Cloning of the human skeletal muscle alpha 1 subunit of the dihydropyridine-sensitive L-type calcium channel (CACNL1A3)

Arg(615)Cys substitution in pig skeletal ryanodine receptors increases activation of single channels by a segment of the skeletal DHPR II-III loop

The effect of peptides, corresponding to sequences in the skeletal muscle dihydropyridine receptor II-III loop, on Ca(2+) release from sarcoplasmic reticulum (SR) and on ryanodine receptor (RyR) calcium release channels have been compared in preparations from normal and malignant hyperthermia (MH)-susceptible pigs. Peptide A (Thr(671)-Leu(690); 36 microM) enhanced the rate of Ca(2+) release from normal SR (SR(N)) and from SR of MH-susceptible muscle (SR(MH)) by 10 +/- 3.2 nmole/mg/min and 76 +/- 9.7 nmole/mg/min, respectively. Ca (2+) release from SR(N) or SR(MH) was not increased by control peptide NB (Gly(689)-Lys(708)). AS (scrambled A sequence; 36 microM) did not alter Ca (2+) release from SR(N), but increased release from SR(MH) by 29 +/- 4.9 nmoles/mg/min. RyR channels from MH-susceptible muscle (RyR(MH)) were up to about fourfold more strongly activated by peptide A (> or =1 nM) than normal RyR channels (RyR(N)) at -40 mV. Neither NB or AS activated RyR(N). RyR(MH) showed an approximately 1.8-fold increase in mean current with 30 microM AS. Inhibition at +40 mV was stronger in RyR(MH) and seen with peptide A (> or = 0.6 microM) and AS (> or = 0.6 microM), but not NB. These results show that the Arg(615)Cys substitution in RyR(MH) has multiple effects on RyRs. We speculate that enhanced DHPR activation of RyRs may contribute to increased Ca(2+) release from SR in MH-susceptible muscle.

Molecular cloning and functional expression of a skeletal muscle dihydropyridine receptor from Rana catesbeiana

In skeletal muscle the dihydropyridine receptor is the voltage sensor for excitation-contraction coupling and an L-type Ca2+ channel. We cloned a dihydropyridine receptor (named Fgalpha1S) from frog skeletal muscle, where excitation-contraction coupling has been studied most extensively. Fgalpha1S contains 5600 base pairs coding for 1688 amino acids. It is highly homologous with, and of the same length as, the C-truncated form predominant in rabbit muscle. The primary sequence has every feature needed to be an L-type Ca2+ channel and a skeletal-type voltage sensor. Currents expressed in tsA201 cells had rapid activation (5-10 ms half-time) and Ca2+-dependent inactivation. Although functional expression of the full Fgalpha1S was difficult, the chimera consisting of Fgalpha1S domain I in the rabbit cardiac Ca channel had high expression and a rapidly activating current. The slow native activation is therefore not determined solely by the alpha1 subunit sequence. Its Ca2+-dependent inactivation strengthens the notion that in rabbit skeletal muscle this capability is inhibited by a C-terminal stretch (Adams, B., and Tanabe, T. (1997) J. Gen. Physiol. 110, 379-389). This molecule constitutes a new tool for studies of excitation-contraction coupling, gating, modulation, and gene expression.

An L-type calcium-channel gene mutated in incomplete X-linked congenital stationary night blindness

The locus for the incomplete form of X-linked congenital stationary night blindness (CSNB2) maps to a 1.1-Mb region in Xp11.23 between markers DXS722 and DXS255. We identified a retina-specific calcium channel alpha1-subunit gene (CACNA1F) in this region, consisting of 48 exons encoding 1966 amino acids and showing high homology to L-type calcium channel alpha1-subunits. Mutation analysis in 13 families with CSNB2 revealed nine different mutations in 10 families, including three nonsense and one frameshift mutation. These data indicate that aberrations in a voltage-gated calcium channel, presumably causing a decrease in neurotransmitter release from photoreceptor presynaptic terminals, are a frequent cause of CSNB2.

