Ion Channels in Biophysics and Physiology: Methods & Challenges to Study Mechanosensitive Ion Channels

In a seminal work published in 1950, Sir B. Katz showed that the electrical response of the frog muscle spindle varies directly with the rate and amplitude of muscle stretch. This observation led him to propose the existence of a piezoelectric substance in this organ, setting the stage for the field of mechanobiology (Katz, J Physiol 111, 261-282, 1950). Despite this early work, the identity of the molecules responsible for the conversion of mechanical stimuli into biological signals has remained hidden for decades. This delay is often attributed to the inherent difficulty to precisely quantify the mechanical deformations of biological samples. In contrast to other forms of stimuli such as ligand concentration and membrane potential, quantifying mechanical deformations of cell membranes is not trivial. Mechanical forces produce a complex array of membrane deformations including bending, thinning, compression, expansion, and shear, and thus, have components in many strain dimensions. In addition, due to the viscoelastic nature of cells, these deformations may have linear and nonlinear components. In spite of these experimental challenges, Sukharev et al. cloned the first mechanosensitive ion channel from the bacteria E. coli in the mid-1990s (Sukharev et al. Nature, 265-268, 1994). Two decades later, several protein families encompassing dozens of eukaryotic mechanosensitive ion channels have been identified, depicting an astonishing diversity of force-activated molecular machines. In this chapter, we intend to provide an overview of the current state of knowledge and technical challenges to study how cell membranes deform upon mechanical stress and how ion channel proteins detect these deformations to engage homeostatic cellular responses.