Notch is required for successive cell decisions in the developing Drosophila retina

The Rap1 small GTPase affects cell fate or survival and morphogenetic patterning during Drosophila melanogaster eye development

The Drosophila melanogaster eye has been instrumental for determining both how cells communicate with one another to determine cell fate, as well as cell morphogenesis and patterning. Here, we describe the effects of the small GTPase Rap1 on the development of multiple cell types in the D. melanogaster eye. Although Rap1 has previously been linked to RTK-Ras-MAPK signaling in eye development, we demonstrate that manipulation of Rap1 activity is modified by increase or decrease of Delta/Notch signaling during several events of cell fate specification in eye development. In addition, we demonstrate that manipulating Rap1 function either in primary pigment cells or in interommatidial cells affects cone cell contact switching, primary pigment cell enwrapment of the ommatidial cluster, and sorting of secondary and tertiary pigment cells. These data suggest that Rap1 has roles in both ommatidial cell recruitment/survival and in ommatidial morphogenesis in the pupal stage. They lay groundwork for future experiments on the role of Rap1 in these events.

Patterning of the Drosophila retina by the morphogenetic furrow

Pattern formation is the process by which cells within a homogeneous epithelial sheet acquire distinctive fates depending upon their relative spatial position to each other. Several proposals, starting with Alan Turing's diffusion-reaction model, have been put forth over the last 70 years to describe how periodic patterns like those of vertebrate somites and skin hairs, mammalian molars, fish scales, and avian feather buds emerge during development. One of the best experimental systems for testing said models and identifying the gene regulatory networks that control pattern formation is the compound eye of the fruit fly, Drosophila melanogaster. Its cellular morphogenesis has been extensively studied for more than a century and hundreds of mutants that affect its development have been isolated. In this review we will focus on the morphogenetic furrow, a wave of differentiation that takes an initially homogeneous sheet of cells and converts it into an ordered array of unit eyes or ommatidia. Since the discovery of the furrow in 1976, positive and negative acting morphogens have been thought to be solely responsible for propagating the movement of the furrow across a motionless field of cells. However, a recent study has challenged this model and instead proposed that mechanical driven cell flow also contributes to retinal pattern formation. We will discuss both models and their impact on patterning.

Hippo pathway and Bonus control developmental cell fate decisions in the Drosophila eye

The canonical function of the Hippo signaling pathway is the regulation of organ growth. How this pathway controls cell-fate determination is less well understood. Here, we identify a function of the Hippo pathway in cell-fate decisions in the developing Drosophila eye, exerted through the interaction of Yorkie (Yki) with the transcriptional regulator Bonus (Bon), an ortholog of mammalian transcriptional intermediary factor 1/tripartite motif (TIF1/TRIM) family proteins. Instead of controlling tissue growth, Yki and Bon promote epidermal and antennal fates at the expense of the eye fate. Proteomic, transcriptomic, and genetic analyses reveal that Yki and Bon control these cell-fate decisions by recruiting transcriptional and post-transcriptional co-regulators and by repressing Notch target genes and activating epidermal differentiation genes. Our work expands the range of functions and regulatory mechanisms under Hippo pathway control.

EyeVolve, a modular PYTHON based model for simulating developmental eye type diversification

Vision is among the oldest and arguably most important sensory modalities for animals to interact with their external environment. Although many different eye types exist within the animal kingdom, mounting evidence indicates that the genetic networks required for visual system formation and function are relatively well conserved between species. This raises the question as to how common developmental programs are modified in functionally different eye types. Here, we approached this issue through EyeVolve, an open-source PYTHON-based model that recapitulates eye development based on developmental principles originally identified in Drosophila melanogaster. Proof-of-principle experiments showed that this program’s animated timeline successfully simulates early eye tissue expansion, neurogenesis, and pigment cell formation, sequentially transitioning from a disorganized pool of progenitor cells to a highly organized lattice of photoreceptor clusters wrapped with support cells. Further, tweaking just five parameters (precursor pool size, founder cell distance and placement from edge, photoreceptor subtype number, and cell death decisions) predicted a multitude of visual system layouts, reminiscent of the varied eye types found in larval and adult arthropods. This suggests that there are universal underlying mechanisms that can explain much of the existing arthropod eye diversity. Thus, EyeVolve sheds light on common principles of eye development and provides a new computational system for generating specific testable predictions about how development gives rise to diverse visual systems from a commonly specified neuroepithelial ground plan.

The early history of the eye-antennal disc of Drosophila melanogaster

A pair of eye-antennal imaginal discs give rise to nearly all external structures of the adult Drosophila head including the compound eyes, ocelli, antennae, maxillary palps, head epidermis, and bristles. In the earliest days of Drosophila research, investigators would examine thousands of adult flies in search of viable mutants whose appearance deviated from the norm. The compound eyes are dispensable for viability and perturbations to their structure are easy to detect. As such, the adult compound eye and the developing eye-antennal disc emerged as focal points for studies of genetics and developmental biology. Since few tools were available at the time, early researchers put an enormous amount of thought into models that would explain their experimental observations-many of these hypotheses remain to be tested. However, these "ancient" studies have been lost to time and are no longer read or incorporated into today's literature despite the abundance of field-defining discoveries that are contained therein. In this FlyBook chapter, I will bring these forgotten classics together and draw connections between them and modern studies of tissue specification and patterning. In doing so, I hope to bring a larger appreciation of the contributions that the eye-antennal disc has made to our understanding of development as well as draw the readers' attention to the earliest studies of this important imaginal disc. Armed with the today's toolkit of sophisticated genetic and molecular methods and using the old papers as a guide, we can use the eye-antennal disc to unravel the mysteries of development.

Tiling mechanisms of the Drosophila compound eye through geometrical tessellation

Tiling patterns are observed in many biological structures. The compound eye is an interesting example of tiling and is often constructed by hexagonal arrays of ommatidia, the optical unit of the compound eye. Hexagonal tiling may be common due to mechanical restrictions such as structural robustness, minimal boundary length, and space-filling efficiency. However, some insects exhibit tetragonal facets.^1-4 Some aquatic crustaceans, such as shrimp and lobsters, have evolved with tetragonal facets.^5-8 Mantis shrimp is an insightful example as its compound eye has a tetragonal midband region sandwiched between hexagonal hemispheres.^9,10 This casts doubt on the naive explanation that hexagonal tiles recur in nature because of their mechanical stability. Similarly, tetragonal tiling patterns are also observed in some Drosophila small-eye mutants, whereas the wild-type eyes are hexagonal, suggesting that the ommatidial tiling is not simply explained by such mechanical restrictions. If so, how are the hexagonal and tetragonal patterns controlled during development? Here, we demonstrate that geometrical tessellation determines the ommatidial tiling patterns. In small-eye mutants, the hexagonal pattern is transformed into a tetragonal pattern as the relative positions of neighboring ommatidia are stretched along the dorsal-ventral axis. We propose that the regular distribution of ommatidia and their uniform growth collectively play an essential role in the establishment of tetragonal and hexagonal tiling patterns in compound eyes.

