NPT088 reduces both amyloid-β and tau pathologies in transgenic mice

Emerging perspectives on precision therapy for Parkinson's disease: multidimensional evidence leading to a new breakthrough in personalized medicine

PD is a prevalent and progressive neurodegenerative disorder characterized by both motor and non-motor symptoms. Genes play a significant role in the onset and progression of the disease. While the complexity and pleiotropy of gene expression networks have posed challenges for gene-targeted therapies, numerous pathways of gene variant expression show promise as therapeutic targets in preclinical studies, with some already in clinical trials. With the recognition of the numerous genes and complex pathways that can influence PD, it may be possible to take a novel approach to choose a treatment for the condition. This approach would be based on the symptoms, genomics, and underlying mechanisms of the disease. We discuss the utilization of emerging genetic and pathological knowledge of PD patients to categorize the disease into subgroups. Our long-term objective is to generate new insights for the therapeutic approach to the disease, aiming to delay and treat it more effectively, and ultimately reduce the burden on individuals and society.

LETC inhibits α-Syn aggregation and ameliorates motor deficiencies in the L62 mouse model of synucleinopathy

Alpha-Synuclein (α-Syn) aggregation is a pathological feature of synucleinopathies, neurodegenerative disorders that include Parkinson's disease (PD). Here, we explored the efficacy of N,N,N',N'-tetraethyl-10H-phenothiazine-3,7-diamine dihydrochloride (LETC), a protein aggregation inhibitor, on α-Syn aggregation. In both cellular models and transgenic mice, α-Syn aggregation was achieved by the overexpression of full-length human α-Syn fused with a signal sequence peptide. α-Syn accumulated in transfected DH60.21 neuroblastoma cells and α-Syn aggregation was inhibited by LETC with an EC50 of 0.066 ± 0.047 μM. Full-length human α-Syn overexpressing Line 62 (L62) mice accumulated neuronal α-Syn that was associated with a decreased motor performance in the open field and automated home cage. LETC, administered orally for 6 weeks at 10 mg/kg significantly decreased α-Syn-positive neurons in multiple brain regions and this resulted in a rescue of movement deficits in the open field in these mice. LETC however, did not improve activity deficits of L62 mice in the home cage environment. The results suggest that LETC may provide a potential disease modification therapy in synucleinopathies through the inhibition of α-Syn aggregation.

Recent trends in treatment strategies for Alzheimer's disease and the challenges: A topical advancement

Alzheimer's Disease (AD) is an irreversible and progressive neurological disease that has affected at least 50 million people around the globe. Considering the severity of the disease and the continuous increase in the number of patients, the development of new effective drugs or intervention strategies for AD has become urgent. AD is caused by a combination of genetic, environmental, and lifestyle factors, but its exact cause has not yet been clarified. Given the current challenges being faced in the clinical treatment of AD, such as complex AD pathological network and insufficient early diagnosis, herein, we have focused on the three core pathological features of AD, including amyloid-β (Aβ) aggregation, tau phosphorylation and tangles, and activation of inflammatory factors. In this review, we have briefly underscored the primary evidence supporting each pathology and discuss AD pathological network among Aβ, tau, and inflammation. We have also comprehensively summarized the most instructive drugs and their treatment strategies against Aβ, tau, or neuroinflammation used in basic research and clinical trials. Finally, we have discussed and outlined the pros and cons of each pathological approach and looked forward to potential personalized diagnosis and treatment strategies that are beneficial to AD patients.

Promising biomarkers and therapeutic targets for the management of Parkinson's disease: recent advancements and contemporary research

Parkinson's disease (PD) is one of the progressive neurological diseases which affect around 10 million population worldwide. The clinical manifestation of motor symptoms in PD patients appears later when most dopaminergic neurons have degenerated. Thus, for better management of PD, the development of accurate biomarkers for the early prognosis of PD is imperative. The present work will discuss the potential biomarkers from various attributes covering biochemical, microRNA, and neuroimaging aspects (α-synuclein, DJ-1, UCH-L1, β-glucocerebrosidase, BDNF, etc.) for diagnosis, recent development in PD management, and major limitations with current and conventional anti-Parkinson therapy. This manuscript summarizes potential biomarkers and therapeutic targets, based on available preclinical and clinical evidence, for better management of PD.

