Oxygen deprivation causes suspended animation in the zebrafish embryo

Coordination of development and metabolism in the pre-midblastula transition zebrafish embryo

To define the mechanisms that coordinate early embryonic development and metabolism, we have examined the response of zebrafish embryos to anoxia before the midblastula transition. Our findings reveal that anoxic pre-midblastula transition embryos slow the cell cycle, arrest before the midblastula transition and can recover normally if restored to a normoxic environment. Analyses of respiratory rates reveal that pre-midblastula transition embryos are less reliant on oxidative phosphorylation than older embryos. Interestingly, arrest in anoxia occurs despite inhibition of zygotic transcription, revealing a central role for maternal factors in the response to energy limitation. Consistent with this concept, we demonstrate that the posttranslational energy-sensing AMP-activated protein kinase pathway is activated in anoxia in pre-midblastula transition embryos. Taken together, these findings demonstrate a maternal program capable of coordinating developmental rate and metabolism in the absence of transcription-based pathways or cell cycle checkpoints.

Reduction in ovulation or male sex phenotype increases long-term anoxia survival in a daf-16-independent manner in Caenorhabditis elegans

Identifying genotypes and phenotypes that enhance an organism's ability to survive stress is of interest. We used Caenorhabditis elegans mutants, RNA interference (RNAi), and the chemical 5-fluorodeoxyuridine (FUDR) to test the hypothesis that a reduction in progeny would increase oxygen deprivation (anoxia) survival. In the hermaphrodite gonad, germ line processes such as spermatogenesis and oogenesis can be simultaneously as well as independently disrupted by genetic mutations. We analyzed genetic mutants [glp-1(q158), glp-4(bn2ts), plc-1(rx1), ksr-1(ku68), fog-2(q71), fem-3(q20), spe-9(hc52ts), fer-15(hc15ts)] with reduced progeny production due to various reproductive defects. Furthermore, we used RNAi to inhibit the function of gene products in the RTK/Ras/MAPK signaling pathway, which is known to be involved in a variety of developmental processes including gonad function. We determined that reduced progeny production or complete sterility enhanced anoxia survival except in the case of sterile hermaphrodites [spe-9(hc52ts), fer-15(hc15ts)] undergoing oocyte maturation and ovulation as exhibited by the presence of laid unfertilized oocytes. Furthermore, the fog-2(q71) long-term anoxia survival phenotype was suppressed when oocyte maturation and ovulation were induced by mating with males that have functional or nonfunctional sperm. The mutants with a reduced progeny production survive long-term anoxia in a daf-16- and hif-1-independent manner. Finally, we determined that wild-type males were able to survive long-term anoxia in a daf-16-independent manner. Together, these results suggest that the insulin signaling pathway is not the only mechanism to survive oxygen deprivation and that altering gonad function, in particular oocyte maturation and ovulation, leads to a physiological state conducive for oxygen deprivation survival.

Oxygen decline in biotesting of environmental samples--is there a need for consideration in the acute zebrafish embryo assay?

Environmental samples such as groundwater, sediment pore water, native or freeze dried sediments may be difficult to analyze for toxic effects with organismic aquatic bioassays. These samples might evoke low oxygen concentration or oxygen depletion during the test. The toxicity assessment could thus be confounded by low oxygen concentrations. The acute zebrafish embryo assay was used to analyze the influence of oxygen deficit on the embryonic development in the first 48 h post fertilization. Embryos were exposed to varying oxygen concentrations ranging from <30 to >80% oxygen saturation of water. A clear concentration dependent retardation of fish embryo development was observed. Because of a retarded development toxic thresholds of environmental samples which might include substances slowing down the development will be altered. For the purpose of identification of critical contaminants in complex environmental samples, it is proposed to actively aerate environmental samples which are likely to be oxygen depleted during the duration of the zebrafish embryo bioassay.

Proteomic analysis of anoxia tolerance in the developing zebrafish embryo

While some species and tissue types are injured by oxygen deprivation, anoxia tolerant organisms display a protective response that has not been fully elucidated and is well-suited to genomic and proteomic analysis. However, such methodologies have focused on transcriptional responses, prolonged anoxia, or have used cultured cells or isolated tissues. In this study of intact zebrafish embryos, a species capable of >24 h survival in anoxia, we have utilized 2D difference in gel electrophoresis to identify changes in the proteomic profile caused by near-lethal anoxic durations as well as acute anoxia (1 h), a timeframe relevant to ischemic events in human disease when response mechanisms are largely limited to post-transcriptional and post-translational processes. We observed a general stabilization of the proteome in anoxia. Proteins involved in oxidative phosphorylation, antioxidant defense, transcription, and translation changed over this time period. Among the largest proteomic alterations was that of muscle cofilin 2, implicating the regulation of the cytoskeleton and actin assembly in the adaptation to acute anoxia. These studies in an intact embryo highlight proteomic components of an adaptive response to anoxia in a model organism amenable to genetic analysis to permit further mechanistic insight into the phenomenon of anoxia tolerance.

A high throughput live transparent animal bioassay to identify non-toxic small molecules or genes that regulate vertebrate fat metabolism for obesity drug development

Background

The alarming rise in the obesity epidemic and growing concern for the pathologic consequences of the metabolic syndrome warrant great need for development of obesity-related pharmacotherapeutics. The search for such therapeutics is severely limited by the slow throughput of animal models of obesity. Amenable to placement into a 96 well plate, zebrafish larvae have emerged as one of the highest throughput vertebrate model organisms for performing small molecule screens. A method for visually identifying non-toxic molecular effectors of fat metabolism using a live transparent vertebrate was developed. Given that increased levels of nicotinamide adenine dinucleotide (NAD) via deletion of CD38 have been shown to prevent high fat diet induced obesity in mice in a SIRT-1 dependent fashion we explored the possibility of directly applying NAD to zebrafish.

Methods

Zebrafish larvae were incubated with daily refreshing of nile red containing media starting from a developmental stage of equivalent fat content among siblings (3 days post-fertilization, dpf) and continuing with daily refreshing until 7 dpf.

Results

PPAR activators, beta-adrenergic agonists, SIRT-1 activators, and nicotinic acid treatment all caused predicted changes in fat, cholesterol, and gene expression consistent with a high degree of evolutionary conservation of fat metabolism signal transduction extending from man to zebrafish larvae. All changes in fat content were visually quantifiable in a relative fashion using live zebrafish larvae nile red fluorescence microscopy. Resveratrol treatment caused the greatest and most consistent loss of fat content. The resveratrol tetramer Vaticanol B caused loss of fat equivalent in potency to resveratrol alone. Significantly, the direct administration of NAD decreased fat content in zebrafish. Results from knockdown of a zebrafish G-PCR ortholog previously determined to decrease fat content in C. elegans support that future GPR142 antagonists may be effective non-toxic anti-obesity therapeutics.

