The Drosophila lethal(2)giant larvae tumor suppressor protein is a component of the cytoskeleton

Damage control of epithelial barrier function in dynamic environments

Epithelial tissues cover the surfaces and lumens of the internal organs of multicellular animals and crucially contribute to internal environment homeostasis by delineating distinct compartments within the body. This vital role is known as epithelial barrier function. Epithelial cells are arranged like cobblestones and intricately bind together to form an epithelial sheet that upholds this barrier function. Central to the restriction of solute and fluid diffusion through intercellular spaces are occluding junctions, tight junctions in vertebrates and septate junctions in invertebrates. As part of epithelial tissues, cells undergo constant renewal, with older cells being replaced by new ones. Simultaneously, the epithelial tissue undergoes relative rearrangement, elongating, and shifting directionally as a whole. The movement or shape changes within the epithelial sheet necessitate significant deformation and reconnection of occluding junctions. Recent advancements have shed light on the intricate mechanisms through which epithelial cells sustain their barrier function in dynamic environments. This review aims to introduce these noteworthy findings and discuss some of the questions that remain unanswered.

Lateral cell polarization drives organization of epithelia in sea anemone embryos and embryonic cell aggregates

One of the first organizing processes during animal development is the assembly of embryonic cells into epithelia. In certain animals, including Hydra and sea anemones, epithelia also emerge when cells from dissociated tissues are aggregated back together. Although cell adhesion is required to keep cells together, it is not clear whether cell polarization plays a role as epithelia emerge from disordered aggregates. Here, we demonstrate that lateral cell polarization is essential for epithelial organization in both embryos and aggregates of the sea anemone Nematostella vectensis. Specifically, knock down of the lateral polarity protein Lgl disrupts epithelia in developing embryos and impairs the capacity of dissociated cells to epithelialize from aggregates. Cells in lgl mutant epithelia lose their columnar shape and have mispositioned mitotic spindles and ciliary basal bodies. Together, our data suggest that in Nematostella, Lgl is required to establish lateral cell polarity and position cytoskeletal organelles in cells of embryos and aggregates during de novo epithelial organization.

Identification of the genes encoding candidate septate junction components expressed during early development of the sea urchin, Strongylocentrotus purpuratus, and evidence of a role for Mesh in the formation of the gut barrier

Septate junctions (SJs) evolved as cell-cell junctions that regulate the paracellular barrier and integrity of epithelia in invertebrates. Multiple morphological variants of SJs exist specific to different epithelia and/or phyla but the biological significance of varied SJ morphology is unclear because the knowledge of the SJ associated proteins and their functions in non-insect invertebrates remains largely unknown. Here we report cell-specific expression of nine candidate SJ genes in the early life stages of the sea urchin Strongylocentrotus purpuratus. By use of in situ RNA hybridization and single cell RNA-seq we found that the expression of selected genes encoding putatively SJ associated transmembrane and cytoplasmic scaffold molecules was dynamically regulated during epithelial development in the embryos and larvae with different epithelia expressing different cohorts of SJ genes. We focused a functional analysis on SpMesh, a homolog of the Drosophila smooth SJ component Mesh, which was highly enriched in the endodermal epithelium of the mid- and hindgut. Functional perturbation of SpMesh by both CRISPR/Cas9 mutagenesis and vivo morpholino-mediated knockdown shows that loss of SpMesh does not disrupt the formation of the gut epithelium during gastrulation. However, loss of SpMesh resulted in a severely reduced gut-paracellular barrier as quantitated by increased permeability to 3-5 ​kDa FITC-dextran. Together, these studies provide a first look at the molecular SJ physiology during the development of a marine organism and suggest a shared role for Mesh-homologous proteins in forming an intestinal barrier in invertebrates. Results have implications for consideration of the traits underlying species-specific sensitivity of marine larvae to climate driven ocean change.

Apocrine secretion in the salivary glands of Drosophilidae and other dipterans is evolutionarily conserved

Apocrine secretion is a transport and secretory mechanism that remains only partially characterized, even though it is evolutionarily conserved among all metazoans, including humans. The excellent genetic model organism Drosophila melanogaster holds promise for elucidating the molecular mechanisms regulating this fundamental metazoan process. Two prerequisites for such investigations are to clearly define an experimental system to investigate apocrine secretion and to understand the evolutionarily and functional contexts in which apocrine secretion arose in that system. To this end, we recently demonstrated that, in D. melanogaster, the prepupal salivary glands utilize apocrine secretion prior to pupation to deliver innate immune and defense components to the exuvial fluid that lies between the metamorphosing pupae and its chitinous case. This finding provided a unique opportunity to appraise how this novel non-canonical and non-vesicular transport and secretory mechanism is employed in different developmental and evolutionary contexts. Here we demonstrate that this apocrine secretion, which is mechanistically and temporarily separated from the exocytotic mechanism used to produce the massive salivary glue secretion (Sgs), is shared across Drosophilidae and two unrelated dipteran species. Screening more than 30 species of Drosophila from divergent habitats across the globe revealed that apocrine secretion is a widespread and evolutionarily conserved cellular mechanism used to produce exuvial fluid. Species with longer larval and prepupal development than D. melanogaster activate apocrine secretion later, while smaller and more rapidly developing species activate it earlier. In some species, apocrine secretion occurs after the secretory material is first concentrated in cytoplasmic structures of unknown origin that we name "collectors." Strikingly, in contrast to the widespread use of apocrine secretion to provide exuvial fluid, not all species use exocytosis to produce the viscid salivary glue secretion that is seen in D. melanogaster. Thus, apocrine secretion is the conserved mechanism used to realize the major function of the salivary gland in fruitflies and related species: it produces the pupal exuvial fluid that provides an active defense against microbial invasion during pupal metamorphosis.

How cells tell up from down and stick together to construct multicellular tissues - interplay between apicobasal polarity and cell-cell adhesion

Polarized epithelia define a topological inside and outside, and hence constitute a key evolutionary innovation that enabled the construction of complex multicellular animal life. Over time, this basic function has been elaborated upon to yield the complex architectures of many of the organs that make up the human body. The two processes necessary to yield a polarized epithelium, namely regulated adhesion between cells and the definition of the apicobasal (top-bottom) axis, have likewise undergone extensive evolutionary elaboration, resulting in multiple sophisticated protein complexes that contribute to both functions. Understanding how these components function in combination to yield the basic architecture of a polarized cell-cell junction remains a major challenge. In this Review, we introduce the main components of apicobasal polarity and cell-cell adhesion complexes, and outline what is known about their regulation and assembly in epithelia. In addition, we highlight studies that investigate the interdependence between these two networks. We conclude with an overview of strategies to address the largest and arguably most fundamental unresolved question in the field, namely how a polarized junction arises as the sum of its molecular parts.

