Early-life Hypoxia Research Articles

Development of Atlantic Salmon (Salmo salar L.) Under Hypoxic Conditions Induced Sustained Changes in Expression of Immune Genes and Reduced Resistance to Moritella viscosa

Atlantic salmon is characterized with high sensitivity to low dissolved oxygen (DO) levels. Hypoxia can affect diverse biological processes with consequences that can be manifested immediately or with delay. Effects of hypoxia on the immune system and the resistance to a bacterial pathogen were investigated. Two groups were reared at, respectively, normal (NO, 80–100%) and low (LO, 60%) levels of DO over 10 months after which both groups were reared at NO. Smoltification was initiated after 13 months by a winter signal for 6 weeks, followed by constant light for 6 weeks. Samples were collected at the start and end of the constant light period. Expression of 92 immune and stress genes was analyzed in the gill, head kidney, and spleen using a Biomark HD. Most of differentially expressed genes showed higher levels in LO fish compared to NO fish; many immune genes were downregulated during smoltification and these changes were stronger in NO fish. A notable exception was pro-inflammatory genes upregulated in gill of NO fish. Further, salmon were challenged with Moritella viscosa, the causative agent of winter ulcer. Mortality was registered from 5 days post infection (dpi) to the end of trial at 36 dpi. Survival was consistently higher in NO than LO fish, reaching a maximum difference of 18% at 21–23 dpi that reduced to 10% at the end. Analyses with a genome-wide microarray at 36 dpi showed strong responses to the pathogen in gill and spleen. Notable features were the stimulation of eicosanoid metabolism, suggesting an important role of lipid mediators of inflammation, and the downregulation of chemokines. Many immune effectors were activated, including multiple lectins and acute phase proteins, enzymes producing free radicals, and matrix metalloproteinases. The transcriptomic changes induced with a bacterial challenge were similar in NO and LO. After the challenge, interferons a and g and panel of genes of innate antiviral immunity showed higher expression in LO, especially in the gill. The results from the present study suggest that chronic hypoxia in early life stimulated immune genes and attenuated their downregulation associated with smoltification. However, these changes did not improve protection against a bacterial pathogen of major concern in salmon aquaculture.

Antenatal Hypoxia Affects Pulmonary Artery Contractile Functions via Downregulating L-type Ca2+ Channels Subunit Alpha1 C in Adult Male Offspring.

BackgroundAntenatal intrauterine fetal hypoxia is a common pregnancy complication that has profound adverse effects on an individual's vascular health later in life. Pulmonary arteries are sensitive to hypoxia, but adverse effects of antenatal hypoxia on pulmonary vasoreactivities in the offspring remain unknown. This study aimed to determine the effects and related mechanisms of antenatal hypoxia on pulmonary artery functions in adult male offspring.Methods and ResultsPregnant Sprague‐Dawley rats were housed in a normoxic or hypoxic (10.5% O2) chamber from gestation days 10 to 20. Male offspring were euthanized at 16 weeks old (adult offspring). Pulmonary arteries were collected for vascular function, electrophysiology, target gene expression, and promoter methylation studies. In pulmonary artery rings, contractions to serotonin hydrochloride, angiotensin II, or phenylephrine were reduced in the antenatal hypoxic offspring, which resulted from inactivated L‐type Ca2+ channels. In pulmonary artery smooth muscle cells, the basal whole‐cell Ca2+ currents, as well as vasoconstrictor‐induced Ca2+ transients were significantly reduced in antenatal hypoxic offspring. In addition, increased promoter methylations within L‐type Ca2+ channel subunit alpha1 C were compatible with its reduced expressions.ConclusionsThis study indicated that antenatal hypoxia programmed long‐lasting vascular hypocontractility in the male offspring that is linked to decreases of L‐type Ca2+ channel subunit alpha1 C in the pulmonary arteries. Antenatal hypoxia resulted in pulmonary artery adverse outcomes in postnatal offspring, was strongly associated with reprogrammed L‐type Ca2+ channel subunit alpha1 C expression via a DNA methylation‐mediated epigenetic mechanism, advancing understanding toward the effect of antenatal hypoxia in early life on long‐term vascular health.

