A calcium-activated sodium conductance produces a long-duration action potential in the egg of a nemertean worm

Resting state structure of the hyperdepolarization activated two-pore channel 3

Voltage-gated ion channels endow membranes with excitability and the means to propagate action potentials that form the basis of all neuronal signaling. We determined the structure of a voltage-gated sodium channel, two-pore channel 3 (TPC3), which generates ultralong action potentials. TPC3 is distinguished by activation only at extreme membrane depolarization (V₅₀ ∼ +75 mV), in contrast to other TPCs and Na_V channels that activate between -20 and 0 mV. We present electrophysiological evidence that TPC3 voltage activation depends only on voltage sensing domain 2 (VSD2) and that each of the three gating arginines in VSD2 reduces the activation threshold. The structure presents a chemical basis for sodium selectivity, and a constricted gate suggests a closed pore consistent with extreme voltage dependence. The structure, confirmed by our electrophysiology, illustrates the configuration of a bona fide resting state voltage sensor, observed without the need for any inhibitory ligand, and independent of any chemical or mutagenic alteration.

Ca2+-activated Cl- current ensures robust and reliable signal amplification in vertebrate olfactory receptor neurons

Activation of most primary sensory neurons results in transduction currents that are carried by cations. One notable exception is the vertebrate olfactory receptor neuron (ORN), where the transduction current is carried largely by the anion [Formula: see text] However, it remains unclear why ORNs use an anionic current for signal amplification. We have sought to provide clarification on this topic by studying the so far neglected dynamics of [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] in the small space of olfactory cilia during an odorant response. Using computational modeling and simulations we compared the outcomes of signal amplification based on either [Formula: see text] or [Formula: see text] currents. We found that amplification produced by [Formula: see text] influx instead of a [Formula: see text] efflux is problematic for several reasons: First, the [Formula: see text] current amplitude varies greatly, depending on mucosal ion concentration changes. Second, a [Formula: see text] current leads to a large increase in the ciliary [Formula: see text] concentration during an odorant response. This increase inhibits and even reverses [Formula: see text] clearance by [Formula: see text] exchange, which is essential for response termination. Finally, a [Formula: see text] current increases the ciliary osmotic pressure, which could cause swelling to damage the cilia. By contrast, a transduction pathway based on [Formula: see text] efflux circumvents these problems and renders the odorant response robust and reliable.

A non-inactivating high-voltage-activated two-pore Na⁺ channel that supports ultra-long action potentials and membrane bistability

Action potentials (APs) are fundamental cellular electrical signals. The genesis of short APs lasting milliseconds is well understood. Ultra-long APs (ulAPs) lasting seconds to minutes also occur in eukaryotic organisms, but their biological functions and mechanisms of generation are largely unknown. Here, we identify TPC3, a previously uncharacterized member of the two-pore channel protein family, as a new voltage-gated Na⁺ channel (Na_V) that generates ulAPs, and that establishes membrane potential bistability. Unlike the rapidly inactivating Na_Vs that generate short APs in neurons, TPC3 has a high activation threshold, activates slowly, and does not inactivate—three properties that help generate long-lasting APs and guard the membrane against unintended perturbation. In amphibian oocytes, TPC3 forms a channel similar to channels induced by depolarization and sperm entry into eggs. TPC3 homologs are present in plants and animals, and they may be important for cellular processes and behaviors associated with prolonged membrane depolarization.

Calcium-activated, voltage-dependent, non-selective cation currents in endosperm plasma membrane from higher plants

Single-channel and whole-cell patch-clamp techniques were used to characterize the electrophysiological behaviour of plasma membranes from freshly isolated, non-enzyme-treated endosperm protoplasts. A non-selective monovalent cation channel with a single-channel conductance of 22 pS in solutions with physiological potassium concentrations was observed in inside-out patches. The channel passes outward current at depolarized potentials and is highly selective for cations over anions, but discriminates poorly between lithium, sodium, potassium, rubidium and caesium ions. Specific potassium channel blockers were ineffective. The channel kinetics were apparently complex, with burst-like openings and rapid closures within a single burst. Single-channel openings were more frequent both for depolarizing pulses and maintained positive potentials. Channel activity was also increased by elevated cytoplasmic concentrations of either calcium or barium. Subsequent exposure of patches to low calcium, EGTA-buffered solutions resulted in large decreases in activity. Under whole-cell current clamp, small negative resting potentials were observed. A slowly developing outward current evoked by depolarizing pulses was seen in whole-cell recordings.