Conclusion

Owing to the apparently high level of evolutionary conservation of signal transduction pathways regulating lipid metabolism, the zebrafish can be useful for identifying non-toxic small molecules or pharmacological target gene products for developing molecular therapeutics for treating clinical obesity. Our results support the promising potential in applying NAD or resveratrol where the underlying target protein likely involves Sirtuin family member proteins. Furthermore data supports future studies focused on determining whether there is a high concentration window for resveratrol that is effective and non-toxic in high fat obesity murine models.

Anoxia-induced suspended animation in budding yeast as an experimental paradigm for studying oxygen-regulated gene expression

A lack of oxygen can force many organisms to enter into recoverable hypometabolic states. To better understand how organisms cope with oxygen deprivation, our laboratory previously had shown that when challenged with anoxia, both the nematode Caenorhabditis elegans and embryos of the zebrafish Danio rerio enter into suspended animation, in which all life processes that can be observed by light microscopy reversibly halt pending the restoration of oxygen (P. A. Padilla and M. B. Roth, Proc. Natl. Acad. Sci. USA 98:7331-7335, 2001, and P. A. Padilla, T. G. Nystul, R. A. Zager, A. C. Johnson, and M. B. Roth, Mol. Biol. Cell 13:1473-1483, 2002). Here, we show that both sporulating and vegetative cells of the budding yeast Saccharomyces cerevisiae also enter into a similar state of suspended animation when made anoxic on a nonfermentable carbon source. Transcriptional profiling using cDNA microarrays and follow-on quantitative real-time PCR analysis revealed a relative derepression of aerobic metabolism genes in carbon monoxide (CO)-induced anoxia when compared to nitrogen (N(2)) gas-induced anoxia, which is consistent with the known oxygen-mimetic effects of CO. We also found that mutants deleted for components of the mitochondrial retrograde signaling pathway can tolerate prolonged exposure to CO but not to N(2). We conclude that the cellular response to anoxia is dependent on whether the anoxic gas is an oxygen mimetic and that the mitochondrial retrograde signaling pathway is functionally important for mediating this response.

An RNA interference screen identifies a novel regulator of target of rapamycin that mediates hypoxia suppression of translation in Drosophila S2 cells

In addition to its central role in energy production, oxygen has pervasive regulatory actions. Hypoxia (oxygen limitation) triggers the shutdown of major cellular processes, including gene expression. We carried out a genome-wide RNA interference (RNAi) screen in Drosophila S2 cells for functions required to down-regulate translation during hypoxia. RNAi knockdown of specific genes allowed induction of a green fluorescent protein (GFP) reporter gene and continued protein synthesis during hypoxia. Among the identified genes, Tsc1 and Tsc2, which together form the tuberose sclerosis complex that negatively regulates target of rapamycin (TOR) kinase, gave an especially strong effect. This finding is consistent with the involvement of TOR in promoting translation. Another gene required for efficient inhibition of protein translation during hypoxia, the protein tyrosine phosphatase 61F (Ptp61F), down-regulates TOR activity under hypoxia. Lack of Ptp61F or Tsc2 improves cell survival under prolonged hypoxia in a TOR-dependent manner. Our results identify Ptp61F as a novel modulator of TOR activity and suggest that its function during hypoxia contributes to the down-regulation of protein synthesis.

The zebrafish embryo as a dynamic model of anoxia tolerance

Developing organisms depend upon a delicate balance in the supply and demand of energy to adapt to variable oxygen availability, although the essential mechanisms determining such adaptation remain elusive. In this study, we examine reversible anoxic arrest and dynamic bioenergetic transitions during zebrafish development. Our data reveal that the duration of anoxic viability corresponds to the developmental stage and anaerobic metabolic rate. Diverse chemical inhibitors of mitochondrial oxidative phosphorylation induce a similar arrest in normoxic embryos, suggesting a pathway responsive to perturbations in aerobic energy production rather than molecular oxygen. Consistent with this concept, arrest is accompanied by rapid activation of the energy-sensing AMP-activated protein kinase pathway, demonstrating a potential link between the sensing of energy status and adaptation to oxygen availability. These observations permit mechanistic insight into energy homeostasis during development that now enable genetic and small molecule screens in this vertebrate model of anoxia tolerance.

Fish embryos are damaged by dissolved PAHs, not oil particles

To distinguish the toxicity of whole oil droplets from compounds dissolved in water, responses of zebrafish embryos exposed to particulate-laden, mechanically dispersed Alaska North Slope crude oil (mechanically dispersed oil (MDO)) were compared to those of embryos protected from direct oil droplet contact by an agarose matrix. Most polycyclic aromatic hydrocarbons (PAHs) in MDO were contained in oil droplets; about 16% were dissolved. The agarose precluded embryo contact with particulate oil but allowed diffusive passage of dissolved PAHs. The incidence of edema, hemorrhaging, and cardiac abnormalities in embryos was dose-dependent in both MDO and agarose and the biological effects in these compartments were identical in character. Although mean total PAH (TPAH) concentrations in MDO were about 5-9 times greater than in agarose, dissolved PAH concentrations were similar in the two compartments. Furthermore, mean differences in paired embryo responses between compartments were relatively small (14-23%, grand mean 17%), typically with a larger response in embryos exposed to MDO. Therefore, the embryos reacted only to dissolved PAHs and the response difference between compartments is explained by diffusion. Averaged over 48 h, the estimated mean TPAH concentration in agarose was about 16% less than the dissolved TPAH concentration in MDO. Thus, PAHs dissolved from oil are toxic and physical contact with oil droplets is not necessary for embryotoxicity.

Effects of nickel chloride and oxygen depletion on behaviour and vitality of zebrafish (Danio rerio, Hamilton, 1822) (Pisces, Cypriniformes) embryos and larvae

We examined acute (2 h exposure of 5-day-old larvae) and subchronic (exposure from fertilization up to an age of 11 days) effects of NiCl(2).6H2O on embryos and larvae of zebrafish (Danio rerio), both alone and in combination with oxygen depletion. The following endpoints were recorded: acute exposure: locomotory activity and survival; subchronic exposure: hatching rate, deformations, locomotory activity (at 5, 8 and 11 days) and mortality. In acute exposures nickel chloride (7.5-15 mg Ni/L) caused decreasing locomotory activity. Oxygen depletion (<or=2.45+/-0.16 mg O2/L) also resulted in significantly reduced locomotory activity. In the subchronic test, exposure to >or=10 mg Ni/L resulted in delayed hatching at an age of 96 h, in decreased locomotory activity at an age of 5 days, and increased mortality at an age of 11 days (LC20=9.5 mg Ni/L). The observed LOEC for locomotory activity (7.5 mg Ni/L) is in the range of environmentally relevant concentrations. Since locomotory activity was already affected by acute exposure, this parameter is recommended to supplement commonly recorded endpoints of toxicity.

