Thapsigargin induces meiotic maturation in surf clam oocytes

T-type Ca2+ current activity during oocyte growth and maturation in the ascidian Styela plicata

Voltage-dependent calcium currents play a fundamental role during oocyte maturation, mostly L-type calcium currents, whereas T-type calcium currents are involved in sperm physiology and cell growth. In this paper, using an electrophysiological and pharmacological approach, we demonstrated, for the first time in oocytes, that T-type calcium currents are present with functional consequences on the plasma membrane of growing immature oocytes of the ascidian Styela plicata. We classified three subtypes of immature oocytes at the germinal vesicle stage on the basis of their size, morphology and accessory cellular structures. These stages were clearly associated with an increased activity of T-type calcium currents and hyperpolarization of the plasma membrane. We also observed that T-type calcium currents oscillate in the post-fertilization embryonic stages, with minimal amplitude of the currents in the zygote and maximal at 8-cell stage. In addition, chemical inhibition of T-type calcium currents, obtained by applying specific antagonists, induced a significant reduction in the rate of cleavage and absence of larval formation. We suggest that calcium entry via T-type calcium channels may act as a potential pacemaker in regulating cytosolic calcium involved in fertilization and early developmental events.

Ion currents modulating oocyte maturation in animals

Growing oocytes are arrested at the first prophase of meiosis which is morphologically identified by the presence of a large and vesicular nucleus, called the germinal vesicle. The dissolution of the germinal vesicle marks the resumption of meiosis during which the oocyte undergoes massive modifications up to the second meiotic block, which is removed at fertilization. The interval between the first and the second meiotic block is defined as maturation and the events occurring during this period are crucial for ovulation, fertilization, and embryo development. Oocytes are excitable cells that react to stimuli by modifying their electrical properties as a consequence of ion currents flowing through ion channels on the plasma membrane. These electrical changes have been largely described at fertilization whereas little information is available during oocyte maturation. The aim of this review is to give an overview on the involvement of ion channels and ion currents during oocyte maturation in species from invertebrates to mammals. The results summarized here point to the possible functional role of ion channels underlying oocyte growth and maturation.

Role of cyclic AMP in the maturation of Ciona intestinalis oocytes

Immature oocytes are arrested at prophase I of the meiotic process and maturation onset is indicated by oocyte nuclear disassembly (germinal vesicle breakdown or GVBD). Signaling pathways that elevate intracellular cyclic AMP (cAMP) may either prevent or induce oocyte maturation depending on the species. In some marine invertebrates and, in particular, in ascidian oocytes, cAMP triggers GVBD rather than blocking it. In this paper, we tested different cAMP elevators in fully grown oocytes at the germinal vesicle stage (GV) of the ascidian Ciona intestinalis. We demonstrated that through the activation of adenylate cyclase or the inhibition and phosphodiesterases the oocyte remained at the GV stage. This effect was reversible as the GV-arrested oocytes, rinsed and incubated in sea water, are able to undergo spontaneous maturation and extrusion of follicle cells. In addition, oocytes acquire the ability to be fertilized and start early development. However, morphology of follicle cells, embryos and larvae from in vitro matured oocytes showed different morphology from those derived from in vivo mature oocytes. The role and the transduction mechanism of cAMP in the regulation of oocyte maturation were discussed. Finally, we indicated a variation of biological mechanisms present in the ascidian species; moreover, we sustain evidence proving that tunicates share some biological mechanisms with vertebrates. This information provided new hints on the importance of ascidians in the evolution of chordates.

Calcium ion currents mediating oocyte maturation events

During maturation, the last phase of oogenesis, the oocyte undergoes several changes which prepare it to be ovulated and fertilized. Immature oocytes are arrested in the first meiotic process prophase, that is morphologically identified by a germinal vesicle. The removal of the first meiotic block marks the initiation of maturation. Although a large number of molecules are involved in complex sequences of events, there is evidence that a calcium increase plays a pivotal role in meiosis re-initiation. It is well established that, during this process, calcium is released from the intracellular stores, whereas less is known on the role of external calcium entering the cell through the plasma membrane ion channels. This review is focused on the functional role of calcium currents during oocyte maturation in all the species, from invertebrates to mammals. The emerging role of specific L-type calcium channels will be discussed.

