Assessment of CA1 injury after global ischemia using supervised 2D analyses of nuclear pyknosis

Selective vulnerability of hippocampal CA1 and CA3 pyramidal cells: What are possible pathomechanisms and should more attention be paid to the CA3 region in future studies?

Transient ischemia and reperfusion selectively damage neurons in brain, with hippocampal pyramidal cells being particularly vulnerable. Even within hippocampus, heterogeneous susceptibility is evident, with higher vulnerability of CA1 versus CA3 neurons described for several decades. Therefore, numerous studies have focused exclusively on CA1. Pediatric cardiac surgery is increasingly focusing on studies of hippocampal structures, and a negative impact of cardiopulmonary bypass on the hippocampus cannot be denied. Recent studies show a shift in selective vulnerability from neurons of CA1 to CA3. This review shows that cell damage is increased in CA3, sometimes stronger than in CA1, depending on several factors (method, species, age, observation period). Despite a highly variable pattern, several markers illustrate greater damage to CA3 neurons than previously assumed. Nevertheless, the underlying cellular mechanisms have not been fully deciphered to date. The complexity is reflected in possible pathomechanisms discussed here, with numerous factors (NMDA, kainate and AMPA receptors, intrinsic oxidative stress potential and various radicals, AKT isoforms, differences in vascular architecture, ratio of pro- and anti-apoptotic Bcl-2 factors, vulnerability of interneurons, mitochondrial dysregulation) contributing to either enhanced CA1 or CA3 vulnerability. Furthermore, differences in expressed genome, proteome, metabolome, and transcriptome in CA1 and CA3 appear to influence differential behavior after damaging stimuli, thus metabolomics-, transcriptomics-, and proteomics-based analyses represent a viable option to identify pathways of selective vulnerability in hippocampal neurons. These results emphasize that future studies should focus on the CA3 field in addition to CA1, especially with regard to improving therapeutic strategies after ischemic/hypoxic brain injury.

Intraarterial anti-leptin therapy via ICA protects ipsilateral CA1 neurons subjected to ischemia and reperfusion

Background

Brain reperfusion following an ischemic event is essential for tissue viability, however, it also involves processes that promote neuronal cell death. We have recently shown that local expression of the hormone leptin in cardiovascular organs drives deleterious remodeling. As cerebral ischemia-reperfusion (IR) lesions derive expression of both the leptin hormone and its receptor, we hypothesized that blocking leptin activity in the injured brain area will reduce the deleterious effects of IR injury.

Methods

C57BL6 male mice underwent bilateral common carotid artery and external carotid artery ligation. The right hemisphere was reperfused after 12 minutes, followed by intraarterial injection of either a low-dose leptin antagonist or saline solution via the ipsilateral ICA. The left common carotid artery remained ligated. Fifteen IR/leptin antagonist-injected and fourteen IR/saline-injected mice completed the experiment. Five days after surgery brains were collected and samples of the hippocampal CA1 region were analyzed for cell viability (H&E) and apoptosis (TUNEL and caspase3), for neuroinflammation (Iba1), and for signaling pathways of pSTAT3 and pSmad2.

Results

The right hemisphere hippocampal CA1 region subjected to IR and saline injection exhibited increased apoptosis and necrosis of pyramidal cells. Also, increased density of activated microglia/macrophages was evident around the CA1 region. Comparatively, leptin antagonist treatment at reperfusion reduced apoptosis and necrosis of pyramidal cells, as indicated by increased number of viable cells (p < 0.01), and reduced TUNEL (p < 0.001) and caspase3-positive cells (p<0.05). Furthermore, this treatment reduced the density of activated microglia/macrophages (p < 0.001) in the CA1 region. Signaling pathway analysis revealed that while pSTAT3 and pSmad2-positive cells were found surrounding the stratum pyramidal in saline-treated animals, pSTAT3 signal was undetected and pSmad2 was greatly reduced in this territory following leptin antagonist treatment (p < 0.01).

Conclusions

Inhibition of leptin activity in hemispheric IR injury preserved the viability of ipsilateral hippocampal CA1 neurons, likely by preventing apoptosis and local inflammation. These results indicate that intraarterial anti-leptin therapy may have clinical potential in reducing hemispheric brain IR injury.

Lung SOD3 limits neurovascular reperfusion injury and systemic immune activation following transient global cerebral ischemia

BACKGROUND AND OBJECTIVES

Studies implicate the lung in moderating systemic immune activation via effects on circulating leukocytes. In this study, we investigated whether targeted expression of the antioxidant extracellular superoxide dismutase (SOD3) within the lung would influence post-ischemic peripheral neutrophil activation and CNS reperfusion injury.

METHODS

Adult, male mice expressing human SOD3 within type II pneumocytes were subjected to 15 min of transient global cerebral ischemia. Three days post-reperfusion, lung and brain tissue was collected and analyzed by immunohistochemistry for inflammation and injury markers. In vitro motility and neurotoxicity assays were conducted to ascertain the direct effects of hSOD3 on PMN activation. Results were compared against C57BL/6 age and sex-matched controls.

