Evidence for lysophosphatidic acid 1 receptor signaling in the early phase of neuropathic pain mechanisms in experiments using Ki-16425, a lysophosphatidic acid 1 receptor antagonist

TMEM120A/TACAN: A putative regulator of ion channels, mechanosensation, and lipid metabolism

TMEM120A (TACAN) is an enigmatic protein with several seemingly unconnected functions. It was proposed to be an ion channel involved in sensing mechanical stimuli, and knockdown/knockout experiments have implicated that TMEM120A may be necessary for sensing mechanical pain. TMEM120A's ion channel function has subsequently been challenged, as attempts to replicate electrophysiological experiments have largely been unsuccessful. Several cryo-EM structures revealed TMEM120A is structurally homologous to a lipid modifying enzyme called Elongation of Very Long Chain Fatty Acids 7 (ELOVL7). Although TMEM120A's channel function is debated, it still seems to affect mechanosensation by inhibiting PIEZO2 channels and by modifying tactile pain responses in animal models. TMEM120A was also shown to inhibit polycystin-2 (PKD2) channels through direct physical interaction. Additionally, TMEM120A has been implicated in adipocyte regulation and in innate immune response against Zika virus. The way TMEM120A is proposed to alter each of these processes ranges from regulating gene expression, acting as a lipid modifying enzyme, and controlling subcellular localization of other proteins through direct binding. Here, we examine TMEM120A's structure and proposed functions in diverse physiological contexts.

LPA1 receptors in the lateral habenula regulate negative affective states associated with alcohol withdrawal

The role of lysophosphatidic acid (LPA) signaling in psychiatric disorders and drug abuse is significant. LPA receptors are widely expressed in the central nervous system, including the lateral habenula (LHb). Recent studies suggest that LHb is involved in a negative emotional state during alcohol withdrawal, which can lead to relapse. The current study examines the role of LHb LPA signaling in the negative affective state associated with alcohol withdrawal. Adult male Long-Evans rats were trained to consume either alcohol or water for eight weeks. At 48 h of withdrawal, alcohol-drinking rats showed anxiety- and depression-like symptoms, along with a significant increase in LPA signaling and related neuronal activation molecules, including autotaxin (ATX, Enpp2), LPA receptor 1/3 (LPA1/3), βCaMKII, and c-Fos. However, there was a decrease in lipid phosphate phosphatase-related protein type 4 (LPPR4) in the LHb. Intra-LHb infusion of the LPA1/3 receptor antagonist ki-16425 or PKC-γ inhibitor Go-6983 reduced the abnormal behaviors and elevated relapse-like ethanol drinking. It also normalized high LPA1/3 receptors and enhanced AMPA GluA1 phosphorylation in Ser831 and GluA1/GluA2 ratio. Conversely, selective activation of LPA1/3 receptors by intra-LHb infusion of 18:1 LPA induced negative affective states and upregulated βCaMKII-AMPA receptor phosphorylation in Naive rats, which were reversed by pretreatment with intra-LHb Go-6983. Our findings suggest that disturbances in LPA signaling contribute to adverse affective disorders during alcohol withdrawal, likely through PKC-γ/βCaMKII-linked glutamate signaling. Targeting LPA may therefore be beneficial for individuals suffering from alcohol use disorders.

Lysophosphatidic acid, a simple phospholipid with myriad functions

Lysophosphatidic acid (LPA) is a simple phospholipid consisting of a phosphate group, glycerol moiety, and only one hydrocarbon chain. Despite its simple chemical structure, LPA plays an important role as an essential bioactive signaling molecule via its specific six G protein-coupled receptors, LPA_1-6. Recent studies, especially those using genetic tools, have revealed diverse physiological and pathological roles of LPA and LPA receptors in almost every organ system. Furthermore, many studies are illuminating detailed mechanisms to orchestrate multiple LPA receptor signaling pathways and to facilitate their coordinated function. Importantly, these extensive "bench" works are now translated into the "bedside" as exemplified by approaches targeting LPA₁ signaling to combat fibrotic diseases. In this review, we discuss the physiological and pathological roles of LPA signaling and their implications for clinical application by focusing on findings revealed by in vivo studies utilizing genetic tools targeting LPA receptors.

Novel Antagonist of the Type 2 Lysophosphatidic Acid Receptor (LPA2), UCM-14216, Ameliorates Spinal Cord Injury in Mice

Spinal cord injuries (SCIs) irreversibly disrupt spinal connectivity, leading to permanent neurological disabilities. Current medical treatments for reducing the secondary damage that follows the initial injury are limited to surgical decompression and anti-inflammatory drugs, so there is a pressing need for new therapeutic strategies. Inhibition of the type 2 lysophosphatidic acid receptor (LPA₂) has recently emerged as a new potential pharmacological approach to decrease SCI-associated damage. Toward validating this receptor as a target in SCI, we have developed a new series of LPA₂ antagonists, among which compound 54 (UCM-14216) stands out as a potent and selective LPA₂ receptor antagonist (E_max = 90%, IC₅₀ = 1.9 μM, K_D = 1.3 nM; inactive at LPA_1,3–6 receptors). This compound shows efficacy in an in vivo mouse model of SCI in an LPA₂-dependent manner, confirming the potential of LPA₂ inhibition for providing a new alternative for treating SCI.

Enhancement of P2X3 Receptor-Mediated Currents by Lysophosphatidic Acid in Rat Primary Sensory Neurons

Lysophosphatidic acid (LPA), a lipid metabolite, plays a role in both neuropathic and inflammatory pain through LPA₁ receptors. P2X3 receptor has also been shown to participate in these pathological processes. However, it is still unclear whether there is a link between LPA signaling and P2X3 receptors in pain. Herein, we show that a functional interaction between them in rat dorsal root ganglia (DRG) neurons. Pretreatment of LPA concentration-dependently enhanced α,β-methylene-ATP (α,β-meATP)-induced inward currents mediated by P2X3 receptors. LPA significantly increased the maximal current response of α,β-meATP, showing an upward shift of the concentration-response curve for α,β-meATP. The LPA enhancement was independent on the clamping-voltage. Enhancement of P2X3 receptor-mediated currents by LPA was prevented by the LPA₁ receptor antagonist Ki16198, but not by the LPA₂ receptor antagonist H2L5185303. The LPA-induced potentiation was also attenuated by intracellular dialysis of either G-protein inhibitor or protein kinase C (PKC) inhibitor, but not by Rho inhibitor. Moreover, LPA significantly changed the membrane potential depolarization and action potential burst induced by α,β-meATP in DRG neurons. Finally, LPA exacerbated α,β-meATP- induced nociceptive behaviors in rats. These results suggested that LPA potentiated the functional activity of P2X3 receptors in rat primary sensory neurons through activation of the LPA₁ receptor and its downstream PKC rather than Rho signaling pathway, indicating a novel peripheral mechanism underlying the sensitization of pain.

