Cost-Effective Protein Production in CHO Cells Following Polyethylenimine-Mediated Gene Delivery Showcased by the Production and Crystallization of Antibody Fabs

Laboratory production of recombinant mammalian proteins, particularly antibodies, requires an expression pipeline assuring sufficient yield and correct folding with appropriate posttranslational modifications. Transient gene expression (TGE) in the suspension-adapted Chinese Hamster Ovary (CHO) cell lines has become the method of choice for this task. The antibodies can be secreted into the media, which facilitates subsequent purification, and can be glycosylated. However, in general, protein production in CHO cells is expensive and may provide variable outcomes, namely in laboratories without previous experience. While achievable yields may be influenced by the nucleotide sequence, there are other aspects of the process which offer space for optimization, like gene delivery method, cultivation process or expression plasmid design. Polyethylenimine (PEI)-mediated gene delivery is frequently employed as a low-cost alternative to liposome-based methods. In this work, we are proposing a TGE platform for universal medium-scale production of antibodies and other proteins in CHO cells, with a novel expression vector allowing fast and flexible cloning of new genes and secretion of translated proteins. The production cost has been further reduced using recyclable labware. Nine days after transfection, we routinely obtain milligrams of antibody Fabs or human lactoferrin in a 25 mL culture volume. Potential of the platform is established based on the production and crystallization of antibody Fabs and their complexes.

Time to Kill and Time to Heal: The Multifaceted Role of Lactoferrin and Lactoferricin in Host Defense

Lactoferrin is an iron-binding glycoprotein present in most human exocrine fluids, particularly breast milk. Lactoferrin is also released from neutrophil granules, and its concentration increases rapidly at the site of inflammation. Immune cells of both the innate and the adaptive immune system express receptors for lactoferrin to modulate their functions in response to it. On the basis of these interactions, lactoferrin plays many roles in host defense, ranging from augmenting or calming inflammatory pathways to direct killing of pathogens. Complex biological activities of lactoferrin are determined by its ability to sequester iron and by its highly basic N-terminus, via which lactoferrin binds to a plethora of negatively charged surfaces of microorganisms and viruses, as well as to mammalian cells, both normal and cancerous. Proteolytic cleavage of lactoferrin in the digestive tract generates smaller peptides, such as N-terminally derived lactoferricin. Lactoferricin shares some of the properties of lactoferrin, but also exhibits unique characteristics and functions. In this review, we discuss the structure, functions, and potential therapeutic uses of lactoferrin, lactoferricin, and other lactoferrin-derived bioactive peptides in treating various infections and inflammatory conditions. Furthermore, we summarize clinical trials examining the effect of lactoferrin supplementation in disease treatment, with a special focus on its potential use in treating COVID-19.

Blockade of TMPRSS2-mediated priming of SARS-CoV-2 by lactoferricin

In addition to vaccines, there is an urgent need for supplemental antiviral therapeutics to dampen the persistent COVID-19 pandemic caused by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). The transmembrane protease serine 2 (TMPRSS2), that is responsible for proteolytic priming of the SARS-CoV-2 spike protein, appears as a rational therapeutic target. Accordingly, selective inhibitors of TMPRSS2 represent potential tools for prevention and treatment of COVID-19. Previously, we identified the human milk glycoprotein lactoferrin as a natural inhibitor of plasminogen conversion to plasmin, a serine protease homologous to TMPRSS2. Here, we tested whether lactoferrin and lactoferricin, a biologically active natural peptide produced by pepsin-mediated digestion of lactoferrin, together with synthetic peptides derived from lactoferrin, were able to block TMPRSS2 and SARS-CoV-2 infection. Particularly, we revealed that both lactoferricin and the N-terminal synthetic peptide pLF1 significantly inhibited: i) proteolytic activity of TMPRSS2 and plasmin, ii) proteolytic processing of the SARS-CoV-2 spike protein, and iii) SARS-CoV-2 infection of SARS-CoV-2-permissive cells. Thus, natural and synthetic peptides derived from lactoferrin represent feasible candidates for supporting prevention and treatment of COVID-19.

Interaction kinetics reveal distinct properties of conformational ensembles of three-repeat and four-repeat tau proteins

Tau protein is an intrinsically disordered protein. Its physiological state is best described as a conformational ensemble (CE) of metastable structures interconverting on the local and molecular scale. The monoclonal antibody DC39C recognizes a linear C-terminal tau epitope, and as the tau interaction partner, its binding parameters report about tau CE. Association kinetics of DC39C binding, together with crosslinking mass spectrometry, show differences in the accessibility of the C-terminus in CEs of tau isoforms. Furthermore, removal of the C-terminus accelerated the aggregation kinetics of three-repeat tau proteins. Our results suggest a novel mechanism of splicing-driven regulation of the tau C-terminal domain with consequences on the specific roles of tau isoforms in microtubule assembly and pathological aggregation.

Monoclonal antibodies targeting two immunodominant epitopes on the Spike protein neutralize emerging SARS-CoV-2 variants of concern

BACKGROUND

The emergence of new SARS-CoV-2 variants of concern B.1.1.7 (Alpha), B.1.351 (Beta), P.1 (Gamma) and B.1.617.2 (Delta) that harbor mutations in the viral S protein raised concern about activity of current vaccines and therapeutic antibodies. Independent studies have shown that mutant variants are partially or completely resistant against some of the therapeutic antibodies authorized for emergency use.

METHODS

We employed hybridoma technology, ELISA-based and cell-based S-ACE2 interaction assays combined with authentic virus neutralization assays to develop second-generation antibodies, which were specifically selected for their ability to neutralize the new variants of SARS-CoV-2.

