Metabolic fuel utilization and pyruvate oxidation during the postnatal period

Edited magnetic resonance spectroscopy in the neonatal brain

J-difference-edited spectroscopy is a valuable approach for the detection of low-concentration metabolites with magnetic resonance spectroscopy (MRS). Currently, few edited MRS studies are performed in neonates due to suboptimal signal-to-noise ratio, relatively long acquisition times, and vulnerability to motion artifacts. Nonetheless, the technique presents an exciting opportunity in pediatric imaging research to study rapid maturational changes of neurotransmitter systems and other metabolic systems in early postnatal life. Studying these metabolic processes is vital to understanding the widespread and rapid structural and functional changes that occur in the first years of life. The overarching goal of this review is to provide an introduction to edited MRS for neonates, including the current state-of-the-art in editing methods and editable metabolites, as well as to review the current literature applying edited MRS to the neonatal brain. Existing challenges and future opportunities, including the lack of age-specific reference data, are also discussed.

Sexual dimorphism in the fetal cardiac response to maternal nutrient restriction

Poor maternal nutrition causes intrauterine growth restriction (IUGR); however, its effects on fetal cardiac development are unclear. We have developed a baboon model of moderate maternal undernutrition, leading to IUGR. We hypothesized that the IUGR affects fetal cardiac structure and metabolism. Six control pregnant baboons ate ad-libitum (CTRL)) or 70% CTRL from 0.16 of gestation (G). Fetuses were euthanized at C-section at 0.9G under general anesthesia. Male but not female IUGR fetuses showed left ventricular fibrosis inversely correlated with birth weight. Expression of extracellular matrix protein TSP-1 was increased (p<0.05) in male IUGR. Expression of cardiac fibrotic markers TGFβ, SMAD3 and ALK-1 were downregulated in male IUGRs with no difference in females. Autophagy was present in male IUGR evidenced by upregulation of ATG7 expression and lipidation LC3B. Global miRNA expression profiling revealed 56 annotated and novel cardiac miRNAs exclusively dysregulated in female IUGR, and 38 cardiac miRNAs were exclusively dysregulated in males (p<0.05). Fifteen (CTRL) and 23 (IUGR) miRNAs, were differentially expressed between males and females (p<0.05) suggesting sexual dimorphism, which can be at least partially explained by differential expression of upstream transcription factors (e.g. HNF4α, and NFκB p50). Lipidomics analysis of fetal cardiac tissue exhibited a net increase in diacylglycerol and plasmalogens and a decrease in triglycerides and phosphatidylcholines. In summary, IUGR resulting from decreased maternal nutrition is associated with sex-dependent dysregulations in cardiac structure, miRNA expression, and lipid metabolism. If these changes persist postnatally, they may program offspring for higher later life cardiac risk.

Neuronal Progenitor Maintenance Requires Lactate Metabolism and PEPCK-M-Directed Cataplerosis

This study investigated the metabolic requirements for neuronal progenitor maintenance in vitro and in vivo by examining the metabolic adaptations that support neuronal progenitors and neural stem cells (NSCs) in their undifferentiated state. We demonstrate that neuronal progenitors are strictly dependent on lactate metabolism, while glucose induces their neuronal differentiation. Lactate signaling is not by itself capable of maintaining the progenitor phenotype. The consequences of lactate metabolism include increased mitochondrial and oxidative metabolism, with a strict reliance on cataplerosis through the mitochondrial phosphoenolpyruvate carboxykinase (PEPCK-M) pathway to support anabolic functions, such as the production of extracellular matrix. In vivo, lactate maintains/induces populations of postnatal neuronal progenitors/NSCs in a PEPCK-M-dependent manner. Taken together, our data demonstrate that, lactate alone or together with other physical/biochemical cues maintain NSCs/progenitors with a metabolic signature that is classically found in tissues with high anabolic capacity.

