Extrinsic and intrinsic mechanisms directing epithelial cell sheet replacement during Drosophila metamorphosis

Role of abd-A and Abd-B in development of abdominal epithelia breaks posterior prevalence rule

Hox genes that determine anteroposterior body axis formation in all bilaterians are often found to have partially overlapping expression pattern. Since posterior genes dominate over anterior Hox genes in the region of co-expression, the anterior Hox genes are thought to have no function in such regions. In this study we show that two Hox genes have distinct and essential functions in the same cell. In Drosophila, the three Hox genes of the bithorax complex, Ubx, abd-A and Abd-B, show coexpression during embryonic development. Here, we show that in early pupal abdominal epithelia, Ubx does not coexpress with abd-A and Abd-B, while abd-A and Abd-B continue to coexpress in the same nuclei. The abd-A and Abd-B are expressed in both histoblast nest cells and larval epithelial cells of early pupal abdominal epithelia. Further functional studies demonstrate that abd-A is required in histoblast nest cells for their proliferation and suppression of Ubx to prevent first abdominal segment like features in posterior segments while in larval epithelial cells it is required for their elimination. We also observed that these functions of abd-A are required in its exclusive as well as the coexpression domain with that of Abd-B. The expression of Abd-B is required in histoblast nest cells for their identity while it is dispensable in the larval epithelial cells. The higher level of Abd-B in the seventh abdominal segment, that down-regulates abd-A expression, leads this segment to be absent in males or of smaller size in females. We also show that abd-A in histoblast nest cells positively regulates expression of wingless for the formation of the abdominal epithelia. Our study reveals an exception to the rule of posterior prevalence and shows that two different Hox genes have distinct functions in the same cell, which is essential for the development of abdominal epithelia.

The spatially non-overlapping function of Hox genes is known to determine Antero-posterior body axis in all the bilaterians. The expression of Hox genes is found to be overlapping in several cases. According to the posterior prevalence rule, posterior Hox genes suppress the function of anterior Hox genes in the overlapping expression domains. Our findings show an exception to the rule of posterior prevalence. We show that in the overlapping expression domains of abd-A and Abd-B in early pupal abdominal epithelia, both the genes have essential roles. While abd-A is required for cell proliferation, Abd-B determines the segmental identity.

Hox targets and cellular functions

Hox genes are a group of genes that specify structures along the anteroposterior axis in bilaterians. Although in many cases they do so by modifying a homologous structure with a different (or no) Hox input, there are also examples of Hox genes constructing new organs with no homology in other regions of the body. Hox genes determine structures though the regulation of targets implementing cellular functions and by coordinating cell behavior. The genetic organization to construct or modify a certain organ involves both a genetic cascade through intermediate transcription factors and a direct regulation of targets carrying out cellular functions. In this review I discuss new data from genome-wide techniques, as well as previous genetic and developmental information, to describe some examples of Hox regulation of different cell functions. I also discuss the organization of genetic cascades leading to the development of new organs, mainly using Drosophila melanogaster as the model to analyze Hox function.

Cell migration

Cell migration is fundamental to establishing and maintaining the proper organization of multicellular organisms. Morphogenesis can be viewed as a consequence, in part, of cell locomotion, from large-scale migrations of epithelial sheets during gastrulation, to the movement of individual cells during development of the nervous system. In an adult organism, cell migration is essential for proper immune response, wound repair, and tissue homeostasis, while aberrant cell migration is found in various pathologies. Indeed, as our knowledge of migration increases, we can look forward to, for example, abating the spread of highly malignant cancer cells, retarding the invasion of white cells in the inflammatory process, or enhancing the healing of wounds. This article is organized in two main sections. The first section is devoted to the single-cell migrating in isolation such as occurs when leukocytes migrate during the immune response or when fibroblasts squeeze through connective tissue. The second section is devoted to cells collectively migrating as part of multicellular clusters or sheets. This second type of migration is prevalent in development, wound healing, and in some forms of cancer metastasis.

The Ecdysone receptor constrains wingless expression to pattern cell cycle across the Drosophila wing margin in a Cyclin B-dependent manner

Background

Ecdysone triggers transcriptional changes via the ecdysone receptor (EcR) to coordinate developmental programs of apoptosis, cell cycle and differentiation. Data suggests EcR affects cell cycle gene expression indirectly and here we identify Wingless as an intermediary factor linking EcR to cell cycle.

Results

We demonstrate EcR patterns cell cycle across the presumptive Drosophila wing margin by constraining wg transcription to modulate CycB expression, but not the previously identified Wg-targets dMyc or Stg. Furthermore co-knockdown of Wg restores CycB patterning in EcR knockdown clones. Wg is not a direct target of EcR, rather we demonstrate that repression of Wg by EcR is likely mediated by direct interaction between the EcR-responsive zinc finger transcription factor Crol and the wg promoter.

Conclusions

Thus we elucidate a critical mechanism potentially connecting ecdysone with patterning signals to ensure correct timing of cell cycle exit and differentiation during margin wing development.

Homeostatic epithelial renewal in the gut is required for dampening a fatal systemic wound response in Drosophila

Effective defense responses involve the entire organism. To maintain body homeostasis after tissue damage, a systemic wound response is induced in which the response of each tissue is tightly orchestrated to avoid incomplete recovery or an excessive, damaging response. Here, we provide evidence that in the systemic response to wounding, an apoptotic caspase pathway is activated downstream of reactive oxygen species in the midgut enterocytes (ECs), cells distant from the wound site, in Drosophila. We show that a caspase-pathway mutant has defects in homeostatic gut cell renewal and that inhibiting caspase activity in fly ECs results in the production of systemic lethal factors after wounding. Our results indicate that wounding remotely controls caspase activity in ECs, which activates the tissue stem cell regeneration pathway in the gut to dampen the dangerous systemic wound reaction.

