Targeting of the myosin-I myr 3 to intercellular adherens type junctions induced by dominant active Cdc42 in HeLa cells

Correlative light and electron microscopy using immunolabeled sections

In correlative microscopy, light microscopy provides the overview and orientation of the complex cells and tissue, while electron microscopy offers the detailed localization and correlation of subcellular structures. In this chapter we offer detailed high-quality electron microscopical preparation methods for optimum preservation of the cellular ultrastructure. From such preparations serial thin sections are collected and used for comparative histochemical, immunofluorescence, and immunogold staining.In light microscopy histological stains identify the orientation of the sample and immunofluorescence labeling facilitates to find the region of interest, namely, the labeled cells expressing the macromolecule under investigation. Sections, labeled with immunogold are analyzed by electron microscopy in order to identify the label within the cellular architecture at high resolution.

Myosin 1e is a component of the glomerular slit diaphragm complex that regulates actin reorganization during cell-cell contact formation in podocytes

Glomerular visceral epithelial cells, also known as podocytes, are critical to both normal kidney function and the development of kidney disease. Podocyte actin cytoskeleton and their highly specialized cell-cell junctions (also called slit diaphragm complexes) play key roles in controlling glomerular filtration. Myosin 1e (myo1e) is an actin-based molecular motor that is expressed in renal glomeruli. Disruption of the Myo1e gene in mice and humans promotes podocyte injury and results in the loss of the integrity of the glomerular filtration barrier. Here, we have used biochemical and microscopic approaches to determine whether myo1e is associated with the slit diaphragm complexes in glomerular podocytes. Myo1e was consistently enriched in the slit diaphragm fraction during subcellular fractionation of renal glomeruli and colocalized with the slit diaphragm markers in mouse kidney. Live cell imaging studies showed that myo1e was recruited to the newly formed cell-cell junctions in cultured podocytes, where it colocalized with the actin filament cables aligned with the nascent contacts. Myo1e-null podocytes expressing FSGS-associated myo1e mutant (A159P) did not efficiently assemble actin cables along new cell-cell junctions. We have mapped domains in myo1e that were critical for its localization to cell-cell junctions and determined that the SH3 domain of myo1e tail interacts with ZO-1, a component of the slit diaphragm complex and tight junctions. These findings suggest that myo1e represents a component of the slit diaphragm complex and may contribute to regulating junctional integrity in kidney podocytes.

Molecular basis of dynamic relocalization of Dictyostelium myosin IB

Class I myosins have a single heavy chain comprising an N-terminal motor domain with actin-activated ATPase activity and a C-terminal globular tail with a basic region that binds to acidic phospholipids. These myosins contribute to the formation of actin-rich protrusions such as pseudopodia, but regulation of the dynamic localization to these structures is not understood. Previously, we found that Acanthamoeba myosin IC binds to acidic phospholipids in vitro through a short sequence of basic and hydrophobic amino acids, BH site, based on the charge density of the phospholipids. The tail of Dictyostelium myosin IB (DMIB) also contains a BH site. We now report that the BH site is essential for DMIB binding to the plasma membrane and describe the molecular basis of the dynamic relocalization of DMIB in live cells. Endogenous DMIB is localized uniformly on the plasma membrane of resting cells, at active protrusions and cell-cell contacts of randomly moving cells, and at the front of motile polarized cells. The BH site is required for association of DMIB with the plasma membrane at all stages where it colocalizes with phosphoinositide bisphosphate/phosphoinositide trisphosphate (PIP(2)/PIP(3)). The charge-based specificity of the BH site allows for in vivo specificity of DMIB for PIP(2)/PIP(3) similar to the PH domain-based specificity of other class I myosins. However, DMIB-head is required for relocalization of DMIB to the front of migrating cells. Motor activity is not essential, but the actin binding site in the head is important. Thus, dynamic relocalization of DMIB is determined principally by the local PIP(2)/PIP(3) concentration in the plasma membrane and cytoplasmic F-actin.

Correlative light and electron microscopy in parasite research

The interaction of a parasite and a host cell is a complex process, which involves several steps: (1) attachment to the plasma membrane, (2) entry inside the host cell, and (3) hijacking of the metabolism of the host. In biochemical experiments, only an event averaged over the whole cell population can be analyzed. The power of microscopy, however, is to investigate individual events in individual cells. Therefore, parasitologists frequently perform experiments with fluorescence microscopy using different dyes to label structures of the parasite or the host cell. Though the resolution of light microscopy has greatly improved, it is not sufficient to reveal interactions at the ultrastructural level. Furthermore, only specifically labeled structures can be seen and related to each other. Here, we want to demonstrate the additional value of electron microscopy in this area of research. Investigation of the different steps of parasite-host cell interaction by electron microscopy, however, is often hampered by the fact that there are only a few cells infected, and therefore it is difficult to find enough cells to study. A solution is to profit from low magnification, hence large overview, and specific location of the players by fluorescence labels in a light microscope with the high power resolution and structural information provided by an electron microscope, in short by correlative light and electron microscopy.

