Phosphodiesterase I, a novel adhesion molecule and/or cytokine involved in oligodendrocyte function

Extracellular Purine Metabolism-Potential Target in Multiple Sclerosis

The purinergic signaling system comprises a complex network of extracellular purines and purine-metabolizing ectoenzymes, nucleotide and nucleoside receptors, ATP release channels, and nucleoside transporters. Because of its immunomodulatory function, this system is critically involved in the pathogenesis of multiple sclerosis (MS) and its best-characterized animal model, experimental autoimmune encephalomyelitis (EAE). MS is a chronic neuroinflammatory demyelinating and neurodegenerative disease with autoimmune etiology and great heterogeneity, mostly affecting young adults and leading to permanent disability. In MS/EAE, alterations were detected in almost all components of the purinergic signaling system in both peripheral immune cells and central nervous system (CNS) glial cells, which play an important role in the pathogenesis of the disease. A decrease in extracellular ATP levels and an increase in its downstream metabolites, particularly adenosine and inosine, were frequently observed at MS, indicating a shift in metabolism toward an anti-inflammatory environment. Accordingly, upregulation of the major ectonucleotidase tandem CD39/CD73 was detected in the blood cells and CNS of relapsing-remitting MS patients. Based on the postulated role of A2A receptors in the transition from acute to chronic neuroinflammation, the association of variants of the adenosine deaminase gene with the severity of MS, and the beneficial effects of inosine treatment in EAE, the adenosinergic system emerged as a promising target in neuroinflammation. More recently, several publications have identified ADP-dependent P2Y12 receptors and the major extracellular ADP producing enzyme nucleoside triphosphate diphosphohydrolase 2 (NTPDase2) as novel potential targets in MS.

Autotaxin in encephalitogenic CD4 T cells as a therapeutic target for multiple sclerosis

Multiple sclerosis (MS) is an immune-mediated inflammatory disease of the CNS. A defining characteristic of MS is the ability of autoreactive T lymphocytes to cross the blood-brain barrier and mediate inflammation within the CNS. Previous work from our lab found the gene Enpp2 to be highly upregulated in murine encephalitogenic T cells. Enpp2 encodes for the protein autotaxin, a secreted glycoprotein that catalyzes the production of lysophosphatidic acid and promotes transendothelial migration of T cells from the bloodstream into the lymphatic system. The present study sought to characterize autotaxin expression in T cells during CNS autoimmune disease and determine its potential therapeutic value. Myelin-activated CD4 T cells upregulated expression of autotaxin in vitro, and ex vivo analysis of CNS-infiltrating CD4 T cells showed significantly higher autotaxin expression compared with cells from healthy mice. In addition, inhibiting autotaxin in myelin-specific T cells reduced their encephalitogenicity in adoptive transfer studies and decreased in vitro cell motility. Importantly, using two mouse models of MS, treatment with an autotaxin inhibitor ameliorated EAE severity, decreased the number of CNS infiltrating T and B cells, and suppressed relapses, suggesting autotaxin may be a promising therapeutic target in the treatment of MS.

Identifying mRNAs Residing in Myelinating Oligodendrocyte Processes as a Basis for Understanding Internode Autonomy

In elaborating and maintaining myelin sheaths on multiple axons/segments, oligodendrocytes distribute translation of some proteins, including myelin basic protein (MBP), to sites of myelin sheath assembly, or MSAS. As mRNAs located at these sites are selectively trapped in myelin vesicles during tissue homogenization, we performed a screen to identify some of these mRNAs. To confirm locations, we used real-time quantitative polymerase chain reaction (RT-qPCR), to measure mRNA levels in myelin (M) and 'non-myelin' pellet (P) fractions, and found that five (LPAR1, TRP53INP2, TRAK2, TPPP, and SH3GL3) of thirteen mRNAs were highly enriched in myelin (M/P), suggesting residences in MSAS. Because expression by other cell-types will increase p-values, some MSAS mRNAs might be missed. To identify non-oligodendrocyte expression, we turned to several on-line resources. Although neurons express TRP53INP2, TRAK2 and TPPP mRNAs, these expressions did not invalidate recognitions as MSAS mRNAs. However, neuronal expression likely prevented recognition of KIF1A and MAPK8IP1 mRNAs as MSAS residents and ependymal cell expression likely prevented APOD mRNA assignment to MSAS. Complementary in situ hybridization (ISH) is recommended to confirm residences of mRNAs in MSAS. As both proteins and lipids are synthesized in MSAS, understanding myelination should not only include efforts to identify proteins synthesized in MSAS, but also the lipids.

Nucleotide pyrophosphatase/phosphodiesterases (NPPs) including NPP1 and NPP2/ ATX as important drug targets: A patent review (2015-2020)

INTRODUCTION

Nucleoside triphosphate diphosphohydrolases (NTPDases), alkaline phosphatases (APs), and ecto-nucleotide pyrophosphatases/phosphodiesterases (NPPs) are nucleotidases found on the cell surface. It is a promising therapeutic target for a range of disorders, including fibrosis, tumour metastasis, pruritus, inflammation, multiple sclerosis, and autoimmune diseases. As a result, therapeutic intervention including selective inhibitors of NPPs is required.

AREA COVERED

Many chemical substances, including pyrazole, pyridine and bicyclic compounds have demonstrated promising inhibitory potential for ecto-nucleotide pyrophosphatase/phosphodiesterases. The chemistry and clinical applications of NPPs inhibitors patented between 2015 and 2020 are discussed in this review.

EXPERT OPINION

: In recent years, there has been a lot of effort put into finding effective and selective inhibitors of NPPs. Despite the fact that a variety of synthetic inhibitors have been created, only a few investigations on their in vivo activity have been published. In addition to IOA-289 which has passed Phase Ia clinical trials; potent ATX inhibitor compounds such as BLD-0409, IPF and BBT-877 have been placed in phase I clinical studies. Some of the most promising ATX inhibitors in recent years are closely related analogs of previously known inhibitors, such as PF-8380. Knowledge of the structure activity relationship of such promising inhibitors can potentially translate into the discovery of more potent and effective inhibitors of NPP with a variety of structural characteristics and favourable therapeutic activities.

ENPP2 Methylation in Health and Cancer

Autotaxin (ATX) encoded by Ectonucleotide Pyrophosphatase/Phosphodiesterase 2 (ENPP2) is a key enzyme in Lysophosphatidic Acid (LPA) synthesis implicated in cancer. Although its aberrant expression has been reported, ENPP2 methylation profiles in health and malignancy are not described. We examined in silico the methylation of ENPP2 analyzing publicly available methylome datasets, to identify Differentially Methylated CpGs (DMCs) which were then correlated with expression at gene and isoform levels. Significance indication was set to be FDR corrected p-value < 0.05. Healthy tissues presented methylation in all gene body CGs and lower levels in Promoter Associated (PA) regions, whereas in the majority of the tumors examined (HCC, melanoma, CRC, LC and PC) the methylation pattern was reversed. DMCs identified in the promoter were located in sites recognized by multiple transcription factors, suggesting involvement in gene expression. Alterations in methylation were correlated to an aggressive phenotype in cancer cell lines. In prostate and lung adenocarcinomas, increased methylation of PA CGs was correlated to decreased ENPP2 mRNA expression and to poor prognosis parameters. Collectively, our results corroborate that methylation is an active level of ATX expression regulation in cancer. Our study provides an extended description of the methylation status of ENPP2 in health and cancer and points out specific DMCs of value as prognostic biomarkers.

