Abstract
Neurotrophic factors play an important role in the development and maintenance of not only neural but also nonneural tissues. Several neurotrophic factors are expressed in dental tissues, but their role in tooth development is not clear. Here, we report that neurotrophic factor neurotrophin (NT)-4 promotes differentiation of dental epithelial cells and enhances the expression of enamel matrix genes. Dental epithelial cells from 3-day-old mice expressed NT-4 and three variants of TrkB receptors for neurotrophins (full-length TrkB-FL and truncated TrkB-T1 and -T2). Dental epithelial cell line HAT-7 expressed these genes, similar to those in dental epithelial cells. We found that NT-4 reduced HAT-7 cell proliferation and induced the expression of enamel matrix genes, such as ameloblastin (Ambn). Transfection of HAT-7 cells with the TrkB-FL expression construct enhanced the NT-4-mediated induction of Ambn expression. This enhancement was blocked by K252a, an inhibitor for Trk tyrosine kinases. Phosphorylation of ERK1/2, a downstream molecule of TrkB, was induced in HAT-7 cells upon NT-4 treatment. TrkB-FL but not TrkB-T1 transfection increased the phosphorylation level of ERK1/2 in NT-4-treated HAT-7 cells. These results suggest that NT-4 induced Ambn expression via the TrkB-MAPK pathway. The p75 inhibitor TAT-pep5 decreased NT-4-mediated induction of the expression of Ambn, TrkB-FL, and TrkB-T1, suggesting that both high affinity and low affinity neurotrophin receptors were required for NT-4 activity. We found that NT-4-null mice developed a thin enamel layer and had a decrease in Ambn expression. Our results suggest that NT-4 regulates proliferation and differentiation of the dental epithelium and promotes production of the enamel matrix.
Highlights
The abbreviations used areAmelogenin; BrdUrd, bromo-2Ј-deoxyuridine; MAPK, mitogen-activated protein kinase; ERK, extracellular signalregulated kinase; Ambn, ameloblastin; Enam, enamelin; Nerve growth factor (NGF), neural growth factor; BDNF, brain-derived neurotrophic factor; NT, neurotrophin; PBS, phosphate-buffered saline; RT, reverse transcriptase; MEK, mitogenactivated protein kinase/extracellular signal-regulated kinase kinase; P1, P3, and P7, postnatal day 1, 3, and 7, respectively
Neurotrophic factors play an important role in the development and maintenance of neural and nonneural tissues
At the initiation stages of tooth germ development, Nerve growth factor (NGF) is expressed in the dental mesenchyme and weakly in the dental epithelium
Summary
Amelogenin; BrdUrd, bromo-2Ј-deoxyuridine; MAPK, mitogen-activated protein kinase; ERK, extracellular signalregulated kinase; Ambn, ameloblastin; Enam, enamelin; NGF, neural growth factor; BDNF, brain-derived neurotrophic factor; NT, neurotrophin; PBS, phosphate-buffered saline; RT, reverse transcriptase; MEK, mitogenactivated protein kinase/extracellular signal-regulated kinase kinase; P1, P3, and P7, postnatal day 1, 3, and 7, respectively. At the initiation stages of tooth germ development, NGF is expressed in the dental mesenchyme and weakly in the dental epithelium. NT-3 expression is gradually restricted to the more cervical parts of the inner enamel epithelium and is no longer detected in postnatal tooth germs [20]. We investigated the roles of NT-4 and TrkB in tooth development in vitro using dental epithelium cultures and in vivo using NT-4 knock-out mice. NT-4 and TrkB receptors were expressed in the dental epithelium of 3-day-old mice and in the HAT-7 dental epithelial cell line. Our findings suggest that NT-4 plays a role in proliferation and differentiation of the dental epithelium and is required for the expression of enamel matrix genes
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