Cdc42: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation and Ectopic Expression | References
Gene name - Cdc42

Synonyms -

Cytological map position - 18E1

Function - signaling

Keywords - cytoskeleton, ectoderm, oogenesis

Symbol - Cdc42

FlyBase ID: FBgn0010341

Genetic map position -

Classification - RAS-type GTPase

Cellular location - cytoplasmic



NCBI links: Precomputed BLAST | Entrez Gene | UniGene |
Recent literature
Kamiyama, D., McGorty, R., Kamiyama, R., Kim, M. D., Chiba, A. and Huang, B. (2015). Specification of dendritogenesis site in Drosophila aCC motoneuron by membrane enrichment of Pak1 through Dscam1. Dev Cell 35: 93-106. PubMed ID: 26460947
Summary:
Precise positioning of dendritic branches is a critical step in the establishment of neuronal circuitry. However, there is limited knowledge on how environmental cues translate into dendrite initiation or branching at a specific position. Through a combination of mutation, RNAi, and imaging experiments, this study found that a Dscam-Dock-Pak1 hierarchical interaction defines the stereotypical dendrite growth site in the Drosophila aCC motoneuron. This interaction localizes the Cdc42 effector Pak1 to the plasma membrane at the dendrite initiation site before the activation of Cdc42. Ectopic expression of membrane-anchored Pak1 overrides this spatial specification of dendritogenesis, confirming its function in guiding Cdc42 signaling. It was further discovered that Dscam1 localization in aCC occurs through an inter-neuronal contact that involves Dscam1 in the partner MP1 neuron. These findings elucidate a mechanism by which Dscam1 controls neuronal morphogenesis through spatial regulation of Cdc42 signaling and, subsequently, cytoskeletal remodeling.

Vogler, G., Liu, J., Iafe, T. W., Migh, E., Mihaly, J. and Bodmer, R. (2014). Vogler, G., Liu, J., Iafe, T. W., Migh, E., Mihaly, J. and Bodmer, R. (2014). Cdc42 and formin activity control non-muscle myosin dynamics during Drosophila heart morphogenesis. J Cell Biol 206: 909-922. PubMed ID: 25267295
Summary:

During heart formation, a network of transcription factors and signaling pathways guide cardiac cell fate and differentiation, but the genetic mechanisms orchestrating heart assembly and lumen formation remain unclear. This study shows that the small GTPase Cdc42 is essential for Drosophila melanogaster heart morphogenesis and lumen formation. Cdc42 genetically interacts with the cardiogenic transcription factor tinman; with dDAAM which belongs to the family of actin organizing formins; and with zipper, which encodes nonmuscle myosin II. Zipper is required for heart lumen formation, and its spatiotemporal activity at the prospective luminal surface is controlled by Cdc42. Heart-specific expression of activated Cdc42, or the regulatory formins dDAAM and Diaphanous caused mislocalization of Zipper and induced ectopic heart lumina, as characterized by luminal markers such as the extracellular matrix protein Slit. Placement of Slit at the lumen surface depends on Cdc42 and formin function. Thus, Cdc42 and formins play pivotal roles in heart lumen formation through the spatiotemporal regulation of the actomyosin network (Vogler, 2014).

