There and Back Again: Transient Microglial Responses to Localized Neuronal Damage In Vivo

Abstract

Purpose : Microglia are believed to be highly dynamic cells that constantly survey their microenvironment for signs of distress and disease, but their cellular behavior is difficult to study because in vitro and ex vivo preparations disrupt native microenvironments. In vivo retinal imaging offers the unique opportunity to non-invasively observe cellular behavior of microglia in health and neurodegeneration. We sought to characterize distribution and dynamics of microglia in response to localized photoreceptor damage in vivo.

Methods : Retinal microglia of Cx3cr1^GFP/GFP and Cx3cr1^+/GFP mice were imaged in vivo with custom-built combined scanning laser ophthalmoscope (SLO) and optical coherence tomography (OCT) system, and adaptive optics SLO (AO-SLO). The same retinal locus was imaged to assess microglial distribution, process motility, and somatic mobility. A 7.8 mW, 860 nm laser was focused onto a spot on the retina for 2 minutes to cause highly localized photoreceptor damage. The microglial response was monitored with SLO, while changes in the retina were monitored with OCT.

Results : In healthy retinas, the density of ramified microglia was highly homogenous across the retina in both strains of mice, but the cellular morphologies varied systematically in different retinal layers, consistent with previous histology. Time-lapse imaging revealed strikingly little movement of the primary or secondary microglial branches, even 1 hr following localized disruption of photoreceptors. Microglia within the immediate vicinity of the disruption migrated into the area of damage relatively slowly, beginning within 8 hours, and continuing over the time scale of days. Over a period of weeks, the cluster of microglia returned to their initial distribution and morphology, concurrent with reduced OCT measures of retinal damage.

Conclusions : These results show that microglial processes in both healthy and damaged retina appear less motile than those reported in cortex, suggesting functional variations between microglial populations depend upon their specific microenvironments within the CNS. The observed restoration of microglial morphology and spatial distribution over time reveals local light damage to be a valuable model system for studying the resolution of neuroinflammation in vivo.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.