Expression of the very low-density lipoprotein receptor (VLDL-r), an apolipoprotein-E receptor, in the central nervous system and in Alzheimer's disease

The shared role of cholesterol in neuronal and peripheral inflammation

Neurodegeneration and its loss of cognitive function is associated with inflammation and an accumulation of lipids. In the periphery, cholesterol's uptake drives a major component of chronic inflammation. In this perspective, we describe the cellular and molecular roles of cholesterol in neuroinflammation and contrast them with those in the periphery. Incorporating shared mechanisms from the periphery, cholesterol emerges as a central signal originating in astrocytes and connecting inflammatory escalation in neurons and microglia. A cholesterol uptake pathway is proposed for neuroinflammation, and we speculate on the binding of cholesterol transport protein apolipoprotein E (apoE), including the Christchurch mutant (R136S), to cell surface receptors as a potential protective modality against uptake of astrocyte cholesterol and escalated neuroinflammation. Lastly, we discuss the molecular basis of cholesterol signaling through nanoscopic clustering and peripheral sources of cholesterol after opening of the blood brain barrier.

Peripheral apolipoprotein E proteins and their binding to LRP1 antagonize Alzheimer’s disease pathogenesis in the brain during peripheral chronic inflammation

C-reactive protein (CRP) impacts apolipoprotein E4 (ApoE4) allele to increase Alzheimer's disease (AD) risk. However, it is unclear how the ApoE protein and its binding to LRP1 are involved. We found that ApoE2 carriers had the highest but ApoE4 carriers had the lowest concentrations of blood ApoE in both humans and mice; blood ApoE concentration was negatively associated with AD risk. Elevation of peripheral monomeric CRP (mCRP) reduced the expression of ApoE in ApoE2 mice, while it decreased ApoE-LRP1 binding in the brains of ApoE4 mice that was characterized by Proximity Ligation Assay. Both serum ApoE and brain ApoE-LRP1 binding were positively associated with the expression of pericytes that disappeared after mCRP treatment, and negatively associated with brain tauopathy and neuroinflammation in the presence of mCRP. In ApoE^-/- mice, mCRP reduced the brain expression levels of synaptophysin and PSD95 and the positive relationship between ApoE-LRP1 binding and synaptophysin or PSD95 expression disappeared. Our study suggests that blood ApoE protects against AD pathogenesis by binding to LRP1 during peripheral chronic inflammation.

Bidirectional Control between Cholesterol Shuttle and Purine Signal at the Central Nervous System

Recent studies have highlighted the mechanisms controlling the formation of cerebral cholesterol, which is synthesized in situ primarily by astrocytes, where it is loaded onto apolipoproteins and delivered to neurons and oligodendrocytes through interactions with specific lipoprotein receptors. The “cholesterol shuttle” is influenced by numerous proteins or carbohydrates, which mainly modulate the lipoprotein receptor activity, function and signaling. These molecules, provided with enzymatic/proteolytic activity leading to the formation of peptide fragments of different sizes and specific sequences, could be also responsible for machinery malfunctions, which are associated with neurological, neurodegenerative and neurodevelopmental disorders. In this context, we have pointed out that purines, ancestral molecules acting as signal molecules and neuromodulators at the central nervous system, can influence the homeostatic machinery of the cerebral cholesterol turnover and vice versa. Evidence gathered so far indicates that purine receptors, mainly the subtypes P2Y2, P2X7 and A2A, are involved in the pathogenesis of neurodegenerative diseases, such as Alzheimer’s and Niemann–Pick C diseases, by controlling the brain cholesterol homeostasis; in addition, alterations in cholesterol turnover can hinder the purine receptor function. Although the precise mechanisms of these interactions are currently poorly understood, the results here collected on cholesterol–purine reciprocal control could hopefully promote further research.

Emerging Evidence of Pathological Roles of Very-Low-Density Lipoprotein (VLDL)

Embraced with apolipoproteins (Apo) B and Apo E, triglyceride-enriched very-low-density lipoprotein (VLDL) is secreted by the liver into circulation, mainly during post-meal hours. Here, we present a brief review of the physiological role of VLDL and a systemic review of the emerging evidence supporting its pathological roles. VLDL promotes atherosclerosis in metabolic syndrome (MetS). VLDL isolated from subjects with MetS exhibits cytotoxicity to atrial myocytes, induces atrial myopathy, and promotes vulnerability to atrial fibrillation. VLDL levels are affected by a number of endocrinological disorders and can be increased by therapeutic supplementation with cortisol, growth hormone, progesterone, and estrogen. VLDL promotes aldosterone secretion, which contributes to hypertension. VLDL induces neuroinflammation, leading to cognitive dysfunction. VLDL levels are also correlated with chronic kidney disease, autoimmune disorders, and some dermatological diseases. The extra-hepatic secretion of VLDL derived from intestinal dysbiosis is suggested to be harmful. Emerging evidence suggests disturbed VLDL metabolism in sleep disorders and in cancer development and progression. In addition to VLDL, the VLDL receptor (VLDLR) may affect both VLDL metabolism and carcinogenesis. Overall, emerging evidence supports the pathological roles of VLDL in multi-organ diseases. To better understand the fundamental mechanisms of how VLDL promotes disease development, elucidation of the quality control of VLDL and of the regulation and signaling of VLDLR should be indispensable. With this, successful VLDL-targeted therapies can be discovered in the future.

Electronegative very-low-density lipoprotein induces brain inflammation and cognitive dysfunction in mice

Epidemiologic studies have indicated that dyslipidemia may facilitate the progression of cognitive dysfunction. We previously showed that patients with metabolic syndrome (MetS) had significantly higher plasma levels of electronegative very-low-density lipoprotein (VLDL) than did healthy controls. However, the effects of electronegative-VLDL on the brain and cognitive function remain unclear. In this study, VLDL isolated from healthy volunteers (nVLDL) or patients with MetS (metVLDL) was administered to mice by means of tail vein injection. Cognitive function was assessed by using the Y maze test, and plasma and brain tissues were analyzed. We found that mice injected with metVLDL but not nVLDL exhibited significant hippocampus CA3 neuronal cell loss and cognitive dysfunction. In mice injected with nVLDL, we observed mild glial cell activation in the medial prefrontal cortex (mPFC) and hippocampus CA3. However, in mice injected with metVLDL, plasma and brain TNF-α and Aβ-42 levels and glial cell activation in the mPFC and whole hippocampus were higher than those in control mice. In conclusion, long-term exposure to metVLDL induced levels of TNF-α, Aβ-42, and glial cells in the brain, contributing to the progression of cognitive dysfunction. Our findings suggest that electronegative-VLDL levels may represent a new therapeutic target for cognitive dysfunction.

Mutations in the Reelin pathway are associated with abnormal expression of microglial IgG FC receptors in the cerebellar cortex

Microglia are the immune cells of the central nervous system involved in a variety of developmental processes, such as regulation of cell death and survival, spatial patterning, and contribute to the development of Purkinje cells (PCs) during migration. Microglia express immunoglobulin G Fc receptors (FcgRs). In this report, we describe microglial FcgR expression and its relation to abnormal PC migration in the cerebellum during development. To detect microglial FcgR, the direct anti-IgG (secondary antisera) and high concentrations of Triton X-100 were applied as a method for labeling microglial cells without the use of any specific primary antiserum. By using Acp2^-/- mice, which show an excessive PC migration into the molecular layer (ml), and 3 different types of mice with a null to alter the Reelin pathway (Reeler-, Dab1 (SCM)-, and Apoer mutant mice), we studied the location of PCs and the expression of FcgRs. Wild type littermates were used as controls in all studies. We show that the expression of microglial FcgRs was absent and PCs were ectopically located in the white matter in the cerebella of all mutant mice, except for the Acp2^-/- mice (PCs were located in the ml). These results suggest a role for FcgRs in the Reelin signaling pathway, not in regulating PC migration, but rather in the adaptation to an environment with a relatively large number of ectopically located PCs. However, the exact correlation between the ectopic location of PCs and lack of FcgRs in Reeler, SCM, and Apoer^-/- mice and the presence of FcgRs and directed PC location in the ml in Acp2^-/- mice are yet to be determined.

