Medaka as a model system for the characterisation of cell cycle regulators: a functional analysis of Ol-Gadd45gamma during early embryogenesis

Gadd45 in Neuronal Development, Function, and Injury

The growth arrest and DNA damage-inducible (Gadd) 45 proteins have been associated with numerous cellular mechanisms including cell cycle control, DNA damage sensation and repair, genotoxic stress, neoplasia, and molecular epigenetics. The genes were originally identified in in vitro screens of irradiation- and interleukin-induced transcription and have since been implicated in a host of normal and aberrant central nervous system processes. These include early and postnatal development, injury, cancer, memory, aging, and neurodegenerative and psychiatric disease states. The proteins act through a variety of molecular signaling cascades including the MAPK cascade, cell cycle control mechanisms, histone regulation, and epigenetic DNA demethylation. In this review, we provide a comprehensive discussion of the literature implicating each of the three members of the Gadd45 family in these processes.

Gadd45g, a novel antidepressant target, mediates metformin-induced neuronal differentiation of neural stem cells via DNA demethylation

Increased neurogenesis elicits antidepressive-like effects. The antidiabetic drug metformin (Met) reportedly promotes hippocampal neurogenesis, which ameliorates spatial memory deficits and depression-like behaviors. However, the precise molecular mechanisms underpinning Met-induced neuronal differentiation of neural stem cells (NSCs) remain unclear. We showed that Met enhanced neuronal differentiation of NSCs via Gadd45g but not Gadd45a and Gadd45b. We further found that Gadd45g increased demethylation of neurogenic differentiation 1 (NeuroD1) promoter by regulating the activity of passive and active DNA demethylation enzymes through an AMPK-independent mechanism in Met-treated NSCs. Importantly, genetic deficiency of Gadd45g decreased hippocampal neurogenesis, which could contribute to spatial memory decline, and depression-like behaviors in the adult mice, whereas forced expression of Gadd45g alleviated the depressive-like behaviors. Our findings provide a model that Gadd45g-mediated DNA demethylation contributes to Met-induced neuronal genesis and its antidepressant-like effects, and propose the concept that targeting Gadd45g regulation of neurogenesis might serve as a novel antidepressant strategy.

Gadd45g initiates embryonic stem cell differentiation and inhibits breast cell carcinogenesis

Many self-renewal-promoting factors of embryonic stem cells (ESCs) have been implicated in carcinogenesis, while little known about the genes that direct ESCs exit from pluripotency and regulate tumor development. Here, we show that the transcripts of Gadd45 family genes, including Gadd45a, Gadd45b, and Gadd45g, are gradually increased upon mouse ESC differentiation. Upregulation of Gadd45 members decreases cell proliferation and induces endodermal and trophectodermal lineages. In contrast, knockdown of Gadd45 genes can delay mouse ESC differentiation. Mechanistic studies reveal that Gadd45g activates MAPK signaling by increasing expression levels of the positive modulators of this pathway, such as Csf1r, Igf2, and Fgfr3. Therefore, inhibition of MAPK signaling with a MEK specific inhibitor is capable of eliminating the differentiation phenotype caused by Gadd45g upregulation. Meanwhile, GADD45G functions as a suppressor in human breast cancers. Enforced expression of GADD45G significantly inhibits tumor formation and breast cancer metastasis in mice through limitation of the propagation and invasion of breast cancer cells. These results not only expand our understanding of the regulatory network of ESCs, but also help people better treatment of cancers by manipulating the prodifferentiation candidates.

β-Catenin signaling dynamics regulate cell fate in differentiating neural stem cells

Stem cells undergo differentiation in complex and dynamic environments wherein instructive signals fluctuate on various timescales. Thus, cells must be equipped to properly respond to the timing of signals, for example, to distinguish sustained signaling from transient noise. However, how stem cells respond to dynamic variations in differentiation cues is not well characterized. Here, we use optogenetic activation of β-catenin signaling to probe the dynamic responses of differentiating adult neural stem cells (NSCs). We discover that, while elevated, sustained β-catenin activation sequentially promotes proliferation and differentiation, transient β-catenin induces apoptosis. Genetic perturbations revealed that the neurogenic/apoptotic fate switch was mediated through cell-cycle regulation by Growth Arrest and DNA Damage 45 gamma (Gadd45γ). Our results thus reveal a role for β-catenin dynamics in NSC fate decisions and may suggest a role for signal timing to minimize cell-fate errors, analogous to kinetic proofreading of stem-cell differentiation.

