Selective loss of AMPA receptor subunits at inhibitory neuron synapses in the cerebellum of the ataxic stargazer mouse

Abstract

AMPA receptor subunits (GluA1-4) are trafficked to membrane synaptic sites by transmembrane AMPA receptor regulatory proteins (TARPs). In the stargazer mutant mouse, expression of TARP-γ2 (stargazin) is severely reduced, resulting in cerebellar ataxia. Stargazer granule cells (GCs) have a complete loss of functional AMPARs, as γ2 is their main TARP; hence mossy fiber (MF)-GC synapses are silent. The aim of the current study was to investigate how the stargazin deficit affects expression levels of AMPAR subunits at output synapses from GC parallel fibers (PF) onto inhibitory neurons in the molecular layer. Cerebella from male litter-pairs of stargazer and control mice were analyzed by post-embedding immunogold-microscopy. Levels of GluA2/3 and GluA4 were evaluated by measuring relative density of immunogold at PF-Purkinje cell (PF-PC) and PF-interneuron (PF-In) synapses respectively. In total, 100 synapses were analyzed in each pair of stargazer and control littermates. GluA2/3 and GluA4 expression was significantly reduced throughout the stargazer cerebellar cortex. GluA2/3 levels were reduced by 52% (p<0.001) at PF-PC synapses, and GluA4 levels by 31% (p<0.001) at PF-In synapses in stargazers. In neither case, however, was there a total loss of synaptic AMPAR subunits as occurs at MF-GC synapses. As the inhibitory neurons of the molecular layer express other TARPs in addition to stargazin, TARP compensation may limit the loss of GluA subunits at these synapses and explain why they are not silent like the MF-GC synapses. These data suggest that the ataxic phenotype in stargazers is primarily due to absence of AMPARs at cerebellar MF-GC synapses.