Interaction between the skeletal muscle type 1 Na+ channel promoter E-box and an upstream repressor element. Release of repression by myogenin

Divergent cis-regulatory evolution underlies the convergent loss of sodium channel expression in electric fish

South American and African weakly electric fish independently evolved electric organs from muscle. In both groups, a voltage-gated sodium channel gene independently lost expression from muscle and gained it in the electric organ, allowing the channel to become specialized for generating electric signals. It is unknown how this voltage-gated sodium channel gene is targeted to muscle in any vertebrate. We describe an enhancer that selectively targets sodium channel expression to muscle. Next, we demonstrate how the loss of this enhancer, but not trans-activating factors, drove the loss of sodium channel gene expression from muscle in South American electric fish. While this enhancer is also altered in African electric fish, key transcription factor binding sites and enhancer activity are retained, suggesting that the convergent loss of sodium channel expression from muscle in these two electric fish lineages occurred via different processes.

New Challenges Resulting From the Loss of Function of Nav1.4 in Neuromuscular Diseases

The voltage-gated sodium channel Na_v1.4 is a major actor in the excitability of skeletal myofibers, driving the muscle force in response to nerve stimulation. Supporting further this key role, mutations in SCN4A, the gene encoding the pore-forming α subunit of Na_v1.4, are responsible for a clinical spectrum of human diseases ranging from muscle stiffness (sodium channel myotonia, SCM) to muscle weakness. For years, only dominantly-inherited diseases resulting from Na_v1.4 gain of function (GoF) were known, i.e., non-dystrophic myotonia (delayed muscle relaxation due to myofiber hyperexcitability), paramyotonia congenita and hyperkalemic or hypokalemic periodic paralyses (episodic flaccid muscle weakness due to transient myofiber hypoexcitability). These last 5 years, SCN4A mutations inducing Na_v1.4 loss of function (LoF) were identified as the cause of dominantly and recessively-inherited disorders with muscle weakness: periodic paralyses with hypokalemic attacks, congenital myasthenic syndromes and congenital myopathies. We propose to name this clinical spectrum sodium channel weakness (SCW) as the mirror of SCM. Na_v1.4 LoF as a cause of permanent muscle weakness was quite unexpected as the Na⁺ current density in the sarcolemma is large, securing the ability to generate and propagate muscle action potentials. The properties of SCN4A LoF mutations are well documented at the channel level in cellular electrophysiological studies However, much less is known about the functional consequences of Na_v1.4 LoF in skeletal myofibers with no available pertinent cell or animal models. Regarding the therapeutic issues for Na_v1.4 channelopathies, former efforts were aimed at developing subtype-selective Na_v channel antagonists to block myofiber hyperexcitability. Non-selective, Na_v channel blockers are clinically efficient in SCM and paramyotonia congenita, whereas patient education and carbonic anhydrase inhibitors are helpful to prevent attacks in periodic paralyses. Developing therapeutic tools able to counteract Na_v1.4 LoF in skeletal muscles is then a new challenge in the field of Na_v channelopathies. Here, we review the current knowledge regarding Na_v1.4 LoF and discuss the possible therapeutic strategies to be developed in order to improve muscle force in SCW.

Physiological and Pathophysiological Insights of Nav1.4 and Nav1.5 Comparison

Mutations in Nav1.4 and Nav1.5 α-subunits have been associated with muscular and cardiac channelopathies, respectively. Despite intense research on the structure and function of these channels, a lot of information is still missing to delineate the various physiological and pathophysiological processes underlying their activity at the molecular level. Nav1.4 and Nav1.5 sequences are similar, suggesting structural and functional homologies between the two orthologous channels. This also suggests that any characteristics described for one channel subunit may shed light on the properties of the counterpart channel subunit. In this review article, after a brief clinical description of the muscular and cardiac channelopathies related to Nav1.4 and Nav1.5 mutations, respectively, we compare the knowledge accumulated in different aspects of the expression and function of Nav1.4 and Nav1.5 α-subunits: the regulation of the two encoding genes (SCN4A and SCN5A), the associated/regulatory proteins and at last, the functional effect of the same missense mutations detected in Nav1.4 and Nav1.5. First, it appears that more is known on Nav1.5 expression and accessory proteins. Because of the high homologies of Nav1.5 binding sites and equivalent Nav1.4 sites, Nav1.5-related results may guide future investigations on Nav1.4. Second, the analysis of the same missense mutations in Nav1.4 and Nav1.5 revealed intriguing similarities regarding their effects on membrane excitability and alteration in channel biophysics. We believe that such comparison may bring new cues to the physiopathology of cardiac and muscular diseases.

