Alix James J P, Fern Robert
Department of Cell Physiology and Pharmacology, University of Leicester, Leicester, England, United Kingdom.
Ann Neurol. 2009 Nov;66(5):682-93. doi: 10.1002/ana.21767.
Ischemic injury of axons is a feature of periventricular leukomalacia, a pathological correlate of cerebral palsy. Recent evidence suggests that axons are damaged before they receive the first layer of compact myelin. Here we examine the cellular mechanisms underlying ischemic-type injury of premyelinated central axons.
Two-thirds of axons in the postnatal day 10 (P10) rat optic nerve are small premyelinated axons (<0.4microm in diameter), and one-third have undergone radial expansion in preparation for glial contact and the onset of myelination. Compound action potential recording and quantitative electron microscopy were used to examine the effect of modeled ischemia (oxygen-glucose deprivation) upon these two axon populations. Glutamate receptor (GluR) expression was investigated using polymerase chain reaction (PCR) and immunostaining approaches at the confocal light and ultrastructural levels.
Oxygen-glucose deprivation produced action potential failure and focal breakdown of the axolemma of small premyelinated axons at sites of contact with oligodendrocyte processes, which were also disrupted. The resulting axon loss was Ca(2+)-dependent, Na(+)- and Cl(-)-independent, and required activation of N-methyl-D-aspartic acid (NMDA) and non-NMDA GluRs. NMDA receptor expression was localized to oligodendrocyte processes at sites of contact with premyelinated axons, in addition to expression within compact myelin. No periaxonal NMDA receptor expression was observed on oligodendrocyte processes ensheathing large premyelinated axons and no protective effect of GluR block was observed in these axons.
NMDA receptor-mediated injury to oligodendrocyte processes navigating along small premyelinated axons precedes damage to the underlying axon, a phenomena that is lost following radial expansion and subsequent oligodendrocyte ensheathment.
轴突缺血性损伤是脑室周围白质软化的一个特征，而脑室周围白质软化是脑性瘫痪的一种病理表现。最近的证据表明，轴突在获得第一层紧密髓鞘之前就已受损。在此，我们研究了未成熟中枢轴突缺血性损伤的细胞机制。
出生后第10天（P10）大鼠视神经中三分之二的轴突是细小的未成熟轴突（直径<0.4微米），三分之一的轴突已发生径向扩张，为与神经胶质细胞接触和髓鞘形成做准备。复合动作电位记录和定量电子显微镜用于研究模拟缺血（氧-葡萄糖剥夺）对这两种轴突群体的影响。使用聚合酶链反应（PCR）以及共聚焦光镜和超微结构水平的免疫染色方法研究谷氨酸受体（GluR）的表达。
氧-葡萄糖剥夺导致未成熟小轴突与少突胶质细胞突起接触部位的动作电位消失和轴膜局灶性破裂，少突胶质细胞突起也受到破坏。由此导致的轴突损失是钙依赖性的，钠和氯非依赖性的，并且需要N-甲基-D-天冬氨酸（NMDA）和非NMDA谷氨酸受体的激活。NMDA受体表达定位于与未成熟轴突接触部位的少突胶质细胞突起，此外在紧密髓鞘内也有表达。在包裹粗大未成熟轴突的少突胶质细胞突起上未观察到轴周NMDA受体表达，并且在这些轴突中未观察到谷氨酸受体阻断的保护作用。
NMDA受体介导的对沿未成熟小轴突导航的少突胶质细胞突起的损伤先于对其下方轴突的损伤，这种现象在径向扩张和随后的少突胶质细胞包裹后消失。
