Pathophysiology of sodium channelopathies. Studies of sodium channel expression by quantitative multiplex fluorescence polymerase chain reaction

Ion channels and neuronal excitability in polyglutamine neurodegenerative diseases

Polyglutamine (polyQ) diseases are a family composed of nine neurodegenerative inherited disorders (NDDs) caused by pathological expansions of cytosine-adenine-guanine (CAG) trinucleotide repeats which encode a polyQ tract in the corresponding proteins. CAG polyQ repeat expansions produce neurodegeneration via multiple downstream mechanisms; among those the neuronal activity underlying the ion channels is affected directly by specific channelopathies or indirectly by secondary dysregulation. In both cases, the altered excitability underlies to gain- or loss-of-function pathological effects. Here we summarize the repertoire of ion channels in polyQ NDDs emphasizing the biophysical features of neuronal excitability and their pathogenic role. The aim of this review is to point out the value of a deeper understanding of those functional mechanisms and processes as crucial elements for the designing and targeting of novel therapeutic avenues.

Dysfunction of NaV1.4, a skeletal muscle voltage-gated sodium channel, in sudden infant death syndrome: a case-control study

BACKGROUND

Sudden infant death syndrome (SIDS) is the leading cause of post-neonatal infant death in high-income countries. Central respiratory system dysfunction seems to contribute to these deaths. Excitation that drives contraction of skeletal respiratory muscles is controlled by the sodium channel NaV1.4, which is encoded by the gene SCN4A. Variants in NaV1.4 that directly alter skeletal muscle excitability can cause myotonia, periodic paralysis, congenital myopathy, and myasthenic syndrome. SCN4A variants have also been found in infants with life-threatening apnoea and laryngospasm. We therefore hypothesised that rare, functionally disruptive SCN4A variants might be over-represented in infants who died from SIDS.

METHODS

We did a case-control study, including two consecutive cohorts that included 278 SIDS cases of European ancestry and 729 ethnically matched controls without a history of cardiovascular, respiratory, or neurological disease. We compared the frequency of rare variants in SCN4A between groups (minor allele frequency <0·00005 in the Exome Aggregation Consortium). We assessed biophysical characterisation of the variant channels using a heterologous expression system.

FINDINGS

Four (1·4%) of the 278 infants in the SIDS cohort had a rare functionally disruptive SCN4A variant compared with none (0%) of 729 ethnically matched controls (p=0·0057).

INTERPRETATION

Rare SCN4A variants that directly alter NaV1.4 function occur in infants who had died from SIDS. These variants are predicted to significantly alter muscle membrane excitability and compromise respiratory and laryngeal function. These findings indicate that dysfunction of muscle sodium channels is a potentially modifiable risk factor in a subset of infant sudden deaths.

FUNDING

UK Medical Research Council, the Wellcome Trust, National Institute for Health Research, the British Heart Foundation, Biotronik, Cardiac Risk in the Young, Higher Education Funding Council for England, Dravet Syndrome UK, the Epilepsy Society, the Eunice Kennedy Shriver National Institute of Child Health & Human Development of the National Institutes of Health, and the Mayo Clinic Windland Smith Rice Comprehensive Sudden Cardiac Death Program.

Understanding the physiology of the asymptomatic diaphragm of the M1592V hyperkalemic periodic paralysis mouse

When muscles become paralyzed in crises of hyperkalemic periodic paralysis, patients do not stop breathing. Here is why.

