Subcellular dynamics and functional activity of the cleaved intracellular domain of the Na+ channel ß1 subunit.

ABSTRACT

The voltage-gated Na+ channel ß1 subunit, encoded by SCN1B, regulates cell surface expression and gating of α subunits and participates in cell adhesion. ß1 is cleaved by α/ß and γ-secretases, releasing an extracellular domain and intracellular domain (ICD), respectively. Abnormal SCN1B expression/function is linked to pathologies including epilepsy, cardiac arrhythmia, and cancer. In this study, we sought to determine the effect of secretase cleavage on ß1 function in breast cancer cells. Using a series of GFP-tagged ß1 constructs, we show that ß1-GFP is mainly retained intracellularly, particularly in the endoplasmic reticulum and endolysosomal pathway, and accumulates in the nucleus. Reduction in endosomal ß1-GFP levels occurred following γ-secretase inhibition, implicating endosomes and/or the preceding plasma membrane as important sites for secretase processing. Using live-cell imaging, we also report ß1ICD-GFP accumulation in the nucleus. Furthermore, ß1-GFP and ß1ICD-GFP both increased Na+ current, whereas ß1STOP-GFP, which lacks the ICD, did not, thus highlighting that the ß1-ICD is necessary and sufficient to increase Na+ current measured at the plasma membrane. Importantly, although the endogenous Na+ current expressed in MDA-MB-231 cells is tetrodotoxin (TTX)-resistant (carried by Nav1.5), the Na+ current increased by ß1-GFP or ß1ICD-GFP was TTX-sensitive. Finally, we found ß1-GFP increased mRNA levels of the TTX-sensitive α subunits SCN1A/Nav1.1 and SCN9A/Nav1.7. Taken together, this work suggests that the ß1-ICD is a critical regulator of α subunit function in cancer cells. Our data further highlight that γ-secretase may play a key role in regulating ß1 function in breast cancer.