Differential effects of anoxia on heart rate in developmental stages of the annual killifish Austrofundulus limnaeus that differ in their tolerance of anoxia

Embryos of the annual killifish Austrofundulus limnaeus can experience oxygen deprivation as part of their normal developmental environment. We exposed embryos to anoxia and monitored heart activity for 48 hr, and subsequent aerobic recovery from anoxia for 40 hr. Embryos were tested at four different developmental stages that differ in their tolerance of anoxia. Our results indicate that high tolerance of anoxia is associated with an arrest of heart contractility during the first 24 hr of anoxia. These embryos recover to normoxic levels of heart rate within 16 hr of aerobic recovery. In contrast, embryos from later developmental stages that have a highly reduced ability to survive long-term anoxia experience a severe bradycardia but not an arrest of heart rate. These data illustrate a new and potentially powerful model for investigating the effects of anoxia on the developing cardiovascular system in vertebrates.

Mitochondrial dysfunction, bioenergetic impairment, and metabolic down-regulation in sepsis

Mitochondrial dysfunction is thought to play an important role in the pathogenesis of many different disease states. It has been proposed that an acquired defect in oxidative phosphorylation prevents cells from using molecular oxygen for adenosine triphosphate production and potentially causes sepsis-induced organ dysfunction. This concept, termed cytopathic hypoxia, however, has been difficult to prove because impaired oxidative phosphorylation has never been shown to cause sepsis-induced organ failure or to be a reversible phenomenon. Presented here is are view of oxidative phosphorylation, evidence of defective electron-transport-chain function in the heart and other organ systems during sepsis, and support for a link between mitochondrial dysfunction and pathologic metabolic down-regulation.

Role of hypoxia in the evolution and development of the cardiovascular system

How multicellular organisms obtain and use oxygen and other substrates has evolved over hundreds of millions of years in parallel with the evolution of oxygen-delivery systems. A steady supply of oxygen is critical to the existence of organisms that depend on oxygen as a primary source of fuel (i.e., those that live by aerobic metabolism). Not surprisingly, a number of mechanisms have evolved to defend against oxygen deprivation. This review highlights evolutionary and developmental aspects of O2 delivery to allow understanding of adaptive responses to O2 deprivation (hypoxia). First, we consider how the drive for more efficient oxygen delivery from the heart to the periphery may have shaped the evolution of the cardiovascular system, with particular attention to the routing of oxygenated and deoxygenated blood in the cardiac outlet. Then we consider the role of O2 in the morphogenesis of the cardiovascular system of animals of increasing size and complexity. We conclude by suggesting areas for future research regarding the role of oxygen deprivation and oxidative stress in the normal development of the heart and vasculature or in the pathogenesis of congenital heart defects.

Environmental and genetic modifiers of squint penetrance during zebrafish embryogenesis

The Nodal-related subgroup of the TGFbeta superfamily of secreted cytokines regulates the specification of the mesodermal and endodermal germ layers during gastrulation. Two Nodal-related proteins - Squint (Sqt) and Cyclops (Cyc) - are expressed during germ-layer specification in zebrafish. Genetic sqt mutant phenotypes have defined a variable requirement for zygotic Sqt, but not for maternal Sqt, in midline mesendoderm development. However a comparison of phenotypes arising from oocytes or zygotes injected with Sqt antisense morpholinos has suggested a novel requirement for maternal Sqt in dorsal specification. In this study we examined maternal-zygotic mutants for each of two sqt alleles and we also compared phenotypes of closely related zygotic and maternal-zygotic sqt mutants. Each of these approaches indicated there is no general requirement for maternal Sqt. To better understand the dispensability of maternal and zygotic Sqt, we sought out developmental contexts that more rigorously demand intact Sqt signalling. We found that sqt penetrance is influenced by genetic modifiers, by environmental temperature, by levels of residual Activin-like activity and by Heat-Shock Protein 90 (HSP90) activity. Therefore, Sqt may confer an evolutionary advantage by protecting early-stage embryos against detrimental interacting alleles and environmental challenges.

Rapid effects of acute anoxia on spindle kinetochore interactions activate the mitotic spindle checkpoint

The dramatic chromosome instability in certain tumors might reflect a synergy of spindle checkpoint defects with hypoxic conditions. In Caenorhabditis elegans and Drosophila melanogaster, spindle checkpoint activation has been implicated in the response to acute anoxia. The activation mechanism is unknown. Our analyses in D. melanogaster demonstrate that oxygen deprivation affects microtubule organization within minutes. The rapid effects of anoxia are identical in wild-type and spindle checkpoint-deficient Mps1 mutant embryos. Therefore, the anoxia effects on the mitotic spindle are not a secondary consequence of spindle checkpoint activation. Some motor, centrosome and kinetochore proteins (dynein, Kin-8, Cnn, TACC, Cenp-C, Nuf2) are rapidly relocalized after oxygen deprivation. Kinetochores congress inefficiently into the metaphase plate and do not experience normal pulling forces. Spindle checkpoint proteins accumulate mainly within the spindle midzone and inhibit anaphase onset. In checkpoint-deficient embryos, mitosis is still completed after oxygen deprivation, although accompanied by massive chromosome missegregation. Inhibitors of oxidative phosphorylation mimic anoxia effects. We conclude that oxygen deprivation impairs the chromosome segregation machinery more rapidly than spindle checkpoint function. Although involving adenosine triphosphate (ATP)-consuming kinases, the spindle checkpoint can therefore be activated by spindle damage in response to acute anoxia and protect against aneuploidies.

Extreme anoxia tolerance in embryos of the annual killifishAustrofundulus limnaeus: insights from a metabolomics analysis

The annual killifish Austrofundulus limnaeus survives in ephemeral pond habitats by producing drought-tolerant diapausing embryos. These embryos probably experience oxygen deprivation as part of their normal developmental environment. We assessed the anoxia tolerance of A. limnaeus embryos across the duration of embryonic development. Embryos develop a substantial tolerance to anoxia during early development, which peaks during diapause II. This extreme tolerance of anoxia is retained during the first 4 days of post-diapause II development and is then lost. Metabolism during anoxia appears to be supported mainly by production of lactate, with alanine and succinate production contributing to a lesser degree. Anoxic embryos also accumulate large quantities of γ-aminobutyrate (GABA), a potential protector of neural function. It appears that the suite of characters associated with normal development and entry into diapause II in this species prepares the embryos for long-term survival in anoxia even while the embryos are exposed to aerobic conditions. This is the first report of such extreme anoxia tolerance in a vertebrate embryo, and introduces a new model for the study of anoxia tolerance in vertebrates.

