Conditional Lpar1 gene targeting identifies cell types mediating neuropathic pain

HSP27 Modulates Neuropathic Pain by Inhibiting P2X3 Degradation

The role of heat shock protein 27 (HSP27), a chaperone, in neuropathic pain after nerve injury has not been systematically surveyed despite its neuroprotective and regeneration-promoting effects. In this study, we found that HSP27 expression in sensory neurons of the dorsal root ganglia (DRG) mediated nerve injury-induced neuropathic pain. Neuropathic pain behaviors were alleviated by silencing HSP27 in the DRG of a rat spinal nerve ligation (SNL) model. Local injection of an HSP27-overexpression construct into the DRG of naïve rats elicited neuropathic pain behaviors. HSP27 interacted with a purinergic receptor, P2X3, and their expression patterns corroborated the induction and reversal of neuropathic pain according to two lines of evidence: colocalization immunohistochemically and immunoprecipitation biochemically. In a cell model cotransfected with HSP27 and P2X3, the degradation rate of P2X3 was reduced in the presence of HSP27. Such an alteration was mediated by reducing P2X3 ubiquitination in SNL rats and was reversed after silencing HSP27 in the DRGs of SNL rats. In summary, the interaction of HSP27 with P2X3 provides a new mechanism of injury-induced neuropathic pain that could serve as an alternative therapeutic target.

Exosome-associated lysophosphatidic acid signaling contributes to cancer pain

ABSTRACT

Pain associated with bone cancer remains poorly managed, and chemotherapeutic drugs used to treat cancer usually increase pain. The discovery of dual-acting drugs that reduce cancer and produce analgesia is an optimal approach. The mechanisms underlying bone cancer pain involve interactions between cancer cells and nociceptive neurons. We demonstrated that fibrosarcoma cells express high levels of autotaxin (ATX), the enzyme synthetizing lysophosphatidic acid (LPA). Lysophosphatidic acid increased proliferation of fibrosarcoma cells in vitro. Lysophosphatidic acid is also a pain-signaling molecule, which activates LPA receptors (LPARs) located on nociceptive neurons and satellite cells in dorsal root ganglia. We therefore investigated the contribution of the ATX-LPA-LPAR signaling to pain in a mouse model of bone cancer pain in which fibrosarcoma cells are implanted into and around the calcaneus bone, resulting in tumor growth and hypersensitivity. LPA was elevated in serum of tumor-bearing mice, and blockade of ATX or LPAR reduced tumor-evoked hypersensitivity. Because cancer cell-secreted exosomes contribute to hypersensitivity and ATX is bound to exosomes, we determined the role of exosome-associated ATX-LPA-LPAR signaling in hypersensitivity produced by cancer exosomes. Intraplantar injection of cancer exosomes into naive mice produced hypersensitivity by sensitizing C-fiber nociceptors. Inhibition of ATX or blockade of LPAR attenuated cancer exosome-evoked hypersensitivity in an ATX-LPA-LPAR-dependent manner. Parallel in vitro studies revealed the involvement of ATX-LPA-LPAR signaling in direct sensitization of dorsal root ganglion neurons by cancer exosomes. Thus, our study identified a cancer exosome-mediated pathway, which may represent a therapeutic target for treating tumor growth and pain in patients with bone cancer.

The Role of Autotaxin and LPA Signaling in Embryonic Development, Pathophysiology and Cancer

Autotaxin (ATX) or Ectonucleotide Pyrophosphatase/Phosphodiesterase 2 (ENPP2) is a secreted enzyme with lysophospholipase D activity, with its primary function being the extracellular hydrolysis of lysophosphatidylcholine (LPC) to lysophosphatidic acid (LPA), a bioactive lipid [...].

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.

Schwann cell stimulation induces functional and structural changes in peripheral nerves

Signal propagation is the essential function of nerves. Lysophosphatidic acid 18:1 (LPA) allows the selective stimulation of calcium signaling in Schwann cells but not neurons. Here, the time course of slowing and amplitude reduction on compound action potentials due to LPA exposure was observed in myelinated and unmyelinated fibers of the mouse, indicating a clear change of axonal function. Teased nerve fiber imaging showed that Schwann cell activation is also present in axon-attached Schwann cells in freshly isolated peripheral rat nerves. The LPA receptor 1 was primarily localized at the cell extensions in isolated rat Schwann cells, suggesting a role in cell migration. Structural investigation of rat C-fibers demonstrated that LPA leads to an evagination of the axons from their Schwann cells. In A-fibers, the nodes of Ranvier appeared unchanged, but the Schmidt-Lanterman incisures were shortened and myelination reduced. The latter might increase leak current, reducing the potential spread to the next node of Ranvier and explain the changes in conduction velocity. The observed structural changes provide a plausible explanation for the functional changes in myelinated and unmyelinated axons of peripheral nerves and the reported sensory sensations such as itch and pain.

Designing Dual Inhibitors of Autotaxin-LPAR GPCR Axis

The ATX-LPA-LPAR1 signaling pathway plays a universal role in stimulating diverse cellular responses, including cell proliferation, migration, survival, and invasion in almost every cell type. The ATX-LPAR1 axis is linked to several metabolic and inflammatory diseases including cancer, fibrosis, and rheumatoid arthritis. Numerous selective ATX or LPAR1 inhibitors have been developed and so far, their clinical efficacy has only been evaluated in idiopathic pulmonary fibrosis. None of the ATX and LPAR1 inhibitors have advanced to clinical trials for cancer and rheumatoid arthritis. Nonetheless, several research groups, including ours, have shown considerable benefit of simultaneous ATX and LPAR1 inhibition through combination therapy. Recent research suggests that dual-targeting therapies are superior to combination therapies that use two selective inhibitors. However, limited reports are available on ATX-LPAR1 dual inhibitors, potentially due to co-expression of multiple different LPARs with close structural similarities at the same target. In this review, we discuss rational design and future directions of dual ATX-LPAR1 inhibitors.

