Omics approaches to discover pathophysiological pathways contributing to human pain

Pain Assessment in Osteoarthritis: Present Practices and Future Prospects Including the Use of Biomarkers and Wearable Technologies, and AI-Driven Personalized Medicine

Osteoarthritis (OA) is a highly prevalent chronic joint disorder affecting ~600 million individuals worldwide and is characterized by complex pain mechanisms that significantly impair patient quality of life. Challenges exist in accurately assessing and measuring pain in OA due to variations in pain perception among individuals and the heterogeneous nature of the disease. Conventional pain assessment methods, such as patient-reported outcome measures and clinical evaluations, often fail to fully capture the heterogeneity of pain experiences among individuals with OA. This review will summarize and evaluate current methods of pain assessment in OA and highlight future directions for standardized pain assessment. We discuss the role of animal models in enhancing our understanding of OA pain pathophysiology and highlight the necessity of translational research to advance pain assessment strategies. Key challenges explored include identifying phenotypes of pain susceptibility, integrating biomarkers into clinical practice, and adopting personalized pain management approaches through the incorporation of multi-modal data and multilevel analysis. We underscore the imperative for continued innovation in pain assessment and management to improve outcomes for patients with OA.

Roles of non-coding RNAs and exosomal non-coding RNAs, particularly microRNAs, long non-coding RNAs, and circular RNAs, in pathogenic mechanisms behind chronic pain: A review

Chronic pain is a significant public health concern that diminishes patients' quality of life and imposes considerable socioeconomic costs. Effective pharmacological treatments for ongoing pain are limited. Recent studies have indicated that various models of chronic pain-such as neuropathic pain, inflammatory pain, and pain associated with cancer-have abnormal levels of long noncoding RNAs (lncRNAs). Research has explored how these abnormal lncRNAs influence the activation of inflammatory cytokines, microRNAs, and other related molecules, which are crucial to the development of chronic pain. These findings suggest that these lncRNAs are vital in chronic pain mechanisms within the spinal cord and dorsal root ganglion following nerve injury. Additionally, exosomes, which can traverse the blood-brain barrier, are considered carriers of noncoding RNAs (ncRNAs) from neurons to systemic circulation. This study aims to summarize the existing knowledge on ncRNAs and exosomal ncRNAs in the context of chronic pain, highlighting potential biomarkers for diagnosis, regulatory roles in disease progression, therapeutic strategies, and clinical implications.

Rare variant association studies: Significance, methods, and applications in chronic pain studies

Rare genetic variants, characterized by their low frequency in a population, have emerged as essential components in the study of complex disease genetics. The biology of rare variants underscores their significance, as they can exert profound effects on phenotypic variation and disease susceptibility. Recent advancements in sequencing technologies have yielded the availability of large-scale sequencing data such as the UK Biobank whole-exome sequencing (WES) cohort empowered researchers to conduct rare variant association studies (RVASs). This review paper discusses the significance of rare variants, available methodologies, and applications. We provide an overview of RVASs, emphasizing their relevance in unraveling the genetic architecture of complex diseases with special focus on chronic pain and Arthritis. Additionally, we discuss the strengths and limitations of various rare variant association testing methods, outlining a typical pipeline for conducting rare variant association. This pipeline encompasses crucial steps such as quality control of WES data, rare variant annotation, and association testing. It serves as a comprehensive guide for researchers in the field of chronic pain diseases interested in rare variant association studies in large-scale sequencing datasets like the UK Biobank WES cohort. Lastly, we discuss how the identified variants can be further investigated through detailed experimental studies in animal models to elucidate their functional impact and underlying mechanisms.

