Nesprin-3 augments peripheral nuclear localization of intermediate filaments in zebrafish

The outer nuclear membrane protein nesprin-3 binds the cytoskeletal linker protein plectin, which are proposed to anchor the intermediate filaments to the nuclear envelope. To investigate the function of nesprin-3 in vivo, we used the zebrafish as a vertebrate model system. Zebrafish nesprin-3 is expressed at the nuclear envelope of epidermal and skeletal muscle cells during development. Unexpectedly, loss of nesprin-3 did not affect embryonic development, viability or fertility. However, nesprin-3-deficient zebrafish embryos showed a reduced concentration of intermediate filaments around the nucleus. Additional analysis revealed the presence of two nesprin-3 isoforms in zebrafish, nesprin-3 alpha and nesprin-3 beta. Nesprin-3 beta is only expressed during early development and lacks seven amino acids in its first spectrin repeat that are crucial for plectin binding and recruitment to the nuclear envelope. These seven amino acids are highly conserved and we showed that residues R43 and L44 within this motif are required for plectin binding. Furthermore, several residues in the actin-binding domain of plectin that are crucial for binding to the integrin beta 4 subunit are also important for the binding to nesprin-3 alpha, indicating partial overlapping binding sequences for nesprin-3 alpha and integrin beta 4. All this shows that nesprin-3 is dispensable for normal development in zebrafish, but important for mediating the association of the intermediate filament system with the nucleus in vivo.