Binding of neomycin and calcium to phospholipids and other anionic compounds

Phospholipase-dependent signalling during the AvrRpm1- and AvrRpt2-induced disease resistance responses in Arabidopsis thaliana

Bacterial pathogens deliver type III effector proteins into plant cells during infection. On susceptible host plants, type III effectors contribute to virulence, but on resistant hosts they betray the pathogen to the plant's immune system and are functionally termed avirulence (Avr) proteins. Recognition induces a complex suite of cellular and molecular events comprising the plant's inducible defence response. As recognition of type III effector proteins occurs inside host cells, defence responses can be elicited by in planta expression of bacterial type III effectors. We demonstrate that recognition of either of two type III effectors, AvrRpm1 or AvrRpt2 from Pseudomonas syringae, induced biphasic accumulation of phosphatidic acid (PA). The first wave of PA accumulation correlated with disappearance of monophosphatidylinosotol (PIP) and is thus tentatively attributed to activation of a PIP specific phospholipase C (PLC) in concert with diacylglycerol kinase (DAGK) activity. Subsequent activation of phospholipase D (PLD) produced large amounts of PA from structural phospholipids. This later wave of PA accumulation was several orders of magnitude higher than the PLC-dependent first wave. Inhibition of phospholipases blocked the response, and feeding PA directly to leaf tissue caused cell death and defence-gene activation. Inhibitor studies ordered these events relative to other known signalling events during the plant defence response. Influx of extracellular Ca(2+) occurred downstream of PIP-degradation, but upstream of PLD activation. Production of reactive oxygen species occurred downstream of the phospholipases. The data presented indicate that PA is a positive regulator of RPM1- or RPS2-mediated disease resistance signalling, and that the biphasic PA production may be a conserved feature of signalling induced by the coiled-coil nucleotide binding domain leucine-rich repeat class of resistance proteins.

Reorganization of the actin cytoskeleton in the protruding lamellae of human fibroblasts

To investigate the mechanisms of protrusion in vertebrate cells, the primary event in cell motility, human fibroblasts were treated with neomycin, an inhibitor of the phosphatidylinositol cycle, to induce protrusion. Changes in cell motility and the cytoskeleton were examined by video, fluorescence, scanning electron, and confocal microscopy and by cytofluorometry. Protrusion in neomycin-treated human fibroblasts is correlated with a transient overall decrease in F-actin followed by an increase in F-actin at the leading edge of the protruding lamella. In growing lamellae, F-actin is organized in a marginal band at the leading edge. Although actin is present in the lamella behind the leading edge, very little of it is F-actin. Scanning electron microscopy of detergent-extracted cells reveals a band of dense filaments at the leading edge, corresponding to the marginal band of F-actin seen in fluorescently labeled cells, and a sparse population of short, fragmented filaments, in the rest of the lamella. Gelsolin is colocalized with F-actin in the marginal band and is also present in the lamella where F-actin is largely absent. The data support the hypothesis that the protrusion is initiated by the breakdown of cortical actin filaments, possibly mediated by gelsolin, whereas expansion of the protrusion requires de novo polymerization of actin filaments at the leading edge.

Inhibition of in vitro endosomal vesicle fusion activity by aminoglycoside antibiotics

