Ischemic preconditioning induces cortical microglial proliferation and a transcriptomic program of robust cell cycle activation

Abstract

Ischemic preconditioning (IPC) is an experimental phenomenon in which a subthreshold ischemic insult applied to the brain reduces damage caused by a subsequent more severe ischemic episode. Identifying key molecular and cellular mediators of IPC will provide critical information needed to develop novel therapies for stroke. Here we report that the transcriptomic response of acutely isolated preconditioned cortical microglia is dominated by marked upregulation of genes involved in cell cycle activation and cellular proliferation. Notably, this transcriptional response occurs in the absence of cortical infarction. We employed ex vivo flow cytometry, immunofluorescent microscopy, and quantitative stereology methods on brain tissue to evaluate microglia proliferation following IPC. Using cellular colocalization of microglial (Iba1) and proliferation (Ki67 and BrdU) markers, we observed a localized increase in the number of microglia and proliferating microglia within the preconditioned hemicortex at 72, but not 24, hours post‐IPC. Our quantification demonstrated that the IPC‐induced increase in total microglia was due entirely to proliferation. Furthermore, microglia in the preconditioned hemisphere had altered morphology and increased soma volumes, indicative of an activated phenotype. Using transgenic mouse models with either fractalkine receptor (CX3CR1)‐haploinsufficiency or systemic type I interferon signaling loss, we determined that microglial proliferation after IPC is dependent on fractalkine signaling but independent of type I interferon signaling. These findings suggest there are multiple distinct targetable signaling pathways in microglia, including CX3CR1‐dependent proliferation that may be involved in IPC‐mediated protection.

Integrated Bioinformatics Analysis of Potential mRNA and miRNA Regulatory Networks in Mice With Ischemic Stroke Treated by Electroacupuncture

“…Among these genes, Mki67, Top2a, and Cenpf, which are cellular markers for proliferation, were increased in the MCAO model and decreased by EA treatment. This was consistent with previous RNA-seq reports of ischemic stroke (40,41). Previous studies also reported that following ischemic injury, the number of reactive astrocytes increased, and subsequently, microglia were activated since oxidative radical imbalance occurred (41).…”

Neuroprotective effects and mechanisms of ischemic/hypoxic preconditioning on neurological diseases

“… 78 Interestingly, IPC could induce cortical microglial proliferation dependent on fractalkine signaling. 79 RIPC inhibited inflammation by decreasing the levels of IL‐1β, IL‐6, and interferon‐γ in the ischemic brain. 80 Astrocytes are another type of glial cells which also exert immune regulation, 81 it mediate inflammatory effects by releasing neurotransmitters such as glutamate, and cytokines such as TNF‐α.…”

“…In addition, conditioned medium from HPC‐treated BMSCs could switch microglia toward anti‐inflammatory polarization and alleviate microglia‐induced injury by inhibiting the levels of pro‐inflammatory cytokines, such tumor necrosis factor (TNF)‐α, and upregulating anti‐inflammatory cytokines, such as IL‐10 78 . Interestingly, IPC could induce cortical microglial proliferation dependent on fractalkine signaling 79 . RIPC inhibited inflammation by decreasing the levels of IL‐1β, IL‐6, and interferon‐γ in the ischemic brain 80 .…”

Reduction of the RNA Binding Protein TIA1 Exacerbates Neuroinflammation in Tauopathy

“…While low-levels of proliferating microglia have been observed in the hippocampus of P301S mice , evidence shows that TIA1 regulates cell-cycle protein translation, with TIA1 reduction increasing cellular proliferation in vitro (Reyes et al, 2009;Sánchez-Jiménez and Izquierdo, 2015). Knockout of TIA1 in neural tissue has been shown to increase expression of CCNF and CDKN1 (Heck et al, 2014), cell cyclin proteins that are expressed during microglial proliferation after ischemic stroke (McDonough et al, 2020). Knockout of TIA1 may not only impact the inflammatory environment, contributing to microglial proliferation, but it may also increase expression of proteins integral for microglial proliferation.…”