Purinergic Signaling in Glioma Progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X₇, is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

Reduced Serum and Cerebrospinal Fluid Levels of Autotaxin in Major Depressive Disorder

BACKGROUND

The autotaxin/lysophosphatidic acid axis is involved in diverse biological processes including neurodevelopment, inflammation, and immunological functioning. The lysophosphatidic acid 1 receptor has been implicated in the pathophysiology of major depressive disorder and in the mechanism of action of antidepressants. However, it is unclear whether central or peripheral autotaxin levels are altered in patients with major depressive disorder.

METHODS

Serum autotaxin levels were measured by an enzyme-linked immunosorbent assay in 37 patients with major depressive disorder diagnosed using DSM-IV-TR who underwent electroconvulsive therapy and were compared with those of 47 nondepressed controls matched for age and sex between January 2011 and December 2015. Patient serum levels of autotaxin before and after electroconvulsive therapy were also compared. In a separate sample set, cerebrospinal fluid autotaxin levels were compared between 26 patients with major depressive disorder and 27 nondepressed controls between December 2010 and December 2015. A potential association was examined between autotaxin levels and clinical symptoms assessed with the Hamilton Depression Rating Scale.

RESULTS

Before electroconvulsive therapy, both serum and cerebrospinal fluidautotaxin levels were significantly lower in major depressive disorder patients than in controls (serum: P = .001, cerebrospinal fluid: P = .038). A significantly negative correlation between serum, but not cerebrospinal fluid, autotaxin levels and depressive symptoms was observed (P = .032). After electroconvulsive therapy, a parallel increase in serum autotaxin levels and depressive symptoms improvement was observed (P = .005).

CONCLUSION

The current results suggest that serum autotaxin levels are reduced in a state-dependent manner. The reduction of cerebrospinal fluidautotaxin levels suggests a dysfunction in the autotaxin/lysophosphatidic acid axis in the brains of patients with major depressive disorder.

Coffee, caffeine, chlorogenic acid, and the purinergic system

Coffee is a drink prepared from roasted coffee beans and is lauded for its aroma and flavour. It is the third most popular beverage in the world. This beverage is known by its stimulant effect associated with the presence of methylxanthines. Caffeine, a purine-like molecule (1,3,7 trymetylxantine), is the most important bioactive compound in coffee, among others such as chlorogenic acid (CGA), diterpenes, and trigonelline. CGA is a phenolic acid with biological properties as antioxidant, anti-inflammatory, neuroprotector, hypolipidemic, and hypoglicemic. Purinergic system plays a key role inneuromodulation and homeostasis. Extracellular ATP, other nucleotides and adenosine are signalling molecules that act through their specific receptors, namely purinoceptors, P1 for nucleosides and P2 for nucleotides. They regulate many pathological processes, since adenosine, for instance, can limit the damage caused by ATP in the excitotoxicity from the neuronal cells. The primary purpose of this review is to discuss the effects of coffee, caffeine, and CGA on the purinergic system. This review focuses on the relationship/interplay between coffee, caffeine, CGA, and adenosine, and their effects on ectonucleotidases activities as well as on the modulation of P1 and P2 receptors from central nervous system and also in peripheral tissue.

Transient Hypothyroidism During Lactation Arrests Myelination in the Anterior Commissure of Rats. A Magnetic Resonance Image and Electron Microscope Study

Thyroid hormone deficiency at early postnatal ages affects the cytoarchitecture and function of neocortical and telencephalic limbic areas, leading to impaired associative memory and in a wide spectrum of neurological and mental diseases. Neocortical areas project interhemispheric axons mostly through the corpus callosum and to a lesser extent through the anterior commissure (AC), while limbic areas mostly project through the AC and hippocampal commissures. Functional magnetic resonance data from children with late diagnosed congenital hypothyroidism and abnormal verbal memory processing, suggest altered ipsilateral and contralateral telencephalic connections. Gestational hypothyroidism affects AC development but the possible effect of transient and chronic postnatal hypothyroidism, as occurs in late diagnosed neonates with congenital hypothyroidism and in children growing up in iodine deficient areas, still remains unknown. We studied AC development using in vivo magnetic resonance imaging and electron microscopy in hypothyroid and control male rats. Four groups of methimazole (MMI) treated rats were studied. One group was MMI-treated from postnatal day (P) 0 to P21; some of these rats were also treated with L-thyroxine (T4) from P15 to P21, as a model for early transient hypothyroidism. Other rats were MMI-treated from P0 to P150 and from embryonic day (E) 10 to P170, as a chronic hypothyroidism group. The results were compared with age paired control rats. The normalized T2 signal using magnetic resonance image was higher in MMI-treated rats and correlated with the number and percentage of myelinated axons. Using electron microscopy, we observed decreased myelinated axon number and density in transient and chronic hypothyroid rats at P150, unmyelinated axon number increased slightly in chronic hypothyroid rats. In MMI-treated rats, the myelinated axon g-ratio and conduction velocity was similar to control rats, but with a decrease in conduction delays. These data show that early postnatal transient and chronic hypothyroidism alters AC maturation that may affect the transfer of information through the AC. The alterations cannot be recovered after delayed T4-treatment. Our data support the neurocognitive delay found in late T4-treated children with congenital hypothyroidism.

Dysregulation of lysophosphatidic acids in multiple sclerosis and autoimmune encephalomyelitis

Abstract

Bioactive lipids contribute to the pathophysiology of multiple sclerosis. Here, we show that lysophosphatidic acids (LPAs) are dysregulated in multiple sclerosis (MS) and are functionally relevant in this disease. LPAs and autotaxin, the major enzyme producing extracellular LPAs, were analyzed in serum and cerebrospinal fluid in a cross-sectional population of MS patients and were compared with respective data from mice in the experimental autoimmune encephalomyelitis (EAE) model, spontaneous EAE in TCR¹⁶⁴⁰ mice, and EAE in Lpar2^-/- mice. Serum LPAs were reduced in MS and EAE whereas spinal cord LPAs in TCR¹⁶⁴⁰ mice increased during the ‘symptom-free’ intervals, i.e. on resolution of inflammation during recovery hence possibly pointing to positive effects of brain LPAs during remyelination as suggested in previous studies. Peripheral LPAs mildly re-raised during relapses but further dropped in refractory relapses. The peripheral loss led to a redistribution of immune cells from the spleen to the spinal cord, suggesting defects of lymphocyte homing. In support, LPAR2 positive T-cells were reduced in EAE and the disease was intensified in Lpar2 deficient mice. Further, treatment with an LPAR2 agonist reduced clinical signs of relapsing-remitting EAE suggesting that the LPAR2 agonist partially compensated the endogenous loss of LPAs and implicating LPA signaling as a novel treatment approach.

