Female mice with apolipoprotein E4 domain interaction demonstrated impairments in spatial learning and memory performance and disruption of hippocampal cyto-architecture.

We have previously reported cognitive impairments in both young and old mice, particularly in female mice expressing mouse Arg-61 apoE, with a point mutation to mimic the domain interaction feature of human apoE4, as compared to the wildtype mouse (C57BL/6J) apoE. In this study, we further evaluated water maze performance in the female Arg-61 mice at an additional time point and then investigated related hippocampal cyto-architecture in these young female Arg-61 apoE mice vs. the wildtype mice. The results of behavioral performance consistently support our previous report that the young female Arg-61 apoE showed cognitive impairment versus C57BL/6J at the same age. The cyto-architectural results showed that volume of the granular cell layer (GCL) was significantly larger in both 5- and 10-month old Arg-61 apoE mice versus C57BL/6J mice. While the number of newborn calretinin-positive neurons was greater in the sub-granular zone (SGZ) in 5-month old Arg-61 mice, this number dropped significantly in 10-month old Arg-61 mice to a lower level than in age-matched C57BL/6J mice. In addition, the amyloid ß species was significantly higher in 5-month old Arg-61 mice versus age-matched C57BL/6J mice. In conclusion, impaired cognitive functions in female Arg-61 apoE mice appear correlated with larger GCL volume and higher calretinin-positive cell number and suggest a compensatory cellular response that may be related to amyloid beta perturbations early in life. Therefore this study suggests a novel cyto-architectural mechanism of apoE4-dependent pathologies and increased susceptibility of APOEε4 subjects to Alzheimer's disease.

Disease-associated polyglutamine stretches in monomeric huntingtin adopt a compact structure.

Abnormal polyglutamine (polyQ) tracts are the only common feature in nine proteins that each cause a dominant neurodegenerative disorder. In Huntington's disease, tracts longer than 36 glutamines in the protein huntingtin (htt) cause degeneration. In situ, monoclonal antibody 3B5H10 binds to different htt fragments in neurons in proportion to their toxicity. Here, we determined the structure of 3B5H10 Fab to 1.9 Å resolution by X-ray crystallography. Modeling demonstrates that the paratope forms a groove suitable for binding two ß-rich polyQ strands. Using small-angle X-ray scattering, we confirmed that the polyQ epitope recognized by 3B5H10 is a compact two-stranded hairpin within monomeric htt and is abundant in htt fragments unbound to antibody. Thus, disease-associated polyQ stretches preferentially adopt compact conformations. Since 3B5H10 binding predicts degeneration, this compact polyQ structure may be neurotoxic.

Structural basis of transfer between lipoproteins by cholesteryl ester transfer protein.

Human cholesteryl ester transfer protein (CETP) mediates the net transfer of cholesteryl ester mass from atheroprotective high-density lipoproteins to atherogenic low-density lipoproteins by an unknown mechanism. Delineating this mechanism would be an important step toward the rational design of new CETP inhibitors for treating cardiovascular diseases. Using EM, single-particle image processing and molecular dynamics simulation, we discovered that CETP bridges a ternary complex with its N-terminal ß-barrel domain penetrating into high-density lipoproteins and its C-terminal domain interacting with low-density lipoprotein or very-low-density lipoprotein. In our mechanistic model, the CETP lipoprotein-interacting regions, which are highly mobile, form pores that connect to a hydrophobic central cavity, thereby forming a tunnel for transfer of neutral lipids from donor to acceptor lipoproteins. These new insights into CETP transfer provide a molecular basis for analyzing mechanisms for CETP inhibition.

Small molecule structure correctors abolish detrimental effects of apolipoprotein E4 in cultured neurons.

