Department of Biology and Volen National Center for Complex Systems, Brandeis University, Waltham, MA 022254; § Present address: Howard Hughes Medical Institute, Univeristy of California, San Franscisco, CA 94143.
Drosophila Appl gene encodes a transmembrane protein that is exclusively expressed in neurons. Amino acid comparisons show that APPL protein is a member of the APP-like family of proteins. Similar to mammalian APP-family proteins, APPL is synthesized as a transmembrane holoprotein and cleaved to release a large secreted amino-terminal domain. Using immunocytochemical methods, we have analyzed the distribution of APPL in the Drosophila central nervous system (CNS). Surprisingly, although APPL is present in all neuronal cell bodies, the neuropil shows stereotypic differential distribution. Double labeling experiments with different neuronal markers were used to distinguish between APPL associated with neuronal processes or extracellular matrix. The distribution of APPL protein produced from transgenes encoding wild type (APPL), secretion defective (APPLsd), and constitutively secreted (APPLs) forms was analyzed in an Appl-deficient background to determine which APPL form is associated with different neuropil regions. We found that APPLsd protein is enriched where APPL-immunoreactivity coincides with neuronal processes. In contrast, APPLs preferentially localizes to those parts of the neuropil that show a diffuse APPL signal that rarely colocalizes with processes, and thus appears to be a component of the extracellular matrix. These data indicate that proteolytic cleavage and trafficking of APPL is differentially regulated in different neuronal populations. Through metamorphosis, APPL is specially abundant in growing axons and in areas where synapses are forming. Interestingly, in adult brains APPL protein is enriched in the mushroom bodies and, to a lesser extent, in the central complex, structures involved in learning and memory. These findings suggest that transmembrane and secreted APPL forms have their respective roles and reinforce our previous finding that Appl is essential for optimal nervous system function. Alternative routes to understand the specific function of the two APPL forms, including ectopic expression through the GAL4/UAS system, are being explored.