Genetic targeting of ERK1 suggests a predominant role for ERK2 in murine pain models

ERK2-topoisomerase II regulatory axis is important for gene activation in immediate early genes

The function of the mitogen-activated protein kinase signaling pathway is required for the activation of immediate early genes (IEGs), including EGR1 and FOS, for cell growth and proliferation. Recent studies have identified topoisomerase II (TOP2) as one of the important regulators of the transcriptional activation of IEGs. However, the mechanism underlying transcriptional regulation involving TOP2 in IEG activation has remained unknown. Here, we demonstrate that ERK2, but not ERK1, is important for IEG transcriptional activation and report a critical ELK1 binding sequence for ERK2 function at the EGR1 gene. Our data indicate that both ERK1 and ERK2 extensively phosphorylate the C-terminal domain of TOP2B at mutual and distinctive residues. Although both ERK1 and ERK2 enhance the catalytic rate of TOP2B required to relax positive DNA supercoiling, ERK2 delays TOP2B catalysis of negative DNA supercoiling. In addition, ERK1 may relax DNA supercoiling by itself. ERK2 catalytic inhibition or knock-down interferes with transcription and deregulates TOP2B in IEGs. Furthermore, we present the first cryo-EM structure of the human cell-purified TOP2B and etoposide together with the EGR1 transcriptional start site (-30 to +20) that has the strongest affinity to TOP2B within -423 to +332. The structure shows TOP2B-mediated breakage and dramatic bending of the DNA. Transcription is activated by etoposide, while it is inhibited by ICRF193 at EGR1 and FOS, suggesting that TOP2B-mediated DNA break to favor transcriptional activation. Taken together, this study suggests that activated ERK2 phosphorylates TOP2B to regulate TOP2-DNA interactions and favor transcriptional activation in IEGs. We propose that TOP2B association, catalysis, and dissociation on its substrate DNA are important processes for regulating transcription and that ERK2-mediated TOP2B phosphorylation may be key for the catalysis and dissociation steps.

Wenshen-Jianpi prescription, a Chinese herbal medicine, improves visceral hypersensitivity in a rat model of IBS-D by regulating the MEK/ERK signal pathway

The goal of the study was to analyze whether WJP can alleviate visceral hypersensitivity in IBS-D model rats. In this study, 36 Sprague–Dawley (SD) rats aged 4 weeks old were randomly divided into two groups: the model group (n = 27) and the control group (n = 9). The rat model of IBS-D was established by modified compound methods for 4 weeks. After the modification, IBS-D rats were randomly divided into three groups, namely, the IBS-D model group (n = 9), the positive drug group (n = 9), and the WJP group (n = 9), with different interventions, respectively. The control group was fed and allowed to drink water routinely. The Bristol stool scale scores were used to assess the severity of diarrhea. Abdominal withdrawal reflex (AWR) scores were used to assess visceral sensitivity. Expression of TNF-α was measured, and histopathological examinations were performed to assess colon inflammation in IBS-D model rats. Key factors of the MEK/ERK signal pathway in the tissue of the colon and hippocampus were measured to analyze the mechanism of WJP. Compared with the control group, the Bristol stool scale scores in the model group were significantly increased (p < 0.0001). The scores of the WJP group were significantly decreased compared with the model group (p = 0.0001). Compared with the control group, AWR scores in the model group at each pressure level were significantly increased (p = 0.0003, p < 0.0001, p = 0.0007, and p = 0.0009). AWR scores of the WJP group were significantly decreased compared with the model group (p = 0.0003, p = 0.0007, p = 0.0007, and p = 0.0009). Compared with the control group, the model group had significantly higher expression of TNF-α in the colon tissue (p < 0.0001). However, the WJP group had significantly lower level of TNF-α compared with the model group (p < 0.0001). Meanwhile, compared with the control group, the relative expression of the proteins of p-MEK1/2, p-ERK1, and p-ERK2 in the colon tissue was significantly increased in the model group (p < 0.0001). Compared with the model group, the relative expression of the proteins in the colon tissue were significantly decreased in the WJP group (p < 0.0001, p = 0.0019, and p = 0.0013). Compared with the control group, the relative expression of the proteins of p-MEK1/2, p-ERK1, and p-ERK2 in the hippocampus tissue were significantly increased in the model group (p < 0.0001). Compared with the model group, the relative expression of the proteins in the hippocampus tissue were significantly decreased in the WJP group (p = 0.0126, p = 0.0291, and p = 0.0145). The results indicated that WJP can alleviate visceral hypersensitivity in IBS-D model rats, possibly mediated by downregulating the expression of TNF-α, p-MEK1/2, p-ERK1, and p-ERK2 in the colon tissue. At the same time, WJP also affects downregulating the expression of p-MEK1/2, p-ERK1, and p-ERK2 in the hippocampus tissue.

A compendium of validated pain genes

The development of novel pain therapeutics hinges on the identification and rigorous validation of potential targets. Model organisms provide a means to test the involvement of specific genes and regulatory elements in pain. Here we provide a list of genes linked to pain-associated behaviors. We capitalize on results spanning over three decades to identify a set of 242 genes. They support a remarkable diversity of functions spanning action potential propagation, immune response, GPCR signaling, enzymatic catalysis, nucleic acid regulation, and intercellular signaling. Making use of existing tissue and single-cell high-throughput RNA sequencing datasets, we examine their patterns of expression. For each gene class, we discuss archetypal members, with an emphasis on opportunities for additional experimentation. Finally, we discuss how powerful and increasingly ubiquitous forward genetic screening approaches could be used to improve our ability to identify pain genes. This article is categorized under: Neurological Diseases > Genetics/Genomics/Epigenetics Neurological Diseases > Molecular and Cellular Physiology.

Central, but not peripheral, nervous system ERK2 is essential for itch signals in murine allergic skin inflammation

BACKGROUND

Itch is a common cutaneous symptom in a variety of dermatological diseases, but detailed neuropathological mechanisms remain to be fully elucidated. This study aimed to assess in vivo ERK2 functions in the nervous system for itch responses.

METHODS

We generated conditional knockout mice deficient in ERK2 of the central nervous system (CNS) or peripheral nervous system (PNS), respectively, and assessed chemical and mechanical itch responses in vivo.

RESULTS

Chemical itch responses to histamine, but not to BAM8-22, were alleviated in CNS Erk2-deficient mice. In contrast, both histamine- and BAM8-22-induced mechanical itch (alloknesis) were alleviated in CNS Erk2-deficient mice. Neither chemical itch nor mechanical itch induced by these pruritogens was affected by PNS ERK2 deficiency. Spontaneous scratching behaviors during acute and chronic contact hypersensitivity were impaired in CNS Erk2-deficient mice, but not PNS Erk2-deficient mice. In addition, CNS ERK2 deficiency attenuated mechanical itch responses during chronic contact hypersensitivity. Again, PNS Erk2-deficient mice showed comparable responses of mechanical itch to control mice. In addition, alleviated mechanical itch in CNS Erk2-deficient mice was observed in IgE-mediated prurigo-like allergic skin inflammation. Mechanical itch induced by IL-31 was also alleviated by CNS ERK2 deficiency. Phosphorylated ERK1/2 was detected in neurokinin B-expressing cells of the spinal dorsal horn of control mice; these cells accumulated during the induction of chronic contact hypersensitivity. Notably, phosphorylated ERK1/2 was also localized in spinal urocortin3-expressing neurons that are known to transmit mechanical itch.

