Wnt1/β-catenin signaling upregulates spinal VGLUT2 expression to control neuropathic pain in mice

Pathology of pain and its implications for therapeutic interventions

Pain is estimated to affect more than 20% of the global population, imposing incalculable health and economic burdens. Effective pain management is crucial for individuals suffering from pain. However, the current methods for pain assessment and treatment fall short of clinical needs. Benefiting from advances in neuroscience and biotechnology, the neuronal circuits and molecular mechanisms critically involved in pain modulation have been elucidated. These research achievements have incited progress in identifying new diagnostic and therapeutic targets. In this review, we first introduce fundamental knowledge about pain, setting the stage for the subsequent contents. The review next delves into the molecular mechanisms underlying pain disorders, including gene mutation, epigenetic modification, posttranslational modification, inflammasome, signaling pathways and microbiota. To better present a comprehensive view of pain research, two prominent issues, sexual dimorphism and pain comorbidities, are discussed in detail based on current findings. The status quo of pain evaluation and manipulation is summarized. A series of improved and innovative pain management strategies, such as gene therapy, monoclonal antibody, brain-computer interface and microbial intervention, are making strides towards clinical application. We highlight existing limitations and future directions for enhancing the quality of preclinical and clinical research. Efforts to decipher the complexities of pain pathology will be instrumental in translating scientific discoveries into clinical practice, thereby improving pain management from bench to bedside.

Answering Big Questions in Pain Medicine

The future of pain medicine is marked by many questions. What can other nations around the world learn from the opioid crisis that is still affecting the United States? The American opioid experience was mischaracterized and wrongly described, and its causes were misdiagnosed from the outset, leading to its mismanagement and the abandonment of many chronic pain patients to their suffering. There are a few new drugs in the analgesic armamentarium. What new targets do we have in pain medicine? There are many breakthroughs, discoveries, and potential new targets that could add to our analgesic prescribing choices. These include sigma receptors, d-amino acid oxidase, endoplasmic reticulum stress receptors, histone deacetylase, and others. Neuromodulation had been used with varying degrees of success for years, but with a simplistic approach based on the gate theory of pain. Despite our familiarity with neuromodulation and spinal cord stimulators, neuromodulation research indicates that the activation of glial cells may activate the immune system and enhance analgesia. Neuromodulation studies have concentrated on how electricity affects neuronal activity rather than how electrical activity could reduce pain. There are still more frontiers in our battle against pain and some promising avenues for treatments. This narrative review will try to summarize what can be done from the perspective of recent technological and pharmacological developments.

Tetrandrine attenuates SNI-induced mechanical allodynia by inhibiting spinal CKLF1

Neuropathic pain (NP) is a prevalent clinical problem for which satisfactory treatment options are unavailable. Tetrandrine (TET), a bisbenzylisoquinoline alkaloid extracted from Stephania tetrandra S. Moore, possesses anti-inflammatory and immune-modulatory properties. Chemokine-like factor 1 (CKLF1) is known to play a crucial role in both peripheral and central inflammatory processes. This study aimed to investigate the potential anti-NP effects of TET and the involvement of CKLF1 in the action of TET. A male C57BL/6J mice model of NP caused by spared nerve injury (SNI) was established and mechanical withdrawal thresholds were measured using von Frey filaments. The results showed that TET improved mechanical allodynia in SNI mice and the propofol-induced sleep assay demonstrated that the TET group did not exhibit central inhibition, while the pregabalin (PGB) group showed significant central inhibition. Western blotting and immunofluorescence staining showed that TET significantly inhibited spinal protein expression levels of CKLF1, p-NF-κB/NF-κB, p-IKK/IKK, pro-inflammatory cytokines IL-1β and TNF-α, and increased protein expression levels of the anti-inflammatory cytokine IL-10, while inhibiting the expression levels of microglia and astrocyte markers IBA-1 and GFAP of SNI mice. Moreover, immunofluorescence double-labeling results revealed that CKLF1 was predominantly colocalized with microglia of the spinal cord (SC) in SNI mice. C19 (an antagonism peptide of CKLF1) alleviated SNI-induced mechanical pain hypersensitivity, while C27 (an analog peptide of CKLF1) induced mechanical allodynia in normal mice. TET significantly attenuated mechanical allodynia induced by C27 in mice. TET may effectively alleviate NP by reducing neuroinflammation and decreasing CKLF1.

