Cryo-EM of Aβ fibrils from mouse models find tg-APPArcSwe fibrils resemble those found in patients with sporadic Alzheimer's disease

Ligands for Protein Fibrils of Amyloid-β, α-Synuclein, and Tau

Amyloid fibrils are characteristic features of many neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. The use of small molecule ligands that bind to amyloid fibrils underpins both fundamental research aiming to better understand the pathology of neurodegenerative disease, and clinical research aiming to develop diagnostic tools for these diseases. To date, a large number of amyloid-binding ligands have been reported in the literature, predominantly targeting protein fibrils composed of amyloid-β (Aβ), tau, and α-synuclein (αSyn) fibrils. Fibrils formed by a particular protein can adopt a range of possible morphologies, but protein fibrils formed in vivo possess disease-specific morphologies, highlighting the need for morphology-specific amyloid-binding ligands. This review details the morphologies of Aβ, tau, and αSyn fibril polymorphs that have been reported as a result of structural work and describes a database of amyloid-binding ligands containing 4,288 binding measurements for 2,404 unique compounds targeting Aβ, tau, or αSyn fibrils.

Alzheimer's disease patient brain extracts induce multiple pathologies in novel vascularized neuroimmune organoids for disease modeling and drug discovery

Alzheimer's Disease (AD) is the most common cause of dementia, afflicting 55 million individuals worldwide, with limited treatment available. Current AD models mainly focus on familial AD (fAD), which is due to genetic mutations. However, models for studying sporadic AD (sAD), which represents over 95% of AD cases without specific genetic mutations, are severely limited. Moreover, the fundamental species differences between humans and animals might significantly contribute to clinical failures for AD therapeutics that have shown success in animal models, highlighting the urgency to develop more translational human models for studying AD, particularly sAD. In this study, we developed a complex human pluripotent stem cell (hPSC)-based vascularized neuroimmune organoid model, which contains multiple cell types affected in human AD brains, including human neurons, microglia, astrocytes, and blood vessels. Importantly, we demonstrated that brain extracts from individuals with sAD can effectively induce multiple AD pathologies in organoids four weeks post-exposure, including amyloid beta (Aβ) plaque-like aggregates, tau tangle-like aggregates, neuroinflammation, elevated microglial synaptic pruning, synapse/neuronal loss, and impaired neural network activity. Proteomics analysis also revealed disrupted AD-related pathways in our vascularized AD neuroimmune organoids. Furthermore, after treatment with Lecanemab, an FDA-approved antibody drug targeting Aβ, AD brain extracts exposed organoids showed a significant reduction of amyloid burden, along with an elevated vascular inflammation response. Thus, the vascularized neuroimmune organoid model provides a unique opportunity to study AD, particularly sAD, under a pathophysiological relevant three-dimensional (3D) human cell environment. It also holds great promise to facilitate AD drug development, particularly for immunotherapies.

Tg-SwDI transgenic mice: A suitable model for Alzheimer's disease and cerebral amyloid angiopathy basic research and preclinical studies

Alzheimer's disease (AD) is the most common neurodegenerative disease and the most frequent cause of dementia. Characteristic features observed in the brain of AD patients are the accumulation of amyloid beta peptide (Aβ) aggregates, neurofibrillary tangles (NFT) composed of hyperphosphorylated Tau protein, neuronal and synaptic loss, and elevated levels of oxidative stress and inflammatory markers. Cerebral amyloid angiopathy (CAA) is another common cause of cognitive decline characterized by the accumulation of Aβ in the cerebral vasculature. The precise overlapping pathogenic mechanisms underlying the co-occurrence of AD and CAA are not very well understood. However, vascular dysfunction observed at early stages is considered a key phenomenon. Tg-SwDI transgenic mice expressing human Aβ precursor protein (AβPP) harboring the Swedish K670N/M671L and vasculotropic Dutch/Iowa E693Q/D694N mutations in the brain have been extensively used to study many pathological features observed in AD/CAA patients and to design biomarkers and therapeutic strategies. The present review summarizes studies addressing different features mimicking human disease in Tg-SwDI mice: parenchymal and cerebral vascular amyloid accumulation, neuroinflammation, complement overactivation, cerebrovascular, mitochondrial and GABAergic system dysfunction, altered NO synthesis, circadian rhythm disruptions, lead exposure effect, among others. Also, reports that evaluated anti-Aβ and anti-inflammatory strategies and compounds capable of delaying or reversing vascular dysfunction and the impairment of GABAergic transmission in Tg-SwDI mice are analyzed. This review may help researchers determine this model's appropriateness for future studies of a particular mechanism or a novel treatment protocol.

