Dual activation of phospholipase C-epsilon by Rho and Ras GTPases

Trichoderma atroviride hyphal regeneration and conidiation depend on cell-signaling processes regulated by a microRNA-like RNA

The ability to respond to injury is essential for the survival of an organism and involves analogous mechanisms in animals and plants. Such mechanisms integrate coordinated genetic and metabolic reprogramming events requiring regulation by small RNAs for adequate healing of the wounded area. We have previously reported that the response to injury of the filamentous fungus Trichoderma atroviride involves molecular mechanisms closely resembling those of plants and animals that lead to the formation of new hyphae (regeneration) and the development of asexual reproduction structures (conidiophores). However, the involvement of microRNAs in this process has not been investigated in fungi. In this work, we explore the participation of microRNA-like RNAs (milRNAs) molecules by sequencing messenger and small RNAs during the injury response of the WT strain and RNAi mutants. We found that Dcr2 appears to play an important role in hyphal regeneration and is required to produce the majority of sRNAs in T. atroviride. We also determined that the three main milRNAs produced via Dcr2 are induced during the damage-triggered developmental process. Importantly, elimination of a single milRNA phenocopied the main defects observed in the dcr2 mutant. Our results demonstrate the essential role of milRNAs in hyphal regeneration and asexual development by post-transcriptionally regulating cellular signalling processes involving phosphorylation events. These observations allow us to conclude that fungi, like plants and animals, in response to damage activate fine-tuning regulatory mechanisms.

Serotonin signals through postsynaptic Gαq, Trio RhoGEF, and diacylglycerol to promote Caenorhabditis elegans egg-laying circuit activity and behavior

Activated Gαq signals through phospholipase-Cβ and Trio, a Rho GTPase exchange factor (RhoGEF), but how these distinct effector pathways promote cellular responses to neurotransmitters like serotonin remains poorly understood. We used the egg-laying behavior circuit of Caenorhabditis elegans to determine whether phospholipase-Cβ and Trio mediate serotonin and Gαq signaling through independent or related biochemical pathways. Our genetic rescue experiments suggest that phospholipase-Cβ functions in neurons while Trio Rho GTPase exchange factor functions in both neurons and the postsynaptic vulval muscles. While Gαq, phospholipase-Cβ, and Trio Rho GTPase exchange factor mutants fail to lay eggs in response to serotonin, optogenetic stimulation of the serotonin-releasing HSN neurons restores egg laying only in phospholipase-Cβ mutants. Phospholipase-Cβ mutants showed vulval muscle Ca2+ transients while strong Gαq and Trio Rho GTPase exchange factor mutants had little or no vulval muscle Ca2+ activity. Treatment with phorbol 12-myristate 13-acetate that mimics 1,2-diacylglycerol, a product of PIP2 hydrolysis, rescued egg-laying circuit activity and behavior defects of Gαq signaling mutants, suggesting both phospholipase-C and Rho signaling promote synaptic transmission and egg laying via modulation of 1,2-diacylglycerol levels. 1,2-Diacylglycerol activates effectors including UNC-13; however, we find that phorbol esters, but not serotonin, stimulate egg laying in unc-13 and phospholipase-Cβ mutants. These results support a model where serotonin signaling through Gαq, phospholipase-Cβ, and UNC-13 promotes neurotransmitter release, and that serotonin also signals through Gαq, Trio Rho GTPase exchange factor, and an unidentified, phorbol 12-myristate 13-acetate-responsive effector to promote postsynaptic muscle excitability. Thus, the same neuromodulator serotonin can signal in distinct cells and effector pathways to coordinate activation of a motor behavior circuit.

Optogenetic control of small GTPases reveals RhoA mediates intracellular calcium signaling

