Multiview confocal super-resolution microscopy

Confocal microscopy1 remains a major workhorse in biomedical optical microscopy owing to its reliability and flexibility in imaging various samples, but suffers from substantial point spread function anisotropy, diffraction-limited resolution, depth-dependent degradation in scattering samples and volumetric bleaching2. Here we address these problems, enhancing confocal microscopy performance from the sub-micrometre to millimetre spatial scale and the millisecond to hour temporal scale, improving both lateral and axial resolution more than twofold while simultaneously reducing phototoxicity. We achieve these gains using an integrated, four-pronged approach: (1) developing compact line scanners that enable sensitive, rapid, diffraction-limited imaging over large areas; (2) combining line-scanning with multiview imaging, developing reconstruction algorithms that improve resolution isotropy and recover signal otherwise lost to scattering; (3) adapting techniques from structured illumination microscopy, achieving super-resolution imaging in densely labelled, thick samples; (4) synergizing deep learning with these advances, further improving imaging speed, resolution and duration. We demonstrate these capabilities on more than 20 distinct fixed and live samples, including protein distributions in single cells; nuclei and developing neurons in Caenorhabditis elegans embryos, larvae and adults; myoblasts in imaginal disks of Drosophila wings; and mouse renal, oesophageal, cardiac and brain tissues.

Hedgehog produced by the Drosophila wing imaginal disc induces distinct responses in three target tissues

Hedgehog (Hh) is an evolutionarily conserved signaling protein that has essential roles in animal development and homeostasis. We investigated Hh signaling in the region of the Drosophila wing imaginal disc that produces Hh and is near the tracheal air sac primordium (ASP) and myoblasts. Hh distributes in concentration gradients in the anterior compartment of the wing disc, ASP and myoblasts, and activates genes in each tissue. Some targets of Hh signal transduction are common to the disc, ASP and myoblasts, whereas others are tissue-specific. Signaling in the three tissues is cytoneme-mediated and cytoneme-dependent. Some ASP cells project cytonemes that receive both Hh and Branchless (Bnl), and some targets regulated by Hh signaling in the ASP are also dependent on Bnl signal transduction. We conclude that the single source of Hh in the wing disc regulates cell type-specific responses in three discreet target tissues.

Rif1 Functions in a Tissue-Specific Manner To Control Replication Timing Through Its PP1-Binding Motif

Replication initiation in eukaryotic cells occurs asynchronously throughout S phase, yielding early- and late-replicating regions of the genome, a process known as replication timing (RT). RT changes during development to ensure accurate genome duplication and maintain genome stability. To understand the relative contributions that cell lineage, cell cycle, and replication initiation regulators have on RT, we utilized the powerful developmental systems available in Drosophila melanogaster We generated and compared RT profiles from mitotic cells of different tissues and from mitotic and endocycling cells of the same tissue. Our results demonstrate that cell lineage has the largest effect on RT, whereas switching from a mitotic to an endoreplicative cell cycle has little to no effect on RT. Additionally, we demonstrate that the RT differences we observed in all cases are largely independent of transcriptional differences. We also employed a genetic approach in these same cell types to understand the relative contribution the eukaryotic RT control factor, Rif1, has on RT control. Our results demonstrate that Rif1 can function in a tissue-specific manner to control RT. Importantly, the Protein Phosphatase 1 (PP1) binding motif of Rif1 is essential for Rif1 to regulate RT. Together, our data support a model in which the RT program is primarily driven by cell lineage and is further refined by Rif1/PP1 to ultimately generate tissue-specific RT programs.

