One of the most vexing issues in the field of spinal cord injury is the accumulation of fibrosis and scar that seems to inhibit the ability of neurons and their axons to repopulate the cord and recover neurological function. The precise role of scarring following spinal cord injury is not really understood, and the cascade of events that occur in the cord are extraordinarily complex and have yet to be fully worked out. Heavy emphasis has been placed on how the prevention of scar accumulation can promote neural regeneration. Indeed, when specific pathways of scar formation following spinal cord injury are inhibited in animal models, neurological recovery is improved. However, a recently published report reminds us that scar formation is a necessary evil that serves to preserve the structural integrity of the spinal cord and limits neuronal loss (Sabelström H, Stenudd M, Réu P, et al. Resident neural stem cells restrict tissue damage and neuronal loss after spinal cord injury in mice. Science. 2013;342(6158):637-640). Thus, strict adherence to an approach of limiting scar formation may not be the most beneficial technique.
One of the first surgical approaches attempted to try and improve neurological function after spinal cord injury was that of stem cell transplantation. This approach has shown promise, and thus some investigators have sought to try and modulate the activity of endogenous stem cells to achieve similar effects. Sabelström et al suggest that implementation of such a strategy requires careful knowledge of the full role of these endogenous stem cells in scar formation. To evaluate how ependymal neural stem cells in the adult mouse spinal cord function after injury, these investigators created a genetically recombinant mouse whereby ependymal neural stem cell proliferation could be turned off by the administration of tamoxifen. Once the mice were created, tamoxifen administration established that ependymal cell progeny were selectively reduced in the mutant mice, but not controls, validating the use of the model. Following an incisional injury to the dorsal cord, ependymal cells migrate to the site of injury and differentiate into scar-forming astrocytes. However, in the recombinant mice, no ependymal stem cell migration was observed.
Next, Sabelström et al observed the pattern of injury following a dorsal incision to the spinal cord over a period of 14 weeks in the recombinant and control mice. All control mice developed dense scar at the site of injury, while 80% of the mutant mice failed to produce a compact scar. Instead, the mutant mice developed varying degrees of tissue defects in the place of a scar. Interestingly, the mutant mice displayed enhanced fibrosis, a process mediated by endogenous pericytes, a process that could be an attempt to compensate for the loss of ependymal cell development. At the end of the 14 weeks, the mutant recombinant mice possessed considerably larger tissue defects at the site of injury. This is distinct from pericyte-mediated scar inhibition where the resultant injury cavity is not meaningfully larger than in controls. Specifically, these investigators evaluated whether their superficial dorsal lesion could extend all the way to the corticospinal tract. In only 1 of the control mice did scar extend to the corticospinal tract, while the tissue defects in the mutant mice extended there in well over half of the mutant mice. This finding was confirmed by serial magnetic resonance imaging (MRI) studies that demonstrated that mutant mice had a 30% increase in the size of their spinal cord lesions after 9 weeks, a difference from control animals that became significant after only 3 weeks. Interestingly, the increased size of the mutant cord injuries was not related to increased inflammation, and there were actually fewer inflammatory cells in the sites.
Not only were the injury sites larger, but atrophy of the spinal cord was meaningfully different in the absence of ependymal stem cells. In the mutant mice, atrophy extended 2 segments rostral and caudal to the lesion. This finding was confirmed again by MRI. Immunohistochemical studies revealed a 20% increase in the loss of neurons in the mutant spinal cords over time. This is in stark contrast to pericyte-mediated scarring, where there was no increased neuronal loss or atrophy. Moreover, there was a substantial number of apoptotic neurons in the mutant injured spinal cord, suggesting a pathway by which the cell loss occurs. Finally, neurotrophic factor expression is upregulated at the site of spinal cord injury, and exogenously added factors, like insulin-like growth factor, promote neuronal survival. To examine differences in neurotrophic factor abundance in this model, immunohistochemical studies were performed. In the absence of ependymal stem cells in the mutant mice, there was a 50 to 80% decrease in the up-regulation of neurotrophic factor mRNA. These findings suggest that the ependymal-derived neural stem cells are likely a primary source of neurotrophic support to the injured cord as well.
Overall, this work by Sabelström et al highlights several key features about spinal cord injury that need to be considered. First, there are a number of cell types that have different roles in regulating scar formation, and interruption of 1 cell type's function can have markedly different effects vs another's. Second, this work suggests that modulation of intrinsic neural stem cell activity may not be the preferred route toward improving results after spinal cord injury. Specifically, in the absence of endogenous ependymal-derived neural stem cells and their scar, the results of spinal cord injury were observed to be worse. Finally, it appears that our true goal may be to maximize the positive effects of scar formation while limiting its impact on neuronal repopulation and axonal growth.