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 one and four weeks in the heart compared to between eight and 24 weeks in the brain. Importantly, we discovered that a second burst of inflammation occurs in the brain between four and eight 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 seven 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 seven 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.

Immune responses to stroke: mechanisms, modulation, and therapeutic potential

“…In about half of stroke survivors, activated microglia and astrocytes, foam cells, and lymphocytes in the stroke are present decades after the event (140,154). Interestingly, ischemia-induced inflammation is longer lasting in the brain than in the heart (155). Chronic inflammation is neurotoxic, and the glial scar that surrounds it is devoid of tight junctions in both humans and mice (155,156).…”

“…Interestingly, ischemia-induced inflammation is longer lasting in the brain than in the heart (155). Chronic inflammation is neurotoxic, and the glial scar that surrounds it is devoid of tight junctions in both humans and mice (155,156). It is permeable to albumin and antibodies, perhaps causing peristroke neuronal injury (156).…”

Revisiting Stem Cell-Based Clinical Trials for Ischemic Stroke

“…This long-lasting inflammatory process results in substantial neurotoxicity, myelin degradation, and glial scarring, as well as releasing a host of neuroinflammatory mediators, including cytokines (TNF-a, IL-1b, IL-6, IL-20),chemokines (MCP-1, MIP1a), cellular adhesion molecules (immunoglobulins, cadherins, integrins), reactive oxygen species, and matrix metalloproteases (Lakhan et al, 2009 ; Ceulemans et al, 2010 ; Stonesifer et al, 2017 ; Chung et al, 2018 ; Zbesko et al, 2018 ). At the liquefactive core of the infarct, hematopoietic lineage (myeloid and lymphoid), mesenchymal lineage (endothelial and other stromal), and neural lineage (neurons, astrocytes, and oligodendrocytes) cells undergo extreme stress, interacting and dying within this inflammatory, acidic, and hypoxic milieu (Chung et al, 2018 ).To address the complex problem of restoring function in ischemic tissue, stem cell transplantation-based therapies have been investigated as potential restorative treatments for chronic stroke. Aligning with the major cell types found within the ischemic brain, stem-cell-based clinical trials for ischemic stroke have fallen under three broad cell lineages: hematopoietic, mesenchymal, and neural ( Table 1 ).…”

A Primeval Mechanism of Tolerance to Desiccation Based on Glycolic Acid Saves Neurons from Ischemia in Mammals by Reducing Intracellular Calcium-Mediated Excitotoxicity

“…This area tended to detach during histological processing (Figure S5). We hypothesized that this area could correspond to the ischemic core undergoing liquefactive necrosis [5, 39, 40]. In such case one would assume that i) contrary to an artefact, there would be a direct correlation between the ischemic volume measured by MRI and the volume of the missing tissue and ii) a tissue undergoing necrotic liquefication would have higher water content compared to the surrounding ischemic tissue and a higher signal in the T2-weighted MRI scan.…”

“…Nissl substance redistributes within the cell body in injured or regenerating neurons, providing a marker for the physiological state of the neuron, thereby identifying the damaged area but cannot differentiate between the part of the penumbra that survived and is regenerating and the part that died within the next days after stroke in a patched manner due to apoptosis and did not liquefy and detach. In any case, one can assume that after 14 days the ischemic core has been partially liquified due to liquefactive necrosis [5] [37] [38] and is detached during our brain slice preparation, considered as “missing tissue” (Figure S5). Stereological analysis (Stereo Investigator, MBF) provided an estimation of cell density inside the Neurotrace + area as well as the volume of the infarct (Figure S5).…”