Calreticulin: Endoplasmic reticulum Ca2+ gatekeeper

Endoplasmic reticulum (ER) luminal Ca2+ is vital for the function of the ER and regulates many cellular processes. Calreticulin is a highly conserved, ER-resident Ca2+ binding protein and lectin-like chaperone. Over four decades of studying calreticulin demonstrate that this protein plays a crucial role in maintaining Ca2+ supply under different physiological conditions, in managing access to Ca2+ and how Ca2+ is used depending on the environmental events and in making sure that Ca2+ is not misused. Calreticulin plays a role of ER luminal Ca2+ sensor to manage Ca2+-dependent ER luminal events including maintaining interaction with its partners, Ca2+ handling molecules, substrates and stress sensors. The protein is strategically positioned in the lumen of the ER from where the protein manages access to and distribution of Ca2+ for many cellular Ca2+-signalling events. The importance of calreticulin Ca2+ pool extends beyond the ER and includes influence of cellular processes involved in many aspects of cellular pathophysiology. Abnormal handling of the ER Ca2+ contributes to many pathologies from heart failure to neurodegeneration and metabolic diseases.

Substrate profiling of the metalloproteinase ovastacin uncovers specific enzyme-substrate interactions and discloses fertilization-relevant substrates

The metalloproteinase ovastacin is released by the mammalian egg upon fertilization and cleaves a distinct peptide bond in zona pellucida protein 2 (ZP2), a component of the enveloping extracellular matrix. This limited proteolysis causes zona pellucida hardening, abolishes sperm binding, and thereby regulates fertility. Accordingly, this process is tightly controlled by the plasma protein fetuin-B, an endogenous competitive inhibitor. At present, little is known about how the cleavage characteristics of ovastacin differ from closely related proteases. Physiological implications of ovastacin beyond ZP2 cleavage are still obscure. In this study, we employed N-terminal amine isotopic labeling of substrates (N-TAILS) contained in the secretome of mouse embryonic fibroblasts to elucidate the substrate specificity and the precise cleavage site specificity. Furthermore, we were able to unravel the physicochemical properties governing ovastacin-substrate interactions as well as the individual characteristics that distinguish ovastacin from similar proteases, such as meprins and tolloid. Eventually, we identified several substrates whose cleavage could affect mammalian fertilization. Consequently, these substrates indicate newly identified functions of ovastacin in mammalian fertilization beyond zona pellucida hardening.

Spatiotemporal profiling of the bovine oviduct fluid proteome around the time of ovulation

Understanding the composition of the oviduct fluid (OF) is crucial to better comprehend the microenvironment in which sperm capacitation, fertilization and early embryo development take place. Therefore, our aim was to determine the spatiotemporal changes in the OF proteome according to the anatomical region of the oviduct (ampulla vs. isthmus), the proximity of the ovulating ovary (ipsilateral vs. contralateral side) and the peri-ovulatory stage (pre-ovulatory or Pre-ov vs. post-ovulatory or Post-ov). Oviducts from adult cyclic cows were collected at a local slaughterhouse and pools of OF were analyzed by nanoLC-MS/MS and label-free protein quantification (n = 32 OF pools for all region × stage × side conditions). A total of 3760 proteins were identified in the OF, of which 65% were predicted to be potentially secreted. The oviduct region was the major source of variation in protein abundance, followed by the proximity of the ovulating ovary and finally the peri-ovulatory stage. Differentially abundant proteins between regions, stages and sides were involved in a broad variety of biological functions, including protein binding, response to stress, cell-to-cell adhesion, calcium homeostasis and the immune system. This work highlights the dynamic regulation of oviduct secretions and provides new protein candidates for interactions between the maternal environment, the gametes and the early embryo.

Challenges and Considerations during In Vitro Production of Porcine Embryos

Genetically modified pigs have become valuable tools for generating advances in animal agriculture and human medicine. Importantly, in vitro production and manipulation of embryos is an essential step in the process of creating porcine models. As the in vitro environment is still suboptimal, it is imperative to examine the porcine embryo culture system from several angles to identify methods for improvement. Understanding metabolic characteristics of porcine embryos and considering comparisons with other mammalian species is useful for optimizing culture media formulations. Furthermore, stressors arising from the environment and maternal or paternal factors must be taken into consideration to produce healthy embryos in vitro. In this review, we progress stepwise through in vitro oocyte maturation, fertilization, and embryo culture in pigs to assess the status of current culture systems and address points where improvements can be made.

Preventing polyspermy in mammalian eggs-Contributions of the membrane block and other mechanisms

The egg's blocks to polyspermy (fertilization of an egg by more than one sperm) were originally identified in marine and aquatic species with external fertilization, but polyspermy matters in mammalian reproduction too. Embryonic triploidy is a noteworthy event associated with pregnancy complications and loss. Polyspermy is a major cause of triploidy with up to 80% of triploid conceptuses being the result of dispermic fertilization. The mammalian female reproductive tract regulates the number of sperm that reach the site of fertilization, but mammals also utilize egg-based blocks to polyspermy. The egg-based blocks occur on the mammalian egg coat (the zona pellucida) and the egg plasma membrane, with apparent variation between different mammalian species regarding the extent to which one or both are used. The zona pellucida block to polyspermy has some similarities to the slow block in water-dwelling species, but the mammalian membrane block to polyspermy differs substantially from the fast electrical block that has been characterized in marine and aquatic species. This review discusses what is known about the incidence of polyspermy in mammals and about the mammalian membrane block to polyspermy, as well as notes some lesser-characterized potential mechanisms contributing to polyspermy prevention in mammals.

Sperm chemorepulsion, a supplementary mechanism to regulate fertilization

STUDY QUESTION

Are human spermatozoa able of chemorepulsive behaviour?

