structures, rounds of cell division, and cytoskeletal remodeling. To understand the contribution of these diverse structures, we adopt the cellular solids model to relate bulk stiffness of a solid foam to the unit size of individual cells, their microstructural organization, and their material properties. Our results indicate that large-scale tissue architecture and cell size are not likely to influence the bulk mechanical properties of early embryonic or progenitor tissues but that F-actin cortical density and composition of the F-actin cortex play major roles in regulating the physical mechanics of embryonic multicellular tissues.

MMP14 Regulates Cranial Neural Crest Epithelial-to-Mesenchymal Transition and Migration

BACKGROUND

Neural crest is a vertebrate specific cell population. Induced at lateral borders of the neural plate, neural crest cells (NCCs) subsequently undergo epithelial-to-mesenchymal transition (EMT) to detach from the neuroepithelium before migrating into various locations in the embryo. Despite the wealth of knowledge of transcription factors involved in this process, little is known about the effectors that directly regulate neural crest EMT and migration.

RESULTS

Here, we examined the activity of matrix metalloproteinase MMP14 in NCCs and found that MMP14 is expressed in both premigratory and migrating NCCs. Overexpression of MMP14 led to premature migration of NCCs, while down-regulation of MMP14 resulted in reduced neural crest migration. Transplantation experiment further showed that MMP14 is required in NCCs, whereas MMP2, which can be activated by MMP14, is required in the surrounding mesenchyme. in vitro explant culture showed that MMP14 is required for neural crest EMT but not for spreading. This is possibly mediated by the changes in cadherin levels, as decreasing MMP14 level led to increased cadherin expression and increasing MMP14 level led to reduced cadherin expression.

CONCLUSIONS

The results demonstrate that MMP14 is critical for neural crest EMT and migration, partially through regulating the levels of cadherins. Developmental Dynamics 247:1083-1092, 2018. © 2018 Wiley Periodicals, Inc.

A model for investigating developmental eye repair in Xenopus laevis

Vertebrate eye development is complex and requires early interactions between neuroectoderm and surface ectoderm during embryogenesis. In the African clawed frog, Xenopus laevis, individual eye tissues such as the retina and lens can undergo regeneration. However, it has been reported that removal of either the specified eye field at the neurula stage or the eye during tadpole stage does not induce replacement. Here we describe a model for investigating Xenopus developmental eye repair. We found that tailbud embryos can readily regrow eyes after surgical removal of over 83% of the specified eye and lens tissues. The regrown eye reached a comparable size to the contralateral control by 5 days and overall animal development was normal. It contained the expected complement of eye cell types (including the pigmented epithelium, retina and lens), and is connected to the brain. Our data also demonstrate that apoptosis, an early mechanism that regulates appendage regeneration, is also required for eye regrowth. Treatment with apoptosis inhibitors (M50054 or NS3694) blocked eye regrowth by inhibiting caspase activation. Together, our findings indicate that frog embryos can undergo successful eye repair after considerable tissue loss and reveals a required role for apoptosis in this process. Furthermore, this Xenopus model allows for rapid comparisons of productive eye repair and developmental pathways. It can also facilitate the molecular dissection of signaling mechanisms necessary for initiating repair.

Ingression-type cell migration drives vegetal endoderm internalisation in the Xenopus gastrula

During amphibian gastrulation, presumptive endoderm is internalised as part of vegetal rotation, a large-scale movement that encompasses the whole vegetal half of the embryo. It has been considered a gastrulation process unique to amphibians, but we show that at the cell level, endoderm internalisation exhibits characteristics reminiscent of bottle cell formation and ingression, known mechanisms of germ layer internalisation. During ingression proper, cells leave a single-layered epithelium. In vegetal rotation, the process occurs in a multilayered cell mass; we refer to it as ingression-type cell migration. Endoderm cells move by amoeboid shape changes, but in contrast to other instances of amoeboid migration, trailing edge retraction involves ephrinB1-dependent macropinocytosis and trans-endocytosis. Moreover, although cells are separated by wide gaps, they are connected by filiform protrusions, and their migration depends on C-cadherin and the matrix protein fibronectin. Cells move in the same direction but at different velocities, to rearrange by differential migration.

