Mitochondria, redox signaling and axis specification in metazoan embryos

Development of a larval nervous system in the sea urchin

This review reports recent findings on the specification and patterning of neurons that establish the larval nervous system of the sea urchin embryo. Neurons originate in three regions of the embryo. Perturbation analyses enabled construction of gene regulatory networks controlling the several neural cell types. Many of the mechanisms described reflect shared features of all metazoans and others are conserved among deuterostomes. This nervous system with a very small number of neurons supports the feeding and swimming behaviors of the larva until metamorphosis when an adult nervous system replaces that system.

C1orf194 deficiency leads to incomplete early embryonic lethality and dominant intermediate Charcot-Marie-Tooth disease in a knockout mouse model

Charcot-Marie-Tooth (CMT) disease is the most common inherited peripheral neuropathy and shows clinical and genetic heterogeneity. Mutations in C1orf194 encoding a Ca2+ regulator in neurons and Schwann cells have been reported previously by us to cause CMT disease. In here, we further investigated the function and pathogenic mechanism of C1or194 by generating C1orf194 knockout (KO) mice. Homozygous mutants of C1orf194 mice exhibited incomplete embryonic lethality, characterized by differentiation abnormalities and stillbirth on embryonic days 7.5-15.5. Heterozygous and surviving homozygous C1orf194 KO mice developed motor and sensory defects at the age of 4 months. Electrophysiologic recordings showed decreased compound muscle action potential and motor nerve conduction velocity in the sciatic nerve of C1orf194-deficient mice as a pathologic feature of dominant intermediate-type CMT. Transmission electron microscopy analysis revealed demyelination and axonal atrophy in the sciatic nerve as well as swelling and loss of mitochondrial matrix and other abnormalities in axons and Schwann cells. A histopathologic examination showed a loss of motor neurons in the anterior horn of the spinal cord and muscle atrophy. Shorter internodal length between nodes of Ranvier and Schmidt-Lanterman incisures was detected in the sciatic nerve of affected animals. These results indicate that C1orf194 KO mice can serve as an animal model of CMT with a severe dominant intermediate CMT phenotype that can be used to investigate the molecular mechanisms of the disease and evaluate the efficacy of therapeutic strategies.

Hypoxia triggers collective aerotactic migration in Dictyostelium discoideum

Using a self-generated hypoxic assay, we show that the amoeba Dictyostelium discoideum displays a remarkable collective aerotactic behavior. When a cell colony is covered, cells quickly consume the available oxygen (O₂) and form a dense ring moving outwards at constant speed and density. To decipher this collective process, we combined two technological developments: porphyrin-based O₂ -sensing films and microfluidic O₂ gradient generators. We showed that Dictyostelium cells exhibit aerotactic and aerokinetic response in a low range of O₂ concentration indicative of a very efficient detection mechanism. Cell behaviors under self-generated or imposed O₂ gradients were modeled using an in silico cellular Potts model built on experimental observations. This computational model was complemented with a parsimonious ‘Go or Grow’ partial differential equation (PDE) model. In both models, we found that the collective migration of a dense ring can be explained by the interplay between cell division and the modulation of aerotaxis.

Genetically encoded thiol redox-sensors in the zebrafish model: lessons for embryonic development and regeneration

Important roles for reactive oxygen species (ROS) and redox signaling in embryonic development and regenerative processes are increasingly recognized. However, it is difficult to obtain information on spatiotemporal dynamics of ROS production and signaling in vivo. The zebrafish is an excellent model for in vivo bioimaging and possesses a remarkable regenerative capacity upon tissue injury. Here, we review data obtained in this model system with genetically encoded redox-sensors targeting H₂O₂ and glutathione redox potential. We describe how such observations have prompted insight into regulation and downstream effects of redox alterations during tissue differentiation, morphogenesis and regeneration. We also discuss the properties of the different sensors and their consequences for the interpretation of in vivo imaging results. Finally, we highlight open questions and additional research fields that may benefit from further application of such sensor systems in zebrafish models of development, regeneration and disease.

Maternal control of early patterning in sea urchin embryos

Sea urchin development has been studied extensively for more than a century and considered regulative since the first experimental evidence. Further investigations have repeatedly supported this standpoint by revealing the presence of inductive mechanisms that alter cell fate decisions at early cleavage stages and flexibility of development in response to environmental conditions. Some features indicate that sea urchin development is not completely regulative, but actually includes determinative events. In 16-cell embryos, mesomeres and macromeres represent multipotency, while the cell fate of most vegetal micromeres is restricted. It is known that the mature sea urchin eggs are polarized by the asymmetrical distribution of some maternal mRNAs and proteins. Spatially-distributed maternal factors are necessary for the orientation of the primary animal-vegetal axis, which is established by both maternal and zygotic mechanisms later in development. The secondary dorsal-ventral axis is conditionally specified later in development. Dorsal-ventral polarity is very liable during the early cleavages, though more recent data argue that its direction may be oriented by maternal asymmetry. In this review, we focus on the role of maternal factors in initial embryonic patterning during the first cleavages of sea urchin embryos before activation of the embryonic genome.