Ca2+ and Na+ current patterns during oocyte maturation, fertilization, and early developmental stages ofCiona intestinalis

Using the whole-cell voltage clamp technique, the electrical changes in oocyte and embryo plasma membrane were followed during different meiotic and developmental stages in Ciona intestinalis. We show, for the first time, an electrophysiological characterization of the plasma membrane in oocytes at the germinal vesicle (GV) stage with high L-type calcium (Ca2+) current activity that decreased through meiosis. Moreover, the absence of Ca2+ reduced germinal vesicle breakdown (GVBD), which is consistent with a role of Ca2+ currents in the prophase/metaphase transition. In mature oocytes at the metaphase I (MI) stage, Ca2+ currents decreased and then disappeared and sodium (Na+) currents first appeared remaining high up to the zygote stage. Intracellular Ca2+ release was higher in MI than in GV, indicating that Ca2+ currents in GV may contribute to fill the stores which are essential for oocyte contraction at fertilization. The fertilization current generated in Na+ free sea water was significantly lower than the control; furthermore, oocytes fertilized in the absence of Na+ showed high development of anomalous "rosette" embryos. Current amplitudes became negligible in embryos at the 2- and 4-cell stage, suggesting that signaling pathways that mediate first cleavage do not rely on ion current activities. At the 8-cell stage embryo, a resumption of Na+ current activity and conductance occurred, without a correlation with specific blastomeres. Taken together, these results imply: (i) an involvement of L-type Ca2+ currents in meiotic progression from the GV to MI stage; (ii) a role of Na+ currents during electrical events at fertilization and subsequent development; (iii) a major role of plasma membrane permeability and a minor function of specific currents during initial cell line segregation events.

Calcium currents correlate with oocyte maturation during the reproductive cycle in Octopus vulgaris

Using the whole-cell voltage clamp technique, we have studied the Ca2+ currents and the steady-state conductance during different oocyte growth stages and during the reproductive cycle of the female of Octopus vulgaris. Evidence is presented that L-type Ca2+ currents are high in small pre-vitellogenic oocytes (80-150 microm diameter) and significantly lower in early vitellogenic oocytes (180-300 microm diameter). Similarly, a significant decrease of the steady-state conductance occurred from the pre to early- vitellogenic oocytes. Octopus oocytes showed larger Ca2+ currents in the reproductive rather than non-reproductive periods. These data indicates that ion and L-type Ca2+ currents play a role in oocyte growth and cytoplasmic maturation, and possibly in preparing the plasma membrane to the interaction with the spermatozoon. By using fluorescent microscopy, we show that oocytes from 80 to 400 microm diameter have the large germinal vesicle characteristic of the immature oocytes. In subsequent stages of growth (up to 1000 microm diameter) the nucleus is no more visible and the metaphase spindle appears. These data demonstrate that Octopus vulgaris oocytes are arrested in the first meiotic prophase up to the early-vitellogenic stage and resume meiosis at this stage up to a second block presumably in metaphase I. We discuss a possible role for progesterone as the hormonal stimulus for the first prophase-metaphase meiotic transition.

Mechanisms of calcium release and sequestration in eggs of Chaetopterus pergamentaceus

Increases in the intracellular free calcium concentration are of great importance to the initiation of development in deuterostomes. Their involvement has not yet been clearly defined in protostomes. We used endogenous ligands (IP3, cADPR, ryanodine and NAADP) and pharmacological agents (thapsigargin [Tg], thimerosal, caffeine and heparin) to study smooth endoplasmic reticulum Ca2+ pump and release mechanisms in eggs of an annelid, Chaetopterus. Oocyte homogenates effectively sequestered Ca2+ and released it in response to IP3 in a concentration-dependent manner. Repeated additions of IP3 were unable to cause further release. Heparin inhibited Ca2+ release in response to IP3. The homogenates also released Ca2+ in response to thimerosal, and this release was sensitive to heparin. Two antibodies to IP3 receptors recognized an appropriate band in Chaetopterus egg lysates. These results indicate that the oocytes possess type-1 IP3-gated Ca2+ channels. Neither calcium itself, nor strontium, cADPR, ryanodine, caffeine nor NAADP released appreciable Ca2+. At low concentrations, Tg caused a slow release of Ca2+; at higher concentrations, it elicited a rapid release. Release of Ca2+ by Tg activated development. Since one theory of fertilization invokes the introduction of a Ca2+ releasing soluble protein into the egg upon sperm-egg fusion, we also tested whether soluble extracts of Chaetopterus sperm could stimulate Ca2+ release in Chaetopterus egg homogenates. There was no Ca2+ release when the sperm extract was added to the homogenate; however, homogenates exposed to sperm extract became refractory to IP3. Thus, Ca2+ release at fertilization in these oocytes occurs through IP3-gated channels.

