Over-expression of PAR-3 suppresses contact-mediated inhibition of cell migration in MDCK cells

BACKGROUND: PAR-3 is one of the PAR proteins, previously named ASIP, which are indispensable for the establishment of cell polarity in the embryo as well as differentiated epithelial cells. In mammalian epithelial cells, it forms a ternary complex with aPKC and PAR-6, and is localized to the tight junction that has been suggested as being important for creating cell polarity. RESULTS: To gain insights into the mode of PAR-3 function in mammalian epithelial cells, we examined the effect of PAR-3 over-expression in MDCK cells. Although exogenous PAR-3-expression does not affect the epithelial polarity of confluent cells, it drastically transforms the morphology of cells at low density into a fibroblastic form with developed membrane protrusions. Time-lapse observations have revealed that PAR-3 over-expressing cells show intense motility, even after they have assembled into loose colonies, suggesting that the contact-mediated inhibition of cell migration (CIM) is suppressed. The expressions of E-cadherin and vimentin do not change with PAR-3 over-expression, suggesting that exogenous PAR-3 only disturbs the endogenous equilibrium of cellular states between a fundamental fibroblastic structure and an epithelial one. The co-expression of a dominant negative mutant of Rac1 and the addition of nocodazole strongly antagonize the effect of PAR-3 over-expression, suggesting the involvement of Rac1 activation and microtubule polymerizations. CONCLUSIONS: : The data presented here suggest an intriguing link between the contact-mediated inhibition of cell migration and the regulation of cell polarity. The putative PAR-3 activities demonstrated here may function endogenously in the epithelial cell polarization process by being sequestered from the cytosol to the cell-cell junctional regions with aPKC and PAR-6 upon cell-cell adhesion.     

Apical localization of ASIP/PAR-3:EGFP in zebrafish neuroepithelial cells involves the oligomerization domain CR1, the PDZ domains, and the C-terminal portion of the protein.

Neurulation in zebrafish (Danio rerio) embryos is characterized by oriented cell divisions and the progressive establishment of cellular polarity. Mitoses in the neural plate and neural tube are planar, but in the neural keel/rod stage, the mitotic spindle rotates by 90 degrees, causing cell divisions to occur perpendicular to the plane of the neuroepithelium. The mechanisms and molecules that establish cellular polarity and cause the stereotypic orientation of the mitotic spindle during neurulation are largely unknown. In Caenorhabditis elegans and Drosophila, the PAR/aPKC complex has been shown to be involved in both establishment of cellular polarity and spindle orientation. Here, we show that the conserved N-terminal oligomerization domain (CR1) and the PDZ domains of ASIP/PAR-3:EGFP are involved in its localization to the apical membrane in zebrafish neuroepithelial cells. We further show that the C-terminal part of ASIP/PAR-3 contributes to proper localization and that the apical localization signals in ASIP/PAR-3 prevent the basolateral localization of a Numb:PAR-3 fusion protein. The parallel orientation of the mitotic spindle in the neural tube, however, is only weakly impaired upon overexpression of various ASIP/PAR-3:EGFP constructs.     

Human homologues of the Caenorhabditis elegans cell polarity protein PAR6 as an adaptor that links the small GTPases Rac and Cdc42 to atypical protein kinase C

BACKGROUND: Asymmetric cell division in the Caenorhabditis elegans embryos requires products of par (partitioning defective) genes 1-6 and atypical protein kinase C (aPKC), whereas Cdc42 and Rac, members of the Rho family GTPases, play an essential role in cell polarity establishment in yeast and mammalian cells. However, little is known about a link between PAR proteins and the GTPases in cell polarization. RESULTS: Here we have cloned cDNAs for three human homologues of PAR6, designated PAR6alpha, beta and gamma, comprising 345, 372 and 376 amino acids, respectively. The PAR6 proteins harbour a PDZ domain and a CRIB-like motif, and directly interact with GTP-bound Rac and Cdc42 via this motif and with the aPKC isoforms PKCiota/lambda and PKCzeta via the N-terminal head-to-head association. These interactions are not mutually exclusive, thereby allowing the PAR6 proteins to form a ternary complex with the GTPases and aPKC, both in vitro and in vivo. When PAR6 and aPKC are expressed with a constitutively active form of Rac in HeLa or COS-7 cells, these proteins co-localize to membrane ruffles, which are known to occur at the leading edge of polarized cells during cell movement. CONCLUSION: Human PAR6 homologues most likely play an important role in the cell polarization of mammalian cells, by functioning as an adaptor protein that links activated Rac and Cdc42 to aPKC signalling.     

