Conditional epithelial sodium channel (ENaC) over-expression in C57Bl6 mouse lung triggers innate immune responses via activation of high mobility group box 1 (HMGB1) signaling
Authors: Zhang Mingyang A. , Zimmerman Elizabeth , Paine Robert , Helms My N. 
Published: May 2024

Background: One of the more sinister examples of a lung disease borne by chronic inflammation is cystic fibrosis (CF). Inherited mutations in the CFTR protein hinders the secretion of Cl−, depolarizing the membrane potential and enabling excessive Na+ flux across epithelial sodium channels (ENaCs). The mechanism by which these electrochemical imbalances contribute to chronic inflammation are unclear. Based on our preliminary observations, we hypothesize that hyperactive ENaC dysfunction plays a pivotal role in initiating sterile inflammation via activation of HMGB1 cytokine activity. Our research group has shown that high HMGB1 levels in CF sputum predicts incidence and recurrence of future acute pulmonary exacerbation. Methods: In support of this new line of investigation, we generated a conditional lung-ENaC overexpression model ( Dox-Scnn1b), which uniquely allows us to control the timing (and thus severity) of ENaC hyperactive dysfunction in C57Bl6 mice. Bronchoalveolar lavage fluid (BALF) and lung tissue samples were collected from WT and Dox-Scnn1b mice (± 625 mg/kg dox chow) in order to evaluate changes in transcript, protein, immune cell differentials, and lung injury following conditional lung ENaC overexpression. Student’s t-tests; ANOVA followed by post-hoc testing; and multivariate linear regression analysis (SigmaPlot 14.0 Systat Software) were used to evaluate statistical significance and determine whether ENaC feedforward signaling to HMGB1 effects pro-inflammatory signaling in lung epithelia. Results: qRT-PCR indicates that high dox chow (625 mg/kg) increased Scnn1b gene expression (encoding for β-ENaC subunit responsible for hyperactive channel dysfunction) by three-log fold over control fed mice, (n=6; p=0.01). Indeed, increase in Scnn1b transcript corresponded with significant increase in β-ENaC protein as detected by western blot analysis and amiloride sensitive current measurements. Importantly, Scnn1b overexpression correlated positively with increased HMGB1 protein in the lung (by 1.5 fold, p=.01 vs control; n= 6 from 2 independent studies) and % neutrophil migration. Dox induction of ENaC and HMGB1 also resulted in increased lung injury scores (from 0.24±0.1 to 0.45±0.06, n=3; p<.01). Conclusions: Conditional lung ENaC overexpression mediates HMGB1 cytokine activity and closely recapitulates the sterile lung injury phenotype observed in CF patients. Significance: The mechanisms identified through this study can be targeted for new drug therapy, which is highly significant, since there are no effective therapies currently available for reversing lung inflammation. Funding: T35DK103596 (MZ) and R01HL137033 (MH). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.