The Myc/Max/Mad1 Network within the Regenerating Peripheral Nervous System
Creators: Poitras, Trevor M
Publisher: name: University of Alberta Library
Year: 2023

<p>The peripheral nervous system (PNS) is often compared to the central nervous system for its ability to regenerate more robustly. While it is true that damaged PNS axons are less restricted in regrowth, outcomes following damage or neuropathy are slow, incomplete and leave patients with motor, sensory or autonomic dysfunction. Clinically, there are no approved therapies for augmenting nerve regrowth after damage. An example of a widespread neuropathy is diabetic polyneuropathy (DPN). This single neuropathy profoundly impacts the quality of life of diabetic patients and places a tremendous burden on the healthcare system, thus exemplifying the need for therapy directed towards promoting PNS regeneration.<br>
    Growth promoting and suppressing mediators within damaged axons are slowly becoming identified within the regenerative milieu. Such targets include molecules like PTEN, Rb1, and RhoA to name a few. Understanding how the axonal microenvironment responds to damage provides critical information that can be utilized for rational drug design for optimizing the axonal response to injury. In this thesis, I focused on the Myc/Max/Mad1 network. This important group of related proteins is crucial during development for growth, proliferation, and differentiation. Myc-associated factor X (Max) binds to Myc and can promote transcription of genes responsible for growth and proliferation. Conversely, Max dimerization protein 1 (Mad1) regulates Myc’s transcriptional activity by competitively binding Max, preventing Myc binding and thereby promoting differentiation and quiescence. </p>

<p>Chapter 1 of this thesis serves as an introductory chapter to familiarize the reader with the peripheral nervous system and challenges PNS axons face when trying to reunite with their targets. In chapter 2, I explore the role of Myc, Mad1 and Max within the regenerating PNS. In this chapter, I used injured and uninjured DRG cultures treated with siRNA to develop an understanding of how each member of the Myc/Max/Mad1 network impacts the growth properties of PNS neurons.  Furthermore, to support our in vitro findings, I used a crush model of nerve injury to assess if local injections of Mad1 siRNA improve indices of regeneration.  Knockdown of Mad1 resulted in enhanced neurite outgrowth and improved recovery following injury suggesting Mad1 is a barricade to regrowth.<br>
Next, chapter 3 shifts focus towards investigating the impact of Mad1 on the repair properties of Schwann cells (SCs). Specifically, Mad1 reduction in the SC population yielded improved motility in addition to enhancing their ability to support the neurite outgrowth of their neuronal counterparts. Interestingly, there was no impact on their proliferative capacity. <br>
In chapter 4, I used long-term type 1 diabetic animals to model DPN to determine if Mad1 knockdown improved deficits associated with DPN. The application of Mad1 siRNA in these diabetic animals demonstrated a return of motor nerve conduction velocity and improved mechanical sensation.  Additionally, in this chapter I use transgenic animals with fluorescent axons to perform live animal imaging to track changes in axon length and branching overtime in response to local injections of Mad1 or scrambled siRNA.<br><br>
Finally, in chapter 5 I discuss future directions and limitations of the work within this thesis. Overall, this thesis aims to develop a thorough understanding of the Myc/Max/Mad1 network in the context of peripheral nerve regeneration.</p>