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Cell migration is fundamental in both animal morphogenesis and disease. The migration of individual cells is relatively well-studied, however in vivo cells often remain joined by cell-cell junctions and migrate in cohesive groups. How such groups of cells coordinate their migration is poorly understood. The planar polarity pathway coordinates the polarity of non-migrating cells in epithelial sheets and is required for cell rearrangements during vertebrate morphogenesis. It is therefore a good candidate to play a role in collective migration of groups of cells. Drosophila border cell migration is a well-characterised and genetically tractable model of collective cell migration, during which a group of about 6-10 epithelial cells detaches from the anterior end of the developing egg chamber and migrates invasively towards the oocyte. We find that the planar polarity pathway promotes this invasive migration, acting both in the migrating cells themselves and in the non-migratory polar follicle cells they carry along. Disruption of planar polarity signalling causes abnormalities in actin rich processes on the cell surface and leads to less efficient migration. This is apparently due in part to loss of regulation of Rho GTPase activity by the planar polarity receptor Frizzled, which itself becomes localised to the migratory edge of the border cells. We conclude that during collective cell migration the planar polarity pathway can mediate communication between motile and non-motile cells, which enhances the efficiency of migration via the modulation of actin dynamics.
The ability of cells to undergo directed migration is a prerequisite for the morphogenesis of complex animal body plans. Such migration can generally be divided into two forms: single cell migration or collective cell migration (Friedl, 2004). In single cell migration, junctional contacts are lost, although cells may still remain loosely associated in small groups or chains. As this can be easily studied in cultured cells, it is relatively well-characterised. During collective cell migration, junctional contacts are maintained between groups of moving cells. Such coordinated movement of cell groups is a key event in organogenesis and has been implicated in disease states such as cancer metastasis (Friedl et al., 2004; Lecaudey and Gilmour, 2006). Well-studied examples of collective cell migration include the movement of relatively small groups of cells such as border cell migration during Drosophila oogenesis (Montell, 2003) and movement of the lateral line primordium in fish and amphibians (Ghysen and Dambly-Chaudière, 2004), as well as the rearrangement of sheets of cells such as occurs during Drosophila dorsal closure (Jacinto et al., 2002) and vertebrate gastrulation (Keller, 2002). A key feature of collective cell migration is that it permits the coordinated movement of both motile and non-motile cells together in a single group. However, as collective cell migration can only be studied in the context of developing organisms, it remains poorly understood.
In recent years a great deal of progress has been made in understanding how cell polarity is coordinated in developing tissues. In many contexts in both Drosophila and vertebrates, a conserved planar polarity pathway mediates local cell-cell interactions to ensure that neighbouring cells adopt the appropriate polarity (Strutt, 2003; Veeman et al., 2003; Klein and Mlodzik, 2005). Central to this is a core group of polarity proteins including the sevenpass transmembrane receptor Frizzled (Fz), the fourpass transmembrane protein Strabismus (Stbm, also known as Van Gogh) and the cytoplasmic proteins Dishevelled (Dsh) and Prickle (Pk). This core is involved in diverse patterning events from bristle and hair polarity on the body surface of flies, to convergent extension and neural crest migration in vertebrate embryos (Strutt, 2003; Wallingford et al., 2002; De Calisto et al., 2005).
Given the requirement of planar polarity pathway function for efficient convergent extension movements, it is a good candidate to coordinate collective migration of cells in other contexts. Therefore we examined its requirement in border cell migration in the Drosophila egg chamber, which represents a particularly well-characterised and genetically tractable model of collective cell migration. The early egg chamber consists of oocyte and nurse cells, of germline origin, surrounded by a single layer follicular epithelium of somatic origin (Figure 1A, Stage 8). At each end of this epithelium is a pair of differentiated cells known as the polar follicle cells (Ruohola et al., 1991). During egg chamber maturation the outer follicle cells undergo a series of stereotypic cell movements (Figure 1A, Stage 9). The anterior polar follicle cells signal to their neighbours inducing a partial epithelial to mesenchymal transition (Silver and Montell, 2001). These cells, known as the border cells, delaminate from the epithelium, invade between the nurse cells and migrate to the anterior border of the oocyte carrying the polar follicle cells along with them. Concurrently the outer follicle cells also rearrange such that they all come into contact with the oocyte (Figure 1A, Stage 10, Montell, 2003).
