Interleukin-1β Plays Key Roles in LPA-Induced Amplification of LPA Production in Neuropathic Pain Model

Lysophosphatidic acid (LPA) is a bioactive lipid mediator that exerts a wide range of biological actions. In recent decades, LPA has been demonstrated as an important initiator of neuropathic pain based on the mechanisms of LPA-induced feed-forward LPA amplification. In this study, we examined the possible involvement of interleukin (IL)-1β in such LPA production. Intrathecal (i.t.) LPA injection rapidly increased the expression of IL-1β mRNA in the spinal dorsal horn as early as 0.5 h after injection, and the level reached peak at 2 h. Through a developed quantitative mass spectrometry for detecting LPA species, the elevated levels of 18:1, 16:0, and 18:0 LPA in the spinal dorsal horn were observed at 3 h after 18:1 LPA injection and this elevation was completely blocked by the pretreatment of IL-1β-neutralizing antibody. Moreover, enzyme assay experiments showed that LPA (i.t.) significantly activated calcium-independent phospholipase A₂ (iPLA₂) and cytosolic phospholipase A₂ (cPLA₂) in the spinal dorsal horn at 1 and 2 h, respectively, and these biochemical changes were also significantly inhibited by IL-1β-neutralizing antibody. Similarly, IL-1β-neutralizing antibody reversed LPA-induced neuropathic pain-like behavior. These findings suggest that the early release of IL-1β is involved in LPA-induced amplification of LPA production, which underlies the initial mechanisms of LPA-induced neuropathic pain.     

LPA-mediated demyelination in ex vivo culture of dorsal root

Lysophosphatidic acid (LPA) causes neuropathic pain with demyelination in sensory fibers. In dorsal root (DR) ex vivo culture, the addition of 0.1 microM LPA caused a characteristic demyelination at 24h in scanning and transmission electron microscopy analyses. Moreover, direct contact between C-fibers due to loss of partition by Schwann cell in Remak bundles was observed. LPA-induced demyelination of DR was concentration-dependent in the range between 0.01 and 1M, and was abolished by BoNT/C3 and Y-27632, a RhoA and Rho kinase inhibitor, respectively. The demyelination was equivalent between the preparations with and without dorsal root ganglion. LPA also caused a down-regulation of myelin proteins, such as myelin basic protein (MBP) and myelin protein zero (MPZ) to approximately 70% of control. All these findings suggest that the demyelination observed in the neuropathic pain due to nerve injury occurs through a direct action of LPA on Schwann cells.     

Lysophosphatidic acid-induced membrane ruffling and brain-derived neurotrophic factor gene expression are mediated by ATP release in primary microglia

We examined the effects of lysophosphatidic acid (LPA) on microglia, which may play an important role in the development and maintenance of neuropathic pain. LPA caused membrane ruffling as detected by scanning electron microscopy, and increased the expression of brain-derived neurotrophic factor (BDNF) in a primary culture of rat microglia, which express LPA(3), but not LPA(1) or LPA(2) receptors. These actions were inhibited by a Galpha(q/11)-antisense oligodeoxynucleotide (AS-ODN), U73122, an inhibitor of phospholipase C (PLC), and apyrase, which specifically degrades ATP and ADP. When ATP release was measured using a luciferin-luciferase bioluminescence assay, LPA was shown to increase it in an LPA(3) and PLC inhibitor-reversible manner. However, LPA-induced ATP release was also blocked by the Galpha(q/11) AS-ODN, but not by pertussis toxin. These results suggest that LPA induces the release of ATP from rat primary cultured microglia via the LPA(3) receptor, Galpha(q/11) and PLC, and that the released ATP or ectopically converted ADP may in turn cause membrane ruffling via P2Y(12) receptors and Galpha(i/o) activation, and BDNF expression via activation of P2X(4) receptors.     

