Name: fm dye cycling at the synapse: comparing high potassium depolarization, electrical and channelrhodopsin stimulation
Uploaded: 2018-05-24T15:00:00
Description: Watch this Scientific Journal Video about FM Dye Cycling at the Synapse: Comparing High Potassium Depolarization, Electrical and Channelrhodopsin Stimulation at JoVE.com

We thank Broadie Lab members for contributions to this article. This work was supported by NIH R01s MH096832 and MH084989 to K.B., and NIH predoctoral fellowship F31 MH111144 to D.L.K.

1. Larval Glue Dissection
<ol>
	<li>Thoroughly mix 10 parts of silicone elastomer base with 1 part of silicone elastomer curing agent from the elastomer kit (Table of Materials).</li>
	<li>Coat 22 x 22 mm glass coverslips with the elastomer and cure on a hot plate at 75 &#730;C for several hours (until no longer sticky to the touch).</li>
	<li>Place a single elastomer-coated glass coverslip into the custom-made plexi glass dissection chamber (Figure 1, bottom) in preparati...

High [K+] saline depolarizing stimulation is by far the easiest of the three options for activity-dependent FM dye cycling, but likely the least physiological29. This simple method depolarizes every accessible cell in the entire animal, and so does not allow directed studies. It may be possible to locally apply high [K+] saline with a micropipette, but this will still depolarize pre/postsynaptic cells and likely synapse-associated glia1. Another major ...

The beautifully-characterized Drosophila larval neuromuscular junction (NMJ) glutamatergic synapse model has been used to study synapse formation and function with a vast spectrum of genetic perturbations1. The motor neuron terminal consists of multiple axon branches, each with many enlarged synaptic boutons. These capacious varicosities (up to 5 &#181;m in diameter) contain all of the neurotransmission machinery, including uniform glutamatergic synaptic vesicles (SVs; ~40 nm in diameter) in cytosolic reserve and readily-releasable pools2. These vesicles dock at the presynaptic plasma membrane fusion site active zones (AZs), where exocytosis mediates the glutamate neurotransmitter release for trans-synaptic communication. Subsequently, the SVs are retrieved from the plasma membrane via kiss-and-run recycling or clathrin-mediated endocytosis (CME) for repeated exo/endocytosis cycles. The Drosophila NMJ is easily accessible and well-suited for both isolating and characterizing SV cycle mutants. Using forward genetic screens, novel mutations have led to the identification of new genes critical for the SV cycle3. Moreover, reverse genetic approaches starting with already known genes have led to the elucidation of new SV cycle mechanisms through the careful description of mutant cycling phenotypes4. The Drosophila NMJ is nearly ideal as an experimental synaptic preparation for dissecting SV endocytosis and exocytosis mechanisms via methods to optically track vesicle cycling during neurotransmission.
A range of fluorescent markers allow visual tracking of vesicles during cycling dynamics, but the most versatile are FM dye analogs which is first synthesized by Mao, F., et al.5. Structurally, FM dyes contain a hydrophilic head and a lipophilic tail connected through an aromatic ring, with a central region conferring spectral properties. These styryl dyes partition reversibly in membranes, do not &#39;flip-flop&#39; between membrane leaflets and so are never free in the cytosol, and are far more fluorescent in membranes than water5. Reversible insertion into a lipid bilayer causes a 40-fold increase in fluorescence6. At neuronal synapses, classic FM dye labeling experiments consist of bathing the synaptic preparation with the dye during depolarizing stimulation to load dye via SV endocytosis. External dye is then washed away and the SV cycle is arrested in a calcium-free ringer solution to image loaded synapses7. A second round of stimulation in a dye-free bath triggers FM release through exocytosis, a process that can be followed by measuring the fluorescence intensity decrease. SV populations from a single vesicle to pools containing hundreds of vesicles can be quantitatively monitored6,7. FM dyes have been used to dissect activity-dependent mobilization of functionally distinct SV pools, and to compare kiss-and-run vs. CME cycling8,9. The method has been modified to separately assay evoked, spontaneous and miniature synaptic cycle activities (with highly sensitive equipment to detect very small fluorescence changes and reduce photobleaching)10. Assays can be extended to the ultrastructural level by photoconverting the fluorescent FM signal into an electron-dense label for transmission electron microscopy11,12,13,14.
Historically, bathing synaptic preparations in a high concentration of potassium (hereafter referred to as &#34;high [K+]&#34;) has been the method of choice for depolarizing stimulation to induce SV cycling; ranging from the frog cholinergic NMJ5, to cultured rodent brain hippocampal neurons15, to the Drosophila glutamatergic NMJ model16,17. This high [K+] approach is simple, requires no specialized equipment, and is therefore accessible to most labs, but has limitations for both application and data interpretation. A much more physiologically appropriate method is to use suction electrode electrical stimulation of the nerve4,5,12. This approach drives action potential propagation for direct stimulation of the presynaptic nerve terminal, and results can be directly compared to electrophysiological assays of neurotransmission function13,14,15, but requires specialized equipment and is technically much more challenging. With the advent of optogenetics, the use of channelrhodopsin neuronal stimulation has additional advantages, including tight spatiotemporal control of channel expression using the binary Gal4/UAS system20. This approach is technically much easier than suction electrode stimulation and requires nothing more than a very cheap LED light source. Here, we employ imaging of FM1-43 (N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide) to both compare and contrast these three different stimulation methods at the Drosophila NMJ: simple high [K+], challenging electrical and new channelrhodopsin approaches.