Leptin Induces Mitosis and Activates the Canonical Wnt/β-Catenin Signaling Pathway in Neurogenic Regions of Xenopus Tadpole Brain

In addition to its well-known role as an adipostat in adult mammals, leptin has diverse physiological and developmental actions in vertebrates. Leptin has been shown to promote development of hypothalamic circuits and to induce mitosis in different brain areas of mammals. We investigated the ontogeny of leptin mRNA, leptin actions on cell proliferation in the brain, and gene expression in the preoptic area/hypothalamus of tadpoles of Xenopus laevis. The level of leptin mRNA was low in premetamorphic tadpoles, but increased strongly at the beginning of metamorphosis and peaked at metamorphic climax. This increase in leptin mRNA at the onset of metamorphosis correlated with increased cell proliferation in the neurogenic zones of tadpole brain. We found that intracerebroventricular (i.c.v.) injection of recombinant Xenopus leptin (rxLeptin) in premetamorphic tadpoles strongly increased cell proliferation in neurogenic zones throughout the tadpole brain. We conducted gene expression profiling of genes induced at 2 h following i.c.v. injection of rxLeptin. This analysis identified 2,322 genes induced and 1,493 genes repressed by rxLeptin. The most enriched Kyoto Encyclopedia of Genes and Genomes term was the canonical Wnt/β-catenin pathway. Using electroporation-mediated gene transfer into tadpole brain of a reporter vector responsive to the canonical Wnt/β-catenin signaling pathway, we found that i.c.v. rxLeptin injection activated Wnt/β-catenin-dependent transcriptional activity. Our findings show that leptin acts on the premetamorphic tadpole brain to induce cell proliferation, possibly acting via the Wnt/β-catenin signaling pathway.

Human amniotic fluid contaminants alter thyroid hormone signalling and early brain development in Xenopus embryos

Thyroid hormones are essential for normal brain development in vertebrates. In humans, abnormal maternal thyroid hormone levels during early pregnancy are associated with decreased offspring IQ and modified brain structure. As numerous environmental chemicals disrupt thyroid hormone signalling, we questioned whether exposure to ubiquitous chemicals affects thyroid hormone responses during early neurogenesis. We established a mixture of 15 common chemicals at concentrations reported in human amniotic fluid. An in vivo larval reporter (GFP) assay served to determine integrated thyroid hormone transcriptional responses. Dose-dependent effects of short-term (72 h) exposure to single chemicals and the mixture were found. qPCR on dissected brains showed significant changes in thyroid hormone-related genes including receptors, deiodinases and neural differentiation markers. Further, exposure to mixture also modified neural proliferation as well as neuron and oligodendrocyte size. Finally, exposed tadpoles showed behavioural responses with dose-dependent reductions in mobility. In conclusion, exposure to a mixture of ubiquitous chemicals at concentrations found in human amniotic fluid affect thyroid hormone-dependent transcription, gene expression, brain development and behaviour in early embryogenesis. As thyroid hormone signalling is strongly conserved across vertebrates the results suggest that ubiquitous chemical mixtures could be exerting adverse effects on foetal human brain development.

Research proceedings on amphibian model organisms

Model organisms have long been important in biology and medicine due to their specific characteristics. Amphibians, especially Xenopus, play key roles in answering fundamental questions on developmental biology, regeneration, genetics, and toxicology due to their large and abundant eggs, as well as their versatile embryos, which can be readily manipulated and developed in vivo. Furthermore, amphibians have also proven to be of considerable benefit in human disease research due to their conserved cellular developmental and genomic organization. This review gives a brief introduction on the progress and limitations of these animal models in biology and human disease research, and discusses the potential and challenge of Microhyla fissipes as a new model organism.