The diaphragm muscle of hyperkalemic periodic paralysis (HyperKPP) patients and of the M1592V HyperKPP mouse model rarely suffers from the myotonic and paralytic symptoms that occur in limb muscles. Enigmatically, HyperKPP diaphragm expresses the mutant NaV1.4 channel and, more importantly, has an abnormally high Na⁺ influx similar to that in extensor digitorum longus (EDL) and soleus, two hindlimb muscles suffering from the robust HyperKPP abnormalities. The objective was to uncover the physiological mechanisms that render HyperKPP diaphragm asymptomatic. A first mechanism involves efficient maintenance of resting membrane polarization in HyperKPP diaphragm at various extracellular K⁺ concentrations compared with larger membrane depolarizations in HyperKPP EDL and soleus. The improved resting membrane potential (EM) results from significantly increased Na⁺ K⁺ pump electrogenic activity, and not from an increased protein content. Action potential amplitude was greater in HyperKPP diaphragm than in HyperKPP soleus and EDL, providing a second mechanism for the asymptomatic behavior of the HyperKPP diaphragm. One suggested mechanism for the greater action potential amplitude is lower intracellular Na⁺ concentration because of greater Na⁺ K⁺ pump activity, allowing better Na⁺ current during the action potential depolarization phase. Finally, HyperKPP diaphragm had a greater capacity to generate force at depolarized EM compared with wild-type diaphragm. Action potential amplitude was not different between wild-type and HyperKPP diaphragm. There was also no evidence for an increased activity of the Na⁺–Ca²⁺ exchanger working in the reverse mode in the HyperKPP diaphragm compared with the wild-type diaphragm. So, a third mechanism remains to be elucidated to fully understand how HyperKPP diaphragm generates more force compared with wild type. Although the mechanism for the greater force at depolarized resting EM remains to be determined, this study provides support for the modulation of the Na⁺ K⁺ pump as a component of therapy to alleviate weakness in HyperKPP.

Physiological basis for muscle stiffness and weakness in a knock-in M1592V mouse model of hyperkalemic periodic paralysis

The mechanisms responsible for the onset and progressive worsening of episodic muscle stiffness and weakness in hyperkalemic periodic paralysis (HyperKPP) are not fully understood. Using a knock‐in HyperKPP mouse model harboring the M1592V Na_V1.4 channel mutant, we interrogated changes in physiological defects during the first year, including tetrodotoxin‐sensitive Na⁺ influx, hindlimb electromyographic (EMG) activity and immobility, muscle weakness induced by elevated [K⁺]e, myofiber‐type composition, and myofiber damage. In situ EMG activity was greater in HyperKPP than wild‐type gastrocnemius, whereas spontaneous muscle contractions were observed in vitro. We suggest that both the greater EMG activity and spontaneous contractions are related to periods of hyperexcitability during which fibers generate action potentials by themselves in the absence of any stimulation and that these periods are the cause of the muscle stiffness reported by patients. HyperKPP muscles had a greater sensitivity to the K⁺‐induced force depression than wild‐type muscles. So, an increased interstitial K⁺ concentration locally near subsets of myofibers as a result of the hyperexcitability likely produced partial loss of force rather than complete paralysis. Na_V1.4 channel protein content reached adult level by 3 weeks postnatal in both wild type and HyperKPP and apparent symptoms did not worsen after the first month of age suggesting (i) that the phenotypic behavior of M1592V HyperKPP muscles results from defective function of mutant Na_V1.4 channels rather than other changes in protein expression after the first month and (ii) that the lag in onset during the first decade and the progression of human HyperKPP symptoms during adolescence are a function of Na_V1.4 channel content.

Loss-of-function mutations in SCN4A cause severe foetal hypokinesia or 'classical' congenital myopathy

See Cannon (doi: 10.1093/brain/awv400 ) for a scientific commentary on this article.

