BBA - Molecular and Cell Biology of Lipids (v.1831, #1)

A compelling question of how phospholipids interact with their target receptors has been of interest since the first receptor-mediated effects were reported. The recent report of a crystal structure for the S1P1 receptor in complex with an antagonist phospholipid provides interesting perspective on the insights that had previously been gained through structure–activity studies of the phospholipids, as well as modeling and mutagenesis studies of the receptors. This review integrates these varied lines of investigation in the context of their various contributions to our current understanding of phospholipid–receptor interactions. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► Phospholipid interactions with LPA and S1P receptors reflected in phospholipid SAR ► Integrated view of phospholipid receptor structures from experiments and modeling ► Protein–protein interactions mediated by C-terminal motifs in the LPA receptors
Keywords: S1P; LPA; GPCR structure; Structure–activity relationships;

Autotaxin in embryonic development by Wouter H. Moolenaar; Anna J.S. Houben; Shyh-Jye Lee; Laurens A. van Meeteren (13-19).
Autotaxin (ATX) is a secreted lysophospholipase D that generates the multifunctional lipid mediator lysophosphatidic acid (LPA). LPA signals through six distinct G protein-coupled receptors, acting alone or in concert to activate multiple effector pathways. The ATX–LPA signaling axis is implicated in a remarkably wide variety of physiological and pathological processes and plays a vital role in embryonic development. Disruption of the ATX-encoding gene (Enpp2) in mice results in intrauterine death due to vascular defects in the extra-embryonic yolk sac and embryo proper. In addition, Enpp2 (−/−) embryos show impaired neural development. The observed angiogenic defects are attributable, at least in part, to loss of LPA signaling through the Gα12/13-linked RhoA-ROCK-actin remodeling pathway. Studies in zebrafish also have uncovered a dual role for ATX in both vascular and neural development; furthermore, they point to a key role for ATX–LPA signaling in the regulation of left–right asymmetry. Here we discuss our present understanding of the role of ATX–LPA signaling in vertebrate development. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► Autotaxin–LPA receptor signaling is vital for vertebrate development. ► Gene targeting studies in mice and zebrafish reveal key roles for ATX in vascular and neural development. ► Studies in zebrafish point to a novel role for ATX and the LPA3 receptor in regulating left–right asymmetry.
Keywords: Autotaxin; Lysophosphatidic acid; Embryonic development; G protein-coupled receptor; Vasculogenesis;

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), two of the best-studied lysophospholipids, are known to influence diverse biological events, including organismal development as well as function and pathogenesis within multiple organ systems. These functional roles are due to a family of at least 11 G protein-coupled receptors (GPCRs), named LPA1–6 and S1P1–5, which are widely distributed throughout the body and that activate multiple effector pathways initiated by a range of heterotrimeric G proteins including Gi/o, G12/13, Gq and Gs, with actual activation dependent on receptor subtypes. In the central nervous system (CNS), a major locus for these signaling pathways, LPA and S1P have been shown to influence myriad responses in neurons and glial cell types through their cognate receptors. These receptor-mediated activities can contribute to disease pathogenesis and have therapeutic relevance to human CNS disorders as demonstrated for multiple sclerosis (MS) and possibly others that include congenital hydrocephalus, ischemic stroke, neurotrauma, neuropsychiatric disorders, developmental disorders, seizures, hearing loss, and Sandhoff disease, based upon the experimental literature. In particular, FTY720 (fingolimod, Gilenya, Novartis Pharma, AG) that becomes an analog of S1P upon phosphorylation, was approved by the FDA in 2010 as a first oral treatment for MS, validating this class of receptors as medicinal targets. This review will provide an overview and update on the biological functions of LPA and S1P signaling in the CNS, with a focus on results from studies using genetic null mutants for LPA and S1P receptors. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► This is a review on CNS aspects of lysophospholipids (LPs) and their receptors. ► LPs are bioactive lipids originating from the cell membrane and include LPA and S1P. ► Cognate GPCRs account for LP biology, pathology and new therapeutics.
Keywords: Lysophosphatidic acid; Sphingosine 1-phosphate; G protein-coupled receptor; Central nervous system; CNS disease;

Current progress in non-Edg family LPA receptor research by Keisuke Yanagida; Yoshitaka Kurikawa; Takao Shimizu; Satoshi Ishii (33-41).
Lysophosphatidic acid (LPA) is the simplest phospholipid yet possesses myriad biological functions. Until 2003, the functions of LPA were thought to be elicited exclusively by three subtypes of the endothelial differentiation gene (Edg) family of G protein-coupled receptors — LPA1, LPA2, and LPA3. However, several biological functions of LPA could not be assigned to any of these receptors indicating the existence of one or more additional LPA receptor(s). More recently, the discovery of a second cluster of LPA receptors which includes LPA4, LPA5, and LPA6 has paved the way for new avenues of LPA research. Analyses of these non-Edg family LPA receptors have begun to fill in gaps to understand biological functions of LPA such as platelet aggregation and vascular development that could not be ascribed to classical Edg family LPA receptors and are also unveiling new biological functions. Here we review recent progress in the non-Edg family LPA receptor research, with special emphasis on the pharmacology, signaling, and physiological roles of this family of receptors. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► Recently identified non-Edg family of LPA receptors (LPA4, LPA5, and LPA6). ► Ligand selectivities of non-Edg family LPA receptors. ► Intracellular signaling evoked by the non-Edg family LPA receptor activation. ► Biological functions of non-Edg family LPA receptors.
Keywords: LPA4; LPA5; LPA6; GPR23/p2y9; GPR92/GPR93; p2y5;

