In-cell NMR suggests that DNA i-motif levels are strongly depleted in living human cells.

I-Motifs (iM) are non-canonical DNA structures potentially forming in the accessible, single-stranded, cytosine-rich genomic regions with regulatory roles. Chromatin, protein interactions, and intracellular properties seem to govern iM formation at sites with i-motif formation propensity (iMFPS) in human cells, yet their specific contributions remain unclear. Using in-cell NMR with oligonucleotide iMFPS models, we monitor iM-associated structural equilibria in asynchronous and cell cycle-synchronized HeLa cells at 37 °C. Our findings show that iMFPS displaying pHT < 7 under reference in vitro conditions occur predominantly in unfolded states in cells, while those with pHT > 7 appear as a mix of folded and unfolded states depending on the cell cycle phase. Comparing these results with previous data obtained using an iM-specific antibody (iMab) reveals that cell cycle-dependent iM formation has a dual origin, and iM formation concerns only a tiny fraction (possibly 1%) of genomic sites with iM formation propensity. We propose a comprehensive model aligning observations from iMab and in-cell NMR and enabling the identification of iMFPS capable of adopting iM structures under physiological conditions in living human cells. Our results suggest that many iMFPS may have biological roles linked to their unfolded states.

A sodium/potassium switch for G4-prone G/C-rich sequences.

Metal ions are essential components for the survival of living organisms. For most species, intracellular and extracellular ionic conditions differ significantly. As G-quadruplexes (G4s) are ion-dependent structures, changes in the [Na+]/[K+] ratio may affect the folding of genomic G4s. More than 11000 putative G4 sequences in the human genome (hg19) contain at least two runs of three continuous cytosines, and these mixed G/C-rich sequences may form a quadruplex or a competing hairpin structure based on G-C base pairing. In this study, we examine how the [Na+]/[K+] ratio influences the structures of G/C-rich sequences. The natural G4 structure with a 9-nt long central loop, CEBwt, was chosen as a model sequence, and the loop bases were gradually replaced by cytosines. The series of CEB mutations revealed that the presence of cytosines in G4 loops does not prevent G4 folding or decrease G4 stability but increases the probability of forming a competing structure, either a hairpin or an intermolecular duplex. Slow conversion to the quadruplex in vitro (in a potassium-rich buffer) and cells was demonstrated by NMR. 'Shape-shifting' sequences may respond to [Na+]/[K+] changes with delayed kinetics.

Towards Profiling of the G-Quadruplex Targeting Drugs in the Living Human Cells Using NMR Spectroscopy.

Recently, the 1H-detected in-cell NMR spectroscopy has emerged as a unique tool allowing the characterization of interactions between nucleic acid-based targets and drug-like molecules in living human cells. Here, we assess the application potential of 1H and 19F-detected in-cell NMR spectroscopy to profile drugs/ligands targeting DNA G-quadruplexes, arguably the most studied class of anti-cancer drugs targeting nucleic acids. We show that the extension of the original in-cell NMR approach is not straightforward. The severe signal broadening and overlap of 1H in-cell NMR spectra of polymorphic G-quadruplexes and their complexes complicate their quantitative interpretation. Nevertheless, the 1H in-cell NMR can be used to identify drugs that, despite strong interaction in vitro, lose their ability to bind G-quadruplexes in the native environment. The in-cell NMR approach is adjusted to a recently developed 3,5-bis(trifluoromethyl)phenyl probe to monitor the intracellular interaction with ligands using 19F-detected in-cell NMR. The probe allows dissecting polymorphic mixture in terms of number and relative populations of individual G-quadruplex species, including ligand-bound and unbound forms in vitro and in cellulo. Despite the probe's discussed limitations, the 19F-detected in-cell NMR appears to be a promising strategy to profile G-quadruplex-ligand interactions in the complex environment of living cells.

Composite 5-methylations of cytosines modulate i-motif stability in a sequence-specific manner: Implications for DNA nanotechnology and epigenetic regulation of plant telomeric DNA.

