Calcium permeates almost every aspect of cellular processes and the pathways associated with them - from proliferation to cell death, contraction and gene expression, hormone and neurotransmitter release, metabolism and synaptic plasticity. Movement of calcium ions in and outside of the cell is paramount to the maintenance of a calcium gradient four order of magnitudes between the extracellular environment and the cytoplasmic concentration of resting cells. The steep gradient allows for fast lo
cal increases in cytoplasmic concentration that underlie the broad range of cellular functions mediated by calcium signaling. A large array of channels at the plasma membrane or the endoplasmic/sarcoplasmic reticulum (ER/SR) mediates calcium entry or release into the cytoplasm; all activated by a variety of stimuli and ligands and in the case of ER/SR channels by calcium itself, also referred to as calcium initiated calcium release (CICR). Extending this initial input, characterized as 'puff', 'blip', 'spark' or 'wave' depending on its spatiotemporal features, a broad spectrum of calcium sensors further carry out the calcium signaling tasks. The increase in calcium ion concentration is transient to prevent its possible toxic effect; in high amount, Ca2+ can aggregate nucleic acids and proteins or impact on the integrity of lipid membranes. Extrusion of calcium ions outside the cell or back into ER/SR is carried out by ATP dependent pumps and exchangers, effectively regulating the calcium signal. Calcium buffers also help maintain the relatively low, ~100nM free calcium concentration of the resting cell. Channels, pumps and exchangers mediating the movement of calcium in and out of cells and organelles control the balance between the provision of the signal and its timely removal; they are shown in the generic diagram and briefly described. As integral components of the 'calcium signaling kit', they are also present in the calcium/calcium-mediated signaling interactive pathway diagram - click here to access it directly ,
(or on the diagram to get to its ontology report).
Prominent players are the voltage operated or voltage-gated, calcium channels (VOCs); they are activated by changes in membrane potential and the flow of calcium initiates signaling events controlling many cellular processes in both excitable and non-excitable cells. In cardiac and muscle cells, calcium entry mediates contraction and also prompts CICR from intracellular stores; in neurons and endocrine cells, it initiates synaptic transmission and hormone release, respectively. Structurally, they are multi-subunit complexes of which alpha is the pore forming subunit and the beta, gamma or delta are ancillary. The alpha subunits differ by the type of Ca2+ currents and can be subdivided into three subfamilies of which the L-type channels are better characterized. The alpha subunit is subject to processing and post-translational modifications that modulate its function. Modulators include Ca2+ sensors and effectors such as calmodulin (CaM), the rather universal Ca2+ sensor.
Transient receptor potential channels (TRPs), when activated depolarize the membrane leading to flow of Na+ and Ca2+ ions into the cell. Overall, the TRP channels are weakly voltage sensitive and mostly nonselective ion channels. The rather large family is subdivided into several subfamilies; Ca2+ selectivity appears to be confined to a subgroup of the 'vanilloid' TRPV family. A member of the mucolipin family, Mcoln1 known as TRPML1, is a Ca2+ and Fe2+ release channel located in lysosomes and late endosomes. Cyclic nucleotide gated channels (CNGs) respond to changes in the intracellular concentration of cGMP and also cAMP to increase Ca2+ concentration in photoreceptor and olfactory cells. Store operated (ORAIs) channels represent a special class as they are not activated by extracellular stimuli but by reduction of calcium in ER with the Stim proteins playing a central role. In the resting state, the EF-hand containing calcium sensor proteins are located in the ER; upon Ca2+ depletion and dissociation of the ion from the EF-hand domain, the proteins oligomerize, translocate to ER-plasma membrane junctions where they couple to and activate the channels. Finally, transmitter/small molecule (acetylcholine, glutamate, ATP) gated ion channels (eg AMPA, NMDA, purinergic) or receptor operated channels (ROCs) are permeable to Ca2+ and other ions and function in neuron-to-neuron signaling, learning, memory and synaptic plasticity, among others; their detailed description is beyond the scope of this synopsis. The link provides a brief description and selected references; they will be individually presented in the context of neurotransmitters/small molecule signaling pathways with which they are associated.
