Distorted Patterns of Dentinogenesis and Eruption in Msx2 Null Mutants

Abstract

The muscle segment homeogenes Msx1 and Msx2 play a major role in tooth and bone formation. Periodontal osteoclast impairment also occurs in Msx2 null mutant mice, which is restored by overexpression of the receptor activator of NF-κB targeted in osteoclast lineage. Here, we investigated the role of Msx2 in dentinogenesis. Experiments were performed on Msx2(-/-) mice and the MDPC-23 odontoblastic cell line. After Msx2 gene silencing, real-time quantitative RT-PCR data showed significant overexpression of Runx2, Bglap, Dspp, and Alpl. Of three inhibitors of Wnt/β-catenin signaling (Dkk1, SostDc1, and Sost/Sclerostin), only Sost was expressed in postnatal teeth and overexpressed in Msx2(-/-) tooth samples. Initial crown dentin formation-primary dentinogenesis-occurred fairly normally in Msx2(-/-) teeth, albeit with distorted cusp patterns. Later stages of tooth development were characterized by a deviation from secondary toward tertiary dentinogenesis with osteodentin formation and impaired dentin deposition leading to limited root elongation. In Msx2(-/-)/receptor activator of NF-κB-transgenic double mutants, the dentin phenotype, notably in the roots, was rescued and sclerostin levels were normalized. These data suggest that Msx2 may act indirectly on dentinogenesis by controlling osteoclast activity and the signaling network related to eruption, supporting and further extending the concept that Msx2 controls formation of mineralized tissues by inhibition of the Wnt/β-catenin pathway; Sost in dentin and Dkk1 in bone, as previously demonstrated.

Experimental periodontitis in Msx2 mutant mice induces alveolar bone necrosis

“…Derived Msx2− /− osteoblastic cell culture showed a higher level of mineralization and expression of bone differentiation markers . In the present study, sclerostin was mainly expressed in osteocytes and cementocytes of cellular cementum, as previously shown . However, sclerostin expression was reduced in the closest osteocytes to alveolar bone surface.…”

“…Clinical dental and periodontal structures have not been studied so far in these specific syndrome features. In mice, Msx2 knock‐out and knock‐in homozygous mutations are associated to pronounced tooth defects, especially root defects. The presence of anatomical defects, such as flattened crowns, shortened roots, and hyperplasia of cellular cementum and epithelial rests of Malassez in first and second maxillary molars appeared to be tooth, root, and site specific .…”

“…For sclerostin immunodetection, non‐specific binding was blocked by incubation 1 hour in 2.5% normal horse serum in PBS. Specimens were incubated for 12 hours at 4°C with a polyclonal goat anti‐mouse sclerostin at a dilution of 1:40 as previously described . Immunostaining was revealed using cell and tissue staining goat kit HRP‐DAB system .…”

“…In human, gain‐and loss‐of‐function associated with MSX2 mutations are correlated to premature cranial suture fusion (Boston‐type2 craniosynostosis) and impaired suture closing in the parietal foramina . Besides these major skeletal manifestations, in Msx2 transgenic mouse model, tooth root abnormalities and delayed tooth eruption are observed in addition to enamel and cranial suture defects . Dental root epithelium and cellular cementum appear hyperplasic and radicular dentinogenesis is prematurely stopped leading to short and deformed roots .…”

“…In these cells, interactions of Msx2 with others transcription factors influence cell differentiation via the control of matrix and bio‐mineralization protein expression, such as osteocalcin, amelogenins, and other proteins essential for periodontal tissue formation and homeostasis such as alkaline phosphatase, dentin sialophosphoprotein, bone sialoprotein BSP, laminine‐5α3, and cytokeratin 5 6 . Msx2 is also a key element of the BMP‐2 and the Wnt‐βcatenin/sclerostin pathways which regulate bone homeostasis . Wnt/βcatenin signaling pathway is involved in normal periodontal tissue formation .…”