Expression profiling in APP23 mouse brain: inhibition of Aβ amyloidosis and inflammation in response to LXR agonist treatment

BACKGROUND:Recent studies demonstrate that in addition to its modulatory effect on APP processing, in vivo application of Liver X Receptor agonist T0901317 (T0) to APP transgenic and non-transgenic mice decreases the level of Aß42. Moreover, in young Tg2576 mice T0 completely reversed contextual memory deficits. Compared to other tissues, the regulatory functions of LXRs in brain remain largely unexplored and our knowledge so far is limited to the cholesterol transporters and apoE. In this study we applied T0 to APP23 mice for various times and examined gene and protein expression. We also performed a series of experiments with primary brain cells derived from wild type and LXR knockout mice subjected to various LXR agonist treatments and inflammatory stimuli.RESULTS:We demonstrate an upregulation of genes related to lipid metabolism/transport, metabolism of xenobiotics and detoxification. Downregulated genes are involved in immune response and inflammation, cell death and apoptosis. Additional treatment experiments demonstrated an increase of soluble apolipoproteins E and A-I and a decrease of insoluble Aß. In primary LXRwt but not in LXRa-/-ß-/- microglia and astrocytes LXR agonists suppressed the inflammatory response induced by LPS or fibrillar Aß.CONCLUSION:The results show that LXR agonists could alleviate AD pathology by acting on amyloid deposition and brain inflammation. An increased understanding of the LXR controlled regulation of Aß aggregation and clearance systems will lead to the development of more specific and powerful agonists targeting LXR for the treatment of AD

ABCA1 Deficiency Affects Basal Cognitive Deficits and Dendritic Density in Mice

Producción CientíficaATP-binding cassette transporter A1 (ABCA1) mediates cholesterol efflux to lipid-free apolipoproteins and regulates the generation of high density lipoproteins. Previously, we have shown that lack of Abca1 significantly increases amyloid deposition and cognitive deficits in Alzheimer’s disease model mice expressing human amyloid-β protein precursor (APP). The goal of this study was to determine if ABCA1 plays a role in memory deficits caused by amyloid-β (Aβ) oligomers and examine neurite architecture of pyramidal hippocampal neurons. Our results confirm previous findings that Abca1 deficiency significantly impairs spatial memory acquisition and retention in the Morris water maze and long-term memory in novel object recognition of APP transgenic mice at a stage of early amyloid pathology. Neither test demonstrated a significant difference between Abca1ko and wild-type (WT) mice. We also examined the effect of intra-hippocampal infused Aβ oligomers on cognitive performance of Abca1ko mice, compared to control infusion of scrambled Aβ peptide. Age-matched WT mice undergoing the same infusions were also used as controls. In this model system, we found a statistically significant difference between WT and Abca1ko mice infused with scrambled Aβ, suggesting that Abca1ko mice are vulnerable to the effect of mild stresses. Moreover, examination of neurite architecture in the hippocampi revealed a significant decrease in neurite length, number of neurite segments, and branches in Abca1ko mice when compared to WT mice. We conclude that mice lacking ABCA1 have basal cognitive deficits that prevent them from coping with additional stressors, which is in part due to impairment of neurite morphology in the hippocampus

Strategies to gain novel Alzheimer’s disease diagnostics and therapeutics using modulators of ABCA transporters

Adenosine-triphosphate-(ATP)-binding cassette (ABC) transport proteins are ubiquitously present membrane-bound efflux pumps that distribute endo- and xenobiotics across intra- and intercellular barriers. Discovered over 40 years ago, ABC transporters have been identified as key players in various human diseases, such as multidrug-resistant cancer and atherosclerosis, but also neurodegenerative diseases, such as Alzheimer’s disease (AD). Most prominent and well-studied are ABCB1, ABCC1, and ABCG2, not only due to their contribution to the multidrug resistance (MDR) phenotype in cancer, but also due to their contribution to AD. However, our understanding of other ABC transporters is limited, and most of the 49 human ABC transporters have been largely neglected as potential targets for novel small-molecule drugs. This is especially true for the ABCA subfamily, which contains several members known to play a role in AD initiation and progression. This review provides up-to-date information on the proposed functional background and pathological role of ABCA transporters in AD. We also provide an overview of small-molecules shown to interact with ABCA transporters as well as potential in silico, in vitro, and in vivo methodologies to gain novel templates for the development of innovative ABC transporter-targeting diagnostics and therapeutics

