HMGCoA reductase potentiates hedgehog signaling in Drosophila melanogaster

The mevalonate pathway contributes to breast primary tumorigenesis and lung metastasis

The mevalonate pathway plays an important role in breast cancer and other tumor types. However, many issues remain obscure as yet regarding its mechanism of regulation and action. In the present study, we report that the expression of mevalonate pathway enzymes is mediated by the RHO guanosine nucleotide exchange factors VAV2 and VAV3 in a RAC1- and sterol regulatory element-binding factor (SREBF)-dependent manner in breast cancer cells. Furthermore, in vivo tumorigenesis experiments indicated that the two most upstream steps of this metabolic pathway [3-hydroxy-3-methylglutaryl-coenzyme A synthase 1 (HMGCS1) and 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR)] are important for primary tumorigenesis, angiogenesis, and cell survival in breast cancer cells. HMGCR, but not HMGCS1, is also important for the extravasation and subsequent fitness of breast cancer cells in the lung parenchyma. Genome-wide expression analyses revealed that HMGCR influences the expression of gene signatures linked to proliferation, metabolism, and immune responses. The HMGCR-regulated gene signature predicts long-term tumor recurrence but not metastasis in cohorts of nonsegregated and chemotherapy-resistant breast cancer patients. These results reveal a hitherto unknown, VAV-catalysis-dependent mechanism involved in the regulation of the mevalonate pathway in breast cancer cells. They also identify specific mevalonate-pathway-dependent processes that contribute to the malignant features of breast cancer cells.

Hedgehog signaling guides migration of primordial germ cells to the Drosophila somatic gonad

In addition to inducing nonautonomous specification of cell fate in both Drosophila and vertebrates, the Hedgehog pathway guides cell migration in a variety of different tissues. Although its role in axon guidance in the vertebrate nervous system is widely recognized, its role in guiding the migratory path of primordial germ cells (PGCs) from the outside surface of the Drosophila embryo through the midgut and mesoderm to the SGPs (somatic gonadal precursors) has been controversial. Here we present new experiments demonstrating (1) that Hh produced by mesodermal cells guides PGC migration, (2) that HMG CoenzymeA reductase (Hmgcr) potentiates guidance signals emanating from the SGPs, functioning upstream of hh and of 2 Hh pathway genes important for Hh-containing cytonemes, and (3) that factors required in Hh receiving cells in other contexts function in PGCs to help direct migration toward the SGPs. We also compare the data reported by 4 different laboratories that have studied the role of the Hh pathway in guiding PGC migration.

Preformation and epigenesis converge to specify primordial germ cell fate in the early Drosophila embryo

A critical step in animal development is the specification of primordial germ cells (PGCs), the precursors of the germline. Two seemingly mutually exclusive mechanisms are implemented across the animal kingdom: epigenesis and preformation. In epigenesis, PGC specification is non-autonomous and depends on extrinsic signaling pathways. The BMP pathway provides the key PGC specification signals in mammals. Preformation is autonomous and mediated by determinants localized within PGCs. In Drosophila, a classic example of preformation, constituents of the germ plasm localized at the embryonic posterior are thought to be both necessary and sufficient for proper determination of PGCs. Contrary to this longstanding model, here we show that these localized determinants are insufficient by themselves to direct PGC specification in blastoderm stage embryos. Instead, we find that the BMP signaling pathway is required at multiple steps during the specification process and functions in conjunction with components of the germ plasm to orchestrate PGC fate.

Proper specification of primordial germ cells (PGCs) is crucial as PGCs serve as the precursors of germline stem cells. To specify PGC fate, invertebrates rely upon cell autonomous preformation involving maternally deposited germ plasm. In Drosophila melanogaster, to insulate newly formed PGCs from the adverse effects of the cell-cell signaling pathways, germ plasm determinants silence transcription and attenuate the cell cycle. However, our data on the BMP signaling pathway challenge this long-held view of PGC specification and suggest that appropriate specification of embryonic PGCs is sensitive to the BMP ligand, decapentaplegic (dpp), and its cognate receptor, thickveins. We find that PGCs are not only capable of responding to BMP signals from the soma, but also that these signals impact the proper determination of the germ cells. Based on these unanticipated similarities between mammals and flies, we propose a model integrating contribution of both the cell-autonomous (preformation) and non-autonomous (epigenesis) pathways during PGC determination. Consistent with the model, we have observed dominant genetic interactions between, oskar, the maternal determinant of PGC fate, and the BMP pathway ligand dpp.

