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Cu(2+) affects amyloid-? (1-42) aggregation by increasing peptide-peptide binding forces.
PUBLISHED: 02-04-2013
The link between metals, Alzheimers disease (AD) and its implicated protein, amyloid-? (A?), is complex and highly studied. AD is believed to occur as a result of the misfolding and aggregation of A?. The dyshomeostasis of metal ions and their propensity to interact with A? has also been implicated in AD. In this work, we use single molecule atomic force spectroscopy to measure the rupture force required to dissociate two A? (1-42) peptides in the presence of copper ions, Cu(2+). In addition, we use atomic force microscopy to resolve the aggregation of A? formed. Previous research has shown that metal ions decrease the lag time associated with A? aggregation. We show that with the addition of copper ions the unbinding force increases notably. This suggests that the reduction of lag time associated with A? aggregation occurs on a single molecule level as a result of an increase in binding forces during the very initial interactions between two A? peptides. We attribute these results to copper ions acting as a bridge between the two peptide molecules, increasing the stability of the peptide-peptide complex.
Authors: Michal S. Shoshan, Edit Y. Tshuva, Deborah E. Shalev.
Published: 12-16-2013
Copper (I) binding by metallochaperone transport proteins prevents copper oxidation and release of the toxic ions that may participate in harmful redox reactions. The Cu (I) complex of the peptide model of a Cu (I) binding metallochaperone protein, which includes the sequence MTCSGCSRPG (underlined is conserved), was determined in solution under inert conditions by NMR spectroscopy. NMR is a widely accepted technique for the determination of solution structures of proteins and peptides. Due to difficulty in crystallization to provide single crystals suitable for X-ray crystallography, the NMR technique is extremely valuable, especially as it provides information on the solution state rather than the solid state. Herein we describe all steps that are required for full three-dimensional structure determinations by NMR. The protocol includes sample preparation in an NMR tube, 1D and 2D data collection and processing, peak assignment and integration, molecular mechanics calculations, and structure analysis. Importantly, the analysis was first conducted without any preset metal-ligand bonds, to assure a reliable structure determination in an unbiased manner.
21 Related JoVE Articles!
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Photo-Induced Cross-Linking of Unmodified Proteins (PICUP) Applied to Amyloidogenic Peptides
Authors: Farid Rahimi, Panchanan Maiti, Gal Bitan.
Institutions: University of California, Los Angeles, University of California, Los Angeles, University of California, Los Angeles.
The assembly of amyloidogenic proteins into toxic oligomers is a seminal event in the pathogenesis of protein misfolding diseases, including Alzheimer's, Parkinson's, and Huntington's diseases, hereditary amyotrophic lateral sclerosis, and type 2 diabetes. Owing to the metastable nature of these protein assemblies, it is difficult to assess their oligomer size distribution quantitatively using classical methods, such as electrophoresis, chromatography, fluorescence, or dynamic light scattering. Oligomers of amyloidogenic proteins exist as metastable mixtures, in which the oligomers dissociate into monomers and associate into larger assemblies simultaneously. PICUP stabilizes oligomer populations by covalent cross-linking and when combined with fractionation methods, such as sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) or size-exclusion chromatography (SEC), PICUP provides snapshots of the oligomer size distributions that existed before cross-linking. Hence, PICUP enables visualization and quantitative analysis of metastable protein populations and can be used to monitor assembly and decipher relationships between sequence modifications and oligomerization1. Mechanistically, PICUP involves photo-oxidation of Ru2+ in a tris(bipyridyl)Ru(II) complex (RuBpy) to Ru3+ by irradiation with visible light in the presence of an electron acceptor. Ru3+ is a strong one-electron oxidizer capable of abstracting an electron from a neighboring protein molecule, generating a protein radical1,2. Radicals are unstable, highly-reactive species and therefore disappear rapidly through a variety of intra- and intermolecular reactions. A radical may utilize the high energy of an unpaired electron to react with another protein monomer forming a dimeric radical, which subsequently loses a hydrogen atom and forms a stable, covalently-linked dimer. The dimer may then react further through a similar mechanism with monomers or other dimers to form higher-order oligomers. Advantages of PICUP relative to other photo- or chemical cross-linking methods3,4 include short (≤1 s) exposure to non-destructive visible light, no need for pre facto modification of the native sequence, and zero-length covalent cross-linking. In addition, PICUP enables cross-linking of proteins within wide pH and temperature ranges, including physiologic parameters. Here, we demonstrate application of PICUP to cross-linking of three amyloidogenic proteins the 40- and 42-residue amyloid β-protein variants (Aβ40 and Aβ42), and calcitonin, and a control protein, growth-hormone releasing factor (GRF).
Cross-linking, Issue 23, PICUP, amyloid β-protein, oligomer, amyloid, protein assembly
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Formation of Ordered Biomolecular Structures by the Self-assembly of Short Peptides
Authors: Sivan Yuran, Meital Reches.
Institutions: The Hebrew University of Jerusalem.
