Nuclear versus perinuclear and cytoplasmic calcium in osteoclasts

Microrobotic Swarms for Intracellular Measurement with Enhanced Signal-to-Noise Ratio

In cell biology, fluorescent dyes are routinely used for biochemical measurements. The traditional global dye treatment method suffers from low signal-to-noise ratios (SNR), especially when used for detecting a low concentration of ions, and increasing the concentration of fluorescent dyes causes more severe cytotoxicity. Here, we report a robotic technique that controls how a low amount of fluorescent-dye-coated magnetic nanoparticles accurately forms a swarm and increases the fluorescent dye concentration in a local region inside a cell for intracellular measurement. Different from existing magnetic micromanipulation systems that generate large swarms (several microns and above) or that cannot move the generated swarm to an arbitrary position, our system is capable of generating a small swarm (e.g., 1 μm) and accurately positioning the swarm inside a single cell (position control accuracy: 0.76 μm). In experiments, the generated swarm inside the cell showed an SNR 10 times higher than the traditional global dye treatment method. The high-SNR robotic swarm enabled intracellular measurements that had not been possible to achieve with traditional global dye treatment. The robotic swarm technique revealed an apparent pH gradient in a migrating cell and was used to measure the intracellular apparent pH in a single oocyte of living C. elegans. With the position control capability, the swarm was also applied to measure calcium changes at the perinuclear region of a cell before and after mechanical stimulation. The results showed a significant calcium increase after mechanical stimulation, and the calcium increase was regulated by the mechanically sensitive ion channel, PIEZO1.

Chemical and physiological characterization of fluo-4 Ca(2+)-indicator dyes

We have developed fluo-4, a new fluorescent dye for quantifying cellular Ca2+ concentrations in the 100 nM to 1 microM range. Fluo-4 is similar in structure and spectral properties to the widely used fluorescent Ca(2+)-indicator dye, fluo-3, but it has certain advantages over fluo-3. Due to its greater absorption near 488 nm, fluo-4 offers substantially brighter fluorescence emission when used with excitation by argon-ion laser or other sources in conjunction with the standard fluorescein filter set. In vitro, fluo-4 exhibited high fluorescence emission, a high rate of cell permeation, and a large dynamic range for reporting [Ca2+] around a Kd(Ca2+) of 345 nM. We have also developed several Ca(2+)-indicators related to fluo-4 having lower affinities for Ca2+ that are useful in cellular studies requiring quantification of higher [Ca2+]. In a variety of physiological studies of live cells, fluo-4 labeled cells more brightly than did fluo-3, when challenged with procedures designed to elevate calcium levels. Fluo-4 is well suited for photometric and imaging applications that make use of confocal laser scanning microscopy, flow cytometry, or spectrofluorometry, or in fluorometric high-throughput microplate screening assays. Because of its higher fluorescence emission intensity, fluo-4 can be used at lower intracellular concentrations, making its use a less invasive practice.

Intracellular Mg2+ surge follows Ca2+ increase during depolarization in cultured neurons

The intracellular magnesium and calcium concentrations in cultured dorsal root ganglion neurons were measured using a fluorescent Mg2+ indicator, Mag-Fura-2 and a Ca2+ indicator, Fura-2, respectively. The magnesium concentration in the cytoplasm was higher than that in the nuclei at rest; 0.68+/-0.10 mM (mean+/-S.E.M., n=7) in the cytoplasm and 0.11+/-0.05 mM in the nucleus. When depolarized by a 60 mM KCl solution, the magnesium concentration increased remarkably in the cytoplasm; 1.52+/-0.26 mM (n=7) in the cytoplasm and 0.25+/-0. 12 mM in the nucleus. This is in contrast to a Ca2+ increase due to depolarization in which the increase was remarkable also in the nucleus. The Mg2+ response displayed a rapid spontaneous recovery even in the presence of the high K+ solution. The Ca2+ response, on the other hand, accompanied a slow recovery 'plateau'. Simultaneous measurements of Mg2+ and Ca2+ by a double-labeling experiment revealed that the Ca2+ concentration started to rise 0.46+/-0.05 s (n=32) earlier, and it reached its peak 1.38+/-0.12 s (n=32) earlier than Mg2+. These results support the scheme of 'calcium induced magnesium release', that the depolarization-induced elevation of the Ca2+ concentration causes an increase in the Mg2+ concentration in the cytoplasm.

Nuclear and cytosolic calcium changes in osteoclasts stimulated with ATP and integrin-binding peptide

Cytosolic calcium modulates the activity of osteoclasts, large multinucleate cells that resorb bone. Nuclear events, such as gene transcription, are also calcium-regulated in these cells, and fluorescence imaging has suggested that calcium signals produced by some stimuli are specifically targeted to, or amplified within, osteoclast nuclei. We used two alternative techniques of dye loading to examine the changes of intracellular calcium induced in rat osteoclasts by three stimuli. Osteoclasts loaded with the calcium indicator Fura-2 by the acetoxymethyl (AM) ester technique appeared to display marked nuclear calcium amplification. During stimulation with integrin-binding peptides, ATP, or high extracellular calcium, fluorescence ratios recorded from the nuclei rose higher than did ratios recorded from extranuclear regions. In contrast, nuclear calcium amplification was not observed after AM loading in the presence of the anion transport inhibitor sulfinpyrazone, nor in osteoclasts injected with Fura-2 conjugated to a high MW dextran. In these cells, nuclear fluorescence ratios were equal to the extranuclear values at all times: upon stimulation by an agonist, the nuclear and cytosolic calcium concentrations increased by the same amount. The calcium changes seen in stimulated osteoclasts can no longer be taken as evidence for the general validity of the phenomenon of nuclear calcium amplification.