An Adeno-Associated Viral vector encoding Neurotrophin 3 injected into affected forelimb muscles modestly improves sensorimotor function after contusive mid-cervical spinal cord injury.

Traumatic spinal cord injury (SCI) in humans occurs most frequently in the cervical spine where it can cause substantial sensorimotor impairments to upper limb function. The altered input to spinal circuits below the lesion leads to maladaptive reorganisation which often leads to hyperreflexia in proprioceptive circuits. Neurotrophin 3 (NT3) is growth factor essential for the development of proprioceptive neurons. We have previously shown that following bilateral corticospinal tract axotomy, intramuscular delivery of an Adeno-Associated Viral vector encoding NT3 (AAV-NT3) induces proprioceptive circuit reorganisation linked to functional recovery. To assess its therapeutic effects following a clinically relevant bilateral C5-C6 contusion in rats, AAV-NT3 was injected intramuscularly into the dominant limb 24 hours after injury and forelimb function was assessed over 13 weeks. The injury generated hyperreflexia of a distal forelimb proprioceptive circuit. There was also loss of fine motor skills during reach-and-grasp and walking on a horizontal ladder. Ex vivo magnetic resonance imaging (MRI) revealed atrophy of the spinal cord and white matter disruption throughout the lesion site together with extensive loss of grey matter. Unexpectedly, animals treated with AAV-NT3 had a slightly smaller lesion in the regions close to the epicentre compared to PBS treated animals. Rats treated with AAV-NT3 showed subtly better performance on the horizontal ladder and transient benefits on reach-and-grasp. AAV-NT3 did not normalise hyperreflexia in a treated muscle. The treatment increased the amount of NT3 in treated muscles but, unexpectedly, serum levels were only elevated in a small subset of animals. These results show that this dose and delivery of AAV-NT3 may generate subtle improvements in locomotion but additional treatments will be required to overcome the widespread sensorimotor deficits caused by contusion injury.