In nature, complex functional structures are formed by the self-assembly of biomolecules under mild conditions. Understanding the forces that control self-assembly and mimicking this process in vitro will bring about major advances in the areas of materials science and nanotechnology. Among the available biological building blocks, peptides have several advantages as they present substantial diversity, their synthesis in large scale is straightforward, and they can easily be modified with biological and chemical entities1,2. Several classes of designed peptides such as cyclic peptides, amphiphile peptides and peptide-conjugates self-assemble into ordered structures in solution. Homoaromatic dipeptides, are a class of short self-assembled peptides that contain all the molecular information needed to form ordered structures such as nanotubes, spheres and fibrils3-8. A large variety of these peptides is commercially available. This paper presents a procedure that leads to the formation of ordered structures by the self-assembly of homoaromatic peptides. The protocol requires only commercial reagents and basic laboratory equipment. In addition, the paper describes some of the methods available for the characterization of peptide-based assemblies. These methods include electron and atomic force microscopy and Fourier-Transform Infrared Spectroscopy (FT-IR). Moreover, the manuscript demonstrates the blending of peptides (coassembly) and the formation of a "beads on a string"-like structure by this process.9 The protocols presented here can be adapted to other classes of peptides or biological building blocks and can potentially lead to the discovery of new peptide-based structures and to better control of their assembly.
Chemistry, Issue 81, Materials (General), self-assembly, peptides, diphenylalanine, atomatic interactions, coassembly, molecular recognition
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Optimized Negative Staining: a High-throughput Protocol for Examining Small and Asymmetric Protein Structure by Electron Microscopy
Authors: Matthew Rames, Yadong Yu, Gang Ren.
Institutions: The Molecular Foundry.
Structural determination of proteins is rather challenging for proteins with molecular masses between 40 - 200 kDa. Considering that more than half of natural proteins have a molecular mass between 40 - 200 kDa1,2, a robust and high-throughput method with a nanometer resolution capability is needed. Negative staining (NS) electron microscopy (EM) is an easy, rapid, and qualitative approach which has frequently been used in research laboratories to examine protein structure and protein-protein interactions. Unfortunately, conventional NS protocols often generate structural artifacts on proteins, especially with lipoproteins that usually form presenting rouleaux artifacts. By using images of lipoproteins from cryo-electron microscopy (cryo-EM) as a standard, the key parameters in NS specimen preparation conditions were recently screened and reported as the optimized NS protocol (OpNS), a modified conventional NS protocol 3 . Artifacts like rouleaux can be greatly limited by OpNS, additionally providing high contrast along with reasonably high‐resolution (near 1 nm) images of small and asymmetric proteins. These high-resolution and high contrast images are even favorable for an individual protein (a single object, no average) 3D reconstruction, such as a 160 kDa antibody, through the method of electron tomography4,5. Moreover, OpNS can be a high‐throughput tool to examine hundreds of samples of small proteins. For example, the previously published mechanism of 53 kDa cholesteryl ester transfer protein (CETP) involved the screening and imaging of hundreds of samples 6. Considering cryo-EM rarely successfully images proteins less than 200 kDa has yet to publish any study involving screening over one hundred sample conditions, it is fair to call OpNS a high-throughput method for studying small proteins. Hopefully the OpNS protocol presented here can be a useful tool to push the boundaries of EM and accelerate EM studies into small protein structure, dynamics and mechanisms.
Environmental Sciences, Issue 90, small and asymmetric protein structure, electron microscopy, optimized negative staining
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The ChroP Approach Combines ChIP and Mass Spectrometry to Dissect Locus-specific Proteomic Landscapes of Chromatin
Authors: Monica Soldi, Tiziana Bonaldi.
Institutions: European Institute of Oncology.
Chromatin is a highly dynamic nucleoprotein complex made of DNA and proteins that controls various DNA-dependent processes. Chromatin structure and function at specific regions is regulated by the local enrichment of histone post-translational modifications (hPTMs) and variants, chromatin-binding proteins, including transcription factors, and DNA methylation. The proteomic characterization of chromatin composition at distinct functional regions has been so far hampered by the lack of efficient protocols to enrich such domains at the appropriate purity and amount for the subsequent in-depth analysis by Mass Spectrometry (MS). We describe here a newly designed chromatin proteomics strategy, named ChroP (Chromatin Proteomics), whereby a preparative chromatin immunoprecipitation is used to isolate distinct chromatin regions whose features, in terms of hPTMs, variants and co-associated non-histonic proteins, are analyzed by MS. We illustrate here the setting up of ChroP for the enrichment and analysis of transcriptionally silent heterochromatic regions, marked by the presence of tri-methylation of lysine 9 on histone H3. The results achieved demonstrate the potential of ChroP in thoroughly characterizing the heterochromatin proteome and prove it as a powerful analytical strategy for understanding how the distinct protein determinants of chromatin interact and synergize to establish locus-specific structural and functional configurations.
Biochemistry, Issue 86, chromatin, histone post-translational modifications (hPTMs), epigenetics, mass spectrometry, proteomics, SILAC, chromatin immunoprecipitation , histone variants, chromatome, hPTMs cross-talks
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In Situ SIMS and IR Spectroscopy of Well-defined Surfaces Prepared by Soft Landing of Mass-selected Ions
Authors: Grant E. Johnson, K. Don Dasitha Gunaratne, Julia Laskin.
Institutions: Pacific Northwest National Laboratory.
