Co-expression of β Subunits with the Voltage-Gated Sodium Channel Na1.7: the Importance of Subunit Association and Phosphorylation and Their Effects on Channel Pharmacology and Biophysics.

The voltage-gated sodium ion channel Na1.7 is crucial in pain signaling. We examined how auxiliary β2 and β3 subunits and the phosphorylation state of the channel influence its biophysical properties and pharmacology. The human Na1.7α subunit was co-expressed with either β2 or β3 subunits in HEK-293 cells. The β2 subunits and the Na1.7α, however, were barely associated as evidenced by immunoprecipitation. Therefore, the β2 subunits did not change the biophysical properties of the channel. In contrast, β3 subunit was clearly associated with Na1.7α. This subunit had a significant degree of glycosylation, and only the fully glycosylated β3 subunit was associated with the Na1.7α. Electrophysiological characterisation revealed that the β3 subunit had small but consistent effects: a right-hand shift of the steady-state inactivation and faster recovery from inactivation. Furthermore, the β3 subunit reduced the susceptibility of Na1.7α to several sodium channel blockers. In addition, we assessed the functional effect of Na1.7α phosphorylation. Inhibition of kinase activity increased channel inactivation, while the blocking phosphatases produced the opposite effect. In conclusion, co-expression of β subunits with Na1.7α, to better mimic the native channel properties, may be ineffective in cases when subunits are not associated, as shown in our experiments with β2. The β3 subunit significantly influences the function of Na1.7α and, together with the phosphorylation of the channel, regulates its biophysical and pharmacological properties. These are important findings to take into account when considering the role of Na1.7 channel in pain signaling.