Blood-endothelial cell and blood-brain transport of L-proline, alpha-aminoisobutyric acid, and L-alanine

Metabolism of Amino Acids in the Brain and Their Roles in Regulating Food Intake

Amino acids (AAs) and their metabolites play an important role in neurological health and function. They are not only the building blocks of protein but are also neurotransmitters. In the brain, glutamate and aspartate are the major excitatory neurotransmitters, whereas γ-aminobutyrate (GABA, a metabolite of glutamate) and glycine are the major inhibitory neurotransmitters. Nitric oxide (NO, a metabolite of arginine), H₂S (a metabolite of cysteine), serotonin (a metabolite of tryptophan) and histamine (a metabolite of histidine), as well as dopamine and norepinephrine (metabolites of tyrosine) are neurotransmitters to modulate synaptic plasticity, neuronal activity, learning, motor control, motivational behavior, emotion, and executive function. Concentrations of glutamine (a precursor of glutamate and aspartate), branched-chain AAs (precursors of glutamate, glutamine and aspartate), L-serine (a precursor of glycine and D-serine), methionine and phenylalanine in plasma are capable of affecting neurotransmission through the syntheses of glutamate, aspartate, and glycine, as well as the competitive transport of tryptophan and tyrosine across from the blood-brain barrier. Adequate consumption of AAs is crucial to maintain their concentrations and the production of neurotransmitters in the central nervous system. Thus, the content and balance of AAs in diets have a profound impact on food intake by animals. Knowledge of AA transport and metabolism in the brain is beneficial for improving the health and well-being of humans and animals.

Influx mechanisms in the embryonic and adult rat choroid plexus: a transcriptome study

The transcriptome of embryonic and adult rat lateral ventricular choroid plexus, using a combination of RNA-Sequencing and microarray data, was analyzed by functional groups of influx transporters, particularly solute carrier (SLC) transporters. RNA-Seq was performed at embryonic day (E) 15 and adult with additional data obtained at intermediate ages from microarray analysis. The largest represented functional group in the embryo was amino acid transporters (twelve) with expression levels 2–98 times greater than in the adult. In contrast, in the adult only six amino acid transporters were up-regulated compared to the embryo and at more modest enrichment levels (<5-fold enrichment above E15). In E15 plexus five glucose transporters, in particular Glut-1, and only one monocarboxylate transporter were enriched compared to the adult, whereas only two glucose transporters but six monocarboxylate transporters in the adult plexus were expressed at higher levels than in embryos. These results are compared with earlier published physiological studies of amino acid and monocarboxylate transport in developing rodents. This comparison shows correlation of high expression of some transporters in the developing brain with higher amino acid transport activity reported previously. Data for divalent metal transporters are also considered. Immunohistochemistry of several transporters (e.g., Slc16a10, a thyroid hormone transporter) gene products was carried out to confirm translational activity and to define cellular distribution of the proteins. Overall the results show that there is substantial expression of numerous influx transporters in the embryonic choroid plexus, many at higher levels than in the adult. This, together with immunohistochemical evidence and data from published physiological transport studies suggests that the choroid plexus in embryonic brain plays a major role in supplying the developing brain with essential nutrients.

System a amino acid transport-targeted brain and systemic tumor PET imaging agents 2-amino-3-[(18)F]fluoro-2-methylpropanoic acid and 3-[(18)F]fluoro-2-methyl-2-(methylamino)propanoic acid

INTRODUCTION

Amino acid based radiotracers target tumor cells through increased uptake by membrane-associated amino acid transport (AAT) systems. In the present study, four structurally related non-natural (18)F-labeled amino acids, (R)- and (S)-[(18)F]FAMP 1 and (R)- and (S)-[(18)F]MeFAMP 2 have been prepared and evaluated in vitro and in vivo for their potential utility in brain and systemic tumor imaging based upon primarily system A transport with positron emission tomography (PET).

METHODS

The transport of enantiomers of [(18)F]FAMP 1 and [(18)F]MeFAMP 2 was measured through in vitro uptake assays in human derived cancer cells including A549 (lung), DU145 (prostate), SKOV3 (ovary), MDA MB468 (breast) and U87 (brain) in the presence and absence of amino acid transporter inhibitors. The in vivo biodistribution of these tracers was evaluated using tumor mice xenografts at 15, 30, 60 and 120 min post injection.

