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Shear Force Sensing of Epithelial Na+ Channel (Enac) Relies on N + Shear force sensing of epithelial Na channel (ENaC) relies on N-glycosylated asparagines in the palm and knuckle domains of αENaC Fenja Knoeppa, Zoe Ashleyb,c, Daniel Barthb,c, Jan-Peter Baldinb,c, Michael Jenningsb, Marina Kazantsevab, Eng Leng Sawb,c, Rajesh Katareb,c, Diego Alvarez de la Rosad, Norbert Weissmanna, and Martin Froniusb,c,1 aExcellence-Cluster Cardio-Pulmonary Institute, Universities of Giessen and Marburg Lung Center, Member of the German Center for Lung Research, Justus- Liebig University Giessen, D-35392 Giessen, Germany; bDepartment of Physiology, University of Otago, 9016 Dunedin New Zealand; cHeartOtago, University of Otago, 9016 Dunedin, New Zealand; and dDepartment of Physiology, Institute of Biomedical Technology, University of Laguna, E-38071 La Laguna, Spain Edited by Martin Chalfie, Columbia University, New York, NY, and approved November 26, 2019 (received for review July 2, 2019) Mechanosensitive ion channels are crucial for normal cell function vanilloid type-1 (TRPV1) channel (11) that belongs to the same and facilitate physiological function, such as blood pressure regula- protein family as NOMPC. Evidence for extracellular tethers as tion. So far little is known about the molecular mechanisms of how constituents of mechanotransduction originates from studies in channels sense mechanical force. Canonical vertebrate epithelial auditory/vestibular hair cells and Caenorhabditis elegans (7, 12). + Na channel (ENaC) formed by α-, β-, and γ-subunits is a shear force In hair cells, extracellular tethers are known as tip links and (SF) sensor and a member of the ENaC/degenerin protein family. ENaC were identified by electron microscopy (6, 13). Tip links are activity in epithelial cells contributes to electrolyte/fluid-homeostasis suggested to be filamentous protein structures that are directly and blood pressure regulation. Furthermore, ENaC in endothelial cells involved in channel gating as identified in neurons that mediate mediates vascular responsiveness to regulate blood pressure. Here, touch sensation (14). Although the existence of tethers is beyond we provide evidence that ENaC’s ability to mediate SF responsiveness dispute, their architecture is largely unknown. relies on the “force-from-filament” principle involving extracellular In C. elegans, mutations of genes encoding ion channel sub- tethers and the extracellular matrix (ECM). Two glycosylated aspara- units (e.g., mechanosensory abnormality-4 [MEC-4] and MEC-10) gines, respectively their N-glycans localized in the palm and knuckle and extracellular proteins (e.g., MEC-5 and MEC-9, which are PHYSIOLOGY α domains of ENaC, were identified as potential tethers. Decreased SF- considered to be part of the ECM) resulted in touch-insensitive induced ENaC currents were observed following removal of the ECM/ animals (15, 16). This indicates that channel proteins and the ECM glycocalyx, replacement of these glycosylated asparagines, or removal — — N- α are connected via tethers to facilitate the mechanical percep- of glycans. Endothelial-specific overexpression of ENaC in mice in- tion of touch in C. elegans. The mechanosensitive MEC channel duced hypertension. In contrast, expression of αENaC lacking these proteins of C. elegans are the founding members of the epithelial glycosylated asparagines blunted this effect. In summary, glycosylated + Na channel (ENaC)/degenerin (DEG) protein family. Members asparagines in the palm and knuckle domains of αENaC are important of this family are widely expressed in the animal kingdom and form for SF sensing. In accordance with the force-from-filament principle, theymayprovideaconnectiontothe ECM that facilitates vascular mechanosensitive ion channels (17). A well-known member of the responsiveness contributing to blood pressure regulation. family constitutes the ENaC in vertebrates (18, 19). Canonical ENaC is a heterotrimer composed of α-, β-, and γ-subunits (20) mechanotransduction | shear force | extracellular tether | N-glycosylation | + epithelial Na channel (ENaC) Significance ll living organisms have developed strategies that enable The ability to sense mechanical forces is essential for all living them to perceive their mechanical environment. This pro- organisms. Extracellular tethers have been proposed to medi- A ate mechanical activation of channels belonging to the epi- cess is known as mechanotransduction and describes how me- + chanical forces that are acting on cells/organisms are translated thelial Na channel (ENaC)/degenerin protein family. The into cellular signals. Among other transmembrane proteins, nature and architecture of the tethers that link the channel mechanosensitive ion channels are key molecules for this process protein with