Roughened: Biological Overview | References
Gene name - Rap1 GTPase
Synonyms - Rap1, Dras3, roughened
Cytological map position 62B7-62B7
Function - signal transduction
Keywords - gastrulation, regulation of cell shape; germ-line stem cell maintenance; dorsal closure; mesoderm - hemocyte migration; Rap protein signal transduction; cell adhesion; establishment of ommatidial planar polarity; substrate-dependent cell migration, cell extension.
Symbol - Rap1
FlyBase ID: FBgn0004636
Genetic map position - chr3L:1,859,114-1,862,234
Classification - Ras_like_GTPase
Cellular location - cytoplasmic
|Recent literature||Marada, S., Truong, A. and Ogden, S. K. (2015). The small GTPase Rap1 is a modulator of Hedgehog signaling. Dev Biol [Epub ahead of print]. PubMed ID: 26481064
During development, the evolutionarily conserved Hedgehog (Hh) morphogen provides instructional cues that influence cell fate, cell affinity and tissue morphogenesis. To do so, the Hh signaling cascade must coordinate its activity with other morphogenetic signals. This can occur through engagement of or response to effectors that do not typically function as core Hh pathway components. Given the ability of small G proteins of the Ras family to impact cell survival, differentiation, growth and adhesion, it was of interest to determine whether Hh and Ras signaling might intersect during development. Genetic modifier tests were performed in Drosophila to examine the ability of select Ras family members to influence Hh signal output, and Rap1 was identified as a positive modulator of Hh pathway activity. The results suggest that Rap1 is activated to its GTP-bound form in response to Hh ligand, and that the GTPase exchange factor C3G likely contributes to this activation. The Rap1 effector Canoe (Cno) also impacts Hh signal output, suggesting that a C3G-Rap1-Cno axis intersects the Hh pathway during tissue morphogenesis.
|Yang, D.S., Roh, S. and Jeong, S. (2016). The axon guidance function of Rap1 small GTPase is independent of PlexA RasGAP activity in Drosophila. Dev Biol [Epub ahead of print]. PubMed ID: 27565025
Plexins (Plexs) comprise a large family of cell surface receptors for semaphorins (Semas) that function as evolutionarily conserved guidance molecules. GTPase activating protein (GAP) activity for Ras family small GTPases has been implicated in plexin signaling cascades through its RasGAP domain. However, little is known about how Ras family GTPases are controlled in vivo by plexin signaling. This study found that Drosophila Rap1, a member of the Ras family of GTPases, plays an important role controlling intersegmental nerve b motor axon guidance during neural development. Gain-of-function studies using dominant-negative and constitutively active forms of Rap1 indicate that Rap1 contributes to axonal growth and guidance. Genetic interaction analyses demonstrate that the Sema-1a/PlexA-mediated repulsive guidance function is regulated positively by Rap1. Furthermore, neuronal expression of mutant PlexA robustly restores defasciculation defects in PlexA null mutants when the catalytic arginine fingers of the PlexA RasGAP domain critical for GAP activity are disrupted. However, deleting the RasGAP domain abolishes the ability of PlexA to rescue the PlexA guidance phenotypes. These findings suggest that PlexA-mediated motor axon guidance is dependent on the presence of the PlexA RasGAP domain, but not on its GAP activity toward Ras family small GTPases.
The establishment and maintenance of apical-basal cell polarity is critical for assembling epithelia and maintaining organ architecture. Drosophila embryos provide a superb model. In the current view, apically positioned Bazooka/Par3 is the initial polarity cue as cells form during cellularization. Bazooka then helps to position both adherens junctions and atypical protein kinase C (aPKC). Although a polarized cytoskeleton is critical for Bazooka positioning, proteins mediating this remained unknown. This study found that the small GTPase Rap1 and the actin-junctional linker Canoe/afadin are essential for polarity establishment, as both adherens junctions and Bazooka are mispositioned in their absence. Rap1 and Canoe do not simply organize the cytoskeleton, as actin and microtubules become properly polarized in their absence. Canoe can recruit Bazooka when ectopically expressed, but they do not obligatorily colocalize. Rap1 and Canoe play continuing roles in Bazooka localization during gastrulation, but other polarity cues partially restore apical Bazooka in the absence of Rap1 or Canoe. The current linear model for polarity establishment was tested. Both Bazooka and aPKC regulate Canoe localization despite being 'downstream' of Canoe. Further, Rap1, Bazooka, and aPKC, but not Canoe, regulate columnar cell shape. These data suggest that polarity establishment is regulated by a protein network rather than a linear pathway (Choi, 2013).
Polarity is a fundamental property of all cells, from polarized cell divisions in bacteria or fungi to the elaborate polarity of neurons. Among the most intensely studied forms of polarity in animal cells is epithelial apical-basal polarity. Polarity of epithelial sheets is key to their function as barriers between body compartments, and is also critical in collective cell migration and cell shape change during morphogenesis, as cytoskeletal and apical-basal polarity often go hand in hand. Loss of apical-basal polarity is a hallmark of metastasis. Significant advances have been made in defining the machinery required for cell polarity in many settings, but fundamental questions remain unanswered (Choi, 2013).
Cadherin-catenin complexes, which assemble into adherens junctions (AJs) near the apical end of the lateral cell interface, are critical polarity landmarks that define the boundary between apical and basolateral domains. Studies in C.elegans and Drosophila identified other key regulators of apical-basal polarity. In the textbook view, the apical domain is defined by the Par3/Par6/aPKC and Crumbs/Stardust(Pals1)/ PATJ complexes, while Scribble, Dlg, Lgl, and Par1 define the basolateral membrane (Choi, 2013).
Complex cross-regulatory interactions between apical and basolateral proteins maintain these mutually exclusive membrane territories. These proteins also regulate other types of polarity during morphogenesis; e.g., fly Par3 (Bazooka; Baz), aPKC, and AJ proteins are planar-polarized during fly convergent extension, thus regulating polarized cell movements (Choi, 2013).
Polarized cytoskeletal networks also play key roles in establishing and maintaining apical-basal and planar polarity. These networks are thought to be physically linked to apical junctional complexes. The earlier model suggesting that cadherin-catenin complexes link directly to actin via α-catenin is now viewed as over-simplified. Instead, different proteins are thought to mediate this connection in different tissues and at different times (Choi, 2013).
Among the linkers is Canoe (Cno)/Afadin, an actin-binding protein that binds transmembrane nectins via its PDZ domain. While originally hypothesized to be essential for cell adhesion, subsequent work supports a model in which afadin modulates adhesive and cytoskeletal machinery during cell migration in vitro (see Fournier, 2011) and the complex events of mouse gastrulation. Afadin has two N-terminal Ras association domains for which the small GTPase Rap1 is the major binding partner (Linnemann, 1999), and Afadin and Rap1 are functionally linked in both flies and mice (Boettner, 2003; Hoshino, 2005). Rap1, Cno, and the Rap1 GEF Dizzy/PDZGEF are all essential for maintaining effective linkage between AJs and the apical actomyosin cytoskeleton during apical constriction of Drosophila mesodermal cells during fly gastrulation (Sawyer, 2009; Spahn, 2012). Rap1 regulates Cno localization to the membrane (Sawyer, 2009). Cno plays a related role during convergent extension, though its role is planar polarized during this process (Sawyer, 2011). Cno also regulates collective cell migration, signaling, and oriented asymmetric divisions. The Rap1/Cno regulatory module is also important in disease, as Afadin and Rap1 are implicated in congenital disorders of the cardiovascular system (Glading, 2007) and cancer metastasis (Fournier, 2011). It remains unclear whether these diverse roles all involve junction-cytoskeletal linkage or whether some are independent functions (Choi, 2013).
The small GTPase Rap1 plays diverse cellular roles. Mammalian Rap1 isoforms are perhaps best known for regulating integrin-based cell matrix adhesion (Bos, 2005; Kim, 2011), but Rap1 also regulates cell-cell AJs in both Drosophila and mice (Kooistra, 2007; Boettner, 2009). In murine endothelial cells, for example, Rap1, its effector Krit1, and VE-cadherin form a complex that regulates endothelial cell junctions and stabilizes apical-basal polarity (Glading, 2007; Lampugnani, 2010; Liu, 2011; Choi, 2013 and references therein).
In Drosophila imaginal disc cells, Rap1 regulates the symmetric distribution of DE-cadherin (DEcad) around the apical circumference of each cell (Knox, 2002). Rap1 carries out these functions via a diverse set of effector proteins, including Krit1, TIAM, RIAM, and Cno/Afadin (Kooistra, 2007; Boettner, 2009). Thus, Rap1 and its effectors are candidate proteins for regulating interactions between AJs, polarity proteins and the cytoskeleton during polarity establishment and maintenance (Choi, 2013).
The early Drosophila embryo provides among the best models of establishing and maintaining apical-basal polarity. Flies start embryogenesis as a syncytium, with 13 rounds of nuclear division without cytokinesis. Membranes then simultaneously invaginate around each nucleus, forming ~6000 cells in a process known as cellularization. Prior to cellularization, the egg membrane is already polarized and serves as a polarity cue for underlying nuclei. This ultimately becomes the apical end of the new cells. Epithelial apical-basal polarity is initiated during cellularization. In the absence of cadherin-catenin complexes, cells form normally but then lose adhesion and polarity as gastrulation begins. These data and earlier work from cell culture suggested AJs are the initial apical cue. However, it was found that Bazooka (Baz)/Par3 acts upstream of AJs in this process. Strikingly, Baz and DEcad apically co-localize in spot AJs from cellularization onset. In the absence of Baz, DEcad loses its apical enrichment and redistributes all along the lateral membrane, while in the absence of AJ proteins, Baz remains apically localized, and a subset of cells retain residual apical-basal polarity, although cell shapes are highly abnormal. Cadherin-catenin and Baz complexes form independently before cellularization, and Baz then helps position DEcad in the apicolateral position where spot AJs will form. This placed Baz atop of the polarization network, raising the question of how it is positioned apically. Two cytoskeletal networks play important roles in initial Baz positioning (Choi, 2013).
Disrupting dynein led to Baz spreading along the lateral membrane, suggesting polarized transport along microtubules (MTs) plays a role. Depolymerizing actin also destabilized apical Baz, as did significantly overexpressing Baz, suggesting an actin-based scaffold with a saturable number of binding sites anchors Baz apically. While both actin and MTs are required for initial Baz polarization, they are not the only cues. Mislocalized Baz is re-recruited or re-stabilized apically at gastrulation onset if either initial cue is disrupted, suggesting a third cue perhaps involving aPKC/Par6 or Par1. Thus, the current model for initial establishment of apical-basal polarity involves a relatively simple pathway in which Baz is positioned apically, and then positions other apical polarity players. However, once initial polarity is established, events become more complex, with a network of mutually reinforcing and inhibitory interactions between apical and basolateral polarity complexes leading to polarity elaboration and maintenance. These were significant advances, but the proteins directing apical accumulation of Baz remained unknown. Work on apical constriction in the fly mesoderm, convergent extension during gastrulation, establishment of anteriorposterior polarity in one cell C. elegans embryos, and on apically constricting Drosophila amnioserosal cells, suggested that a complex network of interactions link AJs, the apical polarity proteins Baz and aPKC, and the actomyosin cytoskeleton. Recent work on Canoe and Rap1's roles in mesoderm apical constriction (Sawyer, 2009) and convergent elongation (Sawyer, 2011) suggested they also fit into this network. These data led to an exploration of whether Rap1 and Cno play roles in initial apical positioning of AJs and Baz and thus in the establishment and early maintenance of polarity (Choi, 2013).
In regulating polarity establishment, Rap1 and Cno could act by several possible mechanisms. Their role in AJ positioning may be solely due to their effects on Baz localization, or alternatively Rap1 and Cno may independently affect the localization of both Baz and AJs. In the latter case, Cno may directly link AJs to the apical actin scaffold, as it was suggested to act in apical constriction (Sawyer, 2009). Rap1 and Cno also clearly regulate Baz positioning. Since Baz apical positioning requires an apical actin scaffold and dynein based MT transport (Harris, 2005), whether Rap1 and Cno act indirectly by regulating cytoskeletal organization was examined. However, the data suggest this is not the case: both the MT and actomyosin cytoskeletons appear normal in mutants. Thus the most likely model is that Rap1 and Cno are required for anchoring Baz apically. Consistent with this, when Cno was ectopically localized to artificial cell-cell contacts in cultured fly cells, it was able to recruit Baz to that site. This could occur directly, for example, by Cno binding Baz, or indirectly, via unknown intermediaries. Strikingly, however, when Baz was over-expressed in cellularizing embryos, presumably saturating its apical binding sites, it accumulated basolaterally and recruited DEcad but not Cno to these ectopic sites. Thus Cno and Baz do not co-localize obligatorily. It likely that each has multiple binding partners and that when pools are limiting, as Cno may be in this latter experiment, ectopic Baz cannot recruit Cno away from a preferred binding site. Of course, it remains possible that Cno and Rap1 also regulate Baz positioning through effects on MT transport or, given Cno's apical localization, unloading at an apical docking site. It will be important to test these possibilities. As is discussed in more detail below, it will also be important to define the Cno- and Rap1-independent mechanisms that partially restore apical Baz localization after gastrulation onset (Choi, 2013).
Since Rap1 is uniformly distributed along the apical-basal axis during cellularization (Sawyer, 2009), the most likely hypothesis is that it is locally activated apically by a GEF. A number of Rap1GEFs exist, many of which are conserved between mammals and flies. Recent work from the Reuter lab demonstrated that, like Cno and Rap1 (Sawyer, 2009), the Rap1 GEF Dizzy (Dzy/PDZ-GEF) plays an important role in coordinated mesodermal apical constriction (Spahn, 2012), suggesting it is the GEF acting upstream of Cno and Rap1 in that process. They also suggest that Rap1 and Dzy help regulate establishment of AJs (Spahn, 2012). While similar in outline, their analysis of AJs differs from this one in detail, as they see strong effects on DEcad localization without similar effects on Arm. This is surprising, since these two proteins of the cadherin-catenin complex generally localize very similarly at the cortex. However, these differences aside, their data are consistent with Dzy acting with Cno and Rap1 in AJ establishment-it will be important to examine the effects of Dzy on Baz localization. It will also be important to determine how pre-existing egg membrane polarity is translated into localized Rap1 activity (Choi, 2013).
In addition to the parallel roles of Rap1 and Cno in regulating initial apical-basal polarization, this study identified a second role for Rap1 in establishing and maintaining columnar cell shape. The data suggest that this is partially or completely Cno-independent, and thus one of the many other Rap1 effectors may play a role in this process. It will be exciting to examine embryos mutant for other Rap1 effectors (Kooistra, 2007), such as Krit1/Bili, TIAM/Stilllife, RIAM/Pico, or RhoL to see if they are required for establishing columnar cell shape. baz and aPKC mutants also had defects in establishing columnar cell architecture. It is possible that each protein provides an independent mechanistic input into this process. This is consistent with the observed differences in the details of how columnar cell shape is disrupted, with Baz and aPKC primarily regulating apical cell area, while Rap1 affects cell shape at multiple apical-basal positions. A more speculative but perhaps less likely possibility is that Rap1 uses Baz and aPKC as effectors in establishing columnar cell shape. Fly Rap1 can form a complex with aPKC and Par6 (Carmena, 2011), and Rap1 acts upstream of cdc42/Par3/aPKC in regulating polarity of cultured neurons (Schwamborn, 2004; Choi, 2013 and references therein).
