Prenatal detection of terminal 9p24.3 microduplication encompassing DOCK8 gene: A variant of likely benign

Trisomy 9p is one of the most common chromosomal partial trisomies in newborns. However, reports on prenatal 9p microduplications are rare in the clinic. This study aimed to examine the genotype-phenotype correlation and assess the clinical significance of 9p24.3 microduplication encompassing the DOCK8 gene. Eight pregnant women underwent amniocentesis for cytogenetic and genetic testing for various indications for prenatal diagnosis from January 2019 to January 2020. Chromosomal karyotypic analysis was performed on G-band metaphases that were prepared from cultured amniotic fluid cells. Chromosomal microarray analysis was carried out to detect chromosomal copy number variations. We also performed a literature review on clinical data on similar 9p24.3 microduplications to determine the genotype-phenotype correlation. We detected 123-248-kb microduplications in the region of 9p24.3 (chr9: 208454-469022), involving part of or the entire DOCK8 gene. The indications for prenatal diagnosis mainly focused on the risk of maternal serum screening for trisomy 21/18, advanced maternal age, and increased nuchal translucency. No evident structural abnormalities were observed for all fetuses, except for case 5 who presented with increased nuchal translucency in prenatal ultrasound findings. Follow-up of postnatal health was performed and showed no apparent abnormalities for cases 1 to 6 after birth. The parents of case 7 chose to terminate the pregnancy while the parents of case 8 chose to continue the pregnancy. We propose that 9p24.3 microduplications that encompass part of or the entire DOCK8 gene are variants that might be benign. However, further large-scale studies are necessary to evaluate the clinical pathogenicity. For prenatal cases with 9p24.3 microduplication, postnatal health and growth should be followed up and assessed regularly from childhood to adulthood.

Expression of FGD3 gene as prognostic factor in young breast cancer patients

The FGD3 gene works as a cell migration inhibitor and seems to be a promising indicator of outcome in some human cancers including breast. In this study, we analysed for the first time the prognostic role of FGD3 in young breast cancer patients. We studied the relationship between traditional prognostic factors, FGD3 expression and outcome in ≤40 years breast cancer patients. We found that lower FGD3 expression decreased the probability of disease-free survival (p = 0.042) and overall survival (p = 0.007). In a multivariate analysis for overall survival AJCC stage (p = 0.005) and FGD3 expression (p = 0.03) resulted independent prognostic factors. Low FGD3 expression increased the risk of death from disease (HR 5.73, p = 0.03). Moreover, low FGD3 expression was associated with more widespread lymph node involvement (p = 0.04) and a lower FGD3 staining intensity was found in positive-lymph-node patients vs negative (p = 0.003) and in patients with ≥10 involved lymph nodes vs <10 (p = 0.05). Our results suggest FGD3 to be a significant independent prognostic factor in young breast cancer patients in terms of disease-free survival and overall survival. A lower expression increased the risk of recurrence and death from disease and was associated with widespread lymph node metastases.

IQSEC2-Associated Intellectual Disability and Autism

Mutations in IQSEC2 cause intellectual disability (ID), which is often accompanied by seizures and autism. A number of studies have shown that IQSEC2 is an abundant protein in excitatory synapses and plays an important role in neuronal development as well as synaptic plasticity. Here, we review neuronal IQSEC2 signaling with emphasis on those aspects likely to be involved in autism. IQSEC2 is normally bound to N-methyl-D-aspartate (NMDA)-type glutamate receptors via post synaptic density protein 95 (PSD-95). Activation of NMDA receptors results in calcium ion influx and binding to calmodulin present on the IQSEC2 IQ domain. Calcium/calmodulin induces a conformational change in IQSEC2 leading to activation of the SEC7 catalytic domain. GTP is exchanged for GDP on ADP ribosylation factor 6 (ARF6). Activated ARF6 promotes downregulation of surface α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors through a c-jun N terminal kinase (JNK)-mediated pathway. NMDA receptors, AMPA receptors, and PSD-95 are all known to be adversely affected in autism. An IQSEC2 transgenic mouse carrying a constitutively active mutation (A350V) shows autistic features and reduced levels of surface AMPA receptor subunit GluA2. Sec7 activity and AMPA receptor recycling are presented as two targets, which may respond to drug treatment in IQSEC2-associated ID and autism.

Biochemical, Biophysical and Cellular Techniques to Study the Guanine Nucleotide Exchange Factor, GIV/Girdin

Canonical signal transduction via heterotrimeric G proteins is spatiotemporally restricted, i.e., triggered exclusively at the plasma membrane, only by agonist activation of G protein-coupled receptors via a finite process that is terminated within a few hundred milliseconds. Recently, a rapidly emerging paradigm has revealed a noncanonical pathway for activation of heterotrimeric G proteins via the nonreceptor guanidine-nucleotide exchange factor, GIV/Girdin. Biochemical, biophysical, and functional studies evaluating this pathway have unraveled its unique properties and distinctive spatiotemporal features. As in the case of any new pathway/paradigm, these studies first required an in-depth optimization of tools/techniques and protocols, governed by rationale and fundamentals unique to the pathway, and more specifically to the large multimodular GIV protein. Here we provide the most up-to-date overview of protocols that have generated most of what we know today about noncanonical G protein activation by GIV and its relevance in health and disease. © 2016 by John Wiley & Sons, Inc.

Novel SIL1 nonstop mutation in a Chinese consanguineous family with Marinesco-Sjögren syndrome and Dandy-Walker syndrome

Marinesco-Sjögren syndrome (MSS) is a rare autosomal recessive disorder, which is characterized by congenital cataracts, cerebellar ataxia, progressive muscle weakness, and delayed psychomotor development. SIL1, which is located at 5q31.2, is the only gene known to cause MSS. Dandy-Walker syndrome (DWS) is defined by hypoplasia, upward rotation of the cerebellar vermis, and cystic dilation of the fourth ventricle; however, its genetic pathogeny remains unclear. Here, we report a Chinese consanguineous family with MSS and DWS. Whole exome sequencing identified a novel nonstop mutation in SIL1. Sanger sequencing revealed that the mutation was segregated in this family according to a recessive mode of inheritance. We found that the mutation changed a stop codon (TGA) to an arginine codon (CGA), and no in-frame termination codon in the 3' untranslated region (UTR) of SIL1 could be found. The mRNA levels of SIL1 were decreased by 56.6% and 37.5% in immortalized lymphoblasts of the patients respectively; the protein levels of SIL1 were substantially decreased. This case study is the first report on Chinese MSS patients, MSS complicated by DWS, and a nonstop mutation in SIL1. Our findings imply the pathogenetic association between DWS and MSS.

The cAMP sensors, EPAC1 and EPAC2, display distinct subcellular distributions despite sharing a common nuclear pore localisation signal

We have identified a conserved nuclear pore localisation signal (NPLS; amino acids 764-838 of EPAC1) in the catalytic domains of the cAMP-sensors, EPAC1 and EPAC2A. Consequently, EPAC1 is mainly localised to the nuclear pore complex in HEK293T cells where it becomes activated following stimulation with cAMP. In contrast, structural models indicate that the cAMP-binding domain of EPAC2A (CNBD1) blocks access to the conserved NPLS in EPAC2A, reducing its ability to interact with nuclear binding sites. Consequently, a naturally occurring EPAC2 isoform, EPAC2B, which lacks CNBD1 is enriched in nuclear fractions, similar to EPAC1. Structural differences in EPAC isoforms may therefore determine their intracellular location and their response to elevations in intracellular cAMP.