Immunoregulatory effects of very low density lipoprotein from healthy individuals and metabolic syndrome patients on glial cells

Epidemiological studies have reported that elderly patients with metabolic syndrome (MetS) are significantly more likely to develop neuronal degenerative diseases than those without MetS. Our previous study showed that patients with MetS had significantly higher levels of negatively charged very low density lipoproteins (VLDLs) in the plasma than healthy controls. Highly electronegative VLDL is a key risk factor for endothelial dysfunction and atrial fibrillation. However, the impact of negatively charged VLDL in brain immunity remains unclear. In this study, VLDLs were isolated from normal healthy (nVLDL) individuals or patients with MetS (metVLDL). Primary astroglia and microglia mixed cell cultures as well as microglial-enriched cultures were used to test the effects of VLDLs. Microglia/astroglia activation as evidenced by their morphological changes and production of pro-inflammatory factors, such as tumor necrosis factor-α (TNF-α) and prostaglandin E2 (PGE2), were assessed by immunofluorescence staining and ELISA, respectively. Our results showed that metVLDLs mainly act on the microglia, and not the astroglia, with low concentration (0.05-0.5 μg/mL) inducing cell morphological changes and decreased cellular processes in the microglia. However, nVLDL treatment at these concentrations had no effects on microglia and astroglia. Most importantly, TNF-α and PGE2 levels significantly increased in the microglia treated with metVLDL via a dose-dependent manner. Together, our data indicate that metVLDLs can contribute to MetS-associated brain disorders through microglia activation and neuroinflammation.

The Reelin Receptors Apolipoprotein E receptor 2 (ApoER2) and VLDL Receptor

Apolipoprotein E receptor 2 (ApoER2) and VLDL receptor belong to the low density lipoprotein receptor family and bind apolipoprotein E. These receptors interact with the clathrin machinery to mediate endocytosis of macromolecules but also interact with other adapter proteins to perform as signal transduction receptors. The best characterized signaling pathway in which ApoER2 and VLDL receptor (VLDLR) are involved is the Reelin pathway. This pathway plays a pivotal role in the development of laminated structures of the brain and in synaptic plasticity of the adult brain. Since Reelin and apolipoprotein E, are ligands of ApoER2 and VLDLR, these receptors are of interest with respect to Alzheimer’s disease. We will focus this review on the complex structure of ApoER2 and VLDLR and a recently characterized ligand, namely clusterin.

Markers of microglia in post-mortem brain samples from patients with Alzheimer's disease: a systematic review

Neuroinflammation is proposed as one of the mechanisms by which Alzheimer's disease pathology, including amyloid-β plaques, leads to neuronal death and dysfunction. Increases in the expression of markers of microglia, the main neuroinmmune cell, are widely reported in brains from patients with Alzheimer's disease, but the literature has not yet been systematically reviewed to determine whether this is a consistent pathological feature. A systematic search was conducted in Medline, Embase and PsychINFO for articles published up to 23 February 2017. Papers were included if they quantitatively compared microglia markers in post-mortem brain samples from patients with Alzheimer's disease and aged controls without neurological disease. A total of 113 relevant articles were identified. Consistent increases in markers related to activation, such as major histocompatibility complex II (36/43 studies) and cluster of differentiation 68 (17/21 studies), were identified relative to nonneurological aged controls, whereas other common markers that stain both resting and activated microglia, such as ionized calcium-binding adaptor molecule 1 (10/20 studies) and cluster of differentiation 11b (2/5 studies), were not consistently elevated. Studies of ionized calcium-binding adaptor molecule 1 that used cell counts almost uniformly identified no difference relative to control, indicating that increases in activation occurred without an expansion of the total number of microglia. White matter and cerebellum appeared to be more resistant to these increases than other brain regions. Nine studies were identified that included high pathology controls, patients who remained free of dementia despite Alzheimer's disease pathology. The majority (5/9) of these studies reported higher levels of microglial markers in Alzheimer's disease relative to controls, suggesting that these increases are not solely a consequence of Alzheimer's disease pathology. These results show that increased markers of microglia are a consistent feature of Alzheimer's disease, though this seems to be driven primarily by increases in activation-associated markers, as opposed to markers of all microglia.

EFAD transgenic mice as a human APOE relevant preclinical model of Alzheimer's disease

Identified in 1993, APOE4 is the greatest genetic risk factor for sporadic Alzheimer's disease (AD), increasing risk up to 15-fold compared with APOE3, with APOE2 decreasing AD risk. However, the functional effects of APOE4 on AD pathology remain unclear and, in some cases, controversial. In vivo progress to understand how the human (h)-APOE genotypes affect AD pathology has been limited by the lack of a tractable familial AD-transgenic (FAD-Tg) mouse model expressing h-APOE rather than mouse (m)-APOE. The disparity between m- and h-apoE is relevant for virtually every AD-relevant pathway, including amyloid-β (Aβ) deposition and clearance, neuroinflammation, tau pathology, neural plasticity and cerebrovascular deficits. EFAD mice were designed as a temporally useful preclinical FAD-Tg-mouse model expressing the h-APOE genotypes for identifying mechanisms underlying APOE-modulated symptoms of AD pathology. From their first description in 2012, EFAD mice have enabled critical basic and therapeutic research. Here we review insights gleaned from the EFAD mice and summarize future directions.

Functional Roles of the Interaction of APP and Lipoprotein Receptors

The biological fates of the key initiator of Alzheimer’s disease (AD), the amyloid precursor protein (APP), and a family of lipoprotein receptors, the low-density lipoprotein (LDL) receptor-related proteins (LRPs) and their molecular roles in the neurodegenerative disease process are inseparably interwoven. Not only does APP bind tightly to the extracellular domains (ECDs) of several members of the LRP group, their intracellular portions are also connected through scaffolds like the one established by FE65 proteins and through interactions with adaptor proteins such as X11/Mint and Dab1. Moreover, the ECDs of APP and LRPs share common ligands, most notably Reelin, a regulator of neuronal migration during embryonic development and modulator of synaptic transmission in the adult brain, and Agrin, another signaling protein which is essential for the formation and maintenance of the neuromuscular junction (NMJ) and which likely also has critical, though at this time less well defined, roles for the regulation of central synapses. Furthermore, the major independent risk factors for AD, Apolipoprotein (Apo) E and ApoJ/Clusterin, are lipoprotein ligands for LRPs. Receptors and ligands mutually influence their intracellular trafficking and thereby the functions and abilities of neurons and the blood-brain-barrier to turn over and remove the pathological product of APP, the amyloid-β peptide. This article will review and summarize the molecular mechanisms that are shared by APP and LRPs and discuss their relative contributions to AD.

Metabolite transport across the mammalian and insect brain diffusion barriers

The nervous system in higher vertebrates is separated from the circulation by a layer of specialized endothelial cells. It protects the sensitive neurons from harmful blood-derived substances, high and fluctuating ion concentrations, xenobiotics or even pathogens. To this end, the brain endothelial cells and their interlinking tight junctions build an efficient diffusion barrier. A structurally analogous diffusion barrier exists in insects, where glial cell layers separate the hemolymph from the neural cells. Both types of diffusion barriers, of course, also prevent influx of metabolites from the circulation. Because neuronal function consumes vast amounts of energy and necessitates influx of diverse substrates and metabolites, tightly regulated transport systems must ensure a constant metabolite supply. Here, we review the current knowledge about transport systems that carry key metabolites, amino acids, lipids and carbohydrates into the vertebrate and Drosophila brain and how this transport is regulated. Blood-brain and hemolymph-brain transport functions are conserved and we can thus use a simple, genetically accessible model system to learn more about features and dynamics of metabolite transport into the brain.

