Calcium and fertilization: the beginning of life

Dithiothreitol Affects the Fertilization Response in Immature and Maturing Starfish Oocytes

Immature starfish oocytes isolated from the ovary are susceptible to polyspermy due to the structural organization of the vitelline layer covering the oocyte plasma membrane, as well as the distribution and biochemical properties of the actin cytoskeleton of the oocyte cortex. After the resumption of the meiotic cycle of the oocyte triggered by the hormone 1-methyladenine, the maturing oocyte reaches fertilizable conditions to be stimulated by only one sperm with a normal Ca2+ response and cortical reaction. This cytoplasmic ripening of the oocyte, resulting in normal fertilization and development, is due to the remodeling of the cortical actin cytoskeleton and germinal vesicle breakdown (GVBD). Since disulfide-reducing agents such as dithiothreitol (DTT) are known to induce the maturation and GVBD of oocytes in many species of starfish, we analyzed the pattern of the fertilization response displayed by Astropecten aranciacus oocytes pre-exposed to DTT with or without 1-MA stimulation. Short treatment of A. aranciacus immature oocytes with DTT reduced the rate of polyspermic fertilization and altered the sperm-induced Ca2+ response by changing the morphology of microvilli, cortical granules, and biochemical properties of the cortical F-actin. At variance with 1-MA, the DTT treatment of immature starfish oocytes for 70 min did not induce GVBD. On the other hand, the DTT treatment caused an alteration in microvilli morphology and a drastic depolymerization of the cortical F-actin, which impaired the sperm-induced Ca2+ response at fertilization and the subsequent embryonic development.

The Effect of Acidic and Alkaline Seawater on the F-Actin-Dependent Ca2+ Signals Following Insemination of Immature Starfish Oocytes and Mature Eggs

In starfish, the addition of the hormone 1-methyladenine (1-MA) to immature oocytes (germinal vesicle, GV-stage) arrested at the prophase of the first meiotic division induces meiosis resumption (maturation), which makes the mature eggs able to respond to the sperm with a normal fertilization response. The optimal fertilizability achieved during the maturation process results from the exquisite structural reorganization of the actin cytoskeleton in the cortex and cytoplasm induced by the maturing hormone. In this report, we have investigated the influence of acidic and alkaline seawater on the structure of the cortical F-actin network of immature oocytes of the starfish (Astropecten aranciacus) and its dynamic changes upon insemination. The results have shown that the altered seawater pH strongly affected the sperm-induced Ca²⁺ response and the polyspermy rate. When immature starfish oocytes were stimulated with 1-MA in acidic or alkaline seawater, the maturation process displayed a strong dependency on pH in terms of the dynamic structural changes of the cortical F-actin. The resulting alteration of the actin cytoskeleton, in turn, affected the pattern of Ca²⁺ signals at fertilization and sperm penetration.

Promising Molecular Targets in Pharmacological Therapy for Neuronal Damage in Brain Injury

The complex etiopathogenesis of brain injury associated with neurodegeneration has sparked a lot of studies in the last century. These clinical situations are incurable, and the currently available therapies merely act on symptoms or slow down the course of the diseases. Effective methods are being sought with an intent to modify the disease, directly acting on the properly studied targets, as well as to contribute to the development of effective therapeutic strategies, opening the possibility of refocusing on drug development for disease management. In this sense, this review discusses the available evidence for mitochondrial dysfunction induced by Ca2+ miscommunication in neurons, as well as how targeting phosphorylation events may be used to modulate protein phosphatase 2A (PP2A) activity in the treatment of neuronal damage. Ca2+ tends to be the catalyst for mitochondrial dysfunction, contributing to the synaptic deficiency seen in brain injury. Additionally, emerging data have shown that PP2A-activating drugs (PADs) suppress inflammatory responses by inhibiting different signaling pathways, indicating that PADs may be beneficial for the management of neuronal damage. In addition, a few bioactive compounds have also triggered the activation of PP2A-targeted drugs for this treatment, and clinical studies will help in the authentication of these compounds. If the safety profiles of PADs are proven to be satisfactory, there is a case to be made for starting clinical studies in the setting of neurological diseases as quickly as possible.

Science CommuniCa2+tion Developing Scientific Literacy on Calcium: The Involvement of CRAC Currents in Human Health and Disease

All human life starts with a calcium (Ca2+) wave. This ion regulates a plethora of cellular functions ranging from fertilisation and birth to development and cell death. A sophisticated system is responsible for maintaining the essential, tight concentration of calcium within cells. Intricate components of this Ca2+ network are store-operated calcium channels in the cells' membrane. The best-characterised store-operated channel is the Ca2+ release-activated Ca2+ (CRAC) channel. Currents through CRAC channels are critically dependent on the correct function of two proteins: STIM1 and Orai1. A disruption of the precise mechanism of Ca2+ entry through CRAC channels can lead to defects and in turn to severe impacts on our health. Mutations in either STIM1 or Orai1 proteins can have consequences on our immune cells, the cardiac and nervous system, the hormonal balance, muscle function, and many more. There is solid evidence that altered Ca2+ signalling through CRAC channels is involved in the hallmarks of cancer development: uncontrolled cell growth, resistance to cell death, migration, invasion, and metastasis. In this work we highlight the importance of Ca2+ and its role in human health and disease with focus on CRAC channels.

Regulation of the Actin Cytoskeleton-Linked Ca2+ Signaling by Intracellular pH in Fertilized Eggs of Sea Urchin

In sea urchin, the immediate contact of the acrosome-reacted sperm with the egg surface triggers a series of structural and ionic changes in the egg cortex. Within one minute after sperm fuses with the egg plasma membrane, the cell membrane potential changes with the concurrent increases in intracellular Ca²⁺ levels. The consequent exocytosis of the cortical granules induces separation of the vitelline layer from the egg plasma membrane. While these cortical changes are presumed to prevent the fusion of additional sperm, the subsequent late phase (between 1 and 4 min after fertilization) is characterized by reorganization of the egg cortex and microvilli (elongation) and by the metabolic shift to activate de novo protein and DNA syntheses. The latter biosynthetic events are crucial for embryonic development. Previous studies suggested that the early phase of fertilization was not a prerequisite for these changes in the second phase since the increase in the intracellular pH induced by the exposure of unfertilized sea urchin eggs to ammonia seawater could start metabolic egg activation in the absence of the cortical granule exocytosis. In the present study, we have demonstrated that the incubation of unfertilized eggs in ammonia seawater induced considerable elongations of microvilli (containing actin filaments) as a consequence of the intracellular pH increase, which increased the egg’s receptivity to sperm and made the eggs polyspermic at fertilization despite the elevation of the fertilization envelope (FE). These eggs also displayed compromised Ca²⁺ signals at fertilization, as the amplitude of the cortical flash was significantly reduced and the elevated intracellular Ca²⁺ level declined much faster. These results have also highlighted the importance of the increased internal pH in regulating Ca²⁺ signaling and the microvillar actin cytoskeleton during the late phase of the fertilization process.

