Integration of calcium and Ras signalling

Calcium is a universal intracellular signal that is responsible for controlling a plethora of cellular processes. Understanding how such a simple ion can regulate so many diverse cellular processes is a key goal of calcium- and cell-biologists. One molecule that is sensitive to changes in intracellular calcium levels is Ras. This small GTPase operates as a binary molecular switch, and regulates cell proliferation and differentiation. Here, we focus on examining the link between calcium and Ras signalling and, in particular, we speculate as to how the complexity of calcium signalling could regulate Ras activity.     

Endocytosis of FcgammaRI is regulated by two distinct signalling pathways

Aggregation by immune complexes of receptors specific for the Fc region of IgG results in their internalisation and disposal by trafficking to lysosomes. We show here that internalisation of FcgammaRI by IFN-gamma treated U937 cells following receptor aggregation by cross-linking antibodies requires the activation of two distinct signalling pathways. The pathways were functionally dissected in streptolysin-O-permeabilised cells by capitalising on their relative dependence on active GTP binding proteins. One pathway required the presence of GTP-gammaS or active betagamma subunits, the other did not. Use of inhibitors revealed that the betagamma-independent pathway required activation of PI 3-kinases and was PKC-independent In contrast, the betagamma-dependent pathway involved activation of phospholipase C-beta and PKC, but was PI 3-kinase-independent. Both these pathways were found to be active in intact cells and are likely to determine receptor trafficking following internalisation.     

Regulation of calcium signalling by the small GTP-binding proteins Ras and Rac1

In order to investigate a possible interaction of the small GTP-binding proteins Ras and Rac1 with Ca²⁺-mediated signalling cascades the effects of dominant negative mutants of Ras and Rac1 on Ca²⁺ signalling have been studied after stimulation of either the EGFR or the nerve growth factor receptor (TRK). Expression of dominant negative Ras blocks the release of Ca²⁺ from internal stores after activation of EGFR whereas the calcium signal elicited by the activated TRK receptor is unaffected. The sensitivity to dominant negative Ras is determined by the structure of the PLCγ-binding sites of the corresponding receptors. Exchange of the PLCγ-binding domain of the EGFR by the PLCγ-binding site of TRK renders the EGFR-induced calcium signal insensitive to the expression of dominant negative Ras. Substitution of the PLCγ-binding site of TRK by the PLCγ-binding region of EGFR renders TRK sensitive to dominant negative Ras. The inhibition of Ca²⁺ release by dominant negative Ras is accompanied by a reduction in PLCγ binding to the EGFR and a concomitant decrease of EGF-induced inositol-1,3,5-trisphosphate (InsP₃) formation. The depression of PLCγ binding to EGFR is explained by a competition of PLCγ with other SH2-domain containing proteins for the same low affinity binding regions of the EGFR. This conclusion is supported by the observation that microinjection of several SH2-domain containing proteins including Ras-GP, lipase-free fragment of PLCγ or Janus kinase binding protein (JAB), reduces the association of PLCγ to the EGFR, not, however, to TRK. In contrast to dominant negative Ras which does not affect the Ca²⁺ transient induced by the activation of the TRK receptor, a dominant negative mutant of Rac significantly depresses the Ca²⁺ signals induced by EGFR as well as by TRK. The different behavior of Rac and Ras supports the notion that the two small GTP-binding proteins act through separate pathways. It is demonstrated that dominant negative Rac significantly reduces the formation of phosphatidylinositol-4,5-bisphosphate (PIP₂), the substrate of PLCγ. This effect is not observed after expression of dominant negative Ras. In summary, the data provide further evidence for a cross-talk between small GTP-binding proteins and Ca²⁺ signalling in which both G-proteins interfere with the formation of InsP₃ although by different mechanisms.     

GTPases in bacterial cell polarity and signalling

In bacteria, large G domain GTPases have well-established functions in translation, protein translocation, tRNA modification and ribosome assembly. In addition, bacteria also contain small Ras-like GTPases consisting of stand-alone G domains. Recent data have revealed that small Ras-like GTPases as well as large G domain GTPases in bacteria function in the regulation of cell polarity, signal transduction and possibly also in cell division. The small Ras-like GTPase MglA together with its cognate GAP MglB regulates cell polarity in Myxococcus xanthus, and the small Ras-like GTPase CvnD9 in Streptomyces coelicolor is involved in signal transduction. Similarly, the large GTPase FlhF together with the ATPase FlhG regulates the localization and number of flagella in polarly flagellated bacteria. Moreover, large dynamin-like GTPases in bacteria may function in cell division. Thus, the function of GTPases in bacteria may be as pervasive as in eukaryotes.     

