Luteinizing hormone stimulates ingression of mural granulosa cells within the mouse preovulatory follicle†

Luteinizing hormone (LH) induces ovulation by acting on its receptors in the mural granulosa cells that surround a mammalian oocyte in an ovarian follicle. However, much remains unknown about how activation of the LH receptor modifies the structure of the follicle such that the oocyte is released and the follicle remnants are transformed into the corpus luteum. The present study shows that the preovulatory surge of LH stimulates LH receptor-expressing granulosa cells, initially located almost entirely in the outer layers of the mural granulosa, to rapidly extend inwards, intercalating between other cells. The cellular ingression begins within 30 min of the peak of the LH surge, and the proportion of LH receptor-expressing cell bodies in the inner half of the mural granulosa layer increases until the time of ovulation, which occurs at about 10 h after the LH peak. During this time, many of the initially flask-shaped cells appear to detach from the basal lamina, acquiring a rounder shape with multiple filipodia. Starting at about 4 h after the LH peak, the mural granulosa layer at the apical surface of the follicle where ovulation will occur begins to thin, and the basolateral surface develops invaginations and constrictions. Our findings raise the question of whether LH stimulation of granulosa cell ingression may contribute to these changes in the follicular structure that enable ovulation.

Granulosa Cells Alone, Without Theca Cells, Can Mediate LH-induced Oocyte Meiotic Resumption

Signaling in the granulosa cells of mammalian ovarian follicles is necessary for maintaining prophase arrest in the oocyte and for mediating the resumption of meiosis in response to luteinizing hormone (LH). However, the follicle also includes an outer layer of theca cells, some of which express receptors for LH. To investigate whether theca cells are required for maintaining meiotic arrest and reinitiating meiosis in response to LH, we mechanically separated the granulosa cells and oocyte from the theca and basal lamina. This was accomplished by cutting a slit in the outer surface of isolated follicles such that the mural granulosa cells and cumulus-oocyte complex were extruded from the theca shell, forming a lawn of cells on an organotypic membrane. The remnant of theca cells and basal lamina was then removed. The separation of the granulosa cells from the theca cells and basal lamina was demonstrated by immunofluorescence localization of endomucin (blood vessels of the theca) and laminin gamma (basal lamina). Cells comprising these granulosa cell-oocyte complexes expressed LH receptors and were connected by gap junctions. Oocytes within these granulosa cell complexes maintained meiotic arrest and resumed meiosis in response to LH, showing that the granulosa cells alone, without theca cells, transduce these signals. This semi-intact and mostly 2-dimensional preparation could facilitate imaging studies of follicle physiology.

Phosphatases modified by LH signaling in ovarian follicles: testing their role in regulating the NPR2 guanylyl cyclase†

In response to luteinizing hormone (LH), multiple proteins in rat and mouse granulosa cells are rapidly dephosphorylated, but the responsible phosphatases remain to be identified. Because the phosphorylation state of phosphatases can regulate their interaction with substrates, we searched for phosphatases that might function in LH signaling by using quantitative mass spectrometry. We identified all proteins in rat ovarian follicles whose phosphorylation state changed detectably in response to a 30-min exposure to LH, and within this list, identified protein phosphatases or phosphatase regulatory subunits that showed changes in phosphorylation. Phosphatases in the phosphoprotein phosphatase (PPP) family were of particular interest because of their requirement for dephosphorylating the natriuretic peptide receptor 2 (NPR2) guanylyl cyclase in the granulosa cells, which triggers oocyte meiotic resumption. Among the PPP family regulatory subunits, PPP1R12A and PPP2R5D showed the largest increases in phosphorylation, with 4-10 fold increases in signal intensity on several sites. Although follicles from mice in which these phosphorylations were prevented by serine-to-alanine mutations in either Ppp1r12a or Ppp2r5d showed normal LH-induced NPR2 dephosphorylation, these regulatory subunits and others could act redundantly to dephosphorylate NPR2. Our identification of phosphatases and other proteins whose phosphorylation state is rapidly modified by LH provides clues about multiple signaling pathways in ovarian follicles.

Luteinizing hormone stimulates ingression of mural granulosa cells within the mouse preovulatory follicle

Luteinizing hormone (LH) induces ovulation by acting on its receptors in the mural granulosa cells that surround a mammalian oocyte in an ovarian follicle. However, much remains unknown about how activation of the LH receptor modifies the structure of the follicle such that the oocyte is released and the follicle remnants are transformed into the corpus luteum. The present study shows that the preovulatory surge of LH stimulates LH receptor-expressing granulosa cells, initially located almost entirely in the outer layers of the mural granulosa, to rapidly extend inwards, intercalating between other cells. The cellular ingression begins within 30 minutes of the peak of the LH surge, and the proportion of LH receptor-expressing cell bodies in the inner half of the mural granulosa layer increases until the time of ovulation, which occurs at about 10 hours after the LH peak. During this time, many of the initially flask-shaped cells appear to detach from the basal lamina, acquiring a rounder shape with multiple filipodia. Starting at about 4 hours after the LH peak, the mural granulosa layer at the apical surface of the follicle where ovulation will occur begins to thin, and the basolateral surface develops invaginations and constrictions. Our findings raise the question of whether LH stimulation of granulosa cell ingression may contribute to these changes in the follicular structure that enable ovulation.

Phosphatases modified by LH signaling in ovarian follicles: testing their role in regulating the NPR2 guanylyl cyclase

In response to luteinizing hormone, multiple proteins in rat and mouse granulosa cells are rapidly dephosphorylated, but the responsible phosphatases remain to be identified. Because the phosphorylation state of phosphatases can regulate their interaction with substrates, we searched for phosphatases that might function in LH signaling by using quantitative mass spectrometry. We identified all proteins in rat ovarian follicles whose phosphorylation state changed detectably in response to a 30-minute exposure to LH, and within this list, identified protein phosphatases or phosphatase regulatory subunits that showed changes in phosphorylation. Phosphatases in the PPP family were of particular interest because of their requirement for dephosphorylating the natriuretic peptide receptor 2 (NPR2) guanylyl cyclase in the granulosa cells, which triggers oocyte meiotic resumption. Among the PPP family regulatory subunits, PPP1R12A and PPP2R5D showed the largest increases in phosphorylation, with 4-10 fold increases in signal intensity on several sites. Although follicles from mice in which these phosphorylations were prevented by serine-to-alanine mutations in either Ppp1r12a or Ppp2r5d showed normal LH-induced NPR2 dephosphorylation, these regulatory subunits and others could act redundantly to dephosphorylate NPR2. Our identification of phosphatases and other proteins whose phosphorylation state is rapidly modified by LH provides clues about multiple signaling pathways in ovarian follicles.

