Proteomic responses of sea urchin embryos to stressful ultraviolet radiation.

Solar ultraviolet radiation (UVR, 290-400 nm) penetrates into seawater and can harm shallow-dwelling and planktonic marine organisms. Studies dating back to the 1930s revealed that echinoids, especially sea urchin embryos, are powerful models for deciphering the effects of UVR on embryonic development and how embryos defend themselves against UV-induced damage. In addition to providing a large number of synchronously developing embryos amenable to cellular, biochemical, molecular, and single-cell analyses, the purple sea urchin, Strongylocentrotus purpuratus, also offers an annotated genome. Together, these aspects allow for the in-depth study of molecular and biochemical signatures of UVR stress. Here, we review the effects of UVR on embryonic development, focusing on the early-cleavage stages, and begin to integrate data regarding single-protein responses with comprehensive proteomic assessments. Proteomic studies reveal changes in levels of post-translational modifications to proteins that respond to UVR, and identify proteins that can then be interrogated as putative targets or components of stress-response pathways. These responsive proteins are distributed among systems upon which targeted studies can now begin to be mapped. Post-transcriptional and translational controls may provide early embryos with a rapid, fine-tuned response to stress during early stages, especially during pre-blastula stages that rely primarily on maternally derived defenses rather than on responses through zygotic gene transcription.

Calcium-mediated inactivation of the MAP kinase pathway in sea urchin eggs at fertilization.

We have evaluated the regulation of a 43-kDa MAP kinase in sea urchin eggs. Both MAP kinase and MEK (MAP kinase kinase) are phosphorylated and active in unfertilized eggs while both are dephosphorylated and inactivated after fertilization, although with distinct kinetics. Reactivation of MEK or the 43-kDa MAP kinase prior to or during the first cell division was not detected. Confocal immunolocalization microscopy revealed that phosphorylated (active) MAP kinase is present primarily in the nucleus of the unfertilized egg, with some of the phosphorylated form in the cytoplasm as well. Incubation of unfertilized eggs in the MEK inhibitor U0126 (0.5 microM) resulted in the inactivation of MEK and MAP kinase within 30 min. Incubation in low concentrations of U0126 (sufficient to inactivate MEK and MAP kinase) after fertilization had no effect on progression through the embryonic cell cycle. Microinjection of active mammalian MAP kinase phosphatase (MKP-3) resulted in inactivation of MAP kinase in unfertilized eggs, as did addition of MKP-3 to lysates of unfertilized eggs. Incubation of unfertilized eggs in the Ca(2+) ionophore A23187 led to inactivation of MEK and MAP kinase with the same kinetics as observed with sperm-induced egg activation. This suggests that calcium may be deactivating MEK and/or activating a MAP kinase-directed phosphatase. A cell-free system was used to evaluate the activation of phosphatase separately from MEK inactivation. Unfertilized egg lysates were treated with U0126 to inactivate MEK and then Ca(2+) was added. This resulted in increased MAP kinase phosphatase activity. Therefore, MAP kinase inactivation at fertilization in sea urchin eggs likely is the result of a combination of MEK inactivation and phosphatase activation that are directly or indirectly responsive to Ca(2+).

Isolation and characterization of sea urchin egg lipid rafts and their possible function during fertilization.

Specialized membrane microdomains called rafts are thought to play a role in many types of cell-cell interactions and signaling. We have investigated the possibility that sea urchin eggs contain these specialized membrane microdomains and if they play a role in signal transduction at fertilization. A low density, TX-100 insoluble membrane fraction, typical of lipid rafts, was isolated by equilibrium gradient centrifugation. This raft fraction contained proteins distinct from cytoskeletal complexes. The fraction was enriched in tyrosine phosphorylated proteins and contained two proteins known to be involved in signaling during egg activation (an egg Src-type kinase and PLC gamma). This fraction was further characterized as a prototypical raft fraction by the release of proteins in response to in vitro treatment of the rafts with the cholesterol binding drug, methyl-beta-cyclodextrin (M beta CD). Furthermore, treatment of eggs with M beta CD inhibited fertilization, suggesting that egg lipid rafts play a physiological role in fertilization. Mol. Reprod. Dev. 59:294-305, 2001.

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.

Evaluation of sequence variation and selection in the bindin locus of the red sea urchin, Strongylocentrotus franciscanus.

