A systematic screen for dominant second-site modifiers of Merlin/NF2 phenotypes reveals an interaction with blistered/DSRF and scribbler

A new diatom species P. hallegraeffii sp. nov. belonging to the toxic genus Pseudo-nitzschia (Bacillariophyceae) from the East Australian Current

A new species belonging to the toxin producing diatom genus Pseudo-nitzschia, P. hallegraeffii sp. nov., is delineated and described from the East Australian Current (EAC). Clonal cultures were established by single cell isolation from phytoplankton net hauls collected as part of a research expedition in the EAC region in 2016 on the RV Investigator. Cultures were assessed for their morphological and genetic characteristics, their sexual compatibility with other Pseudo-nitzschia species, and their ability to produce domoic acid. Light and transmission electron microscopy revealed cells which differed from their closest relatives by their cell width, rows of poroids, girdle band structure and density of band straie. Phylogenetic analyses based on sequencing of nuclear-encoded ribosomal deoxyribonucleic acid (rDNA) regions showed this novel genotype clustered within the P. delicatissima complex, but formed a discrete clade from its closest relatives P. dolorosa, P. simulans, P. micropora and P. delicatissima. Complementary base changes (CBCs) were observed in the secondary structure of the 3’ nuclear ribosomal transcribed spacer sequence region (ITS2) between P. hallegraeffii sp. nov. and its closest related taxa, P. simulans and P. dolorosa. Under laboratory conditions, and in the absence of any zooplankton cues, strains of P. hallegraeffii sp. nov. did not produce domoic acid (DA) and were not sexually compatible with any other Pseudo-nitzschia clones tested. A total of 18 Pseudo-nitzschia species, including three confirmed toxigenic species (P. cuspidata, P. multistriata and P. australis) have now been unequivocally confirmed from eastern Australia.

Invadolysin acts genetically via the SAGA complex to modulate chromosome structure

Identification of components essential to chromosome structure and behaviour remains a vibrant area of study. We have previously shown that invadolysin is essential in Drosophila, with roles in cell division and cell migration. Mitotic chromosomes are hypercondensed in length, but display an aberrant fuzzy appearance. We additionally demonstrated that in human cells, invadolysin is localized on the surface of lipid droplets, organelles that store not only triglycerides and sterols but also free histones H2A, H2Av and H2B. Is there a link between the storage of histones in lipid droplets and the aberrantly structured chromosomes of invadolysin mutants? We have identified a genetic interaction between invadolysin and nonstop, the de-ubiquitinating protease component of the SAGA (Spt-Ada-Gcn5-acetyltransferase) chromatin-remodelling complex. invadolysin and nonstop mutants exhibit phenotypic similarities in terms of chromosome structure in both diploid and polyploid cells. Furthermore, IX-14¹/not¹ transheterozygous animals accumulate mono-ubiquitinated histone H2B (ubH2B) and histone H3 tri-methylated at lysine 4 (H3K4me3). Whole mount immunostaining of IX-14¹/not¹ transheterozygous salivary glands revealed that ubH2B accumulates surprisingly in the cytoplasm, rather than the nucleus. Over-expression of the Bre1 ubiquitin ligase phenocopies the effects of mutating either the invadolysin or nonstop genes. Intriguingly, nonstop and mutants of other SAGA subunits (gcn5, ada2b and sgf11) all suppress an invadolysin-induced rough eye phenotype. We conclude that the abnormal chromosome phenotype of invadolysin mutants is likely the result of disrupting the histone modification cycle, as accumulation of ubH2B and H3K4me3 is observed. We further suggest that the mislocalization of ubH2B to the cytoplasm has additional consequences on downstream components essential for chromosome behaviour. We therefore propose that invadolysin plays a crucial role in chromosome organization via its interaction with the SAGA complex.