BACKGROUND: The i-motif is a tetrameric DNA structure based on the formation of hemiprotonated cytosine-cytosine (C+.C) base pairs. i-motifs are widely used in nanotechnology. In biological systems, i-motifs are involved in gene regulation and in control of genome integrity. In vivo, the i-motif forming sequences are subjects of epigenetic modifications, particularly 5-cytosine methylation. In plants, natively occurring methylation patterns lead to a complex network of C+.C, 5mC+.C and 5mC+.5mC base-pairs in the i-motif stem. The impact of complex methylation patterns (CMPs) on i-motif formation propensity is currently unknown. METHODS: We employed CD and UV-absorption spectroscopies, native PAGE, thermal denaturation and quantum-chemical calculations to analyse the effects of native, native-like, and non-native CMPs in the i-motif stem on the i-motif stability and pKa. RESULTS: CMPs have strong influence on i-motif stability and pKa and influence these parameters in sequence-specific manner. In contrast to a general belief, i) CMPs do not invariably stabilize the i-motif, and ii) when the CMPs do stabilize the i-motif, the extent of the stabilization depends (in a complex manner) on the number and pattern of symmetric 5mC+.5mC or asymmetric 5mC+.C base pairs in the i-motif stem. CONCLUSIONS: CMPs can be effectively used to fine-tune i-motif properties. Our data support the notion of epigenetic modifications as a plausible control mechanism of i-motif formation in vivo. GENERAL SIGNIFICANCE: Our results have implications in epigenetic regulation of telomeric DNA in plants and highlight the potential and limitations of engineered patterning of cytosine methylations on the i-motif scaffold in nanotechnological applications.

Elucidation of protein interactions necessary for the maintenance of the BCR-ABL signaling complex.

Many patients with chronic myeloid leukemia in deep remission experience return of clinical disease after withdrawal of tyrosine kinase inhibitors (TKIs). This suggests signaling of inactive BCR-ABL, which allows the survival of cancer cells, and relapse. We show that TKI treatment inhibits catalytic activity of BCR-ABL, but does not dissolve BCR-ABL core signaling complex, consisting of CRKL, SHC1, GRB2, SOS1, cCBL, p85a-PI3K, STS1 and SHIP2. Peptide microarray and co-immunoprecipitation results demonstrate that CRKL binds to proline-rich regions located in C-terminal, intrinsically disordered region of BCR-ABL, that SHC1 requires pleckstrin homology, src homology and tyrosine kinase domains of BCR-ABL for binding, and that BCR-ABL sequence motif located in disordered region around phosphorylated tyrosine 177 mediates binding of three core complex members, i.e., GRB2, SOS1, and cCBL. Further, SHIP2 binds to the src homology and tyrosine kinase domains of BCR-ABL and its inositol phosphatase activity contributes to BCR-ABL-mediated phosphorylation of SHC1. Together, this study characterizes protein-protein interactions within the BCR-ABL core complex and determines the contribution of particular BCR-ABL domains to downstream signaling. Understanding the structure and dynamics of BCR-ABL interactome is critical for the development of drugs targeting integrity of the BCR-ABL core complex.

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.

In Cell NMR Spectroscopy: Investigation of G-Quadruplex Structures Inside Living Xenopus laevis Oocytes.

G-quadruplexes are inherently polymorphic nucleic acid structures. Their folding topology depends on the nucleic acid primary sequence and on physical-chemical environmental factors. Hence, it remains unclear if a G-quadruplex topology determined in the test tube (in vitro) will also form in vivo. Characterization of G-quadruplexes in their native environment has been proposed as an efficient strategy to tackle this issue. So far, characterization of G-quadruplex structures in living cells has relied exclusively on the use of Xenopus laevis oocytes as a eukaryotic cell model system. Here, we describe the protocol for the preparation of X. laevis oocytes for studies of G-quadruplexes as well as other nucleic acids motifs under native conditions using in-cell NMR spectroscopy.

In-Cell NMR Spectroscopy of Nucleic Acids in Human Cells.

In-cell NMR spectroscopy is a unique tool that enables the study of the structure and dynamics of biomolecules as well as their interactions in the complex environment of living cells at near-to-atomic resolution. In this article, detailed instructions are described for setting up an in-cell NMR experiment for monitoring structures of DNA oligonucleotides introduced into nuclei of living human cells via tailored electroporation. Detailed step-by-step protocols for both the preparation of an in-cell NMR sample as well as protocols for conducting essential control experiments including flow cytometry and confocal microscopy are described. The strengths and limitations of in-cell NMR experiments are discussed. © 2018 by John Wiley & Sons, Inc.

Analysis of binding interfaces of the human scaffold protein AXIN1 by peptide microarrays.