Deficiency of Lipoprotein Lipase in Neurons Decreases AMPA Receptor Phosphorylation and Leads to Neurobehavioral Abnormalities in Mice

Alterations in lipid metabolism have been found in several neurodegenerative disorders, including Alzheimer’s disease. Lipoprotein lipase (LPL) hydrolyzes triacylglycerides in lipoproteins and regulates lipid metabolism in multiple organs and tissues, including the central nervous system (CNS). Though many brain regions express LPL, the functions of this lipase in the CNS remain largely unknown. We developed mice with neuron-specific LPL deficiency that became obese on chow by 16 wks in homozygous mutant mice (NEXLPL-/-) and 10 mo in heterozygous mice (NEXLPL+/-). In the present study, we show that 21 mo NEXLPL+/- mice display substantial cognitive function decline including poorer learning and memory, and increased anxiety with no difference in general motor activities and exploratory behavior. These neurobehavioral abnormalities are associated with a reduction in the 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl) propanoic acid (AMPA) receptor subunit GluA1 and its phosphorylation, without any alterations in amyloid β accumulation. Importantly, a marked deficit in omega-3 and omega-6 polyunsaturated fatty acids (PUFA) in the hippocampus precedes the development of the neurobehavioral phenotype of NEXLPL+/- mice. And, a diet supplemented with n-3 PUFA can improve the learning and memory of NEXLPL+/- mice at both 10 mo and 21 mo of age. We interpret these findings to indicate that LPL regulates the availability of PUFA in the CNS and, this in turn, impacts the strength of synaptic plasticity in the brain of aging mice through the modification of AMPA receptor and its phosphorylation.

More than cholesterol transporters: lipoprotein receptors in CNS function and neurodegeneration

Members of the low-density lipoprotein (LDL) receptor gene family have a diverse set of biological functions that transcend lipid metabolism. Lipoprotein receptors have broad effects in both the developing and adult brain and participate in synapse development, cargo trafficking, and signal transduction. In addition, several family members play key roles in Alzheimer's disease (AD) pathogenesis and neurodegeneration. This Review summarizes our current understanding of the role lipoprotein receptors play in CNS function and AD pathology, with a special emphasis on amyloid-independent roles in endocytosis and synaptic dysfunction.

Expression and regulation of apolipoprotein E receptors in the cells of the central nervous system in culture: A review

The importance of apolipoprotein E (apoE) in the central nervous system (CNS) became increasingly clear since the descovery that apoE ε4 allele is a major risk factor for Alzheimer's disease. ApoE is one of the major apolipoproteins that acts as a ligand for the cellular uptake of lipoproteins via apoE receptors, members of low-density lipoprotein receptor (LDLR) family, in the CNS. Recently, LDLR family has been shown to have new functions that modulate intracellular signalling and affect neuronal and glial functions, survival and regeneration. However, the pattern of expression of apoE receptors in the CNS has not been fully clarified yet. The LDLR, very low density lipoprotein receptor (VLDLR), LDLR-related protein (LRP), and apolipoprotein E receptor 2 (apoER2) are known to bind to and internalize apoE-containing lipoproteins. Here we summarize the expression of apoE receptors in the CNS and demonstrate additional our original data on cell type specific expression and regulation of those receptors in the CNS, using in situ hybridization and RT-PCR. The cells used in our study were highly enriched cultures of neurons, astrocytes, microglia and oligodendrocytes isolated from rat brain and neuroblastoma cell line, Neuro2a. All of these four types of receptors were shown to be expressed in neurons, astrocytes, microglia and oligodendrocytes, while LDLR and LRP were expressed in Neuro2a cells. We further examined the regulation of the expression of these receptors by altering the cholesterol content of the cells, and found that only the LDLR expression was downregulated following internalization of lipoprotein cholesterol and upregulated by cholesterol deprivation, in neuronal and astroglial cells. These data together with previous studies suggest that LDLR, VLDL, LRP, and apoER2 may be involved in apoE-mediated lipid uptake and/or intracellualr signalling in the cells of the CNS cells, i.e., neurons, astrocytes, microglia, and oligodendrocytes.

Similarities and differences in structure, expression, and functions of VLDLR and ApoER2

Very Low Density Lipoprotein Receptor (VLDLR) and Apolipoprotein E Receptor 2 (ApoER2) are important receptors in the brain for mediating the signaling effects of the extracellular matrix protein Reelin, affecting neuronal function in development and in the adult brain. VLDLR and ApoER2 are members of the low density lipoprotein family, which also mediates the effects of numerous other extracellular ligands, including apolipoprotein E. Although VLDLR and ApoER2 are highly homologous, they differ in a number of ways, including structural differences, expression patterns, alternative splicing, and binding of extracellular and intracellular proteins. This review aims to summarize important aspects of VLDLR and ApoER2 that may account for interesting recent findings that highlight the unique functions of each receptor.

Involvement of low-density lipoprotein receptor-related protein and ABCG1 in stimulation of axonal extension by apoE-containing lipoproteins

Apolipoprotein E (apoE)-containing lipoproteins (LpE) are produced by glial cells in the central nervous system (CNS). When LpE are supplied to distal axons, but not cell bodies, of CNS neurons (retinal ganglion cells) the rate of axonal extension is increased. In this study we have investigated the molecular requirements underlying the stimulatory effect of LpE on axonal extension. We show that enhancement of axonal growth by LpE requires the presence of the low-density lipoprotein receptor-related protein-1 (LRP1) in neurons since RNA silencing of LRP1 in neurons, or antibodies directed against LRP, suppressed the LpE-induced axonal extension. In contrast, an alternative LRP1 ligand, α2-macroglobulin, failed to stimulate axonal extension, suggesting that LpE do not exert their growth-stimulatory effect solely by activation of a LRP1-mediated signaling pathway. In addition, although apoE3-containing LpE enhanced axonal extension, apoE4-containing LpE did not. Over-expression of ABCG1 in rat cortical glial cells resulted in production of LpE that increased the rate of axonal extension to a greater extent than did expression of an inactive, mutant form of ABGC1. Furthermore, reconstituted lipoprotein particles containing apoE3, phosphatidylcholine and sphingomyelin, but not cholesterol, stimulated axonal extension, suggesting that sphingomyelin, but not cholesterol, is involved in the stimulatory effect of LpE. These observations demonstrate that LpE and LRP1 promote axonal extension, and suggest that lipids exported to LpE by ABCG1 are important for the enhancement of axonal extension mediated by LpE.

Microglial low-density lipoprotein receptor-related protein 1 modulates c-Jun N-terminal kinase activation

Apolipoprotein E (apoE)-induced activation of low-density lipoprotein receptor (LDL) family members reduces inflammatory responses by suppressing c-Jun N-terminal kinase (JNK) activation. We aimed to identify which specific receptor family member mediates the effect of apoE on inflammation in primary cultures of microglia. Low-density lipoprotein receptor-related protein 1 (LRP1)-deficient (LRP1-/-) microglia were derived from mice using tissue-specific loxP/Cre recombination. Using a peptide formed from the receptor-binding region of apoE (EP), we found that LRP1 mediates the effects of apoE on microglial inflammation. Microglial LRP1 was also essential for EP to suppress JNK activation induced by lipopolysaccharide.

Low-density lipoprotein receptors regulate microglial inflammation through c-Jun N-terminal kinase

Apolipoprotein E (apoE) has been implicated in modulating the central nervous system (CNS) inflammatory response. However, the molecular mechanisms involved in apoE-dependent immunomodulation are poorly understood. We hypothesize that apoE alters the CNS inflammatory response by signaling via low-density lipoprotein (LDL) receptors in glia. To address this hypothesis, we used a small bioactive peptide formed from the receptor-binding domain of apoE, apoE peptide (EP), to study LDL receptor signaling in microglia. To model glial activation, we treated primary mouse microglia and the microglial cell line BV2 with lipopolysaccharide (LPS) and studied two inflammatory responses: an increase in nitric oxide production (NO) and a decrease in apoE production. We found that treatment of primary microglia and BV2 cells with EP attenuated LPS-induced NO accumulation and apoE reduction in a dose-dependent manner. Using the receptor-associated protein to block ligand binding to members of the LDL receptor family, we found that EP attenuated both of these LPS-induced inflammatory responses via LDL receptors. We studied two intracellular signaling cascades associated with apoE: c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK). LPS induced both ERK and JNK activation, whereas EP induced ERK activation, but drastically reduced JNK activation. Inhibition of JNK with SP600125 reduced LPS-induced NO production and apoE reduction in a dose-dependent manner. Treatment of microglia with suboptimal EP in combination with JNK inhibitor enhanced attenuation of LPS-induced NO production. These data suggest that microglial LDL receptors regulate JNK activation, which is necessary for apoE modulation of the inflammatory response.

