Contusion spinal cord injury (SCI) results in devastating life-long debilitation in which there are currently no effective treatments. The primary injury site presents a complex environment marked by subsequent secondary pathophysiological cascades involving excessive reactive oxygen and nitrogen species (ROS/RNS) production, glutamate-induced excitotoxicity, calcium dysregulation, and delayed neuronal apoptosis. Many of these cascades involve mitochondrial dysfunction, thus a single mitochondrial-centric therapy that targets a variety of these factors could be far reaching in its potential benefits after SCI. As such, this dissertation examines whether transplantation of exogenous mitochondria after SCI can attenuate secondary injury cascades to decrease the spread and severity of the injury.
Our first experiment tested the dose-dependent effects of mitochondrial transplantation on the ability to maintain acute overall bioenergetics after SCI. We compared transplantation of mitochondria originating from two different sources-cultured PC12 cells or rat soleus leg muscle. 24 hours after injury, State III oxygen consumption rates were maintained to over 80% of sham levels when 100ug of mitochondria was transplanted, regardless of the origin of the mitochondria. Complex I enzyme activity assays corroborated our findings that the 100ug dosage gave optimal benefits compared to vehicle injection.
We also analyzed the rostral-caudal distribution and cell-type colocalization of transplanted transgenically-labeled tGFP mitochondria after SCI. There were greater volumes and rostral-caudal spread of tGFP mitochondria at the 24 hour time point compared to 7 days post injection. tGFP mitochondria had the greatest propensity to colocalize with macrophages and pericytes. Colocalization was evident in endothelial cells, oligodendrocytes and astrocytes, though no such colabeling was present in neurons. Further, colocalization of tGFP was always greater at the 24 hour time compared to 48 hour or 7days post injection time points. These data indicate that there is a cell-type difference in incorporation potential of exogenous mitochondria which changes over time.
Finally, we tested the effects of mitochondrial transplantation on long term functional recovery. Animals were injected with either vehicle, 100ug cell-derived mitochondria, or 100ug muscle-derived mitochondria immediately after contusion SCI. Functional analyses including BBB overground locomotor scale and von Frey mechanical sensitivity tests did not show any differences between treatment groups. Likewise, there were no differences in tissue sparing when mitochondria were transplanted compared to vehicle injections, though there were higher neuronal cell counts in tGFP mitochondria injected groups caudal of the injury site.
These studies present the potential of mitochondrial transplantation for therapeutic intervention after SCI. While our acute measures do not correspond into long term recovery, we show that at 24 hours transplanted mitochondria do have an effect on bioenergetics and that they are taken into host cells. We believe that further investigation into caveats and technical refinement is necessary at this time to translate the evident acute bioenergetic recovery into long term functional recovery.