Pain-like behavior in the collagen antibody-induced arthritis model is regulated by lysophosphatidic acid and activation of satellite glia cells

Inflammatory and neuropathic-like components underlie rheumatoid arthritis (RA)-associated pain, and lysophosphatidic acid (LPA) is linked to both joint inflammation in RA patients and to neuropathic pain. Thus, we investigated a role for LPA signalling using the collagen antibody-induced arthritis (CAIA) model. Pain-like behavior during the inflammatory phase and the late, neuropathic-like phase of CAIA was reversed by a neutralizing antibody generated against LPA and by an LPA_1/3 receptor inhibitor, but joint inflammation was not affected. Autotaxin, an LPA synthesizing enzyme was upregulated in dorsal root ganglia (DRG) neurons during both CAIA phases, but not in joints or spinal cord. Late-phase pronociceptive neurochemical changes in the DRG were blocked in Lpar1 receptor deficient mice and reversed by LPA neutralization. In vitro and in vivo studies indicated that LPA regulates pain-like behavior via the LPA₁ receptor on satellite glia cells (SGCs), which is expressed by both human and mouse SGCs in the DRG. Furthermore, CAIA-induced SGC activity is reversed by phospholipid neutralization and blocked in Lpar1 deficient mice. Our findings suggest that the regulation of CAIA-induced pain-like behavior by LPA signalling is a peripheral event, associated with the DRGs and involving increased pronociceptive activity of SGCs, which in turn act on sensory neurons.

Generation of an Lpar1-EGFP Fusion Knock-in Transgenic Mouse Line

Lysophosphatidic acid (LPA) is a lysophospholipid that acts as an extracellular signal through the activation of cognate G protein-coupled receptors (GPCRs). There are six known LPA receptors (LPA_1-6). The first such receptor, LPA₁, was identified in the embryonic brain and has been studied extensively for gene expression throughout the body, including through studies of receptor-null mice. However, identifying receptor protein expression in situ and in vivo within living cells and tissues has been difficult because of biologically low receptor expression and variable antibody specificity. To visualize native LPA₁ receptor expression in situ, we generated a knock-in mouse produced by homologous recombination in murine embryonic stem (ES) cells to replace a wildtype Lpar1 allele with a mutant allele created by in-frame fusion of EGFP to the 4th exon of Lpar1 (Lpar1-EGFP knock-in allele). Homozygous knock-in mice appeared normal and the expected mendelian ratios of knock-in allele transmission were present in females and males. Histological assessments of the fetal and adult central nervous system (CNS) demonstrated expression patterns that were consistent with prior in situ hybridization studies. This new mouse line will be useful for studies of LPA₁ in the developing and adult CNS, as well as other tissues, and for receptor assessments in living tissues and disease models.

Critical Roles of Lysophospholipid Receptors in Activation of Neuroglia and Their Neuroinflammatory Responses

Activation of microglia and/or astrocytes often releases proinflammatory molecules as critical pathogenic mediators that can promote neuroinflammation and secondary brain damages in diverse diseases of the central nervous system (CNS). Therefore, controlling the activation of glial cells and their neuroinflammatory responses has been considered as a potential therapeutic strategy for treating neuroinflammatory diseases. Recently, receptor-mediated lysophospholipid signaling, sphingosine 1-phosphate (S1P) receptor- and lysophosphatidic acid (LPA) receptor-mediated signaling in particular, has drawn scientific interest because of its critical roles in pathogenies of diverse neurological diseases such as neuropathic pain, systemic sclerosis, spinal cord injury, multiple sclerosis, cerebral ischemia, traumatic brain injury, hypoxia, hydrocephalus, and neuropsychiatric disorders. Activation of microglia and/or astrocytes is a common pathogenic event shared by most of these CNS disorders, indicating that lysophospholipid receptors could influence glial activation. In fact, many studies have reported that several S1P and LPA receptors can influence glial activation during the pathogenesis of cerebral ischemia and multiple sclerosis. This review aims to provide a comprehensive framework about the roles of S1P and LPA receptors in the activation of microglia and/or astrocytes and their neuroinflammatory responses in CNS diseases.

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.

An Investigation of the Molecular Mechanisms Underlying the Analgesic Effect of Jakyak-Gamcho Decoction: A Network Pharmacology Study

Herbal drugs have drawn substantial interest as effective analgesic agents; however, their therapeutic mechanisms remain to be fully understood. To address this question, we performed a network pharmacology study to explore the system-level mechanisms that underlie the analgesic activity of Jakyak-Gamcho decoction (JGd; Shaoyao-Gancao-Tang in Chinese and Shakuyaku-Kanzo-To in Japanese), an herbal prescription consisting of Paeonia lactiflora Pallas and Glycyrrhiza uralensis Fischer. Based on comprehensive information regarding the pharmacological and chemical properties of the herbal constituents of JGd, we identified 57 active chemical compounds and their 70 pain-associated targets. The JGd targets were determined to be involved in the regulation of diverse biological activities as follows: calcium- and cytokine-mediated signalings, calcium ion concentration and homeostasis, cellular behaviors of muscle and neuronal cells, inflammatory response, and response to chemical, cytokine, drug, and oxidative stress. The targets were further enriched in various pain-associated signalings, including the PI3K-Akt, estrogen, ErbB, neurotrophin, neuroactive ligand-receptor interaction, HIF-1, serotonergic synapse, JAK-STAT, and cAMP pathways. Thus, these data provide a systematic basis to understand the molecular mechanisms underlying the analgesic activity of herbal drugs.

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.