Soft landing of mass-selected ions onto surfaces is a powerful approach for the highly-controlled preparation of materials that are inaccessible using conventional synthesis techniques. Coupling soft landing with in situ characterization using secondary ion mass spectrometry (SIMS) and infrared reflection absorption spectroscopy (IRRAS) enables analysis of well-defined surfaces under clean vacuum conditions. The capabilities of three soft-landing instruments constructed in our laboratory are illustrated for the representative system of surface-bound organometallics prepared by soft landing of mass-selected ruthenium tris(bipyridine) dications, [Ru(bpy)3]2+ (bpy = bipyridine), onto carboxylic acid terminated self-assembled monolayer surfaces on gold (COOH-SAMs). In situ time-of-flight (TOF)-SIMS provides insight into the reactivity of the soft-landed ions. In addition, the kinetics of charge reduction, neutralization and desorption occurring on the COOH-SAM both during and after ion soft landing are studied using in situ Fourier transform ion cyclotron resonance (FT-ICR)-SIMS measurements. In situ IRRAS experiments provide insight into how the structure of organic ligands surrounding metal centers is perturbed through immobilization of organometallic ions on COOH-SAM surfaces by soft landing. Collectively, the three instruments provide complementary information about the chemical composition, reactivity and structure of well-defined species supported on surfaces.
Chemistry, Issue 88, soft landing, mass selected ions, electrospray, secondary ion mass spectrometry, infrared spectroscopy, organometallic, catalysis
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Magnetic Tweezers for the Measurement of Twist and Torque
Authors: Jan Lipfert, Mina Lee, Orkide Ordu, Jacob W. J. Kerssemakers, Nynke H. Dekker.
Institutions: Delft University of Technology.
Single-molecule techniques make it possible to investigate the behavior of individual biological molecules in solution in real time. These techniques include so-called force spectroscopy approaches such as atomic force microscopy, optical tweezers, flow stretching, and magnetic tweezers. Amongst these approaches, magnetic tweezers have distinguished themselves by their ability to apply torque while maintaining a constant stretching force. Here, it is illustrated how such a “conventional” magnetic tweezers experimental configuration can, through a straightforward modification of its field configuration to minimize the magnitude of the transverse field, be adapted to measure the degree of twist in a biological molecule. The resulting configuration is termed the freely-orbiting magnetic tweezers. Additionally, it is shown how further modification of the field configuration can yield a transverse field with a magnitude intermediate between that of the “conventional” magnetic tweezers and the freely-orbiting magnetic tweezers, which makes it possible to directly measure the torque stored in a biological molecule. This configuration is termed the magnetic torque tweezers. The accompanying video explains in detail how the conversion of conventional magnetic tweezers into freely-orbiting magnetic tweezers and magnetic torque tweezers can be accomplished, and demonstrates the use of these techniques. These adaptations maintain all the strengths of conventional magnetic tweezers while greatly expanding the versatility of this powerful instrument.
Bioengineering, Issue 87, magnetic tweezers, magnetic torque tweezers, freely-orbiting magnetic tweezers, twist, torque, DNA, single-molecule techniques
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Nanomanipulation of Single RNA Molecules by Optical Tweezers
Authors: William Stephenson, Gorby Wan, Scott A. Tenenbaum, Pan T. X. Li.
Institutions: University at Albany, State University of New York, University at Albany, State University of New York, University at Albany, State University of New York, University at Albany, State University of New York, University at Albany, State University of New York.
A large portion of the human genome is transcribed but not translated. In this post genomic era, regulatory functions of RNA have been shown to be increasingly important. As RNA function often depends on its ability to adopt alternative structures, it is difficult to predict RNA three-dimensional structures directly from sequence. Single-molecule approaches show potentials to solve the problem of RNA structural polymorphism by monitoring molecular structures one molecule at a time. This work presents a method to precisely manipulate the folding and structure of single RNA molecules using optical tweezers. First, methods to synthesize molecules suitable for single-molecule mechanical work are described. Next, various calibration procedures to ensure the proper operations of the optical tweezers are discussed. Next, various experiments are explained. To demonstrate the utility of the technique, results of mechanically unfolding RNA hairpins and a single RNA kissing complex are used as evidence. In these examples, the nanomanipulation technique was used to study folding of each structural domain, including secondary and tertiary, independently. Lastly, the limitations and future applications of the method are discussed.
Bioengineering, Issue 90, RNA folding, single-molecule, optical tweezers, nanomanipulation, RNA secondary structure, RNA tertiary structure
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The Use of Fluorescent Target Arrays for Assessment of T Cell Responses In vivo
Authors: Benjamin J. C. Quah, Danushka K. Wijesundara, Charani Ranasinghe, Christopher R. Parish.
Institutions: Australian National University.
