Claire Goldsbury

Human Amylin Oligomer Growth and Fibril Elongation Define Two Distinct Phases in Amyloid Formation

Human amylin (hA), a 37-amino-acid polypeptide, is one of a number of peptides with the ability to form amyloid fibrils and cause disease. It is the main constituent of the pancreatic amyloid deposits associated with type 2 diabetes. Increasing interest in early assembly intermediates rather than the mature fibrils as the cytotoxic agent has led to this study in which the smallest hA oligomers have been captured by atomic force microscopy. These are 2.3 ± 1.9 nm in height, 23 ± 14 nm in length, and consist of an estimated 16 hA molecules. Oligomers first grow to a height of about 6 nm before they begin to significantly elongate into fibrils. Congo red inhibits elongation but not the growth in height of hA oligomers. Two distinct phases have thus been identified in hA fibrillogenesis: lateral growth of oligomers followed by longitudinal growth into mature fibrils. These observations suggest that mature fibrils are assembled directly via longitudinal growth of full-width oligomers, making assembly by lateral association of protofibrils appear less likely.

Full-length Rat Amylin Forms Fibrils Following Substitution of Single Residues from Human Amylin

Pancreatic amyloid deposits, composed of the 37 amino acid residue peptide amylin, represent an integral part of type 2 diabetes mellitus pathology. Human amylin (hA) forms fibrils in vitro and is toxic to cultured pancreatic islet beta-cells. In contrast, rat amylin (rA) which differs from hA by only six amino acid residues in the central region of the peptide, residues 18-29, does not form fibrils and is not cytotoxic. To elucidate the role of individual residues in fibril formation, we have generated a series of full-length rA variants and examined their ability to form fibrils in vitro. Single-residue substitutions with amino acids from corresponding positions of the hA sequence, i.e. R18H, L23F, or V26I, were sufficient to render rA competent for fibril formation albeit at a small yield. Combining two or three of these substitutions generally increased the ability to produce fibrils. Variant rA fibril morphologies were examined by negative stain electron microscopy and found to be similar to those generated by hA itself. Bulk assays, i.e. involving thioflavin-T fluorescence and sedimentation, showed that the amount of fibril formation was relatively small for these rA variants when compared to hA under the same conditions. Fibril growth was demonstrated by time-lapse atomic force microscopy, and MALDI-TOF mass spectrometry was used to verify that fibrils consisted of full-length peptide. Our observations confirm previous reports that the three proline residues play a dominant negative role in fibril formation. However, their presence is not sufficient to completely abolish the ability of rA to form fibrils, as each of the other three implicated residues (i.e. R18, L23 and V26) also has a dominant modulating effect.

Inhibition of APP Trafficking by Tau Protein Does Not Increase the Generation of Amyloid‐β Peptides

Amyloid‐β, a peptide derived from the precursor protein APP, accumulates in the brain and contributes to the neuropathology of Alzheimers disease. Increased generation of amyloid‐β might be caused by axonal transport inhibition, via increased dwell time of APP vesicles and thereby higher probability of APP cleavage by secretase enzymes residing on the same vesicles. We tested this hypothesis using a neuronal cell culture model of inhibited axonal transport and by imaging vesicular transport of fluorescently tagged APP and β‐secretase (BACE1). Microtubule‐associated tau protein blocks vesicle traffic by inhibiting the access of motor proteins to the microtubule tracks. In neurons co‐transfected with CFP‐tau, APP‐YFP traffic into distal neurites was strongly reduced. However, this did not increase amyloid‐β levels. In singly transfected axons, APP‐YFP was transported in large tubules and vesicles moving very fast (on average 3 µm/s) and with high fluxes in the anterograde direction (on average 8.4 vesicles/min). By contrast, BACE1‐CFP movement was in smaller tubules and vesicles that were almost 2× slower (on average 1.6 µm/s) with ~18× lower fluxes (on average 0.5 vesicles/min). Two‐colour microscopy of co‐transfected axons confirmed that the two proteins were sorted into distinct carriers. The results do not support the above hypothesis. Instead, they indicate that APP is transported on vesicles distinct from the secretase components and that amyloid‐β is not generated in transit when transport is blocked by tau.

