Lipoproteins, neurobiology, and Alzheimer's disease: structure and function of apolipoprotein E

APOE and Alzheimer's Disease: Evidence Mounts that Targeting APOE4 may Combat Alzheimer's Pathogenesis

Alzheimer's disease (AD) is an immutable neurodegenerative disease featured by the two hallmark brain pathologies that are the extracellular amyloid ß (Aß) and intraneuronal tau protein. People carrying the APOE4 allele are at high risk of AD concerning the ones carrying the ε3 allele, while the ε2 allele abates risk. ApoE isoforms exert a central role in controlling the transport of brain lipid, neuronal signaling, mitochondrial function, glucose metabolism, and neuroinflammation. Regardless of widespread indispensable studies, the appropriate function of APOE in AD etiology stays ambiguous. Existing proof recommends that the disparate outcomes of ApoE isoforms on Aβ accretion and clearance have a distinct function in AD pathogenesis. ApoE-lipoproteins combine diverse cell-surface receptors to transport lipids and moreover to lipophilic Aβ peptide, that is believed to begin deadly events that generate neurodegeneration in the AD. ApoE has great influence in tau pathogenesis, tau-mediated neurodegeneration, and neuroinflammation, as well as α-synucleinopathy, lipid metabolism, and synaptic plasticity despite the presence of Aβ pathology. ApoE4 shows the deleterious effect for AD while the lack of ApoE4 is defensive. Therapeutic strategies primarily depend on APOE suggest to lessen the noxious effects of ApoE4 and reestablish the protective aptitudes of ApoE. This appraisal represents the critical interactions of APOE and AD pathology, existing facts on ApoE levels in the central nervous system (CNS), and the credible active stratagems for AD therapy by aiming ApoE. This review also highlighted utmost ApoE targeting therapeutic tactics that are crucial for controlling Alzheimer's pathogenesis.

Cysteamine revisited: repair of arginine to cysteine mutations

Cysteamine is a small aminothiol endogenously derived from coenzyme A degradation. For some decades, synthetic cysteamine has been employed for the treatment of cystinosis, and new uses of the drug continue to emerge. In this review, we discuss the role of cysteamine in cellular and extracellular homeostasis and focus on the potential use of aminothiols to reconstitute the function of proteins harboring arginine (Arg) to cysteine (Cys) mutations, via repair of the Cys residue into a moiety that introduces an amino group, as seen in basic amino acid residues Lys and Arg. Cysteamine has been utilized in vitro and ex vivo in four different genetic disorders, and thus provides "proof of principle" that aminothiols can modify Cys residues. Other aminothiols such as mercaptoethylguanidine (MEG) with closer structural resemblance to the guanidinium moiety of Arg are under examination for their predicted enhanced capacity to reconstitute loss of function. Although the use of aminothiols holds clinical potential, more studies are required to refine specificity and treatment design. The efficacy of aminothiols to target proteins may vary substantially depending on their specific extracellular and intracellular locations. Redox potential, pH, and specific aminothiol abundance in each physiological compartment are expected to influence the reactivity and turnover of cysteamine and analogous drugs. Upcoming research will require the use of suitable cell and animal models featuring Arg to Cys mutations. Since, in general, Arg to Cys changes comprise about 8% of missense mutations, repair of this specific mutation may provide promising avenues for many genetic diseases.

Molecular neuropathogenesis of Alzheimer's disease: an interaction model stressing the central role of oxidative stress

Alzheimer's disease (AD) exhibits a complex etiology that simultaneously manifests as a complex cellular, neurobiological, molecular, anatomic-physiological and clinical entity. Other significant psychiatric conditions, such as depression and schizophrenia, may also present with complex and concurrent clinical and/or molecular phenotypes. These neuropsychiatric pathologies also originate from both environmental and genetic factors. We analyzed the molecular phenotypes of AD and discuss them with respect to the classical theories, which we integrated into mechanisms that share molecular and/or anatomical connections. Based on these mechanisms, we propose an interaction model and discuss the model in light of studies that refute or support it. Given the spectrum of AD phenotypes, we limit the scope of our discussion to a few, which facilitates concrete analysis. In addition, the study of specific, individual pathogenic phenotypes may be critical to defining the complex mechanisms leading to AD, thereby improving strategies for developing novel therapies.

The effect of acetyl-L-carnitine and R-alpha-lipoic acid treatment in ApoE4 mouse as a model of human Alzheimer's disease

We measured age-dependent effects of human ApoE4 on cerebral blood flow (CBF) using ApoE4 transgenic mice compared to age-matched wild-type (WT) mice by use of [(14)C] iodoantipyrene autoradiography. ApoE4 associated factors reduce CBF gradually to create brain hypoperfusion when compared to WT, and the differences in CBF are greatest as animals age from 6-weeks to 12-months. Transmission electron microscopy with colloidal gold immunocytochemistry showed structural damage in young and aged microvessel endothelium of ApoE4 animals extended to the cytoplasm of perivascular cells, perivascular nerve terminals and hippocampal neurons and glial cells. These abnormalities coexist with mitochondrial structural alteration and mitochondrial DNA overproliferation and/or deletion in all brain cellular compartments. Spatial memory and temporal memory tests showed a trend in improving cognitive function in ApoE4 mice fed selective mitochondrial antioxidants acetyl-l-carnitine and R-alpha-lipoic acid. Our findings indicate that ApoE4 genotype-induced mitochondrial changes and associated structural damage may explain age-dependent pathology seen in AD, indicating potential for novel treatment strategies in the near future.

