Guidance of cell migration by EGF receptor signaling during Drosophila oogenesis

Cell migration in development and disease

A recent meeting at the Max Delbrück Center in Berlin, Germany provided a forum to discuss the molecular mechanisms of cell migration in a broad range of contexts including chemotaxis, development, immunity, and cancer.

Regulation of zebrafish primordial germ cell migration by attraction towards an intermediate target

Migration of primordial germ cells (PGCs) from their site of specification towards the developing gonad is controlled by directional cues from somatic tissues. Although in several animals the PGCs are attracted by signals emanating from their final target, the gonadal mesoderm, little is known about the mechanisms that control earlier steps of migration. We provide evidence that a key step of zebrafish PGC migration, in which the PGCs become organized into bilateral clusters in the anterior trunk, is regulated by attraction of PGCs towards an intermediate target. Time-lapse observations of wild-type and mutant embryos reveal that bilateral clusters are formed at early somitogenesis, owing to migration of PGCs towards the clustering position from medial, posterior and anterior regions. Furthermore, PGCs migrate actively relative to their somatic neighbors and they do so as individual cells. Using mutants that exhibit defects in mesoderm development, we show that the ability to form PGC clusters depends on proper differentiation of the somatic cells present at the clustering position. Based on these findings, we propose that these somatic cells produce signals that attract PGCs. Interestingly, fate-mapping shows that these cells do not give rise to the somatic tissues of the gonad, but rather contribute to the formation of the pronephros. Thus, the putative PGC attraction center serves as an intermediate target for PGCs, which later actively migrate towards a more posterior position. This final step of PGC migration is defective in hands off mutants, where the intermediate mesoderm of the presumptive gonadal region is mispatterned. Our results indicate that zebrafish PGCs are guided by attraction towards two signaling centers, one of which may represent the somatic tissues of the gonad.Movies available on-line

Specificity of FGF signaling in cell migration inDrosophila

We wanted to investigate the relationship between receptor tyrosine kinase (RTK) activated signaling pathways and the induction of cell migration. Using Drosophila tracheal and mesodermal cell migration as model systems, we find that the intracellular domain of the fibroblast growth factor receptors (FGFRs) Breathless (Btl) and Heartless (Htl) can be functionally replaced by the intracellular domains of Torso (Tor) and epidermal growth factor receptor (EGFR). These hybrid receptors can also rescue cell migration in the absence of Downstream of FGFR (Dof), a cytoplasmic protein essential for FGF signaling. These results demonstrate that tracheal and mesodermal cells respond during a specific time window to a receptor tyrosine kinase (RTK) signal with directed migration, independent of the presence or absence of Dof. We discuss our findings in the light of the recent findings that RTKs generate a generic signal that is interpreted in responding cells according to their developmental history.

EGFRs mediate chemotactic migration in the developing telencephalon

Epidermal growth factor receptors (EGFRs) have been implicated in the control of migration in the telencephalon, but the mechanism underlying their contribution is unclear. We show that expression of a threshold level of EGFRs confers chemotactic competence in stem cells, neurons and astrocytes in cortical explants. This level of receptor expression is normally achieved by a subpopulation of cells during mid-embryonic development. Cells that express high levels of EGFR are located in migration pathways, including the tangential pathway to the olfactory bulb via the rostral migratory stream (RMS), the lateral cortical stream (LCS) leading to ventrolateral cortex and the radial pathway from proliferative zones to cortical plate. The targets of these pathways express the ligands HB-EGF and/or TGFα. To test the idea that EGFRs mediate chemotactic migration these pathways, we increased the size of the population of cells expressing threshold levels of EGFRs in vivo by viral transduction. Our results suggest that EGFRs mediate migration radially to the cortical plate and ventrolaterally in the LCS, but not tangentially in the RMS. Within the bulb, however, EGFRs also mediate radial migration. Our findings suggest that developmental changes in EGFR expression, together with changes in ligand expression regulate the migration of specific populations of cells in the telencephalon by a chemoattractive mechanism.