Hypoxia tolerance in reptiles, amphibians, and fishes: life with variable oxygen availability

The ability of fishes, amphibians, and reptiles to survive extremes of oxygen availability derives from a core triad of adaptations: profound metabolic suppression, tolerance of ionic and pH disturbances, and mechanisms for avoiding free-radical injury during reoxygenation. For long-term anoxic survival, enhanced storage of glycogen in critical tissues is also necessary. The diversity of body morphologies and habitats and the utilization of dormancy have resulted in a broad array of adaptations to hypoxia in lower vertebrates. For example, the most anoxia-tolerant vertebrates, painted turtles and crucian carp, meet the challenge of variable oxygen in fundamentally different ways: Turtles undergo near-suspended animation, whereas carp remain active and responsive in the absence of oxygen. Although the mechanisms of survival in both of these cases include large stores of glycogen and drastically decreased metabolism, other mechanisms, such as regulation of ion channels in excitable membranes, are apparently divergent. Common themes in the regulatory adjustments to hypoxia involve control of metabolism and ion channel conductance by protein phosphorylation. Tolerance of decreased energy charge and accumulating anaerobic end products as well as enhanced antioxidant defenses and regenerative capacities are also key to hypoxia survival in lower vertebrates.

Parameters influencing the dissolved oxygen in the boundary layer of rainbow trout (Oncorhynchus mykiss) embryos and larvae

We investigated the influence of oxygen demand (developmental stage) and supply (hypoxia, water flow rate, the chorion and body movements) on the oxygen concentration within the boundary layer next to the chorion of embryos or skin of larvae of rainbow trout (Oncorhynchus mykiss). Oxygen microelectrodes were used to measure dissolved oxygen (DO) within the boundary layer of trout embryos and larvae. As the embryos and larvae developed, the DO gradient and the thickness of the boundary layer increased. The DO concentration within the boundary layer next to the chorion or skin surface decreased as the DO concentration in the free-stream water decreased. A decrease in water flow rate increased the magnitude of the gradient and thickness of the boundary layer. In normoxia, the DO in the perivitelline fluid inside the chorion was 16±3.0% saturation at 31 days post fertilization, indicating that the chorion was a significant barrier to oxygen diffusion. The number of body movements did not change when embryos were exposed to hypoxia before hatching, but after hatching, hypoxia resulted in a decrease in body movements of the larvae. Taken together, our data indicate that the oxygen boundary layer around trout embryos and larvae depends on both the oxygen demand and supply. The factors that significantly impacted boundary layer oxygen were developmental stage, free-stream oxygen levels, water flow rate, and the presence of the chorion.

Ontogenesis of oxygen chemoreception in aquatic vertebrates

In aquatic vertebrates, peripheral O(2) chemoreceptors initiate compensatory physiological and behavioural responses to hypoxia, beginning at very early stages of development, to maintain sufficient gas exchange across the skin or gills. This review highlights the morphological and physiological studies, particularly those of zebrafish, that have contributed to the current understanding of the development of O(2) chemoreception and the response to hypoxic challenges in embryonic and larval stages of fish and amphibians. The gills appear to be the primary site of O(2) chemoreception in developing aquatic vertebrates and initiate ventilatory changes, and adult-like O(2)-sensitive neuroepithelial cells (NECs) are found in the gills in larval stages of zebrafish and Xenopus laevis. However, evidence from zebrafish studies indicates that extrabranchial O(2) chemoreceptors appear before gill NECs and regulate responses to hypoxia that develop earlier. The developmental and evolutionary significance of the internal migration of O(2)-chemoreceptive sites with changes in respiratory organs is also discussed.

Hypoxia induces a complex response of globin expression in zebrafish(Danio rerio)

Unlike most mammals, many fish species live and survive in environments with low or changing levels of oxygen. Respiratory proteins like hemoglobin or myoglobin bind or store oxygen, thus enhancing its availability to the respiratory chain in the mitochondria. Here we investigate by means of quantitative real-time PCR the changes of hemoglobin, myoglobin, neuroglobin,cytoglobin and globin X mRNA in zebrafish (Danio rerio) exposed to mild (PO2=∼8.6 kPa) or severe(PO2=∼4.1 kPa) hypoxia. Neuroglobin and myoglobin protein levels were investigated by western blotting. Whereas mild hypoxia caused only minor changes of mRNA levels, strong hypoxia enhanced mRNA levels of the control genes (lactate dehydrogenase A and phosphoglycerate kinase 1). Surprisingly, levels of hemoglobin α and β mRNA were significantly reduced under severe hypoxia. Myoglobin mRNA and protein in heart mildly increased, in line with its proposed oxygen supply function. Likewise,neuroglobin mRNA and protein significantly increased in brain (up to 5.7-fold at the protein level), but not in eye. This observation, firstly, suggests physiological differences of zebrafish eye and brain under hypoxia, and secondly, indicates an important role of neuroglobin in oxidative metabolism,probably oxygen supply within neurons. There was little change in the expression of the two cytoglobin genes. Globin X mRNA significantly decreased under hypoxia, pointing to a functional linkage to oxygen-dependent metabolism.

Cardiovascular system in larval zebrafish responds to developmental hypoxia in a family specific manner

Background

Genetic and environmental variation are both known to influence development. Evolution of a developmental response that is optimized to the environment (adaptive plasticity) requires the existence of genetic variation for that developmental response. In complex traits composed of integrated sets of subsidiary traits, the adaptive process may be slowed by the existence of multiple possible integrated responses. This study tests for family (sibship) specific differences in plastic response to hypoxia in an integrated set of cardiovascular traits in zebrafish.

Results

Cardiac output, which is the integrated product of several subsidiary traits, varied highly significantly between families, and families differed significantly in the degree and direction of response to developmental oxygen level. The cardiac output response to oxygen environment was entirely family specific with no significant overall trend due to oxygen level. Constituent physiological variables that contribute to cardiac output all showed significant family specific response to hypoxia. Traits that were not directly related to cardiac output, such as arterial and venous diameter, and red blood cell velocities did not respond to hypoxia in a family specific manner.

Conclusion

Zebrafish families vary in their plastic response to hypoxia. Genetic variation in plastic response to hypoxia may therefore provide the basic ingredient for adaptation to a variable environment. Considerable variation in the degree of familial response to hypoxia exists between different cardiovascular traits that may contribute to cardiac output. It is possible that the integration of several subsidiary traits into cardiac output allows the maintenance of genetic variance in cardiac response.

Long term metabolic arrest and recovery of HEK293 spheroids involves NF-kappaB signaling and sustained JNK activation

Understanding how cells withstand a depletion of intracellular water is relevant to the study of longevity, aging, and quiescence because one consequence of air-drying is metabolic arrest. After removal of medium, HEK293 spheroids with intracellular water content of approximately 65% survived partial vacuum, with antistatic control, for weeks in the dark at 25 degrees C. In contrast, only a limited exposure of monolayers to air was lethal; the mitochondrion being a target of this stress. The pathways activated during the long-term arrest and recovery of spheroids depended on both NF-kappaB signaling and sustained JNK activation. A cyclical cascade, presumably originating from an intercellular stress signal, led to endogenous cytokine production (TNF-alpha, IL-1b, and IL-8) and propagation of the cellular stress signal through the co-activation of NF-kappaB and JNK. Increased levels of downstream pathway signaling members, specifically Gadd45beta, c-jun, and ATF3 were observed, as was activation of c-jun (phosphorylation). Activation of these pathways permit cells to survive long-term storage and recovery because chemical inhibition of both NF-kappaB nuclear translocation and JNK phosphorylation led to cell death. The capacity of an immortalized cell to enter, and then exit, a state of long-term quiescence, without genetic or chemical intervention, has implications for the study of cell transformation. In addition, the ability to monitor the relevant signaling pathways at endogenous levels, from effector to transcriptional regulator, emphasizes the utility of multicellular aggregate models in delineating stress response pathways.

