Adhesive but not signaling activity of Drosophila N-cadherin is essential for target selection of photoreceptor afferents

Nematode Extracellular Protein Interactome Expands Connections between Signaling Pathways

Multicellularity was accompanied by the emergence of new classes of cell surface and secreted proteins. The nematode C. elegans is a favorable model to study cell surface interactomes, given its well-defined and stereotyped cell types and intercellular contacts. Here we report our C. elegans extracellular interactome dataset, the largest yet for an invertebrate. Most of these interactions were unknown, despite recent datasets for flies and humans, as our collection contains a larger selection of protein families. We uncover new interactions for all four major axon guidance pathways, including ectodomain interactions between three of the pathways. We demonstrate that a protein family known to maintain axon locations are secreted receptors for insulins. We reveal novel interactions of cystine-knot proteins with putative signaling receptors, which may extend the study of neurotrophins and growth-factor-mediated functions to nematodes. Finally, our dataset provides insights into human disease mechanisms and how extracellular interactions may help establish connectomes.

R7 photoreceptor axon targeting depends on the relative levels of lost and found expression in R7 and its synaptic partners

As neural circuits form, growing processes select the correct synaptic partners through interactions between cell surface proteins. The presence of such proteins on two neuronal processes may lead to either adhesion or repulsion; however, the consequences of mismatched expression have rarely been explored. Here, we show that the Drosophila CUB-LDL protein Lost and found (Loaf) is required in the UV-sensitive R7 photoreceptor for normal axon targeting only when Loaf is also present in its synaptic partners. Although targeting occurs normally in loaf mutant animals, removing loaf from photoreceptors or expressing it in their postsynaptic neurons Tm5a/b or Dm9 in a loaf mutant causes mistargeting of R7 axons. Loaf localizes primarily to intracellular vesicles including endosomes. We propose that Loaf regulates the trafficking or function of one or more cell surface proteins, and an excess of these proteins on the synaptic partners of R7 prevents the formation of stable connections.

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.

N-Cadherin Orchestrates Self-Organization of Neurons within a Columnar Unit in the Drosophila Medulla

Columnar structure is a basic unit of the brain, but the mechanism underlying its development remains largely unknown. The medulla, the largest ganglion of the Drosophila melanogaster visual center, provides a unique opportunity to reveal the mechanisms of 3D organization of the columns. In this study, using N-cadherin (Ncad) as a marker, we reveal the donut-like columnar structures along the 2D layer in the larval medulla that evolves to form three distinct layers in pupal development. Column formation is initiated by three core neurons, R8, R7, and Mi1, which establish distinct concentric domains within a column. We demonstrate that Ncad-dependent relative adhesiveness of the core columnar neurons regulates their relative location within a column along a 2D layer in the larval medulla according to the differential adhesion hypothesis. We also propose the presence of mutual interactions among the three layers during formation of the 3D structures of the medulla columns.SIGNIFICANCE STATEMENT The columnar structure is a basic unit of the brain, but its developmental mechanism remains unknown. The medulla, the largest ganglion of the fly visual center, provides a unique opportunity to reveal the mechanisms of 3D organization of the columns. We reveal that column formation is initiated by three core neurons that establish distinct concentric domains within a column. We demonstrate the in vivo evidence of N-cadherin-dependent differential adhesion among the core columnar neurons within a column along a 2D layer in the larval medulla. The 2D larval columns evolve to form three distinct layers in the pupal medulla. We propose the presence of mutual interactions among the three layers during formation of the 3D structures of the medulla columns.