Developmental regulation of regenerative potential in Drosophila by ecdysone through a bistable loop of ZBTB transcription factors

In many organisms, the regenerative capacity of tissues progressively decreases as development progresses. However, the developmental mechanisms that restrict regenerative potential remain unclear. In Drosophila, wing imaginal discs become unable to regenerate upon damage during the third larval stage (L3). Here, we show that production of ecdysone after larvae reach their critical weight (CW) terminates the window of regenerative potential by acting on a bistable loop composed of two antagonistic Broad-complex/Tramtrack/Bric-à-brac Zinc-finger (ZBTB) genes: chinmo and broad (br). Around mid L3, ecdysone signaling silences chinmo and activates br to switch wing epithelial progenitors from a default self-renewing to a differentiation-prone state. Before mid L3, Chinmo promotes a strong regenerative response upon tissue damage. After mid L3, Br installs a nonpermissive state that represses regeneration. Transient down-regulation of ecdysone signaling or Br in late L3 larvae enhances chinmo expression in damaged cells that regain the capacity to regenerate. This work unveils a mechanism that ties the self-renewing and regenerative potential of epithelial progenitors to developmental progression.

This study finds that the loss of regeneration potential in Drosophila wing imaginal discs is induced by the production of the steroid hormone ecdysone after the larva reaches its critical weight. Manipulating ecdysone signaling or the downstream transcription factors can uncouple regenerative properties from developmental progression.

While some organisms exhibit remarkable regenerative abilities throughout their life, many animals, including mammals, present limited regenerative potential that progressively decreases during development. Understanding the mechanisms underlying this progressive loss is important to devise therapeutic approaches aiming at facilitating the regeneration of a damaged tissue throughout life. The fruitfly Drosophila is a powerful model organism to address such questions. Indeed, while tissues, such as imaginal discs, can fully regenerate if damaged during early development, they fail to do so upon damages during late development. We show here that restriction of regenerative potential occurring during midlarval stages is due to the production of a steroid hormone, named ecdysone. By genetically manipulating ecdysone signaling, we can uncouple regenerative abilities from developmental progression. In particular, we show that ecdysone signaling triggers a switch in the sequential expression of two transcription factors, Chinmo and Broad, that positively and negatively regulate the competence for imaginal disc regeneration, respectively. Our work therefore identifies a key developmental signal that restricts regenerative potential in insects and opens new perspectives on elucidating how regeneration-permissive transcriptional programs are locked as development progresses.

The dynamics of Hippo signaling during Drosophila wing development

Tissue growth needs to be properly controlled for organs to reach their correct size and shape, but the mechanisms that control growth during normal development are not fully understood. We report here that the activity of the Hippo signaling transcriptional activator Yorkie gradually decreases in the central region of the developing Drosophila wing disc. Spatial and temporal changes in Yorkie activity can be explained by changes in cytoskeletal tension and biomechanical regulators of Hippo signaling. These changes in cellular biomechanics correlate with changes in cell density, and experimental manipulations of cell density are sufficient to alter biomechanical Hippo signaling and Yorkie activity. We also relate the pattern of Yorkie activity in older discs to patterns of cell proliferation. Our results establish that spatial and temporal patterns of Hippo signaling occur during wing development, that these patterns depend upon cell-density modulated tissue mechanics and that they contribute to the regulation of wing cell proliferation.

Coordination of Septate Junctions Assembly and Completion of Cytokinesis in Proliferative Epithelial Tissues

How permeability barrier function is maintained when epithelial cells divide is largely unknown. Here, we have investigated how the bicellular septate junctions (BSJs) and tricellular septate junctions (TSJs) are remodeled throughout completion of cytokinesis in Drosophila epithelia. We report that, following cytokinetic ring constriction, the midbody assembles, matures within SJs, and is displaced basally in two phases. In a first slow phase, the neighboring cells remain connected to the dividing cells by means of SJ-containing membrane protrusions pointing to the maturing midbody. Fluorescence recovery after photobleaching (FRAP) experiments revealed that SJs within the membrane protrusions correspond to the old SJs that were present prior to cytokinesis. In contrast, new SJs are assembled below the adherens junctions and spread basally to build a new belt of SJs in a manner analogous to a conveyor belt. Loss of function of a core BSJ component, the Na+/K+-ATPase pump Nervana 2 subunit, revealed that the apical-to-basal spread of BSJs drives the basal displacement of the midbody. In contrast, loss of the TSJ protein Bark beetle indicated that remodeling of TSJs is rate limiting and slowed down midbody migration. In the second phase, once the belt of SJs is assembled, the basal displacement of the midbody is accelerated and ultimately leads to abscission. This last step is temporally uncoupled from the remodeling of SJs. We propose that cytokinesis in epithelia involves the coordinated polarized assembly and remodeling of SJs both in the dividing cell and its neighbors to ensure the maintenance of permeability barrier integrity in proliferative epithelia.