Characterization of sub-nuclear changes in Caenorhabditis elegans embryos exposed to brief, intermediate and long-term anoxia to analyze anoxia-induced cell cycle arrest

BACKGROUND

The soil nematode C. elegans survives oxygen-deprived conditions (anoxia; <.001 kPa O2) by entering into a state of suspended animation in which cell cycle progression reversibly arrests. The majority of blastomeres of embryos exposed to anoxia arrest at interphase, prophase and metaphase. The spindle checkpoint proteins SAN-1 and MDF-2 are required for embryos to survive 24 hours of anoxia. To further investigate the mechanism of cell-cycle arrest we examined and compared sub-nuclear changes such as chromatin localization pattern, post-translational modification of histone H3, spindle microtubules, and localization of the spindle checkpoint protein SAN-1 with respect to various anoxia exposure time points. To ensure analysis of embryos exposed to anoxia and not post-anoxic recovery we fixed all embryos in an anoxia glove box chamber.

RESULTS

Embryos exposed to brief periods to anoxia (30 minutes) contain prophase blastomeres with chromosomes in close proximity to the nuclear membrane, condensation of interphase chromatin and metaphase blastomeres with reduced spindle microtubules density. Embryos exposed to longer periods of anoxia (1-3 days) display several characteristics including interphase chromatin that is further condensed and in close proximity to the nuclear membrane, reduction in spindle structure perimeter and reduced localization of SAN-1 at the kinetochore. Additionally, we show that the spindle checkpoint protein SAN-1 is required for brief periods of anoxia-induced cell cycle arrest, thus demonstrating that this gene product is vital for early anoxia responses. In this report we suggest that the events that occur as an immediate response to brief periods of anoxia directs cell cycle arrest.

CONCLUSION

From our results we conclude that the sub-nuclear characteristics of embryos exposed to anoxia depends upon exposure time as assayed using brief (30 minutes), intermediate (6 or 12 hours) or long-term (24 or 72 hours) exposures. Analyzing these changes will lead to an understanding of the mechanisms required for initiation and maintenance of cell cycle arrest in respect to anoxia exposure time as well as order the events that occur to bring about anoxia-induced cell cycle arrest.

Gene expression profiling of the long-term adaptive response to hypoxia in the gills of adult zebrafish

Low oxygen levels (hypoxia) play a role in clinical conditions such as stroke, chronic ischemia, and cancer. To better understand these diseases, it is crucial to study the responses of vertebrates to hypoxia. Among vertebrates, some teleosts have developed the ability to adapt to extremely low oxygen levels. We have studied long-term adaptive responses to hypoxia in adult zebrafish. We used zebrafish that survived severe hypoxic conditions for 3 wk and showed adaptive behavioral and phenotypic changes. We used cDNA microarrays to investigate hypoxia-induced changes in expression of 15,532 genes in the respiratory organs (the gills). We have identified 367 differentially expressed genes of which 117 showed hypoxia-induced and 250 hypoxia-reduced expressions. Metabolic depression was indicated by repression of genes in the TCA cycle in the electron transport chain and of genes involved in protein biosynthesis. We observed enhanced expression of the monocarboxylate transporter and of the oxygen transporter myoglobin. The hypoxia-induced group further included the genes for Niemann-Pick C disease and for Wolman disease [lysosomal acid lipase (LAL)]. Both diseases lead to a similar intra- and extracellular accumulation of cholesterol and glycolipids. The Niemann-Pick C protein binds to cholesterol from internal lysosomal membranes and is involved in cholesterol trafficking. LAL is responsible for lysosomal cholesterol degradation. Our data suggest a novel adaptive mechanism to hypoxia, the induction of genes for lysosomal lipid trafficking and degradation. Studying physiological responses to hypoxia in species tolerant for extremely low oxygen levels can help identify novel regulatory genes, which may have important clinical implications.

Development, Surface Exposure, and Embryo Behavior Affect Oxygen Levels in Eggs of the Red‐Eyed Treefrog,Agalychnis callidryas

Oxygen stress can slow development, induce hatching, and kill eggs. Terrestrial anamniote embryos face a potential conflict between oxygen uptake and water loss. We measured oxygen levels within eggs to characterize the respiratory environment for embryos of the red-eyed treefrog, Agalychnis callidryas, a Neotropical frog with arboreal egg masses and plastic hatching timing. Perivitelline oxygen partial pressure (Po2) was extremely variable both within and among eggs. Po2 increased with air-exposed surface of the egg and declined over the developmental period before hatching competence. Through the plastic hatching period, however, average Po2 was stable despite continued rapid development. Development was synchronous across a wide range of perivitelline Po2 (0.5-16.5 kPa), and hatching-competent embryos tolerated Po2 as low as 0.5 kPa without hatching. The variation in Po2 measured over short periods of time within individual eggs was as great as that measured across development or surface exposure, including sharp transients associated with embryo movements. There was also a strong gradient of Po2 across the egg from superficial to deep positions. Ciliary circulation of fluid within the egg is clearly insufficient to keep it mixed. Embryos may maintain development under hypoxic conditions by strategic positioning of respiratory surfaces, particularly external gills, to exploit the patchy distribution of oxygen within their eggs.

Ontogeny of cardiovascular control in zebrafish (Danio rerio): Effects of developmental environment

The goal of this symposium paper was to identify and quantify developmental plasticity in the onset of cardiovascular responses in the zebrafish. Developmental plasticity was induced by altering the developmental environment in one of three ways: (1) by developing zebrafish in a constant current of 5 body lengths per second, (2) by developing zebrafish at a colder temperature (20 degrees C), and (3) by developing zebrafish in severe hypoxia (DO=0.8 mg/L). Early morphological development was significantly affected by each of the treatment environments with hypoxia slowing development the most and producing the highest variation in measurements. Development in constant water current did not significantly affect the timing onset of cardiovascular responses to the pharmacological agents applied. Development at 20 degrees C significantly delayed the onset of all cardiovascular responses measured by 2-3 days. Development in hypoxia, however, not only delayed onset of all cardiovascular responses, but also shifted the onset relative to the developmental program. Hypoxia clearly has a profound affect on the onset of cardiovascular regulation and it will take many more studies to elucidate the mechanisms by which hypoxia is having its effect. Furthermore, long term studies are also needed to assess whether the plasticity measured in this study is adaptive in the evolutionary sense.

