Experimental Model Systems for Understanding Human Axonal Injury Responses

Abstract

Neurons are structurally unique and have dendrites and axons that are vulnerable to injury. Some neurons in the peripheral nervous system (PNS) can regenerate their axons after injuries. However, most neurons in the central nervous system (CNS) fail to do so, resulting in irreversible neurological disorders. To understand the mechanisms of axon regeneration, various experimental models have been utilized in vivo and in vitro. Here, we collate the key experimental models that revealed the important mechanisms regulating axon regeneration and degeneration in different systems. We also discuss the advantages of experimenting with the rodent model, considering the application of these findings in understanding human diseases and for developing therapeutic methods.

Keywords: animal models; axonal regeneration; neurodegeneration.

Figure 1

Injury responses in the peripheral nervous system (PNS) [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21].

Figure 2

Experimental models to study axon regeneration. (A) Primary cultured eDRG neurons are axotomized using a blade and the regenerating axons are investigated [13]. (B) eDRG cells are replated to investigate axonal regeneration [37]. (C) Sciatic nerve is dissected after three days of injury then immunostained with anti-SCG10 antibody to identify the regenerating axons [38]. (D) EHL muscle is observed for the target reinnervation [17]. (E) Adult DRGs are trypsinized and dissociated for single-cell culture [14]. (F) Adult DRGs are dissected and explanted to investigate the mechanistic pathway more easily [39]. (G) A laser lesion to the brain and spinal cord for a better understanding of the dynamics of single axons [40]. (H) In vivo imaging is performed by implantation of the spinal window in the spinal cord [41]. (Images created with BioRender.com)

Figure 3

In vivo gene delivery methods to effectively manipulate the target genes. (A) AAV-mediated gene delivery enables efficient gene expression in various tissues. AAV-injected mice are suitable for regeneration-related experiments generally after 4 weeks of husbandry. (B) The direct intrathecal injection is performed on adult mice which can manipulate the genes in the CNS. Contrast to the intravascular injection, injected mice can be used after 2 weeks post-injection. (C) With the electric pulse, plasmids or RNA oligos are introduced into mouse brains at embryonic days 12-17 with the embryos inside the uterus. (D) After the microinjection of plasmids or RNA oligos into the adult DRG, electric pulses are delivered to the target DRG. (Images created with BioRender.com)