Contralateral targeting of the corpus callosum

Abstract

During development, the brain establishes billions of precise connections in order to perform its myriad functions. The largest of these connections in placental mammals is the corpus callosum, which is a large bundle of axons linking the two cortical hemispheres of the brain. Absence or gross malformation of this structure is a relatively common developmental disorder in humans, producing symptoms ranging from severe to mild cognitive, emotional and motor deficits. However, recent evidence suggests that the corpus callosum can also display subtle malformations that are not visible with traditional magnetic resonance imaging methods, and that these may be involved in neurodevelopmental disorders such as autism and schizophrenia. The possibility of callosal misconnectivity arising independently of the widely-studied process of midline crossing highlights the need to investigate other less well-understood stages of callosal development. One such process that may be disrupted after normal midline crossing of the corpus callosum is contralateral targeting, where axons must locate, innervate and stabilize in their appropriate contralateral cortical targets.This thesis focuses on understanding the normal process of contralateral targeting, as well as the nature of its dependence on neuronal activity and the molecular mechanisms that regulate it. To investigate these topics, diverse approaches including in utero electroporation, stereotaxic brain injection, sensory deprivation paradigms, tissuespecific RNA extraction and RNA sequencing were used in the mouse somatosensory cortex model system of callosal connectivity. Novel patterns and processes of normal contralateral targeting were identified, as well as distinct periods of region-specific activitydependent and -independent axonal exuberance. Contralateral callosal targeting was also shown to be not just dependent on neuronal activity, but rather a balance of spatially symmetric activity between the two cortical hemispheres. The molecular mechanisms underlying activity-dependent and -independent stages of contralateral callosal targeting were also investigated, and a novel technique to extract RNA from an electroporated population of cell bodies and/or their callosal axons was developed to facilitate this study in the future.These results constitute fundamental insights into the process of contralateral callosal targeting, as well as some of the mechanisms that may lead to its disruption. This work provides solid foundations for future studies to build upon, and may ultimately help us to better understand subtle human disorders of neuronal connectivity.3