Inducers and modulators of protein aggregation in Alzheimer's disease - Critical tools for understanding the foundations of aggregate structures

Amyloidogenic protein aggregation is a pathological hallmark of Alzheimer's Disease (AD). As such, this critical feature of the disease has been instrumental in guiding research on the mechanistic basis of disease, diagnostic biomarkers and preventative and therapeutic treatments. Here we review identified molecular triggers and modulators of aggregation for two of the proteins associated with AD: amyloid beta and tau. We aim to provide an overview of how specific molecular factors can impact aggregation kinetics and aggregate structure to promote disease. Looking toward the future, we highlight some research areas of focus that would accelerate efforts to effectively target protein aggregation in AD.

How is the Amyloid Fold Built? Polymorphism and the Microscopic Mechanisms of Fibril Assembly

For a given protein sequence, many, up to sometimes hundreds of different amyloid fibril folds, can be formed in vitro. Yet, fibrils extracted from, or found in, human tissue, usually at the end of a long disease process, are often structurally homogeneous. Through monitoring of amyloid assembly reactions in vitro, the scientific community has gained a detailed understanding of the kinetic mechanisms of fibril assembly and the rates at which the different processes involved occur. However, how this kinetic information relates to the structural changes as a protein transforms from its initial, native structure to the canonical cross-β structure of amyloid remain obscure. While cryoEM has yielded a plethora of high-resolution information that portray a vast variety of fibril structures, there remains little knowledge of how and why each particular structure resulted. Recent work has demonstrated that fibril structures can change over an assembly time course, despite the different fibril types having similar thermodynamic stability. This points to kinetic control of the fibrils formed, with structures that initiate or elongate faster becoming the dominant products of assembly. Annotating kinetic assembly mechanisms alongside structural analysis of the fibrils formed is required to truly understand the molecular mechanisms of amyloid formation. However, this is a complicated task. In this review, we discuss how embracing this challenge could open new frontiers in amyloid research and new opportunities for therapy.

Molecular Therapeutics in Development to Treat Alzheimer's Disease

Until recently, only symptomatic therapies, in the form of acetylcholine esterase inhibitors and NMDA-receptor antagonists, have been available for the treatment of Alzheimer's disease. However, advancements in our understanding of the amyloid cascade hypothesis have led to a development of disease-modifying therapeutic strategies. These include immunotherapies based on an infusion of monoclonal antibodies against amyloid-β, three of which have been approved for the treatment of Alzheimer's disease in the USA (one of them, lecanemab, has also been approved in several other countries). They all lead to a dramatic reduction of amyloid plaques in the brain, whereas their clinical effects have been more limited. Moreover, they can all lead to side effects in the form of amyloid-related imaging abnormalities. Ongoing developments aim at facilitating their administration, further improving their effects and reducing the risk for amyloid-related imaging abnormalities. Moreover, a number of anti-tau immunotherapies are in clinical trials, but none has so far shown any robust effects on symptoms or pathology. Another line of development is represented by gene therapy. To date, only antisense oligonucleotides against amyloid precursor protein/amyloid-β and tau have reached the clinical trial stage but a variety of gene editing strategies, such as clustered regularly interspaced short palindromic repeats/Cas9-mediated non-homologous end joining, base editing, and prime editing, have all shown promise on preclinical disease models. In addition, a number of other pharmacological compounds targeting a multitude of biochemical processes, believed to be centrally involved in Alzheimer's disease, are currently being evaluated in clinical trials. This article delves into current and future perspectives on the treatment of Alzheimer's disease, with an emphasis on immunotherapeutic and gene therapeutic strategies.

Dual-ligand fluorescence microscopy enables chronological and spatial histological assignment of distinct amyloid-β deposits

Different types of deposits comprised of amyloid-β (Aβ) peptides are one of the pathological hallmarks of Alzheimer's disease (AD) and novel methods that enable identification of a diversity of Aβ deposits during the AD continuum are essential for understanding the role of these aggregates during the pathogenesis. Herein, different combinations of five fluorescent thiophene-based ligands were used for detection of Aβ deposits in brain tissue sections from transgenic mouse models with aggregated Aβ pathology, as well as brain tissue sections from patients affected by sporadic or dominantly inherited AD. When analyzing the sections with fluorescence microscopy, distinct ligand staining patterns related to the transgenic mouse model or to the age of the mice were observed. Likewise, specific staining patterns of different Aβ deposits were revealed for sporadic versus dominantly inherited AD, as well as for distinct brain regions in sporadic AD. Thus, by using dual-staining protocols with multiple combinations of fluorescent ligands, a chronological and spatial histological designation of different Aβ deposits could be achieved. This study demonstrates the potential of our approach for resolving the role and presence of distinct Aβ aggregates during the AD continuum and pinpoints the necessity of using multiple ligands to obtain an accurate assignment of different Aβ deposits in the neuropathological evaluation of AD, as well as when evaluating therapeutic strategies targeting Aβ aggregates.

