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Picosecond ultrasounds as elasticity probes in neuron-like cells models

Abstract : We report on elasticity measurements in neuron-like cells using picosecond acoustics pump and probe spectroscopy. The stimulated Brillouin oscillations were mapped in PC12 cells to reveal their internal elastic structure. Thanks to a Pearson correlation coefficient mapping, different areas could be distinguished. The nucleus material shows a bulk modulus equal to 12.9 GPa in the case of dry cell. Attenuation of the Brillouin signature gives access to dynamical longitudinal viscosity equal to 10.6 mPa · s, one order magnitude higher than water. The modulus considerably drops to 2.6 GPa in the most physiologically relevant case of a hydrated cell. Keywords: Neuronal cell, elasticity, pump and probe spectroscopy. Cells respond to mechanical signals perceived from the nearest extracellular world 1-4. For instance, it has been suggested that mechanical constraints prevail over biochemical signaling in the early stage of embryogenesis 5. Substrate stiffness has also been identified as a key factor driving cell proliferation and differentiation 6. Both endothelial and smooth muscle cells were shown to proliferate in response to stretching ; however, in the case of endothelial cells this response depends on cell-cell adhesion 7. In the mechanotransduction process, external forces exerted on the cells transit inside them through microscale adhesions domains that serve as anchoring points for the structuration of the cellular cytoskeletal network. This phenomenon allows the cell to sense its surrounding environment and is followed by the activation of fundamental cellular processes involving motility or changes in cell shape 8. Obviously, how this regulation occurs will depend on the cell type and function. In the case of tumors, the increase in rigidity could be related to various factors, including an increase in the modulus of elasticity of transformed cells due to cellular disturbances. This leads to tumors being generally stiffer than normal tissues 9,10. Perturbation of tissue rigidity is associated with different types of pathology. However, it is sometimes difficult to conclude if this change in stiffness of cells or tissue is the effect or the source of the pathologies 11. This is why the characterization of the mechanical properties of cells is essential to understand their behavior during mitosis, apoptosis, adhesion, a) Electronic mail: ahmed.hamraoui@sorbonne-universite.fr mobility and disease development 12-14. However, the complexity of the inner cell composition and the intricate mesh-work formed by molecular mediators of the transmembrane cell-substrate interactions requires non-invasive techniques to probe and quantify local mechanical properties of cells, including modulus of elasticity, viscoelastic properties, adhesion , and forces created at the single-cell scale. Several recent reviews describe tools used to study cell mechanics 15,16 and to apply forces on them 17. The vast majority of conventional methods of measuring the local mechanical properties of cells are based on the use of solid probes, such as AFM, and as a result the measured mechanical properties can strongly depend on the contact/adhesion between the probe and the cell. In contrast, acoustic waves generated by lasers provide a very adequate tool for probing the mechanical properties of biological cells or tissues in a non-contact, non-invasive configuration. In the optical pump probe technique usually called picosecond acoustics (PA), high frequency acoustic pulses (in the 1 − 1000 GHz range) can be generated by the pump beam and detected using a delayed probe beam. Since acoustic waves travel several microns per nanosecond, it is possible to study material on a submicron scale with acoustic waves of 10 GHz or more. Such time resolved measurements are known to achieve sound velocity characterization with an accuracy less than < 5 %, parameter directly related to the elasticity behavior. In addition, by combining the time and space aspects, it is possible to perform 3D elastic investigations with sub micrometer resolution. To finish, the all-optical approach allows to consider complex environments to address issues related to relevant biological conditions, aqueous media, controlled temperature. For more than 30 years, properties of matter, mainly solid thin metallic films and transparent me-This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS
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Alexis Viel, Emmanuel Peronne, Océane Sénépart, Loic Becerra, Claire Legay, et al.. Picosecond ultrasounds as elasticity probes in neuron-like cells models. Applied Physics Letters, American Institute of Physics, 2019, ⟨10.1063/1.5129783⟩. ⟨hal-02376267⟩

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