Lysoglycerophospholipids in chronic inflammatory disorders: The PLA2/LPC and ATX/LPA axes by Ioanna Sevastou; Eleanna Kaffe; Marios-Angelos Mouratis; Vassilis Aidinis (42-60).
Lysophosphatidylcholine (LPC) and lysophosphatidic acid (LPA), the most prominent lysoglycerophospholipids, are emerging as a novel class of inflammatory lipids, joining thromboxanes, leukotrienes and prostaglandins with which they share metabolic pathways and regulatory mechanisms. Enzymes that participate in LPC and LPA metabolism, such as the phospholipase A2 superfamily (PLA2) and autotaxin (ATX, ENPP2), play central roles in regulating LPC and LPA levels and consequently their actions. LPC/LPA biosynthetic pathways will be briefly presented and LPC/LPA signaling properties and their possible functions in the regulation of the immune system and chronic inflammation will be reviewed. Furthermore, implications of exacerbated LPC and/or LPA signaling in the context of chronic inflammatory diseases, namely rheumatoid arthritis, multiple sclerosis, pulmonary fibrosis and hepatitis, will be discussed. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► LPC and LPA signaling is implicated in chronic inflammatory disorders. ► LPA is a pleiotropic growth factor. ► PLA2 is a drug target in MS. ► ATX is a novel player in chronic inflammatory disorders and an emerging drug target.
Keywords: Phospholipase A2 (PLA2); Lysophosphatidylcholine (LPC); Autotaxin (ATX, ENPP2); Lysophosphatidic acid (LPA); Inflammation;

Lysophosphatidic acid: Chemical signature of neuropathic pain by Hiroshi Ueda; Hayato Matsunaga; Omotuyi I. Olaposi; Jun Nagai (61-73).
Acute inflammatory pain signal originates from transient hypersensitivity in afferent fibers when depolarized via injured tissues or proinflammatory cells-derived pronociceptive ligand binding. This pain is sensitive to opioids and NSAIDs. In neuropathic pain, however, damage to the nerve along the pain pathway results in spontaneous generation of action potential and lowered nociceptive threshold, as seen in allodynia and hyperalgesia. This abnormal pain transmission had been linked to LPA production in the spinal cord, through activation of NMDA and NK1 activation by glutamate and SP in iPLA2/cPLA2/ATX-dependent pathway. In a bifurcated response involving Gq/11 and G12/13 coupling, Schwann cell LPA1 mediates degradation and transcriptional suppression of myelin proteins, respectively. The loss of contact inhibition on axonal growth creates cytoskeletal framework for axonal sprouting. LPA causes an amplification of LPA production through activation of LPA3 signaling in microglia immediately after nerve injury. LPA1 deficient mice (LPA 1 −/− ) show no neuropathic-pain behavior or demyelination in response to intrathecal LPA injection or nerve injury. Given these bodies of research evidence, LPA therefore presents as the chemical signature for the initiation of neuropathic pain. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► Roles of LPA-signaling in the development of neuropathic pain and its mechanisms ► LPA1 receptor-mediated mechanisms for demyelination and sprouting ► Roles of early phase of microglial activation in LPA-induced LPA production ► Roles of LPA3 receptor signaling in the feed-forward LPA production.
Keywords: Lysophosphatidic acid; Neuropathic pain; Demyelination; Schwann cell; Axonal sprouting; Calpain;

High expression of autotaxin in cancers is often associated with increased tumor progression, angiogenesis and metastasis. This is explained mainly since autotaxin produces the lipid growth factor, lysophosphatidate (LPA), which stimulates cell division, survival and migration. It has recently become evident that these signaling effects of LPA also produce resistance to chemotherapy and radiation-induced cell death. This results especially from the stimulation of LPA2 receptors, which depletes the cell of Siva-1, a pro-apoptotic signaling protein and stimulates prosurvival kinase pathways through a mechanism mediated via TRIP-6. LPA signaling also increases the formation of sphingosine 1-phosphate, a pro-survival lipid. At the same time, LPA decreases the accumulation of ceramides, which are used in radiation therapy and by many chemotherapeutic agents to stimulate apoptosis. The signaling actions of extracellular LPA are terminated by its dephosphorylation by a family of lipid phosphate phosphatases (LPP) that act as ecto-enzymes. In addition, lipid phosphate phoshatase-1 attenuates signaling downstream of the activation of both LPA receptors and receptor tyrosine kinases. This makes many cancer cells hypersensitive to the action of various growth factors since they often express low LPP1/3 activity. Increasing our understanding of the complicated signaling pathways that are used by LPA to stimulate cell survival should identify new therapeutic targets that can be exploited to increase the efficacy of chemo- and radio-therapy. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► Autotaxin and LPA receptors are upregulated in metastatic cancers. ► LPA receptors, particularly the LPA2 subtype convey resistance against apoptosis. ► LPA is removed from the extracellular compartment by lipid phosphate phosphatases. ► Expression of lipid phosphate phosphatases is often low in cancers. ► The ATX–LPA receptor–LPP axis influences the chemo- and radiation-resistance of cancers.
Keywords: Ceramide; Lipid phosphate phosphatase; Metastasis; Phospholipase D; Sphingosine kinase; LPA2;

Lysophosphatidic acid (LPA), a simple bioactive phospholipid, is present in biological fluids such as plasma and bronchoalveolar lavage (BAL). It appears to have both pro- and anti-inflammatory roles in inflammatory lung diseases. Exogenous LPA promotes inflammatory responses by regulating the expression of chemokines, cytokines, and cytokine receptors in lung epithelial cells. In addition to the modulation of inflammatory responses, LPA regulates cytoskeleton rearrangement and confers protection against lung injury by enhancing lung epithelial cell barrier integrity and remodeling. The biological effects of LPA are mediated through its cell surface G-protein coupled LPA1–7 receptors. The roles of LPA receptors in lung fibrosis, asthma, and acute lung injury have been investigated using genetically engineered LPA receptor deficient mice and there appears to be a definitive role for endogenous LPA and its receptors in the pathogenesis of pulmonary inflammatory diseases. This review summarizes recent reports on the role of LPA and its receptors in the regulation of lung epithelial inflammatory responses and remodeling. This article is part of a Special Issue entitled: Advances in Lysophospholipid Research.► LPA regulates cytokines and cytokine decoy receptors release in lung epithelial cells. ► LPA regulates cell–cell adherens junction and cell motility. ► LPA induces its biological responses via LPA receptors and intracellular signaling pathways. ► LPA receptors contribute to pathogenesis of lung inflammatory diseases.
Keywords: Lysophosphatidic acid (LPA); LPA receptor; Lung epithelial cell; Cytokine; Cell motility; Lung inflammatory disease;

