Reduction of endocytic activity accelerates cell elimination during tissue remodeling of the Drosophila epidermal epithelium.

Epithelial tissues undergo cell turnover both during development and for homeostatic maintenance. Cells that are no longer needed are quickly removed without compromising the barrier function of the tissue. During metamorphosis, insects undergo developmentally programmed tissue remodeling. However, the mechanisms that regulate this rapid tissue remodeling are not precisely understood. Here, we show that the temporal dynamics of endocytosis modulate physiological cell properties to prime larval epidermal cells for cell elimination. Endocytic activity gradually reduces as tissue remodeling progresses. This reduced endocytic activity accelerates cell elimination through the regulation of Myosin II subcellular reorganization, junctional E-cadherin levels, and caspase activation. Whereas the increased Myosin II dynamics accelerates cell elimination, E-cadherin plays a protective role against cell elimination. Reduced E-cadherin is involved in the amplification of caspase activation by forming a positive-feedback loop with caspase. These findings reveal the role of endocytosis in preventing cell elimination and in the cell-property switching initiated by the temporal dynamics of endocytic activity to achieve rapid cell elimination during tissue remodeling.

A local difference in Hedgehog signal transduction increases mechanical cell bond tension and biases cell intercalations along the Drosophila anteroposterior compartment boundary.

Tissue organization requires the interplay between biochemical signaling and cellular force generation. The formation of straight boundaries separating cells with different fates into compartments is important for growth and patterning during tissue development. In the developing Drosophila wing disc, maintenance of the straight anteroposterior (AP) compartment boundary involves a local increase in mechanical tension at cell bonds along the boundary. The biochemical signals that regulate mechanical tension along the AP boundary, however, remain unknown. Here, we show that a local difference in Hedgehog signal transduction activity between anterior and posterior cells is necessary and sufficient to increase mechanical tension along the AP boundary. This difference in Hedgehog signal transduction is also required to bias cell rearrangements during cell intercalations to keep the characteristic straight shape of the AP boundary. Moreover, severing cell bonds along the AP boundary does not reduce tension at neighboring bonds, implying that active mechanical tension is upregulated, cell bond by cell bond. Finally, differences in the expression of the homeodomain-containing protein Engrailed also contribute to the straight shape of the AP boundary, independently of Hedgehog signal transduction and without modulating cell bond tension. Our data reveal a novel link between local differences in Hedgehog signal transduction and a local increase in active mechanical tension of cell bonds that biases junctional rearrangements. The large-scale shape of the AP boundary thus emerges from biochemical signals inducing patterns of active tension on cell bonds.

A histone chaperone, DEK, transcriptionally coactivates a nuclear receptor.

Chromatin reorganization is essential for transcriptional control by sequence-specific transcription factors. However, the molecular link between transcriptional control and chromatin reconfiguration remains unclear. By colocalization of the nuclear ecdysone receptor (EcR) on the ecdysone-induced puff in the salivary gland, Drosophila DEK (dDEK) was genetically identified as a coactivator of EcR in both insect cells and intact flies. Biochemical purification and characterization of the complexes containing fly and human DEKs revealed that DEKs serve as histone chaperones via phosphorylation by forming complexes with casein kinase 2. Consistent with the preferential association of the DEK complex with histones enriched in active epigenetic marks, dDEK facilitated H3.3 assembly during puff formation. In some human myeloid leukemia patients, DEK was fused to CAN by chromosomal translocation. This mutation significantly reduced formation of the DEK complex, which is required for histone chaperone activity. Thus, the present study suggests that at least one histone chaperone can be categorized as a type of transcriptional coactivator for nuclear receptors.

Coordinated sequential action of EGFR and Notch signaling pathways regulates proneural wave progression in the Drosophila optic lobe.