Mitotic spindle misorientation in cancer--out of alignment and into the fire

Mitotic spindle orientation can influence tissue organization and vice versa. Cells orient their spindles by rotating them parallel or perpendicular to the cell--and hence the tissue--axis. Spindle orientation in turn controls the placement of daughter cells within a tissue, influencing tissue morphology. Recent findings implicating tumor suppressor proteins in spindle orientation bring to the forefront a connection between spindle misorientation and cancer. In this Commentary, we focus on the role of three major human tumor suppressors--adenomatous polyposis coli (APC), E-cadherin and von Hippel-Lindau (VHL)--in spindle orientation. We discuss how, in addition to their better-known functions, these proteins affect microtubule stability and cell polarity, and how their loss of function causes spindles to become misoriented. We also consider how other cancer-associated features, such as oncogene mutations, centrosome amplification and the tumor microenvironment, might influence spindle orientation. Finally, we speculate on the role of spindle misorientation in cancer development and progression. We conclude that spindle misorientation alone is unlikely to be tumorigenic, but it has the potential to synergize with cancer-associated changes to facilitate genomic instability, tissue disorganization, metastasis and expansion of cancer stem cell compartments.

Polarity proteins and cell-cell interactions in the testis

In mammalian testes, extensive junction restructuring takes place in the seminiferous epithelium at the Sertoli-Sertoli and Sertoli-germ cell interface to facilitate the different cellular events of spermatogenesis, such as mitosis, meiosis, spermiogenesis, and spermiation. Recent studies in the field have shown that Rho GTPases and polarity proteins play significant roles in the events of cell-cell interactions. Furthermore, Rho GTPases, such as Cdc42, are working in concert with polarity proteins in regulating cell polarization and cell adhesion at both the blood-testis barrier (BTB) and apical ectoplasmic specialization (apical ES) in the testis of adult rats. In this chapter, we briefly summarize recent findings on the latest status of research and development regarding Cdc42 and polarity proteins and how they affect cell-cell interactions in the testis and other epithelia. More importantly, we provide a new model in which how Cdc42 and components of the polarity protein complexes work in concert with laminin fragments, cytokines, and testosterone to regulate the events of cell-cell interactions in the seminiferous epithelium via a local autocrine-based regulatory loop known as the apical ES-BTB-basement membrane axis. This new functional axis coordinates various cellular events during different stages of the seminiferous epithelium cycle of spermatogenesis.

Newly formed E-cadherin contacts do not activate Cdc42 or induce filopodia protrusion in human keratinocytes

BACKGROUND INFORMATION

The appropriate regulation of cell-cell adhesion is an important event in the homoeostasis of different cell types. In epithelial cells, tight adhesion mediated by E-cadherin receptors is essential for the differentiation and functionality of epithelial sheets. Upon assembly of cadherin-mediated cell-cell contacts, it is well established that the small GTPases Rho and Rac are activated and are necessary for junction stability. However, the role of the small GTPase Cdc42 in cadherin adhesion is less clear. Cdc42 can be activated by E-cadherin in a breast tumour cell line, but the requirement for Cdc42 function for new junction assembly or maintenance has been contradictory. Cdc42 participation in cell-cell contacts has been inferred from the presence of filopodia, the typical F-actin structure induced by Cdc42 activation, as cells approach each other to establish cell-cell contacts. Yet, under these conditions, the contribution of migration to filopodia protrusion cannot be excluded and the results are difficult to interpret.

RESULTS

In the present study, we set out to address (a) whether Cdc42 is activated by new E-cadherin cell-cell contacts when junction assembly occurs without prior migration and (b) whether Cdc42 function is necessary for cadherin stability. We found that junction formation in confluent keratinocytes or upon E-cadherin clustering decreased Cdc42-GTP levels. In the absence of serum- and migration-induced Cdc42 activation, we demonstrated that cell-cell contacts do not induce filopodia or require Cdc42 function to assemble.

CONCLUSION

We conclude that Cdc42 does not participate in the early events that initiate stable cadherin adhesion in keratinocytes. Yet, it is feasible that Cdc42 may be activated at later time points or by other receptors. Cdc42 can then participate in additional functions during polarization, such as Golgi re-positioning or basolateral trafficking.

