Transcriptional And Epigenetic Regulation Of Axon Regeneration PDF Download

The mammalian nervous system is a highly intricate network consisting of over a hundred billion specialized cells called neurons. One unique characteristic of neurons is their highly polarized morphology; unlike other cells, neurons project long axonal extensions. These structures allow them to connect and communicate with not only other neurons, but also various cell types in the body and give rise to all motor, sensory, and higher order function. Because axons can extend up to three feet, they are also vulnerable to injury from sources such as traumatic brain and spinal cord injuries, stroke, or neurodegenerative diseases. Indeed, patients who have experienced these injuries often suffer debilitating, irreversible loss of function. Interestingly, whereas neurons which reside in the central nervous system are incapable of regenerating after axon injury, neurons of the peripheral nervous system activate a robust pro-regenerative response capable of promoting long distance regeneration and functional recovery. The molecular mechanisms which underlie this pro-regenerative response may provide key insights into how a pro-regenerative response could be stimulated in injured central nervous system neurons. A comprehensive overview of the known molecular mechanisms involved in this response is reviewed in Chapter 1.As mammals age, the synaptic connections between neurons mature. Following axon injury in peripheral nervous system neurons, the genes involved in synaptic function are turned off and genes required for inducing axon growth are activated. These widespread epigenetic and transcriptional changes require a coordinated effort of epigenetic and transcriptional regulators including epigenetic modifiers, transcription factors, and microRNAs. In Chapter 2, we demonstrated that these changes are, in part, a result of the rapid downregulation of microRNA-9 which occurs following axon injury. At baseline in adult peripheral nervous system neurons, microRNA-9 is highly expressed and actively represses various genes including REST and UHRF1. When microRNA-9 expression decreases following injury, both REST and UHRF1 increase with UHRF1also repressing REST and restricting REST expression to a tight temporal window. During this time, REST binds to and represses various genes involved in synaptic function such as ion channels; a process necessary for peripheral nervous system regeneration. This complete published work can be found in Chapter 2.In coordination with epigenetic modifiers such as UHRF1, various transcription factors are activated following axon injury and promote the expression of pro-growth genes. Various studies have worked to identify the transcription factors involved in this process as exogenous overexpression of transcription factors has been shown to confer specific phenotypes of interest, such as the conversion of one cell type to another, when the correct combination of transcription factors is manipulated. To further this work, in Chapter 3 I used bioinformatics analysis to identify 27 transcription factors putatively involved in the establishment of the pro-regenerative response. Using two complimentary in vitro screens, determined which transcription factors were both necessary for peripheral nervous system axon regeneration and sufficient to drive central nervous system axon regeneration. By pairing these results with network-based bioinformatics analysis, we identified Creb1 as a transcription factor which sits atop the pro-regenerative gene regulatory network. Follow-up studies in which we overexpressed Creb1during optic nerve regeneration demonstrated Creb1 is sufficient to promote central nervous system axon regeneration in vivo. This work provides exciting new insight into the various transcription factors regulating this response as well as their putative genetic relationships.