The direction of border cell migration is determined by gradients of ligands for receptor tyrosine kinases, produced in the oocyte (Duchek and Rørth, 2001; Duchek et al., 2001). Within the migrating cluster the border cells remain attached to one another and to the polar follicle cells by stable epithelial junctions (Niewiadomska et al., 1999). The region of the border cells that is linked neither to the polar follicle cells nor to each other appears to be mesenchymal and is motile.
Here we show that planar polarity gene function is required for efficient border cell migration. Our results support a model in which the planar polarity pathway is required both in the polar follicle cells and the border cells, promoting the production of actin-rich protrusions during migration.
Fly culture and crosses were done at 25°C unless indicated. Strains are described in FlyBase except Upd-Gal4 (Tsai and Sun, 2004). Mutant follicle cell clones were induced using the FLP/FRT system (Xu and Rubin, 1993): 1-3 day old females of genotypes hs-FLP; FRT42 stbm6 / FRT42 arm-lacZ or hs-FLP; fz15 FRT80 / arm-lacZ FRT80 were heatshocked for 1 hour twice daily at 37°C for 3 days, then dissected 3-6 days after the last heat shock.
Border cell migration experiments were scored blind. For wholly mutant egg-chambers, all alleles tested (except for the slbo and Wnt4 controls) were crossed out for 10-20 generations to w1118, to provide a common genetic background and w1118 was used as the control. For GAL4/UAS overexpression and RNAi knockdown experiments the controls were siblings lacking the UAS insert or the GAL4 driver, both these controls being significantly different from the experimental samples but not from each other. To test for defects caused by insertion of the RNAi transgenes, lines containing only the slbo-lacZ marker and the insertion were also scored (data not shown). Significance was scored between an experimental line and its control using the significance test for a difference in two proportions (Statistics at Square One, www.bmj.com). The mosaic data was analysed using Chi squared tests, incorporating the Yates’ correction for small sample sizes. Expected values were calculated by counting the frequency of wildtype to mutant cells in each cluster and calculating the probability of a leading cell being wildtype due to random assortment. To quantitate the actin protrusions in border cell clusters, egg-chambers from control and mutant flies were dissected in a single experiment and processed in parallel. Confocal Z-stacks were captured throughout the entire depth of representative clusters for each genotype and the total number of actin protrusions was then counted for 8 clusters selected at random. There was no statistically significant difference between the numbers of protrusions observed between different mutant genotypes (fz21, stbm6 and dsh1), whereas each mutant genotype showed a highly significant difference from the control w1118 chambers (p<10-7, t-test). As with the border cell migration experiments, the mutant alleles had previously been crossed out to the w1118 control stock for 10-20 generations to provide a common genetic background. Mutant chambers were additionally compared to control chambers processed in parallel in at least two independent experiments, and in each case more protrusions were observed in the control chambers than in the mutant chambers. Levels of GFP-RhoA in the cytoplasm versus the membranes of border cells were quantitated from confocal XY sections through border cell clusters, using NIH Image. The average level of GFP-RhoA fluorescence in the border cell cytoplasm was compared to the peak levels of GFP-RhoA fluorescence in the border cell membrane.
RNAi constructs were made in the pWIZ vector (Lee and Carthew, 2003) against the first exon of fz (bp 682-1332, Accession: AY051808), the second exon of stbm (bp 413-1312, Accession: AF044208) and a 1000 bp segment within dsh (bp 705-1728, Accession: AF044208). Sequence analysis showed no off target matches of more than 17 bp for the fz construct, 18 bp for the stbm construct and a single off target match of 20 bp for the dsh construct. The RNAi lines gave the expected loss-of-function phenotypes in wing, eye and notum, accompanied by loss of Fz/Dsh/Stbm immunolabelling as appropriate. As an additional control for specificity, the fz and stbm RNAi phenotypes were also found to be enhanced in backgrounds heterozygous for fz and stbm gene function respectively. To make Actin-EGFP-RhoA the RhoA ORF was tagged at the N-terminus with EGFP and inserted downstream of the Actin5C promoter in pCasper4. This construct recapitulates known RhoA localisation patterns (Magie et al., 2002).