Genetics of alcohol consumption in Drosophila melanogaster

Individual variation in alcohol consumption in human populations is determined by genetic, environmental, social and cultural factors. In contrast to humans, genetic contributions to complex behavioral phenotypes can be readily dissected in Drosophila, where both the genetic background and environment can be controlled and behaviors quantified through simple high-throughput assays. Here, we measured voluntary consumption of ethanol in ∼3000 individuals of each sex from an advanced intercross population derived from 37 lines of the Drosophila melanogaster Genetic Reference Panel. Extreme quantitative trait loci mapping identified 385 differentially segregating allelic variants located in or near 291 genes at P < 10^-8 . The effects of single nucleotide polymorphisms associated with voluntary ethanol consumption are sex-specific, as found for other alcohol-related phenotypes. To assess causality, we used RNA interference knockdown or P{MiET1} mutants and their corresponding controls and functionally validated 86% of candidate genes in at least one sex. We constructed a genetic network comprised of 23 genes along with a separate trio and a pair of connected genes. Gene ontology analyses showed enrichment of developmental genes, including development of the nervous system. Furthermore, a network of human orthologs showed enrichment for signal transduction processes, protein metabolism and developmental processes, including nervous system development. Our results show that the genetic architecture that underlies variation in voluntary ethanol consumption is sexually dimorphic and partially overlaps with genetic factors that control variation in feeding behavior and alcohol sensitivity. This integrative genetic architecture is rooted in evolutionarily conserved features that can be extrapolated to human genetic interaction networks.

Filopodial dynamics and growth cone stabilization in Drosophila visual circuit development

Filopodial dynamics are thought to control growth cone guidance, but the types and roles of growth cone dynamics underlying neural circuit assembly in a living brain are largely unknown. To address this issue, we have developed long-term, continuous, fast and high-resolution imaging of growth cone dynamics from axon growth to synapse formation in cultured Drosophila brains. Using R7 photoreceptor neurons as a model we show that >90% of the growth cone filopodia exhibit fast, stochastic dynamics that persist despite ongoing stepwise layer formation. Correspondingly, R7 growth cones stabilize early and change their final position by passive dislocation. N-Cadherin controls both fast filopodial dynamics and growth cone stabilization. Surprisingly, loss of N-Cadherin causes no primary targeting defects, but destabilizes R7 growth cones to jump between correct and incorrect layers. Hence, growth cone dynamics can influence wiring specificity without a direct role in target recognition and implement simple rules during circuit assembly.

DOI:http://dx.doi.org/10.7554/eLife.10721.001

Genes encode complicated developmental processes, but it is clear that genetic information cannot encode each and every individual connection that forms between the nerve cells in a brain. Instead, the individual cells and nerve endings must make decisions during brain development. Up until now, few examples were known for how these nerve endings move and choose their paths and partners in a living, developing brain.

The fruit fly Drosophila provides a useful model to explore the ‘wiring’ of nerve cells in the brain, partly because a fruit fly’s brain develops within a few days. However, most previous studies have relied on identifying mutant flies with disrupted brain wiring and studying them using still images. Now, Özel et al. have developed a new imaging method that has enough resolution and speed over sufficiently long periods to track the growing nerve endings in a developing fly brain. The method was applied to a model nerve cell in the fly’s visual system. This revealed that most of this nerve’s dynamic changes are short-lived and random, and appear to help to stabilize the developing nerve ending, rather than guide it to a target. Özel et al. also found that a protein called N-Cadherin, previously thought to be required for the targeting of developing nerve endings, actually plays a role in their stabilization.

These findings uncover the roles of changes in nerve endings during long-term brain development; this was previously largely unknown for any organism. The next stage in this research will involve further analyses of both wild type and mutant flies to try and work out general principles about how the brain develops via the decoding of genetic information.

DOI:http://dx.doi.org/10.7554/eLife.10721.002

N-cadherin negatively regulates collective Drosophila glial migration via actin cytoskeleton remodeling

Cell migration is an essential and highly regulated process. During development, glia and neurons migrate over long distances, in most cases collectively, to reach their final destination and build the sophisticated architecture of the nervous system, the most complex tissue of the body. Collective migration is highly stereotyped and efficient, defects in the process leading to severe human diseases that include mental retardation. This dynamic process entails extensive cell communication and coordination, hence the real challenge is to analyze it in the whole organism and at cellular resolution. We here investigate the impact of the N-cadherin adhesion molecule on collective glial migration using the Drosophila developing wing and cell-type specific manipulation of gene expression. We show that N-cadherin timely accumulates in glial cells and that its levels affect migration efficiency. N-cadherin works as a molecular brake in a dosage dependent manner by negatively controlling actin nucleation and cytoskeleton remodeling through α/β catenins. This is the first in vivo evidence for N-cadherin negatively and cell autonomously controlling collective migration.