FINDINGS

AX290 and AX677, two monoclonal antibodies with non-overlapping epitopes, exhibit subnanomolar or nanomolar affinities to the receptor binding domain of the viral Spike protein carrying amino acid substitutions N501Y, N439K, E484K, K417N, and a combination N501Y/E484K/K417N found in the circulating virus variants. The antibodies showed excellent neutralization of an authentic SARS-CoV-2 virus representing strains circulating in Europe in spring 2020 and also the variants of concern B.1.1.7 (Alpha), B.1.351 (Beta) and B.1.617.2 (Delta). In addition, AX677 is able to bind Omicron Spike protein just like the wild type Spike. The combination of the two antibodies prevented the appearance of escape mutations of the authentic SARS-CoV-2 virus. Prophylactic administration of AX290 and AX677, either individually or in combination, effectively reduced viral burden and inflammation in the lungs, and prevented disease in a mouse model of SARS-CoV-2 infection.

INTERPRETATION

The virus-neutralizing properties were fully reproduced in chimeric mouse-human versions of the antibodies, which may represent a promising tool for COVID-19 therapy.

FUNDING

The study was funded by AXON Neuroscience SE and AXON COVIDAX a.s.

The structure of the unstructured: mosaic of tau protein linear motifs obtained by high-resolution techniques and molecular simulation

Intrinsically disordered proteins are flexible molecules with important physiological functions. Their mode of action often involves short segments, called linear motifs, which may exhibit distinct structural propensities. Tau is intrinsically disordered, microtubule-associated protein involved in the pathogenesis of various tauopathies. In this review we analyze the collection of 3D structures of tau local linear motifs gained from the deposited structures of tau complexes with various binding partners as well as of tau-tau complexes; determined by X-ray and electron crystallography, single-particle electron microscopy, NMR spectroscopy and molecular dynamics simulations. Insights into the partially stabilized conformations of tau linear motifs are valuable for understanding the physiological and pathological processes involving tau protein.

Hypertension does not alter disturbances in leptin signalling observed in experimental model of tauopathy

Neurodegeneration is associated with hypertension and disturbance in fat metabolism. The complex interaction of neurodegenerative processes with both metabolic changes and blood pressure is still not fully elucidated. Here we demonstrate that the experimentally induced tauopathy in hypertensive transgenic animals causes significant downregulation of plasma leptin (53% of control), reduction of body weight by 11%, a 1.2-fold drop of adiposity index, and decrease in HDL cholesterol level, while the fasting glucose and insulin concentration remain unchanged. Despite of these alterations we found the leptin projection circuit including the arcuate nucleus, paraventricular nucleus in hypothalamus, and nucleus tractus solitarius in the brainstem not affected by neurofibrillary pathology. Furthermore, hypertension does not alter disturbances in leptin signalling. The presented data provide further insight into neurodegeneration-induced metabolic alterations relevant for human tauopathies.

NMR Studies of Tau Protein in Tauopathies

Tauopathies, including Alzheimer's disease (AD), are the most troublesome of all age-related chronic conditions, as there are no well-established disease-modifying therapies for their prevention and treatment. Spatio-temporal distribution of tau protein pathology correlates with cognitive decline and severity of the disease, therefore, tau protein has become an appealing target for therapy. Current knowledge of the pathological effects and significance of specific species in the tau aggregation pathway is incomplete although more and more structural and mechanistic insights are being gained using biophysical techniques. Here, we review the application of NMR to structural studies of various tau forms that appear in its aggregation process, focusing on results obtained from solid-state NMR. Furthermore, we discuss implications from these studies and their prospective contribution to the development of new tauopathy therapies.

MIRRAGGE - Minimum Information Required for Reproducible AGGregation Experiments

Reports on phase separation and amyloid formation for multiple proteins and aggregation-prone peptides are recurrently used to explore the molecular mechanisms associated with several human diseases. The information conveyed by these reports can be used directly in translational investigation, e.g., for the design of better drug screening strategies, or be compiled in databases for benchmarking novel aggregation-predicting algorithms. Given that minute protocol variations determine different outcomes of protein aggregation assays, there is a strong urge for standardized descriptions of the different types of aggregates and the detailed methods used in their production. In an attempt to address this need, we assembled the Minimum Information Required for Reproducible Aggregation Experiments (MIRRAGGE) guidelines, considering first-principles and the established literature on protein self-assembly and aggregation. This consensus information aims to cover the major and subtle determinants of experimental reproducibility while avoiding excessive technical details that are of limited practical interest for non-specialized users. The MIRRAGGE table (template available in Supplementary Information) is useful as a guide for the design of new studies and as a checklist during submission of experimental reports for publication. Full disclosure of relevant information also enables other researchers to reproduce results correctly and facilitates systematic data deposition into curated databases.

Plasma Leptin Reflects Progression of Neurofibrillary Pathology in Animal Model of Tauopathy

The close relationship between Alzheimer's disease (AD) and obesity was recognized many years ago. However, complete understanding of the pathological mechanisms underlying the interactions between degeneration of CNS and fat metabolism is still missing. The leptin a key adipokine of white adipose tissue has been suggested as one of the major mediators linking the obesity and AD. Here we investigated the association between peripheral levels of leptin, general metabolic status and stage of the pathogenesis in rat transgenic model of AD. We demonstrate significantly decreased levels of plasma leptin in animals with experimentally induced progressive neurofibrillary pathology, which represents only 62.3% (P = 0.0015) of those observed in normal wild type control animals. More detailed analysis showed a strong and statistically significant inverse correlation between the load of neurofibrillary pathology and peripheral levels of leptin (r = - 0.7248, P = 0.0177). We also observed a loss of body weight during development of neurodegeneration (about 14% less than control animals, P = 0.0004) and decrease in several metabolic parameters such as glucose, insulin, triglycerides and VLDL in plasma of the transgenic animals. Our data suggest that plasma leptin could serve as a convenient peripheral biomarker for tauopathies and Alzheimer's disease. Decrease in gene expression of leptin in fat tissue and its plasma level was found as one of the consequences of experimentally induced neurodegeneration. Our data may help to design rational diagnostic and therapeutic strategies for patients suffering from Alzheimer's disease or other forms of tauopathy.