The homeobox transcription factor cut coordinates patterning and growth during Drosophila airway remodeling

A fundamental question in developmental biology is how tissue growth and patterning are coordinately regulated to generate complex organs with characteristic shapes and sizes. We showed that in the developing primordium that produces the Drosophila adult trachea, the homeobox transcription factor Cut regulates both growth and patterning, and its effects depend on its abundance. Quantification of the abundance of Cut in the developing airway progenitors during late larval stage 3 revealed that the cells of the developing trachea had different amounts of Cut, with the most proliferative region having an intermediate amount of Cut and the region lacking Cut exhibiting differentiation. By manipulating Cut abundance, we showed that Cut functioned in different regions to regulate proliferation or patterning. Transcriptional profiling of progenitor populations with different amounts of Cut revealed the Wingless (known as Wnt in vertebrates) and Notch signaling pathways as positive and negative regulators of cut expression, respectively. Furthermore, we identified the gene encoding the receptor Breathless (Btl, known as fibroblast growth factor receptor in vertebrates) as a transcriptional target of Cut. Cut inhibited btl expression and tracheal differentiation to maintain the developing airway cells in a progenitor state. Thus, Cut functions in the integration of patterning and growth in a developing epithelial tissue.

Beyond apoptosis: caspase regulatory mechanisms and functions in vivo

The caspases, a family of cysteine proteases, function as central regulators of cell death. Recently, caspase activity and caspase substrates identified in the absence of cell death have sparked strong interest in caspase functions in nonapoptotic cellular responses; these functions suggest that caspases may be activated without inducing or before apoptosis, thus leading to the cleavage of a specific subset of substrates. This review focuses primarily on the caspase enzymatic activity. Detailed genetic analyses of caspase-deficient Caenorhabditis elegans, Drosophila, and mice have shown that caspases are essential, not only for controlling the number of cells involved in sculpting or deleting structures in developing animals, but also for dynamic, nonapoptotic cell processes, such as innate immune response, tissue regeneration, cell-fate determination, stem-cell differentiation and neural activation. Our understanding of the spatio-temporal caspase activation mechanisms has advanced, primarily through the study of Drosophila developmental processes. This review will discuss current findings regarding caspase functions in cytoskeletal modification, morphogenetic regulation of cell shape, cell migration and the production of mechanical force during embryogenesis.

Homeotic functions of the Teashirt transcription factor during adult Drosophila development

During Drosophila development region-specific regulation of target genes by Hox proteins is modulated by genetic interactions with various cofactors and genetic collaborators. During embryogenesis one such modulator of Hox target specificity is the zinc-finger transcription factor Teashirt (Tsh) that is expressed in the developing trunk and cooperatively functions with trunk-specific Hox proteins to promote appropriate segment fate. This embryonic function of Tsh is characterized as homeotic since loss of embryonic Tsh activity leads to transformation of trunk segments toward head identity. In addition to this embryonic homeotic role, Tsh also performs vital Hox-independent functions through patterning numerous embryonic, larval and adult structures. Here we address whether the homeotic function of Tsh is maintained throughout development by investigating its contribution to patterning the adult abdomen. We show that Tsh is expressed throughout the developing abdomen and that this expression is dependent on the three Bithorax Hox proteins Ultrabithorax, Abdominal-A and Abdominal-B. Conditional reduction of Tsh activity during pupation reveals broad homeotic roles for this transcription factor throughout the adult abdomen. Additionally we show that, as during embryogenesis, the tsh paralog tiptop (tio) plays a partially redundant role in this homeotic activity.

New emerging roles for epithelial cell extrusion

Epithelia use a unique process called 'cell extrusion' to remove cells from a layer, while preserving their barrier function. Specifically, a cell destined to die triggers formation of an actin and myosin-ring in the live neighboring epithelial cells surrounding it, which squeeze the dying cell out. During extrusion, the surrounding cells expand toward one another and meet to fill the gap left by the extruded cell. Recent studies have revealed new roles of extrusion in controlling developmental morphogenesis, maintaining homeostatic cell numbers, and how this process is usurped during bacterial pathogenesis. Here, we review recent advances in new processes that require cell extrusion and the signaling pathways controlling it.

Apoptotic and nonapoptotic caspase functions in animal development

A developing animal is exposed to both intrinsic and extrinsic stresses. One stress response is caspase activation. Caspase activation not only controls apoptosis but also proliferation, differentiation, cell shape, and cell migration. Caspase activation drives development by executing cell death or nonapoptotic functions in a cell-autonomous manner, and by secreting signaling molecules or generating mechanical forces, in a noncell autonomous manner.

Abdominal segment reduction: development and evolution of a deeply fixed trait

When a new student first begins to push flies, an immediate skill that must be learned is sorting the sexes. In Drosophila melanogaster several sexually dimorphic characters can be used to readily distinguish males from females including abdominal pigmentation, male sex combs and genital morphology. Another, often-overlooked, sexual dimorphism is adult abdominal segment number. Externally, adult Drosophila males possess one fewer abdominal segment than females; the terminal pre-genital segment apparently either absent or fused with the next-most anterior segment. Beyond known roles for the homeotic protein Abdominal-B (Abd-B) and the sex-determining transcription factor Doublesex (Dsx) as key regulators of this trait, surprisingly little is known about either the morphogenetic processes or the downstream genetics responsible for patterning these events. We have explored both and found that rapid epithelial reorganization during pupation eliminates a nascent terminal male segment. We found this Abd-B-dependent process results from sex- and segment-specific regulation of diverse developmental targets including the wingless gene and surprisingly, dsx itself. ( 1) (,) ( 2) Here, I review our observations and discuss this trait as a model to explore both dynamics of epithelial morphogenesis as well as the evolution of developmental mechanisms.

Cell migration

Cell migration is fundamental to establishing and maintaining the proper organization of multicellular organisms. Morphogenesis can be viewed as a consequence, in part, of cell locomotion, from large-scale migrations of epithelial sheets during gastrulation, to the movement of individual cells during development of the nervous system. In an adult organism, cell migration is essential for proper immune response, wound repair, and tissue homeostasis, while aberrant cell migration is found in various pathologies. Indeed, as our knowledge of migration increases, we can look forward to, for example, abating the spread of highly malignant cancer cells, retarding the invasion of white cells in the inflammatory process, or enhancing the healing of wounds. This article is organized in two main sections. The first section is devoted to the single-cell migrating in isolation such as occurs when leukocytes migrate during the immune response or when fibroblasts squeeze through connective tissue. The second section is devoted to cells collectively migrating as part of multicellular clusters or sheets. This second type of migration is prevalent in development, wound healing, and in some forms of cancer metastasis.