Functional interactions of APP with the apoE receptor family

The beta-amyloid precursor protein (APP) is central to the pathogenesis of Alzheimer's disease, but its normal functions in the brain are poorly understood. A number of APP-interacting proteins have been identified: intracellularly, APP interacts with adaptor proteins through its conserved NPXY domain; extracellularly, APP interacts with a component of the extracellular matrix, F-spondin. Interestingly, many of these APP-interacting proteins also interact with the family of receptors for apolipoprotein E (apoE), the Alzheimer's disease risk factor. apoE receptors also share with APP the fact that they are cleaved by the same secretase activities. apoE receptors are shed from the cell surface, a cleavage that is regulated by receptor-ligand interactions, and C-terminal fragments of apoE receptors are cleaved by gamma-secretase. Functionally, both APP and apoE receptors affect neuronal migration and synapse formation in the brain. This review summarizes these numerous interactions between APP and apoE receptors, which provide clues about the normal functions of APP.

Lipoprotein receptors in Alzheimer's disease

Lipoprotein receptors have important roles in pathological processes that lead to Alzheimer's disease (AD). Previously, they were believed to act mainly by modulating the neuronal metabolism of cholesterol and apolipoprotein E, major risk factors for spontaneous AD. However, recent findings point towards an unexpected new function for lipoprotein receptors in regulation of intracellular transport and processing of the amyloid precursor protein (APP) to give amyloid-beta peptide, the principal component of senile plaques. Here, we will discuss how lipoprotein receptors might modulate distinct steps in neuronal trafficking of APP, and how an intricate balance between opposing receptor activities might be a crucial determinant of APP processing, and of onset and progression of neurodegeneration.

The generation and function of soluble apoE receptors in the CNS

More than a decade has passed since apolipoprotein E4 (APOE-ε4) was identified as a primary risk factor for Alzheimer 's disease (AD), yet researchers are even now struggling to understand how the apolipoprotein system integrates into the puzzle of AD etiology. The specific pathological actions of apoE4, methods of modulating apolipoprotein E4-associated risk, and possible roles of apoE in normal synaptic function are still being debated. These critical questions will never be fully answered without a complete understanding of the life cycle of the apolipoprotein receptors that mediate the uptake, signaling, and degradation of apoE. The present review will focus on apoE receptors as modulators of apoE actions and, in particular, explore the functions of soluble apoE receptors, a field almost entirely overlooked until now.

Selectively increased oxidative modifications mapped to detergent‐insoluble forms of Aβ and β‐III tubulin in Alzheimer's disease

Deleterious post-translational modifications (PTMs) to the neuronal cytoskeleton are a proposed mechanistic link between accumulation of amyloid (A) beta peptides and subsequent abnormalities of tau and neurodegeneration in Alzheimer's disease (AD). Here we tested the hypothesis that PTMs on neuronal tubulins selectively accumulate in a pathological protein fraction in AD. We used new software, P-MOD, to identify comprehensively and map PTMs using mass spectral data from soluble (normal) and detergent-insoluble (pathological) protein fractions from AD, as well as total extracts from controls, for selected proteins: Abeta, tau, apolipoprotein (apo) E, glial fibrillary acidic protein (GFAP), alpha-III tubulin, and beta-III tubulin. Our results confirmed direct observations of others by identifying methionine (M) sulfoxides at Abeta position 35 and numerous sites of tau phosphorylation in detergent-insoluble protein from AD, while no PTMs were enriched on primarily astrocyte-derived apoE or GFAP in this fraction. P-MOD mapped several abundant M sulfoxides to neuron-enriched beta-III tubulin but not its heterodimeric partner, neuron-enriched alpha-III tubulin, a result confirmed by selective suppression of CNBr-mediated cleavage of beta-III tubulin. These findings are the first comprehensive assessment of PTMs in AD and point to oxidative modification of beta-III tubulin as a potential contributor to the neuronal cytoskeletal disruption that is characteristic of AD.

Microglial activation induced by neurodegeneration: a proteomic analysis

Neuroinflammation mediated by microglial activation appears to play an essential role in the pathogenesis of Parkinson disease; however, the mechanisms by which microglia are activated are not fully understood. Thus, we first evaluated the effects of two parkinsonian toxicants, manganese ethylene bisdithiocarbamate (Mn-EBDC) and 1-methyl-4-phenylpyridine (MPP+), on microglial activation as well as associated dopaminergic (DAergic) neurotoxicity in primary cell culture systems. The results demonstrated that, when rat primary mesencephalic neuron-enriched or neuron-microglia mixed cultures were treated with Mn-EBDC at 2-8 microm or MPP+ at 0.25-5 microm, respectively, for 7 days, both toxicants were capable of inducing DAergic neurodegeneration as well as activating microglia via a mechanism secondary to DAergic neurodegeneration. Furthermore activated microglia subsequently enhanced DAergic neurotoxicity induced by Mn-EBDC or MPP+. Detailed scrutiny of neuron-microglia interactions identified a fraction of the conditioned media derived from a DAergic cell line treated with Mn-EBDC or MPP+ that potently activated microglia. To further define potential mediators leading to microglial activation secondary to neurodegeneration, we utilized a quantitative proteomic technique termed SILAC (for stable isotope labeling by amino acids in cell culture) to compare the protein profiles of MPP+-treated cellular fraction that mediated microglial activation as compared with controls. The search revealed numerous novel proteins that are potentially important in neurodegeneration-mediated microglial activation, a process believed to be critical in Parkinson disease progression.

Effect of apolipoprotein E (apoE) phenotype on the apoE content of CSF-HDL in children

BACKGROUND

The majority of the lipoprotein in cerebrospinal fluid (CSF) is apolipoprotein E (apoE)-containing HDL. Since neuronal cells express lipoprotein receptors which recognize apoE, apoE in CSF-HDL is believed to be important for the development of central nervous system (CNS) in children. In adults, the apoE phenotype affects the plasma apoE concentration and the epsilon 4 allele is a risk factor for Alzheimer's disease. Due to the requirement for CNS development, we examined whether the apoE phenotype affects the composition and concentration of CSF-HDL in children.

METHODS

We determined the apoE phenotype in 107 neurologically normal subjects, including 67 children (<20 years), by isoelectronic focusing. We also measured apoE, total cholesterol (TC), and phospholipid (PL) concentrations in the CSF.

RESULTS

The respective frequencies of apoE4/3, E3/3 and E3/2 were 16.4%, 77.6%, and 6.0%. The allele frequencies of epsilon 4, epsilon 3, and epsilon 2 were 0.082, 0.888, and 0.030, respectively. There were no significant differences in the CSF-apoE, TC, or PL concentrations or the apoE/PL ratio among the apoE phenotypes. However, the CSF-apoE/PL ratio was significantly higher in children than in adults.

CONCLUSION

The apoE phenotype does not affect the composition or concentration of CSF-HDL in children. We speculate that an apoE4 carrier is prevented in childhood from the impaired development of central nervous system by CSF-HDL enriched with apoE.