CONCLUSIONS

Spinal cord ERK2 could be a potential therapeutic target for intractable itch in pruritic skin diseases.

Extracellular signal-regulated kinases 2 (Erk2) and Erk5 in the central nervous system differentially contribute to central sensitization in male mice

Nervous systems are designed to become extra sensitive to afferent nociceptive stimuli under certain circumstances such as inflammation and nerve injury. How pain hypersensitivity comes about is key issue in the field since it ultimately results in chronic pain. Central sensitization represents enhanced pain sensitivity due to increased neural signaling within the central nervous system (CNS). Particularly, much evidence indicates that underlying mechanism of central sensitization is associated with the change of spinal neurons. Extracellular signal-regulated kinases have received attention as key molecules in central sensitization. Previously, we revealed the isoform-specific function of extracellular signal-regulated kinase 2 (Erk2) in spinal neurons for central sensitization using mice with Cre-loxP-mediated deletion of Erk2 in the CNS. Still, how extracellular signal-regulated kinase 5 (Erk5) in spinal neurons contributes to central sensitization has not been directly tested, nor is the functional relevance of Erk5 and Erk2 known. Here, we show that Erk5 and Erk2 in the CNS play redundant and/or distinct roles in central sensitization, depending on the plasticity context (cell types, pain types, time, etc.). We used male mice with Erk5 deletion specifically in the CNS and found that Erk5 plays important roles in central sensitization in a formalin-induced inflammatory pain model. Deletion of both Erk2 and Erk5 leads to greater attenuation of central sensitization in this model, compared to deletion of either isoform alone. Conversely, Erk2 but not Erk5 plays important roles in central sensitization in neuropathic pain, a type of chronic pain caused by nerve damage. Our results suggest the elaborate mechanisms of Erk signaling in central sensitization.

Exome Sequencing Implicates Impaired GABA Signaling and Neuronal Ion Transport in Trigeminal Neuralgia

Trigeminal neuralgia (TN) is a common, debilitating neuropathic face pain syndrome often resistant to therapy. The familial clustering of TN cases suggests that genetic factors play a role in disease pathogenesis. However, no unbiased, large-scale genomic study of TN has been performed to date. Analysis of 290 whole exome-sequenced TN probands, including 20 multiplex kindreds and 70 parent-offspring trios, revealed enrichment of rare, damaging variants in GABA receptor-binding genes in cases. Mice engineered with a TN-associated de novo mutation (p.Cys188Trp) in the GABA_A receptor Cl⁻ channel γ-1 subunit (GABRG1) exhibited trigeminal mechanical allodynia and face pain behavior. Other TN probands harbored rare damaging variants in Na⁺ and Ca⁺ channels, including a significant variant burden in the α-1H subunit of the voltage-gated Ca²⁺ channel Ca_v3.2 (CACNA1H). These results provide exome-level insight into TN and implicate genetically encoded impairment of GABA signaling and neuronal ion transport in TN pathogenesis.

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Genomic analysis of trigeminal neuralgia (TN) using exome sequencing

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Rare mutations in GABA signaling and ion transport genes are enriched in TN cases

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Generation of a genetic TN mouse model engineered with a patient-specific mutation

The spinal NR2BR/ERK2 pathway as a target for the central sensitization of collagen-induced arthritis pain

Objective

Pain management is a huge challenge in the treatment of rheumatoid arthritis (RA), and central sensitization is reportedly involved in the development of pain. The current study was undertaken to explore the possible role of N-methyl-D-aspartate receptors (NMDARs) in the spinal mechanism of central sensitization in RA using a collagen-induced arthritis (CIA) model.

Methods

Mechanical hypersensitivity was assessed in C57BL/6 mice, before and after the induction of CIA via administration of chick type II collagen. Analgesic drugs, receptor antagonist, and kinase inhibitor were administrated intrathecally in the spinal cord. Protein expression and phosphorylation changes were detected via immunoblotting.

Results

CIA mice developed significant mechanical hypersensitivity, and spinal administration of the NMDAR antagonist D-2-amino-5-phosphonovaleric acid (D-APV) effectively attenuated peripheral pain hypersensitivity. There was specific enhancement of synaptic NR2B-containing NMDAR (NR2BR) expression in the spinal dorsal horns of the mice. Both the increased total protein expression of NR2B subunit and the enhanced total phosphorylation level of NR2B subunit at 1472 tyrosine promoted the synaptic expression of NMDAR in the mice. Intrathecal injection of tramadol suppressed synaptic NMDAR expression mainly by changing the synaptic phosphorylation state of NR2B subunit at Tyr1472. Extracellular signal-regulated protein kinases 2 (ERK2) activity synchronized with the synaptic expression of NR2BR, which was downregulated by the action of tramadol.

Conclusion

Specific enhancement of NR2BR in the spinal dorsal horn may be vital for central sensitization in the CIA model of RA. The NR2BR/ERK2 pathway may be a promising target for pain management in RA patients.

Orai1 Plays a Crucial Role in Central Sensitization by Modulating Neuronal Excitability

Pathological pain is a common and debilitating condition that is often poorly managed. Central sensitization is an important mechanism underlying pathological pain. However, candidate molecules involved in central sensitization remain unclear. Store-operated calcium channels (SOCs) mediate important calcium signals in nonexcitable and excitable cells. SOCs have been implicated in a wide variety of human pathophysiological conditions, including immunodeficiency, occlusive vascular diseases, and cancer. However, the role of SOCs in CNS disorders has been relatively unexplored. Orai1, a key component of SOCs, is expressed in the human and rodent spinal cord dorsal horn, but its functional significance in dorsal horn neurons is poorly understood. Here we sought to explore a potential role of Orai1 in the modulation of neuronal excitability and A-type potassium channels involved in pain plasticity. Using both male and female Orai1 knock-out mice, we found that activation of Orai1 increased neuronal excitability and reduced A-type potassium channels via the protein kinase C-extracellular signal-regulated protein kinase (PKC-ERK) pathway in dorsal horn neurons. Orai1 deficiency significantly decreased acute pain induced by noxious stimuli, nearly eliminated the second phase of formalin-induced nociceptive response, markedly attenuated carrageenan-induced ipsilateral pain hypersensitivity and abolished carrageenan-induced contralateral mechanical allodynia. Consistently, carrageenan-induced increase in neuronal excitability was abolished in the dorsal horn from Orai1 mutant mice. These findings uncover a novel signaling pathway involved in the pain process and central sensitization. Our study also reveals a novel link among Orai1, ERK, A-type potassium channels, and neuronal excitability.SIGNIFICANCE STATEMENT Orai1 is a key component of store-operated calcium channels (SOCs) in many cell types. It has been implicated in such pathological conditions as immunodeficiency, autoimmunity, and cancer. However, the role of Orai1 in CNS disorders remains poorly understood. The functional significance of Orai1 in neurons is elusive. Here we demonstrate that activation of Orai1 modulates neuronal excitability and Kv4-containing A-type potassium channels via the protein kinase C-extracellular signal-regulated protein kinase (PKC-ERK) pathway. Genetic knock-out of Orai1 nearly eliminates the second phase of formalin-induced pain and markedly attenuates carrageenan-induced pain hypersensitivity and neuronal excitability. These findings reveal a novel link between Orai1 and neuronal excitability and advance our understanding of central sensitization.