AADvac1, an Active Immunotherapy for Alzheimer's Disease and Non Alzheimer Tauopathies: An Overview of Preclinical and Clinical Development

Neurofibrillary tau protein pathology is closely associated with the progression and phenotype of cognitive decline in Alzheimer's disease and other tauopathies, and a high-priority target for disease-modifying therapies. Herein, we provide an overview of the development of AADvac1, an active immunotherapy against tau pathology, and tau epitopes that are potential targets for immunotherapy. The vaccine leads to the production of antibodies that target conformational epitopes in the microtubule-binding region of tau, with the aim to prevent tau aggregation and spreading of pathology, and promote tau clearance. The therapeutic potential of the vaccine was evaluated in transgenic rats and mice expressing truncated, non mutant tau protein, which faithfully replicate of human tau pathology. Treatment with AADvac1 resulted in reduction of neurofibrillary pathology and insoluble tau in their brains, and amelioration of their deleterious phenotype. The vaccine was highly immunogenic in humans, inducing production of IgG antibodies against the tau peptide in 29/30 treated elderly patients with mild-to-moderate Alzheimer's. These antibodies were able to recognise insoluble tau proteins in Alzheimer patients' brains. Treatment with AADvac1 proved to be remarkably safe, with injection site reactions being the only adverse event tied to treatment. AADvac1 is currently being investigated in a phase 2 study in Alzheimer's disease, and a phase 1 study in non-fluent primary progressive aphasia, a neurodegenerative disorder with a high tau pathology component.

Humanized tau antibodies promote tau uptake by human microglia without any increase of inflammation

Immunotherapies targeting pathological tau have recently emerged as a promising approach for treatment of neurodegenerative disorders. We have previously showed that the mouse antibody DC8E8 discriminates between healthy and pathological tau, reduces tau pathology in murine tauopathy models and inhibits neuronal internalization of AD tau species in vitro.

Here we show, that DC8E8 and antibodies elicited against the first-in-man tau vaccine, AADvac1, which is based on the DC8E8 epitope peptide, both promote uptake of pathological tau by mouse primary microglia. IgG1 and IgG4 isotypes of AX004, the humanized versions of DC8E8, accelerate tau uptake by human primary microglia isolated from post-mortem aged and diseased brains. This promoting activity requires the presence of the Fc-domain of the antibodies.

The IgG1 isotype of AX004 showed greater ability to promote tau uptake compared to the IgG4 isotype, while none of the antibody-tau complexes provoked increased pro-inflammatory activity of microglia. Our data suggest that IgG1 has better suitability for therapeutic development.

A walk through tau therapeutic strategies

Tau neuronal and glial pathologies drive the clinical presentation of Alzheimer's disease and related human tauopathies. There is a growing body of evidence indicating that pathological tau species can travel from cell to cell and spread the pathology through the brain. Throughout the last decade, physiological and pathological tau have become attractive targets for AD therapies. Several therapeutic approaches have been proposed, including the inhibition of protein kinases or protein-3-O-(N-acetyl-beta-D-glucosaminyl)-L-serine/threonine Nacetylglucosaminyl hydrolase, the inhibition of tau aggregation, active and passive immunotherapies, and tau silencing by antisense oligonucleotides. New tau therapeutics, across the board, have demonstrated the ability to prevent or reduce tau lesions and improve either cognitive or motor impairment in a variety of animal models developing neurofibrillary pathology. The most advanced strategy for the treatment of human tauopathies remains immunotherapy, which has already reached the clinical stage of drug development. Tau vaccines or humanised antibodies target a variety of tau species either in the intracellular or extracellular spaces. Some of them recognise the amino-terminus or carboxy-terminus, while others display binding abilities to the proline-rich area or microtubule binding domains. The main therapeutic foci in existing clinical trials are on Alzheimer's disease, progressive supranuclear palsy and non-fluent primary progressive aphasia. Tau therapy offers a new hope for the treatment of many fatal brain disorders. First efficacy data from clinical trials will be available by the end of this decade.

Neuronal Expression of Truncated Tau Efficiently Promotes Neurodegeneration in Animal Models: Pitfalls of Toxic Oligomer Analysis

Animal models of neurodegeneration induced by neuronal expression of truncated tau protein emerge as an important tool for understanding the pathogenesis of human tauopathies and for therapy development. Here we highlight common features of truncated tau models and make a critical assessment of possible pitfalls in their analysis. Particularly, the amount of soluble tau oligomers, which are suspected to be neurotoxic agents participating on the spreading of pathology inside the brain, may be overestimated due to a post-lysis oxidative tau oligomerization. Using a mouse brain lysate spiked with recombinant truncated and full length tau forms, we show that tau oligomers might inadvertently be produced during the isolation procedure. This finding is further corroborated by the analysis of brain lysates originated from a mouse model expressing truncated tau variant. Our results underline the necessity of thiol-protecting conditions during the analysis of tau oligomers involved in the etiopathogenesis of various tauopathies including Alzheimer's disease.