Cell proliferation and gill morphology in anoxic crucian carp

Is DNA replication/cell proliferation in vertebrates possible during anoxia? The oxygen dependence of ribonucleotide reductase (RNR) could lead to a stop in DNA synthesis, thereby making anoxic DNA replication impossible. We have studied this question in an anoxia-tolerant vertebrate, the crucian carp (Carassius carassius), by examining 5'-bromo-2'-deoxyuridine incorporation and proliferating cell nuclear antigen levels in the gills, intestinal crypts, and liver. We exposed crucian carp to 1 and 7 days of anoxia followed by 7 days of reoxygenation. There was a reduced incidence of S-phase cells (from 12.2 to 5.0%) in gills during anoxia, which coincided with a concomitant increase of G(0) cells. Anoxia also decreased the number of S-phase cells in intestine (from 8.1 to 1.8%). No change in the fraction of S-phase cells ( approximately 1%) in liver was found. Thus new S-phase cells after 7 days of anoxia were present in all tissues, revealing a considerable rate of DNA synthesis. Subsequently, the oxygen-dependent subunit of crucian carp RNR (RNRR2) was cloned. We found no differences in amino acids involved in radical generation and availability of the iron center compared with mouse, which could have explained reduced oxygen dependence. Furthermore, the amount of RNRR2 mRNA in gills did not decrease throughout anoxia exposure. These results indicate that crucian carp is able to sustain some cell proliferation in anoxia, possibly because RNRR2 retains its tyrosyl radical in anoxia, and that the replication machinery is still maintained. Although hypoxia triggers a 7.5-fold increase of respiratory surface area in crucian carp, this response was not triggered in anoxia.

Oxygen-dependent gene expression in fishes

The role of oxygen in regulating patterns of gene expression in mammalian development, physiology, and pathology has received increasing attention, especially after the discovery of the hypoxia-inducible factor (HIF), a transcription factor that has been likened to a "master switch" in the transcriptional response of mammalian cells and tissues to low oxygen. At present, considerably less is known about the molecular responses of nonmammalian vertebrates and invertebrates to hypoxic exposure. Because many animals live in aquatic habitats that are variable in oxygen tension, it is relevant to study oxygen-dependent gene expression in these animals. The purpose of this review is to discuss hypoxia-induced gene expression in fishes from an evolutionary and ecological context. Recent studies have described homologs of HIF in fish and have begun to evaluate their function. A number of physiological processes are known to be altered by hypoxic exposure of fish, although the evidence linking them to HIF is less well developed. The diversity of fish presents many opportunities to evaluate if inter- and intraspecific variation in HIF structure and function correlate with hypoxia tolerance. Furthermore, as an aquatic group, fish offer the opportunity to examine the interactions between hypoxia and other stressors, including pollutants, common in aquatic environments. It is possible, if not likely, that results obtained by studying the molecular responses of fish to hypoxia will find parallels in the oxygen-dependent responses of mammals, including humans. Moreover, novel responses to hypoxia could be discovered through studies of this diverse and species-rich group.

H2S induces a suspended animation-like state in mice

Mammals normally maintain their core body temperature (CBT) despite changes in environmental temperature. Exceptions to this norm include suspended animation-like states such as hibernation, torpor, and estivation. These states are all characterized by marked decreases in metabolic rate, followed by a loss of homeothermic control in which the animal's CBT approaches that of the environment. We report that hydrogen sulfide can induce a suspended animation-like state in a nonhibernating species, the house mouse (Mus musculus). This state is readily reversible and does not appear to harm the animal. This suggests the possibility of inducing suspended animation-like states for medical applications.

Hochachka's "Hypoxia Defense Strategies" and the development of the pathway for oxygen

Hochachka's "Hypoxia Defense Strategies" identify oxygen signalling, metabolic arrest, channel arrest and coordinated suppression of ATP turnover rates as key factors that determine the ability of organisms to survive exposure to chronic hypoxia. In this review, I assess the developmental role played by these phenomena in the morphogenesis of the gas exchange tissues that define the pathway for oxygen transport to cytochrome c oxidase. Key areas of regulation lie in: (I) the suppression of fetal mitochondrial oxidative function in hand with mitochondrial biogenesis (metabolic arrest), (II) the role of hypoxia-driven oxygen signalling pathways in directing the scope of non-differentiated stem cell proliferation in placenta and lung development and (III) the regulation of epithelial fluid secretion/absorption in the lung through the oxygen-dependent modulation of Na+ conductance pathways. The identification of developmental roles for Hochachka's "Hypoxia Defense Strategies" in directing the morphogenesis of gas exchange structures bears with it the implication that these strategies are fundamental to establishing the scope for aerobic metabolic performance throughout life.

The role of ATM and ATR in the cellular response to hypoxia and re-oxygenation

ATM and ATR are stress-response kinases which respond to a variety of insults including ionizing radiation, replication arrest, ultraviolet radiation and hypoxia/re-oxygenation. Hypoxia occupies a unique niche in the study of both ATR- and ATM-mediated checkpoint pathways. Hypoxia is a physiologically significant stress that occurs in virtually all solid tumors and differs from most other stresses in that it does not induce DNA damage. Previous studies have indicated that hypoxia provides a unique way to induce ATR in response to inhibition of DNA replication. During tumor expansion hypoxia is inevitably followed by periods of re-oxygenation which in vitro has been shown to induce significant levels of DNA damage and an ATM response. Therefore both ATR and ATM have a role to play in hypoxia/re-oxygenation.

Insulin-like growth factor-binding protein-1 (IGFBP-1) mediates hypoxia-induced embryonic growth and developmental retardation

Although reduced fetal growth in response to hypoxia has been appreciated for decades, we have a poor understanding of the effects of hypoxia on embryonic development and the underlying cellular and molecular mechanisms. Here we show that hypoxia treatment not only resulted in embryonic growth retardation but also caused significant delay in developmental speed and the timing of morphogenesis in vital organs of zebrafish. Hypoxia strongly induced the expression of insulin-like growth factor (IGF)-binding protein (IGFBP)-1, a secreted protein that binds IGFs in extracellular environments. Hypoxia did not change the expression levels of IGFs, IGF receptors, or other IGFBPs. The hypothesis that elevated IGFBP-1 mediates hypoxia-induced embryonic growth retardation and developmental delay by binding to and inhibiting the activities of IGFs was tested by loss- and gain-of-function approaches. Knockdown of IGFBP-1 significantly alleviated the hypoxia-induced growth retardation and developmental delay. Overexpression of IGFBP-1 caused growth and developmental retardation under normoxia. Furthermore, reintroduction of IGFBP-1 to the IGFBP-1 knocked-down embryos restored the hypoxic effects on embryonic growth and development. When tested in vitro with cultured zebrafish embryonic cells, IGFBP-1 itself had no mitogenic activity, but it inhibited IGF-1- and IGF-2-stimulated cell proliferation. This inhibitory effect was abolished when IGF-1 or IGF-2 was added in molar excess, suggesting that IGFBP-1 inhibits embryonic growth and development by binding to and inhibiting the activities of IGFs. The induction of IGFBP-1 expression may be a conserved physiological mechanism to restrict the IGF-stimulated growth and developmental process under hypoxic stress.