The Analgesic Effect on Neuropathic Pain of Retrogradely Transported botulinum Neurotoxin A Involves Schwann Cells and Astrocytes

In recent years a growing debate is about whether botulinum neurotoxins are retrogradely transported from the site of injection. Immunodetection of cleaved SNAP-25 (cl-SNAP-25), the protein of the SNARE complex targeted by botulinum neurotoxin serotype A (BoNT/A), could represent an excellent approach to investigate the mechanism of action on the nociceptive pathways at peripheral and/or central level. After peripheral administration of BoNT/A, we analyzed the expression of cl-SNAP-25, from the hindpaw’s nerve endings to the spinal cord, together with the behavioral effects on neuropathic pain. We used the chronic constriction injury of the sciatic nerve in CD1 mice as animal model of neuropathic pain. We evaluated immunostaining of cl-SNAP-25 in the peripheral nerve endings, along the sciatic nerve, in dorsal root ganglia and in spinal dorsal horns after intraplantar injection of saline or BoNT/A, alone or colocalized with either glial fibrillar acidic protein, GFAP, or complement receptor 3/cluster of differentiation 11b, CD11b, or neuronal nuclei, NeuN, depending on the area investigated. Immunofluorescence analysis shows the presence of the cl-SNAP-25 in all tissues examined, from the peripheral endings to the spinal cord, suggesting a retrograde transport of BoNT/A. Moreover, we performed in vitro experiments to ascertain if BoNT/A was able to interact with the proliferative state of Schwann cells (SC). We found that BoNT/A modulates the proliferation of SC and inhibits the acetylcholine release from SC, evidencing a new biological effect of the toxin and further supporting the retrograde transport of the toxin along the nerve and its ability to influence regenerative processes. The present results strongly sustain a combinatorial action at peripheral and central neural levels and encourage the use of BoNT/A for the pathological pain conditions difficult to treat in clinical practice and dramatically impairing patients’ quality of life.     

Initiation of neuropathic pain requires lysophosphatidic acid receptor signaling

Lysophosphatidic acid (LPA) is a bioactive lipid with activity in the nervous system mediated by G-protein-coupled receptors. Here, we examined the role of LPA signaling in the development of neuropathic pain by pharmacological and genetic approaches, including the use of mice lacking the LPA(1) receptor. Wild-type animals with nerve injury develop behavioral allodynia and hyperalgesia paralleled by demyelination in the dorsal root and increased expression of both the protein kinase C gamma-isoform within the spinal cord dorsal horn and the alpha(2)delta(1) calcium channel subunit in dorsal root ganglia. Intrathecal injection of LPA induced behavioral, morphological and biochemical changes similar to those observed after nerve ligation. In contrast, mice lacking a single LPA receptor (LPA(1), also known as EDG2) that activates the Rho-Rho kinase pathway do not develop signs of neuropathic pain after peripheral nerve injury. Inhibitors of Rho and Rho kinase also prevented these signs of neuropathic pain. These results imply that receptor-mediated LPA signaling is crucial in the initiation of neuropathic pain.     

PI3K Contributed to Modulation of Spinal Nociceptive Information Related to ephrinBs/EphBs

There is accumulating evidence to implicate the importance of EphBs receptors and ephrinBs ligands were involved in modulation of spinal nociceptive information. However, the downstream mechanisms that control this process are not well understood. In the present study, we investigated whether phosphatidylinositol 3-kinase (PI3K), as the downstream effectors, participates in modulation of spinal nociceptive information related to ephrinBs/EphBs. Intrathecal injection of ephrinB1-Fc produced a dose- and time-dependent thermal and mechanical hyperalgesia, accompanied by the increase of spinal PI3K-p110γ, phosphorylation of AKT (p-AKT) and c-Fos expression. Pre-treatment with PI3K inhibitor wortmannin or LY294002 prevented activation of spinal AKT induced by ephrinB1-Fc. Inhibition of spinal PI3K signaling dose-dependently prevented and reversed pain behaviors and spinal c-Fos protein expression induced by intrathecal injection of ephrinB1-Fc. Inhibition of EphBs receptors by intrathecal injection of EphB1-Fc reduced formalin-induced inflammation and chronic constrictive injury-induced neuropathic pain behaviors accompanied by decreased expression of spinal PI3K,p-AKT and c-Fos protein. Furthermore, pre-treatment with PI3K inhibitor wortmannin or LY294002 prevented ephrinB1-Fc-induced ERK activation in spinal. These data demonstrated that PI3K and PI3K crosstalk to ERK signaling contributed to modulation of spinal nociceptive information related to ephrinBs/EphBs.     

