Decreased, Calcium influx

Biological state: Under physiological resting conditions of the cell, the free intracellular Ca2+ reaches around 100 nM, whereas the extracellular Ca2+ can be found at higher concentrations of 1.2 mM that under certain stimulus may invade the cell (Berridge et al, 2000). Six to seven oxygen atoms surround Ca2+, whereas the protein chelator of Ca2+ is the EF motif that is present in many proteins such as calmodulin (Clapham, 2007). The EF-hand is a helix-loop-helix calcium-binding motif in which two helices pack together at an angle of approximately 90 degrees (Lewit-Bentley and Réty, 2000). The two helices are separated by a loop region where calcium actually binds. The EF notation for the motif is derived from the notation applied to the structure of parvalbumin, in which the E and F helices were originally identified as forming this calcium-binding motif.

Biological compartments: Ca2+ ions accumulate in the cytoplasm, cellular organelles (e.g. mitochondria and endoplasmic reticulum) and nucleus in response to diverse classes of stimuli.

General role in biology: In order to adapt to altered stimulus from exposure to different environmental factors, cells require signal transmission. However, signalling needs messengers whose concentration is modified upon stimulus (Clapham, 2007). Ca2+ ions act as an important intracellular messenger playing the role of ubiquitous signalling molecules and consequently regulate many different cellular functions (Berridge, 2012; Hagenston and Bading, 2011). Given its important role in processes that are fundamental to all cell types, Ca2+ homeostasis is tightly regulated by intracellular and extracellular mechanisms (Barhoumi et al., 2010). Intracellular Ca2+ concentration is regulated by opening or closing channels in the plasma membrane. Additionally, the Ca2+ ions can be released from intracellular stores of the endoplasmic reticulum (ER) through ryanodine receptors (RYRs) or inositol 1,4,5-trisphosphate receptors (InsP3Rs). Ca2+ homeostasis is also regulated by the mechanisms that remove Ca2+ from the cytosol, for example pumps in both cell membrane and ER membrane. In addition, cytosolic Ca2+ regulation involves accumulation of Ca2+ in mitochondria that have the capacity to buffer the excess of cytoplasmic Ca2+ ions. In neurons, Ca2+ ions regulate many critical functions. Firstly, they contribute to dendritic electrical signalling, producing postsynaptic depolarization by the current carried by Ca2+ ions. Secondly, Ca2+ activates Ca2+-sensitive proteins such as different kinases, calcineurin and calpain, triggering signalling pathways critical for cell physiology. Modification of the gene transcription is the final outcome of the Ca2+ ions impact on long-term modifications affecting neurotransmitters release (reviewed in Neher and Sakaba, 2008), neuronal differentiation, synapse function and cell viability (Clapham, 2007; Higley and Sabatini, 2012). Thus, the Ca2+ that enters and accumulates in cytoplasm and nucleus is a central signalling molecule that regulates synapse and neuronal cell function, including learning and memory processes (Berridge, 2012; Hagenston and Bading, 2011).

Methods that have been previously reviewed and approved by a recognized authority should be included in the Overview section above. All other methods, including those well established in the published literature, should be described here. Consider the following criteria when describing each method: 1. Is the assay fit for purpose? 2. Is the assay directly or indirectly (i.e. a surrogate) related to a key event relevant to the final adverse effect in question? 3. Is the assay repeatable? 4. Is the assay reproducible?

No OECD method is available to measure intracellular Ca2+.

The gold standard method for measuring Ca2+ current through NMDA receptor is patch clamp electrophysiology (Blanke and VanDongen, 2009).

In vitro, well-established flow cytometric or high content imaging analysis with specific fluorescent dyes (Ca2+-sensitive fluorophores) such as Fura-2, Oregon Green-BAPTA, Fluo-4 and X-Rhod exist for determination of intracellular Ca2+ concentration. The use of different fluorometric calcium indicators in neuroscience and neurotoxicology have been recently reviewed by Grienberger and Konnerth (2012) and Calvo et al (2015).

Barhoumi et al. 2010 summarised all the methods to measure cytosolic Ca2+ alterations due to exposure to neurotoxic compounds, including steady state, short-term kinetic measurements of stimulated Ca2+ transients and dynamic measurements. This paper further discusses the strengths and weaknesses of each approach in intracellular Ca2+ measurements and its applicability in high throughput screening.

For quantitative estimation of Ca2+ in dendritic spines, besides of Ca2+-sensitive fluorophores the use of two-photon released caged neurotransmitters has been suggested as it allows direct stimulation of visualized spines (Higley and Sabatini, 2012). In Higley and Sabatini 2012 further technical information can be found in relation to study Ca2+ in dendritic spines.