Recent evidence suggests that gamete recognition proteins may be subjected to directed evolutionary pressure that enhances sequence variability. We evaluated whether diversity enhancing selection is operating on a marine invertebrate fertilization protein by examining the intraspecific DNA sequence variation of a 273-base pair region located at the 5' end of the sperm bindin locus in 134 adult red sea urchins (Strongylocentrotus franciscanus). Bindin is a sperm recognition protein that mediates species-specific gamete interactions in sea urchins. The region of the bindin locus examined was found to be polymorphic with 14 alleles. Mean pairwise comparison of the 14 alleles indicates moderate sequence diversity (p-distance = 1.06). No evidence of diversity enhancing selection was found. It was not possible to reject the null hypothesis that the sequence variation observed in S. franciscanus bindin is a result of neutral evolution. Statistical evaluation of expected proportions of replacement and silent nucleotide substitutions, observed versus expected proportions of radical replacement substitutions, and conformance to the McDonald and Kreitman test of neutral evolution all indicate that random mutation followed by genetic drift created the polymorphisms observed in bindin. Observed frequencies were also highly similar to results expected for a neutrally evolving locus, suggesting that the polymorphism observed in the 5' region of S. franciscanus bindin is a result of neutral evolution.

Evidence that Src-type tyrosine kinase activity is necessary for initiation of calcium release at fertilization in sea urchin eggs.

The initiation of Ca(2+) release from internal stores in the egg is a hallmark of egg activation. In sea urchins, PLCgamma activity is necessary for the production of IP(3), which leads to the initial rise in Ca(2+). To examine the possible function of a tyrosine kinase in activating PLCgamma at fertilization, sea urchin eggs were treated with the specific Src kinase inhibitor PP1 or microinjected with recombinant Src-family SH2-domain proteins, which act as dominant interfering inhibitors of Src-family kinase function. Both modes of inhibiting Src-family kinases resulted in a specific and dose-dependent delay in the onset of Ca(2+) release from the endoplasmic reticulum at fertilization. The rise in cytoplasmic pH at fertilization also was inhibited by microinjection of Src-family SH2-domain proteins. Further, an antibody directed against Src-type kinases recognized a protein of ca. M(r) 57K that was enriched in the membrane fraction of eggs. The kinase activity of this protein was stimulated rapidly and transiently at fertilization, as measured by autophosphorylation and by phosphorylation of an exogenous substrate. Together, these data indicate that a Src-type tyrosine kinase is necessary for the initiation of Ca(2+) release from the egg ER at fertilization and identify a Src-type p57 protein as a candidate in the signaling pathway leading to this Ca(2+) release.

The relationship between calcium, MAP kinase, and DNA synthesis in the sea urchin egg at fertilization.

Fertilization releases the brake on the cell cycle and the egg completes meiosis and enters into S phase of the mitotic cell cycle. The MAP kinase pathway has been implicated in this process, but the precise role of MAP kinase in meiosis and the first mitotic cell cycle remains unknown and may differ according to species. Unlike the eggs of most animals, sea urchin eggs have completed meiosis prior to fertilization and are arrested at the pronuclear stage. Using both phosphorylation-state-specific antibodies and a MAP kinase activity assay, we observe that MAP kinase is phosphorylated and active in unfertilized sea urchin eggs and then dephosphorylated and inactivated by 15 min postinsemination. Further, Ca(2+) was both sufficient and necessary for this MAP kinase inactivation. Treatment of eggs with the Ca(2+) ionophore A23187 caused MAP kinase inactivation and triggered DNA synthesis. When the rise in intracellular Ca(2+) was inhibited by injection of a chelator, BAPTA or EGTA, the activity of MAP kinase remained high. Finally, inhibition of the MAP kinase signaling pathway by the specific MEK inhibitor PD98059 triggered DNA synthesis in unfertilized eggs. Thus, whenever MAP kinase activity is retained, DNA synthesis is inhibited while inactivation of MAP kinase correlates with initiation of DNA synthesis.

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.

Evidence that a starfish egg Src family tyrosine kinase associates with PLC-gamma1 SH2 domains at fertilization.