Memory deficits in APP23/Abca1+/− mice correlate with the level of Aβ oligomers

ABCA1, a member of the ATP-binding cassette family of transporters, lipidates ApoE (apolipoprotein A) and is essential for the generation of HDL (high-density lipoprotein)-like particles in the CNS (central nervous system). Lack of Abca1 increases amyloid deposition in several AD (Alzheimer's disease) mouse models. We hypothesized that deletion of only one copy of Abca1 in APP23 (where APP is amyloid precursor protein) AD model mice will aggravate memory deficits in these mice. Using the Morris Water Maze, we demonstrate that 2-year-old Abca1 heterozygous APP23 mice (referred to as APP23/het) have impaired learning during acquisition, and impaired memory retention during the probe trial when compared with age-matched wild-type mice (referred to as APP23/wt). As in our previous studies, the levels of ApoE in APP23/het mice were decreased, but the differences in the levels of Aβ and thioflavin-S-positive plaques between both groups were insignificant. Importantly, dot blot analysis demonstrated that APP23/het mice have a significantly higher level of soluble A11-positive Aβ (amyloid β protein) oligomers compared with APP23/wt which correlated negatively with cognitive performance. To confirm this finding, we performed immunohistochemistry with the A11 antibody, which revealed a significant increase of A11-positive oligomer structures in the CA1 region of hippocampi of APP23/het. This characteristic region-specific pattern of A11 staining was age-dependent and was missing in younger APP23 mice lacking Abca1. In contrast, the levels of Aβ*56, as well as other low-molecular-mass Aβ oligomers, were unchanged among the groups. Overall, the results of the present study demonstrate that in aged APP23 mice memory deficits depend on Abca1 and are likely to be mediated by the amount of Aβ oligomers deposited in the hippocampus

Opposing effects ofApoe/Apoa1double deletion on amyloid-β pathology and cognitive performance in APP mice

Producción CientíficaATP binding cassette transporter A1 (encoded by ABCA1) regulates cholesterol efflux from cells to apolipoproteins A-I and E (ApoA-I and APOE; encoded by APOA1 and APOE, respectively) and the generation of high density lipoproteins. In Abca1 knockout mice (Abca1(ko)), high density lipoproteins and ApoA-I are virtually lacking, and total APOE and APOE-containing lipoproteins in brain substantially decreased. As the ε4 allele of APOE is the major genetic risk factor for late-onset Alzheimer's disease, ABCA1 role as a modifier of APOE lipidation is of significance for this disease. Reportedly, Abca1 deficiency in mice expressing human APP accelerates amyloid deposition and behaviour deficits. We used APP/PS1dE9 mice crossed to Apoe and Apoa1 knockout mice to generate Apoe/Apoa1 double-knockout mice. We hypothesized that Apoe/Apoa1 double-knockout mice would mimic the phenotype of APP/Abca1(ko) mice in regards to amyloid plaques and cognitive deficits. Amyloid pathology, peripheral lipoprotein metabolism, cognitive deficits and dendritic morphology of Apoe/Apoa1 double-knockout mice were compared to APP/Abca1(ko), APP/PS1dE9, and single Apoa1 and Apoe knockouts. Contrary to our prediction, the results demonstrate that double deletion of Apoe and Apoa1 ameliorated the amyloid pathology, including amyloid plaques and soluble amyloid. In double knockout mice we show that (125)I-amyloid-β microinjected into the central nervous system cleared at a rate twice faster compared to Abca1 knockout mice. We tested the effect of Apoe, Apoa1 or Abca1 deficiency on spreading of exogenous amyloid-β seeds injected into the brain of young pre-depositing APP mice. The results show that lack of Abca1 augments dissemination of exogenous amyloid significantly more than the lack of Apoe. In the periphery, Apoe/Apoa1 double-knockout mice exhibited substantial atherosclerosis and very high levels of low density lipoproteins compared to APP/PS1dE9 and APP/Abca1(ko). Plasma level of amyloid-β42 measured at several time points for each mouse was significantly higher in Apoe/Apoa1 double-knockout then in APP/Abca1(ko) mice. This result demonstrates that mice with the lowest level of plasma lipoproteins, APP/Abca1(ko), have the lowest level of peripheral amyloid-β. Unexpectedly, and independent of amyloid pathology, the deletion of both apolipoproteins worsened behaviour deficits of double knockout mice and their performance was undistinguishable from those of Abca1 knockout mice. Finally we observed that the dendritic complexity in the CA1 region of hippocampus but not in CA2 is significantly impaired by Apoe/Apoa1 double deletion as well as by lack of ABCA1. In conclusion: (i) plasma lipoproteins may affect amyloid-β clearance from the brain by the 'peripheral sink' mechanism; and (ii) deficiency of brain APOE-containing lipoproteins is of significance for dendritic complexity and cognition