Schimizzi, G. V., Maher, M. T., Loza, A. J. and Longmore, G. D. (2016). Disruption of the Cdc42/Par6/aPKC or Dlg/Scrib/Lgl polarity complex promotes epithelial proliferation via overlapping mechanisms. PLoS One 11: e0159881. PubMed ID: 27454609
Summary:
The establishment and maintenance of apical-basal polarity is a defining characteristic and essential feature of functioning epithelia. Apical-basal polarity (ABP) proteins are also tumor suppressors that are targeted for disruption by oncogenic viruses and are commonly mutated in human carcinomas. Using the proliferating Drosophila wing disc epithelium, this study demonstrates that disruption of the junctional [Cdc42/Par6/Par3/Atypical PKC (aPKC)] complex vs. basolateral polarity complex [Scribble (Scrib)/Discs Large (Dlg)/Lethal Giant Larvae (Lgl)] complex results in increased epithelial proliferation via distinct downstream signaling pathways. Disruption of the basolateral polarity complex results in JNK-dependent proliferation, while disruption of the junctional complex primarily results in p38-dependent proliferation. Surprisingly, the Rho-Rok-Myosin contractility apparatus appears to play opposite roles in the regulation of the proliferative phenotype based on which polarity complex is disrupted. In contrast, non-autonomous Tumor Necrosis Factor (TNF; Eiger) signaling appears to suppress the proliferation that results from apical-basal polarity disruption, regardless of which complex is disrupted. Finally it was demonstrated that disruption of the junctional polarity complex activates JNK via the Rho-Rok-Myosin contractility apparatus independent of the cortical actin regulator, Moesin.
Colombie, N., Choesmel-Cadamuro, V., Series, J., Emery, G., Wang, X. and Ramel, D. (2017). Non-autonomous role of Cdc42 in cell-cell communication during collective migration. Dev Biol [Epub ahead of print]. PubMed ID: 28143705
Summary:
Collective cell migration is involved in numerous processes both physiological, such as embryonic development, and pathological such as metastasis. Compared to single cell migration, collective motion requires cell behaviour coordination through an as-yet poorly understood but critical cell-cell communication mechanism. Using Drosophila border cell migration, this study shows that the small Rho GTPase Cdc42 regulates cell-cell communication. Indeed, Cdc42 controls protrusion formation in a cell non-autonomous manner. Moreover, the endocytic small GTPase Rab11 was found to control Cdc42 localisation to the periphery of migrating border cell clusters. Accordingly, over-expression of Cdc42 in border cells rescues the loss of Rab11 function. Thus, this study positions Cdc42 as a new key player in cell-cell communication, acting downstream of Rab11.
Couturier, L., Mazouni, K., Bernard, F., Besson, C., Reynaud, E. and Schweisguth, F. (2017). Regulation of cortical stability by RhoGEF3 in mitotic sensory organ precursor cells in Drosophila. Biol Open. PubMed ID: 29101098
Summary:
In epithelia, mitotic cells round up and push against their neighbors to divide. Mitotic rounding results from increased assembly of F-actin and cortical recruitment of Myosin II, leading to increased cortical stability. Whether this process is developmentally regulated is not well known. This study examined the regulation of cortical stability in Sensory Organ precursor cells (SOPs) in the Drosophila pupal notum. SOPs differed in apical shape and actomyosin dynamics from their epidermal neighbors prior to division and appeared to have a more rigid cortex at mitosis. This study identified RhoGEF3 as an actin regulator expressed at higher levels in SOPs and showed that RhoGEF3 had in vitro GTPase Exchange Factor (GEF) activity for Cdc42. Additionally, RhoGEF3 genetically interacted with both Cdc42 and Rac1 when over-expressed in the fly eye. Using a null RhoGEF3 mutation generated by CRISPR-mediated homologous recombination, this study showed using live imaging that the RhoGEF3 gene, despite being dispensable for normal development, contributed to cortical stability in dividing SOPs. It is therefore suggested that cortical stability is developmentally regulated in dividing SOPs of the fly notum.
BIOLOGICAL OVERVIEW

Cdc42, a member of the Rho family of GTP binding proteins, functions in the formation of polarized actin structures, in elongation of cell shape, and in cell signaling. The functions of Cdc42 in developing tissues have been examined using loss-of-function mutations in Drosophila Cdc42. Cdc42 minus epithelial cells fail to elongate into a columnar cell shape and cannot maintain a monolayered epithelial structure. No requirement for Cdc42 in cell division or in activation of the Jun N-terminal kinase pathway is found. In addition, Cdc42 function is not required for cytoplasmic actin filament assembly in the nurse cells during oogenesis, although it may facilitate this process. Furthermore, the results indicate that Cdc42 plays a role in intercellular interactions between the germ line and the somatic follicle cells. These results confirm the role of Cdc42 in actin filament assembly and provide new insights into its functions in epithelial morphogenesis and regulating intercellular signaling events (Genova, 2000).

