A modified density centrifugation gradient-based methodology was utilized to isolate epithelial cells from Rhipicephalus microplus gut tissue. Surface-bound proteins were biotinylated and purified through streptavidin magnetic beads for utilization in downstream applications.
Rhipicephalus microplus – the cattle tick – is the most significant ectoparasite in terms of economic impact on livestock as a vector of several pathogens. Efforts have been dedicated to the cattle tick control to diminish its deleterious effects, with focus on the discovery of vaccine candidates, such as BM86, located on the surface of the tick gut epithelial cells. Current research focuses upon the utilization of cDNA and genomic libraries, to screen for other vaccine candidates. The isolation of tick gut cells constitutes an important advantage in investigating the composition of surface proteins upon the tick gut cells membrane. This paper constitutes a novel and feasible method for the isolation of epithelial cells, from the tick gut contents of semi-engorged R. microplus. This protocol utilizes TCEP and EDTA to release the epithelial cells from the subepithelial support tissues and a discontinuous density centrifugation gradient to separate epithelial cells from other cell types. Cell surface proteins were biotinylated and isolated from the tick gut epithelial cells, using streptavidin-linked magnetic beads allowing for downstream applications in FACS or LC-MS/MS-analysis.
Rhipicephalus microplus, the cattle tick, is the most significant ectoparasite in terms of economic impact on the cattle industry of tropical and sub-tropical regions as it vectors bovine tick fever (babesiosis), anaplasmosis and equine piroplasmosis1,2,3,4. Efforts have been dedicated to cattle tick control, to diminish the deleterious effect, however conventional methods such as the use of chemical acaricides have implicit drawbacks, such as the presence of chemical residues in milk and meat, and the increase in prevalence of chemically resistant ticks5,6,7. Consequently, the development of alternative methods of tick control have been studied, such as the use of natural resistance cattle, biological control (biopesticides) and vaccines4,5,6,7,8,9.
In the pursuit of proteins capable of being utilized as vaccine candidates, current research is focused upon the tick gut. The midgut wall is built from a single layer of epithelial cells resting on a thin basal lamina, with the outside of the basal lamina forming a network of muscle. Light and electron microscope observations indicate that the midgut consists of three types of cells: reserve (undifferentiated), secretory, and digestive. The number of cell types varies considerably depending upon the physiological phase. Secretory and digestive cells both originate from reserve cells18,19,20.
The construction of cDNA libraries to examine the composition of the tick gut has led to the identification of antigenic proteins, such as Bm86, as potential vaccine candidates2,3,4. The glycoprotein Bm86 is localized at the surface of tick gut cells and induces a protective immune response against the cattle tick (R. microplus) in vaccinated cattle. Anti-Bm86 IgGs produced by the immunized host are ingested by the tick, recognize this antigen on the surface of tick gut cells, and subsequently disturb tick gut tissue function and integrity. Vaccines based upon Bm86 antigens have shown effective control of R. microplus and Rhipicephalus annulatus, by reducing the number, weight and reproductive capacity of engorging females, resulting in a reduced larval infestation in subsequent tick generations4. However, Bm86 based vaccines are not effective against all tick stages and have demonstrated unsatisfactory efficacy against some geographical strains of R. microplus, consequently the beef and dairy industries have poorly adopted these vaccines2,4.
The ability to isolate epithelial cells from the tick gut is a significant innovation which would enable the progression of research to determine protein membrane composition including morphology and physiology under different environmental conditions. The method described here utilizes the chelating agent ethylenediaminetetraacetic acid (EDTA) and the reducing agent tris(2-carboxyethyl)phosphine (TCEP) to release the epithelium from its sub-epithelial support tissues10. The epithelium is recovered following mechanical disruption of the tissues by shaking, followed by discontinuous gradient centrifugation in Percoll. This paper describes a feasible and novel technique for the isolation of tick gut epithelial cells. Biotinylated cell surface proteins, isolated from the surface of these epithelial cells can subsequently analyzed in downstream applications such as FACS and/or LC-MS/MS-analysis.