An apocrine mechanism delivers a fully immunocompetent exocrine secretion

Apocrine secretion is a recently discovered widespread non-canonical and non-vesicular secretory mechanism whose regulation and purpose is only partly defined. Here, we demonstrate that apocrine secretion in the prepupal salivary glands (SGs) of Drosophila provides the sole source of immune-competent and defense-response proteins to the exuvial fluid that lies between the metamorphosing pupae and its pupal case. Genetic ablation of its delivery from the prepupal SGs to the exuvial fluid decreases the survival of pupae to microbial challenges, and the isolated apocrine secretion has strong antimicrobial effects in "agar-plate" tests. Thus, apocrine secretion provides an essential first line of defense against exogenously born infection and represents a highly specialized cellular mechanism for delivering components of innate immunity at the interface between an organism and its external environment.

The Drosophila septate junctions beyond barrier function: Review of the literature, prediction of human orthologs of the SJ-related proteins and identification of protein domain families

The involvement of Septate Junctions (SJs) in critical cellular functions that extend beyond their role as diffusion barriers in the epithelia and the nervous system has made the fruit fly an ideal model for the study of human diseases associated with impaired Tight Junction (TJ) function. In this study, we summarized current knowledge of the Drosophila melanogaster SJ-related proteins, focusing on their unconventional functions. Additionally, we sought to identify human orthologs of the corresponding genes as well as protein domain families. The systematic literature search was performed in PubMed and Scopus databases using relevant key terms. Orthologs were predicted using the DIOPT tool and aligned protein regions were determined from the Pfam database. 3-D models of the smooth SJ proteins were built on the Phyre2 and DMPFold protein structure prediction servers. A total of 30 proteins were identified as relatives to the SJ cellular structure. Key roles of these proteins, mainly in the regulation of morphogenetic events and cellular signalling, were highlighted. The investigation of protein domain families revealed that the SJ-related proteins contain conserved domains that are required not only for cell-cell interactions and cell polarity but also for cellular signalling and immunity. DIOPT analysis of orthologs identified novel human genes as putative functional homologs of the fruit fly SJ genes. A gap in our knowledge was identified regarding the domains that occur in the proteins encoded by eight SJ-associated genes. Future investigation of these domains is needed to provide functional information.

Scribble, Lgl1, and myosin II form a complex in vivo to promote directed cell migration

Scribble (Scrib) and Lethal giant larvae 1 (Lgl1) are conserved polarity proteins that play important roles in different forms of cell polarity. The roles of Scrib and Lgl1 in apical-basal cell polarity have been studied extensively, but little is known about their roles in the cell polarity of migrating cells. Furthermore, the effect of Scrib and Lgl1 interaction on cell polarity is largely unknown. In this study, we show that Scrib, through its leucine-rich repeat domain, forms a complex in vivo with Lgl1. Scrib also forms a complex with myosin II, and Scrib, Lgl1, and myosin II colocalize at the leading edge of migrating cells. The cellular localization and the cytoskeletal association of Scrib and Lgl1 are interdependent, as depletion of either protein affects its counterpart. In addition, depletion of either Scrib or Lgl1 disrupts the cellular localization of myosin II. We show that depletion of either Scrib or Lgl1 affects cell adhesion through the inhibition of focal adhesion disassembly. Finally, we show that Scrib and Lgl1 are required for proper cell polarity of migrating cells. These results provide new insights into the mechanism regulating the cell polarity of migrating cells by Scrib, Lgl1, and myosin II.

The septate junction protein Mesh is required for epithelial morphogenesis, ion transport, and paracellular permeability in the Drosophila Malpighian tubule

Septate junctions (SJs) are occluding cell-cell junctions that have roles in paracellular permeability and barrier function in the epithelia of invertebrates. Arthropods have two types of SJs, pleated SJs and smooth SJs (sSJs). In Drosophila melanogaster, sSJs are found in the midgut and Malpighian tubules, but the functions of sSJs and their protein components in the tubule epithelium are unknown. Here we examined the role of the previously identified integral sSJ component, Mesh, in the Malpighian tubule. We genetically manipulated mesh specifically in the principal cells of the tubule at different life stages. Tubules of flies with developmental mesh knockdown revealed defects in epithelial architecture, sSJ molecular and structural organization, and lack of urine production in basal and kinin-stimulated conditions, resulting in edema and early adult lethality. Knockdown of mesh during adulthood did not disrupt tubule epithelial and sSJ integrity but decreased the transepithelial potential, diminished transepithelial fluid and ion transport, and decreased paracellular permeability to 4-kDa dextran. Drosophila kinin decreased transepithelial potential and increased chloride permeability, and it stimulated fluid secretion in both control and adult mesh knockdown tubules but had no effect on 4-kDa dextran flux. Together, these data indicate roles for Mesh in the developmental maturation of the Drosophila Malpighian tubule and in ion and macromolecular transport in the adult tubule.

Structural insights into the aPKC regulatory switch mechanism of the human cell polarity protein lethal giant larvae 2

Metazoan cell polarity is controlled by a set of highly conserved proteins. Lethal giant larvae (Lgl) functions in apical-basal polarity through phosphorylation-dependent interactions with several other proteins as well as the plasma membrane. Phosphorylation of Lgl by atypical protein kinase C (aPKC), a component of the partitioning-defective (Par) complex in epithelial cells, excludes Lgl from the apical membrane, a crucial step in the establishment of epithelial cell polarity. We present the crystal structures of human Lgl2 in both its unphosphorylated and aPKC-phosphorylated states. Lgl2 adopts a double β-propeller structure that is unchanged by aPKC phosphorylation of an unstructured loop in its second β-propeller, ruling out models of phosphorylation-dependent conformational change. We demonstrate that phosphorylation controls the direct binding of purified Lgl2 to negative phospholipids in vitro. We also show that a coil-helix transition of this region that is promoted by phosphatidylinositol 4,5-bisphosphate (PIP₂) is also phosphorylation-dependent, implying a highly effective phosphorylative switch for membrane association.

The Scribble Cell Polarity Module in the Regulation of Cell Signaling in Tissue Development and Tumorigenesis

The Scribble cell polarity module, comprising Scribbled (Scrib), Discs-large (Dlg) and Lethal-2-giant larvae (Lgl), has a tumor suppressive role in mammalian epithelial cancers. The Scribble module proteins play key functions in the establishment and maintenance of different modes of cell polarity, as well as in the control of tissue growth, differentiation and directed cell migration, and therefore are major regulators of tissue development and homeostasis. Whilst molecular details are known regarding the roles of Scribble module proteins in cell polarity regulation, their precise mode of action in the regulation of other key cellular processes remains enigmatic. An accumulating body of evidence indicates that Scribble module proteins play scaffolding roles in the control of various signaling pathways, which are linked to the control of tissue growth, differentiation and cell migration. Multiple Scrib, Dlg and Lgl interacting proteins have been discovered, which are involved in diverse processes, however many function in the regulation of cellular signaling. Herein, we review the components of the Scrib, Dlg and Lgl protein interactomes, and focus on the mechanism by which they regulate cellular signaling pathways in metazoans, and how their disruption leads to cancer.