Notch signaling pathway: architecture, disease, and therapeutics

The NOTCH gene was identified approximately 110 years ago. Classical studies have revealed that NOTCH signaling is an evolutionarily conserved pathway. NOTCH receptors undergo three cleavages and translocate into the nucleus to regulate the transcription of target genes. NOTCH signaling deeply participates in the development and homeostasis of multiple tissues and organs, the aberration of which results in cancerous and noncancerous diseases. However, recent studies indicate that the outcomes of NOTCH signaling are changeable and highly dependent on context. In terms of cancers, NOTCH signaling can both promote and inhibit tumor development in various types of cancer. The overall performance of NOTCH-targeted therapies in clinical trials has failed to meet expectations. Additionally, NOTCH mutation has been proposed as a predictive biomarker for immune checkpoint blockade therapy in many cancers. Collectively, the NOTCH pathway needs to be integrally assessed with new perspectives to inspire discoveries and applications. In this review, we focus on both classical and the latest findings related to NOTCH signaling to illustrate the history, architecture, regulatory mechanisms, contributions to physiological development, related diseases, and therapeutic applications of the NOTCH pathway. The contributions of NOTCH signaling to the tumor immune microenvironment and cancer immunotherapy are also highlighted. We hope this review will help not only beginners but also experts to systematically and thoroughly understand the NOTCH signaling pathway.

Multivalent interactions make adherens junction-cytoskeletal linkage robust during morphogenesis

Embryogenesis requires cells to change shape and move without disrupting epithelial integrity. This requires robust, responsive linkage between adherens junctions and the actomyosin cytoskeleton. Using Drosophila morphogenesis, we define molecular mechanisms mediating junction–cytoskeletal linkage and explore the role of mechanosensing. We focus on the junction–cytoskeletal linker Canoe, a multidomain protein. We engineered the canoe locus to define how its domains mediate its mechanism of action. To our surprise, the PDZ and FAB domains, which we thought connected junctions and F-actin, are not required for viability or mechanosensitive recruitment to junctions under tension. The FAB domain stabilizes junctions experiencing elevated force, but in its absence, most cells recover, suggesting redundant interactions. In contrast, the Rap1-binding RA domains are critical for all Cno functions and enrichment at junctions under tension. This supports a model in which junctional robustness derives from a large protein network assembled via multivalent interactions, with proteins at network nodes and some node connections more critical than others.

Mathematical modeling of Notch dynamics in Drosophila neural development

Notch signalling is a well-conserved signalling pathway that regulates cell fate through cell-cell communication. A typical feature of Notch signalling is ‘lateral inhibition’, whereby two neighbouring cells of equivalent state of differentiation acquire different cell fates. Recently, mathematical and computational approaches have addressed the Notch dynamics in Drosophila neural development. Typical examples of lateral inhibition are observed in the specification of neural stem cells in the embryo and sensory organ precursors in the thorax. In eye disc development, Notch signalling cooperates with other signalling pathways to define the evenly spaced positioning of the photoreceptor cells. The interplay between Notch and epidermal growth factor receptor signalling regulates the timing of neural stem cell differentiation in the optic lobe. In this review, we summarize the theoretical studies that have been conducted to elucidate the Notch dynamics in these systems and discuss the advantages of combining mathematical models with biological experiments.

Hexagonal patterning of the Drosophila eye

A complex network of transcription factor interactions propagates across the larval eye disc to establish columns of evenly-spaced R8 precursor cells, the founding cells of Drosophila ommatidia. After the recruitment of additional photoreceptors to each ommatidium, the surrounding cells are organized into their stereotypical pattern during pupal development. These support cells - comprised of pigment and cone cells - are patterned to encapsulate the photoreceptors and separate ommatidia with an hexagonal honeycomb lattice. Since the proteins and processes essential for correct eye patterning are conserved, elucidating how these function and change during Drosophila eye patterning can substantially advance our understanding of transcription factor and signaling networks, cytoskeletal structures, adhesion complexes, and the biophysical properties of complex tissues during their morphogenesis. Our understanding of many of these aspects of Drosophila eye patterning is largely descriptive. Many important questions, especially relating to the regulation and integration of cellular events, remain.

Modulatory Potential of LncRNA Zfas1 for Inflammation and Neuronal Apoptosis in Temporal Lobe Epilepsy

PURPOSE

This study aimed to elucidate whether lncRNA ZFAS1 is involved in neuronal apoptosis and inflammation in temporal lobe epilepsy (TLE).

MATERIALS AND METHODS

Ninety-six TLE patients were recruited, and their peripheral venous blood was gathered to determine Zfas1 expression with polymerase chain reaction. Neurons were separated from hippocampal tissue of newborn SD rats, and si-Zfas1 or pcDNA3.1-Zfas1 was transfected into the neurons. Inflammatory cytokines released by neurons were determined, and neuronal activities were evaluated through MTT assay, colony formation assay, and flow cytometry.

RESULTS

Serum levels of Zfas1 were higher in TLE patients than in healthy controls (p<0.05). Furthermore, Zfas1 expression in neurons was raised by pcDNA3.1-Zfas1 and declined after silencing of Zfas1 (p<0.05). Transfection of pcDNA-Zfas1 weakened the viability and proliferation of neurons and increased neuronal apoptosis (p<0.05). Meanwhile, pcDNA3.1-Zfas1 transfection promoted lipopolysaccharide-induced release of cytokines, including tumor necrosis factor-α, interleukin (IL)-1, IL-6, and intercellular adhesion molecule-1 (p<0.05), and boosted NF-κB activation by elevating the expression of NF-κB p65, pIκBα, and IKKβ in neurons (p<0.05).

CONCLUSION

Our results indicated that lncRNA ZFAS1 exacerbates epilepsy development by promoting neuronal apoptosis and inflammation, implying ZFAS1 as a promising treatment target for epilepsy.

Comparative transcriptome analyses of the Drosophila pupal eye

Tissue function is dependent on correct cellular organization and behavior. As a result, the identification and study of genes that contribute to tissue morphogenesis is of paramount importance to the fields of cell and developmental biology. Many of the genes required for tissue patterning and organization are highly conserved between phyla. This has led to the emergence of several model organisms and developmental systems that are used to study tissue morphogenesis. One such model is the Drosophila melanogaster pupal eye that has a highly stereotyped arrangement of cells. In addition, the pupal eye is postmitotic that allows for the study of tissue morphogenesis independent from any effects of proliferation. While the changes in cell morphology and organization that occur throughout pupal eye development are well documented, less is known about the corresponding transcriptional changes that choreograph these processes. To identify these transcriptional changes, we dissected wild-type Canton S pupal eyes and performed RNA-sequencing. Our analyses identified differential expression of many loci that are documented regulators of pupal eye morphogenesis and contribute to multiple biological processes including signaling, axon projection, adhesion, and cell survival. We also identified differential expression of genes not previously implicated in pupal eye morphogenesis such as components of the Toll pathway, several non-classical cadherins, and components of the muscle sarcomere, which could suggest these loci function as novel patterning factors.