Apolipoprotein E4 (apoE4), the major genetic risk factor for late onset Alzheimer disease, assumes a pathological conformation, intramolecular domain interaction. ApoE4 domain interaction mediates the detrimental effects of apoE4, including decreased mitochondrial cytochrome c oxidase subunit 1 levels, reduced mitochondrial motility, and reduced neurite outgrowth in vitro. Mutant apoE4 (apoE4-R61T) lacks domain interaction, behaves like apoE3, and does not cause detrimental effects. To identify small molecules that inhibit domain interaction (i.e. structure correctors) and reverse the apoE4 detrimental effects, we established a high throughput cell-based FRET primary assay that determines apoE4 domain interaction and secondary cell- and function-based assays. Screening a ChemBridge library with the FRET assay identified CB9032258 (a phthalazinone derivative), which inhibits domain interaction in neuronal cells. In secondary functional assays, CB9032258 restored mitochondrial cytochrome c oxidase subunit 1 levels and rescued impairments of mitochondrial motility and neurite outgrowth in apoE4-expressing neuronal cells. These benefits were apoE4-specific and dose-dependent. Modifying CB9032258 yielded well defined structure-activity relationships and more active compounds with enhanced potencies in the FRET assay (IC(50) of 23 and 116 nm, respectively). These compounds efficiently restored functional activities of apoE4-expressing cells in secondary assays. An EPR binding assay showed that the apoE4 structure correction resulted from direct interaction of a phthalazinone. With these data, a six-feature pharmacophore model was constructed for future drug design. Our results serve as a proof of concept that pharmacological intervention with apoE4 structure correctors negates apoE4 detrimental effects in neuronal cells and could be further developed as an Alzheimer disease therapeutic.

Identifying polyglutamine protein species in situ that best predict neurodegeneration.

Polyglutamine (polyQ) stretches exceeding a threshold length confer a toxic function to proteins that contain them and cause at least nine neurological disorders. The basis for this toxicity threshold is unclear. Although polyQ expansions render proteins prone to aggregate into inclusion bodies, this may be a neuronal coping response to more toxic forms of polyQ. The exact structure of these more toxic forms is unknown. Here we show that the monoclonal antibody 3B5H10 recognizes a species of polyQ protein in situ that strongly predicts neuronal death. The epitope selectively appears among some of the many low-molecular-weight conformational states assumed by expanded polyQ and disappears in higher-molecular-weight aggregated forms, such as inclusion bodies. These results suggest that protein monomers and possibly small oligomers containing expanded polyQ stretches can adopt a conformation that is recognized by 3B5H10 and is toxic or closely related to a toxic species.

Structure-dependent impairment of intracellular apolipoprotein E4 trafficking and its detrimental effects are rescued by small-molecule structure correctors.

Apolipoprotein (apo) E4 is the major genetic risk factor for Alzheimer disease (AD) and likely contributes to neuropathology through various pathways. Here we report that the intracellular trafficking of apoE4 is impaired in Neuro-2a cells and primary neurons, as shown by measuring fluorescence recovery after photobleaching. In Neuro-2a cells, more apoE4 than apoE3 molecules remained immobilized in the endoplasmic reticulum (ER) and the Golgi apparatus, and the lateral motility of apoE4 was significantly lower in the Golgi apparatus (but not in the ER) than that of apoE3. Likewise, the immobile fraction was larger, and the lateral motility was lower for apoE4 than apoE3 in mouse primary hippocampal neurons. ApoE4 with the R61T mutation, which abolishes apoE4 domain interaction, was less immobilized, and its lateral motility was comparable with that of apoE3. The trafficking impairment of apoE4 was also rescued by disrupting domain interaction with the small-molecule structure correctors GIND25 and PH002. PH002 also rescued apoE4-induced impairments of neurite outgrowth in Neuro-2a cells and dendritic spine development in primary neurons. ApoE4 did not affect trafficking of amyloid precursor protein, another AD-related protein, through the secretory pathway. Thus, domain interaction renders more newly synthesized apoE4 molecules immobile and slows their trafficking along the secretory pathway. Correcting the pathological structure of apoE4 by disrupting domain interaction is a potential therapeutic approach to treat or prevent AD related to apoE4.

Apolipoprotein E4 domain interaction mediates detrimental effects on mitochondria and is a potential therapeutic target for Alzheimer disease.

Apolipoprotein (apo) E4 is the major genetic risk factor for late-onset Alzheimer disease (AD). ApoE4 assumes a pathological conformation through an intramolecular interaction mediated by Arg-61 in the amino-terminal domain and Glu-255 in the carboxyl-terminal domain, referred to as apoE4 domain interaction. Because AD is associated with mitochondrial dysfunction, we examined the effect of apoE4 domain interaction on mitochondrial respiratory function. Steady-state amounts of mitochondrial respiratory complexes were examined in neurons cultured from brain cortices of neuron-specific enolase promoter-driven apoE3 (NSE-apoE3) or apoE4 (NSE-apoE4) transgenic mice. All subunits of mitochondrial respiratory complexes assessed were significantly lower in NSE-apoE4 neurons compared with NSE-apoE3 neurons. However, no significant differences in levels of mitochondrial complexes were detected between astrocytes expressing different apoE isoforms driven by the glial fibrillary acidic protein promoter, leading to our conclusion that the effect of apoE4 is neuron specific. In neuroblastoma Neuro-2A (N2A) cells, apoE4 expression reduced the levels of mitochondrial respiratory complexes I, IV, and V. Complex IV enzymatic activity was also decreased, lowering mitochondrial respiratory capacity. Mutant apoE4 (apoE4-Thr-61) lacking domain interaction did not induce mitochondrial dysfunction in N2A cells, indicating that the effect is specific to apoE4-expressing cells and dependent on domain interaction. Consistent with this finding, treatment of apoE4-expressing N2A cells with a small molecule that disrupts apoE4 domain interaction restored mitochondrial respiratory complex IV levels. These results suggest that pharmacological intervention with small molecules that disrupt apoE4 domain interaction is a potential therapeutic approach for apoE4-carrying AD subjects.

