Sea urchin -l

Sea urchin -l

Slow block to polyspermy is stabilized by:

After initial elevation triggered by mucopolysaccharide swelling, vitelline envelope transformed into fertilization envelope requires chemical hardening to provide durable mechanical barrier against supersperm and environmental stress. Two cortical granule enzymes accomplish covalent stabilization: egg-specific ovoperoxidase catalyzes oxidative crosslinking forming dityrosine bridges between adjacent envelope glycoproteins using hydrogen peroxide, while transglutaminase catalyzes formation of ε-(γ-glutamyl)lysine isopeptide bonds. Together these enzymatic crosslinks convert soluble vitelline envelope into insoluble, tough, impermeable protective coat encasing embryo. Glycosaminoglycans drive swelling but not stabilization, hyalin builds hyaline layer for cell adhesion, Na+ mediates fast electrical block. Thus stabilization depends on peroxidase-transglutaminase system.

Ref: Foerder & Shapiro, PNAS 1977, Peroxidase hardening; Gilbert, Developmental Biology, Chapter 7: Envelope crosslinking.

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IP3 in fertilization is generated by:

Fertilization induces phosphoinositide signaling cascade generating crucial second messenger IP3. Phospholipase C isoforms, including sperm-contributed PLCζ and egg PLCγ activated by Src family tyrosine kinases downstream of sperm-receptor interaction, hydrolyze phosphatidylinositol 4,5-bisphosphate abundant in inner leaflet of plasma membrane. Cleavage yields membrane-retained diacylglycerol that activates protein kinase C and soluble inositol 1,4,5-trisphosphate that diffuses to endoplasmic reticulum. IP3 generation absolutely requires phospholipase C enzymatic catalysis; without PLC activity calcium wave fails. Protein kinase C, dynein ATPase and Na+/H+ exchanger act downstream modulating pH, cytoskeleton and motility but are not synthesizing IP3 upstream.

Ref: Carroll et al., Dev Biol 1997, Phospholipase C and IP3 generation at fertilization; Alberts Chapter 15.

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Which of these is true for bindin protein?

Bindin is not component of egg jelly or fertilization envelope but major cell adhesion protein uniquely sequestered inside intact sperm acrosomal vesicle. Upon Ca2+-triggered acrosome reaction it becomes prominently exposed on surface of extended acrosomal process where it performs dual functions: mediating species-specific attachment to EBR1 receptor complex of vitelline envelope through lectin-like domain, and also possessing hydrophobic fusogenic sequence promoting merger of sperm and egg lipid bilayers after adhesion. Rapid sequence divergence among Strongylocentrotus species establishes potent prezygotic reproductive barrier. Biochemically insoluble collagen-like protein, bindin represents definitive species-specific fusogenic adhesion protein essential for productive gamete recognition and membrane fusion.

Ref: Vacquier & Moy, J Biol Chem 1997, Bindin fusogenic function; Gilbert Chapter 7: Bindin as species-specific fusogen.

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Fertilization cone formation involves:

Immediately after sperm-egg plasma membrane fusion, egg cortex locally remodels beneath bound spermatozoon to form conspicuous fertilization cone. Process begins with transient depolymerization of cortical actin followed by rapid GTPase-dependent actin polymerization involving Arp2/3 complex and formins, pushing plasma membrane outward into blunt protrusion engulfing sperm head and midpiece while drawing sperm nucleus into ooplasm. Myosin II contractility and microtubule-based transport assist later incorporation and pronuclear migration but initial protrusion itself driven by localized actin polymerization triggered by calcium and small GTPases. Cytochalasin B inhibition blocks cone formation preventing sperm internalization, demonstrating dependence on actin dynamics.

Ref: Gilbert, Developmental Biology, Chapter 7: Fertilization cone - actin-based engulfment of sperm.

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Egg jelly in sea urchins primarily functions in:

Egg jelly is thick gelatinous extracellular mucoid coat outside vitelline envelope composed of sulfated fucan polysaccharides intermingled with small peptides such as resact and speract. Its principal biological functions include long-range sperm chemoattraction and preparation for fusion. Diffusible peptides establish chemotactic gradient guiding motile sperm toward egg, while polysaccharides interact with sperm receptors inducing Ca2+-dependent acrosomal reaction ensuring membrane fusogen bindin becomes exposed precisely before reaching vitelline membrane. Although it contributes to species filtering, core specificity arises later at bindin-receptor binding step. It does not block polyspermy nor supply metabolic nutrition; polyspermy prevented by electrical shift and fertilization envelope.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Egg jelly role - sperm attraction and acrosome induction.

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Which enzyme cleaves bindin receptors during slow block?

Permanent elimination of sperm binding capacity during slow block involves enzymatic destruction of specific recognition molecules. Cortical granules discharge large trypsin-like serine protease that cleaves peptide linkages anchoring EBR1 bindin receptor complex to vitelline envelope glycoprotein scaffold and degrades residual fertilizing sperm proteins adhering to envelope. This proteolytic destruction ensures no new bindin-receptor interactions can reform even before envelope hardening physically completes. Acrosomal protease facilitates sperm entry through jelly, ovoperoxidase catalyzes dityrosine crosslinks hardening envelope, phospholipase C generates IP3 for calcium release but does not degrade receptors. Thus enzyme responsible for cleaving bindin receptors is cortical granule serine protease.

