Uptake of [3H]L-serine in rat brain synaptosomal fractions

Chronic postnatal stress induces voluntary alcohol intake and modifies glutamate transporters in adolescent rats

Postnatal stress alters stress responses for life, with serious consequences on the central nervous system (CNS), involving glutamatergic neurotransmission and development of voluntary alcohol intake. Several drugs of abuse, including alcohol and cocaine, alter glutamate transport (GluT). Here, we evaluated effects of chronic postnatal stress (CPS) on alcohol intake and brain glutamate uptake and transporters in male adolescent Wistar rats. For CPS from postnatal day (PD) 7, pups were separated from their mothers and exposed to cold stress (4 °C) for 1 h daily for 20 days; controls remained with their mothers. Then they were exposed to either voluntary ethanol (6%) or dextrose (1%) intake for 7 days (5-7 rats per group), then killed. CPS: (1) increased voluntary ethanol intake, (2) did not affect body weight gain or produce signs of toxicity with alcohol exposure, (3) increased glutamate uptake by hippocampal synaptosomes in vitro and (4) reduced protein levels (Western measurements) in hippocampus and frontal cortex of glial glutamate transporter-1 (GLT-1) and excitatory amino-acid transporter-3 (EAAT-3) but increased glutamate aspartate transporter (GLAST) levels. We propose that CPS-induced decrements in GLT-1 and EAAT-3 expression levels are opposed by activation of a compensatory mechanism to prevent excitotoxicity. A greater role for GLAST in total glutamate uptake to prevent enlarged extracellular glutamate levels is inferred. Although CPS strongly increased intake of ethanol, this had little impact on effects of CPS on brain glutamate uptake or transporters. However, the impact of early life adverse events on glutamatergic neurotransmission may underlie increased alcohol consumption in adulthood.

The response to postnatal stress: amino acids transporters and PKC activity

It is well known that animals exposed to stressful stimuli during their early life develop different neurological disorders when they become adults. In this study, we evaluated the effect of acute cold stress on gamma-aminobutyric acid (GABA) and L-Serine (L-Ser) transporters in vitro, using the uptake of [(3)H]-GABA and [(3)H]L-Ser by synaptosomes-enriched fractions isolated from rat cerebral cortex during postnatal development. GABA and L-Ser uptake studies in vitro will be used in this investigation as a colateral evidence of changes in the expression of transporters of GABA and L-Ser. We observed that the maximum velocity (V (max)) in L-Ser and GABA uptake after stress session increased in all stages studied. In contrast, K (m) values of L-Ser uptake enhancent in almost age calculated, excluding at PD21 after cold stress during development, at the same time as K (m) (uptake affinity) values of GABA increased in just about age considered but not at PD5 compared with the control group. Finally we investigated the mechanism by which cells regulate the substrate affinity of L-Ser and GABA transporters. We demonstrated a significantly increase in total PKC activity to PD5 from PD21. Pretreatment with PKC inhibitor: staurosporine (SP) led to a restoration of control uptake in several postnatal-days suggesting a relationship between amino acids system and PKC activation. These findings suggest that a single exposure to postnatal cold stress at different periods after birth modifies both GABA and L-Ser transporters and the related increase in total PKC activity could be intracellular events that participate in neuronal plasticity by early life stress, which could be relevant to function of transporters in the adult rat brain.

Glutamine synthetase activity and glutamate uptake in hippocampus and frontal cortex in portal hypertensive rats

AIM: To study glutamine synthetase (GS) activity and glutamate uptake in the hippocampus and frontal cortex (FC) from rats with prehepatic portal vein hypertension.

METHODS: Male Wistar rats were divided into sham-operated group and a portal hypertension (PH) group with a regulated stricture of the portal vein. Animals were sacrificed by decapitation 14 d after portal vein stricture. GS activity was determined in the hippocampus and FC. Specific uptake of radiolabeled L-glutamate was studied using synaptosome-enriched fractions that were freshly prepared from both brain areas.

RESULTS: We observed that the activity of GS increased in the hippocampus of PH rats, as compared to control animals, and decreased in the FC. A significant decrease in glutamate uptake was found in both brain areas, and was more marked in the hippocampus. The decrease in glutamate uptake might have been caused by a deficient transport function, significantly and persistent increase in this excitatory neurotransmitter activity.