Sas-4 Colocalizes with the Ciliary Rootlets of the Drosophila Sensory Organs

The Drosophila eye displays peculiar sensory organs of unknown function, the mechanosensory bristles, that are intercalated among the adjacent ommatidia. Like the other Drosophila sensory organs, the mechanosensory bristles consist of a bipolar neuron and two tandemly aligned centrioles, the distal of which nucleates the ciliary axoneme and represents the starting point of the ciliary rootlets. We report here that the centriole associated protein Sas-4 colocalizes with the short ciliary rootlets of the mechanosensory bristles and with the elongated rootlets of chordotonal and olfactory neurons. This finding suggests an unexpected cytoplasmic localization of Sas-4 protein and points to a new underscored role for this protein. Moreover, we observed that the sheath cells associated with the sensory neurons also display two tandemly aligned centrioles but lacks ciliary axonemes, suggesting that the dendrites of the sensory neurons are dispensable for the assembly of aligned centrioles and rootlets.

Notch signaling at the crossroads of innate and adaptive immunity

Notch signaling is an evolutionarily conserved cell-to-cell signaling pathway that regulates cellular differentiation and function across multiple tissue types and developmental stages. In this review, we discuss our current understanding of Notch signaling in mammalian innate and adaptive immunity. The importance of Notch signaling is pervasive throughout the immune system, as it elicits lineage and context-dependent effects in a wide repertoire of cells. Although regulation of binary cell fate decisions encompasses many of the functions first ascribed to Notch in the immune system, recent advances in the field have refined and expanded our view of the Notch pathway beyond this initial concept. From establishing T cell identity in the thymus to regulating mature T cell function in the periphery, the Notch pathway is an essential, recurring signal for the T cell lineage. Among B cells, Notch signaling is required for the development and maintenance of marginal zone B cells in the spleen. Emerging roles for Notch signaling in innate and innate-like lineages such as classical dendritic cells and innate lymphoid cells are likewise coming into view. Lastly, we speculate on the molecular underpinnings that shape the activity and versatility of the Notch pathway.

Mask, a component of the Hippo pathway, is required for Drosophila eye morphogenesis

Hippo signaling is an important regulator of tissue size, but it also has a lesser-known role in tissue morphogenesis. Here we use the Drosophila pupal eye to explore the role of the Hippo effector Yki and its cofactor Mask in morphogenesis. We found that Mask is required for the correct distribution and accumulation of adherens junctions and appropriate organization of the cytoskeleton. Accordingly, disrupting mask expression led to severe mis-patterning and similar defects were observed when yki was reduced or in response to ectopic wts. Further, the patterning defects generated by reducing mask expression were modified by Hippo pathway activity. RNA-sequencing revealed a requirement for Mask for appropriate expression of numerous genes during eye morphogenesis. These included genes implicated in cell adhesion and cytoskeletal organization, a comprehensive set of genes that promote cell survival, and numerous signal transduction genes. To validate our transcriptome analyses, we then considered two loci that were modified by Mask activity: FER and Vinc, which have established roles in regulating adhesion. Modulating the expression of either locus modified mask mis-patterning and adhesion phenotypes. Further, expression of FER and Vinc was modified by Yki. It is well-established that the Hippo pathway is responsive to changes in cell adhesion and the cytoskeleton, but our data indicate that Hippo signaling also regulates these structures.

Making sense out of missense mutations: Mechanistic dissection of Notch receptors through structure-function studies in Drosophila

Notch signaling is involved in the development of almost all organ systems and is required post-developmentally to modulate tissue homeostasis. Rare variants in Notch signaling pathway genes are found in patients with rare Mendelian disorders, while unique or recurrent somatic mutations in a similar set of genes are identified in cancer. The human genome contains four genes that encode Notch receptors, NOTCH1-4, all of which are linked to genetic diseases and cancer. Although some mutations have been classified as clear loss- or gain-of-function alleles based on cellular or rodent based assay systems, the functional consequence of many variants/mutations in human Notch receptors remain unknown. In this review, I will first provide an overview of the domain structure of Notch receptors and discuss how each module is known to regulate Notch signaling activity in vivo using the Drosophila Notch receptor as an example. Next, I will introduce some interesting mutant alleles that have been isolated in the fly Notch gene over the past > 100 years of research and discuss how studies of these mutations have facilitated the understanding of Notch biology. By identifying unique alleles of the fly Notch gene through forward genetic screens, mapping their molecular lesions and characterizing their phenotypes in depth, one can begin to unravel new mechanistic insights into how different domains of Notch fine-tune signaling output. Such information can be useful in deciphering the functional consequences of rare variants/mutations in human Notch receptors, which in turn can influence disease management and therapy.

Structural and Functional Dissection of the 5' Region of the Notch Gene in Drosophila melanogaster

Notch is a key factor of a signaling cascade which regulates cell differentiation in all multicellular organisms. Numerous investigations have been directed mainly at studying the mechanism of Notch protein action; however, very little is known about the regulation of activity of the gene itself. Here, we provide the results of targeted 5'-end editing of the Drosophila Notch gene in its native environment and genetic and cytological effects of these changes. Using the Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR associated protein 9 (CRISPR/Cas9) system in combination with homologous recombination, we obtained a founder fly stock in which a 4-kb fragment, including the 5' nontranscribed region, the first exon, and a part of the first intron of Notch, was replaced by an attachment Phage (attP) site. Then, fly lines carrying a set of six deletions within the 5'untranscribed region of the gene were obtained by ΦC31-mediated integration of transgenic constructs. Part of these deletions does not affect gene activity, but their combinations with transgenic construct in the first intron of the gene cause defects in the Notch target tissues. At the polytene chromosome level we defined a DNA segment (~250 bp) in the Notch5'-nontranscribed region which when deleted leads to disappearance of the 3C6/C7 interband and elimination of CTC-Factor (CTCF) and Chromator (CHRIZ) insulator proteins in this region.

Notch signaling coordinates ommatidial rotation in the Drosophila eye via transcriptional regulation of the EGF-Receptor ligand Argos

In all metazoans, a small number of evolutionarily conserved signaling pathways are reiteratively used during development to orchestrate critical patterning and morphogenetic processes. Among these, Notch (N) signaling is essential for most aspects of tissue patterning where it mediates the communication between adjacent cells to control cell fate specification. In Drosophila, Notch signaling is required for several features of eye development, including the R3/R4 cell fate choice and R7 specification. Here we show that hypomorphic alleles of Notch, belonging to the N^facet class, reveal a novel phenotype: while photoreceptor specification in the mutant ommatidia is largely normal, defects are observed in ommatidial rotation (OR), a planar cell polarity (PCP)-mediated cell motility process. We demonstrate that during OR Notch signaling is specifically required in the R4 photoreceptor to upregulate the transcription of argos (aos), an inhibitory ligand to the epidermal growth factor receptor (EGFR), to fine-tune the activity of EGFR signaling. Consistently, the loss-of-function defects of N^facet alleles and EGFR-signaling pathway mutants are largely indistinguishable. A Notch-regulated aos enhancer confers R4 specific expression arguing that aos is directly regulated by Notch signaling in this context via Su(H)-Mam-dependent transcription.