Cell fate changes induced by a Distal-less enhancer-trap transgene in the Drosophila antennal imaginal disc

The imaginal discs of the genetically tractable model organism Drosophila melanogaster have been used to study cell-fate specification and plasticity, including homeotic changes and regeneration-induced transdetermination. The identity of the reprogramming mechanisms that induce plasticity has been of great interest in the field. Here we identify a change from antennal fate to eye fate induced by a Distal-less-GAL4 (DllGAL4) P-element insertion that is a mutant allele of Dll and expresses GAL4 in the antennal imaginal disc. While this fate change is not induced by tissue damage, it appears to be a hybrid of transdetermination and homeosis as the GAL4 expression causes upregulation of Wingless, and the Dll mutation is required for the fate change. Neither GAL4 expression nor a Dll mutation on its own is able to induce antenna-to-eye fate changes. This plasticity appears to be unique to the DllGAL4 line, possibly due to cellular stress induced by the high GAL4 expression combined with the severity of the Dll mutation. Thus, we propose that even in the absence of tissue damage, other forms of cellular stress caused by high GAL4 expression can induce determined cell fates to change, and selector gene mutations can sensitize the tissue to these transformations.

Frazzled can act through distinct molecular pathways in epithelial cells to regulate motility, apical constriction, and localisation of E-Cadherin

Netrin receptors of the DCC/NEO/UNC-40/Frazzled family have well established roles in cell migration and axon guidance but can also regulate epithelial features such as adhesion, polarity and adherens junction (AJ) stability. Previously, we have shown that overexpression of Drosophila Frazzled (Fra) in the peripodial epithelium (PE) inhibits wing disc eversion and also generates cellular protrusions typical of motile cells. Here, we tested whether the molecular pathways by which Fra inhibits eversion are distinct from those driving motility. We show that in disc proper (DP) epithelial cells Fra, in addition to inducing F-Actin rich protrusions, can affect localization of AJ components and columnar cell shape. We then show that these phenotypes have different requirements for the three conserved Fra cytoplasmic P-motifs and for downstream genes. The formation of protrusions required the P3 motif of Fra, as well as integrins (mys and mew), the Rac pathway (Rac1, wave and, arpc3) and myosin regulatory light chain (Sqh). In contrast, apico-basal cell shape change, which was accompanied by increased myosin phosphorylation, was critically dependent upon the P1 motif and was promoted by RhoGef2 but inhibited by Rac1. Fra also caused a loss of AJ proteins (DE-Cad and Arm) from basolateral regions of epithelial cells. This phenotype required all 3 P-motifs, and was dependent upon the polarity factor par6. par6 was not required for protrusions or cell shape change, but was required to block eversion suggesting that control of AJ components may underlie the ability of Fra to promote epithelial stability. The results imply that multiple molecular pathways act downstream of Fra in epithelial cells.