Small molecules to perform big roles: The search for Parkinson's and Huntington's disease therapeutics

Neurological motor disorders (NMDs) such as Parkinson's disease and Huntington's disease are characterized by the accumulation and aggregation of misfolded proteins that trigger cell death of specific neuronal populations in the central nervous system. Differential neuronal loss initiates the impaired motor control and cognitive function in the affected patients. Although major advances have been carried out to understand the molecular basis of these diseases, to date there are no treatments that can prevent, cure, or significantly delay the progression of the disease. In this context, strategies such as gene editing, cellular therapy, among others, have gained attention as they effectively reduce the load of toxic protein aggregates in different models of neurodegeneration. Nevertheless, these strategies are expensive and difficult to deliver into the patients' nervous system. Thus, small molecules and natural products that reduce protein aggregation levels are highly sought after. Numerous drug discovery efforts have analyzed large libraries of synthetic compounds for the treatment of different NMDs, with a few candidates reaching clinical trials. Moreover, the recognition of new druggable targets for NMDs has allowed the discovery of new small molecules that have demonstrated their efficacy in pre-clinical studies. It is also important to recognize the contribution of natural products to the discovery of new candidates that can prevent or cure NMDs. Additionally, the repurposing of drugs for the treatment of NMDs has gained huge attention as they have already been through clinical trials confirming their safety in humans, which can accelerate the development of new treatment. In this review, we will focus on the new advances in the discovery of small molecules for the treatment of Parkinson's and Huntington's disease. We will begin by discussing the available pharmacological treatments to modulate the progression of neurodegeneration and to alleviate the motor symptoms in these diseases. Then, we will analyze those small molecules that have reached or are currently under clinical trials, including natural products and repurposed drugs.

Testing the amyloid cascade hypothesis: Prevention trials in autosomal dominant Alzheimer disease

OBJECTIVE

The amyloid cascade hypothesis of Alzheimer disease (AD) has been increasingly challenged. Here, we aim to refocus the amyloid cascade hypothesis on its original premise that the accumulation of amyloid beta (Aβ) peptide is the primary and earliest event in AD pathogenesis as based on current evidence, initiating several pathological events and ultimately leading to AD dementia.

BACKGROUND

An ongoing debate about the validity of the amyloid cascade hypothesis for AD has been triggered by clinical trials with investigational disease-modifying drugs targeting Aβ that have not demonstrated consistent clinically meaningful benefits.

UPDATED HYPOTHESIS

It is an open question if monotherapy targeting Aβ pathology could be markedly beneficial at a stage when the brain has been irreversibly damaged by a cascade of pathological changes. Interventions in cognitively unimpaired individuals at risk for dementia, during amyloid-only and pre-amyloid stages, are more appropriate for proving or refuting the amyloid hypothesis. Our updated hypothesis states that anti-Aβ investigational therapies are likely to be most efficacious when initiated in the preclinical (asymptomatic) stages of AD and specifically when the disease is driven primarily by amyloid pathology. Given the young age at symptom onset and the deterministic nature of the mutations, autosomal dominant AD (ADAD) mutation carriers represent the ideal population to evaluate the efficacy of putative disease-modifying Aβ therapies.

MAJOR CHALLENGES FOR THE HYPOTHESIS

Key challenges of the amyloid hypothesis include the recognition that disrupted Aβ homeostasis alone is insufficient to produce the AD pathophysiologic process, poor correlation of Aβ with cognitive impairment, and inconclusive data regarding clinical efficacy of therapies targeting Aβ. Challenges of conducting ADAD research include the rarity of the disease and uncertainty of the generalizability of ADAD findings for the far more common "sporadic" late-onset AD.

LINKAGE TO OTHER MAJOR THEORIES

The amyloid cascade hypothesis, modified here to pertain to the preclinical stage of AD, still needs to be integrated with the development and effects of tauopathy and other co-pathologies, including neuroinflammation, vascular insults, synucleinopathy, and many others.

