The ABC Guide to Fluorescent Toolsets for the Development of Future Biomaterials

In recent decades, diversified approaches using nanoparticles or nano-structured scaffolds have been applied to drug delivery and tissue engineering. Thanks to recent interdisciplinary studies, the materials developed have been intensively evaluated at animal level. Despite these efforts, less attention has been paid to what is really going on at the subcellular level during the interaction between a nanomaterial and a cell. As the proposed concept becomes more complex, the need for investigation of the dynamics of these materials at the cellular level becomes more prominent. For a deeper understanding of cellular events, fluorescent imaging techniques have been a powerful means whereby spatiotemporal information related to cellular events can be visualized as detectable fluorescent signals. To date, several excellent review papers have summarized the use of fluorescent imaging toolsets in cellular biology. However, applying these toolsets becomes a laborious process for those who are not familiar with imaging studies to engage with owing to the skills gap between them and cell biologists. This review aims to highlight the valuable essentials of fluorescent imaging as a tool for the development of effective biomaterials by introducing some cases including photothermal and photodynamic therapies. This distilled information will be a convenient short-cut for those who are keen to fabricate next generation biomaterials.

Focal calcium monitoring with targeted nanosensors at the cytosolic side of endoplasmic reticulum

Ca²⁺ distribution is spatially and temporally non-uniform inside cells due to cellular compartmentalization. However, Ca²⁺ sensing with small organic dyes, such as fura-2 and fluo-4, has been practically applied at a single cell level where the averaged signal from freely diffusing dye molecules is acquired. In this study, we aimed to target azide-functionalized fura-2 (N₃-fura-2) to a specific site of subcellular compartments to realize focal Ca²⁺ sensing. Using scAVD (single-chain avidin)-biotin interaction and a copper-free click reaction system, we linked N₃-fura-2 to specifically-targeted scAVD protein fused with a red fluorescent protein mCherry, so that Ca²⁺ sensors conjugated with four N₃-fura-2 dyes with dibenzocyclooctyne (DBCO)-PEG4-biotin as a linker were generated at subcellular compartments in living cells. In cytoplasm, N₃-fura-2 showed a prolonged retention period after binding to scAVD. Furthermore, the reacted N₃-fura-2 was retained inside cells even after free dyes were washed out by methanol fixation. When scAVD was overexpressed on endoplasmic reticulum (ER) membranes, N₃-fura-2 was accumulated on ER membranes. Upon histamine stimulation, which increases cytosolic Ca²⁺ concentration, ER-localized N₃-fura-2 successfully sensed the Ca²⁺ level changes at the cytosolic side of ER membrane. Our study demonstrated specific targeting of N₃-fura-2 to subcellular compartments and the ability of sensing focal Ca²⁺ level changes with the specifically targeted Ca²⁺ sensors.

Synthesis of an azido-tagged low affinity ratiometric calcium sensor

Changes in high localised concentrations of Ca²⁺ ions are fundamental to cell signalling. The synthesis of a dual excitation, ratiometric calcium ion sensor with a K_d of 90 μM, is described. It is tagged with an azido group for bioconjugation, and absorbs in the blue/green and emits in the red region of the visible spectrum with a large Stokes shift. The binding modulating nitro group is introduced to the BAPTA core prior to construction of a benzofuran-2-yl carboxaldehyde by an allylation–oxidation–cyclisation sequence, which is followed by condensation with an azido-tagged thiohydantoin. The thiohydantoin unit has to be protected with an acetoxymethyl (AM) caging group to allow CuAAC click reaction and incorporation of the KDEL peptide endoplasmic reticulum (ER) retention sequence.

New red-fluorescent calcium indicators for optogenetics, photoactivation and multi-color imaging

Most chemical and, with only a few exceptions, all genetically encoded fluorimetric calcium (Ca(2+)) indicators (GECIs) emit green fluorescence. Many of these probes are compatible with red-emitting cell- or organelle markers. But the bulk of available fluorescent-protein constructs and transgenic animals incorporate green or yellow fluorescent protein (GFP and YFP respectively). This is, in part, not only heritage from the tendency to aggregate of early-generation red-emitting FPs, and due to their complicated photochemistry, but also resulting from the compatibility of green-fluorescent probes with standard instrumentation readily available in most laboratories and core imaging facilities. Photochemical constraints like limited water solubility and low quantum yield have contributed to the relative paucity of red-emitting Ca(2+) probes compared to their green counterparts, too. The increasing use of GFP and GFP-based functional reporters, together with recent developments in optogenetics, photostimulation and super-resolution microscopies, has intensified the quest for red-emitting Ca(2+) probes. In response to this demand more red-emitting chemical and FP-based Ca(2+)-sensitive indicators have been developed since 2009 than in the thirty years before. In this topical review, we survey the physicochemical properties of these red-emitting Ca(2+) probes and discuss their utility for biological Ca(2+) imaging. Using the spectral separability index Xijk (Oheim M., 2010. Methods in Molecular Biology 591: 3-16) we evaluate their performance for multi-color excitation/emission experiments, involving the identification of morphological landmarks with GFP/YFP and detecting Ca(2+)-dependent fluorescence in the red spectral band. We also establish a catalog of criteria for evaluating Ca(2+) indicators that ideally should be made available for each probe. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.