Regulated protein-protein interaction between aPKC and PAR-3 plays an essential role in the polarization of epithelial cells

BACKGROUND: Recent studies have revealed that aPKC (atypical protein kinase C), PAR-3 and PAR-6 play indispensable roles in the regulation of various cell polarization events, from worms to mammals, suggesting that they comprise an evolutionarily conserved protein machinery which is essential for cell polarization. The three proteins interact with each other to form a ternary complex and thus mutually regulate their functionality and localization. Here, we investigated the biochemical nature of the aPKC-PAR-3 interaction in detail to clarify its functional importance in cell polarity. RESULTS: The highly conserved 26 amino acid sequence 816-841, in PAR-3 was found to be necessary and sufficient for the tight association with aPKC. Among several conserved serine/threonine residues within the region, aPKC preferentially phosphorylates serine-827 in vitro, and this phosphorylation reduces the stability of the PAR-3-aPKC interaction. Several analyses using a phospho-serine 827 specific antibody have established that this phosphorylation by aPKC occurs in vivo. Over-expression of a point mutant of PAR-3 (S827A), which is predicted to form a stable complex with aPKC, causes defects in the cell-cell contact-induced cell polarization of epithelial MDCK cells, similarly to a dominant negative mutant of aPKC. CONCLUSIONS: These results imply that serine 827 in the aPKC binding site of PAR-3 is a target of aPKC and that the regulated interaction between a protein kinase, aPKC, and its substrate, PAR-3, plays an essential role in the establishment of cell polarity.     

Cdc42 is crucial for the establishment of epithelial polarity during early mammalian development.

To study the role of Cdc42 in the establishment of epithelial polarity during mammalian development, we generated murine Cdc42-null embryonic stem cells and analyzed peri-implantation development using embryoid bodies (EBs). Mutant EBs developed endoderm and underlying basement membrane, but exhibited defects of cell polarity, cell-cell junctions, survival, and cavitation. These defects corresponded to a decreased phosphorylation and membrane localization of aPKC, a reduced phosphorylation of GSK3beta, and a diminished activity of Rac1. However, neither Rac1 nor the kinase function of GSK3beta seem to contribute to cell polarization and cell-cell contacts. In contrast, EBs expressing dominant-negative (dn) PKCzeta mimicked well the phenotype of Cdc42-null EBs, suggesting a major role of aPKC in mediating cell polarization downstream of Cdc42. Finally, aggregation experiments with endodermal cell lines suggested that Cdc42 might affect formation of adherens and tight junctions by PKCzeta-dependent regulation of the protein levels of p120 catenin and E-cadherin.     

Association of ASIP/mPAR-3 with adherens junctions of mouse neuroepithelial cells.

Polarity proteins play fundamental roles in asymmetric cell division, which is essential for the production of different types of cells in multicellular organisms. Here, we explore the localization of atypical PKC isotype-specific interacting protein (ASIP), a mammalian homologue of the Caenorhabditis elegans polarity protein PAR-3, in embryonic neural tissues. Although ASIP is localized on tight junctions in cultured epithelial cells, it localizes on adherens junctions outlined by beta-catenin and afadin at the luminal surface, an apical end of the neuroepithelium in developing mouse central nervous systems. Mammalian homologues of other C. elegans polarity proteins, mPAR-6 and aPKC, also localize in the adherens junctions. In dorsal root ganglia of the peripheral nervous system, ASIP is found predominantly in the cytoplasm of ganglion cells. In dividing preneural cells at the ventricular (luminal) surface of the embryonic telencephalon, ASIP localize in adherence junctions of luminal surface regardless of the axis of cell division. Therefore, only the daughter cell facing the lumen (apical daughter) may inherit ASIP when the division plate is oriented parallel to the surface. Given the roles of Bazooka, a Drosophila homologue of ASIP/PAR-3, in the asymmetric division of the Drosophila neuroblast, these observations suggest that ASIP, along with other polarity proteins and adherens junction proteins, plays an important role in neural cell differentiation by means of asymmetric cell division.     

