Melting of polymeric DNA double helix at elevated temperature: a molecular dynamics approach

Maturation of siRNA by strand separation: Steered molecular dynamics study

RNA interference, particularly siRNA induced gene silencing is becoming an important avenue of modern therapeutics. The siRNA is delivered to the cells as short double helical RNA which becomes single stranded for forming the RISC complex. Significant experimental evidence is available for most of the steps except the process of the separation of the two strands. We have attempted to understand the pathway for double stranded siRNA (dsRNA) to single stranded (ssRNA) molecules using steered molecular dynamics simulations. As the process is completely unexplored we have applied force from all possible directions restraining all possible residues to convert dsRNA to ssRNA. We found pulling one strand along the helical axis direction restraining the far end of the other strand demands excessive force for ssRNA formation. Pulling a central residue of one strand, in a direction perpendicular to the helix axis, while keeping the base paired residue fixed requires intermediate force for strand separation. Moreover, we found that in this process the force requirement is quite high for the first bubble formation (nucleation energy) and the bubble propagation energies are quite small. We believe the success rate of the design of siRNA sequences for gene silencing may increase if this mechanistic knowledge is utilized for such a design process.Communicated by Ramaswamy H. Sarma.

Antitumor drugs effect on the stability of double-stranded DNA: steered molecular dynamics analysis

Denaturation of the DNA double helix inside the cell is essential for cellular processes such as replication and transcription for the growth of the cells. However, the growth of unwanted cells, which are responsible for cancerous kind of disease, is one of the biggest challenges of modern therapeutics. DNA cross-linking agents may kill cancer cells by damaging their DNA and stopping them from dividing. In the present study, we have carried out steered molecular dynamics simulations to study the effects of rupture and unzipping forces on the stability of dsDNA in the absence and presence of covalently bonded drugs. We have found that the stability of dsDNA increases strongly in the presence of covalently bonded drugs. The microscopic study of disruption of hydrogen-bonds associated with base-pairs of the dsDNA and the study of the variation of stacking overlap parameters gives evidence of symmetry during the rupture and asymmetry in the unzip event. The significance of the mechanism of force-induced melting study of the dsDNA in the absence and presence of antitumor drugs might have a biological relevance as it provides a pathway to open the double helix in a specific position and may help for the pharmaceutical design of drugs.Communicated by Ramaswamy H. Sarma.

Computational assessment of thermostability in miRNA:CNT system using molecular dynamics simulations

BACKGROUND

Carbon nanotubes (CNTs) show great promise as theranostic agents due to their drug delivery properties, intrinsic near-infrared radiation-responsiveness, and magnetic functionalization. However, temperature elevation caused by these external stimuli during drug delivery should be considered for the evaluation of CNT-based systems loaded with temperature-sensitive biomolecules.

METHODS

We examine the thermal stability of a 33 nucleotides long hairpin miRNA encapsulated in (20,20) CNT using all-atom molecular dynamics simulations in explicit water. We systematically increase the temperature as 298, 310, 327, and 343 K, reaching the melting temperature of miRNA. To emphasize the effect of the aromatic confined space, we compare the dynamics of miRNA inside the CNT to its dynamics free in the solution at the same temperatures, reaching a total simulation time of 7.9 μs.

RESULTS

miRNA hairpin mostly maintains its double-stranded structure in the confined CNT, even at elevated temperatures. Binding free energies and potential of mean force calculations also underline the strong π-π interactions between the biomolecule and the CNT for 298-343 K.

CONCLUSION

The let-7 miRNA mimic, which represents a wide family of RNAi-based therapeutics, can be transported in the CNT under medically applied hyperthermic conditions.

GENERAL SIGNIFICANCE

This study shows how the structure and dynamics of miRNA hairpin are affected when encapsulated in an aromatic tube, during a systematic increase of temperature. It also indicates the high potential of CNT-based systems for the delivery of oligonucleotide therapeutics while simultaneous imaging/magnetic field guiding to the target tissue is achieved.

Dimeric aryl-substituted imidazoles may inhibit ALT cancer by targeting the multimeric G-quadruplex in telomere

In 10-15% of cancers, telomere maintenance is provided by a telomerase-independent mechanism known as alternative lengthening of telomere (ALT), making telomerase inhibitors ineffective on these cancers. Ligands that stabilize telomeric G-quadruplex (G4) are considered to be able to inhibit either the ALT process or disrupt the T-loop structure, which would be promising therapeutic agents for ALT cancers. Notably, the 3'-terminal overhang of telomeric DNA might fold into multimeric G4 containing consecutive G4 subunits, which offers an attractive target for selective ligands considering large numbers of G4s widespread in the genome. In this study, a dimeric aryl-substituted imidazole (DIZ-3) was developed as a selective multimeric G4 ligand based on a G4-ligand-dimerizing strategy. Biophysical experiments revealed that DIZ-3 intercalated into the G4-G4 interface, stabilizing the higher-order structure. Furthermore, this ligand was demonstrated to induce cell cycle arrest and apoptosis, and thus inhibited cell proliferation in an ALT cancer cell line. Cancer cells were more sensitive to DIZ-3, relative to normal cells. Notably, DIZ-3 had little effect on the transcription of several G4-dependent oncogenes. This study provides a nice example for discovering dimeric agents to potentially treat ALT cancers via targeting telomeric multimeric G4.

Asymmetric base-pair opening drives helicase unwinding dynamics

The opening of a Watson-Crick double helix is required for crucial cellular processes, including replication, repair, and transcription. It has long been assumed that RNA or DNA base pairs are broken by the concerted symmetric movement of complementary nucleobases. By analyzing thousands of base-pair opening and closing events from molecular simulations, here, we uncover a systematic stepwise process driven by the asymmetric flipping-out probability of paired nucleobases. We demonstrate experimentally that such asymmetry strongly biases the unwinding efficiency of DNA helicases toward substrates that bear highly dynamic nucleobases, such as pyrimidines, on the displaced strand. Duplex substrates with identical thermodynamic stability are thus shown to be more easily unwound from one side than the other, in a quantifiable and predictable manner. Our results indicate a possible layer of gene regulation coded in the direction-dependent unwindability of the double helix.