Richard G. Fehon

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.

Specific EGF Repeats of Notch Mediate Interactions with Delta and Serrate: Implications for Notch as a Multifunctional Receptor

The neurogenic loci Notch and Delta, which both encode EGF-homologous transmembrane proteins, appear to function together in mediating cell-cell communication and have been shown to interact at the cell surface in vitro. To examine the role of the EGF repeats in this interaction, we performed an extensive deletion mutagenesis of the extracellular domain of Notch. We find that of the 36 EGF repeats of Notch, only two, 11 and 12, are both necessary and sufficient to mediate interactions with Delta. Furthermore, this Delta binding ability is conserved in the corresponding two repeats from the Xenopus Notch homolog. We report a novel molecular interaction between Notch and Serrate, another EGF-homologous transmembrane protein containing a region of striking similarity to Delta, and show that the same two EGF repeats of Notch also constitute a Serrate binding domain. These results suggest that Notch may act as a multifunctional receptor whose 36 EGF repeats form a tandem array of discrete ligand-binding units, each of which may potentially interact with several different proteins during development.

Molecular interactions between the protein products of the neurogenic loci Notch and Delta, two EGF-homologous genes in Drosophila

Genetic analyses have raised the possibility of interactions between the gene products of the neurogenic loci Notch and Delta, each of which encodes a transmembrane protein with EGF homology. To examine the possibility of intermolecular association between the products of these two genes, we studied the effects of their expression on aggregation in Drosophila S2 cells. We find that Notch-expressing cells form mixed aggregates specifically with cells that express Delta and that this process is calcium dependent. In addition, we show that Notch and Delta can associate within the membrane of a single cell, and further, that they form detergent-soluble intermolecular complexes. Our analyses suggest that Notch and Delta proteins interact at the cell surface via their extracellular domains.

Organizing the cell cortex: the role of ERM proteins

Specialized membrane domains are an important feature of almost all cells. In particular, they are essential to tissues that have a highly organized cell cortex, such as the intestinal brush border epithelium. The ERM proteins (ezrin, radixin and moesin) have a crucial role in organizing membrane domains through their ability to interact with transmembrane proteins and the cytoskeleton. In doing so, they can provide structural links to strengthen the cell cortex and regulate the activities of signal transduction pathways. Recent studies examining the structure and in vivo functions of ERMs have greatly advanced our understanding of the importance of membrane–cytoskeleton interactions.

Specific truncations of Drosophila Notch define dominant activated and dominant negative forms of the receptor

The Notch gene of Drosophila plays an important role in cell fate specification throughout development. To investigate the functions of specific structural domains of the Notch protein in vivo, a series of deletion mutants have been ectopically expressed under the hsp70 heat shock promoter. Two classes of dominant phenotypes are observed, one suggestive of Notch loss-of-function mutations and the other of Notch gain-of-function mutations. Dominant activated phenotypes result from overexpression of a protein lacking most extracellular sequences, while dominant negative phenotypes result from overexpression of a protein lacking most intracellular sequences. These results support the notion that Notch functions as a receptor whose extracellular domain mediates ligand binding, resulting in the transmission of developmental signals by the cytoplasmic domain. Finally, the phenotypes observed suggest that the cdc 10/ankyrin repeat region within the intracellular domain plays an essential role in the postulated signal transduction events.

Moesin functions antagonistically to the Rho pathway to maintain epithelial integrity

Two prominent characteristics of epithelial cells, apical-basal polarity and a highly ordered cytoskeleton, depend on the existence of precisely localized protein complexes associated with the apical plasma membrane, and on a separate machinery that regulates the spatial order of actin assembly. ERM (ezrin, radixin, moesin) proteins have been proposed to link transmembrane proteins to the actin cytoskeleton in the apical domain, suggesting a structural role in epithelial cells, and they have been implicated in signalling pathways. Here, we show that the sole Drosophila ERM protein Moesin functions to promote cortical actin assembly and apical-basal polarity. As a result, cells lacking Moesin lose epithelial characteristics and adopt invasive migratory behaviour. Our data demonstrate that Moesin facilitates epithelial morphology not by providing an essential structural function, but rather by antagonizing activity of the small GTPase Rho. Thus, Moesin functions in maintaining epithelial integrity by regulating cell-signalling events that affect actin organization and polarity. Furthermore, our results show that there is negative feedback between ERM activation and activity of the Rho pathway.

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 α and β 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.

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.

Ezrin, Radixin and Moesin: key regulators of membrane-cortex interactions and signaling.

The cell cortex serves as a critical nexus between the extracellular environment/cell membrane and the underlying cytoskeleton and cytoplasm. In many cells, the cell cortex is organized and maintained by the Ezrin, Radixin and Moesin (ERM) proteins, which have the ability to interact with both the plasma membrane and filamentous actin. Although this membrane-cytoskeletal linkage function is critical to stability of the cell cortex, recent studies indicate that this is only a part of what ERMs do in many cells. In addition to their role in binding filamentous actin, ERMs regulate signaling pathways through their ability to bind transmembrane receptors and link them to downstream signaling components. In this review we discuss recent evidence in a variety of cells indicating that ERMs serve as scaffolds to facilitate efficient signal transduction on the cytoplasmic face of the plasma membrane.

Merlin and the ERM proteins - regulators of receptor distribution and signaling at the cell cortex

Recent studies highlight the importance of the distribution of membrane receptors in controlling receptor output and in contributing to complex biological processes. The cortical cytoskeleton is known to affect membrane protein distribution but the molecular basis of this is largely unknown. Here, we discuss the functions of Merlin and the ERM proteins both in linking membrane proteins to the underlying cortical cytoskeleton and in controlling the distribution of and signaling from membrane receptors. We also propose a model that could account for the intricacies of Merlin function across model organisms.