A pH-Responsive Topological Switch Based on a DNA Quadruplex-Duplex Hybrid.

A guanine-rich oligonucleotide based on a human telomeric sequence but with the first three-nucleotide intervening stretch replaced by a putative 15-nucleotide hairpin-forming sequence shows a pH-dependent folding into different quadruplex-duplex hybrids in a potassium containing buffer. At slightly acidic pH, the quadruplex domain adopts a chair-type conformation. Upon increasing the pH, a transition with a midpoint close to neutral pH to a major and minor (3+1) hybrid topology with either a coaxially stacked or orthogonally oriented duplex stem-loop occurs. NMR-derived high-resolution structures reveal that an adenine protonation is prerequisite for the formation of a non-canonical base quartet, capping the outer G-tetrad at the quadruplex-duplex interface and stabilizing the antiparallel chair conformation in an acidic environment. Being directly associated with interactions at the quadruplex-duplex interface, this unique pH-dependent topological transition is fully reversible. Coupled with a conformation-sensitive optical readout demonstrated as a proof of concept using the fluorescent dye thiazole orange, the present quadruplex-duplex hybrid architecture represents a potentially valuable pH-sensing system responsive in a physiological pH range of 7±1.

Guiding the folding of G-quadruplexes through loop residue interactions.

A G-rich sequence was designed to allow folding into either a stable parallel or hybrid-type topology. With the parent sequence featuring coexisting species, various related sequences with single and double mutations and with a shortened central propeller loop affected the topological equilibrium. Two simple modifications, likewise introduced separately to all sequences, were employed to lock folds into one of the topologies without noticeable structural alterations. The unique combination of sequence mutations, high-resolution NMR structural information, and the thermodynamic stability for both topological competitors identified critical loop residue interactions. In contrast to first loop residues, which are mostly disordered and exposed to solvent in both propeller and lateral loops bridging a narrow groove, the last loop residue in a lateral three-nucleotide loop is engaged in stabilizing stacking interactions. The propensity of single-nucleotide loops to favor all-parallel topologies by enforcing a propeller-like conformation of an additional longer loop is shown to result from their preference in linking two outer tetrads of the same tetrad polarity. Taken together, the present studies contribute to a better structural and thermodynamic understanding of delicate loop interactions in genomic and artificially designed quadruplexes, e.g. when employed as therapeutics or in other biotechnological applications.

Showcasing Different G-Quadruplex Folds of a G-Rich Sequence: Between Rule-Based Prediction and Butterfly Effect.

In better understanding the interactions of G-quadruplexes in a cellular or noncellular environment, a reliable sequence-based prediction of their three-dimensional fold would be extremely useful, yet is often limited by their remarkable structural diversity. A G-rich sequence related to a promoter sequence of the PDGFR-ß nuclease hypersensitivity element (NHE) comprises a G3-G3-G2-G4-G3 pattern of five G-runs with two to four G residues. Although the predominant formation of three-layered canonical G-quadruplexes with uninterrupted G-columns can be expected, minimal base substitutions in a non-G-tract domain were shown to guide folding into either a basket-type antiparallel quadruplex, a parallel-stranded quadruplex with an interrupted G-column, a quadruplex with a V-shaped loop, or a (3+1) hybrid quadruplex. A 3D NMR structure for each of the different folds was determined. Supported by thermodynamic profiling on additional sequence variants, formed topologies were rationalized by the identification and assessment of specific critical interactions of loop and overhang residues, giving valuable insights into their contribution to favor a particular conformer. The variability of such tertiary interactions, together with only small differences in quadruplex free energies, emphasizes current limits for a reliable sequence-dependent prediction of favored topologies from sequences with multiple irregularly positioned G-tracts.

Thermodynamic Stability of G-Quadruplexes: Impact of Sequence and Environment.