Intrinsically disordered regions (IDRs) are protein regions that lack persistent secondary or tertiary structure under native conditions. IDRs represent >40% of the eukaryotic proteome and play a crucial role in protein-protein interactions. The classical approach for identification of these interaction interfaces is based on mutagenesis combined with biochemical techniques such as coimmunoprecipitation or yeast two-hybrid screening. This approach either provides information of low resolution (large deletions) or very laboriously tries to precisely define the binding epitope via single amino acid substitutions. Here, we report the use of a peptide microarray based on the human scaffold protein AXIN1 for high-throughput and -resolution mapping of binding sites for several AXIN1 interaction partners in vitro For each of the AXIN1-binding partners tested, i.e. casein kinase 1 ϵ (CK1ϵ); c-Myc; peptidyl-prolyl cis/trans isomerase, NIMA-interacting 1 (Pin1); and p53, we found at least three different epitopes, predominantly in the central IDR of AXIN1. We functionally validated the specific AXIN1-CK1ϵ interaction identified here with epitope-mimicking peptides and with AXIN1 variants having deletions of short binding epitopes. On the basis of these results, we propose a model in which AXIN1 competes with dishevelled (DVL) for CK1ϵ and regulates CK1ϵ-induced phosphorylation of DVL and activation of Wnt/ß-catenin signaling.

Nanodiamonds as "artificial proteins": Regulation of a cell signalling system using low nanomolar solutions of inorganic nanocrystals.

The blocking of specific protein-protein interactions using nanoparticles is an emerging alternative to small molecule-based therapeutic interventions. However, the nanoparticles designed as "artificial proteins" generally require modification of their surface with (bio)organic molecules and/or polymers to ensure their selectivity and specificity of action. Here, we show that nanosized diamond crystals (nanodiamonds, NDs) without any synthetically installed (bio)organic interface enable the specific and efficient targeting of the family of extracellular signalling molecules known as fibroblast growth factors (FGFs). We found that low nanomolar solutions of detonation NDs with positive ζ-potential strongly associate with multiple FGF ligands present at sub-nanomolar concentrations and effectively neutralize the effects of FGF signalling in cells without interfering with other growth factor systems and serum proteins unrelated to FGFs. We identified an evolutionarily conserved FGF recognition motif, ∼17 amino acids long, that contributes to the selectivity of the ND-FGF interaction. In addition, we inserted this motif into a de novo constructed chimeric protein, which significantly improved its interaction with NDs. We demonstrated that the interaction of NDs, as purely inorganic nanoparticles, with proteins can mitigate pathological FGF signalling and promote the restoration of cartilage growth in a mouse limb explant model. Based on our observations, we foresee that NDs may potentially be applied as nanotherapeutics to neutralize disease-related activities of FGFs in vivo.

Advances in the cellular structural biology of nucleic acids.

Conventional biophysical and chemical biology approaches for delineating relationships between the structure and biological function of nucleic acids (NAs) abstract NAs from their native biological context. However, cumulative experimental observations have revealed that the structure, dynamics and interactions of NAs might be strongly influenced by a broad spectrum of specific and nonspecific physical-chemical environmental factors. This consideration has recently sparked interest in the development of novel tools for structural characterization of NAs in the native cellular context. Here, we review the individual methods currently being employed for structural characterization of NA structure in a native cellular environment with a focus on recent advances and developments in the emerging fields of in-cell NMR and electron paramagnetic resonance spectroscopy and in-cell single-molecule FRET of NAs.

Evaluation of the Stability of DNA i-Motifs in the Nuclei of Living Mammalian Cells.

C-rich DNA has the capacity to form a tetra-stranded structure known as an i-motif. The i-motifs within genomic DNA have been proposed to contribute to the regulation of DNA transcription. However, direct experimental evidence for the existence of these structures in vivo has been missing. Whether i-motif structures form in complex environment of living cells is not currently known. Herein, using state-of-the-art in-cell NMR spectroscopy, we evaluate the stabilities of i-motif structures in the complex cellular environment. We show that i-motifs formed from naturally occurring C-rich sequences in the human genome are stable and persist in the nuclei of living human cells. Our data show that i-motif stabilities in vivo are generally distinct from those in vitro. Our results are the first to interlink the stability of DNA i-motifs in vitro with their stability in vivo and provide essential information for the design and development of i-motif-based DNA biosensors for intracellular applications.

