Early Surge in Brain GFAP Distribution in Female Hamsters With Mild Peripheral COVID-19.

Mild to moderate respiratory COVID-19 is commonly associated with a range of neurological symptoms. The two primary proposed mechanisms linking this primarily peripheral disease to the brain, include brain entry of peripheral circulating cytokines through a compromised blood-brain barrier, and/or direct brain infection. It has been proposed that the resulting neuroinflammation (i.e., inflammation taking place in the brain) could impair synaptic transmission, causing cognitive dysfunction. One key aspect of neuroinflammation is increased astrocyte reactivity, also known as 'astrogliosis'. Astrogliosis is characterized by altered astrocyte morphology, proliferative capacity, gene expression, and function. Direct investigation of astrogliosis in different brain regions in the context of mild to moderate COVID-19 has been limited. To this end, we quantified changes to astrocytes in a Syrian hamster model of mild to moderate respiratory COVID-19 in five regions of interest: cortex, corpus callosum, hippocampus, bordering the third ventricle, and dorsal striatum. We extracted brains at 1-, 3-, 5-, 7-, and 31-days post-inoculation and from uninfected controls, and immunolabeled sections with astrocyte- (GFAP and SOX9) and neuron- (NEUN) specific markers. We captured tiled confocal micrographs of entire brain sections and quantified these markers in the aforementioned brain regions using a newly-developed unbiased analysis pipeline. At 3-days post-inoculation, we identified a significant, transient increase in GFAP distribution in female hamsters in the cortex, as well as non-significant increases in the hippocampus and corpus callosum. We also found increased GFAP-positive process complexity, an indicator of astrocyte complexity, at 3-days post-inoculation in the female cortex. There were no changes in astrocyte (SOX9), neuron (NEUN) or total cell (Hoechst) numbers. Moreover, there were no changes in male hamsters at any time point in any region of interest. Our findings provide the first spatiotemporal insight into sex differences in astrogliosis in the context of COVID-19.