Sf3b4-depleted Xenopus embryos: A model to study the pathogenesis of craniofacial defects in Nager syndrome

Mandibulofacial dysostosis (MFD) is a human developmental disorder characterized by defects of the facial bones. It is the second most frequent craniofacial malformation after cleft lip and palate. Nager syndrome combines many features of MFD with a variety of limb defects. Mutations in SF3B4 (splicing factor 3b, subunit 4) gene, which encodes a component of the pre-mRNA spliceosomal complex, were recently identified as a cause of Nager syndrome, accounting for 60% of affected individuals. Nothing is known about the cellular pathogenesis underlying Nager type MFD. Here we describe the first animal model for Nager syndrome, generated by knocking down Sf3b4 function in Xenopus laevis embryos, using morpholino antisense oligonucleotides. Our results indicate that Sf3b4-depleted embryos show reduced expression of the neural crest genes sox10, snail2 and twist at the neural plate border, associated with a broadening of the neural plate. This phenotype can be rescued by injection of wild-type human SF3B4 mRNA but not by mRNAs carrying mutations that cause Nager syndrome. At the tailbud stage, morphant embryos had decreased sox10 and tfap2a expression in the pharyngeal arches, indicative of a reduced number of neural crest cells. Later in development, Sf3b4-depleted tadpoles exhibited hypoplasia of neural crest-derived craniofacial cartilages, phenocopying aspects of the craniofacial skeletal defects seen in Nager syndrome patients. With this animal model we are now poised to gain important insights into the etiology and pathogenesis of Nager type MFD, and to identify the molecular targets of Sf3b4.

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.

Endogenous gradients of resting potential instructively pattern embryonic neural tissue via Notch signaling and regulation of proliferation

Biophysical forces play important roles throughout embryogenesis, but the roles of spatial differences in cellular resting potentials during large-scale brain morphogenesis remain unknown. Here, we implicate endogenous bioelectricity as an instructive factor during brain patterning in Xenopus laevis. Early frog embryos exhibit a characteristic hyperpolarization of cells lining the neural tube; disruption of this spatial gradient of the transmembrane potential (Vmem) diminishes or eliminates the expression of early brain markers, and causes anatomical mispatterning of the brain, including absent or malformed regions. This effect is mediated by voltage-gated calcium signaling and gap-junctional communication. In addition to cell-autonomous effects, we show that hyperpolarization of transmembrane potential (Vmem) in ventral cells outside the brain induces upregulation of neural cell proliferation at long range. Misexpression of the constitutively active form of Notch, a suppressor of neural induction, impairs the normal hyperpolarization pattern and neural patterning; forced hyperpolarization by misexpression of specific ion channels rescues brain defects induced by activated Notch signaling. Strikingly, hyperpolarizing posterior or ventral cells induces the production of ectopic neural tissue considerably outside the neural field. The hyperpolarization signal also synergizes with canonical reprogramming factors (POU and HB4), directing undifferentiated cells toward neural fate in vivo. These data identify a new functional role for bioelectric signaling in brain patterning, reveal interactions between Vmem and key biochemical pathways (Notch and Ca(2+) signaling) as the molecular mechanism by which spatial differences of Vmem regulate organogenesis of the vertebrate brain, and suggest voltage modulation as a tractable strategy for intervention in certain classes of birth defects.

An oncologist׳s friend: How Xenopus contributes to cancer research

One of the most striking features of the Xenopus system is the versatility in providing a unique range of both in vitro and in vivo models that are rapid, accessible and easily manipulated. Here we present an overview of the diverse contribution that Xenopus has made to advance our understanding of tumour biology and behaviour; a contribution that goes beyond the traditional view of Xenopus as a developmental model organism. From the utility of the egg and oocyte extract system to the use of whole embryos as developmental or induced tumour models, the Xenopus system has been fundamental to investigation of cell cycle mechanisms, cell metabolism, cell signalling and cell behaviour, and has allowed an increasing appreciation of the parallels between early development and the pathogenesis of tumour progression and metastasis. Although not the prototypical oncological model system, we propose that Xenopus is an adaptable and multifunctional tool in the oncologist׳s arsenal.

