How are small ions involved in the compaction of DNA molecules?

Why Na+ has higher propensity than K+ to condense DNA in a crowded environment

Experimentally, in the presence of the crowding agent polyethylene glycol (PEG), sodium ions compact double-stranded DNA more readily than potassium ions. Here, we have used molecular dynamics simulations and the "ion binding shells model" of DNA condensation to provide an explanation for the observed variations in condensation of short DNA duplexes in solutions containing different monovalent cations and PEG; several predictions are made. According to the model we use, externally bound ions contribute the most to the ion-induced aggregation of DNA duplexes. The simulations reveal that for two adjacent DNA duplexes, the number of externally bound Na+ ions is larger than the number of K+ ions over a wide range of chloride concentrations in the presence of PEG, providing a qualitative explanation for the higher propensity of sodium ions to compact DNA under crowded conditions. The qualitative picture is confirmed by an estimate of the corresponding free energy of DNA aggregation that is at least 0.2kBT per base pair more favorable in solution with NaCl than with KCl at the same ion concentration. The estimated attraction free energy of DNA duplexes in the presence of Na+ depends noticeably on the DNA sequence; we predict that AT-rich DNA duplexes are more readily condensed than GC-rich ones in the presence of Na+. Counter-intuitively, the addition of a small amount of a crowding agent with high affinity for the specific condensing ion may lead to the weakening of the ion-mediated DNA-DNA attraction, shifting the equilibrium away from the DNA condensed phase.

Interactions between Rigid Polyelectrolytes Mediated by Ordering and Orientation of Multivalent Nonspherical Ions in Salt Solutions

Multivalent ions in solutions with polyelectrolytes (PEs) induce electrostatic correlations that can drastically change ion distributions around the PEs and their mutual interactions. Using coarse-grained molecular dynamics simulations, we show how in addition to valency, ion shape and concentration can be harnessed as tools to control rigid like-charged PE-PE interactions. We demonstrate a correlation between the orientational ordering of aspherical ions and how they mediate the effective PE-PE attraction induced by multivalency. The interaction type, strength, and range can thus be externally controlled in ionic solutions. Our results can be used as generic guidelines to tune the self-assembly of like-charged polyelectrolytes by variation of the characteristics of the ions.

Single-molecule compaction of megabase-long chromatin molecules by multivalent cations

To gain insight into the conformational properties and compaction of megabase-long chromatin molecules, we reconstituted chromatin from T4 phage DNA (165 kb) and recombinant human histone octamers (HO). The unimolecular compaction, induced by divalent Mg2+ or tetravalent spermine4+ cations, studied by single-molecule fluorescence microscopy (FM) and dynamic light scattering (DLS) techniques, resulted in the formation of 250-400 nm chromatin condensates. The compaction on this scale of DNA size is comparable to that of chromatin topologically associated domains (TAD) in vivo. Variation of HO loading revealed a number of unique features related to the efficiency of chromatin compaction by multivalent cations, the mechanism of compaction, and the character of partly compact chromatin structures. The observations may be relevant for how DNA accessibility in chromatin is maintained. Compaction of saturated chromatin, in turn, is accompanied by an intra-chain segregation at the level of single chromatin molecules, suggesting an intriguing scenario of selective activation/deactivation of DNA as a result of chromatin fiber heterogeneity due to the nucleosome positioning. We suggest that this chromatin, reconstituted on megabase-long DNA because of its large size, is a useful model of eukaryotic chromatin.

Magnetic Fullerene-DNA/Hyaluronic Acid Nanovehicles with Magnetism/Reduction Dual-Responsive Triggered Release

We created the dual-responsive nanovehicle that can effectively combine and abundantly utilize magnetic and glutathione (GSH)-reductive triggers to control the drug delivery and achieve more intelligent and powerful targeting. In the nanovehicles, paramagnetic fullerene (C₆₀@CTAF) was prepared via one-step modification of fullerene with magnetic surfactant CTAF by hydrophobic interaction for the first time. The perfect conjugation of C₆₀ and CTAF increased the solubility or dispersity of fullerenes and qualified CTAF with more powerful assembly capability with DNA. DNA molecule in the nanovehicles acted as an electrostatic scaffold to load anticancer drug Dox as well as the important building block for assembly with C₆₀@CTAF into C₆₀@CTAF/DNA. The further combination of deshielding and targeting functions in reduction-responsive disulfide modified HA-SS-COOH coating on C₆₀@CTAF/DNA complexes could reduce the agglomeration and regulate the morphology of C₆₀@CTAF/DNA complexes from irregular microstructures to more uniform ones. More importantly, the introduction of HA-SS-COOH provided a response to a simulating reductive extra-tumoral environment by efficient cleavage of disulfide linkages by GSH and site-specific drug delivery to HepG2 cells. Amazingly, the final nanovehicles presented an increased magnetic susceptibility compared with paramagnetic CTAF, and they "walked" under an applied magnetic field. Because of their facile fabrication, rapid responsiveness to extra tumoral environment, and external automatic controllability by external magnet, the drug delivery nanovehicles constructed by magnetic fullerene-DNA/hyaluronic acid might be of great interest for making new functional nucleic-acid-based drug carriers.