Rho/Ras family small GTPases are known to regulate numerous cellular processes, including cytoskeletal reorganization, cell proliferation, and cell differentiation. These processes are also controlled by Ca²⁺, and consequently, cross talk between these signals is considered likely. However, systematic quantitative evaluation has not yet been reported. To fill this gap, we constructed optogenetic tools to control the activity of small GTPases (RhoA, Rac1, Cdc42, Ras, Rap, and Ral) using an improved light-inducible dimer system (iLID). We characterized these optogenetic tools with genetically encoded red fluorescence intensity-based small GTPase biosensors and confirmed these optogenetic tools’ specificities. Using these optogenetic tools, we investigated calcium mobilization immediately after small GTPase activation. Unexpectedly, we found that a transient intracellular calcium elevation was specifically induced by RhoA activation in RPE1 and HeLa cells. RhoA activation also induced transient intracellular calcium elevation in MDCK and HEK293T cells, suggesting that generally RhoA induces calcium signaling. Interestingly, the molecular mechanisms linking RhoA activation to calcium increases were shown to be different among the different cell types: In RPE1 and HeLa cells, RhoA activated phospholipase C epsilon (PLCε) at the plasma membrane, which in turn induced Ca²⁺ release from the endoplasmic reticulum (ER). The RhoA–PLCε axis induced calcium-dependent nuclear factor of activated T cells nuclear translocation, suggesting that it does activate intracellular calcium signaling. Conversely, in MDCK and HEK293T cells, RhoA–ROCK–myosin II axis induced the calcium transients. These data suggest universal coordination of RhoA and calcium signaling in cellular processes, such as cellular contraction and gene expression.

Structure and regulation of phospholipase Cβ and ε at the membrane

Phospholipase C (PLC) β and ε enzymes hydrolyze phosphatidylinositol (PI) lipids in response to direct interactions with heterotrimeric G protein subunits and small GTPases, which are activated downstream of G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). PI hydrolysis generates second messengers that increase the intracellular Ca²⁺ concentration and activate protein kinase C (PKC), thereby regulating numerous physiological processes. PLCβ and PLCε share a highly conserved core required for lipase activity, but use different strategies and structural elements to autoinhibit basal activity, bind membranes, and engage G protein activators. In this review, we discuss recent structural insights into these enzymes and the implications for how they engage membranes alone or in complex with their G protein regulators.

Functional and structural characterization of allosteric activation of phospholipase Cε by Rap1A

Phospholipase Cε (PLCε) is activated downstream of G protein-coupled receptors and receptor tyrosine kinases through direct interactions with small GTPases, including Rap1A and Ras. Although Ras has been reported to allosterically activate the lipase, it is not known whether Rap1A has the same ability or what its molecular mechanism might be. Rap1A activates PLCε in response to the stimulation of β-adrenergic receptors, translocating the complex to the perinuclear membrane. Because the C-terminal Ras association (RA2) domain of PLCε was proposed to the primary binding site for Rap1A, we first confirmed using purified proteins that the RA2 domain is indeed essential for activation by Rap1A. However, we also showed that the PLCε pleckstrin homology (PH) domain and first two EF hands (EF1/2) are required for Rap1A activation and identified hydrophobic residues on the surface of the RA2 domain that are also necessary. Small-angle X-ray scattering showed that Rap1A binding induces and stabilizes discrete conformational states in PLCε variants that can be activated by the GTPase. These data, together with the recent structure of a catalytically active fragment of PLCε, provide the first evidence that Rap1A, and by extension Ras, allosterically activate the lipase by promoting and stabilizing interactions between the RA2 domain and the PLCε core.

Phospholipase C families: Common themes and versatility in physiology and pathology

Phosphoinositide-specific phospholipase Cs (PLCs) are expressed in all mammalian cells and play critical roles in signal transduction. To obtain a comprehensive understanding of these enzymes in physiology and pathology, a detailed structural, biochemical, cell biological and genetic information is required. In this review, we cover all these aspects to summarize current knowledge of the entire superfamily. The families of PLCs have expanded from 13 enzymes to 16 with the identification of the atypical PLCs in the human genome. Recent structural insights highlight the common themes that cover not only the substrate catalysis but also the mechanisms of activation. This involves the release of autoinhibitory interactions that, in the absence of stimulation, maintain classical PLC enzymes in their inactive forms. Studies of individual PLCs provide a rich repertoire of PLC function in different physiologies. Furthermore, the genetic studies discovered numerous mutated and rare variants of PLC enzymes and their link to human disease development, greatly expanding our understanding of their roles in diverse pathologies. Notably, substantial evidence now supports involvement of different PLC isoforms in the development of specific cancer types, immune disorders and neurodegeneration. These advances will stimulate the generation of new drugs that target PLC enzymes, and will therefore open new possibilities for treatment of a number of diseases where current therapies remain ineffective.

