Fly and mammalian lipid phosphate phosphatase isoforms differ in activity both in vitro and in vivo

Origin and establishment of the germline in Drosophila melanogaster

The continuity of a species depends on germ cells. Germ cells are different from all the other cell types of the body (somatic cells) as they are solely destined to develop into gametes (sperm or egg) to create the next generation. In this review, we will touch on 4 areas of embryonic germ cell development in Drosophila melanogaster: the assembly and function of germplasm, which houses the determinants for germ cell specification and fate and the mitochondria of the next generation; the process of pole cell formation, which will give rise to primordial germ cells (PGCs); the specification of pole cells toward the PGC fate; and finally, the migration of PGCs to the somatic gonadal precursors, where they, together with somatic gonadal precursors, form the embryonic testis and ovary.

The journey of a generation: advances and promises in the study of primordial germ cell migration

The germline provides the genetic and non-genetic information that passes from one generation to the next. Given this important role in species propagation, egg and sperm precursors, called primordial germ cells (PGCs), are one of the first cell types specified during embryogenesis. In fact, PGCs form well before the bipotential somatic gonad is specified. This common feature of germline development necessitates that PGCs migrate through many tissues to reach the somatic gonad. During their journey, PGCs must respond to select environmental cues while ignoring others in a dynamically developing embryo. The complex multi-tissue, combinatorial nature of PGC migration is an excellent model for understanding how cells navigate complex environments in vivo. Here, we discuss recent findings on the migratory path, the somatic cells that shepherd PGCs, the guidance cues somatic cells provide, and the PGC response to these cues to reach the gonad and establish the germline pool for future generations. We end by discussing the fate of wayward PGCs that fail to reach the gonad in diverse species. Collectively, this field is poised to yield important insights into emerging reproductive technologies.

The Lipid Phosphate Phosphatase Wunen Promotes Eggshell Formation and Is Essential for Fertility in Drosophila

The eggshell that surrounds insect eggs acts as a barrier, protecting against biotic factors and desiccation. The eggshell is a multi-layered structure which is synthesised by the somatic follicle cells that surround the developing oocyte. Although the temporal order of expression of the protein eggshell components goes someway to explaining how the different layers are built up, but how the precise three-dimensional structure is achieved and how lipid components responsible for desiccation resistance are incorporated are poorly understood. In this paper, we demonstrate that wunen, which encodes a lipid phosphate phosphatase, is necessary for fertility in Drosophila females. Compared to sibling controls, females null for wunen lay fewer eggs which subsequently collapse such that no larvae emerge. We show that this is due to a requirement for wunen in the ovarian follicle cells which is needed to produce an ordered and functional eggshell. Knockdown of a septate junction component also results in collapsed eggs, supporting the idea that similar to its role in embryonic tracheal development, Wunen in follicle cells also promotes septate junction function.

Wun2-mediated integrin recycling promotes apoptotic cell clearance in Drosophila melanogaster

Apoptotic cell (AC) clearance is a complex process in which phagocytes recognize, engulf, and digest ACs during organismal development and tissue homeostasis. Impaired efferocytosis results in developmental defects and autoimmune diseases. In the current study, we performed RNA-sequencing to systematically identify regulators involved in the phagocytosis of ACs by Drosophila melanogaster macrophage-like S2 cells, followed by targeted RNA interference screening. Wunen2 (Wun2), a homolog of mammalian lipid phosphate phosphatase (LPP), was deemed as required for efferocytosis both in vitro and in vivo. However, efferocytosis was independent of Wun2 phosphatase activity. Proteomic analysis further revealed that Rab11 and its effector Rip11 are interaction partners of Wun2. Therefore, Wun2 collaborates with Rip11 and Rab11 to mediate efficient recycling of the phagocytic receptor βν integrin subunit to the plasma membrane. The loss of Wun2 results in the routing of βv integrin subunit (Itgbn) into lysosomes, leading to its degradation. The deficiency of βv integrin subunit on the cell surface leads to aberrant and disorganized actin cytoskeleton, thereby influencing the formation of macrophage pseudopodia toward ACs and thus failure to engulf them. The findings of this study provide insights that clarify how phagocytes coordinate AC signals and adopt a precise mechanism for the maintenance of engulfment receptors at their cell membrane surface to regulate efferocytosis.

Diverse roles of phosphatidate phosphatases in insect development and metabolism

The conversion of the glycerophospholipid phosphatidic acid (PA) into diacylglycerol (DAG) is essential for the biosynthesis of membrane phospholipids and storage fats. Importantly, both PA and DAG can also serve signaling functions in the cell. The dephosphorylation of PA that yields DAG can be executed by two different classes of enzymes, Mg²⁺-dependent lipins and Mg²⁺-independent lipid phosphate phosphatases. Here, I will discuss the current status of research directed at understanding the roles of these enzymes in insect development and metabolism. Special emphasis will be given to studies in the model organism Drosophila melanogaster.

Lipid Phosphate Phosphatases and Cancer

Lipid phosphate phosphatases (LPPs) are a group of three enzymes (LPP1–3) that belong to a phospholipid phosphatase (PLPP) family. The LPPs dephosphorylate a wide spectrum of bioactive lipid phosphates, among which lysophosphatidate (LPA) and sphingosine 1-phosphate (S1P) are two important extracellular signaling molecules. The LPPs are integral membrane proteins, which are localized on plasma membranes and intracellular membranes, including the endoplasmic reticulum and Golgi network. LPPs regulate signaling transduction in cancer cells and demonstrate different effects in cancer progression through the breakdown of extracellular LPA and S1P and other intracellular substrates. This review is intended to summarize an up-to-date understanding about the functions of LPPs in cancers.

Role of the autotaxin-lysophosphatidate axis in the development of resistance to cancer therapy

Autotaxin (ATX) is a secreted enzyme that hydrolyzes lysophosphatidylcholine to produce lysophosphatidate (LPA), which signals through six G-protein coupled receptors (GPCRs). Signaling through LPA is terminated by its degradation by a family of three lipid phosphate phosphatases (LPPs). LPP1 also attenuates signaling downstream of the activation of LPA receptors and some other GPCRs. The ATX-LPA axis mediates a plethora of activities such as cell proliferation, survival, migration, angiogenesis and inflammation, which perform an important role in facilitating wound healing. This wound healing response is hijacked by cancers where there is decreased expression of LPP1 and LPP3 and increased expression of ATX. This maladaptive regulation of LPA signaling also causes chronic inflammation, which has been recognized as one of the hallmarks in cancer. The increased LPA signaling promotes cell survival and migration and attenuates apoptosis, which stimulates tumor growth and metastasis. The wound healing functions of increased LPA signaling also protect cancer cells from effects of chemotherapy and radiotherapy. In this review, we will summarize knowledge of the ATX-LPA axis and its role in the development of resistance to chemotherapy and radiotherapy. We will also offer insights for developing strategies of targeting ATX-LPA axis as a novel part of cancer treatment. This article is part of a Special Issue entitled Lysophospholipids and their receptors: New data and new insights into their function edited by Susan Smyth, Viswanathan Natarajan and Colleen McMullen.

