From mouse to humans: discovery of the CACNG2 pain susceptibility gene

Molecular cloning of the gene promoter encoding the human CaVγ2/Stargazin divergent transcript (CACNG2-DT): characterization and regulation by the cAMP-PKA/CREB signaling pathway

CaVγ2 (Stargazin or TARPγ2) is a protein expressed in various types of neurons whose function was initially associated with a decrease in the functional expression of voltage-gated presynaptic Ca2+ channels (CaV) and which is now known to promote the trafficking of the postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPAR) towards the cell membrane. Alterations in CaVγ2 expression has been associated with several neurological disorders, such as absence epilepsy. However, its regulation at the transcriptional level has not been intensively addressed. It has been reported that the promoter of the Cacng2 gene, encoding the rat CaVγ2, is bidirectional and regulates the transcription of a long non-coding RNA (lncRNA) in the antisense direction. Here, we investigate the proximal promoter region of the human CACNG2 gene in the antisense direction and show that this region includes two functional cAMP response elements that regulate the expression of a lncRNA called CACNG2-DT. The activity of these sites is significantly enhanced by forskolin, an adenylate cyclase activator, and inhibited by H89, a protein kinase A (PKA) antagonist. Therefore, this regulatory mechanism implies the activation of G protein-coupled receptors and downstream phosphorylation. Interestingly, we also found that the expression of CACNG2-DT may increase the levels of the CaVγ2 subunit. Together, these data provide novel information on the organization of the human CACNG2-DT gene promoter, describe modulatory domains and mechanisms that can mediate various regulatory inputs, and provide initial information on the molecular mechanisms that regulate the functional expression of the CaVγ2 protein.

Ion channel long non-coding RNAs in neuropathic pain

Neuropathic pain is one of the primary forms of chronic pain and is the consequence of the somatosensory system's direct injury or disease. It is a relevant public health problem that affects about 10% of the world's general population. In neuropathic pain, alteration in neurotransmission occurs at various levels, including the dorsal root ganglia, the spinal cord, and the brain, resulting from the malfunction of diverse molecules such as receptors, ion channels, and elements of specific intracellular signaling pathways. In this context, there have been exciting advances in elucidating neuropathic pain's cellular and molecular mechanisms in the last decade, including the possible role that long non-coding RNAs (lncRNAs) may play, which open up new alternatives for the development of diagnostic and therapeutic strategies for this condition. This review focuses on recent studies associated with the possible relevance of lncRNAs in the development and maintenance of neuropathic pain through their actions on the functional expression of ion channels. Recognizing the changes in the function and spatio-temporal patterns of expression of these membrane proteins is crucial to understanding the control of neuronal excitability in chronic pain syndromes.

Genetic basis of thermal nociceptive sensitivity and brain weight in a BALB/c reduced complexity cross

Thermal nociception involves the transmission of temperature-related noxious information from the periphery to the CNS and is a heritable trait that could predict transition to persistent pain. Rodent forward genetics complement human studies by controlling genetic complexity and environmental factors, analysis of end point tissue, and validation of variants on appropriate genetic backgrounds. Reduced complexity crosses between nearly identical inbred substrains with robust trait differences can greatly facilitate unbiased discovery of novel genes and variants. We found BALB/cByJ mice showed enhanced sensitivity on the 53.5°C hot plate and mechanical stimulation in the von Frey test compared to BALB/cJ mice and replicated decreased gross brain weight in BALB/cByJ versus BALB/cJ. We then identified a quantitative trait locus (QTL) on chromosome 13 for hot plate sensitivity (LOD = 10.7; p < 0.001; peak = 56 Mb) and a QTL for brain weight on chromosome 5 (LOD = 8.7; p < 0.001). Expression QTL mapping of brain tissues identified H2afy (56.07 Mb) as the top transcript with the strongest association at the hot plate locus (FDR = 0.0002) and spliceome analysis identified differential exon usage within H2afy associated with the same locus. Whole brain proteomics further supported decreased H2AFY expression could underlie enhanced hot plate sensitivity, and identified ACADS as a candidate for reduced brain weight. To summarize, a BALB/c reduced complexity cross combined with multiple-omics approaches facilitated identification of candidate genes underlying thermal nociception and brain weight. These substrains provide a powerful, reciprocal platform for future validation of candidate variants.

