The gene for the alpha 1 subunit of the skeletal muscle dihydropyridine-sensitive calcium channel (Cchl1a3) maps to mouse chromosome 1

Nicardipine-sensitive Na+-mediated single channel currents in guinea-pig sinoatrial node pacemaker cells

1. The Na+-dependent inward currents underlying slow diastolic depolarization of sinoatrial (SA) node cells were examined. Using a Na+-rich, Ca2+-free pipette solution a novel single channel current was recorded in addition to the conventional Na+ and L-type Ca2+ currents. The current (termed ist, as it reflects the whole-cell sustained inward current, Ist) does not show obvious inactivation during a 700 ms depolarization and is unique in having a smaller amplitude (1.1 +/- 0.18 pA at -60 mV, n = 12) than the Na+ current through conventional Na+ ( approximately 3.3 pA) and Ca2+ channels (9.6 +/- 0.32 pA at -60 mV, n = 8). The mean unitary conductance of ist channels was 13.3 pS. 2. The recording of ist was infrequent, was observed only in spontaneously beating SA node cells, and was facilitated by adding Bay-K 8644 to the pipette solution. Overlapping of ist events was observed and ist was abolished by bath application of nicardipine. 3. In the ensemble average, the activation of ist was evident by depolarization beyond -70 mV, and the dynamic voltage range of activation (-70 to -30 mV) encompassed the extent of the slow diastolic depolarization. The current density of ist was 0.33 pA pF-1 at -60 mV, as estimated from the number of channels per membrane patch, the open probability and the unitary amplitude. 4. Cumulative histograms for both open and closed times were fitted with a sum of two exponential components. The slow time constants decreased with depolarization, while the fast time constants and the fraction of the fast component were voltage independent. The number of bursts per sweep increased with depolarization. The time constant of the first latency histogram was about two orders of magnitude larger than those in cardiac L-type Ca2+ channels and decreased with depolarization. 5. It is suggested that the ist channels might be responsible for the whole-cell Ist.

Single-cell RT-PCR and functional characterization of Ca2+ channels in motoneurons of the rat facial nucleus

Voltage-dependent Ca2+ channels are a major pathway for Ca2+ entry in neurons. We have studied the electrophysiological, pharmacological, and molecular properties of voltage-gated Ca2+ channels in motoneurons of the rat facial nucleus in slices of the brainstem. Most facial motoneurons express both low voltage-activated (LVA) and high voltage-activated (HVA) Ca2+ channel currents. The HVA current is composed of a number of pharmacologically separable components, including 30% of N-type and approximately 5% of L-type. Despite the dominating role of P-type Ca2+ channels in transmitter release at facial motoneuron terminals described in previous studies, these channels were not present in the cell body. Remarkably, most of the HVA current was carried through a new type of Ca2+ channel that is resistant to toxin and dihydropyridine block but distinct from the R-type currents described in other neurons. Using reverse transcription followed by PCR amplification (RT-PCR) with a powerful set of primers designed to amplify all HVA subtypes of the alpha1-subunit, we identified a highly heterogeneous expression pattern of Ca2+ channel alpha1-subunit mRNA in individual neurons consistent with the Ca2+ current components found in the cell bodies and axon terminals. We detected mRNA for alpha1A in 86% of neurons, alpha1B in 59%, alpha1C in 18%, alpha1D in 18%, and alpha1E in 59%. Either alpha1A or alpha1B mRNAs (or both) were present in all neurons, together with various other alpha1-subunit mRNAs. The most frequently occurring combination was alpha1A with alpha1B and alpha1E. Taken together, these results demonstrate that the Ca2+ channel pattern found in facial motoneurons is highly distinct from that found in other brainstem motoneurons.

Genetic analysis of voltage-dependent calcium channels

Molecular cloning of calcium channel subunit genes has identified an unexpectedly large number of genes and splicing variants, and a central problem of calcium channel biology is to now understand the functional significance of this genetic complexity. While electrophyisological, pharmacological, and molecular cloning techniques are providing one level of understanding, a complete understanding will require many additional kinds of studies, including genetic studies done in intact animals. In this regard, an intriguing variety of episodic diseases have recently been identified that result from defects in calcium channel genes. A study of these diseases illustrates the kind of insights into calcium channel function that can be expected from this method of inquiry.