Congenital myopathies are a clinically and genetically heterogeneous group of muscle disorders characterized by congenital or early-onset hypotonia and muscle weakness, and specific pathological features on muscle biopsy. The phenotype ranges from foetal akinesia resulting in in utero or neonatal mortality, to milder disorders that are not life-limiting. Over the past decade, more than 20 new congenital myopathy genes have been identified. Most encode proteins involved in muscle contraction; however, mutations in ion channel-encoding genes are increasingly being recognized as a cause of this group of disorders. SCN4A encodes the α-subunit of the skeletal muscle voltage-gated sodium channel (Na _v 1.4). This channel is essential for the generation and propagation of the muscle action potential crucial to muscle contraction. Dominant SCN4A gain-of-function mutations are a well-established cause of myotonia and periodic paralysis. Using whole exome sequencing, we identified homozygous or compound heterozygous SCN4A mutations in a cohort of 11 individuals from six unrelated kindreds with congenital myopathy. Affected members developed in utero - or neonatal-onset muscle weakness of variable severity. In seven cases, severe muscle weakness resulted in death during the third trimester or shortly after birth. The remaining four cases had marked congenital or neonatal-onset hypotonia and weakness associated with mild-to-moderate facial and neck weakness, significant neonatal-onset respiratory and swallowing difficulties and childhood-onset spinal deformities. All four surviving cohort members experienced clinical improvement in the first decade of life. Muscle biopsies showed myopathic features including fibre size variability, presence of fibrofatty tissue of varying severity, without specific structural abnormalities. Electrophysiology suggested a myopathic process, without myotonia. In vitro functional assessment in HEK293 cells of the impact of the identified SCN4A mutations showed loss-of-function of the mutant Na _v 1.4 channels. All, apart from one, of the mutations either caused fully non-functional channels, or resulted in a reduced channel activity. Each of the affected cases carried at least one full loss-of-function mutation. In five out of six families, a second loss-of-function mutation was present on the trans allele. These functional results provide convincing evidence for the pathogenicity of the identified mutations and suggest that different degrees of loss-of-function in mutant Na _v 1.4 channels are associated with attenuation of the skeletal muscle action potential amplitude to a level insufficient to support normal muscle function. The results demonstrate that recessive loss-of-function SCN4A mutations should be considered in patients with a congenital myopathy.

Contractile abnormalities of mouse muscles expressing hyperkalemic periodic paralysis mutant NaV1.4 channels do not correlate with Na+ influx or channel content

Hyperkalemic periodic paralysis (HyperKPP) is characterized by myotonic discharges that occur between episodic attacks of paralysis. Individuals with HyperKPP rarely suffer respiratory distress even though diaphragm muscle expresses the same defective Na(+) channel isoform (NaV1.4) that causes symptoms in limb muscles. We tested the hypothesis that the extent of the HyperKPP phenotype (low force generation and shift toward oxidative type I and IIA fibers) in muscle is a function of 1) the NaV1.4 channel content and 2) the Na(+) influx through the defective channels [i.e., the tetrodotoxin (TTX)-sensitive Na(+) influx]. We measured NaV1.4 channel protein content, TTX-sensitive Na(+) influx, force generation, and myosin isoform expression in four muscles from knock-in mice expressing a NaV1.4 isoform corresponding to the human M1592V mutant. The HyperKPP flexor digitorum brevis muscle showed no contractile abnormalities, which correlated well with its low NaV1.4 protein content and by far the lowest TTX-sensitive Na(+) influx. In contrast, diaphragm muscle expressing the HyperKPP mutant contained high levels of NaV1.4 protein and exhibited a TTX-sensitive Na(+) influx that was 22% higher compared with affected extensor digitorum longus (EDL) and soleus muscles. Surprisingly, despite this high burden of Na(+) influx, the contractility phenotype was very mild in mutant diaphragm compared with the robust abnormalities observed in EDL and soleus. This study provides evidence that HyperKPP phenotype does not depend solely on the NaV1.4 content or Na(+) influx and that the diaphragm does not depend solely on Na(+)-K(+) pumps to ameliorate the phenotype.

Automated sequence screening of the entire dystrophin cDNA in Duchenne dystrophy: point mutation detection

This is the first report of direct sequencing of the complete 11 kb coding sequence of the dystrophin gene affording high sensitivity for all types of mutations of both coding sequence and splicing. Direct automated capillary gel sequence analysis of dystrophin reverse-transcribed polymerase chain reaction (RT-PCR) products was carried out in 15 Duchenne muscular dystrophy (DMD) patient muscle biopsies (170,000 bp sequenced). We identified mutations in 67% of patients tested (10/15); including premature stop codons (n = 5) and small deletions/duplications (n = 5). Mutation-negative patients (n = 5) were also negative for promoter mutations. All were tested for the possibility of transcription abnormalities using quantitative multiplex fluorescence polymerase chain studies (QMF-PCR), however, equal ratios of mRNA transcripts were identified at the 5'and 3' regions, with mild reduction in overall quantity, suggesting that transcription abnormalities were less likely. We suggested that such patients might have a problem with the 3.5 kb 3' UTR, polyA site or undetected stop codons. It is also possible that splicing defects could result in addition of intron sequence which could lead to preferential amplification of low level residual normal transcript skipping.