Kcnip1 a Ca²⁺-dependent transcriptional repressor regulates the size of the neural plate in Xenopus

In amphibian embryos, our previous work has demonstrated that calcium transients occurring in the dorsal ectoderm at the onset of gastrulation are necessary and sufficient to engage the ectodermal cells into a neural fate by inducing neural specific genes. Some of these genes are direct targets of calcium. Here we search for a direct transcriptional mechanism by which calcium signals are acting. The only known mechanism responsible for a direct action of calcium on gene transcription involves an EF-hand Ca²⁺ binding protein which belongs to a group of four proteins (Kcnip1 to 4). Kcnip protein can act in a Ca²⁺-dependent manner as a transcriptional repressor by binding to a specific DNA sequence, the Downstream Regulatory Element (DRE) site. In Xenopus, among the four kcnips, we show that only kcnip1 is timely and spatially present in the presumptive neural territories and is able to bind DRE sites in a Ca²⁺-dependent manner. The loss of function of kcnip1 results in the expansion of the neural plate through an increased proliferation of neural progenitors. Later on, this leads to an impairment in the development of anterior neural structures. We propose that, in the embryo, at the onset of neurogenesis Kcnip1 is the Ca²⁺-dependent transcriptional repressor that controls the size of the neural plate. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

Hippocampal proliferation is increased in presymptomatic Parkinson's disease and due to microglia

Besides dopamine-deficiency related motor symptoms, nonmotor symptoms, including cognitive changes occur in Parkinson's disease (PD) patients, that may relate to accumulation of α-synuclein in the hippocampus (HC). This brain region also contains stem cells that can proliferate. This is a well-regulated process that can, for example, be altered by neurodegenerative conditions. In contrast to proliferation in the substantia nigra and subventricular zone, little is known about the HC in PD. In addition, glial cells contribute to neurodegenerative processes and may proliferate in response to PD pathology. In the present study, we questioned whether microglial cells proliferate in the HC of established PD patients versus control subjects or incidental Lewy body disease (iLBD) cases as a prodromal state of PD. To this end, proliferation was assessed using the immunocytochemical marker minichromosome maintenance protein 2 (MCM2). Colocalization with Iba1 was performed to determine microglial proliferation. MCM2-positive cells were present in the HC of controls and were significantly increased in the presymptomatic iLBD cases, but not in established PD patients. Microglia represented the majority of the proliferating cells in the HC. This suggests an early microglial response to developing PD pathology in the HC and further indicates that neuroinflammatory processes play an important role in the development of PD pathology.

Temperature-independent energy expenditure in early development of the African clawed frog Xenopus laevis

The thermal dissipation of activated eggs and embryos undergoing development from cleavage to the tailbud stage of the African clawed frog Xenopus laevis was measured as a function of incubation time at temperatures ranging from T = 288.2 K to 295.2 K, using a high-precision isothermal calorimeter. A23187-mediated activation of mature eggs induced stable periodic thermal oscillations lasting for 8-34 h. The frequency agreed well with the cell cycle frequency of initial cleavages at the identical temperature. In the developing embryo, energy metabolism switches from embryonic to adult features during gastrulation. The thermal dissipation after gastrulation fit well with a single modified Avrami equation, which has been used for modeling crystal-growth. Both the oscillation frequency of the activated egg and the growth rate of the embryo strongly depend on temperature with the same apparent activation energy of approximately 87 kJ mole(-1). This result suggests that early development proceeds as a single biological time, attributable to a single metabolic rate. A temperature-independent growth curve was derived by scaling the thermogram to the biological time, indicating that the amount of energy expenditure during each developmental stage is constant over the optimal temperature range.

GEF-H1 functions in apical constriction and cell intercalations and is essential for vertebrate neural tube closure

Rho family GTPases regulate many morphogenetic processes during vertebrate development including neural tube closure. Here we report a function for GEF-H1/Lfc/ArhGEF2, a RhoA-specific guanine nucleotide exchange factor that functions in neurulation in Xenopus embryos. Morpholino-mediated depletion of GEF-H1 resulted in severe neural tube defects, which were rescued by GEF-H1 RNA. Lineage tracing of GEF-H1 morphants at different developmental stages revealed abnormal cell intercalation and apical constriction, suggesting that GEF-H1 regulates these cell behaviors. Molecular marker analysis documented defects in myosin II light chain (MLC) phosphorylation, Rab11 and F-actin accumulation in GEF-H1-depleted cells. In gain-of-function studies, overexpressed GEF-H1 induced Rho-associated kinase-dependent ectopic apical constriction - marked by apical accumulation of phosphorylated MLC, γ-tubulin and F-actin in superficial ectoderm - and stimulated apical protrusive activity of deep ectoderm cells. Taken together, our observations newly identify functions of GEF-H1 in morphogenetic movements that lead to neural tube closure.

