Activation of phospholipase D by osmotic cell swelling

An acute decrease in plasma membrane tension induces macropinocytosis via PLD2 activation

Internalization of macromolecules and membrane into cells through endocytosis is critical for cellular growth, signaling and plasma membrane (PM) tension homeostasis. Although endocytosis is responsive to both biochemical and physical stimuli, how physical cues modulate endocytic pathways is less understood. Contrary to the accumulating discoveries on the effects of increased PM tension on endocytosis, less is known about how a decrease of PM tension impacts on membrane trafficking. Here, we reveal that an acute decrease of PM tension results in phosphatidic acid (PA) production, F-actin and phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P₂]-enriched dorsal membrane ruffling and subsequent macropinocytosis in myoblasts. The PA production induced by decreased PM tension depends on phospholipase D2 (PLD2) activation via PLD2 nanodomain disintegration. Furthermore, the 'decreased PM tension-PLD2-macropinocytosis' pathway is prominent in myotubes, reflecting a potential mechanism of PM tension homeostasis upon intensive muscle stretching and relaxation. Together, we identify a new mechanotransduction pathway that converts an acute decrease in PM tension into PA production and then initiates macropinocytosis via actin and PI(4,5)P₂-mediated processes.

Cell culture: complications due to mechanical release of ATP and activation of purinoceptors

There is abundant evidence that ATP (adenosine 5′-triphosphate) is released from a variety of cultured cells in response to mechanical stimulation. The release mechanism involved appears to be a combination of vesicular exocytosis and connexin and pannexin hemichannels. Purinergic receptors on cultured cells mediate both short-term purinergic signalling of secretion and long-term (trophic) signalling such as proliferation, migration, differentiation and apoptosis. We aim in this review to bring to the attention of non-purinergic researchers using tissue culture that the release of ATP in response to mechanical stress evoked by the unavoidable movement of the cells acting on functional purinergic receptors on the culture cells is likely to complicate the interpretation of their data.

Spatial and temporal integration of signalling networks regulating pollen tube growth

The overall function of a cell is determined by its contingent of active signal transduction cascades interacting on multiple levels with metabolic pathways, cytoskeletal organization, and regulation of gene expression. Much work has been devoted to analysis of individual signalling cascades interacting with unique cellular targets. However, little is known about how cells integrate information across hierarchical signalling networks. Recent work on pollen tube growth indicates that several key signalling cascades respond to changes in cell hydrodynamics and apical volume. Combined with known effects on cytoarchitecture and signalling from other cell systems, hydrodynamics has the potential to integrate and synchronize the function of the broader signalling network in pollen tubes. This review will explore recent work on cell hydrodynamics in a variety of systems including pollen, and discuss hydrodynamic regulation of cell signalling and function including exocytosis and endocytosis, actin cytoskeleton reorganization, cell wall deposition and assembly, phospholipid and inositol polyphosphate signalling, ion flux, small G-proteins, fertilization, and self-incompatibility. The combined data support a newly emerging model of pollen tube growth.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

Swelling-induced taurine transport: relationship with chloride channels, anion-exchangers and other swelling-activated transport pathways

Cells have to regulate their volume in order to survive. Moreover, it is now evident that cell volume per se and the membrane transport processes which regulate it, comprise an important signalling unit. For example, macromolecular synthesis, apoptosis, cell growth and hormone secretion are all influenced by the cellular hydration state. Therefore, a thorough understanding of volume-activated transport processes could lead to new strategies being developed to control the function and growth of both normal and cancerous cells. Cell swelling stimulates the release of ions such as K(+) and Cl(-) together with organic osmolytes, especially the beta-amino acid taurine. Despite being the subject of intense research interest, the nature of the volume-activated taurine efflux pathway is still a matter of controversy. On the one hand it has been suggested that osmosensitive taurine efflux utilizes volume-sensitive anion channels whereas on the other it has been proposed that the band 3 anion-exchanger is a swelling-induced taurine efflux pathway. This article reviews the evidence for and against a role of anion channels and exchangers in osmosensitive taurine transport. Furthermore, the distinct possibility that neither pathway is involved in taurine transport is highlighted. The putative relationship between swelling-induced taurine transport and volume-activated anionic amino acid, alpha-neutral amino acid and K(+) transport is also examined.

Cell swelling-induced signaling for insulin secretion bypasses steps involving G proteins and PLA2 and is N-ethylmaleimide insensitive

BACKGROUND

This study was undertaken to examine putative mechanisms of calcium independent signal transduction pathway of cell swelling-induced insulin secretion.

METHODS

The role of phospholipase A(2), G proteins, and soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (SNARE) in insulin secretion induced by 30% hypotonic medium was studied using isolated rat pancreatic islets.

