We describe the regulated transcriptome of CACNA1G, a human gene for T-type Ca(v)3.1 calcium channels that is subject to extensive alternative RNA splicing. Fifteen sites of transcript variation include 2 alternative 5'-UTR promoter sites, 2 alternative 3'-UTR polyadenylation sites, and 11 sites of alternative splicing within the open reading frame. A survey of 1580 fetal and adult human brain full-length complementary DNAs reveals a family of 30 distinct transcripts, including multiple functional forms that vary in expression with development. Statistical analyses of fetal and adult transcript populations reveal patterns of linkages among intramolecular splice site configurations that change dramatically with development. A shift from nearly independent, biased splicing in fetal transcripts to strongly concerted splicing in adult transcripts suggests progressive activation of multiple "programs" of splicing regulation that reorganize molecular structures in differentiating cells. Patch-clamp studies of nine selected variants help relate splicing regulation to permutations of the gating parameters most likely to modify T-channel physiology in expressing neurons. Gating behavior reflects combinatorial interactions between variable domains so that molecular phenotype depends on ensembles of coselected domains, consistent with the observed emergence of concerted splicing during development. We conclude that the structural gene and networks of splicing regulatory factors define an integrated system for the phenotypic variation of Ca(v)3.1 biophysics during nervous system development.

A subset share characteristics that make them effective as antiarrhythmic drugs, ie, they exhibit high affinity, use-dependent block of Na current (I Na ) at high heart rates. Despite extensive study, there remains uncertainty regarding how observed block relates to specific drug-channel conformations. Several vocabularies have emerged to describe block, which in general, have their basis in kinetic models of Na channel gating and assume preferential binding to one or more states that produce no 1 or altered 2 gating. Recent availability of crystal structures in combination with mutagenesis data now allow for linking electrophysiological data, kinetic states, and drug block to specific channel conformations.It is generally accepted that lidocaine and lidocaine-like drugs bind in the inner pore of voltage-gated Na channels. Scanning mutagenesis studies with various Na channel isoforms and multiple lidocaine-like drugs have identified only one amino acid residue, a phenylalanine (Phe) in domain IV, S6 (DIVS6), which, when mutated, alters use-dependent drug affinity by more than ten-fold. When this Phe (1759 in Na V 1.5) is mutated to nonaromatic residues 3-8 or to unnatural amino acids with different electron withdrawing capabilities 9 the mutated channel shows a marked decrease in high-affinity LA block. Homology modeling with K channels predicts that this Phe faces the pore just below the selectivity filter. 10,11 This orientation of Phe is supported by the finding that its cysteine mutant is accessible to MTS reagents applied from inside the pore when the channel is maintained in an open state. 12 Furthermore, it has been shown by us 13 and others 14 that use-dependent block is intimately associated with altered movements of the structurally distant S4 segments in domains III and IV.Block assayed from negative holding potentials at low rates of stimulation is affected very little by channel mutations in contract to their effects on use-dependent block. This lower affinity block is usually called tonic block, although it has also been called rested-state block (or closed-state block)