Distinct Chemical Determinants are Essential for Achieving Ligands for Superior Optical Detection of Specific Amyloid-β Deposits in Alzheimer's Disease

Aggregated forms of different proteins are common hallmarks for several neurodegenerative diseases, including Alzheimer's disease, and ligands that selectively detect specific protein aggregates are vital. Herein, we investigate the molecular requirements of thiophene-vinyl-benzothiazole based ligands to detect a specific type of Aβ deposits found in individuals with dominantly inherited Alzheimer's disease caused by the Arctic APP E693G mutation. The staining of these Aβ deposits was alternated when switching the terminal heterocyclic moiety attached to the thiophene-vinyl-benzothiazole scaffold. The most prevalent staining was observed for ligands having a terminal 3-methyl-1H-indazole moiety or a terminal 1,2-dimethoxybenzene moiety, verifying that specific molecular interactions between these ligands and the aggregates were necessary. The synthesis of additional thiophene-vinyl-benzothiazole ligands aided in pinpointing additional crucial chemical determinants, such as positioning of nitrogen atoms and methyl substituents, for achieving optimal staining of Aβ aggregates. When combining the optimized thiophene-vinyl-benzothiazole based ligands with a conventional ligand, CN-PiB, distinct staining patterns were observed for sporadic Alzheimer's disease versus dominantly inherited Alzheimer's disease caused by the Arctic APP E693G mutation. Our findings provide chemical insights for developing novel ligands that allow for a more precise assignment of Aβ deposits, and might also aid in creating novel agents for clinical imaging of distinct Aβ aggregates in AD.

Alzheimer's Disease: Exploring the Landscape of Cognitive Decline

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by cognitive decline, memory loss, and impaired daily functioning. The pathology of AD is marked by the accumulation of amyloid beta plaques and tau protein tangles in the brain, along with neuroinflammation and synaptic dysfunction. Genetic factors, such as mutations in APP, PSEN1, and PSEN2 genes, as well as the APOE ε4 allele, contribute to increased risk of acquiring AD. Currently available treatments provide symptomatic relief but do not halt disease progression. Research efforts are focused on developing disease-modifying therapies that target the underlying pathological mechanisms of AD. Advances in identification and validation of reliable biomarkers for AD hold great promise for enhancing early diagnosis, monitoring disease progression, and assessing treatment response in clinical practice in effort to alleviate the burden of this devastating disease. In this paper, we analyze data from the CAS Content Collection to summarize the research progress in Alzheimer's disease. We examine the publication landscape in effort to provide insights into current knowledge advances and developments. We also review the most discussed and emerging concepts and assess the strategies to combat the disease. We explore the genetic risk factors, pharmacological targets, and comorbid diseases. Finally, we inspect clinical applications of products against AD with their development pipelines and efforts for drug repurposing. The objective of this review is to provide a broad overview of the evolving landscape of current knowledge regarding AD, to outline challenges, and to evaluate growth opportunities to further efforts in combating the disease.

Antibody engagement with amyloid-beta does not inhibit [11C]PiB binding for PET imaging

The elimination of amyloid-beta (Aβ) plaques in Alzheimer's disease patients after treatment with anti-Aβ antibodies such as lecanemab and aducanumab is supported by a substantially decreased signal in amyloid positron emission tomography (PET) imaging. However, this decreased PET signal has not been matched by a similar substantial effect on cognitive function. There may be several reasons for this, including short treatment duration and advanced disease stages among the patients. However, one aspect that has not been investigated, and the subject of this study, is whether antibody engagement with amyloid plaques inhibits the binding of amyloid-PET ligands, leading to a false impression of Aβ removal from the brain. In the present study, tg-ArcSwe mice received three injections of RmAb158, the murine version of lecanemab or phosphate-buffered saline (PBS) before the administration of the amyloid-PET radioligand [11C]PiB, followed by isolation of brain tissue. Autoradiography showed that RmAb158- and PBS-treated mice displayed similar [11C]PiB binding. Moreover, the total Aβ1-40 levels, representing the major Aβ species of plaques in the tg-ArcSwe model, as well as soluble triggering receptor on myeloid cells 2 (sTREM2) levels, were similar in both groups. Interestingly, the concentration of soluble Aβ aggregates was decreased in the RmAb158-treated group, along with a small but significant decrease in the total Aβ1-42 levels. In conclusion, this study indicates that the binding of [11C]PiB to Aβ accurately mirrors the load of Aβ plaques in the brain, aligning with how amyloid-PET is interpreted in clinical studies of anti-Aβ antibodies. However, early treatment effects on soluble Aβ aggregates and Aβ1-42 levels were not detected.

