Compartmentalized expression of the alpha- and gamma-subunits of the acetylcholine receptor in recently fused myofibers

Methyl-CpG binding proteins identify novel sites of epigenetic inactivation in human cancer

Methyl-CpG binding proteins (MBDs) mediate histone deacetylase-dependent transcriptional silencing at methylated CpG islands. Using chromatin immunoprecitation (ChIP) we have found that gene-specific profiles of MBDs exist for hypermethylated promoters of breast cancer cells, whilst a common pattern of histone modifications is shared. This unique distribution of MBDs is also characterized in chromosomes by comparative genomic hybridization of immunoprecipitated DNA and immunolocalization. Most importantly, we demonstrate that MBD association to methylated DNA serves to identify novel targets of epigenetic inactivation in human cancer. We combined the ChIP assay of MBDs with a CpG island microarray (ChIP on chip). The scenario revealed shows that, while many genes are regulated by multiple MBDs, others are associated with a single MBD. These target genes displayed methylation- associated transcriptional silencing in breast cancer cells and primary tumours. The candidates include the homeobox gene PAX6, the prolactin hormone receptor, and dipeptidylpeptidase IV among others. Our results support an essential role for MBDs in gene silencing and, when combined with genomic strategies, their potential to 'catch' new hypermethylated genes in cancer.

Electrical activity and postsynapse formation in adult muscle: gamma-AChRs are not required

Skeletal muscle fibers will not accept hyperinnervation by foreign motor axons unless they are paralyzed, suggesting that paralysis makes them receptive to innervation, e.g., by upregulating extrasynaptic expression of gamma-AChRs and/or of the agrin receptor MuSK. To examine the involvement of these parameters in paralysis-mediated synapse induction, ectopic expression of agrin, a factor from motor neurons controlling neuromuscular synapse formation, was made dependent on the administration of doxycycline in innervated adult muscle fibers. In response to doxycycline-induced agrin secretion, adult fibers did form ectopic postsynaptic specializations, even when they were electrically active, lacked fetal AChRs, and expressed normal low levels of MuSK. These data demonstrate that paralysis and changes associated with it are not required for agrin-induced postsynapse formation. They suggest that paralyzed muscle induces synapse formation via the release of factors that make motor neurites contact muscle fibers and secrete agrin.

Syne-1, a dystrophin- and Klarsicht-related protein associated with synaptic nuclei at the neuromuscular junction

We describe a novel protein, Syne-1, that is associated with nuclear envelopes in skeletal, cardiac, and smooth muscle cells. Syne-1 contains multiple spectrin repeats similar to those found in dystrophin and utrophin, as well as a domain homologous to the carboxyl-terminal of Klarsicht, a protein associated with nuclei and required for a subset of nuclear migrations in Drosophila. In adult skeletal muscle fibers, levels of Syne-1 are highest in the nuclei that lie beneath the postsynaptic membrane at the neuromuscular junction. These nuclei are transcriptionally specialized, expressing genes for synaptic components at higher levels than extrasynaptic nuclei in the same cytoplasm. Syne-1 is the first protein found to be selectively associated with synaptic nuclei. Syne-1 becomes concentrated in synaptic nuclei postnatally. It remains synaptically enriched following denervation or degeneration/regeneration, and is also present at high levels in the central nuclei of dystrophic myotubes. The location and structure of Syne-1 suggest that it may participate in the migration of myonuclei in myotubes and/or their anchoring at the postsynaptic apparatus. Finally, we identify a homologous gene, syne-2, that is expressed in an overlapping but distinct pattern.

Molecular mechanisms underlying the activity-linked alterations in acetylcholinesterase mRNAs in developing versus adult rat skeletal muscles

The molecular mechanisms underlying the activity-linked plasticity of acetylcholinesterase (AChE) mRNA levels in mammalian skeletal muscle have yet to be established. Here, we demonstrate that denervation of adult muscle induces a dramatic (up to 90%) and rapid (within 24 h) decrease in the abundance of AChE mRNAs. By contrast, denervation of 14-day-old rats leads to a significantly less pronounced reduction (50% of control) in the expression of AChE mRNAs. Assessment of the transcriptional activity of the AChE gene reveals that it remains essentially unchanged in adult denervated muscles, whereas it displays an approximately two- to three-fold increase (p < 0.05) in denervated muscles from 2- to 14-day-old rats. In addition, we observed a higher rate of degradation of in vitro transcribed AChE mRNAs upon incubation with protein extracts from denervated muscles. Finally, UV-crosslinking experiments reveal that denervation increases the abundance of RNA-protein interactions in the 3' untranslated region of AChE transcripts. Taken together, these data suggest that the abundance of AChE transcripts in mature muscles is controlled primarily via posttranscriptional regulatory mechanisms, whereas in neo- and postnatal muscles, both transcriptional and posttranscriptional regulation appears critical in dictating AChE mRNA levels. Accordingly, the activity-linked transcriptional regulation of the AChE gene appears to demonstrate a high level of plasticity during muscle development when maturation of the neuromuscular junctions is still occurring.