Ovaries were generally dissected, fixed and antibody/X-gal stained as described (Verheyen and Cooley, 1994). To preserve the actin cytoskeleton egg-chambers were dissected and fixed as described in (Frydman and Spradling, 2001), except Schneider’s medium was used in place of Grace’s.
Primary antibodies used were 1:4000 rabbit anti-β-galactosidase (Cappel), 1:100 mouse anti-armadillo-N2 7A1 (DSHB), 1:400 rabbit anti-Strabismus (Rawls and Wolff, 2003), 1:50 mouse anti-Rho-p1D9 (DSHB, Magie et al., 2002), 1:1000 rabbit anti-STAT92E (Chen et al., 2003) and 1:10 rat anti-DE-Cad2 (Oda et al., 1994). Rabbit antibodies against Fz were raised using a His-tagged fusion protein containing residues 40-252, and affinity purified using a GST-tagged fusion protein containing residues 40-240. Secondary antibodies used were anti-rabbit-Alexa-568, anti-rabbit-Alexa-488 and anti-mouse-Alexa-488 at 1:1000 (Molecular Probes), anti-rat-Cy2 and anti-mouse-Cy5 at 1:400 (Jackson). Actin was visualised with Phalloidin-Texas-Red and Phalloidin-FITC at 1:200 (Molecular Probes). Fluorescent images were captured on a Leica SP confocal and processed using NIH Image and Adobe Photoshop.
To test the hypothesis that core planar polarity gene function might be involved in border cell migration we examined egg-chambers from flies carrying mutations in a number of these loci. During Stage 9, when border cell migration is in progress (Fig.1A), we found that mutations in the planar polarity genes fz, stbm, dsh, and pk all caused significant delays (Supplementary Figure 1A). In wildtype egg-chambers border cell migration is usually complete by Stage 10, and at this stage most chambers lacking fz or stbm function had also completed migration (Supplementary Figure 1B), indicating that loss of planar polarity pathway function delays but does not block migration. These results suggest that border cell migration is less efficient without planar polarity pathway function. However, as in these experiments gene function is removed from the entire egg-chamber throughout its development, this does not prove a specific function in the border cell cluster itself.
To address whether core planar polarity gene function is required specifically in the border cell cluster, we used RNAi-mediated knockdown of gene function, coupled with tissue-specific expression under control of the GAL4/UAS system (Brand and Perrimon, 1993). The slbo-GAL4 driver expresses specifically in the border cells upon initiation of migration (dark green cells in Fig.1A) (Rørth et al., 1998). Knockdown of fz, stbm or dsh transcripts using this driver results in a significant delay in border cell cluster migration, relative to the concomitant movement of the outer follicle cells over the egg-chamber surface (Fig.1B). This suggests that planar polarity gene function is required in the border cells for cluster migration to occur efficiently. Two aspects of the phenotype are particularly noteworthy: first, most border cells eventually reach the oocyte (not shown but see Supplementary Figure 1B); second, we never observe guidance defects, such that border cell clusters fail to migrate in the correct direction. Hence we conclude that the previously identified RTK-mediated guidance cues (Duchek and Rørth, 2001; Duchek et al., 2001) are intact, but that the ability of the border cell cluster to efficiently migrate in response to these cues is impaired.
An intriguing feature of planar polarity pathway function in other contexts is that overexpression and loss-of-function of pathway components give similar defects (Krasnow and Adler, 1994; Strutt et al., 1997). Consistent with this, we find that overexpressing either fz or stbm in the border cells results in delayed migration (Fig.1C). This suggests that the planar polarity pathway functions in a similar manner in border cells as in other tissues.
Taken together, the observed delays in border cell migration following three independent methods of altering core planar polarity gene function (i.e. classical loss of function mutations, transcript knockdown by RNAi and overexpression of the gene products), for two independent core polarity genes (fz and stbm), provides strong evidence that the planar polarity pathway is required in border cells for efficient migration.