The ability to monitor T cell responses in vivo is important for the development of our understanding of the immune response and the design of immunotherapies. Here we describe the use of fluorescent target array (FTA) technology, which utilizes vital dyes such as carboxyfluorescein succinimidyl ester (CFSE), violet laser excitable dyes (CellTrace Violet: CTV) and red laser excitable dyes (Cell Proliferation Dye eFluor 670: CPD) to combinatorially label mouse lymphocytes into >250 discernable fluorescent cell clusters. Cell clusters within these FTAs can be pulsed with major histocompatibility (MHC) class-I and MHC class-II binding peptides and thereby act as target cells for CD8+ and CD4+ T cells, respectively. These FTA cells remain viable and fully functional, and can therefore be administered into mice to allow assessment of CD8+ T cell-mediated killing of FTA target cells and CD4+ T cell-meditated help of FTA B cell target cells in real time in vivo by flow cytometry. Since >250 target cells can be assessed at once, the technique allows the monitoring of T cell responses against several antigen epitopes at several concentrations and in multiple replicates. As such, the technique can measure T cell responses at both a quantitative (e.g. the cumulative magnitude of the response) and a qualitative (e.g. functional avidity and epitope-cross reactivity of the response) level. Herein, we describe how these FTAs are constructed and give an example of how they can be applied to assess T cell responses induced by a recombinant pox virus vaccine.
Immunology, Issue 88, Investigative Techniques, T cell response, Flow Cytometry, Multiparameter, CTL assay in vivo, carboxyfluorescein succinimidyl ester (CFSE), CellTrace Violet (CTV), Cell Proliferation Dye eFluor 670 (CPD)
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Bead Aggregation Assays for the Characterization of Putative Cell Adhesion Molecules
Authors: Michelle R. Emond, James D. Jontes.
Institutions: Ohio State University.
Cell-cell adhesion is fundamental to multicellular life and is mediated by a diverse array of cell surface proteins. However, the adhesive interactions for many of these proteins are poorly understood. Here we present a simple, rapid method for characterizing the adhesive properties of putative homophilic cell adhesion molecules. Cultured HEK293 cells are transfected with DNA plasmid encoding a secreted, epitope-tagged ectodomain of a cell surface protein. Using functionalized beads specific for the epitope tag, the soluble, secreted fusion protein is captured from the culture medium. The coated beads can then be used directly in bead aggregation assays or in fluorescent bead sorting assays to test for homophilic adhesion. If desired, mutagenesis can then be used to elucidate the specific amino acids or domains required for adhesion. This assay requires only small amounts of expressed protein, does not require the production of stable cell lines, and can be accomplished in 4 days.
Bioengineering, Issue 92, adhesion, aggregation, Fc-fusion, cadherin, protocadherin
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Microwave-assisted Functionalization of Poly(ethylene glycol) and On-resin Peptides for Use in Chain Polymerizations and Hydrogel Formation
Authors: Amy H. Van Hove, Brandon D. Wilson, Danielle S. W. Benoit.
Institutions: University of Rochester, University of Rochester, University of Rochester Medical Center.
One of the main benefits to using poly(ethylene glycol) (PEG) macromers in hydrogel formation is synthetic versatility. The ability to draw from a large variety of PEG molecular weights and configurations (arm number, arm length, and branching pattern) affords researchers tight control over resulting hydrogel structures and properties, including Young’s modulus and mesh size. This video will illustrate a rapid, efficient, solvent-free, microwave-assisted method to methacrylate PEG precursors into poly(ethylene glycol) dimethacrylate (PEGDM). This synthetic method provides much-needed starting materials for applications in drug delivery and regenerative medicine. The demonstrated method is superior to traditional methacrylation methods as it is significantly faster and simpler, as well as more economical and environmentally friendly, using smaller amounts of reagents and solvents. We will also demonstrate an adaptation of this technique for on-resin methacrylamide functionalization of peptides. This on-resin method allows the N-terminus of peptides to be functionalized with methacrylamide groups prior to deprotection and cleavage from resin. This allows for selective addition of methacrylamide groups to the N-termini of the peptides while amino acids with reactive side groups (e.g. primary amine of lysine, primary alcohol of serine, secondary alcohols of threonine, and phenol of tyrosine) remain protected, preventing functionalization at multiple sites. This article will detail common analytical methods (proton Nuclear Magnetic Resonance spectroscopy (;H-NMR) and Matrix Assisted Laser Desorption Ionization Time of Flight mass spectrometry (MALDI-ToF)) to assess the efficiency of the functionalizations. Common pitfalls and suggested troubleshooting methods will be addressed, as will modifications of the technique which can be used to further tune macromer functionality and resulting hydrogel physical and chemical properties. Use of synthesized products for the formation of hydrogels for drug delivery and cell-material interaction studies will be demonstrated, with particular attention paid to modifying hydrogel composition to affect mesh size, controlling hydrogel stiffness and drug release.
Chemistry, Issue 80, Poly(ethylene glycol), peptides, polymerization, polymers, methacrylation, peptide functionalization, 1H-NMR, MALDI-ToF, hydrogels, macromer synthesis
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Rapid Generation of Amyloid from Native Proteins In vitro
Authors: Stephanie M Dorta-Estremera, Jingjing Li, Wei Cao.
Institutions: The University of Texas MD Anderson Cancer Center.
