Tau antibody chimerization alters its charge and binding, thereby reducing its cellular uptake and efficacy

Messenger RNA-encoded antibody approach for targeting extracellular and intracellular tau

Monoclonal antibodies have emerged as a leading therapeutic agent for the treatment of disease, including Alzheimer's disease. In the last year, two anti-amyloid monoclonal antibodies, lecanemab and aducanumab, have been approved in the USA for the treatment of Alzheimer's disease, whilst several tau-targeting monoclonal antibodies are currently in clinical trials. Such antibodies, however, are expensive and timely to produce and require frequent dosing regimens to ensure disease-modifying effects. Synthetic in vitro-transcribed messenger RNA encoding antibodies for endogenous protein expression holds the potential to overcome many of the limitations associated with protein antibody production. Here, we have generated synthetic in vitro-transcribed messenger RNA encoding a tau-specific antibody as a full-sized immunoglobulin and as a single-chain variable fragment. In vitro transfection of human neuroblastoma SH-SY5Y cells demonstrated the ability of the synthetic messenger RNA to be translated into a functional tau-specific antibody. Furthermore, we show that the translation of the tau-specific single-chain variable fragment as an intrabody results in the specific engagement of intracellular tau. This work highlights the utility of messenger RNA for the delivery of antibody therapeutics, including intrabodies, for the targeting of tau in Alzheimer's disease and other tauopathies.

Single-Domain Antibody-Based Protein Degrader for Synucleinopathies

Synucleinopathies are a group of neurodegenerative diseases characterized by the accumulation of α-synuclein (α-syn) in the brain, leading to motor and neuropsychiatric symptoms. Currently, there are no known cures for synucleinopathies, and treatments mainly focus on symptom management. In this study, we developed a single-domain antibody (sdAb)-based protein degrader with features designed to enhance proteasomal degradation of α-syn. This sdAb derivative targets both α-syn and Cereblon (CRBN), a substrate-receptor for the E3-ubiquitin ligase CRL4CRBN, and thereby induces α-syn ubiquitination and proteasomal degradation. Our results indicate that this therapeutic ligand enhances proteasomal degradation of α-syn, in addition to the endogenous lysosomal degradation machinery. By promoting proteasomal degradation of α-syn, we improved clearance of α-syn in primary culture and mouse models of synucleinopathy. These findings indicate that our sdAb-based protein degrader is a promising therapeutic for synucleinopathies. Considering that only a small percentage of antibodies enter the brain, more potent sdAbs with greater brain entry than whole antibodies could enhance clinical benefits of antibody-based therapies.

Tau Immunotherapies for Alzheimer's Disease and Related Tauopathies: Status of Trials and Insights from Preclinical Studies

 The tau protein undergoes pathological changes in Alzheimer's disease and other tauopathies that eventually lead to functional impairments. Over the years, several therapeutic approaches have been examined to slow or halt the progression of tau pathology but have yet to lead to an approved disease-modifying treatment. Of the drugs in clinical trials that directly target tau, immunotherapies are the largest category and mostly consist of antibodies in different stages of development. There is a reasonable optimism that at least some of these compounds will have a clinically meaningful efficacy. This view is based on the significant although modest efficacy of some antibodies targeting amyloid-β in Alzheimer's disease and the fact that tau pathology correlates much better with the degree of dementia than amyloid-β lesions. In Alzheimer's disease, clearing pathological tau may therefore improve function later in the disease process than when removing amyloid-β. This review provides a brief update on the active and passive clinical tau immunization trials with insight from preclinical studies. Various epitopes are being targeted and some of the antibodies are said to target extracellular tau but because almost all of pathological tau is found intracellularly, the most efficacious antibodies should be able to enter the cell.