The syncytial Drosophila embryo as a mechanically excitable medium

Mitosis in the early syncytial Drosophila embryo is highly correlated in space and time, as manifested in mitotic wavefronts that propagate across the embryo. In this paper we investigate the idea that the embryo can be considered a mechanically-excitable medium, and that mitotic wavefronts can be understood as nonlinear wavefronts that propagate through this medium. We study the wavefronts via both image analysis of confocal microscopy videos and theoretical models. We find that the mitotic waves travel across the embryo at a well-defined speed that decreases with replication cycle. We find two markers of the wavefront in each cycle, corresponding to the onsets of metaphase and anaphase. Each of these onsets is followed by displacements of the nuclei that obey the same wavefront pattern. To understand the mitotic wavefronts theoretically we analyze wavefront propagation in excitable media. We study two classes of models, one with biochemical signaling and one with mechanical signaling. We find that the dependence of wavefront speed on cycle number is most naturally explained by mechanical signaling, and that the entire process suggests a scenario in which biochemical and mechanical signaling are coupled.

Maturin is a novel protein required for differentiation during primary neurogenesis

Proliferation and differentiation are tightly controlled during neural development. In the embryonic neural plate, primary neurogenesis is driven by the proneural pathway. Here we report the characterization of Maturin, a novel, evolutionarily conserved protein that is required for normal primary neurogenesis. Maturin is detected throughout the early nervous system, yet it is most strongly expressed in differentiating neurons of the embryonic fish, frog and mouse nervous systems. Maturin expression can be induced by the proneural transcription factors Neurog2, Neurod1, and Ebf3. Maturin overexpression promotes neurogenesis, while loss-of-function inhibits the differentiation of neuronal progenitors, resulting in neural plate expansion. Maturin knockdown blocks the ability of Neurog2, Neurod1, and Ebf3 to drive ectopic neurogenesis. Maturin and Pak3, are both required for, and can synergize to promote differentiation of the primary neurons in vivo. Together, our results suggest that Maturin functions during primary neurogenesis and is required for the proneural pathway to regulate neural differentiation.

The cell cycle and pluripotency

PSCs (pluripotent stem cells) possess two key properties that have made them the focus of global research efforts in regenerative medicine: they have unlimited expansion potential under conditions which favour their preservation as PSCs and they have the ability to generate all somatic cell types upon differentiation (pluripotency). Conditions have been defined in vitro in which pluripotency is maintained, or else differentiation is favoured and is directed towards specific somatic cell types. However, an unanswered question is whether or not the core cell cycle machinery directly regulates the pluripotency and differentiation properties of PSCs. If so, then manipulation of the cell cycle may represent an additional tool by which in vitro maintenance or differentiation of PSCs may be controlled in regenerative medicine. The present review aims to summarize our current understanding of links between the core cell cycle machinery and the maintenance of pluripotency in ESCs (embryonic stem cells) and iPSCs (induced PSCs).

Cell proliferation patterns in early zebrafish development

Although cell proliferation is an essential cell behavior for animal development, a detailed analysis of spatial and temporal patterns of proliferation in whole embryos are still lacking for most model organisms. Zebrafish embryos are particularly suitable for this type of analysis due to their transparency and size. Therefore, the main objective of the present work was to analyze the spatial and temporal patterns of proliferation during the first day of zebrafish embryo development by indirect immunofluorescence against phosphorylated histone H3, a commonly used mitotic marker. Several interesting findings were established. First, we found that mitosis metasynchrony among blastomeres could begin at the 2- to 4-cell stage embryos. Second, mitosis synchrony was lost before the midblastula transition (MBT). Third, we observed a novel pattern of mitotic clusters that coincided in time with the mitotic pseudo "waves" described to occur before the MBT. Altogether, our findings indicate that early development is less synchronic than anticipated and that synchrony is not a requirement for proper development in zebrafish.