In vivo functional analysis of the human NF2 tumor suppressor gene in Drosophila

The proper control of tissue growth is essential during normal development and an important problem in human disease. Merlin, the product of the Neurofibromatosis 2 tumor suppressor gene, has been extensively studied to understand its functions in growth control. Here we describe experiments in which we used Drosophila as an in vivo system to test the functions of the normal human NF2 gene products and patient-derived mutant alleles. Although the predominant NF2 gene isoform, isoform 1, could functionally replace the Drosophila Merlin gene, a second isoform with a distinct C-terminal tail could not. Immunofluorescence studies show that the two isoforms have distinct subcellular localizations when expressed in the polarized imaginal epithelium, and function in genetic rescue assays correlates with apical localization of the NF2 protein. Interestingly, we found that a patient-derived missense allele, NF2L64P, appears to be temperature sensitive. These studies highlight the utility of Drosophila for in vivo functional analysis of highly conserved human disease genes.

The PP1 phosphatase flapwing regulates the activity of Merlin and Moesin in Drosophila

The signalling activities of Merlin and Moesin, two closely related members of the protein 4.1 Ezrin/Radixin/Moesin family, are regulated by conformational changes. These changes are regulated in turn by phosphorylation. The same sterile 20 kinase-Slik co-regulates Merlin or Moesin activity whereby phosphorylation inactivates Merlin, but activates Moesin. Thus, the corresponding coordinate activation of Merlin and inactivation of Moesin would require coordinated phosphatase activity. We find that Drosophila melanogaster protein phosphatase type 1 β (flapwing) fulfils this role, co-regulating dephosphorylation and altered activity of both Merlin and Moesin. Merlin or Moesin are detected in a complex with Flapwing both in-vitro and in-vivo. Directed changes in flapwing expression result in altered phosphorylation of both Merlin and Moesin. These changes in the levels of Merlin and Moesin phosphorylation following reduction of flapwing expression are associated with concomitant defects in epithelial integrity and increase in apoptosis in developing tissues such as wing imaginal discs. Functionally, the defects can be partially recapitulated by over expression of proteins that mimic constitutively phosphorylated or unphosphorylated Merlin or Moesin. Our results suggest that changes in the phosphorylation levels of Merlin and Moesin lead to changes in epithelial organization.

Sip1, the Drosophila orthologue of EBP50/NHERF1, functions with the sterile 20 family kinase Slik to regulate Moesin activity

Organization of the plasma membrane in polarized epithelial cells is accomplished by the specific localization of transmembrane or membrane-associated proteins, which are often linked to cytoplasmic protein complexes, including the actin cytoskeleton. In this study, we identified Sip1 as a Drosophila orthologue of the ezrin-radixin-moesin (ERM) binding protein 50 (EBP50; also known as the Na(+)/H(+) exchanger regulatory factor NHERF1). In mammals, EBP50/NHERF1 is a scaffold protein required for the regulation of several transmembrane receptors and downstream signal transduction activity. In Drosophila, loss of Sip1 leads to a reduction in Slik kinase protein abundance, loss of Moesin phosphorylation and changes in epithelial structure, including mislocalization of E-cadherin and F-actin. Consistent with these findings, Moesin and Sip1 act synergistically in genetic-interaction experiments, and Sip1 protein abundance is dependent on Moesin. Co-immunoprecipitation experiments indicate that Sip1 forms a complex with both Moesin and Slik. Taken together, these data suggest that Sip1 promotes Slik-dependent phosphorylation of Moesin, and suggests a mechanism for the regulation of Moesin activity within the cell to maintain epithelial integrity.

The role of Drosophila Merlin in spermatogenesis

Background

Drosophila Merlin, the homolog of the human Neurofibromatosis 2 (NF2) gene, is important for the regulation of cell proliferation and receptor endocytosis. Male flies carrying a Mer³ allele, a missense mutation (Met¹⁷⁷→Ile) in the Merlin gene, are viable but sterile; however, the cause of sterility is unknown.