Secreted PLA2-III is a possible therapeutic target to treat neuropathic pain

Lysophosphatidic acid (LPA) plays a critical role in developing and maintaining chronic pain in various animal models. Previous studies have reported that cytosolic and calcium-independent phospholipase A2 (PLA2) is involved in the LPA receptor-mediated amplification of LPA production in the spinal dorsal horn (SDH) after nerve injury, while the involvement of secreted PLA2 (sPLA2) remains unclear. The present study revealed that only sPLA2 -III among 11 species of PLA2 showed a significant upregulation of gene expression in the SDH. Intraspinal injection of adeno-associated virus-miRNA targeting sPLA2-III prevented hyperalgesia and unique hypoalgesia in mice treated with partial sciatic nerve ligation. In addition, intrathecal treatment with antisense oligodeoxynucleotide or siRNA targeting sPLA2-III significantly reversed the established thermal hyperalgesia. In the high-throughput screening of sPLA2-III inhibitors from the chemical library, we identified two hit compounds. Through in vitro characterization of PLA2 inhibitor profiles and in vivo assessment of the anti-hyperalgesic effects of known PLA2 inhibitors as well as hit compounds, sPLA2-III was found to be a novel therapeutic target molecule for the treatment of Neuropathic pain.

Lysophosphatidic acid (LPA) receptor modulators: Structural features and recent development

Lysophosphatidic acid (LPA) activates six LPA receptors (LPAR_1-6) and regulates various cellular activities such as cell proliferation, cytoprotection, and wound healing. Many studies elucidated the pathological outcomes of LPA are due to the alteration in signaling pathways, which include migration and invasion of cancer cells, fibrosis, atherosclerosis, and inflammation. Current pathophysiological research on LPA and its receptors provides a means that LPA receptors are new therapeutic targets for disorders associated with LPA. Various chemical modulators are developed and are under investigation to treat a wide range of pathological complications. This review summarizes the physiological and pathological roles of LPA signaling, development of various LPA modulators, their structural features, patents, and their clinical outcomes.

Novel Approaches, Drug Candidates, and Targets in Pain Drug Discovery

Because of the problems associated with opioids, drug discovery efforts have been employed to develop opioids with reduced side effects using approaches such as biased opioid agonism, multifunctional opioids, and allosteric modulation of opioid receptors. Receptor targets such as adrenergic, cannabinoid, P2X3 and P2X7, NMDA, serotonin, and sigma, as well as ion channels like the voltage-gated sodium channels Nav1.7 and Nav1.8 have been targeted to develop novel analgesics. Several enzymes, such as soluble epoxide hydrolase, sepiapterin reductase, and MAGL/FAAH, have also been targeted to develop novel analgesics. In this review, old and recent targets involved in pain signaling and compounds acting at these targets are summarized. In addition, strategies employed to reduce side effects, increase potency, and efficacy of opioids are also elaborated. This review should aid in propelling drug discovery efforts to discover novel analgesics.

Inhibition of autotaxin activity ameliorates neuropathic pain derived from lumbar spinal canal stenosis

Lumbar spinal canal stenosis (LSS) or mechanical compression of dorsal root ganglion (DRG) is one of the causes of low back pain and neuropathic pain (NP). Lysophosphatidic acid (LPA) is a potent bioactive lipid mediator that is produced mainly from lysophosphatidylcholine (LPC) via autotaxin (ATX) and is known to induce NP via LPA1 receptor signaling in mice. Recently, we demonstrated that LPC and LPA were higher in cerebrospinal fluid (CSF) of patients with LSS. Based on the possible potential efficacy of the ATX inhibitor for NP treatment, we used an NP model with compression of DRG (CD model) and investigated LPA dynamics and whether ATX inhibition could ameliorate NP symptoms, using an orally available ATX inhibitor (ONO-8430506) at a dose of 30 mg/kg. In CD model, we observed increased LPC and LPA levels in CSF, and decreased threshold of the pain which were ameliorated by oral administration of the ATX inhibitor with decreased microglia and astrocyte populations at the site of the spinal dorsal horn projecting from injured DRG. These results suggested possible efficacy of ATX inhibitor for the treatment of NP caused by spinal nerve root compression and involvement of the ATX-LPA axis in the mechanism of NP induction.

Role of lysophosphatidic acid and its receptors in health and disease: novel therapeutic strategies

Lysophosphatidic acid (LPA) is an abundant bioactive phospholipid, with multiple functions both in development and in pathological conditions. Here, we review the literature about the differential signaling of LPA through its specific receptors, which makes this lipid a versatile signaling molecule. This differential signaling is important for understanding how this molecule can have such diverse effects during central nervous system development and angiogenesis; and also, how it can act as a powerful mediator of pathological conditions, such as neuropathic pain, neurodegenerative diseases, and cancer progression. Ultimately, we review the preclinical and clinical uses of Autotaxin, LPA, and its receptors as therapeutic targets, approaching the most recent data of promising molecules modulating both LPA production and signaling. This review aims to summarize the most update knowledge about the mechanisms of LPA production and signaling in order to understand its biological functions in the central nervous system both in health and disease.

