Calmodulin binds RalA and RalB and is required for the thrombin-induced activation of Ral in human platelets

A let-7 microRNA-RALB axis links the immune properties of iPSC-derived megakaryocytes with platelet producibility

We recently achieved the first-in-human transfusion of induced pluripotent stem cell-derived platelets (iPSC-PLTs) as an alternative to standard transfusions, which are dependent on donors and therefore variable in supply. However, heterogeneity characterized by thrombopoiesis-biased or immune-biased megakaryocytes (MKs) continues to pose a bottleneck against the standardization of iPSC-PLT manufacturing. To address this problem, here we employ microRNA (miRNA) switch biotechnology to distinguish subpopulations of imMKCLs, the MK cell lines producing iPSC-PLTs. Upon miRNA switch-based screening, we find imMKCLs with lower let-7 activity exhibit an immune-skewed transcriptional signature. Notably, the low activity of let-7a-5p results in the upregulation of RAS like proto-oncogene B (RALB) expression, which is crucial for the lineage determination of immune-biased imMKCL subpopulations and leads to the activation of interferon-dependent signaling. The dysregulation of immune properties/subpopulations, along with the secretion of inflammatory cytokines, contributes to a decline in the quality of the whole imMKCL population.

Regulation of Ras p21 and RalA GTPases activity by quinine in mammary epithelial cells

Quinine, a bitter compound, can act as an agonist to activate the family of bitter taste G protein-coupled receptor family of proteins. Previous work from our laboratory has demonstrated that quinine causes activation of RalA, a Ras p21-related small G protein. Ral proteins can be activated directly or indirectly through an alternative pathway that requires Ras p21 activation resulting in the recruitment of RalGDS, a guanine nucleotide exchange factor for Ral. Using normal mammary epithelial (MCF-10A) and non-invasive mammary epithelial (MCF-7) cell lines, we investigated the effect of quinine in regulating Ras p21 and RalA activity. Results showed that in the presence of quinine, Ras p21 is activated in both MCF-10A and MCF-7 cells; however, RalA was inhibited in MCF-10A cells, and no effect was observed in the case of MCF-7 cells. MAP kinase, a downstream effector for Ras p21, was activated in both MCF-10A and MCF-7 cells. Western blot analysis confirmed the expression of RalGDS in MCF-10A cells and MCF-7 cells. The expression of RalGDS was higher in MCF-10A cells in comparison to the MCF-7 cells. Although RalGDS was detected in MCF-10A and MCF-7 cells, it did not result in RalA activation upon Ras p21 activation with quinine suggesting that the Ras p21-RalGDS-RalA pathway is not active in the MCF-10A cells. The inhibition of RalA activity in MCF-10A cells due to quinine could be as a result of a direct effect of this bitter compound on RalA. Protein modeling and ligand docking analysis demonstrated that quinine can interact with RalA through the R79 amino acid, which is located in the switch II region loop of the RalA protein. It is possible that quinine causes a conformational change that results in the inhibition of RalA activation even though RalGDS is present in the cell. More studies are needed to elucidate the mechanism(s) that regulate Ral activity in mammary epithelial cells.

Association between Downstream Taste Signaling Genes, Oral Microbiome, and Severe Early Childhood Caries

Polymorphisms in taste receptor genes have been shown to play a role in early childhood caries (ECC), a multifactorial, biofilm-mediated disease. This study aimed to evaluate associations between severe-ECC (S-ECC), the oral microbiome, and variants in genes that encode components of the G protein-coupled receptor (GPCR) signaling cascade involved in taste sensation. A total of 176 children (88 caries-free; 88 with S-ECC) were recruited. Analyses of 16S and ITS1 rRNA microbial genes and seven (GNAQ, GNAS, GNAT3, GNAI2, RAC1, RALB, and PLCB2) human genes were pursued using next-generation sequencing. Regression analyses were performed to evaluate associations between genetic variants, S-ECC, and the supragingival plaque microbiome. Results suggest that PLCB2 rs2305645 (T), rs1869901 (G), and rs2305649 (G) alleles had a protective effect on S-ECC (rs2305645, odds ratio (OR) = 0.27 (95% confidence interval (CI): 0.14-0.51); rs1869901, OR = 0.34 (95% CI: 0.20-0.58); and rs2305649, OR = 0.43 (95% CI: 0.26-0.71)). Variants in GNAQ, GNAS, GNAT3, PLCB2, RALB, and RAC1 were associated with oral fungal and bacterial community composition. This study revealed that three loci at PLCB2 are significantly associated with S-ECC. Variants in multiple genes were associated with the composition of dental biofilm. These findings contribute to the current knowledge about the role of genetics in S-ECC.

The RAL Enigma: Distinct Roles of RALA and RALB in Cancer

RALA and RALB are highly homologous small G proteins belonging to the RAS superfamily. Like other small GTPases, the RALs are molecular switches that can be toggled between inactive GDP-bound and active GTP-bound states to regulate diverse and critical cellular functions such as vesicle trafficking, filopodia formation, mitochondrial fission, and cytokinesis. The RAL paralogs are activated and inactivated by a shared set of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) and utilize similar sets of downstream effectors. In addition to their important roles in normal cell biology, the RALs are known to be critical mediators of cancer cell survival, invasion, migration, and metastasis. However, despite their substantial similarities, the RALs often display striking functional disparities in cancer. RALA and RALB can have redundant, unique, or even antagonistic functions depending on cancer type. The molecular basis for these discrepancies remains an important unanswered question in the field of cancer biology. In this review we examine the functions of the RAL paralogs in normal cellular physiology and cancer biology with special consideration provided to situations where the roles of RALA and RALB are non-redundant.