G-quadruplexes have attracted growing interest in recent years due to their occurrence in vivo and their possible biological functions. In addition to being promising targets for drug design, these four-stranded nucleic acid structures have also been recognized as versatile tools for various technological applications. Whereas a large number of studies have yielded insight into their remarkable structural diversity, our current knowledge on G-quadruplex stabilities as a function of sequence and environmental factors only gradually emerges with an expanding collection of thermodynamic data. This minireview provides an overview of general rules that may be used to better evaluate quadruplex thermodynamic stabilities but also discusses present challenges in predicting most stable folds for a given sequence and environment.

Expanding the Topological Landscape by a G-Column Flip of a Parallel G-Quadruplex.

Canonical G-quadruplexes can adopt a variety of different topologies depending on the arrangement of propeller, lateral, or diagonal loops connecting the four G-columns. A novel intramolecular G-quadruplex structure is derived through inversion of the last G-tract of a three-layered parallel fold, associated with the transition of a single propeller into a lateral loop. The resulting (3+1) hybrid fold features three syn⋅anti⋅anti⋅anti G-tetrads with a 3'-terminal all-syn G-column. Although the ability of forming a duplex stem-loop between G-tracts seems beneficial for a propeller-to-lateral loop rearrangement, unmodified G-rich sequences resist folding into the new (3+1) topology. However, refolding can be driven by the incorporation of syn-favoring guanosine analogues into positions of the fourth G-stretch. The presented hybrid-type G-quadruplex structure as determined by NMR spectroscopy may provide for an additional scaffold in quadruplex-based technologies.

A Thermodynamic Perspective on Potential G-Quadruplex Structures as Silencer Elements in the MYC Promoter.

Multiple G-tracts within the promoter region of the c-myc oncogene may fold into various G-quadruplexes with the recruitment of different tracts and guanosine residues for the G-core assembly. Thermodynamic profiles for the folding of wild-type and representative truncated as well as mutated sequences were extracted by comprehensive DSC experiments. The unique G-quadruplex involving consecutive G-tracts II-V with formation of two one-nucleotide and one central two-nucleotide propeller loop, previously proposed to be the biologically most relevant species, was found to be the most stable fold in terms of its Gibbs free energy of formation at ambient temperatures. Its stability derives from its short propeller loops but also from the favorable type of loop residues. Whereas quadruplex folds with long propeller loops are significantly disfavored, a snap-back loop structure formed by incorporating a 3'-terminal guanosine into the empty position of a tetrad seems highly competitive based on its thermodynamic stability. However, its destabilization by extending the 3'-terminus questions the significance of such a species under in vivo conditions.

Enhancement of ABA Sensitivity Through Conditional Expression of the ARF10 Gene in Brassica juncea Reveals Fertile Plants with Tolerance Against Alternaria brassicicola.

Concomitant increase of auxin-responsive factors ARF16 and ARF17, along with enhanced expression of ARF10 in resistant Sinapis alba compared with that in susceptible Brassica juncea upon challenge with Alternaria brassicicola, revealed that abscisic acid (ABA)-auxin crosstalk is a critical factor for resistance response. Here, we induced the ABA response through conditional expression of ARF10 in B. juncea using the A. brassicicola-inducible GH3.3 promoter. Induced ABA sensitivity caused by conditional expression of ARF10 in transgenic B. juncea resulted in tolerance against A. brassicicola and led to enhanced expression of several ABA-responsive genes without affecting the auxin biosynthetic gene expression. Compared with ABI3 and ABI4, ABI5 showed maximum upregulation in the most tolerant transgenic lines upon pathogen challenge. Moreover, elevated expression of ARF10 by different means revealed a direct correlation between ARF10 expression and the induction of ABI5 protein in B. juncea. Through in vitro DNA-protein experiments and chromosome immunoprecipitation using the ARF10 antibody, we demonstrated that ARF10 interacts with the auxin-responsive elements of the ABI5 promoter. This suggests that ARF10 may function as a modulator of ABI5 to induce ABA sensitivity and mediate the resistance response against A. brassicicola.

Spatial Position Regulates Power of Tryptophan: Discovery of a Major-Groove-Specific Nuclear-Localizing, Cell-Penetrating Tetrapeptide.