Amyloid structure and assembly: Insights from scanning transmission electron microscopy

Amyloid fibrils are filamentous protein aggregates implicated in several common diseases such as Alzheimers disease and type II diabetes. Similar structures are also the molecular principle of the infectious spongiform encephalopathies such as Creutzfeldt-Jakob disease in humans, scrapie in sheep, and of the so-called yeast prions, inherited non-chromosomal elements found in yeast and fungi. Scanning transmission electron microscopy (STEM) is often used to delineate the assembly mechanism and structural properties of amyloid aggregates. In this review we consider specifically contributions and limitations of STEM for the investigation of amyloid assembly pathways, fibril polymorphisms and structural models of amyloid fibrils. This type of microscopy provides the only method to directly measure the mass-per-length (MPL) of individual filaments. Made on both in vitro assembled and ex vivo samples, STEM mass measurements have illuminated the hierarchical relationships between amyloid fibrils and revealed that polymorphic fibrils and various globular oligomers can assemble simultaneously from a single polypeptide. The MPLs also impose strong constraints on possible packing schemes, assisting in molecular model building when combined with high-resolution methods like solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR).

Activated actin-depolymerizing factor/cofilin sequesters phosphorylated microtubule-associated protein during the assembly of alzheimer-like neuritic cytoskeletal striations.

In Alzheimers disease (AD), rod-like cofilin aggregates (cofilin–actin rods) and thread-like inclusions containing phosphorylated microtubule-associated protein (pMAP) tau form in the brain (neuropil threads), and the extent of their presence correlates with cognitive decline and disease progression. The assembly mechanism of these respective pathological lesions and the relationship between them is poorly understood, yet vital to understanding the causes of sporadic AD. We demonstrate that, during mitochondrial inhibition, activated actin-depolymerizing factor (ADF)/cofilin assemble into rods along processes of cultured primary neurons that recruit pMAP/tau and mimic neuropil threads. Fluorescence resonance energy transfer analysis revealed colocalization of cofilin-GFP (green fluorescent protein) and pMAP in rods, suggesting their close proximity within a cytoskeletal inclusion complex. The relationship between pMAP and cofilin–actin rods was further investigated using actin-modifying drugs and small interfering RNA knockdown of ADF/cofilin in primary neurons. The results suggest that activation of ADF/cofilin and generation of cofilin–actin rods is required for the subsequent recruitment of pMAP into the inclusions. Additionally, we were able to induce the formation of pMAP-positive ADF/cofilin rods by exposing cells to exogenous amyloid-β (Aβ) peptides. These results reveal a common pathway for pMAP and cofilin accumulation in neuronal processes. The requirement of activated ADF/cofilin for the sequestration of pMAP suggests that neuropil thread structures in the AD brain may be initiated by elevated cofilin activation and F-actin bundling that can be caused by oxidative stress, mitochondrial dysfunction, or Aβ peptides, all suspected initiators of synaptic loss and neurodegeneration in AD.

Oxidative stress increases levels of endogenous amyloid‐β peptides secreted from primary chick brain neurons

Oxidative damage is associated with Alzheimers disease and mild cognitive impairment, but its relationship to the development of neuropathological lesions involving accumulation of amyloid‐β (Aβ) peptides and hyperphosphorylated tau protein remains poorly understood. We show that inducing oxidative stress in primary chick brain neurons by exposure to sublethal doses of H2O2 increases levels of total secreted endogenous Aβ by 2.4‐fold after 20 h. This occurs in the absence of changes to intracellular amyloid precursor protein or tau protein levels, while heat‐shock protein 90 is elevated 2.5‐fold. These results are consistent with the hypothesis that aging‐associated oxidative stress contributes to increasing Aβ generation and up‐regulation of molecular chaperones in Alzheimers disease.

