Effective neuropathic pain relief through sciatic nerve administration of GAD65-expressing rAAV2

Nanotechnology for Pain Management: Current and Future Therapeutic Interventions

Pain is one of the most common medical conditions and affects more Americans than diabetes, heart disease, and cancer combined. Current pain treatments mainly rely on opioid analgesics and remain unsatisfactory. The life-threatening side effects and addictive properties of opioids demand new therapeutic approaches. Nanomedicine may be able to address these challenges as it allows for sensitive and targeted treatments without some of the burdens associated with current clinical pain therapies. This review discusses the physiology of pain, the current landscape of pain treatment, novel targets for pain treatment, and recent and ongoing efforts to effectively treat pain using nanotechnology-based approaches. We highl ight advances in nanoparticle-based drug delivery to reduce side effects, gene therapy to tackle the source of pain, and nanomaterials-based scavenging to proactively mediate pain signaling.

Microglia and Inhibitory Circuitry in the Medullary Dorsal Horn: Laminar and Time-Dependent Changes in a Trigeminal Model of Neuropathic Pain

Craniofacial neuropathic pain affects millions of people worldwide and is often difficult to treat. Two key mechanisms underlying this condition are a loss of the negative control exerted by inhibitory interneurons and an early microglial reaction. Basic features of these mechanisms, however, are still poorly understood. Using the chronic constriction injury of the infraorbital nerve (CCI-IoN) model of neuropathic pain in mice, we have examined the changes in the expression of GAD, the synthetic enzyme of GABA, and GlyT2, the membrane transporter of glycine, as well as the microgliosis that occur at early (5 days) and late (21 days) stages post-CCI in the medullary and upper spinal dorsal horn. Our results show that CCI-IoN induces a down-regulation of GAD at both postinjury survival times, uniformly across the superficial laminae. The expression of GlyT2 showed a more discrete and heterogeneous reduction due to the basal presence in lamina III of 'patches' of higher expression, interspersed within a less immunoreactive 'matrix', which showed a more substantial reduction in the expression of GlyT2. These patches coincided with foci lacking any perceptible microglial reaction, which stood out against a more diffuse area of strong microgliosis. These findings may provide clues to better understand the neural mechanisms underlying allodynia in neuropathic pain syndromes.

Viral Vector-Mediated Gene Transfer of Glutamic Acid Decarboxylase for Chronic Pain Treatment: A Literature Review

Chronic pain is long-lasting nociceptive state, impairing the patient's quality of life. Existing analgesics are generally not effective in the treatment of chronic pain, some of which such as opioids have the risk of tolerance/dependence and overdose death with higher daily opioid doses for increasing analgesic effect. Opioid use disorders have already reached an epidemic level in the United States; therefore, nonopioid analgesic approach and/or use of nonpharmacologic interventions will be employed with increasing frequency. Viral vector-mediated gene therapy is promising in clinical trials in the nervous system diseases. Glutamic acid decarboxylase (GAD) enzyme, a key enzyme in biosynthesis of γ-aminobutyric acid (GABA), plays an important role in analgesic mechanism. In the literature review, we used PubMed and bioRxiv to search the studies, and the eligible criteria include (1) article written in English, (2) use of viral vectors expressing GAD67 or GAD65, and (3) preclinical pain models. We identified 13 eligible original research articles, in which the pain models include nerve injury, HIV-related pain, painful diabetic neuropathy, and formalin test. GAD expressed by the viral vectors from all the reports produced antinociceptive effects. Restoring GABA systems is a promising therapeutic strategy for chronic pain, which provides evidence for the clinical trial of gene therapy for pain in the near future.

Mechanism of dorsal root ganglion stimulation for pain relief in painful diabetic polyneuropathy is not dependent on GABA release in the dorsal horn of the spinal cord

AIMS

It is hypothesized that dorsal root ganglion stimulation (DRGS), sharing some of the mechanisms of traditional spinal cord stimulation (SCS) of the dorsal columns, induces γ-aminobutyric acid (GABA) release from interneurons in the spinal dorsal horn.