Insight on the molecular envelope of lipid-bound apolipoprotein E from electron paramagnetic resonance spectroscopy

Although a high-resolution X-ray structure for the N-terminal domain of apolipoprotein E (apoE) in the lipid-free state has been solved, our knowledge of the structure of full-length apoE in a lipid-bound state is limited to an X-ray model fitting a molecular envelope at 10-A resolution. To add molecular detail to the molecular envelope, we used cysteine mutagenesis to incorporate spin labels for analysis with electron paramagnetic resonance (EPR) spectroscopy. Twelve cysteine residues were introduced singly and in pairs at unique locations throughout apoE4 and labeled with an EPR spin probe. The labeled apoE4 was combined with dipalmitoylphosphatidylcholine, the particles were purified, and spectra were determined for 24 combinations (single and double) of the cysteine mutants. Data on the conformation, mobility, distance, and surface exposure of regions revealed by the cysteine probes were modeled into the molecular envelope of apoE bound to dipalmitoylphosphatidylcholine that had been determined by X-ray analysis. This EPR model of apoE in a native lipid-bound state validates the structural model derived from X-ray analysis and provides additional insight into apoE structure-function relationships.

Apolipoprotein E and cholesterol in aging and disease in the brain

Cholesterol can be detrimental or vital, and must be present in the right place at the right time and in the right amount. This is well known in the heart and the vascular system. However, in the CNS cholesterol is still an enigma, although several of its fundamental functions in the brain have been identified. Brain cholesterol has attracted additional attention owing to its close connection to ApoE, a key polymorphic transporter of extracellular cholesterol in humans. Indeed, both cholesterol and ApoE are so critical to fundamental activities of the brain, that the brain regulates their synthesis autonomously. Yet, similar control mechanisms of ApoE and cholesterol homeostasis may exist on either sides of the blood-brain barrier. One indication is that the APOE ε4 allele is associated with hypercholesterolemia and a proatherogenic profile on the vascular side and with increased risk of Alzheimer's disease on the CNS side. In this review, we draw attention to the association between cholesterol and ApoE in the aging and diseased brain, and to the behavior of the ApoE4 protein at the molecular level. The attempt to correlate in vivo and in vitro observations is challenging but crucial for developing future strategies to address ApoE-related aberrations in cholesterol metabolism selectively in the brain.

AR, apoE, and cognitive function

Reduced androgen levels in aged men and women might be risk factors for age-related cognitive decline and Alzheimer's disease (AD). Ongoing clinical trials are designed to evaluate the potential benefit of estrogen in women and of testosterone in men. In this review, we discuss the potential beneficial effects of androgens and androgen receptors (ARs) in males and females. In addition, we discuss the hypothesis that AR interacts with apolipoprotein (apoE)4, encoded by epsilon4 and a risk factor for age-related cognitive decline and AD, and the potential consequences of this interaction.

Fluorescence resonance energy transfer analysis of apolipoprotein E C-terminal domain and amyloid β peptide (1-42) interaction

The potential neurotoxicity of soluble forms of amyloid beta peptide (Abeta) as a key factor in early pathogenesis of Alzheimer's disease is being recognized. In addition, there is growing evidence of the essential role of apolipoprotein E (apoE) in amyloid formation, although molecular details of apoE/Abeta interaction are poorly understood. We employed apoE C-terminal (CT) domain comprising residues 201-299 to identify binding location of Abeta(1-42) by fluorescence resonance energy transfer (FRET) and quenching analyses. Native tryptophan (Trp) residues in the apoE CT domain served as FRET donor, whereas N-(iodoacetyl)-N'-(5-sulfo-1-naphthyl)ethylenediamine (AEDANS) covalently attached to a unique cysteine residue substituted at position 4 of Abeta(1-42) (AEDANS-F4C-Abeta(1-42)) served as FRET acceptor. Fluorescence analysis verified that the oligomerization behavior of AEDANS-F4C-Abeta(1-42) was not abrogated by covalent attachment of AEDANS and that apoE CT domain/AEDANS-F4C-Abeta(1-42) association results in formation of a soluble complex. A large decrease in Trp fluorescence emission was noted in mixtures containing apoE CT domain and AEDANS-F4C-Abeta(1-42), accompanied by appearance of sensitized fluorescence emission of AEDANS as a result of intermolecular FRET. An average distance of separation of 22.6 Angstroms between donors and acceptor was calculated. Fluorescence quenching by potassium iodide (KI) did not reveal significant differences in apoE CT domain Trp microenvironment in the absence or the presence of Abeta(1-42). A twofold increase in quenching constant was noted for KI quenching of AEDANS fluorescence emission in the presence of apoE CT domain, indicative of alterations in Abeta conformation upon interaction with apoE CT domain. We propose intermolecular FRET analysis as a discriminating approach to examine apoE/Abeta interaction, a potentially critical factor in early events involved in amyloid formation.

Lipid binding ability of human apolipoprotein E N-terminal domain isoforms: correlation with protein stability?