Drosophila Hox and sex-determination genes control segment elimination through EGFR and extramacrochetae activity

The formation or suppression of particular structures is a major change occurring in development and evolution. One example of such change is the absence of the seventh abdominal segment (A7) in Drosophila males. We show here that there is a down-regulation of EGFR activity and fewer histoblasts in the male A7 in early pupae. If this activity is elevated, cell number increases and a small segment develops in the adult. At later pupal stages, the remaining precursors of the A7 are extruded under the epithelium. This extrusion requires the up-regulation of the HLH protein Extramacrochetae and correlates with high levels of spaghetti-squash, the gene encoding the regulatory light chain of the non-muscle myosin II. The Hox gene Abdominal-B controls both the down-regulation of spitz, a ligand of the EGFR pathway, and the up-regulation of extramacrochetae, and also regulates the transcription of the sex-determining gene doublesex. The male Doublesex protein, in turn, controls extramacrochetae and spaghetti-squash expression. In females, the EGFR pathway is also down-regulated in the A7 but extramacrochetae and spaghetti-squash are not up-regulated and extrusion of precursor cells is almost absent. Our results show the complex orchestration of cellular and genetic events that lead to this important sexually dimorphic character change.

Ubiquitous expression of CUG or CAG trinucleotide repeat RNA causes common morphological defects in a Drosophila model of RNA-mediated pathology

Expanded DNA repeat sequences are known to cause over 20 diseases, including Huntington's disease, several types of spinocerebellar ataxia and myotonic dystrophy type 1 and 2. A shared genetic basis, and overlapping clinical features for some of these diseases, indicate that common pathways may contribute to pathology. Multiple mechanisms, mediated by both expanded homopolymeric proteins and expanded repeat RNA, have been identified by the use of model systems, that may account for shared pathology. The use of such animal models enables identification of distinct pathways and their 'molecular hallmarks' that can be used to determine the contribution of each pathway in human pathology. Here we characterise a tergite disruption phenotype in adult flies, caused by ubiquitous expression of either untranslated CUG or CAG expanded repeat RNA. Using the tergite phenotype as a quantitative trait we define a new genetic system in which to examine 'hairpin' repeat RNA-mediated cellular perturbation. Further experiments use this system to examine whether pathways involving Muscleblind sequestration or Dicer processing, which have been shown to mediate repeat RNA-mediated pathology in other model systems, contribute to cellular perturbation in this model.

A cytochrome p450 conserved in insects is involved in cuticle formation

The sequencing of numerous insect genomes has revealed dynamic changes in the number and identity of cytochrome P450 genes in different insects. In the evolutionary sense, the rapid birth and death of many P450 genes is observed, with only a small number of P450 genes showing orthology between insects with sequenced genomes. It is likely that these conserved P450s function in conserved pathways. In this study, we demonstrate the P450 gene, Cyp301a1, present in all insect genomes sequenced to date, affects the formation of the adult cuticle in Drosophila melanogaster. A Cyp301a1 piggyBac insertion mutant and RNAi of Cyp301a1 both show a similar cuticle malformation phenotype, which can be reduced by 20-hydroxyecdysone, suggesting that Cyp301a1 is an important gene involved in the formation of the adult cuticle and may be involved in ecdysone regulation in this tissue.

Hox-mediated regulation of doublesex sculpts sex-specific abdomen morphology in Drosophila

BACKGROUND

Hox transcription factors are deeply conserved proteins that guide development through regulation of diverse target genes. Furthermore, alteration in Hox target cis-regulation has been proposed as a major mechanism of animal morphological evolution. Crucial to understanding how homeotic genes sculpt the developing body and contribute to the evolution of form is identification and characterization of regulatory targets. Because target specificity is achieved through physical or genetic interactions with cofactors or co-regulators, characterizing interactions between homeotic genes and regulatory partners is also critical. In Drosophila melanogaster, sexually dimorphic abdominal morphology results from sex-specific gene regulation mediated by the Hox protein Abdominal-B (Abd-B) and products of the sex-determination gene doublesex (dsx). Together these transcription factors regulate numerous sex-specific characters, including pigmentation, cuticle morphology, and abdominal segment number.

RESULTS

We show Dsx expression in the developing D. melanogaster pupal abdomen is spatiotemporally dynamic, correlating with segments that undergo sexually dimorphic morphogenesis. Furthermore, our genetic analyses show Dsx expression is Abd-B dependent.

CONCLUSIONS

Doublesex and Abd-B are not only requisite co-regulators of sexually dimorphic abdominal morphology. We propose that dsx is itself a transcriptional target of Abd-B. These data present a testable hypothesis about the evolution of sexually dimorphic segment number in Diptera.

Drosophila aPKC is required for mitotic spindle orientation during symmetric division of epithelial cells

Epithelial cells mostly orient the spindle along the plane of the epithelium (planar orientation) for mitosis to produce two identical daughter cells. The correct orientation of the spindle relies on the interaction between cortical polarity components and astral microtubules. Recent studies in mammalian tissue culture cells suggest that the apically localised atypical protein kinase C (aPKC) is important for the planar orientation of the mitotic spindle in dividing epithelial cells. Yet, in chicken neuroepithelial cells, aPKC is not required in vivo for spindle orientation, and it has been proposed that the polarization cues vary between different epithelial cell types and/or developmental processes. In order to investigate whether Drosophila aPKC is required for spindle orientation during symmetric division of epithelial cells, we took advantage of a previously isolated temperature-sensitive allele of aPKC. We showed that Drosophila aPKC is required in vivo for spindle planar orientation and apical exclusion of Pins (Raps). This suggests that the cortical cues necessary for spindle orientation are not only conserved between Drosophila and mammalian cells, but are also similar to those required for spindle apicobasal orientation during asymmetric cell division.

