Calcium-dependent threonine phosphorylation of nonmuscle myosin in stimulated RBL-2H3 mast cells

Adenosine triphosphate acts as a paracrine signaling molecule to reduce the motility of T cells

Organization of immune responses requires exchange of information between cells. This is achieved through either direct cell-cell contacts and establishment of temporary synapses or the release of soluble factors, such as cytokines and chemokines. Here we show a novel form of cell-to-cell communication based on adenosine triphosphate (ATP). ATP released by stimulated T cells induces P2X4/P2X7-mediated calcium waves in the neighboring lymphocytes. Our data obtained in lymph node slices suggest that, during T-cell priming, ATP acts as a paracrine messenger to reduce the motility of lymphocytes and that this may be relevant to allow optimal tissue scanning by T cells.

Non-muscle myosin II regulates neuronal actin dynamics by interacting with guanine nucleotide exchange factors

Background

Non-muscle myosin II (NM II) regulates a wide range of cellular functions, including neuronal differentiation, which requires precise spatio-temporal activation of Rho GTPases. The molecular mechanism underlying the NM II-mediated activation of Rho GTPases is poorly understood. The present study explored the possibility that NM II regulates neuronal differentiation, particularly morphological changes in growth cones and the distal axon, through guanine nucleotide exchange factors (GEFs) of the Dbl family.

Principal Findings

NM II colocalized with GEFs, such as βPIX, kalirin and intersectin, in growth cones. Inactivation of NM II by blebbistatin (BBS) led to the increased formation of short and thick filopodial actin structures at the periphery of growth cones. In line with these observations, FRET analysis revealed enhanced Cdc42 activity in BBS-treated growth cones. BBS treatment also induced aberrant targeting of various GEFs to the distal axon where GEFs were seldom observed under physiological conditions. As a result, numerous protrusions and branches were generated on the shaft of the distal axon. The disruption of the NM II–GEF interactions by overexpression of the DH domains of βPIX or Tiam1, or by βPIX depletion with specific siRNAs inhibited growth cone formation and induced slender axons concomitant with multiple branches in cultured hippocampal neurons. Finally, stimulation with nerve growth factor induced transient dissociation of the NM II–GEF complex, which was closely correlated with the kinetics of Cdc42 and Rac1 activation.

Conclusion

Our results suggest that NM II maintains proper morphology of neuronal growth cones and the distal axon by regulating actin dynamics through the GEF–Rho GTPase signaling pathway.

Regulation of the filament structure and assembly of Acanthamoeba myosin II by phosphorylation of serines in the heavy-chain nonhelical tailpiece

Acanthamoeba myosin II (AMII) has two heavy chains ending in a 27-residue nonhelical tailpiece and two pairs of light chains. In a companion article, we show that five, and only five, serine residues can be phosphorylated both in vitro and in vivo: Ser639 in surface loop 2 of the motor domain and serines 1489, 1494, 1499, and 1504 in the nonhelical tailpiece of the heavy chains. In that paper, we show that phosphorylation of Ser639 down-regulates the actin-activated MgATPase activity of AMII and that phosphorylation of the serines in the nonhelical tailpiece has no effect on enzymatic activity. Here we show that bipolar tetrameric, hexameric, and octameric minifilaments of AMII with the nonhelical tailpiece serines either phosphorylated or mutated to glutamate have longer bare zones and more tightly clustered heads than minifilaments of unphosphorylated AMII, irrespective of the phosphorylation state of Ser639. Although antiparallel dimers of phosphorylated and unphosphorylated myosins are indistinguishable, phosphorylation inhibits dimerization and filament assembly. Therefore, the different structures of tetramers, hexamers, and octamers of phosphorylated and unphosphorylated AMII must be caused by differences in the longitudinal stagger of phosphorylated and unphosphorylated bipolar dimers and tetramers. Thus, although the actin-activated MgATPase activity of AMII is regulated by phosphorylation of Ser639 in loop 2 of the motor domain, the structure of AMII minifilaments is regulated by phosphorylation of one or more of four serines in the nonhelical tailpiece of the heavy chain.

The mechanisms of exocytosis in mast cells

Upon activation through high affinity IgE receptors (FcεRI), mast cells (MCs) can release up to 100% of their content of preformed mediators stored in cytoplasmic secretory granules by compound exocytosis. This causes Type I immediate hypersensitivity reactions and, in the case of inappropriate activation by allergens, the symptoms of allergy. Recent work has uncovered a central role of SNARE (Soluble N-ethylmaleimide-Sensitive Factor (NSF) Attachment Protein (SNAP) Receptors) proteins in regulating the numerous membrane fusion events during exocytosis. This has defined a series of new molecular actors in MC exocytosis that participate in the regulation of membrane fusion and the connection of the fusion machinery with early signaling events. The purpose of this chapter is to describe these proteins and provide a brief overview on their mechanism of action.