The functionality of a therapeutic antibody candidate restored by a single mutation from proline to threonine in the variable region

mAbs play an essential role in the therapeutic arsenal. Our laboratory has patented the Rendomab-B49 mAb targeting the endothelin B receptor (ETB). This G protein-coupled receptor plays a driving role in the progression of numerous cancers. We chimerized our mAb (xiRB49) to evaluate its preclinical therapeutic efficacy in different ETB+ tumor models with an antibody drug conjugate approach. As previously reported, the chimerization process of an antibody can alter its functionality. In this article, we present the chimerization of RB49. xiRB49 purified by Protein A remained perfectly soluble and did not aggregate, but it lost all its ability to recognize ETB. A detailed analysis of its variable region using IMGT tools allowed us to identify an unusual proline at position 125. In silico mAb modeling and in vitro experiments were performed for a better understanding of xiRB49 structure-function relationships. Our results show that the proline in position 125 on the heavy chain alters the xiRB49 CDR3 light chain conformation and its mutation to threonine allows complete functional recovery.

Tau-targeting therapies for Alzheimer disease: current status and future directions

Alzheimer disease (AD) is the most common cause of dementia in older individuals. AD is characterized pathologically by amyloid-β (Aβ) plaques and tau neurofibrillary tangles in the brain, with associated loss of synapses and neurons, which eventually results in dementia. Many of the early attempts to develop treatments for AD focused on Aβ, but a lack of efficacy of these treatments in terms of slowing disease progression led to a change of strategy towards targeting of tau pathology. Given that tau shows a stronger correlation with symptom severity than does Aβ, targeting of tau is more likely to be efficacious once cognitive decline begins. Anti-tau therapies initially focused on post-translational modifications, inhibition of tau aggregation and stabilization of microtubules. However, trials of many potential drugs were discontinued because of toxicity and/or lack of efficacy. Currently, the majority of tau-targeting agents in clinical trials are immunotherapies. In this Review, we provide an update on the results from the initial immunotherapy trials and an overview of new therapeutic candidates that are in clinical development, as well as considering future directions for tau-targeting therapies.

Single-domain antibody-based noninvasive in vivo imaging of α-synuclein or tau pathology

Intracellular deposition of α-synuclein and tau are hallmarks of synucleinopathies and tauopathies, respectively. Recently, several dye-based imaging probes with selectivity for tau aggregates have been developed, but suitable imaging biomarkers for synucleinopathies are still unavailable. Detection of both of these aggregates early in the disease process may allow for prophylactic therapies before functional impairments have manifested, highlighting the importance of developing specific imaging probes for these lesions. In contrast to the β sheet dyes, single-domain antibodies, found in camelids and a few other species, are highly specific, and their small size allows better brain entry and distribution than whole antibodies. Here, we have developed such imaging ligands via phage display libraries derived from llamas immunized with α-synuclein and tau preparations, respectively. These probes allow noninvasive and specific in vivo imaging of α-synuclein versus tau pathology in mice, with the brain signal correlating strongly with lesion burden. These small antibody derivatives have great potential for in vivo diagnosis of these diseases.

A call for better understanding of target engagement in Tau antibody development

Significant efforts have been channeled into developing antibodies for the treatment of CNS indications. Disappointment with the first generation of clinical Tau antibodies in Alzheimer's disease has highlighted the challenges in understanding whether an antibody can reach or affect the target in the compartment where it is involved in pathological processes. Here, we highlight different aspects essential for improving translatability of Tau-based immunotherapy.

Single domain antibodies targeting pathological tau protein: Influence of four IgG subclasses on efficacy and toxicity

BACKGROUND

Eleven tau immunoglobulin G (IgG) antibodies have entered clinical trials to treat tauopathies, including Alzheimer's disease, but it is unclear which IgG subclass/subtype has the ideal efficacy and safety profile. Only two subtypes, with or without effector function, have been examined in the clinic and not for the same tau antibody. The few preclinical studies on this topic have only compared two subtypes of one antibody each and have yielded conflicting results.

METHODS

We selected two single domain antibodies (sdAbs) derived from a llama immunized with tau proteins and utilized them to generate an array of Fc-(sdAb)2 subclasses containing identical tau binding domains but differing Fc region. Unmodified sdAbs and their IgG subclasses were tested for efficacy in primary cultures and in vivo microdialysis using JNPL3 tauopathy mice.