Graphical abstract

Graphical summary of lysophosphatidic signaling in multiple sclerosis

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-017-0446-4) contains supplementary material, which is available to authorized users.

Thyroid Hormone Signaling in Oligodendrocytes: from Extracellular Transport to Intracellular Signal

Thyroid hormone plays an important role in central nervous system (CNS) development, including the myelination of variable axonal calibers. It is well-established that thyroid hormone is required for the terminal differentiation of oligodendrocyte precursor cells (OPCs) into myelinating oligodendrocytes by inducing rapid cell-cycle arrest and constant transcription of pro-differentiation genes. This is well supported by the hypomyelinating phenotypes exhibited by patients with congenital hypothyroidism, cretinism. During development, myelinating oligodendrocytes only appear after the formation of neural circuits, indicating that the timing of oligodendrocyte differentiation is important. Since fetal and post-natal serum thyroid hormone levels peak at the stage of active myelination, it is suspected that the timing of oligodendrocyte development is finely controlled by thyroid hormone. The essential machinery for thyroid hormone signaling such as deiodinase activity (utilized by cells to auto-regulate the level of thyroid hormone), and nuclear thyroid hormone receptors (for gene transcription) are expressed on oligodendrocytes. In this review, we discuss the known and potential thyroid hormone signaling pathways that may regulate oligodendrocyte development and CNS myelination. Moreover, we evaluate the potential of targeting thyroid hormone signaling for white matter injury or disease.

Increased autotaxin activity in multiple sclerosis

Autotaxin (ATX) is an enzyme producing lysophosphatidic acid (LPA) from lysophosphatidyl choline (LPC) and it is up-regulated in inflammatory conditions such as various cancers, arthritis and multiple sclerosis (MS). Numerous studies have shown that the LPA signaling gives rise to angiogenesis, mitosis, cell proliferation and cytokine secretion. On the one hand, an increasing body of evidence suggests that blockade of ATX has anti-inflammatory properties in a variety of diseases. The aim of this study was to measure the enzyme activity of ATX in cerebrospinal fluid (CSF) and serum of patients with MS using an enzymatic photometric method. Twenty definite relapsing remitting MS patients along with 20 patients with other neurological diseases (OND) were recruited. The results showed that ATX activity was significantly higher (p value<0.0001) in MS patients than those patients diagnosed with OND. It is possible that inhibition of the ATX may decrease the rate of MS relapses/progression.

Mechanisms regulating airway nucleotides

In the respiratory system, extracellular nucleotides and nucleosides serve as signaling molecules for a wide spectrum of biological functions regulating airway defenses against infection and toxic material. Their concentrations are controlled by a complex network of cell surface enzymes named ectonucleotidases. This highly integrated metabolic network combines the activities of three dephosphorylating ectonucleotidases, namely nucleoside triphosphate diphosphohydrolases (NTPDases), nucleotide pyrophosphatase/phosphodiesterases (NPPs) and alkaline phosphatases (APs). Extracellular nucleotides are also inter-converted by the transphosphorylating activities of ecto adenylate kinase (ectoAK) and nucleoside diphosphokinase (NDPK). Different cell types use specific combinations of ectonucleotidases to regulate local concentrations of P2 receptor agonists (ATP, UTP, ADP and UDP). In addition, they provide AMP for the activity of ecto 5'-nucleotidase (ecto 5'-NT; CD73), which produces the P1 receptor agonist: adenosine (ADO). Finally, mechanisms are in place to prevent the accumulation of airway ADO, namely adenosine deaminases and nucleoside transporters. This chapter reviews the properties of each enzyme and transporter, and the current knowledge on their distribution and regulation in the airways.

Autotaxin: structure-function and signaling

Autotaxin (ATX), or ecto-nucleotide pyrophosphatase/phosphodiesterase-2, is a secreted lysophospholipase D (lysoPLD) that hydrolyzes extracellular lysophospholipids into the lipid mediator lysophosphatidic acid (LPA), a ligand for specific G protein-coupled receptors. ATX-LPA signaling is essential for development and has been implicated in a great diversity of (patho)physiological processes, ranging from lymphocyte homing to tumor progression. Structural and functional studies have revealed what makes ATX a unique lysoPLD, and how secreted ATX binds to its target cells. The ATX catalytic domain shows a characteristic bimetallic active site followed by a shallow binding groove that can accommodate nucleotides as well as the glycerol moiety of lysophospholipids, and by a deep lipid-binding pocket. In addition, the catalytic domain has an open tunnel of unknown function adjacent to the active site. Here, we discuss our current understanding of ATX structure-function relationships and signaling mechanisms, and how ATX isoforms use distinct mechanisms to target LPA production to the plasma membrane, notably binding to integrins and heparan sulfate proteoglycans. We also briefly discuss the development of drug-like inhibitors of ATX.

Purinergic signaling in glioma progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X(7), is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

The polybasic insertion in autotaxin α confers specific binding to heparin and cell surface heparan sulfate proteoglycans

Autotaxin (ATX) is a secreted lysophospholipase D that generates the lipid mediator lysophosphatidic acid (LPA), playing a key role in diverse physiological and pathological processes. ATX exists in distinct splice variants, but isoform-specific functions remain elusive. Here we characterize the ATXα isoform, which differs from the canonical form (ATXβ) in having a 52-residue polybasic insertion of unknown function in the catalytic domain. We find that the ATXα insertion is susceptible to cleavage by extracellular furin-like endoproteases, but cleaved ATXα remains structurally and functionally intact due to strong interactions within the catalytic domain. Through ELISA and surface plasmon resonance assays, we show that ATXα binds specifically to heparin with high affinity (K(d) ~10(-8) M), whereas ATXβ does not; furthermore, heparin moderately enhanced the lysophospholipase D activity of ATXα. We further show that ATXα, but not ATXβ, binds abundantly to SKOV3 carcinoma cells. ATXα binding was abolished after treating the cells with heparinase III, but not after chondroitinase treatment. Thus, the ATXα insertion constitutes a cleavable heparin-binding domain that mediates interaction with heparan sulfate proteoglycans, thereby targeting LPA production to the plasma membrane.