The effects of two aminoglycoside antibiotics, neomycin and Geneticin, on the endocytic pathway were studied using a cell-free assay that reconstitutes endosome-endosome fusion. Both drugs inhibit the rate and extent of endosome fusion in a dose-dependent manner with IC50 values of approximately 45 microM and approximately 1 mM, respectively. Because the IC50 for neomycin falls within the range of affinities reported for its binding to acidic phospholipids, notably phosphatidylinositol 4,5-bisphosphate (PIP2), these data suggest that negatively charged lipids are required for endosome fusion. A role for negatively charged lipids in membrane traffic has been postulated to involve the activity of a PIP2-dependent phospholipase D (PLD) stimulated by the GTP-binding protein ADP-ribosylation factor (ARF). Although neomycin blocks endosome fusion at a stage of the in vitro reaction that is temporally related to steps inhibited by cytosolic ARFs when they bind guanosine-5'-gamma-thiophosphate (GTPgammaS), these inhibitors appear to act in a synergistic manner. This idea is confirmed by the fact that addition of a PIP2-independent PLD does not suppress neomycin inhibition of endosome fusion; moreover, in vitro fusion activity is not affected by the pleckstrin homology domain of phosphoinositide-specific phospholipase C delta1, which binds to acidic phospholipids, particularly PIP2, with high affinity. Thus, although aminoglycoside-sensitive elements of endosome fusion are required at mechanistic stages that are also blocked by GTPgammaS-bound ARF, these effects are unrelated to inhibition of the PIP2-dependent PLD activity stimulated by this GTP-binding protein. These results argue that there are additional mechanistic roles for acidic phospholipids in the endosomal pathway.

Phosphoinositide hydrolysis by phospholipase C modulated by multivalent cations La(3+), Al(3+), neomycin, polyamines, and melittin

Second messenger production from phosphoinositide hydrolysis is regulated by different pathways, such as G-proteins or tyrosine phosphorylation of phosphoinositide phospholipase C (PI-PLC). Another means of altering the activity of PI-PLC is through cation interaction with the phosphoinositide substrate. A variety of organic and inorganic multi-valent cations were examined for their effects on the activity of purified PI-PLC delta. Surprisingly, the cations produced both stimulation and inhibition of PI-PLC catalyzed phosphoinositide hydrolysis, depending on the substrate and the ion to phosphoinositide stoichiometry. These data support the hypothesis that ionic complexes with phosphoinositides may alter their hydrolysis by PI-PLC.

Spontaneous oscillations of cytoplasmic free calcium ion concentration in cultured smooth muscle cells from guinea pig ileum

The cytoplasmic free calcium ion concentration ([Ca2+]i) of cultured guinea pig ileum longitudinal muscle cells loaded with a fluorescent [Ca2+]i indicator, fura-2, was measured by digital ratio imaging microscopy. Spontaneous [Ca2+]i oscillations were observed in 25% to 80% of the cells, which differed with the batches of the cultured cells after 5 to 8 days in culture. The frequency and amplitude of the [Ca2+]i oscillations in each individual cell were usually regular, but heterogeneity between neighboring cells was observed. The spontaneous [Ca2+]i oscillations were also observed even after incubation of the cells under a serum-free condition for 72 hr. Exchange of extracellular solution to Ca(2+)-free solution containing EGTA or BAPTA immediately stopped the [Ca2+]i oscillations. The ratio of the oscillating cells was dependent on the extracellular calcium ion concentration ([Ca2+]o); and heterogeneity in the range of the [Ca2+]o to generate the [Ca2+]i oscillations was observed. An inorganic Ca(2+)-antagonist, LaCl3, immediately suppressed the [Ca2+]i oscillations, but the treatment with verapamil or nicardipine, Ca(2+)-channel blockers, did not have any effect on the [Ca2+]i oscillations. An inhibitor of the intracellular Ca2+ pump, thapsigargin, induced a transient increase in [Ca2+]i and then inhibited the spontaneous [Ca2+]i oscillations. Neomycin, a compound known to inhibit phosphoinositide turnover, inhibited the [Ca2+]i oscillations.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of cellular signals by calpain

Calpain, an inactive proenzyme, translocates from the cytosol to the membrane upon binding calcium, and is activated at the membrane in the presence of calcium and PIP2. Activated calpain is very unstable and presumably used only once. Thus the primary targets of calpain are considered to be membrane or membrane-associated proteins. Activation of protein kinase C (PKC) occurs concomitantly with calpain at the membrane. Calpain hydrolyzes only the active PKC species leading to downregulation. Calpain participates in the transcriptional regulation by controlling the levels of transcription factors, c-Jun and c-Fos. The calpain gene is a TPA-responsive gene and its expression is stimulated by activation of PKC. Modulation of cellular signal transduction by controlling the levels of the component proteins, such as PKC, c-Jun and c-Fos is one of the important physiological roles of calpain.

