Distribution of Glutamate Transporters in the Human Placenta

Regulation of placental amino acid transport in health and disease

Abnormal fetal growth, i.e., intrauterine growth restriction (IUGR) or fetal growth restriction (FGR) and fetal overgrowth, is associated with increased perinatal morbidity and mortality and is strongly linked to the development of metabolic and cardiovascular disease in childhood and later in life. Emerging evidence suggests that changes in placental amino acid transport may contribute to abnormal fetal growth. This review is focused on amino acid transport in the human placenta, however, relevant animal models will be discussed to add mechanistic insights. At least 25 distinct amino acid transporters with different characteristics and substrate preferences have been identified in the human placenta. Of these, System A, transporting neutral nonessential amino acids, and System L, mediating the transport of essential amino acids, have been studied in some detail. Importantly, decreased placental Systems A and L transporter activity is strongly associated with IUGR and increased placental activity of these two amino acid transporters has been linked to fetal overgrowth in human pregnancy. An array of factors in the maternal circulation, including insulin, IGF-1, and adiponectin, and placental signaling pathways such as mTOR, have been identified as key regulators of placental Systems A and L. Studies using trophoblast-specific gene targeting in mice have provided compelling evidence that changes in placental Systems A and L are mechanistically linked to altered fetal growth. It is possible that targeting specific placental amino acid transporters or their upstream regulators represents a novel intervention to alleviate the short- and long-term consequences of abnormal fetal growth in the future.

Structural bioinformatics studies of glutamate transporters and their AlphaFold2 predicted water-soluble QTY variants and uncovering the natural mutations of L->Q, I->T, F->Y and Q->L, T->I and Y->F

Glutamate transporters play key roles in nervous physiology by modulating excitatory neurotransmitter levels, when malfunctioning, involving in a wide range of neurological and physiological disorders. However, integral transmembrane proteins including the glutamate transporters remain notoriously difficult to study, due to their localization within the cell membrane. Here we present the structural bioinformatics studies of glutamate transporters and their water-soluble variants generated through QTY-code, a protein design strategy based on systematic amino acid substitutions. These include 2 structures determined by X-ray crystallography, cryo-EM, and 6 predicted by AlphaFold2, and their predicted water-soluble QTY variants. In the native structures of glutamate transporters, transmembrane helices contain hydrophobic amino acids such as leucine (L), isoleucine (I), and phenylalanine (F). To design water-soluble variants, these hydrophobic amino acids are systematically replaced by hydrophilic amino acids, namely glutamine (Q), threonine (T) and tyrosine (Y). The QTY variants exhibited water-solubility, with four having identical isoelectric focusing points (pI) and the other four having very similar pI. We present the superposed structures of the native glutamate transporters and their water-soluble QTY variants. The superposed structures displayed remarkable similarity with RMSD 0.528Å-2.456Å, despite significant protein transmembrane sequence differences (41.1%->53.8%). Additionally, we examined the differences of hydrophobicity patches between the native glutamate transporters and their QTY variants. Upon closer inspection, we discovered multiple natural variations of L->Q, I->T, F->Y and Q->L, T->I, Y->F in these transporters. Some of these natural variations were benign and the remaining were reported in specific neurological disorders. We further investigated the characteristics of hydrophobic to hydrophilic substitutions in glutamate transporters, utilizing variant analysis and evolutionary profiling. Our structural bioinformatics studies not only provided insight into the differences between the hydrophobic helices and hydrophilic helices in the glutamate transporters, but they are also expected to stimulate further study of other water-soluble transmembrane proteins.

Abundance of proteins and genes associated with nutrient signaling, protein turnover, and transport of amino acids and glucose in fetuses from lactating Holstein cows

