Temporal dynamics of apoptosis-induced proliferation in pupal wing development: implications for regenerative ability

BACKGROUND

The ability of animals to regenerate damaged tissue is a complex process that involves various cellular mechanisms. As animals age, they lose their regenerative abilities, making it essential to understand the underlying mechanisms that limit regenerative ability during aging. Drosophila melanogaster wing imaginal discs are epithelial structures that can regenerate after tissue injury. While significant research has focused on investigating regenerative responses during larval stages our comprehension of the regenerative potential of pupal wings and the underlying mechanisms contributing to the decline of regenerative responses remains limited.

RESULTS

Here, we explore the temporal dynamics during pupal development of the proliferative response triggered by the induction of cell death, a typical regenerative response. Our results indicate that the apoptosis-induced proliferative response can continue until 34 h after puparium formation (APF), beyond this point cell death alone is not sufficient to induce a regenerative response. Under normal circumstances, cell proliferation ceases around 24 h APF. Interestingly, the failure of reinitiating the cell cycle beyond this time point is not attributed to an incapacity to activate the JNK pathway. Instead, our results suggest that the function of the ecdysone-responsive transcription factor E93 is involved in limiting the apoptosis-induced proliferative response during pupal development.

CONCLUSIONS

Our study shows that apoptosis can prolong the proliferative period of cells in the wing during pupal development as late as 34 h APF, at least 10 h longer than during normal development. After this time point, the regenerative response is diminished, a process mediated in part by the ecdysone-responsive transcription factor E93.

Cell proliferation and Notch signaling coordinate the formation of epithelial folds in the Drosophila leg

The formation of complex three-dimensional organs during development requires precise coordination between patterning networks and mechanical forces. In particular, tissue folding is a crucial process that relies on a combination of local and tissue-wide mechanical forces. Here, we investigate the contribution of cell proliferation to epithelial morphogenesis using the Drosophila leg tarsal folds as a model. We reveal that tissue-wide compression forces generated by cell proliferation, in coordination with the Notch signaling pathway, are essential for the formation of epithelial folds in precise locations along the proximo-distal axis of the leg. As cell numbers increase, compressive stresses arise, promoting the folding of the epithelium and reinforcing the apical constriction of invaginating cells. Additionally, the Notch target dysfusion plays a key function specifying the location of the folds, through the apical accumulation of F-actin and the apico-basal shortening of invaginating cells. These findings provide new insights into the intricate mechanisms involved in epithelial morphogenesis, highlighting the crucial role of tissue-wide forces in shaping a three-dimensional organ in a reproducible manner.

Regulation and coordination of the different DNA damage responses in Drosophila

Cells have evolved mechanisms that allow them to respond to DNA damage to preserve genomic integrity and maintain tissue homeostasis. These responses include the activation of the cell cycle checkpoints and the repair mechanisms or the induction of apoptosis that eventually will eliminate damaged cells. These “life” vs. “death” decisions differ depending on the cell type, stages of development, and the proliferation status of the cell. The apoptotic response after DNA damage is of special interest as defects in its induction could contribute to tumorigenesis or the resistance of cancer cells to therapeutic agents such as radiotherapy. Multiples studies have elucidated the molecular mechanisms that mediate the activation of the DNA damage response pathway (DDR) and specifically the role of p53. However, much less is known about how the different cellular responses such as cell proliferation control and apoptosis are coordinated to maintain tissue homeostasis. Another interesting question is how the differential apoptotic response to DNA damage is regulated in distinct cell types. The use of Drosophila melanogaster as a model organism has been fundamental to understand the molecular and cellular mechanisms triggered by genotoxic stress. Here, we review the current knowledge regarding the cellular responses to ionizing radiation as the cause of DNA damage with special attention to apoptosis in Drosophila: how these responses are regulated and coordinated in different cellular contexts and in different tissues. The existence of intrinsic mechanisms that might attenuate the apoptotic pathway in response to this sort of DNA damage may well be informative for the differences in the clinical responsiveness of tumor cells after radiation therapy.