One reporter for in-cell activity profiling of majority of protein kinase oncogenes.

In-cell profiling enables the evaluation of receptor tyrosine activity in a complex environment of regulatory networks that affect signal initiation, propagation and feedback. We used FGF-receptor signaling to identify EGR1 as a locus that strongly responds to the activation of a majority of the recognized protein kinase oncogenes, including 30 receptor tyrosine kinases and 154 of their disease-associated mutants. The EGR1 promoter was engineered to enhance trans-activation capacity and optimized for simple screening assays with luciferase or fluorescent reporters. The efficacy of the developed, fully synthetic reporters was demonstrated by the identification of novel targets for two clinically used tyrosine kinase inhibitors, nilotinib and osimertinib. A universal reporter system for in-cell protein kinase profiling will facilitate repurposing of existing anti-cancer drugs and identification of novel inhibitors in high-throughput screening studies.

G-quadruplex formation in the Oct4 promoter positively regulates Oct4 expression.

The Oct4 gene codes for a transcription factor that plays a critical role in the maintenance of pluripotency in embryonic and cancer stem cells. Its expression thus has to be tightly regulated. We performed biophysical characterization of the promoter region using a combination of UV absorption, CD, and NMR spectroscopies, native PAGE and chemical probing, which was followed by functional studies involving luciferase reporter assays performed in osteosarcoma and human embryonic stem cell lines. We have shown that the evolutionarily conserved G-rich region close to the Oct4 transcription start site in the non-template strand forms a parallel G-quadruplex structure. We characterized its structure and stability upon point mutations in its primary structure. Functional studies then revealed that whereas the wild type quadruplex sequence ensures high reporter gene expression, the expression of mutated variants is significantly decreased proportionally to the destabilizing effect of the mutations on the quadruplex. A ligand, N-methyl mesoporphyrin IX that increases the stability of formed quadruplex rescued the reporter expression of single-mutated variants to the level of wild-type, but it has no effect on a mutated variant that cannot form quadruplex. These data indicate that the quadruplex acts as a strong, positive regulator of Oct4 expression and as such it might serve as a potential target for therapeutic intervention.

Unique C. elegans telomeric overhang structures reveal the evolutionarily conserved properties of telomeric DNA.

There are two basic mechanisms that are associated with the maintenance of the telomere length, which endows cancer cells with unlimited proliferative potential. One mechanism, referred to as alternative lengthening of telomeres (ALT), accounts for approximately 10-15% of all human cancers. Tumours engaged in the ALT pathway are characterised by the presence of the single stranded 5'-C-rich telomeric overhang (C-overhang). This recently identified hallmark of ALT cancers distinguishes them from healthy tissues and renders the C-overhang as a clear target for anticancer therapy. We analysed structures of the 5'-C-rich and 3'-G-rich telomeric overhangs from human and Caenorhabditis elegans, the recently established multicellular in vivo model of ALT tumours. We show that the telomeric DNA from C. elegans and humans forms fundamentally different secondary structures. The unique structural characteristics of C. elegans telomeric DNA that are distinct not only from those of humans but also from those of other multicellular eukaryotes allowed us to identify evolutionarily conserved properties of telomeric DNA. Differences in structural organisation of the telomeric DNA between the C. elegans and human impose limitations on the use of the C. elegans as an ALT tumour model.

Investigation of quadruplex structure under physiological conditions using in-cell NMR.

In this chapter we describe the application of in-cell NMR spectroscopy to the investigation of G-quadruplex structures inside living Xenopus laevis oocytes and in X. laevis egg extract. First, in-cell NMR spectroscopy of nucleic acids (NA) is introduced and applications and limitations of the approach are discussed. In the following text the application of in-cell NMR spectroscopy to investigation of G-quadruplexes are reviewed. Special emphasis is given to the discussion of the influence of the intracellular environmental factors such as low molecular weight compounds, molecular crowding, and hydration on structural behavior of G-quadruplexes. Finally, future perspectives of in-cell NMR spectroscopy for quantitative characterization of G-quadruplexes and NA are discussed.

Towards characterization of DNA structure under physiological conditions in vivo at the single-molecule level using single-pair FRET.

