Formation of ACh receptor clusters induced by positively charged latex beads

An early event in the formation of neuromuscular junctions in tissue cultures of neurones and muscle cells is an accumulation of acetylcholine receptors (AChRs) in the postsynaptic membrane^1-5. As well as direct neuronal contact, AChR cluster formation can also be induced by certain soluble factors extracted or released from neurones^6-8, by isolated basal lamina material⁹ or by a piece of degenerating nerve¹⁰. These experiments suggest that certain 'trophic factors' released by the neurones or the neuronal cell surface itself may cause the receptor clustering. On the other hand, stable AChR clusters are also present in pure muscle cultures without neuronal influence^1,10-13, which indicates that an interaction between the muscle and an exogenous cue existing on the substrate or in the medium may be sufficient to trigger the mechanism for the clustering of AChRs in the muscle. Here we report the effect of charged latex beads on the formation of AChR clusters in cultured muscle cells. Our results clearly indicate that AChR clusters form specifically at the contacts with polylysine-coated beads. Furthermore, these beads suppressed clusters that formed in the non-contact area before the introduction of the beads.