Local translation highlights novel properties of perisynaptic astrocytic processes, and is modulated by behavior involving the dorsal hippocampus. Local translation highlights novel properties of perisynaptic astrocytic processes, and is modulated by behavior involving the dorsal hippocampus

Local translation is a conserved molecular mechanism. It allows a cell with a complex shape to bypass somatic protein synthesis and transport, and thus to respond quickly to a local stimulus. Local translation also contributes to the establishment of molecular and functional polarity. In the brain, it has been extensively studied in neurons and has also been described more recently in astrocytes - a type of glial cell with specialized extensions that contact vessels and synapses. Here, we studied perisynaptic astrocytic processes (PAPs) in the dorsal hippocampus and showed that they contain RNAs, ribosomes, the endoplasmic reticulum-Golgi intermediate compartment, particles of the Golgi apparatus, and protein synthesis events. We used our recently refined polysome immunoprecipitation technique to characterize the pool of polysomal mRNAs in PAPs (which we refer to as the “PAPome”) from the dorsal hippocampus and compared it with the polysomal mRNAs found in the astrocyte as a whole. The polysomal transcripts that were enriched in the PAPome encoded mostly cytoplasmic proteins and defined an unexpected molecular repertoire with the most enriched transcripts coding for proteins involved in iron homeostasis, translation, cell cycle and cytoskeleton. Interestingly, among them Erz, Fth1, and Rplp1 were enriched in PAPs compared to perivascular astrocytic processes indicating that local translation differ at these two interfaces and may sustain distinct molecular properties. The PAP-enriched transcripts Flt1, Fth1, Ccnd2, Mdm2, Gnb2l1 and Eef1a1 code for proteins involved in memory and learning mechanisms. We therefore studied their local translation in the context of fear conditioning (i.e. behavior involving the dorsal hippocampus). We observed changes in the density and/or distribution of these mRNAs in astrocytes processes as well as a drop of their translation specifically in PAPs. Our results highlight unexpected molecular properties of hippocampal PAPs and suggest for the first time that local translation in this perisynaptic compartment is linked to fear-related memory. Overall design: Perisynaptic Astrocytic Processes (PAPs) versus full astrocytes polysomal transcriptome from dorsal hippocampus in Aldh1l1:L10a-eGFP mice

局部翻译是一种保守的分子机制。它可使具有复杂形态的细胞绕过胞体蛋白质合成与运输过程，从而快速响应局部刺激。局部翻译同样有助于分子与功能极性的建立。在大脑中，该机制已在神经元中得到广泛研究，近年来也在星形胶质细胞——一种具有特化突起以接触血管与突触的胶质细胞——中被报道。本研究针对背侧海马的突触周围星形胶质细胞突起（perisynaptic astrocytic processes, PAPs）展开研究，证实其含有RNA、核糖体、内质网-高尔基体中间室、高尔基体复合物颗粒以及蛋白质合成事件。我们采用近期优化的多聚核糖体免疫沉淀技术，对背侧海马PAPs中的多聚核糖体mRNA库（本研究将其命名为"PAPome"）进行了表征，并将其与整体星形胶质细胞中的多聚核糖体mRNA进行了对比。PAPome中富集的多聚核糖体转录本大多编码胞质蛋白，其构成了一组出人意料的分子特征谱：富集程度最高的转录本所编码的蛋白涉及铁稳态、翻译过程、细胞周期与细胞骨架。有趣的是，与血管周围星形胶质细胞突起相比，Erz、Fth1与Rplp1在PAPs中富集，这表明局部翻译在这两种界面存在差异，并可能维持不同的分子特性。PAPs富集的转录本Flt1、Fth1、Ccnd2、Mdm2、Gnb2l1与Eef1a1所编码的蛋白参与记忆与学习相关机制。因此，本研究在恐惧条件化（即涉及背侧海马的行为范式）背景下，探究了这些转录本的局部翻译情况。我们观察到星形胶质细胞突起中这些mRNA的密度和/或分布发生了变化，且特异性地在PAPs中出现了翻译水平的下降。本研究结果揭示了海马PAPs此前未被认知的分子特性，并首次提出该突触周围区域的局部翻译与恐惧相关记忆存在关联。实验整体设计：在Aldh1l1:L10a-eGFP小鼠的背侧海马中，对比突触周围星形胶质细胞突起（PAPs）与整体星形胶质细胞的多聚核糖体转录组