Hypercapnia in combination with hypoxemia is usually present in severe respiratory disease in the intensive care unit (ICU) and can lead to more severe cognitive dysfunction. Increasing evidence has indicated that the compromised blood–brain barrier (BBB) in the hippocampus in hypoxemia conditions can result in cognitive dysfunction. However, the role and underlying mechanism of hypercapnia in the BBB disruption remains poorly known. A rat model of hypercapnia was first established in this study by intubation and mechanical ventilation with a small-animal ventilator. After this, the cognitive function of the experimental rats was assessed by the Morris water maze test. The BBB permeability was evaluated by the Evans Blue (EB) test and brain water content (BWC). Western blot analysis was carried out to detect the protein expressions of total and nuclear hypoxia-inducible factor-1α (HIF-1α), matrixmetalloproteinase-9 (MMP-9) and Aquaporins-4 (AQP-4) in the hippocampus tissue. Double immunofluorescence further verified the protein expression of different biomarkers was localized in the astrocytes of the hippocampus. Hypercapnia alone did not disrupt the BBB, but it could further enhance the BBB permeability in hypoxemia. Concomitantly, up-regulation of nuclear HIF-1α, AQP-4, MMP-9 protein expression along with increased degradation of the occludin and claudin-5 proteins was found in the hypercapnia rat model, while the total HIF-1α remained unchanged. Interestingly, these changes were independent of the acidosis induced by hypercapnia. Of note, after premedication of 2-Methoxyestradiol (2ME2, an inhibitor of HIF-1α nuclear translocation), the disrupted BBB could be restored resulting in improvement of the cognitive impairment. Meanwhile, accumulation of nuclear HIF-1α, protein expression of AQP-4 and MMP-9 and protein degradation of the occludin and claudin-5 were decreased. Thus, our study demonstrated that hypercapnia can further disrupt the BBB through promoting HIF-1α nuclear translocation and up-regulation of AQP-4 and MMP-9 in hypoxemia. It is therefore suggested that the cascade of hypercapnia-induced nuclear HIF-1α protein translocation in hypoxia-activated astrocytes may be a potential target for ameliorating cognitive impairment.

高碳酸血症和低氧血症通常出现在重症监护室 (ICU) 的严重呼吸系统疾病中，并可能导致更严重的认知功能障碍。越来越多的证据表明，在低氧血症条件下，海马中受损的血脑屏障 (BBB) 可导致认知功能障碍。然而，高碳酸血症在 BBB 破坏中的作用和潜在机制仍然知之甚少。本研究首先采用小动物呼吸机插管和机械通气建立高碳酸血症大鼠模型。在此之后，通过莫里斯水迷宫测试评估实验大鼠的认知功能。通过埃文斯蓝 (EB) 测试和脑含水量 (BWC) 评估 BBB 渗透性。Western blot检测海马组织中总和核缺氧诱导因子1α（HIF-1α）、基质金属蛋白酶9（MMP-9）和水通道蛋白4（AQP-4）的蛋白表达。双免疫荧光进一步验证了不同生物标志物的蛋白表达定位于海马的星形胶质细胞。单独的高碳酸血症不会破坏 BBB，但它可以进一步增强低氧血症时的 BBB 通透性。同时，在高碳酸血症大鼠模型中发现核 HIF-1α、AQP-4、MMP-9 蛋白表达上调以及 occludin 和 claudin-5 蛋白降解增加，而总 HIF-1α 保持不变。有趣的是，这些变化与高碳酸血症引起的酸中毒无关。值得注意的是，在 2-甲氧基雌二醇 (2ME2, HIF-1α 核转位的抑制剂），被破坏的 BBB 可以恢复，从而改善认知障碍。同时，核HIF-1α的积累、AQP-4和MMP-9的蛋白表达以及occludin和claudin-5的蛋白降解减少。因此，我们的研究表明，高碳酸血症可以通过促进 HIF-1α 核转位和在低氧血症中上调 AQP-4 和 MMP-9 来进一步破坏 BBB。因此，提示缺氧激活的星形胶质细胞中高碳酸血症诱导的核 HIF-1α 蛋白易位级联反应可能是改善认知障碍的潜在靶点。AQP-4和MMP-9的蛋白表达以及occludin和claudin-5的蛋白降解减少。因此，我们的研究表明，高碳酸血症可以通过促进 HIF-1α 核转位和在低氧血症中上调 AQP-4 和 MMP-9 来进一步破坏 BBB。因此，提示缺氧激活的星形胶质细胞中高碳酸血症诱导的核 HIF-1α 蛋白易位级联反应可能是改善认知障碍的潜在靶点。AQP-4和MMP-9的蛋白表达以及occludin和claudin-5的蛋白降解减少。因此，我们的研究表明，高碳酸血症可以通过促进 HIF-1α 核转位和在低氧血症中上调 AQP-4 和 MMP-9 来进一步破坏 BBB。因此，提示缺氧激活的星形胶质细胞中高碳酸血症诱导的核 HIF-1α 蛋白易位级联反应可能是改善认知障碍的潜在靶点。