Cellular function and molecular structure of ecto-nucleotidases

Ecto-nucleotidases play a pivotal role in purinergic signal transmission. They hydrolyze extracellular nucleotides and thus can control their availability at purinergic P2 receptors. They generate extracellular nucleosides for cellular reuptake and salvage via nucleoside transporters of the plasma membrane. The extracellular adenosine formed acts as an agonist of purinergic P1 receptors. They also can produce and hydrolyze extracellular inorganic pyrophosphate that is of major relevance in the control of bone mineralization. This review discusses and compares four major groups of ecto-nucleotidases: the ecto-nucleoside triphosphate diphosphohydrolases, ecto-5'-nucleotidase, ecto-nucleotide pyrophosphatase/phosphodiesterases, and alkaline phosphatases. Only recently and based on crystal structures, detailed information regarding the spatial structures and catalytic mechanisms has become available for members of these four ecto-nucleotidase families. This permits detailed predictions of their catalytic mechanisms and a comparison between the individual enzyme groups. The review focuses on the principal biochemical, cell biological, catalytic, and structural properties of the enzymes and provides brief reference to tissue distribution, and physiological and pathophysiological functions.

Blockage of lysophosphatidic acid signaling improves spinal cord injury outcomes

Evidence suggests a proinflammatory role of lysophosphatidic acid (LPA) in various pathologic abnormalities, including in the central nervous system. Herein, we describe LPA as an important mediator of inflammation after spinal cord injury (SCI) in zebrafish and mice. Furthermore, we describe a novel monoclonal blocking antibody raised against LPA that potently inhibits LPA's effect in vitro and in vivo. This antibody, B3, specifically binds LPA, prevents it from interacting with its complement of receptors, and blocks LPA's effects on the neuronal differentiation of human neural stem/progenitor cells, demonstrating its specificity toward LPA signaling. When administered systemically to mice subjected to SCI, B3 substantially reduced glial inflammation and neuronal death. B3-treated animals demonstrated significantly more neuronal survival upstream of the lesion site, with some functional improvement. This study describes the use of anti-LPA monoclonal antibody as a novel therapeutic approach for the treatment of SCI.

Pulmonary autotaxin expression contributes to the pathogenesis of pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, fibrotic form of diffuse lung disease occurring mainly in older adults. Increased lysophosphatidic acid (LPA) concentrations have been reported in the alveolar space of both idiopathic pulmonary fibrosis patients and a corresponding animal model, whereas the genetic deletion or pharmacological inhibition of LPA receptor 1 attenuated the development of the modeled disease, suggesting a direct involvement of LPA in disease pathogenesis. In this report, increased concentrations of autotaxin (ATX; ENPP2), the enzyme largely responsible for extracellular LPA production, were detected in both murine and human fibrotic lungs. The genetic deletion of ATX from bronchial epithelial cells or macrophages attenuated disease severity, establishing ATX as a novel player in IPF pathogenesis. Furthermore, the pharmacological inhibition of ATX attenuated the development of the modeled disease, suggesting that ATX is a possible therapeutic target in IPF.

Modulation of LPA receptor expression in the human brain following neurotrauma

Lysophosphatidic acid (LPA) is involved in physiological and pathological states, including in neural development and inflammation. We assessed the expression pattern of the LPA receptors 1-3 and of LPA-producing enzyme autotaxin in post-mortem human brain tissue, both in normal individuals and in individuals who died following traumatic brain injury. We found that LPA receptors and autotaxin are weakly expressed in the normal control adult brain. Quantitative PCR for the LPA receptors and autotaxin mRNA showed an increase of LPAR(2) and a decrease of autotaxin mRNA expression in the cortex following brain injury. Immunohistochemical analysis showed that LPAR(1) colocalized with astrocytes and that LPAR(2) is present on the ependymal cells lining the lateral ventricle in the brain samples from individuals who died following severe head injury. This work shows for the first time that key components of the LPA pathway are modulated following TBI in humans.

Aiming drug discovery at lysophosphatidic acid targets

Lysophosphatidic acid (LPA, 1-radyl-2-hydroxy-sn-glycero-3-phosphate) is the prototype member of a family of lipid mediators and second messengers. LPA and its naturally occurring analogues interact with G protein-coupled receptors on the cell surface and a nuclear hormone receptor within the cell. In addition, there are several enzymes that utilize LPA as a substrate or generate it as a product and are under its regulatory control. LPA is present in biological fluids, and attempts have been made to link changes in its concentration and molecular composition to specific disease conditions. Through their many targets, members of the LPA family regulate cell survival, apoptosis, motility, shape, differentiation, gene transcription, malignant transformation and more. The present review depicts arbitrary aspects of the physiological and pathophysiological actions of LPA and attempts to link them with select targets. Many of us are now convinced that therapies targeting LPA biosynthesis and signalling are feasible for the treatment of devastating human diseases such as cancer, fibrosis and degenerative conditions. However, successful targeting of the pathways associated with this pleiotropic lipid will depend on the future development of as yet undeveloped pharmacons.

Autotaxin--an LPA producing enzyme with diverse functions

Autotaxin (ATX) is an ecto-enzyme responsible for lysophosphatidic acid (LPA) production in blood. ATX is present in various biological fluids such as cerebrospinal and seminal fluids and accounts for bulk LPA production in these fluids. ATX is a member of the nucleotide pyrophosphatase/phosphodiesterase (NPP) family and was originally isolated from conditioned medium of melanoma cells as an autocrine motility stimulating factor. LPA, a second-generation lipid mediator, binds to its cognate G protein-coupled receptors through which it exerts a number of biological functions including influencing cell motility and proliferation stimulating activity. Some of the biological roles of LPA can be mediated by ATX. However, there are other LPA-producing pathways independent of ATX. The accumulating evidences for physiological and pathological functions of ATX strongly support that ATX is an important therapeutic target. This review summarizes the historical aspects, structural basis, pathophysiological functions identified in mice studies and clinical relevance discovered by measuring the blood ATX level in human. The general features and functions of each NPP family member will be also briefly reviewed. The presence of the ATX gene in other model organisms and recently developed ATX inhibitors, both of which will be definitely useful for further functional analysis of ATX, will also be mentioned.

ATX expression and LPA signalling are vital for the development of the nervous system

Autotaxin (ATX) is a secreted glycoprotein widely present in biological fluids, originally isolated from the supernatant of melanoma cells as an autocrine motility stimulation factor. Its enzymatic product, lysophosphatidic acid (LPA), is a phospholipid mediator that evokes growth-factor-like responses in almost all cell types through G-protein coupled receptors. To assess the role of ATX and LPA signalling in pathophysiology, a conditional knockout mouse was created. Ubiquitous, obligatory deletion resulted to embryonic lethality most likely due to aberrant vascular branching morphogenesis and chorio-allantoic fusion. Moreover, the observed phenotype was shown to be entirely depended on embryonic, but not extraembryonic or maternal ATX expression. In addition, E9.5 ATX null mutants exhibited a failure of neural tube closure, most likely independent of the circulatory failure, which correlated with decreased cell proliferation and increased cell death. More importantly, neurite outgrowth in embryo explants was severely compromised in mutant embryos but could be rescued upon the addition of LPA, thus confirming a role for ATX and LPA signalling in the development of the nervous system. Finally, expression profiling of mutant embryos revealed attenuated embryonic expression of HIF-1a in the absence of ATX, suggesting a novel effector pathway of ATX/LPA.

