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Neurons are elastic...

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Following a pathology or a trauma, the regeneration of the central and peripheral nervous system is today a major societal issue in the context of an aging population. This plasticity, driven by the growth and differentiation of cells into neurons, for example, depends on a large number of parameters. The correlation between these repair mechanisms and the elastic response of the environment or the cells is often mentioned but remains difficult to quantify. Nevertheless, researchers from the INSP in collaboration with the LCMCP and SPPIN have taken a crucial step towards this objective by quantifying, at the micrometric scale, the elastic inhomogeneities into single neuron.

The vast majority of experimental approaches to collect mechanical properties of a single cell are based on the use of proximity microscopy probe, which inevitably leads to considering the interaction between the tip and the cell as well as the role played by the substrate in the interpretation of the responses. Consequently, the development of contactless optical approach is crucial. in addition it opens up the possibility of considering invitro measurements involving often complex environments.

Using time resolved pump probe approaches, elastic waves able to propagate within these biological objects may be generated. Then measuring the optical cell response versus time provides information about the propagation of bulk acoustic waves. Figure 1 shows a typical oscillating response, commonly called the Brillouin oscillation, in a neuronal cell. The measurement of the frequency of this oscillation is directly related to the sound velocity and therefore to the elastic modulus. By reproducing this type of measurement, at all points in a cell, it is possible to get a mapping of the bilk modulus, figure 2. We can thus identify the elastic fluctuations which reveal in particular an increased rigidity into the nucleus cell.

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Figure 1
a) Optical image of a PC12 (model neuron)
b) Typical acoustic response into the cell

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Figure 2
Bulk modulus mapping to identify the nucleus and surrounding cytoskeleton (60µm scale). In the present case, we have rather an accretion of three different cells with their nucleus, feature absent from optical investigations.

Beyond these preliminary measurements on fixed and therefore dead cells, it is advisable to develop our experimental setup in order to reproduce these investigations on living cells and likely to modify their elasticity according to their growth and differentiation cycle. The role of adhesion in these biological processes is another, almost virgin, line of research that will be addressed. The application of external stimuli as an electric field on axonal regeneration and its effects on the intrinsic elasticity of neurons is also under investigations.

Reference
« Picosecond ultrasounds as elasticity probes in neuron-like cells models »
Viel, Alexis ; Peronne, Emmanuel ; Sénépart, Océane ; Becerra, Loic ; Legay, Claire ; Semprez, Fannie ; Trichet, Lea ; Coradin, Thibaud ; Hamraoui, Ahmed ; Belliard, Laurent
APPLIED PHYSICS LETTERS Volume : 115 Issue : 21 Article Number : 213701 DOI : 10.1063/1.5129783

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Contact
Laurent Belliard : laurent.belliard(at)sorbonne-universite.fr