Melanin granules of retinal pigment epithelium are connected with the lysosomal degradation pathway

Melanosomes are closely related to lysosomes and Lipofuscin granules. This paper indicates the potential involvement of lysosomal degradation processe s in retinal pigment epithelial (RPE) cells. RPE cells cultured on Bruch's membrane and choroid were fed with indigestible latex beads. The RPE cells from this preparation were treated with chloroquine to investigate whether membrane swelling, typical for lysosomes under this condition, can be induc ed in melanosomes. To investigate the fate of indigestible material associa ted with rod outer segments (ROS). gold-labeled ROS were injected transscle rally into the subretinal space of Long Evans rats using a 32 gauge Hamilto n syringe. The degradation of labeled ROS was observed after 5 and 12 days by electron microscopy. The Following results were observed. Lates particle s fuse with the melanin granules of the RPE. Following chloroquine treatmen t, the membranes of melanin granules fused, and formed large clusters and v acuoles. Gold granules were detected inside both early stage melanosomes an d mature melanin granules of the RPE cells 5 and 12 days following subretin al injection of the labeled ROS. Higher numbers of gold granules were predo minantly round in immature melanosomes containing still melanofilaments and in small fused mature melanin granules, In conclusion the effect of chloro quine clearly demonstrates that the melanosomes posses active proton pumps which is typical For lysosomes. In RPE cells stressed by overload with rod outer segments or by ingestion of undegradable material (latex beads, gold particles), fusion of these phagosomes with melanosomes of different maturi ty is more a general rule than an exception. Therefore, melanosomes are con nected to lysosomal pathways in RPE cells. (C) 1999 Academic Press.