Three different calcium wave pacemakers in ascidian eggs

Area of Science:

• Cellular and Developmental Biology
• Reproductive Biology
• Calcium Signaling

Background:

• Calcium waves are crucial for fertilization and early development in many species, including ascidians and mice.
• These waves are often initiated by localized sources of calcium release, termed pacemakers.
• Cortical endoplasmic reticulum (ER) accumulations have been previously associated with calcium wave pacemakers in fertilized eggs.

Purpose of the Study:

• To identify and characterize distinct calcium wave pacemakers in fertilized ascidian eggs.
• To investigate the roles of cortical endoplasmic reticulum and sperm aster in pacemaker activity.
• To elucidate the regulatory mechanisms, including inositol trisphosphate (Ins(1,4,5)P3) signaling and cortical contraction, governing these pacemakers.

Main Methods:

• Utilized fertilized ascidian eggs as a model system.
• Employing caged Ins(1,4,5)P3 and caged glycero-myo-PtdIns(4,5)P2 photolysis to induce global Ins(1,4,5)P3 elevations.
• Investigated the effects of microtubule depolymerization and cytochalasin B (an inhibitor of acto-myosin contraction) on pacemaker activity and localization.

Main Results:

• Identified three distinct calcium wave pacemakers (PM1, PM2, PM3) associated with cortical ER.
• Pacemaker PM1 is associated with the sperm aster and moves with cortical contractions, while PM2 is stably localized.
• Pacemaker PM3, located in the animal hemisphere, is highly sensitive to Ins(1,4,5)P3 and can override PM2 activity. PM1 and PM2 activity depend on cortical contraction, unlike PM3.

Conclusions:

• Fertilized ascidian eggs possess multiple, distinct calcium wave pacemakers regulated by different mechanisms.
• Natural pacemakers (PM1, PM2) rely on the proximity of cortical ER to Ins(1,4,5)P3 production and are dependent on cortical contraction.
• The artificial pacemaker (PM3) demonstrates a high sensitivity to Ins(1,4,5)P3 and is independent of cortical contraction, suggesting complex spatiotemporal regulation of calcium signaling during early development.