Summary sentence

Quantitative mass spectrometric analysis of phosphatases whose phosphorylation state is rapidly modified by luteinizing hormone provides clues about how LH signaling dephosphorylates NPR2 as well as a resource for future studies.

Endowed Chairs and Professorships: A New Frontier in Gender Equity

PURPOSE

Endowed chairs and professorships are prestigious and financially important awards that symbolize individual faculty recognition. However, data about the gender distribution of these positions are lacking. The purpose of this study was to examine the gender distribution of endowed positions at U.S. medical schools and identify strategies that have been used to promote investiture of women into these positions.

METHOD

The authors interrogated the websites for all U.S. medical schools for publicly available data. Of 38 schools that listed schoolwide information, they analyzed data from the 30 with at least 10 endowed positions. Then, they conducted interviews with deans of the 10 schools with the highest percentages of women holding endowed positions ("top 10") to understand the strategies they used to increase gender equity in this area.

RESULTS

The percentage of endowed positions held by women at the 30 schools analyzed ranged from 10.8% to 34.6%, with a mean of 21.6%. Themes that emerged from interviews with deans included (1) intentionality to identify women candidates in the selection process, (2) monitoring the numbers of women holding endowed positions, (3) inclusion of endowed positions as part of larger institutional goals on gender equity and diversity, (4) use of endowed positions to recruit, retain, and recognize women faculty, (5) purposeful fundraising to increase the number of endowed positions, and (6) institutional investment of resources to develop women faculty.

CONCLUSIONS

Analysis of the gender distribution of endowed positions across 30 representative U.S. medical schools revealed a significant gender disparity. Interviews with deans at the top 10 schools revealed strategies that they have used to promote equity in this important area. Implementation of a systematic national reporting process could provide schools with comparative data to gauge their progress.

Epitope-tagged and phosphomimetic mouse models for investigating natriuretic peptide-stimulated receptor guanylyl cyclases

The natriuretic peptide receptors NPR1 and NPR2, also known as guanylyl cyclase A and guanylyl cyclase B, have critical functions in many signaling pathways, but much remains unknown about their localization and function in vivo. To facilitate studies of these proteins, we developed genetically modified mouse lines in which endogenous NPR1 and NPR2 were tagged with the HA epitope. To investigate the role of phosphorylation in regulating NPR1 and NPR2 guanylyl cyclase activity, we developed mouse lines in which regulatory serines and threonines were substituted with glutamates, to mimic the negative charge of the phosphorylated forms (NPR1-8E and NPR2-7E). Here we describe the generation and applications of these mice. We show that the HA-NPR1 and HA-NPR2 mice can be used to characterize the relative expression levels of these proteins in different tissues. We describe studies using the NPR2-7E mice that indicate that dephosphorylation of NPR2 transduces signaling pathways in ovary and bone, and studies using the NPR1-8E mice that indicate that the phosphorylation state of NPR1 is a regulator of heart, testis, and adrenal function.

Phosphatase inhibition by LB-100 enhances BMN-111 stimulation of bone growth

Activating mutations in fibroblast growth factor receptor 3 (FGFR3) and inactivating mutations in the natriuretic peptide receptor 2 (NPR2) guanylyl cyclase both result in decreased production of cyclic GMP in chondrocytes and severe short stature, causing achondroplasia (ACH) and acromesomelic dysplasia, type Maroteaux, respectively. Previously, we showed that an NPR2 agonist BMN-111 (vosoritide) increases bone growth in mice mimicking ACH (Fgfr3^Y367C/+). Here, because FGFR3 signaling decreases NPR2 activity by dephosphorylating the NPR2 protein, we tested whether a phosphatase inhibitor (LB-100) could enhance BMN-111–stimulated bone growth in ACH. Measurements of cGMP production in chondrocytes of living tibias, and of NPR2 phosphorylation in primary chondrocytes, showed that LB-100 counteracted FGF-induced dephosphorylation and inactivation of NPR2. In ex vivo experiments with Fgfr3^Y367C/+ mice, the combination of BMN-111 and LB-100 increased bone length and cartilage area, restored chondrocyte terminal differentiation, and increased the proliferative growth plate area, more than BMN-111 alone. The combination treatment also reduced the abnormal elevation of MAP kinase activity in the growth plate of Fgfr3^Y367C/+ mice and improved the skull base anomalies. Our results provide a proof of concept that a phosphatase inhibitor could be used together with an NPR2 agonist to enhance cGMP production as a therapy for ACH.

Kisspeptin-54 injection induces a physiological luteinizing hormone surge and ovulation in mice

Intraperitoneal injection of kisspeptin-54 induces a surge-like release of luteinizing hormone that stimulates ovulation in mice.

Follicle-stimulating hormone and luteinizing hormone increase Ca2+ in the granulosa cells of mouse ovarian follicles†

In mammalian ovarian follicles, follicle stimulating hormone (FSH) and luteinizing hormone (LH) signal primarily through the G-protein Gs to elevate cAMP, but both of these hormones can also elevate Ca2+ under some conditions. Here, we investigate FSH- and LH-induced Ca2+ signaling in intact follicles of mice expressing genetically encoded Ca2+ sensors, Twitch-2B and GCaMP6s. At a physiological concentration (1 nM), FSH elevates Ca2+ within the granulosa cells of preantral and antral follicles. The Ca2+ rise begins several minutes after FSH application, peaks at ∼10 min, remains above baseline for another ∼10 min, and depends on extracellular Ca2+. However, suppression of the FSH-induced Ca2+ increase by reducing extracellular Ca2+ does not inhibit FSH-induced phosphorylation of MAP kinase, estradiol production, or the acquisition of LH responsiveness. Like FSH, LH also increases Ca2+, when applied to preovulatory follicles. At a physiological concentration (10 nM), LH elicits Ca2+ oscillations in a subset of cells in the outer mural granulosa layer. These oscillations continue for at least 6 h and depend on the activity of Gq family G-proteins. Suppression of the oscillations by Gq inhibition does not inhibit meiotic resumption, but does delay the time to 50% ovulation by about 3 h. In summary, both FSH and LH increase Ca2+ in the granulosa cells of intact follicles, but the functions of these Ca2+ rises are only starting to be identified.

