the blood, APOE is mainly associated with VLDLs, IDLs, and chylomicron remnants (2). In the brain and retina, however, the APOE-containing LPPs are different and have densities similar to those of HDLs (3, 4). APOE mediates lipid transfer by being a recognition ligand for specific cellsurface receptors, which bind and internalize the APOEcontaining LPPs (5) after these particles acquire cholesterol from cells with cholesterol excess. The APOE receptors belong to the LDL receptor (LDLR) family and include LDLR, VLDLR, LDLR-related protein 1 (LRP1), LRP1B, LRP2 (megalin), LRP4 (MEGF7), LRP5, LRP6, LRP8 (APOER2), and LRP11 (SORL1) (6) In humans, APOE exists in three isoforms (2, 3, and 4), which differ in their lipid-binding capacity, ability to integrate into LPP, and affinity for the receptors (7). APOE 4 is a risk factor for Alzheimer's disease, whereas APOE 2 is protective (8-10). Conversely, APOE 4 and 2 decrease and increase risks, respectively, for age-related macular Abstract Apolipoprotein E (APOE) is a component of lipid-transporting particles and a recognition ligand for receptors, which bind these particles. The APOE isoform 2 is a risk factor for age-related macular degeneration; nevertheless, APOE absence in humans and mice does not significantly affect the retina. We found that retinal cholesterol biosynthesis and the levels of retinal cholesterol were increased in Apoe / mice, whereas cholesterol elimination by metabolism was decreased. No focal cholesterol deposits were observed in the Apoe / retina. Retinal proteomics identified the most abundant cholesterol-related proteins in WT mice and revealed that, of these cholesterol-related proteins, only APOA4 had increased expression in the Apoe / retina. In addition, there were changes in retinal abundance of proteins involved in proinflammatory and antiinflammatory responses, cellular cytoskeleton maintenance, vesicular traffic, and retinal iron homeostasis. The data obtained indicate that when APOE is absent, particles containing APOA1, APOA4, and APOJ still transport cholesterol in the intraretinal space, but these particles are not taken up by retinal cells. Therefore, cholesterol biosynthesis inside retinal cells increase, whereas metabolism to oxysterols decreases to prevent cells from cholesterol depletion. These and other compensatory changes underlie only a minor retinal phenotype in Apoe / mice.-Saadane, A. A.

Apolipoprotein D (APOD) is an atypical apolipoprotein with unknown significance for retinal structure and function. Conversely, apolipoprotein E (APOE) is a typical apolipoprotein with established roles in retinal cholesterol transport. Herein, we immunolocalized APOD to the photoreceptor inner segments and conducted ophthalmic characterizations of ApoD −/− and ApoD −/− ApoE −/− mice. ApoD −/− mice had normal levels of retinal sterols but changes in the chorioretinal blood vessels and impaired retinal function. The whole body glucose disposal was impaired in this genotype but the retinal glucose metabolism was unchanged. ApoD −/− ApoE −/− mice had altered sterol profile in the retina but apparently normal chorioretinal vasculature and function. The whole body glucose disposal and retinal glucose utilization were enhanced in this genotype. OB-Rb, both leptin and APOD receptor, was found to be expressed in the photoreceptor inner segments and was at increased abundance in the ApoD −/− and ApoD −/− ApoE −/− retinas. Retinal levels of Glut4 and Cd36, the glucose transporter and scavenger receptor, respectively, were increased as well, thus linking APOD to retinal glucose and fatty acid metabolism and suggesting the APOD-OB-Rb-GLUT4/CD36 axis. In vivo isotopic labeling, transmission electron microscopy, and retinal proteomics provided additional insights into the mechanism underlying the retinal phenotypes of ApoD −/− and ApoD −/− ApoE −/− mice. Collectively, our data suggest that the APOD roles in the retina are context-specific and could determine retinal glucose fluxes into different pathways. APOD and APOE do not play redundant, complementary or opposing roles in the retina, rather their interplay is more complex and reflects retinal responses elicited by lack of these apolipoproteins.

Cholesterol excess in the brain is mainly disposed via cholesterol 24-hydroxylation catalyzed by cytochrome P450 46A1 (CYP46A1), a CNS-specific enzyme. CYP46A1 is emerging as a promising therapeutic target for various brain diseases with both enzyme activation and inhibition having a therapeutic potential. The rate of cholesterol 24-hydroxylation determines the rate of brain cholesterol turnover and the rate of sterol flux through the plasma membranes. The latter was shown to affect membrane properties and thereby membrane proteins and membrane-dependent processes. Previously we found that treatment of 5XFAD mice, an Alzheimer’s disease model, with a small dose of an anti-HIV drug efavirenz allosterically activated CYP46A1 in the brain and mitigated several disease manifestations. Herein, we generated Cyp46a1-/-5XFAD mice and treated them, along with 5XFAD animals, with efavirenz to ascertain CYP46A1-dependent and independent drug effects. Efavirenz-treated vs control Cyp46a1-/-5XFAD and 5XFAD mice were compared for the brain sterol and steroid hormone content, amyloid β burden, protein and mRNA expression as well as synaptic ultrastructure. We found that the CYP46A1-dependent efavirenz effects included changes in the levels of brain sterols, steroid hormones, and such proteins as GFAP, Iba1, Munc13-1, PSD-95, gephyrin, synaptophysin, and synapsin-1. Changes in the expression of genes involved in neuroprotection, neurogenesis, synaptic function, inflammation, oxidative stress, and apoptosis were also CYP46A1-dependent. The total amyloid β load was the same in all groups of animals, except lack of CYP46A1 decreased the production of the amyloid β40 species independent of treatment. In contrast, altered transcription of genes from cholinergic, monoaminergic, and peptidergic neurotransmission, steroid sulfation and production as well as vitamin D3 activation was the main CYP46A1-independent efavirenz effect. Collectively, the data obtained reveal that CYP46A1 controls cholesterol availability for the production of steroid hormones in the brain and the levels of biologically active neurosteroids. In addition, CYP46A1 activity also seems to affect the levels of PSD-95, the main postsynaptic density protein, possibly by altering the Camk2n1 (calcium/calmodulin dependent protein kinase II inhibitor 1) expression and activity of GSK3β (glycogen synthase kinase 3β). Even at a small dose, efavirenz likely acts as a transcriptional regulator, yet this regulation may not necessarily lead to functional effects. This study further confirmed that CYP46A1 is a key enzyme for cholesterol homeostasis in the brain and that the therapeutic efavirenz effects on 5XFAD mice are likely realized via CYP46A1 activation.