Table 2.
Main current findings regarding iOPN and associated roles.
| iOPN Expression Pattern/Role | Main Findings | Cell Type | Approach/Methodology | Reference |
|---|---|---|---|---|
| OPN displayed a perinuclear and a perimembraneous distribution | iOPN was associated with stages of osteogenic cell differentiation | Fetal rat calvarial cells | Single-cell analysis, flow cytometry, and confocal microscopy | [62] |
| iOPN localizes in the cytoplasm but outside secretory vesicles | iOPN is translated from a non-AUG codon, accompanied by deletion of the N-terminal 16-aa signal sequence. | Dendritic and T cells | R’ RLM-RACE, in vitro translation, confocal microscopy, Ifna4 promoter reporter assay, isolation of secretory vesicles; in vivo assays, ELISA and intracellular cytokine flow cytometry, immunoblotting, RT-qPCR | [25,26] |
| Soluble OPN altered the actin cytoskeleton of tumor cells | iOPN induces rapid Tyr-418 dephosphorylation of c-Src, with decreases in actin stress fibers and increased binding to the vascular endothelium | Human melanoma and sarcoma cell lines; and patient-derived samples | Functional assays in vitro, in tumor cells, in mouse models, and ex vivo, using exogenous OPN or negative controls, including a site-directed mutant OPN | [72] |
| iOPN co-localizes with fungal PRRs in macrophages stimulated by Pneumocystis | iOPN is essential for generating antifungal innate immune responses in PRR recognition, signal transduction, phagocytosis, clearance of Pneumocystis, and cytokine production | Cells obtained from C57BL/6, Opn−/−, Rag2−/−, and Opn−/− Rag2−/− mice | RT-qPCR, confocal microscopy, immunoprecipitation, immunoblotting, ELISA, and analyses of phagocytosis, ROS production, and Pneumocystis clearance | [73] |
| iOPN expression is enhanced by VSV and SeV virus infection | iOPN acts as a positive regulator in innate antiviral immunity through the stabilization of TRAF3 | iOPN ectopic overexpression or OPN deficiency or knockdown; SPP1 knockout mice; Immunoprecipitation and immunoblotting; ELISA; RT-qPCR; ubiquitination assays | [74] |
OPN: osteopontin; iOPN: intracellular OPN; pattern recognition receptor (PRR); plasmacytoid dendritic cells (pDC); Sendai virus (SeV); vesicular stomatitis virus (VSV); tumor necrosis factor receptor (TNFR)-associated factor 3 (TRAF3); Cytoplasmic OPN has also been reported to be differentially expressed at the pyramidal neurons of the hippocampus of Alzheimer’s disease (AD) compared with age-matched control brains. Moreover, cytoplasmic OPN staining intensity was significantly associated with amyloid-β load and aging among all control and AD subjects [75]. It has also been shown that exhaustion of cytoplasmic OPN expression is associated with the loss of its protective role in the pancreas islets cells and with destructive insulitis and diabetes [76].