Cellular biomechanics investigated by atomic force microscopy

Living biological cells are highly complex, multifunctional systems whose physical attributes are relatively unknown. Critical functions involving plasticity of cell morphology, connectivity and response to stimuli are proposed to be fundamentally related to the micromechanical character and the ability of the cell to exert directed mechanical signaling. Unique abilities of atomic force microscopy in measuring cellular viscoelastic properties (on length scales from nanometers to microns) are explored with specific applications to (1) airway smooth muscle cells and (2) hippocampal neurons. Surface indentation techniques for stiffness mapping, as well as quantitative measurements of frequency-dependent complex rheology are featured. Structural and molecular determinants of dynamic mechanical behavior are identified. In smooth muscle cells, an isotropic fiber network provides strong resistance to deformation. Actin polymerization is largely responsible for stiffening following contractile activation, not myosin cross-bridge formation as expected. On neuronal dendrites, stiffness contrast correlates with known distributions and stability of cytoskeletal elements: microtubules along the shafts and dynamic actin in the spines (micron-sized surface protrusions). Focus is given to dendritic spines as the post-synaptic contact sites for most excitatory transmission between neurons. Large heterogeneity is observed in spine mechanical properties, but stiffer spines appear to be associated with axon-like contacts. Spines may stiffen in response to synaptic stimulation, in agreement with recent observations of actin-based stabilization of spine shape (reduced motility) following excitatory treatments.Remarkably, the frequency dependence of the complex shear moduli (0.5-100 Hz indentations) of both cellular systems is described well by the same rheological model: that of soft glassy materials existing just above the glass transition. The central feature of this model is that storage ( G') and loss moduli (G") scale in parallel as a weak power-law function of frequency. Power-law exponents (alpha), measured to be of the order 0.1, are related to the level of molecular agitations in the cell and determine the degree of solid-like (G' >> G" with a glass transition at alpha = 0) or fluid-like behavior (G' << G" with alpha = 1 for a pure fluid). The soft glassy hypothesis is founded on the concepts of disorder and metastability of structural elements. A Newtonian viscosity (pure fluid) component is also identified with significant effects for high frequency deformations. Together these properties are critical for describing cellular remodeling: contraction in smooth muscle cells or synaptic plasticity at dendritic spines