Cellular Heterogeneity of the Luteinizing Hormone Receptor and Its Significance for Cyclic GMP Signaling in Mouse Preovulatory Follicles

Meiotic arrest and resumption in mammalian oocytes are regulated by 2 opposing signaling proteins in the cells of the surrounding follicle: the guanylyl cyclase natriuretic peptide receptor 2 (NPR2), and the luteinizing hormone receptor (LHR). NPR2 maintains a meiosis-inhibitory level of cyclic guanosine 5'-monophosphate (cGMP) until LHR signaling causes dephosphorylation of NPR2, reducing NPR2 activity, lowering cGMP to a level that releases meiotic arrest. However, the signaling pathway between LHR activation and NPR2 dephosphorylation remains incompletely understood, due in part to imprecise information about the cellular localization of these 2 proteins. To investigate their localization, we generated mouse lines in which hemagglutinin epitope tags were added to the endogenous LHR and NPR2 proteins, and used immunofluorescence and immunogold microscopy to localize these proteins with high resolution. The results showed that the LHR protein is absent from the cumulus cells and inner mural granulosa cells, and is present in only 13% to 48% of the outer mural granulosa cells. In contrast, NPR2 is present throughout the follicle, and is more concentrated in the cumulus cells. Less than 20% of the NPR2 is in the same cells that express the LHR. These results suggest that to account for the LH-induced inactivation of NPR2, LHR-expressing cells send a signal that inactivates NPR2 in neighboring cells that do not express the LHR. An inhibitor of gap junction permeability attenuates the LH-induced cGMP decrease in the outer mural granulosa cells, consistent with this mechanism contributing to how NPR2 is inactivated in cells that do not express the LHR.

The fast block to polyspermy: New insight into a century-old problem

Jaffe underscores new research that identifies key roles for IP3 and TMEM16a in the fast block to polyspermy.

Multiple cAMP Phosphodiesterases Act Together to Prevent Premature Oocyte Meiosis and Ovulation

Luteinizing hormone (LH) acts on the granulosa cells that surround the oocyte in mammalian preovulatory follicles to cause meiotic resumption and ovulation. Both of these responses are mediated primarily by an increase in cyclic adenosine monophosphate (cAMP) in the granulosa cells, and the activity of cAMP phosphodiesterases (PDEs), including PDE4, contributes to preventing premature responses. However, two other cAMP-specific PDEs, PDE7 and PDE8, are also expressed at high levels in the granulosa cells, raising the question of whether these PDEs also contribute to preventing uncontrolled activation of meiotic resumption and ovulation. With the use of selective inhibitors, we show that inhibition of PDE7 or PDE8 alone has no effect on the cAMP content of follicles, and inhibition of PDE4 alone has only a small and variable effect. In contrast, a mixture of the three inhibitors elevates cAMP to a level comparable with that seen with LH. Correspondingly, inhibition of PDE7 or PDE8 alone has no effect on meiotic resumption or ovulation, and inhibition of PDE4 alone has only a partial and slow effect. However, the fraction of oocytes resuming meiosis and undergoing ovulation is increased when PDE4, PDE7, and PDE8 are simultaneously inhibited. PDE4, PDE7, and PDE8 also function together to suppress the premature synthesis of progesterone and progesterone receptors, which are required for ovulation. Our results indicate that three cAMP PDEs act in concert to suppress premature responses in preovulatory follicles.

Shedding light on spawning in jellyfish

An opsin receptor has a central role in the production and release of eggs by female jellyfish.

Luteinizing hormone signaling phosphorylates and activates the cyclic GMP phosphodiesterase PDE5 in mouse ovarian follicles, contributing an additional component to the hormonally induced decrease in cyclic GMP that reinitiates meiosis

Prior to birth, oocytes within mammalian ovarian follicles initiate meiosis, but then arrest in prophase until puberty, when with each reproductive cycle, one or more follicles are stimulated by luteinizing hormone (LH) to resume meiosis in preparation for fertilization. Within preovulatory follicles, granulosa cells produce high levels of cGMP, which diffuses into the oocyte to maintain meiotic arrest. LH signaling restarts meiosis by rapidly lowering the levels of cGMP in the follicle and oocyte. Part of this decrease is mediated by the dephosphorylation and inactivation the NPR2 guanylyl cyclase in response to LH, but the mechanism for the remainder of the cGMP decrease is unknown. At least one cGMP phosphodiesterase, PDE5, is activated by LH signaling, which would contribute to lowering cGMP. PDE5 exhibits increased cGMP-hydrolytic activity when phosphorylated on serine 92, and we recently demonstrated that LH signaling phosphorylates PDE5 on this serine and increases its activity in rat follicles. To test the extent to which this mechanism contributes to the cGMP decrease that restarts meiosis, we generated a mouse line in which serine 92 was mutated to alanine (Pde5-S92A), such that it cannot be phosphorylated. Here we show that PDE5 phosphorylation is required for the LH-induced increase in cGMP-hydrolytic activity, but that this increase has only a modest effect on the LH-induced cGMP decrease in mouse follicles, and does not affect the timing of meiotic resumption. Though we show that the activation of PDE5 is among the mechanisms contributing to the cGMP decrease, these results suggest that another cGMP phosphodiesterase is also activated by LH signaling.

Dephosphorylation of the NPR2 guanylyl cyclase contributes to inhibition of bone growth by fibroblast growth factor

Activating mutations in fibroblast growth factor (FGF) receptor 3 and inactivating mutations in the NPR2 guanylyl cyclase both cause severe short stature, but how these two signaling systems interact to regulate bone growth is poorly understood. Here, we show that bone elongation is increased when NPR2 cannot be dephosphorylated and thus produces more cyclic GMP. By developing an in vivo imaging system to measure cyclic GMP production in intact tibia, we show that FGF-induced dephosphorylation of NPR2 decreases its guanylyl cyclase activity in growth plate chondrocytes in living bone. The dephosphorylation requires a PPP-family phosphatase. Thus FGF signaling lowers cyclic GMP production in the growth plate, which counteracts bone elongation. These results define a new component of the signaling network by which activating mutations in the FGF receptor inhibit bone growth.

Between birth and puberty, the bones of mammals grow drastically in length. This process is controlled by many proteins, and mutations affecting these proteins can cause bones to either be too long or too short. For example, mutations of a protein called the fibroblast growth factor receptor, or FGF for short, and a protein called NPR2, can cause similar forms of dwarfism – a condition characterized by short stature.

The FGF protein controls bone growth, and people with overactive receptors for FGF suffer from a form of dwarfism known as achondroplasia, while people that lack FGF receptors have longer bones. The NPR2 protein, on the other hand, produces a molecule called cGMP, which is necessary for the bones to grow. When NPR2 is blocked, less cGMP is produced, which results in shorter limbs.

