Direct and Intervertebral Disc-Mediated Sensitization of Dorsal Root Ganglion Neurons by Hypoxia and Low pH

Endoplasmic reticulum stress-mediated autophagy alleviates lipopolysaccharide-induced nucleus pulposus cell pyroptosis by inhibiting CHOP signaling in vitro

Pyroptosis, a newly discovered pro-inflammatory programmed necrosis of cells, serves as an initiating and promoting event that leads to intervertebral disc (IVD) degeneration (IDD). Endoplasmic reticulum stress (ERS) and autophagy are vital regulatory mechanisms of cellular homeostasis, which is also closely related to IDD. However, the role and relationship of ERS and autophagy in the pyroptosis of nucleus pulposus cell (NPC) are not well understood. In this research, we aimed to elucidate the role and mechanism of ERS-C/EBP homologous protein (CHOP) in lipopolysaccharide (LPS)-induced cell pyroptosis and determine its interaction with autophagy. ERS and autophagy inducers or inhibitors were used or not in the preconditioning of rat NPCs. Cell viability, pyroptosis-related protein expression, caspase-1 activity assay, and enzyme-linked immunosorbent assay were performed to observe rat NPC pyroptosis after the treatment of LPS. Activation of the ERS pathway and autophagy were assessed by quantitative real-time PCR, western blot analyses, and immunofluorescence staining assay to classify the molecular mechanisms. Our results showed that LPS stimulation induced NPC pyroptosis with concomitant activation of the ERS-CHOP pathway and initiated autophagy. Activation of the ERS-CHOP pathway exacerbated rat NPC pyroptosis, whereas autophagy inhibited cell pyroptosis. LPS-induced cell pyroptosis and CHOP upregulation were negatively regulated by autophagy. LPS-induced autophagy was depressed by the ERS inhibitor but aggravated by the ERS inducer. Taken together, our findings suggested that LPS induced NPC pyroptosis by activating ERS-CHOP signaling and ERS mediated LPS-induced autophagy, which in turn alleviated NPC pyroptosis by inhibiting CHOP signaling.

Achilles' Heel-The Significance of Maintaining Microenvironmental Homeostasis in the Nucleus Pulposus for Intervertebral Discs

The dysregulation of intracellular and extracellular environments as well as the aberrant expression of ion channels on the cell membrane are intricately linked to a diverse array of degenerative disorders, including intervertebral disc degeneration. This condition is a significant contributor to low back pain, which poses a substantial burden on both personal quality of life and societal economics. Changes in the number and function of ion channels can disrupt the water and ion balance both inside and outside cells, thereby impacting the physiological functions of tissues and organs. Therefore, maintaining ion homeostasis and stable expression of ion channels within the cellular microenvironment may prove beneficial in the treatment of disc degeneration. Aquaporin (AQP), calcium ion channels, and acid-sensitive ion channels (ASIC) play crucial roles in regulating water, calcium ions, and hydrogen ions levels. These channels have significant effects on physiological and pathological processes such as cellular aging, inflammatory response, stromal decomposition, endoplasmic reticulum stress, and accumulation of cell metabolites. Additionally, Piezo 1, transient receptor potential vanilloid type 4 (TRPV4), tension response enhancer binding protein (TonEBP), potassium ions, zinc ions, and tungsten all play a role in the process of intervertebral disc degeneration. This review endeavors to elucidate alterations in the microenvironment of the nucleus pulposus during intervertebral disc degeneration (IVDD), with a view to offer novel insights and approaches for exploring therapeutic interventions against disc degeneration.

Updates on Pathophysiology of Discogenic Back Pain

Discogenic back pain, a subset of chronic back pain, is caused by intervertebral disc (IVD) degeneration, and imparts a notable socioeconomic health burden on the population. However, degeneration by itself does not necessarily imply discogenic pain. In this review, we highlight the existing literature on the pathophysiology of discogenic back pain, focusing on the biomechanical and biochemical steps that lead to pain in the setting of IVD degeneration. Though the pathophysiology is incompletely characterized, the current evidence favors a framework where degeneration leads to IVD inflammation, and subsequent immune milieu recruitment. Chronic inflammation serves as a basis of penetrating neovascularization and neoinnervation into the IVD. Hence, nociceptive sensitization emerges, which manifests as discogenic back pain. Recent studies also highlight the complimentary roles of low virulence infections and central nervous system (CNS) metabolic state alteration. Targeted therapies that seek to disrupt inflammation, angiogenesis, and neurogenic pathways are being investigated. Regenerative therapy in the form of gene therapy and cell-based therapy are also being explored.