Analysis of adhesion molecules and basement membrane contributions to synaptic adhesion at the Drosophila embryonic NMJ

Synapse formation and maintenance crucially underlie brain function in health and disease. Both processes are believed to depend on cell adhesion molecules (CAMs). Many different classes of CAMs localise to synapses, including cadherins, protocadherins, neuroligins, neurexins, integrins, and immunoglobulin adhesion proteins, and further contributions come from the extracellular matrix and its receptors. Most of these factors have been scrutinised by loss-of-function analyses in animal models. However, which adhesion factors establish the essential physical links across synaptic clefts and allow the assembly of synaptic machineries at the contact site in vivo is still unclear. To investigate these key questions, we have used the neuromuscular junction (NMJ) of Drosophila embryos as a genetically amenable model synapse. Our ultrastructural analyses of NMJs lacking different classes of CAMs revealed that loss of all neurexins, all classical cadherins or all glutamate receptors, as well as combinations between these or with a Laminin deficiency, failed to reveal structural phenotypes. These results are compatible with a view that these CAMs might have no structural role at this model synapse. However, we consider it far more likely that they operate in a redundant or well buffered context. We propose a model based on a multi-adaptor principle to explain this phenomenon. Furthermore, we report a new CAM-independent adhesion mechanism that involves the basement membranes (BM) covering neuromuscular terminals. Thus, motorneuronal terminals show strong partial detachment of the junction when BM-to-cell surface attachment is impaired by removing Laminin A, or when BMs lose their structural integrity upon loss of type IV collagens. We conclude that BMs are essential to tie embryonic motorneuronal terminals to the muscle surface, lending CAM-independent structural support to their adhesion. Therefore, future developmental studies of these synaptic junctions in Drosophila need to consider the important contribution made by BM-dependent mechanisms, in addition to CAM-dependent adhesion.

Evolution of the cadherin-catenin complex

Adherens junctions are the most common junction type found in animal epithelia. Their core components are classical cadherins and catenins, which form membrane-spanning complexes that mediate intercellular binding on the extracellular side and associate with the actin cytoskeleton on the intracellular side. Junctional cadherin-catenin complexes are key elements involved in driving animal morphogenesis. Despite their ubiquity and importance, comparative studies of classical cadherins, catenins and their related molecules suggest that the cadherin/catenin-based adherens junctions have undergone structural and compositional transitions during the diversification of animal lineages. This chapter describes the molecular diversities related to the cadherin-catenin complex, based on accumulated molecular and genomic information. Understanding when and how the junctional cadherin-catenin complex originated, and its subsequent diversification in animals, promotes a comprehensive understanding of the mechanisms of animal morphological diversification.

Adherens junctions and cadherins in Drosophila development

Drosophila represents a paradigm for the analysis of the cellular, molecular and genetic mechanisms of development and is an ideal model system to study the contribution of Adherens Junctions (AJs) and their major components, cadherins, to morphogenesis. The combination of different techniques and approaches has allowed researchers to identify the requirements of these epithelial junctions in vivo in the context of a whole organism. The functional analysis of mutants for AJ core components, particularly for Drosophila DE-cadherin, has shown that AJs play critical roles in virtually all stages of development. For instance, AJs maintain tissue integrity while allowing the remodelling and homeostasis of many tissues. They control cell shape, contribute to cell polarity, facilitate cell-cell recognition during cell sorting, orient cell divisions, or regulate cell rearrangements, among other activities. Remarkably, these activities require a very fine control of the organisation and turnover of AJs during development. In addition, AJs engage in diverse and complex interactions with the cytoskeleton, signalling networks, intracellular trafficking machinery or polarity cues to perform these functions. Here, by summarising the requirements of AJs and cadherins during Drosophila morphogenesis, we illustrate the capital contribution of this model system to our knowledge of the mechanisms and biology of AJs.