Proteomic analysis and identification of cellular interactors of the giant ubiquitin ligase HERC2

HERC2 is a large E3 ubiquitin ligase with multiple structural domains that has been implicated in an array of cellular processes. Mutations in HERC2 are linked to developmental delays and impairment caused by nervous system dysfunction, such as Angelman Syndrome and autism-spectrum disorders. However, HERC2 cellular activity and regulation remain poorly understood. We used a broad proteomic approach to survey the landscape of cellular proteins that interact with HERC2. We identified nearly 300 potential interactors, a subset of which we validated binding to HERC2. The potential HERC2 interactors included the eukaryotic translation initiation factor 3 complex, the intracellular transport COPI coatomer complex, the glycogen regulator phosphorylase kinase, beta-catenin, PI3 kinase, and proteins involved in fatty acid transport and iron homeostasis. Through a complex bioinformatic analysis of potential interactors, we linked HERC2 to cellular processes including intracellular protein trafficking and transport, metabolism of cellular energy, and protein translation. Given its size, multidomain structure, and association with various cellular activities, HERC2 may function as a scaffold to integrate protein complexes and bridge critical cellular pathways. This work provides a significant resource with which to interrogate HERC2 function more deeply and evaluate its contributions to mechanisms governing cellular homeostasis and disease.

Cyclic AMP and Epac contribute to the genesis of the positive interaction between hypoxia and hypercapnia in the carotid body

Carotid body chemoreceptor cells in response to hypoxic and hypercapnic stimulus increase their resting rate of release of neurotransmitters and their action potential frequency in the carotid sinus sensory nerve. When chemoreceptor activity is assessed at the level of the carotid sinus nerve and on ventilation, there exists an interaction between hypoxic and hypercapnic stimulus so that the response to both stimuli combined is additive or more than additive, over a wide range of stimulation. It is not clear if this interaction occurs at chemoreceptor cell or directly acting on the sensory nerve. In the present work we demonstrate for the first time the existence of a positive interaction between hypoxic and hypercapnic-acidotic stimuli at the level of both, membrane potential depolarization and neurotransmitter release in rat and rabbit carotid body. Inhibition of adenylate cyclase (SQ-22536) abolished the positive interaction between stimuli and the Epac (exchange proteins activated by cAMP) activator 8-pCPT-2'-O-Me-cAMP reversed the effect of adenylate cyclase inhibition. These results suggest that this interaction between the two natural stimuli is mediated by cAMP via an Epac-dependent pathway, at least at the level of neurotransmitter release.

[Influence of lipopolysaccharide on the permeability of rat brain microvascular endothelial cells and the molecular mechanism]

OBJECTIVE

To study the influence of lipopolysaccharide (LPS) on the permeability of rat brain microvascular endothelial cells (BMECs) and possible molecular mechanism.

METHODS

Monolayers of primary rat BMECs were separated and cultured, and then treated with (LPS group) or without LPS (control group). The barrier integrity was measured by transendothelial electrical resistance (TEER) assay. The degrees of RhoA activation were determined by Pull-down assay. The expression levels of p115RhoGEF, zonula occludens-1 (ZO-1), occludin and claudin-5 proteins were detected by Western blot analysis.

RESULTS

The average TEER values of rat BMECs in the LPS group were 108.3±4.2 Ω•cm2 and 85.4±2.5 Ω•cm2 respectively 3 and 12 hrs after LPS treatment, which were significantly lower than that in the control group (159.0±8.6 Ω•cm2). Compared with the control group, the activity of RhoA started to increase 5 minutes after LPS treatment, and the expression of p115RhoGEF protein started to increase 1 hr after LPS treatment and the cellular protein levels of ZO-1, occludin and claudin-5 decreased significantly 3 hrs after LPS treatment in the LPS group (P<0.05).

CONCLUSIONS

LPS may activate the p115RhoGEF/RhoA pathway and decrease protein expression of ZO-1, occludin and claudin-5, resulting in an increased permeability of rat BMECs.

Proteomic analysis of pancreatic juice for the identification of biomarkers of pancreatic cancer

INTRODUCTION

Protein profiles of endoscopically collected pancreatic juice from normal, chronic pancreatitis patients and pancreatic cancer patients were compared to identify diagnostic biomarkers of pancreatic cancer.

METHODS

Secretin was injected intravenously and pancreatic juice was collected via selective cannulation of the pancreatic duct during endoscopic retrograde cholangiopancreatography. Pancreatic juices consisting of three pooled samples for normal control, chronic pancreatitis, and pancreatic cancer patients were compared using two-dimensional gel electrophoresis, and the proteins were subsequently identified using MALDI-TOF-MS.

RESULTS

Thirty-five protein spots were up-regulated twofold in pancreatic cancer compared with the levels in the normal controls, and 85 protein spots were present in pancreatic cancer samples but not in normal controls. After excluding spots that were also expressed in chronic pancreatitis, 26 protein spots that were up-regulated or only expressed in pancreatic cancer samples were identified. Among the identified proteins, we confirmed the expressions of BIG2, PRDX6, and REG1α in pancreatic cancer tissue using immunohistochemistry. ELISA showed that the serum level of REG1α was significantly higher in patients with pancreatic cancer than it was in the normal controls (P = 0.023). With the best cut-off value, the sensitivity and specificity of REG1α to differentiate normal and pancreatic cancer were 82.6 and 81.8%, compared with 69.6 and 100% for CA19-9.

CONCLUSIONS

We have shown that pancreatic juice is a good source of pancreatic cancer tumor markers. Further studies are needed to determine the clinical implications of REG1α and other markers.

Expression of the Rap1 guanine nucleotide exchange factor, MR-GEF, is altered in individuals with bipolar disorder

In the rodent forebrain GABAergic neurons are generated from progenitor cells that express the transcription factors Dlx1 and Dlx2. The Rap-1 guanine nucleotide exchange factor, MR-GEF, is turned on by many of these developing GABAergic neurons. Expression of both Dlx1/2 and MR-GEF is retained in both adult mouse and human forebrain where, in human, decreased Dlx1 expression has been associated with psychosis. Using in situ hybridization studies we show that MR-GEF expression is significantly down-regulated in the forebrain of Dlx1/2 double mutant mice suggesting that MR-GEF and Dlx1/2 form part of a common signalling pathway during GABAergic neuronal development. We therefore compared MR-GEF expression by in situ hybridization in individuals with major psychiatric disorders (schizophrenia, bipolar disorder, major depression) and control individuals. We observed a significant positive correlation between layers II and IV of the dorso-lateral prefrontal cortex (DLPFC) in the percentage of MR-GEF expressing neurons in individuals with bipolar disorder, but not in individuals with schizophrenia, major depressive disorder or in controls. Since MR-GEF encodes a Rap1 GEF able to activate G-protein signalling, we suggest that changes in MR-GEF expression could potentially influence neurotransmission.

Membrane localization of RasGRP1 is controlled by an EF-hand, and by the GEF domain

RasGRP1 is an exchange factor for membrane-localized Ras GTPases. Activation of RasGRP1 requires its translocation to membranes, which can be directly mediated by either its PT or C1 domains. RasGRP1 also has a pair of EF-hands which have been proposed to regulate RasGRP1 by sensing receptor-induced calcium fluxes. We determined that one of these EF-hands, EF1, is required for receptor-induced translocation of RasGRP1 to the plasma membrane in B cell lines. EF1 enables plasma membrane targeting of RasGRP1 by counteracting the SuPT domain, a negative regulator of the PT domain. Contrary to expectations, EF1-mediated translocation of RasGRP1 does not involve antigen receptor-induced intracellular calcium flux. Instead, alternative splicing affecting EF1 serves to modulate RasGRP1 localization. Excision of an exon encoding part of EF1 selectively disables PT domain-mediated plasma membrane targeting of RasGRP1, without affecting C1 domain-mediated localization to endomembranes. While EF1 specifically controls PT-mediated plasma membrane targeting, the Ras binding site in the catalytic GEF domain of RasGRP1 is required for both PT-mediated plasma membrane targeting and C1-mediated localization to endomembranes. Positive feedback between its GEF domain and membrane-binding domains could be important for full activation of RasGRP1, with occupation of the Ras binding sites in the GEF domain resulting in functional liberation of the PT and C1 domains, and membrane binding by these domains serving to maintain the Ras-GEF interaction.