Availability of nutrients in maternal circulation and abundance of nutrient transporters, metabolic enzymes, and nutrient-responsive proteins in fetal tissues coordinate growth. To begin characterizing these mechanisms, we evaluated the abundance of nutrient signaling genes and proteins in bovine fetal tissues. Liver, entire intestine, and semitendinosus muscle were harvested from fetuses (4 female, 2 male) collected at slaughter from 6 clinically-healthy multiparous Holstein dairy cows (167 ± 7 days in milk, 37 ± 6 kg milk/d, 100 ± 3 d gestation). Data were analyzed using PROC MIXED in SAS 9.4. Among proteins measured, abundance of the amino acid (AA) utilization and insulin signaling proteins p-AKT and p-mTOR was greater (P < 0.01) in liver and intestine. The abundance of p-EEF2 (translation elongation) and SLC2A4 (glucose uptake) was greater (P < 0.05) in liver relative to intestine and muscle suggesting this organ has a greater capacity for anabolic processes. In contrast, among mTOR signaling genes, the abundance of IRS1 was greatest (P < 0.01) in muscle and lowest in the intestine, whereas, abundance of AKT1 and mTOR was greater (P < 0.01) in intestine and muscle than liver. Abundance of the protein degradation-related genes UBA1, UBE2G1, and TRIM63 was greater (P < 0.01) in muscle than intestine and liver. Among nutrient transporters, abundance of glucose transporters SLC5A1 and SLC2A2 was greatest (P < 0.01) in the intestine than liver and muscle. Several AA transporters had greater (P < 0.01) abundance in the intestine or liver compared with muscle. Overall, these molecular analyses highlighted important biological differences on various aspects of metabolism in fetal tissues.

Prenatal metabolomic profiles mediate the effect of maternal obesity on early childhood growth trajectories and obesity risk: the Conditions Affecting Neurocognitive Development and Learning in Early Childhood (CANDLE) Study

BACKGROUND

Maternal prepregnancy obesity is an important risk factor for offspring obesity, which may partially operate through prenatal programming mechanisms.

OBJECTIVES

The study aimed to systematically identify prenatal metabolomic profiles mediating the intergenerational transmission of obesity.

METHODS

We included 450 African-American mother-child pairs from the Conditions Affecting Neurocognitive Development and Learning in Early Childhood (CANDLE) Study pregnancy cohort. LC-MS was used to conduct metabolomic profiling on maternal plasma samples of the second trimester. The childhood growth outcomes of interest included BMI trajectories from birth to age 4 y (rising-high-, moderate-, and low-BMI trajectories) as well as overweight/obesity (OWO) risk at age 4 y. Mediation analysis was conducted to identify metabolite mediators linking maternal OWO and childhood growth outcomes. The potential causal effects of maternal OWO on metabolite mediators were examined using the Mendelian randomization (MR) method.

RESULTS

Among the 880 metabolites detected in the maternal plasma during pregnancy, 14 and 11 metabolites significantly mediated the effects of maternal prepregnancy OWO on childhood BMI trajectories and the OWO risk at age 4 y, respectively, and 5 mediated both outcomes. The MR analysis suggested 6 of the 20 prenatal metabolite mediators might be causally influenced by maternal prepregnancy OWO, most of which are from the pathways related to the metabolism of amino acids (hydroxyasparagine, glutamate, and homocitrulline), sterols (campesterol), and nucleotides (N2,N2-dimethylguanosine).

CONCLUSIONS

Our study provides further evidence that prenatal metabolomic profiles might mediate the effect of maternal OWO on early childhood growth trajectories and OWO risk in offspring. The metabolic pathways, including identified metabolite mediators, might provide novel intervention targets for preventing the intrauterine development of obesity in the offspring of mothers with obesity.

Viral model of maternal immune activation alters placental AMPK and mTORC1 signaling in rats

INTRODUCTION

Maternal immune activation (MIA) is associated with neurodevelopmental disorders in offspring. We previously demonstrated that poly(I:C)-mediated MIA alters placental and fetal brain amino acid transporter expression in rats, which could potentially play a role in altered neurodevelopment; however, the mechanism(s) underlying these changes in amino acid transporter expression remain unknown. The objective of the current study was to investigate the mechanism(s) underlying poly(I:C)-mediated changes in the expression of the amino acid transporters in the placenta.

METHODS

Pregnant rats received poly(I:C) on gestational day 14 and placentas were collected 6 h later. Mass spectrometry-based proteomics of placentas was performed followed by pathway enrichment analysis. Activation of mTORC1 and its upstream regulator, AMPK, was investigated using immunoblotting. Finally, the role of mTORC1 and AMPK in regulating the expression and localization of the amino acid transporters EAAT2 and ASCT1 was investigated in the human choriocarcinoma cell line JAR.

