Stapling a G-quadruplex specific peptide

PET Imaging of Solid Tumors with a G-Quadruplex-Targeting 18F-Labeled Peptide Probe

Positron emission tomography (PET) is a common imaging technique and can provide accurate information about the size, shape, and location of tumors. Recent evidence has shown that G-quadruplex structures (G4s) are identified in human oncogenes, and these special structures are recognized as diagnostic cancer markers and drug targets for anticancer therapies. Although a number of techniques for in vivo imaging of G4s have been developed, achieving sufficient sensitivity and selectivity in vivo remains challenging. Herein, we have engineered and developed a radiolabeled peptide probe [18F]AlF-NOTA-RHAU18 targeting mitochondrial DNA G4s for in vivo PET imaging. The results of the study indicate that this probe is able to visualize and detect solid tumors in living homozygous mice. In addition, the distribution of the probe in cancer cells was investigated using FITC-RHAU18. This work may offer new insights into the development of cancer diagnostic tools by targeting in vivo G4s.

Structural Basis for Parallel G-Quadruplex Recognition by an Ankyrin Protein

G-Quadruplex (G4) structures formed by guanine-rich DNA and RNA sequences are implicated in various biological processes. Understanding the mechanisms by which proteins recognize G4 structures is crucial for elucidating their functional roles. Here we present the X-ray crystal structure of an ankyrin protein bound to a parallel G4 structure. Our findings reveal a new specific recognition mode in which a bundle of α-helices and loops of the ankyrin form a flat surface to stack on the G-tetrad core. The protein employs a combination of hydrogen bonds and hydrophobic contacts to interact with the G4, and electrostatic interaction is used to enhance the binding affinity. This binding mechanism provides valuable insights into understanding G4 recognition by proteins.

Why to target G-quadruplexes using peptides: Next-generation G4-interacting ligands

Guanine-rich oligonucleotides existing in both DNA and RNA are able to fold into four-stranded DNA secondary structures via Hoogsteen type hydrogen-bonding, where four guanines self-assemble into a square planar arrangement, which, when stacked upon each other, results in the formation of higher-order structures called G-quadruplexes. Their distribution is not random; they are more frequently present at telomeres, proto-oncogenic promoters, introns, 5'- and 3'-untranslated regions, stem cell markers, ribosome binding sites and so forth and are associated with various biological functions, all of which play a pivotal role in various incurable diseases like cancer and cellular ageing. Several studies have suggested that G-quadruplexes could not regulate biological processes by themselves; instead, various proteins take part in this regulation and can be important therapeutic targets. There are certain limitations in using whole G4-protein for therapeutics purpose because of its high manufacturing cost, laborious structure prediction, dynamic nature, unavailability for oral administration due to its degradation in the gut and inefficient penetration to reach the target site because of the large size. Hence, biologically active peptides can be the potential candidates for therapeutic intervention instead of the whole G4-protein complex. In this review, we aimed to clarify the biological roles of G4s, how we can identify them throughout the genome via bioinformatics, the proteins interacting with G4s and how G4-interacting peptide molecules may be the potential next-generation ligands for targeting the G4 motifs located in biologically important regions.

Understanding the Interactions of Guanine Quadruplexes with Peptides as Novel Strategies for Diagnosis or Tuning Biological Functions

Guanine quadruplexes (G4s) are nucleic acid structures exhibiting a complex structural behavior and exerting crucial biological functions in both cells and viruses. The specific interactions of peptides with G4s, as well as an understanding of the factors driving the specific recognition are important for the rational design of both therapeutic and diagnostic agents. In this review, we examine the most important studies dealing with the interactions between G4s and peptides, highlighting the strengths and limitations of current analytic approaches. We also show how the combined use of high-level molecular simulation techniques and experimental spectroscopy is the best avenue to design specifically tuned and selective peptides, thus leading to the control of important biological functions.

RHAU Peptides Specific for Parallel G-Quadruplexes: Potential Applications in Chemical Biology

G-quadruplexes (G4s) are non-canonical nucleic acid structures formed by guanine (G)-rich sequences, which are ubiquitously found in the human genome and transcriptome. Targeting G4s by specific ligands provides a powerful tool to monitor and regulate G4s-associated biological processes. RHAU peptides, derived from the G4-binding motif of "RNA Helicase associated with AU-rich element" (RHAU), have emerged as extraordinary ligands for specific recognition of parallel G4s. This review highlights the significances of recent studies investigating potential applications of the engineered RHAU peptides incorporated to different functional moieties.

G-Quadruplex Recognition by DARPIns through Epitope/Paratope Analogy

We investigated the mechanisms leading to the specific recognition of Guanine Guadruplex (G4) by DARPins peptides, which can lead to the design of G4 s specific sensors. To this end we carried out all-atom molecular dynamic simulations to unravel the interactions between specific nucleic acids, including human-telomeric (h-telo), Bcl-2, and c-Myc, with different peptides, forming a DARPin/G4 complex. By comparing the sequences of DARPin with that of a peptide known for its high affinity for c-Myc, we show that the recognition cannot be ascribed to sequence similarity but, instead, depends on the complementarity between the three-dimensional arrangement of the molecular fragments involved: the α-helix/loops domain of DARPin and the G4 backbone. Our results reveal that DARPins tertiary structure presents a charged hollow region in which G4 can be hosted, thus the more complementary the structural shapes, the more stable the interaction.

Interaction of a Short Peptide with G-Quadruplex-Forming Sequences: An SRCD and CD Study

G-quadruplex (G4) forming DNA sequences were recently found to play a crucial role in the regulation of genomic processes such as replication, transcription and translation, also related to serious diseases. Therefore, systems capable of controlling DNA and RNA G-quadruplex structures would be useful for the modulation of various cellular events. In particular, peptides represent good candidates for targeting G-quadruplex structures, since they are easily tailored to enhance their functionality. In this work, we analyzed, by circular dichroism and synchrotron radiation circular dichroism spectroscopies, the interaction of a 25-residue peptide deriving from RHAU helicases (Rhau25) with three G-quadruplex-forming oligonucleotide sequences, in both sodium- and potassium-containing buffers, the most relevant monovalent cations in physiological conditions. The peptide displayed greater affinity for the G4 sequences adopting a parallel structure. However, it showed the ability to also interact with antiparallel or hybrid G-quadruplex structures, inducing a conformation conversion to the parallel structure. The stability of the oligonucleotide structure alone or in presence of the Rhau25 peptide was studied by temperature melting and UV denaturation experiments, and the data showed that the interaction with the peptide stabilized the conformation of oligonucleotide sequences when subjected to stress conditions.

A short peptide that preferentially binds c-MYC G-quadruplex DNA

G-quadruplexes (G4s) are non-canonical DNA secondary structures. The identification of selective tools to probe individual G4s over the ∼700 000 found in the human genome is key to unravel the biological significance of specific G4s. We took inspiration from a crystal structure of the bovine DHX36 helicase bound to the G4 formed in the promoter region of the oncogene c-MYC to identify a short peptide that preferentially binds MYC G4 with nM affinity over a small panel of parallel and non-parallel G4s tested.