Reversible conformational switching of i-motif DNA studied by fluorescence spectroscopy

Nanopore-Based Single-Molecule Investigation of Cation Effect on the i-Motif Structure

The i-motif, a secondary structure of a four-helix formed by cytosine-rich DNA (i-DNA) through C-C+ base pairing, is prevalent in human telomeres and promoters. This structure creates steric hindrance, thereby inhibiting both gene expression and protein coding. The conformation of i-DNA is intricately linked to the intracellular ionic environment. Hence, investigating its conformation under various ion conditions holds significant importance. In this study, we explored the impact of cations on the i-motif structure at the single-molecule level using the α-hemolysin (α-HL) nanochannel. Our findings reveal that the ability of i-DNA to fold into the i-motif structure follows the order Cs+ > Na+ > K+ > Li+ for monovalent cations. Furthermore, we observed the interconversion of single-stranded DNA (ss-DNA) and the i-motif structure at high and low concentrations of Mg2+ and Ba2+ electrolyte solutions. This study not only has the potential to extend the application of i-motif-based sensors in complex solution environments but also provides a new idea for the detection of metal ions.

iM-Seeker: a webserver for DNA i-motifs prediction and scoring via automated machine learning

DNA, beyond its canonical B-form double helix, adopts various alternative conformations, among which the i-motif, emerging in cytosine-rich sequences under acidic conditions, holds significant biological implications in transcription modulation and telomere biology. Despite recognizing the crucial role of i-motifs, predictive software for i-motif forming sequences has been limited. Addressing this gap, we introduce 'iM-Seeker', an innovative computational platform designed for the prediction and evaluation of i-motifs. iM-Seeker exhibits the capability to identify potential i-motifs within DNA segments or entire genomes, calculating stability scores for each predicted i-motif based on parameters such as the cytosine tracts number, loop lengths, and sequence composition. Furthermore, the webserver leverages automated machine learning (AutoML) to effortlessly fine-tune the optimal i-motif scoring model, incorporating user-supplied experimental data and customised features. As an advanced, versatile approach, 'iM-Seeker' promises to advance genomic research, highlighting the potential of i-motifs in cell biology and therapeutic applications. The webserver is freely available at https://im-seeker.org.

Comparative Analysis of pH and Target-Induced Conformational Changes of an Oxytetracycline Aptamer in Solution Phase and Surface-Immobilized Form

To date, numerous aptamer-based biosensing platforms have been developed for sensitive and selective monitoring of target analytes, relying on analyte-induced conformational changes in the aptamer for the quantification of the analyte and the conversion of the binding event into a measurable signal. Despite the impact of these conformational rearrangements on sensor performance, the influence of the environment on the structural conformations of aptamers has rarely been investigated, so the link between parameters directly influencing aptamer folding and the ability of the aptamer to bind to the target analyte remains elusive. Herein, the effect a number of variables have on an aptamer's 3D structure was examined, including the pH of the buffering medium, as well as the anchoring of the aptamer on a solid support, with the use of two label-free techniques. Circular dichroism spectroscopy was utilized to study the conformation of an aptamer in solution along with any changes induced to it by the environment (analyte binding, pH, composition and ionic strength of the buffer solution), while quartz crystal microbalance with dissipation monitoring was employed to investigate the surface-bound aptamer's behavior and performance. Analysis was performed on an aptamer against oxytetracycline, serving as a model system, representative of aptamers selected against small molecule analytes. The obtained results highlight the influence of the environment on the folding and thus analyte-binding capacity of an aptamer and emphasize the need to deploy appropriate surface functionalization protocols in sensor development as a means to minimize the steric obstructions and undesirable interactions of an aptamer with a surface onto which it is tethered.

