In situ AFM studies of astrocyte-secreted apolipoprotein E- and J-containing lipoproteins

Divalent cations promote huntingtin fibril formation on endoplasmic reticulum derived and model membranes

Huntington's Disease (HD) is caused by an abnormal expansion of the polyglutamine (polyQ) domain within the first exon of the huntingtin protein (htt). This expansion promotes disease-related htt aggregation into amyloid fibrils and the formation of proteinaceous inclusion bodies within neurons. Fibril formation is a complex heterogenous process involving an array of aggregate species such as oligomers, protofibrils, and fibrils. In HD, structural abnormalities of membranes of several organelles develop. In particular, the accumulation of htt fibrils near the endoplasmic reticulum (ER) impinges upon the membrane, resulting in ER damage, altered dynamics, and leakage of Ca2+. Here, the aggregation of htt at a bilayer interface assembled from ER-derived liposomes was investigated, and fibril formation directly on these membranes was enhanced. Based on these observations, simplified model systems were used to investigate mechanisms associated with htt aggregation on ER membranes. As the ER-derived liposome fractions contained residual Ca2+, the role of divalent cations was also investigated. In the absence of lipids, divalent cations had minimal impact on htt structure and aggregation. However, the presence of Ca2+ or Mg2+ played a key role in promoting fibril formation on lipid membranes despite reduced htt insertion into and association with lipid interfaces, suggesting that the ability of divalent cations to promote fibril formation on membranes is mediated by induced changes to the lipid membrane physicochemical properties. With enhanced concentrations of intracellular calcium being a hallmark of HD, the ability of divalent cations to influence htt aggregation at lipid membranes may play a role in aggregation events that lead to organelle abnormalities associated with disease.

Charge within Nt17 peptides modulates huntingtin aggregation and initial lipid binding events

Toxic aggregation of pathogenic huntingtin protein (htt) is implicated in Huntington's disease and influenced by various factors, including the first seventeen amino acids at the N-terminus (Nt17) and the presence of lipid membranes. Nt17 has a propensity to form an amphipathic α-helix in the presence of binding partners, which promotes α-helix rich oligomer formation and facilitates htt/lipid interactions. Within Nt17 are multiple sites that are subject to post-translational modification, including acetylation and phosphorylation. Acetylation can occur at lysine 6, 9, and/or 15 while phosphorylation can occur at threonine 3, serine 13, and/or serine 16. Such modifications impact aggregation and lipid binding through the alteration of various intra- and intermolecular interactions. When incubated with htt-exon1(46Q), free Nt17 peptides containing point mutations mimicking acetylation or phosphorylation reduced fibril formation and altered oligomer morphologies. Upon exposure to lipid vesicles, changes to peptide/lipid complexation were observed and peptide-containing oligomers demonstrated reduced lipid interactions.

Neurotoxic reactive astrocytes induce cell death via saturated lipids

Astrocytes regulate the response of the central nervous system to disease and injury and have been hypothesized to actively kill neurons in neurodegenerative disease^1-6. Here we report an approach to isolate one component of the long-sought astrocyte-derived toxic factor^5,6. Notably, instead of a protein, saturated lipids contained in APOE and APOJ lipoparticles mediate astrocyte-induced toxicity. Eliminating the formation of long-chain saturated lipids by astrocyte-specific knockout of the saturated lipid synthesis enzyme ELOVL1 mitigates astrocyte-mediated toxicity in vitro as well as in a model of acute axonal injury in vivo. These results suggest a mechanism by which astrocytes kill cells in the central nervous system.

AFM detects the effects of acidic condition on the size and biomechanical properties of native/oxidized low-density lipoprotein

Solution acidification exists under some physiological conditions (e.g. lysosomes in cells) and diseases (e.g. atherosclerosis, tumors, etc.). It is poorly understood whether and how acidification influences the size and biomechanical (stiffness and stickiness) properties of native Low-density lipoprotein (LDL) and its oxidized form (oxLDL) which plays a vital role in atherogenesis and tumorigenesis. Atomic force microscopy (AFM) evaluated that gradient acidification from pH 7.4 to pH 4.4 caused an expanding-first-and-then-shrinking decrease in size and a dramatic decrease in stiffness (but no statistically significant changes in stickiness) of LDL/oxLDL particles by influencing secondary/tertiary structures and lipid release detected by infrared spectral analysis and cholesterol detection, respectively. The smaller and softer characteristics of LDL/oxLDL at acidic conditions versus at the neutral pH partially explains the atherogenic role of acidification. The data may provide important information for a better understanding of LDL/oxLDL and some diseases (e.g. atherosclerosis and tumors).

