The transcription factor NKX2-3 mediates p21 expression and ectodysplasin-A signaling in the enamel knot for cusp formation in tooth development.

Tooth morphogenesis is initiated by reciprocal interactions between the ectoderm and neural crest-derived mesenchyme. During tooth development, tooth cusps are regulated by precise control of proliferation of cell clusters, termed enamel knots, that are present among dental epithelial cells. The interaction of ectodysplasin-A (EDA) with its receptor, EDAR, plays a critical role in cusp formation by these enamel knots, and mutations of these genes is a cause of ectodermal dysplasia. It has also been reported that deficiency in Nkx2-3, encoding a member of the NK2 homeobox family of transcription factors, leads to cusp absence in affected teeth. However, the molecular role of NKX2-3 in tooth morphogenesis is not clearly understood. Using gene microarray analysis in mouse embryos, we found that Nkx2-3 is highly expressed during tooth development and increased during the tooth morphogenesis, especially during cusp formation. We also demonstrate that NKX2-3 is a target molecule of EDA and critical for expression of the cell cycle regulator p21 in the enamel knot. Moreover, NKX2-3 activated the bone morphogenetic protein (BMP) signaling pathway by up-regulating expression levels of Bmp2 and Bmpr2 in dental epithelium and decreased the expression of the dental epithelial stem cell marker SRY box 2 (SOX2). Together, our results indicate that EDA/NKX2-3 signaling is essential for enamel knot formation during tooth morphogenesis in mice.

Nephronectin plays critical roles in Sox2 expression and proliferation in dental epithelial stem cells via EGF-like repeat domains.

Tooth development is initiated by epithelial-mesenchymal interactions via basement membrane (BM) and growth factors. In the present study, we found that nephronectin (Npnt), a component of the BM, is highly expressed in the developing tooth. Npnt localizes in the BM on the buccal side of the tooth germ and shows an expression pattern opposite that of the dental epithelial stem cell marker Sox2. To identify the roles of Npnt during tooth development, we performed knockdown and overexpression experiments using ex vivo organ and dental epithelial cell cultures. Our findings showed that loss of Npnt induced ectopic Sox2-positive cells and reduced tooth germ size. Over expression of Npnt showed increased proliferation, whereas the number of Sox2-positive cells was decreased in dental epithelial cells. Npnt contains 5 EGF-like repeat domains, as well as an RGD sequence and MAM domain. We found that the EGF-like repeats are critical for Sox2 expression and cell proliferation. Furthermore, Npnt activated the EGF receptor (EGFR) via the EGF-like repeat domains and induced the PI3K-Akt signaling pathway. Our results indicate that Npnt plays a critical scaffold role in dental epithelial stem cell differentiation and proliferation, and regulates Sox2 expression during tooth development.

Plakophilin-1, a Novel Wnt Signaling Regulator, Is Critical for Tooth Development and Ameloblast Differentiation.

Tooth morphogenesis is initiated by reciprocal interactions between the ectoderm and neural crest-derived mesenchyme, and the Wnt signaling pathway is involved in this process. We found that Plakophilin (PKP)1, which is associated with diseases such as ectodermal dysplasia/skin fragility syndrome, was highly expressed in teeth and skin, and was upregulated during tooth development. We hypothesized that PKP1 regulates Wnt signaling via its armadillo repeat domain in a manner similar to ß-catenin. To determine its role in tooth development, we performed Pkp1 knockdown experiments using ex vivo organ cultures and cell cultures. Loss of Pkp1 reduced the size of tooth germs and inhibited dental epithelial cell proliferation, which was stimulated by Wnt3a. Furthermore, transfected PKP1-emerald green fluorescent protein was translocated from the plasma membrane to the nucleus upon stimulation with Wnt3a and LiCl, which required the PKP1 N terminus (amino acids 161 to 270). Localization of PKP1, which is known as an adhesion-related desmosome component, shifted to the plasma membrane during ameloblast differentiation. In addition, Pkp1 knockdown disrupted the localization of Zona occludens 1 in tight junctions and inhibited ameloblast differentiation; the two proteins were shown to directly interact by immunoprecipitation. These results implicate the participation of PKP1 in early tooth morphogenesis as an effector of canonical Wnt signaling that controls ameloblast differentiation via regulation of the cell adhesion complex.

[A case of Sjögren's syndrome with subacute transverse myelitis as the initial manifestation].

We report a case of subacute transverse myelitis associated with Sjögren's syndrome free of xerosis. A 62-year-old man was admitted due to dysesthesia of both lower extremities and the left trunk, weakness of the left leg, and urinary disturbance. Neurological examination showed myelopathy at the Th7 level. CSF had increased protein (82 mg/dl) and IgG (23.4 mg/dl) and IgG index (1.03) but an almost normal cell count (7/mm3). T2-weighted MRI showed a high signal intensity lesion at the sixth and seventh thoracic levels. Although he was free of xerosis, typical sialographic findings, as well as the presence of anti-SSA antibody, are consistent with the diagnostic criteria for Sjögren's syndrome decided by the Japanese research group on Sjögren's syndrome. The patient was treated with prednisolone, 60 mg/day, which completely cured his muscle weakness and difficulty in walking, and sensory disturbance was gradually alleviated. Spinal MRI detected a marked reduction in the size of T2-weighted high signal intensity lesion during prednisolone treatment. In Western countries, central nervous system complications are reported in up to 20% of Sjögren's syndrome patients, but myelopathy is a very rare condition. Only 12 cases, including ours, have been reported. The clinical manifestations of myelopathy in Sjögren's syndrome include acute or subacute transverse myelitis (6 cases, including ours), chronic progressive myelopathies (2 cases.), relapsing and remitting cord syndromes (4 cases) and Brown-Séquard syndrome (none). Ten patients were women. In 9 of 12 cases there were sicca symptoms. The level of the myelopathies in 6 of 10 cases was between the third to eighth thoracic level, consistent with the region vulnerable to ischemia. Eight patients were treated successfully with steroids. We speculate that ischemia due to vasculitis is important in the genesis of myelopathy associated with Sjögren's syndrome. In the case of myelopathy, especially in the thoracic cord, it is necessary to look for evidence of Sjögren's syndrome even when xerosis is unremarkable.