Metformin relieves neuropathic pain after spinal nerve ligation via autophagy flux stimulation

Early evidence of beneficial and protective effects of Protectin DX treatment on behavior responses and type-1 diabetes mellitus related-parameters: A non-clinical approach

Protectin DX (PDX), a specialized pro-resolving lipid mediator, presents potential therapeutic applications across various medical conditions due to its anti-inflammatory and antioxidant properties. Since type-1 diabetes mellitus (T1DM) is a disease with an inflammatory and oxidative profile, exploring the use of PDX in addressing T1DM and its associated comorbidities, including diabetic neuropathic pain, depression, and anxiety becomes urgent. Thus, in the current study, after 2 weeks of T1DM induction with streptozotocin (60 mg/kg) in Wistar rats, PDX (1, 3, and 10 ng/animal; i.p. injection of 200 μl/animal) was administered specifically on days 14, 15, 18, 21, 24, and 27 after T1DM induction. We investigated the PDX's effectiveness in alleviating neuropathic pain (mechanical allodynia; experiment 1), anxiety-like and depressive-like behaviors (experiment 2). Also, we studied whether the PDX treatment would induce antioxidant effects in the blood plasma, hippocampus, and prefrontal cortex (experiment 3), brain areas involved in the modulation of emotions. For evaluating mechanical allodynia, animals were repeatedly submitted to the Von Frey test; while for studying anxiety-like responses, animals were submitted to the elevated plus maze (day 26) and open field (day 28) tests. To analyze depressive-like behaviors, the animals were tested in the modified forced swimming test (day 28) immediately after the open field test. Our data demonstrated that PDX consistently increased the mechanical threshold throughout the study at the two highest doses, indicative of antinociceptive effect. Concerning depressive-like and anxiety-like behavior, all PDX doses effectively prevented these behaviors when compared to vehicle-treated T1DM rats. The PDX treatment significantly protected against the increased oxidative stress parameters in blood plasma and in hippocampus and prefrontal cortex. Interestingly, treated animals presented improvement on diabetes-related parameters by promoting weight gain and reducing hyperglycemia in T1DM rats. These findings suggest that PDX improved diabetic neuropathic pain, and induced antidepressant-like and anxiolytic-like effects, in addition to improving parameters related to the diabetic condition. It is worth noting that PDX also presented a protective action demonstrated by its antioxidant effects. To conclude, our findings suggest PDX treatment may be a promising candidate for improving the diabetic condition per se along with highly disabling comorbidities such as diabetic neuropathic pain and emotional disturbances associated with T1DM.

Metformin inhibits paclitaxel-induced mechanical allodynia by activating opioidergic pathways and reducing cytokines production in the dorsal root ganglia and thalamus

It has been shown that AMP-activated protein kinase (AMPK) is involved in the nociceptive processing. This observation has prompted us to investigate the effects of the AMPK activator metformin on the paclitaxel-induced mechanical allodynia, a well-established model of neuropathic pain. Mechanical allodynia was induced by four intraperitoneal (i.p) injections of paclitaxel (2 mg/kg.day) in mice. Metformin was administered per os (p.o.). Naltrexoneandglibenclamide were used to investigate mechanisms mediating metformin activity. Concentrations of cytokines in the dorsal root ganglia (DRG) and thalamus were determined. After a single p.o. administration, the two highest doses of metformin (500 and 1000 mg/kg) attenuated the mechanical allodynia. This response was attenuated by all doses of metformin (250, 500 and 1000 mg/kg) when two administrations, 2 h apart, were carried out. Naltrexone (5 and 10 mg/kg, i.p.), but not glibenclamide (20 and 40 mg/kg, p.o.), attenuated metformin activity. Concentrations of tumor necrosis factor (TNF)-α, interleukin (IL)-1β and CXCL-1 in the DRG were increased after administration of paclitaxel. Metformin (1000 mg/kg) reduced concentrations of TNF-α, IL-1β and CXCL-1 in the DRG. Concentration of IL-6, but not TNF-α, in the thalamus was increased after administration of paclitaxel. Metformin (1000 mg/kg) reduced concentration of IL-6 in the thalamus. In summary, metformin exhibits activity in the model of neuropathic pain induced by paclitaxel. This activity may be mediated by activation of opioidergic pathways and reduced production of TNF-α, IL-1β and CXCL-1 in the DRG and IL-6 in the thalamus.

Employing the sustained-release properties of poly(lactic-co-glycolic acid) nanoparticles to reveal a novel mechanism of sodium-hydrogen exchanger 1 in neuropathic pain

Neuropathic pain is caused by injury or disease of the somatosensory system, and its course is usually chronic. Several studies have been dedicated to investigating neuropathic pain-related targets; however, little attention has been paid to the persistent alterations that these targets, some of which may be crucial to the pathophysiology of neuropathic pain. The present study aimed to identify potential targets that may play a crucial role in neuropathic pain and validate their long-term impact. Through bioinformatics analysis of RNA sequencing results, we identified Slc9a1 and validated the reduced expression of sodium-hydrogen exchanger 1 (NHE1), the protein that Slc9a1 encodes, in the spinal nerve ligation (SNL) model. Colocalization analysis revealed that NHE1 is primarily co-localized with vesicular glutamate transporter 2-positive neurons. In vitro experiments confirmed that poly(lactic-co-glycolic acid) nanoparticles loaded with siRNA successfully inhibited NHE1 in SH-SY5Y cells, lowered intracellular pH, and increased intracellular calcium concentrations. In vivo experiments showed that sustained suppression of spinal NHE1 expression by siRNA-loaded nanoparticles resulted in delayed hyperalgesia in naïve and SNL model rats, whereas amiloride-induced transient suppression of NHE1 expression yielded no significant changes in pain sensitivity. We identified Slc9a1, which encodes NHE1, as a key gene in neuropathic pain. Utilizing the sustained release properties of nanoparticles enabled us to elucidate the chronic role of decreased NHE1 expression, establishing its significance in the mechanisms of neuropathic pain.