To further characterise planar polarity gene function in border cells, we used mitotic recombination to generate genetically mosaic clusters. Border cells, polar follicle cells and the nurse cells through which the cluster migrates are derived from different cell lineages (Margolis and Spradling, 1995), therefore it is possible to generate clusters in which a subset of the migratory border cells lack gene function, but the non-migratory polar follicle cells and the substrate nurse cells retain function. It has recently been demonstrated that the relative position of an individual border cell within the migrating cluster is very fluid, with an individual cell potentially able to occupy leading, lateral and lagging roles during migration (Prasad and Montell, 2007). Furthermore, earlier studies have shown that if cells within a cluster lack activity of a gene which is required for proper motility, then these cells will partition to the lagging (anterior) edge of the cluster, whereas cells that retain gene function are found at the leading (posterior) edge (Niewiadomska et al., 1999; Rørth et al., 2000). We examined clusters in which either fz or stbm activity was removed from a subset of border cells. In both cases mutant border cells were predominantly found at the lagging edge of the clusters whereas cells that retained gene function showed a strong preference to migrate at the leading edge (Fig.1D). This confirms that the planar polarity pathway promotes border cell motility, and furthermore demonstrates that pathway function is required autonomously in the border cells themselves.
The planar polarity pathway has been implicated in the regulation of both gene expression and cell fate, as well as modulation of the cytoskeleton (Strutt, 2003). To investigate how planar polarity signalling promotes border cell migration, we examined the expression of factors previously shown to be important for border cell fate and motility.
The transcription factor encoded by the slow border cells (slbo) gene (Montell et al., 1992; Rørth et al., 2000) and the cell adhesion molecule DE-Cadherin (DE-Cad, Niewiadomska et al., 1999) are both required for efficient migration of individual border cells. Therefore it is possible that the planar polarity pathway could regulate the levels of these proteins. However, in egg chambers wholly lacking planar polarity gene function, slbo expression (Fig.2A-D) and DE-Cad expression and subcellular distribution appear normal (Fig.2E-H).
Activity of the transcription factor STAT92E is also required for border cell migration (Silver and Montell, 2001; Silver et al., 2005), and the JAK/STAT pathway has been previously implicated in planar polarity signalling (Zeidler et al., 1999). However, we found that STAT92E expression and nuclear localisation was also normal in border cell clusters lacking fz and stbm function (Fig.2I-K). Thus, judging from these examples, border cell fate and gene expression is normal.
In contrast, examination of the actin cytoskeleton of border cells from mutant egg chambers did reveal significant defects. Wildtype border cells show prominent actin-rich protrusions (Fig.3A). Removal of fz, stbm or dsh activity results in loss of prominent protrusions and a more even actin distribution over the border cell surface (Fig.3B-D). Whereas border cell clusters from the control w1118 stock showed an average of 94.8 protrusions per cluster (n=8), clusters in a fz21 background showed an average of 38.4 protrusions (n=8), stbm6 showed 37.0 (n=8) and dsh1 showed 42.4 (n=8). We observed no clear directional bias, suggesting that the planar polarity pathway affects the frequency but not the orientation of such actin rich protrusions. However, given the complex morphology of the border cell cluster, an effect on protrusion orientation cannot be ruled out.
A similar phenotype was observed upon knockdown of fz transcripts specifically in border cells (Fig.3E). Furthermore, overexpression of fz and stbm in the border cells also disrupted production of large actin protrusions (Fig.3F,G), suggesting that precise levels or spatial distribution of pathway activity is important for correct production of stable actin structures. These results support the view that the motility defects observed in border cells with altered planar polarity pathway function are due to abnormal cytoskeletal dynamics.
In the Drosophila wing and eye, the planar polarity pathway positively regulates the activity of the cytoskeleton modulator RhoA GTPase. Loss of RhoA function leads to defects in both rotation of ommatidial clusters in the eye and production of actin-rich trichomes in the wing (Strutt et al., 1997). Thus RhoA is a good candidate for mediating the effects of the planar polarity pathway in border cells.