Screening and Identification of Key Genes, Pathways, and Drugs Associated with Neuropathic Pain in Dorsal Horn: Evidence from Bioinformatic Analysis

Purpose

Neuropathic pain is a devastating complex condition occurring post-nervous system damage. Microglia in dorsal horn drives neuropathic pain as a kind of immune cell. We aimed to find potential differentially expressed genes (DEGs) and candidate pathways, which induced neuropathic pain, and to identify some new transcription factors and therapeutic drugs via bioinformatic analysis.

Methods

The microarray profile GSE60670 was downloaded and analyzed. DEGs were screened and analyzed through Gene Ontology (GO), pathway enrichment, and protein-to-protein interaction (PPI) network. Respectively, transcription factors (TFs) and potential therapeutic drugs for DEGs were predicted through NetworkAnalyst and DGIdb databases. At last, we chose top 10 DEGs for external validation.

Results

A total of 100 DEGs were identified. The results of pathway and GO analyses were closely related to malaria inflammatory pathway and inflammatory response. Three necessary PPI modules and 9 hub genes were identified in PPI analysis, and 277 DEG-TF pairs were found among 54 DEGs and 32 TF. Moreover, 22 candidate drugs were found to match 9 hub genes. External validation of 9 of the top 10 DEGs were consistent with bioinformatic analysis.

Conclusion

This study provided comprehensive analyses for the functional gene sets and pathways related to neuropathic pain and promoted our understanding of the mechanism or therapy of neuropathic pain.

Update on the pharmacogenomics of pain management

Pharmacogenomics is the study of genetic variants that impact drug effects through changes in a drug’s pharmacokinetics and pharmacodynamics. Pharmacogenomics is being integrated into clinical pain management practice because variants in individual genes can be predictive of how a patient may respond to a drug treatment. Pain is subjective and is considered challenging to treat. Furthermore, pain patients do not respond to treatments in the same way, which makes it hard to issue a consistent treatment regimen for all pain conditions. Pharmacogenomics would bring consistency to the subjective nature of pain and could revolutionize the field of pain management by providing personalized medical care tailored to each patient based on their gene variants. Additionally, pharmacogenomics offers a solution to the opioid crisis by identifying potentially opioid-vulnerable patients who could be recommended a nonopioid treatment for their pain condition. The integration of pharmacogenomics into clinical practice creates better and safer healthcare practices for patients. In this article, we provide a comprehensive history of pharmacogenomics and pain management, and focus on up to date information on the pharmacogenomics of pain management, describing genes involved in pain, genes that may reduce or guard against pain and discuss specific pain management drugs and their genetic correlations.

Early postoperative neuropathic pain assessed by the DN4 score predicts an increased risk of persistent postsurgical neuropathic pain

BACKGROUND

Acute neuropathic pain can occur in the postoperative period but any link with persistent post-surgical neuropathic pain remains unclear.

OBJECTIVES

The objectives of this study were to prospectively describe the incidence of acute post-surgical neuropathic pain in a large population using the DN4 (clinician administered) questionnaire and to confirm the hypothetical link between acute and persistent neuropathic pain at 2 months after surgery in a large population using the DN2 (self administered) questionnaire.

DESIGN

A multi-centre, prospective and observational trial.

SETTING

Two consecutive days in 27 hospitals in France.

PATIENTS

Six hundred and eight patients undergoing 13 different types of surgery. Fifteen patients were excluded as data were incomplete, and 229 (38.6%) and 260 (43.8%) were not contactable for assessment at 1 and 2 months after surgery, respectively.

MAIN OUTCOME MEASURES

Pain was evaluated at least 2 h postoperatively on the same day (D0),on the second day (D2) and at 1 and 2 months after surgery (M1 and M2). Pain was assessed using a 10-point Numeric Rating Scale. If the Numeric Rating Scale score was greater than 0, neuropathic pain was assessed using a DN4 (clinician administered) questionnaire or using a DN2 (self-administered) questionnaire. Acute and persistent postsurgical neuropathic pain (PPSNP) were defined respectively by a DN4 score at least 4/10 on day 0 and/or day 2 and a DN2 score at least 3/7 at 2 months after surgery.

RESULTS

Of the 593 patients included, 41.2% were in pain before surgery and 8.2% described neuropathic pain. Early after surgery, the majority of the 593 patients (72.2% on the day of surgery and 71.3% on day 2) experienced acute pain. It was neuropathic in nature in 5.6% of patients (95% CI, 3.6 to 8.3) on the day of surgery and 12.9% (95% CI, 9.7 to 16.7) on day 2. Two months after surgery, PPSNP was present in 33.3% of the 333 patients assessed. Multivariate analysis showed that a DN4 score at least 4/10 on the day of surgery or on day 2 was a significant risk factor for PPSNP [odds ratios 4.22 (95% CI, 2.19 to 8.12)].

