Name: multifunctional, micropipette-based method for incorporation and stimulation of bacterial mechanosensitive ion channels in droplet interface bilayers
Uploaded: 2015-11-19T15:00:00
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Research reported in this publication is supported by the Air Force Office of Scientific Research Basic Initiative Grant FA9550-12-1-0464.

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The authors have nothing to disclose.

Mechanosensation signifies one of the first sensory transduction pathways that evolved in living organisms. Using this phenomenon for studying and understanding the mechano-electrical properties of the DIB, is a crucial step toward functional stimuli-responsive materials. It involves the incorporation and activation of a mechanosensitive channel, MscL, in the DIB as a mechanoelectrical transducer and a strain gauge to detect tension increase in the lipid bilayer interface. On another note, the function of MS channels could be regulated through the basic material properties of lipid bilayers including thickness, intrinsic curvature, and compressibility. In light of the aforementioned, the micropipette-based technique provides a valuable tool allowing the researcher the ability to study MS channels in DIBs and provides insights into the structure of the lipid bilayer, as well as the lipid-protein interactions.
Over the past three decades, patch-clamp was the primary method to study MS channels, since it allows clamping of both voltage and tension. However, patch-clamp requires bulky equipment and not suitable for miniaturization, a property required for the engineering of sensory and conversion devices. DIBs due to their simplicity, stability, and compactness represent a suitable environment to study the activity of MscL. Here, we extend previous advances in the DIB formation techniques by proposing a micropipette-based technique, with the ability to control the size of droplets and bilayer interface, the chemical composition of each droplet, and the tension at the interface through dynamic stimulation. The technique consists of anchoring aqueous droplets, containing proteoliposomes, to the tips of coaxially opposing glass capillaries. The droplets are placed in a bath of organic solvent and when brought in contact a lipid bilayer forms at the interface.
The micropipettes are attached to piezoelectric oscillators, allowing horizontal displacement of the droplets. Dynamically compressing the droplets, results in an increase of interfacial tension at the water oil interface and therefore an increase in bilayer tension. Two major aspects differentiate this method from the similar and recently published contact bubble bilayer (CBB) technique37. Using the technique presented herein, the size of the bilayer is controlled using micromanipulators and thus the volumes of the droplets remain constant, unlike in the CBB method. In addition, the CBB technique calls for pressure pumps, which are not needed in the method presented in this paper making it simpler and easier to build.
We are able to incorporate and stimulate bacterial MscL for the first time without the use of a patch pipette or chemical modifications38. Since the system facilitates the formation of robust asymmetric lipid bilayer membranes, it more closely mimics the lipid asymmetry found in biological membranes. This allows us to study the effects of controlled membrane composition or asymmetry on the activity of MscL. Additionally, through image processing techniques, this method helps estimate the tension at the bilayer interface. This technique assists in understanding the principles of interconversion between bulk and surface forces in the DIB, facilitates the measurements of fundamental membrane properties, and improves the understanding of MscL response to membrane tension.
Although this method takes us a step closer toward a biomolecular stimuli-responsive material system and to a different physiological environment to study MscL, there are limitations to the system. Tension in this system cannot be clamped due to the presence of the lipid reservoir in the form of liposomes in each droplet, which tends to relieve tension at the oil/water interface. Therefore, at present mechanosensitive channels can be stimulated in DIBs only in a dynamic regime. The presence of air bubbles in the system significantly affects the precision and reproducibility of the experiments. Air bubbles present in the hydrogels could result loss if electrical connection.
While we describe the use of the micro-pipette based method for the stimulation of MscL, the technique could be used to study other types of MS channels and has the potential to be used by researchers to study a variety of biomolecules. For instance, similar setup has been used in our lab to study the mechanoelectrical response of a channel-free droplet interface bilayer membrane. Various proteins could be reconstituted and activated using this highly controlled setup, taking in consideration that the reconstitution environments of each biomolecule vary. The method described in this article touches on a considerably wider application potential that is only limited to the imagination of the researcher.

