SLIT2 repellent is cleaved by TLL1 protease and promotes sensory axon fasciculation

SLIT2 is a secreted protein that repels axons from the CNS midline. Full-length SLIT2 (SLIT2-FL) is proteolytically cleaved into two fragments, SLIT2-N and SLIT2-C. SLIT2-FL and SLIT2-N have opposing biological effects on cultured dorsal root ganglion (DRG) axons. This study identified SLIT2 cleavage mechanisms and functional significance for DRG axon guidance. The Tolloid-related protease TLL1 cleaved SLIT2 in cultured cells, with TLL1 requiring activation by furin/prohormone convertases. We used CRISPR editing in mice to produce a Slit2ΔTLS allele lacking the TLL1 cleavage site. Slit2ΔTLS embryos retained dorsal repulsion of DRG axons, in contrast to DRG midline invasion in Slit2 knockouts. However, DRG axons in Slit2 knockouts and Slit2ΔTLS mutants showed reduced fasciculation of rootlets and longitudinal DRG projections. In vitro, SLIT2-N promoted fasciculation of DRG axons. These results suggest that proteolytic cleavage generates additional SLIT2 biological functions for organizing DRG central axon projections.