The Receptor Activator of Nuclear Factor-κB Ligand-mediated Osteoclastogenic Pathway Is Elevated in Amelogenin-null Mice

Abstract

Amelogenins, major components of developing enamel, are predominantly involved in the formation of tooth enamel. Although amelogenins are also implicated in cementogenesis, their precise spatial expression pattern and molecular role are not clearly understood. Here, we report for the first time the expression of two alternate splice forms of amelogenins, M180 and the leucine-rich amelogenin peptide (LRAP), in the periodontal region of mouse tooth roots. Lack of M180 and LRAP mRNA expression correlated with cementum defects observed in the amelogenin-null mice. The cementum defects were characterized by an increased presence of multinucleated cells, osteoclasts, and cementicles. These defects were associated with an increased expression of the receptor activator of the nuclear factor-B ligand (RANKL), a critical regulator of osteoclastogenesis. These findings indicate that the amelogenin splice variants, M180 and LRAP, are critical in preventing abnormal resorption of cementum.Ameloblasts synthesize and secrete amelogenins into the dental enamel matrix that undergo systematic proteolysis during enamel mineralization. Numerous mutations were found in the amelogenin coding sequences in patients with the most common genetic disorder affecting enamel, amelogenesis imperfecta (1-5). The targeted disruption of the amelogenin gene locus in mice also showed a hypoplastic enamel phenotype similar to amelogenesis imperfecta, confirming an important role of amelogenins in enamel formation (6).In addition to their role in enamel formation, amelogenins are also believed to play a key role in the formation of root cementum, a mineralized layer on the surface of root dentin (7,8). During cementogenesis, Hertwig's epithelial root sheath dissociates to form cell aggregates (epithelial rests of Malassez) that are located between the alveolar bone and the tooth root. The mesenchyme-derived cementoblasts secrete cementum matrix onto the root surface to form cementum. The presence of amelogenins was detected on the tooth root surface close to the site of acellular cementum (9) and in the epithelial remnants of the root sheath in rat molars (10), indicating their potential role during cementogenesis. Interestingly, amelogenins were also detected in Hertwig's epithelial root sheath cells and the epithelial rests of Malassez (11-13). Therapeutic application of an enamel matrix derivative (EMDOGAIN®, Biora AB, Malmö, Sweden) rich in amelogenins resulted in regeneration of cementum, the surrounding alveolar bone, and periodontal ligament (PDL) 1 in the experimental treatment of periodontitis (14 -17). However, it is not clear from these studies whether the amelogenin or non-amelogenin components of an enamel matrix derivative regulate regeneration of cementum and periodontal tissues.The present study was undertaken to investigate the expression of various alternate splice forms of amelogenins in tooth roots and correlate their expression with the cementum defects observed in amelogenin-null mice teeth. Herein, we report the expression of ...

Amelogenin expression in long bone and cartilage cells and in bone marrow progenitor cells

“…1c), and the reported increase in the number of osteoclasts due to lack of amelogenins in the amelogenin KO mouse, may suggest that amelogenin has a role in the cross-talk between these two cell types controlling the process of osteoclastogenesis. It might be that amelogenin inhibits differentiation toward osteoclasts, as was suggested by Hatakeyama et al (2003Hatakeyama et al ( , 2006 for osteoclasts in the tooth supporting tissues.…”

“…Amelogenin expression in osteocytes, osteoblasts, osteoclasts, bone marrow cells, and cartilage cells, together with the accumulating data indicating amelogenin induction of osteogenesis (Veis et al, 2000) and inhibition of osteoclastogenesis (Hatakeyama et al, 2003), suggest that amelogenin has a crucial role in the processes of bone development and remodeling.…”

“…The defects in cementum were characterized by increased presence of osteoclasts. These defects were also associated with an increased expression of receptor activator of nuclear factor-jB ligand (RANKL) near the cementum, suggesting that amelogenin may play a key role in osteoclastogenesis through the RANKL/RANK-mediated pathway (Hatakeyama et al, 2003(Hatakeyama et al, , 2006.…”

Regulation of CXCL12 expression by canonical Wnt signaling in bone marrow stromal cells

“…Wright et al (Hatakeyama et al, 2003) reported that TGF-β1 down-regulates CXCL12 expression in the stromal cell line MS-5. Similar to our studies, this decrease in CXCL12 production by MS-5 cells correlated with decreased CXCL12-dependent migratory activity (Hatakeyama et al, 2003).…”

“…Wright et al (Hatakeyama et al, 2003) reported that TGF-β1 down-regulates CXCL12 expression in the stromal cell line MS-5. Similar to our studies, this decrease in CXCL12 production by MS-5 cells correlated with decreased CXCL12-dependent migratory activity (Hatakeyama et al, 2003). They indicate that "primitive" mesenchymal This differentiation and maturation is mediated by RANKL, which is expressed on osteoblasts, and by receptor activator of NFkB (RANK), a cognate receptor expressed on osteoclast precursor cells (Theoleyre et al, 2004).…”