<table>
	<tbody>
		<tr>
			<td>SylGard 184 Silicone Elastomer Kit</td>
			<td>Fisher Scientific</td>
			<td>NC9644388</td>
			<td>To put on cover glass for dissections</td>
		</tr>
		<tr>
			<td>Microscope Cover Glass 22x22-1</td>
			<td>Fisherbrand</td>
			<td>12-542-B</td>
			<td>To put SylGard on for dissections</td>
		</tr>
		<tr>
			<td>Aluminum Top Hot Plate Type 2200</td>
			<td>Thermolyne</td>
			<td>HPA2235M</td>
			<td>To cure the SylGard</td>
		</tr>
		<tr>
			<td>Plexi glass dissection chamber</td>
			<td>N/A</td>
			<td>N/A</td>
			<td>Handmade</td>
		</tr>
		<tr>
			<td>Borosilicate Glass Capillaries</td>
			<td>WPI</td>
			<td>1B100F-4</td>
			<td>To make suction and glue micropipettes</td>
		</tr>
		<tr>
			<td>Laser-Based Micropipette Puller</td>
			<td>Sutter Instrument</td>
			<td>P-2000</td>
			<td>To make suction and glue micropipettes</td>
		</tr>
		<tr>
			<td>Tygon E-3603 Laboratory Tubing</td>
			<td>Component Supply Co.</td>
			<td>TET-031A</td>
			<td>For mouth and suction pipette</td>
		</tr>
		<tr>
			<td>P2 pipette tip</td>
			<td>USA Scientific</td>
			<td>1111-3700</td>
			<td>For mouth pipette</td>
		</tr>
		<tr>
			<td>0.6-mL Eppendorf tube cap</td>
			<td>Fisher Scientific</td>
			<td>05-408-120</td>
			<td>Used to put glue in for dissection</td>
		</tr>
		<tr>
			<td>Vetbond 3M</td>
			<td>WPI</td>
			<td>vetbond</td>
			<td>Glue used for dissections</td>
		</tr>
		<tr>
			<td>Potassium Chloride</td>
			<td>Fisher Scientific</td>
			<td>P-217</td>
			<td>Forsaline</td>
		</tr>
		<tr>
			<td>Sodium Chloride</td>
			<td>Millipore Sigma</td>
			<td>S5886</td>
			<td>For saline</td>
		</tr>
		<tr>
			<td>Magnesium Chloride</td>
			<td>Fisher Scientific</td>
			<td>M35-500</td>
			<td>For saline</td>
		</tr>
		<tr>
			<td>Calcium Chloride Dihydrate</td>
			<td>Millipore Sigma</td>
			<td>C7902</td>
			<td>For saline</td>
		</tr>
		<tr>
			<td>Sucrose</td>
			<td>Fisher Scientific</td>
			<td>S5-3</td>
			<td>For saline</td>
		</tr>
		<tr>
			<td>HEPES</td>
			<td>Millipore Sigma</td>
			<td>H3375</td>
			<td>For saline</td>
		</tr>
		<tr>
			<td>HRP:Alexa Fluor 647</td>
			<td>Jackson ImmunoResearch</td>
			<td>123-605-021</td>
			<td>To label neuronal membranes</td>
		</tr>
		<tr>
			<td>Paintbrush</td>
			<td>Winsor &#38; Newton</td>
			<td>94376864793</td>
			<td>To manipulate the larvae</td>
		</tr>
		<tr>
			<td>Dumont Dumostar Tweezers #5</td>
			<td>WPI</td>
			<td>500233</td>
			<td>Used during dissection</td>
		</tr>
		<tr>
			<td>7 cm McPherson-Vannas Microscissors (blades 3 mm)</td>