Intracellular pH increase driven by an Na+/H+ exchanger upon activation of surf clam oocytes

Intracellular pH (pHi) measurements were performed in surf clam (Spisula solidissima) oocytes before and after artificial activation or fertilization [evidenced by germinal vesicle breakdown (GVBD)] by the dimethyloxazolidinedione (DMO) and 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) methods. Results using both methods showed increases of pHi of 0.3 pH unit after activation by excess K+. Using BCECF, we found an increase of similar magnitude after fertilization or after the addition of serotonin. By contrast, GVBD did not occur when the pHi was increased to similar or even higher levels by exposing the oocytes to ammonia. In sodium-free seawater, excess K+ induced GVBD but the pHi of K+-activated oocytes decreased significantly below the resting level of unactivated oocytes. The pHi increases in K+-activated oocytes were otherwise proportional to the external Na+ concentration. The amiloride derivatives dimethylamiloride and hexamethylene amiloride (at 10-50 microM) efficiently inhibited the K+-induced increase of pHi but did not block GVBD. These two derivatives were able, however, to retard K+-induced GVBD, hexamethylene amiloride being the more efficient. This retardation of K+-induced GVBD could be abolished by the simultaneous addition of ammonia. Taken altogether, these results show that a pHi increase, driven by a typical Na+/H+ exchanger, follows activation of surf clam oocytes but that this pHi increase is neither sufficient nor required for GVBD, though it does allow its progression at an optimal rate.

In vivo and in vitro induction of germinal vesicle breakdown in a freshwater bivalve, the zebra mussel Dreissena polymorpha (Pallas)

Oocyte maturation and germinal vesicle breakdown (GVBD) was induced in zebra mussel (Dreissena polymorpha) oocytes by in vivo and in vitro application of serotonin (5-hydroxytryptamine, 5-HT), and in vitro application of 8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT, a 5-HT1A receptor agonist). Oocytes initiated GVBD approximately 30 minutes after exposure to 5-HT (10(-3) M) at 23 degrees C, and by 40-50 minutes after exposure, most oocytes lacked a germinal vesicle. An exposure time to 5-HT as brief as five to ten minutes was required to trigger the maturation process, which terminates in spawning of fertilizable oocytes in nearly all mussels. But, with an exposure time of less than five minutes, spawning was reduced by application of 10(-4) M methiothepin (a potent blocker of 5-HT-induced spawning in zebra mussels). Thus, the sequence of oocyte maturation events in zebra mussels was determined. Oocytes are arrested at the germinal vesicle stage (prophase I) within the ovary. 5-HT reinitiates the maturation process, including GVBD and spawning of metaphase I oocytes, which are further arrested until fertilization. To our knowledge this is the first demonstration of oocyte maturation induction by serotonergic ligands in a freshwater bivalve.

Reception and transduction of the serotonin signal responsible for meiosis reinitiation in oocytes of the Japanese clam Ruditapes philippinarum

Prophase-arrested oocytes of Ruditapes philippinarum are triggered to undergo germinal vesicle breakdown under the influence of the neurohormone serotonin (5HT) and then arrest in metaphase 1. Our data show that these oocytes possess a single class of original 5HT receptors. Their binding parameters have been determined on semipurified membrane preparations incubated with [3H]5HT. No significant differences were observed when comparing 5HT-competent and -incompetent batches as well as prophase- or metaphase-arrested oocytes. Specific experiments including incubation with mastoparan or mas 7, GTP iontophoresis, and IP3 quantification strongly suggest that these receptors must be coupled with G-proteins to be functional. Peak change in IP3 mass occurs at 3 min and is likely to trigger the 5HT-dependent Ca2+ transient that begins at this time. In metaphase-arrested oocytes, binding of 5HT to its receptors no longer produces a Ca2+ surger. This is likely to result from a negative retrocontrol loop which would involve kinase C and exert its effect upstream of the Ca2+ surge. Indeed, the phorbol ester PMA proved able to reduce the Ca2+ response and to block 5HT action when applied during the first 3 min corresponding to the hormone-dependent period. Such an inhibition was reversed in the presence of 5 microM of the C kinase inhibitor GF109203X and could be bypassed by ionophore, ammonia, and thapsigargin, which trigger a receptor-independent Ca2+ surge.