Maitotoxin stimulates phosphoinositide breakdown in neuroblastoma hybrid NCB-20 cells

Intestinal microbiota: A promising therapeutic target for hypertension

Hypertension has developed into an escalating serious global public health problem with multiple and unclear pathophysiological mechanisms. Recent studies have identified intestinal microbiota as a key perpetrator of hypertension through a variety of mechanisms. In this review, we highlight the potential roles of the intestinal microbiota and its metabolites in the development of hypertension, as well as the therapeutic potential for targeting intestinal microbiomes. We also shed light on the main limitations and challenges of the current research and suggest directions for future investigations. Finally, we discuss the development of accurate and personalized preventive and therapeutic strategies for hypotension by the modulation of intestinal microbes and metabolites.

Maitotoxin induces calpain but not caspase-3 activation and necrotic cell death in primary septo-hippocampal cultures

Maitotoxin is a potent toxin that activates voltage and receptor-mediated Ca2+ channels, resulting in Ca2+ overload and rapid cell death. We report that maitotoxin-induced cell death is associated with activation of calpain but not caspase-3 proteases in septo-hippocampal cell cultures. Calpain and caspase-3 activation were examined by accumulation of protease-specific breakdown products to alpha-spectrin. Cell death manifested exclusively necrotic-like characteristics including round, shrunken nuclei, even distribution of chromatin, absence of DNA fragmentation and failure of protein synthesis inhibition to reduce cell death. Necrotic cell death was observed in neurons and astroglia. Calpain inhibitor II inhibited calpain-specific processing of alpha-spectrin and significantly reduced cell death. The pan-caspase inhibitor, Z-D-DCB, nominally attenuated cell death. Results suggest that: (1) calpain, but not caspase-3, is activated as a result of maitotoxin-induced Ca2+ influx; (2) necrotic cell death caused by maitotoxin exposure is partially mediated by calpain activation; (3) maitotoxin is a useful tool to investigate pathological mechanisms of necrosis.

Maitotoxin-induced intracellular calcium rise in PC12 cells: involvement of dihydropyridine-sensitive and omega-conotoxin-sensitive calcium channels and phosphoinositide breakdown

The biological activities of maitotoxin are strictly dependent on the extracellular calcium concentration and are always associated with an increase of the free cytosolic calcium level. We tested the effects of voltage-sensitive calcium channel blockers (nicardipine and omega-conotoxin) on maitotoxin-induced intracellular calcium increase, membrane depolarization, and inositol phosphate production in PC12 cells. Maitotoxin dose dependently increased the cytosolic calcium level, as measured by the fluorescent probe fura 2. This effect disappeared in a calcium-free medium; it was still observed in the absence of extracellular sodium and was enhanced by the dihydropyridine calcium agonist Bay K 8644. Nicardipine inhibited the effect of maitotoxin on intracellular calcium concentration in a dose-dependent manner. The maitotoxin-induced calcium rise was also reduced by pretreating cells with omega-conotoxin. Pretreatment of cells with maitotoxin did not modify 125I-omega-conotoxin and [3H]PN 200-110 binding to PC12 membranes. Nicardipine and omega-conotoxin inhibition of maitotoxin-evoked calcium increase was reduced by pertussis toxin pretreatment. Maitotoxin caused a substantial membrane depolarization of PC12 cells as assessed by the fluorescent dye bisoxonol. This effect was reduced by pretreating the cells with either nicardipine or omega-conotoxin and was almost completely abolished by the simultaneous pretreatment with both calcium antagonists. Maitotoxin stimulated inositol phosphate production in a dose-dependent manner. This effect was reduced by pretreating the cells with 1 microM nicardipine and was completely abolished in a calcium-free EGTA-containing medium. The findings on maitotoxin-induced cytosolic calcium rise and membrane depolarization suggest that maitotoxin exerts its action primarily through the activation of voltage-sensitive calcium channels, the increase of inositol phosphate production likely being an effect dependent on calcium influx. The ability of nicardipine and omega-conotoxin to inhibit the effect of maitotoxin on both calcium homeostasis and membrane potential suggests that L- and N-type calcium channel activation is responsible for the influx of calcium following exposure to maitotoxin, and not that a depolarization of unknown nature causes the opening of calcium channels.

