Versatile probes for the selective detection of vicinal-dithiol-containing proteins: design, synthesis, and application in living cells

Disruption of cyclin D1 degradation leads to the development of mantle cell lymphoma

Cyclin D1 has been recognized as an oncogene due to its abnormal upregulation in different types of cancers. Here, we demonstrated that cyclin D1 is SUMOylated, and we identified Itch as a specific E3 ligase recognizing SUMOylated cyclin D1 and mediating SUMO-induced ubiquitination and proteasome degradation of cyclin D1. We generated cyclin D1 mutant mice with mutations in the SUMOylation site, phosphorylation site, or both sites of cyclin D1, and found that double mutant mice developed a Mantle cell lymphoma (MCL)-like phenotype. We showed that arsenic trioxide (ATO) enhances cyclin D1 SUMOylation-mediated degradation through inhibition of cyclin D1 deSUMOylation enzymes, leading to MCL cell apoptosis. Treatment of severe combined immunodeficiency (SCID) mice grafted with MCL cells with ATO resulted in a significant reduction in tumor growth. In this study, we provide novel insights into the mechanisms of MCL tumor development and cyclin D1 regulation and discover a new strategy for MCL treatment.

Increasing the versatility of the biphenyl-fused-dioxacyclodecyne class of strained alkynes

Biphenyl-fused-dioxacyclodecynes are a promising class of strained alkyne for use in Cu-free 'click' reactions. In this paper, a series of functionalised derivatives of this class of reagent, containing fluorescent groups, are described. Studies aimed at understanding and increasing the reactivity of the alkynes are also presented, together with an investigation of the bioconjugation of the reagents with an azide-labelled protein.

Biarsenical fluorescent probes for multifunctional site-specific modification of proteins applicable in life sciences: an overview and future outlook

Fluorescent modification of proteins of interest (POI) in living cells is desired to study their behaviour and functions in their natural environment. In a perfect setting it should be easy to perform, inexpensive, efficient and site-selective. Although multiple chemical and biological methods have been developed, only a few of them are applicable for cellular studies thanks to their appropriate physical, chemical and biological characteristics. One such successful system is a tetracysteine tag/motif and its selective biarsenical binders (e.g. FlAsH and ReAsH). Since its discovery in 1998 by Tsien and co-workers, this method has been enhanced and revolutionized in terms of its efficiency, formed complex stability and breadth of application. Here, we overview the whole field of knowledge, while placing most emphasis on recent reports. We showcase the improvements of classical biarsenical probes with various optical properties as well as multifunctional molecules that add new characteristics to proteins. We also present the evolution of affinity tags and motifs of biarsenical probes demonstrating much more possibilities in cellular applications. We summarize protocols and reported observations so both beginners and advanced users of biarsenical probes can troubleshoot their experiments. We address the concerns regarding the safety of biarsenical probe application. We showcase examples in virology, studies on receptors or amyloid aggregation, where application of biarsenical probes allowed observations that previously were not possible. We provide a summary of current applications ranging from bioanalytical sciences to allosteric control of selected proteins. Finally, we present an outlook to encourage more researchers to use these magnificent probes.

An Activatable T1-Weighted MR Contrast Agent: A Noninvasive Tool for Tracking the Vicinal Thiol Motif of Thioredoxin in Live Cells

We have synthesized new magnetic resonance imaging (MRI) T₁ contrast agents (CA1 and CA2) that permit the activatable recognition of the cellular vicinal thiol motifs of the protein thioredoxin. The contrast agents showed MR relaxivities typical of gadolinium complexes with a single water molecule coordinated to a Gd³⁺ center (i.e., ~4.54 mM⁻¹s⁻¹) for both CA1 and CA2 at 60 MHz. The contrast agent CA1 showed a ~140% relaxivity enhancement in the presence of thioredoxin, a finding attributed to a reduction in the flexibility of the molecule after binding to thioredoxin. Support for this rationale, as opposed to one based on preferential binding, came from ¹H-¹⁵N-HSQC NMR spectral studies; these revealed that the binding affinities toward thioredoxin were almost the same for both CA1 and CA2. In the case of CA1, T₁-weighted phantom images of cancer cells (MCF-7, A549) could be generated based on the expression of thioredoxin. We further confirmed thioredoxin expression-dependent changes in the T₁-weighted contrast via knockdown of the expression of the thioredoxin using siRNA-transfected MCF-7 cells. The nontoxic nature of CA1, coupled with its relaxivity features, leads us to suggest that it constitutes a first-in-class MRI T₁ contrast agent that allows for the facile and noninvasive monitoring of vicinal thiol protein motif expression in live cells.

