Inactivation of muscle chloride channel by transposon insertion in myotonic mice

An aspartic acid residue important for voltage-dependent gating of human muscle chloride channels

A point mutation (D136G) predicting the substitution of glycine for aspartate in position 136 of the human muscle Cl- channel (hClC-1) causes recessive generalized myotonia. Heterologous expression of a recombinant D136G produces functional Cl- channels with profound alterations in voltage-dependent gating, without concomitant changes in pore properties. The mutant exhibits slowly activating current upon hyperpolarization, in contrast to wild-type channels, which display time-dependent current decay (deactivation) at negative membrane potentials. Steady-state activation of D136G depends upon the transmembrane Cl- gradient, reaching zero at voltages positive to the Cl- reversal potential in physiological Cl- distribution. This explains the reduced sarcolemmal Cl- conductance that causes myotonia. The functional disturbances exhibited by D136G may stem from a defect in the ClC-1 voltage sensor.

A new inherited muscular disorder in Japanese quails (Coturnix coturnix japonica)

Thirteen adult mutant (LWC strain) Japanese quails (Coturnix coturnix japonica), between the ages of 8 and 60 weeks were examined for a progressive muscular disorder. The disorder, inherited as an autosomal dominant trait, was clinically apparent as early as 28 days of age; it was characterized by generalized myotonia, muscle stiffness, and muscle weakness. Affected birds were identified by their inability to lift their wings vertically upward and by their inability to right themselves when placed on their dorsum. Electromyographic studies in two mutant quails showed high-frequency repetitive discharges comparable to those of myotonic runs. These discharges persisted after nerve resection. The distinctive histopathologic changes in the various muscles examined were ring fibers, sarcoplasmic masses, and internal migration of sarcolemmal nuclei. A slight decrease in the size of type IIB muscle fibers and a slight increase in the size of type IIA fibers were observed in the M. pectoralis thoracicus of affected quails. In older affected birds, inter- and intrafascicular fatty infiltration with replacement of type IIB fibers by fat cells was seen in the pectoral muscles. Single fiber necrosis, nonspecific lymphorrages, and variations in the muscle fiber size and shape were also noted. The typical muscle lesions and multisystem involvement, which was manifested by testicular degeneration and atrophy in the male LWC specimens and bilateral lenticular cataracts in 6 of 13 affected mutant quails, suggest resemblance of this new inherited muscular disorder to myotonic dystrophy in man.

Absence of the skeletal muscle sarcolemma chloride channel ClC-1 in myotonic mice

The voltage-dependent chloride channel ClC-1 stabilizes resting membrane potential in skeletal muscle. Mutations in the ClC-1 gene are responsible for both human autosomal recessive generalized myotonia and autosomal dominant myotonia congenita. To understand the tissue distribution and subcellular localization of ClC-1 and to evaluate its role in an animal model of myotonia, antibodies were raised against the carboxyl terminus of this protein. Expression of the 130-kDa ClC-1 protein is unique to skeletal muscle, consistent with its mRNA tissue distribution. Immunolocalization shows prominent ClC-1 antigen in the sarcolemma of both type I and II muscle fibers. Sarcolemma localization is confirmed by Western analysis of skeletal muscle subcellular fractions. The ADR myotonic mouse (phenotype ADR, genotype adr/adr), in which defective ClC-1 mRNA has been identified, is shown here to be absent in ClC-1 protein expression, whereas other skeletal muscle sarcolemma protein expression appears normal. Immunohistochemistry of skeletal muscle from ADR and other mouse models of human muscle disease demonstrate that the absence of ClC-1 chloride channel is a defect specific to ADR mice.

Chloride currents across the membrane of mammalian skeletal muscle fibres

1. Chloride currents through the membrane of rat psoas muscle fibre segments were investigated with a double Vaseline gap under conditions minimizing the currents of other ion species. 2. In Cl(-)-free solutions a time- and voltage-independent conductance of 1.1 +/- 0.4 microS was observed. 3. As with intact fibres, the steady-state Cl- conductance was 2.5 +/- 0.9 mS cm-2; the halide selectivity was Cl- > Br- > I-, and Cl- currents were completely blocked by 0.1 mM 9-anthracene carboxylic acid (9-AC). 4. Voltage steps from -85 mV to between -125 and +55 mV elicited currents with deactivation upon hyperpolarization and activation upon depolarization. Activation was fitted with two exponentials, the smaller time constant increasing from 37.5 ms at +55 mV to 67.0 ms at -5 mV, the larger time constant (450 ms) being independent of potentials more positive than -5 mV. The two deactivation time constants ranged between 30.6 (-105 mV) and 99.3 ms (-35 mV), and 139.4 (-105 mV) and 738.5 ms (-35 mV). 5. The activation curve was fitted with a Boltzmann distribution (half-maximum, -39 mV; slope at inflexion point, 1/17.2 mV). Deactivation was incomplete. At very negative potentials about one-quarter of the maximum number of channels were open. 6. When tested with 5 and 61 mM intracellular Cl- concentration ([Cl-]i) the kinetic parameters were not different. 7. During depolarizations lasting > 5 s, activation was followed by a decline. With progressively longer prepulses going positive to the reversal potential and test pulses going negative, the responses to test and prepulses decreased with similar time constants, suggesting a real inactivation process.