In the past decade, the assembly of artificial lipid bilayers has been substantially advanced through the development of the droplet interface bilayer method. Known as stable and robust, DIBs imposed themselves as alternative model systems to the classical painted (Mueller) and folded (Montal-Mueller) planar bilayers1. Although the idea of using droplets to create lipid bilayers dates back to the 1960s2, it has not gained popularity until recently. The first successful attempt was reported by the Takeushi group3, followed by several studies demonstrating bilayer formation using a network of droplets by the Bayley group4-6. More recently, encapsulation techniques were proposed by the Leo group7-9, who pioneered the concept of using DIBs as building blocks of novel stimuli-responsive material systems10. In previous studies, DIBs have proved their ability to respond to electrical9,11, chemical10,12, and optical stimuli13. Various biomolecules with different stimuli-responsive functionalities have been effectively stimulated when reconstituted in the DIB10,14. In light of these successful attempts an important question is raised: could the DIB respond to mechanical stimulus when appropriate biomolecules are incorporated? The interfacial forces acting on a DIB differ from those in other bilayer system15,16. Therefore, the tension in the bilayer held by the droplets could be controlled by regulating tension at the water-lipid-oil interfaces; a concept not applicable with the painted or folded bilayer systems.
MscL channels, widely known as osmolyte release valves and fundamental elements of the bacterial cytoplasmic membrane, react to increased membrane tension17,18. In the event of hypo-osmotic shocks, several channels residing in the membrane of a small cell19 can generate a massive permeability response to quickly release ions and small molecules, saving bacteria from lysis20. Biophysically, MscL is well studied and characterized primarily through the prominent patch clamp technique21-23. Reliable structural models explaining MscL&#39;s gating mechanism24,25 are proposed based on its homolog&#39;s crystal structure26,27, modeling28, and results of extensive experimentation 24,29-31. Under an applied tension of ~10 mN/m, the closed channel which consists of a tight bundle of transmembrane helices, transforms into a ring of greatly tilted helices forming a ~28 &#197; water-filled conductive pore21,24,32. It has also been established that the hydrophobicity of the tight gate, positioned at the intersection of the inner TM1 domains, determines the activation threshold of the channel33. Correspondingly, it was found that by decreasing the hydrophobicity of the gate, the tension threshold could be lowered22. This property of MscL made possible the design of various controllable valves34, primarily for drug delivery purposes. For all the aforementioned properties and based on its fundamental role of translating cell membrane excessive tensions into electrophysiological activities, MscL makes a great fit as a mechanoelectrical transducer in DIBs.
In this article, we present an original micropipette-based method to form DIBs and measure the activity of the incorporated MscL channels under mechanical stimulation. We report for the first time, the response of DIBs to mechanical stimulus and the functional reconstitution of the V23T low-threshold mutant of MscL in DIBs35.
The experimental system consists of lipid encased aqueous droplets anchored to the tips of two opposing borosilicate glass micropipettes. When droplets are brought into contact a lipid bilayer interface is formed. This technique offers control over the chemical composition and size of each droplet (bulk), as well as the dimensions of the bilayer interface. In addition, asymmetric membranes with various lipid compositions in each leaflet could be easily formed. Having one of the micropipettes attached to a harmonic piezoelectric actuator, provides the ability to apply a pre-programmed single-cycle or oscillatory stimulus. Tension is delivered to the artificial membrane through the compression of both droplets supporting it. As a result of droplet deformation, the areas of water-lipid-oil interfaces increase, and simultaneously the angle between the droplets decreases, causing an increase in membrane tension and transient MscL activation. Through analysis of the shapes of the droplets during deformation, the tension created at the interface could be estimated. Even though the focus in this article is on the mechano-transduction properties of the DIB, we also emphasize that other types of biomolecules, such as alamethicin, can be activated by this multi-functional platform. We present here, all the technical aspects of preparing, assembling, and taking measurements with this new method in a step-by-step manner.