Central Role of Protein Kinase A in Promoting Trigeminal Nociception in an In Vivo Model of Temporomandibular Disorders

AIMS

To investigate cellular changes in the spinal trigeminal nucleus (STN) and trigeminal ganglion (TG) associated with trigeminal nociception mediated by inflammation in the temporomandibular joint (TMJ).

METHODS

Male Sprague-Dawley rats (n = 86) were utilized to investigate cellular and behavioral responses to prolonged TMJ inflammation caused by bilateral injection of Complete Freund's Adjuvant (CFA) in the TMJ capsules. To investigate the cellular effects of protein kinase A (PKA) in the STN, rats were injected intrathecally with the selective PKA inhibitor KT5720 prior to injection of CFA into both TMJ capsules. Levels of calcitonin gene-related peptide (CGRP), active PKA, and ionized calcium-binding adapter molecule 1 (Iba1) in the STN and expression of phosphorylated extracellular regulated kinases (p-ERK) in the TG were determined with immunohistochemistry (n ≥ 3 experiments per test condition). Nocifensive head withdrawal responses to mechanical stimulation of the cutaneous tissue over the TMJ were monitored following CFA injection in the absence or presence of KT5720 (n = 7). Statistical analysis was performed using parametric analysis of variance (ANOVA) tests.

RESULTS

Intrathecal injection of KT5720 significantly inhibited the stimulatory effect of CFA on levels of CGRP, PKA, and Iba1 in the STN. In addition, administration of KT5720 decreased the average number of CFA-induced nocifensive withdrawal responses to mechanical stimulation and the CFA-mediated increase in p-ERK expression in the ganglion.

CONCLUSION

These findings provide evidence that elevated PKA activity in the STN promotes cellular events temporally associated with trigeminal nociception caused by prolonged TMJ inflammation.

Central amygdala activation of extracellular signal-regulated kinase 1 and age-dependent changes in inflammatory pain sensitivity in mice

Aging populations are more sensitive to noxious stimuli as a result of altered somatosensory systems. In these experiments, we examined pain-like behaviors in young, middle-aged, and old mice during peripheral inflammation to determine if the same sensitivity exists in preclinical animal models. Immediately following injury, middle-aged and old mice exhibited more spontaneous pain-like behaviors than young mice, matching pain prevalence in clinical populations. Middle-aged and old mice also developed persistent mechanical hypersensitivity in the injured paw. Furthermore, old mice developed mechanical hypersensitivity in the noninjured paw suggesting age-dependent changes in central nociceptive systems. To address this end, pain-related protein expression was examined in the central nucleus of the amygdala, a limbic brain region that modulates somatic pain. Following injury, increased phosphorylation of extracellular signal-regulated kinase 1, a protein with known nociceptive functions, was observed in the right central nucleus of the amygdala of old mice and not middle-aged or young animals. These findings suggest that age-dependent changes in supraspinal nociceptive systems may account for increased pain-like behaviors in aging populations.

Crosstalk between Smad2/3 and specific isoforms of ERK in TGF-β1-induced TIMP-3 expression in rat chondrocytes

This study investigated the roles of ERK1 and ERK2 in transforming growth factor‐β1 (TGF‐β1)‐induced tissue inhibitor of metalloproteinases‐3 (TIMP‐3) expression in rat chondrocytes, and the specific roles of ERK1 and ERK2 in crosstalk with Smad2/3 were investigated to demonstrate the molecular mechanism of ERK1/2 regulation of TGF‐β1 signalling. To examine the interaction of specific isoforms of ERK and the Smad2/3 signalling pathway, chondrocytes were infected with LV expressing either ERK1 or ERK2 siRNA and stimulated with or without TGF‐β1. At indicated time‐points, TIMP‐3 expression was determined by real‐time PCR and Western blotting; p‐Smad3, nuclear p‐Smad3, Smad2/3, p‐ERK1/2 and ERK1/2 levels were assessed. And then, aggrecan, type II collagen and the intensity of matrix were examined. TGF‐β1‐induced TIMP‐3 expression was significantly inhibited by ERK1 knock‐down, and the decrease in TIMP‐3 expression was accompanied by a reduction of p‐Smad3 in ERK1 knock‐down cells. Knock‐down of ERK2 had no effect on neither TGF‐β1‐induced TIMP‐3 expression nor the quantity of p‐Smad3. Moreover, aggrecan, type II collagen expression and the intensity of matrix were significantly suppressed by ERK1 knock‐down instead of ERK2 knock‐down. Taken together, ERK1 and ERK2 have different roles in TGF‐β1‐induced TIMP‐3 expression in rat chondrocytes. ERK1 instead of ERK2 can regulate TGF‐β/Smad signalling, which may be the mechanism through which ERK1 regulates TGF‐β1‐induced TIMP‐3 expression.

Activation of the Extracellular Signal-Regulated Kinase in the Amygdale Modulates Fentanyl-Induced Hypersensitivity in Rats

Opioid-induced hyperalgesia (OIH) is one of the major problems associated with use of opioids in perioperative and chronic pain management. The mechanism underlying this paradoxical phenomenon needs to be fully elucidated. Laterocapsular division of the central nucleus of amygdale (CeLC) has emerged as an important brain center for pain modulation, so we hypothesize that the activation of extracellular signal-regulated kinase (ERK) in CeLC may modulate OIH through strengthening synaptic transmission between neurons in the CeLC. Phospho-ERK in CeLC was first found to be increased significantly in OIH rats induced by repeated subcutaneous injection of fentanyl. Blockade of this fentanyl-induced ERK activation by microinjection of U0126, an ERK inhibitor, into the CeLC reversed the behavioral hypersensitivity in a dose-dependent manner. In vitro whole-cell recordings evaluating the change in synaptic transmission found that the frequency as well as amplitude of miniature excitatory postsynaptic currents recorded on CeLC neurons from OIH rats were fundamentally increased and were completely reversed by acutely applied U0126 (10 μM in the recording well). In vivo microinjection of U0126 into the CeLC reversed the spinal long-term potentiation in OIH rats. These results showed that fentanyl-induced hypersensitivity may occur partly through the mechanism of ERK activation and followed by the strengthening of synaptic transmission in CeLC neurons.

