P.5.18 Tissue-specific expression of nesprin isoforms and its relevance to muscular dystrophy and dilated cardiomyopathy

Abstract

A combination of isoform-specific, quantitative RT-PCR and site-specific monoclonal antibodies has been used to produce consistent evidence at both RNA and protein levels for the principal isoform products of SYNE1 and SYNE2, the genes encoding nesprin-1 and nesprin-2 proteins. Western blots show that both genes produce three groups of isoforms that are homologous in size and structure: giant, or full-length, isoforms of about 1000 kDa, medium isoforms of about 380 kDa (nesprin-1-beta and nesprin-2-gamma) and short isoforms of about 110 kDa (nesprin-1-alpha and nesprin-2-epsilon). SYNE2 alone produces a very short isoform at 60 kDa (nesprin-2-alpha) in skeletal muscle. Although the giant forms were the dominant isoforms in all tissues, major differences between tissues in expression of shorter isoforms were observed. From SYNE1, nesprin-1-alpha was found in skeletal and cardiac muscle only. Several cell lines, including embryonic stem cells, did not produce any detectable nesprin-1 at all. From SYNE2, we observed a developmental transition from nesprin-2-epsilon-1 at very early stages (embryonic stem cells, embryonal teratocarcinoma and ovary tissue) to nesprin-2-epsilon-2 in adult and fetal tissues. Both skeletal muscle and heart have a high proportion of short isoforms compared with other tissues, but heart expresses the nesprin-2-epsilon-2 isoform where skeletal muscle produces nesprin-2-alpha-1. This study has clarified the major nesprin isoforms expressed in cells and tissues, especially in cardiac and skeletal muscle, which are the major affected tissues in neuromuscular disease. Mutations in nesprin-1 and/or nesprin-2 can cause both Emery-Dreifuss muscular dystrophy and dilated cardiomyopathy. The operative functional unit appears to be the “LINC” complex of emerin, lamin A/C, nesprins and SUN proteins, since mutations in any single component can result in a similar pathology.