Alternative splicing of BCL-x is controlled by RBM25 binding to a G-quadruplex in BCL-x pre-mRNA.

BCL-x is a master regulator of apoptosis whose pre-mRNA is alternatively spliced into either a long (canonical) anti-apoptotic Bcl-xL isoform, or a short (alternative) pro-apoptotic Bcl-xS isoform. The balance between these two antagonistic isoforms is tightly regulated and overexpression of Bcl-xL has been linked to resistance to chemotherapy in several cancers, whereas overexpression of Bcl-xS is associated to some forms of diabetes and cardiac disorders. The splicing factor RBM25 controls alternative splicing of BCL-x: its overexpression favours the production of Bcl-xS, whereas its downregulation has the opposite effect. Here we show that RBM25 directly and specifically binds to GQ-2, an RNA G-quadruplex (rG4) of BCL-x pre-mRNA that forms at the vicinity of the alternative 5' splice site leading to the alternative Bcl-xS isoform. This RBM25/rG4 interaction is crucial for the production of Bcl-xS and depends on the RE (arginine-glutamate-rich) motif of RBM25, thus defining a new type of rG4-interacting domain. PhenDC3, a benchmark G4 ligand, enhances the binding of RBM25 to the GQ-2 rG4 of BCL-x pre-mRNA, thereby promoting the alternative pro-apoptotic Bcl-xS isoform and triggering apoptosis. Furthermore, the screening of a combinatorial library of 90 putative G4 ligands led to the identification of two original compounds, PhenDH8 and PhenDH9, superior to PhenDC3 in promoting the Bcl-xS isoform and apoptosis. Thus, favouring the interaction between RBM25 and the GQ-2 rG4 of BCL-x pre-mRNA represents a relevant intervention point to re-sensitize cancer cells to chemotherapy.

A G-quadruplex-binding platinum complex induces cancer mitochondrial dysfunction through dual-targeting mitochondrial and nuclear G4 enriched genome.

BACKGROUND: G-quadruplex DNA (G4) is a non-canonical structure forming in guanine-rich regions, which play a vital role in cancer biology and are now being acknowledged in both nuclear and mitochondrial (mt) genome. However, the impact of G4-based targeted therapy on both nuclear and mt genome, affecting mt function and its underlying mechanisms remain largely unexplored. METHODS: The mechanisms of action and therapeutic effects of a G4-binding platinum(II) complex, Pt-ttpy, on mitochondria were conducted through a comprehensive approaches with in vitro and in vivo models, including ICP-MS for platinum measurement, PCR-based genetic analysis, western blotting (WB), confocal microscope for mt morphology study, extracellular flux analyzer, JC1 and Annexin V apoptosis assay, flow cytometry and high content microscope screening with single-cell quantification of both ROS and mt specific ROS, as well as click-chemistry for IF study of mt translation. Decipher Pt-ttpy effects on nuclear-encoded mt related genes expression were undertaken via RNA-seq, Chip-seq and CUT-RUN assays. RESULTS: Pt-ttpy, shows a highest accumulation in the mitochondria of A2780 cancer cells as compared with two other platinum(II) complexes with no/weak G4-binding properties, Pt-tpy and cisplatin. Pt-ttpy induces mtDNA deletion, copy reduction and transcription inhibition, hindering mt protein translation. Functional analysis reveals potent mt dysfunction without reactive oxygen species (ROS) induction. Mechanistic study provided first evidence that most of mt ribosome genes are highly enriched in G4 structures in their promoter regions, notably, Pt-ttpy impairs most nuclear-encoded mt ribosome genes' transcription through dampening the recruiting of transcription initiation and elongation factors of NELFB and TAF1 to their promoter with G4-enriched sequences. In vivo studies show Pt-ttpy's efficient anti-tumor effects, disrupting mt genome function with fewer side effects than cisplatin. CONCLUSION: This study underscores Pt-ttpy as a G4-binding platinum(II) complex, effectively targeting cancer mitochondria through dual action on mt and nuclear G4-enriched genomes without inducing ROS, offering promise for safer and effective platinum-based G4-targeted cancer therapy.

Type I arginine methyltransferases are intervention points to unveil the oncogenic Epstein-Barr virus to the immune system.