Identification of key amino acids is required for development of efficient cell-penetrating peptides (CPPs) and has tremendous implications in medicine. Extensive research work has enlightened us about the importance of two amino acids, arginine and tryptophan, in cell penetration. Here, we present a top-down approach to show how spatial positions of two tryptophans regulate the cellular entry and nuclear localization. This enables us to develop short, non-toxic tetrapeptides with excellent potential for cell penetration and nuclear localization. Among them, Glu-Thr-Trp-Trp (ETWW) emerges as the most promising. Results suggest that it enters into cancer cells following an endocytic pathway and binds at the major groove of nuclear DNA, where successive tryptophan plays major role. We subsequently show that it is not a P-glycoprotein substrate and is non-toxic to PC12-derived neurons, suggesting its excellent potential as a CPP. Furthermore, its potential as a CPP is validated in multi-cellular 3D cell culture (spheroid) and in in vivo mice model. This study provides major fundamental insights about the positional importance of tryptophan and opens new avenues toward the development of next-generation CPPs and major-groove-specific anticancer drugs.

Parkia javanica Extract Induces Apoptosis in S-180 Cells via the Intrinsic Pathway of Apoptosis.

Parkia javanica is a leguminous tree, various parts of which are used as food and folklore medicine by the ethnic groups of northeastern India. The present study investigates the in vitro and in vivo anticancer effect of aqueous methanol extract of P. javanica fruit (PJE). HPLC analysis was done to establish the fingerprint chromatogram of PJE and its in vitro radical scavenging activity was measured. PJE caused significant cytotoxicity in sarcoma-180 (S-180), A549, AGS, and MDA-MB435S cancer cells in vitro. Exploration of the mechanistic details in S-180 cells suggested that the reduced cell viability was mediated by induction of apoptosis. Increased expression of proapoptotic proteins such as p53, p21, Bax/Bcl2, cytochrome c (Cyt c), caspase 9, and cleaved poly(ADP-ribose) polymerase, and decrease in proliferative and antiapoptotic markers (Ki-67, Proliferating Cell Nuclear Antigen [PCNA], Bcl-2) validated the anticancer effect of PJE. A decline in the relative fluorescence emission upon staining S-180 cells with Rhodamine 123 (Rh 123), enhanced expression of cytosolic Cyt c and mitochondrial Bax, and inhibition of apoptosis in the presence of caspase-9 inhibitor in PJE-treated cells indicated intrinsic pathway of apoptosis. Liver function test and hepatic antioxidant enzymes demonstrated non-toxicity of PJE. Finally, the detection of PJE in sera by HPLC confirmed its bioavailability.

Plant alkaloid chelerythrine induced aggregation of human telomere sequence--a unique mode of association between a small molecule and a quadruplex.

Small molecules that interact with G-quadruplex structures formed by the human telomeric region and stabilize them have the potential to evolve as anticancer therapeutic agents. Herein we report the interaction of a putative anticancer agent from a plant source, chelerythrine, with the human telomeric DNA sequence. It has telomerase inhibitory potential as demonstrated from telomerase repeat amplification assay in cancer cell line extract. We have attributed this to the quadruplex binding potential of the molecule and characterized the molecular details of the interaction by means of optical spectroscopy such as absorbance and circular dichroism and calorimetric techniques such as isothermal titration calorimetry and differential scanning calorimetry. The results show that chelerythrine binds with micromolar dissociation constant and 2:1 binding stoichiometry to the human telomeric DNA sequence. Chelerythrine association stabilizes the G-quadruplex. Nuclear magnetic resonance spectroscopy ((1)H and (31)P) shows that chelerythrine binds to both G-quartet and phosphate backbone of the quadruplex leading to quadruplex aggregation. Molecular dynamics simulation studies support the above inferences and provide further insight into the mechanism of ligand binding. The specificity toward quartet binding for chelerythrine is higher compared to that of groove binding. MM-PBSA calculation mines out the energy penalty for quartet binding to be -4.7 kcal/mol, whereas that of the groove binding is -1.7 kcal/mol. We propose that the first chelerythrine molecule binds to the quartet followed by a second molecule which binds to the groove. This second molecule might bring about aggregation of the quadruplex structure which is evident from the results of nuclear magnetic resonance.

