Bone marrow stromal cells (BMSCs) are versatile mesenchymal cell populations underpinning the major functions of the skeleton, a majority of which adjoin sinusoidal blood vessels and express C-X-C motif chemokine ligand 12 (CXCL12). However, how these cells are activated during regeneration and facilitate osteogenesis remains largely unknown. Celllineage analysis using Cxcl12-creER mice reveals that quiescent Cxcl12-creER + perisinusoidal BMSCs differentiate into cortical bone osteoblasts solely during regeneration. A combined single cell RNA-seq analysis demonstrate that these cells convert their identity into a skeletal stem cell-like state in response to injury, associated with upregulation of osteoblast-signature genes and activation of canonical Wnt signaling components along the single-cell trajectory. β-catenin deficiency in these cells indeed causes insufficiency in cortical bone regeneration.Therefore, quiescent Cxcl12-creER + BMSCs transform into osteoblast precursor cells in a manner mediated by canonical Wnt signaling, highlighting a unique mechanism by which dormant stromal cells are enlisted for skeletal regeneration.

Formation of functional skeletal tissues requires highly organized steps of mesenchymal progenitor cell differentiation. The dental follicle (DF) surrounding the developing tooth harbors mesenchymal progenitor cells for various differentiated cells constituting the tooth root–bone interface and coordinates tooth eruption in a manner dependent on signaling by parathyroid hormone-related peptide (PTHrP) and the PTH/PTHrP receptor (PPR). However, the identity of mesenchymal progenitor cells in the DF and how they are regulated by PTHrP-PPR signaling remain unknown. Here, we show that the PTHrP-PPR autocrine signal maintains physiological cell fates of DF mesenchymal progenitor cells to establish the functional periodontal attachment apparatus and orchestrates tooth eruption. A single-cell RNA-seq analysis revealed cellular heterogeneity of PTHrP+ cells, wherein PTHrP+ DF subpopulations abundantly express PPR. Cell lineage analysis using tamoxifen-inducible PTHrP-creER mice revealed that PTHrP+ DF cells differentiate into cementoblasts on the acellular cementum, periodontal ligament cells, and alveolar cryptal bone osteoblasts during tooth root formation. PPR deficiency induced a cell fate shift of PTHrP+ DF mesenchymal progenitor cells to nonphysiological cementoblast-like cells precociously forming the cellular cementum on the root surface associated with up-regulation of Mef2c and matrix proteins, resulting in loss of the proper periodontal attachment apparatus and primary failure of tooth eruption, closely resembling human genetic conditions caused by PPR mutations. These findings reveal a unique mechanism whereby proper cell fates of mesenchymal progenitor cells are tightly maintained by an autocrine system mediated by PTHrP-PPR signaling to achieve functional formation of skeletal tissues.

Periodontal disease is one of the most common infectious diseases in adults and is characterized by the destruction of tooth-supporting tissues. Mesenchymal stem cells (MSCs) comprise the mesoderm-originating stem cell population, which has been studied and used for cell therapy. However, because of the lower rate of cell survival after MSC transplantation in various disease models, paracrine functions of MSCs have been receiving increased attention as a regenerative mechanism. The aim of this study was to investigate the regenerative potential of transplanted conditioned medium (CM) obtained from cultured periodontal ligament stem cells (PDLSCs), the adult stem cell population in tooth-supporting tissues, using a rat periodontal defect model. Cell-free CM was collected from PDLSCs and fibroblasts, using ultrafiltration and transplanted into surgically created periodontal defects. Protein content of CM was examined by antibody arrays. Formation of new periodontal tissues was analyzed using microcomputed tomography and histological sections. PDLSC-CM transplantation enhanced periodontal tissue regeneration in a concentration-dependent manner, whereas fibroblast-CM did not show any regenerative function. Proteomic analysis revealed that extracellular matrix proteins, enzymes, angiogenic factors, growth factors and cytokines were contained in PDLSC-CM. Furthermore, PDLSC-CM transplantation resulted in the decreased mRNA level of tumor necrosis factor-α (TNF-α) in healing periodontal tissues. In addition, we found that PDLSC-CM suppressed the mRNA level of TNF-α in the monocyte/macrophage cell line, RAW cells, stimulated with IFN-γ. Our findings suggested that PDLSC-CM enhanced periodontal regeneration by suppressing the inflammatory response through TNF-α production, and transplantation of PDLSC-CM could be a novel approach for periodontal regenerative therapy.

During early mouse embryogenesis, cranial neural crest cells (CNCC) emigrate from the posterior midbrain and rhombomeres 1 and 2 of the anterior hindbrain into the first branchial arch-derived maxillary and mandibular processes and there provide cell lineages for several phenotypes, including cartilage, bone, and tooth. Here, we report that Sox9 and Msx2 were coexpressed in a subpopulation of CNCC during their migration. Because Sox9 is a transactivator of chondrogenesis, and Msx genes can act as transcriptional repressors, we hypothesized that Sox9 expression indicates the determination of CNCC-derived chondrogenic cell lineage and that Msx2 represses chondrogenic differentiation until CNCC migration is completed within the mandibular processes. To test whether Msx2 represses chondrogenesis, we designed experiments to inhibit Msx2 function in migratory CNCC in primary cultures through the expression of loss-of-function Msx2 mutants. We showed that infection of migratory CNCC with adenovirus Msx2 mutants accelerated the rate and extent of chondrogenesis, as indicated by the expression level of type II collagen and aggrecan, and the amount of alcian blue staining. Adenovirus infections did not apparently interfere with CNCC proliferation or migration. These findings suggest that an important early event in craniofacial morphogenesis is a transient expression of both Sox9 and Msx2 during emigration into the forming mandibular processes followed by restricted expression of Sox9 within CNCCderived chondroprogenitor cells. We conclude that Msx2 serves as a repressor of chondrogenic differentiation during CNCC migration.