Hmgcr promotes a long-range signal to attract Drosophila germ cells independently of Hedgehog

During development, many cell types migrate along stereotyped routes determined through deployment of cell surface or secreted guidance molecules. Although we know the identity of many of these molecules, the distances over which they natively operate can be difficult to determine. Here, we have quantified the range of an attractive signal for the migration of Drosophila germ cells. Their migration is guided by an attractive signal generated by the expression of genes in the 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (Hmgcr) pathway, and by a repulsive signal generated by the expression of Wunens. We demonstrate that the attractive signal downstream of Hmgcr is cell-contact independent and acts at long range, the extent of which depends on Hmgcr levels. This range would be sufficient to reach all of the germ cells for their entire migration. Furthermore, Hmgcr-mediated attraction does not require Wunens but can operate simultaneously with Wunen-mediated repulsion. Finally, several papers posit Hedgehog (Hh) as being the germ cell attractant downstream of H mgcr Here, we provide evidence that this is not the case.

Increasing the low lipid phosphate phosphatase 1 activity in breast cancer cells decreases transcription by AP-1 and expressions of matrix metalloproteinases and cyclin D1/D3

Metastasis is the leading cause of mortality in breast cancer patients and lysophosphatidate (LPA) signaling promotes this process. LPA signaling is attenuated by lipid phosphate phosphatase-1 (LPP1) whose activity is decreased in cancers. Consequently, increasing LPP1 levels suppresses breast tumor growth and metastasis. This study shows that increasing LPP1 in breast cancer cells decreases transcription through cFos and cJun. This decreases production of cyclin D1/D3 and matrix metalloproteinases (MMPs), which provides new insights into the role of LPP1 in controlling tumor growth and metastasis. Methods: Invasiveness was determined by a Matrigel invasion assay. MMP expression was measured by qPCR, multiplex LASER bead technology and gelatin zymography. Levels of cJUN, cFOS, FRA1, cyclin D1, and cyclin D3 were determined by qPCR and western blotting. Collagen was determined by Picro-Sirius Red staining. Results: Increasing LPP1 expression inhibited invasion of MDA-MB-231 breast cancer cells through Matrigel. This was accompanied by decreases in expression of MMP-1, -3, -7, -9, -10, -12 and -13, which are transcriptionally regulated by the AP-1 complex. Increasing LPP1 attenuated the induction of mRNA of MMP-1, -3, cFOS, and cJUN by EGF or TNFα, but increased FRA1. LPP1 expression also decreased the induction of protein levels for cFOS and cJUN in nuclei and cytoplasmic fractions by EGF and TNFα. Protein levels of cyclin D1 and D3 were also decreased by LPP1. Although FRA1 in total cell lysates or cytoplasm was increased by LPP1, nuclear FRA1 was not affected. LPP1-induced decreases in MMPs in mouse tumors created with MDA-MB-231 cells were accompanied by increased collagen in the tumors and fewer lung metastases. Knockdown of LPP1 in MDA-MB-231 cells increased the protein levels of MMP-1 and -3. Human breast tumors also have lower levels of LPP1 and higher levels of cJUN, cFOS, MMP-1, -7, -8, -9, -12, -13, cyclin D1, and cyclin D3 relative to normal breast tissue. Conclusion: This study demonstrated that the low LPP1 expression in breast cancer cells is associated with high levels of cyclin D1/D3 and MMPs as a result of increased transcription by cFOS and cJUN. Increasing LPP1 expression provides a novel approach for decreasing transcription through AP-1, which could provide a strategy for decreasing tumor growth and metastasis.

Targeting sphingosine-1-phosphate signaling in lung diseases

Sphingosine-1-phosphate (S1P), a simple, bioactive sphingolipid metabolite, plays a key role, both intracellularly and extracellularly, in various cellular processes such as proliferation, survival, migration, inflammation, angiogenesis, and endothelial barrier integrity. The cellular S1P level is low and is tightly regulated by its synthesis and degradation. Sphingosine Kinases (SphKs) 1 and 2, catalyze the ATP-dependent phosphorylation of sphingosine to S1P, while the degradation is mediated by the reversible dephosphorylation catalyzed by the S1P phosphatases and lipid phosphate phosphatases and the irreversible degradation to hexadecenal and ethanolamine phosphate by sphingosine-1-phosphate lyase (S1PL). As a ligand for specific G-protein-coupled receptors, S1P1-5, which are differentially expressed in different cell types, S1P generates downstream signals that play crucial role in developmental and disease related pathologies. In addition to acting extracellularly on receptors located on the plasma membrane, S1P can also act intracellularly, independently of S1P1-5, affecting calcium homeostasis and cell proliferation. The SphKs /S1P /S1PL metabolic pathway is implicated in numerous human pathologies including respiratory disorders, thereby raising the possibility that manipulating intracellular S1P levels could offer therapeutic potential in ameliorating lung diseases. This review focuses on the prospects of targeting S1P signaling and S1P metabolizing enzymes using small molecule inhibitors, receptor agonists, and antagonists in the treatment of lung diseases.

Drosophila melanogaster as a Model of Muscle Degeneration Disorders

Drosophila melanogaster provides a powerful platform with which researchers can dissect complex genetic questions and biochemical pathways relevant to a vast array of human diseases and disorders. Of particular interest, much work has been done with flies to elucidate the molecular mechanisms underlying muscle degeneration diseases. The fly is particularly useful for modeling muscle degeneration disorders because there are no identified satellite muscle cells to repair adult muscle following injury. This allows for the identification of endogenous processes of muscle degeneration as discrete events, distinguishable from phenotypes due to the lack of stem cell-based regeneration. In this review, we will discuss the ways in which the fruit fly provides a powerful platform with which to study human muscle degeneration disorders.

Endothelial lipid phosphate phosphatase-3 deficiency that disrupts the endothelial barrier function is a modifier of cardiovascular development

Aims

Lipid phosphate phosphatase-3 (LPP3) is expressed at high levels in endothelial cells (ECs). Although LPP3 is known to hydrolyse the phosphate group from lysolipids such as spingosine-1-phosphate and its structural homologues, the function of Lpp3 in ECs is not completely understood. In this study, we investigated how tyrosine-protein kinase receptor (TEK or Tie2) promoter–dependent deletion of Lpp3 alters EC activities.

Methods and results

Lpp3^fl/fl mice were crossed with the tg.Tie2^Cre transgenic line. Vasculogenesis occurred normally in embryos with Tie2^Cre-mediated deletion of Lpp3 (called Lpp3^ECKO), but embryonic lethality occurred in two waves, the first wave between E8.5 and E10.5, while the second between E11.5 and E13.5. Lethality in Lpp3^ECKO embryos after E11.5 was accompanied by vascular leakage and haemorrhage, which likely resulted in insufficient cardiovascular development. Analyses of haematoxylin- and eosin-stained heart sections from E11.5 Lpp3^ECKO embryos showed insufficient heart growth associated with decreased trabeculation, reduced growth of the compact wall, and absence of cardiac cushions. Staining followed by microscopic analyses of Lpp3^ECKO embryos revealed the presence of apoptotic ECs. Furthermore, Lpp3-deficient ECs showed decreased gene expression and protein levels of Cyclin-D1, VE-cadherin, Fibronectin, Klf2, and Klf4. To determine the underlying mechanisms of vascular leakage and barrier disruption, we performed knockdown and rescue experiments in cultured ECs. LPP3 knockdown decreased transendothelial electrical resistance and increased permeability. Re-expression of β-catenin cDNA in LPP3-knockdown ECs partially restored the effect of the LPP3 loss, whereas re-expression of p120ctn cDNA did not.