CONCLUSION: The presence of moderate ammonia blood levels may add to the toxicity of excitotoxic glutamate in the brain, which causes alterations in brain function. Portal vein stricture that causes portal hypertension modifies the normal function in some brain regions.

The monocarboxylate transporter 8 and L-type amino acid transporters 1 and 2 are expressed in mouse skeletons and in osteoblastic MC3T3-E1 cells

BACKGROUND

Several plasma membrane transporters have been shown to mediate the cellular influx and/or efflux of iodothyronines, including the sodium-independent organic anion co-transporting polypeptide 1 (OATP1), the sodium taurocholate co-transporting polypeptide (NTCP), the L-type amino acid transporter 1 (LAT1) and 2 (LAT2), and the monocarboxylate transporter 8 (MCT8). The aim of this study was to investigate if the mRNAs of these transporters were expressed and regulated by thyroid hormone (TH) in mouse calvaria-derived osteoblastic MC3T3-E1 cells and in the fetal and postnatal bones of mice.

METHODS

The mRNA expression of the iodothyronine transporters was investigated with real-time polymerase chain reaction analysis in euthyroid and hypothyroid fetuses and litters of mice and in MC3T3-E1 cells treated with increasing doses of triiodothyronine (T(3); 10(-10) to 10(-6) M) or with 10(-8) M T(3) for 1-9 days.

RESULTS

MCT8, LAT1, and LAT2 mRNAs were detected in fetal and postnatal femurs and in MC3T3-E1 cells, while OATP1 and NTCP mRNAs were not. LAT1 and LAT2 mRNAs were not affected by TH status in vivo or in vitro or by the stage of bone development or osteoblast maturation (analyzed by the expression of osteocalcin and alkaline phosphatase, which are key markers of osteoblastic differentiation). In contrast, the femoral mRNA expression of MCT8 decreased significantly during post-natal development, whereas MCT8 mRNA expression increased as MC3T3-E1 cells differentiated. We also showed that MCT8 mRNA was up-regulated in the femur of hypothyroid animals, and that it was down-regulated by treatment with T(3) in MC3T3-E1 cells.

CONCLUSIONS

This is the first study to demonstrate the mRNA expression of LAT1, LAT2, and MCT8 in the bone tissue of mice and in osteoblast-like cells. In addition, the pattern of MCT8 expression observed in vivo and in vitro suggests that MCT8 may be important to modulate TH effects on osteoblast differentiation and on bone development and metabolism.

Evidence from gene knockout studies implicates Asc-1 as the primary transporter mediating d-serine reuptake in the mouse CNS

In the mammalian central nervous system, transporter-mediated reuptake may be critical for terminating the neurotransmitter action of D-serine at the strychnine insensitive glycine site of the NMDA receptor. The Na(+) independent amino acid transporter alanine-serine-cysteine transporter 1 (Asc-1) has been proposed to account for synaptosomal d-serine uptake by virtue of its high affinity for D-serine and widespread neuronal expression throughout the brain. Here, we sought to validate the contribution of Asc-1 to D-serine uptake in mouse brain synaptosomes using Asc-1 gene knockout (KO) mice. Total [(3)H]D-serine uptake in forebrain and cerebellar synaptosomes from Asc-1 knockout mice was reduced to 34 +/- 5% and 22 +/- 3% of that observed in wildtype (WT) mice, respectively. When the Na(+) dependent transport components were removed by omission of Na(+) ions in the assay buffer, D-serine uptake in knockout mice was reduced to 8 +/- 1% and 3 +/- 1% of that measured in wildtype mice in forebrain and cerebellum, respectively, suggesting Asc-1 plays a major role in the Na(+) independent transport of D-serine. Potency determination of D-serine uptake showed that Asc-1 mediated rapid high affinity Na(+) independent uptake with an IC(50) of 19 +/- 1 microm. The remaining uptake was mediated predominantly via a low affinity Na(+) dependent transporter with an IC(50) of 670 +/- 300 microm that we propose is the glial alanine-serine-cysteine transporter 2 (ASCT2) transporter. The results presented reveal that Asc-1 is the only high affinity D-serine transporter in the mouse CNS and is the predominant mechanism for D-serine reuptake.

