Cathepsin S release from primary cultured microglia is regulated by the P2X7 receptor

Exploring the Association between BDNF related Signaling Pathways and Depression: A Literature Review

Depression is a debilitating mental disease that inflicts significant harm upon individuals and society, yet effective treatment options remain elusive. At present, the pathogenesis of multiple depression is not fully clear, but its occurrence can be related to biological or environmental pathways, among which Brain-derived neurotrophic factor (BDNF) can unequivocally act on two downstream receptors, tyrosine kinase receptor (TrkB) and the p75 neurotrophin receptor (p75NTR), then affect the related signal pathways, affecting the occurrence and development of depression. Accumulating studies have revealed that BDNF-related pathways are critical in the pathophysiology of depression, and their interaction can further influence the efficacy of depression treatment. In this review, we mainly summarized the signaling pathways associated with BDNF and classified them according to different receptors and related molecules, providing promising insights and future directions in the treatment of depression.

Roles of cathepsin S expression levels on the prognosis and tumour microenvironment in clear cell renal cell carcinoma

BACKGROUND

Increasing evidence suggests a link between the enzyme cathepsin S (CTSS) and tumour development. However, the potential involvement and molecular functions of CTSS in clear cell renal cell carcinoma (ccRCC) remain unclear.

METHODS

We downloaded original data from The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) databases and integrated them using R. Kaplan-Meier plots of integrated expression scores were used to analyse survival outcomes. Additionally, we investigated mRNA expression, clinicopathological features, immune infiltrates, and single-cell sequencing analysis of CTSS in ccRCC. In vitro experiments were conducted with qRT-PCR and IHC staining.

RESULTS

CTSS transcriptomic and proteomic levels were higher in ccRCC than in para-cancerous tissues. Low CTSS expression was correlated with poor prognosis in patients with ccRCC. Our data demonstrated that the expression of CTSS was strongly correlated with immune cell infiltration levels and gene markers of immune cells, chemokines, and receptors. Single-cell sequencing analysis demonstrated that CTSS expression was detectable in monocytes/macrophages. Finally, certain chemicals were confirmed to affect CTSS expression.

CONCLUSION

Our findings indicate that CTSS offers promise as a prognostic biomarker and novel immune-related therapeutic target for ccRCC.

Chronic post-surgical pain after total knee arthroplasty: a narrative review

Total knee arthroplasty (TKA) is an efficacious treatment for end-stage knee osteoarthritis, often accompanied by severe postoperative pain. In certain patients, this pain can persist for over 3 months and is referred to as chronic post-surgical pain (CPSP). Postoperative persistent pain has emerged as a significant and noteworthy issue impacting patient quality of life following TKA. The etiology of CPSP after TKA is multifaceted. Peripheral or central sensitizations resulting from inflammatory reactions, nerve injury, and neurobiological mechanisms are the primary mechanisms contributing to chronic persistent pain after TKA. Preoperative, intraoperative, and postoperative factors can induce pain sensitization. Once CPSP occurs after TKA, it significantly hampers patient recovery with challenging treatment options. Currently, among the preventive and therapeutic strategies for chronic pain after TKA, it is widely believed that early comprehensive preventive treatment to prevent acute to chronic pain transition can substantially reduce the incidence of CPSP following TKA. In recent years, studies have investigated perioperative strategies aimed at reducing the occurrence of persistent pain after TKA. This article provides an overview of advancements in understanding the pathogenesis, high-risk factors, and preventive measures for chronic pain following TKA. We hope that this review will guide future research directions on CPSP after TKA while contributing to clinical perioperative pain management.

Dendritic Spines and Pain Memory

Neuropathic pain is a debilitating form of pain arising from injury or disease of the nervous system that affects millions of people worldwide. Despite its prevalence, the underlying mechanisms of neuropathic pain are still not fully understood. Dendritic spines are small protrusions on the surface of neurons that play an important role in synaptic transmission. Recent studies have shown that dendritic spines reorganize in the superficial and deeper laminae of the spinal cord dorsal horn with the development of neuropathic pain in multiple models of disease or injury. Given the importance of dendritic spines in synaptic transmission, it is possible that studying dendritic spines could lead to new therapeutic approaches for managing intractable pain. In this review article, we highlight the emergent role of dendritic spines in neuropathic pain, as well as discuss the potential for studying dendritic spines for the development of new therapeutics.

Differential contribution of THIK-1 K+ channels and P2X7 receptors to ATP-mediated neuroinflammation by human microglia

Neuroinflammation is highly influenced by microglia, particularly through activation of the NLRP3 inflammasome and subsequent release of IL-1β. Extracellular ATP is a strong activator of NLRP3 by inducing K+ efflux as a key signaling event, suggesting that K+-permeable ion channels could have high therapeutic potential. In microglia, these include ATP-gated THIK-1 K+ channels and P2X7 receptors, but their interactions and potential therapeutic role in the human brain are unknown. Using a novel specific inhibitor of THIK-1 in combination with patch-clamp electrophysiology in slices of human neocortex, we found that THIK-1 generated the main tonic K+ conductance in microglia that sets the resting membrane potential. Extracellular ATP stimulated K+ efflux in a concentration-dependent manner only via P2X7 and metabotropic potentiation of THIK-1. We further demonstrated that activation of P2X7 was mandatory for ATP-evoked IL-1β release, which was strongly suppressed by blocking THIK-1. Surprisingly, THIK-1 contributed only marginally to the total K+ conductance in the presence of ATP, which was dominated by P2X7. This suggests a previously unknown, K+-independent mechanism of THIK-1 for NLRP3 activation. Nuclear sequencing revealed almost selective expression of THIK-1 in human brain microglia, while P2X7 had a much broader expression. Thus, inhibition of THIK-1 could be an effective and, in contrast to P2X7, microglia-specific therapeutic strategy to contain neuroinflammation.

Exercise, Spinal Microglia and Neuropathic Pain: Potential Molecular Mechanisms

As one of the most common neuropathic disorders, neuropathic pain often has a negative impact on patients with persistent pain, mood disorders and sleep disturbances. Currently, neuropathic pain is not treated with any specific drug, instead, drugs for other diseases are used as replacements in clinics, but most have adverse effects. In recent years, the role of spinal cord microglia in the pathogenesis of neuropathic pain has been widely recognized, and they are being explored as potential therapeutic targets. Spinal microglia are known to be involved in the pathogenic mechanisms of neuropathic pain through purine signaling, fractalkine signaling, and p38 MAPK signaling. Exercise is a safe and effective treatment, and numerous studies have demonstrated its effectiveness in improving neurological symptoms. Nevertheless, it remains unclear what the exact molecular mechanism is. This review summarized the specific molecular mechanisms of exercise in alleviating neuropathic pain by mediating the activity of spinal microglia and maintaining the phenotypic homeostasis of spinal microglia through purine signaling, fractalkine signaling and p38 MAPK signaling. In addition, it has been proposed that different intensities and types of exercise affect the regulation of the above-mentioned signaling pathways differently, providing a theoretical basis for the improvement of neuropathic pain through exercise.

