Contribution of microglial reactivity to olfactory ensheathing cell migration in vivo
Olfactory ensheathing cells (OECs) are glial cells that are an attractive candidate for neural repair after spinal cord injury and for remyelination of axons in diseases such as multiple sclerosis. OECs appear to migrate within the adult mammalian central nervous system (CNS) in animal models of spinal cord injury, but until recently there has been no systematic examination of the factors inducing or guiding this migration. Previous work in our lab (V.Skihar) implicated microglial reactivity in the generation of a migratory signal(s) inducing OECs to migrate towards an ethidium bromide-induced focal demyelination in the adult rat spinal cord. The long-term objective of this research project was to test the hypothesis that reactive microglial provide a migratory signal(s) driving the migration of OECs within the spinal cord of adult rats.The first set of experiments determined the time-frame in which Wallerian degeneration (WD) induced microglial reactivity occurs in the right dorsal corticospinal tract (dCST) of adult rats at the level of T11 following aspiration of the contralateral sensorimotor cortex. This timing data from this study demonstrated a prominent microglial activation in the right dCST of T11 eight weeks after sensorimotor cortex injury indicating the microglial response to WD of dCST axons was very slow to appear. The second set of experiments determined whether OECs were induced to migrate in response to WD-induced microglial reactivity in the dCST, which based on the first set of experiments was known to occur within 8 weeks of lesioning the left sensorimotor cortex. This second set of experiments also examined the migratory path taken by OECs with respect to the location of reactive microglia (i.e. inside vs outside the right dCST). For these experiments, the left sensorimotor cortex was damaged 8 weeks prior to grafting the OECs at T12. The next group of experiments examined the contribution of TNF-á induced microglial reactivity to generation of a migratory signal. First we identified concentrations of TNF-á that when injected into the DF of the T11 spinal cord segment of an adult rat induced microglial reactivity either along at least a 5 mm distance from the injection site or confined to the immediate vicinity of the injection site. The result of this experiment identified a concentration of 1 ng/µl and 0.01 ng/µl TNF-á as appropriate concentrations to induce the appropriate amount of microglial reactivity, respectively. The final set of experiments used these two concentrations to determine whether TNF-á induced microglial reactivity that is initiated 5 mm rostral to a DiI+ve OEC graft generates a migratory signal(s) inducing OECs to migrate towards the rostral part of T11 and whether the migratory signal(s) was present only if the microglial reactivity extended the full 5 mm distance between the TNF-á injection and the OEC graft. The major findings were: i) there was a significantly higher density of DiI+ve OECs within the right dCST of rats in which there was WD-induced microglial reactivity as compared to the right dCST of rats in which there was no microglial reactivity; ii) the migratory path taken by DiI+ve OECs was preferentially within areas containing reactive microglia (i.e. dCST) and towards the site of TNF-á induced microglial reactivity (i.e. rostral to cell graft as opposed to caudal); iii) significantly more DiI+ve OECs migrated towards the site of a TNF-á injection when the microglia were reactive along the entire length of the migratory path between the cytokine injection and cell graft; and iv) minocycline treatment both dampened microglial reactivity and significantly reduced the number of migrating DiI+ve OECs. The major conclusions are that the migration of OECs within the adult rat spinal cord occurs in response to migratory signal(s) arising as a result of microglial activation and that this migration occurs preferentially along the path of microglial reactivity.
DegreeDoctor of Philosophy (Ph.D.)
DepartmentAnatomy and Cell Biology
ProgramAnatomy and Cell Biology
SupervisorDoucette, J. Ronald
Olfactory Ensheathing Cell