Eukaryotic initiation factor subunit I (EIF3i), also called as p36 or TRIP-1, is a component of the translation initiation complex and acts as a modulator of TGF-β signaling. We demonstrated earlier that this intracellular protein is not only exported to the extracellular matrix via exosomes but also binds calcium phosphate and promotes hydroxyapatite nucleation. To assess other functional roles of TRIP-1, we first examined their phylogeny and showed that it is highly conserved in eukaryotes. Comparing human EIF3i sequence with that of 63 other eukaryotic species showed that more than 50% of its sequence is conserved, suggesting the preservation of its important functional role (translation initiation) during evolution. TRIP-1 contains WD40 domains and predicting its function based on this structural motif is difficult as it is present in a vast array of proteins with a wide variety of functions. Therefore, bioinformatics analysis was performed to identify putative regulatory functions for TRIP-1 by examining the structural domains and post-translational modifications and establishing an interactive network using known interacting partners such as type I collagen. Insight into the function of TRIP-1 was also determined by examining structurally similar proteins such as Wdr5 and GPSß, which contain a ß-propeller structure which has been implicated in the calcification process. Further, proteomic analysis of matrix vesicles isolated from TRIP-1-overexpressing preosteoblastic MC3T3-E1 cells demonstrated the expression of several key biomineralization-related proteins, thereby confirming its role in the calcification process. Finally, we demonstrated that the proteomic signature in TRIP1-OE MVs facilitated osteogenic differentiation of stem cells. Overall, we demonstrated by bioinformatics that TRIP-1 has a unique structure and proteomic analysis suggested that the unique osteogenic cargo within the matrix vesicles facilitates matrix mineralization.

Dental pulp tissue contains mesenchymal stem/progenitor cells that possess high proliferative potential for self-renewal. They are neural-crest derived cells and exhibit multi-lineage differentiation properties. These progenitor stem cells are now recognized as being vital to the dentin regeneration process following injury. Understanding the molecular mechanisms that mediate the differentiation of adult stem cells into odontoblasts and their use in the repair of the dentin–pulp complex is of significant interest in regenerative dental medicine. Dentin Phosphophoryn (DPP), synthesized and processed predominantly by the odontoblasts, functions both as a structural and signaling protein. We had previously demonstrated that DPP activates NF-κB and promotes Wnt5a expression in dental pulp stem cells. In this context, we observed that DPP can activate TAZ, a biologically potent transcriptional coactivator which serves as a downstream element of the NF-κB signaling cascade. Furthermore, binding of NF-κB p65 subunit to the TAZ promoter was facilitated by DPP stimulation, and their interaction was confirmed by ChIP analysis. In addition, DPP-dependent activation of the TAZ/TEAD reporter was confirmed by luciferase activity in DPSCs. Co-immunoprecipitation analysis confirmed the in vivo interaction between TAZ and β-catenin with DPP stimulation. This regulatory complex facilitated TAZ to bind to the conserved TEAD binding motifs of key gene targets involved in odontogenic differentiation such as RUNX2, OSX, OCN, ALP, BMP4, and WNT5A. Some of these genes also contain binding sites for the TCF/LEF transcription factors that interact with the Wnt effector, β-catenin. Activation of TAZ and β-catenin resulted in the upregulation of odontoblast gene expression and reduced expression in the presence of the TAZ–TEAD protein complex inhibitor. Using mandibles of DSPP KO and WT mice, we confirmed reduced TAZ and β-catenin protein levels in the dental pulp cells and in the odontoblasts of DSPP KO mice when compared with WT. Thus, DPP, an extracellular matrix protein, provides biological cues to activate the TAZ signaling pathway that can stimulate the terminal differentiation of DPSCs into functional odontoblasts.