Cell proliferation control by Notch signalling during imaginal discs development in Drosophila

The Notch signalling pathway is evolutionary conserved and participates in numerous developmental processes, including the control of cell proliferation. However, Notch signalling can promote or restrain cell division depending on the developmental context, as has been observed in human cancer where Notch can function as a tumor suppressor or an oncogene. Thus, the outcome of Notch signalling can be influenced by the cross-talk between Notch and other signalling pathways. The use of model organisms such as Drosophila has been proven to be very valuable to understand the developmental role of the Notch pathway in different tissues and its relationship with other signalling pathways during cell proliferation control. Here we review recent studies in Drosophila that shed light in the developmental control of cell proliferation by the Notch pathway in different contexts such as the eye, wing and leg imaginal discs. We also discuss the autonomous and non-autonomous effects of the Notch pathway on cell proliferation and its interactions with different signalling pathways.

JNK and JAK/STAT signalling are required for inducing loss of cell fate specification during imaginal wing discs regeneration in Drosophila melanogaster

The regenerative process after tissue damage relies on a variety of cellular responses that includes compensatory cell proliferation and cell fate re-specification. The identification of the signalling networks regulating these cellular events is a central question in regenerative biology. Tissue regeneration models in Drosophila have shown that two of the signals that play a fundamental role during the early stages of regeneration are the c-Jun N-terminal kinase (JNK) and JAK/STAT signalling pathways. These pathways have been shown to be required for controlling regenerative proliferation, however their contribution to the processes of cellular reprogramming and cell fate re-specification that take place during regeneration are largely unknown. Here, we present evidence for a previously unrecognised function of the cooperative activities of JNK and JAK/STAT signalling pathways in inducing loss of cell fate specification in imaginal discs. We show that co-activation of these signalling pathways induces both the cell fate changes in injured areas, as well as in adjacent cells. We have also found that this function relies on the activity of the Caspase initiator encoded by the gene dronc.

Analysis of the Function of Apoptosis during Imaginal Wing Disc Regeneration in Drosophila melanogaster

Regeneration is the ability that allows organisms to replace missing organs or lost tissue after injuries. This ability requires the coordinated activity of different cellular processes, including programmed cell death. Apoptosis plays a key role as a source of signals necessary for regeneration in different organisms. The imaginal discs of Drosophila melanogaster provide a particularly well-characterised model system for studying the cellular and molecular mechanisms underlying regeneration. Although it has been shown that signals produced by apoptotic cells are needed for homeostasis and regeneration of some tissues of this organism, such as the adult midgut, the contribution of apoptosis to disc regeneration remains unclear. Using a new method for studying disc regeneration in physiological conditions, we have defined the pattern of cell death in regenerating discs. Our data indicate that during disc regeneration, cell death increases first at the wound edge, but as regeneration progresses dead cells can be observed in regions far away from the site of damage. This result indicates that apoptotic signals initiated in the wound spread throughout the disc. We also present results which suggest that the partial inhibition of apoptosis does not have a major effect on disc regeneration. Finally, our results suggest that during disc regeneration distinct apoptotic signals might be acting simultaneously.

The bHLH factors extramacrochaetae and daughterless control cell cycle in Drosophila imaginal discs through the transcriptional regulation of the Cdc25 phosphatase string

One of the major issues in developmental biology is about having a better understanding of the mechanisms that regulate organ growth. Identifying these mechanisms is essential to understand the development processes that occur both in physiological and pathological conditions, such as cancer. The E protein family of basic helix-loop helix (bHLH) transcription factors, and their inhibitors the Id proteins, regulate cell proliferation in metazoans. This notion is further supported because the activity of these factors is frequently deregulated in cancerous cells. The E protein orthologue Daughterless (Da) and the Id orthologue Extramacrochaetae (Emc) are the only members of these classes of bHLH proteins in Drosophila. Although these factors are involved in controlling proliferation, the mechanism underlying this regulatory activity is poorly understood. Through a genetic analysis, we show that during the development of epithelial cells in the imaginal discs, the G2/M transition, and hence cell proliferation, is controlled by Emc via Da. In eukaryotic cells, the main activator of this transition is the Cdc25 phosphatase, string. Our genetic analyses reveal that the ectopic expression of string in cells with reduced levels of Emc or high levels of Da is sufficient to rescue the proliferative defects seen in these mutant cells. Moreover, we present evidence demonstrating a role of Da as a transcriptional repressor of string. Taken together, these findings define a mechanism through which Emc controls cell proliferation by regulating the activity of Da, which transcriptionally represses string.