A plethora of cardiac chloride conductances: molecular diversity or a related gene family

Recent electrophysiologic studies have provided evidence suggesting that as many as six different Cl- conductances can be identified in the sarcolemma of cardiac myocytes isolated from various animal species and areas of the heart. These include Cl- conductances activated by stimulation of protein kinase A, protein kinase C, extracellular ATP, intracellular Ca2+, membrane stretch, and a basally active Cl- conductance. Many basic biophysical and pharmacological properties of these channels are presently unknown, and the only molecular information presently available suggests that the cAMP-activated Cl- conductance is due to cardiac expression of an isoform of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel normally found in epithelial cells. We used the polymerase chain reaction (PCR) to amplify four distinct regions corresponding to the cardiac CFTR gene product from several cardiac tissues to determine if the molecular distribution of CFTR matches the distribution of cAMP-dependent Cl- channels in native myocytes. Amplification of regions corresponding to the first nucleotide binding domain (NBD1), transmembrane segments (TS) VII-XII, and the regulatory (R) domain showed a precise correlation to tissues that electrophysiologically exhibit sarcolemmal cAMP-dependent Cl- channels, whereas region TS I-VI exhibited a distribution independent of the presence of cAMP-dependent Cl- channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Insertional mutation of the motor endplate disease (med) locus on mouse chromosome 15

Homozygous transgenic mice from line A4 have an early-onset progressive neuromuscular disorder characterized by paralysis of the rear limbs, muscle atrophy, and lethality by 4 weeks of age. The transgene insertion site was mapped to distal chromosome 15 close to the locus motor endplate disease (med). The sequence of mouse DNA flanking the insertion site junctions was determined. A small (< 20 kb) deletion was detected at the insertion site, with no evidence of additional rearrangement of the chromosomal DNA. Noncomplementation of the transgene-induced mutation and med was demonstrated in a cross with medJ/+mice. The new allele is designated medTgNA4Bs (medtg). The homologous human locus MED was assigned to chromosome 12. Synaptotagmin 1 and contactin 1 were eliminated as candidate genes for the med mutation. The transgene-induced allele provides molecular access to the med gene, whose function is required for synaptic transmission at the neuromuscular junction and long-term survival of cerebellar Purkinje cells.

Gating of the voltage-dependent chloride channel CIC-0 by the permeant anion

Chloride channels of the ClC family are important for the control of membrane excitability, cell volume regulation, and possibly transepithelial transport. Although lacking the typical voltage-sensor found in cation channels, gating of ClC channels is clearly voltage-dependent. For the prototype Torpedo channel ClC-0 (refs 11-15) we now show that channel opening is strongly facilitated by external chloride. Other less permeable anions can substitute for chloride with less efficiency. ClC-0 conductance shows an anomalous mole fraction behaviour with Cl-/NO3- mixtures, suggesting a multi-ion pore. Gating shows a similar anomalous behaviour, tightly linking permeation to gating. Eliminating a positive charge at the cytoplasmic end of domain D12 changes kinetics, concentration dependence and halide selectivity of gating, and alters pore properties such as ion selectivity, single-channel conductance and rectification. Taken together, our results strongly suggest that in these channels voltage-dependent gating is conferred by the permeating ion itself, acting as the gating charge.

