Wnt signaling from Gli1-expressing apical stem/progenitor cells is essential for the coordination of tooth root development

Missense variants weakening a SOX9 phosphodegron linked to odontogenesis defects, scoliosis, and other skeletal features

SOX9 encodes an SRY-related transcription factor critical for chondrogenesis and sex determination among other processes. Loss-of-function variants cause campomelic dysplasia and Pierre Robin sequence, while both gain- and loss-of-function variants cause disorders of sex development. SOX9 has also been linked to scoliosis and cancers, but variants are undetermined. It is highly expressed in tooth progenitor cells, but its odontogenic roles remain elusive, and tooth defects are unreported in SOX9-related conditions. Here, we performed whole-exome sequencing for nine unrelated children with tooth eruption delay and no known syndromes and identified a 7-year-old girl heterozygous for a SOX9 p.Thr239Pro variant and a 10-year-old boy heterozygous for presumably adjacent p.Thr239Pro and p.Thr240Pro variants. These variants were de novo and rare in control populations. Both cases had primary tooth eruption delay. Additionally, the boy had mesiodens blocking permanent central upper incisor eruption, severe scoliosis, and mild craniofacial and appendicular skeleton abnormalities. p.Thr239 and p.Thr240 occupy variable and obligatory positions, respectively, in a cell division control protein 4 (Cdc4)/FBXW7-targeted phosphodegron motif (CPD) fully conserved in SOX9 vertebrate orthologs and SOX8 and SOX10 paralogs, but functionally uncharacterized in vivo. Structural modeling predicted p.Thr240Pro and p.Thr239Pro/p.Thr240Pro but not p.Thr239Pro to strongly reduce SOX9/FBXW7 interaction. Accordingly, p.Thr240Pro and p.Thr239Pro/p.Thr240Pro but not p.Thr239Pro blocked FBXW7-induced SOX9 degradation in cultured cells. All variants increased SOX9-mediated reporter activation independently of protein stabilization, suggesting that CPD may also modulate the transactivation function of SOX9. Altogether, these findings concur that CPD has critical functions, that SOX9 decisively controls odontogenesis, and that gain-of-function variants may markedly perturb both this process and skeletogenesis.

The Role of Gli1+ Mesenchymal Stem Cells in Craniofacial Development and Disease Treatment

OBJECTIVE

This review summarises the role of Gli1+ (Glioma-associated oncogene homologue 1) mesenchymal stem cells in craniofacial growth and development or tissue repair, and their application in the treatment of some diseases.

DESIGN

The search for this narrative review was conducted in PubMed and Web of Science using relevant keywords, including checking reference lists of journal articles by hand searching.

RESULTS

Gli1+ mesenchymal stem cells play an important role in the growth and development of the skull, tooth, periodontium and mandibular condyle. They can be applied to the treatment of pulp and periodontal diseases, temporomandibular joint osteoarthritis and other diseases.

CONCLUSIONS

Gli1+ mesenchymal stem cells are crucial for the development and repair of craniofacial tissue.

Deciphering the spatial distribution of Gli1-lineage cells in dental, oral, and craniofacial regions

The craniofacial bone, crucial for protecting brain tissue and supporting facial structure, undergoes continuous remodeling through mesenchymal (MSCs) or skeletal stem cells in their niches. Gli1 is an ideal marker for labeling MSCs and osteoprogenitors in this region, and Gli1-lineage cells are identified as pivotal for bone growth, development, repair, and regeneration. Despite its significance, the distribution of Gli1-lineage cells across the dental, oral, and craniofacial (DOC) regions remains to be systematically explored. Utilizing tissue-clearing and light sheet fluorescence microscopy with a Gli1CreER; tdTomatoAi14 mouse model, we mapped the spatial distribution of Gli1-lineage cells throughout the skull, focusing on calvarial bones, sutures, bone marrow, teeth, periodontium, jaw bones, and the temporomandibular joint. We found Gli1-lineage cells widespread in these areas, underscoring their significance in DOC regions. Additionally, we observed their role in repairing calvarial bone defects, providing novel insights into craniofacial biology and stem cell niches and enhancing our understanding of stem cells and their progeny's behavior in vivo.

Osteoblast differentiation of Gli1⁺ cells via Wnt and BMP signaling pathways during orthodontic tooth movement

OBJECTIVES

Factors that induce bone formation during orthodontic tooth movement (OTM) remain unclear. Gli1 was recently identified as a stem cell marker in the periodontal ligament (PDL). Therefore, we evaluated the mechanism of differentiation of Cre/LoxP-mediated Gli1/Tomato+ cells into osteoblasts during OTM.