Results

Testis examination reveals that hemizygous Mer³ mutant males have small seminal vesicles that contain only a few immotile sperm. By cytological and electron microscopy analyses of the Mer³, Mer⁴ (Gln¹⁷⁰→stop), and control testes at various stages of spermatogenesis, we show that Merlin mutations affect meiotic cytokinesis of spermatocytes, cyst polarization and nuclear shaping during spermatid elongation, and spermatid individualization. We also demonstrate that the lethality and sterility phenotype of the Mer⁴ mutant is rescued by the introduction of a wild-type Merlin gene. Immunostaining demonstrates that the Merlin protein is redistributed to the area associated with the microtubules of the central spindle in telophase and its staining is less in the region of the contractile ring during meiotic cytokinesis. At the onion stage, Merlin is concentrated in the Nebenkern of spermatids, and this mitochondrial localization is maintained throughout sperm formation. Also, Merlin exhibits punctate staining in the acrosomal region of mature sperm.

Conclusion

Merlin mutations affect spermatogenesis at multiple stages. The Merlin protein is dynamically redistributed during meiosis of spermatocytes and is concentrated in the Nebenkern of spermatids. Our results demonstrated for the first time the mitochondrial localization of Merlin and suggest that Merlin may play a role in mitochondria formation and function during spermatogenesis.

Contact-dependent inhibition of EGFR signaling by Nf2/Merlin

The neurofibromatosis type 2 (NF2) tumor suppressor, Merlin, is a membrane/cytoskeleton-associated protein that mediates contact-dependent inhibition of proliferation. Here we show that upon cell-cell contact Merlin coordinates the processes of adherens junction stabilization and negative regulation of epidermal growth factor receptor (EGFR) signaling by restraining the EGFR into a membrane compartment from which it can neither signal nor be internalized. In confluent Nf2(-/-) cells, EGFR activation persists, driving continued proliferation that is halted by specific EGFR inhibitors. These studies define a new mechanism of tumor suppression, provide mechanistic insight into the poorly understood phenomenon of contact-dependent inhibition of proliferation, and suggest a therapeutic strategy for NF2-mutant tumors.

Phosphorylation and activity of the tumor suppressor Merlin and the ERM protein Moesin are coordinately regulated by the Slik kinase

Merlin and Moesin are closely related members of the 4.1 Ezrin/Radixin/Moesin domain superfamily implicated in regulating proliferation and epithelial integrity, respectively. The activity of both proteins is regulated by head to tail folding that is controlled, in part, by phosphorylation. Few upstream regulators of these phosphorylation events are known. In this study, we demonstrate that in Drosophila melanogaster, Slik, a Ste20 kinase, controls subcellular localization and phosphorylation of Merlin, resulting in the coordinate but opposite regulation of Merlin and Moesin. These results suggest the existence of a novel mechanism for coordinate regulation of cell proliferation and epithelial integrity in developing tissues.

Sometimes the result is not the answer: the truths and the lies that come from using the complementation test

It is standard genetic practice to determine whether or not two independently obtained mutants define the same or different genes by performing the complementation test. While the complementation test is highly effective and accurate in most cases, there are a number of instances in which the complementation test provides misleading answers, either as a result of the failure of two mutations that are located in different genes to complement each other or by exhibiting complementation between two mutations that lie within the same gene. We are primarily concerned here with those cases in which two mutations lie in different genes, but nonetheless fail to complement each other. This phenomenon is often referred to as second-site noncomplementation (SSNC). The discovery of SSNC led to a large number of screens designed to search for genes that encode interacting proteins. However, screens for dominant enhancer mutations of semidominant alleles of a given gene have proved far more effective at identifying interacting genes whose products interact physically or functionally with the initial gene of interest than have SSNC-based screens.

