Membrane trafficking in morphogenesis and planar polarity

Our understanding of how membrane trafficking pathways function to direct morphogenetic movements and the planar polarization of developing tissues is a new and emerging field. While a central focus of developmental biology has been on how protein asymmetries and cytoskeletal force generation direct cell shaping, the role of membrane trafficking in these processes has been less clear. Here, we review recent advances in Drosophila and vertebrate systems in our understanding of how trafficking events are coordinated with planar cytoskeletal function to drive lasting changes in cell and tissue topologies. We additionally explore the function of trafficking pathways in guiding the complex interactions that initiate and maintain core PCP (planar cell polarity) asymmetries and drive the generation of systematically oriented cellular projections during development.      

Planar polarity and short-range polarization in Drosophila embryos

Planar cell polarity is a common and probably universal feature of epithelial cells throughout their life. It is not only visible in the external parts of adult animals and plants, but also present in newborn cells such as in the primary Drosophila epithelium. It controls not only cell shape and differentiation, but also cell motility, cell shape changes and it directs how animals are shaped. In this review, we report how planar cell polarity arises in Drosophila embryos and thereby illustrate how general and extensive planar polarity is during development, from the very beginning to the end. We present the main features of planar cell polarization in Drosophila embryos, in particular the fact that it occurs over a short range of just a few cell diameters, and within a very short time window. We contrast these with other systems, such as the adult Drosophila wing where planar cell polarity occurs at longer range.     

Cell-cell signalling,microtubule organization and RNA localization: Is PKA a link?

Specification of the anterior-posterior axis of the Drosophila embryo is brought about by the asymmetric localization of specific maternally expressed RNAs and proteins within the oocyte. While many of these localized molecules have been identified and progress has been made towards understanding their functions, how the localization process is instigated remains unclear. A recent paper reports that protein kinase A (PKA) activity is essential for many of these RNA localizations and for the correct polarization of the microtubule cytoskeleton⁽¹⁾. These and other results support a model for anterior-posterior axis establishment which involves intercellular signalling between the oocyte and certain neighbouring somatic cells.     

Planar Cell Polarity: Keeping Hairs Straight Is Not So Simple

The fur on a cat''s back, the scales on a fish, or the bristles on a fly are all beautifully organized, with a high degree of polarization in their surface organization. Great progress has been made in understanding how individual cell polarity is established, but our understanding of how cells coordinate their polarity in forming coherent tissues is still fragmentary. The organization of cells in the plane of the epithelium is known as planar cell polarity (PCP), and studies in the past decade have delineated a genetic pathway for the control of PCP. This review will first briefly review data from the Drosophila field, where PCP was first identified and genetically characterized, and then explore how vertebrate tissues become polarized during development.     

Drosophila melanogaster as a model system for the study of the function of calcium/calmodulin-dependent protein kinase II in synaptic plasticity

Drosophila melanogaster has been used as a biological model system for almost a century. In the last several decades,Drosophila has been used as a system to probe the molecular basis of behavior and discoveries in the fly have been at the forefront of the elucidation of important basic mechanisms. This review will outline the variety of approaches that makeDrosophila an excellent model system with which to study the function of the enzyme calcium/calmodulin-dependent protein kinase II (CaMKII) in synaptic plasticity. CaMKII has a well documented role in behavior and synaptic plasticity in both vertebrates and invertebrates. The behavioral and genetic richness ofDrosophila allow for a multi-level approach to understanding the physiological roles of this enzyme's function.     

The apical/basal-polarity determinant Scribble cooperates with the PCP core factor Stbm/Vang and functions as one of its effectors

Most tissues display several features of cellular polarization. Besides the ubiquitous epithelial polarization in the Apical–Basal (A/B) axis, many epithelia (and associated organs) display a Planar Cell Polarization (PCP). Recently, a crosstalk between the PCP and A/B polarity determinants has been suggested, i.e. the activity or stability of the PCP factor Frizzled is regulated by the A/B determinants aPKC and Bazooka in the Drosophila eye. We have systematically investigated genetic and physical interactions between the Drosophila A/B factors and the core PCP component Strabismus (Stbm)/Van Gogh (Vang). The A/B determinant Scribble was found to interact both genetically and physically with Stbm/Vang. We demonstrate that Scribble binds Stbm/Vang through its PDZ domain 3 and that it cooperates with Stbm/Vang in PCP establishment. Our data indicate that Scribble, in addition to its role in A/B polarity, has a distinct requirement in PCP establishment in the Drosophila eye and wing. We define a scribble allele that is largely PCP specific. Our data show that Scribble is part of the Stbm/Vang PCP complex and further suggest that it might act as an effector of Stbm/Vang during PCP establishment.     

