Expression of PINK1 and Parkin in human apical periodontitis

PTEN-induced putative kinase 1 (PINK1) and Parkin E3 ubiquitin-protein ligase (Parkin) are critical for immune and inflammatory regulation in health and disease. PINK1 and Parkin have been confirmed to be involved in the progression of apical periodontitis by affecting mitophagy-related osteoblast apoptosis; however, the expression of PINK1 and Parkin in macrophages, one of the most important cells in apical periodontitis, remains unknown. This study aimed to investigate the expression of PINK1 and Parkin in human apical periodontitis lesions, as well as their possible localization in macrophages.

Thirty-seven human periapical tissues, including periapical granulomas (PGs, n = 12), radicular cysts (RCs, n = 11) and healthy gingival tissues (n = 14) were examined. The inflammatory infiltrates of lesions were evaluated by haematoxylin staining, and the expression of PINK1 and Parkin was detected by immunohistochemistry. Double immunofluorescence was used to explore the colocalization of microtubule-associated protein 1 light chain 3 (LC3) and TOMM20, as well as the localization of PINK1 and Parkin, in macrophages of human apical periodontitis lesions. The ultrastructural morphology of mitochondria in human apical periodontitis lesions was visualized by transmission electron microscopy (TEM). Data were analysed by one-way anova with Student-Newman-Keul's test and the Mann-Whitney test. p < .05 was considered statistically significant.

Immunohistochemistry demonstrated a significantly higher expression of PINK1 and Parkin proteins in human apical periodontitis lesions than in healthy gingival tissues (p < .0001), but no significant difference was demonstrated between PGs and RCs (p > .05). The higher expression of LC3 and the presence of more LC3-TOMM20 double-positive cells were also observed in human apical periodontitis. Double-labelling analysis of PINK1, Parkin and LC3 with CD68 indicated that macrophage mitophagy might be present in the progression of human apical periodontitis. Finally, the results of TEM morphological analysis revealed the appearance of double-membraned mitophagosomes and vacuolated mitochondria in macrophage-like cells of apical periodontitis lesions.

Our findings indicated that PINK1 and Parkin proteins were highly expressed in clinical apical periodontitis lesions.