Voltage-gated ion channelopathies: inherited disorders caused by abnormal sodium, chloride, and calcium regulation in skeletal muscle

The pathological genetic defects in the inherited myotonias and periodic paralyses were recently elucidated using molecular genetic studies. These disorders are usually transmitted as a dominant trait from an affected parent to a child. The many clinical symptoms include cold-induced uncontrollable contraction of muscle, potassium-induced contraction and paralysis, myotonia with dramatic muscular hypertrophy, muscle stiffness, and insulin-induced paralysis (in males). Horses afflicted with the disorder can suddenly collapse, despite an impressive physique. In the past three years, these clinically defined disorders have been shown to share a common etiology: subtle defects of ion channels in the muscle-fiber membrane. Although the specific ion channel involved varies depending on the disease, most patients have single amino acid changes in the channel proteins, with both normal and mutant channels present in each muscle fiber. For each patient, we can now establish a precise molecular diagnosis in the face of overlapping clinical symptoms and begin specific pharmacological treatment based on the primary problem. These studies have also provided insight into basic muscle biology and emphasize the careful regulation of ions in muscle excitation.

Properties of voltage-gated chloride channels of the ClC gene family

We review the properties of ClC chloride channels, members of an expanding gene family originally discovered by the cloning of the ClC-0 chloride channel from Torpedo electric organ. There are at least nine different ClC genes in mammals, several of which seem to be expressed ubiquitously, while others are expressed in a highly specific manner (e.g. the muscle-specific ClC-1 channel and the kidney-specific ClC-K channels). The newly cloned rat ClC-4 is strongly expressed in liver and brain, but also in heart, muscle, kidney and spleen. ClC chloride channels are structurally unrelated to other channel proteins and have twelve putative transmembrane domains. They function as multimers with probably four subunits. Functional characterization is most advanced with ClC-0, ClC-1 (mutations which cause myotonia) and ClC-2, a swelling-activated chloride channel. Many of the new ClC family members cannot yet be expressed functionally.

Autoimmunity, apoptosis defects and retroviruses

Autoimmunity and AIDS both share the common feature of increased expression of retroviral protein products and abnormal apoptosis of immune cells (81). This leads to a more global immunomodulatory defect (82-84). The challenge in the future will be to devise compounds that can either regulate the effect of the retroviral products on apoptosis, or that can inhibit apoptosis pathways in order to restore normal immune system function.

Intracisternal A-type particle elements as genetic markers: detection by repeat element viral element amplified locus-PCR

We describe a novel, PCR-based technique termed REVEAL-PCR for examining the inheritance of intracisternal A-type particles (IAP). Amplifications use an unlabeled primer to SINE repeats and a radiolabeled primer to the IAP long terminal repeat; labeled products, which can be resolved on sequencing gels, are formed when IAPs lie in proximity to SINEs. With this technique we have identified a total of 124 polymorphisms in the BXH and CXS recombinant inbred strains. We suggest that this method will be equally applicable for examining other gene families present at around a thousand copies per genome.

The spastic mouse: aberrant splicing of glycine receptor beta subunit mRNA caused by intronic insertion of L1 element

Mice homozygous for the spastic mutation (spa) suffer from a complex motor disorder resulting from reduced CNS levels of the adult glycine receptor isoform GlyRA, which is composed of ligand-binding alpha 1 and structural beta polypeptides. The beta subunit-encoding gene (Glyrb) was mapped near the spa locus on mouse chromosome 3. In spa/spa mice, aberrant splicing of the beta subunit pre-mRNA strikingly diminishes the CNS contents of full-length transcripts, whereas truncated beta subunit mRNAs accumulate. This is a result of exon skipping, which causes translational frameshifts and premature stop codons. Intron 5 of the spa Glyrb gene contains an L1 transposable element that apparently is causal for the aberrant splicing of beta subunit transcripts.

Molecular physiology of anion channels

Anion channels have diverse functions, ranging from regulation of cell volume to transepithelial transport and control of excitability. Three well established structural classes of plasma membrane chloride channels now exist: the ligand-gated chloride channels, the cAMP-stimulated cystic fibrosis transmembrane conductance regulator channel, and the voltage-gated (or swelling-activated) members of the CLC chloride channel family. Genetic defects leading to inherited disease are known for each of these classes. A combination of mutagenesis and biophysical analysis has been used to correlate their structure with function. Recently, the role of several molecules has been questioned; rather than being chloride channels themselves, they may be activators of endogenous channels in the cells used for expression.

Glycine receptor beta-subunit gene mutation in spastic mouse associated with LINE-1 element insertion

Congenital myoclonus is a widespread neurologic disorder characterized by hyperexcitability, muscular spasticity and myoclonus associated with marked reduction in neural glycine binding sites. The recessive mouse mutation spastic (spa) is a prototype of inherited myoclonus. Here we show that defects in the gene encoding the beta-subunit of the glycine receptor (Glrb) underlie spa: Glrb maps to the same region of mouse chromosome 3 as spa, and Glrb mRNA is markedly reduced throughout brains of spa mice, most likely as a result of an insertional mutation of a 7.1 kilobase LINE-1 element within intron 6 of Glrb. These results provide evidence that Glrb is necessary for postsynaptic expression of glycine receptor complexes, and suggest Glrb as a candidate gene for inherited myoclonus in other species.