FINDINGS

Unmodified sdAbs were non-toxic, blocked tau toxicity and promoted tau clearance. However, the efficacy/safety profile of their Fc-(sdAb)2 subclasses varied greatly within and between sdAbs. For one of them, all its subtypes were non-toxic, only those with effector function cleared tau, and were more effective in vivo than unmodified sdAb. For the other sdAb, all its subtypes were toxic in tauopathy cultures but not in wild-type cells, suggesting that bivalent binding of its tau epitope stabilizes a toxic conformation of tau, with major implications for tau pathogenesis. Likewise, its subclasses were less effective than the unmodified sdAb in clearing tau in vivo.

INTERPRETATION

These findings indicate that tau antibodies with effector function are safe and better at clearing pathological tau than effectorless antibodies, Furthermore, tau antibodies can provide a valuable insight into tau pathogenesis, and some may aggravate it.

FUNDING

Funding for these studies was provided by the National Institute of Health (R01 AG032611, R01 NS077239, RF1 NS120488, R21 AG 069475, R21 AG 058282, T32AG052909), and the NYU Alzheimer's Disease Center Pilot Grant Program (via P30 AG008051).

Ultrasound-mediated delivery of novel tau-specific monoclonal antibody enhances brain uptake but not therapeutic efficacy

Tau-specific immunotherapy is an attractive strategy for the treatment of Alzheimer's disease and other tauopathies. However, effectively targeting tau in the brain remains a considerable challenge due to the restrictive nature of the blood-brain barrier (BBB), which excludes an estimated >99% of peripherally administered antibodies. However, their transport across the BBB can be facilitated by a novel modality, low-intensity scanning ultrasound used in combination with intravenously injected microbubbles (SUS^+MB). We have previously shown that SUS^+MB-mediated delivery of a tau-specific antibody in a single-chain (scFv) format to tau transgenic mice enhanced brain and neuronal uptake and subsequently, reduced tau pathology and improved behavioural outcomes to a larger extent than either scFv or SUS^+MB on its own. Here we generated a novel tau-specific monoclonal antibody, RNF5, and validated it in its IgG format in the presence or absence of SUS^+MB by treating K369I tau transgenic K3 mice once weekly for 12 weeks. We found that both RNF5 and SUS^+MB treatments on their own significantly reduced tau pathology. In the combination group (RNF5 + SUS^+MB), however, despite increased antibody localization in the brain, there were no further reductions in tau pathology when compared to RNF5 treatment alone. Furthermore, following SUS^+MB, RNF5 accumulated heavily within cells across the pyramidal cell layer of the hippocampus, that were negative for MAP2 and p-tau, suggesting that SUS^+MB may not facilitate enhanced RNF5 engagement of intraneuronal tau. Overall, our new findings reveal the complexities of combining tau immunotherapy with SUS^+MB and challenge the view that this is a straight-forward approach.

Microtubule-modulating Agents in the Fight Against Neurodegeneration: Will it ever Work?

The microtubule skeleton plays an essential role in nerve cells as the most important structural determinant of morphology and as a highway for axonal transport processes. Many neurodegenerative diseases are characterized by changes in the structure and organization of microtubules and microtubule-regulating proteins such as the microtubule-associated protein tau, which exhibits characteristic changes in a whole class of diseases collectively referred to as tauopathies. Changes in the dynamics of microtubules appear to occur early under neurodegenerative conditions and are also likely to contribute to age-related dysfunction of neurons. Thus, modulating microtubule dynamics and correcting impaired microtubule stability can be a useful neuroprotective strategy to counteract the disruption of the microtubule system in disease and aging. In this article, we review current microtubule- directed approaches for the treatment of neurodegenerative diseases with microtubules as a drug target, tau as a drug target, and post-translational modifications as potential modifiers of the microtubule system. We discuss limitations of the approaches that can be traced back to the rather unspecific mechanism of action, which causes undesirable side effects in non-neuronal cell types or which are due to the disruption of non-microtubule-related interactions. We also develop some thoughts on how the specificity of the approaches can be improved and what further targets could be used for modulating substances.

