PIP5KIγ90-generated phosphatidylinositol-4,5-bisphosphate promotes the uptake of Staphylococcus aureus by host cells

Staphylococcus aureus, a Gram-positive pathogen, invades cells mainly in an integrin-dependent manner. As the activity or conformation of several integrin-associated proteins can be regulated by phosphatidylinositol-4,5-bisphosphate (PI-4,5-P₂ ), we investigated the roles of PI-4,5-P₂ and PI-4,5-P₂ -producing enzymes in cellular invasion by S. aureus. PI-4,5-P₂ accumulated upon contact of S. aureus with the host cell, and targeting of an active PI-4,5-P₂ phosphatase to the plasma membrane reduced bacterial invasion. Knockdown of individual phosphatidylinositol-4-phosphate 5-kinases revealed that phosphatidylinositol-4-phosphate 5-kinase γ (PIP5KIγ) plays an important role in bacterial internalization. Specific ablation of the talin and FAK-binding motif in PIP5KIγ90 reduced bacterial invasion, which could be rescued by reexpression of an active, but not inactive PIP5KIγ90. Furthermore, PIP5KIγ90-deficient cells showed normal basal PI-4,5-P₂ levels in the plasma membrane but reduced the accumulation of PI-4,5-P₂ and talin at sites of S. aureus attachment and overall lower levels of FAK phosphorylation. These results highlight the importance of local synthesis of PI-4,5-P₂ by a focal adhesion-associated lipid kinase for integrin-mediated internalization of S. aureus.

Histological comparison of arterial thrombi in mice and men and the influence of Cl-amidine on thrombus formation

Aims

Medical treatment of arterial thrombosis is mainly directed against platelets and coagulation factors, and can lead to bleeding complications. Novel antithrombotic therapies targeting immune cells and neutrophil extracellular traps (NETs) are currently being investigated in animals. We addressed whether immune cell composition of arterial thrombi induced in mouse models of thrombosis resemble those of human patients with acute myocardial infarction (AMI).

Methods and results

In a prospective cohort study of patients suffering from AMI, 81 human arterial thrombi were harvested during percutaneous coronary intervention and subjected to detailed histological analysis. In mice, arterial thrombi were induced using two distinct experimental models, ferric chloride (FeCl₃) and wire injury of the carotid artery. We found that murine arterial thrombi induced by FeCl₃ were highly concordant with human coronary thrombi regarding their immune cell composition, with neutrophils being the most abundant cell type, as well as the presence of NETs and coagulation factors. Pharmacological treatment of mice with the protein arginine deiminase (PAD)-inhibitor Cl-amidine abrogated NET formation, reduced arterial thrombosis and limited injury in a model of myocardial infarction.

Conclusions

Neutrophils are a hallmark of arterial thrombi in patients suffering from acute myocardial infarction and in mouse models of arterial thrombosis. Inhibition of PAD could represent an interesting strategy for the treatment of arterial thrombosis to reduce neutrophil-associated tissue damage and improve functional outcome.

Sphingosine 1-Phosphate Produced by Sphingosine Kinase 2 Intrinsically Controls Platelet Aggregation In Vitro and In Vivo

RATIONALE

Platelets are known to play a crucial role in hemostasis. Sphingosine kinases (Sphk) 1 and 2 catalyze the conversion of sphingosine to the bioactive metabolite sphingosine 1-phosphate (S1P). Although platelets are able to secrete S1P on activation, little is known about a potential intrinsic effect of S1P on platelet function.

OBJECTIVE

To investigate the role of Sphk1- and Sphk2-derived S1P in the regulation of platelet function.

METHODS AND RESULTS

We found a 100-fold reduction in intracellular S1P levels in platelets derived from Sphk2(-/-) mutants compared with Sphk1(-/-) or wild-type mice, as analyzed by mass spectrometry. Sphk2(-/-) platelets also failed to secrete S1P on stimulation. Blood from Sphk2-deficient mice showed decreased aggregation after protease-activated receptor 4-peptide and adenosine diphosphate stimulation in vitro, as assessed by whole blood impedance aggregometry. We revealed that S1P controls platelet aggregation via the sphingosine 1-phosphate receptor 1 through modulation of protease-activated receptor 4-peptide and adenosine diphosphate-induced platelet activation. Finally, we show by intravital microscopy that defective platelet aggregation in Sphk2-deficient mice translates into reduced arterial thrombus stability in vivo.