Human apolipoprotein (apo) E exists as one of three major isoforms, E2, E3 or E4. Individuals carrying the epsilon 4 allele have an increased risk of heart disease and premature onset of Alzheimer's disease. To investigate the molecular basis for this phenomenon, the N-terminal domain of apoE3, apoE2 and apoE4 were expressed in bacteria, isolated and employed in lipid binding and stability studies. Far UV circular dichroism spectroscopy in buffer at pH 7 revealed a similar amount of alpha-helix secondary structure for the three isoforms. By contrast, differences were noted in apoE-NT isoform-specific transformation of bilayer vesicles of dimyristoylphosphatidylglycerol (DMPG) into discoidal complexes. ApoE4-NT induced transformation was most rapid, followed by apoE3-NT and apoE2-NT. To determine if differences in the rate of apoE-NT induced DMPG vesicle transformation is due to isoform-specific differences in helix bundle stability, guanidine HCl denaturation studies were conducted. The results revealed that apoE2-NT was the most stable, followed by apoE3-NT and apoE4-NT, establishing an inverse correlation between helix bundle stability and DMPG vesicle transformation rate at pH 7. When the zwitterionic dimyristoylphosphatidylcholine (DMPC) was employed as the model lipid surface, interaction of apoE-NT isoforms with the lipid substrate was slow. However, upon lowering the pH from 7 to 3, a dramatic increase in the rate of DMPC vesicle transformation rate was observed for each isoform. To evaluate if the increased DMPC vesicle transformation rates observed at low pH is due to pH-dependent alterations in helix bundle stability, guanidine HCl denaturation studies were performed. ApoE2-NT and apoE3-NT displayed increased resistance to denaturation as a function of decreasing pH, while apoE4-NT showed no change in stability. Studies with the fluorescent probe, 8-anilino-1-naphthalene sulfonic acid, indicated an increase in apoE hydrophobic surface exposure upon decreasing the pH to 3.0. Taken together, the data indicate that changes in the stability of secondary structure elements in apoE-NT isoforms are not responsible for pH-induced increases in lipid binding activity. It is likely that pH-induced disruption of inter-helical tertiary contacts may promote helix bundle conformational changes that present the hydrophobic interior of the protein to potential lipid surface binding sites.

Apolipoprotein E: far more than a lipid transport protein

First recognized as a major determinant in lipoprotein metabolism and cardiovascular disease, apolipoprotein (apo) E has emerged as an important molecule in several biological processes not directly related to its lipid transport function, including Alzheimer's disease and cognitive function, immunoregulation, and possibly even infectious diseases. ApoE is a polymorphic protein arising from three alleles at a single gene locus. The three major isoforms, apoE4, apoE3, and apoE2, differ from one another only by single amino acid substitutions, yet these changes have profound functional consequences at both the cellular and molecular levels. ApoE3 seems to be the normal isoform in all known functions, while apoE4 and apoE2 can each be dysfunctional. Isoform (allele)-specific effects include the association of apoE2 with the genetic disorder type III hyperlipoproteinemia and with both increased and decreased risk for atherosclerosis and the association of apoE4 with increased risk for both atherosclerosis and Alzheimer's disease, impaired cognitive function, and reduced neurite outgrowth; isoform-specific differences in cellular signaling events may also exist. Functional differences in the apoE isoforms that affect (or did affect) survival before the reproductive years probably account, at least in part, for the allele frequencies of the present day.

Apolipoprotein-E gene polymorphism and lipid profiles in Alzheimer's disease

In this study, the relationship between lipid profiles of sera and apolipoprotein E (apo E) gene polymorphism was investigated in 35 patients with Alzheimer's disease (AD) and 29 healthy people. Apo E genotypes and allele frequencies of the AD patient group were: apo E2/3, 2 (5.7 percent); apo E2/4, 1 (2.9 percent); apo E3/3, 26 (74.3 percent); apo E3/4, 5 (14.3 percent); apo E4/4, 1 (2.9 percent); epsilon 2, 3(4.2 percent); epsilon 3, 59 (84.2 percent); epsilon 4, 8 (11.4 percent). The healthy group's apo E genotypes and allele frequencies were: apo E2/3, 1 (3.4 percent); apo E3/3, 27 (93.1 percent); apo E3/4, 1 (3.4 percent); epsilon 2, 1 (1.7 percent); epsilon 3, 56 (96.5 percent); epsilon 4, 1 (1.7 percent). In Alzheimer's cases, epsilon 4 allele frequencies increased significantly as compared to the healthy group (p < 0.05). When the effects of the apo E isoforms on lipid profiles were evaluated, a relationship between apo E epsilon 4 allele and high total levels of serum cholesterol was found, whereas of apo E epsilon 2 allele was associated with the low total cholesterol of serum, although the difference was not statistically significant (p > 0.05). This study confirms the association of apo E epsilon 4 allele with lipid profiles in AD patients.

Expression of the apolipoprotein E gene in the skin is controlled by a unique downstream enhancer

A distal enhancer that specifies apolipoprotein E gene expression in the skin was identified and characterized by in situ hybridization in transgenic mice generated with constructs of the human apolipoprotein E/C-I/C-IV/C-II gene cluster. Transgene constructs containing the enhancer expressed high levels of apolipoprotein E mRNA in the germinative cell layer of the sebaceous gland and in epithelial cells of the hair follicle root sheath. Apolipoprotein E mRNA was also detected in basal epithelial cells of the epidermis. Expression of the human apolipoprotein E transgene at these sites was specified by a unique 1.0 kb enhancer domain located 1.7 kb downstream of the apolipoprotein E gene. No transgene expression was detected in skin epithelial cells in transgenic mice when this enhancer was deleted from the apolipoprotein E gene cluster. The enhancer was used to construct a transgene expression vector that faithfully directed a heterologous cDNA to the normal sites of apolipoprotein E gene expression in epithelial cells of the skin.

The lipid-associated conformation of the low density lipoprotein receptor binding domain of human apolipoprotein E

Apolipoprotein E (apoE) is a 34-kDa exchangeable apolipoprotein that regulates metabolism of plasma lipoproteins by functioning as a ligand for members of the LDL receptor family. The receptor-binding region localizes to the vicinity of residues 130-150 within its independently folded 22-kDa N-terminal domain. In the absence of lipid, this domain exists as a receptor-inactive, globular four-helix bundle. Receptor recognition properties of this domain are manifest upon lipid association, which is accompanied by a conformational change in the protein. Fluorescence resonance energy transfer has been used to monitor helix repositioning, which accompanies lipid association of the apoE N-terminal domain. Site-directed mutagenesis was used to replace naturally occurring Trp residues with phenylalanine, creating a Trp-null apoE3 N-terminal domain (residues 1-183). Subsequently, tyrosine residues in helix 2, helix 3, or helix 4 were converted to Trp, generating single Trp mutant proteins. The lone cysteine at position 112 was covalently modified with N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine, which serves as an energy acceptor from excited tryptophan residues. Fluorescence resonance energy transfer analysis of apoE N-terminal domain variants in phospholipid disc complexes suggests that the helix bundle opens to adopt a partially extended conformation. A model is presented that depicts a tandem arrangement of the receptor-binding region of the protein in the disc complex, corresponding to its low density lipoprotein receptor-active conformation.