Members of the Ras superfamily of small GTPase proteins have been shown to act as molecular switches that regulate cell proliferation, cell fate, and a range of other cellular and developmental processes. Within this superfamily, the Rho family, including Rho, Rac, and Cdc42, has been particularly well studied because of its role in regulating cytoskeletal assembly and related cellular events such as membrane trafficking, cell polarity, cell adhesion, and cell elongation. More recently, these proteins have also been suggested to function as switches in signal transduction pathways, such as the Jun N-terminal kinase (JNK) pathway, with functions that extend beyond cytoskeletal regulation. In addition, numerous studies implicate Rho family members in processes associated with oncogenesis, either directly or as effectors of oncogenic pathways. Thus it appears that Rho family proteins play crucial roles in regulating cytoskeletal processes and a range of related cellular and developmental functions (Genova, 2000 and references therein).

Within the Rho family, the Cdc42 gene is unique in that it was originally identified as a cell cycle mutation in the yeast Saccharomyces cerevisiae. In the yeast, CDC42 function is necessary during the assembly of the bud site, an actin-rich structure that forms at one end of the budding yeast cell. Mutations in the yeast CDC42 gene disrupt the assembly of a ring of actin microfilaments that normally forms in the neck of the bud, thereby blocking bud formation and cell division. This phenotype has been interpreted to indicate that CDC42 mutations disrupt the ability of cells to construct localized or 'polarized' assemblies of actin microfilaments (Genova, 2000).

Experiments using cultured cells have implicated human Cdc42 in the formation of filopodia in growth factor stimulated cells, presumably via the assembly of membrane-associated microfilaments in these structures. Interestingly, other Rho family members appear to regulate the assembly of morphologically distinct cellular processes, such as lamellipodia and stress fibers. In addition, several lines of evidence implicate Cdc42 or another Rho family member as functioning upstream of the JNK pathway. Activated forms of both Rac and Cdc42 stimulate the JNK pathway in HeLa, NIH-3T3, and Cos cells. However, constitutively activated forms of Cdc42 and Rho appear to activate the JNK pathway in human kidney 293T cells. These seemingly contradictory results could indicate that the Rho family regulation of the JNK pathway occurs in a tissue-specific manner. Alternatively, it is possible that the mutationally activated forms of these highly related proteins result in nonspecific interactions between the structurally similar members of the Rho family (Genova, 2000).

The developmental functions of Cdc42 and the Rho family members have been investigated in Drosophila using dominant interfering or activating alleles. Dominant active and negative mutations in Rac disrupt axon outgrowth in the embryonic nervous system and affect myoblast fusion. Similar forms of Cdc42 affect neurite outgrowth in the peripheral nervous system but do not affect myoblast fusion. Dominant negative Cdc42 has also been shown to disrupt the process of dorsal closure late in embryonic development. Studies of Drosophila oogenesis using ectopically expressed dominant negative Cdc42 suggest that Cdc42 may function to regulate the assembly of actin microfilaments that are necessary for proper transfer of cytoplasm from the nurse cells to the oocyte. Dominant alleles of Cdc42 also affect the ability of imaginal epithelial cells to elongate into a columnar cell shape and disrupt the planar polarity (rather than the apical-basal polarity) of these epithelial sheets (Genova, 2000).