Apolipoprotein E expression and purification.

Since the discovery of the association of apolipoprotein E (apoE) 4 with Alzheimer's disease 17 years ago, numerous in vitro experiments with the apoE isoforms (apoE2, apoE3, and apoE4) have been performed to try to understand the basis for this association. The majority of these studies used commercial sources for apoE, but some used recombinant protein. In either case, these studies were most often conducted without considering the ramifications of the structural and biophysical differences among the three isoforms or without adequate quality control of the preparations. Here, we present a protocol for producing recombinant apoE that we have used successfully in our laboratory for the last 20 years. We also review the considerations that are critical for obtaining reliable and interpretable results with the end product.

Morphology and structure of lipoproteins revealed by an optimized negative-staining protocol of electron microscopy.

Plasma lipoprotein levels are predictors of risk for coronary artery disease. Lipoprotein structure-function relationships provide important clues that help identify the role of lipoproteins in cardiovascular disease. The compositional and conformational heterogeneity of lipoproteins are major barriers to the identification of their structures, as discovered using traditional approaches. Although electron microscopy (EM) is an alternative approach, conventional negative staining (NS) produces rouleau artifacts. In a previous study of apolipoprotein (apo)E4-containing reconstituted HDL (rHDL) particles, we optimized the NS method in a way that eliminated rouleaux. Here we report that phosphotungstic acid at high buffer salt concentrations plays a key role in rouleau formation. We also validate our protocol for analyzing the major plasma lipoprotein classes HDL, LDL, IDL, and VLDL, as well as homogeneously prepared apoA-I-containing rHDL. High-contrast EM images revealed morphology and detailed structures of lipoproteins, especially apoA-I-containing rHDL, that are amenable to three-dimensional reconstruction by single-particle analysis and electron tomography.

VLDL lipolysis products increase VLDL fluidity and convert apolipoprotein E4 into a more expanded conformation.

Our previous work indicated that apolipoprotein (apo) E4 assumes a more expanded conformation in the postprandial period. The postprandial state is characterized by increased VLDL lipolysis. In this article, we tested the hypothesis that VLDL lipolysis products increase VLDL particle fluidity, which mediates expansion of apoE4 on the VLDL particle. Plasma from healthy subjects was collected before and after a moderately high-fat meal and incubated with nitroxyl-spin labeled apoE. ApoE conformation was examined by electron paramagnetic resonance spectroscopy using targeted spin probes on cysteines introduced in the N-terminal (S76C) and C-terminal (A241C) domains. Further, we synthesized a novel nitroxyl spin-labeled cholesterol analog, which gave insight into lipoprotein particle fluidity. Our data revealed that the order of lipoprotein fluidity was HDL approximately LDL

An optimized negative-staining protocol of electron microscopy for apoE4 POPC lipoprotein.

Apolipoprotein E (apoE), one of the major protein components of lipoproteins in the peripheral and central nervous systems, regulates cholesterol metabolism through its interaction with members of the low density lipoprotein receptor family. One key to understanding apoE function is determining the structure of lipid-bound forms of apoE. Negative-staining (NS) electron microscopy (EM) is an easy and rapid approach for studying the structure and morphology of lipid-bound forms of apoE. However, an artifact of using the conventional NS protocol is that the apoE phospholipid particles form rouleaux. In this study, we used cryo-electron microscopy (cryo-EM) to examine apoE4 palmitoyl-oleoylphosphatidylcholine (POPC) particles in a frozen-hydrated native state. By comparing the particle sizes and shapes produced by different NS protocols to those produced by cryo-EM, we propose an optimized protocol to examine apoE4 POPC particles. Statistical analysis demonstrated that the particle sizes differ by less than 5% between the optimized protocol and the cryo-EM method, with similar shapes. The high contrast and fine detail of particle images produced using this optimized protocol lend themselves to the structural study of lipid-bound forms of apoE.