Importance of spatial activation of Cdc42 and rac small G proteins by frabin for microspike formation in MDCK cells

BACKGROUND: Frabin is an actin filament (F-actin)-binding protein that shows GDP/GTP exchange activity for Cdc42 small G protein (Cdc42). Frabin furthermore induces indirect activation of Rac small G protein (Rac) in intact cells. We have recently shown that in nonepithelial cells, frabin induces the formation of both filopodia- and lamellipodia-like processes through the activation of Cdc42 and Rac, respectively. In epithelial cells such as MDCK cells, Cdc42 and Rac regulate cell-cell adherens junctions (AJs) via the accumulation of F-actin and E-cadherin, although neither Cdc42 nor Rac induces the formation of filopodia or lamellipodia. In this study, we have examined the effects of frabin on the reorganization of the actin cytoskeleton in MDCK cells. RESULTS: Frabin induces the formation of microspikes at the basal area of the lateral membranes through the activation of Cdc42 and Rac in MDCK cells, although a dominant active mutant of Cdc42 or Rac alone, or both, did not induce the formation of microspikes. Furthermore, frabin weakly increased the accumulation of F-actin and E-cadherin at cell-cell AJs and the formation of stress fibres through the activation of Cdc42 and Rac, under conditions where the dominant active mutant of Cdc42 or Rac markedly showed these effects. The Cdc42- and Rac-induced formation of stress fibres was dependent on the activation of Rho small G protein. CONCLUSION: These results indicate that the frabin-dependent spatial activation of Cdc42 and Rac is important for the formation of microspikes.     

ERBIN associates with p0071, an armadillo protein,at cell-cell junctions of epithelial cells

BACKGROUND: ERBIN, an ErbB2 receptor-interacting protein, belongs to a recently described family of proteins termed the LAP [leucine-rich repeats and PSD-95/dLg-A/ZO-1 (PDZ) domains] family which has essential roles in establishment of cell polarity. RESULTS: To identify new ERBIN-binding proteins, we screened a yeast two-hybrid library, using the carboxyl-terminal fragment of ERBIN containing PDZ domain as the bait, and we isolated p0071 (also called plakophilin-4) as an ERBIN-interacting protein. p0071 is a member of the p120 catenin family, which are defined as proteins with 10 armadillo repeats, and localizes along the cell-cell border. The ERBIN PDZ domain binds the COOH-terminus of p0071 containing the PDZ domain-binding sequence. Endogenous ERBIN was co-immunoprecipitated with p0071. In fully polarized Madin-Darby canine kidney (MDCK) cells, ERBIN co-localized largely with beta-catenin and partly with desmoplakin along the lateral plasma membrane domain. At these cell-cell contact regions, ERBIN co-localizes with p0071. Over-expression of the dominant active forms of Cdc42, Rac1 or RhoA, Rho family small GTPases, resulted in a marked accumulation of ERBIN at the cell-cell contacts of MDCK and HeLa cells. CONCLUSION: These results show that ERBIN interacts in vivo with p0071 and that it may be involved in the organization of adherens junctions and the desmosomes of epithelia. In addition, we demonstrated that the subcellular localization of ERBIN might be regulated by Rho family small GTPases.     

Intracellular polarity protein PAR-1 regulates extracellular laminin assembly by regulating the dystroglycan complex

Cell polarity depends on extrinsic spatial cues and intrinsic polarity proteins including PAR-aPKC proteins. In mammalian epithelial cells, cell–cell contacts provide spatial cues that activate the aPKC-PAR-3-PAR-6 complex to establish the landmark of the initial cellular asymmetry. PAR-1, a downstream target of the aPKC-PAR-3-PAR-6 complex, mediates further development of the apical and basolateral membrane domains. However, the relationships between the PAR-aPKC proteins and other extrinsic spatial cues provided by the extracellular matrix (ECM) remain unclear. Here, we show that PAR-1 colocalizes with laminin receptors and is required for the assembly of extracellular laminin on the basal surface of epithelial cells. Furthermore, PAR-1 regulates the basolateral localization of the dystroglycan (DG) complex, one of the laminin receptors essential for basement membrane formation. We also show that PAR-1 interacts with the DG complex and is required for the formation of a functional DG complex. These results reveal the presence of a novel inside-out pathway in which an intracellular polarity protein regulates the ECM organization required for epithelial cell polarity and tissue morphogenesis.     

