A monoclonal antibody against a Na⁺-L-glutamate cotransporter from rat brain

A monoclonal mouse IgM antibody (Z8E9) was raised against the Na⁺-L-glutamate cotransporter from rat brain. In a preparation of brain plasma membrane vesicles, Z8E9 binds specifically to a polypeptide with an apparent molecular weight of 70,000 and inhibits Na⁺ gradient-dependent L-glutamate cotransport (up to 50%) in brain membrane vesicles. In the membrane vesicles, the antibody does not alter the membrane permeability for Na⁺ and K⁺ nor the Na⁺ gradient-dependent uptake of γ-aminobutyric acid. Kinetic experiments showed that Z8E9 does not alter the K_0.5 values for L-glutamate and Na⁺ activation of L-glutamate transport. However, an apparent cooperativity observed for L-glutamate activation was increased, and the V_max of L-glutamate transport was decreased. Immunostaining of rat cerebellum identified antigenic sites of Z8E9 in Golgi epithelial cells and astrocytes (by light and electron microscopy), whereas no labeling at nerve terminals was detected. The data suggest that a component of a Na⁺-L-glutamate cotransporter subtype has been identified that is specific for glia cells in brain.