Conclusion

These findings demonstrate the essential roles of LPP3 in the maturation of EC barrier integrity and normal cardiovascular development.

Repulsive cues combined with physical barriers and cell-cell adhesion determine progenitor cell positioning during organogenesis

The precise positioning of organ progenitor cells constitutes an essential, yet poorly understood step during organogenesis. Using primordial germ cells that participate in gonad formation, we present the developmental mechanisms maintaining a motile progenitor cell population at the site where the organ develops. Employing high-resolution live-cell microscopy, we find that repulsive cues coupled with physical barriers confine the cells to the correct bilateral positions. This analysis revealed that cell polarity changes on interaction with the physical barrier and that the establishment of compact clusters involves increased cell–cell interaction time. Using particle-based simulations, we demonstrate the role of reflecting barriers, from which cells turn away on contact, and the importance of proper cell–cell adhesion level for maintaining the tight cell clusters and their correct positioning at the target region. The combination of these developmental and cellular mechanisms prevents organ fusion, controls organ positioning and is thus critical for its proper function.

The precise positioning of organ progenitor cells is essential for organ development and function. Here the authors use live imaging and mathematical modelling to show that the confinement of a motile progenitor cell population results from coupled physical barriers and cell-cell interactions.

Drosophila gene tao-1 encodes proteins with and without a Ste20 kinase domain that affect cytoskeletal architecture and cell migration differently

Tao-1, the single representative of the Sterile 20 kinase subfamily in Drosophila, is best known for destabilizing microtubules at the actin-rich cortex, regulating the cytoskeletal architecture of cells. More recently, Tao-1 was shown to act in the Salvador–Warts–Hippo pathway by phosphorylating Hippo, regulating cell growth as well as cell polarity. Here, we show that tao-1 encodes two proteins, one with the Sterile 20 kinase domain (Tao-L) and one without it (Tao-S), and that they act in an antagonistic manner. Tao-L expression causes lamellipodia-like cell protrusions, whereas Tao-S expression results in filopodia-like structures that make cells stick to the surface they attach to. Ectopic Tao-1 expression in the anterior region of Drosophila embryos results in pole cell formation as normally observed at the posterior end. Tao-S expression causes primordial germ cells (PGCs) to adhere to the inner wall of the gut primordia and prevents proper transepithelial migration to the gonads. Conversely, RNAi knockdowns of Tao-1 cause disordered migration of PGCs out of the gut epithelium, their dispersal within the embryo and cell death. The results reveal a novel function of Tao-1 in cell migration, which is based on antagonistic activities of two proteins encoded by a single gene.

Cellular and molecular mechanisms of single and collective cell migrations in Drosophila: themes and variations

The process of cell migration is essential throughout life, driving embryonic morphogenesis and ensuring homeostasis in adults. Defects in cell migration are a major cause of human disease, with excessive migration causing autoimmune diseases and cancer metastasis, whereas reduced capacity for migration leads to birth defects and immunodeficiencies. Myriad studies in vitro have established a consensus view that cell migrations require cell polarization, Rho GTPase-mediated cytoskeletal rearrangements, and myosin-mediated contractility. However, in vivo studies later revealed a more complex picture, including the discovery that cells migrate not only as single units but also as clusters, strands, and sheets. In particular, the role of E-Cadherin in cell motility appears to be more complex than previously appreciated. Here, we discuss recent advances achieved by combining the plethora of genetic tools available to the Drosophila geneticist with live imaging and biophysical techniques. Finally, we discuss the emerging themes such studies have revealed and ponder the puzzles that remain to be solved.

Cloning and characterization of three Eimeria tenella lipid phosphate phosphatases

Although lipid phosphate phosphatases (LPPs) play an important role in cellular signaling in addition to lipid biosynthesis, little is thus far known about parasite LPPs. In this study, we characterized three Eimeria tenella cDNA clones encoding LPP named EtLPP1, EtLPP2 and EtLPP3. Key structural features previously described in LPPs, including the three conserved domains proposed as catalytic sites, a single conserved N-glycosylation site, and putative transmembrane domains were discovered in the three resulting EtLPP amino acid sequences. Expression of His6-tagged EtLPP1, -2, and -3 in HEK293 cells produced immunoreactive proteins with variable molecular sizes, suggesting the presence of multiple forms of each of the three EtLPPs. The two faster-migrating protein bands below each of the three EtLPP proteins were found to be very similar to the porcine 35-kDa LPP enzyme in their molecular size and the extent of their N-glycosylation, suggesting that the three EtLPPs are partially N-glycosylated. Kinetic analyses of the activity of the three enzymes against PA, LPA, C1P and S1P showed that Km values for each of the substrates were (in μM) 284, 46, 28, and 22 for EtLPP1; 369, 179, 237, and 52 for EtLPP2; and 355, 83, and 260 for EtLPP3. However, EtLPP3 showed negligible activity on S1P. These results confirmed that the three EtLPPs have broad substrate specificity. The results also indicated that despite structural similarities, the three EtLPPs may play distinct functions through their different models of substrate preference. Furthermore, particularly high expression levels of the three EtLPP genes were detected in the sporozoite stage of the E. tenella life cycle (p<0.001), suggesting that their encoded proteins might play an important biological function in the sporozoite stage.

Lipid phosphate phosphatases and their roles in mammalian physiology and pathology

Lipid phosphate phosphatases (LPPs) are a group of enzymes that belong to a phosphatase/phosphotransferase family. Mammalian LPPs consist of three isoforms: LPP1, LPP2, and LPP3. They share highly conserved catalytic domains and catalyze the dephosphorylation of a variety of lipid phosphates, including phosphatidate, lysophosphatidate (LPA), sphingosine 1-phosphate (S1P), ceramide 1-phosphate, and diacylglycerol pyrophosphate. LPPs are integral membrane proteins, which are localized on plasma membranes with the active site on the outer leaflet. This enables the LPPs to degrade extracellular LPA and S1P, thereby attenuating their effects on the activation of surface receptors. LPP3 also exhibits noncatalytic effects at the cell surface. LPP expression on internal membranes, such as endoplasmic reticulum and Golgi, facilitates the metabolism of internal lipid phosphates, presumably on the luminal surface of these organelles. This action probably explains the signaling effects of the LPPs, which occur downstream of receptor activation. The three isoforms of LPPs show distinct and nonredundant effects in several physiological and pathological processes including embryo development, vascular function, and tumor progression. This review is intended to present an up-to-date understanding of the physiological and pathological consequences of changing the activities of the different LPPs, especially in relation to cell signaling by LPA and S1P.