The initiation of calcium release at fertilization in the eggs of most animals relies on the production of IP3, implicating the activation of phospholipase C. Recent work has demonstrated that injection of PLC-gamma SH2 domain fusion proteins into starfish eggs specifically inhibits the initiation of calcium release in response to sperm, indicating that PLC-gamma is necessary for Ca2+ release at fertilization [Carroll et al. (1997) J. Cell Biol. 138, 1303-1311]. Here we investigate how PLC-gamma may be activated, by using the PLC-gamma SH2 domain fusion protein as an affinity matrix to identify interacting proteins. A tyrosine kinase activity and an egg protein of ca. Mr 58 K that is recognized by an antibody directed against Src family tyrosine kinases associate with PLC-gamma SH2 domains in a fertilization-dependent manner. These associations are detected by 15 s postfertilization, consistent with a function in releasing Ca2+. Calcium ionophore treatment of eggs did not cause association of the kinase activity or of the Src family protein with the PLC-gamma SH2 domains. These data identify an egg Src family tyrosine kinase as a potential upstream regulator of PLC-gamma in the activation of 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.

Surface localization of the sea urchin egg receptor for sperm.

The sea urchin egg receptor for sperm is thought to be involved in species-specific sperm-egg interactions at the egg surface. Recent revisions in the deduced amino acid sequence of the cloned cDNAs indicate that the protein encoded does not possess the common structural hallmarks of a membrane protein. Thus, investigation of the localization and association of the protein with the egg surface is crucial. We describe and characterize a new monoclonal antibody raised against recombinant sperm receptor protein. This antibody, in conjunction with several polyclonal antibodies, was used to study the receptor protein in eggs. Immunoprecipitation studies indicated that the antibodies recognize the high Mr (ca. 350 K) sperm receptor protein which copurified with egg plasma membrane-vitelline layer complexes. The sperm receptor protein was solubilized only by detergents and not by treatments designed to solubilize peripherally associated or lipid-anchored membrane proteins, suggesting a tight association with the membrane fraction. Confocal immunofluorescence microscopy of live eggs indicated surface staining. Finally, lysylendoproteinase C treatment of live eggs resulted in a loss of the high Mr receptor protein epitopes, and the concomitant release of a 70-kDa proteolytic fragment, which correlated with a reduced ability of the eggs to be fertilized. Taken together, these data indicate that at least some fraction of the sperm receptor protein is present on the egg surface, a requisite locale for a sperm binding protein.

Gamete and immune cell recognition revisited.

Fertilization is the result of a series of successful recognition and binding events mediated by gamete surface molecules. Recent advances in the identification and characterization of some of these recognition molecules provide extremely valuable information necessary to understand sperm-egg recognition and subsequent egg activation. We discuss these new data in the context of the model of gamete recognition first proposed by F.R. Lillie in the early part of the 20th century, and revisited periodically in the subsequent literature, which relates fertilization events to those of immune cell recognition and activation events. Here we discuss the principles underlying the molecular recognition and activation mechanisms of gametes and immune cells.

Sperm-binding proteins.

Gamete recognition and binding are mediated by specific proteins on the surface of the sperm and egg. Identification and characterization of some of these proteins from several model systems, particularly mouse and sea urchin, have focused interest on the general properties and functions of gamete recognition proteins. Sperm-binding proteins located in egg extracellular coats as well as sperm-binding proteins that are localized to the egg plasma membrane are presented in the context of their structure and function in gamete binding. Unifying and disparate characteristics are discussed in light of the diverse biology of fertilization among species. Outstanding questions, alternative mechanisms and models, and strategies for future work are presented.

Tyrosine phosphorylation of the egg receptor for sperm at fertilization.

Gamete interaction triggers a variety of responses within the egg, collectively referred to as egg activation. In addition to the hallmarks of calcium release and fertilization envelope elevation, there are cytoskeletal rearrangements, protein tyrosine phosphorylation, and an increase in pH, among others. The ultimate goal of these concerted activation events is entry of the newly fertilized egg into the cell cycle. However, the molecular mechanisms which promote downstream cell activation events remain poorly understood. One model suggests that sperm deliver an "activating factor" upon fusion with the egg plasma membrane, while a second model proposes that the egg receptor for sperm transduces a signal that mediates a cascade of subsequent events. It also is possible that multiple pathways are activated. As a first step toward testing the hypothesis of receptor-mediated signal transduction, we have investigated the tyrosine phosphorylation state of the sea urchin egg receptor for sperm using specific antibodies. The present work indicates that the sperm receptor is phosphorylated by an egg cortical tyrosine kinase in response to sperm or purified ligand (bindin) binding. Maximal phosphorylation was reached within 20 sec. These data support the hypothesis that the sperm receptor is a gamete recognition protein which responds to ligand binding and focus attention on the question of the role of this tyrosine phosphorylation signal in egg activation.

The molecular basis of sea urchin gamete interactions at the egg plasma membrane.

