Identification of Critical Conditions for Immunostaining in the Pea Aphid Embryos: Increasing Tissue Permeability and Decreasing Background Staining

Summary

A protocol of whole-mount immunostaining on aphid embryos is presented, in which critical conditions for decreasing background staining and increasing tissue permeability are addressed. These conditions are specifically developed for effective detection of protein expression in the embryonic tissues of aphids.

Abstract

The pea aphid Acyrthosiphon pisum, with a sequenced genome and abundant phenotypic plasticity, has become an emerging model for genomic and developmental studies. Like other aphids, A. pisum propagate rapidly via parthenogenetic viviparous reproduction, where the embryos develop within egg chambers in an assembly-line fashion in the ovariole. Previously we have established a robust platform of whole-mount in situ hybridization allowing detection of mRNA expression in the aphid embryos. For analyzing the expression of protein, though, established protocols for immunostaining the ovarioles of asexual viviparous aphids did not produce satisfactory results. Here we report conditions optimized for increasing tissue permeability and decreasing background staining, both of which were problems when applying established approaches. Optimizations include: (1) incubation of proteinase K (1 µg/ml, 10 min), which was found essential for antibody penetration in mid- and late-stage aphid embryos; (2) replacement of normal goat serum/bovine serum albumin with a blocking reagent supplied by a Digoxigenin (DIG)-based buffer set and (3) application of methanol rather hydrogen peroxide (H₂O₂) for bleaching endogenous peroxidase; which significantly reduced the background staining in the aphid tissues. These critical conditions optimized for immunostaining will allow effective detection of gene products in the embryos of A. pisum and other aphids.

Introduction

Aphids are hemipteran insects with small (1-10 mm) soft bodies. They feed on plants by sucking phloem sap with piercing mouthparts. Additionally, they rely on an obligate endosymbiotic bacterium, Buchnera aphidicola, to synthesize essential amino acids that are deficient in the phloem sap diet. Aphids have a complex life history that includes parthenogenetic viviparous reproduction during spring and summer long-day photoperiods and sexual oviparous reproduction triggered by short-day photoperiods during which they lay a limited number of overwintering eggs^1,2. In spring these eggs hatch to produce the first generation of all-femal....

Protocol

1. Culture of Aphids

NOTE: The laboratory strain of the parthenogenetic viviparous pea aphid A. pisum was originally collected in the central Taiwan and has been reared on host plants (the garden pea Pisum sativum or broad bean Vicia faba) under long-day photoperiod for more than 300 generations (one generation: ~10 days).

1. Germination of Seeds      
   1. Soak the seeds of host plants in tap water for 3-5 days at RT. Refill with fresh water once per day.
      NOTE: Alternatively, putting seeds of host plants straight into moist soil can induce germination as well.
   2. Grow 10 germina....

Results

In this study, we performed whole-mount immunostaining on embryos of asexual pea aphids (Figure 1A). These females produce offspring parthenogenetically and viviparously. These female embryos develop within egg chambers of the ovarian tubules (ovarioles) (Figure 1B and Figure 2A). Before microscopy, the dissected ovarioles are the staining targets; however, separation of egg chambers is required for observation of embryos under a microsco.......