Lactoferrin is a natural inhibitor of plasminogen activation

The plasminogen system is essential for dissolution of fibrin clots, and in addition, it is involved in a wide variety of other physiological processes, including proteolytic activation of growth factors, cell migration, and removal of protein aggregates. On the other hand, uncontrolled plasminogen activation contributes to many pathological processes (e.g. tumor cells' invasion in cancer progression). Moreover, some virulent bacterial species (e.g. Streptococci or Borrelia) bind human plasminogen and hijack the host's plasminogen system to penetrate tissue barriers. Thus, the conversion of plasminogen to the active serine protease plasmin must be tightly regulated. Here, we show that human lactoferrin, an iron-binding milk glycoprotein, blocks plasminogen activation on the cell surface by direct binding to human plasminogen. We mapped the mutual binding sites to the N-terminal region of lactoferrin, encompassed also in the bioactive peptide lactoferricin, and kringle 5 of plasminogen. Finally, lactoferrin blocked tumor cell invasion in vitro and also plasminogen activation driven by Borrelia Our results explain many diverse biological properties of lactoferrin and also suggest that lactoferrin may be useful as a potential tool for therapeutic interventions to prevent both invasive malignant cells and virulent bacteria from penetrating host tissues.

N-terminal truncation of microtubule associated protein tau dysregulates its cellular localization

Tau protein is a member of microtubule-associated protein family. Under pathological conditions, tau undergoes multiple modifications that lead to the formation of insoluble deposits in neurons, resulting in neuronal dysfunction in several neurodegenerative disorders collectively called tauopathies, with Alzheimer's disease being the most frequent example. This typical cytosolic protein has been shown to translocate into the nucleus and participate in DNA protection upon stress conditions. In our study, we demonstrate that truncated Tau151-391/4R changes its usual behavior and gains constitutive access into the nucleus of both primary rat neurons and human neuroblastoma cells. Our results show that partial/dysregulated nuclear localization of tau results from the removal of the N-terminal (1-150) residues of the protein. Data obtained by cell fractionation data were supported by confocal microscopy analysis of GFP-fused tau proteins. Furthermore, neither addition of the fusion protein, nor increased tau phosphorylation had any effect on the intracellular distribution of truncated tau. Our data further suggest that differential tau phospho-status between cytosolic and nuclear fractions is rather a consequence than a cause of truncated tau nuclear localization. Finally, truncated tau in the nucleus is engaged in interactions with subnuclear structure(s), since it exhibits reduced mobility. We conclude that N-terminal truncation of tau proteins leads to their nonphysiological subcellular distribution as a result of modified tau conformation.

Unravelling viral camouflage: approaches to the study and characterization of conformational epitopes

Antibodies are broadly used in clinical and basic research. Many of monoclonal antibodies are successfully adopted for therapeutic and diagnostic targeting of viral pathogens. Efficacy of antiviral neutralizing or protective antibodies depends on their ability to recognize epitopes interfering with viral infection. However, viruses are able to incessantly change their antigenic determinants to escape surveillance of humoral immune system and therefore the successful antiviral therapies require continuous development. Characterization of interactions of antibodies with prevalently conformational viral epitopes is important for understanding antibody mode of action and can help to identify conserved regions that may be exploited in designing new vaccines and virus neutralizing antibodies. In this article, we are reviewing techniques in use for characterization of conformational epitopes of monoclonal antibodies with focus on viruses.

Identification of structural determinants on tau protein essential for its pathological function: novel therapeutic target for tau immunotherapy in Alzheimer's disease

Introduction

Pathologically modified tau protein is the main feature of Alzheimer’s disease (AD) and related tauopathies. Therefore, immunotherapies that target mis-disordered tau represent a promising avenue for the disease-modifying treatment of AD. In this report, we present our discovery of (1) a novel target for tau immunotherapy; (2) monoclonal antibody DC8E8, which neutralizes this target; and (3) the results of efficacy studies of DC8E8 in a murine model of tauopathy.

Methods

In vitro tau oligomerisation assays were used for the selection of antibodies. The therapeutic efficacy of DC8E8 was evaluated in transgenic mice. The structure of the DC8E8 epitope was determined by X-ray crystallography.

Results

Screening of a panel of monoclonal antibodies for their inhibitory activity in an in vitro pathological tau–tau interaction assay yielded DC8E8, which reduced the amount of oligomeric tau by 84%. DC8E8 recognised all developmental stages of tau pathology in AD human brains, including pretangles and intra- and extracellular tangles. Treatment with DC8E8 in a mouse AD model expressing mis-disordered human tau significantly reduced the amount of insoluble oligomerised tau and the number of early and mature neurofibrillary tangles in the transgenic mouse brains. By using a panel of tau-derived peptides in a competitive enzyme-linked immunosorbent assay, we identified the tau domain essential for pathological tau–tau interaction, which is targeted by DC8E8. The antibody was capable of binding to four highly homologous and yet independent binding regions on tau, each of which is a separate epitope. The X-ray structure of the DC8E8 Fab apo form, solved at 3.0 Å, suggested that the four DC8E8 epitopes form protruding structures on the tau molecule. Finally, by kinetic measurements with surface plasmon resonance, we determined that antibody DC8E8 is highly discriminatory between pathological and physiological tau.

Conclusions

We have discovered defined determinants on mis-disordered truncated tau protein which are responsible for tau oligomerisation leading to neurofibrillary degeneration. Antibody DC8E8 reactive with these determinants is able to inhibit tau–tau interaction in vitro and in vivo. DC8E8 is able to discriminate between the healthy and diseased tau proteome, making its epitopes suitable targets, and DC8E8 a suitable candidate molecule, for AD immunotherapy.

