Impact of anti-viral immunity on neural stem/progenitor cell activity and implications for CNS development
School of Pharmacy
Lauren A O'Donnell
Viral infection and inflammation in the central nervous system (CNS) can cause neuropathology, particularly in the prenatal and neonatal stages. Severe damage to the CNS may result from cytopathic effects of viral infection or from the immune response that may lyse virally-infected cells or release inflammatory mediators to mediate viral clearance. Neural stem/progenitor cells (NPSCs) are multipotent cells in the CNS that are often disrupted by neurotropic viral infections. They may be directly infected by the virus or respond to inflammatory cytokines released from resident as well as infiltrating immune cells. This bystander effect may affect NSPC differentiation and proliferation depending on the milieu of inflammatory mediators. Interferon gamma (IFN), a potent antiviral cytokine required for the control and clearance of many CNS infections, can differentially affect cell survival and cell cycle progression depending upon the cell type and the profile of activated intracellular signaling molecules. Here, we show that IFN inhibits proliferation of primary NSPCs through dephosphorylation of the tumor suppressor Retinoblastoma protein (pRb), which is dependent on activation of Signal Transducers and Activators of Transcription-1 (STAT1) signaling pathways. We observed inhibition of proliferation in wild type NSPCs (WT/NSPCs) as well as a decrease in neurosphere growth. IFN restricted cell cycle progression by inhibiting the G1- to S-phase transition. Cell cycle restriction was associated with decreases in the G1–phase specific cyclin E/CDK2 proteins and in pRb phosphorylation at serine 795 (S795). Together, these results indicate that the NSPC cell cycle was restricted in the late G1-phase. In STAT1-deficient (STAT1-KO) NSPCs, the effects of IFN on NSPC proliferation were lost, demonstrating that IFN signaling is STAT1-dependent. These data define a mechanism by which IFN could contribute to a reduction in NSPC proliferation in inflammatory conditions. Furthermore, this was the first study to implicate the pRb protein in mediating anti-proliferative effects of IFN on NSPCs.
The cellular tropism of neurotropic viruses varies, with NSPCs being targeted by some viruses and spared by others. During a viral infection, microglia are typically the first immune cells to become activated in the brain. Microglia may contribute to the anti-viral program generated against the virus and/or alter other neural cells through the release of inflammatory mediators. Evidence in neonatal brains suggest that microglia can also influence NSPC numbers and differentiation under basal conditions. However, whether microglia affect NSPCs during an anti-viral immune response is an outstanding question. To evaluate the effects of microglial activation on NSPCs, we used a mouse model for measles virus (MV) infection in neurons. In this model, MV infection is restricted to mature CNS neurons expressing the human isoform of CD46, a receptor for MV. NSPCs and microglia are spared from infection. In order to examine the interactions between infected neurons, microglia, and NSPCs, primary microglia were co-cultured with MV-infected CD46+ neurons and the conditioned medium was used to treat primary NSPCs in culture. We found that factors released from the infected neuron/microglia co-cultures increased BrdU-incorporation and neuronal differentiation in NSPCs. Thus, even though the NSPCs are not infected in this model, the cells respond by generating young neurons that could serve as potential replacements for the mature neurons damaged by the virus. These studies provide a novel model system for identifying the signals that microglia use to communicate between infected neurons and responding NSPCs.
Kulkarni, A. (2017). Impact of anti-viral immunity on neural stem/progenitor cell activity and implications for CNS development (Doctoral dissertation, Duquesne University). Retrieved from https://ddc.duq.edu/etd/175
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