Biochemistry of the Rap-specific guanine nucleotide exchange factors PDZ-GEF1 and -2

PDZ-GEFs represent one of four types of highly conserved Rap-specific guanine nucleotide exchange factors. They contain a number of well-known protein domains, including a "related to cyclic nucleotide binding domain" (RCBD), a PDZ-domain, a Ras-associating domain (RA), and, of course, a catalytic domain required for their exchange activity. Since their cloning more than 5 years ago, relatively little has been learned about their mode of regulation. Although their activity may in part depend on regulated membrane localization by means of the RA and/or PDZ domain, it seems highly likely that PDZ-GEFs can be modified by additional mechanisms as well. Based on analogy of the regulatory mechanisms of the cAMP-responsive GEF Epac, in the past we postulated a role for the RCBD domain in this. In this chapter, we give a detailed description of the methods that were used to unravel this mechanism in vitro and in vivo.

Molecular interaction of neurocalcin alpha with alsin (ALS2)

Membrane microdomains (MDs), or lipid rafts, are recently identified dynamic membrane domains on which various signal-transductions are performed. Intracellular Ca(2+)-binding proteins participate in the Ca(2+) signaling through interaction with various proteins. Neurocalcin alpha (NCalpha) is a member of neuronal calcium sensor (NCS) protein family and shows Ca(2+)-dependent binding to the cell membrane through N-terminal myristoyl moiety. Since NCalpha was identified as a Ca(2+)-dependent binding protein to neuronal MDs, its binding proteins may participate in the signal-transduction on the MDs. In an immunoprecipitate using anti-NCalpha antibody, alsin (ALS2), a protein product of one of the responsive genes for amyotrophic lateral sclerosis, was detected through LC-MS/MS. Specific antibody to alsin was produced and immunoprecipitation using this antibody showed co-sedimentation of NCalpha. Some part of alsin bound to brain-derived MD fraction in the presence of Ca(2+) ions and eluted out by the chelation of Ca(2+) ions, as in the case of NCalpha. Immunostaining of cultured neurons showed broad distribution of alsin and NCalpha, and membrane association of these proteins were increased through Ca(2+) loading by maitotoxin. These results suggest that alsin binds cell membrane in a Ca(2+)-dependent manner through NCalpha and regulates membrane dynamics.

Epac-selective cAMP analogs: new tools with which to evaluate the signal transduction properties of cAMP-regulated guanine nucleotide exchange factors

The identification of 2'-O-methyl substituted adenosine-3',5'-cyclic monophosphate (cAMP) analogs that activate the Epac family of cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs, also known as Epac1 and Epac2), has ushered in a new era of cyclic nucleotide research in which previously unrecognized signalling properties of the second messenger cAMP have been revealed. These Epac-Selective Cyclic AMP Analogs (ESCAs) incorporate a 2'-O-methyl substitution on the ribose ring of cAMP, a modification that impairs their ability to activate protein kinase A (PKA), while leaving intact their ability to activate Epac (the Exchange Protein directly Activated by Cyclic AMP). One such ESCA in wide-spread use is 8-pCPT-2'-O-Me-cAMP. It is a cell-permeant derivative of 2'-O-Me-cAMP, and it is a super activator of Epac. A wealth of newly published studies demonstrate that 8-pCPT-2'-O-Me-cAMP is a unique tool with which to asses atypical actions of cAMP that are PKA-independent. Particularly intriguing are recent reports demonstrating that ESCAs reproduce the PKA-independent actions of ligands known to stimulate Class I (Family A) and Class II (Family B) GTP-binding protein-coupled receptors (GPCRs). This topical review summarizes the current state of knowledge regarding the molecular pharmacology and signal transduction properties of Epac-selective cAMP analogs. Special attention is focused on the rational drug design of ESCAs in order to improve their Epac selectivity, membrane permeability, and stability. Also emphasized is the usefulness of ESCAs as new tools with which to assess the role of Epac as a determinant of intracellular Ca2+ signalling, ion channel function, neurotransmitter release, and hormone secretion.

C. elegans RPM-1 regulates axon termination and synaptogenesis through the Rab GEF GLO-4 and the Rab GTPase GLO-1

C. elegans RPM-1 (for Regulator of Presynaptic Morphology) is a member of a conserved protein family that includes Drosophila Highwire and mammalian Pam and Phr1. These are large proteins recently shown to regulate synaptogenesis through E3 ubiquitin ligase activities. Here, we report the identification of an RCC1-like guanine nucleotide exchange factor, GLO-4, from mass spectrometry analysis of RPM-1-associated proteins. GLO-4 colocalizes with RPM-1 at presynaptic terminals. Loss of function in glo-4 or in its target Rab GTPase, glo-1, causes neuronal defects resembling those in rpm-1 mutants. We show that the glo pathway functions downstream of rpm-1 and acts in parallel to fsn-1, a partner of RPM-1 E3 ligase function. We find that late endosomes are specifically disorganized at the presynaptic terminals of glo-4 mutants. Our data suggest that RPM-1 positively regulates a Rab GTPase pathway to promote vesicular trafficking via late endosomes.

Identification and characterization of Asef2, a guanine-nucleotide exchange factor specific for Rac1 and Cdc42

The tumor suppressor adenomatous polyposis coli (APC) is mutated in sporadic and familial colorectal tumors. APC interacts with the Rac1-specific guanine-nucleotide exchange factor (GEF) Asef, which contains an APC-binding region (ABR) in addition to Dbl homology (DH), Pleckstrin (PH) and Src homology 3 (SH3) domains. APC stimulates the GEF activity of Asef, and thereby regulates cell adhesion and migration. Here, we have identified a second Asef, termed Asef2, that shows significant structural and functional similarities to Asef. We found that both the N-terminal ABR and SH3 domains of Asef2 are responsible for its interaction with APC. When expressed in HeLa cells, a mutant Asef2 lacking the ABR and SH3 domains, Asef2-DeltaABR/SH3, induced increases in the levels of the active forms of Rac1 and Cdc42. Full-length Asef2 also showed this activity when co-transfected with truncated mutant APC expressed in colorectal tumor cells. Consistent with this, either Asef2-DeltaABR/SH3 or Asef2 plus truncated mutant APC stimulated lamellipodia formation in MDCK cells and filopodia formation in HeLa cells. Furthermore, RNA interference experiments showed that Asef2 is required for migration of colorectal tumor cells expressing truncated APC. These results suggest that similar to Asef, Asef2 plays an important role in cell migration, and that Asef2 activated by truncated mutant APC is required for aberrant migration of colorectal tumor cells.

Functional analysis of RalGPS2, a murine guanine nucleotide exchange factor for RalA GTPase

RalGPS2 is a murine guanine nucleotide exchange factor of the RalGPS family; it contains a Cdc25-like GEF domain and does not exhibit a Ras-binding domain. The main characteristic of RalGPS2 is its pleckstrin homology (PH) domain, present at the C terminus, that preferentially binds phosphatidylinositol-4,5-biphosphate and in HEK 293 cells localized in membranes, causing ruffling and vesiculation. Moreover, RalGPS2 contains a PxxP motif in the central part of the molecule. This motif binds in vitro and in vivo SH3 domains of Grb2 and PLCgamma. RalGPS2 and its GEF domain activate RalA in vivo while the PH-PxxP domains inhibited it behaving as a dominant negative for the RalA pathway; this activation was not inhibited by co-expression of a dominant negative Ras. RalGPS2 is physiologically expressed in testis and brain; when overexpressed, the whole RalGPS2 causes considerable morphological changes in HEK 293 cells, suggesting its possible role on cytoskeleton reorganization. This is further strengthened by data obtained in NIH3T3 cells where expression of PH-PxxP domain promotes actin depolymerization. Finally, RalGPS2 and its GEF domain induce Ras-independent transcriptional activation of the c-fos promoter in NIH3T3 cells.

