NALCN Ion Channels Have Alternative Selectivity Filters Resembling Calcium Channels or Sodium Channels

Abstract

NALCN is a member of the family of ion channels with four homologous, repeat domains that include voltage-gated calcium and sodium channels. NALCN is a highly conserved gene from simple, extant multicellular organisms without nervous systems such as sponges and placozoans and mostly remains a single gene compared to the calcium and sodium channels which diversified into twenty genes in humans. The single NALCN gene has alternatively-spliced exons at exons 15 or exon 31 that splices in novel selectivity filter residues that resemble calcium channels (EEEE) or sodium channels (EKEE or EEKE). NALCN channels with alternative calcium, (EEEE) and sodium, (EKEE or EEKE) -selective pores are conserved in simple bilaterally symmetrical animals like flatworms to non-chordate deuterostomes. The single NALCN gene is limited as a sodium channel with a lysine (K)-containing pore in vertebrates, but originally NALCN was a calcium-like channel, and evolved to operate as both a calcium channel and sodium channel for different roles in many invertebrates. Expression patterns of NALCN-EKEE in pond snail, Lymnaea stagnalis suggest roles for NALCN in secretion, with an abundant expression in brain, and an up-regulation in secretory organs of sexually-mature adults such as albumen gland and prostate. NALCN-EEEE is equally abundant as NALCN-EKEE in snails, but is greater expressed in heart and other muscle tissue, and 50% less expressed in the brain than NALCN-EKEE. Transfected snail NALCN-EEEE and NALCN-EKEE channel isoforms express in HEK-293T cells. We were not able to distinguish potential NALCN currents from background, non-selective leak conductances in HEK293T cells. Native leak currents without expressing NALCN genes in HEK-293T cells are NMDG+ impermeant and blockable with 10 µM Gd3+ ions and are indistinguishable from the hallmark currents ascribed to mammalian NALCN currents expressed in vitro by Lu et al. in Cell. 2007 Apr 20;129(2):371-83.

T-type Channels Become Highly Permeable to Sodium Ions Using an Alternative Extracellular Turret Region (S5-P) Outside the Selectivity Filter

“…4 was carried out as described previously (14,19,20). RNA was extracted from entire juvenile and adult snails, where sexually immature juveniles have shell lengths of 1.0 to 1.5 cm, and reproductioncapable adults have shell lengths of 2.0 to 2.5 cm (21).…”

“…We discovered alternative exons in domain II of NALCN channels that alter the high field strength residue to generate calcium-like (EEEE) or sodium-like (EKEE) pores (of flatworms, mollusks, annelids, echinoderms, and hemichordates) (19,50). Interestingly, some arthropods generate alternative pores in NALCN channels differently, using an alternative exon in domain III to generate calcium-like (EEEE) or sodium-like (EEKE) pores in their NALCN channels (19,50).…”

“…We discovered alternative exons in domain II of NALCN channels that alter the high field strength residue to generate calcium-like (EEEE) or sodium-like (EKEE) pores (of flatworms, mollusks, annelids, echinoderms, and hemichordates) (19,50). Interestingly, some arthropods generate alternative pores in NALCN channels differently, using an alternative exon in domain III to generate calcium-like (EEEE) or sodium-like (EEKE) pores in their NALCN channels (19,50). Finally, a completely unique approach to altering ion selectivity is the use of alternative translational start sites to generate a truncated isoform in K 2P 2.1 channels that are sodium-permeable, to produce offsetting depolarizing currents in a channel type that typically only serves up hyperpolarizing potassium leak currents (39).…”

“…Ca v 3 T-type channels are members of the 4ϫ6TM (4 heterologous domains of 6 trans-membrane segments) family of voltage-gated ion channels, which includes calcium (Ca v ) and sodium (Na v ) channels and sodium leak conductance channel (NALCN) (1)(2)(3). The four heterologous domains of 4ϫ6TM channels are considered to have evolved from two rounds of duplication of ancestral channels with one domain, such as the voltage-gated potassium channels (4).…”

Dopamine Negatively Modulates the NCA Ion Channels inC. elegans

“…The NALCN channel was proposed to be the major contributor to the sodium leak current that helps set the resting membrane potential of neurons [16], though there is controversy whether NALCN is indeed a sodium leak channel [17][18][19]. In humans, mutations in NALCN or its accessory subunit UNC80 have been associated with a number of neurological symptoms, including cognitive and developmental delay [20][21][22][23][24][25][26][27][28][29][30][31][32][33].…”