The release of exosomes in the medial prefrontal cortex and nucleus accumbens brain regions of chronic constriction injury (CCI) model mice could elevate the pain sensation

Schwann cell-derived extracellular vesicles promote memory impairment associated with chronic neuropathic pain

BACKGROUND

The pathogenesis of memory impairment, a common complication of chronic neuropathic pain (CNP), has not been fully elucidated. Schwann cell (SC)-derived extracellular vesicles (EVs) contribute to remote organ injury. Here, we showed that SC-EVs may mediate pathological communication between SCs and hippocampal neurons in the context of CNP.

METHODS

We used an adeno-associated virus harboring the SC-specific promoter Mpz and expressing the CD63-GFP gene to track SC-EVs transport. microRNA (miRNA) expression profiles of EVs and gain-of-function and loss-of-function regulatory experiments revealed that miR-142-5p was the main cargo of SC-EVs. Next, luciferase reporter gene and phenotyping experiments confirmed the direct targets of miR-142-5p.

RESULTS

The contents and granule sizes of plasma EVs were significantly greater in rats with chronic sciatic nerve constriction injury (CCI)than in sham rats. Administration of the EV biogenesis inhibitor GW4869 ameliorated memory impairment in CCI rats and reversed CCI-associated dendritic spine damage. Notably, during CCI stress, SC-EVs could be transferred into the brain through the circulation and accumulate in the hippocampal CA1-CA3 regions. miR-142-5p was the main cargo wrapped in SC-EVs and mediated the development of CCI-associated memory impairment. Furthermore, α-actinin-4 (ACTN4), ELAV-like protein 4 (ELAVL4) and ubiquitin-specific peptidase 9 X-linked (USP9X) were demonstrated to be important downstream target genes for miR-142-5p-mediated regulation of dendritic spine damage in hippocampal neurons from CCI rats.

CONCLUSION

Together, these findings suggest that SCs-EVs and/or their cargo miR-142-5p may be potential therapeutic targets for memory impairment associated with CNP.

Restoration of the Activity of the Prefrontal Cortex to the Nucleus Accumbens Core Pathway Relieves Fentanyl-Induced Hyperalgesia in Male Rats

Purpose

Functional connectivity between the prelimbic medial prefrontal cortex (PL-mPFC) and the core of the nucleus accumbens (NAc core) predicts pain chronification. Inhibiting the apoptosis of oligodendrocytes in the PL-mPFC prevents fentanyl-induced hyperalgesia in rats. However, the role of prefrontal cortex (PFC)-NAc projections in opioid-induced hyperalgesia (OIH) remains unclear. Herein, we explored the role of the PL-NAc core circuit in fentanyl-induced hyperalgesia.

Methods

An OIH rat model was established, and patch-clamp recording, immunofluorescence, optogenetics, and chemogenetic methods were employed for neuron excitability detection and nociceptive behavioral assessment.

Results

Our results showed decreased activity of the right PL-mPFC layer V output neurons in rats with OIH. Similarly, the excitability of the NAc core neurons receiving glutamatergic projections from the PL-mPFC decreased in OIH rats, observed by the light-evoked excitatory postsynaptic currents/light-excited inhibitory postsynaptic currents ratio (eEPSC/eIPSC ratio). Fentanyl-induced hyperalgesia was reversed by optogenetic activation of the PL-NAc core pathway, and chemogenetic suppression of this pathway induced hyperalgesia in control (saline-treated) rats. However, behavioral hyperalgesia was not aggravated by this chemogenetic suppression in OIH (fentanyl-treated) rats.

Conclusion

Our findings indicate that inactivation of the PL-NAc core pathway may be a cause of OIH and restoring the activity of this pathway may provide a strategy for OIH treatment.

