Accumulation of C-terminally truncated tau protein associated with vulnerability of the perforant pathway in early stages of neurofibrillary pathology in Alzheimer's disease

Abstract

Neurofibrillary pathology is a characteristic hallmark of Alzheimer's disease that is closely correlated with cognitive decline. We have analysed the density and distribution of neurofibrillary tangles (NFTs) that are immunoreactive with the monoclonal antibody (mAb) 423 in a prospectively analysed population of Alzheimer's disease (AD) cases and age-matched controls. NFTs were examined in allocortical and isocortical areas and correlated with Braak pathological stage and clinical severity of dementia. The mAb 423 was used as it recognises a C-terminally truncated tau fragment that is a major constituent of NFTs. Our results show that extracellular NFTs and, to a lesser extent, intracellular NFTs, correlated significantly with both Braak stages and the clinical index of severity. Furthermore, a differential distribution of the two types of tangles indicates that layer II of the entorhinal cortex and the transentorhinal area are particularly vulnerable to neurofibrillary degeneration. These areas serve as a point of connection between isocortex and hippocampus. Our findings, therefore, suggest that the perforant pathway may be substantially affected by the accumulation of truncated tau protein in AD and that this represents a neuropathological predictor for the clinical severity of dementia. When neurofibrillary pathology was examined by combined labelling with mAbs 423 and Alz-50 and the dye thiazin red, we were able to demonstrate various stages of tau aggregation. The different stages may represent a sequence of conformational changes that tau proteins undergo during tangle formation in the allocortex during the early development of dementia in AD.

Introduction

Alzheimer's disease (AD) is a progressive degenerative disorder of insidious onset, initially characterised by memory loss and, in later stages, by severe dementia. Neurofibrillary tangles (NFTs) and dystrophic neurites, occurring in the neuropil and in neuritic plaques, are the major neuropathological features of AD. Their presence is correlated with clinical dementia (Blessed et al., 1968, Kowall and Kosik, 1987, Ball et al., 1988, Van Hoesen et al., 1991). In AD, NFT formation is associated with neuronal loss (Cras et al., 1995, Gómez-Isla et al., 1996, Gómez-Isla et al., 1997) and there is a strong correlation between neuronal degeneration and the transition between intracellular and extracellular NFTs (Bondareff et al., 1993). In the superior temporal sulcus, neuronal loss in AD both correlates with and exceeds NFTs in number (Gómez-Isla et al., 1997).

NFTs and dystrophic neurites are composed of pathological paired helical filaments (PHFs). The PHF consists of the microtubule-associated protein tau as an integral structural component (Kondo et al., 1988, Wischik et al., 1988a). The predominant fragment of tau in pronase-resistant core-PHFs is a 12-kDa species, corresponding to the C-terminal repeat region that normally functions as a microtubule-binding domain. It is truncated C-terminally at Glu-391 and can be recognised by the monoclonal antibody (mAb) 423 (Wischik et al., 1988b, Novak et al., 1993).

The presence of C-terminally truncated tau protein discriminates between elderly cases with and without clinical dementia, and its quantity is also correlated with neurofibrillary pathology (Harrington et al., 1991). In an earlier study, we reported that the progressive accumulation of mAb 423 immunoreactivity in brain tissue is associated with the evolution of neurofibrillary pathology (Mena et al., 1991). Although mAb 423 immunoreactivity becomes more prominent in NFTs as they become extracellular, following the death of NFT-bearing neurons (Bondareff et al., 1990), immunoreactivity is also observed at early stages of pathology (Mena et al., 1991).

Braak proposed that AD can be staged neuropathologically based upon the spread of NFTs within the hippocampus, entorhinal and transentorhinal cortices (Braak and Braak, 1991). Furthermore, the transentorhinal/entorhinal regions were the zones in which hyperphosphorylated tau protein first appeared (Braak et al., 1994). The latter took the form of granular material, which accumulated in the soma and preceded the development of neurofibrillary pathology.

In the present study, we have examined the distribution of mAb 423-immunoreactive NFTs in the allocortex and isocortex of well-characterised cases with AD and age-matched controls. We have examined the density and distribution of tangles in relation to Braak pathological stage and clinical severity of dementia, as measured by CAMDEX (Roth et al., 1986). We report that the presence of extracellular tangles in the transentorhinal and entorhinal cortices serves to predict both Braak stage and clinical severity of dementia in AD cases.

Using confocal microscopy, we have further analysed the immunochemical characteristics of the lesions that are detected by both mAb 423 and mAb Alz-50. The latter antibody recognises a particular conformational state of tau molecule adopted in PHFs (Carmel et al., 1996). Our findings demonstrate that tau protein aggregates exist in different forms within the various neurofibrillary structures that develop within the brain. We interpret these differences as resulting from conformational changes and post-translational modifications of the tau molecules.