Abstract
Purpose :
To investigate the contribution of inflammation to retinal degeneration caused by loss of MERTK.
Methods :
RT-PCR and ELISA were used to quantify inflammatory cytokines in neural retina dissected from pigmented mertk-/- mice from postnatal age P14. Fixed eye tissue paraffin microtome sections of these mice and of mertk mutant Royal College of Surgeons (RCS) rats were labeled with cell marker antibodies and analyzed by immunofluorescence microscopy to assess microglia and retinal structure. Clodronate liposomes applied by intravitreal injection or tamoxifen applied orally and as eyedrops were used to suppress microglia in RCS rats. Retinal function was assessed by recording the scotopic electroretinogram (ERG) followed by quantification of mean a- and b-wave amplitudes. Each individual experiment tested a mix of male and female animals with 6-9 animals per cohort. Differences between treated and untreated animals were established using Student’s t-test.
Results :
We detected CCL4 and CCL5 cytokine increase at early age in morphologically normal mertk-/- retina. Migration of Iba1-positive microglia from their normal localization in the inner retina to an abnormal localization in the outer retina was observed by postnatal day 20, an age before any overt photoreceptor degeneration. To test whether this early migration of microglia had an aggravating or a protective role we decreased microglia in RCS rats either by clodronate liposome intravitreal injection or by oral and eyedrop administration of tamoxifen. Clodronate modestly delayed photoreceptor loss retaining two additional rows of photoreceptor nuclei 15 days after treatment. ERG a-waves of clodronate-treated animals were improved but b-waves were lower than control buffer-injected animals suggesting benefit to photoreceptors but toxicity to inner retina. Tamoxifen also decreased microglia with similar partial efficacy as clodronate but showed no retinal toxicity. Tamoxifen-treated rats showed increased ERG a-waves and similar b-waves compared with control littermates. Like with clodronate, tamoxifen treatment preserved 2 rows of photoreceptor nuclei.
Conclusions :
Our results imply that inflammatory mechanisms and specifically microglia aggravate retinal degeneration in experimental animals lacking MERTK. Therapies targeting inflammatory cells may thus succeed in slowing the course of retinal degenerations in mutant mertk associated retinitis pigmentosa.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.