			<td>WPI</td>
			<td>14177</td>
			<td>Used during dissection&#160;</td>
		</tr>
		<tr>
			<td>FM1-43</td>
			<td>Fisher Scientific</td>
			<td>T35356</td>
			<td>Fluorescent styryl dye</td>
		</tr>
		<tr>
			<td>Digital Timer</td>
			<td>VWR</td>
			<td>62344-641</td>
			<td>For timing FM dye load/unload&#160;</td>
		</tr>
		<tr>
			<td>LSM 510 META laser-scanning confocal microscope</td>
			<td>Zeiss</td>
			<td></td>
			<td>For imaging the fluorescent markers</td>
		</tr>
		<tr>
			<td>Zen 2009 SP2 version 6.0</td>
			<td>Zeiss</td>
			<td></td>
			<td>Software for imaging on confocal</td>
		</tr>
		<tr>
			<td>HeNe 633nm laser</td>
			<td>Lasos</td>
			<td></td>
			<td>To excite HRP:647 during imaging</td>
		</tr>
		<tr>
			<td>Argon 488nm laser</td>
			<td>Lasos</td>
			<td></td>
			<td>To excite the FM dye during imaging</td>
		</tr>
		<tr>
			<td>Micro-Forge</td>
			<td>WPI</td>
			<td>MF200</td>
			<td>To fire polish glass micropipettes</td>
		</tr>
		<tr>
			<td>20mL Syringe Slip Tip</td>
			<td>BD</td>
			<td>301625</td>
			<td>To suck up the axon for electrical stimulation.</td>
		</tr>
		<tr>
			<td>Micro Manipulator (magnetic base)</td>
			<td>Narishige</td>
			<td>MMN-9</td>
			<td>To control the suction electrode for electrical stimulation</td>
		</tr>
		<tr>
			<td>Stimulator</td>
			<td>Grass</td>
			<td>S48</td>
			<td>To control the LED and electrical stimulation</td>
		</tr>
		<tr>
			<td>Zeiss Axioskop Microscope</td>
			<td>Zeiss</td>
			<td></td>
			<td>Used during electrical stimulation.</td>
		</tr>
		<tr>
			<td>40X Achroplan Water Immersion Objective</td>
			<td>Zeiss</td>
			<td></td>
			<td>Used during electrical stimulation and confocal imaging</td>
		</tr>
		<tr>
			<td>All-trans Retinal</td>
			<td>Millipore Sigma</td>
			<td>R2500</td>
			<td>Essential co-factor for ChR2</td>
		</tr>
		<tr>
			<td>Zeiss Stemi Microscope with camera port</td>
			<td>Zeiss</td>
			<td>2000-C</td>
			<td>Used during channelrhodopsin stimulation</td>
		</tr>
		<tr>
			<td>LED 470nm</td>
			<td>ThorLabs</td>
			<td>M470L2</td>
			<td>Used for ChR activation</td>
		</tr>
		<tr>
			<td>T-Cube LED Driver</td>
			<td>ThorLabs</td>
			<td>LEDD1B</td>
			<td>To control the LED</td>
		</tr>
		<tr>
			<td>LED Power Supply</td>
			<td>Cincon Electronics Co.</td>
			<td>TR15RA150</td>
			<td>To power the LED</td>
		</tr>
		<tr>
			<td>Optical Power and Energy Meter</td>
			<td>ThorLabs</td>
			<td>PM100D</td>
			<td>To measure LED intensity</td>
		</tr>
	</tbody>
</table>

