Aquaporin 9 changes in pyramidal cells before and is expressed in astrocytes after delayed neuronal death in the ischemic hippocampal CA1 region of the gerbil

Diabetes mitigates the recovery following intracranial hemorrhage in rats

Intracranial hemorrhage (ICH) is a common subtype of stroke with high morbidity and mortality. However, few studies have examined the effects of diabetes on the recovery from ICH-induced brain injury. Therefore, we examined the effects of diabetes on protein levels of aquaporins, neuronal loss, angiogenesis, blood brain barrier (BBB) integrity, and neurological deficits following intra-DH collagenase-induced ICH in the hippocampus. We found that diabetic rats exhibited enhanced AQP9 expression in the hippocampus relative to non-diabetic rats, which was associated with increased behavioral deficits. Additionally, ICH induced neovascularization, proliferation of brain microvascular endothelial cells, and hippocampal neuronal loss. However, ICH-induced neovascularization and proliferation of brain microvascular endothelial cells was severely impaired in diabetic rats. Furthermore, ICH-induced hippocampal neuronal loss was exaggerated in diabetic rats. Finally, ICH impaired BBB integrity in the ipsilateral hemisphere, which was increased in diabetic rats. Taken together, the attenuated brain angiogenesis, increased hippocampal neuronal loss, and impaired BBB integrity in diabetic rats after ICH were associated with enhanced AQP9 expression. This may suggest that AQP9 is one of the underlying mechanisms that can mitigate the recovery from ICH in diabetic populations.

Aquaporin and brain diseases

BACKGROUND

The presence of water channel proteins, aquaporins (AQPs), in the brain led to intense research in understanding the underlying roles of each of them under normal conditions and pathological conditions.

SCOPE OF REVIEW

In this review, we summarize some of the recent knowledge on the 3 main AQPs (AQP1, AQP4 and AQP9), with a special focus on AQP4, the most abundant AQP in the central nervous system.

MAJOR CONCLUSIONS

AQP4 was most studied in several brain pathological conditions ranging from acute brain injuries (stroke, traumatic brain injury) to the chronic brain disease with autoimmune neurodegenerative diseases. To date, no specific therapeutic agents have been developed to either inhibit or enhance water flux through these channels. However, experimental results strongly underline the importance of this topic for future investigation. Early inhibition of water channels may have positive effects in prevention of edema formation in brain injuries but at later time points during the course of a disease, AQP is critical for clearance of water from the brain into blood vessels.

GENERAL SIGNIFICANCE

Thus, AQPs, and in particular AQP4, have important roles both in the formation and resolution of edema after brain injury. The dual, complex function of these water channel proteins makes them an excellent therapeutic target. This article is part of a Special Issue entitled Aquaporins.

Heterogeneity of astrocytes: from development to injury - single cell gene expression

Astrocytes perform control and regulatory functions in the central nervous system; heterogeneity among them is still a matter of debate due to limited knowledge of their gene expression profiles and functional diversity. To unravel astrocyte heterogeneity during postnatal development and after focal cerebral ischemia, we employed single-cell gene expression profiling in acutely isolated cortical GFAP/EGFP-positive cells. Using a microfluidic qPCR platform, we profiled 47 genes encoding glial markers and ion channels/transporters/receptors participating in maintaining K⁺ and glutamate homeostasis per cell. Self-organizing maps and principal component analyses revealed three subpopulations within 10–50 days of postnatal development (P10–P50). The first subpopulation, mainly immature glia from P10, was characterized by high transcriptional activity of all studied genes, including polydendrocytic markers. The second subpopulation (mostly from P20) was characterized by low gene transcript levels, while the third subpopulation encompassed mature astrocytes (mainly from P30, P50). Within 14 days after ischemia (D3, D7, D14), additional astrocytic subpopulations were identified: resting glia (mostly from P50 and D3), transcriptionally active early reactive glia (mainly from D7) and permanent reactive glia (solely from D14). Following focal cerebral ischemia, reactive astrocytes underwent pronounced changes in the expression of aquaporins, nonspecific cationic and potassium channels, glutamate receptors and reactive astrocyte markers.