Dynamic AFM detection of the oxidation-induced changes in size, stiffness, and stickiness of low-density lipoprotein

Background

Low-density lipoprotein (LDL) is an important plasma lipoprotein transporting lipids to peripheral tissues/cells. The oxidation of LDL plays critical roles in atherogenesis and its oxidized form (oxLDL) is an important risk factor of atherosclerosis. The biomechanical properties of LDL/oxLDL are closely correlated with the disease. To date, however, the oxidation-induced changes in size and biomechanical properties (stiffness and stickiness) of LDL particles are less investigated.

Methods

In this study, copper-induced LDL oxidation was confirmed by detecting electrophoretic mobility, malondialdehyde production, and conjugated diene formation. Then, the topographical and biomechanical mappings of LDL particles before/after and during oxidation were performed by using atomic force microscopy (AFM) and the size and biomechanical forces of particles were measured and quantitatively analyzed.

Results

Oxidation induced a significant decrease in size and stiffness (Young’s modulus) but a significant increase in stickiness (adhesion force) of LDL particles. The smaller, softer, and stickier characteristics of oxidized LDL (oxLDL) partially explains its pro-atherosclerotic role.

Conclusions

The data implies that LDL oxidation probably aggravates atherogenesis by changing the size and biomechanical properties of LDL particles. The data may provide important information for a better understanding of LDL/oxLDL and atherosclerosis.

Comparative investigation on the sizes and scavenger receptor binding of human native and modified lipoprotein particles with atomic force microscopy

BACKGROUND

The size and receptor-binding abilities of plasma lipoproteins are closely related with their structure/functions. Presently, the sizes of native lipoproteins have been measured by various methods including atomic force microscopy (AFM) whereas the sizes of modified lipoproteins are poorly determined and the receptor-binding ability of lipoproteins is less detected and compared at the nanoscale.

METHODS

Here, AFM was utilized to detect/compare the size and scavenger receptor-binding properties of three native human lipoproteins including high-density lipoprotein, low-density lipoprotein (LDL), and very low-density lipoprotein, and two modified human lipoproteins including oxidized and acetylated LDL, as well as bovine serum albumin and their antibodies as negative and positive controls, respectively.

RESULTS

AFM detected that the sizes of these lipoproteins are close to the commonly known values and the previously-reported AFM-detected sizes and that native and modified LDL have different height/size. AFM also revealed that the CD36-binding abilities of the five lipoproteins are different from one another and from their SR-B1-binding abilities and that the anti-CD36/SR-B1 antibodies as positive controls have strong CD36/SR-B1-binding abilities.

CONCLUSIONS

The data provide important information on lipoproteins for better understanding their structures/functions. Moreover, the data certify that besides size measurement AFM also can visualize receptor-lipoprotein binding at the nanoscale, as well as antigen-antibody (scavenger receptors and their antibodies) binding.

In situ AFM imaging of apolipoprotein A-I directly derived from plasma HDL

BACKGROUND AND AIMS

The major apolipoproteins of plasma lipoproteins play vital roles in the structural integrity and physiological functions of lipoproteins. More than ten structural models of apolipoprotein A-I (apoA-I), the major apolipoprotein of high-density lipoprotein (HDL), have been developed successively. In these models, apoA-I was supposed to organize in a ring-shaped form. To date, however, there is no direct evidence under physiological condition.

METHODS

Here, atomic force microscopy (AFM) was used to in situ visualize the organization of apoA-I, which was exposed via depletion of the lipid component of plasma HDL pre-immobilized on functionalized mica sheets.

RESULTS

For the first time, the ring-shaped coarse structure and three detailed structures (crescent-shaped, gapped "O"-shaped, and parentheses-shaped structures, respectively) of apoA-I in plasma HDL, which have the ability of binding scavenger receptors, were directly observed and quantitatively measured by AFM. The three detailed structures probably represent the different extents to which the lipid component of HDL was depleted. Data on lipid depletion of HDL may provide clues to understand lipid insertion of HDL.

CONCLUSIONS

These data provide important information for the understanding of the structure/maturation of plasma HDL. Moreover, they suggest a powerful method for directly visualizing the major apolipoproteins of plasma lipoproteins or the protein component of lipoprotein-like lipid-protein complexes.

Imaging and force measurement of LDL and HDL by AFM in air and liquid

The size and biomechanical properties of lipoproteins are tightly correlated with their structures/functions. While atomic force microscopy (AFM) has been used to image lipoproteins the force measurement of these nano-sized particles is missing. We detected that the sizes of LDL and HDL in liquid are close to the commonly known values. The Young's modulus of LDL or HDL is ∼0.4 GPa which is similar to that of some viral capsids or nanovesicles but greatly larger than that of various liposomes. The adhesive force of LDL or HDL is small (∼200 pN). The comparison of AFM detection in air and liquid was also performed which is currently lacking. Our data may provide useful information for better understanding and AFM detection of lipoproteins.