Quantifying the range of a lipid phosphate signal in vivo

Quantitative information about the range of influence of extracellular signaling molecules is critical for understanding their effects, but is difficult to determine in the complex and dynamic three-dimensional environment of a living embryo. Drosophila germ cells migrate during embryogenesis and use spatial information provided by expression of lipid phosphate phosphatases called Wunens to reach the somatic gonad. However, whether guidance requires cell contact or involves a diffusible signal is not known. We ectopically expressed Wunens in various segmentally repeated ectodermal and parasegmental patterns in embryos otherwise null for Wunens and used germ cell behavior to show that the signal is diffusible and to define its range. We correlated this back to the wild-type scenario and found that the germ cell migratory path can be primarily accounted for by Wunen expression. This approach provides the first quantitative information of the effective range of a lipid phosphate in vivo and has implications for the migration of other cell types that respond to lipid phosphates.

Compartmentalizing the embryo: lipids and septate junction mediated barrier function

Lipid phosphate phosphatases (LPPs) are a class of enzymes that can dephosphorylate a number of lysophopholipids in vitro. Analysis of knockouts of LPP family members has demonstrated striking but diverse developmental roles for these enzymes. LPP3 is required for mouse vascular development while the Drosophila LPPs Wunen (Wun) and Wunen2 (Wun2) are required during embryogenesis for germ cell migration and survival. In a recent publication we examined if these fly LPPs have further developmental roles and found that Wun is required for proper tracheal formation. In particular we highlight a role for Wun in septate junction mediated barrier function in the tracheal system. In this paper we discuss further the possible mechanisms by which LPPs may influence barrier activity.

Genetic elevation of sphingosine 1-phosphate suppresses dystrophic muscle phenotypes in Drosophila

Duchenne muscular dystrophy is a lethal genetic disease characterized by the loss of muscle integrity and function over time. Using Drosophila, we show that dystrophic muscle phenotypes can be significantly suppressed by a reduction of wunen, a homolog of lipid phosphate phosphatase 3, which in higher animals can dephosphorylate a range of phospholipids. Our suppression analyses include assessing the localization of Projectin protein, a titin homolog, in sarcomeres as well as muscle morphology and functional movement assays. We hypothesize that wunen-based suppression is through the elevation of the bioactive lipid Sphingosine 1-phosphate (S1P), which promotes cell proliferation and differentiation in many tissues, including muscle. We confirm the role of S1P in suppression by genetically altering S1P levels via reduction of S1P lyase (Sply) and by upregulating the serine palmitoyl-CoA transferase catalytic subunit gene lace, the first gene in the de novo sphingolipid biosynthetic pathway and find that these manipulations also reduce muscle degeneration. Furthermore, we show that reduction of spinster (which encodes a major facilitator family transporter, homologs of which in higher animals have been shown to transport S1P) can also suppress dystrophic muscle degeneration. Finally, administration to adult flies of pharmacological agents reported to elevate S1P signaling significantly suppresses dystrophic muscle phenotypes. Our data suggest that localized intracellular S1P elevation promotes the suppression of muscle wasting in flies.

Wunen, a Drosophila lipid phosphate phosphatase, is required for septate junction-mediated barrier function

Lipid phosphate phosphatases (LPPs) are integral membrane enzymes that regulate the levels of bioactive lipids such as sphingosine 1-phosphate and lysophosphatidic acid. The Drosophila LPPs Wunen (Wun) and Wunen-2 (Wun2) have a well-established role in regulating the survival and migration of germ cells. We now show that wun has an essential tissue-autonomous role in development of the trachea: the catalytic activity of Wun is required to maintain septate junction (SJ) paracellular barrier function, loss of which causes failure to accumulate crucial luminal components, suggesting a role for phospholipids in SJ function. We find that the integrity of the blood-brain barrier is also lost in wun mutants, indicating that loss of SJ function is not restricted to the tracheal system. Furthermore, by comparing the rescue ability of different LPP homologs we show that wun function in the trachea is distinct from its role in germ cell migration.

A suppressor/enhancer screen in Drosophila reveals a role for wnt-mediated lipid metabolism in primordial germ cell migration

Wnt proteins comprise a large family of secreted ligands implicated in a wide variety of biological roles. WntD has previously been shown to inhibit the nuclear accumulation of Dorsal/NF-κB protein during embryonic dorsal/ventral patterning and the adult innate immune response, independent of the well-studied Armadillo/β-catenin pathway. In this paper, we present a novel phenotype for WntD mutant embryos, suggesting that this gene is involved in migration of primordial germ cells (PGC) to the embryonic gonad. Additionally, we describe a genetic suppressor/enhancer screen aimed at identifying genes required for WntD signal transduction, based on the previous observation that maternal overexpression of WntD results in lethally dorsalized embryos. Using an algorithm to narrow down our hits from the screen, we found two novel WntD signaling components: Fz4, a member of the Frizzled family, and the Drosophila Ceramide Kinase homolog, Dcerk. We show here that Dcerk and Dmulk (Drosophila Multi-substrate lipid kinase) redundantly mediate PGC migration. Our data are consistent with a model in which the activity of lipid phosphate phosphatases shapes a concentration gradient of ceramide-1-phosphate (C1P), the product of Dcerk, allowing proper PGC migration.

Drosophila Chk2 and p53 proteins induce stage-specific cell death independently during oogenesis

In Drosophila, the checkpoint protein-2 kinase (DmChk2) and its downstream effector protein, Dmp53, are required for DNA damage-mediated cell cycle arrest, DNA repair and apoptosis. In this study we focus on understanding the function of these two apoptosis inducing factors during ovarian development. We found that expression of Dmp53, but not DmChk2, led to loss of ovarian stem cells. We demonstrate that expression of DmChk2, but not Dmp53, induced mid-oogenesis cell death. DmChk2 induced cell death was not suppressed by Dmp53 mutant, revealing for the first time that in Drosophila, over-expression of DmChk2 can induce cell death which is independent of Dmp53. We found that over-expression of caspase inhibitors such as DIAP1, p35 and p49 did not suppress DmChk2- and Dmp53-induced cell death. Thus, our study reveals stage-specific effects of Dmp53 and DmChk2 in oogenesis. Moreover, our results demonstrate that although DmChk2 and Dmp53 affect different stages of ovarian development, loss of ovarian stem cells by p53 expression and mid-oogenesis cell death induced by DmChk2 do not require caspase activity.