Structure of phospholipase Cε reveals an integrated RA1 domain and previously unidentified regulatory elements

Phospholipase Cε (PLCε) generates lipid-derived second messengers at the plasma and perinuclear membranes in the cardiovascular system. It is activated in response to a wide variety of signals, such as those conveyed by Rap1A and Ras, through a mechanism that involves its C-terminal Ras association (RA) domains (RA1 and RA2). However, the complexity and size of PLCε has hindered its structural and functional analysis. Herein, we report the 2.7 Å crystal structure of the minimal fragment of PLCε that retains basal activity. This structure includes the RA1 domain, which forms extensive interactions with other core domains. A conserved amphipathic helix in the autoregulatory X-Y linker of PLCε is also revealed, which we show modulates activity in vitro and in cells. The studies provide the structural framework for the core of this critical cardiovascular enzyme that will allow for a better understanding of its regulation and roles in disease.

Direct observation of conformational dynamics of the PH domain in phospholipases Cϵ and β may contribute to subfamily-specific roles in regulation

Phospholipase C (PLC) enzymes produce second messengers that increase the intracellular Ca2+ concentration and activate protein kinase C (PKC). These enzymes also share a highly conserved arrangement of core domains. However, the contributions of the individual domains to regulation are poorly understood, particularly in isoforms lacking high-resolution information, such as PLCϵ. Here, we used small-angle X-ray scattering (SAXS), EM, and functional assays to gain insights into the molecular architecture of PLCϵ, revealing that its PH domain is conformationally dynamic and essential for activity. We further demonstrate that the PH domain of PLCβ exhibits similar dynamics in solution that are substantially different from its conformation observed in multiple previously reported crystal structures. We propose that this conformational heterogeneity contributes to subfamily-specific differences in activity and regulation by extracellular signals.

Colon cancer cell-derived 12(S)-HETE induces the retraction of cancer-associated fibroblast via MLC2, RHO/ROCK and Ca2+ signalling

Retraction of mesenchymal stromal cells supports the invasion of colorectal cancer cells (CRC) into the adjacent compartment. CRC-secreted 12(S)-HETE enhances the retraction of cancer-associated fibroblasts (CAFs) and therefore, 12(S)-HETE may enforce invasivity of CRC. Understanding the mechanisms of metastatic CRC is crucial for successful intervention. Therefore, we studied pro-invasive contributions of stromal cells in physiologically relevant three-dimensional in vitro assays consisting of CRC spheroids, CAFs, extracellular matrix and endothelial cells, as well as in reductionist models. In order to elucidate how CAFs support CRC invasion, tumour spheroid-induced CAF retraction and free intracellular Ca²⁺ levels were measured and pharmacological- or siRNA-based inhibition of selected signalling cascades was performed. CRC spheroids caused the retraction of CAFs, generating entry gates in the adjacent surrogate stroma. The responsible trigger factor 12(S)-HETE provoked a signal, which was transduced by PLC, IP3, free intracellular Ca²⁺, Ca²⁺-calmodulin-kinase-II, RHO/ROCK and MYLK which led to the activation of myosin light chain 2, and subsequent CAF mobility. RHO activity was observed downstream as well as upstream of Ca²⁺ release. Thus, Ca²⁺ signalling served as central signal amplifier. Treatment with the FDA-approved drugs carbamazepine, cinnarizine, nifedipine and bepridil HCl, which reportedly interfere with cellular calcium availability, inhibited CAF-retraction. The elucidation of signalling pathways and identification of approved inhibitory drugs warrant development of intervention strategies targeting tumour–stroma interaction.

Phospholipase Cε Modulates Rap1 Activity and the Endothelial Barrier

The phosphoinositide-specific phospholipase C, PLCε, is a unique signaling protein with known roles in regulating cardiac myocyte growth, astrocyte inflammatory signaling, and tumor formation. PLCε is also expressed in endothelial cells, however its role in endothelial regulation is not fully established. We show that endothelial cells of multiple origins, including human pulmonary artery (HPAEC), human umbilical vein (HUVEC), and immortalized brain microvascular (hCMEC/D3) endothelial cells, express PLCε. Knockdown of PLCε in arterial endothelial monolayers decreased the effectiveness of the endothelial barrier. Concomitantly, RhoA activity and stress fiber formation were increased. PLCε-deficient arterial endothelial cells also exhibited decreased Rap1-GTP levels, which could be restored by activation of the Rap1 GEF, Epac, to rescue the increase in monolayer leak. Reintroduction of PLCε rescued monolayer leak with both the CDC25 GEF domain and the lipase domain of PLCε required to fully activate Rap1 and to rescue endothelial barrier function. Finally, we demonstrate that the barrier promoting effects PLCε are dependent on Rap1 signaling through the Rap1 effector, KRIT1, which we have previously shown is vital for maintaining endothelial barrier stability. Thus we have described a novel role for PLCε PIP₂ hydrolytic and Rap GEF activities in arterial endothelial cells, where PLCε-dependent activation of Rap1/KRIT1 signaling promotes endothelial barrier stability.