Epithelial differentiation in Drosophila

Our understanding of epithelial development in Drosophila has been greatly improved in recent years. Two key regulators of epithelial polarity, Crumbs and DE-cadherin, have been studied at the genetic and molecular levels and a number of additional genes are being analyzed that contribute to the differentiation of epithelial cell structure. Epithelial architecture has a profound influence on morphogenetic movements, patterning and cell-type determination. The combination of embryological and genetic/molecular tools in Drosophila will help us to elucidate the complex events that determine epithelial cell structure and how they relate to morphogenesis and other developmental processes.     

Recent advances in using Drosophila to model neurodegenerative diseases

Neurodegenerative diseases are progressive disorders of the nervous system that affect the function and maintenance of specific neuronal populations. Most disease cases are sporadic with no known cause. The identification of genes associated with familial cases of these diseases has enabled the development of animal models to study disease mechanisms. The model organism Drosophila has been successfully used to study pathogenic mechanisms of a wide range of neurodegenerative diseases. Recent genetic studies in the Drosophila models have provided new insights into disease mechanisms, emphasizing the roles played by mitochondrial dynamics, RNA (including miRNA) function, protein translation, and synaptic plasticity and differentiation. It is anticipated that Drosophila models will further our understanding of mechanisms of neurodegeneration and facilitate the development of novel and rational treatments for these debilitating neurodegenerative diseases.     

Rho-kinase regulates tissue morphogenesis via non-muscle myosin and LIM-kinase during Drosophila development

Background  

The Rho-kinases (ROCKs) are major effector targets of the activated Rho GTPase that have been implicated in many of the Rho-mediated effects on cell shape and movement via their ability to affect acto-myosin contractility. The role of ROCKs in cell shape change and motility suggests a potentially important role for Rho-ROCK signaling in tissue morphogenesis during development. Indeed, in Drosophila, a single ROCK ortholog, DRok, has been identified and has been found to be required for establishing planar cell polarity.     

How do the Fat–Dachsous and core planar polarity pathways act together and independently to coordinate polarized cell behaviours?

Planar polarity describes the coordinated polarization of cells within the plane of a tissue. This is controlled by two main pathways in Drosophila: the Frizzled-dependent core planar polarity pathway and the Fat–Dachsous pathway. Components of both of these pathways become asymmetrically localized within cells in response to long-range upstream cues, and form intercellular complexes that link polarity between neighbouring cells. This review examines if and when the two pathways are coupled, focusing on the Drosophila wing, eye and abdomen. There is strong evidence that the pathways are molecularly coupled in tissues that express a specific isoform of the core protein Prickle, namely Spiny-legs. However, in other contexts, the linkages between the pathways are indirect. We discuss how the two pathways act together and independently to mediate a diverse range of effects on polarization of cell structures and behaviours.     

Phototransduction in Drosophila

The Drosophila visual transduction is the fastest known G protein-coupled signaling cascade and has been served as a model for understanding the molecular mechanisms of other G protein-coupled signaling cascades. Numbers of components in visual transduction machinery have been identified. Based on the functional characterization of these genes, a model for Drosophila phototransduction has been outlined, including rhodopsin activation, phosphoinoside signaling, and the opening of TRP and TRPL channels. Recently, the characterization of mutants, showing slow termination, revealed the physiological significance and the mechanism of rapid termination of light response.     

