Abstract
Sonogenetics has emerged as a tool of therapeutic ultrasound which is revolutionizing the ability to non-invasively modulate the activity of neurons and other excitatory cells. This technology utilizes bioengineering methods to confer or amplify ultrasound sensitivity in target cells using engineered or modified protein mediators. The neuromodulation community has shown a growing interest in sonogenetics due to ultrasound’s ability to penetrate the skull and reach deep brain tissue, enabling non-invasive modulation of neurons. Novel methods of sonogenetics aim to enhance cellular control in humans by leveraging mechanosensitive and thermosensitive cellular mechanisms activated by ultrasound to address cellular dysfunction and degeneration. This mini review summarizes the progress of sonogenetic mediators proposed for neuromodulation and looks at new therapeutic applications of sonogenetics for cancer treatment and vision restoration.
1 Introduction
Focused ultrasound (FUS) has emerged in the biomedical field as a highly effective and widely applicable, non-invasive therapeutic tool. As ultrasound waves pass through the body, energy delivered causes a wide range of bio-effects evoked by different low or high pressure, frequency, and duration parameters. With the advent of FUS guided by magnetic resonance imaging (MRI), it is possible to precisely direct an ultrasound’s beam to a desired treatment location with high spatiotemporal resolution (). This has offered precision targetability of small volumes of cells in deep tissue environments. Clinical applications of FUS have improved patient quality of care by providing non-invasive treatments, such as ablation of cancerous tumors, improved spatial control of drug delivery, and modulation of the activity of excitatory cells such as neurons.
The field of bioengineering has also made vast strides in improving the treatment of diseases and cancers (Tamura and Toda, 2020). Advancements in genomics and gene editing have enabled researchers to rapidly test and identify genes which may be associated with specific disease pathology or biomolecular function (). Genetic engineering and transfection techniques allow for the modification and cross-species transfer of specific genes to improve or alter genetic function. Bioengineering thus offers methods to finely hone cellular mechanisms sensitive to FUS in order to increase its efficacy, improve spatial resolution, or reduce unintended off-target effects (). The pairing of these two technologies is known as sonogenetics, the non-invasive modulation of cellular function using bioengineering techniques to sensitize specific cells to ultrasound stimulation. Though only recently introduced, two mechanisms of sonogenetics have already yielded promising results: The insertion of a gene or protein mediator using genetic engineering to sensitize a cell to ultrasound (), and the use of genetically encoded proteins or acoustic biomolecules to amplify the mechanical effects of ultrasound on targeted cells (; ).
The inception and development of sonogenetics have been primarily influenced by the neuromodulation community. FUS has opened the doorway to access and treat the most complicated organ in the human body; the brain. Neurodegenerative diseases are becoming more prevalent around the world with expanding population and life expectancy (). However, there is currently no effective way to address the underlying pathology causing neurodegeneration (Wareham et al., 2022). Accessing the brain to image, diagnose, and treat neurodegeneration is naturally limited by the skull and blood brain barrier. Symptoms are generally managed through drug administration, but continual systemic delivery of such drugs can cause severe side effects to organs throughout the body (). Techniques to modulate neural activity using light, electrical, or chemical stimulus have been effective in alleviating degeneration symptoms, but require invasive surgical procedures or implants, risking additional brain tissue damage and post surgical complications (). FUS has therefore offered an advantage over other neuromodulation techniques due to its non-invasive nature and has been approved by the FDA to treat Parkinson’s dyskinesia and tremor via thermal ablation. Sonogenetics, on the other hand, shows promising potential to improve ultrasound neuromodulation, incurring minimal cell destruction by specifying and amplifying neural response to FUS through bioengineering.
2 Cellular effects of ultrasound neuromodulation
FUS transducers deliver mechanical and thermal energy via mechanical waves which penetrate through bone and tissue non-invasively, with a scattering effect much less than that of light (). As depicted in Figure 1A, it may also induce stable or inertial intramembrane cavitation of microbubbles if they are present within the lipid bilayer (). Mechanisms of ultrasound have been investigated at the cellular level using a range of frequency, intensity, and pulse parameters. While high intensity focused ultrasound (HIFU) is commonly used to perform thermal ablation of tissue (), low intensity focused ultrasound (LIFU) can be applied thermally or non-thermally and can stimulate or suppress cellular activity (Tyler et al., 2008). This effect was observed in hippocampal neurons where low frequency LIFU was used to stimulate electrical activity in cells by activating membrane bound proteins, triggering increased synaptic transmission (Tyler et al., 2008).
