Microglia/Macrophage Glucagon-like Peptide 1 Receptors are Necessary for GLP-1R Retinal Ganglion Cell Rescue in Glaucoma

Abstract

Purpose : Glucagon-like peptide 1 receptor (GLP1R) agonists including liraglutide promote retinal ganglion cell (RGC) survival in a hypertensive glaucoma mouse model (PMID33147455, 37362000). Retinal macrophage/microglia (m/m) express GLP1R in ocular hypertension (Fig1A). Whether this expression is necessary to mediate RGC survival is unknown. We used a mouse model with inducible m/m-specific GLP1R knockout to answer this question.

Methods : A m/m-specific GLP1R knockout mouse (Glp1RmmKO) is generated by crossing Glp1r^(fl/fl)with Cx3cr1^(CreER). At 8 weeks, Glp1RmmKO mice received either IP tamoxifen 5mg (20mg/ml corn oil) x 5 days to induce KO, or corn oil as controls. After 2 weeks, microbeads were injected into the anterior chamber of the left eye while the right eye received a balanced salt solution (BSS). Eyes with IOP elevation < 21mmHg received a 2nd injection 3 weeks later. Intraocular pressure (IOP) was measured immediately before the first injection and weekly thereafter. For 6 weeks, treatment groups received daily SQ injections of liraglutide (400 µg/kg) while controls received PBS injections. Retina flat mounts were immunolabeled for RBPMS and Iba1 to quantify RGC and myeloid density (per 40x HPF). RGC axons were quantified in optic nerve cross-sections.

Results : Retina m/m (11b+) were isolated using flow cytometry and confirmed to lack GLP1R expression in tamoxifen-treated Glp1RmmKO mice (mmKO-T), in contrast with B6 (WT) and CO-treated Glp1RmmKO mice (mmKO-CO) where GLP1R expression increased in 11b+ cells following IOP elevation (Fig2A). IOP remained elevated for 6 weeks after bead injections (p<0.0001 compared to BSS-injected eyes; Fig1B). In the liraglutide groups, mmKO-T mice demonstrated 12% reduction in RGC density (p=0.0138) following 6 weeks of IOP elevation, whereas mmKO-CO mice showed preservation of RGC density (p=0.1109) (Fig2B). Results of the PBS groups are pending, as are myeloid density and optic nerve axon analyses.

Conclusions : Preliminary results showed significant RGC loss in the m/m-specific GLP1R knockout mouse model of glaucoma compared to controls despite liraglutide rescue, suggesting GLP1R expression on m/m is necessary to produce the full effect of GLP1R agonism-induced RGC protection in ocular hypertension. Pending results from the PBS-treated groups will help quantify the extent to which RGC rescue relies on activation of GLP1R on m/m.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.