Having identified Rap1's direct effector(s) in regulating cell shape, it is necessary to move downstream. Based on analogies with other epithelial tissues in fly development, it is hypothesized establishing columnar cell shape involves regulating apical tension. Other small GTPases play key roles in this; e.g., Rho and cdc42 have striking and opposing roles in apical tension regulation during fly eye development. In that context, Rho acts via separate effectors to maintain AJs and apical tension-it regulates tension via Rok, Diaphanous, and ultimately myosin contractility. It will be interesting to determine whether the defects in apical cell shape in the absence of Rap1, Baz, or aPKC also reflect unbalanced contractility in different nascent cells, and which contractility regulators are involved. However, for now, this is speculative (Choi, 2013).
Previous work has suggested a linear hierarchy regulating polarity establishment, with Baz at the top, positioning AJs and aPKC (Harris, 2004, Harris, 2005). The current work extends this hierarchy, positioning Rap1 and Cno upstream of Baz in this process. However, the data further suggest that viewing polarity establishment as a linear process is significantly over-simplified. It is now known that all of the relevant players -- including the AJ proteins, Baz, Cno and aPKC -- are at the cortex in syncytial embryos, prior to cellularization and the initiation of apical-basal polarity. This places them in position to cross-regulate one another. Consistent with this, the data suggest that viewing relationships with an 'upstream-downstream' point of view misses important reciprocal interactions that occur as polarity is established. Two examples point this out most clearly. First, earlier work suggested that localization of aPKC occurs 'downstream' of Baz, as apical positioning of aPKC at gastrulation onset requires Baz function (Harris, 2005). The new data reveal that Rap1 and Cno are, in turn, 'upstream' of Baz, and thus, if things work in a strictly linear fashion, Rap1 and Cno should be 'upstream' of aPKC. However, in contrast to this simple view, this study found that precise positioning of Cno during cellularization requires aPKC - in its absence, Cno is not cleared from the apical region, and the apical-basal cables of Cno at tricellular junctions are not properly assembled. In a similar fashion, Baz, which in a linear model is 'downstream' of Cno, also regulates precise positioning of Cno during cellularization. aPKC and Baz also play important roles in Cno localization during the early polarity maintenance phase beginning at gastrulation onset. Together, these data suggest that initial positioning of proteins along the apical-basal axis involves a network of protein interactions, similar to that previously suggested to regulate polarity elaboration during the extended germband phase and beyond, as cells develop the full suite of epithelial junctions. It will now be important to define mechanisms by which aPKC and Baz act to precisely position Cno: two broad possibilities are that they act on Cno directly, or that they modulate the fine scale architecture of the actin cytoskeleton, with indirect effects on Cno. It will also be exciting to determine if other polarity determinants, like the basolateral proteins Discs Large, Scribble or Lgl, or the basolateral kinase Par1 also play roles in polarity establishment, as they do in polarity maintenance. Consistent with this possibility, recent work from the Harris lab suggests Par1 is important for the gastrulation onset rescue of Baz localization in embryos in which early cues are disrupted (McKinley, 2012). Finally, it will be interesting to identify the cues that come into play at gastrulation onset, which partially restore apical Baz localization, as part of the increasingly complex network of partially redundant regulatory cues that give polarity its robustness (Choi, 2013).
Localized cell shape change initiates epithelial folding, while neighboring cell invagination determines the final depth of an epithelial fold. The mechanism that controls the extent of invagination remains unknown. During Drosophila gastrulation, a higher number of cells undergo invagination to form the deep posterior dorsal fold, whereas far fewer cells become incorporated into the initially very similar anterior dorsal fold. A decrease in α-catenin activity causes the anterior fold to invaginate as extensively as the posterior fold. In contrast, constitutive activation of the small GTPase Rap1 restricts invagination of both dorsal folds in an α-catenin-dependent manner. Rap1 activity appears spatially modulated by Rapgap1, whose expression levels are high in the cells that flank the posterior fold but low in the anterior fold. A model is proposed whereby distinct activity states of Rap1 modulate α-catenin-dependent coupling between junctions and actin to control the extent of epithelial invagination (Wang, 2013).
This study used the dorsal fold system to investigate whether specific cellular mechanisms actively regulate the extent of epithelial invagination. α-catenin was shown to be required for the restricted invagination caused by constitutive activation of Rap1, and Rapgap1 was identified as a locally expressed modulator of Rap1 that is required for the extensive invagination of the posterior fold. These data suggest a model whereby Rap1 regulates dorsal fold invagination through an α-catenin-dependent process and establish that differential regulation of an active, specific cellular mechanism confers distinct properties to the neighboring cells to control the extent of epithelial invagination (Wang, 2013).
Genetic analysis identifies two separate functions of Rap1 during dorsal fold formation. The early function appears to be a general role required in all cells that is important for junctional positioning. This was established via examination of embryos that lack Rap1 activity, such as embryos that are produced by the germline clones of null alleles of Rap1 or dizzy, which encodes the Drosophila homolog of PDZ-GEF, a known guanine nucleotide exchange factor that activates Rap1 or embryos that overexpress a GDP-locked, dominant-negative form of Rap1, Rap1N17 (see Spahn, 2012). These embryos display normal assembly of the adherens junctions, the initial basal shift of junction positioning in the initiating cells, and attempt to form dorsal folds. Subsequently, however, the junctions relocalize to the apical surface in all dorsal cells, reversing these initial attempts of dorsal fold formation and eliminating all folding structures (Wang, 2013).
Rap1 appears to maintain the junctional positioning by maintaining the junctional levels of Bazooka. This notion is supported by the lower levels of junctional Bazooka in the Rap1 mutant embryos and by suppression of the loss-of-function phenotype of Rap1 by Bazooka overexpression, which restores the apical domain in the initiating cells and the dorsal fold structures in the Rap1 mutant embryos. Because Bazooka levels are uniform across the dorsal epithelium), this early function of Rap1 appears broadly required, independently of the levels of Rapgap1 expression, and operates in addition to Rap1's later role during epithelial invagination. The effective suppression of Rap1 loss of function following Bazooka overexpression suggests that the two separate functions of Rap1 (the maintenance of Bazooka levels and the regulation of junction-actin connection during epithelial invagination) could be decoupled, allowing comparison of the effect of loss of Rap1 function to that of constitutively active Rap1V12 (Wang, 2013).
The later, spatially regulated function of Rap1 is independent of Bazooka and is differentially modulated by the spatially restricted expression of Rapgap1. Since active Rap1 appears to act through α-catenin to inhibit invagination, it seems plausible that distinct Rap1 activity states modulate the coupling strength between junctions and actin, thereby conferring distinct properties of junctional restructuring to the neighboring cells of the anterior and posterior folds. The geometric measurements of the neighboring cells suggest a model whereby constitutively active Rap1 inhibits junctional mobility so that the size of the apical domain remains constant in the cells surrounding the anterior fold where Rapgap1 levels are low. In contrast, Rapgap1 expression modulates Rap1 activity to promote junctional mobility in the neighboring cells of the posterior fold so that their apical domain expands. In this view, both the initiation and invagination processes require active remodeling of the junctions, but differ in their underlying cellular mechanisms. During initiation, the junctional shift is induced by a modification of the epithelial apical-basal polarity as a result of the downregulation of Par-1 in the initiating cells (Wang, 2012). During invagination, since Par-1 levels do not decrease, mechanical stress might be the dominant force that causes the junctions to move in the neighboring cells (Wang, 2013).
How Rap1 modulates α-catenin-dependent junction-actin coupling remains unknown. The intensities, localization, and turnover kinetics (as measured by fluorescent recovery after photobleaching) were examined of the core junctional components (E-Cadherin and Armadillo), α-catenin, two junctional proteins that interact with both α-catenin, and actin, but no difference between the neighboring cells of the anterior and posterior folds was detected. Recent work in mammalian tissue culture cells showed that the FRET (fluorescent resonance energy transfer) intensities of an E-Cadherin tension sensor correlate with the actin-coupling states of adherens junctions (Borghi, 2012). The use of such a sensor in the living Drosophila embryo might help to reveal the difference in junction-actin coupling states between the anterior and posterior fold neighboring cells (Wang, 2013).
Recent work suggests that α-catenin undergoes a conformational change upon mechanical stretch at the cell junctions. Such conformational change could in principle relieve α-catenin from an intramolecular inhibition on actin binding, thereby increasing its affinity to, or stabilizing its interaction with, the junctional actin. Changes in α-catenin conformation thus may determine its ability to mediate the physical coupling between junctions and actin. It is of note that expression of a mutant form of α-catenin that lacks the domain that modulates its conformational change can support static junctional function, but fails to effectively rescue the loss-of-α-catenin phenotype in dynamic morphogenetic processes. It is possible that mechanical forces during morphogenesis dynamically modulate the conformational states of α-catenin, the maintenance of which may require distinct Rap1 activity states. The dynamic changes of the α-catenin conformations and the actin-coupling states of adherens junctions that they confer might be crucial for morphogenetic processes that involve extensive restructuring of cell-cell adhesion (Wang, 2013).
If Rapgap1 dictates the spatial extent of cell invagination, one simple model would envision that elevating the levels of Rapgap1 expression in the anterior region could promote anterior fold invagination. This possibility was explored using a UAS transgene to uniformly express Rapgap1 under the control of a maternal Gal4 driver. Two classes of phenotypes were observed: either a complete loss of dorsal fold formation or a limited degree of invagination similarly in both dorsal folds. The former class suggests that the level of expression may be too high to permit the normal function of Rap1, while the latter class suggests that a reversal of the expression pattern (high in the anterior fold, but low in the posterior fold) might be necessary. Attempts were made to express Rapgap1 in the cells anterior to the anterior fold using a Gal4 driver localized through the 3' UTR of the bicoid gene, and the enhancer of the Kr gene was also used to direct the expression of Rapgap1 in cells that are posterior to the anterior fold. In neither case was there an effect on anterior fold invagination. It is possible that driving extensive invagination for the anterior fold would require that Rapgap1 be expressed only in the surrounding cells of the anterior fold in a manner that mimics the endogenous pattern of Rapgap1 expression in the region of posterior fold. Currently, no cis-regulatory element or a Gal4 driver has been found that can drive gene expression in such a specific pattern. Thus, it remains unresolved whether ectopic expression in the anterior fold region would be sufficient to cause extensive invagination (Wang, 2013).
In summary, the data suggest an exciting conceptual framework in which regulated coupling between junctions and actin has a profound impact on the levels of tissue reorganization and on the cellular responses to mechanical stresses that arise during tissue reorganization. This study has defined a specific molecular pathway that produces drastically different epithelial structures from a morphogenetic process whose initiation mechanism appears similar. The regulatory principles that were unveil for Rap1 and α-catenin might be employed in other contexts of morphogenesis in which a tissue undergoes dramatic remodeling, while unperturbed tissue integrity and cell adhesion must be maintained (Wang, 2013).
Mechanisms that govern cell-fate specification within developing epithelia have been intensely investigated, with many of the critical intercellular signaling pathways identified, and well characterized. Much less is known, however, about downstream events that drive the morphological differentiation of these cells, once their fate has been determined. In the Drosophila wing-blade epithelium, two cell types predominate: vein and intervein. After cell proliferation is complete and adhesive cell-cell contacts have been refined, the vast majority of intervein cells adopt a hexagonal morphology. Within vein territories, however, cell-shape refinement results in trapezoids. Signaling events that differentiate between vein and intervein cell fates are well understood, but the genetic pathways underlying vein/intervein cyto-architectural differences remain largely undescribed. This study shows that the Rap1 (Roughened) GTPase plays a critical role in determining cell-type-specific morphologies within the developing wing epithelium. Rap1, together with its effector Canoe, promotes symmetric distribution of the adhesion molecule DE-cadherin about the apicolateral circumference of epithelial cells. Evidence is provided that in presumptive vein tissue Rap1/Canoe activity is down-regulated, resulting in adhesive asymmetries and non-hexagonal cell morphologies. In particular Canoe levels are reduced in vein cells as they morphologically differentiate. It was also demonstrate that over-expression of Rap1 disrupts vein formation both in the developing epithelium and the adult wing blade. Therefore, vein/intervein morphological differences result, at least in part, from the patterned regulation of Rap1 activity (O'Keefe, 2012).
During the early, proliferative phase of epithelial development each cell strives to maintain adhesive contacts with its neighbors, generating, on average, a field of hexagonal-shaped cells. This uniformity is transient, however, as multiple cell types are frequently specified within a single epithelium, each with a unique function and cyto-architecture. Mechanisms must exist, therefore, for cell-type-specific shapes to emerge as these heterogeneous epithelia begin to morphologically differentiate. This study shows that in the Drosophila wing the regulation of Rap1 activity is one means by which non-hexagonal epithelial cell shapes are generated (O'Keefe, 2012).
These studies have focused on the Drosophila wing vein. Within the wing blade, veins comprise a small subset of cells, and during pupal stages of development it was shown that vein-precursor cells adopt a unique shape (trapezoidal), compared to surrounding intervein cells (hexagonal). Presumptive vein cells are first identified by high levels of Egfr activity, and previous studies have shown that Egfr signaling up-regulates the homophilic adhesion molecule DE-cad in these cells (both transcriptionally and post-translationally) (O'Keefe, 2007). High levels of cadherin generally result in apical constriction, a prominent characteristic of the adult vein. DE-cad is only one component of this morphogenetic process, however, as increased levels of DE-cad did not result in a vein-like trapezoidal shape. It was asked, therefore, what other mechanisms might determine the non-hexagonal morphology of vein precursors (O'Keefe, 2012).
In addition to elevated levels of DE-cad, another distinguishing feature of pupal vein cells is an asymmetric distribution of DE-cad about their apicolateral circumference, a phenotype most apparent when two-cell clones of ectopic veins were examined. As loss of Rap1 leads to asymmetric DE-cad (Knox, 2002; O'Keefe, 2009), it was hypothesized that Rap1 activity is down-regulated in vein precursor cells compared to surrounding intervein precursors. Consistent with this hypothesis, Rap1 over-expression dramatically disrupted pupal vein cell shape without affecting cell fate (i.e., DSRF levels). Rap1 over-expressing vein cells had more symmetric DE-Cad distributions, and did not adopt a trapezoidal morphology. This often led to morphological vein defects in the adult wing. In addition, the localization patterns of Rap1-GFP and Canoe suggested lower levels of Rap1 activity in pupal-vein precursors (compared with surrounding intervein cells). It has been previously demonstrated that the generation of Rap1 loss-of-function clones during larval stages results in vein loss (O'Keefe, 2009). Rap1 activity, therefore, plays a dual role in wing-vein formation. First, during larval and early pupal stages, Rap1 stabilizes adhesive contacts between adjacent epithelial cells, thereby facilitating Egfr signaling and maintaining vein-cell fate. Hours later, as the wing begins to differentiate, down-regulation of Rap1 activity drives the morphological changes necessary for vein formation (O'Keefe, 2012).