Lamellipodia-based migrations of larval epithelial cells are required for normal closure of the adult epidermis of Drosophila

Cell migrations are an important feature of animal development. They are, furthermore, essential to wound healing and tumour progression. Despite recent progress, it is still mysterious how cell migration is spatially and temporally regulated during morphogenesis and how cell migration is coordinated with other cellular behaviours to shape tissues and organs. The formation of the abdominal epithelium of Drosophila during metamorphosis provides an attractive system to study morphogenesis. Here, the diploid adult histoblasts replace the polyploid larval epithelial cells (LECs). Using in vivo 4D microscopy, I show that, besides apical constriction and apoptosis, the LECs undergo extensive coordinated migrations. The migrations follow a transition from a stationary (epithelial) to a migratory mode. The migratory behaviour is stimulated by autocrine Dpp signalling. Directed apical lamellipodia-like protrusions propel the cells. Initially, planar cell polarity determines the orientation of LEC migration. While LECs are migrating they also constrict apically, and changes in activity of the small GTPase Rho1 can favour one behaviour over the other. This study shows that the LECs play a more active role in morphogenesis than previously thought, with their migrations contributing to abdominal closure. It furthermore provides insights into how the migratory behaviour of cells is regulated during morphogenesis.

Conserved repressive function of Krüppel homolog 1 on insect metamorphosis in hemimetabolous and holometabolous species

Insect metamorphosis is regulated by ecdysteroids, which induce molts, and juvenile hormone (JH), which inhibits metamorphic changes. The molecular action of ecdysteroids has been thoroughly studied, but that of JH is poorly understood, with data currently only being available for holometabolous species, like Drosophila melanogaster and Tribolium castaneum. We studied the function of Krüppel homolog 1 (Kr-h1) in Blattella germanica, a hemimetabolous model. Kr-h1 is a Zn finger transcription factor whose function as transductor of the antimetamorphic action of JH has recently been demonstrated in D. melanogaster and T. castaneum. The RNAi experiments reported herein indicated that Kr-h1 transduces the antimetamorphic action of JH also in B. germanica, thereby suggesting that this role is an ancestral condition that has been conserved in insect evolution from hemimetabolous to holometabolous species.

Drosophila pupal abdomen immunohistochemistry

The Drosophila pupal abdomen is an established model system for the study of epithelial morphogenesis and the development of sexually dimorphic morphologies ^1-3. During pupation, which spans approximately 96 hours (at 25 °C), proliferating populations of imaginal cells replace the larval epidermis to generate the adult abdominal segments. These imaginal cells, born during embryogenesis, exist as lateral pairs of histoblast nests in each abdominal segment of the larvae. Four pairs of histoblast nests give rise to the adult dorsal cuticle (anterior and posterior dorsal nests), the ventral cuticle (ventral nests) and the spiracles associated with each segment (spiracle nests) ⁴. Upon puparation, these diploid cells (distinguishable by size from the larger polyploid larval epidermal cells- LECs) begin a stereotypical process of proliferation, migration and replacement of the LECs. Various molecular and genetic tools can be employed to investigate the contributions of genetic pathways involved in morphogenesis of the adult abdomen. Ultimate adult phenotypes are typically analyzed following dissection of adult abdominal cuticles. However, investigation of the underlying molecular processes requires immunohistochemical analyses of the pupal epithelium, which present unique challenges. Temporally dynamic morphogenesis and the interactions of two distinct epithelial populations (larval and imaginal) generate a fragile tissue prone to excessive cell loss during dissection and subsequent processing. We have developed methods of dissection, fixation, mounting and imaging of the Drosophila pupal abdominem epithelium for immunohistochemical studies that generate consistent high quality samples suitable for confocal or standard fluorescent microscopy.

Apoptotic regulation of epithelial cellular extrusion

Cellular extrusion is a mechanism that removes dying cells from epithelial tissues to prevent compromising their barrier function. Extrusion occurs in all observed epithelia in vivo and can be modeled in vitro by inducing apoptosis in cultured epithelial monolayers. We established that actin and myosin form a ring that contracts in the surrounding cells that drives cellular extrusion. It is not clear, however, if all apoptotic pathways lead to extrusion and how apoptosis and extrusion are molecularly linked. Here, we find that both intrinsic and extrinsic apoptotic pathways activate cellular extrusion. The contraction force that drives cellular extrusion requires caspase activity. Further, necrosis does not trigger the cellular extrusion response, but instead necrotic cells are removed from epithelia by a passive, stochastic movement of epithelial cells.

Active participation of cell death in development and organismal homeostasis

The precise, regional execution of programmed cell death is required for the proper patterning and sculpting of the embryonic primordium during animal development. In addition, cell death that is not directly involved in sculpting is also widely observed. The most abundant morphological form of programmed cell death in developing animals is apoptosis, and identification of the apoptotic genetic pathways has enabled the study of apoptosis' regulation and roles during development. Genetic and bio-imaging studies have permitted the study of the active roles of cell death in development and organismal homeostasis.

Apoptotic force: active mechanical function of cell death during morphogenesis

Apoptosis, or programmed cell death, is an essential process for the elimination of unnecessary cells during embryonic development, tissue homeostasis, and certain pathological conditions. Recently, an active mechanical function of apoptosis called apoptotic force has been demonstrated during a tissue fusion process of Drosophila embryogenesis. The mechanical force produced during apoptosis is used not only to force dying cells out from tissues in order to keep tissue integrity, but also to change the morphology of neighboring cells to fill the space originally occupied by the dying cell. Furthermore, the occurrence of apoptosis correlates with tissue movement and tension of the tissue. This finding suggests that apoptotic forces might be harnessed throughout cell death-related morphogenesis; however, this concept remains to be fully investigated. While the investigation of this active mechanical function of apoptosis has just begun, here we summarize the current understandings of this novel function of apoptosis, and discuss some possible developmental processes in which apoptosis may play a mechanical role. The concept of apoptotic force prompts a necessity to rethink the role of programmed cell death during morphogenesis.