Reconstruction of metabolic pathways for the cattle genome

Background

Metabolic reconstruction of microbial, plant and animal genomes is a necessary step toward understanding the evolutionary origins of metabolism and species-specific adaptive traits. The aims of this study were to reconstruct conserved metabolic pathways in the cattle genome and to identify metabolic pathways with missing genes and proteins. The MetaCyc database and PathwayTools software suite were chosen for this work because they are widely used and easy to implement.

Results

An amalgamated cattle genome database was created using the NCBI and Ensembl cattle genome databases (based on build 3.1) as data sources. PathwayTools was used to create a cattle-specific pathway genome database, which was followed by comprehensive manual curation for the reconstruction of metabolic pathways. The curated database, CattleCyc 1.0, consists of 217 metabolic pathways. A total of 64 mammalian-specific metabolic pathways were modified from the reference pathways in MetaCyc, and two pathways previously identified but missing from MetaCyc were added. Comparative analysis of metabolic pathways revealed the absence of mammalian genes for 22 metabolic enzymes whose activity was reported in the literature. We also identified six human metabolic protein-coding genes for which the cattle ortholog is missing from the sequence assembly.

Conclusion

CattleCyc is a powerful tool for understanding the biology of ruminants and other cetartiodactyl species. In addition, the approach used to develop CattleCyc provides a framework for the metabolic reconstruction of other newly sequenced mammalian genomes. It is clear that metabolic pathway analysis strongly reflects the quality of the underlying genome annotations. Thus, having well-annotated genomes from many mammalian species hosted in BioCyc will facilitate the comparative analysis of metabolic pathways among different species and a systems approach to comparative physiology.

Metabolic adaptation at birth

After birth, the neonate must make a transition from the assured continuous transplacental supply of glucose to a variable fat-based fuel economy. The normal infant born at term accomplishes this transition through a series of well-coordinated metabolic and hormonal adaptive changes. The patterns of adaptation in the preterm infant and the baby born after intrauterine growth restriction are, however, different to that of a full-term neonate, with the risk for former groups that there will be impaired counter-regulatory ketogenesis. There is much less precise linkage of neonatal insulin secretion to prevailing blood glucose concentrations. These patterns of metabolic adaptation are further influenced by feeding practices.

Severe transient neonatal hyperinsulinism associated with hyperlactataemia in non-asphyxiated infants

Transient hyperinsulinism (HI) occurs in infants born to diabetic mothers, in infants experiencing perinatal asphyxia and in infants with intrauterine growth retardation. The precise mechanism of transient HI in these different aetiologies is not fully understood. Lactic acidosis is commonly seen in neonates as a secondary phenomenon due to hypoxia, hypovolaemia, anaemia and infection. The combination of transient HI and lactic acidosis is rare. We present the clinical and biochemical features of five infants presenting with transient HI associated with hyperlactataemia in the absence of markers of perinatal stress. This combination lasted for 3-4 weeks with complete resolution except in one patient in whom the hyperinsulinism lasted until 6 months before resolution. The precise mechanism of this association is not clear but may be related either to immaturity of the pyruvate dehydrogenase complex or to the accumulation of abnormal intramitochondrial intermediary metabolites. Infants presenting with HI should have a free flowing blood sample drawn for the measurement of plasma lactate levels.

Cerebrospinal fluid lactate and pyruvate concentrations and their ratio in children: age-related reference intervals

BACKGROUND

Lactate (L) and pyruvate (P) concentrations in cerebrospinal fluid (CSF) and the L/P ratio have diagnostic value in numerous primary and acquired disorders affecting the central nervous system, but age-related reference values are not available for children.

METHODS

We analyzed CSF and blood lactate and pyruvate concentrations and their ratio in a 4-year retrospective survey of a children's hospital laboratory database. Reference intervals (10th-90th percentiles) were established from data on 197 hospitalized children. A recent regression modeling method was used to normalize and smooth values against age. The model equation of best fit was calculated for each variable.

