The small GTPase RAB28 is required for phagocytosis of cone outer segments by the murine retinal pigmented epithelium

• Published in: The Journal of biological chemistry Vol. 293; no. 45; pp. 17546 - 17558
• Main Authors: Ying, Guoxin, Boldt, Karsten, Ueffing, Marius, Gerstner, Cecilia D., Frederick, Jeanne M., Baehr, Wolfgang
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 09.11.2018; American Society for Biochemistry and Molecular Biology
• Subjects: Animals; Cone-Rod Dystrophies - enzymology; Cone-Rod Dystrophies - genetics; Cone-Rod Dystrophies - pathology; Cyclic Nucleotide Phosphodiesterases, Type 6 - genetics; Cyclic Nucleotide Phosphodiesterases, Type 6 - metabolism; Mice; Mice, Knockout; Molecular Bases of Disease; Phagocytosis; Potassium Channels, Inwardly Rectifying - genetics; Potassium Channels, Inwardly Rectifying - metabolism; rab GTP-Binding Proteins - genetics; rab GTP-Binding Proteins - metabolism; Retinal Cone Photoreceptor Cells - enzymology; Retinal Cone Photoreceptor Cells - pathology; Retinal Pigment Epithelium - enzymology; Retinal Pigment Epithelium - pathology; Retinal Rod Photoreceptor Cells - enzymology; Retinal Rod Photoreceptor Cells - pathology; gene knockout; phagocytosis; retinal degeneration; retina; photoreceptor; disc shedding; cone-rod dystrophy; RAB28; retinal pigmented epithelium; small GTPase
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.RA118.005484

RAB28, a member of the RAS oncogene family, is a ubiquitous, farnesylated, small GTPase of unknown function present in photoreceptors and the retinal pigmented epithelium (RPE). Nonsense mutations of the human RAB28 gene cause recessive cone-rod dystrophy 18 (CRD18), characterized by macular hyperpigmentation, progressive loss of visual acuity, RPE atrophy, and severely attenuated cone and rod electroretinography (ERG) responses. In an attempt to elucidate the disease-causing mechanism, we generated Rab28−/− mice by deleting exon 3 and truncating RAB28 after exon 2. We found that Rab28−/− mice recapitulate features of the human dystrophy (i.e. they exhibited reduced cone and rod ERG responses and progressive retina degeneration). Cones of Rab28−/− mice extended their outer segments (OSs) to the RPE apical processes and formed enlarged, balloon-like distal tips before undergoing degeneration. The visual pigment content of WT and Rab28−/− cones was comparable before the onset of degeneration. Cone phagosomes were almost absent in Rab28−/− mice, whereas rod phagosomes displayed normal levels. A protein–protein interaction screen identified several RAB28-interacting proteins, including the prenyl-binding protein phosphodiesterase 6 δ-subunit (PDE6D) and voltage-gated potassium channel subfamily J member 13 (KCNJ13) present in the RPE apical processes. Of note, the loss of PDE6D prevented delivery of RAB28 to OSs. Taken together, these findings reveal that RAB28 is required for shedding and phagocytosis of cone OS discs.