Probing the Competition between Duplex, G-Quadruplex and i-Motif Structures of the Oncogenic c-Myc DNA Promoter Region

Development of probe systems that provide unique spectral signatures for duplex, G-quadruplex (GQ) and i-motif (iM) structures is very important to understand the relative propensity of a G-rich-C-rich promoter region to form these structures. Here, we devise a platform using a combination of two environment-sensitive nucleoside analogs namely, 5-fluorobenzofuran-modified 2'-deoxyuridine (FBF-dU) and 5-fluoro-2'-deoxyuridine (F-dU) to study the structures adopted by a promoter region of the c-Myc oncogene. FBF-dU serves as a dual-purpose probe containing a fluorescent and 19 F NMR label. When incorporated into the C-rich sequence, it reports the formation of different iMs via changes in its fluorescence properties and 19 F signal. F-dU incorporated into the G-rich ON reports the formation of a GQ structure whose 19 F signal is clearly different from the signals obtained for iMs. Rewardingly, the labeled ONs when mixed with respective complementary strands allows us to determine the relative population of different structures formed by the c-Myc promoter by the virtue of the probe's ability to produce distinct and resolved 19 F signatures for different structures. Our results indicate that at physiological pH and temperature the c-Myc promoter forms duplex, random coil and GQ structures, and does not form an iM. Whereas at acidic pH, the mixture largely forms iM and GQ structures. Taken together, our system will complement existing tools and provide unprecedented insights on the population equilibrium and dynamics of nucleic acid structures under different conditions.

Revealing unfolding steps and volume changes of human telomeric i-motif DNA

The i-motif structure of the human telomeric DNA was destabilized by pressure and unfolded with a negative volume change.

Single-molecule analysis of i-motif within self-assembled DNA duplexes and nanocircles

The cytosine (C)-rich sequences that can fold into tetraplex structures known as i-motif are prevalent in genomic DNA. Recent studies of i-motif-forming sequences have shown increasing evidence of their roles in gene regulation. However, most of these studies have been performed in short single-stranded oligonucleotides, far from the intracellular environment. In cells, i-motif-forming sequences are flanked by DNA duplexes and packed in the genome. Therefore, exploring the conformational dynamics and kinetics of i-motif under such topologically constrained environments is highly relevant in predicting their biological roles. Using single-molecule fluorescence analysis of self-assembled DNA duplexes and nanocircles, we show that the topological environments play a key role on i-motif stability and dynamics. While the human telomere sequence (C3TAA)3C3 assumes i-motif structure at pH 5.5 regardless of topological constraint, it undergoes conformational dynamics among unfolded, partially folded and fully folded states at pH 6.5. The lifetimes of i-motif and the partially folded state at pH 6.5 were determined to be 6 ± 2 and 31 ± 11 s, respectively. Consistent with the partially folded state observed in fluorescence analysis, interrogation of current versus time traces obtained from nanopore analysis at pH 6.5 shows long-lived shallow blockades with a mean lifetime of 25 ± 6 s. Such lifetimes are sufficient for the i-motif and partially folded states to interact with proteins to modulate cellular processes.

Fluorescence-based tools to probe G-quadruplexes in cell-free and cellular environments

Biophysical and biochemical investigations provide compelling evidence connecting the four-stranded G-quadruplex (GQ) structure with its role in regulating multiple cellular processes. Hence, modulating the function of GQs by using small molecule binders is being actively pursued as a strategy to develop new chemotherapeutic agents. However, sequence diversity and structural polymorphism of GQs have posed immense challenges in terms of understanding what conformation a G-rich sequence adopts inside the cell and how to specifically target a GQ motif amidst several other GQ-forming sequences. In this context, here we review recent developments in the applications of biophysical tools that use fluorescence readout to probe the GQ structure and recognition in cell-free and cellular environments. First, we provide a detailed discussion on the utility of covalently labeled environment-sensitive fluorescent nucleoside analogs in assessing the subtle difference in GQ structures and their ligand binding abilities. Furthermore, a detailed discussion on structure-specific antibodies and small molecule probes used to visualize and confirm the existence of DNA and RNA GQs in cells is provided. We also highlight the open challenges in the study of tetraplexes (GQ and i-motif structures) and how addressing these challenges by developing new tools and techniques will have a profound impact on tetraplex-directed therapeutic strategies.

