Purpose: In our companion paper we demonstrated that circulating lipoproteins enter the retina via the retinal pigment epithelium (RPE) and possibly Müller cells. In order to understand how these lipids are transported within the retina, expression and localization of the main proteins known to be involved in systemic lipid transport was determined.
Methods: Expression of ABCA1, apoA1 (the major HDL protein), SR-BI, SR-BII, CD36, lecithin:cholesterol acyltransferase (LCAT), and cholesteryl ester transfer protein (CETP) was determined by reverse transcriptase polymerase chain reaction (RT-PCR) and immunoblots. Localization was determined by immunohistochemistry using fresh monkey vibrotome sections and imaged by confocal microscopy.
Results: ABCA1 and apoA1 were localized to the ganglion cell layer, retinal pigment epithelium (RPE), and rod photoreceptor inner segments. ApoA1 was also observed associated with rod photoreceptor outer segments, presumably localized to the interphotoreceptor matrix (IPM). The scavenger receptors SR-BI and SR-BII localized mainly to the ganglion cell layer and photoreceptor outer segments; in the latter they appear to be associated with microtubules. LCAT and CETP localized mainly to the IPM.
Conclusions: The presence and specific localization of these well-known lipid transport proteins suggest that the retina employs an internal lipid transport mechanism that involves processing and maturation of HDL-like particles.