Transport of Amino Acids Across the Blood-Brain Barrier (original) (raw)
Related papers
Structure of the blood-brain barrier and its role in the transport of amino acids
The Journal of nutrition, 2006
Brain capillary endothelial cells form the blood-brain barrier (BBB). They are connected by extensive tight junctions, and are polarized into luminal (blood-facing) and abluminal (brain-facing) plasma membrane domains. The polar distribution of transport proteins mediates amino acid (AA) homeostasis in the brain. The existence of two facilitative transporters for neutral amino acids (NAAs) on both membranes provides the brain access to essential AAs. Four Na(+)-dependent transporters of NAA exist in the abluminal membranes of the BBB. Together these systems have the capability to actively transfer every naturally occurring NAA from the extracellular fluid (ECF) to endothelial cells and from there into circulation. The presence of Na(+)-dependent carriers on the abluminal membrane provides a mechanism by which NAA concentrations in the ECF of brain are maintained at approximately 10% those of the plasma. Also present on the abluminal membrane are at least three Na(+)-dependent system...
American journal of physiology. Endocrinology and metabolism, 2003
Several Na+-dependent carriers of amino acids exist on the abluminal membrane of the blood-brain barrier (BBB). These Na+-dependent carriers are in a position to transfer amino acids from the extracellular fluid of brain to the endothelial cells and thence to the circulation. To date, carriers have been found that may remove nonessential, nitrogen-rich, or acidic (excitatory) amino acids, all of which may be detrimental to brain function. We describe here Na+-dependent transport of large neutral amino acids across the abluminal membrane of the BBB that cannot be ascribed to currently known systems. Fresh brains, from cows killed for food, were used. Microvessels were isolated, and contaminating fragments of basement membranes, astrocyte fragments, and pericytes were removed. Abluminal-enriched membrane fractions from these microvessels were prepared. Transport was Na+ dependent, voltage sensitive, and inhibited by 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid, a particular inhibi...
Synergism between the Two Membranes of the Blood-brain Barrier: Glucose and Amino Acid Transport
Jurnal Pengurusan
Brain capillary endothelial cells, which are connected by extensive tight junctions and are polarized into luminal (blood-facing) and abluminal (brain-facing) plasma membrane domains, form the blood-brain barrier (BBB). The polar distribution of transport proteins mediates glucose and amino acid (AA) homeostasis in the brain. The ability to isolate the luminal and abluminal membranes has permitted the study of each side of the BBB separately in vitro and yielded new information on BBB function. The two membranes have different characteristics. Facilitative transporters were found on both membranes in a position to permit the bidirectional transport of glucose, almost all amino acids and taurine. Na +-dependent transporters were only found on abluminal membranes. The Na +-dependent transporters on the abluminal side are capable of removing virtually all amino acids including acidic AA from the extracellular fluid of brain (ECF). The presence of Na +-dependent carriers on the abluminal membrane provides a mechanism by which the concentrations of AA, glucose and taurine in the ECF of brain may be maintained at optimal levels under physiological and pathophysiological circumstances. Facilitative carriers for glutamine (n) and glutamate (xg-) are found only in the luminal membrane of the BBB. This organization allows the net removal of acidic and nitrogen-rich AA from brain, and explains the low rate of glutamate and glutamine penetration into the central nervous system. The presence of a g-glutamyl cycle at the luminal membrane and Na +-dependent AA transporters at the abluminal membrane may serve to modulate movement of AA from blood to brain. The g-glutamyl cycle is expected to generate pyroglutamate within the endothelial cells. Pyroglutamate stimulates Na +-dependent AA transporters at the abluminal membrane thereby reducing net influx of AA the to brain. It is now clear the BBB may actively participate in the regulation of the AA content of the brain as well as contributing to the control of brain osmolarity.
