Differential expression of proteins in muscle and electric organ, a muscle derivative

The electric organ of electric fish develops from a myogenic lineage. We have used immunohistochemistry and immunoblotting to determine which features of the muscle phenotype are retained and whether any new ones are expressed in mature electrocytes of the electric fish Sternopygus. The muscle-specific intermediate filament desmin was found throughout the electrocytes, and different desmin antibodies detected molecules with different subcellular distributions. Western blots confirm that these antibodies recognize a protein of MW = 53 kD, the molecular weight of desmin. Other muscle proteins were also present within electrocytes: Actin and sarcomeric alpha-actinin were found within the subsynaptic membrane beneath the plasmalemma of the electrocytes, and talin and acetylcholine receptors were detected both at the innervated posterior face and at the non-innervated anterior face. This was confirmed using rhodamine-conjugated alpha-bungarotoxin. Neither myosin heavy chain nor tropomyosin was present in electrocytes. Finally, we detected within electrocytes a type I acidic keratin that forms a filamentous meshwork within each cell. Immunoblots corroborate this result: A keratin-positive doublet of MW = 50 kD and 57 kD was found in both electrocytes and skin. Myosin, actin, talin, tropomyosin, desmin, alpha-actinin, and acetylcholine receptor, but not keratin, were all expressed in fish skeletal muscle fibers. Thus, electrocytes retain some muscle-specific proteins, do not express others, and in addition, express a non-muscle protein.