Precise control of cell proliferation is critical for normal development and tissue homeostasis. Members of the inhibitor of differentiation (Id) family of helix-loop-helix (HLH) proteins are key regulators that coordinate the balance between cell division and differentiation. These proteins exert this function in part by combining with ubiquitously expressed bHLH transcription factors (E proteins), preventing these transcription factors from forming functional hetero- or homodimeric DNA binding complexes. Deregulation of the activity of Id proteins frequently leads to tumour formation. The Daughterless (Da) and Extramacrochaetae (Emc) proteins are the only members of the E and Id families in Drosophila, yet their role in the control of cell proliferation has not been determined. In this study, we show that the elimination of emc or the ectopic expression of da arrests cells in the G2 phase of the cell cycle. Moreover, we demonstrate that emc controls cell proliferation via Da, which acts as a transcriptional repressor of the Cdc25 phosphatase string. These results provide an important insight into the mechanisms through which Id and E protein interactions control cell cycle progression and therefore how the disruption of the function of Id proteins can induce oncogenic transformation.

The bHLH factors Dpn and members of the E(spl) complex mediate the function of Notch signalling regulating cell proliferation during wing disc development

The Notch signalling pathway plays an essential role in the intricate control of cell proliferation and pattern formation in many organs during animal development. In addition, mutations in most members of this pathway are well characterized and frequently lead to tumour formation. The Drosophila imaginal wing discs have provided a suitable model system for the genetic and molecular analysis of the different pathway functions. During disc development, Notch signalling at the presumptive wing margin is necessary for the restricted activation of genes required for pattern formation control and disc proliferation. Interestingly, in different cellular contexts within the wing disc, Notch can either promote cell proliferation or can block the G1-S transition by negatively regulating the expression of dmyc and bantam micro RNA. The target genes of Notch signalling that are required for these functions have not been identified. Here, we show that the Hes vertebrate homolog, deadpan (dpn), and the Enhancer-of-split complex (E(spl)C) genes act redundantly and cooperatively to mediate the Notch signalling function regulating cell proliferation during wing disc development.

The bHLH factor deadpan is a direct target of Notch signaling and regulates neuroblast self-renewal in Drosophila

A defining feature of stem cells is their capacity to renew themselves at each division while producing differentiated progeny. How these cells balance self-renewal versus differentiation is a fundamental issue in developmental and cancer biology. The Notch signaling pathway has long been known to influence cell fate decisions during development. Indeed, there is a great deal of evidence correlating its function with the regulation of neuroblast (NB) self-renewal during larval brain development in Drosophila. However, little is known about the transcription factors regulated by this pathway during this process. Here we show that deadpan (dpn), a gene encoding a bHLH transcription factor, is a direct target of the Notch signaling pathway during type II NB development. Type II NBs undergo repeated asymmetric divisions to self-renew and to produce immature intermediate neural progenitors. These cells mature into intermediate neural progenitors (INPs) that have the capacity to undergo multiple rounds of asymmetric division to self-renew and to generate GMCs and neurons. Our results indicate that the expression of dpn at least in INPs cells depends on Notch signaling. The ectopic expression of dpn in immature INP cells can transform these cells into NBs-like cells that divide uncontrollably causing tumor over-growth. We show that in addition to dpn, Notch signaling must be regulating other genes during this process that act redundantly with dpn.

Upregulation of glypicans in Hippo mutants alters the coordinated activity of morphogens

The function of the conserved Drosophila Hippo signaling pathway has been shown to be required to limit cell proliferation. Several studies have identified different target genes of this pathway that could modulate this function. However, the ectopic expression of these genes cannot account for all of the hyperplasic and pattern defects displayed by Hippo signaling mutants. We have recently identified two new targets of the Hippo pathway, the heparan sulfate proteoglycans (HSPGs) encoded by division abnormally delayed (dally) and dally-like protein (dlp). The function of these glypicans is required to modulate the activity of different signaling pathways triggered by diffusible ligands. Thus, our results link the function of the Hippo pathway with the control of the activity of several signaling pathways required for the definition of the size and pattern of an organ.

