LS-14-P-2945 Pathological modifications of tau protein during the formation of neurofibrillary tangles in Alzheimer´s disease

The progression of the neurofibrillary pathology in Alzheimer´s disease (AD) was early described by Braak and Braak [1]. This progression consists in a predictable appearance and distribution of neurofibrillary tangles (NFTs) and dystrophic neurites (DN) along entorhinal, limbic and isocortical areas in the brain of AD affected individuals. It is well known that in AD, NFTs and DNs are mostly composed of tau protein which has undergone several posttranslational modifications such as abnormal phosphorylation, conformational changes and truncation [2, 3, and 4]. The timing of occurrence and possible cosegregation of these modifications during the formation and progression of the neurofibrillary pathology in AD is still under debate and we aimed to address this issue in the present study. Formalin fixed brain sections from the hippocampus and cerebral cortex of AD cases were processed for multiple immunolabeling with antibodies to distinct modifications occurring in tau protein, such as phosphorylation, truncation, ubiquitination, and conformational changes. The pattern of staining and the degree of colocalization were analyzed by using confocal microscopy. Antibodies and fluorescent markers to detect programmed cell death were also included in some experiments. Brain sections from AD cases versus age-matched control individuals were analyzed and compared. We reported the timing by which the distinct events that tau protein undergoes during its aggregation in the form of NFTs. Hence, abnormal phosphorylation was found as the earliest modification on the tau molecule that may alter its conformation. We also found that proteolytic processing of the C-terminus is a key event that links distinct conformational changes in the molecule. Our results indicate that the C-terminus of tau protein is transiently truncated from the aminoacid aspartic acid-D421 to glutamic acid-E391, leading the tau molecule to adopt distinct conformations. Apoptosis in the brain of AD cases was also associated with tau pathology, mostly when the early truncation of tau protein at the aspartic acid-D421 was the major component of NFTs. Besides this, ubiquitination was associated with this early truncation of tau, which may link two different pathways of proteolytic processing in a single substrate occurring at intermediate stages of NFTs maturation. We may conclude that this cascade of pathological molecular events occurring in the molecule of tau protein may give a better correlation with the neuropathological progression of the disease.

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[4] M. Novak et al., EMBO J. 12 (1993) 365.