Conventional myosins - unconventional functions

While the discovery of unconventional myosins raised expectations that their actions were responsible for most aspects of actin-based cell motility, few anticipated the wide range of cellular functions that would remain the purview of conventional two-headed myosins. The three nonsarcomeric, cellular myosins-M2A, M2B and M2C-participate in diverse roles including, but not limited to: neuronal dynamics, axon guidance and synaptic transmission; endothelial cell migration; cell adhesion, polarity, fusion and cytokinesis; vesicle trafficking and viral egress. These three conventional myosins each take on specific, differing functional roles during development and maturity, characteristic of each cell lineage; exact roles depend on the developmental stage of the cell, cellular location, upstream regulatory controls, relative isoform expression, orientation and associated state of the actin cytoscaffolds in which these myosins operate. Here, we discuss the separate yet related roles that characterise the actions of M2A, M2B and M2C in various cell types and show that these conventional myosins are responsible for functions as unconventional as any performed by unconventional myosins.

Nonmuscle myosin II is responsible for maintaining endothelial cell basal tone and stress fiber integrity

Cultured confluent endothelial cells exhibit stable basal isometric tone associated with constitutive myosin II regulatory light chain (RLC) phosphorylation. Thrombin treatment causes a rapid increase in isometric tension concomitant with myosin II RLC phosphorylation, actin polymerization, and stress fiber reorganization while inhibitors of myosin light chain kinase (MLCK) and Rho-kinase prevent these responses. These findings suggest a central role for myosin II in the regulation of endothelial cell tension. The present studies examine the effects of blebbistatin, a specific inhibitor of myosin II activity, on basal tone and thrombin-induced tension development. Although blebbistatin treatment abolished basal tension, this was accompanied by an increase in myosin II RLC phosphorylation. The increase in RLC phosphorylation was Ca(2+) dependent and mediated by MLCK. Similarly, blebbistatin inhibited thrombin-induced tension without interfering with the increase in RLC phosphorylation or in F-actin polymerization. Blebbistatin did prevent myosin II filament incorporation and association with polymerizing or reorganized actin filaments leading to the disappearance of stress fibers. Thus the inhibitory effects of blebbistatin on basal tone and induced tension are consistent with a requirement for myosin II activity to maintain stress fiber integrity.

TRPM7 regulates myosin IIA filament stability and protein localization by heavy chain phosphorylation

Deregulation of myosin II-based contractility contributes to the pathogenesis of human diseases, such as cancer, which underscores the necessity for tight spatial and temporal control of myosin II activity. Recently, we demonstrated that activation of the mammalian alpha-kinase TRPM7 inhibits myosin II-based contractility in a Ca(2+)- and kinase-dependent manner. However, the molecular mechanism is poorly defined. Here, we demonstrate that TRPM7 phosphorylates the COOH-termini of both mouse and human myosin IIA heavy chains--the COOH-terminus being a region that is critical for filament stability. Phosphorylated residues were mapped to Thr1800, Ser1803 and Ser1808. Mutation of these residues to alanine and that to aspartic acid lead to an increase and a decrease, respectively, in myosin IIA incorporation into the actomyosin cytoskeleton and accordingly affect subcellular localization. In conclusion, our data demonstrate that TRPM7 regulates myosin IIA filament stability and localization by phosphorylating a short stretch of amino acids within the alpha-helical tail of the myosin IIA heavy chain.

Myosin II and mechanotransduction: a balancing act

Adherent cells respond to mechanical properties of the surrounding extracellular matrix. Mechanical forces, sensed at specialized cell-matrix adhesion sites, promote actomyosin-based contraction within the cell. By manipulating matrix rigidity and adhesion strength, new roles for actomyosin contractility in the regulation of basic cellular functions, including cell proliferation, migration and stem cell differentiation, have recently been discovered. These investigations demonstrate that a balance of forces between cell adhesion on the outside and myosin II-based contractility on the inside of the cell controls many aspects of cell behavior. Disturbing this balance contributes to the pathogenesis of various human diseases. Therefore, elaborate signaling networks have evolved that modulate myosin II activity to maintain tensional homeostasis. These include signaling pathways that regulate myosin light chain phosphorylation as well as myosin II heavy chain interactions.

