Spinal cord injury (SCI) at the cervical level compromises the function of both upper and lower extremities, thereby impeding an individual’s ability to complete daily tasks required for independent living and profoundly affecting the overall quality of life among individuals afflicted by SCI and their families. Recovery of spinal cord functions may be attained by promoting the sprouting of non-injured axons and/or the regeneration of damaged axons. The regenerative capacity of neurons differs profoundly between animal lineages and within the mammalian central and peripheral nervous systems (Tedeschi et al., 2017). Whereas axons in the peripheral nervous system are able to mount a successful regenerative response after injury, long-distance axon regeneration fails in the adult mammalian central nervous system. To date, no therapeutic strategy that aims to restore function is currently available for SCI individuals.
To gain a better understanding of the mechanisms underlying axon growth and regenerative competence, we surveyed the transcriptional landscape of mouse dorsal root ganglion (DRG) neurons in both growth competent and incompetent states in diverse cultures and in vivo experimental conditions. Among a few candidates, our unbiased and systematic approach has identified Cacna2d2, the gene encoding the α2δ2 subunit of voltage-gated calcium channels, as a molecular switch limiting axon growth and regeneration in adulthood (Tedeschi et al., 2016; Sun et al., 2020). On one hand, α2δ subunits positively regulate synaptic transmission by increasing plasma membrane expression of voltage-gated calcium channels and vesicle release probability (Hoppa et al., 2012). On the other hand, increased pathological expression of α2δ subunits may lead to neuronal hyperexcitability and axon regeneration failure following trauma and disease (Li et al., 2006; Sun et al., 2020).
Whilst Cacna2d2 overexpression in adult DRG neurons gives rise to short and highly branched neurites, genetic deletion or silencing of Cacna2d2 yields the formation of long and sparsely branched neurites in vitro (Tedeschi et al., 2016). Cacna2d2 overexpression in adult DRG neurons boosts expression of the P/Q voltage-dependent calcium channel Cav2.1 (Tedeschi et al., 2016), that is responsible for initiating synaptic transmission. Presynaptic voltage-dependent calcium channels clustered at the active zone mark the point of entry for calcium to trigger vesicle fusion and thereby mediate neurotransmitter exocytosis. In vitro, axon growth defects in Cacna2d2-overexpressing DRG neurons can be rescued by incubation with N- and P/Q-type channel blockers and by chelating calcium (Tedeschi et al., 2016). In contrast, treating control neurons with the calcium-independent secretagogue Ruthenium Red and P/Q-type channel agonists can cause defects in axon elongation similar to those seen after forcing Cacna2d2 expression (Tedeschi et al., 2016). Thus, it is likely that Cacna2d2 inhibits axon elongation in adult neurons by forcing synaptic transmission and axon branching.
More recently, we found that α2δ2 subunits also inhibit axon growth and regeneration of corticospinal neurons (Sun et al., 2020), the cells that originate the corticospinal tract. Gabapentinoids (e.g., gabapentin and pregabalin), drugs used to treat neurological disorders, bind with high affinity and selectivity to α2δ1/2 subunits (Gee et al., 1996). Systemic administration of gabapentinoids dampens excitatory synaptic transmission and promotes structural plasticity and regeneration after SCI and ischemic stroke in adult mice (Tedeschi et al., 2016, 2022; Sun et al., 2020). Moreover, we have shown that mice administered gabapentin recover upper extremity function after cervical SCI and ischemic stroke (Sun et al., 2020; Tedeschi et al., 2022), and a multi-center cohort study has found that motor recovery is improved in SCI individuals receiving early versus late gabapentinoids administration (Warner et al., 2017).
A combinatorial approach is required to maximize recovery of forelimb function after cervical SCI. Among other promising strategies such as neuroprotection, cell reprogramming/transplantation, and neuromodulation, cardiovascular exercise has proven to stimulate motor recovery by augmenting neuronal plasticity following SCI. Keeping this in mind, we asked whether gabapentinoids administration may synergize with cardiovascular-based efforts like voluntary treadmill training to strengthen synaptic connections within the injured spinal cord. To our surprise, we failed to observe any additive forelimb recovery after combining gabapentin daily administration (beginning 1 hour after SCI) and voluntary treadmill training (3–5 sessions/week) (Rodocker et al., 2022). It is important to note that SCI mice administered gabapentin consistently participated in voluntary treadmill training. In contrast, mice administered vehicle showed a decline in participation (Rodocker et al., 2022), especially at chronic time points when the potential for intrinsic motivation was drained.