Calmodulin activation of calcium-dependent sodium channels in excised membrane patches of Paramecium

Calmodulin is a calcium-binding protein that participates in the transduction of calcium signals. The electric phenotypes of calmodulin mutants of Paramecium have suggested that the protein may regulate some calcium-dependent ion channels. Calcium-dependent sodium single channels in excised patches of the plasma membrane from Paramecium were identified, and their activity was shown to decrease after brief exposure to submicromolar concentrations of calcium. Channel activity was restored to these inactivated patches by adding calmodulin that was isolated from Paramecium to the cytoplasmic surface. This restoration of channel activity did not require adenosine triphosphate and therefore, probably resulted from direct binding of calmodulin, either to the sodium channel itself or to a channel regulator that was associated with the patch membrane.

Sodium-dependent depolarizing potentials in veratrinized crayfish muscle fibres

In the slow abdominal flexor muscle fibers of the crayfish Procambarus clarkii, veratrine or veratridine applied during several minutes produced the persistent transformation of the muscle fibre from a nonspiking into a Ca-dependent spiking one and spikes were followed by a long-duration depolarization. The long-duration potential depends on external Na+ but is not blocked by 30 microM tetrodotoxin (TTX). In solutions containing normal concentrations of Na+ (207 mM) the absence of Ca2+ or the presence of calcium channel blockers abolished both potentials. The results show that alkaloid toxins reveal a Ca2+-dependent, TTX-resistant Na+ conductance in crayfish tonic muscle fibres.

The wave of activation current in the egg of the medaka fish

An extracellular vibrating electrode was used to measure the ring-shaped wave of inward current, the activation current, that propagates at 10 micron/sec across the egg of the medaka fish, Oryzias latipes, from the site of sperm-egg fusion at the animal pole to the vegetal pole. This activation wave is due to a localized increase in the conductance to Na+, K+, and Ca2+ and reflects the propagated opening of these ion channels. The earliest detectable current begins to enter the animal pole 20 sec after the initiation of the fertilization potential, so the first ion movements responsible for the fertilization potential are below the resolution of the vibrating probe system. These channels are present in both the animal and vegetal hemispheres, but the magnitude of the activation current is about seven times greater in the animal hemisphere. An outward current of smaller magnitude and spread out over a larger area precedes and follows the inward current except at the point of fertilization where the current is first inward. The current direction is dependent on the external Na+ concentration, and in the more physiological solution of 10% NaCl-Yamamoto's Ringer's, its direction reverses to become outward, apparently carried by K+ efflux. Raising the external Ca2+ in this same low-Na+ medium reverses the current so that it becomes inward again and increases the propagation velocity of the wave, suggesting a Ca2+ component to the inward current. Current enters a given region on the egg's surface about 16 sec before any vesicle fusion occurs in that region. Iontophoresis of inositol-1,4,5,-trisphosphate immediately triggers egg activation with a minimum activating charge of 0.6 nC.

A calcium-activated sodium conductance contributes to the fertilization potential in the egg of the nemertean worm Cerebratulus lacteus

The fertilization potential of the egg of the nemertean worm Cerebratulus lacteus consisted of a rapid shift from a resting potential of about -65 mV to a peak of about +44 mV; the peak was followed by a positive plateau at about +24 mV, lasting an average of 80 min. Reduction of extracellular calcium reduced the peak of the fertilization potential, indicating that the peak resulted from a calcium conductance, while reduction of extracellular sodium reduced the plateau potential, indicating that the plateau resulted from a sodium conductance. Microinjection of ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) or 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)/CaBAPTA buffers, having a free calcium concentration of less than or equal to about 0.1 microM lowered the fertilization potential plateau. Injection of a BAPTA/CaBAPTA mixture with a free calcium concentration of about 1 microM resulted in a prolonged positive potential at the level of the fertilization potential plateau. These observations indicated that the fertilization potential of the Cerebratulus egg depended on a calcium-activated sodium conductance. The plateau potential was reduced little, if any, when calcium-free seawater was perfused through the bath during the fertilization potential; nor was it reduced in seawater containing cadmium. These observations suggested the possibility that intracellular calcium stores could be important in producing the fertilization potential.