Nicotinic acetylcholine receptor at vertebrate motor endplates: Endocytosis, recycling, and degradation.

At vertebrate motor endplates, the conversion of nerve impulses into muscle contraction is initiated by binding of acetylcholine to its nicotinic receptor (nAChR) at the postsynapse. Efficiency and safety of this process are dependent on proper localization, density, and molecular composition of the receptors. To warrant this, intricate machineries regulating the turnover of nAChR are in place. They control and execute the processes of i) expression, ii) delivery to the postsynaptic membrane, iii) clustering at the plasma membrane, iv) endocytic retrieval, v) activity-dependent recycling, and vi) degradation of nAChR. Concentrating on aspects iv-vi, this review addresses the current status of techniques, concepts, and open questions on endocytosis, recycling, and degradation of nAChR. A picture is emerging, that shows connections between executing machineries and their regulators. The first group includes the actin cytoskeleton, myosin motor proteins, Rab G-proteins, and the autophagic cascade. The second group features protein kinases A and C, Cdk5, and CaMKII as well as other components like the E3-ligase MuRF1 and the membrane shaping regulator, SH3GLB1. Recent studies have started to shed light onto nerve inputs that appear to master the tuning of the postsynaptic protein trafficking apparatus and the expression of critical components for nAChR turnover.