Tau Protein in Oral Mucosa and Cognitive State: A Cross-sectional Study

Abstract

Neurodegenerative diseases are characterized by the presence of abnormal aggregates of proteins in brain tissue. Among them, the presence of aggregates of phosphorylated Tau protein (p-Tau) is the hallmark of Alzheimer's disease (AD) and other major neurodegenerative disorders such as corticobasal degeneration and frontotemporal dementia among others. Although Tau protein has previously been assumed to be exclusive to the central nervous system, it is also found in peripheral tissues. The purpose of this study was to determine whether there is a differential Tau expression in oral mucosa cells according to cognitive impairment. Eighty-one subjects were enrolled in the study and classified per Mini-Mental State Examination test score into control, mild cognitive impairment (MCI), and severe cognitive impairment (SCI) groups. Immunocytochemistry and immunofluorescence revealed the presence of Tau and four p-Tau forms in the cytoplasm and nucleus of oral mucosa cells. More positivity was present in subjects with cognitive impairment than in control subjects, both in the nucleus and cytoplasm, in a speckle pattern. The mRNA expression of Tau by quantitative real-time polymerase chain reaction was higher in SCI as compared with the control group (P < 0.01). A significantly higher percentage of immunopositive cells in the SCI group was found via flow cytometry in comparison to controls and the MCI group (P < 0.01). These findings demonstrate the higher presence of p-Tau and Tau transcript in the oral mucosa of cognitively impaired subjects when compared with healthy subjects. The feasibility of p-Tau quantification by flow cytometry supports the prospective analysis of oral mucosa as a support tool for screening of proteinopathies in cognitively impaired patients.

Keywords: Alzheimer disease; dementia; neurodegenerative diseases; oral mucosa cells; tau protein.

Figure 1

Confocal microscopy. Presence and distribution of p-Tau in oral mucosa cells. Control and cognitive impaired (CI) subject. The two first rows show p-Tau (Ser202 + Thr205) and p-Tau (Ser396) immunostaining. The following two rows depict the juxtanuclear localization of p-Tau (Ser202 + Thr205) in nucleus (Lamin A, blue). The nuclei stained with SYTOX (red) are shown in a box in the first image of each series. The scale bars represent 20 µm for all images.

Figure 2

Immunocytochemistry for Tau and p-Tau in oral mucosa cells. Assay of different monoclonal antibodies. (A) Tau-5. (B) p-Tau (Ser396). (C) p-Tau (Thr231). (D) p-Tau (Ser214). (E) p-Tau (Ser202 + Thr205). The scale bars represent 20 µm for all images.

Figure 3

Immunocytochemistry for p-Tau in oral mucosa cells. (A,B) Immunocytochemistry staining shows p-Tau (Ser202) presence in oral mucosa, (A) cognitive impaired subject, (B) control subject. (C) p-Tau (Ser202) positivity percentage by cognitive state. The scale bars represent 20 µm for all images. Mean plus SD, P = 0.108.

Figure 4

(A) 4R-Tau transcript evaluated by quantitative real-time polymerase chain reaction. 4R-Tau relative expression levels (2^−ΔCq) and normalized to 18s ribosomal subunit. Line: median, whiskers: min–max, box: interquartile range. P < 0.01. (B) p-Tau positivity by flow cytometry. Percentage of p-Tau (Ser202) positive oral mucosa cells by cognitive state. Mean plus SD, **P = 0.0029.