Neutrophils and Neutrophil Extracellular Traps Cause Vascular Occlusion and Delayed Cerebral Ischemia After Subarachnoid Hemorrhage in Mice

BACKGROUND

After subarachnoid hemorrhage (SAH), neutrophils are deleterious and contribute to poor outcomes. Neutrophils can produce neutrophil extracellular traps (NETs) after ischemic stroke. Our hypothesis was that, after SAH, neutrophils contribute to delayed cerebral ischemia (DCI) and worse outcomes via cerebrovascular occlusion by NETs.

METHODS

SAH was induced via endovascular perforation, and SAH mice were given either a neutrophil-depleting antibody, a PAD4 (peptidylarginine deiminase 4) inhibitor (to prevent NETosis), DNAse-I (to degrade NETs), or a vehicle control. Mice underwent daily neurological assessment until day 7 and then euthanized for quantification of intravascular brain NETs (iNETs). Subsets of mice were used to quantify neutrophil infiltration, NETosis potential, iNETs, cerebral perfusion, and infarction. In addition, NET markers were assessed in the blood of aneurysmal SAH patients.

RESULTS

In mice, SAH led to brain neutrophil infiltration within 24 hours, induced a pro-NETosis phenotype selectively in skull neutrophils, and caused a significant increase in iNETs by day 1, which persisted until at least day 7. Neutrophil depletion significantly reduced iNETs, improving cerebral perfusion, leading to less neurological deficits and less incidence of DCI (16% versus 51.9%). Similarly, PAD4 inhibition reduced iNETs, improved neurological outcome, and reduced incidence of DCI (5% versus 30%), whereas degrading NETs marginally improved outcomes. Patients with aneurysmal SAH who developed DCI had elevated markers of NETs compared with non-DCI patients.

CONCLUSIONS

After SAH, skull-derived neutrophils are primed for NETosis, and there are persistent brain iNETs, which correlated with delayed deficits. The findings from this study suggest that, after SAH, neutrophils and NETosis are therapeutic targets, which can prevent vascular occlusion by NETs in the brain, thereby lessening the risk of DCI. Finally, NET markers may be biomarkers, which can predict which patients with aneurysmal SAH are at risk for developing DCI.

Abstract WP220: Platelet Inhibition Prevents Delayed Cerebral Ischemia After Subarachnoid Hemorrhage In Mice

Subarachnoid hemorrhage (SAH) induces delayed cerebral ischemia (DCI) in about 30% of patients which is characterized by functional decline. As microthrombi is a cause of DCI, platelets may be a therapeutic target for preventing microthrombi formation and development of DCI. Clinical trials of anti-platelets have been inconclusive for their effect on outcome. However, clinical trials used different anti-platelets. Since platelets have numerous receptors which can each prevent thrombosis, there is no clear choice to target for SAH. Here, we utilize 6 platelet antagonists to identify if any receptor is a therapeutic target for DCI after SAH. Our hypothesis is that P2Y receptor inhibition will prevent microthrombi and reduce DCI after SAH. Male and female mice were randomly assigned into sham, SAH+vehicle, SAH+P2Y antagonism, SAH+PAR4 antagonism, SAH+PAF antagonism, SAH+TPβ-R antagonism, and SAH+GPIIb/IIIa antagonism. Mice were euthanized 2 (n=10/group) or 7 days (n=15/group) post-SAH. All outcomes were performed and data was analyzed by a blinded investigator. Behavior was assessed daily. Brain microthrombi were counted. Both male and female SAH mice treated with inhibitors of PAF and TPβ-R had significantly better day 1 function than vehicle-treated mice. Significantly better function on day 1 was also observed in only male mice treated the P2Y antagonists or GPIIb/IIIa antagonist. Neither sex had improved behavior for the PAR-4 inhibitor. Mice exhibiting better function had fewer brain microthrombi with the exception of the GPIIb/IIIa antagonist which reduced microthrombi in female mice but did not correspond with improved behavior. This suggests there may be another deleterious mechanism which is more prevalent/detrimental in females that is not prevented by GPIIb/IIIa inhibition, such as platelet degranulation. However, this needs to be further investigated. Additionally, inhibition of TPβ-R reduced the incidence of DCI compared to vehicle-treated mice. In conclusion, an anti-platelet drug for SAH needs to be chosen carefully as not all antagonists have a beneficial effect after SAH; of vital importance is that anti-platelet drugs may not have similar beneficial effects in both sexes.