Discussion

We identified optimal conditions critical to successful immunostaining in the pea aphid A. pisum, an emerging model organism for genomic and developmental studies^3,15. Optimized conditions for increasing tissue permeability and reducing background staining enhanced the intensity and specificity of signals. They differ from standard protocols for immunostaining in other animal models in the steps for creating pores in cell membranes and blocking non-specific antibody binding.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
30% hydrogen perxidase (H2O2)	Sigma-Aldrich	18304	For bleaching endogenous peroxidase of embryos.
4’6-diamidino-2-phenyl-indole dihydrochloride (DAPI)	Sigma-Aldrich	D9542	For labeling the double-strand DNA. TOXIC. Wear gloves, dispense into small aliquots and feeze to minimize exposure
4D9 anti-engrailed/invected mouse monoclonal antibody	Developmental studies hybridoma bank	AB_528224	For labeling the developing segments of embryo.
ApVas1 rabbit polyclonal antibody	Our laboratory	N/A	For labeling the aphid germline specific Vas1 protein in embryos. This antibody was made by our laboratory.
Austerlitz Insect pins	ENTOMORAVIA	N/A	For seperating each egg chambers from an ovariole. Size 000, BLACK ENAMELLED.
Bovine serum albumin (BSA)	Sigma-Aldrich	9048-46-8	For blocking the non-specific binding of antibodies.
Camera connted to compound microscope	Canon	EOS 5D	For photographing of aphid embryos.
Colorimetric 8 cell tray	Kartell Labware	357	For developing the staining signal of aphid embryos. Diameter 95 x 57 mm, cell depth 2 mm.
Compound microscope with DIC optics	Leica Microsystems	DMR	For photographing of aphid embryo mounted on the slides.
DIG Wash and Block Buffer Set	Sigma-Aldrich (Roche)	11585762001	For blocking the non-specific binding of antibodies. We diluted the 10x Blocking solution into 1x as blocking reagent.
Forceps	Ideal-tek	N/A	For dissection of ovaries from aphids. Manufacturer part No 5: 5 SA.
Glass dropper	N/A	N/A	For transfering ovaries of aphids from the splot plate to tubes. 150 mm of total length and 5 3/4" of tip length.
Glycerol	Sigma-Aldrich	G5516	For clearing of aphid embryos and mounting.
Glycine	Bioshop	N/A	For blocking the enzyme activity of proteinase K (PK).
Goat anti-mouse IgG conjugated Alexa Fluor 488	Invitrogen	A11017	For detection of primary antibody from mouse.
Goat anti-rabbit IgG conjugated Alexa Fluor 633	Invitrogen	A21072	For detection of primary antibody from rabbit.
Intelli mixer	ELMI laboratory equipment	RM-2M	For improving the thorughly reaction of reagents with ovaries.
Laser-scanning confocal microscopy	Leica Microsystems	SP5	For photographing of aphid embryo mounted on the slides with florecent tag.
methanol	Burdick and Jackson	AH2304	For bleaching endogenous peroxidase of embryos.
Microscope slides	Thermo scientific	10143560	For mounting of embryos. SUPERFROST ground edges, ca./env. 76 x 26 mm.
Microscope Cover glasses	Marienfeld	0101030	For mounting embryos on the slides. Size 18 x 18 mm. Thickness No. 1 (0.13 to 0.16 mm)
Microscope Cover glasses	Marienfeld	0101050	For mounting embryos on the slides. Size 22 x 22 mm. Thickness No. 1 (0.13 to 0.16 mm)
mouse monoclonal anti-alphaTubulin antibody (DM1A)	Santa Cruz Biotechnology	sc-32293	For labeling the distrbution of alpha-tubulin of cells.
Nail polish	N/A	N/A	For sealing the coverslips on the slides.
Normal goat serum (NGS)	Sigma-Aldrich	G9023	For blocking the non-specific binding of antibodies.
Paraformaldehyde (PFA)	VWR	MK262159	For fixation of aphid ovaries.
Phalloidin-TRITC	Sigma-Aldrich	P1951	For labeling the distrbution of F-actin on embryos.
Phosphate-buffered saline (PBS)	N/A	N/A	As isotonic solution for aphid ovaries.
Plastic dropper	N/A	N/A	For transfering ovaries of aphids from tube to cell tray. Size 3 ml.
Proteinase K	Merck	124678	For creating pores (punching) on the cell membrane and facilitating the access of antibodies.
Pyrex spot plate	N/A	N/A	For dissection and color reactions of aphid ovaries under microscopy. Each cavity with diameter 22 mm and  depth 7 mm.
SIGMAFAST 3,3’-diaminobenzidine (DAB) tablets	Sigma-Aldrich	D4168	For developing of substrated signals. Also called DAB peroxidase substrate tablet set.
Stereo microscope	Leica Microsystems	EZ4	For dissection and observation of aphid ovaries.
Triton-X 100	Sigma-Aldrich	T8787	For creating pores (punching) on the cell membrane.
VECTASHIELD Elite ABC Kit (Rabbit IgG)	Vector laboratories	PK-6101	For enhancement of the signal. Including of biotinylated goat anti-rabbit IgG secondary antibody, A and B reagents.
VECTASHIELD mounting medium	Vector laboratories	H1000	For clearing of aphid embryos and mounting.