CONCLUSIONS

We demonstrate that Sphk2 is the major Sphk isoform responsible for the generation of S1P in platelets and plays a pivotal intrinsic role in the control of platelet activation. Correspondingly, Sphk2-deficient mice are protected from arterial thrombosis after vascular injury, but have normal bleeding times. Targeting this pathway could therefore present a new therapeutic strategy to prevent thrombosis.

Critical role of platelet glycoprotein ibα in arterial remodeling

OBJECTIVE

Arteriogenesis is strongly dependent on the recruitment of leukocytes, especially monocytes, into the perivascular space of growing collateral vessels. On the basis of previous findings that platelets are central players in inflammatory processes and mediate the recruitment of leukocytes, the aim of this study was to assess the role of platelets in a model of arterial remodeling.

APPROACH AND RESULTS

C57Bl6 wild-type mice, IL4-R/Iba mice lacking the extracellular domain of the glycoprotein Ibα (GPIbα) receptor, and mice treated with antibodies to block GPIbα or deplete circulating platelets were studied in peripheral arteriogenesis. Using a novel model of intravital 2-photon and epifluorescence imaging, we visualized and quantified the interaction of platelets with leukocytes and the vascular endothelium in vivo. We found that transient platelet adhesion to the endothelium of collateral vessels was a major event during arteriogenesis and depended on GPIbα. Furthermore, leukocyte recruitment was obviously affected in animals with defective platelet GPIbα function. In IL4-R/Iba mice, transient and firm leukocyte adhesion to the endothelium of collateral vessels, as well as leukocyte accumulation in the perivascular space, were significantly reduced. Furthermore, we detected platelet-leukocyte aggregates within the circulation, which were significantly reduced in IL4-R/Iba animals. Finally, platelet depletion and loss of GPIbα function resulted in poor reperfusion recovery as determined by laser Doppler imaging.

CONCLUSIONS

Thus, GPIbα-mediated interactions between platelets and endothelial cells, as well as leukocytes, support innate immune cell recruitment and promote arteriogenesis-establishing platelets as critical players in this process.

The integrin adhesome: from genes and proteins to human disease

The adhesive interactions of cells with their environment through the integrin family of transmembrane receptors have key roles in regulating multiple aspects of cellular physiology, including cell proliferation, viability, differentiation and migration. Consequently, failure to establish functional cell adhesions, and thus the assembly of associated cytoplasmic scaffolding and signalling networks, can have severe pathological effects. The roles of specific constituents of integrin-mediated adhesions, which are collectively known as the 'integrin adhesome', in diverse pathological states are becoming clear. Indeed, the prominence of mutations in specific adhesome molecules in various human diseases is now appreciated, and experimental as well as in silico approaches provide insights into the molecular mechanisms underlying these pathological conditions.

Induction of membrane circular dorsal ruffles requires co-signalling of integrin-ILK-complex and EGF receptor

Integrin and receptor tyrosine kinase signalling networks cooperate to regulate various biological functions. The molecular details underlying the integration of both signalling networks remain largely uncharacterized. Here we identify a signalling module composed of a fibronectin-α5β1-integrin-integrin-linked-kinase (ILK) complex that, in concert with epidermal growth factor (EGF) cues, cooperatively controls the formation of transient actin-based circular dorsal ruffles (DRs) in fibroblasts. DR formation depends on the precise spatial activation of Src at focal adhesions by integrin and EGF receptor signals, in an ILK-dependent manner. In a SILAC-based phosphoproteomics screen we identified the tumour-suppressor Cyld as being required for DR formation induced by α5β1 integrin and EGF receptor co-signalling. Furthermore, EGF-induced Cyld tyrosine phosphorylation is controlled by integrin-ILK and Src as a prerequisite for DR formation. This study provides evidence for a novel function of integrin-ILK and EGF signalling crosstalk in mediating Cyld tyrosine phosphorylation and fast actin-based cytoskeletal rearrangements.