Nasopharyngeal carcinoma: molecular biomarker discovery and progress

Nasopharyngeal carcinoma (NPC) is a rare malignancy in most part of the world and it is one of the most confusing, commonly misdiagnosed and poorly understood diseases. The cancer is an Epstein-Barr virus-associated malignancy with a remarkable racial and geographical distribution. It is highly prevalent in southern Asia where the disease occurs at a prevalence about a 100-fold higher compared with other populations not at risk. The etiology of NPC is thought to be associated with a complex interaction of genetic, viral, environmental and dietary factors. Thanks to the advancements in genomics, proteomics and bioinformatics in recent decades, more understanding of the disease etiology, carcinogenesis and progression has been gained. Research into these components may unravel the pathways in NPC development and potentially decipher the molecular characteristics of the malignancy. In the era of molecular medicine, specific treatment to the potential target using technologies such as immunotherapy and RNAi becomes formulating from bench to bedside application and thus makes molecular biomarker discovery more meaningful for NPC management. In this article, the latest molecular biomarker discovery and progress in NPC is reviewed with respect to the diagnosis, monitoring, treatment and prognostication of the disease.

EhGEF3, a novel Dbl family member, regulates EhRacA activation during chemotaxis and capping inEntamoeba histolytica

Rho GTPases are critical elements involved in the regulation of signal transduction cascades from extracellular stimuli to cytoskeleton. The Rho guanine nucleotide exchange factors (RhoGEFs) have been implicated in direct activation of these GTPases. Here, we describe a novel RhoGEF, denominated EhGEF3 from the parasite Entamoeba histolytica, which encodes a 110 kDa protein containing the domain arrangement of a Dbl homology domain in tandem with a pleckstrin homology domain, the DH domain of EhGEF3 is closely related with the one of the Vav3 protein. Biochemical analysis revealed that EhGEF3 is capable of stimulating nucleotide exchange on the E. histolytica EhRacA and EhRho1 GTPases in vitro, however only a partial GEF activity toward Cdc42 was observed. Conserved residue analysis showed that the N816 and L817 residues are critical for EhGEF3 activity. Cellular studies revealed that EhGEF3 colocalises with EhRacA in the rear of migrating cells, probably regulating the retraction of the uroid and promoting the activation of these GTPase during the chemotactic response toward fibronectin, and that EhGEF3 also regulates EhRacA activation during the capping of cell receptors. These results suggest that EhGEF3 should have a direct role in activating EhRacA, and in bringing the activated GTPase to specific target sites such as the uroid.

Remedial strategies in structural proteomics: expression, purification, and crystallization of the Vav1/Rac1 complex

The signal transduction pathway involving the Vav1 guanine nucleotide exchange factor (GEF) and the Rac1 GTPase plays several key roles in the immune response mediated by the T cell receptor. Vav1 is also a unique member of the GEF family in that it contains a cysteine-rich domain (CRD) that is critical for Rac1 binding and maximal guanine nucleotide exchange activity, and thus may provide a unique protein-protein interface compared to other GEF/GTPase pairs. Here, we have applied a number of remedial structural proteomics strategies, such as construct and expression optimization, surface mutagenesis, limited proteolysis, and protein formulation to successfully express, purify, and crystallize the Vav1-DH-PH-CRD/Rac1 complex in an active conformation. We have also systematically characterized various Vav1 domains in a GEF assay and Rac1 in vitro binding experiments. In the context of Vav1-DH-PH-CRD, the zinc finger motif of the CRD is required for the expression of stable Vav1, as well as for activity in both a GEF assay and in vitro formation of a Vav1/Rac1 complex suitable for biophysical and structural characterization. Our data also indicate that the isolated CRD maintains a low level of specific binding to Rac1, appears to be folded based on 1D NMR analysis and coordinates two zinc ions based on ICP-MS analysis. The protein reagents generated here are essential tools for the determination of a three dimensional Vav1/Rac1 complex crystal structure and possibly for the identification of inhibitors of the Vav1/Rac1 protein-protein interaction with potential to inhibit lymphocyte activation.

Endogenous ARF6 interacts with Rac1 upon angiotensin II stimulation to regulate membrane ruffling and cell migration

ARF6 and Rac1 are small GTPases known to regulate remodelling of the actin cytoskeleton. Here, we demonstrate that these monomeric G proteins are sequentially activated when HEK 293 cells expressing the angiotensin type 1 receptor (AT(1)R) are stimulated with angiotensin II (Ang II). After receptor activation, ARF6 and Rac1 transiently form a complex. Their association is, at least in part, direct and dependent on the nature of the nucleotide bound to both small G proteins. ARF6-GTP preferentially interacts with Rac1-GDP. AT(1)R expressing HEK293 cells ruffle, form membrane protrusions, and migrate in response to agonist treatment. ARF6, but not ARF1, depletion using small interfering RNAs recapitulates the ruffling and migratory phenotype observed after Ang II treatment. These results suggest that ARF6 depletion or Ang II treatment are functionally equivalent and point to a role for endogenous ARF6 as an inhibitor of Rac1 activity. Taken together, our findings reveal a novel function of endogenously expressed ARF6 and demonstrate that by interacting with Rac1, this small GTPase is a central regulator of the signaling pathways leading to actin remodeling.

Cdc42 GEF Tuba regulates the junctional configuration of simple epithelial cells

Epithelial cells are typically arranged in a honeycomb-like pattern, minimizing their cell–cell contact areas, which suggests that some tension operates for shaping of the cell boundaries. However, the molecular mechanisms that generate such tension remain unknown. We found that Tuba, which is a Cdc42-specific GEF, was concentrated at the apical-most region of cell junctions in simple epithelia via its interaction with ZO-1. RNAi–mediated depletion of Tuba altered the geometrical configuration of cell junctions, resulting in a curved and slack appearance. At the subcellular level, Tuba inactivation modified the assembly pattern of junctional F-actin and E-cadherin. Tuba RNAi also retarded cell junction formation in calcium-switch experiments. Suppression of Cdc42 activity or depletion of N-WASP, which is an effector of Cdc42, mimicked the effects of Tuba depletion. Conversely, overexpression of dominant-active Cdc42 or N-WASP enhanced the junction formation of Tuba-depleted cells. These results suggest that Tuba controls the shaping of cell junctions through the local activation of Cdc42 and its effectors.

Monitoring of cAMP synthesis and degradation in living cells

cAMP is an important second messenger with a plethora of cellular effects and biological roles. To monitor and visualize cAMP in intact living cells, electrophysiological and fluorescent methods have been developed based on activation of all three types of cAMP effectors: protein kinase A, cyclic nucleotide-gated channels, and exchange protein directly activated by cAMP. In this review, we describe and compare these techniques in terms of their robustness, sensitivity and spatio-temporal resolution.

CYK-4/GAP provides a localized cue to initiate anteroposterior polarity upon fertilization

The Caenorhabditis elegans anteroposterior axis is established in response to fertilization by sperm. Here we present evidence that RhoA, the guanine nucleotide-exchange factor ECT-2, and the Rho guanosine triphosphatase-activating protein CYK-4 modulate myosin light-chain activity to create a gradient of actomyosin, which establishes the anterior domain. CYK-4 is enriched within sperm, and paternally donated CYK-4 is required for polarity. These data suggest that CYK-4 provides a molecular link between fertilization and polarity establishment in the one-cell embryo. Orthologs of CYK-4 are expressed in sperm of other species, which suggests that this cue may be evolutionarily conserved.

Differential effects of bryostatin 1 and 12-O-tetradecanoylphorbol-13-acetate on the regulation and activation of RasGRP1 in mouse epidermal keratinocytes

The antitumor agent bryostatin 1 and the tumor-promoting phorbol esters function as structural mimetics of the second lipid messenger diacylglycerol (DAG) by binding to the C1 domain of DAG receptors. However, bryostatin 1 and the phorbol esters often differ in their cellular actions. In mouse skin, the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) is a potent tumor promoter, whereas bryostatin 1 lacks this activity and antagonizes the tumor-promoting effects of TPA. Although protein kinase C mediates many of the effects of DAG on skin, the exact mechanisms responsible for the biology of bryostatin 1 and TPA in the epidermis have not been elucidated. We recently reported that the novel DAG receptor RasGRP1 is expressed in mouse keratinocytes and mediates TPA-induced Ras activation. This finding prompted us to examine the regulation of RasGRP1 by bryostatin 1. We found that whereas TPA induced translocation of RasGRP1 to both the plasma and internal membranes of the keratinocytes, bryostatin 1 recruited RasGRP1 only to internal membranes and the nuclear envelope. In addition, TPA led to a concentration-dependent down-regulation of RasGRP1, whereas bryostatin 1 failed to induce full RasGRP1 down-regulation. Interestingly, bryostatin 1 was less effective than TPA at activating Ras. The results presented here suggest the possibility that a differential modulation of RasGRP1 by bryostatin 1 compared with TPA could participate in the disparate responses of the epidermal cells to both DAG analogues. This result may have implications in the understanding of the antitumor effects of bryostatin 1 in the skin.