Sexually dimorphic regulation of the Wingless morphogen controls sex-specific segment number in Drosophila

Sexual dimorphism is widespread throughout the metazoa and plays important roles in mate recognition and preference, sex-based niche partitioning, and sex-specific coadaptation. One notable example of sex-specific differences in insect body morphology is presented by the higher diptera, such as Drosophila, in which males develop fewer abdominal segments than females. Because diversity in segment number is a distinguishing feature of major arthropod clades, it is of fundamental interest to understand how different numbers of segments can be generated within the same species. Here we show that sex-specific and segment-specific regulation of the Wingless (Wg) morphogen underlies the development of sexually dimorphic adult segment number in Drosophila. Wg expression is repressed in the developing terminal male abdominal segment by the combination of the Hox protein Abdominal-B (Abd-B) and the sex-determination regulator Doublesex (Dsx). The subsequent loss of the terminal male abdominal segment during pupation occurs through a combination of developmental processes including segment compartmental transformation, apoptosis, and suppression of cell proliferation. Furthermore, we show that ectopic expression of Wg is sufficient to rescue this loss. We propose that dimorphic Wg regulation, in concert with monomorphic segment-specific programmed cell death, are the principal mechanisms of sculpting the sexually dimorphic abdomen of Drosophila.

RNA polymerase II kinetics in polo polyadenylation signal selection

Regulated alternative polyadenylation is an important feature of gene expression, but how gene transcription rate affects this process remains to be investigated. polo is a cell-cycle gene that uses two poly(A) signals in the 3' untranslated region (UTR) to produce alternative messenger RNAs that differ in their 3'UTR length. Using a mutant Drosophila strain that has a lower transcriptional elongation rate, we show that transcription kinetics can determine alternative poly(A) site selection. The physiological consequences of incorrect polo poly(A) site choice are of vital importance; transgenic flies lacking the distal poly(A) signal cannot produce the longer transcript and die at the pupa stage due to a failure in the proliferation of the precursor cells of the abdomen, the histoblasts. This is due to the low translation efficiency of the shorter transcript produced by proximal poly(A) site usage. Our results show that correct polo poly(A) site selection functions to provide the correct levels of protein expression necessary for histoblast proliferation, and that the kinetics of RNA polymerase II have an important role in the mechanism of alternative polyadenylation.

Nonautonomous apoptosis is triggered by local cell cycle progression during epithelial replacement in Drosophila

Tissue remodeling involves collective cell movement, and cell proliferation and apoptosis are observed in both development and disease. Apoptosis and proliferation are considered to be closely correlated, but little is known about their coordinated regulation in physiological tissue remodeling in vivo. The replacement of larval abdominal epidermis with adult epithelium in Drosophila pupae is a simple model of tissue remodeling. During this process, larval epidermal cells (LECs) undergo apoptosis and are replaced by histoblasts, which are adult precursor cells. By analyzing caspase activation at the single-cell level in living pupae, we found that caspase activation in LECs is induced at the LEC/histoblast boundary, which expands as the LECs die. Manipulating histoblast proliferation at the LEC/histoblast boundary, either genetically or by UV illumination, indicated that local interactions with proliferating histoblasts triggered caspase activation in the boundary LECs. Finally, by monitoring the spatiotemporal dynamics of the S/G₂/M phase in histoblasts in vivo, we found that the transition from S/G₂ phases is necessary to induce nonautonomous LEC apoptosis at the LEC/histoblast boundary. The replacement boundary, formed as caspase activation is regulated locally by cell-cell communication, may drive the dynamic orchestration of cell replacement during tissue remodeling.

Active deformation of apoptotic intestinal epithelial cells with adhesion-restricted polarity contributes to apoptotic clearance

Dying epithelial cells are thought to be squeezed out of the epithelium by the contraction of an actomyosin ring formed in live neighboring cells, which simultaneously closes any potential gap, thereby maintaining the integrity of the epithelial layer. The shrinkage and contraction of apoptotic cells contribute little to the extrusion process. In contrast, the clearance of dying intestinal columnar epithelial cells in vivo usually leaves a transient gap via an unknown mechanism. By using freshly isolated small intestinal villus units with or without basal lamina, we found that the nucleus of apoptotic enterocytes moved apically until they budded off, leaving the cytoplasmic residue in the transient gap. Apical polarity of nucleus movement was restricted unless the basal lamina was artificially removed. F-actin mainly accumulated in apoptotic cells rather than neighboring live cells, even after the addition of resistance force against extrusion. The actin accumulation in apoptotic cells does not depend on the living state of neighboring cells. Apoptotic cells can complete the shedding process when neighboring a goblet cell, as the majority of space is occupied by mucin granules and the cytoplasm consists of intermediate filaments and microtubules, but lacks F-actin. We found that the elongation and deformation of apoptotic cells depend on the stretching force generated inside the cell, rather than the force generated by neighboring cells extending. Our findings clearly demonstrate that intestinal epithelial shedding does not depend on the formation and contraction of an actomyosin ring in live neighboring cells. Apoptotic epithelial cells may undergo an active process of cell deformation with adhesion-restricted polarity, which may contribute to maintaining barrier function during a high rate of cellular turnover.

Embryonic multipotent progenitors remodel the Drosophila airways during metamorphosis

Adult structures in holometabolous insects such as Drosophila are generated by groups of imaginal cells dedicated to the formation of different organs. Imaginal cells are specified in the embryo and remain quiescent until the larval stages, when they proliferate and differentiate to form organs. The Drosophila tracheal system is extensively remodeled during metamorphosis by a small number of airway progenitors. Among these, the spiracular branch tracheoblasts are responsible for the generation of the pupal and adult abdominal airways. To understand the coordination of proliferation and differentiation during organogenesis of tubular organs, we analyzed the remodeling of Drosophila airways during metamorphosis. We show that the embryonic spiracular branch tracheoblasts are multipotent cells that express the homeobox transcription factor Cut, which is necessary for their survival and normal development. They give rise to three distinct cell populations at the end of larval development, which generate the adult tracheal tubes, the spiracle and the epidermis surrounding the spiracle. Our study establishes the series of events that lead to the formation of an adult tubular structure in Drosophila.