CONCLUSION

Our results suggest that early acute postsurgical neuropathic pain significantly increases the risk of persistent post-surgical neuropathic pain.

TRIAL REGISTRATION

Clinicaltrials.gov identifier: NTC NCT02826317.

Combining Human and Rodent Genetics to Identify New Analgesics

Most attempts at rational development of new analgesics have failed, in part because chronic pain involves multiple processes that remain poorly understood. To improve translational success, one strategy is to select novel targets for which there is proof of clinical relevance, either genetically through heritable traits, or pharmacologically. Such an approach by definition yields targets with high clinical validity. The biology of these targets can be elucidated in animal models before returning to the patients with a refined therapeutic. For optimal treatment, having biomarkers of drug action available is also a plus. Here we describe a case study in rational drug design: the use of controlled inhibition of peripheral tetrahydrobiopterin (BH4) synthesis to reduce abnormal chronic pain states without altering nociceptive-protective pain. Initially identified in a population of patients with low back pain, the association between BH4 production and chronic pain has been confirmed in more than 12 independent cohorts, through a common haplotype (present in 25% of Caucasians) of the rate-limiting enzyme for BH4 synthesis, GTP cyclohydrolase 1 (GCH1). Genetic tools in mice have demonstrated that both injured sensory neurons and activated macrophages engage increased BH4 synthesis to cause chronic pain. GCH1 is an obligate enzyme for de novo BH4 production. Therefore, inhibiting GCH1 activity eliminates all BH4 production, affecting the synthesis of multiple neurotransmitters and signaling molecules and interfering with physiological function. In contrast, targeting the last enzyme of the BH4 synthesis pathway, sepiapterin reductase (SPR), allows reduction of pathological BH4 production without completely blocking physiological BH4 synthesis. Systemic SPR inhibition in mice has not revealed any safety concerns to date, and available genetic and pharmacologic data suggest similar responses in humans. Finally, because it is present in vivo only when SPR is inhibited, sepiapterin serves as a reliable biomarker of target engagement, allowing potential quantification of drug efficacy. The emerging development of therapeutics that target BH4 synthesis to treat chronic pain illustrates the power of combining human and mouse genetics: human genetic studies for clinical selection of relevant targets, coupled with causality studies in mice, allowing the rational engineering of new analgesics.

Molecular Changes Involving MEK3-p38 MAPK Activation in Chronic Masticatory Myalgia

The exact mechanism underlying chronic masticatory myalgia (CMM), a conspicuous symptom in temporomandibular disorders, remains unclear. This investigation compared gene expression profiles between CMM patients and healthy subjects. Peripheral blood leukocytes were collected in 8 cases and 8 controls and subjected to whole genome microarray analyses. Data were analyzed with Gene Ontology and interactive pathways analyses. According to Gene Ontology analysis, categories such as ion transport, response to stimuli, and metabolic process were upregulated. The pathway analysis suggested overexpression of the mitogen-activated protein kinase (MAPK) pathway in CMM patients and to a higher degree in a pathway network. Overexpression of representative members of the MAPK pathway-including MAPK kinase 3 (MEK3), calcium voltage-gated channel auxiliary subunit gamma 2 (CACNG2), and growth arrest and DNA damage-inducible gamma (GADD45G)-was validated with real-time polymerase chain reaction. The upregulation of MEK3 was negatively correlated with the age of the CMM group. In the next step, the authors focused on MEK3, the gene that exhibited the greatest degree of differential expression, and its downstream target protein p38 MAPK. The results revealed upregulation of MEK3, as well as phosphorylated MEK3 and phosphorylated p38 MAPK, in CMM patients. These results provide a "fingerprint" for mechanistic studies of CMM in the future and highlight the importance of MEK3-p38 MAPK activation in CMM.

