Different degradation rates of junctional and extrajunctional acetylcholine receptors of human muscle cultured in monolayer and innervated by fetal rat spinal cord neurons

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Abstract
It is well demonstrated that in intact animals the degradation rate of the junctional acetylcholine receptor (AChR) is significantly slower than that of the extrajunctional receptor. Such data, however, are not available for human AChRs because the required experimentation cannot be performed in humans. We have now studied the degradation rate of the junctional and extrajunctional AChRs, utilizing our tissue culture model, in which well-differentiated neuromuscular junctions (NMJs) form on human muscle cultured in monolayer and innervated long-term by fetal rat spinal cord neurons. Half-life of AChRs was studied by a method utilizing the autoradiography of 125I-αbungarotoxin and computerized video image analysis. Extrajunctional AChRs degraded with a half-life of 1.3 days whereas junctional AChRs degraded with a half-life of 3.5 days. Our studies demonstrate for the first time that in innervated cultured human muscle: 1. (a) the life span of human junctional AChR, is approximately 3 times longer than that of the extrajunctional AChR and 2. (b) the stability of human AChR is neuronally regulated. This system can now be applied to evaluate the influence of pharmacologie agents on the stability of human junctional AChR, which is of potential importance in the treatment of myasthenia gravis and other diseases of the NMJ.

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