Cell death-induced regeneration in wing imaginal discs requires JNK signalling

Regeneration and tissue repair allow damaged or lost body parts to be replaced. After injury or fragmentation of Drosophila imaginal discs, regeneration leads to the development of normal adult structures. This process is likely to involve a combination of cell rearrangement and compensatory proliferation. However, the detailed mechanisms underlying these processes are poorly understood. We have established a system to allow temporally restricted induction of cell death in situ. Using Gal4/Gal80 and UAS-rpr constructs, targeted ablation of a region of the disc could be performed and regeneration monitored without the requirement for microsurgical manipulation. Using a ptc-Gal4 construct to drive rpr expression in the wing disc resulted in a stripe of dead cells in the anterior compartment flanking the anteroposterior boundary, whereas a sal-Gal4 driver generated a dead domain that includes both anterior and posterior cells. Under these conditions, regenerated tissues were derived from the damaged compartment, suggesting that compartment restrictions are preserved during regeneration. Our studies reveal that during regeneration the live cells bordering the domain in which cell death was induced first display cytoskeletal reorganisation and apical-to-basal closure of the epithelium. Then, proliferation begins locally in the vicinity of the wound and later more extensively in the affected compartment. Finally, we show that regeneration of genetically ablated tissue requires JNK activity. During cell death-induced regeneration, the JNK pathway is activated at the leading edges of healing tissue and not in the apoptotic cells, and is required for the regulation of healing and regenerative growth.

Dpp signaling directs cell motility and invasiveness during epithelial morphogenesis

Tissue remodeling in development and disease involves the coordinated invasion of neighboring territories and/or the replacement of entire cell populations. Cell guidance, cell matching, transitions from passive to migratory epithelia, cell growth and death, and extracellular matrix remodeling all impinge on epithelial spreading. Significantly, the extracellular signals that direct these activities and the specific cellular elements and mechanisms regulated by these signals remain in most cases to be identified. To address these issues, we performed an analysis of histoblasts (Drosophila abdominal epithelial founder cells) on their transition from a dormant state to active migration replacing obsolete larval epidermal cells (LECs). We found that during expansion, Decapentaplegic (Dpp) secreted from surrounding LECs leads to graded pathway activation in cells at the periphery of histoblast nests. Across nests, Dpp activity confers differential cellular behavior and motility by modulating cell-cell contacts, the organization and activity of the cytoskeleton, and histoblast attachment to the substrate. Furthermore, Dpp also prevents the premature death of LECs, allowing the coordination of histoblast expansion to LEC delamination. Dpp signaling activity directing histoblast spreading and invasiveness mimics transforming growth factor-beta and bone morphogenetic proteins' role in enhancing the motility and invasiveness of cancer cells, resulting in the promotion of metastasis.

Interaction between Drosophila bZIP proteins Atf3 and Jun prevents replacement of epithelial cells during metamorphosis

Epithelial sheet spreading and fusion underlie important developmental processes. Well-characterized examples of such epithelial morphogenetic events have been provided by studies in Drosophila, and include embryonic dorsal closure, formation of the adult thorax and wound healing. All of these processes require the basic region-leucine zipper (bZIP) transcription factors Jun and Fos. Much less is known about morphogenesis of the fly abdomen, which involves replacement of larval epidermal cells (LECs) with adult histoblasts that divide, migrate and finally fuse to form the adult epidermis during metamorphosis. Here, we implicate Drosophila Activating transcription factor 3 (Atf3), the single ortholog of human ATF3 and JDP2 bZIP proteins, in abdominal morphogenesis. During the process of the epithelial cell replacement, transcription of the atf3 gene declines. When this downregulation is experimentally prevented, the affected LECs accumulate cell-adhesion proteins and their extrusion and replacement with histoblasts are blocked. The abnormally adhering LECs consequently obstruct the closure of the adult abdominal epithelium. This closure defect can be either mimicked and further enhanced by knockdown of the small GTPase Rho1 or, conversely, alleviated by stimulating ecdysone steroid hormone signaling. Both Rho and ecdysone pathways have been previously identified as effectors of the LEC replacement. To elicit the gain-of-function effect, Atf3 specifically requires its binding partner Jun. Our data thus identify Atf3 as a new functional partner of Drosophila Jun during development.

Temporal regulation of Drosophila IAP1 determines caspase functions in sensory organ development

Caspase activation is regulated by the turnover of E3 ubiquitin ligase, DIAP1, and depends on cell type and maturity.

The caspases comprise a family of cysteine proteases that function in various cellular processes, including apoptosis. However, how the balance is struck between the caspases’ role in cell death and their nonapoptotic functions is unclear. To address this issue, we monitored the protein turnover of an endogenous caspase inhibitor, Drosophila IAP1 (DIAP1). DIAP1 is an E3 ubiquitin ligase that promotes the ubiquitination of caspases and thereby prevents caspase activation. For this study, we developed a fluorescent probe to monitor DIAP1 turnover in the external sensory organ precursor (SOP) lineage of living Drosophila. The SOP divides asymmetrically to make the shaft, socket, and sheath cells, and the neuron that comprise each sensory organ. We found that the quantity of DIAP1 changed dramatically depending on the cell type and maturity, and that the temporal regulation of DIAP1 turnover determines whether caspases function nonapoptotically in cellular morphogenesis or cause cell death.

Cell rearrangements, cell divisions and cell death in a migrating epithelial sheet in the abdomen of Drosophila

During morphogenesis, cell movements, cell divisions and cell death work together to form complex patterns and to shape organs. These events are the outcome of decisions made by many individual cells, but how these decisions are controlled and coordinated is elusive. The adult abdominal epidermis of Drosophila is formed during metamorphosis by divisions and extensive cell migrations of the diploid histoblasts, which replace the polyploid larval cells. Using in vivo 4D microscopy, we have studied the behaviour of the histoblasts and analysed in detail how they reach their final position and to what extent they rearrange during their spreading. Tracking individual cells, we show that the cells migrate in two phases that differ in speed, direction and amount of cellular rearrangement. Cells of the anterior (A) and posterior (P) compartments differ in their behaviour. Cells near the A/P border are more likely to change their neighbours during migration. The mitoses do not show any preferential orientation. After mitosis, the sisters become preferentially aligned with the direction of movement. Thus, in the abdomen, it is the extensive cell migrations that appear to contribute most to morphogenesis. This contrasts with other developing epithelia, such as the wing imaginal disc and the embryonic germband in Drosophila, where oriented mitoses and local cell rearrangements appear to direct morphogenesis. Furthermore, our results suggest that an active force created by the histoblasts contributes to the formation of the adult epidermis. Finally, we show that histoblasts occasionally undergo apoptosis.