Membrane extraction by calmodulin underpins the disparate signalling of RalA and RalB

Both RalA and RalB interact with the ubiquitous calcium sensor, calmodulin (CaM). New structural and biophysical characterisation of these interactions strongly suggests that, in the native membrane-associated state, only RalA can be extracted from the membrane by CaM and this non-canonical interaction could underpin the divergent signalling roles of these closely related GTPases. The isoform specificity for RalA exhibited by CaM is hypothesised to contribute to the disparate signalling roles of RalA and RalB in mitochondrial dynamics. This would lead to CaM shuttling RalA to the mitochondrial membrane but leaving RalB localisation unperturbed, and in doing so triggering mitochondrial fission pathways rather than mitophagy.

Calmodulin extracts the Ras family protein RalA from lipid bilayers by engagement with two membrane-targeting motifs

RalA is a small GTPase and a member of the Ras family. This molecular switch is activated downstream of Ras and is widely implicated in tumor formation and growth. Previous work has shown that the ubiquitous Ca2+-sensor calmodulin (CaM) binds to small GTPases such as RalA and K-Ras4B, but a lack of structural information has obscured the functional consequences of these interactions. Here, we have investigated the binding of CaM to RalA and found that CaM interacts exclusively with the C terminus of RalA, which is lipidated with a prenyl group in vivo to aid membrane attachment. Biophysical and structural analyses show that the two RalA membrane-targeting motifs (the prenyl anchor and the polybasic motif) are engaged by distinct lobes of CaM and that CaM binding leads to removal of RalA from its membrane environment. The structure of this complex, along with a biophysical investigation into membrane removal, provides a framework with which to understand how CaM regulates the function of RalA and sheds light on the interaction of CaM with other small GTPases, including K-Ras4B.

The RAL signaling network: Cancer and beyond

The RAL proteins RALA and RALB belong to the superfamily of small RAS-like GTPases (guanosine triphosphatases). RAL GTPases function as molecular switches in cells by cycling through GDP- and GTP-bound states, a process which is regulated by several guanine exchange factors (GEFs) and two heterodimeric GTPase activating proteins (GAPs). Since their discovery in the 1980s, RALA and RALB have been established to exert isoform-specific functions in central cellular processes such as exocytosis, endocytosis, actin organization and gene expression. Consequently, it is not surprising that an increasing number of physiological functions are discovered to be controlled by RAL, including neuronal plasticity, immune response, and glucose and lipid homeostasis. The critical importance of RAL GTPases for oncogenic RAS-driven cellular transformation and tumorigenesis still attracts most research interest. Here, RAL proteins are key drivers of cell migration, metastasis, anchorage-independent proliferation, and survival. This chapter provides an overview of normal and pathological functions of RAL GTPases and summarizes the current knowledge on the involvement of RAL in human disease as well as current therapeutic targeting strategies. In particular, molecular mechanisms that specifically control RAL activity and RAL effector usage in different scenarios are outlined, putting a spotlight on the complexity of the RAL GTPase signaling network and the emerging theme of RAS-independent regulation and relevance of RAL.

Pleiotropic Roles of Calmodulin in the Regulation of KRas and Rac1 GTPases: Functional Diversity in Health and Disease

Calmodulin is a ubiquitous signalling protein that controls many biological processes due to its capacity to interact and/or regulate a large number of cellular proteins and pathways, mostly in a Ca²⁺-dependent manner. This complex interactome of calmodulin can have pleiotropic molecular consequences, which over the years has made it often difficult to clearly define the contribution of calmodulin in the signal output of specific pathways and overall biological response. Most relevant for this review, the ability of calmodulin to influence the spatiotemporal signalling of several small GTPases, in particular KRas and Rac1, can modulate fundamental biological outcomes such as proliferation and migration. First, direct interaction of calmodulin with these GTPases can alter their subcellular localization and activation state, induce post-translational modifications as well as their ability to interact with effectors. Second, through interaction with a set of calmodulin binding proteins (CaMBPs), calmodulin can control the capacity of several guanine nucleotide exchange factors (GEFs) to promote the switch of inactive KRas and Rac1 to an active conformation. Moreover, Rac1 is also an effector of KRas and both proteins are interconnected as highlighted by the requirement for Rac1 activation in KRas-driven tumourigenesis. In this review, we attempt to summarize the multiple layers how calmodulin can regulate KRas and Rac1 GTPases in a variety of cellular events, with biological consequences and potential for therapeutic opportunities in disease settings, such as cancer.

Tropomyosin 1 genetically constrains in vitro hematopoiesis

BACKGROUND

Identifying causal variants and genes from human genetic studies of hematopoietic traits is important to enumerate basic regulatory mechanisms underlying these traits, and could ultimately augment translational efforts to generate platelets and/or red blood cells in vitro. To identify putative causal genes from these data, we performed computational modeling using available genome-wide association datasets for platelet and red blood cell traits.

