Study of the Mechanisms Underlying Hippocampal Neuron Synaptogenesis: The Roles of Neurotrophin Signaling and Micrornas

Synapse formation requires contacts between dendrites and axons. Although this process is often viewed as axon mediated, dendritic filopodia may be actively involved in mediating synaptogenic contacts. Brain-derived neurotrophic factor (BDNF) increases the density of dendritic filopodia and the conditional deletion of tyrosine receptor kinase B (TrkB) reduces synapse density in vivo (Luikart et al., 2005). Here, we report that TrkB associates with dendritic growth cones and filopodia, mediates filopodial motility, and does so via the phosphoinositide 3 kinase (PI3K) pathway. We used genetic and pharmacological manipulations of mouse hippocampal neurons to assess signaling downstream of TrkB. Conditional knock-out of two downstream negative regulators of TrkB signaling, Pten (phosphatase with tensin homolog) and Nf1 (neurofibromatosis type 1), enhanced filopodial motility. This effect was PI3K-dependent and correlated with synapse density. Phosphatidylinositol 3,4,5- trisphosphate (PIP3) was preferentially localized in filopodia and this distribution was enhanced by BDNF application. Thus, intracellular control of filopodial dynamics converged on PI3K activation and PIP3 accumulation, a cellular paradigm conserved for chemotaxis in other cell types. Our results suggest that filopodial movement is not random, but responsive to synaptic guidance cues. In order to further elucidate the mechanisms of BDNF-TrkB-PI3K pathway downstream signaling involved in regulating dendritic filopodial motility, we used a pharmacological approach as well as a gene expression approach to show that Rac1 and RhoA may play a role in this pathway. Rac1 positively regulated dendritic filopodial motility while RhoA had a negative effect. Our data suggest that BDNF-TrkB signaling might function to regulate the balance between Rac1 and RhoA, thus controlling dendritic filopodial motility. The developing nervous system is shaped by highly orchestrated programs of gene expression. This tight regulation is regulated by various transcriptional and post-transcriptional events that control individual gene expression. The recent discovery of small, non-coding RNAs has greatly expanded our understanding of the mechanisms that regulate gene expression at the post-transcriptional level. Here, I characterized the expression pattern of one neuronal microRNA, miR-381, and used in vitro cultured hippocampal neurons to show that miR-381 regulates neurite growth, as overexpression of miR-381 promotes neuronal dendritic branching. The effect of miR-381 on neuronal dendritic branching might be through a net regulation of multiple target genes.