Targeting tau only extracellularly is likely to be less efficacious than targeting it both intra- and extracellularly

Aggregation of the tau protein is thought to be responsible for the neurodegeneration and subsequent functional impairments in diseases that are collectively named tauopathies. Alzheimer's disease is the most common tauopathy, but the group consists of over 20 different diseases, many of which have tau pathology as their primary feature. The development of tau therapies has mainly focused on preventing the formation of and/or clearing these aggregates. Of these, immunotherapies that aim to either elicit endogenous tau antibodies or deliver exogenous ones are the most common approach in clinical trials. While their mechanism of action can involve several pathways, both extra- and intracellular, pharmaceutical companies have primarily focused on antibody-mediated clearance of extracellular tau. As we have pointed out over the years, this is rather surprising because it is well known that most of pathological tau protein is found intracellularly. It has been repeatedly shown by several groups over the past decades that antibodies can enter neurons and that their cellular uptake can be enhanced by various means, particularly by altering their charge. Here, we will briefly describe the potential extra- and intracellular mechanisms involved in antibody-mediated clearance of tau pathology, discuss these in the context of recent failures of some of the tau antibody trials, and finally provide a brief overview of how the intracellular efficacy of tau antibodies can potentially be further improved by certain modifications that aim to enhance tau clearance via specific intracellular degradation pathways.

Bispecific Tau Antibodies with Additional Binding to C1q or Alpha-Synuclein

BACKGROUND

Alzheimer's disease (AD) and other tauopathies are neurodegenerative disorders characterized by cellular accumulation of aggregated tau protein. Tau pathology within these disorders is accompanied by chronic neuroinflammation, such as activation of the classical complement pathway by complement initiation factor C1q. Additionally, about half of the AD cases present with inclusions composed of aggregated alpha-synuclein called Lewy bodies. Lewy bodies in disorders such as Parkinson's disease and Lewy body dementia also frequently occur together with tau pathology.

OBJECTIVE

Immunotherapy is currently the most promising treatment strategy for tauopathies. However, the presence of multiple pathological processes within tauopathies makes it desirable to simultaneously target more than one disease pathway.

METHODS

Herein, we have developed three bispecific antibodies based on published antibody binding region sequences. One bispecific antibody binds to tau plus alpha-synuclein and two bispecific antibodies bind to tau plus C1q.

RESULTS

Affinity of the bispecific antibodies to their targets compared to their monospecific counterparts ranged from nearly identical to one order of magnitude lower. All bispecific antibodies retained binding to aggregated protein in patient-derived brain sections. The bispecific antibodies also retained their ability to inhibit aggregation of recombinant tau, regardless of whether the tau binding sites were in IgG or scFv format. Mono- and bispecific antibodies inhibited cellular seeding induced by AD-derived pathological tau with similar efficacy. Finally, both Tau-C1q bispecific antibodies completely inhibited the classical complement pathway.

CONCLUSION

Bispecific antibodies that bind to multiple pathological targets may therefore present a promising approach to treat tauopathies and other neurodegenerative disorders.

Phosphorylated TAR DNA-Binding Protein-43: Aggregation and Antibody-Based Inhibition

TAR DNA-binding protein-43 (TDP-43) pathology, including fibrillar aggregates and mutations, develops in amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration (FTLD) and limbic-predominant age-related TDP-43 encephalopathy (LATE). Hyperphosphorylation and aggregation of TDP-43 contribute to pathology and are viable therapeutic targets for ALS. In vivo inhibition of TDP-43 aggregation was evaluated using anti-TDP-43 antibodies with promising outcomes. However, the exact mechanism of antibody-based inhibition targeting TDP-43 is not well understood but may lead to the identification of viable immunotherapies. Herein, the mechanism of in vitro aggregation of phosphorylated TDP-43 was explored, and the anti-TDP-43 antibodies tested for their inhibitor efficacies. Specifically, the aggregation of phosphorylated full-length TDP-43 protein (pS410) was monitored by transmission electron microscopy (TEM), turbidity absorbance, and thioflavin (ThT). The protein aggregates were insoluble, ThT-positive and characterized with heterogeneous morphologies (fibers, amorphous structures). Antibodies specific to epitopes 178-393 and 256-269, within the RRM2-CTD domain, reduced the formation of β-sheets and insoluble aggregates, at low antibody loading (antibody: protein ratio = 1 ug/mL: 45 ug/mL). Inhibition outcomes were highly dependent on the type and loading of antibodies, indicating dual functionality of such inhibitors, as aggregation inhibitors or aggregation promoters. Anti-SOD1 and anti-tau antibodies were not effective inhibitors against TDP-43 aggregation, indicating selective inhibition.

