Novel Activities of Adenomatous Polyposis Coli (APC) Protein and Type III Neuregulin 1 in the Developing Nervous System

Cell polarity controls major processes during nervous system development, including axon-dendrite polarity, axon guidance, synaptogenesis and plasticity. The work presented here is dedicated to investigating novel activities of two proteins, APC and Type III Nrg1, which are implicated in cell polarity and neural development. We show that APC immunoprecipitates and partially co-localizes with FMRP, and components of translation machinery. Significantly, we reveal that endogenous APC possesses novel RNA-binding activity in embryonic brain. In cells, a conserved APC C-terminal region directly binds RNA and blocks migration. Stringent purification and sequencing of APC-cross-linked mRNAs from brain reveals targets implicated in neural development, including: axonogenesis, axon guidance, and Wnt/β-catenin signaling. We also present evidence supporting a role for Type III Nrg1 receptors in targeting TrkA+ DRG axons to the dorsal spinal cord in vivo. Analysis of sensory neurons in culture indicates that Type Ill Nrg1 is important for regulating Sema3A receptor levels and sensory axon responsiveness to Sema3A. Our work suggests that Type III Nrg1 may play a novel role in modulating responsiveness of axons to guidance cues at target fields. We discuss our findings in the context of cell polarity, neural development, and known APC and Type III Nrg1 protein functions.