Guidance of cell migration by the Drosophila PDGF/VEGF receptor

Directed cell migrations are important for development, but the signaling pathways and mechanisms responsible for guiding cell migration in vivo are poorly understood. Migration of border cells during Drosophila oogenesis is a simple and attractive model system in which to address these questions. We demonstrate that PVR, a receptor tyrosine kinase related to mammalian PDGF and VEGF receptors, acts in border cells to guide them to the oocyte. The oocyte is the source of a ligand for PVR, PDGF/VEGF factor 1 (PVF1). Intriguingly, the guidance function of PVR is largely redundant with that of EGFR. We present evidence implicating Rac and the Rac activator Mbc/DOCK180/CED-5 as mediators of the guidance signal.

Two signals are better than one: border cell migration in Drosophila

Recent studies of border cell migration during Drosophila oogenesis demonstrate that the EGFR and PDGFR signaling pathways act in a partially redundant manner to guide this process. Evidence presented shows that PDGFR signaling directs border cell migration via Rac and the Rac activator Mbc/CED-5/Dock180.

Axis formation during Drosophila oogenesis

Recent advances shed light on the cellular processes that cooperate during oogenesis to produce a fully patterned egg, containing all the maternal information required for embryonic development. Progress has been made in defining the early steps in oocyte specification and it has been shown that progression of oogenesis is controlled by a meiotic checkpoint and requires active maintenance of the oocyte cell fate. The function of Gurken signalling in patterning the dorsal-ventral axis later in oogenesis is better understood. Anterior-posterior patterning of the embryo requires activities of bicoid and oskar mRNAs, localised within the oocyte. A microtubule motor, Kinesin, is directly implicated in localisation of oskar mRNA to the posterior pole of the oocyte.

Cell migration in invertebrates: clues from border and distal tip cells

Recent studies in two invertebrate systems, border cells in Drosophila melanogaster and distal tip cells in Caenorhabditis elegans, have provided important insight into the mechanisms of directed cell migration. These migrating cells are guided by extracellular signals, such as EGF, TGF-beta and netrin. In addition, metalloproteases alter the extracellular matrix of the tissue through which these cells migrate. Along the migratory path, migrating cells respond to changes in guidance signals by altering the expression of receptor signaling pathways. Finally, Dock180, CrkII and the GTPase Rac link the extracellular signals to the cellular machinery that controls cell motility.

Command and control: regulatory pathways controlling invasive behavior of the border cells

The invasiveness of cancer cells resembles the normal behavior of cells that migrate into surrounding tissues during development. For example, the border cells in the Drosophila ovary undergo a partial epithelial to mesenchymal transition and invade the neighboring cluster of germline cells, migrating to the oocyte border. Once there, they provide patterning information to the oocyte and produce an eggshell specialization known as the micropyle. Border cell migration has been subjected to extensive genetic analyses using a variety of screening approaches. Recent findings demonstrate that conversion of the border cells from a stationary group of epithelial cells to invasive cells requires integration of the activities of at least two transcriptional regulatory pathways. One such pathway requires the slbo gene, which encodes Drosophila C/EBP, a basic region/leucine zipper transcriptional activator that is required for elevated expression of a number of downstream targets, including DE-cadherin and focal adhesion kinase (FAK). An independent pathway requires the activity of the ecdysone receptor and a recently identified co-activator for the ecdysone receptor known as Taiman (abbreviated TAI, pronounced ti-maan', meaning too slow). Ecdysone is produced in the Drosophila ovary in response to adequate nutrition and is required for progression of oogenesis through stage 9, when border cell migration occurs. Border cells mutant for tai accumulate abnormally high levels of adhesion complexes at their surfaces, which may account for their inability to migrate. Thus border cell migration requires a differentiation program mediated by the C/EBP pathway, which is required for elevated expression of a number of proteins required for motility. In addition, migration requires a hormonal signal that relays information regarding nutritional status and appears to be required for regulation of the proper localization of some of the C/EBP targets. These findings suggest that steroid hormones can regulate cell motility relatively directly, independent of the effects on proliferation. This may contribute to the metastatic effects of steroid hormones on certain cancers and the inhibition of metastasis by steroid hormone antagonists such as tamoxifen.