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ON THE COVER
Polyspermy, the fertilization of an egg by multiple sperm, is lethal to most developing embryos. Additional pairs of centrioles delivered by supernumerary sperm compete for chromosomes, which results in incomplete and asymmetric cleave furrows. A mosaic of monospermic (symmetric furrows) and polyspermic Xenopus laevis embryos at the two- and four-cell stages are pictured here. Due to the detrimental effects of polyspermy, most sexually reproducing organisms have derived polyspermy prevention mechanisms. External fertilizers, such as frogs and sea urchins, have an electrically mediated polyspermy block. In their companion papers, Wozniak et al. elucidate the source of increased intracellular Ca2+ and the identity of the Cl− channel mediating the fast, electrical polyspermy block in X. laevis. See pages 1239 and 1249. - PDF Icon PDF LinkTable of Contents
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Research News
Propofol’s paradox, explained
Tandem JGP studies investigate how propofol affects voltage-gated sodium channels.
Commentary
The fast block to polyspermy: New insight into a century-old problem
Jaffe underscores new research that identifies key roles for IP3 and TMEM16a in the fast block to polyspermy.
Progress in understanding slow inactivation speeds up
A new study reveals that conformational flexibility in the pore of a voltage-gated sodium channel may underlie slow inactivation.
Article
PLC and IP3-evoked Ca2+ release initiate the fast block to polyspermy in Xenopus laevis eggs
The fast block to polyspermy is achieved in Xenopus laevis eggs by fertilization-induced depolarization. Wozniak et al. show that fertilization activates a signaling cascade involving phospholipase C, IP3, and intracellular Ca2+ release, which induces depolarization via Ca2+-activated Cl− efflux.
The TMEM16A channel mediates the fast polyspermy block in Xenopus laevis
In their preceding paper, Wozniak et al. show that fertilization increases intracellular Ca2+ in Xenopus laevis eggs by activating an IP3 signaling cascade. Here, they reveal that Ca2+ subsequently opens the Cl− channel TMEM16A to allow Cl− efflux, cell depolarization, and fast block to polyspermy.
All four subunits of HCN2 channels contribute to the activation gating in an additive but intricate manner
HCN pacemaker channels are dually gated by hyperpolarizing voltages and cyclic nucleotide binding. Sunkara et al. show that each of the four binding sites promotes channel opening, most likely by exerting a turning momentum on the tetrameric intracellular gating ring.
Singlet oxygen modification abolishes voltage-dependent inactivation of the sea urchin spHCN channel
Singlet oxygen modifies several different proteins within cells. Idikuda et al. show that, in the case of the sea urchin hyperpolarization-activated cyclic nucleotide–gated channel, a histidine residue in S6 is essential for the abolition of voltage-dependent inactivation by singlet oxygen.
On the role of transient depolarization-activated K+ current in microvillar photoreceptors
The transient K+ current carried by Shaker channels is thought to play a role in low-frequency signal amplification in Drosophila melanogaster photoreceptors. By combining patch-clamp recordings with a physiological variability analysis, Frolov reveals its role in high-frequency signal transmission.
Propofol inhibits prokaryotic voltage-gated Na+ channels by promoting activation-coupled inactivation
Despite extensive use in clinical practice, the mechanisms of propofol action on sodium channels are not fully understood. Yang et al. incorporate complementary biophysical approaches (electrophysiology and molecular dynamics simulations) to demonstrate that propofol inhibits two prokaryotic voltage-gated sodium channels, NaChBac and NavMs, by modulating both activation and inactivation gating.
Propofol inhibits the voltage-gated sodium channel NaChBac at multiple sites
General anesthetics inhibit voltage-gated sodium channels by unknown molecular mechanisms. Using computation-guided NMR and electrophysiology analyses, Wang et al. show that propofol binds to the prokaryotic sodium channel NaChBac at multiple distinct sites.
The voltage-gated sodium channel pore exhibits conformational flexibility during slow inactivation
Voltage-gated sodium channels undergo slow inactivation during prolonged depolarization by means of a mechanism that is poorly understood. Chatterjee et al. study this process spectroscopically and reveal conformational flexibility of the pore region in the slow-inactivated state.
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