Effects of Dithiothreitol on Fertilization and Early Development in Sea Urchin

The vitelline layer (VL) of a sea urchin egg is an intricate meshwork of glycoproteins that intimately ensheathes the plasma membrane. During fertilization, the VL plays important roles. Firstly, the receptors for sperm reside on the VL. Secondly, following cortical granule exocytosis, the VL is elevated and transformed into the fertilization envelope (FE), owing to the assembly and crosslinking of the extruded materials. As these two crucial stages involve the VL, its alteration was expected to affect the fertilization process. In the present study, we addressed this question by mildly treating the eggs with a reducing agent, dithiothreitol (DTT). A brief pretreatment with DTT resulted in partial disruption of the VL, as judged by electron microscopy and by a novel fluorescent polyamine probe that selectively labelled the VL. The DTT-pretreated eggs did not elevate the FE but were mostly monospermic at fertilization. These eggs also manifested certain anomalies at fertilization: (i) compromised Ca2+ signaling, (ii) blocked translocation of cortical actin filaments, and (iii) impaired cleavage. Some of these phenotypic changes were reversed by restoring the DTT-exposed eggs in normal seawater prior to fertilization. Our findings suggest that the FE is not the decisive factor preventing polyspermy and that the integrity of the VL is nonetheless crucial to the egg's fertilization response.

Fertilization and development of Arbacia lixula eggs are affected by osmolality conditions

The sea urchin Arbacia lixula coexist with Paracentrotus lividus in the Mediterranean, but the two sea urchin species are quite different from each other. Concerning the female gamete, A. lixula eggs are much darker than those of P. lividus due to the characteristic pigmentation. Upon insemination, the fertilization envelope formed by A. lixula eggs is remarkably thinner than that of P. livius eggs, which implies that the cortical organization of the eggs in the two species may be quite different. In this communication, we examined the phenotypic plasticity of A. lixula eggs in the changing osmolality. The plasma membrane, cortical actin cytoskeleton and vesicles are extensively altered in the eggs exposed to 40% seawater for 15 min. When fertilized, the Ca²⁺ response in these eggs was significantly compromised and the sperm often failed to enter the eggs. Remarkably, the pattern of the Ca²⁺ response was restored when these eggs were transferring back to the natural seawater before fertilization, while the actin cytoskeleton partially reverted to the original state. Nonetheless, these eggs restored in seawater failed to regain the innate sperm receptivity that allows only one sperm to enter in natural seawater. Thus, the ability to guide monospermic fertilization is lost by water entry into the eggs, and the eggs incorporated either multiple or no sperm. On the other hand, eggs briefly exposed to hypertonic seawater exhibited no evident morphological anomaly. Nonetheless, the monospermic eggs that experienced a brief exposure (15 min) to hypertonic seawater prior to fertilization in natural seawater displayed a subtly altered sperm-induced Ca²⁺ response and morpho-functional anomaly around the pluteus stage. Our results suggest that A. lixula eggs attain only a limited extent of cytological plasticity, and that the osmolality shock affects the physical nature of the egg surface which in turn affects the developmental programming.

Effects of Salinity and pH of Seawater on the Reproduction of the Sea Urchin Paracentrotus lividus

AbstractFertilization and early development are usually the most vulnerable stages in the life of marine animals, and the biological processes during this period are highly sensitive to the environment. In nature, sea urchin gametes are shed in seawater, where they undergo external fertilization and embryonic development. In a laboratory, it is possible to follow the exact morphological and biochemical changes taking place in the fertilized eggs and the developing embryos. Thus, observation of successful fertilization and the subsequent embryonic development of sea urchin eggs can be used as a convenient biosensor to assess the quality of the marine environment. In this paper, we have examined how salinity and pH changes affect the normal fertilization process and the following development of Paracentrotus lividus. The results of our studies using confocal microscopy, scanning and transmission electron microscopy, and time-lapse Ca²⁺ image recording indicated that both dilution and acidification of seawater have subtle but detrimental effects on many aspects of the fertilization process. They include Ca²⁺ signaling and coordinated actin cytoskeletal changes, leading to a significantly reduced rate of successful fertilization and, eventually, to abnormal or delayed embryonic development.

A Microfluidic Probe Integrated Device for Spatiotemporal 3D Chemical Stimulation in Cells

Numerous in vitro studies have been conducted in conventional static cell culture systems. However, most of the results represent an average response from a population of cells regardless of their local microenvironment. A microfluidic probe is a non-contact technology that has been widely used to perform local chemical stimulation within a restricted space, providing elaborated modulation and analysis of cellular responses within the microenvironment. Although microfluidic probes developed earlier have various potential applications, the two-dimensional structure can compromise their functionality and flexibility for practical use. In this study, we developed a three-dimensional microfluidic probe integrated device equipped with vertically oriented microchannels to overcome crucial challenges and tested the potential utility of the device in biological research. We demonstrated that the device tightly regulated spatial diffusion of a fluorescent molecule, and the flow profile predicted by simulation replicated the experimental results. Additionally, the device modulated the physiological Ca²⁺ response of cells within the restricted area by altering the local and temporal concentrations of biomolecules such as ATP. The novel device developed in this study may provide various applications for biological studies and contribute to further understanding of molecular mechanisms underlying cellular physiology.