Proteins carry out crucial tasks in organisms by exerting functions elicited from their specific three dimensional folds. Although the native structures of polypeptides fulfill many purposes, it is now recognized that most proteins can adopt an alternative assembly of beta-sheet rich amyloid. Insoluble amyloid fibrils are initially associated with multiple human ailments, but they are increasingly shown as functional players participating in various important cellular processes. In addition, amyloid deposited in patient tissues contains nonproteinaceous components, such as nucleic acids and glycosaminoglycans (GAGs). These cofactors can facilitate the formation of amyloid, resulting in the generation of different types of insoluble precipitates. By taking advantage of our understanding how proteins misfold via an intermediate stage of soluble amyloid precursor, we have devised a method to convert native proteins to amyloid fibrils in vitro. This approach allows one to prepare amyloid in large quantities, examine the properties of amyloid generated from specific proteins, and evaluate the structural changes accompanying the conversion.
Biochemistry, Issue 82, amyloid, soluble protein oligomer, amyloid precursor, protein misfolding, amyloid fibril, protein aggregate
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Analyzing Protein Dynamics Using Hydrogen Exchange Mass Spectrometry
Authors: Nikolai Hentze, Matthias P. Mayer.
Institutions: University of Heidelberg.
All cellular processes depend on the functionality of proteins. Although the functionality of a given protein is the direct consequence of its unique amino acid sequence, it is only realized by the folding of the polypeptide chain into a single defined three-dimensional arrangement or more commonly into an ensemble of interconverting conformations. Investigating the connection between protein conformation and its function is therefore essential for a complete understanding of how proteins are able to fulfill their great variety of tasks. One possibility to study conformational changes a protein undergoes while progressing through its functional cycle is hydrogen-1H/2H-exchange in combination with high-resolution mass spectrometry (HX-MS). HX-MS is a versatile and robust method that adds a new dimension to structural information obtained by e.g. crystallography. It is used to study protein folding and unfolding, binding of small molecule ligands, protein-protein interactions, conformational changes linked to enzyme catalysis, and allostery. In addition, HX-MS is often used when the amount of protein is very limited or crystallization of the protein is not feasible. Here we provide a general protocol for studying protein dynamics with HX-MS and describe as an example how to reveal the interaction interface of two proteins in a complex.   
Chemistry, Issue 81, Molecular Chaperones, mass spectrometers, Amino Acids, Peptides, Proteins, Enzymes, Coenzymes, Protein dynamics, conformational changes, allostery, protein folding, secondary structure, mass spectrometry
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Preparation of Oligomeric β-amyloid1-42 and Induction of Synaptic Plasticity Impairment on Hippocampal Slices
Authors: Mauro Fa, Ian J. Orozco, Yitshak I. Francis, Faisal Saeed, Yimin Gong, Ottavio Arancio.
Institutions: Columbia University.
Impairment of synaptic connections is likely to underlie the subtle amnesic changes occurring at the early stages of Alzheimer s Disease (AD). β-amyloid (Aβ), a peptide produced in high amounts in AD, is known to reduce Long-Term Potentiation (LTP), a cellular correlate of learning and memory. Indeed, LTP impairment caused by Aβ is a useful experimental paradigm for studying synaptic dysfunctions in AD models and for screening drugs capable of mitigating or reverting such synaptic impairments. Studies have shown that Aβ produces the LTP disruption preferentially via its oligomeric form. Here we provide a detailed protocol for impairing LTP by perfusion of oligomerized synthetic Aβ1-42 peptide onto acute hippocampal slices. In this video, we outline a step-by-step procedure for the preparation of oligomeric Aβ1-42. Then, we follow an individual experiment in which LTP is reduced in hippocampal slices exposed to oligomerized Aβ1-42 compared to slices in a control experiment where no Aβ1-42 exposure had occurred.
JoVE Neuroscience, Issue 41, brain, mouse, hippocampus, plasticity, LTP, amyloid
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Selection of Aptamers for Amyloid β-Protein, the Causative Agent of Alzheimer's Disease
Authors: Farid Rahimi, Gal Bitan.
Institutions: David Geffen School of Medicine, University of California, Los Angeles, University of California, Los Angeles.
Alzheimer's disease (AD) is a progressive, age-dependent, neurodegenerative disorder with an insidious course that renders its presymptomatic diagnosis difficult1. Definite AD diagnosis is achieved only postmortem, thus establishing presymptomatic, early diagnosis of AD is crucial for developing and administering effective therapies2,3. Amyloid β-protein (Aβ) is central to AD pathogenesis. Soluble, oligomeric Aβ assemblies are believed to affect neurotoxicity underlying synaptic dysfunction and neuron loss in AD4,5. Various forms of soluble Aβ assemblies have been described, however, their interrelationships and relevance to AD etiology and pathogenesis are complex and not well understood6. Specific molecular recognition tools may unravel the relationships amongst Aβ assemblies and facilitate detection and characterization of these assemblies early in the disease course before symptoms emerge. Molecular recognition commonly relies on antibodies. However, an alternative class of molecular recognition tools, aptamers, offers important advantages relative to antibodies7,8. Aptamers are oligonucleotides generated by in-vitro selection: systematic evolution of ligands by exponential enrichment (SELEX)9,10. SELEX is an iterative process that, similar to Darwinian evolution, allows selection, amplification, enrichment, and perpetuation of a property, e.g., avid, specific, ligand binding (aptamers) or catalytic activity (ribozymes and DNAzymes). Despite emergence of aptamers as tools in modern biotechnology and medicine11, they have been underutilized in the amyloid field. Few RNA or ssDNA aptamers have been selected against various forms of prion proteins (PrP)12-16. An RNA aptamer generated against recombinant bovine PrP was shown to recognize bovine PrP-β17, a soluble, oligomeric, β-sheet-rich conformational variant of full-length PrP that forms amyloid fibrils18. Aptamers generated using monomeric and several forms of fibrillar β2-microglobulin (β2m) were found to bind fibrils of certain other amyloidogenic proteins besides β2m fibrils19. Ylera et al. described RNA aptamers selected against immobilized monomeric Aβ4020. Unexpectedly, these aptamers bound fibrillar Aβ40. Altogether, these data raise several important questions. Why did aptamers selected against monomeric proteins recognize their polymeric forms? Could aptamers against monomeric and/or oligomeric forms of amyloidogenic proteins be obtained? To address these questions, we attempted to select aptamers for covalently-stabilized oligomeric Aβ4021 generated using photo-induced cross-linking of unmodified proteins (PICUP)22,23. Similar to previous findings17,19,20, these aptamers reacted with fibrils of Aβ and several other amyloidogenic proteins likely recognizing a potentially common amyloid structural aptatope21. Here, we present the SELEX methodology used in production of these aptamers21.
