Transcriptional and Post-Transcriptional Regulation of Synaptic Acetylcholinesterase in Skeletal Muscle

myotubesProper muscle function depends upon the fine tuning of the different molecular components of the neuromuscular junction (NMJ). Synaptic acetylcholinesterase (AChE) is responsible for rapidly terminating neurotransmission. Neuroscientists in the field have elucidated many aspects of synaptic AChE structure, function, and localization during the last 75 years. Nevertheless, how the enzyme is regulated and targeted to the NMJ is not completely understood. In skeletal muscle the synaptic AChE form derives from two separate genes encoding the catalytic and the collagenic tail (ColQ) subunits respectively. ColQ-AChE expression is regulated by muscle activity; however, how this regulation takes place remains poorly understood. We found that over or down-regulation of ColQ is sufficient to change the levels of AChE activity by promoting assembly of higher order oligomeric forms including the collagen-tailed forms. Furthermore, when peptides containing the Proline Rich Attachment Domain (PRAD), the region of ColQ that interacts with the AChE, are fed to muscle cells or cell lines expressing AChE, they are taken up by the cells and retrogradely transported to the endoplasmic reticulum (ER)/Golgi network where they induce assembly of newly synthesize AChE into tetramers. This results in an increase, as a consequence, in total cell associated AChE activity and active tetramer secretion, making synthetic PRAD peptides potential candidates for the treatment of organophosphate pesticides and nerve gas poisoning. To study the developmental regulation of ColQ-AChE we determined the levels of ColQ and ColQ mRNA in primary quail muscle cells in culture and as a function of muscle activity. Surprisingly, we found dissociation between transcription and translation of ColQ from its assembly into ColQ-AChE indicating the importance of posttranslational controls in the regulation of AChE folding and assembly. Furthermore, we found that the vast majority of the ColQ molecules in QMCs are not assembled into ColQ-AChE, suggesting that they can have alternative function(s). Finally, we found that the levels of ER molecular chaperones calnexin, calreticulin, and particularly protein disulfide isomerase are regulated by muscle activity and they correlate with the levels of ColQ-AChE. More importantly, our results suggest that newly synthesized proteins compete for chaperone assistance during the folding process