Chaperone-Like Antibodies in Neurodegenerative Tauopathies: Implication for Immunotherapy

Recent advances in tau-directed immunotherapy against Alzheimer's disease: an overview of pre-clinical and clinical development

Alzheimer's disease (AD) has been a worldwide concern for many years now. This is due to the fact that AD is an irreversible and progressive neurodegenerative disease that affects quality of life. Failure of some Phase II/III clinical trials in AD targeting accumulation of β-amyloid in the brain has led to an increase in interest in studying alternative treatments against tubulin-associated unit (Tau) pathology. These alternative treatments include active and passive immunisation. Based on numerous studies, Tau is reported as a potential immunotherapeutic target for tauopathy-related diseases including AD. Accumulation and aggregation of hyperphosphorylated Tau as neuropil threads and neurofibrillary tangles (NFT) are pathological hallmarks of AD. Both active and passive immunisation targeting Tau protein have shown the capabilities to decrease or prevent Tau pathology and improve either motor or cognitive impairment in various animal models. In this review, we summarise recent advances in active and passive immunisation targeting pathological Tau protein, and will discuss with data obtained from both animal and human trials. Together, we give a brief overview about problems being encountered in these immunotherapies.

Enzyme-containing spin membranes for rapid digestion and characterization of single proteins

Proteolytic digestion is an important step in characterizing protein sequences and post-translational modifications (PTMs) using mass spectrometry (MS). This study uses pepsin- or trypsin-containing spin membranes for rapid digestion of single proteins or simple protein mixtures prior to ultrahigh-resolution Orbitrap MS analysis. Centrifugation of 100 μL of pretreated protein solutions through the functionalized membranes requires less than 1 min and conveniently digests proteins into large peptides that aid in confirming specific protein sequence variations and PTMs. Peptic and tryptic peptides from spin digestion of apomyoglobin and four commercial monoclonal antibodies (mAbs) typically cover 100% of the protein sequences in direct infusion MS analysis. Increasing the spin rate leads to a higher fraction of large peptic peptides for apomyoglobin, and MS analysis of peptic and tryptic peptides reveals mAb PTMs such as N-terminal pyroglutamate formation, C-terminal lysine clipping and glycosylation. Relative to overnight in-solution digestion of mAbs, spin digestion yields higher sequence coverages. Spin-membrane digestion followed by infusion MS readily differentiates a mAb to the Ebola virus from a related antibody that differs by addition of a single amino acid.

Tau-based therapeutics for Alzheimer's disease: active and passive immunotherapy

Pharmacological manipulation of tau protein in Alzheimer's disease included microtubule-stabilizing agents, tau protein kinase inhibitors, tau aggregation inhibitors, active and passive immunotherapies and, more recently, inhibitors of tau acetylation. Animal studies have shown that both active and passive approaches can remove tau pathology and, in some cases, improve cognitive function. Two active vaccines targeting either nonphosphorylated (AAD-vac1) and phosphorylated tau (ACI-35) have entered Phase I testing. Notwithstanding, the recent discontinuation of the monoclonal antibody RG7345 for Alzheimer's disease, two other antitau antibodies, BMS-986168 and C2N-8E12, are also currently in Phase I testing for progressive supranuclear palsy. After the recent impressive results in animal studies obtained by salsalate, the dimer of salicylic acid, inhibitors of tau acetylation are being actively pursued.

Tau-directed approaches for the treatment of Alzheimer's disease: focus on leuco-methylthioninium

Small molecular weight compounds able to inhibit formation of tau oligomers and fibrils have already been tested for Alzheimer's disease (AD) treatment. The most advanced tau aggregation inhibitor (TAI) is methylthioninium (MT), a drug existing in equilibrium between a reduced (leuco-methylthioninium) and oxidized form (MT(+)). MT chloride (also known as methylene blue) was investigated in a 24-week Phase II study in 321 mild-to-moderate AD patients at the doses of 69, 138, and 228 mg/day. This trial failed to show significant positive effects of MT in the overall patient population. The dose of 138 mg/day showed potential benefits on cognitive performance of moderately affected patients and cerebral blood flow in mildly affected patients. A follow-up compound (TRx0237) claimed to be more bioavailable and less toxic than MT, is now being developed. Phase III clinical trials on this novel TAI in AD and in the behavioral variant of frontotemporal dementia are underway.