A β-allyl carbamate fluorescent probe for vicinal dithiol proteins

An unprecedented β-allyl carbamate fluorescent probe for vicinal dithiol proteins (VDPs) was developed. The favourable properties of the probe make it a useful tool for tracing the global changes of VDPs in living systems.

A bis-indole/carbazole based C5-curcuminoid fluorescent probe with large Stokes shift for selective detection of biothiols and application to live cell imaging

A series of heterocyclic C5-Curcuminoid (PJ1–PJ6) having large Stokes shift (Δλ= 104–173 nm) have been synthesized under the microwave irradiation andPJ1has been utilized for selective detection of thiols in A375 cells and apoptosis in AGS cells.

Recognition of Proteins by Metal Chelation-Based Fluorescent Probes in Cells

Fluorescent probes such as thiol-reactive and Ni2+-nitrilotriacetate (NTA) based probes provide a powerful toolbox for real-time visualization of a protein and a proteome in living cells. Herein, we first went through basic principles and applications of thiol-reactive based probes in protein imaging and recognition. We then summarize a family of metal-NTA based fluorescence probes in the visualization of His6-tagged protein and identification of metalloproteins at proteome-wide scale. The pros and cons of the probes, as well as ways to optimize them, are discussed.

Novel naphthalene-enoates: Design and anticancer activity through regulation cell autophagy

Eleven dihydroxy-2-(1-hydroxy-4-methylpent-3-enyl)naphthalene derivatives as anticancer agents through regulating cell autophagy were designed and synthesized. The anticancer activity results indicated that most compounds manifested obvious un-toxic effect on GES-1 and L-02 with IC₅₀ from 0.58 to 1.41 mM. Among them, (S,Z)-1-(5,8-dihydroxy-1,4-dioxo-1,4-dihydronaphthalen-2-yl)-4-methylpent-3-enyl 4-(3,4- dihydroisoquinolin-2(1 H)-yl)-4-oxobut-2-enoate (compound 4i) could induce cancer cells apoptosis. Further experiments showed that autophagy played an important role in the pro-apoptotic effect of this compound. Preliminary mechanism indicated that this compound could inhibit phosphoinositide 3-kinase/protein kinase B and the mammalian target of rapamycin (PI3K/AKT/mTOR) pathway by mediating apoptosis in an autophagy-dependent manner.

Carbohydrate-conjugated 4-(1,3,2-dithiarsolan-2-yl)aniline as a cytotoxic agent against colorectal cancer

Arsenic trioxide (As₂O₃) has been approved for the treatment of acute promyelocytic leukemia (APL); however, its use in the treatment of solid tumors is limited due to its pharmacokinetic properties. Organic arsenic compounds provide better options for pharmaceutical optimization. p-Aminophenyl arsenoxide (p-APAO), an organic arsenic compound, was found to interact with the promyelocytic leukemia–retinoic acid receptor alpha (PML–RARα) fusion protein in a similar manner to arsenic trioxide. Analogs of p-APAO such as 4-(1,3,2-dithiarsolan-2-yl)aniline (p-APDTAs) were recently found to show improved cytotoxicity toward several solid tumor cell lines with lower toxicity to normal cells. Here, we synthesized a carbohydrate-conjugated 4-(1,3,2-dithiarsolan-2-yl)aniline (p-APDTAs) and showed that it exhibited reduced cytotoxicity to normal cells, suggesting a feasible approach to improve the therapeutic index of arsenic-containing compounds as chemotherapeutic agents.

We synthesized a carbohydrate-conjugated 4-(1,3,2-dithiarsolan-2-yl)aniline. It exhibited reduced cytotoxicity to normal cells, suggesting a feasible approach to improve the therapeutic index of arsenic-containing compounds as chemotherapeutic agents.