Hedgehog signaling and Tre1 regulate actin dynamics through PI(4,5)P2 to direct migration of Drosophila embryonic germ cells

The Tre1 G-protein coupled receptor (GPCR) was discovered to be required for Drosophila germ cell (GC) coalescence almost two decades ago, yet the molecular events both upstream and downstream of Tre1 activation remain poorly understood. To gain insight into these events, we describe a bona fide null allele and both untagged and tagged versions of Tre1. We find that the primary defect with complete Tre1 loss is the failure of GCs to properly navigate, with GC mis-migration occurring from early stages. We find that Tre1 localizes with F-actin at the migration front, along with PI(4,5)P₂; dPIP5K, an enzyme that generates PI(4,5)P₂; and dWIP, a protein that binds activated Wiskott-Aldrich syndrome protein (WASP), which stimulates F-actin polymerization. We show that Tre1 is required for polarized accumulation of F-actin, PI(4,5)P₂, and dPIP5K. Smoothened also localizes with F-actin at the migration front, and Hh, through Smo, increases levels of Tre1 at the plasma membrane and Tre1’s association with dPIP5K.

Kim et al. uncover molecular and cellular events upstream and downstream of the Tre1 G-protein coupled receptor (GPCR), which is required for germ cell navigation in Drosophila. Hedgehog signaling through Smoothened localizes Tre1 to activate F-actin assembly through dPIP5K, PI(4,5)P₂, and WASP.

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.

Functional analysis of Niemann-Pick disease type C family protein, NPC1a, in Drosophila melanogaster

During embryonic gonad coalescence, primordial germ cells (PGCs) follow a carefully choreographed migratory route circumscribed by guidance signals towards somatic gonadal precursor cells (SGPs). In Drosophila melanogaster, SGP-derived Hedgehog (Hh), which serves as a guidance cue for the PGCs, is potentiated by mesodermally restricted HMGCoA-reductase (Hmgcr) and the ABC transporter Multi-drug-resistant-49 (Mdr49). Given the importance of cholesterol modification in the processing and long-distance transmission of the Hh ligand, we have analyzed the involvement of the Niemann-Pick disease type C-1a (NPC1a) protein, a cholesterol transporter, in germ cell migration and Hedgehog signaling. We show that mesoderm-specific inactivation of Npc1a results in germ cell migration defects. Similar to Mdr49, PGC migration defects in the Npc1a embryos are ameliorated by a cholesterol-rich diet. Consistently, reduction in Npc1a weakens the ability of ectopic HMG Coenzyme A reductase (Hmgcr) to induce germ cell migration defects. Moreover, compromising Npc1a levels influences Hh signaling adversely during wing development, a process that relies upon long-range Hh signaling. Last, doubly heterozygous embryos (Mdr49/Npc1a) display enhanced germ cell migration defects when compared with single mutants (Npc1a/+ or Mdr49/+), supporting cooperative interaction between the two.

Cells on the move: Modulation of guidance cues during germ cell migration

In Drosophila melanogaster the progenitors of the germ-line stem cells, the primordial germ cells (PGCs) are formed on the outside surface of the early embryo, while the somatic gonadal precursor cells (SGPs) are specified during mid-embryogenesis. To form the primitive embryonic gonad, the PGCs travel from outside of the embryo, across the mid-gut and then migrate through the mesoderm to the SGPs. The migratory path of PGCs is dictated by a series of attractive and repulsive cues. Studies in our laboratory have shown that one of the key chemoattractants is the Hedgehog (Hh) ligand. Although, Hh is expressed in other cell types, the long-distance transmission of this ligand is specifically potentiated in the SGPs by the hmgcr isoprenoid biosynthetic pathway. The distant transmission of the Hh ligand is gated by restricting expression of hmgcr to the SGPs. This is particularly relevant in light of the recent findings that an ABC transporter, mdr49 also acts in a mesoderm specific manner to release the germ cell attractant. Our studies have demonstrated that mdr49 functions in hh signaling likely via its role in the transport of cholesterol. Given the importance of cholesterol in the processing and long distance transmission of the Hh ligand, this observation has opened up an exciting avenue concerning the possible role of components of the sterol transport machinery in PGC migration.