During neurogenesis in the medulla of the Drosophila optic lobe, neuroepithelial cells are programmed to differentiate into neuroblasts at the medial edge of the developing optic lobe. The wave of differentiation progresses synchronously in a row of cells from medial to the lateral regions of the optic lobe, sweeping across the entire neuroepithelial sheet; it is preceded by the transient expression of the proneural gene lethal of scute [l(1)sc] and is thus called the proneural wave. We found that the epidermal growth factor receptor (EGFR) signaling pathway promotes proneural wave progression. EGFR signaling is activated in neuroepithelial cells and induces l(1)sc expression. EGFR activation is regulated by transient expression of Rhomboid (Rho), which is required for the maturation of the EGF ligand Spitz. Rho expression is also regulated by the EGFR signal. The transient and spatially restricted expression of Rho generates sequential activation of EGFR signaling and assures the directional progression of the differentiation wave. This study also provides new insights into the role of Notch signaling. Expression of the Notch ligand Delta is induced by EGFR, and Notch signaling prolongs the proneural state. Notch signaling activity is downregulated by its own feedback mechanism that permits cells at proneural states to subsequently develop into neuroblasts. Thus, coordinated sequential action of the EGFR and Notch signaling pathways causes the proneural wave to progress and induce neuroblast formation in a precisely ordered manner.

Recognition of pre- and postsynaptic neurons via nephrin/NEPH1 homologs is a basis for the formation of the Drosophila retinotopic map.

Topographic maps, which maintain the spatial order of neurons in the order of their axonal connections, are found in many parts of the nervous system. Here, we focus on the communication between retinal axons and their postsynaptic partners, lamina neurons, in the first ganglion of the Drosophila visual system, as a model for the formation of topographic maps. Post-mitotic lamina precursor cells differentiate upon receiving Hedgehog signals delivered through newly arriving retinal axons and, before maturing to extend neurites, extend short processes toward retinal axons to create the lamina column. The lamina column provides the cellular basis for establishing stereotypic synapses between retinal axons and lamina neurons. In this study, we identified two cell-adhesion molecules: Hibris, which is expressed in post-mitotic lamina precursor cells; and Roughest, which is expressed on retinal axons. Both proteins belong to the nephrin/NEPH1 family. We provide evidence that recognition between post-mitotic lamina precursor cells and retinal axons is mediated by interactions between Hibris and Roughest. These findings revealed mechanisms by which axons of presynaptic neurons deliver signals to induce the development of postsynaptic partners at the target area. Postsynaptic partners then recognize the presynaptic axons to make ensembles, thus establishing a topographic map along the anterior/posterior axis.

Nutrient-driven dedifferentiation of enteroendocrine cells promotes adaptive intestinal growth in Drosophila.

Post-developmental organ resizing improves organismal fitness under constantly changing nutrient environments. Although stem cell abundance is a fundamental determinant of adaptive resizing, our understanding of its underlying mechanisms remains primarily limited to the regulation of stem cell division. Here, we demonstrate that nutrient fluctuation induces dedifferentiation in the Drosophila adult midgut to drive adaptive intestinal growth. From lineage tracing and single-cell RNA sequencing, we identify a subpopulation of enteroendocrine (EE) cells that convert into functional intestinal stem cells (ISCs) in response to dietary glucose and amino acids by activating the JAK-STAT pathway. Genetic ablation of EE-derived ISCs severely impairs ISC expansion and midgut growth despite the retention of resident ISCs, and in silico modeling further indicates that EE dedifferentiation enables an efficient increase in the midgut cell number while maintaining epithelial cell composition. Our findings identify a physiologically induced dedifferentiation that ensures ISC expansion during adaptive organ growth in concert with nutrient conditions.

Estimation of crossbridge-state during cardiomyocyte beating using second harmonic generation.

Estimation of dynamic change of crossbridge formation in living cardiomyocytes is expected to provide crucial information for elucidating cardiomyopathy mechanisms, efficacy of an intervention, and others. Here, we established an assay system to dynamically measure second harmonic generation (SHG) anisotropy derived from myosin filaments depended on their crossbridge status in pulsating cardiomyocytes. Experiments utilizing an inheritable mutation that induces excessive myosin-actin interactions revealed that the correlation between sarcomere length and SHG anisotropy represents crossbridge formation ratio during pulsation. Furthermore, the present method found that ultraviolet irradiation induced an increased population of attached crossbridges that lost the force-generating ability upon myocardial differentiation. Taking an advantage of infrared two-photon excitation in SHG microscopy, myocardial dysfunction could be intravitally evaluated in a Drosophila disease model. Thus, we successfully demonstrated the applicability and effectiveness of the present method to evaluate the actomyosin activity of a drug or genetic defect on cardiomyocytes. Because genomic inspection alone may not catch the risk of cardiomyopathy in some cases, our study demonstrated herein would be of help in the risk assessment of future heart failure.

