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The neuronal ISO may originate from astrocyte's calcium wave, which is a cyclic variation of calcium concentration in the cytosol (the intracellular fluid outside of organelles), with oscillation frequency in the infra-slow range 0.005-0.1 Hz (Lorincz et al., 2009). This frequency is mainly controlled by IP3-regulated calcium channels.
We know that an embryo begins with the division of an egg cell into two cells. Subsequently, two are divided into four, four double into eight, etc. However, the egg cell cannot divide on her own. It requires the sperm. How can the sperm trigger such complex and remarkable changes? The answer lies in phospholipase C (PLC) which, other than DNA, is the most important substance in the sperm.
PLC is an enzyme which catalyzes the conversion of phosphatidylinositol 4,5-bisphosphate (PIP2) into diacyl glycerol (DAG) and inositol 1,4,5-trisphosphate (IP3) (Figure 5-4). PLC has several types: β, γ, δ, ε, η and ζ. The intrinsic catalytic activity of PLC-β is low. Interaction with the G protein may enhance its catalytic activity (Chapter 6). However, the PLC-ζ of the sperm has sufficient catalytic activity by itself (Saunders et al., 2002).
The endoplasmic reticulum (ER) in a cell (including egg) stores a large amount of Ca2+ ions. The membrane surrounding ER contains a type of calcium channels that are regulated by IP3. When the channel is open, Ca2+ ions will flow from ER into the cytosol, increasing its Ca2+ concentration. When closed, Ca2+ concentration in cytosol may decrease by other processes. The increase in IP3 can open more calcium channels, thereby increasing Ca2+ concentration in cytosol (Figure 5-5). Shortly after the calcium channels open, they will become "inactivated" (channels closed, even upon IP3 binding) (Foskett et al., 2007). During the inactivation period, Ca2+ ions in the cytosol will decrease. The channels then gradually recover to the resting state, which can be activated by IP3.
Ca2+ ions regulate the activities of many enzymes as well as the gating of several ion channels. In the egg, activated enzymes can promote cell division. In astrocytes, Ca2+ ions may trigger the release of neurotransmitters, leading to ISO in adjacent neurons.