Preparation protocols of aβ(1-40) promote the formation of polymorphic aggregates and altered interactions with lipid bilayers

The appearance of neuritic amyloid plaques comprised of β-amyloid peptide (Aβ) in the brain is a predominant feature in Alzheimer's disease (AD). In the aggregation process, Aβ samples a variety of potentially toxic aggregate species, ranging from small oligomers to fibrils. Aβ has the ability to form a variety of morphologically distinct and stable amyloid fibrils. Commonly termed polymorphs, such distinct aggregate species may play a role in variations of AD pathology. It has been well documented that polymorphic aggregates of Aβ can be produced by changes in the chemical environment and peptide preparations. As Aβ and several of its aggregated forms are known to interact directly with lipid membranes and this interaction may play a role in a variety of potential toxic mechanisms associated with AD, we determine how different Aβ(1-40) preparation protocols that lead to distinct polymorphic fibril aggregates influence the interaction of Aβ(1-40) with model lipid membranes. Using three distinct protocols for preparing Aβ(1-40), the aggregate species formed in the absence and presence of a lipid bilayers were investigated using a variety of scanning probe microscopy techniques. The three preparations of Aβ(1-40) promoted distinct oligomeric and fibrillar aggregates in the absence of bilayers that formed at different rates. Despite these differences in aggregation properties, all Aβ(1-40) preparations were able to disrupt supported total brain lipid extract bilayers, altering the bilayer's morphological and mechanical properties.

Measurement of single low-density lipoprotein particles by atomic force microscopy

Background

The size of lipoprotein particles is relevant to the risk of coronary artery disease (CAD).

Methods

We investigated the feasibility of atomic force microscopy (AFM) for evaluating the size of large low-density lipoprotein (LDL) and small dense LDL (sd-LDL) separated by ultracentrifugation. The measurements by AFM in tapping mode were compared to those by electron microscopy (EM).

Results

There was a significant difference in particle sizes determined by AFM between large LDL (20.6 ± 1.9 nm, mean ± SD) and sd-LDL (16.2 ± 1.4 nm) obtained from six healthy volunteers ( P < 0.05). The particle sizes determined by EM for the same samples were 23.2 ± 1.4 nm for large LDL and 20.4 ± 1.4 nm for sd-LDL. The difference between large LDL and sd-LDL detected by EM was also statistically significant ( P < 0.05). In addition, the particle sizes of each lipoprotein fraction were significantly different between AFM and EM: P < 0.05 for large LDL and P < 0.05 for sd-LDL.

Conclusions

AFM can differentiate between sd-LDL and large LDL particles by their size, and might be useful for evaluating risk for CAD.

Huntingtin disrupts lipid bilayers in a polyQ-length dependent manner

Huntington's Disease (HD) is a neurodegenerative disorder that is defined by the accumulation of nanoscale aggregates comprised of the huntingtin (htt) protein. Aggregation is directly caused by an expanded polyglutamine (polyQ) domain in htt, leading to a diverse population of aggregate species, such as oligomers, fibrils, and annular aggregates. Furthermore, the length of this polyQ domain is directly related to onset and severity of disease. The first 17 N-terminal amino acids of htt have been shown to further modulate aggregation. Additionally, these 17 amino acids appear to have lipid binding properties as htt interacts with a variety of membrane-containing structures present in cells, such as organelles, and interactions with these membrane surfaces may further modulate htt aggregation. To investigate the interaction between htt exon1 and lipid bilayers, in situ atomic force microscopy (AFM) was used to directly monitor the aggregation of htt exon1 constructs with varying Q-lengths (35Q, 46Q, 51Q, and myc-53Q) on supported lipid membranes comprised of total brain lipid extract. The exon1 fragments accumulated on the lipid membranes, causing disruption of the membrane, in a polyQ dependent manner. Furthermore, the addition of an N-terminal myc-tag to the htt exon1 fragments impeded the interaction of htt with the bilayer.

The interaction of polyglutamine peptides with lipid membranes is regulated by flanking sequences associated with huntingtin

Huntington disease (HD) is caused by an expanded polyglutamine (poly(Q)) repeat near the N terminus of the huntingtin (htt) protein. Expanded poly(Q) facilitates formation of htt aggregates, eventually leading to deposition of cytoplasmic and intranuclear inclusion bodies containing htt. Flanking sequences directly adjacent to the poly(Q) domain, such as the first 17 amino acids on the N terminus (Nt17) and the polyproline (poly(P)) domain on the C-terminal side of the poly(Q) domain, heavily influence aggregation. Additionally, htt interacts with a variety of membraneous structures within the cell, and Nt17 is implicated in lipid binding. To investigate the interaction between htt exon1 and lipid membranes, a combination of in situ atomic force microscopy, Langmuir trough techniques, and vesicle permeability assays were used to directly monitor the interaction of a variety of synthetic poly(Q) peptides with different combinations of flanking sequences (KK-Q35-KK, KK-Q35-P10-KK, Nt17-Q35-KK, and Nt17-Q35-P10-KK) on model membranes and surfaces. Each peptide aggregated on mica, predominately forming extended, fibrillar aggregates. In contrast, poly(Q) peptides that lacked the Nt17 domain did not appreciably aggregate on or insert into lipid membranes. Nt17 facilitated the interaction of peptides with lipid surfaces, whereas the poly(P) region enhanced this interaction. The aggregation of Nt17-Q35-P10-KK on the lipid bilayer closely resembled that of a htt exon1 construct containing 35 repeat glutamines. Collectively, this data suggests that the Nt17 domain plays a critical role in htt binding and aggregation on lipid membranes, and this lipid/htt interaction can be further modulated by the presence of the poly(P) domain.