Rescue of the cardiac defect in ErbB2 mutant mice reveals essential roles of ErbB2 in peripheral nervous system development

ErbB2 receptor tyrosine kinase plays a role in neuregulin signaling and is expressed in the developing nervous system. We genetically rescued the cardiac defect of erbB2 null mutant embryos, which otherwise died at E11. These rescued erbB2 mutant mice die at birth and display a severe loss of both motor and sensory neurons. Motor and sensory axons are severely defasciculated and aberrantly projected within their final target tissues. Schwann cells are completely absent in the peripheral nerves. Schwann cell precursors are present within the DRG and proliferate normally, but their ability to migrate is decreased. Acetylcholine receptors cluster within the central band of the mutant diaphragm muscle. However, these clusters are dispersed and morphologically different from those in control muscle. Our results reveal an important role for erbB2 during normal peripheral nervous system development.

Implication of a multisubunit Ets-related transcription factor in synaptic expression of the nicotinic acetylcholine receptor

In adult muscle, transcription of the nicotinic acetylcholine receptor (AChR) is restricted to the nuclei located at the neuromuscular junction. The N-box, a new promoter element, was identified recently and shown to contribute to this compartmentalized synaptic expression of the AChR delta- and epsilon-subunits. We demonstrate that the N-box mediates transcriptional activation in cultured myotubes and identify the transcription factor that binds to the N-box as a heterooligomer in myotubes and adult muscle. The GABP (GA-binding protein) alpha-subunit belongs to the Ets family of transcription factors, whereas the beta-subunit shares homology with IkappaB and Drosophila Notch protein. GABP binding specificity to mutated N-box in vitro strictly parallels the sequence requirement for beta-galactosidase targeting to the endplate in vivo. In situ hybridization studies reveal that the mRNAs of both GABP subunits are abundant in mouse diaphragm, with preferential expression of the alpha-subunit at motor endplates. In addition, heregulin increases GABPalpha protein levels and regulates phosphorylation of both subunits in cultured chick myotubes. Finally, dominant-negative mutants of either GABPalpha or GABPbeta block heregulin-elicited transcriptional activation of the AChR delta and epsilon genes. These findings establish the expected connection with a presynaptic trophic factor whose release contributes to the accumulation of AChR subunit mRNAs at the motor endplate.

Accumulation in fetal muscle and localization to the neuromuscular junction of cAMP-dependent protein kinase A regulatory and catalytic subunits RI alpha and C alpha

Using probes specific for cAMP-dependent protein kinase, we have analyzed by in situ hybridization the patterns of expression of regulatory and catalytic subunits in mouse embryos and in adult muscle. RI alpha transcripts are distributed in muscle fibers exactly as acetylcholinesterase, showing that this RNA is localized at the neuromuscular junction. The transcript levels increase upon denervation of the muscle, but the RNA remains localized, indicating a regulation pattern similar to that of the epsilon subunit of nicotinic acetylcholine receptor. RI alpha transcripts have accumulated in the muscle by day 12 of mouse embryogenesis, and localization is established by day 14, at about the time of formation of junctions. This localization is maintained throughout development and in the adult. Immunocytochemical analysis has demonstrated that RI alpha protein is also localized. In addition, RI alpha recruits C alpha protein to the junction, providing at this site the potential for local responsiveness to cAMP. PKA could be implicated in the establishment and/or maintenance of the unique pattern of gene expression occurring at the junction, or in the modulation of synaptic activity via protein phosphorylation. Embryonic skeletal muscle shows a high level of C alpha transcripts and protein throughout the fiber; the transcripts are already present by day 12 of embryogenesis, and their elevated level is maintained only through fetal life. In the adult, the C alpha hybridization signal of muscle is weak and homogeneous.