Effect of protein kinase A on accumulation of brefeldin A-inhibited guanine nucleotide-exchange protein 1 (BIG1) in HepG2 cell nuclei

Brefeldin A-inhibited guanine nucleotide-exchange proteins, BIG1 and BIG2, are activators of ADP-ribosylation factor GTPases that are essential for regulating vesicular traffic among intracellular organelles. Biochemical analyses and immunofluorescence microscopy demonstrated BIG1 in nuclei as well as membranes and cytosol of serum-starved HepG2 cells. Within 20 min after addition of 8-Br-cAMP, BIG1 accumulated in nuclei, and this effect was blocked by protein kinase A (PKA) inhibitors H-89 and PKI, suggesting a dependence on PKA-catalyzed phosphorylation. BIG2 localization was not altered by cAMP, nor did BIG2 small interfering RNA influence nuclear accumulation of BIG1 induced by cAMP. Mutant BIG1 (S883A) in which Ala replaced Ser-883, a putative PKA phosphorylation site, did not move to the nucleus with cAMP addition, whereas replacement with Asp (S883D) resulted in nuclear accumulation of BIG1 without or with cAMP exposure, consistent with the mechanistic importance of a negative charge at that site. Mutation (712KPK714) of the nuclear localization signal inhibited BIG1 accumulation in nuclei, and PKA-catalyzed phosphorylation of S883, although necessary, was not sufficient for nuclear accumulation, as shown by the double mutation S883D/nuclear localization signal. A role for microtubules in cAMP-induced translocation of BIG1 is inferred from its inhibition by nocodazole. Thus, two more critical elements of BIG1 molecular structure were identified, as well as the potential function of microtubules in a novel PKA effect on BIG1 translocation.

Association of brefeldin A-inhibited guanine nucleotide-exchange protein 2 (BIG2) with recycling endosomes during transferrin uptake

ADP-ribosylation factors (ARFs) are critical in vesicular trafficking. Brefeldin A-inhibited guanine nucleotide-exchange protein (BIG)1 and BIG2 activate ARFs by accelerating replacement of bound GDP with GTP. Additional and differing functions of these approximately 200-kDa proteins are now being recognized, as are their independent intracellular movements. Here, we describe the localization in COS7 cells by immunofluorescence microscopy of BIG2, but not BIG1, with structures that have characteristics of recycling endosomes during transferrin (Tfn) uptake and Tfn receptor (TfnR) recycling. Cell content of BIG2 and Rab11, but not TfnR, BIG1, Rab4, or Exo70, was increased after 60 min of Tfn uptake. BIG2, but not BIG1, appeared in density-gradient fractions containing TfnR, Rab11, and Exo70 after 60 min of Tfn uptake. Treatment of cells with BIG2 small interfering RNA (siRNA), but not BIG1 or control siRNAs, decreased BIG2 protein >90% without affecting BIG1, ARF, or actin content, whereas TfnR was significantly increased as was its accumulation in perinuclear recycling endosomes. Tfn release appeared unaffected by BIG1 siRNA but was significantly slowed from cells treated with BIG2 siRNA alone or plus BIG1 siRNA. We suggest that BIG2 has an important role in Tfn uptake and TfnR recycling, perhaps through its demonstrated interaction with Exo70 and the exocyst complex.

A PDZ-binding motif as a critical determinant of Rho guanine exchange factor function and cell phenotype

We identified a Rho guanine exchange factor (GEF) expressed as two splice variants, which differ only in either having or lacking a Postsynaptic density 95, Disk large, Zona occludens-1 (PDZ) motif. The PDZ adaptor protein synectin bound the longer splice variant, Syx1, which was targeted to the plasma membrane in a synectin-dependent manner. The shorter variant, Syx2, was diffusely distributed in the cytoplasm. Fluorescence resonance energy transfer (FRET) imaging revealed similar differences between the spatial patterns of active RhoA in Syx1 versus Syx2-expressing cells. Expression of Syx1 augmented endothelial cell (EC) migration and tube formation, whereas Syx2 expression did not. It appears, therefore, that synectin-dependent targeting of Syx is critical to its contribution to these EC functions. Although agonist-stimulated global RhoA activity was similar in Syx1- and Syx2-expressing cells, basal RhoA activity was surprisingly higher in the latter. Out of 23 cell types, we found a significant level of endogenous Syx2 expression only in brain tumor cells, which also exhibited high basal RhoA activity. We found that the activity level of JNK, which mediates transcriptional regulation downstream of RhoA, is elevated in a Syx2-dependent manner in these cells, possibly contributing to their tumorigenicity.

Modulation of muscle regeneration, myogenesis, and adipogenesis by the Rho family guanine nucleotide exchange factor GEFT

Rho family guanine nucleotide exchange factors (GEFs) regulate diverse cellular processes including cytoskeletal reorganization, cell adhesion, and differentiation via activation of the Rho GTPases. However, no studies have yet implicated Rho-GEFs as molecular regulators of the mesenchymal cell fate decisions which occur during development and repair of tissue damage. In this study, we demonstrate that the steady-state protein level of the Rho-specific GEF GEFT is modulated during skeletal muscle regeneration and that gene transfer of GEFT into cardiotoxin-injured mouse tibialis anterior muscle exerts a powerful promotion of skeletal muscle regeneration in vivo. In order to molecularly characterize this regenerative effect, we extrapolate the mechanism of action by examining the consequence of GEFT expression in multipotent cell lines capable of differentiating into a number of cell types, including muscle and adipocyte lineages. Our data demonstrate that endogenous GEFT is transcriptionally upregulated during myogenic differentiation and downregulated during adipogenic differentiation. Exogenous expression of GEFT promotes myogenesis of C2C12 cells via activation of RhoA, Rac1, and Cdc42 and their downstream effector proteins, while a dominant-negative mutant of GEFT inhibits this process. Moreover, we show that GEFT inhibits insulin-induced adipogenesis in 3T3L1 preadipocytes. In summary, we provide the first evidence that the Rho family signaling pathways act as potential regulators of skeletal muscle regeneration and provide the first reported molecular mechanism illustrating how a mammalian Rho family GEF controls this process by modulating mesenchymal cell fate decisions.

Rgf1p is a specific Rho1-GEF that coordinates cell polarization with cell wall biogenesis in fission yeast

Rho1p regulates cell integrity by controlling the actin cytoskeleton and cell wall synthesis. We have identified a new GEF, designated Rgf1p, which specifically regulates Rho1p during polarized growth. The phenotype of rgf1 null cells was very similar to that seen after depletion of Rho1p, 30% of cells being lysed. In addition, rgf1(+) deletion caused hypersensitivity to the antifungal drug Caspofungin and defects in the establishment of bipolar growth. rho1(+), but none of the other GTPases of the Rho-family, suppressed the rgf1Delta phenotypes. Moreover, deletion of rgf1(+) suppressed the severe growth defect in rga1(+) null mutants (a Rho1-GAP, negative regulator). Rgf1p and Rho1p coimmunoprecipitated and overexpression of rgf1(+) specifically increased the GTP-bound Rho1p; it caused changes in cell morphology, and a large increase in beta(1,3)-glucan synthase activity. These effects were similar to those elicited when the hyperactive rho1-G15V allele was expressed. A genetic relationship was observed between Rgf1p, Bgs4p (beta[1,3]-glucan synthase), and Pck1p (protein kinase C [PKC] homologue); Bgs4p and Pck1p suppressed the hypersensitivity to Caspofungin in rgf1Delta mutants. Rgf1p localized to the growing ends and the septum, where Rho1, Pck1p, and Bgs4p are known to function. Our results suggest that Rgf1p probably activates the Rho functions necessary for coordinating actin deposition with cell wall biosynthesis during bipolar growth, allowing the cells to remodel their wall without risk of rupture.