Previous studies of bone cells grown in the laboratory have shown that these two proteins are linked by a chain of chemical messages. When the FGF receptor is active, phosphate molecules are removed from the NPR2 protein, which reduces the amount of GMP produced. However, until now it was not known whether this mechanism also controls growth in actual bones.

Here, Shuhaibar et al. used genetically modified mice in which the phosphate group could not be removed from their NPR2 enzyme. As a result, the bones of these mice were longer than usual. Shuhaibar et al. then developed an imaging technique to examine the region in the bone were growth happens. To see whether FGF reduces the amount of cGMP produced by NPR2 in these areas, cGMP was detected with a fluorescent sensor in order to be tracked.

In normal mice, the FGF receptor reduced the rate at which cGMP was produced, but in mice with mutated NPR2, this did not happen. When the cells could not remove the phosphates from NPR2, cGMP levels stayed high and the bones grew longer.

These findings reveal new insights into the molecular causes of dwarfism. The next step will be to identify the enzyme responsible for removing phosphate from NPR2. Blocking its activity could help to enhance bone growth. In the future, this could lead to new drug treatments for achondroplasia.

Regulation of Mammalian Oocyte Meiosis by Intercellular Communication Within the Ovarian Follicle

Meiotic progression in mammalian preovulatory follicles is controlled by the granulosa cells around the oocyte. Cyclic GMP (cGMP) generated in the granulosa cells diffuses through gap junctions into the oocyte, maintaining meiotic prophase arrest. Luteinizing hormone then acts on receptors in outer granulosa cells to rapidly decrease cGMP. This occurs by two complementary pathways: cGMP production is decreased by dephosphorylation and inactivation of the NPR2 guanylyl cyclase, and cGMP hydrolysis is increased by activation of the PDE5 phosphodiesterase. The cGMP decrease in the granulosa cells results in rapid cGMP diffusion out of the oocyte, initiating meiotic resumption. Additional, more slowly developing mechanisms involving paracrine signaling by extracellular peptides (C-type natriuretic peptide and EGF receptor ligands) maintain the low level of cGMP in the oocyte. These coordinated signaling pathways ensure a fail-safe system to prepare the oocyte for fertilization and reproductive success.

Luteinizing Hormone Causes Phosphorylation and Activation of the cGMP Phosphodiesterase PDE5 in Rat Ovarian Follicles, Contributing, Together with PDE1 Activity, to the Resumption of Meiosis

The meiotic cell cycle of mammalian oocytes in preovulatory follicles is held in prophase arrest by diffusion of cGMP from the surrounding granulosa cells into the oocyte. Luteinizing hormone (LH) then releases meiotic arrest by lowering cGMP in the granulosa cells. The LH-induced reduction of cGMP is caused in part by a decrease in guanylyl cyclase activity, but the observation that the cGMP phosphodiesterase PDE5 is phosphorylated during LH signaling suggests that an increase in PDE5 activity could also contribute. To investigate this idea, we measured cGMP-hydrolytic activity in rat ovarian follicles. Basal activity was due primarily to PDE1A and PDE5, and LH increased PDE5 activity. The increase in PDE5 activity was accompanied by phosphorylation of PDE5 at serine 92, a protein kinase A/G consensus site. Both the phosphorylation and the increase in activity were promoted by elevating cAMP and opposed by inhibiting protein kinase A, supporting the hypothesis that LH activates PDE5 by stimulating its phosphorylation by protein kinase A. Inhibition of PDE5 activity partially suppressed LH-induced meiotic resumption as indicated by nuclear envelope breakdown, but inhibition of both PDE5 and PDE1 activities was needed to completely inhibit this response. These results show that activities of both PDE5 and PDE1 contribute to the LH-induced resumption of meiosis in rat oocytes, and that phosphorylation and activation of PDE5 is a regulatory mechanism.

Dephosphorylation and inactivation of NPR2 guanylyl cyclase in granulosa cells contributes to the LH-induced decrease in cGMP that causes resumption of meiosis in rat oocytes

In mammals, the meiotic cell cycle of oocytes starts during embryogenesis and then pauses. Much later, in preparation for fertilization, oocytes within preovulatory follicles resume meiosis in response to luteinizing hormone (LH). Before LH stimulation, the arrest is maintained by diffusion of cyclic (c)GMP into the oocyte from the surrounding granulosa cells, where it is produced by the guanylyl cyclase natriuretic peptide receptor 2 (NPR2). LH rapidly reduces the production of cGMP, but how this occurs is unknown. Here, using rat follicles, we show that within 10 min, LH signaling causes dephosphorylation and inactivation of NPR2 through a process that requires the activity of phosphoprotein phosphatase (PPP)-family members. The rapid dephosphorylation of NPR2 is accompanied by a rapid phosphorylation of the cGMP phosphodiesterase PDE5, an enzyme whose activity is increased upon phosphorylation. Later, levels of the NPR2 agonist C-type natriuretic peptide decrease in the follicle, and these sequential events contribute to the decrease in cGMP that causes meiosis to resume in the oocyte.

Hormonal coordination of natriuretic peptide type C and natriuretic peptide receptor 3 expression in mouse granulosa cells

Natriuretic peptide type C (NPPC) and its receptor natriuretic peptide receptor 2 (NPR2) regulate cGMP in ovarian follicles and participate in maintaining oocyte meiotic arrest. We investigated the regulation of Nppc expression in mouse granulosa cells in vivo and in vitro. In mural granulosa cells (MGCs) in vivo, eCG caused an increase in Nppc mRNA, and subsequent human chorionic gonadotropin (hCG) treatment caused a decrease. A culture system was established for MGCs isolated from follicles not stimulated with equine chorionic gonadotropin to further define the mechanisms controlling Nppc expression. In this system, expression of Nppc mRNA was increased by estradiol (E2), with augmentation by follicle-stimulating hormone (FSH), but FSH or luteinizing hormone (LH) alone had no effect. Thus, estrogens are important for regulating Nppc expression, probably by feedback mechanisms enhancing the action of gonadotropins. In MGCs treated with E2 plus FSH in vitro, subsequent treatment with EGF, but not LH, decreased Nppc mRNA. MGCs express higher levels of both Nppc and Lhcgr mRNAs than cumulus cells. Oocyte-derived paracrine factors suppressed cumulus cell Lhcgr but not Nppc expression. Thus, higher Nppc expression by MGCs is not the result of oocyte suppression of expression in cumulus cells. Another possible regulator of the LH-induced NPPC decrease is NPR3, an NPPC clearance receptor. Human chorionic gonadotropin increased Npr3 expression in vivo and LH increased Npr3 mRNA in cultured MGCs, independently of EGF receptor activation. Interestingly, despite the increase in Npr3 mRNA, the hCG-induced decrease in ovarian NPPC occurred normally in an Npr3 mutant (lgj), thus NPR3 probably does not participate in regulation of ovarian NPPC levels or oocyte development.