Epithelial cell adhesion in the developing Drosophila retina is regulated by Atonal and the EGF receptor pathway

In the Drosophila retina, photoreceptor differentiation is preceded by significant cell shape rearrangements within and immediately behind the morphogenetic furrow. Groups of cells become clustered into arcs and rosettes in the plane of the epithelium, from which the neurons subsequently emerge. These cell clusters also have differential adhesive properties: adherens junction components are upregulated relative to surrounding cells. Little is known about how these morphological changes are orchestrated and what their relevance is for subsequent neuronal differentiation. Here, we report that the transcription factor Atonal and the canonical EGF receptor signalling cascade are both required for this clustering and for the accompanying changes in cellular adhesion. In the absence of either component, no arcs are formed behind the furrow, and all cells show low Armadillo and DE-cadherin levels, although in the case of EGFR pathway mutants, single, presumptive R8 cells with high levels of adherens junction components can be seen. Atonal regulates DE-cadherin transcriptionally, whereas the EGFR pathway, acting through the transcription factor Pointed, exerts its effects on adherens junctions indirectly, at a post-transcriptional level. These observations define a new function for EGFR signalling in eye development and illustrate a mechanism for the control of epithelial morphology by developmental signals.

Atrophin contributes to the negative regulation of epidermal growth factor receptor signaling in Drosophila

Dentato-rubral and pallido-luysian atrophy (DRPLA) is a dominant, progressive neurodegenerative disease caused by the expansion of polyglutamine repeats within the human Atrophin-1 protein. Drosophila Atrophin and its human orthologue are thought to function as transcriptional co-repressors. Here, we report that Drosophila Atrophin participates in the negative regulation of Epidermal Growth Factor Receptor (EGFR) signaling both in the wing and the eye imaginal discs. In the wing pouch, Atrophin loss of function clones induces cell autonomous expression of the EGFR target gene Delta, and the formation of extra vein tissue, while overexpression of Atrophin inhibits EGFR-dependent vein formation. In the eye, Atrophin cooperates with other negative regulators of the EGFR signaling to prevent the differentiation of surplus photoreceptor cells and to repress Delta expression. Overexpression of Atrophin in the eye reduces the EGFR-dependent recruitment of cone cells. In both the eye and wing, epistasis tests show that Atrophin acts downstream or in parallel to the MAP kinase rolled to modulate EGFR signaling outputs. We show that Atrophin genetically cooperates with the nuclear repressor Yan to inhibit the EGFR signaling activity. Finally, we have found that expression of pathogenic or normal forms of human Atrophin-1 in the wing promotes wing vein differentiation and acts as dominant negative proteins inhibiting endogenous fly Atrophin activity.

The Orientation of Cell Divisions Determines the Shape of Drosophila Organs

Organ shape depends on the coordination between cell proliferation and the spatial arrangement of cells during development. Much is known about the mechanisms that regulate cell proliferation, but the processes by which the cells are orderly distributed remain unknown. This can be accomplished either by random division of cells that later migrate locally to new positions (cell allocation) or through polarized cell division (oriented cell division; OCD). Recent data suggest that the OCD is involved in some morphogenetic processes such as vertebrate gastrulation, neural tube closure, and growth of shoot apex in plants; however, little is known about the contribution of OCD during organogenesis. We have analyzed the orientation patterns of cell division throughout the development of wild-type and mutant imaginal discs of Drosophila. Our results show a causal relationship between the orientation of cell divisions in the imaginal disc and the adult morphology of the corresponding organs, indicating a key role of OCD in organ-shape definition. In addition, we find that a subset of planar cell polarity genes is required for the proper orientation of cell division during organ development.

Control of Cell Proliferation in the Drosophila Eye by Notch Signaling

Cell proliferation in animals must be precisely controlled, but the signaling mechanisms that regulate the cell cycle are not well characterized. A regulated terminal mitosis, called the second mitotic wave (SMW), occurs during Drosophila eye development, providing a model for the genetic analysis of proliferation control. We report a cell cycle checkpoint at the G1-S transition that initiates the SMW, and we demonstrate that Notch signaling is required for cells to overcome this checkpoint. Notch triggers the onset of proliferation by multiple pathways, including the activation of dE2F1, a member of the E2F transcription factor family. Delta to Notch signaling derepresses the inhibition of dE2F1 by RBF, and Delta expression depends on the secreted proteins Hedgehog and Dpp. Notch is also required for the expression of Cyclin A in the SMW.