Spatiotemporal phosphoregulation of Lgl: Finding meaning in multiple on/off buttons

Intracellular asymmetries, often termed cell polarity, determine how cells organize and divide to ultimately control cell fate and shape animal tissues. The tumor suppressor Lethal giant larvae (Lgl) functions at the core of the evolutionarily conserved cell polarity machinery that controls apico-basal polarization. This function relies on its restricted basolateral localization via phosphorylation by aPKC. Here, we summarize the spatial and temporal control of Lgl during the cell cycle, highlighting two ideas that emerged from our recent findings: 1) Aurora A directly phosphorylates Lgl during symmetric division to couple reorganization of epithelial polarity with the cell cycle; 2) Phosphorylation of Lgl within three conserved serines controls its localization and function in a site-specific manner. Considering the importance of phosphorylation to regulate the concentration of Lgl at the plasma membrane, we will further discuss how it may work as an on-off switch for the interaction with cortical binding partners, with implications on epithelial polarization and spindle orientation.

aPKC regulates apical localization of Lgl to restrict elongation of microridges in developing zebrafish epidermis

Epithelial cells exhibit apical membrane protrusions, which confer specific functions to epithelial tissues. Microridges are short actin protrusions that are laterally long and form a maze-like pattern in the apical domain. They are widely found on vertebrate squamous epithelia including epidermis and have functions in mucous retention, membrane storage and abrasion resistance. It is largely unknown how the formation of these laterally long actin projections is regulated. Here, we show that antagonistic interactions between aPKC and Lgl–regulators of apical and basolateral domain identity, respectively,–control the length of microridges in the zebrafish periderm, the outermost layer of the epidermis. aPKC regulates the levels of Lgl and the active form of non-muscle myosinII at the apical cortex to prevent actin polymerization-dependent precocious fusion and elongation of microridges. Our data unravels the functional significance of exclusion of Lgl from the apical domain in epithelial cells.

Squamous epithelia present actin-rich microridges on the apical surface, but the mechanism of their formation is not known. Here the authors show that, in zebrafish epidermis, the exclusion of the basolateral regulator Lgl from the apical domain by atypical protein kinase C prevents precocious elongation and fusion of microridges.

Hugl-1 inhibits glioma cell growth in intracranial model

Drosophila lethal (2) giant larvae (lgl) has been reported as a tumor suppressor and could regulate the Drosophila hippo signaling. Human giant larvae-1(Hugl-1), one human homologue of Drosophila lgl, also has been reported to be involved in the development of some human cancers. However, whether Hugl-1 is associated with the pathogenesis of malignant gliomas remains poorly understood. In the present work, we examined the effect of Hugl-1 on glioma cell growth both in vitro and in vivo. Firstly, we found that Hugl-1 protein levels decreased in the human glioma tissues, suggesting that Hugl-1 is involved in glioma progression. Unfortunately, either stably or transiently over-expressing Hugl-1 did not affect glioma cell proliferation in vitro. In addition, Hugl-1 over-expression did not regulate hippo signaling pathway. Interestingly, over-expression of Hugl-1 not only inhibited gliomagenesis but also markedly inhibited cell proliferation and promoted the apoptosis of U251 cells in an orthotopic model of nude mice. Taken together, this study provides the evidence that Hugl-1 inhibits glioma cell growth in intracranial model of nude mice, suggesting that Hugl-1 might be a potential tumor target for glioma therapy.

Changes in organelle position and epithelial architecture associated with loss of CrebA

Drosophila CrebA facilitates high-level secretion by transcriptional upregulation of the protein components of the core secretory machinery. In CrebA mutant embryos, both salivary gland (SG) morphology and epidermal cuticle secretion are abnormal, phenotypes similar to those observed with mutations in core secretory pathway component genes. Here, we examine the cellular defects associated with CrebA loss in the SG epithelium. Apically localized secretory vesicles are smaller and less abundant, consistent with overall reductions in secretion. Unexpectedly, global mislocalization of cellular organelles and excess membrane accumulation in the septate junctions (SJs) are also observed. Whereas mutations in core secretory pathway genes lead to organelle localization defects similar to those of CrebA mutants, they have no effect on SJ-associated membrane. Mutations in tetraspanin genes, which are normally repressed by CrebA, have mild defects in SJ morphology that are rescued by simultaneous CrebA loss. Correspondingly, removal of several tetraspanins gives partial rescue of the CrebA SJ phenotype, supporting a role for tetraspanins in SJ organization.

The unforeseen challenge: from genotype-to-phenotype in cell populations

Biological cells present a paradox, in that they show simultaneous stability and flexibility, allowing them to adapt to new environments and to evolve over time. The emergence of stable cell states depends on genotype-to-phenotype associations, which essentially reflect the organization of gene regulatory modes. The view taken here is that cell-state organization is a dynamical process in which the molecular disorder manifests itself in a macroscopic order. The genome does not determine the ordered cell state; rather, it participates in this process by providing a set of constraints on the spectrum of regulatory modes, analogous to boundary conditions in physical dynamical systems. We have developed an experimental framework, in which cell populations are exposed to unforeseen challenges; novel perturbations they had not encountered before along their evolutionary history. This approach allows an unbiased view of cell dynamics, uncovering the potential of cells to evolve and develop adapted stable states. In the last decade, our experiments have revealed a coherent set of observations within this framework, painting a picture of the living cell that in many ways is not aligned with the conventional one. Of particular importance here, is our finding that adaptation of cell-state organization is essentially an efficient exploratory dynamical process rather than one founded on random mutations. Based on our framework, a set of concepts underlying cell-state organization-exploration evolving by global, non-specific, dynamics of gene activity-is presented here. These concepts have significant consequences for our understanding of the emergence and stabilization of a cell phenotype in diverse biological contexts. Their implications are discussed for three major areas of biological inquiry: evolution, cell differentiation and cancer. There is currently no unified theoretical framework encompassing the emergence of order, a stable state, in the living cell. Hopefully, the integrated picture described here will provide a modest contribution towards a physics theory of the cell.

The heavy chain has its day: regulation of myosin-II assembly

Nonmuscle myosin-II is an actin-based motor that converts chemical energy into force and movement, and thus functions as a key regulator of the eukaryotic cytoskeleton. Although it is established that phosphorylation on the regulatory light chain increases the actin-activated MgATPase activity of the motor and promotes myosin-II filament assembly, studies have begun to characterize alternative mechanisms that regulate filament assembly and disassembly. These investigations have revealed that all three nonmuscle myosin-II isoforms are subject to additional regulatory controls, which impact diverse cellular processes. In this review, we discuss current knowledge on mechanisms that regulate the oligomerization state of nonmuscle myosin-II filaments by targeting the myosin heavy chain.