Pathogenic mechanisms of lipid mediator lysophosphatidic acid in chronic pain

A number of membrane lipid-derived mediators play pivotal roles in the initiation, maintenance, and regulation of various types of acute and chronic pain. Acute pain, comprising nociceptive and inflammatory pain warns us about the presence of damage or harmful stimuli. However, it can be efficiently reversed by opioid analgesics and anti-inflammatory drugs. Prostaglandin E₂ and I₂, the representative lipid mediators, are well-known causes of acute pain. However, some lipid mediators such as lipoxins, resolvins or endocannabinoids suppress acute pain. Various types of peripheral and central neuropathic pain (NeuP) as well as fibromyalgia (FM) are representatives of chronic pain and refractory owing to abnormal pain processing distinct from acute pain. Accumulating evidence demonstrated that lipid mediators represented by lysophosphatidic acid (LPA) are involved in the initiation and maintenance of both NeuP and FM in experimental animal models. The LPAR₁-mediated peripheral mechanisms including dorsal root demyelination, Ca_vα2δ1 expression in dorsal root ganglion, and LPAR₃-mediated amplification of central LPA production via glial cells are involved in the series of molecular mechanisms underlying NeuP. This review also discusses the involvement of lipid mediators in emerging research directives, including itch-sensing, sexual dimorphism, and the peripheral immune system.

A Novel Agonist of the Type 1 Lysophosphatidic Acid Receptor (LPA1), UCM-05194, Shows Efficacy in Neuropathic Pain Amelioration

Neuropathic pain (NP) is a complex chronic pain state with a prevalence of almost 10% in the general population. Pharmacological options for NP are limited and weakly effective, so there is a need to develop more efficacious NP attenuating drugs. Activation of the type 1 lysophosphatidic acid (LPA₁) receptor is a crucial factor in the initiation of NP. Hence, it is conceivable that a functional antagonism strategy could lead to NP mitigation. Here we describe a new series of LPA₁ agonists among which derivative (S)-17 (UCM-05194) stands out as the most potent and selective LPA₁ receptor agonist described so far (E_max = 118%, EC₅₀ = 0.24 μM, K_D = 19.6 nM; inactive at autotaxin and LPA_2-6 receptors). This compound induces characteristic LPA₁-mediated cellular effects and prompts the internalization of the receptor leading to its functional inactivation in primary sensory neurons and to an efficacious attenuation of the pain perception in an in vivo model of NP.

[Lysophosphatidic Acid Receptor Signaling Underlying Chronic Pain and Neuroprotective Mechanisms through Prothymosin α]

For my Ph.D. research topic, I isolated endogenous morphine-like analgesic dipeptide, kyotorphin, which mediates Met-enkephalin release, and discovered kyotorphin synthetase, a putative receptor and antagonist. Furthermore, I succeeded in purifying μ-opioid receptor and functional reconstitution with purified G proteins. After receiving my full professor position at Nagasaki University in 1996, I worked on two topics of research, molecular mechanisms of chronic pain through lysophosphatidic acid (LPA) and identification and characterization of neuroprotective protein, prothymosin α. In a series of studies, we have shown that LPA signaling defines the molecular mechanisms of neuropathic pain and fibromyalgia in terms of development and maintenance. Above all, the discovery of feed-forward system in LPA production and pain memory may contribute to better understanding of chronic pain and future analgesic drug discovery. Regarding prothymosin α, we first discovered it as neuronal necrosis-inhibitory molecule through two independent mechanisms, such as toll-like receptor and F₀/F₁ ATPase, both which protect neurons through indirect mechanisms. Prothymosin α is released by non-classical and non-vesicular mechanisms on various stresses, such as ischemia, starvation, and heat-shock. Thus it may be called a new type of neuroprotective damage-associated molecular patterns (DAMPs)/Alarmins. Heterozygotic mice showed a defect in memory-learning and neurogenesis as well as anxiogenic behaviors. Small peptide, P6Q derived from prothymosin α retains neuroprotective actions, which include blockade of cerebral hemorrhage caused by late treatment with tissue plasminogen activator in the stroke model in mice.

Involvement of lysophosphatidic acid-induced astrocyte activation underlying the maintenance of partial sciatic nerve injury-induced neuropathic pain

We have previously demonstrated that lysophosphatidic acid (LPA) plays key roles in the initial mechanisms for neuropathic pain (NeuP) development. Here, we examined whether LPA receptor mechanisms and LPA production are related to the glial activation at a late stage after partial sciatic nerve ligation (pSNL) by use of microglial inhibitor, Mac1-saporin or astrocyte inhibitor, and L-α-aminoadipate (L-AA). Although single intrathecal injection of LPA1/3 antagonist, Ki-16425 did not affect the pain threshold at day 7 after the spinal cord injury, repeated treatments of each compound gradually reversed the basal pain threshold to the control level. The intrathecal administration of a microglia inhibitor, Mac-1-saporin reversed the late hyperalgesia and LPA production at day 14 after the pSNL, whereas L-AA inhibited the hyperalgesia, but had no effect on LPA production. The involvement of LPA receptors in astrocyte activation in vivo was evidenced by the findings that Ki-16425 treatments abolished the upregulation of CXCL1 in activated astrocytes in the spinal dorsal horn of mice at day 14 after the pSNL, and that Ki-16425 reversed the LPA-induced upregulation of several chemokine gene expressions in primary cultured astrocytes. Finally, we found that significant hyperalgesia was observed with intrathecal administration of primary cultured astrocytes, which had been stimulated by LPA in a Ki-16425-reversible manner. All these findings suggest that LPA production and LPA1/3 receptor activation through differential glial mechanisms play key roles in the maintenance as well as initiation mechanisms in NeuP.

