Effects of Mutation or Deletion (part 2/2)

The role of Rac in dorsal closure

Two Drosophila genes DRacA and DRacB encode proteins with homology to mammalian Rac1 and Rac2. In transgenic flies expressing dominant inhibitory N17DRacA, results in a high frequency of defects in dorsal closure result, due to disruption of cell shape changes in the lateral epidermis. Embryonic expression of N17DRacA also affects germband retraction and head involution. The epidermal cell shape defects caused by expression of N17DRacA are accompanied by disruption of a localized accumulation of actin and myosin thought to be driving epidermal cell shape change (Hardin, 1995).

canoe and polychaetoid genetically interact giving rise to a more severe dorsal closure phenotype than one resulting from canoe mutation alone. canoe3 is an embryonic lethal allele of cno and displays a typical dorsal open phenotype. cnomis1 is a weak hypomorph that yields adult flies with rough eyes and subtle changes in the bristle number. In a search for mutations that interact with cnomis1, polychaetoid was identified as an enhancer of cno phenotypes. Flies doubly homozygous for pydtam1 and cnomis1 die as embryos; this represents a synthetic lethal combination. Examination of embryonic cuticles demonstrates that the cnomis1;pydtam1 double mutant remains open dorsally. Comparisons of cell shape during dorsal closure reveal that cno3 embryos exhibit insufficient elongation of cells. This is most evident in the leading edge cells, which appear square in cno3, in contrast to the oblong cells of wild-type. cnomis1;pydtam1 double mutant embryos exhibit a more extreme phenotype than single mutants: the leading edge cells elongate even less than in cno3 mutants. These results suggest that cno and pyd are required for coordinated cell shape changes in the cells of the leading edge and the lateral ectodermal cells during dorsal closure (Takahashi, 1998).

There is compelling evidence that the small GTPase Drac1 functions in dorsal closure as an upstream (early acting) element of the JNK pathway, which is composed of hemipterous, basket and puckered. To determine if cno is further upstream of Drac1, puckered-lacZ expression was examined in cno3 homozygous embryos: the leading edge of the epidermis in these embryos is driven to express Drac1V12, a constitutively active form of Drac1. If Drac1 is upstream of cno, then the effect of Drac1V12 on puc-lacZ transcription should be blocked by the loss of cno function. Targeted expression of Drac1V12 in the leading edge cells restores puc-lacZ transcription in cno3 homozygotes to a level comparable to that of wild-type. This result is compatible with the hypothesis that cno is upstream of Drac1, or that cno functions in a pathway parallel to that of Drac1 (Takahashi, 1998).

Demonstration of a physical interaction between Cno and Pyd places Pyd similarly upstream of Rac in the dorsal closure pathway. Cno and Pyd exhibit a similar tissue distribution and appear to colocalize at junctional membrane sites within the cell. ZO-1 is a component of both tight junctions and adherens junctions in mammalian cells. Mammalian ZO-1 binds to alpha-spectrin, which cross-links with actin filaments, thereby affecting cell shape. Pyd and mammalin ZO-1 also interact with Drosophila Cortactin and mammalian cortactin respectively. Mammalian Cortactin is known to be a filamentous actin cross-linking protein and a substrate of Src protein tyrosine kinase. Cortactin is phosphorylated at tyrosine residues upon stimulation by extracellular signals. Filamentous actin cross-linking activity of cortactin is attenuated by Src. The intracellular localization of mammalian cortactin is regulated by the activation of Rac1. Cortactin redistributes from the cytoplasm into membrane ruffles as a result of growth factor-induced Rac1 activation, and this translocation is blocked by expression of dominant negative Rac1N17. Thus in mammals, cortactin is a putative target of Rac1-induced signal transduction events involved in membrane ruffling and lamellipodia formation. It would thus seem that Rac signaling is tied to actin dynamics and Polychaetoid/ZO-1 function both in Drosophila and mammals (Takahashi, 1998 and references).

In addition to the defects in myoblast fusion and CNS development, Suppressor of (rac)1 alleles exhibit a dorsal closure defect similar to that reported previously for mbc mutants and embryos expressing dominant-negative rac1 (Harden, 1995). Su(rac)1 alleles were isolated based on their ability to dominantly suppresses the GMR-rac1-induced rough eye surface as well as the underlying retinal morphology. During dorsal closure, two symmetric epithelial monolayers coordinately migrate from their lateral position to fuse along the dorsal midline. The row of cells along the dorsal apical edge, known as the leading up edge (LE) cells, elongate first and remain morphologically distinct from the more ventral cells until the two sheets have nearly met at the midline. Recently, a Rac-mediated signaling pathway thatregulates this process has been elucidated. Rac1 appears to activate the c-Jun amino (N)-terminal kinase (JNK) pathway, which leads to decapentaplegic (dpp) expression in the LE cells of the dorsal epidermis, and several JNK pathway mutants associated with reduced dpp expression exhibit similar dorsal closure defects, including hemipterous (hep; Jun kinase kinase), basket (Jun kinase), Djun, and kayak (c-Fos). To determine whether Mbc mediates the activity of Rac in the activation of JNK during dorsal closure, the expression of DPP mRNA was examined in mbc mutant embryos. In wild-type embryos, dpp is expressed predominantly in the visceral mesoderm and the LE of the dorsal epidermis. In mbc mutant embryos, 50% of which exhibit dorsal closure defects, dpp is expressed at normal levels in the majority of embryos but appears to be mildly reduced specifically in the leading edge cells of some of these embryos. This is in contrast to hep mutant embryos, in which dpp expression in leading edge cells is clearly absent. This result suggests that Mbc is not absolutely required for JNK pathway activation and may play a distinct role in dorsal closure. However, the possiblity cannot be excluded that Mbc contributes to the activation of JNK in the leading edge cells, but the effects of its absence are masked by a redundant function of Cdc42, which is also capable of activation of JNK in the leading edge cells (Nolan, 1998 and references).

The mammalian protein DOCK180, has been demonstrated to interact directly with Rac, but it is unlikely to act as a RacGEF, that is, it is unlikely that DOCK180 functions directly as a Rac activator. There are two models that most simply explain the role of Mbc in dorsal closure. Possibly, Mbc is required for Rac activation in the leading edge cells during dorsal closure, but some functional redundancy for JNK regulation, which takes place in mbc mutants, is provided by Cdc42. In this scenario, Mbc is still required for Rac-dependent cytoskeletal changes, reflecting a more stringent requirement for Rac activity in regulating cell morphology than in regulating transcription. Consistent with such a possibility, it is found that only a small fraction of mbc mutant embryos that exhibit a dorsal closure defect exhibit any detectable reduction in dpp expression. Although this result suggests a lesser role for Mbc in JNK activation than in cytoskeletal regulation, it is observed that overexpression of DOCK180 leads to activation of JNK in transfected mammalian cells, suggesting that Mbc can potentially play a role in activating JNK in vivo. Alternatively, two separate pools of Rac, with different subcellular localizations, may be utilized for distinct biological processes. In this scenario, Mbc promotes activation of a pool of Rac that regulates reorganization of the actin cytoskeleton but does not substantially affect the pool of Rac required for JNK activation. Consistent with this model, it is found that DOCK180 colocalizes with Rac in membrane ruffles, raising the possibility that Rac, and perhaps other GTPases, can regulate distinct biological processes within a single cell by virtue of subcellularly localized activation (Nolan, 1998).

The Rho subfamily of Ras-related small GTPases participates in a variety of cellular events including organization of the actin cytoskeleton and signaling by c-Jun N-terminal kinase and p38 kinase cascades. These functions of the Rho subfamily are likely to be required in many developmental events. The participation of the RHO subfamily in dorsal closure of the Drosophila embryo, a process involving morphogenesis of the epidermis, has been studied. Drac1, a Rho subfamily protein, is required for the presence of an actomyosin contractile apparatus believed to be driving the cell shape changes essential to dorsal closure. Expression of a dominant negative Drac1 transgene causes a loss of this contractile apparatus from the leading edge of the advancing epidermis, and consequently, dorsal closure fails. Two other Rho subfamily proteins, Dcdc42 and RhoA, as well as Ras1 are also required for dorsal closure. Dcdc42 appears to have conflicting roles during dorsal closure: establishment and/or maintenance of the leading edge cytoskeleton versus its down regulation. Down regulation of the leading edge cytoskeleton may be controlled by the serine/threonine kinase DPAK, a potential Drac1/Dcdc42 effector. RhoA is required for the integrity of the leading edge cytoskeleton specifically in cells flanking the segment borders. The interactions of the various small GTPases in regulating dorsal closure have been characterized and no evidence is found for the hierarchy of Rho subfamily activity described in some mammalian cell types. Rather, the results suggest that while all Rho subfamily p21s tested are required for dorsal closure, they act largely in parallel (Harden, 1999).