Bone defects in LPA receptor genetically modified mice by Jean Pierre Salles; Sara Laurencin-Dalicieux; Françoise Conte-Auriol; Fabienne Briand-Mésange; Isabelle Gennero (93-98).
LPA and LPA1 have been shown to increase osteoblastic proliferation and differentiation as well as activation of osteoclasts. Cell and animal model studies have suggested that LPA is produced by bone cells and bone tissues. We obtained data from invalidated mice which support the hypothesis that LPA1 is involved in bone development by promoting osteogenesis. LPA1-invalidated mice demonstrate growth and sternal and costal abnormalities, which highlights the specific roles of LPA1 during bone development. Microcomputed tomography and histological analysis demonstrate osteoporosis in the trabecular and cortical bone of LPA1-invalidated mice. Moreover, bone marrow mesenchymal progenitors from these mice displayed decreased osteoblastic differentiation. Infrared analysis did not indicate osteomalacia in the bone tissue of LPA1-invalidated mice. LPA1 displays opposite effects to LPA4 on the related G proteins Gi and Gs, responsible for decrease and increase of the cAMP level respectively, which itself is essential to the control of osteoblastic differentiation. The opposite effects of LPA1 and LPA4 during osteoblastic differentiation support the possibility that new pharmacological agents derived from the LPA pathways could be found and used in clinical practice to positively influence bone formation and treat osteoporosis. The paracrine effect of LPA is potentially modulated by its concentration in bone tissues, which may result from various intracellular and extracellular pathways. The relevance of LPA1 in bone remodeling, as a receptor able to influence both osteoblast and osteoclast activity, still deserves further clarification. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► Lysophosphatidic acid is present in bone tissues, produced by different pathways and cells. ► LPA1 receptor is involved in bone cells proliferation and differentiation. ► LPA1-invalidated mice display bone abnormalities. ► Microcomputed tomography and histological analysis demonstrate osteoporosis in LPA1-invalidated mice. ► Stem cells from LPA1-invalidated mice demonstrate decreased potentiality for differentiation and mineralization.
Keywords: LPA1; Lysophosphatidic acid; Parathyroid hormone-related peptide; Bone resorption; Osteogenesis; Osteoporosis;

Pleiotropic activity of lysophosphatidic acid in bone metastasis by Olivier Peyruchaud; Raphael Leblanc; Marion David (99-104).
Bone is a common metastatic site for solid cancers. Bone homeostasis is tightly regulated by intimate cross-talks between osteoblast (bone forming cells) and osteoclasts (bone resorbing cells). Once in the bone microenvironment, metastatic cells do not alter bone directly but instead perturb the physiological balance of the bone remodeling process controlled by bone cells. Tumor cells produce growth factors and cytokines stimulating either osteoclast activity leading to osteolytic lesions or osteoblast function resulting in osteoblastic metastases. Growth factors, released from the resorbed bone matrix or throughout osteoblastic bone formation, sustain tumor growth. Therefore, bone metastases are the sites of vicious cycles wherein tumor growth and bone metabolism sustain each other. Lysophosphatidic acid (LPA) promotes the growth of primary tumors and metastatic dissemination of cancer cells. We have shown that by acting on cancer cells via the contribution of blood platelets and the LPA-producing enzyme Autotaxin (ATX), LPA promotes the progression of osteolytic bone metastases in animal models. In the light of recent reports it would appear that the role of LPA in the context of bone metastases is complex involving multiple sources of lipid combined with direct and indirect effects on target cells. This review will present our current knowledge on the LPA/ATX axis involvement in osteolytic and osteoblastic skeletal metastases and will discuss the potential activity of LPA upstream and downstream metastasis seeding of cancer cells to bone as well as its implication in cancer induced bone pain. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► The LPA/ATX axis controls osteolytic and potentially osteoblastic bone metastases. ► Osteotropic cancer cells express LPA receptors. ► Blocking LPA1 signaling impairs bone metastasis formation. ► LPA is produced at the bone metastatic site through multiple mechanisms. ► LPA is a pro-osteoclastic factor.
Keywords: Lysophosphatidic acid; Autotaxin; Osteoclast; Osteoblast; Bone metastasis; Mouse model;

