Figure 6From: All-trans retinoic acid promotes neural lineage entry by pluripotent embryonic stem cells via multiple pathwaysA. Interaction between RA and the ERK pathway during promoted neural specification. FGF receptor tyrosine kinase inhibitor SU5402 and MEK1/2 inhibitors PD184352 and PD0325901, respectively suppresses neural differentiation of ESCs. In RA-treated cultures, PD184352, PD0325901 and RARα-selective antagonist Ro 415253 could inhibit the neural specification of ESCs, while SU5402 shows weak neural inhibition effect. RA probably enhances ERK phosphorylation via MEK1/2 or its upstream but not in a manner that is dependent on FGF signaling, and activated ERK1/2 mediates phosphorylation of the MAPK sites of Smad1 (pSmad1MAPK), which inhibits the inhibitory function of the bone morphogenetic proteins (BMPs)-Smad1 pathway on neural differentiation. Blockade of RA signaling might affect the activation of the upstream of ERK1/2, which leads to failure of neural specification of ESCs even with the stimulation of FGF signaling. B. Possible roles of Wnt signaling in RA-promoted neural differentiation. RA promotes neural specification probably by up-regulating Sfrp2, which inhibits the Wnt-β-catenin anti-neural pathway. Upon adding LiCl or CHIR99021, which stimulates the β-catenin signaling and/or BMP signaling through inhibition of GSK3, the suppression effects on neural specification of β-catenin pathway can be passed down even with the stimuli of RA. RA also may promote neural differentiation by up-regulating RAR, which may decrease β-catenin/LEF/TCF mediated transactivation. There exists a possibility that RA may suppress the inhibition effects on neural differentiation of Wnt-β-catenin pathway by sequestration of β-catenin. pSmad1GSK3 indicates phosphorylation of the GSK3 sites of Smad1.Back to article page