Reduced beta-catenin expression in the hippocampal CA1 region following transient cerebral ischemia in the gerbil

Beta-catenin, a transcription factor, plays a critical role in cell survival and degradation after stroke. In this study, we examined changes of expression in beta-catenin in the hippocampal CA1 region of the gerbil following 5 min of transient cerebral ischemia. We observed neuronal damage using cresyl violet staining, neuronal nuclei immunohistochemistry and Fluro-Jade B immunofluorescence. Four days after ischemia-reperfusion (I-R), most of pyramidal cells in the CA1 region were damaged. In addition, early damage in dendrites was detected 1 day after I-R by immunohistochemical staining for microtubule-associated protein 2 (MAP-2), and MAP-2 immunoreactivity was hardly detected in the CA1 region 4 days after I-R. We found that beta-catenin (a synapse-enriched cell adhesion molecule) was well expressed in dendrites before I-R. Its immunoreactivity was well colocalized with MAP-2. Chronological change of beta-catenin immunoreactivity was novelty in the present study. Twelve hours after I-R, its immunoreactivity was decreased in the stratum radiatum of the CA1 region, however, its immunoreactivity was increased 1 and 2 days after I-R, and decreased sharply 4 days after I-R. However, we did not find any change in beta-catenin immunoreactivity in the CA2 and CA3 region. In brief, we suggest that early change of beta-catenin expression in the stratum pyramidale of ischemic hippocampal CA1 region is associated with early dendrite damage following transient cerebral ischemia.

Aquaporin 9 in rat brain after severe traumatic brain injury

OBJECTIVE

To reveal the expression and possible roles of aquaporin 9 (AQP9) in rat brain, after severe traumatic brain injury (TBI).

METHODS

Brain water content (BWC), tetrazolium chloride staining, Evans blue staining, immunohistochemistry (IHC), immunofluorescence (IF), western blot, and real-time polymerase chain reaction were used.

RESULTS

The BWC reached the first and second (highest) peaks at 6 and 72 hours, and the blood brain barrier (BBB) was severely destroyed at six hours after the TBI. The worst brain ischemia occurred at 72 hours after TBI. Widespread AQP9-positive astrocytes and neurons in the hypothalamus were detected by means of IHC and IF after TBI. The abundance of AQP9 and its mRNA increased after TBI and reached two peaks at 6 and 72 hours, respectively, after TBI.

CONCLUSIONS

Increased AQP9 might contribute to clearance of excess water and lactate in the early stage of TBI. Widespread AQP9-positive astrocytes might help lactate move into neurons and result in cellular brain edema in the later stage of TBI. AQP9-positive neurons suggest that AQP9 plays a role in energy balance after TBI.

Aquaporins in cerebrovascular disease: a target for treatment of brain edema?

In cerebrovascular disease, edema formation is frequently observed within the first 7 days and is characterized by molecular and cellular changes in the neurovascular unit. The presence of water channels, aquaporins (AQPs), within the neurovascular unit has led to intensive research in understanding the underlying roles of each of the AQPs under normal conditions and in different diseases. In this review, we summarize some of the recent knowledge on AQPs, focusing on AQP4, the most abundant AQP in the central nervous system. Several experimental models illustrate that AQPs have dual, complex regulatory roles in edema formation and resolution. To date, no specific therapeutic agents have been developed to inhibit water flux through these channels. However, experimental results strongly suggest that this is an important area for future investigation. In fact, early inhibition of water channels may have positive effects in the prevention of edema formation. At later time points during the course of disease, AQP is important for the clearance of water from the brain into blood vessels. Thus, AQPs, and in particular AQP4, have important roles in the resolution of edema after brain injury. The function of these water channel proteins makes them an excellent therapeutic target.

