The roles of Ca2+, downstream protein kinases, and oscillatory signaling in regulating fertilization and the activation of development

AbstractReviews in Developmental Biology have covered the pathways that generate the all-important intracellular calcium (Ca2+) signal at fertilization [Miyazaki, S., Shirakawa, H., Nakada, K., Honda, Y., 1993a. Essential role of the inositol 1,4,5-trisphosphate receptor/Ca2+ release channel in Ca2+ waves and Ca2+ oscillations at fertilization of mammalian eggs. Dev. Biol. 158, 62–78; Runft, L., Jaffe, L., Mehlmann, L., 2002. Egg activation at fertilization: where it all begins. Dev. Biol. 245, 237–254] and the different temporal responses of Ca2+ in many organisms [Stricker, S., 1999. Comparative biology of calcium signaling during fertilization and egg activation in animals. Dev. Biol. 211, 157–176]. Those reviews raise the importance of identifying how Ca2+ causes the events of egg activation (EEA) and to what extent these temporal Ca2+ responses encode developmental information. This review covers recent studies that have analyzed how these Ca2+ signals are interpreted by specific proteins, and how these proteins regulate various EEA responsible for the onset of development. Many of these proteins are protein kinases (CaMKII, PKC, MPF, MAPK, MLCK) whose activity is directly or indirectly regulated by Ca2+, and whose amount increases during late oocyte maturation. We cover biochemical progress in defining the signaling pathways between Ca2+ and the EEA, as well as discuss how oscillatory or multiple Ca2+ signals are likely to have specific advantages biochemically and/or developmentally. These emerging concepts are put into historical context, emphasizing that key contributions have come from many organisms. The intricate interdependence of Ca2+, Ca2+-dependent proteins, and the EEA raise many new questions for future investigations that will provide insight into the extent to which fertilization-associated signaling has long-range implications for development. In addition, answers to these questions should be beneficial to establishing parameters of egg quality for human and animal IVF, as well as improving egg activation protocols for somatic cell nuclear transfer to generate stem cells and save endangered species

Conotoxins as Tools to Understand the Physiological Function of Voltage-Gated Calcium (CaV) Channels

Voltage-gated calcium (CaV) channels are widely expressed and are essential for the completion of multiple physiological processes. Close regulation of their activity by specific inhibitors and agonists become fundamental to understand their role in cellular homeostasis as well as in human tissues and organs. CaV channels are divided into two groups depending on the membrane potential required to activate them: High-voltage activated (HVA, CaV1.1–1.4; CaV2.1–2.3) and Low-voltage activated (LVA, CaV3.1–3.3). HVA channels are highly expressed in brain (neurons), heart, and adrenal medulla (chromaffin cells), among others, and are also classified into subtypes which can be distinguished using pharmacological approaches. Cone snails are marine gastropods that capture their prey by injecting venom, “conopeptides”, which cause paralysis in a few seconds. A subset of conopeptides called conotoxins are relatively small polypeptides, rich in disulfide bonds, that target ion channels, transporters and receptors localized at the neuromuscular system of the animal target. In this review, we describe the structure and properties of conotoxins that selectively block HVA calcium channels. We compare their potency on several HVA channel subtypes, emphasizing neuronal calcium channels. Lastly, we analyze recent advances in the therapeutic use of conotoxins for medical treatments

Sperm Factor Initiates Capacitance and Conductance Changes in Mouse Eggs That Are More Similar to Fertilization Than IP3- or Ca2+-induced Changes