Establishment of cell polarity by afadin during the formation of embryoid bodies

Afadin directly links nectin, an immunoglobulin-like cell–cell adhesion molecule, to actin filaments (F-actin) at adherens junctions (AJs). The nectin–afadin complex is important for the formation of not only AJs but also tight junctions (TJs) in epithelial cells. Studies using afadin-knockout mice have revealed that afadin is indispensable for embryonic development by organizing the formation of cell–cell junctions. However, the molecular mechanism of cell–cell junction disorganization during embryonic development in afadin-knockout mice is poorly understood. To address this, we took advantage of embryoid bodies (EBs) as a model system. The formation of cell–cell junctions including AJs and TJs was impaired in afadin-null EBs. The proper accumulation of the Par complex and the activation of Cdc42 and atypical PKC (aPKC), which are crucial for the formation of cell polarity, were also inhibited by knockout of afadin. In addition, the disruption of afadin caused the abnormal deposition of laminin and the dislocalization of its receptors integrin α₆ and integrin β₁. These results indicate that afadin organizes the formation of cell–cell junctions by regulating cell polarization in early embryonic development.     

Densin-180, a synaptic protein,links to PSD-95 through its direct interaction with MAGUIN-1

BACKGROUND: Densin-180, a brain-specific protein highly concentrated at the postsynaptic density (PSD), belongs to the LAP [leucine-rich repeats and PSD-95/Dlg-A/ZO-1 (PDZ) domains] family of proteins, some of which play fundamental roles in the establishment of cell polarity. RESULTS: To identify new Densin-180-interacting proteins, we screened a yeast two-hybrid library using the COOH-terminal fragment of Densin-180 containing the PDZ domain as bait, and we isolated MAGUIN-1 as a Densin-180-binding protein. MAGUIN-1, a mammalian homologue of Drosophila connector enhancer of KSR (CNK), is known to interact with PSD-95 and has a short isoform, MAGUIN-2. The Densin-180 PDZ domain bound to the COOH-terminal PDZ domain-binding motif of MAGUIN-1. Densin-180 co-immunoprecipitated with MAGUIN-1 as well as with PSD-95 from the rat brain. In dissociated hippocampal neurones Densin-180 co-localized with MAGUINs and PSD-95, mainly at neuritic spines. In transfected cells, Densin-180 formed a ternary complex with MAGUIN-1 and PSD-95, whereas no association was detected between Densin-180 and PSD-95 in the absence of MAGUIN-1. MAGUIN-1 formed a dimer or multimer via the COOH-terminal leucine-rich region which is present in MAGUIN-1 but not in -2. Among the PDZ domains of PSD-95, the first was sufficient for interaction with MAGUIN-1. CONCLUSION: These results suggest that the potential to dimerize or multimerize allows MAGUIN-1 to bind simultaneously to both Densin-180 and PSD-95, leading to the ternary complex assembly of these proteins at the postsynaptic membrane.     

Topographical significance of membrane skeletal component protein 4.1B in mammalian organs

The polarized architecture of epithelial cells is a fundamental determinant of cell structures and functions. Both formation and orientation of proper epithelial polarity are needed for cell-cell or cell-matrix adhesion, signal transduction and cytoskeletal interactions of multimolecular complexes at apical, lateral and basal cell membranes. These cell membrane domains are usually segregated by some junctional complexes. Recent molecular genetic studies on the anchor structure between myelin sheaths and axons have indicated the specific molecular organization for polarization of axolemma and the myelin sheaths at paranodes, termed 'septate-like junctions'. It was also speculated that other mammalian organs may use a similar junctional system. The protein 4.1 B was originally found to be localized in paranodes and juxtaparanodes of myelinated nerve fibers. Our recent immunohistochemical studies on protein 4.1B have indicated its significance for the cell-cell and/or cell-matrix adhesion in various rodent organs. The protein 4.1 family of proteins have been supposed to possess variable molecular domains relating to cell adhesion, ion balance, receptor responses and signal transduction. Therefore, more precise studies on the molecular structure and the functional domains of protein 4.1B, as well as on its changes under physiological and pathological conditions, may provide a clue for organogenesis in various mammalian organs.     