Coupling between Histone Conformations and DNA Geometry in Nucleosomes on a Microsecond Timescale: Atomistic Insights into Nucleosome Functions

An octamer of histone proteins wraps about 200bp of DNA into two superhelical turns to form nucleosomes found in chromatin. Although the static structure of the nucleosomal core particle has been solved, details of the dynamic interactions between histones and DNA remain elusive. We performed extensively long unconstrained, all-atom microsecond molecular dynamics simulations of nucleosomes including linker DNA segments and full-length histones in explicit solvent. For the first time, we were able to identify and characterize the rearrangements in nucleosomes on a microsecond timescale including the coupling between the conformation of the histone tails and the DNA geometry. We found that certain histone tail conformations promoted DNA bulging near its entry/exit sites, resulting in the formation of twist defects within the DNA. This led to a reorganization of histone-DNA interactions, suggestive of the formation of initial nucleosome sliding intermediates. We characterized the dynamics of the histone tails upon their condensation on the core and linker DNA and showed that tails may adopt conformationally constrained positions due to the insertion of "anchoring" lysines and arginines into the DNA minor grooves. Potentially, these phenomena affect the accessibility of post-translationally modified histone residues that serve as important sites for epigenetic marks (e.g., at H3K9, H3K27, H4K16), suggesting that interactions of the histone tails with the core and linker DNA modulate the processes of histone tail modifications and binding of the effector proteins. We discuss the implications of the observed results on the nucleosome function and compare our results to different experimental studies.

Why double-stranded RNA resists condensation

The addition of small amounts of multivalent cations to solutions containing double-stranded DNA leads to inter-DNA attraction and eventual condensation. Surprisingly, the condensation is suppressed in double-stranded RNA, which carries the same negative charge as DNA, but assumes a different double helical form. Here, we combine experiment and atomistic simulations to propose a mechanism that explains the variations in condensation of short (25 base-pairs) nucleic acid (NA) duplexes, from B-like form of homopolymeric DNA, to mixed sequence DNA, to DNA:RNA hybrid, to A-like RNA. Circular dichroism measurements suggest that duplex helical geometry is not the fundamental property that ultimately determines the observed differences in condensation. Instead, these differences are governed by the spatial variation of cobalt hexammine (CoHex) binding to NA. There are two major NA-CoHex binding modes--internal and external--distinguished by the proximity of bound CoHex to the helical axis. We find a significant difference, up to 5-fold, in the fraction of ions bound to the external surfaces of the different NA constructs studied. NA condensation propensity is determined by the fraction of CoHex ions in the external binding mode.

Lignin nanotubes as vehicles for gene delivery into human cells

Lignin nanotubes (LNTs) synthesized from the aromatic plant cell wall polymer lignin in a sacrificial alumina membrane template have as useful features their flexibility, ease of functionalization due to the availability of many functional groups, label-free detection by autofluorescence, and customizable optical properties. In this report we show that the physicochemical properties of LNTs can be varied over a wide range to match requirements for specific applications by using lignin with different subunit composition, a function of plant species and genotype, and by choosing the lignin isolation method (thioglycolic acid, phosphoric acid, sulfuric acid (Klason), sodium hydroxide lignin), which influences the size and reactivity of the lignin fragments. Cytotoxicity studies with human HeLa cells showed that concentrations of up to 90 mg/mL are tolerated, which is a 10-fold higher concentration than observed for single- or multiwalled carbon nanotubes (CNTs). Confocal microscopy imaging revealed that all LNT formulations enter HeLa cells without auxiliary agents and that LNTs made from NaOH-lignin penetrate the cell nucleus. We further show that DNA can adsorb to LNTs. Consequently, exposure of HeLa cells to LNTs coated with DNA encoding the green fluorescent protein (GFP) leads to transfection and expression of GFP. The highest transfection efficiency was obtained with LNTs made from NaOH-lignin due to a combination of high DNA binding capacity and DNA delivery directly into the nucleus. These combined features of LNTs make LNTs attractive as smart delivery vehicles of DNA without the cytotoxicity associated with CNTs or the immunogenicity of viral vectors.

Nanostructure-induced DNA condensation

The control of the DNA condensation process is essential for compaction of DNA in chromatin, as well as for biological applications such as nonviral gene therapy. This review endeavours to reflect the progress of investigations on DNA condensation effects of nanostructure-based condensing agents (such as nanoparticles, nanotubes, cationic polymer and peptide agents) observed by using atomic force microscopy (AFM) and other techniques. The environmental effects on structural characteristics of nanostructure-induced DNA condensates are also discussed.