RESULTS

Our model identified a joint collection of genomic features enriched at established trait associations and plausible candidate variants. Additional studies associating variation at these loci with change in gene expression highlighted Tropomyosin 1 (TPM1) among our top-ranked candidate genes. CRISPR/Cas9-mediated TPM1 knockout in human induced pluripotent stem cells (iPSCs) enhanced hematopoietic progenitor development, increasing total megakaryocyte and erythroid cell yields.

CONCLUSIONS

Our findings may help explain human genetic associations and identify a novel genetic strategy to enhance in vitro hematopoiesis. A similar trait-specific gene prioritization strategy could be employed to help streamline functional validation experiments for virtually any human trait.

A Non-Canonical Calmodulin Target Motif Comprising a Polybasic Region and Lipidated Terminal Residue Regulates Localization

Calmodulin (CaM) is a Ca²⁺-sensor that regulates a wide variety of target proteins, many of which interact through short basic helical motifs bearing two hydrophobic ‘anchor’ residues. CaM comprises two globular lobes, each containing a pair of EF-hand Ca²⁺-binding motifs that form a Ca²⁺-induced hydrophobic pocket that binds an anchor residue. A central flexible linker allows CaM to accommodate diverse targets. Several reported CaM interactors lack these anchors but contain Lys/Arg-rich polybasic sequences adjacent to a lipidated N- or C-terminus. Ca²⁺-CaM binds the myristoylated N-terminus of CAP23/NAP22 with intimate interactions between the lipid and a surface comprised of the hydrophobic pockets of both lobes, while the basic residues make electrostatic interactions with the negatively charged surface of CaM. Ca²⁺-CaM binds farnesylcysteine, derived from the farnesylated polybasic C-terminus of KRAS4b, with the lipid inserted into the C-terminal lobe hydrophobic pocket. CaM sequestration of the KRAS4b farnesyl moiety disrupts KRAS4b membrane association and downstream signaling. Phosphorylation of basic regions of N-/C-terminal lipidated CaM targets can reduce affinity for both CaM and the membrane. Since both N-terminal myristoylated and C-terminal prenylated proteins use a Singly Lipidated Polybasic Terminus (SLIPT) for CaM binding, we propose these polybasic lipopeptide elements comprise a non-canonical CaM-binding motif.

A novel Microproteomic Approach Using Laser Capture Microdissection to Study Cellular Protrusions

Cell–cell communication is vital to multicellular organisms, and distinct types of cellular protrusions play critical roles during development, cell signaling, and the spreading of pathogens and cancer. The differences in the structure and protein composition of these different types of protrusions and their specific functions have not been elucidated due to the lack of a method for their specific isolation and analysis. In this paper, we described, for the first time, a method to specifically isolate distinct protrusion subtypes, based on their morphological structures or fluorescent markers, using laser capture microdissection (LCM). Combined with a unique fixation and protein extraction protocol, we pushed the limits of microproteomics and demonstrate that proteins from LCM-isolated protrusions can successfully and reproducibly be identified by mass spectrometry using ultra-high field Orbitrap technologies. Our method confirmed that different types of protrusions have distinct proteomes and it promises to advance the characterization and the understanding of these unique structures to shed light on their possible role in health and disease.

Ral signaling pathway in health and cancer

The Ral (Ras-Like) signaling pathway plays an important role in the biology of cells. A plethora of effects is regulated by this signaling pathway and its prooncogenic effectors. Our team has demonstrated the overactivation of the RalA signaling pathway in a number of human malignancies including cancers of the liver, ovary, lung, brain, and malignant peripheral nerve sheath tumors. Additionally, we have shown that the activation of RalA in cancer stem cells is higher in comparison with differentiated cancer cells. In this article, we review the role of Ral signaling in health and disease with a focus on the role of this multifunctional protein in the generation of therapies for cancer. An improved understanding of this pathway can lead to development of a novel class of anticancer therapies that functions on the basis of intervention with RalA or its downstream effectors.

Identification of oral cancer related candidate genes by integrating protein-protein interactions, gene ontology, pathway analysis and immunohistochemistry

In the recent years, bioinformatics methods have been reported with a high degree of success for candidate gene identification. In this milieu, we have used an integrated bioinformatics approach assimilating information from gene ontologies (GO), protein–protein interaction (PPI) and network analysis to predict candidate genes related to oral squamous cell carcinoma (OSCC). A total of 40973 PPIs were considered for 4704 cancer-related genes to construct human cancer gene network (HCGN). The importance of each node was measured in HCGN by ten different centrality measures. We have shown that the top ranking genes are related to a significantly higher number of diseases as compared to other genes in HCGN. A total of 39 candidate oral cancer target genes were predicted by combining top ranked genes and the genes corresponding to significantly enriched oral cancer related GO terms. Initial verification using literature and available experimental data indicated that 29 genes were related with OSCC. A detailed pathway analysis led us to propose a role for the selected candidate genes in the invasion and metastasis in OSCC. We further validated our predictions using immunohistochemistry (IHC) and found that the gene FLNA was upregulated while the genes ARRB1 and HTT were downregulated in the OSCC tissue samples.