Lipid phosphate phosphatase 3 stabilization of beta-catenin induces endothelial cell migration and formation of branching point structures

Endothelial cell (EC) migration, cell-cell adhesion, and the formation of branching point structures are considered hallmarks of angiogenesis; however, the underlying mechanisms of these processes are not well understood. Lipid phosphate phosphatase 3 (LPP3) is a recently described p120-catenin-associated integrin ligand localized in adherens junctions (AJs) of ECs. Here, we tested the hypothesis that LPP3 stimulates beta-catenin/lymphoid enhancer binding factor 1 (beta-catenin/LEF-1) to induce EC migration and formation of branching point structures. In subconfluent ECs, LPP3 induced expression of fibronectin via beta-catenin/LEF-1 signaling in a phosphatase and tensin homologue (PTEN)-dependent manner. In confluent ECs, depletion of p120-catenin restored LPP3-mediated beta-catenin/LEF-1 signaling. Depletion of LPP3 resulted in destabilization of beta-catenin, which in turn reduced fibronectin synthesis and deposition, which resulted in inhibition of EC migration. Accordingly, reexpression of beta-catenin but not p120-catenin in LPP3-depleted ECs restored de novo synthesis of fibronectin, which mediated EC migration and formation of branching point structures. In confluent ECs, however, a fraction of p120-catenin associated and colocalized with LPP3 at the plasma membrane, via the C-terminal cytoplasmic domain, thereby limiting the ability of LPP3 to stimulate beta-catenin/LEF-1 signaling. Thus, our study identified a key role for LPP3 in orchestrating PTEN-mediated beta-catenin/LEF-1 signaling in EC migration, cell-cell adhesion, and formation of branching point structures.

Mechanisms guiding primordial germ cell migration: strategies from different organisms

The regulated migration of cells is essential for development and tissue homeostasis, and aberrant cell migration can lead to an impaired immune response and the progression of cancer. Primordial germ cells (PGCs), precursors to sperm and eggs, have to migrate across the embryo to reach somatic gonadal precursors, where they carry out their function. Studies of model organisms have revealed that, despite important differences, several features of PGC migration are conserved. PGCs require an intrinsic motility programme and external guidance cues to survive and successfully migrate. Proper guidance involves both attractive and repulsive cues and is mediated by protein and lipid signalling.

Programmed cell death of primordial germ cells in Drosophila is regulated by p53 and the Outsiders monocarboxylate transporter

Primordial germ cell development uses programmed cell death to remove abnormal, misplaced or excess cells. Precise control of this process is essential to maintain the continuity and integrity of the germline, and to prevent germ cells from colonizing locations other than the gonads. Through careful analyses of primordial germ cell distribution in developing Drosophila melanogaster embryos, we show that normal germ cell development involves extensive programmed cell death during stages 10-12 of embryogenesis. This germ cell death is mediated by Drosophila p53 (p53). Mutations in p53 result in excess primordial germ cells that are ectopic to the gonads. Initial movements of the germ cells appear normal, and wild-type numbers of germ cells populate the gonads, indicating that p53 is required for germ cell death, but not migration. To our knowledge, this is the first report of a loss-of-function phenotype for Drosophila p53 in a non-sensitized background. The p53 phenotype is remarkably similar to that of outsiders (out) mutants. Here, we show that the out gene encodes a putative monocarboxylate transporter. Mutations in p53 and out show nonallelic noncomplementation. Interestingly, overexpression of p53 in primordial germ cells of out mutant embryos partially suppresses the out germ cell death phenotype, suggesting that p53 functions in germ cells either downstream of out or in a closely linked pathway. These findings inform models in which signaling between p53 and cellular metabolism are integrated to regulate programmed cell death decisions.

Sex-specific DoublesexM expression in subsets of Drosophila somatic gonad cells

BACKGROUND

In Drosophila melanogaster, a pre-mRNA splicing hierarchy controls sexual identity and ultimately leads to sex-specific Doublesex (DSX) transcription factor isoforms. The male-specific DSXM represses genes involved in female development and activates genes involved in male development. Spatial and temporal control of dsx during embryogenesis is not well documented.

RESULTS

Here we show that DSX(M) is specifically expressed in subsets of male somatic gonad cells during embryogenesis. Following testis formation, germ cells remain in contact with DSX(M)-expressing cells, including hub cells and premeiotic somatic cyst cells that surround germ cells during spermatogenesis in larval and adult testes.

CONCLUSION

We show that dsx is transcriptionally regulated in addition to being regulated at the pre-mRNA splicing level by the sex determination hierarchy. The dsx locus is spatially controlled by somatic gonad identity. The continuous expression of DSX(M) in cells contacting the germline suggests an ongoing short-range influence of the somatic sex determination pathway on germ cell development.

Drosophila mojoless, a retroposed GSK-3, has functionally diverged to acquire an essential role in male fertility

Retroposition is increasingly recognized as an important mechanism for the acquisition of new genes. We show that a glycogen synthase kinase-3 gene, shaggy (sgg), retroposed at least 50 MYA in the Drosophila genus to generate a new gene, mojoless (mjl). We have extensively analyzed the function of mjl and examined its functional divergence from the parental gene sgg in Drosophila melanogaster. Unlike Sgg, which is expressed in many tissues of both sexes, Mjl is expressed specifically in the male germ line, where it is required for male germ line survival. Our analysis indicates that mjl has acquired a specific function in the maintenance of male germ line viability. However, it has not completely lost its ancestral biochemical function and can partially compensate for loss of the parental gene sgg when ectopically expressed in somatic cells. We postulate that mjl has undergone functional diversification and is now under stabilizing selection in the Drosophila genus.

In vivo migration: a germ cell perspective

The basic concepts of the molecular machinery that mediates cell migration have been gleaned from cell culture systems. However, the three-dimensional environment within an organism presents migrating cells with a much greater challenge. They must move between and among other cells while interpreting multiple attractive and repulsive cues to choose their proper path. They must coordinate their cell adhesion with their surroundings and know when to start and stop moving. New insights into the control of these remaining mysteries have emerged from genetic dissection and live imaging of germ cell migration in Drosophila, zebrafish, and mouse embryos. In this review, we first describe germ cell migration in cellular and mechanistic detail in these different model systems. We then compare these systems to highlight the emerging principles. Finally, we contrast the migration of germ cells with that of immune and cancer cells to outline the conserved and different mechanisms.

Lipid Phosphate Phosphatases 1 and 3 Are Localized in Distinct Lipid Rafts

Lipid phosphate phosphatases (LPPs), integral membrane proteins with six transmembrane domains, dephosphorylate a variety of extracellular lipid phosphates. Although LPP3 is already known to bind to Triton X-100-insoluble rafts, we here report that LPP1 is also associated with lipid rafts distinct from those harboring LPP3. We found that LPP1 was Triton X-100-soluble, but CHAPS-insoluble in LNCaP cells endogenously expressing LPP1 and several LPP1 cDNA-transfected cells including NIH3T3 fibroblasts. In addition to the non-ionic detergent insolubility, LPP1 further possessed several properties formulated for raft-localizing proteins as follows: first, the CHAPS-insolubility was resistant to the actin-disrupting drug cytochalasin D; second, the CHAPS-insoluble LPP1 floated in an Optiprep density gradient; third, the CHAPS insolubility of LPP1 was lost by cholesterol depletion; and finally, the subcellular distribution pattern of LPP1 exclusively overlapped with that of a raft marker, cholera toxin B subunit. Interestingly, confocal microscopic analysis showed that LPP1 was distributed to membrane compartments distinct from those of LPP3. Analysis using various LPP1/LPP3 chimeras revealed that their first extracellular regions determine the different Triton X-100 solubilities. These results indicate that LPP1 and LPP3 are distributed in distinct lipid rafts that may provide unique microenvironments defining their non-redundant physiological functions.

