Effects of Vitamin D3 and Meso-Zeaxanthin on Human Retinal Pigmented Epithelial Cells in Three Integrated in vitro Paradigms of Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a degenerative retinal disease and one of major causes of irreversible vision loss. AMD has been linked to several pathological factors, such as oxidative stress and inflammation. Moreover, Aβ (1–42) oligomers have been found in drusen, the extracellular deposits that accumulate beneath the retinal pigmented epithelium in AMD patients. Hereby, we investigated the hypothesis that treatment with 1,25(OH) 2D3 (vitamin D3) and meso-zeaxathin, physiologically present in the eye, would counteract the toxic effects of three different insults on immortalized human retinal pigmented epithelial cells (ARPE-19). Specifically, ARPE-19 cells have been challenged with Aβ (1–42) oligomers, H2O2, LPS, and TNF-α, respectively. In the present study, we demonstrated that the combination of 1,25(OH)2D3 and meso-zeaxanthin significantly counteracted the cell damage induced by the three insults, at least in these in vitro integrated paradigms of AMD. These results suggest that combination of 1,25(OH)2D3 and meso-zeaxathin could be a useful approach to contrast pathological features of AMD, such as retinal inflammation and oxidative stress.

MERTK-Mediated LC3-Associated Phagocytosis (LAP) of Apoptotic Substrates in Blood-Separated Tissues: Retina, Testis, Ovarian Follicles

Timely and efficient elimination of apoptotic substrates, continuously produced during one’s lifespan, is a vital need for all tissues of the body. This task is achieved by cells endowed with phagocytic activity. In blood-separated tissues such as the retina, the testis and the ovaries, the resident cells of epithelial origin as retinal pigmented epithelial cells (RPE), testis Sertoli cells and ovarian granulosa cells (GC) provide phagocytic cleaning of apoptotic cells and cell membranes. Disruption of this process leads to functional ablation as blindness in the retina and compromised fertility in males and females. To ensure the efficient elimination of apoptotic substrates, RPE, Sertoli cells and GC combine various mechanisms allowing maintenance of tissue homeostasis and avoiding acute inflammation, tissue disorganization and functional ablation. In tight cooperation with other phagocytosis receptors, MERTK—a member of the TAM family of receptor tyrosine kinases (RTK)—plays a pivotal role in apoptotic substrate cleaning from the retina, the testis and the ovaries through unconventional autophagy-assisted phagocytosis process LAP (LC3-associated phagocytosis). In this review, we focus on the interplay between TAM RTKs, autophagy-related proteins, LAP, and Toll-like receptors (TLR), as well as the regulatory mechanisms allowing these components to sustain tissue homeostasis and prevent functional ablation of the retina, the testis and the ovaries.

Multiple phosphatidylinositol(3)phosphate roles in retinal pigment epithelium membrane recycling

The low-abundance lipid phosphatidylinositol-3-phosphate (PI(3)P) is important for membrane dynamics in autophagy, endosome processing, and phagocytosis. In retinal pigmented epithelial cells (RPE) all three pathways are important, but phagocytosis of disk membranes shed from adjacent photoreceptors is especially important for ensuring health of photoreceptors and preventing retinal degeneration. By eliminating Vps34, the kinase responsible for synthesizing PI(3)P in RPE, we have found that PI(3)P plays distinct roles in each pathway. In phagocytosis it is not required for disk engulfment or phagosome transport but is essential for recruitment and lipidation of LC3. In contrast, initiation of autophagy and LC3 recruitment to autophagosomes does not require PI(3)P, which can be bypassed by an alternative mechanism of ATG16L recruitment that does not require PI(3)P, Rab11a, or WIPI2. In all three pathways, PI(3)P is essential for fusion with lysosomes; autophagosomes, phagosomes, and Rab7-positive late endosomes, as well as enlarged lysosomes, accumulate in large numbers in the absence of Vps34, leading to cell death.