Comparative phenotypic analysis of the two major splice isoforms of phosphatidylinositol phosphate kinase type Iγ in vivo

Localized production of polyphosphoinositides is critical for their signaling function. To examine the biological relevance of specific pools of phosphatidylinositol 4,5-bisphosphate we compared the consequences of genetically ablating all isoforms of phosphatidylinositol phosphate (PIP) kinase type Iγ (PIPKIγ), encoded by the gene Pip5k1c, versus ablation of a specific splice isoform, PIPKIγ_i2, with respect to three reported PIPKIγ functions. Ablation of PIPKIγ_i2 caused a neuron-specific endocytosis defect similar to that found in PIPKIγ(-/-) mice, while agonist-induced calcium signaling was reduced in PIPKIγ(-/-) cells, but was not affected in the absence of PIPKIγ_i2. A reported contribution of PIPKIγ to epithelial integrity was not evident in PIPKIγ(-/-) mice. Given that mice lacking PIPKIγ_i2 live a normal lifespan whereas PIPKIγ(-/-) mice die shortly after birth, we propose that PIPKIγ-mediated metabotropic calcium signaling may represent an essential function of PIPKIγ, whereas functions specific to the PIPKIγ_i2 splice isoform are not essential for survival.

Integrin adhesion and force coupling are independently regulated by localized PtdIns(4,5)2 synthesis

The 90-kDa isoform of the lipid kinase PIP kinase Type I γ (PIPKIγ) localizes to focal adhesions (FAs), where it provides a local source of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)). Although PtdIns(4,5)P(2) regulates the function of several FA-associated molecules, the role of the FA-specific pool of PtdIns(4,5)P(2) is not known. We report that the genetic ablation of PIPKIγ specifically from FAs results in defective integrin-mediated adhesion and force coupling. Adhesion defects in cells deficient in FAPtdIns(4,5)P(2) synthesis are corrected within minutes while integrin-actin force coupling remains defective over a longer period. Talin and vinculin, but not kindlin, are less efficiently recruited to new adhesions in these cells. These data demonstrate that the specific depletion of PtdIns(4,5)P(2) from FAs temporally separates integrin-ligand binding from integrin-actin force coupling by regulating talin and vinculin recruitment. Furthermore, it suggests that force coupling relies heavily on locally generated PtdIns(4,5)P(2) rather than bulk membrane PtdIns(4,5)P(2).

PIPKI gamma 90 negatively regulates LFA-1-mediated adhesion and activation in antigen-induced CD4+ T cells

T cell activation requires the formation and maintenance of stable interactions between T cells and APCs. The formation of stable T cell-APC contacts depends on the activation of the integrin LFA-1 (CD11aCD18). Several positive regulators of LFA-1 activation downstream of proximal TCR signaling have been identified, including talin; however, negative regulators of LFA-1 activity remain largely unexplored. Extended isoform of phosphatidylinositol phosphate kinase type I γ (PIPKIγ90) is a member of the type I phosphatidylinositol phosphate kinase family that has been shown previously to modulate talin activation of integrins through production of phosphatidylinositol 4,5-bisphosphate and direct binding to talin. In this study, we show that PIPKIγ90 negatively regulates LFA-1-mediated adhesion and activation of T cells. Using CD4(+) T cells from PIPKIγ90-deficient mice, we show that CD4(+) T cells exhibit increased LFA-1-dependent adhesion to ICAM-1 and increased rates of T cell-APC conjugate formation with enhanced LFA-1 polarization at the synapse. In addition to increased adhesiveness, PIPKIγ90-deficient T cells exhibit increased proliferation both in vitro and in vivo and increased production of IFN-γ and IL-2. Together, these results demonstrate that PIPKIγ90 is a negative regulator of Ag-induced T cell adhesion and activation.

The tail of integrins, talin, and kindlins

Integrins are transmembrane cell-adhesion molecules that carry signals from the outside to the inside of the cell and vice versa. Like other cell surface receptors, integrins signal in response to ligand binding; however, events within the cell can also regulate the affinity of integrins for ligands. This feature is important in physiological situations such as those in blood, in which cells are always in close proximity to their ligands, yet cell-ligand interactions occur only after integrin activation in response to specific external cues. This review focuses on the mechanisms whereby two key proteins, talin and the kindlins, regulate integrin activation by binding the tails of integrin-beta subunits.