The influence of intervertebral disc overloading on nociceptor calcium flickering

Introduction

Mechanical overloading can trigger a degenerative-like cascade in an organ culture of intervertebral disc (IVD). Whether the overloaded IVD can influence the activation of nociceptors (i.e., the damage sensing neurons) remains unknown. The study aims to investigate the influence of overloaded IVD conditioned medium (CM) on the activation of nociceptors.

Methods

In the static loading regime, force-controlled loading of 0.2 MPa for 20 h/day representing "long-term sitting and standing" was compared with a displacement-controlled loading maintaining original IVD height. In the dynamic loading regime, high-frequency-intensity loading representing degenerative "wear and tear" was compared with a lower-frequency-intensity loading. CM of differently loaded IVDs were collected to stimulate the primary bovine dorsal root ganglion (DRG) cultures. Calcium imaging (Fluo-4) and calcitonin gene-related peptide (CGRP) immunofluorescent labeling were jointly used to record the calcium flickering in CGRP(+) nociceptors.

Results

Force-controlled loading led to a higher IVD cell death compared to displacement-controlled loading. Both static and dynamic overloading (force-controlled and high-frequency-intensity loadings) elevated the frequency of calcium flickering in the subsurface space of CGRP(+) nociceptors compared to their mild loading counterparts.

Conclusion

In the organ culture system, IVD overloading mediated an altered IVD-nociceptor communication suggesting a biological mechanism associated with discogenic pain.

Celecoxib alleviates nociceptor sensitization mediated by interleukin-1beta-primed annulus fibrosus cells

PURPOSE

This study aims to analyze the effect of pro-inflammatory cytokine-stimulated human annulus fibrosus cells (hAFCs) on the sensitization of dorsal root ganglion (DRG) cells. We further hypothesized that celecoxib (cxb) could inhibit hAFCs-induced DRG sensitization.

METHODS

hAFCs from spinal trauma patients were stimulated with TNF-α or IL-1β. Cxb was added on day 2. On day 4, the expression of pro-inflammatory and neurotrophic genes was evaluated using RT-qPCR. Levels of prostaglandin E2 (PGE-2), IL-8, and IL-6 were measured in the conditioned medium (CM) using ELISA. hAFCs CM was then applied to stimulate the DRG cell line (ND7/23) for 6 days. Then, calcium imaging (Fluo4) was performed to evaluate DRG cell sensitization. Both spontaneous and bradykinin-stimulated (0.5 μM) calcium responses were analyzed. The effects on primary bovine DRG cell culture were performed in parallel to the DRG cell line model.

RESULTS

IL-1ß stimulation significantly enhanced the release of PGE-2 in hAFCs CM, while this increase was completely suppressed by 10 µM cxb. hAFCs revealed elevated IL-6 and IL-8 release following TNF-α and IL-1β treatment, though cxb did not alter this. The effect of hAFCs CM on DRG cell sensitization was influenced by adding cxb to hAFCs; both the DRG cell line and primary bovine DRG nociceptors showed a lower sensitivity to bradykinin stimulation.

CONCLUSION

Cxb can inhibit PGE-2 production in hAFCs in an IL-1β-induced pro-inflammatory in vitro environment. The cxb applied to the hAFCs also reduces the sensitization of DRG nociceptors that are stimulated by the hAFCs CM.

Controversies in spine research: Organ culture versus in vivo models for studies of the intervertebral disc

Intervertebral disc degeneration is a common cause of low back pain, the leading cause of disability worldwide. Appropriate preclinical models for intervertebral disc research are essential to achieving a better understanding of underlying pathophysiology and for the development, evaluation, and translation of more effective treatments. To this end, in vivo animal and ex vivo organ culture models are both widely used by spine researchers; however, the relative strengths and weaknesses of these two approaches are a source of ongoing controversy. In this article, members from the Spine and Preclinical Models Sections of the Orthopedic Research Society, including experts in both basic and translational spine research, present contrasting arguments in support of in vivo animal models versus ex vivo organ culture models for studies of the disc, supported by a comprehensive review of the relevant literature. The objective is to provide a deeper understanding of the respective advantages and limitations of these approaches, and advance the field toward a consensus with respect to appropriate model selection and implementation. We conclude that complementary use of several model types and leveraging the unique advantages of each is likely to result in the highest impact research in most instances.