Modeling the mechanics of calcium regulation in T lymphocyte: A finite element method approach

Changes in cellular Ca[Formula: see text] concentration control a variety of physiological activities including hormone and neurotransmitter release, muscular contraction, synaptic plasticity, ionic channel permeability, apoptosis, enzyme activity, gene transcription and reproduction process. Spatial–temporal Ca[Formula: see text] dynamics due to Ca[Formula: see text] release, buffering and re-uptaking plays a central role in studying the Ca[Formula: see text] regulation in T lymphocytes. In most cases, Ca[Formula: see text] has its major signaling function when it is elevated in the cytosolic compartment. In this paper, a two-dimensional mathematical model to study spatiotemporal variations of intracellular Ca[Formula: see text] concentration in T lymphocyte cell is proposed and investigated. The cell is assumed to be a circular shaped geometrical domain for the representation of properties of Ca[Formula: see text] dynamics within the cell including important parameters. Ca[Formula: see text] binding proteins for the dynamics of Ca[Formula: see text] are itself buffer and other physiological parameters located in Ca[Formula: see text] stores. The model incorporates the important biophysical processes like diffusion, reaction, voltage-gated Ca[Formula: see text] channel, leak from endoplasmic reticulum (ER), efflux from cytosol to ER via sarco–ER Ca[Formula: see text] adenosine triphosphate (SERCA) pumps, buffers and Na[Formula: see text] exchanger. The proposed mathematical model is solved using a finite difference method and the finite element method. Appropriate initial and boundary conditions are incorporated in the model based on biophysical conditions of the problem. Computer simulations in MATLAB R2019b are employed to investigate mathematical models of reaction–diffusion equation. The effect of source, buffer, Na[Formula: see text]/Ca[Formula: see text] exchanger, etc. on spatial and temporal patterns of Ca[Formula: see text] in T lymphocyte has been studied with the help of numerical results. From the obtained results, it is observed that, the coordinated combination of the incorporated parameters plays a significant role in Ca[Formula: see text] regulation in T lymphocytes. ER leak and voltage-gated Ca[Formula: see text] channel provides the necessary Ca[Formula: see text] to the cell when required for its proper functioning, while on the other side buffers, SERCA pump and Na[Formula: see text]/Ca[Formula: see text] exchanger makes balance in the Ca[Formula: see text] concentration, so as to prevent the cell from death as higher concentration for longer time is harmful for the cell and can cause cell death.

Oxygen supersaturation mitigates the impact of the regime of contaminated sediment reworking on sea urchin fertilization process

Dismissed industrial plants with chronic environmental contamination globally affect all levels of biological organization in concert with other natural and anthropogenic perturbations. Assessing the impact of such perturbations and finding effective ways to mitigate them have clear ecological and societal implications. Through indoor manipulative experiments, we assessed here the effects of the temporal regime of reworking of contaminated sediment from the Bagnoli-Coroglio brownfield (Tyrrhenian Sea, Italy) on the fertilization process in Paracentrotus lividus. Adult sea urchins were kept for one month in tanks containing contaminated sediment that was re-suspended according to two temporal patterns of water turbulence differing in the time intervals between consecutive events of agitation (mimicking the storms naturally occurring in the study area) in seawater with natural vs. supersaturated oxygenation levels. At the end of the treatment, gametes were collected and used to test the hypothesis that the regime of contaminated sediment reworking negatively, but reversibly, affects morphological and physiological traits of the fertilized eggs. We found that aggregated events of sediment re-suspension had profound negative effects on gamete interactions and Ca²⁺ signaling at fertilization. The same experimental condition also inflicted marked ultrastructural changes in eggs. Importantly, however, such detrimental effects were inhibited by increased oxygenation. By contrast, the regime of sediment re-working with a longer interval between consecutive turbulent events had only marginal effects. Thus, the current and predicted changes of climate-related disturbance appear to modulate the biological effects of chronic contamination in post-industrial areas, suggesting that environmental rehabilitation via restoration of habitat-forming primary producers such as seagrasses or algal canopies could alleviate the pollutants' effects on resident biota.

Reactive oxygen species in reproduction: harmful, essential or both?

The process of embryonic development is crucial and radically influences preimplantation embryo competence. It involves oocyte maturation, fertilization, cell division and blastulation and is characterized by different key phases that have major influences on embryo quality. Each stage of the process of preimplantation embryonic development is led by important signalling pathways that include very many regulatory molecules, such as primary and secondary messengers. Many studies, both in vivo and in vitro, have shown the importance of the contribution of reactive oxygen species (ROS) as important second messengers in embryo development. ROS may originate from embryo metabolism and/or oocyte/embryo surroundings, and their effect on embryonic development is highly variable, depending on the needs of the embryo at each stage of development and on their environment (in vivo or under in vitro culture conditions). Other studies have also shown the deleterious effects of ROS in embryo development, when cellular tissue production overwhelms antioxidant production, leading to oxidative stress. This stress is known to be the cause of many cellular alterations, such as protein, lipid, and DNA damage. Considering that the same ROS level can have a deleterious effect on the fertilizing oocyte or embryo at certain stages, and a positive effect at another stage of the development process, further studies need to be carried out to determine the rate of ROS that benefits the embryo and from what rate it starts to be harmful, this measured at each key phase of embryonic development.

Cellular and molecular aspects of oocyte maturation and fertilization: a perspective from the actin cytoskeleton

ABSTRACT

Much of the scientific knowledge on oocyte maturation, fertilization, and embryonic development has come from the experiments using gametes of marine organisms that reproduce by external fertilization. In particular, echinoderm eggs have enabled the study of structural and biochemical changes related to meiotic maturation and fertilization owing to the abundant availability of large and transparent oocytes and eggs. Thus, in vitro studies of oocyte maturation and sperm-induced egg activation in starfish are carried out under experimental conditions that resemble those occurring in nature. During the maturation process, immature oocytes of starfish are released from the prophase of the first meiotic division, and acquire the competence to be fertilized through a highly programmed sequence of morphological and physiological changes at the oocyte surface. In addition, the changes in the cortical and nuclear regions are essential for normal and monospermic fertilization. This review summarizes the current state of research on the cortical actin cytoskeleton in mediating structural and physiological changes during oocyte maturation and sperm and egg activation in starfish and sea urchin. The common denominator in these studies with echinoderms is that exquisite rearrangements of the egg cortical actin filaments play pivotal roles in gamete interactions, Ca2+ signaling, exocytosis of cortical granules, and control of monospermic fertilization. In this review, we also compare findings from studies using invertebrate eggs with what is known about the contributions made by the actin cytoskeleton in mammalian eggs. Since the cortical actin cytoskeleton affects microvillar morphology, movement, and positioning of organelles and vesicles, and the topography of the egg surface, these changes have impacts on the fertilization process, as has been suggested by recent morphological studies on starfish oocytes and eggs using scanning electron microscopy. Drawing the parallelism between vitelline layer of echinoderm eggs and the zona pellucida of mammalian eggs, we also discuss the importance of the egg surface in mediating monospermic fertilization.

