Axon damage after stroke, traumatic injury or disease disrupts neuronal circuits and can result in disability. Mammalian central axons do not regenerate well, but peripheral axons have a greater, if limited, potential to regrow. Axon regeneration is influenced by extrinsic and intrinsic factors; the intrinsic mechanisms regulating axon regeneration in vertebrates are not well understood. Because the environments of both the central and peripheral nervous systems are permissive for axon regeneration in fish, I can use them to study the role of intrinsic pathways in this process. Dual Leucine Kinase (DLK) is a mitogen-activated protein kinase kinase kinase (MAP3K) that initiates a signaling cascade upon axon damage that promotes axon regeneration. Although DLK is known to be a critical regulator of axon regeneration in worms, flies, and mice, its interactions with other MAPK pathways are still not well understood, the downstream processes it activates are not fully understood, and it is unclear if these pathways are necessary for regeneration of all types of neurites. Vertebrates have two similar DLK-related MAP3K proteins, DLK and LZK (Leucine kinase zipper). Vertebrate DLK shares more sequence identity with worm DLK-1, but only LZK shares a calcium-sensing domain at the c- terminus with DLK-1. While DLK gene trap mutants in mice have significantly slower axon regeneration, regeneration is not prevented altogether, suggesting that LZK may also contribute to axon regeneration (Chen et al, 2016). Although DLK is required for axon regeneration in fly sensory neurons, it is not required for regeneration of their dendrites. Unlike these bipolar Drosophila sensory neurons, somatosensory neurons are unipolar: they have central and peripheral axons, but no dendrites. I established a zebrafish model to study the function of DLK and LZK signaling in somatosensory neuron repair. With this model I will clarify several important questions about these signaling pathways: 1) Are both DLK and LZK required the regeneration of peripheral and central sensory axons? Are their functions redundant or complimentary? 2) What are the specific MAP2Ks and MAPKs involved in this process? 3) Do DLK and LZK regulate physiological changes associated with axon regeneration? Zebrafish larvae provide an amenable model for addressing these questions in live animals, with single cell resolution.
To efficiently treat axon damage resulting from disease, traumatic injury or stroke, it is crucial to understand not only the extrinsic factors influencing this process, but also the intrinsic capabilities of neurons to repair themselves. This project investigates the role of the axon injury sensor Dual leucine kinase (DLK), and its less known relative, Leucine zipper kinase (LZK), in regeneration of zebrafish sensory axons. Thanks to the amenability of zebrafish to live imaging and single cell analysis, I will determine if DLK or LZK are required for axon regeneration, their specific downstream targets in this process, and whether or not they act in a neurite- specific manner.
