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

Summary

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.

Abstract

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 >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.

Introduction

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 µm 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....

Protocol

1. Larval Glue Dissection

1. Thoroughly mix 10 parts of silicone elastomer base with 1 part of silicone elastomer curing agent from the elastomer kit (Table of Materials).
2. Coat 22 x 22 mm glass coverslips with the elastomer and cure on a hot plate at 75 ˚C for several hours (until no longer sticky to the touch).
3. Place a single elastomer-coated glass coverslip into the custom-made plexi glass dissection chamber (Figure 1, bottom) in preparation for the larval dissection.
4. Prepare the glue pipettes from borosilicate glass capillary using a standard microelectrode puller to obtain t....

Results

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 µM solution of FM1-43 (

Discussion

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 .......

Materials

List of materials used in this article

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