P161 Cell-autonomous effect of microglia on the induction of Alzheimer’s disease neuronal cell cycle events

Introduction Massive neuronal loss is the prominent pathological hallmark feature of Alzheimer’s disease (AD). One of the ways post-mitotic neurons die is via their ectopic entry into cell-cycle, which often occurs at very early stages of the disease [1] . We have previously demonstrated occurrence of neuronal cell cycle events (CCEs) in a transgenic mouse model of AD (R1.40) similar to those occur in human AD [1] . Furthermore, these events were dependent upon amyloidogenic processing of amyloid-precursor protein [2] , precede deposition of senile plaques and coincide with elevated neuroinflammation. While we have also demonstrated that Tool-Like Receptor-4 (TLR4) mediated induction of neuroinflammation accelerated neuronal CCEs, and blocking inflammation via NSAIDs completely blocked neuronal CCEs and reduced microglial activation [3] , the source of neuroinflammation and molecular mechanisms for the induction of neuronal CCEs is unclear. Methods We utilized following methods: immunohistochemistry, purification and adoptive transfer of microglia, primary neurons and microglial cultures and generation and analysis of R1.40- Tnf α −/− mice. Results Here we demonstrate that neuroinflammation, cell autonomous to microglia, is capable of inducing neuronal CCEs. First, CD45+ microglia is present at six months of age in the affected region of R1.40 mouse brain and increases with age. Second, the F4/80+ microglia spatially co-exist with cyclin D1+ neurons in the temporal cortex of human AD brain. Notably, these cyclin D1+ neurons in the human brain are also TUNEL positive, suggesting that a portion of them is apoptotic. Third, purification and adoptive transfer of CD11b+ microglia derived form R1.40, but not from non-transgenic mice, was sufficient to induce cyclin D1 expression within the neurons of non-transgenic recipient mouse brain. Fourth, conditioned media (CM) derived from amyloid beta activated primary microglia significantly induced dose-dependent increase in DNA replication within primary cortical neurons and strongly correlated with increased levels of tumor-necrosis factor-α(TNFα) in the CM. Notably, blocking or activating TNFα pathway either via anti-TNFα antibody or recombinant human TNFα, significantly induced cyclin D1 expression and/or DNA replication in in vitro as well as in adoptive transfer studies. Generation of R1.40 mice deficient for TNFα (R1.40- Tnfa −/− ) displayed decreased microglial activation and showed significantly lower levels of cyclin D1+ neurons at six months of age. Finally, analysis of downstream molecules of TNFα receptor pathway suggested that neuronal CCEs is mediated via activation of JNK signaling pathway. Conclusion Together, our results provide first direct evidence that the neuroinflammation, cell-autonomous to microglia, induces AD-related neuronal CCEs and neurodegeneration via alteration of TNFR-MAPK signaling pathway.