GRAPHICAL ABSTRACT

Toward the understanding of biology of oocyte life cycle in Xenopus Laevis: No oocytes left behind

BACKGROUND

For the past more than 25 years, we have been focusing on the developmental and reproductive biology of the female gametes, oocytes, and eggs, of the African clawed frog Xenopus laevis.

METHODS

The events associated with the life cycle of these cells can be classified into the four main categories: first, oogenesis and cell growth in the ovary during the first meiotic arrest; second, maturation and ovulation that occur simultaneously and result in the acquisition of fertilization competence and the second meiotic arrest; third, fertilization, that is sperm-induced transition from egg to zygote; and fourth, egg death after spontaneous activation in the absence of fertilizing sperm.

MAIN FINDINGS

Our studies have demonstrated that signal transduction system involving tyrosine kinase Src and other oocyte/egg membrane-associated molecules such as uroplakin III and some other cytoplasmic proteins such as mitogen-activated protein kinase (MAPK) play important roles for successful ovulation, maturation, fertilization, and initiation of embryonic development.

CONCLUSION

We summarize recent advances in understanding cellular and molecular mechanisms underlying life cycle events of the oocytes and eggs. Our further intention is to discuss and predict potentially promising impact of the recent findings on the challenges facing reproductive biology and medicine, as well as societal contexts.

A Statistical View on Calcium Oscillations

Transient rises and falls of the intracellular calcium concentration have been observed in numerous cell types and under a plethora of conditions. There is now a growing body of evidence that these whole-cell calcium oscillations are stochastic, which poses a significant challenge for modelling. In this review, we take a closer look at recently developed statistical approaches to calcium oscillations. These models describe the timing of whole-cell calcium spikes, yet their parametrisations reflect subcellular processes. We show how non-stationary calcium spike sequences, which e.g. occur during slow depletion of intracellular calcium stores or in the presence of time-dependent stimulation, can be analysed with the help of so-called intensity functions. By utilising Bayesian concepts, we demonstrate how values of key parameters of the statistical model can be inferred from single cell calcium spike sequences and illustrate what information whole-cell statistical models can provide about the subcellular mechanistic processes that drive calcium oscillations. In particular, we find that the interspike interval distribution of HEK293 cells under constant stimulation is captured by a Gamma distribution.

Sodium-mediated fast electrical depolarization does not prevent polyspermic fertilization in Paracentrotus lividus eggs

During sea urchins fertilization, the activating spermatozoon triggers a series of physiological changes that transforms the quiescent egg into a dynamic zygote. It has been suggested that several of these egg activation events, e.g. sperm-induced plasma membrane depolarization and the Ca2+-linked cortical reaction, play additional roles to prevent the entry of supernumerary spermatozoa. In particular, the abrupt shift in egg membrane potential at fertilization, which is sustained by a Na+ influx, has been considered as a fast mechanism to block polyspermy. To test the relevance of the Na+-mediated fast electrical block to polyspermy, we fertilized sea urchin eggs in artificial seawater with a low concentration of Na+; nearly all the eggs were still monospermic, as judged by the number of Hoechst 33422-stained sperm. When fertilized in normal seawater, eggs that were pre-incubated in the low Na+ medium exhibited impaired elevation of the fertilization envelope. Nevertheless, these eggs manifested entry of a single spermatozoon, suggesting that the fertilization envelope was not the primary determinant of the block to polyspermy. Furthermore, we showed that the abnormal cleavage patterns displayed by eggs pre-incubated in low Na+, which were often considered a hallmark of polyspermy, were due to the alterations in the cortical actin filaments dynamics following fertilization, and not to the formation of multipolar spindles associated with supernumerary sperm centrosomes. Hence, our results suggested that Paracentrotus lividus eggs do not utilize Na+ to rapidly prevent additional spermatozoa from entering the egg, at variance with the hypothesis of an electrical fast block to polyspermy.

Altered actin cytoskeleton in ageing eggs of starfish affects fertilization process

Integrity of oocytes is of pivotal interest in the medical and zootechnical practice of in vitro fertilization. With time, oocytes undergo deterioration in quality, and ageing oocytes often exhibit compromised competence in fertilization and the subsequent embryonic development. With ageing oocytes and eggs of starfish (Astropecten aranciacus), we addressed the issue by examining changes of the subcellular structure and their performance at fertilization. Ageing eggs were simulated in two different experimental paradigms: i) oocytes were overmatured by 6 hours stimulation with 1-methyladenine (1-MA); ii) oocytes were removed from the gonad and maintained in seawater for 24 or 48 h before applying the hormonal stimulation (1-MA, 70 min). These eggs were compared with normally matured eggs (stimulated after isolation from the gonad with 1-MA for 70 min) with respect to the sperm-induced intracellular Ca²⁺ signaling and the structural changes of the egg surface. The cytoskeletal and ultrastructural differences in these eggs were assessed by confocal and transmission electron microscopy, respectively. In the two categories of ageing eggs, we have found remarkable structural modifications of the actin cytoskeleton and the cortical vesicles beneath the plasma membrane. At fertilization, these ageing eggs manifested an altered pattern of intracellular Ca²⁺ release, aberrant actin dynamics, and increased rate of polyspermy often despite full elevation of the fertilization envelope. Taken together, our results highlight the importance of spatio-temporal regulation of the actin cytoskeleton in the cortex of the eggs, and we postulate that the status of the actin cytoskeleton is one of the major determinants of the oocyte quality that ensures successful monospermic fertilization.