High intensity interval training attenuates osteoarthritis-associated hyperalgesia in rats

High-intensity interval training (HIIT) is a physical therapy that may benefit patients with osteoarthritis (OA). Cacna2d1 is a calcium channel subunit protein that plays an important role in the activity of nerve cells. However, there is currently no evidence on HIIT relieving OA-associate hyperalgesia by decreased Cacna2d1. Our study established the OA rat models with intra-articular injection of monosodium iodoacetate (MIA). This experiment was divided into two stages. The first stage comprised three groups: the control, OA, and OA-HIIT groups. The second stage comprised two groups, including the AAV-C and AAV-shRNA-Cacna2d1 groups. OA rats were positioned at the L5-L6 segments, and 20 µl of AAV virus was injected intrathecally. The pain threshold, cartilage analysis, Cacna2d1, and pain neurotransmitters were measured and compared. The pain threshold was significantly lower in OA rats than in control rats from the first to the tenth week. Starting from the sixth week, OA-HIIT rats exhibited significantly increased pain thresholds. The expression of Cacna2d1 increased in OA rats. Moreover, the knockdown of Cacna2d1 significantly down-regulated the expression of c-Fos, SP, and Vglut2 in the posterior horn of the spinal cord. In conclusion, HIIT attenuates OA-associated hyperalgesia, which may be related to the down-regulation of Cacna2d1.

Upregulation of β-catenin signaling represents a single common pathway leading to the various phenotypes of spinal degeneration and pain

Spine degeneration is an aging-related disease, but its molecular mechanisms remain unknown, although elevated β-catenin signaling has been reported to be involved in intervertebral disc degeneration. Here, we determined the role of β-catenin signaling in spinal degeneration and in the homeostasis of the functional spinal unit (FSU), which includes the intervertebral disc, vertebra and facet joint and is the smallest physiological motion unit of the spine. We showed that pain sensitivity in patients with spinal degeneration is highly correlated with β-catenin protein levels. We then generated a mouse model of spinal degeneration by transgenic expression of constitutively active β-catenin in Col2⁺ cells. We found that β-catenin-TCF7 activated the transcription of CCL2, a known critical factor in osteoarthritic pain. Using a lumbar spine instability model, we showed that a β-catenin inhibitor relieved low back pain. Our study indicates that β-catenin plays a critical role in maintaining spine tissue homeostasis, its abnormal upregulation leads to severe spinal degeneration, and its targeting could be an avenue to treat this condition.

ARMADILLO REGULATES NOCICEPTIVE SENSITIVITY IN THE ABSENCE OF INJURY

Abnormal pain has recently been estimated to affect ∼50 million adults each year within the United States. With many treatment options for abnormal pain, such as opioid analgesics, carrying numerous deleterious side effects, research into safer and more effective treatment options is crucial. To help elucidate the mechanisms controlling nociceptive sensitivity, the Drosophila melanogaster larval nociception model has been used to characterize well-conserved pathways through the use of genetic modification and/or injury to alter the sensitivity of experimental animals. Mammalian models have provided evidence of β-catenin signaling involvement in neuropathic pain development. By capitalizing on the conserved nature of β-catenin functions in the fruit fly, here we describe a role for Armadillo, the fly homolog to mammalian β-catenin, in regulating baseline sensitivity in the primary nociceptor of the fly, in the absence of injury, using under- and over-expression of Armadillo in a cell-specific manner. Underexpression of Armadillo resulted in hyposensitivity, while overexpression of wild-type Armadillo or expression of a degradation-resistant Armadillo resulted in hypersensitivity. Neither underexpression nor overexpression of Armadillo resulted in observed dendritic morphological changes that could contribute to behavioral phenotypes observed. These results showed that focused manipulation of Armadillo expression within the nociceptors is sufficient to modulate baseline response in the nociceptors to a noxious stimulus and that these changes are not shown to be associated with a morphogenetic effect.

