Mechanotransduction, the conversion of mechanical cues into biochemical signals, is crucial for many biological processes in plants and animals (Arnadóttir and Chalfie, 2010; Ranade et al., 2015). In mammals, some mechanosensory processes such as touch sensation and vascular development are mediated by the PIEZO family of mechanically activated (MA) ion channels (Coste et al., 2010; Coste et al., 2012; Murthy et al., 2017). For hearing, the mechanotransduction complex in hair cells includes TMC1 as the pore-forming ion channel, but also comprises of TMHS and TMIE (Ballesteros et al., 2018; Pan et al., 2018; Qiu and Müller, 2018). However, other MA ion channels that govern processes such as pain sensation await identification.

In plants, the impact of gravity or soil properties on root development, wind on stem growth, and turgor pressure on plant-cell size and shape are proposed to involve activation of MA ion channels (Hamant, 2013; Hamant and Haswell, 2017). Homologues of the bacterial MA channel MscS-Like (MSLs) exist in plants, and MSL8 is shown to be involved in pollen hydration in Arabidopsis thaliana (Cox et al., 2015; Hamilton et al., 2015). Apart from MSL8, the identity of the MA channels required for most mechanotransduction processes in plants has remained elusive. Beyond MSLs, hyperosmolarity-evoked intracellular calcium increase is shown to be dependent on the genes OSCA1.1 and OSCA1.2 in Arabidopsis thaliana (Hou et al., 2014; Yuan et al., 2014); however, the activation mechanism for these proteins and whether they encode a pore-forming ion channel remains unknown.

We synthesized human codon-optimized versions of OSCA1.1 (At4g04340) and OSCA1.2 (At4g22120) cDNA in pIRES2-mCherry vector, heterologously expressed them in mechanically-insensitive PIEZO1-knockout HEK293T cells (HEK-P1KO) (Dubin et al., 2017), and electrophysiologically characterized hyperosmolarity-activated currents. In contrast to published reports (Hou et al., 2014; Yuan et al., 2014), we find that hyperosmolarity-evoked whole-cell currents recorded from OSCA1.1- or OSCA1.2-expressing cells were only modestly larger than baseline currents (Figure 1—figure supplement 1).

We next explored the possibility that OSCA1.1 and OSCA1.2 are mechanosensitive, and that the modest hyperosmolarity-induced currents might be due to osmotic shock causing cell shrinking, and affecting membrane tension (Sachs, 2010). In cells, MA currents are commonly induced by two direct methods: 1) cell-membrane indentation with a glass probe induces macroscopic MA currents in the whole-cell patch clamp mode; 2) cell-membrane stretch induces single-channel or macroscopic MA currents when pressure is applied to a recording pipette in the cell-attached (or excised) patch clamp mode. Surprisingly, MA whole-cell currents recorded from cells transfected with OSCA1.1 or OSCA1.2 were 10- and 100-fold larger than their hyperosmolarity-activated currents, respectively (Figure 1A,B vs. Figure 1—figure supplement 1), and were comparable to those recorded from cells transfected with mouse PIEZO1, a well-characterized mechanosensitive ion channel (Figure 1B). Mechanosensitivity of a channel can be estimated by calculating the apparent threshold for activating MA currents that are elicited by membrane indentation. Threshold is measured as the differential of probe distance that first touches the cell and the probe distance that induces the first channel response. Therefore, it is the minimum distance of indentation required to activate the channel. OSCA1.1 and OSCA1.2 whole-cell MA currents had an apparent activation threshold of 8.6 ± 0.9 µm and 6.3 ± 0.7 µm, and inactivated (channel closure in continued presence of stimulus) with a time constant of 10.0 ± 1.3 ms and 10.4 ± 1.7 ms, respectively (Figure 1B and Table 1). Similarly, robust macroscopic stretch-activated currents were recorded from cells transfected with OSCA1.1 or OSCA1.2 but not from mock-transfected cells (Figure 1C,D). Stretch-activated currents from OSCA1.1 and OSCA1.2 were reversible and inactivated with a time constant of 24 ± 3.4 ms and 24.6 ± 4.8 ms, respectively (Figure 1D). The pressure required for half-maximal activation (P₅₀) of OSCA1.1 and OSCA1.2 was -58.5 ± 3.7 mmHg and -54.5 ± 2.2 mmHg, respectively (Figure 1E). These values are higher than mouse PIEZO1 which has a threshold of -24 ± 3.6 mmHg (Coste et al., 2010; Coste et al., 2015) (Figure 1E and Table 1), demonstrating that at least in HEK-P1KO cells these proteins evoke high-threshold MA currents. These results suggest that OSCA1.1 and OSCA1.2 are involved in mechanotransduction.