RESULTS

All four tracers showed moderate to high levels of uptake (1-9%ID/5×10(5) cells) by the cancer cell lines tested in vitro. AAT cell inhibition assays demonstrated that (R)-[(18)F]1 and (S)-[(18)F]1 entered these tumor cells via mixed AATs, likely but not limited to system A and system L. In contrast, (R)-[(18)F]2 and (S)-[(18)F]2 showed high selectivity for system A AAT. Similar to the results of in vitro cell studies, the tumor uptake of all four tracers was good to high and persisted over the 2 hours time course of in vivo studies. The accumulation of these tracers was higher in tumor than most normal tissues including blood, brain, muscle, bone, heart, and lung, and the tracers with the highest in vitro selectivity for system A AAT generally demonstrated the best tumor imaging properties. Higher uptake of these tracers was observed in the pancreas, kidney and spleen compared to tumors.

CONCLUSIONS

These preclinical studies demonstrate good imaging properties in a wide range of tumors for all four amino acids evaluated with (R)-[(18)F]2 having the highest selectivity for system A AAT.

Radiosynthesis and biological evaluation of alpha-[F-18]fluoromethyl phenylalanine for brain tumor imaging

OBJECTIVES

Radiolabeled amino acids have proven utility for imaging brain tumors in humans, particularly those that target system L amino acid transport. We have prepared the novel phenylalanine analogue, (FMePhe, 9), as part of an effort to develop new system L tracers that can be prepared in high radiochemical yield through nucleophilic [(18)F]fluorination. The tumor imaging properties of both enantiomers of this new tracer were evaluated through cell uptake, biodistribution and microPET studies in the mouse DBT model of high grade glioma.

METHODS

The non-radioactive form of 9 and the cyclic sulfamidate labeling precursor were prepared from commercially available racemic α-benzylserine. Racemic [(18)F]9 was prepared from the labeling precursor in two steps using standard[(18)F]fluoride nucleophilic reaction conditions followed by acidic deprotection. The individual enantiomers [(18)F]9a and [(18)F]9b were isolated using preparative chiral HPLC. In vitro uptake inhibition assays were performed with each enantiomer using DBT cells. Biodistribution and microPET/CT studies were performed with each enantiomer in male BALB/c mice at approximately 2 weeks after implantation of DBT tumor cells.

RESULTS

Radiolabeling of the cyclic sulfamidate precursor 5 provides racemic [(18)F]9 in high radiochemical yield (60%-70%, n=4) and high radiochemical purity (>96%, n=4). In vitro uptake assays demonstrate that both [(18)F]9a and [(18)F]9b undergo tumor cell uptake through system L transport. The biodistribution studies using the single enantiomers [(18)F]9a and [(18)F]9b demonstrated good tumor uptake with lower uptake in most normal tissues, and [(18)F]9a had higher tumor uptake than [(18)F]9b. MicroPET imaging demonstrated good tumor visualization within 10 min of injection, rapid uptake of radioactivity, and tumor to brain ratios of approximately 6:1 at 60 min postinjection.

CONCLUSIONS

The novel PET tracer, [(18)F]FMePhe, is readily synthesized in good yield from a cyclic sulfamidate precursor. Biodistribution and microPET studies in the DBT model demonstrate good tumor to tissue ratios and tumor visualization, with enantiomer [(18)F]9a having higher tumor uptake. However, the brain availability of both enantiomers was lower than expected for system L substrates, suggesting the [(18)F]fluorine group in the β-position affects uptake of these compounds by system L transporters.