We examined the effects of RhoA inactivation and activation in migrating border cells. We find that expression of a dominant negative form of RhoA leads to the normally compact border cell cluster becoming spread out along the anteroposterior axis, with the trailing edges of cells failing to retract towards the cell bodies (Fig.3H). This is consistent with studies in other migrating cells where RhoA is required for retraction of the trailing edge (Raftopoulou and Hall, 2004). Overall border cell migration is strongly delayed and in any particular cluster many cells never reach the oocyte. In contrast, expression of a constitutively active RhoA produced clusters in which the border cells are tightly rounded with no large actin protrusions, indicative of excessive retractive activity (Fig.3K). This also delayed migration, although less severely than expression of dominant negative RhoA, with about 50% of clusters showing an overall delay relative to controls. Consistent with its postive role on RhoA activity in other tissues (Strutt et al., 1997), reduction of planar polarity pathway function is able to ameloriate the effects of RhoA activation, leading to less rounded cells showing obvious actin protrusions (Fig.3L), but does not alter the effects of RhoA inactivation (Fig.3I).
Using either a GFP-RhoA fusion in transgenic flies (Fig.3M), or an antibody against RhoA (data not shown), we observed that in wildtype border cell clusters, RhoA localises with actin at the cell periphery, again consistent with a role regulating the cytoskeleton in these cells. Knockdown of fz activity specifically in the border cells results in a partial redistribution of RhoA to the cytoplasm (compare Fig.3M’ and Fig.3N’). Quantitation showed the relative cytoplasmic levels of Rho-GFP upon fz knockdown to be over 50% higher than in control clusters.
Therefore, we conclude that the planar polarity pathway controls the actin cytoskeleton in border cells and positively regulates RhoA activity, and that RhoA itself is required for normal border cell migration.
Border cell migration depends not only on the motile border cells, but also on the presence of the non-migratory polar follicle cells in the cluster, which form adherens junctions with the border cells and signal to them (Niewiadomska et al., 1999; Han et al., 2000; Silver and Montell, 2001). We investigated whether border cell migration also requires planar polarity gene function in the polar follicle cells, by using RNAi-mediated knockdown of transcripts specifically in these cells. Knockdown of either fz or stbm results in delayed border cell migration, but no delay was observed upon knockdown of dsh (Fig.4A). Recent work in the Drosophila wing has shown that dsh is not required for intercellular communication mediated by the planar polarity pathway, but is necessary to couple such signals to downstream effectors (Strutt and Strutt, 2006). These results suggest that Fz/Stbm-dependent cell-cell communication in the polar follicle cells is required for efficient border cell migration, but that downstream pathway effectors are not required in these non-migratory cells.
To verify the specificity of the requirement for fz and stbm in the polar follicle cells, we again examined genetically mosaic border cell clusters. We obtained border cell clusters in which both polar follicle cells lacked either fz or stbm function, but in which some of the border cells retained activity. The number of such clusters was small, as loss-of-function clones are rarer in the polar follicle cell lineage than in the border cell lineage (Margolis and Spradling, 1995). Contrary to what is observed in mosaic clusters in which both polar cells retain fz or stbm activity, in clusters where both polar follicle cells lack activity we no longer observe a preference for non-mutant border cells to partition to the leading edge of the cluster (Fig.4B). From this we deduce that planar polarity pathway function only confers a migratory advantage on border cells if the polar follicle cells also have fz and stbm function. Therefore, a Fz/Stbm-dependent signal must pass from the polar cells, either directly or indirectly, to the border cells and this signal is required in the border cells for planar polarity pathway function to enhance their migration.
These results indicate a requirement for fz and stbm in the polar follicle cells, but do not address whether the requirement is in one or both cells and whether it requires direct contact between polar follicle cells and responding border cells. Therefore we examined the positions of wildtype and mutant cells within mosaic clusters in which only one polar cell retained fz or stbm activity.
In such mosaic clusters lacking fz activity in one polar follicle cell, we made an important observation (Fig.4C): The polar cell that retains fz function is always positioned towards the leading edge of the cluster contacting the leading border cells, whereas the fz mutant polar cell is always positioned towards the lagging end of the cluster contacting the lagging border cells. This is independent of the genotype of the border cells. Consequently, border cells are positioned within the cluster according to the genotype of the polar cell with which they make junctional contact. Thus we conclude that motility of either wildtype or fz mutant border cells is enhanced by contact with a fz-expressing polar cell.