Nervous system delivery of antilysophosphatidic acid antibody by nasal application attenuates mechanical allodynia after traumatic brain injury in rats

Lysophosphatidic acid (LPA) is a bioactive lipid that impacts neurological outcomes after neurotrauma by inhibiting neuroregeneration, promoting inflammation, and contributing to behavioral deficits. Blocking LPA signaling with a novel anti-LPA monoclonal antibody (mAb) is neuroprotective after traumatic brain injury (TBI) if given to injured animals whose blood-brain barrier (BBB) has been compromised. It is hypothesized that the anti-LPA mAb could improve chronic pain initiated by TBI. However, poor brain penetration after systemic application of the antibody makes access to the central nervous system (CNS) problematic in situations where the BBB is intact. Our experiments investigated whether intranasal delivery of the anti-LPA mAb could bypass the BBB, allowing for direct entry of the antibody to certain areas of the CNS. When the humanized anti-LPA mAb, LT3114, was intranasally applied to injured rats within 30 minutes after mild TBI using the central lateral percussion model, enzyme-linked immunospecific assay and immunohistochemistry demonstrated antibody uptake to several areas in the CNS, including the area of cortical injury, the corpus callosum, cerebellum, and the subventricular region. Compared with control rats that received LT3114 but no TBI, TBI rats demonstrated significantly higher concentrations of intranasally administered LT3114 antibody in some tissues. In behavioral studies, a significant attenuation of mechanical allodynia after TBI was observed in the anti-LPA treatment group (P = 0.0079), when compared with vehicle controls within 14 days after TBI. These results suggest that intranasal application of the anti-LPA antibody directly accesses CNS sites involved in TBI-related pain and that this access attenuates pain sequelae to the neurotrauma.

Identification of compounds acting as negative allosteric modulators of the LPA1 receptor

The Lysophosphatidic Acid 1 Receptor (LPA₁ receptor) has been linked to the initiation and progression of a variety of poorly treated fibrotic conditions. Several compounds that have been described as LPA₁ receptor antagonists have progressed into clinical trials: 1-(4-{4-[3-methyl-4-({[(1R)-1-phenylethoxy]carbonyl}amino)-1,2-oxazol-5-yl]phenyl}phenyl)cyclopropane-1-carboxylic acid (BMS-986202) and 2-{4-methoxy-3-[2-(3-methylphenyl)ethoxy]benzamido}-2,3-dihydro-1H-indene-2-carboxylic acid (SAR-100842). We considered that as LPA₁ receptor function is involved in many normal physiological processes, inhibition of specific signalling pathways associated with fibrosis may be therapeutically advantageous. We compared the binding and functional effects of a novel compound; 4-({(Cyclopropylmethyl)[4-(2-fluorophenoxy)benzoyl]amino}methyl}benzoic acid (TAK-615) with BMS-986202 and SAR-100842. Back-scattering interferometry (BSI) was used to show that the apparent affinity of TAK-615 was enhanced in the presence of LPA. The binding signal for BMS-986202 was not detected in the presence of LPA suggesting competition but interestingly the apparent affinity of SAR-100842 was also enhanced in the presence of LPA. Only BMS-986202 was able to fully inhibit the response to LPA in calcium mobilisation, β-arrestin, cAMP, GTPγS and RhoA functional assays. TAK-615 and SAR-100842 showed different inhibitory profiles in the same functional assays. Further binding studies indicated that TAK-615 is not competitive with either SAR-100842 or BMS-986202, suggesting a different site of binding. The results generated with this set of experiments demonstrate that TAK-615 acts as a negative allosteric modulator (NAM) of the LPA₁ receptor. Surprisingly we find that SAR-100842 also behaves like a NAM. BMS-986202 on the other hand behaves like an orthosteric antagonist.

Lysophosphatidic acid provides a missing link between osteoarthritis and joint neuropathic pain

OBJECTIVE

Emerging evidence suggests that osteoarthritis (OA) has a neuropathic component; however, the identity of the molecules responsible for this peripheral neuropathy is unknown. The aim of this study was to determine the contribution of the bioactive lipid lysophosphatidic acid (LPA) to joint neuropathy and pain.

DESIGN

Male Lewis rats received an intra-articular injection of 50 μg of LPA into the knee and allowed to recover for up to 21 days. Saphenous nerve myelination was assessed by g-ratio calculation from electron micrographs and afferent nerve damage visualised by activation transcription factor-3 (ATF-3) expression. Nerve conduction velocity was measured electrophysiologically and joint pain was determined by hindlimb incapacitance. The effect of the LPA antagonist Ki-16425 was also evaluated. Experiments were repeated in the sodium monoiodoacetate (MIA) model of OA.

RESULTS

LPA caused joint nerve demyelination which resulted in a drop in nerve conduction velocity. Sensory neurones were ATF-3 positive and animals exhibited joint pain and knee joint damage. MIA-treated rats also showed signs of demyelination and joint neuropathy with concomitant pain. Nerve damage and pain could be ameliorated by Ki-16425 pre-treatment.

CONCLUSION

Intra-articular injection of LPA caused knee joint neuropathy, joint damage and pain. Pharmacological blockade of LPA receptors inhibited joint nerve damage and hindlimb incapacitance. Thus, LPA is a candidate molecule for the development of OA nerve damage and the origin of joint neuropathic pain.

Lysophosphatidic acid receptors (LPARs): Potential targets for the treatment of neuropathic pain

Neuropathic pain can arise from lesions to peripheral or central nerve fibres leading to spontaneous action potential generation and a lowering of the nociceptive threshold. Clinically, neuropathic pain can manifest in many chronic disease states such as cancer, diabetes or multiple sclerosis (MS). The bioactive lipid, lysophosphatidic acid (LPA), via activation of its receptors (LPARs), is thought to play a central role in both triggering and maintaining neuropathic pain. In particular, following an acute nerve injury, the excitatory neurotransmitters glutamate and substance P are released from primary afferent neurons leading to upregulated synthesis of lysophosphatidylcholine (LPC), the precursor for LPA production. LPC is converted to LPA by autotaxin (ATX), which can then activate macrophages/microglia and modulate neuronal functioning. A ubiquitous feature of animal models of neuropathic pain is demyelination of damaged nerves. It is thought that LPA contributes to demyelination through several different mechanisms. Firstly, high levels of LPA are produced following macrophage/microglial activation that triggers a self-sustaining feed-forward loop of de novo LPA synthesis. Secondly, macrophage/microglial activation contributes to inflammation-mediated demyelination of axons, thus initiating neuropathic pain. Therefore, targeting LPA production and/or the family of LPA-activated G protein-coupled receptors (GPCRs) may prove to be fruitful clinical approaches to treating demyelination and the accompanying neuropathic pain. This review discusses our current understanding of the role of LPA/LPAR signalling in the initiation of neuropathic pain and suggests potential targeted strategies for its treatment. This article is part of the Special Issue entitled 'Lipid Sensing G Protein-Coupled Receptors in the CNS'.