Finding their way: themes in germ cell migration

Embryonic germ cell migration is a vital component of the germline lifecycle. The translocation of germ cells from the place of origin to the developing somatic gonad involves several processes including passive movements with underlying tissues, transepithelial migration, cell adhesion dynamics, the establishment of environmental guidance cues and the ability to sustain directed migration. How germ cells accomplish these feats in established model organisms will be discussed in this review, with a focus on recent discoveries and themes conserved across species.

Role of the ABC transporter Mdr49 in Hedgehog signaling and germ cell migration

Coalescence of the embryonic gonad in Drosophila melanogaster requires directed migration of primordial germ cells (PGCs) towards somatic gonadal precursor cells (SGPs). It was recently proposed that the ATP-binding cassette (ABC) transporter Mdr49 functions in the embryonic mesoderm to facilitate the transmission of the PGC attractant from the SGPs; however, the precise molecular identity of the Mdr49-dependent guidance signal remained elusive. Employing the loss- and gain-of-function strategies, we show that Mdr49 is a component of the Hedgehog (hh) pathway and it potentiates the signaling activity. This function is direct because in Mdr49 mutant embryos the Hh ligand is inappropriately sequestered in the hh-expressing cells. Our data also suggest that the role of Mdr49 is to provide cholesterol for the correct processing of the Hh precursor protein. Supporting this conclusion, PGC migration defects in Mdr49 embryos are substantially ameliorated by a cholesterol-rich diet.

Mevalonate Pathway Regulates Cell Size Homeostasis and Proteostasis through Autophagy

Balance between cell growth and proliferation determines cell size homeostasis, but little is known about how metabolic pathways are involved in the maintenance of this balance. Here, we perform a screen with a library of clinically used drug molecules for their effects on cell size. We find that statins, inhibitors of the mevalonate pathway, reduce cell proliferation and increase cell size and cellular protein density in various cell types, including primary human cells. Mevalonate pathway effects on cell size and protein density are mediated through geranylgeranylation of the small GTPase RAB11, which is required for basal autophagic flux. Our results identify the mevalonate pathway as a metabolic regulator of autophagy and expose a paradox in the regulation of cell size and proteostasis, where inhibition of an anabolic pathway can cause an increase in cell size and cellular protein density.

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Mevalonate pathway regulates cell size, proliferation, and cellular protein density

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Geranylgeranylation of RAB11 links mevalonate pathway to autophagy

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Mevalonate pathway is required for basal autophagic flux

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Cell size and protein density effects are dependent on RAB11 and autophagy

The Hedgehog Signalling Pathway in Cell Migration and Guidance: What We Have Learned from Drosophila melanogaster

Cell migration and guidance are complex processes required for morphogenesis, the formation of tumor metastases, and the progression of human cancer. During migration, guidance molecules induce cell directionality and movement through complex intracellular mechanisms. Expression of these molecules has to be tightly regulated and their signals properly interpreted by the receiving cells so as to ensure correct navigation. This molecular control is fundamental for both normal morphogenesis and human disease. The Hedgehog (Hh) signaling pathway is evolutionarily conserved and known to be crucial for normal cellular growth and differentiation throughout the animal kingdom. The relevance of Hh signaling for human disease is emphasized by its activation in many cancers. Here, I review the current knowledge regarding the involvement of the Hh pathway in cell migration and guidance during Drosophila development and discuss its implications for human cancer origin and progression.

Modulating effect of synthetic statins against damage induced by doxorubicin in somatic cells of Drosophila melanogaster

The competitive inhibitors of HMG-CoA reductase, popularly known as statins, exert pleiotropic effects, which result from the ability of statins to inhibit the synthesis of isoprenoids, which are fundamental for the functioning of proteins responsible for intracellular signaling. Some recent studies suggest an important role associated with the use of antineoplastic atorvastatin and rosuvastatin, the statins most widely used today. In this study, the Drosophila wing spot test was used to evaluate possible protective effects of atorvastatin and rosuvastatin against damage induced by DXR. Larvae were chronically treated with negative control (ethanol 5%), positive control (DXR 0.125 mg/mL) and five different concentrations of atorvastatin and rosuvastatin. The results demonstrated absence of a mutagenic effect for the two statins tested. The analysis of the descendants co-treated with DXR and atorvastatin/rosuvastatin revealed a modulatory effect of these statins on damage induced by DXR. This effect was verified in all concentrations tested in the descendants of the ST and HB crosses treated with rosuvastatin, and only in descendants of the HB cross treated with atorvastatin. Induction of apoptosis and antioxidant activity appear to be the main mechanisms involved in reducing the frequency of mutant spots and consequent modulation of the damage induced by DXR.