Atf3 links loss of epithelial polarity to defects in cell differentiation and cytoarchitecture

Interplay between apicobasal cell polarity modules and the cytoskeleton is critical for differentiation and integrity of epithelia. However, this coordination is poorly understood at the level of gene regulation by transcription factors. Here, we establish the Drosophila activating transcription factor 3 (atf3) as a cell polarity response gene acting downstream of the membrane-associated Scribble polarity complex. Loss of the tumor suppressors Scribble or Dlg1 induces atf3 expression via aPKC but independent of Jun-N-terminal kinase (JNK) signaling. Strikingly, removal of Atf3 from Dlg1 deficient cells restores polarized cytoarchitecture, levels and distribution of endosomal trafficking machinery, and differentiation. Conversely, excess Atf3 alters microtubule network, vesicular trafficking and the partition of polarity proteins along the apicobasal axis. Genomic and genetic approaches implicate Atf3 as a regulator of cytoskeleton organization and function, and identify Lamin C as one of its bona fide target genes. By affecting structural features and cell morphology, Atf3 functions in a manner distinct from other transcription factors operating downstream of disrupted cell polarity.

Epithelial cells form sheets and line both the outside and inside of our body. Their proper development and function require the asymmetric distribution of cellular components from the top to the bottom, known as apicobasal polarization. As loss of polarity hallmarks a majority of cancers in humans, understanding how epithelia respond to a collapse of the apicobasal axis is of great interest. Here, we show that in the fruit fly Drosophila melanogaster the breakdown of epithelial polarity engages Activating transcription factor 3 (Atf3), a protein that directly binds the DNA and regulates gene expression. We demonstrate that many of the pathological consequences of disturbed polarity require Atf3, as its loss in this context results in normalization of cellular architecture, vesicle trafficking and differentiation. Using unbiased genome-wide approaches we identify the genetic program controlled by Atf3 and experimentally verify select candidates. Given the evolutionary conservation of Atf3 between flies and man, we believe that our findings in the Drosophila model will contribute to a better understanding of diseases stemming from compromised epithelial polarity.

Trans-generational transmission of altered phenotype resulting from flubendiamide-induced changes in apoptosis in larval imaginal discs of Drosophila melanogaster

The eye and wing morphology of Drosophila melanogaster maintain unique, stable pattern of genesis from larval eye and wing imaginal discs. Increased apoptosis in cells of eye and wing discs was found to be associated with flubendiamide (fluoride containing insecticide) exposure (at the range 0.25-10μg/mL) in D. melanogaster larvae. The chemical fed larvae on attaining adulthood revealed alterations in morphology and symmetry of their compound eyes and wings through scanning electron microscopy. Nearly 40% and 30% of flies (P generation) demonstrated alterations in eyes and wings respectively. Transmission electron microscopic study (at the range 1-20μg/mL) also established variation in the rhabdomere and pigment cell orientation as well as in the shape of the ommatidium. Subsequent SEM study with F₁ and F₂ generation flies also revealed structural variation in eye and wing. Decrease in percentage of altered eye and wing phenotype was noted in subsequent generations (P> F₁>F₂). Thus, the diamide insecticide, flubendiamide, expected to be environmentally safe at sub-lethal concentrations was found to increase apoptosis in larvae and thereby cause morphological alteration in the adult D. melanogaster. This study further demonstrated trans-generational transmission of altered phenotype in three subsequent generations of a non-target insect model, D. melanogaster.

Chronic activation of JNK JAK/STAT and oxidative stress signalling causes the loser cell status

Cell competition is a form of cell interaction that causes the elimination of less fit cells, or losers, by wild-type (WT) cells, influencing overall tissue health. Several mutations can cause cells to become losers; however, it is not known how. Here we show that Drosophila wing disc cells carrying functionally unrelated loser mutations (Minute and mahjong) display the common activation of multiple stress signalling pathways before cell competition and find that these pathways collectively account for the loser status. We find that JNK signalling inhibits the growth of losers, while JAK/STAT signalling promotes competition-induced winner cell proliferation. Furthermore, we show that losers display oxidative stress response activation and, strikingly, that activation of this pathway alone, by Nrf2 overexpression, is sufficient to prime cells for their elimination by WT neighbours. Since oxidative stress and Nrf2 are linked to several diseases, cell competition may occur in a number of pathological conditions.Cell competition causes the removal of less fit cells ('losers') but why some gene mutations turn cells into losers is unclear. Here, the authors show that Drosophila wing disc cells carrying some loser mutations activate Nrf2 and JNK signalling, which contribute to the loser status.