Drug-phospholipid interactions: role in aminoglycoside nephrotoxicity

Aminoglycoside antibiotics are known to be transported and accumulated within lysosomes of renal proximal tubular cells and to cause proximal tubular cell injury and necrosis. The pathogenesis of aminoglycoside nephrotoxicity is postulated to be related to the capacity of these organic polycations to interact electrostatically with membrane anionic phospholipids and to disrupt membrane structure and function. Aminoglycoside antibiotics have been shown to bind to anionic phospholipids of model membranes and to alter membrane permeability and promote membrane aggregation. In vivo these drugs induce phospholipiduria and a renal cortical phospholipidosis. The latter reflects the accumulation of phospholipid-containing myeloid bodies within the lysosomal compartment consequent to aminoglycoside-induced inhibition of lysosomal phospholipases. The mechanism of drug-induced inhibition of phospholipases has been shown to be secondary to the binding of these cationic drugs to anionic phospholipids. As the lysosomes became progressively distended with myeloid bodies, they become unstable and eventually rupture, which results in the release of acid hydrolases as well as high concentrations of aminoglycosides into the cytoplasm where they interact with and disrupt the function of other membranes and organelles including mitochondria and microsomes. It is postulated that the redistribution of drug from the lysosomal compartment to organellar membranes is the critical event which triggers the irreversible injury cascade. Polyaspartic acid is a polyanionic peptide which when administered in vitro or in vivo forms electrostatic complexes with aminoglycoside antibiotics and prevents these drugs from interacting with anionic phospholipids, from perturbing phospholipid metabolism and from causing cell injury and necrosis.

ADH-dependent phosphoinositide signalling system and prostaglandin E production in the frog urinary bladder

Antidiuretic hormone (ADH) stimulated formation of inositol 1,4,5-trisphosphate (IP3), 1,2-diaclyglycerol (DAG) and an increase of phosphatidylinositol 4,5-biphosphate (PIP2) breakdown in the frog urinary bladder 20 s after addition. ADH also increased the prostaglandin E (PGE) secretion into serosal medium 3.5-fold and the release of arachidonic acid (AA) from 1,2-DAG, which was intensified in the presence of DAG kinase inhibitor R59022. Neomycin sulphate (10(-5) M) from the serosal side blocked ADH-stimulated PIP2 hydrolysis, IP3 production and increased the hydro-osmotic response to ADH. It also inhibited the ADH-stimulated PGE production (55%) and release of AA from 1,2-DAG. This data suggest that PIP2 breakdown is involved in the mechanism of feedback regulation of ADH action and is associated with PGE production via (i) the increase of AA release from PIP2-generated 1,2-DAG and (ii) possible activation of phospholipase A2 by IP3-induced elevation of cytosol Ca2+.

Polyaspartic acid inhibits gentamicin-induced perturbations of phospholipid metabolism

We investigated whether polyaspartic acid (PAA) can inhibit aminoglycoside-induced perturbations of phospholipid metabolism in cultured renal cells of opossum and rabbit and examined the mechanism involved. Cells incubated in medium containing gentamicin (10(-3) M) manifested a time-dependent increase in total phospholipid in association with the appearance of lysosomal myeloid bodies, impaired degradation of phospholipid, and disruption of the phosphatidylinositol (PI) cascade in response to bradykinin stimulation. These alterations of phospholipid metabolism were either completely or almost completely prevented in cells grown in medium containing gentamicin (10(-3) M) and PAA (3 x 10(-4) M, mol wt 11,000) even though PAA did not inhibit the cellular accumulation of gentamicin (40 +/- 1 vs. 42 +/- 1 micrograms/mg protein). In other in vitro studies, we demonstrated that gentamicin depressed the permeability of phosphatidylcholine (PC)/PI liposomes to glycerol and promoted liposomal aggregation. Both effects were blocked by prior addition of PAA. Methylene blue, a cationic dye, was shown to form an electrostatic complex with PAA; gentamicin competitively displaced methylene blue bound to PAA. Our results support the conclusion that the protective effect of PAA is related to its ability to serve as an anionic substrate that electrostatically binds aminoglycoside antibiotics and, thereby, prevents these polycationic drugs from interacting electrostatically with anionic phospholipid of cell membranes.