Injury Activates a Dynamic Cytoprotective Network to Confer Stress Resilience and Drive Repair

In healthy individuals, injured tissues rapidly repair themselves following damage. Within a healing skin wound, recruited inflammatory cells release a multitude of bacteriocidal factors, including reactive oxygen species (ROS), to eliminate invading pathogens. Paradoxically, while these highly reactive ROS confer resistance to infection, they are also toxic to host tissues and may ultimately delay repair. Repairing tissues have therefore evolved powerful cytoprotective "resilience" machinery to protect against and tolerate this collateral damage. Here, we use in vivo time-lapse imaging and genetic manipulation in Drosophila to dissect the molecular and cellular mechanisms that drive tissue resilience to wound-induced stress. We identify a dynamic, cross-regulatory network of stress-activated cytoprotective pathways, linking calcium, JNK, Nrf2, and Gadd45, that act to both "shield" tissues from oxidative damage and promote efficient damage repair. Ectopic activation of these pathways confers stress protection to naive tissue, while their inhibition leads to marked delays in wound closure. Strikingly, the induction of cytoprotection is tightly linked to the pathways that initiate the inflammatory response, suggesting evolution of a fail-safe mechanism for tissue protection each time inflammation is triggered. A better understanding of these resilience mechanisms-their identities and precise spatiotemporal regulation-is of major clinical importance for development of therapeutic interventions for all pathologies linked to oxidative stress, including debilitating chronic non-healing wounds.

Molecular characterisation of a cellular conveyor belt in Clytia medusae

The tentacular system of Clytia hemisphaerica medusa (Cnidaria, Hydrozoa) has recently emerged as a promising experimental model to tackle the developmental mechanisms that regulate cell lineage progression in an early-diverging animal phylum. From a population of proximal stem cells, the successive steps of tentacle stinging cell (nematocyte) elaboration, are spatially ordered along a "cellular conveyor belt". Furthermore, the C. hemisphaerica tentacular system exhibits bilateral organisation, with two perpendicular polarity axes (proximo-distal and oral-aboral). We aimed to improve our knowledge of this cellular system by combining RNAseq-based differential gene expression analyses and expression studies of Wnt signalling genes. RNAseq comparisons of gene expression levels were performed (i) between the tentacular system and a control medusa deprived of all tentacles, nematogenic sites and gonads, and (ii) between three samples staggered along the cellular conveyor belt. The behaviour in these differential expression analyses of two reference gene sets (stem cell genes; nematocyte genes), as well as the relative representations of selected gene ontology categories, support the validity of the cellular conveyor belt model. Expression patterns obtained by in situ hybridisation for selected highly differentially expressed genes and for Wnt signalling genes are largely consistent with the results from RNAseq. Wnt signalling genes exhibit complex spatial deployment along both polarity axes of the tentacular system, with the Wnt/β-catenin pathway probably acting along the oral-aboral axis rather than the proximo-distal axis. These findings reinforce the idea that, despite overall radial symmetry, cnidarians have a full potential for elaboration of bilateral structures based on finely orchestrated deployment of an ancient developmental gene toolkit.

Epigenetic Modulation of Stem Cells in Neurodevelopment: The Role of Methylation and Acetylation

The coordinated development of the nervous system requires fidelity in the expression of specific genes determining the different neural cell phenotypes. Stem cell fate decisions during neurodevelopment are strictly correlated with their epigenetic status. The epigenetic regulatory processes, such as DNA methylation and histone modifications discussed in this review article, may impact both neural stem cell (NSC) self-renewal and differentiation and thus play an important role in neurodevelopment. At the same time, stem cell decisions regarding fate commitment and differentiation are highly dependent on the temporospatial expression of specific genes contingent on the developmental stage of the nervous system. An interplay between the above, as well as basic cell processes, such as transcription regulation, DNA replication, cell cycle regulation and DNA repair therefore determine the accuracy and function of neuronal connections. This may significantly impact embryonic health and development as well as cognitive processes such as neuroplasticity and memory formation later in the adult.

