Hedgehog activation promotes osteogenic fates of growth plate resting zone chondrocytes through transient clonal competency.

The resting zone of the postnatal growth plate is organized by slow-cycling chondrocytes expressing parathyroid hormone-related protein (PTHrP), which include a subgroup of skeletal stem cells that contribute to the formation of columnar chondrocytes. The PTHrP-Indian hedgehog feedback regulation is essential for sustaining growth plate activities; however, molecular mechanisms regulating cell fates of PTHrP+ resting chondrocytes and their eventual transformation into osteoblasts remain largely undefined. Here, in a mouse model, we specifically activated Hedgehog signaling in PTHrP+ resting chondrocytes and traced the fate of their descendants using a tamoxifen-inducible Pthrp-creER line with patched-1-floxed and tdTomato reporter alleles. Hedgehog-activated PTHrP+ chondrocytes formed large, concentric, clonally expanded cell populations within the resting zone ("patched roses") and generated significantly wider columns of chondrocytes, resulting in hyperplasia of the growth plate. Interestingly, Hedgehog-activated PTHrP+ cell descendants migrated away from the growth plate and transformed into trabecular osteoblasts in the diaphyseal marrow space in the long term. Therefore, Hedgehog activation drives resting zone chondrocytes into transit-amplifying states as proliferating chondrocytes and eventually converts these cells into osteoblasts, unraveling a potentially novel Hedgehog-mediated mechanism that facilitates osteogenic cell fates of PTHrP+ skeletal stem cells.

Growth plate resting zone chondrocytes acquire transient clonal competency upon Hedgehog activation and efficiently transform into trabecular bone osteoblasts.

The resting zone of the postnatal growth plate is organized by slow-cycling chondrocytes expressing parathyroid hormone-related protein (PTHrP), which include a subgroup of skeletal stem cells that contribute to the formation of columnar chondrocytes. The PTHrP-indian hedgehog (Ihh) feedback regulation is essential for sustaining growth plate activities; however, molecular mechanisms regulating cell fates of PTHrP + resting chondrocytes and their eventual transformation into osteoblasts remain largely undefined. Here, in a mouse model, we utilized a tamoxifen-inducible PTHrP-creER line with Patched-1 ( Ptch1 ) floxed and tdTomato reporter alleles to specifically activate Hedgehog signaling in PTHrP + resting chondrocytes and trace the fate of their descendants. Hedgehog-activated PTHrP + chondrocytes formed large concentric clonally expanded cell populations within the resting zone (' patched roses ') and generated significantly wider columns of chondrocytes, resulting in hyperplasia of the growth plate. Interestingly, Hedgehog-activated PTHrP + cell-descendants migrated away from the growth plate and eventually transformed into trabecular osteoblasts in the diaphyseal marrow space in the long term. Therefore, Hedgehog activation drives resting zone chondrocytes into transit-amplifying states as proliferating chondrocytes and eventually converts these cells into osteoblasts, unraveling a novel Hedgehog-mediated mechanism that facilitates osteogenic cell fates of PTHrP + skeletal stem cells.

The fate of early perichondrial cells in developing bones.

In endochondral bone development, bone-forming osteoblasts and bone marrow stromal cells have dual origins in the fetal cartilage and its surrounding perichondrium. However, how early perichondrial cells distinctively contribute to developing bones remain unidentified. Here we show using in vivo cell-lineage analyses that Dlx5+ fetal perichondrial cells marked by Dlx5-creER do not generate cartilage but sustainably contribute to cortical bone and marrow stromal compartments in a manner complementary to fetal chondrocyte derivatives under the regulation of Hedgehog signaling. Postnatally, Dlx5+ fetal perichondrial cell derivatives preferentially populate the diaphyseal marrow stroma with a dormant adipocyte-biased state and are refractory to parathyroid hormone-induced bone anabolism. Therefore, early perichondrial cells of the fetal cartilage are destined to become an adipogenic subset of stromal cells in postnatal diaphyseal bone marrow, supporting the theory that the adult bone marrow stromal compartments are developmentally prescribed within the two distinct cells-of-origins of the fetal bone anlage.

Hypoxia-inducible factor 1α induces osteo/odontoblast differentiation of human dental pulp stem cells via Wnt/ß-catenin transcriptional cofactor BCL9.

Accelerated dental pulp mineralization is a common complication in avulsed/luxated teeth, although the mechanisms underlying this remain unclear. We hypothesized that hypoxia due to vascular severance may induce osteo/odontoblast differentiation of dental pulp stem cells (DPSCs). This study examined the role of B-cell CLL/lymphoma 9 (BCL9), which is downstream of hypoxia-inducible factor 1α (HIF1α) and a Wnt/ß-catenin transcriptional cofactor, in the osteo/odontoblastic differentiation of human DPSCs (hDPSCs) under hypoxic conditions. hDPSCs were isolated from extracted healthy wisdom teeth. Hypoxic conditions and HIF1α overexpression induced significant upregulation of mRNAs for osteo/odontoblast markers (RUNX2, ALP, OC), BCL9, and Wnt/ß-catenin signaling target genes (AXIN2, TCF1) in hDPSCs. Overexpression and suppression of BCL9 in hDPSCs up- and downregulated, respectively, the mRNAs for AXIN2, TCF1, and the osteo/odontoblast markers. Hypoxic-cultured mouse pulp tissue explants showed the promotion of HIF1α, BCL9, and ß-catenin expression and BCL9-ß-catenin co-localization. In addition, BCL9 formed a complex with ß-catenin in hDPSCs in vitro. This study demonstrated that hypoxia/HIF1α-induced osteo/odontoblast differentiation of hDPSCs was partially dependent on Wnt/ß-catenin signaling, where BCL9 acted as a key mediator between HIF1α and Wnt/ß-catenin signaling. These findings may reveal part of the mechanisms of dental pulp mineralization after traumatic dental injury.

HIF1α inhibits LPS-mediated induction of IL-6 synthesis via SOCS3-dependent CEBPß suppression in human dental pulp cells.

Hypoxia-inducible factor 1 alpha (HIF1α) is a transcriptional factor that plays a key role in the regulation of various molecules expressed in hypoxic conditions. Ischemic/hypoxic conditions are regarded as a distinct characteristic of dental pulp inflammation due to the encasement of pulp tissue within the rigid tooth structure. This study was performed to examine the role of HIF1α in the regulation of interleukin (IL)-6, a proinflammatory cytokine expressed in inflamed dental pulp, in lipopolysaccharide (LPS)-stimulated human dental pulp cells (hDPCs). LPS stimulation promoted the expression of IL-6 in hDPCs, while HIF1α suppressed the expression of IL-6. Moreover, HIF1α induced suppressor of cytokine signaling 3 (SOCS3) expression in LPS-stimulated hDPCs, and SOCS3 activity led to downregulate expression of CCAAT enhancer-binding protein beta (CEBPß), an inducer of IL-6. LPS stimulation promoted HIF1α expression in hDPCs and mouse pulp tissue explants cultured under hypoxic conditions. These findings suggest that HIF1α negatively regulates IL-6 synthesis in LPS-stimulated hDPCs via upregulation of SOCS3 and subsequent downregulation of CEBPß.