Specific domains of Aβ facilitate aggregation on and association with lipid bilayers

A hallmark of Alzheimer's disease, a late-onset neurodegenerative disease, is the deposition of neuritic amyloid plaques composed of aggregated forms of the β-amyloid peptide (Aβ). Aβ forms a variety of nanoscale, toxic aggregate species ranging from small oligomers to fibrils. Aβ and many of its aggregate forms strongly interact with lipid membranes, which may represent an important step in several toxic mechanisms. Understanding the role that specific regions of Aβ play in regulating its aggregation and interaction with lipid membranes may provide insights into the fundamental interaction between Aβ and cellular surfaces. We investigated the interaction and aggregation of several Aβ fragments (Aβ1-11, Aβ1-28, Aβ10-26, Aβ12-24, Aβ16-22, Aβ22-35, and Aβ1-40) in the presence of supported model total brain lipid extract (TBLE) bilayers. These fragments represent a variety of chemically unique domains within Aβ, that is, the extracellular domain, the central hydrophobic core, and the transmembrane domain. Using scanning probe techniques, we elucidated aggregate morphologies for these different Aβ fragments in free solution and in the presence of TBLE bilayers. These fragments formed a variety of oligomeric and fibrillar aggregates under free solution conditions. Exposure to TBLE bilayers resulted in distinct aggregate morphologies compared to free solution and changes in bilayer stability dependent on the Aβ sequence. Aβ10-26, Aβ16-22, Aβ22-35, and Aβ1-40 aggregated into a variety of distinct fibrillar aggregates and disrupted the bilayer structure, resulting in altered mechanical properties of the bilayer. Aβ1-11, Aβ1-28, and Aβ12-24 had minimal interaction with lipid membranes, forming only sparse oligomers.

Atomic force microscopy assays for evaluating polyglutamine aggregation in solution and on surfaces

Mutations which cause an expansion of CAG triplet repeats encoding polyglutamine (polyQ) are responsible for the subsequent misfolding of specific proteins that contribute directly to the pathogenesis of at least nine neurodegenerative disorders, including Huntington's disease (HD) and the spinocerebellar ataxias (SCAs). Expansion of polyQ tracts results in the aggregation of these proteins, potentially through a variety of precursor aggregates, into fibrillar structures. There may also be a variety of aggregates formed that are off-pathway to the formation of fibrils. Here, detailed protocols for analyzing the aggregation of mutant huntingtin (htt) fragments (associated with HD) and synthetic polyQ peptides with atomic force microscopy (AFM) are described. Ex situ AFM can be used to characterize htt aggregate formation and morphology. In situ AFM allows for tracking the formation and fate of individual polyQ peptide aggregates on surfaces. The interaction of htt with a variety of surfaces, including lipid bilayers, can also be investigated. Furthermore, the mechanical impact of htt on lipid surfaces can be studied using specialized AFM techniques. Methods to analyze AFM images of htt aggregates are also presented.

Identification of novel potentially toxic oligomers formed in vitro from mammalian-derived expanded huntingtin exon-1 protein

Huntington disease is a genetic neurodegenerative disorder that arises from an expanded polyglutamine region in the N terminus of the HD gene product, huntingtin. Protein inclusions comprised of N-terminal fragments of mutant huntingtin are a characteristic feature of disease, though are likely to play a protective role rather than a causative one in neurodegeneration. Soluble oligomeric assemblies of huntingtin formed early in the aggregation process are candidate toxic species in HD. In the present study, we established an in vitro system to generate recombinant huntingtin in mammalian cells. Using both denaturing and native gel analysis, we have identified novel oligomeric forms of mammalian-derived expanded huntingtin exon-1 N-terminal fragment. These species are transient and were not previously detected using bacterially expressed exon-1 protein. Importantly, these species are recognized by 3B5H10, an antibody that recognizes a two-stranded hairpin conformation of expanded polyglutamine believed to be associated with a toxic form of huntingtin. Interestingly, comparable oligomeric species were not observed for expanded huntingtin shortstop, a 117-amino acid fragment of huntingtin shown previously in mammalian cell lines and transgenic mice, and here in primary cortical neurons, to be non-toxic. Further, we demonstrate that expanded huntingtin shortstop has a reduced ability to form amyloid-like fibrils characteristic of the aggregation pathway for toxic expanded polyglutamine proteins. Taken together, these data provide a possible candidate toxic species in HD. In addition, these studies demonstrate the fundamental differences in early aggregation events between mutant huntingtin exon-1 and shortstop proteins that may underlie the differences in toxicity.