Involvement of microglial P2X7 receptor in pain modulation

BACKGROUND

Pain is a rapid response mechanism that compels organisms to retreat from the harmful stimuli and triggers a repair response. Nonetheless, when pain persists for extended periods, it can lead to adverse changes into in the individual's brain, negatively impacting their emotional state and overall quality of life. Microglia, the resident immune cells in the central nervous system (CNS), play a pivotal role in regulating a variety of pain-related disorders. Specifically, recent studies have shed light on the central role that microglial purinergic ligand-gated ion channel 7 receptor (P2X7R) plays in regulating pain. In this respect, the P2X7R on microglial membranes represents a potential therapeutic target.

AIMS

To expound on the intricate link between microglial P2X7R and pain, offering insights into potential avenues for future research.

METHODS

We reviewed 140 literature and summarized the important role of microglial P2X7R in regulating pain, including the structure and function of P2X7R, the relationship between P2X7R and microglial polarization, P2X7R-related signaling pathways, and the effects of P2X7R antagonists on pain regulation.

RESULTS

P2X7R activation is related to M1 polarization of microglia, while suppressing P2X7R can transfer microglia from M1 into M2 phenotype. And targeting the P2X7R-mediated signaling pathways helps to explore new therapy for pain alleviation. P2X7R antagonists also hold potential for translational and clinical applications in pain management.

CONCLUSIONS

Microglial P2X7R holds promise as a potential novel pharmacological target for clinical treatments due to its distinctive structure, function, and the development of antagonists.

Antinociceptive and neuroprotective effect of echinacoside on peripheral neuropathic pain in mice through inhibiting P2X7R/FKN/CX3CR1 pathway

Clinically, neuropathic pain treatment remains a challenging issue because the major therapy, centred around pharmacological intervention, is not satisfactory enough to patient by reason of low effectiveness and more adverse reaction. Therefore, it is still necessary to find more effective and safe therapy to ameliorate neuropathic pain. The purpose of this study was to explore the antinociceptive effect of Echinacoside (ECH), an active compound of Cistanche deserticola Ma, on peripheral neuropathic pain induced by chronic constriction injury (CCI) in mice, and to demonstrate its potential mechanism in vivo and vitro. In the present study, results showed that intraperitoneal administration of ECH (50, 100, and 200 mg/kg) could alleviate mechanical allodynia, cold allodynia and thermal hyperalgesia via behavioural test. Moreover, the structure and function of injured sciatic nerve by CCI were taken a turn for the better to a certain extent after ECH treatment using histopathological and electrophysiological test. Furthermore, ECH repressed the expression of the P2X7R and FKN and reduced the expression and release of the IL-1β, IL-6 and TNF-α. Besides, ECH could decrease Ca2+ influx and Cats efflux and inhibit phosphorylation of p38MAPK. To sum up, the present study illustrated that ECH could alleviate peripheral neuropathic pain by inhibiting microglia overactivation and inflammation through P2X7R/FKN/CX3CR1 signalling pathway in spinal cord. This study would provide a new perspective and strategy for the pharmacological treatment on neuropathic pain.

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.

Potential role of the hydroxyl carboxylic acid receptor type 2 (HCAR2) in microglia pathophysiology: A possible cross-talk with C-X-C chemokine receptor 1 (CXCR1)

Following insults or injury, microglia cells are activated contributing to the cytotoxic response or by promoting an immune-mediated damage resolution. Microglia cells express HCA2R, a hydroxy carboxylic acid (HCA) receptor, which has been shown to mediate neuroprotective and anti-inflammatory effects. In this study we found that HCAR2 expression levels were increased in cultured rat microglia cells after Lipopolysaccharide (LPS) exposure. In a similar fashion, the treatment with MK 1903, a potent full agonist of HCAR2, increased the receptor protein levels. Moreover, HCAR2 stimulation prevented i) cells viability ii) morphological activation iii) pro/anti-inflammatory mediators production in LPS-treated cells. Likewise, HCAR2 stimulation reduced the proinflammatory mediators mRNA expression induced by neuronal chemokine fractalkine (FKN), a neuronal derived chemokine activating its unique receptor, chemokine receptor 1 (CX3CR1) on microglia surface. Interestingly, electrophysiological recordings in vivo revealed that MK1903 was able to prevent the increase of the nociceptive neurons (NS) firing activity mediated by the spinal FKN application in healthy rats. Collectively, our data demonstrate that HCAR2 is functionally expressed in microglia, by showing its capability to shift microglia toward an anti-inflammatory phenotype. Moreover, we indicated the contribute of HCAR2 in the FKN signaling and suggested a possible HCAR2/CX3CR1 functional interaction. This study paves the way for further investigations aimed at understanding the role HCAR2 as potential target in neuroinflammation-based CNS disorders. This article is part of the Special Issue on "The receptor-receptor interaction as a new target for therapy".

Research progress of p38 as a new therapeutic target against morphine tolerance and the current status of therapy of morphine tolerance

With the development of the medical industry, new painkillers continue to appear in people's field of vision, but so far no painkiller can replace morphine. While morphine has a strong analgesic effect, it is also easy to produce pain sensitivity and tolerance. Due to the great inter-individual differences in patient responses, there are few clear instructions on how to optimise morphine administration regimens, which complicates clinicians' treatment strategies and limits the effectiveness of morphine in long-term pain therapy. P38MAPK is a key member of the MAPK family. Across recent years, it has been discovered that p38MAPK rises dramatically in a wide range of morphine tolerance animal models. Morphine tolerance can be reduced or reversed by inhibiting p38MAPK. However, the role and specific mechanism of p38MAPK are not clear. In this review, we synthesise the relevant findings, highlight the function and potential mechanism of p38MAPK in morphine tolerance, as well as the present status and efficacy of morphine tolerance therapy, and underline the future promise of p38MAPK targeted morphine tolerance treatment.

Cysteine cathepsins: A long and winding road towards clinics

Biomedical research often focuses on properties that differentiate between diseased and healthy tissue; one of the current focuses is elevated expression and altered localisation of proteases. Among these proteases, dysregulation of cysteine cathepsins can frequently be observed in inflammation-associated diseases, which tips the functional balance from normal physiological to pathological manifestations. Their overexpression and secretion regularly exhibit a strong correlation with the development and progression of such diseases, making them attractive pharmacological targets. But beyond their mostly detrimental role in inflammation-associated diseases, cysteine cathepsins are physiologically highly important enzymes involved in various biological processes crucial for maintaining homeostasis and responding to different stimuli. Consequently, several challenges have emerged during the efforts made to translate basic research data into clinical applications. In this review, we present both physiological and pathological roles of cysteine cathepsins and discuss the clinical potential of cysteine cathepsin-targeting strategies for disease management and diagnosis.

Cathepsin S (CTSS) activity in health and disease - A treasure trove of untapped clinical potential

Amongst the lysosomal cysteine cathepsin family of proteases, cathepsin S (CTSS) holds particular interest due to distinctive properties including a normal restricted expression profile, inducible upregulation and activity at a broad pH range. Consequently, while CTSS is well-established as a member of the proteolytic cocktail within the lysosome, degrading unwanted and damaged proteins, it has increasingly been shown to mediate a number of distinct, more selective roles including antigen processing and antigen presentation, and cleavage of substrates both intra and extracellularly. Increasingly, aberrant CTSS expression has been demonstrated in a variety of conditions and disease states, marking it out as both a biomarker and potential therapeutic target. This review seeks to contextualise CTSS within the cysteine cathepsin family before providing an overview of the broad range of pathologies in which roles for CTSS have been identified. Additionally, current clinical progress towards specific inhibitors is detailed, updating the position of the field in exploiting this most unique of proteases.