A role for myosin 1e in cortical granule exocytosis in Xenopus oocytes

Xenopus oocytes undergo dynamic structural changes during maturation and fertilization. Among these, cortical granule exocytosis and compensatory endocytosis provide effective models to study membrane trafficking. This study documents an important role for myosin 1e in cortical granule exocytosis. Myosin 1e is expressed at the earliest stage that cortical granule exocytosis can be detected in oocytes. Prior to exocytosis, myosin 1e relocates to the surface of cortical granules. Overexpression of myosin 1e augments the kinetics of cortical granule exocytosis, whereas tail-derived fragments of myosin 1e inhibit this secretory event (but not constitutive exocytosis). Finally, intracellular injection of myosin 1e antibody inhibits cortical granule exocytosis. Further experiments identified cysteine string proteins as interacting partners for myosin 1e. As constituents of the membrane of cortical granules, cysteine string proteins are also essential for cortical granule exocytosis. Future investigation of the link between myosin 1e and cysteine string proteins should help to clarify basic mechanisms of regulated exocytosis.

Pore forming polyalkylpyridinium salts from marine sponges versus synthetic lipofection systems: distinct tools for intracellular delivery of cDNA and siRNA

Background

Haplosclerid marine sponges produce pore forming polyalkylpyridinium salts (poly-APS), which can be used to deliver macromolecules into cells. The aim of this study was to investigate the delivery of DNA, siRNA and lucifer yellow into cells mediated by poly-APS and its potential mechanisms as compared with other lipofection systems (lipofectamine and N⁴,N⁹-dioleoylspermine (LipoGen)). DNA condensation was evaluated and HEK 293 and HtTA HeLa cells were used to investigate pore formation and intracellular delivery of cDNA, siRNA and lucifer yellow.

Results

Poly-APS and LipoGen were both found to be highly efficient DNA condensing agents. Fura-2 calcium imaging was used to measure calcium transients indicative of cell membrane pore forming activity. Calcium transients were evoked by poly-APS but not LipoGen and lipofectamine. The increases in intracellular calcium produced by poly-APS showed temperature sensitivity with greater responses being observed at 12°C compared to 21°C. Similarly, delivery of lucifer yellow into cells with poly-APS was enhanced at lower temperatures. Transfection with cDNA encoding for the expression enhanced green fluorescent protein was also evaluated at 12°C with poly-APS, lipofectamine and LipoGen. Intracellular delivery of siRNA was achieved with knockdown in beta-actin expression when lipofectamine and LipoGen were used as transfection reagents. However, intracellular delivery of siRNA was not achieved with poly-APS.

Conclusion

Poly-APS mediated pore formation is critical to its activity as a transfection reagent, but lipofection systems utilise distinct mechanisms to enable delivery of DNA and siRNA into cells.

Actin at cell-cell junctions is composed of two dynamic and functional populations

The ability of epithelial cells to polarize requires cell-cell adhesion mediated by cadherin receptors. During cell-cell contact, the mechanism via which a flat, spread cell shape is changed into a tall, cuboidal epithelial morphology is not known. We found that cadherin-dependent adhesion modulates actin dynamics by triggering changes in actin organization both locally at junctions and within the rest of the cell. Upon induction of cell-cell contacts, two spatial actin populations are distinguishable: junctional actin and peripheral thin bundles. With time, the relative position of these two populations changes and becomes indistinguishable to form a cortical actin ring that is characteristic of mature, fully polarized epithelial cells. Junctional actin and thin actin bundles differ in their actin dynamics and mechanism of formation, and interestingly, have distinct roles during epithelial polarization. Whereas junctional actin stabilizes clustered cadherin receptors at cell-cell contacts, contraction of peripheral actin bundle is essential for an increase in the maximum height at the lateral domain during polarization (cuboidal morphology). Thus, both junctional actin and thin bundles are necessary, and cooperate with each other to generate a polarized epithelial morphology.

Axonal isoforms of myosin-I

We have examined spinal motor neurons in Sprague-Dawley rats to further characterize a mechanoenzyme, myosin-Igamma (myr4), which is found in high concentration during axon tract formation in neonates. We raised an antibody to myr4 and made riboprobes for in situ hybridization. Myr4 mRNA was abundant in spinal cord motor neurons (particularly during axon regrowth). Nerves undergoing Wallerian degeneration (from a crush 7 days earlier) showed anti-myr4 labeling of the axolemma and SER--after microtubules, neurofilaments, and F-actin had already been degraded--which is consistent with a described lipid-binding domain in the tail region of myosin-Is. Newly synthesized myr4 was carried in axons by the slow component (SC) of axonal transport at 1-8 mm/day, whereas, none was carried by the fast component (FC). We conclude that SC delivers myr4 to the cytoplasmic surfaces of stationary axonal membranes (SER and axolemma). This positioning would anchor the tail domain of myr4 and leave the catalytic head domain free to interact with F-actin.