Stress/cell death pathways, neuroinflammation, and neuropathic pain

Neuropathic pain is a common and debilitating modality of chronic pain induced by a lesion or disease of the somatosensory nervous system. Albeit the elucidation of numerous pathophysiological mechanisms and the development of potential treatment compounds, safe and reliable therapies of neuropathic pain remain poor. Multiple stress/cell death pathways have been shown to be implicated in neuroinflammation during neuropathic pain. Here, we summarize the current knowledge of stress/cell death pathways and present an overview of the roles and molecular mechanisms of stress/cell death pathways in neuroinflammation during neuropathic pain, covering intrinsic and extrinsic apoptosis, autophagy, mitophagy, ferroptosis, pyroptosis, necroptosis, and phagoptosis. Small molecule compounds that modulate stress/cell death pathways in alleviating neuropathic pain are discussed mainly based on preclinical neuropathic pain models. These findings will contribute to in-depth understanding of the pathological processes during neuropathic pain as well as bridge the gap between basic and translational research to uncover new neuroprotective interventions.

New insights into the effect of VMP1 on the treatment of pressure overload-induced pathological cardiac hypertrophy: Involving SERCA-regulated autophagic flux

Pathological cardiac hypertrophy is an adaptive reaction in response to pressure or volume overload. Autophagy is critical for damage caused by pathological cardiac hypertrophy. Vacuole membrane protein 1 (VMP1) is an endoplasmic reticulum (ER) transmembrane protein that is effective in activating autophagy. However, the role of VMP1 in pathological cardiac hypertrophy and its underlying mechanisms remain elusive. This study was designed to explore the potential mechanisms of VMP1 on pressure overload-induced pathological cardiac hypertrophy. In this work, abdominal aorta constriction (AAC) surgery was used to induce pathological cardiac hypertrophy in male C57BL/6 mice. H9C2 cardiomyocytes were treated with phenylephrine stimulation (PE) to induce the hypertrophic response. The in vivo results revealed that mice with AAC surgery caused pathological cardiac hypertrophy as evidenced by improved cardiac function according to multiple echocardiographic parameters. Moreover, elevated VMP1 expression was also observed in mice after AAC surgery. VMP1 knockdown aggravated changes in cardiac structure, cardiac dysfunction, and fibrosis. Meanwhile, VMP1 knockdown suppressed autophagy and endoplasmic reticulum calcium ATPase (SERCA) activity in heart tissues. H9C2 cardiomyocytes with VMP1 overexpression were used to investigate the specific mechanism of VMP1 in pathological cardiac hypertrophy, and VMP1 overexpression increased autophagic flux by upregulating SERCA activity. In conclusion, these findings revealed that VMP1 protected against pressure overload-induced pathological cardiac hypertrophy by inducing SERCA-regulated autophagic flux. Our results provide valuable insights regarding the pathophysiology of pathological cardiac hypertrophy and clues to a novel target for the treatment of pathological cardiac hypertrophy.

Store-operated calcium entry mediates hyperalgesic responses during neuropathy

Neuropathic pain (NP), resulting from nerve injury, alters neural plasticity in spinal cord and brain via the release of inflammatory mediators. The remodeling of store-operated calcium entry (SOCE) involves the refilling of calcium in the endoplasmic reticulum via STIM1 and Orai1 proteins and is crucial for maintaining neural plasticity and neurotransmitter release. The mechanism underlying SOCE-mediated NP remains largely unknown. In this study, we found SOCE-mediated calcium refilling was significantly higher during neuropathic pain, and the major component Orai1 was specifically co-localized with neuronal markers. Intrathecal injection of SOCE antagonist SKF96365 remarkably alleviated nerve injury- and formalin-induced pain and suppressed c-Fos expression in response to innocuous mechanical stimulation. RNA sequencing revealed that SKF96365 altered the expression of spinal transcription factors, including Fos, Junb, and Socs3, during neuropathic pain. In order to identify the genes critical for SKF96365-induced effects, we performed weighted gene co-expression network analysis (WGCNA) to identify the genes most correlated with paw withdrawal latency phenotypes. Of the 16 modules, MEsalmon module was the most highly correlated with SKF96365 induced effects. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that the enriched genes of MEsalmon module were significantly related to Toll-like receptor signaling, steroid biosynthesis, and chemokine signaling, which may mediate the analgesic effect caused by SKF9636 treatment. Additionally, the SOCE antagonist YM-58483 produced similar analgesic effects in nerve injury- and formalin-induced pain. Our results suggest that manipulation of spinal SOCE signaling might be a promising target for pain relief by regulating neurotransmitter production and spinal transcription factor expression.

Role of autophagy in the pathogenesis and regulation of pain

Pain is a ubiquitous and highly concerned clinical symptom, usually caused by peripheral or central nervous injury, tissue damage, or other diseases. The long-term existence of pain can seriously affect daily physical function and quality of life and produce great torture on the physiological and psychological levels. However, the complex pathogenesis of pain involving molecular mechanisms and signaling pathways has not been fully elucidated, and managing pain remains highly challenging. As a result, finding new targets to pursue effective and long-term pain treatment strategies is required and urgent. Autophagy is an intracellular degradation and recycling process that maintains tissue homeostasis and energy supply, which can be cytoprotective and is vital in maintaining neural plasticity and proper nervous system function. Much evidence has shown that autophagy dysregulation is linked to the emergence of neuropathic pain, such as postherpetic neuralgia and cancer-related pain. Autophagy has also been connected to pain caused by osteoarthritis and lumbar disc degeneration. It is worth noting that in recent years, studies on traditional Chinese medicine have also proved that several traditional Chinese medicine monomers involve autophagy in the mechanism of pain relief. Therefore, autophagy can serve as a potential regulatory target to provide new ideas and inspiration for pain management.

