Biophysical basis for airway hyperresponsiveness

Abstract

Airway hyperresponsiveness is the excessive narrowing of the airway lumen caused by stimuli that would cause little or no narrowing in the normal individual. It is one of the cardinal features of asthma, but its mechanisms remain unexplained. In asthma, the key end-effector of acute airway narrowing is contraction of the airway smooth muscle cell that is driven by myosin motors exerting their mechanical effects within an integrated cytoskeletal scaffolding. In just the past few years, however, our understanding of the rules that govern muscle biophysics has dramatically changed, as has their classical relationship to airway mechanics. It has become well established, for example, that muscle length is equilibrated dynamically rather than statically, and that in a dynamic setting nonclassical features of muscle biophysics come to the forefront, including unanticipated interactions between the muscle and its time-varying load, as well as the ability of the muscle cell to adapt (remodel) its internal microstructure rapidly in response to its ever-changing mechanical environment. Here, we consider some of these emerging concepts and, in particular, focus on structural remodeling of the airway smooth muscle cell as it relates to excessive airway narrowing in asthma.

Figure 1. Magnetic twisting cytometry with optical detection

A. An RGD-coated bead (4.5 μm in diameter) binds to the surface of the adherent cell. B. Such bead (white arrow) becomes well-integrated into underlying actin lattice (phalloidin staining). C. The bead is magnetized horizontally (parallel to the surface on which cells are plated) and then twisted in a vertically aligned homogenous magnetic field that is varying sinusoidally in time. D. This sinusoidal twisting field causes both a rotation and a pivoting displacement of the bead. As the bead moves, the cell develops internal stresses which in turn resist bead motions. Here the ratio of specific torque to lateral bead displacement is computed and is expressed as cell stiffness in Pa/nm. [Reproduced by permission of J. Appl. Physiol., Vol. 91, pp. 986-994 (2001), © The American Physiological Society and of Phys. Rev. Lett., Vol. 87, 14 (2001), © The American Physical Society]

Figure 2. Airway smooth muscle cell exerts traction upon an elastic substrate

A representative changes in traction field of a single airway smooth muscle cell in response to isoproterenol (A, 0 μM; B, 0.1 μM; C, 1 μM; D, 10 μM). The traction field was computed from the displacement field using Fourier transform traction cytometry (FTTC) (Butler et al. 2002; Tolic-Norrelykke et al. 2002; Wang et al. 2002). The cell boundary is shown by the white line. Colors show the magnitude of the tractions in Pascal (Pa) (see color scale). Arrows show the direction and relative magnitude of the tractions. In general, the greatest tractions are at the cell periphery and directed centripetally. Inset: A phase-contrast image of the respective airway smooth muscle cell. Scale bar, 50 μm.

Figure 3. Fisher airway smooth muscle cells stiffen fast and also stiffen more

Airway smooth muscle cells isolated from the relatively hyporesponsive Lewis rat (blue closed circles) and the relatively hyperresponsive Fisher rat (red closed squares) were maximally stimulated with a panel of contractile agonists: A, 5-HT [1 μM]; B, bradykinin [1 μM]; C, acetylcholine [1 μM]; D, carbachol [100 μM]. For each agonist, changes in cell stiffness were normalized to the baseline stiffness of each individual cell before stimulation. [Reproduced by permission of Am. J. Respir. Cell Mole. Biol., Vol. 35, pp. 55-64 (2006), © The American Thoracic Society]

Figure 4. Mechanical plasticity of the airway smooth muscle

Isometric force (F), unloaded shortening velocity (V), and compliance (C) of canine tracheal smooth muscle activated over a range of muscle lengths. Circles represent data modified from Pratusevich et al. (1995) and Kuo et al. (2003), as compiled by Lambert et al. (2004); solid lines are 3rd-order polynominal functions adjusted to the original data (Silveira and Fredberg, 2005). [Reproduced by permission of Can. J. Physiol. Pharmacol. Vol. 83, pp. 923-931 (2005), © NRC Canada]

Figure 5. Cytoskeleton remodeling of the airway smooth muscle cell

A. Spontaneous motions of a representative bead show intermittent dynamics, with periods of confinement alternating with hopping; a bead glued to the coverslip is shown at the bottom left and taken to represent the upper limit of measurement noise. B. MSD_b calculated from Equation 1 is shown for representative beads. Inset:Ensemble average of all MSD_b (MSD) increased with time as ~ t^1.6. C. The histograms of diffusion coefficient D* and exponent α estimated from a least-square fits of a power-law (Equation 2) to the MSD_b data. D. Ensemble average of all MSD_b (MSD) increased faster than linearly with time (~ t^1.6); beads attached to a cell seeded on a micropatterned substrate (50 μm × 50 μm), on which it could adhere but not crawl, exhibited the same anomalous motions. [Reproduced by permission of Biochem. Biophys. Res. Comm., Vol. 355, pp. 324-330 (2007), © Elsevier Inc.]