Antibody development and use in chromogenic and fluorescent immunostaining

Antibody-based detection of protein distribution patterns both in wholemount and on sections revolutionized Xenopus research and ushered in the visual-based era of Xenopus data presentation. The ability to view the distribution of a gene product throughout an embryo makes it possible to rapidly map normal expression profiles and profiles that have been altered by an experimental intervention. The main limiting element in Xenopus immunostaining techniques has always been the availability of antibodies that work well on fixed whole embryos, a problem that persists. However, new antibodies are constantly being generated and improvements in detection systems allow antibodies that were once below the limits of detection to be utilized in multichannel immunofluorescence using tyramide amplification.

Comparative functional analysis of ZFP36 genes during Xenopus development

ZFP36 constitutes a small family of RNA binding proteins (formerly known as the TIS11 family) that target mRNA and promote their degradation. In mammals, ZFP36 proteins are encoded by four genes and, although they show similar activities in a cellular RNA destabilization assay, there is still a limited knowledge of their mRNA targets and it is not known whether or not they have redundant functions. In the present work, we have used the Xenopus embryo, a model system allowing gain- and loss-of-function studies, to investigate, whether individual ZFP36 proteins had distinct or redundant functions. We show that overexpression of individual amphibian zfp36 proteins leads to embryos having the same defects, with alteration in somites segmentation and pronephros formation. In these embryos, members of the Notch signalling pathway such as hairy2a or esr5 mRNA are down-regulated, suggesting common targets for the different proteins. We also show that mouse Zfp36 protein overexpression gives the same phenotype, indicating an evolutionary conserved property among ZFP36 vertebrate proteins. Morpholino oligonucleotide-induced loss-of-function leads to defects in pronephros formation, reduction in tubule size and duct coiling alterations for both zfp36 and zfp36l1, indicating no functional redundancy between these two genes. Given the conservation in gene structure and function between the amphibian and mammalian proteins and the conserved mechanisms for pronephros development, our study highlights a potential and hitherto unreported role of ZFP36 gene in kidney morphogenesis.

Homeoprotein hhex-induced conversion of intestinal to ventral pancreatic precursors results in the formation of giant pancreata in Xenopus embryos

Liver and ventral pancreas develop from neighboring territories within the endoderm of gastrulae. ventral pancreatic precursor 1 (vpp1) is a marker gene that is differentially expressed in a cell population within the dorsal endoderm in a pattern partially overlapping with that of hematopoietically expressed homeobox (hhex) during gastrulation. In tail bud embryos, vpp1 expression specifically demarcates two ventral pancreatic buds, whereas hhex expression is mainly restricted to the liver diverticulum. Ectopic expression of a critical dose of hhex led to a greatly enlarged vpp1-positive domain and, subsequently, to the formation of giant ventral pancreata, putatively by conversion of intestinal to ventral pancreatic precursor cells. Conversely, antisense morpholino oligonucleotide-mediated knockdown of hhex resulted in a down-regulation of vpp1 expression and a specific loss of the ventral pancreas. Furthermore, titration of hhex with a dexamethasone-inducible hhex-VP16GR fusion construct suggested that endogenous hhex activity during gastrulation is essential for the formation of ventral pancreatic progenitor cells. These observations suggest that, beyond its role in liver development, hhex controls specification of a vpp1-positive endodermal cell population during gastrulation that is required for the formation of the ventral pancreas.

The RNA-binding protein XSeb4R regulates maternal Sox3 at the posttranscriptional level during maternal-zygotic transition in Xenopus

The maternal-zygotic transition (MZT) is an embryonic event that overlaps with and plays key roles in primary germ layer specification in vertebrates. During MZT, maternally supplied mRNAs are degraded while zygotic transcripts are synthesized to either reinforce the already specified cell fate or to trigger new cell identity. Here, we show that forced expression of the RNA-binding protein, XSeb4R, in animal pole blastomeres of Xenopus embryos, inappropriately stabilizes transcripts there, including maternal Sox3. This leads to the impaired ability of the ectodermal progenitors to respond to factors regulating brain patterning and their eventual loss by apoptosis. XSeb4R protein binds specifically to the 3'UTR of Sox3 mRNA. XSeb4R gain-of-function in ectodermal explants reveals increased stability of the maternal Sox3 transcripts, associated with a robust Sox3 protein production. Conversely, whereas XSeb4R depletion abolishes VegT expression, the amount of the maternal Sox3 mRNA is rather increased but without augmentation in the amount of Sox3 protein. Moreover, XSeb4R protein knockdown leads to the modification of the ectoderm-mesoderm boundary, marked by expanded/shifted expression of the mesodermal marker genes such as Xbra and Apod, followed by an expression inhibition of Epi. K., an ectodermal marker. Overall, our data suggest XSeb4R as a novel player in gene expression regulation, acting at the posttranscriptional level during ectoderm specification in Xenopus.