The smooth muscle cell-restricted KCNMB1 ion channel subunit is a direct transcriptional target of serum response factor and myocardin

Large conductance calcium-activated potassium (MaxiK) channels play a pivotal role in maintaining normal arterial tone by regulating the excitation-contraction coupling process. MaxiK channels comprise alpha and beta subunits encoded by Kcnma and the cell-restricted Kcnmb genes, respectively. Although the functionality of MaxiK channel subunits has been well studied, the molecular regulation of their transcription and modulation in smooth muscle cells (SMCs) is incomplete. Using several model systems, we demonstrate down-regulation of Kcnmb1 mRNA upon SMC phenotypic modulation in vitro and in vivo. As part of a broad effort to define all functional CArG elements in the genome (i.e. the CArGome), we discovered two conserved CArG boxes located in the proximal promoter and first intron of the human KCNMB1 gene. Gel shift and chromatin immunoprecipitation assays confirmed serum response factor (SRF) binding to both CArG elements. A luciferase assay showed myocardin (MYOCD)-mediated transactivation of the KCNMB1 promoter in a CArG element-dependent manner. In vivo analysis of the human KCNMB1 promoter disclosed activity in embryonic heart and aortic SMCs; mutation of both conserved CArG elements completely abolished in vivo promoter activity. Forced expression of MYOCD increased Kcnmb1 expression in a variety of rodent and human non-SMC lines with no effect on expression of the Kcnma1 subunit. Conversely, knockdown of Srf resulted in decreases of endogenous Kcnmb1. Functional studies demonstrated MYOCD-induced, iberiotoxin-sensitive potassium currents in porcine coronary SMCs. These results reveal the first ion channel subunit as a direct target of SRF-MYOCD transactivation, providing further insight into the role of MYOCD as a master regulator of the SMC contractile phenotype.

Multiple regulatory elements mediating neuronal-specific expression of zebrafish sodium channel gene, scn8aa

Zebrafish scn8aa sodium channels mediate the majority of sodium conductance, which is essential for the embryonic locomotor activities. Here, we investigated the transcriptional regulation of scn8aa in developing zebrafish embryos by constructing a GFP reporter driven by a 15-kb fragment of scn8aa gene designed as scn8aa:GFP. GFP expression patterns of scn8aa:GFP temporally and spatially recapitulated the expression of endogenous scn8aa mRNA during zebrafish embryonic development, with one exception in the inner nuclear layer of the retina. Three novel elements, along with an evolutionarily conserved element shared with mouse SCN8A, modulated neuronal-specific expression of scn8aa. The deletion of each positive element reduced the expression levels in neurons without inducing ectopic GFP expression in non-neuronal cells. Our results demonstrate that these four regulatory elements function cooperatively to enhance scn8aa expression in the zebrafish nervous system.

S. macrurus myogenic regulatory factors (MRFs) induce mammalian skeletal muscle differentiation; evidence for functional conservation of MRFs

The current-producing cells of the electric organ, i.e., electrocytes, in Sternopygus macrurus derive from skeletal muscle fibers. Mature electrocytes are not contractile, but they do retain some muscle proteins, are multinucleated, and receive cholinergic innervation. Electrocytes express the myogenic regulatory factors (MRFs) MyoD, myogenin, Myf5 and MRF4 despite their incomplete muscle phenotype. Although S. macrurus MRFs share functional domains which are highly conserved and their expression is confined to the myogenic lineage, their capability to induce the muscle phenotype has not been determined. To test the functional conservation of S. macrurus MRFs to transcriptionally activate skeletal muscle gene expression and induce the myogenic program, we transiently over-expressed S. macrurus MyoD (SmMyoD) and myogenin (SmMyoG) in mouse C3H/10T1/2 and NIH3T3 embryonic cells. RT-PCR and immunolabeling studies showed that SmMyoD and SmMyoG can efficiently convert these two cell lines into multinucleated myotubes which expressed differentiated muscle markers. The levels of myogenic induction by SmMyoD and SmMyoG were comparable to those obtained with mouse MRF homologs. Furthermore, SmMyoD and SmMyoG proteins were able to induce mouse MyoD and myogenin in C3H/10T1/2 cells. We conclude that S. macrurus MRFs are functionally conserved as they can transcriptionally activate skeletal muscle gene expression and induce the myogenic program in mammalian non-muscle cells. Hence, these data suggest that the partial muscle phenotype of electrocytes is not likely due to differences in the MRF-dependent transcriptional program between skeletal muscle and electric organ.