PERSPECTIVE

This study provides evidence that ERK in the laterocapsular division of the CeLC is a key contributor to the development of fentanyl-induced hypersensitivity. Targeting the superspinal central CeLC can inhibit spinal long-term potentiation and alleviate behavioral hyperreflexia induced by fentanyl.

The mitogen and stress-activated protein kinase 1 regulates the rapid epigenetic tagging of dorsal horn neurons and nocifensive behaviour

Phosphorylation of histone H3 at serine 10 (p-H3S10) is a marker of active gene transcription. Using cognitive models of neural plasticity, p-H3S10 was shown to be downstream of extracellular signal-regulated kinase (ERK) signalling in the hippocampus. In this study, we show that nociceptive signalling after peripheral formalin injection increased p-H3S10 expression in the ipsilateral dorsal horn. This increase was maximal 30 minutes after formalin injection and occurred mainly within p-ERK-positive neurons. Spinal p-H3S10-enhanced expression was also observed in neurokinin 1 receptor (NK1R), c-Fos, and Zif268 positive neurons and was inhibited by ablation of serotonergic descending controls. The mitogen and stress-activated protein kinase 1 (MSK1) is downstream of ERK and can induce p-H3S10. We found that, after formalin injection, most phospho-MSK1 (p-MSK1)-positive cells (87% ± 3%) expressed p-ERK and the majority of p-H3S10-positive cells (85% ± 5%) expressed p-MSK1. Inhibition of ERK activity with the MEK inhibitor SL327 reduced formalin-induced p-ERK, p-MSK1, and p-H3S10, demonstrating that spinal p-MSK1 and p-H3S10 were at least partly downstream of ERK signalling. Crucially, pharmacological blockade of spinal MSK1 activity with the novel MSK1 inhibitor SB727651A inhibited formalin-induced spinal p-H3S10 and nocifensive behaviour. These findings are the first to establish the involvement of p-H3S10 and its main kinase, MSK1, in ERK regulation of nociception. Given the general importance of ERK signalling in pain processing, our results suggest that p-H3S10 could play a role in the response to injury.

Hormonal and molecular effects of restraint stress on formalin-induced pain-like behavior in male and female mice

The evolutionary advantages to the suppression of pain during a stressful event (stress-induced analgesia (SIA)) are obvious, yet the reasoning behind sex-differences in the expression of this pain reduction are not. The different ways in which males and females integrate physiological stress responses and descending pain inhibition are unclear. A potential supraspinal modulator of stress-induced analgesia is the central nucleus of the amygdala (CeA). This limbic brain region is involved in both the processing of stress and pain; the CeA is anatomically and molecularly linked to regions of the hypothalamic pituitary adrenal (HPA) axis and descending pain network. The CeA exhibits sex-based differences in response to stress and pain that may differentially induce SIA in males and females. Here, sex-based differences in behavioral and molecular indices of SIA were examined following noxious stimulation. Acute restraint stress in male and female mice was performed prior to intraplantar injections of formalin, a noxious inflammatory agent. Spontaneous pain-like behaviors were measured for 60min following formalin injection and mechanical hypersensitivity was evaluated 120 and 180min post-injection. Restraint stress altered formalin-induced spontaneous behaviors in male and female mice and formalin-induced mechanical hypersensitivity in male mice. To assess molecular indices of SIA, tissue samples from the CeA and blood samples were collected at the 180min time point. Restraint stress prevented formalin-induced increases in extracellular signal regulated kinase 2 (ERK2) phosphorylation in the male CeA, but no changes associated with pERK2 were seen with formalin or restraint in females. Sex differences were also seen in plasma corticosterone concentrations 180min post injection. These results demonstrate sex-based differences in behavioral, molecular, and hormonal indices of acute stress in mice that extend for 180min after stress and noxious stimulation.

Redundancy in the World of MAP Kinases: All for One

The protein kinases ERK1 and ERK2 are the effector components of the prototypical ERK1/2 mitogen-activated protein (MAP) kinase pathway. This signaling pathway regulates cell proliferation, differentiation and survival, and is essential for embryonic development and cellular homeostasis. ERK1 and ERK2 homologs share similar biochemical properties but whether they exert specific physiological functions or act redundantly has been a matter of controversy. However, recent studies now provide compelling evidence in support of functionally redundant roles of ERK1 and ERK2 in embryonic development and physiology. In this review, we present a critical assessment of the evidence for the functional specificity or redundancy of MAP kinase isoforms. We focus on the ERK1/ERK2 pathway but also discuss the case of JNK and p38 isoforms.

ERK1 and ERK2 Map Kinases: Specific Roles or Functional Redundancy?

The MAP kinase signaling cascade Ras/Raf/MEK/ERK has been involved in a large variety of cellular and physiological processes that are crucial for life. Many pathological situations have been associated to this pathway. More than one isoform has been described at each level of the cascade. In this review we devoted our attention to ERK1 and ERK2, which are the effector kinases of the pathway. Whether ERK1 and ERK2 specify functional differences or are in contrast functionally redundant, constitutes an ongoing debate despite the huge amount of studies performed to date. In this review we compiled data on ERK1 vs. ERK2 gene structures, protein sequences, expression levels, structural and molecular mechanisms of activation and substrate recognition. We have also attempted to perform a rigorous analysis of studies regarding the individual roles of ERK1 and ERK2 by the means of morpholinos, siRNA, and shRNA silencing as well as gene disruption or gene replacement in mice. Finally, we comment on a recent study of gene and protein evolution of ERK isoforms as a distinct approach to address the same question. Our review permits the evaluation of the relevance of published studies in the field especially when measurements of global ERK activation are taken into account. Our analysis favors the hypothesis of ERK1 and ERK2 exhibiting functional redundancy and points to the concept of the global ERK quantity, and not isoform specificity, as being the essential determinant to achieve ERK function.

Antinociceptive Effect of Tephrosia sinapou Extract in the Acetic Acid, Phenyl-p-benzoquinone, Formalin, and Complete Freund's Adjuvant Models of Overt Pain-Like Behavior in Mice

Tephrosia toxicaria, which is currently known as Tephrosia sinapou (Buc'hoz) A. Chev. (Fabaceae), is a source of compounds such as flavonoids. T. sinapou has been used in Amazonian countries traditional medicine to alleviate pain and inflammation. The purpose of this study was to evaluate the analgesic effects of T. sinapou ethyl acetate extract in overt pain-like behavior models in mice by using writhing response and flinching/licking tests. We demonstrated in this study that T. sinapou extract inhibited, in a dose (1-100 mg/kg) dependent manner, acetic acid- and phenyl-p-benzoquinone- (PBQ-) induced writhing response. Furthermore, it was active via intraperitoneal, subcutaneous, and peroral routes of administration. T. sinapou extract also inhibited formalin- and complete Freund's adjuvant- (CFA-) induced flinching/licking at 100 mg/kg dose. In conclusion, these findings demonstrate that T. sinapou ethyl acetate extract reduces inflammatory pain in the acetic acid, PBQ, formalin, and CFA models of overt pain-like behavior. Therefore, the potential of analgesic activity of T. sinapou indicates that it deserves further investigation.