The modulating effect of mechanical changes in lipid bilayers caused by apoE-containing lipoproteins on Aβ induced membrane disruption

A major feature of Alzheimer's disease (AD), a late-onset neurodegenerative disorder, is the ordered aggregation of the β-amyloid peptide (Aβ) into fibrils that comprise extracellular neuritic plaques found in the disease brain. One of many potential pathways for Aβ toxicity may be modulation of lipid membrane function. Here, we show by in situ atomic force microscopy (AFM) that astrocyte secreted lipoprotein particles (ASLPs) containing different isoforms of apolipoprotein E (apoE), of which the apoE4 allele is a major risk factor for the development of AD, can protect total brain lipid extract bilayers from Aβ(1-40) induced disruption. The apoE4 allele was less effective in protecting lipid bilayers from disruption compared with apoE3. Size analysis of apoE-containing ASLPs and mechanical studies of bilayer properties revealed that apoE-containing ASLPs modulate the mechanical properties of bilayers by acquiring some bilayer components (most likely cholesterol and/or oxidatively damaged lipids). Measurement of bilayer mechanical properties was accomplished with scanning probe acceleration microscopy (SPAM). These measurements demonstrated that apoE4 was also less effective in modulating mechanical properties of bilayers in comparison with apoE3. This ability of apoE to alter the mechanical properties of lipid membranes may represent a potential mechanism for the suppression of Aβ(1-40) induced bilayer disruption.

Point mutations in Aβ result in the formation of distinct polymorphic aggregates in the presence of lipid bilayers

A hallmark of Alzheimer's disease (AD) is the rearrangement of the β-amyloid (Aβ) peptide to a non-native conformation that promotes the formation of toxic, nanoscale aggregates. Recent studies have pointed to the role of sample preparation in creating polymorphic fibrillar species. One of many potential pathways for Aβ toxicity may be modulation of lipid membrane function on cellular surfaces. There are several mutations clustered around the central hydrophobic core of Aβ near the α-secretase cleavage site (E22G Arctic mutation, E22K Italian mutation, D23N Iowa mutation, and A21G Flemish mutation). These point mutations are associated with hereditary diseases ranging from almost pure cerebral amyloid angiopathy (CAA) to typical Alzheimer's disease pathology with plaques and tangles. We investigated how these point mutations alter Aβ aggregation in the presence of supported lipid membranes comprised of total brain lipid extract. Brain lipid extract bilayers were used as a physiologically relevant model of a neuronal cell surface. Intact lipid bilayers were exposed to predominantly monomeric preparations of Wild Type or different mutant forms of Aβ, and atomic force microscopy was used to monitor aggregate formation and morphology as well as bilayer integrity over a 12 hour period. The goal of this study was to determine how point mutations in Aβ, which alter peptide charge and hydrophobic character, influence interactions between Aβ and the lipid surface. While fibril morphology did not appear to be significantly altered when mutants were prepped similarly and incubated under free solution conditions, aggregation in the lipid membranes resulted in a variety of polymorphic aggregates in a mutation dependent manner. The mutant peptides also had a variable ability to disrupt bilayer integrity.

Hsp70 and Hsp40 functionally interact with soluble mutant huntingtin oligomers in a classic ATP-dependent reaction cycle

Inclusion bodies of aggregated mutant huntingtin (htt) fragments are a neuropathological hallmark of Huntington disease (HD). The molecular chaperones Hsp70 and Hsp40 colocalize to inclusion bodies and are neuroprotective in HD animal models. How these chaperones suppress mutant htt toxicity is unclear but might involve direct effects on mutant htt misfolding and aggregation. Using size exclusion chromatography and atomic force microscopy, we found that mutant htt fragments assemble into soluble oligomeric species with a broad size distribution, some of which reacted with the conformation-specific antibody A11. Hsp70 associated with A11-reactive oligomers in an Hsp40- and ATP-dependent manner and inhibited their formation coincident with suppression of caspase 3 activity in PC12 cells. Thus, Hsp70 and Hsp40 (DNAJB1) dynamically target specific subsets of soluble oligomers in a classic ATP-dependent reaction cycle, supporting a pathogenic role for these structures in HD.

