Molecular clues to mechanosensitivity

Avian magnetite-based magnetoreception: a physiologist's perspective

It is now well established that animals use the Earth's magnetic field to perform long-distance migration and other navigational tasks. However, the transduction mechanisms that allow the conversion of magnetic field variations into an electric signal by specialized sensory cells remain largely unknown. Among the species that have been shown to sense Earth-strength magnetic fields, birds have been a model of choice since behavioural tests show that their direction-finding abilities are strongly influenced by magnetic fields. Magnetite, a ferromagnetic mineral, has been found in a wide range of organisms, from bacteria to vertebrates. In birds, both superparamagnetic (SPM) and single-domain magnetite have been found to be associated with the trigeminal nerve. Electrophysiological recordings from cells in the trigeminal ganglion have shown an increase in action potential firing in response to magnetic field changes. More recently, histological evidence has demonstrated the presence of SPM magnetite in the subcutis of the pigeon's upper beak. The aims of the present review are to review the evidence for a magnetite-based mechanism in birds and to introduce physiological concepts in order to refine the proposed models.

The Cytoskeletal Connection to Ion Channels as a Potential Mechanosensory Mechanism: Lessons from Polycystin-2 (TRPP2)

Mechanosensitivity of ion channels, or the ability to transfer mechanical forces into a gating mechanism of channel regulation, is split into two main working (not mutually exclusive) hypotheses. One is that elastic and/or structural changes in membrane properties act as a transducing mechanism of channel regulation. The other hypothesis involves tertiary elements, such as the cytoskeleton which, itself by dynamic interactions with the ion channel, may convey conformational changes, including those ascribed to mechanical forces. This hypothesis is supported by numerous instances of regulatory changes in channel behavior by alterations in cytoskeletal structures/interactions. However, only recently, the molecular nature of these interactions has slowly emerged. Recently, a surge of evidence has emerged to indicate that transient receptor potential (TRP) channels are key elements in the transduction of a variety of environmental signals. This chapter describes the molecular linkage and regulatory elements of polycystin-2 (PC2), a TRP-type (TRPP2) nonselective cation channel whose mutations cause autosomal dominant polycystic kidney disease (ADPKD). The chapter focuses on the involvement of cytoskeletal structures in the regulation of PC2 and discusses how these connections are the transducing mechanism of environmental signals to its channel function.

Simulation of MscL gating in a bilayer under stress

The initial stages of the gating of the mechanoselective channel of large conductance from Mycobacterium tuberculosis have been studied in atomic detail using molecular dynamics simulation techniques. A truncated form of the protein complex embedded in a palmitoyloleoylphosphatidylcholine lipid bilayer and surrounded by explicit water was simulated under different pressure conditions to mimic the effects of tension and compression within the membrane on the protein. As a direct result of lateral tension being applied to the membrane, an increase in the tilt of a subset of the transmembrane helices was observed. This in turn led to the enlargement of the pore and the disruption of the hydrophobic gate consisting of residues Ile-14 and Val-21. The simulations suggest that opening occurs in a sequential staged process. Such a mechanism could explain the partial opening or staged conductance observed in patch-clamp experiments using related large conductance mechanosensitive channel complexes.

Regulation of I(Cl,swell) in neuroblastoma cells by G protein signaling pathways

Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) activated the I(Cl,swell) anion channel in N1E115 neuroblastoma cells in a swelling-independent manner. GTPgammaS-induced current was unaffected by ATP removal and broadly selective tyrosine kinase inhibitors, demonstrating that phosphorylation events do not regulate G protein-dependent channel activation. Pertussis toxin had no effect on GTPgammaS-induced current. However, cholera toxin inhibited the current approximately 70%. Exposure of cells to 8-bromoadenosine 3',5'-cyclic monophosphate did not mimic the effect of cholera toxin, and its inhibitory action was not prevented by treatment of cells with an inhibitor of adenylyl cyclase. These results demonstrate that GTPgammaS does not act through Galpha(i/o) GTPases and that Galpha(s)/Gbetagamma G proteins inhibit the channel and/or channel regulatory mechanisms through cAMP-independent mechanisms. Swelling-induced activation of I(Cl,swell) was stimulated two- to threefold by GTPgammaS and inhibited by 10 mM guanosine 5'-O-(2-thiodiphosphate). The Rho GTPase inhibitor Clostridium difficile toxin B inhibited both GTPgammaS- and swelling-induced activation of I(Cl,swell). Taken together, these findings indicate that Rho GTPase signaling pathways regulate the I(Cl,swell) channel via phosphorylation-independent mechanisms.