<table>
	<tbody>
		<tr>
			<td>0.22 &#181;m filter</td>
			<td>Corning</td>
			<td>430624</td>
			<td></td>
		</tr>
		<tr>
			<td>1,2-diphytanoy-sn-glycero-3-phosphocholine (DPhPC)</td>
			<td>Avanti Polar Lipids</td>
			<td>850356P</td>
			<td>Purchased as lyophilized powder</td>
		</tr>
		<tr>
			<td>34-gauge microfil</td>
			<td>World Precision Instruments</td>
			<td>MF24G-5</td>
			<td></td>
		</tr>
		<tr>
			<td>400 mL Centrifuge bottels</td>
			<td>ThermoFisher</td>
			<td>3141</td>
			<td>Nalgene</td>
		</tr>
		<tr>
			<td>Agilent Function/Arbitrary Waveform Generator, 20 MHz</td>
			<td>Keysight Technologies</td>
			<td>33220A</td>
			<td></td>
		</tr>
		<tr>
			<td>Ampicillian</td>
			<td>ThermoFisher</td>
			<td>BP1760</td>
			<td>ACS Grade</td>
		</tr>
		<tr>
			<td>Avanti&#174; Mini-Extruder</td>
			<td>Avanti Polar Lipids</td>
			<td>610000</td>
			<td></td>
		</tr>
		<tr>
			<td>Axio Scope.A1</td>
			<td>Carl Zeiss</td>
			<td>-</td>
			<td></td>
		</tr>
		<tr>
			<td>AxioCam HSm</td>
			<td>Carl Zeiss</td>
			<td>-</td>
			<td></td>
		</tr>
		<tr>
			<td>Axopatch 200B Amplifier&#160;</td>
			<td>Molecular Devices</td>
			<td>-</td>
			<td></td>
		</tr>
		<tr>
			<td>BCA protein assay kit</td>
			<td>Pierce&#160;</td>
			<td>23225</td>
			<td></td>
		</tr>
		<tr>
			<td>BK Precision 4017B 10 MHz DDs Sweep/Function Generator</td>
			<td>Digi-Key</td>
			<td>BK4017B-ND</td>
			<td></td>
		</tr>
		<tr>
			<td>Borosilicate Glass Capillaries</td>
			<td>World Precision Instruments</td>
			<td>1B100F-4</td>
			<td></td>
		</tr>
		<tr>
			<td>Dialysis tubing</td>
			<td>7 Spectra/Por</td>
			<td>132113</td>
			<td>MWCO 8000, 7.5 mm diameter</td>
		</tr>
		<tr>
			<td>DigiData 1440A system</td>
			<td>Molecular Devices</td>
			<td>-</td>
			<td></td>
		</tr>
		<tr>
			<td>DNAse</td>
			<td>Sigma-Aldrich</td>
			<td>DN25</td>
			<td></td>
		</tr>
		<tr>
			<td>DPhPC</td>
			<td>Avanti</td>
			<td>850356C</td>
			<td></td>
		</tr>
		<tr>
			<td>E-625 PZT Servo-Controller&#160;</td>
			<td>Physik Instrumente</td>
			<td>E-526</td>
			<td></td>
		</tr>
		<tr>
			<td>FPLC System</td>
			<td>Pharmacia Biotech</td>
			<td>-</td>
			<td></td>
		</tr>
		<tr>
			<td>HCl</td>
			<td>J.T. Baker</td>
			<td>9535-33</td>
			<td></td>
		</tr>
		<tr>
			<td>Hexadecane, 99%</td>
			<td>Sigma-Aldrich</td>
			<td>544-76-3</td>
			<td></td>
		</tr>
		<tr>
			<td>Homoginizer</td>
			<td>Wheaton</td>
			<td>357426</td>
			<td>15 mL</td>
		</tr>
		<tr>
			<td>Imidazole</td>
			<td>Sigma-Aldrich</td>
			<td>I5513</td>
			<td></td>
		</tr>
		<tr>
			<td>IPTG</td>
			<td>Affymetrix</td>
			<td>17886</td>
			<td></td>
		</tr>
		<tr>
			<td>IRGACURE&#174; 2959</td>
			<td>IRGACURE&#174;</td>
			<td>555047962</td>
			<td></td>
		</tr>
		<tr>
			<td>Isopore Membrane Filters&#160;</td>
			<td>EMD Millipore</td>
			<td>VCTP02500</td>
			<td></td>
		</tr>
		<tr>
			<td>Isopropyl Alcohol</td>
			<td>VWR International</td>
			<td>BDH1133-4LP</td>
			<td></td>
		</tr>
		<tr>
			<td>KCl</td>
			<td>Sigma-Aldrich</td>
			<td>P3911</td>
			<td>ACS Grade</td>
		</tr>
		<tr>
			<td>KH2PO4</td>
			<td>Mallinckrodt</td>
			<td>7100</td>
			<td>ACS Grade</td>
		</tr>
		<tr>
			<td>Kimble-Chase</td>
			<td>Kontes</td>
			<td>420401-1515</td>
			<td>Flex-Column</td>
		</tr>
		<tr>
			<td>LED-100 UV Spot Curing System</td>
			<td>Electro-Lite, corp.</td>
			<td>81170</td>
			<td></td>
		</tr>
		<tr>
			<td>Lysozyme</td>
			<td>Sigma-Aldrich</td>
			<td>L6876</td>
			<td></td>
		</tr>
		<tr>
			<td>Manual Patch-Clamp Micromanipulators</td>
			<td>Thorlabs</td>
			<td>PCS-520N</td>
			<td></td>
		</tr>
		<tr>
			<td>MgCl2</td>
			<td>ThermoFisher</td>
			<td>M33</td>
			<td>ACS Grade</td>
		</tr>
		<tr>
			<td>Microelectrode Holder&#160;</td>
			<td>World Precision Instruments</td>
			<td>MEH1S</td>
			<td></td>
		</tr>
		<tr>
			<td>Micropipette Puller</td>
			<td>Sutter Instruments</td>
			<td>P-1000</td>
			<td></td>
		</tr>
		<tr>
			<td>MOPS, minimum 99.5% titration</td>
			<td>Sigma-Aldrich</td>
			<td>M1254-100G</td>
			<td></td>
		</tr>
		<tr>
			<td>N2 Gas</td>
			<td>Airgas</td>
			<td>UN1066</td>
			<td></td>
		</tr>
		<tr>
			<td>NaCl</td>
			<td>EMD</td>
			<td>SX0420-1</td>
			<td>ACS Grade</td>
		</tr>
		<tr>
			<td>Ni NTA agarose beads</td>
			<td>Qiagen</td>
			<td>1000632</td>
			<td></td>
		</tr>
		<tr>
			<td>Optically Clear Cast Acrylic Tube, 2-1/2&#34; OD x 2&#34; ID</td>
			<td>McMaster-Carr</td>
			<td>8486K545</td>
			<td></td>
		</tr>
		<tr>
			<td>P-601 PiezoMove Flexure-Guided Linear Actuator</td>
			<td>Physik Instrumente</td>
			<td>P-601</td>
			<td></td>
		</tr>
		<tr>
			<td>PAGE gel</td>
			<td>Bio-Rad</td>
			<td>456-9033</td>
			<td></td>
		</tr>
		<tr>
			<td>Parafilm M&#174; All-Purpose Laboratory Film</td>
			<td>Parafilm&#174;</td>
			<td>PM999</td>
			<td></td>
		</tr>
		<tr>
			<td>Phenylmethylsulfonyl fluoride</td>
			<td>Sigma-Aldrich</td>
			<td>P7626</td>
			<td></td>
		</tr>
		<tr>
			<td>Poly(ethylene glycol)1000 dimethacrylate</td>
			<td>Polysciences, Inc.</td>
			<td>15178-100</td>
			<td></td>
		</tr>
		<tr>
			<td>Polycarbonate (PCTE) Membrane Filters, Black, 0.4 Micron, 25mm, 100/Pk</td>
			<td>Sterlitech Corporation</td>
			<td><a href="http://www.sterlitech.com/filters/membrane-disc-filters/polycarbonate-membranes/hydrophilic-polycarbonate-membrane-filters/hydrophilic-polycarbonate-membrane-filter-pctb0425100.html">PCTB0425100</a></td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium Chloride&#160;</td>
			<td>Sigma-Aldrich</td>
			<td>P5405-500G</td>
			<td></td>
		</tr>
		<tr>
			<td>Powder Free Soft Nitrile Examination Gloves&#160;</td>
			<td>VWR International</td>
			<td>CA89-38-272</td>
			<td></td>
		</tr>
		<tr>
			<td>Replacement Gasket 1.0mm</td>
			<td>World Precision Instruments</td>
			<td>GO1-100</td>
			<td></td>
		</tr>
		<tr>
			<td>SDS</td>
			<td>Sigma-Aldrich</td>
			<td>L5750</td>
			<td></td>
		</tr>
		<tr>
			<td>Silver wire</td>
			<td>GoodFellow</td>
			<td>147-346-94</td>
			<td>Different diameters could be used depending on the application&#160;</td>
		</tr>
		<tr>
			<td>Sodium Azide</td>
			<td>Affymetrix</td>
			<td>21610</td>
			<td></td>
		</tr>
		<tr>
			<td>Test tubes</td>
			<td>ThermoFisher</td>
			<td>14-961-27</td>
			<td>12 x 130 mm</td>
		</tr>
		<tr>
			<td>Tryptone</td>
			<td>ThermoFisher</td>
			<td>BP1421</td>
			<td></td>
		</tr>
		<tr>
			<td>Ultracal 30K</td>
			<td>Millipore</td>
			<td>UFC803024</td>
			<td>Amicore Ultra 30 MWCO</td>
		</tr>
		<tr>
			<td>VWR Light-Duty Tissue Wipers</td>
			<td>VWR International</td>
			<td>82003-820</td>
			<td></td>
		</tr>
		<tr>
			<td>VWR Scientific 50D Ultrasonic Cleaner</td>
			<td>VWR International</td>
			<td>13089</td>
			<td></td>
		</tr>
		<tr>
			<td>Water Purifier</td>
			<td>Barnstead</td>
			<td>D11931</td>
			<td></td>
		</tr>
		<tr>
			<td>Yeast</td>
			<td>ThermoFisher</td>
			<td>BP1422</td>
			<td></td>
		</tr>
		<tr>
			<td>&#946;-octylglucopyranoside</td>
			<td>Anatrace</td>
			<td>O311S</td>
			<td></td>
		</tr>
	</tbody>
</table>