Herpes simplex virus-mediated expression of Pax3 and MyoD in embryoid bodies results in lineage-Related alterations in gene expression profiles

The ability of embryonic stem cells to develop into multiple cell lineages provides a powerful resource for tissue repair and regeneration. Gene transfer offers a means to dissect the complex events in lineage determination but is limited by current delivery systems. We designed a high-efficiency replication-defective herpes simplex virus gene transfer vector (JDbetabeta) for robust and transient expression of the transcription factors Pax3 and MyoD, which are known to be involved in skeletal muscle differentiation. JDbetabeta-mediated expression of each gene in day 4 embryoid bodies (early-stage mesoderm) resulted in the induction of unique alterations in gene expression profiles, including the upregulation of known target genes relevant to muscle and neural crest development, whereas a control enhanced green fluorescent protein expression vector was relatively inert. This vector delivery system holds great promise for the use of gene transfer to analyze the impact of specific genes on both regulatory genetic events and commitment of stem cells to particular lineages.

Identification of the sensory neuron specific regulatory region for the mouse gene encoding the voltage-gated sodium channel NaV1.8

Voltage-gated sodium channels (VGSC) are critical membrane components that participate in the electrical activity of excitable cells. The type one VGSC family includes the tetrodotoxin insensitive sodium channel, Na(V)1.8, encoded by the Scn10a gene. Na(V)1.8 expression is restricted to small and medium diameter nociceptive sensory neurons of the dorsal root ganglia and cranial sensory ganglia. To understand the stringent transcriptional regulation of the Scn10a gene, the sensory neuron specific promoter was functionally identified. While identifying the mRNA 5'-end, alternative splicing within the 5'-UTR was observed to create heterogeneity in the RNA transcript. Four kilobases of upstream genomic DNA was cloned and the presence of tissue specific promoter activity was tested by microinjection and adenoviral infection of fluorescent protein reporter constructs into primary mouse and rat neurons, and cell lines. The region contained many putative transcription factor-binding sites and strong homology with the predicted rat ortholog. Homology to the predicted human ortholog was limited to the proximal end and several conserved cis elements were noted. Two regulatory modules were identified by microinjection of reporter constructs into dorsal root ganglia and superior cervical ganglia neurons: a neuron specific proximal promoter region between -1.6 and -0.2 kb of the transcription start site cluster, and a distal sensory neuron switch region beyond -1.6 kb that restricted fluorescent protein expression to a subset of primary sensory neurons.

Basic helix-loop-helix factors recruit nuclear factor I to enhance expression of the NaV 1.4 Na+ channel gene

We have previously shown that the basic helix-loop-helix (bHLH) transcription factors coordinate Na(V) 1.4 Na(+) channel gene expression in skeletal muscle, but the identity of the co-factors they direct is unknown. Using C2C12 muscle cells as a model system, we test the hypothesis that the bHLH factors counteract negative regulation exerted through a repressor E box (-90/-85) by recruiting positive-acting transcription factors to the nucleotides (-135/-57) surrounding the repressor E box. We used electrophoretic mobility shift assays to identify candidate factors that bound the repressor E box or these adjacent regions. Repressor E box-binding factors included the known transcription factor, ZEB/AREB6, and a novel repressor E box-binding factor designated REB. Mutations of the repressor E box that interfere with the binding of these factors prevented repression. The transcription factor, nuclear factor I (NFI), bound immediately upstream and downstream of the repressor E box. Mutation of the NFI-binding sites diminished the ability of myogenin and MRF4 to counteract repression. Based on these observations we suggest that bHLH factors recruit NFI to enhance skeletal muscle Na(+) channel expression.

