The two myostatin genes of Atlantic salmon (Salmo salar) are expressed in a variety of tissues

Two myostatin isoforms were identified in Atlantic salmon (Salmo salar) by RT-PCR, and genomic sequences encoding this negative muscle growth factor w ere for the first time isolated from a nonmammalian species. Salmon myostat in isoform I is transcribed in white skeletal muscle as a 2346-nucleotide m RNA species that encodes a precursor protein of 373 amino acids. Salmon myo statin I shows 93% sequence identity with isoform II which was isolated fro m white muscle as a partial cDNA sequence of 1409 nucleotides. In contrast to the restricted gene expression of myostatin in mammals, salmon myostatin I and II mRNAs were identified by RT-PCR in multiple tissues, including wh ite muscle, intestine, brain, gills, tongue and eye. In addition, isoform I mRNA was found in red skeletal muscle, heart, spleen, and ovarian tissue. Using polyclonal antibodies against both isoforms, a 55-kDa precursor prote in was detected by Western blot analysis in the red and white skeletal musc le, heart, intestine, and brain. Immunoreactive peptides of 35-40 kDa were identified in the gills, tongue, spleen, and head kidney, while the 25-kDa mature myostatin was found in the eye and serum, and in vitro expressed in rabbit reticulocyte lysate. Salmon myostatin was immunohistochemically loca lized in the sarcoplasma of red and white muscle fibres, in intestinal epit helial cells, at the basis of the branchial primary lamellae, and in odonto blasts and ameloblasts of the tongue teeth. The results indicate that the r ole of fish myostatin may not be restricted to muscle growth regulation, bu t may have additional functions similar to the growth/differentiation facto r-11 in mammals.