Faris Hannah, Almasieh Mohammadali, Levin Leonard A
Department of Ophthalmology and Visual Sciences, McGill University, Montreal, Canada.
Department of Neurology and Neurosurgery, McGill University, Montreal, Canada.
Cell Death Discov. 2021 Sep 17;7(1):247. doi: 10.1038/s41420-021-00641-7.
Axonal degeneration is a common feature of multiple neurodegenerative diseases, yet the mechanisms underlying its various manifestations are incompletely understood. We previously demonstrated that axonal degeneration is associated with externalization of phosphatidylserine (PS), which precedes morphological evidence of degeneration, is redox-sensitive, and is delayed in Wallerian degeneration slow (Wld) mutant animals. Phosphatidylethanolamine (PE) is the other major membrane phospholipid in the inner leaflet of the cell membrane, and given that PS signals apoptosis, phagocytosis, and degeneration, we hypothesized that PS and PE membrane dynamics play distinct roles in axonal degeneration. To test this hypothesis, axonal degeneration was induced with calcium ionophores in postnatal rat retinal ganglion cells, and PS- and PE-specific fluorescent probes used to measure their externalization over time. In untreated cells, cell-surface PS was prominent in the cell body alone. Elevation of intracellular calcium with calcium ionophores resulted in significantly increased levels of PS externalization in the cell body, axon, and axon growth cone. Unlike PS, cell-surface PE was diffusely distributed in untreated cells, with comparable levels across the soma, axons, and axon terminals. After exposure to calcium ionophores, PE externalization significantly increased in the cell body and axon. Elevated intracellular calcium also resulted in the formation of axonal blebs which exclusively contained externalized PS, but not PE. Together, these results indicated distinct patterns of externalized PS and PE in normal and degenerating neurons, suggesting a differential role for these phospholipids in transducing neuronal injury.
轴突退变是多种神经退行性疾病的共同特征，但其各种表现背后的机制尚未完全明确。我们之前证实轴突退变与磷脂酰丝氨酸（PS）外化有关，PS外化先于退变的形态学证据，对氧化还原敏感，且在慢 Wallerian 退变（Wld）突变动物中延迟发生。磷脂酰乙醇胺（PE）是细胞膜内小叶中的另一种主要膜磷脂，鉴于PS可发出凋亡、吞噬和退变信号，我们推测PS和PE膜动力学在轴突退变中发挥不同作用。为验证这一假设，我们用钙离子载体在新生大鼠视网膜神经节细胞中诱导轴突退变，并使用PS和PE特异性荧光探针来测量它们随时间的外化情况。在未处理的细胞中，细胞表面PS仅在细胞体中显著。用钙离子载体提高细胞内钙水平会导致细胞体、轴突和轴突生长锥中PS外化水平显著增加。与PS不同，细胞表面PE在未处理的细胞中呈弥散分布，在胞体、轴突和轴突终末中的水平相当。暴露于钙离子载体后，细胞体和轴突中的PE外化显著增加。细胞内钙升高还导致轴突小泡形成，这些小泡仅含有外化的PS，而不含PE。总之，这些结果表明正常和退变神经元中PS和PE的外化模式不同，提示这些磷脂在传导神经元损伤中具有不同作用。