Stress-induced acidification may contribute to formation of unusual structures in C9orf72-repeats

BACKGROUND

Expansion of the C9orf72 hexanucleotide repeat (GGGGCC)_n·(GGCCCC)_n is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Both strands of the C9orf72 repeat have been shown to form unusual DNA and RNA structures that are thought to be involved in mutagenesis and/or pathogenesis. We previously showed that the C-rich DNA strands from the C9orf72 repeat can form four-stranded quadruplexes at neutral pH. The cytosine residues become protonated under slightly acidic pH (pH 4.5-6.2), facilitating the formation of intercalated i-motif structures.

METHODS

Using CD spectroscopy, UV melting, and gel electrophoresis, we demonstrate a pH-induced structural transition of the C-rich DNA strand of the C9orf72 repeat at pHs reported to exist in living cells under stress, including during neurodegeneration and cancer.

RESULTS

We show that the repeats with lengths of 4, 6, and 8 units, form intercalated quadruplex i-motifs at low pH (pH < 5) and monomolecular hairpins and monomolecular quadruplexes under neutral-basic conditions (pH ≥ 8). Furthermore, we show that the human replication protein A (RPA) binds to the G-rich and C-rich DNA strands under acidic conditions, suggesting that it can bind to i-motif structures.

CONCLUSIONS

In the proper sequence context, i-motif structures can form at pH values found in some cells in vivo.

GENERAL SIGNIFICANCE

DNA conformational plasticity exists over broad range of solution conditions.

Preferential targeting of i-motifs and G-quadruplexes by small molecules

i-Motifs and G-quadruplexes are dynamic nucleic acid secondary structures, which are believed to play key roles in gene expression. We herein report two peptidomimetic ligands (PBP1 and PBP2) that selectively target i-motifs and G-quadruplexes over double-stranded DNA. These peptidomimetics, regioisomeric with respect to the position of triazole/prolinamide motifs, have been synthesized using a modular method involving Cu(i)-catalyzed azide and alkyne cycloaddition. The para-isomer, PBP1 exhibits high selectivity for i-motifs while the meta-isomer PBP2 binds selectively to G-quadruplex structures. Interestingly, these ligands have the ability to induce G-quadruplex or i-motif structures from the unstructured single-stranded DNA conformations, as observed using single molecule Förster resonance energy transfer (smFRET) studies. The quantitative real-time polymerase chain reaction (qRT-PCR), western blot, and dual-luciferase assays indicate that PBP1 upregulates and PBP2 downregulates BCL-2 gene expression in cancer cells.

The formation and characteristics of the i-motif structure within the promoter of the c-myb proto-oncogene

C-myb proto-oncogene is a potential therapeutic target for some human solid tumors and leukemias. A long cytosine-rich sequence, which locates the downstream of the transcription initiation site, is demonstrated to fold into an intramolecular i-motif DNA using electrospray ionization mass spectrometry (ESI-MS) and circular dichroism (CD) spectroscopy. Effects of factors, including the pH value, the number of C:C(+) dimers, the concentration of buffer, the molecular crowding condition, and the coexistence of the complementary DNA, on the formation and the structural stability of the i-motif DNA are systematically studied. We have demonstrated that the i-motif folding in the c-myb promoter could be accelerated upon synergistic physiological stimuli including intracellular molecular crowding and low pH values, as well as the large number of the i-motif C:C(+) dimers. Meanwhile, various inputs, such as acids/bases and metal ions, have exhibited their abilities in controlling the conformational switch of the c-myb GC-rich DNA. Acidic pH values and the presence of K(+) ions can induce the dissociation of the double helix. Our present strategy can greatly extend the potential usages of i-motif DNA molecules with specific sequences as conformational switch-controlled devices. Moreover, this work demonstrates the superiority of CD spectroscopy associated with ESI-MS as a rapid, more cost-effective and sensitive structural change responsive method in the research of DNA conformational switching.