Identification of an element crucial for the sub-synaptic expression of the acetylcholine receptor epsilon-subunit gene

The adult neuromuscular junction displays an accumulation of both the acetylcholine receptor (AChR) protein in the subneural domain of the post-synaptic membrane and the mRNAs coding for all its subunits at the level of the subjunctional "fundamental nuclei." In the course of end plate development, the epsilon-subunit, at variance with other subunits, becomes exclusively expressed at the level of the fundamental nuclei, yet at a rather late stage (around birth). To analyze the promoter region of the epsilon-subunit gene which directs its specific expression at the synapse, we used a quantitative transient in vivo expression assay in intact muscle tissue using constructs of the epsilon-subunit promoter placed upstream of the beta-galactosidase reporter gene. One crucial element for synapse-specific expression was detected between the -11 and -6 positions. Disruption of this element, either by a scanning mutation or single base mutations, greatly diminishes, or even completely inhibits, preferential expression of the transgene at the end plate. Gel shift experiments reveal the presence of a complex in nuclear muscle extracts that bind the core sequence of this element. The identification of such a site opens the possibility to identify regulatory factors responsible for compartmentalized expression at the neuromuscular junction.

Neuregulins and their receptors

The recent identification of an activator for the ErbB2/Neu receptor has uncovered a new family of polypeptide growth factors that undoubtedly play a major role in the regulation of neuronal growth and differentiation. These factors, called the neuregulins, are expressed in neural and mesenchymal tissues, and activate members of the epidermal growth factor family of receptor tyrosine kinases. The identification and characterization of the neuregulins and their receptors will facilitate the dissection of the biochemical pathways regulating nervous system development.

Developmental regulation of acetylcholinesterase transcripts in the mouse diaphragm: alternative splicing and focalization

We studied the splicing and compartmentalization of acetylcholinesterase (AchE) mRNAs during muscle differentiation in the mouse, both in vitro and in vivo. We used the polymerase chain reaction (PCR) to analyse AChE mRNAs in cultures of the myogenic C2 and Sol8 cell lines, and in the developing diaphragm, from embryonic day 14 (E14). We characterized three types of alternatively spliced AChE mRNAs, encoding catalytic subunits that differ by their C-terminal regions (R, H and T). The T transcript is predominant in all cases and represents the only AChE mRNA in the adult muscle. We detected the presence of the minor R and H transcripts in the myogenic cell lines, both as myoblasts and differentiated myotubes, and also in the diaphragm from E14 until birth. At E14 the R transcript represents approximately 1% of AChE mRNA and the level of the H transcript is still lower. By in situ hybridization, we found that the T AChE mRNAs begin to preferentially accumulate at the level of the first neuromuscular contacts in the mouse diaphragm and other muscles as early as E14, e.g. concomitantly with mRNAs encoding the receptor subunits. This suggests that a common control mechanism ensures the synaptic focalization of mRNAs encoding the cholinergic proteins AChE and acetylcholine receptor during muscle development.

Cellular expression of alpha 4 subunit mRNA of the nicotinic acetylcholine receptor in the developing rat telencephalon

By contrast to adult brain, little is known on the development of nicotinic acetylcholine receptor (nAChR) expression. Using a digoxigenin-labeled riboprobe for in situ hybridization, alpha 4 nAChR subunit mRNA expression was studied in embryonic and postnatal rat neocortex and hippocampus where it was transiently increased in neuronal subpopulations and preceded cholinergic fiber ingrowth. alpha 4 expression was increased in neocortical layer VIb between E20 and P2 and, about birth, in dentate gyrus granule cells subsequently decreasing to adult levels. nAChR mRNA expression is increased at the developing neuromuscular endplate preceding cholinergic innervation which triggers changes in non-alpha nAChR isoform expression. It has to be elucidated whether similar changes may occur in the telencephalon.

Differential expression patterns of five acetylcholine receptor subunit genes in rat muscle during development

The spatial and temporal expression patterns of five genes which encode the alpha-, beta-, gamma-, delta- and epsilon-subunits of the nicotinic acetylcholine receptor in skeletal muscle were followed during development in the rat by in situ hybridization analysis. Three major developmental phases, characterized by specific expression patterns, could be distinguished. (i) During myogenic differentiation alpha-, beta-, gamma- and delta-subunit genes are activated and transcripts are expressed in muscle precursor cells at embryonic day 12 (E12) and during subsequent cell fusion. (ii) Following innervation of myotubes at approximately E15-E17 the mRNA of the alpha-, beta-, gamma- and delta-subunit genes accumulate in synaptic and decrease in extrasynaptic fibre regions during early synaptogenesis. The mRNA of the epsilon-subunit gene becomes detectable first in subsynaptic nuclei 2-3 days after innervation has occurred. (iii) During postnatal development alpha-, beta- and delta- subunit transcript levels are reduced predominantly in extrasynaptic fibre segments and show significant differences in distribution depending on the muscle subtype whereas the gamma-subunit mRNA disappears completely within the first postnatal week in all muscles. In contrast, the epsilon-subunit gene is transcribed only in subsynaptic myonuclei throughout development and in the adult muscle.

