James Warner Patrick

Protein synthesis and secretion in a myogenic cell line.

Abstract Myogenesis in a clonal myoblast cell line is accompanied by an increase in the specific activities of creatine phosphokinase and myokinase and in the rates of synthesis and accumulation of myosin heavy chain. Exponentially dividing myoblasts synthesize myosin heavy chain at a rate of about 1% of their rate of total protein synthesis; this rate increases 7-fold during the differentiation process. Both myoblasts and myotubes secrete a minimum of 12-soluble proteins. Although there is a quantitative change in the rates of appearance of five of these proteins during myogenesis, no qualitative changes in the profile of the secreted proteins are detected. Three of the secreted proteins share several properties of soluble collagen molecules. Basal laminae and polymerized collagen fibrils are associated with myotubes, but not with exponentially dividing myoblasts.

Differentiation and Interaction of Clonal Cell Lines of Nerve and Muscle

Since some of the basic problems in neurobiology are ultimately biochemical in nature, it follows that clonal populations of neuronal cells will be required to define the underlying mechanisms. Initial progress in this direction was made by Harrison, who was able to maintain primary explants of nervous tissue for a limited time in vitro (Harrison, 1907). With the introduction of more sophisticated tissue culture technology, it became possible to observe nervous tissue in vitro for extended periods of time and to demonstrate functional interaction between individual cells within the population (Crain et al., 1968). Although the viability of these cultures was adequate, the problem of cell heterogeneity was yet to be overcome; two independent directions were taken in search of a solution. One was the fractionation of cell populations from whole brain (Roots and Johnston, 1964; Varon and Raiborn, 1968), and the other consisted of the use of neoplastic tissue as a relatively pure source of nerve cells (Murray and Stout, 1947). These approaches were, however, faced with the common difficulty that the neuronal cells in the population did not divide, and the various fractionation schemes were unable to yield homogeneous collections of cells. Since the requirement for homogeneous or clonal populations of cells necessarily demands cell division, methods which yield dividing nerve cells are clearly required. This problem is not unique to the nervous system but is common to the study of most differentiated functions in cell culture, for the maximally differentiated end cell usually does not divide in vivo (Grobstein, 1959). Perhaps the only way of circumventing this situation at present is by adapting neoplasms of differentiated cells or their immediate developmental precursors to continuous cell culture. Subsequent cloning then yields homogeneous populations of cells. This approach was successfully applied to the study of endocrine (Sato and Yasumura, 1966), hepatic (Thompson et al., 1966), and endoreticular (Cohn, 1967) cell function and has since been extended to many other cell types. Among these is a mouse neoplasm, C1300, of apparently neuronal origin which has been established in continuous cell culture and characterized as a neuroblastoma (Augusti-Tocco and Sato, 1969; Schubert et al., 1969). In addition to the above lines, Yaffe (1968) has established clonal myogenic cells, and it has recently been possible to induce and establish in clonal culture several more nerve cell lines (Carlisle et al., 1973). The availability of clonal neural and myogenic cell lines thus presents a unique opportunity to study the two major classes of electrically excitable cells.