Retinal degeneration and protein mislocalization in MKS6 mutants

Abstract

Purpose : Mutations occurring in transition zone (TZ) genes result in retinal degeneration-associated ciliopathies. The exact role of the TZ gene Meckel Grüber 6 (Mks6 or CC2D2A) is currently unknown within the retina. We have generated two Mks6 knockout (KO) animal models, a conditional KO mouse model and a congenital zebrafish KO model.

Methods : Mouse embryonic stem cells containing LoxP sites flanking exons 6 and 7 of the Mks6 allele were obtained from MMRC. Offspring from chimeric animals were bred to homozygosity (Mks6^F/F) and crossed with CAGG-Cre; Mks6^F/+ mice. To induce Mks6 loss mice in offspring, Mks6^F/F;CAGG-CreER and Mks6^F/F;Cre^- littermates were injected i.p with tamoxifen. We assessed juvenile induced (P21) and adult induced (20 weeks) animals. For IHC, eyes were enucleated, fixed, embedded in OCT, cryosliced, immunostained for rhodopsin, arrestin and transducin and imaged using confocal microscopy. Electroretinography (ERG) was performed weekly to monitor retinal function with adult induced Mks6 KO. To generate Mks6 KO zebrafish, we deleted 7 base pairs (-GCAGCGA) in Exon 7. Animals were euthanized at 5 days post fertilization (dpf) and histology was analyzed by hematoxylin and eosin staining. For dark adaptation studies, animals were dark adapted for 24 hours prior to euthanasia and analysis.

Results : In the dark, juvenile induced Mks6 KO mice show mislocalization of rhodopsin, whereas arrestin and transducin have proper localization. In the light, transducin properly localizes to the inner segment (IS), but rhodopsin and arrestin are mislocalized in the IS and outer nuclear layer (ONL). In contrast, adult induced KO mice show a reduction of scoptopic and photopic retinal responses via ERG and have two rows of nuclei in the ONL. Mks6 KO zebrafish have a curled down tail at 2dpf and retinal degeneration with apoptotic cell death and severely shortened OS.

Conclusions : Mks6 plays an important role in trafficking of proteins and retinal homeostasis. Arrestin is mislocalized in light, whereas rhodopsin is slightly mislocalized in light and dark conditions. In adult induced Mks6 conditional KO mice, progressive loss of scotopic and photopic function was found through ERG, with minimal OS present. In Mks6 KO zebrafish, at 5dpf, severely shortened OS are present. These data support the hypothesis that Mks6 is important within the TZ in maintenance of the primary cilia in the retina as shown in two distinct animal models.

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