Characterization and identification of calmodulin and calmodulin binding proteins in hemocyte of the black tiger shrimp (Penaeus monodon)

Calmodulin (CaM), a ubiquitous intracellular calcium (Ca(2+)) sensor in all eukaryotic cells, is one of the well-known signaling proteins. Previously, CaM gene has shown a high transcriptional level in hemocyte of the pathogen infected shrimp, suggesting that shrimp CaM does not only regulate Ca(2+) metabolism, but is also involved in immune response cascade. In the present study, the CaM gene of shrimp Penaeus monodon was identified and the recombinant P.monodon CaM (rPmCaM) was produced and biochemically characterized. The identification of CaM-binding proteins was also performed. The PmCaM cDNA consisted of an open reading frame of 447 bp encoding for 149 amino acid residues with a calculated mass of 16,810 Da and an isoelectric point of 4.09. Tissue distribution showed that the PmCaM transcript was expressed in all examined tissues. The results of gel mobility shift assay, circular dichroism spectroscopy and fluorescence spectroscopy all confirmed that the conformational changes of the rPmCaM were observed after the calcium binding. According to the gene silencing of PmCaM transcript levels, the shrimp's susceptibility to pathogenic Vibrio harveyi infection increased in comparison with that of the control groups. Protein pull-down assay and LC-MS/MS analysis were performed to identify rPmCaM-binding proteins involved in shrimp immune responses and transglutaminase, elongation factor 1-alpha, elongation factor 2 and actin were found. However, by computational analysis, only the first three proteins contained CaM-binding domain. These findings suggested that PmCaM may play an important role in regulation of shrimp immune system.

Characterization and functional analysis of the calmodulin-binding domain of Rac1 GTPase

Rac1, a member of the Rho family of small GTPases, has been shown to promote formation of lamellipodia at the leading edge of motile cells and affect cell migration. We previously demonstrated that calmodulin can bind to a region in the C-terminal of Rac1 and that this interaction is important in the activation of platelet Rac1. Now, we have analyzed amino acid residue(s) in the Rac1-calmodulin binding domain that are essential for the interaction and assessed their functional contribution in Rac1 activation. The results demonstrated that region 151–164 in Rac1 is essential for calmodulin binding. Within the 151–164 region, positively-charged amino acids K153 and R163 were mutated to alanine to study impact on calmodulin binding. Mutant form of Rac1 (K153A) demonstrated significantly reduced binding to calmodulin while the double mutant K153A/R163A demonstrated complete lack of binding to calmodulin. Thrombin or EGF resulted in activation of Rac1 in CHRF-288-11 or HeLa cells respectively and W7 inhibited this activation. Immunoprecipitation studies demonstrated that higher amount of CaM was associated with Rac1 during EGF dependent activation. In cells expressing mutant forms of Rac1 (K153A or K153A/R163A), activation induced by EGF was significantly decreased in comparison to wild type or the R163A forms of Rac1. The lack of Rac1 activation in mutant forms was not due to an inability of GDP-GTP exchange or a change in subcelllular distribution. Moreover, Rac1 activation was decreased in cells where endogenous level of calmodulin was reduced using shRNA knockdown and increased in cells where calmodulin was overexpressed. Docking analysis and modeling demonstrated that K153 in Rac1 interacts with Q41 in calmodulin. These results suggest an important role for calmodulin in the activation of Rac1 and thus, in cytoskeleton reorganization and cell migration.

Aurora A kinase activity influences calcium signaling in kidney cells

Aurora A is abnormally expressed and activated in cells lining cysts associated with polycystic kidney disease and can phosphorylate and inactivate polycystin 2.

Most studies of Aurora A (AurA) describe it as a mitotic centrosomal kinase. However, we and others have recently identified AurA functions as diverse as control of ciliary resorption, cell differentiation, and cell polarity control in interphase cells. In these activities, AurA is transiently activated by noncanonical signals, including Ca²⁺-dependent calmodulin binding. These and other observations suggested that AurA might be involved in pathological conditions, such as polycystic kidney disease (PKD). In this paper, we show that AurA is abundant in normal kidney tissue but is also abnormally expressed and activated in cells lining PKD-associated renal cysts. PKD arises from mutations in the PKD1 or PKD2 genes, encoding polycystins 1 and 2 (PC1 and PC2). AurA binds, phosphorylates, and reduces the activity of PC2, a Ca²⁺-permeable nonselective cation channel and, thus, limits the amplitude of Ca²⁺ release from the endoplasmic reticulum. These and other findings suggest AurA may be a relevant new biomarker or target in the therapy of PKD.

Rapid calcium-dependent activation of Aurora-A kinase

Oncogenic hyperactivation of the mitotic kinase Aurora-A (AurA) in cancer is associated with genomic instability. Increasing evidence indicates that AurA also regulates critical processes in normal interphase cells, but the source of such activity has been obscure. We report here that multiple stimuli causing release of Ca²⁺ from intracellular endoplasmic reticulum stores rapidly and transiently activate AurA, without requirement for second messengers. This activation is mediated by direct Ca²⁺-dependent calmodulin (CaM) binding to multiple motifs on AurA. On the basis of structure–function analysis and molecular modelling, we map two primary regions of CaM-AurA interaction to unfolded sequences in the AurA N- and C-termini. This unexpected mechanism for AurA activation provides a new context for evaluating the function of AurA and its inhibitors in normal and cancerous cells.

Aurora-A kinase localizes to centrosomes, is involved in the progression through mitosis and is overexpressed in certain cancers. Here, calcium is shown to induce Aurora-A auto-phosphorylation in a calmodulin-dependent manner, suggesting a novel role for Aurora-A in non-mitotic cells.

