Lysophosphatidic acid (LPA) signaling in vertebrate reproduction

LPA signaling has been implicated in post-implantation embryo developmental processes such as vascular formation, vascular maturation and maintenance, heart development, and brain formation (references in [ 2 ]). In a study preceding LPA receptor identification, culturing of embryos from the pronuclear stage in the presence of LPA significantly increased the success rate of the development of 2-cell and 4-cell stage embryos to blastocysts via a G αi -protein receptor mechanism [ 98 ]. A more recent study reported Lpar1 mRNA expression in differentiating mouse blastocysts, expression of Lpar2 in late-stage blastocysts and no expression of Lpar3 at any of the examined stages [ 99 ]. One potential mechanism could be that LPA elevates [Ca 2+ ] i levels to accelerate murine blastocyst differentiation. LPA induces the transient accumulation of heparin-binding EGF-like growth factor (HB-EGF) on the embryo surface, and interfering with HB-EGF signaling through EGF receptors ErbB1 or ErbB4 could attenuate LPA-stimulated blastocyst differentiation [ 99 ]. However, there is no obvious defect in blastocyst development in LPA 3 -deficient or LPA 1 /LPA 2 -double deficient mice [ 32 ]. Delayed post-implantation embryo development in LPA 3 -deficient uteri reflects delayed embryo implantation that is maternal in origin [ 32 ]. Nevertheless, LPA signaling can influence post-implantation embryo development. Deletion of ATX, a key enzyme in LPA production, leads to embryonic lethality due to defects in blood vessel formation, brain development, etc [ 15 , 16 ], suggesting roles for LPA signaling in embryo development. Expression patterns of LPA receptors in embryo suggest potential functions of LPA signaling in organogenesis [ 31 ]. We have observed embryonic hematoma and embryonic lethality with incomplete but increased penetrance in LPA 1 , LPA 1/2 double knockout, and LPA 1/2/3 triple knockout mice [ 34 , 46 , 100 ]. However, these phenotypes can't represent what are observed in ATX knockout mice. Considering the fact that more LPA receptors are being identified, any single or a few LPA receptors may not be able to represent the function of ATX. LPA signaling may play a role in reproduction for other species as well. XLPA 1 and XLPA 2 receptor-mediated LPA signaling is important for early embryo development in Xenopus via the maintenance of overall rigidity and shape of the embryo [ 101 , 102 ]. LPA 3 was detected in porcine concepti of day 12 and 15, and it was suggested that LPA produced in porcine uterine endometrium influences conceptus development during implantation and establishment of porcine pregnancy [ 55 ]. A recent study shows multiple effects of LPA on ovine trophectoderm cells, such as activation of MAPK ERK1/2 phosphorylation, promotion of proliferation and cytoskeletal rearrangement, as well as release of PGF 2α and PGE 2 , suggesting a potential role of LPA signaling in the ovine conceptus at the time of implantation [ 54 ]. Summary Progress over the past decade has revealed the importance of LPA signaling in female and male reproduction, as well as fertilization and pre-implantation embryo development. All examined vertebrate species utilize LPA receptor-mediated mechanisms, and this form of lysophospholipid signaling influences most elements of reproduction directly or indirectly. Numerous issues remain to be addressed in the future (Box 1), a partial list that underscores both the multitude of pathway elements to consider, along with selected questions within each group. The therapeutic potential of LPA signaling represents an area of opportunity that awaits future investigations. Box 1. Outstanding Questions LPA ligand-related issues What chemical forms of LPA exist within the reproductive system? What are local LPA concentrations? Do LPA concentrations vary with reproductive stage or age? Are there LPA concentration gradients? Which cells produce LPA? What is most critical for LPA signaling with respect to ligand: synthesis or degradation? Are there physiological or disease conditions that significantly alter LPA levels or gradients (e.g., obesity)? LPA biosynthetic and degradative enzyme-related issues Which enzymes are most important in the reproductive system? How is their expression and activity controlled? Do LPA precursors and/or products create positive and negative feedback loops affecting activity? Are there unidentified enzymes that contribute to LPA enzymatic pathways? LPA receptor-related issues Have all LPA receptors involved in reproduction been identified? What are rate-limiting mechanisms in controlling LPA receptor activity? Which downstream signaling pathways are dominant for LPA-mediated reproductive effects? What is the relationship between LPA signaling and other lysophospholipid pathways? What is the relationship between LPA signaling and other signaling pathways involved in reproduction? LPA-based therapeutic issues Which molecular targets are tractable for intervention? What physiological or disease indications could be therapeutically and safely accessed? Progress over the past decade has revealed the importance of LPA signaling in female and male reproduction, as well as fertilization and pre-implantation embryo development. All examined vertebrate species utilize LPA receptor-mediated mechanisms, and this form of lysophospholipid signaling influences most elements of reproduction directly or indirectly. Numerous issues remain to be addressed in the future (Box 1), a partial list that underscores both the multitude of pathway elements to consider, along with selected questions within each group. The therapeutic potential of LPA signaling represents an area of opportunity that awaits future investigations. What chemical forms of LPA exist within the reproductive system? What are local LPA concentrations? Do LPA concentrations vary with reproductive stage or age? Are there LPA concentration gradients? Which cells produce LPA? What is most critical for LPA signaling with respect to ligand: synthesis or degradation? Are there physiological or disease conditions that significantly alter LPA levels or gradients (e.g., obesity)? Which enzymes are most important in the reproductive system? How is their expression and activity controlled? Do LPA precursors and/or products create positive and negative feedback loops affecting activity? Are there unidentified enzymes that contribute to LPA enzymatic pathways? Have all LPA receptors involved in reproduction been identified? What are rate-limiting mechanisms in controlling LPA receptor activity? Which downstream signaling pathways are dominant for LPA-mediated reproductive effects? What is the relationship between LPA signaling and other lysophospholipid pathways? What is the relationship between LPA signaling and other signaling pathways involved in reproduction? Which molecular targets are tractable for intervention? What physiological or disease indications could be therapeutically and safely accessed?