1. Dissection of the Gut Epithelium from R. microplus
2. Epithelial Cell Dissociation
3. Isolation of Single Epithelial Cells using a density centrifugation gradient
4. Assessment of Cell Isolation
5. Cell Surface Protein Biotinylation
6. Isolation of Biotinylated Surface Proteins
7. Assessment of Biotinylated Surface Protein
Epithelial cells were isolated from the gut tissues of R. microplus as per the schematic presented in Figure 1. Representative fluorescence microscopy imagery of tick gut epithelial cells prepared using this protocol are shown in Figure 2A and 2B. As the cell isolation is conducted upon semi-engorged R. microplus, cells appear as singular, spherical, smooth surface morphology and a consistent size throughout the sample. Differences in the size and type of gut cell populations, are more evident once the tick proceeds to become fully-engorged adults.
Fluorescent staining of the nucleus of isolated epithelial cells with DAPI assists to visualize the cells. Poor isolations are visualized to contain incomplete dissociation of epithelial cells with varying cell populations identified through the varying cell sizes and morphologies (Figure 2C and 2D)
Utilizing this protocol, approximately 1.2 x 107 cell per/mL from 50 tick gut dissections, with 75-80% viability were successfully isolated from R. microplus tick gut. Cross-contamination from host proteins (Figure 3A) can be minimized by adequately rinsing tick guts until they have a white appearance that result in a white/clear Percoll gradient (Figure 3B).
Surface-bound proteins were isolated through biotinylation of surface-bound proteins, destruction of cellular membranes and the purification of biotinylated surface proteins with magnetic streptavidin beads. A total of 20-24 µg of purified biotinylated surface proteins can be isolated for down-stream applications from an initial dissection of 50x tick guts utilizing this protocol. Comparison of proteins by SDS-PAGE (Figure 4A), Silver stain (Figure 4B), Dot blot (Figure 5) and ELISA (Figure 6) indicates that the described methodology successfully purified biotinylated surface proteins from epithelial cells isolated from R. microplus tick gut.
Figure 1: Schematic representation to isolate biotinylated proteins present in the surface of the R. microplus midgut cells. Please click here to view a larger version of this figure.
Figure 2: Fluorescence microscope visualization (100x) of the R. microplus midgut epithelial cells stained with DAPI. The software was used to conduct the fluorescence overlay. A&B) Represent epithelial cells successfully isolated from the R. microplus midgut. C/D) Epithelial cells isolated from the R. microplus gut contaminated with larger tick tissues.
Figure 3: Percoll gradient containing DMEM: 20% Percoll:40% Percoll. The epithelial cell layers were formed between the DMEM:20% Percoll and the 20:40% Percoll. (A) Tick gut dissection without adequate wash step. (B) Tick gut dissection with adequate wash step. Please click here to view a larger version of this figure.
Figure 4: Electrophoretic separations of biotinylated surface proteins run on 4-20% Tris-MOPS gel, at 140 V for 55 min with 10 µg of sample utilized. (L) PageRuler Prestained protein ladder (1) Unlabelled R. microplus whole gut sample (2) Biotinylated proteins extracted from R. microplus whole gut as per step 6. (3) Proteins from epithelial cells extracted from unlabelled R. microplus gut (4) Biotinylated proteins extracted from R. microplus epithelial cells as per step 6. (A) SDS-PAGE stained by Comassie blue (B) Silver stain.
Figure 5: Dot blot analysis. Streptavidin-HRP conjugated diluted 1/5,000. (A) A total of 10 µg of raw tick gut protein extract (B) Biotinylated surface proteins extracted from purified epithelial cells (10 µg).
Figure 6: Assessment of Biotinylated surface proteins viability utilizing ELISA. ELISA of surface proteins was developed by Strep-HRP conjugated antibody diluted 1/15,000 against different concentrations of biotinylated tick gut proteins (from 0.7 ng to 100 ng). Unlabeled tick gut proteins and bovine serum albumin (BSA) were used as negative control of the ELISA. Please click here to view a larger version of this figure.