A model is given of the control of DC by the Drac1/JNK and Dcdc42/Dpp pathways. Drac1/JNK signaling, initiated by an as yet unknown factor, assembles cytoskeletal components (F-actin, myosin and focal complexes) and other proteins (Dpp, Puckered and Pak) in the leading edge cells and initiates the cellular migration that characterizes DC. Dpp-activated signaling controls the dynamics of epidermal migration, via Dcdc42 and the Dpp pathway, through the serine/threonine kinase Pak, which transiently downregulates the leading edge cytoskeleton at the segmental borders. Transient downregulation of the actin cytoskeleton and focal contacts near the segment border cells is likely to cause local relaxation of the anterior-posterior tension along the LE. Such transient relief of tension may then limit excessive migration of leading edge cells toward each other and prevent the bunching and shearing of epidermal segments that occurs following impairment of Dpp/Dcdc42 signaling. Segment borders cells are potential regions of highest Dpp signaling, because they are adjacent to the highest local concentrations of Dpp protein, and they have high levels of Pak protein and transcripts for the Tkv receptor. Segmental border cells are the only places where transient downregulation of the leading edge cytoskeleton is ever seen in wild-type embryos during DC. As such, it is proposed that the role of Dcdc42/Dpp signaling is the induction of Pak to downregulate the leading edge cytoskeleton at the segment borders, introducing a degree of flexibility to the leading edge during the dorsal closure process (Ricos, 1999).

In the process of dorsal closure, Rac1 appears to be a primary upstream activator of JNK signaling. To position slipper function in the JNK pathway relative to the GTPase, additional genetic epistasis tests were performed. Due to the difficulty in following all relevant chromosomes in the embryo, the adult Drosophila eye was used to evaluate a possible genetic interaction. Expression of wild-type dRac1 in the eye, under regulation by the glass promotor, causes a rough, glazed appearance. This phenotype is dominantly suppressed by 50% reduction in the levels of JNK signal transducers, msn, slpr, hep, and bsk. Heterozygosity at the puc locus, encoding a negative regulator of JNK signaling acting in opposition to bsk, dominantly enhances the Rac1-induced rough eye. To assess whether other putative JNKKKs in Drosophila can interact in this assay, Drosophila TGF-ß activated kinase 1 (Tak1) was included in this analysis. Unlike slpr, removal of one copy of Tak1 has no effect on the Rac1-induced eye phenotype. These data suggest that there is a dosage-sensitive interaction between the JNK pathway and Rac1 function in this tissue whereby increased Rac1 activity can be suppressed by reduction in downstream components, including slpr. Although it is not known whether Tak1 is expressed in the eye or at what level, Tak1 shows no genetic interaction with GMR-dRac1. Thus, slpr appears to be a relevant JNKKK in this assay. Taken together, these epistasis tests are consistent with slpr function being required downstream of Rac1 and upstream of bsk (Stronach, 2002).

Dorsal closure of the Drosophila embryo involves morphological changes in two epithelia, the epidermis and the amnioserosa, and is a popular system for studying the regulation of epithelial morphogenesis. The small GTPase Rac1 has been implicated in the assembly of an actomyosin contractile apparatus, contributing to cell shape change in the epidermis during dorsal closure. Evidence is presented that Rac1 and Crumbs, a determinant of epithelial polarity, are involved in setting up an actomyosin contractile apparatus that drives amnioserosa morphogenesis by inducing apical cell constriction. Expression of constitutively active Rac1 causes excessive constriction of amnioserosa cells and contraction of the tissue, whereas expression of dominant-negative Rac1 impairs amnioserosa morphogenesis. These Rac1 transgenes may be acting through their effects on the amnioserosa cytoskeleton, since constitutively active Rac1 causes increased staining for F-actin and myosin, whereas dominant-negative Rac1 reduces F-actin levels. Overexpression of Crumbs causes premature cell constriction in the amnioserosa, and dorsal closure defects are seen in embryos homozygous for hypomorphic crumbs alleles. The ability of constitutively active Rac1 to cause contraction of the amnioserosa is impaired in a crumbs mutant background. It is proposed that amnioserosa morphogenesis is a useful system for studying the regulation of epithelial morphogenesis by Rac1 (Hardin, 2002).

Expression of dominant negative Drac1N17 in the amnioserosa slows morphogenesis of this tissue which remains as a squamous epithelium for a longer period than in wild-type embryos. In Drac1N17-expressing embryos, where amnioserosa morphogenesis is lagging, the movement of the epidermis is also slowed, and the embryos have a larger dorsal hole than wild-type embryos of similar age. It is thought that the impaired movement of the epidermis in such embryos is caused by lack of morphogenesis in the amnioserosa. These results are strong evidence that active cell shape changes in the amnioserosa are required for normal dorsal closure. Examination of wild-type embryos has shown that this cell shape change in the amnioserosa begins with apical constriction of cells at the anterior and posterior ends of the amnioserosa. These cells have elevated levels of myosin, F-actin and phosphotyrosine, suggesting that an apically localized actomyosin contractile apparatus is driving their constriction. Early in dorsal closure, the middle cells in between the two clusters of apically constricted cells do not show elevated levels of F-actin or myosin but do change shape, losing their original elongation perpendicular to the A-P axis of the embryo. The middle cells may be stretching passively, in response to tension from the cell constrictions occurring at both ends of the amnioserosa. By the end of dorsal closure, the middle cells are both elongated along the A-P axis and apically constricted, and it is conceivable that late in dorsal closure they undergo an active cell shape change as their neighbors did earlier (Hardin, 2002).

Excessive Drac1 activity induces a dramatic contraction of the amnioserosa such that it shrinks to occupy less than half the dorsal hole, and this is accompanied by elevated levels of myosin, F-actin, and phosphotyrosine in this tissue. It is thought that Drac1V12 is driving premature and excessive amnioserosa cell constriction through its effects on the cytoskeleton. It is proposed that Drac1 participates in amnioserosa morphogenesis by driving the assembly of an apical actomyosin contractile apparatus that constricts the amnioserosa cells, first at the ends of the tissue and possibly later in the middle. Contraction of an apical actomyosin belt has been implicated in diverse types of epithelial morphogenesis including Drosophila gastrulation, which shows some similarity to amnioserosa morphogenesis in that both processes involve apical construction of a monolayer of cells that then invaginates (Hardin, 2002).

Cell ablation has been used to address the contributions of the epidermis and amnioserosa to dorsal closure. This work has demonstrated that the amnioserosa is under tension, since ablation of cells in the amnioserosa causes the tissue to recoil away from the wound site, and the leading edge is pushed back away from the dorsal midline. It is concluded that there is active cell shape change in the amnioserosa that contributes to dorsal closure, rather than the tissue being simply compressed by the movement of the leading edge. The finding that the recoiling of the amnioserosa after wounding pushes back the leading edge is consistent with the result that impairing amnioserosa morphogenesis through Drac1N17 expression hinders leading edge migration (Hardin, 2002).

Overexpression of Crb in the amnioserosa leads to contraction of the tissue and failure of dorsal closure. This phenotype was examined in more detail; excessive Crb activity induces a premature constriction of cells at the ends of the amnioserosa. Five P-element-induced crb alleles were identified that are hypomorphic mutations, causing defects in dorsal closure and germband retraction. One of these crb mutations, crbS010409, was characterized in detail. Embryos homozygous for crbS010409 show a dorsal closure defect similar to that seen with expression of Drac1N17 in the amnioserosa: amnioserosa morphogenesis is impaired, but the leading edge cytoskeleton is intact. In contrast to amorphic crb alleles, the epidermis is not disorganized in crbS010409 mutants and it secretes cuticle. Amnioserosa morphogenesis and germband retraction may be particularly sensitive to the level of Crb activity. It is thought that Crb activity in the amnioserosa is required for amnioserosa morphogenesis, although the possibility cannot be excluded that loss of Crb activity elsewhere in the embryo is affecting this process (Hardin, 2002).

Drac1 may act through Crb in regulating the cytoskeleton, since the constitutively active Drac1V12-induced phenotype of excessive contraction of the amnioserosa is weakened in a crbS010409 mutant background. This weaker Drac1V12 phenotype of premature constriction of the end cells of the amnioserosa is very similar to that caused by Crb overexpression. There may be sufficient Crb in the crbS010409 mutant embryos for Drac1V12 to be able to prematurely constrict cells at the ends of the amnioserosa but not to excessively contract the tissue. Crb overexpression does not appear to require Drac1 to cause premature constriction of amnioserosa cells, since it can achieve this in the presence of Drac1N17. The excessive contraction of the amnioserosa caused by Drac1V12 expression in embryos with wild-type Crb activity, and the dumbbell-shaped amnioserosa induced by Crb overexpression, could both result from excessive constriction of amnioserosa cells to produce a tissue that only occupies a fraction of the dorsal hole. Such excessive constriction may be driven by ectopic accumulation of a normally apically localized actomyosin contractile apparatus. A role for Crb in defining the location of the actomyosin contractile apparatus is consistent with the idea that Crb defines the range of the apical membrane cytoskeleton. The actin-crosslinking protein ßHeavyH)-spectrin normally has an apicolateral distribution, but upon overexpression of Crb is also found at the basolateral membrane, indicating a redistribution of the membrane cytoskeleton. ßH-spectrin is required for apical constriction of follicle cells during Drosophila oogenesis and may participate in organization of an actomyosin contractile apparatus. It is conceivable that the ectopic localization of (ßH)-spectrin domain following Crb overexpression could be accompanied by an ectopic accumulation of F-actin and myosin. Future goals in studying Drac1-Crb function in amnioserosa morphogenesis will include addressing the nature of the interaction between the two proteins and defining which portion(s) of the Crb protein are required. The short cytoplasmic domain of Crb appears sufficient to execute all Crb functions studied to date. No definitive role has been found for the large extracellular domain, although there is evidence that the Drosophila and human Crb proteins have non-cell-autonomous functions (Hardin, 2002).