Genetic mutations in Drosophila Cdc42 (Fehon, 1997) were used to examine the effects of loss of Cdc42 function in developing tissues. Reduction of Cdc42 function in embryonic, imaginal, and follicular epithelial cells results in a loss of epithelial character -- mutant cells are unable to maintain a columnar cell shape, lose cohesion necessary to maintain cell sheets, and form multiple cell layers. Reduction of Cdc42 function in the embryonic epidermis leads to a failure in germ-band retraction and a degeneration of the ventral epidermis prior to the process of dorsal closure, yet axon outgrowth in the central and peripheral nervous systems still occurs. Functional Cdc42 is also required for proper differentiation of imaginal epithelial cells and in both the germ-line and the follicle cells for proper specification of the follicular stalk cells. In contrast, actin filament assembly in the developing nurse cells of the ovary is not dependent on Cdc42 function. In addition, contrary to predictions from previous studies, loss of Cdc42 function does not appear to affect activation of the JNK pathway during dorsal closure or cytokinesis in proliferating epithelial cells. Thus, these studies, which are the first to use loss-of-function Cdc42 alleles in a higher eukaryote, provide new insights into the functions of this ubiquitous cytoskeletal regulatory switch (Genova, 2000).

To determine the functions of Cdc42 during Drosophila development, the phenotypes of six previously isolated mutations in the Drosophila Cdc42 gene (Fehon, 1997) were examined. The molecular lesions associated with two of these alleles, Cdc421 and Cdc422, were previously characterized (Fehon, 1997). Of the lethal alleles, two (Cdc421 and Cdc423) are missense mutations while the other (Cdc424) is a mutation in the splice acceptor site (Genova, 2000).

Cdc423 and Cdc424 display a similar range of phenotypes and produce similar degrees of maternal-effect lethality, suggesting that they reduce Cdc42 function to similar levels. Rescue experiments using a Ubiquitin promoter-Cdc421 transgene indicate that Cdc421 is only partially rescuable, while all of the other alleles can be rescued completely, consistent with the notion that Cdc421 has dominant negative functions (Genova, 2000).

Previous studies of Cdc42 in yeast and mammalian cells have indicated that it plays a fundamental role in cellular morphogenesis. It is therefore surprising that lethal Cdc42 mutations in Drosophila display no embryonic phenotype (Fehon, 1997). One possible explanation for this apparent paradox is that Cdc42 is maternally expressed and loaded into the egg, thereby providing sufficient Cdc42 function for embryogenesis. Cdc422 germ-line clones could not be used to address this possibility because Cdc42 is required during oogenesis for the production of fertile eggs. Instead heteroallelic combinations of weak and strong alleles were used to produce females that, while viable, had reduced Cdc42 function. Examination of two such combinations, Cdc423/Cdc426 and Cdc424/Cdc426, demonstrated that both combinations produce over 70% embryonic lethality, even when outcrossed to wild-type males. Given the Mendelian expectation that 25% of the progeny will be hemizygous for the lethal allele, the high embryonic lethality indicates that Cdc42 is maternally contributed to the embryo and is essential (Genova, 2000).

To examine the role of Cdc42 in embryogenesis, the phenotypes of embryos with reduced maternal Cdc42 function were examined. Cuticle preparations and staining with anti-Coracle, which specifically stains ectodermally derived epithelial cells, revealed epidermal defects in these embryos, prior to the process of dorsal closure, that first appeared at the onset of germ-band retraction. In the embryos that failed to hatch, germ-band retraction failed to proceed to completion, leaving embryos open on the dorsal side. In addition, holes frequently appeared in the embryonic epidermis along the ventral midline. In extreme cases, possibly corresponding to embryos that also lacked zygotic Cdc42 function, the ventral epidermis was observed to tear apart along almost the entire anterior-posterior axis of the retracting embryo (Genova, 2000).

