The Astroglial ASCT2 Amino Acid Transporter as a Mediator of Glutamine Efflux

Glutamine release from astrocytes is an essential part of the glutamate-glutamine cycle in the brain. Uptake of glutamine into cultured rat astrocytes occurs by at least four different routes. In agreement with earlier studies, a significant contribution of amino acid transport systems ASC, A, L, and N was detected. It has not been determined whether these systems are also involved in glutamine efflux or whether specific efflux transporters exist. We show here that ASCT2, a variant of transport system ASC, is strongly expressed in rat astroglia-rich primary cultures but not in neuron-rich primary cultures. The amino acid sequence of rat astroglial ASCT2 is 83% identical to that of mouse ASCT2. In Xenopus laevis oocytes expressing rat ASCT2, we observed high-affinity uptake of [U-14C]glutamine (Km = 70 microM) that was Na(+)-dependent, concentrative, and unaffected by membrane depolarization. When oocytes were preloaded with [U-14C]glutamine, no glutamine efflux was detected in the absence of extracellular amino acids. Neither lowering intracellular pH nor raising the temperature elicited efflux. However, addition of 0.1 mM unlabeled alanine, serine, cysteine, threonine, glutamine, or leucine to the extracellular solution resulted in a rapid release of glutamine from the ASCT2-expressing oocytes. Amino acids that are not recognized as substrates by ASCT2 were ineffective in this role. Extracellular glutamate stimulated glutamine release weakly at pH 7.5 but was more effective on lowering pH to 5.5, consistent with the pH dependence of ASCT2 affinity for glutamate. Our findings suggest a significant role of ASCT2 in glutamine efflux from astrocytes by obligatory exchange with extracellular amino acids. However, the relative contribution of this pathway to glutamine release from cells in vivo or in vitro remains to be determined.     

Uptake and Efflux of Quinacrine, a Candidate for the Treatment of Prion Diseases, at the Blood-Brain Barrier

1. A clinical trial of quinacrine in patients with Creutzfeldt-Jakob disease is now in progress. The permeability of drugs through the blood-brain barrier (BBB) is a determinant of their therapeutic efficacy for prion diseases. The mechanism of quinacrine transport across the BBB was investigated using mouse brain endothelial cells (MBEC4). 2. The permeability of quinacrine through MBEC4 cells was lower than that of sodium fluorescein, a BBB-impermeable marker. The basolateral-to-apical transport of quinacrine was greater than its apical-to-basolateral transport. In the presence of P-glycoprotein (P-gp) inhibitor, cyclosporine or verapamil, the apical-to-basolateral transport of quinacrine increased. The uptake of quinacrine by MBEC4 cells was enhanced in the presence of cyclosporine or verapamil. 3. Quinacrine uptake was highly concentrative, this event being carried out by a saturable and carrier-mediated system with an apparent Km of 52.1 microM. Quinacrine uptake was insensitive to Na+-depletion and changes in the membrane potential and sensitive to changes in pH. This uptake was decreased by tetraethylammonium and cimetidine, a substrate and an inhibitor of organic cation transporters, respectively. 4. These findings suggest that quinacrine transport at the BBB is mediated by the efflux system (P-gp) and the influx system (organic cation transporter-like machinery).     

Blood-Brain Barrier Produces Significant Efflux of L-Aspartic Acid but Not D-Aspartic Acid

