In addition to reprogramming fibroblasts into chondrocytes a
In addition to reprogramming fibroblasts into chondrocytes, a recent study also demonstrated a direct conversion of human placental e1 ubiquitin to chondrocytes by a different set of transcription factors (Ishii et al., 2012). Whether osteo-chondrogenic cells could also be formed during this conversion remains to be determined. Nevertheless, we report here the use of defined factors to generate osteo-chondrogenic cells with higher bone-forming potency from skin fibroblasts. This study has paved the way for the generation of patient-specific osteo-chondrogenic cells as a possible new cell source for osteoblasts for the treatment of bone defects.
Acknowledgments We thank Noriyuki Tsumaki for providing dox-inducible lentiviral vectors, Allan Bradley for Plat-E cells, and Shinya Yamanaka for pMX retroviral vectors. We also thank members of K.S.E.C. and D.C. laboratories for their technical advice and assistance; technical staff in the Faculty Core Facility for assistance with cell sorting; and J.A.I. Liu for assistance with figure preparation. This study was supported by grants from the Research Grants Council and University Grants Council of Hong Kong (AoE/M-04/04, X_HKU708/14, GRF_17110715, and GRF_17123016 to Y.W. and M.C., and AoE/M-04/04, T12-708/12-N to K.S.E.C.)
Introduction The organ size, determined by the number and size of cells in the organ, is regulated by genetic and environmental factors (Csibi and Blenis, 2012; Penzo-Mendez and Stanger, 2015). The size of bones determines the height, build, and stability of the skeleton, while the quality of bones, defined by bone matrix and mineral contents and microstructures, determines the bone strength (Whiting et al., 2004). It is estimated that peak bone mass accounts for 60% of the risk of osteoporosis (Baxter-Jones et al., 2011). The maximal bone size, mass, and strength are reached in humans at the age of 23–24 years and in mice at 10–12 weeks (Bonjour and Chevalley, 2014; Farr and Khosla, 2015). Bone growth and remodeling are carried out by coordinated osteoblastic bone formation and osteoclastic bone resorption (Henriksen et al., 2014; Karsenty et al., 2009). Osteoblasts (OBs) are derived from bone marrow-mesenchymal stromal cells (BM-MSCs), which can self-renew and differentiate into OBs, chondrocytes, or adipocytes, whereas osteoclasts (OCs) are derived from monocytes (Edwards and Mundy, 2011; Harada and Rodan, 2003; Lacey et al., 2012). Proliferation of stem cells and progenitors increases the pool of OBs or OCs, while proper maturation determines the activities of these cells. Bone size, mass, and quality can be regulated by the growth hormone-insulin growth factor (IGF) axis, nutrition, steroid hormones, vitamin D, and mechanical stimuli (Bonjour and Chevalley, 2014; Hendrickx et al., 2015; Rosello-Diez and Joyner, 2015; Zemel, 2013). It is generally believed that IGFs promote OB and chondrocyte proliferation and differentiation, which mediate the anabolic effects of IGFs on bone growth (Callewaert et al., 2010; Canalis, 2009; Yakar et al., 2010). The mammalian target of rapamycin (mTOR) pathway is the sensor of growth factors, including IGF1, macrophage colony-stimulating factor (M-CSF), and nutrients (Hay and Sonenberg, 2004; Laplante and Sabatini, 2012), and is expected to play critical roles in bone growth spurt during adolescence. Activated mTOR increases global protein synthesis to promote cell proliferation (Jewell et al., 2013; Johnson et al., 2013). In humans, tuberous sclerosis complex (TSC) patients carry mutations in one allele of TSC1 or TSC2, encoding negative regulators of mTOR, and they develop benign tumors in multiple organs due to loss of heterozygosity of the TSC genes (Laplante and Sabatini, 2012). In addition, TSC patients develop focal sclerotic bone lesions (Avila et al., 2010; Li et al., 2015; Rafal et al., 2013; Umeoka et al., 2008). In mouse, mTor ablation or mTOR activation by ablation of Tsc1 or Tsc2 causes embryonic or neonatal lethality (Laplante and Sabatini, 2012). Studies of mice with Tsc1 or Tsc2 deleted in committed OBs or chondrocytes established roles for mTOR signaling in bone mass accrual and endochondral ossification (Huang et al., 2015; Riddle et al., 2014), with the cartilage studies being limited to embryos or newborns due to survival problems of the mice (Chen and Long, 2014; Yan et al., 2016). Yet, the roles for mTOR signaling in bone size and quality control during adolescent growth, especially from the angle of the stem cells of the skeleton, remain largely unknown.