Molecular control of production and specification of cortical upper layer neurons

Project Leader: Victor Tarabykin e-mail

 

The neocortex contains multiple types of projection neurons, which are ordered in six layers. These neurons are born from the same type of neuronal progenitors named radial glia cells. We have recently shown that young neurons can influence the fate of neuronal progenitors by a feedback mechanism so that they secrete certain factors that act on progenitors and instruct them to give rise to various types of neurons and glia. This feedback signaling plays an important role in neocortical cell fate specification. The transcription factor Sip1 (Smad Interacting Protein-1, also known as Zeb2) is a major factor controlling this process. Sip1 mutation was shown to cause Mowat-Wilson syndrome in humans. Previously we identified and characterized several downstream targets of Sip1: the transcription factors FoxO1 and Ebf1, and the secreted factors Ntf3, Sfrp1 and Cbln4. The main goal of the current funding period was to validate the Sip1 downstream pathway and identify the molecules that transmit Sip1 mediated signaling to neocortical progenitors. Using in vivo gain of function experiments, we were able to identify Cbln4 as a major mediator of the Sip1 controlled feedback. Cbln4 is a poorly characterized cytokine whose receptor is not known. In the developing neocortex, we could show that Cbln4 as well as its homologs Cbln1 and 2 can induce overproduction of upper layer neurons at the expense of deep layer neurons mimicking the phenotype of the Sip1 mutant. On the other hand, we also discovered that Sip1 controls formation of the corpus callosum, the major axonal tract interconnecting the two cortical hemispheres. We were able to show that the corpus callosum is not formed in Sip1 deficient brains due to the abnormal development of radial glia progenitors in the cortical midline.

The first goal of the next funding period is to uncover the molecular basis of Cbln4 function in feedback signaling. We will search for Cbln4 receptors on progenitor cells using two approaches: a modified yeast two hybrid system (“Split-Ubiquitin System”) and a proteomics approach. Once the receptor(s) is identified, we will study its function in the developing neocortex. We would further like to study the effect of Cbln 1,2,4 deletion on neocortical development using transgenic mouse models. The second goal of the next funding period is to uncover the molecular mechanism underlying abnormal development of radial glia progenitors in the cortical midline of Sip1 mutant mice that lead to corpus callosum agenesis. Here we will search for the molecules whose expression is changed in the midline of Sip1 mutants and by genetic rescue experiments we will attempt to repair the corpus callosum agenesis observed in these animals.