RESULTS

The impact of poly(I:C) on the placental proteome was highly sexually dimorphic. While proteomics-based pathway enrichment analysis indicated enrichment of mTOR signaling in male placentas only, further investigation revealed inhibition of mTORC1 in both male and female placentas in addition to activation of AMPK. In vitro, activation of AMPK and inhibition of mTORC1 decreased membrane localization of EAAT2 and ASCT1.

DISCUSSION

Poly(I:C)-mediated MIA activates AMPK and inhibits mTORC1 in rat placenta, both of which decrease expression and membrane localization of EAAT2 and ASCT1 in vitro. Thus, AMPK/mTORC1 signaling could be a novel treatment target for alleviating MIA-mediated changes in placental amino acid transport.

Poly(I:C) alters placental and fetal brain amino acid transport in a rat model of maternal immune activation

PROBLEM

Maternal immune activation (MIA) during pregnancy is associated with increased chances of neurodevelopmental disorders including schizophrenia and autism spectrum disorder (ASD). However, the exact mechanism through which MIA contributes to altered neurodevelopment is unknown. Due to the important role that amino acids play in neurodevelopment, altered amino acid transport could play a role in neurodevelopmental disorders. Indeed, altered plasma concentrations of multiple amino acids have been reported in individuals with ASD or schizophrenia. Therefore, our objective was to determine whether virally mediated MIA induces changes in amino acid transporters in the placenta and fetal brain.

METHOD OF STUDY

Pregnant rats were administered poly(I:C) on gestational day 14, and placental and fetal tissues were collected 6, 24, and 48 hours later. Amino acid transporter expression was measured in the placenta and fetal brain using qPCR, Western blotting, and Simple Western. Free amino acid concentrations in the fetal brain were quantified using HPLC.

RESULTS

Poly(I:C) increased mRNA expression of several amino acid transporters in the placenta and fetal brain over these timepoints. Conversely, poly(I:C) imposed significant decreases in the protein expression of ASCT1 and EAAT2 in placenta and expression of SNAT5, EAAT1, and GLYT1 in fetal brain. Functional consequences of altered transporter expression were demonstrated through widespread changes in the concentrations of free amino acids in the fetal brains.

CONCLUSION

Together, these results represent novel findings with the poly(I:C) MIA model and contribute to the understanding of how MIA during pregnancy potentially leads to neurodevelopmental disorders.

A systems perspective on placental amino acid transport

Placental amino acid transfer is a complex process that is essential for fetal development. Impaired amino acid transfer causes fetal growth restriction, which may have lifelong health consequences. Transepithelial transfer of amino acids across the placental syncytiotrophoblast requires accumulative, exchange and facilitated transporters on the apical and basal membranes to work in concert. However, transporters alone do not determine amino acid transfer and factors that affect substrate availability, such as blood flow and metabolism, may also become rate-limiting for transfer. In order to determine the rate-limiting processes, it is necessary to take a systems approach which recognises the interdependence of these processes. New technologies have the potential to deliver targeted interventions to the placenta and help poorly growing fetuses. While many factors are necessary for amino acid transfer, novel therapies need to target the rate-limiting factors if they are going to be effective. This review will outline the factors which determine amino acid transfer and describe how they become interdependent. It will also highlight the role of computational modelling as a tool to understand this process.

DNA methylation of amino acid transporter genes in the human placenta

INTRODUCTION

Placental transfer of amino acids via amino acid transporters is essential for fetal growth. Little is known about the epigenetic regulation of amino acid transporters in placenta. This study investigates the DNA methylation status of amino acid transporters and their expression across gestation in human placenta.

METHODS

BeWo cells were treated with 5-aza-2'-deoxycytidine to inhibit methylation and assess the effects on amino acid transporter gene expression. The DNA methylation levels of amino acid transporter genes in human placenta were determined across gestation using DNA methylation array data. Placental amino acid transporter gene expression across gestation was also analysed using data from publically available Gene Expression Omnibus data sets. The expression levels of these transporters at term were established using RNA sequencing data.

