differentiation

Hedgehog signaling controls bone homeostasis by regulating osteogenic/adipogenic fate of skeletal stem/progenitor cells in mice

AUTHORS

Liwei Zhang, Xuejie Fu, Li Ni, Cunchang Liu, Yixin Zheng, Hongji You, Meng Li, Chunmei Xiu, Lei Zhang, Tingting Gong, Na Luo, Zunyi Zhang, Guangxu He, Shijun Hu, Huilin Yang, Di Chen, Jianquan Chen

ABSTRACT

Skeletal stem/progenitor cells (SSPCs) can differentiate into osteogenic or adipogenic lineage. The mechanism governing lineage allocation of SSPCs is still not completely understood. Hedgehog (Hh) signaling plays an essential role in specifying osteogenic fate of mesenchymal progenitors during embryogenesis. However, it is still unclear whether Hh signaling is required for lineage allocation of SSPCs in postnatal skeleton, and whether its dysregulation is related to age-related osteoporosis. Here, we demonstrated that Hh signaling was activated in metaphyseal SSPCs during osteogenic differentiation in the adult skeleton, and its activity decreased with aging. Inactivation of Hh signaling by genetic ablation of Smo, a key molecule in Hh signaling, in Osx-Cre-targeted SSPCs and hypertrophic chondrocytes led to decreased bone formation and increased bone marrow adiposity, two key pathological features of age-related osteoporosis. Moreover, we found that the bone-fat imbalance phenotype caused by Smo deletion mainly resulted from aberrant allocation of SSPCs toward adipogenic lineage at the expense of osteogenic differentiation, but not due to accelerated transdifferentiation of chondrocytes into adipocytes. Mechanistically, we found that Hh signaling regulated osteoblast versus adipocyte fate of SSPCs partly through upregulating Wnt signaling. Thus, our results indicate that Hh signaling regulates bone homeostasis and age-related osteoporosis by acting as a critical switch of cell fate decisions of Osx-Cre-targeted SSPCs in mice and suggest that Hh signaling may serve as a potential therapeutic target for the treatment of osteoporosis and other metabolic bone diseases.

Sestrin2 Regulates Osteoclastogenesis via the p62-TRAF6 Interaction

AUTHORS

Sue Young Oh, Namju Kang, Jung Yun Kang, Ki Woo Kim, Jong-Hoon Choi, Yu-Mi Yang1 and Dong Min Shin

ABSTRACT

The receptor activator of nuclear factor-kappa B ligand (RANKL) mediates osteoclast differentiation and functions by inducing Ca2+ oscillations, activating mitogen-activated protein kinases (MAPKs), and activating nuclear factor of activated T-cells type c1 (NFATc1) via the RANK and tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) interaction. Reactive oxygen species (ROS) also plays an important role during osteoclastogenesis and Sestrin2, an antioxidant, maintains cellular homeostasis upon stress injury via regulation of ROS, autophagy, and inflammation. However, the role of Sestrin2 in osteoclastogenesis remains unknown. In this study, we investigated the role of Sestrin2 in the RANKL-RANK-TRAF6 signaling pathway during osteoclast differentiation. Deletion of Sestrin2 (Sesn2) increased bone mass and reduced the number of multinucleated osteoclasts on bone surfaces. RANKL-induced osteoclast differentiation and function decreased in Sesn2 knockout (KO) bone marrow-derived monocytes/macrophages (BMMs) due to inhibition of NFATc1 expression, but osteoblastogenesis was not affected. mRNA expression of RANKL-induced specific osteoclastogenic genes and MAPK protein expression were lower in Sesn2 KO BMMs than wild-type (WT) BMMs after RANKL treatment. However, the Sesn2 deletion did not affect ROS generation or intracellular Ca2+ oscillations during osteoclastogenesis. In contrast, the interaction between TRAF6 and p62 was reduced during osteoclasts differentiation in Sesn2 KO BMMs. The reduction in the TRAF6/p62 interaction and TRAP activity in osteoclastogenesis in Sesn2 KO BMMs was recovered to the WT level upon expression of Flag-Sesn2 in Sesn2 KO BMMs. These results suggest that Sestrin2 has a novel role in bone homeostasis and osteoclasts differentiation through regulation of NFATc1 and the TRAF6/p62 interaction.

MicroRNA miR-23a cluster promotes osteocyte differentiation by regulating TGF-β signalling in osteoblasts

Osteocytes are the terminally differentiated cell type of the osteoblastic lineage and have important functions in skeletal homeostasis. Although the transcriptional regulation of osteoblast differentiation has been well characterized, the factors that regulate differentiation of osteocytes from mature osteoblasts are poorly understood. Here we show that miR-23a∼27a∼24-2 (miR-23a cluster) promotes osteocyte differentiation.

N-cadherin Regulation of Bone Growth and Homeostasis Is Osteolineage Stage–Specific

N-cadherin inhibits osteogenic cell differentiation and canonical Wnt/β-catenin signaling in vitro. However, in vivo both conditional Cdh2 ablation and overexpression in osteoblasts lead to low bone mass. We tested the hypothesis that N-cadherin has different effects on osteolineage cells depending upon their differentiation stage. Embryonic conditional osteolineage Cdh2 deletion in mice results in defective growth, low bone mass, and reduced osteoprogenitor number.

Reciprocal stabilization of ABL and TAZ regulates osteoblastogenesis through transcription factor RUNX2

Cellular identity in metazoan organisms is frequently established through lineage-specifying transcription factors, which control their own expression through transcriptional positive feedback, while antagonizing the developmental networks of competing lineages. Here, we have uncovered a distinct positive feedback loop that arises from the reciprocal stabilization of the tyrosine kinase ABL and the transcriptional coactivator TAZ.