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Extracellular lipid signaling in Drosophila development

Group leader: Andrew Renault

Phone: +49 7071 601 1320

Secretary: Helgard Hege-Baldwin

Phone: +49 7071 601 1309

Fax: +49 7071 601 1308

We currently have space for HiWi and diploma students. Interested persons should e-mail their CV and a cover letter to andrew[dot]renault[at]tuebingen[dot]mpg[dot]de

We also have space for PhD students. Applicants should normally apply via our PhD program which has two rounds per year (usually closing dates are in January and July). Applicants who would like to apply outside of these deadlines should make enquiries to andrew[dot]renault[at]tuebingen[dot]mpg[dot]de

Introduction

Lysophospholipids are derivatives of phospholipids that include the structurally similar lysophosphatidic acid (LPA) and sphingosine 1 phosphate (S1P). Lysophospholipids can act intracellularly as both metabolic intermediates and second messengers, but are also extracellular signaling molecules in their own right. Research in our lab is aimed at understanding how such lipids act extracellularly to regulate cell behaviour including cell proliferation, survival and migration.

Drosophila germ cell migration

In the fruitfly Drosophila melanogaster genetic screens have uncovered a role for lysophospholipids in cell migration through the identification of lipid modifying enzymes that guide migrating germ cells during embryogenesis. The germ cells are the cells that in the adult give rise to sperm and eggs. During embryogenesis, as in most organisms, germ cells form spatially and temporally separate from the somatic cells of the gonad, the somatic gonadal precursors (SGPs). At the beginning of their migration, germ cells are clustered in the midgut pocket. They migrate individually across the midgut epithelium and into the mesoderm during which time they split into two bilateral groups, one towards each SGP cluster (figure 1).

Figure 1: Migration of germ cells during Drosophila embryogenesis. Cartoons show germ cells (yellow), SGPs (green) and midgut (red). Embryos on right show germ cells (brown) and wun2 RNA expression (blue).

Their migratory route is bounded by expression of two related genes wunen (wun) and wunen2 (wun2). Germ cells avoid somatic tissues with wun or wun2 (for example the central nervous system and regions of the gut) and in embryos lacking wun and wun2 in the soma virtually all germ cells fail to reach the SGPs, and instead scatter throughout the embryo. Over-expression of wun or wun2 is sufficient to repel germ cells from otherwise attractive tissues and eventually results in germ cell death. Intriguingly germ cells themselves also express wun2 and germ cells lacking wun2 die. These data suggest that wun and wun2 act in both somatic cells, where they guide germ cells by repulsion and at high levels promote germ cell death, and also in germ cells where their activity protects germ cells from death.
Wun and Wun2 encode lipid phosphate phosphatases (LPPs). LPPs are six-pass transmembrane enzymes with the catalytic domain exposed to the cell surface (figure 2).

Figure 2: Predicted topology of Wun2 in the cell membrane. The catalytic sites (red) face outside of the cell.

Wun2 catalytic activity not only hydrolyses the lysophospholipid but also leads to accumulation of the dephosphorylated lipid product inside cells. Thus one model for Wunen action is that in the soma, Wun/Wun2 mediates hydrolysis of the phospholipid and subsequent uptake of the lipid makes the phospholipid inaccessible to germ cells. This mechanism of substrate elimination would create a phospholipid gradient with the highest levels of phospholipid in regions of lowest Wun/Wun2 concentration. Germ cell Wun2 also mediates hydrolysis and lipid uptake, however germ cells, unlike the soma, require the lipid or its metabolites for survival and migrate toward regions of higher phospholipid concentration (figure 3).

Figure 3: Model for competition between germ cells and somatic cells for phospholipid dephosphorylation and uptake.

Wun and Wun2, in common with their mammalian homologues, can dephosphorylate a number of related phospholipids in vitro including LPA and S1P however it is currently unclear which phospholipid is acting on germ cells in vivo. In vertebrates S1P and LPA are ligands for G protein coupled receptors which couple to a number of heterotrimeric G proteins leading to modulation of one or several downstream targets including phospholipase C, adenylate cyclase, PI3K, Rac and Rho. The vertebrate lysophospholipid GPCRs have no fly homologues, implying the presence of a novel signaling mechanism within germ cells. Our goal is to identify this signaling mechanism, determine the nature of the lipid and what factors establish and maintain a gradient.

Drosophila heart development

The Drosophila heart is a linear structure composed of two cell types: two inner rows of cardioblasts and two outer rows of pericardial cells (figure 4).

Figure 4: Drosophila embryonic heart labeled with GFP, composed of 2 rows of cardioblasts (inner rows) and 2 rows of pericardial cells (outer rows).

These cells are specified at stage 11, at which time the heart precursors form continuous rows on both sides of the embryo. In order to coalesce the heart cells contact the ectoderm layer adjacent to its leading edges and make use of dorsal closure events to move dorsally towards each other. By stage 16 the heart cells meet at the dorsal midline, subsets of cells undergo specific differentiation patterns and the heart starts beating (figure 5).

Figure 5: Beating Drosophila embryonic heart.

Wunens are strongly expressed in the Drosophila heart and in wunen mutants the heart fails to form properly. Our goal is to discern the lipids and their role in heart formation and to understand how defects in lipid signaling impair heart function.

Selected publications:

Renault, A.D., Kunwar, P. and Lehmann, R. (2010) Lipid phosphate phosphatase activity regulates dispersal and bilateral sorting of embryonic germ cells in Drosophila. Development. 137 (11) 1815-23.
Renault, A.D.*, Ricardo, S.*, Kunwar, P.S., Santos, A., Starz-Gaiano, M., Stein, J.A. and Lehmann, R. (2009) Hedgehog does not guide migrating Drosophila germ cells. Developmental Biology, 328 (2) 355-62. * denotes equal contribution
Kunwar, P.S., Sano, H., Renault, A.D., Barbosa, V., Fuse, N. and Lehmann, R. (2008) Tre1 GPCR initiates germ cell transepithelial migration by regulating Drosophila melanogaster E-cadherin. Journal of Cell Biology, 183 157-68
Renault, A.D. and Lehmann, R. (2006) Follow the fatty brick road: extracellular lipids and cell migration. Current Opinion in Genetics and Development, 16 (4) 348-54.
Sano, H., Renault, A.D. and Lehmann, R. (2005) Control of lateral migration and germ cell elimination by the Drosophila melanogaster lipid phosphate phosphatases Wunen and Wunen2. Journal of Cell Biology, 171 (4) 675-83.
Renault, A.D., Sigal, Y., Morris, A. and Lehmann, R. (2004) Soma-germ line competition for lipid phosphate uptake regulates germ cell migration and survival. Science, 305 (5692) 1963-1966.
Renault, A.D., Starz-Gaiano, M. and Lehmann, R. (2002) Metabolism of sphingosine 1-phosphate and lysophosphatidic acid: a genome wide analysis of gene expression in Drosophila. Mechanisms of Development, 119S S293-S301.