Translational Control of Canonical and Non-Canonical Translation Initiation Factors at the Sea Urchin Egg to Embryo Transition

Sea urchin early development is a powerful model to study translational regulation under physiological conditions. Fertilization triggers an activation of the translation machinery responsible for the increase of protein synthesis necessary for the completion of the first embryonic cell cycles. The cap-binding protein eIF4E, the helicase eIF4A and the large scaffolding protein eIF4G are assembled upon fertilization to form an initiation complex on mRNAs involved in cap-dependent translation initiation. The presence of these proteins in unfertilized and fertilized eggs has already been demonstrated, however data concerning the translational status of translation factors are still scarce. Using polysome fractionation, we analyzed the impact of fertilization on the recruitment of mRNAs encoding initiation factors. Strikingly, whereas the mRNAs coding eIF4E, eIF4A, and eIF4G were not recruited into polysomes at 1 h post-fertilization, mRNAs for eIF4B and for non-canonical initiation factors such as DAP5, eIF4E2, eIF4E3, or hnRNP Q, are recruited and are differentially sensitive to the activation state of the mechanistic target of rapamycin (mTOR) pathway. We discuss our results suggesting alternative translation initiation in the context of the early development of sea urchins.

Revisiting the role of metabolism during development

An emerging view emphasizes that metabolism is highly regulated in both time and space. In addition, it is increasingly being recognized that metabolic pathways are tightly connected to specific biological processes such as cell signaling, proliferation and differentiation. As we obtain a better view of this spatiotemporal regulation of metabolism, and of the molecular mechanisms that connect metabolism and signaling, we can now move from largely correlative to more functional studies. It is, therefore, a particularly promising time to revisit how metabolism can affect multiple aspects of animal development. In this Review, we discuss how metabolism is mechanistically linked to cellular and developmental programs through both its bioenergetic and metabolic signaling functions. We highlight how metabolism is regulated across various spatial and temporal scales, and discuss how this regulation can influence cellular processes such as cell signaling, gene expression, and epigenetic and post-translational modifications during embryonic development.

MAPK and GSK3/ß-TRCP-mediated degradation of the maternal Ets domain transcriptional repressor Yan/Tel controls the spatial expression of nodal in the sea urchin embryo

In the sea urchin embryo, specification of the dorsal-ventral axis critically relies on the spatially restricted expression of nodal in the presumptive ventral ectoderm. The ventral restriction of nodal expression requires the activity of the maternal TGF-β ligand Panda but the mechanism by which Panda restricts nodal expression is unknown. Similarly, what initiates expression of nodal in the ectoderm and what are the mechanisms that link patterning along the primary and secondary axes is not well understood. We report that in Paracentrotus lividus, the activity of the maternally expressed ETS-domain transcription factor Yan/Tel is essential for the spatial restriction of nodal. Inhibiting translation of maternal yan/tel mRNA disrupted dorsal-ventral patterning in all germ layers by causing a massive ectopic expression of nodal starting from cleavage stages, mimicking the phenotype caused by inactivation of the maternal Nodal antagonist Panda. We show that like in the fly or in vertebrates, the activity of sea urchin Yan/Tel is regulated by phosphorylation by MAP kinases. However, unlike in the fly or in vertebrates, phosphorylation by GSK3 plays a central role in the regulation Yan/Tel stability in the sea urchin. We show that GSK3 phosphorylates Yan/Tel in vitro at two different sites including a β-TRCP ubiquitin ligase degradation motif and a C-terminal Ser/Thr rich cluster and that phosphorylation of Yan/Tel by GSK3 triggers its degradation by a β-TRCP/proteasome pathway. Finally, we show that, Yan is epistatic to Panda and that the activity of Yan/Tel is required downstream of Panda to restrict nodal expression. Our results identify Yan/Tel as a central regulator of the spatial expression of nodal in Paracentrotus lividus and uncover a key interaction between the gene regulatory networks responsible for patterning the embryo along the dorsal-ventral and animal-vegetal axes.

