Recent studies by our group demonstrated the presence of

Recent studies by our group demonstrated the presence of stem Calcium Ionophore I in the CEP. These cartilage endplate stem cells (CESCs) exhibited superior capacity for chondrogenic and osteogenic differentiation to those of bone marrow mesenchymal stem cells (BM-MSCs) (Liu et al., 2011). This differentiation property attracted our attention because it is likely that CESCs play an important role in the restoration and regeneration of CEP, and the direction of chondrogenesis and osteogenesis in CESCs may be responsible for CEP chondrification and ossification. As an avascular tissue, IVD remains in a hypoxic microenvironment (Boskey, 2008), and the oxygen tension within CEP is as low as 1% (Lee et al., 2007). Hypoxia greatly affects the chondrogenesis and osteogenesis of MSCs (Merceron et al., 2010), which indicates that physiological hypoxia may regulate the chondro-osteogenic differentiation of CESCs to maintain a balance of chondrification and ossification in CEP. The hypoxia-inducible factor-1α subunit/macrophage migration inhibitory factor (HIF1A/MIF) pathway is one of the most important signaling pathways in response to hypoxia. HIF1A is a key cellular regulator in responding to hypoxia; MIF, which has been recognized as a downstream target of HIF1A, acts as a regulator of innate immunity and can regulate many biological activities (Fu et al., 2010; Maity and Koumenis, 2006). MIF is highly involved in cartilage metabolism. MIF knockdown in zebrafish embryos can lead to undeveloped jaw cartilage (Ito et al., 2008). Moreover, MIF was observed to be involved in the degenerative process of CEP (Xiong et al., 2014). In mouse neural progenitor cells, MIF promoted survival and maintenance by upregulating the expression of SOX6, which is also a member of the SOX family and is co-expressed with SOX9 in all chondroprogenitors, indicating that MIF may initiate chondrogenesis in progenitor cells (Ohta et al., 2013). In addition, MIF was also involved in bone metabolism (Onodera et al., 2002). Transgenic mice overexpressing MIF exhibited osteoporosis, implying that the overexpression of MIF may lead to poor osteogenesis capability (Onodera et al., 2006). Obviously, the cartilage and bone metabolism was closely related to the chondrogenic and osteogenic differentiation of stem cells; however, the impact of the HIF1A/MIF pathway upon chondro-osteogenic differentiation is rarely reported. The transcription factor SOX9 is the master regulator of chondrogenesis and is expressed in pre-chondrocytes and differentiated chondrocytes during skeletal development (Healy et al., 1996). The transcription factor RUNX2 acts as the master regulator of skeletogenesis; its expression is necessary for osteoblast differentiation and maturation (Ducy et al., 1997). In this study, we investigated how the HIF1A/MIF pathway regulated SOX9 and RUNX2, which resulted in a change in the chondro-osteogenic differentiation fate of CESCs.
Results
Discussion The CEP arises from embryonic mesoderm-derived tissue, and CESCs share characteristics related to cell-surface immunophenotype and the capacity to differentiate into mesoderm-derived cells, namely osteocytes, chondroblasts, and adipocytes (Liu et al., 2011), CESCs are thus considered as MSCs. Common sources of MSCs in studies usually exist under hypoxic conditions: bone marrow (4%–7%), adipose tissue (3.8%–9.6%), and muscle tissue (1%–10%) (D\'Ippolito et al., 2006; Redshaw and Loughna, 2012; Schiller et al., 2013). The tissue specificity of the differentiation of the MSCs in response to hypoxia can be observed. In BM-MSCs, hypoxia promoted chondrogenesis and inhibited osteogenesis and adipogenesis (Khan et al., 2010; Martin-Rendon et al., 2007; Yang et al., 2011). Adipose-derived MSCs tended to differentiate into adipocytes or chondrocytes, and the ability of osteogenic differentiation was dwarfed under hypoxia (Kim et al., 2014; Merceron et al., 2010). In muscle stem cells, low oxygen concentration favored myogenic differentiation but suppressed adipogenic differentiation (Redshaw and Loughna, 2012). The periodontal ligament stem cells promoted osteogenic differentiation under hypoxia (Zhang et al., 2014). Taken together, stem cells from different tissues show different tissue-specific differentiation fate in the physiological hypoxic microenvironment, which may facilitate the restoration and regeneration function of stem cells.