Characterization of Endogenous Sodium Channels in the ND7-23 Neuroblastoma Cell Line: Implications for Use as a Heterologous Ion Channel Expression System Suitable for Automated Patch Clamp Screening

The rodent neuroblastoma cell line, ND7-23, is used to express voltage-dependent sodium (Na_v) and other neuronal ion channels resistant to heterologous expression in Chinese hamster ovary (CHO) or human embryonic kidney (HEK) cells. Their advantage is that they provide endogenous factors and signaling pathways to promote ion channel peptide folding, expression, and function at the cell surface and are also amenable to automated patch clamping. However, ND7-23 cells exhibit endogenous tetrodotoxin (TTX)-sensitive Na_v currents, and molecular profiling has revealed the presence of Na_v1.2, Na_v1.3, Na_v1.6, and Na_v1.7 transcripts, but no study has determined which subtypes contribute to functional channels at the cell surface. We profiled the repertoire of functional Nav channels endogenously expressed in ND7-23 cells using the QPatch automated patch clamp platform and selective toxins and small molecules. The potency and subtype selectivity of the ligands (Icagen compound 68 from patent US-20060025415-A1-20060202, 4,9 anhydro TTX, and Protoxin-II) were established in human Na_v1.3, Na_v1.6, and Na_v1.7 channel cell lines before application of selective concentrations to ND7-23 cells. Our data confirm previous studies that >97% of macroscopic Na_v current in ND7-23 cells is carried by TTX-sensitive channels (300 nM TTX) and that Na_v1.7 is the predominant channel contributing to this response (65% of peak inward current), followed by Nav1.6 (∼20%) and negligible Na_v1.3 currents (∼2%). In addition, our data are the first to assess the Na_v1.6 potency (50% inhibitory concentration [IC₅₀] of 33 nM) and selectivity (50-fold over Na_v1.7) of 4,9 anhydro TTX in human Na_v channels expressed in mammalian cells, confirming previous studies of rodent Na_v channels expressed in oocytes and HEK cells.