Subtractive cDNA cloning as a tool to analyse secondary effects of a muscle disease. Characterization of affected genes in the myotonic ADR mouse

In myotonic ADR mice that are homozygous for a defect in the muscular chloride channel gene adr/Clc-1, the hyperexcitability of fast muscles is associated with secondary changes in gene expression and fibre type composition. cDNA clones derived from a set of genes down regulated in fast muscles of the myotonic ADR mouse were isolated by a subtractive cloning procedure. A total of 1200 clones were analysed for high expression in fast muscle of wild type and low expression in mutant mouse. Differential transcript levels were verified by northern blot hybridizations. The identities of the corresponding transcripts were determined by sequencing as myosin heavy chain IIB, alpha-tropomyosin, troponin C, a Ca2+ ATPase and parvalbumin mRNAs. Of these, mRNAs for parvalbumin and myosin heavy chain IIB were drastically downregulated in myotonic muscle (to < 10% of control). A full length cDNA clone for skeletal muscle alpha-tropomyosin was homologous to the mouse fibroblast tropomyosin isoform 2, except for the portion encoding the alpha-tropomyosin specific amino acids 258-284. A cDNA derived from the 1100 nucleotide parvalbumin transcript was cloned and the sequence for the as yet unknown 3' extended trailer, generated by alternative polyadenylation, was determined.

Aging and chloride channel regulation in rat fast-twitch muscle fibres

By the use of pharmacological tools, we tested the hypothesis that age-related alterations in the regulatory pathways of chloride channels might contribute to the lowered chloride conductance (GCl) found in skeletal muscle of aged rats. The resting GCl of extensor digitorum longus (EDL) muscles from adult rats either young (3-4 months old) or aged (29 months old) was measured by means of computerized intracellular microelectrode recordings. In EDL muscle from 3 to 4-month-old rats, 4-beta-phorbol 12,13-dibutyrate (4-beta-PDB), a direct activator of protein kinase C (PKC), decreased GCl in a concentration-dependent manner. The same effect was exerted by cholera toxin. The effects of both the phorbol ester and cholera toxin were inhibited by staurosporine, thus indicating that either direct or indirect (via G protein) activation of PKC accounts for the decrease of GCl. An increase of cytosolic Ca2+ by the ionophore A23187 also significantly decreased GCl by 25%. In EDL muscles from aged rats, 4-beta-PDB was 20-fold more potent in blocking GCl than in muscles from younger controls, and the ionophore blocked GCl by 40%. On the other hand, cholera toxin was ineffective. Our findings support the hypothesis that in fast-twitch muscle the regulation of chloride channels by PKC and Ca2+ is a target of the aging process.

Cloning and expression of a protein kinase C-regulated chloride channel abundantly expressed in rat brain neuronal cells

cDNA (CIC-3) encoding a protein kinase C-regulated chloride channel was cloned and characterized. The open reading frame encodes 760 amino acids, which possess significantly amino acid identity with previously cloned CIC chloride channels. The chloride currents expressed in Xenopus oocytes injected with CIC-3 cRNA were completely blocked by activation of protein kinase C by 12-O-tetradecanoylphorbol 13-acetate. Abundant expression of CIC-3 mRNA was observed in rat brain, especially in the olfactory bulb, hippocampus, and cerebellum. These findings suggest that CIC-3 may play an important role in neuronal cell function through regulation of membrane excitability by protein kinase C.

Exploring the mammalian neuromuscular system by analysis of mutations: spinal muscular atrophy and myotonia

Any biological structure can be studied using mutations that interfere either with its emergence or its function. We investigate spontaneous and induced mutations in the mouse that affect neuromuscular development and function. The wobbler mouse (phenotype WR, genotype wr/wr) suffers from muscular atrophy because of the degeneration of 20-40% of the motoneurones; it is also unable to produce functional spermatozoa. As a step towards positional cloning of the wr gene, we have mapped the locus to proximal chromosome 11, thus excluding CNTF and its receptor as candidates, and suggesting the closely-linked Rab 1 gene encoding a GTP-binding protein as a possibility. In the case of the adr (arrested development of righting response) mouse, which shows hyperexcitability of mature muscle fibres due to a reduction of the 'dampening' function of chloride conductance at resting potential, we have shown that the defect is in the chloride channel gene adr/Clc-1 on chromosome 6. This allowed us to predict via synteny the chromosomal location of human Thomsen's and Becker's myotonias as close to the TCRB gene on human chromosome 7q. The combination of these approaches with gene-targeting approaches will allow genetic analysis of the establishment and structure of the neuromuscular system.

