Hedgehog Signaling Controls Bone Homeostasis by Regulating Osteogenic/Adipogenic Fate of Skeletal Stem/Progenitor Cells in Mice

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.

Suppressor of fused-restrained Hedgehog signaling in chondrocytes is critical for epiphyseal growth plate maintenance and limb elongation in juvenile mice

“…These two Hh ligands might activate Hh signaling in the growth plate chondrocytes of juvenile mice. In agreement with this prediction, activity of Hh signaling was indeed detected in the growth plate chondrocytes in juvenile and adult mice (Shi et al, 2017;Haraguchi et al, 2018;Newton et al, 2019;Zhang et al, 2022), implying a direct requirement of Hh signaling in chondrocytes. Consistently, pharmacological inhibition of Hh signaling during or after SOC formation led to premature closure of long bone growth plates (Kimura et al, 2008), confirming the functional significance of Hh signaling in epiphyseal growth plate maintenance during juvenile growth.…”

“…We previously showed that Hh signaling was activated in the chondrocytes of epiphyseal growth plates from 2-and 12-monthold mice (Zhang et al, 2022). To further characterize Hhresponsive cells in the epiphyseal growth plates during postnatal skeletal growth, we first examined the expression of Gli1, a transcriptional target of Hh signaling activity in juvenile and adult mice.…”

“…Paraffin or frozen sections of long bones were prepared as we previously described (Zhang et al, 2022). Hematoxylin and eosin (H&E) staining was performed following the standard protocol.…”

“…In contrast, a recent study showed that modulating Hh signaling by conditionally ablating either Smo or Ptch1 in Col10a1-expressing hypertrophic chondrocytes did not impair growth plate cartilage (Wang et al, 2022), suggesting that Hh signaling might mainly function in immature chondrocytes. To further explore this possibility in our hand, we utilized Osx-Cre to ablate Smo or Sufu in prehypertrophic/hypertrophic chondrocytes, since we have previously showed that Osx-Cre can efficiently mediate recombination in these cells in postnatal growing mice (Zhang et al, 2022). In line with the finding by Wang et al, histological analyses of tibiae from 4-month-old Osx-Cre; Smo f/f (Smo Osx ) and their littermate controls revealed that Osx-Cre-mediated ablation of Smo did not overtly affect tibial growth plates (Figures 8A-D).…”

“…Indian hedgehog (Ihh) signaling plays multiple critical roles in the regulation of chondrocyte proliferation and differentiation during epiphyseal cartilage development at both embryonic and neonatal stages (St-Jacques et al, 1999;Long et al, 2001;Razzaque et al, 2005;Maeda et al, 2007;Amano et al, 2015;Ohba, 2020). Interestingly, Ihh continues to be expressed in prehypertrophic chondrocytes of the growth plates and some osteoblast-like cells at the chondro-osseous junction in juvenile mice (Maeda et al, 2007;Zhang et al, 2022). In addition, Sonic hedgehog (Shh) molecules are expressed by several types of cells residing in the SOC (Newton et al, 2019).…”