Fractalkine-upregulated milk-fat globule EGF factor-8 protein in cultured rat microglia

Fractalkine is the only known member of the CX(3)C-chemokine family, and so is its receptor CX(3)CR1. Fractalkine, typically is expressed by neurons where it is inserted in the plasma membrane ("chemokine on a stalk"). It can, however, be clipped off by a specific enzyme and diffuse into the extracellular space. CX(3)CR1 is primarily expressed by microglia, the phagocytes of the brain. This study was aimed at studying gene expression changes in cultured rat microglia upon fractalkine stimulation using gene chip technology. Six genes turned out to be upregulated, amongst which milk-fat globule EGF factor-8 protein (MFG-E8) was the most surprising, but also the most revealing one. We hypothesize that it serves as a bridging molecule between apoptotic cells (neurons) and microglia. Since the docking to microglia is, in part, mediated by members of the integrin family, six of these molecules have been-post hoc-included in real-time PCR confirmations of chip results. Two of them-integrin alpha(2) and integrin beta(5)-were upregulated as well. These data provide a much closer look into molecular mechanisms involved in apoptosis of neurons and their removal by microglia.

Effect of sterols on beta-amyloid peptide (AbetaP 1-40) channel formation and their properties in planar lipid membranes

We investigate the role played by membrane composition on the interaction and self-assembly of beta-amyloid peptide (AbetaP1-40) during pore formation in planar lipid membranes (PLMs). Incorporation studies showed that AbetaP does not interact with zwitterionic membranes made up of phosphatidylcholine, whereas the addition of cholesterol or ergosterol to the membranes leads to channel formation. Among the PLMs used, a higher propensity of AbetaP to form channels at low applied potential (+/-20 mV) was observed in 7-dehydrocholesterol and in oxidized cholesterol PLMs. These channels present long lifetimes, high-occurrence frequencies, and are voltage dependent. In particular, the AbetaP channel in oxidized cholesterol showed anion selectivity. Thus cholesterol (and sterols in general) could be considered as targets for AbetaP, which prevents the fibrillation process by increasing incorporation into membranes. Furthermore, by switching the channel selectivity versus anions, cholesterol helps to reduce the imbalance of the cellular ions, calcium included, induced by membrane depolarization, which could be one of the factors responsible for cytotoxicity in Alzheimer's disease.

The cellular biochemistry of cholesterol and statins: insights into the pathophysiology and therapy of Alzheimer's disease

The causes of late onset Alzheimer disease (AD) are poorly understood. Although beta-amyloid (Abeta) is thought to play a critical role in the pathophysiology of AD, no genetic evidence directly ties Abeta to late onset AD. This suggests that the accumulation of Abeta and neurodegeneration associated with AD might result from an abnormality that indirectly affects Abeta production or accumulation. Increasing evidence suggests that abnormalities in the metabolism of cholesterol and related molecules, such as cholseterol esters and 24(S) hydroxycholesterol might contribute to the pathophysiology of late onset AD by increasing production of Abeta. 24(S) Hydroxycholesterol is a member of a family of oxidized cholesterol catabolites, termed oxysterols, which function to regulate export of cholesterol from the cell and transcription of genes related to cholesterol metabolism. Cholesterol esters are cholesterol derivatives used for cholesterol storage. Levels of 24(S) hydroxycholesterol increase with AD. Polymorphisms in several different genes important for cholesterol physiology are associated with an increased load or level of Abeta in AD. These genes include apolipoprotein E, cholesterol 24 hydroxylase (Cyp46), acyl-CoA:cholesterol acetyltransferase (ACAT), and the cholesterol transporter ABCA1. Other studies show that levels of cholesterol, or its precursors, are elevated in subjects early in the course of AD. Finally, studies of the processing of amyloid precursor protein show that cholesterol and its catabolites modulate amyloid precursor protein processing and Abeta production. These lines of evidence raise the possibility that genetic abnormalities in cholesterol metabolism might contribute to the pathophysiology of AD.

The Very Low-density Lipoprotein (VLDL) Receptor: Characterization and Functions as a Peripheral Lipoprotein Receptor

The very low-density lipoprotein (VLDL) receptor is a member of the low-density lipoprotein (LDL) receptor family. In vitro and in vivo studies have shown that VLDL receptor binds triglyceride (TG)-rich lipoproteins but not LDL, and functions as a peripheral remnant lipoprotein receptor. VLDL receptor is expressed abundantly in fatty acid-active tissues (heart, skeletal muscle and fat), the brain and macrophages. It is likely that VLDL receptor functions in concert with lipoprotein lipase (LPL), which hydrolyses TG in VLDL and chylomicron. In contrast to the LDL receptor, VLDL receptor binds apolipoprotein (apo) E2/2 VLDL particles as well as apoE3/3 VLDL, and the expression is not down-regulated by intracellular lipoproteins. Recently, various functions of the VLDL receptor have been reported in lipoprotein metabolism, metabolic syndrome/atherosclerosis, cardiac fatty acid metabolism, neuronal migration and angiogenesis/tumor growth. Gene therapy of VLDL receptor into the liver showed a benefit effect for lipoprotein metabolism in both LDL receptor knockout and apoE mutant mice. Beyond its function as a peripheral lipoprotein receptor, possibilities of its physiological function have been extended to include signal transduction, angiogenesis and tumor growth.

Domains of apoE required for binding to apoE receptor 2 and to phospholipids: implications for the functions of apoE in the brain

We have studied the contribution of the carboxy terminal domains of lipid-free apoE isolated from apoE-expressing cell cultures in binding to phospholipids and have determined the affinities of reconstituted POPC-apoE particles for the apoER2. It was found that the initial rate of association of apoE2, apoE3, apoE4, and a mutant form apoE4R158M to multilamellar DMPC vesicles was similar and was reduced and eventually diminished by gradual deletion of the carboxy terminal segments. The truncated apoE forms retained their ability to associate with plasma lipoproteins. Receptor binding studies were performed using the ldlA-7 cells expressing apoER2 and transiently transfected COS-M6 and the appropriate control untransfected cells. Specific binding to apoER2 was obtained by subtracting from the total binding to the receptor-expressing cells the nonspecific binding values of the untransfected cells. POPC-apoE particles generated using apoE3, apoE4, the truncated apoE4-259, apoE4-229, apoE4-202, and apoE-165, and the mutant apoE4R158M all bound tightly to the apoER2 (K(d) range of 12 +/- 3 to 19 +/- 4 microg/mL). POPC-apoE2 bound with reduced affinity (K(d) = 31 +/- 5.3 microg/mL). The findings establish that the apoER2 binding domain of apoE is in the 1-165 amino terminal region, whereas the carboxy terminal 230-299 region of apoE is required for efficient initial association with phospholipids.

Causative and susceptibility genes for Alzheimer's disease: a review

Alzheimer's disease (AD) is the most common type of dementia in the elderly population. Three genes have been identified as responsible for the rare early-onset familial form of the disease: the amyloid precursor protein (APP) gene, the presenilin 1 (PSEN1) gene and the presenilin 2 (PSEN2) gene. Mutations in these genes, however, account for less than 5% of the total number of AD cases. The remaining 95% of AD patients are mostly sporadic late-onset cases, with a complex aetiology due to interactions between environmental conditions and genetic features of the individual. In this paper, we review the most important genes supposed to be involved in the pathogenesis of AD, known as susceptibility genes, in an attempt to provide a comprehensive picture of what is known about the genetic mechanisms underlying the onset and progression of AD. Hypotheses about the role of each gene in the pathogenic pathway are discussed, taking into account the functions and molecular features, if known, of the coded protein. A major susceptibility gene, the apolipoprotein E (APOE) gene, found to be associated with sporadic late-onset AD cases and the only one, whose role in AD has been confirmed in numerous studies, will be included in a specific chapter. As the results reported by association studies are conflicting, we conclude that a better understanding of the complex aetiology that underlies AD may be achieved likely through a multidisciplinary approach that combines clinical and neurophysiological characterization of AD subtypes and in vivo functional brain imaging studies with molecular investigations of genetic components.

