Glutamine Uptake at the Blood‐Brain Barrier Is Mediated by N‐System Transport

Although the blood-brain barrier effects of cerebral ischemia have been extensively examined, less attention has focused on ischemia-induced damage to the choroid plexuses that form the blood-cerebrospinal fluid (CSF) barrier (BSCFB). This study examined the rat lateral ventricle choroid plexuses (LVCP) in three ischemic models, bilateral common carotid artery occlusion (2VO) + hypotension with or without reperfusion and permanent middle cerebral artery (MCA) occlusion with or without a tandem common carotid artery occlusion. Blood flow was assessed using [14 C]-N-isopropyl-p-iodoamphetamine, and LVCP injury by tissue edema, alterations in [ 14 C]glutamine transport and BSCFB disruption (measured with [ 3 H]inulin). 2VO + hypotension caused an 87% reduction in LVCP blood flow (P < 0.01) and a progressive reduction in LVCP glutamine transport. In contrast to cortex, there was no LVCP hyperemia or delayed hypoperfusion on reperfusion, but there was marked BSCFB disruption. After 30 mins of 2VO + hypotension with 6 h of reperfusion, the [ 3 H]inulin entry into CSF was increased threefold (P < 0.05). Blood-CSF barrier rather than blood-brain barrier disruption appeared to be the main cause of enhanced [ 3 H]inulin entry into hippocampus. Middle cerebral artery occlusion with and without a tandem common carotid artery occlusion only caused 53% and 38% reductions in LVCP blood flow but induced LVCP edema. Results suggest that the LVCP is selectively vulnerable to ischemic injury in terms of the absolute blood flows or, for the MCA occlusion models, the % reductions in flows required to induce injury. BCSFB disruption early after ischemia may enhance the movement of compounds from blood to areas close to the ventricular system and participate in delayed neuronal death.

Section: Methodsmentioning

confidence: 99%

“…14 C]glutamine transport since such transport at the CP and the BBB is Na + -dependent (Keep and Xiang, 1995;Ennis et al, 1998). The use of this marker allowed assessment of CP injury during the short ischemic period as well as during reperfusion.…”

Section: Morphologic and Methodological Considerationsmentioning

confidence: 99%

The Effects of Cerebral Ischemia on the Rat Choroid Plexus

Ennis

Keep

2005

“…This value is considerably higher than Table 1. Concentrations (mmol/L) and 13 C enrichments (%) of plasma and brain amino acids during intravenous [U- 13 Glutamine transport in mouse brain P Bagga et al previously reported rates of glutamine influx for physiologic plasma glutamine concentrations (0.42 to 0.48 mmol/L) of 0.0049 mmol/g per minute 26 or 0.0041 mmol/g per minute in parietal cortex 23 as measured with L-[ 14 C]glutamine and in situ carotid artery perfusion. However, under physiologic condition, several amino acids present in blood may compete with glutamine for transport, effectively raising the apparent plasma concentration required for half maximal transport, K m ', and reducing its influx as much as 22-fold.…”

Section: Comparison Of Glutamine Influx With Previously Reported Valuesmentioning

confidence: 99%

“…First, glutamine transport between blood and brain involves several transporter proteins, all of which belong to one or more families of large neutral amino-acid transporters. 26,[38][39][40] Glutamine transport into brain and cerebrospinal fluid from the blood involves both System N, A and L-like transporters, and within brain parenchyma, glutamine transport into neurons and astrocytes involves mainly systems-A and N transporter proteins, respectively. Because the present study involved bulk cortical tissue measurements of glutamine and glutamate, and consequently did not differentiate between the different neural compartments, the measured value of V Gln(in) must reflect an average of all the different transporter isoforms mediating cortical glutamine uptake and metabolism.…”

Section: Limitations Of the Study And Other Observationsmentioning

confidence: 99%

Characterization of Cerebral Glutamine Uptake from Blood in the Mouse Brain: Implications for Metabolic Modeling of¹³C NMR Data