multifunctional, micropipette-based method for incorporation and stimulation of bacterial mechanosensitive ion channels in droplet interface bilayers

MscL, a large conductance mechanosensitive channel (MSC), is a ubiquitous osmolyte release valve that helps bacteria survive abrupt hypo-osmotic shocks. It has been discovered and rigorously studied using the patch-clamp technique for almost three decades. Its basic role of translating tension applied to the cell membrane into permeability response makes it a strong candidate to function as a mechanoelectrical transducer in artificial membrane-based biomolecular devices. Serving as building blocks to such devices, droplet interface bilayers (DIBs) can be used as a new platform for the incorporation and stimulation of MscL channels. Here, we describe a micropipette-based method to form DIBs and measure the activity of the incorporated MscL channels. This method consists of lipid-encased aqueous droplets anchored to the tips of two opposing (coaxially positioned) borosilicate glass micropipettes. When droplets are brought into contact, a lipid bilayer interface is formed. This technique offers control over the chemical composition and the size of each droplet, as well as the dimensions of the bilayer interface. Having one of the micropipettes attached to a harmonic piezoelectric actuator provides the ability to deliver a desired oscillatory stimulus. Through analysis of the shapes of the droplets during deformation, the tension created at the interface can be estimated. Using this technique, the first activity of MscL channels in a DIB system is reported. Besides MS channels, activities of other types of channels can be studied using this method, proving the multi-functionality of this platform. The method presented here enables the measurement of fundamental membrane properties, provides a greater control over the formation of symmetric and asymmetric membranes, and is an alternative way to stimulate and study mechanosensitive channels.