Broadening cell selection criteria with micropallet arrays of adherent cells

A host of technologies exists for the separation of living, nonadherent cells, with separation decisions typically based on fluorescence or immunolabeling of cells. Methods to separate adherent cells as well as to broaden the range of possible sorting criteria would be of high value and complementary to existing strategies. Cells were cultured on arrays of releasable pallets. The arrays were screened and individual cell(s)/pallets were released and collected. Conventional fluorescence and immunolabeling of cells were compatible with the pallet arrays, as were separations based on gene expression. By varying the size of the pallet and the number of cells cultured on the array, single cells or clonal colonies of cells were isolated from a heterogeneous population. Since cells remained adherent throughout the isolation process, separations based on morphologic characteristics, for example cell shape, were feasible. Repeated measurements of each cell in an array were performed permitting the selection of cells based on their temporal behavior, e.g. growth rate. The pallet array system provides the flexibility to select and collect adherent cells based on phenotypic and temporal criteria and other characteristics not accessible by alternative methods.

Phosphorylation of nonmuscle myosin heavy chain IIA on Ser1917 is mediated by protein kinase C beta II and coincides with the onset of stimulated degranulation of RBL-2H3 mast cells

Dynamic remodeling of the actinomyosin cytoskeleton is integral to many biological processes. It is regulated, in part, by myosin phosphorylation. Nonmuscle myosin H chain IIA is phosphorylated by protein kinase C (PKC) on Ser(1917). Our aim was to determine the PKC isoform specificity of this phosphorylation event and to evaluate its potential role in regulated secretion. Using an Ab against the phosphorylated form of Ser(1917), we show that this site is not phosphorylated in unstimulated RBL-2H3 mast cells. The physiological stimulus, Ag, or the pharmacological activators, PMA plus A23187, induced Ser(1917) phosphorylation with a time course coincident with the onset of granule mediator secretion. Dephosphorylation at this site occurred as Ag-stimulated secretion declined from its peak, but dephosphorylation was delayed in cells activated with PMA plus A23187. Phosphate incorporation was also enhanced by PMA alone and by inhibition of protein phosphatase 2A. Gö6976, an inhibitor of conventional PKC isoforms, abolished secretion and Ser(1917) phosphorylation with similar dose dependencies consistent with involvement of either PKCalpha or PKCbeta. Phorbol ester-stimulated Ser(1917) phosphorylation was reconstituted in HEK-293 cells (which lack endogenous PKCbeta) by overexpression of both wild-type and constitutively active PKCbetaII but not the corresponding PKCbetaI or PKCalpha constructs. A similar selectivity for PKCbetaII overexpression was also observed in MIN6 insulinoma cells infected with recombinant PKC wild-type adenoviruses. Our results implicate PKC-dependent phosphorylation of myosin H chain IIA in the regulation of secretion in mast cells and suggest that Ser(1917) phosphorylation might be a marker of PKCbetaII activation in diverse cell types.

Real-time measurement of signaling and motility during T cell development in the thymus

Intracellular signals arising from interactions of immature thymocytes with distinct populations of stromal cells in the thymus are central to T cell development. The characteristics of these signals and the mechanisms underlying thymocyte migration between stromal cell compartments have been difficult to identify from static measurements of fixed tissue. Recent advances in two-photon microscopy and the development of three-dimensional models for real-time studies of T cell development have shed light on how single cells navigate the thymus. These studies reveal crosstalk between thymocyte signaling and motility that may integrate the search for potentially rare self-antigens with the requirement for sustained signaling in T cell maturation.

Calcium oscillations regulate thymocyte motility during positive selection in the three-dimensional thymic environment

The three-dimensional thymic microenvironment and calcium signaling pathways are essential for driving positive selection of developing T cells. However, the nature of calcium signals and the diversity of their effects in the thymus are unknown. We describe here a thymic slice preparation for visualizing thymocyte motility and signaling in real time with two-photon microscopy. Naive thymocytes were highly motile at low intracellular calcium concentrations, but during positive selection cells became immobile and showed sustained calcium concentration oscillations. Increased intracellular calcium was necessary and sufficient to arrest thymocyte motility. The calcium dependence of motility acts to prolong thymocyte interactions with antigen-bearing stromal cells, promoting sustained signaling that may enhance the expression of genes underlying positive selection.

A single class II myosin modulates T cell motility and stopping, but not synapse formation

Upon encountering an antigen, motile T cells stop crawling, change morphology and ultimately form an 'immunological synapse'. Although myosin motors are thought to mediate various aspects of this process, the molecules involved and their exact roles are not defined. Here we show that nonmuscle myosin heavy chain IIA, or MyH9, is the only class II myosin expressed in T cells and is associated with the uropod during crawling. MyH9 function is required for maintenance of the uropod and for T cell motility but is dispensable for synapse formation. Phosphorylation of MyH9 in its multimerization domain by T cell receptor-generated signals indicates that inactivation of this motor may be a key step in the 'stop' response during antigen recognition.