Local Cell Death Changes the Orientation of Cell Division in the Developing Drosophila Wing Imaginal Disc Without Using Fat or Dachsous as Orienting Signals

Drosophila imaginal disc cells exhibit preferred cell division orientations according to location within the disc. These orientations are altered if cell death occurs within the epithelium, such as is caused by cell competition or by genotypes affecting cell survival. Both normal cell division orientations, and their orientations after cell death, depend on the Fat-Dachsous pathway of planar cell polarity (PCP). The hypothesis that cell death initiates a planar polarity signal was investigated. When clones homozygous for the pineapple eye (pie) mutation were made to initiate cell death, neither Dachsous nor Fat was required in pie cells for the re-orientation of nearby cells, indicating a distinct signal for this PCP pathway. Dpp and Wg were also not needed for pie clones to re-orient cell division. Cell shapes were evaluated in wild type and mosaic wing discs to assess mechanical consequences of cell loss. Although proximal wing disc cells and cells close to the dorso-ventral boundary were elongated in their preferred cell division axes in wild type discs, cell shapes in much of the wing pouch were symmetrical on average and did not predict their preferred division axis. Cells in pie mutant clones were slightly larger than their normal counterparts, consistent with mechanical stretching following cell loss, but no bias in cell shape was detected in the surrounding cells. These findings indicate that an unidentified signal influences PCP-dependent cell division orientation in imaginal discs.

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.

[hrs Gene and Borders of Compartments of Imaginal Wing Disc in Drosophila melanogaster]

Hepatocyte growth factor receptor tyrosine kinase substrate (Hrs) is an endosome protein involved in the sorting and transport of receptor tyrosine kinases and other proteins (which are absorbed by the cell during endocytosis) from early endosomes to lysosomes. Since receptor tyrosine kinases are important components of different cellular signaling systems, Hrs protein defects can result in the appearance of a number of developmental anomalies. In particular, it was demonstrated that ectopic Hrs expression in the wing kidney results in the disappearance of one or several rows of marginal wing setae in imago (which indicates hrs gene involvement in the development of the disc D/V border). We previously confirmed Hrs involvement in the developmental process of imaginal wing disc D/V border and demonstrated a change in the apterous gene expression pattern during ectopic hrs expression. The present report is devoted to the clarification and detailing of the Hrs effect on D/V, as well as on A/P borders of the imaginal wing disc compartments in Drosophila melanogaster.

Myoblast cytonemes mediate Wg signaling from the wing imaginal disc and Delta-Notch signaling to the air sac primordium

The flight muscles, dorsal air sacs, wing blades, and thoracic cuticle of the Drosophila adult function in concert, and their progenitor cells develop together in the wing imaginal disc. The wing disc orchestrates dorsal air sac development by producing decapentaplegic and fibroblast growth factor that travel via specific cytonemes in order to signal to the air sac primordium (ASP). Here, we report that cytonemes also link flight muscle progenitors (myoblasts) to disc cells and to the ASP, enabling myoblasts to relay signaling between the disc and the ASP. Frizzled (Fz)-containing myoblast cytonemes take up Wingless (Wg) from the disc, and Delta (Dl)-containing myoblast cytonemes contribute to Notch activation in the ASP. Wg signaling negatively regulates Dl expression in the myoblasts. These results reveal an essential role for cytonemes in Wg and Notch signaling and for a signal relay system in the myoblasts.

