Progressive Long‐term Spatial Memory Loss following Repeat Concussive and Subconcussive Brain Injury in Mice, Associated with Dorsal Hippocampal Neuron Loss, Microglial Phenotype Shift, and Vascular Abnormalities

There is considerable concern about the long-term deleterious effects of repeat head trauma on cognition, but little is known about underlying mechanisms and pathology. To examine this, we delivered four air blasts to the left side of the mouse cranium, a week apart, with an intensity that causes deficits when delivered singly and considered "concussive," or an intensity that does not yield significant deficits when delivered singly and considered "subconcussive." Neither repeat concussive nor subconcussive blast produced spatial memory deficits at 4 months, but both yielded deficits at 14 months, and dorsal hippocampal neuron loss. Hierarchical cluster analysis of dorsal hippocampal microglia across the three groups based on morphology and expression of MHCII, CX3CR1, CD68 and IBA1 revealed five distinct phenotypes. Types 1A and 1B microglia were more common in sham mice, linked to better neuron survival and memory, and appeared mildly activated. By contrast, 2B and 2C microglia were more common in repeat concussive and subconcussive mice, linked to poorer neuron survival and memory, and characterized by low expression levels and attenuated processes, suggesting they were de-activated and dysfunctional. In addition, endothelial cells in repeat concussive mice exhibited reduced CD31 and eNOS expression, which was correlated with the prevalence of type 2B and 2C microglia. Our findings suggest that both repeat concussive and subconcussive head injury engender progressive pathogenic processes, possibly through sustained effects on microglia that over time lead to increased prevalence of dysfunctional microglia, adversely affecting neurons and blood vessels, and thereby driving neurodegeneration and memory decline.