Spinophilin-Targeted Protein Phosphatase-1 Alleviated Inflammatory Pain by Negative Control of MEK/ERK Signaling in Spinal Cord Dorsal Horn of Rats

Protein phosphatase-1 (PP1), anchored by regulatory or targeting proteins at excitatory glutamatergic synapses, controls the phosphorylation of postsynaptic substrates and regulates the neurotransmission and plasticity. Here, we found that spinophilin, an actin-binding protein that targets PP1 at postsynaptic density, served as a scaffold for extracellular signal-regulated kinase (ERK) signaling components. Through the C-terminal PDZ domain, spinophilin directly interacted with ERK and its upstream mitogen-activated protein kinase kinase (MEK). PP1, recruited by spinophilin, gained access to and dephosphorylated these kinases, exerting a tonic inhibition of ERK signaling. The removal of PP1 inhibition by disturbing spinophilin/PP1 interaction allowed a restricted activation of MEK/ERK at synapses, which in turn augmented the synaptic transmission specifically mediated by GluN2B subunit-containing N-methyl-d-aspartate subtype of glutamate receptors. We provided evidence that in pain-related spinal cord dorsal horn, the scaffolding function of spinophilin played an important role in the negative control of ERK-dependent and GluN2B-dependent pain sensitization. Expression of wild-type spinophilin produced an effective analgesic action against chronic inflammatory pain induced by complete Freund's adjuvant in rats.

SIGNIFICANCE STATEMENT

Extracellular signal-regulated kinase (ERK) relays the signals from multiple transmembrane receptors to a wide range of downstream effectors critical for the regulation of neuronal excitability and plasticity. The strength and duration of ERK signaling is spatiotemporally controlled by protein phosphatases. Sustained activation of ERK has been implicated in a variety of pathological processes. The current study revealed that spinophilin, a well characterized protein phosphatase 1 (PP1) synaptic targeting protein, was able to scaffold mitogen-activated protein kinase kinase (MEK) and ERK for dephosphorylation and inactivation by PP1. The loss of PP1 inhibition, as a result of spinophilin/PP1 dissociation, led to aberrant activation of MEK/ERK signaling, which had important implications for the exaggeration of NMDA receptor-dependent nociceptive synaptic transmission in spinal cord dorsal horn.

ERK2 Alone Drives Inflammatory Pain But Cooperates with ERK1 in Sensory Neuron Survival

Extracellular signal-regulated kinases 1 and 2 (ERK1/2) are highly homologous yet distinct components of signal transduction pathways known to regulate cell survival and function. Recent evidence indicates an isoform-specific role for ERK2 in pain processing and peripheral sensitization. However, the function of ERK2 in primary sensory neurons has not been directly tested. To dissect the isoform-specific function of ERK2 in sensory neurons, we used mice with Cre-loxP-mediated deletion of ERK2 in Nav1.8(+) sensory neurons that are predominantly nociceptors. We find that ERK2, unlike ERK1, is required for peripheral sensitization and cold sensation. We also demonstrate that ERK2, but not ERK1, is required to preserve epidermal innervation in a subset of peptidergic neurons. Additionally, deletion of both ERK isoforms in Nav1.8(+) sensory neurons leads to neuron loss not observed with deletion of either isoform alone, demonstrating functional redundancy in the maintenance of sensory neuron survival. Thus, ERK1 and ERK2 exhibit both functionally distinct and redundant roles in sensory neurons.

SIGNIFICANCE STATEMENT

ERK1/2 signaling affects sensory neuron function and survival. However, it was not clear whether ERK isoform-specific roles exist in these processes postnatally. Previous work from our laboratory suggested either functional redundancy of ERK isoforms or a predominant role for ERK2 in pain; however, the tools to discriminate between these possibilities were not available at the time. In the present study, we use new genetic knock-out lines to demonstrate that ERK2 in sensory neurons is necessary for development of inflammatory pain and for postnatal maintenance of peptidergic epidermal innervation. Interestingly, postnatal loss of both ERK isoforms leads to a profound loss of sensory neurons. Therefore, ERK1 and ERK2 display both functionally distinct and redundant roles in sensory neurons.

Transient Blockade of ERK Phosphorylation in the Critical Period Causes Autistic Phenotypes as an Adult in Mice

The critical period is a distinct time-window during the neonatal stage when animals display elevated sensitivity to certain environmental stimuli, and particular experiences can have profound and long-lasting effects on behaviors. Increasing evidence suggests that disruption of neuronal activity during the critical period contributes to autistic phenotype, although the pathogenic mechanism is largely unknown. Herein we show that extracellular signal-regulated protein kinases (ERKs) play important roles in proper formation of neural circuits during the critical period. Transient blockade of ERKs phosphorylation at postnatal day 6 (P6) by intraperitoneal injection of blood-brain barrier-penetrating MEK inhibitor, α-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl)benzeneacetonitrile (SL327) caused significant increase of apoptosis in the forebrain. Furthermore, this induced long-term deleterious effects on brain functioning later in adulthood, resulting in social deficits, impaired memory and reduced long-term potentiation (LTP). Conversely, blockade of ERK phosphorylation at P14 no longer induced apoptosis, nor behavioral deficits, nor the reduced LTP. Thus, surprisingly, these effects of ERKs are strongly age-dependent, indicating that phosphorylation of ERKs during the critical period is absolutely required for proper development of brain functioning. This study provides novel insight into the mechanistic basis for neurodevelopment disorders: various neurodevelopment disorders might be generally linked to defects in ERKs signaling during the critical period.

Nicotine stimulates expression of proteins implicated in peripheral and central sensitization

Pain patients who are nicotine dependent report a significantly increased incidence and severity of pain intensity. The goal of this study was to determine the effects of prolonged nicotine administration on inflammatory proteins implicated in the development of peripheral and central sensitization of the trigeminal system. Behavioral, immunohistochemical, and microarray studies were utilized to investigate the effects of nicotine administered daily for 14 days via an Alzet® osmotic pump in Sprague Dawley rats. Systemic nicotine administration caused a significant increase in nocifensive withdrawals to mechanical stimulation of trigeminal neurons. Nicotine stimulated expression of the pro-inflammatory signal transduction proteins phosphorylated-extracellular signal-regulated kinase (p-ERK), phosphorylated-c-Jun N-terminal kinase (p-JNK), and protein kinase A (PKA) in the spinal trigeminal nucleus. Nicotine also promoted elevations in the expression of glial fibrillary acidic protein (GFAP), a biomarker of activated astrocytes, and the microglia biomarker ionized calcium-binding adapter molecule 1 (Iba1). Similarly, levels of eleven cytokines were significantly elevated with the largest increase in expression of TNF-α. Levels of PKA, p-ERK, and p-JNK in trigeminal ganglion neurons were increased by nicotine. Our findings demonstrate that prolonged systemic administration of nicotine promotes sustained behavioral and cellular changes in the expression of key proteins in the spinal trigeminal nucleus and trigeminal ganglion implicated in the development and maintenance of peripheral and central sensitization.