Although Drac1 and Crb both generate premature contraction of the amnioserosa when their activity is experimentally upregulated in this tissue, their phenotypic effects are not identical. Drac1V12 expression drives constriction of all amnioserosa cells early in closure, whereas, at the same stage, Crb overexpression only promotes constriction of the end cells. A plausible explanation for this is that constriction of the middle cells requires Drac1 to activate Crb-independent processes and that Crb function is necessary but not sufficient for middle cell constriction. Crb overexpression in the amnioserosa causes disruption of the leading edge cytoskeleton and a failure of cell shape change in the epidermis, suggesting that a signal from the amnioserosa required for dorsal closure is disrupted. That communication between the amnioserosa and the epidermis is a component of dorsal closure is demonstrated by the observation that JNK signaling in the amnioserosa is required for phosphotyrosine accumulation at the leading edge and dorsalward migration of the epidermis and by the observation that leading edge cells are specified wherever an interface of amnioserosa and dorsal epidermis occurs. Drac1V12 expression in the amnioserosa does not disrupt the leading edge cytoskeleton or prevent closure of the epidermis, and this result suggests that Drac1V12 cannot activate a function of Crb that influences communication between the amnioserosa and the epidermis (Hardin, 2002).

The Pak kinases are effectors for the small GTPases Rac and Cdc42 and are divided into two subfamilies. Group I Paks possess an autoinhibitory domain that can suppress their kinase activity in trans. In Drosophila, two Group I kinases have been identified, dPak and Pak3. Rac and Cdc42 participate in dorsal closure (DC) of the embryo, a process in which a hole in the dorsal epidermis is sealed through migration of the epidermal flanks over a tissue called the amnioserosa. DC is driven in part by an actomyosin contractile apparatus at the leading edge (LE) of the epidermis, and is regulated by a Jun amino terminal kinase (JNK) cascade. Impairment of dPak function using either loss-of-function mutations or expression of a transgene encoding the autoinhibitory domain (AID) of dPak leads to disruption of the LE cytoskeleton and defects in DC does not affect the JNK cascade. Group I Pak kinase activity in the amnioserosa is required for correct morphogenesis of the epidermis, and may be a component of the signaling known to occur between these two tissues. It is concluded that DC requires Group I Pak function in both the amnioserosa and the epidermis (Conder, 2004).

Three results indicate that Group I Pak kinase activity in the amnioserosa does not make a major contribution to amnioserosa morphogenesis: (1) amnioserosa cell shape change still occurs in embryos lacking maternal and zygotic dPak; (2) inhibition of Group I Pak kinase activity in the amnioserosa does not prevent apical constriction of amnioserosa cells; (3) Rac1V12 is still capable of inducing excessive contraction of the amnioserosa in a dpak6 mutant background. Presumably, Rac uses cytoskeletal effectors other than dPak to drive cell shape change in the amnioserosa (Conder, 2004).

The finding that expression of dPak-AID with either the LE or amnioserosa GAL4 drivers causes a high frequency of failures to form cuticle is curious. These GAL4 drivers do not show expression in the lateral epidermis with the exception of GAL4332.3, which shows scattered epidermal expression after completion of dorsal closure. One possibility is that many embryos are dying in late embryogenesis before completion of cuticle secretion. Another, perhaps less likely explanation, is that Group I Pak kinase activity in the amnioserosa and DME cells is required non-autonomously for cuticle secretion in the epidermis through cell-cell signaling. Whatever the mechanism, a further indication that alterations in Rho family small GTPase/Pak signaling have consequences with regard to cuticle secretion is demonstrated by the finding that the disruption of cuticle secretion by Rac1V12 expression is suppressed by zygotic loss of dPak (Conder, 2004).

Rac promotes epithelial cell rearrangement during tracheal tubulogenesis

Cell rearrangement, accompanied by the rapid assembly and disassembly of cadherin-mediated cell adhesions, plays essential roles in epithelial morphogenesis. Various in vitro and cell culture studies on the small GTPase Rac have suggested that Rac is a key regulator of cell adhesion, but this notion needs to be verified in the context of embryonic development. The tracheal system of Drosophila was used to investigate the function of Rac in the epithelial cell rearrangement, with a special attention to its role in regulating epithelial cadherin activity. A reduced Rac activity leads to an expansion of cell junctions in the embryonic epidermis and tracheal epithelia, which was accompanied by an increase in the amount of Drosophila E-Cadherin-Catenin complexes by a post-transcriptional mechanism. Reduced Rac activity inhibits dynamic epithelial cell rearrangement. In contrast, hyperactivation of Rac inhibits assembly of newly synthesized E-Cadherin into cell junctions and causes loss of tracheal cell adhesion, resulting in cell detachment from the epithelia. Thus, in the context of Drosophila tracheal development, Rac activity must be maintained at a level necessary to balance the assembly and disassembly of E-Cadherin at cell junctions. Together with its role in cell motility, Rac regulates plasticity of cell adhesion and thus ensures smooth remodeling of epithelial sheets into tubules (Chihara, 2003).

Cadherin-based cell adhesions are vital to maintain the morphological and functional features of the epithelium of multicellular organisms. During morphogenesis of the epithelia, cell adhesions must be disrupted and re-assembled in a regulated manner to allow movement of individual cells in the epithelia. In vivo analyses have demonstrated that a reduction in Rac activity prevents cell rearrangement. This phenotype is associated with an increase in the level of E-Cadherin and its associated molecules, and expansion of E-Cadherin localization to the basolateral membrane. It is inferred that increased E-Cadherin expression consolidates cell adhesiveness. Hyperactivation of Rac prevents incorporation of newly synthesized E-Cadherin into cell junctions and reduces cell adhesiveness, transforming the tracheal epithelium into mesenchyme. It is suggested that switching of Rac between active and inactive states promotes turnover of the complex containing E-Cadherin at the cell junction, and maintains the plasticity of the tracheal epithelium to allow branching morphogenesis (Chihara, 2003).

Expression of a dominant-negative form of Rac 1 greatly reduces cell rearrangement required for partitioning cells into the stalk of the dorsal branch. Overproduction of this form, Rac 1N17, would shift the cellular pool of Rac toward the inactive GDP-bound state. It is suggested that turnover of E-Cadherin at a proper level requires a high level of Rac activity. However, since highly active movement of cell extensions in the cells at the tip is still visible, it is suggested that the ability of those cells to move toward their target is mostly intact. A stronger reduction in Rac activity might be required to demonstrate a role for Rac in promoting cell extensions, as proposed from studies on tissue culture cells (Chihara, 2003).

Time-lapse analysis demonstrates that reduced Rac activity inhibits cell rearrangement during branching of tracheal tubules. Under this condition, the amounts of cadherins and catenins were increased and filled the cell membrane. This phenotype is different from the phenotype of E-Cadherin-GFP overexpression, which does not inhibit cell rearrangement. It is suggested that a reduction in Rac promotes the association of cadherin-catenin complexes with the cell membrane and stabilization of these complexes. Activation of Rac results in an opposite phenotype characterized by the loss of E-Cadherin and cell dissociation, and in prevention of E-Cadherin-GFP from accumulating at apical cell junctions. All of these observations are consistent with a hypothesis that Rac regulates the formation of cadherin-catenin complexes at the cell junction. Incorporation of a cadherin-catenin complex into the cellular junction would explain stabilization of the complex when Rac activity is reduced. Possible modes of Rac action on cadherin include apical transport and assembly/stabilization of the complex. It is suggested that the inhibitory action on the cadherin cell adhesion system is a general property of Rac in the Drosophila embryo (Chihara, 2003).

In addition to the defects in cell adhesion and rearrangement, Rac1, 2 mutant embryos show various defects in tracheal cell migration and differentiation. A wide range of tracheal defects are observed in Rac1, 2 mutants. In embryos showing the weak class phenotype, misrouting of the dorsal branch toward the anteroposterior direction is often observed. In intermediate-class embryos, the number of truncated dorsal trunks increases and the germband does not retract completely. The severity of the defects and their frequency increases when the gene dose of rac is progressively reduced. When the maternal expression of Rac1 and Rac2 was reduced by half, the tracheal defects occurred in 25% of the embryos. Rac1 Rac2 Mtl triple mutants laid by Rac1 Rac2 Mtl heterozygous mothers were also analyzed; the mutants showed higher frequency of severe tracheal defects similar to those found in Rac1, 2 mutants. These results suggest that a change in Rac activity within the physiological range significantly affects morphogenetic movement of the tracheal system (Chihara, 2003).

p21-activated kinase (Pak) is known as a mediator of the activity of Rac GTPase. Tracheal defects similar to those of Rac1, 2 mutants are found in pak mutants. Furthermore, Rac1, 2 and Pak mutations synergistically enhance tracheal defects. Such results suggest that Rac and Pak are required for directed movement of tracheal branches (Chihara, 2003).

The loss of Rac activity also causes a defect in cell differentiation. Tips of dorsal branch 1-9 are normally capped with terminal cells that extend terminal branch in the ventral direction. In Rac 1, 2 mutant embryos, the loss of terminal branches was observed with high penetrance. Consistently, serum response factor (SRF), a marker protein for the terminal cell, also disappears, suggesting that terminal cell differentiation does not occur (Chihara, 2003).

Since directed cell migration and terminal cell differentiation are processes requiring FGF signaling, it was asked whether Rac is involved in FGF signaling (a strong genetic interaction). Although tracheal patterning is only mildly affected by half dose reductions of bnl (ligand), btl (receptor) and dof (intracellular effector), the phenotype is strongly enhanced by introducing one copy of Rac1, 2 mutant chromosome from mothers. A similar genetic interaction was found between pak and bnl. These genetic interactions suggest that Rac and Pak are required for the migration of tracheal branches in response to FGF signaling (Chihara, 2003).

