Intraneuronal filamentous tau inclusions such as neurofibrillary tangles (NFTs) are neuropathological hallmarks of Alzheimer's disease (AD) and related sporadic and familial tauopathies. NFTs identical to those found in AD brains have also been detected in the hippocampus and entorhinal cortex of cognitively normal individuals as they age. To recapitulate ageinduced NFT formation in a mouse model, we examined 12-to 24-month-old transgenic (Tg) mice overexpressing the smallest human brain tau isoform. These Tg mice develop congophilic tau inclusions in several brain regions including the hippocampus, amygdala, and entorhinal cortex. NFT-like inclusions were first detected in Tg mice at 18 to 20 months of age and they were detected by histochemical dyes that bind specifically to crossed -pleated sheet structures (eg, Congo red, Thioflavin S). Moreover, ultrastructurally these lesions contained straight tau filaments comprised of both mouse and human tau proteins but not other cytoskeletal proteins (eg, neurofilaments, microtubules). Isolated tau filaments were also recovered from detergent-insoluble tau fractions and insoluble tau proteins accumulated in brain in an agedependent manner. Thus, overexpression of the smallest human brain tau isoform resulted in late onset and age-dependent formation of congophilic tau inclusions with properties similar to those in the tangles of human tauopathies, thereby implicating aging in the pathogenesis of fibrous tau inclusions. Abnormal tau proteins are implicated in mechanisms of brain degeneration in Alzheimer's disease (AD), frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), Pick's disease, and related neurodegenerative tauopathies. Indeed, the neuropathological hallmarks of tauopathies are abundant aggregates of paired helical filaments (PHFs) and/or straight filaments composed of aberrantly phosphorylated tau proteins in central nervous system (CNS) neurons and/or glia.1,2 In addition to neurodegenerative tauopathies, cognitively normal individuals develop a small number of neurofibrillary tangles (NFTs) in hippocampus and limbic structures as they age. 3 Thus, aging is a risk factor for the formation of NFTs in specific brain regions. Six highly soluble tau isoforms are generated from a single gene by alternative splicing and expressed predominantly in axons of the adult human brain. 4 -6 The functions of these proteins include binding to and stabilizing microtubules (MTs) in the polymerized state.7,8 However, it has been shown that abnormally phosphorylated filamentous tau proteins cannot perform these important functions and aggregate in neurons to form insoluble NFTs in AD 9 -11 or similar fibrillary neuronal and/or glial inclusions in other tauopathies.
1,2To study the pathogenesis of neurodegenerative tauopathies, we previously generated transgenic (Tg) mice that overexpressed the shortest human brain tau isoform in CNS neurons, and showed that 3-to 12-monthold Tg mice accumulate insoluble intraneuronal filamentous hyperphosphorylated ta...