T-Type Ca2+ Current Activity during Oocyte Growth and Maturation in the Ascidian Styela plicata

Abstract

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.

CaV3.2 T-type channels mediate Ca2+ entry during oocyte maturation and following fertilization

“…Ca 2+ influx currents consistent with T-type channels have been measured previously in mouse eggs (Day et al, 1998;Kang et al, 2007;Peres, 1987). T-type channels are also important during oocyte growth and maturation in ascidians (Gallo et al, 2013), indicating that the role of these channels might be conserved.…”

Ion currents modulating oocyte maturation in animals

“…L-type calcium channels, however, do not represent the unique type of calcium channels found in ascidian oocytes that may play a role during oogenesis. Recently, in other ascidian species, we observed, for the first time in oocytes, T-type calcium channels [Gallo et al 2012], which are typically involved in sperm physiology [Jagannathan et al 2002]. This novel finding suggests new insights for the mechanism of calcium currents during oocyte maturation.…”

Calcium signals and oocyte maturation in marine invertebrates

“…In addition, the activity of T-type calcium channels increases as immature oocytes of Styela plicata grow larger during prophase I arrest, and in this species, such increased activity is correlated with the ability of some fully-grown prophase-arrested oocytes to undergo normal post-fertilization GVBD and subsequent embryogenesis (Gallo et al, 2013). Furthermore, compared to metaphase-arrested oocytes of Ciona intestinalis, prophase-arrested specimens generate lower amplitude calcium transients in response to either IP 3 injections or ionophore treatments (Cuomo et al, 2006).…”

“…All ascidians are hermaphrodites that produce follicle-enclosed oocytes, and depending on the species examined, fully grown oocytes in the ovary can either remain arrested at prophase I or undergo GVBD (Lambert, 2005). After spawning, ascidian oocytes mature to a metaphase I arrest before being fertilized (Costache et al, 2014), although in vitro fertilization of certain GV-containing ascidian oocytes also yields some normal development (Gallo et al, 2013).…”