How does the down-regulation of Rap1 activity specifically increase DE-cad levels at vein-vein cell contacts? Rap1 recruits Cno to adherens junctions, where Cno forms a physical link between adherens junctions and the actin cytoskeleton (Sawyer, 2009). As such, Cno primarily acts as a non-enzymatic scaffolding protein, which suggests that stoichiometry between DE-cad and Cno is important. Based on immunofluorescence analysis of apicolateral cell junctions in the wing, there is a large disparity between Cno and DE-cad levels in vein cells, as Egfr/Ras signaling both up-regulates DE-cad, and down-regulates Cno. It is inferred from these data that vein cells contain far fewer adherens junction complexes that are associated with a molecule(s) of Cno (compared to intervein cells). As Cno represents the critical Rap1 effector in this context, these Cno-free adherens junction complexes would be functionally dissociated from Rap1 signaling, and free to localize in an asymmetric fashion. Relieved from spatial constraints concerning symmetry, adherens junction complexes would accumulate at vein-vein interfaces, where chances of encountering an intercellular binding partner are highest for two reasons: (1) adjacent vein cells express higher levels of DE-cad than adjacent intervein cells, and (2) adjacent vein cells contain Cno-free adherens junction complexes, which are similarly relieved from symmetry constraints (O'Keefe, 2012).
The formation of asymmetrical adhesive contacts in presumptive vein cells is coincident with changes in apical cell shape. It was asked, therefore, how changes in DE-cad localization might affect vein-cell shape, and have proposed a simple model based on examinations of a timecourse of vein differentiation. The balance between intercellular adhesion and cortical tension is a critical determinant of cell shape. Increased adhesion expands cell contacts, and cortical tension opposes this effect. The data suggest that after ~24 h APF, vein-vein cell contacts are characterized by high levels of adhesion (i.e., DE-cad) and decreased levels of cortical tension (i.e., Cno, which links adherens junctions to the actin cytoskeleton). It is hypothesized that these factors drive the expansion of vein-vein contacts at the expense of one vein-intervein cell contact, resulting in the formation of a pentagon. Real-time imaging of vein differentiation will be used in the future to test this model of morphogenesis (O'Keefe, 2012).
The Egfr/Ras and Dpp signaling pathways act in concert to specify vein-cell fate. At 12-16 h APF, Egfr/Ras activity turns on dpp expression in presumptive vein cells. After this stage of development, Dpp is required to maintain vein identity and high levels of Egfr/Ras signaling in presumptive vein cells (creating a positive feed-back loop). In contrast, these developmental signaling pathways have very different effects on cell adhesion and epithelial cell morphology. It has been shown previously that Egfr/Ras activity up-regulates DE-cad levels in vein precursors, and that it does so in a Dpp-independent fashion (O'Keefe, 2007). Results presented in this study indicate that Egfr/Ras signaling also plays the dominant role in regulating Rap1/Cno. Two-cell clones that express RasV12 phenotypically resembled Rap1 loss-of-function cells (more so than TkvQ235D clones). In addition, RasV12 down-regulated the critical Rap1 effector Cno, whereas this effect was not evident in TkvQ235D-expressing cells. As loss of Cno disassociates actin-myosin contractility from cell shape (Sawyer, 2009), RasV12 two-cell clones were less apically constricted than TkvQ235D-expressing cells. Egfr/Ras signaling is also associated with asymmetric adhesive contacts in other developmental contexts. In the Drosophila eye, for example, Egfr/Ras signaling is required in photoreceptors. Much like vein cells, photoreceptors adhere more tightly to one another than to surrounding cells. This raises the possibility that Egfr down-regulates Rap1 activity in multiple cell types following their specification, enabling them to differentiate appropriate cell shapes. Finally, it will be interesting to determine how the Egfr/Ras and Dpp signaling pathways regulate other aspects of vein-cell morphology (e.g., constriction along the apical/basal axis to generate a lumen) (O'Keefe, 2012).
In the wing, Egfr/Ras signaling does not affect Rap1/Cno activity at every developmental stage. High levels of Egfr/Ras signaling are detected in vein cells at the beginning of the third larval instar, but vein/intervein cell-shape differences are not observed before ~24 h APF. As such, the Rap1/Cno complex likely represents a pupal-specific target of Egfr signaling. This study has shown, therefore, that a single developmental signaling pathway can first determine a cell's fate, and later contribute towards its morphological differentiation. Critical to this process, therefore, are genetic and/or epigenetic factors that temporally regulate the output of Egfr/Ras signaling. In the future it will be important to identify such factors not only for the Egfr/Ras pathway, but other developmental signaling pathways as well (O'Keefe, 2012).
Finally, it is becoming increasingly clear that Rap1 affects cancer progression, often by promoting metastasis. In cancer cells, levels of Rap1 activity are typically high, which stimulates migration and metastasis by up-regulating integrin-based cell adhesion. Such is the case in pancreatic, prostate, and breast cancers. However, loss of Rap1 can also cause metastasis by down-regulating cadherin and disrupting the epithelial integrity of the tumor (e.g., ovarian and prostate cancer). Within this disease context, the Egfr/Ras and Rap1 signaling networks often interact. Most recently, Egfr activation of Rap1 has been shown to promote metastasis of human pancreatic carcinoma cells. The precise mechanisms by which Egfr/Ras signaling affects Rap1 activity (both during normal development and disease) must be deciphered, therefore, if these metastatic processes are to be understood and/or mitigated (O'Keefe, 2012).
The PDZ-GEF protein Dizzy (Dzy) and its downstream GTPase Rap1 have pleiotropic roles during development of the Drosophila embryo. This study shows that maternally provided Dzy and Rap1 first function during ventral furrow formation (VFF) where they are critical to guarantee rapid apical cell constrictions. Contraction of the apical actomyosin filament system occurs independently of Dzy and Rap1, but loss of Dzy results in a delayed establishment of the apical adherens junction (AJ) belt, whereas in the absence of Rap1 only a fragmentary apical AJ belt is formed in the epithelium. The timely establishment of apical AJs appears to be essential for coupling actomyosin contractions to cell shape change and to assure completion of the ventral furrow. Immediately after VFF, the downregulation of Dzy and Rap1 is necessary to allow normal mesodermal development to continue after the epithelial-to-mesenchymal transition, as overexpression of Dzy or of constitutively active Rap1 compromises mesodermal migration and monolayer formation. It is proposes that Dzy and Rap1 are crucial factors regulating the dynamics of AJs during gastrulation (Spahn, 2012).
This study found that both the PDZ-GEF Dzy and its target Rap1 are required for ventral furrow formation (VFF) during Drosophila gastrulation. In the absence of Dzy the establishment of the circumferential adhesion belt is slowed down while in the absence of Rap1 only a fragmentary adhesion belt is formed). In the case of dzy, this slowdown in apical junction assembly translates into a slowdown of apical cell constriction, since the cytoskeleton cannot attach to membranes during early gastrulation. Thus, first actomyosin contractions do not evoke cell shape change. In the case of rap1, junction assembly is much more severely affected, and only a fragmentary apical junction belt forms during gastrulation. This results in a variable capability of mid-ventral cells to undergo constriction since only variable levels of apical AJs are available to connect to the contracting actomyosin. Dzy and Rap1 localize cortically during and after gastrulation consistent with a role in junction assembly and possibly maintenance. Levels of both proteins are diminished in the mesoderm once it has been internalized. Overexpressing Dzy or Rap1V12 in the mesoderm results in an inhibition of mesodermal spreading, possibly by keeping up DE-Cad-mediated adhesion. The findings underline the roles of the PDZ-GEF Dzy and its GTPase Rap1 as critical factors regulating the dynamics of adherens junction (AJ) formation in Drosophila gastrulation (Spahn, 2012).
Apical constriction of ventral cells is known to be a major driving force of VFF and much progress has been made in deciphering the signal cascade leading from ventral fate determinants to an assembly of a contractile actomyosin at the apices of ventral cells. Also, the importance of tight coupling between the contracting actomyosin and the cell membranes mediated by AJs has been previously highlighted (Sawyer, 2009). Although MyoII has been implicated as a downstream target of dzy during dorsal closure (Boettner, 2007), the slowdown in cell shape change during VFF seen in dzy GLC cannot be attributed to a slowdown in apical assembly of the actomyosin apparatus. Unlike what has been reported for dorsal closure, actomyosin exhibits the same relocalization to the apex of ventral cells at the end of cellularization in dzy GLC and in wild-type. Furthermore, MyoII coalesced into balls within unconstricted cells when gastrulation starts, supporting the notion of a contracting actomyosin meshwork. Coalesced MyoII within unconstricted cells has also been reported previously for ventral cells in arm, cno and rap1 GLC (Sawyer, 2009) all of which exhibit defective cell constriction. In these studies, this observation was considered an indication of contracting actomyosin that is detached from cell membranes. The current findings are consistent with this view (Spahn, 2012).
Previous work has revealed that ventral cells are not constricted by continuous contraction and that circumferential actomyosin cables do not contribute significantly to the constriction. Instead, a medially localized actomyosin meshwork is thought to reach out to make contact to AJs at the cell membranes and executes discontinuous contraction pulses to constrict the apex (Martin, 2009). The observation that apical constriction still occurs in dzy GLC, later than in wild-type, but apparently as soon as AJs are in place, are in accordance with these findings. Thus, apical constriction is not irrecoverably affected if AJs are not ready at the onset of gastrulation. Actomyosin contraction appears to take place in a dynamical and repeated pulsed fashion over the entire time-span of gastrulation allowing cells to constrict eventually, despite an initial delay in AJ formation (Spahn, 2012).
A puzzling feature of the dzy phenotype is the failure of the ventral furrow to finally close although ventral cells have undergone complete, albeit delayed, apical constriction. It is proposed that the invagination of the mesoderm has to occur within a critical time slot, which is missed in dzy GLC due to the delay in AJ establishment and, consequently, apical cell constriction. In fact, the ventral furrow of dzy GLC very much resembles the ventral furrow of a wild-type embryo 5 to 10 minutes earlier. Still, the furrow is not properly sealed in the end, less tissue moves inside and often the furrow opens up again. This supports the notion that apical constriction alone is not sufficient to internalize the ventral furrow. Computer simulations have indicated that apical constriction alone is incapable of generating a tissue invagination and have postulated ectodermal pushing as a second source of force to internalize the ventral furrow. Such a force could be exerted by turgor pressure in medio-lateral direction within the cellular blastoderm. The ventral furrow may serve as a 'predetermined breaking line', where the tissue can give in to the inherent pressure. The delay in VFF of dzy GLC leads to a temporal overlap with germband extension and PMG invagination that immediately follow the internalization of the ventral furrow in wild-type. Both processes are likely to reduce the epithelial pressure in medio-lateral dimension since they expand the epithelium in the antero-posterior dimension. Consequently, pressure might have already become too low to generate the force required to push in the mesoderm when the 'breaking line' has finally emerged. In addition, it cannot be ruled out that the ventral furrow is not properly closed and opens up again in dzy GLC because of a failure in sealing the edges of the furrow (Spahn, 2012).
In contrast to dzy GLC, only a fragmentary AJ belt is formed in rap1 GLC as DE-Cad is diffusely distributed in the membranes and shows delayed and incomplete apical accumulation. In addition, DE-Cad reveals a striking cytoplasmic mislocalization to floating particles that are seen in rap1 GLC only. Although the nature of these particles remains to be clarified, it is speculated that they represent DE-Cad rich membrane vesicles originating from the cell membrane. It has been reported earlier (Sawyer, 2009) that initial AJ assembly is unaffected in rap1 GLC, but this conclusion was based on anti-Arm staining which look unaffected in the current analysis as well. Thus, Rap1 seems to act on DE-Cad specifically to assure its proper localization. In mammalian cells regulation of DE-Cad endocytosis has long been recognized as a cellular mechanism to modulate AJs. In this context, Rap1 has been implicated in having a key role in stabilizing DE-Cad in membrane-bound aggregates as it is thought to enhance binding of DE-Cad to p120-catenin, which may serve as a cap protecting DE-Cad from being endocytosed. On the other hand, p120-catenin appears to play only a minor role in Drosophila. Despite the accordance with previous studies (Sawyer, 2009), the unaffected apical accumulation of Arm in rap1 GLC observed in this study was surprising, especially since loss of DE-Cad is reported to entail loss of Arm in various tissues. However, Arm is also involved in many other DE-Cad independent processes, e.g., acting as a signal molecule or transcription factor, so a requirement of DE-Cad for its localization does not appear coercive (Spahn, 2012).
Albeit the precise mechanism remains to be investigated, it is assumed that in the absence of maternal Rap1, confinement of DE-Cad to cell membranes and accumulation into stable apical junctions is severely compromised. Instead, only fragmentary junctions are formed whose stability may vary stochastically. Thus, AJ fragmentation may affect different cells to a different degree. As a consequence, ventral cells show a broad distribution of constriction capability ranging from complete constriction to a total failure of constriction. It may be recognized that apical constriction does not appear to be slowed down in those cells of rap1 GLC that are capable of undergoing constriction. A reason for this could be the lack of constriction in surrounding cells, so constricting cells experience considerably less opposing force from their neighbours in the epithelium. This could allow them to constrict faster and make up the inefficient actomyosin attachment in their membranes. Similarly, the lack of constriction in neighbours may allow constricting cells to constrict uniformly ('isotropically'), rather than become eccentric like wild-type cells. Due to the discontiguous actomyosin meshwork in the ventral epithelium, tension in the anteroposterior axis will be strongly reduced so constricting cells are not forced into an eccentric morphology. Indeed, previous work has shown that mid-ventral cells can undergo isotropic constriction when anteroposterior tension is disrupted by inflicting tears upon the supracellular actomyosin meshwork (Martin, 2010). Surprisingly, in spite of the large fraction of mid-ventral cells with high constriction levels, rap1 GLC do not form a ventral furrow. It is assumed that the minor fraction of unconstricted and bloated mid-ventral cells has an inhibitory influence on VFF, possibly by interrupting the 'predetermined breaking line' (Spahn, 2012).
Thus, rap1 and dzy differ qualitatively in their maternal phenotypes because loss of Dzy only delays establishment of AJs whereas loss of Rap1 additionally entails a fragmentation of the AJ belt and massive cytoplasmic mislocalization of DE-Cad. This discrepancy is not in conflict with the concept of Dzy acting exclusively via Rap1, but strongly argues in favour of Rap1 being regulated by additional GEFs besides Dzy (Spahn, 2012).
It must be emphasized that the effects on AJ assembly seen in dzy and rap1 GLC are not confined to the prospective mesoderm but occur around the entire epithelium consistent with the localization of Dzy and Rap1 in wild-type. dzy and rap1 have been recognized as 'ventral furrow mutants' because apical constriction of ventral cells is the earliest process in embryogenesis requiring a properly built apical AJ belt (Spahn, 2012).