To determine the epistatic relationship between Rac and FGF signaling, the effect of constitutive activation of Rac was tested in btl mutants. In the btl mutant, tracheal branching does not proceed beyond the invagination at stage 11, and MAP kinase activation is absent (Chihara, 2003).

Expression of Rac1V12 partially restores the movement of tracheal cells, and activates MAP kinase, as revealed by staining with the antibody against the diphosphorylated form of MAP kinase (dp-MAPK). These results suggest that Rac activation is an essential downstream event of tracheal cell motility induced by FGF signaling (Chihara, 2003).

Extracellular signals that promote tracheal branching are good candidates for regulators of Rac in tracheal cells. In this regard, the strong genetic interaction between Rac and FGF signaling components observed suggests an intriguing possibility that FGF signaling activates Rac within tracheal cells to promote both cell motility and cell rearrangement. In support of this idea, it was found that activated Rac 1 partially rescues tracheal cell motility and MAP kinase activation in btl mutants. Involvement of Rac in FGF-dependent events may not be limited to cell motility. Expression of SRF, the product of one of the target genes activated by FGF signaling in the tracheal system, is lost in the mutant trachea with reduced Rac activity because of Rac 1, 2 mutation or Rac 1N17. This result suggests that Rac also regulates transcription (Chihara, 2003).

Several lines of evidence suggest that FGF signaling is activated locally at the tip of branches, and activation of FGF signaling in all tracheal cells prevents branching, suggesting that localized activation of FGF signaling is essential for branching. Therefore the proposed function of Rac in transducing FGF signaling must be localized at the tip of branches. How does the proposed function of Rac in transducing FGF signaling relate to the Rac function in regulating cell rearrangement? Since the effect of Rac 1N17 is most clearly observed in cells destined to become tracheal stalk cells, the location of tracheal cells requiring two of the Rac functions appears to be different. One idea is that FGF signaling activated at the tracheal tip is transmitted to tracheal stalk cells by a secondary signal that activates Rac to promote cell rearrangement. It will be important to identify the upstream signal regulating Rac in stalk cells (Chihara, 2003).

Specification of leading and trailing cell features during collective migration in the Drosophila trachea

The role of tip and rear cells in collective migration is still a matter of debate and their differences at the cytoskeletal level are poorly understood. This study analysed these issues in the Drosophila trachea, an organ that develops from the collective migration of clusters of cells that respond to Branchless (Bnl), a FGF homologue expressed in surrounding tissues. Individual cells in the migratory cluster were tracked and their features were characterized; two prototypical types of cytoskeletal organization were unveiled that account for tip and rear cells respectively. Indeed, once the former are specified, they remain as such throughout migration. Furthermore, it was shown that FGF signalling in a single tip cell can trigger the migration of the cells in the branch. Finally, specific Rac activation was found at the tip cells, and how FGF-independent cell features such as adhesion and motility act on coupling the behaviour of trailing and tip cells was analyzed. Thus, the combined effect of FGF promoting leading cell behaviour and the modulation of cell properties in a cluster can account for the wide range of migratory events driven by FGF (Lebreton, 2013).

Among the tracheal branches from each placode, two grow towards the ventral side of the embryo, one in the anterior and the other in the posterior region of the segment, the lateral trunk anterior (LTa) and the lateral trunk posterior (LTp) respectively. By a combination of migration, intercalation and elongation, the tip cell of the LTp migrates towards the central nervous system (CNS), and the resulting ganglionic branch (GB) connects the CNS to the main tracheal tube. Another cell from the LTp migrates towards the LTa of the adjacent posterior metamere and makes a fusion branch that connects the two LT branches. This study focused on this branch (LTp/GB) because its complex morphology and pattern of migration make it particularly appropriate for analysing the morphology and behaviour of the tip and trailing cell during tracheal collective migration (Lebreton, 2013).

The FGF signalling pathway is involved in many morphogenetic events requiring collective migration of cell clusters. However, it is not entirely clear whether in these events FGF signalling is directly involved in triggering cell migration, or alternatively if it is required for other processes such as cell determination which only affect cell migration indirectly. Moreover, while FGF might be required it is not clear either whether all the cells or just a subset of those need to directly receive the signal to sustain the migration of the entire cluster. One well-studied case is the role of FGF in the development of the zebra fish lateral line. In that case, FGF appears to be produced by the leading cells which signal to the trailing cells, the cells where FGF signalling is active. Restriction of FGF signalling is thereafter required for the asymmetric expression of the receptors for the chemokines that guide migration (Lebreton, 2013).

A very different scenario applies in the case of Drosophila tracheal migration. On the one hand, FGF is expressed in groups of cells outside the migrating cluster. On the other hand the results in the LTp/GB indicate that FGF signalling is required and sufficient in the leading cells, and not in the trailing cells, for the migration of the whole cell cluster. Therefore, in spite of its widespread involvement, the mechanisms triggered by FGF signalling in collective migration appear to be quite different (Lebreton, 2013).

Rho inactivation produced breaks and detachment in the LTp/GB cluster while its constitutive activation led these cells to hold together impairing migration. Likewise, upon Cdc42 inactivation LTp/GB cells were associated by thin extensions associated in some cases with breaks, while upon its constitutive activation, the LTp/GB transient pyramidal organisation did not evolve, or evolved much more slowly, towards branch elongation. However, the phenotypes from each RhoGTPase mutants don't look alike and the detailed analysis suggests that Rho impinges primarily on cell adhesion while Cdc42 does so on cell motility (Lebreton, 2013).

These results are consistent with previous findings that show a role for Rho in regulating adherens junctions stability and for Cdc42 as the main mediator of filopodia formation. It is noted, however, that Cdc42 was found to exert in the LTp/GB an opposite effect to the one identified in other systems, as Cdc42DN mutants showed more protrusions and were more actin-enriched basally than wild-type cells and Cdc42ACT mutants showed a reduced the motility of LTp/GB (Lebreton, 2013).

There is an increasing amount of data pointing to the different effect of RhoGTPases in vitro versus in vivo models and also among various cell types. A unidirectional assignment between a specific cellular process in vivo and a single RhoGTPase is probably an oversimplification and this was not the aim of the current study. Rather the study relied on mutant forms of the RhoGTPases to modulate cell features, either individually or collectively, to assess their role in the overall behaviour of the cell cluster. In doing so, the results point to a critical role for a balance between cell adhesion and cell motility for the collective migration of a cell cluster (Lebreton, 2013).

The results support the following model for the specification, features and behaviour of leading cells in the migration of the LTp/GB branch. Upon signalling from the FGF pathway, tip cells reorganise their cytoskeleton features (actin enrichment at the basal membrane, small apical surface and an apicobasal polarity along the proximo-distal axis), thereby enabling them to acquire leading behaviour. Indeed, FGF can induce migratory capacity to the whole cluster by signalling only the tip cells, where a dynamic transition between states of Rac activity is needed to acquire a leading role. How the behaviour of tip cells leads collective migration thereafter depends on the features of the cells in the cluster, which are determined by various regulators (among these, the RhoGTPases) which act, at least in part, in an FGF-independent manner. Ultimately, the balance between individual cell properties such as cell adhesion, motility and apicobasal polarity will (1) determine the net movement of the overall cell bodies or alternatively changes in cell shape in terms of elongation, (2) control the migratory speed and (3) define whether cells will migrate individually or in clusters and whether clusters will bifurcate in different paths. The combined effect of the changes promoting leading cell behaviour and modulation of cell features is likely to be a widely exploited mechanism in collective migration. In particular, the actual balance between these cell features may dictate the specifics of each migratory process and, consequently, the final shape of the tissues and organs they contribute to generate (Lebreton, 2013).

Rac function in glial migration and nerve ensheathement

Peripheral glial cells in both vertebrates and insects are born centrally and travel large distances to ensheathe axons in the periphery. There is very little known about how this migration is carried out. In other cells, it is known that rearrangement of the Actin cytoskeleton is an integral part of cell motility, yet the distribution of Actin in peripheral glial cell migration in vivo has not been previously characterized. To gain an understanding of how glia migrate, the peripheral glia of Drosophila were labelled using an Actin-GFP marker and their development in the embryonic PNS was analyzed. It was found that Actin cytoskeleton is dynamically rearranged during glial cell migration. The peripheral glia were observed to migrate as a continuous chain of cells, with the leading glial cells appearing to participate to the greatest extent in exploring the extracellular surroundings with filopodia-like Actin containing projections. It is hypothesized that the small GTPases Rho, Rac and Cdc42 are involved in Actin cytoskeletal rearrangements that underlie peripheral glial migration and nerve ensheathement. To test this, transgenic forms of the GTPases were ectopically expressed specifically in the peripheral glia during their migration and wrapping phases. The effects on glial Actin-GFP distribution and the overall effects on glial cell migration and morphological development were assessed. It as found that RhoA and Rac1 have distinct roles in peripheral glial cell migration and nerve ensheathement; however, Cdc42 does not have a significant role in peripheral glial development. RhoA and Rac1 gain-of-function and loss-of-function mutants had both disruption of glial cell development and secondary effects on sensory axon fasciculation. Together, Actin cytoskeletal dynamics is an integral part of peripheral glial migration and nerve ensheathement, and is mediated by RhoA and Rac1 (Sepp, 2003).