Crystallization and preliminary X-ray diffraction analysis of two peptides from Alzheimer PHF in complex with the MN423 antibody Fab fragment

The major constituent of the Alzheimer's disease paired helical filaments (PHF) core is the intrinsically disordered protein (IDP) tau. Globular binding partners, e.g. monoclonal antibodies, can stabilize the fold of disordered tau in complexes. A previously published structure of a proteolytically generated tau fragment in a complex with the PHF-specific monoclonal antibody MN423 revealed a turn-like structure of the PHF core C-terminus [Sevcik et al. (2007). FEBS Lett. 581, 5872-5878]. To examine the structures of longer better-defined PHF segments, crystals of the MN423 Fab fragment were grown in the presence of two synthetic peptides derived from the PHF core C-terminus. For each, X-ray diffraction data were collected at 100 K at a synchrotron source and initial phases were obtained by molecular replacement.

Non-robotic high-throughput setup for manual assembly of nanolitre vapour-diffusion protein crystallization screens

Nanolitre-sized drops are characteristic of high-throughput protein crystallization screening. Traditionally, reliable nanolitre drop dispensing has required the use of robotics. This work describes the design and development of a protocol for the reproducible manual assembly of nanolitre-sized protein vapour-diffusion crystallization trials in a 96/192-drop format. The protocol exploits the repetitive-pipetting mode of handheld motorized pipettes together with simple tools available in standard laboratories. The method saves precious protein material without sacrificing the effectiveness of the screening process. To verify the approach, two monoclonal antibody Fab fragments were crystallized alone and in a complex with tau peptide antigens in 0.2–0.5 µl drops. Crystals grown directly from the screen conditions in sitting drops on 96-well plates diffracted up to 1.6 Å resolution on a synchrotron source. The results proved that successful crystallization in nanolitre high-throughput format is affordable even in the absence of expensive robotic instrumentation.

Crystallization and preliminary X-ray diffraction analysis of tau protein microtubule-binding motifs in complex with Tau5 and DC25 antibody Fab fragments

The Alzheimer's disease-associated protein tau is an intrinsically disordered protein with no preferred structure in solution. Under physiological conditions, tau binds to microtubules and regulates their dynamics, whereas during the development of neurodegeneration tau dissociates from microtubules, misfolds and creates highly insoluble deposits. To elucidate the determinants of tau-protein misfolding, tau peptides from microtubule-binding motifs were crystallized in complexes with Fab fragments of specific monoclonal antibodies. The crystals diffracted to 1.69 Å resolution and gave complete data sets using a synchrotron X-ray source. Molecular replacement was used to solve the phase problem.

Hyperphosphorylated truncated protein tau induces caspase-3 independent apoptosis-like pathway in the Alzheimer's disease cellular model

Neurofibrillary degeneration and neuronal loss represent key pathological hallmarks of Alzheimer's disease (AD). It has been demonstrated that the decrease of total neuronal numbers correlates with the presence of neurofibrillary degeneration in AD brain. In order to unravel the mechanism leading to the cell death in AD, we developed a stably transfected human neuroblastoma cellular model with doxycycline-regulatable expression of AD truncated tau protein (AT tau, 151-391 4R). Cells expressing the longest tau isoform (Tau 40) were used as a control. We found that more than 80% of the total amount of AT tau and Tau 40 were phosphorylated. Strikingly, both AT tau and Tau 40 reduced the metabolic activity of the cells in a time-dependent manner (p < 0.0001) suggesting that tau overexpression slows down cell proliferation. However, AT tau showed significantly higher toxicity than Tau 40 (p < 0.0001), which indicates that truncation leads to a toxic gain of function. The analysis of the type of the cell death revealed the characteristic features of apoptosis such as cell shrinkage, nuclear, and DNA fragmentation. However, we did not find either the activation of executive caspase (caspase-3) or the caspase cleavage products (PARP and fodrin). These results show that posttranslationally modified truncated tau protein induces caspase-3-independent apoptosis-like programmed cell death, a phenomenon we term tauoptosis.

Monoclonal antibody MN423 as a stable mold facilitates structure determination of disordered tau protein

Flexibility of intrinsically disordered tau protein is important for performing its functions. It is believed that alteration of the flexibility is instrumental to the assembly of tau protein into paired helical filaments (PHF) in tauopathies. Tau flexibility represents the main obstacle for structure determination of its conformation in physiology and/or pathology. We have alleviated this inherited difficulty by using specific monoclonal antibodies as tau protein surrogate binding partners. In this work we compare two "antibody mold structures": (1) X-ray structure of the free form of the Alzheimer's disease PHF core-specific antibody MN423 and (2) previously solved structure of the complex of MN423 with the PHF core C-terminal tau peptide. We found that MN423 combining site is in both structures identical. As a consequence, recombinant tau assumes in the complex a fold determined by the antibody combining site. Obtained results show that MN423 functions as a molecular mold for the PHF core segment, and opens the way for structure determination of other PHF core segments providing that other conformation-specific antibodies are available. Data from in silico docking of tau peptide into antibody mold, obtained in this study, show that biochemical data and computational approaches provide results comparable to X-ray crystallography.

Transition of tau protein from disordered to misordered in Alzheimer's disease

The neuronal protein tau, a member of the class of intrinsically disordered proteins, is characterized by the absence of any firm 3-D structure and high solubility when free in solution. The tau protein forms insoluble fibrils in the brain of people suffering from Alzheimer's disease (AD) and other tauopathies and plays a key role in the neurodegenerative process. Posttranslational events leading to the transition of tau from a disordered highly soluble protein to the insoluble aggregate seem to be associated with hyperphosphorylation, truncation or a combination of both. These modifications are assumed to change the native disorder of tau into a preaggregation state, which then directly initiates the fibrillization process. Conformation-specific anti-tau antibody DC11 detects pathological truncated tau forms present in AD brains but not in the normal human brain. A truncated tau protein selected from this pool of DC11-positive molecules was sufficient to initiate and drive the complete tau cascade of neurofibrillary pathology in a rat model of tauopathy. Thus, DC11 antibody recognizes the AD-specific conformation of tau not found in the normal human brain. We propose the term 'misdisordered' for this distinct conformational state of tau, which is the first step in the transition of tau from a soluble disordered protein to its insoluble, misordered aggregated form.