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Under physiological ionic conditions, OSCA1.1- and OSCA1.2-dependent stretch-activated currents had single-channel conductance of 184 ± 4 pS and 122 ± 3 pS, respectively (Figure 1F,G). We note that these values are larger than what was previously reported (Yuan et al., 2014). Unlike our measurements, the single-channel conductance described in Yuan et al., 2014 was measured in the absence of a stimulus, and might not be strictly OSCA-dependent. Interestingly, stretch-activated single-channel current traces from either protein revealed a single sub-conductance state, which was half the amplitude of the full open state (Figure 1—figure supplement 2). Sub-conductance states are a hallmark of many ion channels and are indicative of concerted gating of multiple pores from the same channel, or of changes within a single pore (Dani and Fox, 1991; Hartzell and Whitlock, 2016; Miller, 1982). The presence of a single intermediate state in OSCA1.1 and OSCA1.2 could be suggestive of two cooperative subunits. Indeed, this suggestion is confirmed by the cryo-EM structures of OSCA1.1 and OSCA1.2, which reveal a pore within each subunit of a dimeric channel (Jojoa Cruz et al., 2018; Zhang et al., 2018). Characterization of the stretch-activated currents in asymmetrical NaCl solutions revealed that OSCA1.1 and OSCA1.2 evoked non-selective cation currents with some chloride permeability (OSCA1.1: P_Cl/P_Na = 0.21 ± 0.06; OSCA1.2: P_Cl/P_Na = 0.17 ± 0.01) (Figure 1H). In addition, gadolinium, a generic MA cation channel blocker, inhibited OSCA1.1- and OSCA1.2-induced stretch-activated currents (Figure 1—figure supplement 3). Together, these results demonstrate that OSCA1.1 and OSCA1.2 induce MA non-selective cation currents.

In Arabidopsis thaliana, OSCA1.1 and OSCA1.2 belong to a family of genes that include 15 members across four clades (Hou et al., 2014; Yuan et al., 2014) (Figure 2A). OSCA1.1 and OSCA1.2 share 85% sequence identity, while other family members within the same clade are more divergent (50 – 70%). Genes within Clade 2 have about 30% identity to Clade 1, and Clade 3 and 4 share the least homology with Clade 1 (Figure 2A and Table 2). We investigated whether mechanosensitivity might be conserved across this family. We selected one gene from each clade, and tested whether their expression could induce MA currents in a heterologous expression system (Figure 2). In the whole-cell patch clamp mode, mechanical indentation of cells expressing OSCA1.8, OSCA2.3, OSCA3.1, or OSCA4.1 did not elicit MA currents (Figure 2—figure supplement 1). Remarkably, however, distinct stretch-activated currents were recorded from cell expressing OSCA1.8, OSCA2.3, or OSCA3.1, but not OSCA4.1 (Figure 2B,C and Table 1). Although the lack of OSCA4.1-induced MA currents could suggest that this gene is functionally distinct from other members of the family, we cannot rule out that OSCA4.1 might be incorrectly folded or not trafficked to the membrane in HEK-P1KO cells. The three mechanosensitive OSCA proteins had disparate gating kinetics, with different inactivation time constants (Figure 2C). While the pressure of half-maximal activation was comparable among all the mechanosensitive OSCA proteins (Figure 2D), the most striking feature was the diversity in their single-channel conductance (Figure 2E,F). OSCA1.8 and OSCA3.1 channel conductance was approximately four- and six-fold smaller than OSCA1.1, respectively. Furthermore, single-channel amplitude of OSCA2.3 stretch-activated currents were in the sub-picoampere range and unresolvable, which could explain the relatively smaller maximal current responses (Figure 2C). These results provide evidence that OSCAs are a family of MA ion channels with unique biophysical properties. Intriguingly, the different members of the OSCA family have differential responses to the two types of mechanical stimulation. It is noteworthy that OSCA1.1 and OSCA1.2 which were the only channels activated by membrane indentation, had a high threshold (8 µm) compared to mouse PIEZO1 (4 – 5 µm) (Dubin et al., 2017) (Table 1). Perhaps other OSCA channels have a much higher threshold, technically limiting us from recording reliable MA currents, since HEK-P1KO cells often break near 10 – 12 µm of indentation. Alternatively, mechanical indentation and membrane stretch deliver biophysically-distinct forces to the membrane and the different proteins might be tuned to the unique type of force applied.