Examination of mosaic clusters containing one polar follicle cell lacking stbm function revealed a different requirement for stbm activity. In the small number of such clusters we obtained, we found that the polar cell which retained stbm activity could contact either the leading or the lagging border cells, and that stbm mutant border cells always lag at the back of the cluster (Fig.4D). Thus, unlike the situation observed for fz, there is no migratory advantage conferred on a border cell that is in contact with a stbm-expressing polar cell. However, as the wildtype border cells are always at the leading edge of the clusters, unlike in clusters that lack stbm activity in both polar cells (Fig.4B), we can conclude that stbm activity in at least one polar cell does confer increased motility on stbm-expressing border cells but that this effect is not contact-dependent.
We can summarise our findings from the mosaic analysis data as follows: In clusters that retain fz and stbm activity in the non-migratory polar follicle cells, border cell motility is cell-autonomously enhanced by fz and stbm function. Second, this enhancement of border cell motility by planar polarity pathway function requires direct contact with a fz expressing polar follicle cell. Thirdly, this enhancement also requires stbm activity in at least one one polar follicle cell, but this cell does not need to directly contact a border cell to enhance its migration.
In other contexts such as the eye and wing, Fz and Stbm are believed to mediate intercellular communication via formation of asymmetric protein complexes at the adherens junctions, in which Fz in one cell is juxtaposed with Stbm in the neighbouring cell (Strutt, 2001; Bastock et al., 2003). As the polar cells contact each other and the border cells via an adherens junction-like region (Niewiadomska et al., 1999), we asked whether Fz and Stbm localise to this region, which would be consistent with the Fz/Stbm-dependent signalling that our mosaic analysis revealed between these cells.
Immunolabelling of border cell clusters prior to migration (late Stage 8) or during migration (Stage 9) reveals the presence of Fz in the adherens junction region joining the polar and border cells (Fig.5A,C). Similarly, Stbm was also seen localised to the junctional region at these stages (Fig.5E,G), consistent with a role mediating intercellular communication.
We also observe Fz (but not Stbm) localised to the migratory edges of the border cells, both prior to and during migration (Fig.5B,D). Fz localisation has not previously been observed in migrating cells, but tagged forms of the Xenopus Dsh homologue (a Fz-binding partner) are enriched at the actin-rich bilateral tips of elongating cells undergoing convergent extension (Kinoshita et al., 2003). Using available reagents (Shimada et al., 2001; Strutt et al., 2006) we were unable to detect a specific distribution of endogenous Dsh within migrating border cell clusters (data not shown). Therefore we investigated the distribution of a Dsh-GFP fusion protein which accurately reflects the junctional distribution of Dsh in the Drosophila pupal wing (Axelrod, 2001). We found that this protein accumulated at high levels in a punctate pattern in the cytoplasm of both polar follicle cells and border cells (Fig.5F,H). Although there was evidence of enrichment in the junctional region of the clusters, this was partly obscured by the high cytoplasmic levels. Similarly we were unable to determine if Dsh-GFP was specifically localised to the migratory edges of the border cells.
The planar polarity pathway involving Fz/Stbm is implicated in an increasing number of processes in development, organogenesis and disease. These include cell rearrangements during gastrulation, neural crest migration, neuronal pathfinding, sensory hair orientation, heart formation and cell invasion in cancer (Curtin et al., 2003; De Calisto et al., 2005; Heisenberg et al., 2000; Jessen et al., 2002; Montcouquiol et al., 2003; Wallingford et al., 2000; Phillips et al., 2005; Weeraratna et al., 2002). Many of these processes involve coordinated cell movement, and yet this aspect of planar polarity pathway function is poorly understood.
Here we use the Drosophila ovary to study control of coordinated cell movements by the planar polarity pathway, taking advantage of its relative simplicity and the ability to precisely manipulate gene function in individual cell populations. Activity of the core polarity genes facilitates invasive migration of the border cell cluster through the nurse cells. Of particular interest is our observation that migration of the border cells is enhanced by planar polarity activity in the non-migratory epithelial polar follicle cells, suggesting a key role for interactions between migratory and non-migratory cell types.