Metabolic changes in Japanese medaka (Oryzias latipes) during embryogenesis and hypoxia as determined by in vivo 31P NMR

In vivo (31)P nuclear magnetic resonance spectroscopy (NMR) was used to determine phosphometabolite changes in medaka (Oryzias latipes) during embryogenesis and hypoxia. NMR data were acquired using a flow-through NMR tube perfusion system designed to both deliver oxygenated water to embryos and accommodate a hypoxic challenge. Measurements of embryogenesis at 12- and 24-h intervals throughout 8 days of development (n = 3 per time point, 900 embryos per replicate) and during acute hypoxia (n = 6, 900 embryos at Iwamatsu stage 37 per replicate) were performed via NMR, and replicate samples (n = 4, 250 embryos each) were flash frozen for HPLC analysis. The hypoxic challenge experiment consisted of data acquisition with recirculating water (pre-hypoxic control period; 1 h), without recirculating water (hypoxic challenge; 1 h), then again with recirculating water (recovery period; 1.3 h). Concentrations of ATP, phosphocreatine (PCr), orthophosphate (P(i)), phosphomonoesters (PME), phosphodiesters (PDE), and intracellular pH (pH(i)) were determined by NMR, and ATP, ADP, AMP, GTP, GDP, and PCr were also determined via HPLC. During embryogenesis, [ATP] and [PCr] as determined by HPLC increased from 1-day post fertilization (DPF) levels of 0.93+/-0.08 and 2.48+/-0.21 micromol/mg (dry tissue), respectively, to 7.24+/-0.77 and 15.66+/-1.08 micromol/mg, respectively, by day 8. [ATP] and [PCr] measured by both NMR and HPLC fluctuated over 1-3 DPF, then increased significantly (p<0.05) over 3-8 DPF, while [PME] and [PDE] decreased (p<0.05) throughout embryogenesis. NMR and HPLC measurements revealed 1-3, 4-5, and 6-8 DPF as periods of embryogenesis significantly different from each other (p<0.05), and representing important transitions in metabolism and growth. During hypoxic challenge, [ATP] and [PCr] declined (p<0.05), [PME] and [PDE] decreased slightly, and [P(i)] increased (p<0.05). All phosphometabolites returned to pre-hypoxia concentrations during recovery. The pH(i) decreased (p<0.05) from 7.10+/-0.03 to 6.94+/-0.03 as a result of hypoxia, and failed to return to pre-hypoxic levels within the 1.3-h recovery phase. Results demonstrate the utility of in vivo (31)P NMR to detect significant alterations in phosphorylated nucleotides and phosphometabolites at specific developmental stages during medaka development and that late-stage medaka utilize PCr to generate ATP under hypoxic conditions.

Aquatic hypoxia is a teratogen and affects fish embryonic development

Hypoxia occurs over large areas in aquatic systems worldwide, and there is growing concern that hypoxia may affect aquatic animals, leading to population decline and changes in community by elimination of sensitive species. For the first time, we report that sublethal levels of hypoxia can significantly increase (+77.4%) malformation in fish embryonic development. Disruption of apoptotic pattern was clearly evident at 24 h post-fertilization, which may be a major cause of malformation. Furthermore, embryonic development was delayed, and balance of sex hormones (testosterone and estradiol) was disturbed during embryonic stages, implicating that subsequent sexual development may also be affected. Overall, our results imply that hypoxia may have a teratogenic effect on fish and delay fish embryonic development, which may subsequently impair species fitness leading to natural population decline.

Carbon monoxide-induced suspended animation protects against hypoxic damage in Caenorhabditis elegans

Oxygen deprivation is a major cause of cellular damage and death. Here we demonstrate that Caenorhabditis elegans embryos, which can survive both in anoxia (<0.001 kPa O(2)) by entering into suspended animation and in mild hypoxia (0.25-1 kPa O(2)) through a hypoxia-inducible factor 1-mediated response, cannot survive in intermediate concentrations of oxygen, between 0.01 and 0.1 kPa O(2). Moreover, we show that carbon monoxide can protect C. elegans embryos against hypoxic damage in this sensitive range. Carbon monoxide can also rescue the hypoxia-sensitive mutant hif-1(ia04) from lethality in hypoxia. This work defines the oxygen tensions over which hypoxic damage occurs in C. elegans embryos and demonstrates that carbon monoxide can prevent this damage by inducing suspended animation.

Suspended animation in C. elegans requires the spindle checkpoint

In response to environmental signals such as anoxia, many organisms enter a state of suspended animation, an extreme form of quiescence in which microscopically visible movement ceases. We have identified a gene, san-1, that is required for suspended animation in Caenorhabditis elegans embryos. We show that san-1 functions as a spindle checkpoint component in C. elegans. During anoxia-induced suspended animation, embryos lacking functional SAN-1 or a second spindle checkpoint component, MDF-2, failed to arrest the cell cycle, exhibited chromosome missegregation, and showed reduced viability. These data provide a model for how a dynamic biological process is arrested in suspended animation.

Neuroepithelial cells and associated innervation of the zebrafish gill: a confocal immunofluorescence study

Peripheral chemoreceptors responsive to hypoxia have been well characterized in air-breathing vertebrates, but poorly in water-breathers. The present study examined the distribution of five populations of neuroepithelial cells (NECs), putative O(2) chemoreceptors, and innervation patterns in the zebrafish gill using whole-mounts and confocal immunofluorescence. Nerve bundles and fibers of the gill were labeled with zn-12 (a zebrafish-specific neuronal marker) and SV2 antisera and NECs were characterized by serotonin (5-HT) immunoreactivity (IR), SV2-IR and the purinoceptor P2X(3)-IR. A zn-12-IR nerve bundle extended the length of the gill filament and gave rise to a nerve plexus surrounding the efferent filament artery (eFA) and a rich network of fibers that innervated both serotonergic and nonserotonergic NECs of the filament and lamellar epithelium. Three populations of serotonergic, SV2-IR neurons intrinsic to the gill filaments are described, one of which provided innervation to NECs of the filament epithelium. Degeneration of nerve fibers in gill arches maintained in explant culture for 2 days revealed the extrinsic origin of nerve fibers of the plexus and lamellae and the innervation of filament NECs by both intrinsic and extrinsic fibers. Intrinsic innervation surrounding the eFA survived in explant cultures, suggesting a mechanism of local vascular control within the gill. In addition, NECs survived in explants after degeneration of extrinsic nerve fibers. Thus, NECs of the zebrafish gill are organized in a manner reminiscent of O(2) chemoreceptors of mammalian vertebrates, suggesting a role in respiratory regulation.

