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 muscle cell development and regeneration was analysed both in vivo and in vitro. In vivo, R1 was developmentally regulated, with an embryonic 65,00 subunit and a neonatal 67,000 subunit, being replaced progressively by a 135,000 adult form. Lectin R1 was intracellularly localized at birth and in the prenatal period. During development and at the time of myoblast fusion, the antigen was progressively found at the surface, where it remained at low levels in the adult. In vitro, in pure myoblast cultures, only the embryonic form was present. The ultrastructural studies indicated that the lectin could participate in the membrane fusion process during myoblast fusion. The specific role in myoblast fusion, derived from the ultrastructural localization of R1, was evidenced by a strong inhibitory effect of anti-R1 Fab fragments (10-100 micrograms/ml), relative to control Fab fragments. In vivo, the embryonic subunit pattern and subcellular distribution of R1 reappeared in muscle cells after lesion of the adult muscle. This suggested that, as observed in vitro, R1 participated in vivo in the phenomenon of myoblast fusion. Similar modifications in subunit expression were observed in muscles after denervation (the embryonic form of lectin R1 reappearing after lesion), suggesting that R1 could be involved in the process of neuromuscular junction formation. Thus, it is proposed that the carbohydrate-binding protein R1 is an important recognition molecule for the formation of myotubes. Its potential involvement in a recognition process between axons and muscle cells during neuromuscular junction formation is discussed.