Feeling the hidden mechanical forces in lipid bilayer is an original sense

Abstract

Life's origin entails enclosing a compartment to hoard material, energy, and information. The envelope necessarily comprises amphipaths, such as prebiotic fatty acids, to partition the two aqueous domains. The self-assembled lipid bilayer comes with a set of properties including its strong anisotropic internal forces that are chemically or physically malleable. Added bilayer stretch can alter force vectors on embedded proteins to effect conformational change. The force-from-lipid principle was demonstrated 25 y ago when stretches opened purified Escherichia coli MscL channels reconstituted into artificial bilayers. This reductionistic exercise has rigorously been recapitulated recently with two vertebrate mechanosensitive K(+) channels (TREK1 and TRAAK). Membrane stretches have also been known to activate various voltage-, ligand-, or Ca(2+)-gated channels. Careful analyses showed that Kv, the canonical voltage-gated channel, is in fact exquisitely sensitive even to very small tension. In an unexpected context, the canonical transient-receptor-potential channels in the Drosophila eye, long presumed to open by ligand binding, is apparently opened by membrane force due to PIP2 hydrolysis-induced changes in bilayer strain. Being the intimate medium, lipids govern membrane proteins by physics as well as chemistry. This principle should not be a surprise because it parallels water's paramount role in the structure and function of soluble proteins. Today, overt or covert mechanical forces govern cell biological processes and produce sensations. At the genesis, a bilayer's response to osmotic force is likely among the first senses to deal with the capricious primordial sea.

Keywords: bilayer mechanics; channel gating; force sensing; mechanosensitivity; touch.

Fig. 1.

Anisotropic forces of the lipid bilayer and their changes that can reshape embedded proteins. (A) A diagram comparing a protein in the cytoplasm, near-isotropically compressed by the bombardment of neighboring particles (Left) and one embedded in a bilayer, subjected to large lateral tension at the two polar–nonpolar interfaces opposed by compression forces at the other levels of the bilayer (Right). (B) The lateral tension at the interface is balanced by the pressure in the head groups and hydrophobic tails (Upper Left) and this equilibrium can match a certain protein conformation, say, the closed state of a channel protein (Upper Right). External stretch force (Lower, broad red arrows) and/or amphipaths with positive (red triangle) or negative (green triangles) spontaneous curvature asymmetrically added to one leaflet can thin and bend the bilayer at the protein–lipid interface, changing the force vectors (small arrows) and therefore prefer a better matched protein conformation, say, the open state.

Fig. 2.

The atomic structures and the reconstituted activities of TRAAK. (A and B) TRAAK structures from two perspectives showing the two S₂ amphipathic peptides [hydrophobic (green) and charged (purple) residues] and one of the two prominent fenestrations (yellow) (from ref. 32). (C) Monodispersion of column eluate indicates the purity of the TRAAK protein produced. (D) Such protein is reconstituted into liposomes. TRAAK channels in a patch excised from such a proteoliposome are activated (upper traces) in proportion to the suction applied to patch (bottom traces). (E) TRAAKs in a patch excised from an expressing CHO cell, in which they should have the same orientation, respond (upper traces) to either suction (black) or pressure (red). (F) Activation by pressures in either direction are quantitatively similar, indicating that it is the membrane stretch that activates (modified from ref. 46).

Fig. 3.

The mechanosensitivity of Kv. (A–C) The Kv1.2 paddle chimera expressed in an Sf-9 cell responds to voltage steps ranging from −100 to 40 mV. The responses in on-cell mode (A) are drastically different from those in the whole-cell mode (B). The normalized conductances are graphically compared in C. As interpreted in D, the large differences originate from the high pipet tension in the on-cell patch due to lipid–glass adhesion, but the lack of it in the whole-cell membrane, where the cortical cytoskeleton prevents the local membranes from being stretched (from ref. 44). (E) The role of Kv1.1’s mechanosensitivity in countering impact-induced excitation in vivo is made evident when a copy of a dominant-negative allele of Kv1.1 is expressed in the mouse (mceph^+/−) causing it to be overly sensitive to gentle poking, as indicated by the low limb-withdrawal threshold of prod strength (from ref. 58). ns, not significant; **P < 0.01.

Fig. 4.

The photomechanical transduction pathway in the Drosophila compound eye. (A) A diagram showing the pathway, where the G protein-activated PLC converts PIP₂ to DAG, concentrated in the inner leaflet (upper half of the bilayer in this diagram). (B) Lipid structures emphasize that the beheading of the rod-shaped PIP₂ converts it into the cone-shaped DAG, yielding change in both the volume of lipid and spontaneous curvature in the inner leaflet (modified from ref. 64). (C) The arrangement of the cantilever of an atomic force microscope on a retina. (D) Brief flashes (bump on the center line) leads to shrinkages as measured by the atomic force microscope, proportional to light intensity (lower curves). The shrinkage precedes the phototransduction current (upper curves) (from ref. 64). E shows the structure of an ommatidium, the unit of the compound eye, which comprises thousands of microvilli where the phototransduction takes place. It is the sum of the small shrinkage in each microvillus that is measured by the atomic force microscope (modified from ref. 100).