Polarized secretion of lysosomes at the B cell synapse couples antigen extraction to processing and presentation

Engagement of the B cell receptor (BCR) by surface-tethered antigens (Ag) leads to formation of?a synapse that promotes Ag uptake for presentation onto major histocompatibility complex class II (MHCII) molecules. We have highlighted the membrane trafficking events and associated molecular mechanisms involved in Ag extraction and processing at the B cell synapse. MHCII-containing lysosomes are recruited to the synapse where they locally undergo exocytosis, allowing synapse acidification and the extracellular release of hydrolases that promote the extraction of the immobilized Ag. Lysosome recruitment and secretion results from the polarization of the microtubule-organizing center (MTOC), which relies on the cell division cycle (Cdc42)-downstream effector, atypical protein kinase C (aPKCζ). aPKCζ is phosphorylated upon BCR engagement, associates to lysosomal vesicles, and is required for their polarized secretion at the B cell synapse. Regulation of B lymphocyte polarity therefore emerges as?a central mechanism that couples Ag extraction to Ag processing and presentation.     

Influence of cell polarity on early development of the sea urchin embryo

Background: Establishment and maintenance of cell polarity is critical for normal embryonic development. Previously, it was thought that the echinoderm embryo remained relatively unpolarized until the first asymmetric division at the 16‐cell stage. Here, we analyzed roles of the cell polarity regulators, the PAR complex proteins, and how their disruption in early development affects later developmental milestones. Results: We found that PAR6, aPKC, and CDC42 localize to the apical cortex as early as the 2‐cell stage and that this localization is retained through the gastrula stage. Of interest, PAR1 also colocalizes with these apical markers through the gastrula stage. Additionally, PAR1 was found to be in complex with aPKC, but not PAR6. PAR6, aPKC, and CDC42 are anchored in the cortical actin cytoskeleton by assembled myosin. Furthermore, assembled myosin was found to be necessary to maintain proper PAR6 localization through subsequent cleavage divisions. Interference with myosin assembly prevented the embryos from reaching the blastula stage, while transient disruptions of either actin or microtubules did not have this effect. Conclusions: These observations suggest that disruptions of the polarity in the early embryo can have a significant impact on the ability of the embryo to reach later critical stages in development. Developmental Dynamics 244:1469–1484, 2015. © 2015 Wiley Periodicals, Inc.     

Cdc42 and Rac small G proteins activated by trans-interactions of nectins are involved in activation of c-Jun N-terminal kinase,but not in association of nectins and cadherin to form adherens junctions,in fibroblasts

BACKGROUND: Nectins are Ca2+-independent immunoglobulin-like cell-cell adhesion molecules which associate with cadherins to form adherens junctions (AJs) in epithelial cells and fibroblasts. Nectin-1 and -3 are members of the nectin family which most strongly trans-interact, causing cell-cell adhesion. The trans-interaction between nectin-1 and -3 induces the activation of both Cdc42 and Rac small G proteins in epithelial cells. We studied the roles of Cdc42 and Rac activated in this way in L fibroblasts stably expressing both nectin-1 and E-cadherin (nectin-1-EL cells). RESULTS: The trans-interaction between nectin-1 and -3 induced the activation of Cdc42 and Rac in nectin-1-EL cells. Cdc42, and presumably Rac, activated in this way, induced the activation of c-Jun N-terminal kinase (JNK), but not p38 mitogen-activated protein (MAP) kinase or extracellular signal-regulated kinase (ERK). Cdc42 or Rac was not essential for the association of nectin-1 and E-cadherin to form AJs. Reorganization of the actin cytoskeleton was not required for the association of nectin-1 and E-cadherin. CONCLUSION: These results indicate that Cdc42 and Rac activated by the trans-interaction of nectins selectively induce the activation of JNK, but are not essential for the association of nectins and cadherin to form AJs in fibroblasts.     

