Imaging beyond the proteome

A platform for designing hyperpolarized magnetic resonance chemical probes

Hyperpolarization is a highly promising technique for improving the sensitivity of magnetic resonance chemical probes. Here we report [¹⁵N, D₉]trimethylphenylammonium as a platform for designing a variety of hyperpolarized magnetic resonance chemical probes. The platform structure shows a remarkably long ¹⁵N spin–lattice relaxation value (816 s, 14.1 T) for retaining its hyperpolarized spin state. The extended lifetime enables the detection of the hyperpolarized ¹⁵N signal of the platform for several tens of minutes and thus overcomes the intrinsic short analysis time of hyperpolarized probes. Versatility of the platform is demonstrated by applying it to three types of hyperpolarized chemical probes: one each for sensing calcium ions, reactive oxygen species (hydrogen peroxide) and enzyme activity (carboxyl esterase). All of the designed probes achieve high sensitivity with rapid reactions and chemical shift changes, which are sufficient to allow sensitive and real-time monitoring of target molecules by ¹⁵N magnetic resonance.

Hyperpolarization of chemical nuclei is known to greatly increase sensitivity to characterization by magnetic resonance imaging. Here a new platform that allows for the design of a number of hyperpolarized probes for chemical sensing applications is demonstrated.

Cyanine-based fluorescent probe for highly selective detection of glutathione in cell cultures and live mouse tissues

Glutathione (GSH) plays a crucial role in human pathologies. Near-infrared fluorescence-based sensors capable of detecting intracellular GSH in vivo would be useful tools to understand the mechanisms of diseases. In this work, two cyanine-based fluorescent probes, 1 and 2, containing sulfonamide groups were prepared. Evaluation of the fluorescence changes displayed by probe 1, which contains a 2,4-dinitrobenzenesulfonamide group, shows that it is cell-membrane-permeable and can selectively detect thiols such as GSH, cysteine (Cys), and homocysteine (Hcy) in living cells. The response of 1 to thiols can be reversed by treatment with N-methylmaleimide (NMM). Probe 2, which possesses a 5-(dimethylamino)naphthalenesulfonamide group, displays high selectivity for GSH over Cys and Hcy, and its response can be reversed using NMM. The potential biological utility of 2 was shown by its use in fluorescence imaging of GSH in living cells. Furthermore, probe 2 can determine changes in the intracellular levels of GSH modualated by H2O2. The properties of 2 enable its use in monitoring GSH in vivo in a mouse model. The results showed that intravenous injection of 2 into a mouse generates a dramatic image in which strong fluorescence is emitted from various tissues, including the liver, kidney, lung, and spleen. Importantly, 2 can be utilized to monitor the depletion of GSH in mouse tissue cells promoted by excessive administration of the painkiller acetaminophen. The combined results coming from this effort suggest that the new probe will serve as an efficient tool for detecting cellular GSH in animals.

Finding the right (bioorthogonal) chemistry

Bioorthogonal chemistries can be used to tag diverse classes of biomolecules in cells and other complex environments. With over 20 unique transformations now available, though, selecting an appropriate reaction for a given experiment is challenging. In this article, we compare and contrast the most common classes of bioorthogonal chemistries and provide a framework for matching the reactions with downstream applications. We also discuss ongoing efforts to identify novel biocompatible reactions and methods to control their reactivity. The continued expansion of the bioorthogonal toolkit will provide new insights into biomolecule networks and functions and thus refine our understanding of living systems.

An azide-modified nucleoside for metabolic labeling of DNA

Metabolic incorporation of azido nucleoside analogues into living cells can enable sensitive detection of DNA replication through copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) and strain-promoted azide-alkyne cycloaddition (SPAAC) "click" reactions. One major limitation to this approach is the poor chemical stability of nucleoside derivatives containing an aryl azide group. For example, 5-azido-2'-deoxyuridine (AdU) exhibits a 4 h half-life in water, and it gives little or no detectable labeling of cellular DNA. In contrast, the benzylic azide 5-(azidomethyl)-2'-deoxyuridine (AmdU) is stable in solution at 37 °C, and it gives robust labeling of cellular DNA upon addition of fluorescent alkyne derivatives. In addition to providing the first examples of metabolic incorporation into and imaging of azide groups in cellular DNA, these results highlight the general importance of assessing azide group stability in bioorthogonal chemical reporter strategies.