Aurora A triggers Lgl cortical release during symmetric division to control planar spindle orientation

Mitotic spindle orientation is essential to control cell-fate specification and epithelial architecture. The tumor suppressor Lgl localizes to the basolateral cortex of epithelial cells, where it acts together with Dlg and Scrib to organize apicobasal polarity. Dlg and Scrib also control planar spindle orientation, but how the organization of polarity complexes is adjusted to control symmetric division is largely unknown. Here, we show that the Dlg complex is remodeled during Drosophila follicular epithelium cell division, when Lgl is released to the cytoplasm. Lgl redistribution during epithelial mitosis is reminiscent of asymmetric cell division, where it is proposed that Aurora A promotes aPKC activation to control the localization of Lgl and cell-fate determinants. We show that Aurora A controls Lgl localization directly, triggering its cortical release at early prophase in both epithelial and S2 cells. This relies on double phosphorylation within the putative aPKC phosphorylation site, which is required and sufficient for Lgl cortical release during mitosis and can be achieved by a combination of aPKC and Aurora A activities. Cortical retention of Lgl disrupts planar spindle orientation, but only when Lgl mutants that can bind Dlg are expressed. Hence, our work reveals that Lgl mitotic cortical release is not specifically linked to the asymmetric segregation of fate determinants, and we propose that Aurora A activation breaks the Dlg/Lgl interaction to allow planar spindle orientation during symmetric division via the Pins (LGN)/Dlg pathway.

The lethal giant larvae gene in Tribolium castaneum: molecular properties and roles in larval and pupal development as revealed by RNA interference

We identified and characterized the TcLgl gene putatively encoding lethal giant larvae (Lgl) protein from the red flour beetle (Tribolium castaneum). Analyses of developmental stage and tissue-specific expression patterns revealed that TcLgl was constitutively expressed. To examine the role of TcLgl in insect development, RNA interference was performed in early (1-day) larvae, late (20-day) larvae, and early (1-day) pupae. The early larvae injected with double-stranded RNA of TcLgl (dsTcLgl) at 100, 200, and 400 ng/larva failed to pupate, and 100% mortality was achieved within 20 days after the injection or before the pupation. The late larvae injected with dsTcLgl at these doses reduced the pupation rates to only 50.3%, 36.0%, and 18.2%, respectively. The un-pupated larvae gradually died after one week, and visually unaffected pupae failed to emerge into adults and died during the pupal stage. Similarly, when early pupae were injected with dsTcLgl at these doses, the normal eclosion rates were reduced to only 22.5%, 18.0%, and 11.2%, respectively, on day 7 after the injection, and all the adults with abnormal eclosion died in two days after the eclosion. These results indicate that TcLgl plays an essential role in insect development, especially during their metamorphosis.

Cadherin 99C regulates apical expansion and cell rearrangement during epithelial tube elongation

Apical and basolateral determinants specify and maintain membrane domains in epithelia. Here, we identify new roles for two apical surface proteins - Cadherin 99C (Cad99C) and Stranded at Second (SAS) - in conferring apical character in Drosophila tubular epithelia. Cad99C, the Drosophila ortholog of human Usher protocadherin PCDH15, is expressed in several embryonic tubular epithelial structures. Through loss-of-function and overexpression studies, we show that Cad99C is required to regulate cell rearrangement during salivary tube elongation. We further show that overexpression of either Cad99C or SAS causes a dramatic increase in apical membrane at the expense of other membrane domains, and that both proteins can do this independently of each other and independently of mislocalization of the apical determinant Crumbs (Crb). Overexpression of Cad99C or SAS results in similar, but distinct effects, suggesting both shared and unique roles for these proteins in conferring apical identity.

Polarity protein complex Scribble/Lgl/Dlg and epithelial cell barriers

The Scribble polarity complex or module is one of the three polarity modules that regulate cell polarity in multiple epithelia including blood-tissue barriers. This protein complex is composed of Scribble, Lethal giant larvae (Lgl) and Discs large (Dlg), which are well conserved across species from fruitflies and worms to mammals. Originally identified in Drosophila and C. elegans where the Scribble complex was found to work with the Par-based and Crumbs-based polarity modules to regulate apicobasal polarity and asymmetry in cells and tissues during embryogenesis, their mammalian homologs have all been identified in recent years. Components of the Scribble complex are known to regulate multiple cellular functions besides cell polarity, which include cell proliferation, assembly and maintenance of adherens junction (AJ) and tight junction (TJ), and they are also tumor suppressors. Herein, we provide an update on the Scribble polarity complex and how this protein complex modulates cell adhesion with some emphasis on its role in Sertoli cell blood-testis barrier (BTB) function. It should be noted that this is a rapidly developing field, in particular the role of this protein module in blood-tissue barriers, and this short chapter attempts to provide the information necessary for investigators studying reproductive biology and blood-tissue barriers to design future studies. We also include results of recent studies from flies and worms since this information will be helpful in planning experiments for future functional studies in the testis to understand how Scribble-based proteins regulate BTB dynamics and spermatogenesis.

Function and cancer genomics of FAT family genes (review)

FAT1, FAT2, FAT3 and FAT4 are human homologs of Drosophila Fat, which is involved in tumor suppression and planar cell polarity (PCP). FAT1 and FAT4 undergo the first proteolytic cleavage by Furin and are predicted to undergo the second cleavage by γ-secretase to release intracellular domain (ICD). Ena/VAPS-binding to FAT1 induces actin polymerization at lamellipodia and filopodia to promote cell migration, while Scribble-binding to FAT1 induces phosphorylation and functional inhibition of YAP1 to suppress cell growth. FAT1 is repressed in oral cancer owing to homozygous deletion or epigenetic silencing and is preferentially downregulated in invasive breast cancer. On the other hand, FAT1 is upregulated in leukemia and prognosis of preB-ALL patients with FAT1 upregulation is poor. FAT4 directly interacts with MPDZ/MUPP1 to recruit membrane-associated guanylate kinase MPP5/PALS1. FAT4 is involved in the maintenance of PCP and inhibition of cell proliferation. FAT4 mRNA is repressed in breast cancer and lung cancer due to promoter hypermethylation. FAT4 gene is recurrently mutated in several types of human cancers, such as melanoma, pancreatic cancer, gastric cancer and hepatocellular carcinoma. FAT1 and FAT4 suppress tumor growth via activation of Hippo signaling, whereas FAT1 promotes tumor migration via induction of actin polymerization. FAT1 is tumor suppressive or oncogenic in a context-dependent manner, while FAT4 is tumor suppressive. Copy number aberration, translocation and point mutation of FAT1, FAT2, FAT3, FAT4, FRMD1, FRMD6, NF2, WWC1, WWC2, SAV1, STK3, STK4, MOB1A, MOB1B, LATS1, LATS2, YAP1 and WWTR1/TAZ genes should be comprehensively investigated in various types of human cancers to elucidate the mutation landscape of the FAT-Hippo signaling cascades. Because YAP1 and WWTR1 are located at the crossroads of adhesion, GPCR, RTK and stem-cell signaling network, cancer genomics of the FAT signaling cascades could be applied for diagnostics, prognostics and therapeutics in the era of personalized medicine.