Mouse homologues of human hereditary disease

Details are given of 214 loci known to be associated with human hereditary disease, which have been mapped on both human and mouse chromosomes. Forty two of these have pathological variants in both species; in general the mouse variants are similar in their effects to the corresponding human ones, but exceptions include the Dmd/DMD and Hprt/HPRT mutations which cause little, if any, harm in mice. Possible reasons for phenotypic differences are discussed. In most pathological variants the gene product seems to be absent or greatly reduced in both species. The extensive data on conserved segments between human and mouse chromosomes are used to predict locations in the mouse of over 50 loci of medical interest which are mapped so far only on human chromosomes. In about 80% of these a fairly confident prediction can be made. Some likely homologies between mapped mouse loci and unmapped human ones are also given. Sixty six human and mouse proto-oncogene and growth factor gene homologies are also listed; those of confirmed location are all in known conserved segments. A survey of 18 mapped human disease loci and chromosome regions in which the manifestation or severity of pathological effects is thought to be the result of genomic imprinting shows that most of the homologous regions in the mouse are also associated with imprinting, especially those with homologues on human chromosomes 11p and 15q. Useful methods of accelerating the production of mouse models of human hereditary disease include (1) use of a supermutagen, such as ethylnitrosourea (ENU), (2) targeted mutagenesis involving ES cells, and (3) use of gene transfer techniques, with production of 'knockout mutations'.

Low single channel conductance of the major skeletal muscle chloride channel, ClC-1

We expressed the skeletal muscle chloride channel, ClC-1, in HEK293 cells and investigated it with the patch-clamp technique. Macroscopic properties are similar to those obtained after expression in Xenopus oocytes, except that faster gating kinetics are observed in mammalian cells. Nonstationary noise analysis revealed that both rat and human ClC-1 have a low single channel conductance of about 1 pS. This finding may explain the lack of single-channel data for chloride channels from skeletal muscle despite its high macroscopic chloride conductance.

Chromosomal mapping in the mouse of eight K(+)-channel genes representing the four Shaker-like subfamilies Shaker, Shab, Shaw, and Shal

The four Shaker-like subfamilies of Shaker-, Shab-, Shaw-, and Shal-related K+ channels in mammals have been defined on the basis of their sequence homologies to the corresponding Drosophila genes. Using interspecific backcrosses between Mus musculus and Mus spretus, we have chromosomally mapped in the mouse the Shaker-related K(+)-channel genes Kcna1, Kcna2, Kcna4, Kcna5, and Kcna6; the Shab-related gene Kcnb1; the Shaw-related gene Kcnc4; and the Shal-related gene Kcnd2. The following localizations were determined: Chr 2, cen-Acra-Kcna4-Pax-6-a-Pck-1-Kras-3-Kcn b1 (corresponding human Chrs 11p and 20q, respectively); Chr 3, cen-Hao-2-(Kcna2, Kcnc4)-Amy-1 (human Chr 1); and Chr 6, cen-Cola-2-Met-Kcnd2-Cpa-Tcrb-adr/Clc-1-Hox-1.1-Myk - 103-Raf-1-(Tpi-1, Kcna1, Kcna5, Kcna6) (human Chrs 7q and 12p, respectively). Thus, there is a cluster of at least three Shaker-related K(+)-channel genes on distal mouse Chr 6 and a cluster on Chr 2 that at least consists of one Shaker-related and one Shaw-related gene. The three other K(+)-channel genes are not linked to each other. The map positions of the different types of K(+)-channel genes in the mouse are discussed in relation to those of their homologs in man and to hereditary diseases of mouse and man that might involve K+ channels.

Retroviruses, apoptosis and autogenes

Autoimmunity and autoimmune disease are not the same. Autoimmunity is a normal consequence of aging, potentially reversible and possibly physiological. Autoimmune disease is dependent on genetic, viral, hormonal and psychoneuroimmunological factors. Aside from the apparently normal regulation of autoimmune responses by immune response genes, little is known about other genetic factors. Here, Norman Talal and John Mountz propose the term autogene to describe non-MHC genes which directly or indirectly interfere with important immunoregulatory actions. When mutated or otherwise genetically altered (e.g. by retrotransposon insertion), these genes predispose to immune dysregulation, lymphoproliferation and autoimmunity.