The Role of Microglia in Neuroinflammation of the Spinal Cord after Peripheral Nerve Injury

Peripheral nerve injuries induce a pronounced immune reaction within the spinal cord, largely governed by microglia activation in both the dorsal and ventral horns. The mechanisms of activation and response of microglia are diverse depending on the location within the spinal cord, type, severity, and proximity of injury, as well as the age and species of the organism. Thanks to recent advancements in neuro-immune research techniques, such as single-cell transcriptomics, novel genetic mouse models, and live imaging, a vast amount of literature has come to light regarding the mechanisms of microglial activation and alluding to the function of microgliosis around injured motoneurons and sensory afferents. Herein, we provide a comparative analysis of the dorsal and ventral horns in relation to mechanisms of microglia activation (CSF1, DAP12, CCR2, Fractalkine signaling, Toll-like receptors, and purinergic signaling), and functionality in neuroprotection, degeneration, regeneration, synaptic plasticity, and spinal circuit reorganization following peripheral nerve injury. This review aims to shed new light on unsettled controversies regarding the diversity of spinal microglial-neuronal interactions following injury.

P2X7 receptor mediated release of microglial prostanoids and miRNAs correlates with reversal of neuropathic hypersensitivity in rats

BACKGROUND

P2X7 receptor antagonists have potential for treating various CNS diseases, including neuropathic pain, although none have been approved for clinical use. Reasons may include insufficient understanding of P2X7 receptor signaling in pain and the lack of a corresponding preclinical mechanistic biomarker.

METHODS

Lu AF27139 is a highly selective and potent small molecule antagonist at rat, mouse, and human forms of the P2X7 receptor, with excellent pharmacokinetic and CNS permeability properties. In the current experiments, we probed the utility of previously characterized and novel signaling cascades exposed to Lu AF27139 using cultured microglia combined with release assays. Subsequently, we assessed the biomarker potential of identified candidate molecules in the rat chronic constriction injury (CCI) model of neuropathic pain; study design limitations precluded their assessment in spared nerve injury (SNI) rats.

RESULTS

Lu AF27139 blocked several pain-relevant pathways downstream of P2X7 receptors in-vitro. At brain and spinal cord receptor occupancy levels capable of functionally blocking P2X7 receptors, it diminished neuropathic hypersensitivity in SNI rats, and less potently in CCI rats. Although tissue levels of numerous molecules previously linked to neuropathic pain and P2X7 receptor function (e.g. IL-6, IL-1β, cathepsin-S, 2-AG) were unaffected by CCI, Lu AF27139-mediated regulation of spinal PGE2 and miRNA (e.g. rno-miR-93-5p) levels increased by CCI aligned with its ability to diminish neuropathic hypersensitivity.

CONCLUSIONS

We have identified a pain-relevant P2X7 receptor-regulated mechanism in neuropathic rats that could hold promise as a translatable biomarker and by association enhance the clinical progression of P2X7 receptor antagonists in neuropathic pain.

Role of Microglia and Astrocytes in Spinal Cord Injury Induced Neuropathic Pain

Background:

Spinal cord injuries incite varying degrees of symptoms in patients, ranging from weakness and incoordination to paralysis. Common amongst spinal cord injury (SCI) patients, neuropathic pain (NP) is a debilitating medical condition. Unfortunately, there remain many clinical impediments in treating NP because there is a lack of understanding regarding the mechanisms behind SCI-induced NP (SCINP). Given that more than 450,000 people in the United States alone suffer from SCI, it is unsatisfactory that current treatments yield poor results in alleviating and treating NP.

Summary:

In this review, we briefly discussed the models of SCINP along with the mechanisms of NP progression. Further, current treatment modalities are herein explored for SCINP involving pharmacological interventions targeting glia cells and astrocytes.

Key message:

The studies presented in this review provide insight for new directions regarding SCINP alleviation. Given the severity and incapacitating effects of SCINP, it is imperative to study the pathways involved and find new therapeutic targets in coordination with stem cell research, and to develop a new gold-standard in SCINP treatment.

Fractalkine/CX3CR1 Pathway in Neuropathic Pain: An Update

Injuries to the nervous system can result in a debilitating neuropathic pain state that is often resistant to treatment with available analgesics, which are commonly associated with several side-effects. Growing pre-clinical and clinical evidence over the last two decades indicates that immune cell-mediated mechanisms both in the periphery and in the Central Nervous System (CNS) play significant roles in the establishment and maintenance of neuropathic pain. Specifically, following peripheral nerve injury, microglia, which are CNS resident immune cells, respond to the activity of the first pain synapse in the dorsal horn of spinal cord and also to neuronal activity in higher centres in the brain. This microglial response leads to the production and release of several proinflammatory mediators which contribute to neuronal sensitisation under neuropathic pain states. In this review, we collect evidence demonstrating the critical role played by the Fractalkine/CX₃CR₁ signalling pathway in neuron-to-microglia communication in neuropathic pain states and explore how strategies that include components of this pathway offer opportunities for innovative targets for neuropathic pain.

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.

P2X7 receptor in multifaceted cellular signalling and its relevance as a potential therapeutic target in different diseases

P2X7 receptor, a purinergic receptor family member, is abundantly expressed on many cells, including immune, muscle, bone, neuron, and glia. It acts as an ATP-activated cation channel that permits the influx of Ca²⁺, Na⁺ and efflux of K⁺ ions. The P2X7 receptor plays crucial roles in many physiological processes including cytokine and chemokine secretion, NLRP3 inflammasome activation, cellular growth and differentiation, locomotion, wound healing, transcription factors activation, cell death and T-lymphocyte survival. Past studies have demonstrated the up-regulation and direct association of this receptor in many pathophysiological conditions such as cancer, diabetics, arthritis, tuberculosis (TB) and inflammatory diseases. Hence, targeting this receptor is considered a worthwhile approach to lessen the afflictions associated with the disorders mentioned above by understanding the receptor architecture and downstream signalling processes. Here, in the present review, we have dissected the structural and functional aspects of the P2X7 receptor, emphasizing its role in various diseased conditions. This information will provide in-depth knowledge about the receptor and help to develop apt curative methodologies for the betterment of humanity in the coming years.

Distinct roles of GT1b and CSF-1 in microglia activation in nerve injury-induced neuropathic pain

Although microglia activation plays an important role in the development of nerve injury-induced neuropathic pain, the molecular mechanisms of spinal cord microglia activation in nerve injury are not completely understood. Recently, two injured sensory neuron-derived molecules, colony stimulating factor-1 (CSF-1) and GT1b, were proposed to trigger spinal cord microglia activation, yet their relationship and relative contribution to microglia activation have not been addressed. In the present study, the role of GT1b and CSF-1 in microglia activation and proliferation was characterized. GT1b stimulation upregulated proinflammatory mediators such as IL-1β, TNF-α, and NADPH oxidase 2 (Nox2), without microglia proliferation. Conversely, CSF-1 stimulation induced microglia proliferation with minimal proinflammatory gene induction. Notably, neither GT1b nor CSF-1 induced mechanical hypersensitivity in female mice; however, they induced similar microglial proliferation in both male and female mice. Taken together, our data indicate that injured sensory neuron-derived GT1b and CSF-1 activate spinal cord microglia in concert through distinct activation pathways.