With the apical adhesion belt being a prominent feature of ectodermal cells, internalized mesodermal cells show substantially weaker DE-Cad intensity indicating that junctions are disassembled in order to reduce cell-cell adhesion and allow mesenchymal migration. Overexpression of Dzy or Rap1V12 impairs this mesenchymal migration significantly, Rap1V12 noticeably stronger than Dzy alone. This is very plausible given that Dzy works via Rap1 which is considerably reduced in the internalized mesoderm. Migration defects upon Rap1V12 overexpression are accompanied by significantly risen relative amounts of DE-Cad in mesenchymal cells suggesting the possibility that the downregulation of Rap1 is required to allow AJs to become disassembled in the mesoderm. Accordant results have been found in the Drosophila testis where reduction of AJs can be restored to wild-type level through overexpression of constitutively active Rap1 (Wang, 2006). It remains to be seen by what mechanism AJs are disassembled in the internalized mesoderm and how the remarkably fast diminishment of Dzy and Rap1 is triggered. Conceivably, processes accompanying EMT such as mechanical alterations in the cytoskeleton could trigger degradation signals since these processes have been found to have potential signalling ability in other systems (Spahn, 2012).
As discussed above, the discrepancy between the maternal phenotypes of dzy and rap1 implies the necessity of other GEFs acting on Rap1 during gastrulation. C3G is a tempting candidate as it has been shown to interact with Rap1 in mammalian cell culture as well as in Drosophila (Dupuy, 2005; Ishimaru, 1999). Furthermore, it exhibits GEF activity on Drosophila Rap1 in vitro (Shirinian, 2010; Spahn, 2012 and references therein).
In addition to uncovering alternative activators of Rap1 it will be interesting to identify players upstream of Dzy. Despite its cyclic nucleotide binding domain there is no indication so far that Dzy is activated by cAMP signalling. However, like several proteins involved in cell polarity, Dzy bears a PDZ domain through which it possibly binds to a membrane scaffold typically involved in mediating quick linkage between signalling molecules and structural proteins. Indeed, the PDZ protein MAGI-1 has been shown to serve as a scaffold for the vertebrate homologue of Dzy and is a good candidate for a protein giving the relevant spatial cue. Unravelling the architecture of such a signalling scaffold will be key to understanding how an epithelium can be reorganized so rapidly to allow the extraordinarily fast morphogenesis of the ventral furrow (Spahn, 2012).
Drosophila Raf (DRaf) contains an extended N terminus, in addition to three conserved regions (CR1-CR3); however, the function(s) of this N-terminal segment remains elusive. In this study, a novel region within Draf's N terminus that is conserved in BRaf proteins of vertebrates was identified and termed conserved region N-terminal (CRN). The N-terminal segment can play a positive role(s) in the Torso receptor tyrosine kinase pathway in vivo, and its contribution to signaling appears to be dependent on the activity of Torso receptor, suggesting this N-terminal segment can function in signal transmission. Circular dichroism analysis indicates that DRaf's N terminus (amino acids 1-117) including CRN (amino acids 19-77) is folded in vitro and has a high content of helical secondary structure as predicted by proteomics tools. In yeast two-hybrid assays, stronger interactions between DRaf's Ras binding domain (RBD) and the small GTPase Ras1, as well as Rap1, were observed when CRN and RBD sequences were linked. Together, these studies suggest that DRaf's extended N terminus may assist in its association with the upstream activators (Ras1 and Rap1) through a CRN-mediated mechanism(s) in vivo (Ding, 2010).
Amino acids 19-77) within Draf's N terminus, conserved for Raf genes of most invertebrates and BRaf genes of vertebrates, was identified and termed CRN. This conserved region has not been described by others, but potential roles for the extended N terminus have been proposed in two reports. One found that in HeLa cells, the N terminus of BRaf may mediate Raf dimerization to generate BRaf-BRaf or BRaf-CRaf complexes, and play an important regulatory role in calcium-induced BRaf activation. Another study reported that deletion of BRaf's N terminus did not affect BRaf-CRaf dimer formation. Instead, it was found that N-terminal residues appeared to facilitate interaction with HRas in vitro. In accordance with the previous study, stronger interactions between DRaf's RBD (Ras binding domain) and the small GTPase Ras1δCAAX were observed when N-terminal and RBD sequences were linked in a yeast two-hybrid analysis. This suggested that the N terminus might assist in Ras1 binding. Furthermore, the identity of specific residues in the N terminus that might participate in Ras1 binding were mapped to the CRN region (amino acids 19-77). Two known Raf motifs, RBD and CRD, are involved in Raf's interaction with Ras. This studies, and previous results using BRaf, suggest that the N-terminal residues of DRaf and BRaf proteins, particularly the CRN region, might be another element that plays a role(s) in Ras-Raf coupling (Ding, 2010).
The small GTPase Rap shares with Ras nearly identical Raf binding regions that comprise switch 1 and the lipid moiety. Rap functions as an antagonist of Ras in regulating CRaf activity, but can activate BRaf in a parallel way with Ras. Isoform-specific features of different Raf family members may explain their distinct responses to Rap. In flies, both Ras1 and Rap1 can interact with and activate DRaf. Thus, it was reasonable to test whether DRaf's N terminus including CRN might also assist in Rap1 binding. In agreement with this idea, stronger interaction between RBD and Rap1δCAAX was observed when DRaf's CRN and RBD sequences were linked in vitro, further suggesting that the N terminus may contribute to both Ras1 and Rap1 binding potentially through a CRN-mediated mechanism(s) in vivo (Ding, 2010).
No direct interaction between Ras1 or Rap1 and the isolated DRaf N-terminal segment (amino acids 1-117) was detected, or when the N terminus was linked with the Ras1/Rap1 binding-deficient RBDR174L. Thus, the contribution of DRaf's N-terminal residues to Ras1 and Rap1 binding requires the presence of RBD. It is possible that the CRN-containing N terminus may assist in Raf-Ras interaction by making RBD more accessible to Ras1 and/or in a sequential manner, subsequent to RBD-Ras1 interaction, by stabilizing the RBD-Ras1 complex. Deletion of CRN may result in conformational or structural changes that reduce Ras1 binding affinity. Structural analysis of these complexes may provide important clues and help to understand the molecular mechanism(s) by which CRN assists in Ras-Raf interaction. The computational analysis suggested conserved CRN has the propensity to form two α-helical structures (α1 and α2) and contains a putative phosphorylation motif T-S-K located in α2. In agreement, DRaf's N terminus (amino acids 1-117) was folded in vitro and had a high content of helical secondary structure. These findings may help to establish a basis for future determination of molecular structure (Ding, 2010).
Although no verified binding partner(s) for DRaf or BRaf's N terminus has been identified, it is still possible that CRN may interact with other regulatory factors in vivo, that may affect Ras or Rap binding and/or function in activation of DRaf and BRaf. If so, the conserved structural features of CRN most likely relate to these regulatory events in vivo. Site-directed mutagenesis of conserved sites/motifs could provide useful information regarding the molecular mechanism(s) of CRN's role in the activation of DRaf and BRaf (Ding, 2010).
This in vitro study of DRaf's N terminus was initiated on the basis of in vivo findings using both loss- and gain-of-function genetic assays that deletion of N-terminal residues consistently reduces DRaf's signal potential in the Torso pathway. When expressed at high levels, FL DRaf enhanced the gain-of-function effects of the torRL3 allele much more significantly than DRafδN114. In embryos from trk-/- mothers, addition of FL DRaf, but not DRAFδN114, partially restored the A8 denticle belt structure. These findings indicate that the N terminus can play a positive role(s) in Torso RTK signaling. Interestingly, the contribution of DRaf's N terminus in the Torso pathway appeared to be dependent on upstream receptor activity, suggesting its role in transmission of the signal. Together with yeast two-hybrid data it is proposed that the presence of N-terminal residues may facilitate the association of DRaf with the upstream regulators Ras1 and Rap1, thereby assisting in transmission of the RTK signal in vivo (Ding, 2010).
For instance, in the trk- background, a small amount of active GTP-Ras1 and GTP-Rap1 are likely present, mostly due to activation by residual upstream Trunk activity, the presence of Torso-like ligand, and/or the intrinsic activity of the Torso receptor. The trk1 mutation used in this analysis results in protein truncation at the last 16 amino acids. It is possible that overexpression of FL DRaf proteins in this background increases the likelihood of interaction between abundant DRaf proteins and membrane bound GTP-Ras1 or GTP-Rap1. This in turn, could elevate the RTK signal and partially restore development of the A8 denticle belt structure in some embryos. In contrast, deletion of the N terminus could destabilize Ras1-DRaf (or Rap1-DRaf) coupling or decrease the duration of interaction, resulting in reduced DRaf signal transmission. This may explain why expression of DRafδN114 failed to rescue the A8 denticle belt in embryos from trk-/- mothers (Ding, 2010).
Previously, an auto-inhibitory role had been assigned to residues compromising the first half of the DRaf protein, in addition to their functions in promoting its activity. Deletion of the N-terminal amino acids 1-272 (including the N terminus and CR1) or 1-402 (including the N terminus, CR1, and CR2) of DRaf at least partially relieved these negative effects. In this study, although removal of the N-terminal 1-114 residues did not result in constitutive DRafδN114 activity in embryos lacking the maternal Torso receptor, it is still possible that the N terminus may contribute to auto-inhibitory effects. Together with CR1 and CR2, these N-terminal residues (1-114) may help maintain DRaf's inactive conformation. If so, the N terminus might play dual roles, both positively and negatively regulating DRaf. Therefore, its contribution to signaling may be neutralized by this auto-inhibition and consequently result in a subtle in vivo effect. If so, selective mutagenesis of the 'inhibitory' motifs/sites in the N-terminal region or removal of other cofactors involved in its negative regulation may amplify signaling differences between FL DRaf and DRafδN114. Ras binding has been thought crucial to recruit Raf to the membrane and promote its RTK signaling activity. However, the Drosophila Torso pathway appears tolerant of alterations in Ras1-DRaf coupling. Draf C110 has a R174L point mutation in the RBD domain and likely comprised for Ras1 binding. The RBDR174L is Ras binding deficient in the yeast two-hybrid assay. However, tll expression patterns and cuticles of the embryos derived from mothers with Draf C110/Draf C110 germ cells were indistinguishable from those of wild-type embryos, suggesting a mechanism(s) independent of RBD-Ras1 interaction might function in recruiting DRaf to the membrane. In agreement with this model, it has been found that membrane translocation of CRaf could be mediated by its interaction with phosphatidic acid (PA) and independent of Ras binding. This PA binding site is also conserved in ARaf, BRaf, and DRaf. Thus, DrafC110 could be recruited to the cell membranes by associating with PA. Moreover, it is known that Raf's CRD participates in Ras binding through its interaction with the lipid moiety of Ras. Once at the membrane, it is also possible that the interaction between DrafC110's CRD and Ras1 could further promote its membrane attachment and result in relatively normal Torso signal production. In this study, the presence of RBD, CRD, and the potential PA binding site may be sufficient to promote DRaf's activation in Torso signaling. This may explain why at approximately endogenous wild-type protein level maternally expressed DRafδN114 is able to rescue the embryonic terminal defects of Draf11-29 mutants. Together, considering the Torso pathway's tolerance of alterations in Ras1-DRaf coupling and the minor role DRaf's N terminus plays in Ras1 binding, it is reasonable that the phenotypic consequences of removing these N-terminal residues (DRafδN114) are not great in Torso signaling. The subtle phenotypic effects of DRaf's N terminus could also be due to compensation provided by potential autoregulatory feedback or alternative redundant processes in the in vivo system. In this study, the expression of DRaf proteins at a low level appeared to sensitize the assay system. It was found that deletion of the N terminus seemed to increase the threshold of DRaf protein levels required for normal signaling. Furthermore, by adding one copy of the ectopic torRL3 allele or removing wild-type maternal Trunk activity the sensitivity of the Torso pathway was apparently increased. These allowed the embryonic terminal system to display enhanced differences between FL DRaf and DRafδN114 proteins (Ding, 2010).
Why is this N terminus with its 'subtle' functional effects conserved during evolution, and what is its biological relevance? There are numerous RTK pathways functioning in Drosophila cellular and developmental processes. In spite of the identical Ras-Raf-MEK signal cassette they share, these RTK pathways can lead to different biological responses. Previous studies indicated that such specificity might be due to the difference in the intensity and/or duration of the signal. This suggested that the magnitude of Raf signal could function as a critical determinant of biological responses. Participation of multiple DRaf elements in Ras1 or Rap1 binding could be a good strategy to modulate its activity. Normally, tight association with Ras1 or Rap1 through RBD and CRD regions is required and sufficient to initiate the activation of DRaf, while minor adjustments/regulation of interaction by the CRN region could optimize signaling potential and reduce variability. Thus, the extended N terminus including CRN may play a role(s) as one element in a multidomain effort to promote DRaf's interaction with Ras1 and Rap1, participating and assisting in regulation to reliably attain maximal signal output (Ding, 2010).
Migration is a complex, dynamic process that has largely been studied using qualitative or static approaches. As technology has improved, it is now possible to take quantitative approaches towards understanding cell migration using in vivo imaging and tracking analyses. In this manner, a four-step model of mesoderm migration during Drosophila gastrulation was establised: (I) mesodermal tube formation, (II) collapse of the mesoderm, (III) dorsal migration and spreading and (IV) monolayer formation. The data provide evidence that these steps are temporally distinct and that each might require different chemical inputs. To support this, the role was analyzed of fibroblast growth factor (FGF) signaling, in particular the function of two Drosophila FGF ligands, Pyramus and Thisbe, during mesoderm migration. It was determined that FGF signaling through both ligands controls movements in the radial direction. Thisbe is required for the initial collapse of the mesoderm onto the ectoderm, whereas both Pyramus and Thisbe are required for monolayer formation. In addition, it was uncovered that the GTPase Rap1 regulates radial movement of cells and localization of the beta-integrin subunit, Myospheroid, which is also required for monolayer formation. These analyses suggest that distinct signals influence particular movements, since it was found that FGF signaling is involved in controlling collapse and monolayer formation but not dorsal movement, whereas integrins are required to support monolayer formation only and not earlier movements. This work demonstrates that complex cell migration is not necessarily a fluid process, but suggests instead that different types of movements are directed by distinct inputs in a stepwise manner (McMahon, 2010).
Mesoderm migration was found to be a combination of complex three-dimensional movements involving many molecular components. live imaging, coupled with quantitative analyses, is important for studying complex cell movements, as it allowed migration to be decomposed into different movement types and thus has allowed description of subtle phenotypes. First, analysis of the directional movements of mesoderm cells within wild-type embryos was extended, focusing on the temporal sequences of events. Cells were found follow a sequential and distinct set of trajectories: movement in the radial direction (tube collapse: -5 to 15 minutes, 0=onset of germband elongation), followed by movement in the angular direction (dorsal migration: 15 to 75 minutes) and ending with small intercalation movements in the radial direction (monolayer formation: 75 to 110 minutes). These movements appear temporally distinct (i.e. stepwise), and thus molecular signals controlling each process were sought (McMahon, 2010).