Optogenetic inhibition of Delta reveals digital Notch signalling output during tissue differentiation

Spatio-temporal regulation of signalling pathways plays a key role in generating diverse responses during the development of multicellular organisms. The role of signal dynamics in transferring signalling information in vivo is incompletely understood. Here, we employ genome engineering in Drosophila melanogaster to generate a functional optogenetic allele of the Notch ligand Delta (opto-Delta), which replaces both copies of the endogenous wild-type locus. Using clonal analysis, we show that optogenetic activation blocks Notch activation through cis-inhibition in signal-receiving cells. Signal perturbation in combination with quantitative analysis of a live transcriptional reporter of Notch pathway activity reveals differential tissue- and cell-scale regulatory modes. While at the tissue-level the duration of Notch signalling determines the probability with which a cellular response will occur, in individual cells Notch activation acts through a switch-like mechanism. Thus, time confers regulatory properties to Notch signalling that exhibit integrative digital behaviours during tissue differentiation.

Threshold response to stochasticity in morphogenesis

During development of biological organisms, multiple complex structures are formed. In many instances, these structures need to exhibit a high degree of order to be functional, although many of their constituents are intrinsically stochastic. Hence, it has been suggested that biological robustness ultimately must rely on complex gene regulatory networks and clean-up mechanisms. Here we explore developmental processes that have evolved inherent robustness against stochasticity. In the context of the Drosophila eye disc, multiple optical units, ommatidia, develop into crystal-like patterns. During the larva-to-pupa stage of metamorphosis, the centers of the ommatidia are specified initially through the diffusion of morphogens, followed by the specification of R8 cells. Establishing the R8 cell is crucial in setting up the geometric, and functional, relationships of cells within an ommatidium and among neighboring ommatidia. Here we study an PDE mathematical model of these spatio-temporal processes in the presence of parametric stochasticity, defining and applying measures that quantify order within the resulting spatial patterns. We observe a universal sigmoidal response to increasing transcriptional noise. Ordered patterns persist up to a threshold noise level in the model parameters. In accordance with prior qualitative observations, as the noise is further increased past a threshold point of no return, these ordered patterns rapidly become disordered. Such robustness in development allows for the accumulation of genetic variation without any observable changes in phenotype. We argue that the observed sigmoidal dependence introduces robustness allowing for sizable amounts of genetic variation and transcriptional noise to be tolerated in natural populations without resulting in phenotype variation.

Blue light induces a neuroprotective gene expression program in Drosophila photoreceptors

BACKGROUND

Light exposure induces oxidative stress, which contributes to ocular diseases of aging. Blue light provides a model for light-induced oxidative stress, lipid peroxidation and retinal degeneration in Drosophila melanogaster. In contrast to mature adults, which undergo retinal degeneration when exposed to prolonged blue light, newly-eclosed flies are resistant to blue light-induced retinal degeneration. Here, we sought to characterize the gene expression programs induced by blue light in flies of different ages to identify neuroprotective pathways utilized by photoreceptors to cope with light-induced oxidative stress.

RESULTS

To identify gene expression changes induced by blue light exposure, we profiled the nuclear transcriptome of Drosophila photoreceptors from one- and six-day-old flies exposed to blue light and compared these with dark controls. Flies were exposed to 3 h blue light, which increases levels of reactive oxygen species but does not cause retinal degeneration. We identified substantial gene expression changes in response to blue light only in six-day-old flies. In six-day-old flies, blue light induced a neuroprotective gene expression program that included upregulation of stress response pathways and downregulation of genes involved in light response, calcium influx and ion transport. An intact phototransduction pathway and calcium influx were required for upregulation, but not downregulation, of genes in response to blue light, suggesting that distinct pathways mediate the blue light-associated transcriptional response.

CONCLUSION

Our data demonstrate that under phototoxic conditions, Drosophila photoreceptors upregulate stress response pathways and simultaneously, downregulate expression of phototransduction components, ion transporters, and calcium channels. Together, this gene expression program both counteracts the calcium influx resulting from prolonged light exposure, and ameliorates the oxidative stress resulting from this calcium influx. Thus, six-day-old flies can withstand up to 3 h blue light exposure without undergoing retinal degeneration. Developmental transitions during the first week of adult Drosophila life lead to an altered gene expression program in photoreceptors that includes reduced expression of genes that maintain redox and calcium homeostasis, reducing the capacity of six-day-old flies to cope with longer periods (8 h) of light exposure. Together, these data provide insight into the neuroprotective gene regulatory mechanisms that enable photoreceptors to withstand light-induced oxidative stress.

Ribosomal protein L10A and signaling pathway

Ribosome: machinery in control of messenger RNA's (mRNAs) and several ribosomal proteins are in the small and large subunit of the ribosome. Various aspects of ribosomal proteins have related to cell growth, cell cycle, and diseases. Ribosomal protein L10A (RpL10A) in shrimp and fruit fly has been demonstrated to play a role in oogenesis. Interestingly, deletion RpL10A gene (RpL10Ab^-/-) germ line clone of the fruit fly showed a loss of follicle cells surrounding the egg chamber, but nurse cells appeared normal. This phenotype is reminiscent of insulin receptor mutants (InR^-/-). In contrast, over-expression of RpL10A in the eyes of the fruit flies resulted in abnormal ommatidia with a loss of red pigment in the center of the eyes. In this study, the abnormal rearrangements of nuclei and lack of cell membranes in those eyes were demonstrated. Furthermore, the expression of InR gene and the InR protein were extensively increased as determined by real-time PCR and immunohistochemistry, respectively. In addition, some insulin signaling mediators were also detected. The Akt and FOXO proteins were highly phosphorylated in the RpL10A over-expressed mutant. The results revealed that RpL10A induced over-expression of the insulin receptor and consequently activated in insulin signaling pathway which affects cell proliferation and we suggest RpL10A stimulates cell proliferation via the insulin signaling pathway.

The phosphoinositide phosphatase Sac1 regulates cell shape and microtubule stability in the developing Drosophila eye

Epithelial patterning in the developing Drosophila melanogaster eye requires the Neph1 homolog Roughest (Rst), an immunoglobulin family cell surface adhesion molecule expressed in interommatidial cells (IOCs). Here, using a novel temperature-sensitive (ts) allele, we show that the phosphoinositide phosphatase Sac1 is also required for IOC patterning. Sac1^ts mutants have rough eyes and retinal patterning defects that resemble rst mutants. Sac1^ts retinas exhibit elevated levels of phosphatidylinositol 4-phosphate (PI4P), consistent with the role of Sac1 as a PI4P phosphatase. Indeed, genetic rescue and interaction experiments reveal that restriction of PI4P levels by Sac1 is crucial for normal eye development. Rst is delivered to the cell surface in Sac1^ts mutants. However, Sac1^ts mutant IOCs exhibit severe defects in microtubule organization, associated with accumulation of Rst and the exocyst subunit Sec8 in enlarged intracellular vesicles upon cold fixation ex vivo Together, our data reveal a novel requirement for Sac1 in promoting microtubule stability and suggest that Rst trafficking occurs in a microtubule- and exocyst-dependent manner.