Myoblast and myotube nuclei display similar patterns of heterogeneous acetylcholine receptor subunit mRNA expression

Muscle progenitor cells differentiate to myoblasts, and subsequently myotubes, upon expression of muscle specific genes. We and others have previously shown that myotube nuclei, even in the absence of nerve, express AChR alpha subunit RNA at varying levels, with a small subset (about ten percent) of the nuclei expressing at high levels. These findings raised two important questions: 1) is the observed heterogeneity a unique property of the alpha subunits, and 2) when does the heterogeneity begin? In particular, is it induced only at or after the time of fusion, or does it exist at the myoblast stage? We have, therefore, extended our observations to the gamma and delta subunits and we also have examined the distributions of AChR alpha, gamma, and delta subunit RNAs in both myoblasts and myotubes. We used intron and intron-exon probes to detect prespliced transcripts or mature mRNAs in the cells. Because intron-containing transcripts are not transported out of the nuclei, the distributions of these transcripts can indicate their expression patterns among nuclei in the same myotubes. Our results show that both myotubes and myoblasts have distributions of the AChR alpha, gamma, and delta subunit RNAs which differ sharply from that of the U1 RNA or Myo D. Thus, the heterogeneous expression of AChR genes is not only an intrinsic property of muscle cell nuclei (in the sense that it does not require the presence of nerves), but it also exists prior to fusion. Our results suggest that muscle nuclei attain individualized capacities for AChR subunit mRNA production early in their development. Conceptual models consistent with such individuality imply an additional level of regulation beyond the known diffusible transcriptional factors.

Myosin light chain 3F regulatory sequences confer regionalized cardiac and skeletal muscle expression in transgenic mice

The myosin light chain IF/3F locus contains two independent promoters, MLC1F and MLC3F, which are differentially activated during skeletal muscle development. Transcription at this locus is regulated by a 3' skeletal muscle enhancer element, which directs correct temporal and tissue-specific expression from the MLC1F promoter in transgenic mice. To investigate the role of this enhancer in regulation of the MLC3F promoter in vivo, we have analyzed reporter gene expression in transgenic mice containing lacZ under transcriptional control of the mouse MLC3F promoter and 3' enhancer element. Our results show that these regulatory elements direct strong expression of lacZ in skeletal muscle; the transgene, however, is activated 4-5 d before the endogenous MLC3F promoter, at the time of initiation of MLC1F transcription. In adult mice, transgene activity is downregulated in muscles that have reduced contributions of type IIB fibers (soleus and diaphragm). The rostrocaudal positional gradient of transgene expression documented for MLC1F transgenic mice (Donoghue, M., J. P. Merlie, N. Rosenthal, and J. R. Sanes. 1991. Proc. Natl. Acad. Sci. USA. 88:5847-5851) is not seen in MLC3F transgenic mice. Although MLC3F was previously thought to be restricted to skeletal striated muscle, the MLC3F-lacZ transgene is expressed in cardiac muscle from 7.5 d of development in a spatially restricted manner in the atria and left ventricular compartments, suggesting that transcriptional differences exist between cardiomyocytes in left and right compartments of the heart. We show here that transgene-directed expression of the MLC3F promoter reflects low level expression of endogenous MLC3F transcripts in the mouse heart.

A novel synapse-associated noncoding RNA

Synaptic nuclei of innervated muscle transcribe acetylcholine receptor (AChR) genes at a much higher level than extrasynaptic nuclei. To isolate candidate synaptic regulatory molecules responsible for the unique transcriptional potential of synaptic nuclei, we have taken a subtractive hybridization approach. Here, we report the cloning and characterization of a novel synapse-associated RNA, 7H4. 7H4 is expressed selectively in the endplate zone of skeletal muscle and is upregulated during early postnatal development and after denervation. Interestingly, the 7H4 gene has no introns, and yet two different-size RNAs with identical polyadenylated 3' ends are generated. Most intriguingly, the nucleotide sequence does not contain any significant open reading frames, suggesting that 7H4 may function as a noncoding RNA.

A novel synapse-associated noncoding RNA

Synaptic nuclei of innervated muscle transcribe acetylcholine receptor (AChR) genes at a much higher level than extrasynaptic nuclei. To isolate candidate synaptic regulatory molecules responsible for the unique transcriptional potential of synaptic nuclei, we have taken a subtractive hybridization approach. Here, we report the cloning and characterization of a novel synapse-associated RNA, 7H4. 7H4 is expressed selectively in the endplate zone of skeletal muscle and is upregulated during early postnatal development and after denervation. Interestingly, the 7H4 gene has no introns, and yet two different-size RNAs with identical polyadenylated 3' ends are generated. Most intriguingly, the nucleotide sequence does not contain any significant open reading frames, suggesting that 7H4 may function as a noncoding RNA.