Luteinizing hormone reduces the activity of the NPR2 guanylyl cyclase in mouse ovarian follicles, contributing to the cyclic GMP decrease that promotes resumption of meiosis in oocytes

In preovulatory ovarian follicles of mice, meiotic prophase arrest in the oocyte is maintained by cyclic GMP from the surrounding granulosa cells that diffuses into the oocyte through gap junctions. The cGMP is synthesized in the granulosa cells by the transmembrane guanylyl cyclase natriuretic peptide receptor 2 (NPR2) in response to the agonist C-type natriuretic peptide (CNP). In response to luteinizing hormone (LH), cGMP in the granulosa cells decreases, and as a consequence, oocyte cGMP decreases and meiosis resumes. Here we report that within 20 min, LH treatment results in decreased guanylyl cyclase activity of NPR2, as determined in the presence of a maximally activating concentration of CNP. This occurs by a process that does not reduce the amount of NPR2 protein. We also show that by a slower process, first detected at 2h, LH decreases the amount of CNP available to bind to the receptor. Both of these LH actions contribute to decreasing cGMP in the follicle, thus signaling meiotic resumption in the oocyte.

Voltage sensitive phosphoinositide phosphatases of Xenopus: their tissue distribution and voltage dependence

Voltage-sensitive phosphatases (VSPs) are unique proteins in which membrane potential controls enzyme activity. They are comprised of the voltage sensor domain of an ion channel coupled to a lipid phosphatase specific for phosphoinositides, and for ascidian and zebrafish VSPs, the phosphatase activity has been found to be activated by membrane depolarization. The physiological functions of these proteins are unknown, but their expression in testis and embryos suggests a role in fertilization or development. Here we investigate the expression pattern and voltage dependence of VSPs in two frog species, Xenopus laevis and Xenopus tropicalis, that are well suited for experimental studies of these possible functions. X. laevis has two VSP genes (Xl-VSP1 and Xl-VSP2), whereas X. tropicalis has only one gene (Xt-VSP). The highest expression of these genes was observed in testis, ovary, liver, and kidney. Our results show that while Xl-VSP2 activates only at positive membrane potentials outside of the physiological range, Xl-VSP1 and Xt-VSP phosphatase activity is regulated in the voltage range that regulates sperm-egg fusion at fertilization.

Epidermal growth factor receptor kinase activity is required for gap junction closure and for part of the decrease in ovarian follicle cGMP in response to LH

The meiotic cell cycle in mouse oocytes is arrested in prophase, and then restarted when LH acts on the surrounding granulosa cells. The granulosa cells keep meiosis arrested by providing a source of cGMP that diffuses into the oocyte through gap junctions, and LH restarts the cell cycle by closing the junctions and by decreasing granulosa cell cGMP, thus lowering oocyte cGMP. Epidermal growth factor receptor (EGFR) activation is an essential step in triggering LH-induced meiotic resumption, but its relationship to the cGMP decrease in the follicle is incompletely understood, and its possible function in causing gap junction closure has not been investigated. Here, we use EGFR agonists (epiregulin and amphiregulin) and an EGFR kinase inhibitor (AG1478) to study the function of the EGFR in the signaling pathways leading to the release of oocytes from prophase arrest. Our results indicate that the EGFR kinase contributes to LH-induced meiotic resumption in two different ways. First, it is required for gap junction closure. Second, it is required for an essential component of the decrease in follicle cGMP. Our data show that the EGFR kinase-dependent component of the cGMP decrease is required for LH-induced meiotic resumption, but they also indicate that an as yet unidentified pathway accounts for a large part of the cGMP decrease.

Cyclic GMP from the surrounding somatic cells regulates cyclic AMP and meiosis in the mouse oocyte

Mammalian oocytes are arrested in meiotic prophase by an inhibitory signal from the surrounding somatic cells in the ovarian follicle. In response to luteinizing hormone (LH), which binds to receptors on the somatic cells, the oocyte proceeds to second metaphase, where it can be fertilized. Here we investigate how the somatic cells regulate the prophase-to-metaphase transition in the oocyte, and show that the inhibitory signal from the somatic cells is cGMP. Using FRET-based cyclic nucleotide sensors in follicle-enclosed mouse oocytes, we find that cGMP passes through gap junctions into the oocyte, where it inhibits the hydrolysis of cAMP by the phosphodiesterase PDE3A. This inhibition maintains a high concentration of cAMP and thus blocks meiotic progression. LH reverses the inhibitory signal by lowering cGMP levels in the somatic cells (from approximately 2 microM to approximately 80 nM at 1 hour after LH stimulation) and by closing gap junctions between the somatic cells. The resulting decrease in oocyte cGMP (from approximately 1 microM to approximately 40 nM) relieves the inhibition of PDE3A, increasing its activity by approximately 5-fold. This causes a decrease in oocyte cAMP (from approximately 700 nM to approximately 140 nM), leading to the resumption of meiosis.

Luteinizing hormone causes MAP kinase-dependent phosphorylation and closure of connexin 43 gap junctions in mouse ovarian follicles: one of two paths to meiotic resumption

Luteinizing hormone (LH) acts on ovarian follicles to reinitiate meiosis in prophase-arrested mammalian oocytes, and this has been proposed to occur by interruption of a meioisis-inhibitory signal that is transmitted through gap junctions into the oocyte from the somatic cells that surround it. To investigate this idea, we microinjected fluorescent tracers into live antral follicle-enclosed mouse oocytes, and we demonstrate for the first time that LH causes a decrease in the gap junction permeability between the somatic cells, prior to nuclear envelope breakdown (NEBD). The decreased permeability results from the MAP kinase-dependent phosphorylation of connexin 43 on serines 255, 262 and 279/282. We then tested whether the inhibition of gap junction communication was sufficient and necessary for the reinitiation of meiosis. Inhibitors that reduced gap junction permeability caused NEBD, but an inhibitor of MAP kinase activation that blocked gap junction closure in response to LH did not prevent NEBD. Thus, both MAP kinase-dependent gap junction closure and another redundant pathway function in parallel to ensure that meiosis resumes in response to LH.