The simplest signalling lipid Lysophosphatidic acid (LPA) elicits pleiotropic actions upon most mammalian cell types. Although LPA has an established role in many biological processes, particularly wound healing and cancer, the function of LPA for human osteoblast (hOB) biology is still unravelling. Early studies, identified in this review, gave a reliable indication that LPA, via binding to one of several transmembrane receptors, stimulated multiple intracellular signalling networks coupled to changes in cell growth, fibronectin binding, maturation and survival. The majority of studies exploring the actions of LPA on hOB responses have done so using the lipid in isolation. Our own research has focussed on the co-operation of LPA with the active vitamin D3 metabolite, 1α25,dihydroxycholecalciferol (calcitriol), in light of a serendipitous discovery that calcitriol, in a serum-free culture setting, was unable to promote hOB maturation. We subsequently learnt that the serum-borne factor co-operating with calcitriol to enhance hOB differentiation was LPA bound to the albumin fraction of whole serum. Recent studies from our laboratory have identified that LPA and calcitriol are a potent pairing for securing hOB formation from their stem cell progeny. Greater understanding of the ability of LPA to influence, for example, hOB growth, maturation and survival could be advantageous in developing novel strategies aimed at improving skeletal tissue repair and regeneration. Herein this review provides an insight into the diversity of studies exploring the actions of a small lipid on a major cell type key to bone tissue health and homeostasis. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► Human osteoblasts (hOBs) express receptors for the simplest signalling lipid, LPA. ► LPA affects hOB fibronectin binding, growth, morphology, maturation and survival. ► LPA participates in bone development. ► LPA co-operates with calcitriol to secure hOB maturation upon biomaterials. ► LPA could find an application in bone repair via implant bio-functionalisation.
Keywords: Lysophosphatidic acid; Osteoblast; Stem cells; Active vitamin D; Differentiation; Osteoblastogenesis;

Lysophosphatidic acid: A potential mediator of osteoblast–osteoclast signaling in bone by Stephen M. Sims; Nattapon Panupinthu; Danielle M. Lapierre; Alexey Pereverzev; S. Jeffrey Dixon (109-116).
Osteoclasts (bone resorbing cells) and osteoblasts (bone forming cells) play essential roles in skeletal development, mineral homeostasis and bone remodeling. The actions of these two cell types are tightly coordinated, and imbalances in bone formation and resorption can result in disease states, such as osteoporosis. Lysophosphatidic acid (LPA) is a potent bioactive phospholipid that influences a number of cellular processes, including proliferation, survival and migration. LPA is also involved in wound healing and pathological conditions, such as tumor metastasis and autoimmune disorders. During trauma, activated platelets are likely a source of LPA in bone. Physiologically, osteoblasts themselves can also produce LPA, which in turn promotes osteogenesis. The capacity for local production of LPA, coupled with the proximity of osteoblasts and osteoclasts, leads to the intriguing possibility that LPA acts as a paracrine mediator of osteoblast–osteoclast signaling. Here we summarize emerging evidence that LPA enhances the differentiation of osteoclast precursors, and regulates the morphology, resorptive activity and survival of mature osteoclasts. These actions arise through stimulation of multiple LPA receptors and intracellular signaling pathways. Moreover, LPA is a potent mitogen implicated in promoting the metastasis of breast and ovarian tumors to bone. Thus, LPA released from osteoblasts is potentially an important autocrine and paracrine mediator — physiologically regulating skeletal development and remodeling, while contributing pathologically to metastatic bone disease. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► Osteoclasts and osteoblasts play key roles in skeletal development and remodeling. ► Lysophosphatidic acid (LPA) controls cell differentiation and survival. ► LPA can be produced by osteoblasts and serve as an autocrine and paracrine factor. ► LPA may coordinate osteoblast and osteoclast activity. ► LPA enhances osteoclast differentiation and the survival of mature osteoclasts.
Keywords: Apoptosis; Cytosolic calcium; Lysophosphatidic acid; Osteoblast; Osteoclast; Paracrine;

Mitigation of radiation injury by selective stimulation of the LPA2 receptor by Gyöngyi N. Kiss; Sue-Chin Lee; James I. Fells; Jianxiong Liu; William J. Valentine; Yuko Fujiwara; Karin Emmons Thompson; Charles R. Yates; Balázs Sümegi; Gabor Tigyi (117-125).
Due to its antiapoptotic action, derivatives of the lipid mediator lysophosphatidic acid (LPA) provide potential therapeutic utility in diseases associated with programmed cell death. Apoptosis is one of the major pathophysiological processes elicited by radiation injury to the organism. Consequently, therapeutic explorations applying compounds that mimic the antiapoptotic action of LPA have begun. Here we present a brief account of our decade-long drug discovery effort aimed at developing LPA mimics with a special focus on specific agonists of the LPA2 receptor subtype, which was found to be highly effective in protecting cells from apoptosis. We describe new evidence that 2-((3-(1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)propyl)thio)benzoic acid (GRI977143), a prototypic nonlipid agonist specific to the LPA2 receptor subtype, rescues apoptotically condemned cells in vitro and in vivo from injury caused by high-dose γ-irradiation. GRI977143 shows the features of a radiomitigator because it is effective in rescuing the lives of mice from deadly levels of radiation when administered 24 h after radiation exposure. Our findings suggest that by specifically activating LPA2 receptors GRI977143 activates the ERK1/2 prosurvival pathway, effectively reduces Bax translocation to the mitochondrion, attenuates the activation of initiator and effector caspases, reduces DNA fragmentation, and inhibits PARP-1 cleavage associated with γ-irradiation-induced apoptosis. GRI977143 also inhibits bystander apoptosis elicited by soluble proapoptotic mediators produced by irradiated cells. Thus, GRI977143 can serve as a prototype scaffold for lead optimization paving the way to more potent analogs amenable for therapeutic exploration. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► The nonlipid compound GRI977143 is a specific agonist of the LPA2 receptor subtype. ► GRI977143 rescues apoptotically condemned cells from radiation-induced programmed cell death. ► GRI977143 reduces mortality of mice exposed to lethal levels of γ-irradiation.
Keywords: LPA; Radiomitigator; Radioprotection; Apoptosis; Bystander-apoptosis; Distant effects of radiation exposure;

