Sonication-facilitated Immunofluorescence Staining of Late-stage Embryonic and Larval Drosophila Tissues In Situ

Summary

Immunostaining is an effective technique for visualizing specific cell types and proteins within tissues. By utilizing sonication, the protocol described here alleviates the need to dissect Drosophila melanogaster tissues from late-stage embryos and larvae before immunostaining. We provide an efficient methodology for the immunostaining of formaldehyde-fixed whole mount larvae.

Abstract

Studies performed in Drosophila melanogaster embryos and larvae provide crucial insight into developmental processes such as cell fate specification and organogenesis. Immunostaining allows for the visualization of developing tissues and organs. However, a protective cuticle that forms at the end of embryogenesis prevents permeation of antibodies into late-stage embryos and larvae. While dissection prior to immunostaining is regularly used to analyze Drosophila larval tissues, it proves inefficient for some analyses because small tissues may be difficult to locate and isolate. Sonication provides an alternative to dissection in larval Drosophila immunostaining protocols. It allows for quick, simultaneous processing of large numbers of late-stage embryos and larvae and maintains in situ morphology. After fixation in formaldehyde, a sample is sonicated. Sample is then subjected to immunostaining with antigen-specific primary antibodies and fluorescently labeled secondary antibodies to visualize target cell types and specific proteins via fluorescence microscopy. During the process of sonication, proper placement of a sonicating probe above the sample, as well as the duration and intensity of sonication, is critical. Additonal minor modifications to standard immunostaining protocols may be required for high quality stains. For antibodies with low signal to noise ratio, longer incubation times are typically necessary. As a proof of concept for this sonication-facilitated protocol, we show immunostains of three tissue types (testes, ovaries, and neural tissues) at a range of developmental stages.

Introduction

Drosophila embryos and larvae provide an excellent model to study developmental processes in many organs and tissues. Imaging of individual cells is often necessary in these studies in order to ascertain the complex environments in which cells develop. Visualization of cells in tissues can be accomplished through immunostaining. Well-described immunostaining protocols exist for embryonic Drosophila tissues <17 hr after egg laying (AEL)^1-3. However, a protective cuticle forms toward the end of embryogenesis, preventing effective antibody permeation. Thus, these immunostaining protocols are inefficient in the analysis of tissues in late-....

Protocol

1. Preparation of a Collection Cage

1. Anesthetize young, fertile flies with CO₂. Transfer anesthetized flies to a cage. To obtain optimal yield, use 100-120 adult flies ranging from 2-7 days of age at a 4:1 ratio of females to males. Allow flies an appropriate acclimation period, ~24 hr prior to obtaining sample for fixation. If the cage was set up with virgin females mated to males, a use a 36 - 48 hr acclimation period.
2. On the open end of the cage, place a pre-prepared apple juice agar plate with a dime-sized drop of yeast paste in the center over the opening of the cage. Then, secure with tape. Seal the junction between the apple....

Results

To demonstrate the efficacy of sonication-based immunostaining in analysis of late-stage embryonic and larval tissues in situ, wild-type embryos and larvae were processed for immunostaining of testes, ovaries, and neural tissue. Samples were imaged via confocal microscopy and representative results are shown (Figure 1 and Figure 2). Results reveal that the described protocol is effective for visualizing morphological features as well as individual cells in situ during l.......

