Investigator: Alexander Jaworski, Brown University

Scientific Theme: Neuroscience

Abstract: Guidance of axons to their targets is central to nervous system wiring, but the mechanisms of axon guidance are not completely understood. The proposed work aims to elucidate the function of a new axon guidance cue, NELL2, and its receptor Robo3 in neurodevelopment and to define molecular mechanisms of NELL2-Robo3 signaling. While the receptors Robo1 and Robo2 mediate axon repulsion by Slit family guidance cues, Robo3 does not bind Slits but instead silences Slit-Robo1/2 signaling. We identified a novel guidance cue, NELL2, as a repulsive ligand for Robo3. We generated NELL2 knockout mice and found that NELL2/Robo3-mediated repulsion guides commissural axons in the spinal cord. Our findings define a novel axon guidance signaling pathway and raise the possibility that it controls multiple aspects of nervous system wiring. To test this idea, we will analyze NELL2 and Robo3 tissue expression by several means, including new genetic reporter lines. Select neuronal populations will be examined for NELL2 responses in vitro, and Robo3-dependence will be tested using neurons from Robo3 mutant mice. To determine the in vivo roles of NELL2 and Robo3, axonal trajectories in NELL2- and Robo3-deficient mice will be analyzed by various complementary approaches. Robo3 fulfills dual functions in axon guidance, as it inhibits Slit-induced repulsion through Robo1/2 and mediates repulsion from its own ligand NELL2, but it is unclear which intracellular signaling pathways act downstream of NELL2 and how Robo3 can perform its two functions in parallel. To address these questions, we will test if NELL2 repulsion and Slit silencing can be separated in Robo3 structure-function analyses in vitro and in vivo. Using candidate and unbiased biochemical approaches, we will identify novel NELL2 receptors and mediators of NELL2-Robo3 signaling and study their function in vitro and in vivo. Our work will provide important insights into functions of NELL2/Robo3. It will also help elucidate how multiple guidance signals interact within the same neuron, a fundamental but still poorly understood question. This work has the potential to inform therapeutic approaches for neuronal repair, and it can help define the mechanisms underlying diseases that result from nervous system mis-wiring.

Human Health Relevance: Guidance of axons to their correct targets is an important step in nervous system wiring. The proposed study will elucidate the functions of a novel signaling pathway, NELL2-Robo3, in axon guidance and neural circuit assembly. By providing insights into mechanisms of axon pathfinding, this work will improve our understanding of disorders resulting from brain mis-wiring, and it can inform therapeutic approaches aimed at restoring damaged axonal connections after physical injury or onset of neurodegenerative disease.