Understanding the basis for the association of apoE4 with Alzheimer's disease: opening the door for therapeutic approaches.

Apolipoprotein E4 (apoE4) is the major genetic risk factor for Alzheimer's disease (AD) by an as yet to be defined mechanism. Since the structure or biophysical properties of a protein directly determines function, our approach to addressing mechanism is structure:function based. Domain interaction a structural property of apoE4 that distinguishes it from apoE3 is predicted to contribute to the association of apoE4 with AD. We developed a mouse model, the Arg-61 apoE model, which is specific for domain interaction. These mice display synaptic, functional, and cognitive deficits, demonstrating domain interaction is the causative factor. We present evidence that domain interaction results in stressed astrocytes that are dysfunctional and propose that dysfunctional astrocytes are an early player in apoE4-associated AD and that domain interaction is a potential therapeutic target.

Apolipoprotein E4 domain interaction induces endoplasmic reticulum stress and impairs astrocyte function.

Domain interaction, a structural property of apolipoprotein E4 (apoE4), is predicted to contribute to the association of apoE4 with Alzheimer disease. Arg-61 apoE mice, a gene-targeted mouse model specific for domain interaction, have lower brain apoE levels and synaptic, functional, and cognitive deficits. We hypothesized that domain interaction elicits an endoplasmic reticulum (ER) stress in astrocytes and an unfolded protein response that targets Arg-61 apoE for degradation. Primary Arg-61 apoE astrocytes had less intracellular apoE than wild-type astrocytes, and unfolded protein response markers OASIS (old astrocyte specifically induced substance), ATF4, and XBP-1 and downstream effectors were up-regulated. ER stress appears to cause global astrocyte dysfunction as glucose uptake was decreased in Arg-61 apoE astrocytes, and astrocyte-conditioned medium promoted neurite outgrowth less efficiently than wild-type medium in Neuro-2a cell cultures. We showed age-dependent up-regulation of brain OASIS levels and processing in Arg-61 apoE mice. ER stress and astrocyte dysfunction represent a new paradigm underlying the association of apoE4 with neurodegeneration.

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: structure determines function, from atherosclerosis to Alzheimer's disease to AIDS.

Apolipoprotein (apo) E has roles beyond lipoprotein metabolism. The detrimental effects of apoE4 in cardiovascular, neurological, and infectious diseases correlate with its structural features (e.g., domain interaction) that distinguish it from apoE3 and apoE2. Structure/function studies revealed that apoE2 is severely defective in LDL receptor binding because of a structural difference that alters the receptor binding region and helped unravel the mechanism of type III hyperlipoproteinemia. ApoE4 is the major genetic risk factor for Alzheimer's disease and sets the stage for neuropathological disorders precipitated by genetic, metabolic, and environmental stressors. ApoE also influences susceptibility to parasitic, bacterial, and viral infections. In HIV-positive patients, apoE4 homozygosity hastens progression to AIDS and death and increases susceptibility to opportunistic infections. The next phase in our understanding of apoE will be characterized by clinical intervention to prevent or reverse the detrimental effects of apoE4 by modulating its structure or blocking the pathological processes it mediates.

Understanding the association of apolipoprotein E4 with Alzheimer disease: clues from its structure.

Despite intense interest, the molecular mechanisms underlying the association of apoE4 with Alzheimer disease are not clear. Because the function (or dysfunction) of a protein is based on its structure, this review focuses on the effects of the structural differences among the isoforms on neurodegeneration. Understanding how apoE4 structure impacts neurodegeneration is likely to provide mechanistic insight as well as potential therapeutic approaches to blunt or reduce its effects.

Apolipoprotein E4 domain interaction: synaptic and cognitive deficits in mice.

