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RNase A Inhibits Formation of Neutrophil Extracellular Traps in Subarachnoid Hemorrhage.

ABSTRACT: Background: Subarachnoid hemorrhage (SAH) caused by rupture of an intracranial aneurysm, is a life-threatening emergency that is associated with substantial morbidity and mortality. Emerging evidence suggests involvement of the innate immune response in secondary brain injury, and a potential role of neutrophil extracellular traps (NETs) for SAH-associated neuroinflammation. In this study, we investigated the spatiotemporal patterns of NETs in SAH and the potential role of the RNase A (the bovine equivalent to human RNase 1) application on NET burden. Methods: A total number of n=81 male C57Bl/6 mice were operated utilizing a filament perforation model to induce SAH, and Sham operation was performed for the corresponding control groups. To confirm the bleeding and exclude stroke and intracerebral hemorrhage, the animals received MRI after 24h. Mice were treated with intravenous injection of RNase A (42μg/kg body weight) or saline solution for the control groups, respectively. Quadruple-immunofluorescence (IF) staining for cell nuclei (DAPI), F-actin (phalloidin), citrullinated H3, and neurons (NeuN) was analyzed by confocal imaging and used to quantify NET abundance in the subarachnoid space (SAS) and brain parenchyma. To quantify NETs in human SAH patients, cerebrospinal spinal fluid (CSF) and blood samples from day 1, 2, 7, and 14 after bleeding onset were analyzed for double-stranded DNA (dsDNA) via Sytox Green. Results: Neutrophil extracellular traps are released upon subarachnoid hemorrhage in the SAS on the ipsilateral bleeding site 24h after ictus. Over time, NETs showed progressive increase in the parenchyma on both ipsi- and contralateral site, peaking on day 14 in periventricular localization. In CSF and blood samples of patients with aneurysmal SAH, NETs also increased gradually over time with a peak on day 7. RNase application significantly reduced NET accumulation in basal, cortical, and periventricular areas. Conclusion: Neutrophil extracellular trap formation following SAH originates in the ipsilateral SAS of the bleeding site and spreads gradually over time to basal, cortical, and periventricular areas in the parenchyma within 14days. Intravenous RNase application abrogates NET burden significantly in the brain parenchyma, underpinning a potential role in modulation of the innate immune activation after SAH.

SUBMITTER: Fruh A

PROVIDER: S-EPMC8481772 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The inflammatory microenvironment is commonly characterized by extracellular acidosis (pH < 7.35). Sensitivity to pH, CO2 or bicarbonate concentrations allows neutrophils to react to changes in their environment and to detect inflamed areas in the tissue. One important antimicrobial effector mechanism is the production of neutrophil extracellular traps (NETs), which are released during a programmed reactive oxygen species (ROS)-dependent cell death, the so-called NETosis. Although several functions of neutrophils have been analyzed under acidic conditions, the effect of extracellular acidosis on NETosis remains mainly unexplored and the available experimental results are contradictory. We performed a comprehensive study with the aim to elucidate the effect of extracellular acidosis on ROS-dependent NETosis of primary human neutrophils and to identify the underlying mechanisms. The study was performed in parallel in a CO2-bicabonate-buffered culture medium, which mimics in vivo conditions, and under HEPES-buffered conditions to verify the effect of pH independent of CO2 or bicarbonate. We could clearly show that extracellular acidosis (pH 6.5, 6.0, and 5.5) and intracellular acidification inhibit the release of ROS-dependent NETs upon stimulation of neutrophils with phorbol myristate acetate and immobilized immune complexes. Moreover, our findings suggest that the diminished NET release is a consequence of reduced ROS production and diminished glycolysis of neutrophils under acidic conditions. It was suggested previously that neutrophils can sense the border of inflamed tissue by the pH gradient and that a drop in pH serves as an indicator for the progress of inflammation. Following this hypothesis, our data indicate that an acidic inflammatory environment results in inhibition of extracellular operating effector mechanisms of neutrophils such as release of ROS and NETs. This way the release of toxic components and tissue damage can be avoided. However, we observed that major antimicrobial effector mechanisms such as phagocytosis and the killing of pathogens by neutrophils remain functional under acidic conditions.

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RNase A Inhibits Formation of Neutrophil Extracellular Traps in Subarachnoid Hemorrhage.

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