Effects of Bisphenol A and 4-tert-Octylphenol on Embryo Implantation Failure in Mouse

Miscarriage due to blastocyst implantation failure occurs in up to two-thirds of all human miscarriage cases. Calcium ion has been shown to be involved in many cellular signal transduction pathways as well as in the regulation of cell adhesion, which is necessary for the embryo implantation process. Exposure to endocrine-disrupting chemicals (EDs) during early gestation results in disruption of intrauterine implantation and uterine reception, leading to implantation failure. In this study, ovarian estrogen (E2), bisphenol A (BPA), or 4-tert-octylphenol (OP), with or without ICI 182,780 (ICI) were injected subcutaneously from gestation day 1 to gestation day 3 post-coitus. The expression levels of the calcium transport genes were assessed in maternal uteri and implantation sites. The number of implantation sites was significantly low in the OP group, and implantation sites were absent in the E2, ICI and EDs + ICI groups. There were different calcium transient transport channel expression levels in uterus and implantation site samples. The levels of TRPV5 and TRPV6 gene expression were significantly increased by EDs with/without ICI treatment in utero. Meanwhile, TRPV5 and TRPV6 gene expression were significantly lower in implantation sites samples. NCX1 and PMCA1 mRNA levels were significantly decreased by OP and BPA in the implantation site samples. Compared to vehicle treatment in the uterus, both the MUC1 mRNA and protein levels were markedly high in all but the BPA group. Taken together, these results suggest that both BPA and OP can impair embryo implantation through alteration of calcium transport gene expressions and by affecting uterine receptivity.

The functional expression of transient receptor potential channels in the mouse endometrium

STUDY QUESTION

Does mouse endometrial epithelial cells and stromal cells have a similar transient receptor potential (TRP)-channel expression profile and to that found in the human endometrium?

SUMMARY ANSWER

Mouse endometrial epithelial and stromal cells have a distinct TRP channel expression profile analogous to what has been found in human endometrium, and hence suggests the mouse a good model to investigate the role of TRP channels in reproduction.

WHAT IS KNOWN ALREADY

An optimal intercellular communication between epithelial and stromal endometrial cells is crucial for successful reproduction. Members of the TRP family were recently described in the human endometrial stroma; however their functional expression in murine endometrium remains unspecified. Furthermore, epithelial and stromal cells have distinct functions in the reproductive process, implying the possibility for a different expression profile. However, knowledge about the functional expression pattern of TRP channels in either epithelial or stromal cells is not available.

STUDY DESIGN, SIZE, DURATION

In this study, the expression pattern of TRP channels in the murine (C57BL/6 J strain) endometrium was investigated and compared to the human expression pattern. Therefore, expression was examined in uterine tissue isolated during the natural estrous cycle (n = 16) or during an induced menstrual cycle using the menstruating mouse model (n = 28). Next, the functional expression of TRP channels was assessed separately in endometrial epithelial and stromal cell populations.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Quantitative RT-PCR was used to evaluate the relative mRNA expression of TRP channels in murine uterine tissue and cells. To further assess the functional expression in epithelial or stromal cells, primary endometrial cell cultures and Fura2-based calcium-microfluorimetry experiments were performed.

MAIN RESULTS AND THE ROLE OF CHANCE

The expression pattern of TRP channels during the natural estrous cycle or the induced menstrual cycle is analog to what has been shown in human samples. Furthermore, a very distinct expression pattern was observed in epithelial cells compared to stromal cells. Expression of TRPV4, TRPV6 and TRPM6 was significantly higher in epithelial cells whereas TRPV2, TRPC1/4 and TRPC6 were almost exclusively expressed in stromal cells.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Although relevant mRNA levels are detected for TRPV6 and TRPM6, and TRPM4, lack of selective, available pharmacology restricted functional analysis of these ion channels.

WIDER IMPLICATIONS OF THE FINDINGS

Successful reproduction, and more specifically embryo implantation, is a dynamic developmental process that integrates many signaling molecules into a precisely orchestrated program. Here, we describe the expression pattern of TRP channels in mouse endometrium that is similar to human tissue and their restricted functionality in either stromal cells or epithelial cells, suggesting a role in the epithelial-stromal crosstalk. These results will be very helpful to identify key players involved in the signaling cascades required for successful embryo implantation. In addition, these results illustrate that mouse endometrium is a valid representative for human endometrium to investigate TRP channels in the field of reproduction.

STUDY FUNDING/COMPETING INTEREST(S)

The Research Foundation-Flanders (G.0856.13 N to J.V.); the Research Council of the Katholieke Universiteit Leuven (OT/13/113 to J.V. and PF-TRPLe to T.V.); the Planckaert-De Waele fund (to J.V.);  Fonds Wetenschappelijk Onderzoek Belgium (to K.D.C. and A.H.). None of the authors have a conflict of interest.

Fertilization in Starfish and Sea Urchin: Roles of Actin

Marine animals relying on "external fertilization" provide advantageous opportunities to study the mechanisms of gamete activation and fusion, as well as the subsequent embryonic development. Owing to the large number of eggs that are easily available and handled, starfish and sea urchins have been chosen as favorable animal models in this line of research for over 150 years. Indeed, much of our knowledge on fertilization came from studies in the echinoderms. Fertilization involves mutual stimulation between eggs and sperm, which leads to morphological, biochemical, and physiological changes on both sides to ensure successful gamete fusion. In this chapter, we review the roles of actin in the fertilization of starfish and sea urchin eggs. As fertilization is essentially an event that takes place on the egg surface, it has been predicted that suboolemmal actin filaments would make significant contributions to sperm entry. A growing body of evidence from starfish and sea urchin eggs suggests that the prompt reorganization of the actin pools around the time of fertilization plays crucial regulatory roles not only in guiding sperm entry but also in modulating intracellular Ca²⁺ signaling and egg activation.

Comparative Analysis of Spontaneous and Stimulus-Evoked Calcium Transients in Proliferating and Differentiating Human Midbrain-Derived Stem Cells

Spontaneous cytosolic calcium transients and oscillations have been reported in various tissues of nonhuman and human origin but not in human midbrain-derived stem cells. Using confocal microfluorimetry, we studied spontaneous calcium transients and calcium-regulating mechanisms in a human ventral mesencephalic stem cell line undergoing proliferation and neuronal differentiation. Spontaneous calcium transients were detected in a large fraction of both proliferating (>50%) and differentiating (>55%) cells. We provide evidence for the existence of intracellular calcium stores that respond to muscarinic activation of the cells, having sensitivity for ryanodine and thapsigargin possibly reflecting IP₃ receptor activity and the presence of ryanodine receptors and calcium ATPase pumps. The observed calcium transient activity potentially supports the existence of a sodium-calcium antiporter and the existence of calcium influx induced by depletion of calcium stores. We conclude that the cells have developed the most important mechanisms governing cytosolic calcium homeostasis. This is the first comparative report of spontaneous calcium transients in proliferating and differentiating human midbrain-derived stem cells that provides evidence for the mechanisms that are likely to be involved. We propose that the observed spontaneous calcium transients may contribute to mechanisms involved in cell proliferation, phenotypic differentiation, and general cell maturation.

