SEQUENTIAL EXPRESSION OF NICOTINIC ACETYLCHOLINE RECEPTOR SUBUNITS SUPPORTS DEVELOPMENT AND PLASTICITY OF A DROSOPHILA CENTRAL SYNAPSE

Abstract

The central nervous system (CNS) of animals is arguably one of the most sophisticated instruments designed by nature, and one its cardinal components is the postsynaptic specialization. Decades of studies on the cholinergic neuromuscular junction and the central glutamatergic synapses in vertebrate organisms has informed us of just how many factors are at play during postsynaptic development. However, despite its importance, the central cholinergic synapse is one system lacking the same knowledge base as the above models. The thesis work presented here was designed with the aim of understanding if and how nicotinic acetylcholine receptor (nAchR) activity at the postsynapse is used by a developing neuron to shape the structural and functional properties of the synapse and its dendrite arbor during normal periods of maturation. To this end, we employed the ventral lateral neuron (LNv) as a cellular model for Drosophila CNS development. This small group of cells are second-order projection neurons which convey visual activity to higher brain centers and are also critical mediators of adjusting the fly’s internal circadian clock. We report how nAchRs not only play a role in LNv neurophysiology by the end of larval development but show how in fact they actively participate during the formation and refinement of the LNv postsynapse. Our transcriptomic, morphological and physiological approaches reveal that two functionally distinct nAchR subunit genes, Da1 and Da6, are preferentially expressed during separate periods of larval development. Here, young and immature LNvs are characterized by high Da6 expression which facilitates synaptic formation. As the animal grows, Da6 is downregulated and Da1 is upregulated, which is necessary for synaptic stabilization and maturation. We also expand the scope of our primary investigation by identifying promising candidate genes, including transcription factors, molecular chaperones and membrane-associated proteins, that are key to orchestrating the successive stages of nAchR expression, maturation and postsynaptic activity. In summary, our findings will work to clarify in vivo subunit-specific functionalities for the insect nAchR and illustrate how individual nAchR subunits in the CNS are coordinately regulated within a single cell through time to actively regulate distinct properties of the synapse during development.