Dysregulation of Vesicular Glutamate Transporter VGluT2 via BDNF/TrkB Pathway Contributes to Morphine Tolerance in Mice

Morphine is widely used in the treatment of moderate to severe pain. Long-term use of morphine leads to various adverse effects, such as tolerance and hyperalgesia. Vesicular glutamate transporter 2 (VGluT2) accumulates glutamate into synaptic vesicles and plays multiple roles in the central nervous system. However, the specific role of VGluT2 in morphine tolerance has not been fully elucidated. Here, we investigated the regulatory role of VGluT2 in morphine tolerance and assessed the potential role of the brain-derived neurotrophic factor (BDNF)/tyrosine kinase B (TrkB) pathway in VGluT2 mediated morphine antinociceptive tolerance in mice. In the present study, we found that VGluT2 is upregulated in the spinal cord after the development of morphine tolerance. Furthermore, inhibition of VGluT2 with its antagonist (Chicago sky blue 6 B, CSB6B) or knockdown of VGluT2 by lentivirus restored the analgesic effect of morphine, suppressed the activation of astrocytes and microglia, and decreased glial-derived pro-inflammatory cytokines. Overexpression of VGluT2 by lentivirus facilitated morphine tolerance and mechanical hyperalgesia. In addition, we found the expression of BDNF is correlated with VGluT2 expression in the spinal cord after chronic morphine administration. Intrathecal injection of the BDNF/TrkB pathway antagonist K252a attenuated the development of morphine tolerance and decreased the expression of VGluT2 in the spinal cord, which suggested the BDNF/TrkB pathway participates in the regulation of VGluT2 in morphine tolerance. This study elucidates the functional capability of VGluT2 in modulating morphine tolerance and identifies a novel mechanism and promising therapeutic target for morphine tolerance.

Osteoarthritis Pain

Joint pain is the hallmark symptom of osteoarthritis (OA) and the main reason for patients to seek medical assistance. OA pain greatly contributes to functional limitations of joints and reduced quality of life. Although several pain-relieving medications are available for OA treatment, the current intervention strategy for OA pain cannot provide satisfactory pain relief, and the chronic use of the drugs for pain management is often associated with significant side effects and toxicities. These observations suggest that the mechanisms of OA-related pain remain undefined. The current review mainly focuses on the characteristics and mechanisms of OA pain. We evaluate pathways associated with OA pain, such as nerve growth factor (NGF)/tropomyosin receptor kinase A (TrkA), calcitonin gene-related peptide (CGRP), C–C motif chemokine ligands 2 (CCL2)/chemokine receptor 2 (CCR2) and tumor necrosis factor alpha (TNF-α), interleukin-1beta (IL-1β), the NOD-like receptor (NLR) family, pyrin domain-containing protein 3 (NLRP3) inflammasome, and the Wnt/β-catenin signaling pathway. In addition, animal models currently used for OA pain studies and emerging preclinical studies are discussed. Understanding the multifactorial components contributing to OA pain could provide novel insights into the development of more specific and effective drugs for OA pain management.

