Neuroglian-mediated cell adhesion induces assembly of the membrane skeleton at cell contact sites

Transcriptomic analysis of feminizing somatic stem cells in the Drosophila testis reveals putative downstream effectors of the transcription factor Chinmo

One of the best examples of sexual dimorphism is the development and function of the gonads, ovaries and testes, which produce sex-specific gametes, oocytes, and spermatids, respectively. The development of these specialized germ cells requires sex-matched somatic support cells. The sexual identity of somatic gonadal cells is specified during development and must be actively maintained during adulthood. We previously showed that the transcription factor Chinmo is required to ensure the male sexual identity of somatic support cells in the Drosophila melanogaster testis. Loss of chinmo from male somatic gonadal cells results in feminization: they transform from squamous to epithelial-like cells that resemble somatic cells in the female gonad but fail to properly ensheath the male germline, causing infertility. To identify potential target genes of Chinmo, we purified somatic cells deficient for chinmo from the adult Drosophila testis and performed next-generation sequencing to compare their transcriptome to that of control somatic cells. Bioinformatics revealed 304 and 1549 differentially upregulated and downregulated genes, respectively, upon loss of chinmo in early somatic cells. Using a combination of methods, we validated several differentially expressed genes. These data sets will be useful resources to the community.

Glial and Neuronal Neuroglian, Semaphorin-1a and Plexin A Regulate Morphological and Functional Differentiation of Drosophila Insulin-Producing Cells

The insulin-producing cells (IPCs), a group of 14 neurons in the Drosophila brain, regulate numerous processes, including energy homeostasis, lifespan, stress response, fecundity, and various behaviors, such as foraging and sleep. Despite their importance, little is known about the development and the factors that regulate morphological and functional differentiation of IPCs. In this study, we describe the use of a new transgenic reporter to characterize the role of the Drosophila L1-CAM homolog Neuroglian (Nrg), and the transmembrane Semaphorin-1a (Sema-1a) and its receptor Plexin A (PlexA) in the differentiation of the insulin-producing neurons. Loss of Nrg results in defasciculation and abnormal neurite branching, including ectopic neurites in the IPC neurons. Cell-type specific RNAi knockdown experiments reveal that Nrg, Sema-1a and PlexA are required in IPCs and glia to control normal morphological differentiation of IPCs albeit with a stronger contribution of Nrg and Sema-1a in glia and of PlexA in the IPCs. These observations provide new insights into the development of the IPC neurons and identify a novel role for Sema-1a in glia. In addition, we show that Nrg, Sema-1a and PlexA in glia and IPCs not only regulate morphological but also functional differentiation of the IPCs and that the functional deficits are likely independent of the morphological phenotypes. The requirements of nrg, Sema-1a, and PlexA in IPC development and the expression of their vertebrate counterparts in the hypothalamic-pituitary axis, suggest that these functions may be evolutionarily conserved in the establishment of vertebrate endocrine systems.

Collectively stabilizing and orienting posterior migratory forces disperses cell clusters in vivo

Individual cells detach from cohesive ensembles during development and can inappropriately separate in disease. Although much is known about how cells separate from epithelia, it remains unclear how cells disperse from clusters lacking apical-basal polarity, a hallmark of advanced epithelial cancers. Here, using live imaging of the developmental migration program of Drosophila primordial germ cells (PGCs), we show that cluster dispersal is accomplished by stabilizing and orienting migratory forces. PGCs utilize a G protein coupled receptor (GPCR), Tre1, to guide front-back migratory polarity radially from the cluster toward the endoderm. Posteriorly positioned myosin-dependent contractile forces pull on cell-cell contacts until cells release. Tre1 mutant cells migrate randomly with transient enrichment of the force machinery but fail to separate, indicating a temporal contractile force threshold for detachment. E-cadherin is retained on the cell surface during cell separation and augmenting cell-cell adhesion does not impede detachment. Notably, coordinated migration improves cluster dispersal efficiency by stabilizing cell-cell interfaces and facilitating symmetric pulling. We demonstrate that guidance of inherent migratory forces is sufficient to disperse cell clusters under physiological settings and present a paradigm for how such events could occur across development and disease.

Optimized expression of the antimicrobial protein Gloverin from Galleria mellonella using stably transformed Drosophila melanogaster S2 cells

Antimicrobial proteins and peptides (AMPs) are valuable as leads in the pharmaceutical industry for the development of novel anti-infective drugs. Here we describe the efficient heterologous expression and basic characterization of a Gloverin-family AMP derived from the greater wax moth Galleria mellonella. Highly productive single-cell clones prepared by limiting dilution achieved a 100% increase in productivity compared to the original polyclonal Drosophila melanogaster S2 cell line. Comprehensive screening for suitable expression conditions using statistical experimental designs revealed that optimal induction was achieved using 600 µM CuSO4 at the mid-exponential growth phase. Under these conditions, 25 mg/L of the AMP was expressed at the 1-L bioreactor scale, with optimal induction and harvest times ensured by dielectric spectroscopy and the online measurement of optical density. Gloverin was purified from the supernatant by immobilized metal ion affinity chromatography followed by dialysis. In growth assays, the purified protein showed specific antimicrobial activity against two different strains of Escherichia coli.

Comparative and quantitative proteomics reveal the adaptive strategies of oyster larvae to ocean acidification

Decreasing pH due to anthropogenic CO2 inputs, called ocean acidification (OA), can make coastal environments unfavorable for oysters. This is a serious socioeconomical issue for China which supplies >70% of the world's edible oysters. Here, we present an iTRAQ-based protein profiling approach for the detection and quantification of proteome changes under OA in the early life stage of a commercially important oyster, Crassostrea hongkongensis. Availability of complete genome sequence for the pacific oyster (Crassostrea gigas) enabled us to confidently quantify over 1500 proteins in larval oysters. Over 7% of the proteome was altered in response to OA at pHNBS 7.6. Analysis of differentially expressed proteins and their associated functional pathways showed an upregulation of proteins involved in calcification, metabolic processes, and oxidative stress, each of which may be important in physiological adaptation of this species to OA. The downregulation of cytoskeletal and signal transduction proteins, on the other hand, might have impaired cellular dynamics and organelle development under OA. However, there were no significant detrimental effects in developmental processes such as metamorphic success. Implications of the differentially expressed proteins and metabolic pathways in the development of OA resistance in oyster larvae are discussed. The MS proteomics data have been deposited to the ProteomeXchange with identifiers PXD002138 (http://proteomecentral.proteomexchange.org/dataset/PXD002138).