Our homology molecular model of the open/inactivated state of the Na + channel pore predicts, based on extensive mutagenesis data, that the local anaesthetic lidocaine docks eccentrically below the selectivity filter, such that physical occlusion is incomplete. Electrostatic field calculations suggest that the drug's positively charged amine produces an electrostatic barrier to permeation. To test the effect of charge at this pore level on permeation in hNa V 1.5 we replaced Phe-1759 of domain IVS6, the putative binding site for lidocaine's alkylamino end, with positively and negatively charged residues as well as the neutral cysteine and alanine. These mutations eliminated use-dependent lidocaine block with no effect on tonic/rested state block. Mutant whole cell currents were kinetically similar to wild type (WT). Single channel conductance (γ) was reduced from WT in both F1759K (by 38%) and F1759R (by 18%). The negatively charged mutant F1759E increased γ by 14%, as expected if the charge effect were electrostatic, although F1759D was like WT. None of the charged mutations affected Na + /K + selectivity. Calculation of difference electrostatic fields in the pore model predicted that lidocaine produced the largest positive electrostatic barrier, followed by lysine and arginine, respectively. Negatively charged glutamate and aspartate both lowered the barrier, with glutamate being more effective. Experimental data were in rank order agreement with the predicted changes in the energy profile. These results demonstrate that permeation rate is sensitive to the inner pore electrostatic field, and they are consistent with creation of an electrostatic barrier to ion permeation by lidocaine's charge. Local anaesthetic (LA) drugs such as lidocaine interfere with impulse conduction in nerve and muscle by binding to the inner pore of voltage-gated Na + channels and blocking current (Hille, 2001). The major drug mechanism of action is not resolved, with experimental evidence variously favouring steric block, stabilization of a closed state, or some combination of the two. Extensive site-directed mutagenesis experiments have provided strong evidence that lidocaine-like drugs (LA) bind in the inner pore. S6 segment residues in domains I, III and IV (but not II) have been shown to be important for use-dependent LA block (Ragsdale et al. 1994;Wright et al. 1998;Yarov-Yarovoy et al. 2001;Yarov-Yarovoy et al. 2002). Two residues in domain IV S6 are of particular importance -Phe-1759 (following the heart Na V 1.5 isoform numbering, corresponding to Phe-1579 in skeletal Na V 1.4 and Phe-1764 in brain Na V 1.2) and Tyr-1766 (Tyr-1586 in Na V 1.4; Tyr-1771 in Na V 1.2), because their alanine mutants exhibit the greatest changes in LA affinity. Open/inactivated state block of the brain isoform Na V 1.2 by etidocaine was reduced by 130-and 35-fold for the alanine substitutions of the phenylalanine and tyrosine, respectively (Ragsdale et al. 1994). Cysteine accessibility experiments with methanethiosulphonate (MTS) reagents confirm th...

Copper cells were originally identified in Drosophila midgut epithelium by their striking orange fluorescence in copper-fed larvae. Here, we examined copper cell fluorescence in light of the previous observations that (1) a similar fluorescent signal in yeast is produced by a complex between copper and metallothionein, and (2) metallothionein is expressed constitutively in the copper cell region and inducibly in other regions of the Drosophila midgut. Pulse-feeding experiments with 1 mM CuCl2 revealed that fluorescence appeared rapidly in copper cells (<5 min) and slowly in other cells of the midgut (days), suggesting a constitutive cofactor in the former and an inducible cofactor in the latter. Fluorescence was also detected in Drosophila S2 tissue culture cells after induction of metallothionein synthesis by addition of CuCl2 to the growth medium. Thus, fluorescence coincided spatially and temporally with the expression of metallothionein. Fluorescence was also linked to the acid-secreting activity of copper cells. Fluorescence was not observed when acid secretion was inhibited by a mutation in the alpha spectrin gene and acidification was blocked in copper-fed wild-type larvae. However, acidification was restored after a 1-day chase period in which the fluorescent signal became sequestered within a vesicular compartment. We therefore conclude that copper cell fluorescence is most probably attributable to a cytoplasmic copper-metallothionein complex, suggesting an unanticipated role for metallothionein in acid-secreting cells.

Verapamil is a potent phenylalkylamine antihypertensive believed to exert its therapeutic effect primarily by blocking highvoltage-activated L-type calcium channels. It was the first clinically used calcium channel blocker and remains in clinical use, although it has been eclipsed by other calcium channel blockers because of its short half-life and interactions with other channels. In addition to blocking L-type channels, it has been reported to block T-type (low-voltage activated) calcium channels. This type of cross-reactivity is likely to be beneficial in the effective control of blood pressure. Although the interactions of T channels with a number of drugs have been described, the mechanisms by which these agents modulate channel activity are largely unknown. Most calcium channel blockers exhibit state-dependence (i.e., preferential binding to certain channel conformations), but little is known about state-dependent verapamil block of T channels. We stably expressed human Ca v 3.1 T-type channels in human embryonic kidney 293 cells and studied the state-dependence of the drug with macroscopic and gating currents. Verapamil blocked currents at micromolar concentrations at polarized potentials similar to those reported for L-type channels, although unlike for L-type currents, it did not affect current time course. The drug exhibited use-dependence and significantly slowed the apparent recovery from inactivation. Current inhibition was dependent on potential. This dependence was restricted to negative potentials, although all data were consistent with verapamil binding in the pore. Gating currents were unaffected by verapamil. We propose that verapamil achieves its inhibitory effect via occlusion of the channel pore associated with an open/inactivated conformation of the channel.