The miR-146 family consists of two microRNAs (miRNAs), miR-146a and miR-146b, which are both known to suppress a variety of immune responses. Here in this study, we show that miR-146b is abundantly expressed in neuronal cells, while miR-146a is mainly expressed in microglia and astroglia of adult mice. Accordingly, miR-146b deficient (Mir146b-/-) mice exhibited anxiety-like behaviors and enhanced cognition. Characterization of cellular composition of Mir146b-/- mice using flow cytometry revealed an increased number of neurons and a decreased abundancy of astroglia in the hippocampus and frontal cortex, whereas microglia abundancy remained unchanged. Immunohistochemistry showed a higher density of neurons in the frontal cortex of Mir146b-/- mice, enhanced hippocampal neurogenesis as evidenced by an increased proliferation, and survival of newly generated cells with enhanced maturation into neuronal phenotype. No microglial activation or signs of neuroinflammation were observed in Mir146b-/- mice. Further analysis demonstrated that miR-146b deficiency is associated with elevated expression of glial cell line-derived neurotrophic factor (Gdnf) mRNA in the hippocampus, which might be at least in part responsible for the observed neuronal expansion and the behavioral phenotype. This hypothesis is partially supported by the positive correlation between performance of mice in the object recognition test and Gdnf mRNA expression in Mir146b-/- mice. Together, these results show the distinct function of miR-146b in controlling behaviors and provide new insights in understanding cell-specific function of miR-146b in the neuronal and astroglial organization of the mouse brain.

Many studies have demonstrated significant mouse-strain-specific differences in behavior and response to pathogenic and pharmacological agents. This study seeks to characterize possible differences in microglia activation and overall severity of neuroinflammation in two widely used mouse strains, C57BL/6NTac (Bl6) and 129S6/SvEvTac (129Sv), in response to acute lipopolysaccharide (LPS) administration. Locomotor activity within the open field arena revealed similar 24 h motor activity decline in both strains. Both strains also exhibited significant bodyweight loss due to LPS treatment, although it was more severe in the Bl6 strain. Furthermore, LPS induced a hypothermic response in Bl6 mice, which was not seen in 129Sv. We found that 24 h LPS challenge significantly increased the inflammatory status of microglia in 129Sv mice. On the other hand, we observed that, under physiological conditions, microglia of Bl6 seemed to be in a higher immune-alert state. Gene and protein expression analysis revealed that LPS induces a significantly stronger upregulation of MHC-I-pathway-related components in the brain of Bl6 compared to 129Sv mice. The most striking difference was detected in the olfactory bulb, where we observed significant LPS-induced upregulation of MHC-I pathway components in Bl6 mice, whereas no alterations were observed in 129Sv. We observed significant positive correlations between bodyweight decline and expressions of MHC-I components in the olfactory bulbs of Bl6 mice and the frontal cortex of 129Sv, highlighting different brain regions most affected by LPS in these strains. Our findings suggest that the brains of Bl6 mice exist in a more immunocompetent state compared to 129Sv mice.

Anxiety is a known comorbidity and risk factor for conversion to neuroinflammation-mediated dementia in patients with Alzheimer’s disease (AD). Here, we investigated if anxiety occurred as an early endophenotype of mutant familial AD (5 × FAD) male mice and the underlying neuroinflammatory mechanisms. We observed that compared to wildtype (WT) littermates, 5 × FAD mice showed enhanced anxiety at as early as 2 months old (mo). Interestingly, these 5 × FAD male mice had concomitantly increased mRNA levels of pro-inflammatory cytokines such as interleukin 1 beta (Il1b) and tumor necrosis factor (Tnf) in the olfactory bulb (OB) but not the frontal cortex (FC). Increased expression of Tnf in the OB was significantly correlated with the anxious behavior in the FAD but not WT mice. Furthermore, we found more prominent microglial activation and morphological changes in the OB of 2 mo 5 × FAD mice, while only microglial ramification was seen in the FC. To understand if neuroinflammatory changes in the FC could occur at a later stage, we studied 5~6 mo male mice and found that Il1b, interleukin 18 (Il18), and Tnf were upregulated in the FC at this older age. Furthermore, we observed that numbers of microglia and macrophage as well as microglial synaptic pruning, as indicated by phagocytosis of presynaptic component of vesicular glutamate transporter-2, were increased in the OB but not the FC of 5~6 mo 5 × FAD mice. Our findings demonstrated the OB as a more sensitive brain region than the cerebral cortex for microglia-mediated neuroinflammation in association with anxiety in FAD mice and supported the notion that the OB can be an early-stage biomarker in AD.