Acute actions of marine toxin latrunculin A on the electrophysiological properties of cultured dorsal root ganglion neurones

Signaling Enzymes and Ion Channels Being Modulated by the Actin Cytoskeleton at the Plasma Membrane

A cell should deal with the changing external environment or the neighboring cells. Inevitably, the cell surface receives and transduces a number of signals to produce apt responses. Typically, cell surface receptors are activated, and during this process, the subplasmalemmal actin cytoskeleton is often rearranged. An intriguing point is that some signaling enzymes and ion channels are physically associated with the actin cytoskeleton, raising the possibility that the subtle changes of the local actin cytoskeleton can, in turn, modulate the activities of these proteins. In this study, we reviewed the early and new experimental evidence supporting the notion of actin-regulated enzyme and ion channel activities in various cell types including the cells of immune response, neurons, oocytes, hepatocytes, and epithelial cells, with a special emphasis on the Ca²⁺ signaling pathway that depends on the synthesis of inositol 1,4,5-trisphosphate. Some of the features that are commonly found in diverse cells from a wide spectrum of the animal species suggest that fine-tuning of the activities of the enzymes and ion channels by the actin cytoskeleton may be an important strategy to inhibit or enhance the function of these signaling proteins.

CXCL1 activates TRPV1 via Gi/o protein and actin filaments

AIMS

CXCL1 is a chemokine with pleiotropic effects, including pain and itch. Itch, an unpleasant sensation that elicits the desire or reflex to scratch, it is evoked mainly from the skin and implicates activation of a specific subset of IB4+, C-type primary afferents. In previous studies we showed that acute application of CXCL1 induced a Ca²⁺ influx of low amplitude and slow kinetics in a subpopulation of transient receptor potential vanilloid type 1 (TRPV1)+/isolectin B4 (IB4)+dorsal root ganglia neurons which also responded to other itch-inducing agents. In this study we explored the mechanism behind the Ca²⁺ influx to better understand how CXCL1 acts on primary sensitive neurons to induce itch.

MATERIALS AND METHODS

Intracellular Ca²⁺ imaging and patch-clamp recordings on dorsal root ganglia neurons primary cultures and HEK293T cell transiently transfected with TRPV1 and CXCR2 plasmids were used to investigate the acute effect (12min application) of 4nM CXCL1. In primary cultures, the focus was on TRPV1+/IB4+ cells to which the itch-sensitive neurons belong.

KEY FINDINGS

The results showed that the Ca²⁺ influx induced by the acute application of CXCL1 is mediated mainly by TRPV1 receptors and depends on extracellular Ca²⁺ not on intracellular stores. TRPV1 was activated, not sensitized by CXCL1, in a CXCR2 receptors- and actin filaments-dependent manner, since specific blockers and actin depolymerizing agents disrupted the CXCL1 effect.

SIGNIFICANCE

This study brings additional data about the itch inducing mechanism of CXCL1 chemokine and about a new mechanism of TRPV1 activation via actin filaments.

Effects of eslicarbazepine acetate on acute and chronic latrunculin A-induced seizures and extracellular amino acid levels in the mouse hippocampus

Background

Latrunculin A microperfusion of the hippocampus induces acute epileptic seizures and long-term biochemical changes leading to spontaneous seizures. This study tested the effect of eslicarbazepine acetate (ESL), a novel antiepileptic drug, on latrunculin A-induced acute and chronic seizures, and changes in brain amino acid extracellular levels. Hippocampi of Swiss mice were continuously perfused with a latrunculin A solution (4 μM, 1 μl/min, 7 h/day) with continuous EEG and videotape recording for 3 consecutive days. Microdialysate samples were analyzed by HPLC and fluorescence detection of taurine, glycine, aspartate, glutamate and GABA. Thereafter, mice were continuously video monitored for two months to identify chronic spontaneous seizures or behavioral changes. Control EEG recordings (8 h) were performed in all animals at least once a week for a minimum of one month.

Results

Oral administration of ESL (100 mg/kg), previous to latrunculin A microperfusion, completely prevented acute latrunculin A-induced seizures as well as chronic seizures and all EEG chronic signs of paroxysmal activity. Hippocampal extracellular levels of taurine, glycine and aspartate were significantly increased during latrunculin A microperfusion, while GABA and glutamate levels remained unchanged. ESL reversed the increases in extracellular taurine, glycine and aspartate concentrations to basal levels and significantly reduced glutamate levels. Plasma and brain bioanalysis showed that ESL was completely metabolized within 1 h after administration to mainly eslicarbazepine, its major active metabolite.

Conclusion

ESL treatment prevented acute latrunculin A-induced seizures as well as chronic seizures and all EEG chronic signs of paroxysmal activity, supporting a possible anti-epileptogenic effect of ESL in mice.

Actin and Diseases of the Nervous System

Abnormal regulation of the actin cytoskeleton results in several pathological conditions affecting primarily the nervous system. Those of genetic origin arise during development, but others manifest later in life. Actin regulation is also affected profoundly by environmental factors that can have sustained consequences for the nervous system. Those consequences follow from the fact that the actin cytoskeleton is essential for a multitude of cell biological functions ranging from neuronal migration in cortical development and dendritic spine formation to NMDA receptor activity in learning and alcoholism. Improper regulation of actin, causing aggregation, can contribute to the neurodegeneration of amyloidopathies, such as Down's syndrome and Alzheimer's disease. Much progress has been made in understanding the molecular basis of these diseases.