fm dye cycling at the synapse: comparing high potassium depolarization, electrical and channelrhodopsin stimulation

FM dyes are used to study the synaptic vesicle (SV) cycle. These amphipathic probes have a hydrophilic head and hydrophobic tail, making them water-soluble with the ability to reversibly enter and exit membrane lipid bilayers. These styryl dyes are relatively non-fluorescent in aqueous medium, but insertion into the outer leaflet of the plasma membrane causes a &#62;40X increase in fluorescence. In neuronal synapses, FM dyes are internalized during SV endocytosis, trafficked both within and between SV pools, and released with SV exocytosis, providing a powerful tool to visualize presynaptic stages of neurotransmission. A primary genetic model of glutamatergic synapse development and function is the Drosophila neuromuscular junction (NMJ), where FM dye imaging has been used extensively to quantify SV dynamics in a wide range of mutant conditions. The NMJ synaptic terminal is easily accessible, with a beautiful array of large synaptic boutons ideal for imaging applications. Here, we compare and contrast the three ways to stimulate the Drosophila NMJ to drive activity-dependent FM1-43 dye uptake/release: 1) bath application of high [K+] to depolarize neuromuscular tissues, 2) suction electrode motor nerve stimulation to depolarize the presynaptic nerve terminal, and 3) targeted transgenic expression of channelrhodopsin variants for light-stimulated, spatial control of depolarization. Each of these methods has benefits and disadvantages for the study of genetic mutation effects on the SV cycle at the Drosophila NMJ. We will discuss these advantages and disadvantages to assist the selection of the stimulation approach, together with the methodologies specific to each strategy. In addition to fluorescent imaging, FM dyes can be photoconverted to electron-dense signals visualized using transmission electron microscopy (TEM) to study SV cycle mechanisms at an ultrastructural level. We provide the comparisons of confocal and electron microscopy imaging from the different methods of Drosophila NMJ stimulation, to help guide the selection of future experimental paradigms.

Figure 1 shows the work-flow for the activity-dependent FM dye imaging protocol. The experiment always begins with the same larval glue dissection, regardless of the stimulation method subsequently used. Figure 1a is a schematic of a dissected larva, showing the ventral nerve cord (VNC), radiating nerves and repeated hemisegmental muscle pattern. The VNC is removed and the preparation bathed in a 4 &#181;M solution of FM1-43 (<st...

Watch this Scientific Journal Video about FM Dye Cycling at the Synapse: Comparing High Potassium Depolarization, Electrical and Channelrhodopsin Stimulation at JoVE.com

FM Dye Cycling at the Synapse: Comparing High Potassium Depolarization, Electrical and Channelrhodopsin Stimulation

Synaptic vesicle (SV) cycling is the core mechanism of intercellular communication at neuronal synapses. FM dye uptake and release are the primary means of quantitatively assaying SV endo- and exocytosis. Here, we compare all the stimulation methods to drive FM1-43 cycling at the Drosophila neuromuscular junction (NMJ) model synapse.

FM dyes are used to study the synaptic vesicle (SV) cycle. These amphipathic probes have a hydrophilic head and hydrophobic tail, making them ...

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