Effect of potential role of p53 on embryo development arrest induced by H2O2 in mouse

During mammalian embryo development in vitro, mechanism of embryonic development arrest caused by oxidative stress has not been clear so far. The tumor suppressor protein p53 controls cell cycle and programmed cell death by regulating relevant signal pathway. Recent researches revealed that the concentration and distribution of p53 are closely related with reactive oxygen species (ROS). The main objective of this experiment was to explore the role of p53 on embryonic development arrest caused by oxidative stress. Results showed that embryo arrest at two-four-cell stage was significantly increased in the presence of 50 μM H₂O₂ (39.01 ± 2.74 vs. 77.20 ± 5.34%, p < 0.05). Supplementation of N-acetyl-L-cysteine (NAC) obviously reduced the ratio of development arrest (39.01 ± 2.74 vs. 71.18 ± 5.34%, p < 0.05), which was accompanied by an increase in ROS level, and H₂O₂ treatment sharply increased messenger RNA (mRNA) expression and protein levels of p53 and p53-ser15. Further increased transcription of GADD45a and p21, a downstream of p53, has an especially significant effect on the mRNA expression of GADD45a. However, expressions of cdc2 were reduced by H₂O₂. In addition, using Pifithrin-α (PFT-α), the suppresser of p53, the result showed that GADD45a and p21 were significantly downregulated, but the cell cycle gene cdc2 was significantly upregulated, while the protein level of p53 and p53-ser15 was significantly decreased. Taken together, these results demonstrate that ROS could activate p53 and regulate p53 target genes to influence early embryo development in in vitro culture.

Comparative analysis of developmentally regulated expressions of Gadd45a, Gadd45b, and Gadd45g in the mouse and marmoset cerebral cortex

The cerebral cortex is an indispensable region that is involved in higher cognitive function in the mammalian brain, and is particularly evolved in the primate brain. It has been demonstrated that cortical areas are formed by both innate and activity-dependent mechanisms. However, it remains unknown what molecular changes induce cortical expansion and complexity during primate evolution. Active DNA methylation/demethylation is one of the epigenetic mechanisms that can modify gene expression via the methylation/demethylation of promoter regions. Three growth arrest and DNA damage-inducible small nuclear proteins, Gadd45 alpha, beta, and gamma, have been identified as regulators of methylation status. To understand the involvement of epigenetic factors in primate cortical evolution, we started by analyzing expression of these demethylation genes in the developing common marmoset (Callithrix jacchus) and mouse (Mus musculus) brain. In the marmoset brain, we found that cortical expression levels of Gadd45 alpha and gamma were reduced during development, whereas there was high expression of Gadd45 beta in some areas of the adult brain, including the prefrontal, temporal, posterior parietal and insula cortices, which are particularly expanded in greater primates and humans. Compared to the marmoset brain, there were no clear regional differences and constant or reduced Gadd45 expression was seen between juvenile and adult mouse brain. Double staining with a neuronal marker revealed that most Gadd45-expressing cells were NeuN-positive neurons. Thus, these results suggest the possibility that differential Gadd45 expression affects neurons, contributing cortical evolution and diversity.

MicroRNA-383 regulates the apoptosis of tumor cells through targeting Gadd45g

Background

MicroRNAs (miRNAs) are a class of small non-coding single-stranded RNA molecules that inhibit gene expression at post-transcriptional level. Gadd45g (growth arrest and DNA-damage-inducible 45 gamma) is a stress-response protein, which has been implicated in several biological processes, including DNA repair, the cell cycle and cell differentiation.

Results

In this work, we found that miR-383 is a negative regulator of Gadd45g. Forced expression of miR-383 decreased the expression of Gadd45g through binding to the 3′ untranslated region (3′-UTR), whereas inhibition of miR-383 increased Gadd45g expression. The presence of miR-383 increased the cellular sensitivity to DNA damage in breast cancer cells, which was rescued by ectopic expression of Gadd45g without the 3′-UTR. miR-383 also regulates the expression of Gadd45g in embryonic stem (ES) cells, but not their apoptosis under genotoxic stress. miR-383 was further showed to negatively regulate ES cell differentiation via targeting Gadd45g, which subsequently modulates the pluripotency-associated genes. Taken together, our study demonstrates that miR-383 is a negative regulator of Gadd45g in both tumor cells and ES cells, however, has distinct function in regulating cell apoptosis. miR-383 may be used as antineoplastic agents in cancer chemotherapy.