Control of Drosophila eye specification by Wingless signalling

Organ formation requires early specification of the groups of cells that will give rise to specific structures. The Wingless protein plays an important part in this regional specification of imaginal structures in Drosophila, including defining the region of the eye-antennal disc that will become retina. We show that Wingless signalling establishes the border between the retina and adjacent head structures by inhibiting the expression of the eye specification genes eyes absent, sine oculis and dachshund. Ectopic Wingless signalling leads to the repression of these genes and the loss of eyes, whereas loss of Wingless signalling has the opposite effects. Wingless expression in the anterior of wild-type discs is complementary to that of these eye specification genes. Contrary to previous reports, we find that under conditions of excess Wingless signalling, eye tissue is transformed not only into head cuticle but also into a variety of inappropriate structures.

Pointed and Tramtrack69 establish an EGFR-dependent transcriptional switch to regulate mitosis

Cell division in animals must be regulated; during development, for example, proliferation often occurs in spatially and temporally restricted patterns, and loss of mitotic control underlies cancer. The epidermal growth factor receptor (EGFR) has been implicated extensively in the control of cell proliferation in metazoans; in addition, hyperactivity of the EGFR and its three relatives, ErbB2-ErbB4, are implicated in many cancers. But little is known about how these receptor tyrosine kinases regulate the cell cycle. In the developing Drosophila melanogaster imaginal eye disc, there is a single patterned mitosis that sweeps across the eye disc epithelium in the third larval instar. This 'second mitotic wave' is triggered by EGFR signalling and depends on expression of String, the Drosophila homologue of Cdc25 phosphatase, the ultimate regulator of mitosis in all eukaryotic cells. Here we show that two antagonistic transcriptional regulators, Pointed, an activator, and Tramtrack69, a repressor, directly regulate the transcription of string. The activity of at least one of these regulators, Pointed, is controlled by EGFR signalling. This establishes a molecular mechanism for how intercellular signalling can control string expression, and thereby cell proliferation.

Notch signalling and the initiation of neural development in the Drosophila eye

Neural determination in the Drosophila eye occurs progressively. A diffusible signal, Dpp, causes undetermined cells first to adopt a ‘pre-proneural’ state in which they are primed to start differentiating. A second signal is required to trigger the activation of the transcription factor Atonal, which causes the cells to initiate overt photoreceptor neurone differentiation. Both Dpp and the second signal are dependent on Hedgehog (Hh) signalling. Previous work has shown that the Notch signalling pathway also has a proneural role in the eye (as well as a later, opposite function when it restricts the number of cells becoming photoreceptors – a process of lateral inhibition). It is not clear how the early proneural role of Notch integrates with the other signalling pathways involved. We provide evidence that Notch activation by its ligand Delta is the second Hh-dependent signal required for neural determination. Notch activity normally only triggers Atonal expression in cells that have adopted the pre-proneural state induced by Dpp. We also report that Notch drives the transition from pre-proneural to proneural by downregulating two repressors of Atonal: Hairy and Extramacrochaetae.

A primary role for the epidermal growth factor receptor in ommatidial spacing in the Drosophila eye

BACKGROUND

The differentiation of regularly spaced structures within an epithelium is a common feature of developmental pattern formation. The regular spacing of ommatidia in the Drosophila eye imaginal disc provides a good model for this phenomenon. The correct spacing of ommatidia is a central event in establishing the precise hexagonal pattern of ommatidia in the Drosophila compound eye. The R8 photoreceptors are the founder cells of each of the ommatidia that comprise the adult eye and are specified by a bHLH transcription factor, Atonal.

RESULTS

We find that the epidermal growth factor receptor (Egfr) has a primary function in regulating R8 spacing. The receptor's activation within nascent ommatidia induces the expression of a secreted inhibitor that blocks atonal expression, and therefore ommatidial initiation, in nearby cells. The identity of the secreted inhibitor remains elusive but, contrary to previous suggestions, we show that it is not Argos. This Egfr-dependent inhibition acts in parallel to the inhibition of atonal by the secreted protein Scabrous. The activation of the Egfr pathway is dependent on Atonal function via the expression of Rhomboid-1. Our results also allow us to conclude that Egfr's role in promoting cell survival is largely independent of its role in photoreceptor recruitment; even when cell death is blocked, most photoreceptors fail to form.

