Examination of the differential incorporation of 3R and 4R tau isoforms into neurofibrillary pathology in Alzheimer's disease

Abstract

Fibrillar aggregates of tau, a microtubule-associated protein (MAP), are pathological hallmarks in several neurodegenerative diseases including Alzheimer's disease (AD). Tau, expressed in neurons, regulates microtubule polymerization and stabilization.;Tau binds microtubules through several repeat regions that constitute the microtubule-binding domain (MTBD). Exon 10 located in the MTBD is alternatively spliced to produce 3 isoforms of tau that contain 3 repeat regions (3R tau) and 3 isoforms that contain 4 repeat regions (4R tau).;In the adult human all six isoforms of tau are expressed in the brain with a 3R:4R tau ratio of 50:50. It is believed that the 3R:4R tau ratio has a role in maintaining neuronal integrity because mutations that affect the splicing of the tau gene change the delicate balance of 3R to 4R tau and cause the neurodegenerative disease frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17).;In several other neurodegenerative diseases including AD there are no known mutations that affect tau splicing. However, previous studies performed on brain tissue homogenates have shown that 3R or 4R tau isoforms may be preferentially incorporated into inclusions characteristic of these diseases.;To investigate the intracellular distribution of 3R tau and 4R tau isoforms in neurofibrillary pathology in AD, we generated and characterized a 4R tau-specific monoclonal antibody that was then used in conjunction with a previously characterized 3R tau-specific monoclonal antibody. We analyzed the relative expression of 3R and 4R tau in AD cases by immunohistochemistry and comparative biochemical analysis of PHF-tau. Our immunohistochemical study showed that in certain severe AD cases there was an abundance of 3R tau pathology.;Several mechanisms might be involved in generating isoform-specific pathology by influencing the preferential aggregation of 3R or 4R tau isoforms. Therefore, we also developed a cell culture model in order to examine the effect of altered tau isoform ratio, oxidative stress, and hyperphosphorylation in generating the earliest changes in pathological tau leading to tau isoform-specific pathology and cell death.