Department of Anatomy and Cell Biology and Centro de Investigación Biomédica en Red sobre Enfermedades Neurodegenerativas, University of Cantabria, Santander, Spain.
Neurobiol Dis. 2012 Apr;46(1):118-29. doi: 10.1016/j.nbd.2011.12.052. Epub 2012 Jan 10.
Nuclear speckles are essential nuclear compartments involved in the assembly, delivery and recycling of pre-mRNA processing factors, and in the post-transcriptional processing of pre-mRNAs. Oculopharyngeal muscular dystrophy (OPMD) is caused by a small expansion of the polyalanine tract in the poly(A)-binding protein nuclear 1 (PABPN1). Aggregation of expanded PABPN1 into intranuclear inclusions (INIs) in skeletal muscle fibers is the pathological hallmark of OPMD. In this study what we have analyzed in muscle fibers of OPMD patients and in primary cultures of human myoblasts are the relationships between nuclear speckles and INIs, and the contribution of the former to the biogenesis of the latter. While nuclear speckles concentrate snRNP splicing factors and PABPN1 in control muscle fibers, they are depleted of PABPN1 and appear closely associated with INIs in muscle fibers of OPMD patients. The induction of INI formation in human myoblasts expressing either wild type GFP-PABPN1 or expanded GFP-PABPN1-17ala demonstrates that the initial aggregation of PABPN1 proteins and their subsequent growth in INIs occurs at the edges of the nuclear speckles. Moreover, the growing of INIs gradually depletes PABPN1 proteins and poly(A) RNA from nuclear speckles, although the existence of these nuclear compartments is preserved. Time-lapse experiments in cultured myoblasts confirm nuclear speckles as biogenesis sites of PABPN1 inclusions. Given the functional importance of nuclear speckles in the post-transcriptional processing of pre-mRNAs, the INI-dependent molecular reorganization of these nuclear compartments in muscle fibers may cause a severe dysfunction in nuclear trafficking and processing of polyadenylated mRNAs, thereby contributing to the molecular pathophysiology of OPMD. Our results emphasize the potential importance of nuclear speckles as nuclear targets of neuromuscular disorders.
核斑点是参与前体 mRNA 处理因子的组装、传递和回收以及前体 mRNA 转录后加工的重要核区室。眼咽型肌营养不良症（OPMD）是由多聚 A 结合蛋白核 1（PABPN1）中的多聚丙氨酸序列扩展引起的。扩展的 PABPN1 在骨骼肌纤维中聚集形成核内包涵体（INIs）是 OPMD 的病理学标志。在这项研究中，我们分析了 OPMD 患者的肌肉纤维和人成肌细胞的原代培养物中核斑点与 INIs 之间的关系，以及前者对后者生物发生的贡献。虽然核斑点集中了 snRNP 剪接因子和 PABPN1 在对照肌肉纤维中，但它们在 OPMD 患者的肌肉纤维中耗尽了 PABPN1，并且似乎与 INIs 密切相关。在表达野生型 GFP-PABPN1 或扩展型 GFP-PABPN1-17ala 的人成肌细胞中诱导 INI 形成表明，PABPN1 蛋白的初始聚集及其随后在 INIs 中的生长发生在核斑点的边缘。此外，尽管这些核区室仍然存在，但 INIs 的生长逐渐从核斑点中耗尽 PABPN1 蛋白和多聚 A 核糖核酸。在培养的成肌细胞中进行的延时实验证实了核斑点是 PABPN1 包涵体生物发生的部位。鉴于核斑点在后转录处理前体 mRNA 中的功能重要性，这些核区室在肌肉纤维中的 INI 依赖性分子重排可能导致核运输和多聚腺苷酸化 mRNA 处理严重功能障碍，从而导致 OPMD 的分子病理生理学。我们的研究结果强调了核斑点作为神经肌肉疾病核靶标的潜在重要性。
