Monocarboxylic acid transporters, MCT1 and MCT2, in cortical astrocytes in vitro and in vivo

We used sequence-specific antibodies tocharacterize two monocarboxylic acid transporters, MCT1 and MCT2, inastrocytes. Both proteins are expressed in primary cultures of corticalastrocytes, as indicated by immunoblotting and immunofluorescence. BothMCT1 and MCT2 are present in small, punctate structures in thecytoplasm and at the cell membrane. Cells showing very low levels oflabeling for glial fibrillary acidic protein (GFAP) also label moredimly for MCT2, but not for MCT1. In vivo, double-labelimmunofluorescence studies coupled with confocal microscopy indicatethat MCT1 and MCT2 are rare in astrocytes in the cortex. However, theyare specifically labeled in astrocytes of the glial limiting membraneand in white matter tracts. Both transporters are also present in themicrovasculature. Comparison of labeling for MCT1 and MCT2 with markersof the blood-brain barrier shows that the transporters are not alwayslimited to the astrocytic endfeet in vivo. Our results suggestthat the level of expression of monocarboxylic acid transporters MCT1and MCT2 by cortical astrocytes in vivo is significantly lowerthan in vitro but that astrocytes in some other regions of thebrain can express one or both proteins in significant amounts.