Pten regulates endocytic trafficking of cell adhesion and signaling molecules to pattern the retina

The retina is an exquisitely patterned tissue, with neuronal somata positioned at regular intervals to completely sample the visual field. Cholinergic amacrine cells are spectacular exemplars of precision, distributing in two radial layers and tangentially, forming regular mosaics. Here, we investigated how the intracellular phosphatase Pten and the cell adhesion molecule Dscam cooperate to regulate amacrine cell patterning. Using double mutants to test epistasis, we found that Pten and Dscam function in parallel pathways to regulate amacrine cell positioning. Mechanistically, Pten regulates endocytic remodeling of cell adhesion molecules (Dscam, Megf10, Fat3), which are aberrantly redistributed in Pten conditional-knock-out (cKO) cholinergic amacrine cells. Furthermore, extracellular vesicle content, including that of cell adhesion and signaling molecules, is altered in Pten cKO retinas. Consequently, Wnt signalling is attenuated, the pharmacological disruption of which phenocopies Pten cKO amacrine cell patterning defects. Pten thus controls endocytic trafficking of critical cell adhesion/signaling molecules to control amacrine cell spacing.视网膜（retina）是一种具有精细模式化结构的组织，其神经元胞体以规则间隔排布，以完整覆盖视觉视野。胆碱能无长突细胞（cholinergic amacrine cells）便是这种精准排布的绝佳范例：它们分布于两个放射状层中，并沿切线方向延伸，形成规则的镶嵌模式。本研究探讨了细胞内磷酸酶Pten与细胞黏附分子Dscam如何协同调控无长突细胞的模式形成。我们通过构建双突变体（double mutants）进行上位性（epistasis）分析，发现Pten与Dscam通过平行通路共同调控无长突细胞的定位。从机制层面来看，Pten可调控细胞黏附分子（Dscam、Megf10、Fat3）的内吞重塑（endocytic remodeling），而这些分子在Pten条件性敲除（conditional-knock-out，cKO）的胆碱能无长突细胞中出现异常重分布。此外，在Pten cKO视网膜中，细胞外囊泡（extracellular vesicle）的内容物——包括细胞黏附与信号分子的囊泡内容——发生了异常改变。由此导致Wnt信号通路（Wnt signalling）活性减弱，而对该通路进行药理学干预，可表型模拟Pten cKO所引发的无长突细胞模式形成缺陷。综上，Pten通过调控关键细胞黏附/信号分子的内吞运输（endocytic trafficking），从而控制无长突细胞的间距排布。