Mechanisms that regulate adaptor binding to beta-integrin cytoplasmic tails

Cells recognize and respond to their extracellular environment through transmembrane receptors such as integrins, which physically connect the extracellular matrix to the cytoskeleton. Integrins provide the basis for the assembly of intracellular signaling platforms that link to the cytoskeleton and influence nearly every aspect of cell physiology; however, integrins possess no enzymatic or actin-binding activity of their own and thus rely on adaptor molecules, which bind to the short cytoplasmic tails of integrins, to mediate and regulate these functions. Many adaptors compete for relatively few binding sites on integrin tails, so regulatory mechanisms have evolved to reversibly control the spatial and temporal binding of specific adaptors. This Commentary discusses the adaptor proteins that bind directly to the tails of beta integrins and, using talin, tensin, filamin, 14-3-3 and integrin-linked kinase (ILK) as examples, describes the ways in which their binding is regulated.

What mouse mutants teach us about extracellular matrix function

For many years the extracellular matrix was viewed as a benign scaffold for arranging cells within connective tissues, but it is now being redefined as a dynamic, mobile, and flexible key player in defining cellular behavior. Gene targeting, transgene expression, and spontaneous mutations of extracellular matrix proteins in mice have greatly accelerated our mechanistic view of the structural and instructive functions of the extracellular matrix in developmental and regenerative processes. This review summarizes the phenotypes of genetic mouse models carrying mutations in extracellular matrix proteins, with specific emphasis on recent advances. The application of reverse genetics has demonstrated the multifunctionality of matrix proteins in a biological context and, in addition, has brought a novel perspective to the understanding of human pathologies.

Genetic analysis of beta1 integrin "activation motifs" in mice

Akey feature of integrins is their ability to regulate the affinity for ligands, a process termed integrin activation. The final step in integrin activation is talin binding to the NPXY motif of the integrin beta cytoplasmic domains. Talin binding disrupts the salt bridge between the alpha/beta tails, leading to tail separation and integrin activation. We analyzed mice in which we mutated the tyrosines of the beta1 tail and the membrane-proximal aspartic acid required for the salt bridge. Tyrosine-to-alanine substitutions abolished beta1 integrin functions and led to a beta1 integrin-null phenotype in vivo. Surprisingly, neither the substitution of the tyrosines with phenylalanine nor the aspartic acid with alanine resulted in an obvious defect. These data suggest that the NPXY motifs of the beta1 integrin tail are essential for beta1 integrin function, whereas tyrosine phosphorylation and the membrane-proximal salt bridge between alpha and beta1 tails have no apparent function under physiological conditions in vivo.

ILK, PINCH and parvin: the tIPP of integrin signalling

The ternary complex of integrin-linked kinase (ILK), PINCH and parvin functions as a signalling platform for integrins by interfacing with the actin cytoskeleton and many diverse signalling pathways. All these proteins have synergistic functions at focal adhesions, but recent work has indicated that these proteins might also have separate roles within a cell. They function as regulators of gene transcription or cell-cell adhesion.

Integrin-actin interactions

The integrin family of extracellular matrix receptors regulates many aspects of cell life, in particular cell adhesion and migration. These two processes depend on organization of the actin cytoskeleton into adhesive and protrusive organelles in response to extracellular signals. Integrins are important switch points for the spatiotemporal control of actin-based motility in higher eukaryotes. Ligands of integrin cytoplasmic tails are central elements of signalling pathways involving small GTPases as well as protein and lipid kinases in the regulation of Factin crosslinking, actin treadmilling and de novo nucleation of actin filaments. We present an overview of common pathways and discuss recent evidence for their differential use by individual integrin receptors.

The beta-subunit of the signal recognition particle receptor is a novel GTP-binding protein without intrinsic GTPase activity

The beta-subunit of the signal recognition particle receptor (SRbeta), a member of the Ras family of small molecular weight GTPases, is involved in the targeting of nascent polypeptide chains to the protein translocation machinery in the endoplasmic reticulum membrane. We purified SRbeta from an expressing strain of Escherichia coli and investigated the properties of the isolated GTPase. We find that, unlike other Ras family GTPases, most SRbeta purifies bound to GTP, and SRbeta-bound GTP is not easily exchanged with solution GTP. SRbeta possesses no detectable GTPase activity. Although a stable interaction between SRbeta and ribosomes is observed, SRbeta is not stimulated to hydrolyze GTP when incubated with ribosomes or ribosome-nascent chains. A GTPase mutant harboring a mutation in a region predicted to be functionally important, based on observations made in related GTPases, binds GTP with faster kinetics and appears to be a less stable protein but otherwise displays similar properties to the wild-type SRbeta GTPase. Our results demonstrate that as an isolated GTPase, SRbeta functions differently from the Arf- and Ras-type GTPases that it is most closely related to by sequence.