METHODS

We used quantitative immunohistochemical analysis in order to investigate the effect of DRGS on intensity of intracellular GABA-staining levels in the L4-L6 spinal dorsal horn of painful diabetic polyneuropathy (PDPN) animals. To establish the maximal pain relieving effect, we tested for mechanical hypersensitivity to von Frey filaments and animals received 30 minutes of DRGS at day 3 after implantation of the electrode. One day later, 4 Sham-DRGS animals and four responders-to-DRGS received again 30 minutes of DRGS and were perfused at the peak of DRGS-induced pain relief.

RESULTS

No significant difference in GABA-immunoreactivity was observed between DRGS and Sham-DRGS in lamina 1-3 of the spinal levels L4-6 neither ipsilaterally nor contralaterally.

CONCLUSIONS

Dorsal root ganglion stimulation does not induce GABA release from the spinal dorsal horn cells, suggesting that the mechanisms underlying DRGS in pain relief are different from those of conventional SCS. The modulation of a GABA-mediated "Gate Control" in the DRG itself, functioning as a prime Gate of nociception, is suggested and discussed.

Normalizing HDAC2 Levels in the Spinal Cord Alleviates Thermal and Mechanical Hyperalgesia After Peripheral Nerve Injury and Promotes GAD65 and KCC2 Expression

Neuropathic pain is a worldwide health concern with poor treatment outcomes. Accumulating evidence suggests that histone hypoacetylation is involved in development and maintenance of neuropathic pain. Thus, many natural and synthetic histone deacetylase (HDACs) inhibitors were tested and exhibited a remarkable analgesic effect against neuropathic pain in animals. However, studies evaluating specific subtypes of HDACs contributing to neuropathic pain are limited. In this study, using the chronic constriction injury (CCI) rat model, we found that mRNA and protein levels of HDAC2 were increased in the lumbar spinal cord of rats after sciatic nerve injury. Intrathecal injection of TSA, a pan-HDAC inhibitor, suppressed the increase in HDAC2 protein but not mRNA, and showed a dose-dependent pain-relieving effect. By introducing HDAC2-specific shRNA into the spinal cord via a lentivirus vector, we confirmed that HDAC2 mediates mechanical and thermal hyperalgesia after nerve injury. Further examination found two essential participants in neuropathic pain in the inhibitory circuit of the central nervous system: GAD65 and KCC2 were increased in the spinal cord of CCI rats after HDAC2 knockdown. Thus, our research confirmed that HDAC2 was involved in mechanical and thermal hyperalgesia induced by peripheral nerve injury. Furthermore, GAD65 and KCC2 were the possible downstream targets of HDAC2 in pain modulation pathways.

Nonviral vector plasmid DNA encoding human proenkephalin gene attenuates inflammatory and neuropathic pain-related behaviors in mice

Inflammatory pain and neuropathic pain are major clinical health issues that represent considerable social and economic burden worldwide. In the present study, we investigated the anti-nociceptive efficacy of delivery of human proenkephalin gene by a plasmid DNA vector (pVAX1-PENK) on complete Freund's adjuvant (CFA) induced inflammatory pain and spared nerve injury (SNI) induced neuropathic pain in mice. Mice were intramuscularly or intrathecally administered pVAX1 or pVAX1-PENK, respectively. Pain thresholds in the pVAX1-PENK treated mice were significantly higher at day 3, then reached a peak at day 7 and lasted until day 28 after gene transfer, and the analgesic effect of pVAX1-PENK was blocked with naloxone hydrochloride. In contrast, pVAX1 treated mice did not significantly improve pain thresholds. These results indicate that peripheral or spinal delivery of a plasmid encoding human proenkephalin gene provides a potential therapeutic strategy for inflammatory pain and neuropathic pain.