Ref: NCBI Bookshelf, Developmental Biology, Chapter 7: Cortical granule protease cleaves bindin receptor.

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What molecule initiates calcium release in egg cytoplasm after fertilization?

Egg activation calcium wave absolutely requires inositol trisphosphate-mediated release of endoplasmic reticulum calcium stores rather than extracellular influx. Phospholipase C enzyme introduced by sperm factor and activated endogenous PLCγ via Src family tyrosine kinases cleaves abundant phosphatidylinositol 4,5-bisphosphate residing in egg plasma membrane inner leaflet generating two potent second messengers: membrane-bound diacylglycerol activating protein kinase C and soluble inositol 1,4,5-trisphosphate (IP3) that diffuses rapidly to bind IP3 receptors on endoplasmic reticulum opening ligand-gated Ca2+ channels. Resulting localized Ca2+ efflux triggers further calcium-induced calcium release propagating global wave. cAMP, DAG alone, ATP do not directly liberate calcium from internal stores.

Ref: Whitaker & Irvine, Nature 1984, IP3 mediated calcium release; Gilbert Chapter 7: PLC-IP3 pathway.

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Actin polymerization in sperm cells occurs primarily during:

During acrosomal reaction, sperm subacrosomal region contains concentrated pool of globular actin monomers maintained unpolymerized by profilin. Upon Ca2+ influx triggered by egg jelly contact, actin rapidly polymerizes into bundle of filamentous F-actin pushing acrosomal membrane forward forming slender elongated acrosomal process about one micrometer long, thrusting bindin-bearing tip through viscous egg jelly toward vitelline envelope receptor field. Cytochalasin D experiments blocking actin polymerization abolish process extension and prevent fertilization. Fertilization cone actin polymerization occurs in egg cortex, cortical reaction involves egg exocytosis, not sperm actin. Thus actin polymerization in sperm cells occurs primarily during acrosomal reaction creating penetration organelle.

Ref: Tilney & Inoue, J Cell Biol 1985, Acrosomal process actin; Gilbert Chapter 7: Actin polymerization in sperm.

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During cortical granule reaction, glycosaminoglycans:

Among cortical granule components massively released during slow block to polyspermy are large sulfated mucopolysaccharides, hyaline and glycoproteins that hydrate explosively upon discharge into extracellular space. Their high colloid osmotic pressure and water-binding capacity draws surrounding seawater and ovular fluid into perivitelline space between egg plasma membrane and elevated vitelline envelope, causing dramatic space expansion and envelope elevation that physically separates sperm receptors from oolemma. Subsequent hardening by peroxidase-mediated dityrosine crosslinking and transglutaminase isopeptide crosslinking then freezes envelope in raised position. Glycosaminoglycan-driven swelling crucially creates space for lifting envelope, while proteases digest receptors and ovoperoxidase hardens structure independently.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Perivitelline space expansion by mucopolysaccharides.

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Sperm motility in sea urchins is initiated by:

Uncapacitated sea urchin sperm stored within gonads maintains quiescent low-motility state due to relatively acidic intracellular pH around 7.2 and suppressed metabolism. Upon spawning into normal seawater pH approximately 8.0, Na+/H+ exchanger sNHE encoded by SLC9C1 extrudes protons raising internal pH to about 7.6, directly stimulating dynein ATPase activity, adenylate cyclase and flagellar beating. Intracellular pH elevation, potentiated further by resact-induced transient K+ efflux and hyperpolarization that activates sNHE, unlocks flagellar wave propagation. Artificially holding internal pH acidic even with abundant ATP prevents motility. Therefore increased internal pH initiates sperm motility and primes chemotaxis.

Ref: Lee et al., Dev Biol 1983, pH-dependent motility initiation; Gilbert Chapter 7: Sperm activation mechanisms.

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Bindin receptors in sea urchins are located on the:

Molecular cloning of bindin receptor revealed EBR1 glycoprotein complex not localized on plasma membrane but embedded within vitelline envelope surrounding egg. Receptor evolved as extracellular matrix component providing docking platform for acrosomal process containing bindin after penetrating jelly. Upon bindin-EBR1 adhesion, sperm becomes tethered, allowing subsequent lipid bilayer fusion lateral to bound site rather than directly at receptor cluster. Only after secure binding does sperm plasma membrane merge with egg membrane. Cortical granules, jelly and cortical cytoplasm lack primary bindin receptor; vitelline envelope serves exclusive scaffold for species-specific gamete adhesion.

Ref: Kamei & Glabe, PNAS 2003, Bindin receptor localization; Gilbert Chapter 7: Vitelline envelope EBR1 receptor.

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Sea urchin eggs activate upon fertilization primarily due to:

Fertilization triggers awakening termed egg activation converting quiescent oocyte into metabolically active totipotent zygote preparing for mitosis. Central event is massive rise of intracellular free Ca2+ from endoplasmic reticulum after IP3 production by phospholipase C. Propagating calcium wave stimulates cortical granule exocytosis, cytoplasmic alkalinization via Na+/H+ antiporter, increased oxygen consumption, enhanced protein synthesis from maternal mRNAs, and cell cycle resumption through cyclin activation. Without Ca2+ elevation, shown by BAPTA chelation, none of these events occur, proving rise in intracellular Ca2+ drives activation and initiates embryogenesis.

Ref: Stricker, Dev Biol 1999, Egg activation Ca2+; Gilbert Chapter 7: Calcium wave as activation signal.

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