NEURONAL CELL EXPRESSION OF INSERTED ISOFORMS OF VERTEBRATE NONMUSCLE MYOSIN HEAVY-CHAIN II-B

Previous work has demonstrated that unique isoforms of nonmuscle myosin heavy chain II-B (MHC-B) are expressed in chicken and human neuronal cells (Takahashi, M., Kawamoto, S., and Adelstein, R. S. (1992) J. Biol. Chem. 267, 17864-17871). These isoforms, which appear to be generated by alternative splicing of pre-mRNA, differ from the MHC-B isoform present in a large number of nonmuscle cells in that they contain inserted cassettes of amino acids near the ATP binding region and/or near the actin binding region. The insert near the ATP binding region begins after amino acid 211 and consists of either 10 or 16 amino acids. The insert nc ar the actin binding region begins after amino acid 621 and consists of 21 amino acids. Using a variety of techniques, we have studied the distribution and expression of the inserted MHC-B isoforms. In the developing chick;en brain, mRNA encoding the 10-amino acid insert gradually increases after embryonic day 4, peaks in the 10-14-day embryo, and then declines. In contrast, the mRNA encoding the 21-amino acid insert appears just before birth and is abundantly expressed in the adult chicken cerebellum. There is a marked species difference between the distribution of the inserted isoforms in adult tissues. The mRNA encoding MHC-B containing the 10-amino acid insert near the ATP binding region is expressed at low levels in the adult chicken brain, but makes up most of the MHC-B mRNA expressed in the human cerebrum and approximately 90% of MHC-B in the human retina. It is also expressed in neuronal cell lines. The mRNA encoding MHC-B containing the 21-amino acid insert is abundantly expressed in the chicken cerebellum and human cerebrum, but is absent from the retina and cell lines, Employing human retinoblastoma (Y-79) and neuroblastoma (SK-N-SH) cell lines, an increase in expression of mRNA encoding the 10-amino acid inserted isoform was seen following treatment by a number of agonists or by serum deprivation. In each case, expression of the inserted MHC-B isoform correlated with cell differentiation (neuronal phenotype) and inhibition of cell division. Using a rat pheochromocytoma cell line (PC12), we found that prior to treatment with nerve growth factor (NGF), there was no evidence for either inserted isoform, although noninserted MHC-B was present. NGF treatment resulted in the appearance of mRNA encoding MHC-B containing the 10-amino acid insert, concomitant with neurite outgrowth. Both the inserted isoform and neurites disappeared following withdrawal of NGF, showing that the appearance and disappearance of the neurites is coupled to the inclusion and exclusion of this MHC-B insert.