Transcriptomic and behavioural characterisation of a mouse model of burn pain identify the cholecystokinin 2 receptor as an analgesic target

Burn injury is a cause of significant mortality and morbidity worldwide and is frequently associated with severe and long-lasting pain that remains difficult to manage throughout recovery. We characterised a mouse model of burn-induced pain using pharmacological and transcriptomic approaches. Mechanical allodynia elicited by burn injury was partially reversed by meloxicam (5 mg/kg), gabapentin (100 mg/kg) and oxycodone (3 and 10 mg/kg), while thermal allodynia and gait abnormalities were only significantly improved by amitriptyline (3 mg/kg) and oxycodone (10 mg/kg). The need for relatively high opioid doses to elicit analgesia suggested a degree of opioid resistance, similar to that shown clinically in burn patients. We thus assessed the gene expression changes in dorsal root ganglion neurons and pathophysiological mechanisms underpinning burn injury-induced pain using a transcriptomic approach. Burn injury was associated with significantly increased expression of genes associated with axon guidance, neuropeptide signalling, behavioural defence response and extracellular signalling, confirming a mixed neuropathic and inflammatory aetiology. Notably, among the pain-related genes that were upregulated post-injury was the cholecystokinin 2 receptor (Cckbr), a G protein-coupled receptor known as a pain target involved in reducing opioid effectiveness. Indeed, the clinically used cholecystokinin receptor antagonist proglumide (30 mg/kg) was effective at reversing mechanical allodynia, with additional analgesia evident in combination with low-dose oxycodone (1 mg/kg), including significant reversal of thermal allodynia. These findings highlight the complex pathophysiological mechanisms underpinning burn injury-induced pain and suggest that cholecystokinin-2 receptor antagonists may be useful clinically as adjuvants to decrease opioid requirements and improve analgesic management.

HSV vector-mediated GAD67 suppresses neuropathic pain induced by perineural HIV gp120 in rats through inhibition of ROS and Wnt5a

Human immunodeficiency virus (HIV)-related neuropathic pain is a debilitating chronic condition that is severe and unrelenting. Despite the extensive research, the exact neuropathological mechanisms remain unknown, which hinders our ability to develop effective treatments. Loss of GABAergic tone may play an important role in the neuropathic pain state. Glutamic acid decarboxylase 67 (GAD67) is one of isoforms that catalyze GABA synthesis. Here, we used recombinant herpes simplex virus (HSV-1) vectors that encode gad1 gene to evaluate the therapeutic potential of GAD67 in peripheral HIV gp120-induced neuropathic pain in rats. We found that 1) subcutaneous inoculation of the HSV vectors expressing GAD67 attenuated mechanical allodynia in the model of HIV gp120-induced neuropathic pain, 2) the anti-allodynic effect of GAD67 was reduced by GABA-A and-B receptors antagonists, 3) HSV vectors expressing GAD67 reversed the lowered GABA-IR expression, and 4) the HSV vectors expressing GAD67 suppressed the upregulated mitochondrial superoxide and Wnt5a in the spinal dorsal horn. Taken together, our studies support the concept that recovering GABAergic tone by the HSV vectors may reverse HIV-associated neuropathic pain through suppressing mitochondrial superoxide and Wnt5a. Our studies provide validation of HSV-mediated GAD67 gene therapy in the treatment of HIV-related neuropathic pain.

Gene therapy strategies for the treatment of spinal cord injury

Spinal cord injury is a complex pathology often resulting in functional impairment and paralysis. Gene therapy has emerged as a possible solution to the problems of limited neural tissue regeneration through the administration of factors promoting axonal growth, while also offering long-term local delivery of therapeutic molecules at the injury site. Of note, gene therapy is our response to the requirements of neural and glial cells following spinal cord injury, providing, in a time-dependent manner, growth substances for axonal regeneration and eliminating axonal growth inhibitors. Herein, we explore different gene therapy strategies, including targeting gene expression to modulate the presence of neurotrophic growth or survival factors and increase neural tissue plasticity. Special attention is given to describing advances in viral and non-viral gene delivery systems, as well as the available routes of gene delivery. Finally, we discuss the future of combinatorial gene therapies and give consideration to the implementation of gene therapy in humans.

