A novel SLC26A9 inhibitor, S9-A13, reveals a minor contribution of
SLC26A9 to constitutive airway chloride secretion
Background and purpose: The solute carrier 26 family member A9 (SLC26A9)
is an epithelial anion transporter that is assumed to contribute to
basal airway chloride conductance and airway surface hydration. It is a
genetic modifier in cystic fibrosis and therefore may serve as a
possible alternative Cl- secretory pathway in airways. Whether SLC26A9
or CFTR is responsible for airway Cl- transport under basal conditions,
is still unclear, due to the lack of a specific inhibitor for SLC26A9.
Experimental approach: In the present study we report a novel potent and
specific inhibitor for SLC26A9, which was identified by screening of a
drug-like small molecule library and subsequent chemical modifications.
Key results: The most potent compound S9-A13 inhibited SLC26A9 with an
IC50 of 90.9 13.4 nM in YFP fluorescence quenching assay. Notably,
S9-A13 did not inhibit other members of the SLC26-family and showed
little effects on other Cl- channels such as CFTR, TMEM16A, and VRAC.
Although S9-A13 clearly inhibited SLC26A9 when overexpressed in HEK293
cells, it showed little inhibitory effects on ion transport in highly
differentiated human airway epithelial cells or mouse trachea under
basal conditions, despite clear apical expression of SLC26A9 in ciliated
epithelial cells. Conclusions and Implications: Using the potent
inhibitor S9-A13, the present data demonstrate the lack of activity of
SLC26A9 in nonstimulated airways. Because on the contrary, the
CFTR-inhibitor CFTRinh-172 clearly inhibited transport in non-stimulated
airways, we conclude that CFTR rather than SLC26A9 is in charge of
constitutive Cl- transport in airways.