Metal-Free Synthesis of N-Aryl Amides using Organocatalytic Ring-Opening Aminolysis of Lactones

Catalytic ring-opening of bio-sourced non-strained lactones with aromatic amines can offer a straightforward, 100 % atom-economical, and sustainable pathway towards relevant N-aryl amide scaffolds. Herein, the first general, metal-free, and highly efficient N-aryl amide formation is reported from poorly reactive aromatic amines and non-strained lactones under mild operating conditions using an organic bicyclic guanidine catalyst. This protocol has high application potential as exemplified by the formal syntheses of drug-relevant molecules.

Reversible oxidation of vicinal-thiols motif in sarcoplasmic reticulum calcium regulatory proteins is involved in muscle fatigue mechanism

The mechanism underlying fatigue in skeletal muscle (SM) related to the redox-potential hypothesis, ranges from a direct effect of oxygen reactive species, to a number of other free radical intermediates targeting specific amino acids in the Ca(2+)-regulatory proteins of the sarcoplasmic reticulum (SR). In the present study, we investigate the selective oxidation/reduction of the protein motif Cys-(Xn=2-6)-Cys, known as a vicinal thiol group (VTG), present in the SR Ca(2+)-ATPase (SERCA) and in the Ca(2+)-channel ryanodine receptor (RyR) which are modified during muscle fatigue in SM. Selective oxidation of VTG with phenyl arsine oxide (PAO) increases fatigue in rat isolated SM and fatigue is prevented when muscle is previously incubated with a VTG selective reducing agent, 2,3-dimercaptopropanol (British anti-Lewisite (BAL)). In isolated SR membranes, PAO [<0.1mM] modifies SERCA conformation and inhibits ATPase activity but does not affect Ca(2+)-release. However, PAO at [>0.1mM] inhibits SERCA and RyR activities in a reversible manner by selectively reducing them. Interestingly, as observed by differential scanning calorimetry, the conformation of SERCA from fatigued muscle changed in a similar manner as when SERCA VTG where oxidized. The addition of BAL to fatigued muscle restored the structural conformation and activity of SERCA with full recovery of muscle force production after fatigue. We conclude that VTG reversible oxidation of SR Ca(2+) regulatory proteins are involved in muscle contraction/relaxation and are a molecular mechanism to be considered for muscle fatigue.

Naphthalimide Scaffold Provides Versatile Platform for Selective Thiol Sensing and Protein Labeling

Reversible thiol modifications are fundamental of cellular redox regulation. Specific thiol detection, including thiol sensing and protein thiols labeling, is critical to study such modifications. We reported the discovery of 4-methylsulfonyl-N-n-butyl-1,8-naphthalimide (MSBN), a highly selective fluorogenic probe for thiols based on the 1,8-naphthalimide scaffold. Thiols react with MSBN nearly quantitatively via nucleophilic aromatic substitution to replace the methylsulfonyl group and restore the quenched fluorescence (>100-fold increase). MSBN was employed to selectively image thiols in live cells and specifically label protein thiols with a turn-on signal to determine diverse reversible protein thiol modifications. In addition, we introduced a bulky group into the MSBN as a mass tag to create a probe MSBN-TPP, which readily discriminates the reduced thioredoxin from the oxidized one. The specific reaction of MSBN with thiols and the easy manipulation of the naphthalimide unit enable MSBN a versatile scaffold in developing novel probes for thiol-based protein bioconjugation and studying various thiol modifications.

A mitochondria-targeted ratiometric two-photon fluorescent probe for biological zinc ions detection

A mitochondria-targeted ratiometric two-photon fluorescent probe (Mito-MPVQ) for biological zinc ions detection was developed based on quinolone platform. Mito-MPVQ showed large red shifts (68 nm) and selective ratiometric signal upon Zn(2+) binding. The ratio of emission intensity (I488 nm/I420 nm) increases dramatically from 0.45 to 3.79 (ca. 8-fold). NMR titration and theoretical calculation confirmed the binding of Mito-MPVQ and Zn(2+). Mito-MPVQ also exhibited large two-photon absorption cross sections (150 GM) at nearly 720 nm and insensitivity to pH within the biologically relevant pH range. Cell imaging indicated that Mito-MPVQ could efficiently located in mitochondria and monitor mitochondrial Zn(2+) under two-photon excitation with low cytotoxicity.