Conclusion

We demonstrate for the first time that miR-383 can specifically regulates the expression of Gadd45g by directly targeting to the 3-UTR region of Gadd45g mRNA, a regulatory process conserved in human tumor cells and mouse embryonic stem cells. These two compotents can be potentially used as antineoplastic agents in cancer chemotherapy.

Molecular characterization and functional divergence of two Gadd45g homologs in sex determination in half-smooth tongue sole (Cynoglossus semilaevis)

The growth arrest and DNA-damage-inducible protein 45 gamma (Gadd45g) is known to play a major role in embryonic development and sex determination. In this study, two Gadd45g genes were isolated from half-smooth tongue sole (Cynoglossus semilaevis). Using chromosomal fluorescence in situ hybridization (FISH), Gadd45g1 and Gadd45g2 were located on the W and Z chromosomes, respectively. The full-length cDNA sequences of Gadd45g1 (1270bp) and Gadd45g2 (1181bp) were predicted to contain a 480-bp coding sequence that could encode a protein of 159 amino acids residues. A phylogenetic tree showed that the predicted Gadd45g1 and Gadd45g2 amino acid sequences clustered closely in one branch. It is proposed that Gadd45g1 and Gadd45g2 are paralogous genes derived from the divergence of the sex chromosome. Ka/Ks ratios indicated that Gadd45g1 and Gadd45g2 may have undergone a high number of mutations and have a divergence time of only about 68,000years, although Gadd45g homologs are highly conserved. The qRT-PCR demonstrated that Gadd45g1 and Gadd45g2 were highly expressed in ovary, and negligibly expressed in testis of male and neo-male. During development of the ovary (from 80 to 150days), the expression levels of both genes reached high levels. Gadd45g1 was also highly expressed at 50days, the stage just before gonad differentiation in C. semilaevis. All these findings imply functional divergence of the two Gadd45g homologs; Gadd45g1 may be necessary for sex differentiation in the early stage of gonad development, and then Gadd45g1 and Gadd45g2 maintain ovary development and the female character of half-smooth tongue sole.

Zebrafish midbrain slow-amplifying progenitors exhibit high levels of transcripts for nucleotide and ribosome biogenesis

Investigating neural stem cell (NSC) behaviour in vivo, which is a major area of research, requires NSC models to be developed. We carried out a multilevel characterisation of the zebrafish embryo peripheral midbrain layer (PML) and identified a unique vertebrate progenitor population. Located dorsally in the transparent embryo midbrain, these large slow-amplifying progenitors (SAPs) are accessible for long-term in vivo imaging. They form a neuroepithelial layer adjacent to the optic tectum, which has transitory fast-amplifying progenitors (FAPs) at its margin. The presence of these SAPs and FAPs in separate domains provided the opportunity to data mine the ZFIN expression pattern database for SAP markers, which are co-expressed in the retina. Most of them are involved in nucleotide synthesis, or encode nucleolar and ribosomal proteins. A mutant for the cad gene, which is strongly expressed in the PML, reveals severe midbrain defects with massive apoptosis and sustained proliferation. We discuss how fish midbrain and retina progenitors might derive from ancient sister cell types and have specific features that are not shared with other SAPs.

Transcriptional mechanisms of developmental cell cycle arrest: problems and models

Metazoans begin their life as a single cell. Then, this cell enters a more or less protracted period of active cell proliferation, which can be considered as the default cellular state. A crucial event, the developmental cell cycle exit, occurs thereafter. This phenomenon allows for differentiation to happen and regulates the final size of organs and organisms. Its control is still poorly understood. Herein, we review some transcriptional mechanisms of cell cycle exit in animals, and propose to use cellular conveyor belts as model systems for its study. We finally point to evidence that suggests that the mechanisms of developmental cell cycle arrest may have to be maintained in adult tissues.

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.