[Acquired human prion diseases--past and present issues]

Transmissible spongiform encephalopathies, or prion diseases, are fatal neurodegenerative disorders. In aetiological viewpoint, human prion diseases are classified into 1) sporadic Creutzfeldt-Jakob disease (CJD) which comprises 80-90% of the total population of human prion disaeses, 2) inherited forms, and 3) acquired types by prion-contaminated surgical instruments, biopharmaceuticals or foodstuffs. The diseases cause an accumulation of the disease-associated form(s) of prion protein (PrP(Sc)) in the central nervous system. PrP(Sc) is regarded as the entity of prion agents and generally exerts infectivity, irrespective of its origin being from the sporadic cases or the inherited cases. Variant CJD (vCJD), first identified in the United Kingdom (UK) in 1996, is an acquired type of human CJD by oral intake of BSE prion. Cumulative numbers of 215 patients in the world have been reported for definite or probable vCJD cases according to the UK National Creutzfeldt-Jakob Disease Surveillance Unit by September, 2009. Different from sporadic CJD cases, vCJD patients show an accumulation of PrP(Sc) in spleen and tonsils. Such distribution of PrP(Sc) in lymphoid tissues raised clinical concern about the potential infectivity in the blood or blood components used for blood transfusion. To date, five instances of probable transfusion-mediated transmission of vCJD prion have been found in UK. Here we review the past and the present issues about the acquired human prion diseases.

The GTPase RalA regulates different steps of the secretory process in pancreatic beta-cells

Background

RalA and RalB are multifuntional GTPases involved in a variety of cellular processes including proliferation, oncogenic transformation and membrane trafficking. Here we investigated the mechanisms leading to activation of Ral proteins in pancreatic β-cells and analyzed the impact on different steps of the insulin-secretory process.

Methodology/Principal Findings

We found that RalA is the predominant isoform expressed in pancreatic islets and insulin-secreting cell lines. Silencing of this GTPase in INS-1E cells by RNA interference led to a decrease in secretagogue-induced insulin release. Real-time measurements by fluorescence resonance energy transfer revealed that RalA activation in response to secretagogues occurs within 3–5 min and reaches a plateau after 10–15 min. The activation of the GTPase is triggered by increases in intracellular Ca²⁺ and cAMP and is prevented by the L-type voltage-gated Ca²⁺ channel blocker Nifedipine and by the protein kinase A inhibitor H89. Defective insulin release in cells lacking RalA is associated with a decrease in the secretory granules docked at the plasma membrane detected by Total Internal Reflection Fluorescence microscopy and with a strong impairment in Phospholipase D1 activation in response to secretagogues. RalA was found to be activated by RalGDS and to be severely hampered upon silencing of this GDP/GTP exchange factor. Accordingly, INS-1E cells lacking RalGDS displayed a reduction in hormone secretion induced by secretagogues and in the number of insulin-containing granules docked at the plasma membrane.

Conclusions/Significance

Taken together, our data indicate that RalA activation elicited by the exchange factor RalGDS in response to a rise in intracellular Ca²⁺ and cAMP controls hormone release from pancreatic β-cell by coordinating the execution of different events in the secretory pathway.

Guanine exchange factor RalGDS mediates exocytosis of Weibel-Palade bodies from endothelial cells

The small GTP-binding protein Ral has been implicated in regulated exocytosis via its interaction with the mammalian exocyst complex. We have previously demonstrated that Ral is involved in exocytosis of Weibel-Palade bodies (WPBs). Little is known about intracellular signaling pathways that promote activation of Ral in response to ligand binding of G protein-coupled receptors. Here we show that RNAi-mediated knockdown of RalGDS, an exchange factor for Ral, results in inhibition of thrombin- and epinephrine-induced exocytosis of WPBs, while overexpression of RalGDS promotes exocytosis of WPBs. A RalGDS variant lacking its exchange domain behaves in a dominant negative manner by blocking release of WPBs. We also provide evidence that RalGDS binds calmodulin (CaM) via an amino-terminal CaM-binding domain. RalGDS association to CaM is required for Ral activation because a cell-permeable peptide comprising this RalGDS CaM-binding domain inhibits Ral activation and WPB exocytosis. Together our findings suggest that RalGDS plays a vital role in the regulation of Ral-dependent WPB exocytosis after stimulation with Ca(2+)- or cAMP-raising agonists.

Primary platelet signaling cascades and integrin-mediated signaling control ADP-ribosylation factor (Arf) 6-GTP levels during platelet activation and aggregation