Lipid phosphate phosphatase (LPP3) and vascular development by H. Ren; M. Panchatcharam; P. Mueller; D. Escalante-Alcalde; A.J. Morris; S.S. Smyth (126-132).
Lipid phosphate phosphatases (LPP) are integral membrane proteins with broad substrate specificity that dephosphorylate lipid substrates including phosphatidic acid, lysophosphatidic acid, ceramide 1-phosphate, sphingosine 1-phosphate, and diacylglycerol pyrophosphate. Although the three mammalian enzymes (LPP1-3) demonstrate overlapping catalytic activities and substrate preferences in vitro, the phenotypes of mice with targeted inactivation of the Ppap2 genes encoding the LPP enzymes reveal nonredundant functions. A specific role for LPP3 in vascular development has emerged from studies of mice lacking Ppap2b. A meta-analysis of multiple, large genome-wide association studies identified a single nucleotide polymorphism in PPAP2B as a novel predictor of coronary artery disease. In this review, we will discuss the evidence that links LPP3 to vascular development and disease and evaluate potential molecular mechanisms. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► Lipid phosphate phosphatases (LPP) are membrane proteins that dephosphorylate lipids. ► LPP3‐catalyzed dephosphorylation of LPA and S1P renders them receptor inactive. ► A nonredundant role for LPP3 in vasculogenesis was revealed in mice lacking Ppap2b. ► A polymorphism in PPAP2B predicts coronary artery disease. ► LPP3's biologic effects may be due to catalytic or non-catalytic functions.
Keywords: Lysophospholipid; Vascular development; Sphingosine-1-phosphate; Lysophosphatidic acid; Endothelial cell;

Plasticity-related genes (PRGs, Lipid phosphate phosphatase-related proteins LPPRs) are a defined as a subclass of the lipid phosphate phosphatase (LPP) superfamily, comprising so far five brain- and vertebrate-specific membrane-spanning proteins. LPPs interfere with lipid phosphate signaling and are thereby involved in mediating the extracellular concentration and signal transduction of lipid phosphate esters such as lysophosphatidate (LPA) and spingosine-1 phosphate (S1P). LPPs dephosphorylate their substrates through extracellular catalytic domains, thus making them ecto-phosphatases. PRGs/LPPRs are structurally similar to the other LPP family members in general. They are predominantly expressed in the CNS in a subtype specific pattern rather than having a wide tissue distribution. In contrast to LPPs, PRGs/LPPRs may act by modifying bioactive lipids and their signaling pathways, rather than possessing an ecto-phosphatase activity. However, the exact functional roles of PRGs/LPPRs have just begun to be explored. Here, we discuss new findings on the neuron-specific transcriptional regulation of PRG1/LPPR4 and new insights into protein–protein interaction and signaling pathway regulation. Further, we start to shed light on the subcellular localization and the resulting functional modulatory influence of PRG1/LPPR4 expression in excitatory synaptic transmission to the established neural effects such as promotion of filopodia formation, neurite extension, axonal sprouting and reorganization after lesion. This range of effects suggests an involvement in the pathogenesis and/or reparation attempts in disease. Therefore, we summarize available data on the association of PRGs/LPPRs with several neurological and other diseases in humans and experimental animals. Finally we highlight important open questions and emerging future directions of research. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► PRG1/LPPR4 modulates excitatory transmission. ► PRG1/LPPR4 and ‐3/-1 expression are linked to epilepsy. ► A novel PRG/LPPR family member: PRG5/LPPR5 ► Characterization of the PRG1/LPPR4 promoter ► PRG3/LPPR1 and ‐5/-5 induce filopodia formation independent of cdc42.
Keywords: Central nervous system; Transcription; Epilepsy; Filopodia formation; Excitatory synaptic transmission;

Sphingosine kinases 1 and 2 (SK1 and SK2) generate the bioactive lipid mediator sphingosine 1-phosphate and as such play a significant role in cell fate and in human health and disease. Despite significant interest in and examination of the role played by SK enzymes in disease, comparatively little is currently known about the three-dimensional structure and catalytic mechanisms of these enzymes. To date, limited numbers of studies have used site directed mutagenesis and activity determinations to examine the roles of individual SK residues in substrate, calmodulin, and membrane binding, as well as activation via phosphorylation. Assays are currently available that allow for both single and bisubstrate kinetic analysis of mutant proteins that show normal, lowered and enhanced activity as compared to wild type controls. Additional studies will be required to build on this foundation to completely understand SK mediated substrate binding and phosphoryl group transfer. A deeper understanding of the SK catalytic mechanism, as well as SK interactions with potential small molecule inhibitors will be invaluable to the future design and identification of SK activity modulators as research tools and potential therapeutics. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► SK is critical to many cellular functions, including cell survival. ► SK plays an important role in human health and disease. ► Mutagenesis/activity analysis has been used to examine SK structure and function. ► Several activity assays exist for the analysis of SK activity. ► Characterization of SK will enable design of inhibitors as potential therapeutics.
Keywords: Sphingosine kinase; Sphingosine; Sphingosine 1-phosphate; Mutagenesis; Enzyme kinetics; Activity assay;

Post-translational regulation of sphingosine kinases by Huasheng Chan; Stuart M. Pitson (147-156).
Sphingosine kinases (SKs) catalyse the conversion of sphingosine to sphingosine 1-phosphate (S1P), a signalling lipid that is involved in a plethora of cellular processes including proliferation, apoptosis, calcium homeostasis, angiogenesis, vascular and neuronal maturation, cell migration and immune responses. Over the last few years, it has become clear that SKs are subject to various forms of post-translational regulation which play important roles in the function of these enzymes. Moreover, dysregulation of SKs has been implicated in many pathological conditions, such as cancer. Here we review the various mechanisms of post-translational regulation of the SKs with the view that such knowledge may lead to the development of therapeutic strategies to modulate the activities of these enzymes in the treatment of cancer and a range of other conditions. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► SKs are critical regulators of cellular signalling. ► Post-translational regulation of SKs plays an important role in their function. ► SKs are regulated by phosphorylation and interaction with proteins and lipids. ► Subcellular localisation of SKs is important for their signalling function. ► Degradation of SKs may serve as a further regulation of these enzymes.
Keywords: Lipid; Sphingosine kinase; Sphingosine; Sphingosine 1-phosphate; Post-translational regulation; Cancer;