The oncogenic Epstein-Barr virus (EBV) evades the immune system but has an Achilles heel: its genome maintenance protein EBNA1. Indeed, EBNA1 is essential for viral genome maintenance but is also highly antigenic. Hence, EBV seemingly evolved a system in which the glycine-alanine repeat (GAr) of EBNA1 limits the translation of its own mRNA to the minimal level to ensure its essential function, thereby, at the same time, minimizing immune recognition. Therefore, defining intervention points at which to interfere with GAr-based inhibition of translation is an important step to trigger an immune response against EBV-carrying cancers. The host protein nucleolin (NCL) plays a critical role in this process via a direct interaction with G-quadruplexes (G4) formed in the GAr-encoding sequence of the viral EBNA1 mRNA. Here we show that the C-terminal arginine-glycine-rich (RGG) motif of NCL is crucial for its role in GAr-based inhibition of translation by mediating interaction of NCL with G4 of EBNA1 mRNA. We also show that this interaction depends on the type I arginine methyltransferase family, notably PRMT1 and PRMT3: drugs or small interfering RNA that target these enzymes prevent efficient binding of NCL on G4 of EBNA1 mRNA and relieve GAr-based inhibition of translation and of antigen presentation. Hence, this work defines type I arginine methyltransferases as therapeutic targets to interfere with EBNA1 and EBV immune evasion.

BrdU immuno-tagged G-quadruplex ligands: a new ligand-guided immunofluorescence approach for tracking G-quadruplexes in cells.

G-quadruplexes (G4s) are secondary structures forming in G-rich nucleic acids. G4s are assumed to play critical roles in biology, nonetheless their detection in cells is still challenging. For tracking G4s, synthetic molecules (G4 ligands) can be used as reporters and have found wide application for this purpose through chemical functionalization with a fluorescent tag. However, this approach is limited by a low-labeling degree impeding precise visualization in specific subcellular regions. Herein, we present a new visualization strategy based on the immuno-recognition of 5-bromo-2'-deoxyuridine (5-BrdU) modified G4 ligands, functionalized prior- or post-G4-target binding by CuAAC. Remarkably, recognition of the tag by antibodies leads to the detection of the modified ligands exclusively when bound to a G4 target both in vitro, as shown by ELISA, and in cells, thereby providing a highly efficient G4-ligand Guided Immunofluorescence Staining (G4-GIS) approach. The obtained signal amplification revealed well-defined fluorescent foci located in the perinuclear space and RNase treatment revealed the preferential binding to G4-RNA. Furthermore, ligand treatment affected significantly BG4 foci formation in cells. Our work headed to the development of a new imaging approach combining the advantages of immunostaining and G4-recognition by G4 ligands leading to visualization of G4/ligands species in cells with unrivaled precision and sensitivity.

SARS-CoV-2 Nsp3 unique domain SUD interacts with guanine quadruplexes and G4-ligands inhibit this interaction.

The multidomain non-structural protein 3 (Nsp3) is the largest protein encoded by coronavirus (CoV) genomes and several regions of this protein are essential for viral replication. Of note, SARS-CoV Nsp3 contains a SARS-Unique Domain (SUD), which can bind Guanine-rich non-canonical nucleic acid structures called G-quadruplexes (G4) and is essential for SARS-CoV replication. We show herein that the SARS-CoV-2 Nsp3 protein also contains a SUD domain that interacts with G4s. Indeed, interactions between SUD proteins and both DNA and RNA G4s were evidenced by G4 pull-down, Surface Plasmon Resonance and Homogenous Time Resolved Fluorescence. These interactions can be disrupted by mutations that prevent oligonucleotides from folding into G4 structures and, interestingly, by molecules known as specific ligands of these G4s. Structural models for these interactions are proposed and reveal significant differences with the crystallographic and modeled 3D structures of the SARS-CoV SUD-NM/G4 interaction. Altogether, our results pave the way for further studies on the role of SUD/G4 interactions during SARS-CoV-2 replication and the use of inhibitors of these interactions as potential antiviral compounds.