Abstract WP214: Inhibition Of 12/15-Lipoxygenase Attenuates Delayed Vasospasm After Subarachnoid Hemorrhage In Mice

Approximately 30% of subarachnoid hemorrhage (SAH) patients who survive the initial ictus develop delayed cerebral ischemia (DCI) 4 to 10 days following SAH. Multiple clinical studies indicate the cause of DCI is multifactorial and includes vasospasm. 12/15-Lipoxygenase (LOX), enzymes that metabolize arachidonic acid into bioactive lipid metabolites, such as 12-HETE and 15-HETE, are expressed in several cell types of the brain including neurons, and microglia. Previous work has shown that 12/15-LOX has detrimental effects on brain vasculature and neurons, triggers inflammation, and thrombosis. In a previous study, inhibition of 12/15-LOX was beneficial in reducing early brain injury in mice after SAH. We hypothesize that inhibition of 12/15-LOX would attenuate delay vasospasm and reduce DCI in SAH mice. C57BL/6 mice of both sexes were randomized into Sham+Vehicle, SAH+Vehicle, SAH+ML351. Vehicle and ML351 (LOX inhibitor, 25mg/kg) were intravenously injected 15 minutes after SAH. 12/15-LOX -/- mice of both sexes were randomized into sham or SAH. Behavior was assessed daily until euthanasia on day 5 for analysis of vasospasm. Following SAH, there, there was an increased expression of 12/15-LOX in brain endothelial cells. Inhibition of 12/15-LOX in C57BL/6 males reduced large artery vasospasm and microvessel constriction, improved functional outcome, and lowered DCI incidence. In female C57BL/6 mice, inhibition of 12/15-LOX also reduced vasospasm and microvessel constriction, and ameliorated behavioral deficits. 12/15-LOX -/- displayed better behavior and have less delayed vasospasm. The findings of this study suggest that inhibition of 12/15-LOX is therapeutically beneficial for male and female mice after SAH via preventing delay vasospasm and DCI.

Abstract WMP111: Microvascular Occlusion Precedes Delayed Deficits In Mice With Subarachnoid Hemorrhage

Subarachnoid hemorrhage (SAH) induces delayed cerebral ischemia (DCI) in about 30% of patients which is characterized by functional decline. Microvascular occlusion, via microvessel constriction or microthrombi, may be a cause of DCI. Our hypothesis is that microvascular occlusion and reduced cerebral blood flow (CBF) precede the onset of delayed deficits in mice with SAH. Mice were randomly assigned into sham, SAH+saline, and SAH+A3P5P+Clopidogrel (platelet antagonists). Laser speckle imaging (to assess large and microvessel perfusion) and behavior were assessed daily. All outcomes were performed and data was analyzed by a blinded investigator. Both large artery and microvessel/tissue perfusion recover to baseline in SAH mice which do not get delayed deficits. However, in SAH mice which develop delayed deficits, microvessel/tissue perfusion has a prolonged reduction (Figure 1A). Interestingly, large artery perfusion recovers in these mice. Antiplatelets prevent microthrombi, attenuating microvessel occlusion, and reduce delayed deficits (Figure 2B-C). Our findings suggest that 1) microvessel occlusion and prolonged tissue perfusion are causative factors for DCI and 2) preventing platelet activation may be a therapeutic target to reduce the incidence of DCI. Our data also suggests a sex difference in the response to platelet antagonists (data not shown).

Abstract TMP101: Epigenetics in Blood Brain Barrier Formation and Maintenance

The blood brain barrier (BBB), unique to the central nervous system (CNS) vessels, tightly controls the passage of molecules between the blood and the brain. BBB breakdown is documented in many pathological conditions including, stroke, and contributes to the severity of disease and poor outcome. Currently, there is no efficient treatment strategy to prevent BBB breakdown or restore disrupted BBB . Despite BBB’s importance in the CNS function, t he genetic program and mechanism, that regulates BBB formation and maintenance is poorly characterized. To understand the molecular features underlying BBB gene regulation during development we utilize endothelial cell (EC) cultures derived from brain of both embryonic (E-13.5) and adult mice and investigated differences in BBB gene expression and epigenetic status using real time PCR analysis and ChIP-qPCR. We found that a distinct BBB profile exist in the CNS ECs during BBB formation and maintenance . When compared to embryonic ECs, adult ECs showed a significant down regulation in expression of tight junction (TJ) genes such as CLDN-1 , CLDN -11 , ZO-1 and OCLN . Conversely, another important TJ gene CLDN- 5 shows a significant increase in adult ECs. Further, we found that epigenetic-histone modifications are involved in the repression in the TJ gene in adult ECs. To more fully understand the role of Wnt/β-catenin in BBB formation and maintenance we block the downstream of the Wnt/β-catenin signaling pathway using LF3 (block the interaction of β-catenin and transcription factor TCF4) in developing ECs. This results in a adult BBB phenotype in developing ECs indicating, silencing of Wnt/β-catenin pathway is required for BBB maturation . Further, epigenetic component HDAC2 was increased significantly in developing ECs when Wnt/β-catenin signaling is blocked demonstrating this pathway can modulate the epigenetics of CNS ECs .

In summary, we describe that BBB genes have distinguished expression pattern during BBB formation and maintenance, guided by the Wnt/β-catenin pathway through epigenetic modifications. Understanding the fundamental epigenetic and regulatory mechanisms of BBB formation and maintenance is critical for developing therapeutic interventions against BBB breakdown.