Putting bicarbonate on the spot: pharmacological insights for CFTR correction in the airway epithelium

Introduction: Cystic fibrosis (CF) is caused by defective Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) proteins. CFTR controls chloride (Cl-) and bicarbonate (HCO3 -) transport into the Airway Surface Liquid (ASL). We investigated the impact of F508del-CFTR correction on HCO3 - secretion by studying transepithelial HCO3 - fluxes. Methods: HCO3 - secretion was measured by pH-stat technique in primary human respiratory epithelial cells from healthy subjects (WT) and people with CF (pwCF) carrying at least one F508del variant. Its changes after CFTR modulation by the triple combination VX445/661/770 and in the context of TNF-α+IL-17 induced inflammation were correlated to ASL pH and transcriptional levels of CFTR and other HCO3 - transporters of airway epithelia such as SLC26A4 (Pendrin), SLC26A9 and NBCe1. Results: CFTR-mediated HCO3 - secretion was not detected in F508del primary human respiratory epithelial cells. It was rescued up to ∼ 80% of the WT level by VX-445/661/770. In contrast, TNF-α+IL-17 normalized transepithelial HCO3 - transport and increased ASL pH. This was related to an increase in SLC26A4 and CFTR transcript levels. VX-445/661/770 induced an increase in pH only in the context of inflammation. Effects on HCO3 - transport were not different between F508del homozygous and F508del compound heterozygous CF airway epithelia. Conclusion: Our studies show that correction of F508del-CFTR HCO3 - is not sufficient to buffer acidic ASL and inflammation is a key regulator of HCO3 - secretion in CF airways. Prediction of the response to CFTR modulators by theratyping should take into account airway inflammation.

Airway environment drives the selection of quorum sensing mutants and promote Staphylococcus aureus chronic lifestyle

Staphylococcus aureus is a predominant cause of chronic lung infections. While the airway environment is rich in highly sialylated mucins, the interaction of S. aureus with sialic acid is poorly characterized. Using S. aureus USA300 as well as clinical isolates, we demonstrate that quorum-sensing dysfunction, a hallmark of S. aureus adaptation, correlates with a greater ability to consume free sialic acid, providing a growth advantage in an air-liquid interface model and in vivo. Furthermore, RNA-seq experiment reveals that free sialic acid triggers transcriptional reprogramming promoting S. aureus chronic lifestyle. To support the clinical relevance of our results, we show the co-occurrence of S. aureus, sialidase-producing microbiota and free sialic acid in the airway of patients with cystic fibrosis. Our findings suggest a dual role for sialic acid in S. aureus airway infection, triggering virulence reprogramming and driving S. aureus adaptive strategies through the selection of quorum-sensing dysfunctional strains.

Nonsense mutations accelerate lung disease and decrease survival of cystic fibrosis children

RATIONALE

Limited information is available on the clinical status of people with Cystic Fibrosis (pwCF) carrying 2 nonsense mutations (PTC/PTC). The main objective of this study was to compare disease severity between pwCF PTC/PTC, compound heterozygous for F508del and PTC (F508del/PTC) and homozygous for F508del (F508del+/+).

METHODS

Based on the European CF Society Patient Registry clinical data of pwCF living in high and middle income European and neighboring countries, PTC/PTC (n = 657) were compared with F508del+/+ (n = 21,317) and F508del/PTC(n = 4254).CFTR mRNA and protein activity levels were assessed in primary human nasal epithelial (HNE) cells sampled from 22 PTC/PTC pwCF.

MAIN RESULTS

As compared to F508del+/+ pwCF; both PTC/PTC and F508del/PTC pwCF exhibited a significantly faster rate of decline in Forced Expiratory Volume in 1 s (FEV1) from 7 years (-1.33 for F508del +/+, -1.59 for F508del/PTC; -1.65 for PTC/PTC, p < 0.001) until respectively 30 years (-1.05 for F508del +/+, -1.23 for PTC/PTC, p = 0.048) and 27 years (-1.12 for F508del +/+, -1.26 for F508del/PTC, p = 0.034). This resulted in lower FEV1 values in adulthood. Mortality of pediatric pwCF with one or two PTC alleles was significantly higher than their F508del homozygous pairs. Infection with Pseudomonas aeruginosa was more frequent in PTC/PTC versus F508del+/+ and F508del/PTC pwCF. CFTR activity in PTC/PTC pwCF's HNE cells ranged between 0% to 3% of the wild-type level.

