Autotaxin, a Secreted Lysophospholipase D, Is Essential for Blood Vessel Formation during Development

Abstract

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 G␣ 13 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 G␣ 13 .Autotaxin (ATX), also known as ectonucleotide pyrophosphatase-phosphodiesterase 2, belongs to the nucleotide pyrophosphatase (NPP) family of ectoenzymes and exoenzymes, originally defined by their ability to hydrolyze nucleotides in vitro (8,15,44). Full-length ATX is cleaved along the classical export pathway and secreted as a catalytically active glycoprotein (21, 52). ATX was initially isolated as an autocrine motility factor for melanoma cells (45) and later found to promote metastasis and tumor vascularization in nude mice as well as eliciting an angiogenic response in Matrigel assays (31, 32). Hence, ATX may contribute to tumor progression by providing an invasive and/or angiogenic microenvironment for both malignant and stromal cells, a notion supported by growing evidence that ATX expression is upregulated in various invasive and metastatic cancers (4,18,22,28,43,55).The physiological substrate of ATX had remained elusive until it was discovered that ATX is identical to lysophospholipase D (lysoPLD), a secreted enzyme present in plasma and conditioned media that converts lysophosphatidylcholine (LPC) into bioactive lysophosphatidic acid (LPA) (11,47,48). LPA stimulates cell proliferation, migration, and survival by acting on specific G protein-coupled receptors (GPCRs) that are linked to multiple G proteins, including G q/11 , G i/o , and G 12/13 (20,30). LPA promotes wound healing in vivo and has been implicated in tumor progression, inflammation, vascular disease, and neural development (5,23,28,42,51). It has now become clear that LPA production, rather than nucleotide metabolism, accounts for the growth factor-like effects of ATX observed in cell culture. Strikingly, the other NPP family members lack intrinsic lysoPLD activity despite the similarity between their catalytic domain and that of ATX (14), implying that ATX/NPP2 is a unique lysoPLD with no functional redundancy within the NPP family.In addition to converting LPC into LPA, ATX can also hydrolyze sphingosyl-phosphorycholine (...

Expression patterns of the lysophospholipid receptor genes during mouse early development

“…However, as null mutants for autotaxin, an enzyme required for LPA biosynthesis, exhibit severe defects in neural tube formation (Tanaka et al, 2006;van Meeteren et al, 2006), there is no doubt that LPA signaling is required for early brain development. It is, therefore, of considerable interest to determine which LPA receptor(s) mediate LPA signals in early brain primordia.…”

“…Additional defects include malformations in the allantois and neural tube (van Meeteren et al, 2006;Tanaka et al, 2006). We have previously shown that ATX is expressed in a wide range of local signaling centers in mouse embryos, including the MHB, the floor plate of the neural tube, and various organ primordia (Ohuchi et al, 2007).…”

“…G 13 (Gna13)-null mice exhibit a very similar phenotype to that of ATX-null mice (Offermanns et al, 1997;van Meeteren et al, 2006), suggesting that many of the LPA receptors functioning during development may activate G 13 as a main signal transduction pathway in vivo (Meyer zu Heringdorf and Jakobs, 2007).…”

“…The ATX-null mouse has been described as early embryonic lethal secondary to profound defects in vascular and neural tube formation (Tanaka et al, 2006;van Meeteren et al, 2006). Mice heterozygous for the ATX-null mutation have one-half the normal plasma levels of LPA and ATX, but normal levels of S1P, confirming the specificity of ATX for the biosynthesis of LPA in vivo (Tanaka et al, 2006).…”

Targeting sphingosine-1-phosphate for cancer therapy

“…Potential additional sources of S1P like the autotaxin story may explain why sphk1-null mice exhibited no significant changes in tissue S1P levels even though tissue SPHK activity was nearly eliminated . However, autotaxin KO mice exhibited no significant changes in S1P levels either (van Meeteren et al, 2006); so, it remains unclear as to how dominant SPHK is as the sole source of tissue or blood S1P.…”