Tamotsu Zako

Amyloid oligomers: formation and toxicity of Aβ oligomers

Alzheimer’s disease (AD) is an age‐related, progressive degenerative disorder that is characterized by synapse and neuron loss in the brain and the accumulation of protein‐containing deposits (referred to as ‘senile plaques’) and neurofibrillary tangles. Insoluble amyloid β‐peptide (Aβ) fibrillar aggregates found in extracellular plaques have long been thought to cause the neurodegenerative cascades of AD. However, accumulating evidence suggests that prefibrillar soluble Aβ oligomers induce AD‐related synaptic dysfunction. The size of Aβ oligomers is distributed over a wide molecular weight range (from < 10 kDa to > 100 kDa), with structural polymorphism in Aβ oligomers of similar sizes. Recent studies have demonstrated that Aβ can accumulate in living cells, as well as in extracellular spaces. This review summarizes current research on Aβ oligomers, focusing on their structures and toxicity mechanism. We also discuss possible formation mechanisms of intracellular and extracellular Aβ oligomers.

Cyclic RGD peptide-labeled upconversion nanophosphors for tumor cell-targeted imaging

One of the great challenges of oncology is to improve methods for early tumor detection. Thus tumor cell-targeted optical imaging has been intensively studied. Bioimaging with upconversion (UC) phosphors (UCPs) is of considerable interest due to a variety of possible applications taking advantage of infrared-to-visible luminescence. Here we report for the first time tumor cell-targeted UC imaging using UCPs modified with cyclic RGD peptide (RGD-Y2O3). Cyclic RGD peptide binds specifically to integrin alphavbeta3 which is highly expressed in a tumor cell surface of certain cancer types but not in normal tissues. Since UC emission from RGD-Y2O3 was observed for U87MG cancer cell (high integrin alphavbeta3 expression), but not for MCF-7 cancer cell (low integrin alphavbeta3 expression), this UC imaging is considered to be integrin alphavbeta3 specific. The non-invasive imaging of integrin alphavbeta3 expression using UCP-based probes will have great potential in cancer imaging in general in living subjects.

Bovine Insulin Filaments Induced by Reducing Disulfide Bonds Show a Different Morphology, Secondary Structure, and Cell Toxicity from Intact Insulin Amyloid Fibrils

Amyloid fibrils are associated with more than 20 diseases, including Alzheimers disease and type II diabetes. Insulin is a 51-residue polypeptide hormone, with its two polypeptide chains linked by one intrachain and two interchain disulfide bonds, and has long been known to self-assemble in vitro into amyloid fibrils. We demonstrate here that bovine insulin forms flexible filaments in the presence of a reducing agent, Tris (2-carboxyethyl) phosphine. The insulin filaments, possibly formed due to partial reduction of S-S bonds in insulin molecules, differ from intact insulin fibrils in terms of their secondary structure. The insulin filaments were determined to have an antiparallel beta-sheet structure, whereas the insulin fibrils have a parallel beta-sheet structure. Of importance, the cell toxicity of the insulin filaments was remarkably lower than that of the insulin fibrils. This finding supports the idea that cell toxicity of amyloids correlates with their morphology. The remarkably low toxicity of the filamentous structure should shed new light on possible pharmacological approaches to the various diseases caused by amyloid fibrils.

Facilitated release of substrate protein from prefoldin by chaperonin

Prefoldin is a chaperone that captures a protein‐folding intermediate and transfers it to the group II chaperonin for correct folding. However, kinetics of interactions between prefoldin and substrate proteins have not been investigated. In this study, dissociation constants and dissociation rate constants of unfolded proteins with prefoldin were firstly measured using fluorescence microscopy. Our results suggest that binding and release of prefoldin from hyperthermophilic archaea with substrate proteins were in a dynamic equilibrium. Interestingly, the release of substrate proteins from prefoldin was facilitated when chaperonin was present, supporting a handoff mechanism of substrate proteins from prefoldin to the chaperonin.

Formation of highly toxic soluble amyloid beta oligomers by the molecular chaperone prefoldin

Alzheimer’s disease (AD) is a neurological disorder characterized by the presence of amyloid β (Aβ) peptide fibrils and oligomers in the brain. It has been suggested that soluble Aβ oligomers, rather than Aβ fibrils, contribute to neurodegeneration and dementia due to their higher level of toxicity. Recent studies have shown that Aβ is also generated intracellularly, where it can subsequently accumulate. The observed inhibition of cytosolic proteasome by Aβ suggests that Aβ is located within the cytosolic compartment. To date, although several proteins have been identified that are involved in the formation of soluble Aβ oligomers, none of these have been shown to induce in vitro formation of the high‐molecular‐mass (> 50 kDa) oligomers found in AD brains. Here, we examine the effects of the jellyfish‐shaped molecular chaperone prefoldin (PFD) on Aβ(1–42) peptide aggregation in vitro. PFD is thought to play a general role in de novo protein folding in archaea, and in the biogenesis of actin, tubulin and possibly other proteins in the cytosol of eukaryotes. We found that recombinant Pyrococcus PFD produced high‐molecular‐mass (50–250 kDa) soluble Aβ oligomers, as opposed to Aβ fibrils. We also demonstrated that the soluble Aβ oligomers were more toxic than Aβ fibrils, and were capable of inducing apoptosis. As Pyrococcus PFD shares high sequence identity to human PFD and the PFD‐homolog protein found in human brains, these results suggest that PFD may be involved in the formation of toxic soluble Aβ oligomers in the cytosolic compartment in vivo.