Cell mechanics and cell-cell recognition controls by Toll-like receptors in tissue morphogenesis and homeostasis.

Signal transduction by the Toll-like receptors (TLRs) is conserved and essential for innate immunity in metazoans. The founding member of the TLR family, Drosophila Toll-1, was initially identified for its role in dorsoventral axis formation in early embryogenesis. The Drosophila genome encodes nine TLRs that display dynamic expression patterns during development, suggesting their involvement in tissue morphogenesis and homeostasis. Recent progress on the developmental functions of TLRs beyond dorsoventral patterning has revealed not only their diverse functions in various biological processes, but also unprecedented molecular mechanisms in directly regulating cell mechanics and cell-cell recognition independent of the canonical signal transduction pathway involving transcriptional regulation of target genes. In this review, I feature and discuss the non-immune functions of TLRs in the control of epithelial tissue homeostasis, tissue morphogenesis, and cell-cell recognition between cell populations with different cell identities.

Sample Preparation and Imaging of the Pupal Drosophila Abdominal Epidermis.

The epithelium is one of the best studied tissues for morphogenesis, pattern formation, cell polarity, cell division, cell competition, tumorigenesis, and metastatic behaviors. However, it has been challenging to analyze real-time cell interactions or cell dynamics within the epithelia under physiological conditions. The Drosophila pupal abdominal epidermis is a model system that allows to combine long-term real-time imaging under physiological conditions with the use of powerful Drosophila genetics tools. The abdominal epidermis displays a wide range of stereotypical characteristics of the epithelia and cellular behaviors including cell division, cell death, cell rearrangement, apical constriction, and apicobasal/planar polarity, making this tissue a first choice for the study of epithelial morphogenesis and relevant phenomena. In this chapter, I describe the staging and mounting of pupae and the live imaging of the abdominal epidermis. Moreover, methods to combine live imaging with mosaic analysis or drug injection will be presented. The long-term live imaging of the pupal abdominal epidermis is straightforward and opens up the possibility to analyze cell dynamics during epithelial morphogenesis at an unprecedented resolution.

DWnt4 regulates the dorsoventral specificity of retinal projections in the Drosophila melanogaster visual system.

In Drosophila melanogaster, the axons of retinal photoreceptor cells extend to the first optic ganglion, the lamina, forming a topographic representation. Here we show that DWnt4, a secreted protein of the Wnt family, is the ventral cue for the lamina. In DWnt4 mutants, ventral retinal axons misprojected to the dorsal lamina. DWnt4 was normally expressed in the ventral half of the developing lamina and DWnt4 protein was detected along ventral retinal axons. Dfrizzled2 and dishevelled, respectively, encode a receptor and a signaling molecule required for Wnt signaling. Mutations in both genes caused DWnt4-like defects, and both genes were autonomously required in the retina, suggesting a direct role of DWnt4 in retinal axon guidance. In contrast, iroquois homeobox genes are the dorsal cues for the retina. Dorsal axons accumulated DWnt4 and misprojected to the ventral lamina in iroquois mutants; the phenotype was suppressed in iroquois Dfrizzled2 mutants, suggesting that iroquois may attenuate the competence of Dfrizzled2 to respond to DWnt4.

Differential cell adhesion implemented by Drosophila Toll corrects local distortions of the anterior-posterior compartment boundary.

Maintaining lineage restriction boundaries in proliferating tissues is vital to animal development. A long-standing thermodynamics theory, the differential adhesion hypothesis, attributes cell sorting phenomena to differentially expressed adhesion molecules. However, the contribution of the differential adhesion system during tissue morphogenesis has been unsubstantiated despite substantial theoretical support. Here, we report that Toll-1, a transmembrane receptor protein, acts as a differentially expressed adhesion molecule that straightens the fluctuating anteroposterior compartment boundary in the abdominal epidermal epithelium of the Drosophila pupa. Toll-1 is expressed across the entire posterior compartment under the control of the selector gene engrailed and displays a sharp expression boundary that coincides with the compartment boundary. Toll-1 corrects local distortions of the boundary in the absence of cable-like Myosin II enrichment along the boundary. The reinforced adhesion of homotypic cell contacts, together with pulsed cell contraction, achieves a biased vertex sliding action by resisting the separation of homotypic cell contacts in boundary cells. This work reveals a self-organizing system that integrates a differential adhesion system with pulsed contraction of cells to maintain lineage restriction boundaries.

