Molecular mechanisms of neuron motility and axon guidance

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The brain relies for its function on a precise and complex pattern of neuronal connections. The broad long-term goal of this project is to understand molecular mechanisms that set up this pattern of connections during development, and how aberrations of this process lead to neurodevelopmental disorders. This proposal focuses particularly on RNA-based regulatory mechanisms. Key advantages of regulating mRNA translation via RNA-binding proteins (RBPs) are: (1) allowing protein synthesis to be locally regulated in specific subcellular regions where the proteins are needed, and (2) coordinately regulating expression of large networks of functionally related mRNAs. To understand the basic principles of axon guidance, a major model system has been spinal commissural axon guidance at the midline. Navigating this intermediate target requires axons to be attracted and then repelled, and the classic mechanism for this is the `Robo switch' where repellent Robo receptors are upregulated in post-crossing axons; however, the extracellular signal and the mechanism by which it triggers this switch have been unknown. We have now identified a highly novel mechanism for the Robo switch, involving extracellular ligand binding to the transmembrane Amyloid Precursor Protein (APP), which interacts with the RBP CPEB4, to regulate Robo local translation in post-crossing axon segments. This novel APP-CPEB4 pathway has high relevance to disease: in addition to the role of APP in neurodegeneration, CPEB4 is currently of high interest as a cause of Autism Spectrum Disorder (ASD). The proposed studies continue our work on CPEB4, studying it in two developmental systems: (1) Spinal commissural axon midline guidance. Expression of many proteins is known to be locally regulated in axon segments at the midline, and the novel APP-CPEB4 pathway is likely to regulate not only Robo but other proteins. This work is expected to show coordinate regulation of a large gene network at an intermediate target, bringing together many disparate past observations into a unifying model for this major paradigm of axon guidance. (2) Radially migrating neurons in the cerebral cortex. Abnormalities in this phase of development are believed to be a leading cause of ASD. Based on existing evidence, CPEB4 regulates Robo expression in developing cortex, and CPEB4 conditional disruption in mouse cortex at this specific stage causes ASD-like behaviors. Compared to commissural axons, evidence indicates that CPEB4 regulates different processes in cortical development. Studies of CPEB4 in cortical development will therefore uncover novel biological principles, and will also directly inform the understanding of pathogenic mechanisms for ASD and other neurodevelopmental disorders. Approaches include genome-wide target mRNA identification, and functional developmental studies in vitro and in vivo. Additionally, studies of signal transduction mechanisms in the novel APP-CPEB4 pathway will be essential to understand its operation and long-term potential for therapeutic intervention, not only in the neurodevelopmental context but also in other systems in biology and disease.