Fluid-percussion brain injury induces changes in aquaporin channel expression

Edema, the accumulation of excess fluid, is a major pathological change in the brain that contributes significantly to pathology and mortality after moderate to severe brain injury. Edema is regulated by aquaporin (AQP) channels which transport water across cellular membranes. Six AQPs are found in the brain (1, 3, 4, 5, 8, and 9), and previous studies have found that AQP4 is regulated after traumatic brain injury (TBI). To further understand how AQPs contribute to brain edema, we investigated whether expression of AQP1, 3, and 9 are also regulated after TBI. Adult male Sprague Dawley rats received moderate parasagittal fluid-percussion brain injury (FPI) or sham surgery. After induction of FPI, the injured, ipsilateral parietal cortex and hippocampus were dissected and analyzed by Western blotting. We observed a small decrease in AQP3 and 4 levels at 7 days after FPI in the ipsilateral, parietal cortex. Both AQP1 and 9 significantly increased within 30 min post-injury and remained elevated for up to 6 h in the ipsilateral, parietal cortex. Aqp1 and 9 mRNA levels were also significantly increased at 30 min post-FPI. Administration of an AQP1 and 4 antagonist, AqB013, non-significantly increased brain water content in sham, non-injured animals, and did not prevent edema formation 24 h after trauma in either the parietal cortex or hippocampus. These results indicate that Aqp1 and 9 mRNA and protein levels increase after moderate parasagittal FPI and that an inhibitor of AQP1 and 4 does not decrease edema after moderate parasagittal FPI.

Analysis of mice with targeted deletion of AQP9 gene provides conclusive evidence for expression of AQP9 in neurons

AQP9 is an aquaglyceroporin that serves important functions in peripheral organs, including the liver. Reflecting the lack of AQP9 knockout mice, uncertainties still prevail regarding the localization and roles of AQP9 in the central nervous system. Here we present a comprehensive analysis of AQP9 gene expression in brain, based on a quantitative and multipronged approach that includes the use of animals with targeted deletion of the AQP9 gene. We show by real-time PCR that AQP9 mRNA concentration in rat and mouse brain is approximately 3% and approximately 0.5%, respectively, of that in rat and mouse liver, the organ with the highest level of AQP9. By blue native gel analysis it could be demonstrated that the brain contains tetrameric AQP9, corresponding to the functional form of AQP9. The band corresponding to the AQP9 tetramer was absent in AQP9 knockout brain and liver. Immunocytochemistry and in situ hybridization analyses with AQP9 knockout controls show that subpopulations of nigral neurons express AQP9 both at the mRNA and at the protein levels and that populations of cortical cells (including hilar neurons in the hippocampus) contain AQP9 mRNA but no detectable AQP9 immunosignal. The present data provide conclusive evidence for the presence of tetrameric AQP9 in brain and for the expression of AQP9 in neurons.

Folic acid deficiency increases delayed neuronal death, DNA damage, platelet endothelial cell adhesion molecule-1 immunoreactivity, and gliosis in the hippocampus after transient cerebral ischemia

Folic acid deficiency increases stroke risk. In the present study, we examined whether folic acid deficiency enhances neuronal damage and gliosis via oxidative stress in the gerbil hippocampus after transient forebrain ischemia. Animals were exposed to a folic acid-deficient diet (FAD) for 3 months and then subjected to occlusion of both common carotid arteries for 5 min. Exposure to an FAD increased plasma homocysteine levels by five- to eightfold compared with those of animals fed with a control diet (CD). In CD-treated animals, most neurons were dead in the hippocampal CA1 region 4 days after ischemia/reperfusion, whereas, in FAD-treated animals, this occurred 3 days after ischemia/reperfusion. Immunostaining for 8-hydroxy-2'-deoxyguanosine (8-OHdG) was performed to examine DNA damage in CA1 neurons in both groups after ischemia, and it was found that 8-OHdG immunoreactivity in both FAD and CD groups peaked at 12 hr after reperfusion, although the immunoreactivity in the FAD group was much greater than that in the CD group. Platelet endothelial cell adhesion molecule-1 (PECAM-1; a final mediator of neutrophil transendothelial migration) immunoreactivity in both groups increased with time after ischemia/reperfusion: Its immunoreactivity in the FAD group was much higher than that in the CD group 3 days after ischemia/reperfusion. In addition, reactive gliosis in the ischemic CA1 region increased with time after ischemia in both groups, but astrocytosis and microgliosis in the FAD group were more severe than in the CD group at all times after ischemia. Our results suggest that folic acid deficiency enhances neuronal damage induced by ischemia.