Tracking the subcellular fate of 20(s)-hydroxycholesterol with click chemistry reveals a transport pathway to the Golgi

Oxysterols, oxidized metabolites of cholesterol, are endogenous small molecules that regulate lipid metabolism, immune function, and developmental signaling. Although the cell biology of cholesterol has been intensively studied, fundamental questions about oxysterols, such as their subcellular distribution and trafficking pathways, remain unanswered. We have therefore developed a useful method to image intracellular 20(S)-hydroxycholesterol with both high sensitivity and spatial resolution using click chemistry and fluorescence microscopy. The metabolic labeling of cells with an alkynyl derivative of 20(S)-hydroxycholesterol has allowed us to directly visualize this oxysterol by attaching an azide fluorophore through cyclo-addition. Unexpectedly, we found that this oxysterol selectively accumulates in the Golgi membrane using a pathway that is sensitive to ATP levels, temperature, and lysosome function. Although previous models have proposed nonvesicular pathways for the rapid equilibration of oxysterols between membranes, direct imaging of oxysterols suggests that a vesicular pathway is responsible for differential accumulation of oxysterols in organelle membranes. More broadly, clickable alkynyl sterols may represent useful tools for sterol cell biology, both to investigate the functions of these important lipids and to decipher the pathways that determine their cellular itineraries.

Glycotherapy: new advances inspire a reemergence of glycans in medicine

The beginning of the 20(th) century marked the dawn of modern medicine with glycan-based therapies at the forefront. However, glycans quickly became overshadowed as DNA- and protein-focused treatments became readily accessible. The recent development of new tools and techniques to study and produce structurally defined carbohydrates has spurred renewed interest in the therapeutic applications of glycans. This review focuses on advances within the past decade that are bringing glycan-based treatments back to the forefront of medicine and the technologies that are driving these efforts. These include the use of glycans themselves as therapeutic molecules as well as engineering protein and cell surface glycans to suit clinical applications. Glycan therapeutics offer a rich and promising frontier for developments in the academic, biopharmaceutical, and medical fields.

Dual-sugar imaging using isonitrile and azido-based click chemistries

We report the first account of metabolically labelling N-acetylglucosamine, in conjunction with either N-acetylgalactosamine or N-acetylmannosamine using a combination of isonitrile- and azide-based chemistries. With the appropriately labelled fluorescent probe molecules, that react with either the azido or isonitrile groups, the method enabled co-visualisation of cancer cell glycoproteins.

Exterior design: strategies for redecorating the bacterial surface with small molecules

Recombinant techniques for expressing heterologous proteins and sugars on the surface of bacteria have been known since the 1980s, and have proven useful in a variety of settings from biocatalysis to vaccinology. The past decade has also seen the emergence of novel methods that allow modification of bacterial surfaces with small non-biological compounds. Such technologies enable researchers to harness the unique properties of synthetic materials on a live bacterial platform, opening the door to an exciting new set of applications. Here we review strategies for bacterial surface display and describe how they have been applied thus far. We believe that chemical surface display holds great potential for advancing research in basic bacteriology and applied fields of biotechnology and biomedicine.

Strategy for dual-analyte luciferin imaging: in vivo bioluminescence detection of hydrogen peroxide and caspase activity in a murine model of acute inflammation

In vivo molecular imaging holds promise for understanding the underlying mechanisms of health, injury, aging, and disease, as it can detect distinct biochemical processes such as enzymatic activity, reactive small-molecule fluxes, or post-translational modifications. Current imaging techniques often detect only a single biochemical process, but, within whole organisms, multiple types of biochemical events contribute to physiological and pathological phenotypes. In this report, we present a general strategy for dual-analyte detection in living animals that employs in situ formation of firefly luciferin from two complementary caged precursors that can be unmasked by different biochemical processes. To establish this approach, we have developed Peroxy Caged Luciferin-2 (PCL-2), a H(2)O(2)-responsive boronic acid probe that releases 6-hydroxy-2-cyanobenzothiazole (HCBT) upon reacting with this reactive oxygen species, as well as a peptide-based probe, z-Ile-Glu-ThrAsp-D-Cys (IETDC), which releases D-cysteine in the presence of active caspase 8. Once released, HCBT and D-cysteine form firefly luciferin in situ, giving rise to a bioluminescent signal if and only if both chemical triggers proceed. This system thus constitutes an AND-type molecular logic gate that reports on the simultaneous presence of H(2)O(2) and caspase 8 activity. Using these probes, chemoselective imaging of either H(2)O(2) or caspase 8 activity was performed in vitro and in vivo. Moreover, concomitant use of PCL-2 and IETDC in vivo establishes a concurrent increase in both H(2)O(2) and caspase 8 activity during acute inflammation in living mice. Taken together, this method offers a potentially powerful new chemical tool for studying simultaneous oxidative stress and inflammation processes in living animals during injury, aging, and disease, as well as a versatile approach for concurrent monitoring of multiple analytes using luciferin-based bioluminescence imaging technologies.

Peptide-based Targeting of Fluorescent Zinc Sensors to the Plasma Membrane of Live Cells

Combining fluorescent zinc sensors with the facile syntheses and biological targeting capabilities of peptides, we created green- and blue-emitting probes that, (i) are readily prepared on the solid-phase, (ii) retain the photophysical and zinc-binding properties of the parent sensor, and (iii) can be directed to the extracellular side of plasma membranes in live cells for detection of mobile zinc.