Despite the widespread occurrence of axon and synaptic loss in the injured and diseased nervous system, the cellular and molecular mechanisms of these key degenerative processes remain incompletely understood. Wallerian degeneration (WD) is a tightly regulated form of axon loss after injury, which has been intensively studied in large myelinated fibre tracts of the spinal cord, optic nerve and peripheral nervous system (PNS). Fewer studies, however, have focused on WD in the complex neuronal circuits of the mammalian brain, and these were mainly based on conventional endpoint histological methods. Post-mortem analysis, however, cannot capture the exact sequence of events nor can it evaluate the influence of elaborated arborisation and synaptic architecture on the degeneration process, due to the non-synchronous and variable nature of WD across individual axons. To gain a comprehensive picture of the spatiotemporal dynamics and synaptic mechanisms of WD in the nervous system, we identify the factors that regulate WD within the mouse cerebral cortex. We combined single-axon-resolution multiphoton imaging with laser microsurgery through a cranial window and a fluorescent membrane reporter. Longitudinal imaging of > 150 individually injured excitatory cortical axons revealed a threshold length below which injured axons consistently underwent a rapid-onset form of WD (roWD). roWD started on average 20 times earlier and was executed 3 times slower than WD described in other regions of the nervous system. Cortical axon WD and roWD were dependent on synaptic density, but independent of axon complexity. Finally, pharmacological and genetic manipulations showed that a nicotinamide adenine dinucleotide (NAD+)-dependent pathway could delay cortical roWD independent of transcription in the damaged neurons, demonstrating further conservation of the molecular mechanisms controlling WD in different areas of the mammalian nervous system. Our data illustrate how in vivo time-lapse imaging can provide new insights into the spatiotemporal dynamics and synaptic mechanisms of axon loss and assess therapeutic interventions in the injured mammalian brain.

中文翻译：

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受伤的皮层轴突的体内成像揭示了Wallerian变性的快速发作形式

尽管在受损和患病的神经系统中广泛发生轴突和突触损失，但这些关键的退化过程的细胞和分子机制仍未完全了解。沃勒变性（WD）是一种严格调节的损伤后轴突丢失形式，已在脊髓，视神经和周围神经系统（PNS）的大型髓纤维束中进行了深入研究。但是，很少有研究集中在哺乳动物大脑复杂神经元回路中的WD，这些研究主要基于常规的终点组织学方法。但是，事后分析无法捕获事件的确切顺序，也无法评估精心设计的树状结构和突触结构对退化过程的影响，由于WD在各个轴突之间具有非同步性和可变性。为了全面了解WD在神经系统中的时空动态和突触机制，我们确定了调节小鼠大脑皮层中WD的因素。我们将单轴突分辨率多光子成像与通过颅窗和荧光膜报道分子进行的激光显微手术相结合。纵向成像的150多个单独受伤的兴奋性皮层轴突显示阈值长度，在该阈值长度以下，受伤的轴突始终经历快速发作的WD（roWD）形式。roWD的启动时间平均早20倍，执行速度比神经系统其他区域描述的WD慢3倍。皮质轴突WD和roWD取决于突触密度，但独立于轴突复杂性。最后，药理和遗传学操作表明，烟酰胺腺嘌呤二核苷酸（NAD +）依赖性途径可延迟皮质roWD的表达，而不受受损神经元中转录的影响，这表明在哺乳动物神经系统的不同区域，控制WD的分子机制进一步得到保护。我们的数据说明了体内延时成像如何能够为轴突丢失的时空动力学和突触机制提供新的见解，并评估在受伤的哺乳动物脑中的治疗干预措施。