Damage to the adult central nervous system, including both brain and spinal cord, results in profound long-term functional impairment. While the mammalian CNS is notorious for its inability to repair itself after injury, there are two forms of axonal repair that may contribute to some recovery following CNS injury: regeneration of severed axons and compensatory sprouting of uninjured axons. Of these two mechanisms of repair, sprouting can occur spontaneously, and is believed to contribute at least partially to the clinical manifestations of recovery in individuals with milder forms of CNS trauma, although many continue to show profound lifelong deficits in function. Alternately, regeneration is rarely spontaneous and requires manipulation of neuron-intrinsic and extrinsic mechanisms in order to occur in the laboratory setting. Manipulation of various intrinsic and extrinsic signaling pathways to enhance the efficacy of sprouting or regenerative repair mechanisms remains an important goal in promoting functional recovery, although the molecular understanding of these processes is still incomplete.The major goal of this dissertation is to understand the roles of Leucine Zipper-Bearing Kinase (LZK) and Dual Leucine Zipper Kinase (DLK), a pair of evolutionarily conserved mitogen-activated protein kinase kinase kinases (MAP3Ks), in both forms of axonal repair after central nervous system CNS injury, or more specifically after spinal cord injury (SCI). The Jin and Bastiani labs identified DLK-1 as an injury sensor, and a critical regulator of axon regeneration in C. elegans. DLK has been shown to play a role in axon regeneration in the worm motor nerve after laser axotomy (Hammarlund et al., 2009; Nakata et al., 2005). In the mouse, these kinases have been studied in the optic nerve and the sciatic nerve, but the role of DLK in axonal repair in the mammalian spinal cord is not known. Furthermore, nothing is known about neuronal LZK in axonal repair in the mammalian CNS.
My dissertation investigates the roles of these kinases in both sprouting and regeneration using two different surgical models of spinal cord injury. In addition to assessing the roles of these kinases in an unmanipulated genetic background, I leveraged the well described effect of PTEN deletion to enhance basal sprouting and regeneration phenotypes and assessed if further deletion of one or both kinases contributes to any change in repair phenotypes. In these investigations, I found that while deleting both kinases abolishes the effect PTEN-enhanced repair, deletion of LZK alone results in no meaningful changes in either sprouting or regeneration. Furthermore, co-deletion of DLK and LZK in both induced sprouting and regeneration models does not alter the effect of PTEN deletion on AKT/mTOR activity, as evidenced by elevated downstream marker pS6, demonstrating for the first time the failure of axon regeneration in the presence of upregulated AKT/mTOR activity. This implies that regenerative competence, as reflected by pS6 levels, is separate or parallel to the pathway mediated by DLK/LZK. Both pathways must be intact and active in order for regenerative competence to translate to axonal repair