Molecular Mechanisms of Neuron Motility and Axon Guidance

Harvard Medical School, Boston MA

Investigators

Abstract

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 these mechanisms lead to Alzheimer’s Disease (AD) later in life. This project 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 remained 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 intracellularly with the RBP CPEB4, to regulate Robo local translation in post-crossing axon segments. Having identified this novel APP-CPEB4 pathway, the proposed studies are designed to expand our understanding of the pathway’s molecular mechanisms and functions. In commissural axon guidance, expression of many proteins is known to be locally regulated in axon segments at the midline; the proposed studies of the APP-CPEB4 pathway are expected to identify coordinate regulation of a large gene network, bringing together many disparate past observations into a unifying model for this premier paradigm of axon guidance. In addition to axon guidance, preliminary studies reveal an overlapping yet distinct set of functions for the same molecular pathway in another major developmental model system, cortical neuron migration. The novel APP-CPEB4 pathway also has high relevance to disease: in addition to its involvement in developmental processes that lead to Autism Spectrum Disorder (ASD), the pivotal role of APP in our pathway gives it key relevance to AD. Regarding autism, abnormalities at the cortical neuron migration stage are believed to be a leading cause of ASD, and CPEB4 disruption in mouse cortex at this specific stage causes ASD-like behaviors. Moreover, all the components of our pathway from ligands to downstream targets have been implicated in ASD, though not previously linked in a unifying model. Regarding AD, the transmembrane structure of APP has long led to the idea that it is a cell surface receptor, yet despite decades of intensive work no instructive receptor role – where the spatiotemporal pattern of a ligand regulates a downstream developmental or physiological function – has yet been identified for APP. Now identifying a receptor role for APP – including a pathway from ligands through a signaling pathway to functional readouts – opens the door to a qualitatively new level of understanding APP, which is especially important since the challenges of therapeutically targeting Aβ place increased emphasis on understanding the roles of APP itself. Studies of our APP-CPEB4 pathway will uncover novel biological principles, while leading to enhanced understanding of mechanisms underlying neurodevelopmental and neurodegenerative disorders. Approaches include genome-wide target mRNA identification, and functional cellular and 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 its potential for therapeutic intervention.