Specification of the embryonic axes is an essential step during early development of metazoa. In the sea urchin embryo, specification of the dorsal-ventral axis critically relies on the spatial restriction of the expression of the TGF-ß family member Nodal in ventral cells, a process that requires the activity of the maternal determinant Panda. How the spatially restricted expression of nodal is established downstream of Panda is not well understood. We have discovered that, in the Mediterranean sea urchin Paracentrotus lividus, the spatial restriction of nodal on the ventral side of the embryo requires the inhibitory activity of a transcriptional repressor named Yan/Tel. This finding suggests a molecular mechanism for the control of nodal expression by the release of a repression. We found that this release requires the activity of two families of kinases that we identified as the MAP kinases and GSK3, a kinase which, intriguingly, was previously known as a key regulator of patterning along the animal-vegetal axis. We discovered that phosphorylation by MAPK and GSK3 triggers degradation of Yan/Tel by a β-TRCP proteasome pathway. Finally, we find that Yan/Tel likely acts downstream of Panda in the hierarchy of genes required for nodal restriction. Our study therefore identifies Yan/Tel as a new essential regulator of nodal expression downstream of Panda and identifies a novel key interaction between the gene regulatory networks responsible for patterning along the primary and secondary axis of polarity.

Redox stress and signaling during vertebrate embryonic development: Regulation and responses

Vertebrate embryonic development requires specific signaling events that regulate cell proliferation and differentiation to occur at the correct place and the correct time in order to build a healthy embryo. Signaling pathways are sensitive to perturbations of the endogenous redox state, and are also susceptible to modulation by reactive species and antioxidant defenses, contributing to a spectrum of passive vs. active effects that can affect redox signaling and redox stress. Here we take a multi-level, integrative approach to discuss the importance of redox status for vertebrate developmental signaling pathways and cell fate decisions, with a focus on glutathione/glutathione disulfide, thioredoxin, and cysteine/cystine redox potentials and the implications for protein function in development. We present a tissue-specific example of the important role that reactive species play in pancreatic development and metabolic regulation. We discuss NFE2L2 (also known as NRF2) and related proteins, their roles in redox signaling, and their regulation of glutathione during development. Finally, we provide examples of xenobiotic compounds that disrupt redox signaling in the context of vertebrate embryonic development. Collectively, this review provides a systems-level perspective on the innate and inducible antioxidant defenses, as well as their roles in maintaining redox balance during chemical exposures that occur in critical windows of development.

Pancreatic beta cells are a sensitive target of embryonic exposure to butylparaben in zebrafish (Danio rerio)

BACKGROUND

Butylparaben (butyl p-hydroxybenzoic acid) is a common cosmetic and pharmaceutical preservative reported to induce oxidative stress and endocrine disruption. Embryonic development is sensitive to oxidative stress, with redox potentials playing critical roles in progenitor cell fate decisions. Because pancreatic beta cells have been reported to have low antioxidant gene expression, they may be sensitive targets of oxidative stress. We tested the hypotheses that butylparaben causes oxidative stress in the developing embryo, and that pancreatic beta cells are a sensitive target of butylparaben embryotoxicity.

METHODS

Transgenic insulin:GFP zebrafish embryos (Danio rerio) were treated daily with 0, 250, 500, 1,000, and 3,000 nM butylparaben. Pancreatic islet and whole embryo development were examined though 7 days postfertilization, and gene expression was measured by quantitative real-time PCR. Glutathione (GSH) and cysteine redox content were measured at 28 hr postfertilization using HPLC.

RESULTS

Butylparaben exposure caused intestinal effusion, pericardial edema, and accelerated yolk utilization. At 250 nM, beta cell area increased by as much as 55%, and increased incidence of two aberrant morphologies were observed-fragmentation of the islet cluster and ectopic beta cells. Butylparaben concentrations of 500 and 1,000 nM increased GSH by 10 and 40%, respectively. Butylparaben exposure downregulated transcription factor pdx1, as well as genes involved in GSH synthesis, while upregulating GSH-disulfide reductase (gsr).

CONCLUSIONS

The endocrine pancreas is a sensitive target of embryonic exposure to butylparaben, which also causes developmental deformities and perturbs redox conditions in the embryo.

Mitochondria-Specific Monoclonal Antibodies in Eggs and Embryos of the Ascidian Halocynthia roretzi

Ascidian embryos have become an important model for embryological studies, offering a simple example for mechanisms of cytoplasmic components segregation. It is a well-known example that the asymmetric segregation of mitochondria into muscle lineage cells occurs during ascidian embryogenesis. However, it is still unclear which signaling pathway is involved in this process. To obtain molecular markers for studying mechanisms involved in the asymmetric distribution of mitochondria, we have produced monoclonal antibodies, Mito-1, Mito-2 and Mito-3, that specifically recognize mitochondriarich cytoplasm in cells of the ascidian Halocynthia roretzi embryos. These antibodies stained cytoplasm like reticular structure in epidermis cells, except for nuclei, at the early tailbud stage. Similar pattern was observed in vital staining of mitochondria with DiOC2, a fluorescent probe of mitochondria. Immunostaining with these antibodies showed that mitochondria are evenly distributed in the animal hemisphere blastomeres at cleavage stages, whereas not in the vegetal hemisphere blastomeres. Mitochondria were transferred to the presumptive muscle and nerve cord lineage cells of the marginal zone in the vegetal hemisphere more than to the presumptive mesenchyme, notochord and endoderm lineage of the central zone. Therefore, it is suggested that these antibodies will be useful markers for studying mechanisms involved in the polarized distribution of mitochondria during ascidian embryogenesis.

