Yamasaki Miwako, Fukaya Masahiro, Yamazaki Maya, Azechi Hirotsugu, Natsume Rie, Abe Manabu, Sakimura Kenji, Watanabe Masahiko
Department of Anatomy, Hokkaido University Graduate School of Medicine, Sapporo 060-8638, Japan.
Department of Anatomy, Hokkaido University Graduate School of Medicine, Sapporo 060-8638, Japan, Department of Anatomy, Kitasato University School of Medicine, Sagamihara, Kanagawa 252-0374, Japan, and.
J Neurosci. 2016 Apr 13;36(15):4296-312. doi: 10.1523/JNEUROSCI.4178-15.2016.
The number of AMPA-type glutamate receptors (AMPARs) at synapses is the major determinant of synaptic strength and varies from synapse to synapse. To clarify the underlying molecular mechanisms, the density of AMPARs, PSD-95, and transmembrane AMPAR regulatory proteins (TARPs) were compared at Schaffer collateral/commissural (SCC) synapses in the adult mouse hippocampal CA1 by quantitative immunogold electron microscopy using serial sections. We examined four types of SCC synapses: perforated and nonperforated synapses on pyramidal cells and axodendritic synapses on parvalbumin-positive (PV synapse) and pravalbumin-negative interneurons (non-PV synapse). SCC synapses were categorized into those expressing high-density (perforated and PV synapses) or low-density (nonperforated and non-PV synapses) AMPARs. Although the density of PSD-95 labeling was fairly constant, the density and composition of TARP isoforms was highly variable depending on the synapse type. Of the three TARPs expressed in hippocampal neurons, the disparity in TARP γ-2 labeling was closely related to that of AMPAR labeling. Importantly, AMPAR density was significantly reduced at perforated and PV synapses in TARP γ-2-knock-out (KO) mice, resulting in a virtual loss of AMPAR disparity among SCC synapses. In comparison, TARP γ-8 was the only TARP expressed at nonperforated synapses, where AMPAR labeling further decreased to a background level in TARP γ-8-KO mice. These results show that synaptic inclusion of TARP γ-2 potently increases AMPAR expression and transforms low-density synapses into high-density ones, whereas TARP γ-8 is essential for low-density or basal expression of AMPARs at nonperforated synapses. Therefore, these TARPs are critically involved in AMPAR density control at SCC synapses.
Although converging evidence implicates the importance of transmembrane AMPA-type glutamate receptor (AMPAR) regulatory proteins (TARPs) in AMPAR stabilization during basal transmission and synaptic plasticity, how they control large disparities in AMPAR numbers or densities across central synapses remains largely unknown. We compared the density of AMPARs with that of TARPs among four types of Schaffer collateral/commissural (SCC) hippocampal synapses in wild-type and TARP-knock-out mice. We show that the density of AMPARs correlates with that of TARP γ-2 across SCC synapses and its high expression is linked to high-density AMPAR expression at perforated type of pyramidal cell synapses and synapses on parvalbumin-positive interneurons. In comparison, TARP γ-8 is the only TARP expressed at nonperforated type of pyramidal cell synapses, playing an essential role in low-density or basal AMPAR expression.
突触处α-氨基-3-羟基-5-甲基-4-异恶唑丙酸（AMPA）型谷氨酸受体（AMPAR）的数量是突触强度的主要决定因素，且因突触而异。为阐明其潜在的分子机制，通过使用连续切片的定量免疫金电子显微镜，比较了成年小鼠海马CA1区中Schaffer侧支/连合（SCC）突触处AMPAR、突触后致密蛋白95（PSD-95）和跨膜AMPAR调节蛋白（TARP）的密度。我们检查了四种类型的SCC突触：锥体细胞上的穿孔和非穿孔突触以及小白蛋白阳性（PV突触）和小白蛋白阴性中间神经元（非PV突触）上的轴树突突触。SCC突触被分为表达高密度（穿孔和PV突触）或低密度（非穿孔和非PV突触）AMPAR的突触。尽管PSD-95标记的密度相当恒定，但TARP亚型的密度和组成因突触类型而异。在海马神经元中表达的三种TARP中，TARP γ-2标记的差异与AMPAR标记的差异密切相关。重要的是，在TARP γ-2基因敲除（KO）小鼠的穿孔和PV突触处，AMPAR密度显著降低，导致SCC突触之间AMPAR差异几乎消失。相比之下，TARP γ-8是唯一在非穿孔突触处表达的TARP，在TARP γ-8-KO小鼠中，非穿孔突触处的AMPAR标记进一步降至背景水平。这些结果表明，TARP γ-2的突触内吞有力地增加了AMPAR的表达，并将低密度突触转变为高密度突触，而TARP γ-8对于非穿孔突触处AMPAR的低密度或基础表达至关重要。因此，这些TARP在SCC突触处的AMPAR密度控制中起关键作用。
尽管越来越多的证据表明跨膜AMPA型谷氨酸受体（AMPAR）调节蛋白（TARP）在基础传递和突触可塑性期间对AMPAR稳定的重要性，但它们如何控制中枢突触中AMPAR数量或密度的巨大差异在很大程度上仍然未知。我们在野生型和TARP基因敲除小鼠中比较了四种类型的Schaffer侧支/连合（SCC）海马突触中AMPAR与TARP的密度。我们表明，在SCC突触中，AMPAR的密度与TARP γ-2的密度相关，其高表达与锥体细胞穿孔型突触和小白蛋白阳性中间神经元突触处的高密度AMPAR表达相关。相比之下，TARP γ-8是唯一在锥体细胞非穿孔型突触处表达的TARP，在低密度或基础AMPAR表达中起重要作用。