Identification of two eukaryote-like serine/threonine kinases encoded by Chlamydia trachomatis serovar L2 and characterization of interacting partners of Pkn1

Genome sequencing of C. trachomatis serovar D revealed the presence of three putative open reading frames (ORFs), CT145 (Pkn1), CT673 (Pkn5), and CT301 (PknD), encoding eukaryote-like serine/threonine kinases (Ser/Thr kinases). Two of these putative kinase genes, CT145 and CT301, were PCR amplified from serovar L2, cloned, and sequenced. Predicted translation products of the ORFs showed the presence of conserved kinase motifs at the N terminus of the proteins. CT145 and CT301 (encoding Pkn1 and PknD, respectively) were expressed in Escherichia coli as GST fusion proteins. In vitro kinase assays with Escherichia coli-derived glutathione S-transferase fusion proteins showed autophosphorylation of Pkn1 and PknD, indicating that they are functional kinases. Gene expression analysis of these kinase genes in Chlamydia by reverse transcriptase PCR indicated expression of these kinases at the early mid phase of the developmental cycle. Immunoprecipitated native chlamydial Pkn1 and PknD proteins also showed autophosphorylation in an in vitro kinase assay. Phosphoamino acid analysis by thin-layer chromatography confirmed that Pkn1 and PknD are phosphorylated on both serine and threonine residues. Interaction of Pkn1 and PknD with each other as well as interaction of Pkn1 with inclusion membrane protein G (IncG) was demonstrated by using a bacterial two-hybrid system. These interactions were further suggested by phosphorylation of the proteins in in vitro kinase assays. This report is the first description of the existence of functional Ser/Thr kinases in Chlamydia. The results of these findings should lead to a better understanding of how Chlamydia interact and interfere with host signaling pathways, since kinases represent potential mediators of the intimate host-pathogen interactions that are essential to the intracellular life cycle of Chlamydia.

Serine/threonine protein kinases synergistically regulate phospholipase D1 and 2 and secretion in RBL-2H3 mast cells

The role of phospholipase (PL) D in secretion was examined in RBL-2H3 mast cells which contain both PLD1 and 2. The effects of pharmacologic stimulants and inhibitors of Ca(2+)/calmodulin-dependent kinase II, protein kinase C, and protein kinase A suggested that all three kinases synergistically stimulate PLD and, when associated with a calcium signal, secretion as well to indicate a possible linkage between these two events. Overexpression of either PLD1 or 2 markedly enhanced the activation of PLD by pharmacologic stimulants as well as antigen and both isoforms thus appear co-ordinately regulated. As the expressed PLD1 was associated with secretory granules and PLD2 with the plasma membrane, the two isoforms may serve distinct but complementary functions in secretion.

Protein phosphatases 1 and 2A transiently associate with myosin during the peak rate of secretion from mast cells

Mast cells undergo cytoskeletal restructuring to allow secretory granules passage through the cortical actomyosin barrier to fuse with the plasma membrane and release inflammatory mediators. Protein phosphorylation is believed to regulate these rearrangements. Although some of the protein kinases implicated in this phosphorylation are known, the relevant protein phosphatases are not. At the peak rate of antigen-induced granule mediator release (2.5 min), protein phosphatases PP1 and PP2A, along with actin and myosin II, are transiently relocated to ruffles on the apical surface and a band at the peripheral edge of the cell. This leaves an area between the nucleus and the peripheral edge significantly depleted (3-5-fold) in these proteins. Phorbol 12-myristate 13-acetate (PMA) plus A23187 induces the same changes, at a time coincident with its slower rate of secretion. Coimmunoprecipitation experiments demonstrated a significantly increased association of myosin with PP1 and PP2A at the time of peak mediator release, with levels of association decreasing by 5 min. Jasplakinolide, an inhibitor of actin assembly, inhibits secretion and the cytoskeletal rearrangements. Surprisingly, jasplakinolide also affects myosin, inducing the formation of short rods throughout the cytoplasm. Inhibition of PP2A inhibited secretion, the cytoskeletal rearrangements, and led to increased phosphorylation of the myosin heavy and light chains at protein kinase C-specific sites. These findings indicate that a dynamic actomyosin cytoskeleton, partially regulated by both PP1 and PP2A, is required for mast cell secretion.