A previous study using a dominant activated Cdc42 allele has proposed that Cdc42 functions in neuronal cells to regulate neurite outgrowth (Luo, 1994). To determine if Cdc42 is required for normal neurite formation, embryos with reduced maternal Cdc42 contribution were examined for defects in the central and peripheral nervous systems. Because the embryonic defects observed were not completely penetrant and some embryos (<30%) completed embryogenesis normally, embryos with obvious germ-band retraction and epidermal defects were selected as an indication of reduced Cdc42 function. Patterns of neuronal differentiation, as determined by mAb 22C10 and anti-horseradish peroxidase (anti-HRP), were largely normal, even in areas where the integrity of the overlying epidermis was disrupted. In certain embryos defects in the central nervous system (CNS) such as incomplete formation and midline fusion of the longitudinal commissures were observed. However, in all embryos the axons of the peripheral nervous system as detected by antibody 22C10 appeared to extend properly. The defects observed in the CNS could represent a requirement for Cdc42 function in this tissue or they could be secondary to the other embryonic defects, such as failure in germband retraction and epidermal holes. In either case, the formation of axon commissures in the CNS and the apparently correct projection of neurites in the peripheral nervous system suggest that Cdc42 function is not essential for neurite outgrowth. Alternatively, it is possible that the requirement for Cdc42 function is higher in the epidermis than in other tissues or that tissues of the nervous system contain greater amounts of the maternal Cdc42 product (Genova, 2000).

The function of Cdc42 in postembryonic development was examined using analysis of phenotypes caused by partial loss-of-function mutations and somatic mosaic analysis of lethal mutations. Flies homozygous for weak Cdc42 alleles or carrying combinations of weak and strong alleles displayed distinct visible phenotypes. Scanning electron micrographs of Cdc42 mutant eyes revealed minor flaws in the ommatidial array, primarily in the dorsal posterior quadrant. In this region, occasional fusions of the ommatidia were observed and frequent loss or duplication of bristles. Although these phenotypes suggest possible defects in the structure of ommatidial units, examination of histological sections from Cdc42 mutant eyes revealed that all of the photoreceptor cells are present and in the proper orientation (Genova, 2000).

Cdc42 mutant adults also possessed extra crossveins on the wing between veins II and III and veins III and IV. Some allelic combinations displayed blisters in the wings where the dorsal and ventral surfaces were not fully apposed. Extra crossveins were also observed in 12% of flies heterozygous for Cdc421, consistent with the assertion that this allele has dominant negative properties. To examine the function of Cdc42 at the cellular level, somatic mosaic clones were generated to produce cells homozygous for Cdc42 lethal alleles in the wing imaginal discs. Clones generated early in larval development, at 36 h AEL, did not survive to the wandering third-instar stage, leaving only sister clones that mark where a recombination event occurred. Mutant clones that were generated later in larval development, at 72 h AEL, were smaller than their sister clones at the end of larval development. In these discs, mutant clones that occurred in the notum were only slightly reduced in size relative to their sister clones, whereas the mutant clones generated in the blade were either absent or much smaller than their sisters. This clonal pattern is characteristic of a cell competition effect and suggests that Cdc42 mutant cells are at a proliferation disadvantage relative to their wild-type neighbors (Genova, 2000).

As in the wing discs, Cdc422 clones generated in the eye at 36 h AEL were not observed in wandering third-instar larvae. Clones generated throughout the eye later in development, at 72 or 96 h AEL, did survive to the wandering third-instar stage, but did not survive to the adult stage. In histological sections of such eyes, mild pattern disruptions were observed around small scars near the sister clones, but no mutant cells had differentiated into photoreceptors or other recognizable cell types. To overcome the implied competition disadvantage of Cdc42 mutant cells and to examine their ability to differentiate adult structures, eye clones were generated in a Minute background. Control clones generated in the developing eye at 72 or 96 h AEL in a Minute background grew to encompass the majority of the eye. In contrast, clones of Cdc42 mutant cells failed to produce adult ommatidia and instead left scars on the eye. This result suggests that, in this case, the competition effect is due to a cause other than impaired proliferation (Genova, 2000).