Unconventional myosins, actin dynamics and endocytosis: a ménage à trois?

Ever since the discovery of class I myosins, the first nonmuscle myosins, about 30 years ago, the history of unconventional myosins has been linked to the organization and working of actin filaments. It slowly emerged from studies of class I myosins in lower eukaryotes that they are involved in mechanisms of endocytosis. Most interestingly, a flurry of recent findings assign a more active role to class I myosins in regulating the spatial and temporal organization of actin filament nucleation and elongation. The results highlight the multiple links between class I myosins and the major actin nucleator, the Arp2/3 complex, and its newly described activators. Two additional types of unconventional myosins, myosinIX, and Dictyostelium discoideum MyoM, have recently been tied to the signaling pathways controlling actin cytoskeleton remodeling. The present review surveys the links between these three classes of molecular motors and the complex cellular processes of endocytosis and actin dynamics, and concentrates on a working model accounting for the function of class I myosins via recruitment of the machinery responsible for actin nucleation and elongation.

Cell-cell adhesion and signalling

Signalling pathways activated by Rho small GTPases have recently been identified that coordinate junction assembly, stability and function, as well as interactions of adhesive complexes with the underlying cortical cytoskeleton. Particularly exciting is the interplay between adherens junctions, activation of Rho proteins and the dynamics of microtubule, actin and intermediate filaments. This interplay has important implications for functional regulation of cell-cell adhesion, and points to a more integrated view of signalling processes.

The kinetic mechanism of Myo1e (human myosin-IC)

Myo1e is the widely expressed subclass-1 member of the myosin-I family. We performed a kinetic analysis of a truncated myo1e that consists of the motor and the single IQ motif with a bound calmodulin. We determined the rates and equilibrium constants for the key steps in the ATPase cycle. The maximum actin activated ATPase rate (V(max)) and the actin concentration at half-maximum of V(max) (K(ATPase)) of myo1e are similar to those of the native protein. The K(ATPase) is low (approximately 1 microm), however the affinity of myo1e for actin in the presence of ATP is very weak. A weak actin affinity and a rapid rate of phosphate release result in a pathway under in vitro assay conditions in which phosphate is released while myo1e is dissociated from actin. Actin activation of the ATPase activity and the low K(ATPase) are the result of actin activation of ADP release. We propose that myo1e is tuned to function in regions of high concentrations of cross-linked actin filaments. Additionally, we found that ADP release from actomyo1e is > 10-fold faster than other vertebrate myosin-I isoforms. We propose that subclass-1 myosin-Is are tuned for rapid sliding, whereas subclass-2 isoforms are tuned for tension maintenance or stress sensing.

Pitx2a expression alters actin-myosin cytoskeleton and migration of HeLa cells through Rho GTPase signaling

We ectopically expressed the transcription factor Pitx2a, one of the Pitx2 isoforms, in HeLa cells by using a tetracycline-inducible expression system and examined whether Pitx2a was capable of modulating Rho GTPase signaling and altering the cell's cytoskeleton. Ectopic expression of Pitx2a induced actin-myosin reorganization, leading to increased cell spreading, suppression of cell migration, and the strengthening of cell-cell adhesion, marked by the accumulation and localization of beta-catenin and N-cadherin to the sites of cell-cell contacts. Moreover, Pitx2a expression resulted in activation of the Rho GTPases Rac1 and RhoA, and the dominant negative Rac1 mutant N17Rac1 inhibited cell spreading and disrupted localization of beta-catenin to the sites of cell-cell contacts. Both reorganization of actin-myosin and cell spreading require phosphatidylinositol 3-kinase activity, which is also necessary for activation of the Rho GTPase proteins. Pitx2a induced the expression of Trio, a guanine nucleotide exchange factor for Rac1 and RhoA, which preceded cell spreading, and the expression of Trio protein was down-regulated after the changes in cell spreading and cell morphology were initiated. In addition, Pitx2a also induces cell cycle arrest at G0/G1, most likely due to the accumulation of the tumor suppressor proteins p53 and p21. Our data indicate that the transcriptional activities initiated in the nucleus by Pitx2a result in profound changes in HeLa cell morphology, migration, and proliferation.

