Self-Aggregating Tau Fragments Recapitulate Pathologic Phenotypes and Neurotoxicity of Alzheimer's Disease in Mice

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].

The Enigma of Tau Protein Aggregation: Mechanistic Insights and Future Challenges

Tau protein misfolding and aggregation are pathological hallmarks of Alzheimer's disease and over twenty neurodegenerative disorders. However, the molecular mechanisms of tau aggregation in vivo remain incompletely understood. There are two types of tau aggregates in the brain: soluble aggregates (oligomers and protofibrils) and insoluble filaments (fibrils). Compared to filamentous aggregates, soluble aggregates are more toxic and exhibit prion-like transmission, providing seeds for templated misfolding. Curiously, in its native state, tau is a highly soluble, heat-stable protein that does not form fibrils by itself, not even when hyperphosphorylated. In vitro studies have found that negatively charged molecules such as heparin, RNA, or arachidonic acid are generally required to induce tau aggregation. Two recent breakthroughs have provided new insights into tau aggregation mechanisms. First, as an intrinsically disordered protein, tau is found to undergo liquid-liquid phase separation (LLPS) both in vitro and inside cells. Second, cryo-electron microscopy has revealed diverse fibrillar tau conformations associated with different neurodegenerative disorders. Nonetheless, only the fibrillar core is structurally resolved, and the remainder of the protein appears as a "fuzzy coat". From this review, it appears that further studies are required (1) to clarify the role of LLPS in tau aggregation; (2) to unveil the structural features of soluble tau aggregates; (3) to understand the involvement of fuzzy coat regions in oligomer and fibril formation.

Modulation of Tau Pathology in Alzheimer's Disease by Dietary Bioactive Compounds

Tau is a microtubule-associated protein essential for microtubule assembly and stability in neurons. The abnormal intracellular accumulation of tau aggregates is a major characteristic of brains from patients with Alzheimer's disease (AD) and other tauopathies. In AD, the presence of neurofibrillary tangles (NFTs), which is composed of hyperphosphorylated tau protein, is positively correlated with the severity of the cognitive decline. Evidence suggests that the accumulation and aggregation of tau cause synaptic dysfunction and neuronal degeneration. Thus, the prevention of abnormal tau phosphorylation and elimination of tau aggregates have been proposed as therapeutic strategies for AD. However, currently tau-targeting therapies for AD and other tauopathies are limited. A number of dietary bioactive compounds have been found to modulate the posttranslational modifications of tau, including phosphorylation, small ubiquitin-like modifier (SUMO) mediated modification (SUMOylation) and acetylation, as well as inhibit tau aggregation and/or promote tau degradation. The advantages of using these dietary components over synthetic substances in AD prevention and intervention are their safety and accessibility. This review summarizes the mechanisms leading to tau pathology in AD and highlights the effects of bioactive compounds on the hyperphosphorylation, aggregation and clearance of tau protein. The potential of using these bioactive compounds for AD prevention and intervention is also discussed.