Microinjection of follicle-enclosed mouse oocytes

The mammalian oocyte develops within a complex of somatic cells known as a follicle, within which signals from the somatic cells regulate the oocyte, and signals from the oocyte regulate the somatic cells. Because isolation of the oocyte from the follicle disrupts these communication pathways, oocyte physiology is best studied within an intact follicle. Here we describe methods for quantitative microinjection of follicle-enclosed mouse oocytes, thus allowing the introduction of signaling molecules as well as optical probes into the oocyte within its physiological environment.

A G(s)-linked receptor maintains meiotic arrest in mouse oocytes, but luteinizing hormone does not cause meiotic resumption by terminating receptor-G(s) signaling

The maintenance of meiotic prophase arrest in fully grown vertebrate oocytes depends on the activity of a G(s) G-protein that activates adenylyl cyclase and elevates cAMP, and in the mouse oocyte, G(s) is activated by a constitutively active orphan receptor, GPR3. To determine whether the action of luteinizing hormone (LH) on the mouse ovarian follicle causes meiotic resumption by inhibiting GPR3-G(s) signaling, we examined the effect of LH on the localization of Galpha(s). G(s) activation in response to stimulation of an exogenously expressed beta(2)-adrenergic receptor causes Galpha(s) to move from the oocyte plasma membrane into the cytoplasm, whereas G(s) inactivation in response to inhibition of the beta(2)-adrenergic receptor causes Galpha(s) to move back to the plasma membrane. However, LH does not cause a change in Galpha(s) localization, indicating that LH does not act by terminating receptor-G(s) signaling.

Meiotic resumption in response to luteinizing hormone is independent of a Gi family G protein or calcium in the mouse oocyte

The signaling pathway by which luteinizing hormone (LH) acts on the somatic cells of vertebrate ovarian follicles to stimulate meiotic resumption in the oocyte requires a decrease in cAMP in the oocyte, but how cAMP is decreased is unknown. Activation of Gi family G proteins can lower cAMP by inhibiting adenylate cyclase or stimulating a cyclic nucleotide phosphodiesterase, but we show here that inhibition of this class of G proteins by injection of pertussis toxin into follicle-enclosed mouse oocytes does not prevent meiotic resumption in response to LH. Likewise, elevation of Ca2+ can lower cAMP through its action on Ca2+-sensitive adenylate cyclases or phosphodiesterases, but inhibition of a Ca2+ rise by injection of EGTA into follicle-enclosed mouse oocytes does not inhibit the LH response. Thus, neither of these well-known mechanisms of cAMP regulation can account for LH signaling to the oocyte in the mouse ovary.

Regulation of meiotic prophase arrest in mouse oocytes by GPR3, a constitutive activator of the Gs G protein

The arrest of meiotic prophase in mouse oocytes within antral follicles requires the G protein G(s) and an orphan member of the G protein-coupled receptor family, GPR3. To determine whether GPR3 activates G(s), the localization of Galpha(s) in follicle-enclosed oocytes from Gpr3(+/+) and Gpr3(-/-) mice was compared by using immunofluorescence and Galpha(s)GFP. GPR3 decreased the ratio of Galpha(s) in the oocyte plasma membrane versus the cytoplasm and also decreased the amount of Galpha(s) in the oocyte. Both of these properties indicate that GPR3 activates G(s). The follicle cells around the oocyte are also necessary to keep the oocyte in prophase, suggesting that they might activate GPR3. However, GPR3-dependent G(s) activity was similar in follicle-enclosed and follicle-free oocytes. Thus, the maintenance of prophase arrest depends on the constitutive activity of GPR3 in the oocyte, and the follicle cell signal acts by a means other than increasing GPR3 activity.

SH2 domain-mediated activation of an SRC family kinase is not required to initiate Ca2+ release at fertilization in mouse eggs

SRC family kinases (SFKs) function in initiating Ca2+release at fertilization in several species in the vertebrate evolutionary line, but whether they play a similar role in mammalian fertilization has been uncertain. We investigated this question by first determining which SFK proteins are expressed in mouse eggs, and then measuring Ca2+release at fertilization in the presence of dominant negative inhibitors. FYN and YES proteins were found in mouse eggs, but other SFKs were not detected; based on this, we injected mouse eggs with a mixture of FYN and YES Src homology 2 (SH2) domains. These SH2 domains were effective inhibitors of Ca2+release at fertilization in starfish eggs, but did not inhibit Ca2+release at fertilization in mouse eggs. Thus the mechanism by which sperm initiate Ca2+release in mouse eggs does not depend on SH2 domain-mediated activation of an SFK. We also tested the small molecule SFK inhibitor SU6656, and found that it became compartmentalized in the egg cytoplasm, thus suggesting caution in the use of this inhibitor. Our findings indicate that although the initiation of Ca2+release at fertilization of mammalian eggs occurs by a pathway that has many similarities to that in evolutionarily earlier animal groups, the requirement for SH2 domain-mediated activation of an SFK is not conserved.

The Gs-Linked Receptor GPR3 Maintains Meiotic Arrest in Mammalian Oocytes

Mammalian oocytes are held in prophase arrest by an unknown signal from the surrounding somatic cells. Here we show that the orphan Gs-linked receptor GPR3, which is localized in the oocyte, maintains this arrest. Oocytes from Gpr3 knockout mice resume meiosis within antral follicles, independently of an increase in luteinizing hormone, and this phenotype can be reversed by injection of Gpr3 RNA into the oocytes. Thus, the GPR3 receptor is a link in communication between the somatic cells and oocyte of the ovarian follicle and is crucial for the regulation of meiosis.

Maintenance of meiotic prophase arrest in vertebrate oocytes by a G s protein-mediated pathway

Maintenance of meiotic prophase arrest in fully grown vertebrate oocytes depends on an elevated level of cAMP in the oocyte. To investigate how the cAMP level is regulated, we examined whether the activity of an oocyte G protein of the family that stimulates adenylyl cyclase, Gs, is required to maintain meiotic arrest. Microinjection of a dominant negative form of Gs into Xenopus and mouse oocytes, or microinjection of an antibody that inhibits the Gs G protein into zebrafish oocytes, caused meiosis to resume. Together with previous studies, these results support the conclusion that Gs-regulated generation of cAMP by the oocyte is a common mechanism for maintaining meiotic prophase arrest in vertebrate oocytes.

