Ameloblastin expression and putative autoregulation in mesenchymal cells suggest a role in early bone formation and repair

Abstract

Ameloblastin is mainly known as a dental enamel protein, synthesized and secreted into developing enamel matrix by the enamel-forming ameloblasts. The function of ameloblastin in tooth development remains unclear, but it has been suggested to be involved in processes varying from regulating crystal growth to activity as a growth factor or partaking in cell signaling. Recent studies suggest that some enamel matrix proteins also might have important functions outside enamel formation. In this context ameloblastin has recently been reported to induce dentin and bone repair, as well as being present in the early bone and cartilage extracellular matrices during embryogenesis. However, what cells express ameloblastin in these tissues still remain unclear. Thus, the expression of ameloblastin was examined in cultured primary mesenchymal cells and in vivo during healing of bone defects in a “proof of concept” animal study. The real time RT-PCR analysis revealed human ameloblastin (AMBN) mRNA expression in human mesenchymal stem cells and primary osteoblasts and chondrocytes. Expression of AMBN mRNA was also confirmed in human CD34 positive cells and osteoclasts. Western and dot blot analysis of cell lysates and medium confirmed the expression and secretion of ameloblastin from mesenchymal stem cells, primary human osteoblasts and chondrocytes. Expression of ameloblastin was also detected in newly formed bone in experimental bone defects in adult rats. Together these findings suggest a role of this protein in early bone formation and repair.

Ameloblastin is not implicated in bone remodelling and repair

“…On the other hand, they also showed that expression decreased markedly with time and disappeared after completion of bone modelling (Spahr et al, 2006), suggesting no involvement in the physiologic maintenance of adult bone. While no impairment of bone formation is evident in the AMBN mutant mice , recent studies -also reported by Fukumoto et al (2004) -suggested a positive role for AMBN in bone generation in vivo and in vitro (Tamburstuen et al, 2010;Tamburstuen et al, 2011;Iizuka et al, 2011). Tamburstuen et al (2010) showed the expression of AMBN mRNA and protein secretion in human mesenchymal stem cells, primary osteoblasts and chondrocytes.…”

“…While no impairment of bone formation is evident in the AMBN mutant mice , recent studies -also reported by Fukumoto et al (2004) -suggested a positive role for AMBN in bone generation in vivo and in vitro (Tamburstuen et al, 2010;Tamburstuen et al, 2011;Iizuka et al, 2011). Tamburstuen et al (2010) showed the expression of AMBN mRNA and protein secretion in human mesenchymal stem cells, primary osteoblasts and chondrocytes. They also demonstrated, using a microarray technique, that the genes related to immune responses, expression of cytokines and markers of osteogenic cell differentiation were upregulated by the application of recombinant AMBN in cell culture (Tamburstuen et al, 2011).…”

“…There have been some recent reports suggesting that AMBN may be involved in bone formation (Tamburstuen et al, 2010;Tamburstuen et al, 2011;Iizuka et al, 2011) and that, like its sister protein AMEL, it may be capable of promoting bone repair (Bosshardt, 2008;Haze et al, 2009). To test this possibility, we have evaluated the effect of AMBN on bone formation both in vitro and in various animal models.…”

“…Tamburstuen et al (2010) showed the expression of AMBN mRNA and protein secretion in human mesenchymal stem cells, primary osteoblasts and chondrocytes. They also demonstrated, using a microarray technique, that the genes related to immune responses, expression of cytokines and markers of osteogenic cell differentiation were upregulated by the application of recombinant AMBN in cell culture (Tamburstuen et al, 2011). Iizuka et al (2011) concluded, in their in vitro investigation of sarcoma cell lines, that AMBN was expressed in osteoblasts and acted as a promoting factor for osteogenic differentiation via a pathway through the interaction between CD63 and integrin β1.…”

“…Other investigations suggested that the protein has an effect on differentiation of mesenchymal stem cells (Tamburstuen et al, 2010;Tamburstuen et al, 2011), and on osteogenic differentiation (Iizuka et al, 2011). These reports have raised the possibility that AMBN might be implicate not only in amelogenesis but also in osteogenesis.…”