Jessica Meinhardt

Aβ(1-40) Fibril Polymorphism Implies Diverse Interaction Patterns in Amyloid Fibrils

Amyloid fibrils characterize a diverse group of human diseases that includes Alzheimers disease, Creutzfeldt-Jakob and type II diabetes. Alzheimers amyloid fibrils consist of amyloid-beta (Abeta) peptide and occur in a range of structurally different fibril morphologies. The structural characteristics of 12 single Abeta(1-40) amyloid fibrils, all formed under the same solution conditions, were determined by electron cryo-microscopy and three-dimensional reconstruction. The majority of analyzed fibrils form a range of morphologies that show almost continuously altering structural properties. The observed fibril polymorphism implies that amyloid formation can lead, for the same polypeptide sequence, to many different patterns of inter- or intra-residue interactions. This property differs significantly from native, monomeric protein folding reactions that produce, for one protein sequence, only one ordered conformation and only one set of inter-residue interactions.

Directed selection of a conformational antibody domain that prevents mature amyloid fibril formation by stabilizing Aβ protofibrils

The formation of amyloid fibrils is a common biochemical characteristic that occurs in Alzheimers disease and several other amyloidoses. The unifying structural feature of amyloid fibrils is their specific type of β-sheet conformation that differentiates these fibrils from the products of normal protein folding reactions. Here we describe the generation of an antibody domain, termed B10, that recognizes an amyloid-specific and conformationally defined epitope. This antibody domain was selected by phage-display from a recombinant library of camelid antibody domains. Surface plasmon resonance, immunoblots, and immunohistochemistry show that this antibody domain distinguishes Aβ amyloid fibrils from disaggregated Aβ peptide as well as from specific Aβ oligomers. The antibody domain possesses functional activity in preventing the formation of mature amyloid fibrils by stabilizing Aβ protofibrils. These data suggest possible applications of B10 in the detection of amyloid fibrils or in the modulation of their formation.

Structural polymorphism of Alzheimer Aβ and other amyloid fibrils

Deposits of amyloid fibrils characterize a diverse group of human diseases that includes Alzheimer’s disease, Creutzfeldt-Jakob disease and type II diabetes. Amyloid fibrils formed from different polypeptides contain a common cross-β spine. Nevertheless, amyloid fibrils formed from the same polypeptide can occur in a range of structurally different morphologies. The heterogeneity of amyloid fibrils reflects different types of polymorphism: (i) variations in the protofilament number, (ii) variations in the protofilament arrangement and (iii) different polypeptide conformations. Amyloid fibril polymorphism implies that fibril formation can lead, for the same polypeptide sequence, to many different patterns of inter- or intra-residue interactions. This property differs significantly from native, monomeric protein folding reactions that produce, for one protein sequence, only one ordered conformation and only one set of inter-residue interactions.

Early neuronal dysfunction by amyloid β oligomers depends on activation of NR2B-containing NMDA receptors.

Several studies indicate that NMDA receptor signaling is involved in Aβ oligomer-mediated impairment of neuronal function and morphology. Utilizing primary neuronal cell culture and hippocampal slices from rat and mouse, we found that Aβ oligomer administration readily impairs long-term potentiation, reduces baseline synaptic transmission, decreases neuronal spontaneous network activity and induces retraction of synaptic contacts long before major cytotoxic effects are visible. Interestingly, all these effects can be blocked with the NR2B-containing NMDA-receptor antagonist ifenprodil or Ro 25-6981 suggesting that activation of downstream effectors of these receptors is involved in early detrimental actions of Aβ oligomers. In line we found that Jacob, a messenger that can couple extrasynaptic NMDA-receptor activity to CREB dephosphorylation, accumulates in the nucleus after Aβ oligomer administration and that the nuclear accumulation of Jacob can be blocked by a simultaneous application of ifenprodil. We conclude that Aβ oligomers induce early neuronal dysfunction mainly by activation of NR2B-containing NMDA-receptors.

Oligomeric and fibrillar species of β-amyloid (Aβ42) both impair mitochondrial function in P301L tau transgenic mice

We recently provided evidence for a mitochondrial dysfunction in P301L tau transgenic mice, a strain modeling the tau pathology of Alzheimer’s disease (AD) and frontotemporal dementia (FTD). In addition to tau aggregates, the AD brain is further characterized by Aβ peptide-containing plaques. When we addressed the role of Aβ, this indicated a synergistic action of tau and Aβ pathology on the mitochondria. In the present study, we compared the toxicity of different Aβ42 conformations in light of recent studies suggesting that oligomeric rather than fibrillar Aβ might be the actual toxic species. Interestingly, both oligomeric and fibrillar, but not disaggregated (mainly monomeric) Aβ42 caused a decreased mitochondrial membrane potential in cortical brain cells obtained from FTD P301L tau transgenic mice. This was not observed with cerebellar preparations indicating selective vulnerability of cortical neurons. Furthermore, we found reductions in state 3 respiration, the respiratory control ratio, and uncoupled respiration when incubating P301L tau mitochondria either with oligomeric or fibrillar preparations of Aβ42. Finally, we found that aging specifically increased the sensitivity of mitochondria to oligomeric Aβ42 damage indicating that oligomeric and fibrillar Aβ42 are both toxic, but exert different degrees of toxicity.