Competition for Space Is Controlled by Apoptosis-Induced Change of Local Epithelial Topology.

During the initial stage of tumor progression, oncogenic cells spread despite spatial confinement imposed by surrounding normal tissue. This spread of oncogenic cells (winners) is thought to be governed by selective killing of surrounding normal cells (losers) through a phenomenon called "cell competition" (i.e., supercompetition). Although the mechanisms underlying loser elimination are increasingly apparent, it is not clear how winner cells selectively occupy the space made available following loser apoptosis. Here, we combined live imaging analyses of two different oncogenic clones (Yki/YAP activation and Ras activation) in the Drosophila epithelium with computer simulation of tissue mechanics to elucidate such a mechanism. Contrary to the previous expectation that cell volume loss after apoptosis of loser cells was simply compensated for by the faster proliferation of winner cells, we found that the lost volume was compensated for by rapid cell expansion of winners. Mechanistically, the rapid winner-dominated cell expansion was driven by apoptosis-induced epithelial junction remodeling, which causes re-connection of local cellular connectivity (cell topology) in a manner that selectively increases winner apical surface area. In silico experiments further confirmed that repetition of loser elimination accelerates tissue-scale winner expansion through topological changes over time. Our proposed mechanism for linking loser death and winner expansion provides a new perspective on how tissue homeostasis disruption can initiate from an oncogenic mutation.

Planar polarized contractile actomyosin networks in dynamic tissue morphogenesis.

The complex shapes of animal bodies are constructed through a sequence of simple physical interactions of constituent cells. Mechanical forces generated by cellular activities, such as division, death, shape change and rearrangement, drive tissue morphogenesis. By confining assembly or disassembly of actomyosin networks within the three-dimensional space of the cell, cells can localize forces to induce tissue deformation. Tissue-scale morphogenesis emerges from a collective behavior of cells that coordinates the force generation in space and time. Thus, the molecular mechanisms that govern the temporal and spatial regulation of forces in individual cells are elemental to organogenesis, and the tissue-scale coordination of forces generated by individual cells is key to determining the final shape of organs.

Inference of Cell Mechanics in Heterogeneous Epithelial Tissue Based on Multivariate Clone Shape Quantification.

Cell populations in multicellular organisms show genetic and non-genetic heterogeneity, even in undifferentiated tissues of multipotent cells during development and tumorigenesis. The heterogeneity causes difference of mechanical properties, such as, cell bond tension or adhesion, at the cell-cell interface, which determine the shape of clonal population boundaries via cell sorting or mixing. The boundary shape could alter the degree of cell-cell contacts and thus influence the physiological consequences of sorting or mixing at the boundary (e.g., tumor suppression or progression), suggesting that the cell mechanics could help clarify the physiology of heterogeneous tissues. While precise inference of mechanical tension loaded at each cell-cell contacts has been extensively developed, there has been little progress on how to distinguish the population-boundary geometry and identify the cause of geometry in heterogeneous tissues. We developed a pipeline by combining multivariate analysis of clone shape with tissue mechanical simulations. We examined clones with four different genotypes within Drosophila wing imaginal discs: wild-type, tartan (trn) overexpression, hibris (hbs) overexpression, and Eph RNAi. Although the clones were previously known to exhibit smoothed or convoluted morphologies, their mechanical properties were unknown. By applying a multivariate analysis to multiple criteria used to quantify the clone shapes based on individual cell shapes, we found the optimal criteria to distinguish not only among the four genotypes, but also non-genetic heterogeneity from genetic one. The efficient segregation of clone shape enabled us to quantitatively compare experimental data with tissue mechanical simulations. As a result, we identified the mechanical basis contributed to clone shape of distinct genotypes. The present pipeline will promote the understanding of the functions of mechanical interactions in heterogeneous tissue in a non-invasive manner.

Segmentation and Quantitative Analysis of Epithelial Tissues.

Epithelia are tissues that regulate exchanges with the environment. They are very dynamic and can acquire virtually any shape; at the cellular level, they are composed of cells tightly connected by junctions. Most often epithelia are amenable to live imaging; however, the large number of cells composing an epithelium and the absence of informatics tools dedicated to epithelial analysis largely prevented tissue scale studies. Here we present Tissue Analyzer, a free tool that can be used to segment and analyze epithelial cells and monitor tissue dynamics.