Fgfr4 is required for effective muscle regeneration in vivo. Delineation of a MyoD-Tead2-Fgfr4 transcriptional pathway

Fgfr4 has been shown to be important for appropriate muscle development in chick limb buds; however, Fgfr4 null mice show no phenotype. Here, we show that staged induction of muscle regeneration in Fgfr4 null mice becomes highly abnormal at the time point when Fgfr4 is normally expressed. By 7 days of regeneration, differentiation of myotubes became poorly coordinated and delayed by both histology and embryonic myosin heavy chain staining. By 14 days much of the muscle was replaced by fat and calcifications. To begin to dissect the molecular pathways involving Fgfr4, we queried the promoter sequences for transcriptional factor binding sites and tested candidate regulators in a 27-time point regeneration series. The Fgfr4 promoter region contained a Tead protein binding site (M-CAT 5'-CATTCCT-3'), and Tead2 showed induction during regeneration commensurate with Fgfr4 regulation. Co-transfection of Tead2 and Fgfr4 promoter reporter constructs into C2C12 myotubes showed Tead2 to activate Fgfr4, and mutation of the M-CAT motif in the Fgfr4 promoter abolished these effects. Immunostaining for Tead2 showed timed expression in myotube nuclei consistent with the mRNA data. Query of the expression timing and genomic sequences of Tead2 suggested direct regulation by MyoD, and consistent with this, MyoD directly bound to two strong E-boxes in the first intron of Tead2 by chromatin immunoprecipitation assay. Moreover, co-transfection of MyoD and Tead2 intron reporter constructs into 10T1/2 cells activated reporter activity in a dose-dependent manner. This activation was greatly reduced when the two E-boxes were mutated. Our data suggest a novel MyoD-Tead2-Fgfr4 pathway important for effective muscle regeneration.

Intracellular expression profiling by laser capture microdissection: three novel components of the neuromuscular junction

The neuromuscular junction (NMJ) is a regionally specialized area of myofibers defined, in part, by specific gene expression from underlying myonuclei. We sought to obtain a more complete picture of the mRNA transcripts and proteins playing a role in NMJ formation and maintenance using laser capture microdissection (LCM) and to define expression profiles of the nuclear domain at the NMJ. NMJs (800) were isolated from normal mouse tibialis anterior muscle by LCM, with an equal amount of adjacent non-NMJ regions isolated. Many known components of the NMJ were found significantly differentially expressed. Three differentially expressed potential novel components of the NMJ were chosen for further study, and each was validated by immunostaining with and without blocking peptides (3/3), quantitative RT-PCR (3/3), and in situ hybridization (1/3). The three genes validated were dual-specificity phosphatase-6 (DUSP6), ribosomal receptor-binding protein-1 (RRBP1), and vacuolar protein sorting-26 (VPS26). Query of each of these novel components in a 27-time point in vivo muscle regeneration series showed expression commensurate with previously known NMJ markers (nestin, alpha-ACh receptor). Understanding and discovering elements responsible for the integrity and function of NMJs is relevant to understanding neuromuscular diseases such as spinal muscular atrophy. Our LCM-based mRNA expression profiling provided us with new means of identification of specific genes potentially responsible for NMJ stability and function and new candidates for involvement in disease pathogenesis.

