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#sea urchin sperm

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Regeneration -ll

Calcium channels activated by resact in sea urchin sperm are encoded by:

Chemotactic calcium signaling in sea urchin sperm relies on specific ion channels opened downstream of resact binding. Resact activates receptor guanylate cyclase raising cGMP, opening tetraKCNG channels causing hyperpolarization, then depleting cGMP leading to depolarization that activates voltage-dependent calcium entry. Electrophysiology and molecular cloning identified entry path as CatSper channel complex encoded by four homologous CatSper1-4 genes forming heterotetramer in principal piece of flagellum, associated with auxiliary subunits. CatSper null sperm lack calcium pulses and chemotaxis. Bindin, resact, or hyalin genes encode structural adhesion proteins or chemoattractants, not calcium channel subunits responsible for chemosensation.

Ref: EMBO Journal, Seifert et al., The CatSper channel controls chemosensation in sea urchin sperm.

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Regeneration -ll

The primary role of the bindin protein in sea urchin sperm is:

Bindin is a thirty-five kilodalton insoluble protein sequestered within sperm acrosomal vesicle and exposed at tip of acrosomal process after calcium-dependent exocytosis. It contains lectin-like domains recognizing sulfated polysaccharides on Egg Bindin Receptor EBR1 glycoprotein embedded in vitelline envelope. Interaction confers species-specific binding, constituting major barrier to heterospecific fertilization and reproductive isolation. Beyond adhesion, bindin exhibits amphipathic fusogenic activity promoting mixing of sperm and egg lipid bilayers facilitating membrane fusion. It does not power dynein-driven motility, regulate intracellular alkalization, or mediate chromatin condensation occurring during spermiogenesis.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Bindin - species-specific adhesion and membrane fusion in sea urchin.

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Sea urchin -l

Sea urchin sperm propulsion relies on:

Sea urchin sperm propulsion depends on highly conserved flagellar axoneme structure composed of nine outer microtubule doublets surrounding central pair with dynein arms and nexin links. Dynein heavy chain is ATPase motor protein that hydrolyzes ATP to slide adjacent doublet microtubules relative to each other, converting chemical energy into mechanical bending motion of flagellum that propels cell forward. Activation by intracellular pH rise stimulates dynein ATPase catalytic activity. Specific mutations or chemical inhibitors of dynein such as vanadate abolish motility while actin, myosin and tubulin alone provide structural tracks but no motor power. Thus dynein-driven microtubule sliding drives flagellar propulsion.

Ref: Gibbons, J Cell Biol 1981, Dynein in flagellar motility; Alberts, Molecular Biology of Cell, Chapter 20: Axoneme.

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