Liquefaction of the brain following stroke shares a similar molecular and morphological profile with atherosclerosis and mediates secondary neurodegeneration in an osteopontin dependent mechanism

Abstract

Here we used mouse models of heart and brain ischemia to compare the inflammatory response to ischemia in the heart, a protein rich organ, to the inflammatory response to ischemia in the brain, a lipid rich organ. We report that ischemia-induced inflammation resolves between 1 and 4 weeks in the heart compared to between 8 and 24 weeks in the brain. Importantly, we discovered that a second burst of inflammation occurs in the brain between 4 and 8 weeks following ischemia, which coincided with the appearance of cholesterol crystals within the infarct. This second wave shares a similar cellular and molecular profile with atherosclerosis and is characterized by high levels of osteopontin (OPN) and matrix metalloproteinases (MMPs). In order to test the role of OPN in areas of liquefactive necrosis, OPN-/- mice were subjected to brain ischemia. We found that at 7 weeks following stroke, the expression of pro-inflammatory proteins and MMPs was profoundly reduced in the infarct of the OPN-/- mice, although the number of cholesterol crystals was increased. OPN-/- mice exhibited faster recovery of motor function and a higher number of neuronal nuclei (NeuN) positive cells in the peri-infarct area at 7 weeks following stroke. Based on these findings we propose that the brain liquefies after stroke because phagocytic cells in the infarct are unable to efficiently clear cholesterol rich myelin debris, and that this leads to the perpetuation of an OPN-dependent inflammatory response characterized by high levels of degradative enzymes.\n\nSignificance StatementThe inflammatory response to ischemia in the brain is different to the response to ischemic injury in other organs. In the brain, and for unknown reasons, dead tissue liquefies in response to ischemia by the process of liquefactive necrosis. However, the data we present here demonstrate that there is overlap between the pathophysiology of liquefactive necrosis and atherosclerosis. Specifically, we show that chronic stroke infarcts contain foamy macrophages, cholesterol crystals, high levels of OPN and MMPs, and a similar cytokine profile to atherosclerosis. Therefore, because cholesterol is a central component of myelin, liquefactive necrosis in response to stroke may be caused by an inflammatory response to cholesterol-rich myelin debris that is driven in large part by OPN and MMPs.