TrkC-T1, the Non-Catalytic Isoform of TrkC, Governs Neocortical Progenitor Fate Specification by Inhibition of MAP Kinase Signaling.

Neocortical projection neurons are generated by neural progenitor cells (NPCs) within the ventricular and subventricular zone. While early NPCs can give rise to both deep and upper layer neurons, late progenitors are restricted to upper layer neurogenesis. The molecular mechanisms controlling the differentiation potential of early versus late NPCs are unknown. Here, we report a novel function for TrkC-T1, the non-catalytic isoform of the neurotrophin receptor TrkC, that is distinct from TrkC-TK+, the full-length isoform. We provide direct evidence that TrkC-T1 regulates the switch in NPC fate from deep to upper layer neuron production. Elevated levels of TrkC-T1 in early NPCs promote the generation of deep layer neurons. Conversely, downregulation of TrkC-T1 in these cells promotes upper layer neuron fate. Furthermore, we show that TrkC-T1 exerts this control by interaction with the signaling adaptor protein ShcA. TrkC-T1 prevents the phosphorylation of Shc and the downstream activation of the MAP kinase (Erk1/2) pathway. In vivo manipulation of the activity of ShcA or Erk1/2, directly affects cortical neuron cell fate. We thus show that the generation of upper layer neurons by late progenitors is dependent on the downregulation of TrkC-T1 in late progenitor cells and the resulting activation of the ShcA/Erk1/2 pathway.

Bcl11a (Ctip1) Controls Migration of Cortical Projection Neurons through Regulation of Sema3c.

During neocortical development, neurons undergo polarization, oriented migration, and layer-type-specific differentiation. The transcriptional programs underlying these processes are not completely understood. Here, we show that the transcription factor Bcl11a regulates polarity and migration of upper layer neurons. Bcl11a-deficient late-born neurons fail to correctly switch from multipolar to bipolar morphology, resulting in impaired radial migration. We show that the expression of Sema3c is increased in migrating Bcl11a-deficient neurons and that Bcl11a is a direct negative regulator of Sema3c transcription. In vivo gain-of-function and rescue experiments demonstrate that Sema3c is a major downstream effector of Bcl11a required for the cell polarity switch and for the migration of upper layer neurons. Our data uncover a novel Bcl11a/Sema3c-dependent regulatory pathway used by migrating cortical neurons.

Sip1 downstream Effector ninein controls neocortical axonal growth, ipsilateral branching, and microtubule growth and stability.

Sip1 is an important transcription factor that regulates several aspects of CNS development. Mutations in the human SIP1 gene have been implicated in Mowat-Wilson syndrome (MWS), characterized by severe mental retardation and agenesis of the corpus callosum. In this study we have shown that Sip1 is essential for the formation of intracortical, intercortical, and cortico-subcortical connections in the murine forebrain. Sip1 deletion from all postmitotic neurons in the neocortex results in lack of corpus callosum, anterior commissure, and corticospinal tract formation. Mosaic deletion of Sip1 in the neocortex reveals defects in axonal growth and in ipsilateral intracortical-collateral formation. Sip1 mediates these effects through its direct downstream effector ninein, a microtubule binding protein. Ninein in turn influences the rate of axonal growth and branching by affecting microtubule stability and dynamics.

miR-128 regulates neuronal migration, outgrowth and intrinsic excitability via the intellectual disability gene Phf6.

miR-128, a brain-enriched microRNA, has been implicated in the control of neurogenesis and synaptogenesis but its potential roles in intervening processes have not been addressed. We show that post-transcriptional mechanisms restrict miR-128 accumulation to post-mitotic neurons during mouse corticogenesis and in adult stem cell niches. Whereas premature miR-128 expression in progenitors for upper layer neurons leads to impaired neuronal migration and inappropriate branching, sponge-mediated inhibition results in overmigration. Within the upper layers, premature miR-128 expression reduces the complexity of dendritic arborization, associated with altered electrophysiological properties. We show that Phf6, a gene mutated in the cognitive disorder Börjeson-Forssman-Lehmann syndrome, is an important regulatory target for miR-128. Restoring PHF6 expression counteracts the deleterious effect of miR-128 on neuronal migration, outgrowth and intrinsic physiological properties. Our results place miR-128 upstream of PHF6 in a pathway vital for cortical lamination as well as for the development of neuronal morphology and intrinsic excitability.

Ntf3 acts downstream of Sip1 in cortical postmitotic neurons to control progenitor cell fate through feedback signaling.

Cortical progenitors undergo progressive fate restriction, thereby sequentially producing the different layers of the neocortex. However, how these progenitors precisely change their fate remains highly debatable. We have previously shown the existence of cortical feedback mechanisms wherein postmitotic neurons signal back to the progenitors and promote a switch from neurogenesis to gliogenesis. We showed that Sip1 (Zeb2), a transcriptional repressor, controls this feedback signaling. A similar mechanism was also suggested to control neuronal cell type specification; however, the underlying mechanism was not identified. Here, we provide direct evidence that in the developing mouse neocortex, Ntf3, a Sip1 target neurotrophin, acts as a feedback signal between postmitotic neurons and progenitors, promoting both apical progenitor (AP) to basal progenitor (BP) and deep layer (DL) to upper layer (UL) cell fate switches. We show that specific overexpression of Ntf3 in neocortical neurons promotes an overproduction of BP at the expense of AP. This shift is followed by a decrease in DL and an increase in UL neuronal production. Loss of Ntf3, by contrast, causes an increase in layer VI neurons but does not rescue the Sip1 mutant phenotype, implying that other parallel pathways also control the timing of progenitor cell fate switch.

Unc5C and DCC act downstream of Ctip2 and Satb2 and contribute to corpus callosum formation.

The pyramidal neurons of the mammalian neocortex form two major types of long-range connections-corticocortical and cortico-subcortical. The transcription factors Satb2 and Ctip2 are critical regulators of neuronal cell fate that control interhemispheric versus corticofugal connections respectively. Here, we investigate the axon guidance molecules downstream of Satb2 and Ctip2 that establish these connections. We show that the expression of two Netrin1 receptors- DCC and Unc5C is under direct negative regulation by Satb2 and Ctip2, respectively. Further, we show that the Netrin1-Unc5C/DCC interaction is involved in controlling the interhemispherical projection in a subset of early born, deep layer callosal neurons.