AbstractThe periodontal ligament (PDL), a connective tissue located between the cementum of teeth and the alveolar bone of mandibula, plays a crucial role in the maintenance and regeneration of periodontal tissues. The PDL contains fibroblastic cells of a heterogeneous cell population, from which we have established several cell lines previously. To analyze characteristics unique for PDL at a molecular level, we performed cDNA microarray analysis of the PDL cells versus MC3T3‐E1 osteoblastic cells. The analysis followed by validation by reverse transcription‐polymerase chain reaction and immunochemical staining revealed that endoglin, which had been shown to associate with transforming growth factor (TGF)‐β and bone morphogenetic proteins (BMPs) as signaling modulators, was abundantly expressed in PDL cells but absent in osteoblastic cells. The knockdown of endoglin greatly suppressed the BMP‐2‐induced osteoblastic differentiation of PDL cells and subsequent mineralization. Interestingly, the endoglin knockdown did not alter the level of Smad‐1/5/8 phosphorylation induced by BMP‐2, while it suppressed the BMP‐2‐induced expression of Id1, a representative BMP‐responsive gene. Therefore, it is conceivable that endoglin regulates the expression of BMP‐2‐responsive genes in PDL cells at some site downstream of Smad‐1/5/8 phosphorylation. Alternatively, we found that Smad‐2 as well as Smad‐1/5/8 was phosphorylated by BMP‐2 in the PDL cells, and that the BMP‐2‐induced Smad‐2 phosphorylation was suppressed by the endoglin knockdown. These results, taken together, raise a possibility that PDL cells respond to BMP‐2 via a unique signaling pathway dependent on endoglin, which is involved in the osteoblastic differentiation and mineralization of the cells. J. Cell. Physiol. 222: 465–473, 2010. © 2009 Wiley‐Liss, Inc.