Current Status of Clinical Trials on Tau Immunotherapies

Tau immunotherapies have advanced from proof-of-concept studies to over a dozen clinical trials for Alzheimer's disease (AD) and other tauopathies. Mechanistic studies in animal and culture models have provided valuable insight into how these therapies may work but multiple pathways are likely involved. Different groups have emphasized the importance of intracellular vs extracellular antibody-mediated clearance of the tau protein and there is no consensus on which pool of tau should ideally be targeted. Likewise, various normal and disease-selective epitopes are being targeted, and the antibody isotypes either favor phagocytosis of the tau-antibody complex or are neutral in that aspect. Most of the clinical trials are in early stages, thus their efficacy is not yet known, but all have been without any major adverse effects and some have reported target engagement. A few have been discontinued. One in phase I, presumably because of a poor pharmacokinetic profile, and three in phase II for a lack of efficacy although this trial stage is not well powered for efficacy measures. In these phase II studies, trials with two antibodies in patients with progressive supranuclear palsy or other primary tauopathies were halted but are continuing in patients with AD, and one antibody trial was stopped in early-stage AD but is continuing in moderate AD. These three antibodies have been reported to only work extracellularly and tau is not increased in the cerebrospinal fluid of primary tauopathies, which may explain the failures of two of them. In the discontinued AD trial, there are some concerns about how much of extracellular tau contains the N-terminal epitope that is being targeted. In addition, extracellular tau is only a small part of total tau, compared to intracellular tau. Targeting only the former may not be sufficient for functional benefits. Given these outcomes, decision makers within the pharmaceutical companies who green light these trials should attempt to target tau not only extracellularly but also intracellularly to increase their chances of success. Hopefully, some of the ongoing trials will provide some functional benefits to the large number of patients with tauopathies.

Alzheimer's Disease Pathogenesis: Role of Autophagy and Mitophagy Focusing in Microglia

Alzheimer's disease (AD) is a debilitating neurological disorder, and currently, there is no cure for it. Several pathologic alterations have been described in the brain of AD patients, but the ultimate causative mechanisms of AD are still elusive. The classic hallmarks of AD, including amyloid plaques (Aβ) and tau tangles (tau), are the most studied features of AD. Unfortunately, all the efforts targeting these pathologies have failed to show the desired efficacy in AD patients so far. Neuroinflammation and impaired autophagy are two other main known pathologies in AD. It has been reported that these pathologies exist in AD brain long before the emergence of any clinical manifestation of AD. Microglia are the main inflammatory cells in the brain and are considered by many researchers as the next hope for finding a viable therapeutic target in AD. Interestingly, it appears that the autophagy and mitophagy are also changed in these cells in AD. Inside the cells, autophagy and inflammation interact in a bidirectional manner. In the current review, we briefly discussed an overview on autophagy and mitophagy in AD and then provided a comprehensive discussion on the role of these pathways in microglia and their involvement in AD pathogenesis.

Tau antibody isotype induces differential effects following passive immunisation of tau transgenic mice

One of the main pathological hallmarks of Alzheimer's disease (AD) is the intraneuronal accumulation of hyperphosphorylated tau. Passive immunotherapy is a promising strategy for the treatment of AD and there are currently a number of tau-specific monoclonal antibodies in clinical trials. A proposed mechanism of action is to engage and clear extracellular, pathogenic forms of tau. This process has been shown in vitro to be facilitated by microglial phagocytosis through interactions between the antibody-tau complex and microglial Fc-receptors. As this interaction is mediated by the conformation of the antibody's Fc domain, this suggests that the antibody isotype may affect the microglial phagocytosis and clearance of tau, and hence, the overall efficacy of tau antibodies. We therefore aimed to directly compare the efficacy of the tau-specific antibody, RN2N, cloned into a murine IgG1/κ framework, which has low affinity Fc-receptor binding, to that cloned into a murine IgG2a/κ framework, which has high affinity Fc-receptor binding. Our results demonstrate, for RN2N, that although enhanced microglial activation via the IgG2a/κ isotype increased extracellular tau phagocytosis in vitro, the IgG1/κ isoform demonstrated enhanced ability to reduce tau pathology and microgliosis following passive immunisation of the P301L tau transgenic pR5 mouse model.

