Pre-Conditioned Lesion: Inflammatory Effects on CNS Regeneration

Author: Ernest Antonio Aguilar Salegio

Aguilar Salegio, Ernest Antonio, 2009 Pre-Conditioned Lesion: Inflammatory Effects on CNS Regeneration, Flinders University, School of Medicine

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Abstract

In the adult central nervous system (CNS) several factors are implicated in the failure of neurons to regenerate after spinal cord injury (SCI). However, this reduced ability of injured CNS neurons to regenerate can be improved by under certain conditions. For instance, in adult dorsal root ganglion (DRG) neurons, injury to its peripheral branch (unilateral conditioning lesion) prior to injury of its central DRG branch (dorsal column cut) enhances the intrinsic capability of some but not all CNS afferent neurons to regenerate. The exact mechanism mediating this type of response is not known. However, previous studies by other groups have proposed that the regeneration of these CNS afferent neurons might be associated with the inflammatory response following injury to the peripheral DRG branch. Our general aim, was to examine the involvement of the immune response in the regeneration of the CNS DRG branch, as part of the pre-conditioned lesion model. To test this, three questions/hypotheses were investigated. Firstly, we investigated the effects of vaccination in pre-conditioned lesion animals using a peripheral nerve homogenate (PNH, sciatic nerve) as the immunogen. Given the regenerative capabilities of peripheral nerves, we proposed that exposure to this homogenate could enhance the limited regeneration of CNS fibres, after pre-conditioning of DRG neurons. Our results showed that in adult and/or neonatal Sprague Dawley (SD) rats PNH-vaccinated, had greater number of regenerated fibres, as compared to injury matched saline-vaccinated controls. Conversely, passive exposure to PNH through parental vaccination resulted in the suppression of this regenerative trigger. This suppressed competence of CNS fibres to regenerate was indirectly correlated with a reduced number of macrophage cells throughout the SCI epicentre, as compared to greater macrophage numbers found in the adult and/or neonatal treated groups. Secondly, we explored the possibility that a systemic inflammatory effect originating from the peripheral conditioning lesion, might be able to contribute to the regeneration of other injured neurons within the matured CNS. Again, using adult SD rats, we pre-conditioned the peripheral DRG branch as previous and changed the location of the CNS injury from the spinal cord to the optic nerve. Where alike any other injured neuron within the CNS, fails to regenerate. Unfortunately, our results from anterograde or retrograde labelling did not find any regenerated optic nerve fibres, although, we did find macrophage numbers to be higher in pre-conditioned lesion animals as compared to sham-operated animals. Therefore, it is possible that the pre-conditioning peripheral lesion might be allowing for a greater macrophage infiltration into the CNS compartment. Finally, we determined whether an early macrophage infiltration into the CNS compartment could be correlated with the observed CNS regeneration, characteristic of the pre-conditioned lesion model. To test this, we temporarily depleted macrophages before, during and after peripheral nerve lesion, via liposomal clodronate delivery. Our results from anterograde and retrograde labelling of spinal cord fibres revealed no regenerated CNS fibres in macrophage depleted animals, only in injury matched controls. In conclusion, macrophage cells play a beneficial role in the regeneration of CNS afferent fibres of pre-conditioned lesion DRG neurons. This most likely occurs through activation of intrinsic somatic DRG responses, as well as, an increased macrophage activation. We believe this inflammatory response to be of favourable phenotypic characteristic to the regeneration of injured CNS neurons, especially those in proximity to the DRG cell body. In addition, we propose that the conditioning peripheral lesion permits an influx of macrophage cells into the CNS compartment before injury of the CNS DRG branch, which is also likely to be supporting regeneration of afferent fibres. Future studies should evaluate the possibility that activated inflammatory cells might be infiltrating into the CNS under minimal blood-brain barrier disruption. It is clear that a complex communication between the nervous and immune system is occurring after the initial peripheral injury.

Keywords: Spinal cord injury,Pre-conditioned lesion,inflammation,macrophage cells
Subject: Medicine thesis, Physiology thesis

Thesis type: Doctor of Philosophy
Completed: 2009
School: School of Medicine
Supervisor: Prof Xin-fu Zhou