A lethal fungal pathogen directly alters tight junction proteins in the skin of a susceptible amphibian

Abstract

Bacterial and viral pathogens can weaken epithelial barriers by targeting and disrupting tight junction (TJ) proteins. However, comparatively little is known about the direct effects of fungal pathogens on TJ proteins and their expression. The disease chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), is threatening amphibian populations worldwide. Bd is known to infect amphibian skin and disrupt cutaneous osmoregulation. However, exactly how this occurs is poorly understood. This study considered the impact of Bd infection on the barrier properties of the Australian green tree frog (Litoria caerulea) epidermis by examining how inoculation of animals with Bd influenced the paracellular movement of FITC-dextran (4 kDa, FD-4) across the skin in association with alterations in the mRNA and protein abundance of select TJ proteins of the epidermal TJ complex. It was observed that Bd infection increased paracellular movement of FD-4 across the skin linearly with fungal infection load. In addition, Bd infection increased transcript abundance of the tricellular TJ (tTJ) protein tricellulin (Tric) as well as the bicellular TJ (bTJ) proteins occludin (Ocln), claudin (Cldn)-1, Cldn-4 and the scaffolding TJ protein zonula occludens 1 (ZO-1). However, while Tric protein abundance increased in accord with changes in transcript abundance, protein abundance of Cldn-1 was significantly reduced and Ocln protein abundance was unchanged. Data indicate that disruption of cutaneous osmoregulation in L. caerulea following Bd infection occurs, at least in part, by an increase in epidermal paracellular permeability in association with compromised integrity of the epidermal TJ complex.

Mechanistic basis for loss of water balance in green tree frogs infected with a fungal pathogen

“…For epithelial junction-associated genes, we observed an upregulation of mRNA expression for claudins 4 and 15 and e-cadherin, which are responsible for establishing the barrier function of the epithelium and regulating paracellular salt and water movement (29). The increase in junctional protein gene expression may be a response to compensate for the loss of skin integrity in Bd-infected frogs, as seen in Gauberg et al (27) and Wu et al (80). However, changes in mRNA expression alone do not necessarily reflect commensurate changes in the abundance of the functional protein, nor do they directly pertain to changes in protein activity levels (57,63,80).…”

“…Within infected animals, AQP inhibition by HgCl 2 further increased water efflux rates. These data may indicate that for infected animals in particular, HgCl 2 -sensitive AQPs play an important role in reducing the gradient that promotes paracellular water loss in infected frogs (27). Bd-infected frogs show impaired skin integrity (8,11), which has been linked to the increased permeability of the skin (78).…”

“…However, there is evidence that integument dysfunction may increase osmotic leakage (8,12). Bd secretions also disrupt epidermal intercellular junctions (12,27), which can increase the permeability of the paracellular pathway. Water transport rates may be similarly disrupted via indirect impacts on paracellular pathway structure or directly by affecting the activity and/or abundance of AQPs (69).…”

Claudins of sea lamprey (Petromyzon marinus) – organ‐specific expression and transcriptional responses to water of varying ion content

“…Specifically, although the brain-specific expression of cldn-5 is consistent with observations of this gene in the blood-brain barrier of other vertebrates, it contrasts with the broader expression of cldn-5 in derived vertebrates, which is, at least in part, due to the presence of Cldn-5 as a predominant endothelial TJ protein (Gunzel & Yu, 2013). In addition, the presence and salinity-induced alterations of cldn-14 transcript in the kidney of lamprey are consistent with observations from derived vertebrates where Cldn-14 is present in renal tissues and possesses a cation barrier function (Gong et al, 2012;Wilcox et al, 2001).…”

“…Nonetheless, several studies conducted by the authors indicate that TJ proteins in several vertebrates are expressed in the skin as a meshwork around epithelial cells exposed to surrounding water as well as in the deeper layers of the tissue Gauberg et al, 2019). Whether these findings translate to the skin of juveniles requires further investigation.…”