Perinatal Programming - New Insights into the Origins of Neurodegenerative Disorders.

Neurodegenerative diseases, including Alzheimer’s disease (AD), constitute a problem of great significance in aging societies. The origin and underlying causes have not been established yet. The lately discovered phenomenon of foetal programming explains the connection between perinatal episodes and the development of chronic diseases in the later stages of life. The aim of this review is to show that altered foetal programming may connect perinatal asphyxia and the development of AD later in life. It is believed that most cases of AD arise through interactions between genetic and environmental factors. Among all the exposures, transient brain hypoxia has been extensively studied recently. The role of hypoxia in the early developmental period as a trigger for developing AD in adults through altered programming of genes expression cannot be excluded. It is possible that severe hypoxia in early life can cause biochemical changes, including long-lasting alterations in gene expression, leading to neurodegenerative disorders in adults. The prevention of neurodegenerative diseases should focus on events from the earliest periods of life. Better recognition of underlying mechanisms is necessary for further investigations and the development of novel therapeutic methods.

Early-life hypoxia alters adult physiology and reduces stress resistance and lifespan in Drosophila.

In many animals, short-term fluctuations in environmental conditions in early life often exert long-term effects on adult physiology. In Drosophila, one ecologically relevant environmental variable is hypoxia. Drosophila larvae live on rotting, fermenting food rich in microorganisms, an environment characterized by low ambient oxygen. They have therefore evolved to tolerate hypoxia. Although the acute effects of hypoxia in larvae have been well studied, whether early-life hypoxia affects adult physiology and fitness is less clear. Here, we show that Drosophila exposed to hypoxia during their larval period subsequently show reduced starvation stress resistance and shorter lifespan as adults, with these effects being stronger in males. We find that these effects are associated with reduced whole-body insulin signaling but elevated TOR kinase activity, a manipulation known to reduce lifespan. We also identify a sexually dimorphic effect of larval hypoxia on adult nutrient storage and mobilization. Thus, we find that males, but not females, show elevated levels of lipids and glycogen. Moreover, we see that both males and females exposed to hypoxia as larvae show defective lipid mobilization upon starvation stress as adults. These data demonstrate how early-life hypoxia can exert persistent, sexually dimorphic, long-term effects on Drosophila adult physiology and lifespan.

Impact of uteroplacental insufficiency on ovarian follicular pool in the rat

BackgroundA low oxygen supply to the fetus causes intrauterine growth restriction and can affect gonadal development of the offspring, having a potential impact on fertility. We investigated histology and gene expression in the postnatal rat ovary after fetal hypoxia induced by uterine artery ligation.MethodsSprague-Dawley rats underwent uterine artery ligation at day 19 of gestation. Offspring were sacrificed at 5, 20 and 40 days post-partum. Follicles were counted and classified in hematoxylin-eosin stained sections. Gene expression of 90 genes was analyzed by TaqMan® Low Density Array.ResultsA significantly lower number of total and primordial follicles was detected in 20 days post-partum intrauterine growth restricted animals. Follicle density was not different at 40 days post-partum, suggesting that compensatory mechanisms occurred during the pre-pubertal window. Uterine artery ligation modified the expression of 24 genes involved in different cellular functions, among which proliferation, apoptosis and metabolism.ConclusionOvarian follicle pool was affected by fetal hypoxia in early life, but this effect did not persist in puberty. Genes involved in cellular processes were affected at all ages, potentially implying long-term genetic alterations. Further analyses are needed to elucidate later effects of fetal hypoxia on ovarian function and fertility.

Hypoxia during incubation does not affect aerobic performance or haematology of Atlantic salmon (Salmo salar) when re-exposed in later life.