To examine the development of these clones, it was asked if, in a wild type background, Cdc42 mutant cells could initiate neuronal differentiation. As determined by anti-HRP staining for developing neurons, mutant clones generated in eye discs 72 h AEL initiated differentiation as photoreceptor cells in the larval eye imaginal disc. These results indicate that although Cdc42 function is not required to initiate neuronal cell fate in the eye, it is necessary to complete photoreceptor differentiation (Genova, 2000).

Previous studies using ectopically expressed dominant alleles of yeast CDC42 and Drosophila Cdc42 in flies have suggested that Cdc42 may have important functions during Drosophila oogenesis (Murphy, 1996). To examine the effects of loss of Cdc42 function in the somatic and germ-line cells of the developing egg chamber, Cdc422 clones were generated via mitotic recombination. Follicle cell clones had three highly penetrant phenotypes: loss of columnar cell shape, formation of multiple layers of cells at the posterior end of the egg chamber, and apparent fusion of adjacent egg chambers. Germ-line clones displayed minor defects in actin filament assembly and, unexpectedly, defects in a subset of the follicle cells (Genova, 2000).

At stage 9 of egg chamber development, wild-type follicle cells change from a cuboidal cell shape to a more columnar shape. However, in mosaic egg chambers Cdc42 mutant cells remain cuboidal or irregular in cell shape while the wild-type cells abutting the mutant clone become columnar. Anti-Armadillo staining indicated that although the cells are abnormally shaped, the adherens junctions remain intact and cell polarity is maintained. Other apical markers, such as filamentous actin and Notch, also indicated that the apical- basal polarity is normal (Genova, 2000).

Clones of Cdc42 mutant follicle cells appear to lose the characteristics of a monolayered epithelium and form multiple layers of cells, primarily at the poles of the egg chamber. Mutations in alpha Spectrin, which produce a similar multilayered phenotype, have been shown to cause overproliferation in the follicle cells. To determine if the Cdc42 phenotype is due to overproliferation or alternatively is caused by redistribution of mutant cells to the posterior end of the egg chamber, follicle cell number was measured in egg chambers containing over 75% mutant follicle cells. These egg chambers did not appear to accumulate yolk in the oocyte and they degenerated by stage 9. The average follicle cell number in egg chambers containing Cdc42 mutant cells was not significantly different from that of wild-type egg chambers. In addition, cell division, as determined using antiphosphohistone H3, an antibody marker for mitotic cells, ceases at stage 6 in mutant clones as it does in wild-type cells. These results indicate that Cdc42 mutant follicle cells do not overproliferate, but rather display an inability to form a normal, single-layered epithelium (Genova, 2000).

A third phenotype observed in follicle cell clones that comprised the entire egg chamber was that egg chambers often appeared abnormally large, containing germ-line cells of two distinct size classes. These characteristics indicate that adjacent cysts may have fused or failed to separate during oogenesis. To examine this phenotype further, egg chambers were stained with anti-PS1 integrin, a stalk-cell-specific marker. Fused egg chambers appeared to lack stalk cells, suggesting that Cdc42 function is necessary for stalk cell differentiation or placement at the poles of egg chambers (Genova, 2000).

In the female germ line, highly organized bundles of cytoplasmic filaments are rapidly polymerized at stage 10b in the nurse cells. To examine Cdc42 localization during the polymerization of these actin filaments, a Myc-tagged Cdc421 transgene was constructed whose expression was regulated by the Drosophila Ubiquitin promoter. This transgene rescued lethal Cdc42 alleles, indicating that the addition of the Myc epitope tag does not affect Cdc42 function. In nurse cells prior to and during stage 10b, Myc-tagged Cdc42 was observed throughout the cytoplasm and was concentrated at the plasma membrane. In addition, it was observed that during stage 10b Cdc42 accumulates in regions where the cytoplasmic actin filaments intersect the plasma membrane. This localization pattern suggests that Cdc42 is recruited to sites on the plasma membrane where actin assembly is occurring (Genova, 2000).