Novel mutations in TLR genes cause hyporesponsiveness to Mycobacterium avium subsp. paratuberculosis infection

BACKGROUND

Toll like receptors (TLR) play the central role in the recognition of pathogen associated molecular patterns (PAMPs). Mutations in the TLR1, TLR2 and TLR4 genes may change the ability to recognize PAMPs and cause altered responsiveness to the bacterial pathogens.

RESULTS

The study presents association between TLR gene mutations and increased susceptibility to Mycobacterium avium subsp. paratuberculosis (MAP) infection. Novel mutations in TLR genes (TLR1- Ser150Gly and Val220Met; TLR2 - Phe670Leu) were statistically correlated with the hindrance in recognition of MAP legends. This correlation was confirmed subsequently by measuring the expression levels of cytokines (IL-4, IL-8, IL-10, IL-12 and IFN-gamma) in the mutant and wild type moDCs (mocyte derived dendritic cells) after challenge with MAP cell lysate or LPS. Further in silico analysis of the TLR1 and TLR4 ectodomains (ECD) revealed the polymorphic nature of the central ECD and irregularities in the central LRR (leucine rich repeat) motifs.

CONCLUSION

The most critical positions that may alter the pathogen recognition ability of TLR were: the 9th amino acid position in LRR motif (TLR1-LRR10) and 4th residue downstream to LRR domain (exta-LRR region of TLR4). The study describes novel mutations in the TLRs and presents their association with the MAP infection.

Structure solution of misfolded conformations adopted by intrinsically disordered Alzheimer's tau protein

Until now it was impossible to obtain atomic structure of intrinsically disordered protein (IDP) tau and/or its assembly in Alzheimer's paired helical filaments as neither of them could have been prepared in the form amenable to X-ray or NMR techniques. Using IDP tau property to attain regular tertiary structure after binding events during self-assembly or when complexed with its target we propose monoclonal antibodies as surrogate tau protein binding partners to form complexes and crystals for structure solution by X-ray technique.

A global benchmark study using affinity-based biosensors

To explore the variability in biosensor studies, 150 participants from 20 countries were given the same protein samples and asked to determine kinetic rate constants for the interaction. We chose a protein system that was amenable to analysis using different biosensor platforms as well as by users of different expertise levels. The two proteins (a 50-kDa Fab and a 60-kDa glutathione S-transferase [GST] antigen) form a relatively high-affinity complex, so participants needed to optimize several experimental parameters, including ligand immobilization and regeneration conditions as well as analyte concentrations and injection/dissociation times. Although most participants collected binding responses that could be fit to yield kinetic parameters, the quality of a few data sets could have been improved by optimizing the assay design. Once these outliers were removed, the average reported affinity across the remaining panel of participants was 620 pM with a standard deviation of 980 pM. These results demonstrate that when this biosensor assay was designed and executed appropriately, the reported rate constants were consistent, and independent of which protein was immobilized and which biosensor was used.

Preserving free thiols of intrinsically disordered tau protein without the use of a reducing agent

Intrinsically disordered proteins (IDPs) represent key mediators in many physiological as well as pathological processes. Solution-exposed cysteines of IDPs are highly reactive and therefore reducing agents are frequently included during their preparation to prevent formation of nonnative disulfides. However, reductants can potentially interfere with subsequent assays performed on the purified IDPs. Herein we report a method for purification of IDP tau in an atmosphere of inert argon, which eliminates the need for reducing agents. We have used this method for preparing several IDP tau isoforms and found it useful in the investigation of monomeric tau toxicity in rat cerebral neurons.

X-ray structure of the PHF core C-terminus: insight into the folding of the intrinsically disordered protein tau in Alzheimer's disease

The major constituent of Alzheimer's disease paired helical filaments (PHF) core is intrinsically disordered protein (IDP) tau. In spite of a considerable effort, insoluble character of PHF together with inherent physical properties of IDP tau have precluded so far reconstruction of PHF 3D structure by X-ray crystallography or NMR spectroscopy. Here we present first crystallographic study of PHF core C-terminus. Using monoclonal antibody MN423 specific to the tertiary structure of the PHF core, the in vivo PHF structure was imprinted into recombinant core PHF tau. Crystallization of the complex led to determination of the structure of the core PHF tau protein fragment 386TDHGAE391 at 1.65A resolution. Structural analysis suggests important role of the core PHF C-terminus for PHF assembly. It is reasonable to expect that this approach will help to reveal the structural principles underlying the tau protein assembly into PHF and possibly will facilitate rationale drug design for inhibition of Alzheimer neurofibrillary changes.