Peripheral CaV2.2 Channels in the Skin Regulate Prolonged Heat Hypersensitivity during Neuroinflammation

Neuroinflammation can lead to chronic maladaptive pain affecting millions of people worldwide. Neurotransmitters, cytokines, and ion channels are implicated in neuroimmune cell signaling, but their roles in specific behavioral responses are not fully elucidated. Voltage-gated CaV2.2 channel activity in skin controls rapid and transient heat hypersensitivity induced by intradermal (i.d.) capsaicin via IL-1ɑ cytokine signaling. CaV2.2 channels are not, however, involved in mechanical hypersensitivity that developed in the i.d. capsaicin animal model. Here, we show that CaV2.2 channels are also critical for heat hypersensitivity induced by i.d. complete Freund adjuvant (CFA). i.d. CFA, a model of chronic neuroinflammation, involves ongoing cytokine signaling for days leading to pronounced edema and hypersensitivity to sensory stimuli. Peripheral CaV2.2 channel activity in the skin was required for the full development and week-long time course of heat hypersensitivity induced by i.d. CFA, but paw edema and mechanical hypersensitivity were independent of CaV2.2 channel activity. CFA induced increases in several cytokines in hindpaw fluid including IL-6 which was also dependent on CaV2.2 channel activity. Using IL-6-specific neutralizing antibodies in vivo, we show that IL-6 contributes to heat hypersensitivity and that neutralizing both IL-1ɑ and IL-6 was even more effective at reducing the magnitude and duration of CFA-induced heat hypersensitivity. Our findings demonstrate a functional link between CaV2.2 channel activity and the release of IL-6 in the skin and show that CaV2.2 channels have a privileged role in the induction and maintenance of heat hypersensitivity during chronic forms of neuroinflammation in the skin.

Improving neuropathic pain treatment - by rigorous stratification from bench to bedside

Chronic pain is a constantly recurring and persistent illness, presenting a formidable healthcare challenge for patients and physicians alike. Current first-line analgesics offer only low-modest efficacy when averaged across populations, further contributing to this debilitating disease burden. Moreover, many recent trials for novel analgesics have not met primary efficacy endpoints, which is particularly striking considering the pharmacological advances have provided a range of highly relevant new drug targets. Heterogeneity within chronic pain cohorts is increasingly understood to play a critical role in these failures of treatment and drug discovery, with some patients deriving substantial benefits from a given intervention while it has little-to-no effect on others. As such, current treatment failures may not result from a true lack of efficacy, but rather a failure to target individuals whose pain is driven by mechanisms which it therapeutically modulates. This necessitates a move towards phenotypical stratification of patients to delineate responders and non-responders in a mechanistically driven manner. In this article, we outline a bench-to-bedside roadmap for this transition to mechanistically informed personalised pain medicine. We emphasise how the successful identification of novel analgesics is dependent on rigorous experimental design as well as the validity of models and translatability of outcome measures between the animal model and patients. Subsequently, we discuss general and specific aspects of human trial design to address heterogeneity in patient populations to increase the chance of identifying effective analgesics. Finally, we show how stratification approaches can be brought into clinical routine to the benefit of patients.

Peripheral CaV2.2 channels in skin regulate prolonged heat hypersensitivity during neuroinflammation

Neuroinflammation can lead to chronic maladaptive pain affecting millions of people worldwide. Neurotransmitters, cytokines, and ion channels are implicated in neuro-immune cell signaling but their roles in specific behavioral responses are not fully elucidated. Voltage-gated CaV2.2 channel activity in skin controls rapid and transient heat hypersensitivity induced by intradermal capsaicin via IL-1α cytokine signaling. CaV2.2 channels are not, however, involved in mechanical hypersensitivity that developed in the same animal model. Here, we show that CaV2.2 channels are also critical for heat hypersensitivity induced by the intradermal (id) Complete Freund's Adjuvant (CFA) model of chronic neuroinflammation that involves ongoing cytokine signaling for days. Ongoing CFA-induced cytokine signaling cascades in skin lead to pronounced edema, and hypersensitivity to sensory stimuli. Peripheral CaV2.2 channel activity in skin is required for the full development and week-long time course of heat hypersensitivity induced by id CFA. CaV2.2 channels, by contrast, are not involved in paw edema and mechanical hypersensitivity. CFA induced increases in cytokines in hind paws including IL-6 which was dependent on CaV2.2 channel activity. Using IL-6 specific neutralizing antibodies, we show that IL-6 contributes to heat hypersensitivity and, neutralizing both IL-1α and IL-6 was even more effective at reducing the magnitude and duration of CFA-induced heat hypersensitivity. Our findings demonstrate a functional link between CaV2.2 channel activity and the release of IL-6 in skin and show that CaV2.2 channels have a privileged role in the induction and maintenance of heat hypersensitivity during chronic forms of neuroinflammation in skin.