Autophagy: An Emerging Target for Developing Effective Analgesics

Inadequate treatment of acute and chronic pain causes depression, anxiety, sleep disturbances, and increased mortality. Abuse and overdose of opioids and the side effects associated with chronic use of NSAID illustrate the need for development of safer and effective pain medication. Working toward this end, an in silico tool based on an emergent intelligence analytical platform that examines interactions between protein networks was used to identify molecular mechanisms involved in regulating the body's response to painful stimuli and drug treatments. Examining interactions between protein networks associated with the expression of over 20 different pain types suggests that the regulation of autophagy plays a central role in modulation of pain symptoms (see Materials and Methods). Using the topology of this regulatory scheme as an in silico screening tool, we identified that combinations of functions targeted by cannabidiol, myo-inositol, and fish oils with varying ratios of eicosapentaenoic and docosahexaenoic acids are projected to produce superior analgesia. For validating this prediction, we administered combinations of cannabidiol, myo-inositol, and fish oils to rats that received formalin injections in hind paws, prior to substance administration, and showed that analgesic effects produced by these combinations were comparable or superior to known NSAID analgesics, which suggests that these combinations have potential in treatment of pain.

Mesenchymal stem cell-derived extracellular vesicles carrying miR-99b-3p restrain microglial activation and neuropathic pain by stimulating autophagy

Neuropathic pain is a complex condition that seriously affects human quality of life. This study aimed to investigate the therapeutic mechanism of mesenchymal stem cell-derived extracellular vesicles (MSC-EVs) and try to discover new targets for alleviating neuropathic pain. Extracellular vesicles were isolated and identified via ultracentrifugation. BV-2 microglial cells were stimulated with lipopolysaccharide (LPS) in the presence or absence of MSC-EVs. Further, microglial activation and neuroinflammation were evaluated by flow cytometry, RT-qPCR, and ELISA. High-throughput sequencing analysis was performed to reveal the differentially expressed (DE) miRNAs in BV-2 microglia. Autophagy-related regulators were assessed by Western blotting and Immunofluorescence staining. Chronic constriction injury (CCI) model was used to induce neuropathic pain in rats, and the mechanical withdrawal threshold (MWT) was measured. High-throughput sequencing analysis identified 17 DE miRNAs, which were mainly enriched in PI3K-AKT and mTOR signaling pathways. MSC-EVs inhibited the activation of PI3K/AKT/mTOR signaling pathway in LPS-stimulated microglia. Moreover, MSC-EVs treatment enhanced the autophagy level in activated microglia, whereas autophagy inhibitor 3-MA reversed the suppressing effects of MSC-EVs on microglial activation and neuroinflammation. The MSC-EV-mediated transfer of miR-99b-3p was verified to promote microglial autophagy, and miR-99b-3p overexpression suppressed the expression of pro-inflammatory factors in activated microglia. During in vivo studies, intrathecal injection of MSC-EVs significantly up-regulated the expression of miR-99b-3p, and alleviated mechanical allodynia caused by activated microglia in the spinal cord dorsal horn of CCI rats. Moreover, MSC-EVs treatment repaired CCI-induced autophagic impairment by stimulating autophagy in the spinal cord. Collectively, our findings demonstrated that MSC-EVs had an analgesic effect on neuropathic pain via promoting autophagy, and these antinociceptive effects were at least in part caused by MSC-EV-mediated transfer of miR-99b-3p, thereby inhibiting microglial activation and pro-inflammatory cytokines expression.

Review: Myelin clearance is critical for regeneration after peripheral nerve injury

Traumatic peripheral nerve injury occurs frequently and is a major clinical and public health problem that can lead to functional impairment and permanent disability. Despite the availability of modern diagnostic procedures and advanced microsurgical techniques, active recovery after peripheral nerve repair is often unsatisfactory. Peripheral nerve regeneration involves several critical events, including the recreation of the microenvironment and remyelination. Results from previous studies suggest that the peripheral nervous system (PNS) has a greater capacity for repair than the central nervous system. Thus, it will be important to understand myelin and myelination specifically in the PNS. This review provides an update on myelin biology and myelination in the PNS and discusses the mechanisms that promote myelin clearance after injury. The roles of Schwann cells and macrophages are considered at length, together with the possibility of exogenous intervention.

Sex Differences in Neuropathy: The Paradigmatic Case of MetFormin

As a widely prescribed anti-diabetic drug, metformin has been receiving novel attention for its analgesic potential. In the study of the complex etiology of neuropathic pain (NeP), male and female individuals exhibit quite different responses characterized by higher pain sensitivity and greater NeP incidence in women. This "gender gap" in our knowledge of sex differences in pain processing strongly limits the sex-oriented treatment of patients suffering from NeP. Besides, the current investigation of the analgesic potential of metformin has not addressed the "gender gap" problem. Hence, this study focuses on metformin and sex-dependent analgesia in a murine model of NeP induced by chronic constriction injury of the sciatic nerve. We investigated sexual dimorphism in signaling pathways involved by 7 days of metformin administration, such as changes in AMP-activated protein kinase and the positive regulation of autophagy machinery, discovering that metformin affected in a sexually dimorphic manner the immunological and inflammatory response to nerve lesion. These effects were complemented by morphological and adaptive changes occurring after peripheral nerve injury. Altogether these data can contribute to explaining a number of potential mechanisms responsible for the complete recovery from NeP found in male mice, as opposed to the failure of long-lasting recovery in female animals.

The leptomeninges as a critical organ for normal CNS development and function: First patient and public involved systematic review of arachnoiditis (chronic meningitis)

BACKGROUND & IMPORTANCE

This patient and public-involved systematic review originally focused on arachnoiditis, a supposedly rare "iatrogenic chronic meningitis" causing permanent neurologic damage and intractable pain. We sought to prove disease existence, causation, symptoms, and inform future directions. After 63 terms for the same pathology were found, the study was renamed Diseases of the Leptomeninges (DLMs). We present results that nullify traditional clinical thinking about DLMs, answer study questions, and create a unified path forward.

METHODS

The prospective PRISMA protocol is published at Arcsology.org. We used four platforms, 10 sources, extraction software, and critical review with ≥2 researchers at each phase. All human sources to 12/6/2020 were eligible for qualitative synthesis utilizing R. Weekly updates since cutoff strengthen conclusions.