A novel monoclonal antibody DC63 reveals that inhibitor 1 of protein phosphatase 2A is preferentially nuclearly localised in human brain

Abnormal phosphorylation of tau protein represents one of the major candidate pathological mechanisms leading to Alzheimer's disease (AD) and related tauopathies. Altered phosphorylation status of neuronal tau protein may result from upregulation of tau-specific kinases or from inhibition of tau-specific phosphatases. Increased expression of the protein inhibitor 1 of protein phosphatase 2A (I1PP2A) could therefore indirectly regulate the phosphorylation status of tau. As an important step towards elucidation of the role of I1PP2A in the physiology and pathology of tau phosphorylation, we developed a novel monoclonal antibody, DC63, which recognizes I1PP2A. Specificity of the antibody was examined by mass spectrometry and Western blot. This analysis supports the conclusion that the antibody does not recognize any of the other proteins of the 9-member leucine-rich acidic nuclear phosphoprotein family to which I1PP2A belongs. Immunoblot detection revealed that the inhibitor I1PP2A is expressed throughout the brain, including the hippocampus, temporal cortex, parietal cortex, subcortical nuclei and brain stem. The cerebellum displayed significantly higher levels of expression of I1PP2A than was seen elsewhere in the brain. Imunohistochemical analysis of normal human brain showed that I1PP2A is expressed in both neurons and glial cells and that the protein is preferentially localized to the nucleus. We conclude that the novel monoclonal antibody DC63 could be successfully employed as a mass spectrometry-validated molecular probe that may be used for in vitro and in vivo qualitative and quantitative studies of physiological and pathological pathways involving I1PP2A.

Alzheimer’s-disease-associated conformation of intrinsically disordered tau protein studied by intrinsically disordered protein liquid-phase competitive enzyme-linked immunosorbent assay

Tau protein, the major constituent of paired helical filaments in Alzheimer's disease, belongs to the intrinsically disordered proteins (IDPs). IDPs are an emerging group in the protein kingdom characterized by the absence of a rigid three-dimensional structure. Disordered proteins usually acquire a "functional fold" upon binding to their interaction partner(s). This property of IDPs implies the need for innovative approaches to measure their binding affinity. We have mapped and measured the Alzheimer's-disease-associated epitope on intrinsically disordered tau protein with a novel two-step sandwich competitive enzyme-linked immunosorbent assay (ELISA). This approach allowed us to determine the binding affinity of disordered tau protein in liquid phase without any disturbance to the competitive equilibrium and without any need for covalent or noncovalent modification of tau protein. Furthermore, the global fitting method, used for the reconstruction of tau binding curves, significantly improved the assay readout. The proposed novel competitive ELISA allowed us to determine the changes in the standard Gibbs energy of binding, thus enabling measurement of tau protein conformation in the core of paired helical filaments. IDP competitive ELISA results showed, for the first time, that the tau protein C terminus of the Alzheimer's-disease-derived paired helical filaments core subunit adopts beta-turn type I' fold and is accessible from solution.

Preparation, Crystallization and Preliminary X-Ray Analysis of the Fab Fragment of Monoclonal Antibody MN423, Revealing the Structural Aspects of Alzheimers Paired Helical Filaments

Monoclonal antibody (mAb) MN423 recognizes Alzheimer's disease specific conformation of tau protein assembled into paired helical filaments (PHF). Since the three-dimensional structure of PHF is currently unavailable, the structure of MN423 binding site could provide important information about PHF conformation with the consequences for the Alzheimer's disease prevention and cure. Fab fragment of MN423 was prepared and purified. We have identified two different conditions for crystallization of the Fab fragment that yielded two crystal forms. They diffracted to 3.0 and 1.6 A resolution with four and one molecule in the asymmetric unit, respectively. Both crystal forms belonged to the space group P2(1) with unit cell parameters a = 76.4 A, b = 138.4 A, c = 92.4 A, beta = 101.9 degrees , and a = 71.5 A, b = 36.8 A, c = 85.5 A, beta = 113.9 degrees .

Intrinsically Disordered Proteins in the Neurodegenerative Processes: Formation of Tau Protein Paired Helical Filaments and Their Analysis

1. Several intrinsically disordered proteins (IDPs) play principal role in the neurodegenerative processes of various types. Among them, alpha-synuclein is involved in Parkinson's disease, prion protein in transmissible spongiform encephalopathies, and tau protein in Alzheimer's disease (AD) and related tauopathies. Neuronal damage in AD is accompanied by the presence of tau protein fibrils composed of paired helical filaments (PHF). 2. Tau protein represents a typical IDP. IDPs do not exhibit any stable secondary structure in the free form, but they are able to fold after binding to targets and contain regions with large propensity to adopt a defined type of secondary structure. Binding-folding event at tau protein leading to PHF generation is believed to happen in the course of tauopathies. 3. Detailed molecular topology of PHF formation is unknown. There are evidences about the cross-beta structure in PHF core; however the precise arrangement of the tau polypeptide chain is unclear. In this review we summarize current attempts at in vitro PHF reconstruction and the development of methods for PHF structure determination. The emphasis is put on the monoclonal antibodies used as structural molecular probes for research on the role of IDPs in pathogenesis of neurodegenerative diseases.

Truncated tau from sporadic Alzheimer's disease suffices to drive neurofibrillary degeneration in vivo

Truncated tau protein is the characteristic feature of human sporadic Alzheimer's disease. We have identified truncated tau proteins conformationally different from normal healthy tau. Subpopulations of these structurally different tau species promoted abnormal microtubule assembly in vitro suggesting toxic gain of function. To validate pathological activity in vivo we expressed active form of human truncated tau protein as transgene, in the rat brain. Its neuronal expression led to the development of the neurofibrillary degeneration of Alzheimer's type. Furthermore, biochemical analysis of neurofibrillary changes revealed that massive sarcosyl insoluble tau complexes consisted of human Alzheimer's tau and endogenous rat tau in ratio 1:1 including characteristic Alzheimer's disease (AD)-specific proteins (A68). This work represents first insight into the possible causative role of truncated tau in AD neurofibrillary degeneration in vivo.

Intrinsically disordered tau protein in Alzheimer's tangles: a coincidence or a rule?

