Nesprins comprise a family of multi-isomeric scaffolding proteins, forming the linker of nucleoskeleton-and-cytoskeleton complex with lamin A/C, emerin and SUN1/2 at the nuclear envelope. Mutations in nesprin-1/-2 are associated with Emery-Dreifuss muscular dystrophy (EDMD) with conduction defects and dilated cardiomyopathy (DCM). We have previously observed sarcomeric staining of nesprin-1/-2 in cardiac and skeletal muscle, but nesprin function in this compartment remains unknown. In this study, we show that specific nesprin-2 isoforms are highly expressed in cardiac muscle and localize to the Z-disc and I band of the sarcomere. Expression of GFP-tagged nesprin-2 giant spectrin repeats 52 to 53, localized to the sarcomere of neonatal rat cardiomyocytes. Yeast two-hybrid screening of a cardiac muscle cDNA library identified telethonin and four-and-half LIM domain (FHL)-2 as potential nesprin-2 binding partners. GST pull-down and immunoprecipitation confirmed the individual interactions between nesprin-2/telethonin and nesprin-2/FHL-2, and showed that nesprin-2 and telethonin binding was dependent on telethonin phosphorylation status. Importantly, the interactions between these binding partners were impaired by mutations in nesprin-2, telethonin, and FHL-2 identified in EDMD with DCM and hypertrophic cardiomyopathy patients. These data suggest that nesprin-2 is a novel sarcomeric scaffold protein that may potentially participate in the maintenance and/or regulation of sarcomeric organization and function.

Limb-girdle muscular dystrophy (LGMD) is a group of clinically and genetically heterogeneous neuromuscular disorders. LGMD-R7, which is caused by telethonin gene (TCAP) mutations, is one of the rarest forms of LGMD, and only a small number of LGMD-R7 cases have been described and mostly include patients from Brazil. A total of two LGMD-R7 patients were enrolled at a Chinese neuromuscular center. Demographic and clinical data were collected. Laboratory investigations and electromyography were performed. Routine and immunohistochemistry staining of muscle specimens was performed, and a next-generation sequencing panel array for genes associated with hereditary neuromuscular disorders was used for analysis. The patients exhibited predominant muscle weakness. Electromyography revealed myopathic changes. The muscle biopsy showed myopathic features, such as increased fiber size variation, muscle fiber atrophy and regeneration, slight hyperplasia of the connective tissue, and disarray of the myofibrillar network. Two patients were confirmed to have mutations in the open reading frame of TCAP by next-generation sequencing. One patient had compound heterozygous mutations, and the other patient harbored a novel homozygous mutation. Western blotting analysis of the skeletal muscle lysate confirmed the absence of telethonin in the patients. We described two LGMD-R7 patients presenting a classical LGMD phenotype and a novel homozygous TCAP mutation. Our research expands the spectrum of LGMD-R7 due to TCAP mutations based on patients from a Chinese neuromuscular center.

Telethonin anchors the N-terminal region of titin in the Z-disk of the sarcomere by binding to two immunoglobulin-like (Ig) domains (Z1 and Z2) of titin (Z1Z2). Thereby telethonin plays an important role in myofibril assembly and in muscle development and functional regulation. The expression and purification of recombinant telethonin is very challenging. In previous studies, recombinant telethonin expressed from E. coli was refolded in the presence of Z1Z2. Here, we report various strategies to establish a reliable and efficient protocol for the preparation of telethonin and titin Z1Z2 protein. First, a co-expression strategy was designed to obtain soluble Z1Z2/telethonin complexes. The concentration of antibiotics and the type of expression vector were found to be important for achieving high yields of purified complex. Second, the five cysteine residues of telethonin were mutated to serine to avoid severe problems with cysteine oxidation. Third, a short version of telethonin (telethonin1-90) was designed to avoid the proteolytic degradation observed for longer constructs of the protein. The short telethonin formed a highly stable complex with Z1Z2 with no degradation being observed for 30 days at 4 °C. Fourth, an improved refolding protocol was developed to achieve high yields of Z1Z2/telethonin complex. Finally, based on the crystal structure in which Z1Z2 and telethonin1-90 assemble into a 2:1 complex, a single chain fusion protein was designed, comprising two Z1Z2 modules that are connected by flexible linkers N- and C-terminally of the telethonin1-90. Expression of this fusion protein, named ZTZ, affords high yields of soluble expressed and purified protein.