Antitumorigenic potential of linalool is accompanied by modulation of oxidative stress: an in vivo study in sarcoma-180 solid tumor model.

Coriander, used as a common food seasoning, contains linalool as the main constituent of its essential oil. In this study, we tested the effect of linalool vis-à-vis that of a conventional chemotherapeutic drug, cyclophosphamide, against solid S-180 tumor-bearing Swiss albino mice. Tumor volume, cell count, cell cycle phase distribution, apoptosis, and proliferation markers indicate that linalool has potent antitumor activity. In vitro and in vivo data suggest that induction of oxidative stress might be responsible for the anticancer effect of linalool. However, interestingly, unlike cyclophosphamide, linalool did not induce myelosuppression or hepatotoxicity in mice as evident from bone marrow cell count, status of hepatic oxidative stress/antioxidant enzymes, and histopathology. Thus, linalool exerted prooxidant effect in tumor tissue and an antioxidant effect in liver. This is also supported by the expression of Nrf-2 and p21, which are considered to be important players in response to oxidative stress. Moreover, administration of linalool modulated the proliferation of spleen cells in tumor-bearing mice challenged with lipopolysaccharide. Finally, the detection of linalool in sera and tumor tissues by HPLC confirmed its bioavailability. In conclusion, linalool showed differential cytotoxicity towards tumor and normal cells in contrast to cyclophosphamide, which is uniformly toxic to both.

Impact of loop length and duplex extensions on the design of hybrid-type G-quadruplexes.

A G-rich core sequence G3-TCA-G3-T1,2-G3-T1,2-G3 can be designed to fold into a parallel or into two different (3+1) hybrid-type G-quadruplexes, among them an elusive topology with one lateral followed by two propeller loops. Favored folds can be rationalized based on the number of intervening thymidines and on additional complementary flanking sequences.

53BP1 interacts with the RNA primer from Okazaki fragments to support their processing during unperturbed DNA replication.

RNA-binding proteins (RBPs) are found at replication forks, but their direct interaction with DNA-embedded RNA species remains unexplored. Here, we report that p53-binding protein 1 (53BP1), involved in the DNA damage and replication stress response, is an RBP that directly interacts with Okazaki fragments in the absence of external stress. The recruitment of 53BP1 to nascent DNA shows susceptibility to in situ ribonuclease A treatment and is dependent on PRIM1, which synthesizes the RNA primer of Okazaki fragments. Conversely, depletion of FEN1, resulting in the accumulation of uncleaved RNA primers, increases 53BP1 levels at replication forks, suggesting that RNA primers contribute to the recruitment of 53BP1 at the lagging DNA strand. 53BP1 depletion induces an accumulation of S-phase poly(ADP-ribose), which constitutes a sensor of unligated Okazaki fragments. Collectively, our data indicate that 53BP1 is anchored at nascent DNA through its RNA-binding activity, highlighting the role of an RNA-protein interaction at replication forks.

Structural motifs and intramolecular interactions in non-canonical G-quadruplexes.

Guanine(G)-rich DNA or RNA sequences can assemble or intramolecularly fold into G-quadruplexes formed through the stacking of planar G·G·G·G tetrads in the presence of monovalent cations. These secondary nucleic acid structures have convincingly been shown to also exist within a cellular environment exerting important regulatory functions in physiological processes. For identifying nucleic acid segments prone to quadruplex formation, a putative quadruplex sequence motif encompassing closely spaced tracts of three or more guanosines is frequently employed for bioinformatic search algorithms. Depending on the number and type of intervening residues as well as on solution conditions, such sequences may fold into various canonical G4 topologies with continuous G-columns. On the other hand, a growing number of sequences capable of quadruplex formation feature G-deficient guanine tracts, escaping the conservative consensus motif. By folding into non-canonical quadruplex structures, they adopt unique topologies depending on their specific sequence context. These include G-columns with only two guanines, bulges, snapback loops, D- and V-shaped loops as well as interlocked structures. This review focuses on G-quadruplex species carrying such distinct structural motifs. It evaluates characteristic features of their non-conventional scaffold and highlights principles of stabilizing interactions that also allow for their folding into stable G-quadruplex structures.