L1CAM/Neuroglian controls the axon-axon interactions establishing layered and lobular mushroom body architecture

The establishment of neuronal circuits depends on the guidance of axons both along and in between axonal populations of different identity; however, the molecular principles controlling axon-axon interactions in vivo remain largely elusive. We demonstrate that the Drosophila melanogaster L1CAM homologue Neuroglian mediates adhesion between functionally distinct mushroom body axon populations to enforce and control appropriate projections into distinct axonal layers and lobes essential for olfactory learning and memory. We addressed the regulatory mechanisms controlling homophilic Neuroglian-mediated cell adhesion by analyzing targeted mutations of extra- and intracellular Neuroglian domains in combination with cell type-specific rescue assays in vivo. We demonstrate independent and cooperative domain requirements: intercalating growth depends on homophilic adhesion mediated by extracellular Ig domains. For functional cluster formation, intracellular Ankyrin2 association is sufficient on one side of the trans-axonal complex whereas Moesin association is likely required simultaneously in both interacting axonal populations. Together, our results provide novel mechanistic insights into cell adhesion molecule-mediated axon-axon interactions that enable precise assembly of complex neuronal circuits.

Analysis of cell-cell contact mediated by Ig superfamily cell adhesion molecules

Cell-cell adhesion is a fundamental requirement for all multicellular organisms. The calcium-independent cell adhesion molecules of the immunoglobulin superfamily (IgSF-CAMs) represent a major subgroup. They consist of immunoglobulin folds alone or in combination with other protein modules, often fibronectin type-III folds. More than 100 IgSF-CAMs have been identified in vertebrates and invertebrates. Most of the IgSF-CAMs are cell surface molecules that are membrane-anchored either by a single transmembrane segment or by a glycosylphosphatidylinositol (GPI) anchor. Some of the IgSF-CAMs also occur in soluble form, e.g., in the cerebrospinal fluid or in the vitreous fluid of the eye, due to naturally occurring cleavage of the GPI anchor or the membrane-proximal peptide segment. Some IgSF-CAMs, such as NCAM, occur in various forms that are generated by alternative splicing. This unit contains a series of protocols that have been used to study the function of IgSF-CAMs in vitro and in vivo.

Genetic studies of spectrin in the larval fat body of Drosophila melanogaster: evidence for a novel lipid uptake apparatus

Spectrin cytoskeleton defects produce a host of phenotypes affecting the plasma membrane, cell polarity, and secretory membrane traffic. However, many of the underlying molecular mechanisms remain unexplained by prevailing models. Here we used the larval fat body of Drosophila melanogaster as a genetic model system to further elucidate mechanisms of αβ-spectrin function. The results provide unexpected new insights into spectrin function as well as mechanisms of dietary fat uptake and storage. We show that loss of α- or β-spectrin in the fat body eliminated a population of small cortical lipid droplets and altered plasma membrane architecture, but did not affect viability of the organism. We present a novel model in which αβ-spectrin directly couples lipid uptake at the plasma membrane to lipid droplet growth in the cytoplasm. In contrast, strong overexpression of β-spectrin caused fat body atrophy and larval lethality. Overexpression of β-spectrin also perturbed transport of dietary fat from the midgut to the fat body. This hypermorphic phenotype appears to be the result of blocking secretion of the lipid carrier lipophorin from fat cells. However, this midgut phenotype was never seen with spectrin loss of function, suggesting that spectrin is not normally required for lipophorin secretion or function. The β-spectrin hypermorphic phenotype was ameliorated by co-overexpression of α-spectrin. Based on the overexpression results here, we propose that β-spectrin family members may be prone to hypermorphic effects (including effects on secretion) if their activity is not properly regulated.

Transsynaptic coordination of synaptic growth, function, and stability by the L1-type CAM Neuroglian

Experiments in peripheral and central synapses reveal the regulatory mechanisms that enable trans-synaptic control of synapse development and maintenance by the L1-type CAM Neuroglian.

The precise control of synaptic connectivity is essential for the development and function of neuronal circuits. While there have been significant advances in our understanding how cell adhesion molecules mediate axon guidance and synapse formation, the mechanisms controlling synapse maintenance or plasticity in vivo remain largely uncharacterized. In an unbiased RNAi screen we identified the Drosophila L1-type CAM Neuroglian (Nrg) as a central coordinator of synapse growth, function, and stability. We demonstrate that the extracellular Ig-domains and the intracellular Ankyrin-interaction motif are essential for synapse development and stability. Nrg binds to Ankyrin2 in vivo and mutations reducing the binding affinities to Ankyrin2 cause an increase in Nrg mobility in motoneurons. We then demonstrate that the Nrg–Ank2 interaction controls the balance of synapse growth and stability at the neuromuscular junction. In contrast, at a central synapse, transsynaptic interactions of pre- and postsynaptic Nrg require a dynamic, temporal and spatial, regulation of the intracellular Ankyrin-binding motif to coordinate pre- and postsynaptic development. Our study at two complementary model synapses identifies the regulation of the interaction between the L1-type CAM and Ankyrin as an important novel module enabling local control of synaptic connectivity and function while maintaining general neuronal circuit architecture.

The function of neuronal circuits relies on precise connectivity, and processes like learning and memory involve refining this connectivity through the selective formation and elimination of synapses. Cell adhesion molecules (CAMs) that directly mediate cell–cell interactions at synaptic contacts are thought to mediate this structural synaptic plasticity. In this study, we used an unbiased genetic screen to identify the Drosophila L1-type CAM Neuroglian as a central regulator of synapse formation and maintenance. We show that the intracellular Ankyrin interaction motif, which links Neuroglian to the cytoskeleton, is an essential regulatory site for Neuroglian mobility, adhesion, and synaptic function. In motoneurons, the strength of Ankyrin binding directly controls the balance between synapse formation and maintenance. At a central synapse, however, a dynamic regulation of the Neuroglian–Ankyrin interaction is required to coordinate transsynaptic development. Our study identifies the interaction of the L1-type CAM with Ankyrin as a novel regulatory module enabling local and precise control of synaptic connectivity without altering general neuronal circuit architecture. This interaction is relevant for normal nervous system development and disease as mutations in L1-type CAMs cause mental retardation and psychiatric diseases in humans.

Functional genomic screen and network analysis reveal novel modifiers of tauopathy dissociated from tau phosphorylation

A functional genetic screen using loss-of-function and gain-of-function alleles was performed to identify modifiers of tau-induced neurotoxicity using the 2N/4R (full-length) isoform of wild-type human tau expressed in the fly retina. We previously reported eye pigment mutations, which create dysfunctional lysosomes, as potent modifiers; here, we report 37 additional genes identified from ∼1900 genes screened, including the kinases shaggy/GSK-3beta, par-1/MARK, CamKI and Mekk1. Tau acts synergistically with Mekk1 and p38 to down-regulate extracellular regulated kinase activity, with a corresponding decrease in AT8 immunoreactivity (pS202/T205), suggesting that tau can participate in signaling pathways to regulate its own kinases. Modifiers showed poor correlation with tau phosphorylation (using the AT8, 12E8 and AT270 epitopes); moreover, tested suppressors of wild-type tau were equally effective in suppressing toxicity of a phosphorylation-resistant S11A tau construct, demonstrating that changes in tau phosphorylation state are not required to suppress or enhance its toxicity. Genes related to autophagy, the cell cycle, RNA-associated proteins and chromatin-binding proteins constitute a large percentage of identified modifiers. Other functional categories identified include mitochondrial proteins, lipid trafficking, Golgi proteins, kinesins and dynein and the Hsp70/Hsp90-organizing protein (Hop). Network analysis uncovered several other genes highly associated with the functional modifiers, including genes related to the PI3K, Notch, BMP/TGF-β and Hedgehog pathways, and nuclear trafficking. Activity of GSK-3β is strongly upregulated due to TDP-43 expression, and reduced GSK-3β dosage is also a common suppressor of Aβ42 and TDP-43 toxicity. These findings suggest therapeutic targets other than mitigation of tau phosphorylation.

