Axonal extension and synaptic targeting are usually completed during early development, but the axonal length and synaptic integrity need to be actively maintained during later developmental stages and the adult life. Failure in the axonal length maintenance and the subsequent axonal degeneration have been associated with neurological disorders, but currently little is known about the genetic factors controlling this process. Here, we show that regulated intracellular levels of cAMP-dependent protein kinase A (PKA) are critical for the axon maintenance during the transition from the early to the later larval stages in the Drosophila class IV dendritic arborization (da) sensory neurons. Our data indicate that when the intracellular levels of PKA are increased via genetic manipulations, these peripheral neurons initially form synapses with wild-type appearance, at their predicted ventral nerve cord (VNC) target sites (in the first and second instar larval stages), but that their synapses disintegrate, and the axons retract and become fragmented in the subsequent larval stages (third larval stage). The affected axonal endings at the disintegrated synaptic sites still express the characteristic presynaptic and cytoskeletal markers such as Bruchpilot and Fascin, indicating that the synapse had been initially properly formed, but that it later lost its integrity. Finally, the phenotype is significantly more prominent in the axons of the neurons whose cell bodies are located in the posterior body segments. We propose that the reason for this is the fact that during the larval development the posterior neurons face a much greater challenge while trying to keep up with the fast-paced growth of the larval body, and that PKA is critical for this process. Our data reveal PKA as a novel factor in the axonal length and synapse integrity maintenance in sensory neurons. These results could be of help in understanding neurological disorders characterized by destabilized synapses.
Keywords: Drosophila; axon growth and maintenance; cAMP/PKA; synapse formation and stability.