Time filter

Source Type

Sun City Center, United States

Ha T.,Center for the Physics of Living Cells | Ha T.,University of Illinois at Urbana - Champaign | Kozlov A.G.,University of Washington | Lohman T.M.,University of Washington
Annual Review of Biophysics

The advent of new technologies allowing the study of single biological molecules continues to have a major impact on studies of interacting systems as well as enzyme reactions. These approaches (fluorescence, optical, and magnetic tweezers), in combination with ensemble methods, have been particularly useful for mechanistic studies of proteinnucleic acid interactions and enzymes that function on nucleic acids. We review progress in the use of single-molecule methods to observe and perturb the activities of proteins and enzymes that function on flexible single-stranded DNA. These include single-stranded DNA binding proteins, recombinases (RecARad51), and helicasestranslocases that operate as motor proteins and play central roles in genome maintenance. We emphasize methods that have been used to detect and study the movement of these proteins (both ATP-dependent directional and random movement) along the single-stranded DNA and the mechanistic and functional information that can result from detailed analysis of such movement. © 2012 by Annual Reviews. All rights reserved. Source

Ghoshal G.,Acoustic Med System | Wu Z.,Center for the Physics of Living Cells | Bromfield C.R.,Agricultural Animal Care and Use Program | Williams E.M.,Acoustic Med System | And 9 more authors.
ACS Nano

Repurposing of existing cancer drugs to overcome their physical limitations, such as insolubility, represents an attractive strategy to achieve enhanced therapeutic efficacy and broaden the range of clinical applications. Such an approach also promises to offer substantial cost savings in drug development efforts. Here we repurposed FDA-approved topical agent bexarotene (Targretin), currently in limited use for cutaneous manifestations of T-cell lymphomas, and re-engineer it for use in solid tumor applications by forming self-assembling nanobubbles. Physico-chemical characterization studies of the novel prodrug nanobubbles demonstrated their stability, enhanced target cell internalization capability, and highly controlled release profile in response to application of focused ultrasound energy. Using an in vitro model of hepatocellular carcinoma and an in vivo large animal model of liver ablation, we demonstrate the effectiveness of bexarotene prodrug nanobubbles when used in conjunction with catheter-based ultrasound, thereby highlighting the therapeutic promise of this trimodal approach. © 2015 American Chemical Society. Source

Crawled News Article
Site: http://phys.org/biology-news/

Led by Aleksei Aksimentiev, a professor of physics at the University of Illinois, and Taekjip Ha, a professor of biophysics and biophysical chemistry at Johns Hopkins University and an adjunct at the University of Illinois Center for the Physics of Living Cells along with Aksimentiev, the researchers published their work in the journal Nature Communications. "We are still only starting to explore the physical properties of DNA. It's not just a string of letters," Aksimentiev said. "It's a complex molecule with unique characteristics. The prevailing hypothesis is that everything that happens inside the nucleus, the way the DNA is organized, is all the work of proteins. What we show is that direct DNA-DNA interactions may play a role in large-scale chromosome organization as well." Using the Blue Waters supercomputer at the National Center for Supercomputing Applications on the Illinois campus, Aksimentiev and postdoctoral researcher Jejoong Yoo performed detailed simulations of two DNA molecules interacting in a charged solution such as is found in the cell. The supercomputer allowed them to map each individual atom and its behavior, and to measure the forces between the molecules. They found that, though DNA molecules tend to repel each other in water, in a cell-like environment two DNA molecules can interact according to their respective sequences. "In the DNA alphabet, there is A, T, G and C. We found that when a sequence is rich in A and T, there is a stronger attraction," Aksimentiev said. "Then we looked at what actually causes it at the molecular level. We like to think of DNA as a nice symmetrical helix, but actually there's a line of bumps which are methyl groups, which we find are the key to regulating this sequence-dependent attraction." One of the processes for regulating gene expression is methylation, which adds methyl groups to the DNA helix. In further simulations, the researchers found that the methyl groups strengthen the attraction, so sequences heavy in G and C with methyl groups attached will interact just as strongly as sequences rich in A and T. "The key is the presence of charged particles in the solution," Aksimentiev said. "Let's say you have two people who don't like each other, but I like them both, so I can shake hands with both of them and bring them close. The counter-ions work exactly like that. The strength of how they pull the DNA molecules together depends on how many of them are between the molecules. When we have these bumps, we have a lot of counter-ions." Ha and graduate researcher Hajin Kim experimentally verified the findings of the simulations. Using advanced single-molecule imaging techniques, they isolated two DNA molecules inside a tiny bubble, then watched to see how the molecules interacted. The experiments matched well with the data from the simulations, both for the sequence-dependent interactions and for interactions between methylated DNA. "It was wonderful to see the computational predictions borne out exactly in our experiments," Ha said. "It tells us how accurate the atomic-level simulations are and shows that they can guide new research avenues." The researchers posit that the observed interactions between DNA molecules could play a role in how chromosomes are organized in the cell and which ones are expanded or folded up compactly, determining functions of different cell types or regulating the cell cycle. "For example, once you methylate DNA, the chromosome becomes more compact. It prevents the cellular machinery from accessing the DNA," Aksimentiev said. "It's a way to tell which genes are turned on and which are turned off. This could be part of the bigger question of how chromosomes are arranged and how organizational mechanisms can affect gene expression." Explore further: Charged graphene gives DNA a stage to perform molecular gymnastics More information: Jejoong Yoo et al. Direct evidence for sequence-dependent attraction between double-stranded DNA controlled by methylation, Nature Communications (2016). DOI: 10.1038/ncomms11045