Crystal structures of Drosophila N-cadherin ectodomain regions reveal a widely used class of Ca²+-free interdomain linkers

Vertebrate classical cadherins mediate selective calcium-dependent cell adhesion by mechanisms now understood at the atomic level. However, structures and adhesion mechanisms of cadherins from invertebrates, which are highly divergent yet function in similar roles, remain unknown. Here we present crystal structures of three- and four-tandem extracellular cadherin (EC) domain segments from Drosophila N-cadherin (DN-cadherin), each including the predicted N-terminal EC1 domain (denoted EC1') of the mature protein. While the linker regions for the EC1'-EC2' and EC3'-EC4' pairs display binding of three Ca(2+) ions similar to that of vertebrate cadherins, domains EC2' and EC3' are joined in a "kinked" orientation by a previously uncharacterized Ca(2+)-free linker. Biophysical analysis demonstrates that a construct containing the predicted N-terminal nine EC domains of DN-cadherin forms homodimers with affinity similar to vertebrate classical cadherins, whereas deleting the ninth EC domain ablates dimerization. These results suggest that, unlike their vertebrate counterparts, invertebrate cadherins may utilize multiple EC domains to form intercellular adhesive bonds. Sequence analysis reveals that similar Ca(2+)-free linkers are widely distributed in the ectodomains of both vertebrate and invertebrate cadherins.

The cytoskeletal regulator Genghis khan is required for columnar target specificity in the Drosophila visual system

A defining characteristic of neuronal cell type is the growth of axons and dendrites into specific layers and columns of the brain. Although differences in cell surface receptors and adhesion molecules are known to cause differences in synaptic specificity, differences in downstream signaling mechanisms that determine cell type-appropriate targeting patterns are unknown. Using a forward genetic screen in Drosophila, we identify the GTPase effector Genghis khan (Gek) as playing a crucial role in the ability of a subset of photoreceptor (R cell) axons to innervate appropriate target columns. In particular, single-cell mosaic analyses demonstrate that R cell growth cones lacking Gek function grow to the appropriate ganglion, but frequently fail to innervate the correct target column. Further studies reveal that R cell axons lacking the activity of the small GTPase Cdc42 display similar defects, providing evidence that these proteins regulate a common set of processes. Gek is expressed in all R cells, and a detailed structure-function analysis reveals a set of regulatory domains with activities that restrict Gek function to the growth cone. Although Gek does not normally regulate layer-specific targeting, ectopic expression of Gek is sufficient to alter the targeting choices made by another R cell type, the targeting of which is normally Gek independent. Thus, specific regulation of cytoskeletal responses to targeting cues is necessary for cell type-appropriate synaptic specificity.

Evolution: structural and functional diversity of cadherin at the adherens junction

Adhesion between cells is essential to the evolution of multicellularity. Indeed, morphogenesis in animals requires firm but flexible intercellular adhesions that are mediated by subcellular structures like the adherens junction (AJ). A key component of AJs is classical cadherins, a group of transmembrane proteins that maintain dynamic cell-cell associations in many animal species. An evolutionary reconstruction of cadherin structure and function provides a comprehensive framework with which to appreciate the diversity of morphogenetic mechanisms in animals.

Missed connections: photoreceptor axon seeks target neuron for synaptogenesis

Extending axons must choose the appropriate synaptic target cells in order to assemble functional neural circuitry. The axons of the Drosophila color-sensitive photoreceptors R7 and R8 project as a single fascicle from each ommatidium, but their terminals are segregated into distinct layers within their target region. Recent studies have begun to reveal the molecular mechanisms that establish this projection pattern. Both homophilic adhesion molecules and specific ligand-receptor interactions make important contributions to stabilizing R7 and R8 terminals in the appropriate target layers. These cell recognition molecules are regulated by the same transcription factors that control R7 and R8 cell fates. Autocrine and repulsive signaling mechanisms prevent photoreceptor terminals from encroaching on their neighbors, preserving the spatial resolution of visual information.