Alpha-Synuclein Aggregation Pathway in Parkinson's Disease: Current Status and Novel Therapeutic Approaches

Following Alzheimer’s, Parkinson’s disease (PD) is the second-most common neurodegenerative disorder, sharing an unclear pathophysiology, a multifactorial profile, and massive social costs worldwide. Despite this, no disease-modifying therapy is available. PD is tightly associated with α-synuclein (α-Syn) deposits, which become organised into insoluble, amyloid fibrils. As a typical intrinsically disordered protein, α-Syn adopts a monomeric, random coil conformation in an aqueous solution, while its interaction with lipid membranes drives the transition of the molecule part into an α-helical structure. The central unstructured region of α-Syn is involved in fibril formation by converting to well-defined, β-sheet rich secondary structures. Presently, most therapeutic strategies against PD are focused on designing small molecules, peptides, and peptidomimetics that can directly target α-Syn and its aggregation pathway. Other approaches include gene silencing, cell transplantation, stimulation of intracellular clearance with autophagy promoters, and degradation pathways based on immunotherapy of amyloid fibrils. In the present review, we sum marise the current advances related to α-Syn aggregation/neurotoxicity. These findings present a valuable arsenal for the further development of efficient, nontoxic, and non-invasive therapeutic protocols for disease-modifying therapy that tackles disease onset and progression in the future.

Emerging Therapeutic Strategies for Parkinson’s Disease and Future Prospects: A 2021 Update

Parkinson’s disease (PD) is a neurodegenerative disorder pathologically distinguished by degeneration of dopaminergic neurons in the substantia nigra pars compacta. Muscle rigidity, tremor, and bradykinesia are all clinical motor hallmarks of PD. Several pathways have been implicated in PD etiology, including mitochondrial dysfunction, impaired protein clearance, and neuroinflammation, but how these factors interact remains incompletely understood. Although many breakthroughs in PD therapy have been accomplished, there is currently no cure for PD, only trials to alleviate the related motor symptoms. To reduce or stop the clinical progression and mobility impairment, a disease-modifying approach that can directly target the etiology rather than offering symptomatic alleviation remains a major unmet clinical need in the management of PD. In this review, we briefly introduce current treatments and pathophysiology of PD. In addition, we address the novel innovative therapeutic targets for PD therapy, including α-synuclein, autophagy, neurodegeneration, neuroinflammation, and others. Several immunomodulatory approaches and stem cell research currently in clinical trials with PD patients are also discussed. Moreover, preclinical studies and clinical trials evaluating the efficacy of novel and repurposed therapeutic agents and their pragmatic applications with encouraging outcomes are summarized. Finally, molecular biomarkers under active investigation are presented as potentially valuable tools for early PD diagnosis.

Therapeutics in the Pipeline Targeting α-Synuclein for Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder and the fastest growing neurologic disease in the world, yet no disease-modifying therapy is available for this disabling condition. Multiple lines of evidence implicate the protein α-synuclein (α-Syn) in the pathogenesis of PD, and as such, there is intense interest in targeting α-Syn for potential disease modification. α-Syn is also a key pathogenic protein in other synucleionpathies, most commonly dementia with Lewy bodies. Thus, therapeutics targeting this protein will have utility in these disorders as well. Here we discuss the various approaches that are being investigated to prevent and mitigate α-Syn toxicity in PD, including clearing its pathologic aggregates from the brain using immunization strategies, inhibiting its misfolding and aggregation, reducing its expression level, enhancing cellular clearance mechanisms, preventing its cell-to-cell transmission within the brain and perhaps from the periphery, and targeting other proteins associated with or implicated in PD that contribute to α-Syn toxicity. We also discuss the therapeutics in the pipeline that harness these strategies. Finally, we discuss the challenges and opportunities for the field in the discovery and development of therapeutics for disease modification in PD. SIGNIFICANCE STATEMENT: PD is the second most common neurodegenerative disorder, for which disease-modifying therapies remain a major unmet need. A large body of evidence points to α-synuclein as a key pathogenic protein in this disease as well as in dementia with Lewy bodies, making it of leading therapeutic interest. This review discusses the various approaches being investigated and progress made to date toward discovering and developing therapeutics that would slow and stop progression of these disabling diseases.

