Nicotinic synapse formation between neurons: Novel roles for cell adhesion molecules

Nicotinic cholinergic synapses between neurons are thought to be involved in numerous aspects of nervous system development, function, and degeneration. Despite their importance, almost nothing is known about how these neural connections are formed, modulated, and maintained. To address these issues the chick ciliary ganglion (CG), a central component of a classic nicotinic pathway, was used to identify and characterize some of the key transmembrane molecules responsible for formation of interneuronal nicotinic synapses.; Immunostaining and biochemistry were used to characterize the temporal and spatial expression patterns of a recently discovered family of cell adhesion molecules (CAMs) in the CG, protocadherin-alphas (pcdh-alphas). Furthermore, intracellular binding interactions for pcdh-alphas in the CG were defined using a yeast-two-hybrid screen and immunoprecipitations. Previous reports have suggested a role for pcdh-alphas in synaptic specificity during target cell selection, and the data presented here corroborates this hypothesis.; Several other CAMs have been reported to play more central roles in directing synapse formation in glutamatergic pathways. Using complex cell culture assays to dissect aspects of synaptogenesis this work shows that the IgCAM L1 is necessary and sufficient to accumulate and align presynaptic specializations in cholinergic neurons. Further examination revealed that a transsynaptic homophilic interaction between distinct L1 populations likely mediates this affect, and also facilitates nAChR signaling to the nucleus.; The presence of two other synaptogenic CAMs in the CG, neuroligin and SynCAM, is described here. To address the relative roles of L1, neuroligin, and SynCAM in CG neurons a novel electroporation protocol was generated to transfect CG neurons in situ. This technique revealed that L1 and NL are each required for normal presynaptic organization in the ganglion. Furthermore, these two CAMs are not redundant in this function, yet act at the same synapse population and in the same functional pathway.; This work identifies novel functions for CAMs beyond simple adhesion. The results also suggest an emerging theme of multipotency of CAM function, as well as the existence of numerous synaptic CAMs on the same cholinergic neurons. These conclusions suggest that CAMs play crucial, inductive, and complex roles in the formation of cholinergic nicotinic synapses between neurons.