Wnt signaling: A prospective therapeutic target for chronic pain

Despite the rapid advance over the past decades to design effective therapeutic pharmacological interventions, chronic pain remains to be an unresolved healthcare concern. Long term use of opioids, the first line analgesics, often causes detrimental side effects. Therefore, a profound understanding of the mechanisms underlying the development and maintenance of chronic pain states is urgently needed for the management of chronic pain. Substantial evidence indicates aberrant activation of Wnt signaling pathways in sciatic nerve, dorsal root ganglia and spinal cord dorsal horn in rodent models of chronic pain. Moreover, growing evidence shows that pharmacological blockage of aberrant activation of Wnt signaling pathways attenuates pain behaviors in animal models of chronic pain. Importantly, both intrathecal injection of Wnt agonists and Wnt ligands to naïve rats lead to the development of mechanical allodynia, which was inhibited by Wnt inhibitors. In this review, we summarized and discussed the therapeutic potential of pharmacological inhibitors of Wnt signaling in chronic pain in preclinical studies. These evidence showed that aberrant activation of Wnt signaling pathways contributed to chronic pain via enhancing neuroinflammation, regulating synaptic plasticity and reducing intraepidermal nerve fiber density. However, these findings raise further questions. Overall, despite the future challenges, these pioneering studies suggest that Wnt signaling is a promising therapeutic target for chronic pain.

Impaired Transport Activity of Human Organic Anion Transporters (OATs) and Organic Anion Transporting Polypeptides (OATPs) by Wnt Inhibitors

The Wnt/β-catenin signaling pathway is dysregulated in diseases and Wnt inhibitors like PRI-724 are in clinical development. This study evaluated the regulatory actions of PRI-724 and other Wnt inhibitors on the transport activity of human renal Organic anion transporters (OATs) and Organic anion transporting polypeptides (OATPs). The substrate uptake by OAT4 and OATP2B1 was markedly decreased by PRI-724 (V_max/K_m: ∼26% and ∼17% of corresponding control), with less pronounced decreases in OAT1, OAT3 and OAT1A2. PRI-724 decreased the plasma membrane expression of inhibited OATs/OATPs but didn't affect their total cellular expression. Two model Wnt inhibitors - FH535 and 21H7 - were also tested in comparative studies. Like PRI-724, they also strongly decreased the activities and membrane expression of multiple OATs/OATPs. In contrast, FH535 didn't affect the substrate uptake by organic cation transporters. In control studies, the EGFR inhibitor lapatinib did not inhibit the function of some OATs/OATPs. Together these findings suggest that Wnt inhibitors selectively modulate the function of multiple organic anions transporters, so their clinical use may have unanticipated effects on drug entry into cells. These findings are pertinent to current clinical trials that have been designed to understand the safety and efficacy of new Wnt inhibitor drugs.

MiR-216a-5p alleviates chronic constriction injury-induced neuropathic pain in rats by targeting KDM3A and inactivating Wnt/β-catenin signaling pathway

Neuropathic pain is a devastating disease and exists tolerance to current available analgesics. MicroRNAs were reported to be involved in the regulation of neuropathic pain, but the biological role of miR-216a-5p in neuropathic pain remains unclear. In this study, we constructed a CCI rat model of neuropathic pain. Our results showed that the expression of miR-216a-5p was downregulated in CCI rats, and mechanical allodynia and thermal hyperalgesia in CCI rats were improved by miR-216a-5p overexpression, suggesting that miR-216a-5p overexpression alleviated neuropathic pain. Moreover, ELISA showed that miR-216a-5p overexpression inhibited concentration and mRNA expression of IL-6, TNF-α and IL-1β as well as suppressed microglial infiltration, indicating that miR-216a-5p overexpression inhibited neuroinflammation. Besides, we found that miR-216a-5p upregulation inactivated the Wnt/β-catenin signaling pathway. Furthermore, KDM3A was the downstream target of miR-216a-5p and KDM3A knockdown attenuated neuropathic pain. Finally, through rescue assay, we found that KDM3A countervailed miR-216a-5p mediated regulation of neuropathic pain via the Wnt/β-catenin signaling pathway. To sum up, our study confirmed that miR-216a-5p alleviated neuropathic pain in rats by targeting KDM3A and inactivating the Wnt/β-catenin signaling pathway, which may open a new and useful way for treatment of neuropathic pain.