Fibre type-specific expression patterns of myosin heavy chain genes in adult torafugu Takifugu rubripes muscles

Comprehensive in silico studies, based on the total fugu genome database, which was the first to appear in fish, revealed that torafugu Takifugu rubripes contains 20 sarcomeric myosin heavy chain (MYH) genes (MYH genes) (Ikeda et al., 2007). The present study was undertaken to identify MYH genes that would be expressed in adult muscles. In total, seven MYH genes were found by screening cDNA clone libraries constructed from fast, slow and cardiac muscles. Three MYH genes, fast-type MYHM86-1, slow-type MYHM8248 and slow/cardiac-type MYHM880, were cloned exclusively from fast, slow and cardiac muscles, respectively. Northern blot hybridization substantiated their specific expression, with the exception of MYHM880. In contrast, transcripts of fast-type MYHM2528-1 and MYHM1034 were found in both fast and slow muscles as revealed by cDNA clone library and northern blot techniques. This result was supported by in situ hybridization analysis using specific RNA probes, where transcripts of fast-type MYHM2528-1 were expressed in fast fibres with small diameters as well as in fibres of superficial slow muscle with large diameters adjacent to fast muscle. Transcripts of fast-type MYHM86-1 were expressed in all fast fibres with different diameters, whereas transcripts of slow-type MYHM8248 were restricted to fibres with small diameters located in a superficial part of slow muscle. Interestingly, histochemical analyses showed that fast fibres with small diameters and slow fibres with large diameters both contained acid-stable myofibrillar ATPase, suggesting that these fibres have similar functions, possibly in the generation of muscle fibres irrespective of their fibre types.