The NALCN channel complex is voltage sensitive and directly modulated by extracellular calcium

Abstract

The sodium leak channel (NALCN) is essential for survival in mammals: NALCN mutations are life-threatening in humans and knockout is lethal in mice. However, the basic functional and pharmacological properties of NALCN have remained elusive. Here, we found that robust function of NALCN in heterologous systems requires co-expression of UNC79, UNC80, and FAM155A. The resulting NALCN channel complex is constitutively active and conducts monovalent cations but is blocked by physiological concentrations of extracellular divalent cations. Our data support the notion that NALCN is directly responsible for the increased excitability observed in a variety of neurons in reduced extracellular Ca2+. Despite the smaller number of voltage-sensing residues in NALCN, the constitutive activity is modulated by voltage, suggesting that voltage-sensing domains can give rise to a broader range of gating phenotypes than previously anticipated. Our work points toward formerly unknown contributions of NALCN to neuronal excitability and opens avenues for pharmacological targeting.

Structure of human sodium leak channel NALCN in complex with FAM155A

“…1e). These ndings are distinct from other recent studies 40,41 . Moreover, we observed no inactivation of the steady state current by setting the prepulse potential to different levels when NALCN channel was co-expressed with auxiliary subunits ( Supplementary Fig.…”

“…These observations indicate that the auxiliary subunits are essential for a functional NALCN ( Supplementary Fig. 1a), consistent with previous studies 30,31 . The electrophysiological properties of NALCN suggested that it tends to maintain the RMP in a manner reminiscent the Le Chatelier's principle in chemical equilibria.…”

“…Therefore, FAM155B may also be able to form a stable complex with NALCN in vivo. This notion is supported by the nding that FAM155A can be functionally substituted by human FAM155B 30 .…”

“…Other studies suggested that the VSDs of NALCN can exhibit a broader range of gating behaviors. Co-expression of NALCN, UNC79, UNC80, and FAM155A resulted in voltage dependent NALCN current 30,31 . Finally, NALCN was also reported to be only selective for monovalent cations and to be blocked by extracellular divalent cations 30 .…”

“…Despite its physiological and pathological importance, the biophysical properties of the voltage sensitivity and ion selectivity of NALCN are still under debate. Electrophysiological studies on NALCN have been performed in heterologous expression systems such as HEK293 cells, X. laevis oocytes and the neuronal cell line NG108-15 5,30,31 . NALCN expressed in HEK293 cells was reported to exhibit a linear current-voltage relationship, suggesting that NALCN is a voltage-independent ion channel.…”