Damage to connections within the adult Central Nervous System (CNS) by injury or disease is often irreparable. To design therapies to repair CNS damage requires a detailed understanding of the cellular mechanisms underlying CNS development. As the brain develops, neurotrophic cues, such as netrin, guide axons to their postsynaptic targets and induce axon branching to increase synaptic capacity. Both the guided locomotion of axonal growth cones, and their ramification into multiple axon branches, require the same fundamental cellular machinery. F-actin and microtubule (MT) dynamics initiate and steer membrane protrusions. Exocytosis delivers phospholipids and membrane proteins required to supply material to the expanding plasma membrane. Coordination of cytoskeletal dynamics and vesicle trafficking likely plays key roles in axon guidance and branching. However, the molecular mechanisms that mediate such interactions during axon guidance and axon branching are not understood. Our findings place TRIM9 at the junction of netrin/DCC signaling to both the cytoskeletal and vesicle trafficking machinery. Using a combination of mouse genetics, primary cell culture, live cell imaging and neuroanatomical studies, my lab found that TRIM9-deficient cortical neurons show misregulated exocytosis and defective actin and MT dynamics. Furthermore, we found that cortical neurons devoid of TRIM9 have constitutive branching defects, fail to form from branches in response to netrin, and are defective in netrin-based axon guidance. In vivo, we have found that loss of TRIM9 is associated with defective cortical axon fiber tracts. Our findings that TRIM9 interacts with and regulates the exocytic tSNARE, SNAP25, lead us to hypothesis that TRIM9 spatially and temporally regulates exocytosis in the growing axon. Novel interactions identified with multiple cytoskeletal regulators, including Ena/VASP proteins, Lamellipodin, and MAP1B lead us to hypothesize that TRIM9 participates in protein networks that play key roles in F-actin and MTs dynamics. As we have found that TRIM9 binds directly to the netrin receptor, DCC, and is required for functions downstream of the axon guidance cue netrin, we hypothesize that TRIM9 is essential for the coordinated activities of the cytoskeleton and exocytosis that dictate axon branching and guidance in response to netrin/DCC. My lab is in a unique position to determine the molecular mechanism that link guidance cues to local changes in cytoskeletal dynamics and axon branching through live-cell imaging, quantitative image analysis, biochemistry, and mouse models. Our long-term goal is to understand how neurons integrate environmental cues to orchestrate changes in their morphology and movement necessary to establish a functional nervous system. A better understanding of the mechanistic basis of axon guidance and axon branching will provide fundamental insight into how connections in the nervous system are established and how they are remodeled during plasticity. The results of our research plan should be of great value to the development of therapeutic approaches to repair these connections subsequent to disease or injury.
Nerves must reach appropriate targets to establish the neural connectivity underlying function and behavior. Damage to nerves within the adult brain and spinal column by injury or disease is often irreparable. We are working to understand how nerve fibers reach targets during development, expecting this will illuminate the fundamental defects associated with neurological syndromes as well as aid in the development of rehabilitative therapies for spinal cord injury.
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