Non-autonomous Vnd acts with autonomous Appl during mushroom body axon growth in Drosophila

The Amyloid Precursor Protein (APP) is associated with Alzheimer’s disease. Appl is the single Drosophila APP ortholog and is expressed in all neurons throughout development. Appl was previously shown to cell-autonomously modulate axon outgrowth in the mushroom bodies (MBs), the fly olfactory memory center. However, we found that Appl d , the only reported null allele, affects the normal function of vnd , the gene just proximal to Appl . To decipher developmental defects specifically due to a loss of only Appl function, we generated a precise Appl null allele ( Appl C2 . 1 ) by CRISPR/Cas9 genomic engineering. With Appl C2 . 1 , we confirmed the partial requirement for Appl in MB axon outgrowth. We also produced new CRISPR vnd alleles removing either vnd-B or vnd-A function. We report here that vnd is also required for MB β-branch axon outgrowth and to a much greater extent than Appl itself. Moreover, vnd is expressed in neurons close to, but not within, the MB during development and is required non-cell-autonomously for MB axon outgrowth