Dendrimer-targeted immunosuppression of microglia reactivity super-accelerates photoreceptor regeneration kinetics in the zebrafish retina

Abstract

Purpose : We previously showed that microglia are key regulators of retinal regeneration in zebrafish. Microglia reactivity to induced rod photoreceptor death stimulates Müller glia to mount a regenerative response. Ablating microglia or suppressing microglia reactivity with dexamethasone (Dex) prior to induction of cell death inhibited regeneration. However, suppressing microglia reactivity after induction of rod death enhanced regeneration kinetics. These data support the concept that microglia play stage-dependent roles during retinal regeneration and that immunosuppression could be a viable therapeutic approach for promoting repair in retinal degenerative disease. Here, we used intravital time-lapse imaging to investigate how post-injury Dex treatment altered microglia reactivity. We also tested what effect targeted delivery of Dex to reactive immune cells following induction of rod cell death had on regeneration kinetics.

Methods : Metronidazole (Mtz) induced rod cell death was enabled by a transgenic line expressing bacterial nitroreductase (NTR) and a fluorescent reporter in rod photoreceptors. A novel intravital imaging technique, Adaptive Optics-Lattice Light Sheet Microscopy (AO-LLSM), was used to capture fast subcellular dynamics of microglia. Imaris was used to quantify aspects of microglia behavior during regeneration ± Dex. Dendrimer conjugated Dex (Dendrimer-Dex) was evaluated for effects on rod cell regeneration kinetics using an established plate reader assay.

Results : AO-LLSM time-lapse imaging showed that post-injury Dex represses reactivity to rod cell death by reducing the speed of microglia migration rather than altering morphology (sphericity). Compared to free Dex controls, Dendrimer-Dex formulations led to: 1) reduced toxicity, 2) targeted delivery of Dex to reactive microglia, and 3) a further enhancement of regeneration kinetics, from +33% (free Dex controls) to +67% (Dendrimer-Dex treated fish).

Conclusions : These results increase our understanding of the roles microglia play during retinal regeneration and advance the therapeutic potential of immune suppressing drugs and dendrimer-based immune cell targeting in retinal disease settings.

This is a 2021 ARVO Annual Meeting abstract.

 

Figure. Microglia migration speed is suppressed in response to post-ablation Dex treatment. Each dot in E+F represents one fish.