Selection of Bis-Indolyl Pyridines and Triphenylamines as New Inhibitors of SARS-CoV-2 Cellular Entry by Modulating the Spike Protein/ACE2 Interfaces.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the infectious agent that has caused the current coronavirus disease (COVID) pandemic. Viral infection relies on the viral S (spike) protein/cellular receptor ACE2 interaction. Disrupting this interaction would lead to early blockage of viral replication. To identify chemical tools to further study these functional interfaces, 139,146 compounds from different chemical libraries were screened through an S/ACE2 in silico virtual molecular model. The best compounds were selected for further characterization using both cellular and biochemical approaches, reiterating SARS-CoV-2 entry and the S/ACE2 interaction. We report here two selected hits, bis-indolyl pyridine AB-00011778 and triphenylamine AB-00047476. Both of these compounds can block the infectivity of lentiviral vectors pseudotyped with the SARS-CoV-2 S protein as well as wild-type and circulating variant SARS-CoV-2 strains in various human cell lines, including pulmonary cells naturally susceptible to infection. AlphaLISA and biolayer interferometry confirmed a direct inhibitory effect of these drugs on the S/ACE2 association. A specific study of the AB-00011778 inhibitory properties showed that this drug inhibits viral replication with a 50% effective concentration (EC50) between 0.1 and 0.5 µM depending on the cell lines. Molecular docking calculations of the interaction parameters of the molecules within the S/ACE2 complex from both wild-type and circulating variants of the virus showed that the molecules may target multiple sites within the S/ACE2 interface. Our work indicates that AB-00011778 constitutes a good tool for modulating this interface and a strong lead compound for further therapeutic purposes.

Phen-DC3 Induces Refolding of Human Telomeric DNA into a Chair-Type Antiparallel G-Quadruplex through Ligand Intercalation.

Human telomeric G-quadruplex DNA structures are attractive anticancer drug targets, but the target's polymorphism complicates the drug design: different ligands prefer different folds, and very few complexes have been solved at high resolution. Here we report that Phen-DC3 , one of the most prominent G-quadruplex ligands in terms of high binding affinity and selectivity, causes dTAGGG(TTAGGG)3 to completely change its fold in KCl solution from a hybrid-1 to an antiparallel chair-type structure, wherein the ligand intercalates between a two-quartet unit and a pseudo-quartet, thereby ejecting one potassium ion. This unprecedented high-resolution NMR structure shows for the first time a true ligand intercalation into an intramolecular G-quadruplex.

Ligand Binding to Dynamically Populated G-Quadruplex DNA.

Several small-molecule ligands specifically bind and stabilize G-quadruplex (G4) nucleic acid structures, which are considered to be promising therapeutic targets. G4s are polymorphic structures of varying stability, and their formation is dynamic. Here, we investigate the mechanisms of ligand binding to dynamically populated human telomere G4 DNA by using the bisquinolinium based ligand Phen-DC3 and a combination of single-molecule FRET microscopy, ensemble FRET and CD spectroscopies. Different cations are used to tune G4 polymorphism and folding dynamics. We find that ligand binding occurs to pre-folded G4 structures and that Phen-DC3 also induces G4 formation in unfolded single strands. Following ligand binding to dynamically populated G4s, the DNA undergoes pronounced conformational redistributions that do not involve direct ligand-induced G4 conformational interconversion. On the contrary, the redistribution is driven by ligand-induced G4 folding and trapping of dynamically populated short-lived conformation states. Thus, ligand-induced stabilization does not necessarily require the initial presence of stably folded G4s.

Modulation of Cellular Fate of Vinyl Triarylamines through Structural Fine Tuning: To Stay or Not To Stay in the Mitochondria?

Mitochondria are involved in many cellular pathways and dysfunctional mitochondria are linked to various diseases. Hence efforts have been made to design mitochondria-targeted fluorophores for monitoring the mitochondrial status. However, the factors that govern the mitochondria-targeted potential of dyes are not well-understood. In this context, we synthesized analogues of the TP-2Bzim probe belonging to the vinyltriphenylamine (TPA) class and already described for its capacity to bind nuclear DNA in fixed cells and mitochondria in live cells. These analogues (TP-1Bzim, TPn -2Bzim, TP1+ -2Bzim, TN-2Bzim) differ in the cationic charge, the number of vinylbenzimidazolium branches and the nature of the triaryl core. Using microscopy, we demonstrated that the cationic derivatives accumulate in mitochondria but do not reach mtDNA. Under depolarisation of the mitochondrial membrane, TP-2Bzim and TP1+ -2Bzim translocate to the nucleus in direct correlation with their strong DNA affinity. This reversible phenomenon emphasizes that these probes can be used to monitor ΔΨm variations.

Competition of Ligands and the 18-mer Binding Domain of the RHAU Helicase for G-Quadruplexes: Orthosteric or Allosteric Binding Mechanism?