Exploring the complexity of amyloid-beta fibrils: structural polymorphisms and molecular interactions

The aggregation of amyloid-beta (Aβ) peptides into cross-β structures forms a variety of distinct fibril conformations, potentially correlating with variations in neurodegenerative disease progression. Recent advances in techniques such as X-ray crystallography, solid-state NMR, and cryo-electron microscopy have enabled the development of high-resolution molecular structures of these polymorphic amyloid fibrils, which are either grown in vitro or isolated from human and transgenic mouse brain tissues. This article reviews our current understanding of the structural polymorphisms in amyloid fibrils formed by Aβ40 and Aβ42, as well as disease-associated mutants of Aβ peptides. The aim is to enhance our understanding of various molecular interactions, including hydrophobic and ionic interactions, within and among cross-β structures.

Cryo-EM structures of cotton wool plaques' amyloid β and of tau filaments in dominantly inherited Alzheimer disease

Cotton wool plaques (CWPs) have been described as features of the neuropathologic phenotype of dominantly inherited Alzheimer disease (DIAD) caused by some missense and deletion mutations in the presenilin 1 (PSEN1) gene. CWPs are round, eosinophilic amyloid-β (Aβ) plaques that lack an amyloid core and are recognizable, but not fluorescent, in Thioflavin S (ThS) preparations. Amino-terminally truncated and post-translationally modified Aβ peptide species are the main component of CWPs. Tau immunopositive neurites may be present in CWPs. In addition, neurofibrillary tangles coexist with CWPs. Herein, we report the structure of Aβ and tau filaments isolated from brain tissue of individuals affected by DIAD caused by the PSEN1 V261I and A431E mutations, with the CWP neuropathologic phenotype. CWPs are predominantly composed of type I Aβ filaments present in two novel arrangements, type Ic and type Id; additionally, CWPs contain type I and type Ib Aβ filaments. Tau filaments have the AD fold, which has been previously reported in sporadic AD and DIAD. The formation of type Ic and type Id Aβ filaments may be the basis for the phenotype of CWPs. Our data are relevant for the development of PET imaging methodologies to best detect CWPs in DIAD.

Cryo-EM structures of pathogenic fibrils and their impact on neurodegenerative disease research

Neurodegenerative diseases are commonly associated with the formation of aberrant protein aggregates within the brain, and ultrastructural analyses have revealed that the proteins within these inclusions often assemble into amyloid filaments. Cryoelectron microscopy (cryo-EM) has emerged as an effective method for determining the near-atomic structure of these disease-associated filamentous proteins, and the resulting structures have revolutionized the way we think about aberrant protein aggregation and propagation during disease progression. These structures have also revealed that individual fibril conformations may dictate different disease conditions, and this newfound knowledge has improved disease modeling in the lab and advanced the ongoing pursuit of clinical tools capable of distinguishing and targeting different pathogenic entities within living patients. In this review, we summarize some of the recently developed cryo-EM structures of ex vivo α-synuclein, tau, β-amyloid (Aβ), TAR DNA-binding protein 43 (TDP-43), and transmembrane protein 106B (TMEM106B) fibrils and discuss how these structures are being leveraged toward mechanistic research and therapeutic development.

Distinctive contribution of two additional residues in protein aggregation of Aβ42 and Aβ40 isoforms

Amyloid-β (Aβ) is one of the amyloidogenic intrinsically disordered proteins (IDPs) that self-assemble to protein aggregates, incurring cell malfunction and cytotoxicity. While Aβ has been known to regulate multiple physiological functions, such as enhancing synaptic functions, aiding in the recovery of the blood-brain barrier/brain injury, and exhibiting tumor suppression/antimicrobial activities, the hydrophobicity of the primary structure promotes pathological aggregations that are closely associated with the onset of Alzheimer's disease (AD). Aβ proteins consist of multiple isoforms with 37-43 amino acid residues that are produced by the cleavage of amyloid-β precursor protein (APP). The hydrolytic products of APP are secreted to the extracellular regions of neuronal cells. Aβ 1-42 (Aβ42) and Aβ 1-40 (Aβ40) are dominant isoforms whose significance in AD pathogenesis has been highlighted in numerous studies to understand the molecular mechanism and develop AD diagnosis and therapeutic strategies. In this review, we focus on the differences between Aβ42 and Aβ40 in the molecular mechanism of amyloid aggregations mediated by the two additional residues (Ile41 and Ala42) of Aβ42. The current comprehension of Aβ42 and Aβ40 in AD progression is outlined, together with the structural features of Aβ42/Aβ40 amyloid fibrils, and the aggregation mechanisms of Aβ42/Aβ40. Furthermore, the impact of the heterogeneous distribution of Aβ isoforms during amyloid aggregations is discussed in the system mimicking the coexistence of Aβ42 and Aβ40 in human cerebrospinal fluid (CSF) and plasma. [BMB Reports 2024; 57(6): 263-272].