The tumor suppressor Lgl1 regulates NMII-A cellular distribution and focal adhesion morphology to optimize cell migration

The Drosophila tumor suppressor Lethal (2) giant larvae (Lgl) regulates the apical-basal polarity in epithelia and asymmetric cell division. However, little is known about the role of Lgl in cell polarity in migrating cells. In this study we show direct physiological interactions between the mammalian homologue of Lgl (Lgl1) and the nonmuscle myosin II isoform A (NMII-A). We demonstrate that Lgl1 and NMII-A form a complex in vivo and provide data that Lgl1 inhibits NMII-A filament assembly in vitro. Furthermore, depletion of Lgl1 results in the unexpected presence of NMII-A in the cell leading edge, a region that is not usually occupied by this protein, suggesting that Lgl1 regulates the cellular localization of NMII-A. Finally, we show that depletion of Lgl1 affects the size and number of focal adhesions, as well as cell polarity, membrane dynamics, and the rate of migrating cells. Collectively these findings indicate that Lgl1 regulates the polarity of migrating cells by controlling the assembly state of NMII-A, its cellular localization, and focal adhesion assembly.

Asymmetric cell division: recent developments and their implications for tumour biology

The ability of cells to divide asymmetrically is essential for generating diverse cell types during development. The past 10 years have seen tremendous progress in our understanding of this important biological process. We have learned that localized phosphorylation events are responsible for the asymmetric segregation of cell fate determinants in mitosis and that centrosomes and microtubules play important parts in this process. The relevance of asymmetric cell division for stem cell biology has added a new dimension to the field, and exciting connections between asymmetric cell division and tumorigenesis have begun to emerge.

Discs large in the Drosophila testis: an old player on a new task

Gamete development requires a coordinated soma-germ line interaction that ensures renewal and differentiation of germline and somatic stem cells. The physical contact between the germline and somatic cell populations is crucial because it allows the exchange of diffusible signals among them. The tumor suppressor gene discs large (dlg) encodes a septate junction protein with functions in epithelial cell polarity, asymmetric neuroblast division and formation of neuromuscular junctions. Our recent work reveals a new role of dlg in the Drosophila testis, as mutations in dlg lead to testis defects and cell death. Dlg is required throughout spermatogenesis in the somatic lineage and its localization changes from a uniform distribution along the plasma membrane of somatic cells in the testis apex, to a restricted localization on the distally located somatic cell in growing cysts. The extensive defects in dlg testis underline the importance of the somatic cells in the establishment and maintenance of the male stem cell niche and somatic cell differentiation. Here, we discuss our latest findings on the role of dlg in the Drosophila testis, supporting the view that junction proteins are dynamic structures, which can provide guiding cues to recruit scaffold proteins or other signaling molecules.

Epithelial polarity proteins regulate Drosophila tracheal tube size in parallel to the luminal matrix pathway

Regulation of epithelial tube size is critical for organ function. However, the mechanisms of tube size control remain poorly understood. In the Drosophila trachea, tube dimensions are regulated by a luminal extracellular matrix (ECM). ECM organization requires apical (luminal) secretion of the protein Vermiform (Verm), which depends on the basolateral septate junction (SJ). Here, we show that apical and basolateral epithelial polarity proteins interact to control tracheal tube size independently of the Verm pathway. Mutations in yurt (yrt) and scribble (scrib), which encode SJ-associated polarity proteins, cause an expansion of tracheal tubes but do not disrupt Verm secretion. Reducing activity of the apical polarity protein Crumbs (Crb) suppresses the length defects in yrt but not scrib mutants, suggesting that Yrt acts by negatively regulating Crb. Conversely, Crb overexpression increases tracheal tube dimensions. Reducing crb dosage also rescues tracheal size defects caused by mutations in coracle (cora), which encodes an SJ-associated polarity protein. In addition, crb mutations suppress cora length defects without restoring Verm secretion. Together, these data indicate that Yrt, Cora, Crb, and Scrib operate independently of the Verm pathway. Our data support a model in which Cora and Yrt act through Crb to regulate epithelial tube size.

discs large regulates somatic cyst cell survival and expansion in Drosophila testis

Gonad development requires a coordinated soma-germline interaction that ensures renewal and differentiation of germline and somatic stem cells to ultimately produce mature gametes. The Drosophila tumour suppressor gene discs large (dlg) encodes a septate junction protein functioning during epithelial polarization, asymmetric neuroblast division, and formation of neuromuscular junctions. Here, we report the role of dlg in testis development and its critical function in somatic cyst cells (SCCs). In these cells dlg is primarily required for their survival and expansion, and contributes to spermatocyte cyst differentiation. Cell death primarily occurred in SCCs at the end of spermatogonial amplification at a time when Dlg becomes restricted in wild-type (wt) testes to the distal somatic cells capping the growing spermatocyte cysts. RNAi depletion of dlg transcripts in early SCCs fully prevented testis development, whereas depletion in late SCCs resulted in a breakdown of spermatocyte cyst structure and germ cell individualization. Specific dlg expression in SCCs resulted in developmental rescue of dlg mutant testes, whereas its expression in germ cells exerted no such effect. dlg overexpression in wt testes led to spermatocyte cyst expansion at the expense of spermatogonial cysts. Our data demonstrate that dlg is essentially required in SCCs for their survival, expansion, and differentiation, and for the encapsulation of the germline cells.

Differential regulation of Dlg1, Scrib, and Lgl1 expression in a transgenic mouse model of ocular cancer

PURPOSE

Discs large (dlg), scribble (scrib), and lethal giant larvae (lgl) are major suppressor genes in Drosophila melanogaster. They encode proteins that regulate cell polarity and cell proliferation in Drosophila and mammals. However, their basic oncogenic roles have not yet been established in mouse epithelial ocular cancer. We evaluated the potential implication of these proteins in tumorigenesis of adenocarcinomas originating from the retinal pigmented epithelium of the Trp1/Tag transgenic mouse model. We examined the changes in the distribution and levels of these proteins in mouse ocular tissues from the Trp1/Tag mouse model.