Cattle tick infestations constitute a major problem for the cattle industry in tropical and subtropical regions of the world, with the most common method of control reliant upon the use of acaricides1,4. Bm86 was previously identified within the tick gut epithelial surface as a protective antigen against R. microplus infestation10, with limited success as a vaccine strategy due to Bm86 geographic sequence variation and the requirement for regular boosting4.
Previous publications focusing upon epithelial isolation methodologies were principally focused upon vertebrates or insects species9,11,12,13,14. For example, early fractionation attempts to isolate midgut utilizing mammalian techniques, found that the same methodology could not be applied to insects due to the different cell structure and organizations12. Furthermore, mammalian techniques rely upon the use of either dipase or collagenase, to enzymatically digest the cell-cell junction proteins9,13. Dipase and collagenase considerably affect cell surface bound proteins, and therefore techniques dependent on their use are not suitable for cell-surface protein studies15. Insect midgut cell isolations currently focus upon two techniques. The first utilizes mechanical disruption of midgut through ultrasound followed by separation over a continuous linear sucrose gradient8,11,12,14. This mechanical technique produces a samples almost clear of contaminants, however produces a low yield of microvillarmembranes14. The second technique relies upon Tris disruption of membranes to release microvillar membranes8.
The technique outlined within this paper allows for the isolation of epithelial cells, biotinylation of surface proteins and their isolation16, permitting further studies through downstream applications such as mass spectrometry or FACS. The method described here utilizes the chelating agent EDTA and the reducing agent TCEP. EDTA functions to sequester calcium ions, inhibiting cadherins, breaking cadherin-mediated cell-cell junctions whereas TCEP reduces the disulfide bonds that confers viscosity of the glycan-rich mucus-like peritrophic matrix that separates the gut lumen from the epithelium. The epithelium is recovered following mechanical disruption of the tissues by shaking, followed by discontinuous gradient centrifugation in the density centrifugation gradient. Epithelial cells are isolated between the DMEM:20% density centrifugation gradient, and 20%:40% density centrifugation gradient layers.
Utilization of a higher temperature during epithelial cell dissociation will cause large tissues to dissociate from the gut, leading to failure to isolate epithelial cells. Ensuring that epithelial cell dissociation is conducted promptly post tick dissection; the utilization of ice-cold buffers and the avoidance of high centrifugation speeds is critical in retaining live viable cells. Despite this, the dislodgment of larger cells from the basal lamina can provide some contamination. Furthermore, the midgut epithelium of the tick alters dramatically dependent on the time from the last blood meal18,19,20.
As such, this protocol has been designed and accessed on R. microplus ticks starting digestion, and the techniques prescribed may have varying resulting cell populations dependent on when the tick's last blood meal was taken. The protocol has been designed to collect varying epithelial cell populations, however as the ticks utilized were semi-engorged the cells visualized (Figure 1) are of a uniform nature.
In conclusion, the methods utilized in this study successfully isolated epithelial cells from the whole gut of R. microplus. Proteins from the surface of R. microplus epithelial cells were obtained for further analysis and studies. Finally, the protocol developed has demonstrated the potential yield of epithelial cells from the tick gut, and the efficiency of biotinylated surface proteins of the cell. The protocol developed can be utilized in any tick species of economic significance in an effort to investigate tick:host interactions by studying the membrane protein composition of the gut.
The authors have nothing to disclose.
The authors wish to thank the Biosecurity Tick Colony (Queensland Department of Agriculture & Fisheries, Australia) for the provision of Rhipicephalus microplus ticks utilized for this study, and Lucas Karbanowicz for assistance with video filming.