Duplicated downstream enhancers control expression of the human apolipoprotein E gene in macrophages and adipose tissue

Two distal enhancers that specify apolipoprotein (apo) E gene expression in isolated macrophages and adipose tissue were identified in transgenic mice that were generated with constructs of the human apoE/C-I/C-I'/C-IV/C-II gene cluster. One of these enhancers, multienhancer 1, consists of a 620-nucleotide sequence located 3.3 kilobases (kb) downstream of the apoE gene. The second enhancer, multienhancer 2, is a 619-nucleotide sequence located 15.9 kb downstream of the apoE gene and 5.9 kb downstream of the apoC-I gene. The two enhancers are 95% identical in sequence, and they are likely to have arisen as a consequence of the gene duplication event that yielded the apoC-I gene and the apoC-I' pseudogene. Both enhancer sequences appear to have equivalent activity in directing apoE gene expression in peritoneal macrophages and in adipocytes, suggesting that their activity in specific cell types may be determined by common regulatory elements.

A novel signaling molecule for neuropeptide action: activity-dependent neuroprotective protein

The complete coding sequence of a novel protein (828 amino acids, pI 5.99), a potential new mediator of vasoactive intestinal peptide (VIP) activity was recently revealed. The expression of this molecule, activity-dependent neuroprotective protein (ADNP), was augmented in the presence of VIP, in cerebral cortical astrocytes. The mRNA transcripts encoding ADNP were enriched in the mouse hippocampus and cerebellum. The protein deduced sequence contained the following: (1) a unique peptide, NAPVSIPQ, sharing structural and immunological homologies with the previously reported, activity-dependent neurotrophic factor (ADNF) and exhibiting neuroprotection in vitro and in vivo; (2) a glutaredoxin active site; and (3) a classical zinc binding domain. Comparative studies suggested that the peptide, NAPVSIPQ (NAP), was more efficacious than peptides derived from ADNF. ADNP, a potential mediator of VIP-associated neuronal survival, and the new peptide, a potential lead compound for drug design, are discussed below.

Kinetics of apolipoprotein E isoforms-binding to the major glycosaminoglycans of the extracellular matrix

Apolipoprotein E (apoE), a key lipid transport protein, displays a heparin-binding property that is critical in several apoE functions. The kinetics of the interaction between apoE isoforms and glycosaminoglycans (GAGs) were studied using surface plasmon resonance. The dissociation constant of equilibrium K(D) for apoE3-heparin interaction was estimated to be 12 nM for apoE3 and three common apoE isoforms revealed similar affinities for heparin. ApoE binds to GAGs in the following order: heparin>heparan sulfate>dermatan sulfate>chondroitin sulfate. The affinity parameter of the binding of low molecular weight heparins to apoE is correlated with the chain length. The effective number Z of electrostatic interactions between plasma apoE3 and heparin was assessed to be three. Metal chelators were able to diminish apoE-binding to heparin, suggesting some stabilizing effect of metal ions while reconstitution with lipids did not affect binding affinities for heparin, suggesting that the N-terminal heparin-binding site is responsible for apoE-containing lipoprotein interactions with heparin.

Apolipoprotein E: from atherosclerosis to Alzheimer's disease and beyond

Apolipoprotein E is a key regulator of plasma lipid levels. Our appreciation of its role continues to expand as additional aspects of its function are discovered. Apolipoprotein E affects the levels of all lipoproteins, either directly or indirectly by modulating their receptor-mediated clearance or lipolytic processing and the production of hepatic very low density lipoproteins. Furthermore, it plays a critical role in neurobiology. The apolipoprotein E4 allele is the major susceptibility gene related to the occurrence and early age of onset of Alzheimer's disease. It is probable that one of the major functions of apolipoprotein E in the central nervous system is to mediate neuronal repair, remodeling, and protection, with apolipoprotein E4 being less effective than the E3 and E2 alleles. The isoform-specific effects of apolipoprotein E are currently being unraveled through detailed structure and function studies of this protein.

Complete sequence of a novel protein containing a femtomolar-activity-dependent neuroprotective peptide

The vulnerability of neurons and the irreversibility of loss make discoveries of neuroprotective compounds fundamentally important. Here, the complete coding sequence of a novel protein (828 amino acids, pI 5.99), derived from mouse neuroglial cells, is revealed. The sequence contained (1) a neuroprotective peptide, NAPVSIPQ, sharing structural and immunological homologies with the previously reported, activity-dependent neurotrophic factor; (2) a glutaredoxin active site; and (3) a zinc binding domain. Gene expression was enriched in the mouse hippocampus and cerebellum and augmented in the presence of the neuropeptide vasoactive intestinal peptide, in cerebral cortical astrocytes. In mixed neuron-astrocyte cultures, NAPVSIPQ provided neuroprotection at subfemtomolar concentrations against toxicity associated with tetrodotoxin (electrical blockade), the beta-amyloid peptide (the Alzheimer's disease neurotoxin), N-methyl-D-aspartate (excitotoxicity), and the human immunodeficiency virus envelope protein. Daily NAPVSIPQ injections to newborn apolipoprotein E-deficient mice accelerated the acquisition of developmental reflexes and prevented short-term memory deficits. Comparative studies suggested that NAPVSIPQ was more efficacious than other neuroprotective peptides in the apolipoprotein E-deficiency model. A potential basis for rational drug design against neurodegeneration is suggested with NAPVSIPQ as a lead compound. The relative enrichment of the novel mRNA transcripts in the brain and the increases found in the presence of vasoactive intestinal peptide, an established neuroprotective substance, imply a role for the cloned protein in neuronal function.

