A stereological study of glial activation in GFAP-IL6 mice: A model of chronic neuroimmune conditions.

Abstract

Understanding the role of chronic neuroinflammation, described by increased astro- and microglia activation, in the pathogenesis of Alzheimer's disease (AD) is becoming crucial due to its role throughout the disease progression (Deczkowska et al., 2018). In search of a suitable mouse model of chronic glial activation, we characterized the GFAP-IL6 transgenic mouse model in which the pro-inflammatory cytokine interleukine-6 is overexpressed under the control of the glial fibrillary acidic protein (GFAP) promoter (Campbell et al., 1993). The GFAP-IL6 mice display chronic inflammatory conditions in the brain, with the homozygous mice developing an aggressive pathology characterized by tremor, ataxia, and seizures by 6 months of age (Campbell et al., 1993). While the heterozygous phenotype is milder and characterized by astrocytosis, microgliosis, angiogenesis, and neurodegeneration. Moreover, evidences showed that the progressive motor and cognitive impairment is correlated with the level of microgliosis (Heyser et al., 1997). Using immunohistochemically stained brain sections of 3 months old wild-type (WT) (n=4), GFAP-IL6 heterozygous (n=5), and homozygous (n=5) mice, we estimated the absolute number and morphology of Iba1 and GFAP positive cells, microglia and astrocytes respectively. We focused the analysis on cortex, hippocampus, cerebellum, and medial septum (MS) to find a possible connection with the loss of cholinergic neurons during AD. Homozygous GFAP-IL6 mice showed an increase in the number of microglia by 50-80% in all the brain areas compared to the WT, while the increase in the heterozygous was 35-60%. Moreover, homozygous mice increased the number of microglia by 40% in the MS and nearly 50% in the cerebellum compared to the heterozygous. In the MS astrocytes of the homozygous mice are 70% more than in the heterozygous, which are similar to the WT. The glial morphological analysis showed a different ramification and microglial cell body size in the homozygous compared to the heterozygous and WT. The stronger impact of neuroinflammation in the homozygous GFAP-IL6 mice at 3 months old suggests a possible application in evaluating chronic conditions at early stages. Moreover, the results highlight the high vulnerability to inflammation of the MS. Increased cerebellar glial mediated inflammation emphasized the chance to develop motor deficits.