Calcium currents and transients of native and heterologously expressed mutant skeletal muscle DHP receptor alpha1 subunits (R528H)

Rabbit cDNA of the alpha1 subunit of the skeletal muscle dihydropyridine (DHP) receptor was functionally expressed in a muscular dysgenesis mouse (mdg) cell line, GLT. L-type calcium currents and transients were recorded for the wild type and a mutant alpha1 subunit carrying an R528H substitution in the supposed voltage sensor of the second channel domain that is linked to a human disease, hypokalemic periodic paralysis. L-type channels expressed in GLT myotubes exhibited currents similar to those described for primary cultured mdg cells injected with rabbit wild type cDNA, indicating this system to be useful for functional studies of heterologous DHP receptors. Voltage dependence and kinetics of activation and inactivation of L-type calcium currents from mutant and wild type channels did not differ significantly. Intracellular calcium release activation measured by fura-2 microfluorimetry was not grossly altered by the mutation either. Analogous measurements on myotubes of three human R528H carriers revealed calcium transients comparable to controls while the voltage dependence of both activation and inactivation of the L-type current showed a shift to more negative potentials of approximately 6 mV. Similar effects on the voltage dependence of the fast T-type current and changes in the expression level of the third-type calcium current point to factors not primarily associated with the mutation perhaps participating in disease pathogenesis.

Sequence-based exon prediction around the synaptophysin locus reveals a gene-rich area containing novel genes in human proximal Xp

The human Xp11.23-p11.22 interval has been implicated in several inherited diseases including Wiskott-Aldrich syndrome; three forms of X-linked hypercalciuric nephrolithiaisis; and the eye disorders retinitis pigmentosa 2, congenital stationary night blindness, and Aland Island eye disease. In constructing YAC contigs spanning Xp11. 23-p11.22, we have previously shown that the region around the synaptophysin (SYP) gene is refractory to cloning in YACs, but highly stable in cosmids. Preliminary analysis of the latter suggested that this might reflect a high density of coding sequences and we therefore undertook the complete sequencing of a SYP-containing cosmid. Sequence data were extensively analyzed using computer programs such as CENSOR (to mask repeats), BLAST (for homology searches), and GRAIL and GENE-ID (to predict exons). This revealed the presence of 29 putative exons, organized into three genes, in addition to the 7 exons of the complete SYP coding region, all mapping within a 44-kb interval. Two genes are novel, one (CACNA1F) showing high homology to alpha1 subunits of calcium channels, the other (LMO6) encoding a product with significant similarity to LIM-domain proteins. RT-PCR and Northern blot studies confirmed that these loci are indeed transcribed. The third locus is the previously described, but not previously localized, A4 differentiation-dependent gene. Given that the intron-exon boundaries predicted by the analysis are consistent with previous information where available, we have been able to suggest the genomic organization of the novel genes with some confidence. The region has an elevated GC content (>53%), and we identified CpG islands associated with the 5' ends of SYP, A4, and LMO6. The order of loci was Xpter-A4-LMO6-SYP-CACNA1F-Xcen, with intergenic distances ranging from approximately 300 bp to approximately 5 kb. The density of transcribed sequences in this area (>80%) is comparable to that found in the highly gene-rich chromosomal band Xq28. Further studies may aid our understanding of the long-range organization surrounding such gene-enriched regions.

Hypokalemic periodic paralysis: an autosomal dominant muscle disorder caused by mutations in a voltage-gated calcium channel

Hypokalemic periodic paralysis (hypoPP) is an autosomal dominant disorder characterized by acute attacks of muscle weakness concomitant to a drop in blood potassium levels. Recent molecular work has shown that hypoPP is due to mutations in a skeletal muscle voltage-gated calcium channel: the dihydropyridine receptor (DHP receptor). Mutations affect segments S4 of domains II and IV, changing an arginine in position 528 and 1239 into an histidine, or an histidine or a glycine respectively. Surprisingly, expressing in vitro mutants channels in a non-muscular environment resulted in functional calcium channels with minor modifications in electrophysiological properties. Expressing mutant channels in a muscular environment or transgenic mice might help to bridge the gap between the knowledge of the molecular defect and the understanding of the pathophysiology of the disease.