Mutant huntingtin fragments form oligomers in a polyglutamine length-dependent manner in vitro and in vivo

Huntington disease (HD) is caused by an expansion of more than 35-40 polyglutamine (polyQ) repeats in the huntingtin (htt) protein, resulting in accumulation of inclusion bodies containing fibrillar deposits of mutant htt fragments. Intriguingly, polyQ length is directly proportional to the propensity for htt to form fibrils and the severity of HD and is inversely correlated with age of onset. Although the structural basis for htt toxicity is unclear, the formation, abundance, and/or persistence of toxic conformers mediating neuronal dysfunction and degeneration in HD must also depend on polyQ length. Here we used atomic force microscopy to demonstrate mutant htt fragments and synthetic polyQ peptides form oligomers in a polyQ length-dependent manner. By time-lapse atomic force microscopy, oligomers form before fibrils, are transient in nature, and are occasionally direct precursors to fibrils. However, the vast majority of fibrils appear to form by monomer addition coinciding with the disappearance of oligomers. Thus, oligomers must undergo a major structural transition preceding fibril formation. In an immortalized striatal cell line and in brain homogenates from a mouse model of HD, a mutant htt fragment formed oligomers in a polyQ length-dependent manner that were similar in size to those formed in vitro, although these structures accumulated over time in vivo. Finally, using immunoelectron microscopy, we detected oligomeric-like structures in human HD brains. These results demonstrate that oligomer formation by a mutant htt fragment is strongly polyQ length-dependent in vitro and in vivo, consistent with a causative role for these structures, or subsets of these structures, in HD pathogenesis.

Identical oligomeric and fibrillar structures captured from the brains of R6/2 and knock-in mouse models of Huntington's disease

Huntington's disease (HD) is a late-onset neurodegenerative disorder that is characterized neuropathologically by the presence of neuropil aggregates and nuclear inclusions. However, the profile of aggregate structures that are present in the brains of HD patients or of HD mouse models and the relative contribution of specific aggregate structures to disease pathogenesis is unknown. We have used the Seprion ligand to develop a highly sensitive enzyme-linked immunosorbent assay (ELISA)-based method for quantifying aggregated polyglutamine in tissues from HD mouse models. We used a combination of electron microscopy, atomic force microscopy (AFM) and sodium dodecyl sulphate–polyacrylamide gel electrophoresis (SDS–PAGE) to investigate the aggregate structures isolated by the ligand. We found that the oligomeric, proto-fibrillar and fibrillar aggregates extracted from the brains of R6/2 and HdhQ150 knock-in mice were remarkably similar. Using AFM, we determined that the nanometre globular oligomers isolated from the brains of both mouse models have dimensions identical to those generated from recombinant huntingtin exon 1 proteins. Finally, antibodies that detect exon 1 Htt epitopes differentially recognize the ligand-captured material on SDS–PAGE gels. The Seprion-ligand ELISA provides an assay with good statistical power for use in preclinical pharmacodynamic therapeutic trials or to assess the effects of the genetic manipulation of potential therapeutic targets on aggregate load. This, together with the ability to identify a spectrum of aggregate species in HD mouse tissues, will contribute to our understanding of how these structures relate to the pathogenesis of HD and whether their formation can be manipulated for therapeutic benefit.

Apolipoprotein E protects cultured pericytes and astrocytes from D-Abeta(1-40)-mediated cell death

Cerebral amyloid angiopathy (CAA) is a common pathological finding in Alzheimer's disease and hereditary cerebral hemorrhage with amyloidosis of the Dutch type; in this latter condition it is caused by deposition of mutated amyloid beta protein (Abeta Glu22Gln; D-Abeta(1-40)). Previously, we found a dependence of the Abeta-mediated toxicity and apolipoprotein E (apoE) production by cultured pericytes on apoE genotype. Given their close association with the cerebrovascular wall both astrocytes and pericytes may be involved in CAA development, a process that includes Abeta deposition and clearance and that may be affected by interaction with locally produced apolipoprotein E (apoE). Although astrocytes are regarded as the major source of apolipoprotein E (apoE) in the brain, also pericytes produce apoE. In this study we compared the apoE production capacity, the effects of apoE on D-Abeta(1-40) internalization, D-Abeta(1-40) cell surface accumulation and the vulnerability for D-Abeta(1-40)-induced toxicity of either cell type in order to quantify the relative contributions of astrocytes and pericytes in the various processes that contribute to CAA formation. Strikingly, cultured astrocytes produced only 3-10% of the apoE amounts produced by pericytes. Furthermore, pericytes with the apoE epsilon4 allele produced three times less apoE and were more vulnerable to D-Abeta(1-40) treatment than pericytes without an epsilon4 allele. Such relations were not observed with astrocytes in vitro. Both pericytes and astrocytes, however, were protected from Abeta-induced cytotoxicity by high levels of pericyte-derived apoE, but not recombinant apoE. In addition, pericyte-derived apoE dose-dependently decreased both internalization of Abeta and Abeta accumulation at the cell surface in either cell type. The present data suggest that apoE produced by pericytes, rather than astrocyte-produced apoE, modulates Abeta cytotoxicity and Abeta removal near the vasculature in the brain. Furthermore, since apoE production in pericytes is genotype dependent, this may contribute to the apoE genotype-dependent development of CAA in vivo.