Lipid phosphate phosphatase-1 regulates lysophosphatidate-induced fibroblast migration by controlling phospholipase D2-dependent phosphatidate generation

Lysophosphatidate (LPA) stimulates cell migration and division through a family of G-protein-coupled receptors. Lipid phosphate phosphatase-1 (LPP1) regulates the degradation of extracellular LPA as well as the intracellular accumulation of lipid phosphates. Here we show that increasing the catalytic activity of LPP1 decreased the pertussis toxin-sensitive stimulation of fibroblast migration by LPA and an LPA-receptor agonist that could not be dephosphorylated. Conversely, knockdown of endogenous LPP1 activity increased LPA-induced migration. However, LPP1 did not affect PDGF- or endothelin-induced migration of fibroblasts in Transwell chamber and "wound healing" assays. Thus, in addition to degrading exogenous LPA, LPP1 controls signaling downstream of LPA receptors. Consistent with this conclusion, LPP1 expression decreased phospholipase D (PLD) stimulation by LPA and PDGF, and phosphatidate accumulation. This LPP1 effect was upstream of PLD activation in addition to the possible metabolism of phosphatidate to diacylglycerol. PLD(2) activation was necessary for LPA-, but not PDGF-induced migration. Increased LPP1 expression also decreased the LPA-, but not the PDGF-induced activation of important proteins involved in fibroblast migration. These included decreased LPA-induced activation of ERK and Rho, and the basal activities of Rac and Cdc42. However, ERK and Rho activation were not downstream targets of LPA-induced PLD(2) activity. We conclude that the intracellular actions of LPP1 play important functions in regulating LPA-induced fibroblast migration through PLD2. LPP1 also controls PDGF-induced phosphatidate formation. These results shed new light on the roles of LPP1 in controlling wound healing and the growth and metastasis of tumors.

Follow the fatty brick road: lipid signaling in cell migration

Lysophospholipids play important roles in the migration of lymphocytes, smooth muscle cells and germ cells in vertebrates and invertebrates. In vertebrates, the migratory responses are mediated by specific G-protein-coupled receptors. These are expressed in both migrating lymphocyte and smooth muscle cells, and in their surrounding cells. In Drosophila germ cell migration, lipid phosphatases also act in both the surrounding and the migrating cells. In all three scenarios, the contributions of these genes in the stationary and migrating cells are being teased apart by genetic studies and direct observation, with exciting results.

lazaro encodes a lipid phosphate phosphohydrolase that regulates phosphatidylinositol turnover during Drosophila phototransduction

An essential step in Drosophila phototransduction is the hydrolysis of phosphatidylinositol 4,5 bisphosphate PI(4,5)P2 by phospholipase Cbeta (PLCbeta) to generate a second messenger that opens the light-activated channels TRP and TRPL. Although the identity of this messenger remains unknown, recent evidence has implicated diacylglycerol kinase (DGK), encoded by rdgA, as a key enzyme that regulates its levels, mediating both amplification and response termination. In this study, we demonstrate that lazaro (laza) encodes a lipid phosphate phosphohydrolase (LPP) that functions during phototransduction. We demonstrate that the synergistic activity of laza and rdgA regulates response termination during phototransduction. Analysis of retinal phospholipids revealed a reduction in phosphatidic acid (PA) levels and an associated reduction in phosphatidylinositol (PI) levels. Together our results demonstrate the contribution of PI depletion to the rdgA phenotype and provide evidence that depletion of PI and its metabolites might be a key signal for TRP channel activation in vivo.

Control of lateral migration and germ cell elimination by the Drosophila melanogaster lipid phosphate phosphatases Wunen and Wunen 2

In most organisms, primordial germ cells (PGCs) arise far from the region where somatic gonadal precursors (SGPs) are specified. Although PGCs in general originate as a single cluster of cells, the somatic parts of the gonad form on each site of the embryo. Thus, to reach the gonad, PGCs not only migrate from their site of origin but also split into two groups. Taking advantage of high-resolution real-time imaging, we show that in Drosophila melanogaster PGCs are polarized and migrate directionally toward the SGPs, avoiding the midline. Unexpectedly, neither PGC attractants synthesized in the SGPs nor known midline repellents for axon guidance were required to sort PGCs bilaterally. Repellent activity provided by wunen (wun) and wunen-2 (wun-2) expressed in the central nervous system, however, is essential in this migration process and controls PGC survival. Our results suggest that expression of wun/wun-2 repellents along the migratory paths provides faithful control over the sorting of PGCs into two gonads and eliminates PGCs left in the middle of the embryo.

IFITM/Mil/Fragilis Family Proteins IFITM1 and IFITM3 Play Distinct Roles in Mouse Primordial Germ Cell Homing and Repulsion

The family of interferon-induced transmembrane protein (Ifitm/mil/fragilis) genes encodes cell surface proteins that may modulate cell adhesion and influence cell differentiation. Mouse Ifitm1 and -3, which are expressed in primordial germ cells (PGCs), are implicated to have roles in germ cell development, but the specific functions have been unclear. Our results show that Ifitm1 activity is required for PGC transit from the mesoderm into the endoderm, and that it appears to act via a repulsive mechanism, such that PGCs avoid Ifitm1-expressing tissues. In contrast, Ifitm3, which is expressed in migratory PGCs, is sufficient to confer autonomous PGC-like homing properties to somatic cells. These guidance activities are mediated by the N-terminal extracellular domain of the specific IFITM, which cannot be substituted by that of another family member. Complex homo- and/or heterotypic intercellular interactions among various IFITMs in PGCs and neighboring cells may underpin coordinated germ cell guidance in mice.

Integral membrane lipid phosphatases/phosphotransferases: common structure and diverse functions

Phospholipids and sphingolipids play critical roles in signal transduction, intracellular membrane trafficking, and control of cell growth and survival. We discuss recent progress in the identification and characterization of a family of integral membrane proteins with central roles in bioactive lipid metabolism and signalling. These five groups of homologous proteins, which we collectively term LPTs (lipid phosphatases/phosphotransferases), are characterized by a core domain containing six transmembrane-spanning alpha-helices connected by extramembrane loops, two of which interact to form the catalytic site. LPT family members are localized to all major membrane compartments of the cell. The transmembrane topology of these proteins places their active site facing the lumen of endomembrane compartments or the extracellular face of the plasma membrane. Sequence conservation between the active site of the LPPs (lipid phosphate phosphatases), SPPs (sphingosine phosphate phosphatases) and the recently identified SMSs (sphingomyelin synthases) with vanadium-dependent fungal oxidases provides a framework for understanding their common catalytic mechanism. LPPs hydrolyse LPA (lysophosphatidic acid), S1P (sphingosine 1-phosphate) and structurally-related substrates. Although LPPs can dephosphorylate intracellularly generated substrates to control intracellular lipid metabolism and signalling, their best understood function is to regulate cell surface receptor-mediated signalling by LPA and S1P by inactivating these lipids at the plasma membrane or in the extracellular space. SPPs are intracellularly localized S1P-selective phosphatases, with key roles in the pathways of sphingolipid metabolism linked to control of cell growth and survival. The SMS enzymes catalyse the interconversion of phosphatidylcholine and ceramide with sphingomyelin and diacylglycerol, suggesting a pivotal role in both housekeeping lipid synthesis and regulation of bioactive lipid mediators. The remaining members of the LPT family, the LPR/PRGs (lipid phosphatase-related proteins/plasticity-related genes) and CSS2s (type 2 candidate sphingomyelin synthases), are presently much less well studied. These two groups include proteins that lack critical amino acids within the catalytic site, and could therefore not use the conserved LPT reaction mechanism to catalyse lipid phosphatase or phosphotransferase reactions. In this review, we discuss recent ideas about their possible biological activities and functions, which appear to involve regulation of cellular morphology and, possibly, lipid metabolism and signalling in the nuclear envelope.