The brain efflux index method has been used to clarify the mechanism of efflux transport of acidic amino acids such as L-aspartic acid (L-Asp), L-glutamic acid (L-Glu), and D-aspartic acid (D-Asp) across the blood-brain barrier (BBB). About 85% of L-[3H]Asp and 40% of L-[3H]Glu was eliminated from the ipsilateral cerebrum within, respectively, 10 and 20 min of microinjection into the brain. The efflux rate constant of L-[3H]Asp and L-[3H]Glu was 0.207 and 0.0346 min(-1), respectively. However, D-[3H]Asp was not eliminated from brain over a 20-min period. The efflux of L-[3H]Asp and L-[3H]Glu was inhibited in the presence of excess unlabeled L-Asp and L-Glu, whereas D-Asp did not inhibit either form of efflux transport. Aspartic acid efflux across the BBB appears to be stereospecific. Using a combination of TLC and the bioimaging analysis, attempts were made to detect the metabolites of L-[3H]Asp and L-[3H]Glu in the ipsilateral cerebrum and jugular vein plasma following a microinjection into parietal cortex, area 2. Significant amounts of intact L-[3H]Asp and L-[3H]Glu were found in all samples examined, including jugular vein plasma, providing direct evidence that at least a part of the L-Asp and L-Glu in the brain interstitial fluid is transported across the BBB in the intact form. To compare the transport of acidic amino acids using brain parenchymal cells, brain slice uptake studies were performed. Although the slice-to-medium ratio of D-[3H]Asp was the highest, followed by L-[3H]Glu and L-[3H]Asp, the initial uptake rate did not differ for both L-[3H]Asp and D-[3H]Asp, suggesting that the uptake of aspartic acid in brain parenchymal cells is not stereospecific. These results provide evidence that the BBB may act as an efflux pump for L-Asp and L-Glu to reduce the brain interstitial fluid concentration and act as a static wall for D-Asp.     

Efflux of a suppressive neurotransmitter, GABA, across the blood-brain barrier

In this study, GABA efflux transport from brain to blood was estimated by using the brain efflux index (BEI) method. [3H]GABA microinjected into parietal cortex area 2 (Par2) of the rat brain was eliminated from the brain with an apparent elimination half-life of 16.9 min. The blood-brain barrier (BBB) efflux clearance of [3H]GABA was at least 0.153 mL/min/g brain, which was calculated from the elimination rate constant (7.14 x 10(-2) x min(-1)) and the distribution volume in the brain (2.14 mL/g brain). Direct comparison of the apparent BBB influx clearance [3H]GABA (9.29 microL/min/g brain) and the apparent efflux clearance (153 microL/min/g brain) indicated that the efflux clearance was at least 16-fold greater than the influx clearance. In order to reduce the effect of metabolism in the neuronal cells following intracerebral microinjection, we determined the apparent efflux of [3H]GABA in the presence of nipecotic acid, a GABA transport inhibitor in parenchymal cells, using the BEI method. Under such conditions, the elimination of [3H]GABA across the BBB showed saturation and inhibition by probenecid in the presence of nipecotic acid. Furthermore, the uptake of [3H]GABA by MBEC4 cells was inhibited by GABA, taurine, beta-alanine and nipecotic acid in a concentration-dependent manner. It is likely that GABA inhibits the first step in the abluminal membrane uptake by brain endothelial cells, and that probenecid selectively inhibits the luminal membrane efflux transport process from the brain capillary endothelial cells based on the in vivo and in vitro evidence. The BBB acts as the efflux pump for GABA to reduce the brain interstitial fluid concentration.     

Blood-brain barrier is involved in the efflux transport of a neuroactive steroid, dehydroepiandrosterone sulfate, via organic anion transporting polypeptide 2

We have investigated the transport characteristics of dehydroepiandrosterone sulfate (DHEAS), a neuroactive steroid, at the blood-brain barrier (BBB) in a series of functional in vivo and in vitro studies. The apparent BBB efflux rate constant of [(3)H]DHEAS evaluated by the brain efflux index method was 2.68 x 10(-2) min(-1). DHEAS efflux transport was a saturable process with a Michaelis constant (K:(m)) of 32.6 microM: Significant amounts of [(3)H]DHEAS were determined in the jugular venous plasma by HPLC, providing direct evidence that most of the DHEAS is transported in intact form from brain to the circulating blood across the BBB. This efflux transport of [(3)H]DHEAS was significantly inhibited by common rat organic anion-transporting polypeptide (oatp) substrates such as taurocholate, cholate, sulfobromophthalein, and estrone-3-sulfate. Moreover, the apparent efflux clearance of [(3)H]DHEAS across the BBB (118 microl/min-g of brain) was 10.4-fold greater than its influx clearance estimated by the in situ brain perfusion technique (11.4 microl/min-g of brain), suggesting that DHEAS is predominantly transported from the brain to blood across the BBB. In cellular uptake studies using a conditionally immortalized mouse brain capillary endothelial cell line (TM-BBB4), [(3)H]DHEAS uptake by TM-BBB4 cells exhibited a concentration dependence with a K:(m) of 34.4 microM: and was significantly inhibited by the oatp2-specific substrate digoxin. Conversely, [(3)H]digoxin uptake by TM-BBB4 cells was significantly inhibited by DHEAS. Moreover, the net uptake of [(3)H]DHEAS at 30 min was significantly increased under ATP-depleted conditions, suggesting that an energy-dependent efflux process may also be involved in TM-BBB4. RT-PCR and sequence analysis suggest that an oatp2 is expressed in TM-BBB4 cells. In conclusion, DHEAS efflux transport takes place across the BBB, and studies involving in vitro DHEAS uptake and RT-PCR suggest that there is oatp2-mediated DHEAS transport at the BBB.     