Hypoxia in aquatic ecosystems is becoming increasingly prevalent, potentially reducing fish performance and survival by limiting the oxygen available for aerobic activities. Hypoxia is a challenge for conserving and managing fish populations and demands a better understanding of the short- and long-term impacts of hypoxic environments on fish performance. Fish acclimate to hypoxia via a variety of short- and long-term physiological modifications in an attempt to maintain aerobic performance. In particular, hypoxia exposure during early development may result in enduring cardio-respiratory modifications that affect future hypoxia acclimation capacity, yet this possibility remains poorly investigated. We incubated Atlantic salmon (Salmo salar) in normoxia (~100% dissolved oxygen [DO, as percent air saturation]), moderate hypoxia (~63% DO) or cyclical hypoxia (100–25% DO daily) from fertilization until 113 days post-fertilization prior to rearing all groups in normoxia for a further 8 months. At ~11 months of age, subsets of each group were acclimated to hypoxia (50% DO) for up to 44 days prior to haematology, aerobic metabolic rate and hypoxia tolerance measurements. Hypoxia exposure during incubation (fertilization to 113 days post-fertilization) did not affect the haematology, aerobic performance or hypoxia tolerance of juvenile salmon in later life. Juveniles acclimated to hypoxia increased maximum aerobic metabolic rate and aerobic scope by ~23 and ~52%, respectively, when measured at 50% DO but not at 100% DO. Hypoxia-incubated juveniles also increased haematocrit and haemoglobin concentration but did not affect acute hypoxia tolerance (critical oxygen level and DO at LOE). Thus, while Atlantic salmon possess a considerable capacity to physiologically acclimate to hypoxia by improving aerobic performance in low oxygen conditions, we found no evidence that this capacity is influenced by early-life hypoxia exposure.

Impaired Organization of GABAergic Neurons Following Prenatal Hypoxia

Several conditions related to the intrauterine environment are associated with neuropsychiatric conditions in later life. In humans, approximately 2% of infants are exposed to perinatal hypoxia–ischemia or prolonged anoxic insult, a condition to which very low birth weight preterm infants exhibit the highest susceptibility. Analyses of postmortem tissue link some presentations of these conditions to changes in GABA pathway functionality in the brains of affected subjects. Using animal models of early-life hypoxia–ischemia, losses of particular interneuron populations were reported. We hypothesize that the origin of GABAergic cell loss is in the mispositioning of neurons during the formation of the cerebral cortex. Here we report that in C57 black mice exposed to hypoxic conditions (9% O2; 3% CO2), 22–26% of cell loss was detected in the cortical plate as early as four days after the hypoxic event. Moreover, the surviving cells failed to populate the proper layers in the developing cortex. Differential sensitivities were observed in neurons that originated from different germinal zones. A significant effect of GABAergic cell location along the anterior–posterior and medio-lateral axes on neuron sensitivity to hypoxia was detected. Finally, changes in guidance molecules in the developing cortex, including increases in hypoxia-inducible factor 1-alpha levels and intracellular distribution, decreases in reelin levels in the cortical plate and altered organization of radial glia, were observed. These changes in the molecular landscape of the immediate environment of the immature neurons may contribute to the observed outcomes in neuronal migration to, and establishment in, the correct cortical layers. We suggest that the interneuron loss may be related to these early events.

Divergent effects of levetiracetam and tiagabine against spontaneous seizures in adult rats following neonatal hypoxia

Animal models are valuable tools for screening novel therapies for patients who suffer from epilepsy. However, a wide array of models are necessary to cover the diversity of human epilepsies. In humans, neonatal hypoxia (or hypoxia-ischemia) is one of the most common causes of epilepsy early in life. Hypoxia-induced seizures (HS) during the neonatal period can also lead to spontaneous seizures in adulthood. This phenomenon, i.e., early-life hypoxia leading to adult epilepsy − is also seen in experimental models, including rats. However, it is not known which anti-seizure medications are most effective at managing adult epilepsy resulting from neonatal HS. Here, we examined the efficacy of three anti-seizure medications against spontaneous seizures in adult rats with a history of neonatal HS: (1) phenobarbital (PHB), the oldest epilepsy medicine still in use today; (2) levetiracetam (LEV); and (3) tiagabine (TGB). Both LEV and TGB are relatively new anticonvulsant drugs that are ineffective in traditional seizure models, but strikingly effective in other models. We found that PHB and LEV decreased seizures in adult rats with a history of HS, whereas TGB exacerbated seizures. These divergent drug effects indicate that the HS model may be useful for differentiating the clinical efficacy of putative epilepsy therapies.

Metabolic response to hypoxia in European sea bass (Dicentrarchus labrax) displays developmental plasticity.

