NaV1.6 and NaV1.7 channels are major endogenous voltage-gated sodium channels in ND7/23 cells

ND7/23 cells are gaining traction as a host model to express peripheral sodium channels such as Na(V)1.8 and Na(V)1.9 that have been difficult to express in widely utilized heterologous cells, like CHO and HEK293. Use of ND7/23 as a model cell to characterize the properties of sodium channels requires clear understanding of the endogenous ion channels. To define the nature of the background sodium currents in ND7/23 cells, we aimed to comprehensively profile the voltage-gated sodium channel subunits by endpoint and quantitative reverse transcription-PCR and by whole-cell patch clamp electrophysiology. We found that untransfected ND7/23 cells express endogenous peak sodium currents that average - 2.12nA (n = 15) and with kinetics typical of fast sodium currents having activation and inactivation completed within few milliseconds. Furthermore, sodium currents were reduced to virtually nil upon exposure to 100nM tetrodotoxin, indicating that ND7/23 cells have essentially null background for tetrodotoxin-resistant (TTX-R) currents. qRT-PCR profiling indicated a major expression of TTX-sensitive (TTX-S) Na(V)1.6 and Na(V)1.7 at similar levels and very low expression of TTX-R Na(V)1.9 transcripts. There was no expression of TTX-R Na(V)1.8 in ND7/23 cells. There was low expression of Na(V)1.1, Na(V)1.2, Na(V)1.3 and no expression of cardiac or skeletal muscle sodium channels. As for the sodium channel auxiliary subunits, beta 1 and beta 3 subunits were expressed, but not the beta 2 and beta 4 subunits that covalently associate with the alpha-subunits. In addition, our results also showed that only the mouse forms of Na(V)1.6, Na(V)1.7 and Na(V)1.9 sodium channels were expressed in ND7/23 cells that was originally generated as a hybridoma of rat embryonic DRG and mouse neuroblastoma cell-line. By molecular profiling of auxiliary beta- and principal alpha-subunits of the voltage gated sodium channel complex, our results define the background sodium channels expressed in ND7/23 cells, and confirm their utility for detailed functional studies of emerging pain channelopathies ascribed to mutations of the TTX-R sodium channels of sensory neurons.