Glutamine transport in C6 glioma cells shows ASCT2 system characteristics

Previous studies from this laboratory have shown that glutamine (Gln) uptake in a rat astrocytoma-derived C6 cell line shows characteristics similar with the uptake of a model ASC system substrate, threonine, whose pH-dependence and partial tolerance of Li(+) substitution for Na(+) resemble the ASCT2 variant of the system. In support of the previous findings, RT-PCR analysis revealed that C6 cells strongly express ASCT2 mRNA, but not at all GlnT mRNA or NAT2 mRNA, the A and N system variants specifically engaged in Gln transport in normal CNS. Other features of Gln transport in C6 cells indicating the involvement of ASCT2 system included its resistance to ouabain and stimulation of Gln efflux from the cells in the presence of excess Gln or cysteine (Cys), demonstrating that the system operates in the exchange mode. Replacement of NaCl in the incubation medium with isoosmotic sucrose did neither significantly affect the kinetics, nor any other major characteristics of Gln or Thr transport, including its pH-dependence, inhibition by ASCT system substrates or resistance to the model system A substrate-N-methylamino-isobutyric acid (MeAiB).     

Glutamine uptake and expression of mRNA's of glutamine transporting proteins in mouse cerebellar and cerebral cortical astrocytes and neurons

The relative roles of the three sodium-dependent transport systems: A, ASC and N in the uptake of [3H]Gln, and the compatibility of the uptake characteristics with the expression of mRNAs coding for the Gln transporting molecules, were examined in primary cultures of astrocytes and neurons derived from mouse cerebellum, a glutaminergic system-enriched structure, and in cerebral cortex. Gln uptake activity (Vmax) was higher in cerebellar astrocytes or neurons than in their cerebral cortical counterparts. The N-methylamino-isobutyric acid (MeAiB)- and pH-sensitive, system A-mediated component of the uptake, and the uptake of [14C]MeAiB itself, was much more active in neurons than in astrocytes derived from either region. Also, the expression of mRNA for GlnT (SAT1), a system A isoform specific for Gln, was only expressed in neurons derived from both structures, while an alanine (Ala)-preferring system A transporter, SAT2, was expressed in neurons and astrocytes from either region. System ASC-mediated Gln uptake and expression of ASCT2 mRNA were in both structures more pronounced in astrocytes than in neurons, consistent with the postulated role of ASCT2 in the efflux of de novo synthesized Gln from astrocytes. System N-mediated (threonine+MeAiB-inhibitable) Gln uptake showed comparable activities in all four types of cells, which is compatible with the ubiquitous expression of NAT2 mRNA-a mouse brain-specific N-system isoform.     