Neuroscience, Issue 39, Cellular Biology, Aptamer, RNA, amyloid β-protein, oligomer, amyloid fibrils, protein assembly
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Demonstrating the Uses of the Novel Gravitational Force Spectrometer to Stretch and Measure Fibrous Proteins
Authors: James W. Dunn, Douglas D. Root.
Institutions: University of North Texas.
The study of macromolecular structure has become critical to the elucidation of molecular mechanisms and function. There are several limited, but important bioinstruments capable of testing the force dependence of structural features in proteins. Scale has been a limiting parameter on how accurately researchers can peer into the nanomechanical world of molecules, such as nucleic acids, enzymes, and motor proteins that perform life-sustaining work. Atomic force microscopy (AFM) is well tuned to determine native structures of fibrous proteins with a distance resolution on par with electron microscopy. However, in AFM force studies, the forces are typically much higher than a single molecule might experience 1, 2. Optical traps (OT) are very good at determining the relative distance between the trapped beads and they can impart very small forces 3. However, they do not yield accurate absolute lengths of the molecules under study. Molecular simulations provide supportive information to such experiments, but are limited in the ability to handle the same large molecular sizes, long time frames, and convince some researchers in the absence of other supporting evidence2, 4. The gravitational force spectrometer (GFS) fills a critical niche in the arsenal of an investigator by providing a unique combination of abilities. This instrument is capable of generating forces typically with 98% or better accuracy from the femtonewton range to the nanonewton range. The distance measurements currently are capable of resolving the absolute molecular length down to five nanometers, and relative bead pair separation distances with a precision similar to an optical trap. Also, the GFS can determine stretching or uncoiling where the force is near equilibrium, or provide a graded force to juxtapose against any measured structural changes. It is even possible to determine how many amino acid residues are involved in uncoiling events under physiological force loads 2. Unlike in other methods where there is extensive force calibration that must precede any assay, the GFS requires no such force calibration 5. By complementing the strengths of other methods, the GFS will bridge gaps in understanding the nanomechanics of vital proteins and other macromolecules.
Biophysics, Issue 49, Force Spectroscopy, Single Molecule Assays, Myosin, Antibodies, Digital Image Processing, Microscopy, Education, Microspheres, Coiled Coil, Protein
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Multiplexed Single-molecule Force Proteolysis Measurements Using Magnetic Tweezers
Authors: Arjun S. Adhikari, Jack Chai, Alexander R. Dunn.
Institutions: Stanford University .
The generation and detection of mechanical forces is a ubiquitous aspect of cell physiology, with direct relevance to cancer metastasis1, atherogenesis2 and wound healing3. In each of these examples, cells both exert force on their surroundings and simultaneously enzymatically remodel the extracellular matrix (ECM). The effect of forces on ECM has thus become an area of considerable interest due to its likely biological and medical importance4-7. Single molecule techniques such as optical trapping8, atomic force microscopy9, and magnetic tweezers10,11 allow researchers to probe the function of enzymes at a molecular level by exerting forces on individual proteins. Of these techniques, magnetic tweezers (MT) are notable for their low cost and high throughput. MT exert forces in the range of ~1-100 pN and can provide millisecond temporal resolution, qualities that are well matched to the study of enzyme mechanism at the single-molecule level12. Here we report a highly parallelizable MT assay to study the effect of force on the proteolysis of single protein molecules. We present the specific example of the proteolysis of a trimeric collagen peptide by matrix metalloproteinase 1 (MMP-1); however, this assay can be easily adapted to study other substrates and proteases.
Bioengineering, Issue 65, Chemical Engineering, Physics, Single-molecule spectroscopy, magnetic tweezers, force proteolysis, collagen, MMP-1
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Controlling the Size, Shape and Stability of Supramolecular Polymers in Water
Authors: Pol Besenius, Isja de Feijter, Nico A.J.M. Sommerdijk, Paul H.H. Bomans, Anja R. A. Palmans.
Institutions: Westfälische Wilhelms-Universität Münster, Eindhoven University of Technology, Eindhoven University of Technology.