The tumor suppressors Merlin and Expanded function cooperatively to modulate receptor endocytosis and signaling

The precise coordination of signals that control proliferation is a key feature of growth regulation in developing tissues . While much has been learned about the basic components of signal transduction pathways, less is known about how receptor localization, compartmentalization, and trafficking affect signaling in developing tissues. Here we examine the mechanism by which the Drosophila Neurofibromatosis 2 (NF2) tumor suppressor ortholog Merlin (Mer) and the related tumor suppressor expanded (ex) regulate proliferation and differentiation in imaginal epithelia. Merlin and Expanded are members of the FERM (Four-point one, Ezrin, Radixin, Moesin) domain superfamily, which consists of membrane-associated cytoplasmic proteins that interact with transmembrane proteins and may function as adapters that link to protein complexes and/or the cytoskeleton . We demonstrate that Merlin and Expanded function to regulate the steady-state levels of signaling and adhesion receptors and that loss of these proteins can cause hyperactivation of associated signaling pathways. In addition, pulse-chase labeling of Notch in living tissues indicates that receptor levels are upregulated at the plasma membrane in Mer; ex double mutant cells due to a defect in receptor clearance from the cell surface. We propose that these proteins control proliferation by regulating the abundance, localization, and turnover of cell-surface receptors and that misregulation of these processes may be a key component of tumorigenesis.

Membrane organization and tumorigenesis--the NF2 tumor suppressor, Merlin

The NF2 tumor-suppressor gene was cloned more than a decade ago, but the function of its encoded protein, Merlin, remains elusive. Merlin, like the closely related ERM proteins, appears to provide regulated linkage between membrane-associated proteins and the actin cytoskeleton and is therefore poised to function in receiving and interpreting signals from the extracellular milieu. Recent studies suggest that Merlin may coordinate the processes of growth-factor receptor signaling and cell adhesion. Varying use of this organizing activity by different types of cells could provide an explanation for the unique spectrum of tumors associated with NF2 deficiency in mammals.

Modeling early Epstein-Barr virus infection in Drosophila melanogaster: the BZLF1 protein

Epstein-Barr virus (EBV) is the causative agent of infectious mononucleosis and is associated with several forms of cancer, including lymphomas and nasopharyngeal carcinoma. The EBV immediate-early protein BZLF1 functions as a transcriptional activator of EBV early gene expression and is essential for the viral transition between latent and lytic replication. In addition to its role in the EBV life cycle, BZLF1 (Z) also has profound effects upon the host cellular environment, including disruption of cell cycle regulation, signal transduction pathways, and transcription. In an effort to understand the nature of Z interactions with the host cellular environment, we have developed a Drosophila model of early EBV infection, where we have expressed Z in the Drosophila eye. Using this system, we have identified a highly conserved interaction between the Epstein-Barr virus Z protein and shaven, a Drosophila homolog of the human Pax2/5/8 family of genes. Pax5 is a well-characterized human gene involved with B-cell development. The B-cell-specific Pax5 also promotes the transcription of EBV latent genes from the EBV Wp promoter. Our work clearly demonstrates that the Drosophila system is an appropriate and powerful tool for identifying the underlying genetic networks involved in human infectious disease.

Turning behavior in Drosophila larvae: a role for the small scribbler transcript

The Drosophila larva is extensively used for studies of neural development and function, yet the mechanisms underlying the appropriate development of its stereotypic motor behaviors remain largely unknown. We have previously shown that mutations in scribbler (sbb), a gene encoding two transcripts widely expressed in the nervous system, cause abnormally frequent episodes of turning in the third instar larva. Here we report that hypomorphic sbb mutant larvae display aberrant turning from the second instar stage onwards. We focus on the smaller of the two sbb transcripts and show that its pan-neural expression during early larval life, but not in later larval life, restores wild type turning behavior. To identify the classes of neurons in which this sbb transcript is involved, we carried out transgenic rescue experiments. Targeted expression of the small sbb transcript using the cha-GAL4 driver was sufficient to restore wild type turning behavior. In contrast, expression of this sbb transcript in motoneurons, sensory neurons or large numbers of unidentified interneurons was not sufficient. Our data suggest that the expression of the smaller sbb transcript may be needed in a subset of neurons for the maintenance of normal turning behavior in Drosophila larvae.

