Time and voltage windows for reversing the electrical block to fertilization.

Abstract

The electrical block to fertilization of sea urchin eggs can be overcome by very brief periods of inside-negative egg membrane potential. Lytechinus Pictus eggs whose membrane potentials have been clamped at + 15 mV cannot be fertilized. If the membrane potential is repolarized to insidenegative voltages for a brief interval, the egg can be successfully fertilized. By varying the duration and voltage of these brief periods of inside negativity, we have uncovered three general properties of the electrically sensitive step in fertilization.First, a membrane-potential step that becomes rate limiting at inside-positive voltages can be initiated within a few milliseconds of inside negativity (30-60 msec at -60 mV). Second, at the time that the electrically sensitive step is being completed, there are other potential-independent steps with probably slower time constants because the duration of negativity was more effective applied as paired pulses rather than a single long pulse. Third, the permissive state is more quickly established by inside negativity than the nonpermissive state is established by inside positivity because the interval between paired pulses could be a few times longer than the effective single pulse in duration. In these voltage-clamped eggs the intervals from the successful completion of the electrically sensitive step to the next identifiable signs of activation were on the order of several seconds and highly variable.At fertilization in a sea urchin egg, the first successful sperm triggers rapid permeability increases to calcium and sodium that reverse the polarity of the egg membrane potential to inside-positive values (1-4). This rapid depolarization to inside-positive membrane potentials prevents fertilization by secondary sperm (1). This is called the electrical block. These permeability changes do not appear to play a direct role in the activation of development (5). However, the inside-positive values of membrane potential that result from fertilization by the first sperm can serve as the basis for the fast block to polyspermy in the eggs of sea urchins (1), the marine worm Urechis (6), and the frog Xenopus (7). The electrical block can also be artificially imposed by the application of current through an intracellular electrode so that the inside-positive membrane potentials prevent any sperm from completing the fusion and activation process (1).Although there is no direct evidence on how the positive inside values of egg membrane potential block the completion of fusion of the sperm with the egg and the activation of the egg, there is suggestive evidence that the voltage-sensitive component is a property of the sperm and not the egg membrane (8,9). This implies that the electrical block is directly interfering with the fusion process itself, because the voltage-sensitive component of the sperm can be influenced by the egg membrane potential only when it is inserted at least partially across the egg membrane resistance. In the experiments described here, we have attempted to furt...

Polyspermy prevention in marine invertebrates

“…1C). The block acts by reducing the probability of sperm-egg fusion since more positive voltages are necessary to exclude sperm when sperm concentrations are higher (Jaffe, 1976;Jaffe et al, 1982;Shen and Steinhardt, 1984). In eggs held at positive potentials, sperm attach and gyrate on the surface, then gradually lose motility (Lynn and Chambers, 1984).…”

A developmental handshake: Neuronal control of ionic currents and their control of neuronal differentiation

“…Sustained depolarization has been suggested to facilitate insertion of membrane proteins, such as receptors for neuro-transmitters ( Ohmori and Sasaki, 1977 ) . Depolarization of oocytes can block polyspermic fertilization in a reversible manner (Jaffc, 1976;Shen and Steinhardt, 1984). Moreover, the orientation of proteins in artificial membranes can be voltage-dependent (Blumenthal, Kempf, Van Renswoude, Weinstein, and Klausner, 1983).…”

A novel mechanism controls the Ca2+ oscillations triggered by activation of ascidian eggs and has an absolute requirement for Cdk1 activity

“…This sperm factor hypothesis was later confirmed in other mammalian species (Stice and Robl, 1990;Homa and Swann, 1994;Wu et al, 1997), nemerteans (Stricker, 1997) and ascidians (Kyozuka et al, 1998;McDougall et al, 2000;Runft and Jaffe, 2000). The sperm factor model is based upon sperm-egg fusion (allowing delivery of the sperm protein into the egg), and probably explained earlier observations in sea urchin eggs showing that egg activation could be blocked by preventing sperm-egg fusion; sperm-egg binding alone proving insufficient for activation (Shen and Steinhardt, 1984). Although the expectation was that a universal sperm factor would trigger the Ca 2+ increase at fertilisation, this assumption was not supported with the identification of mammalian sperm factor in 2002 by Saunders et al (Saunders et al, 2002).…”