Neuronal free calcium measurement using BTC/AM, a low affinity calcium indicator

Discovery of a novel calcium-sensitive fluorescent probe for α-ketoglutarate

α-Ketoglutarate (α-KG), a pivotal metabolite in energy metabolism, has been implicated in nonalcoholic fatty liver disease (NAFLD) and several cancers. It is recently proposed that plasma α-KG is a surrogate biomarker of NAFLD. Here, we report the development of a novel "turn-on" chemosensor for α-KG that contains a coumarin moiety as a fluorophore. Using benzothiazole-coumarin (BTC) as inspiration, we designed a probe for calcium ion recognition that possesses a unique fluorophore compared with previously reported probes for α-KG measurement. This chemosensor is based on the specific Schiff base reaction and the calcium ion recognition property of the widely used calcium indicator BTC. The probe was synthesized, and a series of parallel experiments were conducted to optimize the chemical recognition process. Compared to the initial weak fluorescence, a remarkable 7.6-fold enhancement in fluorescence intensity (I/I₀ at 495 nm) was observed for the conditions in which the probe (1 μmol/L), α-KG (50 μmol/L), and Ca²⁺ (100 μmol/L) were incubated at 30 °C in EtOH. The probe displayed good selectivity for α-KG even in an environment with an abundance of amino acids and other interfering species such as glutaric acid. We determined that the quantitative detection range of α-KG in EtOH was between 5 and 50 μmol/L. Finally, probe in serum loaded with α-KG (10 mmol/L) showed a 7.4-fold fluorescence enhancement. In summary, a novel probe for detecting the biomarker α-KG through a typical Schiff base reaction has been discovered. With further optimization, this probe may be a good alternative for detecting the physiological metabolite α-KG.

Synthesis and properties of Asante Calcium Red--a novel family of long excitation wavelength calcium indicators

Although many synthetic calcium indicators are available, a search for compounds with improved characteristics continues. Here, we describe the synthesis and properties of Asante Calcium Red-1 (ACR-1) and its low affinity derivative (ACR-1-LA) created by linking BAPTA to seminaphthofluorescein. The indicators combine a visible light (450-540 nm) excitation with deep-red fluorescence (640 nm). Upon Ca2+ binding, the indicators raise their fluorescence with longer excitation wavelengths producing higher responses. Although the changes occur without any spectral shifts, it is possible to ratio Ca(2+)-dependent (640 nm) and quasi-independent (530 nm) emission when using visible (< 490 nm) or multiphoton (∼780 nm) excitation. Therefore, both probes can be used as single wavelength or, less dynamic, ratiometric indicators. Long indicator emission might allow easy [Ca2+]i measurement in GFP expressing cells. The indicators bind Ca2+ with either high (Kd = 0.49 ± 0.07 μM; ACR-1) or low affinity (Kd = 6.65 ± 0.13 μM; ACR-1-LA). Chelating Zn2+ (Kd = 0.38 ± 0.02 nM) or Mg2+ (Kd∼5mM) slightly raises and binding Co2+ quenches dye fluorescence. New indicators are somewhat pH-sensitive (pKa = 6.31 ± 0.07), but fairly resistant to bleaching. The probes are rather dim, which combined with low AM ester loading efficiency, might complicate in situ imaging. Despite potential drawbacks, ACR-1 and ACR-1-LA are promising new calcium indicators.

Determining calcium concentration in heterogeneous model systems using multiple indicators

Intracellular free calcium concentrations ([Ca2+]i) are assessed by measuring indicator fluorescence in entire cells or subcellular regions using fluorescence microscopy. [Ca2+]i is calculated using equations which link fluorescence intensities (or intensity ratios) to calcium concentrations [G. Grynkiewicz, M. Poenie, R.Y. Tsien, A new generation of Ca2+ indicators with greatly improved fluorescence properties, J. Biol. Chem. 260 (1985) 3440-3450]. However, if calcium ions are heterogeneously distributed within a region of interest, then the observed average fluorescence intensity may not reflect average [Ca2+]i. We assessed potential calcium determination errors in mathematical and experimental models consisting of 'low' and 'high' calcium compartments, using indicators with different affinity for calcium. [Ca2+] calculated using average fluorescence intensity was lower than the actual mean concentrations. Low affinity indicators reported higher (more accurate) values than their high affinity counterparts. To estimate compartment dimensions and respective [Ca2+], we extended the standard approach by using different indicator responses to the same [Ca2+]. While two indicators were sufficient to provide a partial characterization of two-compartment model systems, the use of three or more indicators offered full description of the model provided compartmental [Ca2+] were within the indicator sensitivity ranges. These results show that uneven calcium distribution causes underestimation of actual [Ca2+], and offers novel approaches to estimating calcium heterogeneity.

