Slits are large secreted proteins that repel growing axons during nervous system development, primarily by binding Roundabout (Robo) receptors. In addition to axon guidance, Slit-Robo signaling is linked to a wide range of biological processes including cell migration, angiogenesis, and cancer metastasis. The full-length Slit protein (Slit-FL) is proteolytically cleaved into two functionally distinct fragments, Slit-N and Slit-C. The Slit-N fragment has been identified for its therapeutic potential in multiple disease pathologies with diverse beneficial effects such as increasing vascular stability and promoting neuronal survival and outgrowth. Accurately identifying the Slit protease is paramount for understanding Slit processing and necessary for targeting the protease for modulation to therapeutically increase availability of Slit-N. Here we provide evidence to support that the metalloprotease Tolloid-like 1 (Tll1) is the direct protease of Slit, not prohormone convertases as other labs have previously proposed. Furthermore, we propose a pharmacological strategy to indirectly activate Tll1 as a mechanism for increasing Slit-N levels in microbially infected endothelial and lung cells. This therapeutic potential speaks to the critical need for effective strategies to treat SARS-CoV-2-related vascular and lung damage
