EXPRESSION OF THE EXTRACELLULAR FATTY-ACID-BINDING PROTEIN (EX-FABP) DURING MUSCLE-FIBER FORMATION IN-VIVO AND IN-VITRO

We report that Ex-FABP, an extracellular protein belonging to the lipo calin family and involved ill the extracellular transport of long-chai n fatty acids, is expressed in the forming myotubes both in vivo and i n vitro. The presence of the protein and of the mRNA was observed in n ewly formed myotubes at early stages of chick embryo development by im munohistochemistry and by in situ hybridization. At later stages of de velopment myofibers still expressed both the mRNA and the protein. Ex- FABP expression was observed also in the developing myocardium and the muscular layer of large blood vessels. In agreement with these findin gs, an initial expression of the mRNA and protein secretion by culture d chicken myoblasts were observed only after the onset of myoblast fus ion. Double-immunofluorescence staining of these cultured cells reveal ed that multinucleate myotubes were stained by antibodies directed aga inst both the Ex-FABP and the sarcomeric myosin, whereas immature myot ubes and single myoblasts were not. When added to cultured myoblasts, antibodies against the Ex-FABP induced a strong enhancement of the pro duction of the same protein. In all experiments some cell sufferance a nd a transient impairment of myotube formation were also observed. The finding that the continuous removal of the Ex-FABP from the culture m edium of myoblasts, due to the formation of immune complexes, resulted in an overproduction of the protein suggests at feedback (autocrine) control during myotube differentiation and maturation. We propose that the requirement for increased transport and metabolism of free fatty acid released from the membrane phospholipids and storage lipids, medi ated by Ex-FABP, may be essential during differentiation of multinucle ated myotubes or that an increased local demand of fatty acids and met abolites may act as a local hormone in tissues differentiating and und ergoing morphogenesis. (C) 1998 Academic Press.