RACK1 regulates axon outgrowth and point contact formation through local translation in developing neurons

During development, neurons must extend processes and make connections with their appropriate targets. This process is dependent on multiple molecular and cellular mechanisms, and if disrupted, neurodevelopmental disorders can result. We previously demonstrated that Receptor for activated C kinase (RACK1), a ribosomal scaffolding protein, regulates the adhesion and motility of developing neurons through its regulation of point contacts, adhesion points located in the tips of pathfinding axons. Furthemore, RACK1 regulates the local translation of β-actin mRNA, which is necessary for appropriate axon guidance. However, RACK1 has multiple signaling and ribosomal functions, and how the ribosomal binding function of RACK1 contributes to neural development is unknown, Thus, we specifically investigated the ribosomal binding function of RACK1 in point contact formation, axonal outgrowth, and local translation. We overexpressed RACK1-WT, RACK1-DE (a mutant form of RACK1 that cannot bind ribosomes) or a control construct in embryonic mouse cortical neurons. Immunocytochemistry was performed, followed by quantification of point contact formation, axon outgrowth, and local translation of β-actin. Overexpression of RACK1-DE inhibited BDNF-induced point contact formation, and also led to a significant decrease in axonal outgrowth. We are currently examining whether �-actin protein levels decrease following overexpression of RACK1-DE; this is expected because RACK1 mediates the local translation of �-actin. Together, these experiments show that local translation mediated by RACK1 regulates adhesion and axon outgrowth in the developing nervous system. We previously identified aberrant expression of RACK1 in Down syndrome, and thus these results have implications for the pathogenesis of this neurodevelopmental disorder.