Development of a red fluorescent light-up probe for highly selective and sensitive detection of vicinal dithiol-containing proteins in living cells

Vicinal dithiol-containing proteins (VDPs) play a key role in cellular redox homeostasis and are responsible for many diseases. Here, we develop a red fluorescent light-up probe FAsH for the highly selective and sensitive detection of VDPs using the environment-sensitive 2-(4-dimethylaminophenyl)-4-(2-carboxyphenyl)-7-diethylamino-1-benzopyrylium (F1) as the fluorescent reporter and cyclic dithiaarsane as the targeting unit. FAsH is almost nonfluorescent in aqueous solution. However, it exhibits intense fluorescence emission upon binding to reduced bovine serum albumin (rBSA, selected as the model protein). The fluorescence intensity of FAsH is directly proportional to the concentration of rBSA over the range of 0.06-0.9 μM, with a detection limit (3δ) of 0.015 μM. Importantly, the fast kinetics of binding between FAsH and VDPs (∼2.5 min) enables the dynamic tracing of VDPs in biological systems. Preliminary experiments show that FAsH can be used for the no-wash imaging of endogenous VDPs in living cells. In addition, our study shows that F1 presents both high environment-sensitivity and good fluorescence properties, and is promising for the development of no-wash fluorescent light-up probes for target-specific proteins in living cells.

Acid-Activatable Michael-Type Fluorescent Probes for Thiols and for Labeling Lysosomes in Live Cells

A Michael addition is usually taken as a base-catalyzed reaction. Most fluorescent probes have been designed to detect thiols in slightly alkaline solutions (pH 7-9). The sensing reactions of almost all Michael-type fluorescent probes for thiols are faster in a high pH solution than in a low pH solution. In this work, we synthesized a series of 7-substituted 2-(quinolin-2-ylmethylene)malonic acids (QMAs, substituents: NEt2, OH, H, Cl, or NO2) and their ethyl esters (QMEs) as Michael-type fluorescent probes for thiols. The sensing reactions of QMAs and QMEs occur in distinct pH ranges, pH < 7 for QMAs and pH > 7 for QMEs. On the basis of experimental and theoretic studies, we have clarified the distinct pH effects on the sensing reactivity between QMAs and QMEs and demonstrated that two QMAs (NEt2, OH) are highly sensitive and selective fluorescent probes for thiols in acidic solutions (pH < 7) and promising dyes that can label lysosomes in live cells.

Therapeutic and analytical applications of arsenic binding to proteins

Knowledge of arsenic binding to proteins advances the development of bioanalytical techniques and therapeutic drugs.

A two-photon fluorescent probe for detecting endogenous hypochlorite in living cells

A highly selective two-photon fluorescent probe (HQ) for endogenous hypochlorite (ClO⁻) was developed.

A thiol–thiosulfonate reaction providing a novel strategy for turn-on thiol sensing

A thiosulfonate scaffold was applied to design selective and turn-on thiol probes for the first time.

Site-specific protection and dual labeling of human epidermal growth factor (hEGF) for targeting, imaging, and cargo delivery

Well-defined human epidermal growth factor (hEGF) constructs featuring selectively addressable labels are urgently needed to address outstanding questions regarding hEGF biology. A protein-engineering approach was developed to provide access to hEGF constructs that carry two cysteine-based site-specific orthogonal labeling sites in multi-milligram quantities. Also, a site-selective (de)protection and labeling approach was devised, which allows selective modification of these hEGF constructs. The hEGF, featuring three native disulfide bonds, was expressed featuring additional sulfhydryl groups, in the form of cysteine residues, as orthogonal ligation sites at both the N and C termini. Temporary protection of the N-terminal cysteine unit, in the form of a thiazolidine ring, avoids interference with protein folding and enables sequential labeling in conjunction with the cysteine residue at the C terminus. Based on thus-generated hEGF constructs, sequential and site-specific labeling with a variety of molecular probes could be demonstrated, thus leading to a biological fully functional hEGF with specifically incorporated fluorophores or protein cargo and native cellular targeting and uptake profiles. Thus, this novel strategy provides a robust entry to high-yielding access of hEGF and rapid and easy site-specific and multifunctional dual labeling of this growth factor.