Structure, functional regulation and signaling properties of Rap2B

The Ras family small guanosine 5'-triphosphate (GTP)-binding protein Rap2B is is a member of the Ras oncogene family and a novel target of p53 that regulates the p53-mediated pro-survival function of cells. The Rap2B protein shares ~90% homology with Rap2A, and its sequence is 70% identical to other members of the Rap family such as RaplA and RaplB. As a result, Rap2B has been theorized to have similar signaling effectors to the GTPase-binding protein Rap, which mediates various biological functions, including the regulation of sterile 20/mitogen-activated proteins. Since its identification in the early 1990s, Rap2B has elicited a considerable interest. Numerous studies indicate that Rap2B exerts specific biological functions, including binding and stimulating phospholipase C-ε and interferon-γ. In addition, downregulation of Rap2B affects the growth of melanoma cells. The present review summarizes the possible effectors and biological functions of Rap2B. Increasing evidence clearly supports the association between Rap2B function and tumor development. Therefore, it is conceivable that anticancer drugs targeting Rap2B may be generated as novel therapies against cancer.

Ras Conformational Ensembles, Allostery, and Signaling

Ras proteins are classical members of small GTPases that function as molecular switches by alternating between inactive GDP-bound and active GTP-bound states. Ras activation is regulated by guanine nucleotide exchange factors that catalyze the exchange of GDP by GTP, and inactivation is terminated by GTPase-activating proteins that accelerate the intrinsic GTP hydrolysis rate by orders of magnitude. In this review, we focus on data that have accumulated over the past few years pertaining to the conformational ensembles and the allosteric regulation of Ras proteins and their interpretation from our conformational landscape standpoint. The Ras ensemble embodies all states, including the ligand-bound conformations, the activated (or inactivated) allosteric modulated states, post-translationally modified states, mutational states, transition states, and nonfunctional states serving as a reservoir for emerging functions. The ensemble is shifted by distinct mutational events, cofactors, post-translational modifications, and different membrane compositions. A better understanding of Ras biology can contribute to therapeutic strategies.

Cyclin A2: At the crossroads of cell cycle and cell invasion

Cyclin A2 is an essential regulator of the cell division cycle through the activation of kinases that participate to the regulation of S phase as well as the mitotic entry. However, whereas its degradation by the proteasome in mid mitosis was thought to be essential for mitosis to proceed, recent observations show that a small fraction of cyclin A2 persists beyond metaphase and is degraded by autophagy. Its implication in the control of cytoskeletal dynamics and cell movement has unveiled its role in the modulation of RhoA activity. Since this GTPase is involved in both cell rounding early in mitosis and later, in the formation of the cleavage furrow, this suggests that cyclin A2 is a novel actor in cytokinesis. Taken together, these data point to this cyclin as a potential mediator of cell-niche interactions whose dysregulation could be taken as a hallmark of metastasis.

Thrombin promotes sustained signaling and inflammatory gene expression through the CDC25 and Ras-associating domains of phospholipase Cϵ

Phospholipase C-epsilon (PLCϵ) plays a critical role in G-protein-coupled receptor-mediated inflammation. In addition to its ability to generate the second messengers inositol 1,4,5-trisphosphate and diacylglycerol, PLCϵ, unlike the other phospholipase C family members, is activated in a sustained manner. We hypothesized that the ability of PLCϵ to function as a guanine nucleotide exchange factor (GEF) for Rap1 supports sustained downstream signaling via feedback of Rap1 to the enzyme Ras-associating (RA2) domain. Using gene deletion and adenoviral rescue, we demonstrate that both the GEF (CDC25 homology domain) and RA2 domains of PLCϵ are required for long term protein kinase D (PKD) activation and subsequent induction of inflammatory genes. PLCϵ localization is largely intracellular and its compartmentalization could contribute to its sustained activation. Here we show that localization of PLCϵ to the Golgi is required for activation of PKD in this compartment as well as for subsequent induction of inflammatory genes. These data provide a molecular mechanism by which PLCϵ mediates sustained signaling and by which astrocytes mediate pathophysiological inflammatory responses.