Xaml1/Runx1 is required for the specification of Rohon-Beard sensory neurons in Xenopus

Lower vertebrates develop a unique set of primary sensory neurons located in the dorsal spinal cord. These cells, known as Rohon-Beard (RB) sensory neurons, innervate the skin and mediate the response to touch during larval stages. Here we report the expression and function of the transcription factor Xaml1/Runx1 during RB sensory neurons formation. In Xenopus embryos Runx1 is specifically expressed in RB progenitors at the end of gastrulation. Runx1 expression is positively regulated by Fgf and canonical Wnt signaling and negatively regulated by Notch signaling, the same set of factors that control the development of other neural plate border cell types, i.e. the neural crest and cranial placodes. Embryos lacking Runx1 function fail to differentiate RB sensory neurons and lose the mechanosensory response to touch. At early stages Runx1 knockdown results in a RB progenitor-specific loss of expression of Pak3, a p21-activated kinase that promotes cell cycle withdrawal, and of N-tub, a neuronal-specific tubulin. Interestingly, the pro-neural gene Ngnr1, an upstream regulator of Pak3 and N-tub, is either unaffected or expanded in these embryos, suggesting the existence of two distinct regulatory pathways controlling sensory neuron formation in Xenopus. Consistent with this possibility Ngnr1 is not sufficient to activate Runx1 expression in the ectoderm. We propose that Runx1 function is critically required for the generation of RB sensory neurons, an activity reminiscent of that of Runx1 in the development of the mammalian dorsal root ganglion nociceptive sensory neurons.

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.

Pronephric tubulogenesis requires Daam1-mediated planar cell polarity signaling

Canonical β-catenin-mediated Wnt signaling is essential for the induction of nephron development. Noncanonical Wnt/planar cell polarity (PCP) pathways contribute to processes such as cell polarization and cytoskeletal modulation in several tissues. Although PCP components likely establish the plane of polarization in kidney tubulogenesis, whether PCP effectors directly modulate the actin cytoskeleton in tubulogenesis is unknown. Here, we investigated the roles of Wnt PCP components in cytoskeletal assembly during kidney tubule morphogenesis in Xenopus laevis and zebrafish. We found that during tubulogenesis, the developing pronephric anlagen expresses Daam1 and its interacting Rho-GEF (WGEF), which compose one PCP/noncanonical Wnt pathway branch. Knockdown of Daam1 resulted in reduced expression of late pronephric epithelial markers with no apparent effect upon early markers of patterning and determination. Inhibiting various points in the Daam1 signaling pathway significantly reduced pronephric tubulogenesis. These data indicate that pronephric tubulogenesis requires the Daam1/WGEF/Rho PCP pathway.

Negative autoregulation of Oct3/4 through Cdx1 promotes the onset of gastrulation

Gastrulation marks the onset of germ layer formation from undifferentiated precursor cells maintained by a network including the Pou5f1 gene, Oct3/4. Negative regulation of the undifferentiated state is a prerequisite for germ layer formation and subsequent development. A novel cross-regulatory network was characterized including the Pou5f1 and Cdx1 genes as part of the signals controlling the onset of gastrulation. Of particular interest was the observation that, preceding gastrulation, the Xenopus Oct3/4 factors, Oct60, Oct25, and Oct91, positively regulate Cdx1 expression through FGF signaling, and during gastrulation the Oct3/4 factors become repressors of Cdx1. Cdx1 negatively regulates the Pou5f1 genes during gastrulation, thus contributing to the repression of the network maintaining the undifferentiated state and promoting the onset of gastrulation. These regulatory interactions suggest that Oct3/4 initiates its own negative autoregulation through Cdx1 up-regulation to begin the repression of pluripotency in preparation for the onset of gastrulation and germ layer differentiation.