pH and near-infrared light dual-stimuli responsive drug delivery using DNA-conjugated gold nanorods for effective treatment of multidrug resistant cancer cells

A thiolated pH-responsive DNA conjugated gold nanorod (GNR) was developed as a multifunctional nanocarrier for targeted, pH-and near infrared (NIR) radiation dual-stimuli triggered drug delivery. It was further passivated by a thiolated poly(ethylene glycol)-biotin to improve its cancer targeting ability by specific binding to cancer cell over-expressed biotin receptors. Doxorubicin (DOX), a widely used clinical anticancer drug, was conveniently loaded into nanocarrier by intercalating inside the double-stranded pH-responsive DNAs on the GNR surface to complete the construction of the multifunctional nanomedicine. The nanomedicine can rapidly and effectively release its DOX payload triggered by an acidic pH environment (pH~5) and/or applying an 808nm NIR laser radiation. Compared to free DOX, the biotin-modified nanomedicine displayed greatly increased cell uptake and significantly reduced drug efflux by model multidrug resistant (MDR) breast cancer cell lines (MCF-7/ADR). The application of NIR radiation further increased the DOX release and facilitated its nuclear accumulation. As a result, this new DNA-GNR based multifunctional nanomedicine exerted greatly increased potency (~67 fold) against the MDR cancer cells over free DOX.

Berberine as a novel light-up i-motif fluorescence ligand and its application in designing molecular logic systems

Berberine is reported as a light-up fluorescence ligand for i-motif structures, which enables the development of label-free DNA-based logic gates.

Label-free DNA-based biosensors using structure-selective light-up dyes

Label-free biosensors (LFBs) have demonstrated great potential in cost-effective applications. This review collected the latest reported works which employed structure-selective nucleic acid dyes for the development of DNA-based LFBs.

Fluorescent Sensor for PH Monitoring Based on an i-Motif---Switching Aptamer Containing a Tricyclic Cytosine Analogue (tC)

There are cytosine-rich regions in the genome that bind protons with high specificity. Thus protonated C-rich sequence may undergo folding to tetraplex structures called i-motifs. Therefore, one can regard such specific C-rich oligonucleotides as aptamers that recognize protons and undergo conformational transitions. Proper labeling of the aptamer with a fluorescent tag constitutes a platform to construct a pH-sensitive aptasensor. Since the hemiprotonated C-C⁺ base pairs are responsible for the folded tetraplex structure of i-motif, we decided to substitute one of cytosines in an aptamer sequence with its fluorescent analogue, 1,3-diaza-2-oxophenothiazine (tC). In this paper we report on three tC-modified fluorescent probes that contain RET related sequences as a proton recognizing aptamer. Results of the circular dichroism (CD), UV absorption melting experiments, and steady-state fluorescence measurements of these tC-modified i-motif probes are presented and discussed. The pH-induced i-motif formation by the probes resulted in fluorescence quenching of tC fluorophore. Efficiency of quenching was related to the pH variations. Suitability of the sensor for monitoring pH changes was also demonstrated.