Similarities and Differences in the Pattern of Tau Hyperphosphorylation in Physiological and Pathological Conditions: Impacts on the Elaboration of Therapies to Prevent Tau Pathology

Tau protein, a neuronal microtubule-associated protein, becomes hyperphosphorylated in several neurodegenerative diseases called tauopathies. Hyperphosphorylation of tau is correlated to its redistribution from the axon to the somato-dendritic compartment at early stages of tauopathies. Interestingly, tau hyperphosphorylation begins in different regions of the brain in each tauopathy. In some regions, both neurons and glial cells develop tau hyperphosphorylation. Tau hyperphosphorylation is also observed in physiological conditions such as hibernation and brain development. In the first section of present article, we will review the spatiotemporal and cellular distribution of hyperphosphorylated tau in the most frequent tauopathies. In the second section, we will compare the pattern of tau hyperphosphorylation in physiological and pathological conditions and discuss the sites that could play a pivotal role in the conversion of non-toxic to toxic forms of hyperphosphorylated tau. Furthermore, we will discuss the role of hyperphosphorylated tau in physiological and pathological conditions and the fact that tau hyperphosphorylation is reversible in physiological conditions but not in a pathological ones. In the third section, we will speculate how the differences and similarities between hyperphosphorylated tau in physiological and pathological conditions could impact the elaboration of therapies to prevent tau pathology. In the fourth section, the different therapeutic approaches using tau as a direct or indirect therapeutic target will be presented.

Dynamics of Internalization and Intracellular Interaction of Tau Antibodies and Human Pathological Tau Protein in a Human Neuron-Like Model

We and others have shown in various in vivo, ex vivo and cell culture models that several tau antibodies interact with pathological tau within neurons. To further clarify this interaction in a dynamic human model, we differentiated SH-SY5Y cells with retinoic acid and BDNF to create a neuron-like model. Therein, tau antibodies were primarily taken up by receptor-mediated endocytosis, and prevented toxicity of human brain-derived paired helical filament-enriched tau (PHF). Subsequently, we monitored in real-time the interaction of antibodies and PHF within endocytic cellular compartments. Cells were pre-treated with fluorescently-tagged PHF and then incubated with tau antibodies, 4E6, 6B2, or non-specific isotype control IgG1 labeled with a pH sensitive dye. The uptake and binding of the efficacious antibody, 4E6, to PHF occurred mainly within the soma, whereas the ineffective antibody, 6B2, and ineffective control IgG1, were visualized via the processes and showed limited colocalization with PHF within this period. In summary, we have developed a neuron-like model that clarifies the early intracellular dynamics of the interaction of tau antibodies with pathological tau, and identifies features associated with efficacy. Since the model is entirely human, it is suitable to verify the therapeutic potential of humanized antibodies prior to extensive clinical trials.

Increased neuronal activity in motor cortex reveals prominent calcium dyshomeostasis in tauopathy mice

Perturbed neuronal Ca²⁺ homeostasis is implicated in Alzheimer's disease, which has primarily been demonstrated in mice with amyloid-β deposits but to a lesser and more variable extent in tauopathy models. In this study, we injected AAV to express Ca²⁺ indicator in layer II/III motor cortex neurons and measured neuronal Ca²⁺ activity by two photon imaging in awake transgenic JNPL3 tauopathy and wild-type mice. Various biochemical measurements were conducted in postmortem mouse brains for mechanistic insight and a group of animals received two intravenous injections of a tau monoclonal antibody spaced by four days to test whether the Ca²⁺ dyshomeostasis was related to pathological tau protein. Under running conditions, we found abnormal neuronal Ca²⁺ activity in tauopathy mice compared to age-matched wild-type mice with higher frequency of Ca²⁺ transients, lower amplitude of peak Ca²⁺ transients and lower total Ca²⁺ activity in layer II/III motor cortex neurons. While at resting conditions, only Ca²⁺ frequency was increased. Brain levels of soluble pathological tau correlated better than insoluble tau levels with the degree of Ca²⁺ dysfunction in tauopathy mice. Furthermore, tau monoclonal antibody 4E6 partially rescued Ca²⁺ activity abnormalities in tauopathy mice after two intravenous injections and decreased soluble pathological tau protein within the brain. This correlation and antibody effects strongly suggest that the neuronal Ca²⁺ dyshomeostasis is causally linked to pathological tau protein. These findings also reveal more pronounced neuronal Ca²⁺ dysregulation in tauopathy mice than previously reported by two-photon imaging that can be partially corrected with an acute tau antibody treatment.