Cyclin A2 modulates EMT via β-catenin and phospholipase C pathways

We have previously demonstrated that Cyclin A2 is involved in cytoskeletal dynamics, epithelial-mesenchymal transition (EMT) and metastasis. This phenotype was potentiated by activated oncogenic H-Ras. However, the mechanisms governing EMT in these cells have not yet been elucidated. Here, we dissected the pathways that are responsible for EMT in cells deficient for Cyclin A2. In Cyclin A2-depleted normal murine mammary gland (NMuMG) cells expressing RasV12, we found that β-catenin was liberated from the cell membrane and cell-cell junctions and underwent nuclear translocation and activation. Components of the canonical wingless (WNT) pathway, including WNT8b, WNT10a, WNT10b, frizzled 1 and 2 and TCF4 were upregulated at the messenger RNA and protein levels following Cyclin A2 depletion. However, suppression of the WNT pathway using the acetyltransferase porcupine inhibitor C59 did not reverse EMT whereas a dominant negative form of TCF4 as well as inhibition of phospholipase C using U73122 were able to do so. This suggests that a WNT-independent mechanism of β-catenin activation via phospholipase C is involved in the EMT induced by Cyclin A2 depletion. Our findings will broaden our knowledge on how Cyclin A2 contributes to EMT and metastasis.

Phospholipase Cδ1 induces E-cadherin expression and suppresses malignancy in colorectal cancer cells

Colorectal cancer (CRC) is one of the most common causes of cancer-related deaths worldwide, and Kirsten rat sarcoma viral oncogene homolog (KRAS) mutations in CRC predict the ineffectiveness of EGF receptor-targeted therapy. Previous transcriptional microarray analysis suggests the association between phospholipase Cδ1 (PLCδ1) expression and KRAS mutation status in CRC. However, both the roles and the regulatory mechanisms of PLCδ1 in CRC are not known. Here, we found that the expression of PLCδ1, one of the most basal PLCs, is down-regulated in CRC specimens compared with normal colon epithelium by immunohistochemistry. Furthermore, we examined the roles of PLCδ1 in CRC cell lines that harbor an activating KRAS mutation. Ectopic expression of PLCδ1 in CRC cells induced the expression of E-cadherin, whereas knockdown of PLCδ1 repressed the expression of E-cadherin. Moreover, the overexpression of PLCδ1 suppressed the expression of several mesenchymal genes and reduced cell motility, invasiveness, and in vivo tumorigenicity of SW620 CRC cells. We also showed that PLCδ1 expression is repressed by the KRAS/mitogen-activated protein kinase kinase (MEK) pathway. Furthermore, PLCδ1 suppressed the phosphorylation of extracellular signal-regulated kinase (ERK)1/2 through E-cadherin induction in CRC cells, suggesting the presence of a negative regulatory loop between KRAS/MEK/ERK signaling and PLCδ1. These data indicate that PLCδ1 has tumor-suppressive functions in CRC through E-cadherin induction and KRAS/MEK/ERK signal attenuation.

Mechanotransduction in C. elegans morphogenesis and tissue function

Mechanobiology is an emerging field that investigates how living cells sense and respond to their physical surroundings. Recent interest in the field has been sparked by the finding that stem cells differentiate along different lineages based on the stiffness of the cell surroundings (Engler et al., 2006), and that metastatic behavior of cancer cells is strongly influenced by the mechanical properties of the surrounding tissue (Kumar and Weaver, 2009). Many questions remain about how cells convert mechanical information, such as viscosity, stiffness of the substrate, or stretch state of the cells, into the biochemical signals that control tissue function. Caenorhabditis elegans researchers are making significant contributions to the understanding of mechanotransduction in vivo. This review summarizes recent insights into the role of mechanical forces in morphogenesis and tissue function. Examples of mechanical regulation across length scales, from the single-celled zygote, to the intercellular coordination that enables cohesive tissue function, to the mechanical influences between tissues, are considered. The power of the C. elegans system as a gene discovery and in vivo quantitative bioimaging platform is enabling an important discoveries in this exciting field.

