MAGNETIC nanoparticles targeted to nerve cell membranes can be used to remotely control cellular activity and even the simple reflex behaviours of nematode worms, according to research by a team of biophysicists at the University of Buffalo. The new method, which is described in the journal Nature Nanotechnology, could be very useful for investigating how cells interact in neuronal networks, and may eventually lead to new therapies for cancer and diabetes.
Heng Huang and her colleagues synthesized manganese-iron nanoparticles, each just 6 millionths of a millimeter in diameter, and coated with the bacterial protein straptavidin attached to a fluorescent molecule called DyLight549. Strepdavidin binds another molecule, much like a key fits into a lock, enabling specified cells to be targeted, while DyLight549 acts like a molecular thermometer, whose fluoresence intensity changes with temperature.
The researchers first tested whether the nanoparticles could be used to activate specified cells maintained in culture dishes. They inserted the gene encoding TRPV1, one member of a family of temperature-sensitive membrane proteins, into human embryonic kidney cells and neurons isolated from the rat hippocampus. The cells were also made to express a genetically engineered membrane protein ‘marker’, consisting of cyan fluorescent protein and two peptides, one that anchors it to the membrane, and another that binds the streptavidin molecules coating the nanoparticles.
A solution of nanoparticles was added to the culture dishes, and the cells examined under the microscope. Cyan fluorescence was found to be localized exclusively to the membranes, showing that the nanoparticles had targeted only those cells expressing the marker protein. The researchers then applied a small magnetic field to the culture dishes, to heat the nanoparticles, and monitored the intensity of the fluorescence emitted by DyLight549. This revealed a highly localized increase in temperature: as soon as the magnetic field was applied, the fluorescence intensity in the immediate vicinity of the cell surface decreased, indicating a temperature increase of more than 15°C within 15 seconds.
The heat generated by the nanoparticles was sufficient to trigger activation of the TRPV1 proteins expressed by the cultured cells. This was established using a genetically encoded calcium sensor, whose fluoresence signal changes in response to the tiny increases in calcium ion concentration that are characteristic of neuronal activity. The increases in calcium ion concentration were found to be due to influxes of calcium through the activated TRPV1 – they were observed in cells expressing both the membrane marker and TRPV1, but not in control cells expressing the membrane marker alone. Significantly, the calcium influxes were found to elicit nervous impulses in the TRPV1-expressing cells.
Huang and her colleagues then showed that this approach can be adapted to remotely control a simple behavioural response in the nematode worm Caenhorhabditis elegans. When this tiny organism encounters noxious heat, it acts reflexively by moving in the opposite direction, and this heat avoidance response is initiated by TRPV1. The sensory neurons expressing TRPV1 have not been identified, however, so the researchers could not target them directly. Instead, they used nanoparticles coated with polyethylene glycol, a fatty molecule that causes the particles to accumulate in the mucus layer near the mouth.
A magnetic field was then applied, and the crawling movements of individual worms were tracked. As the above film clip shows, the worms stopped their forward movements within 5 seconds of applying the magnetic field. Of the 40 worms used in the experiment, 34 stopped dead in their tracks in response to the field, and of those 27 subseqently moved back in the opposite direction. Worms that were not injected with the nanoparticles, however, continued moving in a forward direction when subjected to the magnetic field.
Nanomagnetic neuronal activation could prove to be very useful for investigating the functioning of cells within complex neural networks, and the authors suggest that it has several advantages over another recently developed technique called optogenetics, which involves using laser light to activate specified cells. Although extremely powerful, optogenetics is limited in its applications, because light has to be targeted precisely to groups of specified cells, and does not penetrate biological tissue very deeply. By contrast, magnetic fields can go straight through tissue virtually unhindered, and can be applied to whole organisms or even groups of them.
Although questions have been raised regarding the safety of using nanoparticles in humans, the method could eventually have various clinical applications, because, as this study demonstrates, it can heat cells without causing them any damage. One possible application is hyperthermic cancer therapy, in which heat is used to kill off rapidly dividing tumour cells. Another is to stimulate pancreatic cells to secrete insulin in diabetics. The first step towards developing any such an application will be to demonstrate that the method works effectively in the brains of rats or mice. In the immediate future though, studies will probably focus on targeting the nanoparticles to specified cells in the nematode worm.
Huang, H., et al. (2010). Remote control of ion channels and neurons through magnetic-field heating of nanoparticles. Nat. Nanotech. DOI: 10.1038/nnano.2010.125.
Hughes, S., et al. (2008). Selective activation of mechanosensitive ion channels using magnetic particles. J. R. Soc. Interface 5: 855-863. [Full text]
Mannix, R. J., et al. (2008). Nanomagnetic actuation of receptor-mediated signal transduction. Nat. Nanotech. 3: 36-40 [PDF]
Wittenburg, N. & Baumeister, R. (1999). Thermal avoidance in Caenorhabditis elegans: An approach to the study of nociception. Proc. Nat. Acad. Sci. 96: 10477-10482. [PDF]