l-serine and GABA uptake by synaptosomes during postnatal development of rat

Postnatal development changes in mechanisms of synaptosomal amino acid transport have been studied in rat cerebral cortex. Specific uptake of radiolabeled L-serine was examined and compared with that of radiolabeled GABA using synaptosomes-enriched fractions freshly prepared from cerebral cortex at different postnatal days from the birth to young adulthood. The preparations were incubated with 10 nM of [3H]L-serine and 10 nM of [3H]-GABA in either the presence or absence of NaCl, KCl or choline chloride, at 2 and 30 degrees C, for different periods up to 30 min. The uptake of [3H]l-serine was temperature dependent in synaptosomal fractions prepared from cerebral cortex of rats in postnatal days 5, 7, 13 and 21, but stronger dependence was observed in adult brain, irrespective of the presence of Na+, K+ or choline ions. At all postnatal ages studied, [3H]-GABA uptake showed a high activity in the presence of Na+ ions and at 30 degrees C. The values of Km were 90-489 microM in L-serine uptake. However, in the uptake of GABA the values of Km were 80-150 microM. The highest values of Vmax were obtained at 5 and 21 postnatal days for both transport systems. These results indicate that the uptake of l-serine and GABA are regulated differentially during postnatal development.

Homocysteine transport by human aortic endothelial cells: identification and properties of import systems

Hyperhomocysteinemia is an independent risk factor for cardiovascular disease. Transport of L-homocysteine into and out of the human vascular endothelium is poorly understood. We hypothesized that cultured human aortic endothelial cells (HAEC) would import L-homocysteine on one or more of the L-cysteine transport systems. Inhibitors of the transporters were used to characterize the uptake of [35S]L-homocysteine, [35S]L-homocystine, and [35S]L-cysteine. We found that L-homocysteine uptake is mediated by the sodium-dependent cysteine transport systems X(AG), ASC, and A, and the sodium-independent transport system L. Thus, HAEC utilize multiple cysteine transporters (X(AG) > or = L > ASC > A) to import L-homocysteine. Kinetic analysis supported the uptake results. Michaelis-Menten constants (Km) for the four systems yielded values of 19.0, 27.1, 112, and 1000 microM for systems L, X(AG), ASC, and A, respectively. The binding and uptake of [35S]L-homocystine, the disulfide homodimer of L-homocysteine, was mediated by systems X(AG), L, and ASC but not by system A. In contrast to [35S]L-homocysteine, system x(c) was active for [35S]L-homocystine uptake. A similar pattern was observed for [35S]L-cysteine. Thus, L-homocysteine and L-homocystine found in hyperhomocysteinemic subjects can gain entry into the vascular endothelium by way of multiple L-cysteine transporters.

Functional analysis of mouse 3-phosphoglycerate dehydrogenase (Phgdh) gene promoter in developing brain

D-3-Phosphoglycerate dehydrogenase (Phgdh; EC 1.1.1.95) is a necessary enzyme for de novo L-serine biosynthesis via the phosphorylated pathway. Targeted disruption of the mouse Phgdh gene has been shown to result in embryonic lethality, accompanied by severe abnormalities in brain development. Phgdh is expressed exclusively by neuroepithelium and radial glia in developing brain and later mainly by astrocytes. To elucidate the molecular mechanism that regulates such cell-type-specific expression of Phgdh in developing brain, an upstream 3.5-kilobase-pair (kbp) region of the gene harboring the promoter was characterized in primary cultures and transgenic mice. Analysis of Phgdh 5'-nested deletions in transfected cultures indicated that overall reporter luciferase levels were higher in glial cultures than those in neuronal cultures. Although basal promoter activity of the gene appeared to depend on an Sp1 binding sequence residing between -193 and -184 in both glial and neuronal cultures, an upstream 5'-flanking region between -1,794 and -1,095 contributed to up-regulation of Phgdh transcription in a glial-cell-specific manner. In the cerebral cortex of transgenic mouse embryos, the Phgdh promoter-LacZ transgene DNA containing -1,794/+4 promoter sequences directed beta-galactosidase (beta-Gal) expression mainly to Phgdh-positive neuroepithelium and radial glia. This glial preference diminished when beta-Gal expression was driven solely by the upstream 0.2-kbp minimal promoter. However, glial preference of beta-Gal expression was restored by placing the 700-base-pair 5'-DNA segment upstream of the minimal promoter. These observations suggest the presence of cis-acting elements that confer the cell type specificity of Phgdh transcription in the distal promoter region.