GABAergic Inhibition Regulated Pain Sensitization through STEP61 Signaling in Spinal Dorsal Horn of Mice

Background:

The reduction of γ-aminobutyric acid (GABA) type A receptor–mediated inhibition has long been implicated in spinal sensitization of nociceptive responses. However, it is largely unknown which signaling cascades in spinal dorsal horn neurons are initiated by the reduced inhibition to trigger pain hypersensitivity.

Methods:

GABAergic inhibition was manipulated by intrathecal application of GABA type A receptor antagonist bicuculline in intact mice or by GABA type A receptor agonist muscimol in complete Freund’s adjuvant–injected mice. Immunoblotting, coimmunoprecipitation, immunohistochemistry, and behavioral tests were used to explore the signaling pathways downstream of the altered GABAergic tone.

Results:

The study data revealed that the 61-kD isoform of striatal-enriched protein phosphatase (STEP61) was a key molecule that relayed the signals from GABAergic neurotransmission. The authors found that STEP61 was highly expressed in dorsal horn neurons. Under physiological conditions, STEP61 tonically interacted with and negatively controlled the activities of extracellular signal–regulated kinase and Src-family protein tyrosine kinases member Fyn, two critical kinases involved in spinal sensitization. Once GABAergic inhibition was impaired, STEP61 interaction with its substrates was substantially disturbed, allowing for activation of extracellular signal–regulated kinase and Fyn (n = 4 to 6). The hyperactivities of extracellular signal–regulated kinase and Fyn, along with STEP61 dysregulation, caused the tyrosine phosphorylation and synaptic accumulation of GluN2B subunit-containing N-methyl-d-aspartate subtype of glutamate receptors (n = 6), leading to GluN2B receptor-dependent pain hypersensitivity. Overexpression of wild-type STEP61 to resume its enzymatic activity significantly blocked the mechanical allodynia evoked by bicuculline and more importantly, alleviated chronic inflammatory pain (n = 6 in each group).

Conclusion:

These data identified STEP61 as a key intermediary for GABAergic inhibition to regulate pain sensitization.

δ-opioid receptor and somatostatin receptor-4 heterodimerization: possible implications in modulation of pain associated signaling

Pain relief is the principal action of opioids. Somatostatin (SST), a growth hormone inhibitory peptide is also known to alleviate pain even in cases when opioids fail. Recent studies have shown that mice are prone to sustained pain and devoid of analgesic effect in the absence of somatostatin receptor 4 (SSTR4). In the present study, using brain slices, cultured neurons and HEK-293 cells, we showed that SSTR4 and δ-Opioid receptor (δOR) exist in a heteromeric complex and function in synergistic manner. SSTR4 and δOR co-expressed in cortical/striatal brain regions and spinal cord. Using cultured neuronal cells, we describe the heterogeneous complex formation of SSTR4 and δOR at neuronal cell body and processes. Cotransfected cells display inhibition of cAMP/PKA and co-activation of SSTR4 and δOR oppose receptor trafficking induced by individual receptor activation. Furthermore, downstream signaling pathways either associated with withdrawal or pain relief are modulated synergistically with a predominant role of SSTR4. Inhibition of cAMP/PKA and activation of ERK1/2 are the possible cellular adaptations to prevent withdrawal induced by chronic morphine use. Our results reveal direct intra-membrane interaction between SSTR4 and δOR and provide insights for the molecular mechanism for the anti-nociceptive property of SST in combination with opioids as a potential therapeutic approach to avoid undesirable withdrawal symptoms.

PTEN/PI3K and MAPK signaling in protection and pathology following CNS injuries

Brain and spinal cord injuries initiate widespread temporal and spatial neurodegeneration, through both necrotic and programmed cell death mechanisms. Inflammation, reactive oxidation, excitotoxicity and cell-specific dysregulation of metabolic processes are instigated by traumatic insult and are main contributors to this cumulative damage. Successful treatments rely on prevention or reduction of the magnitude of disruption, and interfering with injurious cellular responses through modulation of signaling cascades is an effective approach. Two intracellular signaling pathways, the phosphatase and tensin homolog (PTEN)/phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signaling cascades play various cellular roles under normal and pathological conditions. Activation of both pathways can influence anatomical and functional outcomes in multiple CNS disorders. However, some mechanisms involve inhibiting or enhancing one pathway or the other, or both, in propagating specific downstream effects. Though many intracellular mechanisms contribute to cell responses to insult, this review examines the evidence exploring PTEN/PI3K and MAPK signaling influence on pathology, neuroprotection, and repair and how these pathways may be targeted for advancing knowledge and improving neurological outcome after injury to the brain and spinal cord.

Assessment of pain and itch behavior in a mouse model of neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is characterized primarily by tumor formation in the nervous system, but patients report other neurological complications including pain and itch. Individuals with NF1 harbor 1 mutated NF1 allele causing heterozygous expression in all of their cells. In mice, Nf1 heterozygosity leads to hyperexcitability of sensory neurons and hyperproliferation of mast cells, both of which could lead to increased hypersensitivity and scratching in response to noxious and pruritic stimuli. To determine whether Nf1 heterozygosity may increase pain and itch behaviors independent of secondary effects of tumor formation, we used mice with a targeted, heterozygous Nf1 gene deletion (Nf1±) that lack tumors. Nf1± mice exhibited normal baseline responses to thermal and mechanical stimuli. Moreover, similar to wild-type littermates, Nf1± mice developed inflammation-induced heat and mechanical hypersensitivity, capsaicin-induced nocifensive behavior, histamine-dependent or -independent scratching, and chronic constriction injury-induced cold allodynia. However, Nf1± mice exhibited an attenuated first phase of formalin-induced spontaneous behavior and expedited resolution of formalin-induced heat hypersensitivity. These results are not consistent with the hypothesis that Nf1 heterozygosity alone is sufficient to increase pain and itch sensation in mice, and they suggest that additional mechanisms may underlie reports of increased pain and itch in NF1 patients.

PERSPECTIVE

This study assessed whether Nf1 heterozygosity in mice increased hypersensitivity and scratching following noxious and pruritic stimuli. Using Nf1± mice lacking tumors, this study finds no increases in pain or itch behavior, suggesting that there is no predisposition for either clinical symptom solely due to Nf1 heterozygosity.