Unfolding Kinetics of the Human Telomere i-Motif Under a 10 pN Force Imposed by the α-Hemolysin Nanopore Identify Transient Folded-State Lifetimes at Physiological pH

Cytosine (C)-rich DNA can adopt i-motif folds under acidic conditions, with the human telomere i-motif providing a well-studied example. The dimensions of this i-motif are appropriate for capture in the nanocavity of the α-hemolysin (α-HL) protein pore under an electrophoretic force. Interrogation of the current vs time (i–t) traces when the i-motif interacts with α-HL identified characteristic signals that were pH dependent. These features were evaluated from pH 5.0 to 7.2, a region surrounding the transition pH of the i-motif (6.1). When the i-motif without polynucleotide tails was studied at pH 5.0, the folded structure entered the nanocavity of α-HL from either the top or bottom face to yield characteristic current patterns. Addition of a 5′ 25-mer poly-2′-deoxyadensosine tail allowed capture of the i-motif from the unfolded terminus, and this was used to analyze the pH dependency of unfolding. At pH values below the transition point, only folded strands were observed, and when the pH was increased above the transition pH, the number of folded events decreased, while the unfolded events increased. At pH 6.8 and 7.2 4% and 2% of the strands were still folded, respectively. The lifetimes for the folded states at pH 6.8 and 7.2 were 21 and 9 ms, respectively, at 160 mV electrophoretic force. These lifetimes are sufficiently long to affect enzymes operating on DNA. Furthermore, these transient lifetimes are readily obtained using the α-HL nanopore, a feature that is not easily achievable by other methods.

A simple approach to study the conformational switching of i-motif DNA by fluorescence anisotropy

Fluorescence anisotropy is a simple, reliable and sensitive approach to study the conformational switching of the i-motif structure.

Unconventional features of C9ORF72 expanded repeat in amyotrophic lateral sclerosis and frontotemporal lobar degeneration

Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are devastating neurodegenerative diseases that form two ends of a complex disease spectrum. Aggregation of RNA binding proteins is one of the hallmark pathologic features of ALS and FTDL and suggests perturbance of the RNA metabolism in their etiology. Recent identification of the disease-associated expansions of the intronic hexanucleotide repeat GGGGCC in the C9ORF72 gene further substantiates the case for RNA involvement. The expanded repeat, which has turned out to be the single most common genetic cause of ALS and FTLD, may enable the formation of complex DNA and RNA structures, changes in RNA transcription, and processing and formation of toxic RNA foci, which may sequester and inactivate RNA binding proteins. Additionally, the transcribed expanded repeat can undergo repeat-associated non-ATG-initiated translation resulting in accumulation of a series of dipeptide repeat proteins. Understanding the basis of the proposed mechanisms and shared pathways, as well as interactions with known key proteins such as TAR DNA-binding protein (TDP-43) are needed to clarify the pathology of ALS and/or FTLD, and make possible steps toward therapy development.

The transcriptional complex between the BCL2 i-motif and hnRNP LL is a molecular switch for control of gene expression that can be modulated by small molecules

In a companion paper (DOI: 10.021/ja410934b) we demonstrate that the C-rich strand of the cis-regulatory element in the BCL2 promoter element is highly dynamic in nature and can form either an i-motif or a flexible hairpin. Under physiological conditions these two secondary DNA structures are found in an equilibrium mixture, which can be shifted by the addition of small molecules that trap out either the i-motif (IMC-48) or the flexible hairpin (IMC-76). In cellular experiments we demonstrate that the addition of these molecules has opposite effects on BCL2 gene expression and furthermore that these effects are antagonistic. In this contribution we have identified a transcriptional factor that recognizes and binds to the BCL2 i-motif to activate transcription. The molecular basis for the recognition of the i-motif by hnRNP LL is determined, and we demonstrate that the protein unfolds the i-motif structure to form a stable single-stranded complex. In subsequent experiments we show that IMC-48 and IMC-76 have opposite, antagonistic effects on the formation of the hnRNP LL–i-motif complex as well as on the transcription factor occupancy at the BCL2 promoter. For the first time we propose that the i-motif acts as a molecular switch that controls gene expression and that small molecules that target the dynamic equilibrium of the i-motif and the flexible hairpin can differentially modulate gene expression.

Fundamental aspects of the nucleic acid i-motif structures

The latest research on fundamental aspects of i-motif structures is reviewed with special attention to their hypothetical rolein vivo.