Background: Ion selectivity of voltage-gated channels is governed by selectivity filters. Results: Alternative turret region in domain II promotes highly sodium-permeable T-type channels without major changes to gating and kinetic features. Conclusion: T-type channels can generate variable sodium or calcium permeability by gene splicing. Significance: Ion selectivity in T-type channels can be altered using extracellular domains outside the ion selectivity filter.

Dravet Syndrome is an intractable form of childhood epilepsy associated with deleterious mutations in SCN1A, the gene encoding neuronal sodium channel Nav1.1. Earlier studies using human induced pluripotent stem cells (iPSCs) have produced mixed results regarding the importance of Nav1.1 in human inhibitory versus excitatory neurons. We studied a Nav1.1 mutation (p.S1328P) identified in a pair of twins with Dravet Syndrome and generated iPSC-derived neurons from these patients. Characterization of the mutant channel revealed a decrease in current amplitude and hypersensitivity to steady-state inactivation. We then differentiated Dravet-Syndrome and control iPSCs into telencephalic excitatory neurons or medial ganglionic eminence (MGE)-like inhibitory neurons. Dravet inhibitory neurons showed deficits in sodium currents and action potential firing, which were rescued by a Nav1.1 transgene, whereas Dravet excitatory neurons were normal. Our study identifies biophysical impairments underlying a deleterious Nav1.1 mutation and supports the hypothesis that Dravet Syndrome arises from defective inhibitory neurons.DOI: http://dx.doi.org/10.7554/eLife.13073.001

How nature discriminates sodium from calcium ions in eukaryotic channels has been difficult to resolve because they contain four homologous, but markedly different repeat domains. We glean clues from analyzing the changing pore region in sodium, calcium and NALCN channels, from single-cell eukaryotes to mammals. Alternative splicing in invertebrate homologs provides insights into different structural features underlying calcium and sodium selectivity. NALCN generates alternative ion selectivity with splicing that changes the high field strength (HFS) site at the narrowest level of the hourglass shaped pore where the selectivity filter is located. Alternative splicing creates NALCN isoforms, in which the HFS site has a ring of glutamates contributed by all four repeat domains (EEEE), or three glutamates and a lysine residue in the third (EEKE) or second (EKEE) position. Alternative splicing provides sodium and/or calcium selectivity in T-type channels with extracellular loops between S5 and P-helices (S5P) of different lengths that contain three or five cysteines. All eukaryotic channels have a set of eight core cysteines in extracellular regions, but the T-type channels have an infusion of 4–12 extra cysteines in extracellular regions. The pattern of conservation suggests a possible pairing of long loops in Domains I and III, which are bridged with core cysteines in NALCN, Cav, and Nav channels, and pairing of shorter loops in Domains II and IV in T-type channel through disulfide bonds involving T-type specific cysteines. Extracellular turrets of increasing lengths in potassium channels (Kir2.2, hERG, and K2P1) contribute to a changing landscape above the pore selectivity filter that can limit drug access and serve as an ion pre-filter before ions reach the pore selectivity filter below. Pairing of extended loops likely contributes to the large extracellular appendage as seen in single particle electron cryo-microscopy images of the eel Nav1 channel.

Invertebrate LCa V 3 shares the quintessential features of vertebrate Ca V 3 T-type channels, with a low threshold of channel activation, rapid activation and inactivation kinetics and slow deactivation kinetics compared to other known Ca 2+ channels, the Ca V 1 and Ca V 2 channels. Unlike the vertebrates though, Ca V 3 T-type channels in non-cnidarian invertebrates possess an alternative exon 12 spanning the D2L5 extracellular loop, which alters the invertebrate LCa V 3 channel into a higher Na + and lower Ca 2+ current passing channel, more resembling a classical Na V 1 Na + channel. Cnidarian Ca V 3 T-type channels can possess genes with alternative cysteine-rich, D4L6 extracellular loops in a manner reminiscent of the alternative cysteine-rich, D2L5 extracellular loops of non-cnidarian invertebrates. We illustrate here that the preferences for greater Na + or Ca 2+ ion current passing through Ca V 3 T-type channels are contributed by paired cysteines within D2L5 and D4L6 extracellular loops looming above the pore selectivity filter. Swapping of invertebrate tri-and tetra-cysteine containing extracellular loops, generates higher Na + current passing channels in human Ca V 3.2 channels, while corresponding mono-and di-cysteine loop pairs in human Ca V 3.2 generates greater Ca 2+ current passing, invertebrate LCa V 3 channels. Alanine substitutions of unique D2L5 loop cysteines of LCa V 3 channels increases relative monovalent ion current sizes and increases the potency of Zn 2+ and Ni 2+ block by ~ 50× and ~ 10× in loop cysteine mutated channels respectively, acquiring characteristics of the high affinity block of Ca V 3.2 channels, including the loss of the slowing of inactivation kinetics during Zn 2+ block. Charge neutralization of a ubiquitous aspartate residue of calcium passing Ca V 1, Ca V 2 and Ca V 3 channels, in the outer pore of the selectivity filter residues in Domain II generates higher Na + current passing channels in a manner that may resemble how the unique D2L5 extracellular loops of invertebrate Ca V 3 channels may confer a relatively higher peak current size for Na + ions over Ca 2+ The extracellular loops of Ca V 3 channels are not engaged with accessory subunit binding, as the other Na + (Na V 1) and Ca 2+ (Ca V 1/Ca V 2) channels, enabling diversity and expansion of cysteine-bonded extracellular loops, which appears to serve, amongst other possibilities, to alter to the preferences for passage of Ca 2+ or Na + ions through invertebrate Ca V 3 channels. High field strength site D2L5 Extracellular loop spanning the end of transmembrane segment 5 to the start of the pore selectivity filter (S5-P) in Domain II D4L6 Extracellular loop spanning the end of the pore selectivity filter to the start of transmembrane segment 6 (P-S6

Cav3 T-type channels are low-voltage-gated channels with rapid kinetics that are classified among the calcium-selective Cav1 and Cav2 type channels. Here, we outline the fundamental and unique regulators of T-type channels. An ubiquitous and proximally located "gating brake" works in concert with the voltage-sensor domain and S6 alpha-helical segment from domain II to set the canonical low-threshold and transient gating features of T-type channels. Gene splicing of optional exon 25c (and/or exon 26) in the short III-IV linker provides a developmental switch between modes of activity, such as activating in response to membrane depolarization, to channels requiring hyperpolarization input before being available to activate. Downstream of the gating brake in the I-II linker is a key region for regulating channel expression where alternative splicing patterns correlate with functional diversity of spike patterns, pacemaking rate (especially in the heart), stage of development, and animal size. A small but persistent window conductance depolarizes cells and boosts excitability at rest. T-type channels possess an ion selectivity that can resemble not only the calcium ion exclusive Cav1 and Cav2 channels but also the sodium ion selectivity of Nav1 sodium channels too. Alternative splicing in the extracellular turret of domain II generates highly sodium-permeable channels, which contribute to low-threshold sodium spikes. Cav3 channels are more ubiquitous among multicellular animals and more widespread in tissues than the more brain centric Nav1 sodium channels in invertebrates. Highly sodium-permeant Cav3 channels can functionally replace Nav1 channels in species where they are lacking, such as in Caenorhabditis elegans.