Gc1, a Downstream Target for the hmgcr -Isoprenoid Biosynthetic Pathway, Is Required for Releasing the Hedgehog Ligand and Directing Germ Cell Migration Girish Deshpande, Anuradha Godishala, Paul Schedl* Department of Molecular Biology, Princeton University, Princeton, New Jersey, United States of America Abstract The isoprenoid biosynthetic pathway leading from the production of mevalonate by HMGCoA reductase (Hmgcr) to the geranylation of the G protein subunit, Gc1, plays an important role in cardiac development in the fly. Hmgcr has also been implicated in the release of the signaling molecule Hedgehog (Hh) from hh expressing cells and in the production of an attractant that directs primordial germ cells to migrate to the somatic gonadal precursor cells (SGPs). The studies reported here indicate that this same hmgcrRGc1 pathway provides a novel post-translational mechanism for modulating the range and activity of the Hh signal produced by hh expressing cells. We show that, like hmgcr, gc1 and quemao (which encodes the enzyme, geranylgeranyl diphosphate synthetase, that produces the substrate for geranylation of Gc1) are components of the hh signaling pathway and are required for the efficient release of the Hh ligand from hh expressing cells. We also show that the hmgcrRGc1 pathway is linked to production of the germ cell attractant by the SGPs through its ability to enhance the potency of the Hh signal. We show that germ cell migration is disrupted by the loss or gain of gc1 activity, by trans-heterozygous combinations between gc1 and either hmgcr or hh mutations, and by ectopic expression of dominant negative Gc1 proteins that cannot be geranylated. Citation: Deshpande G, Godishala A, Schedl P (2009) Gc1, a Downstream Target for the hmgcr-Isoprenoid Biosynthetic Pathway, Is Required for Releasing the Hedgehog Ligand and Directing Germ Cell Migration. PLoS Genet 5(1): e1000333. doi:10.1371/journal.pgen.1000333 Editor: Eric Rulifson, University of California San Francisco, United States of America Received May 13, 2008; Accepted December 9, 2008; Published January 9, 2009 Copyright: 2009 Deshpande et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: These studies were supported by GM-025976 until the February/October 2008 NIH Dev2 Panels deemed the research it covered unworthy of further consideration. Competing Interests: The authors have declared that no competing interests exist. * E-mail: pschedl@princeton.edu Introduction Two distinct cell types, the primordial germ cells and the somatic gonadal precursor cells (SGPs), coalesce to form the Drosophila embryonic gonad (for review, see [1,2]). These cells arise in different regions of the embryo and are specified by completely different mechanisms. The SGPs are derived from the lateral mesoderm in parasegments 10–13 during mid-embryogenesis and are specified by the input from a combination of cell-cell signaling pathways and zygotic patterning genes [3,4]. By contrast, the primordial germ cells, or pole cells, are formed on the outside surface of the embryo at the posterior end during the syncitial blastoderm stage and are specified by determinants localized in the posterior pole plasm during oogenesis [5,6]. In order for pole cells to assemble into a gonad with the SGPs, they must traverse from the posterior end into the middle of the embryo and then subsequently move to the lateral mesodermal cell layer, which contains the newly formed SGPs. This is a multistep process that begins at gastrulation when the pole cells are carried into the interior of the embryo by the midgut invagination [1,2]. They then pass through the midgut epithelium, and move along the surface of the midgut until they split into two groups. The germ cells in each group migrate laterally and this brings them into contact with the gonadal mesoderm on either side of the embryo. The germ cells align themselves in a row with the SGPs in parasegments 10-13 and these juxtaposed cells coalesce into the embryonic gonad. Analysis of the different migration steps has suggested that a combination of repulsive and attractive cues guide germ cell migration through the midgut and toward the somatic gonadal mesoderm. Repulsive clues, whose production depends upon Wunen and Wunen2, are thought to hasten the movement of the germ cells away from the midgut epithelium [7,8]. Once the germ cells exit the midgut and migrate along its surface, attractive cues produced by the SGPs are thought to entice the germ cells towards that lateral mesoderm and promote their subsequent association with the SGPs. One of the first genes implicated in the production of the germ cell attractant by the SGPs was hmgcr [9]. hmgcr is initially expressed broadly in the embryonic mesoderm; however, by the time germ cells commence their migration into the mesoderm, hmgcr expression is largely restricted to the SGPs [9]. In hmgcr mutants germ cells fail to migrate towards the SGPs and instead either remain associated with the midgut or scatter through the mesoderm. Conversely, ectoptically expressed hmgcr can induce germ cells to migrate towards tissues expressing the Hmgcr protein. Another gene that functions to induce migration towards the SGPs encodes the signaling molecule hedgehog (hh) [10]. Both ectopic expression of Hh and mutations that compromise the production or transmission of the Hh ligand by the SGPs induce mismigration. Since Hh functions as a morphogen in other contexts, one explanation for its effects on germ cell migration is that it acts indirectly by inducing cells to assume a SGP identity so PLoS Genetics | www.plosgenetics.org 1 January 2009 | Volume 5 | Issue 1 | e1000333