Gintonin enhances performance of mice in rotarod test: Involvement of lysophosphatidic acid receptors and catecholamine release

Ginseng has a long history of use as a tonic for restoration of vigor. One example of ginseng-derived tonic effect is that it can improve physical stamina under conditions of stress. However, the active ingredient and the underlying molecular mechanism responsible for the ergogenic effect are unknown. Recent studies show that ginseng contains a novel ingredient, gintonin, which consists of a unique class of herbal-medicine lysophosphatidic acids (LPAs). Gintonin activates G protein-coupled LPA receptors to produce a transient [Ca(2+)]i signal, which is coupled to diverse intra- and inter-cellular signal transduction pathways that stimulate hormone or neurotransmitter release. However, relatively little is known about how gintonin-mediated cellular modulation is linked to physical endurance. In the present study, systemic administration of gintonin, but not ginsenosides, in fasted mice increased blood glucose concentrations in a dose-dependent manner. Gintonin treatment elevated blood glucose to a maximum level after 30min. This elevation in blood glucose level could be abrogated by the LPA1/3 receptor antagonist, Ki16425, or the β-adrenergic receptor antagonist, propranolol. Furthermore, gintonin-dependent enhanced performance of fasted mice in rotarod test was likewise abrogated by Ki16425. Gintonin also elevated plasma epinephrine and norepinephrine concentrations. The present study shows that gintonin mediates catecholamine release through activation of the LPA receptor and that activation of the β-adrenergic receptor is coupled to liver glycogenolysis, thereby increasing the supply of glucose and enhancing performance in the rotarod test. Thus, gintonin acts via the LPA-catecholamine-glycogenolysis axis, representing a candidate mechanism that can explain how ginseng treatment enhances physical stamina.

Lysophosphatidic acid receptor 1 antagonist ki16425 blunts abdominal and systemic inflammation in a mouse model of peritoneal sepsis

Lysophosphatidic acid (LPA) is a bioactive lipid mediator of inflammation via the LPA receptors 1-6. We and others have previously described proinflammatory and profibrotic activities of LPA signaling in bleomycin- or lipopolysaccharide (LPS)-induced pulmonary fibrosis or lung injury models. In this study, we investigated if LPA signaling plays a role in the pathogenesis of systemic sepsis from an abdominal source. We report here that antagonism of the LPA receptor LPA1 with the small molecule ki16425 reduces the severity of abdominal inflammation and organ damage in the setting of peritoneal endotoxin exposure. Pretreatment of mice with intraperitoneal ki16425 eliminates LPS-induced peritoneal neutrophil chemokine and cytokine production, liver oxidative stress, liver injury, and cellular apoptosis in visceral organs. Mice pretreated with ki16425 are also protected from LPS-induced mortality. Tissue myeloperoxidase activity is not affected by LPA1 antagonism. We have shown that LPA1 is associated with LPS coreceptor CD14 and the association is suppressed by ki16425. LPS-induced phosphorylation of protein kinase C δ (PKCδ) and p38 mitogen-activated protein kinase (p38 MAPK) in liver cells and interleukin 6 production in Raw264 cells are likewise blunted by LPA1 antagonism. These studies indicate that the small molecule inhibitor of LPA1, ki16425, suppresses cytokine responses and inflammation in a peritoneal sepsis model by blunting downstream signaling through the LPA1-CD14-toll-like receptor 4 receptor complex. This anti-inflammatory effect may represent a therapeutic strategy for the treatment of systemic inflammatory responses to infection of the abdominal cavity.

Lysophosphatidic acid induces integrin activation in vascular smooth muscle and alters arteriolar myogenic vasoconstriction

In vascular smooth muscle cells (VSMC) increased integrin adhesion to extracellular matrix (ECM) proteins, as well as the production of reactive oxygen species (ROS) are strongly stimulated by lysophosphatidic acid (LPA). We hypothesized that LPA-induced generation of ROS increases integrin adhesion to the ECM. Using atomic force microscopy (AFM) we determined the effects of LPA on integrin adhesion to fibronectin (FN) in VSMC isolated from rat (Sprague-Dawley) skeletal muscle arterioles. In VSMC, exposure to LPA (2 μM) doubled integrin-FN adhesion compared to control cells (P < 0.05). LPA-induced integrin-FN adhesion was reduced by pre-incubation with antibodies against β1 and β3 integrins (50 μg/ml) by 66% (P < 0.05). Inhibition of LPA signaling via blockade of the LPA G-protein coupled receptors LPAR1 and LPAR3 with 10 μM Ki16425 reduced the LPA-enhanced adhesion of VSCM to FN by 40% (P < 0.05). Suppression of ROS with tempol (250 μM) or apocynin (300 μM) also reduced the LPA-induced FN adhesion by 47% (P < 0.05) and 59% (P < 0.05), respectively. Using confocal microscopy, we observed that blockade of LPA signaling, with Ki16425, reduced ROS by 45% (P < 0.05), to levels similar to control VSMC unexposed to LPA. In intact isolated arterioles, LPA (2 μM) exposure augmented the myogenic constriction response to step increases in intraluminal pressure (between 40 and 100 mm Hg) by 71% (P < 0.05). The blockade of LPA signaling, with Ki16425, decreased the LPA-enhanced myogenic constriction by 58% (P < 0.05). Similarly, blockade of LPA-induced ROS release with tempol or gp91 ds-tat decreased the LPA-enhanced myogenic constriction by 56% (P < 0.05) and 55% (P < 0.05), respectively. These results indicate that, in VSMC, LPA-induced integrin activation involves the G-protein coupled receptors LPAR1 and LPAR3, and the production of ROS, and that LPA may play an important role in the control of myogenic behavior in resistance vessels through ROS modulation of integrin activity.