REM Sleep Regulating Mechanisms in the Cholinergic Cell Compartment of the Brainstem

Rapid eye movement (REM) sleep is a highly evolved yet paradoxical behavioral state (highly activated brain in a paralyzed body) in mammalian species. Since the discovery of REM sleep and its physiological distinction from other sleep states¹, a vast number of studies in neurosciences have been dedicated toward understanding the mechanisms and functions of this behavioral state. Collectively, studies have shown that each of the physiological events that characterize the behavioral state of REM sleep is executed by distinct cell groups located in the brainstem. These cell groups are discrete components of a widely distributed network, rather than a single REM sleep center. The final activity within each of these executive cell groups is controlled by the ratio of cholinergic neurotransmission emanating from the pedunculopontine tegmentum (PPT) to aminergic neurotransmission emanating from the locus coeruleus (LC) and raphe nucleus (RN). In this review, we summarize the most recent findings on the cellular and molecular mechanisms in the PPT cholinergic cell compartment that underlie the regulation of REM sleep. This up-to-date review should allow clinicians and researchers to better understand the effects of drugs and neurologic disease on REM sleep.

The antinociceptive effects of nicotinic receptors α7-positive allosteric modulators in murine acute and tonic pain models

The α7 nicotinic acetylcholine receptor (nAChR) subtype is abundantly expressed in the central nervous system and in the periphery. Recent evidence suggests that α7 nAChR subtypes, which can be activated by an endogenous cholinergic tone, comprising acetylcholine and the α7 nAChR agonist choline, play an important role in subchronic pain and inflammation. This study's objective was to test whether α7 nAChR positive allosteric modulators (PAMs) produce antinociception in in vivo mouse models of acute and persistent pain. Testing type I [N-(5-chloro-2-hydroxyphenyl)-N'-[2-chloro-5-(trifluoromethyl)phenyl] (NS1738)] and type II [1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxazol-3-yl) (PNU-120596)] α7 nAChR PAMs in acute and persistent pain, we found that, although neither reduced acute thermal pain, only PNU-120596 dose-dependently attenuated paw-licking behavior in the formalin test. The long-acting effect of PNU-120596 in this test was in discordance with its pharmacokinetic profile in mice, which suggests the involvement of postreceptor signaling mechanisms. Our results with selective mitogen-activated protein kinase kinase inhibitor 1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene monoethanolate (U0126) argues for an important role of extracellular signal-regulated kinase-1/2 pathways activation in PNU-120596's antinociceptive effects. The α7 antagonist MLA, administered intrathecally, reversed PNU-120596's effects, confirming PNU-120596's action, in part, through central α7 nAChRs. Importantly, tolerance to PNU-120596 was not developed after subchronic treatment of the drug. Surprisingly, PNU-120596's antinociceptive effects were blocked by NS1738. Our results indicate that type II α7 nAChR PAM PNU-120596, but not type I α7 nAChR PAM NS1738, shows significant antinociception effects in persistent pain models in mice.

The role of extracellular signal-related kinase during abdominal aortic aneurysm formation

BACKGROUND

It is hypothesized that activation of extracellular signal-related kinase (ERK) is critical in activating matrix metalloproteinases (MMPs) during abdominal aortic aneurysm (AAA) formation.

STUDY DESIGN

C57BL/6 male mice underwent either elastase or heat-inactivated elastase aortic perfusion (n = 9 per group). Mouse aortic smooth muscle cells were transfected with ERK-1 and 2 siRNA along with or without elastase treatment. Mouse and human aortic tissue were analyzed by Western blots, zymograms, and immunohistochemistry, and statistical analysis was done using Graphpad and Image J softwares.

RESULTS

Western blot and immunohistochemistry documented increased phospho-mitogen-activated protein kinase kinase-1/2 (pMEK-1/2; 153%, p = 0.270 by Western) and pERK (171%, p = 0.004 by Western blot) in the elastase perfused aortas. Male ERK-1(-/-) mice underwent elastase perfusion, and aortic diameter was determined at day 14. ERK-1(-/-) mice failed to develop AAA, and histologic analysis depicted intact collagen and elastin fibers in the aortas. Zymography of aortas of elastase-treated ERK-1(-/-) mice showed lower levels of proMMP2 (p < 0.005) and active MMP2 (p < 0.0001), as well as proMMP9 (p = 0.037) compared with C57BL/6 mice. siRNA transfection of ERK-1 and -2 significantly reduced formation of pro- and active MMP2 (p < 0.01 for both isoforms) in aortic smooth muscle cells treated with elastase in vitro. Human AAA tissue had significantly elevated levels of pMEK-1/2 (150%, p = 0.014) and pERK (159%, p = 0.013) compared with control tissues.

CONCLUSIONS

The MAPK (mitogen-activated protein kinase)/ERK pathway is an important modulator of MMPs during AAA formation. Targeting the ERK pathway by reagents that inhibit either the expression or phosphorylation of ERK isoforms could be a potential therapy to prevent AAA formation.

Mechanical allodynia but not thermal hyperalgesia is impaired in mice deficient for ERK2 in the central nervous system

Extracellular signal-regulated kinase (ERK) plays critical roles in pain plasticity. However, the specific contribution of ERK2 isoforms to pain plasticity is not necessarily elucidated. Here we investigate the function of ERK2 in mouse pain models. We used the Cre-loxP system to cause a conditional, region-specific, genetic deletion of Erk2. To induce recombination in the central nervous system, Erk2-floxed mice were crossed with nestin promoter-driven cre transgenic mice. In the spinal cord of resultant Erk2 conditional knockout (CKO) mice, ERK2 expression was abrogated in neurons and astrocytes, but indistinguishable in microglia compared to controls. Although Erk2 CKO mice showed a normal baseline paw withdrawal threshold to mechanical stimuli, these mice had a reduced nociceptive response following a formalin injection to the hind paw. In a partial sciatic nerve ligation model, Erk2 CKO mice showed partially restored mechanical allodynia compared to control mice. Interestingly, thermal hyperalgesia was indistinguishable between Erk2 CKO and control mice in this model. In contrast to Erk2 CKO mice, mice with a targeted deletion of ERK1 did not exhibit prominent anomalies in these pain models. In Erk2 CKO mice, compensatory hyperphosphorylation of ERK1 was detected in the spinal cord. However, ERK1 did not appear to influence nociceptive processing because the additional inhibition of ERK1 phosphorylation using MEK (MAPK/ERK kinase) inhibitor SL327 did not produce additional changes in formalin-induced spontaneous behaviors in Erk2 CKO mice. Together, these results indicate that ERK2 plays a predominant and/or specific role in pain plasticity, while the contribution of ERK1 is limited.

Isozyme-specific effects of protein kinase C in pain modulation

BACKGROUND

Protein kinase C (PKC) is a family of serine/threonine kinases that contains more than 10 isozymes. Evidence suggests that PKC may play important roles in pain modulation, but the isozyme-specific effects of PKC on different aspects of pain modulation are not fully understood. We hypothesize that different PKC isozymes play different roles in different aspects of pain modulation.

METHODS

The nociceptive behaviors of mice with deletion of PKCα, β, γ, or δ in multiple pain models were compared with their respective wild-type littermates. Also, morphine analgesia and the development of morphine tolerance in mice with deletion of PKCγ were compared with their respective wild-type littermates.