RESULTS

Inhibition of DNA methylation in BeWo cells demonstrated that expression of specific amino acid transporters can be inversely associated with DNA methylation. Amino acid transporters expressed in term placenta generally showed low levels of promoter DNA methylation. Transporters with little or no expression in term placenta tended to be more highly methylated at gene promoter regions. The transporter genes SLC1A2, SLC1A3, SLC1A4, SLC7A5, SLC7A11 and SLC7A10 had significant changes in enhancer DNA methylation across gestation, as well as gene expression changes across gestation.

CONCLUSION

This study implicates DNA methylation in the regulation of amino acid transporter gene expression. However, in human placenta, DNA methylation of these genes remains low across gestation and does not always play an obvious role in regulating gene expression, despite clear evidence for differential expression as gestation proceeds.

Glutamate in peripheral organs: Biology and pharmacology

Glutamate is a versatile molecule existing in both the central nervous system and peripheral organs. Previous studies have mainly focussed on the biological effect of glutamate in the brain. Recently, abundant evidence has demonstrated that glutamate also participates in the regulation of physiopathological functions in peripheral tissues, including the lung, kidney, liver, heart, stomach and immune system, where the glutamate/glutamate receptor/glutamate transporter system plays an important role in the pathogenesis of certain diseases, such as myocardial ischaemia/reperfusion injury and acute gastric mucosa injury. All these findings provide new insight into the biology and pharmacology of glutamate and suggest a potential therapeutic role of glutamate in non-neurological diseases.

Potential protective role of endogenous glutamate-oxaloacetate transaminase against glutamate excitotoxicity in fetal hypoxic-ischaemic asphyxia

AIM

Fetal blood contains higher concentrations of glutamate-oxaloacetate transaminase (GOT; a blood enzyme able to metabolize glutamate) than maternal blood. The aim of this study was to determine the relationship between GOT and glutamate levels in arterial blood samples from umbilical cord in control newborn infants and newborn infants with hypoxic-ischaemic insult and/or symptoms of hypoxia-ischemia after delivery.

METHOD

A total of 46 newborn infants (28 females, 18 males) were prospectively included in the study. Twenty-three infants (18 females, five males) were included as control participants and 23 (10 females, 13 males) were included as newborn infants at risk of adverse neurological outcome (defined as umbilical blood with pH <7.1).

RESULTS

Analysis of glutamate concentration and GOT activity in umbilical blood samples showed that newborn infants with pH <7.1 had higher levels of glutamate (142.4 μmol/L [SD 61.4] vs 62.8 μmol/L [SD 25.5]; p<0.001) and GOT (83.1 U/L [SD 60.9] vs 34.9 U/L [SD 18.2]; p<0.001) compared to newborn infants without fetal distress. Analysis of Apgar scores and blood pH values (markers of perinatal distress) showed that conditions of severe distress were associated with higher glutamate and GOT levels.

INTERPRETATION

During fetal development, the ability of GOT to metabolize glutamate suggests that this enzyme can act as an endogenous protective mechanism in the control of glutamate homeostasis.

Glutamate cycling may drive organic anion transport on the basal membrane of human placental syncytiotrophoblast

Key points

The placenta removes waste products, drugs and environmental toxins from the fetal circulation and two of the transport proteins responsible for this are OAT4 and OATP2B1 localised to the basal membrane of placental syncytiotrophoblast.

We provide evidence that OAT4 and OATP2B1 mediate glutamate efflux when expressed in Xenopus oocytes and that in the perfused placenta, bromosulphothalein (an OAT4 and OATP2B1 substrate) stimulates glutamate efflux.

Furthermore the efflux of glutamate can only be seen in the presence of aspartate, which will block glutamate reuptake by the placenta, consistent with cycling of glutamate across the basal membrane.

We propose that glutamate efflux down its transmembrane gradient drives placental uptake via OAT4 and OATP2B1 from the fetal circulation and that reuptake of glutamate maintains this driving gradient.