The lipid kinase PIP5K1C regulates pain signaling and sensitization

Numerous pain-producing (pronociceptive) receptors signal via phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis. However, it is currently unknown which lipid kinases generate PIP2 in nociceptive dorsal root ganglia (DRG) neurons and if these kinases regulate pronociceptive receptor signaling. Here, we found that phosphatidylinositol 4-phosphate 5 kinase type 1C (PIP5K1C) is expressed at higher levels than any other PIP5K and, based on experiments with Pip5k1c(+/-) mice, generates at least half of all PIP2 in DRG neurons. Additionally, Pip5k1c haploinsufficiency reduces pronociceptive receptor signaling and TRPV1 sensitization in DRG neurons as well as thermal and mechanical hypersensitivity in mouse models of chronic pain. We identified a small molecule inhibitor of PIP5K1C (UNC3230) in a high-throughput screen. UNC3230 lowered PIP2 levels in DRG neurons and attenuated hypersensitivity when administered intrathecally or into the hindpaw. Our studies reveal that PIP5K1C regulates PIP2-dependent nociceptive signaling and suggest that PIP5K1C is a therapeutic target for chronic pain.

An LPA species (18:1 LPA) plays key roles in the self-amplification of spinal LPA production in the peripheral neuropathic pain model

BACKGROUND

We previously reported that nerve injury-induced neuropathic pain is initiated by newly produced lysophosphatidic acid (LPA).

RESULTS

In this study, we developed a quantitative mass spectrometry for detecting LPA species by using Phos-tag. Following nerve injury, the levels of 18:1, 16:0 and 18:0 LPA in the spinal dorsal horn significantly increased at 3 h and declined at 6 h. Among them, 18:1 LPA level was the most abundant. In the same preparation, there were significant elevations in the activities of cytosolic phospholipase A2 (cPLA2) and calcium-independent phospholipase A2 (iPLA2), key enzymes for LPA synthesis, at 1 h, while there was no significant change in phospholipase A1 activity. Pharmacological studies revealed that NMDA and neurokinin 1 receptors, cPLA2, iPLA2 and microglial activation, as well as LPA1 and LPA3 receptors were all involved in the nerve injury-induced LPA production, and underlying cPLA2 and iPLA2 activations. In the cells expressing LPA1 or LPA3 receptor, the receptor-mediated calcium mobilization was most potent with 18:1 LPA, compared with 16:0 or 18:0 LPA. Moreover, the intrathecal injection of 18:1 LPA, but not 16:0 or 18:0 LPA, caused a spinal LPA production and neuropathic pain-like behavior.

CONCLUSION

These results suggest that 18:1 LPA is the predominant ligand responsible for LPA1 and LPA3 receptors-mediated amplification of LPA production through microglial activation.

Lysophosphatidic acid receptor inhibition as a new multipronged treatment for rheumatoid arthritis

OBJECTIVE

To investigate the effect of lysophosphatidic acid (LPA) receptor inhibition in a mouse model of autoantibody-mediated arthritis.

METHODS

Arthritis was induced in C57BL/6 mice by K/BxN serum transfer. Arthritic mice were treated with the LPA receptor antagonist, Ki16425 and arthritis severity was assessed clinically and histologically. Expression of inflammatory mediators in joints was identified by a mouse cytokine array and validated by western blot and real-time PCR assays. Effects of treatment with LPA receptor antagonist or with small interfering RNA on bone metabolism were assessed by in vitro assays of osteoclastogenesis, bone resorption, osteoblasts differentiation and bone mineralisation.

RESULTS

Mice treated with the LPA receptor antagonist Ki16425 showed attenuated arthritis characterised by reduction of synovial inflammation, cartilage damage and, more markedly, bone erosion. We detected increased apoptosis, reduction of inflammatory mediators and of bone remodelling proteins in arthritic joints from mice treated with Ki16425. In addition, we demonstrated that inhibition or suppression of LPA1 receptor reduces osteoclast differentiation and bone resorption and, on the contrary, it promotes differentiation of osteoblasts and bone mineralisation.

CONCLUSIONS

Pharmacological inhibition of LPA1 receptor in the K/BxN serum-transfer arthritis model led to reduction of severity of arthritis involving multiple mechanisms, increased apoptosis, reduced inflammatory mediators and proteins involved in bone remodelling, that show LPA1 as a very promising target in rheumatoid arthritis treatment.

Distinct phospholipase C-β isozymes mediate lysophosphatidic acid receptor 1 effects on intestinal epithelial homeostasis and wound closure

Maintenance of the epithelial barrier in the intestinal tract is necessary to protect the host from the hostile luminal environment. Phospholipase C-β (PLC-β) has been implicated to control myriad signaling cascades. However, the biological effects of selective PLC-β isozymes are poorly understood. We describe novel findings that lysophosphatidic acid (LPA) regulates PLC-β1 and PLC-β2 via two distinct pathways to enhance intestinal epithelial cell (IEC) proliferation and migration that facilitate wound closure and recovery of the intestinal epithelial barrier. LPA acting on the LPA1 receptor promotes IEC migration by facilitating the interaction of Gαq with PLC-β2. LPA-induced cell proliferation is PLC-β1 dependent and involves translocation of Gαq to the nucleus, where it interacts with PLC-β1 to induce cell cycle progression. An in vivo study using LPA1-deficient mice (Lpar1(-/-)) shows a decreased number of proliferating IECs and migration along the crypt-luminal axis. Additionally, LPA enhances migration and proliferation of IECs in an LPA1-dependent manner, and Lpar1(-/-) mice display defective mucosal wound repair that requires cell proliferation and migration. These findings delineate novel LPA1-dependent lipid signaling that facilitates mucosal wound repair via spatial targeting of distinct PLC-βs within the cell.

