Calcium signaling

Calcium ions are important for cellular signalling, as once they enter the cytosol of the cytoplasm they exert allosteric regulatory effects on many enzymes and proteins. Calcium can act in signal transduction resulting from activation of ion channels or as a second messenger caused by indirect signal transduction pathways such as G protein-coupled receptors.

Calcium Concentration Regulation

The resting concentration of Ca2+ in the cytoplasm is normally maintained in the range of 10–100 nM. To maintain this low concentration, Ca2+ is actively pumped from the cytosol to the extracellular space and into the endoplasmic reticulum (ER), and sometimes in the mitochondria. Certain proteins of the cytoplasm and organelles act as buffers by binding Ca2+. Signaling occurs when the cell is stimulated to release calcium ions (Ca2+) from intracellular stores, and/or when calcium enters the cell through plasma membrane ion channels.[1]

Phospholipase C Pathway

Phospholipase C cleaving PIP2 into IP3 and DAG

Specific signals can trigger a sudden increase in the cytoplasmic Ca2+ level up to 500–1,000 nM by opening channels in the endoplasmic reticulum or the plasma membrane. The most common signaling pathway that increases cytoplasmic calcium concentration is the phospholipase C pathway.

Depletion of calcium from the endoplasmic reticulum will lead to Ca2+ entry from outside the cell by activation of "Store-Operated Channels" (SOCs). This inflowing calcium current that results after stored calcium reserves have been released is referred to as Ca2+-release-activated Ca2+ current (ICRAC). The mechanisms through which ICRAC occurs are currently still under investigation, although two candidate molecules, Orai1 and STIM1, have been linked by several studies, and a model of store-operated calcium influx, involving these molecules, has been proposed. Recent studies have cited the phospholipase A2 beta,[3] nicotinic acid adenine dinucleotide phosphate (NAADP),[4] and the protein STIM 1[5] as possible mediators of ICRAC.

Movement of calcium ions from the extracellular compartment to the intracellular compartment alters membrane potential. This is seen in the heart, during the plateau phase of ventricular contraction. In this example, calcium acts to maintain depolarization of the heart. Calcium signaling through ion channels is also important in neuronal synaptic transmission.

Calcium as a secondary messenger

Important physiological roles for calcium signaling range widely. These include muscle contraction, neuronal transmission as in an excitatory synapse, cellular motility (including the movement of flagella and cilia), fertilisation, cell growth or proliferation, learning and memory as with synaptic plasticity, and secretion of saliva.[6] High levels of cytoplasmic calcium can also cause the cell to undergo apoptosis.[7] Other biochemical roles of calcium include regulating enzyme activity, permeability of ion channels, activity of ion pumps, and components of the cytoskeleton.[8]

Many of Ca2+-mediated events occur when the released Ca2+ binds to and activates the regulatory protein calmodulin. Calmodulin may activate calcium-calmodulin-dependent protein kinases, or may act directly on other effector proteins.[9] Besides calmodulin, there are many other Ca2+-binding proteins that mediate the biological effects of Ca2+.

In neurons, concomitant increases in cytosolic and mitochondrial calcium are important for the synchronization of neuronal electrical activity with mitochondrial energy metabolism. Mitochondrial matrix calcium levels can reach the tens of micromolar levels, which is necessary for the activation of isocitrate dehydrogenase, one of the key regulatory enzymes of the Krebs cycle.[10][11]

In the neuron, the ER may serve in a network integrating numerous extracellular and intracellular signals in a binary membrane system with the plasma membrane. Such an association with the plasma membrane creates the relatively new perception of the ER and theme of “a neuron within a neuron.” The ER’s structural characteristics, ability to act as a Ca2+ sink, and specific CCa2+ releasing proteins, serve to create a system that may produce regenerative waves of Ca2+ release that may communicate both locally and globally in the cell. These Ca2+ signals, integrating extracellular and intracellular fluxes, have been implicated to play roles in synaptic plasticity and memory, neurotransmitter release, neuronal excitability and long term changes at the gene transcription level. ER stress is also related to Ca2+ signaling and along with the unfolded protein response, can cause ER associated degradation (ERAD) and autophagy.[12]

See also

References

  1. ↑ Clapham, D.E. (2007). "Calcium Signaling". Cell 131 (6): 1047–1058. doi:10.1016/j.cell.2007.11.028. PMID 18083096.
  2. ↑ Alberts; Bray; Hopkin; Johnson; Raff; Lewis; Roberts; Walter (2014). Essential Cell Biology (4th ed.). New York, NY: Garland Science. pp. 548–549. ISBN 978-0-8153-4454-4.
  3. ↑ Csutora, P.; et al. (2006). "Activation Mechanism for CRAC Current and Store-operated Ca2+ Entry". Journal of Biological Chemistry 281 (46): 34926–34935. doi:10.1074/jbc.M606504200. PMID 17003039.
  4. ↑ Moccia, F.; et al. (2003). "NAADP activates a Ca2+ current that is dependent on F-actin cytoskeleton". The FASEB Journal 17 (13): 1907–1909. doi:10.1096/fj.03-0178fje. PMID 12923070.
  5. ↑ Baba, Y.; et al. (2006). "Coupling of STIM1 to store-operated Ca2+ entry through its constitutive and inducible movement in the endoplasmic reticulum". PNAS 103 (45): 16704–16709. doi:10.1073/pnas.0608358103. PMC 1636519. PMID 17075073.
  6. ↑ Berridge, Michael J.; Lipp, Peter; Bootman, Martin D. (October 2000). "The versatility and universality of calcium signalling". Nature Reviews Molecular Cell Biology 1 (1): 11–21. doi:10.1038/35036035. PMID 11413485.
  7. ↑ Joseph, Suresh K.; Hajnóczky, György (2007-02-06). "IP3 receptors in cell survival and apoptosis: Ca2+ release and beyond". Apoptosis 12 (5): 951–968. doi:10.1007/s10495-007-0719-7. ISSN 1360-8185.
  8. ↑ Koolman, Jan; Röhm, Klaus-Heinrich (2005). Color Atlas of Biochemistry. New York: Thieme. ISBN 1-58890-247-1.
  9. ↑ Berg, Jeremy; Tymoczko, John L.; Gatto, Gregory J.; Stryer, Lubert (2015). Biochemistry (Eighth ed.). New York, NY: W.H. Freeman and Company. p. 407. ISBN 978-1-4641-2610-9.
  10. ↑ Ivannikov, M.; et al. (2013). "Mitochondrial Free Ca2+ Levels and Their Effects on Energy Metabolism in Drosophila Motor Nerve Terminals". Biophys. J. 104 (11): 2353–2361. doi:10.1016/j.bpj.2013.03.064. PMC 3672877. PMID 23746507.
  11. ↑ Ivannikov, M.; et al. (2013). "Synaptic vesicle exocytosis in hippocampal synaptosomes correlates directly with total mitochondrial volume". J. Mol. Neurosci. 49 (1): 223–230. doi:10.1007/s12031-012-9848-8. PMID 22772899.
  12. ↑ Berridge, M. (1998). "Neuronal calcium signaling". Neuron 21 (1): 13–26. doi:10.1016/S0896-6273(00)80510-3. PMID 9697848.

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