Discs large 5, an Essential Gene in Drosophila, Regulates Egg Chamber Organization

Discs large 5 (Dlg5) is a member of the MAGUK family of proteins that typically serve as molecular scaffolds and mediate signaling complex formation and localization. In vertebrates, Dlg5 has been shown to be responsible for polarization of neural progenitors and to associate with Rab11-positive vesicles in epithelial cells. In Drosophila, however, the function of Dlg5 is not well-documented. We have identified dlg5 as an essential gene that shows embryonic lethality. dlg5 embryos display partial loss of primordial germ cells (PGCs) during gonad coalescence between stages 12 and 15 of embryogenesis. Loss of Dlg5 in germline and somatic stem cells in the ovary results in the depletion of both cell lineages. Reduced expression of Dlg5 in the follicle cells of the ovary leads to a number of distinct phenotypes, including defects in egg chamber budding, stalk cell overgrowth, and ectopic polar cell induction. Interestingly, loss of Dlg5 in follicle cells results in abnormal distribution of a critical component of cell adhesion, E-cadherin, shown to be essential for proper organization of egg chambers.

Hedgehog and its circuitous journey from producing to target cells

The hedgehog (Hh) signaling protein has essential roles in the growth, development and regulation of many vertebrate and invertebrate organs. The processes that make Hh and prepare it for release from producing cells and that move it to target cells are both diverse and complex. This article reviews the essential features of these processes and highlights recent work that provides a novel framework to understand how these processes contribute to an integrated pathway.

Developmental processes regulated by the 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) pathway: highlights from animal studies

The 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) is the rate-limiting enzyme in the biosynthesis of cholesterol and isoprenoids, which are substrates required for post-translational modification of signalling proteins that can potentially regulate various aspects of embryonic development. The HMGCR transcripts are detectable during early embryogenesis in both invertebrates and vertebrates, which suggests a conserved developmental requirement for mevalonate derivatives. Consistently, recent animal and in vitro studies have yielded valuable insights into potential morphogenic parameters that are modulated by HMGCR activity. These developmental end-points include brain and craniofacial morphogenesis, PGC migration and survival, myocardial epithelial migration and fusion, EC migration and survival, and vascular stabilization. By providing a synthesis of these studies, we hope that this review will highlight the need to comprehensively examine the entire suite of developmental processes regulated by HMGCR.

The hedgehog pathway gene shifted functions together with the hmgcr-dependent isoprenoid biosynthetic pathway to orchestrate germ cell migration

The Drosophila embryonic gonad is assembled from two distinct cell types, the Primordial Germ Cells (PGCs) and the Somatic Gonadal Precursor cells (SGPs). The PGCs form at the posterior of blastoderm stage embryos and are subsequently carried inside the embryo during gastrulation. To reach the SGPs, the PGCs must traverse the midgut wall and then migrate through the mesoderm. A combination of local repulsive cues and attractive signals emanating from the SGPs guide migration. We have investigated the role of the hedgehog (hh) pathway gene shifted (shf) in directing PGC migration. shf encodes a secreted protein that facilitates the long distance transmission of Hh through the proteoglycan matrix after it is released from basolateral membranes of Hh expressing cells in the wing imaginal disc. shf is expressed in the gonadal mesoderm, and loss- and gain-of-function experiments demonstrate that it is required for PGC migration. Previous studies have established that the hmgcr-dependent isoprenoid biosynthetic pathway plays a pivotal role in generating the PGC attractant both by the SGPs and by other tissues when hmgcr is ectopically expressed. We show that production of this PGC attractant depends upon shf as well as a second hh pathway gene gγ1. Further linking the PGC attractant to Hh, we present evidence indicating that ectopic expression of hmgcr in the nervous system promotes the release/transmission of the Hh ligand from these cells into and through the underlying mesodermal cell layer, where Hh can contact migrating PGCs. Finally, potentiation of Hh by hmgcr appears to depend upon cholesterol modification.

