NEUROTROPHINS STIMULATE CHEMOTAXIS OF EMBRYONIC CORTICAL-NEURONS

During mammalian cortical development, neuronal precursors proliferate within ventricular regions then migrate to their target destinations in the cortical plate, where they organize into layers. In the rat, mo st cortical neuronal migration occurs during the final week of gestati on (Bayer et al., 1991; Jacobson, 1991). At this time (E15-E21), rever se transcriptase-polymerase chain reaction demonstrated that cortical homogenates contain mRNA encoding brain derived neurotrophic factor (B DNF) and the catalytic form of its high-affinity receptor, TrkB. Immun ocytochemistry and in situ hybridization of sections revealed that the catalytic TrkB receptors predominantly localize to regions containing migratory cells. Many TrkB+ cells exhibited the classic morphology of migrating neurons, suggesting that TrkB ligands play a role in cortic al neuronal migration. We analysed whether TrkB ligands influence the motility of embryonic cortical cells (from E15-E21) using a quantitati ve in vitro chemotaxis assay. High-affinity TrkB ligands (BDNF and NT4 /5) stimulated chemotaxis (directed migration) of embryonic neurons at concentrations ranging from 1 to 100 ng/ml. NT-3, a low-affinity TrkB ligand, only stimulated significant migration at high concentrations (greater than or equal to 100 ng/ml). Peak migration to BDNF was obser ved at gestational day 18 (E18). BDNF-induced chemotaxis was blocked b y either tyrosine kinase inhibitor, K252a, or the Ca2+-chelator, BAPTA -AM, suggesting that BDNF-induces motility via autophosphorylation of TrkB receptor proteins and involves Ca2+-dependent mechanisms. BDNF-st imulation of increased cytosolic Ca2+ was confirmed with optical recor dings of E18 cortical cells loaded with Ca2+ indicator dye. Thus, sign al transduction through the TrkB receptor complex directs neuronal mig ration, suggesting that, in vivo, BDNF exerts chemotropic effects that are critical to morphogenesis of the cortex.