Effective temperature inside living cells
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1227-JJ05-03
Effective temperature inside living cells Claire Wilhelm Laboratoire Matière et Systèmes Complexes (MSC), UMR 7057 CNRS & Université Paris Diderot, Paris, France.
ABSTRACT The combination of active and passive microrheology using magnetic probes engulfed inside living cells demonstrates the violation of the fluctuation dissipation theorem in cells. It is proposed to quantify the deviation from the in equilibrium situation with an effective temperature. Each magnetic probe then serves as a local thermometer within the cells. The response of pairs of magnetic beads of two diameters (1 and 2.8 µm) to an oscillating magnetic field is analyzed to measure the viscoelastic complex modulus in the beads environment (active measurement). The spontaneous motion of the beads is tracked to compute their mean square displacements (passive measurement). The effective temperature is derived using an extension of the fluctuation dissipation theorem.
INTRODUCTION The viscoelasticity of the cell inside is a fundamental parameter for a complete understanding of many cellular processes such as intracellular transport of vesicles or cell crawling. The recently developed microrheology, based on the tracked motion of embedded micron-scale probes 1, naturally emerges as the convenient tool to explore the cytoplasm viscoelasticity. Microrheology typically deals with either the deformation in response to a force applied at the microscale (termed active microrheology) or with the spontaneous motion of tracer probes (termed passive microrheology). Inside the cells, only a few studies have been reported to measure the intracellular mechanics with active microrheology, and for most of them the probe is a magnetic bead engulfed into the internal cell volume by phagocytosis 2,3. By contrast, a large amount of work has been performed to derive the intracellular mechanical properties from the displacement of internal pre-existing granules or microinjected and endocytosed particles, using the passive microrheological approach 4-9. Probes fluctuations are tracked and a generalized fluctuation-dissipation theorem (FDT) is used to deduce the frequency-dependent viscoelastic shear modulus, G(ω). Complex systems, such as aging systems, glasses or granulars, consume and dissipate energy to their surroundings, creating a state that is far from equilibrium, invalidating in return the use of the FDT. Such active out of equilibrium systems arise as well from biology, examples given with active gels involving molecular motors 10, with active membranes, with self-propelled micro-organisms, and with the continuous intracellular trafficking of organelles inside living cells 11-14. To test the validity of the FDT approach, that is the proximity to thermal equilibrium, one way is to perform simultaneously both passive and active microrheology. If both approaches lead to different complex shear moduli, the FDT is violated. It was then proposed for aging or
glassy systems to associate to the FDT violation a frequency-dependent effective temperature 15
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