The Erk MAP kinase pathway is activated at muscle spindles and is required for induction of the muscle spindle-specific gene Egr3 by neuregulin1.

Muscle spindles are sensory receptors composed of specialized muscle fibers, known as intrafusal muscle fibers, along with the endings of sensory neuron axons that innervate these muscle fibers. Formation of muscle spindles requires neuregulin1 (NRG1), which is released by sensory axons, activating ErbB receptors in muscle cells that are contacted. The transcription factor Egr3 is transcriptionally induced by NRG1, which in turn activates various target genes involved in forming intrafusal fibers. We have previously shown that, in cultured muscle cells, NRG1 signaling activates the Egr3 gene through SRF and CREB, which bind to a composite regulatory element, and that NRG1 signaling targets SRF by stimulating nuclear translocation of SRF coactivators myocardin-related transcription factor (MRTF)-A and MRTF-B and targets CREB by phosphorylation. The current studies examined signaling relays that might function in the NRG1 pathway upstream of SRF and CREB. We found that transcriptional induction of Egr3 in response to NRG1 requires the MAP kinase Erk1/2, which acts upstream of CREB to induce its phosphorylation. MRTFs are targeted by the Rho-actin pathway, yet in the absence of Rho-actin signaling, even though MRTFs fail to be translocated to the nucleus, NRG1 induces Egr3 transcription. In mouse muscle in vivo, activation of Erk1/2 is enhanced selectively where muscle spindles are located. These results suggest that Erk1/2 acts in intrafusal fibers of muscle spindles to induce transcription of Egr3 and that Egr3 induction occurs independently of MRTFs and involves Erk1/2 acting on other transcriptional regulatory targets that interact with the SRF-CREB regulatory element.

Neuregulin1 signaling targets SRF and CREB and activates the muscle spindle-specific gene Egr3 through a composite SRF-CREB-binding site.

Muscle spindles are sensory receptors embedded within muscle that detect changes in muscle length. Each spindle is composed of specialized muscle fibers, known as intrafusal muscle fibers, along with the endings of axons from sensory neurons that innervate these muscle fibers. Formation of muscle spindles requires neuregulin1 (NRG1), which is released by sensory axons, activating ErbB receptors in muscle cells that are contacted. In muscle cells, the transcription factor Egr3 is transcriptionally induced by NRG1, which in turn activates various target genes involved in forming the intrafusal fibers of muscle spindles. The signaling relay within the NRG1-ErbB pathway that acts to induce Egr3 is presumably critical for muscle spindle formation but for the most part has not been determined. In the current studies, we examined, using cultured muscle cells, transcriptional regulatory mechanisms by which Egr3 responds to NRG1. We identified a composite regulatory element for the Egr3 gene, consisting adjacent sites that bind cAMP response element binding protein (CREB) and serum response factor (SRF), with a role in NRG1 responsiveness. The SRF element also influences Egr3 basal expression in unstimulated myotubes, and in the absence of the SRF element, the CREB element influences basal expression. We show that NRG1 signaling, to target SRF, acts on the SRF coactivators myocardian-related transcription factor (MRTF)-A and MRTF-B, which are known to activate SRF-mediated transcription, by stimulating their translocation from the cytoplasm to the nucleus. CREB is phosphorylated, which is known to contribute to its activation, in response to NRG1. These results suggest that NRG1 induces expression of the muscle spindle-specific gene Egr3 by stimulating the transcriptional activity of CREB and SRF.

Chromatin modifications that support acetylcholine receptor gene activation are established during muscle cell determination and differentiation.