R 56865 and flunarizine displace Ca2+ from phosphatidylserine monolayers in a stoichiometric manner

The monovalent cationic drugs, R 56865 and flunarizine added to the subphase, displaced Ca2+ from phosphatidylserine (PS) monolayers at pH 5 with EC50 values of 1.2 X 10(-6) and 1.3 X 10(-6) mol/l. The EC50 of flunarizine for the increase in surface pressure due to incorporation of the drug into the PS film was 1.8 X 10(-6) mol/l. The maximal binding of both drugs to PS monolayers was estimated from experiments in which the drug was added to the spreading solution for the monolayer. The maximal molar binding ratio of R 56865 was identical to twice the maximal Ca2+ binding (2 X 0.13 Ca/PS) to the monolayer. The uptake of [14C]R 56865 into PS monolayers could be measured directly and was well described by a Langmuir isotherm in which the EC50 for 45Ca displacement was taken as the concentration for half-maximal binding, and the molar ratio for twice the Ca2+ binding was taken as maximal molar binding ratio. These results show that two molecules of the univalently charged R 56865 displace one molecule of Ca2+. Maximal binding of the cationic drug flunarizine to PS monolayers showed a similar stoichiometry. 45Ca displacement data may thus be useful for predicting monolayer and perhaps also membrane concentrations of the charged species of cationic drugs.

Purification and characterization of bovine heart phosphoinositide-specific phospholipase C: kinetic analysis of the Ca2+ requirement and La3+ inhibition

Bovine heart contains multiple phosphoinositide-specific phospholipase C (PIC) activities separable by ion-exchange chromatography. One PIC activity was purified to apparent homogeneity and migrated as a single band of Mr 85,000 on SDS-PAGE. The purified PIC was characterized with sonicated suspensions of either pure phosphatidylinositol 4,5-bisphosphate (PIP2) or phosphatidylinositol (PI) as substrates. At pH 7, apparent Vmax and Km values were higher for PIP2 than for PI, but the value of Vmax/Km was similar for the two substrates. PIC required Ca2+ for the hydrolysis of either PI or PIP2, and increasing free Ca2+ concentrations from 20 to 300 nM saturated PIC activity. The requirement of Ca2+ for PIC activity and the sensitivity of PIC to Ca2+ concentrations in the physiological range suggested the ion may be a cofactor. The PIC reaction mechanism was determined by two-substrate kinetic analysis; the data fit a model in which PIC contained single sites for Ca2+ and phosphoinositide, and utilized a rapid-equilibrium, random-order ternary mechanism for phosphoinositide hydrolysis. The KCa value for either PI or PIP2 hydrolysis was approximately 30 nM, suggesting resting intracellular free Ca2+ concentrations are sufficient to saturate the Ca2+ site of PIC. La3+ was used as a calcium analogue to modulate PIC activity. Low concentrations of LaCl3 (0.01-0.3 microM) inhibited PIC activity competitively with respect to calcium, consistent with a Ca2+ binding site on the enzyme.