AbstractWe used patch clamp electrophysiology and concurrent imaging with the Ca2+-sensitive dye, fura-2, to study the temporal relationship between membrane capacitance and conductance and intracellular free Ca2+ concentration ([Ca2+]i) during mouse egg fertilization. We found an ∼2 pF step increase in egg membrane capacitance and a minor increase in conductance with no change in [Ca2+]i at sperm fusion. This was followed ∼1 min later by a rise in [Ca2+]i that led to larger changes in capacitance and conductance. The most common pattern for these later capacitance changes was an initial capacitance decrease, followed by a larger increase and eventual return to the approximate starting value. There was some variation in this pattern, and sub-μM peak [Ca2+]i favored capacitance decrease, while higher [Ca2+]i favored capacitance increase. The magnitude of accompanying conductance increases was variable and did not correlate well with peak [Ca2+]i. The intracellular introduction of porcine sperm factor reproduced the postfusion capacitance and conductance changes with a similar [Ca2+]i dependence. Raising [Ca2+]i by the intracellular introduction of IP3 initiated fertilization-like capacitance changes, but the conductance changes were slower to activate. Capacitance decrease could be induced when [Ca2+]i was increased modestly by activation of an endogenous Ca2+ current, with little effect on resting conductance. These results suggest that net turnover of the mouse egg surface membrane is sensitive to [Ca2+]i and that sperm and the active component of sperm factor may be doing more than initiating the IP3-mediated release of intracellular Ca2+

Partial Characterization of the Calcium-Releasing Activity of Porcine Sperm Cytosolic Extracts

AbstractInjection of sperm cytosolic extracts into mammalian eggs has been shown to elicit intracellular calcium ([Ca2+]i) oscillations that are similar in amplitude, duration, and frequency to those observed following fertilization. Thus, to characterize the Ca2+-release component(s) in porcine sperm cytosolic extracts, a combination of fractionation techniques was used. The fraction with Ca2+releasing activity was precipitated by 50% saturating solutions of ammonium sulfate and Western blot analysis showed that the pellets contained glucosamine-6-phosphate deaminase (gpd)/oscillin, a protein which has been suggested to be the sperm's active component. Single and double isoelectrofocusing (IEF) of porcine sperm extracts generated fractions with different Ca2+-releasing activities. Fractions with maximal Ca2+-releasing activity did not contain material that was immunoreactive with antibodies against gpd/oscillin; adjacent fractions containing gpd/oscillin had no Ca2+-releasing activity. These findings were confirmed by IEF coupled with size exclusion chromatography on Superose 12 and with hydroxyapatite chromatography. These procedures predict an isoelectric point of our active component of 6.5–7.0 and a relative molecular weight ranging from 29 to 68 kDa. In summary, the data show that the Ca2+release-inducing component(s) of porcine sperm extracts can be fractionated and that gpd/oscillin is not the pig sperm Ca2+oscillogen

TRPM7-like Channels are Functionally Expressed in Oocytes and Modulate Post-Fertilization Embryo Development in Mouse

The Transient Receptor Potential (TRP) channels are a family of cationic ion channels widely distributed in mammalian tissues. In general, the global genetic disruption of individual TRP channels result in phenotypes associated with impairment of a particular tissue and/or organ function. An exception is the genetic ablation of the TRP channel TRPM7, which results in early embryonic lethality. Nevertheless, the function of TRPM7 in oocytes, eggs and pre-implantation embryos remains unknown. Here, we described an outward rectifying non-selective current mediated by a TRP ion channel in immature oocytes (germinal vesicle stage), matured oocytes (metaphase II eggs) and 2-cell stage embryos. The current is activated by specific agonists and inhibited by distinct blockers consistent with the functional expression of TRPM7 channels. We demonstrated that the TRPM7-like channels are homo-tetramers and their activation mediates calcium influx in oocytes and eggs, which is fundamental to support fertilization and egg activation. Lastly, we showed that pharmacological inhibition of the channel function delays pre-implantation embryo development and reduces progression to the blastocyst stage. Our data demonstrate functional expression of TRPM7-like channels in mouse oocytes, eggs and embryos that may play an essential role in the initiation of embryo development

Phospholipase Cδ4 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 PLCδ4 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 PLCδ4−/− sperm. Progesterone, another physiological inducer of the acrosome reaction, failed to induce sustained [Ca2+]i increases in PLCδ4−/− 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 PLCδ4−/− sperm. These results indicate that PLCδ4 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

Parthenogenetic activation of bovine oocytes using bovine and murine phospholipase C zeta

