Amyloid precursor protein ortholog Appl acts with Vnd during mushroom body axon growth in <i>Drosophila</i>

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 Appld, the only reported null allele, affects the normal function of vnd, the gene just proximal to Appl. To decipher developmental and memory defects specifically due to a loss of only Appl function, we generated a precise Appl null allele (ApplC2.1) by CRISPR/Cas9 genomic engineering. With ApplC2.1, we confirmed the partial contribution 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. It was previously shown that Appl knockdown in the MBs results in loss of memory following the association of odorants with electric shocks. Differently from Appld flies we found no defects of electric shock avoidance in ApplC2.1 flies, allowing us to test for memory. ApplC2.1 flies showed a complete loss of long-term memory which was fully rescued by MB-restricted expression of Appl+ only during the adult stage. Therefore, we demonstrate that the complete lack of Appl affects memory independently from structural developmental defects.