FIGURE 1
As ultrasound waves pass though tissue, cellular membranes deflect and oscillate (Figure 1B), which drives biophysical transduction mechanisms (
Calcium signaling is shown to act as a primary initiator and indicator of response to FUS stimulation in cortical neurons (Yoo et al., 2022). As calcium ions accumulate, they trigger the opening of calcium-gated cation channels. This leads to depolarization of the cellular membrane and the further opening of voltage-gated sodium and potassium channels (
3 Direct neuromodulatory sonogenetic mediators
Sonogenetics was initially proposed as a method to improve targeted neuromodulation based on the idea of optogenetics, a technique in which neurons are sensitized to light stimulus by artificially overexpressing photosensitive ion channels (
Neuromodulatory sonogenetic mediators are genes which can be modified and inserted into cells to express mechanosensitive or thermosensitive protein, affect membrane elasticity, or confer sensitivity to ultrasound by some other mechanisms. Identifying sonogenetic mediators which are compatible with the human genome is the most essential element for the success of sonogenetics and its advancement to human clinical trials. To be usable in humans, genes identified as potential sonogenetic mediators must be naturally occurring in humans or have a homolog capable of being modified through genetic engineering and inserted into human DNA. While many transmembrane protein ion channel families have now been identified as being mechano- or thermosensitive, only a few have been validated as potential sonogenetic mediators. Table 1 provides a list of the ion channels and other protein mediators highlighted in this review.
TABLE 1
| Mediator | Protein | Biological sample | Ultrasound Frequency (MHz) | References |
|---|---|---|---|---|
| DEG/ENaC/ASIC | MEC-4 | C. Elegans | 10 | |
| TRP | TRP-4 | C. Elegans | 2.25 | |
| hsTRPA1 | HEK293T | 7 | ||
| TRPV1 | HEK293T | 1.7 | Yang et al. (2021) | |
| MscL | MscL-G22s | Mouse DA neurons | 0.5, 0.9 | Xian et al. (2023) |
| Rat retinal ganglion cells | 0.5, 2.25, 15 | |||
| Piezo Prestin | Piezo 1 | HEK293T, CAR-t cells | 2 | |
| mPrestin (N7T, N308S) | Mouse DA neurons | 0.5 | ||
| Gas Vesicle | GV+MscL-G22s | Rat VTA DA neurons | 1 |
Sonogenetic mediators used to confer ultrasound sensitivity to neurons and other cells in order to activate specific cellular responses.
The initial success by Ibsen et al. has led to extensive investigation of the transient receptor potential (TRP) family of cationic channels for potential sonogenetic mediators (
The TRPV subfamily contains thermosensitive channels expressed in neurons that react to temperature increase inducible by FUS. TRPV1 is a ligand-gated nonselective cation channel expressed in sensory neurons and skin. In tissues exposed to dynamic temperature change, it activates by noxious heat above 42°C as an ambient temperature sensor. In sensory neurons it is an initiator of pain response, modulates neurotransmitter release, synaptic efficiency and affects plasticity (
The mechanosensitive (MS) channel family consists of ion channels which translate physical forces applied to the membrane into electrophysical functions. Small conductance (MscS) and Large conductance mechanosensitive ion channels (MscL) are stretch-activated osmotic release valves which act as electromechanical switches, sensing the state of lipid bilayer membranes and activating independent of other proteins or ligands (Teng et al., 2015). When opened, these channels are able to pass ions, water and small proteins. The MscL gene was originally extracted from Escherichia Coli and has been thoroughly analyzed. Its small genetic structure makes it a versatile model for genetic engineering (Ye et al., 2018). MscL channels expressed in vitro in rat hippocampal neurons have shown inherent response to ultrasound stimulation (Ye et al., 2018). Gain-of-function mutants such as the sonic response mutation MscL-192L show increased sensitivity to ultrasound without the need for addition of microbubbles to amplify stimulation (Ye et al., 2018). A notable mutant MscL-G22s has been much more effective than wild-type MscL in conferring ultrasound sensitivity as it has a lower threshold for gating without showing spontaneous activity (Qiu et al., 2020). Further work by the same group expressed MscL-G22s in vivo in the dorsal striatum, subthalamic nucleus, and ventral tegmental area of mouse brains in order to alleviate Parkinsonian motor disfunction and stimulate dopamine release (Xian et al., 2023).