CONCLUSIONS

Based on our data and those of others, we propose a model for R8 spacing that comprises a self-organizing network of signaling molecules. This model describes how successive rows of ommatidia form out of phase with each other, leading to the hexagonal array of facets in the compound eye.

DER signaling restricts the boundaries of the wing field during Drosophila development

Arthropod and vertebrate limbs develop from secondary embryonic fields. In insects, the wing imaginal disk is subdivided early in development into the wing and notum subfields. The activity of the Wingless protein is fundamental for this subdivision and seems to be the first element of the hierarchy of regulatory genes promoting wing formation. Drosophila epidermal growth factor receptor (DER) signaling has many functions in fly development. Here we show that antagonizing DER signaling during the second larval instar leads to notum to wing transformations and wing mirror-image duplications. DER signaling is necessary for confining the wing subregion in the developing wing disk and for the specification of posterior identity. To do so, DER signaling acts by restricting the expression of Wingless to the dorsal-posterior quadrant of wing discs, suppressing wing-organizing activities, and by cooperating in the maintenance of Engrailed expression in posterior compartment cells.

Relationships between extramacrochaetae and Notch signalling in Drosophila wing development

The function of extramacrochaetae is required during the development of the Drosophila wing in processes such as cell proliferation and vein differentiation. extramacrochaetae encodes a transcription factor of the HLH family, but unlike other members of this family, Extramacrochaetae lacks the basic region that is involved in interaction with DNA. Some phenotypes caused by extramacrochaetae in the wing are similar to those observed when Notch signalling is compromised. Furthermore, maximal levels of extramacrochaetae expression in the wing disc are restricted to places where Notch activity is higher, suggesting that extramacrochaetae could mediate some aspects of Notch signalling during wing development. We have studied the relationships between extramacrochaetae and Notch in wing development, with emphasis on the processes of vein formation and cell proliferation. We observe strong genetic interaction between extramacrochaetae and different components of the Notch signalling pathway, suggesting a functional relationship between them. We show that the higher level of extramacrochaetae expression coincides with the domain of expression of Notch and its downstream gene Enhancer of split-m(beta). The expression of extramacrochaetae at the dorso/ventral boundary and in boundary cells between veins and interveins depends on Notch activity. We propose that at least during vein differentiation and wing margin formation, extramacrochaetae is regulated by Notch and collaborates with other Notch-downstream genes such as Enhancer of split-m(beta).

Notch signaling directly controls cell proliferation in the Drosophila wing disc

Notch signaling is involved in cell differentiation and patterning during morphogenesis. In the Drosophila wing, Notch activity regulates the expression of several genes at the dorsal/ventral boundary, and this is thought to elicit wing-cell proliferation. In this work, we show the effect of clones of cells expressing different forms of several members of the Notch signaling pathway, which result in an alteration of Notch activity. The ectopic expression in clones of activated forms of Notch or of its ligands (Delta or Serrate) in the wing causes outgrowths associated with the appearance of ectopic wing margins. These outgrowths consist of mutant territories and of surrounding wild-type cells. However, the ectopic expression of Delta, at low levels in ventral clones, causes large outgrowths that are associated neither with the generation of wing margin structures nor with the expression of genes characteristic of the dorsal/ventral boundary. These results suggest that Notch activity is directly involved in cell proliferation, independently of its role in the formation of the dorsal/ventral boundary. We propose that the nonautonomous effects (induction of extraproliferation and vein differentiation in the surrounding wild-type cells) result from pattern accommodation to positional values caused by the ectopic expression of Notch.

A temporal switch in DER signaling controls the specification and differentiation of veins and interveins in the Drosophila wing

The Drosophila EGF receptor (DER) is required for the specification of diverse cell fates throughout development. We have examined how the activation of DER controls the development of vein and intervein cells in the Drosophila wing. The data presented here indicate that two distinct events are involved in the determination and differentiation of wing cells. (1) The establishment of a positive feedback amplification loop, which drives DER signaling in larval stages. At this time, rhomboid (rho), in combination with vein, initiates and amplifies the activity of DER in vein cells. (2) The late downregulation of DER activity. At this point, the inactivation of MAPK in vein cells is necessary for the maintenance of the expression of decapentaplegic (dpp) and becomes essential for vein differentiation. Together, these temporal and spatial changes in the activity of DER constitute an autoregulatory network that controls the definition of vein and intervein cell types.