Introduction to Atomic Force Microscopy (AFM) in Biology

The atomic force microscope has the unique capability of imaging biological samples with molecular resolution in buffer solution. In addition to providing topographical images of surfaces with nanometer‐ to angstrom‐scale resolution, forces between single molecules and mechanical properties of biological samples can be investigated. Importantly, the measurements are made in buffer solutions, allowing biological samples to stay alive within a physiological‐like environment while temporal changes in structure are measured. This overview provides an introduction to AFM on biological systems and describes specific examples of AFM on proteins. The physical principles of the technique and methodological aspects of its practical use and applications are also described

Microglia show altered morphology and reduced arborization in human brain during aging and Alzheimer's disease

Changes in microglia function are involved in Alzheimers disease (AD) for which ageing is the major risk factor. We evaluated microglial cell process morphologies and their gray matter coverage (arborized area) during ageing and in the presence and absence of AD pathology in autopsied human neocortex. Microglial cell processes were reduced in length, showed less branching and reduced arborized area with aging (case range 52–98 years). This occurred during normal ageing and without microglia dystrophy or changes in cell density. There was a larger reduction in process length and arborized area in AD compared to aged‐matched control microglia. In AD cases, on average, 49%–64% of microglia had discontinuous and/or punctate Iba1 labeled processes instead of continuous Iba1 distribution. Up to 16% of aged‐matched control microglia displayed discontinuous or punctate features. There was no change in the density of microglial cell bodies in gray matter during ageing or AD. This demonstrates that human microglia show progressive cell process retraction without cell loss during ageing. Additional changes in microglia occur with AD including Iba1 protein puncta and discontinuity. We suggest that reduced microglial arborized area may be an aging‐related correlate of AD in humans. These variations in microglial cells during ageing and in AD could reflect changes in neural‐glial interactions which are emerging as key to mechanisms involved in ageing and neurodegenerative disease.

Rapid changes in phospho-MAP/tau epitopes during neuronal stress: cofilin-actin rods primarily recruit microtubule binding domain epitopes.

Abnormal mitochondrial function is a widely reported contributor to neurodegenerative disease including Alzheimers disease (AD), however, a mechanistic link between mitochondrial dysfunction and the initiation of neuropathology remains elusive. In AD, one of the earliest hallmark pathologies is neuropil threads comprising accumulated hyperphosphorylated microtubule-associated protein (MAP) tau in neurites. Rod-like aggregates of actin and its associated protein cofilin (AC rods) also occur in AD. Using a series of antibodies - AT270, AT8, AT100, S214, AT180, 12E8, S396, S404 and S422 - raised against different phosphoepitopes on tau, we characterize the pattern of expression and re-distribution in neurites of these phosphoepitope labels during mitochondrial inhibition. Employing chick primary neuron cultures, we demonstrate that epitopes recognized by the monoclonal antibody 12E8, are the only species rapidly recruited into AC rods. These results were recapitulated with the actin depolymerizing drug Latrunculin B, which induces AC rods and a concomitant increase in the 12E8 signal measured on Western blot. This suggests that AC rods may be one way in which MAP redistribution and phosphorylation is influenced in neurons during mitochondrial stress and potentially in the early pathogenesis of AD.

Mutant human FUS Is ubiquitously mislocalized and generates persistent stress granules in primary cultured transgenic zebrafish cells.

FUS mutations can occur in familial amyotrophic lateral sclerosis (fALS), a neurodegenerative disease with cytoplasmic FUS inclusion bodies in motor neurons. To investigate FUS pathology, we generated transgenic zebrafish expressing GFP-tagged wild-type or fALS (R521C) human FUS. Cell cultures were made from these zebrafish and the subcellular localization of human FUS and the generation of stress granule (SG) inclusions examined in different cell types, including differentiated motor neurons. We demonstrate that mutant FUS is mislocalized from the nucleus to the cytosol to a similar extent in motor neurons and all other cell types. Both wild-type and R521C FUS localized to SGs in zebrafish cells, demonstrating an intrinsic ability of human FUS to accumulate in SGs irrespective of the presence of disease-associated mutations or specific cell type. However, elevation in relative cytosolic to nuclear FUS by the R521C mutation led to a significant increase in SG assembly and persistence within a sub population of vulnerable cells, although these cells were not selectively motor neurons.