Abstract

The organic anion transporter OAT4 (SLC22A11) and organic anion transporting polypeptide OATP2B1 (SLCO2B1) are expressed in the basal membrane of the placental syncytiotrophoblast. These transporters mediate exchange whereby uptake of one organic anion is coupled to efflux of a counter‐ion. In placenta, these exchangers mediate placental uptake of substrates for oestrogen synthesis as well as clearing waste products and xenobiotics from the fetal circulation. However, the identity of the counter‐ion driving this transport in the placenta, and in other tissues, is unclear. While glutamate is not a known OAT4 or OATP2B1 substrate, we propose that its high intracellular concentration has the potential to drive accumulation of substrates from the fetal circulation. In the isolated perfused placenta, glutamate exchange was observed between the placenta and the fetal circulation. This exchange could not be explained by known glutamate exchangers. However, glutamate efflux was trans‐stimulated by an OAT4 and OATP2B1 substrate (bromosulphothalein). Exchange of glutamate for bromosulphothalein was only observed when glutamate reuptake was inhibited (by addition of aspartate). To determine if OAT4 and/or OATP2B1 mediate glutamate exchange, uptake and efflux of glutamate were investigated in Xenopus laevis oocytes. Our data demonstrate that in Xenopus oocytes expressing either OAT4 or OATP2B1 efflux of intracellular [¹⁴C]glutamate could be stimulated by conditions including extracellular glutamate (OAT4), estrone‐sulphate and bromosulphothalein (both OAT4 and OATP2B1) or pravastatin (OATP2B1). Cycling of glutamate across the placenta involving efflux via OAT4 and OATP2B1 and subsequent reuptake will drive placental uptake of organic anions from the fetal circulation.

Upregulation of excitatory amino acid transporters by coexpression of Janus kinase 3

Janus kinase 3 (JAK3) contributes to cytokine receptor signaling, confers cell survival and stimulates cell proliferation. The gain of function mutation JAK3(A572V) is found in acute megakaryoplastic leukemia. Replacement of ATP coordinating lysine by alanine yields inactive JAK3(K855A). Most recent observations revealed the capacity of JAK3 to regulate ion transport. This study thus explored whether JAK3 regulates glutamate transporters EAAT1-4, carriers accomplishing transport of glutamate and aspartate in a variety of cells including intestinal cells, renal cells, glial cells, and neurons. To this end, EAAT1, 2, 3, or 4 were expressed in Xenopus oocytes with or without additional expression of mouse wild-type JAK3, constitutively active JAK3(A568V) or inactive JAK3(K851A), and electrogenic glutamate transport was determined by dual electrode voltage clamp. Moreover, Ussing chamber was employed to determine electrogenic glutamate transport in intestine from mice lacking functional JAK3 (jak3(-/-)) and from corresponding wild-type mice (jak3(+/+)). As a result, in EAAT1, 2, 3, or 4 expressing oocytes, but not in oocytes injected with water, addition of glutamate to extracellular bath generated an inward current (Ig), which was significantly increased following coexpression of JAK3. Ig in oocytes expressing EAAT3 was further increased by JAK3(A568V) but not by JAK3(K851A). Ig in EAAT3 + JAK3 expressing oocytes was significantly decreased by JAK3 inhibitor WHI-P154 (22 µM). Kinetic analysis revealed that JAK3 increased maximal Ig and significantly reduced the glutamate concentration required for half maximal Ig (Km). Intestinal electrogenic glutamate transport was significantly lower in jak3(-/-) than in jak3(+/+) mice. In conclusion, JAK3 is a powerful regulator of excitatory amino acid transporter isoforms.

Influence of prenatal exposure to environmental pollutants on human cord blood levels of glutamate