The impact of chronic pain on brain gene expression

Background

Chronic pain affects one fifth of American adults, contributing significant public health burden. Chronic pain mechanisms can be further understood through investigating brain gene expression.

Methods

We tested differentially expressed genes (DEGs) in chronic pain, migraine, lifetime fentanyl and oxymorphone use, and with chronic pain genetic risk in four brain regions (dACC, DLPFC, MeA, BLA) and imputed cell type expression data from 304 postmortem donors. We compared findings across traits and with independent transcriptomics resources, and performed gene-set enrichment.

Results

We identified two chronic pain DEGs: B4GALT and VEGFB in bulk dACC. We found over 2000 (primarily BLA microglia) chronic pain cell type DEGs. Findings were enriched for mouse microglia pain genes, and for hypoxia and immune response. Cross-trait DEG overlap was minimal.

Conclusions

Chronic pain-associated gene expression is heterogeneous across cell type, largely distinct from that in pain-related traits, and shows BLA microglia are a key cell type.

Genetically Regulated Gene Expression in the Brain Associated With Chronic Pain: Relationships With Clinical Traits and Potential for Drug Repurposing

BACKGROUND

Chronic pain is a common, poorly understood condition. Genetic studies including genome-wide association studies have identified many relevant variants, which have yet to be translated into full understanding of chronic pain. Transcriptome-wide association studies using transcriptomic imputation methods such as S-PrediXcan can help bridge this genotype-phenotype gap.

METHODS

We carried out transcriptomic imputation using S-PrediXcan to identify genetically regulated gene expression associated with multisite chronic pain in 13 brain tissues and whole blood. Then, we imputed genetically regulated gene expression for over 31,000 Mount Sinai BioMe participants and performed a phenome-wide association study to investigate clinical relationships in chronic pain-associated gene expression changes.

RESULTS

We identified 95 experiment-wide significant gene-tissue associations (p < 7.97 × 10-7), including 36 unique genes and an additional 134 gene-tissue associations reaching within-tissue significance, including 53 additional unique genes. Of the 89 unique genes in total, 59 were novel for multisite chronic pain and 18 are established drug targets. Chronic pain genetically regulated gene expression for 10 unique genes was significantly associated with cardiac dysrhythmia, metabolic syndrome, disc disorders/dorsopathies, joint/ligament sprain, anemias, and neurologic disorder phecodes. Phenome-wide association study analyses adjusting for mean pain score showed that associations were not driven by mean pain score.

CONCLUSIONS

We carried out the largest transcriptomic imputation study of any chronic pain trait to date. Results highlight potential causal genes in chronic pain development and tissue and direction of effect. Several gene results were also drug targets. Phenome-wide association study results showed significant associations for phecodes including cardiac dysrhythmia and metabolic syndrome, thereby indicating potential shared mechanisms.