Organic anion transporter 3 is involved in the brain-to-blood efflux transport of thiopurine nucleobase analogs

Thiopurines are used as antileukemic drugs. However, during chemotherapy CNS relapses occur due to the proliferation of leukemic cells in the CNS resulting from restricted drug distribution in the brain. The molecular mechanism for this limited cerebral distribution remains unclear. The purpose of this study was to identify the transporter responsible for the brain-to-blood transport of thiopurines across the blood-brain barrier (BBB) using the brain efflux index method. [14C]6-Mercaptopurine (6-MP) and [3H]6-thioguanine were eliminated from rat brain in a time-dependent manner. The elimination of [14C]6-MP was inhibited by substrates of rat organic anion transporters (rOATs), including indomethacin and benzylpenicillin. rOAT1 and rOAT3 exhibited 6-MP uptake, while benzylpenicillin inhibited rOAT3-mediated uptake, but not that by rOAT1. rOAT3-mediated [14C]6-MP uptake was also inhibited by other thiopurine derivatives. Although methotrexate inhibited rOAT3-mediated [14C]6-MP uptake, the Ki value was 17.5-fold greater than the estimated brain concentration of methotrexate in patients receiving chemotherapy. Accordingly, 6-MP would undergo efflux transport by OAT3 from the brain without any inhibitory effect from coadministered methotrexate in the chemotherapy. In conclusion, rOAT3 is involved in the brain-to-blood transport of thiopurines at the BBB and is one mechanism of limited cerebral distribution.     

Manganese disrupts astrocyte glutamine transporter expression and function

Glutamine (Gln) plays an important role in brain energy metabolism and as a precursor for the synthesis of neurotransmitter glutamate and GABA. Previous studies have shown that astrocytic Gln transport is impaired following manganese (Mn) exposure. The present studies were performed to identify the transport routes and the respective Gln transporters contributing to the impairment. Rat neonatal cortical primary astrocytes treated with Mn displayed a significant decrease in Gln uptake mediated by the principle Gln transporting systems, N and ASC. Moreover, systems N, ASC and L were less efficient in Gln export after Mn treatment. Mn treatment caused a significant reduction of both in mRNA expression and protein levels of SNAT3 (system N), SNAT2 (system A) and LAT2 (system L), and lowered the protein but not mRNA expression of ASCT2 (system ASC). Mn exposure did not affect the expression of the less abundant systems N transporter SNAT5 and the system L transporter LAT1, at either the mRNA or protein level. Hence, Mn-induced decrease of inward and outward Gln transport can be largely ascribed to the loss of the specific Gln transporters. Consequently, deregulation of glutamate homeostasis and its diminished availability to neurons may lead to impairment in glutamatergic neurotransmission, a phenomenon characteristic of Mn-induced neurotoxicity.     

Brain to blood efflux transport of adenosine: blood-brain barrier studies in the rat

The blood-brain barrier (BBB) efflux transport of [(14)C] adenosine was studied using the brain efflux index (BEI) technique. BEI increased linearly over the first 2 min after injection, with deviation from linearity thereafter; 90.12 +/- 1.5% of the injected [(14)C] radioactivity remained within the brain after 20 min. The remaining tracer appears to be mainly intracellular, trapped by phosphorylation, as an almost linear increase of BEI over 20 min was observed after intracerebral injection of [(14)C] adenosine together with 5-iodo tubercidin. The BBB efflux clearance of [(14)C] radioactivity was estimated to be 27.62 +/- 5.2 micro L/min/g, almost threefold higher than the BBB influx clearance estimated by the brain uptake index technique. High-performance liquid chromatography (HPLC) analysis of blood plasma collected from the jugular vein after the intracerebral injection revealed metabolic breakdown of [(14)C] adenosine into nucleobases. The BBB efflux transport was saturable with apparent K(m) = 13.22 +/- 1.75 micro m and V(max) = 621.07 +/- 71.22 pmole/min/g, which indicated that BBB efflux in vivo is 6.2-12p mole/min/g, negligible when compared to the reported rate of adenosine uptake into neurones/glia. However, these kinetic parameters also suggest that under conditions of elevated ISF adenosine in hypoxia/ischaemia, BBB efflux transport could increase up to 25% of the uptake into neurones/glia and become an important mechanism to oppose the rise in ISF concentration. HPLC-fluorometry detected 93.6 +/- 5.25 nm of adenosine in rat plasma, which is 17- to 220-fold lower than the reported K(m) of adenosine BBB influx in rat. Together with the observed rapid degradation inside endothelial cells, this indicated negligible BBB influx of intact adenosine under resting conditions. Cross-inhibition studies showed that unlabelled inosine, adenine and hypoxanthine caused a decrease in BBB efflux of [(14)C] radioactivity in a concentration-dependent manner, with K(i) of 16.7 +/- 4.88, 65.1 +/- 14.1 and 71.1 +/- 16.9 micro m, respectively. This could be due to either competition of unlabelled molecules with [(14)C] adenosine or competition with its metabolites hypoxanthine and adenine for the same transport sites.     