Signals and mechanics shaping compartment boundaries in Drosophila.

During animal development groups of cells with similar fates and functions often stay together and separate from cells with different fates. An example for this cellular behavior is the formation of compartments, groups of cells with similar fates that are separated by sharp boundaries from neighboring groups of cells. Compartments play important roles during patterning by serving as units of growth and gene expression. Boundaries between compartments are associated with organizers that secrete signaling molecules instructing growth and differentiation throughout the tissue. The straight shape of the boundary between compartments is important for maintaining the position and shape of the organizer and thus for precise patterning. The straight shape of compartment boundaries, however, is challenged by cell divisions and cell intercalations that take place in many developing tissues. Early work established a role for selector genes and signaling pathways in setting up and keeping boundaries straight. Recent work in Drosophila has now begun to further unravel the physical and cellular mechanisms that maintain compartment boundaries. Key to the separation of compartments is a local increase of actomyosin-dependent mechanical tension at cell junctions along the boundary. Increased mechanical tension acts as a barrier to cell mixing during cell division and influences cell rearrangements during cell intercalations along the compartment boundary in a way that the straight shape of the boundary is maintained. An important question for the future is how the signaling pathways that maintain the straight shape of compartment boundaries control mechanical tension along these boundaries.

An RNA interference screen for genes required to shape the anteroposterior compartment boundary in Drosophila identifies the Eph receptor.

The formation of straight compartment boundaries separating groups of cells with distinct fates and functions is an evolutionarily conserved strategy during animal development. The physical mechanisms that shape compartment boundaries have recently been further elucidated, however, the molecular mechanisms that underlie compartment boundary formation and maintenance remain poorly understood. Here, we report on the outcome of an RNA interference screen aimed at identifying novel genes involved in maintaining the straight shape of the anteroposterior compartment boundary in Drosophila wing imaginal discs. Out of screening 3114 transgenic RNA interference lines targeting a total of 2863 genes, we identified a single novel candidate that interfered with the formation of a straight anteroposterior compartment boundary. Interestingly, the targeted gene encodes for the Eph receptor tyrosine kinase, an evolutionarily conserved family of signal transducers that has previously been shown to be important for maintaining straight compartment boundaries in vertebrate embryos. Our results identify a hitherto unknown role of the Eph receptor tyrosine kinase in Drosophila and suggest that Eph receptors have important functions in shaping compartment boundaries in both vertebrate and insect development.

Local increases in mechanical tension shape compartment boundaries by biasing cell intercalations.

Mechanical forces play important roles during tissue organization in developing animals. Many tissues are organized into adjacent, nonmixing groups of cells termed compartments. Boundaries between compartments display a straight morphology and are associated with signaling centers that are important for tissue growth and patterning. Local increases in mechanical tension at cell junctions along compartment boundaries have recently been shown to prevent cell mixing and to maintain straight boundaries. The cellular mechanisms by which local increases in mechanical tension prevent cell mixing at compartment boundaries, however, remain poorly understood. Here, we have used live imaging and quantitative image analysis to determine cellular dynamics at and near the anteroposterior compartment boundaries of the Drosophila pupal abdominal epidermis. We show that cell mixing within compartments involves multiple cell intercalations. Frequency and orientation of cell intercalations are unchanged along the compartment boundaries; rather, an asymmetry in the shrinkage of junctions during intercalation is biased, resulting in cell rearrangements that suppress cell mixing. Simulations of tissue growth show that local increases in mechanical tension can account for this bias in junctional shrinkage. We conclude that local increases in mechanical tension maintain cell populations separate by influencing junctional rearrangements during cell intercalation.

Compartment boundaries: sorting cells with tension.

The subdivision of proliferating tissues into groups of non-intermingling sets of cells, termed compartments, is a common process of animal development. Signaling between adjacent compartments induces the local expression of morphogens that pattern the surrounding tissue. Sharp and straight boundaries between compartments stabilize the source of such morphogens during tissue growth and, thus, are of crucial importance for pattern formation. Signaling pathways required to maintain compartment boundaries have been identified, yet the physical mechanisms that maintain compartment boundaries remained elusive. Recent data now show that a local increase in actomyosin-based mechanical tension on cell bonds is vital for maintaining compartment boundaries in Drosophila.