Divergent Age-Dependent Conformational Rearrangement within Aβ Amyloid Deposits in APP23, APPPS1, and AppNL-F Mice

Amyloid plaques composed of fibrils of misfolded Aβ peptides are pathological hallmarks of Alzheimer's disease (AD). Aβ fibrils are polymorphic in their tertiary and quaternary molecular structures. This structural polymorphism may carry different pathologic potencies and can putatively contribute to clinical phenotypes of AD. Therefore, mapping of structural polymorphism of Aβ fibrils and structural evolution over time is valuable to understanding disease mechanisms. Here, we investigated how Aβ fibril structures in situ differ in Aβ plaque of different mouse models expressing familial mutations in the AβPP gene. We imaged frozen brains with a combination of conformation-sensitive luminescent conjugated oligothiophene (LCO) ligands and Aβ-specific antibodies. LCO fluorescence mapping revealed that mouse models APP23, APPPS1, and AppNL-F have different fibril structures within Aβ-amyloid plaques depending on the AβPP-processing genotype. Co-staining with Aβ-specific antibodies showed that individual plaques from APP23 mice expressing AβPP Swedish mutation have two distinct fibril polymorph regions of core and corona. The plaque core is predominantly composed of compact Aβ40 fibrils, and the corona region is dominated by diffusely packed Aβ40 fibrils. Conversely, the AβPP knock-in mouse AppNL-F, expressing the AβPP Iberian mutation along with Swedish mutation has tiny, cored plaques consisting mainly of compact Aβ42 fibrils, vastly different from APP23 even at elevated age up to 21 months. Age-dependent polymorph rearrangement of plaque cores observed for APP23 and APPPS1 mice >12 months, appears strongly promoted by Aβ40 and was hence minuscule in AppNL-F. These structural studies of amyloid plaques in situ can map disease-relevant fibril polymorph distributions to guide the design of diagnostic and therapeutic molecules.

Altered amyloid-β structure markedly reduces gliosis in the brain of mice harboring the Uppsala APP deletion

Deposition of amyloid beta (Aβ) into plaques is a major hallmark of Alzheimer's disease (AD). Different amyloid precursor protein (APP) mutations cause early-onset AD by altering the production or aggregation properties of Aβ. We recently identified the Uppsala APP mutation (APPUpp), which causes Aβ pathology by a triple mechanism: increased β-secretase and altered α-secretase APP cleavage, leading to increased formation of a unique Aβ conformer that rapidly aggregates and deposits in the brain. The aim of this study was to further explore the effects of APPUpp in a transgenic mouse model (tg-UppSwe), expressing human APP with the APPUpp mutation together with the APPSwe mutation. Aβ pathology was studied in tg-UppSwe brains at different ages, using ELISA and immunohistochemistry. In vivo PET imaging with three different PET radioligands was conducted in aged tg-UppSwe mice and two other mouse models; tg-ArcSwe and tg-Swe. Finally, glial responses to Aβ pathology were studied in cell culture models and mouse brain tissue, using ELISA and immunohistochemistry. Tg-UppSwe mice displayed increased β-secretase cleavage and suppressed α-secretase cleavage, resulting in AβUpp42 dominated diffuse plaque pathology appearing from the age of 5-6 months. The γ-secretase cleavage was not affected. Contrary to tg-ArcSwe and tg-Swe mice, tg-UppSwe mice were [11C]PiB-PET negative. Antibody-based PET with the 3D6 ligand visualized Aβ pathology in all models, whereas the Aβ protofibril selective mAb158 ligand did not give any signals in tg-UppSwe mice. Moreover, unlike the other two models, tg-UppSwe mice displayed a very faint glial response to the Aβ pathology. The tg-UppSwe mouse model thus recapitulates several pathological features of the Uppsala APP mutation carriers. The presumed unique structural features of AβUpp42 aggregates were found to affect their interaction with anti-Aβ antibodies and profoundly modify the Aβ-mediated glial response, which may be important aspects to consider for further development of AD therapies.