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.     

Influence of breast cancer resistance protein (Abcg2) and p-glycoprotein (Abcb1a) on the transport of imatinib mesylate (Gleevec) across the mouse blood-brain barrier

Imatinib, a protein tyrosine kinase inhibitor, may prevent the growth of glioblastoma cells. Unfortunately, its brain distribution is restricted by p-glycoprotein (p-gp or multidrug resistance protein Mdr1a), and probably by breast cancer resistance protein (Bcrp1), two efflux pumps expressed at the blood-brain barrier (BBB). We have used in situ brain perfusion to investigate the mechanisms of imatinib transport across the mouse BBB. The brain uptake of imatinib in wild-type mice was limited by saturable efflux processes. The inhibition of p-gp, by valspodar and zosuquidar, increased imatinib uptake (2.5-fold), as did the deficiency of p-gp in Mdr1a/1b(-/-) mice (5.5-fold). Perfusing imatinib with the p-gp/Bcrp1 inhibitor, elacridar, enhanced the brain uptake of imatinib in wild-type (4.1-fold) and Mdr1a/1b(-/-) mice (1.2-fold). However, the brain uptake of imatinib was similar in wild-type and Bcrp1(-/-) mice when it was perfused at a non-saturating concentration. The brain uptake of CGP74588, an active metabolite of imatinib, was low. It was increased by perfusion with elacridar (twofold), but not with valspodar and zosuquidar. CGP74588 uptake was 1.5 times greater in Bcrp1(-/-) mice than in wild-type mice. These data suggest that imatinib transport at the mouse BBB is limited by p-gp and probably by Bcrp1, and that CGP74588 transport is restricted by Bcrp1.     

Dibasic amino acid interactions with Na+-independent transport system asc in horse erythrocytes. Kinetic evidence of functional and structural homology with Na+-dependent system ASC

Amino acid transport in horse erythrocytes is regulated by three co-dominant allelomorphic genes coding for high-affinity transport activity (system asc1), low-affinity transport activity (system asc2) and transport-deficiency, respectively. The asc systems are selective for neutral amino acids of intermediate size, but unlike conventional system ASC, do not require Na+ for activity. In the present series of experiments we have used a combined kinetic and genetic approach to establish that dibasic amino acids are also asc substrates, systems asc1 and asc2 representing the only mediated routes of cationic amino acid transport in horse erythrocytes. Both transporters were found to exhibit a strong preference for dibasic amino acids compared with neutral amino acids of similar size. Apparent Km values (mM) for influx via system asc1 were L-lysine (9), L-ornithine (27), L-arginine (27), L-alanine (0.35). Corresponding Vmax estimates (mmol/l cells per h, 37 degrees C) were L-lysine (1.65), L-ornithine (2.15), L-arginine (0.54), L-alanine (1.69). Apparent Km values for L-lysine and L-ornithine influx via system asc2 were approximately 90 and greater than 100 mM, respectively, with Vmax values greater than 2 and greater than 1 mmol/l cells per h, respectively. Apparent Km and Vmax values for L-alanine uptake by system asc2 were 14 mM and 6.90 mmol/l cells per h. In contrast, L-arginine was transported by system asc2 with the same apparent Km as L-alanine (14 mM), but with a 77-fold lower Vmax. This dibasic amino acid was shown to cause cis- and trans-inhibition of system asc2 in a manner analogous to its interaction with system ASC, where the side-chain guanidinium group is considered to occupy the Na+-binding site on the transporter. Concentrations of extracellular L-arginine causing 50% inhibition of zero-trans L-alanine influx and half-maximum inhibition of L-alanine zero-trans efflux were 14 mM (extracellular L-alanine concentration 15 mM) and 3 mM (intracellular L-alanine concentration 15.5 mM), respectively. We interpret these observations as evidence of structural homology between the horse erythrocyte asc transporters and system ASC. Physiologically, intracellular L-arginine may function as an endogenous inhibitor of system asc2 activity.     