Fluorescence resonance energy transfer (FRET) under in vivo conditions is a well-established technique for the evaluation of populations of protein bound/unbound nucleic acid (NA) molecules or NA hybridization kinetics. However, in vivo FRET has not been applied to in vivo quantitative conformational analysis of NA thus far. Here we explored parameters critical for characterization of NA structure using single-pair (sp)FRET in the complex cellular environment of a living Escherichia coli cell. Our measurements showed that the fluorophore properties in the cellular environment differed from those acquired under in vitro conditions. The precision for the interprobe distance determination from FRET efficiency values acquired in vivo was found lower (≈ 31%) compared to that acquired in diluted buffers (13%). Our numerical simulations suggest that despite its low precision, the in-cell FRET measurements can be successfully applied to discriminate among various structural models. The main advantage of the in-cell spFRET setup presented here over other established techniques allowing conformational analysis in vivo is that it allows investigation of NA structure in various cell types and in a native cellular environment, which is not disturbed by either introduced bulk NA or by the use of chemical transfectants.

Sixteen years and counting: the current understanding of fibroblast growth factor receptor 3 (FGFR3) signaling in skeletal dysplasias.

In 1994, the field of bone biology was significantly advanced by the discovery that activating mutations in the fibroblast growth factor receptor 3 (FGFR3) receptor tyrosine kinase (TK) account for the common genetic form of dwarfism in humans, achondroplasia (ACH). Other conditions soon followed, with the list of human disorders caused by FGFR3 mutations now reaching at least 10. An array of vastly different diagnoses is caused by similar mutations in FGFR3, including syndromes affecting skeletal development (hypochondroplasia [HCH], ACH, thanatophoric dysplasia [TD]), skin (epidermal nevi, seborrhaeic keratosis, acanthosis nigricans), and cancer (multiple myeloma [MM], prostate and bladder carcinoma, seminoma). Despite many years of research, several aspects of FGFR3 function in disease remain obscure or controversial. As FGFR3-related skeletal dysplasias are caused by growth attenuation of the cartilage, chondrocytes appear to be unique in their response to FGFR3 activation. However, the reasons why FGFR3 inhibits chondrocyte growth while causing excessive cellular proliferation in cancer are not clear. Likewise, the full spectrum of molecular events by which FGFR3 mediates its signaling is just beginning to emerge. This article describes the challenging journey to unravel the mechanisms of FGFR3 function in skeletal dysplasias, the extraordinary cellular manifestations of FGFR3 signaling in chondrocytes, and finally, the progress toward therapy for ACH and cancer.

NMR cross-correlated relaxation rates reveal ion coordination sites in DNA.

In this work, a novel NMR method for the identification of preferential coordination sites between physiologically relevant counterions and nucleic acid bases is demonstrated. In this approach, the NMR cross-correlated relaxation rates between the aromatic carbon chemical shift anisotropy and the proton-carbon dipolar interaction are monitored as a function of increasing Na(+), K(+), and Mg(2+) concentrations. Increasing the counterion concentration modulates the residence times of the counterions at specific sites around the nucleic acid bases. It is demonstrated that the modulation of the counterion concentration leads to sizable variations of the cross-correlated relaxation rates, which can be used to probe the site-specific counterion coordination. In parallel, the very same measurements report on the rotational tumbling of DNA, which, as shown here, depends on the nature of the ion and its concentration. This methodology is highly sensitive and easily implemented. The method can be used to cross-validate and/or complement direct but artifact-prone experimental techniques such as X-ray diffraction, NMR analysis with substitutionary ions, and molecular dynamics simulations. The feasibility of this technique is demonstrated on the extraordinarily stable DNA mini-hairpin d(GCGAAGC).

The parallel G-quadruplex structure of vertebrate telomeric repeat sequences is not the preferred folding topology under physiological conditions.

G-quadruplex topologies of telomeric repeat sequences from vertebrates were investigated in the presence of molecular crowding (MC) mimetics, namely polyethylene glycol 200 (PEG), Ficoll 70 as well as Xenopus laevis egg extract by CD and NMR spectroscopy and native PAGE. Here, we show that the conformational behavior of the telomeric repeats in X. laevis egg extract or in Ficoll is notably different from that observed in the presence of PEG. While the behavior of the telomeric repeat in X. laevis egg extract or in Ficoll resembles results obtained under dilute conditions, PEG promotes the formation of high-order parallel topologies. Our data suggest that PEG should not be used as a MC mimetic.