Melanoregulin (MREG) Modulates Lysosome Function in Pigment Epithelial Cells

Abstract

Melanoregulin (MREG), the product of the Mreg dsu gene, is a small highly charged protein, hypothesized to play a role in organelle biogenesis due to its effect on pigmentation in dilute, ashen, and leaden mutant mice. Here we provide evidence that MREG is required in lysosome-dependent phagosome degradation. In the Mreg ؊/؊ mouse, we show that loss of MREG function results in phagosome accumulation due to delayed degradation of engulfed material. Over time, the Mreg ؊/؊ mouse retinal pigment epithelial cells accumulate the lipofuscin component, A2E. MREG-deficient human and mouse retinal pigment epithelial cells exhibit diminished activity of the lysosomal hydrolase, cathepsin D, due to defective processing. Moreover, MREG localizes to small intracellular vesicles and associates with the endosomal phosphoinositide, phosphatidylinositol 3,5-biphosphate. Collectively, these studies suggest that MREG is required for lysosome maturation and support a role for MREG in intracellular trafficking.

Melanosomes on the move: a model to understand organelle dynamics

“…Consistent with this idea, albinism in dilute, ashen and leaden mice is partially suppressed in a dilute suppressor (dsu) mutant background [18]. dsu encodes melanoregulin, a vertebrate-specific protein proposed to regulate membrane fusion [19][20][21]. However, although crossing dsu with dilute restores pigment transfer to the hair shaft, it does not restore melanosome distribution in melanocytes, thus the mechanism of suppression remains unclear [19].…”

Defects in retinal pigment epithelial cell proteolysis and the pathology associated with age-related macular degeneration

“…The primary enzyme involved in the proteolysis of rhodopsin in the RPE phagosomes is Cathepsin D, and a decrease in the activity of Cathepsin D, as is seen in the βA3/A1-crystallin deficient genetic animal models, leads to the accumulation of photoreceptor OS-derived debris in RPE cells (Bosch et al, 1993; Deguchi et al, 1994; Rakoczy et al, 1997; Lai et al, 2000; Hoppe et al, 2004). The degradation of photoreceptor OS is also delayed in Mreg−/− mice, suggesting a role for melanoregulin in the process (Damek-Poprawa et al, 2009). …”

Understanding photoreceptor outer segment phagocytosis: Use and utility of RPE cells in culture

“…For instance, increase in rhodopsin-positive phagosomes from 1 hour prior to 1 hour after light onset is indicative of a synchronized phagocytic burst (Nandrot et al, 2007). Decrease of rhodopsin-positive phagosomes from 1 hour after light onset to 4 hours after light onset is indicative of efficient phagolysosomal digestion (Damek-Poprawa et al, 2009). …”

“…Analysis of albino experimental animals greatly facilitates accurate quantification of toluidine blue-stained phagosomes as RPE melanosomes may in part obscure phagosomes in pigmented animals (Gibbs et al, 2003). Electron microscopy allows identifying phagosomes based on their size, location, and content of outer segment disks regardless of RPE pigmentation (Bosch et al, 1993; Damek-Poprawa et al, 2009; Nandrot et al, 2004). Fluorescence microscopy allows unequivocal identification of immunolabeled phagosomes in cross sections (Nandrot et al, 2007).…”

“…In mice lacking the lysosomal protein melanoregulin (MREG), in contrast, phagosome numbers after light onset and trafficking in the RPE are normal. However, phagosome numbers do not decline as in wild-type mice at later time points indicating that phagolysosomal digestion of rhodopsin phagosome content is markedly delayed (Damek-Poprawa et al, 2009). …”