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Manabu Kurokawa

Publications and source records attributed to Manabu Kurokawa.

8 recordsLinked to original sources
PubMed2003

Release of the Ca(2+) oscillation-inducing sperm factor during mouse fertilization.

A cytosolic sperm protein(s), referred to as the sperm factor (SF), is thought to induce intracellular calcium ([Ca(2+)](i)) oscillations during fertilization in mammalian eggs. These oscillations, which are responsible for inducing complete egg activation, persist for several hours. Nevertheless, whether a protracted release of SF is responsible for the duration of the oscillations is unknown. Using a combination of intracytoplasmic sperm injection (ICSI), in vitro fertilization (IVF), sperm removal, reinjection of the withdrawn sperm, and [Ca(2+)](i) monitoring, we determined that 30 min was necessary for establishing oscillations. Importantly, a significant portion of the Ca(2+) activity became dissociated from the sperm within 15-60 min after entry, and by 120 min post-ICSI or IVF, sperm were unable to induce oscillations. The initiation of oscillations coincided with exposure and solubilization of the perinuclear theca (PT), as evidenced by transmission electron microscopy, although disassembly of the PT was not required for commencement of the [Ca(2+)](i) responses. Remarkably, despite its complete release into the ooplasm, SF associated with nuclear structures at the time of pronuclear formation. Lastly, release of SF was not affected by the cell cycle. We conclude that mouse sperm serves as a carrier for SF, which is rapidly and completely solubilized to establish [Ca(2+)](i) oscillations.

Animals↗
PubMed2003

Reconstitution of Src-dependent phospholipase Cgamma phosphorylation and transient calcium release by using membrane rafts and cell-free extracts from Xenopus eggs.

We reported previously that egg membrane rafts serve as a subcellular microdomain for sperm-dependent tyrosine kinase signaling in Xenopus fertilization. Moreover, we demonstrated that raft-associated Src tyrosine kinase was activated by sperm in vitro. Here we show that egg rafts incubated with sperm or hydrogen peroxide (H2O2) can promote Src-dependent phosphorylation of phospholipase Cgamma (PLCgamma) and transient calcium release in the extracts of unfertilized Xenopus eggs. In vivo egg activation by sperm or H2O2 also promotes tyrosine phosphorylation and raft-translocalization of PLCgamma. Immunodepletion of PLCgamma from the egg extracts inhibits the raft-dependent calcium release. Rafts prepared from H2O2-activated eggs also promote Src-dependent dephosphorylation of p42 mitogen-activated protein kinase and cell cycle transition from metaphase II to interphase in egg extracts. PLCgamma phosphorylation and calcium release in egg extracts can be promoted by rafts prepared from COS-7 cells expressing the Xenopus Src gene. These results demonstrate that the signaling events elicited by fertilization in Xenopus eggs can be reconstituted in vitro. The development of such experimental platforms will allow us to dissect the molecular mechanism of sperm-dependent activation of raft-associated Src and subsequent up-regulation of PLCgamma and egg activation machinery in Xenopus eggs.

Animals↗
PubMed2003

Phospholipase Cdelta4 is required for Ca2+ mobilization essential for acrosome reaction in sperm.

Zona pellucida (ZP)-induced acrosome reaction in sperm is a required step for mammalian fertilization. However, the precise mechanism of the acrosome reaction remains unclear. We previously reported that PLCdelta4 is involved in the ZP-induced acrosome reaction in mouse sperm. Here we have monitored Ca2+ responses in single sperm, and we report that the [Ca2+]i increase in response to ZP, which is essential for driving the acrosome reaction in vivo, is absent in PLCdelta4-/- sperm. Progesterone, another physiological inducer of the acrosome reaction, failed to induce sustained [Ca2+]i increases in PLCdelta4-/- sperm, and consequently the acrosome reaction was partially inhibited. In addition, we observed oscillatory [Ca2+]i increases in wild-type sperm in response to these acrosome inducers. Calcium imaging studies revealed that the [Ca2+]i increases induced by exposure to ZP and progesterone started at different sites within the sperm head, indicating that these agonists induce the acrosome reaction via different Ca2+ mechanisms. Furthermore, store-operated channel (SOC) activity was severely impaired in PLCdelta4-/- sperm. These results indicate that PLCdelta4 is an important enzyme for intracellular [Ca2+]i mobilization in the ZP-induced acrosome reaction and for sustained [Ca2+]i increases through SOC induced by ZP and progesterone in sperm.

Acrosome↗
PubMed2003

ICSI-generated mouse zygotes exhibit altered calcium oscillations, inositol 1,4,5-trisphosphate receptor-1 down-regulation, and embryo development.

