Nanolithography of Amyloid Precursor Protein Cleavage with β-Secretase by Atomic Force Microscopy

Cleavage of the amyloid precursor protein (APP) by secretases is critical in neural cell processes including the pathway for neural cell proliferation and that underlying the pathogenesis of Alzheimer's disease (AD). Understanding the mechanism of APP cleavage and development of a convenient tool for the accurate evaluation of APP cleavage intensity by secretases are very important in the development of new AD therapeutic targets. In this study, we developed a sophisticated technology to evaluate the APP cleavage mechanism at the nano-molecular level by atomic force microscopic (AFM) nanolithography. APP was modified on a glass substrate; nanolithography of APP cleavage by β-secretase-modified AFM probe scanning was achieved. APP cleavage was verified by the AFM imaging and the fluorescent immunostaining. The present method will be very useful in understanding the molecular level of the APP cleavage mechanism by β-secretase in vitro; this method will facilitate inhibitor screening for the therapeutic target of AD.

Ultrastructural studies in APP/PS1 mice expressing human ApoE isoforms: implications for Alzheimer's disease

Alzheimer's disease is characterized in part by extracellular aggregation of the amyloid-β peptide in the form of diffuse and fibrillar plaques in the brain. Electron microscopy (EM) has made an important contribution in understanding of the structure of amyloid plaques in humans. Classical EM studies have revealed the architecture of the fibrillar core, characterized the progression of neuritic changes, and have identified the neurofibrillary tangles formed by paired helical filaments (PHF) in degenerating neurons. Clinical data has strongly correlated cognitive impairment in AD with the substantial synapse loss observed in these early ultrastructural studies. Animal models of AD-type brain amyloidosis have provided excellent opportunities to study amyloid and neuritic pathology in detail and establish the role of neurons and glia in plaque formation. Transgenic mice overexpressing mutant amyloid precursor protein (APP) alone with or without mutant presenilin 1 (PS1), have shown that brain amyloid plaque development and structure grossly recapitulate classical findings in humans. Transgenic APP/PS1 mice expressing human apolioprotein E isoforms also develop amyloid plaque deposition. However no ultrastructural data has been reported for these animals. Here we show results from detailed EM analysis of amyloid plaques in APP/PS1 mice expressing human isoforms of ApoE and compare these findings with EM data in other transgenic models and in human AD. Our results show that similar to other transgenic animals, APP/PS1 mice expressing human ApoE isoforms share all major cellular and subcellular degenerative features and highlight the identity of the cellular elements involved in Aβ deposition and neuronal degeneration.

Neuropathology of the recessive A673V APP mutation: Alzheimer disease with distinctive features

Mutations of three different genes, encoding β-amyloid precursor protein (APP), presenilin 1 and presenilin 2 are associated with familial Alzheimer's disease (AD). Recently, the APP mutation A673V has been identified that stands out from all the genetic defects previously reported in these three genes, since it causes the disease only in the homozygous state (Di Fede et al. in Science 323:1473-1477, 2009). We here provide the detailed neuropathological picture of the proband of this family, who was homozygous for the APP A673V mutation and recently came to death. The brain has been studied by histological and immunohistochemical techniques, at the optical and ultrastructural levels. Cerebral Aβ accumulation and tau pathology were severe and extensive. Peculiar features were the configuration of the Aβ deposits that were of large size, mostly perivascular and exhibited a close correspondence between the pattern elicited by amyloid stainings and the labeling obtained with immunoreagents specific for Aβ40 or Aβ42. Moreover, Aβ deposition spared the neostriatum while deeply affecting the cerebellum, and therefore was not in compliance with the hierarchical topographical sequence of involvement documented in sporadic AD. Therefore, the neuropathological picture of familial AD caused by the APP recessive mutation A673V presents distinctive characteristics compared to sporadic AD or familial AD inherited as a dominant trait. Main peculiar features are the morphology, structural properties and composition of the Aβ deposits as well as their topographic distribution in the brain.

Amyloidogenesis abolished by proline substitutions but enhanced by lipid binding

The influence of lipid molecules on the aggregation of a highly amyloidogenic segment of human islet amyloid polypeptide, hIAPP20–29, and the corresponding sequence from rat has been studied by all-atom replica exchange molecular dynamics (REMD) simulations with explicit solvent model. hIAPP20–29 fragments aggregate into partially ordered β-sheet oligomers and then undergo large conformational reorganization and convert into parallel/antiparallel β-sheet oligomers in mixed in-register and out-of-register patterns. The hydrophobic interaction between lipid tails and residues at positions 23–25 is found to stabilize the ordered β-sheet structure, indicating a catalysis role of lipid molecules in hIAPP20–29 self-assembly. The rat IAPP variants with three proline residues maintain unstructured micelle-like oligomers, which is consistent with non-amyloidogenic behavior observed in experimental studies. Our study provides the atomic resolution descriptions of the catalytic function of lipid molecules on the aggregation of IAPP peptides.

