Processing of Matrix Extracellular Phosphoglycoprotein in Dental Pulp Cells

Objectives: The extracellular matrix of bone and dentin contains several non-collagenous proteins (NCPs), which are believed to play key biological roles in the mineralization of bone and dentin. Matrix extracellular phosphoglycoprotein with ASARM motif (MEPE) is a newly identified NCP, and reported to be expressed in osteoblasts and pulp cells. The specific mechanisms of MEPE in bone and dentin formation are still unknown, but its function should be dependent on the nature and extent of post-translational modifications. Here, we report the proteolytic processing of MEPE in dental pulp cells and tissues.

Methods: Eukaryotic expression vector pEF-mMEPE and pSecTag-mMEPE, which contained the ORF of mouse MEPE cDNA, was transfected into dental pulpal cell lines: MDPC-23 and OLC, and MEPE expression was analyzed by western blotting with anti-V5 or -MEPE antibodies. Natural expression of MEPE in rat pulp tissues was also analyzed.

Results: Enforced expression of MEPE protein was observed both in cell lysate and supernatant. Three bands, 58kDa, 45kDa and 22kDa, appeared in supernatant and single band, 58kDa, was observed in cell lysate. 58kDa band was thought to be the full length of MEPE, and 45kDa and 22kDa bands were thought to be cleavage products of MEPE. These three forms of MEPE protein were also identified In the rat pulp tissue. These results indicated that MEPE protein was cleavaged into two fragments after its secretion from pulp cells.

Conclusion: The proteolytic processing of MEPE protein occurred in dental pulp cells, which might play important roles in the function of pulp cells.