Professor Scott Small, Columbia University, USA
Roles of the endo-lysosomal system in dementia initiation and pathogenesis
Professor Rick Livesey, UCL/Great Ormond Street Institute of Child Health, UK
Endolysosome dysfunction is emerging as an important pathogenic process in many neurodegenerative diseases, including Alzheimer’s disease, frontotemporal dementia, and lysosomal storage disorders. Using human stem cell-derived neurons, the group has found that autosomal dominant mutations in PSEN1, APP and SORL1 causal for Alzheimer’s disease all cause defects in human neuronal endolysosome function, compromising the degradative phase of autophagy. Furthermore, the proteins encoded by all three genes are localised to the endolysosomal network and act in a single pathway to regulate endolysosome function. Endolysosome dysfunction occurs in young neurons, prior to any protein aggregation, affecting endosome size and number, lysosome transport to the neuronal cell body, lysosome maturation (reflected in protease activation) and lysosome function in the degradative phase of autophagy, indicating that endolysosome dysfunction is a primary pathogenic effect of monogenic AD mutations in these genes. Tauopathies, including AD, share a feature of apparent spatial spreading through neural networks, from affected to healthy neurons. Whole genome CRISPR screens in human neurons were used to identify genes and pathways required for neuronal uptake of both monomeric and aggregated tau. In addition to key surface receptors and receptor-mediated endocytosis, those screens found that disruption of different aspects of intracellular vesicular trafficking alters tau uptake, pointing to roles for the endolysosomal system in both neurodegenerative disease initiation and progression.
Dynein promotes retrograde transport of late endosomes in dendrites and is required for degradation of somatodendritic cargos
Dr Bettina Winckler, University of Virginia Medical School, USA
Unlike axons, dendrites in vertebrate neurons have microtubule arrays with mixed polarities. The question of how directional transport is organised in dendrites is thus a longstanding one. Dynein has been reported as either an anterograde or retrograde motor in dendrites. The group has studied endosomal transport in dendrites, in particular the trafficking of the neuronal dendritic membrane proteins NSG1/2. They previously found that NSG1/2 was endocytosed into dendritic early endosomes and then rapidly transported via late endosomes to the soma where most of the lysosomes reside. When Rab7 function was impaired by expression of Rab7-T22N, late endosomes carrying endocytosed NSG1/2 stopped moving and accumulated high levels of NSG1/2 throughout dendrites. The group now asks if movement of late endosomes towards the soma requires kinesin or dynein motors. Since the retrograde transport and subsequent degradation of NSG1/2 depended on Rab7, they first expressed two Rab7 effectors, FYCO1 which recruits kinesin and moves lysosomes to the cell periphery in fibroblasts, and RILP which recruits dynein and moves lysosomes to the cell centre in fibroblasts. RILP overexpression leads to massive re-localisation of late endosomes to the soma whereas FYCO1 overexpression does not. This observation suggests that dynein promotes retrograde movement of late endosomes in dendrites. In order to test this hypothesis more directly, the group carried out live imaging of Rab7-mCherry. Overexpression of DIC2-GFP leads to an increase in net retrograde movements of Rab7-positive compartments, but anterograde discursions are frequent as well. In addition, they live imaged Rab7-mCherry in the presence of dynein inhibitors (CC1-GFP, ciliobrevin). Both of these approaches largely halt Rab7 movements in dendrites. The group then determined if degradation of NSG1/2 was slowed if dynein function is disrupted with CC1 overexpression. They find that NSG1/2 accumulates in dispersed compartments in dendrites and degradation is slowed. In addition, they find reduced accumulation of acidified (Lysotracker-positive) compartments in somata of CC1-GFP expressing neurons. Lastly, DQ-BSA conversion to red (a degradative tracer) is reduced in the somata of CC1-GFP expressing neurons. These data show that late endosomes use dynein motors for net motility to the soma and thus use dendritic plus end-out microtubules as their tracks for motility.
Endosomal trafficking is required for the normal maturation of the Alzheimer’s-associated protein SORLA
Professor Olav Andersen, Aarhus University, Denmark
SORL1 is among the most significant genetic factors that affect development of Alzheimer’s disease (AD). This gene encodes a sorting receptor, SORLA, involved in trafficking of multiple different cargoes between cell surface and golgi/endosomal compartments.
SORLA undegoes posttranslational modifications and maturation with ultimate ectodomain shedding, however knowledge of these processes remains limited. Here it is demonstrated that SORLA exists in two forms at the plasma membrane, an immature and a mature form, characterised by distinct N-glycosylation profiles. The mature receptor form is shed from the cell surface, whereas immature form of sorLA does not undergo shedding. Conversion of the immature to the mature N-glycan profile relies on endocytosis and recycling of the receptor to the cell surface by a retromer-dependent endosomal trafficking pathway.
Understanding the maturation process of SORLA has implications for assessment of genetic SORL1 variants identified in AD patients, as mutations that cause misfolding or trafficking dysfunction are likely to impair maturation. Therefore, the data presented point to impaired maturation as a signature of AD-associated SORLA dysfunction. This can be utilised for future functional studies to assess pathogenicity of genetic SORL1 variants.
Selected short presentation
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