Characterization of archaeal group II chaperonin-ADP-metal fluoride complexes: Implications that group II chaperonins operate as a "two-stroke engine"

Group II chaperonins, found in Archaea and in the eukaryotic cytosol, act independently of a cofactor corresponding to GroES of group I chaperonins. Instead, the helical protrusion at the tip of the apical domain forms a built-in lid of the central cavity. Although many studies on the lids conformation have been carried out, the conformation in each step of the ATPase cycle remains obscure. To clarify this issue, we examined the effects of ADP-aluminum fluoride (AlFx) and ADP-beryllium fluoride (BeFx) complexes on α-chaperonin from the hyperthermophilic archaeum, Thermococcus sp. strain KS-1. Biochemical assays, electron microscopic observations, and small angle x-ray scattering measurements demonstrate that α-chaperonin incubated with ADP and BeFx exists in an asymmetric conformation; one ring is open, and the other is closed. The result indicates that α-chaperonin also shares the inherent functional asymmetry of bacterial and eukaryotic cytosolic chaperonins. Most interestingly, addition of ADP and BeFx induced α-chaperonin to encapsulate unfolded proteins in the closed ring but did not trigger their folding. Moreover, α-chaperonin incubated with ATP and AlFx or BeFx adopted a symmetric closed conformation, and its functional turnover was inhibited. These forms are supposed to be intermediates during the reaction cycle of group II chaperonins.

Contribution of the C-terminal region to the thermostability of the archaeal group II chaperonin from Thermococcus sp. strain KS-1

Chaperonin is a double ring-shaped oligomeric protein complex, which captures a protein in the folding intermediate state and assists its folding in an ATP-dependent manner. The chaperonin from a hyperthermophilic archaeum, Thermococcus sp. strain KS-1, is a group II chaperonin and is composed of two distinct subunits, α and β. Although these subunits are highly homologous in sequence, the homo-oligomer of the β-subunit is more thermostable than that of the α-subunit. To identify the region responsible for this difference in thermostability, we constructed domain-exchange mutants. The mutants containing the equatorial domain of the β-subunit were more resistant to thermal dissociation than the mutants with that of the α-subunit. Thermostability of a β-subunit mutant whose C-terminal 22 residues were replaced with those of the α-subunit decreased to the comparable level of that of the α-subunit homo-oligomer. These results indicate that the difference in thermostability between α- and β-subunits mainly originates in the C-terminal residues in the equatorial domain, only where they exhibit substantial sequence difference.

Prefoldin Protects Neuronal Cells from Polyglutamine Toxicity by Preventing Aggregation Formation

Background: Prefoldin, a molecular chaperone composed of six subunits, prevents misfolding of newly synthesized nascent polypeptides. Results: Prefoldin inhibited aggregation of pathogenic Huntingtin and subsequent cell death. Conclusion: Prefoldin suppressed Huntingtin aggregation at the small oligomer stage. Significance: Prefoldin plays a role in preventing protein aggregation in Huntington disease. Huntington disease is caused by cell death after the expansion of polyglutamine (polyQ) tracts longer than ∼40 repeats encoded by exon 1 of the huntingtin (HTT) gene. Prefoldin is a molecular chaperone composed of six subunits, PFD1–6, and prevents misfolding of newly synthesized nascent polypeptides. In this study, we found that knockdown of PFD2 and PFD5 disrupted prefoldin formation in HTT-expressing cells, resulting in accumulation of aggregates of a pathogenic form of HTT and in induction of cell death. Dead cells, however, did not contain inclusions of HTT, and analysis by a fluorescence correlation spectroscopy indicated that knockdown of PFD2 and PFD5 also increased the size of soluble oligomers of pathogenic HTT in cells. In vitro single molecule observation demonstrated that prefoldin suppressed HTT aggregation at the small oligomer (dimer to tetramer) stage. These results indicate that prefoldin inhibits elongation of large oligomers of pathogenic Htt, thereby inhibiting subsequent inclusion formation, and suggest that soluble oligomers of polyQ-expanded HTT are more toxic than are inclusion to cells.

Effect of the C-terminal Truncation on the Functional Cycle of Chaperonin GroEL IMPLICATION THAT THE C-TERMINAL REGION FACILITATES THE TRANSITION FROM THE FOLDING-ARRESTED TO THE FOLDING-COMPETENT STATE

To elucidate the exact role of the C-terminal region of GroEL in its functional cycle, the C-terminal 20-amino acid truncated mutant of GroEL was constructed. The steady-state ATPase rate and duration of GroES binding showed that the functional cycle of the truncated GroEL is extended by ∼2 s in comparison with that of the wild type, without interfering with the basic functions of GroEL. We have proposed a model for the functional cycle of GroEL, which consists of two rate-limiting steps of ∼3- and ∼5-s duration ( Ueno, T., Taguchi, H., Tadakuma, H., Yoshida, M., and Funatsu, T. (2004) Mol. Cell 14, 423-434 g). According to the model, detailed kinetic studies were performed. We found that a 20-residue truncation of the C terminus extends the time until inorganic phosphate is generated and the time for arresting protein folding in the central cavity, i.e. the lifetime of the first rate-limiting step in the functional cycle, to an ∼5-s duration. These results suggest that the integrity of the C-terminal region facilitates the transition from the first to the second rate-limiting state.

The role of firefly luciferase C-terminal domain in efficient coupling of adenylation and oxidative steps

The N‐terminal domain (N‐domain) of the firefly luciferase from Photinus pyraris has weak luminescence activity, and shows a unique light emitting profile with very long rise time of more than several minutes. Through a sensitive assay of the reaction intermediate luciferyl‐adenylate (LH2‐AMP), we found that the slow increase in the N‐domain luminescence faithfully reflected the concentration of dissociated LH2‐AMP. No such correlation was observed for wild‐type or mutant enzymes with short rise time, except one with longer rise time. The results suggest that the C‐terminal domain plays an indispensable role in efficiently coupling adenylation and oxidative steps.