Past, present and future of therapeutic strategies against amyloid-β peptides in Alzheimer's disease: a systematic review

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease in ageing, affecting around 46 million people worldwide but few treatments are currently available. The etiology of AD is still puzzling, and new drugs development and clinical trials have high failure rates. Urgent outline of an integral (multi-target) and effective treatment of AD is needed. Accumulation of amyloid-β (Aβ) peptides is considered one of the fundamental neuropathological pillars of the disease, and its dyshomeostasis has shown a crucial role in AD onset. Therefore, many amyloid-targeted therapies have been investigated. Here, we will systematically review recent (from 2014) investigational, follow-up and review studies focused on anti-amyloid strategies to summarize and analyze their current clinical potential. Combination of anti-Aβ therapies with new developing early detection biomarkers and other therapeutic agents acting on early functional AD changes will be highlighted in this review. Near-term approval seems likely for several drugs acting against Aβ, with recent FDA approval of a monoclonal anti-Aβ oligomers antibody -aducanumab- raising hopes and controversies. We conclude that, development of oligomer-epitope specific Aβ treatment and implementation of multiple improved biomarkers and risk prediction methods allowing early detection, together with therapies acting on other factors such as hyperexcitability in early AD, could be the key to slowing this global pandemic.

Thiophene-Based Dual Modulators of Aβ and Tau Aggregation

Alzheimer's disease is characterized by the accumulation of amyloid beta (Aβ) and Tau aggregates in the brain, which induces various pathological events resulting in neurodegeneration. There have been continuous efforts to develop modulators of the Aβ and Tau aggregation process to halt or modify disease progression. A few small-molecule-based inhibitors that target both Aβ and Tau pathology have been reported. Here, we report the screening of a targeted library of small molecules to modulate Aβ and Tau aggregation together with their in vitro, in silico and cellular studies. In vitro ThT fluorescence assay, dot blot assay, gel electrophoresis and transmission electron microscopy (TEM) results have shown that thiophene-based lead molecules effectively modulate Aβ aggregation and inhibit Tau aggregation. In silico studies performed by employing molecular docking, molecular dynamics and binding-free energy calculations have helped in understanding the mechanism of interaction of the lead thiophene compounds with Aβ and Tau fibril targets. In cellulo studies revealed that the lead candidate is biocompatible and effectively ameliorates neuronal cells from Aβ and Tau-mediated amyloid toxicity.

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.

Identification of ortho catechol-containing isoflavone as a privileged scaffold that directly prevents the aggregation of both amyloid β plaques and tau-mediated neurofibrillary tangles and its in vivo evaluation

In this study, polyhydroxyisoflavones that directly prevent the aggregation of both amyloid β (Aβ) and tau were expediently synthesized via divergent Pd(0)-catalyzed Suzuki-Miyaura coupling and then biologically evaluated. By preliminary structure-activity relationship studies using thioflavin T (ThT) assays, an ortho-catechol containing isoflavone scaffold was proven to be crucial for preventing both Aβ aggregation and tau-mediated neurofibrillary tangle formation. Additional TEM experiment confirmed that ortho-catechol containing isoflavone 4d significantly prevented the aggregation of both Aβ and tau. To investigate the mode of action (MOA) of 4d, which possesses an ortho-catechol moiety, ¹H-¹⁵N HSQC NMR analysis was thoroughly performed and the result indicated that 4d could directly inhibit both the formation of Aβ₄₂ fibrils and the formation of tau-derived neurofibrils, probably through the catechol-mediated nucleation of tau. Finally, 4d was demonstrated to alleviate cognitive impairment and pathologies related to Alzheimer's disease in a 5XFAD transgenic mouse model.

Feasibility of short imaging protocols for [18F]PI-2620 tau-PET in progressive supranuclear palsy

Purpose

Dynamic 60-min positron emission tomography (PET) imaging with the novel tau radiotracer [¹⁸F]PI-2620 facilitated accurate discrimination between patients with progressive supranuclear palsy (PSP) and healthy controls (HCs). This study investigated if truncated acquisition and static time windows can be used for [¹⁸F]PI-2620 tau-PET imaging of PSP.