Loss of partitioning-defective-3/isotype-specific interacting protein (par-3/ASIP) in the elongating spermatid of RA175 (IGSF4A/SynCAM)-deficient mice

IGSF4a/RA175/SynCAM (RA175) and junctional adhesion molecules (Jams) are members of the immunoglobulin superfamily with a PDZ-binding domain at their C termini. Deficiency of Ra175 (Ra175(-/-)) as well as Jam-C deficiency (Jam-C(-/-)) causes the defect of the spermatid differentiation, oligo-astheno-teratozoospermia. Ra175(-/-) elongating spermatids fail to mature further, whereas Jam-C(-/-) round spermatids lose cell polarity, and most of Jam-C(-/-) elongated spermatids are completely lost. RA175 and Jam-C seem to have similar but distinct functional roles during spermatid differentiation. Here we show that the cell polarity protein Par-3 with PDZ domains, a binding partner of Jams, is one of the associated proteins of the cytoplasmic region of RA175 in testis. Par-3 and Jam-C are partly co-localized with RA175 in the elongating and elongated spermatids; their distributions overlapped with that of RA175 on the tips of the dorsal region of the head of the elongating spermatid (steps 9 to 12) in the wild type. In the Ra175(-/-) elongating spermatid, Par-3 was absent, and Jam-C was absent or abnormally localized. The RA175 formed a ternary complex with Jam-C via interaction with Par-3. The lack of the ternary complex in the Ra175(-/-) elongating spermatid may cause the defect of the specialized adhesion structures, resulting in the oligo-astheno-teratozoospermia.     

Association of frabin with specific actin and membrane structures

BACKGROUND: Frabin is an actin filament (F-actin)-binding protein with GDP/GTP exchange activity specific for Cdc42 small G protein. Expression of frabin forms filopodia-like microspikes through the direct activation of Cdc42, and lamellipodia through indirect activation of Rac small G protein. Frabin consists of the F-actin-binding domain (FAB), the Dbl homology domain (DH), the first pleckstrin homology domain (PH1), the FYVE-finger domain (FYVE), the second PH domain (PH2) from the N-terminus in this order. Although DH and PH1 show exchange activity, FAB, in addition to DH and PH1, is required for the formation of microspikes, whereas FYVE and PH2, in addition to DH and PH1, are required for the formation of lamellipodia. RESULTS: Various truncated mutants of frabin were co-expressed with a dominant active mutant (DA) of Cdc42, Rac1DA, or full-length frabin in L fibroblasts. FAB was recruited to the Cdc42DA-formed filopodia-like microspikes. FAB and a fragment containing DH, PH1, FYVE and PH2 were recruited to the Rac1DA-formed membrane ruffles. Furthermore, each of these fragments served as a dominant negative mutant of frabin when co-expressed with full-length frabin, and inhibited the full-length frabin-formed morphological changes. CONCLUSION: These results suggest that frabin recognizes a specific actin structure(s) through FAB and a specific membrane structure(s) through FAB and the region containing DH, PH1, FYVE and PH2. It is likely that frabin associates with the specific actin and membrane structures and activates Cdc42 and Rac in the vicinity of these structures, eventually leading to morphological changes.     

JNK and ROKalpha function in the noncanonical Wnt/RhoA signaling pathway to regulate Xenopus convergent extension movements.