Fertilization is the result of a series of well-choreographed interactions between molecules located on the surfaces of the egg and the sperm. The recent molecular characterizations of several of these surface molecules has led to a greater understanding of their roles in gamete recognition and binding. We present a review of these recent advances with emphasis on the sea urchin, a classic system for the study of fertilization. In particular, the structure and function of the sea urchin egg plasma membrane receptor for sperm, a novel cell recognition molecule, is discussed.

Sea urchin egg receptor for sperm: sequence similarity of binding domain and hsp70.

Fertilization depends on cell surface recognition proteins that interact and thereby mediate binding and subsequent fusion of the sperm and egg. Overlapping complementary DNA's encoding the egg plasma membrane receptor for sperm from the sea urchin Strongylocentrotus purpuratus were cloned and sequenced. Analysis of the deduced primary structure suggests that the receptor is a transmembrane protein with a short cytoplasmic domain. This domain showed no sequence similarity to known protein sequences. In contrast, the extracellular, sperm binding domain of the receptor did show sequence similarity to the heat shock protein 70 (hsp70) family of proteins. Recombinant protein representing this portion of the receptor bound to the sperm protein, binding, and also inhibited fertilization in a species-specific manner; beads coated with the protein became specifically bound to acrosome-reacted sperm. These data provide a basis for detailed investigations of molecular interactions that occur in gamete recognition and egg activation.

Identification of the sea urchin egg receptor for sperm using an antiserum raised against a fragment of its extracellular domain.

Sea urchin egg fertilization requires the species-specific interaction of molecules on the sperm and egg surfaces. Previously, we isolated an extracellular, 70-kD glycosylated fragment of the S. purpuratus egg receptor for sperm by treating the eggs with lysylendoproteinase C (Foltz, K. R., and W. J. Lennarz. 1990. J. Cell Biol. 111:2951-2959). To characterize the receptor further, we have generated a polyclonal antiserum (anti-70KL) against the purified 70-kD fragment. Anti-70KL was found to react with a single polypeptide of approximately 350 kD on Western blots, presumed to be the intact receptor, in an egg cell surface preparation. This polypeptide appeared to be tightly associated with the plasma membrane/vitelline layer complex, as it was released from these preparations only by detergent treatment. Immunofluorescence microscopy revealed that the receptor was distributed evenly over the egg surface. The anti-70KL was species specific both in its ability to recognize the egg surface protein and to inhibit sperm binding. Fab fragments generated from affinity-purified anti-70KL also bound to the egg surface and inhibited sperm binding in a concentration-dependent manner. Interestingly, treatment with Fabs caused a small percentage of eggs to undergo cortical granule exocytosis, even in the absence of external Ca2+. These results confirm earlier findings indicating that the receptor is a cell surface glycoprotein of high molecular weight that species specifically binds sperm. This antiserum provides a powerful tool for further investigation of gamete interactions and the structure of the sperm receptor.

Purification and characterization of an extracellular fragment of the sea urchin egg receptor for sperm.

Fertilization in the sea urchin involves species-specific interaction between the ligand bindin on the surface of acrosome-reacted sperm and a receptor of high molecular weight on the surface of the egg. Efforts to understand this interaction and the resultant signal transduction events leading to egg activation have been limited because of the large size and extreme insolubility of the intact receptor on the egg surface. Earlier work suggested that an alternative strategy would be to isolate proteolytic fragments of the extracellular domain of this receptor. Consequently, we have treated S. purpuratus eggs with a specific protease, lysylendoproteinase C. This enzyme treatment abolished the ability of eggs to bind sperm and resulted in the release of proteolytic fragments that bound to sperm and showed inhibitory activity in a fertilization bioassay. One of these fragments, presumed to be a fragment of the extracellular domain of the receptor, was purified to homogeneity by gel filtration and anion exchange chromatography and shown to be a 70-kD glycosylated protein. Several lines of evidence support the contention that this fragment is derived from the receptor. First, the fragment inhibited fertilization species specifically. Second, species specific binding of the 70-kD glycoprotein to acrosome-reacted sperm was directly demonstrated by using 125I-labeled receptor fragment. Third, the fragment exhibited the same species specificity in binding to isolated bindin particles. Species specificity was abolished by Pronase digestion of the fragment. This observation supports the hypothesis that although binding is mediated by the carbohydrate moieties, species specificity is dependent on the polypeptide backbone. The availability of a structurally defined fragment of the receptor will facilitate further studies of the molecular basis of gamete interaction.