Modularity in the DCC extracellular domain elicits distinct effects on axon guidance

Complex neuronal circuits arise from a small set of cell-surface receptors that position neurons, promote axon extension, and define synaptic connections. A central receptor is Deleted in Colon Cancer (DCC), which mediates both short- and long-range axon guidance and confines migrating neurons to the central nervous system. DCC's versatility reflects its ability to interact at distinct sites of its extracellular domain with two ligands, Netrin-1 and Draxin, which also bind to each other. Alternative splicing further alters the Netrin-1 binding site and modulates affinity. By generating two mouse lines with mutations that selectively impair DCC binding to Netrin-1 and/or Draxin, we show that molecular modularity within the DCC extracellular domain is essential for precise circuit assembly. An eight-amino acid insertion in the DCC ^long isoform is required for Netrin-1-dependent long-range commissural axon guidance in the spinal cord. Conversely, isoleucine 372 in the Draxin binding site enables DCC clustering and is necessary for all known DCC functions, including axon guidance in the spinal cord and retina and neuronal migration in the brainstem. Draxin also supports long-range commissural guidance, but mutations in its binding site cause stronger defects. These results underscore how DCC's distinct modules drive specific developmental responses.