Sialylation in protostomes: a perspective from <Emphasis Type="Italic">Drosophila</Emphasis> genetics and biochemistry

Numerous studies have revealed important functions for sialylation in both prokaryotes and higher animals. However, the genetic and biochemical potential for sialylation in Drosophila has only been confirmed recently. Recent studies suggest significant similarities between the sialylation pathways of vertebrates and insects and provide evidence for their common evolutionary origin. These new data support the hypothesis that sialylation in insects is a specialized and developmentally regulated process which likely plays a prominent role in the nervous system. Yet several key issues remain to be addressed in Drosophila, including the initiation of sialic acid de novo biosynthesis and understanding the structure and function of sialylated glycoconjugates. This review discusses our current knowledge of the Drosophila sialylation pathway, as compared to the pathway in bacteria and vertebrates. We arrive at the conclusion that Drosophila is emerging as a useful model organism that is poised to shed new light on the function of sialylation not only in protostomes, but also in a larger evolutionary context. K. Koles and E. Repnikova contributed equally to this work.     

Drosophila learning and memory: Recent progress and new approaches

The processes of learning and memory have traditionally been studied in large experimental organisms (Aplysia, mice, rats and humans), where well-characterized behaviors are easily tested. Although Drosophila is one of the most experimentally tractable organisms, it has only recently joined the others as a model organism for learning and memory. Drosophila behavior has been studied for over 20 years; however, most of the work in the learning and memory field has focused on initial learning, because establishing memory in Drosophila has not been as straightforward as in other organisms. A major recent advance in this field has been the development of a training protocol that induces long-term memory in flies. This made possible experiments that implicated the Drosophila CREB gene as a critical component in the consolidation of long-term memory, and paves the way for future experiments utilizing the well developed tools in Drosophila. This review will briefly summarize what is known in the field of Drosophila learning and memory to date, and discuss why the unique aspects of this field make traditional approaches difficult and reward the use of alternative paths of experimentation.     

1.688 g/cm3 satellite‐related repeats: a missing link to dosage compensation and speciation

Despite the important progress that has been made on dosage compensation (DC), a critical link in our understanding of the X chromosome recognition mechanisms is still missing. Recent studies in Drosophila indicate that the missing link could be a family of DNA repeats populating the euchromatin of the X chromosome. In this opinion article, I discuss how these findings add a new fresh twist on the DC problem. In the following sections, I first summarize our understanding of DC in Drosophila and integrate these recent discoveries into our knowledge of the X chromosome recognition problem. Next, I introduce a model according to which, 1.688 g/cm³ satellite‐related (SR) repeats would be the primary recognition elements for the dosage compensation complex. Contrary to the current belief, I suggest that the DC system in Drosophila is not conserved and static, but it is continuously co‐evolving with the target SR repeats. The potential role of the SR repeats in hybrid incompatibilities and speciation is also discussed.     

The mechanism of telomere protection: a comparison between Drosophila and humans

Drosophila telomeres are maintained by transposition of specialized retrotransposons rather than by telomerase activity, and their stability is independent of the sequence of DNA termini. Recent studies have identified several proteins that protect Drosophila telomeres from fusion events. These proteins include the telomere capping factors HP1/ORC-associated protein (HOAP) and heterochromatin protein 1 (HP1), the Rad50 and Mre11 DNA repair proteins that are required for HOAP and HP1 localization at telomeres, and the ATM kinase. Another telomere-protecting factor identified in Drosophila is UbcD1, a polypeptide highly homologous to class I ubiquitin-conjugating E2 enzymes. In addition, it has been shown that HP1 and both components of the Drosophila Ku70/80 heterodimer act as negative regulators of telomere length. Except for HOAP, all these proteins are conserved in humans and are associated with human telomeres. Collectively, these results indicate that Drosophila is an excellent model system for the analysis of the mechanisms of telomere maintenance. In past and current studies, 15 Drosophila genes have been identified that prevent telomeric fusion, and it has been estimated that the Drosophila genome contains at least 40 genes required for telomere protection. We believe that the molecular characterization of these genes will lead to identification of many novel human genes with roles in telomere maintenance.     

