8-Hydroxyquinoline Schiff-base Compounds as Antioxidants and Modulators of Copper-Mediated Aβ Peptide Aggregation

One of the hallmarks of Alzheimer\u27s disease (AD) in the brain are amyloid-β (Aβ) plaques, and metal ions such as copper(II) and zinc(II) have been shown to play a role in the aggregation and toxicity of the Aβ peptide, the major constituent of these extracellular aggregates. Metal binding agents can promote the disaggregation of Aβ plaques, and have shown promise as AD therapeutics. Herein, we describe the syntheses and characterization of an acetohydrazone (8-H2QH), a thiosemicarbazone (8-H2QT), and a semicarbazone (8-H2QS) derived from 8-hydroxyquinoline. The three compounds are shown to be neutral at pH 7.4, and are potent antioxidants as measured by a Trolox Equivalent Antioxidant Capacity (TEAC) assay. The ligands form complexes with CuII, 8-H2QT in a 1:1 metal:ligand ratio, and 8-H2QH and 8-H2QS in a 1:2 metal:ligand ratio. A preliminary aggregation inhibition assay using the Aβ1–40 peptide showed that 8-H2QS and 8-H2QH inhibit peptide aggregation in the presence of CuII. Native gel electrophoresis/Western blot and TEM images were obtained to give a more detailed picture of the extent and pathways of Aβ aggregation using the more neurotoxic Aβ1 −42 in the presence and absence of CuII, 8-H2QH, 8-H2QS and the drug candidate PBT2. An increase in the formation of oligomeric species is evident in the presence of CuII. However, in the presence of ligands and CuII, the results match those for the peptide alone, suggesting that the ligands function by sequestering CuII and limiting oligomer formation in this assay

Tuning the Emission Properties of 2,1,3‐Benzothiadiazoles via Regioselective Substitution

The 2,1,3-benzothiadiazole (BTD) unit is a prominent building block commonly used in various research areas such as optoelectronics and bioimaging. Despite its great versatility, the development of strategies to elaborate BTD has been largely neglected, including exploring its reactivity and understanding how regioselective functionalization can be used to tune the fluorescence emission. Previous focus has primarily been on C4- or C4,C7-substitutions. Here, a series of unsymmetrical mono – and disubstituted BTDs was synthesized and characterized for their photophysical properties. The reaction scope includes all six possible substituent patterns on the BTD benzoid ring (C4-, C5-, C4,C5-, C4,C6-, C4,C7- and C5,C6-substitution), which comprise arrangements that previously been synthetically challenging to access. By introducing a methoxy and/or a phenyl group we demonstrate that the emissive behavior of BTD derivatives strongly depends on the position of the substituent (s). We show that regioselective substitution on BTD can engender long-lived fluorescence and circumvent strong fluorescence quenching in polar protic solvents, which is a limitation of many previously described BTD derivatives

Specific Imaging of Intracellular Lipid Droplets Using a Benzothiadiazole Derivative with Solvatochromic Properties

Altered lipid metabolism and extensive lipid storage in cells have been associated with various medical disorders, including cancer. The development of fluorescent probes that specifically accumulate in lipid deposits is therefore of great interest in order to study pathological processes that are linked to dysregulated lipogenesis. In the present study, we present a small fluorescent benzothiadiazole dye that specifically stains lipid droplets in living and fixated cells. The photophysical characterization of the probe revealed strong solvatochromic behavior, large Stokes shifts, and high fluorescent quantum yields in hydrophobic solvents. In addition, the fluorophore exhibits a nontoxic profile and a high signal-to-noise ratio in cells (i.e., lipid droplets vs cytosol), which make it an excellent candidate for studying lipid biology using confocal fluorescent microscopy.Funding Agencies|Swedish Research Council [350-2012-239]; Swedish Foundation of Strategic Research [ICA14-0018]; Wenner-Gren Foundation</p

Prevalent Bioimaging Scaffolds : Synthesis, Photophysical Properties and Applications

Coumarin, xanthene, BODIPY, and cyanine are four of the most prevalent bioimaging scaffolds in chemical biology. In this review, we summarize the synthesis, structure-photophysical properties relationships, and imaging applications of fluorophores based on these units. We review the advantages and disadvantages of each scaffold in bioimaging as well as the practical issues to consider when using small organic compounds in biological systems. This minireview describes the syntheses, structure-photophysical properties relationship, and bioimaging applications of fluorescent coumarins, xanthenes, BODIPYs, and cyanines. Advantages and disadvantages of the emissive units are also discussed, as are general practical issues that need to be considered when using fluorophores in a biological setting

Synthesis and evaluation of benzothiazole-triazole and benzothiadiazole-triazole scaffolds as potential molecular probes for amyloid-β aggregation.