Figures 1 and 2 display the experimental setup and equipment used to record protein activity in the course of mechanical stimulation of the lipid bilayer membrane. To minimize electrical noise into our measurements, the workstation is placed within a lab-made Faraday cage, grounded to a ground connection on the AxoPatch 200 B Amplifier.
Formation of a stable insulating lipid bilayer is a key step in this study. In this arrangement, a lipid monolayer assembles at the oil/water interface of the aqueous droplets immersed in a bath of an organic solvent. When droplets are placed in contact, excess oil is eliminated, and the opposing lipid monolayers thin to a two-molecule thick lipid bilayer. The most common technique used in bilayer characterization is voltage-clamp. With voltage-clamp, the voltage across the bilayer is maintained at a constant value while the current is measured. Figure 4 portrays a typical real-time current recording of the initial bilayer formation. Knowing the specific capacitance (~0.6 &#956;F/cm2)5 of the DPhPC lipid bilayer, the area of the formed bilayer could be calculated. The bilayer area could be controlled by changing the position of the droplets (Figure 4A). Using the piezoelectric actuator, different types of waveforms (sinusoidal, square, triangular, etc.) at different frequencies, amplitudes, and duty cycles could be applied to the droplets to horizontally and axially oscillate them and thus, bilayer tension and area could be altered (Figure 4B).
When the DIB is mechanically stimulated, while maintaining a constant DC potential across the membrane, a low-threshold (gain-of-function) V23T mutant of MscL generates reliable activities including mainly sub-conductive states and occasionally full opening events (Figure 5). These events are identical to those recorded using the patch-clamp technique from intact inner E. coli membranes and liposomes reconstituted with the purified V23T MscL. The results in Figure 5 prove that gating occurs in response to an increase in tension, since all current spikes are observed at peak compression. At peak compression, the relative areal expansion of the droplets is maximal and therefore, tension at the interface is maximal.
Alamethicin, a voltage-gated ion channel and one of the most studied peptides, increases the membrane permeability when a DC voltage is applied across the membrane36. The ability of the lipid bilayer interface to host transmembrane proteins and peptides is also tested by performing voltage-gating current recordings using alamethicin peptide. Alamethicin is mixed with the phospholipid solution to a final concentration of 100 ng/ml. Figure 6 shows the current measurements under voltage clamp (+115 mV). The droplets in this experiment are pulled apart in order to achieve small bilayer interface and thus higher resistance and smaller capacitance. The gating behavior of the Alamethicin peptide is shown through the discrete steps of current (Figure 6). The histogram on the right side of the plot shows the changes in conductance from the base level (0.0962 nS), which is basically the first conductance level of the channel itself.
<img alt="Figure 1" src="/files/ftp_upload/53362/53362fig1.jpg" /> 
Figure 1: A schematic describing the main parts and dimensions of the oil reservoir. The oil reservoir is manufactured at the machine shop at Virginia Tech. It consists of a machined cylindrical acrylic tube glued to the surface of an acrylic sheet. The dimensions and design can be modified to accommodate different applications or more than two micropipettes. <a href="https://www.jove.com/files/ftp_upload/53362/53362fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53362/53362fig2.jpg" /> 
Figure 2: Experimental setup and micropipettes preparation. (A) The standard workstation for forming, mechanically stimulating, and characterizing the interface bilayers includes a microscope, 3-axis manipulators, a digital camera, piezoelectric oscillator, vibration isolation table, and a Faraday cage (not shown). (B) The experimental setup consists of two opposing PEG-DMA hydrogel filled micropipettes horizontally positioned within a bath of Hexadecane oil. Each of the micropipettes contains an Ag/AgCl electrode to provide electrical connection. A third micropipette filled with proteoliposome solution is used to form the droplets at the tip of the other micropipettes. (C) The DIB current response could be measured using a combination of the patch amplifier and the low-noise data acquisition system. (D) A closed up picture showing the aqueous droplets formed at the tip of the micropipettes. (E) Ag/AgCl electrodes are made by dipping the tip of two 250 &#956;m silver wires in bleach. The electrodes are then fed through two borosilicate glass capillaries filled with PEG-DMA hydrogel, which is cured with UV light to solidify. A straight microelectrode holder with male connector is used to connect one of the micropipettes to the headstage of the patch amplifier.
<img alt="Figure 3" src="/files/ftp_upload/53362/53362fig3.jpg" /> 
Figure 3: Images illustrating the formation of droplet interface bilayers. (A) A 10 &#956;m micropipette filled with proteoliposomes is positioned under the microscope in proximity to the micropipette tips. Using a syringe connected to the micropipette, dispense small volumes of the proteoliposomes to form spherical droplets to desired volume. Let the monolayer form by allowing the droplets to sit for ten min. Bring the droplets into contact; the bilayer will form after all oil at the interface is eliminated. (B) While the bilayer is formed, the chemical composition at both sides of the interface could be controlled by injecting desired chemicals using a micro-sized micropipette. (C) The droplets at the moment of first contact. (D) The droplets when the lipid bilayer is formed.
<img alt="Figure 4" src="/files/ftp_upload/53362/53362fig4.jpg" /> 
Figure 4: Real-time measurements show both the initial thinning and subsequent expansion of the interface. (A) Current measured in the course of bilayer formation through the application of a triangular electrical potential. The magnitude of the measured current is directly proportional to the capacitance, and thus the area of the bilayer interface. The closer the droplets are brought together, the bigger the area of the interface and vice versa. (B) Upon application of mechanical excitation, the area of the bilayer interface increases and decreases at the same frequency as the stimulating signal.
<img alt="Figure 5" src="/files/ftp_upload/53362/53362fig5.jpg" /> 
Figure 5: Real-time measurements show the response of the bilayer to mechanical excitation as well as the gating of the V23T mutant of MscL. The shape of the current response is sinusoidal, which relates to a sinusoidal change in bilayer capacitance as a result of the bilayer area change. The current spikes, occurring at the peak of each cycle, indicate sub-conductance gating of the V23T mutant. A polar plot further indicates that gating occurs at peak compression, which reflects an increase in tension at the bilayer interface.
<img alt="Figure 6" src="/files/ftp_upload/53362/53362fig6.jpg" /> 
Figure 6: Current measurements under voltage clamp and corresponding histogram of conductance levels for gating activity of incorporated Alamethicin channels. The gating behavior of the Alamethicin peptide is shown through the discrete step-wise increase in current. The conductance levels match very well with previous measurements performed by our research group at Virginia Tech7.

Watch this Scientific Journal Video about Multifunctional, Micropipette-based Method for Incorporation And Stimulation of Bacterial Mechanosensitive Ion Channels in Droplet Interface Bilayers at JoVE.

Multifunctional, Micropipette-based Method for Incorporation And Stimulation of Bacterial Mechanosensitive Ion Channels in Droplet Interface Bilayers

Bacterial mechanosensitive channels can be used as mechanoelectrical transducers in biomolecular devices. Droplet interface bilayers (DIBs), cell-inspired building blocks to such devices, represent new platforms to incorporate and stimulate mechanosensitive channels. Here, we demonstrate a new micropipette-based method of forming DIBs, allowing the study of mechanosensitive channels under mechanical stimulation.

