Figure 3: Possible signaling pathways of synaptic transmitters to activate the progression of NG2 cell cycle. Extracellular signals through different pathways, such as VDCCs, activation of GluRs, ligand-gated ion channels, and G-protein-coupled receptors, can finally affect the activities of CAKs or CKIs. CAKs or CKIs then act on CDKs or cyclins to determine NG2 cell cycle progression or exit. Other pathways are not excluded. AC: adenylate cyclase; CAKs: cyclin-dependent kinase activating kinases; CaM-K: calmodulin kinase; CKIs: cyclin-dependent kinases inhibitors; DAG: diacylglycerol; ERK/MAPK: extracellular signal-regulated kinase/mitogen-activated protein kinase; PKC: protein kinase C; PLC: phospholipase C.