Neuropathic pain; what we know and what we should do about it

Neuropathic pain can result from injury to, or disease of the nervous system. It is notoriously difficult to treat. Peripheral nerve injury promotes Schwann cell activation and invasion of immunocompetent cells into the site of injury, spinal cord and higher sensory structures such as thalamus and cingulate and sensory cortices. Various cytokines, chemokines, growth factors, monoamines and neuropeptides effect two-way signalling between neurons, glia and immune cells. This promotes sustained hyperexcitability and spontaneous activity in primary afferents that is crucial for onset and persistence of pain as well as misprocessing of sensory information in the spinal cord and supraspinal structures. Much of the current understanding of pain aetiology and identification of drug targets derives from studies of the consequences of peripheral nerve injury in rodent models. Although a vast amount of information has been forthcoming, the translation of this information into the clinical arena has been minimal. Few, if any, major therapeutic approaches have appeared since the mid 1990's. This may reflect failure to recognise differences in pain processing in males vs. females, differences in cellular responses to different types of injury and differences in pain processing in humans vs. animals. Basic science and clinical approaches which seek to bridge this knowledge gap include better assessment of pain in animal models, use of pain models which better emulate human disease, and stratification of human pain phenotypes according to quantitative assessment of signs and symptoms of disease. This can lead to more personalized and effective treatments for individual patients. Significance statement: There is an urgent need to find new treatments for neuropathic pain. Although classical animal models have revealed essential features of pain aetiology such as peripheral and central sensitization and some of the molecular and cellular mechanisms involved, they do not adequately model the multiplicity of disease states or injuries that may bring forth neuropathic pain in the clinic. This review seeks to integrate information from the multiplicity of disciplines that seek to understand neuropathic pain; including immunology, cell biology, electrophysiology and biophysics, anatomy, cell biology, neurology, molecular biology, pharmacology and behavioral science. Beyond this, it underlines ongoing refinements in basic science and clinical practice that will engender improved approaches to pain management.

Concise review: Current understanding of extracellular vesicles to treat neuropathic pain

Extracellular vesicles (EVs) including exosomes are vesicular vesicles with phospholipid bilayer implicated in many cellular interactions and have the ability to transfer multiple types of cargo to cells. It has been found that EVs can package various molecules including proteins and nucleic acids (DNA, mRNA, and noncoding RNA). The discovery of EVs as carriers of proteins and various forms of RNA, such as microRNAs (miRNA) and long noncoding RNAs (lncRNA), has raised great interest in the field of drug delivery. Despite the underlying mechanisms of neuropathic pain being unclear, it has been shown that uncontrolled glial cell activation and the neuroinflammation response to noxious stimulation are important in the emergence and maintenance of neuropathic pain. Many studies have demonstrated a role for noncoding RNAs in the pathogenesis of neuropathic pain and EVs may offer possibilities as carriers of noncoding RNAs for potential in neuropathic pain treatment. In this article, the origins and clinical application of EVs and the mechanism of neuropathic pain development are briefly introduced. Furthermore, we demonstrate the therapeutic roles of EVs in neuropathic pain and that this involve vesicular regulation of glial cell activation and neuroinflammation.

Mediators of Neuropathic Pain; Focus on Spinal Microglia, CSF-1, BDNF, CCL21, TNF-α, Wnt Ligands, and Interleukin 1β

Intractable neuropathic pain is a frequent consequence of nerve injury or disease. When peripheral nerves are injured, damaged axons undergo Wallerian degeneration. Schwann cells, mast cells, fibroblasts, keratinocytes and epithelial cells are activated leading to the generation of an "inflammatory soup" containing cytokines, chemokines and growth factors. These primary mediators sensitize sensory nerve endings, attract macrophages, neutrophils and lymphocytes, alter gene expression, promote post-translational modification of proteins, and alter ion channel function in primary afferent neurons. This leads to increased excitability and spontaneous activity and the generation of secondary mediators including colony stimulating factor 1 (CSF-1), chemokine C-C motif ligand 21 (CCL-21), Wnt3a, and Wnt5a. Release of these mediators from primary afferent neurons alters the properties of spinal microglial cells causing them to release tertiary mediators, in many situations via ATP-dependent mechanisms. Tertiary mediators such as BDNF, tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β), and other Wnt ligands facilitate the generation and transmission of nociceptive information by increasing excitatory glutamatergic transmission and attenuating inhibitory GABA and glycinergic transmission in the spinal dorsal horn. This review focusses on activation of microglia by secondary mediators, release of tertiary mediators from microglia and a description of their actions in the spinal dorsal horn. Attention is drawn to the substantial differences in the precise roles of various mediators in males compared to females. At least 25 different mediators have been identified but the similarity of their actions at sensory nerve endings, in the dorsal root ganglia and in the spinal cord means there is considerable redundancy in the available mechanisms. Despite this, behavioral studies show that interruption of the actions of any single mediator can relieve signs of pain in experimental animals. We draw attention this paradox. It is difficult to explain how inactivation of one mediator can relieve pain when so many parallel pathways are available.