Tau protein, the major constituent of neurofibrillary tangles in Alzheimer's disease (AD) and related tauopathies, is classified as intrinsically disordered protein (IDP). IDPs in contrast to globular proteins contain high proportion of polar and charged amino acids in their sequence, which results in the absence of a well-defined three-dimensional structure of the free protein. Structural flexibility of IDPs is required to perform their important role in many cellular processes. In the course of tauopathies, highly soluble disordered tau protein acquires rigid fold and forms highly insoluble filaments. Beneficial intrinsic disorder transforms into a fatal order: is it a coincidence, or is there an underlying reason for preferential IDPs assembly? In this review we present the structural characteristics of tau protein filamentous lesions in AD and discuss the tendency of IDPs to assembly and to form amyloid deposits (Ref: 65).

Rapid purification of truncated tau proteins: model approach to purification of functionally active fragments of disordered proteins, implication for neurodegenerative diseases

Truncated tau is of great interest because of its important role in neurofibrillary pathogenesis in Alzheimer's disease (AD). A major obstacle for characterization of detailed biochemical and biological properties of truncated tau species and their fragments has been the lack of reliable and quick purification methods. Uneven distribution of acidic and basic residues in tau determines that the N- and C-terminal tau fragments require entirely different purification conditions. Conventional methods take several days; they do not allow purification of the acidic N-terminal tau fragments and do not prevent aggregation during purification that makes purified truncated tau unusable in functional studies. To prevent these inherent problems, we have designed a two-step, highly efficient purification procedure yielding a fully functional, non-aggregated homogeneous population of truncated tau molecules. Various forms of tau produced in bacteria without the need for a heat pre-treatment step were subjected to anion- and cation-exchange chromatography. Conditions were developed that allowed effective separation and purification of acidic and/or basic tau species. Following the gel filtration step, up to 10mg of tau proteins with 96% purity was obtained within one working day. Purified truncated tau exhibited an unmodified immunoreactivity and allowed its functional activity analysis. Since many neurodegenerative diseases have implicated similar disordered proteins in their pathogenesis, our procedure will allow their detailed analysis and characterization.

Folding of Alzheimer's core PHF subunit revealed by monoclonal antibody 423

At present, the conformation-dependent monoclonal antibodies (mAb) provide the only information on folding of tau in the core PHF. Monoclonal antibody MN423 recognizes all and only those Alzheimer's disease (AD) core paired helical filaments (PHFs) subunits, which terminate at Glu391. Using recombinant analogs of the core PHF subunit corresponding to tau residues tau297-391, we found that the C-terminal pentapeptide (387)DHGAE(391) represented only one component of the structure recognized by mAb 423. Therefore, deletion mutants of the core subunit were generated to identify assembled parts of this conformational structure. We localized two spatially close components in the region 306-325 ((306)VQIVYK(311) and (321)KCGSL(325)) contributing to formation of the structure identified by mAb 423. Thus, the spatial proximity of three subunit segments (306)VQIVYK(311), (321)KCGSL(325) and (387)DHGAE(391) represents constraints for intramolecular folding of the core PHF subunit. Since PHF represents a compelling drug target in AD, structural knowledge presented could contribute to structure-based drug design.

Mapping the C terminal epitope of the Alzheimer's disease specific antibody MN423

The mapping of monoclonal antibody epitopes is now predominantly carried out using molecular diversity techniques, phage display in particular. However, until very recently, phage display methods have been inappropriate for the analysis of epitopes that require a free carboxy terminus. Here we describe the use of two different techniques to analyze the known C terminal epitope specificity of MN423, a monoclonal antibody specifically staining truncated tau in Alzheimer's brain. Using a lambda phage based C-terminal random peptide library, and an intracellular expression library based on truncated tau, we show that this antibody has an absolute requirement for a glycine at position -3 with respect to the C terminus. Both methods give similar results, and identify other important residues in the binding site. However, affinity analysis of synthetic peptides revealed that the affinity of the antibody for identified tripeptides was far lower than the pentapeptide sequence in the native target, and that this in turn was considerably below the affinity for the native target itself. This suggests that molecular diversity methods may define minimum, but not necessarily complete epitopes. The methods described here have a general application to the analysis of antibody epitopes suspected to be found at the C terminus.

Subsite specificity of the proteinase from myeloblastosis associated virus

The subsite requirements of the aspartic proteinase from the myeloblastosis-associated virus (MAV) for the cleavage of peptide substrates were studied with a series of synthetic peptides of general structure Ala-Thr-P4-P3-P2-P1*Nph-Val-Arg-Lys-Ala. The residues in positions P4, P3, P2 and P1 were varied and the kinetic parameters for the cleavage of substrates in 2.0 M NaCl were spectrophotometrically determined at pH 6.0 and 37 degrees C. The acceptance of amino acid residues in particular subsites is similar to that observed with the human immunodeficiency virus type 1 (HIV-1) proteinase in our earlier studies on the same substrate series: hydrophobic or aromatic residues are preferable in P1 position, a broad variety of residues are acceptable in P3 whereas the residues occupying P2 plays the decisive role in the substrate cleavage as evidenced by its dramatic influence on both kcat and Km values. The most remarkable difference between the two enzymes was found in P3 and P4 subsites. In P3, the introduction of negatively charged glutamate increases the substrate binding by the MAV proteinase 12-fold and decreases binding by the HIV-1 proteinase. In P4, Pro in this series is a favourable residue for the MAV proteinase and is strongly inacceptable for HIV-1 the proteinase. The pH profile of the cleavage was studied with a chromogenic substrate and differences between HIV-1 and MAV proteinases are discussed.