Motor domain-dependent localization of myo1b (myr-1)

Myosin-I is the single-headed, membrane binding member of the myosin superfamily that plays a role in membrane dynamics and transport [1-6]. Its molecular functions and its mechanism of regulation are not known. In mammalian cells, myosin-I is excluded from specific microfilament populations, indicating that its localization is tightly regulated. Identifying the mechanism of this localization, and the specific actin populations with which myosin-I interacts, is crucial to understanding the molecular functions of this motor. eGFP chimeras of myo1b [7] were imaged in live and fixed NRK cells. Ratio-imaging microscopy shows that myo1b-eGFP concentrates within dynamic areas of the actin cytoskeleton, most notably in membrane ruffles. Myo1b-eGFP does not associate with stable actin bundles or stress fibers. Truncation mutants consisting of the motor or tail domains show a partially overlapping cytoplasmic localization with full-length myo1b, but do not concentrate in membrane ruffles. A chimera consisting of the light chain and tail domains of myo1b and the motor domain from nonmuscle myosin-IIb (nmMIIb) concentrates on actin filaments in ruffles as well as to stress fibers. In vitro motility assays show that the exclusion of myo1b from certain actin filament populations is due to the regulation of the actomyosin interaction by tropomyosin. Therefore, we conclude that tropomyosin and spatially regulated actin polymerization play important roles in regulating the function and localization of myo1b.

9Rgr; and Rac but not Cdc42 regulate endothelial cell permeability

Endothelial permeability induced by thrombin and histamine is accompanied by actin stress fibre assembly and intercellular gap formation. Here, we investigate the roles of the Ρ family GTPases Rho1, Rac1 and Cdc42 in regulating endothelial barrier function, and correlate this with their effects on F-actin organization and intercellular junctions. RhoA, Rac1 and Cdc42 proteins were expressed efficiently in human umbilical vein endothelial cells by adenovirus-mediated gene transfer. We show that inhibition of Ρ prevents both thrombin- and histamine-induced increases in endothelial permeability and decreases in transendothelial resistance. Dominant-negative RhoA and a Ρ kinase inhibitor, Y-27632, not only inhibit stress fibre assembly and contractility but also prevent thrombin- and histamine-induced disassembly of adherens and tight junctions in endothelial cells, providing an explanation for their effects on permeability. In contrast, dominant-negative Rac1 induces permeability in unstimulated cells and enhances thrombin-induced permeability, yet inhibits stress fibre assembly, indicating that increased stress fibre formation is not essential for endothelial permeability. Dominant-negative Cdc42 reduces thrombin-induced stress fibre formation and contractility but does not affect endothelial cell permeability or responses to histamine. These results demonstrate that Ρ and Rac act in different ways to alter endothelial barrier function, whereas Cdc42 does not affect barrier function.

Regulation and expression of metazoan unconventional myosins

Unconventional myosins are molecular motors that convert adenosine triphosphate (ATP) hydrolysis into movement along actin filaments. On the basis of primary structure analysis, these myosins are represented by at least 15 distinct classes (classes 1 and 3-16), each of which is presumed to play a specific cellular role. However, in contrast to the conventional myosins-2, which drive muscle contraction and cytokinesis and have been studied intensively for many years in both uni- and multicellular organisms, unconventional myosins have only been subject to analysis in metazoan systems for a short time. Here we critically review what is known about unconventional myosin regulation, function, and expression. Several points emerge from this analysis. First, in spite of the high relative conservation of motor domains among the myosin classes, significant differences are found in biochemical and enzymatic properties of these motor domains. Second, the idea that characteristic distributions of unconventional myosins are solely dependent on the myosin tail domain is almost certainly an oversimplification. Third, the notion that most unconventional myosins function as transport motors for membranous organelles is challenged by recent data. Finally, we present a scheme that clarifies relationships between various modes of myosin regulation.

Actin dynamics and cell-cell adhesion in epithelia

Recent advances in the field of intercellular adhesion highlight the importance of adherens junction association with the underlying actin cytoskeleton. In skin epithelial cells a dynamic feature of adherens junction formation involves filopodia, which physically project into the membrane of adjacent cells, catalyzing the clustering of adherens junction protein complexes at their tips. In turn, actin polymerization is stimulated at the cytoplasmic interface of these complexes. Although the mechanism remains unclear, the VASP/Mena family of proteins seems to be involved in organizing actin polymerization at these sites. In vivo, adherens junction formation appears to rely upon filopodia in processes where epithelial sheets must be physically moved closer to form stable intercellular connections, for example, in ventral closure in embryonic development or wound healing in the postnatal animal.