Intracellular and extracellular microtubule associated protein tau as a therapeutic target in Alzheimer disease and other tauopathies

INTRODUCTION

Microtubule associated protein tau, a protein mainly expressed in neurons, plays an important role in several diseases related to dementia, named tauopathies. Alzheimer disease is the most relevant tauopathy. The role of tau protein in dementia is now a topic under discussion, and is the focus of this review.

AREAS COVERED

We have covered two major areas: tau pathology and tau as a therapeutic target. Tau pathology is mainly related to a gain of toxic function due to an abnormal accumulation, aberrant modifications (such as hyperphosphorylation and truncation, among others) and self-aggregation of tau into oligomers or larger structures. Also, tau can be found extracellularly in a toxic form. Tau-based therapy is mainly centered on avoiding the gain of these toxic functions of tau.

EXPERT OPINION

Tau therapies are focused on lowering tau levels, mainly of modified tau species that could be toxic for neurons (phosphorylated, truncated or aggregated tau), in intracellular or extracellular form. Decreasing the levels of those toxic species is a possible therapeutic strategy.

Effects of tau domain-specific antibodies and intravenous immunoglobulin on tau aggregation and aggregate degradation

Tau pathology, including neurofibrillary tangles, develops in Alzheimer's disease (AD). The aggregation and hyperphosphorylation of tau are potential therapeutic targets for AD. Administration of anti-tau antibodies reduces tau pathology in transgenic "tauopathy" mice; however, the optimal tau epitopes and conformations to target are unclear. Also unknown is whether intravenous immunoglobulin (IVIG) products, currently being evaluated in AD trials, exert effects on pathological tau. This study examined the effects of anti-tau antibodies targeting different tau epitopes and the IVIG Gammagard on tau aggregation and preformed tau aggregates. Tau aggregation was assessed by transmission electron microscopy and fluorescence spectroscopy, and the binding affinity of the anti-tau antibodies for tau was evaluated by enzyme-linked immunosorbent assays. Antibodies used were anti-tau 1-150 ("D-8"), anti-tau 259-266 ("Paired-262"), anti-tau 341-360 ("A-10"), and anti-tau 404-441 ("Tau-46"), which bind to tau's N-terminus, microtubule binding domain (MBD) repeat sequences R1 and R4, and the C-terminus, respectively. The antibodies Paired-262 and A-10, but not D-8 and Tau-46, reduced tau fibrillization and degraded preformed tau aggregates, whereas the IVIG reduced tau aggregation but did not alter preformed aggregates. The binding affinities of the antibodies for the epitope for which they were specific did not appear to be related to their effects on tau aggregation. These results confirm that antibody binding to tau's MBD repeat sequences may inhibit tau aggregation and indicate that such antibodies may also degrade preformed tau aggregates. In the presence of anti-tau antibodies, the resulting tau morphologies were antigen-dependent. The results also suggested the possibility of different pathways regulating antibody-mediated inhibition of tau aggregation and antibody-mediated degradation of preformed tau aggregates.

Further understanding of tau phosphorylation: implications for therapy

Tau is a brain microtubule-associated protein that regulates microtubule structure and function. Prominent tau neurofibrillary pathology is a common feature in a number of neurodegenerative disorders collectively referred to as tauopathies, the most common of which is Alzheimer's disease. Beyond its classical role as a microtubule-associated protein, recent advances in our understanding of tau cellular functions have unveiled novel important tau cellular functions that may also play a pivotal role in pathogenesis and render novel targets for therapeutic intervention. Regulation of tau behavior and function under physiological and pathological conditions is mainly achieved through post-translational modifications, especially phosphorylation, which has significant implications for the development of novel therapeutic approaches in a number of neurodegenerative disorders.