The mir-279/996 cluster represses receptor tyrosine kinase signaling to determine cell fates in the Drosophila eye

Photoreceptors in the crystalline Drosophila eye are recruited by receptor tyrosine kinase (RTK)/Ras signaling mediated by Epidermal growth factor receptor (EGFR) and the Sevenless (Sev) receptor. Analyses of an allelic deletion series of the mir-279/996 locus, along with a panel of modified genomic rescue transgenes, show that Drosophila eye patterning depends on both miRNAs. Transcriptional reporter and activity sensor transgenes reveal expression and function of miR-279/996 in non-neural cells of the developing eye. Moreover, mir-279/996 mutants exhibit substantial numbers of ectopic photoreceptors, particularly of R7, and cone cell loss. These miRNAs restrict RTK signaling in the eye, since mir-279/996 nulls are dominantly suppressed by positive components of the EGFR pathway and enhanced by heterozygosity for an EGFR repressor. miR-279/996 limit photoreceptor recruitment by targeting multiple positive RTK/Ras signaling components that promote photoreceptor/R7 specification. Strikingly, deletion of mir-279/996 sufficiently derepresses RTK/Ras signaling so as to rescue a population of R7 cells in R7-specific RTK null mutants boss and sev, which otherwise completely lack this cell fate. Altogether, we reveal a rare setting of developmental cell specification that involves substantial miRNA control.

The Drosophila junctophilin gene is functionally equivalent to its four mammalian counterparts and is a modifier of a Huntingtin poly-Q expansion and the Notch pathway

Members of the Junctophilin (JPH) protein family have emerged as key actors in all excitable cells, with crucial implications for human pathophysiology. In mammals, this family consists of four members (JPH1-JPH4) that are differentially expressed throughout excitable cells. The analysis of knockout mice lacking JPH subtypes has demonstrated their essential contribution to physiological functions in skeletal and cardiac muscles and in neurons. Moreover, mutations in the human JPH2 gene are associated with hypertrophic and dilated cardiomyopathies; mutations in JPH3 are responsible for the neurodegenerative Huntington's disease-like-2 (HDL2), whereas JPH1 acts as a genetic modifier in Charcot–Marie–Tooth 2K peripheral neuropathy. Drosophila melanogaster has a single junctophilin (jp) gene, as is the case in all invertebrates, which might retain equivalent functions of the four homologous JPH genes present in mammalian genomes. Therefore, owing to the lack of putatively redundant genes, a jpDrosophila model could provide an excellent platform to model the Junctophilin-related diseases, to discover the ancestral functions of the JPH proteins and to reveal new pathways. By up- and downregulation of Jp in a tissue-specific manner in Drosophila, we show that altering its levels of expression produces a phenotypic spectrum characterized by muscular deficits, dilated cardiomyopathy and neuronal alterations. Importantly, our study has demonstrated that Jp modifies the neuronal degeneration in a Drosophila model of Huntington's disease, and it has allowed us to uncover an unsuspected functional relationship with the Notch pathway. Therefore, this Drosophila model has revealed new aspects of Junctophilin function that can be relevant for the disease mechanisms of their human counterparts.

Summary: This work reveals that the Drosophila Junctophilin protein has similar functions to its mammalian homologues and uncovers new interactions of potential biomedical interest with Huntingtin and Notch signalling.

AKAP200 promotes Notch stability by protecting it from Cbl/lysosome-mediated degradation in Drosophila melanogaster

AKAP200 is a Drosophila melanogaster member of the “A Kinase Associated Protein” family of scaffolding proteins, known for their role in the spatial and temporal regulation of Protein Kinase A (PKA) in multiple signaling contexts. Here, we demonstrate an unexpected function of AKAP200 in promoting Notch protein stability. In Drosophila, AKAP200 loss-of-function (LOF) mutants show phenotypes that resemble Notch LOF defects, including eye patterning and sensory organ specification defects. Through genetic interactions, we demonstrate that AKAP200 interacts positively with Notch in both the eye and the thorax. We further show that AKAP200 is part of a physical complex with Notch. Biochemical studies reveal that AKAP200 stabilizes endogenous Notch protein, and that it limits ubiquitination of Notch. Specifically, our genetic and biochemical evidence indicates that AKAP200 protects Notch from the E3-ubiquitin ligase Cbl, which targets Notch to the lysosomal pathway. Indeed, we demonstrate that the effect of AKAP200 on Notch levels depends on the lysosome. Interestingly, this function of AKAP200 is fully independent of its role in PKA signaling and independent of its ability to bind PKA. Taken together, our data indicate that AKAP200 is a novel tissue specific posttranslational regulator of Notch, maintaining high Notch protein levels and thus promoting Notch signaling.

AKAP200 belongs to a family of scaffolding proteins best known for their regulation of PKA localization. In this study, we have identified a novel role of AKAP200 in Notch protein stability and signaling. In Drosophila melanogaster, AKAP200’s loss and gain-of-function (LOF/GOF) phenotypes are characteristic of Notch signaling defects. Furthermore, we demonstrated genetic interactions between AKAP200 and Notch. Consistent with this, AKAP200 stabilizes the endogenous Notch protein and limits its ubiquitination. AKAP200 exerts its effects on Notch by antagonizing Cbl-mediated ubiquitination and thus lysosome targeting of Notch. Based on these data, we postulate a novel PKA independent mechanism of AKAP200 to achieve optimal Notch protein levels, with AKAP200 preventing Cbl-mediated lysosomal degradation of Notch.

JNK is antagonized to ensure the correct number of interommatidial cells pattern the Drosophila retina

Apoptosis is crucial during the morphogenesis of most organs and tissues, and is utilized for tissues to achieve their proper size, shape and patterning. Many signaling pathways contribute to the precise regulation of apoptosis. Here we show that Jun N-terminal Kinase (JNK) activity contributes to the coordinated removal of interommatidial cells via apoptosis in the Drosophila pupal retina. This is consistent with previous findings that JNK activity promotes apoptosis in other epithelia. However, we found that JNK activity is repressed by Cindr (the CIN85 and CD2AP ortholog) in order to promote cell survival. Reducing the amount of Cindr resulted in ectopic cell death. Increased expression of the Drosophila JNK basket in the setting of reduced cindr expression was found to result in even more severe apoptosis, whilst ectopic death was found to be reduced if retinas were heterozygous for basket. Hence Cindr is required to properly restrict JNK-mediated apoptosis in the pupal eye, resulting in the correct number of interommatidial cells. A lack of precise control over developmental apoptosis can lead to improper tissue morphogenesis.