Prion-derived tetrapeptide stabilizes thermolabile insulin via conformational trapping.

Unfolding followed by fibrillation of insulin even in the presence of various excipients grappled with restricted clinical application. Thus, there is an unmet need for better thermostable, nontoxic molecules to preserve bioactive insulin under varying physiochemical perturbations. In search of cross-amyloid inhibitors, prion-derived tetrapeptide library screening reveals a consensus V(X)YR motif for potential inhibition of insulin fibrillation. A tetrapeptide VYYR, isosequential to the ß2-strand of prion, effectively suppresses heat- and storage-induced insulin fibrillation and maintains insulin in a thermostable bioactive form conferring adequate glycemic control in mouse models of diabetes and impedes insulin amyloidoma formation. Besides elucidating the critical insulin-IS1 interaction (R4 of IS1 to the N24 insulin B-chain) by nuclear magnetic resonance spectroscopy, we further demonstrated non-canonical dimer-mediated conformational trapping mechanism for insulin stabilization. In this study, structural characterization and preclinical validation introduce a class of tetrapeptide toward developing thermostable therapeutically relevant insulin formulations.

Impact of a Snap-Back Loop on Stability and Ligand Binding to a Parallel G-Quadruplex.

Various genomic DNA sequences including a MYC promoter sequence are amenable to the formation of a G-quadruplex featuring a snap-back loop with the incorporation of a 3'-terminal guanine into the quadruplex core. To evaluate relative stabilities and ligand binding in more detail, optical, microcalorimetric, and NMR structural studies were performed on both a minimal mutant sequence Pu22T that exclusively folds into a snap-back loop quadruplex and a parallel MYC quadruplex proposed to be the most relevant fold of the MYC promoter in a cellular environment. Similar thermal stabilities for Pu22T and MYC suggest the coexistence of both quadruplexes when derived from a sequence able to fold into both topologies. Isothermal titration calorimetry indicates a mostly identical enthalpy-driven strong binding of an indoloquinoline ligand but with a reduced number of high-affinity binding sites in Pu22T in line with a novel modified FRET competitive melting assay. Corroborated by fluorescence titrations using 2-aminopurine as a fluorescent probe, NMR chemical shift footprints show binding of the ligand at the Pu22T 5'-outer tetrad with the formation of a binding pocket. On the other hand, steric restrictions due to the snap-back loop severely restrict ligand stacking on the 3'-outer tetrad of Pu22T.

A Small Molecule Impedes Insulin Fibrillation: Another New Role of Phenothiazine Derivatives.

Protein misfolding is interrelated to several diseases, including neurodegenerative diseases and type II diabetes. Misfolded/unfolded proteins produce soluble oligomers that accumulate into "amyloid plaques". Inhibition of amyloid-plaque formation by those misfolded/unfolded proteins will lead to the invention of new therapeutic approaches for amyloid-related diseases. Herein, methylene blue (MB), a well-defined drug against multiple diseases and disorders, is used to impede insulin fibrillation. In this study, we perform an array of in vitro experiments to monitor the effects of MB on the fibrillation of bovine insulin. Our results confirm that MB distresses the kinetics of insulin fibrillation by interacting with insulin in its monomeric form. A thioflavin T assay indicates that insulin fibrillation is interrupted upon the addition of MB. The same results are confirmed by circular dichroism, dynamic light scattering (DLS), and size-exclusion chromatography (SEC). According to the DLS data, the insulin fibrils are 800 nm in diameter, and the addition of MB reduces the size of the fibrils, which remain 23 nm in size, and this indicates that no fibrillation of insulin occurs in the presence of MB. This data is also supported by SEC. Saturation transfer difference NMR spectroscopy and molecular dynamics simulations demonstrate the interactions between insulin and MB at the atomic level.