Sphingosine 1-phosphate (S1P) is an important bioactive sphingolipid metabolite that has been implicated in numerous physiological and cellular processes. Not only does S1P play a structural role in cells by defining the components of the plasma membrane, but in the last 20 years it has been implicated in various significant cell signaling pathways and physiological processes: for example, cell migration, survival and proliferation, cellular architecture, cell–cell contacts and adhesions, vascular development, atherosclerosis, acute pulmonary injury and respiratory distress, inflammation and immunity, and tumorogenesis and metastasis [1,2]. Given the wide variety of cellular and physiological processes in which S1P is involved, it is immediately obvious why the mechanisms governing S1P synthesis and degradation, and the manner in which these processes are regulated, are necessary to understand. In gaining more knowledge about regulation of the sphingosine kinase (SK)/S1P pathway, many potential therapeutic targets may be revealed. This review explores the roles of the SK/S1P pathway in disease, summarizes available SK enzyme inhibitors and examines their potential as therapeutic agents. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► The SK/S1P pathway is dysregulated in numerous pathologies and disease states. ► Only recently have potent and specific inhibitors of SK begun to be developed. ► Further development of SK inhibitors may provide novel disease treatments.
Keywords: Sphingolipid; Sphingosine kinase (SK); Sphingosine 1-phosphate (S1P); Inhibitor; Disease; Biomarker;

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid whose actions are essential for many physiological processes including angiogenesis, lymphocyte trafficking and development. In addition, S1P serves as a muscle trophic factor that enables efficient muscle regeneration. This is due in part to S1P's ability to activate quiescent muscle stem cells called satellite cells (SCs) that are needed for muscle repair. However, the molecular mechanism by which S1P activates SCs has not been well understood. Further, strategies for harnessing S1P signaling to recruit SCs for therapeutic benefit have been lacking. S1P is irreversibly catabolized by S1P lyase (SPL), a highly conserved enzyme that catalyzes the cleavage of S1P at carbon bond C2–3, resulting in formation of hexadecenal and ethanolamine-phosphate. SPL enhances apoptosis through substrate- and product-dependent events, thereby regulating cellular responses to chemotherapy, radiation and ischemia. SPL is undetectable in resting murine skeletal muscle. However, we recently found that SPL is dynamically upregulated in skeletal muscle after injury. SPL upregulation occurred in the context of a tightly orchestrated genetic program that resulted in a transient S1P signal in response to muscle injury. S1P activated quiescent SCs via a sphingosine-1-phosphate receptor 2 (S1P2)/signal transducer and activator of transcription 3 (STAT3)-dependent pathway, thereby facilitating skeletal muscle regeneration. Mdx mice, which serve as a model for muscular dystrophy (MD), exhibited skeletal muscle SPL upregulation and S1P deficiency. Pharmacological SPL inhibition raised skeletal muscle S1P levels, enhanced SC recruitment and improved mdx skeletal muscle regeneration. These findings reveal how S1P can activate SCs and indicate that SPL suppression may provide a therapeutic strategy for myopathies. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► Skeletal muscle injury induces dynamic changes in S1P signaling and metabolism. ► S1P promotes skeletal muscle satellite cell activation via a STAT3-mediated pathway. ► S1P activates STAT3 via S1P receptor 2 signaling. ► Dystrophic muscle of mdx mice exhibit high S1P lyase expression and S1P deficiency. ► S1P lyase inhibition enhances satellite cell recruitment and muscle regeneration.
Keywords: Satellite cell; Skeletal muscle; Sphingosine phosphate lyase; Sphingosine-1-phosphate; Sphingolipid; STAT3;

Lysophospholipids are bioactive molecules that are implicated in the control of fundamental biological processes such as proliferation, differentiation, survival and motility in different cell types. Here we review the role of sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) in the regulation of skeletal muscle biology. Indeed, a wealth of experimental data indicate that these molecules are crucial players in the skeletal muscle regeneration process, acting by controllers of activation, proliferation and differentiation not only of muscle-resident satellite cells but also of mesenchymal progenitors that originate outside the skeletal muscle. Moreover, S1P and LPA are clearly involved in the regulation of skeletal muscle metabolism, muscle adaptation to different physiological needs and resistance to muscle fatigue. Notably, studies accomplished so far, have highlighted the complexity of S1P and LPA signaling in skeletal muscle cells that appears to be further complicated by their close dependence on functional cross-talks with growth factors, hormones and cytokines. Our increasing understanding of bioactive lipid signaling can individuate novel molecular targets aimed at enhancing skeletal muscle regeneration and reducing the fibrotic process that impairs full functional recovery of the tissue during aging, after a trauma or skeletal muscle diseases. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► S1P and LPA regulate muscle precursor cell biology. ► S1P and LPA regulate growth, migration and proliferation of myoblasts. ► S1P and LPA cross-communicate with growth factors, hormones and cytokines. ► S1P and LPA regulate skeletal muscle metabolism, muscle adaptation and fatigue.
Keywords: Sphingosine 1-phosphate; Lysophosphatidic acid; Skeletal muscle regeneration; Satellite cell; Myoblast; Skeletal muscle metabolism;

