Identification of Thyrotropin-Releasing Hormone (TRH)-Degrading Enzyme as a Biomarker for Dental Pulp Tissue

Regeneration of dental pulp after pulpectomy was accomplished recently by autologous transplantation of dental pulp stem/ progenitor cells into the root canal. The identical patterns of qualitative and quantitative protein and mRNA expression in the regenerated pulp and normal pulp demonstrated complete pulp regeneration. The lack of tissue specific markers in the dental pulp is a major challenge in dental regenerative medicine. In order to identify specific markers in dental pulp we undertook a comparison of the gene expression profile of the dental pulp with that of periodontal ligament and gingiva. This systematic investigation identified thyrotropinreleasing hormone (TRH)-degrading enzyme (DE) as a marker of dental pulp. Expression of TRH-DE mRNA in human dental pulp was higher than that in any other tissue except brain as analyzed by real time RT-PCR. Induction of neural cells enhanced the expression of TRH-DE mRNA in dental pulp stem/progenitor cells (CD105+ and CD31- side population (SP) cells) in vitro. Immunohistochemical and in situ hybridization analyses demonstrated that TRH-DE in the neuronal processes in dental pulp. In canine pulp cells, TRH down-regulated TRH-DE mRNA expression, while neuropeptide Y up-regulated it, suggesting that TRH-DE has functional role in the neuropeptide signaling in dental pulp tissue. It is noteworthy that TRH-DE mRNA was expressed in the regenerated pulp 28 days after transplantation of CD31- SP cells into root canals after pulpectomy. These results demonstrate the utility of TRH-DE as a novel dental pulp biomarker during regeneration of pulp.