The RacGAP β2-Chimaerin Selectively Mediates Axonal Pruning in the Hippocampus

Axon pruning and synapse elimination promote neural connectivity and synaptic plasticity. Stereotyped pruning of axons that originate in the hippocampal dentate gyrus (DG) and extend along the infrapyramidal tract (IPT) occurs during postnatal murine development by neurite retraction and resembles axon repulsion. The chemorepellent Sema3F is required for IPT axon pruning, dendritic spine remodeling, and repulsion of DG axons. The signaling events that regulate IPT axon pruning are not known. We find that inhibition of the small G protein Rac1 by the Rac GTPase-activating protein (GAP) beta 2-Chimaerin (beta 2Chn) mediates Sema3F-dependent pruning. The Sema3F receptor neuropilin-2 selectively binds beta 2Chn, and ligand engagement activates this GAP to ultimately restrain Rac1-dependent effects on cytoskeletal reorganization. beta 2Chn is necessary for axon pruning both in vitro and in vivo, but it is dispensable for axon repulsion and spine remodeling. Therefore, a Npn2/beta 2Chn/Rac1 signaling axis distinguishes DG axon pruning from the effects of Sema3F on repulsion and dendritic spine remodeling.