Unravelling the mechanisms underlying ultrasound stimulation of dorsal root and spiral ganglion neurons
Focused ultrasound (US) is a recently-explored method for neurostimulation. One potential advantage of focused US compared to electrical stimulation is the ability to selectively target restricted subpopulations of neurons. This feature makes US neurostimulation a promising alternative tool for the design of future neural prostheses: for example, to restore hearing in deaf people, because contemporary cochlear implants – which work via electrical stimulation - mainly suffer from a lack of spatial selectivity. Focused US was recently shown to directly stimulate neurons in several in vitro and in vivo studies; however the mechanisms of this effect still remain poorly understood. The aim of this interdisciplinary PhD project is to understand the physical and molecular causes of US-induced neurostimulation. The molecular mechanisms will be identified by combining US, calcium imaging and single-cell RNA sequencing on cultured dorsal root ganglia (DRG) neurons. The physical mechanisms (thermal and mechanical effects of US action) will be measured using temperature monitoring and laser vibrometry and confronted to numerical simulations of the in vitro set-up.
 [Naor et al., J. Neural Eng. 2016] and [Tyler et al. Neurobiology 2018] for reviews
Ultrasound neurostimulation, calcium imaging, single cell RNA sequencing
The PhD student will elucidate the physical and molecular mechanisms by which ultrasound activate DRG neurons and spiral ganglion neurons (SGNs; i.e. primary auditory neurons). Our two labs have developed over the past year an original in vitro set-up designed to measure the effects of focused US on neurons cultured in vitro using calcium imaging. This set-up will be used to (i) identify the candidate molecules responding to US stimuli in DRG neurons, (ii) extend DRG findings to SGNs and (iii) understand the mechanical and/or thermal effects responsible for US neurostimulation.
Proposed approach (experimental / theoretical / computational)
Our in vitro preliminary experiments based on calcium imaging demonstrate that some DRG neurons can be activated by US (20 MHz, peak acoustic pressure 2 MPa, pulse duration 0.1ms, duty cycle 25%). Based on this set-up combining calcium imaging, ultrasonic stimuli and pharmocological stimuli, the molecular mechanisms will be investigated by first identifying the type of DRG neurons responding to US stimulation. The RNA contents of US-responding and non-responding neurons will be subjected to RNA deep sequencing to identify candidate US-responsive genes. The genes expressed in both the DRG and SGNs will be selected and their contribution to US responses further validated using in vitro gain and loss-of-function experiments. Finally, numerical simulations using COMSOL Multiphysics and/or K-wave together with complementary experiments (non-contact laser vibrometer, infrared camera) will be conducted to assess the physical mechanisms by which US activate the neurons in the in vitro set-up. We anticipate that these physical mechanisms could potentially be mechanical (acoustic radiation force or streaming) and/or thermal.
This project will combine the strengths of two Marseille labs at the interface of physics and biology, namely:
- Neurobiologists of the IBDM – Aziz Moqrich (DR CNRS), Ana Reynders (CR, CNRS), Sungjae Yoo (Postdoc). The Moqrich team is a leader in the field of sensory biology with high interest in the understanding of the biology of mechanosensory neurons.
- Acousticians of the LMA – Emilie Franceschini (CR CNRS), Olivier Macherey (CR CNRS), Serge Mensah (MCF, ECM) – with an expertise in ultrasound imaging/stimulation and in electrical stimulation of auditory neurons.
The preliminary results that demonstrated the feasibility of generating US-induced responses in DRG neurons have already involved our two laboratories in the framework of an interdisciplinary grant from the CNRS (Challenge “Osez l’Interdisciplinarité”, 2017-2019). The experimental developments of this PhD project will also be supported by the ANR HearUS 2020-2024 that we recently obtained (coordinator: O. Macherey, partners: LMA, IBDM, LNSC).
The PhD candidate should hold a MSc (or equivalent degree) in biophysics, molecular or cellular biology, neuroscience or a related discipline. We aim to find a highly-motivated student with creative skills and appeal for experimental work. Ability to work in an interdisciplinary environment involving several research teams will be required. Some good English language skills and profusion in data analysis using, e.g. Matlab, will be a strong plus.