Genetic LAMP2 deficiency accelerates the age-associated formation of basal laminar deposits in the retina

Abstract

The early stages of age-related macular degeneration (AMD) are characterized by the accumulation of basal laminar deposits (BLamDs). The mechanism for BLamDs accumulating between the retinal pigment epithelium (RPE) and its basal lamina remains elusive. Here we examined the role in AMD of lysosome-associated membrane protein-2 (LAMP2), a glycoprotein that plays a critical role in lysosomal biogenesis and maturation of autophagosomes/phagosomes. LAMP2 was preferentially expressed by RPE cells, and its expression declined with age. Deletion of the Lamp2 gene in mice resulted in age-dependent autofluorescence abnormalities of the fundus, thickening of Bruch’s membrane, and the formation of BLamDs, resembling histopathological changes occurring in AMD. Moreover, LAMP2-deficient mice developed molecular signatures similar to those found in human AMD—namely, the accumulation of APOE, APOA1, clusterin, and vitronectin—adjacent to BLamDs. In contrast, collagen 4, laminin, and fibronectin, which are extracellular matrix proteins constituting RPE basal lamina and Bruch’s membrane were reduced in Lamp2 knockout (KO) mice. Mechanistically, retarded phagocytic degradation of photoreceptor outer segments compromised lysosomal degradation and increased exocytosis in LAMP2-deficient RPE cells. The accumulation of BLamDs observed in LAMP2-deficient mice was eventually followed by loss of the RPE and photoreceptors. Finally, we observed loss of LAMP2 expression along with ultramicroscopic features of abnormal phagocytosis and exocytosis in eyes from AMD patients but not from control individuals. Taken together, these results indicate an important role for LAMP2 in RPE function in health and disease, suggesting that LAMP2 reduction may contribute to the formation of BLamDs in AMD.

Loss of CIB2 causes non-canonical autophagy deficits and visual impairment

“…Further, we found increased lipid droplets and esterified cholesterol, which is associated with drusen in humans and sub-RPE deposits in mouse models. A recent study using RPE-specific LAMP2 knockout mouse showed faulty processing of ingested OS, lipid accumulation, and similar RPE deposit protein composition 28 as compared to this current study.…”

“…Meanwhile, a recent study showcased that the levels of LAMP2/CD107b (a lysosomal membrane protein) are reduced in AMD RPE/choroid tissues 28 . Observation of similar sub-RPE deposits in TEM micrographs in Cib2 mutant mice prompted us to investigate the molecular composition of the deposits.…”

“…Ten micron cryosections were prepared using standard procedures and filipin staining for esterified cholesterol (EC) and unesterified cholesterol (UC) was performed 28 . For oil Red O staining, cryosections were dipped in 60% isopropanol for 5 seconds, followed by incubation for 15 mins at RT in working solution of Oil Red O (3 parts:2 parts::0.5% Oil Red O solution in isopropanol:water, the working solution was mixed and allowed to stand for 10 mins before filtration with vacuum filtration system containing 0.22 µm filter).…”

Phagocytosis by the Retinal Pigment Epithelium: Recognition, Resolution, Recycling

“…Though mechanisms remain to be elucidated, the LAMP family of lysosome-resident glycoproteins is indeed required for the fusion of lysosomes with the phagosome (103). It is therefore noteworthy, that the loss of just one of these family members, LAMP2, in the murine RPE causes delayed POS degradation and the build-up of POS debris (104). In fact, the loss-of-function mutations in LAMP2 that cause Danon disease in humans is also associated with retinal and macular degeneration (105).…”

CIB2 regulates autophagy via Rheb-mTORC1 signaling axis

“…Autophagy is an essential process for all cells, but its function is especially notable in post-mitotic cells such as those in the RPE 16 that have the highest life-long phagocytic load of any cell-type in the body. Defects in autophagy has been implicated in dry AMD 17,18 . Besides AMD, deficits in autophagy are linked to lifespan and several diseases such as Alzheimer's, Parkinson's, certain cancers, and metabolic disorders amongst others [19][20][21] .…”