Magnetogenetics: Your Brain on Magnets

Developing new research techniques to elucidate the brain’s role in behavior and consciousness is the hurdle that neuroscientists face today as current techniques display their limitations. While the techniques we are equipped with have improved our understanding of the central nervous system, innovation is always required in the pursuit of knowledge and understanding.

A not-so-new but recently refined technique for studying neural systems within the CNS involves the use of magnetic fields (via radiofrequency or magnets) to manipulate neuronal activity. There are currently two approaches to manipulating neuronal activity this way. One employs a heat-sensitive ion channel connected to a magnetic nanoparticle. A magnetic field sent through a neuron with this modified channel would heat the magnetic nanoparticle through thermal relaxation, which would then cause the thermosensitive ion channel to open

There is a low chance of damaging the neuron membrane due to the size and local heating of the magnetic nanoparticle. Ramaswamy et al. found that free-floating magnetic nanoparticles guided in mice brains receiving radiofrequency treatment did not alter neuron cell function or affect any surrounding neurons in a circuit, so neurons were still able to communicate with each other (2015).

Cre recombinase “cuts” the inactive gene between two inverted LoxP sites and converts it to an active orientiation to be transcribed and translated

A second technique uses a mechanically sensitive ion channel connected to a magnetic nanoparticle. A magnetic field sent through this neuron would tug at the nanoparticle and open the ion channel. Both techniques are made possible with transgenic mouse models expressing cre-recombinase and the delivery of adeno-associated viruses that are encoded with these modified channels.

Stanley et al. used the thermosensitive TRPV1 ion channel gene, expressed in all humans to detect scalding hot temperatures, and modified it so that the superparamagnetic ferritin molecule, a protein complex that naturally occurs in humans and stores iron atoms, was attached to the channel by an antibody (2016). According to our understanding of TRPV1 and physics, this modified TRPV1 channel should open once the ferritin molecule is heated by a magnetic field.

The team wanted to use this modified TRPV1 channel to study glucose-sensing neurons in the ventromedial hypothalamus, a cluster of neurons involved in the regulation of eating. They hypothesized that the neurons within the ventromedial hypothalamus are responsible for regulating blood glucose levels. To test his hypothesis, they used transgenic mice expressing cre-recombinase in neurons that can sense glucose (glucokinase-cre mice) and injected an adeno-associated virus containing the modified TRPV1 receptor in a double-floxed inverted open reading frame into the ventromedial hypothalamus. This approach ensured that the gene encoding the modified receptor was only being expressed in glucose sensing neurons that contained the cre recombinase protein (Stanley et al., 2016).

Glucokinase-cre mice that received AAV injections containing the modified TRPV1 receptor displayed increased blood glucose levels following radiofrequency treatment, and wild-type mice that lacked cre-recombinase but received AAV injections did not, suggesting that cre-recombinase successfully expressed the TRPV1/Ferritin channel in glucokinase-cre mice and that the magnetic field activated these neurons.

Radiofrequency treatment increased intracellular Ca2+ levels in a hypothalamic cell line expressing the modified channel and was blocked when a TRPV1 inhibitor was applied, confirming that the neuronal activation by RF treatment was due to the TRPV1/Ferritin channel. Radiofrequency treatment also increased food intake of glucokinase-cre mice, demonstrating remote control of eating behavior in the mice

The team created a separate TRPV1/Ferritin channel modified to allow Cl- ions and not Ca2+ ions into the neuron to inhibit it. This channel displayed increased Cl- levels in a hypothalamic cell line, and radiofrequency treatment of this channel in glucokinase-cre mice reduced blood glucose levels significantly and decreased food intake. These results demonstrate the possibility of remotely activating and inhibiting neurons and manipulating behavior through a magnetically sensitive modified TRPV1 ion channel (Stanley et al., 2016).

A team from the University of Virginia successfully generated a mechanically sensitive TRPV4 channel tethered to a ferritin molecule that was optimized for cell membrane expression to be “tugged” open by a magnetic field. After determining its success in activating neurons in mouse brain slices, they inserted the transgene into Rohon-Beard sensory neurons of zebrafish. A magnetic field caused the zebrafish expressing the channel to coil more often, suggesting the magnetic field was activating their sensory neurons (Wheeler et al., 2016).

This system was then introduced to mice expressing cre-recombinase in neurons with the dopamine 1 receptor (D1R) so the team could examine the role of striatal D1R neurons in rewarding behavior using the conditioned place preference.

Striatal D1R activation is believed to play a large role in the reward system pathway and is the target of most drugs of abuse. Dopamine 1 receptor-cre mice expressing the modified channel displayed a significant increase in D1R neuronal activation following magnetic stimulation and a significant preference for the side of the conditioned place preference chamber lined with magnets than the side with an empty chamber, suggesting the magnetic activation of D1R neurons in these mice had reinforcing effects via the reward pathway. Wild-type mice injected with the transgene showed no preference. These results again demonstrate the ability to remotely control mouse behavior using a non-invasive activator (Wheeler et al., 2016).

Overall, both techniques provide promising methods of remotely controlling specific neuron-type activity. The magnetogenetic approach does not require the invasive placement of LEDs into the brain that optogenetics does, and is capable of activating larger numbers of neurons. However, the mechanosensitive TRPV4 ion channel may be prone to opening unintentionally due to movement and not the magnetic field, in which case it becomes more of a kinetogenetic rather than magnetogenetic technique. While magnetogenetics is less invasive than optogenetics, the studies discussed above had to inject AAV’s into the desired area which will kill neurons either way. Lines of transgenic mice that can stably express the modified receptor will reduce invasive surgery, provide better data, and hopefully reveal more about brain regions that will aide clinical researchers.

Works Cited

Ramaswamy, B., Kulkarni, S. D., Villar, P. S., Smith, R. S., Eberly, C., Araneda, R. C., … Shapiro, B. (2015). Movement of Magnetic Nanoparticles in Brain Tissue: Mechanisms and Safety. Nanomedicine : Nanotechnology, Biology, and Medicine, 11(7), 1821–1829.

Stanley, S. A., Kelly, L., Latcha, K. N., Schmidt, S. F., Yu, X., Nectow, A. R., … Friedman, J. M. (2016). Bidirectional electromagnetic control of the hypothalamus regulates feeding and metabolism. Nature, 531(7596), 647–650.

Wheeler, M. A., Smith, C. J., Ottolini, M., Barker, B. S., Purohit, A. M., Grippo, R. M., … Güler, A. D. (2016). Genetically targeted magnetic control of the nervous system. Nature Neuroscience, 19(5), 756–761.