Characterising changes in pathology and function in clinically relevant models of spinal cord injury and using chondroitinase ABC gene therapy to promote repair
Over recent years the bacterial enzyme chondroitinase ABC (ChABC) has emerged as a promising experimental therapeutic for the treatment of spinal cord injury (SCI). In pre-clinical studies ChABC has repeatedly been shown to enhance functional recovery in a number of SCI models in both rodents and larger animals through its enzymatic degradation of inhibitory chondroitin sulphate proteoglycans (CSPGs). However, ChABC treatment has met with limited success in more traumatic, translational models of SCI, such as contusive or compressive injuries. As such injury models mimic the most common form of SCI in humans, it is important to show efficacy of experimental therapeutics in these models. The key aims of this thesis therefore, are to improve upon current methods of ChABC delivery and to assess the efficacy of optimised ChABC in a clinically relevant contusion injury model. The first series of experiments involved a detailed characterisation of the temporal pattern of functional and anatomical changes that occur following a moderate thoracic contusion injury. Changes in dorsal column sensory axon conduction were associated with early demyelination in the perilesional area and subsequent remyelination mediated primarily by Schwann cells. Further electrophysiological analysis revealed a population of viable dorsal column sensory fibres that remained unable to conduct at chronic post-injury time points, in which conduction could be restored following cooling of the lesion site. This established a reproducible and clinically relevant model, with multiple outcome measures and parameters with which to assess the efficacy of optimised ChABC. A gene delivery approach was applied to optimise the administration of ChABC. Sustained and widespread CSPG degradation was achieved using a lentiviral vector containing genetically engineered ChABC (LV-ChABC). This treatment resulted in significant improvements in injury pathology and functional recovery in the moderate thoracic confusion injury model. LV-ChABC treatment resulted in neuroprotection, improved behavioural function, increased spinal conduction through the contusion injury and enhanced plasticity below the level of the injury. Additionally, ChABC gene therapy was associated with modulation of the early post-injury immune response which may have contributed to its effects on neuroprotection, whilst the effect of long-term CSPG degradation were more likely to be responsible for the observed effects on plasticity and spinal conduction. Since recovery of upper limb function is a top priority for SCI patients, further assessments of LV-ChABC were carried out in a moderate contusion injury performed at the cervical level. This resulted in similar neuroprotective effects and functional recovery. In addition, electrophysiological assessments revealed some improved corticospinal tract function below the level of the injury. Finally, LV-ChABC also resulted in neuroprotection and improved spinal conduction in a more severe model of thoracic contusion injury, illustrating the robust effects of ChABC gene therapy in clinically relevant SCI models of varying severity and at different spinal levels. Thus, ChABC gene therapy achieves sustained and widespread degradation of growth inhibitory CSPG molecules following a single administration, resulting in significantly improved injury pathology and functional repair of the spinal cord in traumatic, clinically relevant models of spinal contusion injury. If safety issues associated with gene therapy can be addressed and efficacy can be demonstrated in experimental SCI models in larger animals, then ChABC gene therapy represents a promising candidate for clinical translation.
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