ISOFORMS OF MYOSIN IN GROWING MUSCLES OF KY (KYPHOSCOLIOTIC) MICE

Kyphoscoliotic (ky) mice are spontaneous mutants of the BDL strain who se postural muscles atrophy during post-natal growth, resulting in ext ensive kryphoscoliosis in adult animals. At 21 days of age, the seven muscles examined were already well differentiated into fast, slow and mixed type on the basis of the proportions of their native myosin isof orms or their subunits. During post-natal growth, from 21 to 120 days of age, the normal pattern of myosin maturation was essentially respec ted by the ky mutation: fast muscles became faster, slow muscles becam e slower and mixed muscles specialized in both directions. However, th e post-natal increases of myosin heavy chain 2B and fast myosin light chain LC(3f) were depressed in ky muscles, whilst there was novel expr ession of slow myosin light chains, LC(1s) and LC(2s), in muscles whic h normally did not express them. Intermediate native myosin IM was abs ent in adult ky soleus, bur it increased in adult ky tibialis anterior . We conclude that the ky mutation depresses the normal post-natal tra nsition towards faster muscles and results in adult muscles whose myos in isoforms are generally shifted in a fast-to-slow direction.