Abstract

The role of the Amyloid Precursor Protein (APP) in the pathology of Alzheimer’s disease (AD) has been well studied. However, the normal function of APP in the nervous system is poorly understood. Here, we characterized the role of the Drosophila homolog (APPL) in the adult giant fiber (GF) neurons. We find that endogenous APPL is transported from the synapse to the soma in the adult. Live-imaging revealed that retrograde moving APPL vesicles co-traffic with L1-type cell adhesion molecule Neuroglian (Nrg). In APPL null mutants, stationary Nrg vesicles were increased along the axon, and the number of Nrg vesicles moving in retrograde but not anterograde direction was reduced. In contrast, trafficking of endo-lysosomal vesicles, which did not co-localize with APPL in GF axons, was not affected. This suggests that APPL loss of function does not generally disrupt axonal transport but that APPL has a selective role in the effectiveness of retrograde transport of proteins it co-traffics with. While the GF terminals of APPL loss of function animals exhibited pruning defects, APPL gain of function had no disruptive effect on GF morphology and function, or on retrograde axonal transport of Nrg. However, cell-autonomous developmental expression of a secretion-deficient form of APPL (APPL-SD), lacking the α-, β-, and, γ-secretase cleavage sites, resulted in progressive retraction of the GF terminals. Conditional expression of APPL-SD in mature GFs caused accumulation of Nrg in normal sized synaptic terminals, which was associated with severely reduced retrograde flux of Nrg labeled vesicles in the axons. Albeit β-secretase null mutants developed GF terminals they also exhibited Nrg accumulations. This suggests that cleavage defective APPL has a toxic effect on retrograde trafficking and that β-secretase cleavage has a function in Nrg sorting in endosomal compartments at the synapse. In summary, our results suggest a role for APPL and its proteolytic cleavage sites in retrograde trafficking, thus our findings are of relevance to the understanding of the endogenous role of APP as well as to the development of therapeutic treatments of Alzheimer’s disease.

Highlights

  • Amyloid Precursor Protein (APP) as well as APP-like protein 1 and 2 (APLP1 and APLP2) are a highly conserved family of type 1 transmembrane proteins

  • Analysis of giant fiber (GF) axons expressing V5-tagged amyloid protein precursor-like (APPL) and GFPtagged Nrg revealed that 39.4 ± 4.5% (N = 8) of all APPL vesicles co-localized with Nrg (Figure 1D)

  • Consistent with the observed accumulation of Nrg in GF terminals, we found that the retrograde flux of NrgGFP vesicles in axons imaged in the cervical connective was significantly reduced in APPL-SD expressing but not in APPL-WT expressing animals when compared to controls (Figures 5B,C and Supplementary Videos S8–S10)

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Summary

Introduction

Amyloid Precursor Protein (APP) as well as APP-like protein 1 and 2 (APLP1 and APLP2) are a highly conserved family of type 1 transmembrane proteins. Subsequent cleavage of C99 by γ-secretase/presenilin in the late endolysosomal compartments generates a cytoplasmic C-terminal fragment as well as small Aβ40 or Aβ42 peptides, which are the primary constituent of amyloid plaques, the pathological hallmark of AD (Masters et al, 1985; Sisodia, 1992; Takahashi et al, 2002; Vieira et al, 2010; Glenner and Wong, 2012; Rajendran and Annaert, 2012; van der Kant and Goldstein, 2015). Little is known about the physiological role of APP proteins, which is in part due to redundant functions of APP, APLP1, and APLP2 and that triple knockout mice are lethal (Herms et al, 2004; Wang et al, 2005; Priller et al, 2006; van der Kant and Goldstein, 2015). We further characterized the function of the sole Drosophila homolog of APP, the β-amyloid protein precursor-like (APPL), in an adult central nervous system neuron

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