Constitutive lymphocyte transmigration across the basal lamina of high endothelial venules is regulated by the autotaxin/lysophosphatidic acid axis

Lymphocyte extravasation from the high endothelial venules (HEVs) of lymph nodes is crucial for the maintenance of immune homeostasis, but its molecular mechanism remains largely unknown. In this article, we report that lymphocyte transmigration across the basal lamina of the HEVs is regulated, at least in part, by autotaxin (ATX) and its end-product, lysophosphatidic acid (LPA). ATX is an HEV-associated ectoenzyme that produces LPA from lysophosphatidylcholine (LPC), which is abundant in the systemic circulation. In agreement with selective expression of ATX in HEVs, LPA was constitutively and specifically detected on HEVs. In vivo, inhibition of ATX impaired the lymphocyte extravasation from HEVs, inducing lymphocyte accumulation within the endothelial cells (ECs) and sub-EC compartment; this impairment was abrogated by LPA. In vitro, both LPA and LPC induced a marked increase in the motility of HEV ECs; LPC's effect was abrogated by ATX inhibition, whereas LPA's effect was abrogated by ATX/LPA receptor inhibition. In an in vitro transmigration assay, ATX inhibition impaired the release of lymphocytes that had migrated underneath HEV ECs, and these defects were abrogated by LPA. This effect of LPA was dependent on myosin II activity in the HEV ECs. Collectively, these results strongly suggest that HEV-associated ATX generates LPA locally; LPA, in turn, acts on HEV ECs to increase their motility, promoting dynamic lymphocyte-HEV interactions and subsequent lymphocyte transmigration across the basal lamina of HEVs at steady state.

Single application of A2 NTX, a botulinum toxin A2 subunit, prevents chronic pain over long periods in both diabetic and spinal cord injury-induced neuropathic pain models

Botulinum toxin type A is a unique candidate for inhibition of pain transmission. In the present study we attempted to see the beneficial actions of A2 neurotoxin (NTX), an active subunit of botulinum toxin type A. Intraplantar injection of A2 NTX significantly suppressed mechanical allodynia and hypersensitivities to A-fiber stimuli in the diabetic neuropathic pain model. Spinal application of A2 NTX also showed a potent suppression of thermal hyperalgesia and mechanical allodynia in the spinal cord injury-induced neuropathic pain model. A2 NTX seems to be a long-lasting treatment for diabetic and spinal cord injury-induced neuropathic pain.

Pre-emptive morphine treatment abolishes nerve injury-induced lysophospholipid synthesis in mass spectrometrical analysis

We have previously demonstrated that lysophosphatidic acid (LPA) production in the spinal cord following partial sciatic nerve injury (SCNI) and its signaling initiate neuropathic pain. In order to examine whether LPA production depends on the intense nociceptive signal, we have attempted to see suppression by pre-emptive treatment with centrally administered morphine, which mainly inhibits nociceptive signal at the level of spinal cord. In the present study, we developed a quantitative mass spectrometry assay to simultaneously analyze several species of lysophosphatidyl choline (LPC). The levels of 16:0-, 18:0- and 18:1-LPC in the spinal cord and dorsal root were maximally increased at 75 min after SCNI and then declined, as LPC is converted to LPA by autotaxin (ATX). In atx(+/-)-mice, on the other hand, these levels were similar to wild-type mice at 75 min, but maximal at 120 min, suggesting that this difference is partly due to the low conversion of LPC to LPA in atx(+/-)-mice. When morphine was centrally administered before SCNI, the injury-induced increase of LPC was completely abolished. These results suggest that LPC (or LPA) is produced by injury-induced nociceptive signal, which is effectively and pre-emptively suppressed by central morphine, possibly through known descending anti-nociceptive pathways.

Involvement of lysophosphatidic acid in bone cancer pain by potentiation of TRPV1 via PKCε pathway in dorsal root ganglion neurons

Background

It has been demonstrated that lysophosphatidic acid (LPA) released from injury tissue and transient receptor potential vanilloid 1 (TRPV1) receptor are implicated in the induction of chronic pain. In the present study we examined whether an interaction between LPA receptor LPA₁ and TRPV1 in dorsal root ganglion (DRG) neurons contributes to the development of bone cancer pain.

Results

Bone cancer was established by injection of mammary gland carcinoma cells into the rat tibia. Following the development of bone cancer pain, the TRPV1 expression and capsaicin-evoked currents were up-regulated in rat DRG neurons at L_4-6 segments. Immunohistochemistry staining revealed a high co-localization of LPA₁ with TRPV1 in DRG neurons. In isolated DRG neurons, whole-cell patch recording showed that capsaicin-induced currents were potentiated by LPA in a dose-dependent manner. The potentiation was blocked by either LPA₁ antagonist, protein kinase C (PKC) inhibitor or PKCϵ inhibitor, but not by protein kinase A (PKA) inhibitor or Rho inhibitor. In the behavioral tests, both mechanical allodynia and thermal hyperalgesia in bone cancer rats were attenuated by LPA₁ antagonist.

Conclusion

LPA potentiates TRPV1 current via a PKCϵ-dependent pathway in DRG neurons of rats with bone cancer, which may be a novel peripheral mechanism underlying the induction of bone cancer pain.

Microglial activation mediates de novo lysophosphatidic acid production in a model of neuropathic pain

We recently demonstrated that de novo lysophosphatidic acid (LPA) production in the spinal cord occurs in the early phase after nerve injury or LPA injection, and underlies the peripheral mechanisms of neuropathic pain. In this study, we examined the possible involvement of spinal cord microglia in such LPA-mediated functions. Intrathecal LPA injection rapidly increased the gene expression of CD11b and protein expression of phosphor-p38, accompanied by a morphological change of microglia from a ramified to amoeboid shape. Although early treatment with minocycline significantly inhibited LPA-induced neuropathic pain-like behavior and microglial activation, late treatment did not. Early treatment with minocycline also blocked LPA-evoked de novo LPA production and the increased activation of cytosolic phospholipase A(2), an LPA synthesis-related enzyme. Similar results were observed when the sciatic nerve was partially injured: early, but not late, treatment with minocycline significantly inhibited the injury-induced neuropathic pain, microglial activation, de novo LPA production and the underlying increased activation of cytosolic phospholipase A(2) as well as calcium-independent phospholipase A(2), another LPA synthesis-related enzyme. These findings suggest that the early phase of microglial activation is involved in de novo LPA production, and that this underlies the initial mechanisms of nerve injury-induced neuropathic pain.