Embryonic myogenesis pathways in muscle regeneration

Embryonic myogenesis involves the staged induction of myogenic regulatory factors and positional cues that dictate cell determination, proliferation, and differentiation into adult muscle. Muscle is able to regenerate after damage, and muscle regeneration is generally thought to recapitulate myogenesis during embryogenesis. There has been considerable progress in the delineation of myogenesis pathways during embryogenesis, but it is not known whether the same signaling pathways are relevant to muscle regeneration in adults. Here, we defined the subset of embryogenesis pathways induced in muscle regeneration using a 27 time-point in vivo muscle regeneration series. The embryonic Wnt (Wnt1, 3a, 7a, 11), Shh pathway, and the BMP (BMP2, 4, 7) pathway were not induced during muscle regeneration. Moreover, antagonists of Wnt signaling, sFRP1, sFRP2, and sFRP4 (secreted frizzled-related proteins) were significantly up-regulated, suggesting active inhibition of the Wnt pathway. The pro-differentiation FGFR4 pathway was transiently expressed at day 3, commensurate with expression of MyoD, Myogenin, Myf5, and Pax7. Protein verification studies showed fibroblast growth factor receptor 4 (FGFR4) protein to be strongly expressed in differentiating myoblasts and newly formed myotubes. We present evidence that FGF6 is likely the key ligand for FGFR4 during muscle regeneration, and further suggest that FGF6 is released from necrotic myofibers where it is then sequestered by basal laminae. We also confirmed activation of Notch1 in the regenerating muscle. Finally, known MyoD coactivators (MEF2A, p/CIP, TCF12) and repressors (Twist, Id2) were strongly induced at appropriate time points. Taken together, our results suggest that embryonic positional signals (Wnt, Shh, and BMP) are not induced in postnatal muscle regeneration, whereas cell-autonomous factors (Pax7, MRFs, FGFR4) involving muscle precursor proliferation and differentiation are recapitulated by muscle regeneration.

Large-scale disruption of microtubule pathways in morphologically normal human spastin muscle

OBJECTIVE

To investigate the molecular pathways disrupted by dominant spastin mutations in apparently unaffected skeletal muscle from patients with motor neuron disease (SPG4).

METHODS

The authors studied muscle of three individuals from two unrelated families affected by spastic paraplegia caused by spastin mutations. The authors compared RNA expression profiles to 7 normal and 13 pathologic muscle U95A profiles (Duchenne dystrophy, acute quadriplegic myopathy, and spinal muscular atrophy). Data were validated with U133A arrays with seven different control specimens. mRNA and protein confirmations were done for a subset of genes.

RESULTS

Both nonsense and missense mutations in the spastin gene disrupted microtubule pathways in nonpathologic tissue, including microtubule dynamics, stability, exocytosis, and endocytosis.

CONCLUSIONS

Normal muscle can be used to uncover biochemical perturbation in motor neuron disease. Altered microtubule metabolism in SPG4-linked hereditary spastic paraplegia patients leads to pathology of the long descending tracks of motor neurons that likely have a stringent need for efficient microtubular transport. As many inherited neurologic conditions show a systemic biochemical defect with disease limited to neurons, our data have broader implications for biochemical pathway studies of many neurologic disorders.

Novel isoforms of the sodium channels Nav1.8 and Nav1.5 are produced by a conserved mechanism in mouse and rat

The voltage-gated sodium channel Na(v)1.8 is only expressed in subsets of neurons in dorsal root ganglia (DRG) and trigeminal and nodose ganglia. We have isolated mouse partial length Na(v)1.8 cDNA clones spanning the exon 17 sequence, which have 17 nucleotide substitutions and 12 predicted amino acid differences from the published sequence. The absence of a mutually exclusive alternative exon 17 was confirmed by sequencing 4.1 kilobases of genomic DNA spanning exons 16-18 of Scn10a. A novel cDNA isoform was identified, designated Na(v)1.8c, which results from alternative 3'-splice site selection at a CAG/CAG motif to exclude the codon for glutamine 1031 within the interdomain cytoplasmic loop IDII/III. The ratio of Na(v)1.8c (CAG-skipped) to Na(v)1.8 (CAG-inclusive) mRNA in mouse is approximately 2:1 in adult DRG, trigeminal ganglion, and neonatal DRG. A Na(v)1.8c isoform also occurs in rat DRG, but is less common. Of the two other tetrodotoxin-resistant channels, no analogous alternative splicing of mouse Na(v)1.9 was detected, whereas rare alternative splicing of Na(v)1.5 at a CAG/CAG motif resulted in the introduction of a CAG trinucleotide. This isoform, designated Na(v)1.5c, is conserved in rat and encodes an additional glutamine residue that disrupts a putative CK2 phosphorylation site. In summary, novel isoforms of Na(v)1.8 and Na(v)1.5 are each generated by alternative splicing at CAG/CAG motifs, which result in the absence or presence of predicted glutamine residues within the interdomain cytoplasmic loop IDII/III. Mutations of sodium channels within this cytoplasmic loop have previously been demonstrated to alter electrophysiological properties and cause cardiac arrhythmias and epilepsy.

