MacDonald Ryen, Barbat-Artigas Sebastien, Cho Chulmin, Peng Huashan, Shang Jijun, Moustaine Ayman, Carbonetto Salvatore, Robitaille Richard, Chalifour Lorraine E, Paudel Hemant
Lady Davis Institute for Medical Research, Jewish General HospitalMontreal, QC, Canada.
Integrated Program in Neuroscience, McGill UniversityMontreal, QC, Canada.
Front Aging Neurosci. 2017 Aug 3;9:258. doi: 10.3389/fnagi.2017.00258. eCollection 2017.
Synaptic transmission requires intricate coordination of the components involved in processing of incoming signals, formation and stabilization of synaptic machinery, neurotransmission and in all related signaling pathways. Changes to any of these components cause synaptic imbalance and disruption of neuronal circuitry. Extensive studies at the neuromuscular junction (NMJ) have greatly aided in the current understanding of synapses and served to elucidate the underlying physiology as well as associated adaptive and homeostatic processes. The heparan sulfate proteoglycan agrin is a vital component of the NMJ, mediating synaptic formation and maintenance in both brain and muscle, but very little is known about direct control of its expression. Here, we investigated the relationship between agrin and transcription factor early growth response-1 (Egr-1), as Egr-1 regulates the expression of many genes involved in synaptic homeostasis and plasticity. Using chromatin immunoprecipitation (ChIP), cell culture with cell lines derived from brain and muscle, and animal models, we show that Egr-1 binds to the gene locus and suppresses its expression. When compared with wild type (WT), mice deficient in Egr-1 (Egr-1-/-) display a marked increase in mRNA and agrin full-length and cleavage fragment protein levels, including the 22 kDa, C-terminal fragment in brain and muscle tissue homogenate. Because agrin is a crucial component of the NMJ, we explored possible physiological implications of the Egr-1-agrin relationship. In the diaphragm, Egr-1-/- mice display increased NMJ motor endplate density, individual area and area of innervation. In addition to increased density, soleus NMJs also display an increase in fragmented and faint endplates in Egr-1-/- vs. WT mice. Moreover, the soleus NMJ electrophysiology of Egr-1-/- mice revealed increased quantal content and motor testing showed decreased movement and limb muscle strength compared with WT. This study provides evidence for the potential involvement of a novel Egr-1-agrin pathway in synaptic homeostatic and compensatory mechanisms at the NMJ. Synaptic homeostasis is greatly affected by the process of aging. These and other data suggest that changes in Egr-1 expression may directly or indirectly promote age-related pathologies.
突触传递需要对参与传入信号处理、突触机制形成与稳定、神经传递以及所有相关信号通路的组件进行复杂协调。这些组件中任何一个发生变化都会导致突触失衡和神经元回路破坏。在神经肌肉接头（NMJ）进行的大量研究极大地促进了目前对突触的理解，并有助于阐明潜在的生理学以及相关的适应性和稳态过程。硫酸乙酰肝素蛋白聚糖聚集蛋白是神经肌肉接头的重要组成部分，介导大脑和肌肉中的突触形成和维持，但对其表达的直接调控知之甚少。在这里，我们研究了聚集蛋白与转录因子早期生长反应-1（Egr-1）之间的关系，因为Egr-1调节许多参与突触稳态和可塑性的基因的表达。使用染色质免疫沉淀（ChIP）、来自大脑和肌肉的细胞系进行细胞培养以及动物模型，我们发现Egr-1与聚集蛋白基因位点结合并抑制其表达。与野生型（WT）相比，Egr-1基因缺陷（Egr-1-/-）小鼠的聚集蛋白mRNA以及全长和裂解片段蛋白水平显著增加，包括大脑和肌肉组织匀浆中的22 kDa C末端片段。由于聚集蛋白是神经肌肉接头的关键组成部分，我们探讨了Egr-1-聚集蛋白关系可能的生理学意义。在膈肌中，Egr-1-/-小鼠的神经肌肉接头运动终板密度、单个面积和支配面积增加。除了密度增加外，与WT小鼠相比，Egr-1-/-小鼠比目鱼肌神经肌肉接头还显示出碎片化和模糊终板增加。此外，Egr-1-/-小鼠比目鱼肌神经肌肉接头的电生理学显示量子含量增加，运动测试显示与WT相比运动和肢体肌肉力量下降。这项研究为一种新的Egr-1-聚集蛋白途径在神经肌肉接头突触稳态和补偿机制中的潜在参与提供了证据。突触稳态受到衰老过程的极大影响。这些以及其他数据表明，Egr-1表达的变化可能直接或间接促进与年龄相关的病理变化。