Some chemicals released into the environment, including mercury and some organochlorine compounds (OCs), are suspected to have a key role on subclinical brain dysfunction in childhood. Alteration of the glutamatergic system may be one mechanistic pathway. We aimed to determine whether mercury and seven OCs, including PCBs 138, 153, and 180, DDT and DDE, hexachlorobenzene (HCB), and beta-hexachlorocyclohexane (β-HCH) influence the cord levels of two excitatory amino acids, glutamate and aspartate. Second, we evaluated if this association was mediated by glutamate uptake measured in human placental membranes. The study sample included 40 newborns from a Spanish cohort selected according to cord mercury levels. We determined the content of both amino acids in cord blood samples by means of HPLC and assessed their associations with the contaminants using linear regression analyses, and the effect of the contaminants on glutamate uptake by means of [(3)H]-aspartate binding in human placenta samples. PCB138, β-HCH, and the sum of the three PCBs and seven OCs showed a significant negative association with glutamate levels (decrease of 51, 24, 56 and 54%, respectively, in glutamate levels for each 10-fold increase in the contaminant concentration). Mercury did not show a significant correlation neither with glutamate nor aspartate levels in cord blood, however a compensatory effect between T-Hg and both PCB138, and 4,4'-DDE was observed. The organo-metallic derivative methylmercury completely inhibited glutamate uptake in placenta while PCB138 and β-HCH partially inhibited it (IC50 values: 4.9±0.8 μM, 14.2±1.2 nM and 6.9±2.9 nM, respectively). We conclude that some environmental toxicants may alter the glutamate content in the umbilical cord blood, which might underlie alterations in human development.

Plasmodium falciparum malaria elicits inflammatory responses that dysregulate placental amino acid transport

Placental malaria (PM) can lead to poor neonatal outcomes, including low birthweight due to fetal growth restriction (FGR), especially when associated with local inflammation (intervillositis or IV). The pathogenesis of PM-associated FGR is largely unknown, but in idiopathic FGR, impaired transplacental amino acid transport, especially through the system A group of amino acid transporters, has been implicated. We hypothesized that PM-associated FGR could result from impairment of transplacental amino acid transport triggered by IV. In a cohort of Malawian women and their infants, the expression and activity of system A (measured by Na⁺-dependent ¹⁴C-MeAIB uptake) were reduced in PM, especially when associated with IV, compared to uninfected placentas. In an in vitro model of PM with IV, placental cells exposed to monocyte/infected erythrocytes conditioned medium showed decreased system A activity. Amino acid concentrations analyzed by reversed phase ultra performance liquid chromatography in paired maternal and cord plasmas revealed specific alterations of amino acid transport by PM, especially with IV. Overall, our data suggest that the fetoplacental unit responds to PM by altering its placental amino acid transport to maintain adequate fetal growth. However, IV more profoundly compromises placental amino acid transport function, leading to FGR. Our study offers the first pathogenetic explanation for FGR in PM.

Malaria infection during pregnancy can cause fetal growth restriction and low birthweight associated with high infant mortality and morbidity rates. The pathogenesis of fetal growth restriction in placental malaria is largely unknown, but in other pathological pregnancies, impaired transplacental amino acid transport has been implicated. In a cohort of Malawian women and their infants, we found that placental malaria, especially when associated with local inflammation, was associated with decreased expression and activity of an important group of amino acid placental transporters. Using an in vitro model of placental malaria with local inflammation, we discovered that maternal monocyte products could impair the activity of amino acid transporters on placental cells. Amino acid concentrations in paired maternal and cord plasmas revealed specific alterations of amino acid transport by placental malaria, especially with local inflammation. Overall, our data suggest that, more than malaria infection per se, the local inflammation it triggers compromises placental amino acid transport function, leading to fetal growth restriction. Greater understanding of the mechanisms involved, combined with interventions to improve fetal growth in malaria, are important priorities in areas of the world where the co-existence of malaria and maternal malnutrition threatens the health and lives of millions of young babies.

Relationship between glutamate, GOT and GPT levels in maternal and fetal blood: a potential mechanism for fetal neuroprotection

BACKGROUND

Excess glutamate in the brain is thought to be implicated in the pathophysiology of fetal anoxic brain injury, yet little is known about the mechanisms by which glutamate is regulated in the fetal brain. This study examines whether there are differences between maternal and fetal glutamate concentrations, and whether a correlation between them exists.

METHODS

10 ml of venous blood was extracted from 87 full-term (>37 weeks gestation) pregnant women in active labor. Immediately after delivery of the neonate, 10 ml of blood from the umbilical artery and vein was extracted. Samples were analyzed for levels of glutamate, glutamate-oxaloacetate transaminase (GOT), and glutamate pyruvate transaminase (GPT).

