A novel marker for vertebrate embryonic heart, the EH-myomesin isoform

Myomesin is a structural component of the M-band that is expressed in all t ypes of striated muscle. Its primary function may be the maintenance of the thick filament lattice and its anchoring to the elastic filament system co mposed of titin. Different myomesin isoforms have been described in chicken and mice, but no particular function has been assigned to them. Here we in vestigate the spatio-temporal expression pattern of myomesin isoforms by me ans of reverse transcriptase-polymerase chain reaction and isoform-specific antibodies. We find that two alternative splicing events give rise to four myomesin isoforms in chicken contrary to only one splicing event with two possible isoforms in mice. A splicing event at the C terminus results in tw o splice variants termed H-myomesin and S-myomesin, which represent the maj or myomesin species in heart and skeletal muscle of avian species, respecti vely. In contrast, in mammalian heart and skeletal muscle only S-myomesin i s expressed. In embryonic heart of birds and mammals, alternative splicing in the central part of the molecule gives rise to the isoform that we terme d EH-myomesin. It represents the major myomesin isoform at early embryonic stages of heart but is rapidly down-regulated around birth. Thus, the stric t developmental regulation of the EH-myomesin makes it an ideally suited ma rker for embryonic heart.