Epigenetic modification of DRG neuronal gene expression subsequent to nerve injury: etiological contribution to complex regional pain syndromes (Part I)

DRG is of importance in relaying painful stimulation to the higher pain centers and therefore could be a crucial target for early intervention aimed at suppressing primary afferent stimulation. Complex regional pain syndrome (CRPS) is a common pain condition with an unknown etiology. Recently added new information enriches our understanding of CRPS pathophysiology. Researches on genetics, biogenic amines, neurotransmitters, and mechanisms of pain modulation, central sensitization, and autonomic functions in CRPS revealed various abnormalities indicating that multiple factors and mechanisms are involved in the pathogenesis of CRPS. Epigenetics refers to mitotically and meiotically heritable changes in gene expression that do not affect the DNA sequence. As epigenetic modifications potentially play an important role in inflammatory cytokine metabolism, neurotransmitter responsiveness, and analgesic sensitivity, they are likely key factors in the development of chronic pain. In this dyad review series, we systematically examine the nerve injury-related changes in the neurological system and their contribution to CRPS. In this part, we first reviewed and summarized the role of neural sensitization in DRG neurons in performing function in the context of pain processing. Particular emphasis is placed on the cellular and molecular changes after nerve injury as well as different models of inflammatory and neuropathic pain. These were considered as the potential molecular bases that underlie nerve injury-associated pathogenesis of CRPS.

Menin regulates spinal glutamate-GABA balance through GAD65 contributing to neuropathic pain

BACKGROUND

Our previous work found that tumor suppressor menin potentiates spinal synaptic plasticity in the context of peripheral nerve injury-induced neuropathic hypersensitivity, but the underlying molecular mechanisms are not clear. We hereby assessed the role of menin in regulating the spinal balance between glutamate and GABA and its contribution to the pathological condition of nerve injury-induced hypersensitivity.

METHODS

In spared nerve injury induced C57BL/6 mice, mechanical withdrawal threshold was measured with von Frey filaments after intrathecal administration of small interfering RNA (siRNA) of MEN1 or/and subcutaneous histone deacetylase (HDAC) inhibitors to control the level of glutamic acid decarboxylase 65 (GAD65). Immunoblotting and high-performance liquid chromatography were used to detect the level of protein expression and spinal glutamate and GABA, respectively.

RESULTS

Genetic knockdown of spinal menin alleviated nerve injury evoked mechanical hypersensitivity, which was strongly associated with the alteration of the spinal level of GAD65 that resulted in an imbalance of glutamate/GABA ratio from the baseline ratio of 5.8 ± 0.9 (×10(-4)) to the peak value of 58.6 ± 11.8 (×10(-4)) at the day 14 after SNI (p < 0.001), which was reversed by MEN1 siRNA to 14.7 ± 2.1 (×10(-4)) at the day 14 after nerve injury (p < 0.01). In further, selective inhibitors of HDACs considerably reversed the ratio of spinal glutamate and GABA, and also alleviated the mechanical withdrawal threshold markedly.

CONCLUSION

Our findings provide mechanistic insight into the contribution of the upregulated spinal menin to peripheral nerve injury induced neuropathic hypersensitivity by regulating glutamate-GABA balance through deactivating GAD65.

Chronic pain epidemiology and its clinical relevance

Chronic pain affects ∼20% of the European population and is commoner in women, older people, and with relative deprivation. Its management in the community remains generally unsatisfactory, partly because of lack of evidence for effective interventions. Epidemiological study of chronic pain, through an understanding of its distribution and determinants, can inform the development, targeting, and evaluation of interventions in the general population. This paper reviews current knowledge of risk markers associated with chronic pain and considers how these might inform management and prevention. Risk factors include socio-demographic, clinical, psychological, and biological factors. These are relevant to our understanding of chronic pain mechanisms and the nature of, and responses to, current and future treatments.