Conformation-selective tau monoclonal antibodies inhibit tau pathology in primary neurons and a mouse model of Alzheimer's disease

Background

The spread of tau pathology in Alzheimer’s disease (AD) is mediated by cell-to-cell transmission of pathological tau seeds released from neurons that, upon internalization by recipient neurons, template the misfolding of naïve cellular tau, thereby propagating fibrillization. We hypothesize that anti-tau monoclonal antibodies (mAbs) that selectively bind to pathological tau seeds will inhibit propagation of tau aggregates and reduce the spread of tau pathology in vivo.

Methods

We inoculated mice with human AD brain-derived extracts containing tau paired helical filaments (AD-tau) and identified two novel mAbs, DMR7 and SKT82, that selectively bind to a misfolded pathological conformation of tau relative to recombinant tau monomer. To evaluate the effects of these mAbs on the spread of pathological tau in vivo, 5xFAD mice harboring significant brain Aβ plaque burden were unilaterally injected with AD-tau in the hippocampus, to initiate the formation of neuritic plaque (NP) tau pathology, and were treated weekly with intraperitoneal (i.p.) injections of DMR7, SKT82, or IgG isotype control mAbs.

Results

DMR7 and SKT82 bind epitopes comprised of the proline-rich domain and c-terminal region of tau and binding is reduced upon disruption of the pathological conformation of AD-tau by chemical and thermal denaturation. We found that both DMR7 and SKT82 immunoprecipitate pathological tau and significantly reduce the seeding of cellular tau aggregates induced by AD-tau in primary neurons by 60.5 + 13.8% and 82.2 + 8.3%, respectively, compared to IgG control. To investigate the mechanism of mAb inhibition, we generated pH-sensitive fluorophore-labeled recombinant tau fibrils seeded by AD-tau to track internalization of tau seeds and demonstrate that the conformation-selective tau mAbs inhibit the internalization of tau seeds. DMR7 and SKT82 treatment reduced hyperphosphorylated NP tau as measured with AT8 immunohistochemistry (IHC) staining, but did not achieve statistical significance in the contralateral cortex and SKT82 significantly reduced tau pathology in the ipsilateral hippocampus by 24.2%; p = 0.044.

Conclusions

These findings demonstrate that conformation-selective tau mAbs, DMR7 and SKT82, inhibit tau pathology in primary neurons by preventing the uptake of tau seeds and reduce tau pathology in vivo, providing potential novel therapeutic candidates for the treatment of AD.

Supplementary information

Supplementary information accompanies this paper at 10.1186/s13024-020-00404-5.

Selection of a Single Domain Antibody, Specific for an HLA-Bound Epitope of the Mycobacterial Ag85B Antigen

T cells recognizing epitopes on the surface of mycobacteria-infected macrophages can impart protection, but with associated risk for reactivation to lung pathology. We aimed to identify antibodies specific to such epitopes, which carry potentials for development toward novel therapeutic constructs. Since epitopes presented in the context of major histocompatibility complex alleles are rarely recognized by naturally produced antibodies, we used a phage display library for the identification of monoclonal human single domain antibody producing clones. The selected 2C clone displayed T cell receptor-like recognition of an HLA-A*0201 bound ₁₉₉KLVANNTRL₂₀₇ peptide from the Ag85B antigen, which is known to be an immunodominant epitope for human T cells. The specificity of the selected domain antibody was demonstrated by solid phase immunoassay and by immunofluorescent surface staining of peptide loaded cells of the T2 cell line. The antibody affinity binding was determined by biolayer interferometry. Our results validated the used technologies as suitable for the generation of antibodies against epitopes on the surface of Mycobacterium tuberculosis infected cells. The potential approaches forward the development of antibody in immunotherapy of tuberculosis have been outlined in the discussion.