Latrunculin A depolarizes starfish oocytes

Depolymerization of the actin cytoskeleton may liberate Ca2+ from InsP3-sensitive stores in some cell types, including starfish oocytes, while inhibiting Ca2+ influx in others. However, no information is available on the modulation of membrane potential (V(m)) by actin. The present study was aimed to ascertain whether the widely employed actin depolymerizing drug, latrunculin A (Lat A), affects V(m) in mature oocytes of the starfish Astropecten aranciacus. Lat A induced a membrane depolarization which was mimicked by cytochalasin D, another popular actin disruptor, and prevented by jasplakinolide, a stabilizer of the actin network. Lat A-elicited depolarization consisted in a positive shift in V(m) which reached the threshold of activation of voltage-gated Ca2+ channels (VGCC), thus triggering an action potential. Lat A-promoted depolarization lacked the action potential in Ca2+-free sea water, while it was abolished upon removal of external Na+. Moreover, membrane depolarization was prevented by pre-injection of BAPTA and heparin, but not ryanodine. These data indicate that Lat A induces a membrane depolarization by releasing Ca2+ from InsP3Rs. The Ca2+ signal in turn activates a Ca2+-dependent Na+ entry, which causes the positive shift in V(m) and stimulates the VGCC.

Effect of eslicarbazepine acetate (BIA 2-093) on latrunculin A-induced seizures and extracellular amino acid concentrations in the rat hippocampus

PURPOSE

Eslicarbazepine acetate (ESL, BIA 2-093) is a novel antiepileptic drug endowed with an anticonvulsant potency similar to that of carbamazepine, and shares with carbamazepine and oxcarbazepine the capability to inhibit voltage-gated sodium channels. ESL is efficacious against maximal electroshock seizure-induced seizures, protects against picrotoxin-induced seizures in mice and rats, and prevents development of kindling in rats. In vivo, latrunculin A microperfusion in the rat hippocampus induces acute epileptic seizures and long-term biochemical changes leading to decreased picrotoxin seizure threshold and spontaneous seizures. We have tested the effect of ESL on latrunculin A-induced seizures, and its effect on the changes in extracellular amino acid levels induced by latrunculin A.

METHODS

Rat hippocampus was continuously perfused with a latrunculin A solution (4 microM) through CMA/12 microdialysis probes at a flow rate of 2 microl/min during 8 h with continuous EEG and videotape recording for 3 consecutive days. The same protocol was repeated after oral administration of ESL (3, 10 and 30 mg/kg). Samples from the microdialysate were collected and analyzed by HPLC using pre-column derivatization with 6 aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) and fluorescence detection.

RESULTS

After the administration of 3 mg/kg of ESL, seizures were completely suppressed in the 66.7% of the rats. 10 and 30 mg/kg of ESL did completely suppressed seizures in the 100% of the animals studied. Hippocampal extracellular levels of glutamate, glycine and aspartate were significantly increased during latrunculin A microperfusion, while GABA levels remained unchanged. At the doses studied, ESL reversed the increases in extracellular glutamate and aspartate concentrations to basal levels and significantly reduced glycine levels.

CONCLUSIONS

ESL, at oral doses of 3, 10 and 30 mg/kg, shows an excellent anticonvulsant effect against seizures induced by latrunculin A microperfusion in the rat, and prevents the increases in glutamate and aspartate induced by latrunculin A.

F-actin-based Ca signaling-a critical comparison with the current concept of Ca signaling

A short comparative survey on the current idea of Ca signaling and the alternative concept of F-actin-based Ca signaling is given. The two hypotheses differ in one central aspect, the mechanism of Ca storage. The current theory rests on the assumption of Ca-accumulating endoplasmic/sarcoplasmic reticulum-derived vesicles equipped with an ATP-dependent Ca pump and IP3- or ryanodine-sensitive channel-receptors for Ca-release. The alternative hypothesis proceeds from the idea of Ca storage at the high-affinity binding sites of actin filaments. Cellular sites of F-actin-based Ca storage are microvilli and the submembrane cytoskeleton. Several specific features of Ca signaling such as store-channel coupling, quantal Ca release, spiking and oscillations, biphasic and "phasic" uptake kinetics, and Ca-induced Ca release (CICR), which are not adequately described by the current concept, are inherent properties of the F-actin system and its dynamic state of treadmilling.

A pyridinium derivative from Red Sea soft corals inhibited voltage-activated potassium conductances and increased excitability of rat cultured sensory neurones

BACKGROUND

Whole cell patch clamp recording and intracellular Ca2+ imaging were carried out on rat cultured dorsal root ganglion (DRG) neurones to characterize the actions of crude extracts and purified samples from Red Sea soft corals. The aim of the project was to identify compounds that would alter the excitability of DRG neurones.

RESULTS

Crude extracts of Sarcophyton glaucum and Lobophyton crassum attenuated spike frequency adaptation causing DRG neurones to switch from firing single action potentials to multiple firing. The increase in excitability was associated with enhanced KCl-evoked Ca2+ influx. The mechanism of action of the natural products in the samples from the soft corals involved inhibition of voltage-activated K+ currents. An active component of the crude marine samples was identified as 3-carboxy-1-methyl pyridinium (trigonelline). Application of synthetic 3-carboxy-1-methyl pyridinium at high concentration (0.1 mM) also induced multiple firing and reduced voltage-activated K+ current. The changes in excitability of DRG neurones induced by 3-carboxy-1-methyl pyridinium suggest that this compound contributes to the bioactivity produced by the crude extracts from two soft corals.

CONCLUSION

Sarcophyton glaucum and Lobophyton crassum contain natural products including 3-carboxy-1-methyl pyridinium that increase the excitability of DRG neurones. We speculate that in addition to developmental control and osmoregulation these compounds may contribute to chemical defenses.