Regulation of Phagolysosomal Digestion by Caveolin-1 of the Retinal Pigment Epithelium Is Essential for Vision

Abstract

Caveolin-1 associates with the endo/lysosomal machinery of cells in culture, suggesting that it functions at these organelles independently of its contribution to cell surface caveolae. Here we explored mice lacking caveolin-1 specifically in the retinal pigment epithelium (RPE Support of the neural retina generally and of adjacent photoreceptor neurons specifically by the retinal pigment epithelium (RPE) 5 is essential for vision (1). A major function of the RPE is its contribution to photoreceptor outer segment renewal, a continuous and life-long rejuvenation process that involves the formation of new membrane disks at the proximal end of the outer segment and diurnal shedding of distal spent outer segment tips (2). Outer segment renewal is critical for photoreceptor function and survival, and any abnormality is thought to impair vision. RPE cells participate in outer segment renewal by clearing shed photoreceptor outer segment fragments (POS) by receptor-mediated phagocytosis (3).Mechanistically, RPE phagocytosis belongs to a family of conserved non-inflammatory clearance phagocytosis pathways that other cell types use to remove apoptotic cells and debris. These pathways have in common that their failure to efficiently clear debris contributes to human disease. However, unlike other forms of phagocytosis, RPE clearance of POS occurs in a strict diurnal rhythm that is regulated by light and circadian mechanisms (4). This is a unique advantage for RPE phagocytosis studies because all steps of the synchronized phagocytic process may be quantified precisely in situ in the intact, undisturbed retinas of experimental animals. Content in the RPE of engulfed rod POS phagosomes peaks shortly after light onset and declines characteristically within several hours as RPE cells complete digestion of their phagocytic load before the next burst of intake (5).Like other phagocytic pathways, ingested phagosomes in the RPE fuse with lysosomal vesicles to form phagolysosomes. In POS phagolysosomes, degradation of opsin, which constitutes ϳ85% of POS protein, requires the aspartic protease cathepsin D and phagosomal acidification (6, 7). Because RPE cells are post-mitotic in the mammalian eye and ingest numerous POS daily, prompt and complete POS engulfment is essential to prevent gradual buildup of undigested debris in the RPE (8). Inefficient RPE lysosomal function causes accumulation of debris in human and experimental animal RPE that can be toxic and

Caveolins and caveolae in ocular physiology and pathophysiology

“…Caveolins and/or caveolae have been observed widely in retinal vascular cells (Caldwell and Slapnick, 1992; Chen et al, 2007; Feng et al, 1999a; Feng et al, 1999b; Gardiner and Archer, 1986a; Gu et al, 2014a; Opreanu et al, 2011; Raviola and Butler, 1983; Sagaties et al, 1987; Stitt et al, 2000a; Stitt et al, 2000b), Müller glia (Gu et al, 2014b; Hauck et al, 2010; Li et al, 2012; Nelson et al, 2011; Reagan et al, 2016; Roesch et al, 2008), RPE (Bridges et al, 2001; Chen et al, 2003; Chowers et al, 2004; Forbes et al, 2007; Kook et al, 2008; Mora et al, 2006; Omri et al, 2011; Sethna et al, 2016), and photoreceptors (Berta et al, 2011; Boesze-Battaglia et al, 2002; Corley and Albert, 2011; Elliott et al, 2003; Elliott et al, 2008; Kachi et al, 2001; Nair et al, 2002; Senin et al, 2004). In the adult mouse retina, Cav-1 is the major detectable caveolin protein and is abundant in retinal and choroidal vascular beds and in Müller glia with detectable but lower expression in the retinal pigmented epithelium (RPE) (Li et al, 2012)(Fig.…”

“…As discussed later, the function of photoreceptor-intrinsic Cav-1 is unclear as photoresponse deficits resulting from Cav-1 ablation do not seem to result from loss of photoreceptor-intrinsic Cav-1 (Li et al, 2012). Instead, it is more likely that the loss of Cav-1 in photoreceptor support cells (e.g., RPE and Müller cells) contribute to secondary photoreceptor functional deficits (Sethna et al, 2016). Cav-1 expression in native and cultured RPE can support the formation of rare ultrastructurally-identifiable caveolae with both apical and basolateral localizations (Mora et al, 2006) but immunogold localization (Bridges et al, 2001) and later functional studies (Sethna et al, 2016) suggest that Cav-1 may function outside of caveolae in these cells.…”

“…7A–H modified from (Sethna et al, 2016)). Ablation of Cav-1 specifically in the RPE resulted in significant reductions in both rod photoreceptor-driven a- and b-wave responses as assessed by ERG (see (Sethna et al, 2016)).…”

“…7A–H modified from (Sethna et al, 2016)). Ablation of Cav-1 specifically in the RPE resulted in significant reductions in both rod photoreceptor-driven a- and b-wave responses as assessed by ERG (see (Sethna et al, 2016)). The reduction in ERG amplitudes in RPE-specific Cav-1 mutants at least partially explains the photoreceptor-extrinsic functional deficits observed in global Cav-1 −/− mice (Li et al, 2012).…”

“…This delayed digestion resulted from defects in the upregulation and activity of the lysosomal enzymes, cathepsin D (Fig. 7K–M) and β -N-acetyl-glucosaminidase (see (Sethna et al, 2016)) which are essential for POS-opsin digestion and deglycosylation, respectively. Intriguingly, the upregulation of cathepsin D immediately after diurnal POS shedding observed in control mouse RPE, was significantly delayed in RPE with specific Cav-1 ablation.…”