Loss of putzig Activity Results in Apoptosis during Wing Imaginal Development in Drosophila

The Drosophila gene putzig (pzg) encodes a nuclear protein that is an integral component of the Trf2/Dref complex involved in the transcription of proliferation-related genes. Moreover, Pzg is found in a complex together with the nucleosome remodeling factor NURF, where it promotes Notch target gene activation. Here we show that downregulation of pzg activity in the developing wing imaginal discs induces an apoptotic response, accompanied by the induction of the pro-apoptotic gene reaper, repression of Drosophila inhibitor of apoptosis protein accumulation and the activation of the caspases Drice, Caspase3 and Dcp1. As a further consequence ‘Apoptosis induced Proliferation’ (AiP) and ‘Apoptosis induced Apoptosis’ (AiA) are triggered. As expected, the activity of the stress kinase Jun N-terminal kinase (JNK), proposed to mediate both processes, is ectopically induced in response to pzg loss. In addition, the expression of the mitogen wingless (wg) but not of decapentaplegic (dpp) is observed. We present evidence that downregulation of Notch activates Dcp1 caspase and JNK signaling, however, neither induces ectopic wg nor dpp expression. In contrast, the consequences of Dref-RNAi were largely indistinguishable from pzg-RNAi with regard to apoptosis induction. Moreover, overexpression of Dref ameliorated the downregulation of pzg compatible with the notion that the two are required together to maintain cell and tissue homeostasis in Drosophila.

Notch inhibits Yorkie activity in Drosophila wing discs

During development, tissues and organs must coordinate growth and patterning so they reach the right size and shape. During larval stages, a dramatic increase in size and cell number of Drosophila wing imaginal discs is controlled by the action of several signaling pathways. Complex cross-talk between these pathways also pattern these discs to specify different regions with different fates and growth potentials. We show that the Notch signaling pathway is both required and sufficient to inhibit the activity of Yorkie (Yki), the Salvador/Warts/Hippo (SWH) pathway terminal transcription activator, but only in the central regions of the wing disc, where the TEAD factor and Yki partner Scalloped (Sd) is expressed. We show that this cross-talk between the Notch and SWH pathways is mediated, at least in part, by the Notch target and Sd partner Vestigial (Vg). We propose that, by altering the ratios between Yki, Sd and Vg, Notch pathway activation restricts the effects of Yki mediated transcription, therefore contributing to define a zone of low proliferation in the central wing discs.

Mutating RBF can enhance its pro-apoptotic activity and uncovers a new role in tissue homeostasis

The tumor suppressor retinoblastoma protein (pRb) is inactivated in a wide variety of cancers. While its role during cell cycle is well characterized, little is known about its properties on apoptosis regulation and apoptosis-induced cell responses. pRb shorter forms that can modulate pRB apoptotic properties, resulting from cleavages at caspase specific sites are observed in several cellular contexts. A bioinformatics analysis showed that a putative caspase cleavage site (TELD) is found in the Drosophila homologue of pRb(RBF) at a position similar to the site generating the p76Rb form in mammals. Thus, we generated a punctual mutant form of RBF in which the aspartate of the TELD site is replaced by an alanine. This mutant form, RBFD253A, conserved the JNK-dependent pro-apoptotic properties of RBF but gained the ability of inducing overgrowth phenotypes in adult wings. We show that this overgrowth is a consequence of an abnormal proliferation in wing imaginal discs, which depends on the JNK pathway activation but not on wingless (wg) ectopic expression. These results show for the first time that the TELD site of RBF could be important to control the function of RBF in tissue homeostasis in vivo.

Cytonemes as specialized signaling filopodia

Development creates a vast array of forms and patterns with elegant economy, using a small vocabulary of pattern-generating proteins such as BMPs, FGFs and Hh in similar ways in many different contexts. Despite much theoretical and experimental work, the signaling mechanisms that disperse these morphogen signaling proteins remain controversial. Here, we review the conceptual background and evidence that establishes a fundamental and essential role for cytonemes as specialized filopodia that transport signaling proteins between signaling cells. This evidence suggests that cytoneme-mediated signaling is a dispersal mechanism that delivers signaling proteins directly at sites of cell-cell contact.