MscL, a large conductance mechanosensitive channel (MSC), is a ubiquitous osmolyte release valve that helps bacteria survive abrupt hypo-osmotic shocks. ...

multifunctional-micropipette-based-method-for-incorporation

Research

JoVE Journal

Biology

MALDI Sample Preparation: the Ultra Thin Layer Method

This video demonstrates the preparation of an ultra-thin matrix/analyte layer for analyzing peptides and proteins by Matrix-Assisted Laser Desorption Ionization Mass Spectrometry (MALDI-MS) 1,2. The ultra-thin layer method involves the production of a substrate layer of matrix crystals (alpha-cyano-4-hydroxycinnamic acid) on the sample plate, which serves as a seeding ground for subsequent crystallization of a matrix/analyte mixture. Advantages of the ultra-thin layer method over other sample deposition approaches (e.g. dried droplet) are that it provides (i) greater tolerance to impurities such as salts and detergents, (ii) better resolution, and (iii) higher spatial uniformity. This method is especially useful for the accurate mass determination of proteins. The protocol was initially developed and optimized for the analysis of membrane proteins and used to successfully analyze ion channels, metabolite transporters, and receptors, containing between 2 and 12 transmembrane domains 2. Since the original publication, it has also shown to be equally useful for the analysis of soluble proteins. Indeed, we have used it for a large number of proteins having a wide range of properties, including those with molecular masses as high as 380 kDa 3. It is currently our method of choice for the molecular mass analysis of all proteins. The described procedure consistently produces high-quality spectra, and it is sensitive, robust, and easy to implement.

This video demonstrates the preparation of an ultra-thin matrix/analyte layer for analyzing peptides and proteins by Matrix-Assisted Laser Desorption Ionization Mass Spectrometry (MALDI-MS).

This video demonstrates the preparation of an ultra-thin matrix/analyte layer for analyzing peptides and proteins by Matrix-Assisted Laser Desorption ...

Patch Clamp Recording of Ion Channels Expressed in  Xenopus Oocytes

Since its development by Sakmann and Neher 1, 2, the patch clamp has become established as an extremely useful technique for electrophysiological measurement of single or multiple ion channels in cells. This technique can be applied to ion channels in both their native environment and expressed in heterologous cells, such as oocytes harvested from the African clawed frog, Xenopus laevis. Here, we describe the well-established technique of patch clamp recording from Xenopus oocytes. This technique is used to measure the properties of expressed ion channels either in populations (macropatch) or individually (single-channel recording). We focus on techniques to maximize the quality of oocyte preparation and seal generation. With all factors optimized, this technique gives a probability of successful seal generation over 90 percent. The process may be optimized differently by every researcher based on the factors he or she finds most important, and we present the approach that have lead to the greatest success in our hands.

This is intended as an introduction to patch clamp recording from Xenopus laevis oocytes. It covers vitelline membrane removal, formation of a gigaohm seal (gigaseal), and the optional conversion of the patch to the outside-out topology.

Since its development by Sakmann and Neher 1, 2, the patch clamp has become established as an extremely useful technique for electrophysiological ...

Supported Planar Bilayers for the Formation of Study of Immunological Synapses and Kinapse

Supported planar bilayers are powerful tools that can be used to model the molecular interactions in an immunological synapse.   To mimic the interactions between lymphocytes and antigen presenting cells, we use Ni2+-chelating lipids to anchor recombinant cell adhesion and MHC proteins to the upper leaflet of a bilayer with poly-histidine tags. Planar bilayers are generated by preparing lipid, treating the glass surfaces where the bilayer will form, and then forming the bilayer in a specialized chamber containing a flow-cell where the lymphocytes will be added.  Then, bilayers are charged with Ni and his-tagged recombinant proteins are added.  Finally,  lymphocytes are injected into the flow cell and TIRF microscopy can be used to image synapse formation and the mechanisms that control T cell locomotion, sites of receptor sorting,  and sites of receptor degradation.

Supported planar bilayers are powerful tools that can be used to model the molecular interactions in an immunological synapse. Here, we show methods for anchoring cell adhesion proteins known to modulate synapse formation to the upper leaflet of the lipid bilyer and visualize synapse formation using TIRF microscopy.

Supported planar bilayers are powerful tools that can be used to model the molecular interactions in an immunological synapse.   To mimic the interactions ...

Preparation of Artificial Bilayers for Electrophysiology Experiments

Planar lipid bilayers, also called artificial lipid bilayers, allow you to study ion-conducting channels in a well-defined environment. These bilayers can be used for many different studies, such as the characterization of membrane-active peptides, the reconstitution of ion channels or investigations on how changes in lipid bilayer properties alter the function of bilayer-spanning channels.  Here, we show how to form a planar bilayer and how to isolate small patches from the bilayer, and in a second video  will also demonstrate a procedure for using gramicidin channels to determine changes in lipid bilayer elastic properties.   We also demonstrate the individual steps needed to prepare the bilayer chamber, the electrodes and how to test that the bilayer is suitable for single-channel measurements.

Planar lipid bilayers, also called artificial lipid bilayers, allow you to study ion-conducting channels in a well-defined environment. Here, we demonstrate the individual steps needed to prepare the bilayer chamber, the electrodes and how to test that the bilayer is suitable for single-channel measurements.

Planar lipid bilayers, also called artificial lipid bilayers, allow you to study ion-conducting channels in a well-defined environment. These bilayers can ...

Mutagenesis and Functional Analysis of Ion Channels Heterologously Expressed in Mammalian Cells

We will demonstrate how to study the functional effects of introducing a point mutation in an ion channel. We study G protein-gated inwardly rectifying potassium (referred to as GIRK) channels, which are important for regulating the excitability of neurons. There are four different mammalian GIRK channel subunits (GIRK1-GIRK4) - we focus on GIRK2 because it forms a homotetramer. Stimulation of different types of G protein-coupled receptors (GPCRs), such as the muscarinic receptor (M2R), leads to activation of GIRK channels. Alcohol also directly activates GIRK channels. We will show how to mutate one amino acid by specifically changing one or more nucleotides in the cDNA for the GIRK channel. This mutated cDNA sequence will be amplified in bacteria, purified, and the presence of the point mutation will be confirmed by DNA sequencing. The cDNAs for the mutated and wild-type GIRK channels will be transfected into human embryonic kidney HEK293T cells cultured in vitro. Lastly, whole-cell patch-clamp electrophysiology will be used to study the macroscopic potassium currents through the ectopically expressed wild-type or mutated GIRK channels. In this experiment, we will examine the effect of a L257W mutation in GIRK2 channels on M2R-dependent and alcohol-dependent activation.