Stabilizing the DNA and RNA structures known as G-quadruplexes (G4s) using specific ligands is a strategy that has been proposed to fight cancer. However, although G-quadruplex:ligand (G4:L) interactions have often been investigated, whether or not ligands are able to disrupt G-quadruplex:protein (G4:P) interactions remains poorly studied. In this study, using native mass spectrometry, we have investigated ternary G4:L:P complexes formed by G4s, some of the highest affinity ligands, and the binding domain of the RHAU helicase. Our results suggest that RHAU binds not only preferentially to parallel G4s, but also to free external G-quartets. We also found that, depending on the G4, ligands could prevent the binding of the peptide, either by direct competition for the binding sites (orthosteric inhibition) or by inducing conformational changes (allosteric inhibition). Notably, the ligand Cu-ttpy (ttpy=4'-tolyl-2,2':6',2''-terpyridine) induced a conformational change that increased the binding of the peptide. This study illustrates that it is important to not only characterize drug-target interactions, but also how the binding to other partners is affected.

Thermodynamically stable and genetically unstable G-quadruplexes are depleted in genomes across species.

G-quadruplexes play various roles in multiple biological processes, which can be positive when a G4 is involved in the regulation of gene expression or detrimental when the folding of a stable G4 impairs DNA replication promoting genome instability. This duality interrogates the significance of their presence within genomes. To address the potential biased evolution of G4 motifs, we analyzed their occurrence, features and polymorphisms in a large spectrum of species. We found extreme bias of the short-looped G4 motifs, which are the most thermodynamically stable in vitro and thus carry the highest folding potential in vivo. In the human genome, there is an over-representation of single-nucleotide-loop G4 motifs (G4-L1), which are highly conserved among humans and show a striking excess of the thermodynamically least stable G4-L1A (G3AG3AG3AG3) sequences. Functional assays in yeast showed that G4-L1A caused the lowest levels of both spontaneous and G4-ligand-induced instability. Analyses across 600 species revealed the depletion of the most stable G4-L1C/T quadruplexes in most genomes in favor of G4-L1A in vertebrates or G4-L1G in other eukaryotes. We discuss how these trends might be the result of species-specific mutagenic processes associated to a negative selection against the most stable motifs, thus neutralizing their detrimental effects on genome stability while preserving positive G4-associated biological roles.

Nucleolin Discriminates Drastically between Long-Loop and Short-Loop Quadruplexes.

We investigate herein the interaction between nucleolin (NCL) and a set of G4 sequences derived from the CEB25 human minisatellite that adopt a parallel topology while differing in the length of the central loop (from nine nucleotides to one nucleotide). It is revealed that NCL strongly binds to long-loop (five to nine nucleotides) G4 while interacting weakly with the shorter variants (loop with fewer than three nucleotides). Photo-cross-linking experiments using 5-bromo-2'-deoxyuridine (BrU)-modified sequences further confirmed the loop-length dependency, thereby indicating that the WT-CEB25-L191 (nine-nucleotide loop) is the best G4 substrate. Quantitative proteomic analysis (LC-MS/MS) of the product(s) obtained by photo-cross-linking NCL to this sequence enabled the identification of one contact site corresponding to a 15-amino acid fragment located in helix α2 of RNA binding domain 2 (RBD2), which sheds light on the role of this structural element in G4-loop recognition. Then, the ability of a panel of benchmark G4 ligands to prevent the NCL-G4 interaction was explored. It was found that only the most potent ligand PhenDC3 can inhibit NCL binding, thereby suggesting that the terminal guanine quartet is also a strong determinant of G4 recognition, putatively through interaction with the RGG domain. This study describes the molecular mechanism by which NCL recognizes G4-containing long loops and leads to the proposal of a model implying a concerted action of RBD2 and RGG domains to achieve specific G4 recognition via a dual loop-quartet interaction.

TWJ-Screen: an isothermal screening assay to assess ligand/DNA junction interactions in vitro.

The quest for chemicals able to operate at selected genomic loci in a spatiotemporally controlled manner is desirable to create manageable DNA damages. Mounting evidence now shows that alternative DNA structures, including G-quadruplexes and branched DNA (or DNA junctions), might hamper proper progression of replication fork, thus triggering DNA damages and genomic instability. Therefore, small molecules that stabilize these DNA structures are currently scrutinized as a promising way to create genomic defects that cannot be dealt with properly by cancer cells. While much emphasis has been recently given to G-quadruplexes and related ligands, we report herein on three-way DNA junctions (TWJ) and related ligands. We first highlight the biological implications of TWJ and their strategic relevance as triggers for replicative stress. Then, we describe a new in vitro high-throughput screening assay, TWJ-Screen, which allows for identifying TWJ ligands with both high affinity and selectivity for TWJ over other DNA structures (duplexes and quadruplexes), in a convenient and unbiased manner as demonstrated by the screening of a library of 25 compounds from different chemical families. TWJ-Screen thus represents a reliable mean to uncover molecular tools able to foster replicative stress through an innovative approach, thus providing new strategic opportunities to combat cancers.