Recruitment of a specific amoeboid myosin I isoform to the plasma membrane in chemotactic Dictyostelium cells

The Dictyostelium class I myosins, MyoA, -B, -C, and -D, participate in plasma membrane-based cellular processes such as pseudopod extension and macropinocytosis. Given the existence of a high affinity membrane-binding site in the C-terminal tail domain of these motor proteins and their localized site of action at the cortical membrane-cytoskeleton, it was of interest to determine whether each myosin I was directly associated with the plasma membrane. The membrane association of a myosin I heavy chain kinase that regulates the activity of one of the class I myosins, MyoD was also examined. Cellular fractionation experiments revealed that the majority of the Dicyostelium MyoA, -B, -C and -D heavy chains and the kinase are cytosolic. However, a small, but significant, fraction (appr. 7. -15%) of each myosin I and the kinase was associated with the plasma membrane. The level of plasma membrane-associated MyoB, but neither that of MyoC nor MyoD, increases up to 2-fold in highly motile, streaming cells. These results indicate that Dictyostelium specifically recruits myoB to the plasma membrane during directed cell migration, consistent with its known role in pseudopod formation.

A new family of Cdc42 effector proteins, CEPs, function in fibroblast and epithelial cell shape changes

Cdc42, a Rho GTPase, regulates the organization of the actin cytoskeleton by its interaction with several distinct families of downstream effector proteins. Here, we report the identification of four new Cdc42-binding proteins that, along with MSE55, constitute a new family of effector proteins. These molecules, designated CEPs, contain three regions of homology, including a Cdc42 binding domain and two unique domains called CI and CII. Experimentally, we have verified that CEP2 and CEP5 bind Cdc42. Expression of CEP2, CEP3, CEP4, and CEP5 in NIH-3T3 fibroblasts induced pseudopodia formation. Fibroblasts coexpressing dominant negative Cdc42 with CEP2 or expressing a Cdc42/Rac interactive binding domain mutant of CEP2 did not induce pseudopodia formation. In primary keratinocytes, CEP2- and CEP5-expressing cells showed reduced F-actin localization at the adherens junctions with an increase in thin stress fibers that extended the length of the cell body. Keratinocytes expressing CEPs also showed an altered vinculin distribution and a loss of E-cadherin from adherens junctions. Similar effects were observed in keratinocytes expressing constitutively active Cdc42, but were not seen with a Cdc42/Rac interactive binding domain mutant of CEP2. These results suggest that CEPs act downstream of Cdc42 to induce actin filament assembly leading to cell shape changes.

Small GTP-binding proteins

Small GTP-binding proteins (G proteins) exist in eukaryotes from yeast to human and constitute a superfamily consisting of more than 100 members. This superfamily is structurally classified into at least five families: the Ras, Rho, Rab, Sar1/Arf, and Ran families. They regulate a wide variety of cell functions as biological timers (biotimers) that initiate and terminate specific cell functions and determine the periods of time for the continuation of the specific cell functions. They furthermore play key roles in not only temporal but also spatial determination of specific cell functions. The Ras family regulates gene expression, the Rho family regulates cytoskeletal reorganization and gene expression, the Rab and Sar1/Arf families regulate vesicle trafficking, and the Ran family regulates nucleocytoplasmic transport and microtubule organization. Many upstream regulators and downstream effectors of small G proteins have been isolated, and their modes of activation and action have gradually been elucidated. Cascades and cross-talks of small G proteins have also been clarified. In this review, functions of small G proteins and their modes of activation and action are described.

Functional role for the class IX myosin myr5 in epithelial cell infection by Shigella flexneri

Efficient control of Shigella-induced, rho-dependent cytoskeletal rearrangements seems to be required to shape the delicate cellular structures associated with bacterial invasion of epithelial cells. We therefore studied a class IX myosin and rho antagonist, the GTPase-activating protein (GAP) myr5, for a potential role in the bacterial entry process. We show that myr5 is recruited into bacterial entry spots. The recruitment pattern resembled that of rhoC or ezrin, but not rhoA, rac or CDC42, while in vitro GAP activity of myr5 was similar for rhoA, B or C. Analysis of myr5 mutants suggested that GTPase- or ATP-binding activites are not required for Shigella-induced recruitment of this atypical myosin to the bacterial entry site. Functional studies revealed a potential dual role of the myosin functions and the GAP module of myr5 for bacterial internalization.