Evidence that the species barrier of human immunodeficiency virus-1 does not extend to uptake by the blood--brain barrier: comparison of mouse and human brain microvessels

HIV-1 within the CNS produces a neuroAIDS syndrome and may act as a reservoir for reinfection of the peripheral tissues. Study of how HIV-1 crosses the blood-brain barrier (BBB) has been hampered by the lack of nonprimate animal models. However, BBB transport of HIV-1 does not involve any of the known steps conferring species specificity, including binding to CD4 receptors. In vivo and in vitro studies show that HIV-1 and its glycoprotein coat, gp120, are taken up and transported across the BBB of the mouse. Here, we compared the ability of gp120 and HIV-1 to be taken up by isolated brain microvessels (IBM) freshly isolated from mice, from post-mortem human brain, and from mice that had been treated in a manner analogous to the human material (mouse post-mortem). Freshly isolated mouse IBM took up more gp120 and HIV-1 than the human or mouse post-mortem cells. We found no difference between the ability of mouse post-mortem and human IBM to take up either gp120 or HIV-1. Wheatgerm agglutinin has been previously shown to stimulate gp120 and HIV-1 uptake by the BBB; here, it stimulated the uptake of gp120 and of HIV-1 by both mouse post-mortem and human IBM, although stimulated uptake was greatest for fresh mouse IBM. These results show that the mouse can be used to study the initial phases of HIV-1 uptake by the BBB.     

Nicotine and cotinine increases the brain penetration of saquinavir in rat

Endothelial tight junctions and efflux transporters of the blood-brain barrier (BBB) significantly limit brain accumulation of many drugs, including protease inhibitors such as saquinavir. The cholinergic agonist nicotine is one of the most commonly used drugs in the world and the incidence is even higher in the human immune deficiency virus population (～ 70%). We examined the ability of nicotine and its primary metabolite cotinine to modify brain uptake of saquinavir in rats. Both nicotine and cotinine at pharmacological concentrations matching those in smokers, increased brain saquinavir uptake by two fold. Co-perfusion with nicotinic receptor antagonists and passive permeability markers showed that the effect was not caused by receptor activation or BBB permeability disruption. Transport inhibition studies demonstrated that brain saquinavir uptake is limited by multiple efflux transporters, P-glycoprotein (P-gp), breast cancer resistance protein and multidrug resistance-associated protein. In situ perfusion and in vitro experiments using a classical P-gp substrate rhodamine 123 linked the effect of nicotine to inhibition of BBB P-gp transport. The effect was confirmed in vivo in chronic 14 day nicotine administration animals. These data suggest nicotine increases antiretroviral drug exposure to brain and may represent a significant in vivo drug-drug interaction at the BBB. Although this may slightly benefit CNS antiretroviral efficacy, it may also expose the brain to potential serious neurotoxicity.     

微生物有机酸转运蛋白的研究进展

转运蛋白是一类膜蛋白,可介导生物膜内外化学物质的跨膜转运及信号交换。有机酸转运蛋白在微生物有机酸代谢的跨膜转运过程中发挥重要作用,根据转运蛋白有机酸转运的方向不同可以分为摄取转运蛋白和外排转运蛋白。在微生物代谢中,有些有机酸可以作为能源直接参与体内代谢,有些是能量转换过程中的重要中间产物;摄取转运蛋白的过表达,可以促进微生物细胞获取能源物质,高效的生产目标产物;有机酸摄取转运蛋白敲除或外排转运蛋白表达,有利于底盘细胞外排更多目标产物,进而促进有机酸的生物合成。研究有机酸转运蛋白的结构和功能,有助于解析微生物细胞有机酸生物合成及利用的机制,对于提高工业微生物对有机酸的利用及生物合成具有重要作用。本文综述了微生物有机酸转运蛋白分类和结构、转运方式和转运功能等方面,重点综述了转运蛋白在有机酸生产中的应用,为工业微生物有机酸的高效生物合成及未来发展提供参考。     