In vivo measurement of intradiscal pressure changes related to thrust and non-thrust spinal manipulation in an animal model: a pilot study

BACKGROUND

The intervertebral disc is a known back pain generator and is frequently the focus of spinal manipulative therapy evaluation and treatment. The majority of our current knowledge regarding intradiscal pressure (IDP) changes related to spinal manual therapy involves cadaveric studies with their inherent limitations. Additional in vivo animal models are needed to investigate intervertebral disc physiological and molecular mechanisms related to spinal manipulation and spinal mobilization treatment for low back disorders.

METHODS

Miniature pressure catheters (Millar SPR-1000) were inserted into either the L4-L5 or L5-L6 intervertebral disc of 3 deeply anesthetized adult cats (Oct 2012-May 2013). Changes in IDP were recorded during delivery of instrument-assisted spinal manipulation (Activator V® and Pulstar®) and motorized spinal flexion with/without manual spinous process contact.

RESULTS

Motorized flexion of 30° without spinous contact decreased IDP of the L4-L5 disc by ~ 2.9 kPa, while physical contact of the L4 spinous process decreased IDP an additional ~ 1.4 kPa. Motorized flexion of 25° with L5 physical contact in a separate animal decreased IDP of the L5-L6 disc by ~ 1.0 kPa. Pulstar® impulses (setting 1-3) increased IDP of L4-L5 and L5-L6 intervertebral discs by ~ 2.5 to 3.0 kPa. Activator V® (setting 1-4) impulses increased L4-L5 IDP to a similar degree. Net changes in IDP amplitudes remained fairly consistent across settings on both devices regardless of device setting suggesting that viscoelastic properties of in vivo spinal tissues greatly dampen superficially applied manipulative forces prior to reaching deep back structures such as the intervertebral disc.

CONCLUSIONS

This study marks the first time that feline in vivo changes in IDP have been reported using clinically available instrument-assisted spinal manipulation devices and/or spinal mobilization procedures. The results of this pilot study indicate that a feline model can be used to investigate IDP changes related to spinal manual therapy mechanisms as well as the diminution of these spinal manipulative forces due to viscoelastic properties of the surrounding spinal tissues. Additional investigation of IDP changes is warranted in this and/or other in vivo animal models to provide better insights into the physiological effects and mechanisms of spinal manual therapy at the intervertebral disc level.

Dynamic Immune Landscape and VZV-Specific T Cell Responses in Patients With Herpes Zoster and Postherpetic Neuralgia

Objectives

Varicella-zoster virus (VZV) can induce herpes zoster (HZ) and postherpetic neuralgia (PHN). Immune cells play an important role in regulating HZ and PHN pathogenesis, but the dynamic immune profiles and molecular mechanisms remain unclear. This study aimed to screen dynamic immune signatures during HZ progression and elucidate the mechanism of VZV-specific T cells in PHN.

Methods

We used cytometry by time-of-flight (CyTOF) to analyze peripheral blood mononuclear cells (PBMC) samples from 45 patients with HZ and eight age-sex-matched healthy controls, eight PHN samples and seven non-PHN samples. Correlations between the immune subsets and clinical pain-related scores were performed. Further, the characteristics of VZV-specific T cells between PHN and non-PHN patients were evaluated by VZV peptide pools stimulation. The expression level of cytokines, including granzyme B, interleukin (IL)-2, interferon (IFN)-γ, and tumor necrosis factor (TNF)-α was performed via cytometric bead array. Finally, we analyzed the alteration of Ca²⁺ signals in dorsal root ganglion (DRG)-derived cells after TNF-α stimulation.