METHODS

After the final administration of tamoxifen to 8-week-old Gli1-CreERT2/ROSA26-loxP-stop-loxP-tdTomato mice for 2 days, nickel-titanium closed coil springs were attached between the upper anterior alveolar bone and the first molar. Immunohistochemical localizations of β-catenin, Smad4, and Runx2 were observed in the PDL on 2, 5, and 10 days after OTM initiation.

RESULTS

In the untreated tooth, few Gli1/Tomato+ cells were detected in the PDL. Two days after OTM initiation, the number of Gli1/Tomato+ cells increased in the PDL on the tension side. On this side, 49.3 ± 7.0% of β-catenin+ and 48.7 ± 5.7% of Smad4+ cells were found in the PDL, and Runx2 expression was detected in some Gli1/Tomato+ cells apart from the alveolar bone. The number of positive cells in the PDL reached a maximum on day 5. In contrast, on the compression side, β-catenin and Smad4 exhibited less immunoreactivity. On day 10, Gli1/Tomato+ cells were aligned on the alveolar bone on the tension side, with some expressing Runx2.

CONCLUSIONS

Gli1+ cells in the PDL differentiated into osteoblasts during OTM. Wnt and bone morphogenetic proteins signaling pathways may be involved in this differentiation.

METTL3-mediated pre-miR-665/DLX3 m6A methylation facilitates the committed differentiation of stem cells from apical papilla

Methyltransferase-like 3 (METTL3) is a crucial element of N6-methyladenosine (m6A) modifications and has been extensively studied for its involvement in diverse biological and pathological processes. In this study, we explored how METTL3 affects the differentiation of stem cells from the apical papilla (SCAPs) into odonto/osteoblastic lineages through gain- and loss-of-function experiments. The m6A modification levels were assessed using m6A dot blot and activity quantification experiments. In addition, we employed Me-RIP microarray experiments to identify specific targets modified by METTL3. Furthermore, we elucidated the molecular mechanism underlying METTL3 function through dual-luciferase reporter gene experiments and rescue experiments. Our findings indicated that METTL3+/- mice exhibited significant root dysplasia and increased bone loss. The m6A level and odonto/osteoblastic differentiation capacity were affected by the overexpression or inhibition of METTL3. This effect was attributed to the acceleration of pre-miR-665 degradation by METTL3-mediated m6A methylation in cooperation with the "reader" protein YTHDF2. Additionally, the targeting of distal-less homeobox 3 (DLX3) by miR-665 and the potential direct regulation of DLX3 expression by METTL3, mediated by the "reader" protein YTHDF1, were demonstrated. Overall, the METTL3/pre-miR-665/DLX3 pathway might provide a new target for SCAP-based tooth root/maxillofacial bone tissue regeneration.

Cross-regulation between SOX9 and the canonical Wnt signalling pathway in stem cells

SOX9, a member of the SRY-related HMG-box transcription factors, has been reported to critically regulate fetal development and stem cell homeostasis. Wnt signalling is a highly conserved signalling pathway that controls stem cell fate decision and stemness maintenance throughout embryonic development and adult life. Many studies have shown that the interactions between SOX9 and the canonical Wnt signalling pathway are involved in many of the physiological and pathological processes of stem cells, including organ development, the proliferation, differentiation and stemness maintenance of stem cells, and tumorigenesis. In this review, we summarize the already-known molecular mechanism of cross-interactions between SOX9 and the canonical Wnt signalling pathway, outline its regulatory effects on the maintenance of homeostasis in different types of stem cells, and explore its potential in translational stem cell therapy.

Activin receptor-like kinase 3: a critical modulator of development and function of mineralized tissues

Mineralized tissues, such as teeth and bones, pose significant challenges for repair due to their hardness, low permeability, and limited blood flow compared to soft tissues. Bone morphogenetic proteins (BMPs) have been identified as playing a crucial role in mineralized tissue formation and repair. However, the application of large amounts of exogenous BMPs may cause side effects such as inflammation. Therefore, it is necessary to identify a more precise molecular target downstream of the ligands. Activin receptor-like kinase 3 (ALK3), a key transmembrane receptor, serves as a vital gateway for the transmission of BMP signals, triggering cellular responses. Recent research has yielded new insights into the regulatory roles of ALK3 in mineralized tissues. Experimental knockout or mutation of ALK3 has been shown to result in skeletal dysmorphisms and failure of tooth formation, eruption, and orthodontic tooth movement. This review summarizes the roles of ALK3 in mineralized tissue regulation and elucidates how ALK3-mediated signaling influences the physiology and pathology of teeth and bones. Additionally, this review provides a reference for recommended basic research and potential future treatment strategies for the repair and regeneration of mineralized tissues.