Blood-to-brain influx transport of nicotine at the rat blood–brain barrier: Involvement of a pyrilamine-sensitive organic cation transport process

Nicotine is the most potent neural pharmacological alkaloid in tobacco, and the modulation of nicotine concentration in the brain is important for smoking cessation therapy. The purpose of this study was to elucidate the net flux of nicotine transport across the blood–brain barrier (BBB) and the major contributor to nicotine transport in the BBB. The in vivo brain-to-blood clearance was determined by a combination of the rat brain efflux index method and a rat brain slice uptake study, and the blood-to-brain transport of nicotine was evaluated by in vivo vascular injection in rats and a conditionally immortalized rat brain capillary endothelial cell line (TR-BBB13 cells) as an in vitro model of the rat BBB. The blood-to-brain nicotine influx clearance was obtained by integration plot analysis as 272 μL/(min g brain), and this value was twofold greater than the brain-to-blood efflux clearance (137 μL/(min g brain)). Thus, it is suggested that the net flux of nicotine transport across the BBB is dominated by blood-to-brain influx transport. In vivo blood-to-brain nicotine transport was inhibited by pyrilamine. [³H]Nicotine uptake by TR-BBB13 cells exhibited time-, temperature-, and concentration-dependence with a K_m value of 92 μM. Pyrilamine competitively inhibited nicotine uptake by TR-BBB13 cells with a K_i value of 15 μM, whereas substrates and inhibitors of organic cation transporters had little effect. These results suggest that pyrilamine-sensitive organic cation transport process(es) mediate blood-to-brain influx transport of nicotine at the BBB, and this is expected to play an important role in regulating nicotine-induced neural responses.     

Asymmetry of glutamine transporters in cultured neural cells

Transfer of glutamine between astrocytes and neurons is an essential part of the glutamate-glutamine cycle in the brain. Transport of glutamine was investigated in primary cultures of astrocytes and neurons and compared to glutamine transport in cell lines with glial and neuronal properties. Glutamine uptake in astrocytes was mainly mediated by general amino acid transporters with properties similar to ASCT2, LAT1, LAT2, SN1 and y(+)LAT2. In cultured neurons, transport activities were detected consistent with the presence of LAT1, LAT2 and y(+)LAT2, but the most prominent activity was a novel Na(+)-dependent glutamine transporter that could be inhibited by D-aspartate. The mRNA for system A isoforms ATA1 and ATA2 was detected in both neurons and astrocytes, but system A activity was only detected in neurons. ASCT2 on the other hand appeared to be astrocyte-specific. The cell lines F98 and 108CC-15, having astroglial and neuronal properties, respectively, expressed sets of glutamine transporters that were unrelated to those of the corresponding primary culture and are thus of limited use as models to study transfer of glutamine between astrocytes and neurons.     

Cysteine and Cystine Transport at the Blood-Brain Barrier

Abstract: The nature of cysteine and cystine uptake from the cerebral capillary lumen was studied in the rat using the carotid injection technique. [³⁵S]-Cysteine uptake was readily inhibited by the synthetic amino acid 2-amino-bicyclo(2,2,1)heptane-2-carboxylic acid (BCH), the defining substrate for the leucine-preferring (L) system in the Ehrlich ascites cell. The addition of non-radioactive alanine or serine, representatives of the alanine, serine, and cysteine-preferring (ASC) system, produced no significant decrease in the uptake of cysteine after cysteine transport by the L system was blocked with BCH. This indicated that the major component of cysteine's transport from the brain capillary lumen was by the L system with no detectable uptake of cysteine by the ASC system. No carrier-mediated transport of cystine, the disulfide form of the amino acid, was detected, nor was there any inhibition by cystine of the transport of the neutral amino acid methionine or the basic amino acid arginine. These results suggest that the ASC system, if present, is not quantitatively important for the transport of neutral amino acids from the brain capillary lumen.