| Description |
The nervous and vascular systems are highly branched cellular networks and track along parallel pathways throughout the body. The similarities between these two systems extend beyond the gross anatomical level all the way to the molecular level, as cells in both networks respond to common guidance cues during development. Netrin, one of the first neural guidance cues to be identified, is an example of one of these shared guidance molecules. In the nervous system, Netrin can attract or repel axons, depending on the type of receptors expressed by its leading structure, the growth cone. Here, I set out to define the effect of Netrin on the developing vasculature. It has been previously demonstrated that Netrin1a is required for the formation of a specific vascular structure in zebrafish, the parachordal chain (PAC). Muscle pioneers have further been identified as the source of Netrin1a required for PAC formation. However, the receptor(s) by which Netrin1a signals to regulate PAC formation had not yet been defined. Furthermore, it is unknown if the effect of Netrin on PAC formation is direct or indirect. In this dissertation, I show that Netrin1a's effect on zebrafish vascular development is mediated by at least the effect of two known Netrin receptors: Unc5b and deleted in colorectal cancer (DCC). First, I show that embryos depleted of Unc5b do not form a PAC, phenocopying Netrin1a morphants, suggesting that Unc5b is a receptor that ! mediates Netrin1a's effect on PAC formation. Second, I demonstrate that the effect ofNetrin1a on PAC development is due at least partly to a requirement for motor neuron axon guidance at the horizontal myoseptum (HMS), which is mediated through DCC. The ongoing controversy about whether Netrin1a is attractive or respulsive for endothelial cells appears most in the case of the PAC, where Netrin1a acts on lymphendothelial cells only indirectly, through motor neuron axons. I show that DCC is expressed in motor neurons whose axons track along the HMS, and that depletion of Netrin1a or DCC disrupts motor neuron axon turning at the HMS. Using genetic and surgical approaches, I further demonstrate that motor axons are necessary for PAC formation at the HMS. These observations provide the first evidence that axons can guide vessel formation and pathfinding. |
