Microglia regulate blood clearance in subarachnoid hemorrhage by heme oxygenase-1
Nils Schallner, … , Leo E. Otterbein, Khalid A. Hanafy
Nils Schallner, … , Leo E. Otterbein, Khalid A. Hanafy
Published May 26, 2015
Citation Information: J Clin Invest. 2015;125(7):2609-2625. https://doi.org/10.1172/JCI78443.
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Research Article Neuroscience

Microglia regulate blood clearance in subarachnoid hemorrhage by heme oxygenase-1

Abstract

Subarachnoid hemorrhage (SAH) carries a 50% mortality rate. The extravasated erythrocytes that surround the brain contain heme, which, when released from damaged red blood cells, functions as a potent danger molecule that induces sterile tissue injury and organ dysfunction. Free heme is metabolized by heme oxygenase (HO), resulting in the generation of carbon monoxide (CO), a bioactive gas with potent immunomodulatory capabilities. Here, using a murine model of SAH, we demonstrated that expression of the inducible HO isoform (HO-1, encoded by Hmox1) in microglia is necessary to attenuate neuronal cell death, vasospasm, impaired cognitive function, and clearance of cerebral blood burden. Initiation of CO inhalation after SAH rescued the absence of microglial HO-1 and reduced injury by enhancing erythrophagocytosis. Evaluation of correlative human data revealed that patients with SAH have markedly higher HO-1 activity in cerebrospinal fluid (CSF) compared with that in patients with unruptured cerebral aneurysms. Furthermore, cisternal hematoma volume correlated with HO-1 activity and cytokine expression in the CSF of these patients. Collectively, we found that microglial HO-1 and the generation of CO are essential for effective elimination of blood and heme after SAH that otherwise leads to neuronal injury and cognitive dysfunction. Administration of CO may have potential as a therapeutic modality in patients with ruptured cerebral aneurysms.

Authors

Nils Schallner, Rambhau Pandit, Robert LeBlanc III, Ajith J. Thomas, Christopher S. Ogilvy, Brian S. Zuckerbraun, David Gallo, Leo E. Otterbein, Khalid A. Hanafy

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Figure 2

Role of microglial HO-1 in neuronal injury after SAH.

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Role of microglial HO-1 in neuronal injury after SAH. (A) PMG isolated f...
(A) PMG isolated from brains of Hmox1fl/fl or LyzM-Cre Hmox1fl/fl mice stained for IBA1 (green) and nuclear counterstaining with Hoechst 33258 (blue). DsRed expression (red; yellow for red/green merged) was exclusively initiated when the LoxP site–flanked Hmox1 gene was deleted in cells with LyzM-Cre gene expression (right panel, original magnification, ×40; n = 3). (B) HO-1 protein analyzed by Western blotting in PMG from brains of Hmox1fl/fl control or LyzM-Cre Hmox1fl/fl mice with or without red blood cell exposure (n = 3). (C) Representative (n = 6) H&E-stained cross sections of the MCA after SAH in Hmox1fl/fl and LyzM-Cre Hmox1fl/fl mice (original magnification, ×40). (D) Quantification of MCA vasospasm, defined as the quotient of LR and WL. *P = 0.018 (2-tailed Student’s t test; n = 6). (E and F) Quantification of TUNEL-positive cells per microscopic field in the DG (E) and cortex (F). *P = 0.0043 (E) and **P = 0.0412 (F) (2-tailed Student’s t test; n = 6). (G) Number of total errors in spatial memory function by Barnes maze assessment. *P = 0.0071 (day 1) and **P = 0.0123 (day 3) (2-way ANOVA; n = 9). (H) Total latency in seconds for spatial memory function by Barnes maze assessment. *P = 0.0033 (day 0), **P = 0.0345 (day 1), ***P = 0.0021 (day 2), and #P = 0.0006 (day 3) (2-way ANOVA; n = 9). (I) Representative MR images (n = 3) of mice with intracranial injection of gadopentetate dimeglumine–labeled blood. (J) Quantification of radiographically assessed hematoma volume. *P = 0.0184 (2-tailed Student’s t test; n = 3).