Calcium Signalling - truly marvelous!

I am currently revising for my upcoming exams and one of the topics we have covered this semester is Calcium Signalling. Calcium is a metal ion that is essential for the correct functioning of a huge range of cellular processes.
Maybe the most well-known of these is its importance for muscle function. In this case, Calcium binds to a molecule called troponin, leading to the movement of protein fibres called myosin and actin, creating a contraction of the muscle cell.
Calcium at high levels in the cell is toxic however, as it readily binds phosphate to create nonsoluble Calciumphosphate, the main constituent of kidney stones. So how does the cell generate levels of Calcium that are high enough to initiate the response it needs without endangering itself?
This is where a collection of clever mechansims come into play: The cell's Calcium-control Toolkit, so to speak. There are molecules called Buffering Proteins that bind and effectively mop up any Calcium they come across. Another type of molecule involved is the Plasma Membrane (i.e. Cell Membrane) Calcium ATPase (or pump). This uses energy to pump Calcium out of the cell. Another such pump, the SERCA, pumps Calcium from the cytoplasm (the inner environment of the cell) into the ER or SR (these are a type of organelle, i.e. membrane-bound compartment, found in any normal cell and specifically in muscle cells, respectively), where the Calcium is stored.
These two organelles play an important role in the control of Calcium Signalling. When a certain type of hormone binds onto a receptor on the cell surface, a signalling cascade (involving the Phosphoinositol pathway) takes place that releases the molecule IP3. And tata tata - the ER (i.e. the afore mentioned organelle that stores Calcium) has a receptor for IP3! Binding triggers the release of the stored Calcium. Now comes the really clever part: some signalling pathways rely on large Calcium waves sweeping through the cell, instead of only local increases in Calcium. Once Calcium has been released by the ER, that Calcium itself can then initiate further release from the ER, which can initiate further release, which initiates further release, which ... you get the picture. It's awesome! :) Once the cell realises there is much too much Calcium in the cell, the release stops and the Toolkit come into play again to decrease the Calcium level to resting state.
This Calcium wave is central to the fertilization of an egg cell: once the first sperm enters the egg, a Calcium wave like this is triggered and the increase in Calcium across the whole cell initiates the formation of a protective protein envelope around the fertilized egg, preventing any more sperm from entering. It also starts off the fertilized egg on the route of development! Isn't it amazing that each one of us started developing from a single fertilized egg cell into who we are today because of this amazing mechanism involving a humble metal ion???!!

References


Berridge MJ, Lipp P, Bootman MD (2000): "The Versality and Universality of Calcium Signalling", Nat Rev Mol Cell Bio 1: 11-21