Department of Pathology and Cell Biology, Columbia University, New York, NY 10032, USA.
Proc Natl Acad Sci U S A. 2011 Jan 4;108(1):131-6. doi: 10.1073/pnas.1000824108. Epub 2010 Dec 20.
Mutations in LMNA, which encodes A-type lamins, result in disparate diseases, known collectively as laminopathies, that affect distinct tissues, including striated muscle and adipose tissue. Lamins provide structural support for the nucleus and sites of attachment for chromatin, and defects in these functions may contribute to disease pathogenesis. Recent studies suggest that A-type lamins may facilitate connections between the nucleus and the cytoskeleton mediated by nuclear envelope nesprin and SUN proteins. In mammalian cells, however, interfering with A-type lamins does not affect the localization of these proteins. Here, we used centrosome orientation in fibroblasts, which requires separate nuclear and centrosome positioning pathways, as a model system to understand how LMNA mutations affect nucleus-cytoskeletal connections. We find that LMNA mutations causing striated muscle diseases block actin-dependent nuclear movement, whereas most that affect adipose tissue inhibit microtubule-dependent centrosome positioning. Genetic deletion or transient depletion of A-type lamins also blocked nuclear movement, showing that mutations affecting muscle exhibit the null phenotype. Lack of A-type lamins, or expression of variants that cause striated muscle disease, did not affect assembly of nesprin-2G and SUN2 into transmembrane actin-associated nuclear (TAN) lines that attach the nucleus to retrogradely moving actin cables. Nesprin-2G TAN lines were less stable, however, and slipped over the nucleus rather than moving with it, indicating that they were not anchored. Nesprin-2G TAN lines also slipped in SUN2-depleted cells. Our results establish A-type lamins as anchors for nesprin-2G-SUN2 TAN lines to allow productive movement and proper positioning of the nucleus by actin.
编码 A 型核纤层蛋白的 LMNA 基因突变导致不同的疾病，统称为核纤层病，这些疾病影响不同的组织，包括横纹肌和脂肪组织。核纤层为核提供结构支撑，并为染色质附着提供附着点，这些功能的缺陷可能导致疾病的发病机制。最近的研究表明，A 型核纤层蛋白可能有助于核包膜 nesprin 和 SUN 蛋白介导的核与细胞骨架之间的连接。然而，在哺乳动物细胞中，干扰 A 型核纤层蛋白不会影响这些蛋白质的定位。在这里，我们使用成纤维细胞中的中心体取向作为模型系统，该系统需要独立的核和中心体定位途径，以了解 LMNA 突变如何影响核-细胞骨架连接。我们发现导致横纹肌疾病的 LMNA 突变会阻断肌动蛋白依赖性核运动，而大多数影响脂肪组织的突变会抑制微管依赖性中心体定位。A 型核纤层蛋白的基因缺失或瞬时耗竭也阻断了核运动，表明影响肌肉的突变表现出无效表型。缺乏 A 型核纤层蛋白，或表达导致横纹肌疾病的变体，并不影响 nesprin-2G 和 SUN2 组装成将核与逆行肌动蛋白电缆连接的跨膜肌动蛋白相关核（TAN）线。然而，nesprin-2G TAN 线不太稳定，在细胞核上滑动而不是随其移动，表明它们没有固定。nesprin-2G TAN 线在 SUN2 耗尽的细胞中也会滑动。我们的结果确立了 A 型核纤层蛋白作为 nesprin-2G-SUN2 TAN 线的锚点，以允许核通过肌动蛋白进行有效运动和正确定位。