RESULTS

Included were 887/14286 sources containing 12721 DLMs patients. Pathology involves the subarachnoid space (SAS) and pia. DLMs occurred in all countries as a contributor to the top 10 causes of disability-adjusted life years lost, with communicable diseases (CDs) predominating. In the USA, the ratio of CDs to iatrogenic causes is 2.4:1, contradicting arachnoiditis literature. Spinal fusion surgery comprised 54.7% of the iatrogenic category, with rhBMP-2 resulting in 2.4x more DLMs than no use (p<0.0001). Spinal injections and neuraxial anesthesia procedures cause 1.1%, and 0.2% permanent DLMs, respectively. Syringomyelia, hydrocephalus, and arachnoid cysts are complications caused by blocked CSF flow. CNS neuron death occurs due to insufficient arterial supply from compromised vasculature and nerves traversing the SAS. Contrast MRI is currently the diagnostic test of choice. Lack of radiologist recognition is problematic.

DISCUSSION & CONCLUSION

DLMs are common. The LM clinically functions as an organ with critical CNS-sustaining roles involving the SAS-pia structure, enclosed cells, lymphatics, and biologic pathways. Cases involve all specialties. Causes are numerous, symptoms predictable, and outcomes dependent on time to treatment and extent of residual SAS damage. An international disease classification and possible treatment trials are proposed.

Inflammatory-associated apoptotic markers: are they the culprit to rheumatoid arthritis pain?

BACKGROUND

Rheumatoid arthritis (RA) is a prolonged inflammatory disease resulting from autoimmune reactions that leads to local and systemic bone erosion, joint defects and functional impairment. Although the inflammation is subsided through the prescription of anti-inflammatory therapeutics, the patients persistently complained of sleepless nights due to flare pain. This indicates the possible contribution of other pathways besides inflammation in leading to RA pain. This review aims to uncover the roles and involvement of several inflammatory-associated apoptotic markers in facilitating pain transmission and processing during the pathogenesis of RA.

MATERIALS AND METHODS

This narrative review focused on the reports from the previous literature based on the search string of "apoptotic marker AND inflammation AND 'chronic pain' OR 'neuropathic pain' and apoptosis AND 'rheumatoid arthritis' OR arthritis from the databases including Science Direct and Scopus, considering the exclusion criteria of the published abstracts, proceedings or articles on other neuropathic pain types such as painful bowel syndrom, insterstitial cystitis, fibrosis and so on.

RESULTS

Several studies in the literature demonstrate a close association between imbalanced apoptotic regulations and an increased number of synovial fibroblasts and inflammatory cells in RA. Cell death or specific cell survival has been linked with increased central hypersensitivity in various types of chronic and neuropathic pain.

CONCLUSION

The RA-related flare pain is possibly contributed by the abnormal regulation of apoptosis through several inflammatory-related pathways, and further studies need to modulate these pathways for the putative anti-nociceptive benefits.

Curcumin and metformin synergistically modulate peripheral and central immune mechanisms of pain

Metformin is a well-tolerated antidiabetic drug and has recently been repurposed for numerous diseases, including pain. However, a higher dose of metformin is required for effective analgesia, which can potentiate its dose-dependent gastrointestinal side effects. Curcumin is a natural polyphenol and has beneficial therapeutic effects on pain. Curcumin has been used as an analgesic adjuvant with several analgesic drugs, allowing synergistic antinociceptive effects. Nevertheless, whether curcumin can exert synergistic analgesia with metformin is still unknown. In the present study, the nature of curcumin-metformin anti-inflammatory interaction was evaluated in in vitro using lipopolysaccharide-induced RAW 264.7 macrophage and BV-2 microglia cells. In both macrophage and microglia, curcumin effectively potentiates the anti-inflammatory effects of metformin, indicating potential synergistic effects in both peripheral and central pathways of pain. The nature of the interaction between curcumin and metformin was further recapitulated using a mouse model of formalin-induced pain. Coadministration of curcumin and metformin at a 1:1 fixed ratio of their ED₅₀ doses significantly reduced the dose required to produce a 50% effect compared to the theoretically required dose in phase II of the formalin test with a combination index value of 0.24. Besides, the synergistic interaction does not appear to involve severe CNS side effects indicated by no motor alterations, no alterations in short-term and long-term locomotive behaviors, and the general well-being of mice. Our findings suggest that curcumin exerts synergistic anti-inflammation with metformin with no potential CNS adverse effects.

Caloric Restriction Alleviates CFA-Induced Inflammatory Pain via Elevating β-Hydroxybutyric Acid Expression and Restoring Autophagic Flux in the Spinal Cord

Inflammatory pain is the most common type of pain encountered in clinical practice; however, the currently available treatments are limited by insufficient efficacy and side effects. Therefore, new methods to relieve inflammatory pain targeting new mechanisms are urgently needed. Preclinical investigations have shown that CR (calorie restriction) exerts analgesic effects in neuropathic and cancer pain; however, the effect of CR on chronic inflammatory pain remains unknown. During calorie restriction, autophagy, a lysosome-dependent degradation process, can be activated to support cell survival. In the present study, we investigated the analgesic effects of CR on complete Freund’s adjuvant (CFA)-induced inflammatory pain. The accumulation of LC3-II and p62 showed impaired autophagic flux in the ipsilateral spinal cord of mice with CFA-induced inflammatory pain. CR alleviated mechanical allodynia and thermal hyperalgesia and reduced paw edema and pro-inflammatory factors following CFA administration. CR exerted an analgesic effect by restoring autophagic flux in the spinal cord. Regarding the mechanisms underlying the analgesic effects of CR, β-hydroxybutyric acid (BHB) was studied. CR increased BHB levels in the ipsilateral spinal cord. Furthermore, exogenous BHB administration exerted an analgesic effect by restoring autophagic flux in the spinal cords of CFA-induced inflammatory pain mice. Taken together, these results illustrated that CR relieved inflammatory pain by restoring autophagic flux in the spinal cord, while BHB controlled the benefits of CR, suggesting that CR or BHB might be a promising treatment for inflammatory pain.