Yoo J.,University of Illinois at Urbana - Champaign | Yoo J.,Center for the Physics of Living Cells | Aksimentiev A.,University of Illinois at Urbana - Champaign | Aksimentiev A.,Center for the Physics of Living Cells | Aksimentiev A.,Beckman Institute for Advanced Science and Technology
Journal of Physical Chemistry B

The concept of "ion atmosphere" is prevalent in both theoretical and experimental studies of nucleic acid systems, yet the spatial arrangement and the composition of ions in the ion atmosphere remain elusive, in particular when several ionic species (e.g., Na+, K+, and Mg 2+) compete to neutralize the charge of a nucleic acid polyanion. Complementing the experimental study of Bai and co-workers (J. Am. Chem. Soc.2007, 129, 14981), here we characterize ion atmosphere around double-stranded DNA through all-atom molecular dynamics simulations. We demonstrate that our improved parametrization of the all-atom model can quantitatively reproduce the experimental ion-count data. Our simulations determine the size of the ion atmosphere, the concentration profiles of ionic species competing to neutralize the DNA charge, and the sites of the cations-preferential binding at the surface of double-stranded DNA. We find that the effective size of the ion atmosphere depends on both the bulk concentration and valence of ions: increasing either reduces the size of the atmosphere. Near DNA, the concentration of Mg2+ is strongly enhanced compared to monovalent cations. Within the DNA grooves, the relative concentrations of cations depend on their bulk values. Nevertheless, the total charge of competing cations buried in the DNA grooves is constant and compensates for about ∼30% of the total DNA charge. © 2012 American Chemical Society. Source

Yoo J.,University of Illinois at Urbana - Champaign | Yoo J.,Center for the Physics of Living Cells | Wilson J.,University of Illinois at Urbana - Champaign | Aksimentiev A.,University of Illinois at Urbana - Champaign | Aksimentiev A.,Center for the Physics of Living Cells

Calcium ions (Ca2+) play key roles in various fundamental biological processes such as cell signaling and brain function. Molecular dynamics (MD) simulations have been used to study such interactions, however, the accuracy of the Ca2+ models provided by the standard MD force fields has not been rigorously tested. Here, we assess the performance of the Ca2+ models from the most popular classical force fields AMBER and CHARMM by computing the osmotic pressure of model compounds and the free energy of DNA–DNA interactions. In the simulations performed using the two standard models, Ca2+ ions are seen to form artificial clusters with chloride, acetate, and phosphate species; the osmotic pressure of CaAc2 and CaCl2 solutions is a small fraction of the experimental values for both force fields. Using the standard parameterization of Ca2+ ions in the simulations of Ca2+-mediated DNA–DNA interactions leads to qualitatively wrong outcomes: both AMBER and CHARMM simulations suggest strong inter-DNA attraction whereas, in experiment, DNA molecules repel one another. The artificial attraction of Ca2+ to DNA phosphate is strong enough to affect the direction of the electric field-driven translocation of DNA through a solid-state nanopore. To address these shortcomings of the standard Ca2+ model, we introduce a custom model of a hydrated Ca2+ ion and show that using our model brings the results of the above MD simulations in quantitative agreement with experiment. Our improved model of Ca2+ can be readily applied to MD simulations of various biomolecular systems, including nucleic acids, proteins and lipid bilayer membranes. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 752–763, 2016. © 2016 Wiley Periodicals, Inc. Source

Discover hidden collaborations