Neuronal expression of the proteolipid protein gene in the medulla of the mouse

The proteolipid protein (PLP) gene (Plp) encodes the major myelin proteins, PLP and DM20. Expression of Plp occurs predominantly in oligodendrocytes, but evidence is accumulating that this gene is also expressed in neurons. In earlier studies, we demonstrated that myelin-deficient (MD) rats, which carry a mutation in the Plp gene, exhibit lethal hypoxic ventilatory depression. Furthermore, we found that, in the MD rat, PLP accumulated in neuronal cell bodies in the medulla oblongata. In the current study, we sought to determine which neurons expressed the Plp gene in the medulla oblongata and whether Plp gene expression changed in neurons with maturation. A transgenic mouse expressing the Plp promoter driving expression of enhanced green fluorescent protein (Plp-EGFP) was used to identify neurons expressing this gene. Plp expression in neurons was confirmed by immunostaining EGFP-positive cells for NeuN and by in situ hybridization for PLP mRNA. The numbers of neurons expressing Plp-EGFP and their distribution increased between P5 and P10 in the medulla. Immunostaining for surface receptors and classes of neurons expressing Plp-EGFP revealed that Plp gene expression in brainstem neurons was restricted to neurons expressing specific ligand-gated channels and biosynthetic enzymes, including glutamatergic NMDA receptors, GABA(A) receptors, and ChAT in defined areas of the medulla. Plp gene expression was rarely found in interneurons expressing GABA and was never found in AMPA receptor- or tyrosine hydroxylase-expressing neurons. Thus, Plp expression in the mouse caudal medulla was found to be developmentally regulated and restricted to specific groups of neurons.

Autotaxin

Autotaxin is a protein of approximately 900 amino acids discovered in the early 1990s. Over the past 15 years, a strong association between cancer cells and autotaxin production has been observed. Recent publications indicate that autotaxin and the capacity of cancer to metastasise are intimately linked. The discovery of new molecular targets in pharmacology is a mixture of pure luck, hard work and industrial strategy. Despite a crucial and desperate need for new therapeutic tools, many targets are approached in oncology, but only a few are validated and end up at the patient bed. Outside the busy domain of kinases, few targets have been discovered that can be useful in treating cancer, particularly metastatic processes. The fortuitous relationship between autotaxin and lysophosphatidic acid renders the results of observations made in the diabetes/obesity context considerably important. The literature provides observations that may aid in redesigning experiments to validate autotaxin as a potential oncology target.

Lysophosphatidic acid can support the formation of membranous structures and an increase in MBP mRNA levels in differentiating oligodendrocytes

During development, differentiating oligodendrocytes progress in distinct maturation steps from premyelinating to myelinating cells. Such maturing oligodendrocytes express both the receptors mediating signaling via extracellular lysophosphatidic acid (LPA) and the major enzyme generating extracellular LPA, namely phosphodiesterase-Ialpha/autotaxin (PD-Ialpha/ATX). However, the biological role of extracellular LPA during the maturation of differentiating oligodendrocytes is currently unclear. Here, we demonstrate that application of exogenous LPA induced an increase in the area occupied by the oligodendrocytes' process network, but only when PD-Ialpha/ATX expression was down-regulated. This increase in network area was caused primarily by the formation of membranous structures. In addition, LPA increased the number of cells positive for myelin basic protein (MBP). This effect was associated by an increase in the mRNA levels coding for MBP but not myelin oligodendrocyte glycoprotein (MOG). Taken together, these data suggest that LPA may play a crucial role in regulating the later stages of oligodendrocyte maturation.

Distribution of ectonucleotidases in the rodent brain revisited

Nucleotides comprise a major class of signaling molecules in the nervous system. They can be released from nerve cells, glial cells, and vascular cells where they exert their function via ionotropic (P2X) or metabotropic (P2Y) receptors. Signaling via extracellular nucleotides and also adenosine is controlled and modulated by cell-surface-located enzymes (ectonucleotidases) that hydrolyze the nucleotide to the respective nucleoside. Extracellular hydrolysis of nucleotide ligands involves a considerable number of enzymes with differing catalytic properties differentially affecting the nucleotide signaling pathway. It is therefore important to investigate which type of ectonucleotidase(s) contributes to the control of nucleotide signaling in distinct cellular and physiological settings. By using a classical enzyme histochemical approach and employing various substrates, inhibitors, and knockout animals, we provide, for the first time, a comparative analysis of the overall distribution of catalytic activities reflecting four ectonucleotidase families: ecto-5'-nucleotidase, alkaline phosphatases, ectonucleoside triphosphate diphosphohydrolases (E-NTPDases), and ectonucleotide pyrophyphatases/phosphodiesterases (E-NPPs). We place into perspective the earlier literature and provide novel evidence for a parenchymal localization of tissue non-specific alkaline phosphatase, E-NPPs, and E-NTPDases in the mouse brain. In addition, we specify the location of ectonucleotidases within the brain vasculature. Most notably, brain vessels do not express ecto-5'-nucleotidase. The preponderance of individual enzymes differs considerably between brain locations. The contribution of all types of ectonucleotidases thus needs to be considered in physiological and pharmacological studies of purinergic signaling in the brain.

Involvement of the lysophosphatidic acid-generating enzyme autotaxin in lymphocyte-endothelial cell interactions

Autotaxin (ATX) is a secreted protein with lysophospholipase D activity that generates lysophosphatidic acid (LPA) from lysophosphatidylcholine. Here we report that functional ATX is selectively expressed in high endothelial venules (HEVs) of both lymph nodes and Peyer's patches. ATX expression was developmentally regulated and coincided with lymphocyte recruitment to the lymph nodes. In adults, ATX expression was independent of HEV-expressed chemokines such as CCL21 and CXCL13, innate immunity signals including those via TLR4 or MyD88, and of the extent of lymphocyte trafficking across the HEVs. ATX expression was induced in venules at sites of chronic inflammation. Receptors for the ATX enzyme product LPA were constitutively expressed in HEV endothelial cells (ECs). In vitro, LPA induced strong morphological changes in HEV ECs. Forced ATX expression caused cultured ECs to respond to lysophosphatidylcholine, up-regulating lymphocyte binding to the ECs in a LPA receptor-dependent manner under both static and flow conditions. Although in vivo depletion of circulating ATX did not affect lymphocyte trafficking into the lymph nodes, we surmise, based on the above data, that ATX expressed by HEVs acts on HEVs in situ to facilitate lymphocyte binding to ECs and that ATX in the general circulation does not play a major role in this process. Tissue-specific inactivation of ATX will verify this hypothesis in future studies of its mechanism of action.

alpha- and beta-substituted phosphonate analogs of LPA as autotaxin inhibitors

Autotaxin (ATX) is an attractive pharmacological target due to its lysophospholipase D activity which leads to the production of lysophosphatidic acid (LPA). Blockage of ATX produced LPA by small molecules could be a potential anticancer chemotherapy. In our previous study, we have identified the two beta-hydroxy phosphonate analogs of LPA (compounds f17 and f18) as ATX inhibitors. With this work, we investigated alpha- and beta-substituted phosphonate analogs of LPA and evaluated them for ATX inhibitory activity. The stereochemistry of beta-hydroxy phosphonates was also studied.

