Cell therapy for spinal cord injury, studies of motor and sensory systems

Abstract

Spinal cord injury represents a significant medical concern, affecting thousands of individuals each year. The majority of victims are young males harmed by traffic accidents, violence, and falls. Severe spinal cord injury interupts nerve fiber pathways connecting the brain and the rest of the body. This leads to paralysis, loss of sensation, and reflex function below the level of injury. It also may significantly affect autonomic activity, including respiratory, vascular, bowel, and bladder control. Over time, spinal cord injury victims are also prone to develop other symptoms such as chronic pain, muscle spasms, and infections.

The first part of the present thesis describes functional and structural alterations that follow a contusion injury in adult rats. We were interested in determining whether the pattern of spontaneous sensory and motor recovery in weight-drop injured rats resembles that observed in human spinal cord injury victims. Using behavioral tests and functional magnetic resonance imaging, we found that rats subjected to a moderate spinal cord injury recovered close to normal hindpaw locomotion in the absence of sensory function. Thus, assessment of locomotor function in the rat is not a good indicator of sensory signaling across the injury site. This is in contrast to humans, where sensory function predicts locomotor outcome.

The failure of nerve fiber regeneration following traumatic injury of the spinal cord has been attributed to the insufficient intrinsic growth capacity and/or growth inhibitory properties of the scar tissue. We characterized sprouting of the severed corticospinal tract using in situ hybridization and stereology. Axotomized pyramidal cells upregulated growth-associated molecules and underwent signifcant sprouting rostral to the injury site. However, outgrowing fibers did not regenerate beyond the lesion but remained rostral to the injury site where they formed a massive "central neurinoma".

The second part of the thesis investigates effects of mesenchymal and neural stem cell grafts on outcome following spinal cord injury. Mesenchymal stem cells grafted into the lesion site 7 days following a weight drop injury. Engrafted MSCs formed bundles that bridged the epicenter of the injury. MSC-aggregates contained host-derived immature astrocytes and provided directional guidance to regrowing nerve fibers. Transplantation of MSCs led to significantly improved recovery of open-field walking. We also grafted adult neural stem cells into the lesion 7 days following a contusion injury of the spinal cord. The majority of engrafted cells gave rise to astrocytes. Grafting of these cells improved locomotor function; however, it also causes aberrant axonal sprouting of CGRP-positive fibers that were associated with allodynia-like hypersensitivity of forepaws.

Transduction of neural stem cells with neurogenin2 prior to transplantation suppressed astrocytic differentiation of engrafted cells and prevented the development of allodynia. Instead, ectopic expression of neurogenin2 allowed for enhanced oligodendroglial differentiation of engrafted cellsand increased amounts of myelin in the injured area. This was correlated to significantly better recovery of skilled hindlimb motor tasks. Importantly, neural stem cells transduced with neurogenin2 also allowed for improved hindlimb sensory function and increased cortical BOLD-signaling in response to hindpaw stimulation.

To conclude, functional MRI and stereology allowed objective quantitative measurents of functional and structural recovery following spinal cord injury. We used these methods to characterize the effects of two types of stem cell engraftment in spinal cord injury models. We also identified a potentially harmful side effect of stem cell grafting and found a remedy. These kinds of analysis of current experimental treatment strategies are needed to select which approaches to translate into clinical trials.