Correlating a protein structure with function of a bacterial mechanosensitive channel

MscL, a mechanosensitive channel found in many bacteria, protects cells from hypotonic shock by reducing intracellular pressure through release of cytoplasmic osmolytes. First isolated from Escherichia coli, this protein has served as a model for how a protein senses and responds to membrane tension. Recently the structure of a functionally uncharacterized MscL homologue from Mycobacterium tuberculosis was solved by x-ray diffraction to a resolution of 3.5 A. Here we demonstrate that the protein forms a functional MscL-like mechanosensitive channel in E. coli membranes and azolectin proteoliposomes. Furthermore, we show that M. tuberculosis MscL crystals, when re-solubilized and reconstituted, yield wild-type channel currents in patch clamp, demonstrating that the protein does not irreversibly change conformation upon crystallization. Finally, we apply functional clues acquired from the E. coli MscL to the M. tuberculosis channel and show a mechanistic correlation between these channels. However, the inability of the M. tuberculosis channel to gate at physiological membrane tensions, demonstrated by in vivo E. coli expression and in vitro reconstitution, suggests that the membrane environment or other additional factors influence the gating of this channel.

Temporary disruption of the plasma membrane is required for c-fos expression in response to mechanical stress

Mechanically stressed cells display increased levels of fos message and protein. Although the intracellular signaling pathways responsible for FOS induction have been extensively characterized, we still do not understand the nature of the primary cell mechanotransduction event responsible for converting an externally acting mechanical stressor into an intracellular signal cascade. We now report that plasma membrane disruption (PMD) is quantitatively correlated on a cell-by-cell basis with fos protein levels expressed in mechanically injured monolayers. When the population of PMD-affected cells in injured monolayers was selectively prevented from responding to the injury, the fos response was completely ablated, demonstrating that PMD is a requisite event. This PMD-dependent expression of fos protein did not require cell exposure to cues inherent in release from cell-cell contact inhibition or presented by denuded substratum, because it also occurred in subconfluent monolayers. Fos expression also could not be explained by factors released through PMD, because cell injury conditioned medium failed to elicit fos expression. Translocation of the transcription factor NF-kappaB into the nucleus may also be regulated by PMD, based on a quantitative correlation similar to that found with fos. We propose that PMD, by allowing a flux of normally impermeant molecules across the plasma membrane, mediates a previously unrecognized form of cell mechanotransduction. PMD may thereby lead to cell growth or hypertrophy responses such as those that are present normally in mechanically stressed skeletal muscle and pathologically in the cardiovascular system.

Role of the actin cytoskeleton on epithelial Na+ channel regulation

The regulatory role of actin filament organization on epithelial Na+ channel activity is reviewed in this report. The actin cytoskeleton, consisting of actin filaments and associated actin-binding proteins, is essential to various cellular events including the maintenance of cell shape, the onset of cell motility, and the distribution and stability of integral membrane proteins. Functional interactions between the actin cytoskeleton and specific membrane transport proteins are, however, not as well understood. Recent studies from our laboratory have determined that dynamic changes in the actin cytoskeletal organization may represent a novel signaling mechanism in the regulation of ion transport in epithelia. This report summarizes work conducted in our laboratory leading to an understanding of the molecular steps associated with the regulatory role of the actin-based cytoskeleton on epithelial Na+ channel function. The basis of this interaction lies on the regulation by actin-binding proteins and adjacent structures, of actin filament organization which in turn, modulates ion channel activity. The scope of this interaction may extend to such relevant cellular events as the vasopressin response in the kidney.

Contemplating the plasmalemmal control center model

An abundant epidermal mechanosensory calcium-selective ion channel appears able not only to detect mechanical stimuli such as those that initiate gravitropism but also to detect thermal, electrical, and various chemical stimuli. Because it responds to multimodal input with a second messenger output, this channel system seems likely to be an integrator that can engage in feedbacks with many other systems of the cell--and feedback is the hallmark of regulation. In general, the mechanical tension required for channel activation is likely transmitted from the relatively rigid cell wall to the plasma membrane system via linkage or adhesion sites that display antigenicities recognized by antibodies to animal beta-1 integrin, vitronectin, and fibronectin and which have mechanical connections to the cytoskeleton. Thus, functionally, leverage exerted against any given adhesion site will tend to control channels within a surrounding domain. Reactions initiated by passage of calcium ions through the channels could presumably be more effectively regulated if channels within the domains were somewhat clustered and if appropriate receptors, kinases, porters, pumps, and some key cytoskeletal anchoring sites were in turn clustered about them. Accumulating evidence suggests not only that activity of clusters of channels may contribute to control of cytoskeletal architecture and of regulatory protein function within their domain, but also that both a variety of regulatory proteins and components of the cortical cytoskeleton may contribute to control of channel activity. The emerging capabilities of electronic optical microscopy are well suited for resolving the spatial distributions of many of these cytoskeletal and regulatory molecules in living cells, and for following some of their behaviors as channels are stimulated to open and cytosolic calcium builds in their vicinity. Such microscopy, coupled with biochemical and physiological probing, should help to establish the nature of the feedback loops putatively controlled by the linkage sites and their channel domains.