How a neuron becomes polarized remains an outstanding question. Here, we report that selection of the future axon among neurites of a cultured hippocampal neuron requires the activity of growth factor receptor tyrosine kinase, phosphatidylinositol 3-kinase (PI 3-kinase), as well as atypical protein kinase C (aPKC). The PI 3-kinase activity, highly localized to the tip of the newly specified axon of stage 3 neurons, is essential for the proper subcellular localization of mPar3, the mammalian homolog of C. elegans polarity protein Par3. Polarized distribution of not only mPar3 but also mPar6 is important for axon formation; ectopic expression of mPar6 or mPar3, or just the N terminus of mPar3, leaves neurons with no axon specified. Thus, neuronal polarity is likely to be controlled by the mPar3/mPar6/aPKC complex and the PI 3-kinase signaling pathway, both serving evolutionarily conserved roles in specifying cell polarity.
The ability to select a better option from multiple acceptable ones is important for animals to optimize their resources. The mechanisms that underlie such decision-making processes are not well understood. We found that selection of egg-laying site in Drosophila melanogaster is a suitable system to probe the neural circuit that governs simple decision-making processes. First, Drosophila females pursue active probing of the environment before depositing each egg, apparently to evaluate site quality for every egg. Second, Drosophila females can either accept or reject a sucrose-containing medium, depending on the context. Last, communication of the "acceptability" of the sucrose-containing medium as an egg-laying option to the reproductive system depends on the function of a group of insulin-like peptide 7 (ILP7)-producing neurons. These findings suggest that selection of egg-laying site involves a simple decision-making process and provide an entry point toward a systematic dissection of this process.
SUMMARY
Mating induces changes in the receptivity and egg-laying behavior in Drosophila females, primarily due to a peptide pheromone called sex peptide which is transferred with the sperm into the female reproductive tract during copulation. Whereas sex peptide is generally believed to modulate fruitless-GAL4-expressing neurons in the central nervous system to produce behavioral changes, we found that six to eight sensory neurons on the reproductive tract labeled by both ppk-GAL4 and fruitless-GAL4 can sense sex peptide to control the induction of postmating behaviors. In these sensory neurons, sex peptide appears to act through Pertussis toxin-sensitive G proteins and suppression of protein kinase A activity to reduce synaptic output. Our results uncover a neuronal mechanism by which sex peptide exerts its control over reproductive behaviors in Drosophila females.
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