De novo assembly of a transcriptome from the eggs and early embryos of Astropecten aranciacus

Starfish have been instrumental in many fields of biological and ecological research. Oocytes of Astropecten aranciacus, a common species native to the Mediterranean Sea and the East Atlantic, have long been used as an experimental model to study meiotic maturation, fertilization, intracellular Ca²⁺ signaling, and cell cycle controls. However, investigation of the underlying molecular mechanisms has often been hampered by the overall lack of DNA or protein sequences for the species. In this study, we have assembled a transcriptome for this species from the oocytes, eggs, zygotes, and early embryos, which are known to have the highest RNA sequence complexity. Annotation of the transcriptome identified over 32,000 transcripts including the ones that encode 13 distinct cyclins and as many cyclin-dependent kinases (CDK), as well as the expected components of intracellular Ca²⁺ signaling toolkit. Although the mRNAs of cyclin and CDK families did not undergo significant abundance changes through the stages from oocyte to early embryo, as judged by real-time PCR, the transcript encoding Mos, a negative regulator of mitotic cell cycle, was drastically reduced during the period of rapid cleavages. Molecular phylogenetic analysis using the homologous amino acid sequences of cytochrome oxidase subunit I from A. aranciacus and 30 other starfish species indicated that Paxillosida, to which A. aranciacus belongs, is not likely to be the most basal order in Asteroidea. Taken together, the first transcriptome we assembled in this species is expected to enable us to perform comparative studies and to design gene-specific molecular tools with which to tackle long-standing biological questions.

From cholesterol to consciousness

The nature of consciousness has been debated for centuries. It can be understood as part and parcel of the natural progression of life from unicellular to multicellular, calcium fluxes mediating communication within and between cells. Consciousness is the vertical integration of calcium fluxes, mediated by the Target of Rapamycin gene integrated with the cytoskeleton. The premise of this paper is that there is a fundamental physiologic integration of the organism with the environment that constitutes consciousness.

Development of Ca2+-release mechanisms during oocyte maturation of the starfish Asterina pectinifera

An important step for successful fertilization and further development is the increase in intracellular Ca2+ in the activated oocyte. It has been known that starfish oocytes become increasingly sensitive to inositol 1,4,5-trisphosphate (IP3) during meiotic maturation to exhibit highly efficient IP3-induced Ca2+ release (IICR) by the time of germinal vesicle breakdown (GVBD). However, we noted that the peak level of intracellular Ca2+ increase after insemination is already high in the maturing oocytes before GVBD. Using maturing oocytes before GVBD, we investigated Ca2+ release mechanisms other than IICR. We report here that Ca2+-release mechanisms dependent on nicotinic acid adenine dinucleotide phosphate (NAADP) and nicotinamide adenine dinucleotide (NADP), the precursor of NAADP, became functional prior to the development of IICR mechanisms. As with IP3, but unlike NAADP, the Ca2+ stores responsive to NADP are sensitized during the meiotic maturation induced by 1-methyladenine (1-MA). This suggests that the process may represent a physiological response to the maturation hormone. NADP-dependent Ca2+ release in immature oocytes, however, did not induce oocyte maturation by itself, but was enhanced by the conditions mimicking the increases of intracellular Ca2+ and pH that take place in the maturing oocytes of starfish.

New insights into negative effects of lithium on sea urchin Paracentrotus lividus embryos

The diffuse use of lithium in a number of industrial processes has produced a significant contamination of groundwater and surface water with it. The increased use of lithium has generated only scarce studies on its concentrations in ambient waters and on its effects on aquatic organisms. Only few contributions have focused on the toxicity of lithium in marine organisms (such as marine animals, algae and vegetables), showing that the toxic effect depends on the animal species. In the present study we describe the morphological and the molecular effects of lithium chloride (LiCl), using the sea urchin Paracentrotus lividus as a model organism. We show that LiCl, if added to the eggs before fertilization, induces malformations in the embryos in a dose-dependent manner. We have also followed by RT qPCR the expression levels of thirty seven genes (belonging to different classes of functional processes, such as stress, development, differentiation, skeletogenesis and detoxifications) to identify the molecular targets of LiCl. This study opens new perspectives for the understanding of the mechanism of action of lithium on marine organisms. The findings may also have relevance outside the world of marine organisms since lithium is widely prescribed for the treatment of human bipolar disorders.

Calcium and actin in the saga of awakening oocytes

The interaction of the spermatozoon with the egg at fertilization remains one of the most fascinating mysteries of life. Much of our scientific knowledge on fertilization comes from studies on sea urchin and starfish, which provide plenty of gametes. Large and transparent, these eggs have served as excellent model systems for studying egg activation and embryo development in seawater, a plain natural medium. Starfish oocytes allow the study of the cortical, cytoplasmic and nuclear changes during the meiotic maturation process, which can also be triggered in vitro by hormonal stimulation. These morphological and biochemical changes ensure successful fertilization of the eggs at the first metaphase. On the other hand, sea urchin eggs are fertilized after the completion of meiosis, and are particularly suitable for the study of sperm-egg interaction, early events of egg activation, and embryonic development, as a large number of mature eggs can be fertilized synchronously. Starfish and sea urchin eggs undergo abrupt changes in the cytoskeleton and ion fluxes in response to the fertilizing spermatozoon. The plasma membrane and cortex of an egg thus represent "excitable media" that quickly respond to the stimulus with the Ca(2+) swings and structural changes. In this article, we review some of the key findings on the rapid dynamic rearrangements of the actin cytoskeleton in the oocyte/egg cortex upon hormonal or sperm stimulation and their roles in the modulation of the Ca(2+) signals and in the control of monospermic fertilization.