The molecular mechanisms underlying directed cell migration have been studied extensively in different biological contexts. Germ cell migration provides an effective model to study the dynamics of in vivo cell migration. The process of germ cell migration in Drosophila melanogaster results in embryonic gonad formation consisting of primordial germ cells (PGCs) and somatic gonadal precursor cells (SGPs). Moreover, it likely involves a complex series of attractive and repulsive cues. Molecular and genetic analysis has been performed to elucidate the nature of the attractive cue(s) and components that guide germ cells to the SGPs in the mesoderm. One current model proposes that 3-Hydroxy-3-Methylglutaryl Coenzyme A reductase (Hmgcr), synthesized in the SGPs, potentiates signaling downstream of Hedgehog (Hh) ligand also emanating from the SGPs. The model pivots on the novel activity of an established morphogen, Hedgehog, to function as a chemoattractant for the migrating germ cells. A variety of ‘loss-’ and ‘gain-of-function’ strategies manipulating different components of this signaling pathway have been successfully employed in support of the proposed model.

Hedgehog on the move: a precise spatial control of Hedgehog dispersion shapes the gradient

Hedgehog (Hh) as morphogen directs cell differentiation during development activating various target genes in a concentration dependent manner. The mechanisms that permit controlled Hh dispersion and gradient formation remain controversial. New research in the Drosophila wing disc epithelium has revealed a crucial role of Hh recycling for its release and transportation from source cells. Lipid modifications on Hh mediate key interactions with different elements of the pathway, which balance the retention and release of the molecule through the basolateral side of the epithelium, allowing its tight spatial control. Dispersion of Hh is also determined by its hydrophobic nature, and the mechanisms that include membrane-tethered transport of Hh are increasingly proposed.

Hedgehog does not guide migrating Drosophila germ cells

In many species, the germ cells, precursors of sperm and egg, migrate during embryogenesis. The signals that regulate this migration are thus essential for fertility. In flies, lipid signals have been shown to affect germ cell guidance. In particular, the synthesis of geranylgeranyl pyrophosphate through the 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (Hmgcr) pathway is critical for attracting germ cells to their target tissue. In a genetic analysis of signaling pathways known to affect cell migration of other migratory cells, we failed to find a role for the Hedgehog (Hh) pathway in germ cell migration. However, previous reports had implicated Hh as a germ cell attractant in flies and suggested that Hh signaling is enhanced through the action of the Hmgcr pathway. We therefore repeated several critical experiments and carried out further experiments to test specifically whether Hh is a germ cell attractant in flies. In contrast to previously reported findings and consistent with findings in zebrafish our data do not support the notion that Hh has a direct role in the guidance of migrating germ cells in flies.

Germ cell migration in zebrafish is cyclopamine-sensitive but Smoothened-independent

Primordial germ cells (PGCs) are the progenitors of reproductive cells in metazoans and are an important model for the study of cell migration in vivo. Previous reports have suggested that Hedgehog (Hh) protein acts as a chemoattractant for PGC migration in the Drosophila embryo and that downstream signaling proteins such as Patched (Ptc) and Smoothened (Smo) are required for PGC localization to somatic gonadal precursors. Here we interrogate whether Hh signaling is required for PGC migration in vertebrates, using the zebrafish as a model system. We find that cyclopamine, an inhibitor of Hh signaling, causes strong defects in the migration of PGCs in the zebrafish embryo. However, these defects are not due to inhibition of Smoothened (Smo) by cyclopamine; rather, we find that neither maternal nor zygotic Smo is required for PGC migration in the zebrafish embryo. Cyclopamine instead acts independently of Smo to decrease the motility of zebrafish PGCs, in part by dysregulating cell adhesion and uncoupling cell polarization and translocation. These results demonstrate that Hh signaling is not required for zebrafish PGC migration, and underscore the importance of regulated cell-cell adhesion for cell migration in vivo.