Furthermore, there are three methods for measuring Ca2+ influx in NMDA receptors that involve the measurement of 1) relative Ca2+ permeability, 2) channel blockage by Ca2+, and 3) fractional Ca2+ currents from whole-cell currents determined in the presence of high concentrations of intracellular Fura-2 (Traynelis et al., 2010).

In vivo, two-photon Ca2+ imaging using Ca2+-sensitive fluorescent indicators that measure changes in intracellular Ca2+ concentration as a readout for suprathreshold and subthreshold neuronal activity has also been used to study learning and memory in live rodents (Chen et al., 2013) The last two decades the neuronal function of the larval and adult zebrafish has been extensively studied using Ca2+ imaging methods. By applying simple Ca2+ indicators such as dextran or acetoxymethyl esters to more powerful genetically encoded Ca2+ indicators, zebrafish provides a transparent model where live Ca2+ imaging can be successfully achieved (Kettunen, 2012).

Fluorescent Ca2+ indicators have been also used as Pb2+ sensors in order to resolve spatiotemporal changes in intracellular Pb2+ in relation to cellular signaling and intracellular divalent metal homeostasis (Vijveberg and Westerink, 2012).

Intra-cellular calcium concentration can be measured in cell cultures with the calcium sensitive fluorescent dye Fura-2 AM and fluorescence microscopy. This technique appeared to be more sensitive than the plate-reader based assay Meijer et al., 2014).

Ca2+ homeostatic system is known to be highly conserved throughout evolution and is present from humans to invertebrates (Case et al., 2007).

Barhoumi R, Qian Y, Burghardt RC, Tiffany-Castiglioni E. (2010) Image analysis of Ca2+ signals as a basis for neurotoxicity assays: promises and challenges. Neurotoxicol Teratol. 32: 16-24.

Berridge MJ, Lipp P, Bootman MD. (2000) The versatility and universality of calcium signalling. Nat Rev Mol Cell Biol. 1:11-21.

Berridge MJ. (2012) Calcium signalling remodelling and disease. Biochem Soc Trans. 40: 297-309.

Blanke ML, VanDongen AMJ. (2009) Activation Mechanisms of the NMDA Receptor. In: Van Dongen AM, editor. Biology of the NMDA Receptor. Boca Raton (FL): CRC Press; Chapter 13. Available from: http://www.ncbi.nlm.nih.gov/books/NBK5274/

Calvo M, Villalobos C, Núñez L. (2015) Calcium imaging in neuron cell death. Methods Mol Biol. 1254: 73-85.

Case RM, Eisner D, Gurney A, Jones O, Muallem S, Verkhratsky A. (2007) Evolution of calcium homeostasis: from birth of the first cell to an omnipresent signalling system. Cell Calcium 42: 345-350.

Chen JL, Andermann ML, Keck T, Xu NL, Ziv Y. (2013) Imaging neuronal populations in behaving rodents: paradigms for studying neural circuits underlying behavior in the mammalian cortex. J Neurosci. 33: 17631-17640.

Clapham DE. (2007) Calcium signaling. Cell 131: 1047-1058.

Grienberger C, Konnerth A. (2012) Imaging calcium in neurons. Neuron 73: 862-885.

Hagenston AM, Bading H. (2011) Calcium Signaling in Synapse-to-Nucleus Communication. Cold Spring Harb Perspect Biol. 3: a004564.

Higley MJ, Sabatini BL. (2012) Calcium signalling in dendritic spines. Cold Spring Harb Perspect Biol. 4: a005686.

Kettunen P. (2012) Calcium imaging in the zebrafish. Adv Exp Med Biol. 740: 1039-1071.

Lewit-Bentley A, Réty S. (2000) EF-hand calcium-binding proteins. Curr Opin Struct Biol. 2000 Dec;10(6):637-43.

Meijer M, Hendriks HS, Heusinkveld HJ, Langeveld WT, Westerink RH. 2014. Comparison of plate reader-based methods with fluorescence microscopy for measurements of intracellular calcium levels for the assessment of in vitro neurotoxicity. Neurotoxicology 45: 31-37.

Neher E., Sakaba T. (2008). Multiple roles of calcium ions in the regulation of neurotransmitter release. Neuron 59: 861-872.

Traynelis S, Wollmuth LP, McBain CJ, Menniti FS, Vance KM, Ogden KK, Hansen KB, Yuan H, Myers SJ, Dingledine R. (2010) Glutamate receptor ion channels: structure, regulation, and function. Pharmacol Rev. 62: 405-496.

Vijverberg HP, Westerink RH. 2012. Sense in Pb2+ sensing. Toxicol Sci 130(1): 1-3.