RESULTS

Slight age-related variations were shown by the model, with an increase in lactate, a decrease in pyruvate, and a resulting increase in the L/P ratio with increasing age. However, the SD did not vary with age. We defined the upper limit of the reference intervals as the 90th percentiles, which from birth to 186 months of age varied continuously from 1.78 to 1.88 mmol/L (6%), 148 to 139 micro mol/L (6%), and 16.9 to 19.2 (14%) for lactate, pyruvate, and the L/P ratio, respectively. At a threshold of 2 (in Z-score units), the sensitivity for a subgroup of inborn errors of metabolism (respiratory chain disorders) was 73%, 42%, and 31% for lactate, pyruvate, and the L/P ratio, respectively.

CONCLUSIONS

In children, CSF lactate and pyruvate concentrations and their ratio appear to vary slightly with age. Average 90th percentile values of 1.8 mmol/L, 147 micro mol/L, and 17, respectively, could be used in infants up to 24 months of age. In older children, age-adjusted reference intervals should be used, especially when values are close to the 90th percentile.

The Oxygen Dependency of Cerebral Oxidative Metabolism in the Newborn Piglet Studied with 31P NMRS and NIRS

Mean cerebral saturation and changes in the oxidation state of the CuA centre of cytochrome oxidase were measured by near infra-red spectroscopy simultaneously with phosphorous metabolites and intracellular pH measured using 31P NMR spectroscopy during transient anoxia (inspired oxygen fraction = 0.0 for 105 seconds) in the newborn piglet brain. By collecting high quality 31P spectra every 10 seconds, it was possible to resolve the delay between the onset of anoxia and the fall in PCr and to show that the CuA centre of cytochrome oxidase reduced simultaneously with the fall in PCr. From these observations it is concluded that, at normoxia, oxygen tension at the mitochondrial level is substantially above a critical value at which oxidative metabolism becomes oxygen dependent.

Astrocyte-synthesized oleic acid behaves as a neurotrophic factor for neurons

Unlike in the adult brain, the newborn brain specifically takes up serum albumin during the postnatal period, coinciding with the stage of maximal brain development. Here we shall summarize our knowledge about the role played by albumin in brain development. The role of this protein in brain development is intimately related to its ability to carry fatty acids. Thus, albumin stimulates oleic acid synthesis by astrocytes from the main metabolic substrates available during brain development. Astrocytes internalize albumin in vesicle-like structures by receptor-mediated endocytosis, which is followed by transcytosis, including passage through the endoplasmic reticulum (ER). The presence of albumin in the ER activates the sterol regulatory element-binding protein-1 (SREBP-1) and increases stearoyl-CoA 9-desaturase (SCD) mRNA, the key enzyme in oleic acid synthesis. Oleic acid released by astrocytes is used by neurons for the synthesis of phospholipids and is specifically incorporated into growth cones. In addition, oleic acid promotes axonal growth, neuronal clustering, and the expression of the axonal growth associated protein, GAP-43. All of these observations indicate neuronal differentiation. The effect of oleic acid on GAP-43 synthesis is brought about by the activation of protein kinase C. The expression of GAP-43 is significantly increased by the presence of albumin in neurons co-cultured with astrocytes, indicating that neuronal differentiation takes place by the presence of oleic acid synthesized and released by astrocytes in situ. In conclusion, during brain development the presence of albumin could play an important role by triggering the synthesis and release of oleic acid by astrocytes, thereby inducing neuronal differentiation.