Previous studies showed that ADP-ribosylation factor 6 (Arf6) is important for platelet function; however, little is known about which signaling events regulate this small GTP-binding protein. Arf6-GTP was monitored in platelets stimulated with a number of agonists (TRAP, thrombin, convulxin, collagen, PMA, thapsigargin, or A23187) and all led to a time-dependent decrease in Arf6-GTP. ADP and U46619 were without effect. Using inhibitors, it was shown that the decrease of Arf6-GTP is a direct consequence of known signaling cascades. Upon stimulation via PAR receptors, Arf6-GTP loss could be blocked by treatment with U-73122, BAPTA/AM, Ro-31-8220, or Gö6976, indicating requirements for phospholipase C, calcium, and protein kinase C (PKC) alpha/beta, respectively. The Arf6-GTP decrease in convulxin-stimulated platelets showed similar requirements and was also sensitive to piceatannol, wortmannin, and LY294002, indicating additional requirements for Syk and phosphatidylinositol 3-kinase. The convulxin-induced decrease was sensitive to both PKCalpha/beta and delta inhibitors. Outside-in signaling, potentially via integrin engagement, caused a second wave of signaling that affected Arf6. Inclusion of RGDS peptides or EGTA, during activation, led to a biphasic response; Arf6-GTP levels partially recovered upon continued incubation. A similar response was seen in beta3 integrin-null platelets. These data show that Arf6-GTP decreases in response to known signaling pathways associated with PAR and GPVI. They further reveal a second, aggregation-dependent, process that dampens Arf6-GTP recovery. This study demonstrates that the nucleotide state of Arf6 in platelets is regulated during the initial phases of activation and during the later stages of aggregation.

The B cell antigen receptor controls AP-1 and NFAT activity through Ras-mediated activation of Ral

Signaling by the BCR involves activation of several members of the Ras superfamily of small GTPases, among which is Ras itself. Ras can control the activity of multiple effectors, including Raf, PI3K, and guanine nucleotide exchange factors for the small GTPase Ral. Ras, Raf, and PI3K have been implicated in a variety of processes underlying B cell development, differentiation, and function; however, the role of Ral in B lymphocytes remains to be established. In this study, we show that Ral is activated upon BCR stimulation in human tonsillar and mouse splenic B lymphocytes and in B cell lines. Using signaling molecule-deficient B cells, we demonstrate that this activation is mediated by Lyn and Syk, Btk, phospholipase C-gamma2, and inositol-1,4,5-trisphosphate receptor-mediated Ca(2+) release. In addition, although Ral can be activated by Ras-independent mechanisms, we demonstrate that BCR-controlled activation of Ral is dependent on Ras. By means of expression of the dominant-negative mutants RasN17 and RalN28, or of RalBPDeltaGAP, a Ral effector mutant which sequesters active Ral, we show that Ras and Ral mediate BCR-controlled transcription of c-fos. Furthermore, while not involved in NF-kappaB activation, Ras and Ral mediate BCR-controlled activation of JUN/ATF2 and NFAT transcription factors. Taken together, our data show that Ral is activated upon BCR stimulation and mediates BCR-controlled activation of AP-1 and NFAT transcription factors. These findings suggest that Ral plays an important role in B cell development and function.

Ral-GTPase interacts with the β1 subunit of Na+/K+-ATPase and is activated upon inhibition of the Na+/K+pumpThis paper is one of a selection of papers published in this Special Issue, entitled The Cellular and Molecular Basis of Cardiovascular Dysfunction, Dhalla 70th Birthday Tribute

Na+/K+-ATPase functions as both an ion pump and a signal transducer. Cardiac glycosides partially inhibit Na+/K+-ATPase, causing activation of multiple interrelated growth pathways via the Na+/K+-ATPase/c-Src/epidermal growth factor receptor complex. Such pathways include Ras/MEK/ERK and Ral/RalGDS cascades, which can lead to cardiac hypertrophy. In search of novel Ral-GTPase binding proteins, we used RalB as the bait to screen a human testes cDNA expression library using the yeast 2-hybrid system. The results demonstrated that 1 of the RalB interacting clones represented the C-terminal region of the β1 subunit of Na+/K+-ATPase. Further analysis using the yeast 2-hybrid system and full-length β1 subunit of Na+/K+-ATPase confirmed the interaction with RalA and RalB. In vitro binding and pull-down assays demonstrated that the β1 subunit of Na+/K+-ATPase interacts directly with RalA and RalB. Ral-GTP pull-down assays demonstrated that short-term ouabain treatment of A7r5 cells, a rat aorta smooth muscle cell line, caused activation of Ral GTPase. Maximal activation was observed 10 min after ouabain treatment. Ouabain-mediated Ral activation was inhibited upon the stimulation of Na+/K+-ATPase activity by Ang II. We propose that Ral GTPase is involved in the signal transducing function of Na+/K+-ATPase and provides a possible molecular mechanism connecting Ral to cardiac hypertrophy during diseased conditions.

A serine kinase associates with the RAL GTPase and phosphorylates RAL-interacting protein 1

A kinase activity that phosphorylated myelin basic protein in vitro was detected in RalA and RalB immunoprecipitates from human platelets. Protein-protein interaction studies using recombinant GST-RalA, GST-RalB and GST-cH-Ras confirmed that the kinase specifically associates with the Ral GTPase. The Ral Interacting Protein 1 (RIP1), a GTPase Activating Protein (GAP) for Cdc42 and Rac1, was found to be the preferred substrate for the Ral Interacting Kinase (RIK). Phosphoamino acid analysis demonstrated that RIK phosphorylated serine residue in RIP1. The Ral-RIK interaction was not dependent on the guanine nucleotide status of Ral. RIK was detected in a variety of rat tissues with testis containing the highest and skeletal muscle the lowest activity. In-gel kinase renaturation assay using RIP1 as the substrate demonstrated that the kinase activity was associated with polypeptides of molecular mass of approximately 36-40 kDa and was detected in most rat tissues with a prominent 38 kDa band in testis and a 40 kDa band in brain. Human platelets contained a single band of approximately 36 kDa. RIK was distinct from MAPKs, CDKs, cyclic AMP dependent protein kinase and Ca2+/calmodulin dependent kinases. To demonstrate in vivo interaction, the endogenous Ral-RIK complex was isolated using a calmodulin affinity column. The Ral-RIK complex co-eluted from this column upon washing with a 13 residue peptide that encompasses the calmodulin-binding domain in RalA. The data suggest that RIK is a serine specific kinase that phosphorylates RIP1 and is constitutively associated with Ral. The current study provides additional support for a link between Ral and the Cdc42/Rac1 signalling pathways in the cell.