Lysophosphatidic acid (LPA) is an extracellular lipid signaling molecule that produces a broad range of cellular influences including survival, differentiation, proliferation, morphological changes, migration, and others. Within vertebrates, LPA signaling has been implicated in numerous physiological and pathological processes affecting most, if not all, organ systems [ 1 - 4 ]. One key area LPA influences is reproduction, both male and female. This review briefly introduces LPA signaling and reviews its effects on reproduction. Additional details on LPA signaling can be found in recent reviews that also note a related lysophospholipid, sphingosine 1-phosphate (S1P) that has other influences on reproduction but will not be discussed here [ 5 - 12 ].

The study of receptor-mediated LPA signaling mechanisms began with the cloning of LPA receptors and the establishment of receptor-mediated functions. So far five bona fide LPA receptors, LPA 1-5 , have been identified [ 1 , 5 , 9 , 12 , 22 ]. Their human genes are designated LPARx with x=1-5 (human genome organization (HUGO)), while mouse names are Lparx , with x also =1-5 (Genome Informatix (MGI)) [ 5 ]. P2Y5 has been confirmed to be a new LPA receptor, LPA 6 [ 23 , 24 ]. Two additional possible LPA receptors, GPR87 and P2Y10, have also been reported [ 25 , 26 ], but await validation. LPA receptors are all cell-surface, seven transmembrane spanning G protein-coupled receptors (GPCRs). They can differentially couple with G α12/13 , G αq , G αi/o , or in one instance, G αs to activate downstream signaling pathways leading to gene regulation and LPA-induced cellular functions ( Table 1 ) [ 1 , 12 , 24 , 27 ]. In addition, LPA receptors may have preferences for certain chemical ligand structures. For example, LPA 3 has a relatively high affinity for 2-acyl-LPA containing unsaturated fatty acids (with the ester-linked fatty acid in the 2 position) whereas other receptors such as LPA 1 and LPA 2 do not discriminate 1-acyl- and 2-acyl-LPA [ 28 - 30 ]. LPA receptors also have overlapping and differential gene expression patterns [ 2 ]. These LPA receptor characteristics contribute to receptor activities and functions [ 10 , 31 ]. A critical, strategic approach in determining the biological roles for LPA signaling has been via the creation and study of mice null for one or more LPA receptors [ 5 - 12 ]. In this way, roles for receptor-mediated signaling in reproduction were identified, whereby LPA 3 was found to affect embryo spacing and embryo implantation [ 32 , 33 ], and three receptors, LPA 1 , LPA 2 , and LPA 3 combined to affect spermatogenesis [ 34 ] ( Table 2 ).