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Brain extract induces synaptic characteristics in the basal lamina of cultured myotubes

The basal lamina (BL) that occupies the synaptic cleft of the skeletal neuromuscular junction is antigenically distinct from extrasynaptic muscle fiber BL, rich in acetylcholinesterase (AChE), and bears projections that form junctional folds in the postsynaptic membrane. We report here that these synapse-specific features of BL are all present at low levels in embryonic rat myotubes cultured without nerve, and that their levels are markedly increased by addition of a soluble extract from adult rat brain. Light and electron microscopic methods show that: (1) antibodies which bind preferentially to synaptic BL in vivo stain small, discrete patches of the myotube's BL; (2) AChE accumulates in patches on the myotube surface; and (3) myotube BL and membrane form invaginations that resemble junctional folds. Patches of BL that bear synaptic antigens, AChE, or folds usually overlie clusters of acetylcholine receptors in the plasma membrane. Myotubes treated with a brain extract bear 5 to 20 times more junctional folds and patches rich in acetylcholine receptors, synaptic BL antigens, and AChE than control myotubes. Together with a previous demonstration that electrical and/or contractile activity can modulate the amount and composition of myotube BL (Sanes, J.R., and J.C. Lawrence, Jr. (1983) Dev. Biol. 97: 123–136), these results suggest that nerves could regulate differentiation of muscle fiber BL by a combination of activity-dependent and -independent mechanisms.