Prostatic acid phosphatase reduces thermal sensitivity and chronic pain sensitization by depleting phosphatidylinositol 4,5-bisphosphate

Prostatic acid phosphatase (PAP) is expressed in nociceptive dorsal root ganglion (DRG) neurons, functions as an ectonucleotidase, and generates adenosine extracellularly. Here, we found that PAP inhibits noxious thermal sensitivity and sensitization that is associated with chronic pain through sustained activation of the adenosine A(1) receptor (A(1)R) and phospholipase C-mediated depletion of phosphatidylinositol 4,5-bisphosphate (PIP(2)). In mice, intrathecal injection of PAP reduced PIP(2) levels in DRGs, inhibited thermosensation through TRPV1, and enduringly reduced thermal hyperalgesia and mechanical allodynia caused by inflammation, nerve injury, and pronociceptive receptor activation. This included inhibitory effects on lysophosphatidic acid, purinergic (ATP), bradykinin, and protease-activated (thrombin) receptors. Conversely, PIP(2) levels were significantly elevated in DRGs from Pap(-/-) mice, and this correlated with enhanced thermal hyperalgesia and mechanical allodynia in Pap(-/-) mice. To directly test the importance of PIP(2) in nociception, we intrathecally injected PIP(2) into mice. This transiently (2 h) elevated PIP(2) levels in lumbar DRGs and transiently (2 h) enhanced thermosensation. Additionally, thermal hyperalgesia and mechanical allodynia were enduringly enhanced when PIP(2) levels were elevated coincident with injury/pronociceptive receptor stimulation. Nociceptive sensitization was not affected if PIP(2) levels were elevated in the absence of ongoing pronociceptive receptor stimulation. Together, our data suggest that PIP(2) levels in DRGs directly influence thermosensation and the magnitude of nociceptive sensitization. Moreover, our data suggest there is an underlying "phosphoinositide tone" that can be manipulated by an adenosine-generating ectonucleotidase. This tone regulates how effectively acute nociceptive insults promote the transition to chronic pain.

Calpain-mediated down-regulation of myelin-associated glycoprotein in lysophosphatidic acid-induced neuropathic pain

Lysophosphatidic acid receptor (LPA(1)) signaling initiates neuropathic pain through demyelination of the dorsal root (DR). Although LPA is found to cause down-regulation of myelin proteins underlying demyelination, the detailed mechanism remains to be determined. In the present study, we found that a single intrathecal injection of LPA evoked a dose- and time-dependent down-regulation of myelin-associated glycoprotein (MAG) in the DR through LPA(1) receptor. A similar event was also observed in ex vivo DR cultures. Interestingly, LPA-induced down-regulation of MAG was significantly inhibited by calpain inhibitors (calpain inhibitor X, E-64 and E-64d) and LPA markedly induced calpain activation in the DR. The pre-treatment with calpain inhibitors attenuated LPA-induced neuropathic pain behaviors such as hyperalgesia and allodynia. Moreover, we found that sciatic nerve injury activates calpain activity in the DR in a LPA(1) receptor-dependent manner. The E-64d treatments significantly blocked nerve injury-induced MAG down-regulation and neuropathic pain. However, there was no significant calpain activation in the DR by complete Freund's adjuvant treatment, and E-64d failed to show anti-hyperalgesic effects in this inflammation model. The present study provides strong evidence that LPA-induced calpain activation plays a crucial role in the manifestation of neuropathic pain through MAG down-regulation in the DR.

Role of PAF receptor in proinflammatory cytokine expression in the dorsal root ganglion and tactile allodynia in a rodent model of neuropathic pain

BACKGROUND

Neuropathic pain is a highly debilitating chronic pain following damage to peripheral sensory neurons and is often resistant to all treatments currently available, including opioids. We have previously shown that peripheral nerve injury induces activation of cytosolic phospholipase A(2) (cPLA(2)) in injured dorsal root ganglion (DRG) neurons that contribute to tactile allodynia, a hallmark of neuropathic pain. However, lipid mediators downstream of cPLA(2) activation to produce tactile allodynia remain to be determined.

PRINCIPAL FINDINGS

Here we provide evidence that platelet-activating factor (PAF) is a potential candidate. Pharmacological blockade of PAF receptors (PAFRs) reduced the development and expression of tactile allodynia following nerve injury. The expression of PAFR mRNA was increased in the DRG ipsilateral to nerve injury, which was seen mainly in macrophages. Furthermore, mice lacking PAFRs showed a reduction of nerve injury-induced tactile allodynia and, interestingly, a marked suppression of upregulation of tumor necrosis factor alpha (TNFalpha) and interleukin-1beta (IL-1beta) expression in the injured DRG, crucial proinflammatory cytokines involved in pain hypersensitivity. Conversely, a single injection of PAF near the DRG of naïve rats caused a decrease in the paw withdrawal threshold to mechanical stimulation in a dose-dependent manner and an increase in the expression of mRNAs for TNFalpha and IL-1beta, both of which were inhibited by pretreatment with a PAFR antagonist.

CONCLUSIONS

Our results indicate that the PAF/PAFR system has an important role in production of TNFalpha and IL-1beta in the DRG and tactile allodynia following peripheral nerve injury and suggest that blocking PAFRs may be a viable therapeutic strategy for treating neuropathic pain.

Lysophosphatidic acid-3 receptor-mediated feed-forward production of lysophosphatidic acid: an initiator of nerve injury-induced neuropathic pain

BACKGROUND

We previously reported that intrathecal injection of lysophosphatidylcholine (LPC) induced neuropathic pain through activation of the lysophosphatidic acid (LPA)-1 receptor, possibly via conversion to LPA by autotaxin (ATX).

RESULTS

We examined in vivo LPA-induced LPA production using a biological titration assay with B103 cells expressing LPA1 receptors. Intrathecal administration of LPC caused time-related production of LPA in the spinal dorsal horn and dorsal roots, but not in the dorsal root ganglion, spinal nerve or sciatic nerve. LPC-induced LPA production was markedly diminished in ATX heterozygotes, and was abolished in mice that were deficient in LPA3, but not LPA1 or LPA2 receptors. Similar time-related and LPA3 receptor-mediated production of LPA was observed following intrathecal administration of LPA. In an in vitro study using spinal cord slices, LPA-induced LPA production was also mediated by ATX and the LPA3 receptor. Intrathecal administration of LPA, in contrast, induced neuropathic pain, which was abolished in mice deficient in LPA1 or LPA3 receptors.

CONCLUSION

These findings suggest that feed-forward LPA production is involved in LPA-induced neuropathic pain.