Nociception and Pain: New Roles for Exosomes

The interchange of information from one cell to another relies on the release of hundreds of different molecules including small peptides, amino acids, nucleotides, RNA, steroids, retinoids, or fatty acid metabolites. Many of them are released to the extracellular matrix as free molecules and others can be part of the cargo of cellular vesicles. Small extracellular vesicles (30-150 nm), also known as exosomes, are a known mechanism of cell-to-cell communication in the nervous system. Exosomes participate in the pathogenesis of several neurological conditions including Alzheimer's and Parkinson's disease. However, exciting emerging evidence demonstrates that exosomes also regulate mechanisms of the sensory process including nociception. The goal of this review is to summarize the literature on exosome biogenesis, methods of small vesicle isolation and purification, and their role in nociception. We also provide insights on the potential applications of exosomes as pain biomarkers or as novel therapeutics.

CCR4 Antagonist (C021) Administration Diminishes Hypersensitivity and Enhances the Analgesic Potency of Morphine and Buprenorphine in a Mouse Model of Neuropathic Pain

Neuropathic pain is a chronic condition that remains a major clinical problem owing to high resistance to available therapy. Recent studies have indicated that chemokine signaling pathways are crucial in the development of painful neuropathy; however, the involvement of CC chemokine receptor 4 (CCR4) has not been fully elucidated thus far. Therefore, the aim of our research was to investigate the role of CCR4 in the development of tactile and thermal hypersensitivity, the effectiveness of morphine/buprenorphine, and opioid-induced tolerance in mice exposed to chronic constriction injury (CCI) of the sciatic nerve. The results of our research demonstrated that a single intrathecal or intraperitoneal administration of C021, a CCR4 antagonist, dose dependently diminished neuropathic pain-related behaviors in CCI-exposed mice. After sciatic nerve injury, the spinal expression of CCL17 and CCL22 remained unchanged in contrast to that of CCL2, which was significantly upregulated until day 14 after CCI. Importantly, our results provide evidence that in naive mice, CCL2 may evoke pain-related behaviors through CCR4 because its pronociceptive effects are diminished by C021. In CCI-exposed mice, the pharmacological blockade of CCR4 enhanced the analgesic properties of morphine/buprenorphine and delayed the development of morphine-induced tolerance, which was associated with the silencing of IBA-1 activation in cells and decrease in CCL2 production. The obtained data suggest that the pharmacological blockade of CCR4 may be a new potential therapeutic target for neuropathic pain polytherapy.

Exosomes as a new pain biomarker opportunity

Exosomes are extracellular microvesicles implicated in intercellular communication with ability to transfer cargo molecules, including protein, lipids, and nucleic acids, at both close and distant target sites. It has been shown that exosomes are implicated in physiological and pathological processes. In recent years, the interest on exosomes' role in many pain states has increased. Their involvements in pain processes have been demonstrated by studies on different chronic pain diseases, both inflammatory and neuropathic, such as osteoarthritis, rheumatoid arthritis, inflammatory bowel diseases, neurodegenerative pathologies, complex regional pain syndrome, and peripheral nerve injury. Animal and clinical studies investigated exosomes-based treatments, showing their ability to improve painful symptoms with fewer side effects, with potential immunoprotective and anti-inflammatory effect. Specific molecular patterns characterize exosomes' cargo according to the cellular origin, epigenetic modifications, environmental state, and stressor factors. Therefore, the identification of specific cargo's profile associated to pain states may lead to recognize specific pathological states and to consider the use of exosomes as biomarkers of diseases. Furthermore, exosomes' ability to transfer information and their presence in many accessible biological fluids suggest a potential use as novel non-invasive therapeutic tools in pain field.