Given the localization of Myc-tagged Cdc42 to the base of cytoplasmic actin filaments, it is reasonable to propose that Cdc42 function is required for the formation of these filaments. To test this hypothesis, ovaries were examined from mutant germ-line clones, produced using the ovoD1 dominant, female-sterile mutation and in females carrying hypomorphic combinations of alleles. In egg chambers containing Cdc42 mutant germ-line clones, the characteristic cytoplasmic actin filaments still form and have essentially normal morphology. However, compared to wild-type nurse cells, Cdc42 mutant nurse cells appear to have fewer cytoplasmic actin filaments. In addition, those actin filaments directly associated with the ring canals appeared longer than normal. Dumping of nurse cell contents into the oocyte is delayed and does not appear to proceed to completion. Although these egg chambers fail to produce functional oocytes, staining with anti-Vasa antibody indicated that their overall anterior-posterior polarity is properly established (Genova, 2000).

Recent studies have shown that numerous interactions occur between the germ-line cells and surrounding somatic follicle cells during oogenesis. To determine if any of these interactions depend on Cdc42 function in the germ line, egg chambers containing mutant germ-line cells and wild-type follicle cells (as determined by the structure of the follicular epithelium) were examined. Staining with anti-PS1 showed a greater number of stalk cells adjacent to Cdc42 mutant germ-line clones than in wild-type controls. Interestingly, a study of the Drosophila toucan gene, which encodes a novel protein, has shown that this gene also functions in the germ line to regulate stalk cell number. This result indicates that interactions between the germ line and the soma are important for the differentiation of the stalk cells and that Cdc42 function is required in the germ line for this process (Genova, 2000).

Cdc42 was initially isolated as a cell division mutation that affects the process of budding in the yeast S. cerevisiae (Adams, 1990). It has also been implicated in the proper formation of the actin and myosin contractile ring during cytokinesis (Drechsel, 1997). Taken together, these results suggest that Cdc42 is essential for cell division in all organisms. However, the results presented in this study indicate that this hypothesis is incorrect: Cdc42 function is not required for proliferation of the follicular epithelium. In the ovary Cdc42 mutant follicle cell clones induced 6 days prior to dissection were frequently observed, that were large enough to comprise almost an entire ovariole. The size of the follicle cell clones and the extent of the ovariole that they cover indicate that the recombination was induced in stem cells that subsequently underwent many rounds of cell division. Given the length of time that these clones persisted and their size, it is unlikely that perdurance played a role in the survival of the mutant cells. Thus, Cdc42 does not appear to be required directly for cytokinesis or other aspects of cell proliferation (Genova, 2000).

Cdc42 mutant epithelial cells display phenotypes indicating an inability to undergo changes in cell shape that normally occur during embryogenesis, oogenesis, and imaginal development. At stage 9 during oogenesis, follicle cells undergo a transition from cuboidal to columnar cell shape. Simultaneously, most of the follicle cells migrate toward the posterior end of the egg chamber, leaving only a few specialized squamous cells covering the nurse cells at the anterior end of the egg chamber. In contrast to wild-type cells, Cdc422 cells fail to undergo this change of cell shape and instead remain more cuboidal or irregular in shape. Defects in cell elongation were also observed when dominant negative Cdc42 was expressed during Drosophila development (Eaton, 1995, 1996; Luo, 1994). Although improperly shaped, Cdc42 mutant follicle cells still migrate, which results in multiple layers of cuboidal cells at the posterior end of the egg chamber. This phenotype indicates that while still competent to migrate, mutant follicle cells are unable to undergo the changes in cell shape necessary for proper epithelial organization. Results using somatic mosaic analysis in the imaginal epithelium differ in some respects from those in the follicular epithelium. Although very large clones of mutant cells have been observed in the follicular epithelium, such clones were never found in the imaginal discs. Instead, Cdc42 mutant clones are lost rapidly from the imaginal epithelium in a manner resembling cell competition. The inability of cells to compete in the imaginal epithelium has been shown to be characteristic of cells that proliferate more slowly than their wild-type neighbors. However, the observation that Cdc422 cells in the follicular epithelium proliferate normally suggests that an inability to differentiate properly is the cause of clone loss. Consistent with this hypothesis, Cdc42 mutant clones induced in the background of a Minute mutation, which slows the growth of neighboring cells thereby rescuing clones with a proliferative disadvantage, still fail to survive to the adult stage. A previous study using ectopic expression of a dominant negative Cdc42 allele in wing imaginal disc cells has shown that these cells are unable to elongate properly to a columnar shape (Eaton, 1995), a phenotype similar to that which has been observed for Cdc422 follicle cells. Thus it is possible that cells that are unable to elongate properly to form the columnar shape that is typical of imaginal disc cells are eliminated from the epithelium by cell competition. If so, cell competition may represent a general response to eliminate cells that do not function normally, rather than a specific response to proliferation defects (Genova, 2000).

