Therefore , if IDOL inhibition increases LDLR, and higher levels of LDLR result in enhanced A clearance and improved AD-like pathology (8), could inhibitingIDOLactivity lead to beneficial outcomes in AD? == Figure 1 . possible pathophysiological mechanisms in AD. == ApoE and ApoE receptors in AD == One of the earliest identified, and still strongest, genetic association with general AD is that of Apolipoprotein E (ApoE). ApoE is a component of lipoproteins such as very low density lipoprotein and high density lipoprotein and has been implicated in cholesterol transport. Its transcription is regulated by liver X receptor (LXR) transcription factors in a tissue-dependent manner (3). Several ApoE isoforms exist, and it is now established that the ApoE2 allele is associated with a reduced AD risk, while carriers of the ApoE4 allele are at an increased risk to develop AD. Moreover, healthy ApoE4 carriers show accelerated aging of the central nervous system (CNS) as assessed by cognitive decline and reduced hippocampal volume (4, 5). Several mechanisms underlie the detrimental role of ApoE4 in AD. Aside from the suggested direct toxic effects of ApoE4 (4), decreased lipidation of ApoE might be also involved. ApoE is lipidated by the ATP binding cassette subfamily A member 1 (ABCA1)which similar to ApoE is under transcriptional control by LTI-291 LXRs. Lipidation by ABCA1 is a central determinant of ApoE-mediated degradation of A (6), and accordingly decreased lipidation of ApoE4 by ABCA1 has been linked to decreased clearance of A (5). In contrast, stimulation of ApoE lipidation enhances A degradation by microglia (6). The lipidation status of ApoE also affects its affinity to its receptors, as more extensively reviewed in (7). The major ApoE receptors in the CNS are members of the Low Density Lipoprotein Receptor (LDLR) family, and binding of ApoE to its receptors is required for cellular clearance and transport of A across the blood brain barrier (7). Evidence for the importance of the LDLR in A clearance was recently demonstrated in a mouse model of AD in which overexpression of the LDLR lead to a reduction of ApoE levels, inhibition of A accumulation, and reduced neuroinflammation (8). These effects appear to rely, at least in part, on A transport across the blood brain barrier (9). Given its well-studied role in peripheral cholesterol metabolism, it is surprising that the function of the LDLR in the CNS is less well characterized. In the periphery, the LDLR is the main receptor for hepatic uptake of low-density lipoprotein from the blood and is a central determinant of lipoprotein metabolism. Accordingly, mutations in this receptor are the leading cause of familial hypercholesterolemia and ensuing atherosclerosis. The level and activity of the LDLR are subject to both transcriptional and post-transcriptional regulation. Transcription of theLDLRis controlled by the Sterol Response Element Binding Protein (SREBP) transcription factors, whereas regulated degradation of the LDLR is induced by proprotein convertase subtilisin kexin 9 (PCSK9) and by LTI-291 the LXR-regulated gene Inducible Degrader of the LDLR (IDOL) (10). == The LXR-IDOL-LDLR axis in AD == The sterol-responsive LXRs are members of the nuclear receptor family of transcription factors. LXRs play an important LTI-291 role in maintenance of sterol homeostasis in the CNS (3), and loss of either LXR or LXR exacerbates AD-like pathology in an A amyloidosis mouse model (11). Conversely, pharmacological activation of LXRs has been shown to decrease Mouse monoclonal to KSHV ORF26 amyloidosis, reduce neuroinflammation and improve cognitive function in mouse models of AD (3, 6, 12, 13). The beneficial outcome of LXR activation in AD has been attributed to (I) repression of proinflammatory signaling; (II) increased expression of ApoE and ABCA1 and (III) increased lipidation of ApoE by ABCA1, which supports enhanced A clearance (Figure 1). IDOL, likeABCA1andApoE, is a transcriptional target of LXRs (14). However , whether IDOL plays a role in the actions of LXRs in AD is not known. IDOLis an E3 ubiquitin ligase that stimulates.