May the remodeling of the Ca²⁺ toolkit in endothelial progenitor cells derived from cancer patients suggest alternative targets for anti-angiogenic treatment?

Endothelial progenitor cells (EPCs) may be recruited from bone marrow to sustain the metastatic switch in a number of solid cancers, including breast cancer (BC) and renal cellular carcinoma (RCC). Preventing EPC mobilization causes tumor shrinkage. Novel anti-angiogenic treatments have been introduced in therapy to inhibit VEGFR-2 signaling; unfortunately, these drugs blocked tumor angiogenesis in pre-clinical murine models, but resulted far less effective in human patients. Understanding the molecular mechanisms driving EPC proliferation and tubulogenesis in cancer patients could outline novel targets for alternative anti-angiogenic treatments. Store-operated Ca²⁺ entry (SOCE) regulates the growth of human EPCs, and it is mediated by the interaction between the endoplasmic reticulum Ca²⁺-sensor, Stim1, and the plasmalemmal Ca²⁺ channels, Orai1 and TRPC1. EPCs do not belong to the neoplastic clone: thus, unlike tumor endothelium and neoplastic cells, they should not remodel their Ca²⁺ toolkit in response to tumor microenvironment. However, our recent work demonstrated that EPCs isolated from naïve RCC patients (RCC-EPCs) undergo a dramatic remodeling of their Ca²⁺ toolkit by displaying a remarkable drop in the endoplasmic reticulum Ca²⁺ content, by down-regulating the expression of inositol-1,4,5-receptors (InsP3Rs), and by up-regulating Stim1, Orai1 and TRPC1. Moreover, EPCs are dramatically less sensitive to VEGF stimulation both in terms of Ca²⁺ signaling and of gene expression when isolated from tumor patients. Conversely, the pharmacological abolition of SOCE suppresses proliferation in these cells. These results question the suitability of VEGFR-2 as a therapeutically relevant target for anti-angiogenic treatments and hint at Orai1 and TRPC1 as more promising alternatives. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

FINITE ELEMENT MODEL TO STUDY THE MECHANICS OF CALCIUM REGULATION IN OOCYTE

At various stages of fertilization specific spatial and temporal patterns of Ca 2+ are required for oocyte maturation. It is crucial to understand the mechanics of Ca 2+ regulation in cytosol of oocytes, in order to have better understanding of fertilization process. In this paper, a finite element model of cytosolic calcium regulation in oocyte has been developed for a two-dimensional unsteady state case. The model incorporates the important biophysical processes like diffusion, reaction, leak from endoplasmic recticulum (ER), efflux from cytosol to ER via sarco-ER calcium adenosine triphosphate (SERCA) pumps, buffers and sodium calcium exchanger. Appropriate boundary conditions have been framed. The effect of source, buffer, sodium calcium exchanger, etc. on spatial and temporal patterns of calcium in oocyte have been studied with the help of numerical results.

Comparative biology of sperm factors and fertilization-induced calcium signals across the animal kingdom

Fertilization causes mature oocytes or eggs to increase their concentrations of intracellular calcium ions (Ca²⁺) in all animals that have been examined, and such Ca²⁺ elevations, in turn, provide key activating signals that are required for non-parthenogenetic development. Several lines of evidence indicate that the Ca²⁺ transients produced during fertilization in mammals and other taxa are triggered by soluble factors that sperm deliver into oocytes after gamete fusion. Thus, for a broad-based analysis of Ca²⁺ dynamics during fertilization in animals, this article begins by summarizing data on soluble sperm factors in non-mammalian species, and subsequently reviews various topics related to a sperm-specific phospholipase C, called PLCζ, which is believed to be the predominant activator of mammalian oocytes. After characterizing initiation processes that involve sperm factors or alternative triggering mechanisms, the spatiotemporal patterns of Ca²⁺ signals in fertilized oocytes or eggs are compared in a taxon-by-taxon manner, and broadly classified as either a single major transient or a series of repetitive oscillations. Both solitary and oscillatory types of fertilization-induced Ca²⁺ signals are typically propagated as global waves that depend on Ca²⁺ release from the endoplasmic reticulum in response to increased concentrations of inositol 1,4,5-trisphosphate (IP₃). Thus, for taxa where relevant data are available, upstream pathways that elevate intraoocytic IP3 levels during fertilization are described, while other less-common modes of producing Ca²⁺ transients are also examined. In addition, the importance of fertilization-induced Ca²⁺ signals for activating development is underscored by noting some major downstream effects of these signals in various animals.

DNA 5-methylcytosine demethylation activities of the mammalian DNA methyltransferases

Methylation at the 5-position of DNA cytosine on the vertebrate genomes is accomplished by the combined catalytic actions of three DNA methyltransferases (DNMTs), the de novo enzymes DNMT3A and DNMT3B and the maintenance enzyme DNMT1. Although several metabolic routes have been suggested for demethylation of the vertebrate DNA, whether active DNA demethylase(s) exist has remained elusive. Surprisingly, we have found that the mammalian DNMTs, and likely the vertebrates DNMTs in general, can also act as Ca(2+) ion- and redox state-dependent active DNA demethylases. This finding suggests new directions for reinvestigation of the structures and functions of these DNMTs, in particular their roles in Ca(2+) ion-dependent biological processes, including the genome-wide/local DNA demethylation during early embryogenesis, cell differentiation, neuronal activity-regulated gene expression, and carcinogenesis.

Bidirectional Ca²⁺ signaling occurs between the endoplasmic reticulum and acidic organelles

After acidic organelles induce signaling to activate ER calcium ion release, local microdomains of high calcium at ER–acidic organelle junctions feed back to activate further acidic organelle calcium release.

The endoplasmic reticulum (ER) and acidic organelles (endo-lysosomes) act as separate Ca²⁺ stores that release Ca²⁺ in response to the second messengers IP₃ and cADPR (ER) or NAADP (acidic organelles). Typically, trigger Ca²⁺ released from acidic organelles by NAADP subsequently recruits IP₃ or ryanodine receptors on the ER, an anterograde signal important for amplification and Ca²⁺ oscillations/waves. We therefore investigated whether the ER can signal back to acidic organelles, using organelle pH as a reporter of NAADP action. We show that Ca²⁺ released from the ER can activate the NAADP pathway in two ways: first, by stimulating Ca²⁺-dependent NAADP synthesis; second, by activating NAADP-regulated channels. Moreover, the differential effects of EGTA and BAPTA (slow and fast Ca²⁺ chelators, respectively) suggest that the acidic organelles are preferentially activated by local microdomains of high Ca²⁺ at junctions between the ER and acidic organelles. Bidirectional organelle communication may have wider implications for endo-lysosomal function as well as the generation of Ca²⁺ oscillations and waves.

