DIFFERENTIAL EXPRESSION OF AN ENDOGENOUS MANNOSE-BINDING PROTEIN R1 DURING MUSCLE DEVELOPMENT AND REGENERATION DELINEATING ITS ROLE IN MYOBLAST FUSION

The role of the endogenous brain carbohydrate-binding protein R1 in mu scle cell development and regeneration was analysed both in vivo and i n vitro. In vivo, R1 was developmentally regulated, with an embryonic 65 000 subunit and a neonatal 67 000 subunit, being replaced progressi vely by a 135 000 adult form. Lectin R1 was intracellularly localized at birth and in the prenatal period. During development and at the tim e of myoblast fusion, the antigen was progressively found at the surfa ce, where it remained at low levels in the adult. In vitro, in pure my oblast cultures, only the embryonic form was present. The ultrastructu ral studies indicated that the lectin could participate in the membran e fusion process during myoblast fusion. The specific role in myoblast fusion, derived from the ultrastructural localization of R1, was evid enced by a strong inhibitory effect of anti-R1 Fab fragments (10-100 m u g/ml), relative to control Fab fragments. In vivo, the embryonic sub unit pattern and subcellular distribution of R1 reappeared in muscle c ells after lesion of the adult muscle. This suggested that, as observe d in vitro, R1 participated in vivo in the phenomenon of myoblast fusi on. similar modifications in subunit expression were observed in muscl es after denervation (the embryonic form of lectin R1 reappearing afte r lesion), suggesting that R1 could be involved in the process of neur omuscular junction formation. Thus, it is proposed that the carbohydra te-binding protein R1 is an important recognition molecule for the for mation of myotubes. Its potential involvement in a recognition process between axons and muscle cells during neuromuscular junction formatio n is discussed.