Transmembrane potential of GlyCl-expressing instructor cells induces a neoplastic-like conversion of melanocytes via a serotonergic pathway

Understanding the mechanisms that coordinate stem cell behavior within the host is a high priority for developmental biology, regenerative medicine and oncology. Endogenous ion currents and voltage gradients function alongside biochemical cues during pattern formation and tumor suppression, but it is not known whether bioelectrical signals are involved in the control of stem cell progeny in vivo. We studied Xenopus laevis neural crest, an embryonic stem cell population that gives rise to many cell types, including melanocytes, and contributes to the morphogenesis of the face, heart and other complex structures. To investigate how depolarization of transmembrane potential of cells in the neural crest's environment influences its function in vivo, we manipulated the activity of the native glycine receptor chloride channel (GlyCl). Molecular-genetic depolarization of a sparse, widely distributed set of GlyCl-expressing cells non-cell-autonomously induces a neoplastic-like phenotype in melanocytes: they overproliferate, acquire an arborized cell shape and migrate inappropriately, colonizing numerous tissues in a metalloprotease-dependent fashion. A similar effect was observed in human melanocytes in culture. Depolarization of GlyCl-expressing cells induces these drastic changes in melanocyte behavior via a serotonin-transporter-dependent increase of extracellular serotonin (5-HT). These data reveal GlyCl as a molecular marker of a sparse and heretofore unknown cell population with the ability to specifically instruct neural crest derivatives, suggest transmembrane potential as a tractable signaling modality by which somatic cells can control stem cell behavior at considerable distance, identify a new biophysical aspect of the environment that confers a neoplastic-like phenotype upon stem cell progeny, reveal a pre-neural role for serotonin and its transporter, and suggest a novel strategy for manipulating stem cell behavior.

Control of DNA replication by cyclin-dependent kinases in development

Cyclin-dependent kinases (CDKs) are required for initiation of DNA replication in all eukaryotes, and appear to act at multiple levels to control replication origin firing, depending on the cell type and stage of development. In early development of many animals, both invertebrate and vertebrate, rapid cell cycling is coupled with transcriptional repression, and replication initiates at closely spaced replication origins with little or no sequence specificity. This organisation of DNA replication is modified during development as cell proliferation becomes more controlled and defined. In all eukaryotic cells, CDKs promote conversion of "licensed" pre-replication complexes (pre-RC) to active initiation complexes. In certain circumstances, CDKs may also control pre-RC formation, transcription of replication factor genes, chromatin remodelling, origin spacing, and organisation of replication origin clusters and replication foci within the nucleus. Although CDK1 and CDK2 have overlapping roles, there is a limit to their functional redundancy. Here, I review these findings and their implications for development.

Induction of vertebrate regeneration by a transient sodium current

Amphibians such as frogs can restore lost organs during development, including the lens and tail. To design biomedical therapies for organ repair, it is necessary to develop a detailed understanding of natural regeneration. Recently, ion transport has been implicated as a functional regulator of regeneration. Whereas voltage-gated sodium channels play a well known and important role in propagating action potentials in excitable cells, we have identified a novel role in regeneration for the ion transport function mediated by the voltage-gated sodium channel, Na(V)1.2. A local, early increase in intracellular sodium is required for initiating regeneration following Xenopus laevis tail amputation, and molecular and pharmacological inhibition of sodium transport causes regenerative failure. Na(V)1.2 is absent under nonregenerative conditions, but misexpression of human Na(V)1.5 can rescue regeneration during these states. Remarkably, pharmacological induction of a transient sodium current is capable of restoring regeneration even after the formation of a nonregenerative wound epithelium, confirming that it is the regulation of sodium transport that is critical for regeneration. Our studies reveal a previously undetected competency window in which cells retain their intrinsic regenerative program, identify a novel endogenous role for Na(V) in regeneration, and show that modulation of sodium transport represents an exciting new approach to organ repair.