A multi-ancestry genetic study of pain intensity in 598,339 veterans

Chronic pain is a common problem, with more than one-fifth of adult Americans reporting pain daily or on most days. It adversely affects the quality of life and imposes substantial personal and economic costs. Efforts to treat chronic pain using opioids had a central role in precipitating the opioid crisis. Despite an estimated heritability of 25-50%, the genetic architecture of chronic pain is not well-characterized, in part because studies have largely been limited to samples of European ancestry. To help address this knowledge gap, we conducted a cross-ancestry meta-analysis of pain intensity in 598,339 participants in the Million Veteran Program, which identified 126 independent genetic loci, 69 of which are new. Pain intensity was genetically correlated with other pain phenotypes, level of substance use and substance use disorders, other psychiatric traits, education level and cognitive traits. Integration of the genome-wide association studies findings with functional genomics data shows enrichment for putatively causal genes (n = 142) and proteins (n = 14) expressed in brain tissues, specifically in GABAergic neurons. Drug repurposing analysis identified anticonvulsants, β-blockers and calcium-channel blockers, among other drug groups, as having potential analgesic effects. Our results provide insights into key molecular contributors to the experience of pain and highlight attractive drug targets.

Finding new analgesics: Computational pharmacology faces drug discovery challenges

Despite the worldwide prevalence and huge burden of pain, pain is an undertreated phenomenon. Currently used analgesics have several limitations regarding their efficacy and safety. The discovery of analgesics possessing a novel mechanism of action has faced multiple challenges, including a limited understanding of biological processes underpinning pain and analgesia and poor animal-to-human translation. Computational pharmacology is currently employed to face these challenges. In this review, we discuss the theory, methods, and applications of computational pharmacology in pain research. Computational pharmacology encompasses a wide variety of theoretical concepts and practical methodological approaches, with the overall aim of gaining biological insight through data acquisition and analysis. Data are acquired from patients or animal models with pain or analgesic treatment, at different levels of biological organization (molecular, cellular, physiological, and behavioral). Distinct methodological algorithms can then be used to analyze and integrate data. This helps to facilitate the identification of biological molecules and processes associated with pain phenotype, build quantitative models of pain signaling, and extract translatable features between humans and animals. However, computational pharmacology has several limitations, and its predictions can provide false positive and negative findings. Therefore, computational predictions are required to be validated experimentally before drawing solid conclusions. In this review, we discuss several case study examples of combining and integrating computational tools with experimental pain research tools to meet drug discovery challenges.

Nerve-myeloid cell interactions in persistent human pain: a reappraisal using updated cell subset classifications

ABSTRACT

The past 20 years have seen a dramatic shift in our understanding of the role of the immune system in initiating and maintaining pain. Myeloid cells, including macrophages, dendritic cells, Langerhans cells, and mast cells, are increasingly implicated in bidirectional interactions with nerve fibres in rodent pain models. However, our understanding of the human setting is still poor. High-dimensional functional analyses have substantially changed myeloid cell classifications, with recently described subsets such as epidermal dendritic cells and DC3s unveiling new insight into how myeloid cells interact with nerve fibres. However, it is unclear whether this new understanding has informed the study of human chronic pain. In this article, we perform a scoping review investigating neuroimmune interactions between myeloid cells and peripheral nerve fibres in human chronic pain conditions. We found 37 papers from multiple pain states addressing this aim in skin, cornea, peripheral nerve, endometrium, and tumour, with macrophages, Langerhans cells, and mast cells the most investigated. The directionality of results between studies was inconsistent, although the clearest pattern was an increase in macrophage frequency across conditions, phases, and tissues. Myeloid cell definitions were often outdated and lacked correspondence with the stated cell types of interest; overreliance on morphology and traditional structural markers gave limited insight into the functional characteristics of investigated cells. We therefore critically reappraise the existing literature considering contemporary myeloid cell biology and advocate for the application of established and emerging high-dimensional proteomic and transcriptomic single-cell technologies to clarify the role of specific neuroimmune interactions in chronic pain.