The Drosophila L1CAM homolog Neuroglian signals through distinct pathways to control different aspects of mushroom body axon development

The spatiotemporal integration of adhesion and signaling during neuritogenesis is an important prerequisite for the establishment of neuronal networks in the developing brain. In this study, we describe the role of the L1-type CAM Neuroglian protein (NRG) in different steps of Drosophila mushroom body (MB) neuron axonogenesis. Selective axon bundling in the peduncle requires both the extracellular and the intracellular domain of NRG. We uncover a novel role for the ZO-1 homolog Polychaetoid (PYD) in axon branching and in sister branch outgrowth and guidance downstream of the neuron-specific isoform NRG-180. Furthermore, genetic analyses show that the role of NRG in different aspects of MB axonal development not only involves PYD, but also TRIO, SEMA-1A and RAC1.

Cell adhesion molecules in context: CAM function depends on the neighborhood

Cell adhesion molecules (CAMs) are now known to mediate much more than adhesion between cells and between cells and the extracellular matrix. Work by many researchers has illuminated their roles in modulating activation of molecules such as receptor tyrosine kinases, with subsequent effects on cell survival, migration, and process extension. CAMs also are known to serve as substrates for proteases which can create diffusible fragments capable of signaling independently from the CAM. The diversity of interactions is further modulated by membrane rafts, which can co-localize or separate potential signaling partners to affect the likelihood of a given signaling pathway being activated. Given the ever-growing number of known CAMs and the fact that their heterophilic binding in cis or in trans can affect their interactions with other molecules, including membrane-bound receptors, one would predict a wide range of effects attributable to a particular CAM in a particular cell at a particular stage of development. The function(s) of a given CAM must therefore be considered in the context of the history of the cell expressing it and the repertoire of molecules expressed both by that cell and its neighbors.

The spectrin-ankyrin-4.1-adducin membrane skeleton: adapting eukaryotic cells to the demands of animal life

The cells in animals face unique demands beyond those encountered by their unicellular eukaryotic ancestors. For example, the forces engendered by the movement of animals places stresses on membranes of a different nature than those confronting free-living cells. The integration of cells into tissues, as well as the integration of tissue function into whole animal physiology, requires specialisation of membrane domains and the formation of signalling complexes. With the evolution of mammals, the specialisation of cell types has been taken to an extreme with the advent of the non-nucleated mammalian red blood cell. These and other adaptations to animal life seem to require four proteins--spectrin, ankyrin, 4.1 and adducin--which emerged during eumetazoan evolution. Spectrin, an actin cross-linking protein, was probably the earliest of these, with ankyrin, adducin and 4.1 only appearing as tissues evolved. The interaction of spectrin with ankyrin is probably a prerequisite for the formation of tissues; only with the advent of vertebrates did 4.1 acquires the ability to bind spectrin and actin. The latter activity seems to allow the spectrin complex to regulate the cell surface accumulation of a wide variety of proteins. Functionally, the spectrin-ankyrin-4.1-adducin complex is implicated in the formation of apical and basolateral domains, in aspects of membrane trafficking, in assembly of certain signalling and cell adhesion complexes and in providing stability to otherwise mechanically fragile cell membranes. Defects in this complex are manifest in a variety of hereditary diseases, including deafness, cardiac arrhythmia, spinocerebellar ataxia, as well as hereditary haemolytic anaemias. Some of these proteins also function as tumor suppressors. The spectrin-ankyrin-4.1-adducin complex represents a remarkable system that underpins animal life; it has been adapted to many different functions at different times during animal evolution.

The Drosophila Anion Exchanger (DAE) lacks a detectable interaction with the spectrin cytoskeleton

Background

Current models suggest that the spectrin cytoskeleton stabilizes interacting ion transport proteins at the plasma membrane. The human erythrocyte anion exchanger (AE1) was the first membrane transport protein found to be associated with the spectrin cytoskeleton. Here we evaluated a conserved anion exchanger from Drosophila (DAE) as a marker for studies of the downstream effects of spectrin cytoskeleton mutations.

Results

Sequence comparisons established that DAE belongs to the SLC4A1-3 subfamily of anion exchangers that includes human AE1. Striking sequence conservation was observed in the C-terminal membrane transport domain and parts of the N-terminal cytoplasmic domain, but not in the proposed ankyrin-binding site. Using an antibody raised against DAE and a recombinant transgene expressed in Drosophila S2 cells DAE was shown to be a 136 kd plasma membrane protein. A major site of expression was found in the stomach acid-secreting region of the larval midgut. DAE codistributed with an infolded subcompartment of the basal plasma membrane of interstitial cells. However, spectrin did not codistribute with DAE at this site or in anterior midgut cells that abundantly expressed both spectrin and DAE. Ubiquitous knockdown of DAE with dsRNA eliminated antibody staining and was lethal, indicating that DAE is an essential gene product in Drosophila.

Conclusions

Based on the lack of colocalization and the lack of sequence conservation at the ankyrin-binding site, it appears that the well-characterized interaction between AE1 and the spectrin cytoskeleton in erythrocytes is not conserved in Drosophila. The results establish a pattern in which most of the known interactions between the spectrin cytoskeleton and the plasma membrane in mammals do not appear to be conserved in Drosophila.

Proteolysis of submembrane cytoskeletal proteins ankyrin-G and αII-spectrin following diffuse brain injury: a role in white matter vulnerability at Nodes of Ranvier

A high membrane-to-cytoplasm ratio makes axons particularly vulnerable to traumatic injury. Posttraumatic shifts in ionic homeostasis promote spectrin cleavage, disrupt ankyrin linkages and destabilize axolemmal proteins. This study contrasted ankyrin-G and αII-spectrin degradation in cortex and corpus callosum following diffuse axonal injury produced by fluid percussion insult. Ankyrin-G lysis occurred preferentially in white matter, with acute elevation of all fragments and long-term reduction of a low kD form. Calpain-generated αII-spectrin fragments increased in both regions. Caspase-3 lysis of αII-spectrin showed a small, acute rise in cortex but was absent in callosum. White matter displayed nodal damage, with horseradish peroxidase permeability into the submyelin space. Ankyrin-G-binding protein neurofascin and spectrin-binding protein ankyrin-B showed acute alterations in expression. These results support ankyrin-G vulnerability in white matter following trauma and suggest that ankyrin-G and αII-spectrin proteolysis disrupts Node of Ranvier integrity. The time course of such changes were comparable to previously observed functional deficits in callosal fibers.