Asymmetric distribution of hypoxia-inducible factor α regulates dorsoventral axis establishment in the early sea urchin embryo

Hypoxia signaling is an ancient pathway by which animals can respond to low oxygen. Malfunction of this pathway disturbs hypoxic acclimation and can result in various diseases, including cancers. The role of hypoxia signaling in early embryogenesis remains unclear. Here, we show that in the blastula of the sea urchin Strongylocentrotus purpuratus, hypoxia-inducible factor α (HIFα), the downstream transcription factor of the hypoxia pathway, is localized and transcriptionally active on the future dorsal side. This asymmetric distribution is attributable to its oxygen-sensing ability. Manipulations of the HIFα level entrained the dorsoventral axis, as the side with the higher level of HIFα tends to develop into the dorsal side. Gene expression analyses revealed that HIFα restricts the expression of nodal to the ventral side and activates several genes encoding transcription factors on the dorsal side. We also observed that intrinsic hypoxic signals in the early embryos formed a gradient, which was disrupted under hypoxic conditions. Our results reveal an unprecedented role of the hypoxia pathway in animal development.

p38 MAPK as an essential regulator of dorsal-ventral axis specification and skeletogenesis during sea urchin development: a re-evaluation

Dorsal-ventral axis formation in the sea urchin embryo relies on the asymmetrical expression of the TGFβ Nodal. The p38-MAPK pathway has been proposed to be essential for dorsal-ventral axis formation by acting upstream of nodal expression. Here, we report that, in contrast to previous studies that used pharmacological inhibitors of p38, manipulating the activity of p38 by genetic means has no obvious impact on morphogenesis. Instead, we discovered that p38 inhibitors strongly disrupt specification of all germ layers by blocking signalling from the Nodal receptor and by interfering with the ERK pathway. Strikingly, while expression of a mutant p38 that is resistant to SB203580 did not rescue dorsal-ventral axis formation or skeletogenesis in embryos treated with this inhibitor, expression of mutant Nodal receptors that are resistant to SB203580 fully restored nodal expression in SB203580-treated embryos. Taken together, these results establish that p38 activity is not required for dorsal-ventral axis formation through nodal expression nor for skeletogenesis. Our results prompt a re-evaluation of the conclusions of several recent studies that linked p38 activity to dorsal-ventral axis formation and to patterning of the skeleton.

Analysis of gene expression in the nervous system identifies key genes and novel candidates for health and disease

The incidence of neurodegenerative diseases in the developed world has risen over the last century, concomitant with an increase in average human lifespan. A major challenge is therefore to identify genes that control neuronal health and viability with a view to enhancing neuronal health during ageing and reducing the burden of neurodegeneration. Analysis of gene expression data has recently been used to infer gene functions for a range of tissues from co-expression networks. We have now applied this approach to transcriptomic datasets from the mammalian nervous system available in the public domain. We have defined the genes critical for influencing neuronal health and disease in different neurological cell types and brain regions. The functional contribution of genes in each co-expression cluster was validated using human disease and knockout mouse phenotypes, pathways and gene ontology term annotation. Additionally a number of poorly annotated genes were implicated by this approach in nervous system function. Exploiting gene expression data available in the public domain allowed us to validate key nervous system genes and, importantly, to identify additional genes with minimal functional annotation but with the same expression pattern. These genes are thus novel candidates for a role in neurological health and disease and could now be further investigated to confirm their function and regulation during ageing and neurodegeneration.

Nrf2 and Nrf2-related proteins in development and developmental toxicity: Insights from studies in zebrafish (Danio rerio)

Oxidative stress is an important mechanism of chemical toxicity, contributing to developmental toxicity and teratogenesis as well as to cardiovascular and neurodegenerative diseases and diabetic embryopathy. Developing animals are especially sensitive to effects of chemicals that disrupt the balance of processes generating reactive species and oxidative stress, and those anti-oxidant defenses that protect against oxidative stress. The expression and inducibility of anti-oxidant defenses through activation of NFE2-related factor 2 (Nrf2) and related proteins is an essential process affecting the susceptibility to oxidants, but the complex interactions of Nrf2 in determining embryonic response to oxidants and oxidative stress are only beginning to be understood. The zebrafish (Danio rerio) is an established model in developmental biology and now also in developmental toxicology and redox signaling. Here we review the regulation of genes involved in protection against oxidative stress in developing vertebrates, with a focus on Nrf2 and related cap'n'collar (CNC)-basic-leucine zipper (bZIP) transcription factors. Vertebrate animals including zebrafish share Nfe2, Nrf1, Nrf2, and Nrf3 as well as a core set of genes that respond to oxidative stress, contributing to the value of zebrafish as a model system with which to investigate the mechanisms involved in regulation of redox signaling and the response to oxidative stress during embryolarval development. Moreover, studies in zebrafish have revealed nrf and keap1 gene duplications that provide an opportunity to dissect multiple functions of vertebrate NRF genes, including multiple sensing mechanisms involved in chemical-specific effects.