We will demonstrate how to study the effect of a single point mutation on the function of an ion channel.

We will demonstrate how to study the functional effects of introducing a point mutation in an ion channel. We study G protein-gated inwardly rectifying ...

Patch Clamp and Perfusion Techniques for Studying Ion Channels Expressed in Xenopus oocytes

The protocol presented here is designed to study the activation of the large conductance, voltage- and Ca2+-activated K+ (BK) channels. The protocol may also be used to study the structure-function relationship for other ion channels and neurotransmitter receptors1. BK channels are widely expressed in different tissues and have been implicated in many physiological functions, including regulation of smooth muscle contraction, frequency tuning of inner hair cells and regulation of neurotransmitter release2-6. BK channels are activated by membrane depolarization and by intracellular Ca2+ and Mg2+ 6-9. Therefore, the protocol is designed to control both the membrane voltage and the intracellular solution. In this protocol, messenger RNA of BK channels is injected into Xenopus laevis oocytes (stage V-VI) followed by 2-5 days of incubation at 18&deg;C10-13. Membrane patches that contain single or multiple BK channels are excised with the inside-out configuration using patch clamp techniques10-13. The intracellular side of the patch is perfused with desired solutions during recording so that the channel activation under different conditions can be examined. To summarize, the mRNA of BK channels is injected into Xenopus laevis oocytes to express channel proteins on the oocyte membrane; patch clamp techniques are used to record currents flowing through the channels under controlled voltage and intracellular solutions.

Patch Clamp and Perfusion Techniques for Studying Ion Channels Expressed in Xenopus oocytes

Ionic current of BK channels is recorded using patch clamp techniques. BK channels are expressed in Xenopus oocytes by injecting messenger RNA. The intracellular solution during patch clamp recordings is controlled by a perfusion system.

The protocol presented here is designed to study the activation of the large conductance, voltage- and Ca2+-activated K+ (BK) channels. The protocol may ...

Optimized Transfection Strategy for Expression and Electrophysiological Recording of Recombinant Voltage-Gated Ion Channels in HEK-293T Cells

The in vitro expression and electrophysiological recording of recombinant voltage-gated ion channels in cultured human embryonic kidney cells (HEK-293T) is a ubiquitous research strategy. HEK-293T cells must be plated onto glass coverslips at low enough density so that they are not in contact with each other in order to allow for electrophysiological recording without confounding effects due to contact with adjacent cells. Transfected channels must also express with high efficiency at the plasma membrane for whole-cell patch clamp recording of detectable currents above noise levels. Heterologous ion channels often require long incubation periods at 28&deg;C after transfection in order to achieve adequate membrane expression, but there are increasing losses of cell-coverslip adhesion and membrane stability at this temperature. To circumvent this problem, we developed an optimized strategy to transfect and plate HEK-293T cells. This method requires that cells be transfected at a relatively high confluency, and incubated at 28&deg;C for varying incubation periods post-transfection to allow for adequate ion channel protein expression. Transfected cells are then plated onto glass coverslips and incubated at 37&deg;C for several hours, which allows for rigid cell attachment to the coverslips and membrane restabilization. Cells can be recorded shortly after plating, or can be transferred to 28&deg;C for further incubation. We find that the initial incubation at 28&deg;C, after transfection but before plating, is key for the efficient expression of heterologous ion channels that normally do not express well at the plasma membrane. Positively transfected, cultured cells are identified by co-expressed eGFP or eGFP expressed from a bicistronic vector (e.g. pIRES2-EGFP) containing the recombinant ion channel cDNA just upstream of an internal ribosome entry site and an eGFP coding sequence. Whole-cell patch clamp recording requires specialized equipment, plus the crafting of polished recording electrodes and L-shaped ground electrodes from borosilicate glass. Drug delivery to study the pharmacology of ion channels can be achieved by directly micropipetting drugs into the recording dish, or by using microperfusion or gravity flow systems that produce uninterrupted streams of drug solution over recorded cells.

Reliable method for highly efficient in vitro expression and subsequent electrophysiological recording of recombinant voltage-gated ion channels in cultured human embryonic kidney cells (HEK-293T).

The in vitro expression and electrophysiological recording of recombinant voltage-gated ion channels in cultured human embryonic kidney cells (HEK-293T) ...