Complex interactions amongst N-cadherin, DLAR, and Liprin-alpha regulate Drosophila photoreceptor axon targeting

The formation of stable adhesive contacts between pre- and post-synaptic neurons represents the initial step in synapse assembly. The cell adhesion molecule N-cadherin, the receptor tyrosine phosphatase DLAR, and the scaffolding molecule Liprin-alpha play critical, evolutionarily conserved roles in this process. However, how these proteins signal to the growth cone and are themselves regulated remains poorly understood. Using Drosophila photoreceptors (R cells) as a model, we evaluate genetic and physical interactions among these three proteins. We demonstrate that DLAR function in this context is independent of phosphatase activity but requires interactions mediated by its intracellular domain. Genetic studies reveal both positive and, surprisingly, inhibitory interactions amongst all three genes. These observations are corroborated by biochemical studies demonstrating that DLAR physically associates via its phosphatase domain with N-cadherin in Drosophila embryos. Together, these data demonstrate that N-cadherin, DLAR, and Liprin-alpha function in a complex to regulate adhesive interactions between pre- and post-synaptic cells and provide a novel mechanism for controlling the activity of Liprin-alpha in the developing growth cone.

Optic lobe development

The optic lobes comprise approximately half of the fly's brain. In four major synaptic ganglia, or neuropils, the visual input from the compound eyes is received and processed for higher order visual functions like motion detection and color vision. A common characteristic of vertebrate and invertebrate visual systems is the point-to-point mapping of the visual world to synaptic layers in the brain, referred to as visuotopy. Vision requires the parallel extraction of numerous parameters in a visuotopic manner. Consequently, the optic neuropils are arranged in columns and perpendicularly oriented synaptic layers that allow for the selective establishment of synapses between columnar neurons. How this exquisite synaptic specificity is established during approximately 100 hours of brain development is still poorly understood. However, the optic lobe contains one of the best characterized brain structures in any organism-both anatomically and developmentally. Moreover, numerous molecules and their function illuminate some of the basic mechanisms involved in brain wiring. The emerging picture is that the development of the visual system of Drosophila is (epi-)genetically hard-wired; it supplies the emerging fly with vision without requiring neuronal activity for fine tuning of neuronal connectivity. Elucidating the genetic and cellular principles by which gene activity directs the assembly of the optic lobe is therefore a fascinating task and the focus of this chapter.

More than just glue: the diverse roles of cell adhesion molecules in the Drosophila nervous system

Cell adhesion is the fundamental driving force that establishes complex cellular architectures, with the nervous system offering a striking, sophisticated case study. Developing neurons adhere to neighboring neurons, their synaptic partners, and to glial cells. These adhesive interactions are required in a diverse array of contexts, including cell migration, axon guidance and targeting, as well as synapse formation and physiology. Forward and reverse genetic screens in the fruit fly Drosophila have uncovered several adhesion molecules that are required for neural development, and detailed cell biological analyses are beginning to unravel how these factors shape nervous system connectivity. Here we review our current understanding of the most prominent of these adhesion factors and their modes of action.

Expression patterns of cadherin genes in Drosophila oogenesis

In Drosophila oogenesis, the follicular epithelium that envelops the oocyte is patterned by a small set of inductive signals and gives rise to an elaborate three-dimensional eggshell. Several eggshell structures provide sensitive readouts of the patterning signals, but the formation of these structures is still poorly understood. In other systems, epithelial morphogenesis is guided by the spatial patterning of cell adhesion and cytoskeleton genes. As a step towards developing a comprehensive description of patterning events leading to eggshell morphogenesis, we report the expression of Drosophila cadherins, calcium-dependent adhesion molecules that are repeatedly used throughout development. We found that 9/17 of Drosophila cadherins are expressed in the follicular epithelium in dynamic patterns during oogenesis. In late oogenesis, the expression patterns of cadherin genes in the main body follicle cells is summarized using a compact set of simple geometric shapes, reflecting the integration of the EGFR and DPP inductive signals. The multi-layered composite patterning of the cadherins is hypothesized to play a key role in the formation of the eggshell. Of particular note is the complex patterning of the region of the follicular epithelium that gives rise to the dorsal appendages, which are tubular structures that serve as respiratory organs for the developing embryo.

