The Role of Heme Oxygenase 1 and Carbon Monoxide in Erythrophagocytosis

蛛网膜下腔出血（SAH）导致神经元损伤和认知功能障碍后，血红素诱导的脑炎症所造成的积累含血红素的血液成分。血红素的消除是通过血红素加氧酶（HO）将血红素降解为胆绿素、铁和一氧化碳（CO）而发生的。诱导型HO-1同工型的诱导在包括脑在内的许多疾病模型中发挥强的细胞保护作用。外源性应用低剂量的CO可以概括HO-1诱导的细胞保护作用，但只有少数报道研究了CO在体外和体内对神经细胞的保护作用。事实上，大多数报告都认为一氧化碳具有潜在的神经毒性。与这一教条直接相反，我们最近发现了HO-1亚型在小胶质细胞中对SAH的反应中的新作用。我们在小鼠、分离的原发性小胶质细胞和人脑脊液（CSF）中的实验研究表明，HO-1的小胶质细胞表达在对SAH的响应中是必不可少的，其通过介导血液的清除和消除促氧化血红素负担，而且还通过产生气体CO，该气体CO反过来调节红细胞吞噬作用。HO-1特异性在小胶质细胞中的缺失导致神经元损伤增加和功能结局恶化。缺乏HO-1的原代小胶质细胞表现出红细胞吞噬功能受损，这反过来又增加了共培养实验中的神经元细胞死亡。在人类SAH患者中，脑池血肿体积与HO-1表达和活性升高相关。HO-1-CO轴如何通过小胶质细胞调节红细胞吞噬作用的确切机制仍有待阐明。红细胞和细胞组分被一系列同源受体识别，包括CD 163、CD 36和TLR 4。这些受体中的一种或多种是否参与吞噬作用和血液清除尚未评估。AMP活化蛋白激酶（AMPK）参与吞噬作用。由于我们以前已经表明，巨噬细胞增加产生的活性氧（ROS）在响应CO和ROS信号被称为是连接到AMPK活性，我们认为，内源性或外源性CO的能力，以提高RBC吞噬作用是部分由于ROS依赖性激活AMPK。清道夫受体CD 36的小胶质细胞表达与脑出血和阿尔茨海默病中的小胶质细胞吞噬作用有关。我们发现，CO迅速增加小胶质细胞中的CD 36表达，HO-1缺陷的小胶质细胞显示低CD 36表达。这些初步的数据表明，HO-1和CO调节红细胞吞噬部分通过CD 36和ROS依赖的AMPK信号轴。我们希望进一步探索细胞内ROS-AMPK信号传导和CD 36表面表达之间的可能联系，并确定其在HO-1/CO介导的红细胞吞噬作用和SAH后神经元结局中的作用。

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.