Expression and function of the LIM homeodomain protein Apterous during embryonic brain development of Drosophila.

We analyzed the expression and function of the LIM-homeodomain transcription factor Apterous (Ap ) in embryonic brain development of Drosophila. Expression of Ap in the embryonic brain begins at early stage 12 and is subsequently found in approximately 200 protocerebral neurons and in 4 deutocerebral neurons. Brain glia do not express Ap. Most of the Ap-expressing neurons are interneurons and project their axons across the midline to the contralateral hemisphere; a smaller subset projects their axons into the ventral nerve cord. A few Ap-expressing neurons project to the ring gland, suggesting that they are neurosecretory cells. In ap loss-of-function mutants, some of the protocerebral and deutocerebral interneurons that express Ap in the wild type show axon pathfinding errors and fasciculation defects in the brain, notably in the fascicles of the brain commissure. In contrast, the interneurons that project to the ring gland do not appear to be affected in ap mutants. Thus, in brain development, Ap is required for correct axon guidance and fasciculation of interneurons, and Ap-expressing cells may also be involved in the brain neuroendocrine system.