Phosphodiesterase-Ialpha/autotaxin's MORFO domain regulates oligodendroglial process network formation and focal adhesion organization

Development of a complex process network by maturing oligodendrocytes is a critical but currently poorly characterized step toward myelination. Here, we demonstrate that the matricellular oligodendrocyte-derived protein phosphodiesterase-Ialpha/autotaxin (PD-Ialpha/ATX) and especially its MORFO domain are able to promote this developmental step. In particular, the single EF hand-like motif located within PD-Ialpha/ATX's MORFO domain was found to stimulate the outgrowth of higher order branches but not process elongation. This motif was also observed to be critical for the stimulatory effect of PD-Ialpha/ATX's MORFO domain on the reorganization of focal adhesions located at the leading edge of oligodendroglial protrusions. Collectively, our data suggest that PD-Ialpha/ATX promotes oligodendroglial process network formation and expansion via the cooperative action of multiple functional sites located within the MORFO domain and more specifically, a novel signaling pathway mediated by the single EF hand-like motif and regulating the correlated events of process outgrowth and focal adhesion organization.

Autotaxin and lysophosphatidic acid stimulate intestinal cell motility by redistribution of the actin modifying protein villin to the developing lamellipodia

Autotaxin (ATX) is a potent tumor cell motogen that can produce lysophosphatidic acid (LPA) from lysophosphatidylcholine. LPA is a lipid mediator that has also been shown to modulate tumor cell invasion. Autotaxin mRNA is expressed at significant levels in the intestine. Likewise, LPA2 receptor levels have been shown to be elevated in colon cancers. The molecular mechanism of ATX/LPA-induced increase in intestinal cell migration however, remains poorly understood. Villin is an intestinal and renal epithelial cell specific actin regulatory protein that modifies epithelial cell migration. In this study we demonstrate that both Caco-2 (endogenous villin) and MDCK (exogenous villin) cells, which express primarily LPA2 receptors, show enhanced cell migration in response to ATX/LPA. ATX and LPA treatment results in the rapid formation of lamellipodia and redistribution of villin to these cell surface structures, suggesting a role for villin in regulating this initial event of cell locomotion. The LPA-induced increase in cell migration required activation of c-src kinase and downstream tyrosine phosphorylation of villin by c-src kinase. LPA stimulated cell motility was determined to be insensitive to pertussis toxin, but was regulated by activation of PLC-gamma 1. Together, our results show that in epithelial cells ATX and LPA act as strong stimulators of cell migration by recruiting PLC-gamma 1 and villin, both of which participate in the initiation of protrusion.

Autotaxin: a secreted autocrine/paracrine factor that promotes glioma invasion

Glioblastoma multiforme (GBM) is inherently invasive, and it is from the invasive cell population that the tumor recurs. The GBM invasion transcriptome reveals over-expression of various autocrine factors that could act as motility drivers, such as autotaxin (ATX). Some of these factors could also have paracrine roles, modulating the behavior of cells in the peri-tumoral brain parenchyma. ATX generates lysophosphatidic acid (LPA), which signals through LPA receptors expressed by GBM as well as in astrocytes, oligodendrocytes (ODC) and microglia; their activation manifest cell specific effects. ATX stimulates invasion of GBM cells in vitro and ex vivo invasion assays. ATX activity enhances GBM adhesion in cells expressing the LPA1 receptor, as well as stimulating rac activation. GBM secreted ATX can also have paracrine effects: ATX activity results in reduced ODC adhesion. ODC monolayer invasion showed that U87 and U251 GBM cells expressing ATX invaded through an ODC monolayer significantly more than cells depleted of ATX or cells expressing inactive ATX, suggesting that GBM cells secreting ATX find ODCs less of a barrier than cells that do not express ATX. Secreted factors that drive GBM invasion can have autocrine and paracrine roles; one stimulates GBM motility and the other results in ODC dis-adhesion.

The N-terminal hydrophobic sequence of autotaxin (ENPP2) functions as a signal peptide

Autotaxin, also known as ENPP2, was originally isolated from the culture medium of melanoma cells as a cell-motility promoting protein. It regulates cell growth, motility, and angiogenesis through the production of lysophosphatidic acid and sphingosine 1-phosphate. Because autotaxin shows overall structural similarity to the well-characterized PC-1, it has been assumed to be a type II transmembrane protein that is expressed on the cell surface and is released into the extracellular space after proteolytic cleavage. We found, however, that while autotaxin was efficiently secreted into the extracellular space both in vitro and in vivo, it was expressed neither on the surfaces of autotaxin-transfected cells nor on those of the autotaxin-expressing choroid plexus epithelium cells. N-terminal sequencing of the secreted autotaxin revealed that it was cleaved at two N-terminal sites that match the consensus sequences for cleavage by a signal peptidase and furin. In addition, when translated in vitro, autotaxin was co-translationally translocated into microsome membranes, and its N-terminal 3-kDa fragment corresponding to a signal sequence was cleaved. These data demonstrate that the N-terminal hydrophobic sequence of autotaxin functions as a signal peptide, not as a transmembrane segment, and thus autotaxin is synthesized as a secreted protein.

Regulation and biological activities of the autotaxin-LPA axis

Autotaxin (ATX), or nucleotide pyrophosphatase/phosphodiesterase 2 (NPP2), is an exo-enzyme originally identified as a tumor cell autocrine motility factor. ATX is unique among the NPPs in that it primarily functions as a lysophospholipase D, converting lysophosphatidylcholine into the lipid mediator lysophosphatidic acid (LPA). LPA acts on specific G protein-coupled receptors to elicit a wide range of cellular responses, ranging from cell proliferation and migration to neurite remodeling and cytokine production. While LPA signaling has been studied extensively over the last decade, we are only now beginning to explore the properties and biological importance of ATX as the major LPA-producing phospholipase. In this review, we highlight recent advances in our understanding of the ATX-LPA axis, giving first an update on LPA action and then focusing on ATX, in particular its regulation, its link to cancer and its vital role in vascular development.