On the therapeutic targets and pharmacological treatments for pain relief following spinal cord injury: A mechanistic review

Spinal cord injury (SCI) is globally considered as one of the most debilitating disorders, which interferes with daily activities and life of the affected patients. Despite many developments in related recognizing and treating procedures, post-SCI neuropathic pain (NP) is still a clinical challenge for clinicians with no distinct treatments. Accordingly, a comprehensive search was conducted in PubMed, Medline, Scopus, Web of Science, and national database (SID and Irandoc). The relevant articles regarding signaling pathways, therapeutic targets and pharmacotherapy of post-SCI pain were also reviewed. Data were collected with no time limitation until November 2020. The present study provides the findings on molecular mechanisms and therapeutic targets, as well as developing the critical signaling pathways to introduce novel neuroprotective treatments of post-SCI pain. From the pathophysiological mechanistic point of view, post-SCI inflammation activates the innate immune system, in which the immune cells elicit secondary injuries. So, targeting the critical signaling pathways for pain management in the SCI population has significant importance in providing new treatments. Indeed, several receptors, ion channels, excitatory neurotransmitters, enzymes, and key signaling pathways could be used as therapeutic targets, with a pivotal role of n-methyl-D-aspartate, gamma-aminobutyric acid, and inflammatory mediators. The current review focuses on conventional therapies, as well as crucial signaling pathways and promising therapeutic targets for post-SCI pain to provide new insights into the clinical treatment of post-SCI pain. The need to develop innovative delivery systems to treat SCI is also considered.

Reduction of prefrontal purinergic signaling is necessary for the analgesic effect of morphine

Morphine is commonly used to relieve moderate to severe pain, but repeated doses cause opioid tolerance. Here, we used ATP sensor and fiber photometry to detect prefrontal ATP level. It showed that prefrontal ATP level decreased after morphine injection and the event amplitude tended to decrease with continuous morphine exposure. Morphine had little effect on prefrontal ATP due to its tolerance. Therefore, we hypothesized that the analgesic effect of morphine might be related to ATP in the medial prefrontal cortex (mPFC). Moreover, local infusion of ATP partially antagonized morphine analgesia. Then we found that inhibiting P2X7R in the mPFC mimicked morphine analgesia. In morphine-tolerant mice, pretreatment with P2X4R or P2X7R antagonists in the mPFC enhanced analgesic effect. Our findings suggest that reduction of prefrontal purinergic signaling is necessary for the morphine analgesia, which help elucidate the mechanism of morphine analgesia and may lead to the development of new clinical treatments for neuropathic pain.

P2X7 receptor and the NLRP3 inflammasome: Partners in crime

Adenosine triphosphate (ATP) is a molecule that on one hand plays a central role in cellular energetics and which on the other is a ubiquitous signaling molecule when released into the extracellular media. Extracellular ATP accumulates in inflammatory environments where it acts as a damage-associated molecular pattern and activates the purinergic P2X receptor 7 (P2X7) in immune cells. P2X7 receptor activation induces the formation of the nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing 3 (NLRP3) inflammasome and the activation of the inflammatory caspase-1. Caspase-1 causes an inflammatory type of cell death called pyroptosis through the release of pro-inflammatory cytokines and intracellular content. Consequently, intense research efforts have been devoted to the design of novel anti-inflammatory therapies, focusing in particular on the P2X7 receptor and the NLRP3 pathway and the introduction of new blocking molecules in early phase clinical trials.

Calcium Increase and Substance P Release Induced by the Neurotoxin Brevetoxin-1 in Sensory Neurons: Involvement of PAR2 Activation through Both Cathepsin S and Canonical Signaling

Red tides involving Karenia brevis expose humans to brevetoxins (PbTxs). Oral exposition triggers neurotoxic shellfish poisoning, whereas inhalation induces a respiratory syndrome and sensory disturbances. No curative treatment is available and the pathophysiology is not fully elucidated. Protease-activated receptor 2 (PAR2), cathepsin S (Cat-S) and substance P (SP) release are crucial mediators of the sensory effects of ciguatoxins (CTXs) which are PbTx analogs. This work explored the role of PAR2 and Cat-S in PbTx-1-induced sensory effects and deciphered the signaling pathway involved. We performed calcium imaging, PAR2 immunolocalization and SP release experiments in monocultured sensory neurons or co-cultured with keratinocytes treated with PbTx-1 or P-CTX-2. We demonstrated that PbTx-1-induced calcium increase and SP release involved Cat-S, PAR2 and transient receptor potential vanilloid 4 (TRPV4). The PbTx-1-induced signaling pathway included protein kinase A (PKA) and TRPV4, which are compatible with the PAR2 biased signaling induced by Cat-S. Internalization of PAR2 and protein kinase C (PKC), inositol triphosphate receptor and TRPV4 activation evoked by PbTx-1 are compatible with the PAR2 canonical signaling. Our results suggest that PbTx-1-induced sensory disturbances involve the PAR2-TRPV4 pathway. We identified PAR2, Cat-S, PKA, and PKC that are involved in TRPV4 sensitization induced by PbTx-1 in sensory neurons.

Purinergic Signaling in Endometriosis-Associated Pain

Endometriosis is an estrogen-dependent gynecological disease, with an associated chronic inflammatory component, characterized by the presence of endometrial tissue outside the uterine cavity. Its predominant symptom is pain, a condition notably altering the quality of life of women with the disease. This review is intended to exhaustively gather current knowledge on purinergic signaling in endometriosis-associated pain. Altered extracellular ATP hydrolysis, due to changes in ectonucleotidase activity, has been reported in endometriosis; the resulting accumulation of ATP in the endometriotic microenvironment points to sustained activation of nucleotide receptors (P2 receptors) capable of generating a persistent pain message. P2X3 receptor, expressed in sensory neurons, mediates nociceptive, neuropathic, and inflammatory pain, and is enrolled in endometriosis-related pain. Pharmacological inhibition of P2X3 receptor is under evaluation as a pain relief treatment for women with endometriosis. The role of other ATP receptors is also discussed here, e.g., P2X4 and P2X7 receptors, which are involved in inflammatory cell-nerve and microglia-nerve crosstalk, and therefore in inflammatory and neuropathic pain. Adenosine receptors (P1 receptors), by contrast, mainly play antinociceptive and anti-inflammatory roles. Purinome-targeted drugs, including nucleotide receptors and metabolizing enzymes, are potential non-hormonal therapeutic tools for the pharmacological management of endometriosis-related pain.

Nociceptive signaling mediated by P2X3, P2X4 and P2X7 receptors

Chronic pain is a debilitating condition that often occurs following peripheral tissue inflammation and nerve injury. This pain, especially neuropathic pain, is a significant clinical problem because of the ineffectiveness of clinically available drugs. Since Burnstock proposed new roles of nucleotides as neurotransmitters, the roles of extracellular ATP and P2 receptors (P2Rs) in pain signaling have been extensively studied, and ATP-P2R signaling has subsequently received much attention as it can provide clues toward elucidating the mechanisms underlying chronic pain and serve as a potential therapeutic target. This review summarizes the literature regarding the role of ATP signaling via P2X3Rs (as well as P2X2/3Rs) in primary afferent neurons and via P2X4Rs and P2X7Rs in spinal cord microglia in chronic pain, and discusses their respective therapeutic potentials.