Which mesoderm movements were FGF-dependent were investigated and, in particular, either Ths- or Pyr-dependent. The interaction between Htl and its two ligands provides a simpler system relative to vertebrates (which exhibit over 120 receptor-ligand interactions) in which to study how and why multiple FGF ligands interact with the same receptor. Previously, it was found that FGF signaling via the Htl FGFR controls collapse of the mesodermal tube but not dorsal-directed spreading (McMahon, 2008). This study demonstrated that FGF signaling is also required for monolayer formation. In addition, distinct, non-redundant roles were defined for the FGF ligands: Ths (but not Pyr) is required for collapse of the mesodermal tube, whereas both Pyr and Ths are required for proper intercalation of mesoderm cells after dorsal spreading (McMahon, 2010).
This analysis raises questions about ligand choice during collapse and monolayer formation. Within the mesodermal tube, cells at the top require a long-range signal in order to orient towards the ectoderm during tube collapse, whereas the signals controlling intercalation during monolayer formation can be of shorter range. It is suggested that the ligands have different activities that are appropriately tuned for these processes. In fact, recent studies of the functional domains of these proteins suggest that Ths has a longer range of action than Pyr, in agreement with the analysis that Pyr does not support tube collapse, but does have a hand in monolayer formation (McMahon, 2010).
This study has demonstrated that Rap1 mutants have a similar mesoderm phenotype to the FGFR htl mutant, with defects in collapse and monolayer formation. It was not possible to establish whether Rap1 acts downstream of FGF signaling, as the complete loss of Mys in Rap1 mutants is more severe than the patchy expression of Mys seen in htl mutants. Therefore, Rap1 could be working in parallel to or downstream of FGF signaling during mesoderm migration. Rap1 has been implicated in several morphogenetic events during Drosophila gastrulation and probably interacts with many different signaling pathways. Further study of Rap1, along with other GTPases, will shed light onto their role during mesoderm migration, how they interact with one another and what signaling pathways control them (McMahon, 2010).
Focus was placed on the more specific phenotype of mys mutants, as its localization is affected in htl mutants and it exhibits a monolayer defect that is similar to pyr and ths mutants. Integrins are important for cell adhesion, so it is not surprising that cells fail to make stable contact with the ectoderm through intercalation in mys mutants. However, some cells do contribute to monolayer formation in the absence of Mys, implying that other adhesion molecules are involved in maintaining contact between the mesoderm and ectoderm. These other adhesion molecules might be activated downstream of FGF signaling as the htl mutant monolayer phenotype is more severe than the mys mutant. Discovering the downstream targets of Htl, which might regulate cell adhesion properties, will help to shed light on the mechanisms supporting collapse of the mesodermal tube (which is not dependent on Mys) and monolayer formation (which is Mys-dependent) (McMahon, 2010).
Cell protrusions, such as filopodia, are important for sensing chemoattractants and polarizing movement during migration. Previous studies have focused on protrusive activity at the leading edge during mesoderm migration in Drosophila and shown that these protrusions are FGF-dependent. In this study, it was found that protrusions exist in all mesoderm cells, not just the leading edge, and that these protrusions also extend into the ectoderm (McMahon, 2010).
The study demonstrates that FGF signaling, as well as integrin activity, is required to support protrusive activity into the ectoderm; this is a potential mechanism by which FGF signaling and Mys could control movement toward the ectoderm during monolayer formation. The function of protrusions at the leading edge remains unclear, as they appear to be reduced in pyr and mys mutants, but migration in the dorsal direction still occurs in both mutant backgrounds. One interpretation is that FGF and Mys are important for generalized protrusive activity and that extensive protrusions are required for intercalation but not dorsal migration (McMahon, 2010).
Based on this study, it is proposed that mesoderm migration is a stepwise process, with each event requiring different molecular cues to achieve collective migration. Invagination of the mesoderm is the first step in this process and is dependent on Snail, Twist, Concertina, Fog and several other genes. Next, collapse of the mesoderm tube onto the ectoderm requires Htl activation via Ths. Rap1 might be involved in this process as well but the phenotype of Rap1 mutants is complex and it is unclear which phenotypes are primary defects (McMahon, 2010).
Following collapse, mesoderm cells spread dorsally by an unknown mechanism. Dorsal migration is unaffected in pyr and ths mutants and occurs in all cells that contact the ectoderm in htl mutants, implying that FGF signaling is, at most, indirectly involved in this step owing to the earlier tube collapse defect (McMahon, 2008). Whether dorsal migration requires chemoattractive signals or whether the cells simply move in this direction because it is the area of least resistance remains unclear (McMahon, 2010).
Finally, after dorsal spreading is complete, any remaining cells not contacting the ectoderm intercalate to form a monolayer. This process is controlled by a combination of both Pyr and Ths interacting through Htl and also by Rap1 and Mys. In other systems, intercalation can lead to changes in the properties of the cell collective, for instance, lengthening of a body plan. However, this study has shown that dorsal migration and spreading are not a result of intercalation, as intercalation occurs after spreading has finished (McMahon, 2010).
Coordination of these signals to control collective migration enables the mesoderm to form a symmetrical structure, which is essential for embryo survival. This model begins to address the question of how hundreds of cells move in concerted fashion and is relevant for a generalized understanding of embryogenesis and organogenesis. It was found that mesoderm migration is accomplished through sequential movements in different directions, implying that collective migration might be best achieved by distinct phases of movement (McMahon, 2010).
Human immune cells have to penetrate an endothelial barrier during their beneficial pursuit of infection and their destructive infiltration of tissues in autoimmune diseases. This transmigration requires Rap1 GTPase to activate integrin affinity. A new model system for this process has been defined by demonstrating, with live imaging and genetics, that during embryonic development Drosophila melanogaster immune cells penetrate an epithelial, Drosophila E-cadherin (DE-cadherin)-based tissue barrier. A mutant in RhoL, a GTPase homologue that is specifically expressed in haemocytes, blocks this invasive step but not other aspects of guided migration. RhoL mediates integrin adhesion caused by Drosophila Rap1 overexpression and moves Rap1 away from a concentration in the cytoplasm to the leading edge during invasive migration. These findings indicate that a programmed migratory step during Drosophila development bears striking molecular similarities to vertebrate immune cell transmigration during inflammation, and identify RhoL as a new regulator of invasion, adhesion and Rap1 localization. This work establishes the utility of Drosophila for identifying novel components of immune cell transmigration and for understanding the in vivo interplay of immune cells with the barriers they penetrate (Siekhaus, 2010).
C3G is a guanine nucleotide exchange factor (GEF) and modulator of small G-protein activity, which primarily acts on members of the Rap GTPase subfamily. Via promotion of the active GTP bound conformation of target GTPases, C3G has been implicated in the regulation of multiple cellular and developmental events including proliferation, differentiation and apoptosis. The Drosophila C3G orthologue exhibits a domain organization similar to that of vertebrate C3G. Through deletion of the C3G locus, it was observed loss of C3G causes semi-lethality, and that escaping adult flies are characterized by a reduction in lifespan and general fitness. In situ hybridization reveals C3G expression in the developing embryonic somatic and visceral muscles, and indeed analysis of C3G mutants suggests essential functions of C3G for normal body wall muscle development during larval stages. C3G mutants display abnormal muscle morphology and attachment, as well as failure to properly localize betaPS integrins to muscle attachment sites. Moreover, this study shows that C3G stimulates guanine nucleotide exchange on Drosophila Rap GTPases in vitro. Taken together, it is concluded that Drosophila C3G is a Rap1-specific GEF with important functions in maintaining muscle integrity during larval stages (Shirinian, 2010).
The small GTPase Rap1 affects cell adhesion and cell motility in numerous developmental contexts. Loss of Rap1 in the Drosophila wing epithelium disrupts adherens junction localization, causing mutant cells to disperse, and dramatically alters epithelial cell shape. While the adhesive consequences of Rap1 inactivation have been well described in this system, the effects on cell signaling, cell fate specification, and tissue differentiation are not known. This study demonstrates that Egfr-dependent cell types are lost from Rap1 mutant tissue as an indirect consequence of DE-cadherin mislocalization. Cells lacking Rap1 in the developing wing and eye are capable of responding to an Egfr signal, indicating that Rap1 is not required for Egfr/Ras/MAPK signal transduction. Instead, Rap1 regulates adhesive contacts necessary for maintenance of Egfr signaling between cells, and differentiation of wing veins and photoreceptors. Rap1 is also necessary for planar cell polarity in these tissues. Wing hair alignment and ommatidial rotation, functional readouts of planar cell polarity in the wing and eye respectively, are both affected in Rap1 mutant tissue. Finally, this study shows that Rap1 acts through the effector Canoe to regulate these developmental processes (O'Keefe, 2009).
Epithelial morphogenesis is characterized by an exquisite control of cell shape and position. Progression through dorsal closure in Drosophila gastrulation depends on the ability of Rap1 GTPase to signal through the adherens junctional multidomain protein Canoe. This study provides genetic evidence that epithelial Rap activation and Canoe effector usage are conferred by the Drosophila PDZ-GEF (dPDZ-GEF) exchange factor. dPDZ-GEF/Rap/Canoe signaling modulates cell shape and apicolateral cell constriction in embryonic and wing disc epithelia. In dPDZ-GEF mutant embryos with strong dorsal closure defects, cells in the lateral ectoderm fail to properly elongate. Postembryonic dPDZ-GEF mutant cells generated in mosaic tissue display a striking extension of lateral cell perimeters in the proximity of junctional complexes, suggesting a loss of normal cell contractility. Furthermore, the data indicate that dPDZ-GEF signaling is linked to myosin II function. Both dPDZ-GEF and cno show strong genetic interactions with the myosin II-encoding gene, and myosin II distribution is severely perturbed in epithelia of both mutants. These findings provide the first insight into the molecular machinery targeted by Rap signaling to modulate epithelial plasticity. It is proposed that dPDZ-GEF-dependent signaling functions as a rheostat linking Rap activity to the regulation of cell shape in epithelial morphogenesis at different developmental stages (Boettner, 2007).
In developing tissues, Rap has been found to promote various morphogenetic processes, ranging from epithelial migration and invagination in embryogenesis to the maintenance of epithelial integrity in proliferating tissues at later stages. However, the mechanisms by which Rap is regulated and mediates its effects in morphogenetic episodes remain poorly understood. This report delineates a pathway in which the Drosophila GEF dPDZ-GEF links Rap activity to MyoII and the regulation of lateral contractility and cell shape in different epithelial morphogenetic episodes (Boettner, 2007).
This study identified dPDZ-GEF as a putative activator of Rap GTPases in a yeast two hybrid (YTH) screen and subsequently demonstrated that it specifically associates with Rap, but not Ras, GTPases. PDZ-GEF is highly conserved among metazoans, suggesting that it might serve common physiological roles. dPDZ-GEF was found to be highly expressed in epithelial tissues involved in embryonic dorsal closure (DC), and, importantly, the data revealed that it functions as an activator of Rap1/Cno signaling in this process. First, as in the case of Rap1 and cno, loss of zygotic dPDZ-GEF function is associated with an ectodermal failure, which is manifested by dorsal-open phenotypes. Eliminating both zygotic and maternal dPDZ-GEF elevates the frequency of late gastrulation defects. Second, the genetic analysis places dPDZ-GEF upstream of the Rap/Cno GTPase/effector complex, as both Rap1 and Cno were able to rescue the dPDZ-GEF LOF phenotype to a large extent. Third, all three proteins show an overlapping localization at AJs in ectodermal cells involved in DC. Thus, these findings demonstrate that dPDZ-GEF serves as a Rap1/Cno activator to promote late epithelial gastrulation movements. In support of a conserved role of dPDZ-GEF in epithelial morphogenesis, studies with C. elegans demonstrated that pxf-1, the dPDZ-GEF homolog, is vital for epithelial integrity. pxf-1 mutant animals often are confronted with hypodermal malfunctions; the underlying cellular basis of these defects, however, remains to be elucidated (Boettner, 2007).
Epithelial migration processes often entail striking alterations in cell shape, and much effort has been devoted to unraveling the underlying cellular and molecular mechanisms. This study highlights that dPDZ-GEF as a Rap activator adjusts cell shape to the demands of morphogenetic movements and imaginal disc morphogenesis. It was observed that dPDZ-GEF mutant embryos involved in DC often exhibit bunched regions in their leading edge and an incompetence of ectodermal cells to elongate dorsally. These phenotypes also characterize embryos that either overexpress DN Rap1 or are mutant for cno. Thus, signaling through dPDZ-GEF, Rap, and Cno (1) is vital for the organization of a coherently moving leading edge and (2) enables the typical dorsoventral stretching of lateral ectodermal cell sheets. These studies also unveiled a requirement for dPDZ-GEF for the adjustment of epithelial cell shape in the differentiation program of the wing imaginal disc. It was found that a dPDZ-GEF LOF situation generated in a clonal analysis of mosaic wing discs is associated with a decline in apicolateral contractility in the vicinity of junctional complexes. Loss of contractility, as visualized by a widening of apicolateral circumferences, is coupled to a partially compensating gain of contractility in adjacent wild-type tissue. Wild-type cells in close proximity to mutant clones display smaller apicolateral circumferences. Interestingly, overexpression of dPDZ-GEF in restricted areas of the wing disc causes contractile aberrations. When ectopically expressed in the posterior compartment, dPDZ-GEF leads to a loss of apicolateral contractility in cells lining the A/P boundary. Thus, both gain and loss of dPDZ-GEF function compromise normal contractile strength and result in aberrant adult tissue formation. These observations suggest that a finely tuned level of Rap activation is crucial for normal cellular and organismal development to occur. Tight requirements for activation of small GTPases in vivo have been documented previously, e.g., for Rho GTPases and their function in axon guidance. Importantly, this study found in genetic modification experiments that reduced or enhanced dPDZ-GEF activity in the developing wing can be rescued by ectopic Rap1 or lowered cno doses, respectively, suggesting that signaling through the dPDZ-GEF/Rap/Cno module at least partially controls disc morphogenesis. This, together with the vital cooperative roles of all three genes in embryonic cell sheet migration, corroborates the reiterative function of dPDZ-GEF/Rap/Cno signaling during epithelial development (Boettner, 2007).
What are the mechanisms that translate Rap signaling downstream of dPDZ-GEF into the modulation of cell shape? This analysis of dPDZ-GEF LOF situations during gastrulation and wing disc morphogenesis showed that junctional integrity is not corrupted. Both AJ and SJ belts around the apicolateral circumference are seamlessly maintained in dPDZ-GEF LOF tissue. However, the data support a role for the MyoII heavy chain, the product of the zip gene, as an effector. MyoII assembly and disassembly in migrating cells and tissue homeostasis are tightly balanced processes. In epithelial cells, MyoII localizes to cell-cell junctional complexes and is essential for establishing and maintaining intercellular adhesion and tension. This study found that the decline in apicolateral constriction associated with dPDZ-GEF LOF in mitotic clones in the wing disc epithelium is accompanied by a less compact MyoII localization and that adjacent constricted wild-type cells display overassembled MyoII, which concentrates in ectopic focal structures. Also, in the DC paradigm, dPDZ-GEF and cno mutant embryos that are involved in DC exhibited failures of leading-edge cells to properly assemble MyoII. The abundant MyoII localization at the leading edge that characterizes wild-type embryos during DC is significantly diminished, and the bars-on-a-string-like MyoII distribution is lost in these mutants. In particular, regions of the leading edge adjacent to the bunched segments retain only minimal amounts of assembled MyoII. These observations strongly suggest that loss of MyoII control at the leading edge is contributing to the bunching phenotype observed in dPDZ-GEF and cno mutants. Consistent with a spatiotemporal regulation of MyoII in distinct regions of the leading edge are elegant life-imaging studies undertaken with embryos undergoing DC. Dynamic cycles of MyoII-dependent contraction and relaxation occur that are limited to smaller regions within the leading edge during the migration process (Boettner, 2007).