0.4% Trypan Blue | ThermoFisher Scientific | 15250061 | |
1.5 mL microcentrifuge tube | Eppendorf | 3322 | |
100mM Carbonate Buffer | 3.03 g Na2CO3, 6.0 g NaHCO3 1000 ml distilled water pH 9.6 | ||
16 mL centrifuge tubes with sealing cap | Thermo Scientific | 3138-0016 | Cool in ice prior to gradient |
250 µM cell strainer | Thermo Fisher | 87791 | |
3,3′,5,5′-Tetramethylbenzidine (TMB) Liquid Substrate System for ELISA | Sigma | T0440 | Stored at 4C |
30% Hydrogen Peroxide | Labscene | BSPA5.500 | |
4-20% Tris-MOPS Gel | Gen Script | M42015 | |
4-Chloro-1-naphthol tablet | Sigma-Aldrich | C6788 | |
50 mL Falcon Tube | Corning Blue | 30 x 115mm style. Polyproplyene conical tube. | |
70 µM cell strainer | BD Falcon | 352350 | |
AP15 filter paper | Millipore | AO1504200 | |
Biotin (Type A) Conjugation Kit | Abcam | Ab102865 | |
Dissection microscope | Olympus | SZX7 | |
DP Manager | Olympus | 2.2.1.195 | Cell imagery software |
Duct Tape | Home Handyman | 48mm x 25mm Duct Tape | |
Dulbecco’s Modified Eagle Medium | Gibco | 11995-065 | DMEM – ice cold for protocol |
EDTA | Amresco | 0105-500G | |
F96 Maxisorp Immuno Plate | Nunc | 439454 | |
Fetal Bovine Serum | Sigma-Aldrich | 12003C | FCS |
Fluorescence microscope | Olympus | BX51 | |
Fluoroshield with DAPI | Sigma-Aldrich | F6057-20ML | DAPI |
Forceps | Dumont | #9 Dumont – Switerzland | |
Glycerol | Sigma-Aldrich | G5516 | Glycerol for molecular biology >99% |
Glycine | Sigma-Aldrich | 410225 | |
Hand-Held Counter | Officeworks | JA0376230 | |
Hank’s Balanced Salt Solution | Sigma Life Sciences | H9394 | HBSS – ice cold for protocol |
Hemacytometer | Optik Lakor | – | – |
L-Glutathione oxidized | Sigma-Aldrich | G4376 | |
Magnetic Separation Stand | Novagen | – | 4-Tube Magnetic Separation Rack |
Methanol | Sigma-Aldrich | 179337 | |
Milli-Q Water | Millipore | ZRXQ003WW | Integral Water Purification System for Ultrapure Water |
Nitrocellulose Membrane | Life Sciences | 66485 | 30cm x 3M pure nitrocellulose membrane |
PageRuler Prestained protein Ladder | Thermo-Fisher | SM0671 | |
PBS | 1.16 g Na2HPO4, 0.1 g KCl, 0.1 g K3PO4, 4.0 g NaCl (500 ml distilled water) pH 7.4 | ||
Percoll | Sigma-Aldrich | P1644-500ML | |
Peristaltic Pump | Masterflex | 7518-10 | |
Phosphoric Acid | Sigma-Aldrich | P6560 | |
Pierce Protein-Free T20 PBS Blocking Buffer | Thermo-Scientific | 37573 | Stored at 4C. Blocking Buffer |
Protease Inhibitor Cocktail | Sigma-Aldrich | P8215-5ML | PIC – stored at -20 °C |
Quick Start Bradford Dye Reagent 1x | Biorad | 500-0205 | For Bradford Assay |
Quick Start BSA Standards | Biorad | 500-0207 | BSA standards for Bradford Assay |
Scalpel | Lab. Co | Size 11 Scalpel | |
SilverQuest TM Staining Kit | Invitrogen | LC6070 | |
Simply Blue TM Safe Stain | Invitrogen | LC6060 | |
Sorvall C6+ Ultracentrifuge | Thermo Scientific | 46910 | |
Streptavidin (HRP) | Abcam | AB7403 | |
Streptavidin Magnetic Beads | New England Biolabs | S1420S | |
Super Glue – Ultra Fast Mini | UHU | UHU Super Glue 1mg. Ultra Fast mini | |
Table-top Centrifuge | Eppendorf | 22331 | |
TCEP | Thermo Fisher | 20490 | |
Triton X-100 | Biorad | 161-0407 | |
Tween-20 | Sigma | P2287-500ML | |
Vortex Mixer | Ratek | VM1 | |
Water Bath | Grant | GD100 |