CONCLUSIONS

Nonsense mutations decrease the survival and accelerate the course of respiratory disease in children and adolescents with Cystic Fibrosis.

Lung-adapted Staphylococcus aureus isolates with dysfunctional agr system trigger a proinflammatory response

BACKGROUND

Staphylococcus aureus (Sa) dominates the lung microbiota of Cystic Fibrosis (CF) children and persistent clones are able to establish chronic infection for years, having a direct deleterious impact on lung function. However, in this context, the exact contribution of Sa to the decline in respiratory function in CF children is not elucidated.

METHODS

To investigate the contribution of persistent S. aureus clones in CF disease, we undertook the analysis of sequential isogenic isolates recovered from 15 young CF patients.

RESULTS

Using an Air-Liquid infection model, we observed a strong correlation between Sa adaption in the lung (late isolates), low toxicity and pro-inflammatory cytokine secretion. Conversely, early isolates appeared to be highly cytotoxic but did not promote cytokine secretion. We found that cytokine secretion was dependent on Staphylococcal protein A (Spa), which was selectively expressed in late compared to early isolates as a consequence of dysfunctional agr quorum-sensing system. Finally, we demonstrated the involvement of TNF-α receptor 1 signaling in the inflammatory response of airway epithelial cells to these lung-adapted Sa isolates.

CONCLUSION

Our results suggest an unexpected direct role of bacterial lung adaptation in the progression of chronic lung disease by promoting a pro-inflammatory response through acquired agr dysfunction.

Inflammation biomarkers in sputum for clinical trials in cystic fibrosis: current understanding and gaps in knowledge

RATIONALE

Sputum biomarkers hold promise as a direct measure of inflammation within the cystic fibrosis (CF) lung, but variability in study design and sampling methodology have limited their use. A full evaluation of the reliability, validity and clinical relevance of individual biomarkers is required to optimise their use within CF clinical research.

OBJECTIVES

A biomarker Special Interest Working Group was established within the European Cystic Fibrosis Society-Clinical Trials Network Standardisation Committee, to perform a review of the evidence regarding sputum biomarkers in CF.

METHODS

From the 139 included articles, we identified 71 sputum biomarkers to undergo evaluation of their clinimetric properties, responsiveness, discriminant, concurrent and convergent validity.

RESULTS

Current evidence confirms the potential of sputum biomarkers as outcome measures in clinical trials. Inconsistency in responsiveness, concurrent and convergent validity require further research into these markers and processing standardisation before translation into wider use. Of the 71 biomarkers identified, Neutrophil Elastase (NE), IL-8, TNF-α and IL-1β, demonstrated validity and responsiveness to be currently considered for use in clinical trials. Other biomarkers show future promise, including IL-6, calprotectin, HMGB-1 and YKL-40.

CONCLUSION

A concerted international effort across the cystic fibrosis community is needed to promote high quality biomarker trial design, establish large population-based biomarker studies, and work together to create standards for collection, storage and analysis of sputum biomarkers.

New insights into structure and function of bis-phosphinic acid derivatives and implications for CFTR modulation

C407 is a compound that corrects the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) protein carrying the p.Phe508del (F508del) mutation. We investigated the corrector effect of c407 and its derivatives on F508del-CFTR protein. Molecular docking and dynamics simulations combined with site-directed mutagenesis suggested that c407 stabilizes the F508del-Nucleotide Binding Domain 1 (NBD1) during the co-translational folding process by occupying the position of the p.Phe1068 side chain located at the fourth intracellular loop (ICL4). After CFTR domains assembly, c407 occupies the position of the missing p.Phe508 side chain. C407 alone or in combination with the F508del-CFTR corrector VX-809, increased CFTR activity in cell lines but not in primary respiratory cells carrying the F508del mutation. A structure-based approach resulted in the synthesis of an extended c407 analog G1, designed to improve the interaction with ICL4. G1 significantly increased CFTR activity and response to VX-809 in primary nasal cells of F508del homozygous patients. Our data demonstrate that in-silico optimized c407 derivative G1 acts by a mechanism different from the reference VX-809 corrector and provide insights into its possible molecular mode of action. These results pave the way for novel strategies aiming to optimize the flawed ICL4-NBD1 interface.