Exposure of the Amino Terminus of Tau Is a Pathological Event in Multiple Tauopathies

Abstract

Pathological changes to the tau protein, including conformational changes and aggregation, are major hallmarks of a group of neurodegenerative disorders known as tauopathies. Among the conformational changes are alterations involving the extreme amino terminus of the protein, known as the phosphatase-activating domain (PAD). Aberrant PAD exposure induces a signaling cascade that leads to disruption of axonal transport, a critical function for neuronal survival. Conformational display of PAD is an early marker of pathological tau in Alzheimer disease (AD), but its role in other tauopathies has yet to be firmly established. We used a relatively novel N-terminal, conformation-sensitive antibody, TNT2, to determine whether misfolding in the amino terminus (ie, PAD exposure) occurs in non-AD tauopathies. We found that TNT2 specifically labeled pathological tau in post-mortem human brain tissue from Pick disease, progressive supranuclear palsy, corticobasal degeneration, and chronic traumatic encephalopathy, but did not label nonpathological, parenchymal tau. Tau13, another N-terminal antibody, was not sensitive to pathological N-terminal conformations. Tau13 did not readily distinguish between normal (ie, parenchymal tau) and pathological tau species and showed a range of effectiveness at identifying tau pathologies in the non-AD tauopathies. These findings demonstrate that the conformational display of the PAD in tau represents a common pathological event in many tauopathies.

Figure 1

Pick disease pathology contains PAD-exposed tau. Post-mortem human brain tissue slices from PiD were examined with TNT2 and Tau13 antibodies. A–C: TNT2, a marker of PAD exposure, labels Pick bodies (arrowheads) and neuropil threads in the dentate gyrus (A) and pyramidal neurons (B) of the hippocampus, as well as the frontal cortex (C). C: TNT2-reactive pathology resembling ramified astrocytes is present in the frontal cortex (arrow). D and E: Tau13 labels Pick bodies (arrowheads) in the dentate gyrus (D) and pyramidal cell layers (E) and robustly labels parenchymal tau, which tends to obscure thread pathologies. F: Glial pathology is noted in the frontal cortex with Tau13 as well (arrow). n = 4. Scale bar = 50 μm.

Figure 2

PAD exposure occurs in hallmark tau pathologies of corticobasal degeneration. Immunohistochemical staining of corticobasal degeneration (CBD) post-mortem human brain tissue sections was performed with TNT2 and Tau13 antibodies. A: TNT2 clearly labels the hallmark astrocytic plaques (arrow) characteristic of CBD in the frontal cortex, as well as extensive thread pathology. B: In the hippocampus, TNT2 labels diffuse, granular pretangle neuronal pathology (arrowheads), but few classic neurofibrillary tangles (not depicted). C: Some hippocampal neurons display clusters of thread pathology that appears to surround the apical dendrites of pyramidal cells (arrowheads). D: TNT2 stains coiled bodies (arrow) in the white matter of the frontal cortex as well as extensive thread pathology. E: Astrocytic plaques (arrow) are occasionally and weakly identified by Tau13 in the frontal cortex but are not clearly evident because of relatively strong labeling of parenchymal tau. F: Tau13 labels some neuritic threads and neuronal pathology (arrowheads) in the hippocampus. G: Tau13 identifies some cellular pathology (arrowhead) in the frontal cortex but is not effective at labeling thread pathology in this region. n = 4. Scale bar = 50 μm.

Figure 3

Progressive supranuclear palsy pathology is composed of PAD-exposed tau proteins. Post-mortem human brain tissue from progressive supranuclear palsy (PSP) cases were used for immunohistochemistry with TNT2 and Tau13. A: TNT2 labels various forms of tau pathology associated with PSP, including those resembling tufted astrocytes (arrow), coiled bodies (black arrowhead), and diffuse neuronal tau inclusions (white arrowhead) in frontal cortical gray matter. B: TNT2 also labels coiled bodies in frontal cortical white matter (arrowheads). C: Pretangle neuronal inclusions in the pyramidal layer of the hippocampus (white arrowhead) are identified by TNT2 and the pathology extended far into the neuronal processes. D: Tau13 poorly labels only a few coiled bodies (black arrowhead) and neuronal inclusions (white arrowhead) and strongly labels parenchymal tau in the gray matter of the frontal cortex. E: Coiled bodies (arrowheads) are also detected by Tau13 in the white matter of the frontal cortex. F: Neuronal inclusions (white arrowhead) in the hippocampus are also lightly stained by Tau13. n = 5. Scale bar = 50 μm.

Figure 4

PAD exposure is present in the characteristic tau pathology of chronic traumatic encephalopathy. Chronic traumatic encephalopathy human brain tissue was analyzed by TNT2 and Tau13 immunohistochemistry. A: Extensive tangle and thread pathology is detected by TNT2 in the frontal cortex. B: Tau-positive tangles, plaques, and threads in proximity to blood vessels (v) of the frontal cortex are detected by TNT2. C: TNT2 labels tau pathology concentrated at the depth of a sulcus (s) within the frontal cortex. D: Astrocytic plaques (arrowhead) are detected by TNT2 in the frontal cortex. E: TNT2 identifies PAD-exposed tau in thread pathology as well as coiled bodies (arrow) within white matter of the frontal cortex. F: Neuronal tau pathology (arrowhead) is weakly detected by Tau13, but the antibody also labels high levels of parenchymal tau. G: Tau13 faintly identifies tau pathology near a sulcus of the frontal cortex. n = 5. Scale bar = 50 μm.

Supplemental Figure S1

Lower-magnification images of TNT2 and Tau13 reactivity in control and non-Alzheimer disease tauopathies. Images from immunohistochemical staining of frontal cortical sections by TNT2 and Tau13 were obtained using a 10× objective lens. In general, neither TNT2 (A) nor Tau13 (B) identifies tau pathology in control brain sections from the frontal cortex. C: TNT2 specifically labels tau pathology in Pick's disease (PiD) brains. D: Tau13 labels tau pathology and parenchymal tau in the PiD frontal cortex. E: TNT2 detects pathological tau in corticobasal degeneration (CBD). F: Tau13 is less able to recognize tau pathology in the frontal cortex of CBD but displays robust parenchymal tau staining. G: TNT2 identifies a variety of neuronal and glial forms of tau pathology in progressive supranuclear palsy (PSP). H: Tau13 does not demonstrate any specificity for pathological tau in PSP. I: In cases of chronic traumatic encephalopathy (CTE), TNT2 is able to specifically recognize pathognomonic tau pathology J: Tau13 is generally unable to identify tau pathology with any specificity in CTE cases. Scale bar = 100 μm (A–J).