Identification of histological markers for malignant glioma by genome-wide expression analysis: dynein, alpha-PIX and sorcin

Glioblastoma multiforme (GBM), the most malignant class of glial neoplasm (grade IV in WHO criteria), carries the worst clinical prognosis among primary brain tumors in adults. To identify a set of genes involved in the tumorigenesis of GBM, we evaluated expression profiles of GBM tissues from 11 patients using a cDNA microarray representing 25,344 human genes. By comparing the profiles with those of normal brain tissue, we identified a number of differentially expressed genes: 54 with increased expression and 45 with reduced expression in GBMs. Semi-quantitative RT-PCR experiments with 6 of those genes confirmed higher expression of DNCH2, ARHGEF6, NPM1 and SRI and lower expression of NRGN and TM4SF2 in GBM tumors. Immunohistochemical staining for 3 of the respective gene products, dynein (product of DNCH2), alpha-PIX (product of ARHGEF6), and sorcin (product of SRI) indicated that this technique might be useful for histological grading of glial tumors. To establish criteria for this diagnostic approach, we scored glial tumor tissues of different histological grades according to the staining results; the scores were significantly higher in anaplastic astrocytomas and GBMs than in diffuse astrocytomas or normal brain tissues. These findings indicated that levels of these three proteins might serve as histological markers for malignant glioma classification.

Characterization of the activation of the Rap-specific exchange factor Epac by cyclic nucleotides

Epac1 and Epac2 are cAMP-dependent guanine nucleotide exchange factors (GEF) for the small G-proteins Rap1 and Rap2. Epac is inactive in the absence of cAMP, and binding of cAMP to a cyclic nucleotide-binding domain in the N-terminal regulatory region results in activation of the protein. The cAMP-dependent activity of Epac proteins can be analyzed by a fluorescence-based assay in vitro. These kinds of measurements can help to unravel the molecular mechanism by which cAMP binding is translated in activation of the protein. For this purpose, Epac mutants can be analyzed. In addition, the interaction of cAMP itself might be the focus of the research. Thus, modified cAMP analogs can be characterized by their ability to activate Epac. This is of particular interest for the development of Epac-specific analogs, which do not act on other cellular cAMP targets such as protein kinase A (PKA) or for the design of therapeutic agents targeting Epac.

Ypt32p regulates the translocation of Chs3p from an internal pool to the plasma membrane

The transport of the chitin synthase III, Chs3p, to the plasma membrane is temporally and spatially regulated. Chs3p is delivered to the plasma membrane at the beginning of the cell cycle, forming chitin rings, and at the end of the cell cycle, forming the primary septum. During the rest of the cell cycle, it is maintained in intracellular compartments, termed chitosomes that share characteristics with the late Golgi and the early endosomes. Chs5p and Chs6p are required for the cell cycle-dependent delivery of Chs3p to the cell surface, but the mechanisms underlying the temporal regulation are still unknown. The Rab proteins, Ypt31/32p, are required for exit of secretory vesicles from the late Golgi and for recycling of proteins between the late Golgi and early endosomes. Either gain of Ypt32p function, by overexpression, or loss-of-function mutations alter the localization of Chs3p-GFP. Moreover, cells overexpressing Ypt32p accumulate chitin at the cell surface independent of Chs5p. Overexpression of Ypt32p also disrupts the localization of the late Golgi protein Sec7. We propose that Ypt31/32p have a role in regulating the delivery of Chs3p to the plasma membrane and deposition of chitin at the cell surface.

The gene disrupted in Marinesco-Sjögren syndrome encodes SIL1, an HSPA5 cochaperone

We identified the gene underlying Marinesco-Sjögren syndrome, which is characterized by cerebellar ataxia, progressive myopathy and cataracts. We identified four disease-associated, predicted loss-of-function mutations in SIL1, which encodes a nucleotide exchange factor for the heat-shock protein 70 (HSP70) chaperone HSPA5. These data, together with the similar spatial and temporal patterns of tissue expression of Sil1 and Hspa5, suggest that disturbed SIL1-HSPA5 interaction and protein folding is the primary pathology in Marinesco-Sjögren syndrome.

Very-KIND is a novel nervous system specific guanine nucleotide exchange factor for Ras GTPases

The kinase non-catalytic c-lobe domain (KIND) evolved from the catalytic protein kinase fold into a potential protein interaction module for signalling proteins. Spir family actin organizers and the non-receptor phosphatase type 13 (PTP type 13) encode a KIND domain in the very N-terminal parts of the proteins. Here we report the characterization and cloning of a third member of the KIND protein family, which we have named very-KIND (VKIND) because of its two KIND domains. Like the other members of the protein family, VKIND has a KIND domain at the N-terminus. A second KIND domain is located in the central part of the protein. The C-terminal half encodes a guanine nucleotide exchange factor motif for Ras-like GTPases (RasGEF) and a RasGEF N-terminal module (RasGEFN). There is only one VKIND gene in the mammalian genomes and up to now we have found the gene only in vertebrates. During mouse embryogenesis the VKIND gene was specifically expressed in the developing nervous system. In adult mice Northern hybridizations revealed high expression only in brain. Low expression could be detected in ovary. In situ hybridizations showed a specific expression of VKIND in neuronal cells of the granular and Purkinje cell layers of the cerebellum.

Tiam1-IRSp53 complex formation directs specificity of rac-mediated actin cytoskeleton regulation

The exchange factor Tiam1 regulates multiple cellular functions by activating the Rac GTPase. Active Rac has various effects in cells, including alteration of actin cytoskeleton and gene expression, via binding to and modulating the activity of diverse effector proteins. How individual Rac effectors are selected for activation and regulated in response to upstream signals is not well understood. We find that Tiam1 contributes to both of these processes by binding to IRSp53, an adaptor protein that is an effector for both Rac and Cdc42. Tiam1 directs IRSp53 to Rac signaling by enhancing IRSp53 binding to both active Rac and the WAVE2 scaffold. Moreover, Tiam1 promotes IRSp53 localization to Rac-induced lamellipodia rather than Cdc42-induced filopodia. Finally, IRSp53 depletion from cells prevents Tiam1-dependent lamellipodia induced by Tiam1 overexpression or platelet-derived growth factor stimulation. These findings indicate that Tiam1 not only activates Rac but also contributes to Rac signaling specificity through binding to IRSp53.

Modulation of specific protein expression levels by PTEN: identification of AKAP121, DHFR, G3BP, Rap1, and RCC1 as potential targets of PTEN

The tumor suppressor PTEN is mutated in a high percentage of human cancers, and is implicated in pathways regulating cell growth, proliferation, survival, and migration. Despite significant advances, our understanding of its mechanisms of action remains incomplete. We have used a high-throughput proteomic immunoblotting approach to identify proteins whose expression levels are modulated by PTEN. Out of over 800 proteins screened, 22 proteins showed significant changes in expression. Five proteins that exhibited two-fold or greater changes in expression level were further characterized. AKAP121 and G3BP expression was reduced, while dihydrofolate reductase (DHFR), Rap1 and RCC1 expression was elevated in response to PTEN expression in a PTEN-null T-cell leukemia line. The phosphatase activity of PTEN was required for these effects. However, direct inhibition of PI-3 Kinase could mimic PTEN in modulating expression of DHFR, G3BP, Rap1 and RCC1, but not AKAP121. Real-time PCR showed that the effects of PTEN were primarily post-transcriptional, and would not have been revealed by mRNA-based screens. We conclude from these data that PTEN can modulate the expression level of a number of different proteins. The identified proteins have the potential to serve as previously unrecognized effectors of PTEN, and suggest the existence of additional complexity in the modes by which PTEN can regulate cellular biology.

