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Worldwide, around 50 million people have dementia. Alzheimer’s disease (AD) is the most common type of dementia and one of the major causes of disability and dependency among the elderly worldwide. Clinically, AD is characterized by impaired memory accompanied by other deficiencies in the cognitive domain. Neuritic plaques (NPs) and neurofibrillary tangles (NFTs) are histopathological lesions that define brains with AD. NFTs consist of abundant intracellular paired helical filaments (PHFs) whose main constituent is tau protein. Tau undergoes posttranslational changes including hyperphosphorylation and truncation, both of which favor conformational changes in the protein. The sequential pathological processing of tau is illustrated with the following specific markers: pT231, TG3, AT8, AT100, and Alz50. Two proteolysis sites for tau have been described—truncation at glutamate 391 and at aspartate 421—and which can be demonstrated by reactivity with the antibodies 423 and TauC-3, respectively. In this review, we describe the molecular changes in tau protein as pre-NFTs progress to extracellular NFTs and during which the formation of a minimal nucleus of the filament, as the PHF core, occurs. We also analyzed the PHF core as the initiator of PHFs and tau phosphorylation as a protective neuronal mechanism against the assembly of the PHF core.

Background: Progressive supranuclear palsy (PSP) is a neurodegenerative disease with pathological and clinical heterogeneity. There are six tau isoforms expressed in the adult human brain, with the repeated microtubule-binding domains of three (3R) or four (4R) repeats. Under normal conditions, the 4R:3R ratio is 1:1. In PSP, the 4R isoform is predominantly expressed. The lesions in PSP brains are phosphorylated tau aggregates in both neurons and glial cells. These neurodegenerative diseases with abnormal tau inclusions are called tauopathies, including Alzheimer’s disease (AD). AD is characterized by highly insoluble paired helical filaments (PHFs) composed of tau with abnormal post-translational modifications. Objective: Our objective was to evaluate and compare the pathological tau processing in PSP and AD. Methods: Double and triple immunofluorescence with antibodies to specific post-translational tau modifications (phosphorylation, truncation, and conformational changes) and thiazin red (TR) were carried out and analyzed by confocal microscopy. Results: Our results showed that PSP was characterized by phosphorylated tau in neurofibrillary tangles (NFTs) and glial cells. Truncated tau at Glu391 and Asp421 was not observed. Extracellular NFTs (eNFTs) and glial cells in PSP exhibited a strong affinity for TR and the absence of intact or phosphorylated tau. Conclusion: Phosphorylated tau was abundantly evidenced in PSP as in AD. The presence of eNFTs in glial cells and neuronal bodies suggest that other truncated tau species different from those observed in AD could be present in PSP. Additional studies on truncated tau within PSP lesions could improve understanding of tau’s pathological processing and help identify a discriminatory biomarker for AD and PSP.

Alzheimer’s disease (AD) is a neurodegenerative disease, characterized histopathologically by intra-neuronal tau-related lesions and by the accumulation of amyloid β-peptide (Aβ) in the brain parenchyma and around cerebral blood vessels. According to the vascular hypothesis of AD, an alteration in the neurovascular unit (NVU) could lead to Aβ vascular accumulation and promote neuronal dysfunction, accelerating neurodegeneration and dementia. To date, the effects of insoluble vascular Aβ deposits on the NVU and the blood–brain barrier (BBB) are unknown. In this study, we analyze different Aβ species and their association with the cells that make up the NVU. We evaluated post-mortem AD brain tissue. Multiple immunofluorescence assays were performed against different species of Aβ and the main elements that constitute the NVU. Our results showed that there are insoluble vascular deposits of both full-length and truncated Aβ species. Besides, insoluble aggregates are associated with a decrease in the phenotype of the cellular components that constitute the NVU and with BBB disruption. This approach could help identify new therapeutic targets against key molecules and receptors in the NVU that can prevent the accumulation of vascular fibrillar Aβ in AD.