Cathepsin S as a potential therapeutic target for chronic pain

Chronic pain is a distressing yet poorly-treated condition that can arise as a result of diseases and injuries to the nervous system. The development of more efficacious therapies for chronic pain is essential and requires advances in our understanding of its underlying mechanisms. Clinical and preclinical evidence has demonstrated that immune responses play a crucial role in chronic pain. The lysosomal cysteine protease cathepsin S (CatS) plays a key role in such immune response. Here we discuss the preclinical evidence for the mechanistic importance of extracellular CatS in chronic pain focussing on studies utilising drugs and other pharmacological tools that target CatS activity. We also consider the use of CatS inhibitors as potential novel antihyperalgesics, highlighting that the route and timing of delivery would need to be tailored to the initial cause of pain in order to ensure the most effective use of such drugs.

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Cathepsin S plays a key extracellular role in the underlying mechanisms of chronic pain

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Pharmacological tools provide crucial evidence for this role and the therapeutic potential of targeting Cathepsin S

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The route of delivery and timing of cathepsin S inhibitor administration should be tailored to specific causes of chronic pain

Role of the immune system in neuropathic pain

Background Acute pain is a warning mechanism that exists to prevent tissue damage, however pain can outlast its protective purpose and persist beyond injury, becoming chronic. Chronic Pain is maladaptive and needs addressing as available medicines are only partially effective and cause severe side effects. There are profound differences between acute and chronic pain. Dramatic changes occur in both peripheral and central pathways resulting in the pain system being sensitised, thereby leading to exaggerated responses to noxious stimuli (hyperalgesia) and responses to non-noxious stimuli (allodynia). Critical role for immune system cells in chronic pain Preclinical models of neuropathic pain provide evidence for a critical mechanistic role for immune cells in the chronicity of pain. Importantly, human imaging studies are consistent with preclinical findings, with glial activation evident in the brain of patients experiencing chronic pain. Indeed, immune cells are no longer considered to be passive bystanders in the nervous system; a consensus is emerging that, through their communication with neurons, they can both propagate and maintain disease states, including neuropathic pain. The focus of this review is on the plastic changes that occur under neuropathic pain conditions at the site of nerve injury, the dorsal root ganglia (DRG) and the dorsal horn of the spinal cord. At these sites both endothelial damage and increased neuronal activity result in recruitment of monocytes/macrophages (peripherally) and activation of microglia (centrally), which release mediators that lead to sensitisation of neurons thereby enabling positive feedback that sustains chronic pain. Immune system reactions to peripheral nerve injuries At the site of peripheral nerve injury following chemotherapy treatment for cancer for example, the occurrence of endothelial activation results in recruitment of CX3C chemokine receptor 1 (CX3CR1)-expressing monocytes/macrophages, which sensitise nociceptive neurons through the release of reactive oxygen species (ROS) that activate transient receptor potential ankyrin 1 (TRPA1) channels to evoke a pain response. In the DRG, neuro-immune cross talk following peripheral nerve injury is accomplished through the release of extracellular vesicles by neurons, which are engulfed by nearby macrophages. These vesicles deliver several determinants including microRNAs (miRs), with the potential to afford long-term alterations in macrophages that impact pain mechanisms. On one hand the delivery of neuron-derived miR-21 to macrophages for example, polarises these cells towards a pro-inflammatory/pro-nociceptive phenotype; on the other hand, silencing miR-21 expression in sensory neurons prevents both development of neuropathic allodynia and recruitment of macrophages in the DRG. Immune system mechanisms in the central nervous system In the dorsal horn of the spinal cord, growing evidence over the last two decades has delineated signalling pathways that mediate neuron-microglia communication such as P2X4/BDNF/GABAA, P2X7/Cathepsin S/Fractalkine/CX3CR1, and CSF-1/CSF-1R/DAP12 pathway-dependent mechanisms. Conclusions and implications Definition of the modalities by which neuron and immune cells communicate at different locations of the pain pathway under neuropathic pain states constitutes innovative biology that takes the pain field in a different direction and provides opportunities for novel approaches for the treatment of chronic pain.

Modulatory effect of botulinum toxin type A on the microglial P2X7/CatS/FKN activated-pathway in antigen-induced arthritis of the temporomandibular joint of rats

Analgesic mechanism of Botulinum toxin type A (BoNT/A) involves retrograde axonal transport to central nervous system, where it may interact with sensory neurons. Though, some authors suggested that BoNT/A antinociceptive action may also be associated with the inhibition intracellular factors and neuromodulators expressed by immune cells, especially by microglia. Antigen-induced arthritis in the temporomandibular joint (TMJ) of rats is signal by P2X7 receptor/Cathepsin S (CatS)/Fractalkine (FKN) microglia-activated pathway. Thus, we aimed to evaluate the possible modulatory effect of an intra-TMJ injection of BoNT/A on the P2X7/CatS/FKN microglia-activated pathway in the trigeminal subnucleus caudalis of rats with antigen-induced arthritis of the TMJ. A model of antigen-induced arthritis was used on Wistar rats (n = 40) by systemic injections of an emulsion containing complete Freund's adjuvant and methylated bovine serum albumin (mBSA) diluted in PBS. The arthritic condition was stablished by an intra-TMJ injection of mBSA (10 μg/TMJ/week) for 3 weeks. Then, animals were treated with an intra-TMJ injection of BoNT/A (onabotulinumtoxinA, Allergan®; 7U/kg) or vehicle saline. Animals were euthanized 24 h, 7 or 14 days after BoNT/A treatment and their trigeminal nucleus caudalis was harvested to evaluate the protein level of microglial purinergic P2X7 receptor and CX3 chemokine receptor 1 (CX3CR1) by Western blot, and to measure the protein level of microglial modulators CatS, FKN, and the pro-inflammatory cytokines tumor necrosis alfa (TNF-α) and interleukin 1β (IL-1β) by enzyme-linked immunosorbent assay (ELISA). The antigen-induced arthritis in the TMJ significantly increased the protein levels of P2X7, CatS, FKN, TNF-α and IL-1β in the trigeminal subnucleus caudalis (P < 0.05). The intra-TMJ injection of BoNT/A reduced the protein levels of P2X7 in all time points tested. Additionally, BoNT/A significantly reduced the protein levels of CatS, FKN, and TNF-α 14 days after treatment. However, IL-1β was significantly reduced just 24 h after the BoNT/A intra-TMJ treatment. Based on our results, we can suggest that the intra-TMJ injection of BoNT/A may promote a central effect by reducing the P2X7/CatS/FKN microglia-activated pathway in the trigeminal subnucleus caudalis.