Cationic amino acid transport across the blood-brain barrier is mediated exclusively by system y+
AJP: Endocrinology and Metabolism, 2006
Cationic amino acid (CAA) transport is brought about by two families of proteins that are found in various tissues: Cat (CAA transporter), referred to as system y+, and Bat [broad-scope amino acid (AA) transporter], which comprises systems b0,+, B0,+, and y+L. CAA traverse the blood-brain barrier (BBB), but experiments done in vivo have only been able to examine the BBB from the luminal (blood-facing) side. In the present study, plasma membranes isolated from bovine brain microvessels were used to identify and characterize the CAA transporter(s) on both sides of the BBB. From these studies, it was concluded that system y+ was the only transporter present, with a prevalence of activity on the abluminal membrane. System y+ was voltage dependent and had a Km of 470 ± 106 μM (SE) for lysine, a Ki of 34 μM for arginine, and a Ki of 290 μM for ornithine. In the presence of Na+, system y+ was inhibited by several essential neutral AAs. The Ki values were 3–10 times the plasma concentration...
Biochemical and Biophysical Research Communications, 2008
Brain capillary endothelial cells control the uptake and efflux from the brain of many hydrophilic compounds due to highly specialized transporters often localized in a polarized way. Localization of Na +and Cl À -dependent amino acid and carnitine transporter B 0,+ (ATB 0,+ ) was studied in a co-culture of bovine brain capillary endothelial cells (BBCEC) grown on filters above astrocytes (an in vitro blood-brain barrier model). Immunoblotting and three-dimensional immunocytochemistry analysis with anti-B 0,+ antibodies demonstrated the presence of this transporter and its prevalent co-localization with P-glycoprotein i.e. at the apical side. The sensitivity of leucine uptake through the apical membrane to 2-aminobicyclo-[2.2.1]-heptane-2-carboxylic acid (BCH), D-serine as well as sodium and chloride replacement confirm the functioning of ATB 0,+ and suggests an important physiological role of ATB 0,+ in controlling the delivery of amino acids and carnitine to the brain.
Selective Expression of the Large Neutral Amino Acid Transporter at the Blood-Brain Barrier
Proceedings of The National Academy of Sciences, 1999
Amino acid supply in brain is regulated by the activity of the large neutral amino acid transporter (LAT) at the brain capillary endothelial cell, which forms the blood-brain barrier (BBB) in vivo. Bovine BBB poly(A)+ RNA was isolated from 2.0 kg of fresh bovine brain and size fractionated on a sucrose density gradient, and a size-fractionated bovine BBB cDNA library in the pSPORT vector was prepared. The full-length cDNA encoding the bovine BBB LAT was isolated from this library, and the predicted amino acid sequence was 89-92% identical to the LAT1 isoform. The bovine BBB LAT1 mRNA produced a 10-fold enhancement in tryptophan transport into frog oocytes coinjected with bovine BBB LAT1 mRNA and the mRNA for 4F2hc, which encodes the heavy chain of the heterodimer. Tryptophan transport into the mRNA-injected oocytes was sodium independent and was specifically inhibited by other large neutral amino acids, and the Km of tryptophan transport was 31.5± 5.5 μ M. Northern blotting with the bovine BBB LAT1 cDNA showed that the LAT1 mRNA is 100-fold higher in isolated bovine brain capillaries compared with C6 rat glioma cells or rat brain, and the LAT1 mRNA was not detected in rat liver, heart, lung, or kidney. These studies show that the LAT1 transcript is selectively expressed at the BBB compared with other tissues, and the abundance of the LAT1 mRNA at the BBB is manyfold higher than that of transcripts such as the 4F2hc antigen, actin, or the Glut1 glucose transporter.
Experimental Neurology, 2012
Luminal and abluminal plasma membranes were isolated from bovine brain microvessels and used to identify and characterize Na + -dependent and facilitative taurine transport. The calculated transmembrane potential was − 59 mV at time 0; external Na + (or choline under putative zero-trans conditions) was 126 mM (T = 25°C). The apparent affinity constants of the taurine transporters were determined over a range of taurine concentrations from 0.24 μM to 11.4 μM. Abluminal membranes had both Na + -dependent taurine transport as well as facilitative transport while luminal membranes only had facilitative transport. The apparent K m for facilitative and Na + -dependent taurine transport were 0.06 ± 0.02 μM and 0.7 ± 0.1 μM, respectively. The Na + -dependent transport of taurine was voltage dependent over the range of voltages studied (− 25 to − 101 mV). The transport was over 5 times greater at − 101 mV compared to when V m was − 25 mV. The sensitivity to external osmolality of Na + -dependent transport was studied over a range of osmolalities (229 to 398 mOsm/kg H 2 O) using mannitol as the osmotic agent to adjust the osmolality. For these experiments the concentration of Na + was maintained constant at 50 mM, and the calculated transmembrane potential was − 59 mV. The Na + -dependent transport system was sensitive to osmolality with the greatest rate observed at 229 mOsm/kg H 2 O.
