Mushroom body neuronal remodelling is necessary for short-term but not for long-term courtship memory in Drosophila


Thomas Preat and Jean-Maurice Dura, as above.
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The remodelling of neurons during their development is considered necessary for their normal function. One fundamental mechanism involved in this remodelling process in both vertebrates and invertebrates is axon pruning. A well-documented case of such neuronal remodelling is the developmental axon pruning of mushroom body γ neurons that occurs during metamorphosis in Drosophila. The γ neurons undergo pruning of larval-specific dendrites and axons at metamorphosis, followed by their regrowth as adult-specific dendrites and axons. We recently revealed a molecular cascade required for this pruning. The nuclear receptor ftz-f1 activates the expression of the steroid hormone receptor EcR-B1, a key component for γ remodelling, and represses expression of Hr39, an ftz-f1 homologous gene. If ectopically expressed in the γ neurons, HR39 inhibits normal pruning, probably by competing with endogenous FTZ-F1, which results in decreased EcR-B1 expression. The mushroom bodies are a bilaterally symmetric structure in the larval and adult brain and are involved in the processing of different types of olfactory memory. How memory is affected in pruning-deficient adult flies that possess larval-stage neuronal circuitry will help to explain the functional role of neuron remodelling. Flies overexpressing Hr39 are viable as adults and make it possible to assess the requirement for wild-type mushroom body pruning in memory. While blocking mushroom body neuron remodelling impaired memory after short-term courtship conditioning, long-term memory was normal. These results show that larval pruning is necessary for adult memory and that expression of courtship short-term memory and long-term memory may be parallel and independent.