Effect of glucocorticoids on gene expression of cutaneous antimicrobial peptides and susceptibility to chytridiomycosis in the northern leopard frog (Lithobates pipiens)
Chytridiomycosis is an emerging cutaneous fungal disease that contributes to recent global declines and extinction of amphibian species, caused by infection of the skin with a fungus known as Batrachochytrium dendrobatidis (Bd). Many species of frogs secrete antimicrobial peptides onto their skin that are capable of killing Bd. This thesis is an investigation of the effect of corticosteroids on cutaneous innate immunity in frogs, in the context of infection with Bd. The general hypothesis was that injections of glucocorticoids would impair the cutaneous synthesis of these antimicrobial peptides, thereby increasing susceptibility to Bd infection. The objective of the first experiment was to measure and compare gene expression levels of cutaneous AMP’s in frogs treated with glucocorticoids with sham-treated controls. Wild-caught Lithobates pipiens were acclimatized and administered either the corticosteroid methylprednisolone or saline every 48 hours. Norepinephrine-elicited cutaneous secretions were collected prior to the first injection of corticosteroid or saline, and then every 8 days for 40 days. Gene expression of the AMP’s brevinin and ranatuerin in the cutaneous secretions was quantified relative to the reference genes EF1-α and RPL8 using reverse transcription quantitative polymerase chain reaction (RT-qPCR). Corticosteroid treatment was associated with a significant (P<0.027) increase in brevinin gene expression, which was most notable at 24-40 days of corticosteroid administration. Ranatuerin expression followed a similar but nonsignificant trend. The second experiment was a pilot study intended to establish a Bd challenge protocol in L. pipiens. Frogs were immersed in water containing 0, 104, 105 or 106 zoospores of Bd strain JEL 423. Cutaneous swabs were collected prior to challenge and tested for Bd by qPCR; unexpectedly, some tested positive, indicating pre-challenge infection. The analysis was complicated by an identified cross-reactivity of the assay with other fungi. The findings of the first experiment refuted the hypothesis, and suggested that corticosteroids promote rather than impair AMP gene expression in the skin of L. pipiens, under these experimental conditions. Further, the second study demonstrated that none of the frogs showed clinical abnormalities or died, despite exposure to Bd zoospores and despite molecular and histologic evidence of cutaneous Bd infection in some frogs.