TIE-DYE: a combinatorial marking system to visualize and genetically manipulate clones during development in Drosophila melanogaster

Two types of information are particularly valuable in understanding the development of a tissue or an organ from a small population of founder cells. First, it is useful to know the composition of the final structure in terms the contribution of individual founder cells. Second, it is important to understand cell-cell interactions. To facilitate the study of both of these aspects of organ development at a tissue-wide level, we have developed a method, TIE-DYE, that allows simultaneous lineage tracing of multiple cell populations as well as the genetic manipulation of a subset of these populations. Seven uniquely marked categories of cells are produced by site-directed recombination of three independent cassettes. We have used the TIE-DYE method to estimate the number of founder cells that give rise to the wing-imaginal disc during normal development and following compensatory growth caused by X-ray irradiation of the founder cells. We also show that four out of the seven types of marked clones can be genetically manipulated by gene overexpression or RNAi knockdown, allowing an assessment of the consequences of these manipulations on the entire wing disc. We demonstrate the utility of this system in studying the consequences of alterations in growth, patterning and cell-cell affinity.

Muscle-specific overexpression of the catalytic subunit of DNA polymerase γ induces pupal lethality in Drosophila melanogaster

We show the physiological effects and molecular characterization of overexpression of the catalytic core of mitochondrial DNA (mtDNA) polymerase (pol γ-α) in muscle of Drosophila melanogaster. Muscle-specific overexpression of pol γ-α using the UAS/GAL4 (where UAS is upstream activation sequence) system produced more than 90% of lethality at the end of pupal stage at 25°C, and the survivor adult flies showed a significant reduction in life span. The survivor flies displayed a decreased mtDNA level that is accompanied by a corresponding decrease in the levels of the nucleoid-binding protein mitochondrial transcription factor A (mtTFA). Furthermore, an increase in apoptosis is detected in larvae and adults overexpressing pol γ-α. We suggest that the pupal lethality and reduced life span of survivor adult flies are both caused mainly by massive apoptosis of muscle cells induced by mtDNA depletion.

Homeostatic control of Argonaute stability by microRNA availability

Homeostatic mechanisms regulate the abundance of several components in small-RNA pathways. We used Drosophila and mammalian systems to demonstrate a conserved homeostatic system in which the status of miRNA biogenesis controls Argonaute protein stability. Clonal analyses of multiple mutants of core Drosophila miRNA factors revealed that stability of the miRNA effector AGO1 is dependent on miRNA biogenesis. Reciprocally, ectopic transcription of miRNAs within in vivo clones induced accumulation of AGO1, as did genetic interference with the ubiquitin-proteasome system. In mouse cells, we found that the stability of Ago2 declined in Dicer-knockout cells and was rescued by proteasome blockade or introduction of either Dicer plasmid or Dicer-independent miRNA constructs. Notably, Dicer-dependent miRNA constructs generated pre-miRNAs that bound Ago2 but did not rescue Ago2 stability. We conclude that Argonaute levels are finely tuned by cellular availability of mature miRNAs and the ubiquitin-proteasome system.

Dynamic pattern of expression of dlin52, a member of the Myb/MuvB complex, during Drosophila development