Signaling via G-protein coupled (GPCR) or tyrosine kinase (TKR) receptors leads to generation of inositol 1,4,5,-trisphosphate (IP3) and diacylglycerol (DAG) second messengers. IP3 binds IP3 receptors (ITPRs) in ER to prompt, along with Ca2+ , the release of the ion from ER stores. The functional receptor is a tetramer and in higher organisms, diversity is achieved by the presence of three genes, their splice variants and the possible formation of both homo- and heterotetramers. Phosphorylation and dephosphorylation events and interaction with other partners, in turn subject to phosphorylation/dephosphorylation regulation, modulate their activity. Ahcyl1, known as IRBIT, and the anti-apoptotic Bcl2 are important partners; phosphorylated Ahcyl1 binds to the receptor to a site overlapping the IP3 site and inhibits the receptor by competing with IP3 binding. IRBIT has roles beyond ITPR regulation, such as the control of epithelial fluid and bicarbonate secretion. In a similar fashion Ca2+ and cyclic ADP ribose (cADPr) activate the ryanodine receptors (RYRs) to prompt Ca2+ release from SR stores during excitation-contraction coupling in both cardiac and skeletal muscle. Of note is that in skeletal muscle Ryr1 is in physical contact with the Cacna1s (Cav1.1) channel leading to 'voltage-induced' Ca2+ release; in cardiac muscle, Ryr2 mediated Ca2+ release is initiated in response to Ca2+ influx via Cacna1c(Cav1.2), or CICR. The receptors, which are represented by three genes, are homotetramers and are found in protein complexes with several partners that exert various effects; they include Ca2+ buffers such as calsequestrins or stabilizing proteins such as calstabins, among others. The Ca2+ sensor CaM acts as a partial agonist in its apo- (Ca2+ free) form whereas the Ca2+-bound CaM acts as an inhibitor. Mutations in RYR1 and 2 are associated with a number of human diseases. Both the ITPRs and RYRs are large molecules with RYRs being the largest; both possess Ca2+ binding sites that differ in their affinity for and which mediate the stimulatory or inhibitory effect of the metal ion.
Pumps are P-type ATPases that use the energy of ATP to transport Ca2+ outside the cell or back into the ER/SR against its concentration gradient. They exchange protons for two or one Ca2+ pumped into ER/SR or outside the cell, per ATP hydrolyzed, respectively; there are two Ca2+ binding sites in SERCA pumps whereas the plasma membrane PMCA pumps have one. There are three ER/SR (SERCAs, ATP2A1-3) and four plasma membrane (PMCAs, ATP2B1-4) pumps. Other pumps are represented by the Golgi ATPases (SPCA, ATP2C1 and 2) which also transport Mn2+, a feature that probably relates to the Mn2+ requiring enzymes in the lumen of Golgi, such as the glycosyltransferases.
Another mechanism for Ca2+ extrusion outside the cell involves exchangers: the Na+/Ca2+ exchangers (NCX or SLC8A1-3) and the Na+/Ca2+ -K+ exchangers (NCKX or SLC24A1-5) exchange one Ca2+ for three Na+ ions or co-transport one Ca2+ and one K+ in exchange for four Na+, respectively.
In the mitochondria, the uptake of Ca2+ is favored by the electrochemical proton gradient the electron transport chain pathway generates and is mediated by the Mcu mitochondrial uniporter complex - a selective channel that moves the ion across the mitochondrial inner membrane. In the opposite direction, a Na+/Li+/ Ca2+ exchanger (Slc24a6) promotes release of the metal ion. Nicotinic acid adenine dinucleotide phosphate (NAADP)-gated calcium channels, the two-pore segment channels represented by the two members in humans (TPCN1 and 2) and rodents in the family of three in most other mammals, mediate Ca2+ release from acidic stores in endosomic/lysosomic compartments. Whether TPCNs and perhaps ryanodine channels directly bind NAADP or binding is mediated by some other intermediary protein, is not yet a fully settled issue. The identity of the uptake mechanism in the acidic stores has not been elucidated; a putative Ca2+/H+ exchanger has been postulated. Of note is the fact that in mammalian cells, a multifunctional ATP ribosyl cyclase can generate NAADP or cADPr depending on whether the cofactor used is NADP or NAD, respectively.
Channels activated by/responding to increase in intracellular Ca2+ such as members of the potassium or chloride channels are not part of this balanced gradient control and are not shown; likewise not shown are buffers which do play a role but are not involved in calcium transport/movement. These molecules are responders and modulators of calcium signaling, respectively and along with sensors and other players will be presented in the context of calcium/calcium-mediated signaling.
The movement of calcium ion in and out of cells and organelles, the regulated availability of the free metal ion and the interaction with its many sensors, the calcium transport and calcium-mediated signaling pathways are inextricably connected. Together, they are the components of the 'calcium signaling kit' and together, they orchestrate the homeostasis of what is probably the most versatile element in the kingdoms of life. To see the ontology report for annotations, Gviewer and download, click here ...(less)