The Drosophila Brahma (SWI/SNF) chromatin remodeling complex exhibits cell-type specific activation and repression functions

The Brahma (Brm) complex of Drosophila melanogaster is a SWI/SNF-related chromatin remodeling complex required to correctly maintain proper states of gene expression through ATP-dependent effects on chromatin structure. The SWI/SNF complexes are comprised of 8-11 stable components, even though the SWI2/SNF2 (BRM, BRG1, hBRM) ATPase subunit alone is partially sufficient to carry out chromatin remodeling in vitro. The remaining subunits are required for stable complex assembly and/or proper promoter targeting in vivo. Our data reveals that SNR1 (SNF5-Related-1), a highly conserved subunit of the Brm complex, is required to restrict complex activity during the development of wing vein and intervein cells, illustrating a functional requirement for SNR1 in modifying whole complex activation functions. Specifically, we found that snr1 and brm exhibited opposite mutant phenotypes in the wing and differential misregulation of genes required for vein and intervein cell development, including rhomboid, decapentaplegic, thick veins, and blistered, suggesting possible regulatory targets for the Brm complex in vivo. Our genetic results suggest a novel mechanism for SWI/SNF-mediated gene repression that relies on the function of a 'core' subunit to block or shield BRM (SWI2/SNF2) activity in specific cells. The SNR1-mediated repression is dependent on cooperation with histone deacetylases (HDAC) and physical associations with NET, a localized vein repressor.

Diabetic flies? Using Drosophila melanogaster to understand the causes of monogenic and genetically complex diseases

Approximately three-quarters of human disease loci have counterparts in the fruit fly Drosophila melanogaster. This model organism is therefore extremely valuable for using to understand the role of these loci in normal development, and for unravelling genetic pathways in which these loci take part. Important advantages for Drosophila in such studies are its completed genome, the unparalleled collection of mutations already in existence, the relative ease in which new mutations can be generated, the existence of convenient techniques for inactivating or overexpressing genes in dispensable tissues that are easily observed and measured, and the ability to readily carry out second-site modifier genetics. Recent work in Drosophila on the insulin-signaling pathway, a pathway of profound clinical importance, is reviewed as an illustration of how such research can provide fundamental insights into the functions of this pathway in regulating growth and development. Moreover, Drosophila research is now identifying heretofore unknown regulators of insulin signaling, as well as indicating novel functions for this pathway in suppressing benign tumor formation and regulating life span.

Disruption of mCry2 restores circadian rhythmicity in mPer2 mutant mice

Many biochemical, physiological, and behavioral processes display daily rhythms generated by an internal timekeeping mechanism referred to as the circadian clock. The core oscillator driving this clock is located in the ventral part of the hypothalamus, the so called suprachiasmatic nuclei (SCN). At the molecular level, this oscillator is thought to be composed of interlocking autoregulatory feedback loops involving a set of clock genes. Among the components driving the mammalian circadian clock are the Period 1 and 2 (mPer1 and mPer2) and Cryptochrome 1 and 2 (mCry1 and mCry2) genes. A mutation in the mPer2 gene leads to a gradual loss of circadian rhythmicity in mice kept in constant darkness (DD). Here we show that inactivation of the mCry2 gene in mPer2 mutant mice restores circadian rhythmicity and normal clock gene expression patterns. Thus, mCry2 can act as a nonallelic suppressor of mPer2, which points to direct or indirect interactions of PER2 and CRY2 proteins. In marked contrast, inactivation of mCry1 in mPer2 mutant mice does not restore circadian rhythmicity but instead results in complete behavioral arrhythmicity in DD, indicating different effects of mCry1 and mCry2 in the clock mechanism

ERM proteins and merlin: integrators at the cell cortex

A fundamental property of many plasma-membrane proteins is their association with the underlying cytoskeleton to determine cell shape, and to participate in adhesion, motility and other plasma-membrane processes, including endocytosis and exocytosis. The ezrin-radixin-moesin (ERM) proteins are crucial components that provide a regulated linkage between membrane proteins and the cortical cytoskeleton, and also participate in signal-transduction pathways. The closely related tumour suppressor merlin shares many properties with ERM proteins, yet also provides a distinct and essential function.