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The results described so far indicate that OSCAs evoke stretch-activated currents in a heterologous expression system, meeting at least one of the criteria for MA channels. Indeed, these results by themselves only allow us to conclude that they are a component of an ion channel; it is formally possible that they require endogenously expressed components within HEK-P1KO cells to behave as a functional mechanosensitive ion channel (Arnadóttir and Chalfie, 2010). To address this possibility, we purified OSCA1.2 fused to a GFP tag, and reconstituted the protein in liposomes to determine whether stretch-activated currents can be recorded from excised proteoliposome patches (Figure 3A). Purified OSCA1.2-EGFP (117 kDa) appears as a single protein band on a denaturing gel, indicating the absence of other associated proteins (Figure 3—figure supplement 1). Before characterizing OSCA1.2, we first tested whether we could successfully record stretch-activated currents from liposomes reconstituted with the bacterial mechanosensitive ion channel E. coli MscL (Figure 3—figure supplement 1) (Häse et al., 1995). We observed MscL-like stretch-induced channel activity in seven out of eight excised patches (Figure 3B–D). The stretch-activated single-channel currents had a conductance of 3340 ± 180 pS (N = 6), which is in accordance with previous reports (Kloda and Martinac, 2002) (Figure 3C). Remarkably, applying negative pipette-pressure in the range of 0 to 50 mmHg to patches excised from liposomes reconstituted with OSCA1.2-EGFP protein also induced robust macroscopic currents (Figure 3E,F). Applying negative pressure (as high 100 to 120 mmHg) to patches excised from empty liposomes did not change the baseline current. At the single-channel level, the stretch-activated currents exhibited sub-conductance states, were voltage-dependent, and had a conductance of 346 ± 13 pS in 200 mM KCl solution, which matched the single-channel conductance measured from OSCA1.2-expressing cells (304 ± 9 pS) under the same ionic concentration (Figure 3—figure supplement 1 and Figure 3G,H). These results demonstrate that OSCA1.2, like other bona-fide MA ion channels such as MscL, TREK and TRAAK, and PIEZO1, is directly gated by changes in membrane tension, and requires no additional cellular components for activation (Anishkin et al., 2014; Brohawn et al., 2014; Häse et al., 1995; Syeda et al., 2016). This represents conclusive evidence that OSCA1.2 encodes a pore-forming subunit and is inherently mechanosensitive (Jojoa Cruz et al., 2018).

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We next examined if orthologues of OSCAs are also mechanosensitive. Phylogenetic analysis and previous studies have identified the TMEM63 family of proteins as the closest homologues of OSCAs (Hou et al., 2014; Zhao et al., 2016; Yuan et al., 2014) (Figure 4A and Table 2). To determine whether mechanosensitivity was conserved across different species, we selected homologues in fruit fly, mouse, and human, and tested their ability to induce MA currents in HEK-P1KO cells. In the whole-cell patch clamp mode, mechanical stimulation of cells expressing DmTMEM63, MmTMEM63A, MmTMEM63B, MmTMEM63C, or HsTMEM63A did not elicit MA currents (Figure 4—figure supplement 1). However, these clones (with the exception of MmTMEM63C) induced stretch-activated currents when expressed in naïve cells (Figure 4B,C). The phylogenetic tree illustrates that mammalian TMEM63C is divergent from TMEM63A and TMEM63B (Figure 4A), which may explain its lack of mechanosensitivity. Alternatively, it is possible that this protein is incorrectly folded and not trafficked to the membrane, or that TMEM63C is not sufficient by itself to induce MA currents.

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P₅₀ values for the TMEM63-induced stretch-activated currents were in the range of -60 to -80 mmHg, suggesting that like OSCAs, these proteins elicit high-threshold MA currents in HEK-P1KO cells (Figure 4E). The stretch-activated single-channel currents induced by either DmTMEM63 or the mammalian TMEM63s are in the sub-picoampere range and were unresolvable, similar to OSCA2.3 (Figure 4B vs. Figure 2B). However, compared to plant OSCAs, MA currents induced by the TMEM63 family members had unique gating properties with several fold slower activation and inactivation kinetics (Figure 4D and Table 1). MmTMEM63A stretch-activated currents are non-selective cationic, and gadolinium inhibits MmTMEM63A-induced MA currents by 75% (Figure 4—figure supplement 2). The stark difference in MA current properties between OSCA2.3/TMEM63s and other OSCA members is interesting. At least MmTMEM63A is expressed at the membrane (Figure 4—figure supplement 3), therefore trafficking issues cannot account for the relatively small stretch-activated macroscopic currents. However, one cannot exclude the possibility that in the native cellular environment, other auxiliary subunits might interact with OSCA2.3 and TMEM63 proteins to alter the channel’s permeation and gating properties. Future studies will test this hypothesis. Nonetheless, these results demonstrate that orthologues of OSCAs also induce stretch-activated currents when overexpressed in naïve cells, and that mechanosensitivity is conserved across the various members in the OSCA/TMEM63 family.