Alzheimer's disease: genetic studies and transgenic models

Recent advances in a variety of areas of research, particularly in genetics and in transgenic (Tg)/gene targeting approaches, have had a substantial impact on our understanding of Alzheimer's disease (AD) and related disorders. After briefly reviewing the progress that has been made in diagnostic assessments of patients with senile dementia and in investigations of the neuropathology of AD, we discuss some of the genes/proteins that are causative or risk factors for this disease, including those encoding amyloid precursor protein, presenilin 1 and 2, and apolipoprotein E. In addition, we comment on several potential new candidate loci/genes. Subsequently, we review selected recent reports of analyses of a variety of lines of Tg mice that show several neuropathological features of AD, including A beta-amyloid deposits and dystrophic neurites. Finally, we discuss the several important issues in future investigations of Tg mice, with particular emphasis on the influences of genetic strains on phenotype, especially behavior, and strategies for making new models of neurodegenerative disorders. We believe that investigations of these Tg models will (a) enhance understanding of the relationships between impaired performance on memory tasks and the pathological/biochemical abnormalities in brain, (b) help to clarify pathogenic mechanisms in vivo, (c) lead to identification of new therapeutic targets, and (d) allow testing of new treatment strategies first in mice and then, if successful, in humans with AD.

Transgenic animals relevant to Alzheimer's disease

This article reviews the functional studies that have been carried out on transgenic and knockout animals that are relevant to Alzheimer's disease (AD). The discussion focuses upon the functional characterisation of these strains, particularly upon factors that affect synaptic processes that are thought to contribute to memory formation, including hippocampal long-term potentiation. We examine the use of transgenes associated with amyloid precursor protein and presenilin-1, their mutations linked to early onset familial AD, and the recent attempts to establish double transgenic strains that have an AD-like pathology which occurs with a more rapid onset. The development of new transgenic strains relevant to Alzheimer's disease has rapidly outpaced their characterisation for functional deficits in synaptic plasticity. To date most studies have focused on those transgenes linked to the minority of familial early onset rather than late-onset sporadic AD cases, and have focused on those changes linked to the induction of the early-phase of hippocampal long-term potentiation. Future studies will need to address the question of whether the development of AD pathology can be reversed or at least halted and this will be aided by the use of conditional transgenics in which genes linked to AD can either be switched on or off later in development. Furthermore, it remains to be resolved whether the deficits in synaptic function are specific to the hippocampus and whether deficits affect late-phase long-term potentiation. Nonetheless, the recent advances in genome sciences and the development of transgenic technology have provided a unique opportunity to study how genes associated with human cognitive dysfunction alter synaptic transmission between neurones in the mammalian brain.

Genetic neurodegenerative diseases: the human illness and transgenic models

Review The neurodegenerative disorders, a heterogeneous group of chronic progressive diseases, are among the most puzzling and devastating illnesses in medicine. Some of these disorders, such as Alzheimer's disease, amyotrophic lateral sclerosis, the prion diseases, and Parkinson's disease, can occur sporadically and, in some instances, are caused by inheritance of gene mutations. Huntington's disease is acquired in an entirely genetic manner. Transgenic mice that express disease-causing genes recapitulate many features of these diseases. This review provides an overview of transgenic mouse models of familial amyotrophic lateral sclerosis, familial Alzheimer's disease, and Huntington's disease and the emerging insights relevant to the underlying molecular mechanisms of these diseases.

Developmental regulation of LR11 expression in murine brain

Receptors belonging to the low density lipoprotein receptor (LDLR) superfamily play important biological roles in addition to mediating lipoprotein metabolism. The recent discovery of a novel mosaic LDLR family member by us (Yamazaki H., Bujo, H., Kusunoki, J., Seimiya, K., Kanaki, T., Morisaki, N., Schneider, W.J., and Saito, Y. (1996) J. Biol. Chem. 271, 24761-24768) and others, which we termed LR11, offers the opportunity to gain new insights into receptor multifunctionality. The predominant expression of LR11 in brain and the presence of elements found in neural adhesion molecules suggested a function(s) in the central nervous system (CNS). In order to gain information about this complex receptor in an accessible system, we have molecularly characterized the murine LR11 and report on its detailed localization and developmental expression pattern. The primary sequence of the murine protein further establishes that LRlls are among the closest relatives within the LDLR family and that brain is the predominant site of expression. In situ hybridization showed that neuronal bodies such as Purkinje cells in the cerebellum and other neurons in the hippocampal formations and the cerebral cortex are particularly rich in LR11 transcripts. The developmental pattern of LR11 expression in brain, which peaks at 2 weeks, is in contrast to those of two other LDLR family members, the very low density lipoprotein receptor and the LDLR. During early development, murine LR11 expression levels are highly dependent on neural cell types. These findings are compatible with function(s) of LR11 in neural organization and, possibly, pathogenesis of degenerative brain diseases. In addition, detailed knowledge of LR11 biology will help to elucidate the roles of other mosaic proteins that share with LR11 elements whose function is not yet known.

