Requirement for Microglia for the Maintenance of Synaptic Function and Integrity in the Mature Retina

Abstract

Purpose : Microglia in the retina exert significant effects on retinal neurons during development and in retinal disease. However, whether microglia contribute to normal structure and function in the healthy adult retina is not well understood. We investigate if microglia are constitutively required in the adult retina for the maintenance of normal retinal structure and function by sustained microglial depletion using a genetic mouse model.

Methods : Microglia were depleted in young, two-month old CX3CR1CreER; Rosa26-flox-STOP-flox-DTA transgenic mice by inducing a microglial-specific expression of diphtheria toxin subunit alpha by tamoxifen administration. Retinal function was assessed by electroretinography (ERG) and by measurement of elicited optokinetic motor reflexes (OMR). The structure of retinal synapses was evaluated with immunohistochemistry and electron microscopy. Expression of microglial- and macroglial- specific transcripts was monitored with rtPCR and next generation sequencing (NGS).

Results : Microglial depletion (>95%) was sustained in the adult retina over one month, as evidenced by the elimination of immunopositivity to microglial markers (Iba1, CD11b, CX3CR1) and by the reduction in microglial-specific transcripts as assessed by rtPCR and NGS. Chronic microglial depletion did not (1) alter retinal thickness or lamination (as assessed by OCT and histology), (2) induce cellular apoptosis (as assessed by TUNEL labeling), or (3) alter the laminar organization of dendrites and axons of retinal neurons. However, sustained microglial depletion results in the degeneration of photoreceptor synapses in the outer plexiform layer as evident on electron microscopy. These changes are associated with a progressive and significant deterioration in dark- and light-adapted ERG responses, with a declining b-to-a amplitude ratio. OMR responses were slightly but not significantly decreased with microglial depletion.

Conclusions : Microglia are constitutively required for the maintenance of synaptic structure in the adult retina and for synaptic transmission underlying normal visual function. Understanding constitutive microglia function in the retina is relevant to elucidating microglial contributions to pathology and in considering therapeutic interventions that reduce or perturb constitutive microglial function.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.