Roles of specific membrane lipid domains in EGF receptor activation and cell adhesion molecule stabilization in a developing olfactory system

Background

Reciprocal interactions between glial cells and olfactory receptor neurons (ORNs) cause ORN axons entering the brain to sort, to fasciculate into bundles destined for specific glomeruli, and to form stable protoglomeruli in the developing olfactory system of an experimentally advantageous animal species, the moth Manduca sexta. Epidermal growth factor receptors (EGFRs) and the cell adhesion molecules (IgCAMs) neuroglian and fasciclin II are known to be important players in these processes.

Methodology/Principal Findings

We report in situ and cell-culture studies that suggest a role for glycosphingolipid-rich membrane subdomains in neuron-glia interactions. Disruption of these subdomains by the use of methyl-β-cyclodextrin results in loss of EGFR activation, depletion of fasciclin II in ORN axons, and loss of neuroglian stabilization in the membrane. At the cellular level, disruption leads to aberrant ORN axon trajectories, small antennal lobes, abnormal arrays of olfactory glomerul, and loss of normal glial cell migration.

Conclusions/Significance

We propose that glycosphingolipid-rich membrane subdomains (possible membrane rafts or platforms) are essential for IgCAM-mediated EGFR activation and for anchoring of neuroglian to the cytoskeleton, both required for normal extension and sorting of ORN axons.

The interaction between L1-type proteins and ankyrins--a master switch for L1-type CAM function

L1-type cell adhesion molecules (CAMs) are important mediators of neural differentiation, including axonal outgrowth and pathfinding and also of synapse formation and maintenance. In addition, their interactions with cytoskeletal components are highly conserved and regulated. How these different aspects of CAM functionality relate to each other is not well understood. Based on results from our and other laboratories we propose that ankyrin-binding to L1-type CAMs provides a master switch. The interaction with ankyrins directs L1-type adhesive proteins into different functional contexts, either ankyrin-independent functions, such as neurite outgrowth and axonal pathfinding or into ankyrin-dependent functions, such as L1’s role at axon initial segments (AIS), paranodal regions, synapses and in dendrites.

The L1-CAM, Neuroglian, functions in glial cells for Drosophila antennal lobe development

Although considerable progress has been made in understanding the roles of olfactory receptor neurons (ORNs) and projection neurons (PNs) in Drosophila antennal lobe (AL) development, the roles of glia have remained largely mysterious. Here, we show that during Drosophila metamorphosis, a population of midline glial cells in the brain undergoes extensive cellular remodeling and is closely associated with the collateral branches of ORN axons. These glial cells are required for ORN axons to project across the midline and establish the contralateral wiring in the ALs. We find that Neuroglian (Nrg), the Drosophila homolog of the vertebrate cell adhesion molecule, L1, is expressed and functions in the midline glial cells to regulate their proper development. Loss of Nrg causes the disruption in glial morphology and the agenesis of the antennal commissural tract. Our genetic analysis further demonstrates that the functions of Nrg in the midline glia require its ankyrin-binding motif. We propose that Nrg is an important regulator of glial morphogenesis and axon guidance in AL development.

The lateral mobility of cell adhesion molecules is highly restricted at septate junctions in Drosophila

BACKGROUND

A complex of three cell adhesion molecules (CAMs) Neurexin IV(Nrx IV), Contactin (Cont) and Neuroglian (Nrg) is implicated in the formation of septate junctions between epithelial cells in Drosophila. These CAMs are interdependent for their localization at septate junctions and e.g. null mutation of nrx IV or cont induces the mislocalization of Nrg to the baso-lateral membrane. These mutations also result in ultrastructural alteration of the strands of septate junctions and breakdown of the paracellular barrier. Varicose (Vari) and Coracle (Cora), that both interact with the cytoplasmic tail of Nrx IV, are scaffolding molecules required for the formation of septate junctions.

RESULTS

We conducted photobleaching experiments on whole living Drosophila embryos to analyze the membrane mobility of CAMs at septate junctions between epithelial cells. We show that GFP-tagged Nrg and Nrx IV molecules exhibit very stable association with septate junctions in wild-type embryos. Nrg-GFP is mislocalized to the baso-lateral membrane in nrx IV or cont null mutant embryos, and displays increased mobile fraction. Similarly, Nrx IV-GFP becomes distributed to the baso-lateral membrane in null mutants of vari and cora, and its mobile fraction is strongly increased. The loss of Vari, a MAGUK protein that interacts with the cytoplasmic tail of Nrx IV, has a stronger effect than the null mutation of nrx IV on the lateral mobility of Nrg-GFP.

CONCLUSION

The strands of septate junctions display a stable behavior in vivo that may be correlated with their role of paracellular barrier. The membrane mobility of CAMs is strongly limited when they take part to the multimolecular complex forming septate junctions. This restricted lateral diffusion of CAMs depends on both adhesive interactions and clustering by scaffolding molecules. The lateral mobility of CAMs is strongly increased in embryos presenting alteration of septate junctions. The stronger effect of vari by comparison with nrx IV null mutation supports the hypothesis that this scaffolding molecule may cross-link different types of CAMs and play a crucial role in stabilizing the strands of septate junctions.

Caveolin regulates kv1.5 trafficking to cholesterol-rich membrane microdomains

The targeting of ion channels to cholesterol-rich membrane microdomains has emerged as a novel mechanism of ion channel localization. Previously, we reported that Kv1.5, a prominent cardiovascular K(+) channel alpha-subunit, localizes to caveolar microdomains. However, the mechanisms regulating Kv1.5 targeting and the functional significance of this localization are largely unknown. In this study, we demonstrate a role for caveolin in the trafficking of Kv1.5 to lipid raft microdomains where cholesterol modulates channel function. In cells lacking endogenous caveolin-1 or -3, the association of Kv1.5 with low-density, detergent-resistant membrane fractions requires coexpression with exogenous caveolin, which can form channel-caveolin complexes. Caveolin is not required for cell surface expression, however, and caveolin-trafficking mutants sequester Kv1.5, but not Kv2.1, in intracellular compartments, resulting in a loss of functional cell surface channel. Coexpression with wild type caveolin-1 does not alter Kv1.5 current density; rather, it induces depolarizing shifts in steady-state activation and inactivation. These shifts are analogous to those produced by elevation of membrane cholesterol. Together, these results show that caveolin modulates channel function by regulating trafficking to cholesterol-rich membrane microdomains.

Drosophila as a genetic and cellular model for studies on axonal growth

One of the most fascinating processes during nervous system development is the establishment of stereotypic neuronal networks. An essential step in this process is the outgrowth and precise navigation (pathfinding) of axons and dendrites towards their synaptic partner cells. This phenomenon was first described more than a century ago and, over the past decades, increasing insights have been gained into the cellular and molecular mechanisms regulating neuronal growth and navigation. Progress in this area has been greatly assisted by the use of simple and genetically tractable invertebrate model systems, such as the fruit fly Drosophila melanogaster. This review is dedicated to Drosophila as a genetic and cellular model to study axonal growth and demonstrates how it can and has been used for this research. We describe the various cellular systems of Drosophila used for such studies, insights into axonal growth cones and their cytoskeletal dynamics, and summarise identified molecular signalling pathways required for growth cone navigation, with particular focus on pathfinding decisions in the ventral nerve cord of Drosophila embryos. These Drosophila-specific aspects are viewed in the general context of our current knowledge about neuronal growth.