Arf6 plays an early role in platelet activation by collagen and convulxin

Small GTPases play critical roles in hemostasis, though the roster of such molecules in platelets is not complete. In this study, we report the presence of Ras-related GTPases of the ADP-ribosylation factor (Arf) family. Platelets contain Arf1 or 3 and Arf6, with the latter being predominantly membrane associated. Using effector domain pull-down assays, we show, counter to other GTPases, that Arf6-GTP is present in resting platelets and decreases rapidly upon activation with collagen or convulxin. This decrease does not completely rely on secondary agonists (ADP and thromboxane A2) or require integrin signaling. The decrease in free Arf6-GTP temporally precedes activation of Rho family GTPases (RhoA, Cdc42, and Rac1). Using a membrane-permeant, myristoylated peptide, which mimics the N-terminus of Arf6, we show that the Arf6-GTP decrease is essential for collagen- and convulxin-induced aggregation, platelet adherence, and spreading on collagen-coated glass. Treatment with this peptide also affects the activation of Rho family GTPases, but has little effect on RalA and Rap1 or on agonist-induced calcium mobilization. These data show that Arf6 is a key element in activation through GPVI, and is required for activation of the Rho family GTPases and the subsequent cytoskeletal rearrangements needed for full platelet function.

Calmodulin binding to the small GTPase Ral requires isoprenylated Ral

Ral, a member of the Ras-p21 superfamily of small GTPases, has been shown to require the calcium-signaling protein calmodulin (CaM) for activation. In the present work, we investigated the properties of the Ral-CaM interaction. Using CaM affinity binding assay with lysates from mammalian cells overexpressing various Ral mutants, we found that RalB(V23, DeltaCAAX) lacking the C-terminal isoprenylation region bound significantly less efficiently to CaM. Binding of other mutants containing critical amino acid changes in the nucleotide or substrate binding regions (residues 23, 28, and 49) was not affected. In addition, all mutants bound significantly better in the presence of calcium versus the calcium chelator EGTA. Using in vitro transcription-translation in the presence of geranylgeranyl pyrophosphate, we demonstrate enhanced Ral binding to CaM. Inhibition of isoprenylation in cells in culture with lovastatin resulted in decreased binding of CaM to Ral. The present results show that post-translational isoprenylation of Ral is important in Ral-CaM interaction.

Reversible intracellular translocation of KRas but not HRas in hippocampal neurons regulated by Ca2+/calmodulin

The Ras/MAPK pathway regulates synaptic plasticity and cell survival in neurons of the central nervous system. Here, we show that KRas, but not HRas, acutely translocates from the plasma membrane (PM) to the Golgi complex and early/recycling endosomes in response to neuronal activity. Translocation is reversible and mediated by the polybasic-prenyl membrane targeting motif of KRas. We provide evidence that KRas translocation occurs through sequestration of the polybasic-prenyl motif by Ca²⁺/calmodulin (Ca²⁺/CaM) and subsequent release of KRas from the PM, in a process reminiscent of GDP dissociation inhibitor–mediated membrane recycling of Rab and Rho GTPases. KRas translocation was accompanied by partial intracellular redistribution of its activity. We conclude that the polybasic-prenyl motif acts as a Ca²⁺/CaM-regulated molecular switch that controls PM concentration of KRas and redistributes its activity to internal sites. Our data thus define a novel signaling mechanism that differentially regulates KRas and HRas localization and activity in neurons.

The Small GTPase RalA controls exocytosis of large dense core secretory granules by interacting with ARF6-dependent phospholipase D1

RalA and RalB constitute a family of highly similar Ras-related GTPases widely distributed in different tissues. Recently, active forms of Ral proteins have been shown to bind to the exocyst complex, implicating them in the regulation of cellular secretion. Since RalA is present on the plasma membrane in neuroendocrine chromaffin and PC12 cells, we investigated the potential role of RalA in calcium-regulated exocytotic secretion. We show here that endogenous RalA is activated during exocytosis. Expression of the constitutively active RalA (G23V) mutant enhances secretagogue-evoked secretion from PC12 cells. Conversely, expression of the constitutively inactive GDP-bound RalA (G26A) or silencing of the RalA gene by RNA interference led to a strong impairment of the exocytotic response. RalA was found to co-localize with phospholipase D1 (PLD1) at the plasma membrane in PC12 cells. We demonstrate that cell stimulation triggers a direct interaction between RalA and ARF6-activated PLD1. Moreover, reduction of endogenous RalA expression level interfered with the activation of PLD1 observed in secretagogue-stimulated cells. Finally, using various RalA mutants selectively impaired in their ability to activate downstream effectors, we show that PLD1 activation is essential for the activation of secretion by GTP-loaded RalA. Together, these results provide evidence that RalA is a positive regulator of calcium-evoked exocytosis of large dense core secretory granules and suggest that stimulation of PLD1 and consequent changes in plasma membrane phospholipid composition is the major function RalA undertakes in calcium-regulated exocytosis.

