Brain-derived neurotrophic factor and endocannabinoid functions in gabaergic interneuron development

Abstract

Formation of the cerebral cortex is controlled by tightly regulated processes: the proliferation and survival decisions of neural progenitors, radial and tangential migration of neuronal precursors, axon guidance and dendritic growth underpinning the formation of synapses. Patterning of the neocortex has been extensively studied and significant progress has been made in understanding the principles of cortical neuronal circuit formation. However, the cellular and molecular factors that coordinate the establishment of specialized neuronal subnetworks remain unclear.

In this thesis the roles of brain-derived neurotrophic factor (BDNF) and endocannabinoids, critical regulators of synaptic plasticity, were studied for their functions in corticogenesis. We showed that BDNF, along with membrane depolarization, is critical for fast-spiking interneurons (FS cells) to establish functional inhibitory microcircuits. In enriched FS cell cultures, BDNF promoted interneuron differentiation by increasing the somatic diameter, neurite outgrowth and branching, and the frequency of action potential firing. BDNF treatment led to a significant up-regulation of synaptophysin and vesicular GABA transporter expressions reflecting the accelerated maturation of functional synapses. Next, we addressed the role of BDNF and its receptor TrkB in the differentiation of GABAergic interneurons in the main olfactory bulb in vivo. We used mice lacking BDNF, mice carrying neurotrophin-3 (NT3) in the place of BDNF, and TrkB signaling mutant mice with a receptor that exclusively activates phospholipase Cgamma (PLCgamma). The absence of BDNF resulted in a compressed olfactory bulb. A significant loss of parvalbumin (PV) immunoreactive GABAergic interneurons in the external plexiform layer was dependent on the recruitment of the adaptor proteins Shc/Frs2 to the TrkB receptor. In contrast, PLCgamma signaling was sufficient for dendrite growth.

Since in utero exposure to delta9 -tetrahydrocannabinol, the active component from marijuana, induces cognitive deficits, we asked whether endocannabinoids can alter the BDNF-dependent maturation of cortical GABAergic interneurons. We identified endocannabinoids as chemoattractants for migrating GABAergic interneurons with their effect mediated by Src-dependent TrkB receptor transactivation. Simultaneously, endocannabinoids suppressed BDNF-stimulated neurite outgrowth. Based on the findings that interneurons arbors were affected by endocannabinoids, we studied the mechanisms involved in endocannabinoid-regulated axonal growth and guidance. We identified a selective enrichment of CB 1Rs in isolated growth cone particles. Agonist stimulation induced CB 1R trafficking in motile growth cones and activated the Erk1/2 pathway. Endocannabinoids were identified as repellant cues for cultured rodent GABAergic interneurons through the activation of the small GTPase RhoA. Similarly, endocannabinoids diminished galvanotropism of Xenopus laevis spinal neurons. These results are consistent with the increased density of inhibitory afferents in the neocortex in mice lacking CB 1Rs in forebrain GABAergic interneurons.

Overall, this thesis provides compelling data showing that the antagonism of BDNF and endocannabinoid signaling during the late embryonic and perinatal periods of cortical development is essential for the establishment of the cortical microarchitecture.