Using Terminal Transferase-mediated dUTP Nick End-labelling (TUNEL) and Caspase 3/7 Assays to Measure Epidermal Cell Death in Frogs with Chytridiomycosis

Summary

We quantify epidermal cell death in frogs with chytridiomycosis using two methods. First, we use terminal transferase-mediated dUTP nick end-labelling (TUNEL) in situ histology to determine differences between clinically infected and uninfected animals. Second, we conduct a time series analysis of apoptosis over infection using a caspase 3/7 protein analysis.

Abstract

Amphibians are experiencing a great loss in biodiversity globally and one of the major causes is the infectious disease chytridiomycosis. This disease is caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), which infects and disrupts frog epidermis; however, pathological changes have not been explicitly characterized. Apoptosis (programmed cell death) can be used by pathogens to damage host tissue, but can also be a host mechanism of disease resistance for pathogen removal. In this study, we quantify epidermal cell death of infected and uninfected animals using two different assays: terminal transferase-mediated dUTP nick end-labelling (TUNEL), and caspase 3/7. Using ventral, dorsal, and thigh skin tissue in the TUNEL assay, we observe cell death in the epidermal cells in situ of clinically infected animals and compare cell death with uninfected animals using fluorescent microscopy. In order to determine how apoptosis levels in the epidermis change over the course of infection we remove toe-tip samples fortnightly over an 8-week period, and use a caspase 3/7 assay with extracted proteins to quantify activity within the samples. We then correlate caspase 3/7 activity with infection load. The TUNEL assay is useful for localization of cell death in situ, but is expensive and time intensive per sample. The caspase 3/7 assay is efficient for large sample sizes and time course experiments. However, because frog toe tip biopsies are small there is limited extract available for sample standardization via protein quantification methods, such as the Bradford assay. Therefore, we suggest estimating skin surface area through photographic analysis of toe biopsies to avoid consuming extracts during sample standardization.

Introduction

Amphibians are currently experiencing one the greatest losses of global biodiversity of any vertebrate taxa¹. A major cause of these declines is the fatal skin disease chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis, Bd². The pathogen superficially infects the epidermis, which can lead to the disruption of skin function resulting in severe electrolyte loss, cardiac arrest, and death³. Various potential host immune mechanisms against Bd are currently being studied, such as antimicrobial peptides^4,5....

Protocol

James Cook University approved animal ethics in applications A1875 for P. corroboree and A1897 and A2171 for L. v. alpina.

1. Animal Husbandry and Monitoring

1. House animals individually, in an environment appropriate for the species, with an appropriate water, feeding and cleaning schedule. Check animals daily.
   1. Use adult individuals of the critically endangered Pseudophryne corroboree (for the TUNEL Assay, Section 4) and the threatened Litoria verreauxii alpina (for the Caspase 3/7 assay, Section 5), donated from captive breeding facilities. Maintain the animals at 15 - 18 °C, ....

Results

TUNEL Assay

There were more TUNEL positive cells in the infected animals than in the uninfected control animals. The in situ location of TUNEL positive cells differed in infected and control animals. In control animals, there was an even distribution of TUNEL positive cells throughout the dermal and epidermal skin layers at low levels (See Figure 1A), but in the infected animals, t.......

Discussion

We explored epidermal apoptosis and cell death as a potential mechanism of pathology of the deadly disease chytridiomycosis or a mechanism of disease resistance in Bd susceptible species. We used two methods of assessing cell death in the epidermis, TUNEL assay for in situ epidermal cell death analysis, and caspase 3/7 assay for monitoring epidermal cell death throughout the progress of infection. We found that cell death and apoptosis are correlated with infection load and cell death is significantly h.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
POLARstar Omega	BMG Labtech		Luminescent plate reader
384 well flat clear bottom plate	Corning	3707
384 well low flange white flat bottom plate	Corning	3570
Agar Bacteriological (Oxoid)	Fisher	OXLP0011B
Formal-Fixx 10% Neutral Buffered Formalin	Fisher	6764254
Lactose Broth (Oxoid)	Fisher	OXCM0137B
Sodium Bicarbonate	Fisher	BP328-500
Tricane-S (MS-222)	Fisher	NC0872873
Tryptone	Fisher	BP1421-500
Bovine Serum Albumin	Invitrogen	15561020
Sterile rayon swab	Medical Wire & Equipment	MW-113
ApopTag Red In Situ Apoptosis Detection Kit	Merck Millipore	S7165
Coomassie Bradford reagent	Pierce	23200
Caspase Glo  3/7	Promega	G8090
HEPES buffer	Sigma Aldrich	H0887-20ML
Magnesium chloride	Sigma Aldrich	1374248-1G
Gelatin hydrolysate Enzymatic	Sigma-Aldrich	G0262
PBS (Phosphate Buffered Saline), pH 7.2 (1X)	Thermo/Life	20-012-043
Prepman	Thermo/Life	4318930
TaqMan Fast Advanced Master Mix	ThermoFisher	4444556
Parafilm	Bemis	PM996
Clorox bleach	Clorox
Ethanol, 200 Proof, Molecular Grade	Fisher	BP2818500
ZEISS Axio Scan florescent miscroscope	Carl Zeiss		Florescent microscope
3.2mm stainless steel beads	BioSpec	11079132SS
Primer ITSI-3 Chytr (5′-CCTTGAT ATAATACAGTGTGCCATATGTC-3′)	Taqman		Individual design for primers and probe
Primer 5.8S Chytr (5′-TCGGTT CTCTAGGCAACAGTTT-3′)	Taqman		Individual design for primers and probe
Minor groove binder probe Chytr MGB2(5′-CGAGTCGAAC-3′)	Taqman		Individual design for primers and probe
Rotor-Gene qPCR Instruments	Qiagen		qPCR machine
Microcentrifuge tubes 1.5ml	Fisher	02-681-372
Cell culture petri plates	Nunc	263991
Mini-beadBeater Zircornia-Silicate Beads, 0.5mm	BioSpec	11079105Z