Gamma secretase orthologs are required for lysosomal activity and autophagic degradation in Dictyostelium discoideum, independent of PSEN (presenilin) proteolytic function

Devdutt Sharma,Grant Otto,Eleanor C Warren,Philip Beesley,Jason S King,Robin S B Williams

doi:10.1080/15548627.2019.1586245

Devdutt Sharma, Grant Otto + Show 4 more

Open Access

https://doi.org/10.1080/15548627.2019.1586245

Copy DOI

Abstract

ABSTRACT Mutations in the γ-secretase complex are strongly associated with familial Alzheimer disease. Both proteolytic and non-proteolytic functions for the γ-secretase complex have been previously described in mammalian model organisms, but their relative contributions to disease pathology remain unclear. Here, we dissect the roles of orthologs of the γ-secretase components in the model system Dictyostelium, focusing on endocytosis, lysosomal activity and autophagy. In this model, we show that the orthologs of PSEN (psenA and psenB), Ncstn (nicastrin) and Aph-1 (gamma-secretase subunit Aph-1), are necessary for optimal fluid-phase uptake by macropinocytosis and in multicellular development under basic pH conditions. Disruption of either psenA/B or Aph-1 proteins also leads to disrupted phagosomal proteolysis as well as decreased autophagosomal acidification and autophagic flux. This indicates a general defect in lysosomal trafficking and degradation, which we show leads to the accumulation of ubiquitinated protein aggregates in cells lacking psenA/B and Aph-1 proteins. Importantly, we find that all the endocytic defects observed in Dictyostelium PSEN ortholog mutants can be fully rescued by proteolytically inactive Dictyostelium psenB and human PSEN1 proteins. Our data therefore demonstrates an evolutionarily conserved non-proteolytic role for presenilin, and γ-secretase component orthologs, in maintaining Dictyostelium lysosomal trafficking and autophagy. Abbreviations Atg8: autophagy protein 8a; Aph-1: gamma-secretase subunit Aph-1; crtA: calreticulin; ER: endoplasmic reticulum; GFP: green fluorescent protein; GSK3B: glycogen synthase kinase 3 beta; Ncstn: nicastrin; PSEN1: presenilin 1; psenA and psenB: Dictyostelium presenilin A and B; TRITC; tetramethylrhodamine isothiocyanate.

Highlights

Many studies have sought to explore a role for the γ-secretase complex in the pathology of Alzheimer disease [1,2,3,4]
Since the γ-secretase complex has been implicated in endocytosis in mammalian model organisms [5,6,42], we initially set out to investigate if orthologous components of this complex play roles in Dictyostelium macropinocytosis – the dominant form of endocytosis and fluid-phase uptake in this organism
Recent studies in mammalian models of Alzheimer disease have investigated the role of these components in vesicular trafficking [7,11,13,15,48,51,52,53], and suggest that dysfunction of these processes may contribute to the progression and pathology of Alzheimer disease

Summary

Introduction

Many studies have sought to explore a role for the γ-secretase complex (and PSEN [presenilin] proteins) in the pathology of Alzheimer disease [1,2,3,4]. One function of the mammalian complex (consisting of PSEN1 [presenilin 1], APH1 [aph-1 homolog, gamma-secretase subunit], NCSTN [nicastrin] and PSENEN/ PEN2 [presenilin enhancer, gamma-secretase subunit]) or PSEN1 proteins alone is to regulate endocytosis, lysosomal acidification, and autophagy [5,6,7] This role has been implicated in disease pathology, since mutations in PSEN1 proteins associated with familial Alzheimer disease result in elevated lysosomal pH, and aberrant autophagy in mouse models [7,8,9]. Protease-independent functions of the γ-secretase complex have been observed in evolutionarily diverse species, our understanding of the mechanistic significance of these functions and relevance to Alzheimer pathology remains limited

Methods

Results

Conclusion