Can we achieve pain stratification in musculoskeletal conditions? Implications for clinical practice

In the last few years there has been an increased appreciation that pain perception in rheumatic and musculoskeletal diseases (RMDs) has several mechanisms which include nociceptive, inflammatory, nociplastic and neuropathic components. Studies in specific patient groups have also demonstrated that the pain experienced by people with specific diagnoses can present with distinctive components over time. For example, the pain observed in rheumatoid arthritis has been widely accepted to be caused by the activation of nociceptors, potentiated by the release of inflammatory mediators, including prostaglandins, leukotrienes and cytokine networks in the joint environment. However, people with RA may also experience nociplastic and neuropathic pain components, particularly when treatments with disease modifying anti-rheumatic drugs (DMARDs) have been implemented and are insufficient to control pain symptoms. In other RMDs, the concept of pain sensitisation or nociplastic pain in driving ongoing pain symptoms e.g. osteoarthritis and fibromyalgia, is becoming increasingly recognised. In this review, we explore the hypothesis that pain has distinct modalities based on clinical, pathophysiological, imaging and genetic factors. The concept of pain stratification in RMD is explored and implications for future management are also discussed.

Psycho-Neuroendocrinology in the Rehabilitation Field: Focus on the Complex Interplay between Stress and Pain

Chronic stress and chronic pain share neuro-anatomical, endocrinological, and biological features. However, stress prepares the body for challenging situations or mitigates tissue damage, while pain is an unpleasant sensation due to nociceptive receptor stimulation. When pain is chronic, it might lead to an allostatic overload in the body and brain due to the chronic dysregulation of the physiological systems that are normally involved in adapting to environmental challenges. Managing stress and chronic pain (CP) in neurorehabilitation presents a significant challenge for healthcare professionals and researchers, as there is no definitive and effective solution for these issues. Patients suffering from neurological disorders often complain of CP, which significantly reduces their quality of life. The aim of this narrative review is to examine the correlation between stress and pain and their potential negative impact on the rehabilitation process. Moreover, we described the most relevant interventions used to manage stress and pain in the neurological population. In conclusion, this review sheds light on the connection between chronic stress and chronic pain and their impact on the neurorehabilitation pathway. Our results emphasize the need for tailored rehabilitation protocols to effectively manage pain, improve treatment adherence, and ensure comprehensive patient care.

Plasma proteomic profiles of pain subtypes in adolescents and young adults with endometriosis

STUDY QUESTION

What are the similarities and differences in the systemic proteomic profiles by endometriosis-associated pain subtypes among adolescents and young adults with endometriosis?

SUMMARY ANSWER

Endometriosis-associated pain subtypes exhibited distinct plasma proteomic profiles.

WHAT IS KNOWN ALREADY

Endometriosis patients, especially those diagnosed in adolescents and young adults, are often plagued by various pain symptoms. However, it is not clear what biological processes underlie this heterogeneity.

STUDY DESIGN, SIZE, DURATION

We conducted a cross-sectional analysis using data and plasma samples from 142 adolescent or young adult participants of the Women's Health Study: From Adolescence to Adulthood cohort with laparoscopically confirmed endometriosis.

PARTICIPANTS/MATERIALS, SETTING, METHODS

We measured 1305 plasma protein levels by SomaScan. We classified self-reported endometriosis-associated pain into subtypes of dysmenorrhea, acyclic pelvic pain, life impacting pelvic pain, bladder pain, bowel pain, and widespread pain phenotype. We used logistic regression to calculate the odds ratios and 95% confidence intervals for differentially expressed proteins, adjusting for age, BMI, fasting status, and hormone use at blood draw. Ingenuity Pathway Analysis identified enriched biological pathways.

MAIN RESULTS AND THE ROLE OF CHANCE

Our study population consisted mainly of adolescents and young adults (mean age at blood draw = 18 years), with nearly all (97%) scored as rASRM stage I/II at laparoscopic diagnosis of endometriosis, which is a common clinical presentation of endometriosis diagnosed at a younger age. Pain subtypes exhibited distinct plasma proteomic profiles. Multiple cell movement pathways were downregulated in cases with severe dysmenorrhea and life impacting pelvic pain compared to those without (P < 7.5×10-15). Endometriosis cases with acyclic pelvic pain had upregulation of immune cell adhesion pathways (P < 9.0×10-9), while those with bladder pain had upregulation of immune cell migration (P < 3.7×10-8) and those with bowel pain had downregulation (P < 6.5×10-7) of the immune cell migration pathways compared to those without. Having a wide-spread pain phenotype involved downregulation of multiple immune pathways (P < 8.0×10-10).