Basolateral junctions are sufficient to suppress epithelial invasion duringDrosophilaoogenesis

Epithelial junctions play crucial roles during metazoan evolution and development by facilitating tissue formation, maintenance, and function. Little is known about the role of distinct types of junctions in controlling epithelial transformations leading to invasion of neighboring tissues. Discovering the key junction complexes that control these processes and how they function may also provide mechanistic insight into carcinoma cell invasion. Here, using the Drosophila ovary as a model, we show that four proteins of the basolateral junction (BLJ), Fasciclin-2, Neuroglian, Discs-large, and Lethal-giant-larvae, but not proteins of other epithelial junctions, directly suppress epithelial tumorigenesis and invasion. Remarkably, the expression pattern of Fasciclin-2 predicts which cells will invade. We compared the apicobasal polarity of BLJ tumor cells to border cells (BCs), an epithelium-derived cluster that normally migrates during mid-oogenesis. Both tumor cells and BCs differentiate a lateralized membrane pattern that is necessary but not sufficient for invasion. Independent of lateralization, derepression of motility pathways is also necessary, as indicated by a strong linear correlation between faster BC migration and an increased incidence of tumor invasion. However, without membrane lateralization, derepression of motility pathways is also not sufficient for invasion. Our results demonstrate that spatiotemporal patterns of basolateral junction activity directly suppress epithelial invasion by organizing the cooperative activity of distinct polarity and motility pathways.

Distinct functions of alpha-Spectrin and beta-Spectrin during axonal pathfinding

Cell-shape changes during development require a precise coupling of the cytoskeleton with proteins situated in the plasma membrane. Important elements controlling the shape of cells are the Spectrin proteins that are expressed as a subcortical cytoskeletal meshwork linking specific membrane receptors with F-actin fibers. Here, we demonstrate that Drosophila karussell mutations affect beta-spectrin and lead to distinct axonal patterning defects in the embryonic CNS. karussell mutants display a slit-sensitive axonal phenotype characterized by axonal looping in stage-13 embryos. Further analyses of individual, labeled neuroblast lineages revealed abnormally structured growth cones in these animals. Cell-type-specific rescue experiments demonstrate that beta-Spectrin is required autonomously and non-autonomously in cortical neurons to allow normal axonal patterning. Within the cell, beta-Spectrin is associated with alpha-Spectrin. We show that expression of the two genes is tightly regulated by post-translational mechanisms. Loss of beta-Spectrin significantly reduces levels of neuronal alpha-Spectrin expression, whereas gain of beta-Spectrin leads to an increase in alpha-Spectrin protein expression. Because the loss of alpha-spectrin does not result in an embryonic nervous system phenotype, beta-Spectrin appears to act at least partially independent of alpha-Spectrin to control axonal patterning.

Transformation of Drosophila cell lines: an alternative approach to exogenous protein expression

Techniques and experimental applications are described for exogenous protein expression in Drosophila cell lines. Ways in which the Drosophila cell lines and the baculovirus expression vector system differ in their applications are emphasized.

A postsynaptic spectrin scaffold defines active zone size, spacing, and efficacy at the Drosophila neuromuscular junction

Synaptic connections are established with characteristic, cell type–specific size and spacing. In this study, we document a role for the postsynaptic Spectrin skeleton in this process. We use transgenic double-stranded RNA to selectively eliminate α-Spectrin, β-Spectrin, or Ankyrin. In the absence of postsynaptic α- or β-Spectrin, active zone size is increased and spacing is perturbed. In addition, subsynaptic muscle membranes are significantly altered. However, despite these changes, the subdivision of the synapse into active zone and periactive zone domains remains intact, both pre- and postsynaptically. Functionally, altered active zone dimensions correlate with an increase in quantal size without a change in presynaptic vesicle size. Mechanistically, β-Spectrin is required for the localization of α-Spectrin and Ankyrin to the postsynaptic membrane. Although Ankyrin is not required for the localization of the Spectrin skeleton to the neuromuscular junction, it contributes to Spectrin-mediated synapse development. We propose a model in which a postsynaptic Spectrin–actin lattice acts as an organizing scaffold upon which pre- and postsynaptic development are arranged.

Functional links between membrane transport and the spectrin cytoskeleton

Membrane transporters precisely regulate which molecules cross the plasma membrane and when they can cross. In many cases it is also important to regulate where substances can cross the plasma membrane. Consequently, cells have evolved mechanisms to confine and stabilize membrane transport proteins within specific subdomains of the plasma membrane. A number of different transporters (including ion pumps, channels and exchangers) are known to physically associate with the spectrin cytoskeleton, a submembrane complex of spectrin and ankyrin. These proteins form a protein scaffold that assembles within discrete subdomains of the plasma membrane in polarized cells. Recent genetic studies in humans and model organisms have provided the opportunity to test the hypothesis that the spectrin cytoskeleton has a direct role in restricting transporters to specialized domains. Remarkably, genetic defects in spectrin and ankyrin can produce effects on cell physiology that are comparable to knockouts of the transporters themselves.

Phosphorylation of L1-type cell-adhesion molecules--ankyrins away!

Neural cell-adhesion molecules (CAMs) are powerful initiators of neurite outgrowth during neural differentiation. In addition, their interactions with cytoskeletal components are often conserved and highly regulated. How these two aspects of neural CAM function relate to each other is not well understood. However, a recent publication from the Felsenfeld laboratory ( http://www.mssm.edu/labs/felsenfeld) fills a gap in our knowledge of how the interaction of L1-type CAMs with the membrane skeleton adaptor protein ankyrin is severed by phosphorylation and suggests a feedback mechanism whereby the neurite-stimulating activity of L1-CAM is inversely connected to its cytoskeleton binding.

Axonal ensheathment and septate junction formation in the peripheral nervous system of Drosophila

Axonal insulation is critical for efficient action potential propagation and normal functioning of the nervous system. In Drosophila, the underlying basis of nerve ensheathment is the axonal insulation by glial cells and the establishment of septate junctions (SJs) between glial cell membranes. However, the details of the cellular and molecular mechanisms underlying axonal insulation and SJ formation are still obscure. Here, we report the characterization of axonal insulation in the Drosophila peripheral nervous system (PNS). Targeted expression of tau-green fluorescent protein in the glial cells and ultrastructural analysis of the peripheral nerves allowed us to visualize the glial ensheathment of axons. We show that individual or a group of axons are ensheathed by inner glial processes, which in turn are ensheathed by the outer perineurial glial cells. SJs are formed between the inner and outer glial membranes. We also show that Neurexin IV, Contactin, and Neuroglian are coexpressed in the peripheral glial membranes and that these proteins exist as a complex in the Drosophila nervous system. Mutations in neurexin IV, contactin, and neuroglian result in the disruption of blood-nerve barrier function in the PNS, and ultrastructural analyses of the mutant embryonic peripheral nerves show loss of glial SJs. Interestingly, the murine homologs of Neurexin IV, Contactin, and Neuroglian are expressed at the paranodal SJs and play a key role in axon-glial interactions of myelinated axons. Together, our data suggest that the molecular machinery underlying axonal insulation and axon-glial interactions may be conserved across species.

