Department of Anatomy, Brain Health Research Centre, Otago School of Medical Sciences, University of Otago, PO Box 913, Dunedin, New Zealand.
Brain Res. 2012 Jan 3;1427:54-64. doi: 10.1016/j.brainres.2011.10.022. Epub 2011 Oct 19.
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.
AMPA 受体亚基 (GluA1-4) 通过跨膜 AMPA 受体调节蛋白 (TARPs) 转运到膜突触位点。在星爆突变小鼠中，TARP-γ2（星爆蛋白）的表达严重减少，导致小脑共济失调。星爆颗粒细胞 (GC) 完全丧失功能性 AMPAR，因为 γ2 是它们的主要 TARP；因此，苔藓纤维 (MF)-GC 突触是沉默的。本研究的目的是研究星爆蛋白缺陷如何影响 GC 平行纤维 (PF) 上 AMPAR 亚基在分子层上的抑制性神经元的输出突触的表达水平。通过后嵌入免疫金显微镜分析雄性星爆和对照小鼠的 litter-pairs 小脑。通过测量 PF-浦肯野细胞 (PF-PC) 和 PF-中间神经元 (PF-In) 突触的免疫金相对密度，评估 GluA2/3 和 GluA4 的水平。在每对星爆和对照 littermates 中，总共分析了 100 个突触。在整个星爆小脑皮层中，GluA2/3 和 GluA4 的表达明显降低。星爆者 PF-PC 突触处 GluA2/3 水平降低 52%（p<0.001），PF-In 突触处 GluA4 水平降低 31%（p<0.001）。然而，在这两种情况下，都没有像 MF-GC 突触那样发生突触 AMPAR 亚基的完全缺失。由于分子层中的抑制性神经元除了星爆蛋白之外还表达其他 TARPs，因此 TARP 补偿可能会限制这些突触中 GluA 亚基的丢失，并解释为什么它们不像 MF-GC 突触那样沉默。这些数据表明，星爆者的共济失调表型主要是由于小脑 MF-GC 突触处缺乏 AMPAR。