Immunotherapy for Alzheimer's disease: from anti-β-amyloid to tau-based immunization strategies

The exact mechanisms leading to Alzheimer's disease (AD) are largely unknown, limiting the identification of effective disease-modifying therapies. The two principal neuropathological hallmarks of AD are extracellular β-amyloid (Aβ), peptide deposition (senile plaques) and intracellular neurofibrillary tangles containing hyperphosphorylated tau protein. During the last decade, most of the efforts of the pharmaceutical industry were directed against the production and accumulation of Aβ. The most innovative of the pharmacological approaches was the stimulation of Aβ clearance from the brain of AD patients via the administration of Aβ antigens (active vaccination) or anti-Aβ antibodies (passive vaccination). Several active and passive anti-Aβ vaccines are under clinical investigation. Unfortunately, the first active vaccine (AN1792, consisting of preaggregate Aβ and an immune adjuvant, QS-21) was abandoned because it caused meningoencephalitis in approximately 6% of treated patients. Anti-Aβ monoclonal antibodies (bapineuzumab and solanezumab) are now being developed. The clinical results of the initial studies with bapineuzumab were equivocal in terms of cognitive benefit. The occurrence of vasogenic edema after bapineuzumab, and more rarely brain microhemorrhages (especially in Apo E ε4 carriers), has raised concerns on the safety of these antibodies directed against the N-terminus of the Aβ peptide. Solanezumab, a humanized anti-Aβ monoclonal antibody directed against the midregion of the Aβ peptide, was shown to neutralize soluble Aβ species. Phase II studies showed a good safety profile of solanezumab, while studies on cerebrospinal and plasma biomarkers documented good signals of pharmacodynamic activity. Although some studies suggested that active immunization may be effective against tau in animal models of AD, very few studies regarding passive immunization against tau protein are currently available. The results of the large, ongoing Phase III trials with bapineuzumab and solanezumab will tell us if monoclonal anti-Aβ antibodies may slow down the rate of deterioration of AD. Based on the new diagnostic criteria of AD and on recent major failures of anti-Aβ drugs in mild-to-moderate AD patients, one could argue that clinical trials on potential disease-modifying drugs, including immunological approaches, should be performed in the early stages of AD.

Monoclonal antibodies against β-amyloid (Aβ) for the treatment of Alzheimer's disease: the Aβ target at a crossroads

Several second-generation active β-amyloid (Aβ) vaccines and passive Aβ immunotherapies are under clinical investigation with the aim of boosting Aβ clearance from the brain of the Alzheimer's disease (AD) patients. However, the preliminary cognitive efficacy of bapineuzumab, a humanized anti-Aβ monoclonal antibody, appears uncertain. Moreover, the occurrence of vasogenic edema and, more rarely, brain microhemorrhages, especially in apolipoprotein E ϵ4 carriers, have led to abandoning of the highest dose of the drug. Solanezumab, another humanized anti-Aβ monoclonal antibody, was shown to neutralize soluble Aβ oligomers, which is believed to be the more neurotoxic Aβ species. Phase II studies showed a good safety profile of solanezumab while studies on cerebrospinal and plasma biomarkers documented good signals of pharmacodynamic activity. However, the preliminary equivocal cognitive results obtained with bapineuzumab as well as the detrimental cognitive effects observed with semagacestat, a potent γ-secretase inhibitor, raise the possibility that targeting Aβ may not be clinically efficacious in AD. The results of four ongoing large Phase III trials on bapineuzumab and two Phase III trials on solanezumab will tell us if passive anti-Aβ immunization is able to alter the course of this devastating disease, and if Aβ is still a viable target for anti-AD drugs.

Hsp90 regulates tau pathology through co-chaperone complexes in Alzheimer's disease

Alzheimer's disease is a tauopathy which involves the deposition of microtubule-associated tau proteins into neurofibrillary tangles. Post-translational modifications, in particular site-specific phosphorylations, affect the conformation of tau protein which is an intrinsically disordered protein. These structural changes significantly increase the affinity of tau protein for certain molecular chaperones. Hsp90 is a major cellular chaperone which assembles large complexes with a variety of co-chaperones. The main function of Hsp90 complexes is to maintain protein quality control and assist in protein degradation via proteasomal and autophagic-lysosomal pathways. Tau protein is a client protein for these Hsp90 complexes. If the tau protein is in an abnormal or modified form, then it can trigger the recruitment of CHIP protein, a co-chaperone with E3 activity, to the complex which induces the ubiquitination of tau protein and activates its downstream degradation processes. Large immunophilins, FKBP51 and FKBP52 are also co-chaperones of Hsp90-tau complexes. These proteins contain peptidylprolyl cis/trans isomerase activity which catalyzes phosphorylation-dependent rotation in pSer/Thr-Pro peptide bond. The proline switch in the tau conformation triggers dephosphorylation of Ser/Thr residues phosphorylated, e.g. by two well-known tau kinases Cdk5 and GSK-3β. Binding of PP5 protein phosphatase to Hsp90 complex, can also dephosphorylate tau protein. Subsequently, dephosphorylated tau protein can be shuttled back to the microtubules. It seems that high-affinity binding of abnormal tau to Hsp90 complexes may have some counteracting effects on the aggregation process, since Hsp90 inhibitors can ameliorate the aggregation process in several neurodegenerative diseases. We will review the role of Hsp90 chaperone network in the regulation of tau biology and pathology in Alzheimer's disease.

