Region-specific changes in aquaporin 4 induced by hyperglycemia underlie the differences in cell swelling in the cortex and striatum after cerebral ischemia-reperfusion

Highlights

• AQP4 immunoreactive intensity differences in the cortical and striatal penumbra were assessed.

• Hyperglycemia had more serious effects on striatal penumbra after ischemia.

• Hyperglycemia induced more serious brain edema on striatal penumbra after ischemia.

• These findings may explain the role of diabetes in cerebral ischemic injury.

Abstract

Brain edema is a major cause of death in patients who suffer an ischemic stroke. Diabetes has been shown to aggravate brain edema after cerebral ischemia-reperfusion, but few studies have focused on the heterogeneity of this response across different brain regions. Aquaporin 4 plays an important role in the formation and regression of brain edema. Here, we report that hyperglycemia mainly affects the continuity of aquaporin 4 distribution around blood vessels in the cortical penumbra after ischemia-reperfusion; however, in the striatal penumbra, in addition to affecting the continuity of distribution, it also substantially affects the fluorescence intensity and the polarity distribution in astrocytes. Accordingly, hyperglycemia induces a more significant increase in the number of swelling cells in the striatal penumbra than in the cortical penumbra. These results can improve our understanding of the mechanism underlying the effects of diabetes in cerebral ischemic injury and provide a theoretical foundation for identification of appropriate therapeutic modalities.

Introduction

Diabetes and stroke are both major global diseases that seriously threaten human health. A large number of studies have confirmed that diabetes can aggravate the injury in ischemic stroke. In comparison with non-diabetic patients, diabetic patients are 2–3 times more likely to have stroke, and the mortality and severity of sequelae in diabetes patients are significantly higher than those in normoglycemic individuals [8,12]. Animal experiments have also shown that hyperglycemia can aggravate the cerebral infarction volume and neuronal apoptosis in a cerebral ischemia model and aggravate neurological impairment [19,32]. However, the specific mechanisms underlying the aggravation of cerebral ischemic injury by diabetes require elucidation.

Hyperglycemia can significantly increase the degree of brain edema in middle cerebral artery occlusion (MCAO) rats [34]. Brain edema is one of the earliest pathological changes in ischemic stroke, and its progression can lead to apoptosis or necrosis [10]. The cerebral hernia caused by severe brain edema is the main cause of death in patients [7]. Previous studies on brain edema mainly focused on the cerebral cortex, partly because damage to the cerebral cortex was closely related to a variety of neurological defects after ischemia. However, the striatum may be more likely to swell than the cortex after cerebral ischemia, and the edema in the striatum is less likely to subside with drug treatment [18]. A recent study reported neurogenesis in the striatum to promote the recovery of neurological function after cerebral ischemia, which is not found in the cerebral cortex [25]. According to another clinical study, reduced volumes in the subcortical striatum, but not in the cortex, are associated with cognitive impairment in diabetes [29]. These findings suggest that a thorough exploration of the differences and mechanisms associated with the effects of hyperglycemia on cortical and striatal edema is of great significance for further understanding the mechanism and clinical treatment of cerebral ischemic injury aggravated by diabetes.

Aquaporin 4 (AQP4) is the most abundant member of the aquaporin family in the brain. It is mainly distributed in astrocytes and ependymal cells, and it plays an important role in the formation and regression of brain edema [3]. AQP4 is polarized in astrocytes, that is, it is mainly distributed in the membrane of the astrocytic endfeet surrounding the vascular region [23]. In some pathological conditions, this polarity disappears, resulting in dysfunction of water transport. AQP4 upregulation after cerebral ischemia is generally believed to promote the formation of brain edema. The transcriptional and translational levels of AQP4 in a permanent MCAO model were upregulated after 30 min of ischemia [15]. In a transient cerebral ischemia rat model, AQP4 expression in the penumbra began to increase at 24 h after ischemia and returned to normal at least 72 h and 28 d after ischemia, which was consistent with the degree of brain edema shown on MRI [2].

AQP4 knockdown can reduce the degree of brain edema after 24 h of ischemia [22], and siRNA-based interference in AQP4 expression can reduce neuronal edema after 6 h of MCAO [13]. However, a recent global cerebral ischemia model showed that the AQP4 expression did not significantly change within 48 h after ischemia [1]. These differences in the findings may be related to the different models used or the different brain regions observed in these studies, since gene expression and function of astrocytes in the cerebral cortex and striatum have been reported to show significant heterogeneity [6,24]. A new single-cell sequencing study found that even within the same brain region, the expression of genes and signal pathways in astrocytes were almost completely different in different environments [33]. Thus, we speculate that hyperglycemia has different effects on AQP4 expression and distribution in astrocytes in the cortex and striatum, resulting in regional differences in the extent of brain edema.

To confirm this hypothesis, we established a cerebral ischemia-reperfusion (I/R) rat model with diabetes by intraperitoneal injection of streptozocin (STZ) and MCAO, and observed the differences in the AQP4 expression and distribution and brain edema between the cortical and striatal penumbra by morphological methods. These results will improve our understanding of the mechanism by which diabetes aggravates cerebral ischemic injury and facilitate the development of targeted treatment strategies.