UCLA

Neural circuits are established when axons navigate to their synaptic targets using guidance cues in the environment to direct the path of extension. Netrin1 is best known as an axonal guidance cue, directing commissural axons toward the ventral midline of the developing spinal cord. However, its functions extend beyond axon guidance, and how Netrin1 acts at distinct stages of spinal cord development remains incompletely understood. In this dissertation, I examine distinct functions of Netrin1 during spinal cord development, with a primary focus on defining how floor plate (FP)-derived Netrin1 regulates commissural axon guidance.First, we examined a previously unrecognized role for Netrin1 in dorsal spinal cord patterning. Using genetic and in vitro approaches in chicken, mouse, and mouse embryonic stem cell models, we found that Netrin1 influences the specification of Bmp-dependent dorsal interneurons and modulates Bmp signaling activity, supporting a role for Netrin1 in spatially restricting Bmp signaling during dorsal interneuron development.Next, I investigated the mechanism by which FP-derived Netrin1 guides commissural axons. Using a novel chicken model, I found that FP-derived Netrin1 remains highly localized and is largely dispensable for commissural axon midline crossing. Reconstituting Netrin1 at the FP in a Netrin1-null mouse partially restored commissural axon growth toward the FP, but rescue was frequently incomplete and accompanied by aberrant hyperfasciculation. Moreover, membrane-tethered Netrin1 was sufficient to promote directed and fasciculated commissural axon outgrowth in vitro.Together, these findings reveal that FP-derived Netrin1 does not require a long-range diffusible gradient to promote commissural axon guidance, but instead can function as a local adhesive cue. We propose that membrane-associated Netrin1 provides a short-range anchor that stabilizes and organizes fasciculated axon growth toward and across the midline. This adhesive mechanism represents a previously unrecognized mode of Netrin1 function and expands the canonical view of Netrin1 from a diffusible guidance cue to a local regulator of axonal organization. More broadly, these findings highlight the reiterative use of Netrin1 during spinal cord development, with distinct functions in neuronal patterning and axon guidance contributing to the establishment of spinal neural circuits.