Biochemical analysis of ecto-nucleotide pyrophosphatase phosphodiesterase activity in brain membranes indicates involvement of NPP1 isoenzyme in extracellular hydrolysis of diadenosine polyphosphates in central nervous system

Synaptosomes and plasma membranes obtained from rat brain display ectoenzymatic hydrolytic activity responsible for hydrolysis of the neurotransmitter/neuroregulatory nucleotides diadenosine polyphosphates. Intact synaptosomes and plasma and synaptic membranes isolated by sucrose-gradient ultracentrifugation from several brain regions (hypothalamus, hippocampus, temporal cortex, frontal cortex striatum and cerebellum) degraded the fluorogenic substrates diethenoadenosine polyphosphates up to ethenoadenosine as by-product. Purified ectoenzyme cleaved substrates always releasing the mononucleotide moieties ethenoadenosine 5'-monophosphate and the corresponding ethenoadenosine (n-1) 5'-phosphate. Ectoenzymatic hydrolysis reached maximal activity at pH 9.0 (pH range 6.5-9.0) and was activated by Ca(2+) and Mg(2+) ions, with maximal effects around 2.0 mM cation. EDTA drastically reduced activity and Zn(2+) was required for enzyme reactivation. Hydrolysis of substrates followed hyperbolic kinetics with K(m) values in the 3-10 microM range. Diadenosine polyphosphates and heparin behaved as competitive inhibitors in the enzymatic hydrolysis of diethenoadenosine polyphosphates and AMP, ATP, alpha,beta-methyleneADP, ADPbetaS ATPgammaS, beta,gamma-methyleneATP, suramin and diethyl pyrocarbonate were also inhibitors. Ectoenzymatic activity shared the typical characteristics of members of the ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) family and inhibition data suggest that NPP1 ectoenzyme is involved in the cleavage of extracellular diadenosine polyphosphates in brain. Synaptic membranes from cerebellum, hypothalamus and hippocampus presented the highest activities and no activity differences were observed between young and aged animals. However, plasma membranes showed a more homogeneous distribution of ectoenzymatic activity but a general increase was detected in aged animals. Enhancement of ectoenzymatic diadenosine polyphosphate cleaving activity found in plasma membranes from old animals could play a deleterious role in aged brain by limiting neuroprotective effects reported for extracellular diadenosine tetraphosphate.

Immunolocalization of ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) in the rat forebrain

Immunohistochemical study was performed to determine distribution of ecto-nucleotide pyrophosphatase/phosphodiesterase1 (NPP1) in adult rat forebrain. The study revealed widespread distribution of NPP1 in rat forebrain, yet with regional differences in the expression pattern and abundance. Strong NPP1 immunoreaction was detected in pyramidal cell layer of cerebral cortex and hippocampus, and in the midline regions of hypothalamus and thalamus. In many immunopositive forebrain areas, NPP1 was mainly localized at neuronal cell bodies. However, prominent immunoreaction was also detected at ependymal cells, tanycytes, endothelial cells of the capillaries and cells of the choroid plexus, suggesting that NPP1 could be involved in some highly specialized transport process.

Purification and characterization of lysophospholipase D from rat brain

A lysophospholipase D (lysoPLD) was purified to apparent homogeneity from rat brain nuclear fractions using 1-[(14)C]palmitoyl-glycerophosphorylcholine as a substrate. The abundance of autotaxin (ATX), a secretory lysoPLD, was also estimated for each fraction. The nuclear fraction had relatively high levels of lysoPLD activity but weak immunoreactivity with an anti-ATX antibody. LysoPLD activity was further purified 5550-fold by sequential chromatography. The final preparation migrated as a single band with a molecular weight of 35,000. Anti-ATX antibodies did not cross-react with the purified enzyme. Moreover, enzyme activity was highest at pH 7.0-7.5 and requires Mg(2+). The Km and Vmax values for 1-palmitoyl-glycerophosphorylcholine were 176 microM and 0.3 micromol/min/mg, respectively. The purified enzyme hydrolyzed saturated forms of LPC more robustly than unsaturated forms. The enzyme could hydrolyze platelet-activating factor (PAF) to the same extent as 16:0-LPC, and showed a higher activity toward lysoPAF (1-O-hexadecyl-2-lyso-glycerophosphorylcholine). These results suggested that the lysoPLD purified from rat brain nuclear fractions in this work is a novel enzyme that hydrolyzes lysoPAF, PAF, and LPC to liberate choline.

Autotaxin, a secreted lysophospholipase D, is essential for blood vessel formation during development

Autotaxin (ATX), or nucleotide pyrophosphatase-phosphodiesterase 2, is a secreted lysophospholipase D that promotes cell migration, metastasis, and angiogenesis. ATX generates lysophosphatidic acid (LPA), a lipid mitogen and motility factor that acts on several G protein-coupled receptors. Here we report that ATX-deficient mice die at embryonic day 9.5 (E9.5) with profound vascular defects in yolk sac and embryo resembling the Galpha13 knockout phenotype. Furthermore, at E8.5, ATX-deficient embryos showed allantois malformation, neural tube defects, and asymmetric headfolds. The onset of these abnormalities coincided with increased expression of ATX and LPA receptors in normal embryos. ATX heterozygous mice appear healthy but show half-normal ATX activity and plasma LPA levels. Our results reveal a critical role for ATX in vascular development, indicate that ATX is the major LPA-producing enzyme in vivo, and suggest that the vascular defects in ATX-deficient embryos may be explained by loss of LPA signaling through Galpha13.

Modulation of purinergic signaling by NPP-type ectophosphodiesterases

Extracellular nucleotides can elicit a wide array of cellular responses by binding to specific purinergic receptors. The level of ectonucleotides is dynamically controlled by their release from cells, synthesis by ectonucleoside diphosphokinases and ectoadenylate kinases, and hydrolysis by ectonucleotidases. One of the four structurally unrelated families of ectonucleotidases is represented by the NPP-type ectophosphodiesterases. Three of the seven members of the NPP family, namely NPP1–3, are known to hydrolyze nucleotides. The enzymatic action of NPP1–3 (in)directly results in the termination of nucleotide signaling, the salvage of nucleotides and/or the generation of new messengers like ADP, adenosine or pyrophosphate. NPP2 is unique in that it hydrolyzes both nucleotides and lysophospholipids and, thereby, generates products that could synergistically promote cell motility. We review here the enzymatic properties of NPPs and analyze current evidence that links their nucleotide-hydrolyzing capability to epithelial and neural functions, the immune response and cell motility.

Nucleotide signaling in nervous system development

The development of the nervous system requires complex series of cellular programming and intercellular communication events that lead from the early neural induction to the formation of a highly structured central and peripheral nervous system. Neurogenesis continuously takes place also in select regions of the adult mammalian brain. During the past years, a multiplicity of cellular control mechanisms has been identified, ranging from differential transcriptional mediators to inducers or inhibitors of cell specification or neurite outgrowth. While the identification of transcription factors typical for the stage-specific progression has been a topic of key interest for many years, less is known concerning the potential multiplicity of relevant intercellular signaling pathways and the fine tuning of epigenetic gene regulation. Nucleotide receptors can induce a multiplicity of cellular signaling pathways and are involved in multiple molecular interactions, thus opening the possibility of cross talk between several signaling pathways, including growth factors, cytokines, and extracellular matrix components. An increasing number of studies provides evidence for a role of nucleotide signaling in nervous system development. This includes progenitor cell proliferation, cell migration, neuronal and glial cellular interaction and differentiation, and synaptic network formation.