Bagga

Behar

Mason

et al. 2014

13 C Nuclear Magnetic Resonance (NMR) studies of rodent and human brain using [1-13 C]/[1,6-13 C 2 ]glucose as labeled substrate have consistently found a lower enrichment (B25% to 30%) of glutamine-C4 compared with glutamate-C4 at isotopic steady state. The source of this isotope dilution has not been established experimentally but may potentially arise either from blood/brain exchange of glutamine or from metabolism of unlabeled substrates in astrocytes, where glutamine synthesis occurs. In this study, the contribution of the former was evaluated ex vivo using 1 H-[ 13 C]-NMR spectroscopy together with intravenous infusion of [U-13 C 5 ]glutamine for 3, 15, 30, and 60 minutes in mice. 13 C labeling of brain glutamine was found to be saturated at plasma glutamine levels 41.0 mmol/L. Fitting a blood-astrocyte-neuron metabolic model to the 13 C enrichment time courses of glutamate and glutamine yielded the value of glutamine influx, V Gln(in) , 0.036±0.002 mmol/g per minute for plasma glutamine of 1.8 mmol/L. For physiologic plasma glutamine level (B0.6 mmol/L), V Gln(in) would be B0.010 mmol/g per minute, which corresponds to B6% of the glutamine synthesis rate and rises to B11% for saturating blood glutamine concentrations. Thus, glutamine influx from blood contributes at most B20% to the dilution of astroglial glutamine-C4 consistently seen in metabolic studies using [1-13 C]glucose. Keywords: glutamate; neurotransmitter; nuclear magnetic resonance spectroscopy INTRODUCTION Glucose, the primary source of energy in the mature brain, is mainly oxidized in neurons to support neuronal firing, and the release and recycling of neurotransmitters, glutamate, and GABA, through the neuron-astrocyte glutamate-glutamine and GABAglutamine cycles. The metabolic pathways comprising these cycles have been revealed in vivo by 13 C Nuclear Magnetic Resonance (NMR) spectroscopy combined with infusions of 13 C-labeled substrates, 1,2 permitting detailed investigation of the metabolism of glucose and alternative substrates in the brain.A consistent finding in NMR studies of rodent and human brain metabolism using [1-13 C] or [1,6-13 C 2 ]glucose as precursor is the lower fractional 13 C enrichment of brain glutamate and glutamine at isotopic steady state compared with blood glucose, 3-5 revealing the presence of constant 'dilutional' flows of unlabeled substrates into the respective tricarboxylic acid (TCA) cycles of neurons and astroglia. For glutamate, this dilution amounts to 25% to 30% in rodents 5,6 (B14% in human brain 7 ) of the theoretical maximum expected from the 13 C-labeled glucose, and is thought to arise within neurons mainly from metabolism of blood-borne substrates such as lactate or ketone bodies and within the brain from glutamine produced in astrocytes through the glutamate-glutamine cycle. [8][9][10] In addition to the glutamate-C4 dilution (relative to glucose-C1), 13 C enrichment of glutamine-C4 is further B20% to 30% less than that of glutamate-C4 at isotopic steady state. 4,5,[11][12][13] As the majority of...

“…In addition, early in vivo studies by Oldendorf and others (reviewed in the study by Smith et al (1987)) measured a low but significant brain uptake for glutamine that was attributed to a neutral AA carrier system. Later, Ennis et al (1998) showed a Na + -dependent, pH-sensitive glutamine luminal transport that was fully inhibited by histidine but not by cystine (system A or ASC inhibitors) or 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH) (system L inhibitor) and therefore identified as system N. In addition to our present report showing Snat3 protein expression, our previous study additionally found a robust and regulated gene Snat1 is localized on the luminal membrane of brain vasculature, but not on BBB capillaries. Fresh PFA-fixed mouse brain tissue sections were imaged by confocal microscopy after staining with anti-Snat1 (green) and anti-Glut1 (blue) antibody; the endothelial nucleus is indicated as '*' and pericyte as 'P'.…”

Section: Potential Roles Of Snat1 and Snat3 In The Cerebral Vasculaturementioning

confidence: 98%

Differential axial localization along the mouse brain vascular tree of luminal sodium-dependent glutamine transporters Snat1 and Snat3

Ruderisch

Virgintino

Makrides

et al. 2011

A specialized brain vasculature is key for establishing and maintaining brain interstitial fluid homeostasis, which for most amino acids (AAs) are B10% plasma levels. Indeed, regulation of AA homeostasis seems critical for normal central nervous system functions, and disturbances in brain levels have both direct and indirect roles in several neuropathologies. One mechanism contributing to the plasma to brain AA gradients involves polarized expression of solute carrier (SLC) family transporters on blood-brain barrier (BBB) endothelial cells. Of particular interest is the localization of sodium-dependent transporters that can actively move substrates against their concentration gradient. In this study, the in vivo endothelial membrane localization of the sodium-dependent glutamine transporters Snat3 (Slc38a3) and Snat1 (Slc38a1) was investigated in the mouse brain microvasculature using immunofluorescent colocalization with cellular markers. In addition, luminal membrane expression was probed by in vivo biotinylation. A portion of both Snat3 and Snat1 vascular expressions was localized on luminal membranes. Importantly, Snat1 expression was restricted to larger cortical microvessels, whereas Snat3 was additionally expressed on BBB capillary membranes. This differential expression of system A (Snat1) versus system N (Snat3) transporters suggests distinct roles for Snats in the cerebral vasculature and is consistent with Snat3 involvement in net transendothelial BBB AA transport.