Strategies for Promoting Axon Regeneration and Functional Recovery in the Nervous System

促進神經系統軸突再生和功能回復的醫療策略

Student thesis: Doctoral Thesis

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Award date18 Feb 2019

Abstract

Injuries to the nervous system are often devastating to patients due to incomplete functional recovery which greatly affect their quality of life. In central nervous system (CNS), the injured neurons failed to extend their axons across the lesion for target reinnervation, largely due to their limited intrinsic growth capacity. The presence of extrinsic growth inhibitory molecules such as chondroitin sulfate proteoglycan (CSPG) further limited axonal regrowth following CNS injuries. In contrast, neurons in peripheral nervous system (PNS) can regrow their injured axons at a slow rate (i.e. 1-2mm/day). After proximal nerve injury, the injured axons require to regenerate over a long distance. Although these injured axons can fully extend into the distal muscle targets; however, chronic denervation impairs the reformation of functional synapses at the motor end plate, resulting in incomplete motor functional recovery. These collectively account for the poor clinical outcomes in patients with PNS and CNS injuries. Currently, there are no effective therapies to improve functional restoration after nervous system injuries.

The current study aimed to develop novel strategies to promote axon regeneration and functional recovery in the nervous system. In recent years, emerging evidence suggested that low dose of ionizing radiation (LDIR) including alpha particle and X-ray is beneficial to the irradiated individuals via induction of adaptive responses, activation of DNA repair mechanisms and enhancement of innate immunity. The first part of the study aimed to explore the potential growth stimulating effects of LDIR in promoting axon regeneration and functional recovery in a mouse model of PNI. Low dose (LD) alpha particle and X-ray irradiation induced robust neurite outgrowth in axotomized dorsal root ganglion (DRG) neurons, respectively. LD whole body X-ray irradiation at 2Gy accelerated axon regeneration in vivo, and promoted sensory and motor functional recovery after sciatic nerve crush injury. In-depth genome-wide methylation profiling on alpha particle- and X-ray-irradiated primary DRG neurons identified a gene-specific hypermethylation at Fmn2 promoter region which correlated well with its down-regulation of Fmn2 mRNA. Therefore, the second part of the study aimed to investigate the role of Fmn2 in promoting axon regeneration. We found that genetic ablation of Fmn2 markedly enhanced neurite outgrowth from axotomized DRG neurons in vitro, promoted axonal regrowth in vivo, and improved sensory and motor functional recovery following PNI.

The third part of study aimed to define the role of activator protein 4 (AP4) in regulation of axon regeneration. Following PNI, AP4 was markedly up-regulated in both mRNA and protein levels. Knockdown of AP4 not only reduced neurite formation in axotomized DRG neurons, but also reduced the intrinsic regenerative capacity in pre-conditioned DRG neurons. In vivo gene silencing of AP4 impaired sensory and motor functional recovery following PNI, suggestive a crucial role in axon regeneration. Overexpression of AP4 markedly increased the extent of axonal regrowth following PNI.

Finally, we extended our study to examine the hormetic effects of LD X-ray irradiation on promoting CNS repair in a mouse model of traumatic brain injury (TBI). LD X-ray irradiation induced microglial M1-to-M2 phenotypic switch, promoted microglial migration and enhanced microglial phagocytic capacity. Using cortical stab wound injury as TBI model, LD X-ray irradiation promoted microglial migration towards the injury site associated with the reduced CSPG deposition at the injury site. In addition, fewer apoptotic cells were observed in the injury site of X-ray-irradiated mice, accompanied by a smaller wound size after cortical stab wound injury.

Taken together, we believe the novel strategies described in the current study will be of high therapeutic potential to promote axon regeneration and functional recovery in the nervous system. Bioinformatic analysis helps identify small molecules that activate signaling pathways similar to LDIR, AP4 overexpression and Fmn2 ablation. This will shed new light on the development of strategies to promote functional recovery in patients after injuries to the nervous system.

    Research areas

  • Axon regeneration, Functional recovery, Low dose of ionizing radiation, Radiation hormesis, Formin-2, Activator Protein 4, Traumatic brain injury