Filamin and Phospholipase C-ε are required for calcium signaling in the Caenorhabditis elegans Spermatheca

Mechanical properties of the microenvironment are fundamental in orchestrating normal tissue function, disease progression, and organismal development. Studies of mechanotransduction in cultured cells on artificial substrates have revealed underlying principles, but the in vivo roles of mechanotransduction remain unclear. We recently reported that the Caenorhabditis elegans spermatheca—a myoepithelial tube composed of a cell monolayer—may be mechanosensitive. Live imaging with the genetically encoded calcium indicator GCaMP revealed that oocyte-induced stretching of the spermatheca resulted in calcium oscillations and constriction of the tube. FLN-1/filamin, a mechanosensitive cytoskeletal scaffolding protein, is required to correctly trigger the calcium transients. PLC-1/phospholipase C-epsilon and ITR-1/IP₃ receptor are required to produce the calcium transients, and may function downstream of filamin. In addition to providing important insights into the biology of C. elegans, our studies offer a novel and genetically tractable model for studying mechanotransduction in a myoepithelial tissue.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Filamin and phospholipase C-ε are required for calcium signaling in the Caenorhabditis elegans spermatheca

The Caenorhabditis elegans spermatheca is a myoepithelial tube that stores sperm and undergoes cycles of stretching and constriction as oocytes enter, are fertilized, and exit into the uterus. FLN-1/filamin, a stretch-sensitive structural and signaling scaffold, and PLC-1/phospholipase C-ε, an enzyme that generates the second messenger IP₃, are required for embryos to exit normally after fertilization. Using GCaMP, a genetically encoded calcium indicator, we show that entry of an oocyte into the spermatheca initiates a distinctive series of IP₃-dependent calcium oscillations that propagate across the tissue via gap junctions and lead to constriction of the spermatheca. PLC-1 is required for the calcium release mechanism triggered by oocyte entry, and FLN-1 is required for timely initiation of the calcium oscillations. INX-12, a gap junction subunit, coordinates propagation of the calcium transients across the spermatheca. Gain-of-function mutations in ITR-1/IP₃R, an IP₃-dependent calcium channel, and loss-of-function mutations in LFE-2, a negative regulator of IP₃ signaling, increase calcium release and suppress the exit defect in filamin-deficient animals. We further demonstrate that a regulatory cassette consisting of MEL-11/myosin phosphatase and NMY-1/non-muscle myosin is required for coordinated contraction of the spermatheca. In summary, this study answers long-standing questions concerning calcium signaling dynamics in the C. elegans spermatheca and suggests FLN-1 is needed in response to oocyte entry to trigger calcium release and coordinated contraction of the spermathecal tissue.

During organism development and normal physiological function cells sense, integrate, and respond to a variety of cues or signals including biochemical and mechanical stimuli. In this study we used Caenorhabditis elegans, a small transparent worm, to study filamin (FLN-1), a structural protein that may act as a molecular strain gauge. The C. elegans spermatheca is a contractile tube that is stretched during normal function, making it an ideal candidate for study of how cells respond to stretch. Oocytes are ovulated into the spermatheca, fertilized, and then pushed into the uterus by constriction of the spermatheca. The ability of the spermatheca to constrict depends on inositol 1,4,5-triphosphate (IP₃), a signaling molecule produced by the enzyme phospholipase C (PLC-1) that triggers calcium release within cells. In animals with mutated FLN-1 or PLC-1 the spermathecal cells fail to constrict. Using genetic analysis and a calcium-sensitive fluorescent protein, we show that FLN-1 functions with PLC-1 to regulate IP₃ production, calcium release, and contraction of the spermatheca. Filamin may function to sense stretch caused by entering oocytes and to trigger constriction. These findings establish a link between filamin and calcium signaling that may apply to similar signaling pathways in other systems.

Expression of Phosphoinositide-specific phospholipase C enzymes in human osteosarcoma cell lines

The definition of the number and nature of signal transduction pathways networking in the pathogenesis of osteosarcoma raised great interest. Intracellular calcium ions are important second messengers implicated in the control of cell death. The calcium concentration is regulated by signal transduction pathways, including the Phosphoinositides (PI) signaling. Phosphatydil inositol (4,5) bisphosphate (PIP2) is critical for many cellular activities. The levels of PIP2 are regulated by means of Phosphoinositide-specific Phospholipase C (PI-PLC) family of enzymes. We delineated the panel of expression of PI-PLC enzymes in four human osteosarcoma cell lines. In MG-63 cell line, PI-PLC β1, β2, β3, β4, γ1, γ2, δ1, δ3 and ε resulted expressed. In 143B cell line, PI-PLC β1, β2, β3, β4, γ1, γ2, δ1, δ3 and ε were expressed. In SaOS-2 cell line, PI-PLC β1, β3, β4, γ1, γ2, δ1, δ3, ε and η1. In Hs888 cell line, PI-PLC β1, β3, β4, γ1, δ1, δ3, δ4, ε and η1 the administration of U-73122 to cultures briefly modifies the levels of PI-PLC transcripts. The obtained complete expression panel of PI-PLC isoforms will be a useful tool for further functional studies about the role of the PI signal transduction pathway in osteosarcoma.