Analysis of transient hypermorphic activity of E(spl)D during R8 specification

Drosophila atonal (ato) is required for the specification of founding R8 photoreceptors during retinal development. ato is regulated via dual eye-specific enhancers; ato-3’ is subject to initial induction whereas 5’-ato facilitates Notch-mediated autoregulation. Notch is further utilized to induce bHLH repressors of the E(spl) locus to restrict Ato from its initial broad expression to individual cells. Although Notch operates in two, distinct phases, it has remained unclear how the two phases maintain independence from one another. The difference in these two phases has attributed to the hypothesized delayed expression of E(spl). However, immunofluorescence data indicate that E(spl) are expressed during early Ato patterning, suggesting a more sophisticated underlying mechanism. To probe this mechanism, we provide evidence that although E(spl) exert no influence on ato-3’, E(spl) repress 5’-ato and deletion of the E(spl) locus elicits precocious 5’-ato activity. Thus, E(spl) imposes a delay to the timing in which Ato initiates autoregulation. We next sought to understand this finding in the context of E(spl)^D, which encodes a dysregulated variant of E(spl)M8 that perturbs R8 patterning, though, as previously reported, only in conjunction with the mutant receptor N^spl. We established a genetic interaction between E(spl)^D and roughened eye (roe), a known modulator of Notch signaling in retinogenesis. This link further suggests a dosage-dependence between E(spl) and the proneural activators Ato and Sens, as indicated via interaction assays in which E(spl)^D renders aberrant R8 patterning in conjunction with reduced proneural dosage. In total, the biphasicity of Notch signaling relies, to some degree, on the post-translational regulation of individual E(spl) members and, importantly, that post-translational regulation is likely necessary to modulate the level of E(spl) activity throughout the progression of Ato expression.

The Regulation of Notch Signaling in Retinal Development and Regeneration

Purpose of review

Notch signaling is an important component of retinal progenitor cell maintenance and MG specification during development, and its manipulation may be critical for allowing MG to re-enter the cell cycle and regenerate neurons in adults. In mammals, MG respond to retinal injury by undergoing a gliotic response rather than a regenerative one. Understanding the complexities of Notch signaling may allow for strategies that enhance regeneration over gliosis.

Recent findings

Notch signaling is regulated at multiple levels, and is interdependent with various other signaling pathways in both the receptor and ligand expressing cells. The precise spatial and temporal patterning of Notch components is necessary for proper retinal development. Regenerative species undergo a dynamic regulation of Notch signaling in MG upon injury, whereas non-regenerative species fail to productively regulate Notch.

Summary

Notch signaling is malleable, such that the altered composition of growth and transcription factors in the developing and mature retinas result in different Notch mediated responses. Successful regeneration will require the manipulation of the retinal environment to foster a dynamic rather than static regulation of Notch signaling in concert with other reprogramming and differentiation factors.

The Notch pathway regulates the Second Mitotic Wave cell cycle independently of bHLH proteins

Notch regulates both neurogenesis and cell cycle activity to coordinate precursor cell generation in the differentiating Drosophila eye. Mosaic analysis with mitotic clones mutant for Notch components was used to identify the pathway of Notch signaling that regulates the cell cycle in the Second Mitotic Wave. Although S phase entry depends on Notch signaling and on the transcription factor Su(H), the transcriptional co-activator Mam and the bHLH repressor genes of the E(spl)-Complex were not essential, although these are Su(H) coactivators and targets during the regulation of neurogenesis. The Second Mitotic Wave showed little dependence on ubiquitin ligases neuralized or mindbomb, and although the ligand Delta is required non-autonomously, partial cell cycle activity occurred in the absence of known Notch ligands. We found that myc was not essential for the Second Mitotic Wave. The Second Mitotic Wave did not require the HLH protein Extra macrochaetae, and the bHLH protein Daughterless was required only cell-nonautonomously. Similar cell cycle phenotypes for Daughterless and Atonal were consistent with requirement for neuronal differentiation to stimulate Delta expression, affecting Notch activity in the Second Mitotic Wave indirectly. Therefore Notch signaling acts to regulate the Second Mitotic Wave without activating bHLH gene targets.

Patterning the eye: A role for the cell cycle?

Although highly regulated cell cycle behavior accompanies specification of cell types in the Drosophila retina, no evidence has previously existed that cell cycle phase influences cell fate choice. Meserve and Duronio have now used genetic techniques to trace the fate of a sub-population of cells that accumulate in G2-phase of the cell cycle, discovering that they contribute to a particular fate, the precursor of the sensory inter-ommatidial bristles. Meserve and Duronio further show that G2 cells have an advantage acquiring inter-ommatidial bristle fate. This is the first evidence for a functional contribution of cell cycle phase to cell fate determination in the Drosophila eye and indicates that the fate of one population of retinal cells is established much earlier than previously recognized, during the larval stages when cell-cell interactions influence cell cycle phase. Inter-ommatidial bristle fate had been thought to arise much later, in the pupal stage, and models for how these neuronal eye structures arise will now have to be revised.

Molecular Chaperone Hsp70 and Its Constitutively Active Form Hsc70 Play an Indispensable Role During Eye Development of Drosophila melanogaster

In the present study, we demonstrate that molecular chaperone Hsp70 and Hsc70 is essential for normal organization and development of ommatidial cells in Drosophila melanogaster eye. An exogenously expressed dominant negative mutant of Hsp70 (K71E) and Hsc70.4 (K71S and D206S) in an eye-specific manner resulted in eye degeneration that includes loss of eye pigment, disorganized ommatidia, abnormality in bristle cell arrangement and reduction in the eye size. The developmental organization of ommatidial cells (cone, photoreceptor, pigment, and bristle cell complex) was disturbed in Hsp70 and Hsc70 mutants. Acridine orange (AO) and caspase 3 staining showed an increased cell death in Hsp70 and Hsc70 mutant eyes. Genetic interaction study of Hsp70 and Hsc70 mutants with candidate genes of JNK signaling pathway and immunocytochemistry study using phospho-JNK antibody suggested that mutation in Hsp70 and Hsc70 results in ectopic activation of JNK signaling in fly eye. Further, anti-PH3 staining in Hsp70 and Hsc70 mutant eyes revealed a reduced number of mitotic cells in second mitotic wave (SMW) of developing eye and anti-Rh1 staining showed reduced Rh1 expression, accumulation of Rh1 in the cytoplasm, and rhabdomere degeneration. Thus, on the basis of results, it was concluded that molecular chaperone Hsp70 and Hsc70 play an indispensable role during Drosophila eye development.

Eye development and photoreceptor differentiation in the cephalopod Doryteuthis pealeii

Photoreception is a ubiquitous sensory ability found across the Metazoa, and photoreceptive organs are intricate and diverse in their structure. Although the morphology of the compound eye in Drosophila and the single-chambered eye in vertebrates have elaborated independently, the amount of conservation within the 'eye' gene regulatory network remains controversial, with few taxa studied. To better understand the evolution of photoreceptive organs, we established the cephalopod Doryteuthis pealeii as a lophotrochozoan model for eye development. Utilizing histological, transcriptomic and molecular assays, we characterize eye formation in Doryteuthis pealeii Through lineage tracing and gene expression analyses, we demonstrate that cells expressing Pax and Six genes incorporate into the lens, cornea and iris, and the eye placode is the sole source of retinal tissue. Functional assays demonstrate that Notch signaling is required for photoreceptor cell differentiation and retinal organization. This comparative approach places the canon of eye research in traditional models into perspective, highlighting complexity as a result of both conserved and convergent mechanisms.