Piracetam inhibits the lipid-destabilising effect of the amyloid peptide Abeta C-terminal fragment

Amyloid peptide (Abeta) is a 40/42-residue proteolytic fragment of a precursor protein (APP), implicated in the pathogenesis of Alzheimer's disease. The hypothesis that interactions between Abeta aggregates and neuronal membranes play an important role in toxicity has gained some acceptance. Previously, we showed that the C-terminal domain (e.g. amino acids 29-42) of Abeta induces membrane permeabilisation and fusion, an effect which is related to the appearance of non-bilayer structures. Conformational studies showed that this peptide has properties similar to those of the fusion peptide of viral proteins i.e. a tilted penetration into membranes. Since piracetam interacts with lipids and has beneficial effects on several symptoms of Alzheimer's disease, we investigated in model membranes the ability of piracetam to hinder the destabilising effect of the Abeta 29-42 peptide. Using fluorescence studies and 31P and 2H NMR spectroscopy, we have shown that piracetam was able to significantly decrease the fusogenic and destabilising effect of Abeta 29-42, in a concentration-dependent manner. While the peptide induced lipid disorganisation and subsequent negative curvature at the membrane-water interface, the conformational analysis showed that piracetam, when preincubated with lipids, coats the phospholipid headgroups. Calculations suggest that this prevents appearance of the peptide-induced curvature. In addition, insertion of molecules with an inverted cone shape, like piracetam, into the outer membrane leaflet should make the formation of such structures energetically less favourable and therefore decrease the likelihood of membrane fusion.

ApoE isoforms affect neuronal N-methyl-d-aspartate calcium responses and toxicity via receptor-mediated processes

Apolipoprotein E (apoE) alters the pathophysiology of Alzheimer's disease, but its mechanism is not fully understood. We examined the effects of recombinant human apoE3 and apoE4 on the neuronal calcium response to N-methyl-D-aspartate (NMDA), and compared them to their toxicity. ApoE4 (100 nM) significantly increased the resting calcium (by 70%) and the calcium response to NMDA (by 185%), whereas similar changes were not obtained in apoE3-treated neurons. ApoE4, but not apoE3, also significantly increased neurotoxicity, as evidenced by enhanced lactate dehydrogenase release (by 53%) and reduced 3-(4,5-dimethylthiazol-2-yl)-2,5,diphenyltetrazolium bromide levels (by 32%). ApoE4-induced changes in the calcium response to NMDA and associated neurotoxicity were blocked by coincubation with MK-801. Both the receptor-associated protein, which inhibits interaction of apoE with members of the LDL receptor family, including the low-density lipoprotein receptor-related protein (LRP), and activated alpha2-macroglobulin, another LRP ligand, prevented apoE4-induced enhancement of the calcium response to NMDA, resting calcium levels, and neurotoxicity. A tandem apoE peptide (100 nM) containing only the receptor binding region residues also eliminated the enhanced calcium signaling and neurotoxicity by apoE4. Taken together, our data demonstrate that differential effects of apoE3 and apoE4 on the calcium signaling in neurons correlate with their effect on neurotoxicity, which are secondary to receptor binding.

Membrane destabilization induced by beta-amyloid peptide 29-42: importance of the amino-terminus

Increasing evidence implicates interactions between Abeta-peptides and membrane lipids in Alzheimer's disease. To gain insight into the potential role of the free amino group of the N-terminus of Abeta29-42 fragment in these processes, we have investigated the ability of Abeta29-42 unprotected and Abeta29-42 N-protected to interact with negatively-charged liposomes and have calculated the interaction with membrane lipids by conformational analysis. Using vesicles mimicking the composition of neuronal membranes, we show that both peptides have a similar capacity to induce membrane fusion and permeabilization. The fusogenic effect is related to the appearance of non-bilayer structures where isotropic motions occur as shown by 31P and 2H NMR studies. The molecular modeling calculations confirm the experimental observations and suggest that lipid destabilization could be due to the ability of both peptides to adopt metastable positions in the presence of lipids. In conclusion, the presence of a free or protected (acetylated) amino group in the N-terminus of Abeta29-42 is therefore probably not crucial for destabilizing properties of the C-terminal fragment of Abeta peptides.

Increased extracellular amyloid deposition and neurodegeneration in human amyloid precursor protein transgenic mice deficient in receptor-associated protein

The low-density lipoprotein receptor-related protein (LRP) is an abundant neuronal cell surface receptor that regulates amyloid beta-protein (Abeta) trafficking into the cell. Specifically, LRP binds secreted Abeta complexes and mediates its degradation. Previously, we have shown in vitro that the uptake of Abeta mediated by LRP is protective and that blocking this receptor significantly enhances neurotoxicity. To further characterize the effects of LRP and other lipoprotein receptors on Abeta deposition, an in vivo model of decreased LRP expression, receptor-associated protein (RAP)-deficient (RAP-/-) mice was crossed with human amyloid protein precursor transgenic (hAPP tg) mice, and plaque formation and neurodegeneration were analyzed. We found that, although the age of onset for plaque formation was the same in hAPP tg and hAPP tg/RAP-/- mice, the amount of amyloid deposited doubled in the hAPP tg/RAP-/- background. Moreover, these mice displayed increased neuronal damage and astrogliosis. Together, these results further support the contention that LRP and other lipoprotein receptors might be neuroprotective against Abeta toxicity and that this receptor might play an integral role in Abeta clearance.

Multiplexed, Targeted Gene Expression Profiling and Genetic Analysis on Electronic Microarrays

Background: Electronic microarrays comprise independent microelectrode test sites that can be electronically biased positive or negative, or left neutral, to move and concentrate charged molecules such as DNA and RNA to one or more test sites. We developed a protocol for multiplexed gene expression profiling of mRNA targets that uses electronic field-facilitated hybridization on electronic microarrays.

Methods: A multiplexed, T7 RNA polymerase-mediated amplification method was used for expression profiling of target mRNAs from total cellular RNA; targets were detected by hybridization to sequence-specific capture oligonucleotides on electronic microarrays. Activation of individual test sites on the electronic microarray was used to target hybridization to designated subsets of sites and allow comparisons of target concentrations in different samples. We used multiplexed amplification and electronic field-facilitated hybridization to analyze expression of a model set of 10 target genes in the U937 cell line during lipopolysaccharide-mediated differentiation. Performance of multiple genetic analyses (single-nucleotide polymorphism detection, gene expression profiling, and splicing isoform detection) on a single electronic microarray was demonstrated using the ApoE and ApoER2 genes as a model system.

Results: Targets were detected after a 2-min hybridization reaction. With noncomplementary capture probes, no signal was detectable. Twofold changes in target concentration were detectable throughout the (∼64-fold) range of concentrations tested. Levels of 10 targets were analyzed side by side across seven time points. By confining electronic activation to subsets of test sites, polymorphism detection, expression profiling, and splicing isoform analysis were performed on a single electronic microarray.

Conclusions: Microelectronic array technology provides specific target detection and quantification with advantages over currently available methodologies for targeted gene expression profiling and combinatorial genomics testing.

Cholesterol is an important factor affecting the membrane insertion of beta-amyloid peptide (A beta 1-40), which may potentially inhibit the fibril formation

beta-Amyloid peptide (A beta), a normal constituent of neuronal and non-neuronal cells, has been proven to be the major component of extracellular plaque of Alzheimer's disease. Interactions between A beta and neuronal membranes have been postulated to play an important role in the neuropathology of Alzheimer's disease. Here we show that A beta is able to insert into lipid bilayer. The membrane insertion ability of A beta is critically controlled by the ratio of cholesterol to phospholipids. In a low concentration of cholesterol A beta prefers to stay in membrane surface region mainly in a beta-sheet structure. In contrast, as the ratio of cholesterol to phospholipids rises above 30 mol%, A beta can insert spontaneously into lipid bilayer by its C terminus. During membrane insertion A beta generates about 60% alpha-helix and removes almost all beta-sheet structure. Fibril formation experiments show that such membrane insertion can reduce fibril formation. Our findings reveal a possible pathway by which A beta prevents itself from aggregation and fibril formation by membrane insertion.