RESULTS

Fetal blood glutamate concentrations in both the umbilical artery and vein were found to be significantly higher than maternal blood (p<0.001). Similarly, fetal serum GOT levels in the umbilical artery and vein were found to be significantly higher than maternal GOT levels (p<0.001). The difference in GPT levels between maternal and fetal serum was not statistically significant. There was no difference in fetal glutamate, GOT or GPT between the umbilical artery and vein. There was an association observed between glutamate levels in maternal blood and glutamate levels in both venous (R=0.32, p<0.01) and arterial (R=0.33, p<0.05) fetal blood.

CONCLUSIONS

This study demonstrated that higher baseline concentrations of blood glutamate are present in fetal blood compared with maternal blood, and this was associated with elevated GOT, but not GPT levels. An association was observed between maternal and fetal blood glutamate levels.

Blood glutamate scavenging: insight into neuroprotection

Brain insults are characterized by a multitude of complex processes, of which glutamate release plays a major role. Deleterious excess of glutamate in the brain's extracellular fluids stimulates glutamate receptors, which in turn lead to cell swelling, apoptosis, and neuronal death. These exacerbate neurological outcome. Approaches aimed at antagonizing the astrocytic and glial glutamate receptors have failed to demonstrate clinical benefit. Alternatively, eliminating excess glutamate from brain interstitial fluids by making use of the naturally occurring brain-to-blood glutamate efflux has been shown to be effective in various animal studies. This is facilitated by gradient driven transport across brain capillary endothelial glutamate transporters. Blood glutamate scavengers enhance this naturally occurring mechanism by reducing the blood glutamate concentration, thus increasing the rate at which excess glutamate is cleared. Blood glutamate scavenging is achieved by several mechanisms including: catalyzation of the enzymatic process involved in glutamate metabolism, redistribution of glutamate into tissue, and acute stress response. Regardless of the mechanism involved, decreased blood glutamate concentration is associated with improved neurological outcome. This review focuses on the physiological, mechanistic and clinical roles of blood glutamate scavenging, particularly in the context of acute and chronic CNS injury. We discuss the details of brain-to-blood glutamate efflux, auto-regulation mechanisms of blood glutamate, natural and exogenous blood glutamate scavenging systems, and redistribution of glutamate. We then propose different applied methodologies to reduce blood and brain glutamate concentrations and discuss the neuroprotective role of blood glutamate scavenging.

Glutamate in vitro effects on human term placental mitochondria

OBJECTIVE

Oxidative stress may affect the functionality of placental mitochondria, thus contributing to serious complications. For this reason research of protective substances is of great importance. Our aim was to evaluate, in mitochondria isolated from human term placentas, the effect of in vitro glutamate supplementation on their susceptibility to oxidation, on the chemico-physical characteristics of mitochondrial membranes, and on peroxidase and nitric oxide synthase (NOS) activities.

METHODS

The study was performed on mitochondria isolated from 20 healthy human term placentas. Specific exclusion criteria were: conception by assisted reproduction, chromosomal or other fetal, uterine or placental anomalies, gestational diabetes, preeclampsia, intrauterine growth restriction (IUGR), a history of smoking and hypertension, proteinuria, renal, cardiovascular, hepatic, and endocrine disease, metabolic disorders, and current infection or history of all types of infection.

RESULTS

Incubation with glutamate determined a reduced susceptibility to oxidative stress, an increase in mitochondrial membrane fluidity, and a decrease of both peroxidase and NOS activities.

CONCLUSIONS

On the basis of the observed results, we can hypothesize a role for glutamate in the control of lipid peroxidation extent in physiological pregnancies, as well as in the prevention of free radical-linked complications that can affect the health of both mother and fetus.

