Separating the contributions of SLC26A9 and CFTR to anion secretion in primary human bronchial epithelia (HBE)
Aberrant anion secretion across the bronchial epithelium is associated with airway disease, most notably in cystic fibrosis. While the cystic fibrosis transmembrane conductance regulator (CFTR) is recognized as the primary source of airway anion secretion, alternative anion transport mechanisms play a contributing role. An alternative anion transporter of growing interest is SLC26A9, a constitutively active chloride channel which has been shown to interact with CFTR and may also contribute to bicarbonate secretion. Interest in SLC26A9 has been fueled by genome-wide association studies which suggest it is a significant modifier of CF disease severity. In spite of this growing evidence that SLC26A9 plays an important role in the airway, its presence and function in bronchial epithelia remains poorly understood, in part because its activity is difficult to separate from the activity of CFTR. Here we present results using primary, human bronchial epithelia (HBE) from multiple patient sources to confirm that SLC26A9 mRNA is present in HBE, and that its constitutive channel activity is unaffected by knock down of CFTR. Furthermore, SLC26A9 and CFTR show differential responses to common inhibitors of anion secretion. Finally, we assess the impact of bicarbonate on the activity of SLC26A9 and CFTR. These results confirm that SLC26A9 is the primary source of constitutive anion secretion across HBE, and should inform future studies focused on activation of SLC26A9 as an alternative anion channel in CF. These results should provide a strong foundation to investigate how single nucleotide polymorphisms in SLC26A9 modulate airway disease.