Spinal microglia-neuron interactions in chronic pain

Current deficiency in our understanding of acute-to-chronic pain transition remains a hurdle for developing effective treatments against chronic pain. Whereas neurocentric mechanisms alone are insufficient to provide satisfactory explanation for such transition, neuro-immune crosstalk has attracted attention in recent pain research. In contrast to brain microglia, spinal microglia are activated immediately in various pain states. The fast-responsive enrichment and activation of spinal microglia among different pain conditions have highlighted the crucial role of neuroinflammation caused by microglia-neuron crosstalk in pain initiation. Recent studies have revealed spinal microglia-neuron interactions are also involved in chronic pain maintenance, albeit, with different anatomic distribution, cellular and molecular mechanisms, and biologic functions. Delineating the exact temporal discrepancies of spinal microglia distribution and functions along acute-to-chronic pain transition may provide additional mechanistic insights for drug development to prevent deterioration of acute pain into the chronic state. This narrative review summerizes the longitudinal alterations of spinal microglia-neuron interactions in the initiation of pain hypersensitivity, acute-to-chronic pain progression, and chronic pain maintenance, followed by an overview of current clinical translation of preclinical studies on spinal microglia. This review highlights the crucial role of the interaction between spinal microglia and neighboring neurons in the initiation and maintenance of pain hypersensitivity, in relation to the release of cytokines, chemokines, and neuroactive substances, as well as the modulation of synaptic plasticity. Further exploration of the uncharted functions of spinal microglia-neuron crosstalk may lead to the design of novel drugs for preventing acute-to-chronic pain transition.

Selective inhibition of peripheral cathepsin S reverses tactile allodynia following peripheral nerve injury in mouse

Cathepsin S (CatS) is a cysteine protease found in lysosomes of hematopoietic and microglial cells and in secreted form in the extracellular space. While CatS has been shown to contribute significantly to neuropathic pain, the precise mechanisms remain unclear. In this report, we describe JNJ-39641160, a novel non-covalent, potent, selective and orally-available CatS inhibitor that is peripherally restricted (non-CNS penetrant) and may represent an innovative class of immunosuppressive and analgesic compounds and tools useful toward investigating peripheral mechanisms of CatS in neuropathic pain. In C57BL/6 mice, JNJ-39641160 dose-dependently blocked the proteolysis of the invariant chain, and inhibited both T-cell activation and antibody production to a vaccine antigen. In the spared nerve injury (SNI) model of chronic neuropathic pain, in which T-cell activation has previously been demonstrated to be a prerequisite for the development of pain hypersensitivity, JNJ-39641160 fully reversed tactile allodynia in wild-type mice but was completely ineffective in the same model in CatS knockout mice (which exhibited a delayed onset in allodynia). By contrast, in the acute mild thermal injury (MTI) model, JNJ-39641160 only weakly attenuated allodynia at the highest dose tested. These findings support the hypothesis that blockade of peripheral CatS alone is sufficient to fully reverse allodynia following peripheral nerve injury and suggest that the mechanism of action likely involves interruption of T-cell activation and peripheral cytokine release. In addition, they provide important insights toward the development of selective CatS inhibitors for the treatment of neuropathic pain in humans.

High levels of cathepsin S and cystatin C in patients with fibromyalgia syndrome

OBJECTIVES

Although the etiopathogenesis of fibromyalgia syndrome (FM) is not yet clear, central sensitization is thought to be responsible for the pathogenesis of FM. The aim of this study was to compare the serum cathepsin S (CatS) and cystatin C (CysC) levels between patients with FM and healthy control subjects.

METHODS

This study was conducted in the Physical Medicine and Rehabilitation Clinic between January 2019 and October 2019. The study included 145 FM patients newly diagnosed with primary FM according to the 2010 American College of Rheumatology criteria and 129 healthy volunteers. The age, gender, and body mass index (BMI) of the participants were recorded. Venous blood samples were collected from both groups for the measurement of the levels of serum CatS and CysC. The functional status of FM patients was evaluated using the Fibromyalgia Impact Questionnaire (FIQ).

RESULTS

No statistically significant difference was determined between the patient and control groups in terms of age, gender, and BMI (P > .05). A comparison of the serum CatS and CysC levels of the FM and control groups revealed a statistically significant difference (P = .001). No correlation was determined between FIQ and serum CatS and CysC levels (P > .05).

CONCLUSION

Serum CatS and CysC levels were found to be higher in FM patients. However, there was no correlation between the functional status of FM patients and serum CatS and CysC levels. These results can be of guidance for further clinical studies of the etiopathogenesis and treatment of FM.

Changes in vascular permeability in the spinal cord contribute to chemotherapy-induced neuropathic pain

Chemotherapy-induced neuropathic pain is a dose-limiting side effect of many cancer therapies due to their propensity to accumulate in peripheral nerves, which is facilitated by the permeability of the blood-nerve barrier. Preclinically, the chemotherapy agent vincristine (VCR) activates endothelial cells in the murine peripheral nervous system and in doing so allows the infiltration of monocytes into nerve tissue where they orchestrate the development of VCR-induced nociceptive hypersensitivity. In this study we demonstrate that VCR also activates endothelial cells in the murine central nervous system, increases paracellular permeability and decreases trans endothelial resistance. In in vivo imaging studies in mice, VCR administration results in trafficking of inflammatory monocytes through the endothelium. Indeed, VCR treatment affects the integrity of the blood-spinal cord-barrier as indicated by Evans Blue extravasation, disrupts tight junction coupling and is accompanied by the presence of monocytes in the spinal cord. Such inflammatory monocytes (Iba-1⁺ CCR₂⁺ Ly6C⁺ TMEM119^- cells) that infiltrate the spinal cord also express the pro-nociceptive cysteine protease Cathepsin S. Systemic treatment with a CNS-penetrant, but not a peripherally-restricted, inhibitor of Cathepsin S prevents the development of VCR-induced hypersensitivity, suggesting that infiltrating monocytes play a functional role in sensitising spinal cord nociceptive neurons. Our findings guide us towards a better understanding of central mechanisms of pain associated with VCR treatment and thus pave the way for the development of innovative antinociceptive strategies.

Cigarette smoke induces overexpression of active human cathepsin S in lungs from current smokers with or without COPD

Cigarette smoking has marked effects on lung tissue, including induction of oxidative stress, inflammatory cell recruitment, and a protease/antiprotease imbalance. These effects contribute to tissue remodeling and destruction resulting in loss of lung function in chronic obstructive pulmonary disease (COPD) patients. Cathepsin S (CatS) is a cysteine protease that is involved in the remodeling/degradation of connective tissue and basement membrane. Aberrant expression or activity of CatS has been implicated in a variety of diseases, including arthritis, cancer, cardiovascular, and lung diseases. However, little is known about the effect of cigarette smoking on both CatS expression and activity, as well as its role in smoking-related lung diseases. Here, we evaluated the expression and activity of human CatS in lung tissues from never-smokers and smokers with or without COPD. Despite the presence of an oxidizing environment, CatS expression and activity were significantly higher in current smokers (both non-COPD and COPD) compared with never-smokers, and correlated positively with smoking history. Moreover, we found that the exposure of primary human bronchial epithelial cells to cigarette smoke extract triggered the activation of P2X7 receptors, which in turns drives CatS upregulation. The present data suggest that excessive CatS expression and activity contribute, beside other proteases, to the deleterious effects of cigarette smoke on pulmonary homeostasis.