Epithelial cell shape: cadherins and small GTPases

Cadherins are cell-cell adhesion receptors that are essential for the establishment of the epithelial cell shape and maintenance of the differentiated epithelial phenotype. In order to show efficient adhesion, cadherin receptors require an association with actin filaments and the activity of RHO proteins. The RHO family of small GTPases is primarily involved in the reorganization of the cytoskeleton. In different cell types, each member of the family can induce specific types of organization of actin filaments: stress fibers (Rho), lamellae/ruffles (Rac), or filopodia (Cdc42). This review focuses on how the function of small GTPases may impinge on the regulation of cadherin-dependent adhesion. In particular, it discusses the impact that the above cytoskeletal structures induced by RHO proteins have on the development of epithelial morphology. Finally, the participation of small GTPase-interacting proteins is considered during the remodeling of cell shape that follows cell-cell contact formation.

Regulation of the enzymatic and motor activities of myosin I

Myosins I were the first unconventional myosins to be purified and they remain the best characterized. They have been implicated in various motile processes, including organelle translocation, ion channel gating and cytoskeletal reorganization but their exact cellular functions are still unclear. All members of the myosin I family, from yeast to man, have three structural domains: a catalytic head domain that binds ATP and actin; a tail domain believed to be involved in targeting the myosins to specific subcellular locations and a junction or neck domain that connects them and interacts with light chains. In this review we discuss how each of these three domains contributes to the regulation of myosin I enzymatic activity, motor activity and subcellular localization.

Functions of unconventional myosins

To date, fourteen classes of unconventional myosins have been identified. Recent reports have implicated a number of these myosins in organelle transport, and in the formation, maintenance and/or dynamics of actin-rich structures involved in a variety of cellular processes including endocytosis, cell migration, and sensory transduction. Characterizations of organelle dynamics in pigment cells and neurons have further defined the contributions made by unconventional myosins and microtubule motors to the transport and distribution of organelles. Several studies have provided evidence of complexes through which cooperative organelle transport may be coordinated. Finally, the myosin superfamily has been shown to contain at least one processive motor and one backwards motor.

Directed actin polymerization is the driving force for epithelial cell-cell adhesion

We have found that epithelial cells engage in a process of cadherin-mediated intercellular adhesion that utilizes calcium and actin polymerization in unexpected ways. Calcium stimulates filopodia, which penetrate and embed into neighboring cells. E-cadherin complexes cluster at filopodia tips, generating a two-rowed zipper of embedded puncta. Opposing cell surfaces are clamped by desmosomes, while vinculin, zyxin, VASP, and Mena are recruited to adhesion zippers by a mechanism that requires alpha-catenin. Actin reorganizes and polymerizes to merge puncta into a single row and seal cell borders. In keratinocytes either null for alpha-catenin or blocked in VASP/Mena function, filopodia embed, but actin reorganization/polymerization is prevented, and membranes cannot seal. Taken together, a dynamic mechanism for intercellular adhesion is unveiled involving calcium-activated filopodia penetration and VASP/Mena-dependent actin reorganization/polymerization.

Rho-like GTPases: their role in epithelial cell-cell adhesion and invasion

The family members of small Rho-like GTPases, RhoA, Rac1 and Cdc42Hs, are regulators of diverse cellular signalling pathways, including cytoskeletal organisation, transcription and cell-cycle progression. Recent research has given insight into the complex regulation of cell-cell adhesion and migratory responses of epithelial cells. The Rho-like GTPases RhoA, Rac1 and Cdc42Hs as major determinants of cytoskeletal organisation have been identified as key regulators of epithelial architecture, as well as of cell migration. These findings highlight the complex regulation and cross-talk of GTPase-dependent signalling pathways arising from cell-cell and cell-matrix interactions. The molecular mechanism of how Rho-like GTPases couple to molecules mediating either cell-cell adhesion or cell migration will be of particular interest to understand the invasive phenotype of epithelial tumours.

Overlapping distribution of the 130- and 110-kDa myosin I isoforms on rat liver membranes