Fertilization in echinoderms

For more than 150 years, echinoderm eggs have served as overly favored experimental model systems in which to study fertilization. Sea urchin and starfish belong to the same phylum and thus share many similarities in their fertilization patterns. However, several subtle but fundamental differences do exist in the fertilization of sea urchin and starfish, reflecting their phylogenetic bifurcation approximately 500 million years ago. In this article we review some of the seminal and recent findings that feature similarities and differences in sea urchin and starfish at fertilization.

Calcium pathway machinery at fertilization in echinoderms

Calcium signaling in cells directs diverse physiological processes. The calcium waves triggered by fertilization is a highly conserved calcium signaling event essential for egg activation, and has been documented in every egg tested. This activity is one of the few highly conserved events of egg activation through the course of evolution. Echinoderm eggs, as well as many other cell types, have three main intracellular Ca(2+) mobilizing messengers - IP3, cADPR and NAADP. Both cADPR and NAADP were identified as Ca(2+) mobilizing messengers using the sea urchin egg homogenate, and this experimental system, along with the intact urchin and starfish oocyte/egg, continues to be a vital tool for investigating the mechanism of action of calcium signals. While many of the major regulatory steps of the IP3 pathway are well resolved, both cADPR and NAADP remain understudied in terms of our understanding of the fundamental process of egg activation at fertilization. Recently, NAADP has been shown to trigger Ca(2+) release from acidic vesicles, separately from the ER, and a new class of calcium channels, the two-pore channels (TPCs), was identified as the likely targets for this messenger. Moreover, it was found that both cADPR and NAADP can be synthesized by the same family of enzymes, the ADP-rybosyl cyclases (ARCs). In this context of increasing amount of information, the potential coupling and functional roles of different messengers, intracellular stores and channels in the formation of the fertilization calcium wave in echinoderms will be critically evaluated.

Update on vascular endothelial Ca2+ signalling: A tale of ion channels, pumps and transporters

A monolayer of endothelial cells (ECs) lines the lumen of blood vessels and forms a multifunctional transducing organ that mediates a plethora of cardiovascular processes. The activation of ECs from as state of quiescence is, therefore, regarded among the early events leading to the onset and progression of potentially lethal diseases, such as hypertension, myocardial infarction, brain stroke, and tumor. Intracellular Ca²⁺ signals have long been know to play a central role in the complex network of signaling pathways regulating the endothelial functions. Notably, recent work has outlined how any change in the pattern of expression of endothelial channels, transporters and pumps involved in the modulation of intracellular Ca²⁺ levels may dramatically affect whole body homeostasis. Vascular ECs may react to both mechanical and chemical stimuli by generating a variety of intracellular Ca²⁺ signals, ranging from brief, localized Ca²⁺ pulses to prolonged Ca²⁺ oscillations engulfing the whole cytoplasm. The well-defined spatiotemporal profile of the subcellular Ca²⁺ signals elicited in ECs by specific extracellular inputs depends on the interaction between Ca²⁺ releasing channels, which are located both on the plasma membrane and in a number of intracellular organelles, and Ca²⁺ removing systems. The present article aims to summarize both the past and recent literature in the field to provide a clear-cut picture of our current knowledge on the molecular nature and the role played by the components of the Ca²⁺ machinery in vascular ECs under both physiological and pathological conditions.

Effects of ionomycin on egg activation and early development in starfish

Ionomycin is a Ca²⁺-selective ionophore that is widely used to increase intracellular Ca²⁺ levels in cell biology laboratories. It is also occasionally used to activate eggs in the clinics practicing in vitro fertilization. However, neither the precise molecular action of ionomycin nor its secondary effects on the eggs' structure and function is well known. In this communication we have studied the effects of ionomycin on starfish oocytes and zygotes. By use of confocal microscopy, calcium imaging, as well as light and transmission electron microscopy, we have demonstrated that immature oocytes exposed to ionomycin instantly increase intracellular Ca²⁺ levels and undergo structural changes in the cortex. Surprisingly, when microinjected into the cells, ionomycin produced no Ca²⁺ increase. The ionomycin-induced Ca²⁺ rise was followed by fast alteration of the actin cytoskeleton displaying conspicuous depolymerization at the oocyte surface and in microvilli with concomitant polymerization in the cytoplasm. In addition, cortical granules were disrupted or fused with white vesicles few minutes after the addition of ionomycin. These structural changes prevented cortical maturation of the eggs despite the normal progression of nuclear envelope breakdown. At fertilization, the ionomycin-pretreated eggs displayed reduced Ca²⁺ response, no elevation of the fertilization envelope, and the lack of orderly centripetal translocation of actin fibers. These alterations led to difficulties in cell cleavage in the monospermic zygotes and eventually to a higher rate of abnormal development. In conclusion, ionomycin has various deleterious impacts on egg activation and the subsequent embryonic development in starfish. Although direct comparison is difficult to make between our findings and the use of the ionophore in the in vitro fertilization clinics, our results call for more defining investigations on the issue of a potential risk in artificial egg activation.

The eggshell in the C. elegans oocyte-to-embryo transition

In egg-laying animals, embryonic development takes place within the highly specialized environment provided by the eggshell and its underlying extracellular matrix. Far from being simply a passive physical support, the eggshell is a key player in many early developmental events. Herein, we review current understanding of eggshell structure, biosynthesis, and function in zygotic development of the nematode, C. elegans. Beginning at sperm contact or entry, eggshell layers are produced sequentially. The earlier outer layers are required for secretion or organization of inner layers, and layers differ in composition and function. Developmental events that depend on the eggshell include polyspermy barrier generation, high fidelity meiotic chromosome segregation, osmotic barrier synthesis, polar body extrusion, anterior-posterior polarization, and organization of membrane and cortical proteins. The C. elegans eggshell is proving to be an excellent, tractable system to study the molecular cues of the extracellular matrix that instruct cell polarity and early development.