Enhancement of Asynchronous and Train-Evoked Exocytosis in Bovine Adrenal Chromaffin Cells Infected With a Replication Deficient Adenovirus

Bovine adrenal chromaffin cells share many characteristics with neurons and are often used as a simple model system to study ion channels and neurotransmitter release. We infected bovine adrenal chromaffin cells with a replication deficient adenovirus that induces expression of the common reporters β-galactosidase and Green Fluorescent Protein via a bicistronic sequence. In perforated-patch recordings performed 48-h postinfection, peak calcium currents were reduced 32%, primarily due to loss of ω-conotoxin-GVIA-sensitive current. In contrast, sodium currents were increased 17%. Exocytosis, detected as an increase in membrane capacitance immediately after a single step depolarization, was reduced in proportion to the decrease in calcium influx. However, capacitance continued to increase for seconds after the depolarization. The amplitude of this poststimulus drift, or asynchronous exocytosis, was approximately three times that which occurred in a small fraction of control cells. Exocytosis evoked by repetitive stimulation with a train of brief depolarizations was increased 50%. Intracellular calcium levels measured during and after stimulation were lower, not higher, in adenovirus-infected cells. Electroporated cells showed reduced calcium currents but no enhancement of exocytosis. Cells infected with UV-irradiated virus showed reduced calcium currents and enhancement of exocytosis, but the changes were smaller than those caused by intact virus. Our results are consistent with the idea that adenovirus capsid and adenoviral DNA contribute to a Ca2+influx- and [Ca2+]i-independent enhancement of exocytosis in bovine chromaffin cells.

A selective role for MRF4 in innervated adult skeletal muscle: Na(V) 1.4 Na+ channel expression is reduced in MRF4-null mice

The factors that regulate transcription and spatial expression of the adult skeletal muscle Na+ channel, Na(V) 1.4, are poorly understood. Here we tested the role of the transcription factor MRF4, one of four basic helix-loop-helix (bHLH) factors expressed in skeletal muscle, in regulation of the Na(V) 1.4 Na+ channel. Overexpression of MRF4 in C2C12 muscle cells dramatically elevated Na(V) 1.4 reporter gene expression, indicating that MRF4 is more efficacious than the other bHLH factors expressed at high levels endogenously in these cells. In vivo, MRF4 protein was found both in extrajunctional and subsynaptic muscle nuclei. To test the importance of MRF4 in Na(V) 1.4 gene regulation in vivo, we examined Na+ channel expression in MRF4-null mice using several techniques, including Western blotting, immunocytochemistry, and electrophysiological recording. By all methods, we found that expression of the Na(V) 1.4 Na+ channel was substantially reduced in MRF4-null mice, both in the surface membrane and at neuromuscular junctions. In contrast, expression of the acetylcholine receptor, and in particular its alpha subunit, was unchanged, indicating that MRF4 regulation of Na+ channel expression was selective. Expression of the bHLH factors myf-5, MyoD, and myogenin was increased in MRF4-null mice, but these factors were not able to fully maintain Na(V) 1.4 Na+ channel expression either in the extrajunctional membrane or at the synapse. Thus, MRF4 appears to play a novel and selective role in adult muscle.

Rab3A negatively regulates activity-dependent modulation of exocytosis in bovine adrenal chromaffin cells

Members of the Rab family of monomeric GTPases have been implicated in vesicle trafficking, and Rab3A, located on synaptic vesicles in neurones and secretory vesicles in neuroendocrine cells, is likely to be involved in vesicle fusion leading to neurotransmitter release. A hydrolysis-deficient mutant of Rab3A, Rab3AQ81L, has been shown to potently inhibit hormone release. Here we show that the inhibition of hormone release by Rab3AQ81L is activity-dependent. Bovine adrenal chromaffin cells were induced to express Rab3AQ81L and green fluorescent protein by adenoviral gene transfer of a bicistronic construct. Fluorescent cells were stimulated with single depolarizations and trains of depolarizing pulses in whole cell perforated patch clamp recordings, and exocytosis was detected with cell capacitance measurements and carbon fibre amperometry. When single depolarizations were used to evoke exocytosis, cells expressing Rab3AQ81L showed a 50% reduction in response amplitude. When trains of brief depolarizations (10 or 40 ms) were used to evoke exocytosis, responses rapidly declined to zero in cells expressing Rab3AQ81L. Wild-type Rab3A had effects similar to Rab3AQ81L, causing significant inhibition of exocytosis only during repetitive stimulation. Expression of Rab5A did not alter exocytosis evoked by single depolarizations or repetitive stimulation. Applying a long duration depolarization in the middle of a stimulus train revealed that exocytotic efficacy (capacitance increase per amount of calcium influx) was not decreased in Rab3AQ81L-expressing cells. Instead, the activity-dependent increase in exocytotic efficacy observed in control cells did not occur in Rab3AQ81L-expressing cells. Our results suggest that Rab3A in the GTP bound conformation prevents activity-dependent facilitation.