METHODS

The expression patterns of theses genes and their corresponding proteins in normal mouse ocular tissues were studied by in situ hibridization and immunohistofluorescence experiments. In addition, variations in mRNA and proteins levels and protein distributions for Dlg1, Scrib, and Lgl1 were analyzed in the ocular tissues from Trp1/Tag transgenic mouse model by reverse transcription polymerase chain reaction (RT-PCR), western blot analysis, and immunohistofluorescence.

RESULTS

We found that mouse Dlg1, Scrib, and Lgl1 are widely distributed in normal ocular tissues, particularly in retinal neurons. We found that the three proteins are mislocalized in retinal layers during ocular carcinogenesis. These mislocalizations were correlated to the early dysplastic stages of ocular tumorigenesis. Additionally, the mislocalization of each protein was associated with its downregulation. Decreased levels of these proteins may be considered as late-stage markers of the disease but also as markers of the invasive stage of this cancerous process. This downregulation may be involved in epithelial-mesenchymal transition in this mouse ocular tumoral model. This would be consistent with the downregulation of E-cadherin and upregulation of N-cadherin expression observed in this model.

CONCLUSIONS

This is the first study to demonstrate the involvement of Dlg1, Scrib, and Lgl1 in a mouse with ocular adenocarcinoma and the simultaneous involvement of these proteins in the same cancer. Our results indicate that both the mislocalization and downregulation of these proteins may be involved together in ocular carcinogenesis.

Physical stress primes the immune response of Galleria mellonella larvae to infection by Candida albicans

Larvae of the greater wax moth (Galleria mellonella) that had been subjected to physical stress by shaking in cupped hands for 2 min showed reduced susceptibility to infection by Candida albicans when infected 24 h after the stress event. Physically stressed larvae demonstrated an increase in haemocyte density and elevated mRNA levels of galiomicin and an inducible metalloproteinase inhibitor (IMPI) but not transferrin or gallerimycin. In contrast, previous work has demonstrated that microbial priming of larvae resulted in the induction of all four genes. Examination of the expression of proteins in the insect haemolymph using 2D electrophoresis and MALDI TOF analysis revealed an increase in the intensity of a number of peptides showing some similarities with proteins associated with the insect immune response to infection. This study demonstrates that non-lethal physical stress primes the immune response of G. mellonella and this is mediated by elevated haemocyte numbers, increased mRNA levels of genes coding for two antimicrobial peptides and the appearance of novel peptides in the haemolymph. This work demonstrates that physical priming increases the insect immune response but the mechanism of this priming is different to that induced by low level exposure to microbial pathogens.

Lgl and its phosphorylation by aPKC regulate oocyte polarity formation in Drosophila

Specification of the anteroposterior (AP) axis in Drosophila oocytes requires proper organization of the microtubule and actin cytoskeleton. The establishment and regulation of cytoskeletal polarity remain poorly understood, however. Here, we show important roles for the tumor suppressor Lethal (2) giant larvae (Lgl) and atypical protein kinase C (aPKC) in regulating microtubule polarity and setting up the AP axis of the oocyte. Lgl in the germline cells regulates the localization of axis-specifying morphogens. aPKC phosphorylation of Lgl restricts Lgl activity to the oocyte posterior, thereby dividing the cortex into different domains along the AP axis. Active Lgl promotes the formation of actin-rich projections at the oocyte cortex and the posterior enrichment of the serine/threonine kinase Par-1, a key step for oocyte polarization. Our studies suggest that Lgl and its phosphorylation by aPKC may form a conserved regulatory circuitry in polarization of various cell types.

Interactions of tight junctions with membrane channels and transporters

Tight junctions are unique organelles in epithelial cells. They are localized to the apico-lateral region and essential for the epithelial cell transport functions. The paracellular transport process that occurs via tight junctions is extensively studied and is intricately regulated by various extracellular and intracellular signals. Fine regulation of this transport pathway is crucial for normal epithelial cell functions. Among factors that control tight junction permeability are ions and their transporters. However, this area of research is still in its infancy and much more needs to be learned about how these molecules regulate tight junction structure and functions. In this review we have attempted to compile literature on ion transporters and channels involved in the regulation of tight junctions.

Spatial regulation of exocytosis and cell polarity: yeast as a model for animal cells

Exocytosis is the major mechanism by which new membrane components are delivered to the cell surface. In most, if not all, eukaryotic cells this is also a highly spatially regulated process that is tightly coordinated with the overall polarity of a cell. The Rho/Cdc42 family of GTPases and the lethal giant larvae/Sro7 family are two highly conserved families of proteins which appear to have dual functions both in cell polarity and exocytosis. Analysis of their functions has begun to unravel the coordination between these processes and propose a model for polarized vesicle docking and fusion at the site of asymmetric cell growth.

Preparation of membrane proteins from Drosophila embryos

INTRODUCTIONDrosophila embryos provide a unique system for the study of membrane proteins because of an extensive database in both genetics and biochemistry. Drosophila early embryos have an extraordinary peak in membrane biogenesis with a 23-fold increase in membrane area over ~45 minutes. This protocol describes purification of Drosophila membranes using equilibrium sedimentation in a sucrose density step gradient (0.5 M, 2.0 M, 2.5 M). It also includes two methods to determine the membrane association of a protein of interest: (1) alkaline wash and (2) Triton X-114 phase separation.

Drosophila Varicose, a member of a new subgroup of basolateral MAGUKs, is required for septate junctions and tracheal morphogenesis

Epithelial tubes are the functional units of many organs, but little is known about how tube sizes are established. Using the Drosophila tracheal system as a model, we previously showed that mutations in varicose (vari) cause tubes to become elongated without increasing cell number. Here we show vari is required for accumulation of the tracheal size-control proteins Vermiform and Serpentine in the tracheal lumen. We also show that vari is an essential septate junction (SJ) gene encoding a membrane associated guanylate kinase (MAGUK). In vivo analyses of domains important for MAGUK scaffolding functions demonstrate that while the Vari HOOK domain is essential, the L27 domain is dispensable. Phylogenetic analyses reveal that Vari helps define a new MAGUK subgroup that includes mammalian PALS2. Importantly, both Vari and PALS2 are basolateral, and the interaction of Vari with the cell-adhesion protein Neurexin IV parallels the interaction of PALS2 and another cell-adhesion protein, Necl-2. Vari therefore bolsters the similarity between Drosophila and vertebrate epithelial basolateral regions, which had previously been limited to the common basolateral localization of Scrib, Dlg and Lgl, proteins required for epithelial polarization at the beginning of embryogenesis. However, by contrast to Scrib, Dlg and Lgl, Vari is not required for cell polarity but rather is part of a cell-adhesion complex. Thus, Vari fundamentally extends the similarity of Drosophila and vertebrate basolateral regions from sharing only polarity complexes to sharing both polarity and cell-adhesion complexes.

