Receptor MER Tyrosine Kinase Proto-oncogene (MERTK) Is Not Required for Transfer of Bis-retinoids to the Retinal Pigmented Epithelium

Abstract

Edited by George CarmanAccumulation of bis-retinoids in the retinal pigmented epithelium (RPE) is a hallmark of aging and retinal disorders such as Stargardt disease and age-related macular degeneration. These aberrant fluorescent condensation products, including di-retinoid-pyridinium-ethanolamine (A2E), are thought to be transferred to RPE cells primarily through phagocytosis of the photoreceptor outer segments. However, we observed by twophoton microscopy that mouse retinas incapable of phagocytosis due to a deficiency of the c-Mer proto-oncogene tyrosine kinase (Mertk) nonetheless contained fluorescent retinoid condensation material in their RPE. Primary RPE cells from Mertk ؊/؊ mice also accumulated fluorescent products in vitro.Finally, quantification of A2E demonstrated the acquisition of retinal condensation products in Mertk ؊/؊ mouse RPE prior to retinal degeneration. In these mice, we identified activated microglial cells that likely were recruited to transport A2E-like condensation products to the RPE and dispose of the dying photoreceptor cells. These observations demonstrate a novel transport mechanism between photoreceptor cells and RPE that does not involve canonical Mertk-dependent phagocytosis.Maintaining the health of the vertebrate retina requires a delicate interaction between retinal photoreceptor cells, which collect and transmit visual stimuli, and the adjacent retinal pigmented epithelium (RPE), 4 a cell layer that forms part of the blood-retina barrier (1). Disturbances in this interaction, whether through genetic defects, environmental insults, or trauma can lead to retinal degeneration and ultimate loss of vision (2, 3). The RPE performs many vital functions, including the daily removal of the older distal portions of photoreceptor outer segments (4 -6) and the delivery of both nutrients and the 11-cis-retinal chromophore to rod and cone photoreceptor cells that sustain vision (7). Roughly 10% of each photoreceptor cell outer segment is shed daily throughout an animal's life, distinguishing the RPE as perhaps the most active phagocytic cell layer in the body (5). Although RPE phagocytosis was discovered over 40 years ago (4), the molecular mechanisms underlying this process have not been delineated. However, certain molecular components that support classic phagocytosis have been identified, including the tyrosine receptor kinase Mertk (8, 9). The RPE not only removes toxic photo-oxidation products that accumulate in the retina, but it also recycles usable materials such as proteins, lipids, and small molecules. Retinoid condensation products such as bis-retinoids (di-retinoidpyridinium-ethanolamine (A2E), retinoid dimers, and others) generated in photoreceptors are presumably transported along with photoreceptor membranes to the RPE during phagocytosis. Highly fluorescent A2E accumulation is a pathogenic hallmark of Stargardt disease, the most common juvenile form of human macular degeneration (10). A2E accumulation also is thought to contribute to the pathogenesis of age-related mac...

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“…High-performance liquid chromatography was used in a blinded assay to quantify all- trans- retinyl esters and 11- cis -retinal in the eyes of dark-adapted mice at P21 (prior to degeneration but after significant retinal maturation) 14 , 21 , 57 , 58 . Retinyl esters are notoriously difficult to extract and quantify, thus we developed a novel procedure that allows extraction and quantification of the esters with more than 90 percent accuracy 59 . Interestingly, we found that all- trans- retinyl ester levels were significantly lower in Mertk −/− relative to WT mice (Fig.…”

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“…These dots may represent outer segment debris, RPE cell migration or/and phagocytes (i.e., macrophages and activated microglia) also seen in RCS rats (Dowling & Sidman, ; LaVail, Pinto, & Yasumura, ) and Mertk knockout mice (Duncan et al., ). In keeping with this, a recent study applying two photon microscopy to various Mertk‐/‐ mutant models suggested that microglial cells could be recruited to transport byproducts of the phototransduction cascade to the RPE and be involved in the elimination of dying photoreceptor cells (Palczewska et al., ). Macrophages may also account for the hyperflective dots seen in the choroid.…”