Apolipoprotein E and multiple sclerosis: a biochemical and genetic investigation

Apolipoprotein E (apo E) is postulated to be a major lipid carrier protein in the brain involved in brain development and repair. Multiple sclerosis (MS) is a major demyelinating disease characterized by destruction of myelin and marked alteration of myelin cholesterol and lipid metabolism. We have determined serum and cerebrospinal fluid (CSF) apo E concentrations using an original time-resolved immunofluorometric assay and calculated intrathecal apo E concentration. Apo E concentrations were determined in 13 control subjects and 129 neurological patients: 34 definite MS patients, 25 with Guillain-Barré syndrome (GBS), 32 with amyotrophic lateral sclerosis (ALS) and 38 with other neurological diseases. Seven clinical parameters (sex, age, age at MS onset, duration of the disease, course, clinical status and disability score) were considered in MS patients. Significant (P < 0.01) decrease in CSF apo E was observed in MS, linked to a decrease in intrathecal apo E. The decreased CSF apo E concentration in MS patients occur independent of the apo E genotype. Apo E is considered as a neurotrophic factor in the brain. Any decrease in intrathecal apo E synthesis would thus contribute to progression of neurological diseases, such as MS.

Synergy between the genes for butyrylcholinesterase K variant and apolipoprotein E4 in late-onset confirmed Alzheimer's disease

The allelic frequency of the gene for the K variant of butyrylcholinesterase (BCHE-K) was 0.17 in 74 subjects with late-onset (age > 65 years) histopathologically diagnosed Alzheimer's disease (AD), which was higher than the frequencies in 104 elderly control subjects (0.09), in 14 early-onset cases of confirmed AD (0.07) and in 29 confirmed cases of other dementia (0.10). The association of BCHE-K with late-onset AD was limited to carriers of the epsilon 4 allele of the apolipoprotein E gene (APOE), among whom the presence of BCHE-K gave an odds ratio of confirmed late-onset AD of 6.9 (95% C.I. 1.65-29) in subjects > 65 years and of 12.8 (1.9-86) in subjects > 75 years. In APOE epsilon 4 carriers over 75 years, only 1/22 controls, compared with 10/24 confirmed late-onset AD cases, had BCHE-K. We suggest that BCHE-K, or a nearby gene on chromosome 3, acts in synergy with APOE epsilon 4 as a susceptibility gene for late-onset AD.

Animal models of cerebral beta-amyloid angiopathy

Cerebral amyloid angiopathy (CAA) is a significant risk factor for hemorrhagic stroke in the elderly, and occurs as a sporadic disorder, as a frequent component of Alzheimer's disease, and in several rare, hereditary conditions. The most common type of amyloid found in the vasculature of the brain is beta-amyloid (A beta), the same peptide that occurs in senile plaques. A paucity of animal models has hindered the experimental analysis of CAA. Several transgenic mouse models of cerebral beta-amyloidosis have now been reported, but only one appears to develop significant cerebrovascular amyloid. However, well-characterized models of naturally occurring CAA, particularly aged dogs and non-human primates, have contributed unique insights into the biology of vascular amyloid in recent years. Some non-human primate species have a predilection for developing CAA; the squirrel monkey (Saimiri sciureus), for example, is particularly likely to manifest beta-amyloid deposition in the cerebral blood vessels with age, whereas the rhesus monkey (Macaca mulatta) develops more abundant parenchymal amyloid. These animals have been used to test in vivo beta-amyloid labeling strategies with monoclonal antibodies and radiolabeled A beta. Species-differences in the predominant site of A beta deposition also can be exploited to evaluate factors that direct amyloid selectively to a particular tissue compartment of the brain. For example, the cysteine protease inhibitor, cystatin C, in squirrel monkeys has an amino acid substitution that is similar to the mutant substitution found in some humans with a hereditary form of cystatin C amyloid angiopathy, possibly explaining the predisposition of squirrel monkeys to CAA. The existing animal models have shown considerable utility in deciphering the pathobiology of CAA, and in testing strategies that could be used to diagnose and treat this disorder in humans.

Protection against developmental retardation in apolipoprotein E-deficient mice by a fatty neuropeptide: implications for early treatment of Alzheimer's disease

Stearyl-Nle17-VIP (SNV) is a novel agonist of vasoactive intestinal peptide (VIP) exhibiting a 100-fold greater potency than the parent molecule and specificity for a receptor associated with neuronal survival. Here, mice deficient in apolipoprotein E (ApoE), a molecule associated with the etiology of Alzheimer's disease, served as a model to investigate the developmental and protective effects of SNV. In comparison to control animals, the deficient mice exhibited (a) reduced amounts of VIP messenger RNA; (b) decreased cholinergic activity (c) significant retardation in the acquisition of developmental milestones: forelimb placing behavior and cliff avoidance behavior; and (d) learning and memory impairments. Daily injections of SNV to ApoE-deficient newborn pups resulted in increased cholinergic activity and marked improvements in the time of acquisition of behavioral milestones, with peptide-treated animals developing as fast as control animals and exhibiting improved cognitive functions after cessation of peptide treatment. Specificity was demonstrated in that treatment with a related peptide (PACAP), pituitary adenylate cyclase-activating peptide, produced only limited amelioration. As certain genotypes of ApoE increase the probability of Alzheimer's disease, early counseling and preventive treatments may now offer an important route for therapeutics design.