Perspectives and Trends in Pharmacological Approaches to the Modulation of Pain

Pharmacological approaches to our understanding and treatment of pain have had a long history and have traditionally relied on very few drugs that either have significant side effects and abuse liability, such as the nonsteroidal anti-inflammatory drugs or the opioids, respectively, or those that have been developed for other conditions such as the tricyclic antidepressants. The pathophysiology of pain is undoubtedly complex, complicated in part by the fact that it is not a singular condition, and has a variety of etiologies and a number of associated comorbidities that make treatment interventions challenging. Moreover, there are changes in the central nervous system during the course of the development of chronic pain that, in a manner parallel to neurodegenerative disorders, likely require different pharmacological approaches in the early stages of acute pain compared to those that would be effective when pain has become chronic. This chapter reviews the current status of the field of pain research focusing on some relatively underdeveloped areas, such as pain and its associated comorbidities, and the use of transgenic animals and drug self-administration procedures in the context of analgesic assessment. This chapter also incorporates more recent developments and emerging trends in the area of epigenetics, biomarkers, and the use of induced pluripotent stem cells for pharmacological evaluation, target identification, and validation. Recent progress in the study of "organs-on-a-chip" will also be included in this overview, setting expectations for future progress that integrates these advances for deeper insights into mechanisms, novel treatments, and facilitated efforts in drug discovery.

Human pain and genetics: some basics

Human pain causes untold misery and suffering, with major impact on functioning and resources. Recent advances in genetics have revealed that subtle changes in DNA could partly explain the variation in individual differences in pain. Various genes encoding for receptors are now known to play a major role in the sensitivity, perception and expression of pain. The fields of epigenetics and proteomics hold promises in the way pain could be treated and managed in future.

Modulation of nociceptive ion channels and receptors via protein-protein interactions: implications for pain relief

In the last 2 decades biomedical research has provided great insights into the molecular signatures underlying painful conditions. However, chronic pain still imposes substantial challenges to researchers, clinicians and patients alike. Under pathological conditions, pain therapeutics often lack efficacy and exhibit only minimal safety profiles, which can be largely attributed to the targeting of molecules with key physiological functions throughout the body. In light of these difficulties, the identification of molecules and associated protein complexes specifically involved in chronic pain states is of paramount importance for designing selective interventions. Ion channels and receptors represent primary targets, as they critically shape nociceptive signaling from the periphery to the brain. Moreover, their function requires tight control, which is usually implemented by protein-protein interactions (PPIs). Indeed, manipulation of such PPIs entails the modulation of ion channel activity with widespread implications for influencing nociceptive signaling in a more specific way. In this review, we highlight recent advances in modulating ion channels and receptors via their PPI networks in the pursuit of relieving chronic pain. Moreover, we critically discuss the potential of targeting PPIs for developing novel pain therapies exhibiting higher efficacy and improved safety profiles.

Down-regulation of Stargazin inhibits the enhanced surface delivery of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor GluR1 subunit in rat dorsal horn and ameliorates postoperative pain

BACKGROUND

Stargazin is the first transmembrane protein known to regulate synaptic targeting of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors. However, it is unclear whether regulation of the surface delivery of spinal AMPA receptor subunits by stargazin contributes to postoperative pain development.

METHODS

Western blot analysis was used to examine changes in the surface delivery of AMPA receptor subunits, GluR1 and GluR2, in rat dorsal horn. The interaction between stargazin and GluR1 and GluR2 was examined by coimmunoprecipitation. Expression of stargazin was suppressed by intrathecal administration of small interfering RNA311.

RESULTS

Membrane-bound GluR1, but not GluR2, in ipsilateral dorsal horn was increased at 3 h (1.49 ± 0.15-fold of β-tubulin, mean ± SEM) and 1 day (1.03 ± 0.25) after incision, as compared with that in control rats (naive, 0.63 ± 0.23, P < 0.05, n = 6 per group). The amount of GluR1 coimmunoprecipitated with stargazin was greater at 3 h after incision (1.48 ± 0.31-fold of input) than that in control animals (0.45 ± 0.24, P < 0.05, n = 6 per group). Importantly, the increase in membrane GluR1 at 3 h after incision was normalized to near control level (0.72 ± 0.20-fold of β-tubulin) by pretreatment with intrathecal stargazin small interfering RNA311 (0.87 ± 0.09), but not scrambled small interfering RNA (1.48 ± 0.24) or vehicle (1.25 ± 0.13, P < 0.05, n = 6 per group). Stargazin small interfering RNA311 pretreatment prevented the increase in stargazin-GluR1 interaction and decreased postoperative pain after incision.

CONCLUSIONS

This study suggests a critical role of stargazin-mediated surface delivery of GluR1 subunit in the development of postoperative pain. A better therapeutic strategy for postoperative pain may involve selectively down-regulating spinal stargazin to inhibit synaptic targeting of GluR1 subunit.