The data suggest that RhoA and Rac1 are both involved in peripheral glial cell migration and nerve ensheathement, and have distinct effects on Actin rearrangement. For example, constitutively active Rac1 (V12) and RhoA (V14) expression results in halted migration of cell bodies as well as disrupted cytoplasmic process extension. The phenotypes of the two mutants are very different from one another. Rac1 (V12) mutants show ball-shaped collapsed glia, while RhoA (V14) mutants have very long, spike-shaped actin processes emanating from the cell bodies. The distinct and extreme phenotypes from these mutants suggest that there is a balance of RhoA and Rac1 activity in wild-type peripheral glia to generate normal migration and cytoplasmic process extension. The concept of a balance of GTPase function being necessary for glial cell migration is also supported by observations that glial cell migration is stalled in both the gain-of-function and loss-of-function mutations. These observations are interpreted as suggesting that there is a balance of GTPase activities that is necessary for glial cell migration. In other words, anything that affects this balance either through a loss of function or gain of function, affects the ability of glial cells to migrate (Sepp, 2003).

The well-characterized cultured fibroblast model has shown that Rac is involved in lamellipodia formation, while Rho mediates stress fiber polymerization and Cdc42 is involved in the extension of filopodia. It is possible that Rac1 and RhoA mediate the assembly of similar structures in peripheral glia. The long, straight actin fibers seen in constitutively active RhoA (V14) mutants could represent overextended stress fibers. Furthermore, the massive glial lamellar-like structures that are stimulated by Rac1L89 expression appear very similar to the lamellipodia of cultured fibroblasts. The biochemical activity of the Rac1L89 mutation is not known, and can act as either a dominant-negative or constitutively active form, depending on the cell type. The Rac1L89 phenotype in peripheral glia is most similar to overexpression of wild-type Rac1, suggesting that the ectopic lamellar structures are a result of moderate increase in Rac1 activity. Thus, it is possible that the Rac1L89 mutation causes Rac1 to be overactive but not as much as in the Rac1V12 mutation (Sepp, 2003).

It was interesting to note that the ectopic actin-containing projections of RhoAV14 and Rac1L89 mutants did not always reach over axon tracts, which are the normal peripheral glial migrational substrates in the wild type. For the steering of a migrating cell, large amounts of actin polymerization occur at the contact between the leading edge of the cell and the attractive migrational substrate. Perhaps the hyperactivity of the mutant GTPases enable the peripheral glia to extend processes out on less adhesive substrates compared with axons. It was also interesting to note that ectopic projections of peripheral glia (in the RhoAV14 and Rac1L89 mutants) do not interfere with axon pathfinding in the periphery. The ectopic glial projections could be a result of failed glial pathfinding instead. Interestingly, axons are capable of correctly migrating in the absence of glial sheaths (in the RhoAV14 and Rac1V12 mutants). Peripheral glia are know to be able to mediate sensory axon guidance to the CNS. Thus, peripheral glia most probably mediate sensory axon migration to the CNS using secreted cues (Sepp, 2003).

Control of dendritic development by the Drosophila fragile X-related gene involves Rac1

Fragile X syndrome is caused by loss-of-function mutations in the fragile X mental retardation 1 gene. How these mutations affect neuronal development and function remains largely elusive. Specific point mutations or small deletions have been generated in the Drosophila fragile X-related (Fmr1) gene, and the roles of Fmr1 in dendritic development of dendritic arborization (DA) neurons have been examined in Drosophila larvae. Fmr1 can be detected in the cell bodies and proximal dendrites of DA neurons and Fmr1 loss-of-function mutations increase the number of higher-order dendritic branches. Conversely, overexpression of Fmr1 in DA neurons dramatically decreases dendritic branching. In dissecting the mechanisms underlying Fmr1 function in dendrite development, it was found that the mRNA encoding small GTPase Rac1 is present in the Fmr1-messenger ribonucleoprotein complexes in vivo. Mosaic analysis with a repressor cell marker (MARCM) and overexpression studies reveals that Rac1 has a cell-autonomous function in promoting dendritic branching of DA neurons. Furthermore, Fmr1 and Rac1 genetically interact with each other in controlling the formation of fine dendritic branches. These findings demonstrate that Fmr1 affects dendritic development and that Rac1 is partially responsible for mediating this effect (Lee, 2003).

Each abdominal hemisegment of Drosophila contains 44 sensory neurons that can be grouped into dorsal, lateral and ventral clusters. To test whether Fmr1 affects the dendritic growth of DA neurons, the expression of Fmr1 in these neurons was confirmed. Fmr1 mRNA is expressed at high levels in the embryonic nervous system and in body wall muscles. To examine the subcellular localization of Fmr1 in DA neurons of live larvae, a UAS-Fmr1-GFP transgenic fly line was generated. When Fmr1-GFP was expressed in DA neurons driven by Gal4 109(2)80, Fmr1 expression was observed in the cytoplasm of DA neurons and in particle-like structures in dendrites. To further confirm that the endogenous Fmr1 is expressed in DA neurons, immunostaining analysis was performed on dissected larvae using a monoclonal antibody raised against Fmr1. Fmr1 is present in DA neurons and is expressed predominantly in the cytoplasm. The expression of Fmr1 in the proximal dendrites of DA neurons and in body wall muscle fibers was also detectable. Owing to the high level of Fmr1 expression in muscles, the localization of endogenous Fmr1 in distal dendrites was barely visible with confocal microscopy. The subcellular localization of Fmr1 in DA neurons is consistent with the subcellular localization of FMR1 in mammalian neurons (Lee, 2003).

Because Fmr1 mutants are viable, it is possible to directly examine the effects of Fmr1 mutations on dendritic development of specific neurons in a large number of live flies. To label all dendritic processes, UAS-mCD8::GFP, which targets to the cell membrane, was expressed in all DA neurons. Third instar larvae were selected 4-5 days after egg laying (AEL) and the images of dendrites of ventral DA neurons were recorded from segments 5 and 6 in live animals. The Fmr1 mutant larvae exhibit more dendritic processes than wild-type larvae. To quantify the difference, the number of ends of all dendritic terminal processes were counted. On average, Fmr1 mutations increase the number of terminal dendritic processes of ventral DA neurons by 25%. To demonstrate that the increased number of terminal dendritic processes in Fmr1 mutants are indeed due to the absence of Fmr1 activity, one copy of the wild-type Fmr1 gene was introduced into the Fmr14 mutant background; the transgene rescues the dendritic defects in Fmr1 mutants. A large number of segments in wild-type and Fmr1 mutant larvae exhibit a similar number of terminal dendritic processes, indicating that there is a large variation among individual larvae of a given genotype and that Fmr1 mutations cause subtle changes in neuronal morphology (Lee, 2003)

To further understand the function of Fmr1 in regulating dendritic growth, Fmr1 was overexpressed in all DA neurons of wild-type wandering larvae. To do so, UAS-Fmr1 flies were crossed with Gal4 109(2)80 flies and the third-instar larvae were examined 4 days AEL. The numbers of terminal dendritic processes were dramatically reduced in both ventral and dorsal DA neurons when Fmr1 was overexpressed. The length of remaining terminal processes was also greatly reduced. This phenotype caused by Fmr1 overexpression is 100% penetrant (Lee, 2003)

Drosophila larvae increase their body surface over 50-fold from the first to the third instar larval stages. Correspondingly, the dendritic fields of DA neurons increase substantially during this period of development. In larvae overexpressing Fmr1, the major dendritic branches are still capable of extending more than fivefold during larval development. However, most terminal processes fail to form or fully extend even at the first instar stage. This demonstrates that overexpression of Fmr1 blocks the formation of higher-order dendritic branches and reduces the complexity of DA neuron dendrites during development (Lee, 2003)

The KH domains of Fmr1 share more than 70% identity with the mammalian FMR1 proteins. Indeed, Fmr1 and human FMR1 have similar RNA-binding properties in vitro. A number of studies have identified a large number of mRNAs that are associated with FMR1 in mammalian systems. However, systematic identification of Fmr1-binding targets in flies has not been carried out. To gain mechanistic insights into Fmr1 function in controlling dendritic growth in flies, co-immunoprecipitation experiments were carried out to identify mRNAs that are associated with the Fmr1-mRNP complex in vivo. In this study, using primers specific for genes encoding small GTPase Rac1, alpha-tubulin, and the voltage-gated K+ channel molecule Hyperkinetic, RT-PCR analyses was performed on either total RNAs or the RNAs that were immunoprecipitated by an anti-Fmr1 monoclonal antibody, from lysates derived from third instar larvae. All three mRNAs could be readily detected from total RNAs, while only the Rac1 mRNA was associated with Fmr1 in lysates derived from wild-type larvae as shown by coimmunoprecipitation experiments. These studies demonstrate that Rac1 mRNA is associated with Fmr1-mRNP complexes in vivo (Lee, 2003)

