Regulation of Nicotinic Acetylcholine Receptor Stability at the Mouse Neuromuscular Junction.

Abstract

The effectiveness of synaptic transmission at most mammalian synapses depends largely on the maintenance of a high density of postsynaptic receptors. In a mature synapse, this density is highly dynamic and can be regulated by several factors including synaptic activity, post-translational modifications of receptors, and scaffold proteins. In my thesis work, I focused on the regulation of AChR clustering, which is the hallmark of a neuromuscular junction, a well characterized cholinergic synapse between the motor neuron and the skeletal muscle. Among several pathways, I first focused on the role of alpha-syntrophin (syn), a member of the dystrophin glycoprotein complex (DGC), in the development and modulation of nAChR dynamics of the mouse NMJ. Using syn knock-out mice, I showed that syn is not required for synapse formation, but it is essential for synapse maturation. Particularly, I demonstrated that during the maturation of synapses, the integrity of the postsynaptic apparatus is altered, the turnover rate of AChRs increases significantly, and the number/density of AChRs is impaired. The synaptic alterations observed in this mouse mutant were explained by the loss of tyrosine phosphorylated alpha-dystrobrevin (dbn). Interestingly, when GFP-dbn1 was electroporated into sternomastoid muscles of syn mutant, most of synaptic abnormalities were partially restored. In the second part of my thesis work, I investigated the role of serine/threonine kinases, particularly PKC and PKA on the regulation of nAChR trafficking. We found that PKC accelerates nAChR removal and inhibits recycling at the NMJ, while PKA has the opposite effect. Finally, I begin to address the role of the Wnt/beta-catenin pathway in the adult NMJ, and we show that beta-catenin interacts with the DGC in mature synapses, via rapsyn. Taken together, these results provide new insights into the cellular and molecular underlying signaling of the regulation of nAChR trafficking and dynamics.