Fallini Claudia, Rouanet Jeremy P, Donlin-Asp Paul G, Guo Peng, Zhang Honglai, Singer Robert H, Rossoll Wilfried, Bassell Gary J
Department of Cell Biology, Emory University School of Medicine, Atlanta, GA 30322, USA.
Department of Neurology, UMASS Medical School, Worcester, MA 01605, USA.
Dev Neurobiol. 2014 Mar;74(3):319-332. doi: 10.1002/dneu.22111. Epub 2013 Oct 4.
Spinal muscular atrophy (SMA) is a lethal neurodegenerative disease specifically affecting spinal motor neurons. SMA is caused by the homozygous deletion or mutation of the survival of motor neuron 1 (SMN1) gene. The SMN protein plays an essential role in the assembly of spliceosomal ribonucleoproteins. However, it is still unclear how low levels of the ubiquitously expressed SMN protein lead to the selective degeneration of motor neurons. An additional role for SMN in the regulation of the axonal transport of mRNA-binding proteins (mRBPs) and their target mRNAs has been proposed. Indeed, several mRBPs have been shown to interact with SMN, and the axonal levels of few mRNAs, such as the β-actin mRNA, are reduced in SMA motor neurons. In this study we have identified the β-actin mRNA-binding protein IMP1/ZBP1 as a novel SMN-interacting protein. Using a combination of biochemical assays and quantitative imaging techniques in primary motor neurons, we show that IMP1 associates with SMN in individual granules that are actively transported in motor neuron axons. Furthermore, we demonstrate that IMP1 axonal localization depends on SMN levels, and that SMN deficiency in SMA motor neurons leads to a dramatic reduction of IMP1 protein levels. In contrast, no difference in IMP1 protein levels was detected in whole brain lysates from SMA mice, further suggesting neuron specific roles of SMN in IMP1 expression and localization. Taken together, our data support a role for SMN in the regulation of mRNA localization and axonal transport through its interaction with mRBPs such as IMP1.
脊髓性肌萎缩症（SMA）是一种致命的神经退行性疾病，专门影响脊髓运动神经元。SMA由运动神经元存活1（SMN1）基因的纯合缺失或突变引起。SMN蛋白在剪接体核糖核蛋白的组装中起重要作用。然而，目前仍不清楚普遍表达的低水平SMN蛋白如何导致运动神经元的选择性退化。有人提出SMN在调节mRNA结合蛋白（mRBPs）及其靶mRNA的轴突运输中具有额外作用。事实上，已显示几种mRBPs与SMN相互作用，并且在SMA运动神经元中，少数mRNA（如β-肌动蛋白mRNA）的轴突水平降低。在本研究中，我们已确定β-肌动蛋白mRNA结合蛋白IMP1/ZBP1是一种新型的与SMN相互作用的蛋白。通过在原代运动神经元中结合生化分析和定量成像技术，我们表明IMP1在运动神经元轴突中被积极运输的单个颗粒中与SMN相关联。此外，我们证明IMP1的轴突定位取决于SMN水平，并且SMA运动神经元中SMN的缺乏导致IMP1蛋白水平急剧降低。相比之下，在SMA小鼠的全脑裂解物中未检测到IMP1蛋白水平的差异，这进一步表明SMN在IMP1表达和定位中具有神经元特异性作用。综上所述，我们的数据支持SMN通过与IMP1等mRBPs相互作用在调节mRNA定位和轴突运输中的作用。