Several physiological functions in fish are shaped by environmental stimuli received during early life. In particular, early-life hypoxia has been reported to have long-lasting effects on fish metabolism, with potential consequences for fish life history traits. In the present study, we examine whether the synergistic stressors hypoxia (40% and 100% air saturation) and temperature (15° and 20°C), encountered during early life, could condition later metabolic response in European sea bass (Dicentrarchus labrax) juveniles. Growth rate and metabolic parameters related to carbohydrate and lipid metabolism in the liver were investigated at the juvenile stage under normoxic and chronic hypoxic conditions. Juvenile growth rates were significantly lower (p<1×10-6) under hypoxic conditions and were not improved by prior early-life exposure to hypoxia. Growth was 1.3 times higher (p<5×10-3) in juveniles reared at 15°C during the larval stage than those reared at 20°C, suggesting that compensatory growth had occurred. Early-life exposure to hypoxia induced higher (p<2×10-6) glycogen stores in juveniles even though there was no apparent regulation of their carbohydrate metabolism. In the liver of juveniles exposed to chronic hypoxia, lower glycogen content combined with stimulation of phosphoenolpyruvate carboxykinase gene expression and higher lactate concentration indicated a stimulation of the anaerobic glycolytic pathway. Furthermore, hypoxia only induced lower (p<1×10-3) lipid content in the liver of juveniles that had experienced 15°C at the larval stage. The present study provides evidence that environmental conditions experienced during early life shape the metabolic traits of D. labrax with potential consequences for juvenile physiological performance.

An early life hypoxia event has a long-term impact on protein digestion and growth in juvenile European sea bass.

Ocean warming, eutrophication and the consequent decrease in oxygen lead to smaller average fish size. Although such responses are well known in an evolutionary context, involving multiple generations, this appears to be incompatible with current rapid environmental change. Instead, phenotypic plasticity could provide a means for marine fish to cope with rapid environmental changes. However, little is known about the mechanisms underlying plastic responses to environmental conditions that favour small phenotypes. Our aim was to investigate how and why European sea bass that had experienced a short episode of moderate hypoxia during their larval stage subsequently exhibited a growth depression at the juvenile stage compared with the control group. We examined whether energy was used to cover higher costs for maintenance, digestion or activity metabolisms, as a result of differing metabolic rate. The lower growth was not a consequence of lower food intake. We measured several respirometry parameters and we only found a higher specific dynamic action (SDA) duration and lower SDA amplitude in a fish phenotype with lower growth; this phenotype was also associated with a lower protein digestive capacity in the intestine. Our results contribute to the understanding of the observed decrease in growth in response to climate change. They demonstrate that the reduced growth of juvenile fishes as a consequence of an early life hypoxia event was not due to a change of fish aerobic scope but to a specific change in the efficiency of protein digestive functions. The question remains of whether this effect is epigenetic and could be reversible in the offspring.

Why Are High Altitude Natives So Strong at High Altitude? Nature vs. Nurture: Genetic Factors vs. Growth and Development.

Among high-altitude natives there is evidence of a general hypoxia tolerance leading to enhanced performance and/or increased capacity in several important domains. These domains likely include an enhanced physical work capacity, an enhanced reproductive capacity, and an ability to resist several common pathologies of chronic high-altitude exposure. The "strength" of the high-altitude native in this regard may have both a developmental and a genetic basis, although there is better evidence for the former (developmental effects) than for the latter. For example, early-life hypoxia exposure clearly results in lung growth and remodeling leading to an increased O2 diffusing capacity in adulthood. Genetic research has yet to reveal a population genetic basis for enhanced capacity in high-altitude natives, but several traits are clearly under genetic control in Andean and Tibetan populations e.g., resting and exercise arterial O2 saturation (SaO2). This chapter reviews the effects of nature and nurture on traits that are relevant to the process of gas exchange, including pulmonary volumes and diffusion capacity, the maximal oxygen consumption (VO2max), the SaO2, and the alveolar-arterial oxygen partial pressure difference (A-aDO2) during exercise.