Sphingosine-1-phosphate as a mediator involved in development of fibrotic diseases by Yoh Takuwa; Hitoshi Ikeda; Yasuo Okamoto; Noriko Takuwa; Kazuaki Yoshioka (185-192).
Fibrosis is a pathological process characterized by massive deposition of extracellular matrix (ECM) such as type I/III collagens and fibronectin that are secreted by an expanded pool of myofibroblasts, which are phenotypically altered fibroblasts with more contractile, proliferative, migratory and secretory activities. Fibrosis occurs in various organs including the lung, heart, liver and kidney, resulting in loss of normal tissue architecture and functions. Myofibroblasts could originate from multiple sources including tissue-resident fibroblasts, epithelial and endothelial cells through mechanisms of epithelial/endothelial-mesenchymal transition (EMT/EndMT), and bone marrow-derived circulating progenitors called fibrocytes. Emerging evidence in recent years shows that sphingosine-1-phosphate (S1P) acts on several types of target cells and is engaged in pro-fibrotic inflammatory process and fibrogenic process through multiple mechanisms, which include vascular permeability change, leukocyte infiltration, and migration, proliferation and myofibroblast differentiation of fibroblasts. Many of these S1P actions are receptor subtype-specific. In these actions, S1P has multiple cross-talks with other cytokines, particularly transforming growth factor-β (TGFβ), which plays a major role in fibrosis. The cross-talks include the regulation of S1P production through altered expression and activity of sphingosine kinases in fibrotic lesions, altered expression of S1P receptors, and S1P receptor-mediated transactivation of TGFβ signaling pathway. These cross-talks may give rise to a feed-forward, amplifying loop between S1P and TGFβ, and possibly with other cytokines in stimulating fibrogenesis. Another lysophospholipid mediator lysophosphatidic acid has also been recently implicated in fibrosis. The lysophospholipid signaling pathways represent novel, promising therapeutic targets for treating refractory fibrotic diseases. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► S1P stimulates migration and proliferation of fibroblasts. ► S1P stimulates myofibroblast differentiation of fibroblasts. ► TGFβ and other cytokines alter expression of sphingosine kinases and S1P receptors. ► S1P may transactivate TGFβ signaling pathway. ► The above two mechanisms may constitute a feed-forward amplifying loop.
Keywords: Sphingosine-1-phosphate; Lysophosphatidic acid; Fibrosis; Myofibroblast; Fibrocyte; Epithelial-mesenchymal transition;

Shaping the landscape: Metabolic regulation of S1P gradients by Ana Olivera; Maria Laura Allende; Richard L. Proia (193-202).
Sphingosine-1-phosphate (S1P) is a lipid that functions as a metabolic intermediate and a cellular signaling molecule. These roles are integrated when compartments with differing extracellular S1P concentrations are formed that serve to regulate functions within the immune and vascular systems, as well as during pathologic conditions. Gradients of S1P concentration are achieved by the organization of cells with specialized expression of S1P metabolic pathways within tissues. S1P concentration gradients underpin the ability of S1P signaling to regulate in vivo physiology. This review will discuss the mechanisms that are necessary for the formation and maintenance of S1P gradients, with the aim of understanding how a simple lipid controls complex physiology. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► S1P is both a lipid metabolite and a signaling molecule. ► An S1P concentration gradient exists between circulation (high) and tissues (low). ► S1P gradients are produced by differential expression of metabolic pathways. ► S1P receptor signaling regulates normal physiology and pathogenesis. ► S1P gradients underlie the regulation of S1P receptor signaling.
Keywords: Sphingosine-1-phosphate; Sphingolipid; Signaling; Gradient; Receptor; Metabolism;

Activation of sphingosine kinase/sphingosine 1-phosphate (SK/S1P)‐mediated signaling has emerged as a critical cardioprotective pathway in response to acute ischemia/reperfusion injury. S1P is released in both ischemic pre- and post-conditioning. Application of exogenous S1P to cultured cardiac myocytes subjected to hypoxia or treatment of isolated hearts either before ischemia or at the onset of reperfusion exerts prosurvival effects. Synthetic congeners of S1P such as FTY720 mimic these responses. Gene targeted mice null for the SK1 isoform whose hearts are subjected to ischemia/reperfusion injury exhibit increased infarct size and respond poorly either to ischemic pre- or postconditioning. Measurements of cardiac SK activity and S1P parallel these observations. Experiments in SK2 knockout mice have revealed that this isoform is necessary for survival in the heart. High density lipoprotein (HDL) is a major carrier of S1P, and studies of hearts in which selected S1P receptors have been inhibited implicate the S1P cargo of HDL in cardioprotection. Inhibition of S1P lyase, an endogenous enzyme that degrades S1P, also leads to cardioprotection. These observations have considerable relevance for future therapeutic approaches to acute and chronic myocardial injury. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► The S1P/SK pathway is a key participant in endogenous cardioprotection. ► Both isoforms of SK are necessary for optimal cardioprotection. ► Ischemic pre-conditioning preserves both SK activity and S1P levels. ► The immunomodulator FTY720 mimics S1P effects in the heart.
Keywords: Sphingosine 1-phosphate; FTY720; Sphingosine kinase; S1P lyase; Ischemia/reperfusion injury; Cardioprotection;

Lysophospholipid receptor activation of RhoA and lipid signaling pathways by Sunny Yang Xiang; Stephanie S. Dusaban; Joan Heller Brown (213-222).
The lysophospholipids sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) signal through G-protein coupled receptors (GPCRs) which couple to multiple G-proteins and their effectors. These GPCRs are quite efficacious in coupling to the Gα12/13 family of G-proteins, which stimulate guanine nucleotide exchange factors (GEFs) for RhoA. Activated RhoA subsequently regulates downstream enzymes that transduce signals which affect the actin cytoskeleton, gene expression, cell proliferation and cell survival. Remarkably many of the enzymes regulated downstream of RhoA either use phospholipids as substrates (e.g. phospholipase D, phospholipase C-epsilon, PTEN, PI3 kinase) or are regulated by phospholipid products (e.g. protein kinase D, Akt). Thus lysophospholipids signal from outside of the cell and control phospholipid signaling processes within the cell that they target. Here we review evidence suggesting an integrative role for RhoA in responding to lysophospholipids upregulated in the pathophysiological environment, and in transducing this signal to cellular responses through effects on phospholipid regulatory or phospholipid regulated enzymes. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► Lysophospholipid receptors couple to G12/13 and activation of RhoA. ► RhoA stimulates phospholipases and lipid regulated enzymes. ► Phospholipase D and C (epsilon) and protein kinase D mediate RhoA signaling. ► S1P and LPA elicit pathophysiological responses through these RhoA effectors.
Keywords: Lysophospholipid; S1P; LPA; RhoA;