RESULTS

Relative to wild-type controls, hSOD3 heterozygous mice exhibited a reduction in lung inflammation, blood-brain barrier damage, and post-ischemic neuronal injury within the hippocampus and cortex. PMNs harvested from hSOD3 mice were also resistant to LPS priming, slower-moving, and less toxic to primary neuronal cultures.

CONCLUSIONS

Constitutive, focal expression of hSOD3 is neuroprotective in a model of global cerebral ischemia-reperfusion injury. The underlying mechanism of SOD3-dependent protection is attributable in part to effects on the activation state and toxic potential of circulating neutrophils. These results implicate lung-brain coupling as a determinant of cerebral ischemia-reperfusion injury and highlight post-stroke lung inflammation as a potential therapeutic target in acute ischemic cerebrovascular injuries.

Transient Oxygen/Glucose Deprivation Causes a Delayed Loss of Mitochondria and Increases Spontaneous Calcium Signaling in Astrocytic Processes

Recently, mitochondria have been localized to astrocytic processes where they shape Ca(2+) signaling; this relationship has not been examined in models of ischemia/reperfusion. We biolistically transfected astrocytes in rat hippocampal slice cultures to facilitate fluorescent confocal microscopy, and subjected these slices to transient oxygen/glucose deprivation (OGD) that causes delayed excitotoxic death of CA1 pyramidal neurons. This insult caused a delayed loss of mitochondria from astrocytic processes and increased colocalization of mitochondria with the autophagosome marker LC3B. The losses of neurons in area CA1 and mitochondria in astrocytic processes were blocked by ionotropic glutamate receptor (iGluR) antagonists, tetrodotoxin, ziconotide (Ca(2+) channel blocker), two inhibitors of reversed Na(+)/Ca(2+) exchange (KB-R7943, YM-244769), or two inhibitors of calcineurin (cyclosporin-A, FK506). The effects of OGD were mimicked by NMDA. The glutamate uptake inhibitor (3S)-3-[[3-[[4-(trifluoromethyl)benzoyl]amino]phenyl]methoxy]-l-aspartate increased neuronal loss after OGD or NMDA, and blocked the loss of astrocytic mitochondria. Exogenous glutamate in the presence of iGluR antagonists caused a loss of mitochondria without a decrease in neurons in area CA1. Using the genetic Ca(2+) indicator Lck-GCaMP-6S, we observed two types of Ca(2+) signals: (1) in the cytoplasm surrounding mitochondria (mitochondrially centered) and (2) traversing the space between mitochondria (extramitochondrial). The spatial spread, kinetics, and frequency of these events were different. The amplitude of both types was doubled and the spread of both types changed by ∼2-fold 24 h after OGD. Together, these data suggest that pathologic activation of glutamate transport and increased astrocytic Ca(2+) through reversed Na(+)/Ca(2+) exchange triggers mitochondrial loss and dramatic increases in Ca(2+) signaling in astrocytic processes.

SIGNIFICANCE STATEMENT

Astrocytes, the most abundant cell type in the brain, are vital integrators of signaling and metabolism. Each astrocyte consists of many long, thin branches, called processes, which ensheathe vasculature and thousands of synapses. Mitochondria occupy the majority of each process. This occupancy is decreased by ∼50% 24 h after an in vitro model of ischemia/reperfusion injury, due to delayed fragmentation and mitophagy. The mechanism appears to be independent of neuropathology, instead involving an extended period of high glutamate uptake into astrocytes. Our data suggest that mitochondria serve as spatial buffers, and possibly even as a source of calcium signals in astrocytic processes. Loss of mitochondria resulted in drastically altered calcium signaling that could disrupt neurovascular coupling and gliotransmission.

The role of microclot formation in an acute subarachnoid hemorrhage model in the rabbit

Background. Microvascular dysfunction and microthrombi formation are believed to contribute to development of early brain injury (EBI) after aneurysmal subarachnoid hemorrhage (SAH). Objective. This study aimed to determine (i) extent of microthrombus formation and neuronal apoptosis in the brain parenchyma using a blood shunt SAH model in rabbits; (ii) correlation of structural changes in microvessels with EBI characteristics. Methods. Acute SAH was induced using a rabbit shunt cisterna magna model. Extent of microthrombosis was detected 24 h post-SAH (n = 8) by fibrinogen immunostaining, compared to controls (n = 4). We assessed apoptosis by terminal deoxynucleotidyl transferase nick end labeling (TUNEL) in cortex and hippocampus. Results. Our results showed significantly more TUNEL-positive cells (SAH: 115 ± 13; controls: 58 ± 10; P = 0.016) and fibrinogen-positive microthromboemboli (SAH: 9 ± 2; controls: 2 ± 1; P = 0.03) in the hippocampus after aneurysmal SAH. Conclusions. We found clear evidence of early microclot formation in a rabbit model of acute SAH. The extent of microthrombosis did not correlate with early apoptosis or CPP depletion after SAH; however, the total number of TUNEL positive cells in the cortex and the hippocampus significantly correlated with mean CPP reduction during the phase of maximum depletion after SAH induction. Both microthrombosis and neuronal apoptosis may contribute to EBI and subsequent DCI.