In the developing embryonic epithelium, phenotypes were also observed indicating a role for Cdc42 in epithelial morphogenesis. In embryos derived from females that are hypomorphic for Cdc42 function, early embryogenesis through germ-band extension occurs normally, presumably by using maternally encoded Cdc42. However, holes appear in the embryonic epithelium at the onset of germ-band retraction, particularly along the ventral midline. Epidermal cells along the ventral midline undergo extensive cell shape changes and rearrangements during germband retraction. Taken together, the phenotypes observed in developing Drosophila epithelial cells demonstrate a role for Cdc42 in cell elongation and other dynamic cell shape changes. In addition, the presence of epithelial holes may indicate that Cdc42 mutant cells are unable to either form or maintain normal adhesive contacts (Genova, 2000).

How does Cdc42 function in cell shape and morphogenesis? In epithelial cells, Cdc42 is essential for elongation into a columnar cell shape, a process that is likely to be dependent on actin filament assembly. Although the small size of Drosophila epithelial cells did not allow a direct analysis of cytoskeletal structure in Cdc42 mutant cells, the cortical actin filaments that form in the nurse cells during stage 10b of oogenesis are much larger and are readily observable. In Cdc422 nurse cells the number of cortical actin filaments is typically reduced relative to wild-type cells, but those that form appear morphologically normal. Myc-tagged Cdc42 protein localizes to the plasma membrane in these cells and is concentrated at the base of the clusters of cortical actin filaments. These results suggest that Cdc42 functions at the plasma membrane to facilitate the nucleation of actin filaments, but is not absolutely required for their formation (Genova, 2000).

This view of Cdc42 function in vivo agrees well with evidence from studies in vitro. Recent studies indicate that Cdc42 plays a primary role in the nucleation of new actin filaments by regulating the activity of an actin filament nucleating complex composed of the Wiskott-Aldrich syndrome protein (WASP: see Drosophila WASP) and the Arp2/3 complex (Rohatgi, 1999). Although Arp2/3 and WASP can nucleate new actin filaments in the absence of Cdc42, this activity increases dramatically in the presence of activated Cdc42 and phospholipids. Thus the reduction (rather than complete loss) of assembly of cortical actin filaments observed is consistent with the hypothesis that Cdc42 functions by regulating the rate of actin filament assembly, presumably in response to activation of an as yet unknown signaling pathway (Genova, 2000).


GENE STRUCTURE

cDNA clone length - 1443

Bases in 5' UTR - 191

Exons - 3

Bases in 3' UTR - 676


PROTEIN STRUCTURE

Amino Acids - 191

Structural Domains

See SMART (Simple Modular Architecture Research Tool) for information on Rho-family small GTPases.


Cdc42: Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation and Ectopic Expression | References

date revised: 27 January 2003

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