The Maternal Maverick/GDF15-like TGF-β Ligand Panda Directs Dorsal-Ventral Axis Formation by Restricting Nodal Expression in the Sea Urchin Embryo

Specification of the dorsal-ventral axis in the highly regulative sea urchin embryo critically relies on the zygotic expression of nodal, but whether maternal factors provide the initial spatial cue to orient this axis is not known. Although redox gradients have been proposed to entrain the dorsal-ventral axis by acting upstream of nodal, manipulating the activity of redox gradients only has modest consequences, suggesting that other factors are responsible for orienting nodal expression and defining the dorsal-ventral axis. Here we uncover the function of Panda, a maternally provided transforming growth factor beta (TGF-β) ligand that requires the activin receptor-like kinases (Alk) Alk3/6 and Alk1/2 receptors to break the radial symmetry of the embryo and orient the dorsal-ventral axis by restricting nodal expression. We found that the double inhibition of the bone morphogenetic protein (BMP) type I receptors Alk3/6 and Alk1/2 causes a phenotype dramatically more severe than the BMP2/4 loss-of-function phenotype, leading to extreme ventralization of the embryo through massive ectopic expression of nodal, suggesting that an unidentified signal acting through BMP type I receptors cooperates with BMP2/4 to restrict nodal expression. We identified this ligand as the product of maternal Panda mRNA. Double inactivation of panda and bmp2/4 led to extreme ventralization, mimicking the phenotype caused by inactivation of the two BMP receptors. Inhibition of maternal panda mRNA translation disrupted the early spatial restriction of nodal, leading to persistent massive ectopic expression of nodal on the dorsal side despite the presence of Lefty. Phylogenetic analysis indicates that Panda is not a prototypical BMP ligand but a member of a subfamily of TGF-β distantly related to Inhibins, Lefty, and TGF-β that includes Maverick from Drosophila and GDF15 from vertebrates. Indeed, overexpression of Panda does not appear to directly or strongly activate phosphoSmad1/5/8 signaling, suggesting that although this TGF-β may require Alk1/2 and/or Alk3/6 to antagonize nodal expression, it may do so by sequestering a factor essential for Nodal signaling, by activating a non-Smad pathway downstream of the type I receptors, or by activating extremely low levels of pSmad1/5/8. We provide evidence that, although panda mRNA is broadly distributed in the early embryo, local expression of panda mRNA efficiently orients the dorsal-ventral axis and that Panda activity is required locally in the early embryo to specify this axis. Taken together, these findings demonstrate that maternal panda mRNA is both necessary and sufficient to orient the dorsal-ventral axis. These results therefore provide evidence that in the highly regulative sea urchin embryo, the activity of spatially restricted maternal factors regulates patterning along the dorsal-ventral axis.

Glutathione redox dynamics and expression of glutathione-related genes in the developing embryo

Embryonic development involves dramatic changes in cell proliferation and differentiation that must be highly coordinated and tightly regulated. Cellular redox balance is critical for cell fate decisions, but it is susceptible to disruption by endogenous and exogenous sources of oxidative stress. The most abundant endogenous nonprotein antioxidant defense molecule is the tripeptide glutathione (γ-glutamylcysteinylglycine, GSH), but the ontogeny of GSH concentration and redox state during early life stages is poorly understood. Here, we describe the GSH redox dynamics during embryonic and early larval development (0-5 days postfertilization) in the zebrafish (Danio rerio), a model vertebrate embryo. We measured reduced and oxidized glutathione using HPLC and calculated the whole embryo total glutathione (GSHT) concentrations and redox potentials (Eh) over 0-120 h of zebrafish development (including mature oocytes, fertilization, midblastula transition, gastrulation, somitogenesis, pharyngula, prehatch embryos, and hatched eleutheroembryos). GSHT concentration doubled between 12h postfertilization (hpf) and hatching. The GSH Eh increased, becoming more oxidizing during the first 12h, and then oscillated around -190 mV through organogenesis, followed by a rapid change, associated with hatching, to a more negative (more reducing) Eh (-220 mV). After hatching, Eh stabilized and remained steady through 120 hpf. The dynamic changes in GSH redox status and concentration defined discrete windows of development: primary organogenesis, organ differentiation, and larval growth. We identified the set of zebrafish genes involved in the synthesis, utilization, and recycling of GSH, including several novel paralogs, and measured how expression of these genes changes during development. Ontogenic changes in the expression of GSH-related genes support the hypothesis that GSH redox state is tightly regulated early in development. This study provides a foundation for understanding the redox regulation of developmental signaling and investigating the effects of oxidative stress during embryogenesis.