The effect of gentamicin on the biophysical properties of phosphatidic acid liposomes is influenced by the O-C = O group of the lipid

We previously reported that gentamicin binds to liposomes composed of anionic phospholipids and depresses glycerol permeability and raises the activation energy for glycerol permeation in these liposomes. We postulated that these changes in the glycerol permeability and in the activation energy (Ea) for glycerol permeation were due to hydrogen bonding between O-C = O groups in the hydrogen belt and one or more amino groups of gentamicin. To test this hypothesis, we examined the effects of gentamicin on the membrane surface potential, the glycerol permeability coefficient (p), the Ea for glycerol permeation, and the aggregation of liposomes composed of 1:1 phosphatidylcholine (PC) and phosphatidic acid with the acyl chains of phosphatidic acid in either an ester (PA) or an ether (PA*) linkage. Gentamicin depressed the membrane surface electrostatic potential, measured by the partitioning of methylene blue between the bulk solution and the liposomal membrane, to an equivalent degree in PC-PA and PC-PA* liposomes, which indicates that substitution of the ether for the ester linkage did not interfere with the electrostatic interaction between the cationic drug and the negatively charged phosphate head group. Gentamicin caused a temperature-dependent decrease of p and raised Ea for glycerol permeation from 17.7 +/- 0.3 to 21.6 +/- 0.4 kcal/mol in PC-PA liposomes but had little or no effect on these parameters in PC-PA* liposomes. In contrast, gentamicin induced a significantly greater degree of aggregation of PC-PA* liposomes compared to that of PC-PA liposomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Aluminum affects phosphoinositide hydrolysis by phosphoinositidase C

A single phosphoinositidase C (PIC) activity has been purified 500-fold from bovine heart cytosol to a specific activity of 3 mumol phosphatidylinositol (Ptdlns) hydrolyzed/(min.mg protein). The enzyme required Ca2+ for the hydrolysis of polyphosphoinositides or Ptdlns; PIC activity was saturable at micromolar free Ca2+ concentrations with an apparent KCa of 230nM. None of the multivalent cations tested could effectively substitute for Ca2+ in the activation of PIC, although several ions inhibited PIC activity in the presence of Ca2+. AlCl3 had a differential effect on PIC activity, stimulating Ptdlns hydrolysis while inhibiting Ptdlns-(4,5)P2 hydrolysis in a concentration-dependent manner from 1-100 microM. The effect of AlCl3 was attributed to the free ion, Al3+. Complexation of Al3+ with phosphate, citrate, fluoride, or hydroxide could block the stimulatory effect on Ptdlns hydrolysis. Only fluoride or hydroxide could partially reverse the inhibition of Ptdlns-(4,5)P2 hydrolysis by AlCl3.

Pharmacokinetics of aminoglycoside antibiotics in blood, inner-ear fluids and tissues and their relationship to ototoxicity

This review critically evaluates the literature on aminoglycoside pharmacokinetics in order to answer the question how fluid and tissue levels of the drugs relate to the development of ototoxic and nephrotoxic side effects. We will summarize the evidence that: (1) aminoglycosides do not accumulate in inner-ear fluids; (2) aminoglycoside levels in fluids do not correlate with the ototoxic potential of a drug, and (3) selective toxicity cannot be explained by selective tissue penetration of the drugs. We suggest that studies of drug disposition at the cellular level after chronic aminoglycoside treatment be conducted to establish whether a cell-specific uptake contributes to the selective toxicity of the aminoglycoside antibiotics. A sequence of biochemical events that may lead to the development of toxicity at the molecular level is briefly described.

Evidence that neomycin inhibits HSV 1 infection of BHK cells

The effect of neomycin on the Herpes Simplex Virus (HSV) type 1 and 2 infection of baby hamster kidney cells was studied. Neomycin concentrations of 3 mM or more caused a more than 90% inhibition of HSV 1 proliferation, while it had no effect on HSV 2 proliferation, measured as plaque-forming units. Furthermore, neomycin must be present at the time of infection in order to exert full effect, addition 1 hour postinfection was comparable to untreated cells. This indicates that neomycin may specifically interfere with very early stages of HSV 1 infection.