Phosphorylated Histone 3 at Serine 10 Identifies Activated Spinal Neurons and Contributes to the Development of Tissue Injury-Associated Pain

Elevated 18:0 lysophosphatidylcholine contributes to the development of pain in tissue injury

ABSTRACT

Tissue injuries, including burns, are major causes of death and morbidity worldwide. These injuries result in the release of intracellular molecules and subsequent inflammatory reactions, changing the tissues' chemical milieu and leading to the development of persistent pain through activating pain-sensing primary sensory neurons. However, the majority of pain-inducing agents in injured tissues are unknown. Here, we report that, amongst other important metabolite changes, lysophosphatidylcholines (LPCs) including 18:0 LPC exhibit significant and consistent local burn injury-induced changes in concentration. 18:0 LPC induces immediate pain and the development of hypersensitivities to mechanical and heat stimuli through molecules including the transient receptor potential ion channel, vanilloid subfamily, member 1, and member 2 at least partly via increasing lateral pressure in the membrane. As levels of LPCs including 18:0 LPC increase in other tissue injuries, our data reveal a novel role for these lipids in injury-associated pain. These findings have high potential to improve patient care.

Extracellular signal-regulated kinase-dependent phosphorylation of histone H3 serine 10 is involved in the pathogenesis of traumatic brain injury

Traumatic brain injury (TBI) induces a series of epigenetic changes in brain tissue, among which histone modifications are associated with the deterioration of TBI. In this study, we explored the role of histone H3 modifications in a weight-drop model of TBI in rats. Screening for various histone modifications, immunoblot analyses revealed that the phosphorylation of histone H3 serine 10 (p-H3S10) was significantly upregulated after TBI in the brain tissue surrounding the injury site. A similar posttraumatic regulation was observed for phosphorylated extracellular signal-regulated kinase (p-ERK), which is known to phosphorylate H3S10. In support of the hypothesis that ERK-mediated phosphorylation of H3S10 contributes to TBI pathogenesis, double immunofluorescence staining of brain sections showed high levels and colocalization of p-H3S10 and p-ERK predominantly in neurons surrounding the injury site. To test the hypothesis that inhibition of ERK-H3S10 signaling ameliorates TBI pathogenesis, the mitogen-activated protein kinase–extracellular signal-regulated kinase kinase (MEK) 1/2 inhibitor U0126, which inhibits ERK phosphorylation, was administered into the right lateral ventricle of TBI male and female rats via intracerebroventricular cannulation for 7 days post trauma. U0126 administration indeed prevented H3S10 phosphorylation and improved motor function recovery and cognitive function compared to vehicle treatment. In agreement with our findings in the rat model of TBI, immunoblot and double immunofluorescence analyses of brain tissue specimens from patients with TBI demonstrated high levels and colocalization of p-H3S10 and p-ERK as compared to control specimens from non-injured individuals. In conclusion, our findings indicate that phosphorylation-dependent activation of ERK-H3S10 signaling participates in the pathogenesis of TBI and can be targeted by pharmacological approaches.

CRISPR/Cas9-Based Mutagenesis of Histone H3.1 in Spinal Dynorphinergic Neurons Attenuates Thermal Sensitivity in Mice

Burn injury is a trauma resulting in tissue degradation and severe pain, which is processed first by neuronal circuits in the spinal dorsal horn. We have recently shown that in mice, excitatory dynorphinergic (Pdyn) neurons play a pivotal role in the response to burn-injury-associated tissue damage via histone H3.1 phosphorylation-dependent signaling. As Pdyn neurons were mostly associated with mechanical allodynia, their involvement in thermonociception had to be further elucidated. Using a custom-made AAV9_mutH3.1 virus combined with the CRISPR/cas9 system, here we provide evidence that blocking histone H3.1 phosphorylation at position serine 10 (S10) in spinal Pdyn neurons significantly increases the thermal nociceptive threshold in mice. In contrast, neither mechanosensation nor acute chemonociception was affected by the transgenic manipulation of histone H3.1. These results suggest that blocking rapid epigenetic tagging of S10H3 in spinal Pdyn neurons alters acute thermosensation and thus explains the involvement of Pdyn cells in the immediate response to burn-injury-associated tissue damage.