Direct transcriptional induction of Gadd45gamma by Ascl1 during neuronal differentiation

The basic helix-loop-helix transcription factor Ascl1 plays a critical role in the intrinsic genetic program responsible for neuronal differentiation. Here, we describe a novel model system of P19 embryonic carcinoma cells with doxycycline-inducible expression of Ascl1. Microarray hybridization and real-time PCR showed that these cells demonstrated increased expression of many neuronal proteins in a time- and concentration-dependent manner. Interestingly, the gene encoding the cell cycle regulator Gadd45gamma was increased earliest and to the greatest extent following Ascl1 induction. Here, we provide the first evidence identifying Gadd45gamma as a direct transcriptional target of Ascl1. Transactivation and chromatin immunoprecipitation assays identified two E-box consensus sites within the Gadd45gamma promoter necessary for Ascl1 regulation, and demonstrated that Ascl1 is bound to this region within the Gadd45gamma promoter. Furthermore, we found that overexpression of Gadd45gamma itself is sufficient to initiate some aspects of neuronal differentiation independent of Ascl1.

Idiopathic-type scoliosis is not exclusive to bipedalism

Human familial/idiopathic-type scoliosis (IS) is a complex genetic disorder for which the cause is unknown. The curve phenotype characteristically demonstrates pronounced morphological and developmental variability that is likely a consequence of biomechanical, environmental, and genetic differences between individuals. In addition, risk factors that affect the propensity for curves to progress to severity are unknown. Progress in understanding the fundamental biology of idiopathic-type scoliosis has been limited by the lack of a genetic/developmental animal model. Prior to consideration of teleosts, developmental idiopathic-type scoliosis has been considered to be exclusive to humans. Consequently, there is the notion that the syndrome is a result of bipedalism, and many studies try to explain the deformity from this anthrocentric viewpoint. This perspective has been reinforced by the choice of animals used for study, in that chickens and bipedal rats and mice demonstrate idiopathic-type curvature when made melatonin-deficient, but quadrupedal animals do not. Overlooked is the fact that teleosts also demonstrate similar curvature when made melatonin-deficient. Our characterization of the guppy curveback has demonstrated that non-induced idiopathic-type curvature is not exclusive to humans, nor bipedalism. We hypothesize that unique morphological, developmental and genetic parallels between the human and guppy syndromes are due to common molecular pathways involved in the etiopathogenesis of both phenotypes. We explore established gene conservation between human and teleost genomes that are in pathways hypothesized to be involved in the IS syndrome. We present non-induced vertebral wedging as a unique shared feature in IS and curveback that suggests a similar interaction between a molecular phenotype on the level of the vertebral anatomy, and biomechanics. We propose that rather than bipedalism per se, expression of idiopathic-type scoliosis is dependent on normal spinal loading applied along the cranio-caudal axis that interacts with an unknown factor causing the primary curve. In this regard, a comparative biological approach using a simplified teleost model will promote discovery of basic processes integral to idiopathic-type scoliosis in teleosts and humans, and highlight human-specific aspects of the deformity.

Expression of the Drosophila melanogaster GADD45 homolog (CG11086) affects egg asymmetric development that is mediated by the c-Jun N-terminal kinase pathway

The mammalian GADD45 (growth arrest and DNA-damage inducible) gene family is composed of three highly homologous small, acidic, nuclear proteins: GADD45alpha, GADD45beta, and GADD45gamma. GADD45 proteins are involved in important processes such as regulation of DNA repair, cell cycle control, and apoptosis. Annotation of the Drosophila melanogaster genome revealed that it contains a single GADD45-like protein (CG11086; D-GADD45). We found that, as its mammalian homologs, D-GADD45 is a nuclear protein; however, D-GADD45 expression is not elevated following exposure to genotoxic and nongenotoxic agents in Schneider cells and in adult flies. We showed that the D-GADD45 transcript increased following immune response activation, consistent with previous microarray findings. Since upregulation of GADD45 proteins has been characterized as an important cellular response to genotoxic and nongenotoxic agents, we aimed to characterize the effect of D-GADD45 overexpression on D. melanogaster development. Overexpression of D-GADD45 in various tissues led to different phenotypic responses. Specifically, in the somatic follicle cells overexpression caused apoptosis, while overexpression in the germline affected the dorsal-ventral polarity of the eggshell and disrupted the localization of anterior-posterior polarity determinants. In this article we focused on the role of D-GADD45 overexpression in the germline and found that D-GADD45 caused dorsalization of the eggshell. Since mammalian GADD45 proteins are activators of the c-Jun N-terminal kinase (JNK)/p38 mitogen-activated protein kinase (MAPK) signaling pathways, we tested for a genetic interaction in D. melanogaster. We found that eggshell polarity defects caused by D-GADD45 overexpression were dominantly suppressed by mutations in the JNK pathway, suggesting that the JNK pathway has a novel, D-GADD45-mediated, function in the Drosophila germline.