Protectin DX Attenuates Lumbar Radicular Pain of Non-compressive Disc Herniation by Autophagy Flux Stimulation via Adenosine Monophosphate-Activated Protein Kinase Signaling

Background: Neuroinflammation plays a crucial role in initiating and sustaining lumbar radicular pain (LRP). Protectin DX (PDX) has been experimentally verified to possess pro-resolving properties and anti-inflammatory effects. This study aimed to observe the analgesic effects of PDX and its potential mechanisms in LRP rats with non-compressive lumbar disc herniation (NCLDH). Method: Only male rats were selected to avoid gender-related interferences. Rat models of NCLDH were established, and rats were randomly divided into four groups: the sham group, the vehicle group, the PDX (10 ng PDX) group, and the PDX (100 ng PDX) group. Changes in the mechanical withdrawal threshold and thermal withdrawal latency were observed for 7 days. The mRNAs of pro-inflammatory and anti-inflammatory mediators were evaluated via real-time polymerase chain reaction, whereas western blot and immunohistochemistry were separately conducted to assess the expression levels of autophagy-related proteins and adenosine monophosphate-activated protein kinase (AMPK) signaling. Results: Intrathecal delivery of PDX reduced interleukin (IL)-6 and IL-1β mRNA levels and facilitated mRNA transcription of transforming growth factor-β₁, with attenuation of mechanical and thermal hyperalgesia in LRP rat models. With the application of nucleus pulposus to the dorsal root ganglion, autophagy flux and AMPK signaling were severely disrupted in the spinal dorsal horns, and intrathecal treatment with PDX could dose-dependently restore the dysfunction of autophagy flux and AMPK signaling. Conclusion: These data suggest that PDX possesses pro-resolving properties and exerts potent analgesic effects in LRP by affecting autophagy flux via AMPK signaling.

Abnormal Insulin-like Growth Factor 1 Signaling Regulates Neuropathic Pain by Mediating the Mechanistic Target of Rapamycin-Related Autophagy and Neuroinflammation in Mice

Neuropathic pain is a chronic condition with little specific treatment. Insulin-like growth factor 1 (IGF1), interacting with its receptor, IGF1R, serves a vital role in neuronal and brain functions such as autophagy and neuroinflammation. Yet, the function of spinal IGF1/IGF1R in neuropathic pain is unclear. Here, we examined whether and how spinal IGF1 signaling affects pain-like behaviors in mice with chronic constriction injury (CCI) of the sciatic nerve. To corroborate the role of IGF1, we injected intrathecally IGF1R inhibitor (nvp-aew541) or anti-IGF1 neutralizing antibodies. We found that IGF1 (derived from astrocytes) in the lumbar cord increased along with the neuropathic pain induced by CCI. IGF1R was predominantly expressed on neurons. IGF1R antagonism or IGF1 neutralization attenuated pain behaviors induced by CCI, relieved mTOR-related suppression of autophagy, and mitigated neuroinflammation in the spinal cord. These findings reveal that the abnormal IGF1/IGF1R signaling contributes to neuropathic pain by exacerbating autophagy dysfunction and neuroinflammation.

Role of the Ubiquitin System in Chronic Pain

As a significant public health issue, chronic pain, mainly neuropathic pain (NP) and inflammatory pain, has a severe impact. The underlying mechanisms of chronic pain are enigmatic at present. The roles of ubiquitin have been demonstrated in various physiological and pathological conditions and underscore its potential as therapeutic targets. The dysfunction of the component of the ubiquitin system that occurs during chronic pain is rapidly being discovered. These results provide insight into potential molecular mechanisms of chronic pain. Chronic pain is regulated by ubiquitination, SUMOylation, ubiquitin ligase, and deubiquitinating enzyme (DUB), etc. Insight into the mechanism of the ubiquitin system regulating chronic pain might contribute to relevant therapeutic targets and the development of novel analgesics.

Interactions between Autophagy, Proinflammatory Cytokines, and Apoptosis in Neuropathic Pain: Granulocyte Colony Stimulating Factor as a Multipotent Therapy in Rats with Chronic Constriction Injury

Our previous studies have shown that early systemic granulocyte colony-stimulating factor (G-CSF) treatment can attenuate neuropathic pain in rats with chronic constriction injury (CCI) by modulating expression of different proinflammatory cytokines, microRNAs, and proteins. Besides the modulation of inflammatory mediators' expression, previous studies have also reported that G-CSF can modulate autophagic and apoptotic activity. Furthermore, both autophagy and apoptosis play important roles in chronic pain modulation. In this study, we evaluated the temporal interactions of autophagy, and apoptosis in the dorsal root ganglion (DRG) and injured sciatic nerve after G-CSF treatment in CCI rats. We studied the behaviors of CCI rats with or without G-CSF treatment and the various levels of autophagic, proinflammatory, and apoptotic proteins in injured sciatic nerves and DRG neurons at different time points using Western blot analysis and immunohistochemical methods. The results showed that G-CSF treatment upregulated autophagic protein expression in the early phase and suppressed apoptotic protein expression in the late phase after nerve injury. Thus, medication such as G-CSF can modulate autophagy, apoptosis, and different proinflammatory proteins in the injured sciatic nerve and DRG neurons, which have the potential to treat neuropathic pain. However, autophagy-mediated regulation of neuropathic pain is a time-dependent process. An increase in autophagic activity in the early phase before proinflammatory cytokines reach the threshold level to induce neuropathic pain can effectively alleviate further neuropathic pain development.