Function of a sea urchin egg Src family kinase in initiating Ca2+ release at fertilization

Egg activation at fertilization requires the release of Ca(2+) from the egg's endoplasmic reticulum, and recent evidence has indicated that a Src family kinase (SFK) may function in initiating this signaling pathway in echinoderm eggs. Here, we identify and characterize a SFK from the sea urchin Strongylocentrotus purpuratus, SpSFK1. SpSFK1 RNA is present in eggs, and an antibody made against a SpSFK1 peptide recognizes an approximately 58-kDa egg membrane-associated protein in eggs of S. purpuratus as well as another sea urchin Lytechinus variegatus. Injection of both species of sea urchin eggs with dominant-interfering Src homology 2 domains of SpSFK1 delays and reduces the release of Ca(2+) at fertilization. Injection of an antibody against SpSFK1 into S. purpuratus eggs also causes a small increase in the delay between sperm-egg fusion and Ca(2+) release. In contrast, when injected into eggs of L. variegatus, this same antibody has a dramatic stimulatory effect: it causes PLCgamma-dependent Ca(2+) release like that occurring at fertilization. Correspondingly, in lysates of L. variegatus eggs, but not S. purpuratus eggs, the antibody stimulates SFK activity. Injection of L. variegatus eggs with another antibody that recognizes the L. variegatus egg SFK also causes PLCgamma-dependent Ca(2+) release like that at fertilization. These results indicate that activation of a Src family kinase present in sea urchin eggs is necessary to cause Ca(2+) release at fertilization and is capable of stimulating Ca(2+) release in the unfertilized egg via PLCgamma, as at fertilization.

A receptor linked to a Gi-family G-protein functions in initiating oocyte maturation in starfish but not frogs

The stimulation of oocyte maturation by 1-methyladenine in starfish, and by a steroid in frogs, has been proposed to involve G-protein-coupled receptors. To examine whether activation of receptors linked to G(i) or G(z) was sufficient to cause oocyte maturation, we expressed mammalian G(i)- and G(z)-linked receptors in starfish and frog oocytes. Application of the corresponding agonists caused meiosis to resume in the starfish but not the frog oocytes. We confirmed that the receptors were effectively expressed in the frog oocytes by using a chimeric G-protein, G(qi), that converts input from G(i)- and G(z)-linked receptors to a G(q) output and results in a contraction of the oocyte's pigment. These results argue against G(i) or G(z) functioning to cause maturation in frog oocytes. Consistently, maturation-inducing steroids did not cause pigment contraction in frog oocytes expressing G(qi), and G(z) protein was not detectable in frog oocytes. For starfish oocytes, however, our results support the conclusion that G(i) functions in 1-methyladenine signaling and suggest the possibility of using frog oocyte pigment contraction as an assay to identify the 1-methyladenine receptor. To test this concept, we coexpressed G(qi) and a starfish adenosine receptor in frog oocytes and showed that applying adenosine caused pigment contraction.

Meiotic arrest in the mouse follicle maintained by a Gs protein in the oocyte

The mammalian ovarian follicle consists of a multilayered complex of somatic cells that surround the oocyte. A signal from the follicle cells keeps the oocyte cell cycle arrested at prophase of meiosis I until luteinizing hormone from the pituitary acts on the follicle cells to release the arrest, causing meiosis to continue. Here we show that meiotic arrest can be released in mice by microinjecting the oocyte within the follicle with an antibody that inhibits the stimulatory heterotrimeric GTP-binding protein Gs. This indicates that Gs activity in the oocyte is required to maintain meiotic arrest within the ovarian follicle and suggests that the follicle may keep the cell cycle arrested by activating Gs.

Egg activation at fertilization: where it all begins

A centrally important factor in initiating egg activation at fertilization is a rise in free Ca(2+) in the egg cytosol. In echinoderm, ascidian, and vertebrate eggs, the Ca(2+) rise occurs as a result of inositol trisphosphate-mediated release of Ca(2+) from the endoplasmic reticulum. The release of Ca(2+) at fertilization in echinoderm and ascidian eggs requires SH2 domain-mediated activation of a Src family kinase (SFK) and phospholipase C (PLC)gamma. Though some evidence indicates that a SFK and PLC may also function at fertilization in vertebrate eggs, SH2 domain-mediated activation of PLC gamma appears not to be required. Much work has focused on identifying factors from sperm that initiate egg activation at fertilization, either as a result of sperm-egg contact or sperm-egg fusion. Current evidence from studies of ascidian and mammalian fertilization favors a fusion-mediated mechanism; this is supported by experiments indicating that injection of sperm extracts into eggs causes Ca(2+) release by the same pathway as fertilization.

Evidence that phospholipase C from the sperm is not responsible for initiating Ca(2+) release at fertilization in mouse eggs

Release of Ca(2+) from intracellular stores at fertilization of mammalian eggs is mediated by inositol 1,4,5-trisphosphate (IP3), but the mechanism by which the sperm initiates IP3 production is not yet understood. We tested the hypothesis that phospholipase C (PLC) activity introduced into the mouse egg as a consequence of sperm-egg fusion is responsible for causing Ca(2+) release. We demonstrated that microinjecting purified, recombinant PLCgamma1 protein into mouse eggs caused Ca(2+) oscillations like those seen at fertilization. However, the PLC activity in the minimum amount of purified PLCgamma1 protein needed to elicit Ca(2+) release when injected into eggs was approximately 500-900 times the PLC activity contained in a single sperm. This indicates that a single mouse sperm does not contain enough PLC activity to be responsible for causing Ca(2+) release at fertilization. We also examined whether phosphatidylinositol 3-kinase (PI3K) could have a role in this process, and found that several inhibitors of PI3K-mediated signaling had no effect on Ca(2+) release at fertilization.

Ca2+ signalling during fertilization of echinoderm eggs

The Ca2+ rise at fertilization of echinoderm eggs is initiated by a process requiring the sequential activation of a Src family kinase, phospholipase C gamma, and the inositol trisphosphate receptor/channel in the endoplasmic reticulum. The consequences of the Ca2+ rise include exocytosis of cortical granules, which establishes a block to polyspermy, and inactivation of MAP kinase, which functions in linking the Ca2+ rise to the reinitiation of the cell cycle.

Sperm extract injection into ascidian eggs signals Ca(2+) release by the same pathway as fertilization

Injection of eggs of various species with an extract of sperm cytoplasm stimulates intracellular Ca(2+) release that is spatially and temporally like that occurring at fertilization, suggesting that Ca(2+) release at fertilization may be initiated by a soluble factor from the sperm. Here we investigate whether the signalling pathway that leads to Ca(2+) release in response to sperm extract injection requires the same signal transduction molecules as are required at fertilization. Eggs of the ascidian Ciona intestinalis were injected with the Src-homology 2 domains of phospholipase C gamma or of the Src family kinase Fyn (which act as specific dominant negative inhibitors of the activation of these enzymes), and the effects on Ca(2+) release at fertilization or in response to injection of a sperm extract were compared. Our findings indicate that both fertilization and sperm extract injection initiate Ca(2+) release by a pathway requiring phospholipase C gamma and a Src family kinase. These results support the hypothesis that, in ascidians, a soluble factor from the sperm cytoplasm initiates Ca(2+) release at fertilization, and indicate that the activating factor from the sperm may be a regulator, directly or indirectly, of a Src family kinase in the egg.