Conformational diseases: looking into the eyes

Conformational diseases, a general term comprising more than 40 disorders are caused by the accumulation of unfolded or misfolded proteins. Improper protein folding (misfolding) as well as accrual of unfolded proteins can lead to the formation of disordered (amorphous) or ordered (amyloid fibril) aggregates. The gradual accumulation of protein aggregates and the acceleration of their formation by stress explain the characteristic late or episodic onset of the diseases. The best studied in this group are neurodegenerative diseases and amyloidosis accompanied by the deposition of a specific aggregation-prone proteins or protein fragments and formation of insoluble fibrils. Amyloidogenic protein accumulation often occurs in the brain tissues, e.g. in Alzheimer's disease with the deposition of amyloid-beta and Tau, in scrapie and bovine spongiform encephalopathy with the accumulation of prion protein, in Parkinson's disease with the deposition of alpha-synuclein. Other examples of amyloid proteins are transthyretin, immunoglobulin light chain, gelsolin, etc. In addition to the brain, the accumulation of unfolded or misfolded proteins leading to pathology takes place in a wide variety of organs and tissues, including different parts of the eye. The best studied ocular conformational diseases are cataract in the lens and retinitis pigmentosa in the retina, but accumulation of misfolded proteins also occurs in other parts of the eye causing various disorders. Furthermore, ocular manifestation of systemic amyloidosis often causes the deposition of amyloidogenic proteins in different ocular tissues. Here we present the data regarding naturally unfolded and misfolded proteins in eye tissues, their structure-function relationships, and molecular mechanisms underlying their involvement in diseases. We also summarize the etiology of ocular conformational diseases and discuss approaches to their treatment.

Treating lysosomal storage diseases with pharmacological chaperones: from concept to clinics

Lysosomal storage diseases (LSDs) are a group of genetic disorders due to defects in any aspect of lysosomal biology. During the past two decades, different approaches have been introduced for the treatment of these conditions. Among them, enzyme replacement therapy (ERT) represented a major advance and is used successfully in the treatment of some of these disorders. However, ERT has limitations such as insufficient biodistribution of recombinant enzymes and high costs. An emerging strategy for the treatment of LSDs is pharmacological chaperone therapy (PCT), based on the use of chaperone molecules that assist the folding of mutated enzymes and improve their stability and lysosomal trafficking. After proof-of-concept studies, PCT is now being translated into clinical applications for Fabry, Gaucher and Pompe disease. This approach, however, can only be applied to patients carrying chaperone-responsive mutations. The recent demonstration of a synergistic effect of chaperones and ERT expands the applications of PCT and prompts a re-evaluation of their therapeutic use and potential. This review discusses the strengths and drawbacks of the potential therapies available for LSDs and proposes that future research should be directed towards the development of treatment protocols based on the combination of different therapies to improve the clinical outcome of LSD patients.

Mediators of tau phosphorylation in the pathogenesis of Alzheimer's disease

The need for disease-modifying drugs for Alzheimer's disease has become increasingly important owing to escalating disease prevalence and the associated socio-economic burden. Until recently, reducing brain amyloid accumulation has been the main therapeutic focus; however, increasing evidence suggests that targeting abnormal tau phosphorylation could be beneficial. Tau is phosphorylated by several protein kinases and this is balanced by dephosphorylation by protein phosphatases. Phosphorylation at specific sites can influence the physiological functions of tau, including its role in binding to and stabilizing the neuronal cytoskeleton. aberrant phosphorylation of tau could render it susceptible to potentially pathogenic alterations, including conformational changes, proteolytic cleavage and aggregation. While strategies that reduce tau phosphorylation in transgenic models of disease have been promising, our understanding of the mechanisms through which tau becomes abnormally phosphorylated in disease is lacking.