Malignant-hyperthermia susceptibility is associated with a mutation of the alpha 1-subunit of the human dihydropyridine-sensitive L-type voltage-dependent calcium-channel receptor in skeletal muscle

Malignant hyperthermia susceptibility (MHS) is characterized by genetic heterogeneity. However, except for the MHS1 locus, which corresponds to the skeletal muscle ryanodine receptor (RYR1) and for which several mutations have been described, no direct molecular evidence for a mutation in another gene has been reported so far. In this study we show that the CACNL1A3 gene encoding the alpha 1-subunit of the human skeletal muscle dihydropyridine-sensitive L-type voltage-dependent calcium channel (VDCC) represents a new MHS locus and is responsible for the disease in a large French family. Linkage analysis performed with an intragenic polymorphic microsatellite marker of the CACLN1A3 gene generated a two-point LOD score of 4.38 at a recombinant fraction of 0. Sequence analysis of the coding region of the CACLN1A3 gene showed the presence of an Arg-His substitution at residue 1086, resulting from the transition of A for G3333, which segregates perfectly with the MHS phenotype in the family. The mutation is localized in a very different part of the alpha 1-subunit of the human skeletal muscle VDCC, compared with previously reported mutations found in patients with hypokalemic periodic paralysis, and these two diseases might be discussed in terms of allelic diseases. This report is the first direct evidence that the skeletal muscle VDCC is involved in MHS, and it suggests a direct interaction between the skeletal muscle VDCC and the ryanodine receptor in the skeletal muscle sarcoplasmic reticulum.

Construction of cDNA libraries from small amounts of total RNA using the suppression PCR effect

Here we describe a method for preparing high-quality cDNA libraries from total RNA. By this method, double-stranded (ds) cDNA ligated with a specially designed ds adaptor is amplified by PCR using a modified T-primer and another primer corresponding to the outer part of the adaptor. The suppression PCR effect strongly inhibits the amplification of poly(A) RNA, thereby reducing background. This method leads to amplification of high-quality cDNA, facilitating the construction of representative cDNA libraries from as little as 10-100 ng of total RNA.

Electrophysiological properties of the hypokalaemic periodic paralysis mutation (R528H) of the skeletal muscle alpha 1s subunit as expressed in mouse L cells

Hypokalaemic periodic paralysis (HypoPP) is an autosomal dominant muscle disease which has been linked to point mutations in the skeletal muscle L-type calcium channel alpha 1 subunit (alpha 1s). Here, we have introduced one of the point mutations causing HypoPP (R528H) into cDNA of the rabbit alpha 1s. Expression of either the wild-type alpha 1s or the mutant R528H alpha 1s (alpha 1s-R528H) subunits was obtained in mouse Ltk- cells using a selectable expression vector. The alpha 1s-R528H subunit led to the expression of functional L-type Ca2+ channels. Corresponding whole-cell Ba2+ currents exhibit very slow activation and inactivation kinetics, typical for recombinant skeletal Ca2+ channel currents. Voltage-dependent activation and inactivation properties were similar for alpha 1s- and alpha 1s-R528H, as well as their sensitivity to the dihydropyridine agonist Bay K 8644. Differences in alpha 1s- and alpha 1s-R528H-directed channels reside in the Ba2+ current density, which was significantly reduced 3.2 fold in cells expressing alpha 1s-R528H. It was concluded that the R528H mutation af alpha 1s results in minor differences in the electrophysiological properties but significantly reduces the whole-cell Ca2+ channel current in its amplitude.