Indiana University School of Medicine-Muncie and Ball State University, 2000 University Avenue, Muncie, IN 47306, USA.
Exp Cell Res. 2013 Mar 10;319(5):718-30. doi: 10.1016/j.yexcr.2013.01.001. Epub 2013 Jan 11.
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.
肌梭是嵌入肌肉中的感觉受体，可检测肌肉长度的变化。每个肌梭由专门的肌纤维组成，称为梭内肌纤维，以及支配这些肌纤维的感觉神经元轴突的末端。肌梭的形成需要神经调节素 1（NRG1），它由感觉轴突释放，激活与之接触的肌肉细胞中的 ErbB 受体。在肌肉细胞中，转录因子 Egr3 被 NRG1 转录诱导，Egr3 转而激活参与形成肌梭梭内纤维的各种靶基因。NRG1-ErbB 途径内的信号转导中继对于肌梭的形成可能至关重要，但在很大程度上尚未确定。在当前的研究中，我们使用培养的肌肉细胞研究了 Egr3 对 NRG1 反应的转录调节机制。我们确定了 Egr3 基因的复合调节元件，该元件由结合 cAMP 反应元件结合蛋白（CREB）和血清反应因子（SRF）的相邻位点组成，在 NRG1 反应性中起作用。SRF 元件也影响未受刺激的肌管中的 Egr3 基础表达，并且在没有 SRF 元件的情况下，CREB 元件影响基础表达。我们表明，NRG1 信号通过刺激其从细胞质易位到细胞核，靶向 SRF，作用于已知激活 SRF 介导的转录的心肌相关转录因子（MRTF）-A 和 MRTF-B 等 SRF 共激活因子，从而发挥作用。CREB 被磷酸化，已知这有助于其激活，以响应 NRG1。这些结果表明，NRG1 通过刺激 CREB 和 SRF 的转录活性诱导肌肉梭特异性基因 Egr3 的表达。