Regulatory mechanisms of EGFR signalling during Drosophila eye development

EGFR signalling is a well-conserved signalling pathway playing major roles during development and cancers. This review explores what studying the EGFR pathway during Drosophila eye development has taught us in terms of the diversity of its regulatory mechanisms. This model system has allowed the identification of numerous positive and negative regulators acting at specific time and place, thus participating to the tight control of signalling. EGFR signalling regulation is achieved by a variety of mechanisms, including the control of ligand processing, the availability of the receptor itself and the transduction of the cascade in the cytoplasm. Ultimately, the transcriptional responses contribute to the establishment of positive and negative feedback loops. The combination of these multiple mechanisms employed to regulate the EGFR pathway leads to specific cellular outcomes involved in functions as diverse as the acquisition of cell fate, proliferation, survival, adherens junction remodelling and morphogenesis.

Periodic patterning of the Drosophila eye is stabilized by the diffusible activator Scabrous

Generation of periodic patterns is fundamental to the differentiation of multiple tissues during development. How such patterns form robustly is still unclear. The Drosophila eye comprises ∼750 units, whose crystalline order is set during differentiation of the eye imaginal disc: an activation wave sweeping across the disc is coupled to lateral inhibition, sequentially selecting pro-neural cells. Using mathematical modelling, here we show that this template-based lateral inhibition is highly sensitive to spatial variations in biochemical parameters and cell sizes. We reveal the basis of this sensitivity, and suggest that it can be overcome by assuming a short-range diffusible activator. Clonal experiments identify Scabrous, a previously implicated inhibitor, as the predicted activator. Our results reveal the mechanism by which periodic patterning in the fly eye is stabilized against spatial variations, highlighting how the need to maintain robustness shapes the design of patterning circuits.

Patterning in the Drosophila eye is achieved by a series of signalling cascades over several cell distances. Here Gavish et al. model lateral inhibition in the developing eye to understand how developmental noise refines such patterning, identifying a novel activator required for buffering spatial variability.

ato-Gal4 fly lines for gene function analysis: Eya is required in late progenitors for eye morphogenesis

The Gal4/UAS system is one of the most powerful tools for the study of cellular and developmental processes in Drosophila. Gal4 drivers can be used to induce targeted expression of dominant-negative and dominant-active proteins, histological markers, activity sensors, gene-specific dsRNAs, modulators of cell survival or proliferation, and other reagents. Here, we describe novel atonal-Gal4 lines that contain regions of the regulatory DNA of atonal, the proneural gene for photoreceptors, stretch receptors, auditory organ, and some olfactory sensilla. During neurogenesis, the atonal gene is expressed at a critical juncture, a time of transition from progenitor cell to developing neuron. Thus, these lines are particularly well suited for the study of the transcription factors and signaling molecules orchestrating this critical transition. To demonstrate their usefulness, we focus on two visual organs, the eye and the Bolwig. We demonstrate the induction of predicted eye phenotypes when expressing the dominant-negative EGF receptor or a dsRNA against Notch in the developing eye disc. In another example, we show the deletion of the Bolwig's organ using the proapoptotic factor Hid. Finally, we investigate the function of the eye specification factor Eyes absent or Eya in late retinal progenitors, shortly before they begin morphogenesis. We show that Eya is still required in these late progenitors to promote eye formation, and show failure to induce the target gene atonal and consequent lack of neuron formation.

Acute retinal injury and the relationship between nerve growth factor, Notch1 transcription and short-lived dedifferentiation transient changes of mammalian Müller cells

Our aim is to define related molecular events on how dormant Müller glia cells re-enter the cell cycle, proliferate and produce new retinal neurons from initial injury to glial scar formation. Sodium iodate (NaIO3) was used to induce acute retinal injury. Long-Evans rats were administered with NaIO3 or phosphate-buffered saline by intraperitoneal injection. The proliferation, dedifferentiation and neurogenesis of Müller cells were analyzed by double-labeled fluorescence immunohistochemistry with primary antibodies - against Müller cells and specific cell markers. Possible molecules that limit the regenerative potential of Müller cells were also determined by immunofluorescence staining, quantitative RT-PCR, protein array, ELISA and Western blot. In the first 3-7days after NaIO3 administration, Müller cells were activated and underwent a fate switch, including transient proliferation, dedifferentiation and neurogenesis. Nerve growth factor (NGF) signaling concomitantly increased with the downregulation of p27(Kip1) in Müller cells, which may promote Müller cells to re-enter the cell cycle. The transient increase of NGF signaling and the transient decrease of Notch signaling inhibited Hes1, which might enhance the neuronal differentiation of dedifferentiated Müller cells and suppress gliosis. Upregulated Notch and decreased NGF expressions limit dedifferentiation and neurogenesis, but induces retinal Müller cell gliosis at a later stage. We conclude that transient NGF upregulation and Notch1 downregulation may activate the transient proliferation, dedifferentiation and neurogenesis of Müller cells during NaIO3-induced acute retinal injury; which could be a therapeutic target for overcoming Müller cell gliosis. Such therapy could be potentially used for treating retinal-related diseases.

Drosophila PS2 and PS3 integrins play distinct roles in retinal photoreceptors-glia interactions

Cellular migration and differentiation are important developmental processes that require dynamic cellular adhesion. Integrins are heterodimeric transmembrane receptors that play key roles in adhesion plasticity. Here, we explore the developing visual system of Drosophila to study the roles of integrin heterodimers in glia development. Our data show that αPS2 is essential for retinal glia migration from the brain into the eye disc and that glial cells have a role in the maintenance of the fenestrated membrane (Laminin-rich ECM layer) in the disc. Interestingly, the absence of glial cells in the eye disc did not affect the targeting of retinal axons to the optic stalk. In contrast, αPS3 is not required for retinal glia migration, but together with Talin, it functions in glial cells to allow photoreceptor axons to target the optic stalk. Thus, we present evidence that αPS2 and αPS3 integrin have different and specific functions in the development of retinal glia.

Cell death in development: Signaling pathways and core mechanisms

Programmed cell death eliminates unneeded and dangerous cells in a timely and effective manner during development. In this review, we examine the role cell death plays during development in worms, flies and mammals. We discuss signaling pathways that regulate developmental cell death, and describe how they communicate with the core cell death pathways. In most organisms, the majority of developmental cell death is seen in the nervous system. Therefore we focus on what is known about the regulation of developmental cell death in this tissue. Understanding how the cell death is regulated during development may provide insight into how this process can be manipulated in the treatment of disease.

