Expression of the exon 9-skipping form of EAAT2 in astrocytes of rats.

mRNA for the exon 9-skipping form of the glutamate transporter excitatory amino acid transporter (EAAT) 2 (glutamate transporter 1, GLT-1) is known to be expressed in brain and spinal cord, and such expression was initially proposed to be associated with motor neuron disease. Surprisingly, a protein corresponding to the size of this splice variant has not previously been detected when using antibodies against one of the possible carboxyl terminal regions of EAAT2. This has been construed as indicating that little of the exon 9-skipping protein is expressed, or that such protein is not stable. We have now made selective antibodies against the splice site of this form of EAAT2. We show that in the adult rat brain and spinal cord, it is expressed primarily in populations of white matter astrocytes. Astrocytes expressing this splice variant also expressed glial fibrillary acidic protein. Expression was developmentally regulated, being expressed in a small number of astrocytes at postnatal day 7, but strongly expressed by large populations of white matter astrocytes by 25 days postnatum and into adulthood. Only a subset of gray matter astrocytes and radial glia expressed exon 9-skipping EAAT2. We suggest that exon 9-skipping EAAT2 may have a role in regulating extracellular glutamate in white matter tracts, either by interacting with normally spliced EAAT2 and modifying its targeting or transport activity, or by acting as a transporter itself. Conversely, the limited expression in gray matter suggests it is unlikely to be important for modulating synaptic levels of glutamate.