Culturing of Human Nasal Epithelial Cells at the Air Liquid Interface

In vitro models using human primary epithelial cells are essential in understanding key functions of the respiratory epithelium in the context of microbial infections or inhaled agents. Direct comparisons of cells obtained from diseased populations allow us to characterize different phenotypes and dissect the underlying mechanisms mediating changes in epithelial cell function. Culturing epithelial cells from the human tracheobronchial region has been well documented, but is limited by the availability of human lung tissue or invasiveness associated with obtaining the bronchial brushes biopsies. Nasal epithelial cells are obtained through much less invasive superficial nasal scrape biopsies and subjects can be biopsied multiple times with no significant side effects. Additionally, the nose is the entry point to the respiratory system and therefore one of the first sites to be exposed to any kind of air-borne stressor, such as microbial agents, pollutants, or allergens. 
Briefly, nasal epithelial cells obtained from human volunteers are expanded on coated tissue culture plates, and then transferred onto cell culture inserts. Upon reaching confluency, cells continue to be cultured at the air-liquid interface (ALI), for several weeks, which creates more physiologically relevant conditions. The ALI culture condition uses defined media leading to a differentiated epithelium that exhibits morphological and functional characteristics similar to the human nasal epithelium, with both ciliated and mucus producing cells. Tissue culture inserts with differentiated nasal epithelial cells can be manipulated in a variety of ways depending on the research questions (treatment with pharmacological agents, transduction with lentiviral vectors, exposure to gases, or infection with microbial agents) and analyzed for numerous different endpoints ranging from cellular and molecular pathways, functional changes, morphology, etc. 
In vitro models of differentiated human nasal epithelial cells will enable investigators to address novel and important research questions by using organotypic experimental models that largely mimic the nasal epithelium in vivo.

Nasal epithelial cells, obtained through superficial scrape biopsy of human volunteers, are expanded and transferred onto tissue culture inserts. Upon reaching confluency, cells are grown at air liquid interface, yielding cultures of ciliated and non-ciliated cells. Differentiated nasal epithelial cell cultures provide viable experimental models for studying the respiratory mucosa.

In vitro models using human primary  epithelial cells are essential in understanding key functions of the respiratory  epithelium in the context of ...

Measurement of Extracellular Ion Fluxes Using the Ion-selective Self-referencing Microelectrode Technique

Cells from animals, plants and single cells are enclosed by a barrier called the cell membrane that separates the cytoplasm from the outside. Cell layers such as epithelia also form a barrier that separates the inside from the outside or different compartments of multicellular organisms. A key feature of these barriers is the differential distribution of ions across cell membranes or cell layers. Two properties allow this distribution: 1) membranes and epithelia display selective permeability to specific ions; 2) ions are transported through pumps across cell membranes and cell layers. These properties play crucial roles in maintaining tissue physiology and act as signaling cues after damage, during repair, or under pathological condition. The ion-selective self-referencing microelectrode allows measurements of specific fluxes of ions such as calcium, potassium or sodium at single cell and tissue levels. The microelectrode contains an ionophore cocktail which is selectively permeable to a specific ion. The internal filling solution contains a set concentration of the ion of interest. The electric potential of the microelectrode is determined by the outside concentration of the ion. As the ion concentration varies, the potential of the microelectrode changes as a function of the log of the ion activity. When moved back and forth near a source or sink of the ion (i.e. in a concentration gradient due to ion flux) the microelectrode potential fluctuates at an amplitude proportional to the ion flux/gradient. The amplifier amplifies the microelectrode signal and the output is recorded on computer. The ion flux can then be calculated by Fick&#8217;s law of diffusion using the electrode potential fluctuation, the excursion of microelectrode, and other parameters such as the specific ion mobility. In this paper, we describe in detail the methodology to measure extracellular ion fluxes using the ion-selective self-referencing microelectrode and present some representative results.

Transporters in cell membranes allow differential segregation of ions across cell membranes or cell layers and play crucial roles during tissue physiology, repair and pathology. We describe the ion-selective self-referencing microelectrode that allows the measurement of specific ion fluxes at single cells and tissues in vivo.

Cells from animals, plants and single cells are enclosed by a barrier called the cell membrane that separates the cytoplasm from the outside. Cell layers ...

Determination of the Relative Cell Surface and Total Expression of Recombinant Ion Channels Using Flow Cytometry

Inherited or de novo mutations in cation-selective channels may lead to sudden cardiac death. Alteration in the plasma membrane trafficking of these multi-spanning transmembrane proteins, with or without change in channel gating, is often postulated to contribute significantly in this process. It has thus become critical to develop a method to quantify the change of the relative cell surface expression of cardiac ion channels on a large scale. Herein, a detailed protocol is provided to determine the relative total and cell surface expression of cardiac L-type calcium channels CaV1.2 and membrane-associated subunits in tsA-201 cells using two-color fluorescent cytometry assays. Compared with other microscopy-based or immunoblotting-based qualitative methods, flow cytometry experiments are fast, reproducible, and large-volume assays that deliver quantifiable end-points on large samples of live cells (ranging from 104 to 106 cells) with similar cellular characteristics in a single flow. Constructs were designed to constitutively express mCherry at the intracellular C-terminus (thus allowing a rapid assessment of the total protein expression) and express an extracellular-facing hemagglutinin (HA) epitope to estimate the cell surface expression of membrane proteins using an anti-HA fluorescence conjugated antibody. To avoid false negative, experiments were also conducted in permeabilized cells to confirm the accessibility and proper expression of the HA epitope. The detailed procedure provides: (1) design of tagged DNA (deoxyribonucleic acid) constructs, (2) lipid-mediated transfection of constructs in tsA-201 cells, (3) culture, harvest, and staining of non-permeabilized and permeabilized cells, and (4) acquisition and analysis of fluorescent signals. Additionally, the basic principles of flow cytometry are explained and the experimental design, including the choice of fluorophores, titration of the HA antibody and control experiments, is thoroughly discussed. This specific approach offers objective relative quantification of the total and cell surface expression of ion channels that can be extended to study ion pumps and plasma membrane transporters.

Inherited cardiac arrhythmias are often caused by mutations that alter the surface delivery of one or more ion channels. Here, we adapt flow cytometry assays to provide a quantification of the relative total and cell surface protein expression of recombinant ion channels expressed in tsA-201 cells.

Inherited or de novo mutations in cation-selective channels may lead to sudden cardiac death. Alteration in the plasma membrane trafficking of these ...