Simultaneous imaging of redox states in dystrophic neurites and microglia at Aβ plaques indicate lysosome accumulation not microglia correlate with increased oxidative stress

The inter-relationship between microglia dynamics and oxidative stress (Ox-stress) in dystrophic neurites (DNs) at Alzheimer's Disease (AD) plaques may contribute to the pathological changes in neurons. We developed new in vivo imaging strategies to combine EGFP expression in microglia with neuronal expression of genetically encoded ratiometric redox sensors (rogRFP2 or roGFP1), and immunohistochemistry to investigate how microglia influence Ox-stress at amyloid plaques in 5xFAD AD mice. By simultaneously imaging microglia morphology and neuronal Ox-stress over time in vivo and in fixed brains we found that microglia preferentially enwrapped DNs exhibiting the greatest degree of Ox-stress. After microglia were partially depleted with the CSF1 receptor antagonist PLX3397, Ox-stress in DNs increased in a manner that was inversely correlated to the extent of coverage of the adjacent Aβ plaques by the remaining microglia. These data suggest that microglia do not create Ox-stress at Aβ plaques but instead create protective barriers around Aβ plaques possibly reducing the spread of Aβ. Intracranial injection of Aβ was sufficient to induce neuronal Ox-stress suggesting it to be the initial trigger of Ox-stress generation. Although Ox-stress is increased in DNs, neuronal survival is enhanced following microglia depletion indicating complex and multifactorial roles of microglia with both neurotoxic and neuroprotective components. Increased Ox-stress of DNs was correlated with higher LAMP1 and ubiquitin immunoreactivity supporting proposed mechanistic links between lysosomal accumulation in DNs and their intrinsic generation of Ox-stress. Our results suggest protective as well as neurotoxic roles for microglia at plaques and that the generation of Ox-stress of DNs could intrinsically be generated via lysosomal disruption rather than by microglia. In Brief: Simultaneous imaging of microglia and neuronal Ox-stress revealed a double-edged role for microglia in 5xFAD mice. Plaque associated microglia were attracted to and enwrapped Aβ plaques as well as the most highly oxidized DNs. After partial depletion of microglia, DNs were larger with greater levels of Ox-stress. Despite increased Ox-stress after microglia removal neuronal survival improved. Greater Ox-stress was correlated with increased levels of LAMP1 and ubiquitin thereby linking lysosome accumulation and Ox-stress in DNs.