Interest in the PGE EP pathway
Interest in the PGE2/EP4 pathway is increasing given its diverse capability of regulating central nervous system activity . EP4 has a protective function by reducing cerebral injury and improving functional outcome after stroke , and in suppressing SMER 3 molecular inflammation . The EP4 receptor has been suggested to contribute to PGE2-induced changes in body temperature . Furthermore, EP4 activation can decrease the level of amyloid-beta in the brain and improve behavioral performance in a murine model of Alzheimer\'s disease . The EP4 receptor along with EP3C (EP3γ) mediates PGE2-induced sensitization of sensory neurons . PGE2-prolonged sensitization of nociceptive dorsal root ganglion neurons may also contribute to the transition from acute to chronic pain by facilitating EP4 receptor synthesis and anterograde axonal trafficking . We have previously found that the PGE2/EP4 pathway plays an inhibitory role in regulating the intracellular calcium homeostasis in mouse neuroblastoma (Neuro-2a) cells via PI3K mechanism . Expression of the EP4 receptor is higher during early neurogenesis as compared to later embryonic stages in mouse embryos suggesting its importance in the developing nervous system .
Recent research shows that the subcellular trafficking of the EP4 receptor may have functional implications. It has been shown that PGE2-induced EP4 externalization to the plasma membrane in dorsal root ganglion neurons is important for the inflammatory pain response . The goal of this study was to determine whether PGE2 can also induce EP4 receptor trafficking in neuroectodermal (NE-4C) stem cells used as an experimental model system for early neuronal development. We show that PGE2 causes translocation of the EP4 receptor from its normal location in the Golgi apparatus  to the plasma membrane in undifferentiated NE-4C stem cells. This was confirmed with a specific EP4 receptor agonist. We also show for the first time that PGE2 can enhance trafficking of the EP4 receptor to growth cones of differentiated neuronal NE-4C cells. This study shows that PGE2 can influence the subcellular localization of the EP4 receptor in neuronal stem cells and differentiated neuronal cells.
Discussion The results of this study show that 10µM PGE2 induces EP4 externalization from its previously characterized localization in the Golgi apparatus  to the plasma membrane in undifferentiated NE-4C stem cells. 1µM PGE2 did not have a significant effect on undifferentiated NE-4C stem cells, indicating that EP4-translocation may be PGE2 concentration-dependent. Interestingly, we also found that in differentiated NE-4C neuronal cells, 1 or 10µM PGE2 enhanced EP4 trafficking to the growth cones. The observed subcellular translocation of the EP4 receptor from its normal location in the Golgi apparatus to the plasma membrane and growth cones indicates that it may play an important role in early function of neuronal cells. The functional importance of PGE2-induced subcellular localization of the EP4 receptor in the developing nervous system is still largely unknown. Whether EP4 signaling in various subcellular compartments of NE-4C cells is regulated by Gs and/or Gi proteins still needs further investigation. PGE2-mediated EP4 activation of either Gs or Gi may result in increased or decreased cAMP-PKA signaling respectively . EP4 activity may also stimulate cAMP-independent signaling through PI3K activation . However, there is growing evidence for the functional importance of the PGE2/EP4 signaling in the developing nervous system. The PGE2/EP4 pathway has been suggested to play a role in the transition from acute to chronic pain in nociceptive dorsal root ganglion neurons . St. Jacques and Ma found that PGE2-prolonged sensitization of neurons facilitated the synthesis and anterograde axonal trafficking of EP4 receptors . Our previous study shows that EP4 is involved in PGE2-dependent regulation of intracellular calcium level through a novel PI3K inhibitory mechanism and it also reduces neurite lengths in differentiated Neuro-2a cells . This is interesting because calcium ions are key mediators to multiple cellular processes in early neuronal development. For example, in neuronal growth cones calcium contributes to axonal growth and guidance, which must be strictly regulated during neuronal development , , . Interestingly, we also showed that the level of the EP4 receptor is higher in the mouse embryo (embryonic stage 7, 11 and 15) as compared to the later stage E17 , indicating its important role in early development.