TARPs γ-2 and γ-7 are essential for AMPA receptor expression in the cerebellum

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors require auxiliary subunits termed transmembrane AMPA receptor regulatory proteins (TARPs), which promote receptor trafficking to the cell surface and synapses and modulate channel pharmacology and gating. Of six TARPs, γ-2 and γ-7 are the two major TARPs expressed in the cerebellum. In the present study, we pursued their roles in synaptic expression of cerebellar AMPA receptors. In the cerebellar cortex, γ-2 and γ-7 were preferentially localized at various asymmetrical synapses. Using quantitative Western blot and immunofluorescence, we found severe reductions in GluA2 and GluA3 and mild reduction in GluA4 in γ-2-knockout (KO) cerebellum, whereas GluA1 and GluA4 were moderately reduced in γ-7-KO cerebellum. GluA2, GluA3 and GluA4 were further reduced in γ-2/γ-7 double-KO (DKO) cerebellum. The large losses of GluA2 and GluA3 in γ-2-KO mice and further reductions in DKO mice were confirmed at all asymmetrical synapses examined with postembedding immunogold. Most notably, the GluA2 level in the postsynaptic density fraction, GluA2 labeling density at parallel fiber-Purkinje cell synapses, and AMPA receptor-mediated currents at climbing fiber-Purkinje cell synapses were all reduced to approximately 10% of the wild-type levels in DKO mice. On the other hand, the reduction in GluA4 in γ-7-KO granular layer reflected its loss at mossy fiber-granule cell synapses, whereas that of GluA1 and GluA4 in γ-7-KO molecular layer was caused, at least partly, by their loss in Bergmann glia. Therefore, γ-2 and γ-7 cooperatively promote synaptic expression of cerebellar AMPA receptors, and the latter also promotes glial expression.