Evidence that fertilization activates starfish eggs by sequential activation of a Src-like kinase and phospholipase cgamma

Recent evidence has indicated a requirement for a Src family kinase in initiating Ca(2+) release at fertilization in starfish eggs (Giusti, A. F., Carroll, D. J., Abassi, Y. A., Terasaki, M., Foltz, K. R., and Jaffe, L. A. (1999) J. Biol. Chem. 274, 29318-29322). We now show that injection of Src protein into starfish eggs initiates Ca(2+) release and DNA synthesis, as occur at fertilization. These responses depend on the phosphorylation state of the Src protein; only the kinase active form is effective. Like Ca(2+) release at fertilization, the Ca(2+) release in response to Src protein injection is inhibited by prior injection of the SH2 domains of phospholipase Cgamma. These findings support the conclusion that in starfish, sperm-egg interaction causes egg activation by sequential activation of a Src-like kinase and phospholipase Cgamma. Injection of the SH2 domain of Src, which inhibits Ca(2+) release at fertilization, does not inhibit Ca(2+) release caused by Src protein injection. This indicates that the requirement for a Src SH2 domain interaction is upstream of Src activation in the pathway leading to Ca(2+) release at fertilization.

Calcium release at fertilization of Xenopus eggs requires type I IP(3) receptors, but not SH2 domain-mediated activation of PLCgamma or G(q)-mediated activation of PLCbeta

Elevation of intracellular Ca2+ at fertilization is essential for the initiation of development in the Xenopus egg, but the pathway between sperm-egg interaction and Ca2+ release from the egg's endoplasmic reticulum is not well understood. Here we show that injection of an inhibitory antibody against the type I IP(3) receptor reduces Ca2+ release at fertilization, indicating that the Ca2+ release requires IP(3). We then examine how IP(3) production is initiated. Xenopus eggs were injected with specific inhibitors of the activation of two phospholipase C isoforms, PLCgamma and PLCbeta. The Src-homology 2 (SH2) domains of PLCgamma were used to inhibit SH2-mediated activation of PLCgamma, and an antibody against G(q) family G-proteins was used to inhibit G(q)-mediated activation of PLCbeta. Though the PLCgamma SH2 domains inhibited platelet-derived growth factor (PDGF)-induced Ca2+ release in eggs with exogenously expressed PDGF receptors, they did not inhibit the Ca2+ rise at fertilization. Similarly, the G(q) family antibody blocked serotonin-induced Ca2+ release in eggs with exogenously expressed serotonin 2C receptors, but not the Ca2+ rise at fertilization. A mixture of PLCgamma SH2 domains and the G(q) antibody also did not inhibit the Ca2+ rise at fertilization. These results indicate that Ca2+ release at fertilization of Xenopus eggs requires type I IP(3)-gated Ca2+ channels, but not SH2 domain-mediated activation of PLCgamma or G(q)-mediated activation of PLCbeta.

Requirement of a Src family kinase for initiating calcium release at fertilization in starfish eggs

Signal transduction leading to calcium release in echinoderm eggs at fertilization requires phospholipase Cgamma-mediated production of inositol trisphosphate (IP(3)), indicating that a tyrosine kinase is a likely upstream regulator. Because previous work has shown a fertilization-dependent association between the Src homology 2 (SH2) domains of phospholipase Cgamma and a Src family kinase, we examined whether a Src family kinase was required for Ca(2+) release at fertilization. To inhibit the function of kinases in this family, we injected starfish eggs with the SH2 domains of Src and Fyn kinases. This inhibited Ca(2+) release in response to fertilization but not in response to injection of IP(3). We further established the specificity of the inhibition by showing that the SH2 domains of several other tyrosine kinases (Abl, Syk, and ZAP-70), and the SH3 domain of Src, were not inhibitory. Also, a point-mutated Src SH2 domain, which has reduced affinity for phosphotyrosine, was a correspondingly less effective inhibitor of fertilization-induced Ca(2+) release. These results indicate that a Src family kinase, by way of its SH2 domain, links sperm-egg interaction to IP(3)-mediated Ca(2+) release at fertilization in starfish eggs.

Identification of PLCgamma-dependent and -independent events during fertilization of sea urchin eggs

At fertilization, sea urchin eggs undergo a series of activation events, including a Ca2+ action potential, Ca2+ release from the endoplasmic reticulum, an increase in intracellular pH, sperm pronuclear formation, MAP kinase dephosphorylation, and DNA synthesis. To examine which of these events might be initiated by activation of phospholipase Cgamma (PLCgamma), which produces the second messengers inositol trisphosphate (IP3) and diacylglycerol, we used recombinant SH2 domains of PLCgamma as specific inhibitors. Sea urchin eggs were co-injected with a GST fusion protein composed of the two tandem SH2 domains of bovine PLCgamma and (1) Ca2+ green dextran to monitor intracellular free Ca2+, (2) BCECF dextran to monitor intracellular pH, (3) Oregon Green dUTP to monitor DNA synthesis, or (4) fluorescein 70-kDa dextran to monitor nuclear envelope formation. Microinjection of the tandem SH2 domains of PLCgamma produced a concentration-dependent inhibition of Ca2+ release and also inhibited cortical granule exocytosis, cytoplasmic alkalinization, MAP kinase dephosphorylation, DNA synthesis, and cleavage after fertilization. However, the Ca2+ action potential, sperm entry, and sperm pronuclear formation were not prevented by injection of the PLCgammaSH2 domain protein. Microinjection of a control protein, the tandem SH2 domains of the phosphatase SHP2, had no effect on Ca2+ release, cortical granule exocytosis, DNA synthesis, or cleavage. Specificity of the inhibitory action of the PLCgammaSH2 domains was further indicated by the finding that microinjection of PLCgammaSH2 domains that had been point mutated at a critical arginine did not inhibit Ca release at fertilization. Additionally, Ca2+ release in response to microinjection of IP3, cholera toxin, cADP ribose, or cGMP was not inhibited by the PLCgammaSH2 fusion protein. These results indicate that PLCgamma plays a key role in several fertilization events in sea urchin eggs, including Ca2+ release and DNA synthesis, but that the action potential, sperm entry, and male pronuclear formation can occur in the absence of PLCgamma activation or Ca2+ increase.