Activation of epidermal growth factor receptor mediates receptor axon sorting and extension in the developing olfactory system of the moth Manduca sexta

During development of the adult olfactory system of the moth Manduca sexta, olfactory receptor neurons extend axons from the olfactory epithelium in the antenna into the brain. As they arrive at the brain, interactions with centrally derived glial cells cause axons to sort and fasciculate with other axons destined to innervate the same glomeruli. Here we report studies indicating that activation of the epidermal growth factor receptor (EGFR) is involved in axon ingrowth and targeting. Blocking the EGFR kinase domain pharmacologically leads to stalling of many axons in the sorting zone and nerve layer as well as abnormal axonal fasciculation in the sorting zone. We also find that neuroglian, an IgCAM known to activate the EGFR through homophilic interactions in other systems, is transiently present on olfactory receptor neuron axons and on glia during the critical stages of the sorting process. The neuroglian is resistant to extraction with Triton X-100 in the sorting zone and nerve layer, possibly indicating its stabilization by homophilic binding in these regions. Our results suggest a mechanism whereby neuroglian molecules on axons and possibly sorting zone glia bind homophilically, leading to activation of EGFRs, with subsequent effects on axon sorting, pathfinding, and extension, and glomerulus development.

Presynaptic spectrin is essential for synapse stabilization

BACKGROUND

Precise neural circuitry is established and maintained through a regulated balance of synapse stabilization and disassembly. Currently, little is known about the molecular mechanisms that specify synapse stability versus disassembly.

RESULTS

Here, we demonstrate that presynaptic spectrin is an essential scaffold that is required to maintain synapse stability at the Drosophila neuromuscular junction (NMJ). Loss of presynaptic spectrin leads to synapse disassembly and ultimately to the elimination of the NMJ. Synapse elimination is documented through light-level, ultrastructural, and electrophysiological assays. These combined assays reveal that impaired neurotransmission is secondary to synapse retraction. We demonstrate that loss of presynaptic, but not postsynaptic, spectrin leads to the disorganization and elimination of essential synaptic cell-adhesion molecules. In addition, we provide evidence of altered axonal transport and disrupted synaptic microtubules as events that contribute to synapse retraction in animals lacking presynaptic spectrin.

CONCLUSIONS

Our data suggest that presynaptic spectrin functions as an essential presynaptic scaffold that may link synaptic cell adhesion with the stabilization of the underlying microtubule cytoskeleton.

L1-mediated branching is regulated by two ezrin-radixin-moesin (ERM)-binding sites, the RSLE region and a novel juxtamembrane ERM-binding region

We investigated how the neural cell adhesion molecule L1 mediates neurite outgrowth through L1-L1 homophilic interactions. Wild-type L1 and L1 with mutations in the cytoplasmic domain (CD) were introduced into L1 knock-out neurons, and transfected neurons were grown on an L1 substrate. Neurite length and branching were compared between wild-type L1 and L1CD mutations. Surprisingly, the L1CD is not required for L1-mediated neurite outgrowth but plays a critical role in neurite branching, through both the juxtamembrane region and the RSLE region. We demonstrate that both regions serve as ezrin-moesin-radixin-binding sites. A truncation mutant that deletes 110 of 114 amino acids of the L1CD still supports neurite outgrowth on an L1 substrate, suggesting that a coreceptor binds to L1 in cis and mediates neurite outgrowth and that L1-ankyrin interactions are not essential for neurite initiation or outgrowth. These data are consistent with a model in which L1 can influence L1-mediated neurite outgrowth and branching through both the L1CD and a coreceptor.

A novel epilepsy mutation in the sodium channel SCN1A identifies a cytoplasmic domain for beta subunit interaction

A mutation in the sodium channel SCN1A was identified in a small Italian family with dominantly inherited generalized epilepsy with febrile seizures plus (GEFS+). The mutation, D1866Y, alters an evolutionarily conserved aspartate residue in the C-terminal cytoplasmic domain of the sodium channel alpha subunit. The mutation decreased modulation of the alpha subunit by beta1, which normally causes a negative shift in the voltage dependence of inactivation in oocytes. There was less of a shift with the mutant channel, resulting in a 10 mV difference between the wild-type and mutant channels in the presence of beta1. This shift increased the magnitude of the window current, which resulted in more persistent current during a voltage ramp. Computational analysis suggests that neurons expressing the mutant channels will fire an action potential with a shorter onset delay in response to a threshold current injection, and that they will fire multiple action potentials with a shorter interspike interval at a higher input stimulus. These results suggest a causal relationship between a positive shift in the voltage dependence of sodium channel inactivation and spontaneous seizure activity. Direct interaction between the cytoplasmic C-terminal domain of the wild-type alpha subunit with the beta1 or beta3 subunit was first demonstrated by yeast two-hybrid analysis. The SCN1A peptide K1846-R1886 is sufficient for beta subunit interaction. Coimmunoprecipitation from transfected mammalian cells confirmed the interaction between the C-terminal domains of the alpha and beta1 subunits. The D1866Y mutation weakens this interaction, demonstrating a novel molecular mechanism leading to seizure susceptibility.

Copper cells and stomach acid secretion in the Drosophila midgut

Copper cells in the Drosophila midgut were originally named for their ability to accumulate dietary copper. Recent studies have uncovered a number of intriguing similarities between copper cells and the acid-producing gastric parietal cells of the mammalian stomach. In addition to their shared roles in stomach acidification, they share a peculiar invaginated morphology in which the apical cell surface is buried deep within the cytoplasm. These shared properties of morphology and function portend the identification of shared molecular mechanisms that account for their specialized roles in digestive physiology.

Activation of EGF receptor kinase by L1-mediated homophilic cell interactions

Neural cell adhesion molecules (CAMs) are important players during neurogenesis and neurite outgrowth as well as axonal fasciculation and pathfinding. Some of these developmental processes entail the activation of cellular signaling cascades. Pharmacological and genetic evidence indicates that the neurite outgrowth-promoting activity of L1-type CAMs is at least in part mediated by the stimulation of neuronal receptor tyrosine kinases (RTKs), especially FGF and EGF receptors. It has long been suspected that neural CAMs might physically interact with RTKs, but their activation by specific cell adhesion events has not been directly demonstrated. Here we report that gain-of-function conditions of the Drosophila L1-type CAM Neuroglian result in profound sensory axon pathfinding defects in the developing Drosophila wing. This phenotype can be suppressed by decreasing the normal gene dosage of the Drosophila EGF receptor gene. Furthermore, in Drosophila S2 cells, cell adhesion mediated by human L1-CAM results in the specific activation of human EGF tyrosine kinase at cell contact sites and EGF receptors engage in a physical interaction with L1-CAM molecules. Thus L1-type CAMs are able to promote the adhesion-dependent activation of EGF receptor signaling in vitro and in vivo.

