Reactivity and increased proliferation of NG2 cells following central nervous system infection with Theiler’s murine encephalomyelitis virus

Abstract

Background Neuron-glial antigen 2 (NG2) cells are a glial cell type tiled throughout the gray and white matter of the central nervous system (CNS). NG2 cells are known for their ability to differentiate into oligodendrocytes and are commonly referred to as oligodendrocyte precursor cells. However, recent investigations have begun to identify additional functions of NG2 cells in CNS health and pathology. NG2 cells form physical and functional connections with neurons and other glial cell types throughout the CNS, allowing them to monitor and respond to the neural environment. Growing evidence indicates that NG2 cells become reactive under pathological conditions, though their specific roles are only beginning to be elucidated. While reactive microglia and astrocytes are well-established contributors to neuroinflammation and the development of epilepsy following CNS infection, the dynamics of NG2 cells remain unclear. Therefore, we investigated NG2 cell reactivity in a viral-induced mouse model of temporal lobe epilepsy. Methods C57BL6/J mice were injected intracortically with Theiler’s murine encephalomyelitis virus (TMEV) or PBS. Mice were graded twice daily for seizures between 3 and 7 days post-injection (dpi). At 4 and 14 dpi, brains were fixed and stained for NG2, the microglia/macrophage marker IBA1, and the proliferation marker Ki-67. Confocal z stacks were acquired in both the hippocampus and the overlying cortex. Total field areas stained by each cell marker and total field area of colocalized pixels between NG2 and Ki67 were compared between groups. Results Both NG2 cells and microglia/macrophages displayed increased immunoreactivity and reactive morphologies in the hippocampus of TMEV-injected mice. While increased immunoreactivity for IBA1 was also present in the cortex, there was no significant change in NG2 immunoreactivity in the cortex following TMEV infection. Colocalization analysis for NG2 and Ki-67 revealed a significant increase in overlap between NG2 and Ki-67 in the hippocampus of TMEV-injected mice at both time points, but no significant differences in cortex. Conclusions NG2 cells acquire a reactive phenotype and proliferate in response to TMEV infection. These results suggest that NG2 cells alter their function in response to viral encephalopathy, making them potential targets to prevent the development of epilepsy following viral infection.

Screening of prototype antiseizure and anti‐inflammatory compounds in the Theiler's murine encephalomyelitis virus model of epilepsy

“…Both male and female C57Bl/6J mice infected with TMEV have behavioral seizures. 1,[67][68][69] Female mice may be differentially responsive to ASMs, 70 which may similarly be present in this seizure model. Future studies comparing prototype compounds in this model may suggest potential sex-dependent differences.…”

Double‐edged effects of tamoxifen‐in‐oil‐gavage on an infectious murine model for multiple sclerosis

“…It is postulated that TMEV‐infection interferes with the differentiation of NG‐2 positive OPCs into myelinating oligodendrocytes, resulting in limited remyelination due to preferential astroglial differentiation ( 24 , 64 ). Recent studies in resistant C57BL / 6 mice have indicated a stimulation of NG‐2 positive cells after TMEV infection, causing a reactive phenotype with increased proliferation ( 65 ). It cannot be excluded that the increased inflammatory response, as shown by increased T cell infiltration in the brain of tamoxifen treated mice at 14 dpi, might had an influence on the NG‐2 positive cell proliferation in the spinal cord.…”

“…It cannot be excluded that the increased inflammatory response, as shown by increased T cell infiltration in the brain of tamoxifen treated mice at 14 dpi, might had an influence on the NG‐2 positive cell proliferation in the spinal cord. However, the beneficial effect on NG‐2 positive cell proliferation appears to be spatially restricted to the hippocampus with associated high virus load and inflammation in C57BL / 6 mice, and not in the neighboring cortex ( 65 ). Higher numbers of mature oligodendrocytes in tamoxifen‐treated mice imply that differentiation of progenitor cells and progression until the expression of NoGo‐A was positively affected in this experiment.…”

Inflammation Unleashed in Viral-Induced Epileptogenesis

“…The 2 primary innate immune cells that participate in the immune response to TMEV infection of the CNS are microglia and macrophages. However, other glial cells such as astrocytes 20 and NG2 cells 21 , 22 can also influence the innate response. Looking at microglia and macrophages in TMEV-infected chimeric mice, Cusick et al 23 demonstrated that while IL-6 is mainly produced by infiltrating macrophages, microglia produce high levels of TNF-α during the acute phase of the infection.…”

“…Reactive NG2-glia are shown to undergo morphological and functional changes that may contribute to epileptogenesis following TMEV infection. 21 Recent studies indicate that NG2-glia are in part responsible for maintaining microglia homeostasis, and dysregulation of NG2-glia signaling to microglia during inflammation and disease heavily exacerbates the pro-inflammatory response, 22 a key factor driving seizure development. Additionally, NG2-glia are known to deposit the highly negatively charged NG2 protein, otherwise known as chondroitin sulfate proteoglycan 4 (CSPG4), which, when embedded in the dense extracellular matrix of the glial scar, likely contributes to long-term disruptions in intracellular and extracellular ion homeostasis.…”