People diagnosed with diabetes have increased from 30 million to 246 million over the last two decades. One hallmark of type 2 diabetes is the formation of amyloid in the pancreatic islet, which is composed of human islet amyloid polypeptide (90%) and lipid molecules (10%). In the long-lasting endeavors against the disease, it is important to understand, at the atomic level, the interaction between peptide aggregation and lipid molecules. In this study, we use molecular dynamics simulations to explore the influence of lipid molecules on the self-assembly process of toxic peptide segments. Moreover, a negative control simulation, employing the non-amyloidogenic rodent sequence, is also performed to evaluate the robustness of the simulation protocol. Our study provides a generic picture of the catalytic role of lipid molecules in the process of amyloidogenesis.

Presenilin-1-mediated Retention of APP Derivatives in Early Biosynthetic Compartments

Processing of the amyloid precursor protein (APP) leads to the production of amyloid-beta (Abeta), the major component of extracellular plaques in the brains of Alzheimer's disease (AD) patients. Presenilin-1 (PS-1) plays a key role in the final step of Abeta formation, the gamma-secretase cleavage. Previously, we showed that PS-1 is retained in pre-Golgi compartments by incorporation into COPI-coated membranes of the vesicular tubular clusters (VTCs) between endoplasmic reticulum (ER) and Golgi complex. Here, we show that PS-1 also mediates the retention of the beta-cleavage-derived APP-C-terminal fragment (CTFbeta) and/or Abeta in pre-Golgi membranes. Overexpression of PS-1 increased the percentage of CTFbeta and/or Abeta in VTCs as well as their distribution to COPI-coated VTC membranes. By contrast, overexpression of the dominant-negative aspartate mutant PS-1(D257A) or PS-knockout decreased incorporation of these APP derivatives into COPI-coated membranes. Sorting of APP derivatives to COPI-coated VTC membranes was not depending on the APP cytosolic tail. In post-Golgi compartments, PS-1 expression enhanced the association of full-length APP/APPs with endosomal compartments at the expense of plasma membrane-bound APP. We conclude that PS-1, in addition to its role in gamma-secretase cleavage, is also required for the subcellular routing of APP and its derivatives. Malfunctioning of PS-1 in this role may have important consequences for the progress of AD.

Structural role of glycine in amyloid fibrils formed from transmembrane alpha-helices

Amyloid fibrils associated with diseases such as Alzheimer's are often derived from the transmembrane helices of membrane proteins. It is known that the fibrils have a cross-beta-sheet structure where main chain hydrogen bonding occurs between beta-strands in the direction of the fibril axis. However, the structural basis for how the membrane-spanning helix is converted into a beta-sheet or how protofibrils associate into fibrils is not known. Here, we use a model peptide corresponding to a portion of the single transmembrane helix of glycophorin A to investigate the structural role of glycine in amyloid-like fibrils formed from transmembrane helices. Glycophorin A contains a GxxxG motif that is found in many transmembrane sequences including that of the amyloid precursor protein and prion protein. We propose that glycine, which mediates helix interactions in membrane proteins, also provides key packing motifs when it occurs in beta-sheets. We show that glycines in the glycophorin A transmembrane helix promote extended beta-strand formation when the helix partitions into aqueous environments and stabilize the packing of beta-sheets in the formation of amyloid-like fibrils. We demonstrate that fibrillization can be disrupted with a new class of inhibitors that target the molecular grooves created by glycine.

Intra- and extracellular Abeta and PHF in clinically evaluated cases of Alzheimer's disease

Temporal cortical sections from postmortem brains of individuals without any dementing condition and with different degrees of severity of Alzheimer's disease (AD) evaluated by the Clinical Dementia Rating scale (CDR 0-CDR 3) were analyzed using immunohistochemical procedures. To demonstrate the amyloid-beta-peptide (Abeta) deposition and the neurofibrillary pathology, two monoclonal antibodies were used, a human CERAD Abeta (10D5) antibody raised against the N-terminal region of the Abeta-peptide, and an antibody raised against paired helical filaments (PHF-1). The neuron cell bodies and the glial cells were also recognized by two polyclonal antibodies raised, respectively, against the protein gene peptide (PGP 9.5) and glial fibrillary acidic protein (GFAP). Directly related to severity of AD, progressive deposits of Abeta-peptide were found within cortical pyramidal-like neurons and forming senile plaques. Ultrastructurally, Abeta-peptide deposits were related to neuronal intracytoplasmic organelles, such as the ER, the mitochondria, the Nissl bodies and lipofuscin. We have also found that the intracellular deposition of the Abeta peptide is a neuropathological finding prior to the appearance of PHF-immunoreactive structures. We suggest that the intracellular Abeta deposition in cortical pyramidal neurons is a first neurodegenerative event in AD development and that it is involved in cell dysfunction, neuronal death, and plaque formation.