Recent Preclinical Insights Into the Treatment of Chronic Traumatic Encephalopathy

Chronic traumatic encephalopathy (CTE) is a neurodegenerative condition associated with significant mortality and morbidity. The central pathophysiological mechanisms by which repetitive cranial injury results in the neurodegeneration of CTE are poorly understood. Current well-established working models emphasize a central role for trauma-induced excessive phosphorylation and accumulation of insoluble tangles of Tau protein. In this review, we summarize recent data from preclinical animal models of CTE where a series of candidate treatments have been carefully evaluated, including kinase inhibitors, antibody therapy, and anti-inflammatory therapies. We discuss the overall translational potential of these approaches and provide recommendations for future bench-to-bedside treatment strategies.

Tau immunotherapies: Lessons learned, current status and future considerations

The majority of clinical trials targeting the tau protein in Alzheimer's disease and other tauopathies are tau immunotherapies. Because tau pathology correlates better with the degree of dementia than amyloid-β lesions, targeting tau is likely to be more effective in improving cognition than clearing amyloid-β in Alzheimer's disease. However, the development of tau therapies is in many ways more complex than for amyloid-β therapies as briefly outlined in this review. Most of the trials are on humanized antibodies, which may have very different properties than the original mouse antibodies. The impact of these differences are to a large extent unknown, can be difficult to decipher, and may not always be properly considered. Furthermore, the ideal antibody properties for efficacy are not well established and can depend on several factors. However, considering the varied approaches in clinical trials, there is a general optimism that at least some of these trials may provide functional benefits to patients suffering of various tauopathies. This article is part of the special issue entitled 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

Neuronally expressed anti-tau scFv prevents tauopathy-induced phenotypes in Drosophila models

We have derived single-chain variable fragments (scFv) from tau antibody hybridomas and previously shown their promise as imaging diagnostic agents. Here, we examined the therapeutic potential of anti-tau scFv in transgenic Drosophila models that express in neurons wild-type (WT) human tau (htau) or the human tauopathy mutation R406W. scFv expressing flies were crossed with the tauopathy flies and analyzed. Overall, the survival curves differed significantly (p < .0001). Control flies not expressing htau survived the longest, whereas R406W expressing flies had the shortest lifespan, which was greatly prolonged by co-expressing the anti-tau scFv (p < .0001). Likewise, htau WT expressing flies had a moderately short lifespan, which was prolonged by co-expressing the anti-tau scFv (p < .01). In addition, the htau expression impaired wing expansion after eclosion (p < .0001), and caused progressive abdomen expansion (p < .0001). These features were more severe in htau R406W flies than in htau WT flies. Importantly, both phenotypes were prevented by co-expression of the anti-tau scFv (p < .01-0.0001). Lastly, brain analyses revealed scFv-mediated tau clearance (p < .05-0.01), and its prevention of tau-mediated neurotoxicity (p < .05-0.001). In summary, these findings support the therapeutic potential of an anti-tau scFv, including as gene therapies, and the use of Drosophila models for such screening.

Alzheimer's therapy development: A few points to consider

Development of therapies for Alzheimer's disease has only resulted in a few approved drugs that provide some temporary symptomatic relief in certain patients. None of these compounds in clinical use halts or slows the progression of the disease. To date, several drugs targeting the amyloid-β peptide, and some against the tau protein, have failed in clinical trials. While there are various reasons for these failures, considering the following points may aid in improving the outcome of future trials. First, the tau protein should ideally be targeted intracellularly because most of tau pathology is within cells, neurons in particular. Second, an overriding emphasis in recent years has been on implementing drug-screening models that focus on prevention of seeding/spread of aggregates. Much less attention has been paid to identify compounds that inhibit neurotoxicity of these aggregates, which may not necessarily relate to their seeding/spread propensity. Ideally, all these markers should be readouts in the same assay or model. Third, diversity in conformers/strains of aggregates complicates drug development of small molecule aggregation inhibitors but is likely to be less of an issue for antibody-based treatments. Lastly, other more general targets associated with neurodegeneration should continue to be pursued but are in many ways more difficult to address than clearing amyloid-β and tau, the defining hallmarks of AD.