Methods

Thirty-seven patients with PSP Richardson syndrome (PSP-RS) were evaluated together with ten HCs. [¹⁸F]PI-2620 PET was performed by a dynamic 60-min scan. Distribution volume ratios (DVRs) were calculated using full and truncated scan durations (0–60, 0–50, 0–40, 0–30, and 0–20 min p.i.). Standardized uptake value ratios (SUVrs) were obtained 20–40, 30–50, and 40–60 min p.i.. All DVR and SUVr data were compared with regard to their potential to discriminate patients with PSP-RS from HCs in predefined subcortical and cortical target regions (effect size, area under the curve (AUC), multi-region classifier).

Results

0–50 and 0–40 DVR showed equivalent effect sizes as 0–60 DVR (averaged Cohen’s d: 1.22 and 1.16 vs. 1.26), whereas the performance dropped for 0–30 or 0–20 DVR. The 20–40 SUVr indicated the best performance of all static acquisition windows (averaged Cohen’s d: 0.99). The globus pallidus internus discriminated patients with PSP-RS and HCs at a similarly high level for 0–60 DVR (AUC: 0.96), 0–40 DVR (AUC: 0.96), and 20–40 SUVr (AUC: 0.94). The multi-region classifier sensitivity of these time windows was consistently 86%.

Conclusion

Truncated and static imaging windows can be used for [¹⁸F]PI-2620 PET imaging of PSP. 0–40 min dynamic scanning offers the best balance between accuracy and economic scanning.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05391-3.

A review on advances of treatment modalities for Alzheimer's disease

Alzheimer's disease (AD) is a multifactorial neurodegenerative disease which is mainly characterized by progressive impairment in cognition, emotion, language and memory in older population. Considering the impact of AD, formulations of pharmaceutical drugs and cholinesterase inhibitors have been widely propagated, receiving endorsement by FDA as a form of AD treatment. However, these medications were gradually discovered to be ineffective in removing the root of AD pathogenesis but merely targeting the symptoms so as to improve a patient's cognitive outcome. Hence, a search for better disease-modifying alternatives is put into motion. Having a clear understanding of the neuroprotective mechanisms and diverse properties undertaken by specific genes, antibodies and nanoparticles is central towards designing novel therapeutic agents. In this review, we provide a brief introduction on the background of Alzheimer's disease, the biology of blood-brain barrier, along with the potentials and drawbacks associated with current therapeutic treatment avenues pertaining to gene therapy, immunotherapy and nanotherapy for better diagnosis and management of Alzheimer's disease.

Tau in Alzheimer's Disease: Pathological Alterations and an Attractive Therapeutic Target

Alzheimer's disease (AD) is an age-related neurodegenerative disease with two major hallmarks: extracellular amyloid plaques made of amyloid-β (Aβ) and intracellular neurofibrillary tangles (NFTs) of abnormally hyperphosphorylated tau. The number of NFTs correlates positively with the severity of dementia in AD patients. However, there is still no efficient therapy available for AD treatment and prevention so far. A deeper understanding of AD pathogenesis has identified novel strategies for the generation of specific therapies over the past few decades. Several studies have suggested that the prion-like seeding and spreading of tau pathology in the brain may be a key driver of AD. Tau protein is considered as a promising candidate target for the development of therapeutic interventions due to its considerable pathological role in a variety of neurodegenerative disorders. Abnormal tau hyperphosphorylation plays a detrimental pathological role, eventually leading to neurodegeneration. In the present review, we describe the recent research progresses in the pathological mechanisms of tau protein in AD and briefly discuss tau-based therapeutic strategies.

Passive immunotherapies targeting Aβ and tau in Alzheimer's disease

Amyloid-β (Aβ) and tau proteins currently represent the two most promising targets to treat Alzheimer's disease. The most extensively developed method to treat the pathologic forms of these proteins is through the administration of exogenous antibodies, or passive immunotherapy. In this review, we discuss the molecular-level strategies that researchers are using to design an effective therapeutic antibody, given the challenges in treating this disease. These challenges include selectively targeting a protein that has misfolded or is pathological rather than the more abundant, healthy protein, designing strategic constructs for immunizing an animal to raise an antibody that has the appropriate conformational selectivity to achieve this end, and clearing the pathological protein species before prion-like cell-to-cell spread of misfolded protein has irreparably damaged neurons, without invoking damaging inflammatory responses in the brain that naturally arise when the innate immune system is clearing foreign agents. The various solutions to these problems in current clinical trials will be discussed.