In further support of the notion that dPDZ-GEF signaling acts on MyoII, evidence was obtained that dPDZ-GEF and cno genetically interact with zip in late gastrulation and, moreover, that cno is genetically linked to zip in wing morphogenesis. In particular, the data show a strong enhancement of dorsal-open frequencies in embryos that are double transheterozygous for hypomorphic combinations of zip and either dPDZ-GEF or cno. Combined mutations at the zip and cno loci were also found to cause a malformation of wings. Together, these findings imply that signaling through the dPDZ-GEF/Rap/Cno module is required for MyoII function at different stages of epithelial development. Future experimentation will be required to determine the precise biochemical link between this module and MyoII regulation. Of note, a recent study demonstrated that the mammalian Cno homolog, AF-6/Afadin, in a two-dimensional tissue culture system moves together with MyoII at the edge of wounds induced by laser ablation. At the onset of wound closure, a subpopulation of MyoII resides apically in the lateral membranes of cells lining the wound. However, when closure progresses into advanced stages, MyoII, together with AF-6/Afadin, migrates basalward to constrict both the wound perimeter and the apicobasal membranes facing the wound (Boettner, 2007).
These data support a model in which dPDZ-GEF, through Rap activation and MyoII regulation, contributes to the adjustment of lateral cell contractility in epithelial cells of the embryo and the developing wing. In a previous study, the analysis of Rap1 mutant clones in the wing imaginal disc revealed a direct effect of Rap1 on the reorganization of AJs at the end of cytokinesis, where resealing of their belts has to occur between daughter cells. Since the data showed that AJ integrity is unperturbed in clones comprised of dPDZ-GEF LOF cells, it is surmised that dPDZ-GEF either is not relevant for Rap1 activation in the reconstitution of a seamless AJ belt during cytokinesis or is compensated for by a still-unknown factor conferring the necessary exchange activity. In contrast, the apicolateral constriction defects detected as a consequence of clonal loss of dPDZ-GEF function so far have not been described for Rap1 mutant clones in the same scenario. It is presumed either that they have escaped scrutiny or, more likely, that Rap1 acts redundantly with its close homolog Rap2l in adjusting apicolateral constriction, while the reorganization of AJs during cytokinesis relies solely on Rap1. In fact, Rap1 and Rap2l have been shown to compensate for each other in the male stem cell niche. In this context, both Rap proteins cooperate downstream of dPDZ-GEF to anchor germ line stem cells to their niche. In future experiments, it is planned to generate Rap1 and Rap2l mitotic clones in parallel and to examine and compare their effects on cell shape and contractility (Boettner, 2007).
A picture is emerging in which specialized GEFs activate Rap GTPases and selective effectors in different morphogenetic scenarios and cellular processes. For example, Rap1 signaling has been implicated in cell/extracellular matrix-dependent force transduction at focal adhesion sites of cultured cells. In this scenario, Rap1 is regulated by an Src/p130Cas/C3G-triggered mechanism. Also, apical constriction during neural tube closure in the Xenopus blastula has been demonstrated to depend on Rap1 function downstream of the Shroom protein; however, the relevant GEF in this scenario remains to be identified. The notion that Rap activation in distinct developmental processes is specified by dedicated GEFs also suggests that Rap effectors are selected in order to fulfill pathway requirements. In light of this, dPDZ-GEF and Rap1 have been implicated in the regulation of mitogen-activated protein kinase activity during differentiation of the Drosophila compound eye, and another reported that D-Raf relays a signal from Rap to mitogen-activated protein kinase in Torso-receptor-dependent terminal differentiation of the early Drosophila embryo. Together, these findings suggest the possibility that dPDZ-GEF could trigger the activation of the Rap/D-Raf pathway to regulate certain differentiation processes. The data reveal a novel function for dPDZ-GEF as an activator of Rap in the implementation of epithelial cell shape changes required for sheet migration and homeostatic cell shape maintenance in the genesis of the wing imaginal disc epithelium. Evidence is provided that Cno functions as a relevant effector of Rap downstream of dPDZ-GEF in these events and that the dPDZ-GEF/Rap/Cno module is connected to the regulation of MyoII and the generation and modulation of appropriate lateral cell contractility. Thus, these findings have unveiled a pathway linking the Rap activator dPDZ-GEF to MyoII and the regulation of lateral contractility and cell shape in epithelium migration and homeostasis. Further elucidation of dPDZ-GEF-interacting proteins and the molecular underpinnings of MyoII regulation downstream of this module in epithelial cells will be key to understanding these aspects of tissue morphogenesis (Boettner, 2007).
In Drosophila embryos, macrophages originate from the cephalic mesoderm and perform a complex migration throughout the entire embryo. The molecular mechanisms regulating this cell migration remain largely unknown. This study identified the Drosophila PDZ G-nucleotide exchange factor (PDZ-GEF) Dizzy as a component essential for normal macrophage migration. In mutants lacking Dizzy, macrophages have smaller cellular protrusions, and their migration is slowed down significantly. This phenotype appears to be cell-autonomous, as it is also observed in embryos with a dsRNA-induced reduction of dizzy function in macrophages. In a complementary fashion, macrophages overexpressing Dizzy are vastly extended and form very long protrusions. These cell shape changes depend on the function of the small GTPase Rap1: in rap1 mutants, Dizzy is unable to induce the large protrusions. Furthermore, forced expression of a dominant-active form of Rap1, but not of the wild-type form, induces similar cell shape changes as Dizzy does overexpression. These findings suggest that Dizzy acts through Rap1. It is further proposed that integrin-dependent adhesion is a Rap1-mediated target of Dizzy activity: in integrin mutants, neither Dizzy nor Rap1 can induce cell shape changes in macrophages. These data provide the first link between a PDZ-GEF, the corresponding small GTPase and integrin-dependent cell adhesion during cell migration in embryonic development (Huelsmann, 2006).
Stem cells will undergo self-renewal to produce new stem cells if they are maintained in their niches. The regulatory mechanisms that recruit and maintain stem cells in their niches are not well understood. In Drosophila testes, a group of 12 nondividing somatic cells, called the hub, identifies the stem cell niche by producing the growth factor Unpaired (Upd). This study shows that Rap-GEF/Rap signaling controls stem cell anchoring to the niche through regulating DE-cadherin-mediated cell adhesion. Loss of function of a Drosophila Rap-GEF (Gef26) results in loss of both germline and somatic stem cells. The Gef26 mutation specifically impairs adherens junctions at the hub-stem cell interface, which results in the stem cells 'drifting away' from the niche and losing stem cell identity. Thus, the Rap signaling/E-cadherin pathway may represent one mechanism that regulates polarized niche formation and stem cell anchoring (Wang, 2006).
Rap1 is a Ras-related GTPase that is principally involved in integrin- and E-cadherin-mediated adhesion. Rap1 is transiently activated in response to many incoming signals via a large family of guanine nucleotide exchange factors (GEFs). The lack of potent Rap1 dominant-negative mutants has limited the ability to decipher Rap1-dependent pathways. In this study a procedure was developed to generate such mutants consisting in the oligonucleotide-mediated mutagenesis of residues 14-19, selection of mutants presenting an enhanced interaction with Epac2 by yeast two-hybrid screening and counter-screening for mutants still interacting with Rap effectors. In detail analysis of their interaction capacity with various Rap-GEFs in the yeast two-hybrid system revealed that mutants of residues 15 and 16 interacted with Epacs, C3G and CalDAG-GEFI, whereas mutants of position 17 had selectively lost their ability to bind CalDAG-GEFI as well as, for some, C3G. In cellular models where Rap1 is activated via endogenous GEFs, the Rap1[S17A] mutant inhibits both the cAMP-Epac and EGF-C3G pathways, whereas Rap1[G15D] selectively interferes with the latter. Finally, Rap1[S17A] is able to act as a bona fide dominant-negative mutant in vivo since it phenocopies the eye-reducing and lethal effects of D-Rap1 deficiency in Drosophila, effects that are overcome by the overexpression of D-Epac or D-Rap1 (Dupuy, 2005).
Rap1 belongs to the highly conserved Ras subfamily of small GTPases. In Drosophila, Rap1 plays a critical role in many different morphogenetic processes, but the molecular mechanisms executing its function are unknown. Canoe (Cno), the Drosophila homolog of mammalian junctional protein AF-6, has been shown to act as an effector of Rap1 in vivo. Cno binds to the activated form of Rap1 in a yeast two-hybrid assay, the two molecules colocalize to the adherens junction, and they display very similar phenotypes in embryonic dorsal closure (DC), a process that relies on the elongation and migration of epithelial cell sheets. Genetic interaction experiments show that Rap1 and Cno act in the same molecular pathway during DC and that the function of both molecules in DC depends on their ability to interact. Rap1 acts upstream of Cno, but Rap1, unlike Cno, is not involved in the stimulation of JNK pathway activity, indicating that Cno has both a Rap1-dependent and a Rap1-independent function in the DC process (Boettner, 2003).
Rap1 cycles between an inactive GDP-bound and an active GTP-bound state, eliciting distinct downstream responses in the active state. Mammalian Rap proteins were originally identified as antagonists of oncogenic Ras, but more recent studies suggest that the function of Rap1 is largely independent of Ras. While Ras is mainly localized at the plasma membrane, Rap1 has been found in different membrane compartments, depending on the cell type. Further, Rap1 activation appears to be stimulated by numerous exchange factors that do not act on the prototypic Ras GTPases. Rap1 has been shown to act in a Ras-independent manner in the production of superoxide, in cAMP-induced neurite outgrowth, and, most recently, in the regulation of integrin-mediated cell adhesion and AMPA receptor trafficking during synaptic plasticity (Boettner, 2003 and references therein).
Perhaps the most important insights into the function of Rap1 are emerging from studies in Drosophila. Loss-of-function (lof) mutations in Drosophila Rap1 cause severe morphogenetic abnormalities during embryonic development, while cell proliferation and cell fate determination, processes that rely heavily on regulation by Ras, appear to be unaffected. Specifically, the ventral invagination and migration of mesodermal precursors in the embryo are severely impaired, as are head involution, dorsal closure, and the migration of gonadal precursors (Asha, 1999). More recently, Rap1 has been shown to play a role in cell adhesion, specifically in the positioning of adherens junctions in proliferating epithelial cells (Knox, 2002). These findings strongly suggest that Rap1 plays a largely Ras-independent role in cell migration and morphogenesis (Boettner, 2003 and references therein).
Little is currently known about the signaling pathways mediating the downstream effects of Rap1 in vertebrates or Drosophila. A number of molecules that were originally identified in vertebrates as Ras-interacting proteins, including B-Raf, members of the RalGEF family, and AF-6, were subsequently shown to associate with Rap1 as well. However, the relevance of these interactions for Rap1 function in vivo remains largely unknown; to date, none of these molecules have been shown to act as Rap1 targets in an in vivo context (Boettner, 2003 and references therein).
This study reports that Canoe (Cno), the Drosophila ortholog of AF-6, acts as an effector of Rap1 during dorsal closure (DC) of the Drosophila embryo. DC is a morphogenetic process that occurs during midembryogenesis and involves the dorsalward movement of the lateral ectoderm over the amnioserosa, a transient structure that covers the dorsal aspect of the embryo, to enclose the embryo. This process relies entirely on the migration and elongation of ectodermal cells, without cell recruitment or proliferation, and is akin to the epithelial cell sheet movements that occur during wound healing. Among the genes identified as necessary for normal DC are proteins associated with the cytoskeleton and/or cell junctions and components of the Drosophila Jun N-terminal kinase (JNK) and Decapentaplegic (Dpp) pathways. cno is required for DC; its protein is localized to the adherens junction and feeds into the JNK pathway by an unknown mechanism. Apart from the fact that it interacts with the ZO-1 homolog Tamou, nothing is known about the regulation of Cno activity at the adherens junction (Boettner, 2003).
Cno has been identified as a protein that interacts with activated Rap1 in a yeast two-hybrid screen. To address the physiological relevance of this interaction, localization studies, a comparative phenotypic analysis, and genetic interaction experiments were undertaken for the two proteins. Rap1 and cno loci are shown to interact synergistically in DC and the physical interaction between Rap1 and Cno is required for DC. The role of Canoe in promoting JNK pathway activity is independent of Rap1 and Canoe therefore has two separate functions in DC (Boettner, 2003).
In Drosophila, embryos lacking both zygotic and maternal Rap1 display strong defects in diverse morphological aspects of embryogenesis, such as ventral invagination, migration of mesodermal precursors, head involution, and DC. A key question is which effector pathways mediate the morphogenetic functions of Rap1. The yeast two-hybrid system was used to identify Drosophila Rap1-specific effector molecules from an embryonic library and several cDNAs encoding Cno were retrieved. Both N-terminal Ras-binding domains (RA1 and RA2) of Cno possess Rap1-binding potential and they interact only with a constitutively active Rap1 mutant, Rap1V12, but not with a dominant negative version of Rap1, Rap1N17, suggesting that Cno may act as an effector for Rap1 (Boettner, 2003).
Several lines of evidence are provided confirming this hypothesis. Rap1 and Cno partially colocalize at the adherens junction in the two tissues that are involved in DC, the amnioserosa and the lateral ectoderm, with Rap1 being present at the entire lateral membrane and also showing vesicular expression throughout the cytoplasm. Moreover, loss of function of the two molecules leads to similar phenotypes, at both the cuticular and the cellular level. To directly address the question whether Rap1 utilizes Cno as an effector during DC, a series of genetic experiments were conducted. They demonstrate that the two molecules act in the same pathway and their physical interaction is essential for their function in DC: (1) Removal of zygotic Rap1 strongly enhances the phenotype of a weak heteroallelic cno combination; (2) removal of the RA-interaction domains and, thus, removal of the ability to bind Rap1, reduces the ability of cno transgenes to rescue the cno lof phenotype, and (3) removal of the RA-interaction domains eliminates the ability of cno to rescue Rap1N17. Finally, the finding that activated Rap1V12 fails to rescue the cno lof defects indicates that Rap1 acts upstream of Cno. Taken together, the yeast two-hybrid data, colocalization results, and genetic interaction experiments provide comprehensive evidence that Cno functions as a downstream effector of Rap1 in the DC process. These findings represent the first demonstration of a protein acting as a Rap1 effector in vivo (Boettner, 2003).
