Abstract
Depolarization of the myometrial smooth muscle cell (MSMC) resting membrane potential is necessary for the transition of the uterus from a quiescent state to a contractile state. The molecular mechanisms involved in this transition are not completely understood. Here, we report a novel coupled system between the Na+-activated K+ channel (SLO2.1) and the non-selective Na+ leak channel (NALCN) which determines the MSMC membrane potential. We show that SLO2.1 currents are activated by an inward Na+ leak current carried by the NALCN channel leading to MSMC hyperpolarization. These results show an unanticipated role for the Na+ leak currents in activating a negative feedback system countering the excitable effects of Na+ currents. This is a novel role for the NALCN channel in which Na+ acts as an intracellular signaling molecule. In fact, we report here that the net effect of Na+ entry through NALCN channels is a hyperpolarization of the MSMCs plasma membrane because of the activation of SLO2.1 K channel. Importantly, we also report that a decrease in NALCN/SLO2.1 activity triggers both Ca2+ entries through VDCCs, promoting myometrial contraction. Consistently, with a functional coupling, our data show that NALCN and SLO2.1 are in proximity to one another in human MSMCs. We propose that the spatial arrangement of SLO2.1 and NALCN permits these channels to functionally interact in order to regulate human MSMC membrane potential and cell excitability to modulate uterine contractile activity.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Previously, due to the lack of specific inhibitors for NALCN and the lack of success of several techniques (siRNA, CRISPR, etc) to knock down NALCN channels in the human cells. We could not isolate the particular contribution of NALCN to the proposed model. But in May 2020 a new manuscript bringing to light new NALCN channel inhibitors was published by Suyun Hahn and collaborators. Our new updated version of the manuscript entitled: SLO2.1 and NALCN form a functional complex to modulate myometrial cell excitability, includes new data with these specific inhibitors (Particularly CP96345) (Figure 1 F-J) elucidated specifically the involvement of NALCN in the activation of Na+ activated K+ currents (SLO2.1). Supplemental figure 2 shows the inhibition by CP96345 of the endogenous Na+ leak currents in human myometrial cells, and Suppl. figure 3 D-F shows that CP96345 has no direct effects over the SLO2.1 current. This new data allowed us to investigate clearly the involvement of NALCN in the proposed model and strengthen our conclusions.