Local uncoordinated gene 5H2 contributes to nerve injury-induced mechanical allodynia associated to its role in autophagy

Lesions of the peripheral nerves can lead to lifelong neuropathic pain (NP). Autophagic deficiency in the Schwann cells (SCs) is an early event in the origin of NP chronification. Uncoordinated gene 5H2 (UNC5H2), one of the repulsive netrin receptors, mediated the effect of netrin-1 on autophagic activation and cell survival in endothelial cells. However, its role on autophagy regulation in peripheral nerves during NP process remains unidentified. Chronic constriction injury (CCI) of the left sciatic nerve was induced in Sprague-Dawley rats, and UNC5H2 small interfering RNA was transfected to the ipsilateral sciatic nerve immediately after injury. Mechanical allodynia was assessed. Sciatic UNC5H2 and netrin-1 protein levels were investigated. Autophagy in the ipsilateral sciatic nerves was evaluated by detecting punctate light chain 3(LC3) and autophagosomes, as well as the levels of LC3 II, p62 and phosphorylated UNC51-like kinase (ULK1). After CCI, UNC5H2 of the sciatic nerves was upregulated, exclusively expressed in SCs. Small interfering RNA transfection resulted in significant decrease of UNC5H2 and netrin-1 protein, leading to exaggeration of mechanical allodynia through 14 days after CCI. Autophagy was activated but autophagic influx was interfered within a week after CCI, shown by the elevated levels of both LC3II and p62, which was further deteriorated with UNC5H2 knockdown. In addition, the injury-induced augmentation of phosphorylated ULK1 was significantly diminished by UNC5H2 knockdown. Altogether, the results suggest that local UNC5H2 of the peripheral nerve plays a significant role in the process of injury-induced mechanical allodynia, probably associated to its contribution to autophagic regulation.

P2X7R-mediated autophagic impairment contributes to central sensitization in a chronic migraine model with recurrent nitroglycerin stimulation in mice

BACKGROUND

Central sensitization is an important pathophysiological mechanism of chronic migraine (CM). According to our previous studies, microglial activation and subsequent inflammation in the trigeminal nucleus caudalis (TNC) contribute to the central sensitization. The P2X7 receptor (P2X7R) is a purinergic receptor expressed in microglia and participates in central sensitization in chronic pain, but its role in CM is unclear. Numerous studies have shown that P2X7R regulates the level of autophagy and that autophagy affects the microglial activation and inflammation. Recently, autophagy has been shown to be involved in neuropathic pain, but there is no information about autophagy in CM. Therefore, the current study investigated the role of P2X7R in CM and its underlying mechanism, focusing on autophagy regulation.

METHODS

The CM model was established by repeated intraperitoneal injection of nitroglycerin (NTG) in mice. A Von Frey filament and radiant heat were used to assess the mechanical and thermal hypersensitivity. Western blotting and immunofluorescence assays were performed to detect the expression of P2X7R, autophagy-related proteins, and the cellular localization of P2X7R. To determine the role of P2X7R and autophagy in CM, we detected the effects of the autophagy inducer, rapamycin (RAPA) and P2X7R antagonist, Brilliant Blue G (BBG), on pain behavior and the expression of calcitonin gene-related peptide (CGRP) and c-fos. In addition, the effect of RAPA and BBG on microglial activation and subsequent inflammation were investigated.

RESULTS

The expression of P2X7R was increased and was mainly colocalized with microglia in the TNC following recurrent NTG administration. The autophagic flux was blocked in CM, which was characterized by upregulated LC3-II, and accumulated autophagy substrate protein, p62. RAPA significantly improved the basal rather than acute hyperalgesia. BBG alleviated both basal and acute hyperalgesia. BBG activated the level of autophagic flux. RAPA and BBG inhibited the activation of microglia, limited the inflammatory response, and reduced the expression of CGRP and c-fos.

CONCLUSIONS

Our results demonstrate the dysfunction of the autophagic process in CM. Activated autophagy may have a preventive effect on migraine chronification. P2X7R contributes to central sensitization through mediating autophagy regulation and might become a potential target for CM.

Metformin: A Prospective Alternative for the Treatment of Chronic Pain

Metformin (biguanide) is a drug widely used for the treatment of type 2 diabetes. This drug has been used for 60 years as a highly effective antihyperglycemic agent. The search for the mechanism of action of metformin has produced an enormous amount of research to explain its effects on gluconeogenesis, protein metabolism, fatty acid oxidation, oxidative stress, glucose uptake, autophagy and pain, among others. It was only up the end of the 1990s and beginning of this century that some of its mechanisms were revealed. Metformin induces its beneficial effects in diabetes through the activation of a master switch kinase named AMP-activated protein kinase (AMPK). Two upstream kinases account for the physiological activation of AMPK: liver kinase B1 and calcium/calmodulin-dependent protein kinase kinase 2. Once activated, AMPK inhibits the mechanistic target of rapamycin complex 1 (mTORC1), which in turn avoids the phosphorylation of p70 ribosomal protein S6 kinase 1 and phosphatidylinositol 3-kinase/protein kinase B signaling pathways and reduces cap-dependent translation initiation. Since metformin is a disease-modifying drug in type 2 diabetes, which reduces the mTORC1 signaling to induce its effects on neuronal plasticity, it was proposed that these mechanisms could also explain the antinociceptive effect of this drug in several models of chronic pain. These studies have highlighted the efficacy of this drug in chronic pain, such as that from neuropathy, insulin resistance, diabetic neuropathy, and fibromyalgia-type pain. Mounting evidence indicates that chronic pain may induce anxiety, depression and cognitive impairment in rodents and humans. Interestingly, metformin is able to reverse some of these consequences of pathological pain in rodents. The purpose of this review was to analyze the current evidence about the effects of metformin in chronic pain and three of its comorbidities (anxiety, depression and cognitive impairment).

