In vivo microglia dynamics in hypertensive mice
Grimoin E., Chazalviel L., Bernaudin M., Touzani O
Normandie University, UNICAEN, CEA, CNRS, ISTCT/CERVOxy Group, 14000 Caen, France
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Chronic arterial hypertension (cAHT) is a major global health issue and its prevalence is very high over the world. Moreover, cAHT is one of the most important risk factor for cerebrovascular diseases such as stroke (Lewington et al., 2002). Indeed, it is well known that cAHT alters the structure and function of the cerebral vasculature, which in turns predispose to stroke and aggravate the resulting brain lesion (Schiffrin et al., 2004). Evidence from the literature suggests that cAHT is associated with vascular inflammation but little is known about its impact on brain cortical parenchyma. Consequently, we hypothesized that cAHT induces chronic cerebral inflammation which sensitises the brain to stroke.
Microglia, the resident immune cells of the brain, use their highly motile processes to continuously scan the entire extracellular space and to trigger a fast, and dynamic response to acute lesions (Nimmerjahn et al., 2012). Hence microglial cells are increasingly used for imaging neuroinflammation. Here we examined the effects of cAHT on cortical brain inflammation using in vivo two-photon microscopy and transgenic mice which express GFP selectively in microglial cells (CX3CR1+/GFP mice). Arterial hypertension was induced through the use of the 2Kidney/1Clip approach (n=18) on CX3CR1+/GFP mice compared to normotensive sham mice (n=20). Arterial blood pressure was measured twice a week using the tail-cuff method. Two weeks after hypertension induction, blood pressure was increased by 20 % and remained stable throughout the experiment. During the first phase (1 month) and the chronic phase (6 months) of hypertension, a reinforced thinned skull window, which is minimally invasive, was mounted over the somatosensory cortex. We quantified, in the cerebral cortex, the microglia 3D morphology by measuring their domain size and density. These parameters were similar between hypertensive and normotensive mice. In the same way, the somata volume was comparable in hypertensive mice relative to normotensive ones in both phases of hypertension. To analyse the microglia dynamics, we quantified the de novo formation and withdrawal velocities of individual processes. In basal conditions, processes of microglia in hypertensive mice displayed extension and retraction rates of 0.81 ± 0.29 µm/min and 1.66 ± 0.68 µm/min respectively and was comparable to what of normotensive mice (0.96 ± 0.36 µm/min for extension and 1.57 ± 0.58 µm/min for retraction). These data suggest that renovascular hypertension did not alter the morphology and the dynamics of microglia. Accordingly, the vulnerability of brain tissue to stroke may not be attributed to cAHT-induced changes of microglia.