Developmental exposure to second-hand smoke increases adult atherogenesis and alters mitochondrial DNA copy number and deletions in apoE(-/-) mice

Cardiovascular disease is a major cause of morbidity and mortality in the United States. While many studies have focused upon the effects of adult second-hand smoke exposure on cardiovascular disease development, disease development occurs over decades and is likely influenced by childhood exposure. The impacts of in utero versus neonatal second-hand smoke exposure on adult atherosclerotic disease development are not known. The objective of the current study was to determine the effects of in utero versus neonatal exposure to a low dose (1 mg/m(3) total suspended particulate) of second-hand smoke on adult atherosclerotic lesion development using the apolipoprotein E null mouse model. Consequently, apolipoprotein E null mice were exposed to either filtered air or second-hand smoke: (i) in utero from gestation days 1-19, or (ii) from birth until 3 weeks of age (neonatal). Subsequently, all animals were exposed to filtered air and sacrificed at 12-14 weeks of age. Oil red-O staining of whole aortas, measures of mitochondrial damage, and oxidative stress were performed. Results show that both in utero and neonatal second-hand smoke exposure significantly increased adult atherogenesis in mice compared to filtered air controls. These changes were associated with changes in aconitase and mitochondrial superoxide dismutase activities consistent with increased oxidative stress in the aorta, changes in mitochondrial DNA copy number and deletion levels. These studies show that in utero or neonatal exposure to second-hand smoke significantly influences adult atherosclerotic lesion development and results in significant alterations to the mitochondrion and its genome that may contribute to atherogenesis.

Evolutionary crossroads in developmental biology: sea urchins

Embryos of the echinoderms, especially those of sea urchins and sea stars, have been studied as model organisms for over 100 years. The simplicity of their early development, and the ease of experimentally perturbing this development, provides an excellent platform for mechanistic studies of cell specification and morphogenesis. As a result, echinoderms have contributed significantly to our understanding of many developmental mechanisms, including those that govern the structure and design of gene regulatory networks, those that direct cell lineage specification, and those that regulate the dynamic morphogenetic events that shape the early embryo.

Maternal Oct1/2 is required for Nodal and Vg1/Univin expression during dorsal-ventral axis specification in the sea urchin embryo

The TGFβ family member Nodal is expressed early in the presumptive ventral ectoderm of the early sea urchin embryo and its activity is crucial for dorsal-ventral (D/V) axis specification. Analysis of the nodal promoter identified a number of critical binding sites for transcription factors of different families including Sox, Oct, TCF and bZIP, but in most cases the specific factors that regulate nodal expression are not known. In this study, we report that the maternal factor Oct1/2 functions as a positive regulator of nodal and that its activity is essential for the initiation of nodal expression. Inhibition of Oct1/2 mRNA translation produced embryos with severe axial defects similar to those observed following inhibition of Nodal function. We show that perturbing Oct1/2 function specifically disrupted specification of the ventral and dorsal ectodermal regions and that these effects were caused by the failure of nodal to be expressed early in development. Furthermore, we identified the key gene vg1/univin, which is also necessary for nodal expression, as an additional factor that was completely dependent on Oct1/2 for its zygotic expression. These data demonstrate that the maternal Oct1/2 protein plays an early and essential role in D/V axis specification by initiating the expression of nodal and vg1/univin, two genes that act at the top of the D/V ectoderm gene regulatory network.

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

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

Redox control in mammalian embryo development

The development of an embryo constitutes a complex choreography of regulatory events that underlies precise temporal and spatial control. Throughout this process the embryo encounters ever changing environments, which challenge its metabolism. Oxygen is required for embryogenesis but it also poses a potential hazard via formation of reactive oxygen and reactive nitrogen species (ROS/RNS). These metabolites are capable of modifying macromolecules (lipids, proteins, nucleic acids) and altering their biological functions. On one hand, such modifications may have deleterious consequences and must be counteracted by antioxidant defense systems. On the other hand, ROS/RNS function as essential signal transducers regulating the cellular phenotype. In this context the combined maternal/embryonic redox homeostasis is of major importance and dysregulations in the equilibrium of pro- and antioxidative processes retard embryo development, leading to organ malformation and embryo lethality. Silencing the in vivo expression of pro- and antioxidative enzymes provided deeper insights into the role of the embryonic redox equilibrium. Moreover, novel mechanisms linking the cellular redox homeostasis to gene expression regulation have recently been discovered (oxygen sensing DNA demethylases and protein phosphatases, redox-sensitive microRNAs and transcription factors, moonlighting enzymes of the cellular redox homeostasis) and their contribution to embryo development is critically reviewed.