Results

We investigated the dynamic characteristics of the immune landscape of peripheral blood samples of patients with HZ and PHN, and depicted two major dynamic signatures in NK, CD4⁺ and CD8⁺ T subsets in patients with HZ, which closely correlated with clinical pain-related scores. The frequency of PD-1⁺CD4⁺ T cells, VZV-specific PD-1⁺CD4⁺ T cells, and the amount of TNF-α produced by VZV-specific T cells were higher in patients with PHN than without PHN. Furthermore, we showed that TNF-α could induce calcium influx in DRG-derived cells in a dose-dependent manner.

Conclusions

Our results profiled the dynamic signatures of immune cells in patients with HZ and highlighted the important role of VZV-specific T cells in the pathogenesis of PHN.

Current Knowledge and Future Therapeutic Prospects in Symptomatic Intervertebral Disc Degeneration

Intervertebral disc (IVD) degeneration is the main source of intractable lower back pain, and symptomatic IVD degeneration could be due to different degeneration mechanisms. In this article, we describe the molecular basis of symptomatic IVD degenerative disc diseases (DDDs), emphasizing the role of degeneration, inflammation, angiogenesis, and extracellular matrix (ECM) regulation during this process. In symptomatic DDD, pro-inflammatory mediators modulate catabolic reactions, resulting in changes in ECM homeostasis and, finally, neural/vascular ingrowth-related chronic intractable discogenic pain. In ECM homeostasis, anabolic protein-regulating genes show reduced expression and changes in ECM production, while matrix metalloproteinase gene expression increases and results in aggressive ECM degradation. The resultant loss of normal IVD viscoelasticity and a concomitant change in ECM composition are key mechanisms in DDDs. During inflammation, a macrophage-related cascade is represented by the secretion of high levels of pro-inflammatory cytokines, which induce inflammation. Aberrant angiogenesis is considered a key initiative pathologic step in symptomatic DDD. In reflection of angiogenesis, vascular endothelial growth factor expression is regulated by hypoxia-inducible factor-1 in the hypoxic conditions of IVDs. Furthermore, IVD cells undergoing degeneration potentially enhance neovascularization by secreting large amounts of angiogenic cytokines, which penetrate the IVD from the outer annulus fibrosus, extending deep into the outer part of the nucleus pulposus. Based on current knowledge, a multi-disciplinary approach is needed in all aspects of spinal research, starting from basic research to clinical applications, as this will provide information regarding treatments for DDDs and discogenic pain.

In Vitro Model to Investigate Communication between Dorsal Root Ganglion and Spinal Cord Glia

Chronic discogenic back pain is associated with increased inflammatory cytokine levels that can influence the proximal peripheral nervous system, namely the dorsal root ganglion (DRG). However, transition to chronic pain is widely thought to involve glial activation in the spinal cord. In this study, an in vitro model was used to evaluate the communication between DRG and spinal cord glia. Primary neonatal rat DRG cells were treated with/without inflammatory cytokines (TNF-α, IL-1β, and IL-6). The conditioned media were collected at two time points (12 and 24 h) and applied to spinal cord mixed glial culture (MGC) for 24 h. Adult bovine DRG and spinal cord cell cultures were also tested, as an alternative large animal model, and results were compared with the neonatal rat findings. Compared with untreated DRG-conditioned medium, the second cytokine-treated DRG-conditioned medium (following medium change, thus containing solely DRG-derived molecules) elevated CD11b expression and calcium signal in neonatal rat microglia and enhanced Iba1 expression in adult bovine microglia. Cytokine treatment induced a DRG-mediated microgliosis. The described in vitro model allows the use of cells from large species and may represent an alternative to animal pain models (3R principles).

Early radiation-induced oral pain signaling responses are reduced with pentoxifylline treatment