Cell growth-dependent coordination of lipid signaling and glycosylation is mediated by interactions between Sac1p and Dpm1p

The integral membrane lipid phosphatase Sac1p regulates local pools of phosphatidylinositol-4-phosphate (PtdIns(4)P) at endoplasmic reticulum (ER) and Golgi membranes. PtdIns(4)P is important for Golgi trafficking, yet the significance of PtdIns(4)P for ER function is unknown. It also remains unknown how localization of Sac1p to distinct organellar membranes is mediated. Here, we show that a COOH-terminal region in yeast Sac1p is crucial for ER targeting by directly interacting with dolicholphosphate mannose synthase Dpm1p. The interaction with Dpm1p persists during exponential cell division but is rapidly abolished when cell growth slows because of nutrient limitation, causing translocation of Sac1p to Golgi membranes. Cell growth–dependent shuttling of Sac1p between the ER and the Golgi is important for reciprocal control of PtdIns(4)P levels at these organelles. The fraction of Sac1p resident at the ER is also required for efficient dolichol oligosaccharide biosynthesis. Thus, the lipid phosphatase Sac1p may be a key regulator, coordinating the secretory capacity of ER and Golgi membranes in response to growth conditions.

Inhibitory control of TGF-beta1 on the activation of Rap1, CD11b, and transendothelial migration of leukocytes

Beta2-integrins are a family of dimeric adhesion molecules expressed on leukocytes. Their capacity to bind ligand is regulated by their state of activation. CD11b, an alphaMbeta2 integrin, is implicated in a number of physiological and pathological events such as inflammation, thrombosis, or atherosclerosis. The GTPase Rap1 is essential for its activation and could therefore play a strategic role in the regulation of leukocyte functioning. Because low levels of circulating TGF-beta have been linked with severe atherosclerosis, we have assessed the role of this cytokine in the regulation of Rap1 and CD11b activation in differentiated U937 cells and in human peripheral blood monocytes. TGF-beta1 caused a significant reduction in the expression of CD11b but not in the expression of other integrins tested. More importantly, TGF-beta1 greatly reduced the capacity of PMA or chemokines to activate CD11b and Rap1, a phenomenon paralleled by a loss of the Epac transcript and a reduction in 8-pCPT-2'-O-Me-cAMP-mediated activation of Rap1. This inhibition diminished the capacity of monocytes to migrate across a monolayer of endothelial cells. The inhibitory effect of TGF-beta1 on Rap1 activity may exert a general protective influence against aberrant transendothelial migration, thereby holding inflammatory responses in check.

Interaction of BIG2, a brefeldin A-inhibited guanine nucleotide-exchange protein, with exocyst protein Exo70

Guanine nucleotide-exchange proteins activate ADP-ribosylation factors by accelerating the replacement of bound GDP with GTP. Mammalian brefeldin A-inhibited guanine nucleotide-exchange proteins, BIG1 and BIG2, are important activators of ADP-ribosylation factors for vesicular trafficking. To identify proteins that interact with BIG2, we used cDNA constructs encoding BIG2 sequences in a yeast two-hybrid screen of a human heart library. Clone p2-5-3, encoding a form of human exocyst protein Exo70, interacted with BIG2 amino acids 1-643 and 1-832, but not 644-832, which was confirmed by coimmunoprecipitation of in vitro-translated BIG2 N-terminal segments and 2-5-3. By immunofluorescence microscopy, endogenous BIG2 and Exo70 in HepG2 cells were visualized at Golgi membranes and apparently at the microtubule-organizing center (MTOC). Both were identified in purified centrosomes. Immunoreactive Exo70 and BIG2 partially or completely overlapped with gamma-tubulin at the MTOC in cells inspected by confocal microscopy. In cells incubated with brefeldin A, most of the BIG2, Exo70, and trans-Golgi protein p230 were widely dispersed from their perinuclear concentrations, but small amounts always remained, apparently at the MTOC. After disruption of microtubules with nocodazole, BIG2 and Exo70 were widely distributed in cells and remained only partially colocalized with p230, BIG2 more so than Exo70. We conclude that in HepG2 cells BIG2 and Exo70 interact in trans-Golgi network and centrosomes, as well as in exocyst structures or complexes that move along microtubules to the plasma membrane, consistent with a functional association in both early and late stages of vesicular trafficking.

Functional Assay of EFA6A, a Guanine Nucleotide Exchange Factor for ADP‐Ribosylation Factor 6 (ARF6), in Dendritic Formation of Hippocampal Neurons

EFA6A is a guanine nucleotide exchange factor (GEF) that can activate ADP-ribosylation factor 6 (ARF6) in vitro, with prominent expression in the forebrain including the hippocampal formation. In this section, we describe the neuronal transfection method and show that the overexpression of a catalytically inactive mutant of EFA6A induces a prominent dendritic formation of the primary hippocampal neurons, suggesting the intimate involvement of EFA6A in the regulation of neuronal dendritic development. This reliable and consistent neuronal transfection method will also be applicable for the vector-based RNA interference method.

A novel somatodendritic marker defined by a peptide derived from the ALS2 protein

We characterized an antibody raised against a peptide derived from the ALS2 protein, the product of a gene linked to recessive juvenile forms of amyotrophic lateral sclerosis, defining the ALS2 RLD-derived antigen (ARDA). In cultured rat cortical neurons, ARDA is localized in perikarya and neurites, but absent from nuclei. Double immunostaining of rodent cultured neurons and spinal cord sections for ARDA and MAP2, a marker of dendrites, or tau and phosphorylated neurofilaments, markers of axons, demonstrated that the ARDA antigen was localized in the somatodendritic compartment and excluded from axons. ARDA was also selectively localized to the somatodendritic compartment of motor neurons in human spinal cord. The ARDA antigen therefore represents a novel somatodendritic marker for embryonic and adult neurons.

SGEF, a RhoG guanine nucleotide exchange factor that stimulates macropinocytosis

SGEF (SH3-containing Guanine Nucleotide Exchange Factor) is a RhoGEF of unknown function. We found the SGEF protein to be expressed in many established cell lines and highly expressed in human liver tissue. SGEF stimulated the formation of large interconnected membrane ruffles across dorsal surfaces when expressed in fibroblasts. SGEF required its proline-rich amino-terminus to generate dorsal, but not lateral, membrane ruffles and a functional SH3 domain to colocalize with filamentous actin at sites of membrane protrusion. Full-length SGEF activated RhoG, but not Rac, when expressed in fibroblasts. Further, recombinant SGEF DH/PH protein exchanged nucleotide on RhoG, but not on Rac1 or Rac3, in vitro. Scanning electron microscopy of fibroblasts demonstrated that SGEF induced dorsal ruffles that were morphologically similar to those generated by constitutively active RhoG, but not constitutively active Rac1. Transient expression of SGEF stimulated fibroblast uptake of 10-kDa dextran, a marker of macropinocytosis. This required the full-length protein and a catalytically active DH domain. Finally, activated RhoG was found to be more effective than activated Rac, and comparable to SGEF, in its ability to trigger dextran uptake. Together, this work establishes SGEF as a RhoG exchange factor and provides evidence that both SGEF and RhoG regulate membrane dynamics in promotion of macropinocytosis.

Somatodendritic localization of the mRNA for EFA6A, a guanine nucleotide exchange protein for ARF6, in rat hippocampus and its involvement in dendritic formation

EFA6A is a guanine nucleotide exchange protein (GEP) that can specifically activate ADP-ribosylation factor 6 (ARF6) in vitro. A recent study has demonstrated that ARF6 is involved in the dendritic formation of developing hippocampal neurons [Hernandez-Deviez et al. (2002) Nature Neurosci., 5, 623-624]. This study examined a potential role for EFA6A in hippocampal development in Wistar rats. Our results provided definitive evidence for somatodendritic localization of EFA6A mRNA in both cultured and in vivo hippocampal neurons by nonradioactive in situ hybridization. During postnatal development, EFA6A mRNA was dramatically increased and its dendritic localization was most evident between P7 and P14. In contrast, ARF6 mRNA was confined to the neuronal layers of the hippocampus throughout development. In addition, the overexpression of a GEP-defective mutant of EFA6A enhanced the dendritic formation of the primary hippocampal neurons. The present findings suggest that EFA6A is intimately involved in the regulation of the dendritic development of hippocampal neurons.