Assembly strategies and GTPase regulation of the eukaryotic and Escherichia coil translocons

The translocation of most proteins across the endoplasmic reticulum or bacterial inner membrane occurs through an aqueous pore that spans the membrane. Substrates that are translocated co-translationally across the membrane are directed to the translocation pore via an interaction between the cytosolic signal recognition particle and its membrane-bound receptor. Together the translocation pore and the receptor are referred to as a translocon. By studying the biogenesis of the translocon a number of alternate targeting and membrane-integration pathways have been discovered that operate independently of the signal recognition particle (SRP) pathway. The novel assembly strategies of the translocon and the ways in which these components interact to ensure the fidelity and unidirectionality of the targeting and translocation process are reviewed here.

Assembly strategies and GTPase regulation of the eukaryotic and Escherichia coli translocons

The translocation of most proteins across the endoplasmic reticulum or bacterial inner membrane occurs through an aqueous pore that spans the membrane. Substrates that are translocated co-translationally across the membrane are directed to the translocation pore via an interaction between the cytosolic signal recognition particle and its membrane-bound receptor. Together the translocation pore and the receptor are referred to as a translocon. By studying the biogenesis of the translocon a number of alternate targeting and membrane-integration pathways have been discovered that operate independently of the signal recognition particle (SRP) pathway. The novel assembly strategies of the translocon and the ways in which these components interact to ensure the fidelity and unidirectionality of the targeting and translocation process are reviewed here.Key words: protein translocation, translocon, SRP receptor, GTPases.

Nucleotide-dependent binding of the GTPase domain of the signal recognition particle receptor beta-subunit to the alpha-subunit

The signal recognition particle (SRP) receptor (SR) is a heterodimer of two polypeptides (SRalpha and SRbeta) that each contain a GTP-binding domain. The GTP-binding domain in the peripheral membrane SRalpha subunit has a well defined role in regulating targeting of SRP-ribosome-nascent chain complexes to the translocon. The only well established function for the transmembrane SRbeta subunit is anchoring SRalpha on the endoplasmic reticulum membrane. Deletion of the amino-terminal transmembrane domain of SRbeta did not affect receptor dimerization, but revealed a cryptic translocation signal that overlaps the GTPase domain. We demonstrate that the domain of SRalpha that binds SRbeta does so by binding directly to the nucleotide-bound form of the GTPase domain of SRbeta. An SRbeta mutant containing an amino acid substitution that allows the GTPase domain to bind XTP dimerized with SRalpha most efficiently in the presence of XTP or XDP, but not ATP. Our results suggest an additional level of regulation of SRP receptor function based on regulated dissociation of the receptor subunits.

An amino-terminal domain containing hydrophobic and hydrophilic sequences binds the signal recognition particle receptor alpha subunit to the beta subunit on the endoplasmic reticulum membrane

The signal recognition particle receptor consists of two subunits of 72 kDa (SR alpha) and 30 kDa (SR beta). Assembly of SR alpha on the endoplasmic reticulum membrane can occur independent of the signal recognition particle-mediated translocation pathway. To identify the sequences within SR alpha necessary for membrane binding, a series of amino-terminal and internal deletion mutants was constructed and translated in a cell-free system. In addition, nascent SR alpha polypeptides of varying lengths were generated by cycloheximide treatment of translation reactions. Microsome binding assays performed on these polypeptides revealed a membrane binding domain consisting of the amino-terminal 140 residues of SR alpha. This domain includes the two hydrophobic sequences originally proposed to bind to membranes and a highly charged region not previously implicated in membrane assembly. Furthermore, the domain forms a protease-resistant folding unit that after proteolysis can target and anchor onto microsomes. Extraction of microsomal SR alpha at high pH supplemented with 1 M NaSCN suggests that SR alpha and the membrane binding domain are not integrated in the endoplasmic reticulum membrane. The membrane binding domain is also the major site of tight binding with SR beta, suggesting that SR beta plays a role in the membrane assembly of SR alpha.