Subcellular dynamics and functional activity of the cleaved intracellular domain of the Na+ channel beta 1 subunit

The voltage-gated Na+ channel beta 1 subunit, encoded by SCN1B, regulates cell surface expression and gating of alpha subunits and participates in cell adhesion. beta 1 is cleaved by alpha/beta and gamma-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 beta 1 function in breast cancer cells. Using a series of GFP-tagged beta 1 constructs, we show that beta 1-GFP is mainly retained intracellularly, particularly in the endoplasmic reticulum and endolysosomal pathway, and accumulates in the nucleus. Reduction in endosomal beta 1-GFP levels occurred following gamma-secretase inhibition, implicating endosomes and/or the preceding plasma membrane as important sites for secretase processing. Using live-cell imaging, we also report beta 1ICD-GFP accumulation in the nucleus. Furthermore, beta 1-GFP and beta 1ICD-GFP both increased Na+ current, whereas beta 1STOP-GFP, which lacks the ICD, did not, thus highlighting that the beta 1-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 Na(v)1.5), the Na+ current increased by beta 1-GFP or beta 1ICD-GFP was TTX-sensitive. Finally, we found beta 1-GFP increased mRNA levels of the TTX-sensitive a subunits SCN1A/Na(v)1.1 and SCN9A/Na(v)1.7. Taken together, this work suggests that the beta 1-ICD is a critical regulator of alpha subunit function in cancer cells. Our data further highlight that gamma-secretase may play a key role in regulating beta 1 function in breast cancer.