We provide evidence that members of OSCA family are bona-fide, pore-forming mechanosensitive ion channels: (1) transfection of various members of the family from plants, flies, and mammals give rise to robust MA currents. Importantly, single-channel conductance of individual OSCAs are quite distinct, arguing that the OSCAs contribute to pore properties of these currents. (2) We directly show in vitro (proteoliposomes) that OSCA1.2 is an inherently mechanosensitive ion channel in the absence of other proteins. (3) The accompanying paper describes the high-resolution structure of OSCA1.2 (Jojoa Cruz et al., 2018), and demonstrates that this protein has similar architecture to the TMEM16 family of ion channels (Whitlock and Hartzell, 2017). Furthermore, structure-guided mutagenesis verifies that a residue within the putative pore-forming region contributes to single-channel conductance (Jojoa Cruz et al., 2018). Therefore, with 15 different members present just in Arabidopsis thaliana (5/6 members we tested were mechanosensitive), OSCA/TMEM63 proteins potentially represent the largest family of mechanosensitive ion channels known to date. Although other eukaryotic ion channels have been previously described, none are conserved from plants to humans. PIEZOs are also present in plants and unicellular organisms; however, evidence that they are mechanosensitive in these species is still lacking.

The existence of 15 members of OSCAs in Arabidopsis thaliana raises the possibility that there might be redundancy, and will make it a challenge to assign function to these proteins individually. However, Yuan et al., 2014reported that under hyperosmotic stress, mutant OSCA1.1 plants had stunted leaf and root growth (Yuan et al., 2014). These phenotypes could indeed be a consequence of impaired mechanotransduction. Expression profile of mouse TMEM63A in public databases (BioGPS.org) suggests expression in tissues that experience mechanical forces such as kidney and stomach, while TMEM63B is high in the nervous system (including dorsal root ganglia) as well as in the heart, skeletal muscle, and stomach. Future studies will investigate the contribution of members of OSCA and TMEM63 families to mechanotransduction in various species.

We note the publication of a recent complementary study (Zhang et al., 2018). Our manuscript was previously deposited on BioRxiv (doi: https://doi.org/10.1101/408732).

All data generated or analyzed during this study are included in the manuscript.

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Author details

1. Swetha E Murthy

   Department of Neuroscience, Dorris Neuroscience Center, The Scripps Research Institute, California, United States

   Contribution

   Conceptualization, Data curation, Formal analysis, Methodology, Writing—original draft, Writing—review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0001-9580-3380

2. Adrienne E Dubin

   Department of Neuroscience, Dorris Neuroscience Center, The Scripps Research Institute, California, United States

   Contribution

   Data curation, Funding acquisition, Methodology, Writing—review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-4683-7175

3. Tess Whitwam

   Department of Neuroscience, Dorris Neuroscience Center, The Scripps Research Institute, California, United States

   Contribution

   Methodology, Writing—review and editing

   Competing interests

   No competing interests declared

4. Sebastian Jojoa-Cruz

   Department of Integrative Structural and Computational Biology, The Scripps Research Institute, Howard Hughes Medical Institute, California, United States

   Contribution

   Methodology, Writing—review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-4392-3898

5. Stuart M Cahalan

   Department of Neuroscience, Dorris Neuroscience Center, The Scripps Research Institute, California, United States

   Present address

   Vertex Pharmaceuticals, California, United States

   Contribution

   Methodology, Writing—review and editing

   Competing interests

   No competing interests declared

6. Seyed Ali Reza Mousavi

   Department of Neuroscience, Dorris Neuroscience Center, The Scripps Research Institute, California, United States

   Contribution

   Methodology, Writing—review and editing

   Competing interests

   No competing interests declared

7. Andrew B Ward

   Department of Integrative Structural and Computational Biology, The Scripps Research Institute, Howard Hughes Medical Institute, California, United States

   Contribution

   Supervision, Funding acquisition, Writing—review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0001-7153-3769

8. Ardem Patapoutian

   Department of Neuroscience, Dorris Neuroscience Center, The Scripps Research Institute, California, United States

   Contribution

   Conceptualization, Supervision, Funding acquisition, Writing—review and editing

   For correspondence

   ardem@scripps.edu

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-0726-7034

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