Apolipoprotein E and apolipoprotein E receptors modulate A beta-induced glial neuroinflammatory responses

Large numbers of activated glia are a common pathological feature of many neurodegenerative disorders, including Alzheimer's disease (AD). Several different stimuli, including lipopolysaccharide (LPS), dibutyryl (db)cAMP, and aged amyloid-beta 1-42 (A beta), can induce glial activation in vitro, as measured by morphological changes and the production of pro-inflammatory cytokines and oxidative stress molecules. Only A beta-induced activation is attenuated by the addition of exogenous apolipoprotein E (apoE)-containing particles. In addition, only A beta also induces an increase in the amount of endogenous apoE, the primary apolipoprotein expressed by astrocytes in the brain. The functional significance of the increase in apoE appears to be to limit the inflammatory response. Indeed, compared to wild type mice, glial cells cultured from apoE knockout mice exhibit an enhanced production of several pro-inflammatory markers in response to treatment with A beta and other activating stimuli. The mechanism for both the A beta-induced glial activation and the increase in apoE appears to involve apoE receptors, a variety of which are expressed by both neurons and glia. Experiments using receptor associated protein (RAP), an inhibitor of apoE receptors with a differential affinity for the low-density lipoprotein receptor (LDLR) and the LDLR-related protein (LRP), revealed that LRP mediates A beta-induced glial activation, while LDLR mediates the A beta-induced changes in apoE levels. In summary, both an apoE receptor agonist (apoE) and an antagonist (RAP) inhibit A beta-induced glial cell activation. Thus, apoE receptors appear to translate the presence of extracellular A beta into cellular responses, both initiating glial cell activation and limiting its scope by inducing apoE, an anti-inflammatory agent.

Role of apoe/Abeta interactions in the pathogenesis of Alzheimer's disease and cerebral amyloid angiopathy

The epsilon4 allele of apolipoprotein E (apoE) is a risk factor for Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA). The mechanism underlying this increased risk is not completely clear, yet mounting evidence supports the idea that the ability of apoE to interact with the amyloid-beta (Abeta) peptide and influence its conformation and clearance plays a major role. Evidence to support this concept comes from in vitro and in vivo studies of apoE/Abeta interactions and the effects of these interactions on Abeta conformation and cellular clearance. Recent studies on the effect of murine and human apoE in APP transgenic mice provide direct evidence that apoE is critically involved in the in vivo converstion of Abeta into forms which contain high 5-sheet content and associated cellular toxicity (neuritic plaques and CAA). These studies also suggest a role for human apoE in Abeta clearance in vivo.

Role of apolipoprotein E receptors in regulating the differential in vivo neurotrophic effects of apolipoprotein E

Apolipoprotein E (apoE) is known to bind to at least five receptors, including the low-density lipoprotein (LDL) receptor-related protein (LRP), very low density LDL receptor (VLDL-R), LDL-R, apoE receptor 2 (apoER2), and megalin/gp330. In this context, the main objective of the present study was to better understand the contributions of LRP and LDL-R to the in vivo neurotrophic effects of apoE. For this purpose, apoE-deficient and receptor-associated protein (RAP)-deficient mice were infused with recombinant apoE3, RAP, or saline. Infusion of apoE3 into apoE-deficient mice resulted in amelioration of degenerative alterations of pyramidal neurons, but had no effect on somatostatin-producing interneurons. In contrast, infusion of apoE3 into RAP-deficient mice resulted in amelioration of degenerative alterations of somatostatin-producing interneurons. LRP and LDL-R levels were significantly reduced in RAP-deficient mice, but significantly increased in the apoE-deficient mice. In contrast, levels of apoE were reduced in the RAP-deficient mice compared to wildtype controls, suggesting that neurotrophic effects of apoE3 in the RAP-deficient mice were related to a combined deficit in endogenous apoE and selected apoE receptors. Furthermore, in apoE-deficient mice, infusion of apoE3 had a neurotrophic effect on somatostatin-producing interneurons only when combined with RAP, suggesting that increased expression of apoE receptors in apoE-deficient mice prevented apoE from rescuing somatostatin-producing neurons. This study supports the contention that some of the in vivo neurotrophic effects of apoE are mediated by LRP and LDL-R and that a critical balance between levels of apoE and its receptors is necessary for the differential neurotrophic effects to appear.

VLDL receptor polymorphism, cognitive impairment, and dementia

OBJECTIVE

Clinical, epidemiologic, and pathologic observations suggest that vascular risk factors are associated with impaired cognition. Previous studies supported an association between cognitive decline and APOE. Although the underlying mechanism is not clear, it might involve apoE receptors, such as the very low density lipoprotein receptor.

METHODS

The impact of a polymorphic triplet repeat in the very low density lipoprotein receptor gene (VLDLR) on cognitive function was examined in two independent studies: a population study involving 221 demented subjects compared with 249 control subjects and a clinical study involving 124 demented subjects compared with 179 control subjects.

RESULTS

In the population study, the presence of the VLDLR-5-repeat allele was associated with a relative risk of dementia (OR, 1.9; 95% CI, 1.2 to 3.0). This result was confirmed in the clinical study (OR, 8.1; 95% CI, 4.4 to 15.1) and was more pronounced in subjects with mixed or vascular dementia than in patients with AD.

CONCLUSION

The VLDLR-5-repeat allele may constitute a genetic susceptibility factor for dementia, particularly in the presence of vascular risk factors. This observation suggests the influence of vascular risk factors in the occurrence of dementia.

Investigation of association of 13 polymorphisms in eight genes in southeastern African American Alzheimer disease patients as compared to age-matched controls

Alzheimer disease (AD) is an emotionally devastating and exceptionally costly disease. Apolipoprotein E (APOE) is a major risk factor gene for AD regardless of age of onset or family history. However, this association may not be as strong or consistent in ethnic groups such as African Americans, raising the possibility of other modifier gene(s). In a group of African American AD patients, a significantly increased risk of AD was associated with two E4 alleles (OR = 5.6; 95% CI = 1.5-21.0) or one E4 allele (OR = 2.5; 95% CI = 1.3-5.0) when compared to E3/E3 genotype, and there was a significant lowering of age of onset for affecteds with E4/E4 genotype as compared to one E2 allele (P = 0.02) or all others (P = 0.03). We also found a significant increase in age of onset with the -308 #2 (A) allele of TNF when compared to AD cases with no #2 allele. A significant increase in age was also demonstrated with the #2 allele (99 base pairs) of the microsatellite TNFa, located approximately 10.5 kb upstream of TNF. When these two alleles were combined with the TNF -238G (#1) allele to give a haplotype, the significant increase in age was still demonstrated. Polymorphisms in the APOE promoter and six other candidate genes did not appear to demonstrate any significant association with our African American AD patients. Our results confirm the established association of APOE4 to AD observed in several ethnic groups, including African Americans. In addition, TNF appears to have some modifying effect in AD, primarily on age of onset, or it could be in linkage disequilibrium with a modifier locus nearby.

Apolipoprotein E deposition and astrogliosis are associated with maturation of beta-amyloid plaques in betaAPPswe transgenic mouse: Implications for the pathogenesis of Alzheimer's disease

A transgenic mouse expressing the human beta-amyloid precursor protein with the 'Swedish' mutation, Tg2576, was used to investigate the mechanism of beta-amyloid (Abeta) deposition. Previously, we have reported that the major species of Abeta in the amyloid plaques of Tg2576 mice are Abeta1-40 and Abeta1-42. Moreover, Abeta1-42 deposition precedes Abeta1-40 deposition, while Abeta1-40 accumulates in the central part of the plaques later in the pathogenic process. Those data indicate that Abeta deposits in Tg2576 mice have similar characteristics to those in Alzheimer's disease. In the present study, to understand more fully the amyloid deposition mechanism implicating Alzheimer's disease pathogenesis, we examined immunohistochemically the distributions of apolipoprotein E (apoE) and Abeta in amyloid plaques of aged Tg2576 mouse brains. Our findings suggest that Abeta1-42 deposition precedes apoE deposition, and that Abeta1-40 deposition follows apoE deposition during plaque maturation. We next examined the relationship between apoE and astrogliosis associated with amyloid plaques using a double-immunofluorescence method. Extracellular apoE deposits were always associated with reactive astrocytes whose processes showed enhancement of apoE-immunoreactivity. Taken together, the characteristics of amyloid plaques in Tg2576 mice are similar to those in Alzheimer's disease with respect to apoE and astrogliosis. Furthermore, apoE deposition and astrogliosis may be necessary for amyloid plaque maturation.