Progress and challenges in understanding planar cell polarity signaling

During development, epithelial cells in some tissues acquire a polarity orthogonal to their apical–basal axis. This polarity, referred to as planar cell polarity (PCP), or tissue polarity, is essential for the normal physiological function of many epithelia. Early studies of PCP focused on insect epithelia (Lawrence, 1966 [1]), and the earliest genetic analyses were carried out in Drosophila (Held et al., 1986; Gubb and Garcia-Bellido, 1982 [2,3]). Indeed, most of our mechanistic understanding of PCP derives from the ongoing use of Drosophila as a model system. However, a range of medically important developmental defects and physiological processes are under the control of PCP mechanisms that appear to be at least partially conserved, driving considerable interest in studying PCP both in Drosophila and in vertebrate model systems. Here, I present a model of the PCP signaling mechanism based on studies in Drosophila. I highlight two areas in which our understanding is deficient, and which lead to current confusion in the literature. Future studies that shed light on these areas will substantially enhance our understanding of the fascinating yet challenging problem of understanding the mechanisms that generate PCP.     

Symbiotic bacteria attenuate Drosophila oviposition repellence to alkaline through acidification

Metazoans harbor a wealth of symbionts that are ever-changing the environment by taking up resources and/or excreting metabolites.One such common environmental modification is a change in pH.Conventional wisdom holds that symbionts facilitate the survival and production of their hosts in the wild,but this notion lacks empirical evidence.Here,we report that symbiotic bacteria in the genus Enterococcus attenuate the oviposition avoidance of alkaline environments in Drosophila.We studied the effects of alkalinity on oviposition preference for the first time,and found that flies are robustly disinclined to oviposit on alkali-containing substrates.This innate repulsion to alkaline environments is explained,in part,by the fact that alkalinity compromises the health and lifespan of both offspring and parent Drosophila.Enterococcus dramatically diminished or even completely reversed the ovipositional avoidance of alkalinity in Drosophila.Mechanistically,Enterococcus generate abundant lactate during fermentation,which neutralizes the residual alkali in an egg-laying substrate.In conclusion,Enterococcus protects Drosophila from alkali stress by acidifying the ovipositional substrate,and ultimately improves the fitness of the Drosophila population.Our results demonstrate that symbionts are profound factors in the Drosophila ovipositional decision,and extend our understanding of the intimate interactions between Drosophila and their symbionts.     

Invertebrates yield a plethora of atypical guanylyl cyclases

Invertebrate model systems have a long history of generating new insights into neuronal signaling systems. This review focuses on cyclic GMP signaling and describes recent advances in understanding the properties and functions of guanylyl cyclases in invertebrates. The sequencing of three invertebrate genomes has provided a complete catalog of the guanylyl cyclases in C. elegans, Drosophila, and the mosquito Anopheles gambiae. Using this data and that from cloned guanylyl cyclases in Manduca sexta, C. elegans, and Drosophila, plus predictions and models from vertebrate guanylyl cyclases, evidence is presented that there is a much broader array of properties for these enzymes than previously realized. In addition to the classic homodimeric receptor guanylyl cyclases, C. elegans has at least two receptor guanylyl cyclases that are predicted to require heterodimer formation for activity. Soluble guanylyl cyclases are generally recognized as being obligate heterodimers that are activated by nitric oxide (NO). Some of the soluble guanylyl cyclases in C. elegans may heterodimeric, but all appear to be insensitive to NO. The β2 soluble guanylyl cyclase subunit in mammals and similar ones in Manduca and Drosophila are active in the absence of additional subunits and there is evidence that Drosophila and Anopheles also express an additional subunit that enhances this activity.     

Intestinal stem cells in the adult Drosophila midgut

Drosophila has long been an excellent model organism for studying stem cell biology. Notably, studies of Drosophila's germline stem cells have been instrumental in developing the stem cell niche concept. The recent discovery of somatic stem cells in adult Drosophila, particularly the intestinal stem cells (ISCs) of the midgut, has established Drosophila as an exciting model to study stem cell-mediated adult tissue homeostasis and regeneration. Here, we review the major signaling pathways that regulate the self-renewal, proliferation and differentiation of Drosophila ISCs, discussing how this regulation maintains midgut homeostasis and mediates regeneration of the intestinal epithelium after injury.