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Jesse B Schallek, Aby Joseph; Time-lapse imaging of retinal microglia in vivo show dynamic process motility at rest. Invest. Ophthalmol. Vis. Sci. 2017;58(8):316. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Microglia are immune cells in the central nervous system that have a ramified appearance and undergo rapid remodeling in response to neural injury. Brain microglia are also dynamic at rest as their processes provide constant surveillance of tissue health. While reports have shown that retinal microglia somas migrate in response to injury, less is known about microscopic process remodeling in the uninjured retina. Here we image processes motility without injury using adaptive optics ophthalmoscopy over the course of minutes-to-hours using transgenic mice with fluorescent microglia.
CX3CR1-GFP transgenic mice (Jackson labs, stock 005582) express green fluorescent protein in microglia, monocytes and other immune cells that express the CX3CR1 chemokine receptor that is important for immune cell adhesion and migration. Healthy, 10 week old mice were anesthetized and imaged with an adaptive optics scanning light ophthalmoscope (AOSLO) with fluorescence capabilities. Time-lapse imaging was conducted at the same retinal location using 188 uW of 796 nm light for image registration and 53 uW of 488 nm light for fluorescence excitation (520/35 nm collection). Such light intensities are not expected to create phototoxic damage. Videos were captured at 25 Hz at fixed intervals to produce time-lapse sequences over >1.5 hr.
Dynamic process motility was observed in microglia in the uninjured retina (N=7 cells, 2 mice). While somas remained stationary over the 1.5 hr imaging interval, we observed highly active process remodeling in the healthy retina (Fig. 1). All imaged microglia showed some form of process motility, including extensions, retractions and de novo sprouts in the en face plane. Tracking the same cells over time, we observed a single process retraction that migrated 111 μm over 55 minutes, net velocity 2.0 μm/min (Fig. 2). On shorter time scales, movement of branches showed accelerations and decelerations. The fastest measured process velocity was 6.38 μm/min, the slowest showed no change, indicating process permanence over 1.5 hr.
Consistent with reports in the brain, we observed rapid process motility in the CX3CR1-GFP mouse retina. In vivo retinal process motility was comparable to velocities reported in the brain and from ex vivo retinal preparations. This dynamism suggests that constant surveillance of the neural parenchyma is a feature of the healthy retina in vivo.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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