Ol-insm1b, a SNAG family transcription factor involved in cell cycle arrest during medaka development

Through whole-mount in situ hybridisation screen on medaka (Oryzias latipes) brain, Ol-insm1b, a member of the Insm1/Mlt1 subfamily of SNAG-domain containing genes, has been isolated. It is strongly expressed during neurogenesis and pancreas organogenesis, with a pattern that suggests a role in cell cycle exit. Here, we describe Ol-insm1b expression pattern throughout development and in adult brain, and we report on its functional characterisation. Our data point to a previously unravelled role for Ol-insm1b as a down-regulator of cell proliferation during development, as it slows down the cycle without triggering apoptosis. Clonal analysis demonstrates that this effect is cell-autonomous, and, through molecular dissection studies, we demonstrate that it is likely to be non-transcriptional, albeit mediated by zinc-finger domains. Additionally, we report that Ol-insm1b mRNA, when injected in one cell of two-cell stage embryos, exhibits a surprising behaviour: it does not spread uniformly amongst daughter cells but remains cytoplasmically localised in the progeny of the injected blastomere. Our experiments suggest that Insm1 is a negative regulator of cell proliferation, possibly through mechanisms that do not involve modulation of transcription.

The mutant guppy syndrome curveback as a model for human heritable spinal curvature

STUDY DESIGN

This study investigated the morphology, pathogenesis, and inheritance of idiopathic-like spinal curvature in the guppy syndrome, curveback.

OBJECTIVE

To determine whether curveback could be applied as a model for the primary factors that contribute to heritable spinal curvature in humans, specifically, the etiopathogenesis of human familial idiopathic scoliosis.

SUMMARY OF BACKGROUND DATA

Although a genetic basis is accepted, phenotypic complexity and the lack of an animal model with noninduced curvature have made identification of idiopathic scoliosis etiology difficult. It is well established that humans and fish share many genes with similar tissue and temporal expression characteristics, and comparisons between human and fish genomes have proven to be valuable for understanding the genetics of diseases affecting humans.

METHODS

The curveback lineage of guppies was constructed from a single curved male crossed to a normal female. Offspring (103) from the original cross were scored from birth until death for the presence and magnitude of spinal curvature. Genetic architecture was investigated through selective inbreeding, analysis of the distribution of curve magnitude in the mature population, and assessment of curve dynamics during development. Computed tomography assessed vertebral detail.

RESULTS

Computed tomography reveals that vertebral breakage or fusion is not associated with the curveback syndrome. Inbreeding demonstrates a strong genetic influence on curveback, and the distribution of curve magnitude among adult fish suggests polygenic inheritance. There is a female bias for curves of high magnitude and curves that resolve before maturity. There is developmental variability for the age of curve onset, curve progression, and final curve magnitude.

CONCLUSIONS

Observed parallels between the curveback syndrome and human idiopathic scoliosis suggest that the guppy model is an unexploited resource for the identification of primary etiological factors involved in curvature. As models for biomedical research, teleosts offer great potential regarding spinal stability and deformity.