Nrf-2 transcriptionally activates P21Cip/WAF1 and promotes A549 cell survival against oxidative stress induced by H2O2.

Cancer cells possess elevated ROS coupled with increased levels of antioxidant enzymes which render them resistant against cytotoxic chemotherapies. Therefore, an understanding of the interaction between key molecules involved in stress adaptive mechanisms is important to innovate strategies against cancer cell chemoresistance. Here, the lung adenocarcinoma cell line A549 with constitutively expressed Nrf2 was found to be more tolerant to H2O2 (0.1, 0.2, 0.5 and 1 mM) than normal lung cell line L132 or p53 null lung cancer cell line H1299. Maximum cytoprotection was observed at 0.2 mM H2O2 accompanied by a significant increase in p21, Nrf2 and antioxidant enzymes in A549 cells. The increased p21 expression was independent of p53 but dependent on Nrf2 as evident from qPCR, Western blotting and dual luciferase assays after silencing Nrf-2 and p53 genes. Highly conserved Nrf-2 binding sites were identified in p21 promoter by bioinformatics and homology modeling which was further confirmed by ChIP and reporter assay.

Chelerythrine down regulates expression of VEGFA, BCL2 and KRAS by arresting G-Quadruplex structures at their promoter regions.

A putative anticancer plant alkaloid, Chelerythrine binds to G-quadruplexes at promoters of VEGFA, BCL2 and KRAS genes and down regulates their expression. The association of Chelerythrine to G-quadruplex at the promoters of these oncogenes were monitored using UV absorption spectroscopy, fluorescence anisotropy, circular dichroism spectroscopy, CD melting, isothermal titration calorimetry, molecular dynamics simulation and quantitative RT-PCR technique. The pronounced hypochromism accompanied by red shifts in UV absorption spectroscopy in conjunction with ethidium bromide displacement assay indicates end stacking mode of interaction of Chelerythrine with the corresponding G-quadruplex structures. An increase in fluorescence anisotropy and CD melting temperature of Chelerythrine-quadruplex complex revealed the formation of stable Chelerythrine-quadruplex complex. Isothermal titration calorimetry data confirmed that Chelerythrine-quadruplex complex formation is thermodynamically favourable. Results of quantative RT-PCR experiment in combination with luciferase assay showed that Chelerythrine treatment to MCF7 breast cancer cells effectively down regulated transcript level of all three genes, suggesting that Chelerythrine efficiently binds to in cellulo quadruplex motifs. MD simulation provides the molecular picture showing interaction between Chelerythrine and G-quadruplex. Binding of Chelerythrine with BCL2, VEGFA and KRAS genes involved in evasion, angiogenesis and self sufficiency of cancer cells provides a new insight for the development of future therapeutics against cancer.

LINCRNA00273 promotes cancer metastasis and its G-Quadruplex promoter can serve as a novel target to inhibit cancer invasiveness.

Discovery of anti-metastatic drugs is of immense clinical significance as metastasis is responsible for 90% of all cancer deaths. Here we report the inhibitory effect of a bis schiff base (M2) on cancer cell migration and invasion in vitro and in vivo. M2 has shown good solubility and permeability across the intestinal cell wall and hence can be classified as BCS (Biopharmaceutical classification system) class I. Microarray studies identified a long non coding intergenic RNA, LINC00273 as a novel molecular target of M2. We report that LINC00273 harbors a unique (4n-1) parallel G-Quadruplex structure in its promoter as validated by DMS footprint. M2 is proposed to stabilize this G-quadruplex structure resulting in the down-regulation of LINC00273 expression. Dual Luciferase reporter assay also suggests inhibition of LINC00273 promoter activity by M2. Involvement of this linc in metastasis is proven by siRNA and shRNA mediated knock down of LINC00273 in vitro and in vivo in nude mice which significantly decelerates cancer cell migration and invasion and also makes the cells unresponsive to TGF-ß's pro-metastatic effects. Furthermore, the real time expression of LINC00273 in thirty seven human clinical samples is found to be positively correlated with the histopathological staging of metastasis.