Retinoids vitamin A are important

Retinoids (vitamin A) are important for a number of physiological and developmental processes including reproduction. Retinoic acid, one of the retinoids, plays an essential role in early development and maintenance of specific tissue types. Khillan and colleagues demonstrated that retinol can be used to maintain mESCs in a pluripotent state in feeder-free cultures (Chen et al., 2007). Retinol increases the expression of NANOG and promotes self-renewal by activating PI3K/AKT and the IGF-1/IRS-1 pathway (Chen and Khillan, 2010; Chen et al., 2007). Wang et al. (2011) showed that retinoic fak pathway receptor gamma, liver receptor homolog 1, and retinoic acid analogs promote faster reprogramming of mouse and human somatic cells. Vaajasaari (2009) and Rajala et al. (2011) have also reported that retinol increases the expression of OCT4 and NANOG in HuESCs. These results show that retinol and some of its nonoxidized derivatives can have a significant role in the self-renewal of pluripotent stem cells. mESCs in media with serum or serum-replacements have very few lipid bodies and low levels of blue fluorescence. mESCs do not take up retinol or retinyl palmitate at levels seen in primed cells, although the retinol typically present at ∼200 nM or less is actively sequestered by HPSCs. Prolonged incubation of mESCs with retinyl palmitate did not increase blue fluorescence, whereas retinol caused a slight increase. Examination of available transcriptomes of mouse and HuESCs indicate significant differences in transcripts related to retinol uptake and metabolism. RBP, STRA6, LRAT that encode proteins that bind, transport, and convert retinol to its ester, are expressed many fold more in human pluripotent cells than in mESCs (http://amazonia.transcriptome.eu, U133A [Enver et al., 2005]; NCBI GEO accession number GSM87830, MoES.C57B). STRA6 is a G protein-coupled receptor and suggested to induce lipogenesis when activated by retinol (Muenzner et al., 2013). This may explain the presence of fluorescent lipid bodies only in epiblast-like or primed pluripotent human cells, and why HuESCs in E8 media lack lipid bodies and when supplemented with retinol acquire them. STRA6 mutations in humans also have varied and strong phenotypes (Chassaing et al., 2009). Retinol could play a significant role in pluripotency and it may be useful to have retinol/retinyl esters in the media. We considered the possibility that lipid bodies are only present in primed or epiblast-like cells. Examination of preimplantation (3.5 dpc) and postimplantation (6.5 dpc) mouse embryos showed the presence of blue fluorescent lipid bodies only in the latter. Retinol/retinoic acid are involved at this developmental stage (Huang et al., 2001). Mouse epiblast-like stem cells derived from postimplantation embryos also led to colonies similar to HuESC colonies, with fluorescent lipid bodies. In media that shift human pluripotent stem cells to the naive state or mESCs to the primed state, the cells show the appropriate shift in blue fluorescent lipid bodies. These results suggest lipid bodies that sequester retinoids to mark the epiblast-like state provide opportunities to explore their functional roles and can distinguish naive from primed pluripotent stem cells. Lipid bodies are also associated with specific metabolic states, and may mark a primed metabolic state because changing the media dramatically alters their levels. The role of lipid bodies, in cellular physiology and development, is becoming increasingly evident. It may have a role to play in primed pluripotent cells. In most somatic cells, lipid bodies, usually present in small numbers, are heterogeneous in composition within and between cell types (Ducharme and Bickel, 2008; Li et al., 2012; Walther and Farese, 2012). They consist of mostly neutral lipids associated with specific proteins that vary between cell types. Cells with lipid bodies that sequester retinyl esters are uncommon. Retinal pigmented epithelium and hepatic stellate cells are two examples. Lipid bodies are also sites of lipogenesis, lipid storage, and synthesis of lipid signaling molecules, such as arachadonic acid, which has been implicated in the maintenance of pluripotency. Recent studies have implicated retinol to activate the JAK/STAT pathway and to regulate lipid accumulation via SOCS3 and PPARγ (Berry and Noy, 2012; Berry et al., 2012). Electron micrographs of primate pluripotent stem cells portray structures that could be lipid bodies but were assumed to be lysosomes or glycogen granules (Johkura et al., 2004). Lipid bodies, although reported in primate pluripotent stem cells, were not associated with epiblast stem cells (Cibelli et al., 2002; Stringari et al., 2012). Oleate, a lipid, has also been related to pluripotency (Ben-David et al., 2013). E8 media does not contain any lipid or retinoids and HPSCs in E8 remain pluripotent, so retinoids are not essential for pluripotency. The lipids necessary for cell growth are synthesized endogenously. HPSCs grown in E8 with retinol acquire lipid bodies in 24–48 hr, indicating their ability to acquire these bodies. It is currently not clear what advantages retinol provides in the propagation of pluripotent stem cells other than the increased expression of NANOG and OCT4 in HuESCs (Rajala et al., 2011).