Human apolipoprotein E4 domain interaction. Arginine 61 and glutamic acid 255 interact to direct the preference for very low density lipoproteins

Human apolipoprotein (apo) E contains an amino- and a carboxyl-terminal domain, which are connected by a hinge region (approximately residues 165 to 215). The interaction of the two domains has been suggested to be responsible for the apoE4-binding preference for very low density lipoproteins (VLDL). In the absence of this interaction in apoE3, the preference is for high density lipoproteins (HDL). To exclude the possibility that the interaction of apoE with other apolipoproteins on the native particles may contribute to the isoform-specific preferences, VLDL-like emulsion particles were incubated with apoE, and the lipid-bound apoE was separated from free apoE on a Superose 6 column. The apoE4 bound more effectively to these particles than did apoE3, indicating that the apoE4 preference for VLDL is due not to interactions with other apolipoproteins but to an intrinsic property of apoE4, likely related to domain interaction. Previously, arginine 61 was shown to be critical for the isoform preferences, suggesting that it interacted with an acidic residue(s) in the carboxyl terminus. Substitution of arginine 61 with lysine did not alter the preference of apoE4 for VLDL, demonstrating that a positive charge rather than a specific requirement for arginine is critical for domain interaction. To identify the acidic residue(s) in the carboxyl terminus interacting with arginine 61, the six acidic residues (244, 245, 255, 266, 270, and 271) in a region known to be important for both lipoprotein association and isoform-specific preferences were substituted individually with alanine in apoE4. Only substitution of glutamic acid 255 altered the preference of apoE4 from VLDL to HDL, indicating that this was the sole residue in the carboxyl terminus that interacts with arginine 61. The participation of the hinge region in domain interaction was examined with internal deletion mutants. Deletion of the residues 186-202 or 186-223, representing major portions of the hinge region, had no effect on the apoE4 preference for VLDL. This suggests that the hinge region may act as a spacer that connects the two domains. Further deletion into the carboxyl-terminal domain (to residue 244) results in a loss of apoE4 VLDL binding. These studies establish that interaction of arginine 61 and glutamic acid 255 mediates apoE4 domain interaction.

Biological markers for the clinical diagnosis of Alzheimer's disease

There is a compelling need to develop biological marker(s) to confirm a clinical diagnosis of Alzheimer's disease (AD) during life in order to unequivocally identify AD patients for emerging therapeutic interventions. This review describes recent advances in the development of diagnostic marker(s) for AD. They include polymorphism of apolipoprotein E (ApoE) and alpha 1-antichymotrypsin as well as cerebrospinal fluid (CSF) tau and CSF-amyloid beta-protein levels, skin biopsy, and pupil dilatation assay by anti-cholinergic agent. In conclusion, ApoE genotyping should not be used as a sole diagnostic test for AD, and that monitoring of CSF-tau appeared to be most promising and reliable diagnostic aid.

Interaction between human amphipathic apolipoproteins and amyloid beta-peptide: surface plasmon resonance studies

Several apolipoproteins including apoE and apoA-I are known to be associated with amyloid beta-peptide, a major component of senile plaques in Alzheimer's disease. In the present study the interaction between three human amphipathic apolipoproteins apoE3, apoA-I and apoA-II and immobilized amyloid beta-peptide (1-40) was quantified by plasmon resonance. The interactions were saturable and reversible. The results demonstrated a high affinity of the binding of amphipathic apolipoproteins to amyloid beta-peptide. On the other hand, only a small population of synthetic amyloid beta-peptide participated in the interaction. The apparent equilibrium dissociation constants K(D) were 10 nM for apoE3, 25 nM for apoA-I and 80 nM for apoA-II under physiological conditions. The affinity of the apoE3-amyloid beta-peptide binding was not affected by pH in the range 6.0-8.0 but was significantly increased by high salt concentration. ApoA-I mainly followed similar patterns. A major participation of hydrophobic forces in the binding of apoE3 and apoA-I to amyloid beta-peptide was suggested.

Apolipoprotein E. Structure, function, and possible roles in Alzheimer's disease

Apolipoprotein (apo) E is associated with the two characteristic neuropathologic lesions of Alzheimer's disease--extracellular neuritic plaques representing deposits of amyloid beta (A beta) peptide and intracellular neurofibrillary tangles representing filaments of a microtubule-associated protein called tau. Incubation of the apoE4 isoform with the A beta peptide in vitro results in the formation of a dense, stable network of very long monofibrils, while incubation of apoE3 with the A beta peptide results in the formation of a less dense, less stable network. The more complex nature of the plaques formed with the A beta peptide in the presence of apoE4 in vivo may impair the normal clearance process and enhance plaque formation. Alternatively or additionally, apoE may alter the cytoskeletal structure and function and, under certain conditions, may promote the formation of the neurofibrillary tangles. Our studies have demonstrated that apoE3 and apoE4 exert differential effects on neuronal growth (i.e., neurite extension and branching) in vitro. When combined with a source of lipid, apoE3 stimulated neurite extension in peripheral nervous system neurons (dorsal root ganglia), whereas apoE4 inhibited it. Similar results were obtained with central nervous system neurons (murine neuroblastoma Neuro-2a cells). Addition of free apoE3 or apoE4 without beta-VLDL had no effect on neurite outgrowth. There was also differential accumulation of apoE3 and apoE4 by the neuroblastoma cells: apoE3 accumulated within cell bodies and neurites to a greater extent than apoE4. Thus, apoE3 may facilitate cytoskeletal activity, whereas apoE4 may inhibit it, which would be detrimental during synaptic remodeling.