Alpha-dystroglycan can mediate arenavirus infection in the absence of beta-dystroglycan

Dystroglycan (DG) is a highly versatile cell surface molecule that provides a molecular link between the extracellular matrix (ECM) and the actin-based cytoskeleton. Encoded by a single gene, DG is posttranslationally processed to form alpha-DG, a peripheral protein identified as the cellular receptor for lymphocytic choriomeningitis virus (LCMV) and Lassa fever virus (LFV), and the membrane-spanning subunit beta-DG. The link of beta-DG to the actin-based cytoskeleton and its association with the cellular signal transduction network suggest that it may function as an essential cofactor for the activity of alpha-DG as a virus receptor. To address this issue, we constructed a deletion mutant lacking the cytoplasmic domain of beta-DG and a C-terminal fusion between alpha-DG and the transmembrane domain of PDGF receptor. Both mutants were functional as virus receptors, indicating that beta-DG does not act as a cofactor with alpha-DG for arenavirus binding and entry. These observations are in agreement with the fact that LCMV infection is independent from the structural integrity of the actin-based cytoskeleton and suggest that alpha-DG functions primarily in the attachment of arenaviruses to the cell surface.

The Na+/K+ ATPase is required for septate junction function and epithelial tube-size control in the Drosophila tracheal system

Although the correct architecture of epithelial tubes is crucial for the function of organs such as the lung, kidney and vascular system, little is known about the molecular mechanisms that control tube size. We show that mutations in the ATPalpha alpha and nrv2 beta subunits of the Na+/K+ ATPase cause Drosophila tracheal tubes to have increased lengths and expanded diameters. ATPalpha and nrv2 mutations also disrupt stable formation of septate junctions, structures with some functional and molecular similarities to vertebrate tight junctions. The Nrv2 beta subunit isoforms have unique tube size and junctional functions because Nrv2, but not other Drosophila Na+/K+ ATPase beta subunits, can rescue nrv2 mutant phenotypes. Mutations in known septate junctions genes cause the same tracheal tube-size defects as ATPalpha and nrv2 mutations, indicating that septate junctions have a previously unidentified role in epithelial tube-size control. Double mutant analyses suggest that tube-size control by septate junctions is mediated by at least two discernable pathways, although the paracellular diffusion barrier function does not appear to involved because tube-size control and diffusion barrier function are genetically separable. Together, our results demonstrate that specific isoforms of the Na+/K+ ATPase play a crucial role in septate junction function and that septate junctions have multiple distinct functions that regulate paracellular transport and epithelial tube size.

Ankyrin binding mediates L1CAM interactions with static components of the cytoskeleton and inhibits retrograde movement of L1CAM on the cell surface

The function of adhesion receptors in both cell adhesion and migration depends critically on interactions with the cytoskeleton. During cell adhesion, cytoskeletal interactions stabilize receptors to strengthen adhesive contacts. In contrast, during cell migration, adhesion proteins are believed to interact with dynamic components of the cytoskeleton, permitting the transmission of traction forces through the receptor to the extracellular environment. The L1 cell adhesion molecule (L1CAM), a member of the Ig superfamily, plays a crucial role in both the migration of neuronal growth cones and the static adhesion between neighboring axons. To understand the basis of L1CAM function in adhesion and migration, we quantified directly the diffusion characteristics of L1CAM on the upper surface of ND-7 neuroblastoma hybrid cells as an indication of receptor-cytoskeleton interactions. We find that cell surface L1CAM engages in diffusion, retrograde movement, and stationary behavior, consistent with interactions between L1CAM and two populations of cytoskeleton proteins. We provide evidence that the cytoskeletal adaptor protein ankyrin mediates stationary behavior while inhibiting the actin-dependent retrograde movement of L1CAM. Moreover, inhibitors of L1CAM-ankyrin interactions promote L1CAM-mediated axon growth. Together, these results suggest that ankyrin binding plays a crucial role in the anti-coordinate regulation of L1CAM-mediated adhesion and migration.

Neuroglian, Gliotactin, and the Na+/K+ ATPase are essential for septate junction function in Drosophila

One essential function of epithelia is to form a barrier between the apical and basolateral surfaces of the epithelium. In vertebrate epithelia, the tight junction is the primary barrier to paracellular flow across epithelia, whereas in invertebrate epithelia, the septate junction (SJ) provides this function. In this study, we identify new proteins that are required for a functional paracellular barrier in Drosophila. In addition to the previously known components Coracle (COR) and Neurexin (NRX), we show that four other proteins, Gliotactin, Neuroglian (NRG), and both the alpha and beta subunits of the Na+/K+ ATPase, are required for formation of the paracellular barrier. In contrast to previous reports, we demonstrate that the Na pump is not localized basolaterally in epithelial cells, but instead is concentrated at the SJ. Data from immunoprecipitation and somatic mosaic studies suggest that COR, NRX, NRG, and the Na+/K+ ATPase form an interdependent complex. Furthermore, the observation that NRG, a Drosophila homologue of vertebrate neurofascin, is an SJ component is consistent with the notion that the invertebrate SJ is homologous to the vertebrate paranodal SJ. These findings have implications not only for invertebrate epithelia and barrier functions, but also for understanding of neuron-glial interactions in the mammalian nervous system.

Neuroglian activates Echinoid to antagonize the Drosophila EGF receptor signaling pathway

echinoid (ed) encodes an cell-adhesion molecule (CAM) that contains immunoglobulin domains and regulates the EGFR signaling pathway during Drosophila eye development. Based on our previous genetic mosaic and epistatic analysis, we proposed that Ed, via homotypic interactions, activates a novel, as yet unknown pathway that antagonizes EGFR signaling. In this report, we demonstrate that Ed functions as a homophilic adhesion molecule and also engages in a heterophilic trans-interaction with Drosophila Neuroglian (Nrg), an L1-type CAM. Co-expression of ed and nrg in the eye exhibits a strong genetic synergy in inhibiting EGFR signaling. This synergistic effect requires the intracellular domain of Ed, but not that of Nrg. In addition, Ed and Nrg colocalize in the Drosophila eye and are efficiently co-immunoprecipitated. Together, our results suggest a model in which Nrg acts as a heterophilic ligand and activator of Ed, which in turn antagonizes EGFR signaling.