Monitoring DNA-Ligand Interactions in Living Human Cells Using NMR Spectroscopy.

Studies on DNA-ligand interactions in the cellular environment are problematic due to the lack of suitable biophysical tools. To address this need, we developed an in-cell NMR-based approach for monitoring DNA-ligand interactions inside the nuclei of living human cells. Our method relies on the acquisition of NMR data from cells electroporated with preformed DNA-ligand complexes. The impact of the intracellular environment on the integrity of the complexes is assessed based on in-cell NMR signals from unbound and ligand-bound forms of a given DNA target. This technique was tested on complexes of two model DNA fragments and four ligands, namely, a representative DNA minor-groove binder (netropsin) and ligands binding DNA base-pairing defects (naphthalenophanes). In the latter case, we demonstrate that two of the three in vitro-validated ligands retain their ability to form stable interactions with their model target DNA in cellulo, whereas the third one loses this ability due to off-target interactions with genomic DNA and cellular metabolites. Collectively, our data suggest that direct evaluation of the behavior of drug-like molecules in the intracellular environment provides important insights into the development of DNA-binding ligands with desirable biological activity and minimal side effects resulting from off-target binding.

Short loop length and high thermal stability determine genomic instability induced by G-quadruplex-forming minisatellites.

G-quadruplexes (G4) are polymorphic four-stranded structures formed by certain G-rich nucleic acids, with various biological roles. However, structural features dictating their formation and/or function in vivo are unknown. In S. cerevisiae, the pathological persistency of G4 within the CEB1 minisatellite induces its rearrangement during leading-strand replication. We now show that several other G4-forming sequences remain stable. Extensive mutagenesis of the CEB25 minisatellite motif reveals that only variants with very short (≤ 4 nt) G4 loops preferentially containing pyrimidine bases trigger genomic instability. Parallel biophysical analyses demonstrate that shortening loop length does not change the monomorphic G4 structure of CEB25 variants but drastically increases its thermal stability, in correlation with the in vivo instability. Finally, bioinformatics analyses reveal that the threat for genomic stability posed by G4 bearing short pyrimidine loops is conserved in C. elegans and humans. This work provides a framework explanation for the heterogeneous instability behavior of G4-forming sequences in vivo, highlights the importance of structure thermal stability, and questions the prevailing assumption that G4 structures with short or longer loops are as likely to form in vivo.

Probing ligand and cation binding sites in G-quadruplex nucleic acids by mass spectrometry and electron photodetachment dissociation sequencing.

Mass spectrometry provides exquisite details on ligand and cation binding stoichiometries with a DNA target. The next important step is to develop reliable methods to determine the cation and ligand binding sites in each complex separated by using a mass spectrometer. To circumvent the caveat of ligand derivatization for cross-linking, which may alter the ligand binding mode, we explored a tandem mass spectrometry (MS/MS) method that does not require ligand derivatization, and is therefore also applicable to localize metal cations. By putting more negative charge states on the complexes using supercharging agents, and by creating radical ions by electron photodetachment, oligonucleotide bonds become weaker than the DNA-cation or DNA-ligand noncovalent bonds upon collision-induced dissociation of the radicals. This electron photodetachment (EPD) method allows one to locate the binding regions of cations and ligands by top-down sequencing of the oligonucleotide target. The very potent G-quadruplex ligands 360A and PhenDC3 were found to replace a potassium cation and bind close to the central loop of 4-repeat human telomeric sequences.

Selectivity of Terpyridine Platinum Anticancer Drugs for G-quadruplex DNA.

Guanine-rich DNA can form four-stranded structures called G-quadruplexes (G4s) that can regulate many biological processes. Metal complexes have shown high affinity and selectivity toward the quadruplex structure. Here, we report the comparison of a panel of platinum (II) complexes for quadruplex DNA selective recognition by exploring the aromatic core around terpyridine derivatives. Their affinity and selectivity towards G4 structures of various topologies have been evaluated by FRET-melting (Fluorescence Resonance Energy Transfert-melting) and Fluorescent Intercalator Displacement (FID) assays, the latter performed by using three different fluorescent probes (Thiazole Orange (TO), TO-PRO-3, and PhenDV). Their ability to bind covalently to the c-myc G4 structure in vitro and their cytotoxicity potential in two ovarian cancerous cell lines were established. Our results show that the aromatic surface of the metallic ligands governs, in vitro, their affinity, their selectivity for the G4 over the duplex structures, and platination efficiency. However, the structural modifications do not allow significant discrimination among the different G4 topologies. Moreover, all compounds were tested on ovarian cancer cell lines and normal cell lines and were all able to overcome cisplatin resistance highlighting their interest as new anticancer drugs.