Polycomb group genes are required to maintain a binary fate choice in the Drosophila eye

Background

Identifying the mechanisms by which cells remain irreversibly committed to their fates is a critical step toward understanding and being able to manipulate development and homeostasis. Polycomb group (PcG) proteins are chromatin modifiers that maintain transcriptional silencing, and loss of PcG genes causes widespread derepression of many developmentally important genes. However, because of their broad effects, the degree to which PcG proteins are used at specific fate choice points has not been tested. To understand how fate choices are maintained, we have been analyzing R7 photoreceptor neuron development in the fly eye. R1, R6, and R7 neurons are recruited from a pool of equivalent precursors. In order to adopt the R7 fate, these precursors make three binary choices. They: (1) adopt a neuronal fate, as a consequence of high receptor tyrosine kinase (RTK) activity (they would otherwise become non-neuronal support cells); (2) fail to express Seven-up (Svp), as a consequence of Notch (N) activation (they would otherwise express Svp and become R1/R6 neurons); and (3) fail to express Senseless (Sens), as a parallel consequence of N activation (they would otherwise express Sens and become R8 neurons in the absence of Svp). We were able to remove PcG genes specifically from post-mitotic R1/R6/R7 precursors, allowing us to probe these genes' roles in the three binary fate choices that R1/R6/R7 precursors face when differentiating as R7s.

Results

Here, we show that loss of the PcG genes Sce, Scm, or Pc specifically affects one of the three binary fate choices that R7 precursors must make: mutant R7s derepress Sens and adopt R8 fate characteristics. We find that this fate transformation occurs independently of the PcG genes' canonical role in repressing Hox genes. While N initially establishes Sens repression in R7s, we show that N is not required to keep Sens off, nor do these PcG genes act downstream of N. Instead, the PcG genes act independently of N to maintain Sens repression in R1/R6/R7 precursors that adopt the R7 fate.

Conclusions

We conclude that cells can use PcG genes specifically to maintain a subset of their binary fate choices.

Live-imaging of the Drosophila pupal eye

Inherent processes of Drosophila pupal development can shift and distort the eye epithelium in ways that make individual cell behavior difficult to track during live cell imaging. These processes include: retinal rotation, cell growth and organismal movement. Additionally, irregularities in the topology of the epithelium, including subtle bumps and folds often introduced as the pupa is prepared for imaging, make it challenging to acquire in-focus images of more than a few ommatidia in a single focal plane. The workflow outlined here remedies these issues, allowing easy analysis of cellular processes during Drosophila pupal eye development. Appropriately-staged pupae are arranged in an imaging rig that can be easily assembled in most laboratories. Ubiquitin-DE-Cadherin:GFP and GMR-GAL4-driven UAS-α-catenin:GFP are used to visualize cell boundaries in the eye epithelium (1-3). After deconvolution is applied to fluorescent images captured at multiple focal planes, maximum projection images are generated for each time point and enhanced using image editing software. Alignment algorithms are used to quickly stabilize superfluous motion, making individual cell behavior easier to track.

Cell adhesion in the assembly of the Drosophila eye

Differential adhesion provides a mechanical force to drive cells into stable configurations during the assembly of tissues and organs. This is well illustrated in the Drosophila eye where differential adhesion plays a role in sequential recruitment of all support cells. Cell adhesion, on the other hand, is linked to the cytoskeleton and subject to regulation by cell signaling. The integration of cell adhesion with the cytoskeleton and cell signaling may provide a more thorough explanation for the diversity of forms and shapes seen in tissues and organs.

Bar represses dPax2 and decapentaplegic to regulate cell fate and morphogenetic cell death in Drosophila eye

The coordinated regulation of cell fate and cell survival is crucial for normal pattern formation in developing organisms. In Drosophila compound eye development, crystalline arrays of hexagonal ommatidia are established by precise assembly of diverse cell types, including the photoreceptor cells, cone cells and interommatidial (IOM) pigment cells. The molecular basis for controlling the number of cone and IOM pigment cells during ommatidial pattern formation is not well understood. Here we present evidence that BarH1 and BarH2 homeobox genes are essential for eye patterning by inhibiting excess cone cell differentiation and promoting programmed death of IOM cells. Specifically, we show that loss of Bar from the undifferentiated retinal precursor cells leads to ectopic expression of Prospero and dPax2, two transcription factors essential for cone cell specification, resulting in excess cone cell differentiation. We also show that loss of Bar causes ectopic expression of the TGFβ homolog Decapentaplegic (Dpp) posterior to the morphogenetic furrow in the larval eye imaginal disc. The ectopic Dpp expression is not responsible for the formation of excess cone cells in Bar loss-of-function mutant eyes. Instead, it causes reduction in IOM cell death in the pupal stage by antagonizing the function of pro-apoptotic gene reaper. Taken together, this study suggests a novel regulatory mechanism in the control of developmental cell death in which the repression of Dpp by Bar in larval eye disc is essential for IOM cell death in pupal retina.

Notch controls cell adhesion in the Drosophila eye

Sporadic evidence suggests Notch is involved in cell adhesion. However, the underlying mechanism is unknown. Here I have investigated an epithelial remodeling process in the Drosophila eye in which two primary pigment cells (PPCs) with a characteristic ‘kidney’ shape enwrap and eventually isolate a group of cone cells from inter-ommatidial cells (IOCs). This paper shows that in the developing Drosophila eye the ligand Delta was transcribed in cone cells and Notch was activated in the adjacent PPC precursors. In the absence of Notch, emerging PPCs failed to enwrap cone cells, and hibris (hbs) and sns, two genes coding for adhesion molecules of the Nephrin group that mediate preferential adhesion, were not transcribed in PPC precursors. Conversely, activation of Notch in single IOCs led to ectopic expression of hbs and sns. By contrast, in a single IOC that normally transcribes rst, a gene coding for an adhesion molecule of the Neph1 group that binds Hbs and Sns, activation of Notch led to a loss of rst transcription. In addition, in a Notch mutant where two emerging PPCs failed to enwrap cone cells, expression of hbs in PPC precursors restored the ability of these cells to surround cone cells. Further, expression of hbs or rst in a single rst- or hbs-expressing cell, respectively, led to removal of the counterpart from the membrane within the same cell through cis-interaction and forced expression of Rst in all hbs-expressing PPCs strongly disrupted the remodeling process. Finally, a loss of both hbs and sns in single PPC precursors led to constriction of the apical surface that compromised the ‘kidney’ shape of PPCs. Taken together, these results indicate that cone cells utilize Notch signaling to instruct neighboring PPC precursors to surround them and Notch controls the remodeling process by differentially regulating four adhesion genes.

In developing tissues, one way to isolate a group of cells from the rest of the tissue is to induce a few neighboring cells to surround them. How centrally localized cells communicate with neighboring cells and how neighboring cells respond to signaling is not well understood. This work describes a mechanism underlying an epithelial remodeling process in the Drosophila eye in which two primary pigment cells (PPCs) with a characteristic ‘kidney’ shape enwrap and isolate a group of cone cells from inter-ommatidial cells (IOCs). This paper shows that cone cells utilize Notch signaling to communicate with neighboring PPC precursors. In response to Notch signaling, PPC precursors activate transcription of hbs and sns, two genes coding for adhesion molecules of the Nephrin group that bind Rst and Kirre, adhesion molecules of the Neph1 group. At the same time, PPC precursors inactivate transcription of rst and kirre genes. In addition, binding of Hbs or Rst to its counterpart from the same cell (cis-interaction) destabilizes the protein complex and promotes removal of the counterparts from the membrane, leading to complementary distribution of four adhesion molecules within two populations of cells. Thus, Notch controls epithelial remodeling by differentially regulating four adhesion genes.