Origin and turnover of microglial cells in fibrillar plaques of APPsw transgenic mice

Activated microglial cells are an integral component of fibrillar plaques in brains of subjects with Alzheimer's disease (AD) and in brains of transgenic mice overexpressing amyloidogenic fragments of human amyloid precursor protein (APP). The aim of this ultrastructural study of fibrillar plaques was to characterize the origin of microglial cells involved in cored plaque formation. Computer-aided three-dimensional reconstruction of plaques and microvessels in APPsw transgenic mice shows perivascular development of cored plaques. Perivascular location of almost all examined plaques and the infiltration at the interface between vessels and plaques with cells of monocyte/microglia lineage indicates that plaques are formed by inflammatory cells of blood origin. The increase in the number of microglial cells from 1 or 2 in an early plaque to more than 100 in a several-month-old plaque does not result in plaque degradation, but is associated with amyloid core growth and progression of neuronal degeneration, and suggests that recruitment of inflammatory cells of blood origin sustains plaque growth. Infiltration of the plaque with cells of blood origin and degeneration of 10-46% of inflammatory cells in large plaques, which is especially frequent at the interface between capillary wall and plaque, suggest their accelerated turnover.

Diverse fibrillar peptides directly bind the Alzheimer's amyloid precursor protein and amyloid precursor-like protein 2 resulting in cellular accumulation

The Alzheimer's disease Abeta peptide can increase the levels of cell-associated amyloid precursor protein (APP) in vitro. To determine the specificity of this response for Abeta and whether it is related to cytotoxicity, we tested a diverse range of fibrillar peptides including amyloid-beta (Abeta), the fibrillar prion peptides PrP106-126 and PrP178-193 and human islet-cell amylin. All these peptides increased the levels of APP and amyloid precursor-like protein 2 (APLP2) in primary cultures of astrocytes and neurons. Specificity was shown by a lack of change to amyloid precursor-like protein 1, tau-1 and cellular prion protein (PrP(c)) levels. APP and APLP2 levels were elevated only in cultures exposed to fibrillar peptides as assessed by electron microscopy and not in cultures treated with non-fibrillogenic peptide variants or aggregated lipoprotein. We found that PrP106-126 and the non-toxic but fibril-forming PrP178-193 increased APP levels in cultures derived from both wild-type and PrP(c)-deficient mice indicating that fibrillar peptides up-regulate APP through a non-cytotoxic mechanism and irrespective of parental protein expression. Fibrillar PrP106-126 and Abeta peptides bound recombinant APP and APLP2 suggesting the accumulation of these proteins was mediated by direct binding to the fibrillated peptide. This was supported by decreased APP accumulation following extensive washing of the cultures to remove fibrillar aggregates. Pre-incubation of fibrillar peptide with recombinant APP18-146, the putative fibril binding site, also abrogated the accumulation of APP. These findings show that diverse fibrillogenic peptides can induce accumulation of APP and APLP2 and this mechanism could contribute to pathogenesis in neurodegenerative disorders.

Amyloid cored plaques in Tg2576 transgenic mice are characterized by giant plaques, slightly activated microglia, and the lack of paired helical filament-typed, dystrophic neurites