Estimation of gluconeogenesis in newborn infants

The rate of glucose turnover (R(a)) and gluconeogenesis (GNG) via pyruvate were quantified in seven full-term healthy babies between 24 and 48 h after birth and in twelve low-birth-weight infants on days 3 and 4 by use of [(13)C(6)]glucose and (2)H(2)O. The preterm babies were receiving parenteral alimentation of either glucose or glucose plus amino acid with or without lipids. The contribution of GNG to glucose production was measured by the appearance of (2)H on C-6 of glucose. Glucose R(a) in full-term babies was 30 +/- 1.7 (SD) micromol. kg(-1). min(-1). GNG via pyruvate contributed approximately 31% to glucose R(a). In preterm babies, the contribution of GNG to endogenous glucose R(a) was variable (range 6-60%). The highest contribution was in infants receiving low rates of exogenous glucose infusion. In an additional group of infants of normal and diabetic mothers, lactate turnover and its incorporation into glucose were measured within 4-24 h of birth by use of [(13)C(3)]lactate tracer. The rate of lactate turnover was 38 micromol. kg(-1). min(-1), and lactate C, not corrected for loss of tracer in the tricarboxylic acid cycle, contributed approximately 18% to glucose C. Lactate and glucose kinetics were similar in infants that were small for their gestational age and in normal infants or infants of diabetic mothers. These data show that gluconeogenesis is evident soon after birth in the newborn infant and that, even after a brief fast (5 h), GNG via pyruvate makes a significant contribution to glucose production in healthy full-term infants. These data may have important implications for the nutritional support of the healthy and sick newborn infant.

Age-dependent variations in white adipose tissue glycerol and lactate production after surgery measured by microdialysis in neonates and children

In previous studies, we observed that lactate concentrations in interstitial white adipose tissue are higher in small infants than in adults. Moreover, no lipolysis following catecholamine challenge has been reported in neonates and small infants. Our aim was to determine with microdialysis whether the above mentioned age-dependent changes could be detected in situ after surgery. A microdialysis catheter was introduced into the abdominal subcutaneous tissue in 13 neonates and 12 children undergoing surgery. Interstitial concentrations of glucose, lactate and glycerol were measured hourly during the first 20 postoperative hours. The concentrations of lactate in interstitial white adipose tissue were consistently higher in neonates compared to older children, with a significant difference during the 9-18 h postoperative period (P < 0.05). A significant difference in the lactate:glucose ratio was observed at 1-2, 8-10, 15 and 18 h postoperatively (P < 0.05). No significant differences were observed between the two groups with respect to glycerol and glucose concentrations. Interstitial lactate concentrations in white adipose tissue were higher in neonates compared with children in the early postoperative period. No age-dependent difference in postoperative lipolysis, measured as interstitial glycerol concentrations, was observed. Thus, an age-dependent difference in interstitial lactate production, but not lipolysis, was detected in the early postoperative period.

Regulation of kynurenic acid levels in the developing rat brain

Several brain-specific mechanisms control the formation of the endogenous excitatory amino acid receptor antagonist kynurenic acid (KYNA) in the adult rat brain. Two of these, dopaminergic neurotransmission and cellular energy metabolism, were examined in the brain of immature (postnatal day 7) rats. The results indicate that during the early postnatal period cerebral KYNA synthesis is exceptionally amenable to modulation by dopaminergic mechanisms but rather insensitive to fluctuations in cellular energy status. These findings may be of relevance for the role of KYNA in the function and dysfunction of the developing brain.

Cardiovascular function and brain metabolites in normal weight and intrauterine growth restricted newborn piglets--effect of mild hypoxia

