Propiedades mecanicas de axones

PRL 99, 018301 (2007)

PHYSICAL REVIEW LETTERS

week ending 6 JULY 2007

Mechanical Properties of Axons
Roberto Bernal,1 Pramod A. Pullarkat,2 and Francisco Melo1
´ Departamento de Fısica, Universidad de Santiago de Chile, and CIMAT, Avenida Ecuador 3493, Casilla 307, Correo 2, Santiago, Chile 2 Experimentalphysik I, University of Bayreuth, D95440 Bayreuth, Germany (Received 26 January2007; published 3 July 2007) The mechanical response of PC12 neurites under tension is investigated using a microneedle technique. Elastic response, viscoelastic relaxation, and active contraction are observed. The mechanical model proposed by Dennerll et al. [J. Cell Biol. 109, 3073 (1989).], which involves three mechanical devices—a stiff spring  coupled with a Voigt element that includes a lessstiff spring k and a dashpot
—has been improved by adding a new element to describe the main features of the contraction of axons. This element, which represents the action of molecular motors, acts in parallel with viscous forces defining a global _ tension response of axons T against elongation rates k . Under certain conditions, axons show a transition from a viscoelastic elongation to activecontraction, suggesting the presence of a negative elongation rate _ sensitivity in the curve T vs k .
DOI: 10.1103/PhysRevLett.99.018301 PACS numbers: 83.80.Lz, 83.60.ÿa, 87.19.La
1

The cytoskeleton —a crosslinked biopolymer network—provides mechanical strength to the cell and also drives vital functions such as locomotion, division, and intracellular transport [1]. These remarkableproperties arise from the ability of the filaments to reorganize through a polymerization-depolymerization process and through the action of molecular motors that generate forces and motion using chemical energy [1]. Thus, the cytoskeleton is a complex ‘‘active gel’’ with fascinating properties. In recent years, the advent of novel techniques has allowed progress in our understanding of live cellmechanics [2 –5]. However, modeling cytoskeletal mechanics at the level of a cell is hindered by cellular heterogeneity and the complex interaction of the cell with the extracellular matrix. In contrast to most cells, axons can be approximated as a uniform, uniaxial structure containing all the main ingredients of the cell cytoskeleton. Typically, the axonal cytoskeleton is made up of a cortex of actinfilaments attached to the plasma membrane, a core of neurofilaments, and aligned and bundled microtubules [1]. As in other cells, and as our results will demonstrate, molecular motors provide the axon with the ability to generate active contractile stresses [6]. The active as well as viscoelastic properties of axons are expected to be important physiologically in axonal retraction after injury orduring rewiring [6], and in stretching during limb movement [7]. Using PC12 neurites as a model system for axons [8,9], we perform a systematic investigation of their mechanical response. We employ an axon-pulling setup and a microneedle to achieve accurate determination of force and strain of axons. We present measurements of the elastic response and viscous dissipation in axons. Furthermore, wecharacterize the active tension in axons and a transition from a passive viscoelastic relaxation to an active contraction. We model our data using a novel mechanical representation for the molecular motors, introducing a minimal 0031-9007=07=99(1)=018301(4)

set of experimentally validated components. The transition from viscous elongation to active contraction is described in the framework of anegative strain rate sensitivity. PC12 cells were grown on collagen-coated coverslides using standard techniques [9]. The cells were maintained at 37  C and observations made using a Nikon Eclipse TE300 microscope with a 40  =0:6 objective in phase contrast mode. Images were recorded using an analog CCD camera and a frame grabber card. Glass microneedles were fabricated using a pipette puller...