Block copolymer micelles as nanocontainers for controlled release of proteins from biocompatible oil phases

Biocompatible oils are used in a variety of medical applications ranging from vaccine adjuvants to vehicles for oral drug delivery. To enable such nonpolar organic phases to serve as reservoirs for delivery of hydrophilic compounds, we explored the ability of block copolymer micelles in organic solvents to sequester proteins for sustained release across an oil-water interface. Self-assembly of the block copolymer, poly(-caprolactone)-block-poly(2-vinyl pyridine) (PCL-b-P2VP), was investigated in toluene and oleic acid, a biocompatible naturally occurring fatty acid. Micelle formation in toluene was characterized by dynamic light scattering (DLS) and atomic force microscopy (AFM) imaging of micelles cast onto silicon substrates. Cryogenic transmission electron microscopy confirmed a spherical morphology in oleic acid. Studies of homopolymer solubility implied that micelles in oleic acid consist of a P2VP corona and a PCL core, while P2VP formed the core of micelles assembled in toluene. The loading of two model proteins (ovalbumin (ova) and bovine serum albumin (BSA)) into micelles was demonstrated with loadings as high as 7.8% wt of protein per wt of P2VP in oleic acid. Characterization of block copolymer morphology in the two solvents after protein loading revealed spherical particles with similar size distributions to the as-assembled micelles. Release of ova from micelles in oleic acid was sustained for 12-30 h upon placing the oil phase in contact with an aqueous bath. Unique to the situation of micelle assembly in an oily phase, the data suggest protein is sequestered in the P2VP corona block of PCL-b-P2VP micelles in oleic acid. More conventionally, protein loading occurs in the P2VP core of micelles assembled in toluene.

Sterols in the central nervous system

PURPOSE OF REVIEW

Aberrations in cerebral cholesterol homeostasis can lead to severe neurological diseases and have been linked to Alzheimer's disease. Many proteins involved in peripheral cholesterol metabolism are also present in the brain. Yet, brain cholesterol metabolism is very different from that in the remainder of the body. This review reports on present insights into the regulation of cerebral cholesterol homeostasis, focusing on cholesterol trafficking between astrocytes and neurons.

RECENT FINDINGS

Astrocytes are a major site of cholesterol synthesis. They secrete cholesterol in the form of apolipoprotein E-containing HDL-like particles. After birth, neurons are thought to reduce their cholesterol synthesis and rely predominantly on astrocytes for their cholesterol supply. How exactly neurons regulate their cholesterol supply is largely unknown. A role for the brain-specific cholesterol metabolite, 24(S)-hydroxycholesterol, in this process was recently proposed. Recent findings strengthen the link between brain cholesterol metabolism and factors involved in synaptic plasticity, a process essential for learning and memory functions, as well as regeneration, which are affected in Alzheimer's disease.

SUMMARY

Insight into the regulation of cerebral cholesterol homeostasis will provide possibilities to modulate the key steps involved and may lead to the development of therapies for the prevention as well as treatment of neurodegenerative diseases such as Alzheimer's disease.

Relationships between expression of apolipoprotein E and beta-amyloid precursor protein are altered in proximity to Alzheimer beta-amyloid plaques: potential explanations from cell culture studies

Theories regarding the initiation and progression of Alzheimer disease (AD) often consider potential roles played by elevations of beta-amyloid precursor protein (betaAPP). Because it is the source of amyloid beta-peptide, betaAPP may simply contribute more pathogenic stimulus when elevated; some analyses have, however, reported a decline in betaAPP in AD. We found a progressive increase in neuronal betaAPP expression with increasing age in the brains of nondemented individuals, whereas in AD patient samples, betaAPP antigenicity decreased in neuronal somata in a manner that correlated with accumulation of mature amyloid beta-peptide plaques. In contrast, apolipoprotein E (ApoE) expression correlated with accumulation of plaques, and even greater amounts of ApoE were detected in plaques. Induction of betaAPP by glutamate in neuronal cell cultures was found to depend upon ApoE levels or activity. Thus, elevations in expression of ApoE and betaAPP by cellular stresses are likely normally linked in vivo, and uncoupling of this link, or other pathologic events in AD initiation, may leave neurons with diminished betaAPP expression, which might in turn reduce their resistance to stressors.