Metformin protects from oxaliplatin induced peripheral neuropathy in rats

Oxaliplatin is a commonly used drug to treat cancer, extending the rate of disease-free survival by 20% in colorectal cancer. However, oxaliplatin induces a disabling form of neuropathy resulting in more than 60% of patients having to reduce or discontinue oxaliplatin, negatively impacting their chance of survival. Oxaliplatin-induced neuropathies are accompanied by degeneration of sensory fibers in the epidermis and hyperexcitability of sensory neurons. These morphological and functional changes have been associated with sensory symptoms such as dysesthesia, paresthesia and mechanical and cold allodynia. Various strategies have been proposed to prevent or treat oxaliplatin-induced neuropathies without success. The anti-diabetic drug metformin has been recently shown to exert neuroprotection in other chemotherapy-induced neuropathies, so here we aimed to test if metformin can prevent the development of oxaliplatin-induced neuropathy in a rat model of this condition. Animals treated with oxaliplatin developed significant intraepidermal fiber degeneration, a mild gliosis in the spinal cord, and mechanical and cold hyperalgesia. The concomitant use of metformin prevented degeneration of intraepidermal fibers, gliosis, and the altered sensitivity. Our evidence further supports metformin as a new approach to prevent oxaliplatin-induced neuropathy with a potential important clinical impact.

p66shc siRNA-Encapsulated PLGA Nanoparticles Ameliorate Neuropathic Pain Following Spinal Nerve Ligation

p66shc, a member of the shc adaptor protein family, has been shown to participate in regulation of mitochondrial homeostasis, apoptosis, and autophagosome formation. The present study was performed to investigate whether p66shc siRNA-encapsulated poly(d,l-lactic-co-glycolic acid) nanoparticles (p66shc siRNA-PLGA NPs) can attenuate spinal nerve ligation (SNL)-induced neuropathic pain in rats. The SNL-induced pain behavior was decreased in the p66shc siRNA-PLGA NP-treated group compared with the scrambled siRNA-PLGA NP-treated group. In the L5 spinal cord of the p66shc siRNA-PLGA NP-treated group, expression levels of phosphorylated p66shc, cleaved caspase-3, p62, and PINK1, as well as microglial activation, were also decreased. In addition, p66shc knockdown using p66shc siRNA reduced the expression levels of cleaved caspase-3, p62, and PINK1, as well as proinflammatory mediators in the H₂O₂-treated HT22 neuronal cells. These results suggest that downregulation of p66shc expression in the spinal cord using p66shc siRNA-PLGA NPs could reduce the SNL-induced neuropathic pain by attenuating the SNL-induced aberrant autophagic, mitophagic, and neuroinflammatory processes in rats.

Metformin limits osteoarthritis development and progression through activation of AMPK signalling

Objectives

In this study, we aim to determine the effect of metformin on osteoarthritis (OA) development and progression.

Methods

Destabilisation of the medial meniscus (DMM) surgery was performed in 10-week-old wild type and AMP-activated protein kinase (AMPK)α1 knockout (KO) mice. Metformin (4 mg/day in drinking water) was given, commencing either 2 weeks before or 2 weeks after DMM surgery. Mice were sacrificed 6 and 12 weeks after DMM surgery. OA phenotype was analysed by micro-computerised tomography (μCT), histology and pain-related behaviour tests. AMPKα1 (catalytic alpha subunit of AMPK) expression was examined by immunohistochemistry and immunofluorescence analyses. The OA phenotype was also determined by μCT and MRI in non-human primates.

Results

Metformin upregulated phosphorylated and total AMPK expression in articular cartilage tissue. Mild and more severe cartilage degeneration was observed at 6 and 12 weeks after DMM surgery, evidenced by markedly increased Osteoarthritis Research Society International scores, as well as reduced cartilage areas. The administration of metformin, commencing either before or after DMM surgery, caused significant reduction in cartilage degradation. Prominent synovial hyperplasia and osteophyte formation were observed at both 6 and 12 weeks after DMM surgery; these were significantly inhibited by treatment with metformin either before or after DMM surgery. The protective effects of metformin on OA development were not observed in AMPKα1 KO mice, suggesting that the chondroprotective effect of metformin is mediated by AMPK signalling. In addition, we demonstrated that treatment with metformin could also protect from OA progression in a partial medial meniscectomy animal model in non-human primates.

Conclusions

The present study suggests that metformin, administered shortly after joint injury, can limit OA development and progression in injury-induced OA animal models.

Exploration of metformin as novel therapy for osteoarthritis: preventing cartilage degeneration and reducing pain behavior

BACKGROUND

Metformin could activate adenosine monophosphate-activated protein kinase (AMPK) which was postulated as a potential therapeutic target for osteoarthritis. This study aimed to examine the effects of metformin on cartilage and pain in osteoarthritis mouse model.

METHODS

Eighty 10-week-old male C57BL/6 mice were randomized to 6 groups: non-operation, sham-operation, destabilization of the medial meniscus (DMM)-operation with intragastric saline/metformin, and DMM-operation with intraarticular saline/metformin. Articular cartilage degeneration was examined by scanning electron microscopy (SEM) and graded using the scoring system recommended by Osteoarthritis Research Society International (OARSI). Mechanical withdrawal threshold and hind paw weight distribution were measured to assess the pain-related behavior. Cell Counting Kit-8 assay, quantificational real-time polymerase chain reaction, and western blot analysis were conducted to examine the anabolic and anti-catabolic effect of metformin and the role of AMPK in mediating its effects on interleukin-1β stimulated primary mice chondrocytes.

RESULTS

Compared with mice receiving intragastric and intraarticular saline, mice in both intragastric and intraarticular metformin displayed attenuated articular cartilage degeneration, indicated by less cartilage damage under SEM and significantly lower OARSI scores. A higher paw withdrawal threshold and a decreased weight-bearing asymmetry were observed in the intragastric and intraarticular metformin mice compared with their corresponding saline groups in DMM model of osteoarthritis. In vitro experiments showed that metformin not only decreased the level of matrix metalloproteinase 13, but also elevated type II collagen production through activating AMPK pathway.

CONCLUSIONS

Metformin attenuates osteoarthritis structural worsening and modulates pain, suggesting its potential for osteoarthritis prevention or treatment.

Metformin Enhances Functional Recovery of Peripheral Nerve in Rats with Sciatic Nerve Crush Injury

Background

The aim of this study was to explore the effect of metformin by inducing autophagy for enhancing functional recovery of peripheral nerve in rats with sciatic nerve crush injury.

Material/Method

Autophagy was determined by electron microscopy, immunofluorescence, and Western blot analysis. Motor function recovery was studied by the footprint intensity method. Axonal growth and regeneration were detected through Western blot while axonal remyelination was analysed through immunocytochemistry. Sensory and functional recovery were assessed by reflexive motor function analysis.