Identification of optimal fluorescent probes for G-quadruplex nucleic acids through systematic exploration of mono- and distyryl dye libraries.

A library of 52 distyryl and 9 mono-styryl cationic dyes was synthesized and investigated with respect to their optical properties, propensity to aggregation in aqueous medium, and capacity to serve as fluorescence "light-up" probes for G-quadruplex (G4) DNA and RNA structures. Among the 61 compounds, 57 dyes showed preferential enhancement of fluorescence intensity in the presence of one or another G4-DNA or RNA structure, while no dye displayed preferential response to double-stranded DNA or single-stranded RNA analytes employed at equivalent nucleotide concentration. Thus, preferential fluorimetric response towards G4 structures appears to be a common feature of mono- and distyryl dyes, including long-known mono-styryl dyes used as mitochondrial probes or protein stains. However, the magnitude of the G4-induced "light-up" effect varies drastically, as a function of both the molecular structure of the dyes and the nature or topology of G4 analytes. Although our results do not allow to formulate comprehensive structure-properties relationships, we identified several structural motifs, such as indole- or pyrrole-substituted distyryl dyes, as well as simple mono-stryryl dyes such as DASPMI [2-(4-(dimethylamino)styryl)-1-methylpyridinium iodide] or its 4-isomer, as optimal fluorescent light-up probes characterized by high fluorimetric response (I/I 0 of up to 550-fold), excellent selectivity with respect to double-stranded DNA or single-stranded RNA controls, high quantum yield in the presence of G4 analytes (up to 0.32), large Stokes shift (up to 150 nm) and, in certain cases, structural selectivity with respect to one or another G4 folding topology. These dyes can be considered as promising G4-responsive sensors for in vitro or imaging applications. As a possible application, we implemented a simple two-dye fluorimetric assay allowing rapid topological classification of G4-DNA structures.

Topology-Selective, Fluorescent "Light-Up" Probes for G-Quadruplex DNA Based on Photoinduced Electron Transfer.

Six novel probes were prepared by covalent attachment of a G4-DNA ligand (bis(quinolinium) pyridodicarboxamide; PDC) to various coumarin or pyrene fluorophores. In the absence of DNA, the fluorescence of all probes is quenched due to intramolecular photoinduced electron transfer (PET), as evidenced by photophysical and electrochemical studies, molecular modeling, and DFT calculations. All probes demonstrate similarly high thermal stabilization of various G4-DNA substrates belonging to different folding topologies, as assessed by fluorescence melting experiments; however, their fluorimetric response is strongly heterogeneous with respect to the structures of the probes and G4-DNA targets. Thus, the probes containing the 7-diethylaminocoumarin fluorophore demonstrate significant fluorescence enhancement in the presence of G4-DNA, with the strongest "light-up" response (20- to 180-fold) observed for antiparallel G4 structures as well as for hybrid G4 structures, formed by the variants of human telomeric sequence and capable of a conformation change to the antiparallel isoform. These results shed light on the influence of the linker and electronic properties of fluorophores on the efficiency of G4-DNA "light-up" probes operating via PET.

Mitochondria-targeted cationic porphyrin-triphenylamine hybrids for enhanced two-photon photodynamic therapy.

The proof of concept for two-photon activated photodynamic therapy has already been achieved for cancer treatment but the efficiency of this approach still heavily relies on the availability of photosensitizers combining high two-photon absorption and biocompatibility. In this line we recently reported on a series of porphyrin-triphenylamine hybrids which exhibit high singlet oxygen production quantum yield as well as high two-photon absorption cross-sections but with a very poor cellular internalization. We present herein new photosensitizers of the same porphyrin-triphenylamine hybrid series but bearing cationic charges which led to strongly enhanced water solubility and thus cellular penetration. In addition the new compounds have been found localized in mitochondria that are preferential target organelles for photodynamic therapy. Altogether the strongly improved properties of the new series combined with their specific mitochondrial localization lead to a significantly enhanced two-photon activated photodynamic therapy efficiency.