LIMITATIONS, REASONS FOR CAUTION

Our study was limited by the lack of an independent validation cohort. We were also only able to explore any presence of a pain subtype and could not evaluate multiple combinations by pain subtypes. Further mechanistic studies are warranted to elucidate the differences in pathophysiology by endometriosis-pain subtype.

WIDER IMPLICATIONS OF THE FINDINGS

The observed variation in plasma protein profiles by pain subtypes suggests different underlying molecular mechanisms, highlighting the need for potential consideration of pain subtypes for effectively treating endometriosis patients presenting with various pain symptoms.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by the Department of Defense W81XWH1910318 and the 2017 Boston Center for Endometriosis Trainee Award. Financial support for establishment of and data collection within the A2A cohort were provided by the J. Willard and Alice S. Marriott Foundation. N.S., A.F.V., S.A.M., and K.L.T. have received funding from the Marriott Family Foundation. C.B.S. is funded by an R35 MIRA Award from NIGMS (5R35GM142676). S.A.M. and K.L.T. are supported by NICHD R01HD094842. S.A.M. reports serving as an advisory board member for AbbVie and Roche, Field Chief Editor for Frontiers in Reproductive Health, personal fees from Abbott for roundtable participation; none of these are related to this study. Other authors report no conflict of interest.

TRIAL REGISTRATION NUMBER

N/A.

The therapeutic potential of natural killer cells in neuropathic pain

Novel disease-modifying treatments for neuropathic pain are urgently required. The cellular immune response to nerve injury represents a promising target for therapeutic development. Recently, the role of natural killer (NK) cells in both CNS and PNS disease has been the subject of growing interest. In this opinion article, we set out the case for NK cell-based intervention as a promising avenue for development in the management of neuropathic pain. We explore the potential cellular and molecular targets of NK cells in the PNS by contrasting with their reported functional roles in CNS diseases, and we suggest strategies for using the beneficial functions of NK cells and immune-based therapeutics in the context of neuropathic pain.

Rethinking the use of NSAIDs in early acute pain

Non-steroidal anti-inflammatory drugs (NSAIDs) are the most widely used analgesics to treat inflammatory pain. Despite their efficacy, recent studies show that NSAID use in early acute pain can prolong pain and inflammation and delay their resolution. We suggest using analgesics without inflammation-related properties in early acute pain instead of NSAIDs.

The genetic architecture of pain intensity in a sample of 598,339 U.S. veterans

Chronic pain is a common problem, with more than one-fifth of adult Americans reporting pain daily or on most days. It adversely affects quality of life and imposes substantial personal and economic costs. Efforts to treat chronic pain using opioids played a central role in precipitating the opioid crisis. Despite an estimated heritability of 25-50%, the genetic architecture of chronic pain is not well characterized, in part because studies have largely been limited to samples of European ancestry. To help address this knowledge gap, we conducted a cross-ancestry meta-analysis of pain intensity in 598,339 participants in the Million Veteran Program, which identified 125 independent genetic loci, 82 of which are novel. Pain intensity was genetically correlated with other pain phenotypes, level of substance use and substance use disorders, other psychiatric traits, education level, and cognitive traits. Integration of the GWAS findings with functional genomics data shows enrichment for putatively causal genes (n = 142) and proteins (n = 14) expressed in brain tissues, specifically in GABAergic neurons. Drug repurposing analysis identified anticonvulsants, beta-blockers, and calcium-channel blockers, among other drug groups, as having potential analgesic effects. Our results provide insights into key molecular contributors to the experience of pain and highlight attractive drug targets.