Restoration of Oligodendrocyte Pools in a Mouse Model of Chronic Cerebral Hypoperfusion

Chronic cerebral hypoperfusion, a sustained modest reduction in cerebral blood flow, is associated with damage to myelinated axons and cognitive decline with ageing. Oligodendrocytes (the myelin producing cells) and their precursor cells (OPCs) may be vulnerable to the effects of hypoperfusion and in some forms of injury OPCs have the potential to respond and repair damage by increased proliferation and differentiation. Using a mouse model of cerebral hypoperfusion we have characterised the acute and long term responses of oligodendrocytes and OPCs to hypoperfusion in the corpus callosum. Following 3 days of hypoperfusion, numbers of OPCs and mature oligodendrocytes were significantly decreased compared to controls. However following 1 month of hypoperfusion, the OPC pool was restored and increased numbers of oligodendrocytes were observed. Assessment of proliferation using PCNA showed no significant differences between groups at either time point but showed reduced numbers of proliferating oligodendroglia at 3 days consistent with the loss of OPCs. Cumulative BrdU labelling experiments revealed higher numbers of proliferating cells in hypoperfused animals compared to controls and showed a proportion of these newly generated cells had differentiated into oligodendrocytes in a subset of animals. Expression of GPR17, a receptor important for the regulation of OPC differentiation following injury, was decreased following short term hypoperfusion. Despite changes to oligodendrocyte numbers there were no changes to the myelin sheath as revealed by ultrastructural assessment and fluoromyelin however axon-glial integrity was disrupted after both 3 days and 1 month hypoperfusion. Taken together, our results demonstrate the initial vulnerability of oligodendroglial pools to modest reductions in blood flow and highlight the regenerative capacity of these cells.

慢性脑低灌注（chronic cerebral hypoperfusion）指脑血流量持续轻度降低，其与髓鞘轴突损伤及衰老相关性认知衰退密切相关。少突胶质细胞（oligodendrocytes，即髓鞘生成细胞）及其前体细胞（oligodendrocyte precursor cells，简称OPCs）对低灌注的损伤作用较为易感；在部分损伤模型中，OPCs可通过增殖与分化增强来响应并修复损伤。本研究采用脑低灌注小鼠模型，对胼胝体（corpus callosum）内少突胶质细胞与OPCs在低灌注后的急性及长期响应特征进行了系统表征。低灌注持续3天后，与对照组相比，OPCs与成熟少突胶质细胞的数量均显著降低。但低灌注持续1个月后，OPCs库得以恢复，且少突胶质细胞数量有所增加。采用增殖细胞核抗原（proliferating cell nuclear antigen，简称PCNA）进行增殖评估发现，两个时间点的组间差异均无统计学意义，但在低灌注3天时，增殖性少突胶质细胞数量减少，这与OPCs丢失的结果一致。累积溴脱氧尿苷（5-bromo-2'-deoxyuridine，简称BrdU）标记实验显示，低灌注组动物的增殖细胞数量高于对照组，且其中一部分新生细胞在部分受试动物体内分化为了少突胶质细胞。GPR17是调控损伤后OPCs分化的重要受体，其表达在短期低灌注后出现下调。尽管少突胶质细胞数量发生了改变，但超微结构评估与氟髓鞘染色（fluoromyelin）结果均显示髓鞘无明显变化；不过，低灌注3天及1个月后，轴突-胶质细胞完整性均遭到破坏。综上，本研究结果证实了少突胶质细胞群对轻度血流量降低的初始易感性，并凸显了此类细胞的再生潜能。