Sodium leak channels in neuronal excitability and rhythmic behaviors

Abstract

Extracellular K⁺, Na⁺, and Ca²⁺ ions all influence the resting membrane potential of the neuron. However, the mechanisms by which extracellular Na⁺ and Ca²⁺ regulate basal neuronal excitability are not well understood. Recent findings suggest that NALCN, in association with UNC79 and UNC80, contributes a basal Na⁺ leak conductance in neurons. Mutations in Nalcn, Unc79, or Unc80 lead to severe phenotypes that include neonatal lethality and disruption in rhythmic behaviors. This review discusses the properties of the NALCN complex, its regulation, and its contribution to neuronal function and animal behavior.

Figure 1. Ion channel families sharing high sequence similarities with Na_Vs and Ca_Vs

The 24-TM (four-repeat, I, II, III & IV) channel superfamily (upper) can be divided into three branches: NALCN (voltage-independent, non-selective, one member in mammals), Ca_Vs (voltage-gated, Ca²⁺ selective, 10 members) and Na⁺-selective channels (9 Na_Vs and one Na_x). Three families of 6-TM channels (lower) also have the T/S-x-E/D-x-W pore motif (see Fig. 2) and have high sequence similarity with the 24-TM channels. They are the alkalinization-activated CatSper Ca²⁺ channels (CatSper1-4), bacterial voltage-activated Na⁺ channels (Na_vBac) and putative bacterial voltage-activated Ca²⁺ channels Ca_vBac (see Fig. 2.)

Figure 2. Illustrative current-voltage relationships of the voltage-independent NALCN channel in comparison with those of voltage-gated channels Na_vs and Ca_vs

Figure 3. Unique sequences of NALCN in the S4 and the pore filter regions

(A) Alignment of the IVS4 regions between human NALCN, representative Ca_V (Ca_v1.1) and Na_V (Na_v1.1) channels. Charged residues (K/R) are colored. (B) Alignment of the two helices (P, P2) in the pore filter regions of representative 24-TM, four-repeat (I, II, III, IV) channels (NALCN, Ca_v and Na_v) and 6-TM, single repeat channels (CatSper, CavBac, NavAB). The T/SxE/DxW pore signature is boxed. NavAB is the only one whose structure has been determined at high resolution (Payandeh et al., 2011). The CavBac sequence is from psychrophilic bacteria Colwellia psychrerythraea (strain 34H, accession # AAR26732). Its sequences in the P2 helix region (FEDWTD) predict that it’s a Ca²⁺ channel based on mutagenesis study in NaChBac (Yue et al., 2002). Many bacteria strains have been found to have CavBac proteins, but they have not been electrophysiologically characterized.

Figure 4. A Model illustrating the G protein –dependent and G protein–independent activation of NALCN by GPCRs

The NALCN complex in the brain consists of NALCN, UNC80 and UNC79. UNC80 directly associates with NALCN and UNC79 forms part of the complex by its interaction with UNC80. There is a tonic suppression of I_NALCN by activated CaSR at normal [Ca²⁺]_e level, which involves G-proteins, UNC80, the last amino acids of NALCN and perhaps those of CaSR. Lowering [Ca²⁺]_e releases the suppression, increases I_NALCN, generates a low [Ca²⁺]_e-activated inward current (I_LCA) and excites neurons. NALCN can also be activated by substance P (SP) receptor TACR1 in a G protein-independent manner that requires the Src family of kinases (SFKs) and UNC80. The interplay between the positive regulation of NALCN through SFKs and the negative control via G proteins may determines the basal levels of Na⁺ leak in neurons (modified from (Lu et al., 2010)).

Figure 5. Synergism between the G protein –dependent and –independent activation of NALCN

A decrease in [Ca²⁺]_e from 2 mM to 0.1 mM or an application of substance P (1 µM) each activates a small NALCN-dependent inward current in a hippocampal neuron. Simultaneous application of both the stimuli generates a large synergistic excitatory action (modified from (Lu et al., 2010)).