Evidence that E-box promoter elements and MyoD transcription factors play a role in the induction of cathepsin B gene expression during human myoblast differentiation

HB13 human myoblasts express physiological and biochemical markers associated with myoblast differentiation in non-human cell culture model systems. During differentiation, HB13 myoblasts also demonstrate fusion-related increases in cathepsin B activity and protein levels. These increases are associated with an increase in levels of cathepsin B mRNA suggesting the involvement of transcriptional regulatory mechanisms. To examine these mechanisms human myoblasts were transfected with cathepsin B nested deletion promoter constructs within the 1.8 kb 5' promoter 1 region of the human catB gene. Transfected myoblasts that were maintained under differentiating conditions demonstrated higher promoter activity than those maintained in proliferating conditions. The highest activity was obtained with pSCB2-3 (-1279/+56 bp), a construct containing two putative upstream E-box elements. Co-transfection experiments demonstrated that MyoD and myogenin transactivate cathepsin B promoter activity. Electrophoretic mobility shift assays of nuclear extracts incubated with an oligonucleotide containing two upstream E-box elements found within the cathepsin B promoter demonstrated two band shifts. The band shifts were abolished using an oligonucleotide with mutations in both E-box elements. Moreover, the shifted bands were super-shifted and abolished when incubated with anti-myogenin and anti-MyoD, respectively. Collectively, these data support myogenic transcription factor-mediated activation of cathepsin B expression during myogenesis.

USF-1 and USF-2 trans-repress IL-1beta-induced iNOS transcription in mesangial cells

Transcriptional activation of the inducible nitric oxide synthase (iNOS) gene requires multiple interactions of cis elements and trans-acting factors. Previous in vivo footprinting studies (Goldring CE, Reveneau S, Algarte M, and Jeannin JF. Nucleic Acids Res 24: 1682-1687, 1996) of the murine iNOS gene demonstrated lipopolysaccharide-inducible protection of guanines in the region -904/-883, which includes an E-box motif. In this report, by using site-directed mutagenesis of the -893/-888 E-box and correlating functional assays of the mutated iNOS promoter with upstream stimulatory factor (USF) DNA-binding activities, we demonstrate that the -893/-888 E-box motif is functionally required for iNOS regulation in murine mesangial cells and that USFs are in vivo components of the iNOS transcriptional response complex. Mutation of the E-box sequence augmented the iNOS response to interleukin-1beta (IL-1beta) in transiently transfected mesangial cells. Gel mobility shift assays demonstrated that USFs cannot bind to the -893/-888 E-box promoter region when the E-box is mutated. Cotransfection of USF-1 and USF-2 expression vectors with iNOS promoter-luciferase reporter constructs suppressed IL-1beta-simulated iNOS promoter activity. Cotransfection of dominant-negative USF-2 mutants lacking the DNA binding domain or cis-element decoys containing concatamers of the -904/-883 region augmented IL-1beta stimulation of iNOS promoter activity. Gel mobility shift assays showed that only USF-1 and USF-2 supershifted the USF protein-DNA complexes. These results demonstrated that USF binding to the E-box at -893/-888 serves to trans-repress basal expression and IL-1beta induction of the iNOS promoter.

