Signal Transduction Pathway - Phospholipids and Calcium Ion Signaling

Phosphatidylinositol 4,5-bisphosphate abbreviated as PIP2 is a phospholipid present in the inner leaflet of the bilayer of the plasma membrane. The second messengers are derived from this small component (phospholipid) and the pathway is based on these messengers.

How does hydrolysis of PIP2 takes place? The hydrolysis of PIP2 takes place by the enzyme phospholipase C as can be seen in the adjacent figure. It is interesting to note that the enzyme phospholipase C is ultimately activated by G-protein coupled receptors (GPCRs) or protein tyrosine kinases. This is so because one form of phospholipase C (PLC-β) is stimulated by G proteins while another form of phospholipase C (PLC-γ) contains SH2 domains (as can be seen in the figure shown below) and hence it associates with activated receptor protein tyrosine kinases. This interaction helps PLC-γ to localize to plasma membrane and also leads to its phosphorylation. This tyrosine phosphorylation increases PLC-γ activity, which in turn stimulates hydrolysis of PIP2.

The hydrolysis of PIP2 produces two distinct second messengers as diacylglycerol and inositol 1,4,5-triphosphate which is abbreviated as IP3. Both these messengers stimulate different downstream signaling pathways thereby triggering two distinct cascades of intracellular signaling. Diacylglycerol stimulates protein kinase C mobilization while IP3 stimulates Ca2+(ions) mobilization. The diacylglycerol as second messenger activates serine/threonine kinases which belongs to the protein kinase C family which play an important role in cell growth and differentiation.

IP3, another second messenger is released into the cytosol and it acts to release the  Ca2+(ions) from intracellular stores. The level of the Ca2+(ions) inside the cell is very low and is maintained by pumping through Ca2+(ion) pumps across the plasma membrane.
The Ca2+(ions) are pumped into the ER and hence ER is considered to be the store of intracellular Ca2+(ions). Here, IP3 binds to the receptors in the ER membrane as can be seen in the adjacent diagram. These receptors are ligand-gated ion channels and hence, there is efflux of Ca2+(ions) into the cytosol. This increase of Ca2+(ions) in the cytosol has an effect on variety of proteins like protein kinases. For example, there are some members of protein kinase C (PKC) family that requires Ca2+(ions) as well as diacylglycerol for their functioning. Hence, these PKC family members are regulated by both IP3 and diacylglycerol.

Calmodulin is another very important protein to mention while we are studying about Ca2+(ions). The word 'calmodulin' means - cal(cium) + modul(ate) + in(g). Thus, calmodulin is 'calcium modulating' protein that mediates most of the activities of Ca2+(ions). Calmodulin is dumbbell shaped protein which has four Ca2+(ions) binding sites (figure is shown below). When the Ca2+(ions) concentration in the cell increases, calmodulin is activated. This active Ca2+/calmodulin complex then binds to a variety of target proteins, like Ca2+ion/calmodulin -dependent protein kinases thereby rendering them active. The examples of Ca2+ion/calmodulin dependent-protein kinases are: myosin light-chain kinase and members of CaM kinase family.

Lets understand how the regulation of Ca2+ ions is important in regulating electrically excitable cells? When there is a change in plasma membrane's potential i.e.; when there is membrane depolarization, the voltage-gated Ca2+ ion channels are opened in the plasma membrane. Because of the opening, there is influx of Ca2+(ions) from the extracellular fluid into the cytosol of the cell. This increase in the levels of Ca2+(ions) further triggers the opening of the another receptor called the ryanodine receptor in the plasma membrane which further releases the Ca2+(ions) from the intracellular stores. This increase in the Ca2+(ions) results in triggering the release of neurotransmitter. Hence, we can say that Ca2+ ion plays an important role in converting electric signals to chemical signals.
In muscle cells, the ryanodine receptors on the sarcoplasmic reticulum. These receptors maybe opened directly when there is membrane depolarization.