Phospholipase C epsilon links G protein-coupled receptor activation to inflammatory astrocytic responses

Neuroinflammation plays a major role in the pathophysiology of diseases of the central nervous system, and the role of astroglial cells in this process is increasingly recognized. Thrombin and the lysophospholipids lysophosphatidic acid and sphingosine 1-phosphate (S1P) are generated during injury and can activate G protein-coupled receptors (GPCRs) on astrocytes. We postulated that GPCRs that couple to Ras homolog gene family, member A (RhoA) induce inflammatory gene expression in astrocytes through the small GTPase responsive phospholipase Cε (PLCε). Using primary astrocytes from wild-type and PLCε knockout mice, we demonstrate that 1-h treatment with thrombin or S1P increases cyclooxygenase 2 (COX-2) mRNA levels ∼10-fold and that this requires PLCε. Interleukin-6 and interleukin-1β mRNA levels are also increased in a PLCε-dependent manner. Thrombin, lysophosphatidic acid, and S1P increase COX-2 protein expression through a mechanism involving RhoA, catalytically active PLCε, sustained activation of protein kinase D (PKD), and nuclear translocation of NF-κB. Endogenous ligands that are released from astrocytes in an in vitro wounding assay also induce COX-2 expression through a PLCε- and NF-κB-dependent pathway. Additionally, in vivo stab wound injury activates PKD and induces COX-2 and other inflammatory genes in WT but not in PLCε knockout mouse brain. Thus, PLCε links GPCRs to sustained PKD activation, providing a means for GPCR ligands that couple to RhoA to induce NF-κB signaling and promote neuroinflammation.

Mammalian phospholipase C

Phospholipase C (PLC) converts phosphatidylinositol 4,5-bisphosphate (PIP(2)) to inositol 1,4,5-trisphosphate (IP(3)) and diacylglycerol (DAG). DAG and IP(3) each control diverse cellular processes and are also substrates for synthesis of other important signaling molecules. PLC is thus central to many important interlocking regulatory networks. Mammals express six families of PLCs, each with both unique and overlapping controls over expression and subcellular distribution. Each PLC also responds acutely to its own spectrum of activators that includes heterotrimeric G protein subunits, protein tyrosine kinases, small G proteins, Ca(2+), and phospholipids. Mammalian PLCs are autoinhibited by a region in the catalytic TIM barrel domain that is the target of much of their acute regulation. In combination, the PLCs act as a signaling nexus that integrates numerous signaling inputs, critically governs PIP(2) levels, and regulates production of important second messengers to determine cell behavior over the millisecond to hour timescale.

Fluorescent phosphatidylinositol 4,5-bisphosphate derivatives with modified 6-hydroxy group as novel substrates for phospholipase C

The capacity to monitor spatiotemporal activity of phospholipase C (PLC) isozymes with a PLC-selective sensor would dramatically enhance understanding of the physiological function and disease relevance of these signaling proteins. Previous structural and biochemical studies defined critical roles for several of the functional groups of the endogenous substrate of PLC isozymes, phosphatidylinositol 4,5-bisphosphate (PIP(2)), indicating that these sites cannot be readily modified without compromising interactions with the lipase active site. However, the role of the 6-hydroxy group of PIP(2) for interaction and hydrolysis by PLC has not been explored, possibly due to challenges in synthesizing 6-hydroxy derivatives. Here, we describe an efficient route for the synthesis of novel, fluorescent PIP(2) derivatives modified at the 6-hydroxy group. Two of these derivatives were used in assays of PLC activity in which the fluorescent PIP(2) substrates were separated from their diacylglycerol products and reaction rates quantified by fluorescence. Both PIP(2) analogues effectively function as substrates of PLC-δ1, and the K(M) and V(max) values obtained with one of these are similar to those observed with native PIP(2) substrate. These results indicate that the 6-hydroxy group can be modified to develop functional substrates for PLC isozymes, thereby serving as the foundation for further development of PLC-selective sensors.

