Japanese Encephalitis Virus Infected Microglial Cells Secrete Exosomes Containing let-7a/b that Facilitate Neuronal Damage via Caspase Activation

Abstract

Extracellular microRNAs (miRNAs) are essential for the cell to cell communication in the healthy and diseased brain. MicroRNAs released from the activated microglia upon neurotropic virus infection may exacerbate CNS damage. Here, we identified let-7a and let-7b (let-7a/b) as the overexpressed miRNAs in Japanese Encephalitis virus (JEV) infected microglia and assessed their role in JEV pathogenesis. We measured the let-7a/b expressions in JEV infected post-mortem human brains, mice brains and in mouse microglial N9 cells by the qRT-PCR and in situ hybridization assay. The interaction between let-7a/b and NOTCH signaling pathway further examined in Toll-like receptor 7 knockdown (TLR7 KD) mice to assess the functions. Exosomes released from JEV infected or let-7a/b mimic transfected N9, and HEK-293 cells were isolated and evaluated their function. We observed an upregulation of let-7a/b in the infected brains as well as in microglia. Knockdown of TLR7 or Inhibition of let-7a/b suppressed the JEV induced NOTCH activation possibly via NF-{kappa}B dependent manner and subsequently, attenuated JEV induced TNF production in microglial cells. Further, exosomes secreted from JEV-infected microglial cells specifically contained let-7a/b. Exosomes overexpressed with let-7a/b were injected into BALB/c mice as well as co-incubated with mouse neuronal (Neuro2a) cells, or primary cortical neuron resulted in caspase activation leading to neuronal damage in the brain. Thus, our results provide evidence for the multifaceted role of let-7a/b miRNAs and unravel the exosomes mediated mechanism for JEV induced pathogenesis.