Cardiac and skeletal muscle troponin I isoforms are encoded by a dispersed gene family on mouse chromosomes 1 and 7

We mapped the locations of the genes encoding the slow skeletal muscle, fast skeletal muscle, and cardiac isoforms of troponin I (Tnni) in the mouse genome by interspecific hybrid backcross analysis of species-specific (C57BL/6 vs Mus spretus) restriction fragment length polymorphisms (RFLPs). The slow skeletal muscle troponin I locus (Tnni1) mapped to Chromosome (Chr) 1. The fast skeletal muscle troponin I locus (Tnni2), mapped to Chr 7, approximately 70 cM from the centromere. The cardiac troponin I locus (Tnni3) also mapped to Chr 7, approximately 5-10 cM from the centromere and unlinked to the fast skeletal muscle troponin I locus. Thus, the troponin I gene family is dispersed in the mouse genome.

Myotube driven myogenic recruitment of cells during in vitro myogenesis

Muscular dysgenesis (mdg) is a recessive lethal mutation in the mouse which drastically affects skeletal muscle development during embryonic life. Physiologically, the disease is characterized by a complete paralysis resulting from a lack of excitation-contraction coupling. Existing electrophysiological, biochemical, and genetic evidence shows that mdg/mdg mice express a basic alteration of L-type voltage-sensitive Ca2+ channels in skeletal muscle. Studies on mdg/mdg myotubes in primary culture have shown that +/+ fibroblasts or +/+ Schwann cells may fuse with them and correct their functional deficiency by genetic complementation. As the spontaneous formation of heterocaryons is thought to be an exclusive property of myoblasts, we asked whether fibroblasts may have changed their properties before fusion occurred. We used primary cells issued from sciatic nerves dissected from newborn transgenic mice carrying the pHuDes1-nls-LacZ transgene (Des-LacZ cells) as non-muscle cells. These cells were mainly fibroblasts (80%) positive for Thy1.1 and Schwann cells positive for S100. The cultures were negative for myogenic markers (desmin, troponin T), did not form myotubes long-term, and did not display significant activation of the muscle reporter gene (pHuDes1-nls-LacZ). After a few days in coculture with dysgenic or normal myotubes, the muscle reporter gene (beta-galactosidase) was detected both within dysgenic myotubes, correlating with the restoration of normal contractile activity, and normal myotubes. As well as confirming that fusion takes place, this shows that Des-LacZ cells nuclei incorporated into recipient myotubes express their own myogenic genes. Moreover, individual mononucleated Des-LacZ cells expressing beta-galactosidase were observed, indicating that myogenic genes were being expressed before fusion. This suggests a mechanism of myotube driven myogenic recruitment of cells during the in vitro myogenesis. Analysis of the distribution of the induced Des-LacZ cells (positive for beta-galactosidase) in compartmentalized muscle cocultures showed that in the presence of dysgenic myotubes, these cells were equally distributed in both myotube free and enriched areas, whereas in the presence of normal myotubes, the positive cells remained in close vicinity of the myotubes. This difference could be explained by the fact that the dysgenic phenotype might include release of the induction process from its normal controls. Our results are consistent with the idea of a transcellular mechanism triggering myogenic differentiation in non-muscle cells, and that myotubes themselves are able to drive myogenic recruitment of cells during the in vitro myogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning of a murine N-type calcium channel alpha 1 subunit. Evidence for isoforms, brain distribution, and chromosomal localization

A cDNA encoding a N-type Ca2+ channel has been cloned from the murine neuroblastoma cell line N1A103. The open reading frame encodes a protein of 2,289 amino acids (257 kDa). Analysis of different clones provided evidence for the existence of distinct isoforms of N-type channels. High levels of mRNA were found in the pyramidal cell layers CA1, CA2 and CA3 of the hippocampus, in the dentate gyrus, in the cortex layers 2 and 4, in the subiculum and the habenula. The N-type Ca2+ channel gene has been localized on the chromosome 2, band A.

The gene coding for the alpha 1 subunit of the skeletal dihydropyridine receptor (Cchl1a3 = mdg) maps to mouse chromosome 1 and human 1q32

Using both chromosomal in situ hybridization and molecular techniques, we report the genetic localization of the gene coding for the alpha 1 subunit of the skeletal slow Ca2+ current channel/DHP receptor gene (Cchl1a3) on human Chromosome (Chr) 1 (1q31-1q32 region) and on mouse Chr 1 (region (F-G)). On the basis of single-strand conformation polymorphism (SSCP-PCR) analysis in an interspecific backcross, we have determined that the Cchl1a3 = mdg (muscular dysgenesis) locus is very closely linked to the myogenin (Myog) locus.