Structural Basis of β‐Amyloid‐Dependent Synaptic Dysfunctions

Aggregation of b-amyloid (Ab) peptide into oligomers and protofibrils is a hallmark of Alzheimer s disease (AD). Increasing evidence shows that the primary insult in AD is caused by oligomeric species that impair the ordered function of synaptic networks. Consistent with this view, oligomers were shown to affect synaptic plasticity, and they impair the long-term potentiation (LTP) in living brain tissues, a widely used model system of brain memory functions. Using solidstate NMR spectroscopy, we here determined the residuespecific molecular conformation of a highly synaptotoxic bamyloid oligomer structure. Our measurements reveal a stable N-terminal b strand that controls the partitioning between oligomer and protofibril formation, whereas targeting the peptide N-terminus ameliorates Ab-dependent neuronal dysfunctions. The presently investigated, chemically well-defined Ab oligomers faithfully reproduce the hallmark characteristics of AD-related oligomers. Living hippocampal brain slices were exposed to different Ab conformers (Figure 1A), and a series of tetanic electrical stimuli were applied to evoke a longlasting increase of the synaptic transmission, termed LTP. Oligomers, but not freshly dissolved, that is, primarily monomeric, Ab peptide or fibrils, reduce the LTP response and therefore disturb the brain memory functions within these tissue samples (Figure 1B). A similar oligomer-specificity is seen with cultured primary neurons, which present a significant oligomer-dependent decrease ( 40%) of their

Similarities in the thermodynamics and kinetics of aggregation of disease‐related Aβ(1–40) peptides

Increasing evidence indicates that polypeptide aggregation often involves a nucleation and a growth phase, although the relationship between the factors that determine these two phases has not yet been fully clarified. We present here an analysis of several mutations at different sites of the Aβ(1–40) peptide, including those associated with early onset forms of the Alzheimers disease, which reveals that the effects of specific amino acid substitutions in the sequence of this peptide are strongly modulated by their structural context. Nevertheless, mutations at different positions perturb in a correlated manner the free energies of aggregation as well as the lag times and growth rates. We show that these observations can be rationalized in terms of the intrinsic propensities for aggregation of the Aβ(1–40) sequence, thus suggesting that, in the case of this peptide, the determinants of the thermodynamics and of the nucleation and growth of the aggregates have a similar physicochemical basis.

Aβ Mediated Diminution of MTT Reduction—An Artefact of Single Cell Culture?

The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazoliumbromide (MTT) reduction assay is a frequently used and easily reproducible method to measure beta-amyloid (Aβ) toxicity in different types of single cell culture. To our knowledge, the influence of Aβ on MTT reduction has never been tested in more complex tissue. Initially, we reproduced the disturbed MTT reduction in neuron and astroglia primary cell cultures from rats as well as in the BV2 microglia cell line, utilizing four different Aβ species, namely freshly dissolved Aβ (25-35), fibrillar Aβ (1-40), oligomeric Aβ (1-42) and oligomeric Aβ (1-40). In contrast to the findings in single cell cultures, none of these Aβ species altered MTT reduction in rat organotypic hippocampal slice cultures (OHC). Moreover, application of Aβ to acutely isolated hippocampal slices from adult rats and in vivo intracerebroventricular injection of Aβ also did not influence the MTT reduction in the respective tissue. Failure of Aβ penetration into the tissue cannot explain the differences between single cells and the more complex brain tissue. Thus electrophysiological investigations disclosed an impairment of long-term potentiation (LTP) in the CA1 region of hippocampal slices from rat by application of oligomeric Aβ (1-40), but not by freshly dissolved Aβ (25-35) or fibrillar Aβ (1-40). In conclusion, the experiments revealed a glaring discrepancy between single cell cultures and complex brain tissue regarding the effect of different Aβ species on MTT reduction. Particularly, the differential effect of oligomeric versus other Aβ forms on LTP was not reflected in the MTT reduction assay. This may indicate that the Aβ oligomer effect on synaptic function reflected by LTP impairment precedes changes in formazane formation rate or that cells embedded in a more natural environment in the tissue are less susceptible to damage by Aβ, raising cautions against the consideration of single cell MTT reduction activity as a reliable assay in Alzheimers drug discovery studies.

Phosphate and HEPES buffers potently affect the fibrillation and oligomerization mechanism of Alzheimer’s Aβ peptide

The oligomerization of Aβ peptide into amyloid fibrils is a hallmark of Alzheimers disease. Due to its biological relevance, phosphate is the most commonly used buffer system for studying the formation of Aβ and other amyloid fibrils. Investigation into the characteristics and formation of amyloid fibrils frequently relies upon material formed in vitro, predominantly in phosphate buffers. Herein, we examine the effects on the fibrillation and oligomerization mechanism of Aβ peptide that occur due solely to the influence of phosphate buffer. We reveal that significant differences in amyloid fibrillation are observed due to fibrillation being initiated in phosphate or HEPES buffer (at physiological pH and temperature). Except for the differing buffer ions, all experimental parameters were kept constant. Fibril formation was assessed using fluorescently monitored kinetic studies, microscopy, X-ray fiber diffraction and infrared and nuclear magnetic resonance spectroscopies. Based on this set up, we herein reveal profound effects on the mechanism and speed of Aβ fibrillation. The three histidine residues at positions 6, 13 and 14 of Aβ(1-40) are instrumental in these mechanistic changes. We conclude that buffer plays a more significant role in fibril formation than has been generally acknowledged.

Structure and biomedical applications of amyloid oligomer nanoparticles.

Amyloid oligomers are nonfibrillar polypeptide aggregates linked to diseases, such as Alzheimers and Parkinsons. Here we show that these aggregates possess a compact, quasi-crystalline architecture that presents significant nanoscale regularity. The amyloid oligomers are dynamic assemblies and are able to release their individual subunits. The small oligomeric size and spheroid shape confer diffusible characteristics, electrophoretic mobility, and the ability to enter hydrated gel matrices or cells. We finally showed that the amyloid oligomers can be labeled with both fluorescence agents and iron oxide nanoparticles and can target macrophage cells. Oligomer amyloids may provide a new biological nanomaterial for improved targeting, drug release, and medical imaging.