Lamin A/C assembly defects in Emery-Dreifuss muscular dystrophy can be regulated by culture medium composition

Lamin A mutation impairs interaction with nucleoporin NUP155 and disrupts nucleocytoplasmic transport in atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia. Here, we show the identification and functional characterization of one AF-associated mutation p.Arg399Cys in lamin A/C. Co-immunoprecipitation and GST pull-down assays demonstrate that lamin A/C interacts with NUP155, which is a nucleoporin and causes AF when mutated. Lamin A/C mutation p.Arg399Cys impairs the interaction between lamin A/C and NUP155, and increases extractability of NUP155 from the nuclear envelope (NE). Mutation p.Arg399Cys leads to aggregation of lamin A/C in the nucleus, although it does not impair the integrity of NE upon cellular stress. Mutation p.Arg399Cys inhibits the export of HSP70 mRNA and the nuclear import of HSP70 protein. Electrophysiological studies show that mutation p.Arg399Cys decreases the peak cardiac sodium current by decreasing the cell surface expression level of cardiac sodium channel Na_v 1.5, but does not affect I_Kr potassium current. In conclusion, our results indicate that lamin A/C mutation p.Arg399Cys weakens the interaction between nuclear lamina (lamin A/C) and the nuclear pore complex (NUP155), leading to the development of AF. The findings provide a novel molecular mechanism for the pathogenesis of AF.

Nesprins: tissue-specific expression of epsilon and other short isoforms

Nesprin-1-giant and nesprin-2-giant regulate nuclear positioning by the interaction of their C-terminal KASH domains with nuclear membrane SUN proteins and their N-terminal calponin-homology domains with cytoskeletal actin. A number of short isoforms lacking the actin-binding domains are produced by internal promotion. We have evaluated the significance of these shorter isoforms using quantitative RT-PCR and western blotting with site-specific monoclonal antibodies. Within a complete map of nesprin isoforms, we describe two novel nesprin-2 epsilon isoforms for the first time. Epsilon isoforms are similar in size and structure to nesprin-1-alpha. Expression of nesprin isoforms was highly tissue-dependent. Nesprin-2-epsilon-1 was found in early embryonic cells, while nesprin-2-epsilon-2 was present in heart and other adult tissues, but not skeletal muscle. Some cell lines lack shorter isoforms and express only one of the two nesprin genes, suggesting that either of the giant nesprins is sufficient for basic cell functions. For the first time, localisation of endogenous nesprin away from the nuclear membrane was shown in cells where removal of the KASH domain by alternative splicing occurs. By distinguishing between degradation products and true isoforms on western blots, it was found that previously-described beta and gamma isoforms are expressed either at only low levels or with a limited tissue distribution. Two of the shortest alpha isoforms, nesprin-1-alpha-2 and nesprin-2-alpha-1, were found almost exclusively in cardiac and skeletal muscle and a highly conserved and alternatively-spliced exon, available in both nesprin genes, was always included in these tissues. These "muscle-specific" isoforms are thought to form a complex with emerin and lamin A/C at the inner nuclear membrane and mutations in all three proteins cause Emery-Dreifuss muscular dystrophy and/or inherited dilated cardiomyopathy, disorders in which only skeletal muscle and/or heart are affected.