Based on the finding that Rac1 mRNA is present in Fmr1-mRNP complexes in vivo, it was hypothesized that the effect of Fmr1 on dendritic development in DA neurons may be partially mediated by Rac1. This hypothesis was tested genetically. First, the function of Rac1 in dendritic growth and branching of DA neurons was examined in Drosophila embryos. A null allele, Rac1J11, was tested. Gal4 109(2)80 was used to drive the expression of GFP in DA neurons in Rac1J11 mutant embryos and no gross defects were observed in dendritic branching patterns in later embryogenesis stages. DA neuron dendrites develop in discrete phases from the embryonic to larval stages. In embryos, dorsal dendrites of DA neurons extend from cell bodies first, and stop elongation 16-17 hours AEL, falling short of the dorsal midline. The lateral dendrites start to extend toward adjacent segment boundaries and cover the hemisegment before hatching (22-23 hours AEL). These findings in Rac1J11 mutant embryos suggest that Rac1 is not required for the initial growth of dorsal dendrites during embryogenesis (Lee, 2003)

During larval stages, the dendritic fields of DA neurons expand many-fold in accordance with the increase of larval body size. Higher-order dendritic branches further develop to cover the whole epidermal surface of each hemisegment. The MARCM technique was used to examine the role of endogenous Rac1 in dendritic growth in the third instar larval stage. Single GFP-labeled wild-type or Rac1 mutant DA neurons were generated in abdominal segments and the number of terminal dendritic branches was counted. Rac1 mutant ddaC neurons fewer dendritic branches than wild-type neurons, a phenotype similar to that caused by Fmr1 overexpression. Different Rac1J11 mutant ddaC neurons exhibit varying severities of dendritic defects. On average, there was a 23% reduction in the number of dendritic branches due to the Rac1 mutation. Similar dendritic defects were also found in other DA neurons. These findings demonstrate that Rac1 is required for normal dendritic branching of DA neurons in vivo, consistent studies that rely on the ectopic expression of dominant mutant forms of Rac1 (Lee, 2003)

To support the notion further that Rac1 is partially responsible for the effect of Fmr1 on dendritic development, Rac1 was overexpressed in DA neurons in third instar larvae with the UAS-Gal4 system. Consistent with the finding that Rac1 loss-of-function results in a decreased number of terminal dendritic branches, overexpression of Rac1 promotes dendritic branching of DA neurons with 100% penetrance. This result is also in line with previous studies that ectopic expression of the constitutively active form of Rac1 promotes dendritic branching. The enhanced dendritic branching caused by Rac1 overexpression is much more dramatic than that caused by Fmr1 loss-of-function, and this is presumably due to the high level of ectopic expression of Rac1 (Lee, 2003)

Because Fmr1 (or its mammalian homolog FMR1) can function as a translation inhibitor, it was of interest to enquire whether the elevated Rac1 expression obtained by using the UAS-Gal4 system would partially rescue the dendritic phenotype caused by Fmr1 overexpression. To test this hypothesis, Fmr1 and Rac1 were expressed simultaneously in DA neurons driven by Gal4 109(2)80. Overexpression of Fmr1 decreases the number of higher-order dendritic branches, but could be partially rescued by co-expression of Rac1. In addition, the number of terminal dendritic branches in Fmr14 mutants with a reduced rac1 dosage was significantly lower that that in Fmr14 mutants. These findings support the notion that Rac1 is one of the downstream components of Fmr1 function in controlling dendritic development (Lee, 2003)

Condensation of the central nervous system in embryonic Drosophila is inhibited by blocking hemocyte migration or neural activity: Removing Pvr or disrupting Rac1 function inhibits CNS condensation

Condensation is a process whereby a tissue undergoes a coordinated decrease in size and increase in cellular density during development. Although it occurs in many developmental contexts, the mechanisms underlying this process are largely unknown. This study investigated condensation in the embryonic Drosophila ventral nerve cord (VNC). Two major events coincide with condensation during embryogenesis: the deposition of extracellular matrix by hemocytes, and the onset of central nervous system activity. Preventing hemocyte migration by removing the function of the Drosophila VEGF receptor homologue, Pvr, or by disrupting Rac1 function in these cells, inhibits condensation. In the absence of hemocytes migrating adjacent to the developing VNC, the extracellular matrix components Collagen IV, Viking and Peroxidasin are not deposited around this tissue. Blocking neural activity by targeted expression of tetanus toxin light chain or an inwardly rectifying potassium channel also inhibits condensation. Disrupting Rac1 function in either glia or neurons, including those located in the nerve cord, causes a similar phenotype. These data suggest that condensation of the VNC during Drosophila embryogenesis depends on both hemocyte-deposited extracellular matrix and neural activity, and suggest a mechanism whereby these processes work together to shape the developing central nervous system (Olofsson, 2005).

Thus, disrupting hemocyte migration inhibits VNC condensation in the embryo. Lack of hemocyte migration is associated with a severe reduction of ECM components (Collagen IV and Peroxidasin) throughout the embryo and more particularly a loss of these components around the VNC. This leads to a proposal that correct assembly of the ECM depends on hemocytes, and that basement membrane is required for condensation. Supporting a role for ECM in VNC condensation, defects are observed in loss-of-function mutants of integrins, which are ECM receptors and appear themselves to be required for correct assembly of basement membranes. Mutants in integrins or the ECM component Laminin A share at least one other phenotype with embryos in which hemocyte migration has been inhibited: gut morphogenesis is impaired. Thus, a dysfunctional ECM may explain several of the morphogenetic defects seen in embryos with defective hemocyte migration (Olofsson, 2005).

How might basement membrane contribute to VNC condensation? Basement membrane may serve as a substrate for cellular movements involved in condensation and/or regulate signaling events relevant to condensation. Basement membrane is also required for normal neuromuscular junction development, and might be part of the functional blood-brain barrier in Drosophila. Hence, neural function may be disrupted when basement membrane formation is inhibited. However, condensation phenotypes in embryos with impeded hemocyte migration are more severe than in embryos in which neural activity has been blocked. This argues that the condensation phenotype seen in hemocyte migration-blocked embryos cannot be explained simply by a loss of neural activity (Olofsson, 2005).

Although animals in which hemocyte migration is blocked fail to deposit Collagen IV appropriately, it has not been demonstrated that Collagen IV function is required for condensation. However, embryos expressing a dominant negative form of Collagen IV under the control of a heatshock promoter fail to condense their nerve cord. While these data point towards a functional role of Collagen IV in condensation, further studies will be necessary to clarify the specific role of Collagen IV during condensation (Olofsson, 2005).

This study has not investigated whether phagocytosis of cells within the VNC contributes to condensation. pvr mutants show a perdurance of unengulfed cells at the ventral surface of the CNS at stage 14. The majority of these cells seem to disappear later, possibly engulfed by epidermal cells. pvr mutants also maintain some very restricted points of attachment between the epidermis and the VNC. This phenotype is not observed when hemocyte migration is blocked using mutant Rac1 expressed by crq-GAL4. This likely reflects failure of hemocyte migration at a later stage, after the two tissues have separated (Olofsson, 2005).

The major cell type that engulfs apoptotic corpses within the CNS is the subperineural glia. In the absence of macrophages (in the Bic-D mutants), apoptotic cells are still expelled from the CNS but accumulate at the ventral surface, similar to the observations in the pvr mutant. Hemocytes are required for normal CNS morphogenesis: at stage 16, pvr mutants and Crq RNAi treated embryos have mispositioned glia and minor axon scaffolding defects. These data were interpreted to reflect a failure of engulfment of cell corpses. In the context of these findings, an additional cause for glial mispositioning in pvr mutant embryos could be a loss of basement membrane components and the failure to condense (Olofsson, 2005).

VNC condensation correlates with the onset of neural activity in the CNS, and it is found that expressing tetanus toxin light chain or the inwardly rectifying K+ channel Kir2.1 pan-neuronally impairs condensation. This suggests that neural activity influences normal condensation. Neural activity could contribute to condensation in multiple ways. It could directly regulate cellular events relevant to condensation, such as adhesion or actin-based motility, or activity could influence the transcription of genes relevant to such events. Alternatively, neural activity could maintain synaptic connectivity among cells necessary for condensation, rather than directing changes in cellular behavior leading to condensation. Some condensation occurs before neural activity begins, and the condensation phenotypes resulting from impeding hemocyte migration are more severe than those resulting from blocking neural activity. This suggests that there may be multiple stages of condensation, including an earlier activity-independent stage and a later stage that is influenced by activity (Olofsson, 2005).

VNC condensation can be inhibited by expressing mutant Rac1 in lateral glia or neurons. In glia, migration and ensheathing behaviors require cytoskeletal integrity. When mutant Rac1 is expressed in peripheral glia, the formation of cellular extensions is disrupted, and this is accompanied by glia migration and axon ensheathment defects. Similarly, ensheathment of longitudinal axon tracts by longitudinal glia is disrupted in htl loss of function embryos. The VNC condensation phenotype in these embryos is interpreted as indication that glia need dynamic actin cytoskeleton to generate a condensing force. Two types of VNC glia are particularly well placed to generate such a force: longitudinal glia associated with VNC longitudinal connectives, and perineural glia, which ensheath the cortex of the VNC. Cell-cell contacts and cell-ECM contacts among these cells accompanied by remodeling of extracellular matrix could help generate a condensing force within and across neuromeres through changes in cell shape, adhesion or migration. A similar process occurs during mesenchymal condensation (Olofsson, 2005).

In neurons, neurite extension requires normal Rac GTPase activity. Expressing mutant Rac1 in these cells causes defects in axonal outgrowth. In wild type animals, VNC axons are arranged into longitudinal connectives that extend along the length of the nerve cord, and these are well placed to generate an anteroposterior condensing force. This could happen through differential cell adhesion of neurites within the longitudinal connectives or overall shortening of the axons. The observation that axons in VNC longitudinal connectives loop out during condensation in metamorphic insects is consistent with this idea. It is interesting to note that condensation is inhibited in embryos in which mutant Rac1 is expressed in glia, but longitudinal axon tracts appear normal in these animals. This suggests that if axons help generate a condensing force, they likely do this with the help of glia, possibly using these cells as a substrate (Olofsson, 2005).

