The pathogen Batrachochytrium dendrobatidis (Bd), which causes the skin disease chytridiomycosis, is one of the few highly virulent fungi in vertebrates and has been implicated in worldwide amphibian declines. However, the mechanism by which Bd causes death has not been determined. We show that Bd infection is associated with pathophysiological changes that lead to mortality in green tree frogs (Litoria caerulea). In diseased individuals, electrolyte transport across the epidermis was inhibited by >50%, plasma sodium and potassium concentrations were respectively reduced by approximately 20% and approximately 50%, and asystolic cardiac arrest resulted in death. Because the skin is critical in maintaining amphibian homeostasis, disruption to cutaneous function may be the mechanism by which Bd produces morbidity and mortality across a wide range of phylogenetically distant amphibian taxa.

We investigated the relationship between gut health, visceral fat dysfunction and metabolic disorders in diet-induced obesity. C57BL/6J mice were fed control or high saturated fat diet (HFD). Circulating glucose, insulin and inflammatory markers were measured. Proximal colon barrier function was assessed by measuring transepithelial resistance and mRNA expression of tight-junction proteins. Gut microbiota profile was determined by 16S rDNA pyrosequencing. Tumor necrosis factor (TNF)-α and interleukin (IL)-6 mRNA levels were measured in proximal colon, adipose tissue and liver using RT-qPCR. Adipose macrophage infiltration (F4/80+) was assessed using immunohistochemical staining. HFD mice had a higher insulin/glucose ratio (P = 0.020) and serum levels of serum amyloid A3 (131%; P = 0.008) but reduced circulating adiponectin (64%; P = 0.011). In proximal colon of HFD mice compared to mice fed the control diet, transepithelial resistance and mRNA expression of zona occludens 1 were reduced by 38% (P<0.001) and 40% (P = 0.025) respectively and TNF-α mRNA level was 6.6-fold higher (P = 0.037). HFD reduced Lactobacillus (75%; P<0.001) but increased Oscillibacter (279%; P = 0.004) in fecal microbiota. Correlations were found between abundances of Lactobacillus (r = 0.52; P = 0.013) and Oscillibacter (r = −0.55; P = 0.007) with transepithelial resistance of the proximal colon. HFD increased macrophage infiltration (58%; P = 0.020), TNF-α (2.5-fold, P<0.001) and IL-6 mRNA levels (2.5-fold; P = 0.008) in mesenteric fat. Increased macrophage infiltration in epididymal fat was also observed with HFD feeding (71%; P = 0.006) but neither TNF-α nor IL-6 was altered. Perirenal and subcutaneous adipose tissue showed no signs of inflammation in HFD mice. The current results implicate gut dysfunction, and attendant inflammation of contiguous adipose, as salient features of the metabolic dysregulation of diet-induced obesity.

The epithelial sodium channel (ENaC) plays an important role in transepithelial Na ؉ absorption; hence its function is essential for maintaining Na ؉ and fluid homeostasis and regulating blood pressure. Insulin is one of the hormones that regulates activity of ENaC. In this study, we investigated the contribution of two related protein kinases, Akt (also known as protein kinase B) and the serum-and glucocorticoid-dependent kinase (Sgk), on insulin-induced ENaC activity in Fisher rat thyroid cells expressing ENaC. Overexpression of Akt1 or Sgk1 significantly increased ENaC activity, whereas expression of a dominant-negative construct of Akt1, Akt1 K179M , decreased basal activity of ENaC. Inhibition of the endogenous expression of Akt1 and Sgk1 by short interfering RNA not only inhibited ENaC but also disrupted the stimulatory effect on ENaC of insulin and of the downstream effectors of insulin, phosphatidylinositol 3-kinase and PDK1. Conversely, overexpression of Akt1 or Sgk1 increased expression of ENaC at the cell membrane and overcame the inhibitory effect of Nedd4-2 on ENaC. Furthermore, mutation of consensus phosphorylation sites on Nedd4-2 for Akt1 and Sgk1, Ser 342 and Ser 428 , completely abolished the inhibitory effect of Sgk1 and Akt1 on Nedd4-2 action. Together these data suggest that both Akt and Sgk are components of an insulin signaling pathway that increases Na ؉ absorption by upregulating membrane expression of ENaC via a regulatory system that involves inhibition of Nedd4-2.Transepithelial Na ϩ absorption across the kidney distal collecting duct, the distal colon, and the ducts of the exocrine glands occurs via the epithelial sodium channels (ENaC) 4 and is tightly regulated by hormones such as aldosterone, insulin, and arginine vasopressin. These hormones play an important role in Na ϩ and fluid homeostasis, and their activities are critical in the regulation of blood pressure. Comprehensive knowledge of the signal transduction pathways that underlie the effect of these hormones on ENaC is therefore essential for our understanding of the development of homeostatic abnormalities such as essential hypertension.Insulin is a peptide hormone that manifests a stimulatory effect on Na ϩ transport in a variety of epithelia (1-5). The immediate natriferic effect of insulin is attributed to an increase in the open probability of ENaC (5, 6) or an increase in the number of active ENaC at the apical membrane (4, 7). Acting through insulin receptor substrate-1, insulin activates phosphatidylinositol 3-kinase (PI3K), a heterodimeric enzyme that catalyzes the formation of phosphatidylinositol 3,4,5-trisphosphate. This activation is essential for mediating several insulin responses, and PI3K has been identified as integral for mediating the effect of insulin on ENaC (8). For instance, insulin treatment increases colocalization of PI3K with ENaC, thereby promoting translocation of ENaC to the apical membrane (4), whereas inhibition of PI3K by LY294002 prevents insulin-induced translocation of the channel (4) an...

Nedd4 and Nedd4-2 are ubiquitin-protein ligases known to regulate a number of membrane proteins including receptors and ion transporters. Regulation of the epithelial Na ؉ channel by Nedd4 and Nedd4-2 is mediated via interactions between the PY motifs of the epithelial sodium channel subunits and the Nedd4/Nedd4-2 WW domains. This example serves as a model for the regulation of other PY motif-containing ion channels by Nedd4 and Nedd4-2. We found that the carboxyl termini of the six voltage-gated Na ؉ (Na v ) channels contain typical PY motifs (PPXY), and a further Na v contains a PY motif variant (LPXY). Not only did we demonstrate by Far-Western analysis that Nedd4 and Nedd4-2 interact with the PY motif-containing Na v channels, but we also showed that these channels have conserved WW domain binding specificity. We further showed that the carboxyl termini fusion proteins of one central nervous system and one peripheral nervous system-derived Na ؉ channel (Na v 1.2 and Na v 1.7, respectively) are readily ubiquitinated by Nedd4-2. In Xenopus oocytes, Nedd4-2 strongly inhibited the activities of all three Na v s (Na v 1.2, Na v 1.7, and Na v 1.8) tested. Interestingly, Nedd4 suppressed the activity of Na v 1.2 and Na v 1.7 but was a poor inhibitor of Na v 1.8. Our results provide evidence that Nedd4 and Nedd4-2 are likely to be key regulators of specific neuronal Na v channels in vivo.