For aqueous based supramolecular polymers, the simultaneous control over shape, size and stability is very difficult1. At the same time, the ability to do so is highly important in view of a number of applications in functional soft matter including electronics, biomedical engineering, and sensors. In the past, successful strategies to control the size and shape of supramolecular polymers typically focused on the use of templates2,3, end cappers4 or selective solvent techniques5. Here we disclose a strategy based on self-assembling discotic amphiphiles that leads to the control over stack length and shape of ordered, chiral columnar aggregates. By balancing electrostatic repulsive interactions on the hydrophilic rim and attractive non-covalent forces within the hydrophobic core of the polymerizing building block, we manage to create small and discrete spherical objects6,7. Increasing the salt concentration to screen the charges induces a sphere-to-rod transition. Intriguingly, this transition is expressed in an increase of cooperativity in the temperature-dependent self-assembly mechanism, and more stable aggregates are obtained. For our study we select a benzene-1,3,5-tricarboxamide (BTA) core connected to a hydrophilic metal chelate via a hydrophobic, fluorinated L-phenylalanine based spacer (Scheme 1). The metal chelate selected is a Gd(III)-DTPA complex that contains two overall remaining charges per complex and necessarily two counter ions. The one-dimensional growth of the aggregate is directed by π-π stacking and intermolecular hydrogen bonding. However, the electrostatic, repulsive forces that arise from the charges on the Gd(III)-DTPA complex start limiting the one-dimensional growth of the BTA-based discotic once a certain size is reached. At millimolar concentrations the formed aggregate has a spherical shape and a diameter of around 5 nm as inferred from 1H-NMR spectroscopy, small angle X-ray scattering, and cryogenic transmission electron microscopy (cryo-TEM). The strength of the electrostatic repulsive interactions between molecules can be reduced by increasing the salt concentration of the buffered solutions. This screening of the charges induces a transition from spherical aggregates into elongated rods with a length > 25 nm. Cryo-TEM allows to visualise the changes in shape and size. In addition, CD spectroscopy permits to derive the mechanistic details of the self-assembly processes before and after the addition of salt. Importantly, the cooperativity -a key feature that dictates the physical properties of the produced supramolecular polymers- increases dramatically upon screening the electrostatic interactions. This increase in cooperativity results in a significant increase in the molecular weight of the formed supramolecular polymers in water.
Chemical Engineering, Issue 66, Chemistry, Physics, Self-assembly, cryogenic transmission electron microscopy, circular dichroism, controlled architecture, discotic amphiphile
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Purification and Aggregation of the Amyloid Precursor Protein Intracellular Domain
Authors: Amina El Ayadi, Emily S. Stieren, José M. Barral, Andres F. Oberhauser, Darren Boehning.
Institutions: University of Texas Medical Branch , University of Texas Medical Branch .
Amyloid precursor protein (APP) is a type I transmembrane protein associated with the pathogenesis of Alzheimer's disease (AD). APP is characterized by a large extracellular domain and a short cytosolic domain termed the APP intracellular domain (AICD). During maturation through the secretory pathway, APP can be cleaved by proteases termed α, β, and γ-secretases1. Sequential proteolytic cleavage of APP with β and γ-secretases leads to the production of a small proteolytic peptide, termed Aβ, which is amyloidogenic and the core constituent of senile plaques. The AICD is also liberated from the membrane after secretase processing, and through interactions with Fe65 and Tip60, can translocate to the nucleus to participate in transcription regulation of multiple target genes2,3. Protein-protein interactions involving the AICD may affect trafficking, processing, and cellular functions of holo-APP and its C-terminal fragments. We have recently shown that AICD can aggregate in vitro, and this process is inhibited by the AD-implicated molecular chaperone ubiquilin-14. Consistent with these findings, the AICD has exposed hydrophobic domains and is intrinsically disordered in vitro5,6, however it obtains stable secondary structure when bound to Fe657. We have proposed that ubiquilin-1 prevents inappropriate inter- and intramolecular interactions of AICD, preventing aggregation in vitro and in intact cells4. While most studies focus on the role of APP in the pathogenesis of AD, the role of AICD in this process is not clear. Expression of AICD has been shown to induce apoptosis8, to modulate signaling pathways9, and to regulate calcium signaling10. Over-expression of AICD and Fe65 in a transgenic mouse model induces Alzheimer's like pathology11, and recently AICD has been detected in brain lysates by western blotting when using appropriate antigen retrieval techniques12. To facilitate structural, biochemical, and biophysical studies of the AICD, we have developed a procedure to produce recombinantly large amounts of highly pure AICD protein. We further describe a method for inducing the in vitro thermal aggregation of AICD and analysis by atomic force microscopy. The methods described are useful for biochemical, biophysical, and structural characterization of the AICD and the effects of molecular chaperones on AICD aggregation.
Medicine, Issue 66, Neuroscience, Cellular Biology, Molecular Biology, Amyloid precursor protein, APP, AICD, Alzheimer's Disease, Atomic Force Microscopy, Aggregation, Ubiquilin-1, Molecular Chaperone
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Reconstitution of a Kv Channel into Lipid Membranes for Structural and Functional Studies
Authors: Sungsoo Lee, Hui Zheng, Liang Shi, Qiu-Xing Jiang.