ICSI bypasses not only fusion of the gametes but also a series of signalling events that occur in the sperm prior to and during interaction with the oocyte's vestments. The effect of this altered encounter of the gametes on the fertilization-associated intracellular calcium ([Ca2+]i) oscillations has not been thoroughly investigated. Here, ICSI and IVF were performed using gametes from two mouse strains, B6D2F1 and CD1, and in-vitro development, pattern of [Ca2+]i oscillations and down-regulation of inositol 1,4,5-trisphosphate receptor-1 (IP3R-1) in the produced embryos were compared. ICSI and IVF resulted in comparable rates of activation and pre-implantation development. However, ICSI-generated zygotes cleaved at a slower rate, had lower cell numbers and lower hatching rates. The deleterious effects of ICSI could not be exclusively attributed to the injury by the injection since sham-injected IVF zygotes only exhibited delayed progression to the blastocyst stage. ICSI and IVF induced similar initial [Ca2+]i responses, although ICSI zygotes exhibited shorter durations of [Ca2+]i oscillations and showed diminished degradation of IP3R-1. Importantly, sperm manipulation affected the pattern of oscillations, which further decreased pre-implantation developmental rates. Our results demonstrate that ICSI-induced [Ca2+]i responses are not equivalent to those initiated by IVF and that this may have developmental consequences.

Animals↗
PubMed2002

Modifications of the Ca2+ release mechanisms of mouse oocytes by fertilization and by sperm factor.

A cytosolic factor from sperm (SF) is thought to be responsible for the generation of intracellular calcium oscillations ([Ca2+]i) associated with fertilization in mammalian oocytes. Whether or not mouse oocytes injected with SF exhibit modifications of their Ca2+ release mechanisms similar to those observed in fertilized oocytes is not known and this was investigated here by injecting porcine SF (pSF). First, pSF-activated oocytes injected with CaCl2 showed persistent sensitization of the Ca2+-induced Ca2+ release mechanism, but this sensitization was absent in SrCl2-activated oocytes. Second, pSF-injected oocytes re-initiated oscillations when fused with untreated oocytes, although the Ca2+ responses were short-lived compared to those initiated by fertilization. Likewise, in the presence of colcemid, pSF-initiated oscillations were prolonged but ceased in advance of those in fertilized zygotes. Also, pronuclear envelope breakdown induced by okadaic acid was not associated with Ca2+ release in pSF-generated zygotes, whereas it was observed in fertilized zygotes. Finally, roscovitine, an inhibitor of maturation promoting factor, blocked pSF-induced [Ca2+]i oscillations. Together, these results show that pSF-induced [Ca2+]i responses exhibit properties similar to those triggered by the sperm, although the SF's Ca2+ active component(s) may be less stable or more susceptible to degradation, resulting in shorter modification of the oocyte's Ca2+ release mechanisms.

Animals↗
PubMed2002

Intracellular calcium oscillations signal apoptosis rather than activation in in vitro aged mouse eggs.

We have previously demonstrated that initiation of intracellular calcium ([Ca2+]i) oscillations in mouse eggs signals activation or apoptotic death depending on the age of the eggs in which the oscillations are induced. To extend these studies, mouse eggs were aged in vitro to 24, 32, and 40 h post-hCG and injected with sperm cytosolic factor (SF), adenophostin A, or sperm (intracytoplasmic sperm injection), and the times at which signs of apoptosis first appeared were examined. These treatments, which induced [Ca2+]i oscillations, caused fragmentation and other signs of programmed cell death in eggs as early as 32 h post-hCG. The susceptibility of aged eggs to apoptosis appeared to be due to cytoplasmic deficiencies, because fusion of recently ovulated eggs with aged, SF-injected eggs prevented fragmentation. Evaluation of mRNA and protein levels of the apoptotic regulatory proteins Bcl-2 and Bax showed a prominent decrease in the amounts of Bcl-2 mRNA and protein in aged eggs, whereas Bax mRNA levels did not appear to be changed. Lastly, the Ca2+ responses induced by the aforementioned Ca2+ agonists ceased in advance in aged eggs. Together, these results suggest that one or several critical cytosolic molecules involved in the regulation of Ca2+ homeostasis, and in maintaining the equilibrium between anti- and proapoptotic proteins, is either lost or inactivated during postovulatory egg aging, rendering the fertilizing Ca2+ signal into an apoptosis-inducing signal.

Adenosine↗
PubMed2002

Mechanisms underlying oocyte activation and postovulatory ageing.

Mammalian oocytes undergo significant growth during oogenesis and experience extensive cytoplasmic and nuclear modifications immediately before ovulation in a process commonly referred to as oocyte maturation. These changes are intended to maximize the developmental success after fertilization. Entry of a spermatozoon into the oocyte, which occurs a few hours after ovulation, initiates long-lasting oscillations in the free intracellular calcium ([Ca(2+)](i)) that are responsible for all events of oocyte activation and the initiation of the developmental programme that often culminates in the birth of young. Nevertheless, the cellular and molecular changes that occur during maturation to optimize development are transient, and exhibit rapid deterioration. Moreover, fertilization of oocytes after an extended residence in the oviduct (or in culture) initiates a different developmental programme, one that is characterized by fragmentation, programmed cell death, and abnormal development. Inasmuch as [Ca(2+)](i) oscillations can trigger both developmental programmes in mammalian oocytes, this review addresses one of the mechanism(s) possibly used by spermatozoa to initiate these persistent [Ca(2+)](i) responses, and the cellular and molecular changes that may underlie the postovulatory cellular fragmentation of ageing mammalian oocytes.

Animals↗