The events downstream of Rap1 and Cno, however, appear to be more complex. Several independent findings suggest that Cno's role in DC can be separated into Rap1-independent and Rap1-dependent functions: removal of the RA-interaction domains does not affect the ability of the remainder of the protein to localize to the adherens junction, and the mutant protein retains the capacity to partially rescue the DC defect of a cno lof mutant. Further, Cno feeds into the JNK pathway, while Rap1 does not: dpp expression levels in the LE are significantly reduced in cno lof embryos at later stages of DC, but appear unaffected in Rap1 mutants. In addition, cno lof is partially rescued by overexpressing bsk (DJNK), whereas the Rap1N17 defect is not. Given the multidomain structure of Cno, it is not surprising that the molecule would participate in multiple pathways. Such a bifurcation of the pathway would also explain the lack of transitivity observed in rescue experiments: Rap1 lof is (partially) rescued by cno overexpression, cno lof is (partially) rescued by bsk overexpression, but Rap1 lof is not rescued by bsk overexpression. The fact that both cnoDeltaN and bsk are unable to rescue Rap1 lof demonstrates that the Rap1-independent function of Cno cannot compensate for the loss of Rap1. This leaves the reciprocal question of whether Rap1 may have a second, Cno-independent function in DC. The fact that the DC phenotype of Rap1N17 is as severe as that of cno lof without affecting JNK pathway signaling might suggest that Rap1 has additional effectors in DC (as does the fact that the phenotype of Rap1N17 is more severe than that of cno2; ptcGAL4 UAScnoDeltaN). However, no conclusive evidence has been found to support this idea, since the additional effectors of Rap1 identified in the yeast two-hybrid screen have not been investigated for their role in DC (Boettner, 2003).
One obstacle in investigating the function of Rap1 is its pleiotropy. A detailed analysis of DC defects, in particular, is difficult to perform in Rap1 null embryos, due to the severe disruption of multiple aspects of embryonic development prior to DC. Therefore, use was made of the dominant negative Rap1N17 mutant. When expressed at appropriate stages in the epithelial cells that are involved in the DC process, this transgene results in robust DC defects. However, early in vitro studies appeared to show that the Rap1N17 mutant does not compete well with normal Rap1 for the GEF C3G, calling into question whether this mutant protein can be regarded as a Rap1 dominant negative. But in vivo studies using mammalian Rap1 and now the current study clearly show that Rap1N17 acts as a dominant negative mutant in Rap1 signaling. The successful rescue of Rap1N17 with a concomitantly expressed Rap1wt transgene demonstrates the specificity of the mutant. Further, dominant negative versions of Drosophila Ras1 and Ras2, the counterparts of the mammalian H, K, and N-Ras and of the R-Ras proteins, respectively, do not disrupt DC when they are examined under the same conditions. This shows that the interaction between DRas1 and Cno detected in vitro and the genetic interaction between DRas1 and Cno that influences cone cell formation in the Drosophila eye, have no role during DC (Boettner, 2003).
On which cellular processes might Rap1 and Cno act? Cno is a multidomain protein consisting of several known and putative protein-interaction domains, including the two RA domains and a PDZ domain, which targets proteins to specific cell membranes and assembles proteins into supramolecular signaling complexes, but no catalytic domain. Cno localizes to the adherens junction and may act by localizing and clustering signal transduction components at the junction or by modulating the mechanical resistance of the adherens junction, and thus, directly or indirectly, influence JNK signaling. Since Cno is found at the adherens junctions under Rap1 lof conditions as well as in the absence of its RA domains, Rap1 cannot be required for the initial localization of the Cno protein, suggesting that Rap1 influences the activity of Cno by changing its conformation. However, another possibility is suggested by a study by Knox (2002), where it was found that Rap1 function is required for evenly (re-)distributing adherens junction components in wing disc epithelial cells after mitosis. It is likely that the adherens junctions in the cells that undergo stretching in the embryonic ectoderm during DC are similarly subject to dynamic reorganization, which may in part be regulated by the Rap1/Cno complex. This idea would be consistent with the observation that in Rap1 and cno lof mutants the lateral ectoderm begins its dorsal stretching, but is then unable to complete the process. Interestingly, Rap1 in mammalian cells has been shown to be activated in cell-stretching assays. In this system, force initiation apparently results in the activation of the JNK kinase family member p38, suggesting the existence of a Rap1-dependent 'mechanosensory' pathway. The data fit this idea. Future studies using fluorescently tagged Rap1 and Cno proteins and live imaging will shed light on dynamic aspects of their localization and function during DC (Boettner, 2003).
Cell-cell junctions are distributed evenly around the lateral circumference of cells within an epithelium. This study found that the even distribution of adherens junctions is an active process that requires the small guanosine triphosphatase Rap1. Cells mutant forRap1 condensed their adherens junctions to one side of the cell. This disrupted normal epithelial cell behavior, and mutant cell clones dispersed into the surrounding wild-type tissue. Rap1 is enriched at adherens junctions, particularly between newly divided sister cells where it may reseal the adherens junction ring. The regulation of adherens junction positioning could play a role in cell mobility and cell division (Knox, 2002).
Cells within an epithelium are linked by several types of junctions. Encircling the apical ends of cells are adherens junctions, which link to the actin cytoskeleton intracellularly and can thereby transmit force across the lateral plane of the epithelium. Although much attention has been paid to the regulation of apico-basal localization of adherens junctions, little is known about the mechanisms that underlie their even distribution around the cell circumference. Rap1 is a small guanosine triphosphatase (GTPase) of the Ras familythat has a role in regulating Drosophila morphogenesis through an undetermined mechanism. During Drosophila wing development, epithelial cells related by lineage normally stay in a coherent group. However, clones of cells mutant for Rap1 dispersed into surrounding wild-type tissue, indicating that loss of Rap1 function disrupts the normal cell-cell adhesion mechanism that keeps lineage-related cells in a coherent group. This phenotype has not been observed for other mutations studied by clonal analysis, including loss-of-function mutations in related GTPases such as Rho1 and Ras85D. Cells lacking Rap1 function still respect the lineage restriction at the anterior-posterior compartment boundary. Observations of shape defects in Rap1 mutant cells suggested that Rap1 might regulate apical cell-cell adhesion. Pupal wing cells mutant for Rap1 lacked the normal hexagonal shape, and the area of the apical, but not the basal, surface was reduced relative to that of wild-type cells. Dispersed mutant cells were often observed in pairs or groups of four cells (Knox, 2002).
To assess the role of Rap1 in cell-cell adhesion, the subcellular localization was examined of adherens junctions and the adjacent, more basal, septate junctions. In contrast to their even distribution around the apical circumference of wild-type epithelial cells, adherens junction components -- including the cell-surface adhesion protein DE-cadherin and two cytoskeletal proteins, α-catenin and β-catenin [visualized with a green fluorescent protein (GFP)] -- were found predominantly on one side ofRap1 mutant pupal wing cells. In a count of 856 cells containing such clusters of adherens junction components, 702 cells (82%) had adherens junctions condensed into a contact with just one neighboring cell. Clusters were also seen between a mutant cell and 2 other mutant cells, or connecting a mutant cell with 3, 4, or 5 neighboring mutant cells. Clusters of adherens junction proteins were observed only at interfaces between mutant cells, and not between a mutant and a wild-type cell. At interfaces between mutant and wild-type cells, normal levels of adherens junctions were observed (Knox, 2002).
Two proteins that may form a molecular link between Rap1 and adherens junctions are the multidomain cytoskeletal linker proteins AF6/canoe and ZO-1. Both AF6 and its Drosophila ortholog canoe bind to activated Rap1, and canoe interacts with ZO-1. Vertebrate ZO-1 binds to the adherens junction component α-catenin, thus completing a possible link from Rap1 to adherens junctions. Both canoe and ZO-1 localize to adherens junctions in normal Drosophila epithelia and like the other adherens junction components, they distributed primarily to one side of Rap1 mutant cells. Although ZO-1 also participates in vertebrate tight junctions and may be present in Drosophila septate junctions, there was not a comparable alteration in septate junction-associated proteins in Rap1 mutant cells. The MAGUK protein Discs large and the band 4.1 ortholog coracle were evenly distributed around the circumference of Rap1 mutant cells. Thus, loss of Rap1 function specifically impairs even distribution of adherens junctions around the cell circumference. The maintenance of septate junctions could explain how Rap1 mutant cells still retain enough cell adhesion to remain within the epithelium (Knox, 2002).
The misplacement of adherens junctions in Rap1 mutant clones suggests that dispersion could be due to sorting caused by differential adhesion. L fibroblasts transfected with P-cadherin sort according to their level of cadherin expression, and such differential adhesion plays a role in Drosophila oocyte positioning at the posterior of the egg chamber. The Rap1 mutant cell-dispersal phenotype may be an additional in vivo example of cell sorting according to differential DE-cadherin-mediated adhesion, although in this case, the amount of adhesion is altered by the failure to distribute adherens junctions evenly around the cell circumference, rather than by altered overall cadherin expression. Provided that the quantity of adherens junction components reflects the strength of adhesion, Rap1 mutant cells could have adhered most strongly to mutant cells on the sides of the cell containing adherens junction clusters, very weakly to other mutant cells, and at normal strength to adjacent wild-type cells. Adhesion between mutant and wild-type cells that was stronger than adhesion between most Rap1 mutant cells could have drawn small groups of mutant cells into wild-type tissue. These results suggest that regulation of the subcellular distribution of cell-cell junctions could play a role in the mobility and invasiveness of cells within an epithelium (Knox, 2002).
Because adherens junctions are also misplaced in undispersed Rap1mutant cells, misplacement is likely to be the cause rather than the consequence of cell dispersal. In this case, mislocalization of adherens junctions during wing development should precede cell dispersal. Clonal cells mutant for Rap1 in the late (wandering) third-instar imaginal disc did not disperse, yet the adherens junction component α-catenin was already mislocalized, indicating that adherens junction mislocalization precedes dispersal. The larvae pupariate within a few hours of this time, and dispersal of Rap1 mutant cells was first observed 2 hours after pupariation. Evagination of the disc during this time period requires extracellular protease activity, which is thought to loosen cell-cell and cell-extracellular matrix contacts, allowing cell rearrangements and shape changes to occur. Cell rearrangements can be observed as the elongation of marked clones; therefore, cells normally exchange neighbors even if they do not normally mix. Loosening of extracellular contacts likely allows Rap1 mutant cells to mix with their wild-type neighbors. Consistent with this, cell dispersion was initially more pronounced at the distal end of the evaginating wing, where cell rearrangements are first initiated (Knox, 2002).
To investigate whether Rap1 recruitment to adherens junctions is involved in aberrant junction distribution in mutant cells, a transgene was expressed encoding a GFP-Rap1 fusion protein. This fusion protein is under the control of the endogenous Rap1 promoter and was expressed ubiquitously throughout development. In normal wing imaginal disc cells, GFP-Rap1 was broadly distributed in the cytoplasm and basolateral membrane and highly concentrated at the position of the adherens junctions, consistent with the possible interaction of Rap1 with adherens junction proteins canoe and ZO-1. Despite its own polarized distribution, Rap1 was not required for normal apico-basal distribution of adherens junctions; α-catenin was located apically in Rap1 mutant imaginal disc clones. β-Catenin and DE-cadherin also did not mislocalize along the apico-basal axis in Rap1 mutant pupal wing clones (Knox, 2002).
The distribution of GFP-Rap1 in dividing cells suggests a mechanism by which Rap1 might normally act to ensure even adherens junction distribution. Dividing cells in the wing imaginal disc retain their adherens junctions with surrounding cells, and the localization of GFP-Rap1 was not altered during division. However, GFP-Rap1 was consistently enriched at the junction between newly formed sister cells. A transient enrichment of GFP-Rap1 between sister cells in the epidermis of living embryos was also observed. Hence, Rap1 may reorganize the adherens junction ring subsequent to or during late cytokinesis to ensure that appropriate amounts of adherens junctions are maintained around the circumference of new cells (Knox, 2002).
One model explaining how loss of Rap1 function during cytokinesis leads to adherens junction clustering is as follows. Maintenance of adherens junction distribution throughout cell division requires a mechanism to convert the single adherens junction ring into two rings, involving breaking and resealing of the ring during cytokinesis. Rap1 could be essential for this process. Failure to reseal the adherens junction ring could allow it to recoil to one side of the cell, driven by contraction of the actin and myosin present in the ring. This would cause rearrangement of cadherin contacts into clusters on sides adjacent to mutant cells with a similar defect, but not on the sides of the cell contacting wild-type cells, where cadherin distribution is stabilized at a normal density. Clusters would most likely form at the interface between sister cells, because both cells' rings recoil at the same time. However, clusters could also form between two adjacent mutant cells that are not sisters if they were in a similar state at the same time. Accordingly, the 14% of clusters between one mutant cell and two others demonstrates that clusters were present at interfaces between cells that are not sisters from their most recent division. Further rounds of division could lead to segregation of clusters into just one daughter cell, producing cells with few adherens junctions, as seen within some Rap1 mutant clones (Knox, 2002).
Rap1 maintains circumferential adherens junction distribution in cells and thus shares with Rho GTPase family members the ability to regulate the cytoskeleton and cell adhesion. Thus, its demonstrated role in morphogenetic processes that are driven by adhesion-dependent cell shape changes and movements may involve regulation of the link between the cytoskeleton and adherens junctions (Knox, 2002).
PDZ-GEF is a novel guanine nucleotide exchange factor for Rap1 GTPase. This study isolated Drosophila melanogaster PDZ-GEF (dPDZ-GEF), which contains the all-conserved domains of mammalian and nematode PDZ-GEF including cyclic nucleotide monophosphate-binding, Ras exchange motif, PDZ, RA, and GEF domains. dPDZ-GEF loss-of-function mutants were defective in the development of various organs including eye, wing, and ovary. Many of these phenotypes are strikingly similar to the phenotype of the rolled mutant, implying that dPDZ-GEF functions upstream of the mitogen-activated protein (MAP) kinase pathway. Indeed, it was found that dPDZ-GEF is specifically involved in photoreceptor cell differentiation, facilitating its neuronal fate via activation of the MAP kinase pathway. Rap1 was found to link dPDZ-GEF to the MAP kinase pathway; however, Ras was not involved in the regulation of the MAP kinase pathway by dPDZ-GEF and actually had an inhibitory function. The analyses of ovary development in dPDZ-GEF-deficient mutants also demonstrated another role of dPDZ-GEF independent of the MAP kinase signaling pathway. Collectively, these findings identify dPDZ-GEF as a novel upstream regulator of various morphogenetic pathways and demonstrate the presence of a novel, Ras-independent mechanism for activating the MAP kinase signaling pathway (Lee, 2002).
The Ras-related Rap GTPases are highly conserved across diverse species but their normal biological function is not well understood. Initial studies in mammalian cells suggested a role for Rap as a Ras antagonist. More recent experiments indicate functions in calcium- and cAMP-mediated signaling and it has been proposed that protein kinase A-mediated phosphorylation activates Rap in vivo. This study shows that Ras1-mediated signaling pathways in Drosophila are not influenced by Rap1 levels, suggesting that Ras1 and Rap1 function via distinct pathways. Moreover, a mutation that abolishes the putative cAMP-dependent kinase phosphorylation site of Drosophila Rap1 can still rescue the Rap1 mutant phenotype. These experiments show that Rap1 is not needed for cell proliferation and cell-fate specification but demonstrate a critical function for Rap1 in regulating normal morphogenesis in the eye disk, the ovary and the embryo. Rap1 mutations also disrupt cell migrations and cause abnormalities in cell shape. These findings indicate a role for Rap proteins as regulators of morphogenesis in vivo (Asha, 1999).