Levels and alternative splicing of amyloid beta protein precursor (APP) transcripts in brains of APP transgenic mice and humans with Alzheimer's disease

Abnormal expression of human amyloid precursor protein (hAPP) gene products may play a critical role in Alzheimer's disease (AD). Recently, a transgenic model was established in which platelet-derived growth factor (PDGF) promoter-driven neuronal expression of an alternatively spliced hAPP minigene resulted in prominent AD-type neuropathology (Games, D., Adams, D., Alessandrini, R., Barbour, R., Berthelette, P., Blackwell, C., Carr, T., Clemens, J., Donaldson, T., Gillespie, F., Guido, T., Hagopian, S., Johnson-Wood, K., Khan, K., Lee, M., Leibowitz, P., Lieberburg, I., Little, S., Masliah, E., McConlogue, L., Montoya-Zavala, M., Mucke, L., Paganini, L., and Penniman, E. (1995) Nature 373, 523-527). Here we compared the levels and alternative splicing of APP transcripts in brain tissue of hAPP transgenic and nontransgenic mice and of humans with and without AD. PDGF-hAPP mice showed severalfold higher levels of total APP mRNA than did nontransgenic mice or humans, whereas their endogenous mouse APP mRNA levels were decreased. This resulted in a high ratio of mRNAs encoding mutated hAPP versus wild-type mouse APP. Modifications of hAPP introns 6, 7, and 8 in the PDGF-hAPP construct resulted in a prominent change in alternative splice site selection with transcripts encoding hAPP770 or hAPP751 being expressed at substantially higher levels than hAPP695 mRNA. Frontal cortex of humans with AD showed a subtle increase in the relative abundance of hAPP751 mRNA compared with normal controls. These data identify specific intron sequences that may contribute to the normal neuronspecific alternative splicing of APP pre-mRNA in vivo and support a causal role of hAPP gene products in the development of AD-type brain alterations.

Stable expression and secretion of apolipoproteins E3 and E4 in mouse neuroblastoma cells produces differential effects on neurite outgrowth

Previously, we demonstrated in cultured dorsal root ganglion neurons that, in the presence of beta-migrating very low density lipoproteins (beta-VLDL), apolipoprotein (apo) E4, but not apoE3, suppresses neurite outgrowth. In the current studies, murine neuroblastoma cells (Neuro-2a) were stably transfected with human apoE3 or apoE4 cDNA, and the effect on neurite outgrowth was examined. The stably transfected cells secreted nanogram quantities of apoE (44-89 ng/mg of cell protein in 48 h). In the absence of lipoproteins, neurite outgrowth was similar in the apoE3- and apoE4-secreting cells. The apoE4-secreting cells, when incubated with beta-VLDL, VLDL, cerebrospinal fluid lipoproteins (d < 1.21 g/ml), or with triglyceride/phospholipid (2.7:1 (w/w)) emulsions, showed a reduction in the number of neurites/cell, a decrease in neurite branching, and an inhibition of neurite extension, whereas in the apoE3-secreting cells in the presence of a lipid source, neurite extension was increased. Uptake of beta-VLDL occurred to a similar extent in both the apoE3- and apoE4-secreting cells. With low density lipoproteins or with dimyristoylphosphatidylcholine emulsions, either alone or complexed with cholesterol, no differential effect on neurite outgrowth was observed. A slight differential effect was observed with apoE-containing high density lipoproteins. The differential effect of apoE3 and apoE4 in the presence of beta-VLDL was blocked by incubation of the cells with heparinase and chlorate, with lactoferrin, or with receptor-associated protein, all of which prevent the uptake of lipoproteins by the low density lipoprotein receptor-related protein (LRP). The data suggest that the secreted and/or cell surface-bound apoE interact with the lipoproteins and facilitate their internalization via the heparan sulfate proteoglycan-LRP pathway. The mechanism by which apoE3 and apoE4 exert differential effects on neurite outgrowth remains speculative. However, the data suggest that apoE4, which has been shown to be associated with late onset familial and sporadic Alzheimer's disease, may inhibit neuronal remodeling and contribute to the progression of the disease.

Apolipoprotein B and E basic amino acid clusters influence low-density lipoprotein association with lipoprotein lipase anchored to the subendothelial matrix

Lipoprotein accumulation in the subendothelial matrix is an important step in atherogenesis. We have previously shown that addition of lipoprotein lipase (LPL) markedly increased binding of apolipoprotein B (apoB)-containing lipoproteins to an endothelial cell-derived matrix, and this enhanced lipoprotein binding was inhibited by apoE. In the present studies we examined the role of various regions of apoB in the binding of LDL to LPL-containing endothelial cell matrix and the ability of various apoE domains to decrease lipoprotein retention. We studied three apoB epitope-specific monoclonal antibodies for their ability to block the binding of 125I-LDL to LPL-containing matrix. Of these, monoclonal antibody 4G3, which recognizes an arginine-containing epitope in apoB, was the most effective in reducing LDL binding. Chemical modification of LDL apoB lysines or arginines markedly reduced the ability of the lipoprotein to block the binding of 125I-LDL to LPL-containing matrix, suggesting that apoB positively charged amino acids are involved in the interaction. Furthermore, polyarginine or polylysine markedly decreased 125I-LDL binding to LPL-containing matrix, whereas polyleucine was ineffective. These data suggest that apoB positively charged regions are important in LDL binding. To explore the role of charge modifications on apoE by single arginine-cysteine interchanges, we examined the effects of the three major human apoE isoforms (apoE2, apoE3, and apoE4). ApoE3 was the most effective in decreasing 125I-LDL retention, followed by apoE4; apoE2 was the least effective. Similarly, apoE2-containing HDL was much less effective than apoE3-containing HDL in decreasing 125I-LDL retention.(ABSTRACT TRUNCATED AT 250 WORDS)

Apolipoprotein E and Alzheimer's disease

The past year has seen widespread confirmation that the epsilon 4 allele of the apolipoprotein E gene is a major risk factor for Alzheimer's disease. The epsilon 4 allele also appears to correlate with life expectancy. This allele has been found to be present in over 50% of Alzheimer patients, regardless of whether or not they have a family history of dementia. It is not yet clear how the epsilon 4 allele mediates its actions; however, recent evidence suggests that apolipoprotein E4 may be responsible for the accelerated formation of beta-pleated amyloid from soluble beta-amyloid peptide, as is seen in the neuritic plaques of Alzheimer patients, as well as interacting with intraneuronal microtubular transport mechanisms.