History and development
Applying magnetic theory to the study of biology is a biophysical technique that started to appear in Germany in the early 1920’s. Possibly the first demonstration was published by Alfred Heilbronn in 1922; his work looked at viscosity of protoplasts.[1] The following year, Freundlich and Seifriz explored rheology in echinoderm eggs. Both studies included insertion of magnetic particles into cells and resulting movement observations in a magnetic field gradient.[2]
In 1949 at Cambridge University, Francis Crick and Arthur Hughes demonstrated a novel use of the technique, calling it "The Magnetic Particle Method." The idea, which originally came from Dr. Honor Fell, was that tiny magnetic beads, phagocytoced by whole cells grown in culture, could be manipulated by an external magnetic field The tissue culture was allowed to grow in the presence of the magnetic material, and cells that contained a magnetic particle could be seen with a high power microscope. As the magnetic particle was moved through the cell by a magnetic field, measurements about the physical properties of the cytoplasm were made.[3] Although some of their methods and measurements were self-admittedly crude, their work demonstrated the usefulness of magnetic field particle manipulation and paved the way for further developments of this technique. The magnetic particle phagocytosis method continued to be used for many years to research cytoplasm rheology and other physical properties in whole cells.[4][5]
An innovation in the 1990's lead to an expansion of the technique's usefulness in a way that was similar to the then-emerging optical tweezers method. Chemically linking an individual DNA molecule between a magnetic bead and a glass slide allowed researchers to manipulate a single DNA molecule with an external magnetic field. Upon application of torsional forces to the molecule, deviations from free-form movement could be measured against theoretical standard force curves or Brownian motion analysis. This provided insight into structural and mechanical properties of DNA, such as elasticity.[6][7]
Magnetic tweezers as an experimental technique has become exceptionally diverse in use and application. More recently, the introduction of even more novel methods have been discovered or proposed. Since 2002, the potential for experiments involving many tethering molecules and parallel magnetic beads has been explored, shedding light on interaction mechanics, especially in the case of DNA-binding proteins.[8] A technique was published in 2005 that involved coating a magnetic bead with a molecular receptor and the glass slide with its ligand. This allows for a unique look at receptor-ligand dissociation force.[9] In 2007, a new method for magnetically manipulating whole cells was developed by Kollmannsberger and Fabry. The technique involves attaching beads to the extracellular matrix and manipulating the cell from the outside of the membrane to look at structural elasticity.[2] This method continues to be used as a means of studying rheology, as well as cellular structural proteins.[10] A study appeared in a 2013 that used magnetic tweezers to mechanically measure the unwinding and rewinding of a single neuronal SNARE complex by tethering the entire complex between a magnetic bead and the slide, and then using the applied magnetic field force to pull the complex apart.[11]
Biological applications
Magnetic tweezer rheology
Magnetic tweezers can be used to measure mechanical properties such as rheology, the study of matter flow and elasticity, in whole cells. The phagocytosis method previously described is useful for capturing a magnetic bead inside a cell. Measuring the movement of the beads inside the cell in response to manipulation from the external magnetic field yields information on the physical environment inside the cell and internal media rheology: viscosity of the cytoplasm, rigidity of internal structure, and ease of particle flow.[3][4][5]
A whole cell may also be magnetically manipulated by attaching a magnetic bead to the extracellular matrix via fibronectin-coated magnetic beads. Fibronectin is a protein that will bind to extracellular membrane proteins. This technique allows for measurements of cell stiffness and provides insights into the functioning of structural proteins.[2] The schematic shown at right depicts the experimental setup devised by Bonakdar and Schilling, et al. (2015)[10] for studying the structural protein plectin in mouse cells. Stiffness was measured as proportional to bead position in response to external magnetic manipulation.
Single-molecule experiments
Magnetic tweezers as a single-molecule method is decidedly the most common use in recent years. Through the single-molecule method, molecular tweezers provide a close look into the physical and mechanical properties of biological macromolecules. Similar to other single-molecule methods, such as optical tweezers, this method provides a way to isolate and manipulate an individual molecule free from the influences of surrounding molecules.[8] Here, the magnetic bead is attached to a tethering surface by the molecule of interest. DNA or RNA may be tethered in either single-stranded or double-stranded form, or entire structural motifs can be tethered, such as DNA Holliday junctions, DNA hairpins, or entire nucleosomes and chromatin. By acting upon the magnetic bead with the magnetic field, different types of torsional force can be applied to study intra-DNA interactions[12], as well as interactions with topoisomerases or histones in chromosomes .[8]
Single-complex studies
Magnetic tweezers go beyond the capabilities of other single-molecule methods, however, in that interactions between and within complexes can also be observed. This has allowed recent advances in understanding more about DNA-binding proteins, receptor-ligand interactions[9], and restriction enzyme cleavage.[8] A more recent application of magnetic tweezers is seen in single-complex studies. With the help of DNA as the tethering agent, an entire molecular complex may be attached between the bead and the tethering surface. In exactly the same way as with pulling a DNA hairpin apart by applying a force to the magnetic bead, an entire complex can be pulled apart and force required for the dissociation can be measured.[11] This is also similar to the method of pulling apart receptor-ligand interactions with magnetic tweezers to measure dissociation force.[9]