Microglia-neuron interactions in the models of neuropathic pain

Chronic pain is a debilitating condition that often emerges as a clinical symptom of inflammatory diseases. It has therefore been widely accepted that the immune system critically contributes to the pathology of chronic pain. Microglia, a type of immune cell in the central nervous system, has attracted researchers' attention because in rodent models of neuropathic pain that develop strong mechanical and thermal hypersensitivity, histologically activated microglia are seen in the dorsal horn of spinal cord. Several kinds of cytokines are generated by damaged peripheral neurons and contribute to microglial activation at the distal site of the injury where damaged neurons send their projections. Microglia are known as key players in the surveillance of the local environment in the central nervous system and have a significant role of circuit remodeling by physical contact to synapses. Key molecules for the pathology of neuropathic pain exist in the activated microglia, but the factors driving pain-inducible microglial activation remain unclear. Therefore, to find the key molecules inducing activation of spinal microglia and to figure out the precise mechanism of how microglia modulate neuronal circuits in the spinal cord to form chronic pain state is a critical step for developing effective treatment of neuropathic pain.

P2X7 Interactions and Signaling - Making Head or Tail of It

Extracellular adenine nucleotides play important roles in cell-cell communication and tissue homeostasis. High concentrations of extracellular ATP released by dying cells are sensed as a danger signal by the P2X7 receptor, a non-specific cation channel. Studies in P2X7 knockout mice and numerous disease models have demonstrated an important role of this receptor in inflammatory processes. P2X7 activation has been shown to induce a variety of cellular responses that are not usually associated with ion channel function, for example changes in the plasma membrane composition and morphology, ectodomain shedding, activation of lipases, kinases, and transcription factors, as well as cytokine release and apoptosis. In contrast to all other P2X family members, the P2X7 receptor contains a long intracellular C-terminus that constitutes 40% of the whole protein and is considered essential for most of these effects. So far, over 50 different proteins have been identified to physically interact with the P2X7 receptor. However, few of these interactions have been confirmed in independent studies and for the majority of these proteins, the interaction domains and the physiological consequences of the interactions are only poorly described. Also, while the structure of the P2X7 extracellular domain has recently been resolved, information about the organization and structure of its C-terminal tail remains elusive. After shortly describing the structure and assembly of the P2X7 receptor, this review gives an update of the identified or proposed interaction domains within the P2X7 C-terminus, describes signaling pathways in which this receptor has been involved, and provides an overlook of the identified interaction partners.

Corneal pain and experimental model development

The cornea is a valuable tissue for studying peripheral sensory nerve structure and regeneration due to its avascularity, transparency, and dense innervation. Somatosensory innervation of the cornea serves to identify changes in environmental stimuli at the ocular surface, thereby promoting barrier function to protect the eye against injury or infection. Due to regulatory demands to screen ocular safety of potential chemical exposure, a need remains to develop functional human tissue models to predict ocular damage and pain using in vitro-based systems to increase throughput and minimize animal use. In this review, we summarize the anatomical and functional roles of corneal innervation in propagation of sensory input, corneal neuropathies associated with pain, and the status of current in vivo and in vitro models. Emphasis is placed on tissue engineering approaches to study the human corneal pain response in vitro with integration of proper cell types, controlled microenvironment, and high-throughput readouts to predict pain induction. Further developments in this field will aid in defining molecular signatures to distinguish acute and chronic pain triggers based on the immune response and epithelial, stromal, and neuronal interactions that occur at the ocular surface that lead to functional outcomes in the brain depending on severity and persistence of the stimulus.

Cerebrovascular inflammation: A critical trigger for neurovascular injury?

The cerebrovascular system is not only inert bystandard that support the metabolic demands of the brain but also elicit the barrier functions against risk factors mediated neurovascular injury. The onsets of cerebrovascular inflammation are considered as stimuli that can provoke the host defense system and trigger the development of neurological disorders. Homeostasis of the brain function is regulated by the movement of endothelial, glial, and neuronal cells within the neurovascular unit (NVU), which acts as a "platform" for the coordinated action of anti- and pro-inflammatory mechanisms. The cerebrovascular system plays an integral role in the inflammatory response by either producing or expressing a variety of cytokines, adhesion molecules, metalloproteinases, and serine proteases. Excessive inflammatory cytokine production can further be affecting the blood-brain barrier (BBB) integrity and lead to brain tissue damage. In this review, we summarize the more recent evidence highlighting the importance of cerebrovascular injury in terms of risk prediction, and the mechanisms mediating the upregulation of inflammatory mediators in cerebrovascular dysfunction and neurodegeneration.

Intrathecal delivery of a palmitoylated peptide targeting Y382-384 within the P2X7 receptor alleviates neuropathic pain

Pain hypersensitivity resulting from peripheral nerve injury depends on pathological microglial activation in the dorsal horn of the spinal cord. This microglial activity is critically modulated by P2X7 receptors (P2X7R) and ATP stimulation of these receptors produces mechanical allodynia, a defining feature of neuropathic pain. Peripheral nerve injury increases P2X7R expression and potentiates its cation channel function in spinal microglia. Here, we report a means to preferentially block the potentiation of P2X7R function by delivering a membrane permeant small interfering peptide that targets Y_382-384, a putative tyrosine phosphorylation site within the P2X7R intracellular C-terminal domain. Intrathecal administration of this palmitoylated peptide (P2X7R_379-389) transiently reversed mechanical allodynia caused by peripheral nerve injury in both male and female rats. Furthermore, targeting Y_382-384 suppressed P2X7R-mediated release of cytokine tumor necrosis factor alpha and blocked the adoptive transfer of mechanical allodynia caused by intrathecal injection of P2X7R-stimulated microglia. Thus, Y_382-384 site-specific modulation of P2X7R is an important microglial mechanism in neuropathic pain.

Lipid transporter Spns2 promotes microglia pro-inflammatory activation in response to amyloid-beta peptide

Accumulating evidence indicates that neuroinflammation contributes to the pathogenesis and exacerbation of neurodegenerative disorders, such as Alzheimer's disease (AD). Sphingosine-1-phosphate (S1P) is a pleiotropic bioactive lipid that regulates many pathophysiological processes including inflammation. We present evidence here that the spinster homolog 2 (Spns2), a S1P transporter, promotes microglia pro-inflammatory activation in vitro and in vivo. Spns2 knockout (Spns2KO) in primary cultured microglia resulted in significantly reduced levels of pro-inflammatory cytokines induced by lipopolysaccharide (LPS) and amyloid-beta peptide 1-42 oligomers (Aβ42) when compared with littermate controls. Fingolimod (FTY720), a S1P receptor 1 (S1PR1) functional antagonist and FDA approved drug for relapsing-remitting multiple sclerosis, partially blunted Aβ42-induced pro-inflammatory cytokine generation, suggesting that Spns2 promotes microglia pro-inflammatory activation through S1P-signaling. Spns2KO significantly reduced Aβ42-induced nuclear factor kappa B (NFκB) activity. S1P increased, while FTY720 dampened, Aβ42-induced NFκB activity, suggesting that Spns2 activates microglia inflammation through, at least partially, NFκB pathway. Spns2KO mouse brains showed significantly reduced Aβ42-induced microglia activation/accumulation and reduced levels of pro-inflammatory cytokines when compared with age-matched controls. More interestingly, Spns2KO ameliorated Aβ42-induced working memory deficit detected by Y-Maze. In summary, these results suggest that Spns2 promotes pro-inflammatory polarization of microglia and may play a crucial role in AD pathogenesis.