Regulation of Imaginal Disc Growth by Tumor-Suppressor Genes inDrosophila

Inactivating mutations in the Drosophila tumor-suppressor genes result in tissue overgrowth. This can occur because the mutant tissue either grows faster than wild-type tissue and/or continues to grow beyond a time when wild-type tissue stops growing. There are three general classes of tumor-suppressor genes that regulate the growth of imaginal disc epithelia. Mutations in the hyperplastic tumor-suppressor genes result in increased cell proliferation but do not disrupt normal tissue architecture. These genes include pten, Tsc1, Tsc2, and components of the hippo/salvador/warts pathway. Mutations in a second class of genes, the neoplastic tumor-suppressor genes, disrupt proteins that function either as scaffolds at cell-cell junctions (scribble, discs large, lgl) or as components of the endocytic pathway (avalanche, rab5, ESCRT components). For the third group, the nonautonomous tumor-suppressor genes, mutant cells stimulate the proliferation of adjacent wild-type cells. Understanding the interactions between these three classes of genes will improve our understanding of how cell and tissue growth are coordinated during organismal development and perturbed in disease states such as cancer.

Induced paternal effects mimic cytoplasmic incompatibility in Drosophila

Wolbachia is an intracellular microbe found in a wide diversity of arthropod and filarial nematode hosts. In arthropods these common bacteria are reproductive parasites that manipulate central elements of their host's reproduction to increase their own maternal transmission in one of several ways. Cytoplasmic incompatibility (CI) is one such manipulation where sperm are somehow modified in infected males and this modification must be rescued by the presence of the same bacterial strain in the egg for normal development to proceed. The molecular mechanisms involved in the expression of CI are unknown. Here we show that Wolbachia infection results in increased mRNA and protein expression of the Drosophila simulans nonmuscle myosin II gene zipper. Induced overexpression of zipper in Wolbachia-free transgenic D. melanogaster males results in paternal-effect lethality that mimics the fertilization defects associated with CI. Likewise, overexpression of the tumor suppressor gene, lethal giant larvae [l(2)gl], results in egg lethality and a CI phenotype. Stoichiometric levels of zipper and l(2)gl are required for proper segregation of cellular determinants during neuroblast stem cell division. Taken together these results form the basis of a working hypothesis whereby Wolbachia induces paternal effects in sperm by manipulating the expression of key regulators of cytoskeletal activity during spermatogenesis.

Expression of Hugl-1 is strongly reduced in malignant melanoma

The human gene Hugl-1 (Llgl/Lgl1) has significant homology to the Drosophila tumor suppressor gene lethal(2)giant larvae (lgl). The lgl gene codes for a cortical cytoskeleton protein, Lgl, that is involved in maintaining cell polarity and epithelial integrity. We speculate that Hugl-1 might play a role in epithelial-mesenchymal transition (EMT) and that loss of Hugl-1 expression plays a role in the development or progression of malignant melanoma. Thus, we evaluated melanoma cell lines and tissue samples of malignant melanoma for loss of Hugl-1 transcription. We found that Hugl-1 was downregulated or lost in all cell lines and in most of the tumor samples analysed, and that these losses were associated with advanced stage of the disease. Reduced Hugl-1 expression occurred as early as in primary tumors detected by both immunohistochemical and reverse transcription-polymerase chain reaction (RT-PCR) analysis. Functional assays with stable Hugl-1-transfected cell lines revealed that Hugl-1 expression increased cell adhesion and decreased cell migration. Further, downregulation of MMP2 and MMP14 (MT1-MMP) and re-expression of E-cadherin was found in the Hugl-1-expressing cell clones supporting a role of Hugl-1 in EMT. Our studies thus indicate that loss of Hugl-1 expression contributes to melanoma progression.

Hox genes and their candidate downstream targets in the developing central nervous system

1. Homeobox (Hox) genes were originally discovered in the fruit fly Drosophila, where they function through a conserved homeodomain as transcriptional regulators to control embryonic morphogenesis. Since then over 1000 homeodomain proteins have been identified in several species. In vertebrates, 39 Hox genes have been identified as homologs of the original Drosophila complex, and like their Drosophila counterparts they are organized within chromosomal clusters. Vertebrate Hox genes have also been shown to play a critical role in embryonic development as transcriptional regulators. 2. Both the Drosophila and vertebrate Hox genes have been shown to interact with various cofactors, such as the TALE homeodomain proteins, in recognition of consensus sequences within regulatory elements of their target genes. These protein-protein interactions are believed to contribute to enhancing the specificity of target gene recognition in a cell-type or tissue- dependent manner. The regulatory activity of a particular Hox protein on a specific regulatory element is highly variable and dependent on its interacting partners within the transcriptional complex. 3. In vertebrates, Hox genes display spatially restricted patterns of expression within the developing CNS, both along the anterioposterior and dorsoventral axis of the embryo. Their restricted gene expression is suggestive of a regulatory role in patterning of the CNS, as well as in cell specification. Determining the precise function of individual Hox genes in CNS morphogenesis through classical mutational analyses is complicated due to functional redundancy between Hox genes. 4. Understanding the precise mechanisms through which Hox genes mediate embryonic morphogenesis requires the identification of their downstream target genes. Although Hox genes have been implicated in the regulation of several pathways, few target genes have been shown to be under their direct regulatory control. Development of methodologies used for the isolation of target genes and for the analysis of putative targets will be beneficial in establishing the genetic pathways controlled by Hox factors. 5. Within the developing CNS various cell adhesion molecules and signaling molecules have been identified as candidate downstream target genes of Hox proteins. These targets play a role in processes such as cell migration and differentiation, and are implicated in contributing to neuronal processes such as plasticity and/or specification. Hence, Hox genes not only play a role in patterning of the CNS during early development, but may also contribute to cell specification and identity.

Zebrafish penner/lethal giant larvae 2 functions in hemidesmosome formation, maintenance of cellular morphology and growth regulation in the developing basal epidermis

Epithelial cells are equipped with junctional complexes that are involved in maintaining tissue architecture, providing mechanical integrity and suppressing tumour formation as well as invasiveness. A strict spatial segregation of these junctional complexes leads to the polarisation of epithelial cells. In vertebrate epithelia, basally localised hemidesmosomes mediate stable adhesion between epithelial cells and the underlying basement membrane. Although components of hemidesmosomes are relatively well known, the molecular machinery involved in governing the formation of these robust junctions, remains elusive. Here, we have identified the first component of this machinery using a forward genetic approach in zebrafish as we show that the function of penner (pen)/lethal giant larvae 2 (lgl2) is necessary for hemidesmosome formation and maintenance of the tissue integrity in the developing basal epidermis. Moreover, in pen/lgl2 mutant, basal epidermal cells hyper-proliferate and migrate to ectopic positions. Of the two vertebrate orthologues of the Drosophila tumour suppressor gene lethal giant larvae, the function of lgl2 in vertebrate development and organogenesis remained unclear so far. Here, we have unravelled an essential function of lgl2 during development of the epidermis in vertebrates.