Histone post-translational modifications as potential therapeutic targets for pain management

Effective pharmacological management of pain associated with tissue pathology is an unmet medical need. Transcriptional modifications in nociceptive pathways are pivotal for the development and the maintenance of pain associated with tissue damage. Accumulating evidence has shown the importance of the epigenetic control of transcription in nociceptive pathways via histone post-translational modifications (PTMs). Hence, histone PTMs could be targets for novel effective analgesics. Here, we discuss the current understanding of histone PTMs in the modulation of gene expression affecting nociception and pain phenotypes following tissue injury. We also provide a critical view of the translational implications of preclinical models and discuss opportunities and challenges of targeting histone PTMs to relieve pain in clinically relevant tissue injuries.

Spinal Excitatory Dynorphinergic Interneurons Contribute to Burn Injury-Induced Nociception Mediated by Phosphorylated Histone 3 at Serine 10 in Rodents

The phosphorylation of serine 10 in histone 3 (p-S10H3) has recently been demonstrated to participate in spinal nociceptive processing. However, superficial dorsal horn (SDH) neurons involved in p-S10H3-mediated nociception have not been fully characterized. In the present work, we combined immunohistochemistry, in situ hybridization with the retrograde labeling of projection neurons to reveal the subset of dorsal horn neurons presenting an elevated level of p-S10H3 in response to noxious heat (60 °C), causing burn injury. Projection neurons only represented a small percentage (5%) of p-S10H3-positive cells, while the greater part of them belonged to excitatory SDH interneurons. The combined immunolabeling of p-S10H3 with markers of already established interneuronal classes of the SDH revealed that the largest subset of neurons with burn injury-induced p-S10H3 expression was dynorphin immunopositive in mice. Furthermore, the majority of p-S10H3-expressing dynorphinergic neurons proved to be excitatory, as they lacked Pax-2 and showed Lmx1b-immunopositivity. Thus, we showed that neurochemically heterogeneous SDH neurons exhibit the upregulation of p-S10H3 shortly after noxious heat-induced burn injury and consequential tissue damage, and that a dedicated subset of excitatory dynorphinergic neurons is likely a key player in the development of central sensitization via the p-S10H3 mediated pathway.

Leptin and fractalkine: novel subcutaneous cytokines in burn injury

Burn injury is a pathology underpinned by progressive and aberrant inflammation. It is a major clinical challenge to survival and quality of life. Although the complex local and disseminating pathological processes of a burn injury ultimately stem from local tissue damage, to date relatively few studies have attempted to characterise the local inflammatory mediator profile. Here, cytokine content and associated transcriptional changes were measured in rat skin for three hours immediately following induction of a scald-type (60°C, 2 min) burn injury model. Leptin (P=0.0002) and fractalkine (P=0.0478) concentrations were significantly elevated post-burn above pre-burn and control site values, coinciding with the development of burn site oedema and differential expression of leptin mRNA (P=0.0004). Further, gene sequencing enrichment analysis indicated cytokine-cytokine receptor interaction (P=1.45×10-6). Subsequent behavioural studies demonstrated that, following subcutaneous injection into the dorsum of the paw, both leptin and fractalkine induced mechanical allodynia, heat hyperalgesia and the recruitment of macrophages. This is the first report of leptin elevation specifically at the burn site, and the first report of fractalkine elevation in any tissue post-burn which, together with the functional findings, calls for exploration of the influence of these cytokines on pain, inflammation and burn wound progression. In addition, targeting these signalling molecules represents a therapeutic potential as early formative mediators of these pathological processes.

Intracerebroventricular Coadministration of Protoxin-II and Trace Elements in Rats Enhances the Analgesic Effect of the 1.7 Voltage-Gate Sodium Channel Blocker

Pain continues to be a global unmet medical need, and the current recommendations for its management require a constant exploration of new drugs that target multiple pain mechanisms, with an improved safety profile and increased treatment adherence. Currently, the enriched distribution and localization within nociceptors of the selective channel blockers and the critical role played by sodium channels in neuronal excitability nominate isoforms as specific targets to generate innovative compounds. In the present report, we verified the hypothesis that coadministration of Protoxin-II, a selective sodium channel inhibitor, and trace elements has direct and improved antinociceptive effects. Groups of seven Wistar rats were treated intracerebroventricularly with a combination of MgCl₂, CdCl₂, and ZnCl₂ and Protoxin-II, respectively, and with Protoxin-II alone (positive) or saline (negative) for controls. Evaluations were performed by nociception assay. Coadministration of these drugs caused an increase in the maximum possible effect of up to 40% as compared with the control groups. Our findings indicate that selective channel blockers continue to be an important nociception target and that the use of trace elements may provide simple but effective means of control over sodium channel blockers' risks, potentially lowering the necessary analgesic doses, thus improving the efficacy and safety profile.

