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Content archived on 2024-06-18

Regeneration and Target Reinnervation after Spinal Cord Injury

Final Report Summary - AXON REGENERATION (Regeneration and Target Reinnervation after Spinal Cord Injury)

Recent studies have indicated that combinatorial treatments that target multiple mechanisms are necessary to address the limited regenerative capacity in the adult nervous system. We have shown that chemotropic guidance by growth factor gradients, activation of cell-intrinsic regenerative programs and suitable cellular bridges at a lesion site allow for axons to extend across a lesion site in the injured spinal cord. Further, regenerated axons form ultrastructurally identified synapses with appropriate target neurons. However, several challenges that limit the relevance of current approaches remain: Regenerating axons only extend for limited distances beyond the lesion site and axons are not functional possibly due to an insufficient number of synapses or a lack of myelination.
The proposed experiments addressed these limitations in 3 specific aims. We examined spatially and temporally controlled expression of neurotrophic factors (NT-3) to enhance the distance of regeneration and target reinnervation by ascending sensory axons after adult spinal cord injury. We have further defined the requirements for persistent target innervation using transient neurotrophin expression. Finally, we determined whether Schwann cell and/or oligodendrocyte precursor cell grafts can re-myelinate regenerating axons.

In the last 4 years of support, we have made significant progress in several of these goals:

First, we have shown that transient NT-3 expression is insufficient to sustain all axons that have regenerated beyond a site of spinal cord injury. Changes in the expression of extracellular matrix components and cell adhesion molecules might underlie the decline in axonal growth (Hou et al. J Neurosci 2012).

Second, we have evaluated the potential of adult neural progenitors (NPCs) to diffierentiate into oligodendrocytes in the injured spinal cord and the influence of adult bone marrow stromal cells (MSCs) on oligodendroglial differentiation. These data have shown that oligodendroglial differentiation of subventricular zone-derived NPCs is strongly enhanced in vitro by co-cultivation of NPCs with MSCs or MSC-conditioned medium. However, in vivo co-grafting of syngeneic NPCs mixed with MSCs did not enhance oligodendroglial differentiation of co-grafted NPCs six weeks post-injury and grafting. The failure of MSCs to induce oligodendroglial differentiation in vivo coincided with a rapid upregulation of bone morphogenetic protein 2/4 (BMP2/4) around the injury site and in vitro data demonstrated that BMP2/4 can override the oligodendrogenic effects of MSCs. Thus, neutralization of BMP or BMP signaling might be required to allow for MSC-induced oligodendroglial differentiation of grafted NPCs in the injured spinal cord. (Sandner et al. 2013).

Finally, we have examined the ability of neural stem cells to act as functional relays in the injured spinal cord. These experiments demonstrated that fetal stem cells can improve sensorimotor recovery after a complete spinal cord transection. We have further established a rat model to examine changes in cardiovascular parameters after spinal cord injury and investigated the influence of fetal neural stem cells after complete spinal cord transection. We could demonstrate that basal cardiovascular paprameters can be normalized and autonomic dysreflexia partially ameliorated (Hu et al. 2013).