Functional expression of mitochondrial KCa3.1 channels in non-small cell lung cancer cells

Lung cancer is one of the leading causes of cancer-related deaths worldwide. The Ca²⁺-activated K⁺ channel K_Ca3.1 contributes to the progression of non-small cell lung cancer (NSCLC). Recently, K_Ca3.1 channels were found in the inner membrane of mitochondria in different cancer cells. Mitochondria are the main sources for the generation of reactive oxygen species (ROS) that affect the progression of cancer cells. Here, we combined Western blotting, immunofluorescence, and fluorescent live-cell imaging to investigate the expression and function of K_Ca3.1 channels in the mitochondria of NSCLC cells. Western blotting revealed K_Ca3.1 expression in mitochondrial lysates from different NSCLC cells. Using immunofluorescence, we demonstrate a co-localization of K_Ca3.1 channels with mitochondria of NSCLC cells. Measurements of the mitochondrial membrane potential with TMRM reveal a hyperpolarization following the inhibition of K_Ca3.1 channels with the cell-permeable blocker senicapoc. This is not the case when cells are treated with the cell-impermeable peptidic toxin maurotoxin. The hyperpolarization of the mitochondrial membrane potential is accompanied by an increased generation of ROS in NSCLC cells. Collectively, our results provide firm evidence for the functional expression of K_Ca3.1 channels in the inner membrane of mitochondria of NSCLC cells.

Co-staining of KCa 3.1 Channels in NSCLC Cells with a Small-Molecule Fluorescent Probe and Antibody-Based Indirect Immunofluorescence

The Ca2+ activated potassium channel 3.1 (KCa 3.1) is involved in critical steps of the metastatic cascade, such as proliferation, migration, invasion and extravasation. Therefore, a fast and efficient protocol for imaging of KCa 3.1 channels was envisaged. The novel fluorescently labeled small molecule imaging probes 1 and 2 were synthesized by connecting a dimethylpyrrole-based BODIPY dye with a derivative of the KCa 3.1 channel inhibitor senicapoc via linkers of different length. Patch-clamp experiments revealed the inhibition of KCa 3.1 channels by the probes confirming interaction with the channel. Both probes 1 and 2 were able to stain KCa 3.1 channels in non-small-cell lung cancer (NSCLC) cells following a simple, fast and efficient protocol. Pre-incubation with unlabeled senicapoc removed the punctate staining pattern showing the specificity of the new probes 1 and 2. Staining of the channel with the fluorescently labeled senicapoc derivatives 1 or 2 or with antibody-based indirect immunofluorescence yielded identical or very similar densities of stained KCa 3.1 channels. However, co-staining using both methods did not lead to the expected overlapping punctate staining pattern. This observation was explained by docking studies showing that the antibody used for indirect immunofluorescence and the probes 1 and 2 label different channel populations. Whereas the antibody binds at the closed channel conformation, the probes 1 and 2 bind within the open channel.

Synthesis of Small-Molecule Fluorescent Probes for the In Vitro Imaging of Calcium-Activated Potassium Channel KCa 3.1

Small-molecule probes for the in vitro imaging of KCa 3.1 channel-expressing cells were developed. Senicapoc, showing high affinity and selectivity for the KCa 3.1 channels, was chosen as the targeting component. BODIPY dyes 15-20 were synthesized and connected by a CuI -catalyzed azide-alkyne [3+2]cycloaddition with propargyl ether senicapoc derivative 8, yielding fluorescently labeled ligands 21-26. The dimethylpyrrole-based imaging probes 25 and 26 allow staining of KCa 3.1 channels in NSCLC cells. The specificity was shown by removing the punctate staining pattern by pre-incubation with senicapoc. The density of KCa 3.1 channels detected with 25 and by immunostaining was identical. The punctate structure of the labeled channels could also be observed in living cells. Molecular modeling showed binding of the senicapoc-targeting component towards the binding site within the ion channel and orientation of the linker with the dye along the inner surface of the ion channel.