Opposing Nodal/Vg1 and BMP signals mediate axial patterning in embryos of the basal chordate amphioxus

The basal chordate amphioxus resembles vertebrates in having a dorsal, hollow nerve cord, a notochord and somites. However, it lacks extensive gene duplications, and its embryos are small and gastrulate by simple invagination. Here we demonstrate that Nodal/Vg1 signaling acts from early cleavage through the gastrula stage to specify and maintain dorsal/anterior development while, starting at the early gastrula stage, BMP signaling promotes ventral/posterior identity. Knockdown and gain-of-function experiments show that these pathways act in opposition to one another. Signaling by these pathways is modulated by dorsally and/or anteriorly expressed genes including Chordin, Cerberus, and Blimp1. Overexpression and/or reporter assays in Xenopus demonstrate that the functions of these proteins are conserved between amphioxus and vertebrates. Thus, a fundamental genetic mechanism for axial patterning involving opposing Nodal and BMP signaling is present in amphioxus and probably also in the common ancestor of amphioxus and vertebrates or even earlier in deuterostome evolution.

Reactive oxygen species: A radical role in development?

Reactive oxygen species (ROS), mostly derived from mitochondrial activity, can damage various macromolecules and consequently cause cell death. This ROS activity has been characterized in vitro, and correlative evidence suggests a role in various pathological conditions. In addition to this passive ROS activity, ROS also participate in cell signaling processes, though the relevance of this function in vivo is poorly understood. Throughout development, elevated cell activity is probably accompanied by highly active metabolism and, consequently, the production of large amounts of ROS. To allow proper development, cells must protect themselves from these potentially damaging ROS. However, to what degree ROS could participate as signaling molecules controlling fundamental and developmentally relevant cellular processes such as proliferation, differentiation, and death is an open question. Here we discuss why available data do not yet provide conclusive evidence on the role of ROS in development, and we review recent methods to detect ROS in vivo and genetic strategies that can be exploited specifically to resolve these uncertainties.

Redox signaling (cross-talk) from and to mitochondria involves mitochondrial pores and reactive oxygen species

This review highlights the important role of redox signaling between mitochondria and NADPH oxidases. Besides the definition and general importance of redox signaling, the cross-talk between mitochondrial and Nox-derived reactive oxygen species (ROS) is discussed on the basis of 4 different examples. In the first model, angiotensin-II is discussed as a trigger for NADPH oxidase activation with subsequent ROS-dependent opening of mitochondrial ATP-sensitive potassium channels leading to depolarization of mitochondrial membrane potential followed by mitochondrial ROS formation and respiratory dysfunction. This concept was supported by observations that ethidium bromide-induced mitochondrial damage suppressed angiotensin-II-dependent increase in Nox1 and oxidative stress. In another example hypoxia was used as a stimulator of mitochondrial ROS formation and by using pharmacological and genetic inhibitors, a role of mitochondrial ROS for the induction of NADPH oxidase via PKCvarepsilon was demonstrated. The third model was based on cell death by serum withdrawal that promotes the production of ROS in human 293T cells by stimulating both the mitochondria and Nox1. By superior molecular biological methods the authors showed that mitochondria were responsible for the fast onset of ROS formation followed by a slower but long-lasting oxidative stress condition based on the activation of an NADPH oxidase (Nox1) in response to the fast mitochondrial ROS formation. Finally, a cross-talk between mitochondria and NADPH oxidases (Nox2) was shown in nitroglycerin-induced tolerance involving the mitochondrial permeability transition pore and ATP-sensitive potassium channels. The use of these redox signaling pathways as pharmacological targets is briefly discussed.

Mitochondria and metazoan epigenesis

In eukaryotes, mitochondrial activity controls ATP production, calcium dynamics, and redox state, thereby establishing physiological parameters governing the transduction of biochemical signals that regulate nuclear gene expression. However, these activities are commonly assumed to fulfill a 'housekeeping' function: necessary for life, but an epiphenomenon devoid of causal agency in the developmental flow of genetic information. Moreover, it is difficult to perturb mitochondrial function without generally affecting cell viability. For these reasons little is known about the extent of mitochondrial influence on gene activity in early development. Recent discoveries pertaining to the redox regulation of key developmental signaling systems together with the fact that mitochondria are often asymmetrically distributed in animal embryos suggests that they may contribute spatial information underlying differential specification of cell fate. In many cases such asymmetries correlate with localization of genetic determinants (i.e., mRNAs or proteins), particularly in embryos that rely heavily on cell-autonomous means of cell fate specification. In such embryos the localized genetic determinants play a dominant role, and any developmental information contributed by the mitochondria themselves is likely to be less obvious and more difficult to isolate experimentally. Hence, 'regulative' embryos that make more extensive use of conditional cell fate specification are better suited to experimental investigation of mitochondrial impacts on developmental gene regulation. Recent studies of the sea urchin embryo, which is a paradigmatic example of such a system, suggest that anisotropic distribution of mitochondria provides a source gradient of spatial information that directs epigenetic specification of the secondary axis via Nodal-Lefty signaling.