The DREAM (DP, RB, E2F and MuvB) complex is required in humans to arrest the expression of cell cycle genes during quiescence. One of its members LIN52 has been isolated from the repressor complex but little is known about its molecular function. It has been reported recently that the serine residue 28 of LIN52 is phosphorylated by DYRK1A, and point mutation of this residue or down regulation of DYRK1A (which phosphorylates LIN52) leads to disruption of DREAM complex assembly, which is needed for G(0) arrest. Function of all the members of the dMyb complex (homologue of DREAM complex) in Drosophila melanogaster is not well characterized. We have studied the Drosophila orthologue of LIN52, known as dlin52, which is strongly conserved across various taxa from worms to human. dlin52 is reported to be present in a large protein complex containing important transcriptional regulators of cell proliferation and cell death like dE2F1, dMyb and dRbf. We have examined the expression of dlin52 transcripts and protein during development. Strong nuclear expression of dlin52 is seen in larval eye-antennal discs, brain, fat body, wing discs and salivary glands. dlin52 is abundantly expressed in endoreplicated tissues like salivary glands, fat body, and certain regions of the gut, and the nurse cells from adult ovaries. dlin52 is also expressed in the larval optic lobe, as well as in the developing neurons of ventral ganglion, indicating that this gene has an important role to play in cell cycle regulation and neuronal development. Robust expression of dlin52 protein was observed in quiescent cells like that of the imaginal cells of larval salivary gland, while marginal expression was seen in the germarium of adult ovary. Study of the spatial and temporal pattern of expression of this gene will help in better understanding of the function of this protein during various developmental processes.

Dynamics and mechanical stability of the developing dorsoventral organizer of the wing imaginal disc

Shaping the primordia during development relies on forces and mechanisms able to control cell segregation. In the imaginal discs of Drosophila the cellular populations that will give rise to the dorsal and ventral parts on the wing blade are segregated and do not intermingle. A cellular population that becomes specified by the boundary of the dorsal and ventral cellular domains, the so-called organizer, controls this process. In this paper we study the dynamics and stability of the dorsal-ventral organizer of the wing imaginal disc of Drosophila as cell proliferation advances. Our approach is based on a vertex model to perform in silico experiments that are fully dynamical and take into account the available experimental data such as: cell packing properties, orientation of the cellular divisions, response upon membrane ablation, and robustness to mechanical perturbations induced by fast growing clones. Our results shed light on the complex interplay between the cytoskeleton mechanics, the cell cycle, the cell growth, and the cellular interactions in order to shape the dorsal-ventral organizer as a robust source of positional information and a lineage controller. Specifically, we elucidate the necessary and sufficient ingredients that enforce its functionality: distinctive mechanical properties, including increased tension, longer cell cycle duration, and a cleavage criterion that satisfies the Hertwig rule. Our results provide novel insights into the developmental mechanisms that drive the dynamics of the DV organizer and set a definition of the so-called Notch fence model in quantitative terms.

During development, tissues are shaped in order to form organs with specific functionalities. This process relies on mechanisms that control cell segregation and migration. These concepts are beautifully illustrated by the morphogenetic events that the imaginal discs of Drosophila undergo during metamorphosis. In particular, the cellular populations that will give rise to the dorsal (D) and ventral (V) parts on the wing blade are segregated and do not intermingle. The so-called organizer, a cellular population that becomes specified by the boundary of the D and V cellular domains, is responsible for this. Yet, how does the DV organizer robustly deal with the cellular growth in order to prevent cell mixing? Moreover, how can the organizer be conveniently scaled as the tissue grows? Herein we address these questions using a computational approach that takes into account the available experimental data. Thus, our study unveils the elements that are necessary and sufficient for understanding in a quantitative and predictive manner the dynamics, structure, and stability of a robust growing DV organizer: distinctive mechanical properties of cells, differences in cell cycle duration, and a well-defined cleavage criterion.

[Nuclear texture in mitotic cells]

Changes of nuclear texture in mitotic cells of Drosophila melanogaster imaginal discs were studied. The distribution of voxels DAPI fluorescence intensities was used as the quantitative measure of the nuclear texture. The integral characteristics such as the portion of voxels with a given fluorescent signal level and autocorrelation of pixel intensities were used. We showed the nuclear texture has specific changes at different mitotic stages and this can be used for more precise staging of mitosis. Colchicines treatment pathologies, connected to abnormal mitoses, by nuclear-texture approach.