The role of amino acid transporters in GSH synthesis in the blood-brain barrier and central nervous system

Glutathione (GSH) plays a critical role in protecting cells from oxidative stress and xenobiotics, as well as maintaining the thiol redox state, most notably in the central nervous system (CNS). GSH concentration and synthesis are highly regulated within the CNS and are limited by availability of the sulfhydryl amino acid (AA) l-cys, which is mainly transported from the blood, through the blood-brain barrier (BBB), and into neurons. Several antiporter transport systems (e.g., x(c)(-), x(-)(AG), and L) with clearly different luminal and abluminal distribution, Na(+), and pH dependency have been described in brain endothelial cells (BEC) of the BBB, as well as in neurons, astrocytes, microglia and oligodendrocytes from different brain structures. The purpose of this review is to summarize information regarding the different AA transport systems for l-cys and its oxidized form l-cys(2) in the CNS, such as expression and activity in blood-brain barrier endothelial cells, astrocytes and neurons and environmental factors that modulate transport kinetics.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

Compartmentalized coculture of rat brain endothelial cells and astrocytes: a syngenic model to study the blood-brain barrier

The specific structure of the blood-brain barrier (BBB) is based on the partnership of brain endothelial cells and astrocytes. In the last decade, cocultures of these two cell types have been developed as in vitro models. However, these studies did not allow close contacts between both cell types. We report here a syngenic coculture model using rat endothelial cells on one side of a polyethylene terephtalate filter and rat astrocytes on the other. Endothelial cells retain their typical morphology and are factor VIII and OX 26 positive. We optimized the diameter of the membrane pores to establish very close contacts between the cells through the membrane pores without mixing the two cell types. Transmission electron microscopy showed evidence of tight junction formation between the endothelial cells and few pinocytic vesicles. The cocultures reached high electrical resistances up to 1000 Omegacm(2) showing their ability to limit the passage of ions. A 15-fold increase in gamma-glutamyl transpeptidase activity was measured in the endothelial cells in coculture compared to endothelial cell monoculture. Our syngenic coculture represents a useful in vitro model of the rat BBB that may prove to be valuable for studying the passage of substances across the barrier as well as other aspects of the BBB function.

ATA2 is predominantly expressed as system A at the blood-brain barrier and acts as brain-to-blood efflux transport for L-proline

Although system A is present at the blood-brain barrier (BBB), the physiological roles of system A have not been clarified. The efflux transport of the substrates of system A, such as L-proline (L-Pro), glycine (Gly), and alpha-methylaminoisobutyric acid (MeAIB), across the BBB was investigated using the in vivo Brain Efflux Index method. Over a period of 40 min, L-[(3)H]Pro and [(3)H]Gly underwent efflux from the brain, whereas [(3)H]MeAIB did not. The efflux of L-[(3)H]Pro was inhibited by the presence of unlabeled L-Pro and MeAIB, suggesting that carrier-mediated efflux transport of L-Pro across the BBB is involved in system A. L-[(3)H]Pro uptake by TR-BBB cells, used as an in vitro BBB model, was Na(+)-dependent with high-affinity (K(m1) = 425 microM) and low-affinity (K(m2) = 10.8 mM) saturable processes. The manner of inhibition of L-[(3)H]Pro uptake for amino acids was consistent with system A. Although GlnT, ATA2, and ATA3 mRNA were all expressed in TR-BBB cells, ATA2 mRNA was predominant. Under hypertonic conditions, ATA2 mRNA in TR-BBB cells was induced by up to 373%, and it activated [(3)H]MeAIB uptake. In light of these observations, our results indicate that L-Pro and Gly are transported from the brain across the BBB, whereas MeAIB is retained in the brain. System A is involved in efflux transport for L-Pro at the BBB. The predominantly expressed ATA2 mRNA at the BBB may play a role in maintaining the concentration of small neutral amino acids and cerebral osmotic pressure in the brain under pathological conditions.

Comparison of blood-brain barrier permeability in mice and rats using in situ brain perfusion technique

Here we present a method for measuring the permeability coefficient-surface area product (PS) values at the blood-brain barrier in mice, using the in situ brain perfusion technique originally developed for rats by Takasato et al. (Am J Physiol Heart Circ Physiol 247: H484-H493, 1984). Retrograde infusion into the right external carotid artery increased the carotid perfusion pressure in proportion to the perfusion rate. Intravascular volume and cerebral perfusion fluid flow at a perfusion rate of 1.0 ml/min in mice were similar to those in rats. In addition, the contribution of systemic blood to total flow in the hemisphere was small (only 3. 2%). These findings indicated that this perfusion rate is suitable for mice. The PS values of more than 20 different compounds were determined in mice by using the in situ brain perfusion technique, and comparisons were made with data from rats. There was a close relationship (1:1) between the PS values in mice and rats, indicating that brain capillary permeabilities are similar in mice and rats.

