Role of voltage-gated L-type Ca2+ channel isoforms for brain function.

Voltage-gated LTCCs (L-type Ca2+ channels) are established drug targets for the treatment of cardiovascular diseases. LTCCs are also expressed outside the cardiovascular system. In the brain, LTCCs control synaptic plasticity in neurons, and DHP (dihydropyridine) LTCC blockers such as nifedipine modulate brain function (such as fear memory extinction and depression-like behaviour). Voltage-sensitive Ca2+ channels Cav1 .2 and Cav1.3 are the predominant brain LTCCs. As DHPs and other classes of organic LTCC blockers inhibit both isoforms, their pharmacological distinction is impossible and their individual contributions to defined brain functions remain largely unknown. Here, we summarize our recent experiments with two genetically modified mouse strains, which we generated to explore the individual biophysical features of Cav1.2 and Cav1.3 LTCCs and to determine their relative contributions to various physiological peripheral and neuronal functions. The results described here also allow predictions about the pharmacotherapeutic potential of isoform-selective LTCC modulators.

alpha 1D (Cav1.3) subunits can form l-type Ca2+ channels activating at negative voltages.

In cochlea inner hair cells (IHCs), L-type Ca(2+) channels (LTCCs) formed by alpha1D subunits (D-LTCCs) possess biophysical and pharmacological properties distinct from those of alpha1C containing C-LTCCs. We investigated to which extent these differences are determined by alpha1D itself by analyzing the biophysical and pharmacological properties of cloned human alpha1D splice variants in tsA-201 cells. Variant alpha1D(8A,) containing exon 8A sequence in repeat I, yielded alpha1D protein and L-type currents, whereas no intact protein and currents were observed after expression with exon 8B. In whole cell patch-clamp recordings (charge carrier 15-20 mm Ba(2+)), alpha1D(8A) - mediated currents activated at more negative voltages (activation threshold, -45.7 versus -31.5 mV, p < 0.05) and more rapidly (tau(act) for maximal inward currents 0.8 versus 2.3 ms; p < 0.05) than currents mediated by rabbit alpha1C. Inactivation during depolarizing pulses was slower than for alpha1C (current inactivation after 5-s depolarizations by 90 versus 99%, p < 0.05) but faster than for LTCCs in IHCs. The sensitivity for the dihydropyridine (DHP) L-type channel blocker isradipine was 8.5-fold lower than for alpha1C. Radioligand binding experiments revealed that this was not due to a lower affinity for the DHP binding pocket, suggesting that differences in the voltage-dependence of DHP block account for decreased sensitivity of D-LTCCs. Our experiments show that alpha1D(8A) subunits can form slowly inactivating LTCCs activating at more negative voltages than alpha1C. These properties should allow D-LTCCs to control physiological processes, such as diastolic depolarization in sinoatrial node cells, neurotransmitter release in IHCs and neuronal excitability.

Three new familial hemiplegic migraine mutants affect P/Q-type Ca(2+) channel kinetics.

Missense mutations in the pore-forming human alpha(1A) subunit of neuronal P/Q-type Ca(2+) channels are associated with familial hemiplegic migraine. We studied the functional consequences on P/Q-type Ca(2+) channel function of three recently identified mutations, R583Q, D715E, and V1457L after introduction into rabbit alpha(1A) and expression in Xenopus laevis oocytes. The potential for half-maximal channel activation of Ba(2+) inward currents was shifted by > 9 mV to more negative potentials in all three mutants. The potential for half-maximal channel inactivation was shifted by > 7 mV in the same direction in R583Q and D715E. Biexponential current inactivation during 3-s test pulses was significantly faster in D715E and slower in V1457L than in wild type. Mutations R583Q and V1457L delayed the time course of recovery from channel inactivation. The decrease of peak current through R583Q (30.2%) and D715E (30. 1%) but not V1457L (18.7%) was more pronounced during 1-Hz trains of 15 100-ms pulses than in wild type (18.2%). Our data demonstrate that the mutations R583Q, D715E, and V1457L, like the previously reported mutations T666M, V714A, and I1819L, affect P/Q-type Ca(2+) channel gating. We therefore propose that altered channel gating represents a common pathophysiological mechanism in familial hemiplegic migraine.

High-conductance calcium-activated potassium channels in rat brain: pharmacology, distribution, and subunit composition.

In rat brain, high-conductance Ca2+-activated K+ (BK) channels are targeted to axons and nerve terminals [Knaus, H. G., et al. (1996) J. Neurosci. 16, 955-963], but absolute levels of their regional expression and subunit composition have not yet been fully established. To investigate these issues, an IbTX analogue ([125I]IbTX-D19Y/Y36F) was employed that selectively binds to neuronal BK channels with high affinity (Kd = 21 pM). Cross-linking experiments with [125I]IbTX-D19Y/Y36F in the presence of a bifunctional reagent led to covalent incorporation of radioactivity into a protein with an apparent molecular mass of 25 kDa. Deglycosylation and immunoprecipitation studies with antibodies raised against alpha- and smooth muscle beta-subunits of the BK channel suggest that the beta-subunit that is associated with the neuronal BK channel is a novel protein. Quantitative receptor autoradiography reveals the highest levels of BK channel expression in the outer layers of the neocortex, hippocampal perforant path projections, and the interpeduncular nucleus. This distribution pattern has also been confirmed in immunocytochemical experiments with a BK channel-selective antibody. Taken together, these findings imply that neuronal BK channels exhibit a restricted distribution in brain and have a subunit composition different from those of their smooth muscle congeners.