Institutions: University of Texas Southwestern Medical Center at Dallas.
To study the lipid-protein interaction in a reductionistic fashion, it is necessary to incorporate the membrane proteins into membranes of well-defined lipid composition. We are studying the lipid-dependent gating effects in a prototype voltage-gated potassium (Kv) channel, and have worked out detailed procedures to reconstitute the channels into different membrane systems. Our reconstitution procedures take consideration of both detergent-induced fusion of vesicles and the fusion of protein/detergent micelles with the lipid/detergent mixed micelles as well as the importance of reaching an equilibrium distribution of lipids among the protein/detergent/lipid and the detergent/lipid mixed micelles. Our data suggested that the insertion of the channels in the lipid vesicles is relatively random in orientations, and the reconstitution efficiency is so high that no detectable protein aggregates were seen in fractionation experiments. We have utilized the reconstituted channels to determine the conformational states of the channels in different lipids, record electrical activities of a small number of channels incorporated in planar lipid bilayers, screen for conformation-specific ligands from a phage-displayed peptide library, and support the growth of 2D crystals of the channels in membranes. The reconstitution procedures described here may be adapted for studying other membrane proteins in lipid bilayers, especially for the investigation of the lipid effects on the eukaryotic voltage-gated ion channels.
Molecular Biology, Issue 77, Biochemistry, Genetics, Cellular Biology, Structural Biology, Biophysics, Membrane Lipids, Phospholipids, Carrier Proteins, Membrane Proteins, Micelles, Molecular Motor Proteins, life sciences, biochemistry, Amino Acids, Peptides, and Proteins, lipid-protein interaction, channel reconstitution, lipid-dependent gating, voltage-gated ion channel, conformation-specific ligands, lipids
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Protein WISDOM: A Workbench for In silico De novo Design of BioMolecules
Authors: James Smadbeck, Meghan B. Peterson, George A. Khoury, Martin S. Taylor, Christodoulos A. Floudas.
Institutions: Princeton University.
The aim of de novo protein design is to find the amino acid sequences that will fold into a desired 3-dimensional structure with improvements in specific properties, such as binding affinity, agonist or antagonist behavior, or stability, relative to the native sequence. Protein design lies at the center of current advances drug design and discovery. Not only does protein design provide predictions for potentially useful drug targets, but it also enhances our understanding of the protein folding process and protein-protein interactions. Experimental methods such as directed evolution have shown success in protein design. However, such methods are restricted by the limited sequence space that can be searched tractably. In contrast, computational design strategies allow for the screening of a much larger set of sequences covering a wide variety of properties and functionality. We have developed a range of computational de novo protein design methods capable of tackling several important areas of protein design. These include the design of monomeric proteins for increased stability and complexes for increased binding affinity. To disseminate these methods for broader use we present Protein WISDOM (, a tool that provides automated methods for a variety of protein design problems. Structural templates are submitted to initialize the design process. The first stage of design is an optimization sequence selection stage that aims at improving stability through minimization of potential energy in the sequence space. Selected sequences are then run through a fold specificity stage and a binding affinity stage. A rank-ordered list of the sequences for each step of the process, along with relevant designed structures, provides the user with a comprehensive quantitative assessment of the design. Here we provide the details of each design method, as well as several notable experimental successes attained through the use of the methods.
Genetics, Issue 77, Molecular Biology, Bioengineering, Biochemistry, Biomedical Engineering, Chemical Engineering, Computational Biology, Genomics, Proteomics, Protein, Protein Binding, Computational Biology, Drug Design, optimization (mathematics), Amino Acids, Peptides, and Proteins, De novo protein and peptide design, Drug design, In silico sequence selection, Optimization, Fold specificity, Binding affinity, sequencing
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Specificity Analysis of Protein Lysine Methyltransferases Using SPOT Peptide Arrays
Authors: Srikanth Kudithipudi, Denis Kusevic, Sara Weirich, Albert Jeltsch.
Institutions: Stuttgart University.
Lysine methylation is an emerging post-translation modification and it has been identified on several histone and non-histone proteins, where it plays crucial roles in cell development and many diseases. Approximately 5,000 lysine methylation sites were identified on different proteins, which are set by few dozens of protein lysine methyltransferases. This suggests that each PKMT methylates multiple proteins, however till now only one or two substrates have been identified for several of these enzymes. To approach this problem, we have introduced peptide array based substrate specificity analyses of PKMTs. Peptide arrays are powerful tools to characterize the specificity of PKMTs because methylation of several substrates with different sequences can be tested on one array. We synthesized peptide arrays on cellulose membrane using an Intavis SPOT synthesizer and analyzed the specificity of various PKMTs. Based on the results, for several of these enzymes, novel substrates could be identified. For example, for NSD1 by employing peptide arrays, we showed that it methylates K44 of H4 instead of the reported H4K20 and in addition H1.5K168 is the highly preferred substrate over the previously known H3K36. Hence, peptide arrays are powerful tools to biochemically characterize the PKMTs.
Biochemistry, Issue 93, Peptide arrays, solid phase peptide synthesis, SPOT synthesis, protein lysine methyltransferases, substrate specificity profile analysis, lysine methylation
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