Effect of maitotoxin analogues on calcium influx and phosphoinositide breakdown in cultured cells

Maitotoxin (MTX) and the analogues, bis-desulfated-MTX (didesulfo-MTX), mono-desulfated-MTX (monodesulfo-MTX), and hydrogenated-MTX (H-MTX) were examined on 45Ca2+ influx and phosphoinositide breakdown with hamster insulinoma HIT cells and rat glioma C6 cells. The activity of MTX was greatly reduced either by desulfation or by hydrogenation. Didesulfo-MTX weakly stimulated calcium influx in HIT cells, but had no stimulatory effect on either calcium influx or phosphoinositide breakdown in C6 cells. All the analogues inhibited MTX-induced calcium influx in either HIT or C6 cells. Didesulfo-MTX inhibited the calcium influx elicited by 3 ng/ml MTX in C6 cells with an IC50 of 7.0 +/- 0.7 ng/ml. The data suggest that the sulfate groups in MTX are important for stimulation of calcium influx and phosphoinositide breakdown, but are not essential for binding to a receptor-site on cell membranes. Although catalytic reduction of double bonds in MTX reduced activity by nearly 100-fold, a tritiated H-MTX still represents a potential radioligand for identification of MTX-binding sites.

Maitotoxin induces phosphoinositide turnover and modulates glutamatergic and muscarinic cholinergic receptor function in cultured cerebellar neurons

Maitotoxin (MTX) stimulated inositol phosphate (IP) formation in primary cultures of rat cerebellar granule cells. MTX-induced IP production was dependent on extracellular Ca2+ but independent of extracellular Na+. The stimulation of IP formation elicited by MTX was unaffected by pretreatment of cells with phorbol dibutyrate, pertussis toxin, and a variety of Ca2+ entry blockers, such as nimodipine, nisoldipine, Co2+, and Mn2+. The presence of MTX markedly attenuated IP production induced by carbachol and glutamate, with no apparent effect on the responses to norepinephrine (NE), histamine, 5-hydroxytryptamine (5-HT), and endothelin-1. The inhibition of the carbachol- and glutamate-induced responses by MTX was dose dependent with IC50 values of 1.2 and 0.5 ng/ml, respectively. Pretreatment of cells with a lower concentration of MTX (0.3 ng/ml) also attenuated carbachol- and glutamate-induced IP formation, in a time-dependent manner, with a decrease observed after 30 min prestimulation, but failed to affect NE-, histamine-, 5-HT-, endothelin-1, and sarafotoxin S6b-induced responses. Thus, MTX elicited a marked Ca2(+)-dependent phosphoinositide (PI) turnover in cerebellar granule cells and selectively inhibited carbachol- and glutamate-induced PI hydrolysis. Possible mechanisms underlying these selective modulations are discussed.

Maitotoxin-induced liver cell death involving loss of cell ATP following influx of calcium

Maitotoxin, one of the most potent marine toxins known, produced cell death in cultures of rat hepatocytes with a TD50 of 80 pM at 24 hr. The cell death, as indicated by a dose- and time-dependent leakage of lactate dehydrogenase (LDH), was also associated with the leakage of [14C]adenine nucleotides from hepatocytes prelabeled with [14C]-adenine. The toxic effect of maitotoxin was completely abolished by the omission of calcium from the culture medium. The cell death induced by maitotoxin increased with increasing concentrations of calcium in the medium. Treatment of hepatocytes with low concentrations of the toxin (less than 0.5 ng/ml) resulted in increases in 45Ca influx into the cells. At higher concentrations of maitotoxin (greater than 1ng/ml), the initial increase in 45Ca influx was followed by the release of the 45Ca from the cells into the medium. Since the 45Ca release paralleled the LDH leakage, the release of calcium was due to cell death. The 45Ca influx, [14C]adenine nucleotide leakage, and LDH leakage were effectively inhibited by verapamil, a calcium channel blocker. Maitotoxin also induced a time- and dose-dependent loss of ATP from hepatocytes, which preceded the [14C]adenine nucleotide and LDH leakage. Thus, it appears that the cell death resulting from maitotoxin treatment is caused by the elevated intracellular calcium, which in turn inhibits mitochondrial oxidative phosphorylation causing depletion of cell ATP. Loss of cell ATP may be the causative event in the maitotoxin-induced cell death.

