Figure 9. Proposed mechanism of lipid transport in the retina

Circulating high density lipoprotein (HDL) and low density lipoprotein (LDL) enter the retina via the SR-Bs and low density lipoprotein receptor (LDLR) in the retinal pigment epithelium (RPE). The RPE breaks-up the LDL and reassembles HDL-like particles using apoA1 and apoE, which are secreted into the interphotoreceptor matrix (IPM) via the ABCA1 transporter. The HDL particles take up additional lipids with the help of lecithin:cholesterol acyltransferase and cholesteryl ester transfer protein in the IPM. Lipids move back and forth between the RPE and the photoreceptors using HDL-like particles as intermediates and the SR-Bs and CD36 as receptors. The SR-Bs and CD36 may help to sort lipid particles with high oxidized lipid content. Müller cells may also play a role in delivering and accepting lipoprotein particles from the RPE and photoreceptors. The RPE may also secrete LDL and HDL-like particles back to the circulation to maintain homeostasis.