Qeios
The blood brain barrier present in brain capillaries constitutes an essential barrier mechanism for normal functioning and development of the brain of structural integrity besides neuronal function. The structure and function of the BBB are summarised besides the physical barrier formed by the endothelial tight junctions, and the transport barrier resulting from membrane transporters and vesicular mechanisms. The presence of tight junctions between adjacent endothelial cells restricts the permeability and movement of molecules between extracellular fluid and plasma. It is divided into luminal and abluminal where each solute must cross both membranes. The roles of the neurovascular unit are outlined, especially the astrocyte endfeet, pericytes, and microglia. Five different systems of facilitative transport are found in the luminal membrane and are specific for a few substrates. Nonetheless, two major facilitative carriers (System L and y +) are located in both membranes asymmetrically. In contrast, several Na + dependent transport systems transport amino acids against its concentration gradient present in the abluminal membrane, where the sodium pump Na + /K +-ATPase is highly expressed. The trojan horse mechanism is also favoured in drug delivery by employing molecular tools to bind the drug and its formulations. In the current work, we have revised the prevailing knowledge on the cellular structure of the BBB and the transport systems present exclusively for each substrate, and the need to find transporters with modifications that facilitate the transport of various drugs. Nevertheless, the blending of the classical pharmacology with nanotechnology needs to be focussed on promising results to rule out the BBB passage for the new generation of neuroactive drugs.
Asymmetrical Transport of Amino Acids Across the Blood-Brain Barrier in Humans
Journal of Cerebral Blood Flow & Metabolism, 1990
Blood-brain barrier permeability to four large neutral and one basic amino acid was studied in 30 pa tients with the double indicator technique. The resultant 64 venous outflow curves were analyzed by means of two models that take tracer backflux and capillary heteroge neity into account. The first model considers the blood brain barrier as a double membrane where amino acids from plasma enter the endothelial cell. When an endothe lial cell volume of 0.001 ml/g was assumed, permeability from the blood into the endothelial cell was, for most amino acids, about 10-20 times larger than the permeabil ity for the reverse direction. The second model assumes that the amino acids, after intracarotid injection, cross a single membrane barrier and enter a well-mixed compart ment, the brain extracellular fluid, i.e., the endothelial cell is assumed to behave as a single membrane. With this model, for large neutral amino acids, the permeability out
Cell Biochemistry and Function, 2008
Brain trafficking of amino acids is mainly mediated by amino acids transport machineries of the blood-brain barrier (BBB), where astrocytes play a key maintenance role. However, little is known about astrocytes impacts on such transport systems, in particular system L that consists of large and small neutral amino acids (NAAs) transporters, that is, LAT1/4F2hc and LAT2/ 4F2hc, respectively. In the current investigation, functionality and expression of system L were studied in the immortalized mouse brain microvascular endothelial b.End3 cells cocultured with astrocytes or treated with astrocyte-conditioned media (ACM). LAT2/4F2hc mediated luminal uptake of L-phenylalanine and L-leucine resulted in significantly decreased affinity of system L in b.End3 cells treated with ACM, while LAT2/4F2hc mediated luminal uptake of L-alanine remained unchanged. Gene expression analysis revealed marked upregulation of LAT1 and 4F2hc, but downregulation of LAT2 in b.End3 cells cultured with ACM. The basal to apical transport of L-phenylalanine and L-alanine appeared to be significantly greater than that of the apical to basal direction in b.End3 cells indicating an efflux functionality of system L. No marked influence was observed for transport of L-phenylalanine in b.End3 cells cocultured with astrocytes, while a slight decrease was seen for L-alanine in the basal to apical direction. Based on our findings, we propose that system L functions as influx and/or efflux transport machinery displaying a greater propensity for the outward transport of large and small NAAs. Astrocytes appeared to modulate the transcriptic expression and uptake functionalities of system L, but not the transport activities.