Glial scars are permeable to the neurotoxic environment of chronic stroke infarcts

Abstract

Following stroke, the damaged tissue undergoes liquefactive necrosis, a stage of infarct resolution that lasts for months although the exact length of time is currently unknown. One method of repair involves reactive astrocytes and microglia forming a glial scar to compartmentalize the area of liquefactive necrosis from the rest of the brain. The formation of the glial scar is a critical component of the healing response to stroke, as well as other central nervous system (CNS) injuries. The goal of this study was to evaluate the toxicity of the extracellular fluid present in areas of liquefactive necrosis and determine how effectively it is segregated from the remainder of the brain. To accomplish this goal, we used a mouse model of stroke in conjunction with an extracellular fluid toxicity assay, fluorescent and electron microscopy, immunostaining, tracer injections into the infarct, and multiplex immunoassays. We confirmed that the extracellular fluid present in areas of liquefactive necrosis following stroke is toxic to primary cortical and hippocampal neurons for at least 7 weeks following stroke, and discovered that although glial scars are robust physical and endocytic barriers, they are nevertheless permeable. We found that molecules present in the area of liquefactive necrosis can leak across the glial scar and are removed by a combination of paravascular clearance and microglial endocytosis in the adjacent tissue. Despite these mechanisms, there is delayed atrophy, cytotoxic edema, and neuron loss in regions adjacent to the infarct for weeks following stroke. These findings suggest that one mechanism of neurodegeneration following stroke is the failure of glial scars to impermeably segregate areas of liquefactive necrosis from surviving brain tissue.

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

“…Even though liquefactive necrosis is imperfectly contained by glial scarring and may intensify post-stroke injury (Zbesko et al, 2018), it is currently unknown how long it takes for liquefactive necrosis to resolve in the brain after stroke. Therefore, C57BL/6 mice underwent DH stroke and were sacrificed 24 weeks later.…”

“…In response to ischemia, the brain degenerates by the process of liquefactive necrosis while the heart degenerates by coagulative necrosis (Robbins et al, 2010). Liquefactive necrosis is a chronic inflammatory response that persists for months following a stroke and occurs for unknown reasons (Doyle et al, 2015;Nguyen et al, 2016;Zbesko et al, 2018). Cholesterol is a central structural component of myelin.…”

Alzheimer’s associated amyloid and tau deposition co-localizes with a homeostatic myelin repair pathway in two mouse models of post-stroke mixed dementia

“…Distal MCAO plus hypoxia (DH stroke) was performed in mice, as previously described [ 22 , 63 , 108 ]. Mice were anesthetized by isoflurane inhalation, and an incision was made to expose the temporalis muscle.…”

Regenerative Medicine Therapies for Targeting Neuroinflammation After Stroke

“…In parallel, reactive astrocytes form a glial scar around the ischemic infarct by 14 days ( 33 ). Astrocytes initially proliferate and then migrate toward the site of ischemic injury that becomes surrounded by multiple layers of reactive astrocytes interspersed with activated microglia and a dense network of ECM proteins such as laminin, fibronectin, and chondroitin sulfate proteoglycans, resulting in the formation of a very tight glial scar ( 34 ). Angiogenesis is also a mechanism of recovery induced by inflammation after an ischemic stroke ( 35 ) that is essential for the reoxygenation of post-ischemic brain tissue, and is also an essential step for BBB repair, neurogenesis, and neuronal synaptic plasticity ( 11 ).…”