Genetic causes of PD: A pathway to disease modification

The underline neuropathology of Parkinson disease is pleiomorphic and its genetic background diverse. Possibly because of this heterogeneity, no effective disease modifying therapy is available. In this paper we give an overview of the genetics of Parkinson disease and explain how this is relevant for the development of new therapies. This article is part of the special issue entitled 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

Current and Future Treatments in Alzheimer Disease: An Update

Disease-modifying treatment strategies for Alzheimer disease (AD) are still under extensive research. Nowadays, only symptomatic treatments exist for this disease, all trying to counterbalance the neurotransmitter disturbance: 3 cholinesterase inhibitors and memantine. To block the progression of the disease, therapeutic agents are supposed to interfere with the pathogenic steps responsible for the clinical symptoms, classically including the deposition of extracellular amyloid β plaques and intracellular neurofibrillary tangle formation. Other underlying mechanisms are targeted by neuroprotective, anti-inflammatory, growth factor promotive, metabolic efficacious agents and stem cell therapies. Recent therapies have integrated multiple new features such as novel biomarkers, new neuropsychological outcomes, enrollment of earlier populations in the course of the disease, and innovative trial designs. In the near future different specific agents for every patient might be used in a “precision medicine” context, where aberrant biomarkers accompanied with a particular pattern of neuropsychological and neuroimaging findings could determine a specific treatment regimen within a customized therapeutic framework. In this review, we discuss potential disease-modifying therapies that are currently being studied and potential individualized therapeutic frameworks that can be proved beneficial for patients with AD.

Novel approaches to counter protein aggregation pathology in Parkinson's disease

The primary neuropathological characteristics of the Parkinsonian brain are the loss of nigral dopamine neurons and the aggregation of alpha synuclein protein. Efforts to development potentially disease-modifying treatments have largely focused on correcting these aspects of the condition. In the last decade treatments targeting protein aggregation have entered the clinical pipeline. In this chapter we provide an overview of ongoing clinical trial programs for different therapies attempting to reduce protein aggregation pathology in Parkinson's disease. We will also briefly consider various novel approaches being proposed-and being developed preclinically-to inhibit/reduce aggregated protein pathology in Parkinson's.

Advances and considerations in AD tau-targeted immunotherapy

The multifactorial and complex nature of Alzheimer’s disease (AD) has made it difficult to identify therapeutic targets that are causally involved in the disease process. However, accumulating evidence from experimental and clinical studies that investigate the early disease process point towards the required role of tau in AD etiology. Importantly, a large number of studies investigate and characterize the plethora of pathological forms of tau protein involved in disease onset and propagation. Immunotherapy is one of the most clinical approaches anticipated to make a difference in the field of AD therapeutics. Tau –targeted immunotherapy is the new direction after the failure of amyloid beta (Aß)-targeted immunotherapy and the growing number of studies that highlight the Aß-independent disease process. It is now well established that immunotherapy alone will most likely be insufficient as a monotherapy. Therefore, this review discusses updates on tau-targeted immunotherapy studies, AD-relevant tau species, updates on promising biomarkers and a prospect on combination therapies to surround the disease propagation in an efficient and timely manner.