BACKGROUND: Apolipoprotein E4 (apoE4), the major genetic risk factor for Alzheimer's disease (AD) and other neurodegenerative diseases, has three structural and biophysical properties that distinguish it from the other isoforms-domain interaction, reduced stability, and lack of cysteine. Assessing their relative contributions to effects of apoE4-associated pathogenesis in AD is important from a mechanistic and therapeutic perspective, that is not possible using human apoE transgene or knock-in models. METHODS: We analyzed Arg-61 apoE mice, a gene-targeted model that selectively displays domain interaction. RESULTS: The mice displayed age-dependent loss of the synaptic protein synaptophysin in neocortex and hippocampus and had lower levels of the postsynaptic neuroligin-1. Activation of dentate gyrus granule neurons increased Arc expression 3.5-fold in wildtype mice but only 2.3-fold in Arg-61 mice. The losses of synaptic proteins caused a mild memory deficit in Arg-61 mice in the water-maze test. Since synaptic integrity requires efficient glutamate uptake, we measured astrocyte glutamate transporter 1 in the hippocampus. The level was reduced in Arg-61 mice, suggesting that inefficient glutamate uptake by astrocytes causes chronic excitotoxicity. Consistent with the reduced secretion of Arg-61 apoE by astrocytes in this model, cholesterol secretion was also reduced 34%. This reduction could also contribute to the synaptic deficits by limiting the availability of cholesterol for neuronal repair. CONCLUSIONS: Domain interaction in the absence of other structural characteristics of apoE4 is sufficient to cause synaptic pathology and functional synaptic deficits, potentially associated with astrocyte dysfunction and impaired maintenance of neurons. Therapeutic targeting of domain interaction might blunt effects of apoE4 in neurodegenerative disease.

Genes contributing to prion pathogenesis.

Prion diseases are caused by conversion of a normally folded, non-pathogenic isoform of the prion protein (PrP(C)) to a misfolded, pathogenic isoform (PrP(Sc)). Prion inoculation experiments in mice expressing homologous PrP(C) molecules on different genetic backgrounds displayed different incubation times, indicating that the conversion reaction may be influenced by other gene products. To identify genes that contribute to prion pathogenesis, we analysed incubation times of prions in mice in which the gene product was inactivated, knocked out or overexpressed. We tested 20 candidate genes, because their products either colocalize with PrP, are associated with Alzheimer's disease, are elevated during prion disease, or function in PrP-mediated signalling, PrP glycosylation, or protein maintenance. Whereas some of the candidates tested may have a role in the normal function of PrP(C), our data show that many genes previously implicated in prion replication have no discernible effect on the pathogenesis of prion disease. While most genes tested did not significantly affect survival times, ablation of the amyloid beta (A4) precursor protein (App) or interleukin-1 receptor, type I (Il1r1), and transgenic overexpression of human superoxide dismutase 1 (SOD1) prolonged incubation times by 13, 16 and 19 %, respectively.

Apolipoprotein E*dipalmitoylphosphatidylcholine particles are ellipsoidal in solution.

Apolipoprotein E (apoE) is a major protein component of cholesterol-transporting lipoprotein particles in the central nervous system and in plasma. Polymorphisms of apoE are associated with cardiovascular disease and with a predisposition to Alzheimer's disease and other forms of neurodegeneration. For full biological activity, apoE must be bound to a lipoprotein particle. Complexes of apoE and phospholipid mimic many of these activities. In contrast to a widely accepted discoidal model of apoA-I bound to dimyristoylphosphatidylcholine, which is based on solution studies, an X-ray diffraction study of apoE bound to dipalmitoylphosphatidylcholine (DPPC) indicated that apoE*DPPC particles are quasi-spheroidal and that the packing of the phospholipid core is similar to a micelle. Using small-angle X-ray scattering, we show that apoE*DPPC particles in solution are ellipsoidal and that the shape of the phospholipid core is compatible with a twisted-bilayer model. The proposed model is consistent with the results of mass spectrometric analysis of products of limited proteolysis. These revealed that the nonlipid-bound regions of apoE in the particle are consistent with an alpha-helical hairpin.

Detrimental effects of apolipoprotein E4: potential therapeutic targets in Alzheimer's disease.

As the major genetic risk factor for Alzheimer's disease, the apolipoprotein (apo) E4 isoform is a promising therapeutic target. ApoE4 likely contributes to Alzheimer's disease pathology by interacting with multiple factors through various pathways. Interactions with the amyloid beta peptide and the amyloid cascade, for example, may lead to cognitive decline and neurodegeneration. Alternatively, apoE4 might act independently of the amyloid beta peptide. Our working hypothesis is that apoE has isoform-specific effects on neuronal repair and remodeling. One or more injurious agents could result in neuronal damage, requiring neuronal repair or remodeling. The injurious agents (or "second hits") may be genetic, metabolic, or environmental. Potential therapeutic strategies include changing the structure of apoE4 to be more apoE3-like, inhibiting the protease that cleaves apoE4 into toxic fragments, and protecting mitochondria from apoE4 toxicity. Structural features that distinguish apoE4 and apoE3 determine their functional differences and hold the key to understanding how apoE4 is involved in Alzheimer's disease.