Sydney-Smith, J. D. | bioRxiv Science

bioRxiv2022

Delayed viral vector mediated delivery of neurotrophin-3 improves skilled hindlimb function and stability after thoracic contusion in rats

It has been reported that intramuscular injection of an Adeno-associated viral vector serotype 1 (AAV1) encoding Neurotrophin-3 (NT3) into hindlimb muscles 24 hours after a severe T9 contusion in rats induced lumbar spinal neuroplasticity, partially restored locomotive function and reduced spasms during swimming. Here we investigated whether a targeted delivery of NT3 to lumbar and thoracic motor neurons 48 hours following a severe contusive injury aids locomotive recovery in rats. AAV1-NT3 was injected into the tibialis anterior, gastrocnemius and rectus abdominus muscles 48-hours following trauma, persistently elevating serum levels of the neurotrophin. NT3 improved trunk stability, accuracy of stepping during skilled locomotive tasks, and alternation of the hindlimbs during swimming, but it had no effect on gross locomotion function in the open field. The number of vGlut1+ (likely proprioceptive afferent) boutons on gastrocnemius -motor neurons was increased after injury but normalised following NT3 treatment suggestive of a mechanism in which the functional effects may be mediated through proprioceptive feedback. Ex vivo MRI revealed substantial loss of grey and white matter at the lesion epicentre but no effect of delayed NT3 treatment to induce neuroprotection or prevent secondary damage. Spasms and hyperreflexia were not reliably induced in this severe injury model suggesting a more complex anatomical or physiological cause to their induction. We have shown that delayed intramuscular AAV-NT3 treatment can promote recovery in skilled stepping and coordinated swimming supporting a role for NT3 as a therapeutic strategy for spinal injuries potentially through modulation of somatosensory feedback. Key PointsO_LITargeted delivery of NT3 to hindlimb and trunk muscles at a clinically relevant 48h following a severe thoracic contusion aids fine locomotor control and synchronised movement. C_LIO_LINT3 mediated improvements in trunk stability, accuracy of stepping during skilled locomotive tasks, and alternation of the hindlimbs during swimming through the normalisation of vGlut1+ boutons on presumptive proprioceptive afferents innervating these muscles. C_LIO_LI250kDyn thoracic contusion does not reliably result in measurable signs of spasticity. C_LI

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bioRxiv2021

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.

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