Meiosis reinitiation in Ruditapes philippinarum (Mollusca): involvement of L-calcium channels in the release of metaphase I block

Abstract

SummaryProphase-arrested oocytes of Ruditapes philippinarum cannot be fertilised or stimulated by excess KCl, in contrast to the situation found in other bivalve species such as Barnea and Spisula. However, these oocytes can be triggered to reinitiate meiosis following treatment by serotonin or several pharmacological agents (calcium ionophores, thapsigargin, weak bases) which promote an intracellular calcium surge. Ruditapes oocytes further arrest in metaphase I, at which stage they can be activated either by sperm or by excess KCl. This suggests that functional voltage-operated calcium channels are expressed in this species during the course of maturation. Using pharmacological tools and direct binding of specific dihydropyridines, we demonstrate that these channels are L-type calcium channels which become functional after serotonin stimulation, their number increasing before germinal vesicle breakdown. Moreover we establish that: (1) the addition of 20 μM S(—)Bayκ8644, an agonist of L-type calcium channels, to metaphase-arrested oocytes releases them from metaphase block; (2) incubating these oocytes with PN200-110, a potent blocker of L-type calcium channels, inhibits their activation by both excess KCl and fertilisation. Taken together these data suggest that the absence of L-type calcium channels in the membrane of prophase-arrested oocytes of Ruditapes may account for their inability to be fertilised.

Ion currents modulating oocyte maturation in animals

“…This is also the case for the oocytes of the surf clam, Ruditapes and Crassostrea [Dubè 1992;Leclerc et al 2000]. The present hypothesis is that an early calcium influx is a prerequisite to allow nuclear maturation in either PI- [Colas and Dubè 1998] or MI-arrested oocytes of bivalves [Moreau et al 1996]. This current seems to be mediated by L-type calcium channels [Guerrier et al 1993;Tomkowiak et al 1997].…”

Serotonergic-induced ion currents in cleaving sea urchin embryos

“…L-type calcium channels also disappear in micromeres at the 4th cleavage division (Dale et al, 1997), and this is correlated with a decreased sensitivity of micromeres to 5HT-antagonists (Buznikov, 1990). A correlation between calcium influx, dihydropyridine-sensitive calcium channels and 5HT stimulation through serotonin receptors has been moreover well described during oocyte maturation process in some marine invertebrates (Leclerc et al, 2000;Moreau et al, 1996;Guerrier et al, 1996). Our data, on one hand, support this period as one in which the membrane reorganization is accompanied by an increase in the number of receptors in the furrow area or an increase in the expression of other sensitive channels; on the other hand, it suggests coupling between serotonergic mechanisms and Ca2+-channel activity.…”

Ion current activity and molecules modulating maturation and growth stages of ascidian (Ciona intestinalis) oocytes

“…The role of Ca 2þ as second messenger involved in oogenesis and oocyte maturation is well described (Whitaker and Swann, 1993;Homa, 1995;Carroll et al, 1996;Stricker, 1999). However, the influence of Ca 2þ entry through plasma membrane channels during maturation has been described in only few mammalian species (Murnane and De Felice, 1993;Tosti et al, 2000;Lee et al, 2004) and mollusks (Dub e, 1992;Guerrier et al, 1993;Moreau et al, 1996;Tomkoviak et al, 1997;Colas and Dub e, 1998;Leclerc et al, 2000;Cuomo et al, 2005). In the majority of cases, the channels responsible for Ca 2þ influx have been shown to be L-type Ca 2þ channels.…”