Regulation of PTEN by Rho small GTPases

PTEN (phosphatase and tensin homologue) is a phosphatase that dephosphorylates both protein and phosphoinositide substrates. It is mutated in a variety of human tumours and has important roles in a diverse range of biological processes, including cell migration and chemotaxis. PTEN's intracellular localization and presumably activity are regulated by chemoattractants in Dictyostelium and mouse neutrophils. However, the mechanisms for its regulation remain elusive. Here we show that RhoA and Cdc42, members of the Rho family of small GTPases, regulate the intracellular localization of PTEN in leukocytes and human transfected embryonic kidney cells. In addition, active RhoA is able to stimulate the phospholipid phosphatase activity of PTEN in human embryonic kidney cells and leukocytes, and this regulation seems to require RhoA's downstream effector, RhoA-associated kinase (Rock). Furthermore, we have identified key residues on PTEN that are required for its regulation by the small GTPase, and show that small GTPase-mediated regulation of PTEN has a significant role in the regulation of chemotaxis.     

A history of stress alters drought calcium signalling pathways in Arabidopsis

Environmental stresses commonly encountered by plants lead to rapid transient elevations in cytosolic free calcium concentration ([Ca2+]cyt) (Bush, 1995; Knight et al., 1991). These cellular calcium (Ca2+) signals lead ultimately to the increased expression of stress-responsive genes, including those encoding proteins of protective function (Knight et al., 1996; Knight et al., 1997). The kinetics and magnitude of the Ca2+ signal, or 'calcium signature', differ between different stimuli and are thought to contribute to the specificity of the end response (Dolmetsch et al., 1997; McAinsh and Hetherington, 1998). We measured [Ca2+]cyt changes during treatment with mannitol (to mimic drought stress) in whole intact seedlings of Arabidopsis thaliana. The responses of plants which were previously exposed to osmotic and oxidative stresses were compared to those of control plants. We show here that osmotic stress-induced Ca2+ responses can be markedly altered by previous encounters with either osmotic or oxidative stress. The nature of the alterations in Ca2+ response depends on the identity and severity of the previous stress: oxidative stress pre-treatment reduced the mannitol-induced [Ca2+]cyt response whereas osmotic stress pretreatment increased the [Ca2+]cyt response. Therefore, our data show that different combinations of environmental stress can produce novel Ca2+ signal outputs. These alterations are accompanied by corresponding changes in the patterns of osmotic stress-induced gene expression and, in the case of osmotic stress pre-treatment, the acquisition of stress-tolerance. This suggests that altered Ca2+ responses encode a 'memory' of previous stress encounters and thus may perhaps be involved in acclimation to environmental stresses.     

CDX2 can be regulated through the signalling pathways activated by IL-6 in gastric cells

The inflammatory infiltrate of the gastric mucosa associated with Helicobacter pylori infection increases the presence of the pro-inflammatory cytokine IL-6 that activates both the SHP-2/ERK/MAPK and the JAK/STAT signalling pathways. Furthermore, the ectopic expression of CDX2 is detected in pre-neoplasic lesions associated with decreased levels of SOX2, and we found that in gastric adenocarcinomas their expression is inversely correlated. To determine the role of IL-6 in the regulation of CDX2, MKN45 that constitutively expresses p-STAT3, and NUGC-4 gastric cancer cell lines were treated with IL-6, which induced the CDX2 up-regulation and SOX2 down-regulation. ChIP assays determined that in IL-6-treated cells, c-JUN and p-STAT3 bound to CDX2 promoter in MKN45 cells whereas in NUGC-4 cells, p-STAT3 binds to and c-JUN releases from the CDX2 promoter. Specific inhibition of STAT3 and ERK1/2 phosphorylation through AG490 and U0126, respectively, and STAT3 down-regulation using shRNA verified that the SHP-2/ERK/MAPK pathway regulates the expression of CDX2 in basal conditions, and the CDX2 up-regulation by IL-6 is through the JAK/STAT pathway in NUGC-4 cells whereas in MKN45 cells both pathways contribute to the CDX2 up-regulation. In conclusion, the signalling pathways activated by IL-6 have a crucial role in the regulation of CDX2 that is a key factor in the process of gastric carcinogenesis, suggesting that the inflammatory infiltrate in the gastric mucosa is relevant in this process and a potential target for new therapeutic approaches.     