The good and bad of therapeutic strategies that directly target α-synuclein

Synucleinopathies are neurodegenerative diseases characterized by the accumulation of either neuronal/axonal or glial insoluble proteinaceous aggregates mainly composed of α-synuclein (α-syn). Among them, the most common disorders are Parkinson's disease, dementia with Lewy bodies, multiple system atrophy, and some forms of familial parkinsonism. Both α-syn fibrils and oligomers have been found to exert toxic effects on neurons or oligodendroglial cells, can activate neuroinflammatory responses, and mediate the spreading of α-syn pathology. This poses the question of which is the most toxic α-syn species. What is worst, α-syn appears as a very peculiar protein, exerting multiple physiological functions in neurons, especially at synapses, but without acquiring a stable tertiary structure. Its conformation is particularly plastic, and the protein can exist in a natively unfolded state (mainly in solution), partially α-helical folded state (when it interacts with biological membranes), or oligomeric state (tetramers or dimers with debated functional profile). The extent of α-syn expression impinges on the resilience of neuronal cells, as multiplications of its gene locus, or overexpression, can cause neurodegeneration and onset of motor phenotype. For these reasons, one of the main challenges in the field of synucleinopathies, which still nowadays can only be managed by symptomatic therapies, has been the development of strategies aimed at reducing α-syn levels, oligomer formation, fibrillation, or cell-to-cell transmission. This review resumes the therapeutic approaches that have been proposed or are under development to counteract α-syn pathology by direct targeting of this protein and discuss their pros and cons in relation to the current state-of-the-art α-syn biology.

Disease-modifying strategies in primary tauopathies

Tauopathies are neurodegenerative brain diseases that are characterized by the formation of intraneuronal inclusions containing the microtubule-associated protein tau. This major hallmark defines tau pathology which is predominant in primary tauopathies, while in secondary forms additional driving forces are involved. In the course of the disease, different brain areas degenerate and lead to severe defects of language, behavior and movement. Although neuropathologically heterogeneous, primary tauopathies share a common feature, which is the generation of abnormal tau species that aggregate and progress into filamentous deposits in neurons. Mechanisms that are involved in this disease-related process offer a broad range of targets for disease-modifying therapeutics. The present review provides an up-to-date overview of currently known targets in primary tauopathies and their possible therapeutic modulation. It is structured into four major targets, the post-translational modifications of tau and tau aggregation, protein homeostasis, disease propagation, and tau genetics. Chances, as well as obstacles in the development of effective therapies are highlighted. Some therapeutic strategies, e.g., passive or active immunization, have already reached clinical development, raising hopes for affected patients. Other concepts, e.g., distinct modulators of proteostasis, are at the ready to be developed into promising future therapies. This article is part of the special issue entitled 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

Four-repeat tauopathies

Tau is a microtubule-associated protein with versatile functions in the dynamic assembly of the neuronal cytoskeleton. Four-repeat (4R-) tauopathies are a group of neurodegenerative diseases defined by cytoplasmic inclusions predominantly composed of tau protein isoforms with four microtubule-binding domains. Progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease or glial globular tauopathy belong to the group of 4R-tauopathies. The present review provides an introduction in the current concept of 4R-tauopathies, including an overview of the neuropathological and clinical spectrum of these diseases. It describes the genetic and environmental etiological factors, as well as the contemporary knowledge about the pathophysiological mechanisms, including post-translational modifications, aggregation and fragmentation of tau, as well as the role of protein degradation mechanisms. Furthermore, current theories about disease propagation are discussed, involving different extracellular tau species and their cellular release and uptake mechanisms. Finally, molecular diagnostic tools for 4R-tauopathies, including tau-PET and fluid biomarkers, and investigational therapeutic strategies are presented. In summary, we report on 4R-tauopathies as overarching disease concept based on a shared pathophysiological concept, and highlight the challenges and opportunities on the way towards a causal therapy.

Progress of Pharmacological Approaches in Parkinson's Disease

The progressive neurodegenerative process in Parkinson's disease (PD) is not restricted to dopaminergic midbrain neurons but involves the entire nervous system. In this review, we outline established treatment options at different disease stages and address new therapeutic approaches. These include, based on recent advances in the understanding of the pathophysiology of PD, genetic and disease-modifying approaches to reduce abnormal accumulation and aggregation of alpha-synuclein (aSYN), mitochondrial dysfunction, and dysfunction of lysosomal proteins. Moreover, we highlight clinical trials to reduce neuroinflammation and increase neurorestoration.