The WldS protein protects against axonal degeneration: a model of gene therapy for peripheral neuropathy

The WldS mouse is a spontaneous mutant that is characterized by the phenotype of delayed degeneration of transected nerves (slow Wallerian degeneration). Molecular genetic analysis identified a mutation in this animal that codes for a unique protein expressed in brain tissue of WldS mice. We asked whether the WldS phenotype, in addition to delaying axonal degeneration after axotomy, might provide neuroprotection against toxic neuropathy. In dorsal root ganglia (DRG) cultures, neurites from WldS transiently exposed to vincristine not only resisted axonal degeneration but resumed growth after withdrawal of the toxin. Neurites from wild type mice died rapidly and did not recover. To prove that the identified mutation and its protein product are responsible for the WldS phenotype, we used an adenoviral gene transfer system to deliver the WldS to rat DRG neurons. Rat neurons expressing the WldS protein were resistant to vincristine-induced axonal degeneration, confirming the functional significance of the identified gene mutation. These data provide evidence that the WldS protein can be neuroprotective against vincristine neuropathy, and possibly other disorders characterized by axonal degeneration. In addition, delivery of this gene to wild type cells can transfer the WldS phenotype, providing the possibility of "gene therapy" for peripheral neuropathy.

Regulatory elements governing transcription in specialized myofiber subtypes

Skeletal myofibers of vertebrates acquire specialized metabolic and physiological properties as a consequence of developmental cues in the embryo and different patterns of contractile activity in the adult. The myoglobin gene is regulated stringently in muscle fibers, such that high myoglobin expression is observed in mitochondria-rich, oxidative myofibers (Types I and IIa) compared with glycolytic fibers (Type IIb). Using germ-line transgenesis and somatic cell gene transfer methods, we defined discrete regions of the murine and human genes encoding myoglobin that are sufficient to confer muscle- and fiber type-specific expression to reporter genes. Mutational analysis confirms the importance of A/T-rich, MEF2-binding motifs in myoglobin gene regulation, as suggested by previous studies using different experimental approaches. In addition, we demonstrated a previously unsuspected role for an intragenic E-box motif as a negative regulatory element contributing to the tightly regulated variation in myoglobin gene expression among particular myofiber subtypes.

Regulation and functional significance of utrophin expression at the mammalian neuromuscular synapse

Duchenne muscular dystrophy (DMD) is caused by the absence of full-length dystrophin molecules in skeletal muscle fibers. In normal muscle, dystrophin is found along the length of the sarcolemma where it links the intracellular actin cytoskeleton to the extracellular matrix, via the dystrophin-associated protein (DAP) complex. Several years ago, an autosomal homologue to dystrophin, termed utrophin, was identified and shown to be expressed in a variety of tissues, including skeletal muscle. However, in contrast to the localization of dystrophin in extrajunctional regions of muscle fibers, utrophin preferentially accumulates at the postsynaptic membrane of the neuromuscular junction in both normal and DMD adult muscle fibers. Since it has recently been suggested that the upregulation of utrophin might functionally compensate for the lack of dystrophin in DMD, considerable interest is now directed toward the elucidation of the various regulatory mechanisms presiding over expression of utrophin in normal and dystrophic skeletal muscle fibers. In this review, we discuss some of the most recent data relevant to our understanding of the impact of myogenic differentiation and innervation on the expression and localization of utrophin in skeletal muscle fibers.

Expression of the utrophin gene during myogenic differentiation

The process of myogenic differentiation is known to be accompanied by large increases ( approximately 10-fold) in the expression of genes encoding cytoskeletal and membrane proteins including dystrophin and the acetylcholine receptor (AChR) subunits, via the effects of transcription factors belonging to the MyoD family. Since in skeletal muscle (i) utrophin is a synaptic homolog to dystrophin, and (ii) the utrophin promoter contains an E-box, we examined, in the present study, expression of the utrophin gene during myogenic differentiation using the mouse C2 muscle cell line. We observed that in comparison to myoblasts, the levels of utrophin and its transcript were approximately 2-fold higher in differentiated myotubes. In order to address whether a greater rate of transcription contributed to the elevated levels of utrophin transcripts, we performed nuclear run-on assays. In these studies we determined that the rate of transcription of the utrophin gene was approximately 2-fold greater in myotubes as compared to myoblasts. Finally, we examined the stability of utrophin mRNAs in muscle cultures by two separate methods: following transcription blockade with actinomycin D and by pulse-chase experiments. Under these conditions, we determined that the half-life of utrophin mRNAs in myoblasts was approximately 20 h and that it remained largely unaffected during myogenic differentiation. Altogether, these results show that in comparison to other synaptic proteins and to dystrophin, expression of the utrophin gene is only moderately increased during myogenic differentiation.