Gene profiling in spinal cord injury shows role of cell cycle in neuronal death

Spinal cord injury causes secondary biochemical changes leading to neuronal cell death. To clarify the molecular basis of this delayed injury, we subjected rats to spinal cord injury and identified gene expression patterns by high-density oligonucleotide arrays (8,800 genes studied) at 30 minutes, 4 hours, 24 hours, or 7 days after injury (total of 26 U34A profiles). Detailed analyses were limited to 4,300 genes consistently expressed above background. Temporal clustering showed rapid expression of immediate early genes (30 minutes), followed by genes associated with inflammation, oxidative stress, DNA damage, and cell cycle (4 and 24 hours). Functional clustering showed a novel pattern of cell cycle mRNAs at 4 and 24 hours after trauma. Quantitative reverse transcription polymerase chain reaction verified mRNA changes in this group, which included gadd45a, c-myc, cyclin D1 and cdk4, pcna, cyclin G, Rb, and E2F5. Changes in their protein products were quantified by Western blot, and cell-specific expression was determined by immunocytochemistry. Cell cycle proteins showed an increased expression 24 hours after injury and were, in part, colocalized in neurons showing morphological evidence of apoptosis. These findings suggest that cell cycle-related genes, induced after spinal cord injury, are involved in neuronal damage and subsequent cell death.

Slug is a novel downstream target of MyoD. Temporal profiling in muscle regeneration

Temporal expression profiling was utilized to define transcriptional regulatory pathways in vivo in a mouse muscle regeneration model. Potential downstream targets of MyoD were identified by temporal expression, promoter data base mining, and gel shift assays; Slug and calpain 6 were identified as novel MyoD targets. Slug, a member of the snail/slug family of zinc finger transcriptional repressors critical for mesoderm/ectoderm development, was further shown to be a downstream target by using promoter/reporter constructs and demonstration of defective muscle regeneration in Slug null mice.

Integrin alpha 7 beta 1 in muscular dystrophy/myopathy of unknown etiology

To investigate the role of integrin alpha 7 in muscle pathology, we used a "candidate gene" approach in a large cohort of muscular dystrophy/myopathy patients. Antibodies against the intracellular domain of the integrin alpha 7A and alpha 7B were used to stain muscle biopsies from 210 patients with muscular dystrophy/myopathy of unknown etiology. Levels of alpha 7A and alpha 7B integrin were found to be decreased in 35 of 210 patients (approximately 17%). In six of these patients no integrin alpha 7B was detected. Screening for alpha 7B mutation in 30 of 35 patients detected only one integrin alpha 7 missense mutation (the mutation on the second allele was not found) in a patient presenting with a congenital muscular dystrophy-like phenotype. No integrin alpha 7 gene mutations were identified in all of the other patients showing integrin alpha 7 deficiency. In the process of mutation analysis, we identified a novel integrin alpha 7 isoform presenting 72-bp deletion. This isoform results from a partial deletion of exon 21 due to the use of a cryptic splice site generated by a G to A missense mutation at nucleotide position 2644 in integrin alpha 7 cDNA. This spliced isoform is present in about 12% of the chromosomes studied. We conclude that secondary integrin alpha 7 deficiency is rather common in muscular dystrophy/myopathy of unknown etiology, emphasizing the multiple mechanisms that may modulate integrin function and stability.

