Three cholinergic neuroblastoma hybrid cell lines that form few synapses on myotubes are deficient in acetylcholine receptor aggregation molecules and large dense core vesicles.

Three neuroblastoma X glioma hybrid cell lines that synthesize and release acetylcholine but that form few or no synapses with cultured skeletal muscle cells lack two characteristics of neuroblastoma or hybrid cell lines that do form many synapses with myotubes: large dense core vesicles and the ability to increase the number of nicotinic acetylcholine receptor clusters on co-cultured myotube membranes. Functional synapse formation on myotubes was increased by co-culturing myotubes and cells from one of the defective lines with neuroblastoma cells that induce myotube acetylcholine receptor aggregation but which synthesize little or no acetylcholine.

Direct observation of the rapid aggregation of acetylcholine receptors on identified cultured myotubes after exposure to embryonic brain extract.

We have directly observed the redistribution of acetylcholine receptors (AChR) on the surface of cultured myotubes, induced by a soluble brain extract. The AChR were fluorescently labeled with rhodamine-conjugated alpha-bungarotoxin and viewed under low incident illumination with a video image intensification system. The results of our sequential observations indicate that AChR aggregates can be assembled rapidly (30-120 min) from mobile, diffuse AChR. This assembly was characterized by the initial formation of microaggregates (less than 1 micron diameter) that increased in number and coalesced or grew to form larger aggregates. The redistribution of fluorescently labeled AChR was completely inhibited by illumination of cells at levels used for conventional fluorescence micrography and could be observed only by using low light levels.

Characterization and partial purification of a neuronal factor which increases acetylcholine receptor aggregation on cultured muscle cells.

Medium conditioned by NG108-15 neuroblastoma x glioma hybrid cells contains a factor which increases the number of acetylcholine receptor (AChR) aggregates on cultured myotubes. Protease digestion indicates that the AChR aggregation factor is a protein, and the molecular weight is from 150,000 to 250,000 daltons as estimated by ultrafiltration and gel filtration. Preparative isoelectrofocusing indicates that the aggregation factor has a pI of about 4.7. The factor is found in the soluble cytoplasmic fraction but not in the plasma membrane fraction of NG108-15 cells. Aggregation activity is not detected in the cytoplasm of liver cells or in the cytoplasm of C6BU-1 glioma cells. A possible developmental role for the aggregation factor is suggested by its presence in embryonic rat brain but not in adult rat brain. AChR aggregation factors found in the cytoplasm or conditioned medium of NG108-15 cells or in the cytoplasmic fraction of fetal brain have similar molecular weights and isoelectric points.