Modulation of calcium signalling by mitochondria

In this review we will attempt to summarise the complex and sometimes contradictory effects that mitochondria have on different forms of calcium signalling. Mitochondria can influence Ca²⁺ signalling indirectly by changing the concentration of ATP, NAD(P)H, pyruvate and reactive oxygen species — which in turn modulate components of the Ca²⁺ signalling machinery i.e. buffering, release from internal stores, influx from the extracellular solution, uptake into cellular organelles and extrusion by plasma membrane Ca²⁺ pumps. Mitochondria can directly influence the calcium concentration in the cytosol of the cell by importing Ca²⁺ via the mitochondrial Ca²⁺ uniporter or transporting Ca²⁺ from the interior of the organelle into the cytosol by means of Na⁺/Ca²⁺ or H⁺/Ca²⁺ exchangers. Considerable progress in understanding the relationship between Ca²⁺ signalling cascades and mitochondrial physiology has been accumulated over the last few years due to the development of more advanced optical techniques and electrophysiological approaches.     

Mitochondria regulate autophagy by conserved signalling pathways

Autophagy is a conserved degradative process that is crucial for cellular homeostasis and cellular quality control via the selective removal of subcellular structures such as mitochondria. We demonstrate that a regulatory link exists between mitochondrial function and autophagy in Saccharomyces cerevisiae. During amino-acid starvation, the autophagic response consists of two independent regulatory arms-autophagy gene induction and autophagic flux-and our analysis indicates that mitochondrial respiratory deficiency severely compromises both. We show that the evolutionarily conserved protein kinases Atg1, target of rapamycin kinase complex I, and protein kinase A (PKA) regulate autophagic flux, whereas autophagy gene induction depends solely on PKA. Within this regulatory network, mitochondrial respiratory deficiency suppresses autophagic flux, autophagy gene induction, and recruitment of the Atg1-Atg13 kinase complex to the pre-autophagosomal structure by stimulating PKA activity. Our findings indicate an interrelation of two common risk factors-mitochondrial dysfunction and autophagy inhibition-for ageing, cancerogenesis, and neurodegeneration.     

Nitric oxide-induced mitochondrial fission is regulated by dynamin-related GTPases in neurons

Mitochondria are present as tubular organelles in neuronal projections. Here, we report that mitochondria undergo profound fission in response to nitric oxide (NO) in cortical neurons of primary cultures. Mitochondrial fission by NO occurs long before neurite injury and neuronal cell death. Furthermore, fission is accompanied by ultrastructural damage of mitochondria, autophagy, ATP decline and generation of free radicals. Fission is occasionally asymmetric and can be reversible. Strikingly, mitochondrial fission is also an early event in ischemic stroke in vivo. Mitofusin 1 (Mfn1) or dominant-negative Dynamin related protein 1 (Drp1(K38A)) inhibits mitochondrial fission induced by NO, rotenone and Amyloid-beta peptide. Conversely, overexpression of Drp1 or Fis1 elicits fission and increases neuronal loss. Importantly, NO-induced neuronal cell death was mitigated by Mfn1 and Drp1(K38A). Thus, persistent mitochondrial fission may play a causal role in NO-mediated neurotoxicity.     

Regulation of cardiac hypertrophy by intracellular signalling pathways

The mammalian heart is a dynamic organ that can grow and change to accommodate alterations in its workload. During development and in response to physiological stimuli or pathological insults, the heart undergoes hypertrophic enlargement, which is characterized by an increase in the size of individual cardiac myocytes. Recent findings in genetically modified animal models implicate important intermediate signal-transduction pathways in the coordination of heart growth following physiological and pathological stimulation.     