Maturation of vulnerability to excitotoxicity: intracellular mechanisms in cultured postnatal hippocampal neurons

Neuronal vulnerability to excitotoxicity changes dramatically during postnatal maturation. To study the intracellular mechanisms by which maturation alters vulnerability in single neurons, we developed techniques to maintain hippocampal neurons from postnatal rats in vitro. After establishing their neuronal phenotype with immunohistochemistry and electrophysiology, we determined that these neurons exhibit developmentally regulated vulnerability to excitotoxicity. At 5 days in vitro, NMDA-induced cell death at 24 h increased from 3.6% in 3-day-old rats to >90% in rats older than 21 days. Time-lapse imaging of neuronal morphology following NMDA demonstrated increasingly prevalent and severe injury as a function of postnatal age. Neither high- nor low-affinity calcium dyes demonstrated differences in peak NMDA-induced [Ca(2+)](i) increases between neurons from younger and older animals. However, neurons from older animals were uniformly distinguished from those from younger animals by their subsequent loss of [Ca(2+)](i) homeostasis. Because of the role of mitochondrial Ca(2+) buffering in [Ca(2+)](i) homeostasis, we measured NMDA-induced changes in mitochondrial membrane potential (DeltaPsi) as a function of postnatal age. NMDA markedly dissipated DeltaPsi in neurons from mature rats, but minimally in those from younger rats. These data demonstrate that, in cultures of postnatal hippocampal neurons, (a) vulnerability to excitotoxicity increases as a function of the postnatal age of the animal from which they were harvested, and (b) developmental regulation of vulnerability to NMDA occurs at the level of the mitochondrion.

Ionic selectivity of low-affinity ratiometric calcium indicators: mag-Fura-2, Fura-2FF and BTC

Accurate measurement of elevated intracellular calcium levels requires indicators with low calcium affinity and high selectivity. We examined fluorescence spectral properties and ionic specificity of three low-affinity, ratiometric indicators structurally related to Fura-2: mag-Fura-2 (furaptra), Fura-2FF, and BTC. The indicators differed in respect to their excitation wavelengths, affinity for Ca2+ (Kd approximately 20 microM, 6 microM and 12 microM respectively) and selectivity over Mg2+ (Kd approximately 2 mM for mag-Fura-2, > 10 mM for Fura-2FF and BTC). Among the tested indicators, BTC was limited by a modest dynamic range upon Ca2+ binding, susceptibility to photodamage, and sensitivity to alterations in pH. All three indicators bound other metal ions including Zn2+, Cd2+ and Gd3+. Interestingly, only in the case of BTC were spectral differences apparent between Ca2+ and other metal ions. For example, the presence of Zn2+ increased BTC fluorescence 6-fold at the Ca2+ isosbestic point, suggesting that this dye may be used as a fluorescent Zn2+ indicator. Fura-2FF has high specificity, wide dynamic range, and low pH sensitivity, and is an optimal low-affinity Ca2+ indicator for most imaging applications. BTC may be useful if experimental conditions require visible wavelength excitation or sensitivity to other metal ions including Zn2+.

Measurement of intracellular calcium

To a certain extent, all cellular, physiological, and pathological phenomena that occur in cells are accompanied by ionic changes. The development of techniques allowing the measurement of such ion activities has contributed substantially to our understanding of normal and abnormal cellular function. Digital video microscopy, confocal laser scanning microscopy, and more recently multiphoton microscopy have allowed the precise spatial analysis of intracellular ion activity at the subcellular level in addition to measurement of its concentration. It is well known that Ca2+ regulates numerous physiological cellular phenomena as a second messenger as well as triggering pathological events such as cell injury and death. A number of methods have been developed to measure intracellular Ca2+. In this review, we summarize the advantages and pitfalls of a variety of Ca2+ indicators used in both optical and nonoptical techniques employed for measuring intracellular Ca2+ concentration.

Blockade of mitochondrial Ca2+ uptake by mitochondrial inhibitors amplifies the glutamate-induced calcium response in cultured cerebellar granule cells

The objective of this study was to evaluate the role of mitochondrial Ca2+ uptake (MCU) in modulation (shaping) of the glutamate (Glu)-induced changes in neuronal cytoplasmic Ca2+ ([Ca2+]i). In order to block MCU, nerve cells were treated with mitochondrial inhibitors (MI) inducing collapse of the mitochondrial potential (Delta Psim). Measurements of changes in [Ca2+]i were performed using either the low-affinity (fura-2FF) or high-affinity (fura-2) Ca2+ indicators. Loading of nerve cells with rhodamine 123 made it possible to monitor changes in Delta Psim. In the first series of experiments it was shown that blockade of MCU in fura-2FF-loaded cells with a cocktail of rotenone (2 microM)+oligomycin (2.5 microg/ml) greatly (2.53+/-0.4 times, n=61) increased the [Ca2+]i response to a 1-min Glu (100 microM) pulse. In fura-2-loaded cells, this increase was small (less than 1.3 times) or absent. In the second series of experiments, cocktails of rotenone+oligomycin or FCCP (1 microM)+oligomycin were applied during a prolonged Glu application. This produced strong mitochondrial depolarisation and an additional [Ca2+]i increase. In most cells the latter could be reversed or prevented by a removal of external Ca2+. The MI-induced additional [Ca2+]i increase was especially pronounced in cells loaded with fura-2FF. In some neurones a removal of external Ca2+ did not produce a decrease in [Ca2+]i during combined Glu+MI application, suggesting an impairment of [Ca2+]i extrusion mechanisms of these cells. The conclusion is drawn that MCU makes a considerable contribution to regulation of [Ca2+]i responses caused by Ca2+ influx via Glu-activated ionic channels. The reasons for a quantitative difference between [Ca2+]i responses observed in fura-2- and fura-2FF-loaded neurones are discussed.