Radiation-induced acute oral mucositis is associated with inflammation and pain. In other realms of pain research, nociceptors are known to be activated by inflammatory cytokines; for example, tumor necrosis factor alpha (TNF-α) can activate transient receptor potential ion channels on sensory neurons. But there is an unclear relationship between inflammatory cytokines and molecular mediators of pain in radiation-induced mucositis (RIM) and radiation-associated pain (RAP). In this prospective, analytical, experimental pilot study, a common drug (pentoxifylline [PTX]) was used with the goal of inhibiting TNF-α signaling in mice that underwent lingual irradiation to induce severe acute oral RIM/RAP. Body weight and glossitis scores were recorded daily. Eye wiping behaviors were assayed as a surrogate measure of oral discomfort (which is possible due to cross-sensitization of the mandibular and ophthalmic branches of the trigeminal nerve). Quantitative real-time reverse transcription polymerase chain reaction was performed on irradiated tongue tissue to measure changes in expression of TNF-α, its receptor, nuclear factor kappa-light-chain-enhancer of activated B cells, transient receptor potential vanilloid type 1 (TRPV1), and transient receptor potential vanilloid type 4 (TRPV4). Responsiveness of afferent sensory trigeminal neurons to TNF-α, a TRPV1 agonist (capsaicin), and a partial TRPV4 agonist (histamine) was measured via calcium imaging. Although PTX treatment did not reduce glossitis severity or mitigate weight loss in mice with RIM/RAP, it did inhibit the upregulation of TNF-α's receptor that normally accompanies RIM, and it also reduced neuronal responsiveness to each of the aforementioned chemical stimuli. These results provide provisional evidence that inhibition of TNF-α signaling with PTX treatment may serve as a useful tool for reducing pain in head and neck cancer patients.

Peripheral mechanisms of arthritic pain: A proposal to leverage large animals for in vitro studies

Pain arising from musculoskeletal disorders such as arthritis is one of the leading causes of disability. Whereas the past 20-years has seen an increase in targeted therapies for rheumatoid arthritis (RA), other arthritis conditions, especially osteoarthritis, remain poorly treated. Although modulation of central pain pathways occurs in chronic arthritis, multiple lines of evidence indicate that peripherally driven pain is important in arthritic pain. To understand the peripheral mechanisms of arthritic pain, various in vitro and in vivo models have been developed, largely in rodents. Although rodent models provide numerous advantages for studying arthritis pathogenesis and treatment, the anatomy and biomechanics of rodent joints differ considerably to those of humans. By contrast, the anatomy and biomechanics of joints in larger animals, such as dogs, show greater similarity to human joints and thus studying them can provide novel insight for arthritis research. The purpose of this article is firstly to review models of arthritis and behavioral outcomes commonly used in large animals. Secondly, we review the existing in vitro models and assays used to study arthritic pain, primarily in rodents, and discuss the potential for adopting these strategies, as well as likely limitations, in large animals. We believe that exploring peripheral mechanisms of arthritic pain in vitro in large animals has the potential to reduce the veterinary burden of arthritis in commonly afflicted species like dogs, as well as to improve translatability of pain research into the clinic.

Immortalized Dorsal Root Ganglion Neuron Cell Lines

Pain is one of the most significant causes of suffering and disability world-wide, and arguably the most burdensome global health challenge. The growing number of patients suffering from chronic pain conditions such as fibromyalgia, complex regional pain syndrome, migraine and irritable bowel syndrome, not only reflect the complexity and heterogeneity of pain types, but also our lack of understanding of the underlying mechanisms. Sensory neurons within the dorsal root ganglia (DRG) have emerged as viable targets for effective chronic pain therapy. However, DRG's contain different classes of primary sensory neurons including pain-associated nociceptive neurons, non-nociceptive temperature sensing, mechanosensory and chemoreceptive neurons, as well as multiple types of immune and endothelial cells. This cell-population heterogeneity makes investigations of individual subgroups of DRG neurons, such as nociceptors, difficult. In attempts to overcome some of these difficulties, a limited number of immortalized DRG-derived cell lines have been generated over the past few decades. In vitro experiments using DRG-derived cell lines have been useful in understanding sensory neuron function. In addition to retaining phenotypic similarities to primary cultured DRG neurons, these cells offer greater suitability for high throughput assays due to ease of culture, maintenance, growth efficiency and cost-effectiveness. For accurate interpretation and translation of results it is critical, however, that phenotypic similarities and differences of DRG-derived cells lines are methodically compared to native neurons. Published reports to date show notable variability in how these DRG-derived cells are maintained and differentiated. Understanding the cellular and molecular differences stemming from different culture methods, is essential to validate past and future experiments, and enable these cells to be used to their full potential. This review describes currently available DRG-derived cell lines, their known sensory and nociceptor specific molecular profiles, and summarize their morphological features related to differentiation and neurite outgrowth.