Comparative effectiveness of antinociceptive gene therapies in animal models of diabetic neuropathic pain

Peripheral neuropathic pain is one of the most common and debilitating complications of diabetes. Several genes have been shown to be effective in reducing neuropathic pain in animal models of diabetes after transfer to the dorsal root ganglion using replication-defective herpes simplex virus (HSV)1-based vectors, yet there has never been a comparative analysis of their efficacy. We compared four different HSV1-based vectors engineered to produce one of two opioid receptor agonists (enkephalin or endomorphin), or one of two isoforms of glutamic acid decarboxylase (GAD65 or GAD67), alone and in combination, in the streptozotocin-induced diabetic rat and mouse models. Our results indicate that a single subcutaneous hindpaw inoculation of vectors expressing GAD65 or GAD67 reduced diabetes-induced mechanical allodynia to a degree that was greater than daily injections of gabapentin in rats. Diabetic mice that developed thermal hyperalgesia also responded to GAD65 or endomorphin gene delivery. The results suggest that either GAD65 or GAD67 vectors are the most effective in the treatment of diabetic pain. The vector combinations, GAD67+endomorphin, GAD67+enkephalin or endomorphin+enkephalin also produced a significant antinociceptive effect but the combination did not appear to be superior to single gene treatment. These findings provide further justification for the clinical development of antinociceptive gene therapies for the treatment of diabetic peripheral neuropathies.

Intraganglionic AAV6 results in efficient and long-term gene transfer to peripheral sensory nervous system in adult rats

We previously demonstrated safe and reliable gene transfer to the dorsal root ganglion (DRG) using a direct microinjection procedure to deliver recombinant adeno-associated virus (AAV) vector. In this study, we proceed to compare the in vivo transduction patterns of self-complementary (sc) AAV6 and AAV8 in the peripheral sensory pathway. A single, direct microinjection of either AAV6 or AAV8 expressing EGFP, at the adjusted titer of 2×10⁹ viral particle per DRG, into the lumbar (L) 4 and L5 DRGs of adult rats resulted in efficient EGFP expression (48±20% for AAV6 and 25±4% for AAV8, mean ± SD) selectively in sensory neurons and their axonal projections 3 weeks after injection, which remained stable for up to 3 months. AAV6 efficiently transfers EGFP to all neuronal size groups without differential neurotropism, while AAV8 predominantly targets large-sized neurons. Neurons transduced with AAV6 penetrate into the spinal dorsal horn (DH) and terminate predominantly in superficial DH laminae, as well as in the dorsal columns and deeper laminae III-V. Only few AAV8-transduced afferents were evident in the superficial laminae, and spinal EGFP was mostly present in the deeper dorsal horn (lamina III-V) and dorsal columns, with substantial projections to the ventral horn. AAV6-mediated EGFP-positive nerve fibers were widely observed in the medial plantar skin of ipsilateral hindpaws. No apparent inflammation, tissue damage, or major pain behaviors were observed for either AAV serotype. Taken together, both AAV6 and AAV8 are efficient and safe vectors for transgene delivery to primary sensory neurons, but they exhibit distinct functional features. Intraganglionic delivery of AAV6 is more uniform and efficient compared to AAV8 in gene transfer to peripheral sensory neurons and their axonal processes.

Gene-based approaches in pain research and exploration of new therapeutic targets and strategies

Large panel of gene-based techniques is used for many years specifically in the pain research field. From the first identification (cloning) of some "mythic" genes, such as those encoding opioid or capsaicin receptors allowing then the creation of first-generation knockout mice, to the today conditional (time, tissue, cell-type and even pathology-dependent) and regulatable modulation of a gene function, these approaches largely contributed to fundamental leaps forward in our understanding of the function of some proteins and of their interest as possible druggable targets. Perhaps one of the most remarkable evolution in the last years is the passage of these approaches from the bench to the patient; whether it concerns the identification of genes involved in inherited pain insensibility/susceptibility, the search for genetic markers of pain types, the individual pharmacogenomics or even the first gene therapy trials. From many possible variants of gene-grounded techniques used in pain research we focus here on gene knockouts and some recent developments, on viral vectors-based gene transfer and on transgenic models for the tracing of pain pathways. Through these selected examples we attempted to emphasize the immense potential of these approaches and their already well-recognized contribution in both the basic and clinical pain research.