It is also possible that at least part of the force required for condensation may come from outside the VNC. Somatic muscles connect to the VNC during embryogenesis, and embryonic muscle activity toward the end of embryogenesis is well timed for generating such a force. Also, the methods used to manipulate glia or neuron development in this study may affect neuromuscular activity by disrupting blood-brain barrier formation, or by affecting the Rac-dependent formation of synaptic structures. However, the observation that the CNS can condense in mutants in which muscles do not form normally argues against a major contribution from muscle activity (Olofsson, 2005).

These data identify several areas for further investigation. By following the behavior of small populations of cells in the VNC it may possible to analyze in vivo changes associated with the condensation process and get insight into how changes in organ shape are generated and coordinated. It will also be interesting to examine the contributions made by components of the ECM to normal blood-brain barrier function. Finally, it may be possible to use VNC condensation in embryonic Drosophila to investigate the molecular and cellular basis of how neural activity is translated into a morphogenetic event (Olofsson, 2005).

DRK/DOS/SOS converge with Crk/Mbc/dCed-12 to activate Rac1 during glial engulfment of axonal debris

Nervous system injury or disease leads to activation of glia, which govern postinjury responses in the nervous system. Axonal injury in Drosophila results in transcriptional upregulation of the glial engulfment receptor Draper; there is extension of glial membranes to the injury site (termed activation), and then axonal debris is internalized and degraded. Loss of the small GTPase Rac1 from glia completely suppresses glial responses to injury, but upstream activators remain poorly defined. Loss of the Rac guanine nucleotide exchange factor (GEF) Crk/myoblast city (Mbc)/dCed-12 has no effect on glial activation, but blocks internalization and degradation of debris. This study shows that the signaling molecules Downstream of receptor kinase (DRK) and Daughter of sevenless (DOS) (mammalian homologs, Grb2 and Gab2, respectively) and the GEF Son of sevenless (SOS) (mammalian homolog, mSOS) are required for efficient activation of glia after axotomy and internalization/degradation of axonal debris. At the earliest steps of glial activation, DRK/DOS/SOS function in a partially redundant manner with Crk/Mbc/dCed-12, with blockade of both complexes strongly suppressing all glial responses, similar to loss of Rac1. This work identifies DRK/DOS/SOS as the upstream Rac GEF complex required for glial responses to axonal injury, and demonstrates a critical requirement for multiple GEFs in efficient glial activation after injury and internalization/degradation of axonal debris (Lu, 2014).

Rac function in salivary gland epithelial tube morphogenesis

Epithelial cell migration and morphogenesis require dynamic remodeling of the actin cytoskeleton and cell-cell adhesion complexes. Numerous studies in cell culture and in model organisms have demonstrated the small GTPase Rac to be a critical regulator of these processes; however, little is known about Rac function in the morphogenic movements that drive epithelial tube formation. This study used the embryonic salivary glands of Drosophila to understand the role of Rac in epithelial tube morphogenesis. Inhibition of Rac function, either through loss of function mutations or dominant-negative mutations, disrupts salivary gland invagination and posterior migration. In contrast, constitutive activation of Rac induces motile behavior and subsequent cell death. Rac regulation of salivary gland morphogenesis occurs through modulation of cell-cell adhesion mediated by the E-cadherin/ß-catenin complex, and shibire, the Drosophila homolog of dynamin, functions downstream of Rac in regulating ß-catenin localization during gland morphogenesis. These results demonstrate that regulation of cadherin-based adherens junctions by Rac is critical for salivary gland morphogenesis and that this regulation occurs through dynamin-mediated endocytosis (Pirraglia, 2006).

This study shows that the Rac GTPases regulate salivary gland morphogenesis through modulation of cadherin/catenin-based cell–cell adhesion, likely by dynamin-mediated endocytosis. The characterization of the Rac mutant phenotypes suggests a model where Rac normally regulates cadherin-mediated cell–cell adhesion in salivary gland cells to allow enough plasticity for its invagination and migration yet keep the cells of the tube adhered to one another so that the gland can migrate as a cohesive tube. One mechanism by which cell surface cadherin levels are regulated is through selective endocytosis of E-cadherin from the apical–lateral membrane in a dynamin-mediated process. When Rac function is compromised through loss-of-function mutations or expression of dominant-negative mutations, the balance between E-cadherin at the plasma membrane and internalized E-cadherin appears to be abrogated so that more E-cadherin remains at the plasma membrane resulting in increased cell–cell adhesion and causing the gland to sever. These studies reveal the importance of precise regulation of adherens junction remodeling during cell migration in the context of a developing organ (Pirraglia, 2006).

In all stage 14 Rac1L89 mutant embryos examined, the salivary gland broke apart close to its approximate mid-point. Reduction in cadherin levels rescues the mutant Rac severing phenotype, suggesting that severing occurs because loss of Rac leads to an increase in cadherin-mediated cell–cell adhesion. At least two possible explanations for the midpoint severing phenotype are envisioned. In the first scenario, levels of cadherin remodeling may differ throughout the gland such that in Rac1L89 embryos the cells in the distal tip are least affected and cells in the mid-region of the gland are most affected by the increase in cadherin function. In this situation, when the distal cells begin to migrate posteriorly, the increased adhesivity of the mid-region cells prevents their migration and causes the gland to sever in the middle. In the second scenario, movement of the mid-region and the distal region of the gland may occur through different mechanisms. It is possible that while the distal most cells migrate by undergoing cell shape change and extending prominent protrusions in the direction of migration, cells in the middle of the gland may follow the distal cells by rearranging their positions along the gland, such as occurs during the convergence extension movements observed in epithelial morphogenesis. Dynamic remodeling of E-cadherin may be particularly important for proper rearrangement of the mid-region cells and an inability to rearrange when E-cadherin adhesion is increased may cause severing of the gland and subsequent separation of the migrating distal portion from the rest of the gland. Alternatively, it is possible that both of these scenarios are at play during normal salivary gland migration. Currently it is not possible to distinguish between these possibilities. In the developing tracheal tubes, Rac1 is required for cell rearrangements; in tracheal cells expressing a dominant-negative Rac1 mutation, the dorsal branch was shorter than that of wild-type embryos. Therefore, it will be important to determine whether cell rearrangement plays a role during salivary gland migration and to further elucidate the role of the Rac genes in this process (Pirraglia, 2006).

When Rac1 function is over-activate, dynamin-mediated endocytosis of E-cadherin may be increased, resulting in decreased cadherin at the plasma membrane, and decreased cell–cell adhesion. The loss of adhesion leads to the dispersal of salivary gland cells and ultimately cell death. Preventing Rac1V12-induced cell death led to the formation of abnormally shaped glands demonstrating that the Rac1V12 salivary gland phenotype is primarily due to abrogation of gland morphogenesis and not to activation of the apoptotic pathway. Moreover, since wild-type full-length E-cadherin is sufficient to rescue the Rac1V12 salivary gland phenotype, loss of cadherin function appears to be the primary cause for salivary gland defects. Thus, the Rac genes function in salivary gland cells to regulate E-cadherin-mediated cell–cell adhesion during tube morphogenesis (Pirraglia, 2006).

During salivary gland morphogenesis, gland integrity is kept intact while cells perform extensive cell shape changes and movements. Rac-regulated endocytosis of E-cadherin is one mechanism by which cell–cell adhesion is likely to be downregulated temporarily. After E-cadherin is endocytosed, it can be recycled back to the cell surface, sequestered transiently inside the cell or routed to late endosomes and lysosomes for degradation. Once salivary gland migration is complete and the gland has reached its final position, cell–cell adhesion may then need to be strengthened again in the mature gland and Rac activity may be downregulated to promote increase in surface cadherins (Pirraglia, 2006).

In addition to endocytosis, studies in mammalian cultured cells have shown that Rac can regulate levels of cell surface E-cadherin by other mechanisms, such as cleavage by presenilins and metalloproteinases, or tyrosine phosphorylation of the cadherin adhesion complex in a process involving reactive oxygen species. Thus, it will be interesting to determine whether additional mechanisms of E-cadherin regulation exist in salivary gland cells during gland morphogenesis (Pirraglia, 2006).

Numerous studies in cell culture have demonstrated that recycling of E-cadherin occurs in both a clathrin-dependent and caveolin-dependent manner. Since dynamin mediates both clathrin- and caveolin-dependent endocytosis, these studies do not allow distinguishing which type is involved in cadherin endocytosis during salivary gland migration. Alternatively, both types of endocytosis may mediate Rac1 regulation of E-cadherin in salivary gland cells (Pirraglia, 2006).