Regulation of the glutamate transporter EAAT3 by mammalian target of rapamycin mTOR

The serine/threonine kinase mammalian target of rapamycin (mTOR) is stimulated by insulin, growth factors and nutrients and confers survival of several cell types. The kinase has previously been shown to stimulate amino acid uptake. In neurons, the cellular uptake of glutamate by the excitatory amino-acid transporters (EAATs) decreases excitation and thus confers protection against excitotoxicity. In epithelia, EAAT3 accomplishes transepithelial glutamate and aspartate transport. The present study explored, whether mTOR regulates EAAT3 (SLC1A1). To this end, cRNA encoding EAAT3 was injected into Xenopus oocytes with or without cRNA encoding mTOR and the glutamate induced current (I(glu)), a measure of glutamate transport, determined by dual electrode voltage clamp. Moreover, EAAT3 protein abundance was determined utilizing chemiluminescence. As a result, I(glu) was observed in Xenopus oocytes expressing EAAT3 but not in water injected oocytes. Coexpression of mTOR significantly increased I(glu), an effect reversed by rapamycin (100 nM). mTOR coexpression increased EAAT3 protein abundance in the cell membrane. The decay of I(glu) following inhibition of carrier insertion with brefeldin A in oocytes coexpressing EAAT3 with mTOR was similar in the presence and absence of rapamycin (100 nM). In conclusion, mTOR is a novel powerful regulator of EAAT3 and may thus contribute to protection against neuroexcitotoxicity.

Regulation of the glutamate transporters by JAK2

The Janus-activated kinase-2 JAK2 is involved in the signaling of leptin and erythropoietin receptors and mediates neuroprotective effects of the hormones. In theory, JAK2 could be effective through modulation of the glutamate transporters, carriers accounting for the clearance of glutamate released during neurotransmission. The present study thus elucidated the effect of JAK2 on the glutamate transporters EAAT1, EAAT2, EAAT3 and EAAT4. To this end, cRNA encoding the carriers was injected into Xenopus oocytes with or without cRNA encoding JAK2 and glutamate transport was estimated from glutamate induced current (I(glu)). I(glu) was observed in Xenopus oocytes expressing EAAT1 or EAAT2 or EAAT3 or EAAT4, but not in water injected oocytes. Coexpression of JAK2 resulted in an increase of I(glu) by 83% (EAAT1), 67% (EAAT2), 42% (EAAT3) and 126% (EAAT4). As shown for EAAT4 expressing Xenopus oocytes, the effect of JAK2 was mimicked by gain of function mutation (V617F)JAK2 but not by the inactive mutant (K882E)JAK2. Incubation with JAK2 inhibitor AG490 (40 μM) resulted in a gradual decrease of I(glu) by 53%, 79% and 92% within 3, 6 and 24 hours. Confocal microscopy and chemiluminescence analysis revealed that JAK2 coexpression increased EAAT4 protein abundance in the cell membrane. Disruption of transcription did not appreciably modify the up-regulation of I(glu) in EAAT4 expressing oocytes. The decay of I(glu) following inhibition of carrier insertion with brefeldin A was similar in oocytes expressing EAAT4 + JAK2 and oocytes expressing EAAT4 alone, indicating that JAK2 did not appreciably affect carrier retrieval from the membrane. In conclusion, JAK2 is a novel powerful regulator of glutamate transporters and thus participates in the protection against excitotoxicity.

Effect of maternal dexamethasone and alpha-ketoglutarate administration on skeletal development during the last three weeks of prenatal life in pigs

BACKGROUND

The effect of dexamethasone (Dex) on postnatal bone formation processes is known to decrease the synthesis of collagen and bone matrix, but the effect of alpha-ketoglutarate (AKG) is to induce positive effects on growth and skeletal development during postnatal life. However, the effects of Dex and AKG treatment on the prenatal processes of skeletal development have not been investigated so far.

OBJECTIVE

The aim of this study was to determine the effect of Dex and AKG administered separately or simultaneously to sows during the last three weeks of pregnancy on the skeletal development in fetuses.

METHODS

Immediately after birth blood samples were collected from non-suckling piglets for alkaline phosphatase and osteocalcin determinations, and the humeri were isolated. Bone mineral density (BMD) and bone mineral content (BMC) of humeri and the geometric and mechanical properties were evaluated.

RESULTS

Dex and AKG administered separately to pregnant sows during the last 24 days of prenatal life decreased BMD, BMC, and geometric and mechanical parameters of humeri in the newborns. Simultaneous administration of Dex and AKG significantly increased the analyzed properties of humeri.

CONCLUSION

The bone mineral density and mechanical and geometric properties of humeri indicate an inverse effect of maternal separate or simultaneous administration of AKG and Dex to sows on bone development during the last 24 days of prenatal life.