SUMMARY ANSWER

Capacitated human spermatozoa are able to be chemorepelled by synthetic Progesterone Receptor Ligands (sPRL, known as contraceptives) and zinc (a cation released by the oocyte upon fertilization).

WHAT IS KNOWN ALREADY

Moving cells can be oriented towards or against a molecular gradient, processes called chemoattraction and chemorepulsion, respectively, which have been described in unicellular organisms such as amoebas and bacteria, to organismic cells such macrophages and developmental cells. In the case of spermatozoa, chemoattraction may help the finding of an oocyte and has been widely studied in various invertebrate and mammalian species; however, chemorepulsion has not yet been verified in spermatozoa.

STUDY DESIGN, SIZE, DURATION

This is an in vitro study involving human, rabbit and mouse spermatozoa which were used to perform 3-30 experiments per treatment.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Human sperm samples were obtained by masturbation from healthy donors who gave written consent. Only those samples exhibiting normal semen parameters according to current WHO criteria were included in the study. Rabbit spermatozoa were obtained by artificial vagina whereas mice spermatozoa were obtained from epididymis. The sperm selection assay (SSA), originally designed to evaluate sperm chemoattraction towards progesterone (P), and a video-microscopy and computer motion analysis system were used to test sperm chemorepulsion. Additional kinetic parameters were also determined by video-microscopy and computer motion analysis. In some experiments, the level of induced acrosome-reacted spermatozoa was determined. Rabbit mating manipulation was achieved to perform the sperm-oocyte co-incubation assay.

MAIN RESULTS AND THE ROLE OF CHANCE

Sperm accumulation in the well containing 100 pg/ml of sPRL was lower than the culture medium negative control (P < 0.05). The percentage of sperm persistence against the well containing 100 pg/ml ulipristal acetate (UPA) (P = 0.001), and the percentage of sperm showing a repulsive pattern of movement (a linear trajectory followed by a transitional one after turning against the UPA), were higher than the culture medium negative control (P = 0.049). Sperm accumulation was diminished when spermatozoa where exposed to a homogeneous distribution of 100 pg/ml sPRL combined with a chemotactic gradient of progesterone (P), with respect to the culture medium negative control (P < 0.05). These results were reverted when non-capacitated spermatozoa were used to perform the same experimental settings. The accumulation of spermatozoa against 100 pg/ml sPRL was lower than the culture medium negative control also in rabbits and mice (P < 0.05). The relative number of rabbit spermatozoa arriving to the vicinity of the oocyte was diminished under the presence of 100 pg/ml UPA (P = 0.004). Sperm accumulation in the well containing zinc was decreased compared to the culture medium negative control (P < 0.05). A homogeneous distribution of zinc combined with a gradient of 10 pM P, was lower than the culture medium negative control (P = 0.016). The results were quite reproducible with two different methodologies (accumulation assay and video-microscopy combined with computer motion analysis), in three mammalian species.

LIMITATIONS REASONS FOR CAUTION

The experiments were performed in vitro. Even though a quite complete characterization of sperm chemorepulsion was provided, the molecular mechanism that governs sperm repulsion is currently under investigation.

WIDER IMPLICATIONS OF THE FINDINGS

Since the chemorepelled spermatozoa are those physiologically ready to fertilize the oocyte, these findings may have both biological and clinical implications, preventing either polyspermy under natural conditions or fertilization under pharmacological treatment with sPRL.

STUDY FUNDING/COMPETING INTEREST(S)

The study was financed by the Universidad Nacional de Cordoba (Argentina). The authors declare that they do not have competing financial interests.

TRIAL REGISTRATION NUMBER

N/A.

Calcineurin role in porcine oocyte activation

Calcineurin is required for oocyte exit from meiotic block in metaphase II (MII) stage in invertebrates and also in lower vertebrates. However, the role of calcineurin in mammalian oocyte activation is still unclear. The aim of this study was to determine whether calcineurin is involved in the processes regulating porcine oocyte activation. Indirect immunofluorescence demonstrated localization of both calcineurin subunits, CnA and CnB, especially in the cortex area of MII oocytes, in vitro fertilized and also parthenogenetically activated oocytes. After activation, the fluorescence intensity of the protein in the cortex area of oocytes remains unchanged; the protein calcineurin in the cytoplasm was recorded mainly around the pronuclei. Treatment of matured oocytes with calcineurin inhibitors, cyclosporin A (CsA) and hymenistatin I (HS-I), followed by activation with calcium ionophore A23187, significantly decreased the rate of activated oocytes compared to oocytes that were treated only with calcium ionophore (Ca-Io), (CsA+Ca-Io 25.0% v. Ca-Io 83.3%; HS-I+Ca-Io 32.5% v. Ca-Io 85.0%). Compared to the control, CsA treatment of matured oocytes followed by activation with Ca-Io did not affect the activity level of metaphase-promoting factor (MPF) and mitogen-activated protein kinase (MAPK) in activated oocytes evaluated by kinase activity assay. Simultaneous staining of calcineurin and cortical granule content in matured oocytes showed that calcineurin distributed in the cortical area of the oocyte has not been colocalized with cortical granules content. On the other hand, the calcineurin inhibition before parthenogenetic activation leads to a reduction of the cortical reaction level compared to oocytes that were not treated with CsA (complete exocytosis: CsA+Ca-Io 2.6% v. Ca-Io 83.9%; sum of cortical granule brightness: CsA + Ca-Io 0.69 v. Ca-Io 0.15). Our results showed that calcineurin is involved in the process of pig oocyte activation and cortical granule exocytosis; however this regulation seems to be MPF and MAPK independent.