Bone is a dynamic organ that is continuously turned over during growth, even in adults. During bone remodeling, homeostasis is regulated by the balance between bone formation by osteoblasts and bone resorption by osteoclasts. However, in pathological conditions such as osteoporosis, osteopetrosis, arthritic joint destruction, and bone metastasis, this equilibrium is disrupted. Since osteoclasts are excessively activated in osteolytic diseases, the inhibition of osteoclast function has been a major therapeutic strategy. It has recently been demonstrated that sphingosine-1-phosphate (S1P), a biologically active lysophospholipid that is enriched in blood, controls the trafficking of osteoclast precursors between the circulation and bone marrow cavities via G protein-coupled receptors, S1PRs. While S1PR1 mediates chemoattraction toward S1P in bone marrow, where S1P concentration is low, S1PR2 mediates chemorepulsion in blood, where the S1P concentration is high. The regulation of precursor recruitment may represent a novel therapeutic strategy for controlling osteoclast-dependent bone remodeling. By means of intravital multiphoton imaging of bone tissues, we have recently revealed that the reciprocal action of S1P controls the migration of osteoclast precursors between bone tissues and blood stream. Imaging technologies have enabled us to visualize the in situ behaviors of different cell types in intact tissues. In this review we also discuss future perspectives on this new method in the field of bone biology and medical sciences in general. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► Osteoclast precursors are circulating in blood and migrating into bone surface. ► Sphingosine-1-phosphate (S1P) controls osteoclast precursor migration in vivo. ► Osteoclast precursors express bidirectional S1P receptors, S1PR1 and S1PR2. ► Osteoclast precursor migration is regulated reciprocally by these two receptors. ► Intravital multiphoton imaging is useful for revealing S1P action in vivo.
Keywords: Sphingosine-1-phosphate; Bone remodeling; Osteoclast; Imaging;

Role of sphingosine 1-phosphate and lysophosphatidic acid in fibrosis by Nigel J. Pyne; Gerald Dubois; Susan Pyne (228-238).
This review highlights an emerging role for sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) in many different types of fibrosis. Indeed, both LPA and S1P are involved in the multi-process pathogenesis of fibrosis, being implicated in promoting the well-established process of differentiation of fibroblasts to myofibroblasts and the more controversial epithelial–mesenchymal transition and homing of fibrocytes to fibrotic lesions. Therefore, targeting the production of these bioactive lysolipids or blocking their sites/mechanisms of action has therapeutic potential. Indeed, LPA receptor 1 (LPA1) selective antagonists are currently being developed for the treatment of fibrosis of the lung as well as a neutralising anti-S1P antibody that is currently in Phase 1 clinical trials for treatment of age related macular degeneration. Thus, LPA- and S1P-directed therapeutics may not be too far from the clinic. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► The lysolipids S1P and LPA contribute to development of fibrosis. ► Intracellular S1P is anti-fibrotic whereas extracellular S1P is pro-fibrotic. ► Extracellular S1P and LPA, acting via their specific GPCR, exert cross-talk with TGFβ. ► S1P and LPA receptor antagonists have therapeutic potential. ► Inhibition of S1P and LPA formation could also be exploited for treatment of fibrosis.
Keywords: Sphingosine 1-phosphate; Lysophosphatidic acid; Fibrosis; TGFβ; Signalling; Connective tissue growth factor;

Sphingosine-1-phosphate: A Janus-faced mediator of fibrotic diseases by Stephanie Schwalm; Josef Pfeilschifter; Andrea Huwiler (239-250).
Sphingosine-1-phosphate (S1P) is a pleiotropic lipid mediator that acts either on G protein-coupled S1P receptors on the cell surface or via intracellular target sites. In addition to the well established effects of S1P in angiogenesis, carcinogenesis and immunity, evidence is now continuously accumulating which demonstrates that S1P is an important regulator of fibrosis. The contribution of S1P to fibrosis is of a Janus-faced nature as S1P exhibits both pro- and anti-fibrotic effects depending on its site of action. Extracellular S1P promotes fibrotic processes in a S1P receptor-dependent manner, whereas intracellular S1P has an opposite effect and dampens a fibrotic reaction by yet unidentified mechanisms. Fibrosis is a result of chronic irritation by various factors and is defined by an excess production of extracellular matrix leading to tissue scarring and organ dysfunction. In this review, we highlight the general effects of extracellular and intracellular S1P on the multistep cascade of pathological fibrogenesis including tissue injury, inflammation and the action of pro-fibrotic cytokines that stimulate ECM production and deposition. In a second part we summarize the current knowledge about the involvement of S1P signaling in the development of organ fibrosis of the lung, kidney, liver, heart and skin. Altogether, it is becoming clear that targeting the sphingosine kinase-1/S1P signaling pathway offers therapeutic potential in the treatment of various fibrotic processes. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.► Most important mechanisms involved in the pathogenesis of fibrosis ► S1P and the fibrotic sequela: tissue damage, endothelial barrier function and inflammation ► Cross-communication between S1P and TGF-β signaling, and between S1P and PDGF signaling ► The role of S1P in the regulation of myofibroblast activity in vitro ► The role of S1P in fibrosis of lung, kidney, liver, heart and skin
Keywords: Sphingosine-1-phosphate; Sphingosine kinase-1; Fibrosis; TGF-β; Inflammation; Myofibroblast;