Tissue factor pathway inhibitor inhibits endothelial cell proliferation via association with the very low density lipoprotein receptor

Tissue factor pathway inhibitor (TFPI) contains three Kunitz-type proteinase inhibitor domains and is a potent inhibitor of tissue factor-mediated coagulation. Here, we report that TFPI inhibits the proliferation of basic fibroblast growth factor-stimulated endothelial cells. A truncated form of TFPI, containing only the first two Kunitz-type proteinase inhibitor domains, has very little antiproliferative activity, suggesting that the carboxyl-terminal region of TFPI is responsible for this activity. Binding studies revealed that full-length TFPI, but not the truncated TFPI molecule, is recognized by the very low density lipoprotein receptor (VLDL receptor) indicating that this receptor is a novel high affinity endothelial cell receptor for TFPI. The antiproliferative activity of TFPI on endothelial cells is inhibited by the receptor-associated protein, a known antagonist of ligand binding by the VLDL receptor, and by anti-VLDL receptor antibodies. These results confirm that the antiproliferative activity of TFPI is mediated by the VLDL receptor and suggest that this receptor-ligand system may be a useful target for the development of new anti-angiogenic and antitumor agents.

Expression of scavenger receptor class B, type I, by astrocytes and vascular smooth muscle cells in normal adult mouse and human brain and in Alzheimer's disease brain

In Alzheimer's disease (AD), fibrillar beta-amyloid protein (fAbeta) accumulates in the walls of cerebral vessels associated with vascular smooth muscle cells (SMCs), endothelium, and pericytes, and with microglia and astrocytes in plaques in the brain parenchyma. Scavenger receptor class A (SR-A) and class B, type I (SR-BI) mediate binding and ingestion of fAbeta by cultured human fetal microglia, microglia from newborn mice, and by cultured SMCs. Our findings that SR-BI participates in the adhesion of cultured microglia from newborn SR-A knock-out mice to fAbeta-coated surfaces, and that microglia secrete reactive oxygen species when they adhere to these surfaces prompted us to explore expression of SR-BI in vivo. We report here that astrocytes and SMCs in normal adult mouse and human brains and in AD brains express SR-BI. In contrast, microglia in normal adult mouse and human brains and in AD brains do not express SR-BI. These findings indicate that SR-BI may mediate interactions between astrocytes or SMCs and fAbeta, but not of microglia and fAbeta, in AD, and that expression of SR-BI by rodent microglia is developmentally regulated. They suggest that SR-BI expression also is developmentally regulated in human microglia.

Apolipoprotein E receptors mediate the effects of beta-amyloid on astrocyte cultures

We have previously shown that beta-amyloid (Abeta) induces astrocyte activation in vitro and that this reaction is attenuated by the addition of exogenous apolipoprotein E (apoE)-containing particles. However, the effects of Abeta on endogenous apoE and apoJ levels and the potential role of apoE receptors in astrocyte activation have not been addressed. Three activating stimuli (lipopolysaccharide, dibutyryl cAMP, and aged Abeta 1-42) were used to induce activation of rat astrocyte cultures, as assessed by changes in morphology and an increase in interleukin-1beta. However, only Abeta also induced approximately 50% reduction in the amount of released apoE and apoJ and an 8-fold increase in the levels of cell-associated apoE and apoJ. Experiments using two concentrations of receptor-associated protein, an inhibitor of apoE receptors with a differential affinity for the low density lipoprotein receptor (LDLR) and the LDLR-related protein (LRP), suggest that LRP mediates Abeta-induced astrocyte activation, whereas LDLR mediates the Abeta-induced changes in apoE levels. Receptor-associated protein had no effect on apoJ levels or on activation by either dibutyryl cAMP or lipopolysaccharide. These data suggest that apoE receptors translate the presence of extracellular Abeta into cellular responses, both initiating and modulating the inflammatory response induced by Abeta.

The endocytic receptor protein LRP also mediates neuronal calcium signaling via N-methyl-D-aspartate receptors

The low density lipoprotein receptor-related protein (LRP) is an endocytic receptor that is a member of the low density lipoprotein receptor family. We report that the LRP ligand, activated alpha(2)-macroglobulin (alpha(2)M*), induces robust calcium influx in cultured primary neurons, but not in nonneuronal LRP-containing cells in the same culture. The calcium influx is mediated through N-methyl-d-aspartate receptor channels, which explains the neuron specificity of the response. Microapplication of alpha(2)M* leads to a localized response at the site of application that dissipates rapidly, suggesting that the calcium signal is temporally and spatially discrete. Calcium influx to alpha(2)M* is blocked by the physiological LRP inhibitor, receptor-associated protein. Bivalent antibodies to the extracellular domain of LRP, but not Fab fragments of the same antibody, cause calcium influx, indicating that the response is specific to LRP and may require dimerization of the receptor. Thus, LRP is an endocytic receptor with a novel signaling role.

Astrocyte lipoproteins, effects of apoE on neuronal function, and role of apoE in amyloid-beta deposition in vivo

The genetic association between the E4 isoform of apolipoprotein E (apoE) and increased risk for Alzheimer's disease (AD) has prompted interest in the neurobiology of apoE and the possible relationship between lipoprotein metabolism in the brain and neurodegenerative disease. ApoE, a product of astrocytes, is abundant in brain and in cerebrospinal fluid (CSF) where it is found in lipoproteins the size of large plasma high-density lipoproteins (HDL). Cultured astrocytes also secrete apoE/HDL, although the lipid and apoprotein composition of these nascent particles differs from that found in CSF, suggesting possible functional differences. In vitro studies have demonstrated isoform-specific effects of apoE on neurite outgrowth, neuronal plasticity, neurotoxicity, lipid peroxidation, oxidative injury, binding to cytoskeletal proteins, and interactions with amyloid-beta (Abeta), a primary component of senile plaques in AD. A number of these proposed functions have also been assessed in apoE -/- mice and transgenic mice expressing human apoE3 or apoE4. Importantly, analysis of transgenic mice overexpressing a mutant form of the human amyloid precursor protein (APP(V717F)) in the presence of mouse apoE, no apoE, or human apoE3 or E4 has demonstrated a critical and isoform-specific role for apoE in neuritic plaque formation, a pathologic hallmark of AD. Together, these data have provided important clues as to possible mechanism(s) by which apoE genotype modifies AD risk.

Apolipoprotein E gene promoter polymorphisms in Alzheimer's disease

Alzheimer's disease, the most frequent form of senile dementia, presents in the vast majority of cases as a multifactorial trait, where a series of genetic and environmental risk factors converge. The increasing body of data, both epidemiological and functional, is strengthening the evidence that apolipoprotein E (APOE, gene; apoE, protein) is a true susceptibility factor for the onset of the common form of Alzheimer's disease. The E4 isoform of apoE remains to date as the main genetic risk factor for the disease, although the mechanisms responsible for this association are not well understood. It is also clear that apoE4 is not necessary or sufficient to cause the disease, indicating that other risk and protecting factors exist. ApoE is upregulated in response to nervous system injury, suggesting that it could have a neuroprotective role; on the other hand, there is evidence indicating that apoE is neurotoxic when present at high levels. Thus, apoE levels seem to be relevant for the functionality of the protein. The APOE proximal promoter hosts numerous regulatory elements, raising the possibility that polymorphisms in this region could produce variation in apoE levels by altering APOE transcriptional activity, which could finally result in AD susceptibility. We will review here the current evidence on the relationship between APOE proximal promoter polymorphisms, APOE gene transcriptional activity and apoE protein levels, and risk for AD.