A universal description for the experimental behavior of salt-(in)dependent oligocation-induced DNA condensation

We report a systematic study of the condensation of plasmid DNA by oligocations with variation of the charge, Z, from +3 to +31. The oligocations include a series of synthetic linear ε-oligo(L-lysines), (denoted εKn, n = 3–10, 31; n is the number of lysines with the ligand charge Z = n+1) and branched α-substituted homologues of εK10: εYK10, εLK10 (Z = +11); εRK10, εYRK10 and εLYRK10 (Z = +21). Data were obtained by light scattering, UV absorption monitored precipitation assay and isothermal titration calorimetry in a wide range concentrations of DNA and monovalent salt (KCl, CKCl). The dependence of EC50 (ligand concentration at the midpoint of DNA condensation) on C(KCl) shows the existence of a salt-independent regime at low C(KCl) and a salt-dependent regime with a steep rise of EC50 with increase of C(KCl). Increase of the ligand charge shifts the transition from the salt-independent to salt-dependent regime to higher C(KCl). A novel and simple relationship describing the EC50 dependence on DNA concentration, charge of the ligand and the salt-dependent dissociation constant of the ligand–DNA complex is derived. For the ε-oligolysines εK6–εK10, the experimental dependencies of EC50 on C(KCl) and Z are well-described by an equation with a common set of parameters. Implications from our findings for understanding DNA condensation in chromatin are discussed.

Electrostatic origin of salt-induced nucleosome array compaction

The physical mechanism of the folding and unfolding of chromatin is fundamentally related to transcription but is incompletely characterized and not fully understood. We experimentally and theoretically studied chromatin compaction by investigating the salt-mediated folding of an array made of 12 positioning nucleosomes with 177 bp repeat length. Sedimentation velocity measurements were performed to monitor the folding provoked by addition of cations Na(+), K(+), Mg(2+), Ca(2+), spermidine(3+), Co(NH(3))(6)(3+), and spermine(4+). We found typical polyelectrolyte behavior, with the critical concentration of cation needed to bring about maximal folding covering a range of almost five orders of magnitude (from 2 μM for spermine(4+) to 100 mM for Na(+)). A coarse-grained model of the nucleosome array based on a continuum dielectric description and including the explicit presence of mobile ions and charged flexible histone tails was used in computer simulations to investigate the cation-mediated compaction. The results of the simulations with explicit ions are in general agreement with the experimental data, whereas simple Debye-Hückel models are intrinsically incapable of describing chromatin array folding by multivalent cations. We conclude that the theoretical description of the salt-induced chromatin folding must incorporate explicit mobile ions that include ion correlation and ion competition effects.

Cation-induced polyelectrolyte-polyelectrolyte attraction in solutions of DNA and nucleosome core particles

The paper reviews our current studies on the experimentally induced cation compaction and aggregation in solutions of DNA and nucleosome core particles and the theoretical modelling of these processes using coarse-grained continuum models with explicit mobile ions and with all-atom molecular dynamics (MD) simulations. Recent experimental results on DNA condensation by cationic oligopeptides and the effects of added salt are presented. The results of MD simulations modelling DNA-DNA attraction due to the presence of multivalent ions including the polyamine spermidine and fragments of histone tails, which exhibit bridging between adjacent DNA molecules, are discussed. Experimental data on NCP aggregation, using recombinantly prepared systems are summarized. Literature data and our results of studying of the NCP solutions are compared with predictions of coarse-grained MD simulations, including the important ion correlation as well as bridging mechanisms. The importance of the results to chromatin folding and aggregation is discussed.

A universal description for the experimental behavior of salt-(in)dependent oligocation-induced DNA condensation

We report a systematic study of the condensation of plasmid DNA by oligocations with variation of the charge, Z, from +3 to +31. The oligocations include a series of synthetic linear epsilon-oligo(l-lysines), (denoted epsilonKn, n = 3-10, 31; n is the number of lysines equal to the ligand charge) and branched alpha-substituted homologues of epsilonK10: epsilonYK10, epsilonLK10 (Z = +10); epsilonRK10, epsilonYRK10 and epsilonLYRK10 (Z = +20). Data were obtained by light scattering, UV absorption monitored precipitation assay and isothermal titration calorimetry in a wide range concentrations of DNA and monovalent salt (KCl, C(KCl)). The dependence of EC(50) (ligand concentration at the midpoint of DNA condensation) on C(KCl) shows the existence of a salt-independent regime at low C(KCl) and a salt-dependent regime with a steep rise of EC(50) with increase of C(KCl). Increase of the ligand charge shifts the transition from the salt-independent to salt-dependent regime to higher C(KCl). A novel and simple relationship describing the EC(50) dependence on DNA concentration, charge of the ligand and the salt-dependent dissociation constant of the ligand-DNA complex is derived. For the epsilon-oligolysines epsilonK3-epsilonK10, the experimental dependencies of EC(50) on C(KCl) and Z are well-described by an equation with a common set of parameters. Implications from our findings for understanding DNA condensation in chromatin are discussed.