Department of Biological Sciences Center for Cancer, Genetic Diseases, and Gene Regulation, Fordham University, Larkin Hall, 441 East Fordham Road, Bronx, NY, 10458, USA.
Section on Epithelial and Retinal Physiology and Disease, National Eye Institute, National Institutes of Health, Bethesda, MD, 20892, USA.
Mol Neurobiol. 2018 May;55(5):4320-4332. doi: 10.1007/s12035-017-0649-5. Epub 2017 Jun 17.
Semaphorins form a family of secreted and membrane-bound molecules that were identified originally as axonal guidance factors during neuronal development. Given their wide distribution in many including mature tissues, semaphorin 4D (sema4D) and its main functional receptor plexin B1 (plxnB1) are expected to fulfill additional functions that remain to be uncovered. A main characteristic of the plexin receptor family is its ability to reorganize the cytoskeleton. PlxnB1 specifically may directly interact with Rho family GTPases to regulate F-actin driven pathways in cells in culture. Diurnal clearance phagocytosis by the retinal pigment epithelium (RPE) of photoreceptor outer segment fragments (POS) is critical for photoreceptor function and longevity. In this process, rearrangement of RPE cytoskeletal F-actin via activation of the Rho family GTPase Rac1 is essential for POS internalization. Here, we show a novel role in POS phagocytosis by RPE cells in culture and in vivo for plexin B1 and its ligand sema4D. Exogenous sema4D abolishes POS internalization (but not binding) by differentiated RPE cells in culture by decreasing the GTP load of Rac1. In the rat eye, sema4D localizes to retinal photoreceptors, while PlxnB1 is expressed by neighboring RPE cells. At the peak of diurnal retinal phagocytosis after light onset, plxnB1 phosphorylation and sema4D levels are reduced in wild-type rat retina in situ but not in mutant RCS rat retina in which the RPE lacks phagocytic activity. Finally, increased POS phagosome content after light onset is observed in the RPE in situ of mice with either plxnB1 or sema4D gene deletion. Altogether, our results demonstrate a novel physiological function for sema4D/plxnB1 signaling in RPE phagocytosis serving as attenuating brake prior to light onset whose release enables the diurnal phagocytic burst.
信号蛋白形成了一个分泌型和膜结合型分子家族，最初在神经元发育过程中被鉴定为轴突导向因子。鉴于它们在包括成熟组织在内的许多组织中的广泛分布，信号蛋白 4D（sema4D）及其主要功能受体神经丛蛋白 B1（plxnB1）预计将发挥尚未揭示的其他功能。神经丛受体家族的一个主要特征是其能够重组细胞骨架。PlxnB1 特别可能直接与 Rho 家族 GTPase 相互作用，以调节细胞培养物中 F-肌动蛋白驱动的途径。视网膜色素上皮 (RPE) 对光感受器外节碎片 (POS) 的昼夜清除吞噬作用对于光感受器功能和寿命至关重要。在这个过程中，通过激活 Rho 家族 GTPase Rac1 对 RPE 细胞骨架 F-肌动蛋白的重排对于 POS 的内化是必不可少的。在这里，我们展示了培养的 RPE 细胞和体内对 plexin B1 和其配体 sema4D 在 POS 吞噬中的新作用。外源性 sema4D 通过降低 Rac1 的 GTP 负荷，消除了分化的 RPE 细胞对 POS 的内化（但不结合）。在大鼠眼中，sema4D 定位于视网膜感光器，而 PlxnB1 则由相邻的 RPE 细胞表达。在光起始后的昼夜视网膜吞噬作用高峰期，野生型大鼠视网膜中 plxnB1 磷酸化和 sema4D 水平降低，但在 RCS 大鼠视网膜中没有降低，在 RCS 大鼠中，RPE 缺乏吞噬活性。最后，在 plxnB1 或 sema4D 基因缺失的小鼠中，光起始后 POS 吞噬体含量增加。总之，我们的结果表明，sema4D/plxnB1 信号在 RPE 吞噬作用中具有新的生理功能，作为光起始前的减弱刹车，其释放使昼夜吞噬爆发成为可能。