We examined the brains of Tg2576 transgenic mice carrying human amyloid precursor protein with the Swedish mutation and Alzheimer's disease (AD) by means of immunohistochemistry and electron microscopy to clarify the characteristics of amyloid-associated pathology in the transgenic mice. In 12- to 29-month-old Tg2576 mice, congophilic cored plaques in the neocortex and hippocampus were labeled by all of the Abeta1-, Abeta40- and 42-specific antibodies, as seen in the classical plaques in AD. However, large-sized (>50 micro m in core diameter) plaques were seen more frequently in the older mice (18-29 months) than in those with AD (approximately 20% vs 2% in total cored plaques), and Tg2576 mice contained giant plaques (>75 micro m in core diameter), which were almost never seen in the brain of those with AD. Neither thread-like structures nor peripheral coronas were observed in the cored plaques of the transgenic mice in the silver impregnations. Immunohistochemically, plaque-accompanied microglia showed a slight enlargement of the cytoplasm with consistent labeling of Mac-1 and macrosialin (murine CD68), and with partial labeling of Ia antigen and macrophage-colony stimulating factor receptor. Ultrastructurally, the microglia surrounding the extracellular amyloid fibrils in the large, cored plaques showed some organella with phagocytic activity, such as secondary lysosomal, dense bodies, but intracellular amyloid fibrils were not evident. Dystrophic neurites in the plaques of the transgenic mice contained many dense multilaminar bodies, but no paired helical filaments. Our results suggest that giant cored plaques without coronas or paired helical filament-typed, dystrophic neurites are characteristic in Tg2576 mice, and that plaque-associated microglia in transgenic mice are activated to be in phagocytic function but not sufficient enough to digest extracellularly deposited amyloid fibrils.

[Presenilins as a marker of Alzheimer's disease]

Although the sporadic form of Alzheimer's disease is the most common, rare familial variants exist. Approximately 50% of these cases are caused by a mutation in the presenilin genes. Mutations in presinilin genes give rise to Alzheimer's disease in a dominant pattern of inheritance with an early age of onset (< 60 years). Both presenilins (PS-1 and PS-2) are transmembrane proteins localized in the intracellular membranes of the endoplasmatic reticulum and Golgi apparatus. This suggests they play a role in transport or sorting of proteins in the cell. Different lines of evidence directly link presenilin to the formation of beta-amyloid, an important constituent of senile plaques. PS-1 has an essential function during development: mice lacking intact PS-1 are not viable. In addition, structural and functional homologies have been identified between presenilins and Notch signal transduction pathways, which play a role in development. The discovery of the presenilin mutations has provided a new angle to Alzheimer's disease research. Eventually, this will probably greatly contribute to knowledge of the pathogenesis of the disease and in time support the development of novel therapeutic strategies.

Filament heterogeneity within the dystrophic neurites of senile plaques suggests blockage of fast axonal transport in Alzheimer's disease

In this study, the direct comparison of biopsy and autopsy tissue by morphological and immunocytochemical techniques, respectively, was used to document cytoskeletal changes of dystrophic neurites (DN) of senile plaques in Alzheimer's disease. This dual approach demonstrated several unreported abnormalities which, together with analogous findings in several experimental models, suggest that DN are associated with deficiencies in fast axonal transport and replacement of the cytoskeleton by an array of related abnormal filaments.

Amyloid beta protein in rat soleus muscle in chloroquine-induced myopathy using end-specific antibodies for A beta 40 and A beta 42: immunohistochemical evidence for amyloid beta protein

Previous immunohistochemical studies from this laboratory demonstrated that monoclonal antibodies raised against various regions of amyloid precursor protein (APP) (i.e., N-terminus, amyloid beta protein (A beta), and C-terminus) strongly labeled vacuoles in chloroquine-induced myopathy-affected muscle in rats. In this study, we used antibodies end specific for the A beta 40 and A beta 42 species, and a monoclonal antibody to A beta 1-9 which reacts with APP and A beta. Most vacuoles clearly reacted with anti-A beta 1-9, while about half reacted with anti-A beta 42, and only a few reacted with anti-A beta 40. These results demonstrate that vacuoles in chloroquine-induced myopathy-affected muscle contain cleaved A beta, and that distribution of the two major A beta species is similar to what is observed in A beta deposition in Alzheimer's disease (AD)-affected brain. This provides further evidence that chloroquine-induced myopathy in rats provides a suitable model to understand APP processing into A beta, and the role of APP in terms of the pathogenesis of AD.

Soluble multimeric Alzheimer beta(1-40) pre-amyloid complexes in dilute solution

Aqueous solutions of beta(1-40) peptide spontaneously associate to form pentameric/hexameric complexes that can be demonstrated by SDS-PAGE following treatment with glutaraldehyde and borohydride reduction. Under amyloidogenic conditions of pH and high peptide concentration these aggregates can further associate to form pentameric/hexameric complexes that can be demonstrated by SDS-PAGE following treatment with glutaraldehyde and borohydride reduction. Under amyloidogenic conditions of pH and high peptide concentration these aggregates can further associate to form sedimentable and filterable structures with beta-sheet amyloid characteristics of Thioflavine T fluorescence. The presence of such preamyloid structures at low peptide concentration suggests a mechanism by which amyloid plaques can accrete additional material by a cooperative rather than monomeric growth. The existence of a monomer<==>multimer equilibrium may partly explain the divergence of biological consequences with respect to neurotoxicity.