Modulation of Disordered Proteins with a Focus on Neurodegenerative Diseases and Other Pathologies

Intrinsically disordered proteins (IDPs) do not have rigid 3D structures, showing changes in their folding depending on the environment or ligands. Intrinsically disordered proteins are widely spread in eukaryotic genomes, and these proteins participate in many cell regulatory metabolism processes. Some IDPs, when aberrantly folded, can be the cause of some diseases such as Alzheimer′s, Parkinson′s, and prionic, among others. In these diseases, there are modifications in parts of the protein or in its entirety. A common conformational variation of these IDPs is misfolding and aggregation, forming, for instance, neurotoxic amyloid plaques. In this review, we discuss some IDPs that are involved in neurodegenerative diseases (such as beta amyloid, alpha synuclein, tau, and the “IDP-like” PrP), cancer (p53, c-Myc), and diabetes (amylin), focusing on the structural changes of these IDPs that are linked to such pathologies. We also present the IDP modulation mechanisms that can be explored in new strategies for drug design. Lastly, we show some candidate drugs that can be used in the future for the treatment of diseases caused by misfolded IDPs, considering that cancer therapy has more advanced research in comparison to other diseases, while also discussing recent and future developments in this area of research. Therefore, we aim to provide support to the study of IDPs and their modulation mechanisms as promising approaches to combat such severe diseases.

Pathological Changes of Tau Related to Alzheimer's Disease

Alzheimer's disease (AD), the most common form of dementia, is characterized by extracellular β-amyloid plaques and intracellular neurofibrillary tangles (NFTs), which are considered as major targets for AD therapies. However, no effective therapy is available to cure or prevent the progression of AD up until now. Accumulation of NFTs, which consist of abnormally hyperphosphorylated tau, is directly correlated with the degree of dementia in AD patients. Emerging evidence indicates that the prion-like seeding and spreading of tau pathology may be the key driver of AD. In the past decades, greater understanding of tau pathway reveals new targets for the development of specific therapies. Here, we review the recent research progress in the mechanism underlying tau pathology in AD and briefly introduce tau-based therapeutics.

Dementia Therapy Targeting Tau

Tau is a microtubule-associated tau proteins but it has also non-microtubular functions. It aggregates in Alzheimer's disease and many neurodegenerative disorders referred to as tauopathies. Such aggregation may result from mutations on the tau gene, MAPT, dysregulation in alternative splicing, post-translational modifications or truncation. This final chapter addresses some of the various researches on a therapeutic potential around the tau protein and its gene, MAPT. Many therapeutic strategies are ongoing but they are hampered by the lack of knowledge on tau physiological functions.

Emerging disease-modifying strategies targeting α-synuclein for the treatment of Parkinson's disease

Parkinson's disease is the most common neurodegenerative movement disorder. It arises as a result of neuronal cell death in specific brain regions, notably the substantia nigra pars compacta, and is characterized by the accumulation of α-synuclein in these brain regions. Current pharmacological therapies alleviate the motor symptoms of the disease and are particularly effective in the early stages of the disease. Ongoing drug development efforts focus on disease-modifying strategies that aim to halt or slow disease progression. In this review, we explore a number of emerging disease-modifying strategies with a focus on direct and indirect targeting of α-synuclein dysfunction. We summarize newer classes of small molecules and biological agents intended to attenuate protein aggregation or to target enzymes that may increase the degradation of the pathogenic forms of α-synuclein. Finally, we discuss emerging strategies that are demonstrating the potential for disease modification at the preclinical stage.

Designer protein disaggregases to counter neurodegenerative disease

Protein misfolding and aggregation unify several devastating neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. There are no effective therapeutics for these disorders and none that target the reversal of the aberrant protein misfolding and aggregation that cause disease. Here, I showcase important advances to define, engineer, and apply protein disaggregases to mitigate deleterious protein misfolding and counter neurodegeneration. I focus on two exogenous protein disaggregases, Hsp104 from yeast and gene 3 protein from bacteriophages, as well as endogenous human protein disaggregases, including: (a) Hsp110, Hsp70, Hsp40, and small heat-shock proteins; (b) HtrA1; and (c) NMNAT2 and Hsp90. I suggest that protein-disaggregase modalities can be channeled to treat numerous fatal and presently incurable neurodegenerative diseases.