Expression of multiple Na+/H+ exchanger isoforms in rat parotid acinar and ductal cells

Several members of the Na+/H+ exchanger gene family (NHE1, NHE2, NHE3, and NHE4) with unique functional properties have been cloned from rat epithelial tissues. The present study examined the molecular and pharmacological properties of Na+/H+ exchange in rat parotid salivary gland cells. In acinar cells superfused with a physiological salt solution (145 mM Na+), Na+/H+ exchanger activity was inhibited by low concentrations of the amiloride derivative ethylisopropyl amiloride (EIPA; IC50 = 0.014 +/- 0.005 microM), suggesting the expression of amiloride-sensitive isoforms NHE1 and/or NHE2. Semiquantitative RT-PCR confirmed that NHE1 transcripts are most abundant in this cell type. In contrast, the intermediate sensitivity of ductal cells to EIPA indicated that inhibitor-sensitive and -resistant Na+/H+ exchanger isoforms are coexpressed. Ductal cells were about one order of magnitude more resistant to EIPA (IC50 = 0.754 +/- 0.104 microM) than cell lines expressing NHE1 or NHE2 (IC50 = 0.076 +/- 0.013 or 0.055 +/- 0.015 microM, respectively). Conversely, ductal cells were nearly one order of magnitude more sensitive to EIPA than a cell line expressing the NHE3 isoform (IC50 = 6.25 +/- 1.89 microM). Semiquantitative RT-PCR demonstrated that both NHE1 and NHE3 transcripts are expressed in ducts. NHE1 was immunolocalized to the basolateral membranes of acinar and ductal cells, whereas NHE3 was exclusively seen in the apical membrane of ductal cells. Immunoblotting, immunolocalization, and semiquantitative RT-PCR experiments failed to detect NHE2 expression in either cell type. Taken together, our results demonstrate that NHE1 is the dominant functional Na+/H+ exchanger in the plasma membrane of rat parotid acinar cells, whereas NHE1 and NHE3 act in concert to regulate the intracellular pH of ductal cells.

Two E-boxes are the focal point of muscle-specific skeletal muscle type 1 Na+ channel gene expression

We have characterized a group of cis-regulatory elements that control muscle-specific expression of the rat skeletal muscle type 1 sodium channel (SkM1) gene. These elements are located within a 3. 1-kilobase fragment that encompasses the 5'-flanking region, first exon, and part of the first intron of SkM1. We sequenced the region between -1062 and +311 and determined the start sites of transcription; multiple sites were identified between +1 and +30. The basal promoter (-65/+11) lacks cell-type specificity, while an upstream repressor (-174/-65) confers muscle-specific expression. A positive element (+49/+254) increases muscle-specific expression. Within these broad elements, two E boxes play a pivotal role. One E box at -31/-26 within the promoter, acting in part through its ability to bind the myogenic basic helix-loop-helix proteins, recruits additional factor(s) that bind elsewhere within the SkM1 sequence to control positive expression of the gene. A second E box at -90/-85 within the repressor controls negative regulation of the gene and acts through a different complex of proteins. Several of these cis-regulatory elements share both sequence and functional similarities with cis-regulatory elements of the acetylcholine receptor delta-subunit; the different arrangement of these elements may contribute to unique expression patterns for the two genes.

Role of the nerve in determining fetal skeletal muscle phenotype

To determine the role of the nerve on the establishment of myofiber diversity in skeletal muscles, the lumbosacral spinal cord of 14-day gestation mice (E14) was laser ablated, and the accumulation of the myosin alkali light chains (MLC) mRNAs in crural (hindleg) muscles was evaluated just prior to birth with in situ hybridization. Numbers of molecules of each alkali MLC/ng total RNA in the extensor digitorum longus (EDL) and soleus muscles were determined with competitive polymerase chain reaction. Transcripts for all four alkali MLCs accumulate in aneural muscles. Data suggest that: (1) the absence of the nerve to either future fast or slow muscles results in less accumulation of MLC1V transcript. Moreover, the presence of the nerve is required for the enhanced accumulation of this transcript in future slow muscles; (2) the absence of innervation of future slow, but not fast, muscles decreases the accumulation of MLC1A transcript. Since increased accumulation of MLC1A and MLC1V transcripts are found in future slow muscles at birth, the nerve is necessary for the development of the slow phenotype during myogenesis; (3) MLC1F and MLC3F transcripts do not display any preferential accumulation in future fast muscles during the fetal period. Therefore, the establishment of the differential distribution of these mRNAs, based on fiber type, is a postnatal phenomenon. The nerve is required during the fetal period to allow accumulation of MLC3F messages above a basal level in future fast as well as slow muscles; whereas, the absence of the innervation to future fast, but not slow, muscles reduces the accumulation of MLC1F. Thus, the accumulation of the various alkali MLC mRNAs shows a differential, rather than coordinate, response to the absence of the nerve, and this response may vary depending on the future fiber type of the muscles.