Dual role of extramacrochaetae in cell proliferation and cell differentiation during wing morphogenesis in Drosophila

The Extramacrochaetae (emc) gene encodes a transcription factor with an HLH domain without the basic region involved in interaction with DNA present in other proteins that have this domain. EMC forms heterodimers with bHLH proteins preventing their binding to DNA, acting as a negative regulator. The function of emc is required in many developmental processes during the development of Drosophila, including wing morphogenesis. Mitotic recombination clones of both null and gain-of-function alleles of emc, indicate that during wing morphogenesis, emc participates in cell proliferation within the intervein regions (vein patterning), as well as in vein differentiation. The study of relationships between emc and different genes involved in wing development reveal strong genetic interactions with genes of the Ras signalling pathway (torpedo, vein, veinlet and Gap), blistered, plexus and net, in both adult wing phenotypes and cell behaviour in genetic mosaics. These interactions are also analyzed as variations of emc expression patterns in mutant backgrounds for these genes. In addition, cell proliferation behaviour of emc mutant cells varies depending on the mutant background. The results show that genes of the Ras signalling pathway are co-operatively involved in the activity of emc during cell proliferation, and later antagonistically during cell differentiation, repressing EMC expression.

Genetic interactions and cell behaviour in blistered mutants during proliferation and differentiation of the Drosophila wing

In this work, we analyse the blistered function in wing vein development by studying genetic mosaics of mutant cells, genetic interactions with other genes affecting vein development and blistered expression in several mutant backgrounds. blistered encodes for a nuclear protein homologous to the mammalian Serum Response Factor and is expressed in presumptive intervein cells of third larval instar and pupal wing discs. Clones of blistered mutant cells proliferate normally but tend to grow along veins and always differentiate as vein tissue. These observations indicate that vein-determined wing cells show a particular behaviour that is responsible for their allocation to vein regions. We observe strong genetic interactions between blistered, veinlet and genes of the Ras signaling cascade. During disc proliferation, blistered expression is under the control of the Ras signal transduction pathway, but its expression is independent of veinlet. During the pupal period, blistered and veinlet expression become interdependent and mutually exclusive. These results link the activity of the Ras pathway to the process of early determination of intervein cells, by the transcriptional control of the blistered nuclear factor.

Wing surface interactions in venation patterning in Drosophila

The adult wing of Drosophila consists of two wing surfaces apposed by their basal membranes which first came into contact following disc eversion at metamorphosis. Veins appear in these surfaces in a dorsal-ventral symmetric pattern, but are 'corrugated' (vein cells are more compacted and more pigmented) in a dorsal-ventral asymmetric pattern. We prevented dorsal-ventral contact apposition during wing imaginal disc morphogenesis by implanting fragments of discs into metamorphosing hosts. In these implants, longitudinal veins differentiate but with wider corrugation and in both surfaces. These results and those of genetic mosaics of mutants removing veins or causing ectopic veins reveal mutual dorso-ventral induction/inhibition at work to modulate the final vein differentiation pattern and corrugation.

Behavior of extramacrochaetae mutant cells in the morphogenesis of the Drosophila wing

The gene extramacrochaetae (emc) encodes a non-basic Helix-loop-helix (HLH) protein that interacts and antagonises other basic-HLH proteins. The expression pattern of emc, and the phenotype of lethal emc alleles indicate that this gene is operative in several developmental processes. Here we study the requirements for emc during cell proliferation and vein differentiation in the wing. Mosaic analysis of hypomorphic conditions of emc reveals the tendency of mutant cells to proliferate along the veins as long stripes. Large clones abuting two adjacent veins obliterate the corresponding inter-vein, affecting the size and shape of the whole wing. Thus, the emc gene participates in the control of cell proliferation within inter-vein regions in the wing. Similar effects were found in the haltere and in the leg. The behavior of emc cells in genetic mosaics indicate that (1) proliferation is locally controlled within inter-vein sectors, (2) cells proliferate according to their genetic activity along preferential positions in the wing morphogenetic landscape and (3) cell proliferation in the wing is integrated by 'accommodation' between mutant and wild type cells.