Ggamma1, a downstream target for the hmgcr-isoprenoid biosynthetic pathway, is required for releasing the Hedgehog ligand and directing germ cell migration

The isoprenoid biosynthetic pathway leading from the production of mevalonate by HMGCoA reductase (Hmgcr) to the geranylation of the G protein subunit, Ggamma1, 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 hmgcr-->Ggamma1 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, ggamma1 and quemao (which encodes the enzyme, geranylgeranyl diphosphate synthetase, that produces the substrate for geranylation of Ggamma1) 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 hmgcr-->Ggamma1 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 ggamma1 activity, by trans-heterozygous combinations between ggamma1 and either hmgcr or hh mutations, and by ectopic expression of dominant negative Ggamma1 proteins that cannot be geranylated.

Inhibition of HMG CoA reductase reveals an unexpected role for cholesterol during PGC migration in the mouse

BACKGROUND

Primordial germ cells (PGCs) are the embryonic precursors of the sperm and eggs. Environmental or genetic defects that alter PGC development can impair fertility or cause formation of germ cell tumors.

RESULTS

We demonstrate a novel role for cholesterol during germ cell migration in mice. Cholesterol was measured in living tissue dissected from mouse embryos and was found to accumulate within the developing gonads as germ cells migrate to colonize these structures. Cholesterol synthesis was blocked in culture by inhibiting the activity of HMG CoA reductase (HMGCR) resulting in germ cell survival and migration defects. These defects were rescued by co-addition of isoprenoids and cholesterol, but neither compound alone was sufficient. In contrast, loss of the last or penultimate enzyme in cholesterol biosynthesis did not alter PGC numbers or position in vivo. However embryos that lack these enzymes do not exhibit cholesterol defects at the stage at which PGCs are migrating. This demonstrates that during gestation, the cholesterol required for PGC migration can be supplied maternally.

CONCLUSION

In the mouse, cholesterol is required for PGC survival and motility. It may act cell-autonomously by regulating clustering of growth factor receptors within PGCs or non cell-autonomously by controlling release of growth factors required for PGC guidance and survival.

Distinct contributions of CXCR4b and CXCR7/RDC1 receptor systems in regulation of PGC migration revealed by medaka mutants kazura and yanagi

Migratory pathways of PGCs to the gonad vary depending on the vertebrate species, yet the underlying regulatory mechanisms guiding PGCs are believed to be largely common. In teleost medaka embryo, PGC migration follows two major steps before colonizing in gonadal areas: (1) bilateral lineup in the trunk and (2) posterior drift of PGCs. kazura (kaz) and yanagi (yan) mutants of medaka isolated in mutagenesis screening were defective in the first and second steps, respectively. kaz(j2-15D) was identified as a missense mutation in chemokine receptor gene cxcr4b expressed in PGCs. Embryonic injection of cxcr4b mRNA with vasa 3' UTR rescued the PGC phenotype of kaz mutant, indicating a cell-autonomous function of cxcr4b in PGCs. yan(j6-29C) was identified as a nonsense mutation in the cxcr7/rdc1 gene encoding another chemokine receptor. cxcr7 transgene with genomic flanking sequences rescued the yan mutant phenotype efficiently at the G0 generation. cxcr7 was expressed in somites rather than PGCs. cxcr7-expressing somitic domain expanded posteriorly with its margin immediately anterior of posteriorly drifting PGCs, as if PGCs were thrusted toward the gonadal area. kaz and yan mutants are also defective in lateral line positioning, suggesting combined employment of these receptor systems in various cell migratory processes.

The development of germline stem cells in Drosophila

Germline stem cells (GSCs) in Drosophila are a valuable model to explore of how adult stem cells are regulated in vivo. Genetic dissection of this system has shown that stem cell fate is determined and maintained by the stem cell's somatic microenvironment or niche. In Drosophila gonads, the stem cell niche -- the cap cell cluster in females and the hub in males -- acts as a signaling center to recruit GSCs from among a small population of undifferentiated primordial germ cells (PGCs). Short-range signals from the niche specify and regulate stem cell fate by maintaining the undifferentiated state of the PGCs next to the niche. Germline cells that do not receive the niche signals because of their location assume the default fate and differentiate. Once GSCs are specified, adherens junctions maintain close association between the stem cells and their niche and help to orient stem cell division so that one daughter is displaced from the niche and differentiates. In females, stem cell fate depends on bone morphogenetic protein (BMP) signals from the cap cells; in males, hub cells express the cytokine-like ligand Unpaired, which activates the Janus kinase-signal transducers and activators of transcription (Jak-Stat) pathway in stem cells. Although the signaling pathways operating between the niche and stem cells are different, there are common general features in both males and females, including the arrangement of cell types, many of the genes used, and the logic of the system that maintains stem cell fate.

