Staging of neurofibrillary pathology in Alzheimer's disease: a study of the BrainNet Europe Consortium

Abstract

It has been recognized that molecular classifications will form the basis for neuropathological diagnostic work in the future. Consequently, in order to reach a diagnosis of Alzheimer's disease (AD), the presence of hyperphosphorylated tau (HP-tau) and beta-amyloid protein in brain tissue must be unequivocal. In addition, the stepwise progression of pathology needs to be assessed. This paper deals exclusively with the regional assessment of AD-related HP-tau pathology. The objective was to provide straightforward instructions to aid in the assessment of AD-related immunohistochemically (IHC) detected HP-tau pathology and to test the concordance of assessments made by 25 independent evaluators. The assessment of progression in 7-microm-thick sections was based on assessment of IHC labeled HP-tau immunoreactive neuropil threads (NTs). Our results indicate that good agreement can be reached when the lesions are substantial, i.e., the lesions have reached isocortical structures (stage V-VI absolute agreement 91%), whereas when only mild subtle lesions were present the agreement was poorer (I-II absolute agreement 50%). Thus, in a research setting when the extent of lesions is mild, it is strongly recommended that the assessment of lesions should be carried out by at least two independent observers.

Figure 1

In both (A) and (B) the assessment of Alzheimer's disease (AD)‐related pathology is based primarily on the counts of senile/neuritic plaques visualized by silver stains. As the formation of pathology spans decades and is also seen in the brains of unimpaired subjects to obtain a definite diagnosis of AD, both the age of the subject and the clinical symptoms are of significance.

Figure 2

The diagnosis of Alzheimer's disease (AD) is given as a likelihood that both of the AD‐related neuropathological lesions, neurofibrillary tangles and neuritic plaques, are causative regarding clinical symptoms. NIA‐RI = the National Institute on Aging and Reagan Institute; CERAD = the Consortium to Establish a Registry for Alzheimer's disease.

Figure 3

Flowchart delineating the structure of the study. IHC = immunohistochemistry; AD = Alzheimer's disease.

Figure 4

Gross view of the neuroanatomical regions included. 1‐occipital cortex including calcarine fissure; 2‐temporal cortex including middle temporal gyrus and at least a part of superior temporal gyrus, 3‐anterior hippocampus at the level of uncus and 4‐posterior hippocampus at the level of lateral geniculate nucleus.

Figure 5

Density of AT‐8 immunopositive neuropil threads at magnifications × 100 and × 200. A. Low (+), that is, immunoreactive (IR) structures are barely noted at low magnification. B. Moderate (++), that is, IR structures are easily seen at both magnifications. C. High (+++), that is, IR structures are seen even without the microscope. Sections are taken from occipital cortex.

Figure 6

Scanned immunohistochemically stained sections applying AT8 antibody. Section from: Block 1 – occipital cortex including calcarine fissure; Block 2 – temporal cortex including middle temporal gyrus and at least a part of superior temporal gyrus; Block 3 – anterior hippocampus at the level of uncus; and Block 4 – posterior hippocampus at the level of lateral geniculate nucleus. The regions are given from left to right in the suggested order of assessment. The arrowheads indicate borders for the relevant neuroanatomical regions for each given stage: Braak I – transentorhinal region; Braak II – entorhinal region; Braak III – temporo‐occipital gyrus; Braak IV – temporal cortex; Braak V – peristriatal cortex; and Braak VI striatal cortex.