Results

The present study deciphered the role of autophagy induction by metformin in motor functions and peripheral nerve regeneration following sciatic nerve crush injury in rats. The process was detected by measuring autophagosomes and the expression of microtubule-associated protein 1A/1B-light chain 3 upon metformin treatment of sciatic nerve crush-injured rats. Neurobehavioral recovery by metformin was tested by CatWalk gait analysis, and we quantified expression of myelin basic protein MBP and neurofilament NF200 at the damage sight by immunoblotting. In metformin-treated injured rats, autophagy was upregulated, by which the number of dead cells was decreased. Motor function was also recovered after metformin treatment, which was accompanied by upregulation of MBP and NF200 through autophagy induction. Surprisingly, the motor regenerative capability was reduced by treatment with 3-methyl adenine (an autophagy inhibitor) in nerve-injured rats.

Conclusions

Our study revealed that pharmacological induction of autophagy has an important and active role in the regeneration of nerve and motor function regain.

Cannabinoid receptor 2‑selective agonist JWH015 attenuates bone cancer pain through the amelioration of impaired autophagy flux induced by inflammatory mediators in the spinal cord

Bone cancer pain (BCP) is a severe complication of advanced bone cancer. Although cannabinoid receptor 2 (CB2) agonists may have an analgesic effect, the underlying mechanism remains unclear. CB2 serves a protective role in various pathological states through the activation of autophagy. Therefore, the present study aimed to determine whether the analgesic effects of the selective CB2 agonist JWH015 was mediated by the activation of autophagy in BCP. BCP was induced by the intra‑femur implantation of NCTC2472 fibrosarcoma cells in C3H/HeN mice. The pain behaviors were assessed on the following postoperative days. The selective CB2 agonist JWH015 (1 and 2 µg) was intrathecally administered on day 14 following implantation. AM630 (1 µg), a CB2 antagonist, was injected 30 min before JWH015 administration. Lipopolysaccharide (LPS; 100 nM)‑stimulated primary neurons were treated with JWH015 (1 µM) and AM630 (1 µM) to further verify the mechanism by which CB2 affects autophagy. The results demonstrated that autophagy flux was impaired in spinal neurons during BCP, as indicated by the increased ratio of microtubule‑associated protein 1 light chain 3β (LC3B)‑II/LC3B‑I and increased expression of p62. Intrathecal administration of JWH015 attenuated BCP, which was accompanied by the amelioration of impaired autophagy flux (decreased LC3B‑II/LC3B‑I ratio and decreased p62expression). In addition, the activation of glia cells and upregulation of the glia‑derived inflammatory mediators, interleukin (IL)‑1β and IL‑6 were suppressed by JWH015. In LPS‑stimulated primary neurons, IL‑1β and IL‑6 were increased, and autophagy flux was impaired; whereas treatment with JWH015 decreased the expression of IL‑1β and IL‑6, LC3B‑II/LC3B‑I ratio and expression of p62. These effects were by pretreatment with the CB2‑selective antagonist AM630. The results of the present study suggested that the impairment of autophagy flux was induced by glia‑derived inflammatory mediators in spinal neurons. Intrathecal administration of the selective CB2 agonist JWH015 ameliorated autophagy flux through the downregulation of IL‑1β and IL‑6 and attenuated BCP.

Metformin reduces c-Fos and ATF3 expression in the dorsal root ganglia and protects against oxaliplatin-induced peripheral sensory neuropathy in mice

Oxaliplatin is a third-generation platinum drug commonly used as the first line treatment of metastatic colorectal cancer. Oxaliplatin-based anticancer regimens course with dose-limiting neurotoxicity. The pharmacological strategies used to manage such side effect are not totally effective. Metformin is an anti-diabetic drug that is described to negatively modulate painful diabetic neuropathy. Then, this study aimed to assess the effect of metformin in the oxaliplatin-induced peripheral sensory neuropathy in mice. For that purpose, Swiss male mice were injected with oxaliplatin (1, 2 or 4 mg/kg, i.v., twice a week with a total of nine injections) alone or in combination with daily administration of metformin (250 mg/kg, p.o.). Thermal and mechanical nociceptive tests were performed once a week for five weeks. Then, the animals were euthanized on day 35 post-first injection of oxaliplatin and the dorsal root ganglia were harvested for the assessment of c-Fos and ATF3 expressions. Oxaliplatin caused a nociceptive response accompanied by the increased expression of c-Fos and ATF3 in the dorsal root ganglia and spinal cord. In addition, the oxaliplatin-associated nociception was significantly attenuated by metformin (P < 0.05), which also reduced the expression of c-Fos and ATF3 (P < 0.05). Therefore, metformin protected from the peripheral sensory neuropathy induced by oxaliplatin, which was confirmed by the reduction of c-Fos and ATF3 expression, two known neuronal activation and damage markers, respectively.

Metformin relieves neuropathic pain after spinal nerve ligation via autophagy flux stimulation

Neuropathic pain is a well‐known type of chronic pain caused by damage to the nervous system. Autophagy is involved in the development and/or progression of many diseases, including neuropathic pain. Emerging evidence suggests that metformin relieves neuropathic pain in several neuropathic pain models; however, metformin's cellular and molecular mechanism for pain relief remains unknown. In this study, we investigated the therapeutic effects of metformin on pain relief after spinal nerve ligation (SNL) and its underlying mechanism of autophagy regulation. Behavioural analysis, histological assessment, expression of c‐Fos and molecular biological changes, as well as ultrastructural features, were investigated. Our findings showed that the number of autophagosomes and expression of autophagy markers, such as LC3 and beclin1, were increased, while the autophagy substrate protein p62, as well as the ubiquitinated proteins, were accumulated in the ipsilateral spinal cord. However, metformin enhanced the expression of autophagy markers, while it abrogated the abundance of p62 and ubiquitinated proteins. Blockage of autophagy flux by chloroquine partially abolished the apoptosis inhibition and analgesic effects of metformin on SNL. Taken together, these results illustrated that metformin relieved neuropathic pain through autophagy flux stimulation and provided a new direction for metformin drug development to treat neuropathic pain.