In the Drosophila wing, the planar polarity pathway regionalises cells through formation of proximal and distal domains at the level of the adherens junctions. The distal domain contains Fz (Strutt, 2001) which acts via the downstream factors Dsh (Axelrod, 2001) and RhoA (Strutt et al., 1997) to ensure local production of a single actin-rich trichome, while in the proximal domain, Stbm (Bastock et al., 2003) recruits factors that locally inhibit trichome formation (Adler et al., 2004) (Fig.6). During border cell migration, the coordinated movement of the non-migratory polar follicle cells and the migratory border cells is achieved in part by the border cells retaining epithelial character in the region contacting the polar follicle cells, but also having an actin-rich partly mesenchymal migratory region (Niewiadomska et al., 1999). Taking these observations together, we propose that in border cells localised Fz in the migratory region and localised Stbm in the junctional region might promote production of actin-rich structures (Fig.6), which in turn would increase the motility of both individual cells and the cluster as a whole.
Our mosaic analyses suggest a mechanism for how this localised Fz and Stbm activity is established within the border cells. Fz and Stbm mediate intercellular communication between the polar cells and the border cells via production of junctional complexes. As contact with a Fz-expressing polar cell enhances migration of border cells, we surmise that Fz in each polar cell interacts with Stbm in the contacting border cell. Junctionally localised Stbm in the border cell can then act as a cue to indirectly promote actin rich protrusion formation in the migratory region, at least in part through localisation of Fz (Fig.6).
Although the planar polarity pathway has been known for some years to promote cell rearrangements during vertebrate gastrulation (Wallingford et al., 2002), surprisingly little is understood about its mechanisms of action in cell movement and the particular roles of cell-cell communication. We have demonstrated that Fz/Stbm-mediated intercellular communication can enhance the invasive migration of a group of cells. Migration of groups of cells, sometimes including both motile and non-motile types, is important for many processes in animal morphogenesis and in disease processes such as cancer metastasis (Friedl, 2004; Lecaudey and Gilmour, 2006). Our work provides evidence that planar polarity pathway function could be generally important in coordinated cell migration, providing a mechanism by which cells within a group can communicate and establish the proper regional production of actin structures required for efficient movement.
(A) Chart showing extent of border cell migration relative to outer follicle cell rearrangement at Stage 9 in different mutant backgrounds. Loss of function alleles of core planar polarity genes were crossed out to a w1118 “wildtype” strain for several generations to provide a common genetic background. Border cell migration was scored relative to rearrangement of the outer follicle cells (the scoring scheme is as described in legend to Fig.1). Both a medium strength (fz25) and strong allele (fz21) of fz show a significant increase in slower migrating clusters (“behind”). Similarly, the dsh1 planar polarity specific allele of dsh and the stbm6 and pkpk-sple-13 null alleles also show significant delays in border cell migration.
As controls we examined the effect on border cell migration of loss of Wnt4 (using the heteroallelic combination of Wnt4C1/Wnt4EMS23) or slow border cells (slbo). Wnt4 has previously been reported to affect earlier stages of ovary morphogenesis via the frizzled homologue Dfrizzled2, but does not affect planar polarity signalling (Cohen et al., 2002). Consistent with this, border cell migration is not delayed in this genotype. Conversely, slbo function is absolutely required for border cell migration (Montell et al., 1992), and in this genotype all clusters remain in an anterior position.
(B) Chart showing proportion of border cell clusters that have completed posterior migration by Stage 10 in egg-chambers from w1118 (n = 595), fz21 (n = 350) or stbm6 (n = 301) mutant individuals. Thus planar polarity gene function is not absolutely required for completion of posterior border cell migration.
We are grateful to Paul Adler, Jeff Axelrod, Richard Carthew, Steven Hou, Tadashi Uemura, Tanya Wolff, the Bloomington Stock Centre and the Developmental Studies Hybridoma Bank for providing reagents. We thank Jenifer Mundy and Dawn Biram for assistance with antibody preparation and Katy Cooper for Act-EGFP-RhoA flies. Advice and helpful comments on the manuscript were provided by Nick Brown, Andrew Furley, Mary Ann Price, Helen Strutt, Steve Winder and Martin Zeidler. This work was supported by an MRC Studentship to R.B., D.S. is a Wellcome Trust Senior Fellow, confocal facilities were provided by Yorkshire Cancer Research.