Cadherins in neuronal morphogenesis and function

Classic cadherins represent a family of calcium-dependent homophilic cell-cell adhesion molecules. They confer strong adhesiveness to animal cells when they are anchored to the actin cytoskeleton via their cytoplasmic binding partners, catenins. The cadherin/catenin adhesion system plays key roles in the morphogenesis and function of the vertebrate and invertebrate nervous systems. In early vertebrate development, cadherins are involved in multiple events of brain morphogenesis including the formation and maintenance of the neuroepithelium, neurite extension and migration of neuronal cells. In the invertebrate nervous system, classic cadherin-mediated cell-cell interaction plays important roles in wiring among neurons. For synaptogenesis, the cadherin/catenin system not only stabilizes cell-cell contacts at excitatory synapses but also assembles synaptic molecules at synaptic sites. Furthermore, this system is involved in synaptic plasticity. Recent studies on the role of individual cadherin subtypes at synapses indicate that individual cadherin subtypes play their own unique role to regulate synaptic activities.

The L1-type cell adhesion molecule Neuroglian is necessary for maintenance of sensory axon advance in the Drosophila embryo

Background

Cell adhesion molecules have long been implicated in the regulation of axon growth, but the precise cellular roles played by individual cell adhesion molecules and the molecular basis for their action are still not well understood. We have used the sensory system of the Drosophila embryo to shed light on the mechanism by which the L1-type cell adhesion molecule Neuroglian regulates axon growth.

Results

We have found a highly penetrant sensory axon stalling phenotype in neuroglian mutant embryos. Axons stalled at a variety of positions along their normal trajectory, but most commonly in the periphery some distance along the peripheral nerve. All lateral and dorsal cluster sensory neurons examined, except for the dorsal cluster neuron dbd, showed stalling. Sensory axons were never seen to project along inappropriate pathways in neuroglian mutants and stalled axons showed normal patterns of fasciculation within nerves. The growth cones of stalled axons possessed a simple morphology, similar to their appearance in wild-type embryos when advancing along nerves. Driving expression of the wild-type form of Neuroglian in sensory neurons alone rescued the neuroglian mutant phenotype of both pioneering and follower neurons. A partial rescue was achieved by expressing the Neuroglian extracellular domain. Over/mis-expression of Neuroglian in all neurons, oenocytes or trachea had no apparent effect on sensory axon growth.

Conclusion

We conclude that Neuroglian is necessary to maintain axon advance along axonal substrates, but is not required for initiation of axon outgrowth, axon fasciculation or recognition of correct growth substrates. Expression of Neuroglian in sensory neurons alone is sufficient to promote axon advance and the intracellular region of the molecule is largely dispensable for this function. It is unlikely, therefore, that Nrg acts as a molecular 'clutch' to couple adhesion of F-actin within the growth cone to the extracellular substrate. Rather, we suggest that Neuroglian mediates sensory axon advance by promoting adhesion of the surface of the growth cone to its substrate. Our finding that stalling of a pioneer sensory neuron is rescued by driving Neuroglian in sensory neurons alone may suggest that Neuroglian can act in a heterophilic fashion.

The transmembrane protein Golden goal regulates R8 photoreceptor axon-axon and axon-target interactions

During Drosophila visual system development, photoreceptor (R) axons choose their correct paths and targets in a step-wise fashion. R axons with different identities make specific pathfinding decisions at different stages during development. We show here that the transmembrane protein Golden goal (Gogo), which is dynamically expressed in all R neurons and localizes predominantly to growth cones, is required in two distinct steps of R8 photoreceptor axon pathfinding: Gogo regulates axon-axon interactions and axon-target interactions in R8 photoreceptor axons. gogo loss-of-function and gain-of-function phenotypes suggest that Gogo mediates repulsive axon-axon interaction between R8 axons to maintain their proper spacing, and it promotes axon-target recognition at the temporary layer to enable R8 axons to enter their correct target columns in the medulla. From detailed structure-function experiments, we propose that Gogo functions as a receptor that binds an unidentified ligand through its conserved extracellular domain.