Accelerating amyloid-beta fibrillization reduces oligomer levels and functional deficits in Alzheimer disease mouse models

Many proteins suspected of causing neurodegenerative diseases exist in diverse assembly states. For most, it is unclear whether shifts from one state to another would be helpful or harmful. We used mutagenesis to change the assembly state of Alzheimer disease (AD)-associated amyloid-beta (Abeta) peptides. In vitro, the "Arctic" mutation (AbetaE22G) accelerated Abeta fibrillization but decreased the abundance of nonfibrillar Abeta assemblies, compared with wild-type Abeta. In human amyloid precursor protein (hAPP) transgenic mice carrying mutations adjacent to Abeta that increase Abeta production, addition of the Arctic mutation markedly enhanced the formation of neuritic amyloid plaques but reduced the relative abundance of a specific nonfibrillar Abeta assembly (Abeta*56). Mice overexpressing Arctic mutant or wild-type Abeta had similar behavioral and neuronal deficits when they were matched for Abeta*56 levels but had vastly different plaque loads. Thus, Abeta*56 is a likelier determinant of functional deficits in hAPP mice than fibrillar Abeta deposits. Therapeutic interventions that reduce Abeta fibrils at the cost of augmenting nonfibrillar Abeta assemblies could be harmful.

Synthesis and in situ atomic force microscopy characterization of temperature-responsive hydrogels based on poly(2-(dimethylamino)ethyl methacrylate) prepared by atom transfer radical polymerization

Well-defined copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and benzophenone methacrylate (BPMA) with different compositions were synthesized via atom transfer radical polymerization. The molecular weights of these copolymers were Mn approximately 30 000 g/mol, while the BPMA content varied from 2.5 to 10 mol %. The copolymers with a low content of BPMA (2.5 and 5 mol %) exhibited a sharp thermal transition at 33-36 degrees C in aqueous solution. A hydrogel was immobilized and patterned on a silicon wafer via UV treatment of the spin-coated polymer layer using a photomask technique. The thermoresponsive behavior of the patterned polymer gel was quantitatively investigated by variable temperature in situ contact mode atomic force microscopy, which revealed the presence of two lower critical solution temperature regions. One region was between 25 and 30 degrees C, corresponding to the topmost layer of the hydrogel film, and the other region, around 40 degrees C, corresponded to the bulk of the hydrogel. Concurrent lateral force microscopy measurements revealed that, just above the transition temperature, the bulk region exhibited enhanced friction.

Method of imaging low density lipoproteins by atomic force microscopy

This short paper reports a simple method to image low density lipoproteins (LDL) using atomic force microscopy (AFM). This instrument allows imaging of biological samples in liquid and presents the advantage of needing no sample preparation such as staining or fixation that may affect their general structure. Dimensions (diameter and height) of individual LDL particles were successfully measured. AFM imaging revealed that LDL have a quasi-spherical structure on the x and y axis with an oblate spheroid structure in the z axis (i.e., height). LDLs were found to have an average diameter of 23 +/- 3 nm. The obtained mean height was 10 +/- 2 nm.

Green tea (-)-epigallocatechin-gallate modulates early events in huntingtin misfolding and reduces toxicity in Huntington's disease models

Huntington's disease (HD) is a progressive neurodegenerative disorder for which only symptomatic treatments of limited effectiveness are available. Preventing early misfolding steps and thereby aggregation of the polyglutamine (polyQ)-containing protein huntingtin (htt) in neurons of patients may represent an attractive therapeutic strategy to postpone the onset and progression of HD. Here, we demonstrate that the green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG) potently inhibits the aggregation of mutant htt exon 1 protein in a dose-dependent manner. Dot-blot assays and atomic force microscopy studies revealed that EGCG modulates misfolding and oligomerization of mutant htt exon 1 protein in vitro, indicating that it interferes with very early events in the aggregation process. Also, EGCG significantly reduced polyQ-mediated htt protein aggregation and cytotoxicity in an yeast model of HD. When EGCG was fed to transgenic HD flies overexpressing a pathogenic htt exon 1 protein, photoreceptor degeneration and motor function improved. These results indicate that modulators of htt exon 1 misfolding and oligomerization like EGCG are likely to reduce polyQ-mediated toxicity in vivo. Our studies may provide the basis for the development of a novel pharmacotherapy for HD and related polyQ disorders.

Nanotechnologies in proteomics

Progress in proteomic researches is largely determined by development and implementation of new methods for the revelation and identification of proteins in biological material in a wide concentration range (from 10(-3) M to single molecules). The most perspective approaches to address this problem involve (i) nanotechnological physicochemical procedures for the separation of multicomponent protein mixtures; among these of particular interest are biospecific nanotechnological procedures for selection of proteins from multicomponent protein mixtures with their subsequent concentration on solid support; (ii) identification and counting of single molecules by use of molecular detectors. The prototypes of biospecific nanotechnological procedures, based on the capture of ligand biomolecules by biomolecules of immobilized ligate and the concentration of the captured ligands on appropriate surfaces, are well known; these are affinity chromatography, magnetic biobeads technology, different biosensor methods, etc. Here, we review the most promising nanotechnological approaches for selection of proteins and kinetic characterization of their complexes based on these biospecific methods with subsequent MS/MS identification of proteins and protein complexes. Two major groups of methods for the analysis and identification of individual molecules and their complexes by use of molecular detectors will be reviewed: scanning probe microscopy (SPM) (including atomic-force microscopy) and cryomassdetector technology.