CX3CR1 deficiency aggravates amyloid driven neuronal pathology and cognitive decline in Alzheimer's disease

Background Despite its identification as a key checkpoint regulator of microglial activation in Alzheimer's disease, the overarching role of CX3CR1 signaling in modulating mechanisms of A beta driven neurodegeneration, including accumulation of hyperphosphorylated tau is not well understood. Methodology Accumulation of soluble and insoluble A beta species, microglial activation, synaptic dysregulation, and neurodegeneration is investigated in 4- and 6-month old 5xFAD;Cx3cr1(+/+) and 5xFAD;Cx3cr1(-/-) mice using immunohistochemistry, western blotting, transcriptomic and quantitative real time PCR analyses of purified microglia. Flow cytometry based, in-vivo A beta uptake assays are used for characterization of the effects of CX3CR1-signaling on microglial phagocytosis and lysosomal acidification as indicators of clearance of methoxy-X-04(+) fibrillar A beta. Lastly, we use Y-maze testing to analyze the effects of Cx3cr1 deficiency on working memory. Results Disease progression in 5xFAD;Cx3cr1(-/-) mice is characterized by increased deposition of filamentous plaques that display defective microglial plaque engagement. Microglial A beta phagocytosis and lysosomal acidification in 5xFAD;Cx3cr1(-/-) mice is impaired in-vivo. Interestingly, Cx3cr1 deficiency results in heighted accumulation of neurotoxic, oligomeric A beta, along with severe neuritic dystrophy, preferential loss of post-synaptic densities, exacerbated tau pathology, neuronal loss and cognitive impairment. Transcriptomic analyses using cortical RNA, coupled with qRT-PCR using purified microglia from 6 month-old mice indicate dysregulated TGF beta-signaling and heightened ROS metabolism in 5xFAD;Cx3cr1(-/-) mice. Lastly, microglia in 6 month-old 5xFAD;Cx3cr1(-/-) mice express a 'degenerative' phenotype characterized by increased levels of Ccl2, Ccl5, Il-1 beta, Pten and Cybb along with reduced Tnf, Il-6 and Tgf beta 1 mRNA. Conclusions Cx3cr1 deficiency impairs microglial uptake and degradation of fibrillar A beta, thereby triggering increased accumulation of neurotoxic A beta species. Furthermore, loss of Cx3cr1 results in microglial dysfunction typified by dampened TGF beta-signaling, increased oxidative stress responses and dysregulated pro-inflammatory activation. Our results indicate that A beta-driven microglial dysfunction in Cx3cr1(-/-) mice aggravates tau hyperphosphorylation, neurodegeneration, synaptic dysregulation and impairs working memory.

Automated summary

This study reveals that Cx3cr1 deficiency impairs the uptake and degradation of A beta by microglia, leading to increased accumulation of neurotoxic A beta species. Additionally, the loss of Cx3cr1 results in dysfunctional microglia characterized by dampened TGF beta signaling, increased oxidative stress responses, and dysregulated pro-inflammatory activation. These findings highlight the significant role of Cx3cr1 in modulating neurodegeneration and cognitive function.