Gene therapy for the treatment of chronic peripheral nervous system pain

Chronic pain is a major health concern affecting 80 million Americans at some time in their lives with significant associated morbidity and effects on individual quality of life. Chronic pain can result from a variety of inflammatory and nerve damaging events that include cancer, infectious diseases, autoimmune-related syndromes and surgery. Current pharmacotherapies have not provided an effective long-term solution as they are limited by drug tolerance and potential abuse. These concerns have led to the development and testing of gene therapy approaches to treat chronic pain. The potential efficacy of gene therapy for pain has been reported in numerous pre-clinical studies that demonstrate pain control at the level of the spinal cord. This promise has been recently supported by a Phase-I human trial in which a replication-defective herpes simplex virus (HSV) vector was used to deliver the human pre-proenkephalin (hPPE) gene, encoding the natural opioid peptides met- and leu-enkephalin (ENK), to cancer patients with intractable pain resulting from bone metastases (Fink et al., 2011). The study showed that the therapy was well tolerated and that patients receiving the higher doses of therapeutic vector experienced a substantial reduction in their overall pain scores for up to a month post vector injection. These exciting early clinical results await further patient testing to demonstrate treatment efficacy and will likely pave the way for other gene therapies to treat chronic pain.

Decreased intracellular GABA levels contribute to spinal cord stimulation-induced analgesia in rats suffering from painful peripheral neuropathy: the role of KCC2 and GABA(A) receptor-mediated inhibition

Elevated spinal extracellular γ-aminobutyric acid (GABA) levels have been described during spinal cord stimulation (SCS)-induced analgesia in experimental chronic peripheral neuropathy. Interestingly, these increased GABA levels strongly exceeded the time frame of SCS-induced analgesia. In line with the former, pharmacologically-enhanced extracellular GABA levels by GABA(B) receptor agonists in combination with SCS in non-responders to SCS solely could convert these non-responders into responders. However, similar treatment with GABA(A) receptor agonists and SCS is known to be less efficient. Since K⁺ Cl⁻ cotransporter 2 (KCC2) functionality strongly determines proper GABA(A) receptor-mediated inhibition, both decreased numbers of GABA(A) receptors as well as reduced KCC2 protein expression might play a pivotal role in this loss of GABA(A) receptor-mediated inhibition in non-responders. Here, we explored the mechanisms underlying both changes in extracellular GABA levels and impaired GABA(A) receptor-mediated inhibition after 30 min of SCS in rats suffering from partial sciatic nerve ligation (PSNL). Immediately after cessation of SCS, a decreased spinal intracellular dorsal horn GABA-immunoreactivity was observed in responders when compared to non-responders or sham SCS rats. One hour later however, GABA-immunoreactivity was already increased to similar levels as those observed in non-responder or sham SCS rats. These changes did not coincide with alterations in the number of GABA-immunoreactive cells. C-Fos/GABA double-fluorescence clearly confirmed a SCS-induced activation of GABA-immunoreactive cells in responders immediately after SCS. Differences in spinal dorsal horn GABA(A) receptor-immunoreactivity and KCC2 protein levels were absent between all SCS groups. However, KCC2 protein levels were significantly decreased compared to sham PSNL animals. In conclusion, reduced intracellular GABA levels are only present during the time frame of SCS in responders and strongly point to a SCS-mediated on/off GABAergic release mechanism. Furthermore, a KCC2-dependent impaired GABA(A) receptor-mediated inhibition seems to be present both in responders and non-responders to SCS due to similar KCC2 and GABA(A) receptor levels.

Gene therapy: a potential approach for cancer pain

Chronic pain is experienced by as many as 90% of cancer patients at some point during the disease. This pain can be directly cancer related or arise from a sensory neuropathy related to chemotherapy. Major pharmacological agents used to treat cancer pain often lack anatomical specificity and can have off-target effects that create new sources of suffering. These concerns establish a need for improved cancer pain management. Gene therapy is emerging as an exciting prospect. This paper discusses the potential for viral vector-based treatment of cancer pain. It describes studies involving vector delivery of transgenes to laboratory pain models to modulate the nociceptive cascade. It also discusses clinical investigations aimed at regulating pain in cancer patients. Considering the prevalence of pain among cancer patients and the growing potential of gene therapy, these studies could set the stage for a new class of medicines that selectively disrupt nociceptive signaling with limited off-target effects.