Visualizing subcellular modules for learning and memory: Axonal bouton-like specializations in Kenyon cells as functional units
The question where and how learned information is physically manifested in the brain as memory traces is one of the most intensely studied topics in the neurosciences. The mushroom body of the insect brain represents a unique model system to address this question because neurons mediating associative odor learning, and their connectivity, are well described. Using Ca2+ imaging we could demonstrate that axonal boutons of Kenyon cells become independently modulated in the course of associative learning, implicating that these boutons represent functional units that collectively process and store learned information. This poses the question which molecular machineries confine neuronal signals, such as Ca2+ and cAMP, to axonal boutons. We propose to analyze this at the sub-cellular level using microscopic approaches. First, we will use high-resolution STED microcopy in combination with protein tagging to localize the cAMP-synthesizing adenylate cyclase rutabaga, the cAMP-degrading phosphodiesterase dunce, and the Ca2+-removing pump PMCA within axonal Kenyon cell boutons. Second, we will use functional in vivo Ca2+ and cAMP-imaging in axonal boutons of individual Kenyon cells to determine their spatio-temporal dynamics, their learning-induced change, and how this is affected by downregulating rutabaga, dunce and PMCA. Third, we will study how plasticity across synaptic axonal boutons mediates complex forms of associative learning. Overall, the project aims at uncovering how axonal boutons can act independently, and how they collectively acquire and store learned information.