RESULTS

In contrast to glucose stimulation, osmotically induced insulin secretion from pancreatic islets was not inhibited by 10 micromol/l bromoenol lactone, an iPLA(2) (Ca(2+) independent phospholipase) inhibitor. Similarly, preincubation of islets for 20 hours with 25 microg/ml mycophenolic acid to inhibit GTP synthesis fully abolished glucose-induced insulin secretion but was without effect on hypotonicity stimulated insulin release. Glucose-induced insulin secretion was prevented by preincubation with 20 nmol/l tetanus toxin (TeTx), a metalloprotease inactivating soluble SNARE. Cell swelling-induced insulin secretion was inhibited by TeTx in the presence of calcium ions but not in calcium depleted medium. The presence of N-ethylmaleimide (NEM, 5 mmol/l, another inhibitor of SNARE proteins) in the medium resulted in high basal insulin secretion and lacking response to glucose stimulation. In contrast, high basal insulin secretion from NEM treated islets further increased after hypotonic stimulation.

CONCLUSION

G proteins and iPLA(2) - putative mediators of Ca(2+) independent signaling pathway participate in glucose but not in hypotonicity-induced insulin secretion. Hypotonicity-induced insulin secretion is sensitive to clostridial neurotoxin TeTx but is resistant to NEM.

Life under pressure: hydrostatic pressure in cell growth and function

H(2)O is one of the most essential molecules for cellular life. Cell volume, osmolality and hydrostatic pressure are tightly controlled by multiple signaling cascades and they drive crucial cellular functions ranging from exocytosis and growth to apoptosis. Ion fluxes and cell shape restructuring induce asymmetries in osmotic potential across the plasma membrane and lead to localized hydrodynamic flow. Cells have evolved fascinating strategies to harness the potential of hydrodynamic flow to perform crucial functions. Plants exploit hydrodynamics to drive processes including gas exchange, leaf positioning, nutrient acquisition and growth. This paradigm is extended by recent work that reveals an important role for hydrodynamics in pollen tube growth.

Effects of hyposmotic stress on exocytosis in isolated turbot, Scophthalmus maximus, hepatocytes

The effect of hyposmotic shock on exocytosis was examined in isolated hepatocytes of turbot, a marine flatfish, using the molecular probe FM1-43. Sudden exposure to a reduced osmolality caused an increase in cell exocytic activity related to the osmotic gradient between intra- and extracellular fluids. Cytoskeletal microtubules could contribute to this hyposmotic-induced exocytosis since colchicine inhibited the process. Protein kinase C, phosphatidylinositol-3 kinase, phospholipases A2, C and D could constitute key enzymes in the mechanism since their inhibition by specific agents altered the hyposmotic-induced exocytic activity. Moreover, arachidonic acid and derivates from the 5-lipoxygenase pathway as well as calcium could participate in the process. As regulatory volume decrease (RVD) exhibited by turbot hepatocytes following hyposmotic stimulation involves similar features, a potential role of exocytosis in volume regulation is suggested. In particular, exocytosis could serve RVD by contributing to ATP release since this latter process similarly appeared to be phospholipase D-dependent and related to the osmotic gradient. This study provides the first evidence of a volume-sensitive exocytosis that could aim at volume constancy in a marine teleost fish cell type.

Hyposmolarity evokes norepinephrine efflux from synaptosomes by a depolarization- and Ca2+ -dependent exocytotic mechanism

Osmolarity reduction (20%) elicited 3H-norepinephrine (NE) efflux from rat cortical synaptosomes. The hyposmotic NE release resulted from the following events: (i) a Na+-dependent and La3+-, Gd3+- and ruthenium red-sensitive depolarization; (ii) a cytosolic Ca2+ ([Ca2+]i) rise with contributions from external Ca2+ influx and internal Ca2+ release, probably through the mitochondrial Na+-Ca2+ exchanger; and (iii) activation of a [Ca2+]i-evoked, tetanus toxin (TeTX)-sensitive, PKC-modulated NE efflux mechanism. This sequence was established from results showing a drop in the hyposmotic [Ca2+]i rise by preventing depolarization with La3+, and by the inhibitory effects of Ca2+-free medium (EGTA; 50%), CGP37157 (the mitochondrial Na+-Ca2+ exchanger blocker; 48%), EGTA + CGP37157 or by EGTA-AM (> 95% in both cases). In close correspondence with these effects, NE efflux was 92% decreased by Na+ omission, 75% by La3+, 47% by EGTA, 50% by CGP37157, 90% by EGTA + CGP37157 and 88% by EGTA-AM. PKC influenced the intracellular Ca2+ release and, mainly through this action, modulated NE efflux. TeTX suppressed NE efflux. The K+-stimulated NE release, studied in parallel, was unaffected by Na+ omission, or by La3+, Gd3+ or ruthenium red. It was fully dependent on external Ca2+, insensitive to CGP37157 and abolished by TeTX. These results suggest that the hyposmotic events, although different from the K+-evoked depolarization and [Ca2+]i rise mechanisms, are able to trigger a depolarization-dependent, Ca2+-dependent and TeTX-sensitive mechanism for neurotransmitter release.