Turnover of Acetylcholine Receptors: Mechanisms of Regulation

The synthesis, insertion and degradation of acetylcholine receptors (AChRs) of skeletal muscle cells is closely regulated both by the muscle cells and by the motor nerves that supply them. The goal of this project is to elucidate the mechanisms of regulation of the AChRs, both at the neuromuscular junction and at extrajunctional regions. We have used Iodine 125-alpha-bungarotoxon (alpha-BuTx) as a label to follow the metabolic turnover of AChRs both in vivo and in tissue culture. Recent experiments have used cDNA that specifically binds to the mRNA for the alpha-subunit of the AChR to measure transcription of the gene, as well. To date, our experiments have shown: That AChRs at normally innervated rodent neuromuscular junctions turn over at two different rates; The rapidly turning over AChRs (RTOs) are synthesized and inserted into the junction at an extremely rapid rate, sufficient to maintain a steady state; This rapid rate of resynthesis can account for the rapid recovery of neuromuscular junctions after blockade with irreversible blocking agents such as alpha-BuTx; A proportion of the RTOs serve as precursors for the stable AChRs; Synthesis of extrajunctional AChRs in cultured skeletal muscle can be down-regulated by the cations lithium, calcium, and sodium; Denervation results in a marked increase in expression of the gene for the alpha-subunit for AChR, in skeletal muscle. (kt)