Mechanisms controlling distinct modes of adult axon growth

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 DESCRIPTION (provided by applicant): Affecting axonal growth as a means to enhance recovery and alleviate pathology in conditions of nervous system injury, insult, or disease is a major goal for the healthcare and biomedical research endeavors. Significant effort is directed at inducing neural plasticity to enhance axonal growth to establish functionally- adaptive connections. However, these efforts must also prevent, and not induce, maladaptive plasticity, a balance which requires a clear understanding of the processes regulating axon growth. A major factor confounding efforts to understand neural plasticity is that the traumatically-injured nervou system contains both directly-injured axons and the NON-injured axons. This project examines the long-standing question and controversy regarding the mechanisms of the two major forms of axon growth in the adult nervous system - growth of injured axons (Regeneration) and that of non-injured axons (Collateral Sprouting - CS). We aim to objectively determine the degree to which the intrinsic molecular mechanisms controlling these processes are similar or different. Doing so will enable identification of sets of genes which control a specific mode and may thus be targeted to affect just that one mode, or may be shared between modes and thus targeted to affect both, and could identify an entire new set of genes capable of regulating adult axonal plasticity. Axonal Regeneration and CS are both relatively robust in the PNS, and the injured and non-injured neurons can be clearly separated. Nerve crush provides a model of successful axon regeneration. Using the spared dermatome model (where intact non-injured neurons of a single dorsal root ganglion are induced to grow by denervating the skin bordering their dermatome) we have generated a transcriptomic profile of genes regulated during CS. Bioinformatic analyses indicate that the genes involved in regeneration and CS are highly distinct. Preliminary data using mice with genetic deletion of transcription factors (TFs) that appear to be specific for each axon growth mode supports the concept that the growth modes involve separate genetic programs. Aim 1 will use mice with mode-specific-TF knockout to thoroughly examine the impact of the gene- deletions on the different modes of axon growth using behavioral and histological assessments of axon growth. This will determine if the flagship mode-specific-TFs are indeed responsible for controlling that single mode. We have data demonstrating 1) a mutually-exclusive expression of the mode-specific TFs and 2) that "conditioning" with one mode of growth appears to influence the functional execution of the other mode. This is rational considering that there must be a large change in which genes are expressed, and proper orchestration of such a significant change could be delayed or prevented. Aim 2 will test the hypothesis that the modes negatively-influence each other and involve mutually-exclusive genetic programs by alternately applying the different models to the same neurons (i.e., regeneration-then-sprouting or sprouting-then-regeneration). This will determine how execution of one mode influences the other. For both Aims, experimental outcomes in accord with preliminary data would strongly support the concept that there are indeed two different growth modes, each with distinct genetic control. Outcomes contrary to preliminary data could include 1) effects on some neural populations but not others, which would suggest that there may still be distinct modes with distinct genetic control, but that these modes may be based not on injury- status, but on cell-type, and/or 2) that the apparent mode-specific genetic control systems act as facilitators but are not necessary (i.e., in their absence the processes occur anyway, but much more slowly), which would suggest that there are not necessarily two fully-distinct modes. All outcomes will serve to address both the conceptual framework and the specific molecular control regarding axon growth in the adult nervous system.