Phosphodiesterase-Ialpha/autotaxin (PD-Ialpha/ATX): a multifunctional protein involved in central nervous system development and disease

Phosphodiesterase-Ialpha/autotaxin (PD-Ialpha/ATX) was originally identified as a cell-motility-stimulating factor secreted by a variety of tumor cells. Thus, studies related to its potential functional roles have traditionally focused on tumorigenesis. PD-Ialpha/ATX's catalytic activity, initially defined as nucleotide pyrophosphatase/phosphodiesterase, was soon recognized as being necessary for its tumor cell-motility-stimulating activity. However, only the discovery of PD-Ialpha/ATX's identity with lysophospholipase D, an extracellular enzyme that converts lysophosphatidylcholine into lysophosphatidic acid (LPA) and potentially sphingosylphosphoryl choline into sphingosine 1-phosphate (S1P), revealed the actual effectors responsible for PD-Ialpha/ATX's ascribed motogenic functions, i.e., its catalytic products. PD-Ialpha/ATX has also been detected during normal development in a number of tissues, in particular, the central nervous system (CNS), where expression levels are high. Similar to tumor cells, PD-Ialpha/ATX-expressing CNS cells secrete catalytically active PD-Ialpha/ATX into the extracellular environment. Thus, it appears reasonable to assume that PD-Ialpha/ATX's CNS-related functions are mediated via lysophospholipid, LPA and potentially S1P, signaling. However, recent studies identified PD-Ialpha/ATX as a matricellular protein involved in the modulation of oligodendrocyte-extracellular matrix interactions and oligodendrocyte remodeling. This property of PD-Ialpha/ATX was found to be independent of its catalytic activity and to be mediated by a novel functionally active domain. These findings, therefore, uncover PD-Ialpha/ATX, at least in the CNS, as a multifunctional protein able to induce complex signaling cascades via distinct structure-function domains. This Mini-Review describes PD-Ialpha/ATX's multifunctional roles in the CNS and discusses their potential contributions to CNS development and pathology.

Immunolocalization of ectonucleotidases along the rat nephron

Evidence is accumulating that extracellular nucleotides act as autocrine/paracrine agents in most tissues, including the kidneys. Several families of surface-located enzymes, collectively known as ectonucleotidases, can degrade nucleotides. Using immunohistochemistry, we have examined the segmental distribution of five ectonucleotidases along the rat nephron. Perfusion-fixed kidneys were obtained from anesthetized male Sprague-Dawley rats. Cryostat sections of cortical and medullary regions were incubated with antibodies specific to the following enzymes: ectonucleoside triphosphate diphosphohydrolase (NTPDase) 1, NTPDase2, NTPDase3, ectonucleotide pyrophosphatase phosphodiesterase 3 (NPP3), and ecto-5'-nucleotidase. Sections were then costained with Phaseolus vulgaris erythroagglutinin (for identification of proximal tubules) and antibodies against Tamm-Horsfall protein (for identification of thick ascending limb), calbindin-D(28k) (for identification of distal tubule), and aquaporin-2 (for identification of collecting duct). The tyramide signal amplification method was used when the ectonucleotidase and marker antibody were raised in the same species. The glomerulus expressed NTPDase1 and NPP3. The proximal tubule showed prominent expression of NPP3 and ecto-5'-nucleotidase in most, but not all, segments. NTPDase2 and NTPDase3, but not NPP3 or ecto-5'-nucleotidase, were expressed in the thick ascending limb and distal tubule. NTPDase3, with some low-level expression of ecto-5'-nucleotidase, was also found in cortical and outer medullary collecting ducts. Inner medullary collecting ducts displayed low-level staining for NTPDase1, NTPDase2, NTPDase3, and ecto-5'-nucleotidase. We conclude that these ectonucleotidases are differentially expressed along the nephron and may play a key role in activation of purinoceptors by nucleotides and nucleosides.

Identification of autotaxin as a neurite retraction-inducing factor of PC12 cells in cerebrospinal fluid and its possible sources

Abstract Cerebrospinal fluid (CSF) induced neurite retraction of differentiated PC12 cells; the action was observed in 15 min (a rapid response) and the activity further increased until 6 h (a long-acting response) during exposure of CSF to the cells. The CSF action was sensitive to monoglyceride lipase and diminished by homologous desensitization with lysophosphatidic acid (LPA) and by pretreatment with an LPA receptor antagonist Ki16425. Although fresh CSF contains LPA to some extent, the LPA content in the medium was increased during culture of PC12 cells with CSF. The rapid response was mimicked by exogenous LPA, and a long-acting response was duplicated by a recombinant autotaxin, lysophospholipase D (lyso-PLD). Although the lyso-PLD substrate lysophosphatidylcholine (LPC) was not detected in CSF, lyso-PLD activity and an approximately 120-kDa autotaxin protein were detected in CSF. On the other hand, LPC but not lyso-PLD activity was detected in the conditioned medium of a PC12 cell culture without CSF. Among neural cells examined, leptomeningeal cells expressed the highest lyso-PLD activity and autotaxin protein. These results suggest that leptomeningeal cells may work as one of the sources for autotaxin, which may play a critical role in LPA production and thereby regulate axonal and neurite morphological change.

Inhibition of autotaxin by lysophosphatidic acid and sphingosine 1-phosphate

Autotaxin (ATX) or nucleotide pyrophosphatase/phosphodiesterase 2 (NPP2) is an NPP family member that promotes tumor cell motility, experimental metastasis, and angiogenesis. ATX primarily functions as a lysophospholipase D, generating the lipid mediator lysophosphatidic acid (LPA) from lysophosphatidylcholine. ATX uses a single catalytic site for the hydrolysis of both lipid and non-lipid phosphodiesters, but its regulation is not well understood. Using a new fluorescence resonance energy transfer-based phosphodiesterase sensor that reports ATX activity with high sensitivity, we show here that ATX is potently and specifically inhibited by LPA and sphingosine 1-phosphate (S1P) in a mixed-type manner (Ki approximately 10(-7) M). The homologous ecto-phosphodiesterase NPP1, which lacks lysophospholipase D activity, is insensitive to LPA and S1P. Our results suggest that, by repressing ATX activity, LPA can regulate its own biosynthesis in the extracellular environment, and they reveal a novel role for S1P as an inhibitor of ATX, in addition to its well established role as a receptor ligand.

Modulation of adaptive immune responses by sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) has long been recognized as a mediator of a variety of cell functions. A growing body of evidence has accumulated demonstrating its role in cell migration and as a mediator of growth factor-induced events. In recent years, it has become apparent that S1P also mediates many cytokine and chemokine functions. Cells of the immune system function and migrate in response to a complex network of cytokines and chemokines, and the outcome is determined by the interplay of the effects of these molecules on the target cell. S1P may be a bona fide component of these networks and influence the responses of cells to these immune modulators.