Overexpression of an ADP-ribosylation factor-guanine nucleotide exchange factor, BIG2, uncouples brefeldin A-induced adaptor protein-1 coat dissociation and membrane tubulation

BIG2 is a guanine nucleotide exchange factor (GEF) for the ADP-ribosylation factor (ARF) family of small GTPases, which regulate membrane association of COPI and adaptor protein (AP)-1 coat protein complexes. A fungal metabolite, brefeldin A (BFA), inhibits ARF-GEFs and leads to redistribution of coat proteins from membranes to the cytoplasm and membrane tubulation of the Golgi complex and the trans-Golgi network (TGN). To investigate the function of BIG2, we examined the effects of BIG2-overexpression on the BFA-induced redistribution of ARF, coat proteins, and organelle markers. The BIG2 overexpression blocked BFA-induced redistribution from membranes of ARF1 and the AP-1 complex but not that of the COPI complex. These observations indicate that BIG2 is implicated in membrane association of AP-1, but not that of COPI, through activating ARF. Furthermore, not only BIG2 but also ARF1 and AP-1 were found as queues of spherical swellings along the BFA-induced membrane tubules emanating from the TGN. These observations indicate that BFA-induced AP-1 dissociation from TGN membranes and tubulation of TGN membranes are not coupled events and suggest that a BFA target other than ARF-GEFs exists in the cell.

KRP3A and KRP3B: candidate motors in spermatid maturation in the seminiferous epithelium

We have identified KRP3, a novel kinesin-related protein expressed in the mammalian testis, and have examined the tissue distribution and subcellular localization of isoforms of this protein. Isolation of KRP3 clones, using the head domain identified in a previous PCR screen as probe, identified at least two KRP3 isoforms in the rat. We have isolated coding sequences of two highly related cDNAs from the rat testis that we have termed KRP3A and KRP3B (kinesin-related protein 3, A and B). Both cDNAs code for predicted polypeptides with the three-domain structure typical of kinesin superfamily members; namely a conserved motor domain, a region capable of forming a limited coiled-coil secondary structure, and a globular tail domain. Although almost identical in their head and stalk domains, these motors diverge in their tail domains. This group of motors is found in many tissues and cell types. The KRP3B motor contains DNA-binding motifs and an RCC1 (regulator of chromosome condensation 1) consensus sequence in its tail domain. Despite this similarity, KRP3B is not associated with the same structures as RCC1. Instead, KRP3 isoforms localize with the nuclei of developing spermatids, and their immunolocalization in the testis overlaps with that of the small GTPase Ran. Like Ran, KRP3 motors are associated in a polarized fashion with the nucleus of maturing spermatids at various stages of elongation. Our findings suggest a possible role for KRP3 motor isoforms in spermatid maturation mediated by possible interaction with the Ran GTPase.

Localization of large ADP-ribosylation factor-guanine nucleotide exchange factors to different Golgi compartments: evidence for distinct functions in protein traffic

Activation of several ADP-ribosylation factors (ARFs) by guanine nucleotide exchange factors (GEFs) regulates recruitment of coat proteins (COPs) on the Golgi complex and is generally assumed to be the target of brefeldin A (BFA). The large ARF-GEFs Golgi-specific BFA resistance factor 1 (GBF1) and BFA-inhibited GEFs (BIGs) localize to this organelle but catalyze exchange preferentially on class II and class I ARFs, respectively. We now demonstrate using quantitative confocal microscopy that these GEFs show a very limited overlap with each other (15 and 23%). In contrast, GBF1 colocalizes with the cis-marker p115 (86%), whereas BIGs overlap extensively with TGN38 (83%). Consistent with these distributions, GBF1, but not BIG1, partially relocalized to peripheral sites after incubation at 15 degrees C. The new GBF1 structures represent peripheral vesicular tubular clusters (VTCs) because 88% of structures analyzed stained for both GBF1 and p115. Furthermore, as expected of VTCs, they rapidly reclustered to the Golgi complex in a microtubule-dependent manner upon warm-up. These observations suggest that GBF1 and BIGs activate distinct subclasses of ARFs in specific locations to regulate different types of reactions. In agreement with this possibility, COPI overlapped to a greater extent with GBF1 (64%) than BIG1 (31%), whereas clathrin showed limited overlap with BIG1, and virtually none with GBF1.

Identification and characterization of RA-GEF-2, a Rap guanine nucleotide exchange factor that serves as a downstream target of M-Ras

The Ras family small GTPase Rap is regulated by an array of specific guanine nucleotide exchange factors (GEFs) in response to upstream stimuli. RA-GEF-1 was identified as a novel Rap GEF, which possesses a Ras/Rap1-associating (RA) domain. Here we report a protein closely related to RA-GEF-1, named RA-GEF-2. Like RA-GEF-1, a putative cyclic nucleotide monophosphate-binding domain, a Ras exchanger motif, a PSD-95/DlgA/ZO-1 domain, and an RA domain in addition to the GEF catalytic domain are found in RA-GEF-2. However, RA-GEF-2 displays a different tissue distribution profile from that of RA-GEF-1. RA-GEF-2 stimulates guanine nucleotide exchange of both Rap1 and Rap2, but not Ha-Ras. The RA domain of RA-GEF-2 binds to M-Ras in a GTP-dependent manner, but not to other Ras family GTPases tested, including Ha-Ras, N-Ras, Rap1A, Rap2A, R-Ras, RalA, Rin, Rit, and Rheb, in contrast to the RA domain of RA-GEF-1, which specifically binds to Rap1. In accordance with this, RA-GEF-2 colocalizes with activated M-Ras in the plasma membrane in COS-7 cells, suggesting a role of RA-GEF-2 in the regulation of Rap1 and Rap2, particularly in the plasma membrane. In fact, an increase in the level of the GTP-bound form of plasma membrane-located Rap1 was observed when coexpressed with RA-GEF-2 and activated M-Ras. Thus, RA-GEF-2 acts as a GEF for Rap1 and Rap2 downstream of M-Ras in the plasma membrane, whereas RA-GEF-1 exerts Rap GEF function in perinuclear compartments including the Golgi apparatus.

Guanine nucleotide exchange factors CalDAG-GEFI and CalDAG-GEFII are colocalized in striatal projection neurons

CalDAG-GEFI and CalDAG-GEFII (identical to RasGRP) are novel, brain-enriched guanine nucleotide exchange factors (GEFs) that can be stimulated by calcium and diacylglycerol and that can activate small GTPases, including Ras and Rap1, molecules increasingly recognized as having signaling functions in neurons. Here, we show that CalDAG-GEFI and CalDAG-GEFII mRNAs, detected by in situ hybridization analysis, have sharply contrasting forebrain-predominant distributions in the mature brain: CalDAG-GEFI is expressed mainly in the striatum and olfactory structures and deep cortical layers, whereas CalDAG-GEFII is expressed widely in the forebrain. Within the striatum, however, the two CalDAG-GEF mRNAs have nearly identical distributions: they are coexpressed in striatal projection neurons that give rise to the direct and indirect pathways of the basal ganglia. Subcellular fractionation analysis of the substantia nigra with monoclonal antibodies against CalDAG-GEFI suggests that CalDAG-GEFI protein is present not only in the cell bodies of striatal projection neurons but also in their axons and axon terminals. These results suggest that the CalDAG-GEFs may be key intracellular regulators whereby calcium and diacylglycerol signals can regulate cellular functions through small GTPases in the basal ganglia circuits.