Chemokines and the signaling modules regulating integrin affinity

Integrin-mediated adhesion is a general concept referring to a series of adhesive phenomena including tethering–rolling, affinity, valency, and binding stabilization altogether controlling cell avidity (adhesiveness) for the substrate. Arrest chemokines modulate each aspect of integrin activation, although integrin affinity regulation has been recognized as the prominent event in rapid leukocyte arrest induced by chemokines. A variety of inside-out and outside-in signaling mechanisms have been related to the process of integrin-mediated adhesion in different cellular models, but only few of them have been clearly contextualized to rapid integrin affinity modulation by arrest chemokines in primary leukocytes. Complex signaling processes triggered by arrest chemokines and controlling leukocyte integrin activation have been described for ras-related rap and for rho-related small GTPases. We summarize the role of rap and rho small GTPases in the regulation of rapid integrin affinity in primary leukocytes and provide a modular view of these pro-adhesive signaling events. A potential, albeit still speculative, mechanism of rho-mediated regulation of cytoskeletal proteins controlling the last step of integrin activation is also discussed. We also discuss data suggesting a functional integration between the rho- and rap-modules of integrin activation. Finally we examine the universality of signaling mechanisms regulating integrin triggering by arrest chemokines.

The phospholipase C isozymes and their regulation

The physiological effects of many extracellular neurotransmitters, hormones, growth factors, and other stimuli are mediated by receptor-promoted activation of phospholipase C (PLC) and consequential activation of inositol lipid signaling pathways. These signaling responses include the classically described conversion of phosphatidylinositol(4,5)P(2) to the Ca(2+)-mobilizing second messenger inositol(1,4,5)P(3) and the protein kinase C-activating second messenger diacylglycerol as well as alterations in membrane association or activity of many proteins that harbor phosphoinositide binding domains. The 13 mammalian PLCs elaborate a minimal catalytic core typified by PLC-d to confer multiple modes of regulation of lipase activity. PLC-b isozymes are activated by Gaq- and Gbg-subunits of heterotrimeric G proteins, and activation of PLC-g isozymes occurs through phosphorylation promoted by receptor and non-receptor tyrosine kinases. PLC-e and certain members of the PLC-b and PLC-g subclasses of isozymes are activated by direct binding of small G proteins of the Ras, Rho, and Rac subfamilies of GTPases. Recent high resolution three dimensional structures together with biochemical studies have illustrated that the X/Y linker region of the catalytic core mediates autoinhibition of most if not all PLC isozymes. Activation occurs as a consequence of removal of this autoinhibition.

Inhibition of MLC phosphorylation restricts replication of influenza virus--a mechanism of action for anti-influenza agents

Influenza A viruses are a severe threat worldwide, causing large epidemics that kill thousands every year. Prevention of influenza infection is complicated by continuous viral antigenic changes. Newer anti-influenza agents include MEK/ERK and protein kinase C inhibitors; however, the downstream effectors of these pathways have not been determined. In this study, we identified a common mechanism for the inhibitory effects of a significant group of anti-influenza agents. Our studies showed that influenza infection activates a series of signaling pathways that converge to induce myosin light chain (MLC) phosphorylation and remodeling of the actin cytoskeleton. Inhibiting MLC phosphorylation by blocking RhoA/Rho kinase, phospholipase C/protein kinase C, and HRas/Raf/MEK/ERK pathways with the use of genetic or chemical manipulation leads to the inhibition of influenza proliferation. In contrast, the induction of MLC phosphorylation enhances influenza proliferation, as does activation of the HRas/Raf/MEK/ERK signaling pathway. This effect is attenuated by inhibiting MLC phosphorylation. Additionally, in intracellular trafficking studies, we found that the nuclear export of influenza ribonucleoprotein depends on MLC phosphorylation. Our studies provide evidence that modulation of MLC phosphorylation is an underlying mechanism for the inhibitory effects of many anti-influenza compounds.

Phospholipase C isozymes as effectors of Ras superfamily GTPases

The physiological effects of many extracellular stimuli are initiated through receptor-promoted activation of phospholipase C and inositol lipid signaling pathways. The historical view that phospholipase C-promoted signaling primarily occurs through activation of heterotrimeric G proteins or tyrosine kinases has expanded in recent years with the realization that at least three different mammalian phospholipase C isozymes are directly activated by members of the Ras superfamily of GTPases. Thus, Ras, Rap, Rac, and Rho GTPases all specifically regulate certain phospholipase C isozymes, and insight into the physiological significance of these signaling responses is beginning to accrue. High resolution three-dimensional structures of phospholipase C isozymes also are beginning to shed light on their mechanism of activation.