Discussion

This protocol provides a method to successfully immunostain target Drosophila embryonic and larval tissues in situ, thus eliminating the need for dissection. As per prior protocols for staining early embryos^1,2,3, the chorionic membrane is removed using 50% bleach (NaOCl). Samples are fixed in formaldehyde and methanol. Because the larval cuticle causes older sample to float, the entire sample is then passed through a cell-strainer to ensure larval retention. Sample is stored, if desired, in .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Table 1: Reagents and Buffers
Phosphate buffer Triton X-100 (PBTx)			For 5 L: 500 ml PBS 10X, 4.45 L ddH2O, 50 ml Triton 10%. Store at RT.
Phosphate buffer saline 10X (PBS 10X)			For 1 L in dH2O: 80 g NaCl, 2 g KCl,14.4g Na2HPO4, 2.4 g KH2PO4. Add components and fill to appropriate volume. Store at 4 °C.
Triton 10%			For 50 ml: 5 ml of Triton, 5 ml of PBS 10X, 45 ml of ddH2O. Rock to mix. Store at RT.
PEMS			0.1 M Pipes (pH 6.9), 2.0 mM MgSO4, 1.0 mM EGTA. Store at RT.
Pipes			For a 400 ml of a 0.25 M solution (pH 6.9): 30.24 g Pipes dH20 NaOH. Dissolve Pipes in 300 ml dH2O and then adjust to pH 6.9 with NaOH. Bring the total volume to 400 ml with dH2O and autoclave. Store at RT.
Formaldehyde			37% formaldehyde by weight in methanol. Store at RT. Store formaldehyde, heptane, and methanol waste mixture in a tightly sealed container in fume hood before disposal as per institutional guidelines.
Heptane		CAS 142-82-5	n-Heptane. Store at RT. Store formaldehyde, heptane, and methanol waste mixture in a tightly sealed container in fume hood before disposal as per institutional guidelines.
Methanol		CAS 67-56-1	Store at RT. Store formaldehyde, heptane, and methanol waste mixture in a tightly sealed container in fume hood before disposal as per institutional guidelines.
Phosphate buffer Tween (PBTw)			To make 1 L: 100 ml PBS 10X, 890 ml ddH2O, 10 ml Tween 10%. Filter sterilize after adding all components. Store at 4 °C.
Tween 10%			For 50 ml: 5 ml Tween, 5 ml of PBS 10X, 40 ml of ddH2O. Rock to mix. Store at RT.
Bovine serum albumin/phosphate buffer Tween (BBTw)			To make 1 L: 100 ml PBS 10X, 890 ml ddH2O, 10 ml Tween 10%, 1 g Bovine Serum Albumin (BSA). Add BSA then sterilize using a 0.2 μm vacuum filter unit. Store at 4 °C.
Normal goat serum (NGS)	ackson ImmunoResearch Laboratories	005-000-121	To make 10 ml: Normal goat serum 10 ml ddH2O. Add ddH2O to vial of NGS and sterilize using a 0.2 μm syrninge filter. Store aliquots at -20 °C.
1,4-diazabicyclo[2.2.2]octane (DABCO)		CAS: 281-57-9	To make 100 ml: 25 ml ddH2O, 1 ml Tris HCl (1M, pH 7.5), 2.5 g of DABCO solid, 3.5 ml 6N HCl, 250 μl 10N NaOH, 70 ml glycerol. In 250 ml beaker with stir bar, add ddH2O, Tris HCl and DABCO. Stir and then add 6N HCl, 10 N NaOH, and glycerol. Then add 10N NaOH dropwise until solution reaches pH 7.5. Aliquot. Store aliquots at -20 °C.
DABCO + p-phenylenediamine (PPD) solution			1.765 ml NaHCO3, 0.353 Na2CO3, 0.02 g PPD (CAS: 106-50-3). Dissolve PPD in NaHCO3 and NaCO3 solution. Add 60 μl of PPD solution to 500 μl of DABCO. Store aliquots at -20 °C.
Apple juice plates			To make ~200 plates: 45 g agar (CAS#9002-18-0), 45 g granulated sugar (store bought), 500 ml apple juice (store bought), 15 ml Tegosept 10%, 1.5 ml ddH2O. Add agar to ddH2O in 4 L flask then autoclave for 30 min. Mix apple juice and sugar on heated stir plate. Gradually add apple juice mixture to autoclaved agar. Mix on heated stir plate then aliquot 10 ml volumes into 35 mm petri dishes and let stand at RT to solidify. Store at 4 °C.
Tegosept 10%			To make 100 ml: 10 g Tegosept, 100 ml ethanol. Store aliquots at -20 °C.
Yeast paste			~50 g dry active yeast. Gradually add ddH2O to beaker containing yeast while stirring until paste-like consistency reached. Store at 4 °C.
Table 2: Staining Materials
DAPI	Invitrogen	D3571	1:1000, stock at 5 mg/ml.
Rabbit anti-Vasa			1:250, a gift from Ruth Lehmann.
Mouse anti-Fasciclin III	Developmental Studies Hybridoma Bank (DSHB)	7G10	1:10
Mouse anti-1B1	Developmental Studies Hybridoma Bank (DSHB)	1B1	1:4
Guniea pig anti-Traffic Jam			1:2500, a gift from Dorthea Godt (Li et al, 2003).
Mouse anti-Prospero	Developmental Studies Hybridoma Bank (DSHB)	Prospero MR1A	1:10
Rat anti-Elav	Developmental Studies Hybridoma Bank (DSHB)	7EBA10	1:30
mouse anti-Repo	Developmental Studies Hybridoma Bank (DSHB)	8D12	1:10
Goat anti-rabbit Alexa546	Invitrogen	A11010	1:500
Goat anti-mouse Alexa488	Invitrogen	A11029	1:500
Goat anti-guniea pig Alexa633	Invitrogen	A21105	1:500
Goat anti-rat Alexa488	Invitrogen	A11006	1:500
Table 3: Materials and Equipment
Fly Cages	Hand-made; Genesee Scientific Corporation	Not applicable; Bottles: 32-130; Pre-made cage: 59-101	Made by cutting clear cast acrylic tubing (1 3/4 inch in diameter) into 4 inch tall segments with a compound miter saw at 400 rpm. Ultrafine stainless steel screening (was attached to one end of the tub with acrylic compund glue. An alternate method using an empty fly food bottle can be found in Drosophila Protocols ISBN 0-87969-584-4. Cages may also be purchased from the Genesee Scientific Corporation.
Sonicator: Branson 250 Digital Sonifier	Branson	Model: Branson Digital Sonifier 250
Sonicator probe	Branson	Model #: 102C (CE)	EDP: 101-135-066; S/N: OBU06064658
Syringe filter	Nalgene	190-25-20	0.2 μm cellulose, acetate membrane filter
Imaging system: Spinning disc confocal microscope with multichromatic light source, digital CCD camera, and imaging software	Microscope: Olympus, Light source: Lumen Dynamics, Camera: Q-Imaging, Imaging Software: Intelligent Imaging Inc.		Microscope: BX51 equipped with DSU spinning disc, Light source: X-Cite 120Q, Camera: RETIGA-SRV, Imaging Software: Slidebook 5.0
Vacuum filter unit	Nalgene	450-0020	0.2 μm cellulose nitrate membrane filter