Initiation of IP3-mediated Ca2+ waves in Xenopus oocytes

Abstract

Inositol (1,4,5)-trisphosphate (IP(3)) evokes Ca(2+) liberation in Xenopus oocytes as elementary events (Ca(2+) puffs) that become coupled to propagate Ca(2+) waves with increasing [IP(3)]. To investigate this transition between local and global Ca(2+) signaling, we developed an optical method for evoking rapid subcellular Ca(2+) elevations, while independently photoreleasing IP(3) and simultaneously recording confocal Ca(2+) images. Focal Ca(2+) elevations triggered waves within 100 ms of photoreleasing IP(3), compared with latencies of seconds following photorelease of IP(3) alone. Wave velocity varied with [IP(3)] but was independent of time after photorelease of IP(3), indicating that delayed wave initiation did not involve slow binding of IP(3) to its receptors. The amount of Ca(2+) required to trigger a wave was approximately 10-fold greater than the average size of puffs, and puffs showed no progressive increase in magnitude before waves initiated. Instead, Ca(2+) puffs contributed to a slow rise in basal free [Ca(2+)], which further increased puff frequency and sensitized IP(3) receptors so that individual events then triggered waves. Because the wave threshold is much greater than the size of the elementary puff, cells can employ both local and global signaling mechanisms, and the summation of stochastic behavior of elementary events allows generation of reproducible periodic waves.

Spatial Control of Ca2+ Signaling by Nicotinic Acid Adenine Dinucleotide Phosphate Diffusion and Gradients

“…Even though the Ca 2ϩ wave is propagated by NAADP diffusion, the wave does not decrease in velocity or amplitude because NAADP is continuously added to the other half of the egg. This result shows that a diffusive wave propagates by CICR (9,14,25). Because cADPR acts by sensitizing ryanodine receptors to CICR (13), it should elicit a regenerative rather than diffusive Ca 2ϩ wave similar to that induced by IP 3 (9,14,25).…”

“…This result shows that a diffusive wave propagates by CICR (9,14,25). Because cADPR acts by sensitizing ryanodine receptors to CICR (13), it should elicit a regenerative rather than diffusive Ca 2ϩ wave similar to that induced by IP 3 (9,14,25). To provide further evidence that NAADP-mediated Ca 2ϩ waves are dependent on a non-regenerative diffusive mechanism, the responses of cADPR and NAADP were compared at their threshold concentrations for evoking Ca 2ϩ release.…”

“…The sensitivity of the IP 3 receptor and the ryanodine receptor to positive feedback by Ca 2ϩ is modulated by IP 3 (1,10,12) and cyclic ADP-ribose (5, 13), respectively. An increase in IP 3 turns the cytoplasm into an excitable medium such that any increase in Ca 2ϩ of sufficient magnitude is propagated as a regenerative Ca 2ϩ wave (7,9,14). IP 3 -mediated Ca 2ϩ waves can propagate even when the concentration of IP 3 is constant and spatially uniform (7,9), demonstrating that they do not have an absolute requirement for IP 3 diffusion.…”

Derivation of Ca 2+ signals from puff properties reveals that pathway function is robust against cell variability but sensitive for control

“…Fluorescence imaging studies and model simulations reveal a cascade of events leading to a Ca 2þ spike: Openings of single Ca 2þ channels (blips) are followed by collective openings of channels in a cluster (puffs). Ca 2þ from a puff diffusing to neighboring clusters can activate them by CICR, eventually leading to a global Ca 2þ spike (7)(8)(9)(10). Channels within a cluster are strongly coupled by Ca 2þ diffusion, whereas coupling between clusters is weak because of steep concentration gradients (11,12).…”

Calcium signalling in early embryos

“…One proof of this is that the wave can propagate in the absence of external calcium (Chambers 1980;Chambers & Angeloni 1981;Schmidt et al 1982;Crossley et al 1988). Calcium waves are built up from many individual calcium release events from one or a very few clustered calcium channels in the ER membrane, be they InsP 3 or RyRs (Cheng et al 1993;Lipp et al 1997;Keizer & Smith 1998;Marchant et al 1999). Propagation occurs by reaction (calciumtriggered activation of the release channel) and diffusion (of calcium to neighbouring receptors), a mechanism of CICR (Endo et al 1970;Keizer & Smith 1998;Solovyova et al 2002).…”