Lipid phosphate phosphatase-1 regulates lysophosphatidic acid-induced calcium release, NF-kappaB activation and interleukin-8 secretion in human bronchial epithelial cells

LPA (lysophosphatidic acid), a potent bioactive phospholipid, elicits diverse cellular responses through activation of the G-protein-coupled receptors LPA1-LPA4. LPA-mediated signalling is partially regulated by LPPs (lipid phosphate phosphatases; LPP-1, -2 and -3) that belong to the phosphatase superfamily. This study addresses the role of LPPs in regulating LPA-mediated cell signalling and IL-8 (interleukin-8) secretion in HBEpCs (human bronchial epithelial cells). Reverse transcription-PCR and Western blotting revealed the presence and expression of LPP-1-3 in HBEpCs. Exogenous [3H]oleoyl LPA was hydrolysed to [3H]-mono-oleoylglycerol. Infection of HBEpCs with an adenoviral construct of human LPP-1 for 48 h enhanced the dephosphorylation of exogenous LPA by 2-3-fold compared with vector controls. Furthermore, overexpression of LPP-1 partially attenuated LPA-induced increases in the intracellular Ca2+ concentration, phosphorylation of IkappaB (inhibitory kappaB) and translocation of NF-kappaB (nuclear factor-kappaB) to the nucleus, and almost completely prevented IL-8 secretion. Infection of cells with an adenoviral construct of the mouse LPP-1 (R217K) mutant partially attenuated LPA-induced IL-8 secretion without altering LPA-induced changes in intracellular Ca2+ concentration, phosphorylation of IkappaB, NF-kappaB activation or IL-8 gene expression. Our results identify LPP-1 as a key regulator of LPA signalling and IL-8 secretion in HBEpCs. Thus LPPs could represent potential targets in regulating leucocyte infiltration and airway inflammation.

Lysophospholipids in development: Miles apart and edging in

Sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) are endogenous bioactive lipids that participate in the regulation of mammalian cell proliferation, apoptosis, migration, and angiogenesis. These processes are each critical for successful embryogenesis, raising the possibility that lysophospholipid signaling may contribute to normal animal development. In fact, recent studies in developmental model systems have established that S1P and LPA are necessary for diverse developmental programs including those required for morphogenesis of vertebrate reproductive, cardiovascular and central and peripheral nervous systems (PNS), as well as the establishment of maternal-fetal circulation and the immune system. Genetic, morphological, and biochemical characterization of developmental model systems offer powerful approaches to elucidating the molecular mechanisms of lysophospholipid signaling and its contributions to animal development and postnatal physiology. In this review, the routes of S1P and LPA metabolism and our current understanding of lysophospholipid-mediated signal transduction in mammalian cells will be summarized. The evidence implicating lysophospholipid signaling in the development of specific vertebrate systems will then be reviewed, with an emphasis on signals mediated through G protein-coupled receptors of the Edg family. Lastly, recent insights derived from the study of simple metazoan models and implications regarding lysophospholipid signaling in organisms in which Edg receptors are not conserved will be explored.

Lipid phosphate phosphatases and related proteins: signaling functions in development, cell division, and cancer

Lipid phosphates initiate key signaling cascades in cell activation. Lysophosphatidate (LPA) and sphingosine 1-phosphate (S1P) are produced by activated platelets. LPA is also formed from circulating lysophosphatidylcholine by autotaxin, a protein involved tumor progression and metastasis. Extracellular LPA and S1P stimulate families of G-protein coupled receptors that elicit diverse responses. LPA is involved in wound repair and tumor growth. Exogenous S1P is a potent stimulator of angiogenesis, a process vital in development, tissue repair and the growth of aggressive tumors. Inside the cell, phosphatidate (PA), ceramide 1-phosphate (C1P), LPA, and S1P act as signaling molecules with distinct functions including the stimulation of cell division, cytoskeletal rearrangement, Ca(2+) transients, and membrane movement. These observations imply that phosphatases that degrade lipid phosphates on the cell surface, or inside the cell, regulate cell signaling under physiological and pathological conditions. This occurs through attenuation of signaling by the lipid phosphates and by the production of bioactive products (diacylglycerol, ceramide, and sphingosine). Three lipid phosphate phosphatases (LPPs) and a splice variant dephosphorylate LPA, PA, CIP, and S1P. Two S1P phosphatases (SPPs) act specifically on S1P. In addition, there is family of four LPP-related proteins (LPRs, or plasticity-related genes, PRGs). PRG-1 expression in neurons has been reported to increase extracellular LPA breakdown and attenuate LPA-induced axonal retraction. It is unclear whether the LRPs dephosphorylate LPA directly, stimulate LPP activity, or bind LPA and S1P. Also, the importance of extra- versus intra-cellular actions of the LPPs and SPPs, and the individual roles of different isoforms is not firmly established. Understanding the functions and regulation of the LPPs, SPPs and related proteins will hopefully contribute to interventions to correct dysfunctions in conditions such as wound repair, inflammation, angiogenesis, tumor growth, and metastasis.

Death and taxis: what non-mammalian models tell us about sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a signaling molecule that regulates critical events including mammalian cell proliferation, survival, migration and cell-cell interactions. Most of these signals are triggered by engagement of sphingosine-1-phosphate receptors of the Edg family. However, accumulating evidence derived from investigation of non-mammalian models that lack Edg receptors suggests that sphingosine-1-phosphate-like molecules can act through alternative mechanisms and thereby contribute to morphogenesis, development, reproduction and survival. This review provides an overview of sphingosine-1-phosphate metabolism, the isolation of genes in this pathway employing yeast genetics, the evidence for its influence on non-mammalian development, and the pertinence of these findings to human disease.

Lysophosphatidic acid and sphingosine 1-phosphate biology: the role of lipid phosphate phosphatases

The biological actions of the lysolipid agonists sphingosine 1-phosphate and lysophosphatidic acid, in addition to other bioactive lipid phosphates such as phosphatidic acid and ceramide 1-phosphate, can be influenced by a family of lipid phosphate phosphatases (LPP), including LPP1, LPP2, LPP3, the Drosophila homologues Wunen (Wun) and Wunen2 (Wun2) and sphingosine 1-phosphate phosphatases 1 and 2 (SPP1, SPP2). This review describes the characteristic of these enzymes and their potential physiological roles in regulating intracellular and extracellular actions and amounts of these lipids in addition to the involvement of these phosphatases in development.