Evidence for genetic homogeneity in autosomal recessive generalised myotonia (Becker)

Generalised myotonia Becker (GM) is an autosomal recessively inherited muscle disorder. Affected subjects exhibit myotonic muscle stiffness in all skeletal muscles with marked hypertrophy in the legs. A transient muscle weakness is particularly pronounced in the arms and hands and is a typical symptom of the disorder. Recently, we showed complete linkage of the disorder GM to the gene (CLCN1) coding for the skeletal muscle chloride channel CLC-1 and the TCRB gene on chromosome 7 in German families. In the study presented here we performed linkage analysis on 14 new GM families. The GM locus was again completely linked to both the CLCN1 and the TCRB gene in all families with a combined lod score of Z = 9.26 at a recombination fraction of theta = 0.00. This confirms our previous data and supports the hypothesis that GM is a genetically homogeneous disorder. The previously detected T to G missense mutation is found on 15% of the 66 GM chromosomes counted so far.

A genetic linkage map of the mouse: current applications and future prospects

Technological advances have made possible the development of high-resolution genetic linkage maps for the mouse. These maps in turn offer exciting prospects for understanding mammalian genome evolution through comparative mapping, for developing mouse models of human disease, and for identifying the function of all genes in the organism.

The defect in Fas mRNA expression in MRL/lpr mice is associated with insertion of the retrotransposon, ETn

Fas is a cell surface protein of the tumor necrosis factor receptor, nerve growth factor receptor, CD40 family, and is involved in the control of lymphocyte apoptosis. A mutation in the Fas gene in MRL/lpr mice results in massive lymphoproliferation (lpr) and accelerated autoimmunity. To further study the nature of this defect, Fas mRNA expression was evaluated by reverse transcriptase polymerase chain reaction as well as by Northern blotting. These studies revealed that the wild-type Fas message was produced at approximately 10-fold lower levels in the lpr compared with the ++ substrain of MRL mice. In addition to the wild-type transcript, lpr mice also synthesized chimeric transcripts containing an insertion of the early retrotransposon (ETn). Molecular cloning and nucleotide sequencing of a Fas-ETn chimeric cDNA suggested that the striking reduction in wild-type Fas mRNA levels and the presence of aberrant transcripts in MRL/lpr mice are most likely explained by the insertion of the ETn retrotransposon into an intron of the Fas gene and induction of alternative splicing involving the 5' ETn long terminal repeat.

Development of electrical myotonia in the ADR mouse: role of chloride conductance in myotubes and neonatal animals

In the ADR mouse, the homozygous condition of the autosomal mutation adr, "arrested development of righting response", leads to the symptoms of myotonia. The adr mutation is caused by an insertion of a retroposon into a gene for a chloride channel (adr = Clc-1) that is expressed in adults, but only at very low levels in neonate rodent muscle. In the present study, we investigated the earliest stages of the ADR myotonia. In muscle from 7-day-old ADR mice that can be recognized by inspection, electrical after-activities are distinct by their low frequency (1-5 Hz) and long duration (several minutes) from those recorded in adult muscle. Similar myotonic symptoms could be evoked in muscle fibres from 7 day wildtype mice after substitution of the external chloride with impermeant anions or by activators of protein kinase C. The genotypes of 3-day-old mice cannot be inferred from inspection and, thus, were identified by Southern blotting with a ClC-1 probe. Although no +/+ animal showed characteristic myotonic series, these were seen both in adr/adr and in most adr/+ animals. Thus, due to the low dosage of chloride channels in 3-day-old mouse muscle, the adr mutation appears to be partially dominant rather than fully recessive, as in adult mice. No indication of electrical myotonia could be demonstrated in cultured myotubes, although their pattern of excitability depended on the presence of external chloride ions. We conclude that the low Cl(-)-conductance of myotubes influences excitability but is not controlled by the adr/Clc-1 gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Chloride channels

Cl- channels have various functions such as regulation of cell volume, transepithelial transport, and control of excitability in nerve and muscle. Several different structural classes of Cl- channels have been identified recently by molecular cloning. The importance of these different classes of Cl- channels can be seen from the inherited diseases resulting from mutations in some of the genes encoding them. Mutagenesis studies are beginning to shed light on their structure-function relationships.