Localization of acetylcholine receptors (AChRs) to the postsynaptic region of muscle is mediated in part by transcriptional mechanisms. An important way of regulating transcription is through targeting histone modifications on chromatin to distinct gene loci. Using chromatin immunoprecipitation, we examined the developmental regulation of certain histone modifications at the AChR epsilon subunit locus, including methylations at lysine residues K4 and K27 and acetylations at K9 and K14. We modeled various stages of muscle development in cell culture, including pre-determined cells, committed but undifferentiated myoblasts, and differentiated myotubes, and modeled synaptic myotube nuclei by stimulating myotubes with neuregulin (NRG) 1. We found that a pattern of histone modifications associated with transcriptional activation is targeted to the AChR epsilon subunit locus in myotubes prior to stimulation with NRG1 and does not change upon addition of NRG1. Instead, we found that during muscle cell determination and differentiation, specific histone modifications are targeted to the AChR epsilon subunit locus. Within the gene, at K4, dimethylation is induced during muscle cell determination, while trimethylation is induced during differentiation. At K27, loss of trimethylation and appearance of monomethylation occurs during determination and differentiation. In addition, in a region upstream of the gene, K4 di- and trimethylation, and K9/14 acetylation are induced in a distinct developmental pattern, which may reflect a functional regulatory element. These results suggest synaptic signaling does not directly target histone modifications but rather the histone modification pattern necessary for transcriptional activation is previously established in a series of steps during muscle development.

Neuregulin-1 induces acetylcholine receptor transcription in the absence of GABPalpha phosphorylation.

Localization of acetylcholine receptors (AChRs) to the postsynaptic region of muscle is mediated in part by transcriptional mechanisms, because the genes encoding AChR subunits are transcribed selectively in synaptic myofiber nuclei. Neuregulin-1 (NRG-1) is a synaptic signal and induces transcription of AChRs in muscle cells. Signaling by NRG-1 is thought to involve the transcription factor GA-binding protein (GABP), a heterodimer of GABPalpha, which is a member of the Ets family, and GABPbeta. Phosphorylation of certain other Ets proteins outside the Ets DNA-binding domain serves to stimulate transcriptional activation in response to extracellular signals. According to previous studies, NRG-1 stimulates phosphorylation of GABPalpha at threonine 280 in the N-terminal region adjacent to the Ets domain, suggesting that GABPalpha phosphorylation might contribute to NRG-1 responsiveness. To determine the functional importance of the N-terminal region of GABPalpha and whether its function is regulated by phosphorylation, we generated muscle cell lines in which the endogenous GABPalpha gene was deleted and replaced by variants of GABPalpha mutated in the N-terminal region. We found that NRG-1 can induce transcription in cells with mutated T280 phosphorylation site, indicating that T280 phosphorylation does not contribute to NRG-1 responsiveness. We also found that NRG-1-induced transcription occurs in cells missing the entire N-terminal region of GABPalpha. Because NRG-1 signaling is not expected to alter the function of the C-terminal region, which remains in these cells, these results suggest that GABPbeta, or other interacting components, rather than GABPalpha directly, is targeted by NRG-1 signaling.

Directing RNA interference specifically to differentiated muscle cells.

A common approach for mediating RNA interference (RNAi) is to introduce DNA that encodes short hairpin RNA (shRNA), which is often contained in a plasmid that can express a shRNA in a wide variety of cell types. Muscle cells and certain other cell types grown in culture can exist in both a dividing state and in a post-mitotic, differentiated state, and it is sometimes useful to induce RNAi selectively in terminally differentiated cells to study the function of a gene, particularly when the gene is also required for propagation of dividing cells. We describe two methods for studying gene function by RNAi specifically in terminally differentiated skeletal muscle cells in culture. We developed a shRNA expression vector, based on myosin light chain 1f gene regulatory sequences, which is designed to induce shRNA expression specifically after differentiation has been initiated. We show that this vector can mediate RNAi and is only active in differentiated muscle cells. Also, we developed an adenoviral vector that is designed to be able to deliver shRNAs directly to post-mitotic muscle cells. We show that adenoviruses produced using this vector mediate RNAi in differentiated muscle cells. These methods add to the repertoire of RNAi tools that can be used for identifying genes involved in any event of interest that occurs in differentiated muscle cells.