The biochemical and mechanochemical properties and localization of myosin I suggest the involvement of these small members of the myosin superfamily in some aspects of intracellular motility in higher cells. We have determined by quantitative immunoblotting with isoform-specific antibodies that the 130-kDa myosin I (myr 1 gene product) and 110-kDa myosin I (myr 2 gene product) account for 0.5 and 0.4%, respectively, of total rat liver protein. Immunoblot analyses reveal that the 130- and 110-kDa myosins I are found in several purified subcellular fractions from rat liver. The membrane-associated 130-kDa myosin I is found at the highest concentration in the plasma membrane (28 ng/microg plasma membrane protein) followed by the endoplasmic reticulum-like mitochondria-associated membrane fraction (MAM; 10 ng/microg MAM protein), whereas the 110-kDa myosin I is found at the highest concentration in Golgi (50 ng/¿g Golgi protein) followed by plasma membrane (20 ng/microg) and MAM (7 ng/microg). Our analyses indicate that myosin I is peripherally associated with Golgi and MAM and its presence in these fractions is not a consequence of myosin I bound to contaminating actin filaments. Although found in relatively low concentrations in microsomes, because of the abundance of microsomes, in liver most of the membrane-associated myosin I is associated with microsomes. Neither myosin I isoform is detected in purified mitochondria. This is the first quantitative analysis addressing the cellular distribution of these mammalian class I myosins.

Rho-like GTPases: their role in epithelial cell-cell adhesion and invasion

The family members of small Rho-like GTPases, RhoA, Rac1 and Cdc42Hs, are regulators of diverse cellular signalling pathways, including cytoskeletal organisation, transcription and cell-cycle progression. Recent research has given insight into the complex regulation of cell-cell adhesion and migratory responses of epithelial cells. The Rho-like GTPases RhoA, Rac1 and Cdc42Hs as major determinants of cytoskeletal organisation have been identified as key regulators of epithelial architecture, as well as of cell migration. These findings highlight the complex regulation and cross-talk of GTPase-dependent signalling pathways arising from cell-cell and cell-matrix interactions. The molecular mechanism of how Rho-like GTPases couple to molecules mediating either cell-cell adhesion or cell migration will be of particular interest to understand the invasive phenotype of epithelial tumours.

The arm-repeat protein NPRAP (neurojungin) is a constituent of the plaques of the outer limiting zone in the retina, defining a novel type of adhering junction

In the retina, special plaque-bearing adhering junctions are aligned to form a planar system (the "outer limiting zone," OLZ) of heterotypic connections between the photoreceptor cells and the surrounding glial cells ("Müller cells"), together with homotypic junctions. In the plaques of these junctions, which contain N-cadherin-and possibly also related cadherins-we have identified, by immunolocalization techniques, a recently discovered neural tissue-specific protein, neurojungin, a member of the plakoglobin/armadillo protein family. In these plaques we have also detected other adherens plaque proteins, such as alpha- and beta-catenin, protein p120, and vinculin, as well as proteins known as constituents of tight junction plaques, such as symplekin and protein ZO-1, and the desmosomal plaque protein plakophilin 2. This unusual combination of proteins and the demonstrated absence of plakoglobin define the OLZ junctions as a new and distinct category of adhering junction, which probably has special architectural functions.

Involvement of Cdc42 small G protein in cell-cell adhesion, migration and morphology of MDCK cells

The Rho small G protein family consists of the Rho, Rac, and Cdc42 subfamilies and regulates various cell functions through reorganization of the actin cytoskeleton. We previously showed that the Rho subfamily regulates the formation of stress fibers and focal adhesions whereas the Rac subfamily regulates the E-cadherin-based cell-cell adhesion in MDCK cells. We studied here the function of the Cdc42 subfamily, consisting of two members, Cdc42Hs and G25k, in cell adhesion, migration, and morphology of MDCK cells. For this purpose, we made and used MDCK cell lines stably expressing each of dominant active mutants of Cdc42Hs (sMDCK-Cdc42HsDA) and G25K (sMDCK-G25KDA). Actin filaments at the cell-cell adhesion sites increased in both sMDCK-Cdc42HsDA and -G25KDA cells. Both E-cadherin and beta-catenin, adherens junctional proteins, at the cell-cell adhesion sites also increased in both sMDCK-Cdc42HsDA and -G25KDA cells. Electron microscopic analysis revealed that sMDCK-Cdc42HsDA cells tightly contacted with each other throughout the lateral membranes. Moreover, both the HGF- and TPA-induced disruption of the cadherin-based cell-cell adhesion and the subsequent cell migration were inhibited in both sMDCK-Cdc42HsDA and -G25KDA cells. Co-expression of the dominant negative mutant of Rac1, a member of the Rac subfamily, with the dominant active mutant of Cdc42Hs did not inhibit the increased accumulation of actin filaments at the cell-cell adhesion sites. These results suggest that the Cdc42 subfamily is involved in the cadherin-based cell-cell adhesion in a manner independent of the Rac subfamily. Furthermore, the cells were frequently enveloped by the large multinuclear cells in both sMDCK-Cdc42HsDA and -G25KDA cells. Video microscopic analysis revealed that the cells were engulfed by the large cells during cytokinesis.