Does a familial subtype of complex regional pain syndrome exist? Results of a systematic review

Background and Objective: Complex regional pain syndrome (CRPS) is a chronic condition characterized by severe regional pain, allodynia, hyperalgesia, and functional impairment. The aim of this systematic review is to investigate whether a familial subtype of CRPS (fCRPS) exists and to determine whether people with fCRPS have specific characteristics.

Methods: Databases CINAHL, Medline, PsycINFO, and PubMed were searched with no date limitation. Quality of reporting was assessed using the Scottish Intercollegiate Guidelines Network scale and the Joanna Briggs Institute’s checklists.

Results: Eight studies were included. Family relationships were defined as any immediate (i.e., parents or siblings) or blood relatives. A combination of participants with known or unknown causes for CRPS was recruited. The studies in this review support the potential for the existence of fCRPS, although this included less than 25% of those affected. People with potential fCRPS showed more severe symptoms, more sites involved, a higher percentage of spontaneous onset, and earlier age at onset. An elevated sibling recurrence risk ratio of 5.6 (95% confidence interval [CI], 3.0 to 9.8) was reported for people under 50. None of the studies established a pattern of heritability. Therefore, the most likely explanation for heritability would be a multifactorial model in which cumulative and interactive Gene × Environment effects may be involved.

Conclusions: This systematic review supports the potential for the existence of fCRPS; however, all identified studies used uncontrolled case reports, case series, and case–control designs that cannot provide evidence of causation. Further studies are required to reveal the heritability and genetic structure of fCRPS.

The NAv1.7 blocker protoxin II reduces burn injury-induced spinal nociceptive processing

Controlling pain in burn-injured patients poses a major clinical challenge. Recent findings suggest that reducing the activity of the voltage-gated sodium channel Na_v1.7 in primary sensory neurons could provide improved pain control in burn-injured patients. Here, we report that partial thickness scalding-type burn injury on the rat paw upregulates Na_v1.7 expression in primary sensory neurons 3 h following injury. The injury also induces upregulation in phosphorylated cyclic adenosine monophosphate response element-binding protein (p-CREB), a marker for nociceptive activation in primary sensory neurons. The upregulation in p-CREB occurs mainly in Na_v1.7-immunopositive neurons and exhibits a peak at 5 min and, following a decline at 30 min, a gradual increase from 1 h post-injury. The Na_v1.7 blocker protoxin II (ProTxII) or morphine injected intraperitoneally 15 min before or after the injury significantly reduces burn injury-induced spinal upregulation in phosphorylated serine 10 in histone H3 and phosphorylated extracellular signal-regulated kinase 1/2, which are both markers for spinal nociceptive processing. Further, ProTxII significantly reduces the frequency of spontaneous excitatory post-synaptic currents in spinal dorsal horn neurons following burn injury. Together, these findings indicate that using Na_v1.7 blockers should be considered to control pain in burn injury.

KEY MESSAGES

• Burn injury upregulates Na_v1.7 expression in primary sensory neurons. • Burn injury results in increased activity of Na_v1.7-expressing primary sensory neurons. • Inhibiting Na_v1.7 by protoxin II reduces spinal nociceptive processing. • Na_v1.7 represents a potential target to reduce pain in burn injury.

MSK1 regulates transcriptional induction of Arc/Arg3.1 in response to neurotrophins

The immediate early gene activity‐regulated cytoskeletal protein (Arc)/Arg3.1 and the neurotrophin brain‐derived neurotrophic factor (BDNF) play important roles in synaptic plasticity and learning and memory in the mammalian brain. However, the mechanisms by which BDNF regulates the expression of Arc/Arg3.1 are unclear. In this study, we show that BDNF acts via the ERK1/2 pathway to activate the nuclear kinase mitogen‐ and stress‐activated protein kinase 1 (MSK1). MSK1 then induces Arc/Arg3.1 expression via the phosphorylation of histone H3 at the Arc/Arg3.1 promoter. MSK1 can also phosphorylate the transcription factor cyclic‐AMP response element‐binding protein (CREB) on Ser133. However, this is not required for BDNF‐induced Arc.Arg3.1 transcription as a Ser133Ala knockin mutation had no effect on Arc/Arg3.1 induction. In parallel, ERK1/2 directly activates Arc/Arg3.1 mRNA transcription via at least one serum response element on the promoter, which bind a complex of the Serum Response Factor (SRF) and a Ternary Complex Factor (TCF).