The<i>Drosophila</i>β-Amyloid Precursor Protein Homolog Promotes Synapse Differentiation at the Neuromuscular Junction

TLDR

APP’s normal function is poorly understood, despite its link to Alzheimer’s disease. The study aimed to elucidate APPL’s role in synapse differentiation by manipulating its structure and levels in Drosophila motoneurons and examining hyperexcitable mutants. The authors expressed wild‑type and mutant APPL in motoneurons and assessed hyperexcitable mutants to determine APPL’s effect on neuromuscular junction synapse formation. Overexpression of APPL dramatically increased synaptic bouton number with normal components, while an Appl null mutant reduced boutons; cytoplasmic domain motifs GYENPTY and a G_o‑binding site mediate these effects, and hyperexcitable mutants partially suppress APPL‑dependent synapse formation, indicating APPL cooperates with activity‑dependent mechanisms to regulate synaptic structure.

Abstract

Although abnormal processing of β-amyloid precursor protein (APP) has been implicated in the pathogenic cascade leading to Alzheimer's disease, the normal function of this protein is poorly understood. To gain insight into APP function, we used a molecular-genetic approach to manipulate the structure and levels of the <i>Drosophila</i>APP homolog APPL. Wild-type and mutant forms of APPL were expressed in motoneurons to determine the effect of APPL at the neuromuscular junction (NMJ). We show that APPL was transported to motor axons and that its overexpression caused a dramatic increase in synaptic bouton number and changes in synapse structure. In an<i>Appl</i> null mutant, a decrease in the number of boutons was found. Examination of NMJs in larvae overexpressing APPL revealed that the extra boutons had normal synaptic components and thus were likely to form functional synaptic contacts. Deletion analysis demonstrated that APPL sequences responsible for synaptic alteration reside in the cytoplasmic domain, at the internalization sequence GYENPTY and a putative G_o-protein binding site. To determine the likely mechanisms underlying APPL-dependent synapse formation, hyperexcitable mutants, which also alter synaptic growth at the NMJ, were examined. These mutants with elevated neuronal activity changed the distribution of APPL at synapses and partially suppressed APPL-dependent synapse formation. We propose a model by which APPL, in conjunction with activity-dependent mechanisms, regulates synaptic structure and number.