ABCA1 has been suggested as a therapeutic approach to increase apoE levels and/or lipidation in the brain and therefore lower Aβ levels and other pathology to improve neuron function and cognition in AD patients. Evidence for this approach includes that ABCA1 loss-of-function mutations increase AD risk [
56] and the reduced ability of lipoproteins isolated from the cerebrospinal fluid (CSF) of AD patients to accept cholesterol from ABCA1 compared to lipoproteins from age-matched controls [
57]. In addition, ABCA1 knockout increases and ABCA1 overexpression reduces Aβ pathology [
33,
34]. These human and genetic studies led to a focus on identifying compounds that activate ABCA1. To date, most efforts have centered on transcriptional upregulation of
APOE and ABCA1 via nuclear receptor agonists (NR) as a therapy for AD [
58,
59]. NRs such as LXR and RXR alter the expression of multiple genes involved in inflammation and importantly, lipid metabolism including
APOE and ABCA1 [
60]. In general, agonists of the nuclear receptors RXR (e.g., bexarotene), LXR, and PPARγ have been shown to increase ABCA1 levels, increase apoE levels/lipidation, and reduce Aβ pathology and cognition in familial AD (FAD) mice [
25,
61‐
67]. However, there are concerns of the side effects of NR receptor agonists including induction of hepatic lipogenesis leading to hepatomegaly [
25,
68] that could explain in part the lack of reproducibility [
69‐
73]. In fact, in E4FAD mice we found bexarotene treatment lowered Aβ levels in the hippocampus but not the cortex possibly by induced hepatic encephalopathy [
25]. To circumvent these issues, researchers are developing novel NR agonists that specifically increase brain apoE levels/lipidation without detrimental peripheral side effects [
66,
74]. An alternative approach is to directly target ABCA1 activity. CS-6253 was developed to stabilize ABCA1, prevent its degradation, and facilitate translocation of ABCA1 to the plasma membrane in turn increasing cholesterol efflux to acceptor lipoproteins [
36,
75]. We found that in young male E3FAD mice, CS increased brain apoE levels (soluble and lipid associated) without an induction of hepatomegaly resulting in lower Aβ pathology and improved cognition as well as markers of neuron function and memory. Although there are caveats (see below), our data in EFAD mice support the overall concept that targeting ABCA1 is a potential therapy for
APOE3 carriers at early stages of Aβ pathology.
Increasing apoE levels and/or lipidation via ABCA1 is proposed as particularly beneficial for
APOE4 carriers. Indeed,
APOE4 is associated with low levels of apoE in the brain, CSF, and plasma in healthy controls and AD patients compared to
APOE3 [
76]. In addition, apoE4-lipoproteins are less lipidated and preferentially degraded compared to apoE3-lipoproteins in vitro and in vivo [
17,
32,
77]. Human apoE4-lipoproteins also have a reduced ability to accept cholesterol from ABCA1 compared to apoE3-lipoproteins when isolated from human CSF [
47]. Lower apoE4 lipidation is thought to result in several functional consequences, including less efficient clearance of Aβ compared to apoE3 [
78]. Thus, therapeutic efforts have focused on targeting apoE4 lipoproteins to resemble apoE3-lipoproteins. In fact, NR agonists have been shown to lower Aβ levels in mice that express apoE4 including E4FAD mice [
25,
62]. Therefore, targeting ABCA1 activation is also proposed to be beneficial for
APOE4 carriers in AD. In support, CS-6253 has been demonstrated to increase apoE4 lipidation, reduce intracellular Aβ42 levels, and improve cognitive performance in α-synuclein-KO/
APOE4-targeted replacement mice [
24]. However, in this current study, CS-6253 treatment did not modify apoE4-lipoproteins, Aβ levels or behavior in the E4FAD mice. One potential explanation is that the combination of
APOE4 and Aβ elevations in the E4FAD model disrupts lipid metabolism to such an extent that ABCA1 stabilization by CS-6253 is no longer sufficient to increase apoE4 lipidation. Previous studies have found that
APOE4 is independently associated with lipid dysregulation including impaired ABCA1 translocation to the plasma membrane [
47], reduced fatty acid transport from neurons to astrocytes [
79], and lipid droplet accumulation in astrocytes [
80] and microglia [
81]. Aβ has also been found to disrupt lipid metabolism, including limiting cholesterol availability. For example, Aβ induces lipid droplet accumulation in glia [
82,
83], which would reduce the availability of free cholesterol for efflux. Aβ has also been shown to upregulate cholesterol biosynthesis via increasing 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCoA) and ABCA1 in glia [
84,
85], without improving cholesterol efflux [
86] suggesting a direct interaction between Aβ and ABCA1 [
84]. In addition, soluble Aβ oligomers can interact with lipid membranes, compromising its integrity and function [
40,
87‐
90], which may interfere with ABCA1 activity. Thus, the combination of high Aβ levels and
APOE4 may have impeded lipid metabolism and therefore the ability of CS to be beneficial for
APOE4 mice. Indeed, in this study, ABCA1 and Aβ levels appeared higher in E4FAD mice, potentially as a compensatory mechanism and therefore it may not be possible to further increase ABCA1 activity. In addition, previous studies demonstrating the beneficial effects of targeting ABCA1 were in models or ages of low Aβ pathology and normal Aβ production [
24,
25,
91]. Therefore, utilizing Aβ directed co-therapies when targeting cholesterol metabolism and ABCA1 in
APOE4 carriers may be beneficial.