P115 RhoGEF and microtubules decide the direction apoptotic cells extrude from an epithelium

Communication between microtubules and actin/myosin networks determine epithelial cell extrusion polarity.

To preserve epithelial barrier function, dying cells are squeezed out of an epithelium by “apoptotic cell extrusion.” Specifically, a cell destined for apoptosis signals its live neighboring epithelial cells to form and contract a ring of actin and myosin II that squeezes the dying cell out of the epithelial sheet. Although most apoptotic cells extrude apically, we find that some exit basally. Localization of actin and myosin IIA contraction dictates the extrusion direction: basal extrusion requires circumferential contraction of neighboring cells at their apices, whereas apical extrusion also requires downward contraction along the basolateral surfaces. To activate actin/myosin basolaterally, microtubules in neighboring cells reorient and target p115 RhoGEF to this site. Preventing microtubule reorientation restricts contraction to the apex, driving extrusion basally. Extrusion polarity has important implications for tumors where apoptosis is blocked but extrusion is not, as basal extrusion could enable these cells to initiate metastasis.

Dynamic control of cell cycle and growth coupling by ecdysone, EGFR, and PI3K signaling in Drosophila histoblasts

Regulation of cell proliferation has been extensively studied in cultured cell systems that are characterized by coordinated growth and cell-cycle progression and relatively uniform cell size distribution. During the development of multicellular organisms, however, growth and division can be temporally uncoupled, and the signaling pathways that regulate these growth programs are poorly understood. A good model for analyzing proliferation control in such systems is the morphogenesis of the Drosophila adult abdominal epidermis by histoblasts. These cells undergo a series of temporally regulated transitions during which neither cell size nor division rate is constant. The proliferation of histoblasts during metamorphosis is uniquely amenable to clonal analysis in combination with live imaging. Thereby, we show that abdominal histoblasts, which grow while in G2 arrest during larval stages, enter a proliferative stage in the pupal period that is initiated by ecdysone-dependent string/Cdc25 phosphatase transcription. The proliferating histoblasts have preaccumulated stores of Cyclin E, which trigger an immediate S phase onset after mitosis. These rapid cell cycles lack a G1 phase and result in a progressive reduction of cell size. Eventually, the histoblasts proceed to a stage of slower proliferation that, in contrast to the preceding, depends on epidermal growth factor receptor (EGFR) signaling for progression through the G2/M transition and on insulin receptor/PI3K-mediated signaling for growth. These results uncover the developmentally programmed changes coupling the growth and proliferation of the histoblasts that form the abdominal epidermis of Drosophila. Histoblasts proceed through three distinct stages: growth without division, division without growth, and growth-coupled proliferation. Our identification of the signaling pathways and cell-cycle regulators that control these programs illustrates the power of in vivo time-lapse analyses after clone induction. It sets the stage for the comprehensive understanding of the coordination of cell growth and cell-cycle progression in complex multicellular eukaryotes.

A fundamental issue in biology is the question of how the rate of cell division is coupled to cell growth. Coordination of these processes has been studied extensively in cultured cell systems but to a much lesser extent in intact organisms. To study this phenomenon in a physiological setting, we developed a methodology to observe and manipulate cell division and growth in a population of Drosophila abdominal cells called histoblasts. The various developmental stages of histoblast morphogenesis include exit from cell-cycle arrest, initially rapid growth in the absence of cell division, and subsequent coupling of proliferation and growth. We identified several critical developmental signaling pathways (including signaling via ecdysone, the EGF receptor, and PI 3-kinase) that regulate and coordinate cell growth and division cycles during these different types of cell-cycle phenomena. We propose that the internal logic of the Drosophila histoblast system may serve as a basic framework for understanding not only how coordinated cell growth and division operate in a number of other developmental contexts, but also how misregulation of cell growth and division occurs in contexts such as cancer cell populations.

Integration of the ecdysone, EGF receptor, and PI 3-kinase signaling pathways determines the relative rates of growth and cell division duringDrosophila abdominal cell morphogenesis.

Quantification of cell edge velocities and traction forces reveals distinct motility modules during cell spreading

Actin-based cell motility and force generation are central to immune response, tissue development, and cancer metastasis, and understanding actin cytoskeleton regulation is a major goal of cell biologists. Cell spreading is a commonly used model system for motility experiments -- spreading fibroblasts exhibit stereotypic, spatially-isotropic edge dynamics during a reproducible sequence of functional phases: 1) During early spreading, cells form initial contacts with the surface. 2) The middle spreading phase exhibits rapidly increasing attachment area. 3) Late spreading is characterized by periodic contractions and stable adhesions formation. While differences in cytoskeletal regulation between phases are known, a global analysis of the spatial and temporal coordination of motility and force generation is missing. Implementing improved algorithms for analyzing edge dynamics over the entire cell periphery, we observed that a single domain of homogeneous cytoskeletal dynamics dominated each of the three phases of spreading. These domains exhibited a unique combination of biophysical and biochemical parameters -- a motility module. Biophysical characterization of the motility modules revealed that the early phase was dominated by periodic, rapid membrane blebbing; the middle phase exhibited continuous protrusion with very low traction force generation; and the late phase was characterized by global periodic contractions and high force generation. Biochemically, each motility module exhibited a different distribution of the actin-related protein VASP, while inhibition of actin polymerization revealed different dependencies on barbed-end polymerization. In addition, our whole-cell analysis revealed that many cells exhibited heterogeneous combinations of motility modules in neighboring regions of the cell edge. Together, these observations support a model of motility in which regions of the cell edge exhibit one of a limited number of motility modules that, together, determine the overall motility function. Our data and algorithms are publicly available to encourage further exploration.

