Biogenesis of acetylcholinesterase is impaired, although its mRNA level remains normal, in the glucocorticoid-treated rat skeletal muscle

Acetylcholinesterase (AChE) is responsible for the hydrolysis of acetylcholine in the neuromuscular junction and other cholinergic synapses. Insight into the mechanisms controlling AChE expression in skeletal muscle is important for understanding formation, plasticity, and various dysfunctions of the neuromuscular junction. We have investigated the mechanisms responsible for the decreased AChE activity in the fast rat sternomastoideus muscle after chronic glucocorticoid treatment. Under such conditions fast skeletal muscles become atrophic and loose 30-40% of their AChE activity, In order to establish at which level synthesis of AChE is affected by glucocorticoids, we studied the effects of chronic dexamethasone treatment at both AChE mRNA and mature enzyme levels. Reduced rate of AChE recovery after subtotal irreversible AChE inhibition was observed during the first week of dexamethasone treatment, but not later. Statistical analyses of four independent northern blots revealed unchanged AChE mRNA levels. At the same time, we observed more than 60% decrease in the (G(1)+G(2))/A(12) ratio of molecular forms at the expense of G forms. It has been generally accepted that globular G(1) and G(2) molecular forms are synthesized in the rough endoplasmic reticulum as precursors of asymmetric (A) AChE forms, assembled in the Golgi apparatus. Reduced levels of G(1) and G(2) AChE forms. in combination with unchanged AChE mRNA, are therefore consistent with the reports demonstrating that glucocorticoids downregulate muscle protein synthesis at the translational level. Our findings support but not entirely prove the concept that impaired translation and/or posttranslational control are the primary cause of decreased AChE activity in the glucocorticoid-treated muscle.