Piezo 1 is a mechanosensitive ion channel involved in touch, support of the auto-immune system and skeletal structure. These channels naturally occur in many human tissues including sensory neurons, blood vessels, lung, heart, bladder, cartilage, and bone cells. Low intensity pulsed ultrasound was shown to activate Piezo 1 in dental stem cells (
4 Secondary sonogenetic actuators
It must be noted that some of the proposed ion channels which show sensitivity to ultrasound must still be considered during sonogenetic stimulation. For example, TRP and ASIC channels are activated at very low stress levels, likely to spontaneously activate in response to environmental changes and may be unintentionally activated by ultrasound if expressed within the focal region (
Prestin is a transmembrane electromotility protein found in ultrasound hearing mammals. This protein naturally occurs in hair follicle cells of ultrasound-hearing mammals and is involved in these animals’ high frequency auditory processing. Prestin responds to a limited bandwidth of ultrasound frequency near 0.5 MHz. Two mutations through amino acid substitution commonly found in echolocating species, mPrestin N7T, and N308S, have been shown to amplify sensitivity to ultrasound when transfected into mammalian cells (
Gas-filled nanostructures have also been used as effective amplifiers of mechanical stimulation. Gas vesicles, genetically encodable hollow-shell protein structures extracted from cyanobacteria, function as force actuators similar to microbubbles (
5 Discussion
While the early stages of sonogenetic research were primarily centered around mechanosensitive channel mediators naturally occurring in neurons, there is a growing interest in discovering mechanosensitive channels that can be applied to other excitable cells found throughout the central and peripheral nervous system, as well as in the heart and other organs within the body. Many TRP channels are naturally expressed in the intestines, which may have implications for sonogenetic control of gut biome proliferation (
Sonogenetic activation of overexpressed mechanosensitive ion channels in retinal cells may help in developing new brain-machine interfaces for visual restoration (
Sonogenetically mediated CAR-t immunotherapy has been explored (Wang et al., 2020; Wu et al., 2023) with exciting implications for targeted non-invasive cancer treatment in complex tissue environments. Piezo 1 shows promise in controlling CAR-t cells in cancer immunotherapy. Piezo 1 engineered into a genetic circuit (Figure 1F) was inserted into HEK293T cells as an actuator for CAR-t cell immunotherapy cancer treatment activated by focused ultrasound (
Mammalian heat shock promoters and bacterial thermal bioswitches have been used in the bioengineering community to transcribe control of biochemical responses in cells. This entails the production of slight heating by pulsed low intensity ultrasound to trigger the activation of therapeutic protein production in implanted cells (
In conclusion, sonogenetics is still in an infant stage of research and much remains uncharted of its full effects or potential. Neuromodulatory sonogenetics, though promising, requires more in vivo verification of potential mediators and investigation into unintentional effects. Methods to compile how different ultrasound pulsing schemes influence ion channel dynamics and neural firing, such as the computational sonogenetics model proposed by Liu et al. (
Statements
Author contributions
KB: Writing–original draft. HH: Writing–review and editing. YJ: Supervision, Writing–review and editing.
Funding
The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This work is supported by Dr. Jing’s Penn State startup funding.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.
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Summary
Keywords
sonogenetics, focused ultrasound, thermosensitive, mechanosensitive, neuromodulation
Citation
Bell K, Heo H and Jing Y (2023) Sonogenetics: a mini review. Front. Acoust. 1:1269867. doi: 10.3389/facou.2023.1269867
Received
31 July 2023
Accepted
06 September 2023
Published
21 September 2023
Volume
1 - 2023
Edited by
Chengzhi Shi, Georgia Institute of Technology, United States
Reviewed by
Dingjie Suo, Beijing Institute of Technology, China
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© 2023 Bell, Heo and Jing.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Yun Jing, yqj5201@psu.edu
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.