Hypoxia in early life is associated with lasting changes in left ventricular structure and function at maturity in the rat

BackgroundThere is a growing population of adults with repaired cyanotic congenital heart disease. These patients have increased risk of impaired cardiac health and premature death. We hypothesized that hypoxia in early life before surgical intervention causes lasting changes in left ventricular structure and function with physiological implications in later life. MethodsSprague–Dawley rats reared initially hypoxic conditions (FiO2=0.12) for days 1–10 of life were compared to rats reared only in ambient air. Cellular morphology and viability were compared among LV cardiomyocytes and histological analyses were performed on LV myocardium and arterioles. Intracellular calcium transients and cell shortening were measured in freshly-isolated cardiomyocytes, and mitochondrial hexokinase 2 (HK2) expression and activity were determined. Transthoracic echocardiography was used to assess LV function in anesthetized animals. ResultsCardiomyocytes from adult animals following hypoxia in early life had greater cellular volumes but significantly reduced viability. Echocardiographic analyses revealed LV hypertrophy and diastolic dysfunction, and alterations in cardiomyocyte calcium transients and cell shortening suggested impaired diastolic calcium reuptake. Histological analyses revealed significantly greater intima–media thickness and decreased lumen area in LV arterioles from hypoxic animals. Alterations in mitochondrial HK2 protein distribution and activity were also observed which may contribute to cardiomyocyte fragility. ConclusionsHypoxia in early life causes lasting changes in left ventricular structure and function that may negatively influence myocardial and vascular responses to physiological stress in later life. These data have implications for the growing population of adults with repaired or palliated cyanotic congenital heart disease.

Systemic Arterial Pressure at Maturity in Rats Following Chronic Hypoxia in Early Life

The effect of prolonged hypoxemia in early life on systemic arterial blood pressure at maturity was assessed in Sprague-Dawley rats. Animals hypoxic in early life (12 males, 10 females) were raised in hypoxia (FiO₂ = 0.12) for the first 10 days of life and subsequently raised in normoxia, along with age-matched controls (11 males, 9 females). At 2 months of age, arterial blood pressure was recorded intravascularly using telemetry in awake and unrestrained animals over two 12-h night-time (active) and daytime (resting) periods. Aortic pulse wave velocity was assessed in six additional hypoxic pretreated and five control anesthetized 2-month-old male rats. Systolic, mean, and pulse pressures were significantly greater in the hypoxic pretreated group compared to the control group during resting and active periods in both sexes (P ≤ 0.05). Diastolic pressure and heart rate did not differ between the two groups. Hypoxic pretreated males displayed significantly increased blood pressure variability during the resting period. Aortic pulse wave velocity was also found to be elevated in the hypoxic pretreated rats. Prolonged hypoxic stress in early life in the rat is associated with increased systolic arterial pressure at maturity very likely due to decreased arterial compliance. These findings suggest that a nutrient-independent, postnatal stress may lead to long-lasting vascular alterations predisposing to increased arterial pressure at maturity. This raises the possibility that adult survivors of congenital cyanotic cardiac disease may be at risk for secondary cardiovascular morbidity unrelated to surgical repair or residual cardiac defects.

Thyroarytenoid muscle activity during hypoxia in awake lambs

It is generally accepted that hypoxia in early life results in active laryngeal braking of expiratory airflow via the recruitment of glottic adductor muscles. We examined the electromyogram expiratory activity of the thyroarytenoid muscle in seven 11- to 18-day-old awake nonsedated lambs exposed to an inspired O2 fraction of 0.08 for 18 min. The lambs breathed through a face mask and a pneumotachograph. During baseline prehypoxic breathing, the thyroarytenoid muscle was largely inactive in each awake lamb. Unexpectedly, no recruitment of the thyroarytenoid muscle was recorded during hypoxia in any of the seven lambs; simultaneous examination of the flow-volume curves revealed an absence of expiratory airflow braking. Also unexpectedly, marked expiratory activity of the thyroarytenoid muscle was recorded, with each expiration occurring within less than 10 s after the return to room air. The resulting delay of expiration was apparent in the flow-volume loops. Thus, in awake 11- to 18-day-old lambs, 1) active expiratory glottic adduction is absent during hypoxia and 2) a return from hypoxia to room air results in prolonged expiration as well as active glottic adduction that controls end-expiratory lung volume.