Astrocytes Protect Neurons in the Hippocampal CA3 Against Ischemia by Suppressing the Intracellular Ca2+ Overload

In the hippocampus, delayed neuronal death is normally seen in neurons of the CA1 region but not in those of the CA3 region. Astrocytes have been reported to play multiple supporting or pathological roles in neuronal functioning. While evidence indicates that astrocytes could exert neuroprotective effects following ischemia, the possible underlying mechanisms remain unclear. We aimed to investigate the roles of astrocytes in the process of delayed neuronal death following transient forebrain ischemia. L-α-aminoadipic acid (L-α-AAA), an astrocyte-selective gliotoxin, was injected into the hippocampal CA3 region of rats through a cranial window to selectively damage astrocytes. Immunofluorescence staining of glial fibrillary acidic protein (GFAP) was used to evaluate the effect of L-α-AAA on astrocyte numbers. Three days after the L-α-AAA injection, transient forebrain ischemia was induced by a modification of the four-vessel occlusion procedure. Seven days after transient forebrain ischemia, hematoxylin-eosin staining was performed to reveal the morphology of hippocampal pyramidal neurons. In rats with ischemia and reperfusion, regional cerebral blood flow (rCBF) and change in intracellular Ca²⁺ concentration ([Ca²⁺]_i) were separately measured in CA1 and CA3 regions. L-α-AAA injection significantly decreased the number of astrocytes in CA3, but did not affect the pattern of rCBF changes upon ischemia/reperfusion. Seven days after transient forebrain ischemia, in rats receiving L-α-AAA, delayed neuronal death comparable with that in CA1 was observed in the CA3 region. In addition, the pattern of increase in [Ca²⁺]_i due to transient forebrain ischemia was completely changed in the hippocampal CA3. The loss of astrocytes induced a persistent increase in [Ca²⁺]_i in the CA3 region following transient ischemia, similar to what is observed in the CA1 region. Our study indicates that astrocytes in the hippocampal CA3 region exert neuroprotective effects following transient forebrain ischemia and act by suppressing the intracellular Ca²⁺ overload. Furthermore, our study will most likely provide a new therapeutic strategy for brain ischemic diseases, targeted to astrocytes.

Improving Microcirculatory Reperfusion Reduces Parenchymal Oxygen Radical Formation and Provides Neuroprotection

BACKGROUND AND PURPOSE

Reperfusion is the most significant determinant of good outcome after ischemic stroke. However, complete reperfusion often cannot be achieved, despite satisfactory recanalization. We hypothesized that microvascular protection was essential for achieving effective reperfusion and, hence, neuroprotection. To test this hypothesis, we have developed an in vivo model to differentially monitor parenchymal and vascular reactive oxygen species (ROS) formation. By comparing the ROS-suppressing effect of N-tert-butyl-α-phenylnitrone (PBN) with its blood-brain barrier impermeable analog 2-sulfo-phenyl-N-tert-butylnitrone (S-PBN), we assessed the impact of vascular ROS suppression alone on reperfusion and stroke outcome after recanalization.

METHODS

The distal middle cerebral artery was occluded for 1 hour by compressing with a micropipette and then recanalized (n=60 Swiss mice). ROS formation was monitored for 1 hour after recanalization by intravital fluorescence microscopy in pial vasculature and cortical parenchyma with topically applied hydroethidine through a cranial window. PBN (100 mg/kg) or S-PBN (156 mg/kg) was administered shortly before recanalization, and suppression of the vascular and parenchymal hydroethidine fluorescence was examined (n=22). Microcirculatory patency, reperfusion, ischemic tissue size, and neurological outcome were also assessed in a separate group of mice 1 to 72 hours after recanalization (n=30).

RESULTS

PBN and S-PBN completely suppressed the reperfusion-induced increase in ROS signal within vasculature. PBN readily suppressed ROS produced in parenchyma by 88%. S-PBN also suppressed the parenchymal ROS by 64% but starting 40 minutes later. Intriguingly, PBN and S-PBN comparably reduced the size of ischemic area by 65% and 48% (P>0.05), respectively. S-PBN restored the microvascular patency and perfusion after recanalization, suggesting that its delayed parenchymal antioxidant effect could be secondary to improved microcirculatory reperfusion.

CONCLUSIONS

Promoting microvascular reperfusion by protecting vasculature can secondarily reduce parenchymal ROS formation and provide neuroprotection. The model presented can be used to directly assess pharmacological end points postulated in brain parenchyma and vasculature in vivo.

Deep insights: intravital imaging with two-photon microscopy

Intravital multiphoton microscopy is widely used to assess the structure and function of organs in live animals. Although different tissues vary in their accessibility for intravital multiphoton imaging, considerable progress has been made in the imaging quality of all tissues due to substantial technical improvements in the relevant imaging components, such as optics, excitation laser, detectors, and signal analysis software. In this review, we provide an overview of the technical background of intravital multiphoton microscopy. Then, we note a few seminal findings that were made through the use of multiphoton microscopy. Finally, we address the technical limitations of the method and provide an outlook for how these limitations may be overcome through future technical developments.

Molecular imaging of ischemia and reperfusion in vivo with mitochondrial autofluorescence

Ischemia and reperfusion (IR) injury constitutes a pivotal mechanism of tissue damage in pathological conditions such as stroke, myocardial infarction, vascular surgery, and organ transplant. Imaging or monitoring of the change of an organ at a molecular level in real time during IR is essential to improve our understanding of the underlying pathophysiology and to guide therapeutic strategies. Herein, we report molecular imaging of a rat model of hepatic IR with the autofluorescence of mitochondrial flavins. We demonstrate a revelation of the histological characteristics of a liver in vivo with no exogenous stain and show that intravital autofluorescent images exhibited a distinctive spatiotemporal variation during IR. The autofluorescence decayed rapidly from the baseline immediately after 20-min ischemia (approximately 30% decrease in 5 min) but recovered gradually during reperfusion (to approximately 99% of the baseline 9 min after the onset of reperfusion). The autofluorescent images acquired during reperfusion correlated strongly with the reperfused blood flow. We show further that the autofluorescence was produced predominantly from mitochondria, and the distinctive autofluorescent variation during IR was mechanically linked to the altered balance between the flavins in the oxidized and reduced forms residing in the mitochondrial electron-transport chain. Our approach opens an unprecedented route to interrogate the deoxygenation and reoxygenation of mitochondria, the machinery central to the pathophysiology of IR injury, with great molecular specificity and spatiotemporal resolution and can be prospectively translated into a medical device capable of molecular imaging. We envisage that the realization thereof should shed new light on clinical diagnostics and therapeutic interventions targeting IR injuries of not only the liver but also other vital organs including the brain and heart.

The role of free radical generation in increasing cerebrovascular permeability

The brain endothelium constitutes a barrier to the passive movement of substances from the blood into the cerebral microenvironment, and disruption of this barrier after a stroke or trauma has potentially fatal consequences. Reactive oxygen species (ROS), which are formed during these cerebrovascular accidents, have a key role in this disruption. ROS are formed constitutively by mitochondria and also by the activation of cell receptors that transduce signals from inflammatory mediators, e.g., activated phospholipase A₂ forms arachidonic acid that interacts with cyclooxygenase and lipoxygenase to generate ROS. Endothelial NADPH oxidase, activated by cytokines, also contributes to ROS. There is a surge in ROS following reperfusion after cerebral ischemia and the interaction of the signaling pathways plays a role in this. This review critically evaluates the literature and concludes that the ischemic penumbra is a consequence of the initial edema resulting from the ROS surge after reperfusion.