Regulation of Phospholipase C-δ1 through Direct Interactions with the Small GTPase Ral and Calmodulin

Second messengers generated from membrane lipids play a critical role in signaling and control diverse cellular processes. Despite being one of the most evolutionarily conserved of all the phosphoinositide-specific phospholipase C (PLC) isoforms, a family of enzymes responsible for hydrolysis of the membrane lipid phosphatidylinositol bisphosphate, the mechanism of PLC-delta1 activation is still poorly understood. Here we report a novel regulatory mechanism for PLC-delta1 activation that involves direct interaction of the small GTPase Ral and the universal calcium-signaling molecule calmodulin (CaM) with PLC-delta1. In addition, we have identified a novel IQ type CaM binding motif within the catalytic region of PLC-delta1 that is not found in other PLC isoforms. Binding of CaM at the IQ motif inhibits PLC-delta1 activity, while addition of Ral reverses the inhibition. The overexpression of various Ral mutants in cells potentiates PLC-delta1 activity. Thus, the Ral-CaM complex defines a multifaceted regulatory mechanism for PLC-delta1 activation.

Integration of signalling pathways regulated by small GTPases and calcium

The Ras superfamily of small GTPases constitutes a large group of structurally and functionally related proteins. They function as signalling switches in numerous signalling cascades in the cell. During the recent years, an increased awareness of a communication between signalling systems employing Ras-like GTPases and signalling systems employing calcium has emerged. For instance, the intensity of the activation of Ras-like GTPases is regulated by calcium-dependent mechanisms, acting on proteins that facilitate the activation or inactivation of the small GTPases. Other Ras-like GTPases have a direct influence on calcium signalling by regulating the activity of certain calcium channels. In addition, several small GTPases collaborate with calcium signalling in regulating cellular processes, such as cell adhesion, cell migration and exocytosis.

Echinococcus multilocularis: identification and molecular characterization of a Ral-like small GTP-binding protein

In mammals, Ral (Ras-like) GTPases have been implicated in the regulation of several cellular key processes such as oncogenic transformation, endocytosis, and actin-cytoskeleton dynamics. Here we provide, for the first time, molecular data on a Ral homologue from a parasitic helminth. We have cloned and characterized the complete cDNA molecule and the chromosomal locus encoding a novel GTP binding protein, EmRal, of the human parasite Echinococcus multilocularis. The encoded protein contained all highly conserved amino acid residues of the protein family at corresponding positions and shared significant sequence homologies with human RalA (53% identity) and RalB (54%). Upon heterologous expression of EmRal in Escherichia coli, the recombinant protein was able to bind GTP, thus indicating functionality of the Echinococcus factor. Using an in vitro prenylation assay, the purified protein was shown to be geranylgernylated, but not farnesylated, in both rabbit reticulocyte and Echinococcus cell extracts. The EmRal mRNA was found to be processed via trans-splicing and, using RT-PCR and virtual Northern blot experiments, expression of the factor could be demonstrated for the larval stages metacestode and protoscolex during an infection of the intermediate host. The data presented herein provide a solid basis for further investigations on Ras-Ral signaling mechanisms in Echinococcus.

Ral-GTPases: approaching their 15 minutes of fame

Andy Warhol, the famous pop artist, once claimed that "in the future everyone will be famous for 15 minutes". The same, it seems, can be said of proteins, because at any given time some proteins become more "fashionable" to study than others. But most proteins have been highly conserved throughout millions of years of evolution, which implies that they all have essential roles in cell biology. Thus, each one will no doubt enter the limelight if the right experiment in the right cell type is done. A good example of this is the Ras-like GTPases (Ral-GTPases), which until recently existed in the shadow of their close cousins--the Ras proto-oncogenes. Recent studies have yielded insights into previously unappreciated roles for Ral-GTPases in intensively investigated disciplines such as vesicle trafficking, cell morphology, transcription and possibly even human oncogenesis.

Ca2+/calmodulin binds and dissociates K-RasB from membrane

We have investigated the interaction of calmodulin (CaM) with Ras-p21 and the significance of this association. All Ras-p21 isoforms tested (H-, K-, and N-Ras) were detected in the particulate fraction of human platelets and MCF-7 cells, a human breast cancer cell line. In MCF-7 cells, H- and N-Ras were also detected in the cytosolic fraction. K-RasB from platelet and MCF-7 cell lysates was found to bind CaM in a Ca2+ -dependent but GTPgammaS-independent manner. The yeast two-hybrid analysis demonstrated that K-RasB binds to CaM in vivo. Incubation of isolated membranes from platelet and MCF-7 cells with CaM caused dissociation of only K-RasB from membranes in a Ca2+ -dependent manner. CaM antagonist, W7, inhibited dissociation of K-RasB. Addition of platelet or MCF-7 cytosol alone to isolated platelet membranes did not cause dissociation of K-RasB and only addition of exogenous CaM caused dissociation. The results suggest a potential role for Ca2+/CaM in the regulation of K-RasB function.