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.

Fyx is associated with two missense point mutations in its gene and can be detected by PCR-SSP

The Duffy blood group antigens are encoded by the Duffy gene with its three major alleles: Fy*A (Fya+), Fy*B (Fyb+), and a nonexpressed Fy*Fy (Fya-b-), which is most commonly found among black people. Additionally, a fourth allele, Fyx, is found among white people and defined as weak Fyb not detectable by all anti-Fyb. Three polymerase chain reactions (PCRs) using sequence-specific priming (SSP) for detection of the major FY alleles were developed. Eighteen Fy(a-b-) samples of Tanzanian origin were correctly typed and of 300 random donors of Caucasian origin with known Fy phenotype, only four out of 59 Fy(a+b-) donors showed the discrepant DNA-type Fy(a+b+). Serologic reinvestigation by adsorption and elution techniques confirmed weakly expressed Fyb antigen in these cases and DNA sequencing of the entire Duffy gene revealed identical point mutations in all of them. Specific PCR reactions were used to reinvestigate the C265T (Arg89Cys) and G298A (Ala100Thr) substitution in the 300 samples. A298 was found to be present in both FY*X and FY*B alleles, pointing to an allelic variation among FY*B alleles. T265 was encountered exclusively in FY*X and is thought to be FY*X specific. Combining the T265 specific reaction with the three PCR-SSPs described above, we were able to correctly DNA-type all phenotypes investigated in our study.

Molecular mechanism of diltiazem interaction with L-type Ca2+ channels

Benzothiazepine Ca2+ antagonists (such as (+)-cis-diltiazem) interact with transmembrane segments IIIS6 and IVS6 in the alpha1 subunit of L-type Ca2+ channels. We investigated the contribution of individual IIIS6 amino acid residues for diltiazem sensitivity by employing alanine scanning mutagenesis in a benzothiazepine-sensitive alpha1 subunit chimera (ALDIL) expressed in Xenopus laevis oocytes. The most dramatic decrease of block by 100 microM diltiazem (ALDIL 45 +/- 4.8% inhibition) during trains of 100-ms pulses (0.1 Hz, -80 mV holding potential) was found after mutation of adjacent IIIS6 residues Phe1164(21 +/- 3%) and Val1165 (8.5 +/- 1.4%). Diltiazem delayed current recovery by promoting a slowly recovering current component. This effect was similar in ALDIL and F1164A but largely prevented in V1165A. Both mutations slowed inactivation kinetics during a pulse. The reduced diltiazem block can therefore be explained by slowing of inactivation kinetics (F1164A and V1165A) and accelerated recovery from drug block (V1165A). The bulkier diltiazem derivative benziazem still efficiently blocked V1165A. From these functional and from additional radioligand binding studies with the dihydropyridine (+)-[3H]isradipine we propose a model in which Val1165 controls dissociation of the bound diltiazem molecule, and where bulky substituents on the basic nitrogen of diltiazem protrude toward the adjacent dihydropyridine binding domain.

Molecular basis of drug interaction with L-type Ca2+ channels

Different types of voltage-gated Ca2+ channels exist in the plasma membrane of electrically excitable cells. By controlling depolarization-induced Ca2+ entry into cells they serve important physiological functions, such as excitation-contraction coupling, neurotransmitter and hormone secretion, and neuronal plasticity. Their function is fine-tuned by a variety of modulators, such as enzymes and G-proteins. Block of so-called L-type Ca2+ channels by drugs is exploited as a therapeutic principle to treat cardiovascular disorders, such as hypertension. More recently, block of so-called non-L-type Ca2+ channels was found to exert therapeutic effects in the treatment of severe pain and ischemic stroke. As the subunits of different Ca2+ channel types have been cloned, the modulatory sites for enzymes, G-proteins, and drugs can now be determined using molecular engineering and heterologous expression. Here we summarize recent work that has allowed us to determine the sites of action of L-type Ca2+ channel modulators. Together with previous biochemical, electrophysiological, and drug binding data these results provide exciting insight into the molecular pharmacology of this voltage-gated Ca2+ channel family.

Familial hemiplegic migraine mutations change alpha1A Ca2+ channel kinetics

Missense mutations in the pore-forming human alpha1A subunit of neuronal P/Q-type Ca2+ channels are associated with familial hemiplegic migraine (FHM). The pathophysiological consequences of these mutations are unknown. We have introduced the four single mutations reported for the human alpha1A subunit into the conserved rabbit alpha1A (R192Q, T666M, V714A, and I1819L) and investigated possible changes in channel function after functional expression of mutant subunits in Xenopus laevis oocytes. Changes in channel gating were observed for mutants T666M, V714A, and I1819L but not for R192Q. Ba2+ current (IBa) inactivation was slightly faster in mutants T666M and V714A than in wild type. The time course of recovery from channel inactivation was slower than in wild type in T666M and accelerated in V714A and I1819L. As a consequence, accumulation of channel inactivation during a train of 1-Hz pulses was more pronounced for mutant T666M and less pronounced for V714A and I1819A. Our data demonstrate that three of the four FHM mutations, located at the putative channel pore, alter inactivation gating and provide a pathophysiological basis for the postulated neuronal instability in patients with FHM.

Molecular mechanism of use-dependent calcium channel block by phenylalkylamines: role of inactivation

The role of channel inactivation in the molecular mechanism of calcium (Ca2+) channel block by phenylalkylamines (PAA) was analyzed by designing mutant Ca2+ channels that carry the high affinity determinants of the PAA receptor site [Hockerman, G. H., Johnson, B. D., Scheuer, T., and Catterall, W. A. (1995) J. Biol. Chem. 270, 22119-22122] but inactivate at different rates. Use-dependent block by PAAs was studied after expressing the mutant Ca2+ channels in Xenopus oocytes. Substitution of single putative pore-orientated amino acids in segment IIIS6 by alanine (F-1499-A, F-1500-A, F-1510-A, I-1514-A, and F-1515-A) gradually slowed channel inactivation and simultaneously reduced inhibition of barium currents (I(Ba)) by (-)D600 upon depolarization by 100 ms steps at 0.1 Hz. This apparent reduction in drug sensitivity was only evident if test pulses were applied at a low frequency of 0.1 Hz and almost disappeared at the frequency of 1 Hz. (-)D600 slowed I(Ba) recovery after maintained membrane depolarization (1-3 sec) to a comparable extent in all channel constructs. A drug-induced delay in the onset of I(Ba) recovery from inactivation suggests that PAAs promote the transition to a deep inactivated channel conformation. These findings indicate that apparent PAA sensitivity of Ca2+ channels is not only defined by drug interaction with its receptor site but also crucially dependent on intrinsic gating properties of the channel molecule. A molecular model for PAA-Ca2+ channel interaction that accounts for the relationship between drug induced inactivation and channel block by PAA is proposed.