RUBCN/rubicon and EGFR regulate lysosomal degradative processes in the retinal pigment epithelium (RPE) of the eye

Abstract

Macroautophagy/autophagy is an intracellular stress survival and recycling system whereas phagocytosis internalizes material from the extracellular milieu; yet, both pathways utilize lysosomes for cargo degradation. Whereas autophagy occurs in all cells, phagocytosis is performed by cell types such as macrophages and the retinal pigment epithelial (RPE) cells of the eye where it is supported by the noncanonical autophagy process termed LC3-associated phagocytosis (LAP). Autophagy and LAP are distinct pathways that use many of the same mediators and must compete for cellular resources, suggesting that cells may regulate both processes under homeostatic and stress conditions. Our data reveal that RPE cells promote LAP through the expression of RUBCN/Rubicon (RUN domain and cysteine-rich domain containing Beclin 1-interacting protein) and suppress autophagy through the activation of EGFR (epidermal growth factor receptor). In the morning when photoreceptor outer segments (POS) phagocytosis and LAP are highest, RUBCN expression is increased. At the same time, outer segment phagocytosis activates the EGFR resulting in MTOR (mechanistic target of rapamycin [serine/threonine kinase]) stimulation, the accumulation of SQSTM1/p62, and the phosphorylation of BECN1 (Beclin 1, autophagy related) on an inhibitory residue thereby suppressing autophagy. Silencing Rubcn, preventing EGFR activity or directly inducing autophagy in RPE cells by starvation inhibits phagocytic degradation of POS. Thus, RPE cells regulate lysosomal pathways during the critical period of POS phagocytosis to support retinal homeostasis.

Phagocytosed photoreceptor outer segments activate mTORC1 in the retinal pigment epithelium

“…Receptor-mediated phagocytosis was required for mTORC1 activation, which could not be achieved by phagocytosis of latex beads. During the revision of this manuscript, Ferguson’s group similarly reported that S6K phosphorylation can be stimulated by POS phagocytosis in RPE-J cells (36). Their data independently validate POS-induced mTORC1 activation in the RPE.…”

“…Starvation stimulates RPE autophagy in mice and suppresses POS degradation (36), suggesting that autophagy and LAP may counteract each other. mTORC1 directly phosphorylates ULK1 at Ser 758 (50) to suppress macroautophagy.…”

Light‐responsive microRNA miR‐211 targets Ezrin to modulate lysosomal biogenesis and retinal cell clearance

“…It is well established that two independent autophagy pathways support diurnal waste clearance in the RPE and both are finely regulated by circadian and non‐circadian processes, with a daily induction that relies on and occurs after the peak period of POS disk‐shedding and can be evoked by a light pulse (Kim et al , ; Muniz‐Feliciano et al , ). Although our understanding of the regulation of autophagy pathways has improved, a gap remains in our understanding on how the diurnal activation of both lysosomal biogenesis and function is generated, maintained, and successively repressed daily upon a switch from dark to light conditions in the RPE.…”

Oxidative Stress and Dysfunctional Intracellular Traffic Linked to an Unhealthy Diet Results in Impaired Cargo Transport in the Retinal Pigment Epithelium (RPE)

“…Consequently, the daily internalization of POS, particularly in aged RPE, adds another factor which requires regulation between LAP, canonical autophagy, and phagocytosis to achieve homeostatic balance. Indeed, under starvation, RPE cells have been shown to activate autophagy in favor of phagocytosis . The central role played by lysosomes under these conditions is therefore of growing interest.…”

“…Indeed, under starvation, RPE cells have been shown to activate autophagy in favor of phagocytosis. [67] The central role played by lysosomes under these conditions is therefore of growing interest. For instance, elimination of CRYBA1/βA3/A1-crstallin which regulates endo-lysosomal acidification, disrupted both autophagy, and phagocytosis.…”