TRANSDIFFERENTIATION OF MUSCLE TO ELECTRIC ORGAN - REGULATION OF MUSCLE-SPECIFIC PROTEINS IS INDEPENDENT OF PATTERNED NERVE ACTIVITY

Transdifferentiation is the conversion of one differentiated cell type into another. The electric organ of fishes transdifferentiates from m uscle but little is known about how this occurs. To begin to address t his question, we studied the expression of muscle- and electrocyte-spe cific proteins with immunohistochemistry during regeneration of the el ectric organ. In the early stages of regeneration, a blastema forms. B lastemal cells cluster, express desmin, fuse into myotubes, and then e xpress a-actinin, tropomyosin, and myosin. Myotubes in the periphery o f the blastema continue to differentiate as muscle; those in the cente r grow in size, probably by fusing with each other, and lose their sar comeres as they become electrocytes. Tropomyosin is rapidly down-regul ated while desmin, alpha-actinin, and myosin continue to be diffusely expressed in newly formed electrocytes despite the absence of organize d sarcomeres. During this time an isoform of keratin that is a marker for mature electrocytes is expressed. One week later, the immunoreacti vities of myosin disappears and cu-actinin weakens, while that of desm in and keratin remain strong. Since nerve fibers grow into the blastem a preceding the appearance of any differentiated cells, we tested whet her the highly rhythmic nerve activity associated with electromotor in put plays a role in transdifferentiation and found that electrocytes d evelop normally in the absence of electromotor neuron activity. (C) 19 97 Academic Press.