A network of PDZ-containing proteins regulates T cell polarity and morphology during migration and immunological synapse formation

T cell shape is dictated by the selective recruitment of molecules to different regions of the cell (polarity) and is integral to every aspect of T cell function, from migration to cytotoxicity. This study describes a mechanism for the regulation of T cell polarity. We show that T cells contain a network of asymmetrically distributed proteins with the capacity to dictate the subcellular localization of both cell surface receptors and morphological determinants in T cells. Proteins from the Scribble, Crumbs3, and Par3 complexes, previously shown to regulate epithelial polarity, were polarized in T cells containing either uropods or immunological synapses. Reduction in Scribble expression prevented the polarization of cell surface receptors and prevented morphological changes associated with uropod formation, migration, and antigen presentation. By dynamically coordinating molecular distribution throughout the T cell, this network provides a mechanism by which T cell function and polarity are linked.

Structurally Conserved Interaction of Lgl Family with SNAREs Is Critical to Their Cellular Function

The Lethal giant larvae (Lgl) tumor suppressor family is conserved from yeast to mammals and plays a critical yet controversial role in cell polarity. Studies on Drosophila Lgl suggest that its function in polarity is through regulation of the acto-myosin cytoskeleton. In contrast, studies on the yeast Lgl homologs, Sro7/Sro77, suggest a function in exocytosis through interaction with the t-SNARE Sec9. Using yeast/mammalian Lgl chimeras, we demonstrate that the overall architecture of Lgl proteins is highly conserved and that the C-terminal domain is the major site of SNARE interaction within both yeast and mammalian homologs. Importantly, we find that the ability of Lgl chimeras to function as the only source of Lgl in yeast correlates precisely with the ability to interact with the yeast t-SNARE. We report a novel interaction between Sro7 and the yeast myosin V, Myo2. However, we find that interactions with either Myo2 or Myo1 (myosin II) cannot account for the dramatic functional differences observed for these chimeras in yeast. These results provide the first demonstration that the interaction of an Lgl family member with a specific effector is critical to its function in vivo. These data support the model that the Lgl family functions in cell polarity, at least in part, by regulating SNARE-mediated membrane delivery events at the cell surface.

Reduced expression of Hugl-1, the human homologue of Drosophila tumour suppressor gene lgl, contributes to progression of colorectal cancer

The human gene, human giant larvae (Hugl-1/Llg1/Lgl1) has significant homology to the Drosophila tumour suppressor gene lethal(2)giant larvae (lgl). The lgl gene codes for a cortical cytoskeleton protein, Lgl, that binds Myosin II and is involved in maintaining cell polarity and epithelial integrity. The human protein, Hugl-1 contains several conserved functional domains found in Lgl, suggesting that these proteins may have closely related functions. Whether loss of Hugl expression plays a role in human tumorigenesis has so far not been extensively investigated. Thus, we evaluated tumour tissues from 94 patients undergoing surgery for colorectal cancer (CRC) for loss of Hugl-1 transcription and compared our findings with the clinical data from each of these patients. We found that Hugl-1 was lost in 75% of tumour samples and these losses were associated with advanced stage and particularly with lymph node metastases. Reduced Hugl-1 expression during the adenoma-carcinoma sequence occurring as early as in colorectal adenomas was detected by both immunohistochemical and reverse transcription-polymerase chain reaction analysis. Functional assays with ecdysone-inducible cell lines revealed that Hugl-1 expression increased cell adhesion and decreased cell migration. Our studies thus indicate that downregulation of Hugl-1 contributes to CRC progression.

Regulation of membrane localization of Sanpodo by lethal giant larvae and neuralized in asymmetrically dividing cells of Drosophila sensory organs

In Drosophila, asymmetric division occurs during proliferation of neural precursors of the central and peripheral nervous system (PNS), where a membrane-associated protein, Numb, is asymmetrically localized during cell division and is segregated to one of the two daughter cells (the pIIb cell) after mitosis. numb has been shown genetically to function as an antagonist of Notch signaling and also as a negative regulator of the membrane localization of Sanpodo, a four-pass transmembrane protein required for Notch signaling during asymmetric cell division in the CNS. Previously, we identified lethal giant larvae (lgl) as a gene required for numb-mediated inhibition of Notch in the adult PNS. In this study we show that Sanpodo is expressed in asymmetrically dividing precursor cells of the PNS and that Sanpodo internalization in the pIIb cell is dependent cytoskeletally associated Lgl. Lgl specifically regulates internalization of Sanpodo, likely through endocytosis, but is not required for the endocytosis Delta, which is a required step in the Notch-mediated cell fate decision during asymmetric cell division. Conversely, the E3 ubiquitin ligase neuralized is required for both Delta endocytosis and the internalization of Sanpodo. This study identifies a hitherto unreported role for Lgl as a regulator of Sanpodo during asymmetric cell division in the adult PNS.

Phosphorylation-induced autoinhibition regulates the cytoskeletal protein Lethal (2) giant larvae

During asymmetric cell division, cell fate determinants localize asymmetrically and segregate into one of the two daughter cells. In Drosophila neuroblasts, the asymmetric localization of cell fate determinants to the basal cell cortex requires aPKC. aPKC localizes to the apical cell cortex and phosphorylates the cytoskeletal protein Lethal (2) giant larvae (Lgl). Upon phosphorylation, Lgl dissociates from the cytoskeleton and becomes inactive. Here, we show that phosphorylation regulates Lgl by allowing an autoinhibitory interaction of the N terminus with the C terminus of the protein. We demonstrate that interaction with the cytoskeleton is mediated by a C-terminal domain while the N terminus is not required. Instead, the N terminus can bind to the C terminus and can compete for binding to the cytoskeleton. Interaction between the N- and C-terminal domains requires phosphorylation of Lgl by aPKC. Our results suggest that unphosphorylated, active Lgl exists in an open conformation that interacts with the cytoskeleton while phosphorylation changes the protein to an autoinhibited state.

A junctional problem of apical proportions: epithelial tube-size control by septate junctions in the Drosophila tracheal system

The size of epithelial tubes is critical for the function of organs such as the lung, kidney and vascular system. However, the molecular mechanisms regulating tube size are largely unknown. Recent work in the Drosophila tracheal system reveals that septate junctions play a previously unsuspected role in tube-size control. Surprisingly, this tube-size function is distinct from the established diffusion barrier function of septate junctions, and involves regulation of cell shape rather than cell number. Possible tube-size functions of septate junctions include patterning of the apical extracellular matrix and regulation of conserved cell polarity genes such as Scribble and Discs Large.