In order to clarify the influence of intrauterine growth restriction on systemic hemodynamics, catecholamine response, and regional distribution of brain energy metabolites per se and during mild hypoxic episodes a study was performed in thirty newborns with a well-characterized state of intrauterine and intra-natal development. Thirty animals were divided into fifteen normal weight piglets (NW) and fifteen intrauterine growth restricted (IUGR) piglets according to their birth weight. Category "NW" covered animals with a birth weight of > 40th percentile; IUGR category covered animals with a birth weight of > 5th and < 10th percentiles. Animals were anesthetized with halothane in 70% nitrous oxide and 30% oxygen and after immobilization artificially ventilated. The acid-base balance and blood gas values at baseline conditions were similar within the different groups investigated and consistent with other data obtained from anesthetized and artificially ventilated newborn piglets. Mild hypoxic hypoxia which was induced by lowering the FiO2 from 0.35 to 0.15 resulted in reduced arterial pO2 (NW: from 115 +/- 37 mmHg to 39 +/- 7 mmHg; IUGR: from 117 +/- 23 mmHg to 39 +/- 3 mmHg; p < 0.05), but arterial pH and pCO2 remained unchanged. Under baseline conditions arterial blood pressure, cardiac output, and myocardial contractility, expressed as dp/dt(max) and plasma catecholamine values were similar in all groups studied. Heart rate was slightly increased in IUGR (p < 0.05). Mild hypoxia led to a strong increase of myocardial contractility in NW as well as IUGR piglets to 2.4 and 2.7 fold and remained increased during recovery (p < 0.05). Moreover, total peripheral resistance was enhanced at the end of recovery period in IUGR animals (p < 0.05). There was a significant increase of epinephrine (E) in NW animals in comparison to sham-operated animals (p < 0.05). Interestingly, during reoxygenation the further increase in E and norepinephrine (NE) levels were enhanced in the animals which suffered from mild hypoxia (p < 0.05). Regional distribution of brain tissue metabolites was partly affected by intrauterine growth restriction. In particular, brain tissue glucose content was strongly reduced by 65 to 72 per cent in all brain regions investigated. Mild hypoxia led to an increase of about 30 percent in NW animals (p < 0.05). In IUGR piglets the percentage increase of brain glucose content was on an average more pronounced but with considerably higher variance. Also, a strong increase of brain lactate content appeared here (p < 0.05). In contrast, brain tissue ATP was quite similar in all groups studied, but brain creatine phosphate was significantly reduced in some forebrain structures of IUGR piglets after mild hypoxia (figure 2, p < 0.05). In summary, this investigation provides information on cardiovascular functions and brain metabolites of normal weight and naturally occurring growth restricted newborn piglets. Mild hypoxemia was well-tolerated from both animal groups. It is suggested that lactate may play a significant role as a source for brain energy production in the newborn IUGR piglets.

Metabolic intermediates in lactic acidosis: compounds, samples and interpretation

A number of acquired conditions including infections, severe catabolic states, tissue anoxia, severe dehydration and poisoning can give rise to hyperlactacidaemia. All these causes should be ruled out before considering inborn errors of metabolism. Carefully collected samples are necessary if artefacts that result in spuriously increased lactate/pyruvate (L/P) and 3-hydroxybutyrate/acetoacetate (B/A) ratios are to be avoided. When properly performed, 24-h studies of L/P and B/A ratios provide a useful tool in making a diagnosis. A few metabolic profiles when present are specific or highly suggestive of a given disorder. When the L/P ratio is normal or low, pyruvate dehydrogenase (PDH) deficiency is highly probable whatever the lactate concentration, which is often only moderately elevated after meal, may be. When the L/P ratio is very high in association with post-prandial hyperketonaemia and in contrast to a normal or low B/A ratio, pyruvate carboxylase (PC) deficiency and alpha-ketoglutarate dehydrogenase (KGDH) deficiency are the most likely diagnoses. The distinction between the two disorders relies upon amino acid and organic acid profiles (glutamate and alpha-ketoglutarate accumulations in KGDH deficiency and hyperammonaemia and hypercitrullinaemia in PC deficiency). When both L/P and B/A ratios are elevated and associated with significant post-prandial hyperketonaemia, respiratory-chain disorders should first be suspected. All other profiles, especially a high L/P ratio without hyperketonaemia, are compatible with respiratory-chain disorders but are not specific; all acquired anoxic conditions should also be ruled out. Clearly, the clinical utility of these profiles needs to be interpreted cautiously in very ill patients in relation to the cardiocirculatory condition and to therapy. Finally, a normal profile, even after stress and loading, does not rule out an inborn error of lactate/pyruvate oxidation.