Calcium imaging

Calcium imaging is a microscopy technique to optically measure the calcium (Ca2+) status of an isolated cell, tissue or medium. Calcium imaging takes advantage of calcium indicators, fluorescent molecules that respond to the binding of Ca2+ ions by changing their fluorescence properties. Two main classes of calcium indicators exist: chemical indicators and genetically encoded calcium indicators (GECI). This technique has allowed studies of calcium signalling in a wide variety of cell types. In neurons, electrical activity is always accompanied by an influx of Ca2+ ions. Thus, calcium imaging can be used to monitor the electrical activity in hundreds of neurons in cell culture or in living animals, which has made it possible to dissect the function of neuronal circuits.

Chemical indicators
Schematic of a typical setup for calcium fluorescence imaging of isolated cardiac myocytes

Chemical indicators are small molecules that can chelate calcium ions. All these molecules are based on an EGTA homologue called BAPTA, with high selectivity for calcium (Ca2+) ions versus magnesium (Mg2+) ions.

This group of indicators includes fura-2, indo-1, fluo-3, fluo-4, Calcium Green-1.

These dyes are often used with the chelator carboxyl groups masked as acetoxymethyl esters, in order to render the molecule lipophilic and to allow easy entrance into the cell. Once this form of the indicator is in the cell, cellular esterases will free the carboxyl groups and the indicator will be able to bind calcium. The free acid form of the dyes (i.e. without the acetoxymethyl ester modification) can also be directly injected into cells via a microelectrode or micropipette which removes uncertainties as to the cellular compartment holding the dye (the acetoxymethyl ester can also enter the endoplasmic reticulum and mitochondria). Binding of a Ca2+ ion to a fluorescent indicator molecule leads to either an increase in quantum yield of fluorescence or emission/excitation wavelength shift. Individual chemical Ca2+ fluorescent indicators are utilized for cytosolic calcium measurements in a wide variety of cellular preparations. The first real time (video rate) Ca2+ imaging was carried out in 1986 in cardiac cells using intensified video cameras.[1] Later development of the technique using laser scanning confocal microscopes revealed sub-cellular Ca2+ signals in the form of Ca2+ sparks and Ca2+ blips. Relative responses from a combination of chemical Ca2+ fluorescent indicators were also used to quantify calcium transients in intracellular organelles such as mitochondria.[2]

Calcium imaging, also referred to as calcium mapping, is also used to perform research on myocardial tissue.[3] Calcium mapping is a ubiquitous technique used on whole, isolated hearts such as mouse, rat, and rabbit species.

Genetically encoded calcium indicators

These indicators are fluorescent proteins derived from green fluorescent protein (GFP) or its variants (e.g. circularly permuted GFP, YFP, CFP), fused with calmodulin (CaM) and the M13 domain of the myosin light chain kinase, which is able to bind CaM. Alternatively, variants of GFP are fused with the calcium binding protein troponin C (TnC), applying the mechanism of FRET (Förster Resonance Energy Transfer) for signal modulation.

Genetically encoded indicators do not need to be loaded onto cells, instead the genes encoding for these proteins can be easily transfected to cell lines. It is also possible to create transgenic animals expressing the dye in all cells or selectively in certain cellular subtypes. GECIs have been used in the studies of neuron[4],[5] T-cell,[6] cardiomyocyte,[7] etc.

GECI Year Sensing Reporting Precursor
Cameleons[8] 1997 Calmodulin FRET pair: BFP or CFP, and GFP or YFP -
FIP-CBSM[9] 1997 Calmodulin FRET pair: BFP and RFP -
Pericams[10] 2000 Calmodulin cpGFP -
GCaMP[11][12] 2000 Calmodulin cpEGFP -
TN-L15[13] 2004 Modified chicken skeletal muscle troponin C FRET pair: YFP (Citrine) and CFP (Cerulean) -
TN-humTnC[13] 2004 Human cardiac troponin C FRET pair: YFP (Citrine) and CFP (Cerulean) -
TN-XL[14] 2006 Modified chicken skeletal muscle troponin C FRET pair: permuted YFP (Citrine) and CFP (Cerulean) TN-L15
TN-XXL[15] 2008 Modified csTnC in TN-XL FRET pair: permuted YFP (Citrine) and CFP (Cerulean) TN-XL
Twitch's[16] 2014 Troponin C FRET pair (various of two FPs) -
RCaMP1[17] 2013 Calmodulin mRuby (red FP) -
jRGECO1a[18] 2016 Calmodulin mApple (red FP) R-GECO[19]

A special class of genetically encoded calcium indicators are designed to form a permanent fluorescent tag in active neurons. They are based on the photoswitchable protein mEos which turns from green to red when illuminated with violet light. Combined with the calcium sensor calmodulin, violet light photoconverts only neurons that have elevated calcium levels. SynTagMA is a synapse-targeted version of CaMPARI2.

GECI Year Sensing Reporting Precursor
CaMPARI[20] 2015 Calmodulin + violet light mEos: green to red conversion -
CaMPARI2[21] 2018 Calmodulin + violet light mEos: green to red conversion CaMPARI
SynTagMA[22] 2020 Calmodulin + violet light mEos: green to red conversion CaMPARI2

Usage

Regardless of the type of indicator used the imaging procedure is generally very similar. Cells loaded with an indicator, or expressing it in the case of a GECI, can be viewed using a fluorescence microscope and captured by a Scientific CMOS (sCMOS)[23] camera or CCD camera. Confocal and two-photon microscopes provide optical sectioning ability so that calcium signals can be resolved in microdomains such as dendritic spines or synaptic boutons, even in thick samples such as mammalian brains. Images are analyzed by measuring fluorescence intensity changes for a single wavelength or two wavelengths expressed as a ratio (ratiometric indicators). If necessary, the derived fluorescence intensities and ratios may be plotted against calibrated values for known Ca2+ levels to measure absolute Ca2+ concentrations. Light field microscopy methods[24] extend functional readout of neural activity capabilities in 3D volumes.

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Further reading
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