Hydrogen sulfide inhibits ATP-induced neuroinflammation and Aβ1-42 synthesis by suppressing the activation of STAT3 and cathepsin S

Neuroinflammation and excessive β-amyloid_1-42 (Aβ_1-42) generation contribute to the pathogenesis of Alzheimer's disease (AD). Emerging evidence has demonstrated that hydrogen sulfide (H₂S), an endogenous gasotransmitter, produces therapeutic effects in AD; however, the underlying mechanisms remain largely elusive. In the present study, we investigated the effects of H₂S on exogenous ATP-induced inflammation and Aβ_1-42 production in both BV-2 and primary cultured microglial cells and analyzed the potential mechanism(s) mediating these effects. Our results showed that NaHS, an H₂S donor, inhibited exogenous ATP-stimulated inflammatory responses as manifested by the reduction of pro-inflammatory cytokines, ROS and activation of nuclear factor-κB (NF-κB) pathway. Furthermore, NaHS also suppressed the enhanced production of Aβ_1-42 induced by exogenous ATP, which is probably due to its inhibitory effect on exogenous ATP-boosted expression of amyloid precursor protein (APP) and activation of β- and γ-secretase enzymes. Thereafter, we found that exogenous ATP-induced inflammation and Aβ_1-42 production requires the activation of signal transducer and activator of transcription 3 (STAT3) and cathepsin S (Cat S) as inhibition of the activity of either proteins attenuated the effect of exogenous ATP. Intriguingly, NaHS suppressed exogenous ATP-induced phosphorylation of STAT3 and the activation of Cat S. In addition, we observed that NaHS led to the persulfidation of Cat S at cysteine-25. Importantly, mutation of cysteine-25 into serine attenuated the activity of Cat S stimulated by exogenous ATP and subsequent inflammation and Aβ_1-42 production, indicating its involvement in H₂S-mediated effect. Taken together, our data provide a novel understanding of H₂S-mediated effect on neuroinflammation and Aβ_1-42 production by suppressing the activation of STAT3 and Cat S.

Z-FL-COCHO, a cathepsin S inhibitor, enhances oxaliplatin-mediated apoptosis through the induction of endoplasmic reticulum stress

Multiple cancer cells highly express cathepsin S, which has pro-tumoral effects. However, it was previously unknown whether knockdown or a pharmacological inhibitor (ZFL) of cathepsin S acts as an inducer of ER stress. Here, ZFL and knockdown of cathepsin S markedly induced ER stress through the up-regulation of calcium levels in the cytosol. Induction of calcium levels by inhibition of cathepsin S is markedly blocked by an inhibitor of the IP3 receptor and the ryanodine receptor Ca²⁺ channel in the ER, but an inhibitor of a mitochondrial Ca²⁺ uniporter had no effect on ZFL-induced calcium levels. Furthermore, production of mitochondrial ROS by ZFL was associated with an increase in cytosolic calcium levels. ZFL-mediated ER stress enhanced anti-cancer drug-induced apoptotic cell death, and pretreatment with chemical chaperones or down-regulation of ATF4 and CHOP by small interfering RNA markedly reduced ZFL plus oxaliplatin-induced apoptosis. Taken together, our findings reveal that inhibition of cathepsin S is an inducer of ER stress; these findings may contribute to the enhancement of therapeutic efficiency in cancer cells.

A drug that inhibits a key cancer enzyme could be used in combination with anti-cancer drugs to improve sensitivity to treatment. The intracellular endoplasmic reticulum (ER) is involved in several vital processes in cells, including folding and processing proteins. Taeg Kyu Kwon at Keimyung University, Daegu, South Korea, and co-workers have demonstrated how inhibition of cathepsin S, which is expressed in many cancer cells, induces ER stress. In trials on human kidney cancer cells grafted onto mice and in vitro, the team found that ZFL (cathepsin S inhibitor) triggered transient ER stress by increasing calcium levels inside cells. Subsequent treatment with the anti-cancer drug oxaliplatin resulted in increased cancer cell death.

Microglial pannexin-1 channel activation is a spinal determinant of joint pain

Chronic joint pain such as mechanical allodynia is the most debilitating symptom of arthritis, yet effective therapies are lacking. We identify the pannexin-1 (Panx1) channel as a therapeutic target for alleviating mechanical allodynia, a cardinal sign of arthritis. In rats, joint pain caused by intra-articular injection of monosodium iodoacetate (MIA) was associated with spinal adenosine 5'-triphosphate (ATP) release and a microglia-specific up-regulation of P2X7 receptors (P2X7Rs). Blockade of P2X7R or ablation of spinal microglia prevented and reversed mechanical allodynia. P2X7Rs drive Panx1 channel activation, and in rats with mechanical allodynia, Panx1 function was increased in spinal microglia. Specifically, microglial Panx1-mediated release of the proinflammatory cytokine interleukin-1β (IL-1β) induced mechanical allodynia in the MIA-injected hindlimb. Intrathecal administration of the Panx1-blocking peptide ¹⁰panx suppressed the aberrant discharge of spinal laminae I-II neurons evoked by innocuous mechanical hindpaw stimulation in arthritic rats. Furthermore, mice with a microglia-specific genetic deletion of Panx1 were protected from developing mechanical allodynia. Treatment with probenecid, a clinically used broad-spectrum Panx1 blocker, resulted in a striking attenuation of MIA-induced mechanical allodynia and normalized responses in the dynamic weight-bearing test, without affecting acute nociception. Probenecid reversal of mechanical allodynia was also observed in rats 13 weeks after anterior cruciate ligament transection, a model of posttraumatic osteoarthritis. Thus, Panx1-targeted therapy is a new mechanistic approach for alleviating joint pain.

A refined rat primary neonatal microglial culture method that reduces time, cost and animal use

BACKGROUND

Primary microglial cultures have been used extensively to facilitate the development of therapeutic strategies for a variety of CNS disorders including neurodegeneration and neuropathic pain. However, existing techniques for culturing these cells are slow and costly.

NEW METHOD

Here, we report a refined protocol based on our previously published methods described by Clark et al., which reduces in the time, reagents and the number of animals used for each culture whilst yielding high number and excellent quality microglial cells.

RESULTS

Our refined protocol offers an isolation of >96% microglia from a mixed glial culture after only four days of incubation. It results in a high yield of microglia, in excess of one million cells per cortex with predominantly resting morphology and a low level of cell activation.

COMPARISON WITH EXISTING METHOD(S)

Compared to conventional procedures our refined protocol requires only one third of the time to prepare high quality microglial cultures, cuts the cost more than four-fold, and significantly reduces the number of animals used per culture.

CONCLUSION

Our consistent, reliable, and time/cost effective microglial culture protocol is crucial for efficient in vitro screening of potential therapeutics. By dramatically reducing the culture time from 2 weeks to just 4 days and increasing the laboratory research output it has implications for the Reduction, Refinement and Replacement policies endorsed by many government funding agencies and animal research regulatory bodies.