Regulation of resting ionic conductances in frog skeletal muscle

The membrane electrical properties and resting ionic conductances of frog semitendinosus muscle fibres were studied in vitro at 25 degrees C with the two-micro-electrode cable technique, in the presence of an activator or inhibitor of protein kinase C (PKC) or in the presence of an activator of adenylate cyclase. The PKC activator, 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB), reduced chloride conductance (GCl) at concentrations greater than 1 microM and did not affect potassium conductance (GK). At 150 microM, the maximum concentration of 4 beta-PDB tested, GCl was reduced by 42%. The "inactive" phorbol ester 4 alpha-phorbol 12,13-dibutyrate did not affect GCl or GK. The inhibitory effect of 4 beta-PDB on GCl was prevented by pretreatment of the muscle preparation with the PKC inhibitor staurosporine. The adenylate cyclase activator forskolin (1.5-8 microM) significantly increased the GK of the fibres, without affecting GCl. Thus, we conclude that frog skeletal muscle GCl, unlike rat muscle GCl, is relatively insensitive to activators of PKC. Moreover, in frog muscle, protein kinase A is a likely modulator of GK, but not GCl.

Molecular basis of Thomsen's disease (autosomal dominant myotonia congenita)

Thomsen's disease (autosomal dominant myotonia congenita) has recently been linked to chromosome 7q35 in the region of the human skeletal muscle chloride channel gene (HUMCLC). Single strand conformation polymorphism analysis (SSCP) was used to screen DNA from members of four unrelated pedigrees with this disorder for mutations in HUMCLC. Abnormal bands were detected in all affected, but no unaffected individuals in three of the families. Direct sequencing revealed a G to A transition that results in the substitution of a glutamic acid for a glycine residue located between the third and fourth predicted membrane spanning segments. This glycine residue is conserved in all known members of this class of chloride channel proteins. These findings establish HUMCLC as the Thomsen's disease gene.

Abnormal fatty acid composition in sarcolemma and sarcoplasmic reticulum from myotonic ADR mouse muscle

The fatty acid composition of membrane lipids from sarcolemma and sarcoplasmic reticulum isolated from biceps and gastrocnemius muscle has been compared in normal (wildtype, +/adrmto or +/+) and affected (adrmto/adrmto) myotonic mice. The adrmto mouse exhibits an arrested development of the righting response, and arose spontaneously from the SWR/J strain. These mice exhibit classical myotonia similar to the human disease, Becker's myotonia [1]. Significant alterations, characterized by a decrease in the saturated fatty acid, palmitic acid (16:0), and the polyunsaturated fatty acid, arachidonic acid (20:4), and an increase in stearic (18:0) and linoleic (18:2) acids, were observed between sarcolemma and sarcoplasmic reticulum from normal and affected mice. These changes in fatty acid composition of muscle membrane from ADR mice may be adequate to cause an alteration in membrane fluidity and affect the function of ion channels. The fatty acid composition of erythrocytes ghosts was also examined, as a potential marker for alterations in muscle membranes. In erythrocyte ghosts isolated from affected mice, the only alteration observed was a decrease in the proportion of oleic acid (18:1), an effect completely different from those observed in muscle membranes. Therefore, erythrocyte ghosts do not serve as an adequate indicator of changes in fatty acid composition of muscle membranes in this model of myotonia.

Periodic paralysis, myotonia congenita and sarcolemmal ion channels: a success of the candidate gene approach

The classification of periodic paralyses and myotonic syndromes has been a subject of debates for the last 40 yr. Recent advances in molecular biology have led geneticists to reconsider this old question, using a candidate gene approach. Two groups of disorders have now emerged: (1) muscle sodium channel-associated diseases which include hyperkalemic periodic paralysis and its clinical variants, as well as paramyotonia congenita; (2) muscle chloride channel-associated disorders which comprise both the dominant and recessive form of myotonia congenita. This review is focussed on these recent discoveries.

Genetic analysis of diabetes and insulitis in an interspecific cross of the nonobese diabetic mouse with Mus spretus

The nonobese diabetic (NOD) mouse is a widely used model for genetic studies of insulin-dependent diabetes mellitus due to the similarities between the murine and human diseases. To aid in the localization and identification of diabetes-related susceptibility genes, we have constructed an interspecific backcross between NOD and Mus spretus (SEG/Pas) mice. Although no diabetic animals were observed in the first backcross generation of (SEG/Pas x NOD) x NOD (BC1), the incidence of insulitis (lymphocyte infiltration of the islets of Langerhans) exceeded 20% after injections of cyclophosphamide, a treatment that provokes an acute form of diabetes in NOD mice. Insulitis, a prediabetic condition, is a useful phenotype in studies of diabetes susceptibility. In the second backcross (BC2) generation, 8% of the animals became diabetic and 76% were found to have insulitis. Genetic mapping studies in the BC2 families confirmed the importance of the major histocompatibility complex region on the severity of insulitis and suggested that additional susceptibility loci were linked to markers on mouse chromosomes 3, 6, and 15. Mus spretus crosses have been an important tool in recent advances in murine genetics, and our results extend their usefulness to the study of a multifactorial disease.