Manipulation of host signalling pathways by anthrax toxins

Infectious microbes face an unwelcoming environment in their mammalian hosts, which have evolved elaborate multicelluar systems for recognition and elimination of invading pathogens. A common strategy used by pathogenic bacteria to establish infection is to secrete protein factors that block intracellular signalling pathways essential for host defence. Some of these proteins also act as toxins, directly causing pathology associated with disease. Bacillus anthracis, the bacterium that causes anthrax, secretes two plasmid-encoded enzymes, LF (lethal factor) and EF (oedema factor), that are delivered into host cells by a third bacterial protein, PA (protective antigen). The two toxins act on a variety of cell types, disabling the immune system and inevitably killing the host. LF is an extraordinarily selective metalloproteinase that site-specifically cleaves MKKs (mitogen-activated protein kinase kinases). Cleavage of MKKs by LF prevents them from activating their downstream MAPK (mitogen-activated protein kinase) substrates by disrupting a critical docking interaction. Blockade of MAPK signalling functionally impairs cells of both the innate and adaptive immune systems and induces cell death in macrophages. EF is an adenylate cyclase that is activated by calmodulin through a non-canonical mechanism. EF causes sustained and potent activation of host cAMP-dependent signalling pathways, which disables phagocytes. Here I review recent progress in elucidating the mechanisms by which LF and EF influence host signalling and thereby contribute to disease.     

Differential regulation of growth-promoting signalling pathways by E-cadherin

Background

Despite the well-documented association between loss of E-cadherin and carcinogenesis, as well as the link between restoration of its expression and suppression of proliferation in carcinoma cells, the ability of E-cadherin to modulate growth-promoting cell signalling in normal epithelial cells is less well understood and frequently contradictory. The potential for E-cadherin to co-ordinate different proliferation-associated signalling pathways has yet to be fully explored.

Methodology/Principal Findings

Using a normal human urothelial (NHU) cell culture system and following a calcium-switch approach, we demonstrate that the stability of NHU cell-cell contacts differentially regulates the Epidermal Growth Factor Receptor (EGFR)/Extracellular Signal-Regulated Kinase (ERK) and Phosphatidylinositol 3-Kinase (PI3-K)/AKT pathways. We show that stable cell contacts down-modulate the EGFR/ERK pathway, whilst inducing PI3-K/AKT activity, which transiently enhances cell growth at low density. Functional inactivation of E-cadherin interferes with the capacity of NHU cells to form stable calcium-mediated contacts, attenuates E-cadherin-mediated PI3-K/AKT induction and enhances NHU cell proliferation by allowing de-repression of the EGFR/ERK pathway and constitutive activation of β-catenin-TCF signalling.

Conclusions/Significance

Our findings provide evidence that E-cadherin can differentially and concurrently regulate specific growth-related signalling pathways in a context-specific fashion, with direct, functional consequences for cell proliferation and population growth. Our observations not only reveal a novel, complex role for E-cadherin in normal epithelial cell homeostasis and tissue regeneration, but also provide the basis for a more complete understanding of the consequences of E-cadherin loss on malignant transformation.     

The regulation of phospholipase D by inositol phospholipids and small GTPases

Phospholipase D1 and D2 (PLD1, PLD2) both have PX and PH domains in their N-terminal regions with these inositol lipid binding domains playing key roles in regulating PLD activity and localisation. The activity of PLD1 is also regulated by protein kinase C and members of the Rho and Arf families of GTPases. Each of these proteins binds to unique sites; however, there appears to be little in vitro discrimination between individual family members. In agonist-stimulated cells, however, there is specificity, with, for example in RBL-2H3 cells, antigen stimulating the activation of PLD1 by association with Arf6, Rac1 and protein kinase Calpha. PLD2 appears to be less directly regulated by GTPases and rather is primarily controlled through interaction with phosphatidylinositol 4-phosphate 5-kinase that generates the activating phosphatidylinositol 4,5-bisphosphate.     

A novel type of myosin implicated in signalling by rho family GTPases.

A novel widely expressed type of myosin (fifth unconventional myosin from rat: myr 5) from rat tissues, defining a ninth class of myosins, was identified. The predicted amino acid sequence of myr 5 exhibits several features not found previously in myosins. The myosin head domain contains a unique N-terminal extension and an insertion of 120 amino acids at a postulated myosin-actin contact site. Nevertheless, myr 5 is able to bind actin filaments in an ATP-regulated manner. The head domain is followed by four putative light chain binding sites. The tail domain of myr 5 contains a region which coordinates two atoms of zinc followed by a region that stimulates GTP hydrolysis of members of the ras-related rho subfamily of small G-proteins. Myr 5 therefore provides the first direct link between rho GTPases which have been implicated in the regulation of actin organization and the actin cytoskeleton. It is also the first unconventional myosin for which a tail binding partner(s), namely members of the rho family, has been identified.