Mitochondrial function and redox state in mammalian embryos

Mitochondria play a central and multifaceted role in the mammalian egg and early embryo, contributing to many different aspects of early development. While the contribution of mitochondria to energy production is fundamental, other roles for mitochondria are starting to emerge. Mitochondria are central to intracellular redox metabolism as they produce reactive oxygen species (ROS, the mediators of oxidative stress) and they can generate TCA cycle intermediates and reducing equivalents that are used in antioxidant defence. A high cytosolic lactate dehydrogenase activity coupled with dynamic levels of cytosolic pyruvate is responsible for a very dynamic intracellular redox state in the oocyte and embryo. Mammalian embryos have a low glucose metabolism during the earliest stages of development, as both glycolysis and the pentose phosphate pathway are suppressed. The mitochondrial TCA cycle is therefore the major source of reducing equivalents in the cytosol so that any change in mitochondrial function in the embryo will be reflected in changes in the intracellular redox state. In the mouse, the metabolic substrates used by the oocyte and early embryo each have a different impact on the intracellular redox state. Pyruvate which oxidises the cytosolic redox state, acts as an energetic and redox substrate whereas lactate, which reduces the cytosolic redox state, acts only as a redox substrate. Mammalian early embryos are very sensitive to oxidative stress which can cause permanent developmental arrest before zygotic genome activation and apoptosis in the blastocyst. The oocyte stockpiles antioxidant defence for the early embryo to cope with exogenous and endogenous oxidant insults arising during early development. Mitochondria provide ATP for glutathione (GSH) production during oocyte maturation and also participate in the regeneration of NADPH and GSH during early development. Finally, a number of pathological conditions or environmental insults impair early development by altering mitochondrial function, illustrating the centrality of mitochondrial function in embryo development.

The surprising complexity of the transcriptional regulation of the spdri gene reveals the existence of new linkages inside sea urchin's PMC and Oral Ectoderm Gene Regulatory Networks

During sea urchin embryogenesis the spdri gene participates in two separate Gene Regulatory Networks (GRNs): the Primary Mesenchyme Cells' (PMCs) and the Oral Ectoderm's one. In both cases, activation of the gene follows initial specification events [Amore, G., Yavrouian, R., Peterson, K., Ransick, A., McClay, D., Davidson, E., 2003. Spdeadringer, a sea urchin embryo gene required separately in skeletogenic and oral ectoderm gene regulatory networks. Dev. Biol. 261, 55-81.]. We identified a portion of genomic DNA ("4.7IL" -3456;+389) which is sufficient to replicate sdpri's expression pattern in experiments of transgenesis, using a GFP reporter. In our experiments, the activation kinetic of 4.7IL-GFP was similar to that of the endogenous gene and the reporter responded to known spdri's transcriptional regulators (Ets1, Alx1, Gsc and Dri). Here we present a dissection of this regulatory region and a description of the modules involved in spdri's transcriptional regulation. Both in the PMCs' and Oral Ectoderm's expression phases, activation of spdri is obtained through the integration of three kinds of inputs: positive and globally distributed ones; negative ones (that prevent ectopic expression); positive and tissue-specific ones. Our results allow to expand the map of the regulatory connections at the spdri node, both in the PMCs and in the Oral Ectoderm Gene Regulatory Networks (GRNs).

cis-Regulatory sequences driving the expression of the Hbox12 homeobox-containing gene in the presumptive aboral ectoderm territory of the Paracentrotus lividus sea urchin embryo

Embryonic development is coordinated by networks of evolutionary conserved regulatory genes encoding transcription factors and components of cell signalling pathways. In the sea urchin embryo, a number of genes encoding transcription factors display territorial restricted expression. Among these, the zygotic Hbox12 homeobox gene is transiently transcribed in a limited number of cells of the animal-lateral half of the early Paracentrotus lividus embryo, whose descendants will constitute part of the ectoderm territory. To obtain insights on the regulation of Hbox12 expression, we have explored the cis-regulatory apparatus of the gene. In this paper, we show that the intergenic region of the tandem Hbox12 repeats drives GFP expression in the presumptive aboral ectoderm and that a 234 bp fragment, defined aboral ectoderm (AE) module, accounts for the restricted expression of the transgene. Within this module, a consensus sequence for a Sox factor and the binding of the Otx activator are both required for correct Hbox12 gene expression. Spatial restriction to the aboral ectoderm is achieved by a combination of different repressive sequence elements. Negative sequence elements necessary for repression in the endomesoderm map within the most upstream 60 bp region and nearby the Sox binding site. Strikingly, a Myb-like consensus is necessary for repression in the oral ectoderm, while down-regulation at the gastrula stage depends on a GA-rich region. These results suggest a role for Hbox12 in aboral ectoderm specification and represent our first attempt in the identification of the gene regulatory circuits involved in this process.