Identification of a critical ankyrin-binding loop on the cytoplasmic domain of erythrocyte membrane band 3 by crystal structure analysis and site-directed mutagenesis

The cytoplasmic domain of erythrocyte membrane band 3 (cdb3) serves as a center of membrane organization, interacting with such proteins as ankyrin, protein 4.1, protein 4.2, hemoglobin, several glycolytic enzymes, a tyrosine phosphatase, and a tyrosine kinase, p72(syk). The crystallographic structure of the cdb3 dimer has revealed that residues 175-185 assume a beta-hairpin loop similar to a putative ankyrin-binding motif at the cytoplasmic surface of the Na(+)/K(+)-ATPase. To test whether this hairpin loop constitutes an ankyrin-binding site on cdb3, we have deleted amino acids 175-185 and substituted the 11-residue loop with a Gly-Gly dipeptide that bridges the deletion without introducing strain into the structure. Although the deletion mutant undergoes the same native conformational changes exhibited by wild type cdb3 and binds other peripheral proteins normally, the mutant exhibits no affinity for ankyrin. This suggests that the exposed beta-hairpin turn indeed constitutes a major ankyrin-binding site on cdb3. Other biochemical studies suggest that ankyrin also docks at the NH(2) terminus of band 3. Thus, antibodies to the NH(2) terminus of cdb3 block ankyrin binding to the cdb3, and ankyrin binding to cdb3 prevents p72(syk) phosphorylation of cdb3 at its NH(2) terminus (predominantly at Tyr-8). However, a truncation mutant of cdb3 lacking the NH(2)-terminal 50 residues displays the same binding affinity as wild type cdb3. These data thus suggest that the NH(2) terminus of cdb3 is proximal to but not required for the cdb3-ankyrin interaction.

A new activity of doublecortin in recognition of the phospho-FIGQY tyrosine in the cytoplasmic domain of neurofascin

Doublecortin is a cytoplasmic protein mutated in the neuronal migration disorder X-linked lissencephaly. This study describes a novel activity of doublecortin in recognition of the FIGQY-phosphotyrosine motif present in the cytoplasmic domain of the L1 cell adhesion molecule neurofascin. Phospho-FIGQY-neurofascin (186 kDa) coimmunoprecipitated with doublecortin from detergent extracts of embryonic brain membranes, and this doublecortin-phospho-FIGQY neurofascin complex was disassociated by a synthetic phospho-FIGQY neurofascin peptide but not by a dephospho-FIGQY peptide. Doublecortin specifically recognized the phospho-FIGQY tyrosine in the context of a synthetic phospho-FIGQY neurofascin peptide and in phospho-FIGQY neurofascin isolated from cells treated with pervanadate. Mutations of doublecortin causing lissencephaly (R59H, D62N, and G253D) abolished binding to the phospho-FIGQY peptide and to phospho-FIGQY neurofascin. Finally, phospho-FIGQY neurofascin and doublecortin colocalize in developing axon tracts and in zones enriched in migrating neurons in the embryonic cerebral cortex. In the adult rostral migratory stream, doublecortin colocalizes in migrating neurons with a phospho-FIGQY bearing L1 CAM different from neurofascin. The finding that doublecortin associates with FIGQY-phosphorylated neurofascin provides the first connection of doublecortin with the plasma membrane and could be important for a function of doublecortin in directing neuronal migration.

A new activity of doublecortin in recognition of the phospho-FIGQY tyrosine in the cytoplasmic domain of neurofascin

Doublecortin is a cytoplasmic protein mutated in the neuronal migration disorder X-linked lissencephaly. This study describes a novel activity of doublecortin in recognition of the FIGQY-phosphotyrosine motif present in the cytoplasmic domain of the L1 cell adhesion molecule neurofascin. Phospho-FIGQY-neurofascin (186 kDa) coimmunoprecipitated with doublecortin from detergent extracts of embryonic brain membranes, and this doublecortin-phospho-FIGQY neurofascin complex was disassociated by a synthetic phospho-FIGQY neurofascin peptide but not by a dephospho-FIGQY peptide. Doublecortin specifically recognized the phospho-FIGQY tyrosine in the context of a synthetic phospho-FIGQY neurofascin peptide and in phospho-FIGQY neurofascin isolated from cells treated with pervanadate. Mutations of doublecortin causing lissencephaly (R59H, D62N, and G253D) abolished binding to the phospho-FIGQY peptide and to phospho-FIGQY neurofascin. Finally, phospho-FIGQY neurofascin and doublecortin colocalize in developing axon tracts and in zones enriched in migrating neurons in the embryonic cerebral cortex. In the adult rostral migratory stream, doublecortin colocalizes in migrating neurons with a phospho-FIGQY bearing L1 CAM different from neurofascin. The finding that doublecortin associates with FIGQY-phosphorylated neurofascin provides the first connection of doublecortin with the plasma membrane and could be important for a function of doublecortin in directing neuronal migration.

Posterior midgut epithelial cells differ in their organization of the membrane skeleton from other drosophila epithelia

In epithelial cells, the various components of the membrane skeleton are segregated within specialized subregions of the plasma membrane, thus contributing to the development and stabilization of cell surface polarity. It has previously been shown that, in various Drosophila epithelia, the membrane skeleton components ankyrin and alphabeta-spectrin reside at the lateral surface, whereas alphabeta(H)-spectrin is restricted to the apical domain. By use of confocal immunofluorescence microscopy, the present study characterizes the membrane skeleton of epithelial cells in the posterior midgut, leading to a number of unexpected results. First, ankyrin and alphabeta-spectrin are not detected on the entire lateral surface but appear to be restricted to the apicolateral area, codistributing with fasciclin III at smooth septate junctions. The presumptive ankyrin-binding proteins neuroglian and Na(+),K(+)-ATPase, however, do not colocalize with ankyrin. Second, alphabeta(H)-spectrin is enriched at the apical domain but is also present in lower amounts on the entire lateral surface, colocalizing apicolaterally with ankyrin/alphabeta-spectrin. Finally, despite the absence of zonulae adherentes, F-actin, beta(H)-spectrin, and nonmuscle myosin-II are enriched in the midlateral region. Thus, the model established for the organization of the membrane skeleton in Drosophila epithelia does not hold for the posterior midgut, and there is quite some variability between the different epithelia with respect to the organization of the membrane skeleton.

Spectrin and ankyrin-based pathways: metazoan inventions for integrating cells into tissues

The spectrin-based membrane skeleton of the humble mammalian erythrocyte has provided biologists with a set of interacting proteins with diverse roles in organization and survival of cells in metazoan organisms. This review deals with the molecular physiology of spectrin, ankyrin, which links spectrin to the anion exchanger, and two spectrin-associated proteins that promote spectrin interactions with actin: adducin and protein 4.1. The lack of essential functions for these proteins in generic cells grown in culture and the absence of their genes in the yeast genome have, until recently, limited advances in understanding their roles outside of erythrocytes. However, completion of the genomes of simple metazoans and application of homologous recombination in mice now are providing the first glimpses of the full scope of physiological roles for spectrin, ankyrin, and their associated proteins. These functions now include targeting of ion channels and cell adhesion molecules to specialized compartments within the plasma membrane and endoplasmic reticulum of striated muscle and the nervous system, mechanical stabilization at the tissue level based on transcellular protein assemblies, participation in epithelial morphogenesis, and orientation of mitotic spindles in asymmetric cell divisions. These studies, in addition to stretching the erythrocyte paradigm beyond recognition, also are revealing novel cellular pathways essential for metazoan life. Examples are ankyrin-dependent targeting of proteins to excitable membrane domains in the plasma membrane and the Ca(2+) homeostasis compartment of the endoplasmic reticulum. Exciting questions for the future relate to the molecular basis for these pathways and their roles in a clinical context, either as the basis for disease or more positively as therapeutic targets.