Germ cell-autonomous Wunen2 is required for germline development in Drosophila embryos

In many animals, primordial germ cells (PGCs) migrate through the embryo towards the future gonad, a process guided by attractive and repulsive cues provided from surrounding somatic cells. In Drosophila, the two related lipid phosphate phosphatases (LPPs), Wunen (Wun) and Wun2, are thought to degrade extracellular substrates and to act redundantly in somatic cells to provide a repulsive environment to steer the migration of PGCs, or pole cells. Wun and Wun2 also affect the viability of pole cells, because overexpression of either one in somatic cells causes pole cell death. However, the means by which they regulate pole cell migration and survival remains elusive. We report that Wun2 has a maternal function required for the survival of pole cells during their migration to the gonad. Maternal wun2 RNA was found to be concentrated in pole cells and pole cell-specific expression of wun2 rescued the pole cell death phenotype of the maternal wun2 mutant, suggesting that wun2 activity in pole cells is required for their survival. Furthermore, we obtained genetic evidence that pole cell survival requires a proper balance of LPP activity in pole cells and somatic cells. We propose that Wun2 in pole cells competes with somatic Wun and Wun2 for a common lipid phosphate substrate, which is required by pole cells to produce their survival signal. In somatic cells, Wun and Wun2 may provide a repulsive environment for pole cell migration by depleting this extracellular substrate.

Soma-germ line competition for lipid phosphate uptake regulates germ cell migration and survival

Lipid phosphates can act as signaling molecules to influence cell division, apoptosis, and migration. wunen and wunen2 encode Drosophila lipid phosphate phosphohydrolases, integral membrane enzymes that dephosphorylate extracellular lipid phosphates. wun and wun2 act redundantly in somatic tissues to repel migrating germ cells, although the mechanism by which germ cells respond is unclear. Here, we report that wun2 also functions in germ cells, enabling them to perceive the wun/wun2-related signal from the soma. Upon Wun2 expression, cultured insect cells dephosphorylate and internalize exogenously supplied lipid phosphate. We propose that Drosophila germ cell migration and survival are controlled by competition for hydrolysis of a lipid phosphate between germ cells and soma.

Germ Cell Specification and Migration in Drosophila and beyond

The passage of an individual's genome to future generations is essential for the maintenance of species and is mediated by highly specialized cells, the germ cells. Genetic studies in a number of model organisms have provided insight into the molecular mechanisms that control specification, migration and survival of early germ cells. Focusing on Drosophila, we will discuss the mechanisms by which germ cells initially form and remain transcriptionally silent while somatic cells are transcriptionally active. We will further discuss three separate attractive and repellent guidance pathways, mediated by a G-protein coupled receptor, two lipid phosphate phosphohydrolases, and isoprenylation. We will compare and contrast these findings with those obtained in other organisms, in particular zebrafish and mice. While aspects of germ cell specification are strikingly different between these species, germ cell specific gene functions have been conserved. In particular, mechanisms that sense directional cues during germ cell migration seem to be shared between invertebrates and vertebrates.

Isoprenoids control germ cell migration downstream of HMGCoA reductase

3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCoAr) provides attractive cues to Drosophila germ cells, guiding them toward the embryonic gonad. However, it remains unclear how HMGCoAr mediates this attraction. In a genomic analysis of the HMGCoAr pathway, we found that the fly genome lacks several enzymes required for cholesterol biosynthesis, ruling out cholesterol and cholesterol-derived proteins as mediators of PGC migration. Genetic analysis of the pathway revealed that two enzymes, farnesyl-diphosphate synthase and geranylgeranyl-diphosphate synthase, required for the production of isoprenoids, act downstream of HMGCoAr in germ cell migration. Consistent with a role in geranylgeranylation, embryos deficient in geranylgeranyl transferase type I show germ cell migration defects. Our data, together with similar findings in zebrafish, implicate an isoprenylated protein in germ cell attraction. The specificity and evolutionary conservation of the HMGCoAr pathway for germ cells suggest that an attractant common to invertebrates and vertebrates guides germ cells in early embryos.

Lipid phosphate phosphatases dimerise, but this interaction is not required for in vivo activity

Background

Lipid phosphate phosphatases (LPPs) are integral membrane proteins believed to dephosphorylate bioactive lipid messengers, so modifying or attenuating their activities. Wunen, a Drosophila LPP homologue, has been shown to play a pivotal role in primordial germ cell (PGC) migration and survival during embryogenesis. It has been hypothesised that LPPs may form oligomeric complexes, and may even function as hexamers. We were interested in exploring this possibility, to confirm whether LPPs can oligomerise, and if they do, whether oligomerisation is required for either in vitro or in vivo activity.

Results

We present evidence that Wunen dimerises, that these associations require the last thirty-five C-terminal amino-acids and depend upon the presence of an intact catalytic site. Expression of a truncated, monomeric form of Wunen in Drosophila embryos results in perturbation of germ cell migration and germ cell loss, as observed for full-length Wunen. We also observed that murine LPP-1 and human LPP-3 can also form associations, but do not form interactions with Wunen or each other. Furthermore, Wunen does not form dimers with its closely related counterpart Wunen-2. Finally we discovered that addition of a trimeric myc tag to the C-terminus of Wunen does not prevent dimerisation or in vitro activity, but does prevent activity in vivo.

Conclusion

LPPs do form complexes, but these do not seem to be specifically required for activity either in vitro or in vivo. Since neither dimerisation nor the C-terminus seem to be involved in substrate recognition, they may instead confer structural or functional stability through dimerisation. The results indicate that the associations we see are highly specific and occur only between monomers of the same protein.

Tre1, a G protein-coupled receptor, directs transepithelial migration of Drosophila germ cells

In most organisms, germ cells are formed distant from the somatic part of the gonad and thus have to migrate along and through a variety of tissues to reach the gonad. Transepithelial migration through the posterior midgut (PMG) is the first active step during Drosophila germ cell migration. Here we report the identification of a novel G protein-coupled receptor (GPCR), Tre1, that is essential for this migration step. Maternal tre1 RNA is localized to germ cells, and tre1 is required cell autonomously in germ cells. In tre1 mutant embryos, most germ cells do not exit the PMG. The few germ cells that do leave the midgut early migrate normally to the gonad, suggesting that this gene is specifically required for transepithelial migration and that mutant germ cells are still able to recognize other guidance cues. Additionally, inhibiting small Rho GTPases in germ cells affects transepithelial migration, suggesting that Tre1 signals through Rho1. We propose that Tre1 acts in a manner similar to chemokine receptors required during transepithelial migration of leukocytes, implying an evolutionarily conserved mechanism of transepithelial migration. Recently, the chemokine receptor CXCR4 was shown to direct migration in vertebrate germ cells. Thus, germ cells may more generally use GPCR signaling to navigate the embryo toward their target.

A novel G protein-coupled receptor (GPCR) is shown to be essential for transepithelial migration of Drosophila germ cells. Leukocyte transepithelial migration also requires GPCR signaling, suggesting a conserved mechanism.