Rediscovering good old friend IGF-I in the new millenium: possible usefulness in Alzheimer's disease and stroke

Much research has been done over the past two decades on the role of insulin-like growth factors I and II (IGF) in the maintenance of normal body homeostasis, especially in regard to various endocrine functions, growth and aging. For example, IGF-I is a well established promoter of tissue growth and has been used in the clinics for the treatment of growth related disorders, even being abused by athletes to enhance performance in competitions. In contrast, comparatively limited attention has been given to the potential significance of the IGFs in the central nervous system. Over the past few years, we have studied the trophic as well as neuromodulatory roles of the IGFs in the brain. IGF-I and IGF-II are potent modulators of acetylcholine release, IGF-I inhibiting release while IGF-II is a potent stimulant. Moreover, only the internalization of the IGF-I receptor complex was blocked by an inhibitor of phosphotyrosylation. This is in accordance with the differential nature of the IGF-I and IGF-II receptors, the former being a tyrosine kinase receptor while the later is a single transmembrane domain protein bearing binding sites for 6-mannose phosphate containing residues. The activation of IGF-I receptors protected neurons against cell death induced by amyloidogenic derivatives likely by an intracellular mechanism distinct from those involved in the regulation of acetylcholine release and neuronal growth. The stimulation of IGF-I receptors can activate intracellular pathways implicating a PI3/Akt kinase and CREB phosphorylation or modulate the production of free radicals. The effects, particularly those of IGF-I on key markers of the Alzheimer's (AD) brains namely cholinergic dysfunction, neuronal amyloid toxicity, tau phosphorylation and glucose metabolism suggest the potential usefulness of this growth factor in the treatment of neurodegenerative diseases. However, the poor bioavailability, enzymatic stability and brain penetration of IGF-I hamper progress in this regard. The recent development of a small, non-peptidyl mimetic of insulin able to directly activate the insulin receptor [Zhang, B., Salituro, G., Szalkowski, D., Li, Z., Zhang, Y., Royo, I., Vilella, D., Diez, M.T., Pelaez, F., Ruby, C., Kendall, R.L., Mao, X., Griffin, P., Calaycay, J., Zierath, J.R., Heck, J. V., Smith, R.G., Moller, D.E., 1999. Science, 284, 974-977] suggests that a similar strategy could be used for IGF-I and the IGF-I receptor leading to the characterization of IGF-I mimics of potential clinical usefulness.

Measurement of blood-brain barrier permeability of rats with alpha-aminoisobutyric acid during microdialysis: possible application to behavioral studies

The purpose of this study was to determine the suitability of alpha-aminoisobutyric acid (AIB) as a marker of the integrity of the blood-brain barrier (BBB) during studies in which amino acid content in dialysates collected by microdialysis probes and behavior of the rat are studied concurrently. AIB (200 mg/kg) was injected intraperitoneally at 1800 h into 24-h fasted, ketamine-acepromazine anesthetized rats on either 10 or 30 days following guide cannula implantation. Dialysates from the medial preoptic area and blood from the tail vein were collected 1 h before and after the AIB injection. Analysis of amino acids, including AIB, in the collections was conducted by reverse-phase HPLC. In 21 of 24 rats, AIB in dialysates averaged less than 3% of plasma at 10, 30, and 50 min after AIB injection. In those rats unidirectional blood-to-brain transfer constant, K(i), of AIB was constant and a good relationship was found between dialysate amino acid concentrations and those predicted from calculations of transport based on the brain uptake index (BUI) for some amino acids. AIB concentration in dialysates was greater than 10% of plasma at 30 min in only 3 of 24 rats. We conclude that AIB can be used as a marker to monitor the integrity of the BBB during serial measurements of dialysate concentrations of amino acids and has application in behavioral studies.