The role of fascin and drebrin in neuroblast migration in the postnatal brain

After birth, stem cells in the subventricular zone (SVZ) generate neuroblasts that migrate along he rostral migratory stream (RMS) to become inerneurons in the olfactory bulb (OB). This migration is a fundamental event controlling the proper integration of new neurons in a pre-existing synaptic network. Many regulators of neuroblast migration have been identified; however, still very little is known about the intracellular molecular mechanisms controlling this process. We have investigated the function of fascin and drebin, two actin-binding proteins highly expressed by RMS neuroblasts in the postnatal mammalian brain. We show that fascin-1 ko mice display an abnormal RMS and a smaller OB. Bromodeoxyuridine labeling experiments show that lasck of fascin significantly impairs neuroblast migration, but does not affect cell proliferation. Moreover, fascin depletion alters the polarized morphology of rat neuroblasts. Protein kinase C (PKC)-dependent posphoylation of fascin on Ser39 regulates its actin-bundling function. Postnatal electroporation of phosphomimetic (S39D) or nonphosphorylatable (S39A) fascin variants followed by time-lapse imaging microscopy (FLIM) studies in rat neuroblasts reveal that the interaction between fascin and PKC can be modulated by cannabinoid signaling, which controls neuroblast migration in vivo. These findings identify fascin as a crucial regulator of neuroblast motility. We propose that a tightly regulated phospho/dephosphofascin cycle modulated by extracellular signals is required for the polarized migration of stem cell-derived neuroblasts. Depletion of drebrin using different RNAi approaches affects neuroblast morphology and impairs neuroblast migration both in vitro and in vivo. Drebrin phosphorylation on Ser142 by Cdk5 promotes actin bundling and microtubule binding. Electroporation of phosphomimetic (S142D) or non-phosphorylatable (S142A) drebrin followed by time-lapse imaging shows decreased neuroblast migration compared to control. Our findings demonstrate that drebrin is necessary for efficient neuroblast migration and suggest that its phosphorylation on Ser142 plays an important role in regulating neuroblast orientation along the RMS.