Subunit composition of brain voltage-gated potassium channels determined by hongotoxin-1, a novel peptide derived from Centruroides limbatus venom.

Five novel peptidyl inhibitors of Shaker-type (Kv1) K+ channels have been purified to homogeneity from venom of the scorpion Centruroides limbatus. The complete primary amino acid sequence of the major component, hongotoxin-1 (HgTX1), has been determined and confirmed after expression of the peptide in Escherichia coli. HgTX1 inhibits 125I-margatoxin binding to rat brain membranes as well as depolarization-induced 86Rb+ flux through homotetrameric Kv1.1, Kv1. 2, and Kv1.3 channels stably transfected in HEK-293 cells, but it displays much lower affinity for Kv1.6 channels. A HgTX1 double mutant (HgTX1-A19Y/Y37F) was constructed to allow high specific activity iodination of the peptide. HgTX1-A19Y/Y37F and monoiodinated HgTX1-A19Y/Y37F are equally potent in inhibiting 125I-margatoxin binding to rat brain membranes as HgTX1 (IC50 values approximately 0.3 pM). 125I-HgTX1-A19Y/Y37F binds with subpicomolar affinities to membranes derived from HEK-293 cells expressing homotetrameric Kv1.1, Kv1.2, and Kv1.3 channels and to rat brain membranes (Kd values 0.1-0.25 pM, respectively) but with lower affinity to Kv1.6 channels (Kd 9.6 pM), and it does not interact with either Kv1.4 or Kv1.5 channels. Several subpopulations of native Kv1 subunit oligomers that contribute to the rat brain HgTX1 receptor have been deduced by immunoprecipitation experiments using antibodies specific for Kv1 subunits. HgTX1 represents a novel and useful tool with which to investigate subclasses of voltage-gated K+ channels and Kv1 subunit assembly in different tissues.

Complex subunit assembly of neuronal voltage-gated K+ channels. Basis for high-affinity toxin interactions and pharmacology.

Neurons require specific patterns of K+ channel subunit expression as well as the precise coassembly of channel subunits into heterotetrameric structures for proper integration and transmission of electrical signals. In vivo subunit coassembly was investigated by studying the pharmacological profile, distribution, and subunit composition of voltage-gated Shaker family K+ (Kv1) channels in rat cerebellum that are labeled by 125I-margatoxin (125I-MgTX; Kd, 0.08 pM). High-resolution receptor autoradiography showed spatial receptor expression mainly in basket cell terminals (52% of all cerebellar sites) and the molecular layer (39% of sites). Sequence-directed antibodies indicated overlapping expression of Kv1. 1 and Kv1.2 in basket cell terminals, whereas the molecular layer expressed Kv1.1, Kv1.2, Kv1.3, and Kv1.6 proteins. Immunoprecipitation experiments revealed that all 125I-MgTX receptors contain at least one Kv1.2 subunit and that 83% of these receptors are heterotetramers of Kv1.1 and Kv1.2 subunits. Moreover, 33% of these Kv1.1/Kv1.2-containing receptors possess either an additional Kv1.3 or Kv1.6 subunit. Only a minority of the 125I-MgTX receptors (<20%) seem to be homotetrameric Kv1.2 channels. Heterologous coexpression of Kv1.1 and Kv1.2 subunits in COS-1 cells leads to the formation of a complex that combines the pharmacological profile of both parent subunits, reconstituting the native MgTX receptor phenotype. Subunit assembly provides the structural basis for toxin binding pharmacology and can lead to the association of as many as three distinct channel subunits to form functional K+ channels in vivo.

[125I]Iberiotoxin-D19Y/Y36F, the first selective, high specific activity radioligand for high-conductance calcium-activated potassium channels.

Iberiotoxin (IbTX), a selective peptidyl ligand for high-conductance Ca2(+)-activated K+ (maxi-K) channels cannot be radioiodinated in biologically active form due to the importance of Y36 in interacting with the channel pore. Therefore, an IbTX double mutant (IbTX-D19Y/Y36F) was engineered, expressed in Escherichia coli, purified to homogeneity, and radiolabeled to high specific activity with 125I. IbTX-D19Y/Y36F and [127I]IbTX-D19Y/Y36F block maxi-K channels expressed in Xenopus laevis oocytes with equal potency as wild-type IbTX (Kd approximately 1 nM). Under low ionic strength conditions, [125I]IbTX-D19Y/Y36F binds with high affinity to smooth muscle sarcolemmal maxi-K channels (Kd of 5 pM as determined by either equilibrium binding or kinetic binding analysis), and with a binding site density of 0.45 pmol/mg of protein. Competition studies with wild-type IbTX, IbTX-D19Y/Y36F or charybdotoxin (ChTX) result in complete inhibition of binding whereas toxins selective for voltage-gated K+ channels (margatoxin (MgTX) or alpha-dendrotoxin (alpha-DaTX) do not have any effect on IbTX binding. Indole diterpene alkaloids, which are selective inhibitors of maxi-K channels, and potassium ions both modulate [125I]IbTX-D19Y/Y36F binding in a complex manner. This pattern is also reflected during covalent incorporation of the radiolabeled toxin into the 31 kDa beta-subunit of the maxi-K channel in the presence of a bifunctional cross-linking reagent. In rat brain membranes, IbTX-D19Y/Y36F does not displace binding of [125I]MgTX or [125I]-alpha-DaTX to sites associated with voltage-gated K+ channels, nor do these latter toxins inhibit [125I]IbTX-D19Y/Y36F binding. Taken together, these results demonstrate that [125I]IbTX-D19Y/Y36F is the first selective radioligand for maxi-K channels with high specific activity.