Connecting left and right brain: wiring mechanisms and functional diversity of cross-hemispheric projections. – CONNECTBRAIN

The experimental approach used in this project is divided into three main areas. A first axis seeks to identify the guidance mechanisms allowing callosal fibers to cross the midline between the two brain hemispheres during embryonic development. This work is being pursued by a 4D analysis of the migration of axons in order to determine, in a tissue context, how receptor activation regulates axonal dynamics. The final objective is the molecular and functional characterization of the different subtypes of nerve fibers comprising the CC . This part of the work focusses on the study of a contingent of heterotopic callosal fibers that connect the cortex to the contralateral striatum, and on their involvement in the lateralization of locomotor behavior.

The first results indicate profound differences in how CC axons respond to axon guidance signal before and after crossing the brain midline. Membrane trafficking of axonal receptors is studied as a mechanism that may regulate these changes. Finally, we have identified a gene encoding an axon guidance receptor which is specifically expressed in crossed cortico-striatal neurons.

The results of this study will provide information about the basic principles of brain development and lead to a better understanding of pathologies associated with commissural malformations.

Chauvet S, Mire E, Mann F. 2014. Characterizing Semaphorin signalling using isolated neurons in culture. Meth Mol Biol. In press.

This mono-partner project investigates how the left and right hemispheres of the mammalian brain become interconnected during development and how subtypes of inter-hemispheric projections control brain function and behaviour. We will approach this question by focusing on a single model system, the corpus callosum, which constitutes the largest commissure connecting the two cerebral hemispheres.
The first part of the scientific program is built around a previous study from our team implicating the chemoattractant Semaphorin (Sema) 3C in midline crossing of the corpus callosum in the mouse. Taking this example, we will explore how guidance signals are detected by growth cones, processed within neurons and translated into changes in axonal dynamics. The first objective will be to define the receptor complex to which Sema3C binds on callosal axons. Using a combination of genetic epistasis analysis, cell culture and biochemical experiments, we will explore a plausible cooperation between Neuropilin-1 and Ephrin-B proteins in specifying Sema3C attraction. We will pursue this work by studying the hypothesis that proper axon guidance not only depends on the ability of extracellular cues to regulate growth cone cytoskeletal dynamics, but also to modulate nuclear functions such as gene transcription. Although a growing body of evidence suggests transcriptional events in the control of growth cone pathways, thus far, however, only few transcription factors have been implicated and no transciptional targets have been identified in the mammalian brain. Our original data implicating beta-catenin-TCF/Lef function in the response of cortical axons to Sema3C in vitro will provide the rationale for experiments aimed at (a) studying the retrograde communication between axon tips and distant nucleus, (b) characterizing TCF/Lef function during axon guidance in the corpus callosum and (c) identifying Sema3C-regulated genes. Finally, to gain novel insight into the mechanisms of corpus callosum development, we will use live imaging of axons in semi-intact brain preparations from mouse embryos to describe the motile behavior of callosal axons extending across the brain midline and evaluate the effect of Sema3C signalling on this dynamics. This imaging approach will be extended to another key decision point– the sharp medial turn of callosal axons upon entry in the cortical white matter- and efforts will be made to identify regulatory molecules.
The second aspect of the project addresses the issue of diversity in the connective patterns and functionality of callosal projections. Based on the recent discovery that the semaphorin receptor PlexinD1 is expressed by specific callosal neurons dually projecting to the cortex and striatum, we plan (a) to explore how Semaphorin-Plexin signalling influence dendritic and/or axonal morphogenesis of this neuronal population and (b) to generate animal models with targeted ablation of crossed corticostriatal neurons to study the function of these heterotopic callosal projections in control of emotional, locomotor and lateralized behaviors.
This program should yield crucial and novel information about the cellular and molecular principles underlying brain circuits’ development and, ultimately, should lead to a better understanding of the molecular etiology of human disorders that arise from abnormal inter-hemispheric connectivity.