Mechanically Activated Ion Channels

Abstract

Mechanotransduction, the conversion of physical forces into biochemical signals, is essential for various physiological processes such as the conscious sensations of touch and hearing, and the unconscious sensation of blood flow. Mechanically activated (MA) ion channels have been proposed as sensors of physical force, but the identity of these channels and an understanding of how mechanical force is transduced has remained elusive. A number of recent studies on previously known ion channels along with the identification of novel MA ion channels have greatly transformed our understanding of touch and hearing in both vertebrates and invertebrates. Here, we present an updated review of eukaryotic ion channel families that have been implicated in mechanotransduction processes and evaluate the qualifications of the candidate genes according to specified criteria. We then discuss the proposed gating models for MA ion channels and highlight recent structural studies of mechanosensitive potassium channels.

Figure 1. Sensory Receptors

(A) The high levels of rhodopsin expressed in rod cells allowed for biochemical isolation and characterization before the advent of many molecular biology cloning techniques. (B and C) Olfactory and Taste receptor cells are also activated by the binding of small molecule ligands to GPCRs. (D and E) Somatosensory neurons detect a wide array of mechanical and thermal inputs. Un-myelinated DRG neurons with free nerve endings embedded in the skin express ion channels such as TRPM8 for cool, innocuous temperature sensing. Some DRG neurons, classified as high threshold mechanoreceptors (HTMR), express TRPV1 and TRPA1 and detect noxious temperature or mechanical forces. At present, the sensor for noxious mechanical force is unknown (illustrated as a “?” in the figure legend). Light touch is mediated by activation of low threshold mechanoreceptors (LTMR) such as the Merkel cell-neurite complex or the Meissner corpuscle. Piezo2 has been shown to play a major role for light touch sensation in mice (Ranade et al., 2014b; Woo et al., 2014a).

Figure 2. Mechanotransduction Assays

(A) To generate shear stress, a perfusion tube with a small opening at the tip is placed near the cell and bath solution is then injected onto the cell membrane (McCarter et al., 1999; Olesen et al., 1988). (B and C) Membrane stretch can either be applied generally to the entire basal side of a cell or a liposome that has been seeded onto a flexible material, or, more locally, to a small patch of membrane using a glass pipette to generate a protrusion or “bleb” (Bhattacharya et al., 2008; Maingret et al., 1999a; Sukharev and Sachs, 2012). (D) Physical indentation, or poking of the cell membrane or a lipid bilayer by a glass pipette controlled by a piezoelectric device, can also be used to apply force onto a local region of the membrane (McCarter et al., 1999). (E and F) Isolated epithelial hair cells can be stimulated with either a glass probe or fluid shear stress. Both methods serve to bend the stereocilia but only the shear stress method can be used for applying force in both positive and negative directions. (G) Changes to the osmotic potential can cause an influx of water and lead to cell swelling. (H) New assays such as the elastomeric pillars have been developed to more precisely deliver mechanical strain and can be precise enough to apply force to specific parts of a cell, such as the nerve endings. (I) The classic setup for the saphenous nerve preparation used to record action potential firing upon stimulation with a von Frey filament. (J) A variant form of the skin-nerve preparation that can be used to monitor the activity of DRG neurons along the entire spinal cord.

Figure 3. Eukaryotic Ion Channel Subunits

The topology of ion channel subunits implicated in eukaryotic mechanotransduction illustrates the vast diversity compared to other sensory receptors. (A) DEG/ENaC proteins have 2 TM domains and associate as trimeric complexes. (B) The general structure of many TRP channels is a tetrameric complex where each subunit contains 4 TM domains and a pore loop domain between TM5 and TM6. An extraordinary amount of structural diversity exists between TRP channel members, particularly within the structure of the N and C terminal soluble domains of the proteins. (C – E) At present, four proteins (TMC1/2, TMIE and TMHS) have been shown to be necessary for mechanotransducer currents in hair cells. The exact composition of the various subunits to the channel complex and whether this composition varies along the tonotopic axis is unknown. (F) K₂P channels are unique among other ion channel complexes in that each subunit contains 2 poor loop helices and the formation of a functional pore requires an association of dimers. (G and H) Biochemical and computational evidence suggest that Piezo ion channels contain at least 18 TM domains and potentially up to 38 TM domains.