Showing papers by "Michael J. Berridge published in 1996"

Phospholipase C in mouse oocytes: characterization of beta and gamma isoforms and their possible involvement in sperm-induced Ca2+ spiking.

Abstract: This study involved an investigation of the role of phospholipase C (PLC) in generating repetitive Ca#+ spikes at fertilization. Using a PCR-based strategy we have demonstrated that mouse oocytes have mRNA coding for PLCb1, PLCb3 and PLCc isoenzymes. Furthermore, immunodetection of PLCc1 using monoclonal antibodies reveals that PLCc1 protein is present in mature mouse oocytes, ruling out the possibility that mRNA was being transcribed but not expressed. We were unsuccessful at detecting the presence of PLCb protein, but the presence of this isoform can be inferred from functional studies. The PLC

A direct measurement of increased divalent cation influx in fertilised mouse oocytes

Abstract: On fertilisation of mouse oocytes, the fusing spermatozoon triggers a series of repetitive calcium (Ca2+) spikes. The Ca2+ spikes seem to be necessary for successful progression through the cell cycle and are regulated in a cell-cycle-dependent manner. The spikes appear to require the linkage of continuous Ca2+ influx to the periodic release of Ca2+ from intracellular stores by a process of Ca(2+)-induced Ca2+ release. The precise role of Ca2+ influx was explored using the manganese (Mn2+)-quench technique to monitor unidirectional cation influx into single mouse oocytes. There was a marked stimulation of cation influx associated closely with the upsweep of the first and subsequent fertilisation Ca2+ spikes. A smaller but significant increase in the rate of cation influx persisted in the interspike period in fertilised oocytes. Spike-associated entry was not as apparent in oocytes stimulated to spike repetitively by thimerosal or acetylcholine application. Instead, there was a continuous increase in cation influx underlying Ca2+ spiking which commenced with the onset of the first spike. Using the specific microsomal inhibitor thapsigargin and the Ca2+ ionophore ionomycin, we found evidence for a capacitative entry mechanism in mouse oocytes. We propose that the persistent influx of Ca2+ observed in response to all stimuli examined is controlled by a capacitative mechanism and sets the frequency of spiking by determining the time taken to refill the internal stores to a point where they are again sensitive enough to initiate the next spike.

Extracellular calcium concentration controls the frequency of intracellular calcium spiking independently of inositol 1,4,5-trisphosphate production in HeLa cells.

Abstract: Stimulation of single HeLa cells with histamine evoked repetitive increases of the intracellular calcium ion concentration (Ca2+ spikes). The frequency of Ca2+ spiking increased as the extracellular hormone concentration was elevated. In addition, the frequency of Ca2+ spiking could be accelerated by increasing the extracellular Ca2+ concentration ([Ca2+]0) in the presence of a constant hormone concentration. The range of [Ca2+]0 over which the spiking frequency could be titrated was nominally-zero to 10mM, being half-maximally effective at approx. 1 and 2.5mM for 37 and 22 degrees C respectively. The effect of [Ca2+]0 on inositol phosphates production was also examined. Changes of [Ca2+]0 over a range which had been found to affect the frequency of Ca2+ spiking did not have any effect on the rate of myo-inositol 1,4,5-trisphosphate (InsP3) production, although an increase in inositol phosphates production was observed as [Ca2+]0 was increased from zero to values giving less than half-maximal Ca2+ spike frequency. These data suggest that at low Ca2+ spike frequency, Ca2+-stimulated activation of phospholipase C may contribute to Ca2+ spiking in HeLa cells, but under some conditions the availability of Ca2+ to the intracellular stores, rather than changes in the rate of InsP3 production, determines the Ca2+ spike frequency.

Capacitative calcium entry is colocalised with calcium release in Xenopus oocytes: Evidence against a highly diffusible calcium influx factor

Abstract: Depletion of intracellular calcium stores activates the plasma membrane capacitative calcium entry pathway in many cell types. The nature of the signal that couples the depletion of the intracellular calcium stores to the activation of the plasma membrane calcium influx pathway is as yet unknown. It has recently been suggested that a highly diffusible calcium influx factor is involved in the activation of capacitative calcium entry, and that its action is potentiated by the protein phosphatase inhibitor okadaic acid. Depletion of intracellular calcium stores in a localised region of a Xenopus oocyte was found to evoke capacitative calcium entry exclusively colocalised across the stimulated area of the plasma membrane, arguing against the involvement of a highly diffusible calcium influx factor. Equally, no evidence could be found for the presence of a soluble calcium influx factor in the bulk cytosol of Xenopus oocytes. The potentiation of capacitative calcium entry by okadaic acid resembled that mediated by the activation of protein kinase C, thus suggesting that okadaic acid activity may not necessarily be related to the action of a putative calcium influx factor.

Multiple, coordinated Ca2+ -release events underlie the inositol trisphosphate-induced local Ca2+ spikes in mouse pancreatic acinar cells.

Abstract: Ca2 + wave initiation and non-propagating Ca2 + spikes occur as a result of localized Ca2 + release from the more sensitive intracellular Ca2 + stores. Using high spatial and temporal Ca2 + -imaging techniques we have investigated inositol 1,4,5 triphosphate (InsP3)-induced local Ca2 + spiking, which occurs at the site of Ca2 + wave initiation in pancreatic acinar cells. The spatial and temporal organization of a single spike suggested discrete hot spots of Ca2 + release. Further analysis of long trains of Ca2 + spikes demonstrated that these hot spots showed regenerative Ca2 + -release events which were consistently active from spike to spike. Regions adjacent to these hot spots also showed regenerative Ca2 + -release events of similar amplitude but with a much lower frequency of occurrence. We conclude that the InsP3-induced non-propagating Ca2 + spikes can be devolved into smaller components of release. Our results are consistent with a model of coordinated activity of pacemaker hot spots of Ca2 + release that recruit and entrain active Ca2+ -release events from surrounding regions.

Calcium oscillations in human oocytes Regulation of calcium spiking in mammalian oocytes through a combination of inositol trisphosphate dependent entry and release

Abstract: ryanodine, and sperm induced Ca 2+ release increase during meiotic maturation of mouse oocytes../. 2+ release channels underlying Ca 2+ waves and oscillations in exocrine pancreas. (1995) Reorganization of the endoplasmic reticulum during meiotic maturation of the mouse oocyte. calsequestrin, is the major calcium binding protein of smooth muscle sarcoplasmic reticulum and liver endoplasmic reticulum.