Localization of the Na(+)-coupled neutral amino acid transporter 2 in the cerebral cortex.
We studied the distribution and cellular localization of Na(+)-coupled neutral amino acid transporter 2, a member of the system A family of amino acid transporters, in the rat and human cerebral cortex using immunocytochemical methods. Na(+)-coupled neutral amino acid transporter 2-positive neurons were pyramidal and non-pyramidal, and Na(+)-coupled neutral amino acid transporter 2/GABA double-labeling studies revealed that Na(+)-coupled neutral amino acid transporter 2 was highly expressed by GABAergic neurons. Double-labeling studies with the synaptophysin indicated that rare axon terminals express Na(+)-coupled neutral amino acid transporter 2. Na(+)-coupled neutral amino acid transporter 2-immunoreactivity was also found in astrocytes, leptomeninges, ependymal cells and choroid plexus. Electron microscopy showed robust Na(+)-coupled neutral amino acid transporter 2-immunoreactivity in the somato-dendritic compartment of neurons and in glial processes, but, as in the case of double-labeling studies, failed to reveal Na(+)-coupled neutral amino acid transporter 2-immunoreactivity in terminals. To rule out the possibility that the absence of Na(+)-coupled neutral amino acid transporter 1- and Na(+)-coupled neutral amino acid transporter 2-positive terminals was due to insufficient antigen detection, we evaluated Na(+)-coupled neutral amino acid transporter 1/synaptophysin and Na(+)-coupled neutral amino acid transporter 2/synaptophysin coexpression using non-standard immunocytochemical procedures and found that Na(+)-coupled neutral amino acid transporter 1 and Na(+)-coupled neutral amino acid transporter 2+ terminals were rare in all conditions. These findings indicate that Na(+)-coupled neutral amino acid transporter 1 and Na(+)-coupled neutral amino acid transporter 2 are virtually absent in cortical terminals, and suggest that they do not contribute significantly to replenishing the Glu and GABA transmitter pools through the glutamate-glutamine cycle. The strong expression of Na(+)-coupled neutral amino acid transporter 2 in the somato-dendritic compartment and in non-neuronal elements that are integral parts of the blood-brain and brain-cerebrospinal fluid barrier suggests that Na(+)-coupled neutral amino acid transporter 2 plays a role in regulating the levels of Gln and other amino acids in the metabolic compartment of cortical neurons.
Melone
M
M
Department of Neuroscience (Section of Physiology), Università Politecnica delle Marche, Via Tronto 10/A, Torrette di Ancona, I-60020 Ancona, Italy.
Varoqui
H
H
Erickson
J D
JD
Conti
F
F
D026924:Amino Acid Transport System A / Q000378:metabolism*; D000818:Animals; D015153:Blotting, Western / Q000379:methods; D002135:Calcium-Binding Proteins / Q000378:metabolism; D002540:Cerebral Cortex / Q000166:cytology* / Q000378:metabolism; D005260:Female; D005904:Glial Fibrillary Acidic Protein / Q000378:metabolism; D006801:Humans; D007150:Immunohistochemistry / Q000379:methods; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism; D046529:Microscopy, Electron, Transmission / Q000379:methods; D008875:Middle Aged; D009124:Muscle Proteins / Q000378:metabolism; D009474:Neurons / Q000378:metabolism* / Q000648:ultrastructure; D010751:Phosphopyruvate Hydratase / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D026924:Amino Acid Transport System A; D002135:Calcium-Binding Proteins; D005904:Glial Fibrillary Acidic Protein; D008565:Membrane Proteins; D009124:Muscle Proteins; C498797:SLC38A2 protein, human; C091825:syntrophin alpha1; D005680:gamma-Aminobutyric Acid; D010751:Phosphopyruvate Hydratase