Gintonin, a ginseng-derived novel ingredient, evokes long-term potentiation through N-methyl-D-aspartic acid receptor activation: Involvement of LPA receptors

Ginseng has been shown to have memory-improving effects in human. However, little is known about the active components and the molecular mechanisms underlying its effects. Recently, we isolated novel lysophosphatidic acids (LPAs)-ginseng protein complex derived from ginseng, gintonin. Gintonin activates G protein-coupled LPA receptors with high affinity. Gintonin activated Ca²⁺-activated Clchannels in Xenopus oocytes through the activation of endogenous LPA receptor. In the present study, we investigated whether the activation of LPA receptor by gintonin is coupled to the regulation of N-methyl-d-aspartic acid (NMDA) receptor channel activity in Xenopus oocytes expressing rat NMDA receptors. The NMDA receptor-mediated ion current (I _NMDA ) was measured using the two-electrode voltage-clamp technique. In oocytes injected with cRNAs encoding NMDA receptor subunits, gintonin enhanced I _NMDA in a concentration-dependent manner. Gintonin-mediated I _NMDA enhancement was blocked by Ki16425, an LPA1/3 receptor antagonist. Gintonin action was blocked by a PLC inhibitor, IP₃ receptor antagonist, Ca²⁺ chelator, and a tyrosine kinase inhibitor. The site-directed mutation of Ser1308 of the NMDA receptor, which is phosphorylated by protein kinase C (PKC), to an Ala residue, or co-expression of receptor protein tyrosine phosphatase with the NMDA receptor attenuated gintonin action. Moreover, gintonin treatment elicited a transient elevation of [Ca²⁺]_i in cultured hippocampal neurons and elevated longterm potentiation (LTP) in both concentration-dependent manners in rat hippocampal slices. Gintonin-mediated LTP induction was abolished by Ki16425. These results indicate that gintonin-mediated I _NMDA potentiation and LTP induction in the hippocampus via the activation of LPA receptor might be responsible for ginseng-mediated improvement of memory-related brain functions.     

Lysophosphatidic acid stimulates astrocyte proliferation through LPA1

Lysophosphatidic acid (LPA) is an extracellular lipid mediator that regulates nervous system development and functions through multiple types of LPA receptors. Here we explore the role of LPA receptor subtypes in cortical astrocyte functions. Astrocytes cultured under serum-free conditions were found to express the genes of five LPA receptor subtypes, lpa1 to lpa5. When astrocytes were treated with dibutyryl cyclic adenosine monophosphate, a reagent inducing astrocyte differentiation or activation, lpa1 expression levels remained unchanged, but those of other LPA receptor subtypes were relatively reduced. LPA stimulated DNA synthesis in both undifferentiated and differentiated astrocytes, but failed to do so in astrocytes prepared from mice lacking lpa1 gene. LPA also inhibited [3H]-glutamate uptake in both undifferentiated and differentiated astrocytes; and LPA-induced inhibition of glutamate uptake was still observed in lpa1-deficient astrocytes. Taken together, these observations demonstrate that LPA1 mediates LPA-induced stimulation of cell proliferation but not inhibition of glutamate uptake in astrocytes.     

Identification and Characterization of a Novel Lysophosphatidic Acid Receptor, p2y5/LPA6

p2y5 is an orphan G protein-coupled receptor that is closely related to the fourth lysophosphatidic acid (LPA) receptor, LPA₄. Here we report that p2y5 is a novel LPA receptor coupling to the G₁₃-Rho signaling pathway. “LPA receptor-null” RH7777 and B103 cells exogenously expressing p2y5 showed [³H]LPA binding, LPA-induced [³⁵S]guanosine 5′-3-O-(thio)triphosphate binding, Rho-dependent alternation of cellular morphology, and G_s/13 chimeric protein-mediated cAMP accumulation. LPA-induced contraction of human umbilical vein endothelial cells was suppressed by small interfering RNA knockdown of endogenously expressed p2y5. We also found that 2-acyl-LPA had higher activity to p2y5 than 1-acyl-LPA. A recent study has suggested that p2y5 is an LPA receptor essential for human hair growth. We confirmed that p2y5 is a functional LPA receptor and propose to designate this receptor LPA₆.     

c-Src-mediated phosphorylation of TRIP6 regulates its function in lysophosphatidic acid-induced cell migration

