The Function and Regulation of LIM Domain Mineralization Protein (LMP) in Periodontal Ligament Progenitor Cells.

Abstract

New methodologies to target and deliver osteogenic factors offer significant potential for craniofacial tissue engineering. LIM domain mineralization protein (LMP) appears to be such a candidate for periodontal bone regeneration. The main purpose of this dissertation was to explore the function and regulation of LMP1 in periodontium, specifically in periodontal ligament (PDL) cell, and to evaluate the potential of LMP gene therapy in promoting periodontal bone formation. Using laser capture microdissection, LMP1 was found to be highly expressed in PDL and gingival tissue, and at lower level in mature alveolar bone. During tooth extraction socket healing, LMP1 expression modestly increased over time. However, in the healing of osteotomy defects around implants, LMP1 expression was gradually decreased. In experimental periodontitis model, LMP1 gene expression was upregulated in the inflamed gingival tissue. The physiological function of LMP1 was also investigated by a loss-of-function strategy. Stable knockdown of LMP1 in PDL cell resulted in impaired cell proliferation and subsequent delay in mineralization. Adenoviral gene delivery of LMP1 and LMP3 (a truncated transcription variant without any LIM domain) was performed to assess the potential of LMP gene transduction in enhancing bone formation. AdLMP3 but not AdLMP1 significantly induced matrix mineralization in PDL cell and bone marrow stromal cell in vitro. However, AdLMP3 transduced-PDL cells failed to induce ectopic bone formation in immunocompromised animals. Interestingly, AdLMP1 and AdBMP7 combinatory gene therapy led to increased bone formation above that of AdBMP7 treatment alone. More studies are needed to understand the mechanisms underlying this synergistic effect. The regulatory mechanism of LMP1 gene expression was identified in this thesis as well. LMP1 gene expression is regulated by TGF-β1 in PDL cell and other preosteoblast. TAK1-JNK/p38 kinase cascade was involved in this regulation event. Gene knockdown LMP1 affected the TGF-β1 effect on PDL proliferation. In summary, this dissertation established the gene expression profiles of LMP1 in normal, diseased, and regenerating periodontium, determined the function of LMP1 on PDL cell proliferation and differentiation, investigated the potential of LMP gene therapy in periodontal regeneration, and characterized a regulatory mechanism of LMP1 gene expression.