Background - During natural fertilization, sperm fusion with the oocyte induces long lasting intracellular calcium oscillations which in turn are responsible for oocyte activation. PLCZ1 has been identified as the factor that the sperm delivers into the egg to induce such a response. We tested the hypothesis that PLCZ1 cRNA injection can be used to activate bovine oocytes. Results - Mouse and bovine PLCZ1 cRNAs were injected into matured bovine oocytes at different concentrations. Within the concentrations tested, mouse PLCZ1 injection activated bovine oocytes at a maximum rate when the pipette concentration of cRNA ranged from 0.25 to 1 μg/μL, while bovine PLCZ1 was optimal at 0.1 μg/μL. At their most effective concentrations, PLCZ1 induced parthenogenetic development at rates similar to those observed using other activation stimuli such as Ionomycin/CHX and Ionomycin/DMAP. Injection of mouse and bovine PLCZ1 cRNA induced dose-dependent sperm-like calcium oscillations whose frequency increased over time. Injection of bovine and mouse PLCZ1 cRNA also induced IP3R-1 degradation, although bovine PLCZ1 cRNA evoked greater receptor degradation than its mouse counterpart. Conclusion - Injection of PLCZ1 cRNA efficiently activated bovine oocytes by inducing a sperm-like calcium oscillatory pattern. Importantly, the high rate of aneuploidy encountered in parthenogenetic embryos activated by certain chemical means was not observed in PLCZ1 activated embryos

Inositol 1,4,5-trisphosphate receptor 1, a widespread Ca2+ channel, is a novel substrate of polo-like kinase 1 in eggs

AbstractTo initiate embryo development, the sperm induces in the egg release of intracellular calcium ([Ca2+]i). During oocyte maturation, the inositol 1,4,5-trisphosphate receptor (IP3R1), the channel implicated, undergoes modifications that enhance its function. We found that IP3R1 becomes phosphorylated during maturation at an MPM-2 epitope and that this persists until the fertilization-associated [Ca2+]i responses cease. We also reported that maturation without ERK activity diminishes IP3R1 MPM-2 reactivity and [Ca2+]i responses. Here, we show that IP3R1 is a novel target for Polo-like kinase1 (Plk1), a conserved M-phase kinase, which phosphorylates it at an MPM-2 epitope. Plk1 and IP3R1 interact in an M-phase preferential manner, and they exhibit close co-localization in the spindle/spindle poles area. This co-localization is reduced in the absence of ERK activity, as the ERK pathway regulates spindle organization and IP3R1 cortical re-distribution. We propose that IP3R1 phosphorylation by Plk1, and possibly by other M-phase kinases, underlies the delivery of spatially and temporally regulated [Ca2+]i signals during meiosis/mitosis and cytokinesis

Beneficial Effect of Young Oocytes for Rabbit Somatic Cell Nuclear Transfer

Abstract This study was designed to examine the effect of the age of rabbit oocytes on the developmental potential of cloned embryos. The metaphase II oocytes used for nuclear transfer (NT) were collected at 10, 12, 14, and 16h post-hCG injection (hpi). The total number of oocytes collected per donor (21.4-23.7) at 12 to 16 hpi was similar, but significantly higher than that collected at 10 hpi (16.2). Additionally, a significant improvement in blastocyst development was achieved with embryos generated by electrically mediated cell fusion (56.0%), compared to those from nuclear injection (13.1 %) (Experiment 1). Markedly higher blastocyst development (45.8-54.5%) was also achieved with oocytes collected at 10-12 hpi than from those collected 14-16 hpi (8.3-14.3%) (Experiment 2). In Experiment 3, the blastocyst rates of NT embryos derived from oocytes harvested 12 hpi (39.2-42.8 %) were significantly higher than from those collected at 16 hpi (6.8-8.4 %) (p<0.05), regardless of the donor cell age. Kinase activity assays showed variable changes of activity in rabbit oocytes over the period of 10-16 hpi; however, there was no correlation with preimplantational development (blastocyst rate vs. MPF, R=0.326; blastocyst rate vs. MAPK, R=0.131). Embryo transfer of NT embryos utilizing 12 hpi oocytes resulted in one full-term but stillborn, and one live cloned rabbit; thus, an efficiency of 1.7 % (n=117) (Experiment 4). These results demonstrated that NT utilizing relatively young rabbit oocytes, harvested at 10-12h after hCG injection, was beneficial for the development of NT embryos.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78139/1/clo.2008.0042.pd