Juvenile mice are susceptible to infarct-induced neurodegeneration that causes delayed cognitive decline in a new model of pediatric ischemic stroke

Over half of pediatric stroke survivors have permanent neurologic and cognitive deficits, and some children develop new or worsening cognitive impairment late after stroke. Their cognitive symptoms are associated with chronic alterations of brain structure in regions distant from the infarct, including in the uninjured hemisphere. However, the mechanisms that cause childrens late cognitive symptoms and disrupted brain growth outside the infarcted tissue remain unknown. We therefore developed and validated a mouse model of pediatric ischemic stroke that causes infarct-induced, delayed cognitive decline accompanied by chronic innate and adaptive neuroinflammation in the infarct and in uninjured subcortical regions that are connected to the infarct. MethodsMale and female C57BL/6J mice were randomized to stroke or sham surgery at p28 to model stroke in late childhood and compared to adult 6-month-old mice. We used permanent distal MCAO followed by 60 minutes of hypoxia, an established model for focal ischemia that generates a purely cortical stroke. Mice underwent behavioral testing at 1- and 7-weeks post stroke Barnes Maze protocol adapted for reversal learning. We performed immunostaining to quantify stroke size, atrophy, and innate and adaptive neuroinflammation at 3 days and 7 weeks after surgery. ResultsAt 7 weeks post-stroke, juvenile stroke mice performed significantly worse on reversal learning on Barnes Maze testing than sham mice (n = 7 stroke, 7 sham, p = 0.0265 2-way ANOVA with repeated measures). At 3 days after stroke, juvenile mice had greater innate immune cell activation at subcortical sites of secondary neurodegeneration; however, by 7 weeks after surgery, this relationship was reversed, and adults had greater chronic innate immune cell activation at these uninjured subcortical sites (corpus callosum, corticospinal tract, thalamus). Like adult mice, pediatric mice exhibited chronic innate and adaptive neuroimmunity in the infarct scar; the inflammation in the stroke scar did not differ by age. ConclusionsIn a model of pediatric ischemic stroke, juvenile mice develop a delayed cognitive deficit that is associated with chronic innate and adaptive neuroinflammation in uninjured subcortical structures that undergo secondary neurodegeneration. Our results suggest that infarct-induced neurodegeneration occurs after stroke in juvenile mice and that juvenile and adult mice have divergent trajectories in the innate immune response to stroke.