Apoptotic force and tissue dynamics during Drosophila embryogenesis

Understanding cell morphogenesis during metazoan development requires knowledge of how cells and the extracellular matrix produce and respond to forces. We investigated how apoptosis, which remodels tissue by eliminating supernumerary cells, also contributes forces to a tissue (the amnioserosa) that promotes cell-sheet fusion (dorsal closure) in the Drosophila embryo. We showed that expression in the amnioserosa of proteins that suppress or enhance apoptosis slows or speeds dorsal closure, respectively. These changes correlate with the forces produced by the amnioserosa and the rate of seam formation between the cell sheets (zipping), key processes that contribute to closure. This apoptotic force is used by the embryo to drive cell-sheet movements during development, a role not classically attributed to apoptosis.

Larval cells become imaginal cells under the control of homothorax prior to metamorphosis in the Drosophila tracheal system

In Drosophila melanogaster, one of the most derived species among holometabolous insects, undifferentiated imaginal cells that are set-aside during larval development are thought to proliferate and replace terminally differentiated larval cells to constitute adult structures. Essentially all tissues that undergo extensive proliferation and drastic morphological changes during metamorphosis are thought to derive from these imaginal cells and not from differentiated larval cells. The results of studies on metamorphosis of the Drosophila tracheal system suggested that large larval tracheal cells that are thought to be terminally differentiated may be eliminated via apoptosis and rapidly replaced by small imaginal cells that go on to form the adult tracheal system. However, the origin of the small imaginal tracheal cells has not been clear. Here, we show that large larval cells in tracheal metamere 2 (Tr2) divide and produce small imaginal cells prior to metamorphosis. In the absence of homothorax gene activity, larval cells in Tr2 become non-proliferative and small imaginal cells are not produced, indicating that homothorax is necessary for proliferation of Tr2 larval cells. These unexpected results suggest that larval cells can become imaginal cells and directly contribute to the adult tissue in the Drosophila tracheal system. During metamorphosis of less derived species of holometabolous insects, adult structures are known to be formed via cells constituting larval structures. Thus, the Drosophila tracheal system may utilize ancestral mode of metamorphosis.

Live imaging of epidermal morphogenesis during the development of the adult abdominal epidermis of Drosophila

During larval stages of Drosophila development, the abdominal epidermis is composed of histoblasts (adult precursors) and larval epidermal cells (LECs). During metamorphosis, histoblasts proliferate and colonize the territories occupied by the LECs, which die and become engulfed by macrophages. This morphogenetic process is an excellent model for in vivo analysis of epithelial migration, cell division, cell death, patterning and differentiation. Here, we describe a protocol for time-lapse recording of the developing epidermis during metamorphosis. The protocol describes the removal of the pupal case (which acts as an opaque barrier to effective imaging) and mounting and imaging of specimens of different stages so that normal developmental processes are preserved. This method enables high-resolution studies over long time periods using fluorescent markers and confocal microscopy. The protocol requires 1 h for pupal dissection and mounting and, depending on the stages and genotypes to be analyzed, several more hours for preprocessing and aging and developmental staging of flies and pupae.

Drosophila importin alpha1 performs paralog-specific functions essential for gametogenesis

Importin alpha's mediate nuclear transport by linking nuclear localization signal (NLS)-containing proteins to importin beta1. Animal genomes encode three conserved groups of importin alpha's, alpha1's, alpha2's, and alpha3's, each of which are competent to bind classical NLS sequences. Using Drosophila melanogaster we describe the isolation and phenotypic characterization of the first animal importin alpha1 mutant. Animal alpha1's are more similar to ancestral plant and fungal alpha1-like genes than to animal alpha2 and alpha3 genes. Male and female importin alpha1 (Dalpha1) null flies developed normally to adulthood (with a minor wing defect) but were sterile with defects in gametogenesis. The Dalpha1 mutant phenotypes were rescued by Dalpha1 transgenes, but not by Dalpha2 or Dalpha3 transgenes. Genetic interactions between the ectopic expression of Dalpha1 and the karyopherins CAS and importin beta1 suggest that high nuclear levels of Dalpha1 are deleterious. We conclude that Dalpha1 performs paralog-specific activities that are essential for gametogenesis and that regulation of subcellular Dalpha1 localization may affect cell fate decisions. The initial expansion and specialization of the animal importin alpha-gene family may have been driven by the specialized needs of gametogenesis. These results provide a framework for studies of the more complex mammalian importin alpha-gene family.

Krüppel homolog 1 (Kr-h1) mediates juvenile hormone action during metamorphosis of Drosophila melanogaster

Juvenile hormone (JH) given at pupariation inhibits bristle formation and causes pupal cuticle formation in the abdomen of Drosophila melanogaster due to its prolongation of expression of the transcription factor Broad (BR). In a microarray analysis of JH-induced gene expression in abdominal integument, we found that Krüppel homolog 1 (Kr-h1) was up-regulated during most of adult development. Quantitative real-time PCR analyses showed that Kr-h1 up-regulation began at 10h after puparium formation (APF), and Kr-h1 up-regulation occurred in imaginal epidermal cells, persisting larval muscles, and larval oenocytes. Ectopic expression of Kr-h1 in abdominal epidermis using T155-Gal4 to drive UAS-Kr-h1 resulted in missing or short bristles in the dorsal midline. This phenotype was similar to that seen after a low dose of JH or after misexpression of br between 21 and 30 h APF. Ectopic expression of Kr-h1 prolonged the expression of BR protein in the pleura and the dorsal tergite. No Kr-h1 was seen after misexpression of br. Thus, Kr-h1 mediates some of the JH signaling in the adult abdominal epidermis and is upstream of br in this pathway. We also show for the first time that the JH-mediated maintenance of br expression in this epidermis is patterned and that JH delays the fusion of the imaginal cells and the disappearance of Dpp in the dorsal midline.