Small-molecule ligands that bind to misfolded protein aggregates are essential tools for the study and detection of pathological hallmarks in neurodegenerative disorders, such as Alzheimer's disease (AD). In the present study, three compounds (one benzothiazole-triazole, L1, and two benzothiadiazole-triazoles, L2 and L3) were synthesized via a modular approach (azide–alkyne cycloaddition) and evaluated as potential ligands for amyloid-β (Aβ) aggregates. The binding to amyloid-like fibrils, generated from recombinant Aβ1–42, were studied and the binding specificity to amyloid deposits was evaluated in brain sections from transgenic mice with AD pathology. All three derivatives showed significant reduced emission in the presence of recombinant Aβ1–42 amyloid fibrils. In addition, the observed binding to Aβ deposits in tissue sections suggests that the benzothiazole-triazole and benzothiadiazole-triazole structures are promising molecular scaffolds that can be modified for binding to specific protein aggregates. [ABSTRACT FROM AUTHOR

Photoinduced ring‐opening and phototoxicity of an indolin‐3‐one derivative

The study of a fluorescent indolin-3-one derivative is reported that, as opposed to its previously described congeners, selectively undergoes photoactivated ring-opening in apolar solvents. The excited state involved in this photoisomerization was partially deactivated by the formation of singlet oxygen. Cell studies revealed lipid droplet accumulation and efficient light-induced cytotoxicity

Significant off-target effects of postulated IGF-1R inhibitor picropodophyllin (PPP) in vitro

The insulin-like growth factor-1 (IGF-1) and its receptors play an important role in the transformation and progression of several malignancies. Several inhibitors of this pathway have been developed and evaluated in clinical trials, a majority of which with discouraging results and several drug candidates have been abandoned. The cyclolignan picropodophyllin (PPP) has been described as a potent and selective inhibitor of IGF-1R, and is currently investigated in clinical trials from which early reports suggest low toxicity and signs of clinical activity. In this report the cytotoxic activity of PPP and sets of standard agents were examined, compared and put in relation to the phosphorylation and expression levels of IGF-1R. The activity of PPP was unrelated to presence or spontaneous phosphorylation of IGF-1R, but correlated in all systems to activity of tubulin inhibitors. This led us to hypothesize that PPP’s effects in these cells were related to tubulin interaction rather than IGF-1R inhibition. Indeed PPP could destabilize microtubule assembly at concentrations needed to induce cytotoxicity cells, and at concentrations achievable in patients. Interestingly, PPP did also inhibit downstream signaling from tyrosine kinase receptors, including the serine/threonine kinase Akt. We could demonstrate that this surprising activity, also reported in previous reports on PPP but then attributed to IGF-1R inhibition, was associated with downregulation of the EGF receptor (EGFR). In summary, this study challenges previous reports on the cyclolignan PPP as an IGF-1R tyrosine kinase inhibitor and instead suggests that PPP targets, directly or indirectly, the microtubule network

Photophysical characterization and fluorescence cell imaging applications of 4-N-substituted benzothiadiazoles

In this work, a series of fluorescent 2,1,3-benzothiadiazole derivatives with various N-substituents in the 4- position was synthesized and photophysically characterized in various solvents. Three compounds emerged as excellent fluorescent probes for imaging lipid droplets in cancer cells. A correlation between their high lipophilicity and lipid droplet specificity could be found, with log P ≥ 4 being characteristic for lipid droplet accumulation

Indolylbenzothiadiazoles as highly tunable fluorophores for imaging lipid droplet accumulation in astrocytes and glioblastoma cells

We present an extensive photophysical study of a series of fluorescent indolylbenzothiadiazole derivatives and their ability to specifically image lipid droplets in astrocytes and glioblastoma cells. All compounds in the series displayed positive solvatochromism together with large Stokes shifts, and π-extended derivatives exhibited elevated brightness. It was shown that the fluorescence properties were highly tunable by varying the electronic character or size of the N-substituent on the indole motif. Three compounds proved capable as probes for detecting small quantities of lipid deposits in healthy and cancerous brain cells. In addition, all twelve compounds in the series were predicted to cross the blood–brain barrier, which raises the prospect for future in vivo studies for exploring the role of lipid droplets in the central nervous system

Photophysical Characteristics of Polarity-Sensitive and Lipid Droplet-Specific Phenylbenzothiadiazoles

In this study, we present a series of solvatochromic phenylbenzothiadiazoles that display dual emission from the locally excited (LE) and intramolecular charge transfer (ICT) excited states. The donor-acceptor derivatives are highly sensitive to polarity changes, which can be monitored by differences in emission efficiency, spectroscopic shifts and variations of the LE/ICT ratio. One of the compounds in the series, containing a thiomethyl substituent, emerged as an excellent blue emitting stain for intracellular lipid droplets, a biomarker for various types of cancer. In addition, a non-emissive nitro derivative becomes fluorescent upon bioreduction in hypoxic cancer cells and accumulates in lipid droplets with a high signal-to-background ratio