RESULTS

Thermal hyperalgesia induced by complete Freund's adjuvant injection was significantly attenuated by the deletion of PKCβ, γ, or δ, but not PKCα. Deletion of PKCγ significantly attenuated neuropathic mechanical allodynia induced by spared nerve injury, whereas deletion of PKCα enhanced this allodynia. Baseline thermal and mechanical sensitivity, nociceptive behaviors induced by formalin, mechanical allodynia induced by complete Freund's adjuvant injection, were not altered by deletion of PKCα, β, γ, or δ. Finally, morphine analgesia and the development of morphine tolerance were not altered in PKCγ-deficient mice.

CONCLUSIONS

PKC has isozyme-specific effects in pain modulation.

Selective inhibition of extracellular signal-regulated kinases 1/2 blocks nerve growth factor to brain-derived neurotrophic factor signaling and suppresses the development of and reverses already established pain behavior in rats

Brain-derived neurotrophic factor (BDNF) plays a key role in the development of pathological pain. Although it is known that nerve growth factor (NGF) induces BDNF mRNA through extracellular signal-regulated kinases (ERK), whether ERK1/2 or ERK5, two closely related members of the ERK family, mediate this signal is still unclear because classical MEK inhibitors block both pathways. We studied the involvement of ERK-signaling in NGF induction of BDNF in PC12 cells, cultured dorsal root ganglia neurons, and in rats subjected to neuropathic pain models using ERK1/2- and ERK5-specific tools. Selective activation of ERK1/2 upregulated BDNF mRNA in PC12 cells, whereas selective ERK5 activation did not. AZD6244, a potent selective inhibitor of ERK1/2 activation, blocked NGF induction of BDNF mRNA in vitro suggesting that NGF induction of BDNF is mediated by ERK1/2. siRNA experiments indicated that both ERK1 or ERK2 can signal suggesting that both pathways must be blocked to prevent NGF-induced increase in BDNF mRNA. I.p. injection of AZD6244 prevented the development of pain in rats subjected to the chronic constriction injury and reversed already established pain in the spared nerve injury model. Immunohistochemical studies showed decreased phospho-ERK1/2-immunoreactivity in dorsal root ganglia and BDNF immunoreactivity in ipsilateral spinal dorsal horn in the drug-treated rats. Our results suggest the possible use of AZD6244, already in human clinical trials as an anticancer agent, for the treatment of pathological pain.

ERK pathway activation bidirectionally affects visual recognition memory and synaptic plasticity in the perirhinal cortex

ERK 1,2 pathway mediates experience-dependent gene transcription in neurons and several studies have identified its pivotal role in experience-dependent synaptic plasticity and in forms of long term memory involving hippocampus, amygdala, or striatum. The perirhinal cortex (PRHC) plays an essential role in familiarity-based object recognition memory. It is still unknown whether ERK activation in PRHC is necessary for recognition memory consolidation. Most important, it is unknown whether by modulating the gain of the ERK pathway it is possible to bidirectionally affect visual recognition memory and PRHC synaptic plasticity. We have first pharmacologically blocked ERK activation in the PRHC of adult mice and found that this was sufficient to impair long term recognition memory in a familiarity-based task, the object recognition task (ORT). We have then tested performance in the ORT in Ras-GRF1 knock-out (KO) mice, which exhibit a reduced activation of ERK by neuronal activity, and in ERK1 KO mice, which have an increased activation of ERK2 and exhibit enhanced striatal plasticity and striatal mediated memory. We found that Ras-GRF1 KO mice have normal short term memory but display a long term memory deficit; memory reconsolidation is also impaired. On the contrary, ERK1 KO mice exhibit a better performance than WT mice at 72 h retention interval, suggesting a longer lasting recognition memory. In parallel with behavioral data, LTD was strongly reduced and LTP was significantly smaller in PRHC slices from Ras-GRF1 KO than in WT mice while enhanced LTP and LTD were found in PRHC slices from ERK1 KO mice.

TRPV1-dependent and -independent alterations in the limbic cortex of neuropathic mice: impact on glial caspases and pain perception

During neuropathic pain, caspases are activated in the limbic cortex. We investigated the role of TRPV1 channels and glial caspases in the mouse prelimbic and infralimbic (PL-IL) cortex after spared nerve injury (SNI). Reverse transcriptase-polymerase chain reaction, western blots, and immunfluorescence showed overexpression of several caspases in the PL-IL cortex 7 days postinjury. Caspase-3 release and upregulation of AMPA receptors in microglia, caspase-1 and IL-1β release in astrocytes, and upregulation of Il-1 receptor-1, TRPV1, and VGluT1 in glutamatergic neurons, were also observed. Of these alterations, only those in astrocytes persisted in SNI Trpv1(-/-) mice. A pan-caspase inhibitor, injected into the PL-IL cortex, reduced mechanical allodynia, this effect being reduced but not abolished in Trpv1(-/-) mice. Single-unit extracellular recordings in vivo following electrical stimulation of basolateral amygdala or application of pressure on the hind paw, showed increased excitatory pyramidal neuron activity in the SNI PL-IL cortex, which also contained higher levels of the endocannabinoid 2-arachidonoylglycerol. Intra-PL-IL cortex injection of mGluR5 and NMDA receptor antagonists and AMPA exacerbated, whereas TRPV1 and AMPA receptor antagonists and a CB(1) agonist inhibited, allodynia. We suggest that SNI triggers both TRPV1-dependent and independent glutamate- and caspase-mediated cross-talk among IL-PL cortex neurons and glia, which either participates or counteracts pain.

Activation of extracellular signal-regulated kinase signaling in the pedunculopontine tegmental cells is involved in the maintenance of sleep in rats

Considerable evidence suggests that receptor-mediated excitation and inhibition of brainstem pedunculopontine tegmental (PPT) neurons are critically involved in the regulation of sleep-wake states. However, the molecular mechanisms operating within the PPT-controlling sleep-wake states remain relatively unknown. This study was designed to examine sleep-wake state-associated extracellular-signal-regulated kinase 1 and 2 (ERK1/2) transduction changes in the PPT of freely moving rats. The results of this study demonstrate that the levels of ERK1/2 expression, phosphorylation, and activity in the PPT increased with increased amount of time spent in sleep. The sleep-associated increases in ERK1/2 expression, phosphorylation, and activity were not observed in the cortex, or in the immediately adjacent medial pontine reticular formation. The results of regression analyses revealed significant positive relationships between the levels of ERK1/2 expression, phosphorylation, and activity in the PPT and amounts of time spent in slow-wave sleep, rapid eye movement sleep, and total sleep. Additionally, these regression analyses revealed significant negative relationships between the levels of ERK1/2 expression, phosphorylation, and activity in the PPT and amounts of time spent in wakefulness. Collectively, these results, for the first time, suggest that the increased ERK1/2 signaling in the PPT is associated with maintenance of sleep via suppression of wakefulness.