TRIP6 (thyroid receptor-interacting protein 6), also known as ZRP-1 (zyxin-related protein 1), is a member of the zyxin family that has been implicated in cell motility. Previously we have shown that TRIP6 binds to the LPA₂ receptor and associates with several components of focal complexes in an agonist-dependent manner and, thus, enhances lysophosphatidic acid (LPA)-induced cell migration. Here we further report that the function of TRIP6 in LPA signaling is regulated by c-Src-mediated phosphorylation of TRIP6 at the Tyr-55 residue. LPA stimulation induces tyrosine phosphorylation of endogenous TRIP6 in NIH 3T3 cells and c-Src-expressing fibroblasts, which is virtually eliminated in Src-null fibroblasts. Strikingly, both phosphotyrosine-55 and proline-58 residues of TRIP6 are required for Crk binding in vitro and in cells. Mutation of Tyr-55 to Phe does not alter the ability of TRIP6 to localize at focal adhesions or associate with actin. However, it abolishes the association of TRIP6 with Crk and p130^cas in cells and significantly reduces the function of TRIP6 to promote LPA-induced ERK activation. Ultimately, these signaling events control TRIP6 function in promoting LPA-induced morphological changes and cell migration.     

Activation of GRs–Akt–nNOs–NR2B signaling pathway by second dose GR agonist contributes to exacerbated hyperalgesia in a rat model of radicular pain

Central Akt, neuronal nitric oxide synthase (nNOS) and N-methyl-d-aspartate receptor subunit 2B (NR2B) play key roles in the development of neuropathic pain. Here we investigate the effects of glucocorticoid receptors (GRs) on the expression and activation of spinal Akt, nNOS and NR2B after chronic compression of dorsal root ganglia (CCD). Thermal hyperalgesia test and mechanical allodynia test were used to measure rats after intrathecal injection of GR antagonist mifepristone or GR agonist dexamethasone for 21 days postoperatively. Expression of spinal Akt, nNOS, NR2B and their phosphorylation state after CCD was examined by western blot. The effects of intrathecal treatment with dexamethasone or mifepristone on nociceptive behaviors and the corresponding expression of Akt, nNOS and NR2B in spinal cord were also investigated. Intrathecal injection of mifepristone or dexamethasone inhibited PWMT and PWTL in CCD rats. However, hyperalgesia was induced by intrathecal injection of dexamethasone on days 12 to 14 after surgery. Treatment of dexamethasone increased the expression and phosphorylation levels of spinal Akt, nNOS, GR and NR2B time dependently, whereas administration of mifepristone downregulated the expression of these proteins significantly. GRs activated spinal Akt–nNOS/NR2B pathway play important roles in the development of neuropathic pain in a time-dependent manner.     

Sensory discrimination between innocuous and noxious cold by TRPM8-expressing DRG neurons of rats

Background

In our present study, we studied the role of demyelination of the trigeminal nerve root in the development of prolonged nociceptive behavior in the trigeminal territory.

Results

Under anesthesia, the Sprague-Dawley rats were mounted onto a stereotaxic frame and 3 ??L of lysophosphatidic acid (LPA, 1 nmol) was injected into the trigeminal nerve root to produce demyelination. This treatment decreased the air-puff thresholds, persisted until postoperative day 130, and then returned to the preoperative levels 160 days after LPA injection. The LPA-treated rats also showed a significant hyper-responsiveness to pin-prick stimulation. We further investigated the antinociceptive and neuroprotective effects of progesterone in rats undergoing demyelination of the trigeminal nerve root. Progesterone (8, 16 mg/kg/day) was administered subcutaneously, beginning on the operative day, for five consecutive days in the LPA-treated rats. Treatment with progesterone produced significant early anti-allodynic effects and delayed prolonged anti-allodynic effects. The expression of protein zero (P0) and peripheral myelin protein 22 (PMP22) were significantly down-regulated in the trigeminal nerve root on postoperative day 5 following LPA injection. This down-regulation of the P0 and PMP22 levels was blocked by progesterone treatment.

Conclusions

These results suggest that progesterone produces antinociceptive effects through neuroprotective action in animals with LPA-induced trigeminal neuropathic pain. Moreover, progesterone has potential utility as a novel therapy for trigeminal neuropathic pain relief at an appropriate managed dose and is therefore a possible future treatment strategy for improving the recovery from injury.     