Maitotoxin, a potent, general activator of phosphoinositide breakdown

Maitotoxin (MTX), a potent marine toxin, elicits a calcium-dependent activation of cells that can be inhibited by calcium channel blockers like nifedipine. MTX also stimulates phosphoinositide breakdown in smooth muscle cells, NCB-20 cells and PC12 cells through a nifedipine-insensitive mechanism. We now report that MTX stimulates phosphoinositide breakdown in a wide variety of cells, and appears to represent the first general activator of this second messenger-generating system. MTX-induced stimulation of phosphoinositide breakdown is dependent in every cell line on the presence of extracellular calcium. In differentiated HL60 cells, in which a chemotactic peptide (fMLP) activates phosphoinositide breakdown via a pertussis toxin-sensitive mechanism, MTX-induced stimulation is not affected by pertussis toxin treatment. A phorbol ester has no effect on the response to MTX. Thus, MTX stimulates phosphoinositide breakdown through a calcium-dependent mechanism that at least in three cell lines (PC12, NCB20 and HL60) is not mediated by a pathway that involves a pertussis toxin-sensitive guanine nucleotide-binding protein.

Histopathological studies on experimental marine toxin poisoning--4. Pathogenesis of experimental maitotoxin poisoning

Repeated injections of 45 ng/kg of maitotoxin into the peritoneal cavities of male ICR mice resulted in marked atrophy of lymphoid tissues, a reduction of lymphocytes in the circulating blood, reduced immunoglobulin M in serum, and an increase of calcium content in the adrenal glands. A single injection of 200 ng/kg of maitotoxin induced a marked increase in total calcium content of the adrenal glands as well as in plasma cortisol concentration (about seven times control) within 1 hr. In contrast, mice pretreated with CoCl2, a calcium channel inhibitor, and/or adrenalectomized mice, showed no discernible changes in the lymphoid tissues after repeated injections of maitotoxin. It is thus suggested that maitotoxin first stimulates calcium influx in the adrenal glands, which then causes the release of cortisol into the blood. The excess amount of cortisol in serum produces acute involution of the thymus and other lymphoid tissues.

Differential effects of maitotoxin on ATP secretion and on phosphoinositide breakdown in rat pheochromocytoma cells

Maitotoxin (MTX) induced exocytotic secretion of ATP from PC12 rat pheochromocytoma cells. The threshold for stimulation of secretion was at concentrations of about 2 ng/ml of MTX. Maximal release occurred at 40 ng/ml. MTX-induced ATP release required the presence of calcium in the extracellular medium and could be inhibited by nifedipine, a specific blocker of voltage-dependent calcium channels. In addition to the effects on ATP secretion from PC12 cells, MTX stimulated the breakdown of phosphoinositides, as measured by the accumulation of [3H]inositol phosphates. Maximal stimulation of phosphoinositide breakdown was reached at only 0.5-1.0 ng/ml MTX. MTX at concentrations required to evoke ATP release (greater than 2 ng/ml) had lesser or no effect on phosphoinositide breakdown. Although stimulation of phosphoinositide breakdown by MTX was dependent on extracellular calcium, it was insensitive to the calcium channel blockers nifedipine, D-600 and cobalt ions. The different concentration range required to elicit these responses and the varying sensitivity to calcium channel blockers indicate that MTX-evoked secretion and MTX-stimulated phosphoinositide breakdown are independent phenomena in PC12 cells.