The cellular signal transduction pathways by which C3G, a RAS family guanine nucleotide exchange factor, mediates v-crk transformation are not well understood. This study reports the identification of Drosophila C3G, which, like its human cognate, specifically binds to CRK but not DRK/GRB2 adaptor molecules. During Drosophila development, constitutive membrane binding of C3G, which also occurs during v-crk transformation, results in cell fate changes and overproliferation, mimicking overactivity of the RAS-MAPK pathway. The effects of C3G overactivity can be suppressed by reducing the gene dose of components of the RAS-MAPK pathway and of RAP1. These findings provide the first in vivo evidence that membrane localization of C3G can trigger activation of RAP1 and RAS resulting in the activation of MAPK, one of the hallmarks of v-crk transformation previously thought to be mediated through activation of SOS (Ishimaru, 1999).
The Drosophila fat facets gene encodes a deubiquitinating enzyme that regulates a cell communication pathway essential very early in eye development, prior to facet assembly, to limit the number of photoreceptor cells in each facet of the compound eye to eight. The Fat facets protein facilitates the production of a signal in cells outside the developing facets that inhibits neural development of particular facet precursor cells. Novel gain-of-function mutations in the Drosophila Rap1 and Ras1 genes are described that interact genetically with fat facets mutations. Analysis of these genetic interactions reveals that Fat facets has an additional function later in eye development involving Rap1 and Ras1 proteins. Moreover, the results suggest that undifferentiated cells outside the facet continue to influence facet assembly later in eye development (Li, 1997).
The activity of Ras family proteins is modulated in vivo by the function of GTPase activating proteins, which increase their intrinsic rate of GTP hydrolysis. This study has isolated cDNAs encoding a GAP for the Drosophila Rap1 GTPase. Drosophila Rapgap1 encodes an 850-amino acid protein with a central region that displays substantial sequence similarity to human RapGAP. This domain, when expressed in Escherichia coli, potently stimulates Rap1 GTPase activity in vitro. Unlike Rap1, which is ubiquitously expressed, Rapgap1 expression is highly restricted. Rapgap1 is expressed at high levels in the developing photoreceptor cells and in the optic lobe. Rapgap1 mRNA is also localized in the pole plasm in an oskar-dependent manner. Although mutations that completely abolish Rapgap1 function display no obvious phenotypic abnormalities, overexpression of Rapgap1 induces a rough eye phenotype that is exacerbated by reducing Rap1 gene dosage. Thus, Rapgap1 can function as a negative regulator of Rap1-mediated signaling in vivo (Chen, 1997).
This study has characterized two new ras-related genes rap1 and rap2 from a human cDNA library, by hybridization with the Drosophila Dras3 gene at low stringency conditions. The rap1 and rap2 genes encode proteins of 184 and 183 amino acid respectively with molecular weights of 20.9 kd and 20.7 kd. These proteins are 53% and 46% identical to the human K-ras protein and share several properties with the classical ras proteins. The C-terminal cysteine involved in the membrane anchoring as well as the GTP binding regions of the p21 ras proteins are present in the rap proteins suggesting that these proteins could bind GTP/GDP and have a membrane localization. The most striking difference between the rap and ras proteins resides in their 61st amino acid. As in the Drosophila Dras3 protein, both rap proteins have a threonine instead of the glutamine found at position 61 of the classical ras proteins. Furthermore the putative effector domain of the ras proteins is strictly conserved in the rap1 protein whereas only one amino acid difference is found in the rap2 protein. This suggests that the rap proteins might interact with the same effector as the ras proteins (Pizon, 1988).
Search PubMed for articles about Rap1
Asha, H., de Ruiter, N. D., Wang, M. G. and Hariharan, I. K. (1999). The Rap1 GTPase functions as a regulator of morphogenesis in vivo. EMBO J 18: 605-615. PubMed ID: 9927420
Boettner, B., Harjes, P., Ishimaru, S., Heke, M., Fan, H. Q., Qin, Y., Van Aelst, L. and Gaul, U. (2003). The AF-6 homolog canoe acts as a Rap1 effector during dorsal closure of the Drosophila embryo. Genetics 165: 159-169. PubMed ID: 14504224
Boettner, B. and Van Aelst, L. (2007). The Rap GTPase activator Drosophila PDZ-GEF regulates cell shape in epithelial migration and morphogenesis. Mol Cell Biol 27: 7966-7980. PubMed ID: 17846121
Borghi, N., Sorokina, M., Shcherbakova, O. G., Weis, W. I., Pruitt, B. L., Nelson, W. J. and Dunn, A. R. (2012). E-cadherin is under constitutive actomyosin-generated tension that is increased at cell-cell contacts upon externally applied stretch. Proc Natl Acad Sci U S A 109: 12568-12573. PubMed ID: 22802638
Bos, J. L. (2005). Linking Rap to cell adhesion. Curr Opin Cell Biol 17: 123-128. PubMed ID: 15780587
Carmena, A., Speicher, S. and Baylies, M. (2006). The PDZ protein Canoe/AF-6 links Ras-MAPK, Notch and Wingless/Wnt signaling pathways by directly interacting with Ras, Notch and Dishevelled. PLoS One 1: e66. PubMed ID: 17183697
Chen, F., Barkett, M., Ram, K. T., Quintanilla, A. and Hariharan, I. K. (1997). Biological characterization of Drosophila Rapgap1, a GTPase activating protein for Rap1. Proc Natl Acad Sci U S A 94: 12485-12490. PubMed ID: 9356476
Choi, W., Harris, N. J., Sumigray, K. D. and Peifer, M. (2013). Rap1 and Canoe/afadin are essential for establishment of apical-basal polarity in the Drosophila embryo. Mol Biol Cell 24: 945-963. PubMed ID: 23363604
Ding, J., Tchaicheeyan, O. and Ambrosio, L. (2010). Drosophila Raf's N terminus contains a novel conserved region and can contribute to torso RTK signaling. Genetics 184: 717-729. PubMed ID: 20008569
Dupuy, A. G., L'Hoste, S., Cherfils, J., Camonis, J., Gaudriault, G. and de Gunzburg, J. (2005). Novel Rap1 dominant-negative mutants interfere selectively with C3G and Epac. Oncogene 24: 4509-4520. PubMed ID: 15856025
Fournier, G., Cabaud, O., Josselin, E., Chaix, A., Adelaide, J., Isnardon, D., Restouin, A., Castellano, R., Dubreuil, P., Chaffanet, M., Birnbaum, D. and Lopez, M. (2011). Loss of AF6/afadin, a marker of poor outcome in breast cancer, induces cell migration, invasiveness and tumor growth. Oncogene 30: 3862-3874. PubMed ID: 21478912
Glading, A., Han, J., Stockton, R. A. and Ginsberg, M. H. (2007). KRIT-1/CCM1 is a Rap1 effector that regulates endothelial cell cell junctions. J Cell Biol 179: 247-254. PubMed ID: 17954608
Harris, T. J. and Peifer, M. (2004). Adherens junction-dependent and -independent steps in the establishment of epithelial cell polarity in Drosophila. J Cell Biol 167: 135-147. PubMed ID: 15479740
Harris, T. J. and Peifer, M. (2005). The positioning and segregation of apical cues during epithelial polarity establishment in Drosophila. J Cell Biol 170: 813-823. PubMed ID: 16129788
Hoshino, T., Sakisaka, T., Baba, T., Yamada, T., Kimura, T. and Takai, Y. (2005). Regulation of E-cadherin endocytosis by nectin through afadin, Rap1, and p120ctn. J Biol Chem 280: 24095-24103. PubMed ID: 15857834
Huelsmann, S., Hepper, C., Marchese, D., Knoll, C. and Reuter, R. (2006). The PDZ-GEF dizzy regulates cell shape of migrating macrophages via Rap1 and integrins in the Drosophila embryo. Development 133: 2915-2924. PubMed ID: 16818452
Ishimaru, S., Williams, R., Clark, E., Hanafusa, H. and Gaul, U. (1999). Activation of the Drosophila C3G leads to cell fate changes and overproliferation during development, mediated by the RAS-MAPK pathway and RAP1. EMBO J 18: 145-155. PubMed ID: 9878058
Kim, C., Ye, F. and Ginsberg, M. H. (2011). Regulation of integrin activation. Annu Rev Cell Dev Biol 27: 321-345. PubMed ID: 21663444
Knox, A. L. and Brown, N. H. (2002). Rap1 GTPase regulation of adherens junction positioning and cell adhesion. Science 295: 1285-1288. PubMed ID: 11847339
Kooistra, M. R., Dube, N. and Bos, J. L. (2007). Rap1: a key regulator in cell-cell junction formation. J Cell Sci 120: 17-22. PubMed ID: 17182900
Lampugnani, M. G., Orsenigo, F., Rudini, N., Maddaluno, L., Boulday, G., Chapon, F. and Dejana, E. (2010). CCM1 regulates vascular-lumen organization by inducing endothelial polarity. J Cell Sci 123: 1073-1080. PubMed ID: 20332120
Lee, J. H., Cho, K. S., Lee, J., Kim, D., Lee, S. B., Yoo, J., Cha, G. H. and Chung, J. (2002). Drosophila PDZ-GEF, a guanine nucleotide exchange factor for Rap1 GTPase, reveals a novel upstream regulatory mechanism in the mitogen-activated protein kinase signaling pathway. Mol Cell Biol 22: 7658-7666. PubMed ID: 12370312
Li, Q., Hariharan, I. K., Chen, F., Huang, Y. and Fischer, J. A. (1997). Genetic interactions with Rap1 and Ras1 reveal a second function for the fat facets deubiquitinating enzyme in Drosophila eye development. Proc Natl Acad Sci U S A 94: 12515-12520. PubMed ID: 9356481
Linnemann, T., Geyer, M., Jaitner, B. K., Block, C., Kalbitzer, H. R., Wittinghofer, A. and Herrmann, C. (1999). Thermodynamic and kinetic characterization of the interaction between the Ras binding domain of AF6 and members of the Ras subfamily. J Biol Chem 274: 13556-13562. PubMed ID: 10224125
Liu, J. J., Stockton, R. A., Gingras, A. R., Ablooglu, A. J., Han, J., Bobkov, A. A. and Ginsberg, M. H. (2011). A mechanism of Rap1-induced stabilization of endothelial cell--cell junctions. Mol Biol Cell 22: 2509-2519. PubMed ID: 21633110
Martin, A. C., Kaschube, M. and Wieschaus, E. F. (2009). Pulsed contractions of an actin-myosin network drive apical constriction. Nature 457: 495-499. PubMed ID: 19029882
McKinley, R. F. and Harris, T. J. (2012). Displacement of basolateral Bazooka/PAR-3 by regulated transport and dispersion during epithelial polarization in Drosophila. Mol Biol Cell 23: 4465-4471. PubMed ID: 23015757
McMahon, A., Supatto, W., Fraser S. E. and Stathopoulos, A. (2008). Dynamic analyses of Drosophila gastrulation provide insights into collective cell migration. Science 322: 1546-1550. PubMed ID: 19056986
McMahon, A., Reeves, G. T., Supatto, W. and Stathopoulos, A. (2010). Mesoderm migration in Drosophila is a multi-step process requiring FGF signaling and integrin activity. Development 137(13): 2167-75. PubMed ID: 20530544
O'Keefe, D. D., Prober, D. A., Moyle, P. S., Rickoll, W. L. and Edgar, B. A. (2007). Egfr/Ras signaling regulates DE-cadherin/Shotgun localization to control vein morphogenesis in the Drosophila wing. Dev Biol 311: 25-39. PubMed ID: 17888420
O'Keefe, D. D., Gonzalez-Nino, E., Burnett, M., Dylla, L., Lambeth, S. M., Licon, E., Amesoli, C., Edgar, B. A. and Curtiss, J. (2009). Rap1 maintains adhesion between cells to affect Egfr signaling and planar cell polarity in Drosophila. Dev Biol 333: 143-160. PubMed ID: 19576205
O'Keefe, D. D., Gonzalez-Nino, E., Edgar, B. A. and Curtiss, J. (2012). Discontinuities in Rap1 activity determine epithelial cell morphology within the developing wing of Drosophila. Dev Biol 369: 223-234. PubMed ID: 22776378
Pizon, V., Chardin, P., Lerosey, I., Olofsson, B. and Tavitian, A. (1988). Human cDNAs rap1 and rap2 homologous to the Drosophila gene Dras3 encode proteins closely related to ras in the 'effector' region. Oncogene 3: 201-204. PubMed ID: 3045729
Sawyer, J. K., Harris, N. J., Slep, K. C., Gaul, U. and Peifer, M. (2009). The Drosophila afadin homologue Canoe regulates linkage of the actin cytoskeleton to adherens junctions during apical constriction. J. Cell Biol. 186(1): 57-73. PubMed Citation: 19596848
Sawyer, J. K., Choi, W., Jung, K. C., He, L., Harris, N. J. and Peifer, M. (2011). A contractile actomyosin network linked to adherens junctions by Canoe/afadin helps drive convergent extension. Mol Biol Cell 22: 2491-2508. PubMed ID: 21613546
Schwamborn, J. C. and Puschel, A. W. (2004). The sequential activity of the GTPases Rap1B and Cdc42 determines neuronal polarity. Nat Neurosci 7: 923-929. PubMed ID: 15286792
Shirinian, M., Popovic, M., Grabbe, C., Varshney, G., Hugosson, F., Bos, H., Rehmann, H. and Palmer, R. H. (2010). The Rap1 guanine nucleotide exchange factor C3G is required for preservation of larval muscle integrity in Drosophila melanogaster. PLoS One 5: e9403. PubMed ID: 20209136
Siekhaus, D., Haesemeyer, M., Moffitt, O. and Lehmann, R. (2010). RhoL controls invasion and Rap1 localization during immune cell transmigration in Drosophila. Nat Cell Biol 12: 605-610. PubMed ID: 20495554
Spahn, P., Ott, A. and Reuter, R. (2012). The PDZ-GEF protein Dizzy regulates the establishment of adherens junctions required for ventral furrow formation in Drosophila. J Cell Sci 125: 3801-3812. PubMed ID: 22553205
Wang, H., Singh, S. R., Zheng, Z., Oh, S. W., Chen, X., Edwards, K. and Hou, S. X. (2006). Rap-GEF signaling controls stem cell anchoring to their niche through regulating DE-cadherin-mediated cell adhesion in the Drosophila testis. Dev Cell 10: 117-126. PubMed ID: 16399083
Wang, Y. C., Khan, Z. and Wieschaus, E. F. (2013). Distinct Rap1 activity states control the extent of epithelial invagination via α-Catenin. Dev Cell 25: 299-309. PubMed ID: 23623612
date revised: 22 June 2013
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