The Wnt/planar cell polarity (PCP) pathway plays a critical role in wing, eye, and sensory bristle development of Drosophila and in convergent extension (CE) movements during vertebrate gastrulation. In Drosophila, Jun N-terminal kinase (JNK) and Rho-associated kinase (ROK) participate in RhoA-mediated PCP pathway during eye and wing development. In mammalian cells, Rac1 and Cdc42 but not RhoA are required for JNK activation by Wnt/PCP signals. However, there has been no evidence that Rho GTPases regulate JNK activation in Wnt/PCP pathway during Xenopus CE movements. Here, we report that Xenopus RhoA (XRhoA), but not Xenopus Cdc42 (XCdc42), is essential for JNK activation downstream of the Wnt/PCP pathway during Xenopus CE movements, and the phenotypic effect of loss of XRhoA function was rescued by Xenopus JNK1 (XeJNK1). In addition, XRhoA rescues the inhibition of CE movements by the DEP domain deletion mutant of Xenopus Dsh (Xdsh-DeltaDEP), which has dominant negative (DN) effects on JNK activation, and the PDZ domain deletion mutant of Xdsh (Xdsh-DeltaPDZ). Moreover, we demonstrate that Xenopus Rho-associated kinase alpha (xROKalpha), which is expressed mainly in mesoderm and ectoderm that undergo extensive cell rearrangements, regulates CE movements without affecting gene expression, and injection of xROKalpha rescued the inhibition of CE movements caused by DN XRhoA. Finally, we show that ROKalpha and JNK synergistically rescued embryos overexpressing DN XRhoA, which exhibit gastrulation defects, although ROKalpha is not required for JNK activation. Together, these data suggest that JNK and ROKalpha function in the noncanonical Wnt/RhoA pathway to regulate Xenopus CE movements.     

Fgd1, the Cdc42 guanine nucleotide exchange factor responsible for faciogenital dysplasia, is localized to the subcortical actin cytoskeleton and Golgi membrane

FGD1, the gene responsible for the inherited disease faciogenital dysplasia, encodes a guanine nucleotide exchange factor (GEF) that specifically activates the p21 GTPase Cdc42. In order, FGD1 is composed of a proline-rich N-terminal region, adjacent GEF and pleckstrin homology (PH) domains, a FYVE-finger domain and a second C-terminal PH domain (PH2), structural motifs involved in signaling and subcellular localization. Fgd1, the mouse FGD1 ortholog, is expressed in regions of active bone formation within osteoblasts and in the osteoblast-like cell line MC3T3-E1, a finding consistent with its role in skeletal formation. Here, we use subcellular fractionation studies to show that endogenous Fgd1 protein is localized in the cytosolic and Golgi and plasma membrane fractions of mouse calvarial cells. Immunocytochemical studies performed with osteoblast-like MC3T3-E1 cells and other mammalian cell lines confirm the localization of Fgd1 and show that the proline-rich N-terminal region is necessary and sufficient for Fgd1 subcellular localization to the plasma membrane and Golgi complex. In contrast, the FYVE-finger and PH2 domains do not appear to direct the localization of Fgd1 or the activation of Cdc42. In addition, microinjection studies indicate that the N-terminal Fgd1 domain inhibits filopodia formation, suggesting that this region down-regulates GEF function. These results characterize the function of the Fgd1 domains for both protein localization and Cdc42 activation and indicate that the Fgd1 Cdc42GEF protein is involved in the regulation of Cdc42 activity at the subcortical actin cytoskeleton and Golgi complex.     

PX-RICS mediates ER-to-Golgi transport of the N-cadherin/beta-catenin complex

Cadherins mediate Ca2+-dependent cell-cell adhesion. Efficient export of cadherins from the endoplasmic reticulum (ER) is known to require complex formation with beta-catenin. However, the molecular mechanisms underlying this requirement remain elusive. Here we show that PX-RICS, a beta-catenin-interacting GTPase-activating protein (GAP) for Cdc42, mediates ER-to-Golgi transport of the N-cadherin/beta-catenin complex. Knockdown of PX-RICS expression induced the accumulation of the N-cadherin/beta-catenin complex in the ER and ER exit site, resulting in a decrease in cell-cell adhesion. PX-RICS was also required for ER-to-Golgi transport of the fibroblast growth factor-receptor 4 (FGFR4) associated with N-cadherin. PX-RICS-mediated ER-to-Golgi transport was dependent on its interaction with beta-catenin, phosphatidylinositol-4-phosphate (PI4P), Cdc42, and its novel binding partner gamma-aminobutyric acid type A receptor-associated protein (GABARAP). These results suggest that PX-RICS ensures the efficient entry of the N-cadherin/beta-catenin complex into the secretory pathway, and thereby regulates the amount of N-cadherin available for cell adhesion and FGFR4-mediated signaling.