Microglial activation in the right amygdala-entorhinal-hippocampal complex is associated with preserved spatial learning in mice

Background: In Alzheimer's disease (AD), regional heterogeneity of beta-amyloid burden and microglial activation of individual patients is a well-known phenomenon. Recently, we described a high incidence of inter-individual regional heterogeneity in terms of asymmetry of plaque burden and microglial activation in beta-amyloid mouse models of AD as assessed by positron-emission-tomography (PET). We now investigate the regional associations between amyloid plaque burden, microglial activation, and impaired spatial learning performance in transgenic mice in vivo. Methods: In 30 App(NL-G-F) mice (15 female, 15 male) we acquired cross-sectional 18 kDa translocator protein (TSPO-PET, F-18-GE-180) and beta-amyloid-PET (F-18-florbetaben) scans at ten months of age. Control data were obtained from age- and sex-matched C57BI/6 wild-type mice. We assessed spatial learning (i.e. Morris water maze) within two weeks of PET scanning and correlated the principal component of spatial learning performance scores with voxel-wise beta-amyloid and TSPO tracer uptake maps in App(NL-G-F) mice, controlled for age and sex. In order to assess the effects of hemispheric asymmetry, we also analyzed correlations of spatial learning performance with tracer uptake in bilateral regions of interest for frontal cortex, entorhinal/piriform cortex, amygdala, and hippocampus, using a regression model. We tested the correlation between regional asymmetry of PET biomarkers with individual spatial learning performance. Results: Voxel-wise analyses in App(NL-G-F) mice revealed that higher TSPO-PET signal in the amygdala, entorhinal and piriform cortices, the hippocampus and the hypothalamus correlated with spatial learning performance. Region-based analysis showed significant correlations between TSPO expression in the right entorhinal/piriform cortex and the right amygdala and spatial learning performance, whereas there were no such correlations in the left hemisphere. Right lateralized TSPO expression in the amygdala predicted better performance in the Morris water maze (beta = -0.470, p = 0.013), irrespective of the global microglial activation and amyloid level. Region-based results for amyloid-PET showed no significant associations with spatial learning. Conclusion: Elevated microglial activation in the right amygdala-entorhinal-hippocampal complex of App(NL-G-F) mice is associated with better spatial learning. Our findings support a protective role of microglia on cognitive function when they highly express TSPO in specific brain regions involved in spatial memory.