The Role of Heme Oxygenase 1 and Carbon Monoxide in Erythrophagocytosis

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Subarachnoid hemorrhage (SAH) leads to neuronal injury and cognitive impairment following heme-induced cerebral inflammation resulting from the accumulation of heme-containing blood components. Elimination of heme occurs through the heme oxygenase (HO) enzymes that degrade heme into biliverdin, iron, and carbon monoxide (CO). Induction of the inducible HO-1 isoform exerts strong cytoprotective effects in numerous disease models including the brain. Exogenous application of low doses of CO can recapitulate the cytoprotective effects of HO-1 induction, but only a few reports have examined the protective effects of CO on neuronal cells in vitro and in vivo. In fact, the majority of reports contend that CO is potently neurotoxic. In direct contrast to this dogma, we have recently identified a novel role for the HO-1 isoform in microglia in response to SAH. Our experimental study in mice, isolated primary microglia and human cerebrospinal fluid (CSF) demonstrated that microglial expression of HO-1 is essential in the response to SAH by mediating clearance of blood and eliminating the pro-oxidant heme burden, but also via the generation of the gas CO that in turn regulates erythrophagocytosis. Absence of HO-1 specifically in microglia resulted in increased neuronal injury and worsened functional outcome. Primary microglia lacking HO-1 demonstrated impaired erythrophagocytosis, which in turn increased neuronal cell death in co-culture experiments. In human SAH patients cisternal hematoma volume was associated with elevations in HO-1 expression and activity. The exact mechanisms as to how the HO-1-CO axis modulates erythrophagocytosis by microglia remains to be elucidated. Red cells and the cellular components are recognized by a series of cognate receptors including CD163, CD36 and TLR4. Whether one or more of these receptors is involved in phagocytosis and blood clearance has not been evaluated. AMP-activated protein kinase (AMPK) is involved in phagocytosis. Since we have previously shown that macrophages increase the generation of reactive oxygen species (ROS) in response to CO and ROS-signaling is known to be linked to AMPK-activity, we posit that the ability of endogenous or exogenous CO to enhance RBC-phagocytosis is in part due to a ROS-dependent activation of AMPK. Microglial expression of the scavenger receptor CD36 has been linked to microglial phagocytosis in intracerebral hemorrhage and Alzheimer's disease. We found that CO rapidly increases CD36 expression in microglia and that microglia deficient in HO-1 show low CD36 expression. These preliminary data suggest that HO-1 and CO regulate erythrophagocytosis partly though a CD36 and ROS-dependent AMPK-signaling axis. We would like to further explore this possible link between intracellular ROS-AMPK signaling and CD36 surface expression and determine its role in HO-1/CO-mediated erythrophagocytosis and neuronal outcome after SAH.