Abstract
BackgroundL-Citrulline is a neutral amino acid and a major precursor of L-arginine in the nitric oxide (NO) cycle. Recently it has been reported that L-citrulline prevents neuronal cell death and protects cerebrovascular injury, therefore, L-citrulline may have a neuroprotective effect to improve cerebrovascular dysfunction. Therefore, we aimed to clarify the brain transport mechanism of L-citrulline through blood-brain barrier (BBB) using the conditionally immortalized rat brain capillary endothelial cell line (TR-BBB cells), as an in vitro model of the BBB.MethodsThe uptake study of [14C] L-citrulline, quantitative real-time polymerase chain reaction (PCR) analysis, and rLAT1, system b0,+, and CAT1 small interfering RNA study were performed in TR-BBB cells.ResultsThe uptake of [14C] L-citrulline was a time-dependent, but ion-independent manner in TR-BBB cells. The transport process involved two saturable components with a Michaelis–Menten constant of 30.9 ± 1.0 μM (Km1) and 1.69 ± 0.43 mM (Km2). The uptake of [14C] L-citrulline in TR-BBB cells was significantly inhibited by neutral and cationic amino acids, but not by anionic amino acids. In addition, [14C]L-citrulline uptake in the cells was markedly inhibited by 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH), which is the inhibitor of the large neutral amino acid transporter 1 (LAT1), B0, B0,+ and harmaline, the inhibitor of system b0,+. Gabapentin and L-dopa as the substrates of LAT1 competitively inhibited the uptake of [14C] L-citrulline. IC50 values for L-dopa, gabapentin, L-phenylalanine and L-arginine were 501 μM, 223 μM, 68.9 μM and 33.4 mM, respectively. The expression of mRNA for LAT1 was predominantly increased 187-fold in comparison with that of system b0,+ in TR-BBB cells. In the studies of LAT1, system b0,+ and CAT1 knockdown via siRNA transfection into TR-BBB cells, the transcript level of LAT1 and [14C] L-citrulline uptake by LAT1 siRNA were significantly reduced compared with those by control siRNA in TR-BBB cells.ConclusionsOur results suggest that transport of L-citrulline is mainly mediated by LAT1 in TR-BBB cells. Delivery strategy for LAT1-mediated transport and supply of L-citrulline to the brain may serve as therapeutic approaches to improve its neuroprotective effect in patients with cerebrovascular disease.
Highlights
L-Citrulline is a neutral amino acid and a major precursor of L-arginine in the nitric oxide (NO) cycle
[14C] L-citrulline uptake by TR-blood-brain barrier (BBB) cells showed no significant difference in the absence of Na+ or Cl− in the uptake buffer (Table 1)
These results suggested that the transport of L-citrulline in TR-BBB cells was mediated by a sodium- and chlorideindependent transporter
Summary
L-Citrulline is a neutral amino acid and a major precursor of L-arginine in the nitric oxide (NO) cycle. L-Citrulline is a neutral and non-protein amino acid which was first identified in the juice of watermelon, Citrullus vulgaris Schrad [1]. L-citrulline has been investigated with a focus on L-citrulline as a product of the nitric oxide (NO) cycle and as a precursor for arginine by nitric oxide synthase (NOS) [2, 3]. As L-arginine can be recycled from L-citrulline through the NO cycle in some cells such as intestinal cells [5], L-citrulline plays an important role in NO metabolism and regulation [3]. In the central nervous system (CNS), NO plays an important role in the cell death or survival mechanisms in brain cells [6, 7]. Neuronal NOS (nNOS) is expressed in neuronal tissues such as neurons and synaptic spines
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