APOEε4 and risk of Alzheimer’s disease – time to move forward

The inheritance of Apolipoprotein E4 (APOEε4) brings the highest genetic risk of Alzheimer’s disease (AD), arguably the highest genetic risk in human pathology. Since the discovery of the association, APOE protein isoforms have been at the center of tens of thousands of studies and reports. While, without a doubt, our knowledge about the normal physiological function of APOE isoforms in the brain has increased tremendously, the questions of how the inheritance of the APOEε4 allele translates into a risk of AD, and the risk is materialized, remain unanswered. Moreover, the knowledge about the risk associated with APOEε4 has not helped design a meaningful preventative or therapeutic strategy. Animal models with targeted replacement of Apoe have been generated and, thanks to the recent NIH/NIA/Alzheimer’s disease Association initiative, are now freely available to AD researchers. While helpful in many aspects, none of the available models recapitulates normal physiological transcriptional regulation of the human APOE gene cluster. Changes in epigenetic regulation of APOE alleles in animal models in response to external insults have rarely been if ever, addressed. However, these animal models provide a useful tool to handle questions and investigate protein–protein interactions with proteins expressed by other recently discovered genes and gene variants considered genetic risk factors of AD, like Triggering Receptor expressed on Myeloid cells 2 (TREM2). In this review, we discuss genetic and epigenetic regulatory mechanisms controlling and influencing APOE expression and focus on interactions of APOE and TREM2 in the context of microglia and astrocytes’ role in AD-like pathology in animal models

The Role of APOE and TREM2 in Alzheimer′s Disease—Current Understanding and Perspectives

Alzheimer’s disease (AD) is the leading cause of dementia worldwide. The extracellular deposits of Amyloid beta (Aβ) in the brain—called amyloid plaques, and neurofibrillary tangles—intracellular tau aggregates, are morphological hallmarks of the disease. The risk for AD is a complicated interplay between aging, genetic risk factors, and environmental influences. One of the Apolipoprotein E (APOE) alleles—APOEε4, is the major genetic risk factor for late-onset AD (LOAD). APOE is the primary cholesterol carrier in the brain, and plays an essential role in lipid trafficking, cholesterol homeostasis, and synaptic stability. Recent genome-wide association studies (GWAS) have identified other candidate LOAD risk loci, as well. One of those is the triggering receptor expressed on myeloid cells 2 (TREM2), which, in the brain, is expressed primarily by microglia. While the function of TREM2 is not fully understood, it promotes microglia survival, proliferation, and phagocytosis, making it important for cell viability and normal immune functions in the brain. Emerging evidence from protein binding assays suggests that APOE binds to TREM2 and APOE-containing lipoproteins in the brain as well as periphery, and are putative ligands for TREM2, thus raising the possibility of an APOE-TREM2 interaction modulating different aspects of AD pathology, potentially in an isoform-specific manner. This review is focusing on the interplay between APOE isoforms and TREM2 in association with AD pathology