the extracellular matrix are unknown. Our study (1, 2). Currently, two principles have been put forward to explain provides experimental evidence that glycosylated asparagines N how force can activate mechanosensitive ion channels: 1) The and their -glycans are part of tethers for mechanical activa- “force-from-lipids” (3) identified by studying stretch-activated tion of ENaC by shear force. The identified asparagines are also channels in bacteria (4), and 2) the “force-from-filaments” prin- important for arterial blood pressure regulation in vivo. These ciple (5) that originates from studies of hair cells (6). The force- findings provide insights into how mechanical forces are from-filaments principle (also known as the “tethered model”) sensed by mechanosensitive ENaC channels to regulate blood proposes the transmission of mechanical force through tethering pressure. of the ion channel proteins to internal (cytoskeletal filaments) or “ Author contributions: F.K. and M.F. designed research; F.K., Z.A., D.B., J.-P.B., and M.J. external filaments (extracellular matrix, ECM) as sensing struc- performed research; M.K., R.K., D.A.d.l.R., and N.W. contributed new reagents/analytic tures” (7–9). Deformation or deflection of the sensing structure tools; F.K., Z.A., D.B., J.-P.B., E.L.S., R.K., and M.F. analyzed data; and F.K. and M.F. wrote will be transduced via tethers to the channel to change its con- the paper. formation and thereby its activity. The authors declare no competing interest. There is emerging evidence for intracellular and extracellular This article is a PNAS Direct Submission. tethers mediating mechanical activation of channels. For exam- This open access article is distributed under Creative Commons Attribution-NonCommercial- ple, mechanical activation of the no mechanoreceptor potential NoDerivatives License 4.0 (CC BY-NC-ND). C (NOMPC) channel of Drosophila melanogaster depends on 1To whom correspondence may be addressed. Email: [email protected]. ankyrin repeats within its N terminus that form an intracellular This article contains supporting information online at https://www.pnas.org/lookup/suppl/ tether (10). The ankyrin repeats facilitate the connection to doi:10.1073/pnas.1911243117/-/DCSupplemental. microtubules as identified for the transient receptor potential First published December 23, 2019. www.pnas.org/cgi/doi/10.1073/pnas.1911243117 PNAS | January 7, 2020 | vol. 117 | no. 1 | 717–726 Downloaded by guest on September 24, 2021 and is highly abundant in absorbing epithelia. In the kidney epi- A B thelium, ENaC is crucial for electrolyte/fluid-homeostasis (21) and SF blood pressure regulation (22). Soon after discovering the molec- SF (dyn/cm2) ular identity of ENaC (23), evidence emerged that ENaC is mechanosensitive and activated by membrane stretch (24–27), which would be in accordance with the force-from-lipids principle. However, these studies were critically discussed (28), suggesting that there is not enough evidence to conclude that ENaC is mechanically activated by stretch. In contrast to the controversial findings about the activation by membrane stretch, there is compelling evidence that ENaC is mechanically regulated by shear force (SF) (29–31), a frictional force caused by particles moving parallel to a surface. ENaC subunits are expressed in endothelial cells of arteries (32), ar- terial baroreceptors (33, 34), and distal kidney epithelium (35) where they are permanently exposed to SF due to the fluids SF (dyn/cm2) flowing over the surface of the cells. SF has been shown to ac- tivate ENaC by increasing the channel’s open probability (36, 37). Moreover, SF was identified to regulate ENaC in isolated kidney tubules (38) and arteries (39). SF sensation in both tissues C D is of considerable importance for blood pressure regulation (22). SF However, knowledge about the mechanisms of how ENaC and ENaC expressing related ENaC/DEG channels senses SF is yet unknown. Since ENaC proteins were identified to be related to the 0 0.2 mechanosensitive proteins of C. elegans, it was hypothesized that a mechanical activation involves the ECM as well as extracellular tethers that facilitate the interdependent activity (40, 41), a hy- 20 nA pothesis that is in agreement with the force-from-filament prin- 1 min ciple. However, despite the observations from touch-insensitive C. elegans strains, experimental evidence for the function and E architecture of extracellular tethers is scarce. Therefore, our water-injected study focuses on two aspects: 1) Providing functional evidence 0 0.2 for the interdependent activity of the channel and the ECM a for mechanical activation; and 2) identifying anchoring points for n=8n=8 n=11 n=11 n=13 tethers within the channel, which facilitate the interaction with 20 nA 0.2 1.3 0.01 0.04
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