Aberrant transcription caused by the insertion of an early transposable element in an intron of the Fas antigen gene of lpr mice

The mouse lpr (lymphoproliferation) mutation carries a rearrangement in the chromosomal gene for the Fas antigen, which mediates apoptosis. Isolation and characterization of mouse Fas antigen chromosomal gene from wild-type and lpr mice indicated an insertion of an early transposable element (ETn) in intron 2 of the Fas antigen gene of lpr mice. Hybrid transcripts carrying the Fas antigen and ETn sequences were expressed in the thymus and liver of the mutant. This indicated that premature termination and aberrant splicing of the Fas antigen transcript caused by the insertion of the ETn in the intron are responsible for the lymphoproliferation and autoimmune phenotype of the mutant mouse. On the other hand, an insertion of the ETn into an intron of a mammalian expression vector dramatically but not completely reduced the expression efficiency. These findings suggest that lpr mice are able to express a very low level of the Fas antigen.

Chloride channels with reduced single-channel conductance in recessive myotonia congenita

Myoballs were cultured from muscle biopsies of patients having Becker-type recessive generalized myotonia (RGM). Single-channel recordings showed that the conductance of the "intermediate" Cl- channel, the Cl- channel having the highest membrane density and accounting for most of the Cl- conductance in myoballs, was reduced to about 50%. All other channel parameters tested, such as gating properties, ion selectivity, and regulatory pathways, were not different from controls. Two other Cl- channel types were only rarely observed and did not seem different from controls. Also, the macroscopic Cl- conductance measured with tight-seal whole-cell recording was smaller than controls, particularly in the positive membrane potential domain. These results might explain myotonia in RGM on the single-channel level.

Splicing of a human endogenous retrovirus to a novel phospholipase A2 related gene

As part of an investigation into the effects of endogenous retroviruses on adjacent genes, we have isolated a cDNA clone derived from the human teratocarcinoma cell line NTera2D1 representing a chimeric transcript in which an endogenous retrovirus-like element, RTVL-H, has been spliced to downstream cellular sequences. The 5' terminus of this clone, termed AF-5, occurs one bp downstream of the predicted transcriptional start site in the RTVL-H long terminal repeat (LTR). AF-5 contains an open reading frame of 689 amino acids beginning within RTVL-H sequences that has two domains of homology with phospholipase A2 (PLA2). These domains, of approximately 120 amino acids each, are 30-38% identical to secreted PLA2s and contain sequence features of both group I and II enzymes. The corresponding AF-5 transcript is 2.5 kb and is derived from a single copy novel gene termed PLA2L. Southern analysis indicates that the RTVL-H element is normally present in human DNA upstream of the PLA2L gene. RTVL-H/PLA2L chimeric transcripts were detected in two independent teratocarcinoma cell lines but not in several other cell lines or primary human tissues. Characterization of additional cDNA clones and PCR analysis indicates that multiple RTVL-H/PLA2L alternatively spliced transcripts are expressed. No evidence has been found for transcription from a non-LTR promoter. These findings strongly suggest that the endogenous LTR promotes expression of the human PLA2L gene in teratocarcinoma cells.

Immunological identification of a Cl- channel protein in electric organs of Narke japonica

A Cl- channel in electric organs of Narke japonica was reconfirmed, using a polyclonal antibody, to be a 180k protein composed of two identical 90k units. The specific antibody against the 180k protein from electric organs reduced, when added in the cis side solution, the open probability of the Cl- channel in a planar bilayer membrane without affecting the single channel conductance, ion selectivity nor voltage dependency. Furthermore, the antibody added in the trans side also affected the channel to increase the open probability. The eluate of the immuno-affinity chromatography was found to contain only the 180k protein. These results indicate that 180k protein to be an integral membrane protein and to form the Cl- channel.

The periodic paralyses

These mysterious attacks of muscle weakness are difficult to treat because the different forms are hard to define, and potassium plays a different role in each one. New genetic probes should finally resolve these issues. In the meantime, attacks can be prevented in at least some patients. A multicenter study is now under way to determine the best treatment for each subtype.