Structural Biochemistry/Cell Signaling Pathways/Calcium Signaling

Calcium is a common signaling mechanism, because once it enters the cytoplasm it exerts allosteric regulatory affects on many enzymes and proteins. Calcium can also be a second messenger caused by indirect signal transduction pathways such as g-protein coupled receptors.

Properties of Calcium Ion (Ca²⁺)

There are two major properties that allow Calcium (Ca²⁺) Ion to work effectively as a signaling mechanism

• Ca²⁺ levels inside the cell are readily detectable. This is because the levels of Ca²⁺ are highly regulated by transport systems that expel Ca²⁺ from the cell. The level of Ca²⁺ in the cytoplasm is approximately 100nM, which is several orders of magnitude lower than outside the cell. This must be regulated in order to keep salts from forming between Ca²⁺ and carboxylated and phosphorylated compounds, as the salts formed are mostly insoluble. Because of all these factors, any small increases of the Ca²⁺ inside the cell as a signal is readily and quickly detectable, making it a useful signaling mechanism.^[1]

• Ca²⁺ can readily bind to proteins and cause conformational changes. Ca²⁺ is attracted to the negatively charged oxygen atoms in the side chains of glutamate and asparagine, and the uncharged oxygens in both the side chains and main chains of glutamine and asparagine. Ca²⁺ is readily able to cause large conformational changes due to the fact that it can form ligands with up to eight oxygen atoms. This can lead to cross linking of amino acids in a protein that did not exist before Ca²⁺ was introduced.^[2]

Calcium Concentration Oscillations

Calcium influx within a cell is a fundamental process within many types of cells, namely, nerves and muscle cells. In nerves, calcium entry drives the release of neurotransmitters, while in muscle cells, calcium is directly involved in actin-myosin sliding filament theory[1]. However, merely stating that calcium influx causes these responses would be undermining the process of calcium signal integration cells undergo.

There are three modes upon which a cell can integrate a calcium oscillation signal: frequency, amplitude, and spatial. Realizing that Calcium oscillations follow sinusoidal waveforms makes it easier to see what these characteristics refer to. The frequency of oscillation is integrated by Protein Kinase C, and Ca/calmodulin-dependent protein kinase II. Here, frequency refers to how many cycles per second the oscillation occures, namely Hertz. Additionally, the amplitude of the signal is a main attribute of the oscillation that is integrated by the cell. To accomplish this, calcium receptors of varying affinities are located within the cell. Since amplitude refers to the magnitude of calcium oscillation, different magnitudes of calcium will activate specific receptors. This allows essentially allows specific cellular functions to taker place based upon the magnitude of calcium spark [2].

A more recently discovered signal integration method involves spatial layout of the calcium flux. Since calcium enters the cell via membrane channels, it is inevitable that on a small timescale, calcium concentration will vary spatially throughout the cell. The amplitude of calcium spark will decay radially from the channel's location. The versatile calmodulin is responsible for integrating these Calcium micro-domains and global-domains signals. Here micro-domains refer to calcium sparks in a small volume relative to the cytoplasm, where global-domain refers to complete cytoplasmic calcium concentration changes. Calmodulin's amino and carboxyl terminals collaborate to carry out spatial integration [2].

[1] Silverthorn, U. D. Human Physiology. Pearson, 2010. pg 400-444. [2] Parekh, A. B. Decoding Cytosolic Calcium Oscillations. Cell Press. TIBS-804. pg 1-10. Picture By: Jjbeard

Calcium Signaling

The /calcium signaling/Calcium Signaling mechanism begins when an agonist molecule binds to its receptor, which is located on a G protein. This docking dissociates GDP from the G protein complex, leading to the dissociation of G-alpha subunit and G-beta/G-gamma complex. The G-alpha is bound to GTP to form the G-alpha-GTP complex, which then moves to PLC (phospholipase C).

The docking of G-alpha-GTP on PLC causes conformation change of PLC, activating an enzyme that is involved in hydrolyzing PIP2 into DAG and IP3. The IP3 moves to the ER and docks on the receptor of IP3, opening the calcium channel and allowing calcium ions exit the ER. Calcium ions then bind to buffer, or enter Mitochondria through calcium uniporter. They could also simply become suspended in the cytoplasm.

IP3 can be phosphorylated or hydrolyzed, increasing the amount of IP3. When Calcium ions in ER are nearly depleted, a signal is sent out of the ER and to activate the SOC channel, which then opens and allows calcium ions from extracellular fluid to enter the cytoplasm. These Calcium ions can enter either ER through the SERCA pump or exit the cell through the PMCA pump.

When the agonist leaves the receptor on the G protein, the recovery process begins. GTP on G-alpha-GTP complex is hydrolyzed to GDP and reassociates with G-alpha, G-beta/G-gamma attaching on G-protein. Calcium ions enter the mitochondria via ion exchanger with sodium ions. SERCA and PMCA pumps slowly allow Calcium ions to move in and out. Consequently, the amount of calcium ions in the ER increases.

Here is a link to an animation of calcium signaling. Open animation link with Internet Explorer. Calcium Signaling Animation

References