Transcription of hepatitis B virus in peripheral blood mononuclear cells from persistently infected patients

Hepatitis B virus (HBV) has been reported to exist in peripheral blood mononuclear cells (PBMC), but it is not clear whether it replicates there. A precondition for replication should be the formation of covalently closed viral DNA and transcription of all essential viral mRNAs. The mRNAs of HBV form a nested box with common 3' ends. In order to detect even low levels of potential replication, we developed a quantitative reverse transcription-PCR method for detection of a smaller HBV mRNA species in the presence of the larger ones. All three highly viremic patients tested so far had mRNAs for the large and the small surface proteins and the X protein of the virus within PBMC but not in the virus from their sera. Furthermore, we detected by PCR covalently closed viral DNA in their PBMC. These data suggest that HBV may be not only taken up but also replicated by mononuclear blood cells and that these cells may be an extrahepatic site of viral persistence. X mRNA was detected in the largest amount. Possibly, X protein interferes with functions of the mononuclear cells during the immune response against the virus.

Congenital muscular dystrophy with primary laminin alpha2 (merosin) deficiency presenting as inflammatory myopathy

Ten laminin alpha2-deficient patients were identified by both immunofluorescence and immunoblotting (30% of congenital muscular dystrophy patients tested). Three of the laminin alpha2-deficient patients were carrying a diagnosis of infantile polymyositis prior to immunostaining studies. The clinical features in the 10 merosin-deficient patients were homogeneous, with severe floppiness at birth, delay in achievement of motor milestones, and magnetic resonance imaging findings of white matter changes with normal intelligence. The 10-kb laminin alpha2-coding sequence was screened for causative mutations by reverse transcriptase-polymerase chain reaction/single-stranded conformational polymorphism analysis in muscle biopsy specimens from 5 patients, followed by automatic sequencing of aberrant conformers. Clear loss-of-function deletion mutations were identified in both alleles of 1 patient. Muscle histopathology in this patient showed a striking inflammatory infiltrate of T cells and B cells. Reexamination of biopsy specimens from other laminin alpha2-deficient patients showed minor signs of inflammation in each. Based on these findings and the histological and clinical picture suggesting failure of muscle regeneration, a pathogenesis model for this major subset of congenital muscular dystrophy is proposed. Our data show that muscle histopathology showing a neonatal inflammatory process should be considered consistent with congenital muscular dystrophy.

Accurate and absolute quantitative measurement of gene expression by single-tube RT-PCR and HPLC

We report a method that allows accurate, absolute quantification of gene expression in a single reverse transcriptase (RT)-PCR reaction. This method makes use of novel high-performance liquid chromatography (HPLC) technology to resolve and quantify the products of competitive, mutant RNA PCRs. The HPLC technique allows rapid, high resolution of reaction products. On-line UV detection eliminates the need for radiolabel or other tracers. The HPLC technique also demonstrates that these competition reactions readily generate heteroduplex products. The ability of HPLC to resolve and quantify heteroduplex products is fundamental to the accuracy of the technique. Accurate measurements of gene expression have been obtained over four orders of magnitude and experiments employing predetermined quantities of specific native RNA input have demonstrated the ability of the system to provide absolute estimates of gene expression. Large size differences between native and mutant RNA inputs affected reverse transcriptase (RT) efficiency, but not PCR amplification efficiency. However, the magnitude of the RT efficiency effect can be estimated, is reproducible, and can therefore be adjusted by a calculated correction factor. The RT efficiency difference can been eliminated by reduction in the magnitude of the sequence difference between native and mutant RNA so that no correction factor is required. The application of the technique to quantification of expression of the alpha 1 subunit of sodium, potassium-ATPase in microdissected nephron segments is demonstrated.