Genetic models in applied physiology: invited review: effect of oxygen deprivation on cell cycle activity: a profile of delay and arrest

One of the most fascinating fields that have emanated in the past few decades is developmental biology. This is not only the case from a research point of view but also from the angle of clinical care and treatment strategies. It is now well demonstrated that there are many diseases (some believe all diseases) that have their roots in embryogenesis or in early life, where nature and environment often team up to facilitate the genesis of disease. There is probably no better example to illustrate the interactions between nature and environment than in early life, as early as in the first several cell cycles. As will be apparent in this review, the cell cycle is a very regulated activity and this regulation is genetic in nature, with checkpoint proteins playing an important role in controlling the timing, the size, and the growth of daughter cells. However, it is also very clear, as will be discussed in this work, that the microenvironment of the first dividing cells is so important for the outcome of the organism. In this review, we will focus on the effect of one stress, that of hypoxia, on the young embryo and its cell division and growth. We will first review some of the cell cycle definitions and stages and then review briefly our current knowledge and its gaps in this area.

Gene expression profile of zebrafish exposed to hypoxia during development

Understanding how vertebrates respond to hypoxia can have important clinical implications. Fish have evolved the ability to survive long exposure to low oxygen levels. However, little is known about the specific changes in gene expression that result from hypoxia. In this study we used a zebrafish cDNA microarray to examine the expression of >4,500 genes in zebrafish embryos exposed to 24 h of hypoxia during development. We tested the hypotheses that hypoxia changes gene expression profile of the zebrafish embryos and that these changes can be reverted by reexposure to a normoxic (20.8% O(2)) environment. Our data were consistent with both of these hypotheses: indicating that zebrafish embryos undergo adaptive changes in gene expression in response to hypoxia. Our study provides a striking genetic portrait of the zebrafish embryos' adaptive responses to hypoxic stress and demonstrates the utility of the microarray technology as a tool for analyzing complex developmental processes in the zebrafish.

Non-invasive imaging of blood cell concentration and blood distribution in zebrafish Danio rerio incubated in hypoxic conditions in vivo

This is the first study to use a combination of digital imaging techniques and vital video microscopy to study hypoxia-induced changes in blood cell concentration, angiogenesis and blood redistribution in entire animals. Zebrafish Danio rerio, which are known to be independent of convective oxygen transport until about 2 weeks post-fertilization, were raised under chronic hypoxia (P(O(2))=8.7 kPa) starting at 1 day after fertilization (d.p.f.) until 15 d.p.f. In control animals, the concentration of red cells (i.e. the number of red cells per nl blood) remained constant until 7 d.p.f., and than decreased by approximately 70% until 15 d.p.f. In hypoxic animals, however, the concentration of red cells remained significantly elevated compared to control animals at 12 and 15 d.p.f. Assuming that the hemoglobin content of the red cells is similar, hypoxic animals have a higher oxygen carrying capacity in their blood. Red cell distribution within the various parts of the circulatory system, taken as an indicator for blood distribution, revealed a significant modification in the number of blood cells perfusing the organs in hypoxic animals. At 12 d.p.f., gut perfusion was reduced by almost 50% in hypoxic animals, while perfusion of the segmental muscle tissue was increased to 350% of control values. No significant changes in brain perfusion were observed under these conditions. At 15 d.p.f., the reduction in gut perfusion was abolished, although muscle perfusion was still significantly elevated. At this time, growth of hypoxic animals was less compared to control animals, revealing that hypoxia had become deleterious for further development. The vascular bed of various organs was not obviously different in hypoxic animals compared to normoxic animals.

Nitric oxide-induced suspended animation promotes survival during hypoxia

Oxygen plays a key role in energy metabolism. However, there are organisms that survive severe shortfalls in oxygen. Drosophila embryos rapidly arrest development upon severe hypoxia and recover upon restoration of oxygen, even days later. Stabilization of the normally unstable engrailed RNA and protein preserved the localized striped pattern of this embryonic patterning gene during 3 days in hypoxia. Severe hypoxia blocked expression of a heat-shock-inducible lacZ transgene. Cyanide, a metabolic poison, did not immediately block gene expression or turnover, arguing against a passive response to energy limitation. In contrast, nitric oxide, a putative hypoxia signal, induced a reversible arrest of development, gene expression and turnover. Reciprocally, a nitric oxide scavenger allowed continued gene expression and turnover during hypoxia, but it reduced hypoxia tolerance. We suggest that hypoxia-induced stasis preserves the status quo of embryonic processes and promotes survival. Our data implicate nitric oxide as a mediator of this response and provide a system in which to investigate its action.

From Zebrafish to human: modular medical models

Genetic screens in Drosophila melanogaster, Caenorhabditis elegans, and Danio rerio clarified the logic of metazoan development by revealing critical unitary steps and pathways to embryogenesis. Can genetic screens similarly organize medicine? We here examine human diseases that resemble mutations in Danio rerio, the zebrafish, the one vertebrate species for which large-scale genetic screens have been performed and extensively analyzed. Zebrafish mutations faithfully phenocopy many human disorders. Each mutation, once cloned, provides candidate genes and pathways for evaluation in the human. The collection of mutations in their entirety potentially provides a medical taxonomy, one based in developmental biology and genetics.

Regulation of protein synthesis by hypoxia via activation of the endoplasmic reticulum kinase PERK and phosphorylation of the translation initiation factor eIF2alpha

Hypoxia profoundly influences tumor development and response to therapy. While progress has been made in identifying individual gene products whose synthesis is altered under hypoxia, little is known about the mechanism by which hypoxia induces a global downregulation of protein synthesis. A critical step in the regulation of protein synthesis in response to stress is the phosphorylation of translation initiation factor eIF2alpha on Ser51, which leads to inhibition of new protein synthesis. Here we report that exposure of human diploid fibroblasts and transformed cells to hypoxia led to phosphorylation of eIF2alpha, a modification that was readily reversed upon reoxygenation. Expression of a transdominant, nonphosphorylatable mutant allele of eIF2alpha attenuated the repression of protein synthesis under hypoxia. The endoplasmic reticulum (ER)-resident eIF2alpha kinase PERK was hyperphosphorylated upon hypoxic stress, and overexpression of wild-type PERK increased the levels of hypoxia-induced phosphorylation of eIF2alpha. Cells stably expressing a dominant-negative PERK allele and mouse embryonic fibroblasts with a homozygous deletion of PERK exhibited attenuated phosphorylation of eIF2alpha and reduced inhibition of protein synthesis in response to hypoxia. PERK(-/-) mouse embryo fibroblasts failed to phosphorylate eIF2alpha and exhibited lower survival after prolonged exposure to hypoxia than did wild-type fibroblasts. These results indicate that adaptation of cells to hypoxic stress requires activation of PERK and phosphorylation of eIF2alpha and suggest that the mechanism of hypoxia-induced translational attenuation may be linked to ER stress and the unfolded-protein response.