Is the "preamyloid" of diffuse plaques in Alzheimer's disease really nonfibrillar?

Several papers have described an 'amorphous' component of the amyloid in diffuse plaques and it has been suggested that this is 'preamyloid,' which is not organized into fibrils. Because most of the studies have been performed on autopsy tissue it was the purpose of this study to compare the ultrastructure of diffuse amyloid deposits in well preserved Alzheimer's disease biopsy specimens with autopsy tissues from patients with Alzheimer's disease and Down's syndrome. A postembedding immunogold technique with anti-beta/A4 protein demonstrated gold particles exclusively on extracellular amyloid fibrils in both biopsy and autopsy brains. We have presented evidence that suggests the claim for the existence of an amorphous component within the beta/A4 protein-positive material is unconvincing.

The influence of amino acid sequence on the fibrillogenicity and amyloidogenicity of the carboxy-terminus of beta-amyloid precursor protein

C-APP, a synthetic peptide corresponding to the C-terminal 20 amino acids of beta-amyloid precursor protein, forms amyloid fibrils in vitro. We investigated the effect of altering the C-APP sequence or deleting part of it on its ability to form amyloid fibrils. Substituting any single amino acid in the C-APP sequence with alanine did not prevent the formation of CAPP-like fibrils. Peptides with single or multiple substitutions that included T11, F14, F15, or Q19 showed reduced fibril-forming capacity while those with K1 and/or K13 replaced with alanine or glutamic acid showed enhanced capacity. When P10 or F14 was replaced with alanine, the fibrils were less congophilic than C-APP fibrils. All of the truncated peptides that were able to form fibrils contained at least 9 amino acids from the N-terminus of C-APP or amino acids 7-20 from the C-terminus. However, several peptides that met these criteria, but started at Q3 or contained only 2-4 amino acids C-terminal to P-10, failed to form many or typical fibrils. Peptides that contained the C-APP sequence plus 5-20 adjacent amino acids from the beta-amyloid precursor protein formed fibrils less readily than C-APP and most of the fibrils were not congophilic. The exception was CAPP-30, which formed moderate amounts of congophilic fibrils resembling C-APP fibrils morphologically. Therefore, proteolysis which releases C-APP from these peptides (except CAPP-30) would be predicted to enhance their amyloidogenicity. These results suggest that several features of C-APP peptide may be important in fibril formation. One of these features is the length of the peptide, with lengths of about 10, 20, or 30 amino acids, favoring fibril formation.

Identification of the Alzheimer beta/A4 amyloid precursor protein in clathrin-coated vesicles purified from PC12 cells

The Alzheimer beta/A4 amyloid precursor protein (APP) can be proteolytically processed by at least two separate pathways in PC12 cells: chloroquine-insensitive secretory cleavage and chloroquine-sensitive intracellular degradation, presumably in the endosomal/lysosomal system. To further investigate the possibility of APP processing in the endosomal/lysosomal system, we have examined whether APP is present in clathrin-coated vesicles (CCVs), which mediate the transport of many proteins to the endosomal compartment. Using a procedure derived from established protocols for the purification of CCVs from mammalian organs, we obtained from PC12 cells highly purified CCVs that displayed the same morphological features as described for CCVs purified from other sources. The CCVs were enriched in full-length mature (fully post-translationally modified) forms of APP, as well as in the carboxyl-terminal APP fragment produced by the secretory cleavage pathway. As CCVs are known to be involved in only two intracellular pathways (trafficking from the plasma membrane to early endosomes, and from the trans-Golgi network to late endosomes/prelysosomes), these findings provide direct evidence that APP is transported to the endosomal/lysosomal system. Furthermore, the presence in CCVs of the carboxyl-terminal fragment resulting from APP secretory cleavage suggests that APP secretory processing occurs in a pre-CCV compartment.

Amyloid-like properties of a synthetic peptide corresponding to the carboxy terminus of beta-amyloid protein precursor

A synthetic peptide whose sequence corresponds to the 20 carboxy-terminal amino acids of beta-amyloid protein precursor (APP) was found to form fibrils in vitro. These fibrils showed birefringence in polarized light when stained with Congo red, fluoresced when bound with thioflavin S, were resistant to proteases, and had a cross-beta conformation. By contrast, peptides with other sequences from the intracellular domain of APP and a peptide corresponding to this entire domain did not exhibit the full range of beta-amyloid properties. These results suggest that a fragment from the C-terminus of the beta-amyloid protein precursor could bind to intraneuronal paired helical filaments and account for some of its amyloid-like properties.