Expression of the Rac1V12 mutation in salivary gland cells leads to loss of expression of salivary gland specific proteins, apical–basal polarity proteins and E-cadherin/β-catenin. Concomitant with changes in gene expression, Rac1V12 mutant salivary gland cells lose adhesion to each other and subsequently migrate away or die by apoptosis. The data suggest that overactivation of Rac1 primarily affects E-cadherin/β-catenin-mediated adhesion and salivary gland cell fate and that the observed cell death is a secondary consequence of these earlier changes. When cell death was prevented in Rac1V12 embryos by expressing p35, more cells expressed the salivary gland specific protein PH4αSG1 than in Rac1V12 embryos; however, the expression level was drastically reduced compared to wild-type, suggesting that even in the Rac1V12p35 cells, cell differentiation was still mostly altered. Moreover, Rac1V12p35 salivary gland cells did not form a normal gland, demonstrating a role for Rac1 in gland morphogenesis. It is possible that apoptosis of Rac1V12 cells is brought about by the loss or reduction of Forkhead (Fkh) function. Fkh is expressed early in the salivary gland placode and its expression is maintained throughout embryogenesis. In the absence of fkh function, salivary gland cells die by apoptosis during the invagination stage. Since expression of dCREB-A and PH4αSG1 is reduced in Rac1V12 mutant salivary gland cells, it is possible that Fkh expression is also similarly reduced, thereby, causing the cells to undergo apoptotic cell death (Pirraglia, 2006).

Many human cancers are due to epithelial-derived tumors. When epithelial cells metastasize, they first undergo an epithelial to mesenchymal transition (EMT) before migrating away from the primary tumor to invade surrounding tissues. EMT is characterized by the loss of epithelial polarity and cell–cell adhesion. When Rac1V12 was expressed in salivary gland cells, expression of apical membrane proteins, Crumbs and aPKC and adherens junction proteins E-cadherin and β-catenin, was either lost or mislocalized. Based on these criteria, activation of Rac1 function induces features characteristic of early changes in EMT and metastasis. Interestingly, the expression levels of Rho GTPases are found to be elevated in a number of human cancers. For example, increased Rac protein levels and fast-cycling Rac mutations have been correlated with colorectal and breast tumors. Expression of constitutively active Rac1 causes some salivary gland cells to lose polarity and adhesion to neighboring cells and migrate away in a manner similar to EMT. These findings suggest that Rac1-regulated endocytosis of E-cadherin in the Drosophila salivary glands may be critical in maintaining epithelial character and preventing the loss of cell–cell adhesion and cell polarity. The Drosophila salivary gland might thus be powerful as a simple system to identify and characterize mechanisms that regulate cadherin-based cell–cell adhesion and certain aspects of EMT (Pirraglia, 2006).

Distinct molecular underpinnings of Drosophila olfactory trace conditioning

Trace conditioning is valued as a simple experimental model to assess how the brain associates events that are discrete in time. This study adapted an olfactory trace conditioning procedure in Drosophila by training fruit flies to avoid an odor that is followed by foot shock many seconds later. The molecular underpinnings of the learning are distinct from the well-characterized simultaneous conditioning, where odor and punishment temporally overlap. First, Rutabaga adenylyl cyclase (Rut-AC), a putative molecular coincidence detector vital for simultaneous conditioning, is dispensable in trace conditioning. Second, dominant-negative Rac expression, thought to sustain early labile memory, significantly enhances learning of trace conditioning, but leaves simultaneous conditioning unaffected. It was further shown that targeting Rac inhibition to the mushroom body (MB) but not the antennal lobe (AL) suffices to achieve the enhancement effect. Moreover, the absence of trace conditioning learning in D1 dopamine receptor mutants is rescued by restoration of expression specifically in the adult MB. These results suggest the MB as a crucial neuroanatomical locus for trace conditioning, which may harbor a Rac activity-sensitive olfactory 'sensory buffer' that later converges with the punishment signal carried by dopamine signaling. The distinct molecular signature of trace conditioning revealed in this study should contribute to the understanding of how the brain overcomes a temporal gap in potentially related events (Shuai, 2011).

In trace conditioning, the conditional stimulus (CS) and the unconditional stimulus (US) are separated in time by a stimulus- free interval. This so-called 'trace interval' can last for a fraction of a second in eyeblink conditioning but many seconds in fear conditioning, which poses a challenging question: how does the brain overcome this temporal gap to form the association between the CS and US? Intriguingly, trace conditioning in mammals engages neural substrates fundamentally different from delay conditioning, where the CS precedes but also temporally overlaps with the US. Early evidence comes from lesion studies with experimental animals showing that acquisition of trace conditioning requires intact hippocampal formation and medial prefrontal cortex, whereas delay conditioning can occur even with the entire forebrain removed. Later studies involving human subjects further validate the involvement of different brain circuits in these two conditioning variants and even suggest, more surprisingly, that conscious awareness might be a prerequisite for trace but not delay conditioning. It is then hypothesized that the participation of hippocampus and neocortex, as well as the associated higher cognitive function, is necessary in trace conditioning to maintain a representation of the CS or CS/US contingency so as to bridge the temporal gap. However, little is known about what form this representation takes and how it eventually converges with the US (Shuai, 2011 and references therein).

This study characterized trace conditioning in the fruit fly and used mutant analyses to show that it is distinct from the well-characterized simultaneous conditioning at the molecular level. These data complement the mammalian circuit-level studies and, more importantly, open up a molecular understanding of the internal trace that the brain uses to bridge the temporal gap (Shuai, 2011).

Odor footshock pairing elicits robust learning in fruit flies. The current study adapted this assay to study trace conditioning simply by modifying the timing relationship between the CS+ odor and the US punishment. To mimic the widely used simultaneous conditioning paradigm, CS- presentation is kept at 45 s after the punishment. Single-trial training is sufficient to elicit considerable learning performance; the learning index for OCT and 4-methycyclohexanol (MCH) is ~35 for trace conditioning at a trace interval of 30 s. Although a portion of the score (~10) might be attributed to attraction to the CS- via backward conditioning, the behavioral results clearly indicate a marked ability of fruit flies to associate events that are temporally discrete (Shuai, 2011).

One remarkable finding of the current study is that flies devoid of Rut-AC perform normally in trace conditioning. This result is interesting in view of the belief that dually regulated adenylyl cyclase plays a central role in invertebrate associative learning. The function of Rut-AC is best described as a molecular coincidence detector that is synergistically activated by the CS-evoked calcium entry and the US-evoked G protein-coupled receptor activation. It has been hypothesized that the stimulus-free gap in trace conditioning can be bridged by the temporal integration property of Rut-AC. However, the current results disagree with this hypothesis. The normal or even higher performance of rut-deficient mutants suggests that CS-US association in trace conditioning may recruit separate molecular machineries or occur in a distinct group of neurons. Also pertinent to this study is that cAMP levels in the prefrontal cortex negatively influence working memory performance. Therefore, whereas cAMP signaling is essential for some learning tasks, it is dispensable or even detrimental for others (Shuai, 2011).

Another intriguing finding is that induced expression of dominant-negative Rac enhances the learning of trace but not simultaneous conditioning. Notably, no learning enhancement was observed in a number of simultaneous conditioning variants with altered training parameters, including lowered odor concentration and conditioned intensity discrimination in the current work, as well as reduced shock pulses and lowered shock voltage in a previous report. Thus, the differential effects are not explained by a ceiling effect or other ancillary factors. Trace conditioning testing was performed almost immediately (within 3 min) after the training, rendering a better retention of the acquired associative memory also unlikely. Trace conditioning becomes less efficient as trace interval increases, indicating that an inner trace of the odor gradually degrades with time. It is therefore speculated that inhibition of Rac activity might preserve this transient 'sensory buffer' so as to facilitate trace conditioning. In the learning of simultaneous conditioning, the co-occurrence of odor and shock makes it possible to process the CS and US information automatically, e.g., via simple convergence on coincidence detection molecules like Rut-AC; hence the requirement of an olfactory sensory buffer is superfluous, which explains the lack of enhancement from Rac inhibition. The above speculation is particularly attractive considering a recently established role of Rac in the forgetting of a cold-shock sensitive early associative memory. It appears that the perdurance of two short-lived memory forms, one registered after a passive olfactory experience and lasting tens of seconds and the other registered after an associative reinforcement and lasting several hours, are both sensitive to Rac signaling manipulation (Shuai, 2011).

Drac1(N17) takes effect in the MB, the center for olfactory learning and sensory integration in insects. The localization of the Drac1 (N17) effect, combined with the full rescue of the dDA1 mutant phenotype in the MB, implies a possible trace conditioning model in which the MB bridges the temporal gap by holding a short-term sensory buffer of the odor, which later converges with the reinforcement signal carried by dopamine signaling. In accordance with this model, two recent studies in fruit fly and honey bee found no correlation between trace conditioning behavior and the postodor calcium response patterns in olfactory sensory neurons and projection neurons of the AL. Both studies pointed out the likelihood that the sensory buffer relevant to trace conditioning is in neurons downstream of the AL, most likely in the MB. Nonetheless, the AL may still retain odor information in biochemical signals other than calcium or in shortterm synaptic plasticityThe rapidly evolving molecular imaging techniques in fruit flies may help to delineate the nature of the putative sensory buffer and how it interacts later with a biologically significant stimulus (Shuai, 2011).

Another remaining puzzle is that both simultaneous and trace conditioning, although recruiting different molecular mechanisms, rely on the MB as a mutual crucial site. This seems at variance with the view from mammalian studies, where trace conditioning recruits neural circuits distinct from delay conditioning. Species or paradigm differences might explain the discrepancy, but it awaits to be fully addressed by future studies exploring whether brain regions outside the MB are additionally engaged in trace conditioning in fruit flies and, more importantly, whether various MB subdivisions contribute differentially to these two conditioning variants (Shuai, 2011).

back to Rac1 Effects of mutation part 1/2

Rac1: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | References

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