toutvelu, a regulator of heparan sulfate proteoglycan biosynthesis, controls guidance cues for germ-cell migration

The primitive embryonic gonad in Drosophila melanogaster is composed of germ cells and somatic gonadal precursor cells (SGPs). The assembly of a functional gonad involves a complex series of germ-cell migration events, which are thought to be guided by attractive and repulsive cues. Here, we demonstrate a novel role for toutvelu (ttv), a regulator of heparan sulfate proteoglycan biosynthesis during this process. Germline clonal analysis suggests that maternal deposition of ttv is required for proper germ-cell migration. Conversely, ectopic expression of ttv in early embryos results in severe germ-cell migration defects and inappropriate spreading of Hh protein. Moreover, overexpression of ttv in only the receiving cells, rather than in the sending cells, leads to phenotypic consequences. Finally, supporting the claim that the signaling molecule Hedgehog (Hh) may function as a chemoattractant to guide germ cells, errant germ cells are found localized near pockets containing high concentrations of Hh protein.

Hmgcr in the corpus allatum controls sexual dimorphism of locomotor activity and body size via the insulin pathway in Drosophila

The insulin signaling pathway has been implicated in several physiological and developmental processes. In mammals, it controls expression of 3-Hydroxy-3-Methylglutaryl CoA Reductase (HMGCR), a key enzyme in cholesterol biosynthesis. In insects, which can not synthesize cholesterol de novo, the HMGCR is implicated in the biosynthesis of juvenile hormone (JH). However, the link between the insulin pathway and JH has not been established. In Drosophila, mutations in the insulin receptor (InR) decrease the rate of JH synthesis. It is also known that both the insulin pathway and JH play a role in the control of sexual dimorphism in locomotor activity. In studies here, to demonstrate that the insulin pathway and HMGCR are functionally linked in Drosophila, we first show that hmgcr mutation also disrupts the sexual dimorphism. Similarly to the InR, HMGCR is expressed in the corpus allatum (ca), which is the gland where JH biosynthesis occurs. Two p[hmgcr-GAL4] lines were therefore generated where RNAi was targeted specifically against the HMGCR or the InR in the ca. We found that RNAi-HMGCR blocked HMGCR expression, while the RNAi-InR blocked both InR and HMGCR expression. Each RNAi caused disruption of sexual dimorphism and produced dwarf flies at specific rearing temperatures. These results provide evidence: (i) that HMGCR expression is controlled by the InR and (ii) that InR and HMGCR specifically in the ca, are involved in the control of body size and sexual dimorphism of locomotor activity.

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.

The social lives of migrating cells in Drosophila

Studies of cell migration in Drosophila are yielding insights into the complex interactions migrating cells have with each other and with the cells in their environment. Intriguing links between factors that promote cell migration and those that control cell survival have been reported recently. For example, migrating germ cells compete with the surrounding somatic tissue for the substrate of the lipid phosphate phosphatases encoded by the genes Wunen and Wunen2. Germ cells take up the dephosphorylated lipid and require it for their survival. In addition, the secreted growth factors called PVFs, previously thought to guide the migrations of hemocytes in the embryo, were found to function instead predominantly as survival factors. And in border cells, DIAP1 and Dronc, two proteins known mainly for their ability to regulate cell death, were found to control cell migration.

Attraction rules: germ cell migration in zebrafish

The migration of zebrafish primordial germ cell towards the region where the gonad develops is guided by the chemokine SDF-1a. Recent studies show that soon after their specification, the cells undergo a series of morphological alterations before they become motile and are able to respond to attractive cues. As migratory cells, primordial germ cells move towards their target while correcting their path upon exiting a cyclic phase in which morphological cell polarity is lost. In the following stages, the cells gather at specific locations and move as cell clusters towards their final target. In all of these stages, zebrafish germ cells respond as individual cells to alterations in the shape of the sdf-1a expression domain, by directed migration towards their target - the position where the gonad develops.