Conditional RAC1 knockout in motor neurons restores H-reflex rate-dependent depression after spinal cord injury

Abstract

A major complication with spinal cord injury (SCI) is the development of spasticity, a clinical symptom of hyperexcitability within the spinal H-reflex pathway. We have previously demonstrated a common structural motif of dendritic spine dysgenesis associated with hyperexcitability disorders after injury or disease insults to the CNS. Here, we used an adeno-associated viral (AAV)-mediated Cre-Lox system to knockout Rac1 protein expression in motor neurons after SCI. Three weeks after AAV9-Cre delivery into the soleus/gastrocnemius of Rac1-“floxed” adult mice to retrogradely infect spinal alpha-motor neurons, we observed significant restoration of RDD and reduced H-reflex excitability in SCI animals. Additionally, viral-mediated Rac1 knockdown reduced presence of dendritic spine dysgenesis on motor neurons. In control SCI animals without Rac1 knockout, we continued to observe abnormal dendritic spine morphology associated with hyperexcitability disorder, including an increase in mature, mushroom dendritic spines, and an increase in overall spine length and spine head size. Taken together, our results demonstrate that viral-mediated disruption of Rac1 expression in ventral horn motor neurons can mitigate dendritic spine morphological correlates of neuronal hyperexcitability, and reverse hyperreflexia associated with spasticity after SCI. Finally, our findings provide evidence of a putative mechanistic relationship between motor neuron dendritic spine dysgenesis and SCI-induced spasticity.

Preclinical Evidence for the Role of Botulinum Neurotoxin A (BoNT/A) in the Treatment of Peripheral Nerve Injury

“…A time-dependent recovery of CMAP amplitude was observed in both saline- and BoNT/A-injected mice, however, the values in the latter group were significantly greater at times ≥21 days following CCI. Moreover, in saline-injected mice, the ratio of the Hoffmann reflex (H-reflex) to the direct motor (M) response increased from day 14 to 28 following CCI, consistent with the development of hyperreflexia [ 126 ]. However, a single injection of BoNT/A in the plantar region on day 5 counteracted this abnormal condition.…”

Molecular Identification of Pro-Excitogenic Receptor and Channel Phenotypes of the Deafferented Lumbar Motoneurons in the Early Phase after SCT in Rats

“…Spasticity caused by SCI is often marked by a slow increase in excitation and over-activity of both flexors and extensors, with reactions possibly occurring many segments away from the stimulus [ 4 ]. In experimental SCI in rodents predominantly used for SCI modeling, the onset of spasticity can be seen as early as one-week postinjury [ 5 , 6 , 7 ]. After a mild contusive SCI at the lumbar L2 level in adult mice, signs of spasticity of hindlimb muscles were observed 1-week postinjury [ 6 ].…”

“…In experimental SCI in rodents predominantly used for SCI modeling, the onset of spasticity can be seen as early as one-week postinjury [ 5 , 6 , 7 ]. After a mild contusive SCI at the lumbar L2 level in adult mice, signs of spasticity of hindlimb muscles were observed 1-week postinjury [ 6 ]. Similarly, in a tail-spasticity model in an adult rat with a complete spinal cord transection (SCT) at the sacral S2 segment, the first signs of spasticity of the tail were observed 1-week postinjury, which was sustained at 2–3 weeks and gradually worsened until 2 months [ 5 ].…”

“…The importance of dendritic spine dysgenesis in the development of spasticity is also emerging [ 14 ]. In a recent study, Benson and co-workers [ 6 ] provided the first evidence for a mechanistic relationship between MN dendritic spine dysgenesis, hyperexcitability, and SCI-induced spasticity. Till now, evidence for dendritic dysgenesis and molecular changes in neurotransmission-related molecules in MNs in spinal animals has been obtained mostly for the late postlesion phase (3–12 weeks), when hyperexcitability is well established.…”

“…Our recent data, which showed a decrease in transcript levels of Grin1 NMDA and Gria2 AMPA receptor subunits after spinalization and nerve implantation, support this assumption [ 27 ]. Here we aimed to disclose the direction and extent of changes in their transcript levels in the hindlimb MNs in the subacute phase after SCI (second-week postinjury) when the spinal shock is resolved and the spasticity starts to develop [ 5 , 6 , 9 ]. To better understand the molecular underpinnings of developing hyperexcitability early postlesion, we sought to investigate also whether SCI leads to a concomitant decrease of transcript expression of the inhibitory GABA and Glycinergic receptors and altered expression of serotoninergic (5-HT) and noradrenergic (NA) receptors in response to the loss of descending sources of 5-HT and NA, respectively [ 28 , 29 ].…”