In an effort to rescue participation, we later introduced an external motivator into our experimental scheme. Rather than running alone, we allowed each cohort of mice to run on the treadmill together. While SCI mice administered gabapentin maintained consistent participation within the training regimen, mice administered vehicle displayed an increase in participation almost immediately following the implementation of group runs (Rodocker et al., 2022). Although modest, mice administered vehicle also achieved additional recovery of forelimb function following the group runs (Rodocker et al., 2022), highlighting the importance of group-based intervention strategies in the overall path to recovery following SCI.
SCI not only leads to long-term neurological deficits but also to the deterioration of mental well-being (Brakel et al., 2021). In fact, SCI individuals are more subjected to psychological distress and are more likely to develop depressive disorders and anxiety when compared to the general population. Given that increased inflammation has been shown to positively correlate with depression in both animal and human studies and that gabapentinoids are often prescribed off-label to treat depression and anxiety, we tested the extent to which gabapentinoids counteract the deterioration of mental health by lowering neuroinflammation after SCI in adult mice. By comparing neuroinflammatory gene expression profiles, no major differences were found that could explain changes in participation in the rehabilitation program (Rodocker et al., 2022). Of interest, we found nine genes that were enhanced 2 weeks after SCI at the lesion site of mice administered gabapentin. Of these, Birc5 encodes the baculoviral IAP repeat containing 5 (also known as Survivin). A member of the inhibitor of apoptosis gene family, Survivin silencing inhibits the proliferation and differentiation of neural precursor cells in the dentate gyrus of the hippocampus after brain trauma, leading to poor functional recovery (Zhang et al., 2015). Fpr1 encodes the formylpeptide receptor 1 (FPR1), a member of the family of G protein-coupled chemoattractant receptors that promote the migration and differentiation of neural stem cells (Zhang et al., 2017). Thus far, the transcriptional or post-transcriptional-dependent mechanisms underlying changes in Birc5 and Fpr1 expression in mice administered gabapentin remain unknown.
Dividing progenitor cells in the subgranular zone of the dentate gyrus of the adult mammalian hippocampus give rise to neurons. Impairment of neurogenesis within the adult hippocampus is directly connected to anxiety-related behaviors and overall mental health abnormalities (Revest et al., 2009). Given that mice administered gabapentin displayed a better recovery profile and are consistently engaged in the SCI rehabilitation program at subacute and chronic time points (Rodocker et al., 2022), we questioned whether or not gabapentin administration increases hippocampal neurogenesis after SCI. As a matter of fact, SCI mice administered gabapentin displayed enhanced hippocampal neurogenesis as illustrated by the surge in overall bromodeoxyuridine-positive cells representing neural progenitor cells and immature neurons in the subgranular zone (Rodocker et al., 2022). Of note, gabapentinoids have previously been shown to positively modulate adult hippocampal neurogenesis in experimental models of chronic restraint stress (Valente et al., 2012). Building on these findings, we asked if increased neurogenesis in mice administered gabapentin is associated with changes in the expression of the tyrosine receptor kinase B (TrkB), given that TrkB plays a crucial role in hippocampal neurogenesis. We found a remarkable ~50% increase in TrkB expression in the subgranular zone of the dentate gyrus in mice administered gabapentin at 14 days after cervical SCI (Rodocker et al., 2022). To date, it is not clear whether TrkB expression in SCI mice administered gabapentin will consistently be elevated at chronic time points. The molecular mechanisms underlying changes of TrkB expression as well as other potential mechanisms sponsoring neurogenesis in mice administered gabapentin require further investigation. Finally, we discovered reduced anxiety-like behavior in SCI mice administered gabapentin as compared to vehicle control (Rodocker et al., 2022).
Taken together, experimental evidence underscores the beneficial action of gabapentinoids to promote structural plasticity and regeneration and address SCI psychopathology (Figure 1). Further, this evidence highlights the need to consider repurposing gabapentinoids as a novel treatment for central nervous system repair. Overcoming side effects associated with prolonged gabapentinoids administration and polypharmacy in SCI individuals must ultimately be investigated prior to embarking on clinical trials in humans.
We would like to thank Dr. Wenjing Sun (The Ohio State University, Wexner Medical Center) for critically reading. Research in the Tedeschi laboratory was supported by the National Institute of Neurological Disorders (R01NS110681 and R21NS109787 (to AT)), the Chronic Brain Injury Program (to AT), and The Ohio State University/Wexner Medical Center. We apologize to all colleagues whose relevant work was not included due to space limitations.
C-Editors: Zhao M, Liu WJ, Li CH; T-Editor: Jia Y
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