Discovery of novel non-carboxylic acid 5-amino-4-cyanopyrazole derivatives as potent and highly selective LPA1R antagonists

High throughput screening (HTS) of our chemical library identified 3-alkylamino-2-aryl-5H-imidazo[1,2,b]pyrazol-7-carbonitrile 1 as a potent antagonist of the LPA1 receptor (LPA1R). Further evaluation of this class of compounds indicated that LPA1R antagonist activity originated from the degradation of the parent molecule in DMSO during the assay conditions. Here, we describe the isolation and characterization of the degradation products and their LPA1R antagonist activity. We further profiled these novel non-carboxylic acid LPA1R antagonists and demonstrated their inhibition of LPA-induced proliferation and contraction of normal human lung fibroblasts (NHLF).     

RAFTK/Pyk2 mediates LPA-induced PC12 cell migration

The phospholipid lysophosphatidic acid (LPA) is a normal constituent of serum that functions as a lipid growth factor and intracellular signaling molecule. In this report, we have investigated the signaling mechanism and function of the tyrosine kinase RAFTK/Pyk2 in LPA-induced cell migration. Analysis of tyrosine phosphorylation upon LPA stimulation in neuroendocrine PC12 cells revealed 6 major tyrosine-phosphorylated proteins with estimated sizes of 180, 120, 115, 68, 44, and 42 kDa. These proteins were identified as epidermal growth factor receptor (EGFR), focal adhesion kinase, RAFTK/Pyk2, paxillin, Erk 1, and Erk 2, respectively. Using specific pharmacological inhibitors, we found that the tyrosine phosphorylation of RAFTK/Pyk2 was intracellular Ca2+-dependent, but not EGFR-dependent, during LPA stimulation of these cells. Moreover, the cytoskeletal and signal scaffolding protein, paxillin, associated with and was regulated by RAFTK/Pyk2 in a Ca2+-dependent manner. Characterization of LPA receptors showed that LPA1 (Edg2) and LPA2 (Edg4) are major receptors for LPA, while LPA3 receptor (Edg7) expression was limited. Upon using the LPA1/LPA3 receptor-specific antagonist VPC 32179, we observed that inhibition of the LPA1/LPA3 receptors had no effect on the LPA-induced phosphorylation of RAFTK, strongly suggesting that the LPA2 receptor is a key mediator of RAFTK phosphorylation. Furthermore, LPA induced PC12 cell migration, which was subsequently blocked by the dominant-negative form of FAK, FRNK. Expression of a dominant-negative form of the small GTPase Ras also blocked LPA-induced cell migration and RAFTK phosphorylation. Taken together, these results indicate that RAFTK is a key signaling molecule that mediates LPA-induced PC12 cell migration in a Ras-dependent manner.     

Role of Lipocalin-2-Chemokine Axis in the Development of Neuropathic Pain following Peripheral Nerve Injury

Lipocalin 2 (LCN2), which is also known as 24p3 and neutrophil gelatinase-associated lipocalin (NGAL), binds small, hydrophobic ligands and interacts with cell surface receptor 24p3R to regulate diverse cellular processes. In the present study, we examined the role of LCN2 in the pathogenesis of neuropathic pain using a mouse model of spared nerve injury (SNI). Lcn2 mRNA levels were significantly increased in the dorsal horn of the spinal cord after SNI, and LCN2 protein was mainly localized in neurons of the dorsal and ventral horns. LCN2 receptor 24p3R was expressed in spinal neurons and microglia after SNI. Lcn2-deficient mice exhibited significantly less mechanical pain hypersensitivity during the early phase after SNI, and an intrathecal injection of recombinant LCN2 protein elicited mechanical pain hypersensitivity in naive animals. Lcn2 deficiency, however, did not affect acute nociceptive pain. Lcn2-deficient mice showed significantly less microglial activation and proalgesic chemokine (CCL2 and CXCL1) production in the spinal cord after SNI than wild-type mice, and recombinant LCN2 protein induced the expression of these chemokines in cultured neurons. Furthermore, the expression of LCN2 and its receptor was detected in neutrophils and macrophages in the sciatic nerve following SNI, suggesting the potential role of peripheral LCN2 in neuropathic pain. Taken together, our results indicate that LCN2 plays a critical role in the development of pain hypersensitivity following peripheral nerve injury and suggest that LCN2 mediates neuropathic pain by inducing chemokine expression and subsequent microglial activation.