Timing the generation of distinct retinal cells by homeobox proteins

The reason why different types of vertebrate nerve cells are generated in a particular sequence is still poorly understood. In the vertebrate retina, homeobox genes play a crucial role in establishing different cell identities. Here we provide evidence of a cellular clock that sequentially activates distinct homeobox genes in embryonic retinal cells, linking the identity of a retinal cell to its time of generation. By in situ expression analysis, we found that the three Xenopus homeobox genes Xotx5b, Xvsx1, and Xotx2 are initially transcribed but not translated in early retinal progenitors. Their translation requires cell cycle progression and is sequentially activated in photoreceptors (Xotx5b) and bipolar cells (Xvsx1 and Xotx2). Furthermore, by in vivo lipofection of "sensors" in which green fluorescent protein translation is under control of the 3' untranslated region (UTR), we found that the 3' UTRs of Xotx5b, Xvsx1, and Xotx2 are sufficient to drive a spatiotemporal pattern of translation matching that of the corresponding proteins and consistent with the time of generation of photoreceptors (Xotx5b) and bipolar cells (Xvsx1 and Xotx2). The block of cell cycle progression of single early retinal progenitors impairs their differentiation as photoreceptors and bipolar cells, but is rescued by the lipofection of Xotx5b and Xvsx1 coding sequences, respectively. This is the first evidence to our knowledge that vertebrate homeobox proteins can work as effectors of a cellular clock to establish distinct cell identities.

Expression domains suggest cell-cycle independent roles of growth-arrest molecules in the adult brain of the medaka, Oryzias latipes

Teleost fish are unique for their enormous potential to produce new neurons in the adult brain. Nevertheless, the regulation of this adult neurogenesis remains to be characterized. Does it resort to the same molecular mechanisms as those at play in embryonic development? Here, we analyse the expression of the neurogenic gene Ol-DeltaA in the brain of medaka (Oryzias latipes) embryos and adults. To determine the relationships between neurogenic and growth-arrest genes in the adult brain, we compare the expression domains of Ol-DeltaA with those of Ol-KIP and Ol-Gadd45gamma, two well-characterized genes involved in cell-cycle arrest and growth inhibition. While it is widely assumed that genes controlling cell-cycle exit show restricted expression domains next to proliferating cells (in the sites of prospective cell differentiation), we observe highly particular expression domains of Ol-KIP and Ol-Gadd45gamma not associated to proliferating areas of the adult brain, suggesting locally different and cell-cycle independent roles of these molecules in the adult brain.

An automated in situ hybridization screen in the medaka to identify unknown neural genes

Despite the fact that a large body of factors that play important roles in development are known, there are still large gaps in understanding the genetic pathways that govern these processes. To find previously unknown genes that are expressed during embryonic development, we optimized and performed an automated whole-mount in situ hybridization screen on medaka embryos at the end of somitogenesis. Partial cDNA sequences were compared against public databases and identified according to similarities found to other genes and gene products. Among 321 isolated genes showing specific expression in the central nervous system in at least one of five stages of development, 55.14% represented genes whose functions are already documented (in fish or other model organisms). Additionally, 16.51% were identified as conserved unknown genes or genes with unknown function. We provide new data on eight of these genes that presented a restricted expression pattern that allowed for formulating testable hypotheses on their developmental roles, and that were homologous to mammalian molecules of unknown function. Thus, gene expression screening in medaka is an efficient tool for isolating new regulators of embryonic development, and can complement genome-sequencing projects that are producing a high number of genes without ascribed functions.

Zebrafish GADD45beta genes are involved in somite segmentation

Somites in vertebrates are periodic segmented structures that give rise to the vertebrae and muscles of body. Somites are generated from presomitic mesoderm (PSM), but it is not fully understood how cellular differentiation and segment formation are achieved in the anterior PSM. We report here that zebrafish gadd45beta1 and gadd45beta2 genes are periodically expressed as paired stripes adjacent to the neural tube in the anterior PSM region where presomitic cells mature. In mammals, it is known that GADD45 (growth arrest and DNA damage) family proteins play a role in cell-cycle control. We found that both knockdown and overexpression of gadd45beta genes caused somite defects with different consequences for marker gene expression. Knockdown of gadd45beta genes with antisense morpholino oligonucleotides caused a broad expansion of mesp-a in the PSM, and both cyclic expression of her1 and segmented expression of MyoD were disorganized. On the other hand, injection of gadd45beta1 or gadd45beta2 suppressed expression of mesp-a and her1 in anterior PSM and MyoD in paraxial mesoderm. These results indicate that regulated expression of gadd45beta genes in the anterior PSM is required for somite segmentation.